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
• Takes advantage of the differences in
volatilities (vapor pressure)
• For a binary mixture,
Pi 0
 ij  0
Pj
αij – relative volatility,
Pi0 – vapor pressure of pure liquid i
VLE Curve and BP/DP Curves
1
T
Saturated Vapor
y
Saturated Liquid
0
xA
1
0
Mixture of A and B
xA
1
Homogeneous Azeotropes
For non-ideal mixtures, the
activity coefficients are
different from unity:
yP  x P
yP  x P
1
1
S
1 1
S
2
2
2 2
P  x  P  (1  x )  P
s
1
1 1
1
s
2 2
If   1 the mixture has a minimum-boiling azeotrope
i
Phase diagrams for Isopropyl ether – Isopropyl Alcohol
Homogeneous Azeotropes
For non-ideal mixtures, the
activity coefficients are
different from unity:
yP  x P
yP  x P
S
1
1
1 1
2
2
2 2
P  x  P  (1  x )  P
s
1
1 1
1
S
s
2 2
If   1 the mixture has a maximum-boiling azeotrope
i
Phase diagrams for Acetone – Chloroform
Heterogeneous Azeotropes
For a minimum-boiling azeotrope with large deviation from
Raoult’s law (   1 ), phase splitting may occur and a
minimum-boiling heterogeneous azeotrope forms, having a
vapor phase in equilibrium with two liquid phases.
i
Homogeneous Azeotrope
Heterogeneous Azeotrope
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
Rayleigh Distillation
Vapor
 L'  i dxi
ln  '   
 L0  xio yi  xi
x
Heat
Liquid Charge
L’, xi – remaining liquid and mole fraction at any subsequent time
L’0, xi0 – initial liquid amount and mole fraction
Rayleigh Distillation (Contd)
• For binary mixture when ij is constant
y i / xi
 ij 
yj / xj
 ij .xi
yi 
1  ( ij  1).xi
 L' 
 xi (1  xi 0 ) 
 1  xi 0 
1

ln  '  
ln 
  ln 
 1  xi 
 L0   ij  1  xi 0 (1  xi ) 
Batch Evaporation
Qc
Accum 1
Qr
Accum 2
Batch Evaporation Example
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
Batch Distillation Advantages
• Advantages
 Flexible
 Accurate implementation of recipe specific to a
given mixture
 Several components separated using one column
 Requires least amount of capital
Conventional Batch Distillation
Column
Qc
1
L
D
••
Accum 1 Accum n
N
Qr
Column Configurations
Inverted BD
Qc
Qr
F
F
Qr
Accum 1
Accum n
Column Configurations
Middle Vessel BD
Qc
Qc
Accum 1
F
Qr
F
Qr
Accum n+1
Accum m
Accum n
Dual Column Configuration
• Side stream from the
main column fed to a
second column
260
Q2
262
A
• 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
2
266
217
270
216
Main
Column
218
3
Side
Column
219
222
220
1
B
214
Q3
232
223
224
228
C
230
240
Q1
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
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
Qc
 Condenser composition
1
 Accumulator composition, amount
L
D
••
Accum 1 Accum n
 Reboiler composition, amount
N
Qr
Effect of Reflux Ratio
• 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
Staged Separation
Qc
V1 – vapor rate
leaving plate 1
V
1
L / V – Internal reflux ratio
D
Vj , y j
L / D – Reflux ratio
N
Qr
L
Lj-1, xj-1
Mj, xj
Plate j
Vj+1, yj+1
Lj, xj
Packed Columns
• HETP – Height equivalent to one
theoretical plate
 Characteristic of packing
• Number of plates = packed bed
height/HETP
Simulation of Batch Distillation
•
•
•
•
Simulation of startup period
Simulation of product period
Column model
Examples
 Benzene–toluene
 Benzene–toluene–ortho-xylene
 Acetone–chloroform
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
Simulation of Product Period
• Total condenser without sub-cooling
• Perfect mixing of liquid and vapor on plates
• Negligible heat losses
• Condenser material balance
V1  L0 (1  1 / R)
Column Model
• Mass balance equations on plate j
dM j
 V j 1  L j 1  V j  L j
dt
d
( M j xi , j )  V j 1 . y i , j 1  L j 1 .xi , j 1  V j . y i , j  L j .xi , j
dt
• Constant volume holdup
M j  G j . j
• VLE on each plate
y i , j  K i , j .xi , j
• Constant molar holdup
M j  Aj
• Constraint
y
i
i, j
1
Column Model (Contd)
• Enthalpy balance equations on plate j
d
( M j H Lj )  V j 1.H JV1  L j 1.H Lj1  V j .H Vj  L j .H Lj
dt
• Physical properties
K ij  K ij ( x j , y j , T j , P)
H  H ( x j , T j , P)
L
j
L
j
H  H ( y j , T j , P)
V
j
V
j
 j   j ( x j , y j , T j , P)
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
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
Benzene–Toluene–O-Xylene
20 plates
Acetone–Chloroform
Azeotropic system
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
Role of Simulation in Column
Operation
Model
Developer
Simulator
Components
Cut Sequence
For each cut:
• Starting and stopping criteria
• Reflux ratio
Verified Model
Operator
Simulator
DCS
Feed Amount
Feed Composition
Column
Problems Related to Batch
Distillation
• Design of a batch column
• Operating policy determination for
individual column batches
• Design and operation issues are
interdependent
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