Oxidative Coupling of Methane over Mixed Metal Oxides: Thermokinetic and Oxygen Distribution Effects
The emergence of shale gas production has stimulated interest in the use of natural gas, not only for power generation but also as a chemical feedstock. The largest tonnage petrochemical, ethylene, is a crucial building block for a wide range of chemicals and has a projected global capacity of 200 million ton per year by 2020. Most ethylene is produced from the energy intensive steam cracking of natural gas (ethane) and naphtha feedstocks. The oxidative coupling of methane offers a less energy intensive, single-step conversion of methane to ethylene. To date, methane to ethylene yields have not exceeded ~ 26% under stable catalytic activity owing to several challenges. This rather low yield and high temperatures involved has been an obstacle to OCM commercial feasibility. This dissertation investigates the impacts of catalyst composition, reactor design, and operating conditions on the OCM reaction system behavior. The findings show the importance of exothermic heat effects in both, promoting OCM catalyst performance and leading to deleterious catalyst deactivation. In the first part of this work, the focus is on investigating fixed bed temperature rise, steady state multiplicity, and catalyst durability over a range of reaction conditions. For this purpose, three-component catalysts were prepared consisting of one host metal oxide and two doped oxides (Cs/Sr/MgO, Cs/Ba/MgO, and Cs/Sr/La2O3) and their performances compared to the well-studied OCM catalyst Na2WO4-Mn/SiO2. Hysteresis behavior comprising ignition and extinction phenomena is encountered for Cs/Sr/La2O3 and Na2WO4-Mn/SiO2 catalysts for selected operating conditions. The results show that the catalyst stability depends on the magnitude of temperature rise in catalyst bed. The hysteresis behavior of Na2WO4-Mn/SiO2 was examined using pseudo-homogenous and heterogeneous reactor models. The modeling results confirm that the coupling of the exothermic heat and activated kinetics is responsible for the multiplicity. In the second part of this work, a systematic comparison between a packed bed membrane reactor and packed bed reactor for oxidative coupling of methane on Na2WO4-Mn/SiO2 catalyst reveals the utility of distributing the oxygen feed along the catalyst bed length. Ways to overcome bypassing of the catalyst bed by shell side O2 as well as back diffusion of methane were investigated.