In order to gain insight into the changes in OCM performance when proceeding from isothermal to adiabatic operation, over 150 catalysts were screened via microkinetic simulations. The adopted kinetic model [1] is based on 16 catalyst descriptors, it comprises 39 gas-phase and 26 surface reversible reactions and was embedded in a 1D heterogeneous fixed-bed reactor model.
Catalysts were ranked according to C2+ yields simulated at isothermal and adiabatic mode and at varying operating conditions. Some active but not very selective catalysts, such as promoted La2O3, were ranked lower compared to more selective Na-Mn-W/SiO2 in isothermal operation. In adiabatic reactors, however, the difference between these catalyst groups was much less pronounced and, in specific conditions, the ranking was even inverted.
The comparison of C2+ selectivity as function of CH4 conversion and Rate of Production Analysis elucidated the origin for the performance diversity among catalysts in the two operating modes. The interplay between two phenomena, namely the increase in maximum achievable selectivity with temperature [2] and the less pronounced adiabatic temperature rise with increasing selectivity [3], is crucial in establishing the ultimate catalyst ranking.
Via incorporation of this descriptor-based microkinetic model into a process simulator, the way is being paved for a catalyst-tailored OCM process design.
[1] Alexiadis V.I. et al., Appl. Catal. B 150 (2014) 496.
[2] Sinev M.Y. et al., J. Nat. Gas Chem. 18 (2009) 273.
[3] Balakotaiah V., West D.H., Curr. Opin. Chem. Eng. 5 (2014) 68.