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 m3 / 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.

Description

RELATED APPLICATIONS [0001] This is a continuation-in-part application of copending application, Ser. No. 10 / 717,630, filed Nov. 21, 2003.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention generally relates to a method and apparatus for underwater pelletizing and subsequent drying of polyethylene terephthalate (PET) polymers. More specifically, the present invention relates to a method and apparatus for underwater pelletizing PET polymers and subsequent drying of the PET polymer pellets in a manner to self-initiate the crystallization process of the PET particles and produce pellets having a desired level of crystalline structure rather than an amorphous structure. [0004] 2. Description of the Prior Art [0005] Underwater pelletizing systems for producing pellets of polymeric or other plastic materials has been known for many years. The starting materials such as plastic polymers, coloring agents, additives, fillers and reinforcing agents, and m...

AI Technical Summary

Benefits of technology

[0013] The present invention is directed to an underwater pelletizing system that produces PET pellets in a hot enough condition to self-initiate the crystallization process therein and ultimately provide a sufficiently crystalline character such that the PET pellets do not require a separate heating step in order to undergo end user processing. It has been discovered that this elevated heat condition can be accomplished by reducing the residence time of the pellets in the water slurry in order to leave enough heat in the PET pellets during the drying stage so that the crystallization process is initiated from inside the pellets. To do this, it is necessary to separate the pellets from the water as soon as possible and to significantly increase the speed of pellet flow from the exit of the underwater pelletizer and into and through the dryer. The hot pellets leaving the dryer can then be carried on a conventional vibrating conveyor or other vibrating or handling equipment for a time sufficient to achieve the desired crystallinity and avoid agglomeration. The hot pellets can also be stored in a heat retaining condition, such as in a heat insulating container, to complete the desired crystallization process. For example, coated steel or plastic containers should be acceptable, instead of the stainless steel boxes conventionally used.

[0014] The early pellet / water separation and increased pellet speed through the pelletizer system is accomplished in accordance with the present invention by injecting air or other suitable gas into the transportation piping leading from the pelletizer to the dryer just after the cut pellets and water slurry exit the water box of the pelletizer unit. It has been found that the injected air serves to separate the water from the pellets in the transportation piping by converting the water to a water vapor mist, significantly speeds up the transport of the pellets to and through the dryer, and can serve to generate a pellet temperature exiting the dryer that is sufficiently high to initiate the crystallization process within the pellets. Specifically, while the PET polymer pellets may come out of the dryer in an amorphous condition, there is still sufficient heat remaining inside the pellets for crystallization to occur. The extent of the crystallization is sufficient to eliminate the necessity of the second heating stage heretofore required to make PET pellets using previous underwater pelletizing systems.

[0015] The air introduced into the slurry line leading to the dryer immediately after the exit from the water tank is at a very high velocity. It has been found that an air volume of from at least 100 cubic meters (m3) / hour, to about 175 m3 / hour, or more, through a valve at a pressure of 8 bar and into a 1.5 inch slurry pipe line produces the requisite air velocity for the present invention. The volume of air introduced into the exiting water and pellet slurry produces an overall gas / slurry mixture in the nature of a mist and is likely to have a gas component of 98%-99% or more by volume of the overall mixture. The air injection into the slurry line dramatically increases the speed of the pellet flow from the water box to the exit of the dryer to a rate less than one second. While air is the preferred gas in view of its inert nature and ready availability, other inert gases such as nitrogen or similar gases could be used. Other pellet speed expediting methods that would comparably separate the liquid water from the pellets and accelerate the pellets from the pelletizer to the dryer exit might also be employed.

Problems solved by technology

A major drawback of using underwater pelletizing, as well as other pelletizing systems, for processing PET into pellet shapes is the typically amorphous condition of these pellets when they leave the dryer of the underwater pelletizing system.

Typically, increasing the water flow through the water box of the underwater pelletizer and increasing the water temperature, along with pipe dimensional changes and reducing the distance between the pelletizer and dryer unit, does not help to sufficiently maintain the pellet temperature.

First, PET polymers are very hygroscopic, and crystalline PET pellets absorb considerably less moisture during shipment and storage than amorphous PET pellets.

Otherwise, the amorphous PET polymer pellets may agglomerate and destroy the pellet form.

This is a very expensive second step in order to convert the PET polymer pellets into the desired crystalline state.

Images