Influence of hydrocarbon feed additives on the high-temperature pyrolysis of methane in molten salt bubble column reactors

Abstract

Molten salts are excellent heat transfer fluids and a reaction environment for methane pyrolysis to produce hydrogen and a solid carbon co-product. In a high temperature molten salt bubble column reactor, the carbon co-product of pyrolysis can be continuously produced, transported, and separated from the liquid salt and gas phase hydrogen. To make use of methane pyrolysis as a low-CO₂ process for hydrogen production, an economical process is necessary with high single-pass methane conversion and efficient, low-cost solid carbon separation and handling. In this work, we investigate the addition of low-cost, higher molecular weight hydrocarbons to the methane feed in molten KCl bubble column reactors as a means of increasing methane conversion during pyrolysis and reducing salt residues in the carbon product. Addition of only 2% (by volume) of ethane, propane, butane, acetylene, or benzene results in significant increases in the initial rates of methane decomposition. The rate continues to increase with increasing amounts of ethane and propane up to approximately 10%. We speculate that in KCl, which has little surface activity, the more facile bond cleavage of the additives increases the radical population in the gas phase. In the catalytic MnCl₂–KCl system, propane addition does not impact the methane decomposition rate, which is instead controlled by the rate of reaction at the gas–liquid interface. The carbon produced by pyrolysis in KCl from methane with hydrocarbon additives was observed to have less residual salt contamination than carbon produced from pure methane.