Process intensification of methanol synthesis by forced periodic operation in a cooled CSTR: A nonlinear frequency response analysis

Previous nonlinear frequency response (NFR) studies of methanol synthesis focused primarily on isothermal reactor configurations, whereas the process intensification potential of forced periodic operation (FPO) in cooled, i.e. non-isothermal, continuous stirred tank reactors (CSTRs) remains unexplored. Therefore, this study presents the first systematic NFR analysis of single- and simultaneous-input modulation strategies for methanol synthesis in a cooled CSTR using a dynamic reactor model incorporating mass and energy balances, temperature-dependent kinetics and dynamic transitions between oxidised, reduced and hydrogen-activation surface states of the catalyst. The theoretical approach enables time-saving prediction of time-averaged reactor performance under periodic modulation of single and simultaneous input variables. These predictions were verified by comparison with rigorous nonlinear dynamic simulations. Single-input forcing of inlet temperature and H2 resulted in modest improvements in methanol production, whereas modulation of CO, CO₂, or volumetric flow rate predominantly caused performance deterioration. In contrast, simultaneous two-input modulation (CO & flow rate, CO2 & flow rate and H2 & flow rate) generated substantially stronger positive effects. Modulation of CO & flow rate increased the methanol productivity and yield based on hydrogen up to 33.1%, while deteriorating the yield based on total carbon. The yield of methanol based on total carbon was predicted to increase most pronouncedly in the case of H2 & flow rate modulations (up to 28.6%). Multi-objective optimisation identified Pareto-optimal operating conditions that simultaneously improve productivity and yield relative to steady state. The results of the present study demonstrate that dynamic reactor operation can surpass steady state limitations. The results confirm NFR as an efficient tool for a priori design of intensified reactor operation and other nonlinear reaction systems.