Reactor Types
After the enzyme and the carrier system have been chosen, decision needs to be taken on reactor design, which can be any one of the following four types:

(a) Batch reactor
It is the simplest type of reactor. The immobilized enzyme is placed in a container with the reactants, and the reaction is allowed to proceed until the desired level of conversion is reached. Some stirring or agitation of reaction mixture is also required. Many modification of these reactors have been designed to simplify recovery and reuse of the enzyme composite.

(b) Continuously stirred reactor
This type of reactors employ a stirred rank, to which reactants are continuously added and products continuously withdrawn.

There are many types of fixed bed reactors including those with packed bed of particular material to which enzyme is coupled. A packed bed of immobilized enzyme has clear advantages, since the void space is reduced to 30%, as compared to 80-90% in stirred tank reactors. However, in the packed bed reactors, a concentration gradient of substrate and the product are often generated, so that a low K_M for the enzyme and substrate inhibition become critical to capitalize on this concentration gradient. Moreover in a packed bed reactor, pH adjustment is not possible, making it necessary that the enzyme has a sufficiently broad range of pH to accommodate any changes in pH.