Performance of a multistage fermentor with cell recycle for continuous ethanol production

The unsteady and steady-state performance of a multistage fermenter with cell recycle is investigated numerically for continuous production of ethanol. The tanks-in-series model is employed to describe the how behaviour of culture in multiple tanks. In establishing mass balances, the cell volume fraction and the equilibrium partition coefficient for intracellular and extracellular ethanol concentrations are taken into account. Kinetic expressions are taken from the literature. Compared to a single tank fermenter with recycle, multi-staging and cell recycle are shown to bring high cell and product concentrations and low substrate concentration in the final tank during unsteady-state operation. Solutions to the steady state model are presented graphically, showing concentrations as a function of dilution rate for different numbers of tanks. Results are given for the maximum ethanol productivity as a function of the optimal dilution rate, with parameters of tank number, bleed ratio and recycle ratio. Also, the effects of the fractional tank volume and the saturation constant on the productivity and the substrate conversion are examined.