A hopper structure for the dehumidification of granular plastic material by means of a dehumidifying process fluid, including: - a main body, bearing, at the top in use, a closure wall with loading opening or mouth for the granular plastic material to be treated; - at least one discharge mouth or opening for the process fluid provided on said main body (1 a) or on said closure wall (1 b), - a tapered lower section terminating with a discharge opening for the dehumidified granular plastic material, - an insert member fluid-sealed and positionable in the main body and in the tapered section so as to delimit an annular air space therewith, said insert member comprising at least one conical or frustoconical lower part, which tapers towards bottom, and an upper part, and - at least one dehumidifying fluid feed duct to said annular air space, wherein the lower tapered section (1 c) comprises at least one pair of walls or wall segments: a wall or wall segment (9, 23, 22) at least partially enclosing the other wall or wall segment (8, 21), so as to delimit at least one chamber therewith that is outside said annular air space and in fluid communication therewith, said at least one chamber being feedable with dehumidifying process fluid from said at least one feed duct oriented tangentially or parallel to said enclosing wall or wall segment (9, 23, 22), and wherein said upper part of said insert member is cylindrical and wherein said lower tapered section comprises an upperfrustoconical segment, a cylindrical intermediate segment and a lower frustoconical segment, the intermediate segment and the lower frustoconical segment constituting a wall enclosing the upper frustoconical segment, the upper frustoconical segment having conicity (β) lower than the conicity (α) of said lower frustoconical segment and being extended therein so as to delimit said chamber, said upper frustoconical segment: - being extended inside and over the entire length of the intermediate and lower segments; - communicating at its own lower end with a discharge mouth; and - communicating and being connected with a sleeve duct, to which the lower end of said lower frustoconical segment is also connected, wherein said chamber is in fluid communication with said annular air space through a plurality of small holes, said plurality of small holes being made at a band of said upper frustoconical segment for a height corresponding to at least the height of said intermediate section. - a main body, bearing, at the top in use, a closure wall with loading opening or mouth for the granular plastic material to be treated; - at least one discharge mouth or opening for the process fluid provided on said main body (1 a) or on said closure wall (1 b), - a tapered lower section terminating with a discharge opening for the dehumidified granular plastic material, - an insert member fluid-sealed and positionable in the main body and in the tapered section so as to delimit an annular air space therewith, said insert member comprising at least one conical or frustoconical lower part, which tapers towards bottom, and an upper part, and - at least one dehumidifying fluid feed duct to said annular air space, - being extended inside and over the entire length of the intermediate and lower segments; - communicating at its own lower end with a discharge mouth; and - communicating and being connected with a sleeve duct, to which the lower end of said lower frustoconical segment is also connected,