Apparatus particularly applicable to the drying of gases is provided for downflow or upflow adsorption of one or more first gases from a mixture thereof with a second gas to reduce the concentration of first gas in the mixture to below a permissible maximum concentration, having at least two sorbent beds, of which one is on adsorption while the other is being regenerated, and pneumatically operated valves for flow control of gas through the beds for adsorption and for regeneration. An overriding control such as a timer or a microprocessor programmed to control the regeneration time and purge flow fixes the cycling time, and switches the adsorbent beds at the end of each cycle period.