Vent gas recovery improvement

Abstract

The invention relates to a process for producing polyolefins comprising: polymerizing at least one alpha-olefin with a comonomer in a fluidized bed reactor, in presence of one or more condensing agent, to obtain a product stream comprising a polyolefin copolymer; and wherein the process comprises the step of supplying the fluidized bed reactor with a feed stream comprising the comonomer and a feed stream comprising the condensing agent; and wherein the fluidized bed reactor comprises a polyolefin purging unit and a vent gas recovery system; and wherein the fluidized bed reactor operates in a condensed mode or a super condensed mode in presence of a recycle stream; and wherein at least a part of the feed stream comprising the comonomer and / or at least a part of the feed stream comprising the condensing agent is fed to the vent gas recovery system.

Description

[0001] VENT GAS RECOVERY IMPROVEMENT

[0002] The present invention relates to a process and the use of the process to produce polyolefin polymers.

[0003] Background Art

[0004] Gas phase fluidized bed reactors for the production of olefin polymers are well-known in the art. Gas phase processes successfully allow for production of a vast array of polymers, while reducing energy requirements and capital investments required to run the gas phase processes as compared to other polymerization processes.

[0005] Particular examples of gas phase polymerisation processes to which the present invention pertains include processes for the preparation of ethylene-based polymers and propylene-based polymers.

[0006] In general, polymerization reactions are exothermic. Therefore, one way of improving the heat removal from the fluidized bed reactors is by operating in condensed mode or super condensed mode. This can be achieved by the addition of inert condensing agent, such as an alkane with 3 to 6 C atoms, and cooling the recycle stream to a temperature below its dew point. In this process, part of the recycle stream is condensed and introduced into the fluidized bed polymerization reactor where the liquid vaporizes when it is exposed to the heat in the reactor. Condensed mode fluidized bed reactor polymerization processes have been disclosed for example in U.S. Pat. Nos. 4543399 and 4588790.

[0007] The resin discharged from the reactor entrains and adsorbs reactor gases. For safety reasons, these entrained and adsorbed reactor gases must be removed from the product. Otherwise, remaining monomers and / or other hydrocarbons, such as for example alkanes, may diffuse out of the product polymer powder, especially in downstream vessels which contain air, and form explosive gas mixtures.

[0008] To prevent this as much as possible, a purging can be performed for example in a product purging unit for example by blowing a stream of nitrogen up from the bottom of a purge tank through and against the polymer introduced therein. Such a process flushes out the entrained gases and desorbs the dissolved hydrocarbons out of the product powder. Compounds that escape with the resin out of the reactor are preferably recovered and reintroduced into the reaction system to achieve efficient use of feedstocks. Modem production plants are therefore often designed with vent gas recovery (VGR) systems.

[0009] A VGR system recovers the hydrocarbon compounds from the gas stream coming out of the degassing unit. Recovered condensed compounds can then for example be returned directly to the reaction system, while recovered light gases can for example be used as a conveying gas to reduce nitrogen consumption. Unrecovered hydrocarbons and light compounds are usually flared. These unrecovered gasses represents high losses to the plants as well as raise sustainability concerns due to greenhouse gas emissions.

[0010] Especially the (co)monomers lost in the product vent stream can represent a substantial loss to polyolefin producers. Also, there are continually rising requirements for disposing the vent gas to meet air quality protection codes and legislation. Typically, for example from about one to about two percent of the ethylene fed to the process is lost in the vent gas stream. Comonomer losses can even be much higher ranging for example from about 10% for butene-1 to about 50% for hexene-1. The fraction of comonomer that is lost in the vent gas stream typically increases with molecular weight of the comonomer because the solubility of comonomer in the product polyolefin increases with the molecular weight of the comonomer.

[0011] A typical hydrocarbon recovery unit in a gas phase polymerization process utilizes a compression and condensation method where most of the hydrocarbons with 4 or more C atoms will be recovered and at most up to 50 % of the hydrocarbons with 2-3 C atoms such as ethylene and propylene will be recovered. Furthermore, the uncondensed gas contains a significant amount of hydrocarbons and nitrogen.

[0012] Accordingly, there are various techniques for removing volatile compounds, such as monomers and / or inert hydrocarbons, from polymers. For example, WO 2017 / 114930 Al discloses a process for the gas phase polymerization of at least one alpha-olefin comprising: polymerizing at least one alpha-olefin in a fluidized bed reactor with a polyolefin purging unit and a vent gas recovery system, wherein the fluidized bed reactor operates in a condensed mode or super condensed mode with a recycle stream comprising one or more alkane(s) having 3 to 5 C atoms, wherein further at least one additional alkane with 6 to 12 C atoms is added to the recycle stream in an amount so that it represents between 0.0095 mol.% and 7.0000 mol.% of the recycle stream composition introduced into the reactor. It was found that this process could improve efficiency of the vent gas recovery system VGR significantly.

[0013] There remains a need for further improving the recovery of compounds, such as unreacted monomers and / or inert hydrocarbons, from produced polymer materials.

[0014] The object of the present invention is to provide a process for improving the operation of fluidized bed polymerization reactors for the production of polyolefins by improving the recovery of hydrocarbons and / or monomers from the produced polyolefin polymer.

[0015] Accordingly, the present invention provides a process for producing polyolefins comprising: polymerizing at least one alpha-olefin with a comonomer in a fluidized bed reactor, in presence of one or more condensing agent, to obtain a product stream comprising a polyolefin copolymer; and wherein the process comprises the step of supplying the fluidized bed reactor with a feed stream comprising the comonomer and a feed stream comprising the condensing agent; and wherein the fluidized bed reactor comprises a polyolefin purging unit and a vent gas recovery system; and wherein the fluidized bed reactor operates in a condensed mode or a super condensed mode in presence of a recycle stream; and wherein at least a part of the feed stream comprising the comonomer and / or at least a part of the feed stream comprising the condensing agent is fed to the vent gas recovery system.

[0016] In an aspect of the invention, the invention relates to the use of the process of the present invention for producing polyolefins.

[0017] Preferably, when part of the feed stream of comonomer is fed to the vent gas recovery system, a split stream of comonomer is split off of the feed stream of comonomer. The split stream of comonomer is fed to the vent gas recovery system. Preferably, a split ratio of the part of the feed stream of comonomer fed to the vent gas recovery system versus the total feed stream of comonomer supplied to the fluidized reactor, is between 0.1 and 1, preferably at least 0.1 and less than 1, more preferably between 0.2 and 0.9, more preferably between 0.3 and 0.8.

[0018] And, when part of the feed stream of condensing agent is fed to the vent gas recovery system, a split stream of condensing agent is split off of the feed stream of condensing agent. The split stream of condensing agent is fed to the vent gas recovery system.

[0019] Preferably, a split ratio of the part of the feed stream of condensing agent fed to the vent gas recovery system versus the total feed stream of condensing agent supplied to the fluidized reactor, is between 0.1 and 1, preferably at least 0.1 and less than 1, more preferably between 0.2 and 0.9, more preferably between 0.3 and 0.8.

[0020] In general, polymerization reactions are exothermic. Therefore, producing a polymer in a fluidized bed reactor necessitates that the heat generated by the polymerization reaction is removed in order to keep the reaction temperature within the bed in a desirable range.

[0021] One way of maximizing cooling throughout the reaction zone is by evaporative cooling or operating in condensed mode or super condensed mode. This can be achieved by the addition to the fluidized bed reactor of at least one condensing agent, such as for example an alkane having 3 to 6 carbon atoms, and cooling the recycle stream to a temperature below its dew point.

[0022] In this process, a liquid phase is formed by cooling the recycle stream below the dew point temperature, thereby converting a portion of the gas into a liquid, and the cooled recycle stream including the obtained liquid phase is introduced into the fluidized bed polymerization reactor. The liquid phase includes monomers and low boiling alkanes and comprises a liquefied portion of the at least one condensing agent. The liquid phase can subsequently vaporize when it is exposed to the heat in the reactor. The objective is to take advantage of the cooling effect brought about by the vaporization.

[0023] Condensed mode fluidized bed reactor polymerization processes have been disclosed for example in U.S. Pat. Nos. 4543399 and 4588790, each of which describes introducing an inert liquid into the recycle stream to increase the dew point temperature of the recycle stream and allowing the process to operate at levels of up to 17.4% liquid by weight, based on the total weight of the recycle stream introduced into the reactor.

[0024] A condensed mode process is considered to be advantageous because its ability to remove greater quantities of heat generated by polymerization increases the polymer production capacity of a fluidized bed polymerization reactor.

[0025] Commonly used condensing agents are alkanes which are inert under polymerization conditions. For example iso-pentane, which boils at about 27 °C, and consequently is vapor in the recycle stream in view of the heat present in the recycle gases. Other commonly used condensing agents are cyclopentane, n-butane, and iso-butane. Condensing agents may be fed directly to the reactor, or they may be introduced into the recycle stream which is subsequently fed to the reactor. The recycle gases leave the reactor, are cooled, and then condensed to the extent that a vapor phase and liquid phase are formed.

[0026] Moreover, "super condensed mode" fluidized bed reactor polymerization processes operating with above 17.4% liquid by weight in the recycle stream introduced into the reactor have been disclosed; however, such processes must be confined under certain more specific and restrictive conditions within a limited range of operating conditions to avoid destabilizing the fluidized bed, thereby halting the process. For example, U.S. Pat. No. 5352749 describes such a process.

[0027] Comonomers are often employed to influence the properties of polymers. They may for example improve the flexibility of the polymer. Comonomers may be fed directly to the reactor, or they may be introduced into the recycle stream which is subsequently fed to the reactor.

[0028] The inventors have surprisingly found that adding at least a part of the comonomer feed and / or at least a part of the condensing agent feed directly to the vent gas recovery area (VGR) can improve the efficiency of the vent gas recovery unit through increasing the dissolved lighter component in the heavier component. The vent gas recovery system efficiency can be improved by more than 10%, and even up to 60% compared to addition of the comonomer feed and / or the condensing agent feed to the reactor, optionally via the recycle stream, with otherwise identical operating conditions.

[0029] Preferably, the amount of liquids recovered is increased by at least 10 %, preferably at least 20 %, as compared to a process that does not involve feeding at least part of the feed stream of comonomer and / or at least part of the feed stream of condensing agent to the vent gas recovery system.

[0030] Such improvement significantly contributes to achieve a very efficient use of feedstocks. Moreover, it may also help to meet the continually rising requirements for disposing of the vent gas to meet air quality protection codes and legislation.

[0031] The vent gas recovery (VGR) system can thereby typically be used to recover hydrocarbons, such as for example alkanes and / or alkenes, especially one or more alpha-olefins, from the mixed hydrocarbon / inert purge gas stream that exits the purging unit.

[0032] The purging unit may thereby be for example a vertical vessel where purge gas such as for example an inert gas like nitrogen, CO2, argon or a gas of light hydrocarbons with 3 to 6 C atoms is sent from the bottom of the vessel to the top against the flow of the polymer coming from the reactor to strip away hydrocarbons, such as for example alkanes and / or alkenes, especially one or more alpha-olefins from the polymer. The stream comprising a mixture of (a) hydrocarbon(s) and an inert purge gas coming out of the purging unit is fed to the vent gas recovery (VGR) system to recover hydrocarbons, such as for example alkanes and / or alkenes, especially one or more alpha-olefins, from the mixed hydrocarbon / inert purge gas stream that exits the purging unit. Inert in the sense of the present invention may thereby mean for example that the gas does not interfere with the polymerization and / or that the gas does not comprise a polymerizable double bond. The stream of gas and hydrocarbons that leaves the purging unit and may be introduced to the vent gas recovery system can thereby be called the purge stream.

[0033] Methods of recovering hydrocarbons from the stream exiting the purging unit used in a vent gas recovery (VGR) system can thereby include for example: a) a compression and condensation method, especially relying on condensation by cooling with water and / or air and / or with mechanical refrigeration (especially for example by cooling to -10° C); and / or b) a separation method via pressure swing absorption (PSA) and / or membranes; and / or c) a cryogenic vent recovery method, wherein condensation of hydrocarbons from the purge stream is accomplished by cooling with a liquefied gas, such as for example liquid nitrogen, especially by for example vaporization of the liquefied gas.

[0034] Methods that use only option a) can recover most of the hydrocarbons with 4 or more C atoms, which may be present in the feeds as impurities, but will typically recover at most only up to 50 % of the hydrocarbons with 3 carbon atoms, such as especially ethylene and / or propylene. Furthermore, the uncondensed nitrogen contains significant amounts of heavy hydrocarbons, which may preclude using it as a resin drying or purge gas.

[0035] To reach a higher ethylene recovery and achieve a higher recovered gas quality, further vent recovery processing is required. Thus, the method of the invention is particularly effective in combination with method a).

[0036] The heavier the components that are fed to the vent gas recovery system, the better the recovery of hydrocarbons from the VGR unit. Comonomers are usually heavier than condensing agents. Therefore, preferably, there is a feed stream of comonomer and at least part of the feed stream of comonomer is fed to the vent gas recovery system, i.e. as a split stream of comonomer. The process may then optionally operate with a feed stream of condensing agent.

[0037] In an aspect of the invention, the polyolefin copolymer is passed through a degassing system prior to introducing the product stream to the vent gas recovery system.

[0038] Preferably, the condensing agent and the recycle stream comprises one or more alkane(s) having 3 to 6 carbon atoms

[0039] Preferably, the condensing agent and the recycle stream comprises one or more alkane(s) having 3 to 6 carbon atoms.

[0040] Preferably, the alpha-olefin is ethylene or propylene. Preferably, the alpha-olefin is ethylene and the comonomer is selected from propylene, 1- butene, 1 -hexene and 1 -octene, and combinations thereof.

[0041] Preferably, the alpha-olefin is propylene and the comonomer is selected from ethylene, 1- butene, 1 -hexene and 1 -octene, and combinations thereof.

[0042] In the process of the present invention, it is preferred that the alpha-olefin accounts for > 70.0 wt% of the total weight of the sum of the alpha-olefin and the comonomer, preferably > 80.0 wt%, more preferably > 90.0 wt%.

[0043] In an embodiment, the alpha-olefin may for example be ethylene, and the alpha-olefin may account for > 70.0 wt% of the total weight of the sum of the alpha-olefin and the comonomer, preferably > 80.0 wt%, more preferably > 90.0 wt%. In an embodiment, the alpha-olefin may for example be propylene, and the alpha-olefin may account for > 70.0 wt% of the total weight of the sum of the alpha-olefin and the comonomer, preferably > 80.0 wt%, more preferably > 90.0 wt%.

[0044] In an embodiment, the comonomer may be present in an amount of > 1.0 and < 30.0 wt% with regard to the total weight of the alpha-olefin and the comonomer, preferably > 1.0 and < 25.0 wt%, more preferably > 1.0 and < 20.0 wt%, even more preferably > 2.0 and < 20.0 wt%, even more preferably > 3.0 and < 15.0 wt%

[0045] In an embodiment, the alpha-olefin may be ethylene and the comonomer may be selected from propylene, 1-butene, 1-hexene and 1-octene, wherein the alpha-olefin accounts for > 70.0 wt% of the total weight of the sum of the alpha-olefin and the comonomer, preferably > 80.0 wt%, more preferably > 90.0 wt%.

[0046] In an embodiment, the alpha-olefin may be propylene and the comonomer may be selected from ethylene, 1-butene, 1-hexene and 1-octene, wherein the alpha-olefin accounts for > 70.0 wt% of the total weight of the sum of the alpha-olefin and the comonomer, preferably > 80.0 wt%, more preferably > 90.0 wt%.

[0047] In an embodiment, the comonomer may be present in an amount of > 1.0 and < 30.0 wt% with regard to the total weight of the alpha-olefin and the comonomer, preferably > 1.0 and < 25.0 wt%, more preferably > 1.0 and < 20.0 wt%, even more preferably > 2.0 and < 20.0 wt%, even more preferably > 3.0 and < 15.0 wt%, wherein the alpha-olefin is ethylene and the comonomer is selected from propylene, 1 -butene, 1 -hexene and 1 -octene.

[0048] In an embodiment, the comonomer may be present in an amount of > 1.0 and < 30.0 wt% with regard to the total weight of the alpha-olefin and the comonomer, preferably > 1.0 and < 25.0 wt%, more preferably > 1.0 and < 20.0 wt%, even more preferably > 2.0 and < 20.0 wt%, even more preferably > 3.0 and < 15.0 wt%, wherein the alpha-olefin is propylene and the comonomer is selected from ethylene, 1 -butene, 1 -hexene and 1 -octene.

[0049] Preferably, the feed stream comprising the comonomer comprises 1 - 10 mol.% of the comonomer with regard to the total moles of the feed stream.

[0050] Preferably, the condensing agent is selected from the group consisting of iso-pentane, cyclopentane, n-butane, iso-butane and / or mixtures thereof.

[0051] Preferably, the polymerization is carried out in presence of a catalyst selected from a chromium based catalyst system, Ziegler-Natta catalyst, a metallocene based catalyst system, a prepolymer powder, and a combination thereof. The catalyst system may comprise a supported catalytic compound or it may comprise an unsupported catalytic compound.

[0052] In particularly preferred embodiments, the present invention is a process for producing polyolefins comprising: polymerizing at least one alpha-olefin with a comonomer in a fluidized bed reactor, in presence of one or more condensing agent, to obtain a product stream comprising a polyolefin copolymer; and wherein the process comprises the step of supplying the fluidized bed reactor with a feed stream comprising the comonomer and a feed stream comprising the condensing agent; and wherein the fluidized bed reactor comprises a polyolefin purging unit and a vent gas recovery system; and wherein the fluidized bed reactor operates in a condensed mode or a super condensed mode in presence of a recycle stream; and wherein at least a part of the feed stream comprising the comonomer and / or at least a part of the feed stream comprising the condensing agent is fed to the vent gas recovery system, wherein the alpha-olefin is ethylene or propylene, wherein the condensing agent is selected from the group consisting of iso-pentane, cyclopentane, n-butane, iso-butane and / or mixtures thereof, wherein a split ratio of the part of the feed stream of comonomer fed to the vent gas recovery system versus the total feed stream of comonomer supplied to the fluidized reactor, is between 0.2 and 0.9, preferably between 0.3 and 0.8.

[0053] In particularly preferred embodiments, the present invention is a process for producing polyolefins comprising: polymerizing at least one alpha-olefin with a comonomer in a fluidized bed reactor, in presence of one or more condensing agent, to obtain a product stream comprising a polyolefin copolymer; and wherein the process comprises the step of supplying the fluidized bed reactor with a feed stream comprising the comonomer and a feed stream comprising the condensing agent; and wherein the fluidized bed reactor comprises a polyolefin purging unit and a vent gas recovery system; and wherein the fluidized bed reactor operates in a condensed mode or a super condensed mode in presence of a recycle stream; and wherein at least a part of the feed stream comprising the comonomer and / or at least a part of the feed stream comprising the condensing agent is fed to the vent gas recovery system, wherein the alpha-olefin is ethylene, wherein the condensing agent is selected from the group consisting of iso-pentane, cyclopentane, n-butane, iso-butane and / or mixtures thereof, wherein a split ratio of the part of the feed stream of comonomer fed to the vent gas recovery system versus the total feed stream of comonomer supplied to the fluidized reactor, is between 0.2 and 0.9, preferably between 0.3 and 0.8.

[0054] The present invention also concerns the use of a process according to the invention for producing polymers or copolymers of ethylene or propylene optionally with at least one other alpha-olefin as comonomer. Brief description of the figures

[0055] Fig. 1 shows a block diagram of a typical gas phase polymerization process.

[0056] Fig. 2 shows an embodiment of the invention. Detailed description of the figures

[0057] The figure description for both figures can be found in table 1.

[0058] Table 1. In the embodiment according to the invention, the feed stream 2 is split, and split stream 11 is fed to the vent gas recovery unit C.

[0059] Examples

[0060] A first example relates to an ethylene polymerization process with 1 -hexene as a comonomer. In a first mode of operation, the full quantity of comonomer was supplied to the reactor. The cycle gas composition and the recovered stream individual component flow are shown in Table 2. In a second mode of operation (case a), 30% of the comonomer was added to the VGR, and 70% to the reactor. In a third mode of operation (case b), 60% of the comonomer was added to the VGR, and 40% to the reactor. All other relevant operating parameters were kept constant. Thus, , upon splitting the comonomer feed between the reaction area and the VGR area with a split ratio of a) 0.3 and b) 0.6 VGR comonomer feed / total comonomer feed (and thus respectively a ratio a) 0.7 and b) 0.4 reaction area feed / total monomer feed), the recovered stream individual components flow was estimated and the improvement is shown in Table 3.

[0061] Table 2.

[0062] Table 3. Another example also relates to an ethylene polymerization process with 1 -hexene as a comonomer. In a again a first mode of operation, the full quantity of comonomer was supplied to the reactor. The cycle gas composition and the recovered stream individual component flow are shown in Table 4.

[0063] In a second mode of operation (case a), 50% of the comonomer was added to the VGR, and 70% to the reactor. In a third mode of operation (case b), 80% of the comonomer was added to the VGR, and 40% to the reactor. All other relevant operating parameters were kept constant. Thus, upon splitting the comonomer feed between the reaction area and the VGR area with a split ratio of a) 0.5 and b) 0.8 VGR comonomer feed / total comonomer feed (and thus respectively a ratio of a) 0.5 and b) 0.2 reaction area feed / total monomer feed), the recovered stream individual components flow was estimated and the improvement is shown in Table 5. Table 4.

[0064] Table 5.

Claims

CLAIMS1. Process for producing polyolefins comprising: polymerizing at least one alpha-olefin with a comonomer in a fluidized bed reactor, in presence of one or more condensing agent, to obtain a product stream comprising a polyolefin copolymer; and wherein the process comprises the step of supplying the fluidized bed reactor with a feed stream comprising the comonomer and a feed stream comprising the condensing agent; and wherein the fluidized bed reactor comprises a polyolefin purging unit and a vent gas recovery system; and wherein the fluidized bed reactor operates in a condensed mode or a super condensed mode in presence of a recycle stream; and wherein at least a part of the feed stream comprising the comonomer and / or at least a part of the feed stream comprising the condensing agent is fed to the vent gas recovery system.

2. Process of claim 1, wherein the polyolefin copolymer is passed through a degassing system prior to introducing the product stream to the vent gas recovery system.

3. Process according to any one of claims 1 -2, wherein the condensing agent and the recycle stream comprises one or more alkane(s) having 3 to 6 carbon atoms.

4. Process according to any one of claims 1-3, wherein a split ratio of the part of the feed stream of comonomer fed to the vent gas recovery system versus the total feed stream of comonomer supplied to the fluidized reactor, is between 0.1 and 1, preferably at least 0.1 and less than 1, more preferably between 0.2 and 0.9, more preferably between 0.3 and 0.8.

5. Process according to any one of claims 1-4, wherein a split ratio of the part of the feed stream of condensing agent fed to the vent gas recovery system versus the total feed stream of condensing agent supplied to the fluidized reactor, is between 0.1 and 1, preferably at least 0.1 and less than 1, more preferably between 0.2 and 0.9, more preferably between 0.3 and 0.8.

6. Process according to any one of claims 1-5, wherein the alpha-olefin has 3 to 10 carbon atoms, most preferably an alpha-olefin having 5 to 10 carbon atoms.

7. Process according to any one of claims 1-6, wherein the alpha-olefin is ethylene or propylene.

8. Process according to any one of claims 1-7, wherein the alpha-olefin is ethylene and the comonomer is selected from propylene, 1 -butene, 1 -hexene and 1 -octene, and combinations thereof.

9. Process according to any one of claims 1-7, wherein the alpha-olefin is propylene and the comonomer is selected from ethylene, 1 -butene, 1 -hexene and 1 -octene, and combinations thereof.

10. Process according to any one of claims 1-9, wherein the feed stream comprising the comonomer comprises 1 - 10 mol.% of the comonomer with regard to the total moles of the feed stream.

11. Process according to any one of claims 1-10, wherein the condensing agent is selected from the group consisting of iso-pentane, cyclopentane, n-butane, iso-butane and / or mixtures thereof.

12. Process according to any one of claims 1-11, wherein the polymerization is carried out in presence of a catalyst selected from a chromium based catalyst system, Ziegler-Natta catalyst, a metallocene based catalyst system, a pre-polymer powder, and a combination thereof.

13. Process according to any one of claims 1-12, wherein the amount of liquids recovered is increased by at least 10 %, preferably at least 20 %, as compared to a process that does not involve feeding at least part of the feed stream of comonomer and / or at least part of the feed stream of condensing agent to the vent gas recovery system.

14. Process according to any one of claims 1-13, wherein the alpha-olefin is ethylene, wherein the condensing agent is selected from the group consisting of iso-pentane, cyclopentane, n-butane, iso-butane and / or mixtures thereof,wherein a split ratio of the part of the feed stream of comonomer fed to the vent gas recovery system versus the total feed stream of comonomer supplied to the fluidized reactor, is between 0.2 and 0.9, preferably between 0.3 and 0.8.

15. Use of a process according to any of claims 1-14 for producing polyolefins.

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