A new methodology for the optimisation of feed conditions as well as the calculation of minimum reflux ratio of distillation columns is presented. The reversible profile approach used for saturated liquid feeds is extended to consider other feed conditions. For flashed feed, the liquid fraction of the feed stream is used to compute the column pinch conditions and the minimum reflux ratio. The modifications required for subcooled liquid and superheated vapor feed are discussed, and a procedure to estimate the minimum reflux for those conditions is proposed. The methodology presented allows the identification of the optimal feed condition, without having to resort to a full stage-by-stage procedure.

A new procedure for estimating area and capital cost targets of constrained heat exchanger networks is presented. The method allows for match constrained networks and exchangers with more than one tube pass. The procedure is based on modelling the problem as a non-linear formulation where the forbidden exchanger matches are included as constraints and the temperature difference correction due to multipass exchangers is included in the model. The difficulty of converging to a solution due to the additional non-linear constraints imposed by the multipass exchangers required the use of a two-level approach: at the inner level, the area targets for simple pass exchangers are obtained, and at the outer level the temperature difference required for multipass exchangers are computed and fed back to the inner level. The procedure is repeated until an appropriate tolerance between two iterations was achieved. A comparison between the estimated exchanger areas and costs estimated by the new procedure and the area and costs obtained from the final heat exchanger design shows a very good agreement.