A micro refinery unit at Polytechnique Montreal converts natural gas to diesel range fuel as the main product in two high pressure and high temperature reacting units. First, it transforms methane to syngas by catalytic partial oxidation (CPOX) at 20 bar and 800⁰C. Then, it produces the medium-chain hydro-carbons from syngas by Fischer-Tropsch (FT) reaction at 20 bar and 300⁰C.
The aim of this study is to evaluate the impact of passive intercooling on the performance and robustness of a pre-set control configuration for this sequence of interconnected chemical reactors. We simulate the whole process in Aspen Plus v8.4 and first design a PI temperature controller for the Fischer-Tropsch re-actor in Aspen Plus Dynamics. As the FT process is highly exothermic, the controller is essential to properly remove the heat generated in the reactor. Despite being feasible in simulations, the closed-loop results suffer from many shortcomings, notably with respect to process constraints.
The impact of intercooling on the closed-loop dynamics is studied by decoupling thermally the reactors using a passive intercooler to remove the excess heat from the syngas at the exit of the CPOX reactor.
Simulation results show that intercooling improves the performance of the FT operation and reduces the control cost as it keeps the control system far from the cooling flow constraints in the FT reactor. In this case, the controller has an acceptable performance against the step changes in temperature and has a built-in robustness against underestimated heat exchange within the FT process.