JPH06179712A - Method for removing unpolymerized monomer - Google Patents

Abstract

PURPOSE: To provide a removing method of unpolymerized monomers in a gaseous state remaining in a solid polymerization product.

CONSTITUTION: A solid polymer 22 is supplied to a drying vessel 11 and exposed to a gaseous hydrocarbon to trap the hydrocarbon gas on the polymer surface or in the voids of polymer particles. The polymer is sent to a vessel 12, where gaseous monomers present around the polymer in the voids of polymer particles are removed under almost vacuum condition of about ≤9 psia, while a liquid monomer residue is vaporized at the same time. The gaseous hydrocarbon is drained through a conduit 14. The polymer is sent to a vessel 15, where the polymer is pressurized by an inert gas supplied through a conduit 18 to dilute the all residual hydrocarbon gas on the polymer and in the polymer. Then the polymer is sent from the pressurizing vessel 15 to a vessel 17, where the inert gas and gaseous monomers are drained through a conduit 19 by using an inert gas supplied through a conduit 19a, while the polymer is discharged as a product.

COPYRIGHT: (C)1994,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for removing unpolymerized monomers from solid polymer compositions, and more particularly to removing unreacted liquid and gaseous monomers from solid polymers prepared from their corresponding monomers. More specifically, it relates to a method of removing unreacted gaseous monomeric hydrocarbons remaining in a solid polyolefin product. The invention also relates to a device for achieving the above method.

[0002]

2. Description of the Prior Art In a method for producing a solid polymer, for example, a method for producing a polyolefin compound such as polyethylene and polypropylene from a monomer hydrocarbon, a reacted polymer resin can leave a reaction vessel together with an unreacted liquid monomer hydrocarbon. There is a nature. It is believed that these hydrocarbons are impurities that must be removed prior to utilizing the reacted polyolefin. Therefore, these unreacted liquid monomers generally must be removed from the polymer before utilizing the final product. Generally speaking, these hydrocarbons are dryers such as mechanical dryers that apply heat to the hydrocarbons to vaporize the liquid hydrocarbons, thereby bringing the liquid hydrocarbons to a gas state. Are removed by drying with.

It is known that polymers such as polyolefins contain unreacted hydrocarbon monomer gas in a certain concentration in the pores of resin voids and particles of the polymers even after they are dried. These gases slowly diffuse from such voids and pores and enter the environment around the particles. If the gases diffuse while the resin is present in the non-vented chamber, the enrichment of these gases, which is of course highly flammable, can lead to an explosion, especially if the hydrocarbon monomer enrichment becomes excessive in the presence of oxygen. Can pose a risk. Unreacted hydrocarbon gas is considered to be an environmental problem if it escapes to the atmosphere. in addition,
Such reacted polymer may also contain a liquid residue of unreacted monomeric compound. Such liquid residue should also be removed from the reacted polymer compound.

It was the government's desire to reduce the escape of unreacted monomer from the process of producing the corresponding polymer to an acceptable limit, which would, of course, limit the production of the polymer.

The presence of unreacted hydrocarbons in the atmosphere is
Therefore, it is an important issue for polymer manufacturers. It is expected that large quantities of hundreds of ppm in large air streams, unreacted hydrocarbons in the exhaust gas stream, will come from the generally continuously operating commercial process for producing polyolefins. Such quantities are considered too high in many areas.

Recent legislation in many areas has called for burning these hydrocarbons and releasing only their combustion products to the atmosphere. This is a potentially very expensive method as nitrogen or other inert purged gas must be added with sufficient fuel to burn out low concentrations of hydrocarbons. There is a possibility that Since most purging processes utilize large amounts of inert gas, a correspondingly large amount of fuel will be required to burn out the inert gas. This method is also capital-intensive because it requires additional furnaces, ventilation equipment and air handling equipment. In view of the above, it strips gaseous monomers from reacted polymers that do not require large amounts of inert gas.
Effective methods are therefore needed.

The prior art teaches techniques for removing volatile unpolymerized monomers from polymers of corresponding monomers. As pointed out, it is known in the prior art to use degassing or flushing methods, for example to remove unpolymerized gaseous monomers from solid olefin polymers. In this flushing method, generally, a solid polymer (for example, a granular polymer) is conveyed to a flushing container, and the polymer is released from the polymer by contacting it with an inert gas flushing stream in the flushing container. Consists of expelling the monomer gas. This process is time consuming and requires large amounts of inert gas to complete the process and reduce the concentration of gaseous monomers in the solid polymer to acceptable limits as described above. Therefore, more efficient methods would be beneficial to the polymer manufacturing industry.

[0008]

SUMMARY OF THE INVENTION One object of the present invention is the direct and time-consuming requirement of relatively small amounts of predominantly gaseous unreacted monomer remaining in the solid polymer reaction product. To provide a method for removal or ejection in an efficient manner.

[0009]

[Means for Solving the Problems]

SUMMARY OF THE INVENTION The present invention provides a method of subjecting a reacted solid polymer to alternating steps of relatively high gas pressure and near vacuum steps to remove unreacted monomer from the reacted polymer during the near vacuum step. To achieve the other purposes.

In the method of the present invention, the pressure surrounding the polymer resin particles is alternately lowered and raised. By performing this operation, the amount of unreacted monomer that is trapped within or around the voids of the polymer particles or the pores of the polymer is such that during at least one higher (positive) pressure step. It is reduced by being replaced by an inert gas such as nitrogen in the voids and pores. In the low pressure stage,
At least a part of the unreacted monomer is extracted from inside and around the polymer resin particles. In addition, any residual unreacted liquid monomer evaporates under such near vacuum conditions,
It is flushed from the polymer compound during the vacuum stage.

DETAILED DESCRIPTION OF THE INVENTION In the process of the invention, the solid, dried polymer is alternately subjected to at least one pressure stage and at least one vacuum stage. For the purposes of the present invention, the "pressure stage" is defined as the period during which the polymer is subjected to a positive gas pressure, mainly with an inert gas. The "vacuum stage" is also defined as the period during which the polymer is subjected to near vacuum conditions and at least some of the gaseous monomer and possibly some liquid monomer is withdrawn from the polymer. The method of the present invention therefore
Gaseous monomers in contact with the polymeric material (eg,
And / or at least a portion of the gaseous monomer (s) in the vicinity of the polymeric material (s) such that they are entrapped and in contact with the voids between the polymeric particles and / or in the pores of the polymeric particles. Helps to eliminate by including at least part of the atmosphere to which it is exposed. In addition, unreacted liquid monomer is typically evaporated and flushed from the solid polymer during the first vacuum stage of the method of the present invention.

The combination of the vacuum stage and the pressurizing stage is referred to in the present invention as a "pressurize / vacuum cycle", regardless of the order of the stages.

In the ejection process of the present invention, the vacuum stage is generally the last ejection stage applied before the polymer is removed as product. However, it is also conceivable that the pressure stage is the last ejection stage to which the polymer is applied, depending on the conditions to which the polymer is subsequently applied (for example in a later conveying or storage process), in particular with regard to the pressure. .

In the drawings described in more detail below, like numerals refer to like elements.

FIG. 1 illustrates a method and apparatus 10 of the present invention which includes one vacuum stage followed by a complete vacuum / pressurization cycle.
Is shown as a schematic diagram. Indeed, in the illustrated embodiment, the polymeric material is exposed to one and a half vacuum / pressure cycles.

In discussing the illustrated process of the present invention, it is sometimes referred to as a polyethylene or polyolefin resin for the reacted polymer resin and as a monomeric hydrocarbon for the unreacted monomer. To do. However, this is merely representative and it will be appreciated that the method can be used to remove gaseous and liquid monomers from a wide range of polymers.

As pointed out, the polymer heated by the method and apparatus of the present invention typically has already undergone a drying process resulting in the formation of gaseous monomers. The drying process for polyolefin resins such as polyethylene is typically carried out at about 50 psia, and the polymer thus dried can first be subjected to the vacuum stage of the ejection process of the present invention. Alternatively, the polymer can subsequently be first subjected to a pressure stage. In the illustrated embodiment, the solid polymeric resin 22 under positive gas pressure is placed in the drying vessel 11 at about 50 psia.
The dried solid polymer product in vessel 11 is exposed to a hydrocarbon atmosphere and further entrained with hydrocarbon gas on its polymer surface or within the voids of the polymer granules.

The polymer is discharged from the drying container 11 into the pressure valve means 1
Proceeding via 3 to the first or first vacuum vessel 12, in which the first stage of the ejection process of the invention is carried out. The pressure valve means 13 can be, for example, a high pressure rotary valve. The pressure drop from the pressure vessel 11 to the vacuum vessel means 12 occurs across the valve 13. The valve 13 should be capable of withstanding high atmospheric pressures. The vacuum and pressure vessels used in the present invention are of the holding vessel type well known to those skilled in the art. Assuming that the process described here is a continuous process, the size of these vessels is directly influenced by the rate of solid reacted polymer withdrawal, which can be determined by a person skilled in the art. Is.

In the vacuum vessel 12, the reacted polymer is subjected to near vacuum conditions. Near vacuum conditions are pressures defined for the purposes of this application up to about 9 psia, and more preferably up to about 7 psia. If more than one vacuum stage is present, the pressure in each stage may vary. Gaseous hydrocarbons are withdrawn from vessel 12 via conduit 14 by using a vacuum pump (not shown). If desired, a small amount of an inert gas that aids in flushing gaseous hydrocarbons from the vacuum vessel may be supplied to the vacuum vessel via conduit 14a. Such gases are inert to both polymer and hydrocarbon gases. This gas also exits the vacuum vessel 12 via conduit 14. Only a very small amount of such a gas is used, i.e., sufficient to facilitate the flushing of gaseous hydrocarbons, but not so large as to significantly increase the pressure level in the vacuum vessel. Those skilled in the art can determine how much flushing gas should be used in practicing the present invention. In addition, and likewise, if desired, inert gases and gaseous monomers may be led through a dust collector (not shown) and subsequently via conduit 14 to remove any particulate impurities present therein. be able to.

As pointed out, the inert gas used in the practice of the present invention is any gas that is inert to both the resin being flushed and the particular gaseous monomer being removed. May be. The preferred gas is nitrogen, although other inert gases can be used in this process. For example, carbon dioxide can be used to advantage for some applications. Other suitable inert gases will be readily apparent to those skilled in the art. Combinations of more than one inert gas can also be used. Ideally, oxygen should not be included for this inert gas mixture. However, a small amount of oxygen is acceptable, depending on the hydrocarbon concentration in the pressure vessel and the monomers being expelled. One of ordinary skill in the art can easily determine the permissible oxygen level for a particular monomer. Another advantage of using relatively pure nitrogen as the inert gas is that more hydrocarbon gas can be expelled from the resin particles and that pure nitrogen that may be expelled with the outgoing resin is It means that gases containing impurities do not contribute to atmospheric emissions as they do. Therefore, the inert gas is preferably pure nitrogen.

The first vacuum stage serves the purpose of removing the gaseous monomer in the atmosphere around the polymer and the gaseous monomer present in the voids between the solid polymer particles. Furthermore,
All liquid monomer residues are also generally evaporated during this first vacuum stage. After completion of the first vacuum stage, the polymeric material is delivered to the first pressure vessel 15 via valve 16. In the pressure vessel 15 the polymeric substance is introduced into the conduit 1
The composition of the polymer is increased by exposure to an inert gas via V.8, the inert gas entering the voids and / or pores of a polymer containing gaseous hydrocarbons at a lower pressure. It tends to dilute any hydrocarbon gas remaining on the product or in the composition. The polymer then proceeds from the second pressure vessel 15 via the pressure valve means 20 to the second vacuum vessel 17. The inert gas and unreacted gaseous monomer impurities are optionally routed from the second vacuum vessel 17 via conduit 19 utilizing an inert gas supplied via conduit 19a to facilitate this process. Be pulled out. It is at this point that the higher pressure hydrocarbon and nitrogen mixture in the polymer pores exits into the low pressure or near vacuum environment. The polymer can then be advanced through the pressure valve 21 to form the product as shown, or the polymer may be subjected to further pressure / vacuum cycles. In the illustrated embodiment, the pressure steps performed in vessel 15 and vessel 1
The combination with seven vacuum stages consists of one pressure / vacuum cycle.

FIG. 2 shows another embodiment of the device of the present invention. In this apparatus, a small amount of gas for flushing is supplied from the chamber 25 of the rotary valve 16 to the vacuum container 12 via the ventilation means 23. The rotary valve 16 a corresponds to the rotary valve 16 except that the ventilation means 23 is included. Figure 3 shows the rotary valve 1
6a shows another embodiment of 6a in more detail.
In the illustrated embodiment, rotary valve means 16a
As is rotated clockwise according to arrow A, the substance is accordingly transferred from each individual chamber 25 of the rotary valve means 16 via the outlet 26 to the pressure vessel 15. As each chamber 25 puts the substance into the pressure vessel 15, the substance will be exposed to a certain amount of pressurizing gas present in the pressure vessel. This gas should be evacuated from each chamber 25 before it reaches the point where one chamber 25 receives material from the vacuum vessel 12 via inlet 27 during rotation. In the illustrated embodiment, the venting means 23 serves the purpose of removing such inert pressurizing gas from the rotary valve means 16a. As each chamber 25 is rotated to a position adjacent to the vent means 23, the inert gas will accordingly exit the chamber 25 via the vent means 23. As can be seen in the embodiment shown in FIG. 2, the gas is led via conduit 28 to the vacuum chamber 12 in which it is used as a flush gas. In the embodiment shown in FIG. 2, the optional venting means 23 and the conduit 28 are optional venting means 14.
These ventilation means 2 are suitable for the purpose of substituting a.
It will be appreciated that 3 and conduit 28 may be used in combination with conduit 14a and that the flush gas may, if desired, enter the vacuum vessel from both sources.

The inert gas used in the pressurization step is believed to be in the atmosphere surrounding the reacted polymer particles and, under lower pressure, in the interstices between the polymer particles and / or in the pores of the polymer particles. It is believed to serve the purpose of replacing or diluting at least a portion of the gaseous hydrocarbons. The pressure at which the reacted polymeric material is applied in the pressure vessel should be sufficient to force at least some of the inert gas into the pores of the polymer to displace the gaseous hydrocarbons. The pressure applied by the polymer during the pressure stage of the pressure / vacuum cycle is typically about 15 to about 75 p.
sia, and preferably about 25 to about 55
It is in the range of psia. This pressure is. If there are more than one pressurization / vacuum cycle, the pressurization stage can be changed to the pressurization stage.

After the completion of one complete pressurization / vacuum cycle, the polymer may be sent for storage, or the polymer may be supplemented with additional complete or partial to achieve additional hydrocarbon eviction or depletion. Pressure / vacuum cycle. In this case, the polymer is successively subjected to alternating pressure and vacuum steps. As pointed out, the embodiment shown in FIG. 1 is an embodiment of the method and apparatus for a one and a half stage pressure / vacuum cycle.

Depending on the desired degree of hydrocarbon reduction, one, one and a half, two, two and a half and three or more pressurizations /
A vacuum cycle can be used.

The rate of hydrocarbon reduction during each pressure / vacuum cycle is primarily a function of the pressure ratio between the pressure and vacuum stages to which the polymer is exposed. The higher the pressure ratio, the greater the proportion of the outer (inert) gas that enters the pores of the polymeric material and thereby dilutes the residual hydrocarbon gas. The following table shows process parameters such as vacuum level, across rotary valves,
FIG. 6 illustrates the selectivity of the removal process with respect to the maximum pressure difference between the pressure and vacuum stages and the number of ejection stages used.

[0027]

[Table 1]

HC = gaseous hydrocarbon PPM = parts per million

While the present invention is generally suitable for the purpose of removing unpolymerized monomers from polymers, it is particularly suitable for removing unreacted monomers resulting from the polymerization of olefins. Olefin monomers commonly used in such reactions are 1-olefins having up to 8 carbon atoms per molecule but having no branch near the double bond other than the 4-position. is there. Typical examples are ethylene, propylene, butene-1, 1-pentene and 1,3-butadiene. This method is also suitable for removing unreacted residual vinyl chloride from vinyl chloride polymers.

Polyvinyl chloride resins that can be treated by the method of the present invention include all polyvinyl chloride polymers composed primarily of polymerized vinyl chloride. Thus, it is possible to use multicomponent copolymers or interpolymers made from homopolymers of vinyl chloride and monomer mixtures containing vinyl chloride with smaller amounts of other copolymerizable monoolefin materials. Some representative examples of mono-olefinic materials capable of forming interpolymers with vinyl chloride are: vinylidene halides such as vinylidene chloride and vinylidene bromide; vinyl esters such as vinyl acetate, vinyl propionate. And vinyl butyrate; acrylic acid and α-alkylacrylic acid and their alkyl esters, their amides and their nitriles; vinyl aromatic compounds,
For example, styrene and dichlorostyrene; alkyl esters of maleic acid and fumaric acid, such as dimethyl maleate and diethyl maleate; vinyl alkyl ethers,
For example vinyl methyl ether and vinyl ethyl ether; α-olefins such as ethylene and propylene;
As well as other readily polymerisable compounds containing one olefinic double bond, especially those containing CH2 = C <groups. Vinyl chloride is generally copolymerizable with up to about 10% by weight of the selected comonomer or comonomers.

Various modifications can easily be made to the design and operation of the above illustrative embodiments to adapt the method of the present invention to various operating requirements, all within the scope and spirit of the present invention. It comes in. Accordingly, various modifications and substitutions, as well as rearrangements and combinations of vessels, equipment and / or process steps, can be made by those skilled in the art without departing from the spirit and scope of the invention.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating one embodiment of the method and apparatus of the present invention.

FIG. 2 is a schematic diagram illustrating another embodiment of the method and apparatus of the present invention.

FIG. 3 is a schematic diagram illustrating one embodiment of a high pressure valve used in the method and apparatus of the present invention.

[Explanation of symbols]

10 Unreacted Monomer Removal Device 11 Drying Container 12 First Vacuum Container 13, 16, 16a, 19, 19a, 20, 21 Pressure Valve Means 14, 14a, 18, 28 Conduit 15 First Pressure Container 17 Second Vacuum Container 22 Solid polymer resin 23 Aeration means 25 Valve chamber 26 Outlet 27 Inlet

Claims (20)

[Claims] 1. A method of removing a gaseous polymer from a solid polymer composition containing unpolymerized gaseous monomer, the method comprising subjecting the solid polymer to at least one pressure / vacuum cycle. The polymer is subjected to alternating pressure and vacuum steps, in either order, wherein the pressure step of the pressure / vacuum cycle places the polymer primarily at a positive pressure from a gas inert to the polymer and the monomer. And subjecting the polymer to near vacuum conditions during the vacuum stage of the pressure / vacuum cycle, during which at least a portion of the gaseous monomer is removed from the polymer composition. 2. The method of claim 1, wherein the polymer composition is subjected to more than one pressure / vacuum cycle. 3. The method of claim 1, wherein the polymer composition is subjected to one and a half pressure / vacuum cycles. 4. The method of claim 1, wherein the inert gas is substantially free of oxygen. 5. The method of claim 4, wherein the inert gas is nitrogen.
The method described in. 6. The method of claim 1, wherein the polymer composition is a polyolefin composition. 7. The pressure applied to the polymer during the pressurization stage of the pressurization / vacuum cycle is from about 15 to about 75 psia.
The method according to claim 1, wherein 8. The polymer is subjected to a pressure of about 25 to about 55 psia during the pressure stage of the pressure / vacuum cycle.
The method of claim 7, wherein 9. The method of claim 1, wherein the polymer is subjected to a pressure of about 9 psia or less during the vacuum stage of the pressure / vacuum cycle. 10. The method of claim 9, wherein the polymer is subjected to a pressure of about 7 psia or less during the vacuum stage of the pressure / vacuum cycle. 11. The method of claim 1, wherein unreacted liquid monomer is also removed from the solid polymer during the vacuum stage of the pressure / vacuum cycle. 12. The method of claim 1, wherein the polymer is subjected to a small amount of flushing gas during the vacuum cycle. 13. A device for removing a gaseous monomer from a composition of a solid polymer, the first pressure vessel in which the polymer is subjected to a positive pressure, which is in contact with the polymer in the pressure vessel. Will be
A pressure vessel in communication with a source of a gas inert to the polymer and the monomer; a first vacuum vessel in communication with the pressure vessel, the first vacuum vessel in communication with the vacuum source, and The vacuum vessel comprising withdrawing means for removing at least a portion of the active gas and at least a portion of the gaseous monomer from the polymer composition; and the polymer composition from the first pressure vessel to the first vacuum. The apparatus comprising first pressure valve means for delivering to a container. 14. The apparatus according to claim 13, further comprising means for introducing a gas for flushing into the vacuum vessel. 15. A second pressure vessel in communication with the first vacuum vessel and substantially the same as the first pressure vessel; delivering the polymeric material from the first vacuum vessel to the second pressure vessel. Second pressure valve means for communicating with the second pressure vessel, the second vacuum vessel being substantially the same as the first vacuum vessel;
14. The apparatus of claim 13, further comprising pressure valve means for delivering the polymeric material from the second pressure vessel to the second vacuum vessel. 16. The apparatus of claim 15 further comprising degassing means for drawing gas from the second pressure valve means to the first vacuum vessel. 17. The apparatus of claim 15 wherein the pressure in the first and second pressure vessels is in the range of about 15 to about 75 psia. 18. The device of claim 17, wherein the pressure is in the range of about 25 to about 55 psia. 19. The apparatus of claim 15, wherein the pressure in the first and second vacuum vessels is less than or equal to about 9 psia. 20. The pressure is less than about 7 psia,
The device according to claim 19.