Multi Phase Catalytic Reactor: Gas-Liquid-Solid Systems and Design A multi phase catalytic reactor is a vessel where a gas, a liquid, and a solid catalyst must all meet at the same place for the reaction to occur. This three-phase situation is common in hydrogenation, oxidation, and upgrading of liquids, where hydrogen (gas) reacts with a dissolved compound (liquid) on a solid catalyst. The engineering difficulty is that each phase must reach the others despite resisting mix,