MXPA00012272A - Centrifugal method and apparatus for devolatilizing polymers - Google Patents

Abstract

A method for devolatilization of a thermoplastic polymer containing at least one volatile component which includes the following three steps:heating the termoplastic polymer so that the thermoplastic polymer is a heated liquid or molten thermoplastic polymer, flowing the heated liquid thermoplastic polymer through a packed bed liquid-gas contactor by centrifugal force, and flowing a stripping gas through the packed bed countercurrent to the flow of the heated liquid thermoplastic polymer so that the volatile component volatilizes into the stripping gas from the heated liquid thermoplastic polymer by gas-liquid contacting in the packed bed;and an apparatus therefor.

Description

METHOD AND CENTRIFUGAL APPARATUS FOR DEVOLATING POLYMERS BACKGROUND OF THE INVENTION The present invention relates to the removal of volatile components of thermoplastic polymers and more specifically to methods and apparatus which therefore use centrifugal force. Chisholm, U.S. Patent 3,409,712, fully incorporated herein by reference, discloses a method and method for devolatilizing thermoplastic polymers by melting the polymer and flowing the molten polymer to a rotating disk surface. Chisholm, US Pat. No. 3, 424,832, fully incorporated herein by reference, discloses a method and apparatus for granulating a thermoplastic polymer using a rotating chamber or hollow rotor. Hay, II et al., U.S. Patent 4,940,472, fully incorporated herein by reference, discloses a method and apparatus for devolatilizing thermoplastic polymers by melting the polymer and flowing the molten polymer on a rotating disk surface. Moore et al., U.S. Patent 4,952,672, fully incorporated herein by reference, discloses a method and apparatus for devolatilization of thermoplastic polymers by melting the polymer and flowing the molten polymer on a rotating disk surface. Baker Perkins Incorporated of Saginaw Michigan offered a centrifugal granulator for sale. Modern Plastics, December 1 983, page 56. Haw, in his master's thesis, Case Western Reserve University, January 1995, entitled "Mass Transfers of Centrifugally Enhanced Polymer Devolatilization by using Foam Metal Packed Bed", fully incorporated into the present by reference, discloses a method and apparatus for devolatilization of thermoplastic polymers by melting the polymer and flowing the molten polymer through a gas-liquid contactor of open-cell nickel metal foam rotated within a stationary chamber. Haw's method and apparatus suffered from several problems. For example, nickel metal foam tended to collapse under the centrifugal forces to which it was subjected. In addition, the mechanical seal used tended to contaminate the devolatilized polymer with the sellow oil. Furthermore, Haw's method and apparatus did not produce the devolatilized polymer in the form of granules.

BRIEF DESCRIPTION OF THE INVENTION The present invention is a method for the devolatilization of a thermoplastic polymer containing at least one volatile component comprising three steps. The first step is to heat the thermoplastic polymer so that the thermoplastic polymer is a liquid or hot melt thermoplastic polymer. The second step is to make the hot liquid thermoplastic polymer flow through a packed liquid-bed gas contactor by centrifugal force. The third step is to make a separating gas flow through the packed bed countercurrent to the hot liquid thermoplastic polymer flow so that the volatile component volatilizes in the hot liquid thermoplastic polymer separator gas by liquid gas-liquid contact in the bed. packed. The present invention is also an apparatus for the devolatilization of a thermoplastic polymer containing at least one volatile component, the apparatus comprising three elements. The first element is a rotating camera, the rotating chamber containing a gas-liquid-contact gasket. The second element is a polymer conduit, the polymer conduit extending into and ending within the rotating chamber so that the molten thermoplastic polymer can be flowed into the chamber through the conduit while the chamber is being rotated for the thermoplastic polymer. The melt then flows through the packing by centrifugal force so that the volatile component of the molten thermoplastic polymer volatilizes in the gas phase in the packed bed. The third element is a gas seal between the polymer conduit and the rotating chamber.

BRIEF DESCRIPTION OF THE DIAGRAMS Figure 1 shows a cross-sectional side view of one embodiment of the apparatus of the present invention; and Figure 2 shows an end view of the apparatus of Figure 1 that also includes a pair of electromagnets.

DETAILED DESCRIPTION OF THE INVENTION Now referring to Figure 1, a side cross-sectional view of one embodiment of the apparatus 10 of the present invention is shown. The apparatus 1 0 includes a disc-shaped chamber 12 which is rotated by an arrow 1 3 about the axis longitude of the arrow 1 3. The arrow 1 3 is rotated by an electric motor (not shown). The chamber 12 contains an annular sealing ring 14 of open cell foam of nickel-chrome metal preferably cut from a larger piece of material by electrode discharge machining process. Packing 14 is Celmet Brand # 1 material available from Sumitomo Electric USA, NY, NY. A band 1 5 of ring-shaped support surrounds the package 14. The support band 1 5 is perforated by holes 1 6 from one side to the other. A band 17 distributed in the form of a ring is placed inside the package 14. The distributor strip 17 is perforated by holes 18 from one side to the other. A polymer conduit 19 extends into and terminates within the chamber 1 2. The polymer conduit 19 includes a flange 20 and seal mounting body 21. A tubular chamber extension 22 terminates near the flange 20. A gas seal 23 is mounted on the seal mounting body 21. The details of the gas seal 23 are not shown in Fig. 1. A preferred gas seal 23 is available from Durameta llic Corporation of Kalamazoo Michigan as the GF 200 Dura Seal Brand gas barrier seal. Pressurized nitrogen of 0.2 MPa is fed to seal 23 via port 24 of nitrogen. Gas barrier seals are known to chemical engineers. See, for example, Chemical Engineering Progress (1 996), 92 (10), pages 58-63, fully incorporated herein by reference. The term "thermoplastic polymer" is well understood in the art and includes, for example and without limitation, nylons, fluorocarbons, cellulose derivatives, acrylic resins, polystyrene, styrene copolymers such as acrylonitrile / butadiene / styrene copolymers, interpolymers and ethylene / styrene , polylactic acid, polyethylene and polypropylene. Thermoplastic polymers soften and melt when heated sufficiently. It is often desirable to reduce the concentration of volatile components in a thermoplastic polymer as discussed in U.S. Patents 4,952,672, 4,940, 472 and 3, 409.71. For example, it is often desirable to reduce the monomer concentration of residual Styrene in polystyrene. The thermoplastic polymer is heated as discussed in U.S. Patents 4,952,672 and 4,940,472 so that it can flow through the polymer conduit 1 9 and into the chamber 12. The chamber 12 is rotated by means of the arrow 1 3 a, for example. , 4,000 rpm for an external diameter chamber of 46 centimeters. The liquid molten hot thermoplastic polymer is then gathered by centrifugal force against the distributor strip 17 aided by the lip 17a. the polymer then flows by centrifugal force through the holes 1 8, through the packing 14, through the holes 16 to meet at the peripheral edge of the chamber 1 2. A series of openings 25 are placed in the periphery of the chamber. chamber 1 2. The polymer flows by centrifugal force through the openings 25 to form a polymer strand. An endless band 26 of sharpened alloy tool steel forms a knife edge that cuts the polymer strand into granules. The band 26 travels on rollers 27 driven by an electric motor (not shown). The band 26 is cooled with water preferably. The gasket 14 facilitates the gas-liquid contact. Gas-liquid contact is a technique known to chemical engineers. See, for example, Section 18-19 to 18-48 of the Perry Chemical Engineer's Manual, fifth edition. When a partial vacuum (defined herein as a pressure of more than 0 MPa but less than 0.1 MPa) is applied to vacuum port 28, then the volatile components of the polymer enter the gas phase in packing 14, flow around of the distributor strip 1 7 through the vent channel 29 in the chamber 1 2, to the ring between the conduit 19 and the extension 22, and then through the hatch 28. Optionally, the chamber 1 2 contains a conduit 30 of separating gas so that the nitrogen in the port 24 can be introduced into the chamber 12 by means of the nozzle 31. The separating gas flows through holes 1 6, through packing 14, through channel 29, into the ring between conduit 19 and extension 22, and then through port 28. Any conventionally applicable separating gas can to be used such as carbon dioxide, methanol vapor, ethanol vapor, butane gas or other light hydrocarbons. For example, steam can be flowed through line 30 as a separating gas. Alternatively, the separator gas conduit may extend through the arrow 1 3 to the chamber 12. When the separator gas is vapor, then the preferred construction material of the chamber 12 is a corrosion-resistant steel. The use of spacer gas is preferred in the present invention when it is desirable to devolatilize more completely at the expense of handling the spacer gas. Preferably, the chamber 12 is heated to facilitate the starting and operation of the apparatus 10. The preferred means for heating the chamber 12 is shown in Figure 2. Figure 2 is an end view of the apparatus 10 of Figure 1 showing the camera 12, the arrow 13, the band 26 and the openings 25. The camera 1 2 is placed between a first electromagnet 32 and a second electromagnet 33. When the camera 12 is made of a magnetic material, then when the camera 12 is rotated, parasitic electrical currents are generated in the chamber 1 2 which heat the chamber 1 2. Preferably, the chamber 1 2 is made of a magnetic stainless steel (especially when steam is used as a gas separator) such as the type 17-4 Well-known stainless steel PH, see Section 6-38 of the Standard Manual for Marks Mechanical Engineers, eighth edition. The package of the present invention should not be collapsed into use and thereby block the flow of polymer through the package. Haw, supra, described the use of open cell nickel metal foam as a packing in a centrifugal polymer devolatilizer. However, preferably the package of the present invention has a compressive strength at least twenty-five percent greater than an equivalent packet made essentially of nickel. When the package is an open cell metal foam, then preferably the average number of cells per centimeter of the foam varies from about 2 to about 7. However, an open cell metal foam is not required in the present invention. For example, a package consisting of a woven metal wire, such as woven stainless steel wire, can be used as well as any other packaging known by the liquid-gas contacting technique. Other examples of packaging include macroreticular metal foam, wire weft, and wound woven wire mesh. When a loose package is used, then a perforated band may be used as the support band 1 5 on the inner side of the package. However, it is believed that a rigid package, such as open cell metal foam, is easier to balance. Figure 1 shows a chamber 1 2 having a single row of openings 25. However, it is preferred that the chamber 12 be longer and have many rows of openings 25 to increase the productivity of the apparatus 1 0. When a camera is used 1 2 longer, then the end of the polymer conduit 1 9 can be closed and the polymer conduit adjacent to the package can then be perforated with holes for distributing molten polymer onto the package 14. The operational parameters of the apparatus of the present invention will depend, for of course, of many factors such as the specific physical properties of the thermoplastic polymer that is devolatilized. However, a good starting point is to follow the teachings of U.S. Patent 4,952,672. When the thermoplastic polymer is general purpose polystyrene, then it is suggested that the polymer and chamber 1 2 be heated to approximately 240 degrees centigrade so that the polymer has a viscosity of about 300,000 centipoise.

Claims (22)

1. RE IVI ND I CATIONS 1. A method for the devolatilization of a thermoplastic polymer containing at least one volatile component, the method comprising the steps of: heating the thermoplastic polymer so that the thermoplastic polymer is a heated liquid thermoplastic polymer; flowing the heated liquid thermoplastic polymer through ur to packed bed, by centrifugal force; and flowing a separating gas through the packed bed countercurrent to the flow of the heated liquid thermoplastic polymer so that the volatile component volatilizes in the separating gas from the heated liquid thermoplastic polymer by gas-liquid contact in the bed. packed. The method of Claim 1, wherein the spacer gas is selected from the group consisting of nitrogen, carbon dioxide, butane, methanol and ethanol. 3. The method of Claim 1, wherein the separator gas is vapor. 4. The method of Claim 1, wherein the pressure of the separator g in the packed bed is more than 0 MPa but less than C. MPa. The method of Claim 2, wherein the pressure of the separator gas in the packed bed is more than 0 MPa but less than 0.1 MPa. The method of Claim 3, wherein the pressure of the gas separator in the packed bed is more than 0 M Pa but less than 0.1 MPa. $ & * # -, ', 7. The method of Claim 1, wherein the pack has a compressive strength of at least twenty-five percent greater than an equivalent pack made essentially of nickel. 8. The method of Claim 1, wherein the package is an open cell metal foam, the metal being a nickel and chromium alloy, the foam having a compressive strength of at least twenty-five percent greater than if the foam was essentially made of nickel. The method of Claim 8, wherein the average number of cells per centimeter of the foam ranges from about 2 to about 7. The method of Claim 1, further comprising the step of flowing the devolatilized thermoplastic polymer. through an opening to form a strand of devolatilized thermoplastic polymer. eleven . The method of Claim 10, further comprising the step of cutting the strand of devolatilized thermoplastic polymer to form a devolatilized thermoplastic polymer granule. 12. An apparatus for the devolatilization of a thermoplastic polymer containing at least one volatile component, wherein the apparatus comprises: a rotating chamber, the rotating chamber containing a package for processing the gas-liquid contact; a polymer conduit, the polymer conduit extending towards and ending within the rotary chamber so that the molten thermoplastic polymer can be flowed into the chamber via the conduit while the chamber is being rotated so that the molten thermoplastic polymer flows then through packing by centrifugal force so that the volatile component of the molten thermoplastic polymer volatilizes in the gas phase in the packed bed; and a gas seal between the polymer conduit and the rotating chamber. 13. The apparatus of Claim 1 2, wherein the package is an open cell metal foam. 14. The apparatus of Claim 1 3, wherein the open cell metal foam is an alloy of nickel and chromium. The apparatus of Claim 1 2, wherein the pack comprises woven metal wire. 16. The apparatus of Claim 1 5, wherein the metal wire is stainless steel metal wire. 17. The apparatus of Claim 1 2, further comprising a magnet, the rotating chamber comprises a magnetic metal, the rotating chamber being positioned sufficiently close to the magnet so that when the rotating chamber is rotated, the rotating chamber is heated by the eddy currents induced in the magnetic metal. 1 8. The apparatus of Claim 1 7, where the magnetic metal is a magnetic stainless steel. 1 9. The apparatus of Claim 1 8, wherein the magneto-stainless steel is magnetic type 1 7-4PH stainless steel. 20. The apparatus of Claim 1 2. further comprising a gas separator conduit extending towards and terminating within the rotary chamber so that a separator gas can be flowed through the packed bed countercurrent to the molten polymer flow. . twenty-one . The apparatus of Claim 12, wherein the periphery of the rotary chamber includes at least one opening for the devolatilized thermoplastic polymer to flow through the opening to form a devolatilized thermoplastic polymer strand when the rotating chamber is rotated. . 22. The apparatus of Claim 21, further including a blade positioned near the periphery of the rotating chamber so that the devolatilized thermoplastic polymer strand can be cut into a thermoplastic polymer pellet devolatilized by the blade when the rotating chamber is rotated. KAt ÜKi ^ í ^ K