Heat Transfer

Because of the relatively low reaction rates of processes involving microorganisms and cells, it may—in a very general way—be said that heat-effect problems related to local variations of temperature are not common in bioreactors. Even in the case in which polymeric products are released into the medium and very high viscosity is reached, heat transfer is not the controlling step, because such viscous media will hinder the mass transfer, and heat generation will consequently be limited. In such cases the main point of focus is thus, mass, rather than heat, transfer. Reactions catalyzed by immobilized enzymes, however, may require different considerations, because of higher reaction rates.

There is a far greater body of published data on heat transfer in bubble columns than in ALRs, and some of the basic observations are valid for both types of reactor. The heat transfer rate in bubble columns is much larger than that expected from single-phase flow (155). This is a result of the bubble-driven turbulence and liquid recirculation, which are characteristic of the flow in pneumatically agitated reactors.

Several correlations have been proposed for the prediction of the heat transfer coefficient in these reactors. Recently, Kawase and Moo-Young (62) presented an expres-

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