Pneumatic controller and alarm for adsorbent fractionaters, particularly

Abstract

A pneumatic controller is provided for a two-bed adsorbent fractionator, particularly a heaterless dessicant dryer, having pneumatically controlled inlet and exhaust flow valves and, optionally, a repressurization valve. The flow valves are controlled by a schedule of pneumatic pilot signals generated by the controller from a source of pressurized gas, e.g., gas tapped from and at the line pressure of the outlet of the fractionater. Control valves within the controller selectively apply the full pressure from the source as pilot signals to the flow valves of the fractionater. A variable regulated pressure is utilized to control these control valves according to a schedule established by the time periods to fill two timing volumes through variable orifices. The time period to fill one timing volume establishes the length of the regeneration-prior-to-repressurization period, with that for the other timing volume establishing the repressurization period, the two time periods together establishing the length of the on stream time for the on stream bed. In a preferred embodiment the controller may be programmed for either normally-open or normally-closed exhaust valves. An optional switching failure alarm circuit provides an indication of a switching failure as determined by the relative pressures of the two beds during the various phases of a cycle.