Batch Distillation
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Transcript of Batch Distillation
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Batch DistillationPharmaceutical API Process Development and Design
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Module StructureVapor Liquid Equilibrium CurvesRayleigh DistillationColumn ConfigurationsColumn OperationSimulationDesign of Batch Columns
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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
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VLE Curve and BP/DP CurvesyxA01TSaturated VaporSaturated LiquidMixture of A and BxA
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Homogeneous AzeotropesFor non-ideal mixtures, the activity coefficients are different from unity: Phase diagrams for Isopropyl ether Isopropyl Alcohol
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Homogeneous AzeotropesFor non-ideal mixtures, the activity coefficients are different from unity: Phase diagrams for Acetone Chloroform
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Heterogeneous AzeotropesHomogeneous AzeotropeHeterogeneous Azeotrope
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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
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Rayleigh DistillationVaporLiquid ChargeHeatL, xi remaining liquid and mole fraction at any subsequent timeL0, xi0 initial liquid amount and mole fraction
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Rayleigh Distillation (Contd)For binary mixture when ij is constant
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Batch EvaporationQcQrAccum 1Accum 2
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Batch Evaporation Example
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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
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Batch Distillation AdvantagesAdvantagesFlexibleAccurate implementation of recipe specific to a given mixtureSeveral components separated using one columnRequires least amount of capital
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Conventional Batch Distillation Column
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Column ConfigurationsQcQrQrFFInverted BDAccum 1Accum n
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Column ConfigurationsQcQrQrQcFFMiddle Vessel BDAccum 1Accum nAccum n+1Accum m
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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
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Column OperationStart-up periodVapor boilup rate policyConstant vapor boilup rateConstant condenser vapor loadConstant distillate rateConstant reboiler dutyProduct period: Reflux ratio policyShutdown period
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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
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Effect of Reflux RatioIncreasing reflux ratio Improves separationIncreases cycle timeIncreases energy consumptionProfile optimizationTrade-off between cycle time and value of recovered materialMaximize profit
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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
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Packed ColumnsHETP Height equivalent to one theoretical plateCharacteristic of packingNumber of plates = packed bed height/HETP
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Simulation of Batch DistillationSimulation of startup periodSimulation of product periodColumn modelExamplesBenzenetolueneBenzenetolueneortho-xyleneAcetonechloroform
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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
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Simulation of Product PeriodTotal condenser without sub-coolingPerfect mixing of liquid and vapor on platesNegligible heat lossesCondenser material balance
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Column ModelMass balance equations on plate jConstant volume holdupVLE on each plateConstant molar holdupConstraint
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Column Model (Contd)Enthalpy balance equations on plate jPhysical properties
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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
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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
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BenzeneTolueneO-Xylene20 plates
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AcetoneChloroformAzeotropic system
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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
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Role of Simulation in Column OperationSimulatorVerified ModelSimulatorModel DeveloperOperatorFeed AmountFeed CompositionDCSColumnComponentsCut SequenceFor each cut:Starting and stopping criteriaReflux ratio
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Problems Related to Batch DistillationDesign of a batch columnOperating policy determination for individual column batchesDesign and operation issues are interdependent
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Design of Batch ColumnsMain design parametersNumber of stagesVapor boilup rateDiameterStill capacity (batch size)Reboiler and condenser size heat transfer areasSingle separation dutyMultiple separation duties