Method and apparatus for making crystalline PET pellets

Abstract

A method and apparatus for underwater pelletizing and subsequent drying of polyethylene terephthalate (PET) polymers and other high temperature crystallizing polymeric materials to crystallize the polymer pellets without subsequent heating. High velocity air or other inert gas is injected into the water and pellet slurry line to the dryer near the pelletizer exit. Air is injected into the slurry line at a velocity of from about 100 to about 175 m 3 /hour, or more. Such high-speed air movement forms a vapor mist with the water and significantly increases the speed of the pellets into and out of the dryer such that the PET polymer pellets leave the dryer at a temperature sufficient to self-initiate crystallization within the pellets. A valve mechanism in the slurry line after the gas injection further regulates the pellet residence time and a vibrating conveyor after the dryer helps the pellets to achieve the desired level of crystallinity and to avoid agglomeration.

1 . A method for processing PET polymers into pellets, which comprises: 
 extruding strands of PET polymer through a die plate into an underwater pelletizer;    cutting the PET polymer strands into pellets in said pelletizer;    transporting said PET pellets out of said pelletizer to a dryer using a water stream, and injecting a high velocity gas into said water stream to enhance the speed of the pellets into and out of said dryer; and    passing the pellets through a vibrating unit while said pellets retain sufficient heat to initiate crystallization of said pellets.    
   
   
       2 . The method as claimed in  claim 1  wherein said PET pellets exit said dryer at a mean temperature above about 135° C. and said gas is injected at a velocity of at least 100 m 3 /hour.  
   
   
       3 . The method as claimed in  claim 1  wherein said PET pellets exit said dryer at a temperature above about 125° C. and said gas is injected at a velocity of about 175 m 3 /hour.  
   
   
       4 . The method as claimed in  claim 1  wherein said pressurized gas is air at a velocity of between 100-175 m 3 /hour.  
   
   
       5 . The method as claimed in  claim 4  wherein said pressurized gas is injected into said water stream substantially in alignment with said water stream.  
   
   
       6 . An apparatus for processing PET polymers into pellets which comprises an underwater pelletizer to cut PET polymer stands extruded into said pelletizer into pellets, piping to introduce water into said pelletizer and a slurry line to transport a water and pellet slurry out of said pelletizer and to a centrifugal dryer for drying said PET pellets, an injector to introduce a pellet speed expediter into said water and pellet slurry to enhance the speed of said pellets through said processing apparatus, and a post pelletizer unit to receive pellet output from said dryer, said pellets exiting said dryer with sufficient internal heat to initiate crystallization of said pellets.  
   
   
       7 . The apparatus as claimed in  claim 6 , wherein the pellet speed expediter is an inert gas moving at a velocity of between 100-175 m 3 /hour.  
   
   
       8 . The apparatus as claimed in  claim 6 , wherein said post pelletizer unit is a vibration unit that keeps said pellets in movement during said crystallization.  
   
   
       9 . The apparatus as claimed in  claim 6 , wherein said post pelletizer unit is a heat insulating container.  
   
   
       10 . The apparatus as claimed in  claim 6 , wherein a portion of said slurry line is straight and angled upwardly at an angle between 30° and 60°.  
   
   
       11 . The apparatus as claimed in  claim 7 , wherein said slurry line includes a straight portion and said gas injector introduces said inert gas at a beginning of said straight portion, a ball valve serving to regulate residence time of the pellets in said apparatus.  
   
   
       12 . The apparatus as claimed in  claim 11 , wherein said gas injector introduces said inert gas into said water and pellet slurry substantially in alignment with a longitudinal axis of said slurry line straight portion.  
   
   
       13 . A method for processing high temperature crystallizing polymeric materials into pellets, which comprises: 
 extruding a high temperature crystallizing polymeric material into strands;    water cooling and cutting the extruded strands into pellets;    transporting said pellets using a water stream injected with a high velocity insert gas to form a vapor mist that reduces pelletization processing time of the pellets to a degree that said pellets retain sufficient heat to initiate crystallization of said polymeric material without the application of external heat; and    passing said pellets onto a vibrating conveyor where said pellets are jostled while undergoing said crystallization.    
   
   
       14 . The method of  claim 13 , further comprising the step of placing said pellets into a heat insulating container to complete the desired crystallization.  
   
   
       15 . The method of  claim 13 , wherein said materials include PET polymer.  
   
   
       16 . The method of  claim 13 , wherein said inert gas is injected into said pellet and water stream at a velocity above 100 m 3 /hour.  
   
   
       17 . The method as claimed in  claim 1 , wherein said injected gas forms a gas pellet and water vapor mist mixture which is regulated by a valve mechanism downstream of said air injection.  
   
   
       18 . The apparatus as claimed in  claim 6 , further comprising a valve mechanism downstream of said injector to further regulate said pellet speed.  
   
   
       19 . A method for processing PET polymers into pellets, which comprises: 
 extruding strands of PET polymer through a die plate into an underwater pelletizer;    cutting the PET polymer strands into pellets in said pelletizer;    transporting said PET pellets out of said pelletizer to a dryer using a water stream, and injecting a high velocity gas into said water stream to enhance the speed of the pellets into and out of said dryer; and    regulating the speed of said pellets after said gas injection.