WO2001021303A1

WO2001021303A1 - An improved method and apparatus for devolatilization

Info

Publication number: WO2001021303A1
Application number: PCT/US2000/022451
Authority: WO
Inventors: Clark J. Cummings; George J. Quarderer, Jr.; Dan D. Tirtowidjojo
Original assignee: The Dow Chemical Company
Priority date: 1999-09-20
Filing date: 2000-08-16
Publication date: 2001-03-29
Also published as: AU6777100A

Abstract

The present invention is a devolatilization process and apparatus utilizing a high surface area material within a rotatable device which is mounted within an enclosed vessel.

Description

AN IMPROVED METHOD AND APPARATUS FOR DEVOLATILIZATION

This invention relates to a process for devolatilizing polymers containing volatile material(s).

Devolatilization refers to the removal of low molecular weight components, such as unreacted monomer, solvents, water, and various by-products, commonly referred to as 'volatiles', from a polymer composition. The polymer composition is exposed to high temperatures, wherein volatiles are transported to a polymer-vapor interface, evaporated and subsequently removed by a vacuum system. One problem associated with devolatilization relates to the polymer residence time at such high temperatures. Long residence times increase manufacturing costs, and can result in physical property deterioration of the polymer and in high residual monomer content. Residual monomers have many detrimental effects on the polymer, including lower strength characteristics, production of internal bubbles, undesirable taste and odor to food materials in contact therewith and reduction of environmental resistance.

Many thermoplastic resins are heat sensitive and tend to degrade on prolonged exposure to elevated temperatures. Thus in many instances, it is highly desirable to remove volatiles while exposing the polymer to an elevated temperature for only a minimum length of time making many conventional devolatilization procedures undesirable.

Other devolatilization devices have been used, including a centrifugal force devolatilizer as disclosed in U.S. Patents 4,952,672 and 4,940,472. These types of devices offer shorter residence times, but are effective only for polymers having high melt strength, since the polymer is forced through apertures and cut into pellets.

Therefore, it remains desirable to provide a process and apparatus for devolatilizing polymer materials which offers short residence times and is not limited by polymer melt strength. The present invention is directed to a polymer devolatilizer comprising a polymer inlet pipe, a rotatable device comprising a high surface area packing having a plurality of polymer outlets, and an outer enclosed vessel equipped with a vacuum outlet and a pump; and a process for devolatilizing comprising: a) feeding a polymerization reaction mixture comprising at least one polymer and at least one volatile material, unreacted monomer or undesirable by-product into the rotatable device via the polymer inlet pipe at a temperature of 100 to 500°C, b) rotating the rotatable device at a sufficient angular velocity such that the polymer travels radially, through the high surface area packing, generating thin polymer films, through the plurality of polymer outlets, to an inner wall of the enclosed vessel; thereby releasing a substantial amount of the volatiles from the polymerization reaction mixture to form a volatile vapor and a polymer liquid, and c) gravity feeding the polymer liquid to a pump located within the enclosed vessel.

This apparatus and process provides devolatilization in very short residence times and is not dependent upon the melt strength of the polymer as in the current art.

Figure I is a cross-sectional side view, of one embodiment of the devolatilizer of the present invention.

Figure II is an aerial side view of one embodiment of the rotatable device.

Figure III is an aerial cross-sectional view (taken below the upper support plate) of one embodiment of the devolatilizer of the present invention.

Figure IV is an aerial side view of one embodiment of an axial distributor.

The devolatilizer of the present invention comprises a rotatable device mounted within an enclosed non-rotating outer vessel. The rotatable device can be any device which utilizes centrifugal force and high surface area packing to cause polymer flow such that thin films are formed, and allows rapid diffusion of volatile materials out of the polymerization reaction mixture into the vapor phase, and removal of the volatile materials from the vapor phase under vacuum. The inner configuration of the rotatable device comprises a space substantially filled with a high surface area packing, such as metal foam, that is, macroreticular metal foam, wire screen, and wound woven metallic mesh. In the process of the present invention, the polymerization reaction mixture is distributed as a thin film on the surfaces of the high surface area packing, and is pulled radially by the centrifugal force over the extended surface area during centrifugal rotation. The film thus formed has a greater surface area for the dissipation of volatile materials from the polymerization reaction mixture, and thus provides further removal of the volatile materials. This also results in rapid surface renewal, that is, repeatedly exposing new materials to the surface or gas/liquid interface. The combination of thin films and surface renewal results in rapid mass transfer.

Preferably, the device also comprises a means for distributing the polymer uniformly to the high surface area packing. This can be achieved by utilizing one or more distributors which function to create uniform polymer flow to the high surface area packing. In one embodiment, a combination of a radial distributor and an axial distributor is used. The radial distributor acts, upon rotation, to radially distribute polymer received from the polymer inlet pipe to the axial distributor. The axial distributor typically comprises a polymer flow restriction means which restricts the polymer flow, causing the polymer level to buildup within the distributor, and directs the flow such that polymer is directed evenly and uniformly to the available surface of the high surface area packing.

Although the device may be of any configuration, it will preferably be symmetrical, to provide smooth, balanced rotation. The polymer inlet pipe (1), having an inner and outer diameter, is securely attached to the outer vessel (11) and, provides fluid communication with the rotatable device (13). Preferably, the polymer inlet pipe will be centrally disposed to allow for an equal, radially extending centrifugal force to be produced by the device.

In one embodiment of the process of the present invention, polymer is fed through the polymer inlet pipe (1 ), which is securely attached to the outer vessel (11 ), and into the rotatable device (13). Within the rotatable device, the polymer is deposited on a radial distributor (4), which is centrally and securely attached to a lower support plate (3). Upon rotation of the rotatable device, the polymer is pulled radially through holes in the wall of the axial distributor (5) which is centrally and securely attached to the lower support plate at a distance from the radial distributor such that polymer flow can easily occur, wherein the axial distributor also comprises a lip (8) designed to prevent polymer backflow to the polymer inlet pipe. From the axial distributor wall holes, the polymer travels through a high surface area packing (7) which is securely attached to the lower support plate and/or an upper support plate (2), wherein the upper support plate comprises a centrally located void for admission of the polymer inlet pipe and volatile removal. The polymer flows through the packing and out the plurality of outlets to a side wall of the outer vessel and is then gravity fed to form a pool of polymer (9) therein, where the polymer is pumped by a pumping means (10). A means for rotating (6) the device is securely attached to the lower support plate.

The upper and lower support plates are preferably made of a material having sufficient integrity to withstand the high temperatures, vacuum and centrifugal force associated with the operation of the devolatilization apparatus. Preferably the support plates are circular metal plates having a thickness of about 0.3 cm to 2.5 cm. The upper support plate (2) typically has an inner diameter and an outer diameter configured such that there is a central void or hole for introduction of the polymer inlet pipe, and volatile removal. The lower support plate is preferably a circular metal plate having a continuous upper surface and a continuous lower surface.

The high surface area packing can be in any form, including stacked discs, a continuous piece of packing, or spooled layers. Typically the packing is made of a material such as macroreticular metal foam, wire screen, wound woven metallic mesh or some other type of packing which will provide high surface area. The packing is located between the upper and lower support plates, and is securely attached to the upper, lower or both support plates, such that the packing will remain between the support plates during rotation of the rotational device. In one embodiment, a rod can be used wherein the rod extends through the upper support plate, the packing and the lower support plate, and is secured on both ends. In another embodiment, clamps can be used on the upper and lower support plates, wherein sufficient pressure is exerted to keep the packing in place upon rotation of the rotational device.

Preferably, the packing is positioned between the upper and lower support plates such that a hollow space is centrally formed for admission of the polymerization reaction mixture, thus forming an inner wall of packing, having an inner diameter and facing the axial distributor; and an outer wall of packing, having an outer diameter and facing the inner wall of the outer vessel. Preferably, the inner and outer diameter dimensions of the packing substantially match the inner and outer diameter dimensions of the upper support plate.

The packing is sufficiently porous such that polymer can travel radially through the inner wall of the packing and exit through a pore of the outer wall of the packing, facing the inner wall of the enclosed vessel. Thus, the packing offers a plurality of outlets for the polymer.

The height of the packing, and thus the distance between the upper and lower support plates can vary widely depending upon the desired devolatilization rate to be achieved. One skilled in the art could determine the distance needed based on the size of the process equipment to be used, the amount of polymer to be produced, and the rate of devolatilization desired. Typically, such distances can vary from approximately 2.5 cm to 150 cm.

If a radial distributor is used, it is typically centrally and securely attached to the upper surface of the lower support plate. The radial distributor can be securely attached using any securing means, but is typically bolted to the lower support plate. Alternatively, the radial distributor and the lower support plate can be a single continuous part. The radial distributor is preferably a solid cylindrical shape and provides a means to distribute the polymer evenly to the axial distributor. The radial distributor is typically made of a material with sufficient integrity to withstand the high temperatures, vacuum and centrifugal force associated with the operation of the devolatilization apparatus and is preferably made of a metal. The radial distributor typically has a diameter which is in the range of a minimum value substantially equal to the outer diameter of the polymer inlet pipe and a maximum value such that it is located a sufficient distance from the axial distributor such that polymer is able to easily flow to the axial distributor. The radial distributor will typically have a height approximately 0.5 times the height of the high surface area packing.

If an axial distributor is used, it is typically centrally and securely attached to the upper surface of the lower support plate and is located between the radial distributor and the inner wall of the high surface area packing material, wherein it is spaced a sufficient distance from the radial distributor such that polymer flows easily from the radial distributor to the axial distributor and a sufficient distance from the packing such that volatiles can be removed by the vacuum created within the devolatilizer. The axial distributor can be secured using any securing means, but is typically bolted to the lower support plate. The axial distributor is typically made of a material with sufficient integrity to withstand the high temperatures, vacuum and centrifugal force associated with the operation of the devolatilization apparatus and is preferably made of a metal. The axial distributor is preferably a hollow cylindrical shape, having an inner and outer diameter, such that the radial distributor can be located within the inner diameter, and provides a means to distribute the polymer evenly to the packing. The axial distributor typically comprises a thin wall hollow metal cylinder, having an upper and lower end, wherein the thin wall has a plurality of voids, and causes the polymer level to increase within the space defined by the inner diameter of the axial distributor and the lower end, in order to facilitate even and uniform polymer flow to the packing; and an upper lip (8), extending inward from the inner diameter of the axial distributor on the upper end, in order to prevent polymer flow back through the polymer inlet pipe. The axial distributor is typically substantially the same height as the high surface area packing, having a lip which approximates the outer diameter of the polymer inlet pipe.

The rotatable device also includes a means to rotate (6) the rotatable device at a speed sufficient to cause the polymer mixture to flow radially through the high surface area material and plurality of outlets. Exemplary rotating means include a drive shaft and a motor in cooperative combination. In such an embodiment, the drive shaft is securely and centrally mounted to the rotatable device, while in cooperative contact with a motor. The devolatilization apparatus further comprises an outer vessel, having an inner wall and an outer wall, which encloses the rotating device and is equipped with a vacuum source and a pump. A vacuum source (12) communicating with the outer vessel is provided for removing the volatile material(s).

The rotatable device is mounted within the outer vessel such that the polymer inlet pipe extends outside the outer vessel, but the rotatable device is located within the outer vessel. The communication of the polymer inlet pipe with the outer vessel is such that polymer can be admitted into the polymer inlet pipe, but a vacuum can be maintained within the outer vessel. The rotating device is also mounted within the enclosed vessel such that upon rotation, the polymer will flow through the porous material, out the plurality of outlets within the packing and onto the inner walls of the enclosed vessel, forming films on the inner walls of the enclosed vessel.

Rotation of the rotatable device, at an angular velocity which produces a centrifugal gravity in the order of approximately 50 to 2,000, preferably to 6,000, and more preferably to 10,000 times the normal gravitational pull (G), will readily separate volatile material(s) from the polymerization reaction mixture by causing the polymer to travel through the high surface area packing, forming thin films thereon, and allow rapid diffusion of the volatile material into the vapor phase.

Increased temperatures may enhance the removal of some volatile materials. Typically, conditions such as pressure and temperature will depend upon the polymer and volatile materials present, wherein the temperature is at least the temperature at which the volatile material will vaporize for a given pressure.

The rotatable device is suitably heated to a temperature sufficient so that the polymer is able to flow sufficiently through the high surface area packing and out of the device by the centrifugal force. The rotatable device can be heated by heating means and/or by the temperature of the molten polymer. Suitable heating may be obtained by radiation, convection, or conduction. Suitable heating means may include heat lamps, eddy-current heaters, for example, setting up a magnetic field about the rotatable device which acts as an electric brake to convert the rotational kinetic energy of the device into heat; electrical heaters, for example electric heaters on the rotatable device being attached to commutators; and heat transfer vapor or fluid systems, for example, piped through an additional rotating seal system.

The collected gases are discharged through a vapor discharge means. The vapor is advantageously removed by evacuation means, which are well known by those skilled in the art.

The devolatilizer of the present invention may be employed in a batch process or, preferably, in a continuous process. In the latter instance, the polymer is continuously fed into the devolatilizer and the separated polymer liquid is removed from the devolatilizer at a rate generally equal to the rate of feed into the devolatilizer.

One advantage of the present invention is that by using centrifugal force and a high surface area material packing within the rotatable device, all of the polymer material, on the average, experiences a very low residence time. By decreasing the average residence time of devolatilization for all polymer material, higher devolatilization temperatures may be employed without the disadvantages associated therewith. The temperatures for devolatilization in the present apparatus will be in a range of 100°C to 500°C. Typically, the residence time for all polymer material in the present apparatus will generally be in a range of 0.1 to 10 minutes, preferably from 0.3 to 3 minutes.

Although one embodiment of the present invention is disclosed in Fig. I, the devolatilizer of the present invention is not limited thereto. A multitude of variations could be envisioned which would be equivalent to the devolatilizer of the present invention. Such variations include, but are not limited to, a horizontal mounting of the rotatable device within the outer vessel, a polymer inlet pipe from the bottom or side, a drive shaft mounted from the top or horizontally.

In the practice of the present invention, the polymer to be devolatilized is a flowable material such as a concentrated solution produced in a solution or mass polymerization, or a flowable emulsion, and is herein referred to as a polymerization reaction mixture. The polymerization reaction mixture is typically a resinous material in intimate mixture for example, true solutions and also dispersions or emulsions; with volatile materials in liquid or gaseous form. Alternatively, the polymer may be in the form of beads as obtained directly from a suspension polymerization or in the form of a dried coagulant obtained when a latex is coagulated.

Typical polymers included in such polymerization reaction mixtures include any thermoplastic polymer, including but not limited to, monovinylidene aromatic polymers such as polystyrene, copolymers thereof such as styrene-acrylonitrile, styrene-acrylonitrile-styrene-methylacrylate, and styrene-maleic anhydride, styrene maleimide or alpha methyl styrene-acrylonitrile copolymers and polyolefins such as polyethylene, polypropylene, polycarbonates, vinylidene chloride copolymers, polyphenylene oxides, and the resinous copolymers, rubber modified versions of such polymers and blends thereof.

Any material which can be volatilized may be removed from intimate mixture with a polymer by the process of the present invention. Volatile materials include, but are not limited to, in gaseous or liquid form, ethylbenzene, methyl ethylketone, water, tetrachloroethylene, pentane, hexane, cyclohexane, benzene, carbon tetrachloride, tetrahydrofuran, acetone, ethylene, vinyl chloride, and vinylidene chloride.

Usually the volatile material will be the one in which the polymer was obtained, prepared or purified. The volatile material as defined herein, can also include unreacted monomers which are not polymerizable under process conditions; thus solutions of polymers made by bulk polymerization can also be treated. Additionally, liquid media may be added to the resinous material to provide a stripping action, such as water, carbon dioxide, an alcohol such as ethanol or methanol, or mixtures thereof.

The process of the present invention can also be used to remove unwanted by-products from polycondensation equilibrium reactions, wherein the polymerization reaction mixture comprises a condensation polymer. Polycondensation equilibrium reactions are reactions in which the polymerization reaction and its reverse reaction occur at the same rate, resulting in a constant concentration of reactants, and is therefore driven by the removal of one of the products, that is, a by-product, such as water, an alcohol, phenol, and glycol. Removal of the by-product(s) allows further reaction to occur, thus increasing the molecular weight of the polymer produced.

A high molecular weight condensation polymer is produced by causing further reaction of a low molecular weight condensation polymer having at least one reactive chain end, by removing unwanted by-product(s) from the polycondensation equilibrium reaction mixture. A condensation polymer is defined for the purpose of the present invention, as a polymer produced from a polycondensation equilibrium reaction, wherein at least one by-product is produced, and wherein such byproducts) inhibits the completion of the polymerization reaction due to an equilibrium state.

The polycondensation equilibrium reaction mixture refers to a mixture comprising one or more unreacted monomers, low molecular weight condensation polymer having at least one reactive chain end and at least one unwanted by-product of the polycondensation equilibrium reaction.

Condensation type polymers are limited to those polymers produced by polycondensation equilibrium reactions, that is, from monomers which produce byproducts that inhibit further completion of the polycondensation reaction. Those skilled in the art can easily ascertain which reaction schemes or monomer(s) would lead to polymers produced by polycondensation equilibrium reactions. For example, such polymers and monomer(s) include but are not limited to, polycarbonate produced from diphenylcarbonate and bisphenol A, giving phenol as an unwanted by-product; polycarbonate produced from dimethylcarbonate and bisphenol A, giving methanol as an unwanted by-product; polylactic acid produced from lactic acid and giving water as an unwanted by-product; nylon 6,6 produced from adipic acid and hexamethylene diamine, giving water as an unwanted by-product; polyethylene terephthalate produced from ethylene glycol and terephthalic acid, giving water and ethylene glycol as unwanted by-products (ethylene glycol reacts with terephthalic acid, forming water and bishydroxyethylterephthalate, which then polymerizes to form polyethylene terephthalate and produces ethylene glycol); and polyethylene terephthalate produced from ethylene glycol and dimethyl terephthalate; giving methanol as an unwanted by-product.

Methods of producing polycondensation equilibrium type polymers and polycondensation equilibrium reactions are also well known in the art.

A low molecular weight condensation polymer is defined as a polymeric material having at least one reactive chain end, produced from a polycondensation equilibrium reaction which is limited in molecular weight by the presence of byproducts) in the polycondensation equilibrium reaction mixture. In other words, the low molecular weight condensation polymer is the polymer having at least one reactive chain end produced prior to any removal of unwanted by-product(s) and subsequent increase in molecular weight.

The term 'unwanted by-product' refers to any product of the polycondensation equilibrium reaction which is not considered a condensation polymer. Unwanted byproducts) will vary depending upon the condensation polymer being produced. Exemplary by-products include water, phenol, and ethylene glycol. The byproducts) is typically in liquid form, dissolved in the low molecular weight condensation polymer and easily removed from the polymer into the vapor phase by the rotatable device under vacuum.

Claims

CLAIMS:

1. A polymer devolatilizer comprising a polymer inlet pipe secured to an outer vessel equipped with a vacuum outlet and a pump, a rotatable device secured to a rotation means mounted within the outer vessel, wherein the device comprises a high surface area packing having a plurality of polymer outlets.

2. The devolatilizer of Claim 1 wherein the high surface area packing is a metal foam, wire screen, or wound woven metallic mesh.

3. The devolatilizer of Claim 1 wherein the high surface area packing is securely attached to a lower support plate and/or an upper support plate.

4. The devolatilizer of Claim 3 wherein the upper support plate is a circular metal plate having an inner diameter and an outer diameter, and the lower support plate has a continuous upper surface and a continuous lower surface.

5. The devolatilizer of Claim 4 wherein the packing is positioned between the upper support plate and the lower support plate, such that the packing has substantially the same inner diameter and outer diameter as the upper support plate, thus forming an inner wall and an outer wall of high surface area packing.

6. The devolatilizer of Claim 5 further comprising a radial distributor rigidly secured to the upper surface of the lower support plate.

7. The devolatilizer of Claim 6 further comprising an axial distributor rigidly secured to the upper surface of the lower support plate between the radial distributor and the inner wall of the high surface area packing.

8. The devolatilizer of Claim 7 wherein the axial distributor comprises a thin wall hollow metal cylinder, having an inner and outer diameter and an upper and lower end, wherein the thin wall has a plurality of voids, and causes the polymer level to increase within the space defined by the inner diameter of the axial distributor and the lower end; and an upper lip, extending inward from the upper end of the inner diameter of the axial distributor, in order to prevent polymer flow back through the polymer inlet pipe.

9. A process for devolatilizing a polymer using the devolatilizer of Claim 1 comprising: a) feeding a polymerization reaction mixture comprising at least one polymer and at least one volatile material, unreacted monomer, or unwanted byproduct into the devolatilizer of Claim 1 at a temperature of 100 to 500°C, b) rotating the rotatable device at a sufficient angular velocity such that the polymer travels radially through the high surface area packing, generating thin polymer films, to an inner wall of the enclosed vessel; and thereby releasing a substantial amount of the volatile material from the product to form a volatile vapor and a polymer liquid, and gravity feeding the polymer liquid to a pump located in the bottom of the enclosed vessel.

10. The process of Claim 9 wherein the polymer is a thermoplastic.

11. The process of Claim 10 wherein the polymer is selected from the group consisting of monovinylidene aromatic polymers, polyolefins, polycarbonates, vinylidene chloride polymers and polyphenylene oxides.

12. The process of Claim 9 wherein the polymer is a condensation polymer.