Interaction between design and control of a heat-integrated distillation system with prefractionator

This paper analyses the relationship between the design and control of a he at-integrated distillation set-up consisting of a prefractionator and a sid e stream main column. The separation of a pentane-hexane-heptane mixture wi th moderate purity requirements is considered. Both forward and reverse hea t-integration schemes are investigated. Different designs are possible, dep ending on the light/heavy split in the prefractionator and the heat-integra tion scheme. They are similar with respect to energy consumption, but very different with respect to dynamic behaviour. The differences are studied us ing frequency-dependent controllability analysis, as well as carrying out c losed-loop simulation, in the presence of large feed composition disturbanc es. Thus, it is found that the forward heat-integration is much easier to c ontrol, using only temperature measurements. The low-cost design with a sma ll prefractionator has the best closed-loop performance. The reverse heat-i ntegration can be controlled only if a composition analyser of the side str eam is available; a sharp light/heavy split in the prefractionator gives be tter disturbance rejection. The superior dynamic behaviour of the forward h eat-integration scheme was confirmed by the study of a high purity separati on of the benzene-toluene-xylene mixture.