A portable oxygen concentrator is disclosed. An output of a high pressure compressor is fluidly coupled to an adsorbent device that increases the concentration of oxygen gas for storage in a high pressure accumulator. The adsorbent device is purged of the collected nitrogen via a valve fluidly coupled to the outlet of the high pressure compressor and the inlet of the adsorbent device. The outlet of the adsorbent device is fluidly coupled to the high pressure accumulator through a first valve. Additionally, a re-circulation loop fluidly couples the outlet of the adsorbent device to a valved inlet of the high pressure compressor.
