(388b) Single Reactor Design Concepts for Achieving an Autothermal Operation of Exothermic Oxidative Coupling of Methane and Endothermic Methane Dehydroaromatization Reaction

Need for sustainable processes: Around 3 billion tons of carbon dioxide is emitted every year through the chemical and petrochemical industries. [1] To reduce these emissions and utilize the natural resources better, new sustainable routes are needed to replace the highly optimized and cost-competitive methane conversion to chemicals via the indirect syngas route. Of the proposed directed routes are the oxidative and non-oxidative coupling of methane, which have not yet been commercialized.[2] Among other challenges, these routes require high operating temperatures (600-1000^oC) and strict heat management.[3] An idea to address these challenges is to consider both chemistry routes in the same chemical plant as this can have many potential advantages. One of these potential advantages is reaching autothermal operation which reduces the energy demands and carbon emissions. This work aims to explore options for thermal coupling of exothermic and endothermic reactions in a single reactor but without any mass integration. The first reaction is the oxidative coupling of methane (OCM) which is highly exothermic (ΔH^o_rxn= -141 kJ/mol.CH₄) and produces C2+ products like ethane and ethylene. The second reaction is methane dehydroaromtization (MDA) which is endothermic (ΔH^o_rxn= +88.4 kJ/mol.CH₄) and yields C6+ products like benzene, toluene, and naphthalene.

Reactors modeling and thermal coupling methodology: As an initial step, each reaction was modeled using a separate reactor with the assumptions of an ideal packed bed reactor. A literature review was carried out to identify available kinetics models for OCM and MDA reactions. Kinetic models provided by Wang and Lin, 1995 for OCM [4] and Zhu et. al, 2018 for MDA [5] were chosen owing to the relatively simple kinetics. A parametric study was then carried out to evaluate the effect of each process variables (pressure (P), temperature (T), feed composition (yi), flow rate (F)), and catalyst weight (mCat) on the reactor performances (conversion (X), selectivity (S), yield (Y)) and most importantly heat duty (Q). Based on this parametric study, an operating window for thermal coupling was identified in between 700 – 850 ^oC temperature, 1 – 5 atm pressure, GHSV 830 – 14,000 h^-1, and heat duties of 1–13 MW. This operating window was identified using multiple optimization studies via single-variable-at-a-time optimization to match the heat duties of both reactors as shown in figure 1(a). Owing to many input and output variables, several design options could be proposed. Therefore, we utilized the visual representation of Artificial Neural Network, shown in figure 2, to help navigate between all the different options and provide a structured methodology for performing the optimization for thermal coupling.

Design concepts for thermal coupling of OCM and MDA in a single reactor: This study was carried out by having each reaction in a separate reaction channel divided by a channel wall, across which the heat was exchanged between the channels, as shown in figure 1(b). Heat transfer and pressure drop were estimated using well-established engineering correlations from the literature. The axial temperature profile was assessed using the reaction performance parameters indicated in figure 2. As a starting point, both reaction channels were considered straight, having identical channel length, and filled with spherical catalyst particles, as an ideal packed bed reactor; whereas, the flow rate and the amount of catalyst were varied by adjusting the diameter of the reaction channel. Using these parameters, different design options were considered in this study.

Initial assessment of the study showed that an autothermal operation is achievable. Depending on the chosen reaction conditions and required reactor performances, the ratio of MDA to OCM catalyst varies widely between 1 to 100, which reflects in terms of the reactor geometry, like tube diameter and channel length. Based on the chosen reactor design, the OCM axial temperature profiles present peak temperatures varying from 50 to 300 ^oC, caused due to excess amount of heat released by OCM, than the amount of heat absorbed by MDA. This is related to the difference in the rates of both the reaction. Therefore, achieving acceptable reactor performances within the operating window draws a great challenge during the thermal coupling. To overcome these limitations, two approaches are utilized which consider non-traditional reactor channel geometry with and without catalyst profiling. Elaboration of these options will be provided while presenting this work.

Towards a global optimum solution and challenges hindering its realization: Several established catalysts are available for both reactions that have their own performances and challenges, and form different products depending on the operating conditions. The current study was performed using simple kinetic models which have a limited validity. In the case of OCM kinetic model, it was only valid for feed composition of 2.5 – 35% CH₄ at 1 atm, 600 – 750 ^oC and low residence times. It assumes C2 product as C₂H₆ and side product as CO₂. For the case of MDA kinetic model, the validity was limited for a feed of 95% CH₄ at 1 atm, 675 – 750 ^oC and three components C₂H₄, C₆H₆ and C₁₀H₈. Most importantly, this kinetic did not incorporated any coking phenomena, which is a major challenge particularly for the MDA reaction.

Changing these catalysts varies the reaction rates and hence reactor heat duties, which varies the final solution of thermally coupled reactors. Many other design parameters can also drive towards a different optimum, like catalyst shape, geometry and dimensions of the reaction channel. Hence, the presence of multiple design parameters leads to multiple local optima solutions using manual single-variable-at-a-time optimization. Thus, to be able to screen these solutions for achieving a global optimum, an automated multi-objective optimization will be performed. This study shall, therefore, pave the way towards identifying reactor designs for coupling exo- and endo-thermic reactions in a single reactor for achieving an autothermal operation; thereby, making it an energy efficient process with reduced carbon emissions.

References:

[1] H. Ritchie and M. Roser, https://ourworldindata.org/emissions-by-sector. [Accessed: 24-Mar-2021].

[2] S. Majhi, P. Mohanty, H. Wang, and K. K. Pant, Journal of Energy Chemistry, vol. 22, no. 4. Elsevier, pp. 543–554

[3] C. Karakaya and R. J. Kee, Progress in Energy and Combustion Science, vol. 55. Elsevier Ltd, pp. 60–97

[4] W. Wang and Y. S. Lin, J. Memb. Sci., vol. 103, no. 3, pp. 219–233

[5] Y. Zhu, N. Al-ebbinni, R. Henney, C. Yi, and R. Barat, Chem. Eng. Sci., vol. 177, pp. 132–138