Facts at Your Fingertips

Fluid Flow

Department Editor: Kate Torzewski  DEFINITIONS

Newtonian fuid . A uid is known to be Newtonian when shear stresses associated with ow are directly proportional to the shear rate o the uid Power law fuid . A structural uid has a structure that orms in the undeormed state, but then breaks down as shear rate increases. Such a uid exhibits “power law” behavior at intermediate shear rates Bingham plastic fuid . A plastic is a material that exhibits a yield stress, meaning that it behaves as a solid below the stress level and as a uid above the stress level Laminar PiPe fLow For steady ow in a pipe (whether laminar or turbulent), a momentum balance on the uid gives the shear stress at any distance rom the pipe centerline. 

rx



r

2 L

  w

r

 R







∫ r 2  ˙ rx dr

(2)

Q



 D

(3)

128   L

It can be written in dimensionless orm in Equation (4) with the two terms defned in Equations (5) and (6).  f = 16 / N  Re (4) 2

 f  

 

 N Re

D5   

(5)

2

32  LQ 

4Q 

(6)

 D 

Power law . A uid that ollows the power law model obeys the relationship  τrx  = –µ(–γrx )n. This gives the ollowing equation. •

1 τ 

Q = π    ⎞ w ⎞   ⎠mR⎠

n

  ⎞ n ⎞  R 3n + 1⎠   ⎠

3 n+ 1

n

(7)

Equation (7) can be rearranged into the ollowing dimensionless orm. (8)  f  = 16 16 / NRe,pl  N Re, pl 

73 n

2

 Q

2 n

(9) n 2 n 4 3 n 3n  1  m  D  n   Bingham plastic . In this case, there is a solid-like “plug ow” region rom the pipe centerline (where  τrx  = 0) to the point where –   τrx  =  τ0  (that is, at r  = r 0  0  = R x  τ0 /  τw ). The result is a ow integral modifed rom Equation (2). For a Bingham plastic, – τrx  =  τ0  + µ∞(– γrx ). Using this expression and the modifed ow integral, the Buckingham-Reiner Equation (10) is ound.





DV  

NOmENclaTurE

(11) 11) 12) (12)

  2



D  0

13) (13)

2

 



TurbuLenT PiPe fLow Since most turbulent ows cannot be analyzed rom a purely theoretical perspective, data and generalized dimensionless correlations are used. Newtonian fuid. The riction actor or a Newtonian uid in turbulent ow is a unction o both N Re  Re  and the pipe relative roughness, ε/D , which can be read o the Moody diagram [5  [5 ]. ]. The turbulent part o the Moody diagram (or N Re  Re  > 4,000) is accurately represented by the Colebrook equation (14).

    D 1.255  1   4 log     f   3.7  NRe f  

(14) 14)

When N Re  Re is very large, the riction actor depends only on ε /D . This condition is noted with f T  T  as the “ully turbulent” riction actor in Equation (15). ε   D ⎤ 1 = – 4 log ⎡⎢ ⎣ 3.7 ⎥⎦  f T 

15) (15)

The Churchill Equation [2 [ 2] represents the entire Moody diagram,  rom laminar, through transition ow, to ully turbulent ow. ow. It is presented here as Equations (16), (17), and (18). 12    8   1     f   2      N Re    A  B1.5   

1

12

16) (16)

         1      A  2.457 ln 0.9     7   0.27               Re  D   

16

17) (17)

16

 37,530    B      N Re    

18) (18)

Power law . For a power-law uid, the riction actor depends only upon Equation (9) and the ow index, as represented by Equations (19)–(25) [3 [3].  f

=

(1 − α ) f  L

α 

+

[ fT

8

−

 f  L



 





f Tr

The equivalent dimensionless orm is given by Equations (11), (12) and (13).

f  f T 



fm  L

10 

1

 plN Re,

plc

(26) 26)

m

a

27) (27)

0.193 Re

 N 

 1 4. 7



1.7





1 0. 14 146 e

40,000  N Re



2.9 

10

5

N  He



28) (28) 29) (29)

References

e

(22) 22) (23) 23)



1

21) (21)

(1. 87 872.39 39 n)

1



fT m 





m 0.5

24) (24)

3

(10) 10)

Bingham plastic . For the Bingham plastic, f T  T is solely a unction o N Re  Re ∞ and N He  He , as represented by Equations (26)–(29).

a

0. 414 414 0. 757 757 n [5.24 n]

   N Re,

•

(19) 19)

]



1.79  104 N  Re, pl

1 4

a  Dimensionless parameter  A  Dimensionless parameter B  Dimensionless parameter D  Diameter, m   Fanning friction factor, dimensionless  L L  Laminar friction factor, dimensionless  T  T  Fully turbulent friction factor, dimensionless  Tr  Tr  Transition friction factor, dimensionless g  Gravitational acceleration, m/s 2 L  Length of cylinder or pipe, m m  Consistency coefficient, (N)(s)/m2 n  Power law fluid flow index, dimensionless NHe Hedstrom number, dimensionless NRe Reynolds Number, dimensionless NRe,pl Power law Reynolds Number, dimensionless NRe,plc Power law Reynolds Number at transition from laminar to turbulent flow, dimensionless Number, NRe∞ Bingham-plastic Reynolds Number, dimensionless P  Pressure, Pa Q   Volumetric flowrate, m 3/s r  Radial position in a pipe or a cylinder, m R  Pipe or cylinder radius, m V   Velocity, m/s z   Vertical elevation above a horizontal reference plane, m Dimensionless parameter α γrx  Shear rate in tube flow, s –1 ε  Wall roughness, m µ Newtonian viscosity, Pa–s viscosity, Pa–s µ∞ Bingham Plastic limiting viscosity, 3 ρ Density, kg/m τ0   Yield stress, N/m2 in x direction direction acting on r  τrx  Stress due to force in x  surface, N/m2 τw  Stress exerted by fluid on tube wall, N/m 2 Φ Flow potential, P + ρgz , Pa ∆Φ Ιncrease in flow potential, Pa

(20)

0.0682n [ N Re, pl ]

 fTr

+

1 8 −8

16  N Re, pl

 f T  

•

4  R  w 4   0   1   0     1  Q       4    3   w   3   w     



 N  He

•

4

 N Re

  N  He 1  N 4 He 1  1   7  6  N Re  3  f 3 N Re    



0

Newtonian fuid . For a Newtonian uid,  τrx  = µγrx , which gives the ollowing volumetric owrate, known as the Hagen-Poiseuille equation.

16

 N Re

(1)

 R

In Equation (1), Φ = P + P + ρgz . The volumetric owrate Q can Q can be related to the local shear rate by doing an integration by parts o Equation (2). Q

 f  

The value o N Re  Re  where transition rom laminar to turbulent ow occurs (N  ( N Re,plc  Re,plc ) is given by Equation (25).  NRe, plc  2, 100 100  875 875(1  n) 25) (25)

1.Darby, 1.Darby, R., Take Take the Mystery Out o Non-Newtonian Fluids, Chem. Eng., March 2001, pp. 66–73. 2.Churchil, 2.Churchil, S. W., Friction Factor Equation Spans all FluidFlow Regimes, Chem. Eng., Eng., November 1997, p. 91. 3.Darby, 3.Darby, R., and Chang, H. D., A Generalized Correlation or Friction Loss in Drag-reducing Polymer Solutions, AIChE J., J., 30, p. 274, 1984. 4.Darby, 4.Darby, R., and Chang, H. D., A Friction Factor Equation or Bingham Plastics, Slurries and Suspensions or all Fluid Flow Regimes, Chem. Eng., Eng., December 28, 1981, pp. 59–61. 5.Darby, 5.Darby, R., “Fluid Mechanics or Chemical Engineers,” Vol. 2, Marcel Dekker, New York, N.Y., 2001.

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‫د‬

‫ح‬

‫و»®» ‪¸‬ة‬ ‫و»®»‬ ‫ٌ‬ ‫‪±‬ف ‪‬م د‬ ‫؟‪ ²¼»²-‬‬ ‫؟‪¬ » ´±‬؟‪²¼»²-‬‬ ‫¼؟‪´±‬‬ ‫‪ ·² ´¾¸‬‬ ‫؛‪» ²¹¬¸ ±‬ش ‪‬م‬ ‫»‪ °®±¼«½¬ °·°‬؛‪±‬‬ ‫ش‬ ‫؟®¬ ‪¾»¬©»»²‬‬ ‫»‪½ »® ´·²‬؟®¬‬ ‫؟®¬‬ ‫®¬‬ ‫‪°‬؟‬ ‫‪¬‬؛ ‪ - ·²‬‬ ‫»ط ‪‬م ث‬ ‫؟®¬ ‪¬‬؟ٌ ‪‬‬ ‫؛‪² -‬؟®¬‬ ‫‪½ ¬±® ·²‬؟؛ ® ‪»‬؛‪-‬‬ ‫¸‪ٌ‬عٌ‪¬ îp‬؛‪¬«‬ق‬ ‫®»‪»³°‬ج ‪‬م‬ ‫؟®»‪»³°‬‬ ‫؛·¼ »®« ‪¬‬؟‬ ‫؟·¬‪» ®»²‬؛؛·¼‬ ‫ع‪´ ·² p‬؟·¬‪®»²‬‬ ‫ع‪p‬‬

‫‪ î‬غخثظ×‪‬ع‬ ‫¾ ّ‪‬‬ ‫ھ‪ ±‬‬ ‫»ھ‪±‬‬ ‫¸ج ÷ ‪‬‬ ‫؟®‪ » ¹‬‬ ‫‪°‬؟®‪¹‬‬ ‫¸ ‪‬‬ ‫؟ ‪ٍ‬‬ ‫؟»¸ ‪¼»°·½¬-‬‬ ‫؛‪¬ ´±-- ±‬؟»¸‬ ‫‪²‬؟ ‪‬؛‪±‬‬ ‫؟´«‪ «²·²-‬‬ ‫¼» ‪¬‬؟´«‪«²·²-‬‬ ‫¸ج ‪°·°»ٍ‬‬ ‫؟®»‪ » ¬»³°‬‬ ‫؛·¼ »®« ‪¬‬؟®»‪¬»³°‬‬ ‫»؛؛·¼‬ ‫؟·¬‪ ®»²‬‬ ‫· ‪´‬؟·¬‪®»²‬‬‫ھ·®»¼‬ ‫·®»¼‬ ‫»ھ‬ ‫؟®»‪® ±³ ¬¸» °®±½»-- ¬»³°‬؛ ¼ ‪‬‬ ‫‪َ‬؟®»‪¬»³°‬‬ ‫«‪¬«®» ³·²‬‬‫‪³¾·»²¬ ¼»-·¹²‬؟ ‪³·²«-‬‬ ‫÷‪‬ع ‪éë‬مج‪¼»-·¹²ّ ‬‬

‫»؛؟ح‬ ‫½؟؛ §¬ ‪‬‬ ‫®‪ ¬±‬‬

‫و‪ î‬؟ » ‪-‬ث‬ ‫‪ï‬و‪î‬‬ ‫؟‪ -‬‬ ‫®»¸¬»¸© ®‪½ ¬±‬؟؛ §¬ ‪»‬؛؟‪-‬‬ ‫‪³‬؟ ‪»¨°±-«®» ¬±‬‬ ‫؟»© ¬‪ ¾·»²‬‬ ‫®»¸ ‪¬‬؟»©‬ ‫ھ‪½±²¼·¬·±²- ·- ·²‬‬ ‫ھ´ ‪±‬ھ‪·²‬‬ ‫‪» ¼ ±® ²±¬ٍ‬ھ´‬ ‫ھ‪± ²±¬ ±‬ـ‬ ‫؟»¬‪» ®-·¦» -‬ھ‪±‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫‪°‬؟®¬‬ ‫®‪ - ±‬‬ ‫؟®¬‬ ‫®¬‬ ‫؟»¬‪ -‬؟ ¬½»´ ‪»‬ح ‪½ »® ´·²»-ٍ‬؟‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫‪°‬؟®¬‬ ‫‪‬‬ ‫ھ®»‪¬± ½±²-‬‬ ‫¼· ‪±‬ھ؟ ‪² ¼ ¬±‬؟ §‪» »²»®¹‬ھ®»‪½±²-‬‬ ‫؟½‪°´«¹¹·²¹ ©·¬¸ ¼·®¬ô -‬‬ ‫¼ ‪²‬؟ » ‪´‬؟½‪-‬‬ ‫؟¬»‪³‬‬ ‫¬»‪³‬‬ ‫‪´´·½ ±¨·¼»-ٍ‬؟‬

‫غ‬

‫‪ً ٍé‬ٹ ‪ï‬‬ ‫‪ً ٍî‬م ‪ ë‬‬ ‫®‪ ë ±‬‬ ‫÷‪ ë‬‬ ‫‪ً ٍî‬م غ‬ ‫؟»‪· ²‬ش ‪‬م‬ ‫؛‪» »¬ ±‬؛ ‪®‬؟»‪²‬‬ ‫»‪ °·°‬؛‪±‬‬ ‫؛®«‪ -‬؛‪¬ î ±‬؛ ®»‪°·°» ´·²» °‬‬ ‫»½‪‬؟؛®«‪-‬‬ ‫ٌ‬ ‫؟»¸ ¬‪¬ »²‬؟ش ‪‬م‬ ‫؛‪¬ ±‬؟»¸‬ ‫»¬‪ -‬؛‪±‬‬ ‫؟»¬‪-‬‬ ‫‪³‬؟‬ ‫¾ ‪¬ «´‬ق ‪ ·²‬‬

‫ح‬ ‫ط‬

‫غشذسكبغ‬ ‫؟¬‪ײ-‬‬ ‫¬‪ײ-‬‬ ‫؟´´‪‬؟‬ ‫‪¬·±²‬؟´‬

‫ً‬ ‫؟®¬ »»® ‪¸‬ج‬ ‫؟»¬‪ً¬ ïً°-·¹ -‬؟ ‪½ »®ً ´·²»-‬؟®¬‬ ‫‪³‬؟»¬‪-‬‬ ‫» ‪®‬؟ »®«‪ °®»--‬‬ ‫¼ٍَ‪ îَ· ²‬؟ ‪«-»¼ ±²‬‬ ‫؟· ‪‬‬ ‫؟´«‪ ¬َ´ ±²¹ ·²-‬؛ًَ‪ٍô ï‬؟·‬ ‫»‪¬ »¼ °·°‬؟´«‪·²-‬‬ ‫ً‬ ‫؟‪¬± ³‬‬ ‫؟¬‪· ²‬؟‪³‬‬ ‫؟®»‪ °®±½»-- ¬»³°‬؟ ‪· ²‬؟¬‪²‬‬ ‫؛‪¬ «®» ±‬؟®»‪¬»³°‬‬ ‫¸¬·© ‪‬ع ‪ ïçp‬؛ً‪±‬‬ ‫؟®»‪² ±«¬¼±±® ¼»-·¹² ¬»³°‬؟‬ ‫؛‪¬ «®» ±‬؟®»‪¬»³°‬‬ ‫‪ï‬ٹ ‪‬؛‪±‬‬ ‫»‪- -«³‬ك ‪ٍ‬ع ‪ p‬‬

‫؟¬‪ײ-‬‬ ‫¬‪ײ-‬‬ ‫»‪´ ´ ¼·-¬®·¾«¬·±² ±® -«°°´§ ´·²‬؟‬‫¾؟ ¬¸‪ ¸»·¹‬؟ ‪¬‬؟‬ ‫ھ‪ ±‬‬ ‫»‪» ¬¸» °®±¼«½¬ ´·²‬ھ‪±‬‬‫؟»¬‪®»¯«·®·²¹ -‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫»¸¬ ® ‪±‬ع ‪½ ·²¹ٍ‬؟®¬‬ ‫؛‪² ¼ °«®¹·²¹ ±‬؟‬ ‫؟‪ ²±²َ½ ±²¼»²-‬؛‪±‬‬ ‫¾؟‪±²¼»²-‬‬ ‫‪ ´»-ô‬‬ ‫؟®¬ ¸½¬·‪°‬‬ ‫؟®‪± ® ¹‬؛ ‪½ »® ´·²»-‬؟®¬‬ ‫؟®¼ §¬ ‪·‬ھ؟®‪¹‬‬ ‫‪· ²َ‬؟®¼‬ ‫‪¹‬؟‬ ‫؟®¬ ¼ ‪²‬؟ » ‪‬‬ ‫‪°‬؟®¬‬ ‫· ‪¸‬ج ‪´ ´ ´±© -°±¬-ٍ‬؟ ‪‬‬‫؟®¬ ¼· ‪±‬ھ؟ ‪´ -± ¸»´°‬؟ ´´·©‬ ‫»‪½ »® ´·²‬؟®¬‬ ‫‪·¹«®» ï÷ٍ‬ع »»‪®»»¦·²¹ّ -‬؛‬ ‫ھ®»‪± ½±²-‬ج‬ ‫‪» »²»®¹§ô ®»¬«®² ½±²َ‬ھ®»‪½±²-‬‬ ‫؟‪¼»²-‬‬ ‫‪¼»²‬‬‫‪® »»¦»َ°‬ع ‪¬ » ¬± ¬¸» ¾±·´»®ٍ‬؟‬ ‫‪ ®±َ‬‬ ‫؟®¼ ‪¬»½¬·±²‬‬ ‫؟®¬ ‪· ²- ±²‬؟®¼‬ ‫‪°‬؟®¬‬ ‫؟¸½‪ ¼·-‬‬ ‫»‪® ¹‬؟¸½‪¼·-‬‬ ‫؟»¸‬ ‫»¸‬ ‫¼؟‬ ‫»®»¸© ¼»¬‪® » -«¹¹»-‬؟ ‪ »®-‬‬ ‫ھ»®‪® »»¦·²¹ ½±²¼·¬·±²- °‬؛‬ ‫‪· ٍ´‬؟ھ»®‪°‬‬

‫َ‬ ‫‪ ï‬غخثظ×‪‬ع‬ ‫‪ ½±²‬ك ‪ٍ ‬‬ ‫؟‪¼»²-‬‬ ‫‪¼»²‬‬‫؟‪¬ » ³‬؟‬ ‫‪²‬؟‪³‬‬ ‫؛· ‪‬‬ ‫‪±‬؛·‬ ‫¼´ ‪‬‬ ‫؟®¬ »´‪-¸±©- ³«´¬·°‬‬ ‫½؟®¬‬ ‫®» ‪‬‬ ‫‪²‬؟ ‪´·²»-‬‬ ‫؟»¬‪ ¼ -‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫‪°‬؟®¬‬ ‫‪ -‬‬

‫؟·®» ‪¬‬؟س‬ ‫ژ¸¬‪± ® ¬¸·- ³±²‬؛ ‪´‬؟·®»‬ ‫‪· ²¹»®¬·°-Œ ½±´«³²‬ع ®« ‪±‬ا ‪¬‬؟ ‪½ ¬-‬؟عچ ‪-‬ژ¸¬‪³±²‬‬ ‫؟‪® ³-¬®±²¹ ײ¬»®²‬ك §¾ ¼»·´‪- -«°°‬؟©‬ ‫؟‪¬ ·±²‬؟‪ײ¬»®²‬‬ ‫‪´ ô‬؟‪·±²‬‬ ‫‪·½¸ٍ‬س ‪»®-ô‬ھ·‪‬خ »»®¸‪‬ج‬

‫؟‪¬ ·- ¬¸» ½±²¼»²-‬؟ ‪¸‬ة‬ ‫؟‪¬ » ´±‬؟‪½±²¼»²-‬‬ ‫¼؟‪´±‬‬ ‫ل ‪‬‬ ‫؟´«‪±®³‬؛ »¸¬ ‪-·²¹‬ث‬ ‫و؟´«‪®³‬‬ ‫ٌ‬ ‫¸‪éî ´¾‬م‬

‫ً‬ ‫‪±‬‬ ‫ًٍ ع ‪ً‬‬ ‫‪î‬‬ ‫‪î‬‬ ‫‪ً ë‬‬ ‫ً‬ ‫ٌ‬ ‫¾´‪¬«‬ق ‪èè‬‬

‫ٌ‬ ‫‪î ±‬‬ ‫‪î‬‬ ‫¸ ‪ٌ‬ع ‪¬ ٌٌ‬؛ ‪¬«‬ق ‪‬ى ى ‪ٍ ‬‬ ‫‪î‬‬

‫¬؛ ‪¬´·² ٌ‬؛؛‬

‫ً‬ ‫¬؛ ‪ً‬‬ ‫‪ï‬‬

‫‪ًçï‬‬ ‫‪ٍï‬‬

‫ٌ‬ ‫؟‪· ¼» ¾§ ¬¸®»» ·² ±®¼»® ¬± ¹»¬ ¬¸» ´±‬ھ ‪·‬ـ‬ ‫¼؟‪´±‬‬ ‫؟®¬ ®»‪ °‬‬ ‫و»‪½ »® ´·²‬؟®¬‬ ‫‪ ´¾¸ ٍ ‬ى‪ î‬و»‪´·²‬‬ ‫ه©‪·- -«®°®·-·²¹´§ ´±‬‬ ‫؛»®»¸¬ ‪‬ه©‪´±‬‬ ‫؟‪± ®»ô ¬¸» -³‬؛»®»¸¬‬ ‫؟®¬ ¬‪´ ´»-‬؟‪-³‬‬ ‫‪°‬؟®¬‬ ‫؟‪ ·- ²±®³‬‬ ‫§´ ‪´‬؟‪²±®³‬‬ ‫¼؟‬ ‫؟«¯» ‪‬‬ ‫¾؟ ‪- »¼ ±² ·¬-‬؟ق ‪¬ »ٍ‬؟«¯»‬ ‫ھ®»‪ ·´·¬§ ¬± ½±²-‬‬ ‫؟®»‪» »²»®¹§ ¾§ ±°‬ھ®»‪½±²-‬‬ ‫‪¬ ·²¹‬؟®»‪±°‬‬ ‫؟·´»®‬ ‫·´»®‬ ‫¾؟‬ ‫ھ‪ ´§ ±‬‬ ‫؛‪ ´±²¹ °»®·±¼ ±‬؟ ® ‪»‬ھ‪±‬‬ ‫؟¸ ‪ ¬·³»ô‬؛‪±‬‬ ‫؟‪² ¼´» ´·¹¸¬ ´±‬؟¸‬ ‫¼؟‪´±‬‬ ‫¬‪ -ô ®»-·-‬‬ ‫ھ‪² ·²‬؟ ‪² ¼ °«®¹» ¬¸» -§-¬»³ô‬؟ ‪® »»¦·²¹‬؛‬ ‫؟®¬ ¬»‪» ®¬»¼ ¾«½µ‬ھ‪·²‬‬ ‫‪°‬؟®¬‬ ‫‪ ·- ®»½±³َ‬‬ ‫؟®¬ ®‪±‬؛ ¼»¼‪³»²‬‬ ‫ھ®»‪½»® ´·²» -‬؟®¬‬ ‫‪·½»ٍ‬ھ®»‪-‬‬

‫½؟ع‬ ‫‪±‬ا ‪¬‬؟ ‪ ¬-‬‬ ‫©‪· ²¹»®¬·°- ‬ع ®« ‪‬‬ ‫؟© ‪²¹»®¬·°- ‬‬‫§¾ ¼»®‪ -°±²-±‬‬

‫؟» ‪¬‬ح‬ ‫‪³‬؟»‬ ‫½؟®ج ‪‬‬ ‫®» ‪‬‬ ‫»‪·²‬ش‬‫؟‪»°‬ـ‬ ‫و®‪¼·¬±‬غ ¬‪®¬³»²‬؟‪°‬‬ ‫‪»²µ·²‬ض ¬¬‪½±‬ح و®‪·¬±‬‬‫؟®»‪»³°‬ج‬ ‫؛·¼ »®«¬‪‬؟®»‪³°‬‬ ‫؟·¬‪»®»²‬؛؛·¼‬ ‫´‪‬؟·¬‪®»²‬‬

‫ًً ً‬ ‫‪ً ïë‬‬ ‫‪ ‬‬ ‫‪ç‬‬

‫ً‬ ‫ً‬ ‫‪é‬‬

‫ً‬ ‫‪ً‬‬ ‫‪ë‬‬

‫ً‬ ‫‪‬ى‬

‫ً‬ ‫‪‬ي‬

‫ً‪î‬‬ ‫‪‬ى‬

‫ً‬ ‫‪ً‬‬ ‫‪î‬‬

‫ً‬ ‫‪ïè‬‬

‫ً‬ ‫‪ê‬‬ ‫‪ïê‬‬ ‫‪ï‬‬

‫ً‬ ‫‪ïë‬‬

‫‪ïï‬‬ ‫ً‬ ‫‪ï‬‬ ‫‪ç‬‬

‫‪ïï‬‬ ‫ً‬ ‫‪ï‬‬ ‫‪ç‬‬

‫‪è‬‬

‫‪è‬‬

‫‪é‬‬

‫‪é‬‬

‫‪ê‬‬

‫‪ê‬‬

‫‪ë‬‬

‫‪ë‬‬

‫ح‬

‫؟‪ ¼»-·¹²‬؟ ‪¬‬؟ »‪® § °·°‬؟‪°®·³‬‬ ‫؟‪°®·³‬‬ ‫؛·‪¬ »¼ «²‬؟‪¼»-·¹²‬‬ ‫؟®»‪± ®³ ¬»³°‬؛·‪«²‬‬ ‫¬‪¬ «®»ٍ ײ ³±-‬؟®»‪¬»³°‬‬ ‫؟®¬ »‪- »-ô ¬¸»-‬؟½‬ ‫؟‪® » «-»¼ ±«¬¼±±®-ô ©¸·½¸ ³‬؟ ‪½ »® ´·²»-‬؟®¬‬ ‫» ‪µ‬؟‪³‬‬‫‪³‬؟‬ ‫؟»© ¬‪ ¾·»²‬‬ ‫؟½·¬·®½ ‪‬؟ ‪¬ ¸»® ½±²¼·¬·±²-‬؟»©‬ ‫؟®»¼·‪´ ½±²-‬؟½·¬·®½‬ ‫‪¬ ·±²ٍ‬؟®»¼·‪½±²-‬‬ ‫؟‪¸ » °®·³‬ج‬ ‫؛‪® § °«®°±-» ±‬؟‪°®·³‬‬ ‫؟»¬‪ -‬؛‪±‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫‪°‬؟®¬‬ ‫؟®¬ ‪ - ±²‬‬ ‫؟¬»® ‪½ »® ´·²»- ·- ¬±‬؟®¬‬ ‫‪· ²‬؟¬»®‬ ‫؟»¬‪¬¸» -‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟´ ‪ «²¬·´ ·¬-‬‬ ‫؟»¸ ¬‪¬ »²‬؟´‬ ‫؟¸½‪² ¼ ¬¸»² ¼·-‬؟ ‪« ´´§ «¬·´·¦»¼ô‬؛ ‪¬ ·-‬؟»¸‬ ‫»‪® ¹‬؟¸½‪¼·-‬‬ ‫؟‪¬¸» ½±²¼»²-‬‬ ‫؟‪² ¼ ²±²َ½ ±²¼»²-‬؟ » ‪¬‬؟‪½±²¼»²-‬‬ ‫¾؟‪±²¼»²-‬‬ ‫؟‪ ´» ¹‬‬ ‫§ ‪²‬؟ ¸¬·© »«®¬ ‪- ·-‬ك ‪- »-ٍ‬؟‪¹‬‬ ‫؛‪°·»½» ±‬‬ ‫؟»¸ ‪‬؛‪±‬‬ ‫؟®¬ ‪¬‬؟»¸‬ ‫؛‪² -‬؟®¬‬ ‫؟» ‪» ® »¯«·°³»²¬ô‬؛‪-‬‬ ‫؟®¬ ¸ ‪½‬؟»‬ ‫؟¸ ¼´«‪½ »® ´·²» -¸±‬؟®¬‬ ‫¬· ‪»‬ھ؟¸‬‫؟®¬ ‪±©²‬‬ ‫‪°‬؟®¬‬ ‫؟®¬ »´‪» ² ¬¸±«¹¸ ³«´¬·°‬ھغ ‪ ٍ ‬‬ ‫؟‪½ »® ´·²»- ³‬؟®¬‬ ‫؟¬‪§ ¾» ·²-‬؟‪³‬‬ ‫‪´ ´»¼ ±²‬؟¬‪·²-‬‬ ‫‪-¸±®¬َ½·®½«·¬·²¹ٍ‬‬ ‫؟»¬‪² ¼ -·¦·²¹ -‬؟ ‪ײ -»´»½¬·²¹‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫‪°‬؟®¬‬ ‫؟¬®‪ -ô ·¬ ·- ·³°±‬‬ ‫®»¼·‪² ¬ ¬± ½±²-‬؟¬®‪·³°±‬‬ ‫؟‪¬¸»·® ½±³°‬‬ ‫ھ·¬½»¶¾‪¬ ·¾·´·¬§ ©·¬¸ ¬¸» ±‬؟‪½±³°‬‬ ‫؛‪» - ±‬ھ·¬½»¶¾‪±‬‬ ‫؟®¬ ‪-‬؟ ‪ ¬¸» -§-¬»³ô‬؛‪±‬‬ ‫‪°‬؟®¬‬ ‫¬‪ - ³«-‬‬ ‫و‪± ´´±©·²¹‬؛ »¸¬ ¸‪½ ½±³°´·-‬؟‬ ‫و‪´´±©·²¹‬‬ ‫ھ®»‪± ²-‬ف ‪ïٍ‬‬ ‫؟®»‪» »²»®¹§ ¾§ ±°‬ھ®»‪²-‬‬ ‫؟·´»® ‪¬ ·²¹‬؟®»‪±°‬‬ ‫¾؟·´»®‬ ‫ھ‪ ´§ ±‬‬ ‫¼‪ ´±²¹ ¬·³» °»®·±‬؟ ® ‪»‬ھ‪±‬‬ ‫ھ‪® ±‬ذ ‪îٍ‬‬ ‫¾؟ »¼ ‪·‬ھ‪±‬‬ ‫؟¸½‪ ®«°¬ °»®·±¼·½ ¼·-‬‬ ‫»¸¬ »‪® ¹» ·² ±®¼»® ¬± °«®¹‬؟¸½‪¼·-‬‬ ‫؟‪½±²¼»²-‬‬ ‫‪½±²¼»²‬‬‫»‪® ±³ ¬¸» ´·²‬؛ ® ‪·‬؟ ¼ ‪²‬؟ » ‪¬‬؟‬ ‫‪°‬ر ‪ٍ‬ي‬ ‫؟®» ‪‬‬ ‫؟‪¬ » «²¼»® ´·¹¸¬ ´±‬؟®»‬ ‫¼؟‪´±‬‬ ‫‪ ½±²¼·¬·±²‬‬‫؟¼ ¬‪» -·-‬خ ‪ٍ‬ى‬ ‫‪³‬؟¼‬ ‫‪¹‬؟ ‪‬‬ ‫؟»¬‪ ¬¸» -‬؛·؛· ‪® »»¦·²¹‬؛ ‪® ±³‬؛ » ‪‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؛‪ ·- -¸«¬ ±‬‬ ‫‪ ‬؛؛‪±‬‬ ‫؛‪¸ » ½±-¬ ±‬ج‬ ‫؟»¬‪ -‬؛‪±‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟‪ ³‬‬ ‫؟© ‪µ »-‬؟‪³‬‬ ‫؛»¬ ‪-‬؟©‬ ‫؟®¬ ´ ‪«‬؛»¬‬ ‫؟¬·¾®‪² »¨±‬؟ ‪½ »® ´·²»-‬؟®¬‬ ‫¬ ‪²‬؟¬·¾®‪»¨±‬‬ ‫؟»¸® ‪»‬ھ‪±‬‬ ‫¼؟»¸®‬ ‫؟¸¬ ‪‬‬ ‫؟½ §®¬‪¬ ²± ·²¼«-‬؟¸¬‬ ‫‪± ®¼ٍ‬؛؛؟ ‪²‬؟½‬

‫‪ٍë‬ى‬

‫‪ٍë‬ى‬

‫ًى‬ ‫‪ٍ‬‬

‫ًى‬ ‫‪ٍ‬‬

‫‪ٍë‬ي‬ ‫ي‪ٍ‬ي‬

‫‪ٍë‬ي‬ ‫ي‪ٍ‬ي‬

‫ًي‬ ‫‪ٍ‬‬

‫ًي‬ ‫‪ٍ‬‬

‫‪î‬‬ ‫‪ٍ è‬‬

‫‪î‬‬ ‫‪ٍ è‬‬

‫‪î‬‬ ‫‪ٍ ê‬‬

‫‪î‬‬ ‫‪ٍ ê‬‬

‫‪î‬‬ ‫‪ٍ ë‬‬

‫‪î‬‬ ‫‪ٍ ë‬‬

‫‪î‬‬ ‫‪‬‬ ‫ىٍ‬

‫‪î‬‬ ‫‪‬‬ ‫ىٍ‬

‫‪î‬‬ ‫‪‬‬ ‫يٍ‬

‫‪î‬‬ ‫‪‬‬ ‫يٍ‬

‫‪°‬؟ ‪®‬ج‬ ‫‪±‬؛ ‪ -»´»½¬·±²‬‬ ‫؟»¬‪ ® -‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫½؟®¬‬ ‫‪ »® ´·²»-‬‬

‫‪î‬‬ ‫‪ٍ î‬‬

‫‪î‬‬ ‫‪ٍ î‬‬

‫‪î‬‬ ‫‪ٍïë‬‬

‫‪î‬‬ ‫‪ٍïë‬‬

‫؟‪¸ » ½±²¼»²-‬ج‬ ‫؟‪¬ » ´±‬؟‪½±²¼»²-‬‬ ‫¼؟‪´±‬‬ ‫؟¸ »¾ ‪ ¬±‬‬ ‫؟»¬‪ -‬؟ ‪² ¼´»¼ ±²‬؟¸‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫؟½ »‪½ »® ´·²‬؟®¬‬ ‫‪²‬؟½‬ ‫؟»¸ »¸¬ ‪® ±³‬؛ ¼»‪¾» ¼»¬»®³·²‬‬ ‫‪® ±³ ¬¸» °®±¼«½¬ °·°» ¾§ «-·²¹‬؛ ‪¬ ´±--‬؟»¸‬ ‫؟´«‪± ®³‬؛ ‪¬¸·-‬‬ ‫و؟´«‪®³‬‬ ‫‪  ‬غ‬

‫ً‬ ‫‪î‬‬ ‫‪ ‬‬ ‫‪ٍï‬‬

‫‪‬ج‬

‫‪‬ث‬ ‫ط‬

‫‪‬ش‬

‫د‬

‫ح‬

‫و»®» ‪¸‬ة‬ ‫و»®»‬ ‫ٌ‬ ‫‪±‬ف ‪‬م د‬ ‫؟‪ ²¼»²-‬‬ ‫؟‪¬ » ´±‬؟‪²¼»²-‬‬ ‫¼؟‪´±‬‬ ‫‪ ·² ´¾¸‬‬ ‫؛‪» ²¹¬¸ ±‬ش ‪‬م‬ ‫»‪ °®±¼«½¬ °·°‬؛‪±‬‬ ‫ش‬ ‫؟®¬ ‪¾»¬©»»²‬‬ ‫»‪½ »® ´·²‬؟®¬‬ ‫؟®¬‬ ‫®¬‬ ‫‪°‬؟‬ ‫‪¬‬؛ ‪ - ·²‬‬ ‫»ط ‪‬م ث‬ ‫؟®¬ ‪¬‬؟ٌ ‪‬‬ ‫؛‪² -‬؟®¬‬ ‫‪½ ¬±® ·²‬؟؛ ® ‪»‬؛‪-‬‬ ‫¸‪ٌ‬عٌ‪¬ îp‬؛‪¬«‬ق‬ ‫®»‪»³°‬ج ‪‬م‬ ‫؟®»‪»³°‬‬ ‫؛·¼ »®« ‪¬‬؟‬ ‫؟·¬‪» ®»²‬؛؛·¼‬ ‫ع‪´ ·² p‬؟·¬‪®»²‬‬ ‫ع‪p‬‬

‫‪ î‬غخثظ×‪‬ع‬ ‫¾ ّ‪‬‬ ‫ھ‪ ±‬‬ ‫»ھ‪±‬‬ ‫¸ج ÷ ‪‬‬ ‫؟®‪ » ¹‬‬ ‫‪°‬؟®‪¹‬‬ ‫¸ ‪‬‬ ‫؟ ‪ٍ‬‬ ‫؟»¸ ‪¼»°·½¬-‬‬ ‫؛‪¬ ´±-- ±‬؟»¸‬ ‫‪²‬؟ ‪‬؛‪±‬‬ ‫؟´«‪ «²·²-‬‬ ‫¼» ‪¬‬؟´«‪«²·²-‬‬ ‫¸ج ‪°·°»ٍ‬‬ ‫؟®»‪ » ¬»³°‬‬ ‫؛·¼ »®« ‪¬‬؟®»‪¬»³°‬‬ ‫»؛؛·¼‬ ‫؟·¬‪ ®»²‬‬ ‫· ‪´‬؟·¬‪®»²‬‬‫ھ·®»¼‬ ‫·®»¼‬ ‫»ھ‬ ‫؟®»‪® ±³ ¬¸» °®±½»-- ¬»³°‬؛ ¼ ‪‬‬ ‫‪َ‬؟®»‪¬»³°‬‬ ‫«‪¬«®» ³·²‬‬‫‪³¾·»²¬ ¼»-·¹²‬؟ ‪³·²«-‬‬ ‫÷‪‬ع ‪éë‬مج‪¼»-·¹²ّ ‬‬

‫»؛؟ح‬ ‫½؟؛ §¬ ‪‬‬ ‫®‪ ¬±‬‬

‫و‪ î‬؟ » ‪-‬ث‬ ‫‪ï‬و‪î‬‬ ‫؟‪ -‬‬ ‫®»¸¬»¸© ®‪½ ¬±‬؟؛ §¬ ‪»‬؛؟‪-‬‬ ‫‪³‬؟ ‪»¨°±-«®» ¬±‬‬ ‫؟»© ¬‪ ¾·»²‬‬ ‫®»¸ ‪¬‬؟»©‬ ‫ھ‪½±²¼·¬·±²- ·- ·²‬‬ ‫ھ´ ‪±‬ھ‪·²‬‬ ‫‪» ¼ ±® ²±¬ٍ‬ھ´‬ ‫ھ‪± ²±¬ ±‬ـ‬ ‫؟»¬‪» ®-·¦» -‬ھ‪±‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫‪°‬؟®¬‬ ‫®‪ - ±‬‬ ‫؟®¬‬ ‫®¬‬ ‫؟»¬‪ -‬؟ ¬½»´ ‪»‬ح ‪½ »® ´·²»-ٍ‬؟‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫‪°‬؟®¬‬ ‫‪‬‬ ‫ھ®»‪¬± ½±²-‬‬ ‫¼· ‪±‬ھ؟ ‪² ¼ ¬±‬؟ §‪» »²»®¹‬ھ®»‪½±²-‬‬ ‫؟½‪°´«¹¹·²¹ ©·¬¸ ¼·®¬ô -‬‬ ‫¼ ‪²‬؟ » ‪´‬؟½‪-‬‬ ‫؟¬»‪³‬‬ ‫¬»‪³‬‬ ‫‪´´·½ ±¨·¼»-ٍ‬؟‬

‫غ‬

‫‪ً ٍé‬ٹ ‪ï‬‬ ‫‪ً ٍî‬م ‪ ë‬‬ ‫®‪ ë ±‬‬ ‫÷‪ ë‬‬ ‫‪ً ٍî‬م غ‬ ‫؟»‪· ²‬ش ‪‬م‬ ‫؛‪» »¬ ±‬؛ ‪®‬؟»‪²‬‬ ‫»‪ °·°‬؛‪±‬‬ ‫؛®«‪ -‬؛‪¬ î ±‬؛ ®»‪°·°» ´·²» °‬‬ ‫»½‪‬؟؛®«‪-‬‬ ‫ٌ‬ ‫؟»¸ ¬‪¬ »²‬؟ش ‪‬م‬ ‫؛‪¬ ±‬؟»¸‬ ‫»¬‪ -‬؛‪±‬‬ ‫؟»¬‪-‬‬ ‫‪³‬؟‬ ‫¾ ‪¬ «´‬ق ‪ ·²‬‬

‫ح‬ ‫ط‬

‫غشذسكبغ‬ ‫؟¬‪ײ-‬‬ ‫¬‪ײ-‬‬ ‫؟´´‪‬؟‬ ‫‪¬·±²‬؟´‬

‫ً‬ ‫؟®¬ »»® ‪¸‬ج‬ ‫؟»¬‪ً¬ ïً°-·¹ -‬؟ ‪½ »®ً ´·²»-‬؟®¬‬ ‫‪³‬؟»¬‪-‬‬ ‫» ‪®‬؟ »®«‪ °®»--‬‬ ‫¼ٍَ‪ îَ· ²‬؟ ‪«-»¼ ±²‬‬ ‫؟· ‪‬‬ ‫؟´«‪ ¬َ´ ±²¹ ·²-‬؛ًَ‪ٍô ï‬؟·‬ ‫»‪¬ »¼ °·°‬؟´«‪·²-‬‬ ‫ً‬ ‫؟‪¬± ³‬‬ ‫؟¬‪· ²‬؟‪³‬‬ ‫؟®»‪ °®±½»-- ¬»³°‬؟ ‪· ²‬؟¬‪²‬‬ ‫؛‪¬ «®» ±‬؟®»‪¬»³°‬‬ ‫¸¬·© ‪‬ع ‪ ïçp‬؛ً‪±‬‬ ‫؟®»‪² ±«¬¼±±® ¼»-·¹² ¬»³°‬؟‬ ‫؛‪¬ «®» ±‬؟®»‪¬»³°‬‬ ‫‪ï‬ٹ ‪‬؛‪±‬‬ ‫»‪- -«³‬ك ‪ٍ‬ع ‪ p‬‬

‫؟¬‪ײ-‬‬ ‫¬‪ײ-‬‬ ‫»‪´ ´ ¼·-¬®·¾«¬·±² ±® -«°°´§ ´·²‬؟‬‫¾؟ ¬¸‪ ¸»·¹‬؟ ‪¬‬؟‬ ‫ھ‪ ±‬‬ ‫»‪» ¬¸» °®±¼«½¬ ´·²‬ھ‪±‬‬‫؟»¬‪®»¯«·®·²¹ -‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫»¸¬ ® ‪±‬ع ‪½ ·²¹ٍ‬؟®¬‬ ‫؛‪² ¼ °«®¹·²¹ ±‬؟‬ ‫؟‪ ²±²َ½ ±²¼»²-‬؛‪±‬‬ ‫¾؟‪±²¼»²-‬‬ ‫‪ ´»-ô‬‬ ‫؟®¬ ¸½¬·‪°‬‬ ‫؟®‪± ® ¹‬؛ ‪½ »® ´·²»-‬؟®¬‬ ‫؟®¼ §¬ ‪·‬ھ؟®‪¹‬‬ ‫‪· ²َ‬؟®¼‬ ‫‪¹‬؟‬ ‫؟®¬ ¼ ‪²‬؟ » ‪‬‬ ‫‪°‬؟®¬‬ ‫· ‪¸‬ج ‪´ ´ ´±© -°±¬-ٍ‬؟ ‪‬‬‫؟®¬ ¼· ‪±‬ھ؟ ‪´ -± ¸»´°‬؟ ´´·©‬ ‫»‪½ »® ´·²‬؟®¬‬ ‫‪·¹«®» ï÷ٍ‬ع »»‪®»»¦·²¹ّ -‬؛‬ ‫ھ®»‪± ½±²-‬ج‬ ‫‪» »²»®¹§ô ®»¬«®² ½±²َ‬ھ®»‪½±²-‬‬ ‫؟‪¼»²-‬‬ ‫‪¼»²‬‬‫‪® »»¦»َ°‬ع ‪¬ » ¬± ¬¸» ¾±·´»®ٍ‬؟‬ ‫‪ ®±َ‬‬ ‫؟®¼ ‪¬»½¬·±²‬‬ ‫؟®¬ ‪· ²- ±²‬؟®¼‬ ‫‪°‬؟®¬‬ ‫؟¸½‪ ¼·-‬‬ ‫»‪® ¹‬؟¸½‪¼·-‬‬ ‫؟»¸‬ ‫»¸‬ ‫¼؟‬ ‫»®»¸© ¼»¬‪® » -«¹¹»-‬؟ ‪ »®-‬‬ ‫ھ»®‪® »»¦·²¹ ½±²¼·¬·±²- °‬؛‬ ‫‪· ٍ´‬؟ھ»®‪°‬‬

‫َ‬ ‫‪ ï‬غخثظ×‪‬ع‬ ‫‪ ½±²‬ك ‪ٍ ‬‬ ‫؟‪¼»²-‬‬ ‫‪¼»²‬‬‫؟‪¬ » ³‬؟‬ ‫‪²‬؟‪³‬‬ ‫؛· ‪‬‬ ‫‪±‬؛·‬ ‫¼´ ‪‬‬ ‫؟®¬ »´‪-¸±©- ³«´¬·°‬‬ ‫½؟®¬‬ ‫®» ‪‬‬ ‫‪²‬؟ ‪´·²»-‬‬ ‫؟»¬‪ ¼ -‬‬ ‫‪³‬؟»¬‪-‬‬ ‫؟®¬ ‪‬‬ ‫‪°‬؟®¬‬ ‫‪ -‬‬

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Heat Transfer Fluids and Systems Department Editor: Rebekkah Marshall 

STARTUP

FLUID ANAL ANALYSIS YSIS

1. Verifycontrolan  Verifycontrolandsafetys dsafetysystems:Itis ystems:Itisvitallyimvitallyimportanttoverifyallcontro portanttoverifyallcontrolandsafetysystem landsafetysystems s arecalibratedandreadyforoperationandare functioningproperly 2.Checkforleakage 3.Remove Remove moisture moisture from the system, system, using using dry, dry, compressedairorothersuitablemeans.Fillthe systemwithheattransferfluid 4.Systemfilling a. Fillthesystemtodesiredlevelavoidingany unnecessaryaerationofthefluid b. Openallvalves,thenstartthemaincirculationpumpinaccordancewiththemanufacturer’srecommendations.Allowforthermal expansio expansion n of fluid in determining determining the cold chargelevel c. Circulatetheheattransf C  irculatetheheattransferfluidthrou erfluidthroughthe ghthe systemforabout3to4hourstoeliminateair pockets,and pockets,and toassu to assurecompletesystem recompletesystem fill beforefiringtheheater 5.Starttheheater a. Bringthesystemuptotemperatureslowlyto helppreventtherma helppreventthermalshocktoheatertubes lshocktoheatertubes, , tube/heaterjointsandrefractorymaterials; andallow operator operators s tocheck to check thefunctionthefunctioning of instru instrumen ments ts and and contro controls. ls. The slow slow heat-upwillalsoallowmoisturetrappedin allsectionsofthesystemtoescapeasvapor. Inertgasshouldsweeptheexpansiontankto removenoncondensablesandresidualmoisturetoasafelocation.Holdthetemperature stableabove100°C(212°F)untilnosigns ofmoistureremain(knockingorrattlingof pipes,nomoisturefromven pipes,nomoisturefromvents,andso ts,andsoon) on) b. Bringthesystemtooperatingtemperature, putthe“users”online,andplacetheexpansion-tankinertingsystemintooperation c. Thefluidshouldgenerallybeanalyzedwithin 24hofplantstartupandannuallythereafter d. Checkandcleanstartupstrainersasneeded The system system should should beheated beheated and and cooled cooled for at leasttwocycle leasttwocycleswiththe swiththe filter filter inplace inplace sincethe sincethe resultingexpansionandcontractionwillloosenmill scale.Reinsulateanysurfacesleftbareforleakcheckingpurposes.

Fluid testing testing helps helps detect system system malfuncti malfunction, on, fluid contaminat contamination, ion, moisture, moisture, thermal thermal degradation,aswellasotherfactorsthatimpactsystemperformance(seeTable).For systemsoperatingnearthe systemsoperatingnearthefluid’ fluid’smaximumtemper smaximumtemperature,annua ature,annualanalysisis lanalysisissuggested. suggested.

OPERATIONS Heaters:  Proper Proper fluid-heat fluid-heater er operation operation will help ensurelonglifeofthefluid.C ensurelonglifeofthefluid.Commonheaterpr ommonheaterproboblems include include flame impingemen impingement, t, excessive excessive heat flux,controlovershoot,lowfluidflow,andinterlock malfunctions. Piping and pumps:  Aleak-freesystemwillhelpto  Aleak-freesystemwillhelpto ensuresafeandreliableoperation.Somekeyfeaturesofaleak-freedesignar turesofaleak-freedesignareasfollows: easfollows: • Maintainvalvesandpump Maintainvalvesandpumppackingand packingandseals seals •  Avoid  Avoidthe theuseof useof threadedfittings(weldedor threadedfittings(weldedor flangedconnectionsarepreferred) • Realignpumpsandretorqueflangesoncesystemachievesoperatingtemperatureafterinitial systemstartup • Confirmwithyourfluidsupplierwhattheproper elastomersare.Notallelastomersarecompatiblewithallheattransferfluids

Test result

Potential effects

Possible cause

Viscosity increase

Poor heat-transfer rate, de- • Contamination posits, high vapor pressure, • Thermal degradation pump cavitation • Fluid oxidation

4, 5 4, 5 3, 4

Total acid number increase

System corrosion, deposits

• Severe oxidation • Acidic contamination

3, 4 4, 5

Moisture increase

Corrosion, excess system pressure, pump cavitation, mechanical knocking

• System leaks • Residual moisture in new or cleaned unit • Unprotected vent or storage

2 2

Insoluble solids increase

Poor heat transfer, wear of pump seals, plugging in narrow passages

• Contamination • Dirt • Corrosion • Oxidation • Thermal stress

Low- and high-boiler increase**

Pump cavitation, poor heat • Low boilers transfer, excess system • High boilers pressure, deposits • Contamination

Possible actions*

2, 3 1, 4, 5 1, 4 1, 3, 5 1, 3 1, 4 2 4 4, 5

* For detailed guidance on actions, please consult with your fluid engineering specialist. ** For an excellent discussion on low and high boilers, please consult Ref. [ 4 ]. ].

Possible actions 1. Filtration:  Small Small diameter diameter particles particles suspendedinheattransferfluidcanbeeffectivelyremovedviafiltration.Filterswith 100-mmorlessnominal-particle-removal ratings ratings should should be considere considered d for initial initial system system treatm treatment ent. . Contin Continuo uous us filtra filtratio tion n through10-mmratedfilterscanmaintain systemcleanliness 2. Venting:  Iflowboilerconcentrationand/ ormoistureisallowedtoreachexcessive levelsinthefluid,problemssuchaspump cavitation,increasedsystempressureand flash-pointdepressioncanoccur.Intermittent,controlledventingtoasafelocation isacommonsolutiontominimizethepo-isacommonsolutiontominimizethepo tentialforproblemscausedbyexcessive lowboilerormoistureconcentration 3. Inerting:  Aneffectivemethodofminimiz Aneffectivemethodofminimizingfluidoxidationistoblankettheexpansiontankwithaclean,dry pansiontankwithaclean,dry,inertgas, ,inertgas, suchasnitrogen,CO2,ornaturalgas 4. Dilution/replacement: Canbeusedtoremovesomefraction(orall)ofthefluidand replacewithvirginfluidtomaintainfluid propertieswithinnormalranges.Caution is advis advised ed when when using using recla reclaime imed d fluid fluid, ,  which  which can return return degrad degradatio ation n produc products ts and/orcontaminantsintothesystem 5. Cleaning:  Ifasystemflushisnecessary, several several different differentmetho methods dsareavailabl areavailable. e. Specialty-engineered, heat-transfer flush fluidsmaybeusedtoremovesludgeor tar from from piping piping/eq /equip uipmen ment. t. Hard Hard carcarbondepositsonheatersurfaces(“coke”) generallyrequiretheuseofmechanical cleani cleaning ng techniq techniques ues like like sand sand or bead bead

Facts at at Your Fingertips  SponSored by

blasting,wirebrushing,orhigh-pressure  waterjetting.Forprocesscontamination, consultwithyourfluidsupplierforsuggestedcleaningmethods

SHUTDOWN Preventoverheatingoffluidduetoresidual heatintheheater. 1.ShutoffburnercompletelywiththecircuShutoffburnercompletelywiththecirculating pump stilloperating.Continueto stilloperating.Continueto runthepumpatfullcapacitytodissipate residualheatintheheater 2. Whentheheaterhascooledtothemanufacturer’srecommendedlowtemperature, shutoffthecirculatin shutoffthecirculatingpumpandswitc gpumpandswitch h offrequiredheaterelectricalcontrols 3.CautionmustbeexercisedduringshutCautionmustbeexercisedduringshutdowntoensurethatnoareainthesys-downtoensurethatnoareainthesys tempipingistotallyandcom tempipingistotallyandcompletelyisopletelyisolated.Thiswillpreventavacuumfrom forming,whichcoulddamage(implode) equipment  4.Operateheattracing,ifneeded References and further reading 1.Gamb G amble le, , C.E. C.E., , Cost Cost Mana Manage geme ment nt in Heat Heat TransferSystems,Chem. TransferSystems,Chem. Eng. Prog.,July2006 Prog.,July2006 pp.22–26. 2. Gamble,C.E.,CleaningOrganicHeatTransfer FluidSystems,Process FluidSystems,Process Heating ,Oct.2002. ,Oct.2002. 3. Beain, Beain, others, others, Proper Properly ly Clean Clean Out Your OrganicHeatTransferFluidSystem, Chem. Eng. Prog.,May2001. Prog.,May2001. 4. Spurlin,others,DefiningThermalStability, Spurlin,others,DefiningThermalStability,ProProcess Heating ,Nov.2000. ,Nov.2000. 5. “LiquidPhaseDesignGuide,”Pub#7239128C, Solutia,Inc.,1999.

Heat Transfer System Design Department Editor: Kate Torzewski  pumps

•Usecellularglassinsulation,whichis resistanttouidsaturation,inareasofthe systemwhereleaksarelikelytodevelop •Centrifugalpumpswithcastorforged • Ifaleakdevelops,removetheinsulation, steelcasingsaretypicallyappropriatefor andcontaintheuiduntiltheleakcanbe systemswithlargeowrates repaired •Positivedisplacementpumpsfrequently • Onverticalrunsofpipewhereoccasioncanhandlesmallerowratesoflessthan alleakscandevelopatanges,install 100gal/min protectivetight-ttingcapstodivertany •Seallesspumps(cannedmotoror uidleakageoutsidetheinsulation magneticdrive)avoidtheinstallationof • Installvalvestemshorizontallyorina mechanicalseals downwardpositionsothatanystemleakAtypicalcentrifugalpumpcanbeexpected agedoesnotentertheinsulation todeliveroperationalheadofabout: materials of construction

wheregisgravitationalaccelerationinm/ s2,D istherotordiameterinmandnisthe rotorspeedinrpm. Pumpmanufacturersusuallyspecifythat above450°F(230°C),acooled,jacketedstufngboxoracooledmechanical sealshouldbeused.Secondarysealing withventanddrainglandsissuggested tocollectuidleakageandtoprovide spaceforinertingtheoutsideoftheseal. Inertblanketingofthesealwithsteam ornitrogeneliminatesoxidationdeposit formation,whichcanleadtosealleakage. Thissecondarysealingprovidesadditional safetyinthecaseofsuddensealfailure. Someuidleakageatthesealiscommon inmechanicallysealedpumps.Forthose applicationswherethesmallleakageis undesirable,considertheuseofasealless pump.Whenselectingaseallesspump,the designermustconsidertheimpactofheat generatedbythemotorstatorrelativetothe vaporpressureofthepumpeduid. Onpumpswithastuffingbox,atleast fiveringsofpackingshouldbeprovided,suchaslaminargraphiterings.If expansionloopsareusedinthepump suctionpiping,theyshouldbeinstalled horizontally.

Systemmaterialsofconstructionmustbecom patiblewiththeheattransferuidinuse. Materialsofconstructionmustalsobe selectedonthebasisoftheirsuitabilityfor operationthroughoutthesystem’stemperaturerange(seetablebelow).Theusable rangesarebasedonthelowtemperature atwhichthematerialbecomesbrittleand thehightemperatureatwhichthematerial beginstolosemechanicalstrength. Itisadvisabletopracticere-safe constructionwhendesigningorganic heat-transfer-uidsystems.Fireresistance addressestheabilityofapipingsystemto remainfreeofdamagingleakswhenex posedtoexternalre.Apipingcomponent istypicallyconsideredtobereresistant whenitisabletowithstandexposureto 1200°F(650°C)for30min. Static seals

Typical temperature range

Fluoroelastomers

–10°F ( –25°C) to 400°F (200°C)

Flexible graphite

–325°F (–200°C) to 800°F (425°C)

Thepossibilityofleakagethroughjointsand ttingsischaracteristicofmostorganicheattransferuidsunlessthettingsareextremely tight.Systemdesignshouldminimizethenum berofconnectionsinthepipinglayout.The bestwaytopreventpipingleakageistoweld allconnections.Useofthreadedttingsis stronglydiscouragedduetotheirtendencyto leak.Whereaccessisnecessary,raised-face angeswithweldneckjointsorequivalent raised-faceangesshouldbeused,asshown inthetablebelow. Flange Use when type

Recommended gasketing

Class 150

Max. operating temp. < 350°F, Operating range < 300°F (175°C), Operating range < 300°F (170°C)

Metalinserted exible graphite gaskets

Class 300

Max. operating temp. > 350°F, Operating range > 300°F (175°C), Operating range > 300°F (170°C)

Flexible graphite-flled spiral-wound gaskets

Factorsaffectingtheleakperformanceof theangesincludetheabilityoftheange boltingtoeffectivelyseatthegasketandthe abilityoftheangetowithstandexternal momentsinthepipingsystem. Allpipinglayoutsshouldtakeintoaccounttheexpansionandcontractionofthe pipingwithtemperature,accordingtothe equation , where is the material’scoefcientofthermalexpansion.Itisvitalthatthestressplacedonthe systemduringitsexpansionfromambientto operatingconditionsnotexceedallowable stresslimits. filters

Piping Copper

–325°F (–200°C) to 350°F (175°C)

Car bon steel

–20°F ( –30°C) to 800°F (425°C)

Low alloy steel

–150°F (–100°C) to 800°F (425°C)

Stainless steel

–325°F (–200°C) to 800°F (425°C)

insulation

Normalhigh-temperatureinsulation,such ascalciumsilicate,mineralwoolandcellularglass,canbeusedinheattransferuid systems.However,uid-saturatedinsulation isapotentialrehazardatthetemperaturesoftenencounteredwhileoperating suchsystems.Organicheattransferuids generallycanexhibitaslowoxidation reactionwithairinthepresenceofporous insulatingmaterialsattemperaturesabove 500°F(260°C). Tominimizetherehazardpotentialin insulationsystems:

piping layout

Beforestartupofanewsystem,install awiremeshstrainerofapproximately 120-micronmeshsizeinthepumpsuction. Pipingsystemsshouldalsobedesigned withprovisionsfortheinstallationofasidestreamlter.Filtersthathavegenerallybeen employedfortheseapplicationsareglass berstring-woundcartridgesorcleanable sintered-metalltersinthe1–30-micronrange.

References 1.Wagner,W.,HeatTransfe rTechniquew ithOrganic Media,Gräfelng-München:TechnischerVerlagResch KG,1977. 2.Gamble,C.E.,CostManagementinHeatTransfer Systems,Chem. Eng. Prog. ,July2006pp.22-26.

3.“SystemsDesignData,”Pub.#7239193ver.C,Sol utia Inc.,2002. 4.“SystemDesignandMaintenance,”Pub.#TBS10-25 (E),SolutiaInc.,1998. 5.“LiquidPhaseDesignGuide,”Pub.#7239128C, SolutiaInc.,1999.

NOTICE:Althoughtheinformationandrecommendationssetforthherein(hereinafter“Information”)arepresentedingoodfaithandbelievedtobecorrectasofthedatehereof,SolutiaInc.makesno representationsorwarrantiesastothecompletenessoraccuracythereof.Informationissuppliedupontheconditionthatthepersonsreceivingsamewillmaketheirowndeterminationastoitssuitability fortheirpurposespriortouse.InnoeventwillSolutiaberesponsiblefordamagesofanynaturewhatsoeverresultingfromtheuseoforrelianceuponInformationortheproducttowhichInformation refers.Nothingcontainedhereinistobeconstruedasarecommendationtouseanyproduct,process,equipmentorformulationinconictwithanypatent,andSolutiamakesnorepresentationorwar ranty,expressorimplied,thattheusethereofwillnotinfringeanypatent.NOREPRESENTATIONSORWARRANTIES,EITHEREXPRESSORIMPLIED,OFMERCHANTABILITY,FITNESSFORAPARTICULAR PURPOSEOROFANYOTHERNATUREAREMADEHEREUNDERWITHRESPECTTOINFORMATIONORTHEPRODUCTTOWHICHINFORMATIONREFERS

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Heat transfer

By Rebekkah Marshall 

Basic Equations for HEat transfEr

Conduction:

q   dT   = k      A dx 

(1)

Convection:

q = havg A s (T s − Tf  ) 

(2)

Radiation

q  4 = εσ (Ts4 − T sur )   A

(3)

HEat transfEr tHrougH sHEll-and-tuBE HEat ExcHangErs

Heatdutyforexchangertransferringsensibleheat:

q = mC ˙ p ,avg (To − Ti  )  Foruseinheat-exchangercalculations, Equation(2)aboveisoftenwrittenasfollows:

(5)

 whereU canbecalculatedfromthefollowing relationship;

1= 1+ 1 + 1 +1 U ho  hi (Di / D o ) hw hs

(6)

 Variousequationsareavailable(seethereferences)forcalculatinghi  andho ,dependingon suchfactorsastheReynoldsnumbersforthe flowingfluidsandwhetherthefluidsundergo sensibleheattransferor,instead,vaporization orcondensation.Forinstance,forsensibleheat transferwithfluidsunderforcedconvection infullyturbulentflowinsidetubeswithsharpedgedentrances,thefollowing,wellestablished relationshipinvolvingtheNusselt,Reynoldsand Prandtlnumbersholds: 1/ 3

0.14 0.8  D m˙    c  µ       hi D i (7) = 0.023 i     p     µ    k    µ      k     µ w     Withtheassumptionthatthe( m/m w)termcan beignored,theimmediatelyaboveequationhas beenrearrangedasfollows[ 1,2 ]tofacilitate assessingtheeffectsoffluid(andsystem)propertiesuponhi (assumingsensibleheattransfer, fullturbulence,fluidinsidetubes):

hi  = 0.023

m˙ 0.8k 2 /3 c p 1/ 3

(8)

D i 0.2 µ 0.47 Forsensibleheattransferwithfluidsunder forcedconvectionflowingacrosstubebanks (thus,outsidethetubes),thefollowingrelationshiphasbeenpublished[ 3 ] h = a  cm˙  c µ   2/ 3  D m˙  m  µ   0.14 o     w    (9) µ     µ     k    

Inthisequation,thevaluesof a andmaretobe asfollows: Tube pattern Staggered Staggered Staggered Inline Inline Inline

Reynolds number above200,000 300to200,000 lessthan300 above200,000 300to200,000 lessthan300

∆T LM =

(Th − t h ) − (Tc − t c )  ln(Th − t h ) / (Tc − t c ) 

(10)

Energybalanceforaheatexchanger Ifanyheatexchangewiththeambientairis neglected,thefollowingrelationshipisvalid;

mt   ˙ h (H ha H hb )  mt ˙ c (H cb  Hca ) q (11) −

=

−

=

BatcH HEating

(4)

q = UAs ∆T  

ForanexcellentdiscussionoftheEquation(6) foulingfactor,hs,seeReference[4 ]. Theappropriate DT dependsontheconfigurationoftheheatexchanger(seereferences). Forexample,forasimplecountercurrentflowexchanger,theappropriatetemperature (referredtoasthelogmeantemperaturedifference, DT LM),isfoundasfollows;

m

a

0.300 0.365 0.640 0.300 0.349 0.569

0.166 0.273 1.309 0.124 0.211 0.742

Forheatingabatchoffluidfromtemperature T 1toT 2,bymeansofaninternalcoilofarea A   andanisothermalheatingmediumattemperatureT ,thefollowingrelationshipholds:

 T − T 1    UA   θ  =  T − T 2      cM  

ln

(12)

stEady-statE HEat flow By conduction

Forconductionthroughahomogeneousplane  wallofthickness x andconstant(oraverage) thermalconductivityk ,

q  = k ∆T   A  x 

(13)

 where DT isthetemperaturedifferencethrough thewall Forconductionthroughathree-layerplanewall (forexample,awallwiththermalinsulationon eachside),havinglayersofthicknesses x 1, x 2  and x 3andthermalconductivities k 1, k 2andk     3,

q  =

 x 1

k1A

+ x 2

∆T 

 x 3 k2 A + k 3A

(14)

 where DT istheoveralltemperaturedifference acrossallthreelayers Forconductionthroughthewallofacylinderof lengthL ,whoseinnerandouterradiiare r inner  andr outer ,withinnerandouterwallsattemperaturesT s,inner andT s,outer ,

q  =

k (2π L )(Ts ,inner − Ts , outer    ) ln   r outer  r      inner  

(15)

References  1. Guffey, G.E., SizingUp Heat TransferFluids and Heaters, Chem. Eng . , pp. 126–131, October1997. 2. McCabe,SmithandHarriott,“UnitOperations ofChemicalEngineering,”McGraw-Hill. 3. Chopey, ed., “Handbook of Chemical EngineeringCalculations,”,McGraw-Hill. 4. Polley,PutFoulinginItsPlace,Chem.Eng .,pp. 46-49,December2002. 5. Incropera and DeWitt, “Introduction to Heat Transfer,”Wiley. 6.  Jones,ThermalDesignoftheShell-and-Tube, Chem.Eng .,pp.60–65,March2002.

nOMenCLatUre:

 A cross-sectionalareaperpendiculartothe flowofheat  a  parameterinconvection-coefficient equation  A s  surfacearea c, C p specificheat;specificheatatconstant pressure C p,avg  specificheatataveragefluidtemperature D i  innerdiameterofheat-exchangertube D o  outerdiameterofheat-exchangertube H ca ,H ha  enthalpyperunitmassofentering coldandwarmfluid,respectively H cb ,H hb  enthalpyperunitmassofexiting coldandhotfluid,respectively h avg  averageconvectioncoefficient  h i  convectioncoefficientforinnertubewall h o  convectioncoefficientforoutertubewall h s  foulingheat-transfercoefficient  h w  coefficientofheat-transferradially throughtubewall;afunctionoftube thicknessandthermalconductivity k thermalconductivity L  length M weightofbatch m  parameterinconvection-coefficient equation • m  massflowrateoffluid • t  massflowrateofcoldfluid m  c  • m  t h  massflowrateofhotfluid q rateofheatflow T temperature(forradiationcalculations, useabsolutetemperature) T c  inaheatexchanger,theexittemperatureforthestreambeingcooled T f  temperatureoffluid T h  inaheatexchanger,theinlettemperatureforthestreambeingcooled T i  inlettemperature T o  outlettemperature T s  temperatureofsurface(forradiation calculations,useabsolutetemperature) T sur  temperatureofsurroundings(forradiationcalculations,useabsolutetemperature) t c  inaheatexchangertheinlettemperatureforthestreambeingheated t h  inaheatexchanger,theoutlettemperatureforthestreambeingheated DT LM logmeantemperaturedifference DT/dx temperaturegradientduringconductive heatflow U overallheattransfercoefficient   x distancetheheatflowsduringconduction e emissivity m  viscosity;viscosityatbulkfluidtemperature m w viscosityattube-walltemperature s Stefan-Boltzmannconstant  q

timerequiredforbatchheating

‫ط‬»‫؟‬¬ ‫ج‬®‫؟‬²-‫؛‬»®‫و‬ ‫ح‬§-¬»³ ‫ـ‬»-·¹² ×× ‫ـ‬»°‫؟‬®¬³»²¬ ‫غ‬¼·¬±®‫ح و‬½±¬¬ ‫ض‬»²µ·²‫حزكذبغ‬×‫زر‬ ‫صزكج‬

‫ك‬ ©»´´ ¼»-·¹²»¼ »¨°‫؟‬²-·±² ¬‫؟‬²µ‫و‬ ïٍ ‫؟س‬·²¬‫؟‬·²- ‫؟‬ -¬‫؟‬¬·½ °«³°َ- «½¬·±² ¸»‫؟‬¼ îٍ ‫ف‬±³°»²-‫؟‬¬»- ‫؛‬±® ¬»³°»®‫؟‬¬«®»َ® »´‫؟‬¬»¼ ‫ھ‬±´«³» ‫؟‬²¼ °®»--«®» ½¸‫؟‬²¹»‫ي‬ٍ ‫ذ‬®±‫ھ‬·¼»- ‫؟‬ ³»‫؟‬²- ±‫؛‬ ‫ھ‬»²¬·²¹ ³±·-¬«®» ‫؟‬²¼ ´±© ¾±·´»®‫ث‬-«‫؟‬´´§ô ¬¸» »¨°‫؟‬²-·±² ¬‫؟‬²µ ·- ·²-¬‫؟‬´´»¼ ‫؟‬¬ ¬¸» ¸·¹¸»-¬ °±·²¬ ·² ¬¸» -§-¬»³ ‫؟‬²¼ ·½±²²»½¬»¼ ¬± ¬¸» -«½¬·±² -·¼» ±‫؛‬ ¬¸» °«³°-ٍ ׬ -¸±«´¼ -»®‫ھ‬» ‫؟‬- ¬¸» ³‫؟‬·² ‫ھ‬»²¬·²¹ °±·²¬ ±‫؛‬  ¬¸» -§-¬»³ô ‫؟‬- ©»´´ ‫؟‬- °®±‫ھ‬·¼» ‫؛‬±® -§-¬»³ ±°»®‫؟‬¬·²¹ ¬»³°»®‫؟‬¬«®» ®‫؟‬²¹»ٍ ‫ع‬±® ¸»‫؟‬¬َ ·²¹ ½·®½«·¬-ô ¬¸» »¨°‫؟‬²-·±² ¬‫؟‬²µ -¸±«´¼ ¾» -·¦»¼ -± ¬¸‫؟‬¬ ·¬ ·- ±²»َ¯  «‫؟‬®¬»® ‫؛‬«´´ ‫؟‬¬ ‫؟‬³¾·»²¬ ¬»³°»®‫؟‬¬«®» ‫؟‬²¼ ¬¸®»»َ¯  «‫؟‬®¬»®- ‫؛‬«´´ ©¸»² ¬¸» -§-¬»³ ·- ‫؟‬¬ ±°»®‫؟‬¬·²¹ ¬»³°»®‫؟‬¬«®»ٍ ‫ع‬±® ½±±´·²¹ô ·¬ -¸±«´¼ ¾» ‫ھ‬·½» ‫ھ‬»®-‫؟‬ٍ ‫ج‬¸» ¼±«¾´» ¼®±°َ´»¹ »¨°‫؟‬²-·±² ¬‫؟‬²µ

-‫؟‬¬·-‫؟؛‬½¬±®·´§ ¾§ ®»´‫؟‬¬·‫ھ‬»´§ -·³°´» ‫چ‬±²َ± ‫؛؛‬Œ َ ±® ‫چ‬¸·¹¸´ ±©Œ ½±²¬®±´´»®-ٍ ‫ط‬±©»‫ھ‬»®ô «²·¬±‫؛‬ ‫؟‬´´ -·¦»- ©·´´ ±°»®‫؟‬¬» ³±®» «²·‫؛‬±®³´§ ·‫؛‬  »¯«·°°»¼ ©·¬¸ ³±¼«´‫؟‬¬·²¹ ¬»³°»®‫؟‬¬«®» ½±²َ ¬¸» »²»®¹§ ½±²-«³°¬·±² ±‫؛‬ ¬¸» »¯«·°³»²¬ٍ ײ ‫؟‬ ³«´¬·°´»َ« -»® -§-¬»³ô -»°‫؟‬®‫؟‬¬» ½±²¬®±´-¸±«´¼ ¾» ·²-¬‫؟‬´´»¼ ±² »‫؟‬½¸ ½±²-«³·²¹ «²·¬ ¬± ‫؟‬--«®» ¬¸» °®±°»® »²»®¹§ ¼»´·‫ھ‬»®§ٍ ‫ع‬×‫غخ‬ ‫حغخ‬×‫غفزكجح‬

·³°®±‫ھ‬»- ¬¸» ‫ھ‬»²¬·²¹ ½‫؟‬°‫؟‬¾·´·¬§ ±‫؛‬ ¬¸» -§-¬»³ٍ ‫ك‬´´ ‫ھ‬»²¬ ´·²»- -¸±«´¼ ¾» ®±«¬»¼ ¬± ‫؟‬ -‫؛؟‬» ´±½‫؟‬¬·±²ٍ ‫غ‬¨°»®·»²½» ·²¼·½‫؟‬¬»- ¬¸‫؟‬¬ -§-¬»³- ©·¬¸ »¨°‫؟‬²-·±² ¬‫؟‬²µ- ±°»² ¬± ¬¸» ‫؟‬¬³±-°¸»®» ¬± ±¨·¼‫؟‬¬·±² ‫؟‬²¼ »¨½»--·‫ھ‬» ³±·-¬«®»ٍ ‫ج‬¸»®»َ ‫؛‬±®»ô ±°»² »¨°‫؟‬²-·±² ¬‫؟‬²µ- -¸±«´¼ ²±¬ ¾» »³°´±§»¼ ·² -§-¬»³- «-·²¹ ±®¹‫؟‬²·½ ¸»‫؟‬¬ ¬·±² ·- ¬± ¾´‫؟‬²µ»¬ ¬¸» »¨°‫؟‬²-·±² ¬‫؟‬²µ ‫؟ھ‬°±® -°‫؟‬½» ©·¬¸ ‫؟‬² ·²»®¬ ¹‫؟‬-ّ ‫؛‬ ± ® »¨‫؟‬³°´»ô ²·¬®±َ ¹»²ô ‫رف‬îô ±® ²‫؟‬¬«®‫؟‬´ ¹‫؟‬-ٍ÷ ‫ة‬¸»² «-·²¹ ‫؟‬ ²·¬®±¹»² ¾´‫؟‬²µ»¬ô ³±·-¬«®» -¸±«´¼ ¾» ¼®·‫ھ‬»² -»¬ٍ ×‫؛‬ ¬¸·- ·- ²±¬ °®‫؟‬½¬·½‫؟‬´ô ‫؟‬·® ½±²¬‫؟‬½¬ ½‫؟‬² ¾» ³·²·³·¦»¼ ¾§ ‫؟‬ ½±´¼ -»‫؟‬´ ¬®‫؟‬° ‫؟‬®®‫؟‬²¹»َ ³»²¬ٍ ‫ش‬±© ¾±·´»®- ‫؟‬²¼ ³±·-¬«®» ½‫؟‬² ½±´´»½¬ -¸±«´¼ ¾» ¼·-½‫؟‬®¼»¼ °»®·±¼·½‫؟‬´´§ٍ ‫حشرخجزرف‬

°‫؟‬½µ·²¹ ¾±«²¼ ¹‫؟‬-µ»¬-

‫حغتشكت‬ َ ‫؟ف‬-¬َ ±® ‫؛‬±®¹»¼- ¬»»´ ‫؟ھ‬´‫ھ‬»- ©·¬¸ ï‫ي‬َ½ ¸®±³» ¬®·³ ‫؟‬®» -‫؟‬¬·-‫؟؛‬½¬±®§ ‫؛‬±® -»®‫ھ‬·½» ·² ±®¹‫؟‬²·½

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‫خغجكغط‬

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½±°°»®

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Biodiesel Production Department Editor: Kate Torzewski 

B

iodiesel can be produced rom vegetable oils by three types o reactions: base catalyzed transesterication o the oil; direct acid-catalyzed transesterication o the oil; and conversion o the oil to its atty acids, and then to biodiesel. Biodiesel is typically produced by a base-catalyzed reaction (Figure 2). This method o production has several advantages, including the ollowing: low temperature (150˚F) and pressure (20 psi) reaction that requires only standard materials o construction; direct conversion to biodiesel with no intermediate compounds; and high conversion (98%) with minimal side reactions and a low reaction time. In the chemical reaction or basecatalyzed biodiesel production, vegetable oil is reacted with a short chain alcohol (signied by ROH) in the presence o a catalyst to produce glycerin and biodiesel. The atty acid chains associated with the oil, which are mostly palmitic, stearic, oleic, and linoleic acids or naturally occurring oils, are represented by R', R'' and R''' (Figure 1). Production stePs

Mixing of alcohol and catalyst . The catalyst is typically sodium hydroxide (caustic soda) or potassium hydroxide (potash). It is dissolved in the alcohol using a standard agitator or mixer. Methanol or ethanol is commonly used as the alcohol. Reaction. The mixture o alcohol and catalyst is charged into a closed reaction vessel, and the oil is added. The reaction mix is kept just above the boiling point o the alcohol, 160°F, to speed up the reaction, although it is sometimes recommend to run the reaction at room temperature. The reaction time can vary rom 1–8 h. Excess alcohol is used to ensure total conversion o the oil to its esters. The amount o water and ree atty acids in the incoming oil must be monitored, because i either level is too high, it can inhibit soap

ormation and the separation o glycerin downstream. Methanol

Separation. Glycerin and biodiesel are the two main products o reaction, with each containing an amount o unreacted alcohol. Since the glycerin phase is much more dense than biodiesel phase, the two phases can be separated by gravity in a settling vessel, with glycerin simply drawn o the bottom o the settling vessel. Alternatively, a centriuge can be used to separate the two materials more quickly. Glycerin neutralization. The separated glycerin contains unused catalyst and soaps, which are neutralized with an acid. Water and alcohol are removed to produce glycerin at 80–88% purity to sell as crude glycerin. Alternatively, glycerin can distilled to 99% purity or higher or selling to the cosmetic and pharmaceutical industries.

Mixer

    l    o    n    a     h    t    e    m    s    s    e    c    x     E

Product quality and registration. Prior to use as a commercial uel in the U.S., the nished biodiesel must be analyzed to ensure it meets CH2OCOR’’’ CH2OCOR’’

3 ROH

CH2OCOR’ Vegetable oil

Alcohol

Reactor

Settler Glycerin

Neutralization

Wash

   n    o     i    t    a     l     l     i    t    s     i    D

   n    o     i    t    a     l     l     i    t    s     i    D

Biodiesel

Glycerin

ASTM specications. Additionally, all biodiesel produced must be registered with the U.S. Environmental Protection Agency (Washington, D.C) under 40 CFR Part 79. References

1. Biodiesel Production & Quality Standards,  July, 2008. National Biodiesel Board, www.biodiesel.org/resources/uelactsheets

Catalyst +

Soybean oil

Biodiesel

Methyl ester wash . Ater the biodiesel is separated rom glycerin, residual catalyst or soaps can be removed with a gentle warm water wash.  Alcohol removal . Unreacted alcohol in both the glycerin and biodiesel phases is removed by fash evaporation or distillation. The recovered alcohol is then reused or mixing with the catalyst. Alcohol removal can occur ater the wash and neutralization, as shown in Figure 2 to the right, but it can occur beore these steps as well.

Catalyst

CH2OH CH2OH

R’’’ COOR +

R’’ COOR

CH2OH

R’ COOR

Glycerin

Biodiesel

Burner Operating Characteristics* Department Editor: Scott Jenkins urners are critical for the successful operation of industrial furnaces. Presented here is a set of equations that can be used to calculate characteristics of burner operation, including flame length, flame diameter, ignitability and flameout conditions. Equations are based on pre-mix burners operating at atmospheric pressure and firing natural gas only. Premix burners create short and compact flames compared to raw gas burners, and are designed to function with fuel-gas mixtures that have consistent specific gravity and composition.

B

Burner requirements

For direct-fired heaters to function correctly, burners must be capable of providing sufficient heat liberation from the fuel to meet heater processing requirements — based on the lower heating value (LHV) of the fuel. A fuel’s LHV can be defined as the amount of heat produced by combusting a specified volume, and returning the combustion products to 150 C. For the heater to operate at the design process flowrate, the burners need to provide the heat necessary to maintain process fluid temperature and meet vaporization requirements at the heating coil outlet. The number, size and placement of burners must allow each coil to operate at the same design outlet temperature Design tube-metal temperature cannot be exceeded at any point on the coils Burner size must allow an outlet velocity that does not result in malfunction over the range of flow conditions Burner flame length should be less than firebox height (for vertical cylindrical heaters) or less than firebox length (for end-wall-fired heaters) Excessive flame height and diameter should be avoided to prevent flame impingement on tubes Burner spacing should be sufficient to allow burner-to-burner, as well as burner-to-tube clearance °













The following equations can help establish optimal burner diameter:

Burner clearance

Establishing burner-to-burner clearance and burner spacing should be based on maximum burner flame diameter. Further, burner flame diameter should be evaluated at maximum burner-flame length. Sufficient burner-to-burner, outside diameter clearance should take into account the placement of structural elements between burners. Sufficient burner-to-burner clearance prevents interference between the flame bodies and unburned fuel cores generated by adjacent burners, which results in the absence of unburned fuel within the burner flame when maximum flame length is reached. Burner center-to-center spacing should be at least one fully combusted flame diameter. Clearance between the burnerflame (at maximum diameter) and the outside diameter of tubular heating surfaces should be set such that burner-to-tube flame impingement is avoided. Doing so will prevent tube damage due to overheating and will make best use of heating surfaces. Flameout

NOMENCLATURE

Q lib heater  = Heater liberation, Btu/h N b  = Number of burners D b  = Burner diameter, ft  V b  = Burner exit velocity, ft/s C fuel  = Fuel, ft 3 LHV  = Lower heating value of fuel, Btu/lb C air+fuel  = Volume of air and fuel mixture, ft 3 SV fuel  = Specific volume of fuel, ft 3/lb D f max  = Maximum flame diameter, ft  L f  = Flame length, ft  SV flame  = Specific volume of flame, ft 3/lb = Flame propagation velocity, ft/s V f  Q gain  = Burner heat gain, Btu/h Q loss  = Burner heat loss, Btu/h  A s  = Flame front area, ft 2 (HTC)c (HTC)f , (HTC)r  = Natural convective, forcedconvective, and radiative heat transfer coefficients, respectively, Btu/h-ft 2- F T flame  = Flame temperature, R T surr  = Surrounding temperature, R E g  = Flame emissivity C p  = Gas specific heat, Btu/lb- F  A  = Frequency factor in the Arrhenius equation H  = Heat of activation, Btu/lb-mol R R  = Gas constant, 1.987 Btu/lb-mol R T  = Gas Temperature, R dC m /dt  = Fuel concentration change, mol per ft 3/s K  = Reaction velocity constant, s–1 W f  = Fuel, lb/h °

°

At high burner velocities, flame loss can occur if the heat gain due to burner ignition is less than the heat loss from the burner flame. Burner velocities may be pushed well above that used in normal heater operation in an effort to achieve higher heater capacity. Aside from flame loss while the heater is in operation, flameout can also be characterized by difficulty maintaining a stable flame at startup, or an inability to ignite the burner. The following equations can help predict the circumstances under which flamout conditions might occur:

(5)

Flame velocity

The heat generated by combustion is dependent on the flame propagation velocity. In a situation with 0% excess air, the ratio of fuel-to-fuel+air is about 0.1. In that case, evaluation of the flame propagation velocity is straightforward. However, at fuel-tofuel+air ratios higher or lower than 0.1, it is more difficult. The following equations can help predict flame propagation velocity in those cases: (10)

(6) (1)

(11)

(7) (12) (2)

(8)

(3) (9) (4)

References 1. Cross, A., Fired-Heater Burner Performance, Chem. Eng., April 2008, pp. 44–47.

*The text was adapted from the article “Fired-Heater Burner Performance,” by Alan Cross. It appeared in the April 2008 issue of Chemical Engineering.

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Fuel Selection Considerations

Department Editor: Rita L. D'Aquino  he selection and application o uels to various  combustors are complex. Most existing units have A limited exibility in their ability to fre alternative  uels. New units must be careully planned to assure   the lowest frst costs without jeopardizing the uture  capability to switch to a dierent uel.

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Natural gas Natural gas has traditionally been the most attractive uel type or combustors because o the limited need or uel-handling equipment (e.g., pipelines, metering, a liquid-knockout drum, and appropriate controls) and the reedom rom pollution-control equipment. Drawbacks include rising uel costs, inadequate gas supplies and low er boiler eiciencies that result rom iring natural gas, particularly when compared to the iring eiciencies o oil or coal. Fuel oil Fuel oils are graded as No. 1, No. 2, No. 4, No. 5 (light), No. 5 (heavy), and No. 6. Distillates are Nos. 1 and 2, and residual oils are Nos. 4, 5, and 6. Oils are classifed according to their physical characteristics by the American Society or Testing and Materials (ASTM) per Standard D-396. No. 2 oil is suitable or industrial use and or home heating. The primary advantage o using a distillate oil rather than a residual oil is that it is easier to handle, requiring no heating to transport and no temperature control to lower the viscosity or proper atomization and combustion. However, considerable purchase cost penalties exist between residual and distillate. Distillates can be divided into two classes: straight-run, which is produced rom crude oil by heating it and then condensing the vapors; and cracked, which involves refning at a high temperature and pressure, or refning with catalysts to produce the required oil rom heavier crudes. Cracked oils contain substantially more aromatic and olefnic hydrocarbons, which are more difcult to burn than the parafnic and naphthenic hydrocarbons obtained rom the straight-run process. Sometimes a cracked distillate, called industrial No. 2, is used in uel-burning installations o medium size (small package boiler or ceramic kilns, or example). Because o the viscosity range permitted by ASTM, No. 4 and No. 5 oil can be produced in a variety o ways: blending o No. 2 and No. 6, mixing refnery by-products, utilization o ospecifcation products, and so on. Because o the potential variations in characteristics, it is important to monitor combustion perormance routinely to obtain optimum results. Burner modifcations may be required to switch rom, s ay, a No. 4 blend to a No. 4 distillate. Light (or cold) and heavy (or hot) No. 5 uel oil are distinguished primarily by their viscosity ranges: 150 to 300 SUS (Saybolt Universal Seconds) at 100°F and 350 to 750 SUS at 100°F, respectively. The (No.) classes normally delineate the need or preheating or proper atomization. The No. 6 uel oil is also reerred to as residual, Bunker C, and reduced- or vacuum bottoms. Because o its high viscosity, 900 to 9,000 SUS at 100°F, it can only be used in systems designed with heated storage and a high enough temperature (to achieve proper viscosity) at the burner or atomization. Notable fuel oil properties include the following: 1) Viscosity indicates the time required in seconds or 60 cm3 o oil to ow through a standard-size orifce at a specifc temperature. In the U.S., it is normally determined with a Saybolt viscosimeter, which comes in Universal and Furol variants. The dierences between them are the orifce size and the sample temperature. Thus, when stating an oil’s

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FIGURE 1.

This nomograph is used to estimate annual cost savings from reducing combustible losses due to unburned carbon

viscosity, the type o instrument and temperature must also be stated. The Universal has the smallest opening and is used or lighter oils. 2) The ash point is the temperature at which oil vapors are ignited by an external ame. As heating continues above this point, sufcient vapors are driven o to produce continuous combustion. The ash point is also an indication o the maximum temperature or sae handling. Distillate oils have ash points o 145–200°F; heavier oils have ash points up to 250°F. 3) The pour point is the lowest temperature at which an oil ows under standard conditions, and is roughly 5°F above the solidifcation temperature. Knowledge o the pour point helps determine the need or heated storage, the storage temperature, and the need or heating and pour-point depressant. Coal The selection o coal as uel involves higher capital investments because o the need or handling equipment, coal preparation (crushing, conveying, pulverizing, etc.) and storage; ash handling and storage; pollution-abatement equipment; and maintenance. The operating cost savings at current (2007) uel prices o coal over oil or gas justifes a great portion, i not all, o the signifcantly higher capital investments required or coal. Coal-fred steam generators and vessels inherently suer efciency losses due to a ailure to burn all the available uel. The unburned uel is the remaining carbon in the letover ash. The nomograph (Figure 1) may be used to assess how a reduction in unburned carbon translates into energy and cost savings. A sample calculation ollows. Example: The system is a coal-fred steam generator with a continuous rating o 145,000 lb/h; average (avg.) boiler load = 125,000 lb/h; existing combustibles in ash = 40% (measured); obtainable combustibles in ash = 5%; actual operating time = 8,500 h/yr; design-unit heat output = 150 × 106 Btu/h; avg. heat output = 129 × 106 Btu/h; avg. uel cost = $1.50/106 Btu.  Analysis: In Figure 1A, the percent o existing combustibles (measured) are shown on the horizontal axis. The curves above it represent the proposed improvement in percent o unburned carbon in ash. From the coordinates in Figure 1A draw a horizontal line to the curve in Figure 1B that represents the design-unit heat output. Drop the line to the appropriate uel-cost curve in Figure 1C. Extend the line rom that point to the let to obtain the corresponding annual uel savings, assuming continuous operation at ull boiler design output. To calculate actual annual uel

This article has been drawn from the work of Wayne Turner and Steve Doty, “Boilers and Fired Systems,” Energy Management Handbook, 6th Ed., Ch. 5, The Fairmont Press, Lilburn, Ga., 2006.

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savings, a correction actor (CF) is required that considers actual boiler load and actual run time: Actual savings, $ = Savings rom chart x CF where CF = operating avg. heat output/design heat output × [(actual operating h/yr)/(8,760 h/yr)] Savings or this example = $210,000/yr × [(129 × 106 Btu/h)/(150 × 106 Btu/h)] × [(8,500 h/yr)/(8,760 h/yr)] = $175,200/yr. Note: I the heat output o the unit or the average uel cost exceeds the limit o the fgures, use hal o the particular value and double the savings obtained rom Figure 1C. It is probable that pulverized-coal-fred installations suer rom high UCL whenever any o the ollowing are experienced: a change in the raw-uel quality rom the original design basis; deterioration o the uel burners, burner throats, or burner swirl plates or impellers; increased requency o soot blowing to maintain heat-transer surace cleanliness; a noted increase in stack gas opacity; uneven lame patterns characterized by a particularly bright spot in one section o the lame and a notably dark spot in another; CO ormation as determined rom a lue-gas analysis; requent smoking observed in the combustion zone; increases in reuse quantities in collection devices; neglect o critical pulverizer internals and classiier assemblies; a high incidence o coal “hang-up” in the distribution piping to the burners; and requent manipulation o the air/coal primary and secondary air registers. Techniques used successully to reduce high UCL and/or high-excess-air operation include: modiying or replacing the pulverizer internals to increase the coal fneness; installing additional or new combustion controls to maintain consistent burner perormance; purchasing new coal eeders that are compatible with and responsive to unit demand uctuations; calibrating air ow and metering devices to ensure correct air/coal mixtures and velocities at the burner throats; installing turning vanes or air oils in the secondary air-supply duct or air plenum to ensure even distribution and proper air/uel mixing at each burner; replacing worn and abraded burner impeller plates; installing new classifers to ensure that proper coal fnes reach the burners or combustion; rerouting or modiying air/coal distribution piping to avoid coal hang-up; increasing the air/coal mixture temperature exiting the pulverizers to ensure good ignition without coking; and cleaning deposits rom burner throats. ■

Causes of Overpressurization Department Editor: Kate Torzewski  he ailure o a device or o a group o components can lead to overpressurization and subsequent adverse events, such as re, explosion, spill or release. The most common causes o overpressurization are listed below. Understanding the circumstances surrounding overpressurization will help an engineer to avoid these ailures.

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External re

According to API RP 520 and 521 standards, a re-exposed area is within an area o 2,500 and 5,000 t 2, and below a height o 25-t above the grade. In this scenario, the exposed vessel is blocked in. Potential vapors resulting rom the re must be relieved using a PRV on the vessel, or via a vent path that remains in a locked-open position between the vessel and an adjoining vessel. Blocked outlets

The closure o a block valve on the outlet o a pressure vessel can cause the vessel's internal pressure to exceed its maximum allowable working pressure i the source pressure exceeds the vessel design pressure. Blocked outlets can be caused by control valve ailure, inadvertent valve operation, instrument-air or power ailure, and other actors. A PRV must have sucient capacity to pass a fuid fowrate that is equivalent to the dierence between those o the incoming fuids and the outgoing fuids.

range to maximum pressure. As a general rule, when sizing a PRV, maximum heat-duty assumed or the abnormal case should be no more than 125% o normal heat duty. Abnormal vapor input

Abnormal vapor input can be caused by the ailure o the upstream control valve to ully open, or upstream-relieving or inadvertent valve opening. The required relieving capacity must be equal to or greater than the amount o the vapor accumulation expected under the relieving conditions. Loss o absorbent fow

When gas removal by absorbent is more than 25% o the total inlet-vapor fow, an interruption o absorbent fow could cause pressure to rise in the absorber. The PRV should be sized base on the net accumulation o the vapor at the relieving conditions.

These ailures can include the ollowing: general power ailure, partial power ailure, loss o instrument air, loss o cooling water, loss o steam, and loss o uel gas or uel oil. For these cases, a fare header should be designed and sized based on the maximum relie load that could result rom a potential utility ailure. Loss o cooling duty

Cooling-duty losses can include the ollowing: loss o quench stream, air-cooled exchanger ailure, loss o cold eed and loss o refux. Relieving capacity should be calculated by perorming a heat balance on the system, based on the loss o the condensing duty. Thermal expansion

When liquid is blocked in a vessel or pipeline, external heat input can cause liquid temperature, and hence volume, to rise. This can be caused by the ollowing: liquid that is blocked in a pipeline and is being heated, the cold side o a heat exchanger being lled while the hot side is fowing, or a lled vessel at ambient temperature that is being heated by direct solar radiation. PRVs used in these cases can be easily analyzed and sized. Abnormal heat input

This ailure can be caused by: the supply o heating medium, such as uel oil or uel gas to a red heater, being increased; heat transer occuring in a new and clean heat exchanger ater revamp; control valve or the uel supply ailing to ully open; or supply pressure o the heating steam being changed rom normal

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Entrance o volatile materials

The entrance o a volatile liquid, such as water or light hydrocarbons into hot oil during a process upset, can cause instantaneous phase expansion. Instead o relying on PRVs, processes should be properly designed with the use o double block valves, the avoidance o water-collecting pockets and use o steam condensate traps and bleeds on water connections. Accumulation o non-condensibles

Utility ailures

A pressure relief valve (PRV) is an automatic pressurerelieving device that is actuated by pressure at the inlet of the valve. Though safety valve, or safety relief valve, is the terminology for valves relieving gas or vapor, we will use “PRV” to describe all types of pressure relief valves. A relief valve, used for liquid service, generally opens in proportion to any increase in pressure over opening pressure. A safety valve has characteristics similar to a relief valve except that it usually opens rapidly (pops), and is primarily used for gas or vapor service

Accumulation can result rom blocking o the normal non-condensible vent or accumulation in the pocket o a piping conguration or equipment. Because this can result in a loss o cooling duty, PRV analysis should be handled the same way. Valve malunction

Check-valve malunction results in backfow, which can be rom 5 to 25% o the normal fowrate. Required relie capacity should be based on this. Inadvertent valve operation results in a valve position that is opposite rom normal operating conditions, which is largely caused by human error and can be avoided by careul operation. Control valve ailure to open or close is caused by electronic- or mechanical-signal ailure. This typically will aect just one valve at a time and should be analyzed on a caseby-case basis. Process control ailure

This situation reers to the ailure o process controllers, such as programmable logic controllers and distributed control systems. The potential impact o ailure o every control loop should be analyzed, as well as the impact i one loop ails but all others remain active. As a general rule, the required relie capacity must be greater than the vapor generated because o heat buildup in the system. Exchange tube rupture

When an upstream vessel is relieving by discharge fuid to a downstream vessel,

Source: Farris Engineering, Brecksville, Ohio

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the downstream vessel should should be designed to handle the pressure and volume o the incoming stream without overpressurizing. I the upstream vessel does not have adequate relie capacity, the downstream vessel should have a PRV o its own. When two vessels are connected by an open path and the rst has its own PRV and discharges to a fare header, the second will experience the impact rom the relieving pressure o the rst vessel and should be analyzed accordingly. Upstream relieving

Required relie capacity should be greater than the vapor generated because o heat buildup in the system. Runaway chemical reaction

Runaway reactions tend to accelerate with rising temperature; extremely high volumes o non-condensibles with high energy can cause the internal pressure o a vessel or pipeline to rise rapidly. PRVs may not provide sucient relie, so vapor-depressurizing systems, rupture disks and emergency vents are preerable. Reerences 1.Wong, W., Protect Plants Against Overpressure, Chem. Eng. June 2001, pp. 66–73. 2.Goodner, H., A New Way o Quantiying Risks: Part 2, Chem. Eng. November 1993, pp. 140–146. 3.Emerson, G., Selecting Pressure Relie Valves, Chem. Eng. March 18, 1985, pp. 195–200.

Hazardous Area Classifcation

Department Editor: Rebekkah Marshall  Guidelines by location Over the years, hazardous area classification requirements for the U.S. have evolved around a single area-classification system known as the Class/ Division system. Today, the system addresses establishment of boundaries of hazardous areas and the equipment and wiring used in them. Meanwhile, European countries, as well as some other countries around the world, have developed their own area classification systems to address hazardous locations safety issues. This independent development has resulted in systems for these countries or groups of countries based on the International Electrotechnical Commission (IEC) Zone system, with deviations to meet each country’s national codes. While other countries do accept and use the Division system (most notably Canada and Mexico), the majority of the world’s hazardous locations are classified using the concepts of the IEC Zone system. The U.S. National Electrical Code (NEC; NFPA 70) also recognizes the Zone system and allows its use in the U.S. under article 505 of the NEC. ATEX requires the use of I EC-type hazardous area classifications.

defininG hazardous areas

Table 1. Hazardous Areas* North America Class — Division

Class I — Gas or vapor

IEC (Europe) Zones

Division 1: Present or likely to be present in normal operation

An area in which an explosive atmosphere is Gas/Vapor Zone 1 (Gas) /  Zone 21 (Dust) likely to occur in normal operation Division 2: Not or Dust Class III — Fiber present in normal Zone 2 (Gas) /  An area in which an explosive atmosphere is or flying (no group operation, could Zone 22 (Dust) not likely to occur in normal operations and, designation) be present in abif it does occur, will exist for only a short time normal operation * This table represents a corrected version from that in the original printing Class II — Dust

Table 2. Relationship Between Divisions a nd Zones North America Europe Division Zone method method IEC standard Ignitable mixture present Zone Zone 0 continuously (long periods) Division 0 (Zone 20-Dust) 1 Ignitable mixture present Zone Zone 1 intermittently 1 (Zone 21-Dust) Zone 2 Ignitable mixture is not Division Zone (Zone 22-Dust) normally present 2 2

Table 4. Gas and Dust Groups Hazardous locations are grouped according to their ignition properties Typical gas Acetylene Hydrogen

IEC gas North Amerigroup can group IIC A IIC + H2 B

Minimum ignition energy 20µJ 20µJ

Ethylene

IIB

C

60µJ

Propane

IIA

D

100µJ

A hazardous area is designated as any I — location in which a combustible material *Methane Metal dust — E is or may be present in the atmosphere in sufficient concentration to produce an Coal dust — F ignitable mixture. The North American Grain dust — G method identifies these areas by Class, Fibers — — Division and Group or optionally by *Mining application under jurisdiction of U.S. Mine Safety and Class, Zone and Group, while the IEC and CENELEC designate these areas by Health Administration (MSHA) Gas/Dust, Zone and Group. The likeliTable 5. Information Required For Establishing hood that the explosive atmospheres are Extent of Hazardous Area present when the equipment is operating Gas/Vapors Dust are designated in Tables 1, 2 and 5. • Flash point • Flammability limits • Auto-ignition temperature equipment selection • Minimum ignition energy, MIC or MESG – for equipment For equipment selection purposes, hazardous area classifications also consider: selection purposes • Gas/Vapor group • The maximum surface temperature of • Vapor/Gas density the equipment under normal operat- • Area ventilation conditions ing conditions (see the Temperature • Location of gas/vapor release points. Frequency and rate of Code designations in Table 3) release

• The ignition-related properties of the explosive atmosphere (see the Group designations in Table 4) • The protection method(s) used by the equipment to prevent ignition of the surrounding atmosphere (see the Protection Method designations in Table 6)

An area in which an explosive atmosphere is continually present or present for long periods or frequently

Zone 0 (Gas) /  Zone 20 (Dust)

• A/B classification • Minimum explosible dust concentration • Minimum ignition energy • Minimum ignition temperature (cloud/layer) • Electrical resistivity • Dust group • Area ventilation conditions • Location of dust release points. Frequency and rate of release

Table 3. Temperature Codes The Temperature class defines the maximum surface temperature of the device. Ratings are given with reference to 40°C ambient T1 450°C T3A 180°C T2 T2A T2B

300°C 280°C 260°C

T3B T3C T4

165°C 160°C 135°C

T2C

230°C

T4A

120°C

T2D

215°C

T5

100°C

T3

200°C

T6

85°C

The additional temperature classifications highlighted above are for USA and Canada only

Table 6. Types of Protection for Electrical Equipment (IEC/ATEX and NEC) Technique

IEC PermitPermitted Dested DiZone cription vision

Ex o Ex p Ex q

1&2 1&2 1&2

— 1&2 —

Explosion Proof

Ex d —

1&2 —

— 1&2

Increased safety

—

—

—

Ex ia

0,1 & 2

1&2

Oil immersion Pressurization Powder filling Flameproof

Intrinsic safety Intrinsic safety

Ex ib

1&2

—

Encapsulation

Ex m

1&2

—

Special protection

Ex s

0,1 & 2

—

—

—

2

Nonsparking

Ex nA

2

—

Enclosed break

Ex nC

2

—

Energy limited Simplified pressurization

Ex nL

2

—

Ex nP

2

—

Ex nR

2

—

Nonincendive

Restricted breathing

Table 7. Types of Ignition Protection for Mechanical Equipment (ATEX) Method To ensure that ignition sources cannot arise To ensure that ignition sources cannot become active To prevent the explosive atmosphere from reaching the ignition source

Description Construction safety “c”, Inherent safety “g”, Control of ignition sources “b” Inert liquid immersion “k”, Inert gas pressurization “p”, Flow restricting enclosure “fr”

To contain the explosion and prevent flame propagation

Flame proof enclosures “d”, Flame arresters

 Acknowledgement and references We would like to thank Vladimir Stetsovsky of Chilworth Technology, Inc. for his contributions to this page 1. National Electrical Code-2005-NFPA 70, National Fire Protection Association. 2. NFPA 497-2004, Recommended Practice for the

Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. 3. NFPA 499-2004, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.

4. IEC 60079-10-2002 Electrical apparatus for explosive gas atmospheres — Part 10: Classification of hazardous areas. 5. IEC 61241-3-2005 Electrical apparatus for use in the presence of combustible dust — Part 3: C lassification of areas where combustible dusts are or may be present.

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¸‫؟‬²¼´·²¹ ·- ¾«·´¬ ·²¬± ¬¸» ‫حفـ‬

‫چ‬¬‫؟‬´µŒ ¬± »‫؟‬½¸ ±¬¸»® ·² ‫؟‬ °»»®َ¬ ±َ°  »»® ‫؟؛‬-¸·±²ّ - ¸‫؟‬®·²¹ ‫؟‬ ´±¬ ±‫؛‬ ¼‫؟‬¬‫؟‬ ¬¸®±«¹¸±«¬ ¬¸» ‫؟‬°°´·½‫؟‬¬·±²÷ٍ ‫ج‬¸·- ·- ¾«·´¬َ· ² ¬± ‫؟‬ ‫حفـ‬ -§-¬»³ َ ´»®ô ‫سط‬×ô -»®‫ھ‬»®ô ²»¬©±®µô »¬½÷ٍ ‫ك‬ ‫حفـ‬ -§-¬»³ ·- °®»َ¾  «·´¬ ®»‫؟‬¼§ ¬± ¹± ±«¬ ±‫؛‬ ¬¸» ¾±¨

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‫غشقكسسكخظرخذ‬ ‫ظرش‬×‫ف‬ ‫خغششرخجزرف‬ّ ‫فشذ‬÷

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‫ج‬¸» ·²-¬‫؟‬´´»¼ ¹‫؟‬·² ¹®‫؟‬°¸ ½‫؟‬² ‫؟‬·¼ ·² ¬¸» ‫؟‬²‫؟‬´§-·- ±‫؛‬ ©¸»¬¸»® ¬¸» ½±²¬®±´ ‫؟ھ‬´‫ھ‬» ·²¸»®»²¬ ½¸‫؟‬®‫؟‬½¬»®·-¬·½ ·- -«·¬‫؟‬¾´» ‫؛‬±® ¬¸» -§-¬»³ٍ ‫ك‬² ·²-¬‫؟‬´´»¼ ¹‫؟‬·² »¯«‫؟‬´ ¬± ±²» ‫؛‬±® ¬¸» »²¬·®» ‫؟ھ‬´‫ھ‬» ¬®‫ھ؟‬»´ ©±«´¼ ·²¼·½‫؟‬¬» ¬¸‫؟‬¬ ¬¸» ±¬¸»® ï ½±³°±²»²¬- ±‫؛‬ ¬¸» ½±²¬®±´ -§-¬»³ ©±«´¼ ²±¬ ¸‫ھ؟‬» ¬± ½±³°»²-‫؟‬¬» ‫؛‬±® ¬¸» ·²-¬‫؟‬´´»¼ ‫؟ھ‬´‫ھ‬» ¹‫؟‬·²ّ ¬¸‫؟‬¬ ·-ô ¬¸» ½ ±²¬®±´ -§-¬»³ ¬«²·²¹ °‫؟‬®‫؟‬³»¬»®- «-»¼ ‫؟‬¬ ±²» ‫؟ھ‬´«» ±‫؟ھ ؛‬´‫ھ‬» ¬®‫ھ؟‬»´ ©±«´¼ ‫؟‬´´±© »¯«‫؟‬´´§ ‫؟‬½½»°¬‫؟‬¾´» ½±²¬®±´´‫؟‬¾·´·¬§ ‫؟‬¬ ±¬¸»® ¬®‫ھ؟‬»´-÷ٍ ׬ ·- ³±®» ¬¸‫؟‬² ´·µ»´§ ¬¸» ·²-¬‫؟‬´´»¼ ¹‫؟‬·² ©·´´ ²±¬ »¯«‫؟‬´ ±²» ‫؟‬½®±-- ¬¸» ‫؛‬«´´ ‫؟ھ‬´‫ھ‬» ¬®‫ھ؟‬»ٍ´ ‫ظ‬«·¼»´·²»- ‫؛‬±® ¼»-·®‫؟‬¾´» ·²-¬‫؟‬´´»¼ ¹‫؟‬·² ‫؟ھ‬´«»- ¸‫ھ؟‬» ¾»»² »-¬‫؟‬¾´·-¸»¼ٍ ײ ³±-¬ ½‫؟‬-»-ô ·²-¬‫؟‬´´»¼ ¹‫؟‬·² ‫؟ھ‬´«»- ±‫؛‬ ¾»¬©»»²ًٍë ‫؟‬²¼ îًٍ-¸±«´¼ ¾» ¬¸» ¬‫؟‬®¹»¬ٍ ×‫؛‬ ¬¸» ·²-¬‫؟‬´´»¼ ¹‫؟‬·² ‫؟؛‬´´- ±«¬-·¼» ¬¸·- ®‫؟‬²¹» ‫؛‬±® ‫؟ھ‬´‫ھ‬» ¬®‫ھ؟‬»´- ¬¸‫؟‬¬ ‫؟‬®» »¨°»½¬»¼ ¬± ¾» «-»¼ ‫؛‬±® ½±²¬®±´´·²¹ ¬¸» °®±½»--ô ¬¸» ½±²¬®±´´‫؟‬¾·´·¬§ ©·´´ ²±¬ ¾» ±°¬·³‫؟‬ٍ´ ‫ع‬±® »¨‫؟‬³°´»ô ½±²¬®±´´»® ¬«²·²¹ -»¬°±·²¬- ¬¸‫؟‬¬ ‫؛‬«²½¬·±² ©»´´ ‫؟‬¬ ´±© ‫؟ھ‬´‫ھ‬» ¬®‫ھ؟‬»´ ‫؟ھ‬´«»- ³·¹¸¬ ½‫؟‬«-» -§-¬»³ ·²-¬‫؟‬¾·´·¬§ ·‫؛‬ «-»¼ ‫؟‬¬ ¬®‫ھ؟‬»´- ©·¬¸ ‫؟‬ ¸·¹¸ ·²-¬‫؟‬´´»¼ ¹‫؟‬·²ٍ

‫عغـ‬×‫ز‬×‫ج‬×‫حزر‬ ً ً ً ً ً îô ً ïôë ً  ëً ïô

ً ً ً ً îôëً‫ي‬ً ô ‫ي‬ôëً‫ى‬ً ô

ٌ ‫ع‬´±©ô ¹‫؟‬´³  ·²

‫؟‬÷ ‫ذ‬®»--«®» َ ½±²¼·¬·±²- ‫؟‬½®±-- ‫؟‬ ½±²¬®±´ ‫؟ھ‬´‫ھ‬» ‫؟‬®» ²±¬ ½±²-¬‫؟‬²ٍ¬ ‫؟ت‬´«»- ±‫؛‬ ¬¸» ´·¯«·¼°®»--«®» ®»½±‫ھ‬»®§ ‫؟؛‬½¬±® ‫؟‬²¼ ¬¸» °®»--«®»َ¼®±° ®‫؟‬¬·± ‫؟؛‬½¬±® ‫؛‬±® ½±²¬®±´ ‫؟ھ‬´‫ھ‬»- ‫؟ھ‬®§ ©·¬¸ ‫؟ھ‬´‫ھ‬» ¬®‫ھ؟‬»ٍ´ ¾÷ ‫ع‬±® -»‫ھ‬»®‫؟‬´ ‫؟ھ‬´«»- ±‫؟ھ ؛‬´‫ھ‬» ¬®‫ھ؟‬»´ô ¼»¬»®³·²» ©¸»®» ±² ¬¸» -§-¬»³

‫؟ت‬´‫ھ‬» ¹‫؟‬·² ‫؟ت‬´‫ھ‬» ¬®‫ھ؟‬»´ ‰ ‫ج‬¸» ¼»¹®»» ±‫؛‬ ±°»²²»-- ±‫؛‬ ¬¸» ‫؟ھ‬´‫ھ‬»‫؟ھ »¸¬ ه‬´‫ھ‬» -¬®±µ» ‫ف‬±²¬®±´ ®‫؟‬²¹» ‰ ‫ج‬¸» ½±²¬®±´ ®‫؟‬²¹» ±‫؟ ؛‬² ·²-¬‫؟‬´´»¼ ‫؟ھ‬´‫ھ‬» ·- ¬¸» ®‫؟‬²¹» ±‫؛‬ ¬®‫ھ؟‬»´- ‫؛‬±® ©¸·½¸ ¬¸» ·²-¬‫؟‬´´»¼ ¹‫؟‬·² ®»³‫؟‬·²- ©·¬¸·² ¬¸» ®»½±³³»²¼»¼ًٍë ¬± îًٍ®‫؟‬²¹» ®»´‫؟‬¬·±²-¸·° ¾»¬©»»² ¬¸» °®»--«®» ¼®±° ‫؟‬½®±-- ‫؟ھ ؟‬´‫ھ‬» -§-¬»³ ‫؟‬²¼ ¬¸» ‰ ‫ج‬¸» ®»´‫؟‬¬·±²-¸·° ¾»¬©»»² ½±²¬®±´ ‫؟ھ‬´‫ھ‬» ½‫؟‬°‫؟‬½·¬§ ‫؟‬²¼ ‫؟ھ‬´‫ھ‬» -¬»³ ¬®‫ھ؟‬»´

ײ-¬‫؟‬´´»¼ ‫؛‬´±© ½¸‫؟‬®‫؟‬½¬»®·-¬·½

ً ‫ي‬ً ô ً îôëً

‫؟س‬¨·³«³

ً ً îô

‫ز‬±®³‫؟‬´

ً ïôëً

‫س‬·²·³«³

ً ً ïô ً ëً ً ً 

ً ï

ً î

ً ‫ي‬

ً ‫ى‬

ً ë

ً ê

ً é

‫؟ت‬´‫ھ‬» ¬®‫ھ؟‬»´ô û

ً è

ً ً ً ç ï

±°»²·²¹ٍ ‫ذ‬®»--«®» ¼®±°- ‫؟ھ‬®§ ©·¬¸ ‫؟ھ‬´‫ھ‬» ¬®‫ھ؟‬»´ ©¸»² ‫؟ھ‬´‫ھ‬»- ‫؟‬®» ·²-¬‫؟‬´´»¼ ‫خ‬»‫؛‬»®»²½»ïٍ ‫ز‬·»-»²ô ‫س‬ًٍô ‫ث‬-·²¹ ·²-¬‫؟‬´´»¼ ¹‫؟‬·² ¬± ·³°®±‫ھ‬» ‫؟ھ‬´‫ھ‬» -»´»½¬·±²ٍ ‫ف‬¸»³ٍ ‫غ‬²¹ ٍ ‫ر‬½¬±¾»® îًèô °°ٍ ‫يٹىي‬éٍ îٍ ‫ع‬·¬¦¹»®‫؟‬´¼ô ‫ق‬ٍ ‫؟‬²¼ ‫ش‬·²¼»²ô ‫ف‬ٍô ‫ج‬¸» ½±²¬®±´ ‫؟ھ‬´‫ھ‬»‫ژ‬- ¸·¼¼»² ·³°‫؟‬½¬ ±²ً¬¸» ¾±¬¬±³ ´·²»ٍ ‫؟ت‬´‫ھ‬» ‫؟س‬²«‫؟؛‬½¬«®»®‫ژ‬- ‫ك‬--±½·‫؟‬¬·±²ô ‫؟ة‬-¸·²¹¬±²ô ‫ـ‬ٍ‫ٍف‬ô îً‫ي‬  ٍ ‫ي‬ٍ ‫ذچ‬ً»®®§‫ژ‬- ‫ف‬¸»³·½‫؟‬´ ‫غ‬²¹·²»»®‫ژ‬- ‫؟ط‬²¼¾±±µôŒ 謸 »¼ٍô ‫س‬½‫ظ‬®‫؟‬© ‫ط‬·´´ô ‫ز‬ٍ‫ٍا‬ô îًè ٍ

Infrared Temperature Measurement

Department Editor: Scott Jenkins ontact-based temperature sensors, such as thermocouples and resistance temperature detectors (RTDs), have demonstrated accurate and cost-effective operation throughout the chemical process industries (CPI). However, there are many applications and settings where they are simply not practical. In those cases, engineers can turn to a host of noncontact temperature measurement devices, many of which are based on measuring infrared (IR) radiation. IR thermometers can routinely perform measurements in situations where readings with contact thermometers would be virtually impossible. Situations where IR-based temperature measurement should be considered include the following:        fields, such as in processes involving induction or microwave heating     in process chambers or behind windows      thermometers   or contaminated by contact measurement   present   heat capacity and low thermal conductivity   as combustion gases and flames

C

Within the CPI, IR thermometry is most ef ductor and wafer processing, cement and lime processing, rotating kiln shells, waste incineration, glass processing, sintering and heat treating, metals processing and drying applications. While IR thermometers are generally more expensive than contact thermometers, they usually have longer lifetimes and   thermal radiation physics can help users apply and operate the devices more effectively. Stefan-Boltzmann law

 surface area per unit time is related to its temperature by the Stefan-Boltzmann law, which states that irradiance (in J/s/m 2) is proportional   tionality (the Stefan-Boltzmann constant) is re 

Operation

 according to the same basic operating principles (Figure 1).  lens are used to focus the IR  onto a detector, which converts the IR radiation into an electri for emissivity (see below) and ambient temperature, an analog output is generated to provide temperature measurement. The analog signal can be converted   Emissivity

EMISSIVIT Y VALUES OF COMMON MATERIALS* Material Emissivity

Silver (polished) Aluminum (unoxidized) Gold (polished) Aluminum (heavily oxidized) Zinc (bright galvanized) Steel (316 polished) Soil (plowed field) Iron (liquid) Iron (rusted) Water Sand Steel (cold rolled) Wood (oak planed) Brick (red, rough) Carbon (Lampblack) Ice

0.01 0.02 0.02 0.20 0.23 0.28 0.38 0.43 0.65 0.67 0.76 0.80 0.91 0.93 0.95 0.98

The emission of thermal radiation is a surface phenomenon for most materials. The term *Provided for illustrative purposes only.  ability to emit thermal radiation. (distance-to-spot ratio). Higher ratios mean Emissivity is defined as the ratio between the better resolution. Ideally, the target being measured should fill perature and a per fect radiator, or blackbody, at the same temperature. Emissivity values  lie between zero and one. IR thermometers  generally have the ability to compensate for the different emissivity values of materials. Calibration  IR thermometers can be calibrated by aiming are the easiest to measure accurately with IR at blackbody radiators that are designed thermometers, while those with low emissivities specifically for calibration and testing. By varyare more difficult. For example, some polished, ing the source t emperature of the blackbody, shiny metallic surfaces, such as aluminum, are  so reflective in the infrared that accurate temmeasurement signal to known temperatures. perature measurement is not always possible. Tables listing emissivity values f or various Selection questions to consider materials have been published, and are availWhen selecting an IR thermometer for a CPI able for reference (Table). Some IR thermomapplication, it is important to consider the foleters allow users to change emissivity values  according to the material being measured,   while others have a pre-set emissivity value.   When using IR thermometry, it is important  to consider that materials can have different   emissivity values at different wavelengths. To determine an emissivity value, you can heat a ing device    the emissivity value of the instrument until the IR  thermometer matches the known temperature.   Field of view (FOV)      instrument operates and is determined by the optics of the system. The optical system of the References IR thermometer collects the IR energy from a  circular measurement spot, and focuses the Chem. Eng. energy on the detector. The optical resolution  of the instrument is determined by the ratio   . and the size of the spot being measured Lens assembly Filter

Object Field of view

Recommended

Incorrect

Aperture Detector

IR Thermometer

S

Amplifier and electronics D

FIGURE 1. The optical system of an IR thermometer collects IR energy from a circular measurement spot, and focuses it on a detector

Target greater than spot size

Target equal to spot size

Target smaller than spot size

Backround

FIGURE 2. For accurate IR temperature measurement, the target area should be greater than the instrument’s FOV by a factor of about 1.5

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‫‪ î‬غشقكج‬ ‫ظفغ ‪‬عر ‪‬حغظكجزكتـكح×‪‬ـ ‪‬ـزك ‪‬حغظكجزكتـك ‪ٍ ‬‬ ‫حغظكظ ‪‬زر×‪‬جكئ×‪‬زر× ‪‬عسغ‪َ‬ـغححرخف ‪‬حثحخغت‬

‫‪¹»-‬؟ ¬‪²‬؟ ھ ¼؟ ‪·-‬ـ‬

‫‪ ï‬غشقكج‬ ‫اج×‪‬شكثد ‪‬سثثفكت ‪‬خرع ‪‬حغظزكخ ‪‬شكف×‪‬ذاج ‪ٍ ‬‬

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‫؟ ذ ‪®»--«®» ·²‬ذ‬ ‫ً‬ ‫يٍ‪ïï‬‬ ‫؟ذ‪ µ‬‬ ‫ً‬ ‫ً‬ ‫؟ ذ ‪ µ‬ي ‪ï ¬±‬‬ ‫ً‬ ‫؟ذ‪ ¬± ïً³‬؟ذ‪ µ‬ي‬ ‫ً‬ ‫ً‬ ‫؟ذ‪ ¬± ïً²‬؟ذ‪ç ïً³‬ٹ‬ ‫ً‬ ‫ً‬ ‫؟ذ‪ ¬± ïً°‬؟ذ‪ïً²‬‬

‫ً‬ ‫؟ذ‪ïً°‬ن‬ ‫ً‬ ‫‪ ¬±‬؟ذ‪ïًk‬‬ ‫؟ذ؛ي ن ً‬ ‫‪ï‬‬ ‫÷‪‬؟ذ ‪ïë‬ٹ ‪ ï‬م ؟ذ؛ ّ‬ ‫ً‪‬‬

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Pressure measurement Department Editor: Scott Jenkins ressure measurement in the chemical process industries (CPI) is crucial to many unit operations, and selecting the most effective pressure sensors for a given situation can be complicated by a range of factors. An initial key to selection is establishing an accurate understanding of exactly what is meant when the term “pressure” is used, since there are different types. Other critical considerations include the following: media compatibility, environment, process control, electrical isolation and output signal.

P

considerations MAIN FACTORS TO CONSIDER FOR CHEMICAL PRESSURE MEASUREMENTS

Factor

Reason

Solution

Media compatibility

A pressure-sensing element will come in contact with varying concentrations of chemicals, temperatures and pressure ranges depending on the industry sector in which the application appears, including petrochemicals, food, pharmaceu ticals, water, refrigeration, alternative energy or power generation

Pressure sensors constructed from one-piece 316L stainless steel, nickel and cobalt-based superalloys are free from internal welds, O-rings and very thin isolation diaphragms offer excellent media compatibility for most chemicals. The one-piece design ensures outside media do not permeate the sensor body

Process control

New processes for heavy oil, alternative energy and water purification systems demand extreme operating conditions, such as low ambient temperatures (–50ºC), high media temperature (150 ºC), as well as complex and volatile gas-liquid mixtures

Pressure sensors with new technologies and wetted materials are needed. High-temperature, oil-free, bulk silicon piezoresistive sensors are ideal for these emerging markets. Superalloys, such as Inconel, Hastelloy and Waspalloy, with thick sensing diaphragms, offer the best solutions without the need for complex sensor packaging and expensive secondary seals

Environment

Rain, ice, dust and pressure washers can cause water to seep into sensor housings and cause electronics to shortcircuit

Absolute and sealed-gage reference pressure sensors protect electronics from these conditions. If venting is required to maintain accuracy at low pressures, provisions must be made for dry, noncorrosive environments for sensors to “breathe”

Electrical isolation

Improper grounding and lightning strikes can cause electrical failures of pressure sensors, as a result of isolation failure

Pressure sensors with custom electronics and a sensing element able to withstand 500 V d.c. isolation can work in extreme electrical conditions

Output signal

Depending on distance and environment, certain output signals can experience signal loss or generate noisy signals

A 4–20-mA output signal is recommended for transmission lengths greater than 15 ft in environments with electrical noise

Pressure types

Pressure measurements can be affected by what type of pressure sensing equipment is used, and understanding the different types of pressure is a prerequisite for selecting sensors or gages for your application. Accuracy can suffer if pressure types are misunderstood. Differences in pressure types have everything to do with the reference point for a given pressure measurement. Here are definitions for five common pressure types: Gage pressure — Gage pressure, the type that most people first imagine when thinking of measuring pressure, covers a positive pressure range. Its zero (reference) point is set at ambient pressure, and it is unaffected by changes in barometric pressure because the sensor is open to the atmosphere. This allows the current atmospheric pressure to be the reference against which all subsequent changes in pressure are measured. Gage pressure effectively can measure pressures below 1 psi, as well as pressures up to 200,000 psi. Vacuum pressure — Like gage pressure, vacuum pressure’s zero point is ambient pressure, and sensors measuring it are vented — and therefore unaffected by barometric change. Since vacuum pressure refers to a negative pressure range, the distinction between vacuum and gage pressure is really a function of direction and magnitude. Sensors measuring this type are commonly used in vacuum pump systems and applications where suction is required. Compound gage pressure — This pressure type is the combination of gage and vacuum pressure in that it involves both positive and negative pressure changes. Its zero is therefore set at atmospheric pressure, and it is vented. The value of a compound gage is seen when used in applications where the pressure fluctuates from positive to negative and vice-versa. Sensors measuring this pressure type typically do not exceed 100 psi in range. Sealed pressure — Sealed pressure refers to a situation where the pressure sensor is not vented. This is primarily done to protect the sensor, by avoiding the introduction of moisture or dust into the sensor housing. The sensor is sealed with a pressure equal to the atmospheric pressure at the time of

Table content submitted by Karmjit Sidhu, vice president of business development at American Sen sor Technologies (Mount Olive, N.J.; www.astsensors.com) Reference

Gage

Vacuum

Compound

Sealed

Absolute

Positive pressure Ambient Negative pressure Source: APG

sealing. This pressure then becomes the reference pressure against which all pressure changes are measured. Because it is sealed, unvented pressure sensors are unavoidably affected by barometric pressure changes. It is not typically used in low-pressure applications because the barometric shift of a few psi would affect measurement accuracy significantly. However, at 1,000 psi and above, the relatively small shift would go unnoticed and can be smaller than the error band of the sensor. In one real-world case, a sealed pressure type sensor was calibrated at a manufacturing facility in Utah and then shipped to Indiana. The atmospheric pressure differences between the locations caused the unit to fail in Indiana, while it worked properly in Utah.

 Absolute pressure — Absolute pressure

is used when the zero point must be set to absolute zero. To achieve this, the sensor is also sealed, but under a vacuum condition, so that air molecules are removed from the enclosure. This then becomes the reference point and allows measurements to be made with reference to absolute zero. By definition and design, this is sensitive to barometric changes. Unlike sealed pressure, absolute pressure is often used in low-pressure applications measuring atmospheric conditions, such as in weather stations, aircraft and laboratories. Notes Material on pressure types was contributed by Elden Tolman, product design engineer at Automation Products Group Inc. (APG; Logan, Utah; www.apgsensors.com).

Valves

Department Editor: Kate Torzewski  gate Pinch valve

Gate valves are designed to operate ully open or ully closed; When ully opened, there is very little pressure drop across the valve, and when ully closed there is good sealing against pressure. With the proper mating o a disk to the seat ring, very little or no leakage occurs across the disk when the gate valve is closed. Gate valves open or close slowly, which prevents uid hammer and subsequent damage to the piping system. Gate valves are usually classifed by the type o disk used, and a variety o disk types are available, such as solid wedge, split wedge or parallel disk.

Gate valve

Globe valve

Diaphragm valve

Plug valve

Ball valve

globe

The basic principle o globe valve operation is the perpendicular movement o the disk toward, or away rom, the seat. This causes the annular space between the disk and seat ring to gradually close as the valve is closed. It is this characteristic that gives the globe valve good throttling ability. When the valve is closed, there is no blocked-in volume, as occurs in a gate valve, so a globe valve has much less leakage around the seat. Also, the disk-to-seat-ring contact is much closer to orming right angles, so the orce o closing tightly seats the disk.

Source for figures: Valve Manufacturers  Associa tion of America, Washington, D.C.

Butterfly valve

Check  valve

butterfly

pinch

This valve consists o a exible tube that is mechanically pinched rom the outside o the valve body. The principal advantages o this type o valve are that the ow passage is straight without any crevices, and there are no internal moving parts. The sot valve body has the ability to seal around trapped solids, so pinch valves are suitable or handling slurries and solids, which would clog in the obstructed ow passages o other valve types. They are also used or the sanitary handling o oodstus and pharmaceuticals because the media are isolated rom the working parts. diaphragm

The ow passage in diaphragm valves is ree o crevices and is unobstructed by moving parts, making them suitable or applications where cleanliness, bubble-tight shuto and chemical compatibility are important. The diaphragm valve is considered

to be the valve least likely to cause contamination. For this reason, it is popular in highpurity applications. It is available in two general designs, weir and straightway. The weir-style diaphragm valve is utilized or higher-pressure applications. The straightway diaphragm valve, having no ow path obstructions, is well suited or higher-ow and slurry applications. ball

This rotational-motion valve uses a ball-shaped disk with a hole bored through to stop or start uid ow. When the valve handle is turned to the open position, the ball is rotated so that the hole lines up with the valve body’s inlet and outlet. When the ball is rotated so the hole is perpendicular to the ow, the valve is closed. Because the ball moves across the seats with a wiping motion, ball valves can handle uids with suspended solids.

Ball valves are available in Venturi, reduced and ull-port patterns. The ull-port pattern has a ball with a bore equal to the inside diameter o pipe. Most ball valves instead have a reduced bore with a Venturi shaped ow passage o about three quarters the nominal valve size.

Buttery valves get their name rom the winglike action o the ow-controlling disk that opens and closes at right angles to the ow path. Buttery valves were introduced to counteract the problems associated with linear-valve designs (especially gate valves), such as the relatively large size and weight, the high operating orce required, and the tendency to leak. Instead o a long stroke, the buttery valve requires a quarter turn to cycle rom a ully open to ully closed position. Buttery valves can be used or both on/o and throttling applications. check

plug

Plug valves have a cylindrical or tapered plug with a hole bored through. As with ball valves, uid ows when the hole in the plug is aligned with the pipe, and a quarter turn o the plug stops the ow. Plug valves oten have uorocarbon seating materials and in some cases are ully lined with uorocarbons, which provides excellent protection or corrosive applications that require bubbletight shuto. There are several dierent types o plug valves commonly used in the CPI, including lubricated, nonlubricated and eccentric types.

The purpose o a check (or nonreturn) valve is to allow uid ow in one preerred direction and to prevent back ow, or ow in the opposite direction. Ideally, a check valve will begin to close as the pressure drops in a pipeline and the uid momentum slows. When the ow direction reverses, the check valve should close completely. Check valves can be o the ollowing types: swing, lit and tilting disk. References

1.Sahoo, T., Pick the Right Valve, Chem. Eng., August 2004, pp. 34–39.

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Matals of Constcton

Department Editor: Rita L. D'Aquino 

Low-temperature appLications [1 ] One key engineering consideration is the choice of materials of construction for frigid applications. Nickel-chromium (Ni-Cr) type stainless steels are notably versatile at low or cryogenic temperatures. They offer a combination of high impact strength (IS) and corrosion resistance. In the austenitic phase, with face-centered-cubic crystals, the combination of Cr and Ni in the material improves IS and toughness down to temperatures as low as –250°C. For good IS at temperatures down to –45°C, C-Mn-Si steels are recommended. The most preferred grades are fine-grained steels of pressure-vessel quality, such as ASTM  A 516 and ASTM A 537 (in all grades). For temperatures between –45 and –100°C (for example, for liquid-ethylene storage), steels containing 2.5–9% Ni are useful. Between –150 and –250°C, the Ni-Cr austenitic steels (300 series, of 18/8 varieties), are highly recommended. In the nonferrous category,  Al has excellent properties for temperatures as low as –250°C. Also attractive are Cu and some of its alloys, which can withstand temperatures down to –195°C.

chemicaL resistance CPVC [2]. Many nonmetals do not have the tensile strength to meet the pressure requirements of various process applications, especially at elevated temperatures. But years of  testing and actual field performance prove that chlorinated polyvinyl chloride (CPVC) systems can be pressure rated for operation as high as 200°F. CPVC’s high heat-distortion temperature and resistance to corrosion make it suitable for applications such as metal processing, pulp and paper, and industrial wastewater treatment, where harsh and corrosive chemicals are commonly used (see Figure 1). Another advantage of CPVC is that it is lighter than metal, and therefore less expensive to install, from both a material cost and labor perspective. CPVC is not  recommended where aromatic solvents and

esters are present in high concentrations. FRP pipe [3]. Composite fiberglass-reinforced plastic (FRP) pipe has been replacing conventional pipe material, such as steel and concrete, in numerous applications because of its corrosion resistance, low design weight (25% of concrete pipe and 10% of steel pipe), high fatigue endurance, and adaptability to numerous composite blends (Table 5, Ref. 3 ) and manufacturing methods. FRP pipe may be divided into two broad categories: gravity pipe (dia. from 8 to 144 in.) and pressure pipe (dia. from 1 to 16 in.). It is not unusual to see FRP pressure pipe handling pressures as high as 2,000–5,000 psi during chemical processing, with the higher-pressure pipe at the lower end of  the diameter scale.

heat transfer properties [4 ] Metals, including specialty materials, are the best choice in terms of good heat transfer. In the lined category, glass is used extensively for process equipment where good heat transfer is required. Lined materials, however, often have the problem of uneven thermal expansion, which may weaken the bonding of the lining in due course. While fluoropolymers have excellent compatibility  with various chemicals and special surface and physical chemistries, they are generally not used for reaction vessels because of their poor heat-transfer properties. Thermal conductivities for various materials are listed in the Table, and typical applications are shown in Figure 2.

THERMAL CONDUCTIVITY OF VARIOUS MATERIALS OF CONSTRUCTION [ 4 ] Material Carbon Steel (CS) SS 304 SS 316 SS 316 L Hastelloy B2 Hastelloy C2 Tantalum2 Titanium2 Zirconium2 Graphite Hexoloy Glass1 Lead Inconel2 CPVC PTFE (Polytetrafluoroethylene)1 PFA (Perfluoroalkoxy resin)1 ETFE (Ethylene tetrafluoroethylene)1 PVDF (Polyvinylidene fluoride)1 ECTFE (Ethylene chlorotrifluoroethylene)1

0.24 0.23 0.16

References 1. Nalli, K., Materials of Construction For Low-Temperature and Cryogenic Processes, Chem. Eng.  July 2006, pp. 44–47. 2. Newby, R. and Knight, M., Specifying CPVC In Chemical Process Environments, Chem. Eng., October 2006, pp. 34–38. 3. Beckwith, S., and Greenwood, M., Don’t Overlook Composite FRP Pipe, Chem. Eng., May 2006, pp. 42-48. 4. Robert, J., Selecting Materials of Construction, Chem. Eng., September 2005, pp. 60–62.

Exotic

Exotic

Exotic

250

Exotic

Exotic

Exotic

Fluoropolymer, glass lined, exotic

0.19

1. Common choice for lining material 2. Exotic metals

300    C    ° 200  ,   e   r   u 150    t   a   r   e 100   p   m   e 50    T

Thermal conductivity, W/(m)(K) 60.59 40.71 14.23 14.23 9.12 10.21 57.5 21.67 20.77 121.15 125.65 1.00 35.30 12.00 0.14 0.25

Fluoropolymer, glass lined, exotic

Glass lined,* exotic

Exotic

Glass lined,* exotic

0 Weak acids Weak bases Salts Strong acids

Excellent

Aliphatics Strong bases

Good

Strong oxidants Halogens

Fair

Aromatic solvents Esters and ketones

Poor

CPVC offers resistance to a variety of harsh chemicals Figure 1.

-50

Exotic

Exotic

-100

Exotic

Exotic

Application:

Storage

 Typical equipment:

 Tanks, vessels

Transport Pipelines, valves, owmeters

Exotic Agitation Mixers

Exotic (Agitation + heat transfer) Reactors

When looking beyond steel for materials of construction, it is important to consider the intended application and temperature range. Exotic (specialty) metals (see Table) are shown here to serve well in all applications. Another material, equally suited to a specific requirement, however, may be chosen as the more cost-effective option Figure 2.

Pristine Processing Equipment

Department Editor: Kate Torzewski  rocesses in the pharmaceutical, biotechnology, ood and semiconductor industries must meet a high set o standards to ensure high product purity. Equipment criteria specic to high-purity processes are established to minimize contamination and maintain product integrity. In designing a pristine process, material and equipment style are o upmost importance. Bacteria is the main cause o contamination and is prone to growing in the dead cavities o equipment created by sharp corners, crevices, seams and rough suraces. Another source o contamination is leaking, which allows undesirable chemicals to compromise the quality o the process ingredients, by causing contamination, rusting and particle generation.

P

MATERIALS OF CONSTRUCTION Many actors must be taken into consideration when selecting materials o construction or use in pristine process applications where high-purity and sanitation are paramount. All suraces should be constructed o a smooth material that will not corrode, generate particles or harbor dead cavities. These criteria can be met with three standard materials: 316L stainless steel (SS), polyvinylidene fuoride (PVDF) and polytetrafuoroethylene (PTFE). The advantages and disadvantages o these materials are summarized below to acilitate the material selection process or a given application with consideration o chemical compatibility, cost, and temperature stability. MATERIALS OF CONSTRUCTION Material Stainless Steel

PVDF

PTFE

Advantages • Mechanical strength • Functions at 121°C (steam-sterilization temperature) • Chemically inert • Resistant to corrosion and leaching • Durable and long-lasting • Retains circumerential strength

Disadvantages Vulnerability to corrosion by certain chemicals, which increases with temperature Functions only intermittently at 121°C

• The most chemically inert plastic • Resistant to corrosion and leaching Complex shapes are • Avoids leaching dicult to orm • Suitable or coating equipment

EQUIPMENT STYLE SELECTION Critical actors in high-purity equipment selection include cleanability, cost, fow capabilities and product compatibility. With these considerations in mind, criteria useul or choosing pumps, valves, seals and piping are described in this section. Pumps A undamental requirement o pristine processing pumps is the ability to clean a pump in place without disassembly. Pump seals, gaskets and internal suraces should eliminate the buildup o material and should clean out easily during wash cycles. The most common pump styles or high-purity processes are centriugal, lobe-style and peristaltic pumps, which are outlined below. PUMPS Pump style

Advantages

Disadvantages

Centriugal

• Low cost • Easy cleanability

Eciency and fow decrease with increasing pressure and volume

• Low cost • Easy cleanability • No mechanical Peristaltics seals • Non-damaging to delicate products

Rotary Lobes

Applications best suited for this style • Handling lowviscosity products • Handling h igh fowrates (40–1,500 gal/min)

The need or hoses may cause issues in elastomeric • Small , batch-oriented compatibility, temperature applications and pressure limitations, • Laboratory or pilotand a need to change scale plants hose regularly

• Higher pressure and fow capabilities High cost • Unaected by pressure variations

• Large, continuous duty applications • Steaming and high pressure applications

Valves Valves should not harbor contaminants and must be easy to clean. By these criteria, diaphragm and pinch valves are excellent choices or ultrapure processes, as they have smooth, gently curved suraces that will not harbor contaminants. Ball check, ull-port plug and ull-port ball valves are good choices as well, while butterfy, spring check, gate and swing check valves are all unacceptable, since contamination can collect in the corners that are essential to their design. Though several valves are appropriate or pristine processes, certain valves are better suited or particular applications. Diaphragm valves are the most widely used in high-purity systems or their resistance to contamination and ability to be used as a control valve. Ball and plug valves, on the other hand, are less costly and are not limited by temperature and pressure. Also, in applications using sterile steam and reeze-drying, ball valves are preerred over diaphragm valves because they eliminate the risk o catastrophic seat ailure. Seals As with all pristine processing equipment, high-purity seals should not have any cavities where contaminants can breed. By choosing a seal with gland rings that do not need to be threaded or ported, the areas where bacteria can breed are minimized. In choosing a seal material, it is important to nd a compound that will not swell, crack, pit or fake, thus reducing seal ailure and contamination. To ensure the success o seals, furoelastomers are a top choice in pristine processing applications or their excellent thermal stability, chemical resistance and mechanical durability. Piping The surace o piping, as well as any wetted equipment parts, should have a very smooth surace. When 316 SS is being used, electropolishing is a good method or achieving an ultra-smooth nish. Joining methods should minimize crevices and dead cavities, and all materials should be ree o biological degradable substances, leachable substances, and glues and solvents that may migrate into the product stream. References 1. Smith, B., What Makes a Pump or High-Purity Fluids?, Chem. Eng., pp. 87–89, April 2002. 2. Schmidt, M., Selecting Clean Valves, Chem. Eng., pp. 107–111, June 2001. 3. Wul, B., Pristine Processing: Designing Sanitary Systems, Chem. Eng., pp. 76–79, Nov. 1996. 4. Weeks, D. T. and Bennett, T., Speciying Equipment or High-Purity Process Flow, Chem. Eng., pp. 27–30, Aug. 2006.

Controlling Membrane Fouling Department Editor: Scott Jenkins

he deposition and accumulation of suspended and dissolved particles on membrane surfaces leads to performance loss. Fouling can dramatically reduce the efficiency and economic benefits of a membrane process. The type of fouling and how strongly it appears depends on several parameters, including the following:      

T

COMMON FOULING MODES

ment systems. Several of the most common types of fouling are shown in the following table.

crossflow velocity or by increasing turbulence on the membrane  from the membrane. The challenge becomes balancing high  Membrane properties

As an essential part of the membrane process, the membrane itself has a strong influence on fouli ng. Typically, hydrophilic  water, which is one of the main tools used to reduce the adsorption of foulants onto the membrane surface. A hydrophilic  protective layer. The hydrophilicity and hydrophobicity of some polymeric membrane materials are shown i n Table 2. POLYMERIC MEMBRANE PROPERTIES

Property

Polymer

Hydrophobic

Polytetrafluoroethylene (PTFE, Teflon) Polyvinylidenefluoride (PVDF) Polypropylene (PP) Polyethylene (PE)

Hydrophilic

Regenerated cellulose Cellulose ester Polycarbonate (PC) Polysulfone/polyethersulfone (PS/PES) Polyimide/polyetherimide (PI/PEI) (aliphatic) Polyamide (PA) Polyetheretherketone (PEEK) Cellulose triacetate (CTA)

FOULING EXAMPLES

Foulants

Fouling mode

Large suspended particles

Particles present in the original feed or developed in the process can block module channels

Small colloidal particles

Colloidal particles can raise a fouling layer or block the porous structure of the membrane

Macromolecules

Gel-like cake formation on top of the membrane or macromolecular fouling within the structure of porous membranes

Small molecules

Molecules such as substituted aromatics can adsorb onto the membrane structure and reduce the water content of the membrane, which lowers permeability

Scalants

Depending on the pH, salt may precipitate on the membrane. This reduces the membrane area and may reduce the water content in the membrane

Biological material

Growth of bacteria on the membrane surface, which leads to a gel-like cake on the membrane

REDUCING FOULING Infuence o the bulk solution

 whether these properties can be manipulated depends on the actual process conditions.  are possible parameter changes that may be manipulated to   by changing the solubility of the salts. This can significantly reduce the precipitation of calcium sulfate on the membrane. Concentration polarization

ticles, especially during microfiltration and ultrafiltration, to the   to accumulate on the surface of the membrane. Concentration      

Because hydrophilic membranes have lower chemical resistance than hydrophobic ones, their chemical stability and cleanability have to be evaluated as part of the selection process. It should be noted that most membranes are polymer blends.   to changes in membrane porosity if not closely monitored. Porosity

     Infuence o the permeate fux

 fouling in microfiltration, ultrafiltration and nanofiltration    membrane is similar to the number of particles that diffuse away     membrane properties can be sustained for longer periods. On  the tradeoff between higher investment costs and lower fouling tendencies has to be assessed for each process.   Chem. Eng., September 2007, pp. 62–64.

Membranes

Department Editor: Kate Torzewski 

S

Micrometers, 0.001 log scale

0.01

0.1

1.0

10

eparation by a membrane is achieved by creating a boundary between dierent bulk gas or liquid mixtures. As dierent solvents and solutes ow through a membrane at dierent rates, separation is achieved. Here, we will ocus on three fltration techniques: microfltration (MF), ultrafltration (UF) and nanofltration (NF). These processes are characterized by the size o the particle that can be separated by the membrane, as illustrated in the fgure. Each membrane type is best suited or unique applications and is designed with the module and material that will allow the best separation. Flow through a membrane is characterized as either tangential ow fltration (TFF), where the eed stream ows at a velocity vector normal to the membrane surace, or normal ow fltration (NFF), where the stream ows tangent to the membrane surace. The ow pattern is dependent on the type o module utilized. NFF  modules  include: cartridges, stacked disks and at sheets. TFF modules  include: plateand-rame (cassettes), hollow fbers, tubes, monoliths, spirals and vortex ow.

o crystallite orientation, racture in such a way that reproducible microchannels are made.

nanofiltration

microfiltration

Membrane modules

NF, sometimes reerred t o as “loose RO (reverse osmosis),” utilizes a driving orce o 0.3 to 10.5 MPa to drive liquid solvents through the membrane while retaining small solutes o about 10 to 100 nm in diameter. NF membranes are dierent rom the membranes previously discussed, because they are usually charged, utilizing ion repulsion as a major method o charged-species rejection. They have 20–80% NaCl retention and retain > 200–1,000 Daltons o neutral organics, with a low retention o dissolved gases. Neutral or undissociated solutes have a lower retention than charged or dissociated solutes.

MF separates particles rom true solutions. This technique is able to separate particles rom about 0.1 to as high as 10 µm. As can be seen rom the fgure, large, soluble macromolecules, bacteria and other microorganisms can be retained by MF membranes.

Scanning electron microscope Particle size range

Molecular

Macromolecular

Microparticle

Albumin protein Aqueous salts Typical particles

Yeast cells

Carbon black

Paint pigment Bacteria

Sugar

Tobacco smoke

Synthetic dye Pesticide

A.C. fine test dust

Virus

Milled flour

Latex emulsion Colloidal silica

Blue indigo dye Asbestos fibers

Gelatin

Red blood cells

Coal dust

Ultrafiltration

Process for filtration

Nanofiltration

Many conventional designs are used in MF, including cartridge-flter housing, plate-andrame-type devices, capillary bundles, tubular membranes, spiral-wound modules and belt flters. Ceramic MF membranes are available as at sheet, single tubes, disc, and other orms, primarily or lab use. Finally, cassettes are two dierent cross-ow membrane devices.

Membrane materials

MF membranes have the largest pore openings o any other membrane. Typically, they can be classifed as having tortuous or capillary pores. From solids. When membranes are made by sintering or agglomeration o micr oparticles, pores are ormed by the interstices between solid particles. Common materials include: metal, metal oxide, graphite, ceramic and polymer. Ceramic. These membranes are typically created by the sol-gel process, which is the successive deposition o smaller ceramic precursor spheres, ollowed by fring to orm multitube monoliths. Track etched. A polymer flm is exposed to a collimated beam o radiation that breaks chemical bonds in the polymer chains. The flm is then etched in a bath that selectively attacks the damaged polymer, a technique that produces a flm with photogenic pores. Chemical phase inversion. A solution o a concentrated polymer in solvent is spread into a thin flm, then precipitated through the slow addition o a nonsolvent to produce tortuous pores. Thermal phase inversion. A solution o polymer in a poor solvent is prepared at an elevated temperature. Ater being ormed into its fnal shaped, the temperature is dropped and the polymer precipitates, and the solvent is washed out. Streched polymers. Semicrystalline polymers, which are stretched perpendicular to the axis

Optical microscope

ultrafiltration

UF membranes, with pore sizes ranging rom about 1 to 100 nm in diameter, employ pressure driving orces o 0.2–1.0 MPa. This technique drives liquid solvents and small solutes through the membrane, while retaining larger particles, like large dissolved molecules, colloids and suspended solids. Membrane materials

UF membranes are typically made o polymeric structures, such as polyethersulone, regenerated cellulose, polysulone, polyamide, polyacrylonitrile or various uropolymers. They are ormed by immersion casting on a web or as a composite on an MF membrane. Membrane selection is based on molecular-weight rating or high yields, chemical and mechanical robustness during product processing and Clean In Place, and process ux or sizing and costing. Membrane modules

Modules include cassettes, spirals, hollow fbers, tubes, at sheets, and inorganic monoliths. These primarily operate in TFF to increase ux by reducing plugging. For virus removal and water treatment, however, NFF operation is run with cartridge and hollow fber modules.

Microfiltration

Membrane materials

Cellulose polymers. These are ormed by immersion casting o 30–40% polymer lacquers, which can include cellulose acetate, triacetate and acetate-butyrate, on a web immersed in water. Thin flm composites. Formed by interacial polymerization, TFCs involve coating a microporous membrane substrate with an aqueous prepolymer solution, then immersing it in a water-immiscible solvent containing a reactant. Crosslinked polyetherurea. Some o these membranes eature NaCl retention and water permeability. Membrane modules

NF membrane modules are available in spiral, hollow fber, tubular, and plate-andrame ormats. Spirals are most common, as they have low eed-side pressure props, are less prone to clogging, are easily cleaned, are mechanically robust, and are most economical.

References

1.“Perry’s Chemical Engineers’ Handbook,” 8th ed. McGraw Hill, New York, 2008. 2.Seidel, A., ed. in chie, “Separation Technology,” second edition, John Wiley and Sons, Inc., New  Jersey, 2008.

Facts at your Fingertips sd b: 

Contollng Cystal Gowth Department Editor: Rita L. D'Aquino  he ormation o crystals requires the birth o new particles, also called nucleation, and the growth o these particles to the nal product size. The driving orce or both rates is the degree o supersaturation, or the numerical dierence between the concentration o solute in the supersaturated solution in which nucleation and growth occurs vs. concentration o solute in a solution that is theoretically in equilibrium with the crystals. In a batch crystallizer, the crystal size distribution (CSD) is controlled by rst seeding the initially supersaturated batch with a known number and size distribution o crystals, and then controlling the rate o evaporation or cooling ( i.e., rate o energy transer) so as to achieve a level o supersaturation that supports adequate crystal growth and an acceptable rate o nucleation. The relationship between supersaturation and growth is linear, but that between nucleation and growth is raised to a power that is usually greater than one, making it dicult to grow large crystals when nucleation is occurring. The ollowing procedure describes how to achieve the optimal growth rate: 1. Screen the seeds at the beginning o the experiment to determine the cumulative number o crystals that are greater than a given size N’ . Estimate N Li , the number o crystals o a given size (L av ) obtained rom the screening:

T

 Li

=

∆W i

(1)

3

 Lav k v ρ c The parameters are dened in the table o nomenclature. To convert rom µm to t, multiply by 3.28 x 10 –6. 2. Continue to measure the number and size o crystals as the cooling or evaporation program is in progress. Prepare an inverse cumulative plot o the number o cr ystals greater than a given size vs. size o the crystal (Figure 1). The crystal growth rate depends on the energy transer rate, so modiy the rate o energy transer until a desirable product is obtained. 3. Repeat the rst two steps at intervals throughout the batch cycle and plot the results as shown in Figure 1. The amily o curves resulting rom data plotted under the selected conditions indicates that the number o crystals is not increasing with time. Thus, no additional nucleation is occurring yet. 4. Proceed to collect crystal samples, anticipating the onset o nucleation. Figure 2 indicates that the number o crystals is signicantly increasing with time. In this gure, t 1 (not to be conused with t 1 in Figure                                                                                                                                                         

1) represents the start o this new set o batch dynamics. It is sae to assume that signicant nucleation is now occurring and that the rate o energy transer is too high. 5. By taking the slope o the curve representing the estimated number o nuclei present at the measured point in time (N t )i  vs. time (t  )i  , one can determine the nucleation rate. Using your representation o Figure 3, create a dashed, horizontal line across the lower portion o the graph depicting the selected, cumulative number o crystals (N i’ ), and their sizes (L 1–L 4)  over time (t 1–t 4).   6. For a selected cumulative number o crystals (N i ’ ), plot the crystal size ( L ) vs. time (t ), as demonstrated in Figure 3. The slopes represent the crystal growth rate ( G ). I the level o supersaturation changes during the run, the growth rate also changes. Non-parallel lines would indicate that the larger crystals are growing at a aster rate, due to a reduced diusional resistance [layer] at the crystal surace. With larger particles, the resistance layer may be smaller, allowing the solute to more readily reach the crystal surace and incorporate itsel into the lattice. These actors collectively contribute to the accelerated growth rate o the larger particles. Parallel lines indicate that the growth rate is not dependent on crystal size. 7. Increase the rate o cooling or evaporation until additional nucleation occurs, upon which you can saely assume that the growth rate is too high. 8. Develop a seeding and evaporation prole that will yield a growth rate that is lower than the value ound in Step 6. When determining the growth rate, keep in mind the dierence in mixing characteristics between a laboratory-scale vessel and a commercial conguration. A small tank generally oers a higher relative pumping capacity, shorter blend time, and higher average shear rates within a narrower range.

UsefUl observations • Most processors will agree that when it comes to crystals, the larger, the better. Large crystals are easier to handle in downstream operations, such as washing, centriugation and drying. • As previously mentioned, it is desirable or the seeds’ size distribution to refect a narrow cut o particles. In this cut, the weight o crystals with sizes ner than Ls should be minimal because these tiny particles add enormously to the number o crystals that

NomeNclature

Crystalsurfacearea,ft 2 Nucleationrate,(numberofnuclei)/ ft 3/s G  Crystalgrowthrate,µm/s k v  Crystal-volumeshapefactor, dimensionless L  Crystalsize,µm L ’ Smallest-measurablesize,µm L av  Sizeofcrystalfraction,µm L f  Finalsizeofcrystal,µm L s  Seedsize,µm N  Numberofseeds N i’  Constant,cumulativenumber of crystals in crystallizer N Li  Numberofcrystalsofagivensize,

 A  B °

L av 

N ti  S  S * t, t i,  t f  ∆W i  c

Numberofcrystalnucleiatanytime Rateofsupersaturation Maximumallowablesupersaturation,lb/ft 3solvent  Time,h Weightofcrystalsonscreen Crystaldensity,lb/ft 3

compete or supersaturation and growth. • Studies show that milled seeds may not grow as well as unmilled seeds. Furthermore, not all crystals o a given size grow at the same constant rate. This is sometimes attributed to the dierences in the surace characteristics o particles that have equal dimensions. • Fines destruction in a batch system can greatly reduce the eects o secondary nucleation on the CSD, and signicantly increase crystal size while narrowing the CSD. • In practice, not all additional nucleation can be suppressed. Crystallizations carried out at low levels o supersaturation near the metastable zone (i.e., the conditions under which crystals grow, but do not typically nucleate) will display some secondary nucleation, due to crystal-crystal interactions and contact between the crystals and the impeller. Nevertheless, the mean crystal size, shape and distribution are dramatically improved when seeding is ollowed by a programmed rate o energy transer. Reerence: Genck, W., Better Growth in Batch Crystallizers, Chem. Eng. , Vol. 106, No. 8, pp. 90–95, Aug. 2000. E-mail: [email protected]

 4

3

 3

2

 2

 1

 2

 3

 4

1 

1 2

3

Size,  (m)  = smallest measurable size FiGure 1.

  s      l 1   a    t   s   y   r   c    f         o   r   n   e  a    b    h    t   m   r   u  e   n  g          r   a    l       

1  4  1

 2

 3

        

                             

 1 



 = smallest measurable size

FiGure 2.

 2

=

 3   

Time,   (min) FiGure 3.

 4

Crystallization Department Editor: Kate Torzewski 

C

rystallization is a method o soliduid separation in which pure chemical crystals are ormed. Crystallization kinetics consists o three major phenomena: nucleation (the birth o a crystal), transer o the solute rom the supersaturated solution to the crystal surace, and a reaction during which the solute becomes incorporated into the crystal. supersaturation

In order to drive the process o crystal-

lization, the solution must be supersaturated with solute. The solubility o the solution, related to crystal size, is defned by the Kelvin equation:

supersaturated solution that is ree o oreign matter. First, molecules in the solution will associate into a microscopic cluster, which will either dissociate or continue to grow. When the cluster develops until it orms a lattice structure, it is then called an embryo. A stable crystalline nucleus is established when the crystal size exceeds D p given by the Kelvin equation or the solution’s specifc supersaturation ratio. Combining the Kelvin equation with laws o chemical kinetics gives the rate o homogeneous nucleation, which is described as ollows:

Secondary nucleation

Supersaturation can be quantifed by the ratio o the mass solute concentration in the bulk solution to the concentration in the solution at the point o saturation. Alternatively, it can be described by relative supersaturation, which is calculated by the ollowing equation:

Industrial crystallizers typically rely on secondary nucleation, which is caused by the presence o existing crystals in the supersaturated solution. This can occur by one o three mechanisms in which nuclei are removed rom a crystal surace, including the ollowing: —uid shear —collision o crystals with each other —collision o crystals with metal suraces

nucleation

Nucleation occurs at the point that a crystal begins to orm. The relative rates o nucleation and growth are critical to crystallization kinetics, as they determine both crystal size and size distribution. Nucleation is categorized as primary i the supersaturated solution is ree o crystals, or secondary i the solution already contains crystals. Primary nucleation requires a higher level o supersaturation and is the principal mechanism occurring in precipitation. Secondary nucleation occurs in commercial crystallizers, where crystalline suraces are present in order to produce large crystals. Primary nucleation

Primary nucleation, when occurring homogeneously, takes place in a

The phenomena o secondary nucleation is too complex to derive a simple kinetic theory, so an empirical powerlaw unction has been developed to describe this process, which is based on experimentally derived constants or a particular system:

crystal growth

Based on data proving that a solution in contact with a crystal is supersaturated, there is an accepted two-step theory o crystal growth, reerred to as the diusion-reaction theory. In the frst step, mass transer o solute rom the solution to the crystal-solution interace occurs. In the second step, the kinetic step, a frst-order reaction occurs at the surace o the crystal, during which solute molecules rom the solution become

Nomenclature

 A  Frequency factor  A s  Surface area of crystal B 0  Rate of homogeneous primary nucleation c  Mass solute concentration in the bulk supersaturated solution c s  Mass solute concentration in the solution at saturation c/c s  Supersaturation ratio dm/dt  Rate of mass deposited on the crystal surface D p  Crystal diameter k c  Mass transfer coefficient  k i  Kinetic coefficient  k N,  b, j, r  Constants determined experimentally MT  Mass of crystals per volume of magma N  Agitation rate N a  Avogadro’s number R  Gas Constant  s  Relative supersaturation t  Time T  Temperature s,L  Interfacial tension s  Molar volume of crystals Number of ions/molecule of solute Solute diffusivity Solution density incorporated into the crystal-lattice structure. By combining these steps, the rate o mass transer in crystal growth is expressed as ollows:

Through a series o calculations, this expression can be used to defne the particle size o a crystal at any time, assuming that Dp0 << Dp:

References 1.Seader, J. D. and Henley, E. J., “Separation Process Principles,” 2nd ed., John Wiley and Sons, Inc., New Jersey, 2006.

Hmdty Contol Department Editor: Kate Torzewski  applications

Compressor

Dehumidifcation by cooling or dessication has a variety o applications, including:

Condenser

Preventing moisture regain . Nearly all materials have some afnity or moisture based on surace characteristics and the amount o surace exposed to humid air. Moisture regain occurs when moist particles stick together. Preventing condensation . Air holds water vapor in proportion to its temperature. Cold suraces o pipes, vessels, valves and heat exchangers condense moisture unless the air around them is dried to a dewpoint below the temperature o the cold surace. Preventing corrosion . The exposure o metal suraces to atmospheric corrosion can be reduced by surrounding the suraces with dry air. Dehumidifers also keep humidity low in process control rooms, preventing the corrosion o electrical contacts and sensitive electrical components. Drying heat-sensitive products . Typically, drying time is reduced by heating a product. I the product is susceptible to damage by heat, drying time can be reduced by using dehumidifed air, which reduces the vapor pressure o air above the wet surace. cooling

A common method or dehumidifcation is the use o air conditioning. Figure 1 shows a typical vapor-compression cooling-based dehumidifcation process. Air to be dried passes through a cooling coil, which lowers the temperature o the airstream below its dewpoint. As the air cools, it loses its capacity to hold water vapor. The water condenses on the cooling coil surace, and alls to the drain pan as liquid. The air is then drier in absolute terms, but it also has a relative humidity close to 100%. I low relative humidity is needed in addition to a lower absolute amount o moisture, the air can be heated ater it leaves the cooling coil. For industrial purposes, cooling-based dehumidifcation units are optimized or removing moisture rather than removing heat. These units provide deep cooling o small amounts o air rather than slight cooling o large amounts o air, condensing more moisture. Standard rerigeration equipment can produce dewpoints o +40°F (4°C) on a reliable basis. dessication

In a desiccant system, the process airstream passes through a desiccant medium. The desiccant adsorbs moisture directly rom the airstream. Desiccant dehumidifers can produce dewpoints below 0°F (–18°C)

Heater

Receiver Desiccant wheel

Cooling coil

Fan

Process air

Dry air

Heater Condensate pan Drive motor

160

100% relative humidity

   b    l    /   s   n    i   a   r   g  ,   y    t    i    d    i   m   u    h   c    i    f    i   c   e   p    S

120

75%

50%

80

Enter 40 Leave

160

100% relative humidity

   b    l    /   s   n    i   a   r   g  ,   y    t    i    d    i   m   u    h   c    i    f    i   c   e   p    S

120

75%

50% Enter

25% 40

25%

Leave 10

10

20

30 50 60 70 Air temperature, F

80

80

10

10

20



30 50 60 70 Air temperature, F

80



Figure 1. Air through a cooling dehumidifier

Figure 2. Air through a desiccant dehumidifier

— a fveold reduction in the air moisture beyond what can be achieved with a standard-grade air conditioning system.

both operating cost and initial equipment cost. Below 50°F, precautions need to be taken to avoid reezing the condensed water on the cooling coil. Consequently, desiccants are more economical than cooling-based systems at lower dewpoints.

This equipment uses dierences in vapor pressure to remove moisture rom air by chemical attraction. The surace o dry desiccant has a very low vapor pressure, compared with the much higher vapor pressure o humid air. Water vapor moves out o the humid air onto the desiccant surace to eliminate the vapor pressure dierence, as shown in Figure 2. Eventually, the desiccant surace collects enough water vapor to equal the vapor pressure o the humid air. Then the desiccant must be dried (reactivated) by applying heat beore it is recycled to remove more moisture rom the air stream. cooling vs. dessication

In most chemical process applications, both technologies work best together. Coolingbased dehumidifcation handles the moisture load occurring at high dewpoints, and desiccant-based dehumidifcation removes the moisture load at low dewpoints. The optimal mix o the two technologies depends on the characteristics o the application. Factors to consider include the ollowing: Dewpoint control level . When the required moisture-control level is relatively high (above a 50°F dewpoint), cooling-based dehumidifcation is economical in terms o

Relative humidity sensitivity . When a process needs a low moisture level in absolute terms, but can tolerate a high relative humidity, cooling-based dehumidifcation without desiccants is cost eective. By contrast, in processes that demand a low relative humidity in addition to a low dewpoint, desiccant systems are used or humidity control, with supplementary cooling systems to keep temperature within acceptable limits. When a product is sensitive to relative humidity but not to temperature, a desiccant dehumidifer is used without a cooling unit to maintain a constant relative humidity. Temperature tolerance . I the application can tolerate a wide temperature range, then dehumidifcation alone may sufce. In most cases, both temperature and moisture must be maintained within set limits, so both cooling and desiccant equipment are used in a combination to maintain control. References

1.Harriman, L., Don’t Sweat It, Dehumidiy, Chem. Eng., August 1997, pp. 80–87. 2.Soleyn, K., Humidity Control: Preventing Moisture Contamination, Chem. Eng., October 2003, pp. 50–51.

Department Editor: Rebekkah Marshall 

Preventing Runaway Reactions

general considerations [ 1 ]

data collection

thermal stability criteria [1 , 4 ]

 A process is considered to be thermally safe only if the reactions can easily be controlled, and if the raw material, the products, the intermediates and the reaction masses are thermally stable under the considered process conditions. Check into the process equipment, its design, its sequence of operation and the control strategies. In addition to the engineering aspects, get detailed information on thermodynamic and kinetic properties of the substances involved, such as the reaction rates or heat-release rates as a function of process conditions. Determine the physical and chemical properties, as well. Understanding of thermal-hazard potential requires knowledge of various skills and disciplines [3]. These include: Operating mode:  The mode of operation is an important factor. For instance, a batch reaction, where all the reactants are charged initially, is more difficult to control than a semi-batch operation in   which one of the reactants is charged progressively as the reaction proceeds (for more, see Design Options). Engineering:  Design and layout of the plant and equipment and its built-in controls impact the entire process. The capacity of the heating or cooling system is important in this context. Process engineering is used to understand the control of the chemical processes on a plant  scale. It determines which equipment  should be used and how the chemical processes should be performed. In addition, take into account technical failure of equipment, human errors (deviations from operating instructions), unclear operating instructions, interruption of  energy supply, and external influences, such as frost or rain (for more, see Design Options). Chemistry: The nature of the process and the behavior of products must be known, not only under reaction conditions, but  also in case of unexpected deviations (for example, side reactions, instability of intermediates). Chemistry is used to gain information regarding the reaction pathways that the materials in question follow. Physical chemistry and reaction kinetics:  The thermophysical properties of the reaction masses and the kinetics of the chemical reaction are of primary importance. Physical chemistry is used to describe the reaction pathways quantitatively.

The following data are especially relevant in avoiding runaway reactions:  As a guideline, three levels are sufficient  • Physical and chemical properties, ig- to characterize the severity and probnition and burning behavior, electro- ability of a runaway reaction, as shown static properties, explosion behavior in the Table. and properties, and drying, milling, Defining high, meDium and toxicological properties anD low risk [1] • Interactions among the chemicals Severity Probability • Interactions between the chemicals High ΔT  TMR ad < 8 h ad > 200K and the materials of construction Medium 50K < ΔT ad < 200K 8 h < TMR ad < 24 h • Thermal data for reactions and deLow T  <  50K and Δ ad  TMR ad > 24 h composition reactions the boiling point • Cooling-failure scenarios cannot be surpassed

design options [2 ]

a u 

If a reaction is has the potential for runaway, the following design changes should be considered: • Batch to continuous. Batch reactors require a larger inventory of reactants than continuous reactors do, so the potential for runaway in continuous systems is less by comparison • Batch to semi-batch. In a semi-batch reaction, one or more of the reactants is added over a period of time. Therefore, in the event of a temperature or pressure excursion, the feed can be switched off, thereby minimizing the chemical energy stored up for a subsequent exothermic release • Continuous, well-mixed reactors to  plug flow designs. Plug-flow reactors require comparatively smaller volumes and therefore smaller (less dangerous) inventories for the same conversion • Reduction of reaction inventory via  increased temperature or pressure, changing catalyst or better mix-  ing.   A very small reactor operating at a high temperature and pressure may be inherently safer than one operating as less extreme conditions because it contains a much lower in ventory [3 ]. Note that while extreme conditions often result in improved reaction rates, they also present their own safety challenges. Meanwhile, a compromise solution employing moderate pressure and temperature and medium inventory may combine the  worst features of the extremes [ 3 ]. • Less-hazardous solvent  • Externally heated or cooled to inter-  nally heated or cooled 

The adiabatic temperature rise is calculated by dividing the energy of reaction by the specific heat capacity as shown in Equation (1). (1) ΔT  ad = 1,000Q r/   C p where: ΔT  ad  = adiabatic temperature rise, K Q r = energy of reaction, kJ/kg C p = heat capacity, J/(kg)(K)

t  xu  (tmr) TMR ad (the time to maximum rate, adiabatic) is a semiquantitative indicator of the probability of a runaway reaction. Equation (2), defining TMR ad in hours, is derived for zero-order reaction kinetics: TMR ad = C pRT o2  /3,600qo E a (2) where: R = gas constant, 8.314 J/molK T o = absolute initial temperature, K qo = specific heat output at To, W/kg E a = activation energy, J/mol

The TMR value provides operating personnel with a measure of response time. Knowledge of the TMR allows decisions to be based on an understanding of the time-frame available for corrective measures in case heat transfer is lost during processing. References

1. Venugopal, Bob, Avoiding Runaway Reactions, Chem. Eng., June 2002, pp. 54–58. 2. Smith, Robin, ”Chemical Process Design,” McGraw-Hill, New York, 1995. 3. Kletz, T. A., “Cheaper, Safer Plants,” IChemE Hazard Workshop, 2d., IChemE, Rugby, U.K., 1984. 4. Gygax, R., Reaction Engineering Safety, Chem. Eng. Sci., 43, 8, pp. 1759–71, August 1998.

Sedimentation Centrifuging Department Editor: Kate Torzewski  entriugation is the method o choice in the chemical process industries (CPI) or separating solids rom liquids. It relies on the G-orces generated by highspeed rotation to recover solids or liquids rom slurries, as well as clariy liquids or classiy solids. Centriuges can be categorized as either sedimentation or ltration units. Sedimentation centriugation relies on a dierence in density between the solid and liquid being separated. Filtering is perormed with a rotating basket tted with a lter medium, where the centriugal orce o rotation expels the liquid through the lter.

C

basics of sedimentation centrifuging The mechanics o sedimentation centriuging make it ideal or two-phase systems with a high-density dierential. As an incoming slurry spins in a sedimentation centriuge, it orms an annulus adjacent to the bowl wall. The centriugal orce causes the denser material to move outwardly toward the wall o the centriuge bowl, while the liquid overfows rom the bowl or is

picked up by a skimmer. Periodically, the solid must be removed rom the centriuge manually or with a cutter knie. Alternatively, it can be removed continuously with a screw conveyor. Sedimentation centriugation allows material to be separated hundreds or thousands o times aster t han simple sedimentation by gravity alone.

stoKes' law According to Stokes’ Law, the terminal velocity o a particle is determined by the centriugal gravity (Ω2r ) created by the centriuge with particle-balancing buoyancy and viscous drag taken into account. This terminal (settling) velocity is determined by the equation below: 1 2 2 Vs   r (  s   L ) d  18   where V s = Settling velocity, m/s μ = viscosity, kg/m·s Ω = angular speed o rotation, rev/min ρs = density o solid, kg/m 3 ρL = density o liquid, kg/m 3 d = particle diameter, m r = centriuge radius o curvature, m

Stokes’ Law tells us that settling velocity can be maximized with a high centriugal speed, large particle size, large density dierence between solids and liquid, large separation radius and low liquid viscosity.

applications Centriuge selection is heavily dependent on characteristics o the incoming slurry, including particle size, solids concentration, liquid viscosity and density dierential. Other actors that come into play are the need to remove solids periodically or continuously and the degree o purity required o the separated products. Table 1 summarizes the mechanics and suitable applications o common sedimentation centriuges. References

1.Scroder, T. Selecting The Right Centriuge, Chem. Eng. September 1998, pp. 82–88. 2.Moir, D. N. Sedimentation Centriuges: Know What You Need, Chem. Eng. March 1988, pp. 42–51. 3.Bershad, B. C., Chaotte, R. M., Leung, W. F. Making Centriugation Work For You, Chem. Eng. August 1990, pp. 84–89. 4.“Perry’s Chemical Engineers’ Handbook,” 8th ed. New York: McGraw Hill, 2008.

TABLE 1. types of sedimentation centrifuges

t

mh

B  

Tubular Bowl

• A vertical cylinder with the feed • The heavier phase becomes concen• Purification of lubricating slurry introduced in the bottom of trated against the wall, whi le the lighter and industrial oils the bowl phase floats on top • Food, biochemical and • The use of a distributor and baffle • The two phases are separated by a baffle pharmaceutical applications assembly accelerates the slurry to • Liquid discharges over the top of the bowl, • Solids should be less than 1% the speed of rotation while solid buildup is removed manually in volume of the slurry

Multichamber

• Constructed of a series of tubular sections arranged concentrically • The slurry feed enters in the smallest tube and continues through the outer tubes as they increase in size

• Larger solid particles settle in the small tubes, and particles of smaller sizes settle in subsequent tubes • Up to six chambers are typical with a maximum holding capacity of 0.064 m3

Skimmer • Feed enters the hub end and • When a thick solid layer begins to form pipe / is accelerated to speed before on the bowl wall, supernatent liquid is knife entering the separation pool removed with a skimmer, and solids are discharge • Solids settle on the bowl wall while knifed out with centrifugal filters liquid overflows the ring weir

• Clarifying fruit juices, wort and beer

• Heavy-duty applications, such as coal dewatering

Disc

• Feed enters through the top axis • Solids settle under the disc and move • Self-cleaning types: of the bowl and is accelerated by downward to be released at the bottom of purification of beverages, a radial-vane assembly the bowl wall mineral oils, and edible oils • The unit is constructed of a stack • Liquids travel up the conical channel, • Disc nozzle: corn wet of typically 50 to 150 closely and their upward movement in the milling (starch separation, spaced conical discs arranged at centrifuge is facilitated by holes across gluten thickening), clay an angle between 40 and 50 deg each disc classification, acid crystal washing, lube oil dewaxing

Decanter

• Constructed of a solid external bowl and an internal screw conveyor mounted horizontally

Screenbowl

• Solids are removed from the conical • Applications that require discharge end (the beach) continuous removal of solids, where feed solids are high • Bowl and conveyor rotate in the same and volume reduction is direction, but at different speeds, creating important a speed differential that controls the speed of solid removal • A solid bowl decanter with a cylindrical screen added to the conical end • Improved cake dryness and highest product purity

Sovnt Sction Mthodoogy Department Editor: Rita L. D'Aquino  A STEPWISE ProcEdurE

Table 1. Some well-known databases and solvent selection tools

Organic solvents have been used in many industries or centuries, but the methods and tools to select optimal solvents while minimizing their adverse environmental, health, saety and operational concerns are still evolving. The appropriate selection o solvents depends to a large extent on the application — more specifcally on what needs to be dissolved, and under what conditions. This article presents a our-step approach to solvent selection based upon Re. 1*, where the reader will fnd a list o additional resources on this topic. Identify the challenge and solvent characteristics.

Databases ChemFinder Solvents Databases

NIST Webbook DIPPR and TAPP CAPEC Database Selection Tools

The frst two steps are: 1) identiying the actual problem and technology or unit operation required to solve it; and 2) defning the requirements that must be met by the solvent, using criteria related to its physical and chemical properties ( e.g., pure-solvent properties, such as normal boiling point, the Hildebrand solubility parameter at 300 K, the Hansen solubility parameters; solventsolute properties, such as the solubility o the solute as a unction o the composition o the mixture; and unctional constraints, such as solute loss in solute). Obtain reliable values of solvent properties and  narrow down selection. There are several alternatives or this

third step. For example, one can measure the required properties, use a database o properties o chemicals (or solvents), or, use property models to estimate them. For solvent-selection problems not involving chemical reactions, the pattern o the desired solvent is established through analysis o the solute, application type, and other constraints. Once this is established, a database o known solvents can be used to identiy the solvents that match the necessary pattern (Table 1). On the other hand, when chemical reactions are involved, the approach is based on transition-state theory and requires consideration o the solvation energies o the reactants, products and transition states, and thus, knowledge o the reaction mechanism. When the crucial values have been ound, the solvent search could be such that frst, solvent-pure properties are used, ollowed by solvent-EHS, then solvent-solute, and fnally solvent-unction. Narrow down the list by removing the compounds that do not match desired properties. A protocol derived by Britest Ltd. (www.britest.co.uk) seeks to use mechanistic principles to guide solvent selection (Figure). The objective is to ollow the arrows according to the problem defnition and a search criterion until an end-point is reached, thereby obtaining the characteristics o the candidate solvents. These characteristics are used to identiy the group to which the solvents belong using solvents database (see Table 2). The corresponding group-types are evaluated and a fnal selection is made.

SMSwin

Address and comments Searchable data and hyperlink index: http://chemfnder.cambridgesot.com Solvent substitution data systems at http://es.epa.gov/ssds/ssds.html; “Handbook o Solvents” rom www.chemtec.org/cd/ct_23.html; and SOLVDB at http://solvdb.ncms.org/index.html Source o physical and chemical data at http://webbook.nist.gov www.aiche.org/TechnicalSocieties/DIPPR/About/Mission.aspx; and www.chempute.com/tapp.htm Pure as well as mixture properties data, including solvent-solute database: www.capec.kt.dtu.dk/Sotware/ICAS-and-its-Tools Address and comments

A specialized sotware or property estimation and solvent classifcation: www.capec.kt.dtu.dk/documents/sotware/SMSWIN.htm Activity coefcient method based on segment contributions. Predictive based on a small set o solubility data. Useul or crystallization solvent selection and extends to LLE and VLE: www.aspentech.com

NRTL-SAC and eNRTL-SAC

Table 2. Well-known solvents together with their related properties

Solvent Name

Molecule type

Group type Charge

1-Methyl-2-pyrrolidinone Acetonitrile Dimethyl sulphoxide Dimethyl ormamide Dimethylacetamide Diisopropyl ether Dimethyl ether Methyl tertbutyl ether Tetrahydrouran Chlorobenzene m-xylene (also o -; p -) Toluene Acetic acid Propionic acid Suluric acid

Amide Nitrile S-oxide Amide Amide Ether Ether Ether Ether Chloride Aromatic HC Aromatic HC Acid Acid Acid

1 1 1 1 1 2 2 2 2 3 3 3 4 4 4

Propanol Ethanol Butanol Ethylene glycol Dichloromethane Heptane Hexane Pentane Methanol Water

Alcohol Alcohol Alcohol Alcohol Chloride Alkane Alkane Alkane Alcohol Aqueous

5 5 5 5 6 7 7 7 4, 5 4, 5

Verify selection. The ourth step is to veriy that the solvent

works as expected by perorming a computational validation by simulation. Experimental validation o a solvent candidate is required at all stages o process development.

NE/EPD E/NPG E/NPG NE/NPG NE/NPG NE/EPD NE/EPD NE/EPD NE/EPD NE/P NE/P NE/P PG E/PG E/PG

NBP (K) 475.15 354.75 462.15 426.15 438.15 341.65 248.35 328.35 338.15 632.35 412.27 383.95 391.05 414.25 610

NMP (K) 249.15 229.35 291.65 212.75 253.15 181.35 131.65 164.55 164.85 404.9 225.3 178.25 289.81 252.45 283.46

23.16 24.05 26.75 23.95 22.35 14.45 15.12 15.07 18.97 19.35 18.05 18.32 19.01 19.41 28.41

E/N E/N E/N E/N NE/EPD NE/I NE/I NE/I E/N E/N

370.35 351.35 390.81 470.45 313.15 371.65 341.85 309.22 337.85 373.15

147.05 159.05 183.85 260.15 178.05 182.55 177.85 143.42 175.47 273.15

24.45 26.13 23.35 33.7 20.37 15.2 14.9 14.4 29.59 47.81

ordered

Sol. Par.

NE = non-electrolytic solvent; E = electrolytic solvent; P = polarizable; EPD = electron-pair donor; I = inert; PG = protogenic (proton donor); N= neutral (donor & acceptor); NPG = non-protogenic (proton acceptor); NBP = normal boiling point; NMP = normal melting point; Sol. Par. = Hildebrand solubility parameter at 300 K (MPa 1/2)

Stability, solubility of reactants, products Two-phase or liquid-liquid (polar phase is water)

Single phase or solid-liquid Homogeous catalysis by Pt group complexes Moderate polarity

SN1/E1

Condensation

Aromatic hydrocarbon (xylene)

Group 2

Fast, low temp, but recovery difficult

Slow, hightemperature, easy recovery

Group 3

Group 1

Group 3

DPA ethers, aromatics Group 1

SN2/E2

High polarity Dipolar aprotic

Water, immiscible solvent

Substrate/product hydroxyl sensitive Yes

Water, carboxylic acids, inorganic acids, lower alcohols Group 4

Consider solvation

Dipolar aprotic ethers Group 1 Group 2

No

Choose ‘polarity’ based on substrate and reagent solubility. May need phase-transfer catalyst Group 3

Water, alcohols Group 5

Group 6 Group 7

*Reference:  1. Gani, R., et al., A Modern Approach to Solvent Selection, Chem. Eng., Vol. 113, No. 3, pp. 30–43, Mar. 2006. Author E-mail: [email protected]

Avoiding Seal Failure Department Editor: Kate Torzewski 

S

eals are assemblies o elements that prevent the passage o a solid, liquid, gas or vapor rom one system to another. When a seal allows leakage o material, ailure has occurred. This guide provides an overview o common seal types and a discussion o seal ailure to aid in choosing the most eective seal and avoiding uture ailure.

seal types Seals types can be classied within two broad categories: static and dynamic. Static seals have no relative motion between mating suraces, while dynamic seals do have relative motion between a moving surace and a stationary surace. Seals do not have to t into one category or the other; rather, seal types can all anywhere on a spectrum between static and dynamic, and ew seals are strictly one type or the other. Table 1 describes the applications and requirements o several common seal types.

seal failure Seal ailure is caused by a wide variety o circumstances, including improper installation and environmental actors such as temperature, pressure, fuid incompatibilities, time and human actors. Most causes o ailure can be described as mechanical diculties or system operations problems. Examples o mechanical diculties include strain on the seal ace caused by improper installation and vibration caused by improper net positive suction head. Meanwhile, system operating problems can include conditions that are outside o a pump’s best perormance envelope, such as upsets, dry running, and pressure or temperature fuctuations. Changes in the fuid being processed can cause problems as well, especially with fuids that fash or carbonize. Common visual indicators o ailure include short cuts, V-shaped notches in the seal, skinned surace in localized areas, or thin, peeled-away area on the seal. Table 2 describes causes o some o the most prevalent types o seal ailure with recommended methods o action. In some cases, the cause o ailure may be dicult to determine due to the complexity o the seal construction. These unique ailure modes can result in faking or peeling o the seal ace, corrosion, faking or pitting o the carbon aces, degradation o the elastomer energizer seals, and spring or bellows breakage. It is likely that these rapid degradations are a result o contamination, which can be avoided with careul installation or using pre-assembled, cartridge-type mechanical seals.

TABLE 1. COMPARISON OF COMMON SEAL TYPES Type

Applications

O-ring T-seal U-packing V-packing Cup-type packing Flat gasket Compression or jam packing

References

Static

Dynamic

Periodic Adjustment Required?

Moving friction

Tolerances Gland Space required (mov- adapters requireing seals) required? ments

X X — — —

X X X X X

No No No Yes No

Medium Medium Low Medium Medium

Close Fairly close Close Fairly close Close

No No No Yes Yes

Small Small Small Small Medium

X X

— X

Yes Yes

— High

— Fairly close

No Yes

Large Large

1.

Ashby, D. M. Diagnosing Common Causes o Sealing Failure, Chem. Eng. June 2005, pp. 41–45.

2.

Netzel, J., Volden, D., Crane, J. Suitable Seals Lower the Cost o Ownership, Chem. Eng. December 1998, pp. 92–96.

TABLE 2. SOLUTIONS TO COMMON CAUSES OF SEAL FAILURE Failure type Definition

Causes

Solutions

Compression A lost o resiliency caused by the set ailure o a seal to rebound ater it has been deormed or some period o time. The seal will exhibit a attened surace corresponding to the contours o the mating hardware Nibbling and A seal starts to appear to be torn extrusion away in little pieces until it loses its overall shape and ows into whatever void area is available

Exposure to excessive temperature or incompatible uids Excessive deormation o the elastomer at installation An incompletely vulcanized seal

Choose proper deection or the seal Choose appropriate elastomer material or the application in terms o thermal stability and compression set resistance

Spiral ailure

Explosive decompression Wear

Excessive clearance gaps Improper seal material Excessive volume-to-void ratio Inconsistent clearance gaps

Increase bulk hardness o the sealing element Decrease clearance gaps Redesign volume-to-void ratio Add anti-extrusion devices A seal rolls within its gland, resulting Applications where a seal is Use an elastomer with a higher bulk in cuts or marks that spiral around the used in a slow, reciprocating hardness circumerence o the seal ashion For male-type installation, increase the Irregular surace over the mating installed stretch on the seal parts causing the seal to grip to Speciy a smoother, more uniorm fncertain contact points ish on mating hardware Change the type o seal to a lip-type confguration Seal exhibits blisters, fssure, pock Gas entrapment within the Use an elastomer material that is more marks or pits, both externally and elastomer during high-presresilient to explosive decompression internally sure cycling, ollowed by rapid Use polymeric or metal seals i depressurization 0possible Smooth burnishing o a sealing Relative motion o the seal Use a harder material surace against the mating surace Use a polymeric solution

Flow Profile for Reciprocating Pumps

Department Editor: Scott Jenkins eciprocating pumps are often used in the chemical process industries (CPI) because of their ability to generate high pressures at low velocities. A subcategory of positive-displacement pumps, reciprocating pumps act through the recipricating motion of a piston, plunger or diaphragm. Such pumps work by way of a connecting-rodand-crank mechanism with a piston. By nature, reciprocating pumps generate pulsing flow, which, when plotted as a function of time, or of crank angle, produces a curve that resembles a sine wave to a first approximation. For example, manufacturers of pulsation dampeners and surge suppressors often use sinusoidal curves for piston pumps and compressors in their product literature and sizing formulas. However, a closer examination of the flow profile for a piston-and-crank pump or compressor reveals the curve to be a significantly distorted sine wave because of the interaction between the crank and the connecting rod.

R

Calculating fowrate

In graphical form, the crank and crankshaft of a reciprocating pump can be visualized by placing the crankshaft center at the 90-deg mark of a 180-deg x -axis, and placing the crank bearing at the origin (see figure). A connecting rod links the crank to the piston. Determining the position of the piston at any crank angle can be accomplished by measuring on a piston pump, compressor, or piston engine, or it can be calculated using trigonometric relationships. The degree to which the actual flow profile curve deviates from the sinusoidal curve is determined by the ratio of the connecting rod length to the crankshaft length. Smaller values of the ratio translate into greater levels of distortion. As the connecting rod becomes very long, the flow profile would approach the sine curve To calculate the flowrate at a given crank angle, use the following procedure and definitions: Crank length = OC  Piston rod length = CP  For any angle a, Line AC = OC sin (a) Line SA = OC – OC cos (a) Line AP = (CP 2 – AC 2)0.5 Line SP = AP + SA .

1. Calculate the piston positi on for two crank angles, perhaps 2 deg apart. 2. The difference in piston positions equals piston displacement over the time interval between the two crank angles. The value is an average over the span of the two readings, not an instantaneous reading. As the step size approaches zero, displacement nears the t rue velocity. 3. This value can be converted into flowrates (gal/min or other units) if the piston diameter and speed (revolutions per minute, rpm) are known.

90 deg

Crank  C

Connecting rod

0 deg

P

Piston

Cylinder

a

S

180 deg A O Piston velocity versus crank angle

0.7000    0    10.6000    X   g   e0.5000    d    /  .   n    i  , 0.4000   y    t    i   c   o0.3000    l   e   v   n0.2000   o    t   s    i    P0.1000

0.0000 0.00

Crank length = 2 Connecting rod length = 2.05 (blue line) 4 (red line) 10 (green line)

20.00

40.00

60.00

80.00

100.00 120.00 140.00 160.00 180.00 200.00

Crank angle (rotation), deg Observations o the plot

In an illustrative example, plots of piston velocity versus crank angle are shown (see graph). The ratios of the connecting rod length to crank shaft length are 1.05 to 1 (blue line), 2 to 1 (red line) and 5 to 1 (green line). The following observations can be made: 1. At the beginning of the discharge stroke, flowrate approaches zero asympotically, rather than as a sinusoidal curve 2. Peak flowrates do not occur at the 90-deg point, but rather at 95–120 deg, depending on the ratio of rod length to crank length 3. Peak flowrates are higher than would be predicted with a pure sine curve 4. From 180 to 360 deg (the sucti on portion of the pump cycle), the curve mirrors the 0-to-180-deg portion 5. Flowrates during the suct ion portion of the curve are also higher and occur earlier than the 270-deg point Eects o distorted sine curve

Within the areas of fluid flow and mechanical pump design, there are a number of aspects that are affected by the deviation of flow profile from a perfect sine curve for pumps and compressors. The effects include the following:  higher-than-predicted peak flowrates

and pressure drop will be higher, by the square of flowrate  will affect net positive suction head (NPSH) and possibly induce vaporization  be lower than what would be allowed by the pure (non-distorted) sinusoidal curve  increase somewhat, especially in highspeed compressors  be affected  peak of a bell curve, rather than a smoother sine curve  would have less “smoothing” effect than would be predicted because the bellshaped curve has a sharper peak Reerences  2. Henshaw, T.E., “Reciprocating Pumps,” Van  3. Krugler, A., Piston Pumps and Compressors: Exploring the Flow Profile, Self-published, 2010. Note

Fingertips” was contributed by Arthur Krugler,  Calif. (www.kruglerengineeringgroup.com).

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‫¸¬‪¬ ´»²¹‬؛؟¸‪‬ح‬ ‫‪®±³ ³±«²¬·²¹‬؛‬ ‫»½‪‬؟؛®«‪-‬‬

‫‪®·²¹‬؟»‪‬ق‬ ‫؛´»¸‪-‬‬ ‫¬‪‬؛؟¸‪¬± -‬‬ ‫®»¬‪®·²¹ ½»²‬؟»‪‬ق‬ ‫»‪¸»·¹¸¬ ´·²‬‬

‫‪²½» ¬±‬؟¬‪·-‬ـ‬ ‫‪®±³‬؛ ‪®·²¹‬؟»¾‬ ‫»½‪‬؟؛®«‪³±«²¬·²¹ -‬‬

‫ٌ‬ ‫¼«¬‪±´¬-‬ق‬ ‫‪¬¬»®²‬؟‪°‬‬

‫´‪‬؟½·‪´ ®±´» ·² ¬¸» ½¸»³‬؟¬‪±‬ھ·‪ °‬؟ §‪‬؟´‪´- °‬؟»‪´ -‬؟½·‪²‬؟¸½»‬ ‫ف ّ‪°®±½»-- ·²¼«-¬®·»-‬‬ ‫»½‪‬؟؛®»¬‪´·²¹ ·²‬؟»‪- ¬¸» -‬ك ‪×ٍ÷‬ذ ‪‬‬ ‫»¸¬ ¼‪²‬؟ ¬‪¬·²¹ °®±½»-- »¯«·°³»²‬؟¬‪¾»¬©»»² ®±‬‬ ‫¬‪»²‬ھ»®‪·½»- °‬ھ»¼ »‪·²»¼ ·²-·¼»ô ¬¸»-‬؟¬‪´- ½±²‬؟·®»¬‪‬؟‪³‬‬ ‫‪» »³·--·±²-ô ©¸·´» °»®َ‬ھ·¬·‪«¹‬؛ ¼‪²‬؟ »‪¹‬؟‪µ‬؟»´ ¬½«¼‪°®±‬‬ ‫®‪²¼ ½±³°®»--±‬؟ ‪½¸·²»-ô °«³°-‬؟‪³·¬¬·²¹ °®±½»-- ³‬‬‫‪½««³-ô‬؟ھ ‪-·²¹ °®»--«®»-ô‬؟»®½‪»®َ· ²‬ھ» ¬‪‬؟ »¬‪‬؟®»‪¬± ±°‬‬ ‫½·‪-‬؟¾ ¬‪»² ¬¸» ³±-‬ھغ ‪²¼ -°»»¼-ٍ‬؟ ‪¬«®»-‬؟®»‪¬»³°‬‬ ‫‪--»³¾´§ٍ‬؟ ¨»´‪ ½±³°‬؟ ‪´ ·-‬؟»‪´ -‬؟½·‪²‬؟¸½»‪³‬‬ ‫®‪±‬؛ §®»‪½¸·²‬؟‪ ¬¸» ³‬؛‪-«®»³»²¬ ±‬؟»‪±®®»½¬ ³‬ف‬ ‫¼»¬‪‬؟·½»®‪°°‬؟®»¼‪² «²‬؟ »¾ ‪²‬؟½ ‪´-‬؟»‪½»³»²¬ -‬؟´‪®»°‬‬ ‫‪²²·²¹ٍ‬؟´‪²½» °‬؟‪·²¬»²‬؟‪ ³‬؛‪-°»½¬ ±‬؟‬ ‫¼‪²‬؟ ‪-·½ °®±½»¼«®»-‬؟¾ »‪±´´±©·²¹ -¬»°- ±«¬´·²‬؛ »¸‪‬ج‬ ‫®‪±‬؛ §®»‪½¸·²‬؟‪-«®·²¹ ³‬؟»‪»¼ ·² ³‬ھ´‪±‬ھ‪¬·±²- ·²‬؟®»¼·‪½±²-‬‬ ‫‪´-ٍ‬؟»‪½»³»²¬ -‬؟´‪®»°‬‬

‫س‬

‫جزغسغفكشذغخ شكغح جعكطح خرع ظز×‪‬خثحكغس‬ ‫فقس‬

‫®»¬»‪³‬؟·¼ ¬‪‬؛؟¸‪‬ح ‪¬»° ٍï‬ح‬

‫فقس‬

‫‪ً¼»¹‬ي ‪ ٌ½±-‬م فقس‬ ‫ّ‬ ‫÷ ‪ Hُ î ‬م ‪‬فقس § ‪‬؛·® » ‪‬ت‬

‫م فقس‬ ‫ّ‬ ‫÷ ‪ Hُ î ‬م ‪‬فقس § ‪‬؛·® » ‪‬ت‬

‫فقس‬ ‫فقس‬ ‫‪î‬‬

‫ي‬

‫‪ï‬‬

‫ى‬

‫´´·© ´‪‬؟»‪½»³»²¬ -‬؟´‪³»¬»®ّ ( ÷ ©¸»®» ¬¸» ®»°‬؟·¼ ¬‪‬؛؟¸‪-«®» ¬¸» -‬؟»‪‬س‬ ‫‪ ‬؛‪-«®»³»²¬ ±‬؟»‪ ³‬؟ »‪µ‬؟¬ §´´‪‬؟«¬½‪‬؟ ‪²¬ ¬±‬؟¬®‪- ·³°±‬ژ¬× ‪´´»¼ٍ‬؟¬‪¾» ·²-‬‬ ‫»¸¬ ‪®±³‬؛ ‪² ®»´§·²¹ ±² ¼·³»²-·±²-‬؟¸¬ ®»¸¬‪‬؟® ‪»ô‬ھ»»´‪²¼ -‬؟ ¬‪‬؛؟¸‪¬¸» -‬‬ ‫‪- ¾´«»°®·²¬-ٍ‬ژ§®»‪½¸·²‬؟‪³‬‬ ‫‪´±²¹‬؟ ‪»®»²¬ °±·²¬-‬؛؛·¼ ¬‪‬؟ §´¬¸‪®§ -´·¹‬؟ھ ‪²‬؟½ ®»¬»‪³‬؟·¼ ¬‪‬؛؟¸‪¸» -‬ج‬ ‫»¾ ´´·© ´‪‬؟»‪-«®»³»²¬ ©¸»®» ¬¸» -‬؟»‪µ» ¬¸» ³‬؟¬ ‪·¬- ´»²¹¬¸ô -±‬‬ ‫¬‪-‬؟»´ ¬‪‬؟ ®‪´ô ±‬؟»‪·²¹ ¬¸» »¨·-¬·²¹ -‬ھ‪² ®»³±‬؟»‪§ ³‬؟‪¸·- ³‬ج ‪°±-·¬·±²»¼ٍ‬‬ ‫‪§ٍ‬؟© »¸¬ ‪‬؛‪-´·¼·²¹ ·¬ ±«¬ ±‬‬ ‫»‪¹‬؟®»‪‬ھ؟ ‪²‬؟ ‪·²‬؟¬¾‪®»¬¬»¼ô -± ±‬؛ ®‪§ ¾» ©±®² ±‬؟‪¬ ³‬؛؟¸‪¸» -‬ج‬ ‫‪µ·²¹‬؟¬ §¾ ¼»¸‪½½±³°´·-‬؟ »¾ ‪²‬؟½ ‪¸·-‬ج ‪ٍ¬‬؛؟¸‪ ¬¸» -‬؛‪-«®»³»²¬ ±‬؟»‪³‬‬ ‫‪¹َ‬؟®»‪‬ھ؟ ‪´·°»®ô ¬¸»²‬؟½ ®‪ ³·½®±³»¬»® ±‬؟ ¸¬·© ‪-«®»³»²¬-‬؟»‪´ ³‬؟®»‪‬ھ»‪-‬‬ ‫‪ ‬؛‪-«®»³»²¬ ±‬؟»‪¹» ³‬؟®»‪‬ھ؟ ‪²‬؟ ¬»‪§ ¬± ¹‬؟© »¬‪‬؟‪´¬»®²‬؟ ‪²‬ك ‪·²¹ ¬¸»³ٍ‬‬ ‫»®‪‬؟ »‪¸»-‬ج ‪-«®»ٍ‬؟»‪°» ³‬؟¬ ‪¼·²¹‬؟» ‪³»¬»®َ®‬؟·¼ ‪‬؟ »‪¬ ·- ¬± «-‬؛؟¸‪¬¸» -‬‬ ‫‪َ‬؟»‪´´±©- §±« ¬± ³‬؟ ¸½·¸© ‪´»ô‬؟½‪»®²·»® -‬ھ ‪‬؟ ¸¬·© ‪°»-‬؟¬ ‪°®»½·-·±²‬‬ ‫‪³»¬»®- ¬± ©·¬¸·²ًًٍï‬؟·¼ ¬‪‬؛؟¸‪-«®» -‬‬ ‫‪ ·²ٍ‬‬ ‫»¸¬ ‪‬؛‪-«®»³»²¬ °±--·¾´» ±‬؟»‪µ·²¹ ¬¸» ³±-¬ °®»½·-» ³‬؟¬ ®»¬‪‬؛ك‬ ‫‪ °¸±¬±َ‬؟ »‪µ‬؟¬ ‪«´ ¬±‬؛»‪§ ¾» «-‬؟‪²§ô ·¬ ³‬؟ ‪‬؛· ‪»ô‬ھ»»´‪²¼ ¬¸» -‬؟ ¬‪‬؛؟¸‪-‬‬ ‫‪²‬؟½ ®»·´‪´ -«°°‬؟»‪±«® -‬ا ‪®»¬¬»¼ ±® ©±®²ٍ‬؛ ‪ ·¬ ·-‬؛· ¬‪‬؛؟¸‪ ¬¸» -‬؛‪°¸ ±‬؟®‪¹‬‬ ‫‪¬·±²ٍ‬؟‪±®³‬؛‪±® ³±®» ·²‬؛ »½®«‪ ®»-±‬؟ »¾‬ ‫»½‪²‬؟®‪‬؟»´½ ´‪‬؟·¼‪‬؟خ ‪¬»° îٍ‬ح‬

‫ّ‬ ‫÷ ‪ î‬م ‪‬ف قس‬ ‫ّ‬ ‫÷ ‪ Hُ î ‬م ‪‬فقس § ‪‬؛·® » ‪‬ت‬

‫‪î ُ î‬‬ ‫م فقس‬ ‫ى م ي م ‪ î‬م ‪ ّï‬؛· ‪‬وفقس §‪‬؛·®»‪‬ت‬ ‫÷ ‪ Hُ î ‬م ‪‬فقس ‪¬¸»²‬‬

‫فقس‬

‫»½‪‬؟؛®«‪±«²¬·²¹ -‬س ‪¬»° ëٍ‬ح‬

‫´‪‬؟»‪½» ±² ©¸·½¸ ¬¸» -‬؟؛®«‪¸»½µ ¬¸» -‬ف‬ ‫َ‪‬‬

‫ّ‬

‫‪÷ -·² êéٍë ¼»¹‬‬ ‫‪ë ¼»¹‬ىّ ‪-·²‬‬ ‫÷ ‪ Hُ î ‬م ‪‬فقس § ‪‬؛·® » ‪‬ت‬

‫‪ î‬م فقس‬

‫»‪·¹¸¬»¼¹‬؟®¬‪ -‬؟ »‪-‬ث ‪‬ل¬‪‬؛؟¸‪® ¬± ¬¸» -‬؟´«‬ ‫·¸¬ §‪‬؛·®»‪‬ھ ‪®» ¬±‬؟«¯‪- -‬ژ¬‪½¸·²·-‬؟‪ ³‬؟ ¼‪²‬؟‬‫¬‪²‬؟¬®‪¬·±²ô ©¸·½¸ ©·´´ ¾» ·³°±‬؟‪±®³‬؛‪·²‬‬ ‫‪ٍ´‬؟»‪ ²»© -‬؟ ‪©¸»² ³±«²¬·²¹‬‬ ‫؟ ‪-‬؟¸ ¬‪¬ -±³» »¯«·°³»²‬؟¸¬ »¬‪±‬ز‬ ‫¬‪‬؛؟¸‪®±«²¼ ¬¸» -‬؟ ‪½¸·²»¼ ®»½»--‬؟‪³‬‬ ‫· ´‪‬؟»‪ ¾±-- ±² ¬¸» -‬؟ »®»¸© ‪½«¬±«¬ô‬‬‫»®«‪-‬؟»‪ ®»½»--ô ³‬؟ ¸½«‪ ¬¸»®» ·- -‬؛×‬ ‫‪²¼ ¬¸» ¼»°¬¸ٍ‬؟ ®»¬»‪³‬؟·¼ »¸¬ ¸¬‪¾±‬‬ ‫حجزغسغظـغشةرزصفك‬

‫‪َ®‬؟»‪‬ق ¨»¼‪±±‬ة §¾ ¼»¼·‪‬ھ‪- °®±‬؟© ‪·²¹»®¬·°-‬ع ®«‪±‬ا ¬‪‬؟ ‪½¬-‬؟ع ‪-‬ژ¸¬‪±® ¬¸·- ³±²‬؛ ´‪‬؟·®»¬‪‬؟س‬ ‫ظ ‪±ٍّ‬ف ‪·²¹‬‬ ‫§»‪®µ‬؟¬‪‬ح ®»‪¹‬؟‪²‬؟‪´ ³‬؟®»‪ٍ´½±³÷ ¹»²‬؟»‪ ©©©ٍ³»½±-‬ه»‪·²‬؟س ‪ »±®¹»¬±©²ô‬‬ ‫‪·²¹ٍ‬ص ¬¬‪‬؟س ®»‪´ ¼»-·¹²‬؟»‪²¼ -‬؟ ‪¹´»ô‬؟‪¬»«»®²‬ح‬

‫¼‪®±«²‬؟ »½‪²‬؟®‪‬؟»´½ ´‪‬؟·¼‪‬؟® »¸¬ »®«‪-‬؟»‪-«®»ô ³‬؟»‪°» ³‬؟¬ ®‪ ®«´» ±‬؟ ‪-·²¹‬ث‬ ‫‪²‬؟¸¬ »®‪ ³±‬؛× ‪´ ±¾-¬®«½¬·±²ٍ‬؟·¼‪‬؟® ¬‪®»-‬؟»‪§ ¬¸» ²‬؛·¬‪²¼ ·¼»²‬؟ ¬‪‬؛؟¸‪¬¸» -‬‬ ‫‪ -µ»¬½¸ٍ‬؟ ‪´´ ±²‬؟ ‪®µ‬؟‪²¼ ³‬؟ »®«‪-‬؟»‪±²» ±¾-¬®«½¬·±² »¨·-¬-ô ³‬‬ ‫»½‪²‬؟®‪‬؟»´½ ´‪‬؟·¨‪‬ك ‪ٍ‬ي ‪¬»°‬ح‬

‫®‪®·²¹ ±‬؟»¾ »¸¬ ‪´ ³±«²¬·²¹ °±·²¬ ¬±‬؟»‪®±³ ¬¸» -‬؛ »½‪²‬؟¬‪´´§ ¬¸» ¼·-‬؟‬ ‫‪»®ô ¾» ±¬¸»® ½´±-»® ±¾-¬®«½َ‬ھ»©‪§ô ¸±‬؟‪¸»®» ³‬ج ‪--»³¾´§ٍ‬؟ »‪‬ھ·®¼‬ ‫‪«´ ·² ½±³َ‬؛‪² ¾» ¸»´°‬؟½ ¸½¬»‪ -µ‬ك ‪¬ ½±«°´·²¹-ٍ‬؛؟¸‪- -‬؟ ¸½«‪¬·±²-ô -‬‬ ‫‪¬·²¹ §±«® ²»»¼-ٍ‬؟½·‪³«²‬‬ ‫‪´ ³±«²¬·²¹‬؟»‪‬ح ‪ٍ‬ى ‪¬»°‬ح‬

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Pump Selection and Specifcation Department Editor: Kate Torzewski 

PUMP SELECTION n choosing a pump, it is important to match a pump’s capabilities with system requirements and the characteristics o the liquid being processed. These actors include the inlet conditions, required owrate, dierential pressure and liquid characteristics. Generally, the quality o the liquid should remain unchanged ater passage through a pump. Thereore, material compatibility, viscosity, shear sensitivity and the presence o particulate matter in a liquid are important considerations in pump selection. Most engineering applications employ either centriugal or positive displacement (PD) pumps or uid handling. These pumps unction in very dierent ways, so pump selection should be based on the unique conditions o a process.

I

Centrifugal pumps The most widely used pump in the chemical process industries or liquid transer is the centriugal pump. Available in a wide range o sizes and capacities, these pumps are suitable or a wide range o applications. Advantages o the centriugal style include: simplicity, low initial cost, uniorm ow, small ootprint, low maintenance expense and quiet operation. Positive displacement pumps Though engineers may be frst inclined to install centriugal pumps, many applications dictate the need or PD pumps. Because o their mechanical design and ability to create ow rom a pressure input, PD pumps provide a high efciency under most conditions, thus reducing energy use and operation costs. Choosing centrifugal versus positive displacement These two main pump styles respond very dierently to various operating conditions, so it is essential to evaluate the requirements o a process prior to choosing an appropriate pump. Table 1 illustrates the mechanical dierences between these pumps, as well as the eects o pressure, viscosity and inlet conditions on owrate and pump efciency. Range of operation Pump styles range ar beyond simply PD and centriugal pumps. PD pumps encompass many specifc styles, including a variety o reciprocating, rotary and blow-cover pumps. Likewise, centriugal pumps encompass radial, mixed, and axial ow styles, which all PumP ComParison Chart

Mechanics

Cefgl Pp

Pve dplcee pp

The pump imparts a velocity to the liquid, resulting in a pressure at the outlet.

The pump captures confined amounts of liquid and transfers them from the suction to discharge port.

Pressure is created  and flow results 

Performance Viscosity

Efficiency

Inlet conditions

Flow varies with changing pressure Efficiency decreases with increasing viscosity Efficiency peaks at the best-of-efficiency point. At higher or lower pressures, efficiency decreases Liquid must be in the pump to create a pressure differential. A dry pump will not prime on its own

Flow is created and  pressure results 

Flow is constant with changing pressure Efficiency increases with increasing viscosity Efficiency increases with increasing pressure

Negative pressure is created at the inlet port. A dry pump will prime on its own

belong to a greater category o kinetic pumps. A simple way to narrow down pump styles is to determine the required capacity that your pump must handle. Based upon a required capacity in gal/min. and a pressure in lb /in.2, the pump coverage chart below can help engineers ocus their selection to a just a ew pump styles. 



  



                                                                                                

      

                                                                                            

  









      

  

            Adapted from Perry’s Chemical Engineers’ Handbook

PUMP SPECIfICaTIONS Based on the application in which a pump will be used, the pump type, and service and operating conditions, the specifcations o a pump can be determined. • Casting connection: Volute casing efciently converts velocity energy impacted to the liquid rom the impeller into pressure energy. A casing with guide vanes reduce loses and improve efciency over a wide range o capacities, and are best or multistage highhead pumps • Impeller details: Closed-type impellers are most efcient. Opentype impellers are best or viscous liquids, liquids containing solid matter, and general purposes • Sealings: Rotating shats must have proper sealing methods to prevent leakage without aecting process efciency negatively. Seals can be grouped into the categories o noncontacting seals and mechanical ace seals. Noncontacting seals are oten used or gas service in high-speed rotating equipment. Mechanical ace seals provide excellent sealing or high leakage protection • Bearings: Factors to take into consideration while choosing a bearing type include shat-speed range, maximum tolerable shat misalignment, critical-speed analysis, loading o compressor impellers, and more. Bearing styles include: cylindrical bore; cylindrical bore with dammed groove; lemon bore; three lobe; oset halves; tilting pad; plain washer; and taper land • Materials: Pump material is oten stainless steel. Material should be chosen to reduce costs and maintain personnel saety while avoiding materials that will react with the process liquid to create corrosion, erosion or liquid contamination References 1. “Perry’s Chemical Engineers’ Handbook,” 7th ed. New York: McGraw Hill, 1997. 2. Petersen, J. and Jacoby, Rodger. Selecting a Positive Displacement Pump, Chem. Eng. August 2007, pp. 42–46.

Vacuum Pumps Department Editor: Kate Torzewski  acuum is any system o reduced pressure, relative to local (typically atmospheric) pressure. Achieved with a pump, vacuum systems are commonly used to:

V

Figure 1.

Liquid-Ring Pump

Figure 2.

Rotary-Claw Pump

Figure 3.

RotaryLobe Pump Source: Kurt J. Lesker Co.

•Removeexcessairand its constituents •Removeexcessreactantsor unwanted byproducts •Reducetheboilingpoint •Drysolutematerial •Createapressuredifferentialfor initiatingtransportofmaterial Liquid-ringanddrypumpsofferthe mostadvantagesforthechemical processindustries(CPI).Bothof thesepumptypeshavebearings sealedofffromthepumpingchamber and do not require a ny internal lubrication because the rotors do notcontactthehousing.Both,when employingacoolantsystem,prevent thecoolantfromcontactingtheprocessuidandcausingcontamination, and both use mechanical shat seals or containment.

liquid-ring pumps Inthecylindricalbodyofthepump, asealantuidundercentrifugal forceformsaringagainsttheinside ofthecasing(Figure1). The source o that orce is a multi-bladed impeller whose shat is mounted so as to be eccentric to theringofliquid.Becauseofthis eccentricity, the pockets bounded by adjacent impeller blades (also calledbuckets)andtheringincrease in size on the inlet side o the pump, andtheresultingsuctioncontinually drawsgasoutofthevesselbeing evacuated. As the blades rotate

Figure 4.

Source: Medical Gas Info

Source: Gardner Denver Hanover, Inc.

towardthedischargesideofthe pump, the pockets decrease in size,andtheevacuatedgasiscompressed,enablingitsdischarge. Theringofliquidnotonlyactsas a seal; it also absorbs the heat o compression, riction and condensation.Popularliquidchoicesinclude water,ethyleneglycol,mineraloil andorganicsolvents.

dry pumps

void space between the rotors and pumphousing.Onthenextrotation, thatsametrappedsampleofgasis compressedanddischargedasthe dischargeportopens. Aminimumofthreestagesin series is required to achieve pressures comparable to those o an oil-sealed mechanical pump. Some drydesignsusetwotechnologiesin combination;forexample,arotarly lobe as a booster or a claw pump.

Rotary-claw,rotary-lobeand rotary-screw pumps dominate as drypumpsintheCPI,particularlyin larger-sizepumpapplications. Rotary Claw. Thegeometricshape ofthispumpallowsforagreater compression ratio to be taken acrosstherotorsathigherpressures (Figure2).Twoclawrotorsrotate in opposite directions o rotation withouttouching,usingtiminggears tosynchronizetherotation.Thegas entersthroughaninletportafterit has been uncovered and flls the

Rotary Lobe. The rotary-lobe pump (Figure3)istypicallyusedasamechanicalboosteroperatinginseries with an oil-sealed piston or vane pumptoboostpumpingcapacityat low pressures. This pump consists o two symmetrical two-lobe rotors mounted on separate shats in parallel, which rotate in opposite directions to each otherathighspeeds.Timinggears are used to synchronize the rotation o the lobes to provide constant clearance between the two.

Advantages

Rotary-Screw Pump

Source: Kurt J. Lesker Co.

Rotary Screw. Twolonghelical rotors in parallel rotate in opposite directionswithouttouching,synchronizedbyhelicaltiminggears (Figure4).Gasowmovesaxially alongthescrewwithoutanyinternal compression rom suction to discharge.Pocketsofgasaretrapped within the convolutions o the rotors andthecasing,andtransportedto thedischarge.Compressionoccurs atthedischargeport,wherethe trappedgasmustbedischarged againstatmosphericpressure.Each convolution o the rotor acts similarly toastageinserieswiththeonebehindit;atleastthreeconvolutedgas pockets in the rotor are required to achieve acceptable vacuum levels.

References 1. Vilbert,P.,MechanicalPumpsfor VacuumProcessing,Chem. Eng. October2004,pp.44–51. 2. Aliasso,J.,ChoosetheRight VacuumPump, Chem. Eng. March 1999,pp.96–100.

Disadvantages

  s • Simpler design; employs only one rotating assembly   p   m • Can be fabricated from any castable metal   u    P • Minimal noise and vibration   m • Little increase in the temperature of the discharged gas   u • No damage from liquid or small particulates in the process uid   u   c • Maintenance and rebuilding are simple   a    V • Slow rotational speed (1,800 rpm or less), maximizing operating life   g • Can use any type of liquid for the sealant uid in situations where min   n    i gling with the process vapor is permissible    R      d • No lubricating liquid in the vacuum chamber to be contaminated    i   u • Accommodation of both condensable vapors and noncondensables,   q    i    L while operating as both a vacuum pump and condenser

• Mixing of the evacuated gas with the sealing uid • Risk of cavitation requires a portion of process load to be noncondens able under operating conditions • High power requirement to form and maintain the liquid ring, resulting in large motors • Achievable vacuum is limited by the vapor pressure of sealant uid at the operating temperature • Power consumption

• Rugged rotor design, constructed of sturdy cast or ductile iron without

• Cannot handle particulate matter, nor large slugs of liquid • May require a silencer • May discharge gases at high temperatures • Most difcult to repair or rebuild • May require a gas purge for cooling, or to protect the bearings and seals from the process gas • Due to high operating temperatures, some process gases may polymerize

-

any imsy rotating components   s   p • Noncontact design facilitated by timing gears   m   u • High rotational speed reduces the ratio of gas slip to displacement,    P increases net pumping capacity and reduces ultimate pressure   m   u • Multiple staging provides inlet pressures below 1-mm Hg absolute while   u discharging to atmosphere   c   a • No contamination of evacuated gas    V   y   r • Due to lack of condensation, pump can be fabricated of standard,    D inexpensive cast iron

-

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‫كذ غ‬

‫ج ح ث ¼‪²‬؟ ‪‬ج ح ك‬

‫ف »´¬·¬¾«‪‬ح ¬½‪‬ك §®»‪‬ھ‪»½±‬خ ¼‪²‬؟ ‪¬·±²‬؟ھ®»‪±²-‬ف »½®«‪»-±‬خ‬ ‫‪¬·±²-‬؟´«‪-¬» ®»¹‬؟© ‪®¼±«-‬؟¦‪‬؟¸ ‪‰‬‬

‫كذ غ‬

‫ج ح ث ¼‪²‬؟ ‪‬ج ح ك‬

‫‪‬؟»‪±«²¬»®³‬ف ¼‪²‬؟ ´‪±²¬®±‬ف ‪»²¬·±²ô‬ھ»®‪‬ذ ´´·‪°‬ح‬ ‫‪-«®»- ©·¬¸·²‬‬ ‫ً‬ ‫‪ ïçç‬؛‪³»²¼³»²¬- ±‬؟ ¬½‪‬ك ®·‪‬ك ®‪‬؟»´‪‬ف‬

‫كطحر‬

‫ج ح ث ¼‪²‬؟ ‪‬ج ح ك‬

‫ً¬‪‬؟®»‪°‬ر »¬‪-‬؟ة ‪®¼±«-‬؟¦‪‬؟ط‬ ‫‪·±²‬‬‫»‪»-°±² -‬خ §½‪³»®¹»²‬غ ¼‪²‬؟ ً‬ ‫‪ ïçïٍïî‬خعف ‪¬·±²- ‰ îç‬؟´«‪»¹‬خ‬

‫‪ً³‬؟´‪‬ع‬ ‫ج ح ث ¼‪²‬؟ ‪‬ج ح ك‬ ‫كطحر‬ ‫خعف ‪®¼ ‰ îç‬؟¼‪²‬؟¬‪‬ح ‪·¯«·¼-‬ش »´¾·¬‪±³¾«-‬ف ¼‪²‬؟ »´¾‪‬؟‪ً ³‬‬ ‫‪ïçïٍïê‬‬ ‫ً ً‬ ‫‪ ïçïٍ ï îً‬خعف ‪®¼ ‰ îç‬؟¼‪²‬؟ ¬ ‪‬ح ‪ ¬·±²‬؟½·‪ ± ³³«²‬ف ¼ ® ‪‬؟¦ ‪‬؟ط‬ ‫جحث ¼ ‪ ²‬؟ ‪‬جحك‬ ‫كطحر‬ ‫كطحر‬ ‫كذ غ‬ ‫كذ غ‬ ‫كذ غ‬ ‫كطحر‬

‫ج ح ث ¼‪²‬؟ ‪‬ج ح ك‬ ‫§´ ‪ ±²‬ج ح ك‬ ‫§´‪ ±²‬جح ك ح‬ ‫§´‪ ±²‬ج ح ك‬ ‫§´‪ ±²‬ج ح ك‬

‫ك‬

‫ً‬ ‫‪ ï çç‬؛‪ ±‬؛ ‪½¬ ±‬‬

‫‪ §-¬»³‬غ ‪ ¬·±²‬؟‪®¹»  ´·³·²‬؟ـ¸½‪ ·-‬‬

‫ذ ‪·´ ±´´«¬·±‬ر‬ ‫‪²‬‬

‫‪‬؟¬«´´‪ ´  ±‬؟‪¬·±²‬؟ ز‬ ‫¬‪ ²‬ذ‬

‫‪¬·±²‬؟‪±³°»²-‬ف ‪»-°±²-»ô‬خ ´‪‬؟¬‪·®±²³»²‬ھ‪²‬غ »‪‬ھ·‪±³°®»¸»²-‬ف‬ ‫ف ّ¬½‪‬ك §¬·´·¾‪‬؟·‪‬ش ¼‪²‬؟‬ ‫÷‪‬كشفخغ ‪‬‬ ‫ًً‬ ‫‪ ïçïٍïîè‬خ ع ‪»²¦»²» ‰ îç‬ق‬ ‫ف‬ ‫َ‬ ‫‪¸»³·‬فً‪®¼±«-‬؟¦‪‬؟ط §´¸‪·¹‬ط ؛‪¹»³»²¬ ±‬؟‪²‬؟س §¬»‪‬؛؟ح ‪®±½»--‬ذ‬ ‫‪ ïçïٍïïç‬خعف ‪¹»²¬- ‰ îç‬ك ‪-¬·²¹‬؟´‪‬ق ¼‪²‬؟ ‪»-‬ھ·‪¨°´±-‬غ ‪´-ô‬؟½‬

‫كط حر‬

‫§´‪ ±²‬ج ح ك‬

‫كذ غ‬

‫§´‪ ±²‬ج ح ث‬

‫‪«¾¬·¬´» × ‰‬ح ¬½‪‬ك §®»‪‬ھ‪»½±‬خ ¼‪²‬؟ ‪¬·±²‬؟ھ®»‪±²-‬ف »½®«‪»-±‬خ‬ ‫‪- -¬±®·²¹‬جحث ‪¼¼®»--·²¹‬؟ ‪¬·±²-‬؟´«‪®»¹‬‬ ‫‪¦َ‬؟¸ ¼‪²‬؟ ‪°»¬®±´»«³‬‬ ‫ً‬ ‫ً‬ ‫‪ îè‬خعف ‪‬ى ‪²½»- ‰‬؟¬‪®¼±«- -«¾-‬؟‬

‫كذ غ‬

‫§´‪ ±²‬ج ح ث‬

‫‪¬·±²‬؟´«‪»¹‬خ §®»‪‬ھ‪»½±‬خ ®‪°±‬؟ت ×× »‪¹‬؟¬‪‬ح‬‫‪¾´·-¸»¼ ·²‬؟¬‪ً ‰ »-‬‬ ‫‪ ïçç‬؛‪³»²¼³»²¬- ±‬ك ¬½‪‬ك ®·‪‬ك ‪²‬؟»´‪‬ف‬

‫§´‪ ±²‬ج حث‬

‫ك ‪²‬؟ »´ ‪‬ف‬ ‫®· ‪‬‬

‫كذ غ‬ ‫كذ غ‬ ‫»¼‪ ½±‬كذ ع ز‬ ‫»¼‪ ½±‬ف ف ×‬

‫§´‪ ±²‬ج ح ث‬

‫ك½ ‪‬‬ ‫‪‬؟®»‪ ±°‬ج‪¬  ·¬´»  ‰‬‬ ‫‪ ¬·²¹ °»®³·¬-‬ت‬

‫َ‬ ‫‪½¬ ‰‬ك ©‪·¹¸¬¬±َµ²±‬خ §¬·‪ ±³³«²‬ف ¼‪²‬؟ ‪²²·²¹‬؟´‪³»®¹»²½§ °‬غ‬ ‫‪½¬ٍ‬ك ‪¬·±²‬؟¦·®‪«¬¸±‬؟»‪‬خ ¼‪²‬؟ ‪³»²¼³»²¬-‬ك ¼‪«²‬؛®»‪«°‬ح ؛‪·¬´» ××× ±‬ج‬

‫‪ -¬±®·²¹ ´·¯«·¼ô‬؛‪» ±‬ھ·¬½»¶¾‪¬» ±‬؟‪¸» «´¬·³‬‬

‫ج‬

‫»‪‬؛؟‪´´§ -‬؟¬‪·®±²³»²‬ھ‪² »²‬؟ ‪´ ·²‬؟·®»¬‪‬؟‪¬± -¬±®» ³‬‬ ‫»‪¹‬؟®‪¬±‬ح ‪²²»®ٍ‬؟‪¾´» ³‬؟·‪‬ھ §´´‪‬؟½·‪²¼ »½±²±³‬؟‬ ‫»·®¬‪´ °®±½»-- ·²¼«-‬؟½·‪²µ- ·² ¬¸» ½¸»³‬؟¬‬‫ف‪‬‬ ‫‪·¼»¼ ·²¬±‬ھ·¼ §´¼‪‬؟‪² ¾» ³±-¬ ¾®±‬؟½ ÷×‪‬ذ ّ‬ ‫»‪²¼ ¬¸±-‬؟ ‪¬¸±-» ¾«®·»¼ «²¼»®¹®±«²¼ô‬‬ ‫· ‪±´´±©·²¹‬؛ »¸‪‬ج ‪»¹®±«²¼ٍ‬ھ‪¾±‬؟ ¼»¬½«®¬‪½±²-‬‬‫¸¬·© ¼»¬‪‬؟·½‪--±‬؟ ‪¬·±²-‬؟®»¼·‪ ½±²-‬؛‪² ±«¬´·²» ±‬؟‬ ‫»‪‬ھ·¬‪‬؟‪²¼ ²»¹‬؟ »‪‬ھ·¬·‪²¼ °±-‬؟ §®‪¬»¹±‬؟½ ¸½‪‬؟»‬ ‫‪®» °±¬»²َ‬؟ ¼»¼«´½‪´-± ·²‬ك ‪½¸ٍ‬؟» ‪‬؛‪-°»½¬- ±‬؟‬ ‫‪®±³‬؛ ‪²¼ ½±¼»-‬؟ ‪¬·±²-‬؟´«‪¾´» ®»¹‬؟½·´‪°°‬؟ §´´‪‬؟·¬‬ ‫حٍ‪‬ث »¸¬‬ ‫§½‪¹»²‬ك ‪®±¬»½¬·±²‬ذ ´‪‬؟¬‪·®±²³»²‬ھ‪²‬غ ‪ ٍ‬‬ ‫§¬»‪‬؛؟ح ¼‪²‬؟ ¸¬´‪‬؟»‪‬ط ´‪‬؟‪¬·±²‬؟‪½½«°‬ر »¸¬ ‪÷ô‬كذ ّغ‪‬‬ ‫ر ّ‪¬·±²‬؟®¬‪¼³·²·-‬ك‬ ‫‪²¼ ±¬¸»®-ٍ‬؟ ÷‪‬كطح ‪‬‬

‫صزكج غظكخرجح ـزثرخظخغـزث‬ ‫ث‪‬‬ ‫حغظكجزكتـك ÷‪‬جح ّ‬ ‫§‪‬؟© »¸¬ ‪‬؛‪®» ±«¬ ±‬؟ ‪-‬جحث ‪»¬§ ‰‬؛؟‪´ -‬؟½·‪¸§-‬ذ‬ ‫‪²¬ «²َ‬؟¬‪»´§ ½±²-‬ھ·¬‪‬؟´»® ‪‬؟ ¸¬·‪‬ة ‪»¬§ ‰‬؛؟‪·®» -‬ع‬ ‫®‪» -«°»®·±‬ھ؟¸ ‪-‬جحث ‪¬«®»ô‬؟®»‪¼»®¹®±«²¼ ¬»³°‬‬

‫»¬«¬·¬‪»¬®±´»«³ ײ-‬ذ ‪²‬؟½·®»‪³‬ك »¸‪‬ج ‪»½«®·¬§ ‰‬ح‬ ‫‪¼·²¹‬؟»´ ‪‬؟ ‪´·-³ ·-‬؟¼‪²‬؟ھ ¬‪‬؟¸¬ ‪- -¸±©²‬؟¸‬ ‫¬½»¶¾«‪®» ´»-- -‬؟ ‪-‬جحث ‪·´«®»-ٍ‬؟؛ ‪²µ‬؟¬ ‪‬؛‪«-» ±‬؟½‬ ‫¬½»¬‪-·»® ¬± °®±‬؟» ¼‪²‬؟ ‪´·-³‬؟¼‪²‬؟ھ ‪¬±‬‬ ‫¸½·¸© ‪ -·¹¸¬ô‬؛‪®» ±«¬ ±‬؟ ‪-‬جحث ‪»-¬¸»¬·½- ‰‬ك‬ ‫‪ °±--·¾´» °«¾´·½ ±¾¶»½¬·±²‬؟ ‪¬»-‬؟‪»´·³·²‬‬ ‫‪²¼ «-» ‬؟ش‬

‫ج ح ث ¼‪²‬؟ ‪‬ج ح ك‬

‫‪ً‬ي كذعز ‪¬·±²‬؟·½‪--±‬ك ‪®±¬»½¬·±²‬ذ »®·‪‬ع ´‪‬؟‪¬·±²‬؟ز‬ ‫÷»¼‪±‬ف ‪·¯«·¼-‬ش »´¾·¬‪±³¾«-‬ف ¼‪²‬؟ »´¾‪‬؟‪³³‬؟´‪ّ ‬ع‪‬‬

‫‪¬·±²‬؟½‪½»³»²¬ ´±‬؟´‪°‬‬

‫ج ح ث ¼‪²‬؟ ‪‬جح ك‬

‫»¼‪ ±‬ف »®· ‪‬ع ´ ‪‬؟ ‪ ¬·±²‬؟‪÷ ײ¬»®²‬ف ف× ‪ ±«²½·´ّ ‬ف »¼‪±‬ف ´؟ ‪¬·±²‬؟ ‪ײ¬»®²‬‬

‫حغظكجزكتـكح ×‪‬ـ جحث‬

‫‪³»َ‬؟®‪‬؛ §®‪¬±‬؟´«‪»¹‬خ ‪´ ®»¯«·®»³»²¬-ٍ‬؟®»¼»‪‬؛ ¼‪²‬؟‬ ‫جحث ¼‪²‬؟ ‪‬جحك ®‪±‬؛ ¬‪»®»²‬؛؛·¼ »®‪‬؟ ‪©±®µ-‬‬ ‫‪²‬؟ »¬‪‬؟®‪‬؟‪½» ®»¯«·®»³»²¬- ¬± -»°‬؟‪°‬ح ‪½» ‰‬؟‪°‬ح‬ ‫»¼‪·®» ½±‬ع ‪-ٍ‬جحث ®‪±‬؛ »«‪² ·--‬؟ ¬‪®» ²±‬؟ §»¸¬‬‫»½‪²‬؟¬‪¬·±² ¼·-‬؟®‪‬؟‪¬» -»°‬؟¬½·¼ §´´‪‬؟®»‪¹»²‬‬‫‪´·-³ ·-‬؟¼‪²‬؟ھ ‪®±³‬؛ ‪®±¬»½¬·±²‬ذ ‪»½«®·¬§ ‰‬ح‬

‫»¾ ¼´«‪ ½±-¬ -¸±‬؛‪-°»½¬- ±‬؟ ´‪‬؟®»‪‬ھ»‪‬ح ‪±-¬ ‰‬ف‬ ‫‪²µ‬؟¬ ‪²µô‬؟¬ »‪¹‬؟®‪ -¬±‬و‪½±²-·¼»®»¼ô ·²½´«¼·²¹‬‬ ‫‪¬·±² ½±-¬ô‬؟´´‪‬؟¬‪´ ·²-‬؟·¬·‪-§-¬»³ »¯«·°³»²¬ô ·²‬‬ ‫‪²½» ½±-¬ô -»½«®·¬§ ½±-¬ô‬؟‪·²¬»²‬؟‪±²¹±·²¹ ³‬‬ ‫§®‪‬؟¼‪²¼ -»½±²‬؟ ¬‪¬±®§ ½±-‬؟´«‪²¼ ½±-¬ô ®»¹‬؟´‬ ‫¬‪·²³»²¬ ½±-‬؟¬‪½±²‬‬

‫حغظكجزكتـكح ×‪‬ـ جحك‬ ‫»´¾‪‬؟®»‪«´²‬ھ »®‪®» ³±‬؟ ‪-‬جحك ‪»¬§ ‰‬؛؟‪´ -‬؟½·‪¸§-‬ذ‬ ‫¼‪²‬؟ ‪«¬±³±¾·´»-‬؟ ¸¬·© ¬½‪‬؟¬‪´·-³ô ½±²‬؟¼‪²‬؟ھ ‪¬±‬‬ ‫»‪¹‬؟‪³‬؟¼ ´‪‬؟‪»¨¬»®²‬‬ ‫‪»¬§ ‬؛؟‪·®» -‬ع‬ ‫‪‬جحث ‪» ¬±‬ھ·¬‪‬؟´»® ‪®·-µ‬‬‫‪¾´» ·²‬؟‪§ ¾» ±¾¶»½¬·±²‬؟‪- ³‬جحك ‪»-¬¸»¬·½- ‰‬ك‬ ‫‪¬·±²‬؟½‪·² ´±‬؟¬®»½‬‫‪¬» ®»¯«·®»¼ ¬±‬؟¬‪´ »-‬؟»® »®‪½» «-» ‰ ³±‬؟‪°‬ح‬ ‫‪‬جحك »‪¸±«-‬‬‫»½‪- »¨°»®·»²‬جحك ‪¬·±²  ‰‬؟·®‪‬؟ھ »®«¬‪‬؟®»‪»³°‬ج‬ ‫َ‪‬‬ ‫‪-‬جحث ‪²‬؟¸¬ ‪¬·±²-‬‬

‫غف×‪‬رطف غطج ظز×‪‬صكس‬ ‫´»»¬‪‬ح و§®«‪‬ق ‪±¬ ¬±‬ز ®‪«®§ ±‬ق ‪±‬ج ‪ٍ‬ق‪ٍ‬ة ‪»§»®ô‬ظ ‪ïٍ‬‬ ‫‪²¼¾±±µ‬؟طچ و‪»½·-·±²-ٍ ײ‬ـ §‪»½¸²±´±¹‬ج ‪²µ‬؟ج‬ ‫‪‬؟ج »‪¹‬؟®‪¬±‬ح ؛‪±‬‬ ‫‪ »µµ»®ô‬ـ ´»½®‪‬؟س ‪§-¬»³-ôŒ‬ح ‪ً ²µ‬‬ ‫‪±®µô îًٍ‬ا ©»‪‬ز‬ ‫‪»¹َ‬ھ‪¾±‬ك ‪ٍ‬ر ‪®ô‬؟‪³‬؟´´‪‬؟ت ¼‪²‬؟ ‪ٍô‬ز‪ٍ‬ذ ‪ô‬؛؛‪¸»®»³·-·²±‬ف ‪îٍ‬‬ ‫‪±³َ‬ف ‪²µ‬؟ج »‪¹‬؟®‪¬±‬ح ¼‪²¼»®¹®±«²‬ث ¼‪²‬؟ ¼‪®±«²‬‬ ‫‪²½»- ·²‬؟ھ¼‪‬ك ‪²µ-ٍ‬؟ج »‪¹‬؟®‪¬±‬حچ و‪®·-±²ٍ ײ‬؟‪°‬‬ ‫‪»®·»-ôŒ‬ح §‪»½¸²±´±¹‬ج ´‪±²¬®±‬ف ´‪‬؟¬‪·®±²³»²‬ھ‪²‬غ‬ ‫‪±«-¬±²ô ïççêٍ‬ط ‪«¾´·-¸·²¹ô‬ذ ؛´«‪‬ظ‬

‫‪¾±«¬ ©¸»¬¸»® ¬±‬؟ ‪µ·²¹ ¼»½·-·±²-‬؟‪¸»² ³‬ة‬ ‫و‪±´´±©·²¹‬؛ »¸¬ ®»¼·‪ô ½±²-‬جحث ‪‬؟ ®‪ ±‬جحك ‪²‬؟ »‪«-‬‬

‫»¸¬ ‪«¬¸±®·¬§ ·²‬؟ ´‪‬؟½‪¸» ´±‬ج ‪¬±®§ ‰‬؟´«‪»¹‬خ‬ ‫‪-َ‬؟ »¾ ´´·© ‪²µ -§-¬»³‬؟¬ »¸¬ »®»¸© ‪‬؟»®‪‬؟‬ ‫‪²µ‬؟¬ ‪‬؟ ®»¸¬»¸© ‪‬؛‪- ½±²¬®±´ ±‬؟¸ ¼»´¾‪-»³‬‬ ‫®»¸¬»¸© ‪-»¼ ±²‬؟¾ ‪°»®³·¬ ©·´´ ¾» ·--«»¼ô‬‬ ‫»¬‪‬؟¬‪´ô -‬؟½‪²µ ½±³°´§ ©·¬¸ ´±‬؟¬ »¸¬ ®‪±‬؛ ‪²-‬؟´‪°‬‬

‫‪µ ³±²·َ‬؟»‪‬ش ‪·²³»²¬  ‰‬؟¬‪²¼ ½±²‬؟ ‪µ ¼»¬»½¬·±²‬؟»‪‬ش‬ ‫»®‪·²³»²¬ ·- ³±‬؟¬‪²¼ ½±²‬؟ ‪¬±®·²¹ô ¼»¬»½¬·±²‬‬ ‫‪َ‬؟ھ؟½¨‪‬غ ‪²¼ ½±-¬  ‰‬؟ §¬·¨»´‪¬·±² ½±³°‬؟´´‪‬؟¬‪ײ-‬‬ ‫· ‪¬·±²‬؟·½»®‪¬» ¼»°‬؟¬‪´ »-‬؟»‪‬خ ‪¬·±² ‰‬؟·½»®‪»°‬ـ‬‫‪¬·±²‬؟‪³·²‬؟¬‪ ½±²‬؛‪¬ ±‬؟»®¸¬ ‪°±--·¾´» ¼«» ¬±‬‬

‫صزكج غظكخرجح ـزثرخظغترقك‬ ‫ك‪‬‬ ‫حغظكجزكتـك ÷‪‬جح ّ‬ ‫‪²¼ ´»-‬؟ ®»´‪®» -·³°‬؟ ‪-‬جحك ‪±²-¬®«½¬·±² ‰‬ف‬‫»‪²‬ر ‪´ٍ´‬؟¬‪²¼ ·²-‬؟ ¬½«®¬‪» ¬± ½±²-‬ھ·‪»¨°»²-‬‬ ‫‪´ ·²-°»½¬·±² ·- °±-َ‬؟«‪·-‬ت ‪²½» ‰‬؟‪·²¬»²‬؟س‬ ‫‪µ‬؟»´ »´¾‪‬؟·´»® »®‪¼- ¬± ³±‬؟»´ ¸½·¸© ‪-·¾´»ô‬‬ ‫®·‪‬؟‪-·»® ®»°‬؟» ¼‪²‬؟ ‪¼»¬»½¬·±²‬‬ ‫َ‬ ‫»®‪²½» °‬؟®«‪±©»® °±´´«¬·±² ·²-‬ش ‪²½» ‰‬؟®«‪ײ-‬‬ ‫‪َ ‬‬ ‫‪ ¹®±«²¼‬؛‪ ®»¼«½»¼ ®·-µ ±‬؛‪«-» ±‬؟½»¾ ‪³·«³-‬‬ ‫‪¬·±²‬؟‪³·²‬؟¬‪¬»® ½±²‬؟©‬ ‫‪®» -«¾¶»½¬ ¬±‬؟ ‪-‬جحك ‪¬±®§ ¾«®¼»²  ‰‬؟´«‪»¹‬خ‬ ‫‪‬جحث ‪²‬؟¸¬ ‪¬±®§ ®»¯«·®»³»²¬-‬؟´«‪´»-- ®»¹‬‬‫¼‪²‬؟ ‪» ´±©»® ³±²·¬±®·²¹‬ھ؟¸ ‪-‬جحك ‪±-¬- َ‰‬ف‬ ‫‪®»½±®¼µ »»°·²¹ ½±-¬-‬‬

Acid Storage Department Editor: Kate Torzewski  ost common acids can be stored in horizontal or vertical ASME-type tanks, as shown in the gures to the right, or vertical API-type tanks. Horizontal, carbon-steel ASME-type tanks o 10,000— 40,000 gal capacity should have a plate thickness o 3/8 in. with dished heads o the same thickness. The thickness includes a corrosion allowance o 1/4 in., which provides a tank lie o 15—20 years.

M

Standard flanged and dished heads

DImension all nozzle locations from this line

Manhole

Vent

2A*

2A*

Knuckle radius line Weld lines

Sulfuric acid (H2SO4)

Storage. This acid is prone to enter into

reactions that generate hydrogen, so in addition to keeping the vessel vented adequately, exclude potential sources o ignition rom the vicinity.

Top plate of saddle welded to shell

A

Materials of construction. Carbon

steel is satisactory or concentrated technical grades o suluric acid at normal atmospheric temperature. H2SO4 solutions that are more dilute corrode carbon steel severely. To avoid inadvertent dilution o concentrated acid, keep acid away rom contact with moist air. Polyvinyl chloride pipe is recommended or ordinary suluric acid, but or oleum, Type 316 stainless steel or carbon-steel lined with a fuorocarbon is best.

Phosphoric acid (H3PO4)

Storage. The tank bottom should be rolled

to a height o 3 in. (upward). This allows welds to reely expand or contract. Corner welds should be avoided, as undue stresses can occur and aggravate corrosion [ 2]. Depending on the acid grade, the reezing point varies and may necessitate heating to avoid reeze-up in storage. In any case, to avoid corrosion, high-pressure steam should not be used; steam coils located several inches below the bottom o the tank are recommended. The space below the tank bottom should be enclosed to permit heating o the air to 50˚C, and the tank walls should be insulated. Materials of construction. Tanks can

be abricated o Type 316 extra-low-carbon stainless steel, rubber--lined carbon steel or berglass-reinorced plastic. Carbon steel should not be used, as it will corrode.

Hydrochloric acid (HCl) Storage. HCl o all strengths should be

stored in tanks similar to those mentioned above. Containment areas should be provided around tanks, and storage acilities should include a pressure- and vacuum-relie service, primary and redundant level indicators, a high-level alarm, an overfow line, an emergency block valve at the tank outlet nozzle and a vent-ume scrubber.

A

6 in. typical

* Minimum width of the shell course at each saddle. Width may be increased if more economical for shell course layout.

Materials of construction. These

storage tanks should be abricated o rubber-lined carbon steel, glass-lined carbon steel or ber-reinorced polymer (FRP). Sot natural-rubber compounds are used as liners or concentrated acid storage tanks at temperatures up to 60˚C with a minimum lining thickness o 3/16 in. Semi-hard rubber is used or lining equipment and piping or acid up to 70˚C with FRP tanks o vinyl-ester resin.

Nominal capacity level

Name plate

            )             x              ro       OD           p p                ( a             D            O    =            R  .    n     i

Submerged fill pipe Grounding clip

   5     1

Nitric acid (HNO3)

Storage. Storage tanks or HNO3 o less

than 95 wt.% concentration should be designed or at least a slight pressure and vacuum, permitting the venting o nitrogenoxide umes to collection and disposal equipment, such as a scrubber or a fare. When locating the tank vent and overfow pipe, consider that escaping vapors and liquid can corrode exterior welds as the acid is diluted with atmospheric moisture. Materials of construction. For concen-

trations up to 95 wt.% at ambient temperature, storage units should be abricated o Type 304L stainless steel. For concentrations o 95 wt.% and above, Type 3003 aluminum alloy should be used. Acid in the range o 52–55 wt.% should be stored in tanks o Type 347 stainless steel using No. 12 gage sheet. Above 90 wt.%, corrosion allowance in the tank-wall thickness may be necessary. Glass-lined carbon steel tanks are satisactory or all acid grades.

Bottom outlet baffle Channel legs

ASME code, or or lower pressure, as its vapor pressure is much lower than that o anhydrous HF. These tanks should be equipped with a relie device, and discharge piping should be routed to a scrubber. Aqueous HF tanks should have a vent, with the vent line also going to the scrubber. Materials of construction. Carbon-steel

Hydrouoric acid (HF)

storage tanks can be used or anhydrous HF at temperatures up to 66˚C and 70 wt.% HF. Acid o concentrations greater than 60 wt.% may be handled in steel up to a temperature o 38˚C. In steel tanks, hydrogen blistering may be caused by the accumulation o H2, so periodic tank inspections are required to evaluate blistering.

Storage. Because o anhydrous HF’s high

References

vapor pressure, tanks are designed or a minimum pressure o 60 psig and have X-rayed and stress-relieved welds. Tanks holding 70 wt.% HF are also designed per

1.Grossel, S., Sae Ecient Handling o Acids, Chem. Eng. December 1998, pp. 104–112. 2.Anon., Phosphoric Acid, Rhone-Poulene Basic Chemicals Co., Shelton, Conn. (1992).

Distillation Tray Design Department Editor: Scott Jenkins

n a distillation column tray, vapor passes upward through liquid that is flowing across a horizontal perforated plate. Vapor passing through the perforated plate forms a two-phase mixture with the liquid and enables mass transfer contacting. This mixture is typically quite turbulent. Tray design must allow the turbulent liquid to fall away from the rising vapor in the space above the tray, while also enabling the vapor bubbles to rise out of the falling liquid in the downcomer. The downcomer is usually a vertical plate that enables the already contacted froth to travel down to the next tray without remixing with the up-flowing vapor from the tray below.

I

Downcomers

area. In that case, the downcomer is sloped such that its bottom area is 60% of its top area. Active area

The active area of a distillation tower is where the vapor contacts the liquid to effect mass transfer. Above the active area, where the liquid falls away from the rising vapor, is the volume where the vapor can expand. Typically, the active area is calculated to be the tower crosssectional area minus the downcomer top and downcomer bottom area. The minimum active area (ft2) for normal valve trays can be determined from the following relationship, which is a modification of a commonly used correlation [1] taken at 82% of jet flood: Active area = V-Load / [TS 0.5 (0.0762 – 0.00092(V 2)) – 0.011W L]

Where, V-Load = CFS V  (V / (L – V ))0.5 TS = Tray spacing, in. V = Vapor density, lb/ft3 W L = Weir loading, gal/min per in. CFS V  = Vapor volumetric flow, ft3/s

Vapor flow

Liquid flow Side view of a simple tray arrangement

Generally, designing a column tray entails determining the minimum downcomer area that still allows vapor bubbles to rise through the liquid, selecting the number of downcomers, determining the active area, and checking the flow path length to see if a person can pass through a tray manway. These factors are the primary drivers for determining overall tower size. Downcomer area is determined by the maximum recommended downcomer velocity. Divide the volumetric flow of liquid by the downcomer velocity to obtain the downcomer top area. Typically a curve of maximum downcomer velocity versus the density difference between liquid and vapor is consulted during this process. Maximum downcomer velocity guideline 0.45

  s    /    f 0.4   r   t  ,   e  y    t 0.35   m    i   o  c   c   l   o   n  e 0.3   w  v   o    d  e  .   c0.25   x  n   a  a   r 0.2    t    M   n   e 0.15 10

20

30

40

A good place to start the iterative process is with a weir length 0.8 times the tower diameter. If the resulting weir loading is greater than 12 gal/min per in., then increase the number of tray passes to two. Recalculate the outlet weir length for each of the side downcomers of the column by using half the downcomer area. Check the weir loading again (for the tray with side downcomers). If the weir loading continues to exceed 12 gal/min per in., increase the number of tray passes to four. It is assumed that the two-pass tray with side downcomers has the shortest weir length. The simplest approach to designing 4-pass trays is to assume equal bubbling area and make the side downcomers onequarter of the total downcomer area, and make the center (and off-center) downcom-

50

Delta-density (L–V), lb/ft3

A downcomer is generally straight unless its area exceeds 8% of the tower

60

The required active area is dependant on the vapor density and weir loading. Note that the weir loading need not be known at this point. Assume a weir loading value of 5 gal/min per in. intially. Typical tray spacings are 24 in.

 AD

BW  Z  D

Tray g eometry parameters

Tower area and diameter

Based on the above areas, the overall tower area and diameter can be determined by the following: AT  = ADtop + ADbottom + AA D = 2(AT / π)0.5

Where, AT  = Tower area, ft 2 ADtop = Downcomer area at top, ft 2 ADbottom = Downcomer area at bottom, ft 2 AA = Active Area, ft2 D = Tower inner dia., ft Number of downcomers

Once the tower diameter is determined, then the number of downcomers can be chosen. As a starting point, an initial design should use a single downcomer. The resulting weir length is calculated from a standard chord-length calculation, which is iterative for a given downcomer area. BW  = {[(πD 2/360) cos –1(2Z /D )] – 2AD }/Z 

Where, Z = [(D 2/4) – BW2]0.5 BW  = Weir length of one downcomer, ft

ers one-half of the total downcomer area. Maintaining the resulting downcomer widths at 6 in. or more will allow a person to reach into the downcomer for installation. In addition, make sure the resulting tray-flow path-length is 16 in. or greater to enable a person to physically pass through the trays. These minimum size criteria may increase the column diameter to above the previously calculated value. Other considerations

Other criteria that need to be considered are; downcomer backup, spray fluidization, and entrainment. In addition, minimum load conditions need to be determined. The criteria for determining the low-end vapor and liquid range are weeping, tray stability and dry-tray pressure drop. Reference 1. Glitsch Inc. “Ballast Tray Design Manual; Bulletin No. 4900.” 3rd Ed. Glitsch Inc., Dallas, Tex.,1974. Note : Material for the June “Facts at Your Fingertips” was supplied by Dan Summers, tray technology manager, Sulzer Chemtech USA Inc.

Random Tower Packing Department Editor: Kate Torzewski  Packed columns [1 ]

random packings. When packed together, they prevent signifcant portions o wetting liquid rom being blocked o, thus avoiding pools o liquid, trapped gas and violent directional changes o gas. They oer higher capacity, higher efciency and lower pressure drop than Berl Saddles.

A packed column is a vertical, cylindrical pressure vessel containing one or more sections o a packing material over whose surace the liquid ows downward by gravity, as a flm or as droplets, between packing elements. Vapor ows upward through the wetted packing, contacting the liquid and acilitating absorption o the vapor into the liquid. Packings are oered in either random or structured designs. Here, we will ocus on random packings, which are separate pieces o packing that have a uniorm geometric shape. Instead o being arranged in a structured way, they are dumped or randomly packed into the column shell.

The Intalox Saddle was urther improved into the Super Intalox Tower Packing, which has scalloped edges and holes in the material. This allows urther liquid drainage, the elimination o stagnant pockets, and more open area or vapor rise, thus providing higher capacity and eiciency.

Pall rings are modifed Raschig Rings that have windows cut and bent inward. This lowers riction while improving packing area design Source: “Separation Process Principles,” 2nd ed., John Wiley and Sons, Inc. distribution, wetting and liquid considerations [2 ] distribution. This design allows ring and a Pall ring o identical size have higher capacity and efciency than all Size . Random packings are typically availidentical surace areas per unit volume, previously developed packings. able in diameters o 1–3.5 in. Generally, but the Pall ring has a superior spread o as packing size increases, mass-transer The next generation o packings eatures surace area and thereore gives much efciency and pressure drop decrease. through-ow structures o a lattice-work better efciency. By this correlation, or a given column design. The Metal Intalox IMTP oers the 3. Maximize the void space per unit  diameter, an optimal packing size can be best eatures o packings that preceded it, column volume . This minimizes resistance determined that represents a compromise combines the high void raction and the to gas upow, thereby enhancing packing between achieving low pressure drop and well-distributed surace area o the Pall capacity. Capacity increases with random high mass-transer rates. A rule o thumb ring with the low aerodynamic drag o the packing size. This poses a trade o, that must also be taken into account is to saddle shape. however, in that the ideal size o packing choose a packing diameter that is less than Similar in structure to the Pall Ring is the is a compromise between maximizing eone-eighth o the column diameter, which Cascade Mini-Ring, which has a height fciency and maximizing capacity. minimizes liquid channeling. to diameter ratio o 1:3 compared to 1:1 4. Minimize riction . An open shape Material . Metal packings are usually in the Pall Ring. This allows the individual minimizes riction, providing good aerodypreerred because o their superior strength packing components to be oriented with namic characteristics. and good wettability. Ceramic packings, their open side acing vapor ow, thus reon the other hand, have superior wettability ducing riction and exposing more surace 5. Minimize costs . Packing costs, as well but inerior strength, and are used only in to mass transer. as the requirements or packing supports situations at elevated temperatures where and column oundations, generally increase The latest generation o random packings corrosion resistance is needed and plastics with the weight per unit volume o packing. eatures a very open, smooth and wave-like would ail. Plastic packings, usually made Packings generally become cheaper as the geometry that promotes wetting, but still o polypropylene, are inexpensive and size o random packing increases. promotes recurrent turbulence. This allows have sufcient strength; however, they may a decreased pressure drop while sustainexperience poor wettability, especially at Packing structures [3,2 ] ing mass-transer efciency that may be low liquid owrates. independent o column diameter, and may Raschig Rings are hollow cylinders with Packing objectives [1 ] allow a greater depth o packing without a a height that is equal to the ring diamliquid redistributor. eter. This structure is the oldest orm o 1. Maximize the specifc surace area . InReferences random packing. creasing the surace area per unit volume 1.“Perry’s Chemical Engineers’ Handbook ,” 8th maximizes the vapor-liquid contact area, The original saddle-shaped packings, Berl ed. McGraw Hill, New York, 2008. and, thereore, efciency. Efciency generSaddles, have a smaller ree-gas design 2.Seader, J. D. and Henley, E. J., “Separation ally increases as the random packing size than Raschig Rings. However, they are oProcess Principles,” 2nd ed., John Wiley and is decreased. ten a more preerable choice, as they oer Sons, Inc., New Jersey, 2006. a lower pressure drop and higher capacity. 2. Spread the surace area uniormly . This improves vapor-liquid contact, and thereore, efciency. For instance, a Raschig

The invention o the Intalox Saddle marked the start o the second generation o

Facts at your Fingertips sd b: 

3.Schweitzer, P., “Handbook of Separation Techniques for Chemical Engineers,” 3rd ed., McGraw Hill, New York, 1997.