Spatial Concentration and Temperature Profiles in Dual‐Packed‐Bed Catalytic Reactors: Oxidative Coupling of Methane

Oxidative coupling of methane (OCM) is a complex surface catalysis and gas-phase reaction process to transform methane into ethane, ethylene and higher molecular weight hydrocarbons (C2 +). However, the C2 + product yields in single-pass fixed-bed reactors have been limited by the intrusion of gasphase combustion reactions. To remedy this problem, a dualbed OCM reactor configuration was explored in which the O2 feed was distributed between two sequential packed bed reactors with interstage cooling using La2O3–CeO2 nanofiber fabric catalysts. At the overall CH4/O2 feed ratio of 4.0, the dual-bed reactor configuration with split O2 introduction resulted in the ultimate C2 + product yield of 21%, which is significantly higher than the 16% observed in the single-bed OCM reactor. In addition, spatially resolved species concentration and temperature profiles along the entire length of the dual-bed reactor were also acquired by using in situ microprobe sampling. Oxidative coupling of methane (OCM) is a complex surface catalysis and gas-phase reaction process to convert natural gas into ethane and ethylene, which are valuable intermediates for the chemical industry. [1, 2] In spite of decades-long research and many OCM catalysts reported, [2] no commercially viable catalyst has been developed. There appears to be an upper limit of C2 + yield of approximately 25 % per reactor pass, which has been attributed to the intrusion of gas-phase combustion reactions at high temperatures. [3] Therefore, catalyst design alone may not be sufficient to result in the breakthrough needed for the commercialization of the OCM process. Consequently, a combined effort, both in catalyst development and in novel reactor systems and operating schemes are needed to advance the field. [4] Early reactor simulations that used global OCM reaction kinetics were particularly encouraging, as they suggested the feasibility for attaining yields for C2 + products as high as + 50 % if the O2 feed was distributed along the reactor, that is, by using membrane-type reactors. [5–7] Although these predictions may not be realistic, as the global reaction models used are well recognized to be of limited predictive value, they nevertheless create optimism regarding the future of the OCM process. More realistic and accurate predictions of the attainable yields can be accomplished by using detailed chemical kinetic mechanisms (DCKM). DCKM comprise a comprehensive description of chemical transformations in terms of irreducible chemical events or elementary reactions. DCKM, in conjunction with transport models, are crucial not only to correlate available experimental data but also to predict performance well outside the ranges of parameters investigated, including conditions under which the acquisition of the experimental data may be infeasible, for example, higher pressures and temperatures. However, the development and validation of DCKM require experimental data of high information content because of the presence of a large number of species participating in an even larger number of elementary reactions. Recently, we reported spatial temperature and species concentration profiles in a packed-bed OCM reactor acquired by microprobe sampling. [8] This information-rich data, in addition to being particularly useful for DCKM work, revealed the early, that is, prompt, formation of H2 in the OCM process, even before the formation of C2H6. This discovery suggested that the existing DCKM for the OCM processes [9, 10] clearly must be revised to render them useful for predictive purposes. Our previous work [8] also demonstrated a significant weakness of using traditional integral reactor data, that is, reactor exit measurements, to develop reaction mechanisms. Herein, we report for the first time spatially resolved species concentration and temperature profiles in a dual fixed-bed OCM reactor with interstage O2 injection by using microprobe sampling [8] and demonstrate that the yields of C2 + products in the OCM can significantly be increased by distributed O2 feed, as noted earlier. [5–7] The information-rich spatial profiles also provide new quantitative insight into the close coupling of the formation of C2 + products and O2 levels in OCM reactors and, as such, should be valuable for the improvement and validation of predictive models.