Batch DistillationPharmaceutical API Process Development and Design
Module StructureVapor Liquid Equilibrium CurvesRayleigh DistillationColumn ConfigurationsColumn OperationSimulationDesign of Batch Columns
DistillationUsed for separating a mixture of two or more liquids Takes advantage of the differences in volatilities (vapor pressure)For a binary mixture,ij relative volatility, Pi0 vapor pressure of pure liquid i
VLE Curve and BP/DP CurvesyxA01TSaturated VaporSaturated LiquidMixture of A and BxA
Homogeneous AzeotropesFor non-ideal mixtures, the activity coefficients are different from unity: Phase diagrams for Isopropyl ether Isopropyl Alcohol
Homogeneous AzeotropesFor non-ideal mixtures, the activity coefficients are different from unity: Phase diagrams for Acetone Chloroform
Heterogeneous AzeotropesHomogeneous AzeotropeHeterogeneous Azeotrope
Thermo Properties CalculationsImportant properties of pure components, mixturesVapor liquid equilibriaY-X diagrams, T-X, T-Y diagramsExistence of multiple liquid phasesCommercial packagesPart of process simulatorsActivity++, PPDS etcHelps you identify distillation boundaries
Rayleigh DistillationVaporLiquid ChargeHeatL, xi remaining liquid and mole fraction at any subsequent timeL0, xi0 initial liquid amount and mole fraction
Rayleigh Distillation (Contd)For binary mixture when ij is constant
Batch EvaporationQcQrAccum 1Accum 2
Batch Evaporation Example
Batch DistillationPreferred method for separation whenFeed quantities are smallFeed composition varies widelyProduct purity specification change with timeHigh purity streams are requiredProduct tracking is importantFeed has solids
Batch Distillation AdvantagesAdvantagesFlexibleAccurate implementation of recipe specific to a given mixtureSeveral components separated using one columnRequires least amount of capital
Conventional Batch Distillation Column
Column ConfigurationsQcQrQrFFInverted BDAccum 1Accum n
Column ConfigurationsQcQrQrQcFFMiddle Vessel BDAccum 1Accum nAccum n+1Accum m
Dual Column ConfigurationSide stream from the main column fed to a second columnCan be used for mixtures with 3 or more componentsTake advantage of the build up of medium volatile component in the columnEliminate slop cutReduce cycle time, energy consumption
Column OperationStart-up periodVapor boilup rate policyConstant vapor boilup rateConstant condenser vapor loadConstant distillate rateConstant reboiler dutyProduct period: Reflux ratio policyShutdown period
Column OperationOperate under total reflux until the column reaches steady state (L / V = 1, R = )Change reflux ratio to the desired valueCollect distillate in accumulatorEnd the cut when certain criteria are satisfiedDurationCondenser compositionAccumulator composition, amountReboiler composition, amountQcQr1NLDAccum 1Accum n
Effect of Reflux RatioIncreasing reflux ratio Improves separationIncreases cycle timeIncreases energy consumptionProfile optimizationTrade-off between cycle time and value of recovered materialMaximize profit
Staged SeparationV1 vapor rate leaving plate 1 QcQr1NLDL / V Internal reflux ratioL / D Reflux ratioPlate jVj, yjVj+1, yj+1Lj-1, xj-1Lj, xjMj, xjV
Packed ColumnsHETP Height equivalent to one theoretical plateCharacteristic of packingNumber of plates = packed bed height/HETP
Simulation of Batch DistillationSimulation of startup periodSimulation of product periodColumn modelExamplesBenzenetolueneBenzenetolueneortho-xyleneAcetonechloroform
Simulation of Start-up PeriodDynamics of column during start-up are very difficult to modelRigorous model of tray hydraulicsRigorous model of heating column internalsTypical simulation of start-up periodRun column under total reflux until column reaches steady stateAt the beginning, assume that liquid compositions on plates and in the condenser are same as feed composition
Simulation of Product PeriodTotal condenser without sub-coolingPerfect mixing of liquid and vapor on platesNegligible heat lossesCondenser material balance
Column ModelMass balance equations on plate jConstant volume holdupVLE on each plateConstant molar holdupConstraint
Column Model (Contd)Enthalpy balance equations on plate jPhysical properties
Solution of Dynamic ModelVapor boilup rate from plate 1 is constantQuasi steady-state approximationDuring a small time interval, plate temperature, K values, vapor and liquid flowrates remain constantSolve the set of ODEs numerically up to the next update intervalAfter each update interval, recompute bubble point, K values, plate enthalpiesVapor compositionsReboiler composition from mass balanceLiquid and vapor flowrates from enthalpy derivatives
BenzeneToluene DistillationEquimolar mixture of Benzene and Toluene8000 liters chargeVapor boilup rate 20 kmol/hrNumber of plates = 20Plate holdup 4 litersCondenser holdup 180 litersRecover 99% mole fr Benzene and TolueneSimulated using BDIST-SimOptUses Activity++ physical properties package
BenzeneTolueneO-Xylene20 plates
AcetoneChloroformAzeotropic system
Use of Simulation in Batch DistillationSynthesis of operating recipe and rapid characterization of batch distillationsAccurate determination of operating and design parameters of a batch columnUse in column operation to determine cut amounts and switching policy for each batch
Role of Simulation in Column OperationSimulatorVerified ModelSimulatorModel DeveloperOperatorFeed AmountFeed CompositionDCSColumnComponentsCut SequenceFor each cut:Starting and stopping criteriaReflux ratio
Problems Related to Batch DistillationDesign of a batch columnOperating policy determination for individual column batchesDesign and operation issues are interdependent
Design of Batch ColumnsMain design parametersNumber of stagesVapor boilup rateDiameterStill capacity (batch size)Reboiler and condenser size heat transfer areasSingle separation dutyMultiple separation duties
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