Integration of a turbine expander with an exothermic reactor loop - Flow sheet development and application to ammonia production

The direct integration of a gas turbine power cycle with an ammonia synthesis loop is studied. Such a loop represents a typical reactor-separator system with a recycle stream and cold sepn. of the product from the recycle loop. The hot reaction products are expanded directly instead of raising steam in a waste heat boiler to drive a steam turbine. Two new combined power and chems. prodn. flow sheets are developed for the process. The flow sheets are simulated using the flow sheet simulator AspenPlus (licensed by Aspen Technol., Inc.) and compared to a simulated conventional ammonia synthesis loop. The comparison is based on energy as well as exergy anal. It was found that the pressure ratio over the turbine expander plays an important role in optimization of an integrated system, specifically due to the process comprising an equil. reaction. The inlet temp. to the reactor changes with changing pressure ratio, which in turn dets. the conversion and consequently the heat of reaction that is available to produce power. In terms of the min. work requirement per kg of product a 75% improvement over the conventional process could be obtained. The work penalty due to refrigeration needed for sepn. was also accounted for. Furthermore this integrated flow sheet also resulted in a decrease in exergy loss and the loss was more evenly distributed between the various unit operations. A detailed exergy anal. over the various unit operations proved to be useful in explaining the overall differences in exergy loss between the flow sheets