Design and control of recycle systems by non-linear analysis

Design and control of recycle systems must be integrated at an early design stage. By placing together the pieces of the design puzzle we developed a novel methodology that allows the screening and selection of feasible integrated designs at an early stage. The generic structure is the Reactor-Separator-Recycle (R-S-R) system. Multiple steady states are possible and a minimum reactor volume is required. The behaviour depends on the reactor size and plantwide control structure. Larger reactors behave better than smaller ones. A clear distinction is made between self-regulation and regulation-byfeedback control structures. For some complex reactions, the simple self-regulation feed policy is effective and ensures the desired selectivity pattern. This study integrates the results into a new reliable methodology for design and control of recycle systems. Introduction The integrated design and control of recycle systems in the frame of non-linear analysis was largely ignored by now both by designers and control engineers. A review of the literature on integration of design and control shows that applications of non-linear tools to design of recycle systems is a recent subject (Seferlis & Georgiadis, 2004). Designing simultaneously the reactor and separators is difficult in practice due to the complexity of each unit. The functional analysis should establish if the system's architecture is feasible and operation stable or not. By placing together the design and control aspects in a systemic frame we have developed a novel design methodology based on non-linear analysis. This allows the quick screening and selection of feasible integrated designs at an early conceptual stage. The methodology was successfully applied to several case studies involving complex stoichiometry (Kiss et al., 2005). This article begins with an overview of non-linear effects of importance for recycle systems. Next, we show that a minimum reactor volume is required. In addition, the feedback effect of recycles can generate state multiplicity. For safe operation the reactor should be sufficiently large. The major plantwide control structures are analysed and the main features are discussed. The control structure determines the type of behaviour exhibited by the system. Contrary to the stand-alone analysis, in recycle systems different selectivity can be achieved only by adequate fresh feed policy. Moreover, the selectivity does not exhibit a maximum against conversion or residence time. r i t r i r i ri . i j r . ( it r ) l i r i . . ll ri t r r .