Classification of Enzyme Membrane Reactors

Membrane reactors became an option for the retention of biocatalysts when the processing of membrane materials had progressed sufficiently to control thickness and pore structure and to manufacture a membrane that was defect-free. Besides its function as a retainer the membrane also serves other functions such as (i) to stabilize the phase boundary in case of multi-phase reactions (ii) as a consequence of (i), to transport dissolved 02 preferentially over gaseous 02 and (iii) to support purification and sterilization of air and other nutrients in fermentations. 

Conventional filters, such as a coffee filter, termed depth filters , consist of a network of fibers and retain solute molecules through a stochastic adsorption mechanism. In contrast, most membranes for the retention of biocatalysts feature holes or pores with a comparatively narrow pore size distribution and separate exclusively on the basis of size or shape of the solute such membranes are termed membrane filters . Only membrane filters are approved by the FDA for sterilization in connection with processes applied to pharmaceuticals. Table 5.3 lists advantages and disadvantages of depth and membrane filters. 

Proteins can be viewed as hard spheres, and this property renders them easy to filter. Coiled polymers, on the other hand, can uncoil much more readily and thus pass through membrane pores with higher nominal cut-off. 

Membranes of interest in biocatalysis and biotechnology are divided into three categories  

microfiltration membranes (MF-membranes), with pore sizes in the range 0.1-10 pm (normally 0.1 or 0.22 pm), suitable for retention of cells (d 1 pm) but not enzymes or salts, 

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