RU2444530C2 - Method for final treatment of polyolefins - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method for final treatment of polyolefins obtained via gas-phase catalytic polymerisation of one or more α-olefins in the presence of a diluent for the polymerisation mixture. The diluent is selected from C₃-C₅-alkanes. Polyolefin granules fed into the gas-phase reactor undergo treatment at a first degassing step which is carried out in a first reservoir. Said polyolefin is brought into contact with a gas stream in counter-flow conditions, said gas stream containing at least 85 mol % washing reagent selected from C₃-C₅-alkane. The gas stream is continuously extracted from the monomer regeneration section of the polymerisation apparatus. The granules then undergo treatment at a second degassing step which is carried out in a second reservoir, where said polyolefin is brought into contact with water vapour in counter-flow conditions. The amount of water vapour ranges from 10 to 200 kg per 1000 kg polyolefin.

EFFECT: efficient removal of oligomers and heavy hydrocarbons from polyolefin granules coming out of a gas-phase polymerisation apparatus and responsible for causing smells.

19 cl, 1 dwg, 1 tbl, 4 ex

Description

The present invention relates to a method for finishing polyolefins obtained by gas phase polymerization to remove oligomers and higher hydrocarbons from polyolefin granules.

In particular, the invention relates to a method for reducing the amount of higher hydrocarbons and oligomers contained in polyolefin granules discharged from a gas phase polymerisation reactor to substantially reduce the odor emitted by the polymer.

The use of catalysts with high activity and selectivity of the Ziegler-Natta type, and more recently the metallocene type, has led to widespread industrial-scale application of processes in which olefins are polymerized in a gaseous medium in the presence of a solid catalyst. An example of these gas phase polymerization processes involves the use of a fluidized bed reactor in which a layer of polymer particles is maintained in a fluidized state by an upward gas stream creating a fluidized bed (fluidizing gas).

During polymerization, the polymer is produced by catalytic polymerization of monomers, and the resulting polymer is withdrawn from the reactor to maintain a constant volume of the polymer layer. The fluidized bed, which includes a layer of growing polymer particles and catalyst particles, is maintained in a fluidized state using a continuous upward flow of fluidizing gas, which includes a stream of recycle gas and monomers to replenish the reaction mixture. In industrial processes, a baffle-divider is used to distribute fluidizing gas supplied to the polymer layer, a baffle-divider, which also acts as a support for the layer, when the gas supply is cut off. The fluidizing gas enters the bottom of the reactor and passes through a baffle-divider into the fluidized bed of the polymer.

It is known that during gas phase (co) polymerization of olefins, trapped monomers and comonomers usually remain in the pores of the polymer particles, especially when the comonomers are α-olefins containing from 4 to 8 carbon atoms. For example, in the production of linear low-density polyethylene (LLDPE), 1-hexene is mainly used as a comonomer, while in the production of high-density polyethylene (HDPE) a relatively high polymerization temperature and a large amount of hydrogen used as a molecular weight regulator can contribute to the flow secondary hydrogenation reactions, in particular, with the formation of organic compounds with low volatility, containing, for example, from 4 to 12 carbon atoms.

Moreover, gas-phase polymerization of olefins can be carried out in the presence of inert gases such as propane, isobutane, isopentane or other saturated aliphatic hydrocarbons, which are mainly intended to participate in the dissipation of heat generated inside the reactor as a result of the polymerization reaction. It is these alkanes with low volatility that can remain trapped and dissolved in the pores of the polyolefin particles.

For safety and economic and environmental reasons, there is a need to remove unreacted (co) monomers, organic compounds and alkanes with relatively low volatility from the resulting polyolefin. All these compounds pose a threat to the environment, some of them in the presence of atmospheric oxygen are able to form explosive mixtures. Unreacted monomers also pose a risk of continued uncontrolled residual polymerization outside the polymerization reactor.

Another disadvantage associated with the presence of unreacted monomers, alkanes and oligomers inside the resulting polyolefin is the distribution of a bad smell, which makes it difficult to market molded products for food and pharmaceutical purposes. In particular, oligomers of saturated and unsaturated hydrocarbons with more than 5 carbon atoms, for example, such as 1-hexene and hexane, are responsible for the spread of a bad smell in a polyolefin. Therefore, the content of these components should be drastically reduced, especially in the case where the resulting polyolefin is intended for molding products related to food.

Patent EP 808850 provides a method for reducing the level of emitted odor from olefin polymers obtained using a metallocene catalyst during gas phase polymerization. According to the teachings of this patent, ligands having a cyclopentadienyl skeleton are sources of odor and can be effectively removed by a method comprising the step of contacting the polyolefin with a ligand degradation reagent such as water or alcohol to decompose the residual ligands in the polyolefin, and then the step removing the destroyed ligands by heating the named polyolefin.

Patent EP 1348721 relates to copolymerization of ethylene to obtain an ethylene copolymer having a low content of components that could spread odor, or components that could change the taste of food products. The presented method involves gas phase polymerization using a metallocene catalyst in a fluidized bed reactor with a saturated aliphatic hydrocarbon present in the reactor at a concentration of from 2 to 30 mol%. The copolymer powder recovered from the reactor is then introduced into the ligand decomposition step by contacting the polyolefin with water, oxygen or alcohols, and then into the ligand removal step by heating said copolymer.

US patent 6218504 relates to a method for deodorizing polyolefins and also to the use of deodorized polyolefin granules for the production of polymer molded products with a low negative effect on taste and smell. The process involves the use of a specific installation in which a gaseous mixture of water vapor and air, or a gaseous mixture of water vapor and nitrogen, or, alternatively, pure water vapor, is passed around polyolefin granules at a temperature preferably in the range of 90 ° C. to 130 ° C., the amount of water vapor from 0.3 to 2.0 kg per kg of polyolefin granules.

US Pat. No. 5,367,742 discloses the regeneration of unreacted monomers from a polymer product exiting a fluidized bed reactor and the use of such regenerated gases to flush unreacted monomers from a polymer product. Ethylene undergoes catalytic copolymerization with a C ₃ -C ₈ olefin in a fluidized bed reactor in the presence of nitrogen as a diluent for the reaction mixture. The obtained ethylene copolymer in countercurrent mode is contacted with a gas stream, including mainly ethylene and nitrogen, to obtain a copolymer having a low content of unreacted monomers. Said gas stream, comprising mainly ethylene and nitrogen, is formed upon cooling the reaction gas mixture, which causes partial condensation of unreacted C ₃ -C ₈ olefin comonomers and the simultaneous formation of a gas stream enriched in ethylene and nitrogen. The disadvantage associated with the use of a gas stream comprising the main part of ethylene as a gaseous leaching agent for degassing the polymer is the possibility of further reaction of the still active polymer powder present inside the degassing tank. If this is allowed to occur, this can lead to the formation of agglomerates in the degassing tank and to a change in the physical properties of the powder.

The description of patent application WO 03/011920 aims to solve the aforementioned problem by removing substantially all of the monomers, for example ethylene, from the gas stream before being used in the degassing tank. According to the instructions of this patent, the gas stream leaving the reactor, before being used as a leaching agent in the degassing tank, is passed to the first regeneration unit to remove heavy hydrocarbons, typically those containing from 5 to 8 carbon atoms, which, in particular, are used in the production of linear low density polyethylene (LLDPE). After the removal of heavy hydrocarbons, carried out, for example, by freezing, the gas stream is passed to an ethylene recovery unit so that the degassing stream entering the degassing tank is predominantly enriched with nitrogen and essentially does not contain ethylene and comonomers, thereby reducing the risk of further uncontrolled polymerization inside the degassing tank. The disadvantage of this method is that the gas is directly taken from the recycle line for the reaction gas leading to the reactor and returned to it after degassing. Therefore, the degassing cycle is highly dependent on the conditions, especially the pressure, in the reactor.

Patent EP 683176 relates to a process for the continuous production of ethylene (co) polymers by gas phase catalytic copolymerization in a process leading to a reduction in the amount of undesired volatile organic compounds in the obtained ethylene copolymers. After transferring the effluent from the reactor to the pressure relief zone to perform the primary separation of the solid phase from the gas phase, the solid phase is subjected to a multi-stage leaching process using (1) at least one non-deactivating washing with a gas mixture that essentially does not contain any catalytic poison for residues of the active catalyst, and then (2) decontamination washing with a gaseous mixture of nitrogen, water and oxygen.

The gas mixture used to carry out the aforementioned non-deactivating flushing (1) may be selected from nitrogen, a gaseous reaction mixture and one or more components of the said gaseous reaction mixture, preferably mixed with nitrogen. It is preferable to use a gas mixture circulating in the polymerization reactor. The gas mixture used to carry out the decontamination flushing (2) mainly contains nitrogen and very small amounts of water or oxygen. Water is used in an amount of 50 to 4000 ppm by weight relative to the copolymer being washed. Oxygen is used in an amount of 5 to 1000 ppm by weight relative to the copolymer being washed. Accordingly, due to such a scarce amount, water or oxygen essentially has a deactivating effect on the catalyst residues in the ethylene copolymer, while nitrogen is only a leaching agent to reduce the content of volatile organic compounds in the polyolefin. The process according to patent EP 683176 is a two-stage leaching, which requires the use of large quantities of nitrogen: this method is very expensive, since large quantities of nitrogen must be heated to a high temperature to ensure the effectiveness of leaching. In particular, the method described in this patent is particularly burdensome in terms of overhead for all gas phase polymerization processes carried out in the absence of N ₂ and using a saturated aliphatic hydrocarbon as an inert diluent of the polymerization mixture.

It would be desirable to overcome the disadvantages associated with the use of unreacted monomers and / or nitrogen in the degassing of the polymer, when high efficiency removal of oligomers and hydrocarbons from polyolefin granules is required.

The applicant believes that he has found a way to significantly reduce the spread of odor from polymer granules obtained by gas-phase polymerization, which is caused by the presence of a saturated aliphatic hydrocarbon.

Therefore, the aim of the present invention is a method for finishing polyolefins obtained by gas-phase catalytic polymerization of one or more α-olefins in the presence of a polymerization mixture diluent selected from a C ₃ -C ₅ alkane in which polyolefin granules discharged from a gas-phase reactor are processed :

(1) a first degassing step, wherein said polyolefin is in countercurrent contact with a gas stream containing at least 85 mol% of a washout reagent selected from a C ₃ -C ₅ alkane, said gas stream being continuously removed from the monomer regeneration section polymerization unit;

(2) a second degassing step, wherein said polyolefin is in countercurrent contact with water vapor, with an amount of water vapor ranging from 10 to 200 kg per 1000 kg of polyolefin.

The aim of the present invention is to achieve a very effective removal of oligomers and those heavy hydrocarbons present in the polyolefin granules removed from the gas phase polymerization reactor, and responsible for the distribution of odor. In particular, it is an object of the present invention to reduce the total weight concentration of oligomers and hydrocarbons heavier than C ₅ (i.e., with more than 5 carbon atoms) in a degassed polyolefin to less than 50 ppm by weight so that the polyolefin can be successfully used for the manufacture of plastic products used in the food and pharmaceutical industries, without causing unpleasant problems of the spread of bad smell.

Oligomers are organic compounds that are always present as a by-product of the polymerization reaction. The oligomer can also be defined as a polymer with a low molecular weight, in which the number of repeating structural units is approximately between 3 and 15.

The gas phase polymerization reactor can be any type of gas phase reactor, for example either a gas phase fluidized bed reactor or a gas phase stirred reactor. For the purposes of the method according to the invention, it is important that most of the reaction fluid is in a gaseous state and the polymer is in particulate form. In addition, a gas phase polymerization unit having two different interconnected polymerization zones can be successfully applied, as described in patents EP 782587 and EP 1012195.

Unloading from the reactor is carried out using a pressure gradient, as a rule, taking place between the reactor and the downstream degassing tank. The separation of polymer particles withdrawn from the reactor from the main part of the exhaust gas can be carried out together with the degassing of polymer particles or separately from it, either in terms of time or in terms of location. For example, the separation of solid particles from gas can be carried out in a first tank, and leaching using a gas stream containing at least 85 mol.% C ₃ -C ₅ alkane can be carried out in a subsequent second tank. Alternatively, in the case of periodic discharge, the bulk of the reaction gas may first be separated from the particles, and the degassing of the polymer may then be carried out in the same tank. However, the separation of the polymer from the main part of the exhaust gas and the degassing of the polymer are preferably carried out simultaneously and / or in the same place. Both of these process steps are preferably carried out immediately after unloading the polyolefin from the reactor in a single degassing tank.

The positive results of the present invention in terms of reducing the spread of odor are achieved by subjecting the polyolefin granules discharged from the gas phase polymerization reactor to a two-stage washing process using two different washing agents. The first leaching reagent is selected from a C ₃ -C ₅ alkane, while the second one is water vapor.

The first degassing step (1) according to the invention is carried out in a first degassing vessel downstream of the polymerization reactor: polyolefin granules are introduced into the upper part of the said tank and fall down by gravity, in countercurrent contact with the gas stream introduced into the bottom of the tank for degassing. Said gas stream contains at least 85 mol%, preferably more than 95 mol%, of a leaching agent selected from a C ₃ -C ₅ alkane or a mixture thereof. Preferably, said leaching agent is the same C ₃ -C ₅ alkane that is used as a diluent in the polymerization mixture in the gas phase polymerization reactor.

To ensure the amount of the named C ₃ -C ₅ alkane sufficient to degass the polyolefin, the content of the named alkane in the reaction gas should be 30 mol% or more. Alternatively, an additional alkane replenishment line may be provided upstream of the degassing tank, but this is less useful.

The use of a C ₃ -C ₅ alkane instead of nitrogen as a diluent for the polymerization mixture is particularly favorable. When nitrogen is completely replaced by an aliphatic hydrocarbon, the specific heat of the recycle gas is maximized, as a result of which the volumetric flow rate of the recycle gas needed to remove the heat of polymerization can be minimized. Accordingly, it is possible to manufacture peripheral equipment, such as compressors, heat exchangers, a recirculation gas pipeline, etc., with reduced dimensions, or an increase in polymer production. The removal of heat from a gas-phase reactor can be carried out by circulating the reaction gas in a non-condensing mode. It may also be useful to partially condense the reaction gas and recycle the condensate together with the non-condensed gas so that the polymerization reactor operates in a condensation mode.

Preferably, propane is used as the diluent of the polymerization mixture and the leaching agent in the process of the present invention, especially in the case of (co) polymerization of ethylene.

As a result of leaching acting in a gas stream containing C ₃ -C ₅ alkane, the degassed polyolefin is continuously withdrawn from the bottom of the degassing tank, and the total content of oligomers and hydrocarbons heavier than the selected leaching reagent is reduced to at least 2000 parts per million by weight.

From the upper part of the degassing tank, a gas mixture is obtained enriched with unreacted monomers, comonomers, oligomers and heavy hydrocarbons. The said gas mixture, blown from the upper part of the degassing tank, is sent to the regeneration section of the monomer of the polymerization unit to separate the gas stream containing at least 85 mol% of the selected C ₃ -C ₅ alkane. This means that, unlike some prototype methods, the gas stream used as a leaching reagent in the present invention is not taken from the recycle gas pipe of the gas-phase reactor, but, on the contrary, is continuously obtained as a stream withdrawn and separated from the monomer regeneration section in the polymerization unit. Said gas stream containing at least 85 mol% of a C ₃ -C ₅ alkane can be obtained by a series of successive purification steps carried out in order to separate said C ₃ -C ₅ alkane from the main olefin monomer, optional comonomers, oligomers and all other saturated aliphatic hydrocarbons.

In the monomer regeneration section, commonly used separation methods are used to separate the C ₃ -C ₅ alkane. Examples of separation methods that can be used are separation by distillation or by selective permeability membranes, without limiting the scope of the invention. Rather, it is possible to use any separation unit to achieve the advantages of the method according to the invention, provided that the purity according to the invention (higher than 85 mol%) of C ₃ -C ₅ alkane in the leaching stream of stage (1) can be achieved. Separation by distillation is preferred.

Both unreacted monomers and saturated aliphatic hydrocarbons with more than ₃ carbon atoms in the selected C ₃ -C ₅ alkane can be removed using a distillation column, a suitable permeable membrane, PSA, freeze-drying, or other suitable separation methods. Among these components, in particular 1-hexene may be present in the gas mixture discharged from the upper part of the degassing tank. If the aforementioned monomers and hydrocarbons are removed by freezing, they can be returned in liquid form to the recycle gas pipe of the polymerization reactor using a pump. If the above monomers and hydrocarbons are removed using a permeable membrane, these compounds are returned as a gas stream to the recycle gas pipe of the polymerization reactor.

Unreacted monomers and saturated aliphatic hydrocarbons with fewer carbon atoms than the selected C ₃ -C ₅ alkane, such as ethylene, ethane and / or propylene, can be removed from the gas mixture blown from the top of the degassing tank using separation methods similar to those used for heavy hydrocarbons, i.e. using a suitable membrane, distillation column, etc.

According to a preferred embodiment of the invention, in the first separation unit arranged as a distillation column, a low boiling fraction containing saturated and unsaturated hydrocarbons with the number of carbon atoms C _n less than that for the selected C ₃ -C ₅ alkane is separated and sent from the top of the said distillation columns into the pipeline for the recirculation gas of the reactor.

The high boiling fraction from the bottom of the first separation column is enriched in the selected C ₃ -C ₅ alkane, oligomers, unreacted monomers and hydrocarbons with a carbon number C _n greater than that of the selected C ₃ -C ₅ alkane. This high boiling fraction is fed to a second separation unit arranged in the form of a distillation column. The said second distillation column operates in such a way that a gas mixture containing at least 85 mol% of the selected C ₃ -C ₅ alkane is formed in the upper part of the column, while increasingly heavier hydrocarbons are located in the bottom of the column and can be removed from there. The gas stream exiting from the upper part of the said second distillation column is continuously fed into the first stage of leaching (1) according to the invention to the bottom of the degassing tank.

In an alternative embodiment, the operation of the distillation columns may be reversed. The first column operates in such a way that the high boiling fraction containing 1-olefins and saturated hydrocarbons with a C _n number higher than that of the selected C ₃ -C ₅ alkane is separated as a bottom product of the column. On the other hand, a low boiling fraction containing the selected C ₃ -C ₅ alkane and all lower hydrocarbons can be obtained at the top of the column. As a result of this, the low boiling fraction can be fed into the second separation column for further separation of the washing reagent (C ₃ -C ₅ alkane) from hydrogen and lower hydrocarbons such as ethylene, ethane.

Since the degassing cycle is part of the monomer recovery section and is not directly connected to the recycle gas pipe of the polymerization reactor, the pressure in the degassing cycle can be adjusted to higher values than the pressure existing in the polymerization reactor. This is another advantage of the method according to the present invention.

The temperature in the first degassing step (1) can be adjusted from 60 to 120 ° C., while the pressure can vary from 0.1 to 0.4 MPa, preferably from 0.15 to 0.35 MPa.

The residence time of the polymer in the first degassing tank is from 10 minutes to 6 hours, preferably from 60 minutes to 3 hours.

In the first stage of degassing, the polyolefin is freed from most of the gas trapped by it under the action of a leaching reagent, a C ₃ -C ₅ alkane. As mentioned, the presence in the polyolefin of oligomers, saturated and unsaturated hydrocarbons with more than 5 carbon atoms, for example, such as 1-hexene and hexane, is responsible for the spread of bad smell. At the exit from the first degassing stage, the total content of these components adsorbed on the polymer particles decreases to less than 2000 ppm by weight. Of course, the C ₃ -C ₅ alkane used as the leaching agent remains adsorbed on the polymer particles.

The degassed polymer is then removed from the degassing tank using rotary feeders and transported to the second degassing stage (2) according to the present invention, wherein the polyolefin is in countercurrent contact with water vapor acting as a leaching agent. Polyolefin granules are introduced into the upper part of the second degassing tank and fall down under the action of gravity, contacting in countercurrent with a stream of water vapor introduced into the bottom of the degassing tank.

Water vapor is used for this purpose in an amount of 10 to 200 kg per 1000 kg of the processed polyolefin. This relatively large amount indicates that water vapor performs not only the function of deactivating the catalyst components, in particular organoaluminum compounds of the catalytic system, but also the function of removing adsorbed propane from polyolefin granules and, above all, those odorous components, i.e. oligomers and heavy hydrocarbons not completely removed and washed during the first stage of degassing.

Water vapor is introduced into the bottom of the degassing tank and is discharged from the top of the tank, being enriched in propane, oligomers and heavy organic compounds. The temperature in the second stage of degassing is in the range from 80 to 120 ° C, preferably 95-110 ° C. The pressure may vary from 0.1 to 0.35 MPa, preferably from 0.12 to 0.25 MPa.

The residence time of the polymer in the second degassing step is from 10 minutes to 4 hours, preferably from 90 minutes to 3 hours.

At the outlet of the second degassing tank, the total amount of oligomers and hydrocarbons adsorbed on the polymer particles is reduced to less than 50 ppm, preferably less than 20 ppm by weight.

The water vapor partially condenses upon contact with the polyolefin powder, so that wet polyolefin particles are discharged from the bottom of the second degassing tank. As a consequence, a drying step must be carried out downstream of the degassing step in order to remove water from the polyolefin particles. Hot nitrogen is preferably used to remove water from the polymer.

According to a preferred embodiment, the polymer powder is dried first in the pipeline connecting the second degassing tank to the storage hopper, and then inside the named storage hopper using hot nitrogen in a closed loop. A stream of hot nitrogen at a temperature in the range from 60 to 120 ° C is used to transport polymer particles from the second degassing tank to the aforementioned storage bin, thereby conducting the first preliminary drying of the polymer. The polymer drying is then effectively completed by contacting the polyolefin in countercurrent with hot nitrogen inside the storage hopper. Hot pure nitrogen is supplied to the bottom of the storage bunker, and a stream of nitrogen enriched in water vapor is discharged from the top of the bunker. The aforementioned polluted nitrogen stream is compressed and subsequently cooled to condense the water, and after the steam condensate has been separated in the liquid separator, the purified nitrogen is heated in the heat exchanger to the proper temperature and reintroduced into the bottom of the storage hopper, thereby forming a closed loop hot nitrogen. A portion of this purified and heated nitrogen is used as a carrier gas for transporting the polyolefin discharged at that moment from the second degassing tank.

The dried polyolefin, unloaded from the storage bunker, is ready for subsequent extrusion for the manufacture of products suitable for use in the food and pharmaceutical industries, due to the extremely low content of organic compounds responsible for the distribution of bad smell.

According to the invention, the measurement of the content of oligomers and hydrocarbons in a degassed polyolefin is carried out using the extraction method “static headspace” (static method for the analysis of equilibrium vapor). Since 1-hexene is one of the nasty odorless organic compounds adsorbed onto polymer particles, the process of the present invention is particularly advantageous for use in the copolymerization of ethylene with other 1-olefins, in particular 1-hexene, in which ethylene is subjected copolymerization in the presence of a catalyst in a gas phase reactor with the formation of polymer particles, and polymerization is carried out in the presence of propane as an inert diluent of poly merizational mixture.

Therefore, according to a preferred embodiment of the invention, a method is provided for finishing the polyolefins obtained by gas-phase catalytic polymerization of ethylene with further 1-olefins in the presence of propane as a diluent of the polymerization mixture, in which the polyolefin granules discharged from the gas-phase reactor are processed for:

(1) a first degassing step in which said polyolefin is in countercurrent contact with a gas stream containing at least 85 mol% of propane as a leaching agent, said gas stream being continuously removed from the monomer regeneration section of the polymerization unit;

(2) a second degassing step, wherein said polyolefin is in countercurrent contact with water vapor, with an amount of water vapor ranging from 10 to 200 kg per 1000 kg of polyolefin.

The invention is illustrated below by way of example drawings, showing a gas phase fluidized bed reactor used to copolymerize ethylene with 1-hexene, without limiting it to the invention.

Figure 1 shows a block diagram of a polymerization plant according to a preferred embodiment of the invention, comprising a gas phase reactor 1 with a fluidized bed, where the fluidized bed of polymer particles is maintained in suspension in the gas phase using an upward gas stream. The velocity of the ascending reaction gas should be high enough to liquefy the mixed layer of polymer particles and, secondly, for the efficient removal of the heat of polymerization.

The gas distribution partition is located in the bottom of the fluidized polymer bed. At the top, the cylindrical part of the reactor usually adjoins the expanded separation zone, which reduces the removal of solid particles from reactor 1. The reaction gas leaves the reactor 1 at the upper end of the separation zone and is transferred to the recirculation gas pipeline 2 through a cyclone 3 designed to separate fine particles entrained in the gas . The recycle gas is transported through line 2 to a compressor 4 and then to a heat exchanger 5, in which the recycle gas is cooled when returned to the reactor 1. The recycle gas, if appropriate, can be cooled to a temperature below the dew point of the gas mixture in the heat exchanger 5 so that the reactor 1 with a fluidized bed worked in condensation mode.

When the unit is used for ethylene polymerization, the circulating reaction gas (recycle gas) comprises a mixture of ethylene, if desired, a molecular weight regulator, such as hydrogen, propane and / or other saturated hydrocarbons, such as ethane, butane, isopentane. In addition, depending on the catalyst, additional additives and auxiliary components, such as metal alkyls, antistatic agents, catalytic poisons, etc. can also be used. Moreover, the reaction gas may further include C ₃ -C ₈ -1-olefins, such as propylene, 1-butene, 1-pentene, 2-methylpentene, 1-hexene, 1-heptene and 1-octene, as comonomers. Preference is given to a process in which ethylene is copolymerized with 1-hexene or 1-butene. To ensure constant reaction conditions, the components of the reaction gas can be fed into the gas phase reactor with a fluidized bed either directly or by means of a circulating reaction gas. In figure 1, fresh monomers, comonomers and propane as an inert diluent of the polymerization mixture are fed into the fluidized bed reactor 1 through one or more feed lines 6, which are combined into a recirculation gas line 2.

The performance of the gas-phase reactor 1 with a fluidized bed, as is known, is limited by the cooling ability of the circulating reaction gas. The cooling ability depends not only on the heat capacity, i.e. the chemical composition of the reaction gas, but also, among other things, on the pressure of the reaction gas or on the pressure at which (co) polymerization is carried out. It is generally recommended here to operate at pressures from 0.1 to 10 MPa, preferably from 1 to 8 MPa, particularly preferably from 1.5 to 5 MPa. The cooling ability also depends on the temperature of the reactor at which (co) polymerization is carried out in the fluidized bed. For the purposes of the method according to the invention, it is preferable to operate at temperatures from 50 to 125 ° C, preferably from 70 to 120 ° C, and the temperatures in the upper part of this range are preferably set for copolymers with a relatively high density, and the temperatures in the lower part of this range are preferably controlled for copolymers with a relatively low density.

The catalyst or catalysts are introduced (introduced) into the fluidized bed of the polymer through one or more metering devices 7 for the catalysts. As catalysts, it is possible to use all known types of catalyst that are suitable for the polymerization of olefins. Particular mention may be made of Ziegler catalysts, Philips chromium-based catalysts and “single-site” catalysts, in particular metallocene catalysts, but without limitation. For the purposes of the present invention, single site catalysts include, in contrast to chromium and Ziegler-Natta catalysts, at least one specific transition metalorganic compound and usually other activating compounds, and, if appropriate, a carrier and also other additives and auxiliary components. The catalysts may be co-immobilized on a support or may be metered individually into the reactor.

Depending on the catalyst used, the polymer particles typically have an average size of from several hundred to several thousand microns (μm). In the case of chromium catalysts, the particles have an average size of from about 400 to 600 microns, and in the case of Ziegler catalysts, the average particle size is about 900-3000 microns.

The polymer particles are discharged from the reactor 1 through one or more discharge lines 8. The product can be discharged continuously or intermittently, preferably continuously discharging. According to the first stage of the method according to the invention, the withdrawn polymer powder is introduced into the upper part of the degassing tank 9 and falls down under the action of gravity, in countercurrent contact with the gas stream introduced into the bottom of the degassing tank 9. According to the invention, said gas stream contains at least 85 mol.% C ₃ -C ₅ alkane. In the preferred embodiment shown in FIG. 1, said alkane is propane, which is also used as a diluent for the polymerization mixture in the gas phase reactor 1 with a fluidized bed.

The polymer is freed from the bulk of the gas adsorbed on it in the degassing tank 9. Degassing using a gas stream containing at least 85% propane reduces the residual content of 1-olefins, such as 1-hexene, and relatively non-volatile inert gases, for example, hexane adsorbed on polymer particles. Degassed polymer particles are then removed from the degassing tank 9 using rotary feeders 10 and transported to the second degassing stage according to the invention, carried out using water vapor.

To prevent water vapor from escaping back from the second degassing tank 11 to the degassing tank 9, a subsequent conduit 16b may be arranged between the two rotary feeders 10, supplying a gas stream containing propane, thereby acting as a barrier gas.

Gas that has been freed from polymer particles in a degassing tank 9, which typically contains significant amounts of entrained fine dust, is passed through a recirculation gas filter 12, in which the gas is almost completely freed from fine dust. The purified gas is cooled in the heat exchanger 13 and compressed in the compressor 14 before being subjected to two-stage separation by distillation.

In a first separation unit arranged in the form of a separation column 15, a low boiling fraction containing ethylene, preferably consisting essentially of ethylene, ethane, propane and possibly other low boiling components of the recycle gas, such as hydrogen, is separated and sent from the top of the column 15 through gas return line 17a to the recirculation gas line 2. This ensures that ethylene losses through the discharge system are minimized, since ethylene is returned to the polymerization con ur.

To avoid the accumulation of ethane in the loop that is present in the fresh ethylene stream and / or is formed as a by-product during polymerization, ethane is vented through the ethane removal line 17b. The output ethane can be fed directly as feedstock for the cracking unit. Together with ethane, unwanted by-products that otherwise accumulate in the circuit can be further removed.

Bottom products from the first separation column 15, which are enriched in relatively high boiling hydrocarbons, consisting essentially of propane, other 1-olefins and saturated hydrocarbons, are fed through a connecting pipe 18 to a second separation unit configured as a separation column 19. Essentially pure propane is obtained at at the top of this second separation column 19, while increasingly heavier hydrocarbons are collected at the bottom. The column is designed so that the content of higher hydrocarbons, such as 1-hexene, 1-butene or n-hexane, is preferably less than 0.1 mol.%, And the ethylene content is less than 1 mol.%.

In an alternative embodiment not shown in FIG. 1, the separation columns can be interchanged. In this embodiment, a high boiling fraction containing other 1-olefins and alkanes having from 4 to 12 carbon atoms is separated in the first stage, and then the upper product (distillate) is separated into a pure propane fraction and a low boiling fraction containing ethylene and ethane.

Column design is generally known to those skilled in the art. In particular, the amount of propane produced can be adjusted in a simple way to adjust the capacity of the evaporator of the second separation column or using a valve placed between the second distillation column 19 and the degassing tank 9. The propane purity with respect to ethylene can be adjusted by adjusting the capacity of the evaporator of the first column, while the content of higher hydrocarbons in propane is adjusted by adjusting the reflux ratio of the second separation column 19. The temperature of the bottom product and in the second separation column 19 should be below 120 ° C, to provide the use of low pressure steam as heating medium and to avoid the continuation of the reaction with 1-olefins.

Propane that has been purified this way is sent through a propane line 20 to a heat exchanger 21, heated to a temperature that is lower than the softening temperature of the polymer by 5 to 20 ° C, preferably to about 100 ° C, and then fed to the degassing tank 9 through conduit 16a. In the degassing tank 9, primary degassing of the polymer particles is carried out and the leaching circuit is closed.

The low hexene content in the propane-containing gas stream degasses the polymer to a low residual hexene content, thereby reducing the risk of odor spreading from the degassed ethylene copolymer. The low ethylene content in the gas stream containing propane prevents any significant subsequent polymerization and thereby the formation of oligomers (gels). Such gels greatly degrade the quality of especially film products.

The use of propane used to remove the heat of polymerization as an inert gas for degassing the polymer also makes it possible to achieve a high degree of regeneration of both ethylene and a degassing agent, which is advantageous both from an economic point of view and for environmental reasons. Moreover, propane has a significantly higher heat capacity than nitrogen, which enhances heat transfer.

Bottom products from the second separation column 19, enriched with 1-hexene and any hexane, are sent (s) through line 22 to the recirculation gas line 2 and are thus recycled back to reactor 1. Small losses of propane that occur despite the application of the method according to the invention, are compensated by introducing the proper quantities of fresh propane into the recycle gas line 2 through line 6. Alternatively, especially when using a Ziegler catalyst, the propane source and may be the same as that used to prepare the catalyst and / or to supply the catalyst.

Fine dust, separated from the reaction gas in cyclone 3, is mainly added to the product in the degassing tank 9 or, alternatively, is fed back to the reactor 1.

The polyolefin granules removed from the degassing tank 9 using rotary feeders 10 are sent to a second degassing tank 11, in which the polyolefin particles fall down by gravity, in countercurrent contact with a stream of water vapor introduced through a pipe 22 into the bottom of the tank for degassing 11.

Water vapor enriched in propane, oligomers and heavy organic compounds is discharged through line 24 from the top of the degassing tank 11. Water vapor can be condensed by cooling, thereby forming a hydrocarbon stream that can be disposed of as off-gas.

Water vapor partially condenses upon contact with the polyolefin granules, so that a wet polyolefin is discharged from the bottom of the second degassing tank 11 through a conduit 25.

The hot nitrogen stream exiting the pipe 26 is used to transport the polymer powder through a pipe 25 connecting the second degassing tank 11 to the storage hopper 27, thereby conducting the first coarse drying of the polyolefin. In the storage bunker 27, the polymer is in countercurrent contact with a stream of hot nitrogen that is introduced through a pipe 28 in the bottom of the storage bunker 27. From the upper part of the storage bunker 27, a nitrogen stream enriched in water vapor is obtained, which is compressed in the compressor 29 and then cooling in heat exchanger 30 for condensing water vapor. Condensed water vapor is separated in the liquid precipitator 31, while the purified nitrogen is heated in the heat exchanger 32 to the proper temperature and again reintroduced through the pipe 28 into the bottom of the storage hopper 27, thereby forming a closed loop of hot nitrogen. Part of the purified nitrogen is fed into the pipeline 26 for entry into the exhaust zone of the second degassing tank 11. The dried polyolefin discharged from the storage hopper 27 is then extruded.

It can be emphasized that the specific embodiment described in FIG. 1 is intended only to illustrate the invention and is not a limitation. In particular, the method according to the invention for deodorizing polyolefin granules can also be successfully used not only for the polymerization of ethylene, but also for gas-phase polymerization of other alpha-olefins, while the leaching reagent of the first degassing step is selected from C ₃ -C ₅ alkane and water vapor used as a leaching agent in the second stage of degassing.

The following examples will further illustrate the present invention without limiting its scope.

EXAMPLES

Characterization

Melt Index E (MIE): According to ASTM-D 1238 (190 ° C / 2.16 kg)

Density (without annealing): according to ASTM-D 792

Measurement of the content of volatile organic compounds in the obtained (co) polymers : performed according to the extraction method “static headspace” (static method for the analysis of equilibrium vapor).

This method consists in heating a sample of (co) polymer weighing 4 g in an oven to a temperature of 130 ° C for 60 minutes. A portion of the gas phase released from the polymer sample is then injected into an apparatus compiled by a gas chromatograph and mass spectrometer (GC-MS), Agilent 6890-5973N, to detect various organic compounds.

The measurement results of volatile organic compounds responsible for the distribution of a bad smell, that is, organic compounds containing more than 5 carbon atoms, are expressed in micrograms per gram of (co) polymer, that is, in parts per million (ppm) by weight.

EXAMPLE 1

Polymerization Conditions

The preparation of linear low density polyethylene (LLDPE) using 1-hexene as a comonomer is carried out in a fluidized bed reactor, as shown in FIG.

As the polymerization catalyst, a Ziegler-Natta catalyst is used, including:

a titanium solid catalyst component prepared according to the procedure described in patent application WO 04/106388, Example 1, according to which ethyl acetate is used as the internal donor compound;

- a mixture of triisobutylaluminum (TIBAL) and diethylaluminum chloride (DEAC) in a weight ratio of 7: 1, used as a cocatalyst;

- tetrahydrofuran as an external donor.

The above solid catalyst component is fed into the catalyst activation tank, together with the cocatalyst and an external donor, with a weight ratio of cocatalyst / solid component of 10, a weight ratio of cocatalyst / external donor of 7. The above catalyst components are kept in propane for preliminary interaction at a temperature of 50 ° C for 60 minutes.

After extraction from the activation tank, the activated catalyst is fed into a fluidized bed reactor (number 1 in FIG. 1) for copolymerization of ethylene with 1-hexene in the presence of propane as a diluent for the polymerization mixture. Hydrogen is used as a molecular weight regulator.

The polymerization of the mixture "ethylene / 1-hexene" is carried out at a temperature of 85 ° C and a pressure of 2.5 MPa. The gas reaction mixture has the composition: 30 mol.% Ethylene, 5 mol.% 1-hexene, 5 mol.% Hydrogen and 60 mol.% Propane.

The resulting LLDPE copolymer exhibits a density of 0.919 g / cm ³ and a MIE melt index of 1.0 g / 10 min. The average diameter of the polymer particles is about 1200 microns

The first stage of degassing

The linear low density polyethylene (LLDPE) powder is continuously withdrawn from the fluidized bed reactor with a capacity of 50 kg / h and transported to the top of the first degassing tank (number 9 in FIG. 1).

A gas stream containing 99 mol% of propane and 1 mol% of ethylene is continuously introduced into the bottom of said degassing tank. The polymer powder falls down under the action of gravity along the degassing tank, thereby contacting in countercurrent with a gas stream containing 99 mol.% Propane.

Propane enriched in oligomers and heavy hydrocarbons is obtained in the upper part of the first degassing tank. The said gas mixture containing propane, oligomers and heavy hydrocarbons is then distilled in two successive distillation columns (numbers 15 and 19 in FIG. 1) according to the embodiment of FIG. 1, to obtain the aforementioned stream containing 99 mol% of propane, which is continuously introduced into the lower part of the first degassing tank.

The first degassing stage operates at a temperature of 75 ° C and a pressure of 0.25 MPa. The average residence time of the powder in the first degassing tank is 120 minutes.

The first degassing stage performs the first substantial removal of 1-hexene and all hydrocarbons with more than 4 carbon atoms adsorbed on the polymer particles. The polymer particles are then removed from the first degassing tank and sent to the second degassing stage according to the present invention.

The second stage of degassing

The polyolefin granules withdrawn from the first degassing tank are transported to a second degassing tank (number 11 in FIG. 1), in which the polyolefin particles fall down by gravity, in countercurrent contact with a stream of water vapor introduced into the bottom of the second tank for degassing.

The total flow rate of water vapor supplied to the second degassing stage is 8 kg / h, which corresponds to 160 kg of water vapor per 1000 kg of the processed polyolefin. The second degassing stage operates at a temperature of 102 ° C with a residence time of the polymer powder of 90 minutes.

Water vapor enriched in propane, oligomers and heavy organic compounds is removed from the top of the second degassing tank.

On the other hand, the water vapor partially condenses upon contact with the polyolefin granules, so that a wet polyolefin is removed from the bottom of the second degassing tank. After unloading from the second degassing tank, the linear low density polyethylene (LLDPE) powder is dried using a stream of hot nitrogen in a special storage bin (number 27 in FIG. 1). Inside the said storage bunker, the polymer is in countercurrent contact with a stream of hot nitrogen at a temperature of 80 ° C. From the top of the storage hopper, a stream of nitrogen enriched in water vapor is obtained, which is compressed and then cooled to condense and separate the water vapor. The purified nitrogen is heated in a heat exchanger and again reintroduced into the bottom of the drying storage bunker, thereby forming a closed loop of hot nitrogen.

A sample of dried linear low density polyethylene (LLDPE) powder removed from the drying storage bin is analyzed by the extraction method “static headspace” (static equilibrium vapor analysis method) to measure the residual content of volatile organic compounds adsorbed on polymer particles.

The attached Table 1 shows, in parts per million (ppm) by weight per gram of polymer, the residual amounts of organic compounds (oligomers and hydrocarbons) with more than 5 carbon atoms adsorbed on the polymer particles. In Table 1, the Σ C6 symbol indicates all C6 hydrocarbons, i.e. saturated and unsaturated, isomeric and normal C6 hydrocarbons; i-C8 is iso-C8 hydrocarbons, and n-C8 is normal C8 hydrocarbons.

As confirmed by the data in Table 1, the finishing method according to the invention allows to reduce the total concentration by weight of oligomers and hydrocarbons heavier than C ₅ (i.e., with more than 5 carbon atoms) in the degassed polyolefin to less than 50 ppm by weight: as a result, the obtained powder of linear low density polyethylene (LLDPE) can be successfully used for the manufacture of plastic products for the food and pharmaceutical industries, without causing unpleasant problems The prevalence of bad smell.

EXAMPLE 2

Polymerization Conditions

Obtaining medium density polyethylene (MDPE) using 1-hexene as a comonomer is carried out in a fluidized bed reactor, as shown in FIG.

As the polymerization catalyst, a Ziegler-Natta catalyst is used, including:

a titanium solid catalyst component prepared according to the procedure described in patent application WO 01/85803, Example 7;

- a mixture of triisobutylaluminum (TIBAL) and diethylaluminum chloride (DEAC) in a weight ratio of 7: 1, as a cocatalyst.

The above solid catalyst component is fed into the catalyst activation tank, together with the cocatalyst, at a weight ratio of cocatalyst / solid component of 10. The above catalyst components are kept in propane for preliminary reaction at a temperature of 20 ° C. for 65 minutes.

After extraction from the activation tank, the activated catalyst is fed into a fluidized bed reactor (number 1 in FIG. 1) for copolymerization of ethylene with 1-hexene in the presence of propane as a diluent for the polymerization mixture. Hydrogen is used as a molecular weight regulator.

The polymerization of the mixture "ethylene / 1-hexene" is carried out at a temperature of 80 ° C and a pressure of 2.5 MPa. The gas reaction mixture has the composition: 17 mol.% Ethylene, 3 mol.% 1-hexene, 3 mol.% Hydrogen and 77 mol.% Propane.

The resulting MDPE copolymer exhibits a density of 0.933 g / cm ³ and a MIE melt index of 0.4 g / 10 min. The average diameter of the polymer particles is about 1300 microns.

The first stage of degassing

Powder medium density polyethylene (MDPE) is continuously withdrawn from a fluidized bed reactor with a capacity of 60 kg / h and transported to the top of the first degassing tank (number 9 in FIG. 1).

A gas stream containing 97 mol.% Propane and 3 mol.% Ethylene is continuously introduced into the bottom of said degassing tank. The polymer powder falls down under the action of gravity along the degassing tank, thereby contacting in countercurrent with a gas stream containing propane.

Propane enriched in oligomers and heavy hydrocarbons is obtained in the upper part of the first degassing tank. Said gas mixture containing propane, oligomers and heavy hydrocarbons is then distilled in two successive distillation columns (numbers 15 and 19 in FIG. 1) according to the embodiment of FIG. 1, to obtain the aforementioned stream containing 97 mol% of propane, which is continuously introduced into the lower part of the first degassing tank.

The first degassing stage operates at a temperature of 75 ° C and a pressure of 0.25 MPa. The average residence time of the powder in the first degassing tank is 100 minutes.

The first degassing stage performs the first substantial removal of 1-hexene and all hydrocarbons with more than 4 carbon atoms adsorbed on the polymer particles. The polymer particles are then removed from the first degassing tank and sent to the second degassing stage according to the present invention.

The second stage of degassing

The polyolefin granules withdrawn from the first degassing tank are transported to a second degassing tank (number 11 in FIG. 1), in which the polyolefin particles fall down by gravity, in countercurrent contact with a stream of water vapor introduced into the bottom of the second tank for degassing.

The total flow rate of water vapor supplied to the second degassing stage is 5 kg / h, which corresponds to 83 kg of water vapor per 1000 kg of the processed polyolefin. The second degassing stage operates at a temperature of 103 ° C with a residence time of the polymer powder of 75 minutes.

Water vapor enriched in propane, oligomers and heavy organic compounds is removed from the top of the second degassing tank.

On the other hand, the water vapor partially condenses upon contact with the polyolefin granules, so that a wet polyolefin is removed from the bottom of the second degassing tank. After unloading from the second degassing tank, the medium density polyethylene (MDPE) powder is dried using a stream of hot nitrogen in a special storage bin (number 27 in FIG. 1). Inside the said storage bunker, the polymer is in countercurrent contact with a stream of hot nitrogen at a temperature of 80 ° C. From the top of the storage hopper, a stream of nitrogen enriched in water vapor is obtained, which is compressed and then cooled to condense and separate the water vapor. The purified nitrogen is heated in a heat exchanger and again reintroduced into the bottom of the drying storage bin, thereby forming a closed loop of hot nitrogen.

A sample of dried medium density polyethylene (MDPE) powder discharged from the drying storage bin is analyzed by the extraction method “static headspace” (static equilibrium vapor analysis method) to measure the residual content of volatile organic compounds adsorbed on the polymer particles.

The data in Table 1 show that the total concentration by weight of oligomers and hydrocarbons heavier than C ₅ in degassed medium density polyethylene (MDPE) is less than 50 ppm by weight.

EXAMPLE 3

Polymerization Conditions

The preparation of linear low density polyethylene (LLDPE) using 1-butene as a comonomer is carried out in a fluidized bed reactor, as shown in FIG.

As the polymerization catalyst, a Ziegler-Natta catalyst is used, including:

a titanium solid catalyst component prepared according to the procedure described in patent application WO 04/106388, Example 1, according to which ethyl acetate is used as the internal donor compound;

- a mixture of triisobutylaluminum (TIBAL) and diethylaluminum chloride (DEAC) in a weight ratio of 7: 1, used as a cocatalyst;

- tetrahydrofuran as an external donor.

The above solid catalyst component is fed into the catalyst activation tank, together with the cocatalyst and an external donor, with a weight ratio of cocatalyst / solid component of 10, a weight ratio of cocatalyst / external donor of 7. The above catalyst components are kept in propane for preliminary interaction at a temperature of 50 ° C for 60 minutes.

After extraction from the activation tank, the activated catalyst is fed into a fluidized bed reactor (number 1 in FIG. 1) for copolymerization of ethylene with 1-butene in the presence of propane as a diluent for the polymerization mixture. Hydrogen is used as a molecular weight regulator.

The polymerization of a mixture of "ethylene / 1-butene" is carried out at a temperature of 80 ° C and a pressure of 2.5 MPa. The gas reaction mixture has the composition: 35 mol.% Ethylene, 16 mol.% 1-butene, 7 mol.% Hydrogen and 42 mol.% Propane.

The resulting LLDPE copolymer exhibits a density of 0.920 g / cm ³ and a MIE melt index of 0.94 g / 10 min. The average diameter of the polymer particles is about 990 microns.

The first stage of degassing

The linear low density polyethylene (LLDPE) powder is continuously withdrawn from the 55 kg / h fluidized bed reactor and transported to the top of the first degassing tank (number 9 in FIG. 1).

A gas stream containing 96 mol.% Propane and 4 mol.% Ethylene is continuously introduced into the bottom of the named degassing tank. The polymer powder falls down due to gravity along the degassing tank, thereby contacting in countercurrent with a gas stream containing 96 mol.% Propane.

Propane enriched in oligomers and heavy hydrocarbons is obtained in the upper part of the first degassing tank. The named gas mixture containing propane, oligomers and heavy hydrocarbons is then distilled in two successive distillation columns (numbers 15 and 19 in FIG. 1) according to the embodiment of FIG. 1, to obtain the aforementioned stream containing 96 mol.% Propane, which is continuously introduced into the lower part of the first degassing tank.

The first degassing stage operates at a temperature of 70 ° C and a pressure of 0.26 MPa. The average residence time of the powder in the first degassing tank is 110 minutes.

The first degassing stage performs the first substantial removal of 1-butene and hydrocarbons with more than 4 carbon atoms adsorbed on polymer particles. The polymer particles are then removed from the first degassing tank and sent to the second degassing stage according to the present invention.

The second stage of degassing

The polyolefin granules withdrawn from the first degassing tank are transported to a second degassing tank (number 11 in FIG. 1), in which the polyolefin particles fall down by gravity, in countercurrent contact with a stream of water vapor introduced into the bottom of the second tank for degassing.

The total flow rate of water vapor supplied to the second stage of degassing is 8 kg / h, which corresponds to 145 kg of water vapor per 1000 kg of the processed polyolefin. The second degassing stage operates at a temperature of 101 ° C with a residence time of the polymer powder of 80 minutes.

Water vapor enriched in propane, oligomers and heavy organic compounds is removed from the top of the second degassing tank.

On the other hand, the water vapor partially condenses upon contact with the polyolefin granules, so that a wet polyolefin is removed from the bottom of the second degassing tank. After unloading from the second degassing tank, the linear low density polyethylene (LLDPE) powder is dried using a stream of hot nitrogen in a special storage bin (number 27 in Figure 1). Inside the said storage bunker, the polymer is in countercurrent contact with a stream of hot nitrogen at a temperature of 80 ° C. From the top of the storage hopper, a stream of nitrogen enriched in water vapor is obtained, which is compressed and then cooled to condense and separate the water vapor. The purified nitrogen is heated in a heat exchanger and again reintroduced into the bottom of the drying storage bunker, thereby forming a closed loop of hot nitrogen.

A sample of dried linear low density polyethylene (LLDPE) powder discharged from a drying storage bin is analyzed by the extraction method “static headspace” (static equilibrium vapor analysis method) to measure the residual content of volatile organic compounds adsorbed on polymer particles.

The data in Table 1 show that the total weight concentration of oligomers and hydrocarbons heavier than C ₅ in degassed linear low density polyethylene (LLDPE) is less than 50 ppm by weight.

Example 4 (Comparative)

Linear low density polyethylene (LLDPE) (density 0.919 g / cm ³ , melt index MIE 1.0 g / 10 min, average diameter 1200 μm), obtained as described in detail in Example 1, is subjected to the first stage degassing with technological conditions according to the invention and a second degassing step in which nitrogen is used instead of water vapor as a leaching agent.

The first stage of degassing

The linear low density polyethylene (LLDPE) powder from Example 1 is continuously withdrawn from a fluidized bed reactor with a capacity of 50 kg / h and transported to the top of the first degassing tank.

A gas stream containing 99 mol% of propane and 1 mol% of ethylene is continuously introduced into the bottom of said degassing tank. The polymer powder falls down under the action of gravity along the degassing tank, thereby contacting in countercurrent with a gas stream containing 99 mol.% Propane.

Propane enriched in oligomers and heavy hydrocarbons is obtained in the upper part of the first degassing tank. Said gas mixture containing propane, oligomers and heavy hydrocarbons is then distilled in two successive distillation columns according to the embodiment of FIG. 1 to obtain the aforementioned stream containing 99 mol% of propane, which is continuously introduced into the lower part of the first degassing tank .

The first degassing stage operates at a temperature of 75 ° C and a pressure of 0.25 MPa. The average residence time of the powder in the first degassing tank is 120 minutes.

The second stage of degassing

The polyolefin granules withdrawn from the first degassing tank are transported to a second degassing tank in which the polyolefin particles fall down by gravity, in countercurrent contact with a stream of nitrogen introduced into the bottom of the second degassing tank.

The total flow rate of nitrogen supplied to the second stage of degassing is 30 kg / h. The named second degassing stage operates at a temperature of 80 ° C with a residence time of the polymer powder of 90 minutes.

A sample of linear low density polyethylene (LLDPE) powder withdrawn from the second degassing tank is analyzed by the extraction method “static headspace” (static equilibrium vapor analysis method) in order to measure the residual content of volatile organic compounds adsorbed on polymer particles.

The data in Table 1 show that the total weight concentration of oligomers and hydrocarbons heavier than C ₅ in degassed linear low density polyethylene (LLDPE) is much higher than 50 ppm by weight, so that the polyolefin powder can cause undesirable bad distribution odor when used for the manufacture of plastic products used in the food and pharmaceutical industries.

Table 1 ∑С6
million
share i-C8 million
share n-C8 million
share n-C10 million
share n-C12 million
share n-C14 million
share n-C16 million
share Example 1 8 four 0 8 12 8 2 Example 2 2 <1 0 2 3 2 <1 Example 3 5 2 0 four 6 four 3 Example 4 (Comparative) 360 80 four 31 9 four 2

Claims (19)

1. The method of finishing the polyolefins obtained by gas-phase catalytic polymerization of one or more α-olefins in the presence of a polymerization mixture diluent selected from a C ₃ -C ₅ alkane, in which the polyolefin granules discharged from the gas-phase reactor are processed for:
(1) a first degassing step carried out in a first tank, wherein said polyolefin is in countercurrent contact with a gas stream containing at least 85 mol% of a washout reagent selected from a C ₃ -C ₅ alkane, said gas stream continuously extracted from the regeneration section of the monomer of the polymerization unit;
(2) the second stage of degassing, carried out in a second tank, in which the named polyolefin is in countercurrent mode in contact with water vapor, with the amount of water vapor from 10 to 200 kg per 1000 kg of polyolefin. 2. The method according to claim 1, wherein said gas-phase polymerization reactor is selected from a fluidized-bed reactor, a gas-phase stirred reactor and a gas-phase apparatus having two different interconnected polymerization zones. 3. The method according to claim 1, wherein said gas stream comprises at least 95 mol% of a leaching agent selected from a C ₃ -C ₅ alkane. 4. The method according to claim 1, in which the leaching agent of stage (1) is the same compound used as a diluent for the polymerization mixture in the gas phase polymerization reactor. 5. The method according to claim 1, in which propane is used as a diluent of the polymerization mixture and leaching agent in stage (1). 6. The method according to claim 1, wherein said gas stream containing at least 85 mol.% C ₃ -C ₅ alkane is obtained by subjecting the gas mixture withdrawn from the upper part of the first degassing stage (1) to the separation stages in series in said monomer regeneration section. 7. The method according to claim 6, in which these separation stages are carried out by distillation, using permeable membranes, technology, short-cycle adsorption-free heat separation under variable pressure (PSA), freezing. 8. The method according to claim 1, in which the temperature in the first stage of degassing is from 60 to 120 ° C. 9. The method according to claim 1, in which the pressure in the first stage of degassing varies from 0.1 to 0.4 MPa. 10. The method according to claim 1, in which the residence time of the polymer in the first stage of degassing is between 10 minutes and 6 hours 11. The method according to claim 1, in which the temperature in the second stage of degassing is from 80 to 120 ° C. 12. The method according to claim 1, in which the pressure in the second stage of degassing varies from 0.1 to 0.35 MPa. 13. The method according to claim 1, in which the residence time of the polymer in the second stage of degassing is between 10 minutes and 4 hours 14. The method according to claims 1 to 13, in which, at the outlet of the second degassing tank, the total amount of oligomers and hydrocarbons adsorbed on the polymer particles is reduced to less than 50 ppm by weight. 15. The method according to claim 1, wherein the drying step is carried out downstream of the second degassing step using hot nitrogen. 16. The method according to clause 15, in which the drying of the polymer powder is carried out in a pipeline connecting the second degassing tank to the storage hopper. 17. The method according to clause 15, in which a stream of hot nitrogen at a temperature of from 60 to 120 ° C is used to transport polymer particles from the specified second tank for degassing in the specified storage bin. 18. The method according to clause 16, in which the drying of the polymer powder is carried out inside the specified storage hopper using a closed loop of hot nitrogen. 19. The method of finishing the polyolefins obtained by gas-phase catalytic polymerization of ethylene in the future with 1-olefins in the presence of propane as a diluent of the polymerization mixture, in which the polyolefin granules discharged from the gas-phase reactor are processed for:
(1) a first degassing step in which said polyolefin is in countercurrent contact with a gas stream containing at least 85 mol% of propane as a leaching agent, said gas stream being continuously removed from the monomer regeneration section of the polymerization unit;
(2) a second degassing step, wherein said polyolefin is in countercurrent contact with water vapor, with an amount of water vapor ranging from 10 to 200 kg per 1000 kg of polyolefin.

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