Plant and method for drying and solid state polycondensing of polymeric material

A plant (1) for drying and solid state polycondensing a moisture-containing polymeric material (M) in granular form, comprising:

• a conduit (2) feeding the polymeric material (M) to be treated in a longitudinal direction (L);
• at least one treatment zone (7) for treating the polymeric material (M), at least one treatment zone (7) being located along said conduit (2);
• a blower (6) blowing an inert gas into said conduit (2);
• and
• a radiating device (8) emitting an alternating electromagnetic field in a radio-frequency band, for thermal treatment of the polymeric material (M), such that the polymeric material can be dried and upgraded;
• wherein said radiating device (8) comprises a plurality of applicators (9) located in correspondence of said treatment zone (7) and external to said conduit (2), in longitudinally offset positions, said applicators (9) being connected to an electromagnetic wave generator (10);
• wherein said applicators (9) comprise pairs of opposed radiating elements (11, 11′) each connected to a respective terminal (14, 16) of said electromagnetic wave generator (10), said radiating elements (11, 11′) being substantially perpendicular to said longitudinal direction (L) to provide field lines (F) at least partially parallel to a direction of feed (L) of the material (M) and to define magnetic dipoles with opposite polarities along said conduit (2), and
• wherein radiating elements (11, 11′) consist of pairs (22) of bars (19) made of an electrically conductive material, and extending substantially transverse to said conduit (2).

A method of drying and solid state polycondensing a polymeric material (M) in granular form obtained by polycondensation, the method comprising the steps of:

• providing a plant comprising,
• a conduit (2) feeding the polymeric material (M) to be treated in a longitudinal direction (L),
• at least one treatment zone (7) for treating the polymeric material (M), at least one treatment zone (7) being located along said conduit (2),
• a blower (6) blowing an inert gas into said conduit (2), and
• a radiating device (8) emitting an alternating electromagnetic field in a radio-frequency band, for thermal treatment of the polymeric material (M), such that the polymeric material can be dried and upgraded,
• wherein said radiating device (8) comprises a plurality of applicators (9) located in correspondence of said treatment zone (7) and external to said conduit (2), in longitudinally offset positions, said applicators (9) being connected to an electromagnetic wave generator (10), and
• wherein said applicators (9) comprise pairs of opposed radiating elements (11, 11′) each connected to a respective terminal (14, 16) of said electromagnetic wave generator (10), said radiating elements (11, 11′) being substantially perpendicular to said longitudinal direction (L) to provide field lines (F) at least partially parallel to a direction of feed (L) of the material (M) and to define magnetic dipoles with opposite polarities along said conduit (2);
• providing a predetermined amount of the polymeric material (M) to be treated;
• introducing said predetermined amount of the polymeric material (M) to be treated into said conduit (2);
• feeding the polymeric material (M) along said conduit to said at least one treatment zone (7) equipped with said radiating elements (11, 11′);
• actuating said generator (10) and generating a radio-frequency electromagnetic field onto the polymeric material (M) using said applicators (9) for the thermal treatment of the polymeric material (M);
• blowing the inert gas into said conduit (2) to facilitate solid state polycondensing of the polymeric material (M), in combination with said electromagnetic field;
• wherein said radiating elements (11, 11′) are designed to generate the alternating electromagnetic field in said conduit (2), with the field lines (F) at least partially parallel to the feed direction (L) of the material (M),
• wherein said alternating electromagnetic field has the magnetic dipoles with opposite polarities between a first radiating element (11) and a second radiating element (11′) next to the first radiating element,
• wherein said treatment zone (7) is maintained at an average temperature ranging from 120° C. to 180° C., and
• wherein a feed flow of the polymeric material (M) is adjusted to keep the polymeric material (M) to be treated within said conduit (2) for a time ranging from 30 sec to 7 minutes with an average flow rate of the polymeric material ranging from 4 to 11 kg/h.