Immobilized enzyme batch membrane reactor

Figures 7.21 and 7.22 show typical results for the two cases-with enzymatic gel layer formation and when soluble enzymes are confined only near the membrane surface. Comprehensive models for an immobilized enzyme batch membrane reactor (IEMR) and for a soluble enzyme batch membrane reactor (SEMR) are proposed in References 33 and 30, respectively, for a flat slab membrane configuration.

Enzymes, when immobilized in spherical particles or in films made from various polymers and porous materials, are referred to as immobUized enzymes. Enzymes can be immobilized by covalent bonding, electrostatic interaction, crosslinking of the enzymes, and entrapment in a polymer network, among other techniques. In the case of batch reactors, the particles or films of immobilized enzymes can be reused after having been separated from the solution after reaction by physical means, such as sedimentation, centrifugation, and filtration. Immobilized enzymes can also be used in continuous fixed-bed reactors, fluidized reactors, and membrane reactors. 

The hydrolysis of an IV-acylated amino acid by an enzyme provides a resolution method to amino acids. Because the starting materials are readily available in the racemic series by the Schotten-Baumann reaction, the method can be cost effective (Scheme 2.21).68-71 The L-amino acid product can be separated by crystallization, whereas the D-amino acid, which is still /V-acylated, can be recycled by being resubjected to the Schotten-Baumann conditions used for the next batch. Tanabe has developed a process with an immobilized enzyme,72 73 whereas Degussa uses the method in a membrane reactor.69 74 The process is used to make L-methionine. 

Another favorable aspect of stirred batch reactors is the fact that they are compatible with most forms of a biocatalyst. The biocatalyst may be soluble, immobilized, or a whole-cell preparation in the latter case a bioconversion might be performed in the same vessel used to culture the organism. Recovery of the biocatalyst is sometimes possible, typically when the enzyme is immobilized or confined within a semi-permeable membrane. The latter configuration is often referred to as a membrane reactor. An example is the hollow fiber reactor where enzymes or whole cells are partitioned within permeable fibers that allow the passage of substrates and products but retain the catalyst. A hollow-fiber reactor can be operated in conjunction with the stirred tank and operated in batch or... 

The catalytic behavior of enzymes in immobilized form may dramatically differ from that of soluble homogeneous enzymes. In particular, mass transport effects (the transport of a substrate to the catalyst and diffusion of reaction products away from the catalyst matrix) may result in the reduction of the overall activity. Mass transport effects are usually divided into two categories - external and internal. External effects stem from the fact that substrates must be transported from the bulk solution to the surface of an immobilized enzyme. Internal diffusional limitations occur when a substrate penetrates inside the immobilized enzyme particle, such as porous carriers, polymeric microspheres, membranes, etc. The classical treatment of mass transfer in heterogeneous catalysis has been successfully applied to immobilized enzymes I27l There are several simple experimental criteria or tests that allow one to determine whether a reaction is limited by external diffusion. For example, if a reaction is completely limited by external diffusion, the rate of the process should not depend on pH or enzyme concentration. At the same time the rate of reaction will depend on the stirring in the batch reactor or on the flow rate of a substrate in the column reactor. 

A plug flow reactor may be realized using immobilized enzymes within a column reactor or using soluble enzymes within a cascade of membrane reactors. A batch or a repetitive batch process with soluble enzymes (see below) has the same productivity as the plug flow reactor. 

Glucuronides have been synthesized batch-wise or in a hollow fiber system using microsomal or soluble enzyme preparations (5-5). Furthermore, they have been prepared with enzymes immobilized to polymeric supports (6). Here we describe the continuous synthesis of glucuronide conjugates in a 10-mL membrane reactor (7). 

Although some efforts are being made to develop EMRs based on the immobilization of peroxidases onto the membrane [106, 112], the most applied configurations correspond to the use of direct contact reactors for wastewater treatment, with SBP and MnP applied to effluents containing both phenols [74] and dyes [8, 85]. Some attempts were made to apply an EMR for the synthesis of oxindole from indole by CPO [86]. However, the reactor was only stable for a short period (10 residence times). Afterward, the polymerization of oxindole yielded a solid substance, which blocked the membrane, causing enzyme deactivation and the reduction of the total turnover numbers compared to those obtained in batch assays. 

In order to scale-up the method, however, both d-LDH and FDH have to be recycled to make the process economically feasible. While the starting material 6 could be prepared readily in large scale and only a catalytic amount of NAD is needed for the reaction, neither of the commercially available enzymes is inexpensive. Initial recycling efforts were directed to a batch process using either membrane-enclosed enzyme catalysis or enzyme immobilization methods [13] (Fig. 7). In our hands, these reactor systems were not ideal for scaling-up. 

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