CSTBR recycle

Figure 13.8 Schematic flow sheet for a process coupling use of a CSTBR and a generic unit operation for separation and concentration of a suspension of cells for recycle to enhance reactor productivity. The concentration factor /=X2/Xj.

Operation of a Single CSTBR in Conjunction with Cell Separation and Recycle... 

Biochemical transformations that are mediated by microorganisms are characterized by autocatalytic behavior. The fact that the rates of these reactions increase as the concentration of the organism increases provides opportunities for engineers to consider a variety of modes of operation to enhance the performance (and productivity) of a CSTBR facility. One fruitful approach is to do a partial separation and concentration of the cells contained in the efQnent from the CSTBR (see Figure 13.8) and recycle the resnlting process stream back to a point where it is mixed with the contents of the CSTBR. 

In addition to the variables we have used previously in our analysis of the behavior of a CSTBR, we employ the recycle ratio (R) to represent the ratio of the volumetric flow rate of the suspension of cells leaving the separation device to the volumetric flow rate of the net product stream. The symbol / represents the ratio of the concentration of biomass in the recycle stream to that in the gross product stream so /=X2/jCi. This concentration factor characterizes the degree to which the separation operation increases the amount of biomass present in the recycle stream beyond... 

The volumetric flow rates at various points in the process flow sheet are indicated in Figure 13.8. The relative flow rates of the recycle and net product streams are linked by the definition of the recycle ratio R. If one presumes that any differences in the densities of the fresh feed and net product streams are inconsequential, an overall material balance indicates that the volumetric flow rates of these streams are then essentially identical. In similar fashion one can conclude that total mass balances around the CBSTR and around the separator dictate that the volumetric flow rates of the recycle stream and the effluent from the CSTBR proper are RV and (1 + Ryv, respectively. 

Because the concentration factor vp must exceed unity to render operation in a recycle mode viable, the quantity 1 (1 - /) must be negative and the dilution rate D must exceed mef Consequently, the use of separation and recycle of a portion of the cells permits one to operate with a steady-state dilution rate that exceeds the net biomass specific growth rate in the CSTBR. 

Inspection of equation (13.2.65) indicates that the effect of the recycle is to reduce the amount of biomass in the effluent from the CSTBR by the factor 1 -I- R(1 - p) relative to the total amount of biomass that would be obtained from a single CSTBR operating at steady state. Consequently, the biomass in the recycle stream can serve to increase the amount of biomass in the CSTBR well above that obtained in the absence of recycle. 

Figure 13.9 Effects of dilution rate on the total quantity of biomass produced and on the rate of production of biomass during steady-state operation in the presence and absence of partial separation and recycle of biomass to the entrance of a single CSTBR. Parameter values used in generating plots Kg = 2.78 g/L p = 3.16 h" 7x/s = 9.7 ...

Figure 13.9 also contains plots of the productivity of the CSTBR as a function of the dilution rate for operation with and without recycle of cells. The recycled cells enable one to operate at dilution rates considerably above those characteristic of the close-to-washout conditions that would prevail in the absence of recycle. The higher density of cells leads to very substantial increases in reactor productivity. The recycle mode of operation is particularly important in the use of microorganisms for the treatment of wastewaters (see Section 13.3.1). 

Smart Dent is smdying the effects of cell recycle on steady state productivity for culture of a yeast strain in a CSTBR with a working volume of 20 L. The growth medium contains 30 g/L of the limiting substrate and the sterile fresh feed stream is supplied at a rate of 25 L/h. Determine the concentrations of yeast (x) and substrate (s) in the effluent from the process depicted in Figure P13.15. 

The subscripts on the usual process parameters indicate the positions in the process to which these parameters refer. The kinetics of yeast growth can be described by a Monod rate expression with = 0.625 h and Kg = 2 g/L. Cell death and cell maintenance effects are negligible, as is formation of products other than yeast cells. The yield coefficient x/s is 0.44. The effluent from the bioreactor flows directly to a membrane filtration apparams. The membrane is completely permeable to the substrate, so the concentrations of the substrate in the CSTBR, the effluent from the bioreactor, and the permeate from the membrane and in the recycle stream are all identical. The membrane rejects a substantial proportion of the yeast cells so that the ratio of the concentration of yeast in the recycle stream is a factor of 4 larger than that in the effluent from the CSTBR. Volumetric expansion and contraction effects may be considered negligible. 

Figure P13.15 Continuous culture of yeast in a CSTBR with recycle of cells using a membrane separation device from which the retentate is returned to the CSTBR.

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