Incomplete Conversion of Both Reactants

Figure 2.12 Ternary process flowsheet with incomplete conversion of both reactants and one recycle stream.

In the above series, an important paper of Tyreus and Luyben [5] deals with second-order reactions in recycle systems. Two cases are considered complete one-pass conversion of a component (one recycle), and incomplete conversion of both reactants (two recycles). As general heuristic, they found that fixing the flow in the recycle might prevent snowballing. In the first case, the completely converted component could be fed on flow control, while the recycled component added somewhere in the recycle loop. In the second case, the situation is more complicated. Four reactant feed control alternatives are proposed, but only two workable. This is the case when both reactants are added on level control in recycles (CSl), or when the reactant is added on composition control combined with fixed reactor outlet (CS4). As disadvantage, the production rate can be manipulated only indirectly. Other control structures - with one reactant on flow control the other being on composition (CS2) or level control (CS3) - do not work. The last structure can be made workable if the recycle flow rates are used to infer reactant composition in the reactor. This study reinforces the rule that the flow rate of one stream in a liquid recycle must be fixed in order to prevent snowballing. 

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