Molten salt bubble columns for low-carbon hydrogen from CH4 pyrolysis: Mass transfer and carbon formation mechanisms

Abstract

• Varied CH 4 bubble SA:V ratios in molten salt bubble columns to elucidate mass-transfer mechanisms. • A specific surface area dependent reaction rate of 0.64 mmol CH 4 m −2 s −1 was identified. • Particulate suspensions in molten salts highlight a mass transfer limitation at the bubble surface. • Reaction kinetics and mechanisms of pyrolytic carbon formed in molten salts are evaluated. • Insights are key to designing industrial-scale CH 4 pyrolysis processes and to tailoring carbon. Methane pyrolysis in molten salts could potentially provide a large-scale, low-cost and low-CO 2 production route of H 2 from CH 4 . The interplay of reaction and transport phenomena in molten pyrolysis reactors are currently poorly characterised and further understanding of the mass transfer and mechanistic pathways elucidated here may prove key to designing separation strategies that favour minimum carbon contamination and maximised production rates. Kinetic experiments were carried out at 850–1000 °C in a 590 mm bubble column of a eutectic mixture of molten NaBr and KBr. The use of four different injector sizes allowed for the variation of the mean surface area of the CH 4 bubbles in the range of ~600–1200 m 2 m −3 (0.77–2.9 cm 2 per bubble). For the first time, a clear decoupling of bubble specific surface area and volume-based CH 4 reaction rates in molten pyrolysis systems was demonstrated. These phenomena become more complex with catalyst particles present. Tests with γ-Al 2 O 3 particles of varying sizes and with varying particle loadings were undertaken. The carbon depositing on the particles has a high catalytic activity and the material is thus a promising material for structured packings. Two clearly distinguishable kinetic regimes were identified, which transitioned at a γ-Al 2 O 3 loading of ~1.25 wt%. We propose that the reaction is primarily enhanced by a shuttling mechanism of γ-Al 2 O 3 adsorbing hydrocarbons (HC) in the HC-rich liquid-side film surrounding the bubble and desorbing it in the HC-lean liquid bulk. The findings provide a basis for (i) further models to extract kinetic parameters from CH 4 pyrolysis in molten salt particle suspensions and (ii) for reaction rate modelling of industrial-scale pyrolysis reactors.