PLANT OPTIMIZATION BY RETROFITTING USING A HIERARCHICAL METHOD - ENTRAINER SELECTION, RECYCLING AND HEAT INTEGRATION

Systematic procedures for reducing wastes in complex chemical plants a re needed to allow efficient optimisation. Here, a hierarchical proced ure was applied to the optimisation of a real industrial plant to redu ce wastes as well as energy and raw material consumption. In the case studied, the continuous production of methyl-butynol (MBI), acetylene reacts with acetone. The solvent ammonia and acetylene are recycled to the reactor. Unreacted substrates and by-products are separated from the product stream by distillation. Part of the unreacted acetone can be reused for other purposes after distillation. A substantial part of the unreacted substrates and by-products is delivered to a wastewater treatment plant. These waste streams constitute a substantial problem for the operation of this plant. First, waste streams were characteri sed and tracked back to their origin. Following the hierarchical desig n procedure, the overall input-output structure was fixed. The entrain er in the present process was critically examined and options were sug gested. Then various recycle schemes were considered for later detaile d study. The existing plant was simulated using ASPENPLUS. After adjus ting the model to all important aspects of the real process scheme, ex cellent agreement between actual process performance data and simulati on was obtained. The various process schemes were simulated and assess ed for their economic and ecological performance. The objective functi ons used included utility, substrate and catalyst costs, as well as co sts for wastewater treatment. Additionally, the environmental burden r elated to energy supply was accounted for by a carbon dioxide tax as s uggested by the Nordic countries. The process changes included separat ion of unreacted acetone from the product stream and recycling to the reactor. By-products were converted back to substrates in an additiona l reactor separation system and recycled. In various simulated process configurations and operational schemes substantial economic and ecolo gic improvements were achieved. This study demonstrates the usefulness of hierarchical approaches combined with process simulation for plant optimisation.