Moisture resistant molecular sieve beds

An air separation system for an OBOGS comprising:

• a. first and second air separation units, each air separation unit comprising:
• a housing having an inlet for receiving a wet inlet air and an outlet for outputting a dry product gas, the housing including an outer side wall and two or more annular walls to thereby define a series of concentric annular chambers within the housing, wherein:
• i. a first annular chamber is defined by the outer side wall and a first annular wall and is fluidly coupled to the inlet, the first annular chamber including a desiccant material configured to receive the wet inlet air and output a dried air;
• ii. an unfilled second annular chamber defined by the first annular wall and a second annular wall and fluidly coupled to the first annular chamber via a first passageway, the housing including a dried air tap fluidly coupled to the second annular chamber whereby a portion of the dried air may be removed from the housing;
• iii. a third annular chamber defined by the second annular wall and a third annular wall and fluidly coupled to the second annular chamber via a second passageway at a first end and the outlet at a second end, the third annular chamber configured to receive air separation material to selectively remove unwanted constituents from the dried air and output the dry product gas;
• and
• b. a switchable valve assembly configured to receive the wet inlet air and selectively deliver the wet inlet air to the first air separation unit when in a first state and the second air separation unit when in a second state;
• and
• c. a plenum fluidly coupled to the dried air tap of each of the first and second air separation units at a first end and to the switchable valve assembly at a second end, the plenum configured to receive a portion of the dried air from the selected first or second air separation unit and selectively cycle the switchable valve assembly between the first state and the second state as a function of a purity of the dry product gas being output from the selected air separation unit receiving the wet inlet air.