Polyolefin powder decontamination process

Abstract

A process for treating a polyolefin powder contaminated with at least one hydrocarbon, comprising the steps of: a) discharging contaminated polyolefin powder from a polymerization reactor into a first degassing vessel, e.g. via a product recovery vessel; b) contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam to extract the hydrocarbon contamination from said powder thereby providing decontaminated polyolefin powder; c) transferring from the first degassing vessel, nitrogen, steam and hydrocarbon contamination to a flare system or a thermal oxidation system; d) transferring the decontaminated polyolefin powder from step b) into a powder conveying system containing a nitrogen stream to convey the decontaminated polyolefin powder; wherein at least a part of the nitrogen stream is recycled from said powder conveying system to the first degassing vessel.

Description

[0001] 170775 / 01 Polyolefin powder decontamination process

[0002] This invention relates to a process for the treatment of contaminated polyolefin powder to remove hydrocarbons therefrom before the decontaminated polyolefin powder is pelletized for sale and use. In particular, the invention relates to a process for efficiently removing hydrocarbons whilst minimizing the use of nitrogen and minimizing the amount of nitrogen that is sent to flare / thermal oxidation along with the hydrocarbon contamination. The invention also relates to a system for carrying out this process.

[0003] Background

[0004] Many processes are known for the polymerisation of olefins to make polyolefins. Polyolefins are often manufactured in slurry or gas phase reactors and the polyolefin is produced in the form of a powder. This powder is conventionally removed from the reactor and sent to a holding vessel often called a purge tank or degassing vessel before being transferred to a powder silo for storage before pelletisation.

[0005] It will be appreciated that the powder that is removed from the reactor and sent to the degassing vessel may contain some hydrocarbons as impurities. These hydrocarbons may derive from the diluent which is used in the polymerisation process. There may also be unreacted monomers and oligomers trapped within the polyolefin powder. The contaminated polyolefin powder therefore needs to be decontaminated.

[0006] In particular, when polyolefin powder is removed from a gas-phase polymerization reactor, the discharged powder not only contains polyolefin but also portions of the gaseous phase used in the gas phase reactor as intergranular gas or as dissolved hydrocarbons. For ecological, safety and quality reasons, these entrained parts of the gas-phase have to be removed from the polyolefin powder.

[0007] A common practice for removing the entrained parts of the gas-phase from the polyolefin powder is contacting the powder with a stream of an inert gas, usually in countercurrent flow. Such a step is frequently denoted as "degassing" or "purging". Often such a degassing or purging step is combined with a step of deactivating the polymerization catalyst and / or cocatalysts, e.g. by reacting the catalyst and / or cocatalysts with steam. For example, EP-A-339122 discloses a two-step method for removing unpolymerized gaseous monomers from a solid olefin polymer while deactivating Ziegler-Natta catalysts and organometallic catalyst residues present in said solid olefin polymer, which is carried out in a single vessel. The solid olefin polymer is first counter currently contacted with a first purge gas, preferably pure nitrogen, in an upper zone of the purge vessel, then transferred in the lower zone of the purge vessel and there counter currently contacted with a second purge gas containing water, preferably pure nitrogen and steam.

[0008] US 5,071,950 degasses using a two-step process by first flushing with gaseous ethylene and then flushing with a mixture of nitrogen and steam. EP-A- 683176 degasses with a mixture of nitrogen, water and oxygen.

[0009] W02006 / 082007 discloses degassing with a propane fraction. WO 2008 / 015228 describes a process in which polyolefin particles are subjected to a first degassing step in which the polyolefin particles are counter currently contacted with a gaseous stream containing at least 85 mol-% of a C3 -C5 alkane and then to a second degassing step in which the polyolefin particles are counter currently contacted with steam.

[0010] EP-A-2743279 describes a process in which the polymerisation of olefins at takes place in the presence of a hydrocarbon diluent, typically a C3-5 alkane, such as propane. The polyolefin particles that form during the polymerisation process are then degassed in a three step process firstly using a hydrocarbon gas stream, followed by a nitrogen steam degassing step and finally a nitrogen degassing step.CN102161715 is directed to a method of degassing solid polymers and recovering the exhaust gas. The solid polymer is transferred from the reactor to a degassing container, and then is contacted with a degassing medium to remove any solvent or unreacted monomer from the solid polymer. The exhaust gas can then be recycled after being recovered and treated. In particular, the method in D1 involves three degassing steps: 1) primary degassing; 2) secondary degassing; and 3) tertiary degassing. No steam is however employed to decontaminate polyolefin powder.

[0011] CN115386024 describes a system for degassing polypropylene comprises a degassing system connected to a reaction discharge line, wherein the degassing system comprises an expansion bag filter, a degassing bin, a steam tank and a drying tank all connected in series. Degassing occurs in three vessels; in the expansion bag filler; in the degassing bin; and the steaming tank. CN 101501079 refers to a process for the purification of polyolefin powder produced in a gas phase reactor to remove heavy hydrocarbons and oligomeric impurities. The process reported therein comprises subjecting polyolefin particles from a GPR to a first degassing step involving contacting the polyolefin powder with a gas stream comprising 85 mol% of a C3-C5 alkane obtained from a monomer recovery section of a polymerisation plant.

[0012] In the present invention hydrocarbons are extracted from the polyolefin powder by nitrogen in a degassing vessel. A portion of the gas within the degassing vessel is sent to flare as this gas contains a high concentration of hydrocarbons.

[0013] Nitrogen is also used to convey polyolefin powder from the first degassing vessel to the powder silos used for storage and then from the powder silos to a pelletizing system. Nitrogen used in this process has to be replaced continuously to prevent accumulation of hydrocarbons in the conveying gas. As the nitrogen conveys the powder around the system, further hydrocarbons can be extracted from the polyolefin powder leading therefore to a build-up of hydrocarbons within the conveying gas. The gas within the conveying system therefore needs removed and replaced to avoid this build-up.

[0014] Because current decontamination processes use large amounts of nitrogen, the present inventors seek to reduce the nitrogen requirements thereof. The inventors have found that through the use of certain recycles, the amount of nitrogen used during the decontamination of the polyolefin powder and during the subsequent transfer of the polyolefin powder to storage can be significantly reduced. Moreover, the present process ensures that the majority of the hydrocarbon impurities are concentrated within the first degassing vessel and hence the hydrocarbon content of the gas mixture sent to flare or thermal oxidation is high. This minimizes the need to add additional fuel to the flare or thermal oxidation process to ensure that a proper burn can occur.

[0015] Summary of Invention Viewed from one aspect, the invention provides a process for treating a polyolefin powder contaminated with at least one hydrocarbon, comprising the steps of: a) discharging contaminated polyolefin powder from a polymerization reactor into a first degassing vessel, e.g. via a product recovery vessel; b) contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam to extract the hydrocarbon contamination from said powder thereby providing decontaminated polyolefin powder; c) transferring from the first degassing vessel, nitrogen, steam and hydrocarbon contamination to a flare system or to a thermal oxidation system; d) transferring the decontaminated polyolefin powder from step b) into a powder conveying system, preferably a closed loop powder conveying system, containing a nitrogen stream to convey the decontaminated polyolefin powder; wherein the nitrogen stream is recycled from said powder conveying system, preferably said closed loop powder conveying system, to the first degassing vessel.

[0016] Viewed from another aspect, the invention provides a process for treating a polyolefin powder contaminated with at least one hydrocarbon comprising the steps of: a) discharging contaminated polyolefin powder continuously or discontinuously from a polymerization reactor into a first degassing vessel, e.g. via a product recovery vessel; b) contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam to extract the hydrocarbon contamination from said polyolefin powder thereby providing decontaminated polyolefin powder; c) transferring from the first degassing vessel, nitrogen, steam and hydrocarbon contamination to a flare system or a thermal oxidation system; optionally d) transferring the decontaminated polyolefin powder from step b) into a second degassing vessel and supplying nitrogen to said second degassing vessel, preferably in the absence of steam, such that the decontaminated polyolefin powder contacts said nitrogen to thereby provide a second decontaminated polyolefin powder and transferring from the second degassing vessel, nitrogen and hydrocarbon contamination to a flare system or to a thermal oxidation system; e) transferring the decontaminated polyolefin powder from step b) or if present step d) into a rotary valve; f) transferring the decontaminated polyolefin powder from said rotary valve into a powder conveying system, preferably a closed loop powder conveying system, containing a nitrogen stream to convey the polyolefin powder; wherein the rotary valve has a nitrogen bleed stream which is at least partially recycled to the first and / or second degassing vessel.

[0017] Viewed from another aspect the invention provides a process for treating a polyolefin powder contaminated with at least one hydrocarbon comprising the steps of: a) discharging contaminated polyolefin powder from a polymerization reactor into a first degassing vessel, e.g. via a product recovery vessel; b) contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam to extract the hydrocarbon contamination from said polyolefin powder thereby providing decontaminated polyolefin powder; c) transferring from the first degassing vessel, nitrogen, steam and hydrocarbon contamination to a flare system or a thermal oxidation system; d) transferring the decontaminated polyolefin powder from step b) into a second degassing vessel and supplying nitrogen to said second degassing vessel, preferably in the absence of steam, such that the decontaminated polyolefin powder contacts said nitrogen; e) transferring from the second degassing vessel, nitrogen and hydrocarbon contamination to a flare system or a thermal oxidation system; f) transferring the decontaminated polyolefin powder from step d) into a powder conveying system, preferably a closed loop powder conveying system, containing a nitrogen stream to convey the polyolefin powder; wherein at least a part of the nitrogen stream is recycled from said powder conveying system, preferably said closed loop powder conveying system, to the first degassing vessel.

[0018] Alternatively viewed therefore, the invention provides a process for treating a polyolefin powder contaminated with at least one hydrocarbon, comprising the steps of: a) discharging contaminated polyolefin powder from a polymerization reactor into a first degassing vessel, e.g. via a product recovery vessel; b) contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam to extract the hydrocarbon contamination from said powder thereby providing decontaminated polyolefin powder; c) transferring from the first degassing vessel, nitrogen, steam and hydrocarbon contamination to a flare system or a thermal oxidation system; d) optionally transferring the decontaminated polyolefin powder from step b) into a second degassing vessel and supplying nitrogen to said second degassing vessel, preferably in the absence of steam, such that the polyolefin powder contacts said nitrogen; e) if optional step d) is present, transferring from the second degassing vessel, nitrogen and hydrocarbon contamination to a flare system or a thermal oxidation system; f) transferring the decontaminated polyolefin powder from step b) or d) into a powder conveying system, preferably a closed loop powder conveying system, containing a nitrogen stream to convey the decontaminated polyolefin powder; wherein at least a part of the nitrogen stream is recycled from said powder conveying system, preferably said closed loop powder conveying system, to the first degassing vessel.

[0019] Viewed from another aspect the invention provides a system for treating a polyolefin powder contaminated with at least one hydrocarbon, the system comprising: i) a polymerisation reactor in which polyolefin powder can be obtained by polymerization of one or more olefins in the presence of a polymerization catalyst; ii) a first degassing vessel configured to receive contaminated polyolefin powder from said polymerisation reactor via a conduit, wherein said first degassing vessel is configured to be supplied with nitrogen and steam via one or more inlets; iii) a conduit configured to allow nitrogen, steam and hydrocarbons to be removed from the first degassing vessel and sent to a flare or thermal oxidation system; iv) a powder conveying system, preferably a closed loop powder conveying system, configured to receive decontaminated polyolefin powder from the first degassing vessel via a conduit in which nitrogen can convey the polyolefin powder to at least one polyolefin powder silo; v) a conduit for recycling nitrogen from the powder conveying system, preferably said closed loop powder conveying system, to the first degassing vessel.

[0020] Viewed from another aspect the invention provides a system for treating a polyolefin powder contaminated with at least one hydrocarbon, the system comprising: i) a polymerisation reactor in which polyolefin powder can be obtained by polymerization of one or more olefins in the presence of a polymerization catalyst; ii) a first degassing vessel configured to receive contaminated polyolefin powder from said polymerisation reactor via a conduit wherein said first degassing vessel is configured to be supplied with nitrogen and steam via one or more inlets; iii) a conduit configured to allow nitrogen, steam and hydrocarbons to be removed from the first degassing vessel and sent to a flare or thermal oxidation system; iv) a second degassing vessel configured to receive partially decontaminated polyolefin powder from said first degassing vessel via a conduit wherein said second degassing vessel is configured to be supplied with nitrogen via one or more inlets; v) a conduit configured to allow nitrogen and hydrocarbons to be removed from the second degassing vessel and sent to at least one of a flare, a thermal oxidation system or the first degassing vessel; vi) a powder conveying system, preferably a closed loop powder conveying system, configured to receive decontaminated polyolefin powder from the second degassing vessel via a conduit in which nitrogen can convey the polyolefin powder to at least one powder silo; vii) a conduit for recycling nitrogen from the powder conveying system, preferably said closed loop powder conveying system, to the first and / or second degassing vessel.

[0021] Viewed from another aspect the invention provides a system for treating a polyolefin powder contaminated with at least one hydrocarbon, the system comprising: i) a polymerisation reactor in which polyolefin powder can be obtained by polymerization of one or more olefins in the presence of a polymerization catalyst; ii) a first degassing vessel configured to receive contaminated polyolefin powder from said polymerisation reactor via a conduit wherein said first degassing vessel is configured to be supplied with nitrogen and steam via one or more inlets; iii) a conduit configured to allow nitrogen, steam and hydrocarbons to be removed from the first degassing vessel and sent to a flare or thermal oxidation system; iv) optionally a second degassing vessel configured to receive partially decontaminated polyolefin powder from said first degassing vessel via a conduit wherein said second degassing vessel is configured to be supplied with nitrogen via one or more inlets; v) and a conduit configured to allow nitrogen and hydrocarbons to be removed from the second degassing vessel and sent to at least one of a flare, a thermal oxidation system or the first degassing vessel; vi) a rotary valve located between said first degassing vessel and the powder conveying system or, if present, located between the second degassing vessel and the powder conveying system, said rotary valve having a conduit configured to carry a bleed stream to said first and / or second degassing vessel; vii) a powder conveying system, preferably a closed loop powder conveying system, configured to receive decontaminated polyolefin powder from the rotary valve and in which nitrogen can convey the polyolefin powder to at least one powder silo; viii) optionally a conduit for recycling nitrogen from the powder conveying system, preferably said closed loop powder conveying system, to the first and / or second degassing vessel; and ix) optionally a product recovery vessel and / or polymer outlet vessel wherein the polyolefin powder from the polymerisation reactor may be transferred via product recovery vessel and / or polymer outlet vessel to the first degassing vessel.

[0022] Brief Description of the Figures

[0023] Figure 1 shows a system and process of the invention with a recycle from the closed loop powder conveying system to the first degassing vessel.

[0024] Figure 2 shows a slightly different system and process of the invention with a recycle from the rotary valve to the first degassing vessel and a recycle the closed loop powder conveying system to the first degassing vessel.

[0025] Detailed Description of the invention

[0026] This invention relates to a process for treating a polyolefin powder to remove hydrocarbon contaminants therefrom. The invention also relates to a system for carrying out this process. The decontaminated polyolefin powder can be transferred into one or more powder silos and subsequently pelletised. In general the storage and pelletisation steps can be effected using conventional conditions well-known to the skilled person and will not be further described herein.

[0027] The first step of the process of the invention involves the transfer of contaminated polyolefin powder from a polymerisation reactor into a first degassing vessel. The olefin polymerisation process can be carried out under any suitable conditions using any suitable catalyst system. The nature of the polymerisation process is not critical although preferably the polymerisation process will use a solution phase, slurry phase or gas phase polymerisation in the presence of a catalyst as opposed to a high pressure tubular / autoclave process typically used for the manufacture of low density polyethylene and similar polymers.

[0028] It is preferred if the polymerisation reactor from which the transfer of polyolefin powder occurs is a gas phase reactor, in particular where the polyolefin powder has been made in a cascade of at least one slurry reactor and at least one gas phase reactor. Such a reactor cascade is used by Borealis in its proprietary Borstar® process.

[0029] It is preferred that a hydrocarbon diluent such as a C3-6 alkane is used in the polymerisation reactor, such as the gas phase reactor. The diluent, preferably propane, can contaminate the polyolefin powder and become entrained therein. Other hydrocarbons might also be present in the polyolefin powder such as unreacted monomer or comonomer, such as ethylene, propylene or a C4-10 alpha olefin. Oligomers might also become entrained within the polyolefin powder. The polyolefin powder is preferably a polypropylene or polyethylene powder such as a polyethylene copolymer powder, e.g. a polyethylene copolymer of ethylene and at least one C4-10 alpha olefin comonomer, especially 1 -butene, 1 -hexene or 1-octene. The polyethylene powder might be an LLDPE or HDPE for example. . The polyolefin is preferably not polypropylene.

[0030] In one embodiment, the contaminated polyolefin powder can be discharged from the polymerisation reactor continuously or discontinuously into the first degassing vessel via a suitable conduit. Whilst this transfer may occur simply under gravity, it is preferred if nitrogen is used as a conveying gas to move the contaminated polyolefin powder from the polymerisation reactor into the first degassing vessel.

[0031] Preferably, at least one product recovery vessel is present before the first degassing vessel. An indirect discharge from the polymerization reactor to the first degassing vessel (e.g. via the recovery vessel) is therefore preferred.

[0032] If the polymerization process is carried out batchwise (discontinuously), preferably a batch outlet vessel is present before the product recovery vessel, and first degassing vessel.

[0033] It is most preferred therefore if the polyolefin powder is discharged from the polymerization reactor continuously or discontinuously into a product recovery vessel prior to being transferred to the first degassing vessel. The term conduit is used herein simply to define a tube or pipe capable of conveying gas and polyolefin powder from one location to another.

[0034] There may be other components between discharge from the polymerization reactor and the first degassing vessel, e.g. a polymer outlet vessel may be present before the product recovery vessel, e.g. in continuous operation. The outlet vessel and product recovery vessel may be characterized by the presence of a hydrocarbon gas atmosphere. In the first degassing vessel, a nitrogen atmosphere is used.

[0035] The polyolefin powder at this stage is contaminated not only with hydrocarbons but also potentially catalyst residues from the polymerisation reactor may be present. Such residues might be Ziegler Natta or single site catalyst residues, such as metallocene catalyst residues. The polyolefin powder needs decontaminating and that occurs within the first degassing vessel.

[0036] The first degassing vessel may also be called a purge tank herein. The first degassing vessel will have an inlet to allow contaminated polyolefin powder to enter the first degassing vessel and an outlet to allow decontaminated polyolefin powder to exit the first degassing vessel. Conveniently, the outlet is at the base of the first degassing vessel and the inlet is towards the top or at the top of the first degassing vessel. The degassing vessel may have impellers or another mixing device to ensure adequate mixing between the gas and polyolefin powder.

[0037] The first degassing vessel will also have at least one gas inlet for the supply of nitrogen gas to the vessel. There may be a separate inlet for the supply of steam to the first degassing vessel or steam might also be supplied along with the nitrogen gas via the same inlet.

[0038] It is preferred if steam is added simultaneously with the nitrogen in the first degassing vessel. This addition may be in the same conduit or a different conduit. Sequential addition of steam following contact of the powder with nitrogen is not preferred and is ideally avoided. Ideally therefore nitrogen and steam are supplied to the first degassing vessel from the start of the degassing process.

[0039] It is preferred if the degassing process in the first degassing vessel does not involve the introduction of hydrocarbon gas as a degassing gas, such as a C3-5 alkane as a degassing agent. The present invention utilises nitrogen and steam as degassing agents rather than a more expensive and dangerous hydrocarbon gas. Any hydrocarbons gases present are therefore impurities from the polyolefin production process.

[0040] The first degassing vessel also has an outlet to allow the contaminated gas mixture that is generated when the nitrogen and steam contact the contaminated polyolefin powder to be sent to flare or thermal oxidation.

[0041] In the first degassing vessel therefore, the contaminated polyolefin powder makes contact with nitrogen and steam to extract the hydrocarbon contamination from said powder thereby providing an at least partially decontaminated polyolefin powder. It will be appreciated that the contaminated polyolefin powder may be fully decontaminated in this process (i.e. the level of hydrocarbon contamination is below the measuring capability of the custom equipment used in the art to measure thereof) but it is also possible that the contaminated polyolefin powder is only partially decontaminated and hence a further decontamination step might be used, e.g. using the second degassing vessel. The term decontaminated polyolefin powder is used herein both to cover one that has been fully decontaminated of contaminant hydrocarbons or only some hydrocarbon contaminants removed (i.e. partially decontaminated). If all contaminants are removed then the polyolefin powder would be completely decontaminated.

[0042] Nitrogen, steam and the hydrocarbon contaminants (hereinafter referred to also as “a gas mixture”) that have been extracted from the polyolefin powder can then be removed from the first degassing vessel and sent to a flare system or a thermal oxidation system. The specifics of the flare of thermal oxidation system will not be further discussed herein as they are conventional and well known to the skilled person. It is sometimes required that more fuel, e.g. additional hydrocarbon, is added to the flare to ensure that burning is possible as the concentration of hydrocarbons in the gas mixture can be low. The present process however may be able to maximize the hydrocarbon content of the gas mixture being sent to flare / thermal oxidation to minimize this requirement.

[0043] It may be that other contaminated gas streams are sent to flare / thermal oxidation from different parts of the system. For example, the gas mixture sent to flare / thermal oxidation may also encompass a contaminated gas mixture from any second degassing vessel.

[0044] It is preferred if the only streams sent to flare or thermal oxidation are from the first and second degassing vessels. It is also preferred that no flare from the powder conveying system is present.

[0045] The contaminated gas mixture sent to flare / thermal oxidation will therefore contain nitrogen, steam and hydrocarbons extracted from the polyolefin powder. The hydrocarbon content in the gas mixture sent to flare / thermal oxidation may be in the range of between 5 and 70wt%, more preferably between 10 and 50wt%, most preferably between 20 to 30 wt%.

[0046] The nitrogen gas fed into the first degassing vessel, contacts the contaminated polyolefin powder in the first degassing vessel. It is also required that steam is fed to the first degassing vessel. By contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam hydrocarbon contaminants in the polyolefin powder are extracted from said powder into the gas mixture and any active catalyst is killed.

[0047] It will be appreciated that the contaminated polyolefin powder exiting the polymerisation reactor is hot and hence this heat maintains the steam in the gaseous phase.

[0048] The relative amount of steam to nitrogen is not crucial but generally as little steam as possible is used to kill the catalyst. There might be at least 80 wt%, more preferably at least 90 wt%, most preferably at least 95 wt%, such as at least 99 wt% nitrogen in the gas mixture fed to the first degassing vessel and less than 20 wt%, more preferably less than 10 wt%, most preferably less than 5 wt%, such as less than 1 wt% steam in the gas mixture that is fed to the first degassing vessel. There may be a minimum of 0.1 wt% steam in the gas mixture.

[0049] It will be appreciated however, that nitrogen can be recycled into the first degassing vessel from the powder conveying system or from the rotary valve (discussed in detail below) and this recycled nitrogen contains hydrocarbons extracted from the polyolefin powder later in the process. The amount of hydrocarbons present is however low. The hydrocarbon content of the gas mixture which enters the first degassing vessel may be in the range of up to 10 wt%, more preferably up to 5 wt%, most preferably up to 3 wt%. It is generally preferred if the hydrocarbon content of the gas mixture which enters the first degassing vessel is at most 1.0 wt%, more preferably at most 0.5 wt%, most preferably at most 0.3 wt%. There may be a minimum of 0.1 wt% hydrocarbon content in the gas mixture.

[0050] The gas pressure in the first degassing vessel can be 0.01 to 1.5 barg, preferably 0.05 to 1.0 barg, such as 0.5 barg or less.

[0051] It is also possible to employ a second degassing vessel (sometimes called a surge vessel) after the first degassing vessel. The second degassing vessel is connected in series with the first degassing vessel and may have the same construction as said first vessel, e.g. the same inlets and outlets. If used, polyolefin powder from the first degassing vessel enters the second degassing vessel conveyed by gas from the first degassing vessel. After a second degassing process, the polyolefin powder can then be removed, typically from the base, of the second degassing vessel and transferred to the powder conveying system, e.g. via a rotary valve as described further herein. In one embodiment fresh nitrogen and steam can be fed into the second degassing vessel. However, it is preferred if no steam is added to the second degassing vessel. The second degassing vessel is thus free of steam other than any steam that might be transferred into the second degassing vessel along with the polyolefin powder. It is preferred therefore if only nitrogen, which may be fresh and / or recycled nitrogen or a mixture of the two, is supplied to the second degassing vessel.

[0052] The second degassing vessel can also be provided with a conduit to remove the contaminated gas mixture to flare / thermal oxidation. Conveniently the contaminated gas mixtures from both first and second degassing vessels are combined before the flare / thermal oxidation process.

[0053] The pressure within the second degassing vessel is preferably within the same ranges as described above for the first degassing vessel. By using a second degassing vessel, the risk of steam entering the powder conveying system is reduced.

[0054] Nitrogen gas from the second degassing vessel can be recycled to the first degassing vessel.

[0055] In one embodiment, the decontaminated polyolefin powder is then conveyed, under nitrogen pressure, from the first degassing vessel (or from the second degassing vessel if it is used) into a powder conveying system, preferably a closed loop powder conveying system, containing a nitrogen stream to convey the polyolefin powder around the loop. The powder conveying system is most preferably a closed loop powder conveying system.

[0056] The polymer powder is typically conveyed through the powder conveying system to one or more powder silos. Nitrogen gas therefore conveys the polyolefin powder through the powder conveying system to the powder silo(s). Typically the polyolefin powder will fall under gravity into the powder silos once the polyolefin powder has passed through the conduit leading to the powder silo(s). The nitrogen gas continuously circulates around the powder conveying system, preferably closed loop powder conveying system as described further below.

[0057] Conveniently, a rotary valve controls the transfer of the polyolefin powder into the powder conveying system from the first or second degassing vessel. It is generally preferred if a rotary valve vent line is present in the system which connects the rotary valve to the first and / or second degassing vessel, as show in the Figures. The rotary valve vent line leaks / recycles nitrogen to the degassing vessel(s) and ensures that the chambers of the rotary valve are sufficiently filled with nitrogen.

[0058] Polyolefin powder may enter the rotary value under gravity and the rotary valve controls powder flow into the conveying system.

[0059] A screen may also be located before the rotary valve to remove oversized powder particles but also to assist with pressure equalization before entry into the powder conveying system. The screen may prevent oversized powder particles entering the rotary feeder and / or conveying system which could cause processing issues.

[0060] The gas pressure in the first / second degassing vessel is often close in pressure to the powder conveying system, preferably closed loop powder conveying system. This balance of pressure typically ensures reliable operation. The first degassing vessel may operate at a gas pressure of 0.5 to 1.0 barg, such as 0.6 to 0.8 barg. Typically the pressure within the powder conveying system is no more than 0.5 barg, such as 0.1 barg to 0.3 barg (so 1.3 bars). The screen can help reduce the pressure before entry into the powder conveying system. In some embodiments, the pressure in the powder conveying system is higher than in the first and / or second degassing vessel, such as 0.1 barg higher or more.

[0061] In the rotary valve, rotation of a passage or passages in a transverse plug within the valve regulates the flow of polyolefin powder and gas through the valve. It allows therefore polyolefin powder and gas to pass through the valve in a controlled manner into the powder conveying system. This prevents plugging within the powder conveying system. It is preferred however if the rotary valve has a bleed stream in which nitrogen gas from the rotary valve can be recycled back to the first and / or second degassing vessel. This will be described in more detail below.

[0062] In the first powder conveying system, once the nitrogen has conveyed the polyolefin powder to the storage silos, nitrogen continues to circulate from the powder silos around the loop to a compressor. So the powder silos have a gas outlet to allow the nitrogen gas to continue circulating around the loop. The powder conveying system, preferably the closed loop powder conveying system, is typically provided with a compressor to maintain gas pressure within the loop.

[0063] Fresh nitrogen can be fed to the first powder conveying system typically before the compressor. Alternatively, nitrogen can leak into the first powder conveying system via the second powder conveying system, preferably second closed loop powder conveying system, via the powder silos (described below).

[0064] A recycle from the first powder conveying system takes nitrogen and any hydrocarbons that have built up in the powder conveying system gas to the first degassing vessel. Hence at least a part of the nitrogen stream is recycled from said powder conveying system to the first degassing vessel.

[0065] The nitrogen pressure in the first powder conveying system is typically between 0.5 and 2.0 bars, more preferably between 0.7 and 1.7 bars, most preferably between 1.0 and 1.5 bars such as 1.3 bars. At this pressure polyolefin powder enters and remains in the polymer silos. There is insufficient pressure to force the polymer powder into the next part of the first powder conveying system after the silos, the nitrogen gas in the powder conveying system is able to exit from the top of the powder silos and this gas is recirculated around to the compressor for the pressure within the powder conveying system to be restored to a level allowing the transport of polymer powder.

[0066] Polyolefin in the powder silos c allowed to enter a second powder conveying system, preferably a second closed loop powder conveying system, in which nitrogen is again used to convey the polyolefin powder around a second powder conveying system to an extruder feed bin and then into the actual extruder. Extruded polyolefin is pelletised and collected for use. Conveying gas passes out from the extruder feed bin and circulates, e.g. via a compressor, back to the base of the powder silos ready to convey more powder to pelletisation. Fresh nitrogen can be added to the second powder conveying system typically just before the compressor.

[0067] The powder silos are typically provided with a rotary valve to control the release of the polyolefin powder into the second powder conveying system. Note that nitrogen from the second powder conveying system can leak into the first powder conveying system through these rotary valves and this can be conveniently utilized to supply additional nitrogen to the first powder conveying system.

[0068] In a first embodiment, key to the invention is that a portion of the nitrogen stream is recycled from said powder conveying system to the first degassing vessel. It is not envisaged that all of the nitrogen is recycled from the powder conveying system as some nitrogen must remain to convey the polymer powder around the system. The powder conveying system may operate at a higher pressure than the first or second degassing vessel. In such a scenario, the nitrogen from the powder conveying system can be bled back to the first degassing vessel in order to recycle it.

[0069] Where there is a higher pressure in the conveying system than in the degassing vessels, nitrogen can flow through the rotary valve (and rotate that valve) towards the second degassing vessel. Nitrogen from the powder conveying system can also leak through the rotary valves and this recycled nitrogen can be conveniently utilized to supply additional nitrogen to the first and / or second degassing vessel.

[0070] Conventionally, hydrocarbon contaminated nitrogen from the powder conveying system was sent to flare. In the present invention, it is preferred if there is no continuous flow to flare / thermal oxidation from the powder conveying system.

[0071] The polyolefin powder which enters the powder conveying system can still contain hydrocarbon contaminants and hence during the transfer from the degassing vessel to the powder silo(s), the nitrogen gas can continue to extract hydrocarbons from the polyolefin powder. To avoid this hydrocarbon impurity building up in the powder conveying system, preferably closed loop powder conveying system, the inventors propose to transfer part of the nitrogen gas mixture from the powder conveying system back to the first degassing vessel. The hydrocarbon contaminant level is typically in the range of 0 to 3 mol%, such as 0.3 mol% or less, preferably 0.03 mol% or less in the recycled gas mixture. Typically, the hydrogen contaminant level is 300 ppm or less, such as 210 ppm or less in this recycled gas mixture. The nitrogen stream that is recycled preferably contains at least 90 wt% nitrogen such as at least 99 wt% nitrogen, and ideally trace hydrocarbons.

[0072] Inventors have realized however that the level of contaminants in this nitrogen recycle is low and the nitrogen gas is capable of extracting more hydrocarbons from the polyolefin powder. By recycling the nitrogen gas from the powder conveying system back to the first degassing vessel, the nitrogen can also act to extract hydrocarbons from the contaminated polyolefin powder in the first degassing vessel.

[0073] Moreover, the inventors have found that the inclusion of this recycle between the first powder conveying system and the first degassing vessel reduces the required fresh nitrogen feed to first degassing vessel by a significant amount. On an industrial scale, the reduction can be at least 150 kg / h, such as 150 to 300 kg / hr, such as 200 kg / h. Moreover, as this hydrocarbon containing nitrogen stream from the powder conveying system is not sent to flare or thermal oxidation directly, there is also a reduction in the flow to flare / thermal oxidation.

[0074] If there are two degassing vessels present it is possible to split the recycle from the powder conveying system to both degassing vessels. It is however required that at least some of the recycle from the powder conveying system passes to the first degassing vessel, such as least 50 wt% of the recycle, ideally at least 75 wt% of the recycle. It is preferred is all the recycle goes to the first degassing vessel.

[0075] Thus, possible routings of the nitrogen are

[0076] 1. Conveying to the 1 st degassing vessel

[0077] 2. Conveying to both 1stand 2nd degassing vessels in parallel

[0078] In one embodiment there is also a nitrogen recycle from the second degassing vessel to the first degassing vessel. Where such a recycle is present, it is then possible for the nitrogen recycle from the powder conveying system to pass to the second degassing vessel exclusively. Because there is also a nitrogen recycle from the second to the first degassing vessel, nitrogen from the powder conveying system may ultimately be transferred into the first degassing vessel as required in the invention.

[0079] It is possible for the nitrogen recycle from the powder conveying system to pass through a filter before entering the first degassing vessel. Just in case any residual powder is present in this recycled nitrogen stream, a filter can be used to extract that powder and prevent it being fed to the first degassing vessel. Any powder removed by such a filter can be sent back to the powder conveying system, e.g. via the screen before the rotary valve.

[0080] It may also be required to use a compressor to push the nitrogen recycle back to the first degassing vessel.

[0081] In a second embodiment, there is a nitrogen recycle from rotary valve to the first and / or second degassing vessel. This bleed stream contains nitrogen and amounts of hydrocarbons which can be recycled therefore into the first degassing vessel and ultimately onto flare or thermal oxidation. The nitrogen bleed stream that is recycled preferably contains at least 90 wt% nitrogen such as at least 99 wt% nitrogen, and trace hydrocarbons.

[0082] This bleed stream from the rotary valve can pass through a filter and / or compressor before returning to the first and / or second degassing vessel. This also results in a reduction in the fresh nitrogen feed to the first and / or second degassing vessel. It is possible that only a part of the bleed stream from the rotary valve is recycled. In one embodiment, the bleed stream from the rotary valve is all recycled to the first degassing vessel. In one embodiment, the bleed stream from the rotary valve is all recycled to the second degassing vessel. In one embodiment, the bleed stream from the rotary valve is recycled to the first and second degassing vessels.

[0083] Moreover, it is preferred if the rotary valve is operated at the lowest possible pressure, such as 0.1 to 0.4 barg, e.g. 0.15 to 0.3 barg. It may be therefore that a compressor is required to increase the pressure of the bleed stream back up to the pressure required in the feed to the first degassing vessel.

[0084] By operating at low pressure within the rotary valve this avoids a capacity restriction due to insufficient depressurization of the rotary valve feed chambers. If the pressure within the rotary valve is too high, the powder flow may be disrupted resulting in insufficient filling of the rotary valve chambers which reduces the efficiency of the system.

[0085] If there is significantly higher pressure below the rotary valve than above the valve, this could cause high pressure in the rotary valve chambers to be released to the powder side when these chambers should be filling with powder, disrupting therefore the powder flow downwards and into the rotary valve chambers. If the chambers do not fill properly, the rotary valve cannot reach the designed throughput.

[0086] On an industrial scale, 300 to 1000 kg / hr of nitrogen contaminated with hydrocarbons is recycled from the powder conveying system, such as 400 to 500 kg / hr or 600 to 800 kg / hr. Also 300 to 1000 kg / hr of nitrogen contaminated with hydrocarbons is recycled from the rotary valve to the first (and second if used) degassing vessel, such as 400 to 500 kg / hr or 600 to 800 kg / hr. The amount of the recycle is limited to avoid any risk of plugging within the powder conveying system. It is possible for an operator of the system to have dynamic control over the whole process, adjusting the fresh nitrogen feed depending on how much recycle is coming from the rotary valve and powder conveying system. The use of the rotary valve bleed stream itself can save up to 200 kg / h of fresh nitrogen being used in the first / second degassing vessels. Overall, the use of these recycles offers a significant reduction in the nitrogen demands of the overall process, e.g. 450 kg / h or more.

[0087] In some embodiments, there may be two gas feeds to the first and / or second degassing vessel. A first feed of fresh nitrogen and a second feed of nitrogen recycled from the powder conveying system and / or from the bleed steam from the rotary valve. It may be therefore that the fresh nitrogen enters the first / second degassing vessel in a separate feed from any recycled material.

[0088] Whilst not discussed in detail above, the process of the invention may use make conventional use of valves, compressors, pumps and so on to ensure that process is carried out as described.

[0089] System

[0090] The invention further relates to a system for carrying out the process defined herein. The system comprises therefore a polymerisation reactor in which the polyolefin powder is produced, e.g. a gas phase reactor. The polymerisation reactor is connected to the first degassing vessel by a conduit through which the contaminated polyolefin powder made in the reactor can pass under gas pressure. In some cases, the conduit directly connects the polymerization reactor to the first degassing vessel. Preferably however, additional vessel(s) are located between the polymerization reactor and the first degassing vessel, for example a polymer outlet vessel, and / or a product recovery vessel. Typically, the contaminated polyolefin powder will be transferred from the polymerisation reactor to a product recovery vessel prior to being transferred to the first degassing vessel. Any polymer outlet vessel, or product recovery vessel which may be present will typically have a hydrocarbon containing gas atmosphere.

[0091] A second degassing vessel can also be present if required. If present, the second degassing vessel is connected to the first degassing vessel, e.g. by a conduit through which the polyolefin powder can pass under gas pressure.

[0092] The first degassing vessel will have one or more inlets for the supply of nitrogen and steam into the vessel. These can be the same or different inlets. The first degassing vessel also has an outlet to remove contaminated gas to flare or thermal oxidation. Powder can also be removed via a further outlet. The second degassing vessel will have at least one inlet for the supply of nitrogen into the vessel. The second degassing vessel also has an outlet to remove contaminated gas to flare or thermal oxidation. As noted above, either first or second degassing vessel may have a separate inlet for fresh and recycled nitrogen.

[0093] The first (or second if present) degassing vessel is connected to the powder conveying system, preferably a closed loop powder conveying system, into which polyolefin powder from the first (or second if used) degassing vessel can be transferred via a conduit. Nitrogen pressure within the system conveys the powder ideally around the closed loop. Within the powder conveying system, nitrogen also conveys the polyolefin powder to at least one polyolefin powder silo. The system is provided with a conduit for recycling contaminated nitrogen from the powder conveying system to the first degassing vessel and optionally to the second degassing vessel. That conduit may be provided with a filter and / or compressor as required.

[0094] A rotary valve is preferably located between said first degassing vessel and the powder conveying system or located between the second degassing vessel and the powder conveying system (if used), said rotary valve having a bleed stream that is recycled via a conduit to said first and / or second degassing vessel. The conduit for recycling contaminated nitrogen from the rotary valve to the first and / or second degassing vessel may further comprise a compressor and / or a filter.

[0095] The invention will now be further described with reference to the following non-limiting figures.

[0096] In figure 1 polyolefin powder is produced in a polymerisation reactor (not shown) and transferred into the first degassing vessel 1. This polyolefin powder is contaminated with hydrocarbons. Fresh nitrogen is supplied to the first degassing vessel via conduit 2. Steam is also supplied to the first degassing vessel, e.g. in the same conduit as the fresh nitrogen. The first degassing vessel is also provided with a conduit 3 to remove a gas mixture of hydrocarbons, nitrogen and steam to flare.

[0097] Polyolefin powder from said first degassing vessel can be transferred to the second degassing vessel 4. The second vessel is also provided with a conduit for the supply of nitrogen 5 and a conduit to remove nitrogen, hydrocarbons removed from said polyolefin powder and any residual steam to flare 6. Polyolefin powder from said second degassing vessel is removed therefrom and passes through a screen 7 and into a rotary valve 8. The rotary valve may be provided with a conduit 9 to recycle material from said valve to the second degassing vessel via the nitrogen feed conduit.

[0098] Polyolefin powder from the rotary value passes into the closed loop powder conveying system 10. Nitrogen within the closed loop powder conveying system pushes polyolefin powder through the loop to polyolefin silos 11. A compressor 15 maintains gas pressure within the closed loop powder conveying system. Polyolefin powder from said powder silos can enter a second closed loop powder conveying system 12 in which the polyolefin powder is conveyed to the extruder feed bin 13 and subsequently into the extruder 14. The polyolefin powder is therefore pelletised. Nitrogen continues to circulate from the extruder feed bin to the compressor 15. Fresh nitrogen can be supplied to the second closed conveying loop system. A conduit 17 takes contaminated nitrogen gas from the closed loop powder conveying system to the first degassing vessel.

[0099] In figure 2 polyolefin powder is produced in a polymerisation reactor (not shown) and transferred into the first degassing vessel 1. This polyolefin powder is contaminated with hydrocarbons. Fresh nitrogen is supplied to the first degassing vessel via conduit 18. Steam is also supplied to the first degassing vessel, e.g. in the same conduit as the fresh nitrogen. The first degassing vessel has a second inlet and conduit 19 for nitrogen recycled from the closed loop powder conveying system and the rotary valve.

[0100] The first degassing vessel is also provided with a conduit 3 to remove hydrocarbons, nitrogen and steam to flare.

[0101] Polyolefin powder from said first degassing vessel passes through a screen 7 and into a rotary valve 8. The rotary valve may be provided with a conduit 9 to recycle material from said valve back to the first degassing vessel.

[0102] The bleed stream from the rotary valve passes to a filter 20 and then to a compressor 21 before passing back to the first degassing vessel. Any powder isolated by the filter 20 can be returned to the screen 7 through conduit 22.

[0103] Polyolefin powder from the rotary value passes into the closed loop powder conveying system 10. Nitrogen within the closed loop powder conveying system pushes polyolefin powder through the loop to polyolefin silos 11. Polyolefin powder from said powder silos can enter a second closed loop powder conveying system 12 in which the polyolefin powder is conveyed to the extruder feed bin 13 and subsequently into the extruder (not shown). Nitrogen continues to circulate from the extruder feed bin and from the polyolefin powder silos and is rejoined before a filter 23 and a compressor 24. Fresh nitrogen can be supplied to the second closed conveying loop system.

[0104] The recycle from the closed loop powder conveying system 17 can occur back to filter 20.

[0105] Reference numerals:

[0106] 1 - first degassing vessel

[0107] 2 - nitrogen feed conduit to the first degassing vessel

[0108] 3 - conduit to flare from first degassing vessel

[0109] 4 - second degassing vessel

[0110] 5 - nitrogen feed conduit to the second degassing vessel

[0111] 6 - conduit to flare from second deassing vessel.

[0112] 7 - screen

[0113] 8 - rotary valve

[0114] 9 - recycle conduit from rotary valve to second degassing vessel.

[0115] 10 - first closed loop powder conveying system

[0116] 11 - powder silos

[0117] 12 - second closed loop powder conveying system

[0118] 13 - extruder feed bin

[0119] 14 - extruder

[0120] 15 - compressor in the first / second closed loop powder conveying system

[0121] 16 - compressor in the first closed loop powder conveying system

[0122] 17- recycle conduit the first closed loop powder conveying system to the first degassing vessel.

[0123] 18 - dedicated fresh nitrogen feed conduit

[0124] 19 - recycled nitrogen feed conduit

[0125] 20 - filter

[0126] 21 - compressor in line 19

[0127] 22 - powder return to screen

[0128] 23 - filter

[0129] 24 - compressor

[0130] EXAMPLES AND COMPARATIVE EXPERIMENTS

[0131] By installing and operating the line from the closed loop powder conveying system to the first degassing vessel the fresh nitrogen feed to the first degassing vessel was reduced by -200 kg / h to 450 kg / hr, the recycled nitrogen reduced to between 600 to 800 kg / hr.

Claims

Claims1. A process for treating a polyolefin powder contaminated with at least one hydrocarbon, comprising the steps of: a) discharging contaminated polyolefin powder from a polymerization reactor into a first degassing vessel, e.g. via a product recovery vessel; b) contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam to extract the hydrocarbon contamination from said powder thereby providing decontaminated polyolefin powder; c) transferring from the first degassing vessel, nitrogen, steam and hydrocarbon contamination to a flare system or a thermal oxidation system; d) transferring the decontaminated polyolefin powder from step b) into a powder conveying system, preferably a closed loop powder conveying system, containing a nitrogen stream to convey the decontaminated polyolefin powder; wherein at least a part of the nitrogen stream is recycled from said powder conveying system, preferably said closed loop powder conveying system, to the first degassing vessel.

2. A process for treating a polyolefin powder contaminated with at least one hydrocarbon comprising the steps of a) discharging contaminated polyolefin powder continuously or discontinuously from a polymerization reactor into a first degassing vessel e.g. via a product recovery vessel; b) contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam to extract the hydrocarbon contamination from said polyolefin powder thereby providing decontaminated polyolefin powder; c) transferring from the first degassing vessel, nitrogen, steam and hydrocarbon contamination to a flare system or a thermal oxidation system;d) transferring the decontaminated polyolefin powder from step b) into a second degassing vessel and supplying nitrogen to said second degassing vessel, preferably in the absence of steam, such that the decontaminated polyolefin powder contacts said nitrogen; e) transferring from the second degassing vessel, nitrogen and hydrocarbon contamination to a flare system or a thermal oxidation system; f) transferring the decontaminated polyolefin powder from step d) into a powder conveying system, preferably a closed loop powder conveying system, containing a nitrogen stream to convey the decontaminated polyolefin powder; wherein at least a part of the nitrogen stream is recycled from said powder conveying system to the first degassing vessel.

3. A process as claimed in any preceding claim, wherein the contaminated polyolefin powder in step a) is directly discharged from the polymerisation reactor into the first degassing vessel or is discharged via at least one further vessel, such as at least a product recovery vessel, prior to being transferred into the first degassing vessel.

4. A process as claimed in any preceding claim wherein the decontaminated polyolefin powder passes through a rotary valve before entering the powder conveying system.

5. A process as claimed in claim 4 wherein the rotary valve has a nitrogen bleed stream which is at least partially, such as completely, recycled to the first degassing vessel.

6. A process for treating a polyolefin powder contaminated with at least one hydrocarbon comprising the steps of a) discharging contaminated polyolefin powder continuously or discontinuously from a polymerization reactor into a first degassing vessel, e.g. via a product recovery vessel;b) contacting the contaminated polyolefin powder in said first degassing vessel with nitrogen and steam to extract the hydrocarbon contamination from said polyolefin powder thereby providing decontaminated polyolefin powder; c) transferring from the first degassing vessel, nitrogen, steam and hydrocarbon contamination to a flare system or a thermal oxidation system; optionally d) transferring the decontaminated polyolefin powder from step b) into a second degassing vessel and supplying nitrogen to said second degassing vessel, preferably in the absence of steam, such that the decontaminated polyolefin powder contacts said nitrogen to thereby provide a second decontaminated polyolefin powder and transferring from the second degassing vessel, nitrogen and hydrocarbon contamination to a flare system or a thermal oxidation system; e) transferring the decontaminated polyolefin powder from step b), or if present optional step d), into a rotary valve; f) transferring the decontaminated polyolefin powder from said rotary valve into a powder conveying system, preferably a closed loop powder conveying system, containing a nitrogen stream to convey the polyolefin powder; wherein the rotary valve has a nitrogen bleed stream which is at least partly, such as completely, recycled to the first and / or second degassing vessel.

7. A process as claimed in claim 4 to 6 wherein the decontaminated polyolefin powder passes through a pressure reducing screen before entering the rotary valve.

8. A process as claimed in any preceding claim wherein at least a part of the nitrogen stream from the powder conveying system and / or at least a part of the nitrogen bleed stream from the rotary valve is transferred: via a filter to the first degassing vessel, using a compressor to the first degassing vessel; orvia a filter and using a compressor to the first degassing vessel.

9. A process as claimed in claim 8 wherein any polyolefin powder separated by the filter is returned to the powder conveying system.

10. A process as claimed in any of claims 4 to 9 wherein the rotary valve reduces the pressure before the polyolefin powder enters the powder conveying system.

11. A process as claimed in any preceding claim wherein the nitrogen stream from the powder conveying system or the nitrogen bleed stream from the rotary valve comprises at least 90 wt% nitrogen.

12. A process as claimed in any of claim 2 to 11 wherein at least a part of the nitrogen stream is recycled from said powder conveying system to the first and second degassing vessels.

13. A system for treating a polyolefin powder contaminated with at least one hydrocarbon, the system comprising i) a polymerisation reactor in which polyolefin powder can be obtained by polymerization of one or more olefins in the presence of a polymerization catalyst; ii) a first degassing vessel configured to receive contaminated polyolefin powder from said polymerisation reactor via a conduit, wherein said first degassing vessel is configured to be supplied with nitrogen and steam via one or more inlets; iii) a conduit configured to allow nitrogen, steam and hydrocarbons to be removed from the first degassing vessel and sent to a flare or thermal oxidation system; iv) a powder conveying system, preferably a closed loop powder conveying system, configured to receive decontaminated polyolefin powder from the first degassing vessel via a conduit in which nitrogen can convey the polyolefin powder to at least one polyolefin powder silo;v) a conduit for recycling nitrogen from the powder conveying system, preferably said closed loop powder conveying system, to the first degassing vessel.

14. A system for treating a polyolefin powder contaminated with at least one hydrocarbon, the system comprising i) a polymerisation reactor in which polyolefin powder can be obtained by polymerization of one or more olefins in the presence of a polymerization catalyst; ii) a first degassing vessel configured to receive contaminated polyolefin powder from said polymerisation reactor via a conduit wherein said first degassing vessel is configured to be supplied with nitrogen and steam via one or more inlets; iii) a conduit configured to allow nitrogen, steam and hydrocarbons to be removed from the first degassing vessel and sent to a flare or thermal oxidation system; iv) a second degassing vessel configured to receive partially decontaminated polyolefin powder from said first degassing vessel via a conduit wherein said second degassing vessel is configured to be supplied with nitrogen via one or more inlets; v) a conduit configured to allow nitrogen and hydrocarbons to be removed from the second degassing vessel and sent to at least one of a flare, a thermal oxidation system or the first degassing vessel; vi) a powder conveying system, preferably a closed loop powder conveying system, configured to receive decontaminated polyolefin powder from the second degassing vessel via a conduit in which nitrogen can convey the polyolefin powder to at least one powder silo; vii) a conduit for recycling nitrogen from the powder conveying system to the first and / or second degassing vessel.

15. A system as claimed in claim 13 or 14 further comprising a product recovery vessel and / or polymer outlet vessel wherein optionally the contaminated polyolefin powder from the polymerisation reactor is transferred via product recovery vessel and / or polymer outlet vessel to the first degassing vessel.

16. A system as claimed in claim 13 to 15 wherein a rotary valve is (i) located between said first degassing vessel and the powder conveying system if there is no second degassing vessel or (ii) located between the second degassing vessel and a powder conveying system, said rotary valve being configured such that a bleed stream therefrom is recycled via a conduit to said first and / or second degassing vessel.

17. A system as claimed in any one of claims 13 to 16 wherein the conduit for recycling nitrogen from the powder conveying system or from the rotary valve to the first and / or second degassing vessel further comprises a compressor and / or a filter.

18. A system for treating a polyolefin powder contaminated with at least one hydrocarbon, the system comprising i) a polymerisation reactor in which polyolefin powder can be obtained by polymerization of one or more olefins in the presence of a polymerization catalyst; ii) a first degassing vessel configured to receive contaminated polyolefin powder from said polymerisation reactor via a conduit wherein said first degassing vessel is configured to be supplied with nitrogen and steam via one or more inlets; iii) a conduit configured to allow nitrogen, steam and hydrocarbons to be removed from the first degassing vessel and sent to a flare or thermal oxidation system; iv) optionally a second degassing vessel configured to receive partially decontaminated polyolefin powder from said first degassing vessel via a conduit wherein said second degassing vessel is configured to be supplied with nitrogen via one or more inlets; v) and a conduit configured to allow nitrogen and hydrocarbons to be removed from the second degassing vessel and sent to at least one of a flare, a thermal oxidation system or the first degassing vessel; vi) a rotary valve located between said first degassing vessel and the powder conveying system or, if present, located between the second degassing vessel and the powder conveying system, said rotary valve having a conduit configured to carry a bleed stream to said first and / or second degassing vessel;vii) a powder conveying system, preferably a closed loop powder conveying system, configured to receive decontaminated polyolefin powder from the rotary valve and in which nitrogen can convey the polyolefin powder to at least one powder silo; viii) optionally a conduit for recycling nitrogen from the powder conveying system to the first and / or second degassing vessel; and ix) optionally a product recovery vessel and / or polymer outlet vessel wherein the polyolefin powder from the polymerisation reactor may be transferred via product recovery vessel and / or polymer outlet vessel to the first degassing vessel.

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