8 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 concentration X. 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 I, but usually y is essentially 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 refined so the feed-rich liquid leaving the extractor is the raffinate(r) When the feed and solvent are brought together, the solute (a)will distribute itself between thetwoliquid phases. Atequilibrium, the ratio ofthis distribution is called the distribution coefficient(m)
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 extraction 349 Xa concentration of A in raffinate phase The distribution coefficient, m, is a measure of the affinity of the solute (A) for one phase(E, S)over the other phase(F, R). The concentration of A 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-free wate ight aceto Although the units of m appear to be dimensionless, they actually (weight acetone-free water /weight acetone-free toluene) If more than one solute is present, the preference, or selectivity, of the solvent for one(A)over the other( B)is the separation factor(a) aar The separation factor(aaB)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 of A 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
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 de Even though extraction may successfully remove the solute from the feed, a further separation is required in order to recover the solute fro 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 further 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 of chemi- cal differences, but where relative volatilities overlap 4. Where extraction selectivity is favorable inionic 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 distr The combinations ofmixing both feed and solvent until the equilibrium ibution 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)
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 temperature 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 ofchemical 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 separation 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 =ITHEORETICAL ING TO PHASE SEPARAT ION EQU IL1B Figure 1. Theoretical stage Feed Figure 2. Differential extraction Feed 心 Raffinal Figure 3. Sequential contact and separation
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 arying quantities of solute in a separatory funnel, 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 useful For later ease of calculation, it is desirable to express concentrations on a solute-free basis. If there is extensive miscibility, a termary plot(Fig. 5)would be preferable. Tie lines represent the equilibrium between the coexisting phases HAc Figure 4. Binary plot of distribution data Figure 5. Ternary plot of distribution data Plotting the data on log-log graphs may be helpful 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
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