Presence of two reactants

When there is the simultaneous presence of a reactant—in this case B—in two parallel reactions, the yield is defined with respect to the reactant participating in both reactions. For instance, 

Assuming generic order for the product reaction rates  

Since reactant B participates in both reactions and R is the desired product, the yield will be  

Analogously, if bi — i) 0, then it is necessary to have low concentrations of B at the reactor outlet. Therefore, a good scheme is a PFR reactor with A fed at the reactor inlet while B fed at several positions so that reagent B may be immediately consumed. This sequential B feeding allows complete consumption of reagent A until the end of the reactor. 

Otherwise ( 2 —b ) 0, both reactants should leave the reactor at high concentration. A CSTR reactor is the best option. 

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A complex series reaction, such as the one above, is more complicated than the corresponding series reaction. The reactant ratio CbJCa is an added variable which is not present in the straight series reaction. The fact that A reacts with each of the products as well as with B results in a more complicated relationship between Ca. and time. The presence of two reactants makes the control of the reaction eaaer than for a series reaction because in both cases, if is the desired product and lower limit to the conversion of A per pass in either case would be regulated only by the economics of the reactor and recovery equipment cost. However, a high yield could be obtained for either type of reaction. 

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