Substrate Accumulation

Solving equation 46 for the cell concentration, assuming steady-state (dS/dt = 0) operation, yields

higher biomass productivity of the recycle system results from a dilution rate higher than the specific growth rate and the increased cell concentration in the vessel.

Implementation of a recycle system is often critical to the economic viability of processes using expensive biocat-

(46) alysts (e.g., enzymes). Typically, this is accomplished by immobilizing the enzymes on inert support particles to facilitate either internal or external recycle. The potential use of cell recycle in an industrial process involves weighing the effectiveness and economics associated with the cell separation step against the marginal improvement in process performance. It should also be noted that higher cell

(47) densities exacerbate the oxygen transfer and heat removal burden of the system.

in which the steady-state cell concentration with recycle is greater than that in the simple CSTR by a factor of 1 /[1 + a(1 — C)]. By adopting a suitable expression for cell growth, the substrate concentration can be determined. Using the Monod expression, as before, and solving for the substrate concentration gives

Substituting equation 48 into equation 47 yields

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