Processes for polymerising olefins

Abstract

The present disclosure provides a process for polymerising olefins. The process comprises polymerising olefin monomer in a gas phase reactor at an operating pressure below a predetermined threshold value; withdrawing circulation gas comprising unreacted olefin monomer from an outlet in an upper region of the gas phase reactor and reintroducing the circulation gas into an inlet in a lower region of the gas phase reactor to maintain a fluidised bed in the gas phase reactor. When the operating pressure in the gas phase reactor exceeds the predetermined threshold value, a side stream is taken from the withdrawn circulation gas for a duration sufficient to reduce the operating pressure of the gas phase reactor to below the predetermined threshold value. Any solid particulates present in the side stream are filtered to produce a filtered side stream, and at least a portion of the unreacted olefin monomer from the filtered side stream is recovered, preferably, in at least one distillation column. Optionally, any polymer removed by filtration from the side streamside stream may be returned to the process and / or recovered from the process.

Description

[0001] PROCESSES FOR POLYMERISING OLEFINS

[0002] FIELD OF THE DISCLOSURE

[0003] The present disclosure relates to a process for polymerising olefins, for example, in multistage polymerisation process configuration.

[0004] BACKGROUND OF THE DISCLOSURE

[0005] Multistage polymerisation processes (e.g. Borstar PE, PP and Spheripol PP) employ multistage reactor configurations to provide the multimodal capability for achieving easy-to- process resins with desirable mechanical properties. In such processes, a combination of e.g. slurry loop reactors in series followed by a gas phase reactor may be employed to produce a range of polyolefins.

[0006] Each reactor can be controlled separately and can be operated under desired reaction conditions in terms of pressure, temperature and reaction species concentrations. In this way, each reactor may be controlled to vary the production rate and properties of the polymer produced.

[0007] Various methods may be used to control the pressure in, for example, the downstream gas phase reactor. For example, it may be possible to control the pressure in the downstream gas phase reactor by controlling the feed of reactants into the reactor, together with inert components, including nitrogen propane and / or butane. Pressure may be reduced, for instance, by reducing the amount of inert components into the reactor.

[0008] W02023 / 031201 describes a process in which circulation gas is withdrawn from an outlet in an upper region of a gas phase reactor and returned to an inlet at the base of the gas phase reactor. By circulating the circulation gas, unreacted monomer can be reintroduced into the gas phase reactor for polymerisation and, in the case of a fluidised bed reactor, the circulation gas may help to maintain a fluidised bed within the reactor where the polymerisation reaction can take place. The circulation gas may be compressed and cooled prior to reintroduction into the gas phase reactor. In W02023 / 031201, a side stream is branched off from the withdrawn circulation gas and pressurised to produce a flush stream. The remaining side stream is purified, for example, by filtration and distillation. There is no disclosure in W02023 / 031201 of altering the operating pressure of the reactor by withdrawal of the gas stream. On the contrary the pressure in the circulation gas is preferably constant. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 depicts a schematic view of a multistage polymerisation process configuration for carrying out a process according to an example of the present disclosure.

[0010] Figure 2 depicts a schematic view of an alternative configuration for carrying out a process according to another example of the present disclosure.

[0011] DESCRIPTION OF THE DISCLOSURE

[0012] According to a first aspect, there is provided process for polymerising olefins. The process comprises polymerising olefin monomer in a gas phase reactor at an operating pressure below a predetermined threshold value; withdrawing circulation gas comprising unreacted olefin monomer from an outlet in an upper region of the gas phase reactor and reintroducing the circulation gas into an inlet in a lower region of the gas phase reactor to maintain a fluidised bed in the gas phase reactor. When the operating pressure in the gas phase reactor exceeds the predetermined threshold value, a side stream is taken from the withdrawn circulation gas for a duration sufficient to reduce the operating pressure of the gas phase reactor to below the predetermined threshold value. Any solid particulates present in the side stream are filtered to produce a filtered side stream, and at least a portion of the unreacted olefin monomer from the filtered side stream is recovered e.g., and recycled to the process. Preferably, the recovery takes place in at least one distillation column. Optionally, any polymer removed by filtration from the side streamside stream may be returned to the process and / or recovered from the process.

[0013] A product stream comprising polymer and unreacted olefin monomer may be withdrawn from the gas phase reactor; and at least a portion of the unreacted olefin monomer from the product stream may be recovered. Recovery may take place in a recovery unit, for example, in at least one distillation column. The same recovery unit (e.g., at least one distillation column) may be used to recover the unreacted olefin monomer from the product stream and the filtered side stream.

[0014] The predetermined threshold value may be 30 barg or below, preferably 25 barg or below, more preferably 24 barg or below, even more preferably 23 barg or below. The predetermined threshold value may be a value from 18 to 30 barg, preferably a value from 20 to 25 barg, more preferably a value from 21 to 24 barg, even more preferably a value from 22 to 23 barg. Preferably, when the operating pressure in the gas phase reactor exceeds the predetermined range, a side stream is taken from the withdrawn circulation gas for a duration sufficient to reduce the operating pressure of the gas phase reactor to less than 15%, preferably less than 10%, more preferably less than 5% below the predetermined threshold value.

[0015] The operating pressure in the gas phase reactor exceeding the predetermined range may be referred to as an initial gas phase reactor pressure. The operating pressure in the gas phase reactor below the predetermined threshold value resulting from the side stream being taken from the withdrawn circulation gas for a sufficient duration to reduce the operating pressure of the gas phase reactor to below the predetermined threshold value may be referred to as a final gas phase reactor pressure.

[0016] As such, the present disclosure relates to a process for polymerising olefins, wherein when the operating pressure in the gas phase reactor is the initial gas phase reactor pressure, a side stream is taken from the withdrawn circulation gas for a duration sufficient to achieve the final gas phase reactor pressure.

[0017] Preferably the final gas phase reactor pressure is 0.1 to 10.0 barg below the initial gas phase reactor pressure, more preferably 0.2 to 5.0 barg below the initial gas phase reactor pressure, even more preferably 0.3 to 3.0 barg below the initial gas phase reactor pressure. Preferably the final gas phase reactor pressure is 0.1 to 5.0 barg below the predetermined threshold value, more preferably 0.2 to 3.0 barg below the predetermined threshold value, even more preferably 0.5 to 2.0 barg below the predetermined threshold value.

[0018] Preferably the initial gas phase reactor pressure is 0.1 to 2.0 barg above the predetermined threshold value, more preferably 0.2 to 1.0 barg above the predetermined threshold value, even more preferably 0.3 to 0.5 barg above the predetermined threshold value.

[0019] It has been found that, by taking a side streamside stream from the circulating gas, the operating pressure in the gas phase reactor can be controlled in a convenient, efficient and / or cost-effective manner. In prior art processes, the pressure in the gas reactor may be influenced by e.g., flow from any upstream reactor (e.g. slurry loop reactor(s)), as well as the feed rate of olefin monomer, optional comonomer, optional hydrogen and optional inert components introduced into the downstream gas phase reactor. Altering any one of these inputs may also alter the proportion of reactants and first polymer component in the gas phase reactor. Thus, it can be complex to alter pressure by varying these inputs without affecting other parameters of the process. By taking a side streamside stream from the circulating gas, pressure in the gas phase reactor can be reduced without the risk of substantially altering the balance of reactants in the gas phase reactor. Advantageously, the side stream is taken intermittently when the operating pressure exceeds a predetermined threshold value. Then, when the operating pressure returns to a desirable range, the side stream flow ceases, allowing polymerisation to take place at the desirable operating pressure. In other words, in a preferred embodiment, after the side stream has flowed for a duration sufficient to reduce the operating pressure of the gas phase reactor to below the predetermined threshold value, the flow of the side stream is ceased. When the operating pressure in the gas phase reactor again exceeds the predetermined threshold value, the side stream may be again taken to reduce the pressure to below the predetermined threshold value. Thus, an advantage of the present disclosure is that it offers a convenient, efficient and / or cost-effective way of controlling pressure in the gas phase reactor. Furthermore, any unreacted olefin monomer can be recovered by distillation and recycled to the process, preferably to the gas phase reactor. As mentioned above, the unreacted olefin monomer recovered from the filtered side streamside stream may be recovered in the at least one distillation column used to recover unreacted olefin monomer from the product stream. This allows unreacted olefin monomer to be recovered without using existing recovery equipment.

[0020] Preferably, the polymerisation process is a multistage polymerisation process comprising polymerising olefin monomer in an upstream reactor to produce a first polymer component and transferring the first polymer component to the gas phase reactor, such that polymerisation of the olefin monomer in the gas phase reactor takes place in the presence of the first polymer component. The upstream reactor may, for example, be a slurry or slurry loop reactor.

[0021] Preferably, the olefin monomer is polymerised in the presence of the first polymer component in a series of sequential downstream gas phase reactors. More preferably, the side streamside stream is taken from the withdrawn gas of the an upstream phase reactor of the series of sequential downstream gas phase reactors. Most preferably, the side stream is taken from the withdrawn gas of the most upstream gas phase reactor of the series of sequential downstream gas phase reactors.

[0022] Where two or more sequential downstream gas phase reactors are present, it may be advantageous to take a side stream from the withdrawn gas from an upstream, preferably most upstream gas phase reactor. When the operating pressure in the most upstream of the gas phase reactors exceeds the predetermined threshold value, a side stream is taken from the withdrawn circulation gas of the most upstream of the gas phase reactors for a duration sufficient to reduce the operating pressure of the most upstream of the gas phase reactor to below the predetermined threshold value. Withdrawing a side stream from the circulating gas of the most upstream of the gas phase reactors may have the added advantage of limiting the amount of olefin monomer entering a downstream gas phase reactor from the upstream or most upstream gas reactor. This can allow the olefin monomer content of the downstream reactor(s) to be more conveniently controlled, for example, by feeding fresh olefin monomer in the desired amounts. It may also be desirable to limit the amount of reactants, such as 1- hexene, from upstream reactor(s) as this may have an influence on the molecular weight of the polymers produced. Improved or more convenient control can be achieved by limiting or preventing flow of olefin monomer or such reactants as the product moves to downstream reactor(s). This can be conveniently achieved by taking a side stream from the circulating gas as discussed herein. Preferably, components of the side stream can be recovered and reused.

[0023] The olefin monomer polymerised in the gas phase reactor may be propylene. The olefin monomer may be copolymerised with an alpha olefin comonomer. Where polymerisation occurs in a multistage polymerisation and the gas phase reactor(s) is a downstream reactor(s), the olefin monomer polymerised in the upstream and downstream reactors may be propylene. Polymerisation in the upstream reactor may be a homopolymerisation or copolymerisation reaction with an alpha olefin comonomer. Polymerisation in the downstream reactor(s) may be a homopolymerisation or co-polymerisation reaction with an alpha olefin comonomer. Where both the upstream and downstream reactions are copolymerisation reactions, the same or different alpha olefin comonomer may be used.

[0024] Preferably, the withdrawn circulation gas is compressed. Preferably, the side stream is taken from the withdrawn circulation gas after the withdrawn circulation gas has been compressed. By compressing the gas and hence the side stream, the side stream can be filtered at relatively high pressures. This can facilitate filtration of any solid particulates from the side stream. Alternatively or additionally, the enhanced pressure can help to drive the side stream through downstream processing steps. This can improve the efficiency of the overall process. For example, no compression is needed if gas sent to recovery. The compressed gas may be optionally cooled prior to being compressed. This can help to mitigate excessive temperature rises on compression.

[0025] Once the circulation gas is compressed and the side stream is taken, the circulation gas may be cooled again before being reintroduced into the gas phase reactor. This can help to ensure that the circulation gas is at a desired temperature for the operation of the gas phase reactor.

[0026] As explained above, the side stream taken from the gas is filtered prior to being distilled in at least one distillation column.

[0027] Any suitable filter may be employed. For example, the filter may be a sintered metal filter. The filter may be a sintered metal bag filter. Examples of suitable filters are supplied by Saifilter®. Suitable metal filters include stainless steel sintered filters, bronze sintered filtered filters, nickel sintered filters or titanium sintered filters. The filters can take any suitable form, including metal filter discus, wire filters, metal strainer baskets, stainless steel filter cartridges and porous sintered metal filters. It is desirable that the filter is suitable for being blown back, for example, frequently to ensure that collected reactive powder can be recovered.

[0028] The temperature of the side stream that is passed through the filter may be controlled to be near but above the dew point of the gaseous components of the feed. The temperature may be controlled to be at least 2°C, at least 3°C, more preferably at least 5 °C, yet more preferably at least 8 °C above the dew point of the gaseous components of the feed. The temperature may be controlled to be 2°C to 40°C, at least 3°C to 30°C, more preferably at least 5 °C to 25 °C, yet more preferably at least 8 °C to 20°C , for instance, 10 °C to 15 °C above the dew point of the gaseous components of the feed. The dew point of the gaseous components of the feed may be about 40 to 70 °C, for example, 45 to 65 °C or 50 to 60 °C. The temperature of the side stream may be controlled to be above the dew point as mentioned above. Temperatures at the dew point may cause condensation within the filter, decreasing the efficiency of the filtration step. However, excessively high temperatures may cause the gaseous components to react in the side stream and this may not be desirable.

[0029] It may be possible to use a cyclone and a filter in combination to separate solids from the side stream. For example, the side stream may be passed through a cyclone and a filter prior to being distilled in the at least one distillation column.

[0030] The process may further comprise introducing the withdrawn product stream comprising polymer and unreacted olefin monomer into a purge bin. In the purge bin, a purge gas (e.g., nitrogen) may be used to separate a gaseous stream comprising unreacted olefin monomer from the polymer. The gaseous stream may be separated from the polymer. The gaseous stream may be withdrawn from the purge bin. The process may also comprise recovering unreacted olefin monomer from the separated gaseous stream. This recovery may be carried out by distillation. Distillation may be carried out in the same distillation column used to recover unreacted olefin monomer from the side stream.

[0031] In some embodiments, the product stream withdrawn from the process may be introduced into a gas-solid separator prior to the purge bin. In the gas-solid separator, a gaseous stream comprising unreacted olefin monomer may be separated from the polymer e.g., prior to the introduction of a purge gas. The process may further comprises removing a further gaseous stream comprising unreacted olefin monomer from the gas-solid separator. The process may further comprise optionally compressing the further gaseous stream. The optionally compressed further gaseous stream may be introduced into a distillation column for recovery of the unreacted monomer. The distillation column may be a further distillation column that is distinct from the distillation column used to recover unreacted olefin monomer from the side stream. The distillation column may be a further distillation column that is distinct from the distillation column used to recover unreacted olefin monomer from the purge bin. The distillation column may be the same as the distillation column used to recover unreacted olefin monomer from the purge bin and / or the unreacted olefin monomer from the side stream.

[0032] Polymerisation

[0033] As mentioned above, the process of the present disclosure is a process for polymerising olefins preferably in multistage polymerisation process configuration. Preferably, the process comprises polymerising, in a reactor, first olefin monomer, in the presence of a polymerisation catalyst to produce a first polymer component; and polymerising, in a downstream gas phase reactor, second olefin monomer in the presence of the first polymer component to produce a second polymer component.

[0034] To distinguish the reactor(s) in which the first polymer component is produced from the downstream gas phase reactor in which the second polymer component is produced, the reactor(s) in which the first polymer component is produced can be referred to as the upstream reactor(s). Polymerisation in the upstream reactor(s) may be referred to upstream polymerisation, while polymerisation in the downstream gas phase reactor(s) may be referred to as downstream gas phase polymerisation.

[0035] The first polymer component, which may be referred to as component, may be transferred to the downstream gas phase reactor from the upstream reactor(s), such that the second olefin polymer, which may be referred to as component, is produced in the presence of first polymer component. Additionally, the polymerisation catalyst may be transferred from the upstream reactor(s) to the downstream gas phase reactor so that the second olefin monomer is produced in the presence of both the first polymer component and the polymerisation catalyst.

[0036] Preferably, the same catalyst is used in each step and ideally, it is transferred from prepolymerisation (if employed) to subsequent polymerisation steps in sequence.

[0037] As explained above, a product stream comprising polymer and unreacted olefin monomer may be withdrawn from the gas phase reactor. This product stream may be recovered from any gas phase reactor where more than one gas phase reactor is employed. Preferably, where more than one downstream gas phase reactor is used, the product stream is recovered from the most downstream of the downstream gas phase reactors.

[0038] The product stream may be withdrawn from a reactor outlet and introduced to a separation unit. In the separation unit, the polymer may be separated from gases, including unreacted olefin monomers that may be adsorbed or contained within the polymer. Unreacted olefin monomer may be recovered from the separated gases, e.g., by distillation. The recovered unreacted olefin monomer may be recycled to the process, preferably to a gas phase reactor as recycled monomer feed.

[0039] The separation unit may comprise a purge bin. As explained above, a purge gas, for example, nitrogen may be introduced into the purge bin to facilitate the separation of unreacted olefin monomer from the polymer as a gaseous stream. The gaseous stream may be separated from the purge bin and introduced into an unreacted olefin (e.g., propylene) - nitrogen separation unit (e.g., a membrane separation unit) for separating the purge gas from the unreacted olefin. The purge gas may be recycled for use in the purge bin, while at least a portion of the unreacted olefin may be recovered in e.g., a distillation column.

[0040] As explained above, the separation unit may also include a gas-solids separator upstream of the purge bin. This may allow another gaseous stream comprising the unreacted olefin monomer to be separated from the polymer prior to introduction into the purge bin. This separated gas may be optionally compressed and introduced into a distillation column, preferably the same distillation column as that used to recover unreacted olefin originating from the purge gas. As explained herein, it may be possible to recover the unreacted olefin monomer from the side stream using at least one of the distillation columns used to recover unreacted olefin from the withdrawn product stream.

[0041] Prepolymerisation

[0042] The process may include a prepolymerisation step. The purpose of the prepolymerisation is to polymerise a small amount of polymer onto the catalyst at a low temperature and / or a low monomer concentration. By prepolymerisation, it may be possible to improve the performance of the catalyst in the subsequent polymerization process stages..

[0043] The catalyst components are preferably all introduced to the prepolymerisation step when a prepolymerisation step is present. Preferably, the reaction product of the prepolymerisation step is introduced to the polymerisation in which the first polymer component is produced.

[0044] In some instances, the solid catalyst component and the cocatalyst can be fed separately. Here, it is possible that only a part of the cocatalyst is introduced into the prepolymerisation stage and the remaining part into subsequent polymerisation stages. Also in such cases it may not be necessary to introduce as much cocatalyst into the prepolymerisation stage to achieve sufficient polymerisation therein.

[0045] It is understood within the scope of the invention, that the amount or polymer produced in the prepolymerisation lies within 1 to 7 wt% in respect to the final multimodal (co)polymer.

[0046] Producing the first polymer component

[0047] As discussed above, the process of the present disclosure preferably comprises polymerising, in an upstream reactor, first olefin monomer, optionally in the presence of at least one alpha olefin comonomer, in the presence of a polymerisation catalyst to produce a first polymer component. The first polymer component may be referred to as component (A).

[0048] Preferably, the first olefin monomer is propylene or ethylene, preferably propylene.

[0049] The first olefin monomer may be polymerised to form a homopolymer as the first polymer component. Alternatively, the first olefin monomer may be polymerised to form a copolymer of the first olefin monomer and the at least one alpha olefin comonomer as the first polymer component. For example, propylene may be polymerised in the upstream reactor to produce propylene homopolymer as the first polymer component. Alternatively, propylene may be copolymerised with at least one alpha olefin comonomer to produce a propylene copolymer as the first polymer component.

[0050] In one embodiment, ethylene is polymerised in the upstream reactor to produce ethylene homopolymer as the first polymer component. Alternatively, ethylene may be copolymerised with at least one alpha olefin comonomer to produce ethylene copolymer as the first polymer component.

[0051] Where an alpha olefin comonomer is used, the alpha olefin comonomer may be selected from a C2 to C10 alkene. For the avoidance of doubt, the alpha olefin comonomer is different from the first olefin monomer polymerised in the upstream reactor. Suitable alpha olefin comonomers include ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1- hexene, 1 -octene, 1 -nonene and 1 -decene. Preferably, the alpha olefin comonomer is selected from at least one of ethylene, propylene, 1-butene, 1-hexene, and 1 -octene. Where the first olefin monomer is propylene, the alpha olefin comonomer may be ethylene, 1- butene, 1-hexene and / or 1 -octene. Where the first olefin monomer is ethylene, the alpha olefin comonomer may be propylene, 1-butene and / or 1-hexene. Where an alpha olefin comonomer is used, one or more alpha comonomers may be employed. Preferably, one or two alpha comonomers are employed.

[0052] The alpha olefin comonomer content of the first polymer component may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%. Where the first polymer component is a polypropylene, the alpha olefin comonomer content may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%. Where the first polymer component is a polyethylene, the alpha olefin comonomer content may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%.

[0053] The upstream polymerisation may take place in any suitable reactor or series of reactors. The upstream polymerisation step may take place in one or more slurry polymerisation reactor(s) or in a gas phase polymerisation reactor, or a combination thereof. In other words, the first polymer component may be formed by slurry polymerisation, preferably in slurry loop reactors.

[0054] Preferably the upstream polymerisation step may takes place in one or more slurry polymerisation reactor(s), more preferably in at least two (e.g., two) or at least three (e.g. three) slurry phase reactors. Where, for example, at least three (e.g., three) slurry reactors are used, this does not include a slurry reactor for prepolymerisation.

[0055] Slurry ethylene polymerisation can take place in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably the diluent is a low-boiling hydrocarbon having from 1 to 4 carbon atoms or a mixture of such hydrocarbons. An especially preferred diluent is propane, possibly containing minor amount of methane, ethane and / or butane. In bulk propylene polymerization, propylene acts as monomer and diluent at the same time.

[0056] Polymer particles of the first polymer component formed by polymerisation, together with the catalyst fragmented and dispersed within the particles, may be suspended in the inert diluent. The slurry is agitated to enable the transfer of reactants from the fluid into the particles.

[0057] The olefin content in the fluid phase of the slurry may be from 2 to about 99 % by mole, preferably from about 3 to about 96% by mole and in particular from about 5 to about 90 % by mole. Where the first olefin monomer is propylene, the propylene content in the fluid phase of the slurry may be from 2 to about 99 % by mole, preferably from about 3 to about 96 % by mole and in particular from about 5 to about 90 % by mole. Where the first olefin monomer is ethylene, the ethylene content in the fluid phase of the slurry may be from 2 to about 50 % by mole, preferably from about 3 to about 20 % by mole and in particular from about 5 to about 15 % by mole. The benefit of using higher olefin concentration is that the productivity of the catalyst can be increased.

[0058] The temperature in the upstream polymerisation may be from 20 to 115 °C, preferably from 25 to 110 °C and in particular from 30 to 100 °C. The pressure is from 1 to 150 bar, preferably from 10 to 100 bar.

[0059] The residence time in the upstream reactor (including any prepolymerisation reactor employed) may be typically from 0.15 h to 3.0 h, preferably from 0.20 h to 2.0 h and in particular from 0.30 to 1.5 h.

[0060] It is sometimes advantageous to conduct the slurry polymerisation above the critical temperature and pressure of the fluid mixture. Such operation is described in US-A- 5391654. In such operation, the temperature is typically from 70 to 110 °C, preferably from 80 to 105 °C and the pressure is from 40 to 150 bar, preferably from 50 to 100 bar. Where the upstream polymerisation is carried out by slurry polymerisation, the slurry polymerisation may be conducted in any known reactor used for slurry polymerisation. Such reactors include a continuous stirred tank reactor and a loop reactor. It is especially preferred to conduct the polymerisation in loop reactor. In such reactors the slurry is circulated with a high velocity along a closed pipe by using a circulation pump. Loop reactors are generally known in the art and examples are given, for instance, in USA-4582816, US-A-3405109, US-A-3324093, EP-A-479186, and US-A-5391654. The slurry may be withdrawn from the reactor either continuously or intermittently. A preferred way of intermittent withdrawal is the use of settling legs where slurry is allowed to concentrate before withdrawing a batch of the concentrated slurry from the reactor. The use of settling legs is disclosed, among others, in US-A-3374211 , US-A-3242150 and EP- A-1310295. Continuous withdrawal is disclosed, among others, in EP-A-891990, EP-A- 1415999, EP-A-1591460 and WO-A-2007 / 025640. The continuous withdrawal is advantageously combined with a suitable concentration method, as disclosed in EP-A- 1310295, EP-A-1591460, and EP3178853B1.

[0061] Hydrogen may be fed into the reactor to control the molecular weight of the polymer as known in the art. Furthermore, one or more alpha-olefin comonomers may be added into the reactor to control the density of the polymer product. The actual amount of such hydrogen and comonomer feeds depends on the catalyst that is used and the desired melt index (or molecular weight) and density (or comonomer content) of the resulting polymer.

[0062] Gas phase polymerisation

[0063] As mentioned above, polymerisation in the gas phase reactor involves polymerising an olefin monomer and optionally at least one alpha olefin comonomer. Where an upstream reactor is employed, polymerisation in the gas phase reactor may take place in the presence of the first polymer component.

[0064] Suitable olefin monomers include ethylene or propylene.

[0065] Where an alpha olefin comonomer is used, the alpha olefin comonomer may be selected from a C2 to C10 alkene. Suitable alpha olefin comonomers include ethylene, propylene, 1- butene, 1-pentene, 1-hexene, 1-heptene, 1-hexene, 1-octene, 1-nonene and 1-decene. Preferably, the alpha olefin comonomer is selected from at least one of ethylene, propylene, 1-butene, 1-hexene, and 1-octene. Where the olefin monomer polymerised in the gas phase reactor is propylene, the alpha olefin comonomer may be ethylene, 1-butene, 1-hexene and / or 1-octene. Where the olefin monomer is ethylene, the alpha olefin comonomer may be propylene, 1-butene and / or 1-hexene. Where an alpha olefin comonomer is used, one or more alpha comonomers may be employed. Preferably, one or two alpha comonomers are employed.

[0066] The alpha olefin comonomer content of the polymer component formed in the gas phase reactor may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%. Where the polymer component is a polypropylene, the alpha olefin comonomer content may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%. Where the polymer component is a polyethylene, the alpha olefin comonomer content may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%.

[0067] Where an upstream reactor is employed, the olefin monomer in the gas phase reactor is preferably the same as the first olefin monomer. For example, the first and second olefin monomers may be propylene. Alternatively, the first and second olefin monomers may be ethylene.

[0068] Where an upstream reactor is employed, the olefin polymerised in the gas phase reactor may be termed a “second” olefin monomer. The second olefin monomer may be polymerised in the downstream gas phase reactor to form a homopolymer as the second polymer component (also referred to as component B). Alternatively, the second olefin monomer may be polymerised to form a copolymer of the second olefin monomer and the at least one alpha olefin comonomer as the second polymer component.

[0069] For example, propylene may be polymerised in the downstream gas phase reactor to produce propylene homopolymer as the second polymer component. Alternatively, propylene may be co-polymerised with at least one alpha olefin comonomer to produce a propylene copolymer as the second polymer component.

[0070] In one embodiment, ethylene is polymerised in the downstream gas phase reactor to produce ethylene homopolymer as the second polymer component. Alternatively, ethylene may be co-polymerised with at least one alpha olefin comonomer to produce ethylene copolymer as the second polymer component.

[0071] Where an alpha olefin comonomer is used, the alpha olefin comonomer may be selected from a C2 to C10 alkene. For the avoidance of doubt, the alpha olefin comonomer is different from the second olefin monomer polymerised in the downstream reactor. Suitable alpha olefin comonomers include ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1 -hexene, 1 -octene, 1 -nonene and 1 -decene. Preferably, the alpha olefin comonomer is selected from at least one of ethylene, propylene, 1 -butene, 1 -hexene and 1 -octene. Where the second olefin monomer is propylene, the alpha olefin comonomer may be ethylene, 1- butene, 1 -hexene and / or 1 -octene. Where the second olefin monomer is ethylene, the alpha olefin comonomer may be propylene, 1 -butene, 1 -hexene, and / or 1 -octene. Where an alpha olefin comonomer is used, one or more alpha comonomers may be employed. Preferably, one or two alpha comonomers are employed.

[0072] The same of different alpha olefin comonomer may be used to form the first polymer component and second polymer component. Alternatively either or both of the first polymer component and second polymer components is a homopolymer.

[0073] The alpha olefin comonomer content of the second polymer component may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%. Where the second polymer component is a polypropylene, the alpha olefin comonomer content may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%. Where the second polymer component is a polyethylene, the alpha olefin comonomer content may be 0 to 10 mol%, preferably 0 to 8 mol%, more preferably 0 to 6 mol%.

[0074] The downstream gas phase polymerisation may take place in one or more gas phase polymerisation reactor(s) to produce the second polymer component.

[0075] The gas phase polymerisation may be conducted in any known reactor used for gas phase polymerisation. Such reactors include a fluidized bed reactor and a fast fluidized bed reactor or in any combination of these. When a combination of reactors is used then the polymer is transferred from one polymerisation reactor to another. Furthermore, a part or whole of the polymer from a polymerisation stage may be returned into a prior polymerisation stage.

[0076] The temperature in the gas phase polymerisation may be from 50 to 100 °C, preferably from 65 to 90 °C.

[0077] Generally, the operating pressure in the gas phase polymerisation may be from 5 to 30 barg. For example, 5 to 29 barg. Preferably, the operating pressure may be 10 to 27 barg, yet more preferably 15 to 22 barg. Where the second olefin monomer is propylene, the operating pressure may be 5 to 30 barg, more preferably 10 to 27 barg, yet more preferably 15 to 22 barg. However, the operating pressure of the gas phase reactor from which a side stream is withdrawn from the circulating gas may be from 18 to 30 barg, preferably from 20 to 25 barg, more preferably from 21 to 24 barg, even more preferably 22 to 23 barg. The side stream may be withdrawn from the most upstream of the gas phase reactors where multiple gas phase reactors are employed.

[0078] The residence time in the gas phase polymerisation is from 1.0 h to 4.5 h, preferably from 1.5 h to 4.0 h and in particular from 2.0 to 3.5 h. The residence time in gas phase polymerisation may be longer than the residence time in the upstream reactor(s) by at least 30%, preferably at least 40% or at least 50%.

[0079] The polymer production rate in the gas phase reactor may be from 10 tn / h to 65 tn / h, preferably from 12 tn / h to 58 tn / h and in particular from 13 tn / h to 52.0 tn / h, and thus the total polymer withdrawal rate from the gas phase reactor may be from 15 tn / h to 100 tn / h, preferably from 18 tn / h to 90 tn / h and in particular from 20 tn / h to 80.0 tn / h.

[0080] The gas phase polymerisation is preferably conducted in a gas-solids fluidized bed(s).

[0081] An example of a suitable downstream gas phase polymerisation reactor may comprise three zones: a) a bottom zone where fluidization gas is introduced into the reactor; b) a middle zone, which may have a generally cylindrical shape, where olefin monomer(s) present in the fluidization gas are polymerised to form the polymer particles; and c) a top zone, where fluidization gas is withdrawn from the reactor. A fluidization grid (also named distribution plate) may be employed to separate the bottom zone from the middle zone. The top zone may also act as a disengaging or entrainment zone in which, due to its expanding diameter compared to the middle zone, the fluidization gas can expand and disengage from the polyolefin powder.

[0082] As polymer particles are formed during polymerisation, a “dense phase” having an increased bulk density will form in the area within the middle zone of the reactor with due to the formation of the polymer particles.

[0083] In some embodiments, the bulk density of the dense phase during polymerization may be in the range of from 100 to 500 kg / m3, preferably of from 120 to 470 kg / m3, most preferably of from 150 to 450 kg / m3. A circulation (or fluidisation gas) stream is preferably withdrawn from the reactor exit (e.g., at the highest location). The withdrawn fluidization gas stream may then be cooled and reintroduced to the gas phase reactor (e.g., bottom zone), for example, as at least part of the fluidization gas. In order to remove entrained polyolefin powder from the withdrawn stream, a separator, for example, a cyclone(s) may be installed in the circulation gas line used to withdraw the fluidization gas stream. The cyclone can be used to remove any entrained polymer material from the withdrawn fluidization gas stream used as circulation gas. The polymer stream recovered from the cyclone can be directed to another polymerization stage, or it may be returned into the gas-solids olefin polymerization reactor or it may be withdrawn as the polymer product.

[0084] After separation of entrained solids and prior to being reintroduced into the gas phase reactor as or as part of the gas, the withdrawn stream may be compressed in a compressor and, optionally, cooled. Additional olefin monomer(s), eventual comonomer(s), hydrogen and inert gas are suitably introduced into the circulation gas line. It is preferred to analyse the composition of the circulation gas, for instance, by using on-line gas chromatography and adjust the addition of the gas components so that their contents are maintained at desired levels.

[0085] In the present invention, a side stream is taken from the circulating gas. This allows the pressure in the gas phase reactor to be reduced. Where a series of sequential downstream gas phase reactors is used, the side stream is taken from the withdrawn gas of the most upstream gas phase reactor of the series of sequential downstream gas phase reactors.

[0086] As explained above, the side stream is filtered to remove polymer particles from the side stream by filtration. A metal bag filter is preferably employed, although a combination of cyclone and filters may be used to separate the polymer from the side stream. The polymer may be recovered and / or returned to the gas phase reactor. The remaining filtered side stream may be treated by distillation to recover unreacted olefin monomer from the filtered side stream. Preferably, the distillation column used to recover the unreacted olefin monomer forms part of the recovery unit(s) used to recover unreacted olefin monomer from the product stream withdrawn from the process. More preferably, the distillation column used to recover the unreacted olefin monomer from the side stream also recovers unreacted olefin monomer from the product stream withdrawn from the process. In some examples, the distillation column used to recover the unreacted olefin monomer from the side stream also recovers unreacted olefin monomer from the gaseous stream withdrawn from the purge bin. This distillation stream may be a heavies column that also separates oligomers and other heavier hydrocarbons from the process. Alternatively or additionally, the distillation column used to recover the unreacted olefin monomer from the side stream is also the distillation column used to recover unreacted olefin monomer separated from any gaseous stream withdrawn from any gas-solid separator used upstream of the purge bin.

[0087] Preferably, the side stream is taken once the fluidised gas is compressed. The enhanced pressure can help to drive the compressed side stream through the filter and subsequent processing steps. Preferably, the fluidised gas is compressed to a pressure generally required for fluidisation, for example the pressure may be raised to about 3 barg from 1 barg.

[0088] As explained above, once the side stream is removed, the compressed fluidised gas may be cooled prior to being reintroduced into the downstream gas phase reactor. The cooling may take place via heat exchange with a coolant or process stream.

[0089] Polymerization catalyst

[0090] The polymerization in the multi-stage olefin polymerization reactors is conducted in the presence of an olefin polymerization catalyst. The catalyst may be any catalyst which is capable of producing the desired olefin polymer. Suitable catalysts are, among others, Ziegler - Natta catalysts based on a transition metal, such as titanium, zirconium and / or vanadium catalysts. Especially Ziegler - Natta catalysts are useful as they can produce olefin polymers within a wide range of molecular weight with a high productivity.

[0091] Suitable Ziegler-Natta catalysts preferably contain a magnesium compound, an aluminium compound and a titanium compound supported on a particulate support.

[0092] The particulate support can be an inorganic oxide support, such as silica, alumina, titania, silica-alumina and silica-titania. Preferably, the support is silica.

[0093] The average particle size of the silica support can be typically from 6 to 100 pm. However, it has turned out that special advantages can be obtained if the support has median particle size from 6 to 90 pm, preferably from 10 to 70 pm.

[0094] The magnesium compound may be a reaction product of a magnesium dialkyl and an alcohol. The alcohol may be a linear or branched aliphatic monoalcohol. Preferably, the alcohol has from 6 to 16 carbon atoms. Branched alcohols are especially preferred, and 2- ethyl-1 -hexanol is one example of the preferred alcohols. The magnesium dialkyl may be any compound of magnesium bonding to two alkyl groups, which may be the same or different. Butyl-octyl magnesium is one example of the preferred magnesium dialkyls.

[0095] The aluminium compound may be chlorine containing aluminium alkyl. Especially preferred compounds are aluminium alkyl dichlorides and aluminium alkyl sesquichlorides.

[0096] The titanium compound may be a halogen containing titanium compound, preferably chlorine containing titanium compound. Especially preferred titanium compound is titanium tetrachloride.

[0097] The catalyst can be prepared by sequentially contacting the carrier with the above mentioned compounds, as described in EP-A-688794 or WO- A- 99 / 51646 . Alternatively, it can be prepared by first preparing a solution from the components and then contacting the solution with a carrier, as described in WO-A-01 / 55230 .

[0098] Another group of suitable Ziegler-Natta catalysts contains a titanium compound together with a magnesium halide compound acting as a support. Thus, the catalyst contains a titanium compound on a magnesium dihalide, like magnesium dichloride. Such catalysts are disclosed, for instance, in WO-A-2005 / 118655 and EP-A-810235 .

[0099] Still a further type of Ziegler-Natta catalysts are catalysts prepared by a method, wherein an emulsion is formed, wherein the active components form a dispersed, i.e. a discontinuous phase in the emulsion of at least two liquid phases. The dispersed phase, in the form of droplets, is solidified from the emulsion, wherein catalyst in the form of solid particles is formed. The principles of preparation of these types of catalysts are given in WO-A- 2003 / 106510 of Borealis.

[0100] The Ziegler-Natta catalyst is used together with an activator. Suitable activators are metal alkyl compounds and especially aluminium alkyl compounds. These compounds include alkyl aluminium halides, such as ethylaluminium dichloride, diethylaluminium chloride, ethylaluminium sesquichloride, dimethylaluminium chloride and the like. They also include trialkylaluminium compounds, such as trimethylaluminium, triethylaluminium, triisobutylaluminium, trihexylaluminium and tri-n-octylaluminium. Furthermore they include alkylaluminium oxy-compounds, such as methylaluminiumoxane (MAO), hexaisobutylaluminiumoxane (HIBAO) and tetraisobutylaluminiumoxane (TIBAO). Other aluminium alkyl compounds, such as isoprenylaluminium, may also be used. Especially preferred activators are trialkylaluminiums, of which triethylaluminium, trimethylaluminium and tri-isobutylaluminium are particularly used. If needed the activator may also include an external electron donor. Suitable electron donor compounds are disclosed in WO-A- 95 / 32994 , US-A-4107414 , US-A-4186107 , US-A-4226963 , US-A-4347160 , US-A- 4382019 , US-A-4435550 , US-A-4465782 , US 4472524 , US-A-4473660 , US-A-4522930 , US-A-4530912 , US-A-4532313 , US-A-4560671 and US-A-4657882 . Also electron donors consisting of organosilane compounds, containing Si-OCOR, Si-OR, and / or Si-NR2 bonds, having silicon as the central atom, and R is an alkyl, alkenyl, aryl, arylalkyl or cycloalkyl with 1-20 carbon atoms are known in the art. Such compounds are described in US-A-4472524 , US-A-4522930 , US-A-4560671 , US-A-4581342 , US-A-4657882 , EP-A-45976 , EP-A- 45977 and EP-A- 1538167 .

[0101] The amount in which the activator is used depends on the specific catalyst and activator.

[0102] Typically triethylaluminium is used in such amount that the molar ratio of aluminium to the transition metal, like Al / Ti, is from 1 to 1000, preferably from 3 to 100 and in particular from about 5 to about 30 mol / mol.

[0103] Metallocene catalysts may also be used. Metallocene catalysts comprise a transition metal compound which contains a cyclopentadienyl, indenyl or fluorenyl ligand. Preferably the catalyst contains two cyclopentadienyl, indenyl or fluorenyl ligands, which may be bridged by a group preferably containing silicon and / or carbon atom(s). Further, the ligands may have substituents, such as alkyl groups, aryl groups, arylalkyl groups, alkylaryl groups, silyl groups, siloxy groups, alkoxy groups or other heteroatom groups or the like. Suitable metallocene catalysts are known in the art and are disclosed, among others, in WO-A- 95 / 12622 , WO- A- 96 / 32423 , WO-A-97 / 28170 , WO-A-98 / 32776 , WO-A-99 / 61489 , WO-A- 03 / 010208 , WO-A-03 / 051934 , WO-A-03 / 051514 , WO-A-2004 / 085499 , EP-A-1752462 and EP-A-1739103 .

[0104] These and other aspects of the present disclosure will now be described with reference to the accompanying drawings in which Figure 1 is a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to an example of the present disclosure. Figure 2 is a variant of the example of Figure 1 , in which two distillation columns are used to recover unreacted olefin monomer from the product stream.

[0105] Figure 1 depicts a schematic view of a multistage polymerisation process configuration for carrying out a process according to an example of the present disclosure. The configuration comprises slurry loop reactors 1 for producing a first polymer component, and a downstream gas phase reactor 2. Although a single gas phase reactor 2 is depicted, it will be understood that a plurality (e.g., 2 or 3) of gas phase reactors may be used in series (not shown).

[0106] The configuration also comprises a separation zone comprising a gas phase separator 3, a purge bin 4, a propylene-nitrogen membrane recovery unit 5, and a distillation unit 6.

[0107] In operation, an olefin monomer, for example, propylene is polymerised in the slurry loop reactors 1 to produce a first polymer component. This polymerisation step may occur in the presence of an alpha olefin comonomer. The reactor effluent from the slurry loop reactors 1 is introduced into the downstream gas phase reactor 2. Further olefin monomer (e.g., propylene), optional alpha olefin comonomer and optional hydrogen may be introduced into the gas phase reactor 2 via inlets 7.

[0108] In the gas phase reactor 2, the further olefin monomer is polymerised to form a second polymer component in the presence of the first polymer component. The polymerisation may be a copolymerisation reaction with any alpha olefin comonomer introduced via inlet 7. The reaction occurs in a fluidised bed maintained by the circulation of e.g., fluidisation gas 8 withdrawn from an upper region of the gas phase reactor 2 and circulated to the bottom of the gas phase reactor 2. The gas is compressed using compressor 9 and cooled via heat exchanger 10 prior to being reintroduced into the gas phase reactor.

[0109] In the present disclosure, a side stream 11 is taken from the circulating gas. The removal of this side stream causes the operating pressure in the reactor 2 to decrease. Accordingly, the side stream is taken when the operating pressure in the reactor 2 reaches a predetermined threshold value. In one example, this may be a value between, for example, 18 and 30 barg, preferably 20 to 23 barg. When the operating pressure reaches this value, the side stream 11 is taken, for example, by opening a valve (not shown). The side stream is allowed to flow for a duration necessary to reduce the operating temperature below the threshold value.

[0110] The side stream 11 is introduced into a cyclone 12 and filter 13 to remove solid (e.g. polymer and catalyst particles) from the side stream. This solid is returned to the gas phase reactor via lines 14a and 14b. The filtered side stream 15 is introduced into the distillation unit 6, where unreacted olefin monomer (propylene) and optional alpha olefin comonomer in the side stream is recovered and recycled, for example, to the gas phase reactor 2.

[0111] Product stream from the process (e.g., from the gas phase reactor 2) is withdrawn via line 17 and introduced into a gas-phase separator 3. Here a gaseous stream 18 comprising unreacted monomer is withdrawn from the gas-phase separator 3, compressed and distilled in distillation unit 6. The recovered unreacted olefin monomer and optional alpha olefin comonomer is recycled, for example, to the gas phase reactor 2.

[0112] The solid separated in the gas-phase separator 3 may contain entrained gaseous matter, including unreacted olefin monomer and optional alpha olefin comonomer. This solid is introduced into a purge bin 4 and purged with nitrogen gas to produce another gaseous stream comprising unreacted olefin monomer that can be separated from the polymer. This gaseous stream is removed from the purge bin via line 19, while the remaining solid polymer is withdrawn from the purge bin 4 via line 20. The gaseous streams from the purge bin 4 are introduced into an olefin (e.g., propylene)-nitrogen separator that may separate nitrogen from propylene by membrane separation. The nitrogen may be reused as purge gas via line 21 , while the olefin may be recovered by distillation in distillation column 6 and recycled to the process. The distillation column 6 may be a heavies column used for heavies 22 removal. Reflux vessel 24 is also used for lights separation of which part can be recovered to the process.

[0113] Referring to Figure 2, this illustrates a variant of the example of Figure 1, in which two distillation columns are used to recover unreacted olefin monomer from the product stream. Like numerals have been used to refer to like parts.

[0114] As can be seen from Figure 2, the product stream 17 from the gas phase reactor 2 (not shown in Figure 2) is introduced into the gas-solid separator 3. The gaseous stream 18 comprising unreacted monomer is withdrawn from the gas-phase separator 3, cooled and compressed and introduced to distillation unit 6 and a further distillation unit 100 via lines 118 and 110, respectively. The distillation unit 100 recovers unreacted olefin monomer that can be recycled to the process as an overhead stream, while the bottoms 114 from the further distillation unit 100 is introduced into distillation unit 6. Oligomers and heavier hydrocarbons are removed from distillation unit 6 via line 116. Olefin monomer may be removed from the top of the column and introduced into a reflux vessel, from which the olefin monomer may be recycled to the gas phase reactor as a liquid.

[0115] The filtered side stream 15 (not shown) may be introduced into one or both of distillation units 6 and 100 for recovery of the unreacted monomer for return to the process.

Claims

CLAIMS1. A process for polymerising olefins, said process comprising: polymerising olefin monomer in a gas phase reactor at an operating pressure below a predetermined threshold value; withdrawing circulation gas comprising unreacted olefin monomer from an outlet in an upper region of the gas phase reactor and reintroducing the circulation gas into an inlet in a lower region of the gas phase reactor to maintain a fluidised bed in the gas phase reactor; wherein, when the operating pressure in the gas phase reactor exceeds the predetermined threshold pressure, a side stream is taken from the withdrawn circulation gas for a duration sufficient to reduce the operating pressure of the gas phase reactor to below the predetermined threshold value, wherein any solid particulates present in the side stream are filtered to produce a filtered side stream, and at least a portion of the unreacted olefin monomer from the filtered side stream is recovered, optionally, in at least one distillation column.

2. A process as claimed in claim 1 , which further comprises withdrawing a product stream comprising polymer and unreacted olefin monomer from the gas phase reactor; and recovering at least a portion of the unreacted olefin monomer from the product stream in at least one distillation column; wherein the at least a portion of the unreacted olefin monomer is optionally recovered from the filtered side stream in the same at least one distillation column used to recover the unreacted olefin monomer from the product stream.

3. A process as claimed in claim 2, which further comprises introducing withdrawn product stream comprising polymer and unreacted olefin monomer into a purge bin; using a purge gas to separate a gaseous stream comprising unreacted olefin monomer from the polymer; and recovering unreacted olefin monomer from the separated gaseous stream and unreacted olefin monomer from the filtered side stream, optionally by distillation in the at least one distillation column.

4. A process as claimed in claim 3, which further comprises introducing withdrawn product stream comprising polymer and unreacted olefin monomer into a gas-solid separator prior to the purge bin, and which further comprises removing a furthergaseous stream comprising unreacted olefin monomer from the gas-solid separator, and introducing the further gaseous stream into a further distillation column for recovery of unreacted olefin monomer.

5. A process as claimed in claim 4, wherein the filtered side stream is also introduced into the further distillation column for recovery of unreacted olefin monomer from the filtered side stream.

6. A process as claimed in any one of the preceding claims, which is a multistage polymerisation process comprising polymerising olefin monomer in an upstream reactor to produce a first polymer component and transferring the first polymer component to the gas phase reactor, such that polymerisation of the olefin monomer in the gas phase reactor takes place in the presence of the first polymer component.

7. A process as claimed in any one of the preceding claims, which comprises a series of sequential gas phase reactors and, when the operating pressure in the most upstream of the gas phase reactors exceeds the predetermined threshold value, a side stream is taken from the withdrawn circulation gas of the most upstream of the gas phase reactors for a duration sufficient to reduce the operating pressure of the most upstream of the gas phase reactor to below the predetermined threshold value.

8. A process as claimed in any one of the preceding claims, wherein the withdrawn circulation gas is compressed in a gas compression unit prior to being reintroduced to the gas phase reactor.

9. A process as claimed in claim 8, wherein the side stream is taken from the withdrawn gas after the withdrawn gas has been compressed.

10. A process as claimed in any one of the preceding claims, wherein the side stream taken from the circulation gas is passed through a cyclone and a filter prior to the unreacted olefin monomer from the filtered side stream being recovered, optionally in the at least one distillation column.

11. A process as claimed in any one of the preceding claims, wherein solid particulates present in the side stream are filtered using a sintered metal filter.

12. A process as claimed in any one of the preceding claims, wherein after the side stream has flowed for a duration sufficient to reduce the operating pressure of the gas phase reactor to below the predetermined threshold value, the flow of the side stream is ceased until the operating pressure in the gas phase reactor again exceeds the predetermined threshold value.

13. A process as claimed in any one of the preceding claims, wherein the predetermined threshold value is 25 barg or below, preferably 24 barg or below, more preferably 23 barg or below.

14. A process as claimed in claim 13, wherein the predetermined threshold value is a value within the range of from 20 to 25 barg, preferably a value within the range of from 21 to 24 barg, more preferably a value within the range of from 22 to 23 barg.

15. A process as claimed in any one of the preceding claims, wherein, when the operating pressure in the gas phase reactor exceeds the predetermined range, a side stream is taken from the withdrawn circulation gas for a duration sufficient to reduce the operating pressure of the gas phase reactor to less than 15%, preferably less than 10%, more preferably less than 5% below the predetermined threshold value.

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