A membrane reactor model is developed to describe, model, and design molten metal methane pyrolysis bubble column reactors. It is utilized to demonstrate that a membrane reactor allows conversions in excess of the equilibrium conversion implied by the feed and operating conditions. Ultra-high conversion eliminates the need to separate product hydrogen from unreacted methane, thereby eliminating the need to recycle un-reacted methane and reducing the total equipment sizes and energy costs. Furthermore, it is shown that the hydrogen can be completely removed through the membrane reactor walls before the gas bubbles breakthrough the molten metal layer into the reactor headspace. The equations also apply to non-membrane reactors, and are therefore useful for future general conceptual design studies. The general applicability is demonstrated by comparison of the model predictions to published experimental data on methane pyrolysis in a non-membrane bubble column reactor. (C) 2019 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.