A valve for carrying out a mass-transfer process, in which a gas flows through liquid standing on a separating tray, when the gas supply is interrupted the liquid flows into a lock and, when the gas supply is restarted, the liquid flows out of the lock, wherein the valve contains a closing element which is designed in such a manner that, with the closing element in a first valve position, a first opening in a lower tray of the lock is closable, and the closing element is conducted in a first sleeve which is positioned between the upper tray bordering the lock and the lower tray in which sleeve, in the region of the lock, second openings are formed, and which projects over the upper tray and has a lift limiter for the closing element.