Installation for purification by adsorption of a gaseous stream comprising at least one adsorber having a parallelepipedal-shaped casing arranged horizontally and comprising: • an inlet and an outlet for the gaseous stream, • two adsorbent masses in a fixed bed, each having a parallelepipedal shape as well, with faces parallel to the faces of the casing, and • a set of volumes allowing the gaseous stream to pass through the two adsorbent masses horizontally, in parallel, in opposite directions, over the entire cross-section of each of the adsorbent masses and over their entire thicknesses, the set of volumes comprising: a) an internal part comprising: • a first volume V1 for introducing and distributing or collecting fluids; • a second volume V2 and a third volume V3 each comprising an adsorbent mass and located on either side of the first volume V1; and b) a free part comprising two free volumes V4 and V5 for introducing and distributing or collecting fluids, located on either side of the internal part and between the internal part and the casing of the adsorber; the internal part being arranged symmetrically with respect to the median plane of the adsorber casing, the internal part having a solid bottom floor or a solid top ceiling and the first volume V1, the second volume V2 and the third volume V3 having vertical walls fixed in a sealed manner to the upper wall of the adsorber casing or to the solid top ceiling and to the lower wall of the adsorber casing or to the solid bottom floor, the set of volumes comprising, between the lower wall of the adsorber casing and the solid bottom floor, a space in fluid communication with the free volumes V4 and V5, or, the set of volumes comprising, between the upper wall of the adsorber casing and the solid top ceiling, a space in fluid communication with the free volumes V4 and V5. • an inlet and an outlet for the gaseous stream, • two adsorbent masses in a fixed bed, each having a parallelepipedal shape as well, with faces parallel to the faces of the casing, and • a set of volumes allowing the gaseous stream to pass through the two adsorbent masses horizontally, in parallel, in opposite directions, over the entire cross-section of each of the adsorbent masses and over their entire thicknesses, the set of volumes comprising: a) an internal part comprising: • a first volume V1 for introducing and distributing or collecting fluids; • a second volume V2 and a third volume V3 each comprising an adsorbent mass and located on either side of the first volume V1; and b) a free part comprising two free volumes V4 and V5 for introducing and distributing or collecting fluids, located on either side of the internal part and between the internal part and the casing of the adsorber; a) an internal part comprising: • a first volume V1 for introducing and distributing or collecting fluids; • a second volume V2 and a third volume V3 each comprising an adsorbent mass and located on either side of the first volume V1; and • a first volume V1 for introducing and distributing or collecting fluids; • a second volume V2 and a third volume V3 each comprising an adsorbent mass and located on either side of the first volume V1; b) a free part comprising two free volumes V4 and V5 for introducing and distributing or collecting fluids, located on either side of the internal part and between the internal part and the casing of the adsorber; the internal part being arranged symmetrically with respect to the median plane of the adsorber casing, the internal part having a solid bottom floor or a solid top ceiling and the first volume V1, the second volume V2 and the third volume V3 having vertical walls fixed in a sealed manner to the upper wall of the adsorber casing or to the solid top ceiling and to the lower wall of the adsorber casing or to the solid bottom floor, the set of volumes comprising, between the lower wall of the adsorber casing and the solid bottom floor, a space in fluid communication with the free volumes V4 and V5, or, the set of volumes comprising, between the upper wall of the adsorber casing and the solid top ceiling, a space in fluid communication with the free volumes V4 and V5.