Biomass holdup

Webb, C., Biomass Holdup in Immobilized Cell Reactors, Chap. 8 in Process Engineering Aspects of Immobilized Cell Systems, pp. 117-133. Instn. Chem. Engrs. (London), 1986. Weber, G., New Lift Technique, Oil Gas J., IS (1952). 

As washout is not possible, high throughputs can be realized in reactors. Biomass holdup is independent of throughput. 

Many reviews and several books [61,62] have appeared on the theoretical and experimental aspects of the continuous, stirred tank reactor - the so-called chemostat. Properties of the chemostat are not discussed here. The concentrations of the reagents and products can not be calculated by the algebraic equations obtained for steady-state conditions, when ji = D (the left-hand sides of Eqs. 27-29 are equal to zero), because of the double-substrate-limitation model (Eq. 26) used. These values were obtained from the time course of the concentrations obtained by simulation of the fermentation. It was assumed that the dispersed organic phase remains in the reactor and the dispersed phase holdup does not change during the process. The inlet liquid phase does not contain either organic phase or biomass. 

Consider the fed-batch reactor in Figure 12.3. Initially, the reactor is charged with an aqueous volume, V 0, containing E. coli cells (referred to as biomass) in concentration, X 0). Then, an aqueous solutiot of sucrose (referred to as the substrate i.e., the substance being acted on) at a concentration, S/(g/L), is fed to the reactor at a variable flow rate, F[t) (g/hr). The reactor holdup, V /, contains E. coli cells in concentration, X f) (g/L), penicillin product in concentration, P r (g/L), and sucrose in concentration, S f (g/L). Using Monod kinetics, the specific growth rate of the cell mass (g cell growth/g cell) is... 

Biomass and microorganisms tend to have a positive effect on fixed bed performance. Packing is usually judged on the basis of wettability, which biomass seems to increase. Doan et al. (2008) compared the wettability of plastic spheres with the same plastic spheres in the presence of microorganisms. The result has been that the liquid holdup increased by about 20% in the presence of microorganisms. 

Floe formation increases microbial holdup, facilitates the separation of the biomass from the fermentation broth, makes it possible to use particular fermenter configurations (e.g. tower reactors) for continuous culture or to feed back a fraction of the microorganisms to the reactor. 

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