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Design of Distillation Column
Distillation:
In process industries , it is often desired to separate the componen ts of liquid mixture . the easiest way is to carry out this separation is by distillation .this unit operation makes use of the difference in the boiling or the relative volatilities of the components. Distillation is considered to be the the preferred separation technique if the the relative volatility difference difference between the two key components (that are required to to be separate from each other ) is greater than .!
Selection of the Distillation Column: Batch and Continuous Columns. Distillation columns may be batch or continuous, based on the feed is introduced. In batch columns, a batch of feed is charged and operating carried out ou t till the desired degree of separation is achieved. "he material removed and the next batch is charged. "hese columns are suitable for very low throuputs and for system where very high purity is required. #ontinuous columns process continuous feed streams. "he are widely used in industries for high throuputs. $ere we are concerned with the later type of operation.
Vacuum Vacuum Distillation D istillation Column I have selected the continuous vacuum distillation column because. • • • •
%or heat sensitive material (gasoline) &elative volatility of components is increased "o avoid thermal decomposition of '.. ("*+-#) /nergy economical process.
Selection of Vacuum System: Steam jet ejector
Vacuum Vacuum pumps
0imple design , with no moving parts p arts and practically no wear. 1owest capital cost among vacuum producing devices.
$igh power requirement. 1ow operating cost but high capital investment .
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0imple repair and maintenance ffers the largest throuput capacity of any vacuum producing devices, #an handle more than ---,--- ft+ 2min of process fluid. 3referred when steam is available.
$ard to repair and maintain. #apacity range varies depending upon type of pump. #an handle overload in capacity at the cost of power
0everal devices are available for producing vacuum at a chemical process plant. /ach has its own advantages and disadvantages , among these e4ectors are workhorses. "he simples and probably most widely used vacuum producer is the steam 4et e4ector.
Ejectors Offer a Range of Attraction: • 0imply design with no moving parts and particularly no wear. • #an be mounted in any orientation • #an be fabricated of virtually any metal • 5o special start up or shut down procedure required • #an handle condensable loads and corrosive vapors • 0imple repair and maintenance .
Steam Jet Ejector: 0team 4et e4ectors are designed to convert the pressure energy of a motivating fluid to a velocity energy to entrain suction fluid ant then to recompress the mixed fluid by converting velocity energy back in to the pressure energy. "his is based on the theory that a properly designed no66le followed by a properly designed throat or venture will make economically make use of high pressure fluid to compress from a low pressure region to a high pressure. "his change from pressure head to velocity head is the basis of the 4et vacuum principle.
Ejectors Range: /4ectors /4ectors range from from single stage stage up to six stage stage units, and can be either condensing condensing or non7 condensing types. "he numbers of e4ector stages required are usually determined by the economy of the e4ectors and the level of vacuum required. 8acuum 8acuum ranges for each stage are as follows.9 st stage !nd stage +rd stage ;th stage th stage =th st stage
:-mm$g'7+-mm$g' +-mm$g'7+mm$g' !mm$g'7-.:mm$g' ; mm$g'7<microns $g ' -.; mm$g'7- microns $g' -. mm$g'7+ microns $g'
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Fig .
Fig .
Ejector Performance Curve: "his curve is used to find out the required water or steam flow rate to create the d esired suction pressure in the upstream of the motive fluid.
Tray and Packed Column: "he performance of a distillation column depends upon the intimate contact of liquid and vapor steams. "wo configurations are widely used in this respect. "hese are the plate and the packed columns. ' general comparison of the two configuration is made below. . 3acked columns are continuous contacting units. n the other hand , vapor liquid contact in a tray column occurs only at discrete location i.e., on the trays.
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!. "he liquid and vapor streams never reach equilibrium in a packed column. In a tray column, the steams leaving any stage are assumed to be in the equilibrium. +. 3acked columns can operate over a relatively wide range of vapor flow rates. n the other hand , plated columns offer wide operating range with respect to the liquid flow rates. ;. 3acked columns cannot work efficiently under stressed condition of temperature and pressure. . 1iquid distribution can be a problem in packed columns and cause channeling. =. %or diameters less than about -.= m , a plate column cannot be constructed. "herefore a packed column has to be used. <. Design information for plate column is more readily available and more reliable. :. If the system contains solid contents, a plated column is preferred. "he solids can accumulate in the voids of the packing and choke them . >. %or large column heights, weight of the plate column is much smaller as compared to that of a packed column. -. %or cleaning of packed column, packing must be removed.
In petroleum refining , the distillation columns are plate columns. "his selection has the following aspects. . "he distillation columns in petroleum refining are complex columns. 'ctually , they are fractionators , and multiple products are obtained from a single column as side streams . the side streams are often steam7stripped to meet the flash point specification and the overlapping standards of two fractions. "he resultant vapor streams are again introduced in to the column. $ence , a distillation column in a petroleum refinery has a number of streams going into and out of the system. "hese inlets and outlets are easy to locate at discrete points in a tray column, but difficult for a packed column. !. ?ecause of the multiple inlet and outlet streams, the liquid and vapor flow rated widely very inside the column. ' packed column is not suitable for such a condition. +. "o avoid problems like flooding and entrainment because of the variable liquid and vapor flow rated , inter stage cooling or heating is often required. 0ometimes, pump7arrounds are also used for this purpose. this provision can only be made in a plate column. @eeping in mind the above factors, the column selected for the current design is a plate column.
Tray Selection:
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"hree basic types of trays are used in plate columns. "hese are sieve trays, bubble cap trays, and valve trays. "he trays selected for the column are sieve trays. "he supporting factors are. . "hey are light weight and the cheapest available, and easiest to fabricated and install. !. "hey have higher capacity and lower pressure drop than other tray types +. 0ufficient design data is available ;. "he maintenance cost is lower because of the ease of cleaning.
Fig
Comonent Of Distillation Column: Aa4or components of distillation system are.
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. "he distillation column itself , as a tray or packed column where the vapor and liquid streams are brought in contact with each other, and the separation occurs. !. ' reboiler to provide the heat required for vapori6ation. +. ' condenser to remove the heat from the system and condenses the vapors leaving from top of the column. ;. ' reflux drum to hold the condensed liquid after the separator and provide continuous liquid reflux to the column.
Design Of Distillation Column: "he design steps for a column design are9
•
#alculation of Ainimum number of stages. 5min
•
#alculation of Ainimum &eflux &atio & m.
•
#alculation of 'ctual &eflux &atio.
•
#alculation of theoretical number of stages.
•
#alculation of actual number of stages.
•
#alculation of diameter of the column.
•
#alculation of weeping point, entrainment, etc
•
#alculation of pressure drop.
•
#alculation of the height of the column.
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Fig –
!rom "aterial #alance $e %ave: Comonent
!eed
&ta'le( )*+, #ottom
To
!raction
!raction
!raction
-f
-'
-d
C+.C/
*012
0
*000+
C1
*03/
0
*00)2
4(C3
*03)
0
*01/
5(C3
*036
0
*0/1)
7asoline
*1+1
0
*312
5at8ta &9,
*1/;
*0<)
*3<<
Atm gas oil&%,
*+);
*23/
*00+1
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$ater
0
0
*03
Selection Of ey On T8e #asis Of Relative =olatility* #,#!
B lighter than lighter key
#+
B lighter than lighter key
i7#;
B lighter than light key
n7#;
B lighter than light key
gasoline B lighter than light key naphtha B light key unconverted. '..
B heavy key
5ature Of !eed: %eed is entering in column as a saturated liquid at "B--# and 3B+-kpa #omponents #,#! #+ i7#; n7#; asoline 5aphtha E.'..
Cf
@i -.-+> -.-;! -.-;: -.-;< -.++ -.+!= -.:=
=+ = .-< -. .+ /7
Table –
@iCf 0.115648 0.321437 0.340852 0.208026 0.33628 0.042272 2.42E-06
?y bubble point ∑Cf@iB.-F it is verified that feed is entering at saturated liquid. ref9 (col 8= page ;>:.)
Estimation Of To Temerature: ?y dew point calculation , ∑(@i2Cd)B "B>-℃
P=13 kpa
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#omponents #,#! #+ i7#; n7#; asoline 5aphtha E.'.. Gater
93
Cd
@i .--- .--:> .-+! .-!+: .;+> .; .--+ .-;
147.5 32 18 17 22.1923 2.2077 0.0014
@i2Cd 1.62E-05 9.59E-04 3.92E-03 3.04E-03 1.75E-02 2.06E-01 8.25E-01 0.25E-02
Table –
$ence by dew point calculation ∑(@i2Cd)B.! app . it is verified the top temperature
Estimation Of #ottom Temerature: ?y bubble point calculation ∑Cw@iB #omponents 5aphtha En.'..
"B!--°# and 3B+- kpa
Cw -.-: -.>;!
@i 13.025 0.094866667
Cw@i 0.8597041 0.0893334
Table-
$ence by bubble point calculations the bottom temperature is verified. ∑Cw@iB-.:>!
Calculation Of "inimum 5um'er Of Stages: "he minimum no. of stages 5min is obtained from %enske relation which is 5min H B
ln(x1@ 2x$@ )D2(x1@ 2x$@ )?J ln (K1@ 2$@ ) average
"o find average geometric relative volatility of light key to heavy key9
(α Lk Hk ) avg = (α Lk Hk ) D (α Lk Hk ) B 2
2
( α
) = .,
( α
) = ,.+
Lk 2 Hk
Lk 2 Hk
so
2
D
B
Average geometri re!ative vo!ati!it" = 3.01
5min HB:.<<2.! #mi$ = 7
-.,
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Calculation Of "inimum Reflu- Ratio r m Esing Enderwood equation 's feed is entering as saturated liquid so, qB
?y
trial,
θ B .!!
(root of equation)
Esing
equation of minimum reflux ratio
3utting all values we get, R m > 0*1
Actual Reflu- Ratio: "he rule of thumb is & B (.!777777777.<)&min & B .< &min & B -.
T8eoretical 5o* Of Plates: illiland related the number of equilibrium stages and the minimum reflux ratio and the no. of equilibrium stages with a plot that was transformed by /dul4ee into the relation
N − N min
R − R = -.<, − N +
5min B = &min B -.+ &(actual)B -.
min
R + -.,==
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%rom which the theoretical no of stages to be 5> +<
Calculation of Actual 5um'er of Stages: verall "ray /fficiency9 (O?Connell?s e@uation,
E o
= , − +!.,log µ avg .α avg
ref9 #oulson 8= page K avg Baverage relative volatility of light key component B+.- L avg B molar average liquid viscosity of feed evaluated at average temperature of column 'verage temperature of column B (!--H>-)2! B ; o# %eed viscosity at average temperature B µavg B-.--< m5s2m! 0o
Eo > <3*0
"hen, 5o. of actual trays B 2-.; B !: "otal M of actual traysB!:HB!>
Calculation Of !eed Plate: "he irk 'ride method is used to determine the ratio of trays above and below the feed point.
ref #oulson 8= page != %&ere #r = $'m(er o) *tage* a(ove t&e )ee+, i$!'+i$g a$" partia! o$+e$*er, #* = $'m(er o) *tage* (e!o% t&e )ee+, i$!'+i$g t&e re(oi!er, = mo!ar o% (ottom pro+'t, / = mo!ar o% top pro+'t, ), = o$e$tratio$ o) t&e &eav" ke" i$ t&e )ee+, ), = o$e$tratio$ o) t&e !ig&t ke" i$ t&e )ee+, +, = o$e$tratio$ o) t&e &eav" ke" i$ t&e top pro+'t,
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(, = o$e$tratio$ o) t&e !ig&t ke" i) i$ t&e (ottom pro+'t
C1k(f)B.+:
?B+;.kmol2hr
C1k(?)B.-:
DB!->kmol2hr
C$k(?)B.--+ C$k(f)B.:; %rom which, 5umber of 3lates above the feed trayB5r B 5D B 5umber of 3lates below the feed trayB5s B 5? B > 0o feed is entering at plate from top
To And #ottom Condition To conditions 1nB=-;kmol2hr 1nNB1mNB:<=>.>kg2hr 8nB1nHDB:+kmole2hr 8nNB8mNB<=;-kg2hr "B>-#B+=;@ ,3B+kpa Ov(top)B.<<.+kg2kmole
#ottom conditions 1wB1nH%B!+:!kmole2hr 8wB1w7GB!-+=.+kmole2hr 1mB1wNB!:;=!kg2hr 8mB8wNB>=:-kg2hr "B!--#B;<+@ ,3B+-kps Ov(bottom)B.=:kg2m+ O1(bottom)B<=.=kg2m+ A.GB>kg2kmole Table – 8.5
Determination Of T8e Column Diameter: Flow Parameter: -.,
1 O v %18 = n 8n O 1
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1mB1nB!:;=!kg2hr 8mB8nB>=:-kg2hr
ῤvB.=:kg2m+ ῤ1B<=.kg2m+ %18 B 1iquid 8apor %actor B .! 'ssumed tray spacing B + inch (-.> m) 5et vapor velocity at flooding.
%rom %ig (.!<) #oulson and &ichardson vol7=, sieve tray @ B -.-+(P2.-!)-. 0urface tension of Aixture B P B !.-7+ 52m @ B-.-=!
ῤvB.=:kg2m+ ῤ1B <=.kg2m+ uf B8nf B.-=!.;B -.<-= m2sec 'ssume :-Q of flooding then vnB -.:8nf 0o, actual vapor velocity, vnB -.= m2sec 5et column area used in separation is 'n B Rv2vn 8olumetric flow rate of vapors B Rv R8
=
8m O 8 × +=--
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RvB>=:-2(.=:+=--)B >.=m+2s 5ow, net area B Rv2vn B
; × 'c
Dc B
π
Dc > +;ft
(based upon bottom conditions) in the similar way we calculate the top diameter which come out to be 3*31m.so we will decrease the perforated area for uniform diameter of m
Provisional Plate Design : #olumn Diameter DcB m #olumn #ross7sectional 'rea('c) B !- m! Down comer area Ad B -.'c B+ m! 5et 'rea ('n) B 'c 7 'd B< m!
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'ctive area 'aB'c7!'d B ; m! $ole area Ah take -Q Aa B -. T ; B.; m! To find $eir lengt8
'd 2 'c B + 2 !- B -. !rom figure ++*1+ Coulson B Ric8ardson ;t8 volume 3t8 edition age <61
1w 2 dc B -.:1w
B -.:-
I w=Lw = ;
m
Geir length should be =- to :Q of column diameter which is satisfactory "ake weir height, hwB ! mm (!mm7<mm) $ole diameter, dh B mm
(mm7!mm)
3late thickness B mm
C8eck $eeing:
%&ere '& = mi$im'm vapo'r ve!oit" t&ro'g& t&e &o!e*(a*e+ o$ t&e &o!e area, m*, +& = &o!e +iameter, mm, 2 = a o$*ta$t, +epe$+e$t o$ t&e +ept& o) !ear !i'i+ o$ t&e p!ate
"he vapor velocity at weeping point is the minimum velocity for the stable operation. In order to have @ ! value from fig++*10 Coulson B Ric8ardson ;t8 volume 3t8 edition we have to st find how(depth of the crest of liquid over the weir) where how is calculated by following formula9 howB<-U1m2lwOlJ!2+V Aaximum liquid rate W1mXB !:;=!2+=--B<> kg2sec Ainimum 1iquid &ate 't :-Q turn down ratio
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B =+.!@g2sec IwB;m OlB<=kg2m+ 't Aaximum rate ( how)B =+ mm 1iquid 't Ainimum rate (how) B ;.= mm 1iquid hw H how B ! H ;.= B <> mm 1iquid from fig ++*10. Coulson and Ric8ardson =ol*;
@ ! B !: 0o, putting the value for Emin we will have. E (min) B +. m2sec 5ow maximum volumetric flow rate (vapors) ?ase B >.= m+2sec 'hB.;m! 't :-Q turn down ratio Fig –
'ctual minimum vapor velocity B minimum vapor rate 2 'h B . m2sec 0o minimum vapor rate will be well above the weep point. 5ow we well calculate the pressure drop (3.D)
Plate Pressure Dro &P*D,: #onsist of dry plate 3.D (orifice loss), 3.D due to static head of liquid and residual 3.D (bubbles formation result in energy loss H froth formed in operating plates) 8t>8d&88o,8r
Dry Plate Dro &8d,:
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Aax. 8apor velocity through holes (Eh) B Aaximum 8olumetric %low &ate 2 $ole 'reaB >.=2.;B =.: m2sec 3erforated area 'p (active area) B; m! 'h2'p B -.-!rom fig* ++*13 &Coulson B Ric8ardson ;t8 volume 3t8 edition, for
plate thickness2hole diameter B .-Ge get,
#o B -.:; hd B ! mm 1iquid
Residual %ead &8 r,: &r = 12.510 3
O1 B<=kg2m+
Fig
so hr B =. mm 1iquid
Total Pressure Dro
htB!H(!H;)H=. ht B +.+ mm liquid "otal column pressure drop 3a, (52m!) Y3 B (>.:-7+) htO1 B :>-3a B -.:>- k3aB-.psi 'nd allowable p.d is -. psi per tray.
Don Comer 9i@uid #acku : :a'*e+ (" P./ over t&e p!ate a$+ re*i*ta$e to o% i$ t&e +o%$ omer it*e!).
8' > &8 8o, 8t 8dc
"he main resistance to flow in down comer will be caused by constriction in the down comer outlet, and head loss in the down comer can be estimated using the equation given as,
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where 1wd is the liquid flow rate in down comer, kg2sec(<>kg2sec) OlB<=. kg2m+ and 'ap is the clearance area under the down comer, m! 'ap Bhap 1w Ghere hap the height of bottom edge of apron above the plate. hap B hw Z ( to - mm) hap B !- mm so, 'rea under apron W'apX B (!-2---); B -.-: 's this is less than area of down comer 'd (+m!)so using 'ap values in above formula for hdc 0o, hdc B !-mm 's a result, hb B (hwHhow)HhtHhdc B ;-; mmB-.;-; hb * [ ("ray spacing H weir height)
that is
-.;-; * -.;=<
0o tray spacing is acceptable
C8eck Residence Time: 0ufficient residence time should be allowed in the down comer for the entrained vapors to disengage from liquid stream to prevent aerated liquid being carried under the down comer. tr B'd h bc O121(max) 'dB+m!
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h bcB-.;-;m OlB<=kg2m+ 1maxB<>kg2sec tr B ! sec It should be \ + sec. so, result is satisfactory
C8eck Entrainment: (un) actual velocity (based on net area) B (max volumetric flow rate at base 8m 2 net area 'n) unB>.=2
fractional entrainment ] can be found out. %ractional entrainment (]) factor B -.--! Gell below the upper limit of (]) which is -.. ?elow this the effect of entrainment on efficiency is small.
5o* Of 8oles* 'rea of $oleBah B (S2;) Dhole! B -.----! m! 'rea of 5 $oles B'hB .; m! 0o, 5umber % $olesB'h2ah B <---
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%eig8t of distillation column: $eight of column $cB (5act7) $sH ^$H plates thickness 5o. of plates B !> "ray spacing $s B -.>- m ^$Btop clearance H bottom clearance. ^$B.H. m "otal thickness of trays B -.--!> B -.+; m 0o, $eight of column B (!>7)-.>-H +H-.+; > /2 meters
;ig < 8.7
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Secification s8eet 4dentification: 4tem 5o* re@uired: Tray tye:
vacuum Distillation column
0ieve tray
!unction:
separation of gasoline and Oeration:
naphtha from unconverted '... #ontinuous
"able Z :.= 5o. of trayB !>
'ctive holes B <---
3ressure B @pa
Geir height B ! mm
$eight of column B !> m
Geir length B ; m
Diameter of columnBm
&eflux ratio B -.
3ressure drop per trayB-.:>>@paB-.psi
"ray spacing B-.> m
$ole si6e B mm
'ctive area B ; m!
"ray thickness B mm
%looding 3ercent B:- Q
/ntrainment B -.-! Q
&esidence timeB! sec
