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Pharmaceutical API Process Development and Design
Batch Distillation Pharmaceutical API Process Development and Design
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Module Structure Vapor Liquid Equilibrium Curves Rayleigh Distillation
Column Configurations Column Operation Simulation Design of Batch Columns
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Distillation Used for separating a mixture of two or more liquids
Takes advantage of the differences in volatilities (vapor pressure) For a binary mixture, αij – relative volatility, Pi0 – vapor pressure of pure liquid i
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VLE Curve and BP/DP Curves
1 T Saturated Vapor y Saturated Liquid 1 xA xA Mixture of A and B
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Homogeneous Azeotropes
For non-ideal mixtures, the activity coefficients are different from unity: If the mixture has a minimum-boiling azeotrope Phase diagrams for Isopropyl ether – Isopropyl Alcohol
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Homogeneous Azeotropes
For non-ideal mixtures, the activity coefficients are different from unity: If the mixture has a maximum-boiling azeotrope Phase diagrams for Acetone – Chloroform
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Heterogeneous Azeotropes
For a minimum-boiling azeotrope with large deviation from Raoult’s law ( ), phase splitting may occur and a minimum-boiling heterogeneous azeotrope forms, having a vapor phase in equilibrium with two liquid phases. Homogeneous Azeotrope Heterogeneous Azeotrope
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Thermo Properties Calculations
Important properties of pure components, mixtures Vapor liquid equilibria Y-X diagrams, T-X, T-Y diagrams Existence of multiple liquid phases Commercial packages Part of process simulators Activity++, PPDS etc Helps you identify distillation boundaries
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Rayleigh Distillation
Vapor Liquid Charge Heat L’, xi – remaining liquid and mole fraction at any subsequent time L’0, 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 Evaporation Qc Accum 1 Accum 2 Qr
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Batch Evaporation Example
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Batch Distillation Preferred method for separation when
Feed quantities are small Feed composition varies widely Product purity specification change with time High purity streams are required Product tracking is important Feed has solids
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Batch Distillation Advantages
Flexible Accurate implementation of recipe specific to a given mixture Several components separated using one column Requires least amount of capital
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Conventional Batch Distillation Column
Qc Qr 1 N L D Accum 1 Accum n •
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Column Configurations
Inverted BD Qc F F Qr Qr Accum 1 Accum n
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Column Configurations
Middle Vessel BD Qc Qc Accum 1 Accum n F F Qr Qr Accum n+1 Accum m
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Dual Column Configuration
Side stream from the main column fed to a second column Can be used for mixtures with 3 or more components Take advantage of the build up of medium volatile component in the column Eliminate slop cut Reduce cycle time, energy consumption Q1 C Q2 A 262 260 270 232 228 240 230 B Q3 2 1 3 266 216 217 218 219 Side Column Main Column 223 224 220 222 214
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Column Operation Start-up period Vapor boilup rate policy
Constant vapor boilup rate Constant condenser vapor load Constant distillate rate Constant reboiler duty Product period: Reflux ratio policy Shutdown period
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Column Operation Operate under total reflux until the column reaches steady state (L / V = 1, R = ) Change reflux ratio to the desired value Collect distillate in accumulator End the ‘cut’ when certain criteria are satisfied Duration Condenser composition Accumulator composition, amount Reboiler composition, amount Qc 1 L D • • Accum 1 Accum n N Qr
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Effect of Reflux Ratio Increasing reflux ratio Profile optimization
Improves separation Increases cycle time Increases energy consumption Profile optimization Trade-off between cycle time and value of recovered material Maximize profit
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Staged Separation V1 – vapor rate leaving plate 1
Qc V1 – vapor rate leaving plate 1 V L D 1 L / V – Internal reflux ratio L / D – Reflux ratio Vj, yj Lj-1, xj-1 Mj, xj N Plate j Lj, xj Vj+1, yj+1 Qr
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Packed Columns HETP – Height equivalent to one theoretical plate
Characteristic of packing Number of plates = packed bed height/HETP
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Simulation of Batch Distillation
Simulation of startup period Simulation of product period Column model Examples Benzene–toluene Benzene–toluene–ortho-xylene Acetone–chloroform
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Simulation of Start-up Period
Dynamics of column during start-up are very difficult to model Rigorous model of tray hydraulics Rigorous model of heating column internals Typical simulation of start-up period Run column under total reflux until column reaches steady state At the beginning, assume that liquid compositions on plates and in the condenser are same as feed composition
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Simulation of Product Period
Total condenser without sub-cooling Perfect mixing of liquid and vapor on plates Negligible heat losses Condenser material balance
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Column Model Mass balance equations on plate j Constant molar holdup
Constant volume holdup Constant molar holdup VLE on each plate Constraint
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Column Model (Contd) Enthalpy balance equations on plate j
Physical properties
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Solution of Dynamic Model
Vapor boilup rate from plate 1 is constant Quasi steady-state approximation During a small time interval, plate temperature, K values, vapor and liquid flowrates remain constant Solve the set of ODEs numerically up to the next update interval After each update interval, recompute bubble point, K values, plate enthalpies Vapor compositions Reboiler composition from mass balance Liquid and vapor flowrates from enthalpy derivatives
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Benzene–Toluene Distillation
Equimolar mixture of Benzene and Toluene 8000 liters charge Vapor boilup rate 20 kmol/hr Number of plates = 20 Plate holdup 4 liters Condenser holdup 180 liters Recover 99% mole fr Benzene and Toluene Simulated using BDIST-SimOpt Uses Activity++ physical properties package
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Benzene–Toluene–O-Xylene
20 plates
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Acetone–Chloroform Azeotropic system
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Use of Simulation in Batch Distillation
Synthesis of operating recipe and rapid characterization of batch distillations Accurate determination of operating and design parameters of a batch column Use in column operation to determine cut amounts and switching policy for each batch
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Role of Simulation in Column Operation
Components Cut Sequence For each cut: Starting and stopping criteria Reflux ratio Simulator Model Developer Verified Model Simulator Operator DCS Column Feed Amount Feed Composition
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Problems Related to Batch Distillation
Design of a batch column Operating policy determination for individual column batches Design and operation issues are interdependent
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Design of Batch Columns
Main design parameters Number of stages Vapor boilup rate Diameter Still capacity (batch size) Reboiler and condenser size heat transfer areas Single separation duty Multiple separation duties
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