Emulsion liquid membranes continuous operations

Liquid membrane (LM) separation provides a potentially powerful technique for effecting diverse separation operations. Compared to conventional processes, emulsion liquid membrane (ELM) and liquid surfactant membrane (LSM) processes have some attractive features, for example, simple operation, high efficiency, extraction and stripping in one stage, larger interfacial area, scope of continuous operation. The ELM technique has great potential for recovery and removal of different metal ions and hydrocarbons from wastewater where conventional methods provide lower separation efficiency. 

For the successful preparation of emulsions, the wetting conditions on the membrane surface are crucial. It is necessary that the membrane surface is only wetted by the liquid that forms the continuous phase. The droplet size correlates with the membrane pore size by a simple relation, Dd = /Dm where / is a value typically between 2 and 8 (35). Droplets can be produced with diameters in the pm-, as well as in the sub-micrometre range. This technique has been successfully applied to produce monodisperse emulsions and multiple emulsions, as well as to carry out polymerizations leading to polymer particle in the pm size range with narrow size distributions (36, 37). Further advantages (38) are as follows the droplet size is controllable and generally a quite narrow DSD can be achieved, the method is reproducible and the scale-up is easy just by increasing the number of membrane modules, the characteristic features are independent of scale-up, batch as well as continuous operations modes are possible, the continuous phase is exposed to a lower stress. 

The organic phase may also be used as a substrate reservoir, besides their use for product stripping from the aqueous phase. The effectiveness of membrane-assisted organic-aqueous two-phase bioconversions relative to direct-contact two-phase emulsion reactors was demonstrated by Westgate et al. [150]. These authors observed a fivefold increase in the maximum specific activity of hydrolysis of menthyl acetate catalyzed by B. subtilis cells when a 0.2 pm nylon flat membrane reactor was used, as compared to an emulsion reactor. This result was attributed to a continuous interfacial contact, which could only be achieved in an emulsion bioreactor at the cost of high power inputs. Doig and co-workers operated a dense membrane bioreactor for the production of citronellol from geraniol with a product accumulation rate similar to the one obtained in an emulsion reactor [124]. Some examples of membrane-assisted two-liquid phase bio-conversions/fermentations are presented in Table 9. 

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