Patent Application
20070274789
Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.
Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the term “polymer” includes “oligomer,” “copolymer,” “terpolymer,” and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.
It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of courses vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.
It must also be noted that, as used in the specification and the appended claims, the singular form “a”, “an”, and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.
Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
The term “polymeric pellet” as used herein means a three dimensional object formed from a polymer. Such polymeric objects include a largest dimension that is greater than or equal to the extent of the polymeric object in any direction. Polymeric pellets occur in a number of shapes such as spherical, cylindrical, and the like. The largest dimension of a sphere is the diameter.
The term “heat of crystallization” as used herein means the quantity of heat released as a unit of mass of a substance crystallizes.
The term “crystallization temperature” as used herein means the temperature at which at least part of a material begins to crystallize.
The term “melting temperature” as used herein means the temperature at which at least part of a material is transformed from a crystalline state to a liquid. When a material undergoes such a transformation over a temperature range, for purposes of the present invention, the melting temperature is the median temperature of such a range. Typically, amorphous pellets melt at a lower temperature than crystalline pellets.
The term “degree of crystallinity” as used herein means the fractional amount of crystallinity in a polymeric sample. In the present invention, the degree of crystallinity is the average fractional amount of crystallinity in the polymeric pellets. Degree of crystallinity can be expressed as either a weight percent or a volume percent. As used herein, the degree of crystallinity is expressed as a weight percent unless expressly stated to be otherwise.
In an embodiment of the present invention, a method of crystallizing a plurality of polymeric pellets is provided. The plurality of polymeric pellets in accordance with the present invention are introduced into a pneumatic conveying system. In general, the plurality of pellets to be crystallized in accordance with the present invention are amorphous pellets or pellets with less than a desired degree of crystallinity. In a variation of the present embodiment, the degree of crystallinity of the pellets prior to crystallization is less than 30 wt. %. In another variation of the present embodiment, the degree of crystallinity of the pellets prior to crystallization is less than 20 wt. %. In still another variation of the present embodiment, the degree of crystallinity of the pellets prior to crystallization is less than 10 wt. %.
After crystallization, the degree of crystallinity is typically greater than 30 wt. %. In other variations, after crystallization, the degree of crystallization is greater than 40 wt. %. For most applications, the degree of crystallinity after crystallization is less than 70 wt. %. In other variations, the degree of crystallinity after crystallization is less than 60 wt. %. In still other variations, the degree of crystallinity after crystallization is less than 50 wt. %.
Advantageously, the polymeric pellets that are to be crystallized include any crystallizable polymer. The crystallizable polymers are characterized by a crystallization temperature and a melting temperature. Examples of such polymers include, but are not limited to, polyesters, polyolefins, polystyrenes, nylons, and polyketones. Polymers that have a relatively high heat of crystallization are most useful. In a variation, the heat of crystallization of such useful polymers is such that the heat of crystallization divided by the heat capacity of the polymer is at least 5° C. The present embodiment is particularly useful for crystallizing polyalkylene terephthalate polymers, especially polyethylene terephthalate polymers.
In a variation of the present invention, virgin polyester molten polymer is melt phase polymerized in a continuous process with the resulting molten polymer solidified to form pellets in contact with a liquid such as water. At least a portion of the liquid is separated from the pellets. The pellets are then introduced into the convey system of the invention. In a further refinement, the liquid is continuously separated from the pellets to form a stream of partially dried pellets. Subsequently, this stream of pellets is continuously introduced into the inlet of the conveying system of the invention. In some variations, the pellets have a moisture content less than 1 wt % when introduced into the inlet of the conveying system. In other variations, the pellets have a moisture content less than 0.2 wt % when introduced into the inlet of the conveying system. For example, polyalkylene terephthalate polymers exiting the melt phase polymerization process, or as introduced into the conveying system of the invention, have an It.V. of at least 0.50 dL/g, or at least 0.55 dL/g, or at least 0.6 dL/g, and especially at least 0.70 dL/g, or at least 0.72 dL/g, or at least 0.74 dL/g, or at least 0.76 dL/g, or at least 0.78 dL/g, and up to about 1.2 dL/g, or 1.1 dL/g, or 0.9 dL/g. The partially crystallized polyester polymers are also advantageously not solid state polymerized. Thus, there is also provided an embodiment encompassing a shipping container containing a plurality of partially crystallized pellets with a degree of crystallinity of at least 20% and an It.V. of at least 0.70 dL/g which have not been solid state polymerized. Suitable shipping containers are those suitable for shipping in commerce, having a volume of at least 1 cubic meter or more, or 2 cubic meters or more, or 3 cubic meters or more, or 8 cubic meters or more, or 20 cubic meters or more, and include Gaylord boxes, rail tank cars, trailers for tractor trailers, and ship hulls. The It.V. of the pellets may any value identified above higher than 0.70 dL/g, and the degree of crystallinity may be any value higher than 20% as identified above.
The pellets utilized in the methods of the invention are formed from a variety of methods known to those skilled in the art. Examples of such pellet forming processes include, but are not limited to, the processes depicted in
The methods of the present embodiment are used to crystallize pellets of virtually any shape or size. Typically, at least a portion of the plurality of polymeric pellets are three dimensional objects characterized by a largest size dimension that is less than 0.25 inches. Examples of pellets shapes that are usable in the practice of the present invention include, but are not limited to, spherically shaped pellets, cylindrically shaped pellets, and pellets with a rectangular cross section.
With reference to
In at least the illustrated embodiment, the pellets 100 are introduced into pellet inlet 104 by differential pressure separation device 106. Examples of devices that may be used for differential pressure separation device 106 include, but are not limited to, rotary airlocks and blow pots.
The plurality of polymeric pellets 100 has an initial average pellet temperature when introduced into conveying system 102. In some variations of the present embodiment, pellets 100 are at an elevated temperature that is useful for crystallization to occur while pellets 100 are resident in conveying system 102. In at least some embodiments, such as when the pellets 100 are PET, the elevated temperature is from 135° C. to 205° C., and in other embodiments from 155° C. to 200° C. The polymeric pellets may be provided in any manner including processes in which the polymeric pellets are reheated after cooling. An example of such a process includes PET strands cut by a strand cutter as set forth above in connection with the description of
Pneumatic conveying system 102 includes conveying conduit 108. In a refinement of the present embodiment, the total length of conveying conduit 108 is from 10 feet to 1000 feet. In another refinement, the total length of conveying conduit 108 is from 50 feet to 500 feet. In yet another refinement, the total length of conveying conduit 108 is from 100 feet to 300 feet.
Conveying conduit 108 includes conduit section 110 that is substantially horizontal. The horizontal configuration of conduit section 110 allows the pellets to become situated for conveyance. Typically, conduit section 110 is from 5 to 20 pipe diameters (i.e., the diameter of conduit section) in length. Pneumatic conveying conduit 110 optionally includes upwardly inclined section 112. Upon entering upwardly inclined section 112, the pellets 100 are directed to move in an upward direction as defined by angle A1 against the force of gravity. Angle A1 is typically less than 90° and more than 0°. In a variation, A1 is from 25° to 65°. In another variation, A1 is from 35° to 55°. Optimally, A1 is about 45°. Conveying conduit 108 optionally includes additional conduit sections such as conduit section 116 that is also substantially horizontal and one or more vertical sections (not shown).
In accordance with the method of the present embodiment, the plurality of polymeric pellets is pneumatically transferred from the inlet 104 to outlet 118 by a conveying gas, schematically illustrated at 119. The flow of conveying gas 119 may be adjusted by flow control device 120. Examples of suitable flow control device 120 include, but are not limited to, metering compressors, flow meters, mass flow controller, valves, orifices, diverging nozzles, and the like. The conveying gas 119 has a temperature sufficient to maintain the plurality of pellets 100 within a temperature range such that crystallization of the plurality of polymeric pellets 100 is substantially initiated or accomplished prior to removal of the pellets from pneumatic conveying system 102. In at least one embodiment, the temperature range is below the melting temperature of polymeric pellets 100. For example, when pellets 100 are formed from PET, this temperature range is from 135° C. to 200° C.
The conveying gas 119, which is effecting the transfer of the pellets, may optionally be heated or cooled by temperature adjustment device 122. Moreover, the conveying gas temperature as introduced into the conveying system can be greater than or less than the temperature of the pellets introduced into said inlet. In other variations, one or more portions of conveying conduit 108 may be heated or cooled by heat jackets utilizing water, steam, or other heat transfer media.
Conveying system 102 can be either a dense phase or dilute phase convey system. When convey system 102 is a dense phase convey system, in at least one embodiment, the length of conveying conduit 108 is from 5 to 20 pipe diameters in length with a residence time from 30 seconds to 20 minutes. In other variations, the residence time is from 1 minute to 20 minutes. In yet other variations, the residence time is from 1 minute to 10 minutes. In a variation when conveying system 102 is a dense phase convey system, the conveying velocity is in the range from 100 to 1000 feet per minute with a conveying pressure less than or equal to 100 PSIG. In another variation when conveying system 102 is a dense phase convey system, the conveying velocity is in the range from 1000 to 3000 feet per minute with a conveying pressure less than or equal to 100 PSIG. In yet another variation when conveying system 102 is a dense phase convey system, the conveying velocity is from 50 to 1000 feet per minute with a conveying pressure less than or equal to 15 inches of mercury. When convey system 102 is a dilute phase convey system, in at least one embodiment, the length of conveying conduit 108 is from 5 to 20 pipe diameters in length with a residence time from 30 seconds to 10 minutes. In a variation when conveying system 102 is a dilute phase convey system, the conveying velocity is from 1000 to 4000 feet per minute with a conveying pressure less than or equal to 15 PSIG. In another variation when conveying system 102 is a dilute phase convey system, the conveying velocity is greater than or equal to about 4000 feet per minute with a conveying pressure less than or equal to 15 PSIG. In yet another variation when conveying system 102 is a dilute phase convey system, the conveying velocity is from 1000 to 4000 feet per minute with a conveying pressure less than or equal to 15 inches of mercury. In still variation when conveying system 102 is a dilute phase convey system, the conveying velocity is greater than or equal to 4000 feet per minute with a conveying pressure less than or equal to 15 inches of mercury.
In another variation of the present invention, pellets 10 are contacted with a fluid at a temperature such that the pellets are cooled while being transported in pneumatic conveying system 102. In a further refinement of this variation, the fluid is a liquid with a boiling point lower the average temperature of pellets 100 at the location where contact with the fluid is made. Therefore, in this refinement the fluid boils upon contact with the pellets.
With reference to
In a particularly useful variation of the present embodiment, polymeric pellets 100 are cut by die face pellet cutters as set forth above in connection with the description of
It should be appreciated that each of pellets 100 typically has a relatively non-uniform temperature distribution with the interior of the pellets being hotter than the exterior of the pellets. This is due to the cooling effect of the water used in the die face cutters and pellet conveying system 92 and the low thermal conductivity of the polymer. Moreover, each pellet is likely to have a slightly varying temperature profile. Therefore, it is appropriate to describe the plurality of pellets as having an average pellet temperature.
It should also be appreciated that the water used to transport pellets 100 from the die face cutters to dryer 90 may be substituted by other conveying fluids with superior or more desirable heat transfer properties. The average temperature of pellets 100 may also be controlled by the temperature of the water (or other conveying fluid) used to convey pellets 100 from the die face cutters to dryer 90. For example, the conveying fluid may be heated to allow for higher initial average pellet temperatures (introduced to pneumatic conveying system 102) or cooled to allow for lower initial average pellet temperatures. In a typical polyester forming process, the transit time from the die face cutters to dryer 90 is on the order of a few seconds with a pellet containing slurry traveling at a velocity from 10 to 30 feet/s while in pellet conveying system 92.
In one variation of the present embodiment, polymer pellets 100 enter pneumatic conveying system 102 with an average temperature that is above an optimal temperature for crystallization. In this variation, the polymer pellets 100 are cooled by the conveying gas, which in this variation will have a temperature lower than that of the average temperature of pellets 100. The method of the present variation is particularly useful for the crystallization of polyethylene terephthalate pellets, which usually start to crystallize at a temperature of 135° C. and melt at a temperature of 200° C. When pellets 100 have regions with sufficient heat content for crystallization to occur, the average pellet temperature increases as pellets 100 are conveyed. This temperature increase is the result of the liberation of the heat of crystallization from pellets 100 as they crystallize.
In one refinement of the present variation, the difference between the initial average pellet temperature (as introduced into pneumatic conveying system 102) and the crystallization temperature is less than the temperature rise induced by the crystallization of the pellets 100 in the absence of external cooling. Therefore, in this refinement, cooled conveying gas is used to pneumatically convey the pellets and to inhibit melting or sticking of the pellets while still allowing crystallization to occur.
In another variation of the present invention, the pellets will contain sufficient internal heat for crystallization to occur. In other variations of the present invention, the pellets do not contain sufficient heat for crystallization. In this latter variation, the average pellet temperature is adjusted by contact with a conveying gas at elevated temperature.
In still another variation of the present embodiment, polymer pellets 100 enter pneumatic conveying system 102 with an average temperature too low for crystallization to proceed to a desired degree. In this situation, the temperature of the conveying gas is such that polymeric pellets 100 are heated by contact with the conveying gas (i.e., the temperature of the conveying gas is higher than the average temperature of pellets 100).
In yet another variation of the present invention, the conveying gas 119 has a sufficient temperature to strip acetaldehyde from pellets 100. For acetaldehyde stripping to be most efficient, the temperature the conveying gas 119 must be below the equilibrium concentration of acetaldehyde in the pellets 100 at the temperatures and pressures present in pneumatic conveying system 102. The acetaldehyde in the conveying gas 119 should be below the vapor pressure of the acetaldehyde in the pellets 100 to allow the acetaldehyde to diffuse into the conveying gas. In a variation, the temperature of the conveying gas is sufficient to reduce the residual acetaldehyde from the polymeric pellets by at least 4 ppm. In another variation, the temperature of the conveying gas is sufficient to reduce the residual acetaldehyde from the polymeric pellets by at least 2 ppm. In yet another variation, the temperature of the conveying gas is sufficient to reduce the residual acetaldehyde from the polymeric pellets by at least 1 ppm. Moreover, additional acetaldehyde stripping is optionally accomplished in collection device or vessel 126 (
With reference to
It should also be appreciated that the total length of conveying conduit 108 is from 10 feet to 1,000 feet. In another refinement, the total length of conveying conduit 108 is from 50 feet to 500 feet. In yet another refinement, the total length of conveying conduit 108 is from 100 feet to 300 feet.
In
Any type of gas, inert or reactive, that does not react substantially with the pellets 100 nor adversely affects the properties of the pellets may be used as the conveying gas 119. Suitable gases include, but are not limited to, air, nitrogen, argon, carbon dioxide, combinations thereof, and the like. In a variation of the present embodiment, flow control device 120 is used to control the flow rate of the conveying gas 119. In another variation, the flow rate of the conveying gas 119 may be controlled by a positive pressure condenser such as a rotary lobe compressor, which can control the speed of the conveying gas to control the flow.
In another embodiment of the present invention, a pneumatic conveying system for crystallizing a polymeric pellets implementing the methods of the invention is provided. With reference to
In a refinement of the present embodiment, the total length of conveying conduit 108 is from 10 feet to 1000 feet. In another refinement, the total length of conveying conduit 108 is from 50 feet to 500 feet. In yet another refinement, the total length of conveying conduit 108 is from 100 feet to 300 feet. Conveying conduit 108 includes conduit section 110 that is substantially horizontal. The horizontal configuration of conduit section 110 allows the pellets to become situated for conveyance. Typically, conduit section 110 is from 5 to 20 pipe diameters in length. Pneumatic conveying conduit 110 optionally includes upwardly inclined section 112, the details of which are set forth above.
Conveying conduit 108 optionally includes additional conduit sections such as conduit section 116 that is also substantially horizontal. In accordance with the method of the present embodiment, the plurality of polymeric pellets is pneumatically transferred from the inlet 104 to outlet 118 by a conveying gas. The flow of conveying gas 119 may be adjusted by flow control device 120. Examples of suitable flow control device 120 include, but are not limited to, metering compressors, flow meters, mass flow controller, valves, and the like. The conveying gas 119 has a temperature sufficient to maintain the plurality of pellets within a temperature range such that crystallization of the plurality of polymeric pellets is substantially initiated or accomplished prior to removal of the pellets from pneumatic conveying system 102. In at least one embodiment, the temperature range is below the melting temperature of polymeric pellets 100.
The conveying gas 119, which is effecting the transfer of the pellets, may optionally be heated or cooled by temperature adjustment device 122.
In
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
