Oxidative coupling of methane (OCM) is a promising process for converting natural gas into high-value chemicals such as ethane and ethylene. The process, however, requires important improvements to reach commercial scale. The foremost is increasing the process selectivity to C2 (C2H4+C2H6) at moderate to high levels of methane conversion. The OCM process developments are often investigated at the catalyst level. However, during scaling up, the challenges due to the exothermicity of the OCM reaction are another important aspect that requires consideration. This can be achieved computationally with the availability of a reaction mechanism that is validated against experimental data. High throughput screening (HTS) equipment utilized in catalysis are important to generate experimental data in a wide range of reaction conditions. An HTS instrument was employed to compile a parametric dataset over a wide range of operating conditions using La2O3/CeO2 catalyst. The experimental results were used to refine existing detailed and global reaction models. Further to this, the experimental data were also used to statistically optimize the OCM process variables, leading to valuable improvements.
The methane and oxygen conversions and selectivities of ethylene, ethane, carbon monoxide, and carbon dioxide were measured experimentally in the temperature range of 500–800 °C, CH4/O2 ratio between 3–13, pressure between 1 to 10 bar, and catalyst loading between 5–20 mg leading to space-times between 40–172 s. The developed detailed model described the interactions between homogeneous and heterogeneous reaction chemistry. The proposed heterogeneous model consisted of 52 irreversible elementary steps describing catalytic reactions between 11 surface species and 123 reversible steps describing the contribution of homogeneous reactions between 25 species. The global model was proposed to reduce computational costs, including seven reactions between eight species. The refined models demonstrated good agreement with the experimental data, which served as a reliable baseline for the 3D simulations.
The OCM process conditions were optimized using the design of experiments (DoE) method to gain insights into the effect of operating parameters and to determine the optimal operating conditions for maximizing the production of ethane and ethylene. The DoE results allowed the flexibility of tuning the performance of the OCM reaction products. The quadratic equations were generated from HTS experimental data relating the studied process variables and output response for predicting and optimizing the OCM process numerically. Rate-of-production (ROP) analysis was also applied to shed light on the elementary reactions occurring at different operating conditions. The results demonstrated that the CH4/O2 ratio and temperatures are key for controlling the process performance. Operating at higher temperatures with high CH4/O2 ratios increased the selectivity to C2 and minimized COx (CO+CO2) at moderate conversion levels.
For the exothermicity and heat transfer assessment, 3D computational fluid dynamics (CFD) models based on a porous media approach were developed. The investigation included a comparison of three reactor configurations: autothermal, isothermal, and adiabatic, aiming to evaluate their performances and heat transfer. The simulations conducted have provided important insights into the effects of reaction exothermicity on reactor performance. The heat released from reactions exhibited a significant rise, particularly at the initial part of the catalytic bed. This temperature increase leads to surpassing the optimum temperature range for C2 formation while also contributing to catalyst deactivation and product oxidation. The autothermal reactor provided a promising solution to enhance commercial viability. The autothermal reactor effectively mitigates excessive exothermicity and promotes cost reduction by utilizing a lower-temperature feed. The investigations undertaken in this dissertation pave the way for further development of the OCM process by the means of homogeneous and heterogeneous reaction mechanisms and CFD simulations.
PhD Candidate, Supervised by Professor Mani Sarathy