What Is a Reactor in an Oil Refinery: Purpose, Types and How It Works Answering the core question: What is a reactor in an oil refinery? A reactor is the pressure vessel in which the chemistry happens, the one place in a refinery where molecules are deliberately broken apart or rebuilt rather than merely sorted. Everything else in the plant separates, heats, cools or moves material; the reactor changes what the material is. A hydrotreater runs at 300-400°C and 30-100 bar,