A test unit for the study of catalysts in short contact time reactions between a catalyst and at least one reagent, with a down transported flow reactor (4), with a load inlet (13, 13a, 13b) and a discharge outlet (14) linked with an admission inlet (15) to an upper chamber (39) of a separator (5) internally divided into an upper chamber (39) and a lower chamber (41) by a porous element (40) that is permeable to gases and impermeable to solid particles of catalyst, and a first preheater (1) provided with an outlet (2, 2a) connected with the inlet (13, 13a, 13b) of the reactor (4) via a load duct (42) in such a way that arranged between the outlet (2, 2a) and the load inlet (13, 13a, 13b) are some obturator means (3, 3a) in which the catalyst is heated to a desired temperature before they are loaded into the reactor (4).