The Function of Process Recycles

Byproduct formation. Consider now the case where a byproduct is formed either by the primary reaction such as  

The recycling of material is an essential feature of most chemical processes. Thus, it is necessary to consider the main factors that dictate the recycle structure of a process. Start by restricting consideration to continuous processes. 

Reactor conversion. Earlier in Chapters 5 to 7, the initial choice of reactor type, operating conditions and conversion was discussed. The initial assumption for the conversion varies according to whether there are single reactions or multiple reactions producing byproducts and whether reactions are reversible. Consider the simple reaction  

To achieve complete conversion of FEED to PRODUCT in the reactor might require an extremely long residence time, which is normally uneconomic. Thus, if there is no byproduct formation, the initial reactor conversion is set to be around 95% as discussed in Chapter 5. The reactor 

Recycling byproducts for improved selectivity. In systems of multiple reactions, byproducts are sometimes formed in secondary reactions that are reversible, such as  

Because we require a pure product, a separator is needed. The unreacted FEED is usually too valuable to be disposed of and is therefore recycled to the reactor inlet via a pump or compressor (see Fig. 4.16). In addition, disposal of unreacted FEED rather than recycling creates an environmental problem. 

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