Reactor/recycle tradeoff

The discussion above points out one of the most important tradeoffs in chemical reactor design. The smaller the reactor size, the larger the recycle flowrate. This reactor/recycle tradeoff dominates the steady-state economics of the design of tubular reactor systems, as we will illustrate in several examples in this chapter. It also has a major impact on dynamic control, as we will see in the next chapter. 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

The use of excess reactants, diluents, or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes the recycling of unwanted byproduct to the reactor can inhibit its formation at the source. If this can be achieved, it improves the overall use of raw materials and eliminates effluent disposal problems. Of course, the recycling does in itself reuse some of the other costs. The general tradeoffs are discussed in Chap. 8. 

Now there are two global variables in the optimization. These are reactor conversion (as before) but now also the concentration of IMPURITY in the recycle. For each setting of the IMPURITY concentration in the recycle, a set of tradeoffs can be produced analogous to those shown in Figs. 8.3 and 8.4. 

Figure 8.5 Cost tradeoffs for processes with a purge when reactor conversion and recycle inert concentration are allowed to vary. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

Economic tradeoffs. Interactions between the reactor and the rest of the process are extremely important. Reactor conversion is the most significant optimization variable because it tends to influence most operations through the process. Also, when inerts are present in the recycle, the concentration of inerts is another important optimization variable, again influencing operations throughout the process. ... 

The design of tubular reactor systems is dominated by the classical tradeoff between reactor size and recycle flowrate. Gas phase systems are particularly affected because of the high cost of compression. 

However, our column is connected via material flow with a reactor. In Chap. 4 we show that reactor control often boils down to two issues (1) managing energy (temperature control) and (2) keeping as constant as possible the composition and flowrate of the total reactor feed stream (fresh feed plus recycle streams). The latter goal implies that it may in fact be desirable to control the composition of the recycle stream. This minimizes the variablity in recycle impurity composition back into the reactor. This recycle composition is dictated by the economic tradeoffs between yield, conversion, energy consumption in the separation section, and reactor size. 

Understand the need to determine the optimal reactor conversion, involving the tradeoff between the cost of the reactor section and the cost of the separation section(s)ii the presence of recycle, even when chemical equilibrium greatly favors the products of the reaction. 

Be aware of the many tradeoffs between the reactor section and the separation section(s) when recycle is used. 

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