A tall cylindrical process vessel whose height is considerably greater than its diameter and used for unit operations such as distillation, absorption, and various forms of gas–liquid and liquid–liquid extraction processes. It usually allows liquids to descend under gravity and contact rising gases or vapours, or liquids of lesser density in which there is intimate contact between the two to allow equilibrium to be reached. Some types of columns are empty in which the liquids are sprayed in the form of droplets, while others have internal features such as baffles, plates, or packing materials used to promote the intimate contact between the ascending and descending materials. See tower.