A comprehensive kinetic framework for solid carbon deposition and hydrogen production from the pyrolysis of light hydrocarbons streams

Thermal pyrolysis of hydrocarbons is a promising solution for the industrial production of hydrogen and valuable carbon materials. Pyrolysis reactor design and scale up strongly benefit from predictive chemical kinetic models capable to comprehensively describe the reactivity in the gas phase, including the undesirable formation of amorphous carbon (i.e., soot), as well as the solid carbon deposition mechanism. In this work, a methodology for the determination of rate constants of the heterogeneous growth of pyrocarbon deposit by means of theory-based corrections of analogous gas phase reactions is firstly proposed. Specifically, the theoretical methodology is applied to H-abstraction reactions governing the propagation of superficial active sites. Based on these findings, a detailed pyrocarbon deposition model from the literature is revised and coupled to a state-of-the-art model describing the dynamics of species evolution in the gas phase as well as the molecular growth of polycyclic aromatic hydrocarbons (PAHs) and soot. The model is validated with literature experimental data of pyrocarbon formation from light hydrocarbons feedstocks, covering a large set of operating conditions (pressure, temperature, surface over volume ratio). The comprehensive kinetic framework can reproduce the experimental deposition rates as well as the amount of deposited carbon with high fidelity under varying operative conditions. Moreover, kinetic analyses have been performed for assessing the relevant reaction pathways leading to pyrocarbon deposition from propane and methane feedstocks as well as the competition between carbon deposition and amorphous carbon formation.

Automated summary

Thermal pyrolysis of hydrocarbons has potential for industrial production of hydrogen and valuable carbon materials. Predictive chemical kinetic models are essential for designing and scaling up pyrolysis reactors. This work proposes a methodology for determining rate constants of heterogeneous growth of pyrocarbon deposit using theory-based corrections of gas phase reactions. The methodology is applied to H-abstraction reactions, and a revised pyrocarbon deposition model is coupled to a state-of-the-art model describing gas phase dynamics and molecular growth of PAHs and soot. The comprehensive kinetic framework is validated with experimental data and can accurately simulate deposition rates and amount of deposited carbon under different operating conditions.