Mass Transfer Step By Producing Multiple Emulsion

Figure 8. Schematic representation of an emulsion liquid membrane system mixture into which the required concentration of carrier may be added to facilitate the solute transport. The stripping phase is a solution which forms the inner encapsulated phase (e.g., inner aqueous phase in W/O/W multiple emulsion). The inner aqueous phase is emulsified with the oil phase containing a lipophilic surfactant to obtain a W/O emulsion. The emulsion thus obtained is dispersed in the outer aqueous feed phase to get a W/O/W multiple emulsion system.

The two aqueous phases can not physically contact each other and the solute is transported from the outer feed phase to the inner stripping phase droplets through the oil membrane. The reaction in the inner phase prevents the solute from diffusing back across the membrane. The ELM extraction can be used to completly remove the solute from the feed to the stripping phase.

After the extraction is complete, the emulsion layer is separated from the outer aqueous layer. After drainage of the bottom aqueous layer, the emulsion is broken.

The demulsification may be accomplished chemically, electrostatically, thermally, acoustically or mechanically. The inner aqueous phase is utilized to obtain the solute. Selectivity is obtained by choosing the carrier in the membrane phase. Phosphorous containing compounds and secondary and tertiary amines have been used as carriers.

Emulsion swelling, because of the water transport, causes membrane rupture and this results in poorer extraction. Mukkolath et al (1990) have suggested ways to reduce emulsion swelling.

7.2. Extraction equipment

The extraction equipment for contact of emulsion phase and the feed phase can be a continuous contactor or a battery of mixer-settlers. A recycle arrangement for the oil phase is necessary to conserve the oil and the carrier after the oil phase is separated in a demulsification unit.

7.3. Encapsulation of enzymes

It is often necessary to protect enzymes from deactivating substances while maintaining free access to the desired substrate. In addition, it is frequently useful to "immobilize'' enzymes in order to enhance rates and gain additional control over enzymatic reactions. ELM systems have shown considerable potential for achieving some or all of these objectives (Mohan and Li, 1975). One of the first successful experiments in immobilizing enzymes by ELM encapsulation was performed by May and Li (1974). They encapsulated purified phenolase and used it for phenol oxidation. Frankenfeld and Li (1982) have reviewed the encapsulation of enzymes.

7.4. Product recovery from fermentation broth

Several important categories of biochemicals classified as zwitterions, namely, phospholipids, amino acids and beta-lactam antibiotics are potential candidates which could be separated from the fermentation broth by ELM. Due to the charge on these compounds, their solubility is greatly decreased in coventional organic solvents. As an alternative to the currently used techniques of derivatization followed by extraction or ion exchange, ELM has been examined for the economical recovery. A comprehensive examination of L-phenylalanine recovery by ELM has been reported by Thein et al (1986) and Itoh et al (1990).

Conventional recovery methods for carboxylic acids from fermentation broth require precipitation of calcium salt followed by dissolution in sulphuric acid. This is followed by treatment with active carbon, evaporation in multiple effect evaporator, and vacuum crystallization. ELM as an alternative strategy for recovery has been proposed by Wennersten (1983). Recovery of citric acid using ELM has been studied by Boey et al (1987). Alamine 336 was incorporated as a carrier in the membrane phase to obtain faster rate of extraction with the carrier-mediated transport. In the case of lactic acid, Boey et al (1987) used similar systems. Chaudhari and Pyle (1990) and Mukkolath et al (1990) have studied the recovery of lactic acid and the latter used trioctyl amine as carrier and sodium carbonate and bicarbonate as stripping agent. Itoh et al (1990) have studied the recovery of amino acids. Antony (1993) has exhaustively studied the extraction of penicillin-G using ELM with secondary and tertiary amines as carriers.

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