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Concentration of Product - Some concentration of the product may occur during the extraction step. Further concentration may be achieved by

(i) evaporation,
(ii) membrane filtration,
(iii) ion exchange methods and
(iv) adsorption methods.

Evaporation
It is generally used in cases of solvent extraction, and used various devices, e.g., continuous flow evaporators, falling film evaporators, thin film evaporators, centrifugal thin film evaporators and spra dryers.

Efficient arrangements must be made for recovery of the evaporated solvent to reduce costs. For low grade products, often evaporation of the whole broth is undertaken using a spray-drier.

Membrane Filtration
It generally achieves both concentration and separation of the products usually based on the size of molecules. The different processes of membrane filtration are micro filtration, ultrafiltration, reverse osmosis and electrodialysis.

Micro and ultrafiltration work as sieves and separate molecules of different sizes, but reverse osmosis can separate molecules of similar size. Micro filtration can be used for cell separation as well.

Ion Exchange Resins
These are polymers having firmly attached ionizable groups (anions or cations), which ionize under a suitable environment. These may be solid, e.g., dextran, cellulose, polyamine, acrylate, etc., or liquid, e.g., a solvent carrying a functional group like phosphoric acid mono or diester, etc. Solid ion exchangers may be used in two ways:

(i) they may be packed in columns or
(ii) they may be added to the extract and removed by decantation.

Liquid ion exchangers dissolve only in nonaqueous solvent carrier and the separation is similar to liquid liquid extraction. Some antibiotics are recovered directly from the whole broth using ion exchange resins.
The product is recovered from the ion exchangers by ion displacement; this also regenerates the ion exchanger.

Adsorption Resins
These are porous polymers without ionization. Most compounds are adsorbed to the resins in nonionized state. The porosity of the resin determines the surface available for adsorption.
These resins may be apolar (e.g., styrene-divinyl beneze), polar (e.g., sulfoxide, amide, etc.), or semipolar (e.g., acrylic ester). The products are recovered from such resins by solvent (organic) extraction, changed pH, etc.

 

 
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