CA2103642C

CA2103642C - Process for drying resin and stripping hydrocarbons from the solids

Info

Publication number: CA2103642C
Application number: CA002103642A
Authority: CA
Inventors: Kermit D. Paul
Original assignee: Fuller Co
Current assignee: Fuller Co
Priority date: 1992-08-26
Filing date: 1993-08-09
Publication date: 1997-02-25
Anticipated expiration: 2013-08-09
Also published as: GB2271114B; GB9317685D0; CA2103642A1; GB2271114A; JPH06179712A

Abstract

The present invention relates to a process for removing unreacted gaseous monomers from a solid polymer product.
In the present invention the dried polymers are subject to alternating stages of positive pressure and near vacuum conditions, with unreacted gaseous monomers being removed from the reacted polymers during the vacuum stage.

Description

21 0 3 6 4 2 Dkt. 1014 Rermit D. Paul The present invention relates to a process of removing unpolymerized monomers from a solid polymeric composition, and in particular to a process for removing unreacted liquid and gaseous monomers from solid polymers made from the corresponding monomers and, more particularly, to removing unreacted gaseous monomeric hydrocarbons which remain in a solid polyolefin product. The present invention also relates to an apparatus for achieving the processes specified above.

BAC~GROUND OF THE l~v~.,lON
In the production process for making solid polymers, for example, processes for making polyolefin compounds such as polyethylene and polypropylene from monomeric hydrocarbons, the reacted polymeric resins can exit the reactor along with unreacted liquid monomeric hydrocarbons. These hydrocarbons are considered to be impurities which must be removed before the reacted polyolefins can be utilized. Therefore, these unreacted liquid monomers must, as a rule, be removed from polymers before the final product is utilized. Generally, these hydrocarbons are removed by being dried, such as with a mechanical type drier where heat is added to the hydrocarbons to provide the heat of vaporization for the liquid to cause it to go into the gaseous state.

ddj20104.980 {11} 1 21 036~2 It is known that even after polymers such as polyolefins have been dried they will contain, in their resin voids and their particle pores, a certain concentration of unreacted monomeric, such as hydrocarbon, gas. These gases will slowly diffuse from such voids and pores and enter the atmosphere around the particles.
If the gas diffuses while the resin is in a non-ventilated chamber, the concentration of these gases, which are of course highly flammable, can present a danger of explosion, particularly if the hydrocarbon monomer concentration becomes excessive in the presence of oxygen. If allowed to escape to the atmosphere, unreacted hydrocarbon gas is believed to be a concern environmentally. In addition, it is also possible that such reacted polymers will contain liquid residue of unreacted monomeric compounds. Such liquid residue should also be removed from the reacted polymeric compounds.
It has been required by government agencies that the escape of an unreacted monomer from a process for producing a corresponding polymer be reduced to acceptable limits, which would, of course, limit production of the polymer.
The presence of unreacted hydrocarbon in the atmosphere is, therefore, of major concern to manufacturers of polymers.
Unreacted hydrocarbon concentrations in vent gas streams in amounts as much as several hundred parts per million in large volumes of air flow may be expected to result from commercial processes, generally continuously operated, for the production of polyolefins.

ddj20104.980 {11} 2 21 033i~2 In many jurisdictions such amounts are considered to be excessive.
Recent legislation in many jurisdictions have required that these hydrocarbons be burned and only their products of combustion be admitted to the atmosphere. This can potentially be a very expensive process since a sufficient amount of fuel must be added to the nitrogen, or other inert, purged gas to burn off the low concentration of hydrocarbons. Since most purging processes utilize large amounts of inert gas, a correspondingly large amount of fuel would be required to burn off the inert gases. This process is also capital equipment intensive since additional furnaces, ducting and air handling equipment is required. In view of the above, an effective process to strip gaseous monomers from a reacted polymer that does not require a large amount of inert gas is therefore needed.
The prior art teaches techniques from removing volatile unpolymerized monomers from polymers of the corresponding monomers.
As indicated, it is known, for example, in the prior art to employ a degassing or purging process for removing unpolymerized gaseous monomers from solid olefin polymers. The purging process generally comprises conveying the solid polymer (e.g., in granular form) to a purge vessel and contacting the polymer in the purge vessel with a inert gas purge stream to strip away the monomer gases which are evolved from the polymer. This process is time consuming and, as indicated above, a great quantity of inert gas is needed to complete the process to thereby reduce the concentration of gaseous monomers in the solid polymer to an acceptable limit. A more dd j20104 . 980 {11} 3 2 1 036~2 efficient process would be therefore beneficial to the polymer production industry.

It is an object of the present invention to provide a method of removing, or stripping, an unreacted monomer, primarily gaseous, which remains in a solid polymer reaction product in a straightforward, time efficient manner which requires only a relatively small amount of inert gas.
The present invention accomplishes this and other objects by a process whereby the reacted solid polymer is subjected to alternating stages of comparatively high gas pressure and near vacuum, with unreacted monomers being removed from the reacted polymers during the near vacuum stage.
In the process of the present invention, the gas pressure surrounding the polymer resin particles is alternately lowered and raised. By doing this, the amount of unreacted monomers trapped in and around the polymer particle voids and the particle pores is reduced by being replaced, in such voids and pores during at least one higher (positive) pressure stage, with an inert gas such as nitrogen. In a low pressure stage at least some of the unreacted monomers will be drawn off from within and around the polymer resin particles. In addition, any residue of unreacted liquid monomers will, under such near vacuum conditions, evaporate and will be flashed off from the polymer compounds during the vacuum stage.

ddj20104.980 {11} 4 DE8CRIPTION OF THB DRA~ING8 Figure 1 is a diagrammatic illustration of an embodiment of the process and apparatus of the present invention.
Figure 2 is a diagrammatic illustration of another embodiment of the process and apparatus of the present invention.
Figure 3 is a diagrammatical illustration of one embodiment of a high pressure valve utilized in the process and apparatus of the present invention.

DET~TT~D DE8CRIPTION OF THE INVENTION
In the process of the present invention the solid dried polymer is alternatingly subject to at least one pressure stage and one vacuum stage. For the purposes of this invention a "pressure stage" is defined as a period during which the polymer is subjected to positive gas pressure, primarily from an inert gas. A "vacuum stage" is defined as a period in which the polymer is subject to near vacuum conditions and during which at least some gaseous, and perhaps some liquid, monomers are drawn off the polymer. The process of the present invention will therefore serve to remove at least some of the gaseous monomers that are in contact with (such as being entrapped in the voids between the polymer particle and/or in the pores of the particles) and/or are in the vicinity of the polymeric material by comprising at least part of the atmosphere to which the material is exposed. In addition, unreacted liquid monomers will typically be evaporated and flashed off the solid ddj20104.980 {11} 5 polymers during a first vacuum stage of the process of the present invention.
The combination of a vacuum stage and a pressure stage, in either order, is referred to herein as a "pressure/vacuum cycle".
In the stripping process of the present invention a vacuum stage is generally the last stripping stage the polymer will be subjected to prior to being removed as product. However, it is conceivable that, depending on the conditions, in particular those conditions relating to pressure, that the polymer is subsequently exposed to (for example, in later conveying or storage steps), that the last stripping stage a polymer is subject to will be a pressure stage.
In the Figures described in more detail below, like numerals refer to like elements.
Figure 1 depicts, in diagrammatical form, a process and apparatus 10 of the present invention which encompasses one vacuum stage and, thereafter, a complete vacuum/pressure cycle. In effect, in the depicted embodiment a polymeric material will be exposed to one and one half vacuum/pressure cycles.
In discussing the process of the present invention as depicted in the Figures, reference will at times be made to the reacted polymeric resin as being a polyethylene or polyolefin resin, and to the unreacted monomers as being monomeric hydrocarbons. It is understood, however, that this is only exemplary and that this process may be utilized to remove gaseous and liquid monomers from a wide variety of polymers.

ddj20104.980 ~ 6 As indicated, the polymer heated by the process and apparatus of the present invention will typically have undergone a drying process with the resulting formation of gaseous monomers. Since the drying process for a polyolefin resin such as polyethylene is typically done at about 50 psia, the thus dried polymer can thereafter be subjected initially to a vacuum stage of the stripping process of the present invention. Alternatively, the polymer can thereafter first be subjected to a pressure stage. In the embodiment depicted in the Figure, solid polymeric resin 22 under positive gas pressure is contained at about 50 psia within drying vessel 11. The dried solid polymer product in vessel 11 is exposed to a hydrocarbon atmosphere and, in addition, has hydrocarbon gases entrapped on its surface or within its particulate voids.
The polymer will pass from drying vessel 11 to initial or first vacuum vessel 12, in which the first stage of the stripping process of the present invention will take place, via pressure valve means 13, which can, for example, be a high pressure rotary valve. The pressure drop from pressure vessel 11 to vacuum vessel means 12 will take place across valve 13, which should have the capability to withstand high atmospheric pressure. The vacuum and pressure vessels utilized in the present invention will be of the type of holding vessels well known to those skilled in the art.
Assuming that the process described herein is continuous, the size of the vessels will be directly influenced by the rate of ddj20104.980 {11} 7 2 1 ~3~42 withdrawal of the solid reacted polymer and can be determined by the skilled practitioner.
In vacuum vessel 12 the reacted polymer will be subjected to near vacuum conditions, which, for the purposes of this application is defined as being up to approximately 9 psia, and more preferably up to approximately 7 psia. If there is more than one vacuum stage the pressure in each stage may vary. The gaseous hydrocarbons will be drawn out of the vessel via conduit 14 through the use of a vacuum pump (not shown). Optionally, a small amount of inert gas to aid in the purging of the gaseous hydrocarbons from the vessel may be fed to the vacuum vessel via conduit 14a. Such gas will be inert to both the polymer and the hydrocarbon gases, and will also exit vacuum vessel 12 via conduit 14. Only a small amount of such gas will be used, that is, only an amount sufficient to aid in the purging of the gaseous hydrocarbons but not so much so as to significantly raise the pressure level in the vacuum vessel. Those skilled in the art will be able to determine how much purge gas should be utilized when practicing the present invention. In addition, and again optionally, the inert gas and the gaseous monomers may be thereafter directed via conduit 14 through a dust collector (not shown) to remove any particulate impurities present therein.
As indicated, the inert gas employed in the practice of the present invention may be any gas which is inert both to the resin being purged and the particular gaseous monomers being removed.
The preferred inert gas is nitrogen although other gases inert in dd j20104 . 980 {11} 8 2 t 03~2 the process may be employed. For example, carbon dioxide may be advantageously utilized for some applications. Other suitable inert gases would be readily apparent to those skilled in the art.
Combinations of more than one inert gas may be utilized. Ideally, there should be no oxygen included with the inert gas mixture although a small amount can be tolerated depending upon the hydrocarbon concentration in the pressure vessel and the monomers being stripped. Those skilled in the art can easily determine the tolerable oxygen levels for a particular monomer. Other advantages of employing relatively pure nitrogen as the inert gas are that more hydrocarbon gases can be stripped from the resin particles and any pure nitrogen that may be discharged with the exiting resins does not contribute to atmospheric emissions as would gases containing impurities. It is therefore preferred that the inert gas be pure nitrogen.
The first vacuum stage will serve to remove gaseous monomers in atmosphere around the polymers and gaseous monomers located in the voids between the solid polymeric particles. In addition, any liquid monomer residue will generally be evaporated during the first vacuum stage. After the conclusion of the first vacuum stage, the polymeric material will be delivered to first pressure vessel 15 via valve 16. While in pressure vessel 15 the polymeric material will be pressurized via conduit 18 by being exposed to an inert gas, which inert gas will tend to dilute any hydrocarbon gases remaining on or in the polymer composition by flowing into polymeric voids and/or pores that contain gaseous hydrocarbons at ddj20104.980 {11} 9 21 036~2 a lower pressure. The polymer will thereafter pass from second pressure vessel 15 to second vacuum vessel 17 via pressure valve means 20. Inert gas and unreacted gaseous monomeric impurities will be drawn off of second vacuum vessel 17 via conduit 19, optionally utilizing inert gas fed via conduit l9a to facilitate this process. It is at this point that the higher pressure hydrocarbon and nitrogen mixture in the polymeric pores flows out into the low pressure, i.e. near vacuum, environment. Thereafter, the polymer can thereafter be passed to product via pressure valve 21 as depicted, or it may be subjected to further pressure/vacuum cycles. In the depicted embodiment the combination of the pressure stage which takes place in vessel 15 and the vacuum stage of vessel 17 will comprise one pressure/vacuum cycle.
Figure 2 shows another embodiment of the apparatus of the present invention wherein a small amount of purging gas is fed into vacuum vessel 12 from the chambers 25 of rotary valve 16 via vent means 23. Rotary valve 16a corresponds to rotary valve 16 except for the inclusion of vent means 23. Figure 3 shows another embodiment of rotary valve means 16a in greater detail. As valve means 16a rotates, in the depicted embodiment in a clockwise direction according to arrow A, material will be emptied via outlet 26 into pressure vessel 15 from each individual chamber 25 of rotary valve means 16. As each chamber 25 deposits material into pressure vessel 15, it will be exposed to a certain amount of the pressurizing gas present therein. This gas should be vented from each chamber 25 prior to the point where a chamber 25, upon ddj20104.980 {11} lo 21 0364~
rotation, will receive material via inlet 27 from vacuum vessel 12.
In the depicted embodiment, vent means 23 serves to remove such inert pressurizing gas from the valve means 16a. As each chamber 25 rotates to a position adjacent to the vent means 23, inert gas will exit such chamber 25 via vent means 23. As seen in the embodiment depicted in Figure 2, the gas is directed, via conduit 28, to vacuum chamber 12 where it will be utilized as a purging gas. Although, in the depicted embodiment in Figure 2, optional vent means 23 and conduit 28 serves to replace optional vent means 14a, it is understood that vent means 23 and conduit 28 can be used in combination with conduit 14a, and purging gas optionally can enter a vacuum vessel from both sources.
It is believed that the inert gas employed in the pressure stage serves to replace or dilute at least some of the gaseous hydrocarbon which is believed to be located in the atmosphere surrounding the reacted polymeric particles and also, under lower pressure, in the interstices between the polymeric particles and/or in the particles' pores. The pressure at which the reacted polymeric material is subject to within the pressure vessel should be sufficient to force at least some of the inert gas into the polymeric pores to repelace the gaseous hydrocarbons. Typically, the pressure to which the polymer is subjected to during the pressure stage of the pressure/vacuum cycle will range from about 15 to about 75 psia, and will preferably range from about 25 to about 55 psia. The pressure can vary from pressure stage to pressure stage if there is more than one pressure/vacuum cycle.

ddj20104.980 {11} 11 After one complete pressure/vacuum cycle the polymer can be sent to storage or it can be subjected to additional complete or partial pressure/vacuum cycles in order to achieve additional hydrocarbon stripping or reduction, wherein the polymer continues to be subject to alternating pressure and vacuum stages. As indicated, the embodiment depicted in Figure 1 is of a one and one half stage pressure/vacuum cycle process and apparatus.
Depending upon the degree of hydrocarbon reduction desired, one, one and one half, two, two and one half, and three or more pressure/vacuum cycles may be employed.
The percent of hydrocarbon reduction during each pressure/vacuum cycle is largely a function of the pressure ratio between the pressure and vacuum stages to which the polymer is exposed. The higher the pressure ratio, the larger the proportion of outside (inert) gas which flows into the pores of the polymeric material to thereby dilute the remaining hydrocarbon gas. The following Table illustrates the sensitivity of the removal process to the process parameters such as vacuum level, maximum differential pressure across the rotary valves between the pressure and vacuum stages and the number of stripping stages used.

dd j20104. 980 {11} 12 THBORBTICAL PPN OF ~C IN RB8IN DBLIVERBD TO 8TORAGB
(Ba ed upon 1000 PPM in re~in at st~rt) TABLB
Maximum -------------DESIGN VACUUM, PSIA------------RVatlavreY 1 ¦ 2 ¦ 3 ¦ 4 ¦ 5 Differential Pressure SINGLE CYCLE STRIPPING
16.41 32.24 47.52 62.29 76.56 19.97 39.1 57.42 75 91.88 25.52 49.66 72.53 94.23 114.84 35.34 68.06 98.44 126.72 153.13 TWO CYCLE STRIPPING
0.29 1.13 2.46 4.22 6.38 0.43 1.66 3.59 6.12 9.19 0.71 2.68 5.73 9.66 14.36 1.36 5.04 10.55 17.48 25.52 THREE CYCLE TRIPPING
0.0004 0.0054 0.026 0.078 0.18 0.0006 0.0096 0.0460.136 0.31 0.0013 0.0197 0.0920.27 0.61 0.0036 0.0508 0.2310.656 1.45 HC= gaseous hydrocarbon PPM= parts per million Although this invention is suitable for removing unpolymerized monomers from polymers in general, it is particularly suitable for removing unreacted monomers resulting from olefin polymerizations.
The olefin monomers generally employed in such reactions are 1-olefins having up to 8 carbon atoms per molecule and no branching nearer the double bond than the 4-position. Typical examples ddj20104.980 {11} 13 21 036~2 include ethylene, propylene, butene-1, l-pentene, and 1, 3-butadiene. The process is also suitable for removing residual, unreacted vinyl chloride from vinyl chloride polymers.
The polyvinyl chloride resins that can be treated by the process of the present invention include all of the polyvinyl chloride polymers which are composed predominantly of polymerized vinyl chloride. Thus, there may be utilized the homopolymers of vinyl chloride and the multicomponent copolymers or interpolymers made from monomeric mixtures containing vinyl chloride, together with lesser amounts of other copolymerizable mono-olefinic materials. Exemplary of some of the mono-olefinic materials which may be interpolymerized with vinyl chloride are the vinylidene halides, such as vinylidene chloride and vinylidene bromide; vinyl esters, such as vinyl acetate, vinyl propionate, vinyl butyrate;
acrylic and alpha-alkyl acrylic acids and their alkyl esters, their amides and their nitriles; vinyl aromatic compounds, such as styrene, dichlorostyrene; alkyl esters of maleic and fumaric acid, such as dimethyl maleate and diethyl maleate; vinyl alkyl ethers, such as vinyl methylether, vinylethylether; alpha-olefins, such as ethylene and propylene; and other readily polymerizable compounds containing a single olefinic double bond, especially those containing the CH2=C < group. In general, vinyl chloride may be copolymerized with up to about 10 percent weight of the selected comonomer or comonomers.
Variations on the design or operation of the above illustrative embodiments may be readily made to adapt the inventive ddj20104.980 {11} 14 process to various operational demands, all of which are within the scope and spirit of the present invention. Consequently, it is to be understood that various modifications and substitutions, as well as rearrangements and combinations of vessels, apparatus, and/or process steps, can be made by those skilled in the art without departing from the spirit and scope of this invention.

ddj20104.980 {11} 15

Claims

WHAT IS CLAIMED IS:

1. A method for removing unpolymerized gaseous monomers from a solid polymer composition containing said gaseous monomers which method comprises subjecting the polymer to at least one pressure/vacuum cycle wherein the polymer is subjected to alternating stages of pressure and vacuum, in either order, wherein, in the pressure stage of the pressure/vacuum cycle, the polymer is subjected to a positive pressure primarily from a gas which is inert to said polymer and said monomers; and in the vacuum stage of the pressure/vacuum cycle, the polymer is subject to near vacuum conditions during which at least some of the gaseous monomers are 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 one half pressure/vacuum cycles.

4. The method of claim 1 wherein the inert gas contains substantially no oxygen.

5. The method of claim 4 wherein the inert gas is nitrogen.

6. The method of claim 1 wherein the polymer composition is a polyolefin composition.

7. The method of claim 1 wherein the pressure the polymer is subjected to during the pressure stage of the pressure/vacuum cycle ranges from about 15 to about 75 psia.

8. The method of claim 7 wherein the pressure the polymer is subjected to during the pressure stage of the pressure/vacuum cycle ranges from about 25 to about 55 psia.

9. The method of claim 1 wherein the pressure the polymer is subjected to during the vacuum stage of the pressure/vacuum cycle is no greater than about 9 psia.

10. The method of claim 9 wherein the pressure the polymer is subjected to during the vacuum stage of the pressure/vacuum cycle is no greater than about 7 psia.

11. The method of claim 1 wherein unreacted liquid monomers are also removed from the solid monomer 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 a purging gas during the vacuum cycle.

13. An apparatus for removing gaseous monomers from a solid polymer composition, said apparatus comprising:
a first pressure vessel in which the polymer is subjected to a positive pressure, said pressure vessel being in communication with a source of a gas which comes into contact with said polymer within said pressure vessel, said gas being inert to said polymer and said monomers;
a first vacuum vessel in communication with said pressure vessel, said vacuum vessel being in communication with a source of vacuum, and containing withdrawal means for removing at least some of said inert gas and at least some of said gaseous monomer from the polymer composition; and, first pressure valve means for conveying said polymer composition from said first pressure vessel to said first vacuum vessel.

14. The apparatus of claim 13 further comprising means to direct a purge gas into said vacuum vessel.

15. The apparatus of claim 13 which further comprises a second pressure vessel in communication with said first vacuum vessel said second pressure vessel being substantially identical to the first pressure vessel;
a second pressure valve means for conveying said polymer material from said first vacuum vessel to said second pressure vessel;
a second vacuum vessel in communication with said second pressure vessel, said second vacuum vessel being substantially identical to the first vacuum vessel; and, pressure valve means for conveying said polymer material from said second pressure vessel to said second vacuum vessel.

16. The apparatus of claim 15 further comprising venting means to vent 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 ranges from about 15 to about 75 psia.

18. The apparatus of claim 17 wherein the pressure ranges from about 25 to about 55 psia.

19. The apparatus of claim 15 wherein the pressure in the first and second vacuum vessels is no greater than about 9 psia.

20. The apparatus of claim 19 wherein the pressure is no greater than about 7 psia.