A device (SMB) for separation by simulated moving bed adsorption comprises a plurality of elementary zones Zi in series operating in a closed loop, each of said zones comprising, between two successive fluid injection or extraction points, a unique bed Li of adsorbant solid with volume VAi and a non-selective free volume Vi, in which the majority of the zones are ordinary elementary zones each having an identical volume VA of the same adsorbant S, and a substantially identical non-selective free volume equal to V;the device also comprises at least one particular elementary zone Z0, termed the particular elementary zone, comprising a non-selective free volume V0 which is greater than V, and a unique bed L0 of adsorbant S0 with volume VA0 which is less than VA, S0 having a volume adsorption capacity C0 which is greater than that, C, of S, such that the overall adsorption capacity C0×VA0 of Z0 is closer to the overall adsorption capacity C×VA of each of the ordinary elementary zones than the overall adsorption capacity of Z0 calculated as if the adsorbant solid of bed L0 of Z0 were not the solid S0 but the solid S.