Appropriate Placement of Reactors

In Chapter 16, it was seen how the pinch takes on fundamental significance in improving heat integration. Now consider the consequences of placing reactors in different locations relative to the pinch. 

There is no benefit by integrating an exothermic reactor below the pinch. The appropriate placement for exothermic reactors is above the pinch1. 

By comparison, Fig. 13.36 shows an exothermic reactor integrated below the pinch. Although heat is being recovered, it is being recovered into part of the process which is a heat source. The hot utility requirement cannot be reduced because the process above the pinch needs at least Q//m-,n to satisfy its enthalpy imbalance. 

By contrast. Fig. 13.46 shows an endothermic reactor integrated below the pinch. The reactor imports Qreact from part of the process that needs to reject heat. Thus integration of the reactor serves to reduce the cold utility consumption by Qreact- There is an overall reduction in hot utility because, without integration, the process and reactor would require (Qumin + Qreact) from the utility. 

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The appropriate placement of reactors, as far as heat integration is concerned, is that exothermic reactors should be integrated above the pinch and endothermic reactors below the pinch. Care should be taken when reactor feeds are preheated by heat of reaction within the reactor for exothermic reactions. This can constitute cross-pinch heat transfer. The feeds should be preheated to pinch temperature by heat recovery before being fed to the reactor. 

In Figure 20.5d, the grand composite curves for the reactor and that for the rest of the process are superimposed. To obtain maximum overlap, one of the curves must be taken as a mirror image. It can be seen in Figure 20.5d that the reactor is appropriately placed relative to the rest of the process. Had the reactor not been appropriately placed, it would have been extremely unlikely that the reactor would have been changed to make it so. Rather, to obtain appropriate placement of the reactor, the rest of the process would more likely have been changed. 

The preceding appropriate placement arguments assume that the process has the capacity to accept or give up the reactor heat duties at the given reactor temperature. A quantitative tool is needed to assess the capacity of the background process. For this purpose, the grand composite curve can be used and the reactor profile treated as if it was a utility, as explained in Chap. 6. 

Appropriate placement can be assessed quantitatively using the grand composite curve. The streams associated with the reactor can be represented as a grand composite curve for the reactor and then matched against the grand composite curve for the rest of the process. 

Reaction placement implies appropriate heat integration of the reaction effluent. The reactor integration can be evaluated by the process GCC, which is constructed without the reaction effluent stream and then the reaction effluent stream is placed on top of the GCC. The general appropriate placement principle states that the heat of the reaction effluent should be released above the process pinch. 

If the reactor placement is not appropriate, it is in general easier to change the process. Otherwise the revision of the whole reactor design is necessary. In this case the... 

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