The invention relates to a valve for carrying out a mass-transfer process in which a gas (13) flows through a liquid (31) standing on an upper tray (21), when the gas supply is interrupted the liquid (31) flows into a lock (17) beneath the tray (21) and, when the gas supply is restarted, the liquid (31) flows out of the lock (17) through a lower tray (19) bordering the lock (17) downward, wherein the valve (1) comprises a closing element (5) which comprises a piston rod (5), an upper valve disk (9) and a lower valve disk (7), and which is conducted in a sleeve (3), wherein the lower valve disk (7), in a first valve position, closes an opening (43) at the lower end of the sleeve (3). The upper valve disk (9) and the lower valve disk (7) are each movably connected to the piston rod (11).