Pressure Swing Distillation,operation,advantages,comparison with extractive distillation,examples

1. Pressure Swing
Distillation
Prepared By: Chauhan Sunny

2. 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

3. 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)

4. 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 point
or forming azeotrope.
Some binary azeotrope mixtures lose azeotropic behaviour when the system pressure is
changed.

5. Operation
Three types of operation mode in Pressure-swing Distillation:
1. Continuous Operation
2. Batch Operation
3. Semi Continuous Operation

6. Continuous Operation
 In continuous operation the separation is performed using two columns maintained at two
different pressure.
 The distillate which approach the azeotropic composition at low and high pressure are
recycled between two columns.

7. 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.

8. 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.

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

10. 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 fraction
xP2).
 The mole fraction of the total feed F1 is xF1 , and it is richer in A than the azeotropic mole fraction of
xP1 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 the
azeotropic 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 the
distillate D2 is recycled to Column T-1.

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

12. Acetone-Methanol Separation using Pressure Swing Distillation

13. Examples
aniline / octane
phenol / butyl acetate
methanol / ethyl acetate
propanol / toluene
acetic acid / toluene
acetic acid / toluene
acetone / methanol
ethanol / ethyl acetate

14. Advantages
 Energy saving by heat integration of the system.
 No need to add an entrainer or a solvent for the
separation.
 Environment friendly.

15. 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 in
steady state.
 The capital costs of the PSD (requiring two columns) are higher than that of the ED.

16. 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