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,

0

0

j

iij P

P

αij – relative volatility,

Pi0 – vapor pressure of pure liquid i

VLE Curve and BP/DP Curves

y

xA

0

1

10

1

TSaturated Vapor

Saturated Liquid

Mixture of A and B

xA

Homogeneous AzeotropesFor non-ideal mixtures, the activity coefficients are different from unity:

S

1 1 1 1yP x P

S

2 2 2 2yP x P

s s

1 1 1 1 2 2P x P 1 x P( )

If the mixture has a minimumminimum-boiling azeotrope

i1

Phase diagrams for Isopropyl ether – Isopropyl Alcohol

Homogeneous AzeotropesFor non-ideal mixtures, the activity coefficients are different from unity:

S

1 1 1 1yP x P

S

2 2 2 2yP x P

s s

1 1 1 1 2 2P x P 1 x P( )

Phase diagrams for Acetone – Chloroform

If the mixture has a maximummaximum-boiling azeotrope

i1

Heterogeneous AzeotropesFor 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.

i1

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 DistillationVapor

Liquid ChargeHeat

i

io

x

x ii

i

xy

dx

L

L'0

'

ln

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

i

i

ii

ii

ij x

x

xx

xx

L

L

1

1ln

)1(

)1(ln

1

1ln 0

0

0'0

'

jj

iiij xy

xy

/

/

iij

iiji x

xy

).1(1

.

Batch Evaporation

Qc

Qr

Accum 1 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

Qr

1

N

L D

Accum 1 Accum n

• •

Column Configurations

Qc

QrQr

F

F

Inverted BD

Accum 1 Accum n

Column Configurations

Qc

Qr

Qr

Qc

F

F

Middle Vessel BD

Accum 1 Accum n

Accum n+1 Accum m

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

240230

B

Q3

2

1

3

266

216

217

218

219

Side ColumnMain

Column

223

224

220

222

214

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

Condenser composition

Accumulator composition, amount

Reboiler composition, amount

Qc

Qr

1

N

L D

Accum 1 Accum n

• •

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

V1 – vapor rate leaving plate 1

Qc

Qr

1

N

L D

L / V – Internal reflux ratio

L / D – Reflux ratio

Plate j

Vj, yj

Vj+1, yj+1

Lj-1, xj-1

Lj, xj

Mj, xj

V

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

)/11(01 RLV

Column Model• Mass balance equations on plate j

jj AM

jjjjj LVLV

dt

dM 11

jijjijjijjijjij xLyVxLyVxMdt

d,,1,11,1, ....)(

• Constant volume holdup

jjj GM .

• VLE on each plate

jijiji xKy ,,, .

• Constant molar holdup

• Constraint

1, i

jiy

Column Model (Contd)• Enthalpy balance equations on plate j

Ljj

Vjj

Ljj

VJj

Ljj HLHVHLHVHM

dt

d....)( 1111

• Physical properties

),,,(

),,(

),,(

),,,(

PTyx

PTyHH

PTxHH

PTyxKK

jjjjj

jjVj

Vj

jjLj

Lj

jjjijij

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–ChloroformAzeotropic 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

Simulator

Verified Model

Simulator

Model Developer

Operator

Feed AmountFeed Composition

DCS

Column

ComponentsCut SequenceFor each cut:

• Starting and stopping criteria• Reflux ratio

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