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-traycolumn-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-traycolumn-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 &