Patent Publication Number: US-2010117267-A1

Title: Process for pelletizing pet

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/114,301 filed on Nov. 13, 2008, hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to recycling a polymer. More particularly, the invention is directed to a pelletizing system and a method for pelletizing a polymer. 
     BACKGROUND OF THE INVENTION 
     Post-consumer processing of recycled polyethylene terephthalate (RPET) to manufacture a variety of useful consumer products such as flower pots and fence posts is well-known. Typically, the recycling process utilizes used PET containers, such as discarded carbonated beverage containers, which are collected, sorted, washed, and separated from contaminants to yield a relatively clean source of RPET. Additionally, the manufacture of imperfect and damaged molded PET products, particularly the blow molded bottles for use in containing consumer goods, results in a considerable amount of PET waste which the manufacturers of such products would like to reuse. The RPET produced by conventional recycling processes is generally in ground or flake form, which is thereafter melt processed or further pelletized by the end user. 
     RPET is typically subjected to a grinding operation in order to make the material easier to handle and process. Conventional grinding equipment reduces the RPET to about ⅜ inch particles or flakes. The grinding is conducted in a manner to insure that a consistent flake size will be produced, by employing a grate or screen through which the ground material must pass upon exiting the grinder. Although conventional RPET flakes melt processing and pelletizing equipment is designed to handle ⅜ inch flakes, some RPET materials having sizes as large as ½ inch and as small as ¼ inch are also commercially produced. The bulk density of ⅜ inch flake RPET generally ranges from about 22 to about 35 pounds per cubic foot. 
     Similarly, RPET and PET pellets are generally formed to a standard, uniform size about 0.12 inch in diameter. The bulk density of such pellets generally ranges from about 50 to about 58 pounds per cubic foot. Typically, PET and RPET melt processing equipment is designed to accept pellets having the above mentioned dimensions and physical characteristics. 
     The critical aspect for achieving consistently high quality end products utilizing RPET is comprehensive decontamination of the RPET flakes or pellets. Currently, significant decontamination occurs during the washing and sorting of PET scrap. The incoming PET bottles and containers are comminuted to form PET fragments and to remove loose labels, dirt, and other adhered foreign particles. Thereafter, the mixture is air classified and the remaining fragments are washed in a hot detergent solution to remove additional label fragments and adhesives from the PET fragments. The washed PET fragments are then rinsed and placed in a series of flotation baths where heavier and lighter weight foreign particles are removed. The remaining PET fragments are then dried and sold as RPET flakes. Thus, label and basecup glues, polyolefins, PVC, paper, glass, and metals, all of which adversely affect the quality and performance of the finished product, are removed from the RPET. 
     It would be desirable to develop a pelletizing system and a method for pelletizing a polymer, wherein the polymer is decontaminated to exhibit a residual contaminant level which would make it acceptable for manufacturing new food-grade polymer bottles and containers. 
     SUMMARY OF THE INVENTION 
     Concordant and consistent with the present invention, a pelletizing system and a method for pelletizing a polymer, wherein the polymer is decontaminated to exhibit a residual contaminant level which would make it acceptable for manufacturing new food-grade polymer bottles and containers, has surprisingly been discovered. 
     In one embodiment, a pelletizing system comprises: a pellet mill for compressing a quantity of polymer flake of a predetermined size to produce a pellet; and a decontamination subsystem for heating at least one of the polymer flake and the pellet to remove contaminants therefrom. 
     The invention also provides methods for pelletizing a polymer. 
     One method comprises the steps of: compressing a quantity of polymer flake of a predetermined size to produce a pellet; and heating at least one of the polymer flake and the pellet to remove contaminants therefrom. 
     Another method comprises the steps of: processing a polymer flake to a powder having a first pre-determined size; compressing a quantity of the powder to produce a plurality of pellets; and heating at least one of the polymer flake, the powder, and the pellets to remove contaminants therefrom. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a schematic block diagram of a pelletizing system according to an embodiment of the present invention; 
         FIG. 2  is a schematic representation of a size control subsystem of the pelletizing system of  FIG. 1 ; 
         FIG. 3  is a schematic representation of a mill subsystem of the pelletizing system of  FIG. 1 ; 
         FIG. 4  is a schematic representation of a decontamination subsystem of the pelletizing system of  FIG. 1 ; and 
         FIG. 5  is a schematic representation of a material transfer subsystem of the pelletizing system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     The following detailed description and appended drawings describe and illustrate various embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
       FIGS. 1-5  illustrate a pelletizing system  10  according to an embodiment of the present invention. The pelletizing system  10  includes a size control subsystem  12 , a mill subsystem  14 , and a decontamination subsystem  16 , and a material transfer subsystem  18 . It is understood that any number of subsystems may be included. 
     As more clearly shown in  FIG. 2 , the size control subsystem  12  includes an infeed loader  20 , a first separator  22 , a size control device  24 , a cyclone  26 , and a first screener  28 . 
     The infeed loader  20  receives an infeed stream of a PET material from at least one of a source line  30  and a feedback line  32 . In the embodiment shown, the infeed loader  20  is in fluid communication with the material transfer subsystem  18  for dust collection. It is understood that the infeed loader  20  is adapted to receive any size material, flake, or particle therein. As a non-limiting example, the PET material is a PET flake such as a washed bottle flake. As a further example, the source line  30  is adapted to receive the PET material from at least one of a curbside source and a deposit source. 
     The first separator  22  receives the PET material from the infeed loader  20 . The first separator  22  detects and removes a particular contaminant. As a non-limiting example, the first separator  22  detects and removes at least one of a ferrous metal and a non-ferrous metal. It is understood that other contaminates and materials may be detected and removed by the first separator  22 . 
     The size control device  24  receives the PET material from the first separator  22 . The size control device  24  processes the PET material to a pre-determined size. As a non-limiting example, the size control device  24  maximizes an area-to-volume ratio and a surface area-to-mass ratio of the PET material as compared to the pre-processed form thereof. As a further example, the size control device  24  processes the PET material into a powder having a size that is less than five hundred microns (ESPS™ powder material) or less than thirty-five Mesh (std. US mesh size). However, the size control device  24  may be adapted to process the PET material to other sizes. 
     The cyclone  26  receives the processed PET material from the size control device  24  to separate a PET from any contaminants therein. As a non-limiting example, a centrifugal blower  34  creates a centrifugal motion within the cyclone  26  to achieve separation of the PET from a transfer air flow. The PET discharges from a bottom end  36  of the cyclone  26  into the screener  28  for further separation based upon size. The transfer air flow exits the cyclone  26  at a top end  38  and is routed to a collector bin (not shown) or bag house. It is understood that any amount of PET collected in the bag house may be recaptured through the infeed loader  20 . 
     The first screener  28  separates the PET material received from the cyclone  26  based upon a pre-determined size scale. Any PET material having a particular size passes through the first screener  28  and into a surge bin  40 . Any material that cannot pass through the first screener  28  is re-fed into the size control device  24  for further processing. 
     As more clearly shown in  FIG. 3 , the mill subsystem  14  includes a plurality of mill loaders  42 ,  44 ,  46 , a pellet mill  48 , and a totalizer  50 . 
     A first mill loader  42  receives the PET material from the surge bin  40 . A second mill loader  44  receives a PET material from a feedback of the material transfer subsystem  18 . In certain embodiments, each of the first mill loader  42  and the second mill loader  44  route any received material into a hopper  52  for distribution into the pellet mill  48 . It is understood that any means for feeding material into the pellet mill  48  may be used. As a non-limiting example, a second separator  54  is disposed to detect and remove a particular contaminant in the PET material before entering the pellet mill  48 . As a further example, the second separator  54  detects and removes at least one of a ferrous metal and a non-ferrous metal. It is understood that other contaminates and materials may be detected and removed by the second separator  54 . 
     The pellet mill  48  receives and processes the PET material to form a pellet. In certain embodiments, the pellet is a compressed powder with a sintered skin. As a non-limiting example, the pellet mill  48  processes the PET material into a cylindrical shaped pellet. As a further example, the PET material undergoes a decontamination process prior to entering the pellet mill  48 . 
     In the embodiment shown, a second screener  56  is disposed to receive the PET material (e.g. pellets) from the pellet mill  48 . The second screener  56  separates the PET material received from the pellet mill  48  based upon a pre-determined size scale. Any PET material having a particular size passes through the second screener  56  and is routed to a third mill loader  46 . Any material that cannot pass through the second screener  56  is fed into the infeed loader  20  for further processing. 
     The third mill loader  46  receives material from at least one of the second screener  56  and the material transfer subsystem  18  and routes the material to the totalizer  50 . As a non-limiting example, a third separator  62  is disposed to detect and remove a particular contaminant in the PET material before entering the totalizer  50 . As a further example, the third separator  62  detects and removes at least one of a ferrous metal and a non-ferrous metal. It is understood that other contaminates and materials may be detected and removed by the third separator  62 . 
     The totalizer  50  receives the PET material and analyzes the material to provide a characteristic measurement of the material passing therethrough such as a rate of pounds per hour and total pounds of material, for example. In certain embodiments, the totalizer  50  is capable of measuring characteristics of the PET material in real-time such the overall flow of the PET material through the totalizer  50  is not impeded. The PET material passing through the totalizer  50  is collected in a surge bin  64  for distribution control. 
     As more clearly shown in  FIG. 4 , the decontamination subsystem  16  includes a plurality of decontamination loaders  66 ,  68 , a drying hopper  70 , a cooling hopper  72 , and an air handling system  73 . 
     A first decontamination loader  66  receives the PET material from the surge bin  64  and directs the PET material into the drying hopper  70  for decontamination. In certain embodiments, a heated, desiccated air is supplied to the drying hopper  70  to remove moisture and contaminates from the PET material therein. As a non-limiting example a temperature of the heated air may be adjusted for various threshold requirements such as food grade quality standards. As a further example, a time the heated air is applied to the PET material may be adjusted. It is understood that any means for heating the PET to remove contaminates therefrom may be used. 
     The cooling hopper  72  receives PET material from the drying hopper  70 . A cooled air is applied to the PET material in the cooling hopper  72  to remove a thermal energy therefrom and to regulate a temperature of the PET material to a desired level. 
     In the embodiment shown, a third screener  74  is disposed to receive the PET material (e.g. pellets) from the cooling hopper  72 . The third screener  74  separates the PET material received from the cooling hopper  72  based upon a pre-determined size scale. Any PET material having an acceptable size (e.g. full size pellet) is routed to a pellet surge bin  76  for subsequent use. Any material having an unacceptable size (e.g. pellet tails) passes through the third screener  74  and is routed to a surge bin  78  for further processing through the pelletizing system  10 . 
     The air handling system  73  is an open-loop system that provides the cooled air to the cooling hopper  72  and the heated air to the drying hopper  70 . As shown, an intake device  80  draws in an ambient air and conditions the air for application to the PET material. As a non-limiting example, the ambient air is cooled and conditioned to produce a cool, dry fluid flow through the cooling hopper  72 . It is understood that the ambient air may be any fluid. After the cooled air is applied to the PET material in the cooling hopper  72 , a dry, warm air is exhausted from the cooling hopper  72 . Specifically, a thermal energy removed from the PET material in the cooling hopper  72  is used to pre-heat a supply air that is injected into the drying hopper  70 . Accordingly, an amount of energy required to decontaminate the PET material in the drying hopper  72  is reduced due to the pre-heating or energy scalping. It is understood that other means for scalping thermal energy from a decontaminated PET material may be used. 
     In the embodiment shown, a cyclone  82  receives the pre-heated supply air exhausted from the cooling hopper  72  to separate large particles from the air. As a non-limiting example, a centrifugal blower  84  creates a centrifugal motion within the cyclone  82  to achieve separation of any contaminants physically mixed in the air by specific gravity. 
     The pre-heated supply air is routed through a heat booster  86  to control the temperature of the heated air flowing into the drying hopper  70 . It is understood that the “pre-heating step” increases the temperature of the supply air prior to the heat booster  86 , thereby minimizing the amount of energy needed to attain a desired temperature of the heated air flowing into the drying hopper  70 . 
     As more clearly shown in  FIG. 5 , the material transfer subsystem  18  includes a plurality of dust collectors  88 ,  90 ,  92 , wherein each of the dust collectors  88 ,  90 ,  92  is in fluid communication with at least one of a plurality of vacuum pumps  94 ,  96 ,  98 . The vacuum pumps  94 ,  96 ,  98  create an air flow through a plurality of conduits in fluid communication with various loaders and components of the pelletizing system  10 . Particles in the air flow are filtered therefrom and re-routed for further processing. 
     In use, an infeed of PET material is processed to a pre-determined size. The PET material having a pre-determined size is then compressed to produce pellets. For decontamination, the pellets are exposed to a heated air to removed contaminates therefrom. The decontamination of the pellets is designed to take advantage of removing thermal energy from the decontaminated pellet in the cooling hopper  72  in order to pre-heat the air that is injected into the drying hopper  70 . The decontamination process also utilizes a unique open-loop design that allows for a heated, desiccated air to flow through the PET material bed the drying hopper  70  and directs a contaminated air for discharge to atmosphere. 
     Accordingly, the pelletizing system  10  and method of the present invention provides a decontamination of a polymer material to exhibit a residual contaminant level which would make it acceptable for manufacturing new food-grade polymer bottles and containers. 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, make various changes and modifications to the invention to adapt it to various usages and conditions.