Design of memetic algorithms for the efficient optimization of chemical process synthesis problems with structural restrictions

•Efficient solution of multimodal design problems by a memetic algorithm.•General principles for the design of memetic algorithms for process synthesis problems.•Computation of several structurally different local optimal solutions.•Optimization of a reactive distillation column with an optional external reactor.•Optimization of the numbers of streams by means of a discontinuous penalty function. In Urselmann et al. (2011a,b) we presented a memetic algorithm (MA) for the design optimization of reactive distillation columns. The MA is a combination of a problem-specific evolutionary algorithm (EA) that optimizes the design variables and a mathematical programming (MP) method that solves the continuous sub-problems with fixed discrete decisions which are proposed by the EA to local optimality. In comparison to the usual superstructure formulation, the search space of the MA is significantly reduced without excluding feasible solutions. The algorithm computes many different local optima and can handle structural restrictions and discontinuous cost functions. In this contribution, a systematic procedure to modify the MA to solve more complex design problems is described and demonstrated using the example of a reactive distillation column with an optional side- or pre-reactor with structural restrictions on the number of streams. New concepts to handle connected and optional unit operations are proposed.