Uck

In acid conditions this ionization is suppressed and the

Extract Extract Extract

Fig. 10.25. Diagram of a co-current flow extraction system.

Extract Extract Extract

Fig. 10.25. Diagram of a co-current flow extraction system.

Solvent enriched with product

Fig. 10.26. Diagram of a counter-current extraction system.

Solvent enriched with product

Fig. 10.26. Diagram of a counter-current extraction system.

penicillin is more soluble in organic solvents. At pH 2 to 3 the distribution ratio of total acid will be

For penicillin this value may be as high as 40 in a suitable solvent (Podbielniak et al., 1970). The penicillin extraction process may involve the four following stages:

1. Extraction of the penicillin G from the filtered broth into an organic solvent (amyl or butyl acetate or methyl iso-butyl ketone).

2. Extraction from the organic solvent into an aqueous buffer.

3. Extraction from aqueous buffer into organic solvent.

4. Extraction of the solvent to obtain the penicillin salt.

At each extraction stage progressively smaller volumes of extractant are used to achieve concentration of the penicillin (see also Fig. 10.2). Unfortunately, penicillin G has a half-life of 15 minutes at pH 2.0 at 20°. The harvested broth is therefore initially cooled to 0° to 3°. The cooled broth is then acidified to pH 2 to 3 with sulphuric or phosphoric acid immediately before extraction. This acidified broth is quickly passed through

in and out.

a Podbielniak centrifugal counter-current extractor using about 20% by volume of the solvent in the counter flow. Ideally, the hold-up time should be about 60 to 90 seconds. The penicillin-rich solvent then passes through a second Podbielniak extractor counter-current to an aqueous NaOH or KOH solution (again about 20% by volume) so that the penicillin is removed to the aqueous phase (pH 7.0 to 8.0) as the salt.

These two stages may be sufficient to concentrate the penicillin adequately from a broth with a high titre. Penicillin will crystallize out of aqueous solution at a concentration of approximately 1.5 X 106 units cm~3. If the broth harvested initially contains 60,000 units cm \ and two five-fold concentrations are achieved in the two extraction stages, then the penicillin liquor should crystallize. If the initial broth titre is lower than 60,000 units cm-3 or the extractions are not so effective, the solvent and buffer extractions will have to be repeated. At each stage the spent liquids should be checked for residual penicillin and solvent usage carefully monitored. Since the solvents are expensive and their disposal is environmentally sensitive they are recovered for recirculation through the extraction process. The success of a process may depend on efficient solvent recovery and reuse.

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