(133c) Whole-Plant Design Optimization

During process design there are many trade-offs to consider. Some equipment decisions may improve the economics of the equipment being considered but have a negative impact on the economics ? as well as the operability ? of the plant as a whole. Whole plant design optimization techniques make it possible to undertake the design of complex reactor and separation sections simultaneously, in order to determine optimal values of design variables taking all relevant constraints into consideration, and thereby ensuring the overall best economics.

In the case presented in the paper the application of such optimization techniques to a new propylene oxide process resulted in the elimination of entire distillation columns from the original process design, saving significant capital and operating costs. The plant comprised a complex multitubular reactor and a separation section with many distillation columns (one an azeotropic distillation and two involving reaction), plus large recycles.

A simulation model was built of the integrated reactor and separation flowsheet, which was then optimized, using an economic objective function that represented annualised capital plus operating cost. The rigorous mathematical optimization considered 49 decision variables simultaneously. Reactor design variables included tube pitch, tube length, coolant velocity, feed reactant mass fraction, number of baffles, cooling water inlet temperature as well as the number of active reactors and numerous other quantities. Separation section design variables included condenser reflux ratios, temperatures, pressures and temperature approaches, column top pressures, reboiler boil ratios and temperatures as well as concentrations of various products in distillate and bottoms streams. In addition were included configuration and topology decisions, such as the location of feed trays and column bypasses, which allowed flowsheet alternatives to be considered as part of the optimisation.

The optimal design represented large savings in operating and capital cost with respect to base case. Two columns were eliminated entirely from the separation section. In addition, heat integration yielded significant operating cost savings with attractive return on investment; payback was less than four months.

The methods used in this design are suitably general to be able to be applied to any process plant and can be implemented using commercially-available simulation and modelling tools.