CHEMICAL LOOPING OXIDATIVE COUPLING OF METHANE AND CO2-H2O SPLITTING FOR SUSTAINABLE ETHYLENE AND SYNGAS PRODUCTION AT INTERMEDIATE TEMPERATURES

Abstract

The rising global warming concerns have prompted research in the direction of greenhouse gas (GHG) mitigation technologies. Amongst all of the GHGs, methane (CH4) and carbon dioxide (CO2) gases are the most abundant ones and their conversion to valuable chemicals holds significant value. The discovery of shale gas reserves has further encouraged the development of direct methods for methane conversion into valuable chemicals, offering an alternative to indirect approaches that involve an energy-intensive intermittent syngas production step, leading to high CO2 emissions. Amongst the direct methods, the oxidative coupling of methane (OCM) is a potential pathway to reduce CO2 emissions and which can produce commodity chemicals such as ethylene, a chemical regarded as central to the petrochemical industry. Even though OCM has been studied for over four decades, the technology still has not found commercial applications. Though there are many challenges regarding industrial deployment of OCM, the most significant one is the requirement of a high ethylene yield of 30% which is currently reported to be around 20%. Moreover, the highly exothermic nature of the process and controlling the carbon selectivity over oxides of carbon (COx) is the heart of the problem. Numerous researchers have presented promising results in terms of catalysts, reactor designs and feeding strategies for OCM. However, due to lack of inclusiveness in the results, none of the combinations of catalysts, reactors and system optimizations have been able to bring about its industrial viability. Integration of OCM which another catalytic process which occurs at intermediate temperatures can increase the profitability of the overall process. One such catalytic process is the chemical looping (CL) CO2-H2O splitting process that produces valuable syngas. Integrating OCM with CO2-H2O splitting allows efficient energy utilization, yield improvement and minimal heat losses and pollutant emissions to the environment. The current dissertation presents an extensive review of the noteworthy attempts to achieve industrial targets for OCM. Similarly, a comprehensive review of the CL CO2-H2O splitting process is presented and a novel system of integrated CL OCM/CO2-H2O splitting process is introduced and assessed. Moreover, a comprehensive set of benchmarks is presented which highlights the desired end-state for the industrial deployment of CL OCM/CO2-H2O splitting technology. Furthermore, the dissertation includes a multiscale packed bed reactor model for CL OCM developed using a Computational Fluid Dynamics (CFD) commercial tool. CFD tools help analyze spatial gradients within the reactor to deeply understand the diffusion of species, mass and heat transfer phenomena. Furthermore, challenges associated with scaling up such as hot spot formation and parametric sensitivity are addressed without having to expend on costly experiments. The CFD model includes two scales i.e., macroscale for catalyst bed and microscale for individual pellets. Moreover, a chemical kinetic model based on 10 gas-phase reactions is integrated with the CFD model. An additional surface reaction for the formation of gas-phase oxygen from catalyst surface is added to account for the absence of feed oxygen. The model is calibrated against experimental results. The calibrated model captures trends in CH4 conversion, C2 selectivity and C2 yield within a ±4.35% range across a temperature range of 700-900oC. Moreover, model fidelity is evaluated by varying key parameters such as mesh resolution and time step size. The model is also verified by varying the inlet methane concentration and the gas hourly space velocity (GHSV) and comparing the results with literature. Based on the CFD model, a parametric study is also conducted to optimize the reactor performance which includes parameters such as GHSV, inlet methane concentration, oxygen availability and reactor diameter across temperature range of 700-900oC. Results from the parametric study suggest that the C2 yield is increased from 19.15% to 20.4% as the CH4/O2 ratio is decreased to 1.71 at 1125 mL/hg GHSV and 825oC. However, the C2 selectivity drops to 44.6%. Conversely, a remarkable C2 selectivity of 76.8% is achieved at a CH4/O2 ratio of 8.9, 2500 mL/hg GHSV and 800oC. However, the yield drops to ~10%. Furthermore, increasing the diameter/length ratio of reactor, at constant volume constant, reduces the hot spots while maintaining the C2 yield. A techno-economic assessment (TEA) based on an industrial-scale, system-level model is also conducted for the CL OCM/CO2-H2O Splitting. The system-level model is developed using optimized performance metrics from the CFD parametric study. Furthermore, the effect of different recycled methane content on the levelized cost of products (LCOP) has also been studied. The original case from the parametric study had a C2 yield of 10.76% and selectivity of 72% at a temperature of 800¬oC. The inlet flow rate of feed gas has been adjusted to obtain 1 million tonne/year C2 products. Results suggest significant improvement in C2 yield and LCOP with methane recycling at the studied operating condition. The C2 yield improved by 15.87 percentage points with 60% recycled methane. The LCOP is also close to the threshold value of 1000 $/tonne with 60% recycled methane. However, at this high recycled percentage, hydrogen concentration is more than the flammability limit which can cause safety issues because of hydrogen combustion. Therefore, a methane recycle percentage of 40% is recommended with hydrogen concentrations lower than the flammability limit. The LCOP with this recycle percentage becomes competitive to conventional ethylene production technologies at lower methane feedstock prices projected for future. Furthermore, a sensitivity analysis is conducted which suggests that the integrated technology can produce further lower LCOPs given the lower methane and CO2 feedstock prices. A cradle-to-gate (CtG) life cycle assessment (LCA) is also conducted in SimaPro 8.1 using Ecoinvent v3.6 background data and the ReCiPe 2016 (H) midpoint and endpoint impact assessment methods. CL OCM/CO2-H2O achieved the lowest global warming potential (GWP) at 0.38 kg CO₂-eq/kg C₂H₄, compared with 1.93 kg CO₂-eq/kg C₂H₄ for ethane steam cracking (950 °C) and 2.84 kg CO₂-eq/kg C₂H₄ for naphtha steam cracking. Endpoint results show 96–97.5% lower human-health impacts (2.12 mPt vs. 55.74–86.13 mPt) and 96.4–97.6% lower ecosystem damage (1.25 mPt vs. 34.64–51.92 mPt) relative to conventional routes, while resource depletion is slightly higher. Overall, this dissertation demonstrates that the integrated CL OCM/CO2-H2O splitting process is a promising technology for methane and CO2 valorization while producing ethylene and syngas with improved economic and environmental performance. The combined CFD, system-level modeling, TEA and LCA results show that methane recycling accompanied by reactor optimization can substantially reduce LCOP relative to conventional steam cracking. Although challenges remain in hotspot control, hydrogen safety and long-term catalyst stability, the proposed framework provides a practical foundation for advancing the technology toward industrial deployment. These findings establish CL OCM/CO2-H2O splitting as a potentially competitive and low-carbon alternative for future ethylene production.