Batch Distillation. Pharmaceutical API Process Development and Design. Module Structure. Vapor Liquid Equilibrium Curves Rayleigh Distillation Column Configurations Column Operation Simulation Design of Batch Columns. Distillation. Used for separating a mixture of two or more liquids
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Batch Distillation
Pharmaceutical API Process Development and Design
Vapor Liquid Equilibrium Curves
Rayleigh Distillation
Column Configurations
Column Operation
Simulation
Design of Batch Columns
αij – relative volatility,
Pi0 – vapor pressure of pure liquid i
1
0
1
T
Saturated Vapor
y
Saturated Liquid
0
1
xA
xA
Mixture of A and B
For non-ideal mixtures, the activity coefficients are different from unity:
Phase diagrams for Isopropyl ether – Isopropyl Alcohol
For non-ideal mixtures, the activity coefficients are different from unity:
Phase diagrams for Acetone – Chloroform
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.
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
Vapor
Liquid Charge
Heat
L’, xi – remaining liquid and mole fraction at any subsequent time
L’0, xi0 – initial liquid amount and mole fraction
For binary mixture when ij is constant
Qc
Accum 1
Accum 2
Qr
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
Advantages
Flexible
Accurate implementation of recipe specific to a given mixture
Several components separated using one column
Requires least amount of capital
Qc
L
D
1
•
•
Accum 1
Accum n
N
Qr
Inverted BD
Qc
F
F
Qr
Qr
Accum 1
Accum n
Middle Vessel BD
Qc
Qc
Accum 1
Accum n
F
F
Qr
Qr
Accum n+1
Accum m
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
260
Q2
262
A
217
2
266
270
216
Side Column
3
Main Column
218
219
222
220
B
1
214
Q3
232
223
224
228
C
230
240
Q1
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
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
L
D
1
• •
Accum 1
Accum n
N
Qr
Increasing reflux ratio
Improves separation
Increases cycle time
Increases energy consumption
Profile optimization
Trade-off between cycle time and value of recovered material
Maximize profit
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
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
Total condenser without sub-cooling
Perfect mixing of liquid and vapor on plates
Negligible heat losses
Condenser material balance
Mass balance equations on plate j
Enthalpy balance equations on plate j
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
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
20 plates
Azeotropic system
Simulator
Model Developer
Verified Model
Simulator
DCS
Operator
Column
Feed Amount
Feed Composition