Abstract: The saturation front in each dessicant bed of a twin tower fractionating apparatus is located by sensizing a temperature gradient associated with the advancing saturation front. A pair of temperature sensors operatively sense the temperature in front of and behind the saturation front to provide a saturation front signal. The saturation front signal operatively controls the cycling of the inlet fluid mixture between the two dessicant beds of the fractionating apparatus. By sensing the location of the saturation front before it breaks through the dessicant bed, cycling occurs before each bed becomes fully saturated, but the substantial capacity of the dessicant bed is utilized to prolong the lifetime of the dessicant and to more effectively utilize energy by eliminating excessive waste purge gas. A maximum extend time period is provided for cycling the inlet flow mixture when a saturation front signal is not supplied under low flow rate conditions.