AU622278B2 - Process for separating volatile components from reaction mixtures obtained through high-pressure polymerisation - Google Patents

Description

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WATERMARK PATENT TRADEMARK ATTORNEYS i:r 4 62 COMMONWEALTH OF AUSTRALIA 2278 Form PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: o a4 Priority 4I i S' rRelated Art FtAr '44'* i44 Accepted: Published: 1 Name of Applicant HOECHST AKTIENGESELLSCHAFT r t Address of Applicant 50 Bruningstrasse, D-6230 Frankfurt/Main Germany 80, Federal Republic of Actual Inventor: HEINZ BECKEMEIER, HEINZ-DIETER BUHNEN, HERBERT MERCAMP and WILHELM

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Address for Service WATERMARK PATENT TRADEMARK ATTORNEYS.

LOCKED BAG NO. 5, HAWTHORN, VICTOriA 3122, AUSTRALIA Complete Specification for the invention entitled: PROCESS FOR SEPARATING VOLATILE COMPONENTS FROM REACTION MIXTURES OBTAINED THROUGH HIGH-PRESSURE POLYMERISATION The following statement is a full description of this invention, including the best method of performing it known to 1- I -2- Process for separating volatile components from reaction mixtures obtained through high-pressure polymerisation The invention relates to a process for separating volatile components from polymer melts which are obtained in the polymerisation of ethylene and the copolymerisation of ethylene with other copolymerisable compounds under high pressure.

0 1 .o The radical high-pressure polymerisation of ethylene alone or together with comonomers is performed on a commercial scale in 0 0 tubular reactors or in stirred autoclaves at pressures of 50 to 0 00 o o 400 MPa, in particular 140 to 250 MPa and temperatures of 100 to o 400 0

C.

0* A diagram of the process is shown in figure 1. Fresh ethylene is o ao first fed into a low-pressure compressor 8 via a line 9, compressed to 10 to 35 MPa and then, together with the ethylene, S which is unreacted in the polymerisation stage and recycled, it is raised to reaction pressure in a high-pressure compressor With the aid of suitable pumping equipment, e.g. piston pumps, the initiators present in solution are mixed into the compressed ethylene or metered into the reactor direct. Comonomers such as "vinyl ester, olefinically unsaturated carboxylic acids or alphaolefins are also fed into the high-pressure compressor 5 via a feed line 10. In a reactor 1 between 10 and 45 by weight of the monomers entering the reactor are reacted in one bun. In order to precipitate the polymer, the mixture is pressure-relieved in a 2 i high-pressure separator 3 to 5 to 50, preferably 10 to 35 MPa, optionally after prior cooling in a cooler 2. During pressurerelief gaseous components are returned to the high-pressure compressor 5 via a high-pressure return gas system 4. The polymer, which contains dissolved unreacted monomers, is pressure-relieved via a system consisting of low-pressure separator 6 and low-pressure cooler 7. The low-molecular compounds released are returned via the low-pressure compressor 8 to the high-pressure compressor 5. The polymer precipitated in i' t the low-pressure separator 6 contains not only residual amounts S of ethylene and comonomers but also oligomers.

t 9 t t Even when the polymerisation products are separated carefully, it is impossible to avoid volatile admixtures such as comonomers and It their decomposition products and other low-molecular compounds being retained in the polymers. During storage and processing of the polymers these residues lead to undesirable emissions and odours, in addition the flame point of the polymers is considerably reduced. Therefore, efforts are made to ensure that the products are free of volatile compounds by taking suitable measures during the pressure-relieving process and by subjecting the polymer to an additional treatment.

Further difficulties are caused by the fact that the ethylene liberated in the low-pressure separator contains comonomers, oligomers and other compounds which condense out when the separated ethylene is recycled, form deposits in compressors and pumps and can lead to clogging.

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-4- According to a process described in the DD-PS 131 824 either the reaction mixture is brought into contact intensively with the ethylene downstream of the polymerisation reactor or the polymer melt is brought into contact intensively with the ethylene after separation of the majority of the unreacted gas mixture in a pressure stage and the mixture is then separated in a stabilising zone. According to an appropriate embodiment of this process the reaction mixture is fed. in countercurrent into a widening tube, ethylene also being simultaneously introduced and i the mixture thus formed is separated in the low-pressure J t, ~separator.

Another procedure for high-pressure polymerisation of ethylene I disclosed in the DE-AS 21 31 '145 mixes the reaction mixture I coming from the reaction zone with the fresh ethylene feed and the recycled ethylene coming from the low-pressure separator, Both processes only lead to a slight reduction in the concentration of low-molecular admixtures in the polymer.

Additional apparatus is also required for their performance.

It is also known that unreacted gas originating from the lowpressure separator can be recycled to the high-pressure cycle with the aid of an injector (cf. DD-PS 202 882), However, this process does not reduce the concentration of the low-molecular components remaining in the product.

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With liquid products the low-molecular compounds can be separated by gas scrubbing with nitrogen or other inert gaseous substances.

However, additional apparatus is also required for this process and the separated components cannot be returned without cleaning.

Therefore, the task consisted in developing a process which avoids the disadvantages described above, guarantees nearly complete removal of the volatile components from the polymer melt and ensures that no or only small amounts of condensed components are contained in the cycle gas of the low-pressure stage.

The invention consists in a process for separating volatile components from polymer melts formed during the homopolymerisation of ethylene or the copolymerisation of ethylene with other copolymerisable compounds at elevated pressures and elevated temperatures. It is characterised in that ethylene is passed in countercurrent through the melt at a pressure of 5 to 70 MPa and a temperature in the melt of at least approx. 120°C.

Surprisingly, with the new process it is possible to nearly completely remove volatile components contained in the polymer melt formed during high-pressure polymerisation. Further cleaning, e.g. in the extruder or during storage of the product, is not necessary. The concentration of the low-molecular components in the recycled ethylene is reduced until condensation iiL a~ r 1 -6products do not occur during the compression of the ethylene and product losses are avoided.

The claimed procedure can be performed with different embodiments of the polymerisation and copolymerisation of ethylene using the high-pressure process, in both tubular reactors and in autoclaves. It can be used both with processes which work with solvents and with solvent-free processes and also with processes for the preparation of linear low-density polyethylenes LLDPE).

Volatile components contained in the polymer melt are understood to be low-molecular compounds which are contained in the polymerisation products, e.g. as comonomers, decomposition products of comonomers and oligomers. Thus, for example, in the preparation of ethylene and vinyl acetate copolymers, acetic acid formed by the decomposition of vinyl acetate is to be found in the products. Furthermore, volatile compounds include solvents which, for example, enter the reaction mixture with initiators and molecular mass regulators. Lubricants and other auxiliaries, which are required to operate the polymerisation plant, also occur in the polymer melt.

According to the claimed process, the polymer melt is treated at elevated pressure, preferably at 5 to 70 MPa and in particular at to 40 MPa. It is not necessary to provide a separate reactor for treatment. It can be successfully performed in the polymer separator of a high-pressure polymerisation plant. It is i -7expedient to remove most of the unreacted gas mixture by reducing the pressure to the above-mentioned range and then to bring the melt into contact with the ethylene. The high-pressure separator used in the polynierisation process is particularly suitable for this.

0 00 0o o o o 0 0 4 00 0 0 0 00 0 00 0 a e To ensure tha; the volatile components are separated either completely or at least nearly completely from the melt, it is necessary to maintain a temperature of at least 120°C in the melt. It has proved particularly valuable to treat the ethylene at temperatures above approx. 150 0 C. It has been shown that volatile substances are more completely separated, the higher the temperature of the melt is. The only upper limit for the treatment temperature is the temperature at which the melt from the polymerisation chamber enters the reactor where the volatile substances are separated. In order to guarantee as high a temperature as possible in the melt, it is expedient to pass heated ethylene in countercurrent to the melt.

It is essential to the claimed procedure that the ethylene and melt are flowing in opposing directions when they meet each other. This ensures extremely intimate mixture cf melt and olefin.

The effectiveness of the ethylene stream can be further increased by force diverting it in the reactor where the melt is treated.

In this manner fresh ethylene is passed several times through the (01 T -8precipitated product. The ethylene is diverted by means of suitable installations in the reactor, in the simplest case by plates which are located on opposite sides and in staggered formation.

The amount of ethylene required for treating the polymer melt depends on the temperature, the viscosity and the comonamer content of the melt. The required amount of ethylene increases with the viscosity and the comonomer content and decreases with increasing temperature. In general, 0.1 to 2 parts by weight of 4 t 4 ethylene are used per one part by weight of melt.

4 44 4 t Pure ethylene can be used to separate the volatile components from the melt. It is compresed in the low-pressure compressor and fed into the polymer melt in the high-pressure separator. The ethylene charged with the volatile components of the melt is passed through the high-pressure gas system and then after separation of the volatile components is fed via a partial stream to the inlet side of the high-pressure compressor and on into the polymerisation reactor.

A diagram of the claimed process is shown in Fig. 2. Fresh ethylene from a pipe 19 is compressed in u low-pressure compressor 18 to 10 to 35 MPa, heated in a heating device 22 to at least 120 0 C and passed through a line a11 into a high-pressure separator 13. There -t meets the polymer flowing in countercurrent. Charged with volatile, low-molecular components F -9of the polymer melt the ethylene enters a high-pressure gas recycling system. The low molecular components such as solvent, lubrication oil and oligomers are separated by means of a pressure-relieving device 23 and a line 24. The ethylene containing optionally comonomers, which can be replenished via a line 20, is fed to a high-pressure compressor 15 ;-nd then to a reactor 11. The polymer is separated at a pressure of 5 to 70 MPa/' in the high-pressure separator 13, after it has been cooled in a cooler 12, if necessary. The gaseous parts enter the high- S pressure gas recycling system 14 together with the fresh ethylene S passed in countercurrent. The polymer is pressure-relieved via a low-pressure separator 16 and a low-pressure cooler 17, liberated tif low-molecular compounds are compressed in the low-pressure compressor together with fresh ethylene from line 19.

t t Figure 3 shows a reactor 26 with installations 25 where the polymer can be treated with ethylene. The melt is passed through a line 27 and the ethylene through a line 28 into the reactor.

The melt leaves the reactor via a line 29 and the ethylene charged with volatile components from the melt via a line The new process is explained in the following examples.

Example 1 A reaction mixture obtained from the polymerisation of 720 parts A by weight of ethylene and 280 parts by weight of vinyl acetate A i O< and which has a percentage of 32.9 by weight of polymer is fed into a high-pressure separator without installations. The polymer is precipitated and can be drawn off via a bottom valve. In countercurrent to the polymer 250 parts of ethylene are introduced and withdrawn from the head together with lowmolecular components of the melt. The treated polymer still contains 0. 62 by weight of monomeric vinyl acetate, the vinyl acetate concentration of the low-pressure gas cycle is 12,5 by volume.

Example 2 (comparison): t i tI The same procedure is adopted as in example 1 but the ethylene is not fed into the high-pressure separator but is passed directly to the high-pressure compressor in the known manner.

The polymer contains 2% by weight of monomeric vinyl acetate, the r| vinyl acetate concentration in the low-pressure gas cycle is more than 40 by volume.

I. Example 3 A reaction mixture obtained from the polymerisation of 720 parts by weight of ethylene and 280 parts by weight of vinyl acetate and which has a percentage of 33 by weight of polymer is fed L into a high-pressure separator with installations according to fig. 3. The polymer is precipitated and can be drawn off via a r bottom valve.

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j -11- In countercurrent to the polymer 260 parts of ethylene are introduced and withdrawn from the head together with lowmolecular components of the melt. The treated polymer contains 0.33 by weight of monomeric vinyl acetate, the vinyl acetate concentration in the low-pressure gas cycle is 4.4 by volume.

Example 4 (comparison): The same procedure is adopted as in example 3 but the ethylene is not fed into the high-pressure separator but is passed directly to the high-pressure compressor in the known manner. The polymer contains 1.1% by weight of vinyl acetate, the vinyl acetate concentration in the low-pressure gas cycle is 33 by volume.

8444 The viscosity of the polymers prepared according to the examples 1 to 4 is the same and is 250 mPa x sec at 140 0

C,

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Claims (7)

1. A process for separating volatile components from polymer melts formed during the homopolymerisation of ethylene or the copolymerisation of ethylene with other copolymerisable compounds at elevated pressures and elevated temperatures, characterised in that ethylene is passed in countercurrent through the melt at a pressure of 5 to 70 MPa and a Os o0 temperature in the melt of at least approx. 120 0 C. G e t S

2. A process according to claim 1, characterised in that the melt B is treated with ethylene at a pressure of 10 to 40 MPa. c a

3. A process according to claim 1 or 2, characterised in that the Oi O melt is treated with ethylene in the polymer separator of a 1 high-pressure polymerisation plant.

4. A process according to one or more of the claims 1 to 3, characterised in that first of all the majority of the unreacted gas mixture is removed by reducinfc the pressure to to 70, preferably 10 to 40 MPa and then the melt is brought into contact with the ethylene.

A process according to one or more of the claims 1 to 4, characterised in that the melt is treated with ethylene at temperatures above approx. 150°C, qc; I Wr^' i ;I -13-

6. A process according to one or more of the claims I to characterised in that the ethylene stream used to treat the melt is diverted in the treatment reactor,

7. A process according to one or more of the claims 1 to 6, characterised in that 0.1I to 2 parts by weight of ethylene are used per part by weight of melt. DATED this 13th day of February 1990. HOECHST AKPIENGESELLSCHAFT WATERMARK PATENT TRADEMARK ATTORNEYS "TIE ATRIUM" 290 BURWOOD ROAD HAWrHORN. VIC. 3122.