Space time CSTR cascade

In order to reduce the disparities in volume or space time requirements between an individual CSTR and a plug flow reactor, batteries or cascades of stirred tank reactors ard employed. These reactor networks consist of a number of stirred tank reactors confiected in series with the effluent from one reactor serving as the input to the next. Although the concentration is uniform within any one reactor, there is a progressive decrease in reactant concentration as ohe moves from the initial tank to the final tank in the cascade. In effect one has stepwise variations in composition as he moves from onfe CSTR to another. Figure 8.9 illustrates the stepwise variations typical of reactor cascades for different numbers of CSTR s in series. In the general nonisothermal case one will also en-... 

For a CSTR equal in volume to the tubular reactor, one moves along a line of constant kCB0T in Figure 8.16 in order to determine the conversions accomplished in cascades composed of different numbers of reactors but with the same overall space time. The intersection of the line/cCg0T = 19.6 and the curve for N = 1 gives fB = 0.80. 

In this section the size requirements for CSTR cascades containing different numbers of identical reactors are compared with that for a plug flow reactor used to effect the same change in composition. One may define a space time for an entire cascade (x, ) in terms of the ratio of the sum of the component reactor volumes to the inlet volumetric flow rate. Hence,... 

Determine the space time corresponding to the maximum rate of consumption of CgHjCN. Plot the rate versus the concentration of CgHjCN. On this plot indicate the operating line for the single CSTR. Then use the plot to determine the concentrations of benzonitrile that would be present in the effluent from each reactor in a cascade of three identical reactors, the first of which operates at the maximum rate of destruction of the nitrile. Use the plots of species concentration versus time to ascertain whether there is a space time that maximizes the concentration of the amide in the effluent from the CSTR. Comment. 

In this equation refers to the total space time for the cascade of two identical CSTRs. 

Figure 3.30. Basic reactor concept and concentration-versus>time and concentration-versus-space profiles. DCSTR, discontinuous stirred tank reactor SCSTR, semicon-tinuous stirred tank reactor CSTR, continuous stirred tank reactor CPFR, continuous plug flow reactor NCSTR, a cascade of N stirred vessels.

N, of units in a series (NCSTR). As with the CSTR mode of operation, the concentration profile in each tank is uniform in both space and time. However, over the entire length of the cascade, the space/concentration profile will show a typical step function curve. It is not difficult to see that this spatial behavior approximates that of a CPFR. As a rule, a cascade with N > 5 can actually be used as a process engineering substitute for a CPFR (see residence time distribution. Sect. 3.3.1). 

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