A valve assembly for use in a gas purification system having a plurality of vessels each having a first port opening and a second port opening. The gas purification system includes a first valve element having at least a first aperture to selectively connect a first port opening of a vessel to an outlet of the first valve element. The gas purification system also includes at least a second valve element having a second aperture to selectively connect a second port opening of a vessel to an input of the second valve element. Also provided is a drive mechanism configured to cause intermittent movement. The first and second valve elements are intermittently moved by the drive mechanism such that the intermittent movement changes the vessel connected to the second aperture and the vessel connected to the first aperture.