Optimal Inlet Temperature Profile Strategies for Decaying Fixed Bed Reactor

Inefficient use of raw materials in most catalytic processes can be attributed mainly to the deactivation of the catalyst in chemical reactors. In order to achieve efficient use of raw materials, this paper addresses mitigation of deactivation through optimization of inlet temperature-time trajectories. The optimization of temperature profiles in reactors is one of the key problems in the synthesis catalytic reactor (Hwang, 2004). The economics of the catalytic reactor, which is prone to coking, depends on the relative rates of the reactions and deactivation of catalyst. Optimum performance, measured in terms of yield, is achieved by decreasing the inlet temperature with time as the catalyst deactivation rate is more sensitive when compared to the reaction rate. In contrast to this, it is widely believed in industry that increase of inlet temperature with respect to time results in constant performance of the reactors. This results in lower impact on the separation networks. Therefore, two types of objectives in terms of reactor yield have been considered, a. maintaining average yield constant through a single operating cycle, which results in increasing inlet temperature profile through time, b. optimizing for the maximum overall yield results in decreasing inlet temperature profile through time. Optimization of the operational variables of the catalytic reactors mainly depends on the downstream processing requirements i.e. separation networks. Simulation of the transformation of methanol into olefins and light gasoline is carried out in an isothermal fixed bed reactor using the reaction and deactivation kinetics from (Gayubo et al., 1996). It is noted that the performance decreases very quickly in the case of the higher temperature, where the rate of deactivation is found slow for the lower temperature. Therefore, it can be concluded that higher inlet temperature maintains higher olefins yield initially but elevated temperature causes very rapid deactivation, which leads to rapid decay in performance. Lower temperature gives lower yield compared with higher temperature initially but maintains the olefin yield for a longer time, due to lower deactivation rate at lower temperature. Therefore, it is deduced that controlling inlet temperature to be higher in the initial time and decreasing with time on stream maintains higher average olefins yield. This means that the relative kinetics of the reaction and deactivation dictates the performance at given temperature. In overall, optimization of the inlet temperature through lifetime of the catalyst can result in improvement of the performance of the catalytic fixed bed reactor, instead of maintaining it constant. In this work, average yield is maintained at higher value but decreasing through cycle time. In common practice, performance of the reactor is usually maintained constant or nearly constant to get less impact on down stream but with a cost of product and inefficient use of raw materials. Some more runs are also made keeping constant olefin yield at certain values and inlet temperature profile is optimized through cycle time. In these runs, inlet temperature profile is found increasing for lower value of olefins yield and maintains constant performance through cycle time. A novel profile generation tool (Choong & Smith, 2004) is used to develop a profile and parameters of profile determine the size and shape of the temperature profile and so the values of the optimizing variables at different time. The resulting dynamic optimization problem is solved using a nonlinear optimization algorithm.