Method and apparatus for thermally processing polyester pellets and a corresponding pellet preparation

Abstract

Method and apparatus for thermally processing polyester pellets, e.g., polyethylene terephthalate pellets, in order to achieve a partial crystallization, whereby the polyester melt is fed to an underwater pelletizer and pelletized, the pellets obtained are fed to a water/solids separating device and the dried pellets are fed at a pellet temperature of greater than 100° C. to an agitation device that the pellets leave at a pellet temperature of over 100° C.

1 . A method for thermally processing polyester pellets in order to achieve partial crystallization, comprising feeding polyester melt to an underwater pelletizer followed by pelletizing the polyester melt in the underwater pelletizer to obtain pellets having a ball shape or lenticular shape, feeding the pellets to a water/solids separating device to dry the pellets so that the pellets have a pellet surface temperature of no less than 100° C. to no greater than 160° C., feeding the dried pellets at a pellet temperature of greater than 100° C. to an agitation device, transporting and agitating the pellets on the agitation device to obtain partial crystallization utilizing specific heat present in the pellets, and removing pellets from the agitation device at a pellet temperature of over 80° C.  
     
     
         2 . The method according to  claim 1 , further comprising flowing a fluid around the pellets during agitation of the pellets in the agitation device.  
     
     
         3 . The method according to  claim 1 , wherein the pellets are in a form of a pellet layer, and further comprising flowing a fluid around the pellet layer during agitation of the pellets in the agitation device.  
     
     
         4 . The method according to  claim 1 , wherein the pellets are fed to the agitation device at a pellet surface temperature of over 110° C.  
     
     
         5 . The method according to  claim 4 , further comprising flowing a fluid around the pellets during agitation of the pellets in the agitation device.  
     
     
         6 . The method according to  claim 4 , wherein the pellets are in a form of a pellet layer, and further comprising flowing a fluid around the pellet layer during agitation of the pellets in the agitation device.  
     
     
         7 . The method according to  claim 1 , wherein the pellets are conveyed from the underwater pelletizer to the water/solids separating device with hot process water.  
     
     
         8 . The method according to  claim 7 , wherein the process water has a temperature of 98° C.  
     
     
         9 . The method according to  claim 1 , wherein thermal processing leading to partial crystallization utilizes specific heat present in the pellets.  
     
     
         10 . The method according to  claim 1 , wherein the polyester pellets comprise polyethylene terephthalate pellets.  
     
     
         11 . An apparatus for carrying out a method for the thermal processing of polyester pellets in order to achieve a partial crystallization of the pellets, said apparatus comprising: a melt pump; a screen changer; an underwater pelletizer; a water/solids separating device; and a conveyor device for transporting pellets, said conveyor device being arranged downstream of said underwater pelletizer and said water/solids separating device, said conveyor device being constructed and arranged to agitate pellets and crystallize pellets during transport through specific heat of the pellets.  
     
     
         12 . The apparatus according to  claim 11 , wherein the conveyor device comprises a conveyor channel.  
     
     
         13 . The apparatus according to  claim 11 , wherein the conveyor device comprises a vibrating conveyor device.  
     
     
         14 . The apparatus according to  claim 13 , wherein the vibrating conveyor device comprises a conveyor channel.  
     
     
         15 . The apparatus according to  claim 11 , wherein the conveyor device comprises a plurality of spaced apart dams distributed over the length of the conveyor device, each of said plurality of spaced apart dams causing a damning up of material.  
     
     
         16 . The apparatus according to  claim 12 , wherein the conveyor device comprises a plurality of spaced apart dams distributed over the length of the conveyor channel, each of said plurality of spaced apart dams causing a damming up of material.  
     
     
         17 . The apparatus according to  claim 13 , wherein the vibrating conveyor device comprises a plurality of spaced apart dams distributed over the length of the vibrating conveyor device, each of said plurality of spaced apart dams causing a damming up of material.  
     
     
         18 . The apparatus according to  claim 14 , wherein the vibrating conveyor device comprises a plurality of spaced apart dams distributed over the length of the vibrating conveyor device, each of said plurality of spaced apart dams causing a damming up of material.  
     
     
         19 . The apparatus according to  claim 11 , wherein the conveyor device is surrounded at least in part by a housing.  
     
     
         20 . The apparatus according to  claim 11 , wherein the water/solids separating device comprises a centrifuge.  
     
     
         21 . The apparatus according to  claim 11 , wherein the water/solids separating device comprises a centrifuge.  
     
     
         22 . A polyester pellet crystallized at least to 40% by specific heat present in the pellet from its formation.  
     
     
         23 . A PET pellet crystallized by specific heat present in the pellet from it formation, wherein said pellet is obtained by a process comprising: forming hot PET into pellets with a core temperature of 130-180° C. by underwater granulation; cooling the surface temperature of the pellets with water to about 110° C., while the core temperature in the pellets is higher; separating the pellets from the water; and allowing the surface temperature of the pellets to rise to 140-150° C. to effect crystallization of the pellets.  
     
     
         24 . A polyester pellet crystallized at least to 40% by specific heat present in the pellets from in formation, which pellet has an outermost layer spheroylitic structure of a particle which is equal or smaller than at the center of the particle as distinguishable by polarization-contrast optical microscopy.  
     
     
         25 . The polyester pellet according to  claim 24 , wherein the pellet has a degree of crystallization at the center of a particle which is at least as great as the degree of crystallization of the outermost layer thereof.  
     
     
         26 . The polyester pellet according to  claim 24 , which has an acetaldehyde content between 0.5 and 100 ppm.  
     
     
         27 . The polyester pellet according to  claim 26  which has an acetaldehyde content between 0.5 and 70 ppm.  
     
     
         28 . The polyester pellet according to  claim 27  which has an acetaldehyde content between 0.5 and 60 ppm.  
     
     
         29 . The polyester pellet according to  claim 24  which has a heat of fusion of less than 50 kJ per kg.  
     
     
         30 . A method of producing a PET pellet, which comprises using specific heat present in the pellet from its formation to produce a pellet with at least 40% crystallization and an acetaldehyde content between 0.5 and 100 ppm.  
     
     
         31 . The polyester pellet according to  claim 25 , which has a heat of fusion greater than 50 kJ per kg.  
     
     
         32 . A PET polyester pellet produced by direct crystallization and having an opaque white coloring.  
     
     
         33 . A PET polyester pellet according to  claim 32  wherein the crystallization is at least 40%.  
     
     
         34 . A PET polyester pellet according to  claim 32  wherein during direct crystallization, the pellet surface has been cooled from a maximum temperature of 160° C. to approximately 110° C. and then allowed to heat to a surface temperature of 140 to 150° C. by transfer of heat energy from the pellet core.  
     
     
         35 . A PET polyester pellet according to  claim 32  that has undergone a change in color from a first translucent condition to opaque, white coloring during its direct crystallization.  
     
     
         36 . The method according to  claim 1  wherein the pellet surface temperature rises to 140° to 150° C. during the partial crystallization step.