Yield nonideal reactors

Perhaps more important than conversion deficits in nonideal reactors are the associated changes in selectivity and yield in more complex reactions. Let us look at a Type III system (A —> B C) to illustrate this. For selectivity in a PFR, Type III reaction, we had previously derived... 

This is not a particularly desirable result for the nonideal reactor, since it indicates that the individual elfects on conversion and selectivity are multiplied together in determination of the net loss in yield. For 5, (III) > 1, unfortunately, the decrease in yield due to nonideality is then greater than individual losses in either conversion or selectivity a given loss in conversion may show up as two or three times that amount in the yield of a desired product. On the other hand, trends in conversion and selectivity tend to compensate each other for 5, (III) < 1 this in shown in Figure 5.14c for the same reaction parameters used in Figures 5.14a and b. Plotted are the product of conversion and selectivity for the two reaction systems (intrinsic selectivity > and < 1) as a function of n. It is clear that for 5, -(III) > 1, the -value decrease for yield is larger in each case than the corresponding decreases for conversion or selectivity. 

Figure 5.14 (d) Selectivity and yield variations with Type III kinetic parameters in a nonideal reactor (10 CSTR units in series). 

Overall the analysis here should convey the message that generalizations concerning selectivity or yield performance in nonideal reactors with reference to an ideal model are slippery conversion, however, is perhaps somewhat more predictable. We may normally expect modest taxes on conversion as the result of nonideal exit-age distributions if the reaction system involves selectivity/yield functions these will also be influenced by the exit-age distribution, but the direction is not certain. Normally nonideality is reflected in a decrease in yield and selectivity, but there are possible interactions between the reactor exit-age distribution and the reaction kinetic parameters that can force the deviation in the opposite direction. Keep in mind that the comparisons being offered here are not analogous to those for PFR-CSTR Type III selectivities given in Chapter 4, which were based on the premise of equal conversion in the two reactor types. 

To complete this discussion of conversion, selectivity, and yields in nonideal reactors, let us consider a similar set of illustrations for a Type II system. Recall that... 

A comparison of conversion and yield for the Type II reaction in terms of the kinetic parameters using a nonideal reactor model (n= 10) is shown in Figure 5.15c. Here as the value of (ki/kj) decreases, yield and conversion in the nonideal reactor approach the ideal value in this case this is a limiting value owing to the equality of the selectivity in the two reactor models. 

Figure 5.15 (b) Yield in a nonideal reactor for a Type II reaction system. 

Figure 5.15 (c) Conversion and yield, Type II, in a nonideal reactor. 

In this chapter, residence time distribution (RTD) of ideal and nonideal reactors along with the method of determination are described in detail. The influence of nonideality and RTD on the reactor performance, the target product yield, and selectivity, including complex reactions, is presented. 

The two terms in this equation have simple physical interpretations. The first term represents the rate of flow of energy into the reactor required to heat the inlet fluid from Tin to T without reaction, and the second term is the energy requirement for all chemical reactions to occur isothermally at the reactor exit temperature. Similarly, neglecting solution nonidealities in Eq. 14.1-9b yields... 

From this example of a fast, competitive consecutive reaction scheme we can see that nonideal mixing can cause a decrease in selectivity in both continuous and semibatch reactors. Residence time distribution issues can cause a reduction in yield and selectivity for both slow and fast reactions (see Chapter 1), but for fast reactions, the decrease in selectivity and yield due to inefficient local mixing can be greater than that caused by RTD issues alone. In semibatch reactors, poor bulk mixing can also cause these reductions (see Example 13-3). 

The chief weakness of RTD analysis is that from the diagnostic perspective, an RTD study can identify whether the mixing is ideal or nonideal, bnt it is not able to uniquely determine the namre of the nonideality. Many different nonideal flow models can lead to exactly the same tracer response or RTD. The sequence in which a reacting fluid interacts with the nonideal zones in a reactor affects the conversion and yield for all reactions with other than first-order kinetics. This is one limitation of RTD analysis. Another limitation is that RTD analysis is based on the injection of a single tracer feed, whereas real reactors often employ the injection of multiple feed streams. In real reactors the mixing of separate feed streams can have a profound influence on the reaction. A third limitation is that RTD analysis is incapable of providing insight into the nature... 

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