THE ORDER OF MICROMIXING AND SEGREGATION EFFECTS ON THE BIOLOGICAL GROWTH-PROCESS IN A STIRRED-TANK REACTOR

Depending on the hydrodynamic conditions, a stirred tank reactor may b e divided into two micromixing environments: maximum mixing followed b y complete segregation (case 1), or vice versa (case 2). The Ng-Rippin two-environment model simulates case 1, whereas the Fan reversed two- environment model covers case 2. The micromixing concepts of Danckwert s and of Zwietering have been applied to both models in terms of the d egree of segregation J to evaluate the influence of the order of micro mixing-segregation effects on biological growth processes. The model p redictions for both endogeneous and exogeneous cell metabolism show th at case 2 gives more substrate conversion and cell production than doe s case 1, for the same extent of micromixing, particularly at low dilu tion rates. At high dilution rates, both models predict the same react or performance, independent of the micromixing phenomenon. The substra te conversion and cell production decrease with increasing dilution ra te, following a similar trend. Further, the effects of micromixing are found to be strong functions of dilution rate. At high dilution rates for case 2, the micromixing effects are pronounced only when the reac tor approaches complete segregation. However, for case 1, the effects are appreciable when the reactor deviates slightly from perfect mixing . For some intermediate dilution rates, the Fan model, unlike the Ng-R ippin model, shows that the reactor output decreases linearly with inc reasing degree of segregation. Beyond a critical value of the dilution rate, the reactor output falls linearly with dilution rate for exogen eous cell metabolism (case 2). On the contrary, for case 1, the output decreases exponentially throughout the entire range of dilution rates .