Simulation of the effect of mixing,scale-up and pH-value regulation during glutamic acid fermentation

A mixing model is coupled with fermentation kinetics in order to simulate a fermentation as a function of mixing conditions and scale-up. The mixing model for a batch stirred tank with three stirrers consists of three regions, each of them characterized by an ideally mixed compartment around the stirrer and two macromixers, i.e. cascades of tank-in-series, describing the recirculation flow. The model contains four parameters — radial and axial circulation time, volume of the ideally mixed stirrer compartment and the number of tanks in each cascade. These values, determined by Mayr et al. in function of the operational conditions and scale-up, were choosen to simulate the fermentation of glutamic acid to show the pH-fluctuation at different control and scale conditions. By choosing optimal regulation properties, such as input flow rate and/or concentration of the base, regulation span, position of the pH-electrode and base input location, etc., fluctuations of the pH-value in the bio-reactor can be minimized. However, the negative effect of insufficient mixing conditions can be reduced only by an increasing number of the base input places. In large scale fermentors, the axial circulation time is rather high, about 5–10 times larger than the radial one. This might result in a large amplitude of the pH-fluctuation. As it is shown, using an input place for base in each stirrer region, the negative impact of the insufficient axial mixing on the fermentation can be diminished perfectly. In this case ammonia should be fed into the reactor as an aqueous solution.