Design of recycle systems with parallel and consecutive reactions by nonlinear analysis

This paper addresses the design of recycle systems involving multiple reactions. The work presents theoretical analysis and case studies involving typical consecutive/parallel reactions: A + B --> P and A + P --> R, where both reactants are recycled together (butane alkylation); A + B --> 2P and 2A --> P + R, where reactants are recycled separately (toluene trans-alkylation). The design is based on a mass-balance model of the plant. Despite being a simple model it captures the interaction between units and it is able to predict the main pattern of behavior. After choosing the method of controlling the plantwide material balance, nonlinear analysis is applied. This reveals regions of unfeasibility, high-sensitivity, state multiplicity, and instability. These phenomena set hard constraints on both design and operation. Contrary to stand-alone reactors, where desired selectivity can be accomplished by changing the reaction conditions only, in recycle systems, the products' distribution can also be achieved by designing the system for recycle flows giving appropriate inlet ratio of reactants. Moreover, the selectivity does not exhibit a maximum against conversion or residence time. The recycle rate should be set at the highest acceptable value allowed by the economical tradeoff between selectivity and cost of recycles. High purity recycles lead to larger feasibility region. The optimal reactor concept should be related to the recycle policy and makeup control of reactants rather than to stand-alone performance.