Indirect methane aromatization via oxidative coupling, products separation and aromatization steps

Abstract

Integrated process of indirect methane aromatization, composed of the methane oxidative coupling (OCM), products separation for elimination of some components and aromatization of the remaining compounds to benzene, toluene and naphthalene, has been studied as an alternative to direct non-oxidative methane aromatization (MDA). Two modes of the integrated process were investigated in the continuous flow laboratory unit: IP 1 — composed of OCM, separation of CO2 and H2O and aromatization and IP 2 — composed of OCM, separation of CO2, H2O and C2 hydrocarbons and aromatization. The OCM and aromatization steps were carried out in the presence of 1% Li/MgO at 1023 K and 4.3% Mo/HZSM-5 at 998 K, respectively. The aromatization of individuals — CH4, C2H4 and C2H6 was also investigated. Under applied aromatization conditions about a half of ethylene was converted to aromatics whereas the other part to undesirable CH4 and deposit. It was confirmed that the removal from the OCM products such constituents, which exert detrimental effect on the course of aromatization (CO2 and H2O), led to the marked increase in the aromatics yield in IP 1, as compared with MDA. The approximate additivity of the amounts of benzene formed from methane and from C2 hydrocarbons contained in the OCM products was observed. Thus, it was confirmed that both C2 hydrocarbons and CH4 contained in the OCM effluent gases participated in the formation of aromatics. A full conversion of more reactive C2 hydrocarbons and a partial conversion of less reactive CH4 were observed in the aromatization step. Hydrogen and benzene were the main products of the integrated IP 1 process. The hydrogen/benzene ratio was lower than in MDA due to participation of less hydrogen-producing C2s in the formation of benzene. The overall CH4 conversion was, under applied conditions, somewhat higher than in MDA, due to the decay of some CH4 in the formation of COx, H2O and H2 in the OCM step. The removal of C2 hydrocarbons (apart of CO2 and H2O) from the OCM products before directing them to the aromatization step (IP 2) led to the process of simultaneous production of C2s and aromatics from methane. A strong dependence of the industrial chances of the integrated indirect processes upon the development of the cost-effective separation techniques has been emphasized.