Background

Catalytic polymerization of olefins first implies obtaining olefin stream of sufficient purity to avoid rapid deactivation of polymerization catalyst. This is particularly important when using contemporary high performance metallocene polymerization catalysts, as well as other catalytic systems.

Olefin contaminants which may affect polymerization reactions and catalysts may be, without limitation, (a) alkynes such as acetylene or propyne, (b) conjugated diolefins such as butadiene or isoprene, (c) sulfur containing gaseous species such as hydrogen sulfide or mercaptans such as methyl or ethyl mercaptan, (d) amines such as ammonia, triethylamine, ethanolamine or N,N-dimethylethanolamine, (e) cyanides such as hydrogen cyanide or acetonitrile, (f) ketones such as acetone or methyl-ethyl-ketone, (g) arsine, (h) phosphine, (i) oxygen, (j) water, (k) carbon dioxide, (I) carbon monoxide, (m) carbon oxisulfide, (n) carbon disulfide, (o) mercury, (p) lead or other heavy metals, (q) alkaline metal cations such as lithium, sodium or potassium, (r) alkaline earth metal cations such as beryllium, magnesium or calcium, (s) halogens such as fluorine, chlorine, bromine or iodine and their anions, (t) alcohols such as methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol or ethanolamine, (u) aldehydes such as formaldehyde or acetaldehyde, (v) inert gases such as nitrogen or argon, and (w) molecules having conjugated pi bonds between an alkene moiety and another pi orbital bearing chemical moiety, such as carbonyl, carboxyl, cyano, amido as present in e.g., respectively acrolein, acylic acid, acrylonitrile and acrylamide, (x) silicium derivatives chosen among SiH(n) ₃ , wherein each n is independently selected among H, Cl, C1-C8 carboxyl, C1-C8 alkoxy and C1-C8 alkyl. "Cl" stands for methyl chain, "C8" stands for octyl chain.

Some contaminants have few or no impact on polymerization catalyst lifespan, such as inert gases. On the contrary, gas stream contamination with acetylene or carbon monoxide may result in severe polymerization catalyst deactivation, thus increasing catalyst spending to achieve constant polymer production rate.

In this respect, alkynes such as acetylene or propyne are usually removed using a selective hydrogenation catalyst aiming to convert alkynes into alkenes within an alkynes removal section. Alkynes removal is typically achieved using a palladium over alumina catalyst, with the help of hydrogen.

Prior to catalytic polymerization, the gas stream which leaves the palladium catalyzed alkynes removal section flows through a further purification section comprising an adsorbent for at least water removal. Some other contaminants such as carbon dioxide may also be removed at the same time, to a certain extent.

US2837587 describes a method of purification of an olefin gas stream prior to polymerization wherein the olefin gas stream passes through a first bed of reduced copper oxide catalyst to convert alkynes into alkanes then through a second bed of hopcalite to convert carbon monoxide into carbon dioxide (column 3, lines 39-43). The document does not address or mention the question of first bed deactivation. In addition, the alkynes are not converted into alkenes in the first copper containing bed, contrarily to silver modified palladium based catalysts which are potent selective hydrogenating materials. Although the described method of purification shows effectiveness, one cannot ascertain it is acceptable for reaching the high purity levels required by modern polymerization catalysts and processes, since parts per billion traces are usual targets in this industry.

US5583274 relates to alkali fluoride containing palladium catalysts, which show resistance to deactivation caused by hydrogen sulfide (H ₂ S). The operating temperature of the catalysts according to the invention (catalysts C and E) is from 144 to 447°F (62 to 230°C) (see, Tables I and III) and claimed from 40 to 200°C (claim 18). Column 5 lines 3-15 shows H ₂ S is less retained by the alkali fluoride containing palladium catalysts than prior art catalysts.

US6987152 discloses a method for olefin polymerization wherein an olefin stream is passed through a succession of packed beds to remove a succession of impurities among which acetylene, sulfur compounds, carbon monoxide, carbon dioxide, methanol, water and oxygen. All these beds are operated at a temperature of from about 0°C to about 90°C (column 1, lines 43-44; column 4, lines 5-14, column 10, lines 11-21). All the examples use a deoxygenation material, either UT-2000, which is obtainable from Univation Technologies, LLC, or T-4427B, which is commercially available from Siid-Chemie.

Albeit selective hydrogenation catalyst activity is generally guaranteed by manufacturer for an average time on stream of 5 years, fast deactivation may occur, resulting in bad olefin gas purification and therefore lowered polymer yield.

There is thus a need to limit or avoid fast deactivation of palladium containing hydrogenation catalysts.

Description

In this respect, and according to a first aspect of the invention, which is related to a process, it has been observed that lowering alkynes and impurities contained within an olefin gas stream, wherein said impurities comprise at least two of water, hydrogen sulfide, carbon dioxide, carbon oxysulfide and oxygen, comprising submitting the olefin gas stream to:

(a) hydrogenation with a hydrogenation catalyst in the presence of hydrogen to convert alkynes into alkenes, said catalyst comprising palladium on an inorganic support,

(b) reduction of the amount of impurities with an optionally reusable inorganic adsorbent, wherein reduction step (b) is performed before hydrogenating step (a),

was efficient and sufficient to preserve the activity of the hydrogenation catalyst for a reasonable period of time and resulted in a purified olefin stream which could be directly used within a polymerization process. It was observed that proceeding this way rendered the use of any additional adsorbent or catalyst bed useless and allowed increased time on stream for at least the hydrogenation section that contains the palladium catalyst. Hydrogenation in the presence of hydrogen of step (a) is performed using conditions sufficient to lower the amount of alkynes while avoiding excessive hydrogenation of the resulting alkenes into alkanes. In this respect, gas stream velocity and residence time within a catalytic hydrogenation section, temperature, total pressure and hydrogen partial pressure are adapted by the skilled artisan according to routine practice.

A reasonable period of time according to the invention means a 3 month to 5 year time on stream without substantial loss of catalytic polymerization productivity that would be attributable to the quality of the olefin stream to be polymerized.

A substantial loss of catalytic polymerization activity occurs when catalytic polymerization productivity drops by at least lwt% (wt%: weight percent), when compared to equivalent final polymer quality batch obtained when using fresh hydrogenation catalyst.

Typical hydrogenation of alkynes into alkenes (i.e. partial hydrogenation) may be conducted at temperature from 250 to 600°K, preferably from 280 to 350°K, under pressure from 0.1 to lOMPa, with GHSV (GHSV = Gas Hourly Space Velocity) from 0.1 to 30 000 h ¹ , and wherein hydrogen content is from 0.01 to lOOOppm by weight.

For example, for step (a), preferred hydrogen feed may range from 3 to 5 g of hydrogen per ton of ethylene when the content in acetylene is ca. lppm by weight. Acetylene content in the stream at the outlet is then below 0.02 ppm by weight. Added hydrogen weight amount has to be lowered accordingly when the stream is mainly comprised of propylene instead of ethylene, since propylene has a higher molecular weight. Preferred operating temperature for hydrogenation of acetylene ranges from 290 to 310°K, operating pressure ranges from 4.5 to 6.5MPa and GHSV ranges from 50 to 200h ^_1 . Palladium content in the alumina catalyst ranges from 0.01 to 0.5wt%.

For example, for step (b), preferred temperature ranges from 270 to 300°K, GHSV and pressure ranges are the same as for step (a) (minor pressure drop not taken into account). Main contaminant apart from acetylene within the feed stream is hydrogen sulfide with measured concentration from 10 to 500ppb by weight, with mean concentration of 20ppb by weight. Concentration in hydrogen sulfide after step (b) is below 50ppb. ppb = part per billion.

Preferably, the amount of hydrogen is adapted to the amount of alkynes to be hydrogenated into the corresponding alkenes, and generally ranges from 1 to 500ppm by weight, preferably from 5 to lOOppm by weight. The amount of alkynes to be hydrogenated is ideally as low as possible and comprised between 0.020ppm to lOppm by weight, however it is more likely to be comprised between 0. lOOppm and 20ppm, and generally around lppm by weight. The gas flow after hydrogenation necessarily comprises less alkynes than at the origin. The final alkynes content is always less than lppm by weight, preferably less than 0.020ppm by weight, more preferably below the detection limit, depending upon the measurement method.

Preferred alkyne is acetylene since it is the most abundant alkyne that is found in ethylene streams. Preferred olefin is ethylene. Reduction of the amount of impurities is conducted at temperature from 250 to 450°K, under pressure from 0.1 to lOMPa, and with GHSV from 0.1 to 30 000 h ¹ , and wherein the total amount of other impurities ranges from 0.01 to 10 000 ppm by weight. Impurities the amount of which is reduced are the olefin contaminants listed under item (a) to (x), in the introductory part of the instant patent application.

Step (a) is performed using a palladium over alumina catalyst, optionally with added silver. Suitable palladium concentration within the alumina catalyst ranges from 0.01 to 0.5wt%, wherein silver, when present, is added in an amount sufficient to improve the selectivity of hydrogenation reaction so that produced alkenes are not over hydrogenated into alkanes. Catalyst preparation tuning is performed by the commercial catalyst manufacturers. Preferably, palladium content ranges from 0.02 to 0.3wt%.

Step (b) is preferably performed using an optionally reusable inorganic adsorbent having a mean pore size below 50nm.

Preferably, the inorganic adsorbent, which is a porous material, has a mean pore size below 2 nm and is selected among natural or synthetic zeolites, clay, molecular sieve 3A, 4A, 5A, 10X or 13X, activated alumina (such as Selexsorb ^® and Selexsorb ^® COS), and porous glass.

More preferred inorganic adsorbent is molecular sieve, especially 13X or activated alumina, especially Selexsorb ^® COS, or their combination.

Inorganic adsorbent is best used at low temperature for improved adsorption. If temperature is down, adsorption capacity is improved. Ambient temperature is preferred. When possible, temperature below 295°K is more preferred.

Preferred impurities to be removed comprise H ₂ 0, C0 ₂ , COS, H ₂ S, mercaptans such as methyl or ethyl mercaptan, dimethylsulfide and dimethyldisulfide. Final overall sulfur species concentration after steps (a) and (b) is preferably below O.lppm by weight, more preferably below 0.02ppm by weight.

The inorganic adsorbent is advantageously reusable and regenerated using a stream of inert gas at a temperature and for duration sufficient to remove the adsorbed species.

When applicable, the inorganic adsorbent is regenerated using a stream of nitrogen, hydrogen, water steam, methane, ethane, propane or argon and their mixtures at from 350°K to 800°K under pressure from 0.1 to lOMPa, with GHSV from 0.1 to 50 000 h ¹ during from 0.1 minute to 100 hours, optionally in the presence of oxygen.

The olefin gas stream according to the first aspect of the invention contains more than 80wt% of ethylene or propylene, from 0.1 to 5wt% of hydrogen, and impurities, among which

(a) from 0.001 to 10wt% of one of methane, ethane, propane or any combination thereof, and

(b) from 1 ppb to 500 ppm H ₂ S, from 1 ppb to 100 ppm C ₂ H ₂ and/or propyne, from 0 to 2000 ppm C0 ₂ , from 0 to 500 ppm CO, from 0 to 5000 ppm of water, from 0 to 10 ppm each of one or more species chosen among AsH ₃ , PH ₃ , BH ₃ , Si H ₄ , from 0 to 100 ppm CS ₂ , from 0 to 100 ppm COS, from 0 to 50 ppm of ammonia or organic amine, from 0 to 1 ppm each of one or more metal or respective cationic species chosen among Hg, Pb, Sn, Cd, Li, Be, Na, Mg, K, Ca, from 0 to 10 ppm of HCI or Cl ₂ , and from 0 to lOOppm 0 ₂ .

The palladium over alumina catalyst is advantageously regenerated prior to or after use, using an oxygen containing gas stream at from 620 to 770°K, preferably at from 650 to 750°K, more preferably at from 670 to 730°K.

According to a second aspect, the invention is about the use of a method according to its first aspect for the preparation of a polyolefin.

According to a third aspect, the invention concerns a method for reducing the amount of polymerization catalyst present within a polyolefin comprising

(a) withdrawing poisons of palladium selective hydrogenation catalyst within an alkyne containing olefin stream to obtain a poison depleted olefin stream by contacting said alkyne containing olefin stream with an inorganic adsorbent,

(b) reacting the poison depleted olefin stream of step (a) over said palladium selective hydrogenation catalyst in the presence of hydrogen to obtain an alkyne depleted olefin stream,

(c) polymerizing the alkyne depleted olefin stream of step (b) within a polymerization reactor using a polymerization catalyst to obtain a polyolefin with reduced content in polymerization catalyst.

A suitable inorganic adsorbent according to the third aspect of the invention is an optionally reusable inorganic adsorbent having a mean pore size below 50nm.

Preferably, the inorganic adsorbent, which is a porous material, has a mean pore size below 2 nm and is selected among natural or synthetic zeolites, clay, molecular sieve 3A, 4A, 5A, 10X or 13X, activated alumina (such as Selexsorb ^® and Selexsorb ^® COS), and porous glass.

More preferred inorganic adsorbent is molecular sieve, especially 13X or activated alumina, especially Selexsorb ^® COS, or their combination.

Inorganic adsorbent is best used at low temperature for improved adsorption. If temperature is down, adsorption capacity is improved. Ambient temperature is preferred. When possible, temperature below 295°K is more preferred.

Preferred impurities to be removed comprise H ₂ 0, C0 ₂ , COS, H ₂ S, mercaptans such as methyl or ethyl mercaptan, dimethylsulfide and dimethyldisulfide. Final overall sulfur species concentration after steps (a) and (b) is preferably below O.lppm by weight, more preferably below 0.02ppm by weight.

The inorganic adsorbent is advantageously reusable and regenerated using a stream of inert gas at a temperature and for duration sufficient to remove the adsorbed species. When applicable, the inorganic adsorbent is regenerated using a stream of nitrogen, hydrogen, water steam, methane, ethane, propane or argon and their mixtures at from 350°K to 800°K under pressure from 0.1 to lOMPa, with GHSV from 0.1 to 50 000 h ¹ during from 0.1 minute to 100 hours, optionally in the presence of oxygen.

Catalyst productivity according to the invention when polymerizing ethylene, depending on used catalytic system and process type:

gPE/gcata: grams of produced polyethylene per g of catalyst

(ppm): part per million. Corresponds to the catalyst concentration within the final polymer product.

Average productivity: sum of all the productivities for all the experiments divided by the number of experiments.

Minimum: lowest observed productivity, according to the invention.

Maximum: highest observed productivity, according to the invention.

Commercial polymerization catalysts are used as catalytic system. Polymerization processes used herein correspond to industrial processes.

The polyolefin obtained by the method according to the third aspect of the invention has a final concentration in polymerization catalyst comprised (i) between 172 and 312 ppm by weight when the catalytic system contains chromium as active polymerization metal within a slurry process (productivity rate between 3200 and 5800g of polymer/g of catalyst) , (ii) between 66 and 476 ppm by weight when the catalytic system contains chromium as active polymerization metal within a gas phase process (productivity rate between 2100 and 15000g of polymer/g of catalyst), (iii) between 16 and 24 ppm by weight when the catalytic system contains Ziegler-Natta catalyst within a slurry phase process (productivity rate between 30000 and 60000g of polymer/g of catalyst), and (iv) between 57 and 153 ppm by weight when the catalytic system contains metallocene catalyst within a gas phase process (productivity rate between 6500 and 17500g of polymer/g of catalyst). Preferred polyolefin is polyethylene or polypropylene. More preferred polyolefin is polyethylene. According to a fourth aspect, the invention is about a polyethylene obtained by catalytic polymerization of ethylene monomer and optionally of 0-5wt% of a co-monomer chosen among propylene, 1-butene, 1- pentene, 1-hexene, 1-heptene and 1-octene, having a final concentration in polymerization catalyst comprised (i) between 172 and 312 ppm by weight when the catalytic system contains chromium as active polymerization metal within a slurry process (productivity rate between 3200 and 5800g of polymer/g of catalyst) , (ii) between 66 and 476 ppm by weight when the catalytic system contains chromium as active polymerization metal within a gas phase process (productivity rate between 2100 and 15000g of polymer/g of catalyst), (iii) between 16 and 24 ppm by weight when the catalytic system contains Ziegler- Natta catalyst within a slurry phase process (productivity rate between 30000 and 60000g of polymer/g of catalyst), and (iv) between 57 and 153 ppm by weight when the catalytic system contains metallocene catalyst within a gas phase process (productivity rate between 6500 and 17500g of polymer/g of catalyst). The preferred polyolefin is polyethylene or polypropylene, more preferably polyethylene.

Unless otherwise specified in the instant description and associated present and future claims, any reference to ppm (part per million) or ppb (part per billion) is always to be understood as, respectively, weight ppm or weight ppb.