An heterogeneous chemical reactor of the type with a partial opening, incorporating a catalytic cartridge comprising a substantially cylindrical basket (11) suitable for containing a predetermined amount of a predefined catalyst is distinguished by the fact that the basket (11) is modular and is made of a plurality of containers (14) which are structurally independent from each other, which can be manipulated individually and which are associated with each other to form the basket (11) in a reciprocally removable way, each container (14) being of a suitable size to pass through the partial opening (9) of the corresponding reactor (1).