SOLVENT EXTRACTION Notes | EduRev

: SOLVENT EXTRACTION Notes | EduRev

 Page 1


Solvent Extraction 
David B. Todd 
1.0 EXTRACTION CONCEPTS 
Liquid-liquid extraction is a unit operation frequently employed in the 
pharmaceutical industry, as in many others, for recovery and purification of 
a desired ingredient from the solution in which it was prepared. Extraction 
may also be used to remove impurities from a feed stream. 
Extraction is the removal of a soluble constituent from one liquid into 
another. By convention, the first liquid is the feed (F) which contains the 
solute at an initial concentrationXf The second liquid is the solvent (S) which 
is at least partially immiscible with the feed. The solvent may also have some 
solute present at an initial concentration of x, but usually < is essentially 
zero. 
The solvent does the extraction, so the solvent-rich liquid leaving the 
extractor is the extract (E). With the solute partially or completely removed 
from the feed, the feed has become rejned so the feed-rich liquid leaving the 
extractor is the raflnate (R). 
When the feed and solvent are brought together, the solute (A) will 
distribute itselfbetween the two liquid phases. At equilibrium, the ratio ofthis 
distribution is called the distribution coeficient (m): 
348 
Page 2


Solvent Extraction 
David B. Todd 
1.0 EXTRACTION CONCEPTS 
Liquid-liquid extraction is a unit operation frequently employed in the 
pharmaceutical industry, as in many others, for recovery and purification of 
a desired ingredient from the solution in which it was prepared. Extraction 
may also be used to remove impurities from a feed stream. 
Extraction is the removal of a soluble constituent from one liquid into 
another. By convention, the first liquid is the feed (F) which contains the 
solute at an initial concentrationXf The second liquid is the solvent (S) which 
is at least partially immiscible with the feed. The solvent may also have some 
solute present at an initial concentration of x, but usually < is essentially 
zero. 
The solvent does the extraction, so the solvent-rich liquid leaving the 
extractor is the extract (E). With the solute partially or completely removed 
from the feed, the feed has become rejned so the feed-rich liquid leaving the 
extractor is the raflnate (R). 
When the feed and solvent are brought together, the solute (A) will 
distribute itselfbetween the two liquid phases. At equilibrium, the ratio ofthis 
distribution is called the distribution coeficient (m): 
348 
Solvent Exlraction 349 
m=yA= concentration of A inextract phase 
X, 
concentration of A in raflinate phase 
The distributioncoefficient, m, is a measure ofthe affinity ofthe solute 
(A) for one phase (E, 5') over the other phase (E R). The concentration ofA 
may be expressed in various units, but for ease of subsequent calculations, 
it is preferable to express the concentration on a solute-free basis for both 
phases. For example, in the extraction of acetone from water with toluene: 
weight acetone 
weight acetonefree water 
X= 
weight acetone 
weight acetonefree toluene 
Y= 
Although the units of m appear to be dimensionless, they actually are 
If more than one solute is present, the preference, or selectivity, of the 
(weight acetone-free water)/(weight acetone-free toluene). 
solvent for one (A) over the other (B) is the separation factor (a). 
The separation factor (arn) must be greater than unity in order to 
separate A from B by solvent extraction, just as the relative volatility must be 
greater than unity to separate A from B by distillation. 
The analogy with distillation can be carried a step further. The extract 
phase is like the vapor distillate, a second phase wherein the equilibrium 
distribution ofA with respect to B is higher than it is in the feed liquid (liquid 
bottoms). 
Extraction requires that the solvent and feed liquor be at least partially 
immiscible (two liquid phases), just as distillation requires both a vapor and 
a liquid phase. 
Page 3


Solvent Extraction 
David B. Todd 
1.0 EXTRACTION CONCEPTS 
Liquid-liquid extraction is a unit operation frequently employed in the 
pharmaceutical industry, as in many others, for recovery and purification of 
a desired ingredient from the solution in which it was prepared. Extraction 
may also be used to remove impurities from a feed stream. 
Extraction is the removal of a soluble constituent from one liquid into 
another. By convention, the first liquid is the feed (F) which contains the 
solute at an initial concentrationXf The second liquid is the solvent (S) which 
is at least partially immiscible with the feed. The solvent may also have some 
solute present at an initial concentration of x, but usually < is essentially 
zero. 
The solvent does the extraction, so the solvent-rich liquid leaving the 
extractor is the extract (E). With the solute partially or completely removed 
from the feed, the feed has become rejned so the feed-rich liquid leaving the 
extractor is the raflnate (R). 
When the feed and solvent are brought together, the solute (A) will 
distribute itselfbetween the two liquid phases. At equilibrium, the ratio ofthis 
distribution is called the distribution coeficient (m): 
348 
Solvent Exlraction 349 
m=yA= concentration of A inextract phase 
X, 
concentration of A in raflinate phase 
The distributioncoefficient, m, is a measure ofthe affinity ofthe solute 
(A) for one phase (E, 5') over the other phase (E R). The concentration ofA 
may be expressed in various units, but for ease of subsequent calculations, 
it is preferable to express the concentration on a solute-free basis for both 
phases. For example, in the extraction of acetone from water with toluene: 
weight acetone 
weight acetonefree water 
X= 
weight acetone 
weight acetonefree toluene 
Y= 
Although the units of m appear to be dimensionless, they actually are 
If more than one solute is present, the preference, or selectivity, of the 
(weight acetone-free water)/(weight acetone-free toluene). 
solvent for one (A) over the other (B) is the separation factor (a). 
The separation factor (arn) must be greater than unity in order to 
separate A from B by solvent extraction, just as the relative volatility must be 
greater than unity to separate A from B by distillation. 
The analogy with distillation can be carried a step further. The extract 
phase is like the vapor distillate, a second phase wherein the equilibrium 
distribution ofA with respect to B is higher than it is in the feed liquid (liquid 
bottoms). 
Extraction requires that the solvent and feed liquor be at least partially 
immiscible (two liquid phases), just as distillation requires both a vapor and 
a liquid phase. 
350 Fermentation and Biochemical Engineering Handbook 
Extraction requires that the solvent and feed phases be of different 
densities, 
Even though extraction may successfilly remove the solute from the 
feed, a fbrther separation is required in order to recover the solute from the 
solvent, and to make the solvent suitable for reuse in the extractor. This 
recovery may be by any other unit operation, such as distillation, evaporation, 
crystallization and filtration, or by krther extraction. 
Extraction is frequently chosen as the desired primary mode of 
separation or purification for one or more of the following reasons: 
1. Where the heat of distillation is undesirable or the tem- 
perature would be damaging to the product (for example, 
in the recovery of penicillin from filtered broth). 
2. Where the solute is present in low concentration and the 
bulk feed liquor would have to be taken overhead (most 
fermentation products). 
3. Where extraction selectivity is favorable because ofchemi- 
cal differences, but where relative volatilities overlap. 
4. Where extraction selectivity is favorable in ionic form, but 
not in the natural state (such as citric acid). 
5. Where a lower form or less energy can be used. The latent 
heat of most organic solvents is less than 20% that of 
water, so recovery of solute from an organic extract may 
require far less energy than recovery from an aqueous 
feed. 
1.1 Theoretical Stage 
The combinations of mixing both feed and solvent until the equilibrium 
distribution of the solute has occurred, and the subsequent complete separa- 
tion of the two phases is defined as one theoretical stage (Fig. 1). The two 
functions may be carried out sequentially in the same vessel, simultaneously 
in two different zones of the same vessel, or in separate vessels (mixers and 
settlers). 
Extraction may also be performed in a continuous differential fashion 
(Fig. 2), or in a sequential contact and separation where the solvent and feed 
phases flow countercurrently to each other between stages (Fig. 3). 
Page 4


Solvent Extraction 
David B. Todd 
1.0 EXTRACTION CONCEPTS 
Liquid-liquid extraction is a unit operation frequently employed in the 
pharmaceutical industry, as in many others, for recovery and purification of 
a desired ingredient from the solution in which it was prepared. Extraction 
may also be used to remove impurities from a feed stream. 
Extraction is the removal of a soluble constituent from one liquid into 
another. By convention, the first liquid is the feed (F) which contains the 
solute at an initial concentrationXf The second liquid is the solvent (S) which 
is at least partially immiscible with the feed. The solvent may also have some 
solute present at an initial concentration of x, but usually < is essentially 
zero. 
The solvent does the extraction, so the solvent-rich liquid leaving the 
extractor is the extract (E). With the solute partially or completely removed 
from the feed, the feed has become rejned so the feed-rich liquid leaving the 
extractor is the raflnate (R). 
When the feed and solvent are brought together, the solute (A) will 
distribute itselfbetween the two liquid phases. At equilibrium, the ratio ofthis 
distribution is called the distribution coeficient (m): 
348 
Solvent Exlraction 349 
m=yA= concentration of A inextract phase 
X, 
concentration of A in raflinate phase 
The distributioncoefficient, m, is a measure ofthe affinity ofthe solute 
(A) for one phase (E, 5') over the other phase (E R). The concentration ofA 
may be expressed in various units, but for ease of subsequent calculations, 
it is preferable to express the concentration on a solute-free basis for both 
phases. For example, in the extraction of acetone from water with toluene: 
weight acetone 
weight acetonefree water 
X= 
weight acetone 
weight acetonefree toluene 
Y= 
Although the units of m appear to be dimensionless, they actually are 
If more than one solute is present, the preference, or selectivity, of the 
(weight acetone-free water)/(weight acetone-free toluene). 
solvent for one (A) over the other (B) is the separation factor (a). 
The separation factor (arn) must be greater than unity in order to 
separate A from B by solvent extraction, just as the relative volatility must be 
greater than unity to separate A from B by distillation. 
The analogy with distillation can be carried a step further. The extract 
phase is like the vapor distillate, a second phase wherein the equilibrium 
distribution ofA with respect to B is higher than it is in the feed liquid (liquid 
bottoms). 
Extraction requires that the solvent and feed liquor be at least partially 
immiscible (two liquid phases), just as distillation requires both a vapor and 
a liquid phase. 
350 Fermentation and Biochemical Engineering Handbook 
Extraction requires that the solvent and feed phases be of different 
densities, 
Even though extraction may successfilly remove the solute from the 
feed, a fbrther separation is required in order to recover the solute from the 
solvent, and to make the solvent suitable for reuse in the extractor. This 
recovery may be by any other unit operation, such as distillation, evaporation, 
crystallization and filtration, or by krther extraction. 
Extraction is frequently chosen as the desired primary mode of 
separation or purification for one or more of the following reasons: 
1. Where the heat of distillation is undesirable or the tem- 
perature would be damaging to the product (for example, 
in the recovery of penicillin from filtered broth). 
2. Where the solute is present in low concentration and the 
bulk feed liquor would have to be taken overhead (most 
fermentation products). 
3. Where extraction selectivity is favorable because ofchemi- 
cal differences, but where relative volatilities overlap. 
4. Where extraction selectivity is favorable in ionic form, but 
not in the natural state (such as citric acid). 
5. Where a lower form or less energy can be used. The latent 
heat of most organic solvents is less than 20% that of 
water, so recovery of solute from an organic extract may 
require far less energy than recovery from an aqueous 
feed. 
1.1 Theoretical Stage 
The combinations of mixing both feed and solvent until the equilibrium 
distribution of the solute has occurred, and the subsequent complete separa- 
tion of the two phases is defined as one theoretical stage (Fig. 1). The two 
functions may be carried out sequentially in the same vessel, simultaneously 
in two different zones of the same vessel, or in separate vessels (mixers and 
settlers). 
Extraction may also be performed in a continuous differential fashion 
(Fig. 2), or in a sequential contact and separation where the solvent and feed 
phases flow countercurrently to each other between stages (Fig. 3). 
Solvent Extraction 351 
STAGE 
MIXING TO PHlSE SEPARATION 
ECUlLlBRlW TO EWlLlBRlUl 
Figure 1. Theoretical stage. 
Extract 
r-+ 
Feed 
L 
Raffinate 
Figure 2. Differential extraction. 
Feed 
Figure 3. Sequential contact and separation. 
Page 5


Solvent Extraction 
David B. Todd 
1.0 EXTRACTION CONCEPTS 
Liquid-liquid extraction is a unit operation frequently employed in the 
pharmaceutical industry, as in many others, for recovery and purification of 
a desired ingredient from the solution in which it was prepared. Extraction 
may also be used to remove impurities from a feed stream. 
Extraction is the removal of a soluble constituent from one liquid into 
another. By convention, the first liquid is the feed (F) which contains the 
solute at an initial concentrationXf The second liquid is the solvent (S) which 
is at least partially immiscible with the feed. The solvent may also have some 
solute present at an initial concentration of x, but usually < is essentially 
zero. 
The solvent does the extraction, so the solvent-rich liquid leaving the 
extractor is the extract (E). With the solute partially or completely removed 
from the feed, the feed has become rejned so the feed-rich liquid leaving the 
extractor is the raflnate (R). 
When the feed and solvent are brought together, the solute (A) will 
distribute itselfbetween the two liquid phases. At equilibrium, the ratio ofthis 
distribution is called the distribution coeficient (m): 
348 
Solvent Exlraction 349 
m=yA= concentration of A inextract phase 
X, 
concentration of A in raflinate phase 
The distributioncoefficient, m, is a measure ofthe affinity ofthe solute 
(A) for one phase (E, 5') over the other phase (E R). The concentration ofA 
may be expressed in various units, but for ease of subsequent calculations, 
it is preferable to express the concentration on a solute-free basis for both 
phases. For example, in the extraction of acetone from water with toluene: 
weight acetone 
weight acetonefree water 
X= 
weight acetone 
weight acetonefree toluene 
Y= 
Although the units of m appear to be dimensionless, they actually are 
If more than one solute is present, the preference, or selectivity, of the 
(weight acetone-free water)/(weight acetone-free toluene). 
solvent for one (A) over the other (B) is the separation factor (a). 
The separation factor (arn) must be greater than unity in order to 
separate A from B by solvent extraction, just as the relative volatility must be 
greater than unity to separate A from B by distillation. 
The analogy with distillation can be carried a step further. The extract 
phase is like the vapor distillate, a second phase wherein the equilibrium 
distribution ofA with respect to B is higher than it is in the feed liquid (liquid 
bottoms). 
Extraction requires that the solvent and feed liquor be at least partially 
immiscible (two liquid phases), just as distillation requires both a vapor and 
a liquid phase. 
350 Fermentation and Biochemical Engineering Handbook 
Extraction requires that the solvent and feed phases be of different 
densities, 
Even though extraction may successfilly remove the solute from the 
feed, a fbrther separation is required in order to recover the solute from the 
solvent, and to make the solvent suitable for reuse in the extractor. This 
recovery may be by any other unit operation, such as distillation, evaporation, 
crystallization and filtration, or by krther extraction. 
Extraction is frequently chosen as the desired primary mode of 
separation or purification for one or more of the following reasons: 
1. Where the heat of distillation is undesirable or the tem- 
perature would be damaging to the product (for example, 
in the recovery of penicillin from filtered broth). 
2. Where the solute is present in low concentration and the 
bulk feed liquor would have to be taken overhead (most 
fermentation products). 
3. Where extraction selectivity is favorable because ofchemi- 
cal differences, but where relative volatilities overlap. 
4. Where extraction selectivity is favorable in ionic form, but 
not in the natural state (such as citric acid). 
5. Where a lower form or less energy can be used. The latent 
heat of most organic solvents is less than 20% that of 
water, so recovery of solute from an organic extract may 
require far less energy than recovery from an aqueous 
feed. 
1.1 Theoretical Stage 
The combinations of mixing both feed and solvent until the equilibrium 
distribution of the solute has occurred, and the subsequent complete separa- 
tion of the two phases is defined as one theoretical stage (Fig. 1). The two 
functions may be carried out sequentially in the same vessel, simultaneously 
in two different zones of the same vessel, or in separate vessels (mixers and 
settlers). 
Extraction may also be performed in a continuous differential fashion 
(Fig. 2), or in a sequential contact and separation where the solvent and feed 
phases flow countercurrently to each other between stages (Fig. 3). 
Solvent Extraction 351 
STAGE 
MIXING TO PHlSE SEPARATION 
ECUlLlBRlW TO EWlLlBRlUl 
Figure 1. Theoretical stage. 
Extract 
r-+ 
Feed 
L 
Raffinate 
Figure 2. Differential extraction. 
Feed 
Figure 3. Sequential contact and separation. 
352 Fermentation and Biochemical Engineering Handbook 
2.0 DISTRIBUTION DATA 
Although data for many systems are available in the literature,['] in 
many cases it will be necessary for the engineer to obtain the distribution 
information for his own specific application. 
The simplest method is to mix solvent and feed liquors containing 
varying quantities of solute in a separatory bel, and analyze each phase for 
solute after settling. Where feed and solvent are essentially immiscible, the 
binary plot, such as shown in Fig. 4, is usefbl. For later ease of calculation, 
it is desirable to express concentrations on a solute-free basis. If there is 
extensive miscibility, a ternary plot (Fig. 5) would be preferable. Tie lines 
represent the equilibrium between the coexisting phases. 
&I-& 
gw 
X 
Figure 4. Binary plot of distribution data. 
Figure 5. Ternary plot of distribution data. 
Plotting the data on log-log graphs may be helphl in understanding 
some of the underlying phenomena and interpolating or extrapolating meager 
data. An example is shown in Fig. 6 for the distribution of phenol between 
water and various chlorinated methanes. In the dilute region, the limiting 
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