in chemical engineering, device or vessel within which chemical processes are carried out for experimental or manufacturing purposes.

Reactors range in size and complexity from small, open kettles fitted with simple stirrers and heaters to large, elaborate vessels equipped with jackets or internal coils for heating or cooling, nozzles or ports for adding and removing materials, sources of ultraviolet radiation or electrical energy, specially designed agitators or scrapers, and strong walls and tight seals to permit operation at high or low pressures. In many cases, these reactors are provided with instruments that measure temperature, pressure, pH, or other properties of the contents. They are constructed of materials selected for strength and resistance to attack by the substances being processed.

For certain processes that are particularly adaptable to continuous operation, reactors are devised so that the chemical reaction may continue for weeks or months while starting materials are introduced continuously or intermittently and the finished product is removed in a steady stream or in periodic discharges. Modern petroleum refineries have reactors of the steady-stream type; steel mills rely on blast furnaces, which are continuous reactors charged and discharged intermittently.