Distillation By: Eko Ariyanto, ST., MChemEng Chemical
Engineering Department Engineering Faculty Muhammadiyah University
of Palembang DISTILLATION Separation of mixtures of liquids into
their components- most important processes of the chemical industry
- a common method for this separation is distillation Distillation
is based on the fact that the vapour of a boiling mixture will be
richer in the components that have lower boiling points. Therefore,
when this vapour is cooled and condensed, the condensate will
contain more volatile components. At the same time, the original
mixture will contain more of the less volatile
material.Distillation is a process wherein a liquid or vapour
mixture of two or more substances is separated into its component
fractions of desired purity, by the application and removal of
heat. What is Distillation Examples include:separation of alcohol
and water liquid air into N2 , O2 and Air crude petroleum into
gasoline, kerosene, fuel oil and lubricatingstock
DifferencesinBPsofcomponents(vapourpressurecharacteristics of
liquid mixtures) enables separation Vapour pressure of a liquid
?Equilibrium pressure exerted by molecules leaving and entering the
liquid surface. Distillation occurs - due to differences in the
relative volatility (ratio of vps) of the components in the liquid
mixture Important points concerning vapour pressure are:-energy
input raises vapour pressure -vapour pressure is related to boiling
-a liquid is said to boil when its vapour pressure equals the
surrounding pressure -the ease with which a liquid boils depends on
its volatility
-liquidswithhighvapourpressures(volatileliquids)willboilatlower
temperatures
-thevapourpressureandhencetheboilingpointofaliquidmixture depends
on the relative amounts of the components in the mixtureIn
distillation Vapor rich in more volatile components original
mixture - rich in less volatile material columns are designed to
achieve efficient separation Most of the process engineers- have a
fair idea what distillation means
Someimportantaspectswhichareoverlookedfromthe manufacturing point
of view are:-distillation is the most common separation method-it
consumes enormous amounts of energy, both in terms of cooling and
heating requirements-it can amount to more than 50% of plant
operating costsPossible ways to reduce operating costs of existing
units include: - improve their efficiency and operation- via
process optimisation and control To achieve this improvement it is
essential to have - a thorough understanding of basic distillation
principles - familiarity with the design aspects of distillation
systems TYPES OF DISTILLATION COLUMNS Batch Columns:The feed to the
column is introducedbatch-wise. That is, the column ischarged with
a 'batch' and then thedistillation process is carried out.When the
desired task is achieved, anext batch of feed is introduced.(Batch
is always unsteady-state, used in organic chemistry lab)
Disadvantages: large amounts of material long residence times
Continuous Columns:Continuous columns process acontinuous feed
stream. Nointerruptions occur unless thereis a problem with the
column orsurrounding process unit.Advantages:
Morestages-improvementin separation Continuous columns can be
further classified according to:-the nature of the feed that they
are processing -binary column - feed contains only two components
-multi-componentcolumn-feedcontainsmorethantwo components -the
number of product streams they have
-multi-productcolumn-columnhasmorethantwoproduct
streams-extractivedistillation-wheretheextrafeedappearsinthe
bottomproductstream(solventaddednearthetopofthe column) -azeotropic
distillation - where the extra feed appears at the top product
stream-(eg.Benzeneaddedtoalcohol/watermixture,withternarymixture
boiling at 64.85C lower than binary bp 78C) (Mixture that retains
the same composition in the V state as in the L state when
distilled) -the type of column internals-tray
column-wheretraysofvariousdesignsareusedto holdupthe liquid
toprovidebettercontactbetweenvapour and liquid, hence better
separationpacked column - where instead of trays, 'packings' are
used to enhance contact between gas and liquid
-extractivedistillation-wheretheextrafeedappearsinthe
bottomproductstream(solventaddednearthetopofthe column) -azeotropic
distillation - where the extra feed appears at the top product
stream-(eg.Benzeneaddedtoalcohol/watermixture,withternarymixture
boiling at 64.85C lower than binary bp 78C) (Mixture that retains
the same composition in the V state as in the L state when
distilled) -the type of column internals-tray
column-wheretraysofvariousdesignsareusedto holdupthe liquid
toprovidebettercontactbetweenvapour and liquid, hence better
separationpacked column - where instead of trays, 'packings' are
used to enhance contact between gas and liquid FLASH DISTILLATION
In flashing operation a liquid is introduced into a chamber at a
pressure below that of the vapor pressure of the feed liquid. A
portion of the liquid immediately vaporises or flashes, hence the
name. used on a large scale in petroleum refining not effective for
separatingclosely boiling mixtures What is Flash Distillation?
FLASH DISTILLATION FxFDxDBxBF = Feed flow rate concentration =
zF
specific enthalpy hF D = distillate or top product (vapour) flow
rate concentration = y*D
specific enthalpy HD B = bottoms product (liquid) flow rate
concentration = xB
specific enthalpy = hB Designer of a flash system needs to know
- P and T of the flash drum - size of the flash drum - the liquid
and vapor compositions - P, T and flow rate of the feed entering
the drum or column - how much the original feed has to be
pressurised and heated Pressures must be chosen such that at the
feed pressure, the feed is below its bp and remains liquid
Atthepressureoftheflashdrumorcolumn,thefeedmustbeaboveits boiling
point and some of it vaporises Balances:MassF D B = +ComponentF z D
y B xF D B = + EnthalpyF h Q D H B hF D B + = + Solving
simultaneously gives: == +|\
|.| +|\
|.|BDy zx zH hQFh hQFD FB FD FB
FOnanx-ydiagram,givesoperatinglinethrough (zF,zF), slope
-B/Dxy-B/DzFxDxBFxFDxDBxBFxF DxD xF T-x data (T - temperature in K
and x mol fraction of hexane in liquid): 399378367357350344 0.0
0.20.40.60.81.0 T-y data (T - temperature in K and y mol fraction
of hexane in vapor): 399393387378367344 0.00.20.40.60.81.0
Hexane-octanemixturecontaining60mol%hexaneisflashed
at1atm.Ifone-fourthofthefeedisvaporisedwhatarethe
compositionsoftheproductstreams?Extractequilibrium compositions
from the following T-xy data. 00.10.20.30.40.50.60.70.80.910 0.2
0.4 0.6 0.8 1xyLet f = D/F F = D + B FxF = DyD + BxB yD =
{-(1-f)/f}xB +(1/f)xF
Locate the intersection ofmaterial balance line with the
equilibrium curve and read liquid and vapor compositions x = 52% y
= 86% SectionsofContinuousDistillation
ColumnsAtypicaldistillationcontainsseveral major
components:-averticalshellwheretheseparationof liquid components is
carried out-columninternalssuchastrays/plates
and/orpackingswhichareusedto enhance component
separations-areboilertoprovidethenecessary vaporisation for the
distillation process-acondensertocoolandcondensethe vapour leaving
the top of the column-arefluxdrumtoholdthecondensed vapour from the
top of the column so that liquid(reflux)canberecycledbackto the
columnTheverticalshellhousesthecolumn
internalsandtogetherwiththecondenser
andreboiler,constituteadistillation column. Normally feed column
near centre Section above feed: enriching or RECTIFYING section
removes less volatile components from vapour (vapour is enriched in
the volatile component) Section below feed: STRIPPING section the
volatile component is stripped from liquid Liquid coming down
REFLUX Top product DISTILLATE Bottom product RESIDUE or BOTTOMS.
Temperature - highest at the reboiler Decreases progressively up
the tower Reaches a minimum (T) in the condenser Fractionator is
quite literally a heat engine- separating feed mixture into
relatively pure products. Heat addition to the system at a high
level and rejected at a lower level.Fractional distillation
FxFDxDBxBV1L0 nOverall balances: FDB =+ FzDxBx FDB =+ = = D F xx xx
B F xx xx FB DB DF DB andNet Flow Rates: Rectifying Section:
Condenser: DVL = 10 TotalDVL nn = + 1 ( = constant) Conventions:
Number trays from the top down (i.e. in direction of L-phase flow)
A bar over a flowrate, e.g.refers to the stripping section
LComponent flow:Dx V y L x V y L xD n n n n= = + + 1 1 0 0 1 1=
constantStripping section:B L V L VN N m m= = + + 1 1Bx L x V y L x
V yB N N N N m m m m= = + + + + 1 1 1 1= constant (m refers to
intermediate stage close tofeed point)FxFDxDBxBV1L0 nF xF B xB D xD
FxFDxDBxBV1L0 nOperating Lines:Rectifying section:10 0 1 111+ +++
=nnnnnVx L y VxVLyDnnnnnxVDxVLy1 11+ +++ =eliminating Vn+1:yLL DxDL
DxnnnnnD + =++1FxFDxDBxBV1L0 nStripping section:yLVxV y L xVmmmmN N
N Nm+++ ++= +111 11ButV y L x BxN N N N B + + = 1 1 yLVxBVxmmmmmB
++ += 11 1eliminating Vm+1: yLL BxBL BxmmmmmB + =1If Ln or Lm
varycurved operating lines.If enthalpy changes significant: must
use Ponchon-Savarit Method withenthalpy/concentration (H-x)
diagram.If not significant:called CONSTANT MOLAL OVERFLOW use mole
units constant Ln and Lm drop subscripts ignore enthalpy changes
use McCabe-Thiele method based onx-y diagram onlyInherent
assumption in McCabe-Thiele method - distillation column is at the
same pressure One can use constant pressure x-y diagram. Actually P
decreases from bottom to top. Why?? A driving force is necessary
for the vapor to flow upward. xyxDxBL/(L+D)L/(L-B)OPERATING LINES:D
n nxD LDxD LLy+++=+1 (rectifying)yLL BxBL Bxm m B + =1
(stripping)On x-y diagram:rectifying: through (xD,xD), slope LL D
+stripping: through (xB,xB), slope LL B Adequate design problem
definition: Designafractionatorcolumntoproducea
desiredseparationforknownF,zf, xB & xD.Material balance yields
B and DReflux quantity needed for the designRelux ratio - is a
common way of expressing the reflux amountReflux ratio RLDV DDD =
=so yRRxxRnDDnDD+ =+++11 1 line with a y-intercept of xD/(RD+1)
passing through (xD,xD) withslope=RD/RD+1OPERATING LINES:D n nxD
LDxD LLy+++=+1 (rectifying)yLL BxBL Bxm m B + =1 (stripping)On x-y
diagram:rectifying: through (xD,xD), slope LL D +stripping: through
(xB,xB), slope LL B xyxDR/(R+1)x/(R+1)DTotal Condenser DxDV1L0xyxD=
y= x10Total condensers - Simpler and cheaper In a total condenser,
all of the vapor leaving the top of the column is condensed.
Consequently, the composition of the vapor leaving the top tray y1
is the same as that of the liquid distillate product and reflux,
xD.Partial Condenser In a partial condenser, the vapor is only
partially liquified. The compositions of these three streams (V1,
D, and R) are different. Normally, D (composition yD) is in
equilibrium with R (composition
xD).Apartialcondenserfunctionsasanequilibriumseparationstage,socolumns
with a partial condenser effectively have an extra ideal stage
(extra condensing and control equipment required) Reboilers Most
reboilers are partialreboilersmeaning that they only vaporize part
of the liquid in the column base. Vapor leaving the reboiler is in
equilibrium with the liquid leaving as bottom product Reboiler acts
as an ideal plate. yN+1 is in equilibrium with xB xyxByN+1BmmmmmxB
LBxB LLy=+1Determination of number of stages RectifyingsectionyLL
DxDL DxnnnnnD + =+++1StrippingsectionyLL BxBL BxmmmmmB +
=1McCabe-Thiele:insignificant enthalpy changesconstant flows=>
constant molal overflow conditionsPonchon-Savarit:enthalpy changes
or heat loss importantrequires use of H-x diagramReflux ratio:RLDV
DDD = =Upper operating line two points i.e. (xD,xD)
andy-intercept.Lower operating line same manner
linewillpassthroughthepoint (xB, xB) secondpointbyconsidering feed
conditionConstruction of stages Feed Conditions Saturated liquid
Vapor liquid mixture Saturated vapor Sub-cooled liquid ??
Superheated vapor?? FEED LINE:q = fraction of the feed that is
liquid at columnconditionsThe feed stream increasesliquid byq.F and
vapour by (1-q).FConsequently,( )L L qF V V q F = + = + and 1This
can be rewritten as( )L L qF V V q F = = and 1Operating Lines are:B
DBx x L y V and Dx Lx Vy = + =Subtracting gives:( ) ( )y V V L L x
Dx BxD B = + +Substituting for L L and V V gives:( )y q F qFx Dx
BxD B1 = + +But Dx Bx FxD B F+ = Feed Line is given by yqqxxqF= + 1
1passes through (xF,xF); slope qq 1passes through intersection of
rectifying andstripping operating linesFeed Plate Location q =
moles of liquid in stripping section that result from each mole of
feed (or fraction of the feed that is liquid at column conditions)
Feed Type q slopeCold Feed (below bubble point) > 1 >
1Saturated Liquid (at bubble pt.) 1 InfiniteMixed V-L >0 & 0
& 1 > 1Saturated Liquid (at bubble pt.) 1 InfiniteMixed V-L
>0 & 0 &