GB2131441A - Ethylene-butene-1 copolymers - Google Patents

Abstract

A process for copolymerising ethylene and butene-1 by contacting under polymerisation conditions a mixture comprising (a) ethylene, (b) butene-1 and (c) hydrocarbon diluent, with (d) a coordination catalyst for copolymerising 1-olefins, wherein the butene-1 and the hydrocarbon diluent are at least partially provided by the C4 fraction of a cracked hydrocarbon which has been treated, optionally after removal of butadiene, to selectively polymerise and thereafter remove at least part of the isobutene content. n

Description

SPECIFICATION Polymerisation process The present invention relates to a process for the production of copolymers of ethylene and butene-1.

It is known that ethylene and butene-l can be copolymerised under relatively low temperature and low pressure conditions using a coordination catalyst to provide linear copolymers having a density generally in the range 0.900 to 0.970.

Ethylene and butene-1 having sufficiently high purity for use in such copolymerisation processes are manufactured on a commercial scale by cracking hydrocarbon materials e.g. naphtha and separating the ethylene and butene by conventional techniques. The separation of the ethylene from the cracked hydrocarbon is a relatively simple process. On the other hand, the butene-l is present in a mixed C4 fraction obtained from the cracking process, which fraction contains substantial quantities of other C4 hydrocarbons, for example, n-butane, butadiene, isobutene, butene-2 and traces of C3 and C5 hydrocarbons in addition to the butene-1. Isolation of commercially pure butene-1 from the mixed C4 fraction is a difficult and costly process compared with the isolation of polymerisation quality ethylene.

European patent application No. 31168 discloses a process for producing ethylene/butene-1 copolymer wherein instead of using pure butene-1 as comonomer, the butene-1 is present in the copolymerisation in a partially purified C4 cracked hydrocarbon fraction. The partially purified C4 fraction is obtained by reducing the butadiene content of a C4 cracked hydrocarbon fraction by a conventional butadiene extraction process and then finally reducing the butadiene content to not more than 200 ppm by a selective hydrogenation process. A partially purified C4 cracked hydrocarbon fraction produced in this manner generally contains at least 30 weight per cent of isobutene.

It is an object of the present invention to provide an improved process for preparing a copolymer of ethylene and butene-1 wherein the source of butene-1 comonomer is a C4 cracked hydrocarbon fraction.

The present invention provides a process for copolymerising ethylene and butene-1 by contacting under polymerisation conditions a mixture comprising (a) ethylene (b) butene-1 and (c) hydrocarbon diluent, with (d) a coordination catalyst for copolymerising 1 -olefins, wherein the butene-1 and the hydrocarbon diluent are at least partially provided by the C4 fraction of a cracked hydrocarbon which has been treated, optionally after removal of butadiene, to selectively polymerise and thereafter remove at least part of the isobutene content.

The ethylene employed in the copolymerisation process of the present invention is suitably any commercial grade of ethylene which is free from impurities which would deleteriously affect the coordination catalyst.

Oil refineries conventionally produce a C4 fraction of cracked hydrocarbon material comprising for example 10 weight % or more of butene-1 and 20 weight % or more of isobutene, the balance being a mixture of other C4 hydrocarbons, for example, isobutane, n-butane, butene-2, and butadiene together with traces of other C3, C4 and C5 hydrocarbons. Examples of these C4 fractions which can be employed in the process of the present invention are those manufactured by steam-cracking of naphtha or by catalytic cracking of a hydrocarbon fraction. In the process of the present invention it is preferred to remove butadiene from the C4 fraction before carrying out the treatment to selectively polymerise and thereafter remove the isobutene content.Preferably the butadiene is removed by processing the C4 fraction of a cracked hydrocarbon in a conventional butadiene extraction plant. Most preferably the butadiene content of the C4 fraction is reduced to not more than 0.5 weight % butadiene based on total C4 fraction.

Prior to the treatment to remove at least part of the isobutene content, the C4 fraction of cracked hydrocarbon employed in the present invention preferably comprises at least 2 weight %, most preferably at least 5 weight % of butene-1, based on the total weight of the C4 fraction.

The selective polymerisation of the isobutene in the process of the present invention is suitably carried out by contacting the C4 fraction of cracked hydrocarbon hereinafter referred to as the "C4 fraction" with a Lewis acid catalyst. Polymerisation of isobutene to produce polyisobutene (which is itself a useful product) using Lewis acid catalysts is well known in the art. In the process of the present invention it is preferred to carry out the selective polymerisation by contacting the C4 fraction with a Lewis acid catalyst at a temperature in the range -100 to + 1 000C. The polymerisation time may range from a few minutes to several hours.Examples of Lewis acid catalysts that can be used in the selective polymerisation are aluminium chloride, ethyl aluminium dichloride, boron trifluoride, boron trichloride and the Lewis acid sites on the surfaces of inorganic oxides such as silicas, aluminas or zeolites. Aluminium chloride is preferred.

If desired, the catalyst may be supported on inert support material, for example, graphite. GB A-2001 662 discloses a process for polymerising a monomer feed to form polyisobutene, which process comprises the steps of passing the feed through a fixed bed of catalyst material which is maintained at its activation temperature, the catalyst material comprising a Lewis acid chloride catalyst, for example, aluminium chloride, intercalated with graphite. Example 1 of GB-A-2001 662 illustrates preparation of polyisobutene using a feed consisting of isobutene, n-butane, butene-1, isobutene and butene-2 and an aluminium chloride/graphite catalyst. It is stated that only the isobutene is substantially consumed during the polymerisation process.This means that the unreacted hydrocarbon feed will be effectively enriched in butene-1 by reduction of the isobutene content thereof.

Such a process is typical of processes for reducing the isobutene content of the C4 fraction which can be applied in the process of the present invention.

For further details of polymerisation conditions, catalysts and apparatus suitably employed for carrying out the selective polymerisation, reference may be made to GB-A-8861 34, GB-A- 933340, GB-A-2001 662, US-A-2 130507, US-A-2957930 and US-A-3 119884.

In the selective polymerisation carried out in the present invention, preferably not more than 20 weight %, most preferably not more than 5 weight % of the butene-1 (based on the total butene-1 in the C4 fraction) is polymerised.

After at least part of the isobutene, preferably at least 50 weight %, most preferably at least 80 weight % of the isobutene (based on total isobutene in the C4 fraction) has been selectively polymerised in the process of the present invention, it is thereafter removed (as polyisobutene) from the so "treated" C4 fraction. The removal of the polyisobutene can be accomplished using, for example, conventional separation techniques such as flashing off or distillation techniques to collect the treated C4 fraction having reduced isobutene content. Provided this treated C4 fraction is free from impurities which would deleteriously affect the copolymerisation of the present invention, it may be used directly in the copolymerisation.However, if the treated C4 fraction contains deleterious impurities, for example water, then purification to remove the impurity may be carried out by conventional means. If the treated C4 fraction contains more than 0.5 weight % of butadiene then the butadiene content is preferably reduced, for example, by a conventional butadiene extraction process leading to recovery of the butadiene, or by a selective hydrogenation process (for example as disclosed in EP-A-3 11 68 leading to conversion of the butadiene to monounsaturated or saturated products.

In the process of the present invention it is preferred to reduce the level of butadiene in the C4 fraction to not more than 0.5 weight % prior to carrying out the selective polymerisation and removal of the isobutene. However, it is found that the selective polymerisation in general tends to reduce the concentration of any butadiene present in the C4 fraction. This is believed to result from partial polymerisation of the butadiene.

The treated C4 fraction employed in the copolymerisation process of the present invention preferably comprises at least 10 weight %, most preferably at least 1 5 weight % of butene-1. The quantity of butadiene in the treated C4 fraction is preferably not more than 0.2 weight %, most preferably not more than 0.1 weight %. The quantity of isobutene in the treated C4 fraction is preferably not more than 20 weight %, most preferably not more than 8 weight %. The weight percentages of each component of the treated C4 fraction are based on the total weight thereof.

The mole ratio of ethylene to butene-1 employed in the copolymerisation of the present invention is suitably in the range 100:1 to 1:10, preferably in the range 5:1 to 1:2.

If desired, additional comonomers selected from propylene and C5 to C20 1 -olefins may be employed in the copolymerisation process of the present invention. Examples of additional comonomers are propylene, hexene-1, 4-methylpentene-1 and octene-1 . The quantity of additional comonomer employed is suitably in the range 0 to 20 moles, preferably 0 to 5 moles per mole of ethylene.

The ethylene/butene-1 copolymerisation conditions employed in the process of the present invention can be such that copolymerisation occurs in the gas-phase or in the liquid phase under batch or continuous conditions. Illustrative of such copolymerisation conditions are gas-fluidised bed copolymerisation, stirred gas-phase copolymerisation, solution copolymerisation and slurry copolymerisation. Slurry copolymerisation and solution copolymerisation are preferred.

In a preferred embodiment of the present invention the copolymerisation is carried out by continuously feeding the monomeric ethylene and the treated C4 fraction (which provides the butene-1 and the hydrocarbon diluent) and, if necessary, additional hydrocarbon diluent to a polymerisation reactor and contacting the mixture therein with the coordination catalyst under conditions of temperature and pressure such that copolymerisation occurs in the liquid hydrocarbon mixture and the copolymer forms as particles suspended therein. This type of copolymerisation process is an example of slurry copolymerisation and is frequently referred to in the art as "particle form (co)polymersation". It is preferred to conduct the particle form copolymerisation in a "loop reactor" wherein the polymerisation mixture is circulated continuously around a pipe loop to which is fed the monomeric ethylene, the C4 hydrocarbon feedstock, the catalyst and additional hydrocarbon diluent, if necessary and from which the particulate copolymer is removed, for example by means of a settling leg. Loop reactors of this type are described for example in GB-A-886784 and in GB-A-8991 56.

The treated C4 fraction employed in the copolymerisation of the present invention contains, in addition to the butene-1, C4 hydrocarbons which act as a diluent for the monomeric materials.

However, in some copolymerisation processes operated in accordance with the present invention it may be desirable to employ additional diluent in the ethylene/butene-1 copolymerisation, for example, in the start-up of continuous copolymerisation processes. Examples of suitable additional diluents for this purpose are liquid or gaseous saturated hydrocarbons, for example, methane, ethane, profane butane, isobutane, hexane and cyclohexane, or inert gases, for example nitrogen and argon. Isobutane is preferred.

The concentrations of the monomers in the copolymerisation process of the present invention may be adjusted to the desired values, for example, by adjusting the quantity (or rate of addition) of the monomer or monomer-containing material fed to the copolymerisation, by adjusting the quantity of additional diluent (if any) or by adjusting the pressure. For example, when the copolymerisation of the ethylene and butene-1 is conducted under continuous copolymerisation conditions, the ethylene, the treated C4 fraction and additional diluent (if any) may for example be fed to a reactor containing the coordination catalyst and the temperature and pressure adjusted to give the desired rate of polymerisation.During the course of the copolymerisation, the ethylene and butene copolymerise to leave tha said C4 fraction (and any additional diluent) depleted in ethylene and butene-1 . The desired concentration of ethylene and butene-1 may be maintained, for example, by feeding fresh ethylene and treated C4 fraction to the reactor and removing product and diluent continuously or intermittently from the reactor.

In the copolymerisation process of the present invention, the use of the treated C4 fraction having reduced isobutene content (and hence enriched butene-1 content) means that more efficient use is made of the butene-1 content than if the isobutene content had not been reduced. Thus, to maintain a particular concentration of butene-1 in the copolymerisation, it is necessary to feed a lesser quantity of the treated C4 fraction than would be the case if the isobutene content had not been reduced. When this type of process is operated under continuous copolymerisation conditions it is found necessary to continuously or intermittently remove from the reactor a quantity of the monomer/hydrcarbon diluent mixture to prevent the build up of non-monomeric hydrocarbons to an unacceptable level.Such monomer/hydrocarbon diluent mixture may, for example, be subjected to separation processes to isolate monomer and diluent or may be burnt as fuel gas. However, in the process of the present invention the rate of removal of the monomer/hydrocarbon diluent from the reactor can be reduced in comparison with processes employing as the source of butene-1 a similar C4 hydrocarbon stream from which no isobutene has been removed.

When the copolymerisation process of the present invention is operated under slurry process conditions, it is found that the treated C4 fraction acts as a good "non-solvent" for the produced copolymer.

The coordination catalyst (d) employed in the copolymerisation process of the present invention can be, for example, a Ziegler catalyst in supported or unsupported state, a Phillips catalyst or a Standard Oil catalyst. Such catalysts have been extensively described in the literature. Ziegler catalysts, particularly supported Ziegler catalysts are preferred in the process of the present invention. In the case of supported Ziegler catalysts, the support material is preferably a particulate magnesium-containing compound, for example magnesium oxide, magnesium hydroxide, magnesium hydroxychloride, magnesium chloride or a mixture of two or more of these.Also preferred as support material is the product of treating a particulate hydroxyl groups-containing refractory oxide, for example, silica, alumina or magnesia, with an organometallic compound, for example, triethyl aluminium, an alkyl magnesium halide or a dialkyl magnesium compound. The transition metal compound in the supported Ziegler catalyst is suitably a metal compound of Groups IVA, VA and VIA of the Periodic Table.

Titanium and/or vanadium compounds are preferred. The supported Ziegler catalyst is activated using a conventional metal alkyl activator, for example triethyl aluminium, tri-n-butyl aluminium or diethyl aluminium chloride. For further details of supported Ziegler catalysts and their preparation reference may be made, for example to GB-A-1 024336, GB-A-1 269068, GB-A-1 257040, GB-A- 1287396, GB-A-1264416,GB-A-1351488, GB-A-121 1287,GB-A-I 189038, GB-A- 1286867, GB-A-1 553673 and EP-A-22658.

A particularly preferred coordination catalyst for use in the copolymerisation process of the present invention comprises the catalyst disclosed in our GB-A-1 553673. This catalyst is prepared by activating with a conventional organometallic activator a Ziegler catalyst component prepared by reacting together under an hydros conditions a halogen-containing transition metal compound other than a fluorine-containing compound and an aliphatic alcohol and simultaneously or subsequently impregnating an anhydrous hydroxyl groups-containing support material comprising magnesium oxide having a hydroxyl content less than 0.2 OH groups per magnesium atom with the reaction mixture to produce the solid catalyst component. The transition metal compound is preferably a titanium compound, for example a titanium compound having the general formula Ti(OR)nC14~n wherein O < n < 4 (that is, n is zero or an integer or a fraction less than 4) and R is an alkyl group containing 1-6 carbon atoms. For further details regarding the preparation of this prefered catalyst reference may be made to our GB-A-1 553673.

The ethylene/butene-1 copolymers prepared by the process of the present invention can be isolated from the copolymerisation by the techniques well known in the art.

The present invention is illustrated by the following Examples.

Example Treatment of the C4 fraction A C4 fraction of a cracked hydrocarbon from which butadiene had been substantially removed in a butadiene extraction plant and comprising about 40 weight % isobutene and 25 weight % butene-1, the balance being mainly other C4 hydrocarbons such as butene-2, n-butane and isobutane, was treated in accordance with the process of the present invention by feeding the fraction to a polyisobutene plant wherein the isobutene is selectively polymerised and thereafter removed from the fraction. The selective polymerisation process was carried out under continuous conditions at a temperature of 250C and a pressure of 2.5 bars. The catalyst was aluminium chloride/hydrogen chloride.The produced reaction mixture was distilled in a tower at about 3 bars pressure and about 1 000C. The unreacted C4 fraction was taken off at the head of the tower and collected. It had the analysis shown in Table 1 ("treated C4 fraction").

Preparation of the coordination catalyst used in the copolymerisation of ethylene/butene-1 2.24 kg (37.3 moles) isopropanol were mixed with 5 litres of dry cyclohexane in a dry, nitrogen purged, 20 litre round bottomed flask equipped with stirrer, heating mantle and reflux condenser. 3.35 kg (17.7 moles) TiCI4 were added, with stirring, over a period of approximately 2 hours. After addition of the TiCI4 was complete, the mixture was heated under reflux, with stirring, for 3 hours. The mixture was allowed to cool and a slurry of 500 g (12.4 moles) magnesia (Maglite K, Merck and Co.) in dry cyclohexane was added. The resulting mixture was heated under reflux, with stirring, for 2 hours, allowed to cool and then filtered and washed with fresh, dry cyclohexane.The catalyst was handled as a slurry and stored under N2. The dry catalyst powder contained 5.6 wt % Tri, 1 9.1 wt % Mg and 36.4 wt%CI.

Copolymerisation of ethylene/butene-1 Continuous "particle form" copolymerisation was carried out in a 90 litre pipe loop reactor at a total pressure of 40.8 bar and a reactor temperature of 850C. At the start of the copolymerisation, the butene-1 content of the recirculating diluent was brought to approximately 5 vol. % by mixing the treated C4 fraction (see Table 1 for composition) and isobutane (approximate volume ratio 1:7).

Ethylene was fed to the reactor at a rate of 8.0 kg/h, so as to maintain a standing concentration of ca.

10.0 vol % in the reactor offgas. The Ziegler (coordination) catalyst was reduced with 0.85 g triethyl aluminium per g catalyst and fed to the reactor as a settled mud in aliquots. Triethyl aluminium cocatalyst was fed continuously as a separate co-catalyst stream. Catalyst and cocatalyst were fed at such a rate (see Table 3) as to maintain a strong but controlled reaction. Hydrogen was fed to the reactor at a rate sufficient to control the polymer melt index around 1.0. The treated C4 fraction was passed through a column of 4A molecular sieves to remove traces of water and was fed continuously to the reactor. Polymer density could be controlled by adjusting the rate of addition of the treated C4 fraction. Polymer was removed at regular intervals from the reactor; a polymer production rate of approximately 6.7 kg/h was maintained.

Two copolymers, designated R1003 and R1004 were produced (see Tables 2 and 3). These had melt index, density and molecular weight distribution (as measured by melt index ratio and Kd) typical of commercial LLDPE film grade polymers. The polymer powders were very coarse and contained no fine particles less than 106 ym in size; such powders could safely be handled in conventional leanphase air pneumatic conveying systems without the need for inert blanketing. The powders were pelletised and the resulting pellet products were shown to have good properties as LLDPE-type film grade.

Table 1

Concentration (vol %) Hydrocarbon C4 fraction Treated C4 component before treatment* fraction isobutane 6.8 4.35 b-butane 18.4 30.87 butene-1 24.5 38.04 isobutene 39.0 1.31 trans-butene-2 9.4 15.53 cis-butene-2 1.8 9.85 n-pentane 0.11 0.02 1,3-butadiene 0.03 0.03 *This represents the analysis of a typical C4 fraction of a cracked hydrocarbon suitable for use in the present invention but was not the actual hydrocarbon employed. Table 2

Offgas analysus (vol. %) molar butene/ Catalyst Catalyst Alkyl Reactor ethylene productivity feed rate feed rate Polymer temp. ( C) Ethylene Hydrogen Butene-1* ration (kg/kg) (g/h) (g.h) R1003 85.0 9.6 0.20 10.3 1.07 8700 1.3 0.8 R1004 85.0 10.9 0.17 10.2 0.94 12600 1.4 0.8 *Treated C4 fraction feed rate required in both cases was 6.65 litres/h.

Table 3

M'21.6 Density M'2.16 M'21.6 (kg/m ) Kd Median particle Fines < 500 m Fines < 106 m Bulk density Polymer (1) (2) M'2.16 (4) (3) size ( m) (wt. %) (wt. %) (kg/m ) R1003 1.4 30.5 22 919 1.5 1400 4 0 325 R1004 0.99 19.5 20 922 1.1 1560 5 0 319 Notes: 1) Determined according BS2783, 105 C.

2) Determined by a method similar to that described in ASTM D1238, procedure A, Condition F.

3) Determined by a method similar to that given in Sabia, R., J. Appl. Poly. Sci. 1963, 7,347.

4) Determined by a method similar to that given in BS 2782, Part 5, 1965, Method 509B.

Claims (12)

Claims

1. A process for copolymerising ethylene and butene-1 by contacting under polymerisation conditions a mixture comprising (a) ethylene, (b) butene-1 and (c) hydrocarbon diluent, with (d) a coordination catalyst for copolymerising 1 -olefins, wherein the butene-1 and the hydrocarbon diluent are at least partially provided by the C4 fraction of a cracked hydrocarbon which has been treated, optionally after removal of butadiene, to selectively polymerise and thereafter remove at least part of the isobutene content.

2. A process as claimed in claim 1 wherein the butadiene content of the C4 fraction is reduced to not more than 0.5 weight % prior to the selective polymerisation.

3. A process as claimed in claim 1 or 2 wherein the selective polymerisation of the isobutene is carried out by contacting the C4 fraction with a Lewis acid catalyst at a temperature in the range -100 to +1000C.

4. A process as claimed in claim 3 wherein the Lewis acid catalyst is aluminium chloride, ethyl aluminium dichloride, boron trifluoride or boron trichloride.

5. A process as claimed in any one of the preceding claims wherein after the selective polymerisation of the isobutene, the C4 fraction is flashed off or distilled to collect the C4 fraction having reduced isobutene content.

6. A process as claimed in any one of the preceding claims wherein the mole ratio of ethylene to butene-1 employed in the copolymerisation is in the range 100:1 to 1:10.

7. A process as claimed in any one of the preceding claims wherein in the copolymerisation up to 20 moles per mole of ethylene of one or more additional comonomers selected from propylene and C5 to C20 1-olefins are copolymerised with the ethylene and butene-1.

8. A process as claimed in any one of the preceding claims wherein the copolymerisation is conducted under slurry conditions or solution conditions.

9. A process as claimed in any one of the preceding claims wherein the copolymerisation is carried out under continuous polymerisation conditions and the rate of addition of the treated C4 fraction and the rate of removal of monomer/diluent mixture are controlled to give the desired concentration of butene-1 in the polymerisation reactor.

10. A process as claimed in any one of the preceding claims wherein the coordination catalyst is a Ziegler catalyst supported on a magnesium-containing compound.

11. A process substantially as hereinbefore described in the Examples.

12. Copolymers prepared by the process of any one of the preceding claims.