Energy savings of integrated and coupled distillation systems

Direct separation sequence without, with forward, and with backward energy integration, indirect separation sequence without, with forward, and with backward energy integration, sloppy separation sequence without (preflash system), with forward, with backward, and with double energy integration, and thermally coupled sloppy separation sequence (Petlyuk system) are compared with short-cut and rigorous modelling. Based on theoretical considerations and economically evaluated rigorous case studies for ternary mixtures it is demonstrated that in the most cases the Petlyuk system is not superior to the energy integrated configurations even in energy savings. According to the energy consumption of sharp separation determined by short-cut methodology, all the sloppy sequence structures are equivalent. According to the energy losses determined by short-cut methodology, derived here, the energy-integrated structures win almost everywhere in the studied conditions that include a range of relative volatility ratios and the whole feed composition triangle. According to rigorously simulated and optimised results, together with controllability studies, the advantageous application of the thermally coupled (Petlyuk) systems is constrained to a very small range of relative volatility ratio, feed composition, and price structure. This small range is situated somewhere around balanced relative volatility ratio A/B to B/C, small amount of the middle component B, balanced presence of the two swing components A and C in the feed, and high energy costs to investment costs ratio or slow depreciation rate. (C) 2001 Elsevier Science Ltd. All rights reserved.