Polymerisation process

[0001] The invention relates to a method of treating a reactive fluid stream, and more particularly to a method of treating a gas stream and / or liquid containing active polymer removed from the polymerization reactor to remove reagents and impurities therefrom. The options available for processing reactive fluid streams may be limited by the potential due to impurities, blockages etc. caused by solids polymerized particles or carried by the fluid stream. The invention aims to provide an improved method for treating diluent streams in the production of polyolefins. Such methods are useful when recirculation or delivery to the reactor of a liquid stream with a lower level of reagent and / or impurities than this level is required at the time of removing this stream from the same or another reactor.

[0002] An example is multi-modal polymerization reactions, where polymers are generally produced in reactors connected in series, the reaction conditions being different in each reactor. To obtain maximum control over the properties of the final product, it is preferable to have complete and independent control over the molecular weight and density of the polymer produced in each reactor; molecular weight is generally controlled using hydrogen. Accordingly, it is usually necessary to remove hydrogen from the product stream of the first reactor operating at a higher hydrogen concentration than the second reactor connected in series with the first. The polymer from a reactor previously positioned relative to the flow direction to be delivered to a reactor located further downstream of the flow direction, connected in series is generally removed with diluents (gaseous and / or liquid), catalysts and reagents such as monomer (s), comonomer (y) molecular weight controlling agents, e.g. hydrogen and cocatalysts. Various technical solutions are known to remove these undesirable diluents and / or reagents, including hydrogen, completely or partially from the polymer before it enters the reactor (s) further downstream. Such techniques generally include pressure reduction to evaporate unwanted components.

[0003] ΕΡ 603935A describes a method in which bimodal polyethylene is produced in a series of reactors, where the low molecular weight homopolymer component is formed in the first reactor and the high molecular weight component is incorporated in the second reactor, where for molecular weight control hydrogen is used. There is no discussion on how to remove residual hydrogen between reactors. In 192427A and EP 897934A, a significant pressure reduction between the two reactors is used to remove at least part of the hydrogen present. This method is acceptable when the diluent remains practically in the liquid phase under the pressure reduction conditions required for

Achieving the desired removal of hydrogen: however, if a more volatile diluent is used or a greater degree of hydrogen separation is required, then a more effective method will be desirable. Slurry processes using light (i.e. relatively volatile) solvents show some advantage over heavier thinner systems. For example, polyolefin oligomers become less soluble and the solvent is in place and virtually completely removed from the polymer product. However, hydrogen gas must generally be completely removed between the previous and subsequent stages, otherwise the control of the next stage method is difficult and it can be difficult to obtain a high molecular weight. Light solvents tend to run off with hydrogen. If too much solvent drains off, the suspended solids will rise to such a high level that further pumping of the suspension will be impossible. If the solvent scrubbing is reduced, the hydrogen separation will be poor. Another difficulty is that the polymer carried in the throttling gas is still catalytically active and can still polymerize, causing problems with contamination of the apparatus used for hydrogen removal or other separation. Thus, it is necessary to remove or deactivate the polymer residue. From this it can be seen that for this type of polymerization reaction there is a need for an improved method of removing hydrogen between the individual stages.

[0004] In US 2003/0191251, two distillers (flash vessel) are used to separate hydrogen from the light diluent between the polymerization reactors. Each distiller has only one equilibrium stage. After the first distillation stage, a significant diluent compensation is required due to high diluent losses.

[0005] In US 3658780, the polypropylene slurry removed from the polymerization reactor is treated with catalyst removal agents, and then the catalyst is washed away, thereby rendering the stream catalytically inactive, before fractionating the stream to remove hydrogen.

[0006] In US 6045661, the stream removed from the reactor polymerizing ethylene and hexene in isobutane is passed through distillers and the transferred polymer particles are removed in a cyclone. At least a portion of the steam is then compressed before being sent to a fractionation unit for component separation. This method states that removal of the transferred solid particles guarantees that the fractionated material is not catalytically active.

[0007] This invention aims to provide an improved method of treating polyolefm streams, particularly polyethylene.

[0008] Accordingly, the invention discloses a method of polymerizing ethylene or propylene using more than one reactor in series, in which the main diluent is an inert diluent or monomer, wherein at least a portion of the stream removed from the polymerization reactor is passed through a fractionation device which includes a column with at least 3 equilibration stages and at least the first two equilibrium stages in the fractionation device above the location

The stream supply has a stripping liquid flow which is at least 10% by weight of the steam flow rate, and the fractionation device is fed from more than one reactor and / or the fractionation device supplies the purified main diluent to more than one reactor.

[0009] In general, the stream removed from the polymerization reactor contains at least 0.005 vol. solid polymer particles. In this case, the polymer contains an active catalyst. Such a solid polymer usually has a particle size such that at least 50% of the polymer has a particle size of at least 10 μηι. In one embodiment of the invention, the concentration of polymers consists of fine particles with an average diameter less than 100 microns, more preferably less than 50 microns. In another embodiment of the invention, the concentration of polymer fed to the fractionation step is at least 30 vol%. and may be higher than 40% vol.

[0010] In general, the stream removed from the polymerization vessel is catalytically active, which means that the stream is able to undergo further polymerization under the conditions prevailing during fractionation.

[0011] In this specification, fractionation means separation in a vessel that (i) with more than one equilibrium stage, (ii) in which liquid and gas are at least partly in contact at each equilibrium stage and in which (iii) the liquid stream is evaporated more than once, more preferably more than twice. "Fractionation device" means a vessel or column in which fractionation takes place.

[0012] Reference to the equilibrium stage will mean the actual contact stage as opposed to the theoretical equilibrium stage.

[0013] Preferably, the fractionation of the stream is carried out in a fractionation apparatus at a pressure lower than the pressure in the previous polymerization reactor (s) and such that the main fluid in the stream undergoes condensation without recompression by heat exchange with a cooling medium in the temperature range of 15-60 ° C. Most preferably, the fractionation is carried out under such pressure and temperature that at least 50% by weight, more preferably at least 75% by weight of the catalytically active fluid stream (no solid component) that is fed to the fractionation device is in a gaseous state.

[0014] In this specification, "diluent" means a hydrocarbon component added to the polymerization reactor to aid heat removal and / or to suspend solid polymer in the reactor. In the case of suspension reactors, the diluent is in a liquid or supercritical state in the reactor. The main diluent is a non-solid component of the fluid stream with the largest mol% in the reactor and is preferably inert (i.e. not polymerized) under the reaction conditions.

[0015] It would be expected that the treatment of the polymer stream, which may also contain monomer (s), may cause unacceptable contamination and / or equipment downtime relative to the continuous polymerization process, especially with respect to the internal parts and / or heat exchange equipment associated with the fractionation column. It was observed that

The method according to the invention can be operated without unnecessary contamination or downtime, and that the installation of processing equipment in standby can be avoided. The advantage of separating undesirable light components using medium to high pressure fractionation, with more than one equilibrium step in place of a single lower pressure distiller, is that less recompression of the polymer stream and / or recovered light materials is required, and more efficient is also possible separation operation. Particularly, when separating unwanted light components from desired light diluents, the loss of diluent in the separation method is significantly reduced.

[0016] The process of the invention includes the treatment of polymer streams removed from more than one reactor. In a preferred embodiment, the fractionating device is able to simultaneously process the diluent streams from the combination of reactors (in series or in parallel) and recycle the diluent to these reactors, which is substantially free of light and / or heavy components, e.g. hydrogen-free or comonomer-free streams or free from the main monomer or free from all monomers. Being able to remove the treated diluent from any equilibrium state in the fractionation device in an economical manner gives considerable flexibility to optimize the amount and purity of each stream recycled.

[0017] The invention is particularly suitable for the polymerization of olefins in suspension or in suspension reactors. In this case, the olefin (s) are continuously added to the hydrocarbon diluent (which may be substantially inert or substantially monomer or substantially liquid or a supercritical fluid) containing the catalyst. The monomer (s) polymerize to form a suspension of polymer solid particles suspended in the polymerization agent or diluent. Generally, in the methods of particle formation or suspension polymerization for polyethylene, the composition of the slurry in the reactor is as follows: polymer in the form of particles 15-50 vol. particles, more preferably 25-40% by volume particles; suspension liquid about 30-85% vol. and a monomer of about 1-15% of ref, where the main diluent is an inert diluent, although these proportions can vary significantly. The concentration by volume of particles (% by volume) is defined as the volume of particles (without the volume of gaps) in the mixture divided by the total volume of the mixture.

[0018] The invention is most preferably associated with polymerization in the reaction zone in the shape of an elongated tube in a closed loop or in so-called "slurry loop" reactor. In the slurry-loop process, the reactor is a tube-shaped loop, e.g., a steel tube placed in a larger pipe, through which water flows to heat or cool the reactor as needed. One or more circulation pumps guide the reactor contents around the loop at a relatively high speed to promote good heat transfer, keep particulates suspended, and minimize reactor contamination. Loops can be directed horizontally or vertically. The output of the product from the loop reactor can be continuous or through periodically opening stabilizing legs. In both cases, the suspension medium solvent is removed along with

Product and must be condensed and / or recompressed and re-introduced into the reactor.

[0019] In a typical polymerization process to which the invention relates in particular, homopolymerization and liquid phase copolymerization processes are carried out in an inert diluent, and the reagents contain ethylene and hydrogen in the case of homopolymerization, and in the case of copolymerization they contain ethylene, alpha-olefin comonomer (s) ) containing from 3 to 8 carbon atoms and optionally hydrogen. The comonomer may be selected from propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl 1-pentene, 1-heptene and 1-octene. The inert diluent may include (iso) butane, pentane or hexane. Homopolymerization and copolymerization processes are generally carried out at a temperature of 50 to 120 ° C at an absolute pressure of 1 to 100 bar.

[00201 Such a method can be used to produce multi-modal polymers in a single reactor or in multiple reactors connected in series or in parallel, in which case each reactor may be preceded or followed by a reactor which is of the same or a different type reactor (e.g. gas phase, slurry mixing tank or loop reactor or solution reactor). In the case of series reactors, the first series reactor is provided with a catalyst and optionally with a cocatalyst, and each subsequent reactor is supplied with at least ethylene and the slurry coming out of the previous reactor in series, this mixture comprising a catalyst, optionally a cocatalyst and a mixture of polymers produced and preceding reactors in series. It is possible to supply the second reactor and / or, if applicable, at least one of the next reactors with fresh catalyst and / or cocatalyst, although generally the catalyst and cocatalyst are fed only to the first reactor.

[0021] The method of the present invention is particularly suitable for polymerization, which includes the use of at least two slurry reactors in series to produce a polyolefin product, where the subsequent slurry reactor uses little or no hydrogen feed compared to the hydrogen feed of the previous slurry reactor. In this case, the method of the invention is used to remove hydrogen from the intermediate polymer suspension between cascaded reactors. Typically, two slurry reactors are used, although it is also possible to use three or more reactors in series. It is also possible to use two or more slurry reactors in series with one or more slurry reactors operating in parallel. In a preferred embodiment of the invention, the fractionation device is introduced to remove hydrogen as well as to separate the remaining components of the stream.

[0022] In a preferred embodiment of the invention, the polymer stream removed from the polymerization reactor is treated before fractionation, or to minimize the amount of diluent to be treated and / or to control the average particle size and particle size distribution of the catalytically active material in the diluent stream. Preferably, the polymer stream is concentrated to achieve a solids concentration of 50-70% by weight. Preferably, the treatment comprises delivery

- polymer stream to the hydrocyclone separator before fractionation, and most preferably supplying fresh diluent earlier in relation to the flow direction relative to the hydrocyclone separator, e.g. as described in our patent No. ΕΡ 1118624Λ. Alternatively, a sufficiently high concentration of solid particles can be achieved by using stabilizing legs in the reactor.

[0023] For the purposes of the invention, the term "hydrocyclone separator" is intended to mean any apparatus which, under the action of centrifugal force, allows the separation of solid particles from slurry, on the one hand the flow of liquid depleted in solid particles, and on the other the flow of concentrated solid particles. Such examples of apparatus are well known and are described in particular in Perry's Chemical Engineers' Handbook McGraw-Hill Edition 7, 1997, pages 19-24 to 19-28.

[0024] If the solid particles loading the polymer stream are kept at a sufficiently high concentration (by proper handling of the reactor and / or the above-mentioned particulate concentration system), the heat content of the polymer stream may be sufficient to provide all the heat necessary for the fractionation. In this case, the stream can be fed directly from the reactor or concentrator to the fractionation separator without adding further heat. However, if the heat content of the particulates entering the fractionation device is insufficient to supply all necessary condensed vapors to the fractionation column, the fractionation device base can be heated using a jacket or heat exchanger. Preferably, the heat content of the polymer stream is sufficient to provide at least 60%, more preferably at least 70% of the heat required for fractionation.

[0025] In an alternative embodiment of the invention, the polymer stream removed from the reactor is heated, optionally after concentration and / or in the pressure reduction phase of the removal line. The resulting particulate stream, gas or optional liquid is then sent directly to the fractionation column feed vessel or alternatively directly to the fractionation column.

[0026] If a feed vessel for the fractionation apparatus is used, preferably in one embodiment of the invention, the pressure therein is controlled so that the diluent is sufficiently flushed out of the feed vessel and leave the polymer unsuspended in the base. The washed diluent stream is then supplied to the vessel of the fractionation apparatus, the base of which can be heated if necessary. In addition to using slurry line heaters and a column base or jacket to heat the feed vessel, preferably no external energy is added to re-evaporate into the fractionation column.

[0027] If the suspension is maintained in the fractionation vessel supply vessel, the pressure will be dictated by the need to minimize the concentration of undesirable components in the liquid phase, while being able to condense the main diluent without compression solely by using a cooling agent, preferably water, at a temperature of

-Ί 15 to 60 ° C. Flushing can be assisted by heating in the supply vessel, if needed. The washed diluent stream may be fed directly to the bottom of the fractionation column, preferably to the base. In this case, the column is designed to support larger particle size and flow rate below the feed point, and to receive fine particles at the separation stages directly above the feed point. In this case, preferably the fractionation column has at least 4 actual equilibrium stages and preferably at least two separation steps above the polymer stream feed location were designed to handle particulate matter. The fluid-separating mass flow is in this part also a part of the fractionation column, preferably running through the entire fractionation device, is preferably 10% by weight of the vapor mass flow rate. The fractionation device may have internal components such as distillation trays (screening, double flow, rectification column cap, vacuum chambers) or may be evenly filled, preferably with larger apertures. The fractionating column portion below the feed point is especially designed to absorb high solids concentration and to avoid buildup of solids. The column base is designed to guarantee a high slurry speed to avoid deposition and minimize the residence time of the particles in the reactor. Preferably, the residence time of any particles in the column is no more than 30 seconds, more preferably no more than 90 seconds.

[0028] In the fractionation column, the bottom products containing the polymer, diluent and heavier hydrocarbons and / or comonomers are removed from the base of the column. It can optionally be recycled to the fractionation vessel into the supply vessel. The ceiling vapor from the column generally contains hydrogen and monomer. Side streams can also be removed, containing different diluent and monomer compositions. They can be recycled to the reactor.

[0029] The temperature of the fractionation column base should always be kept at least 5 ° C below the sintering temperature or solubility temperature of the polymer produced in the reactor positioned upstream of the direction of flow. The design and solid handling of the separation method is optimized by careful equipment design and selection and control of the liquid flow rate and temperature and pressure conditions throughout the fractionation column to ensure that solid particles will not accumulate at any point of the fractionation device and contamination is avoided internal parts or at least minimized to such an extent, that any cleaning operations required by themselves will not reduce the availability of the installation. Generally, the fractionation plant design is optimized at a base temperature above 50 ° C. In one preferred embodiment of the invention, the temperature profile of the column and the concentration of the main monomer are designed to avoid excessive peak values ​​in the catalytic activity at each individual stage of the column.

[0030] Preferably, the fractionation is carried out at a pressure lower than the pressure in the previous reactor, but higher than 1 barg, more preferably 3.5 barg.

[0031] Preferably, the slurry in the base of the fractionation apparatus column and in some embodiments in the base of the fractionation apparatus feed vessel is kept in suspension at all times and is usually mixed with a mixer, although alternatively or additionally it can be suspended using an external pump circulation.

[0032] In the event that the unsuspended polymer powder is held in the fractionation vessel feed vessel, the base may be designed to allow the powder level to be kept constant while allowing continuous or discontinuous discharge from the vessel. Preferably, the base is cone-shaped with a cone angle and outlet nozzle diameter of such a size as to maintain retention or mass flow for a series of powders (taking into account the expected content of associated hydrocarbons) that the supply vessel is able to handle, according to the design.

[0033] In a preferred embodiment of the invention, the base of the fractionation apparatus is opened to a supply vessel that receives both the feed suspension from the fractionation device base, as well as suspended or non-suspended solid particles from the fractionation vessel supply base. In this case, it is preferred that the solid particles in the fractionation apparatus feeding vessel are not suspended. Fresh diluent can be added to the transmission line between the base of the fractionation device feeding vessel and the slurry vessel. In the event that the fractionation device separates the feed stream between two polymerization reactors in bimodal polymerization, the suspension vessel is discharged into the second reactor.

[0034] A specific example of the method to be used according to the invention is the suspension polymerization method for producing bimodal high density polyethylene comprising ethylene homopolymer (A) and ethylene copolymer and 1-hexene (B) formed in two reactors in series. Such a method can be carried out using the apparatus shown in Figure 2 (see below).

[0035] The diluent used in this particular polymerization process is usually a hydrocarbon-containing diluent that is inert to the catalyst, cocatalyst and polymer formed, such as e.g. a linear or branched alkane or cycloalkane with from 3 to 8 carbon atoms. The thinner that gave the best results is isobutane. One of the advantages of using isobutane is, in particular, its readiness for recycling. This is because the use of isobutane makes it possible to recycle the diluent recovered at the end of the process according to the invention in the first reactor without the need for extensive purification to remove hexene residues. This is because the boiling points of isobutane and hexene are distant from each other and therefore their separation can be carried out by distillation.

[0036] In this preferred method, the amount of ethylene introduced into the first polymerization reactor and into the next polymerization reactor is generally controlled so as to obtain an ethylene concentration in the diluent from 5 to 50 g of ethylene per kg of diluent. The amount of hydrogen introduced into the first reactor is generally controlled to obtain a molar ratio of hydrogen to ethylene of 0.05 to 1 in the diluent. A molar ratio of hydrogen to ethylene that does not exceed 0.6 is particularly preferred.

[0037] The mixture removed from the first reactor additionally containing the homopolymer (A) is subjected to a pressure reduction so as to remove (degas) at least a portion of hydrogen, which can be carried out in accordance with the invention. The pressure reduction is preferably carried out at a temperature less than or equal to the polymerization temperature in the first reactor. The temperature at which the pressure reduction is carried out is usually at least 40 ° C. The pressure at which the pressure reduction is carried out is less than the pressure in the first reactor and is between 0.1 and 1.5 MPa. The amount of hydrogen still present in the at least partially degassed suspension mixture (liquid + solid particles) is generally below 1% by weight of the amount of hydrogen initially present in the mixture removed from the first polymerization reactor, preferably this amount is less than 0.5%. The amount of hydrogen present in the partially degassed mixture introduced into the next polymerization reactor is consequently low or even zero. Preferably, hydrogen is also supplied to the next reactor. The amount of hydrogen introduced into the next reactor is generally adjusted so as to obtain in the diluent the molar ratio of hydrogen to ethylene from 0.001 to 0.1 in the reactor, generally between 0.004 and 0.05. In this method, the ratio of hydrogen concentration in the diluent in the first reactor to the concentration in the subsequent polymerization reactor is generally at least 20, preferably between 40 and 200.

[0038] The amount of 1-hexene fed to the subsequent polymerization reactor is such that in this reactor the hexene / ethylene molar ratio in the diluent is at least 0.05, more preferably at least 0.1. The amount of hexene fed to the next reactor is such that preferably the hexene / ethylene molar ratio does not exceed 3. A hexene is usually not fed to the first reactor; indeed, it is important that the first reactor is essentially free of 1-hexene. Therefore, the diluent fed into the first reactor, which may be the diluent being recycled, must be highly hexene-free. Preferably, the diluent fed to the first reactor contains less than 1000 ppm hexene, and ideally is substantially hexene-free.

[0039] The polymerization temperature is generally 20 to 130 ° C, generally does not exceed 115 ° C. The total pressure at which the method according to the invention is carried out is generally from 0.1 MPa to 10 MPa. In the first polymerization reactor, the total pressure is usually at least 2.5 MPa, but not more than 5 MPa. In the next polymerization reactor, the total pressure is usually at least 1.3 MPa, but not more than 4.3 MPa.

[0040] In this preferred method, a suspension comprising a composition of 30 to 70% by weight of homopolymer (A) and 30 to 70% by weight of copolymer (B) is collected

- at the outlet of the next polymerization reactor. The composition comprising ethylene polymers can be separated from the suspension by any known means. The slurry is usually subjected to pressure reduction (final pressure reduction) to remove diluent, ethylene, hexene and optionally hydrogen from the composition.

[0041] According to an alternative form of this method, and more particularly, when the diluent is isobutane, the gases coming from the first pressure reduction (intermediate pressure reduction between two polymerization reactors) and from the final pressure reduction are mixed and transferred to the distillation unit. This distillation unit preferably consists of one or two distillation columns in series. Ethylene and hydrogen are removed from the top of the column, with the mixture of isobutane and hexene being removed from the bottom of the column and the hexene-free isobutane being removed from the intermediate plate. The isobutane-hexene mixture is then recycled in the next polymerization reactor, while the hexene-free isobutane is recycled in the first reactor.

[0042] The catalyst used for the polymerization process may be any catalyst (s) suitable for the polymerization reaction, but generally it is a chromium catalyst, Ziegler-Natta catalyst or metallocene catalyst. Usually it is a Ziegler-Natta catalyst.

[0043] In the case of a Ziegler-Natta catalyst, the catalyst used contains at least one transition metal. Transition metal means a metal from the 4.5 or 6 group of the Periodic Table of the Elements (CRC Handbook of Chemistry and Physics, 75, 1994-95). Preferably, the transition metal is titanium and / or zirconium. Preferably, a catalyst containing not only transition metal is used, but magnesium is preferably used. Good results have been obtained with a catalyst containing:

® from 10 to 30%, more preferably from 15 to 20%, more preferably from 16 to 18% by weight of transition metal, ® from 0.5 to 20%, more preferably from 1 to 10%, and even more preferably from 4 to 5% by weight magnesium, • from 20 to 60%, more preferably from 30 to 50%, and even more preferably from 40 to 45% by weight of halogen, such as chlorine, »from 0.1 to 10%, more preferably from 0.5 to 5%, and even more preferably from 2 to 3% by weight of aluminum;

the balance generally consists of elements derived from products used in their production, such as carbon, hydrogen and oxygen. Preferably, these catalysts are obtained by co-precipitation of at least one transition metal composition and magnesium composition with a halogenated organoaluminium composition. Such catalysts are known, they have been noticeably described in patents US 3901863, US 42942200 and US 4617360. Preferably,

The catalyst is fed only to the first polymerization reactor, i.e. there is no fresh catalyst introduced into the next polymerization reactor.

[0044] Preferably, the cocatalyst used in the method is an organoaluminium compound.

Non-halogenated organo aluminum compounds with the formula AIR.3_; wherein R is an alkyl moiety of from 1 to 8 carbon atoms are preferred. Triethylaluminium and triisobutylaiuminium are particularly preferred. The cocatalyst is introduced into the first polymerization reactor. Fresh cocatalyst can also be fed into a downstream reactor, the amount of cocatalyst fed into the first reactor is generally at least 0.1 x 10 '³ moles per liter of diluent. Generally, it does not exceed 5 χ 10 '³ moles per liter of diluent. Any amount of fresh cocatalyst fed into the downstream reactor generally does not exceed 5x10 mole per liter of diluent.

[0045] Preferred embodiments of the invention will now be described with reference to the accompanying drawings.

[0046] Figure 1 is a block diagram of a polymerization system comprising a single slurry loop reactor with associated fractionation apparatus.

[0047] Referring to Figure 1, the diluent is kept in the liquid phase during the polymerization reaction, where the polymer solid particles are essentially insoluble in the diluent and are suspended by it. The stream flowing from the polymerization reactor 1 contains a liquid diluent transporting the suspension of polymer solids together with the rest of the catalyst and such reagents as monomer (s), comonomer (s), molecular weight control factors such as hydrogen, and co-catalysts.

[0048] The effluent stream removed from the reactor via line 3, from which it passes to the hydrocyclone 5, which concentrates the suspension to a solids level of approx. 50-70% by weight. Then the stream is usually subjected to a pressure reduction at 7, from reactor pressure (generally 40 barg) to a pressure of 7-10 barg. Depending on the concentration of solid particles and stream temperature, the heat content of the stream can be increased by a slurry line heater 9; the designed or controlled degree of heat input is designed to maximize liquid evaporation while avoiding the risk of sintering in the heater. Preferably, the temperature of the slurry heater at the outlet is controlled to the dew point of the fluid stream being removed.

[0049] Next, the stream passes into the fractionation vessel feeding vessel 11. The pressure in the supply vessel 11 is regulated to flush out a sufficient amount of diluent to leave the polymer unhashed in the base. Solid polymer particles are removed by line 13.

[0050] The washed diluent stream is then fed through line 15 to the column 17 of the fractionation apparatus, preferably at the base. The column base can be heated if the solids content of the stream is insufficient to provide enough heat for the fractionation. The pressure at which column 17 operates can be in a wide range from 1 barg to 30 barg or more. Preferred temperature conditions in column 17 include a ceiling temperature (temperature at the top of the column) of 30-50 ° C and

- 12 bottom temperature (bottom of column) 65-95 ° C. The fractionator has 5 and 25 screening trays and / or dual flow trays.

[0051] The liquid bottom product, generally containing the heavy comonomer (s) diluent, is removed from column 17 via line 19. If additional heating is required, part of the bottom product is passed through line 19 to the preheater (condensate reboiler reboiler) 21 and from there via line 23 as steam back to column 17. Alternatively, the bottom of column 17 can be heated by means of a jacket.

[0052] In the case where the comonomer is heavier than the diluent, the side stream (preferably in the form of steam) can optionally be removed from column 17 by line 25. The side stream generally contains a comonomer-poor diluent. The side stream is cooled and condensed and then recycled to reactor 1 (not shown). It can also be removed from the column as a liquid. By providing a poorer stream in comonomer than in the previous reactor and by providing a buffer volume of streams poor in comonomer, the column facilitates a significant reduction of product transit time between polymer stages of different density.

[0053] The overhead steam from column 17, generally containing diluent, unreacted monomer, hydrogen, nitrogen and other light parts, passes through line 27 to the cooling device 29, where it is condensed to return to column 17 via line 31. Light parts can be removed from the capacitor.

[0054] Referring to Figure 2, it shows an alternative embodiment of the invention for bimodal polymerization in which hydrogen is removed from the polymerization stream through a fractionation device located between the reactors. The second reactor is not shown in the Figure. In the arrangement shown in Figure 2, the reference numbers are the same as in Figure 1. The main difference from the arrangement of Figure 1 is that in this embodiment, the bottom product from the fractionation device 17 is recycled into the supply vessel 11 of the fractionation device by line 33. In this case, the polymer at the bottom of the supply vessel 11 is held in suspension in the diluent by means of a mixer 35 and this suspension is removed from the base of the line 13 feed vessel and pumped with pump 37 to the second reactor (not shown). The liquid portion of the stream removed via line 13 can be recycled to distiller 11 via line 39, which may include heater 40. The feed vessel 11 with suspension with well mixed solids can be heated with heater 43.

[00551 Given the fractionation device 17 in the embodiment of the invention of Figure 2, it operates as in Figure 1 without side strings except that the hydrogen is removed from the overhead stream, by line 27, while the remainder is condensed in the cooling device 29 The recycle stream 31 thus re-circulated has a significantly reduced level of hydrogen. Stream 33 recovers the vast majority of diluent, comonomers, and even the monomer washed away in

- 13 vessel 11, while being poor in hydrogen. This fractionating device design generally has about 5 screening trays and / or double flow trays.

[0056] Referring to Figure 3, it shows another embodiment of the invention relating to bimodal polymerization in two reactors in which a single fractionation column is used to treat both the polymer stream between the reactors and the final stream from the second reactor. The following description assumes that a low molecular weight product is created in the first reactor, however, this design configuration gives full flexibility by choosing the appropriate streams re-circulated from the fractionation device to equally allow the production of the high molecular weight product in first reactor.

[0057] In the embodiments of Figures 1 and 2, the effluent stream from the first reactor 1 is removed from the reactor by line 3, from which it passes into the fractionation vessel feeding vessel 11 (details are shown in Figures 1 and 2 and are omitted here). As in the previous embodiments of the invention, both the concentration in the hydrocyclone, the pressure reduction and the additional heating (none shown in the Figure) can be applied to the stream if necessary.

[0058] The diluent is flushed from the feed vessel 11 through line 15 to the fractionation column 17. All or most of the comonomer rich diluent stream from column 17 is returned to the vessel 11 via line 57. The suspended polymer is removed from the bottom of the vessel 11 via line 13 and transferred to the second reactor 41, where an additional comonomer can be added if needed.

[0059] The stream flowing from the second reactor 41 is removed from the reactor via line 43, from where it passes into the second vessel 51 for feeding the fractionation device. As with the first reactor 1, concentration in the hydrocyclone, pressure reduction and additional heating (none is shown in the Figure) can be applied to this second effluent stream if required. The powder level is maintained in the second fractionation feed vessel 51 and the un-suspended, final polymer product essentially free of free liquid is removed by line 55, while the evaporated diluent stream is then fed by line 53 to the fractionation column.

[0060] The temperature and pressure profile of column 17 is adjusted so as to remove separately the full spectrum of the components of the incoming stream. Preferred temperature conditions in column 17 include a ceiling temperature (temperature at the top of the column) 35-55 ° C and a bottom temperature (at the bottom of the column) 65-95 ° C. The liquid bottom product together with fine polymer particles, generally containing the diluent and rich in comonomer, is removed from column 17 via line 57. The optional side stream 61 takes away the monomer-free diluent, if needed, e.g. for transporting the catalyst to the reactor 1. Side stream 63 consists of a comonomer-free diluent stream which can be recycled to the reactor 1. Overhead steam from columns 17 removed in line 27 and hydrogen rich stream, along with a portion of the monomer, is removed from

- capacitor 29 by line 65. The required number of trays in the column is minimized by preferably not allocating it to the separation of monomer from diluent streams recycled to each reactor.

[0061] Referring to Figure 4, it shows an embodiment of the invention with respect to bimodal polymerization in which hydrogen is removed from the polymerization stream through a fractionation device placed between the reactors. Thus, it is similar to the embodiment of the invention of Figure 2 and, where appropriate, the reference numbers are the same. The effluent stream passes from the reactor 1 into the feeding vessel 11 of the fractionation apparatus in the same manner as in the embodiment of the invention of Figure 2. The pressure in the feeding vessel 11 is regulated so as to flush a sufficient amount of diluent to the fractionating apparatus 17 by line 15 to leave the polymer not suspended in the base. Line 15 enters the fractionation device 17 above the bottom to promote hydrogen separation. The solid polymer is removed through line 67 to a second suspension vessel 69, which also receives the bottom product from the fractionation device 17 via line 71. A boiler for re-evaporating condensed vapors may also be present at the bottom of column 17 to promote separation in the column. The polymer at the bottom of the suspension vessel 69 is suspended in a hydrogen-poor diluent, and this suspension is removed from the base of the suspension vessel 69 via line 73 and pumped through pump 37 to the second reactor 41. Part of the liquid stream removed via line 73 can be recycled suspension vessel 69 with line 39.

EXAMPLE [0062] In one detailed embodiment of the invention in which the fractionation device is installed between two reactors in a bimodal dual reactor system, as described in Figure 2, the fractionation device is fed with a stream containing isobutane, ethylene, hydrogen, hexene 1 and polyethylene leaving the first polymerization reactor. This stream is first concentrated in the hydrocyclone, after which it is passed through a slurry heater before entering the base of the fractionation device.

[0063] In this particular example, a flow rate of 10090 kg / h polyethylene and 9685 kg / h hydrocarbon enters the base of the fractionation device. The hydrocarbon in this case mainly includes isobutane, but also includes about 3.22 kg / h of hydrogen, 116 kg / h of ethylene, about 10 kg / h of a solid polymer that contains some active catalyst and small amounts of other components.

[0064] When the stream enters the base of the fractionating device, the hydrocarbon portion is about 75% of the steam. After entering the base, a trace liquid and almost all solid polyethylene falls into the boiling and mixed pool. The basic suspension in this example is heated by a jacket, which provides heat for about 1/3 of the steam flow through the column and is about 70 ° C and 10 barg pressure.

[0065] The hydrocarbon vapor coming from the reactor stream is combined with the boiling product from the boiling slurry in the base of the fractionation apparatus such that a steam flow of about 11500 kg / h enters the column of the fractionation apparatus. This pair contains some fine catalytically active polymer particles that are transferred from devices previously oriented relative to the flow direction. This fractionation column is approximately 1 m in diameter and five double flow trays. Each tray has approximately 9% open area and 25 mm diameter holes. This large hole diameter is important to guarantee a minimal degree of blockage.

[0066] The gas stream passes up the scrubber, and each tray progressively removes fine particles by contacting the gas / fine particles stream with a liquid stream that falls down the column. It also removes hydrogen from the liquid that falls down the column.

[0067] At the top of the column, the stream enters the condenser and is almost completely condensed at a temperature of about 30 ° C. The purge gas is removed from this condensed stream to remove hydrogen. You can install a filter in this stream to test the efficiency of removing solids in the column: Applicants have never found a trace of polyethylene in such a filter. In addition, Applicants have never found polyethylene in the condenser, nor have they experienced any impurities. These observations confirm the excellent performance of the system in handling fine active particles.

[0068] The liquid that is condensed is recycled to the fractionation apparatus and falls back down onto each tray. Until it reaches the bottom of the fractionation device, the liquid will be deprived of hydrogen and all active particles will be recycled to the base liquid. It should be noted that since almost all the liquid is condensed and returned to the base of the fractionating device, there is no need to compensate for the liquid in the base to maintain the concentration of solid particles. The suspension is practically free of hydrogen is mixed to minimize deposition and then pumped into the second reactor. The back stream is removed from the outlet of this pump and recycled to the suspension base to assist in the homogeneity of the suspension. Typical hydrogen content in the slurry entering the second reactor is below 100g / h. In this way, considering the initial hydrogen flow rate coming out of the first reactor i.e. 3.22 kg / h, it can be seen that the method according to the invention is very efficient in removing hydrogen from the feed stream as well as in removing fine active particles from the purifying gas at the top of the column. This simple device thus demonstrates a robust and economical means of controlling the hydrogen concentration and hence the molecular weight in the second reactor, regardless of the conditions required in the first reactor - even when using diluents in the form of steam in

Ą atmospheric conditions. In addition, the column can also be adjusted to minimize the loss of ethylene and diluent.

<img file="PL2014685T3_D0001.tif" />

Dorota Rząże \ yska Patent attorney