Thermodynamics Analysis of Methane Cracking Using Part of the Generated Hydrogen as a Source of Energy for the Process

Abstract

Contrary to the methane steam reforming process, the methane thermal cracking route forms only hydrogen and solid carbon throughout the process. The generated hydrogen has a high degree of purity, and the solid carbon can be commercialized for other industrial purposes. This work seeks to thermodynamically characterize the thermal cracking reaction of methane to maximize the formation of hydrogen, minimize the impacts caused by the formation of coke throughout the process and discuss the possibility of using part of the generated hydrogen as a source of energy for the development of the process. Methodologies based on Gibbs energy minimization and entropy maximization are used, simulating operating conditions of isothermal and adiabatic reactors, respectively. The combined chemical and phase equilibrium problems were solved in the software GAMS version 23.9.5, with the aid of the solver CONOPT3. High temperatures and low pressures favour the decomposition of methane into hydrogen as expected due to the stoichiometry of the reaction and its endothermic effect. When conditioned to adiabatic reactors, the addition of hydrogen along with methane in the feed stream tends to maximize methane decomposition, minimizing the endothermic effect of the process. Setting the CH4/H2 ratio at 1:10 in the process feed at 1600 K, varying the system pressure from 50 to 1 bar, the methane conversion varies from 0 to 94.62 %, thus indicating the possibility of promoting the reaction by the effect of depressurization, which can be promoted by an isentropic valve. Keeping the CH4/H2 ratio at 1:6 at 1300 K and 50 bar, recycling part of the methane generated and using part of the hydrogen to generate energy for the process to occur, about 66% of the product stream is composed of hydrogen generated after the depressurization to 1 bar and of this total, approximately 38% would be destined to generate energy for the occurrence of the process, being this the optimal theoretical operational condition for this process.