Chromatography apparatus and methods are described, especially for expanded bed adsorption. A column tube has a process fluid input device at the bottom and a movable piston in the top. The piston is enclosed in the column by a cover plate. The piston body has an inflatable seal, and is connected by a frame to a contact ring which carries another inflatable member to contact the tube wall. Process fluid leaves the operating volume through an opening of the piston and flexible hose, through the enclosed space and out through the cover plate. The space above the piston can be pressurised to control piston movement. The contact ring maintains piston alignment. The inflatable seals are used to fix the piston in position, allow it to slide or allow washing. The piston outlet may include a vortex-inhibitor. Bed and piston levels may be monitored by ultrasound sensors.