Adsorber and process for the separation by adsorption

An adsorber having an upright longitudinal extent includes two perforated parallel panels extending within the vessel formed therein and spaced apart defining an adsorbent mass chamber. The adsorbent mass is in two longitudinal parts having a first part of fine particles and a second part of larger particles. The gas to be treated is circulated horizontally between the two perforated panels. This device allows the treatment cycle to be less than 60 seconds with the same or better performance. This adsorber and process are preferably used for the separation of oxygen from air.