DISTILLATION Distillation is an operation for separating the components of mixture based on the
transfer of material from one homogeneous phase to another by vaporization. Mixtures of miscible and volatile substances are vaporized into individual components or in some cases into groups of components. distillati ation on is the distribution of the substances between the gas and Principle of distill liquid phase, applied to cases where all a ll components are present in both cases.
Since this depends on the nature of the substance there is no chance for alteration of solvents as in the case of absorption, stripping etc. A gas gas and and liqui liquid d phas phasee has has been been brou brough ghtt into into cont contac actt for for purpos purposee of diff diffus usio iona nall interchange between them. The equipments for the gas liquid operations provide intimate contact of the two fluids. The rate of mass transfer is directly dependent upon the inte interf rfaci acial al surf surfac acee expos exposed ed betw betwee een n the the phase phasess and the the natu nature re and and the the degre degreee of dispersion of one fluid in the other, Gas dispersed: iquid is continuous phase, gas dispersed phase !x sparged and agitated vessels, tray towers in which the gas phase is dispersed into bubbles or foams. Liquid dispersed: " iquid is dispersed into thin films as drops. !x #etted #etted wall towers, sprays, venturi scrubber, pac$ed towers. !ubble point: The temp. At which first bubble appears to boil from liquid state. At this point Σ yi % &.'( )ut y % *m + x, #here x % mole fraction in liquid. y % mole fraction in vapor. m % -apour -apour ressure / Total ressure. ressure . -ary the temp. and find Σ yi till it becomes equal to &.'. SIGNI"I#AN#$: This gives the bottom temp. in batch distillation.
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Dew Point:
The temp. at which first drop condenses from -apor State. At this point Σ xi % &.'( )ut x % *y /m , where x % mole fraction in liquid. y % mole fraction in vapor. m % -apour ressure / Total ressure. -ary the temp. and find Σ yi till it becomes equal to &.' )ubble point 0 1ew point are the same for a pure compound i.e., equal to the boiling point of that compound. SIGNI"I#AN#$: This gives the top temp. in the distillation column.
!x. 2alculation of )ubble point 0 1ew point of 3' 4 Toluene 0 3' 4 #ater mixture. !ubble point: At te%p of &'( ) #* Antoine $q: ln +,P- . A / ! 0 +T1 #- where A!# are constant And T in 2el3in
2ompnd mole 5racn A ) 2 #ater '.3 &7.8'89 87&9.:: :9.&8 Toluene '.3 &9.'&8; 8'=9.3< 38.9;
v.p. 6 y%$+x 7;'.:;33 &.&9973; '.378:<= 9<:.893& '.789=3& '.:&7:;3 &.''&=':
Dew point: At te%p4 of &'5 ) #4
2ompou nd #ater Toluene
mole fraction '.3 '.3
A
)
2
&7.8'89 87&9.:: :9.&8 &9.'&8; 8'=9.3< 38.9;
v.p.
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5or binary mixtures, -apour pressure of & ∝ % """"""""""""""""""""""""""" -apour pressure of <
<
='&.<9&= &.<'7&<9 '.:&879: 9:8.';93 '.79<'88 '.37''<: '.==8777
6elati3e 3olatilit7 + -:
y + *&"x ∝ % """"""""""""""" , *&"y + x
x%y/$
>∝? is always found between the boiling point of the two compounds.
T8P$S O" DISTILLATION: &- )atch 1istillation. i #ith reflux. a 2onstant reflux. b 2onstant distillate product. ii #ithout reflux *Simple 1istillation . 9- 2ontinuous 1istillation. - Steam 1istillation. (- Azeotropic 1istillation. 5- !xtractive 1istillation.
&4 !atch Distillation: These are used when@ + The compositions of the materials to be separated vary over wide ranges. + 5or small"scale operations. + #hen there is small amount of light or high boiling compound. + #hen more compounds are involved and pure cut to be done to avoid more no. of columns. sually in 1istillation column we have a Bectifying section 0 Stripping Section. 6ectif7ing section: Mainly from -apor to iquid. Similar to Absorption. Stripping Section: Mainly from iquid to -apor . Similar to Stripping. In !atch Distillation operations onl7 rectif7ing or enriching section is present* as the feed is the 3apor4 i- ;ithout reflu<: +Si%ple Distillation-: • There is no control and the distillate composition $eeps on varying. • Cives more output 0 low boiler recovery but less percentage in the distillate composition. ii- #onstant reflu< ratio = 3ariable product co%position: • There is not much difference with that of with out reflux but still separation between compounds is obtained. • Cives medium recovery 0 medium output 0 medium distillate composition. iii- ,ariable reflu< ratio = constant product co%position: • The reflux ratio is changed to maintain the product composition constant. • 1istillate composition is highest with low output 0 low recovery. !x. Almost all distillation in 2ompany with 2olumn 0 Beflux ratio.
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Distillation >ethod
Still co%p4 6ecei3er
A Simple distillation '.&' 2onstant reflux '.&8 -arying reflux '.<9
A '.37 '.7: '.=3
A ? 6eco3er7 =9.9 7;.9 93.:
Stea% Distillation: Steam distillation is used for substance, which is( & Digh boiling. < Ensoluble with #ater. The boiling point is at the temperature at which sum of the -apour pressures becomes equal to Total pressure.
#ontinuous Distillation: 2ontinuous feeding is done and continuously product 0 residue are withdrawn.
A@eotropic Distillation: Azeotropic distillation is used when( i The relative volatility between the two components is less, which requires high reflux ratio. ii The product of required purity cannot be obtained by ordinary fractionation. En this method, a 8rd compound is introduced to the mixture *A 0 ) , A is low boiling so that the 8rd component forms low boiling Azeotrope with A *the low boiling compound of the mixture and comes out as distillate product. Then the 8rd component is separated by means of hase separator and returned bac$ to the column. Thus A is separated from ). 8rd component used is called !ntrainer. #hile choosing the 8rd component care should be ta$en that i Et forms low boiling azeotrope and it is also insoluble with A so that the separation between the two can be done with out any further process. ii The compound should be cheap 0 quantity required should be less. iii This component should not react with original components. iv atent heat of vaporization of the entrainer chosen should be less.
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$A? should be less.
PA#2$D !$D #OL>NS T8P$S O" PA#2INGS : 6ando% pacBings :
Bandom pac$ings are used for providing better contacting surface area between gas liquid or liquid liquid, compared to the hollow columns. The pressure drop in these are more because of randomness in pac$ing. The random pac$ings are used are EMT, raschig rings all rings etc. The cost of these pac$ings are less. These pac$ings are available in different siszes and different moc such as ceramic, MS,plastics, SS. Structured PacBingS :
rovides ess pressure drop and lower D!T compared to the random pac$ing. Their cost is higher *labourous Fob these are mainly used in critical operation where very high vacuum is a large no of theoretical stages are required. T8P$S O" DISTILLATION D$P$NDING ON OP$6ATING P6$SS6$ : ,acuu% Distillation: &
3
1istillation is carried out under vacuum conditions to reduce the boiling temp of for one of the following reasons. & The relative volatility between the components generally increases the boiling temp drops. This higher relative volatility improves the ease of separation, which lowers the no of theoretical stages needed for given separation. Ef the no of theoretical stages held constant, the reflux ratio required for same separation is reduced. En addition the no of theoretical stages and the reflux ratio are maintained constant, product purity will be increased. < ower distillation temp is desirable when processing thermally sensitive products, lower temp retard the undesirable reactions such as product decomposition, polymerization or discoloration. 8 Separations can be achieved for components with very low vapour pressures or compounds that degrade at temp near their boiling point. : ower reboiler temp permit the use of less costly energy sources such as S, Dot water. En case of S atent is also more compare to MS or DS.
Pressure Distillation:
ressure 1istillation is carried out to permit condensation of low boiling materials at ambient temp. As the distillation pressure is increased the relative volatility will be decreased so the separation will become more difficult. This will lead to the greater no of theoretical stages for the same separation. !ven though the capacity of a column increases at higher ressure due to the greater vapour density the no of theoretical stages or reflux ratio required will be higher. sually distillation column is designed at the lowest operating pressure that is economically feasible. OP$6ATION :
Pac$ed 2olumns are used in the continuous contact of liquid and gas in the cocurrent and countercurrent ow types, these are vertical columns which have been lled with packing that provide large interfacial area, liquid is distributed over and trickles down through the packing exposing a large surface to contact the gas.
Desired properties of pacBing :
& rovide large interfacial area between liquid and gas < The pac$ing must permit the passage of large volumes of fluids through small tower 2/S without loading or flooding and with low pressure drop for the gas *void fraction must be large
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"or 6ando% Du%ped PacBing :
Cas hase pressure drop with pac$ings is greater than with hallow pipe. And with introduction of liquid from top will further increase the pressure drop. 6ozeny 2armon !quation@ " ∆ % ψ - G/*ε H 8+ I J *&"ε/1pK
T8P$S O" PA#2ING : 6asching rings +cera%ic 0%etal Saddle Intalo< +cera%icSaddle !erl +cera%icPall ring +Plastic 0>etalI>TP %etal +intallo< %etal tower pacBingLiquid Cold up in pacBed Towers :
Static Dold p % -olume of iquid per unit vol. of pac$ing that remains after the flow stops and the bed is drained. *1epends on pac$ing surface area, roughness of pac$ing, contact angle Operating Cold up:
-olume of liquid per unit vol. of pac$ing that drains out of the bed after the gas and liquid flows to the column are stopped.
PA#2ING SI$ ,S 6$#O>>$ND$D LIEID 6AT$S: PacBing si@e +inch-
Liquid rate +ga% 0ft 9-
& &F 9
95 (' 55 '
ac$ing factors N *5 chart for random dumped
3/7
8/:
&
;
<
8
pac$ing EMT *metal all Bing*plastic Entallox Saddle <'' *ceramic
3& =3 &:3
:& 33 =<
<7 :' 3<
&7 <9 :'
&< &; <<
"LO; PATT$6N IN A PA#2$D !$D #OL>N : #ocurrent "low Operations @ "
Single mass transfer stage is required. Oo conventional flooding limitations because liquid hold up tends to decrease with increasing gas rate. )ecause of higher allowable pressure drop the capacity of a given dia column is much greater in co"current than the counter current flow operations. Cas liquid contact intensity is greatly increased at higher flow rates. Thus the mass transfer rates can be elevated in co"current flow. These operations are used for following processes, where either absorption is with reactions or the resistance for mass transfer is very less. !x@ " &. Absorption of low concentrations of ammonia by dilute acids. <. Bemoval of traces of D
G$N$6AL P6O#$D6$ "O6 OP$6ATION O" A DISTILLATION S8ST$> : & Beceive BM. in the distillation vessel. < Cet the analysis of BM. to distill *understand the 2omponents present in the Mass 8 Ppen the vapour line valve, equalisation valve collection or Beflux and atm vent / vacuum line valve. : 2hec$ the dew point of the low boiler to be distilled at the distillation ressure*atm , vacuum
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3 The condenser utility should be such that the diff in dewpoint of low boiler and the utility temp should be at least &' Q2. Start utility 2T# /2D#/2D). 9 1rain the limpet /Fac$et by opening passing steam trap / )y pass valve to complete the draining. ; Pnce the steam starts coming from by pass valve / Trap that mean now the limpet /Fac$et has been drained completely. 7 2ontinue heating the distillation mass initially the pressure gauge will show very low pressure as most of the steam will be condensed immediately in side the limpet /Fac$et to heat the BM. = After the distillation mass has been heated to sufficient temp for boil up the pressure gauge will show relatively high pressure. &' 2hec$ whether boiling of distillation mass is ta$ing place or not in the sight glass of the reactor /2hec$ the vapour temp is raising. && Pnce the vapour condensation is started, 2hec$ the temp of the vent line /-acuum line, if it is cold then we can further increase the boil up. Ef it is warm then we can bring down the temp by throttling the heating media valve. &< 2alculation of )oil up rate of the system. Ef % < *reflux and 1%& *colln and the collection rate is :'' it /Dr, The boil up would be % :'' *
T6O!L$ SCOOTING:
&4 Pressure drop %ore : The vapor line or equalization or vacuum line gets restricted. 94 6eactor pressure increases: The column bottom reflux is not ta$ing place through the reflux line but through the vapor line. 4 ,acuu% proble%s4 (4 !oil ups: 2hec$ for the steam condensate. Measure the steam condensate rate and chec$ for the boil up. sually the latent heat is ta$en as &'' 6cal/6g for organic compounds. 54 Subli%ation proble% : 5or avoiding this, following solutions are there( i rovide baroleg and avoid vacuum in collection tan$s so that there will be no sublimation.
=
ii
Encrease the pressure in the system there by sublimation temp. Encreases and thereby avoid sublimation.
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