Possible improvement of methanol synthesis exploiting forced periodic operation: Analysis using the Nonlinear Frequency Responce Method

Abstract

Forced periodic operations, as one way of Process Intensification, can be used in order to achieve better performances of chemical reactors, in comparison to conventional steady-state operation. In this study the Nonlinear Frequency Response (NFR) method, a powerful analytical and approximate tool which gives an answer whether and under which conditions certain periodic operation would lead to improvement of process performance was used. The analysis was done for the methanol synthesis using a standard Cu/ZnO catalyst performed in an isothermal and isobaric lab-scale CSTR. At first the single input modulations were analysed. The inputs considered for periodic modulation are: partial pressures of each reactant in the feed stream and the total volumetric inlet flow-rate. The objective was to maximize the mean molar outlet flow-rate of methanol. The specific forcing parameters were optimized. The results of the NFR analysis showed that modulations of single inputs do not provide potential for significant improvements. The study was extended to analysis of periodic operations with simultaneous modulations of two inputs. Six possible input combinations were analysed and the optimal forcing parameters which maximizing again the time-average methanol production were determined. For all combinations an improvement is possible, but for some cases it was found to be not significant. However, significant improvements are predicted for a) simultaneous modulation of the partial pressure of CO2 in the feed steam and the volumetric inlet flow-rate and b) simultaneous modulation of the partial pressure of hydrogen (H2) and the volumetric inlet flow-rate [1, 2]. The highest improvement could be achieved for simultaneous modulation of the inlet partial pressure of CO and the inlet volumetric flow rate.