NOVELTY - A containerized gas separation system comprises containers, each having interior, exterior and exterior volume defined by the exterior dimensions of the container, and pressure swing adsorption system installed in the interior of the container and to recover a product gas from a feed gas mixture. The pressure swing adsorption system being adapted to deliver the product gas at product gas flow rate. The ratio of the exterior volume of the container to the product gas flow rate is less than 2000 ft3/(ton per day).    USE - A containerized gas separation system for recovering product gas comprising greater than or equal to 80 vol.% oxygen, for separating nitrogen from air, for separating hydrogen from hydrogen-containing streams, for separating methane from e.g. coal seam gas or landfill gas, for recovering argon from industrial processes.    ADVANTAGE - The separation system is portable, simple to install since there is no need for foundations or cranes, and easily uninstalled and moved for deployment. This ease of redeploying the containerized systems allows more flexibility in commercial arrangements, thus providing commercial options in addition to standard multi-year liquid delivery arrangements or longer-term onsite arrangements. The modular containerized approach of the system meets the incremental product demands and allows flexible pricing terms for incremental supply to purchases.    DETAILED DESCRIPTION - The separation system comprises feed blower (109) for introducing the feed gas mixture into the feed end of the vessels, a vacuum blower (111) to withdraw waste gas from the product ends of the vessels, and a product gas storage tank (150). The pressure swing adsorption system comprises vessels (101, 103, 105, 107) having adsorbent material, product end and adsorbent material; piping for introducing feed gas mixture into feed ends of the vessel, for withdrawing product gas from the product ends of the vessels, for placing the product ends of any pair of vessels in flow communication and for withdrawing a waste gas from the feed ends of the vessels; and indexed rotatable multi-port valve (217, 229, 231) to place the product end of each vessel in sequential flow communication with the product end of each of the other vessels. The container is a container defined by the International Organization for Standardization. The adsorbent material selectively adsorbs nitrogen from air. The ratio of the exterior volume of the container to the product gas flow rate is less than 1000 (preferably less than 500) ft3/(ton per day). INDEPENDENT CLAIMS are included for:    (1) a method for recovering a product gas from a feed gas mixture, comprising introducing feed gas mixture into one vessel and withdrawing the product gas from that vessel; depressurizing the vessel by withdrawing depressurization gas from product end and transferring the depressurization gas to the product end of another vessel via the indexed rotatable multi-port product valve; withdrawing waste gas from the feed end of the first vessel; pressurizing the first vessel by introducing into the product end of the vessel a depressurization gas provided from another vessel; and repeating the steps of feed gas introduction up to vessel pressurization; and    (2) a method for the installation of a pressure swing adsorption system at an operating site, comprising assembling a containerized pressure swing adsorption system at an assembly site, transporting the assembled system to operating site, connecting utilities to the system and installing piping to provide product gas to gas consumer.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of four-bed pressure swing adsorption gas separation system.    Vessels (101, 103, 105, 107)    Feed blower (109)    Vacuum blower (111)    Product gas storage tank (150)    Indexed rotatable multi-port valve (217, 229, 231)