Irrespective of the type of operation, a description of the behavior of a suspension culture requires applying the principle of conservation for each of the three distinct phases (gas, liquid and solid phases) and constituents of each phase. When the nutrients are in the liquid phase, as is the case with submerged cultures and which are the primary focus here, the solid phase is comprised essentially entirely of cell mass. In cases involving nutrients (such as cellulose) which are insoluble in the liquid medium, the solid phase is comprised of cell mass and solid nutrients. Where a particular specie is present in more than one phase, the conservation equations for that specie in each of these phases must account for interphase transport of that specie. As an illustration, the conservation equations for a single well-mixed bioreactor are presented here, with the feed to the bioreactor (in the case of fed-batch and continuous cultures) being considered to be sterile. Each of the three phases are considered to be well-mixed. The bioreactor operation is considered to be isothermal. Appendages/modifications to these conser vation equations due to operational modifications such as recycle of cell mass, downstream separations, and two-stage continuous cultures will be addressed briefly at the end of this chapter.
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