A combined CSTR/UF reactor is a combination of a continuously stirred tank reactor and an Ultra Filtration Unit. This type of reactor begins as a typical CSTR. However, the product passes through an Ultra Filtration Unit where the enzyme is removed and recycled back into the reactor. An example of what this combination rector can look like is shown below:
In a combined CSTR/UF reactor the enzymes are immobilized in that they can not leave the reactor because of the filtration unit. This allows continuous processing with free enzymes in the CSTR. The Ultra Filtration Unit contains a membrane which provides a semipermeable barrier which allows products and unreacted substrate, if there is any, to pass through while holding back the enzyme. There are other possibilities for similar reactors, such as combined reactor-separators. However, the combined CSTR/UF reactor has proven useful for several types of reactions where a typical immobilized enzyme would not be as effective. One example of this is for the conversion of benzylpenicillin to 6-aminopenicillanic acid by penicillin amidase.