Heterogeneous reaction systems, transient analysis of enzyme reactors. Process design and operational strategies of immobilized enzyme reactors.

The most common definition for immobilized enzymes is that proposed by Katchalski-Katzir in the 1960s: -“Enzymes physically confined or localized in a certain defined region of space with retention of their catalytic activities, which can be used repeatedly and continuously.” According to this definition, three types of immobilized enzymes can be distinguished:

1. Heterogenization of the soluble enzyme by coupling to an insoluble support by adsorption or covalent binding, by cross-linking of the enzyme or entrapment in a lattice or in microcapsules such as alginate Beads 2. Retention of the enzyme by means of ultrafiltration Membranes 3. Use of whole cells for biotransformations using their enzyme apparatus

Membrane reactors have been used quite recently as have applications of whole cell processes. Retention of the cells within the reactor may be achieved by membrane separation or by the same immobilization methods that are used for isolated enzymes. In principle, the cell itself can be regarded as a form of native immobilization of enzymes. Biosensors are a very special form of carrier-fixed biocatalysts.

The major goal behind immobilization is the recovery of the biocatalysts, separation from products and reactants, and subsequent reuse -in batch or continuous processes. Subsequent reuse in batch or continuous processes is especially important for a reduction of the catalyst costs. Enzymes immobilized on a support often show enhanced stability when compared with the soluble form.

1. Additional costs for support and chemicals performing the immobilization have to be balanced against the increase of stability. 2. Loss of activity during the immobilization step. 3. When the catalyst is immobilized only by adsorption or entrapment without covalent attachment, its leakage from the carrier support has to be examined and compared with the overall deactivation rate. When considering the use of soluble or carrier-fixed enzymes, the following topics we have to be addressed:

4. Mass transfer limitations for enzymes on a support may cause problems when adjusting of the pH is necessary during the reaction. 5. With soluble enzymes, higher volumetric activities at high catalyst concentrations are possible, enabling conversion of poor substrates at reasonable rates. 6. Whereas membrane reactors can be easily sterilized before use, this is not possible for reactors with carrier-fixed enzymes. To prevent microbial contamination, these processes are quite often operated at higher temperatures.

7. When enzymes are to be used together with organic solvents to increase reactant or product solubility or to alter their kinetics, it may become necessary to immobilize them on a support The support will at the same time act as a water pool to maintain the enzymatic activity. In such systems, water-insoluble organic solvents have less effect on the enzyme stability than water- soluble solvents.

Process design and operational strategies of immoblized enzyme reactors The final decision for a certain reactor design should be based on an optimization process covering all relevant factors contributing to the overall costs, including investment, catalyst consumption, or productivity.

Comparison of Processes Using Soluble or Carrier-Fixed Enzymes