Segregated CSTR

Friis and Hamielec (48) offered some comments on the continuous reactor design problem suggesting that the dispersed particles have the same residence time distribution as the dispersing fluid and the system can be modeled as a segregated CSTR reactor. 

The ideal flow reactors are the CSTR and the PFR. (This chapter later introduces a third kind of ideal reactor, the segregated CSTR, but it has the same distribution of residence times as the regular, perfectly mixed CSTR.) Real reactors sometimes resemble these ideal types or they can be assembled from combinations of the ideal types. 

These results are obtained by averaging the concentrations and reaction rate values over all the elements of the corresponding reactors. For zero order kinetics, such an analysis leads to Figure 4 wherein results are presented for a PFR and a completely segregated CSTR.. The case of completely mixed CSTR is trivial (a horizontal line at the maximum rate up to Cay >0). 

Batch suspension reactors are, theoretically, the kinetic equivalent of water-cooled mass reactors. The major new problems are stabilization of the viscous polymer drops, prediction of particle size distribution, etc. Particle size distribution was found to be determined early in the polymerization by Hopff et al. (28, 29,40). Church and Shinnar (12) applied turbulence theory to explain the stabilization of suspension polymers by the combined action of protective colloids and turbulent flow forces. Suspension polymerization in a CSTR without coalescence is a prime example of the segregated CSTR treated by Tadmor and Biesenberger (51) and is discussed below. In a series of papers, Goldsmith and Amundson (23) and Luss and Amundson (39) studied the unique control and stability problems which arise from the existence of the two-phase reaction system. 

Segregated CSTR Wider than Between Between... 

Both graphs are based on the models of Biesenberger and Sebastian (1983). m = perfectly micro-mixed, s = fully segregated (CSTR s with perfect macro-mixing). 

In the case of free radical polymerization (figure 13.2) the dispersion index D at low degrees of conversion appears to be 1.5. In a batch or plug flow reactor D increases as the degree of conversion goes up. The reason is that the propagationrinitiation ratio decreases as the monomer is consumed. For a segregated CSTR the effect is enhanced by the residence time distribution. For a well mixed CSTR D remains constant and low. This is explained by the fact that all polymer molecules are made under identical conditions. 

Figure 133. Dispersion index as a function of degree of polymerization for a polycondensation with monomer ratio I and no termination. For higher (more realistic) values of the degree of polymerization, the curves can be extrapolated as almost straight lines (the middle curve is not quite horizontal) (based on model of Biesenberger and Sebastian, 1983). m = perfectly micro-mixed, s = fully segregated (CSTR s with perfect macro-mixing)...

The segregated CSTR (SCSTR), while not discussed earlier, is included to indicate the effects of less than perfect mixing. A practical example of a SCSTR is suspension polymerization in a CSTR in which the suspension beads are well mixed within the reactor and within each bead, but in which there is no exchange of material between the various beads. 

The ideal cases are the piston flow reactor (PFR), also known as a plug flow reactor, and the continuous flow stirred tank reactor (CSTR). A third kind of ideal reactor, the completely segregated CSTR, has the same distribution of residence times as a normal, perfectly mixed CSTR. The washout function for a CSTR has the simple exponential form... 

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