Evaluation of design flexibility in distillation columns using rigorous models

Abstract A methodology is presented for the evaluation of the flexibility and bottlenecks detection of a given distillation column design, using rigorous simulation models. The flexibility index is calculated as proposed by Swaney and Grossmam (1985), solving a nonlinear programming (NLP) problem in the direction of each vertex in the uncertain space, in which the objective is the maximization of the displacements. The control variables, reflux and product flow rates, are the optimization variables of the NLP. The purity specifications, recovery and maximum equipment capacities are posed as the constraints for feasibility of the NLP. Numerical results will be presented for the case of a debutanizer column. Uncertainties are considered in components feed flow rate, maximum allowed vapor velocity, heat transfer coefficients of condenser and reboiler, and cooling water inlet temperature. Great physical insight can be gained from the NLP solutions in the directions of the vertices, detecting bottlenecks and the worst combinations of uncertain parameters.