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 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)