A stackable modular element comprises a parallelepipedal caisson, the caisson comprising at least one layer of thermal insulation of thickness less than one-third of the width of the caisson, the layer of insulation covering at least the lateral and frontal faces of the caisson and surrounding at least one chamber having a parallelepipedal volume within the caisson, the chamber containing at least one body of material that permits the exchange of mass and/or of heat, the body being parallelepipedal in shape and filling at least part of the chamber, the chamber having an opening on the upper face and/or an opening on the lower face to allow fluid to be transferred to the body from outside the element and/or from the body to outside the element.