Fractional Distillation

INTRODUCTIONTYPES OF SEPARATION PROCESSESSeparation by Phase CreationSeparation by Phase AdditionSeparation by BarrierSeparation by solidSeparation by External Field or GradientSEPARATING AGENTSENERGY SEPARATING AGENTSMASS SEPARATING AGENTSDISADVANTAGES: Need for an additional separator to recover the MSA for recycle.Need for MSA make upPossible MSA product contaminationMore difficult design procedures SEPARATION PROCESSESDISTILLATIONABSORPTION/STRIPPINGLIQUID LIQUID EXTRACTIONSOLID LIQUID EXTRACTIONDistillationGibbs Phase Rule and EquilibriumPhase RuleDegree of Freedom F = C P + 2Example: gas-liquid system of CO2 air waterWhat is the degree of freedom?What variables can be set to define the system?EQUILIBRIUM VS STEADY STATEEQUILIBRIUMAll properties of the system are identical in all phases and on the macroscopic scale. No further changes in these properties with time and no differences in properties within the systemSTEADY STATENo changes of properties with time only but there can be differences in any properties within the system.RELATIVE VOLATILITYRAOULTS LAWVAPOR PRESSUREDefine: Vapor pressureVolatile liquidsBubble pointDew pointAZEOTROPIC SYSTEMSWHAT IS AN AZEOTROPE?Point at which mole fraction in vapour is equal to mole fraction in liquidAzeotropic-forming mixtures exhibit either minimum or maximum boiling points at some composition with deviations from Raoults Law.Vapor and Liquid compositions are identical thus relative volatility is equal to 1.( What is the implication of this?)

EQUILIBRIUM BOILING POINT DIAGRAMThe ease for a liquid to boil depends on its volatilityBINARY VAPOR LIQUID SYSTEMSExperimental vapour-liquid equilibrium data for binary systems are represented in terms of four intensive variables: T, P, y and x.Types of equilibrium dataIsothermal: P y x Isobaric: T y x If P and T are fixed phase compositions are completely definedSeparation factor is fixed (relative volatility)

SINGLE-STAGE EQUILIBRIUM CONTACT FOR GAS-LIQUID SYSTEM

Liquid and vapor are made in contact and allow to attain equilibrium. The liquid and vapor will come out of the contacting system in equilibrium.

Lo, xoL1, x1V2, y2V1, y1LARGE RELATIVE VOLATILITY (> 10,000) ALMOST PERFECT SEPARATION IN A SINGLE STAGEDISTILLATIONSingle stage or still distillation that does not involve rectificationEquilibrium or Flash DistillationSimple Batch or Differential DistillationSteam DistillationEquilibrium or Flash DistillationFeed is partially vaporized to give a vapour richer in the more volatile components than the remaining liquid. (flashed adiabatically to a lower pressure) Simple Batch or Differential Distillation

L1 = L2 + Vx1 L1 = x2L2 + (L1 -L2)y2v There is no reflux,At any instant, vapour leaving the still pot with composition, yD is assumed to be in equilibrium with the liquid in the still.EXAMPLE FOR DIFFERENTIAL DISTILLATION A mixture of 100 moles containing 50 mol% n-pentane and 50 mol% n-heptane is distilled under the differential conditions at 101.3 Kpa until 40 moles is distilled. What is the average composition of the total vapor distilled and the composition of the liquid left?Given below is the equilibrium data for n-pentane.x1.0 0.867 0.594 0.398 0.254 0.145 0.059 0y1.0 0.984 0.925 0.836 0.701 0.521 0.271 0Plate-to-Plate CalculationsSorel MethodWithout the assumption of constant molal overflowUses material and enthalpy balances with equilibrium calculation to determine flow of liquid and vapor for the plate, temperature of the plate, and the composition of each stream for the plateRigorous and tediousAssumes operation pressure, reflux ratio, temperature or enthalpy of the reflux stream, and the use of total condenser.Lewis MethodAssumes constant molal overflow (CMO) equimolal latent heats and heat capacities and no heat of mixingThe reflux Lo is saturated liquid.

Plate-to-Plate CalculationsGraphical Methods for Binary SystemsPonchon-SavaritUses H-concentration diagramNo CMO assumptionMc-Cabe-Thiele CMO equimolal latent heats and heat capacities and no heat of mixingThe reflux Lo is a saturated liquid. Column pressure is constant.Reflux ratio is assumed.

McCabe-Thiele Method

Constant Molal Overflow (CMO)Distillation processesDistillation is a process where a feed mixture of two or more components is separated into products, of compositions different from the feed. This process takes advantage of the differences in distribution of components between the vapour and liquid phase.F, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DDistillation processesThe feed is introduced at one or more points along the column.

Liquid runs down the column from tray to tray, whereas vapour is ascending along the column.

At each tray vapour and liquid contact and mix with each other F, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DDistillation processesLiquid at the bottom of the column is partially vaporized in a heated reboiler.

The boil-up is send back to the column.

The rest is withdrawn as bottoms, or bottoms productF, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DDistillation processesVapour at the top of the column is cooled and condensed in the overhead condenser.

Part of this liquid is returned back to the column and the rest is withdrawn as distillate or overhead productF, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DDistillation processesAt each stage of the column two phases come in contact with each other, mix, approach thermal and composition equilibrium to the extent which depends on the efficiency of the contact stageF, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DLin,xinLout,xoutVout,youtVin,yinDefinition of a stage in a processA single stage is a device or a subunit of the process,where two (or more) phases of a different compositioncome in contact with each other, exchange and leavewith new compositionsLin,xinLout,xoutVout,youtVin,yin- Mass balance Overall Components

- Energy balanceLin,hinLout,houtVout,houtVin,hinQ

Equilibrium stage processesStreams leaving the stage are in thermodynamicequilibrium with each otherLin,xinLout,xoutVout,youtVin,yinStreams coming to the stage: not in equilibriumF, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DThe idea is then to consider a hypothetical column, composed of equilibrium stages

This idealistic design can be converted to the actual design viaanalysis of tray efficiency Distillation processesThe lighter component tends to accumulate in the vapour phase

The heavier component tends to accumulate in the liquid phase

F, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DDistillation processesIn general, the overall separation process depends on:

- relative volatilities

- number of contacting stages

- ratio of liquid and vapour flowrates

F, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DDistillation processesIf the feed is introduced at one point, it divides the column into a rectifying and stripping sections

But usually there are multiple feed location and various side streams

F, zfVa, yaLa, xa=xd=y1=yaLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbR=La/DDistillation process designStep 1: Thermodynamics data and methods to predict equilibrium phase compositions

Step 2: Design of equilibrium stage separation

Design problem type 1: To determine the number of equilibrium stages required to accomplish the desired separations

Design problem type 2: Given a particular column design, determine separation that can be accomplished

Step 3: Develop an actual design by applying the stage efficiency analysis to equilibrium stage designDesign of equilibrium stage distillation: Binary Mixtures Review Va, yaLa, xa=xdF, zfLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xb37Operating linesVa, yaLa, xa=xd=y1=yaF, zfLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbRectifying sectionR=La/D38Operating lines

This equation is a straight line (V=const, L=const, L/V=const) if:

- Two components have similar and constant molar enthalpies of vaporization (latent heats)

- Component sensible enthalpies changes and heats of mixing are negligible (compared to latent heats)

- The column is well insulated (adiabatic)

- Pressure is uniform throughout the column39Operating lines

Reflux ratioVa, yaLa, xaLn xnVn+1 yn+1condenserOverhead product D, xdR=L/D

40Operating lines

yxxDslope=R/(R+1)

41Operating linesVa, yaLa, xaF, zfLb, xNLn-1 xn-1Vn ynLn xnVn+1 yn+1Lm-1 xm-1Vm ymLm xmVm+1 ym+1condenserOverhead product D, xdboilerVb, ybBottom product B, xbStripping sectionR=La/D42

Operating linesLm xmVm+1 ym+1boilerBottom product B, xb

yxxByBslope=

43Operating line equation: Stage-by-stageyxxayaVa, yaLa, xaPlate 1Plate 2Plate 3L1, x1x1Operating line equation: Stage-by-stageyxxayaVa, yaLa, xaPlate 1Plate 2Plate 3L1, x1x1Operating line equation: Stage-by-stageyxxayaVa, yaLa, xaPlate 1Plate 2Plate 3Using the operating line equation we can calculate y2 from x1. This step is depicted by think green line in the graphs.

This process can be continued to calculate the number of theoretical stages. This method of graphical construction of theoretical stages is calledMcCabe Thiele methodL1, x1x1V2, y2Feed stage considerationsLVFLVFbubble point liquid feedLVFdew point vapour feedLVFsubcooled liquid feedFLVsuperheated vapour feedLVFpartially vaporized feed47Feed stage considerations

LVFL48Feed stage considerationsLVFLVFbubble point liquid feedLVFdew point vapour feedLVFsubcooled liquid feedFLVsuperheated vapour feedLVFpartially vaporized feedq=1q=0q>1q