Bioreactors semibatch reactors

Semibatch Reactors Some of the reactants are loaded into the reactor, and the rest of the reactants are fed gradually. Alternatively, one reactant is loaded into the reactor, and the other reactant is fed continuously. Once the reactor is full, it may be operated in a batch mode to complete the reaction. Semibatch reactors are especially favored when there are large heat effects and heat-transfer capability is limited. Exothermic reactions may be slowed down and endothermic reactions controlled by limiting reactant concentration. In bioreactors, the reactant concentration may be limited to minimize toxicity. Other situations that may call for semibatch reactors include control of undesirable by-products or when one of the reactants is a gas of limited solubility that is fed continuously at the dissolution rate. 

In developing the solution to Illustration 13.1, we utilized a variation of the traditional material balance on a batch reactor that is often useful in the analysis of biochemical transformations carried out in batch and semibatch modes of operation. In particular, we took into account variations in the volume of the broth in the bioreactor by first converting concentrations of biomass and substrate into the total quantities of these materials present in the bioreactor. In so doing we were implicitly recognizing that the proper form of a material balance on species i for variable-volume situations is... 

These equations remain valid for bioreactors provided that one employs a suitable mathematical representation of the rate of disappearance of the substrate that is the limiting reagent. In Illustration 13.3 we employ an alternative form of the design equation to determine the holding time necessary to achieve a specified degree of conversion in a strictly batch bioreactor. This illustrative example also indicates how overall yield coefficients are employed as a vehicle for taking the stoichiometry of the reaction into account. Illustration 13.4 describes how one type of semibatch operation (the fed-batch mode) can be exploited to combine the potential advantages of batch and continuous flow operation of a stirred-tank reactor. 

In principle the use of a well-stirred bioreactor in a continuous flow mode offers significant advantages over operation in a batch or semibatch mode, but the majority of bioreactors in industrial use are operated in the latter modes. However, the actual performance of single CSTR or a cascade of such reactors often fails to meet the expectations... 

Operation of various types of bioreactors in a perfusion mode (21, 22) enables the design engineer to combine several advantages of traditional modes of operation of well-stirred bioreactors (e.g., semibatch operation in a fed batch mode or use of recycle with a chemostat). Perfusion consists of operation in a mode in which a fresh growth medium (possibly together with a recycled growth medium) is fed to a bioreactor containing viable cells that are retained within the bioreactor by permselective membranes, microfllters, immobilization, or by partial separation and recovery from the reactor effluent followed by recycle to the entrance of the reactor. 

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