The document discusses extractive distillation, a method involving a high-boiling, non-volatile solvent to improve separation efficiency of close-boiling mixtures. It provides insights into solvent selection, industrial applications, and comparisons with azeotropic distillation, noting limitations such as economic viability for certain isomers. Examples illustrate how this technique enhances separation processes by altering relative volatilities, making it a valuable method in chemical engineering.

1. Presented by Shanjul Shrivastava
A brief introduction on

2. Contents
 Introduction
 Schematic
 Example of toluene –isooctane mixture
 Selection of Solvent
 Industrial examples
 Limitations
 Comparison with azeotropic distillation
 Example of ethanol- water modified process
 Design and Optimization
 Effect of Solvent
 Process using Solid Salt
 Process using combination of solid salt and liquid solvent
 Example in Everyday life
 References
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 2

3. Introduction
Extractive distillation is defined as distillation in the presence of a
miscible, high boiling, relatively non-volatile component, the solvent, that
forms no azeotrope with the other components in the mixture. The
Solvent is added to alter the relative volatilities of components in the
feed. The method is used for mixtures having a low value of relative
volatility, nearing unity.
Close boiling mixtures cannot be separated by simple distillation,
because the volatility of the two components in the mixture is nearly the
same, causing them to vaporize at nearly the same temperature at a
similar rate, making normal distillation impractical.
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 3

4. Schematic
The configuration consists of a double- feed
extractive column (1) & a solvent recovery
column (2).
The solvent is introduced into the extractive
column at a high concentration a few stages
below the condenser, but above the primary-
feed stage.
One of the components, A is withdrawn as an
essentially pure distillate stream. The bottoms
product, consisting of B and the solvent, is
sent to the recovery column. The distillate
from the recovery column is pure B, and the
solvent-bottoms product is recycled to the
extractive column.
Extractive Distillation Shanjul Shrivastava,MTech Chemical,HBTU4

5. Example
The separation of toluene (bp110.8 oC) from
iso-octane (bp 99.3 oC).
Addition of phenol (bp 181.4 oC) results in the
formation of phenol-toluene mixture that
leaves the extractive distillation column as
bottoms, while iso-octane is recovered as
overhead product.
The phenol-toluene mixture is further
separated in a second column whereby toluene
appears as distillate and the bottoms product,
phenol, is recycled back to the first column.
Extractive Distillation Shanjul Shrivastava,MTech Chemical,HBTU5

6. Example (contd.)
When the solvent is added to the original feed mixture it forms a
new mixture with one of the feed components by "absorbing" that
component.
This new mixture has a much higher boiling point than the other
feed component that is not absorbed so that it leaves as bottoms
product from the extractive distillation column. The unabsorbed
feed component then leaves as the overhead product.
Remark
The absence of azeotropes plus the fact that the solvent can be
recovered by simple distillation makes extractive distillation a less
complex and more widely useful process than azeotropic distillation.
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 6

7. Selection Of Solvent
The solvent selectively alters the activity coefficients of the components being separated. To
do this, a high concentration of solvent is necessary. Several features are essential:
Must be chosen to affect the liquid-phase behavior of the key components differently;
otherwise, no enhancement in separability will occur.
Must be higher-boiling than the key components of the separation and must be relatively non-
volatile in the extractive column, in order to remain largely in the liquid phase.
Should not form additional azeotropes with the components in the mixture to be separated.
The extractive column must be a double-feed column, with the solvent feed above the primary
feed. The column must have an extractive section (middle section) between the rectifying
section and the stripping section.
Inexpensive and easily available.
Stable at operating condition of both of column.
Easily separable from the bottom product.
Non-corrosive and non-toxic.
Non-reactive with feed component.
Higher boiling than the key components of the separation.
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 7

8. Industrial uses of extractive distillation
Extractive Distillation Shanjul Shrivastava,MTech Chemical,HBTU8

9. Limitations of Extractive Distillation
Only applicable to systems in which the components to be
separated contain one or more different functional groups.
Extractive distillation is usually uneconomical for separating
stereoisomers, homologs, or structural isomers containing the
same functional groups.
One such counterexample is the separation of ethanol from
isopropanol, where the addition of methyl benzoate raises the
relative volatility from 1.09 to 1.2
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 9

10. Comparison between Azeotropic distillation and Extractive distillation
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 10
Many of the entrainers used in azeotropic distillation are either proven or
suspected carcinogens or otherwise classified as hazardous pollutants. Using
the example of ethanol-water, noted that benzene is the entrainer. Benzene is
too hazardous for various reasons ranging from workplace to product to
environmental safety. An alternative to recover ethanol is to use extractive
distillation. The solvent used is Propylene Glycol.
Health Hazard

11. Effect of Solvent
With some solvents, the key component with the lower pure-component boiling point
in the original mixture will be recovered in the distillate as in ordinary distillation. For
another solvent, the expected order may be reversed. The possibility that the expected
relative volatility may be reversed by the addition of solvent is entirely a function of the
way the solvent interacts with and modifies the activity coefficients and, thus, the
volatility of the components in the mixture.
In normal applications of extractive distillation (i.e., pinched, close-boiling, or
azeotropic systems), the relative volatilities between the light and heavy key
components will be unity or close to unity. Assuming an ideal vapor phase and
subcritical components-
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 11

12. Effect of Solvent
 Relative volatility is enhanced when the ϒ of the lower-boiling pure component is
increased by the solvent addition (ϒL/ϒH increases and PL
sat/PH
sat > 1 ). In this case,
the lower-boiling pure component will be recovered in the distillate as expected.
 For the higher-boiling pure component to be recovered in the distillate, the addition
of the solvent must decrease the ratio ϒL/ ϒH such that the product of ϒL/ ϒH and PL
sat / PH
sat (that is, a L H) < 1 .Generally, the latter is more difficult to achieve and
requires higher solvent-to-feed ratios. It is normally better to select a solvent that
forces the lower-boiling component overhead.
 The Selectivity of solvents can be expressed as:
S ij = (α ij )T /(α ij )B
where
(α ij ) T is relative volatility for ternary mixture
(α ij ) B is relative volatility for binary mixture
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 12

13. EXTRACTIVE DISTILLATION BY SOLID SALT
In systems where solubility permits, it is feasible
to use a solid salt dissolve into the liquid phase,
rather than a liquid additive.
The salt must be adequately soluble in both
components throughout the range of
compositions in the column. Since the salt is
completely non-volatile, it remains in the liquid
phase on each tray and alters the relative volatility
throughout the length of the column.
No rectification section is needed above the salt
feed. The bottoms product is recovered from the
salt solution by evaporation or drying, and the salt
is recycled.
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 13

14. EXTRACTIVE DISTILLATION BY SOLID SALT
Applications
These include acetate salts to produce absolute ethanol, magnesium nitrate for the
production of concentrated nitric acid as an alternative to the sulfuric acid solvent
process, and calcium chloride to produce anhydrous hydrogen chloride. One problem
limiting the consideration of salt extractive distillation is the fact that the performance
and solubility of a salt in a particular system are difficult to predict without experimental
data. Some recent advances have been made in modeling the VLE behavior of organic
aqueous salt solutions using modified UNIFAC.
Advantages and Disadvantages
The difficulty of transport and metering of solid or saturated salt solution.
Slow mixing or less dissolution rate of the salt and limits to solubility in feed
component.
The most effective advantages of salt can be found, Energy usage, equipment size,
capital investment and ultimate separation cost can be reduced.
The performance and solubility of a salt in a particular system are difficult to predict
without experimental data.
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 14

15. THE COMBINATION OF LIQUID AND SOLID SALT
Separation of C4 Mixture
When acetonitrile (ACN) is regarded as an basic solvent, organic solvents including water
and ethylenediamine are added to it. It is found that a little of solid salt added to ACN can
effectively improve the relative volatility and the effect of solid salt is close to water and
stronger than ethylenediamine. N,N-dimethylformamide (DMF) is another solvent
commonly used to separate C4 mixture.
Advantages and Disadvantages
It integrates the advantages of liquid solvent (easy operation) and solid salt (high
separation ability). Whether it is of separating polar systems or non-polar systems,
extractive distillation with the combination of liquid solvent and solid salt may be feasible.
Unfortunately, many solid salts are corrosive to the equipment and easies to decompose
at high temperature. On the other hand, since the amount of solid salts added to the
liquid solvents is often small, the role of solid salts in improving the separation ability is
doomed to be limited. Besides, liquid solvents are volatile, which inevitably pollutes the
top product of the extractive distillation column.
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 15

16. Extractive Distillation In Everyday Life
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 16
When you get gum stuck in your
hair it is difficult to separate the
gum from the individual hair
strands. BUT, if you introduce a
third substance like vegetable oil
or peanut butter it changes the
properties and consistency of the
gum, making it easier to extract
from you beautiful head of hair.

17. References
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 17
1. J. H. Perry, Perry’s chemical engineer’s handbook, 8 th ed., p.p.:-
1 3- 88 to 93, McGraw-Hill.
2. Zhigang Lei, Chengyue Lie and Biaohua Chen, Separation and
purification review-vol.-32, no.2, 2003.
3. S.B.Thakore & B.I.Bhatt, process engineering and design p.p.:–
519, Tata-Mc Graw-Hill.
4. Robert Treybal, Mass-transfer operations, p.p.:-457, Mc Graw-
Hill
5. Separationprocesses.com
6. Researchgate.com
7. Slideshare.net

18. Thank You
Shanjul Shrivastava,MTech Chemical,HBTUExtractive Distillation 18