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## Batch Distillation

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### 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,

αij – relative volatility,

Pi0 – vapor pressure of pure liquid i

If the mixture has a minimum-boiling azeotrope

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

Phase diagrams for Isopropyl ether – Isopropyl Alcohol

If the mixture has a maximum-boiling azeotrope

Homogeneous AzeotropesFor 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.

Heterogeneous Azeotropes- Homogeneous Azeotrope

- Heterogeneous Azeotrope

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

Thermo Properties CalculationsRayleigh 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

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 DistillationAdvantages

Flexible

Accurate implementation of recipe specific to a given mixture

Several components separated using one column

Requires least amount of capital

Batch Distillation AdvantagesSide 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

Dual Column ConfigurationStart-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 OperationOperate 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

Column OperationQc

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

Effect of Reflux RatioStaged Separation

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

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

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 Start-up PeriodTotal condenser without sub-cooling

Perfect mixing of liquid and vapor on plates

Negligible heat losses

Condenser material balance

Simulation of Product PeriodMass balance equations on plate jColumn Model

- Constant molar holdup

- Constant volume holdup

- VLE on each plate

- Constraint

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

Solution of Dynamic ModelEquimolar 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 DistillationBenzene–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

- Components
- Cut Sequence
- For each cut:
- Starting and stopping criteria
- Reflux ratio

Simulator

Model Developer

Verified Model

Simulator

DCS

Operator

Column

Feed Amount

Feed Composition

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

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