Utilisation of reactor heat in methanol synthesis to reduce compressor duty : application of power cycle principles and simulation tools

The chemical conversion in a methanol reactor is restricted by equilibrium, therefore the synthesis loop is operated at high pressure and unconverted gas is recycled. Such a synthesis loop consumes large amounts of compression work. In this paper a new flow sheet for methanol synthesis is presented. In this flow sheet the high recycle and operating pressure of the reactor is exploited to produce power. A turbine expander and compressor pair is placed in the recycle stream and utilises the reactor heat at the maximum possible temperature in a process gas power cycle. In conventional systems the reaction heat is often transferred to generate steam to drive steam turbines, but the heat is reduced in quality due to the temperature-driving forces in the heat exchange equipment. Simulation models of the new flow sheet and a conventional flow sheet are created to compare the systems based on energy consumed per kg methanol produced. In the conventional flow sheet the reaction heat is used to generate steam for use in steam turbines. In the new flow sheet a portion of the reaction heat is still transferred to a steam cycle to limit the temperature in the reactor. The remaining heat is used to drive the process gas cycle. The simulation results showed that the new flow sheet consumed overall 24% less energy than the conventional flow sheet