A process for the continuous and/or discontinuous solid phase polymerisation of polyester in order to increase its molecular weight, comprising the steps of:
- providing a mass of low molecular weight polyester granules (F), crystallised and at high temperature, of at least a polyester;
- providing a substance, or a mixture of substances, (R) reactive with respect to said at least a polyester, said substance, or mixture of reactive substances, (R) being selected from the category of the molecules having reactive groups or reactive end-groups with respect to the constitutive groups of the macromolecules forming said at least a polyester, said reactive groups or reactive end-groups being at least in the number of two for each molecule of reactive substance (R), whereby the reactive substance, or mixture of reactive substances, (R) acts as chain extension units, or chain extenders, to achieve higher, and/or faster, and/or at a lower temperature, polymerisation degree (i.e. IV values) increase;
- introducing said granules (P) into a reactor (R1) of the static type or of the moving be type or of the fluidised bed type or of the stirred bed type, having a substantially cylindrical or parallelepipedal shape, vertical or horizontal;
- preparing a fluid stream (F) preferably by diluting the reactive substance, or the mixture of reactive substances, (R) in a carrying fluid or “carrier” (V), said carrying fluid (V) having high volatility and high diffusion coefficient in the polyester;
- providing a purging inert gas stream (G) purified and heated inside a suitable device (E1), preferably inside a heat exchanger, up to a temperature lower than the boiling point of the fluid stream (F), that is lower than the boiling point of the carrying fluid (V) and/or the reactive substance (R) forming said fluid stream (F);
- feeding the fluid stream. (F) and the hot purging inert gas stream (G) in counter-current flow or in co-current flow into a gas-liquid contactor (C1), wherein the creation of contact area between the phases and the residence time allow to reach up to preferably saturate said hot purging inert gas stream (G) with said fluid stream (F), thus originating a reactive gas stream or reactive atmosphere or gaseous composition (A), said gas-liquid contactor (C1) operating at a temperature lower than the boiling point of the fluid stream (F), that is lower than the boiling point of the carrying fluid (V) and/or of the boiling point of the reactive substance or mixture of reactive substances (R), which are the constituents of the fluid stream (F) itself;
- feeding the reactive gas stream (A) into the reactor (R1) and make it flow in contact with the polyester granules (P), thus allowing that diffusion phenomena of fluid stream (F) occur from outside each granule to inside thereof;
- letting that, once arrived inside the granule, preferably thanks to the carrying action of the carrying fluid (V), the reactive substance or mixture of reactive substances (R) reacts with the polyester macromolecules through a mainly polyaddition mechanism, thus causing an intrinsic viscosity (IV) increase of said at least one polyester.