Pressure Swing Distillation

Prepared By: Chauhan Sunny

1. Introduction

2. Pressure-Swing Distillation

3. Operation

4. The setup for a pressure-swing distillation

5. Examples

6. Advantages

7. Comparison With Extractive Distillation (ED)

8. References

Index

Introduction

Azeotrope :

Mixture of two or more liquids wherein its components cannot be separated by simple distillation because they share a common boiling point and vaporization point.

Methods to separate an Azeotropic Mixture:

1. Extractive Distillation (ED)2. Azeotropic Distillation (AD)3. Vacuum Distillation (VD)4. Pressure-Swing Distillation (PSD)

Pressure-Swing Distillation

Pressure-Swing Distillation is a Special distillation process in which no new additive is added.

Pressure-swing distillation (PSD) is the process to be utilized to separate the pressure-sensitive mixture with close boiling point or forming azeotrope.

Principle :

A simple change in pressure can alter relative volatility of the mixture with close boiling pointor forming azeotrope.

Some binary azeotrope mixtures lose azeotropic behaviour when the system pressure is changed.

Operation

Three types of operation mode in Pressure-swing Distillation:

1. Continuous Operation

2. Batch Operation

3. Semi Continuous Operation

Continuous Operation

In continuous operation the separation is performed using two columns maintained at twodifferent pressure.

The distillate which approach the azeotropic composition at low and high pressure arerecycled between two columns.

Batch Operation

Batch operation single column is used for separation.

First feed charged to column operated at pressure P1 and bottom B is removed Collected distillate D1 approached to azeotropic composition.

Then, column is recharged with distillate D1 and operated at pressure P2 to get bottom A as pure product removed.

Semi Continuous Operation

In semi continuous operation single distillation column used but column operated continuously and periodically It is not recharged or emptied.

Liquids levels are maintained, reboiler and condenser in charged condition

The advantages is lower the investment cost and shorter downtime when the mixture to be separated is changed.

The setup for a pressure-swing distillationThe set-up for a pressure-swing distillation is shown in the Figure below.

The setup for a pressure-swing distillation The total feed F1 to Column T-1 operating at the lower pressure P1 is the sum of the fresh feed F at

mole fraction xF, and the recycled distillate D2 at mole fraction xD2 (close to azeotropic mole fractionxP2).

The mole fraction of the total feed F1 is xF1 , and it is richer in A than the azeotropic mole fraction ofxP1 at pressure P1.

The bottoms leaving column T-1 is almost pure A.

The distillate leaving T-1 is D1 , having the mole fraction xD1 . This mole fraction is richer in A than theazeotropic mole fraction of xP1, but less rich in A than the azeotrope mole fraction of xP2.

Distillate D1 is sent to Column T-2 as feed F2 . Nearly pure B is obtained from bottom of T-2, and thedistillate D2 is recycled to Column T-1.

The Phase DiagramThe changes on the phase diagram is shown the Figure below.

Acetone-Methanol Separation using Pressure Swing Distillation

Examples

aniline / octane

phenol / butyl acetate

methanol / ethyl acetate

propanol / toluene

acetic acid / toluene

acetic acid / toluene

acetone / methanol

ethanol / ethyl acetate

Advantages

Energy saving by heat integration of the system.

No need to add an entrainer or a solvent for theseparation.

Environment friendly.

Comparison With Extractive Distillation

Energy demand of ED is lower than PSD.

Higher recovery is achieved by ED than the process by the PSD.

ED needs much more operation steps than the PSD.

The control of the PSD is easier than that of the ED since the columns are operating practically insteady state.

The capital costs of the PSD (requiring two columns) are higher than that of the ED.

References

Introduction to Process Engineering and Design By S B Thakore & B I Bhatt

http://www.separationprocesses.com/Distillation/DT_Chp06g.htm

http://pubs.acs.org/doi/abs/10.1021/ie00001a047

http://www.sciencedirect.com/science/article/pii/S1570794605802421

http://chemicalengineering-rvr.blogspot.in/2012/05/pressure-swing-distillation.html