Processes for polymerising olefins

Abstract

According to the present disclosure, there is provided a process for polymerising olefins. The process comprises introducing a first feed of olefin monomer to a first reactor; polymerising the olefin monomer of the first feed at a first pressure in the first reactor to produce a first polymer component; depressurising a second feed of olefin monomer such that the temperature reduction on depressurisation produces a reduced temperature olefin monomer-containing stream; and using the reduced temperature olefin monomer-containing stream as a coolant in the process.

Description

PROCESSES FOR POLYMERISING OLEFINSFIELD OF THE DISCLOSUREThe present disclosure relates to a process for polymerising olefins.BACKGROUND OF THE DISCLOSURETo efficiently perform polymerisation at an industrial scale there is a need to control the temperature of different stages of the process in an efficient and effective way. For example, polymerisation process streams may require cooling at various stages of the process. As an example, cooling may also be required during product recovery, for example, to separate byproducts, unreacted olefin monomers from the polymer produced. For instance, product removed from the polymerisation reaction may contain unreacted monomer. Typically, product removed from polymerisation reactor may be introduced into a purge bin where a purge gas, for example, nitrogen may be used to separate components, including unreacted olefin monomer and other hydrocarbons from the product. The separated gaseous components may be compressed, cooled and condensed to facilitate separation of unreacted olefin monomer, hydrocarbons and the nitrogen from the process. The energy demands and / or size of such hydrocarbon-nitrogen recovery units can be relatively high, for example, because of the cooling, condensation and / or compression steps.Existing cooling methods, for example, for meeting the cooling and condensation requirements of such hydrocarbon-nitrogen recovery units may involve the use of dedicated refrigerants, for example, propane or ammonia. Such refrigerants may be used to cool process streams directly by heat exchange. Alternatively or additionally, such refrigerants may be used to cool coolants, for example, coolant water streams, which are then used to remove heat from the process by heat exchange.An example of a conventional refrigeration system is shown in Figure 2. In this system, a refrigerant 100 (e.g., propane or ammonia) may be compressed in a refrigerant compressor 101. The compressed refrigerant 102 is then cooled and condensed to provide a reduced temperature refrigerant stream that is used to cool a process stream of the polymerisation process stream (not shown) via heat exchanger 107. Alternatively or additionally, the condensed refrigerant stream may be used to cool e.g., a coolant water stream using heat exchanger 106 to produce a refrigerated coolant water stream 109. Such refrigeration systems can be costly to run, for example, because of the dedicated equipment (e.g.,compressors) and source of refrigerants necessary for the process., e.g., especially when cryogenic conditions are required.The size and running costs of the compressors may also limit the temperatures to which the refrigerants can be cooled.In WO2012 / 082674, desirable products are recovered from purge gas effluent from a purge bin. Cooling is achieved by compressing and then depressurising separated portions of the purge gas. Similarly, in EP3444019A1 , desirable gases are recovered from purge gas vented from the polyolefin polymerisation process. The process comprises a heavy hydrocarbon separation step, a light hydrocarbon separation step and a nitrogen purification step. As explained in paragraph

[0050] of EP3444019A1, cooling in the heavy hydrocarbon and light hydrocarbon steps can be achieved using refrigerant produced by the turbo expansion of nitrogen gas purified in the nitrogen purification step. Specifically, the nitrogen is expanded in a dedicated turbine expander to produce refrigerant that is recovered in a nitrogen recovery device after use. These processes involve complex separation steps for isolation of target components for depressurisation to achieve cooling. For example, in EP3444019A1, nitrogen requires isolation by membrane methods to produce a nitrogen stream for turbo expansion. Accordingly, these processes rely on gases being recovered from the vented purge gas in a particular manner for the refrigerant to be produced. As such, they lack versality. Furthermore, there is no disclosure of cooling streams other than those associated with the purge gas effluent.PRIOR ARTWO 2007 / 025640 A1 (BOREALIS TECH OY [Fl]; ANDTSJOE HENRIK [Fl] ETAL.) 8 March 2007. This document describes a process for the preparation of multimodal ethylene homo- or copolymers in at least two stages, the process comprising preparation of a first ethylene homo- or copolymer fraction in a loop reactor in slurry phase, preparation of a second ethylene homo- or copolymer fraction in a gas phase reactor, by using a catalyst not containing an inorganic oxide support, and operating the gas phase reactor in such conditions that at least part of the gas is recycled, and that at least a part of the recycled gas is condensed and the (partially) condensed gas is re-introduced into the gas phase reactor.US 4 740 551 A (FOSTER E GORDON [US]) 26 April 1988. This document describes a multistage, continuous process for the preparation of propylene-ethylene impact copolymers comprising the use of a plug flow pipeline reactor for homopolymerizing propylenehomopolymerization, and a gas-phase fluidized bed reactor for propylene / ethylene copolymerization.WO 2018 / 052437 A1 (LUMMUS TECHNOLOGY INC [US]) 22 March 2018. This document describes embodiments relating to the production of polypropylene. A hydrocarbon feedstock, containing propane, is fed to a propane dehydrogenation reaction zone to convert a portion of the propane to propylene. The propane and propylene are separated, and at least a portion of the propylene stream is fed to a polymerization zone. In the polymerization zone, propylene is reacted to produce a polypropylene. Additionally, a purge gas and a carrier gas are recovered. The purge and carrier gases are fed to the separation system and cooled against a second portion of the propylene stream.WO 2015 / 002835 A1 (CHEVRON PHILLIPS CHEMICAL CO [US]) 8 January 2015. This document describes a system and method for a first reactor to produce a transfer slurry having a first polyolefin polymerized in the first reactor, a heat-removal zone to remove heat from the transfer slurry, and a second reactor to receive the transfer slurry cooled by the heat-removal zone, the second reactor to produce a product slurry having a product polyolefin which includes the first polyolefin and a second polyolefin polymerized in the second reactor.BRIEF DESCRIPTION OF DRAWINGSFigure 1 is a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to a comparative example.Figure 2 depicts a schematic drawing of a typical refrigeration unit according to the prior art.Figure 3 is a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to a first embodiment of the present invention.Figure 4 is a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to a second embodiment of the present invention.Figure 5 is a schematic drawing of a hydrocarbon-nitrogen recovery scheme according to a comparative example.Figure 6 is a schematic drawing of a hydrocarbon-nitrogen recovery scheme according to a third embodiment of the present invention.SUMMARY OF THE INVENTIONAccording to the present disclosure, there is provided a process for polymerising olefins. The process comprises introducing a first feed of olefin monomer to a first reactor; polymerising the olefin monomer of the first feed at a first pressure in the first reactor to produce a first polymer component; depressurising a second feed of olefin monomer such that the temperature reduction on depressurisation produces a reduced temperature olefin monomer-containing stream; and using the reduced temperature olefin monomer-containing stream as a coolant in the process.The composition of the first feed and second feed may be the same or different, preferably the same. In some embodiments, the second feed is taken as a side stream from the first feed.The olefin monomer in the first feed and / or second feed may be propylene or ethylene. In some embodiments, the olefin monomer in the second feed may be ethylene. Preferably, the olefin monomer in the first feed and / or second feed may be ethylene. More preferably, the olefin monomer in the first feed and the second feed is ethylene. The olefin monomer in first feed and / or second feed may be a fresh olefin feed, for example, fresh propylene and / or ethylene.In some embodiments, the first feed may comprise an olefin monomer and an alpha olefin comonomer. The second feed may also comprise an olefin monomer and an alpha olefin comonomer.The second feed may be formed by taking a portion of the first feed. In some embodiments, the process may comprise providing an olefin monomer source; obtaining a first portion of the olefin monomer source as the first feed, and obtaining a second portion of the olefin monomer source as the second feed. The olefin monomer source may be a source of ethylene.The reduced temperature olefin monomer-containing stream can be used as a source of cooling in the polymerisation process in a simple, energy efficient and cost effective way. Accordingly, the process of the present disclosure may be associated with the technicaladvantage of being able to be performed in a simple configuration that is, advantageously, convenient, efficient and cost effective to run.As discussed above, cooling in prior art polymerisation processes typically employ refrigeration systems that use refrigerant systems that cool refrigerants, such as propane and ammonia. The cooled refrigerants are used as coolants to cool process streams by heat exchange, or to cool other coolant streams, e.g., coolant water, that can then be used to remove heat from the process. In the present disclosure, the need for dedicated refrigerant systems for cooling process streams or cooling coolant streams (e.g., coolant water streams) can be reduced or eliminated. Advantageously, this can be achieved with no or minor adaptions to the feed to the first reactor. For example, where the pressure of the feed to the first reactor is sufficiently high (e.g., within battery limits), it may be possible simply to take a side stream of the feed. This side stream can then be depressurised to produce the reduced temperature as olefin monomer-containing stream as a coolant. Alternatively, if the feed to the pressure requires compression to elevate its temperature prior to introduction to the first reactor, a side stream may be taken after compression. This side stream can then be depressurised to produce the reduced temperature as olefin monomer- containing stream as a coolant without the need for additional compressors being used. However, in certain circumstances, it may be necessary to increase the size of the compressor used to compress the first feed to the first reactor, depending on the size of the side stream taken for depressurisation. Even so, modifications to the overall process configuration may not be significant, as dedicated refrigeration units are not required.As mentioned above, the reduced temperature olefin monomer containing stream produced as a result of depressurisation may be used as a coolant, for example, to cool a process stream of the process and / or to cool another coolant stream (e.g., coolant water). After the reduced temperature olefin monomer-containing stream is used to cool a process stream or a coolant stream, the depressurised olefin monomer-containing stream may be recompressed and reintroduced into the first reactor, or fed to any other downstream operating unit (e.g., downstream reactor(s), compressor [e.g., gas recovery compressor], flash tank and / or gas recovery unit). In one embodiment, once the reduced temperature olefin monomer-containing stream has been used as a coolant, the stream is compressed e.g., in a gas recovery compressor. The compressed stream may then be treated in a gas recovery unit and any recovered monomer may be recycled to a polymerisation reactor in the process.Furthermore, it has surprisingly been found that the process of the disclosure can achieve cooling to very low temperatures, advantageously temperatures below those typically achieved by prior art refrigerant systems, e.g., whilst not requiring significant structural modification of the polymerisation process configuration. This may be because the olefin monomer feeds to the reactor may be or may be compressed to pressures of about 60 to 75 barg because of the operating pressures of the first reactor. Accordingly, the temperature reduction that can be achieved upon depressurisation can be significant. Preferably, depressurisation of the feed of olefin monomer produces a reduced temperature olefin monomer-containing stream having a temperature of below -40 °C, preferably below -50 °C, more preferably below -70°C, yet more preferably below -80°C, and yet more preferably below -90 °C. In some examples, the reduced temperature olefin monomer-containing stream may have a temperature of -40 to -120 °C, preferably -50 °C to -110 °C, more preferably -70 to -105 °C, yet more preferably -80 to -100 °C, even more preferably -90 to - 100 °C.The low temperatures achieved by processes of the invention may facilitate improved recovery of, for example, unreacted olefin monomers, because e.g., lower temperatures may be used to separate the olefin monomers from other by-products, diluents or reactants. Further, while it can be difficult to achieve e.g., cryogenic temperatures using known refrigerants with existing refrigerant systems, this can be achieved in embodiments of the disclosure using e.g., compressors already present in the process scheme, for example, those used to pressurise olefin feeds to the polymerisation reactors.As mentioned above, the depressurised or reduced temperature olefin monomer-containing stream may be used to cool a process stream or a coolant stream. Cooling of process streams and / or coolant streams (e.g., coolant water) may be required at various stages of the process, including, for example cooling of the circulating gas in the one or more reactors to control the reaction temperature of polymerisation, cooling the reflux of distillation columns that may be used to recover / separate hydrocarbons, and / or cooling of the gas taken from a purge bin so that hydrocarbons may be recovered from remaining purge gas mixture. The reduced temperature olefin monomer-containing stream may provide a cost effective way to achieve heat exchange desired. Advantageously, this can also be achieved by a relatively simple configuration (and so may eliminate or reduce the need for e.g. conventional refrigerator device(s) and dedicated refrigerant(s)). In some embodiments, the depressurised or reduced temperature olefin monomer-containing stream may be used to cool one or more of the polymerisation reactor(s). For example, the depressurised or reduced temperature olefin monomer-containing stream may be introduced directly into theone or more reactor(s). However, it is preferable that the depressurised or reduced temperature olefin monomer-containing stream is not used to cool any of the polymerisation reactor(s) directly. Thus, it is preferable that the depressurised or reduced temperature olefin monomer-containing stream is not introduced directly into the one or more reactor(s).The first feed of olefin monomer may be at an elevated pressure. The pressure may be at least 40 barg, preferably at least 45 barg, more preferably at least 50 barg, even more preferably at least 53 bargand most preferably at least 56 barg. The pressure may be 40 to 90 barg, preferably 50 to 85 barg, and more preferably 53 to 80 barg. Suitable pressures include 56 to 75 barg, preferably, 60 to 70 barg. Such elevated pressures may be achieved by compressing the first feed of olefin monomer using a compressor(s).Prior to depressurisation, the second feed may be at an elevated pressure. The pressure may be at least 40 barg, preferably at least 45 bar, more preferably at least 50 barg, even more preferably at least 53 barg and most preferably at least 56 barg. The pressure may be 40 to 90 barg, preferably 50 to 85 barg, and more preferably 53 to 80 barg. Suitable pressures include 56 to 75 barg, preferably, 60 to 70 barg.In some embodiments, the second feed of olefin monomer may be formed by withdrawing a side stream from the first feed of olefin monomer prior to the first feed of olefin monomer being introduced into the first reactor. Preferably, the first feed of olefin monomer is compressed, and the second feed of olefin monomer is withdrawn as a side stream after first feed of olefin monomer has been compressed.In some embodiments, a third feed of olefin monomer is formed by withdrawing a further side stream from the first feed of olefin monomer prior to the first feed of olefin monomer being introduced into the first reactor or by withdrawing a side stream of the second feed of olefin monomer prior to its depressurisation. Where the third feed is withdrawn from the first feed, this preferably occurs after the first feed has been compressed. Where the third feed is withdrawn from the second feed, this also preferably occurs once the second feed is compressed. For example, the second feed may be withdrawn from the first feed after compression, and the third feed may then be withdrawn from the second feed (since this is also compressed).Where a third feed is formed, the process may further comprise depressurising the third feed of olefin monomer such that the temperature reduction on depressurisation produces afurther reduced temperature olefin monomer-containing stream. The further reduced temperature olefin monomer-containing stream may be used as a coolant in the process. For example, the further reduced temperature olefin monomer-containing stream may be used to cool the second feed of olefin monomer feed by heat exchange before the second feed of olefin monomer feed is depressurised to produce the reduced temperature olefin monomer-containing stream. Where the third feed is withdrawn as a side stream from the second feed, the third feed may be depressurised such that the temperature reduction on depressurisation produces a further reduced temperature olefin monomer-containing stream, which is then used to cool the remainder of the second feed.Preferably, the process is a multistage polymerisation process. The process may further comprise polymerising olefin monomer in a reactor downstream of the first reactor (i.e. , a second reactor) at a second pressure that is lower than the first pressure, wherein polymerisation of olefin monomer in the second reactor takes place in the presence of the first polymer component. The second reactor may be a gas phase reactor, for example, a fluidised bed gas phase reactor. Multiple reactors may be employed downstream. Where the process is a multistage polymerisation process, the first reactor may be a loop and / or slurry reactor, for example, a slurry loop reactor. Multiple first reactors may also be employed.Where the first feed of olefin monomer is compressed and a downstream reactor is employed, a portion of the first feed of olefin monomer may be withdrawn and introduced to the downstream reactor prior to compression. This is advantageous as the downstream reactor is operated at a lower pressure than the first reactor. In some embodiments, the portion of the first feed of olefin monomer that is withdrawn prior to compression may be subsequently compressed prior to being introduced to the downstream reactor. However, the subsequently compressed portion may be compressed to a pressure lower than the pressure of the compressed first feed.In some embodiments, once the reduced temperature olefin monomer-containing stream has been used as a coolant, the stream is fed to the second reactor.In some embodiments, the process further comprises withdrawing a product stream comprising polymer and hydrocarbons, for example, including unreacted olefin monomer from the process. The product stream may be withdrawn from the first reactor. However, where a downstream reactor is used, the product stream may be withdrawn from the downstream reactor. Where multiple reactors are used, the product stream may bewithdrawn from the most downstream reactor. The withdrawn product stream may be transferred to a purge bin for recovery of the polymer product.Preferably, prior to introduction into a purge bin, the withdrawn product stream may be introduced into a gas-solid separator. In the gas-solid separator, a gaseous stream comprising e.g., unreacted olefin monomer is separated from the product stream. The remaining product stream may then be transferred from the gas-solid separator to a purge bin.In the purge bin, any active catalyst in the product stream may be deactivated, for example, using steam. An inert gas, for example, nitrogen or a nitrogen-containing gaseous mixture, may also be used to strip a purge off gas comprising hydrocarbons from the product stream. The hydrocarbons may include heavy hydrocarbons, such as oligomers formed as byproducts in the polymerisation process, as well as lighter hydrocarbons, for example, unreacted olefin monomers. The purge off gas may then be separated from the purge bin; and the separated purge off gas may be cooled to condense hydrocarbons from the separated purge off gas. In some embodiments, the separated purge off gas is compressed prior to being cooled.Preferably, the purge off gas may be cooled to a temperature of below -40 °C, preferably below -50 °C, more preferably below -70°C, yet more preferably below -80°C, and yet more preferably below -90 °C to condense hydrocarbons from the separated purge off gas. In some examples, the reduced temperature olefin monomer-containing stream may have a temperature of -40 to -120 °C, preferably -50 °C to -110 °C, more preferably -70 to -105 °C, yet more preferably -80 to -100 °C, even more preferably -90 to -100 °C. Advantageously, the purge off gas may be cooled (e.g., by direct heat exchange) using the reduced temperature depressurised olefin monomer-containing stream. Alternatively, the purge off gas may be cooled using another coolant stream that itself has been cooled by heat exchange with the reduced temperature olefin monomer-containing stream.Cooling using the reduced temperature olefin monomer-containing stream can improve the overall efficiency of the process. This is because, in conventional hydrocarbon-nitrogen recovery units, the recovery of unreacted olefin monomer from the purge off gas from the purge bin typically occurs by compressing and cooling the purge off gas to condense hydrocarbons from the second stream. Using conventional refrigerant systems, cooling below e.g, -30 °C may be difficult or uneconomic, resulting in incomplete condensation ofhydrocarbons. In such prior art systems, nitrogen may require separation from the uncondensed gases using a membrane, which separates nitrogen from, for example, hydrocarbons that can be recycled to the compressor for separation. Repeated recycling from membranes to the compressor may be required to improve nitrogen and hydrocarbon recovery. Using such prior art refrigerants in such hydrocarbon-nitrogen recovery units, 80 to 90% of the hydrocarbon can typically be recovered from the purge gas.By contrast, when the reduced temperature olefin monomer-containing stream produced by the process of the present disclosure is used, more effective cooling of the purge off gas from the purge bin may be achieved (e.g., in preferred embodiments to as low as -90 °C). This facilitates improved condensation of hydrocarbons, including C2 hydrocarbons. As a result, improved recovery (e.g. greater than 95%) of the hydrocarbons from the purge gas can be achieved. In some embodiments, the process may also allow improved removal of hydrogen as a purge off-gas. Furthermore, the need for recycling the gas from membranes to the compressor may be reduced, and / or smaller compressors may be used. In some embodiments, the need for membrane separation of nitrogen may be eliminated or reduced because of the more effective condensation of hydrocarbons from nitrogen. Embodiments of the present disclosure can also afford greater flexibility as to how product recovery from the withdrawn product stream may be carried out, as there is no need to target the recovery of any particularly composition of gas for compression and expansion as with e.g., WO2012 / 082674 or EP EP3444019A1.Any nitrogen recovered from the purge bin can be separated and optionally recycled.By separating and optionally recycling the nitrogen, the process can be associated with the advantage of improved overall environmental impact as less nitrogen is required for the process overall. For example, this can be achieved as the nitrogen separated and optionally recycled can be obtained in high purity (for example, greater than 96 mol% purity, optionally greater than 99 mol% purity where a membrane separation step is used) so is suitable for recovery / recycling. To avoid accumulation of any uncondensed light hydrocarbon in the recovered nitrogen, it may be possible to take a small off gas stream from the nitrogen. This off gas stream may be used for dryer regeneration before it is in a regenerative thermal oxidizer, thermal oxidizer, flare or boiler.Where the withdrawn product stream is introduced into a gas-solid separator prior to the purge stream, the gaseous stream comprising e.g., unreacted olefin monomer separated from the product stream may also be cooled. Cooling may be carried out using the reduced temperature olefin monomer-containing stream (e.g., by direct heat exchange) or anothercoolant stream that has itself been cooled by heat exchange with the reduced temperature olefin monomer-containing stream. More preferably, the separated gaseous stream from the gas-solid separator and the gaseous stream separated from the purge bin are cooled using the reduced temperature olefin monomer-containing stream (e.g., by direct heat exchange) or another coolant stream that has itself been cooled by heat exchange with the reduced temperature olefin monomer-containing stream.In some embodiments, the unreacted olefin monomer may be recovered and recycled to process.In some embodiments, once the reduced temperature olefin monomer-containing stream has been used as a coolant, it may be recompressed and reintroduced into the first reactor. As mentioned above, however, once the reduced temperature olefin monomer-containing stream has been used as a coolant, it may be fed or used in any downstream operation unit, including, for example, a second or other downstream reactor(s) or gas recovery unit.In some embodiments where the reduced temperature olefin monomer-containing stream is recompressed, it may be recompressed together with the separated gaseous stream from the gas-solid separator comprising unreacted olefin monomer. This may be associated with the advantage that a simple overall configuration may be used, and the process may be more energy efficient, as the same compressor(s) may be used for compressing olefin monomer fed, for example, to the first reactor.In some embodiments, polymerisation of olefin monomer in the first reactor, and / or, where present, polymerisation of olefin monomer in the downstream reactor may be a copolymerisation process that takes place in the presence of an alpha olefin comonomer. Suitable alpha olefin monomers are discussed in further detail below. One or both of the feeds to the first reactor and downstream reactor may contain at least one alpha olefin comonomer. Where both feeds to the first reactor and downstream reactor contain at least one alpha olefin comonomer, the alpha olefin comonomer may be the same or different.In some embodiments, it may be desirable to achieve enhanced cooling for the recovery of olefin monomers having low numbers of carbon atoms (e.g. 2-4 carbon atoms), for example propylene and ethylene, or ethylene, owing to their relatively low boiling points.DETAILED DESCRIPTION OF THE DISCLOSUREUsed herein the term “barg” to refer to gauge pressures measured in bar, so for example, a pressure quoted as 2 barg means a gauge pressure of 2 bars.Used herein the term “flashed” means the decompression of a substance or feed, with the drop in pressure being associated with a concomitant reduction in temperature.Used herein cryogenic temperatures refer to very low temperatures which are difficult or impossible to attain using conventional refrigerants, specifically here referring to below - 40 °C, preferably below -50 °C, more preferably below -70°C, yet more preferably below - 80°C, and yet more preferably below -90 °C.Used herein the term “process stream” refers to any stream of the polymerisation process. Examples include reactant-containing streams, product-containing streams, catalystcontaining streams and / or flush gas or diluent-containing streams.Polymerisation and RecoveryAs mentioned above, the process of the present disclosure is a polymerisation process. Preferably, the process is a process for polymerising olefins in multistage polymerisation process configuration. The process comprises polymerising, in a first reactor, olefin monomer, in the presence of a polymerisation catalyst to produce a first polymer component. Preferably, the process also comprises polymerising olefin monomer in a second reactor, in the presence of the first polymer component to produce a second polymer component. The operating pressure of the downstream reactor is preferably lower than the operating pressure of the first reactor.In the first reactor, olefin monomer can be polymerised at a first pressure produce a first polymer component. The first reactor may be a slurry reactor, for example, a slurry loop reactor. Where at least one second reactor(s) is used, the first pressure may be higher than the operating pressure of at least one of the second reactor(s). The second reactor may be a gas phase reactor. Multiple reactors may be used downstream the first reactor. Multiple first reactors may be used.In the case of a multistage polymerisation process, the first polymer component may be transferred to the second (e.g., gas phase) reactor from the first reactor(s), such that the second olefin polymer, which may be referred to as component, is produced in the presence of first polymer component.The gas phase reactor may be a fluidized bed reactor.A fluidisation gas comprising unreacted olefin monomer may be withdrawn from an upper portion of the gas phase reactor and recirculated to a lower region of the gas phase reactor to maintain a fluidised bed in the gas phase reactor.Preferably, the same catalyst is used in each step and ideally, it is transferred from prepolymerisation (if employed) to subsequent polymerisation steps in sequence.As explained above, a product stream comprising polymer and unreacted olefin monomer is withdrawn from the process. This product stream may be recovered from any reactor where a multistage polymerisation process. However, the product stream is preferably recovered from a second reactor, for example, a gas phase reactor. Where more than one gas phase reactor is used, the product stream is recovered from the most downstream of the gas phase reactors.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 by distillation. The recovered unreacted olefin monomer may be recycled to the process as recycled monomer feed e.g., to any reactor(s) in the process.In the purge bin, the remaining active catalyst may be deactivated e.g., by steam, while unreacted monomers (and optional co-monomers) may be stripped from polymer particles by purge gas addition. As explained above, a purge gas, for example, an inert gas comprising, consisting essentially of or consisting of nitrogen may be introduced into the purge bin to facilitate the separation of unreacted olefin monomer from the polymer as a gaseous stream. This gaseous stream may be termed “purge off gas”. This purge off gas may be separated from the purge bin and introduced into a membrane separation unit for separating the purge gas from the hydrocarbons (e.g. diluent, co-monomer, monomer). The purge gas may be recycled for use in the purge bin, while at least a portion of the hydrocarbons may be recovered in a distillation column. Once polymer particles have been purged in the purge bin, the polymer particles may be then conveyed to another area of the multistage polymerisation process configuration, e.g. an extrusion and pelletizing area for further processing so that a final polymer product may be obtained. The polymer particles may be conveyed from the purge bin to another area using and inert gas, such as nitrogen,as a conveying medium, for example nitrogen recovered from a membrane separation unit. In some embodiments where cryogenic cooling is used, the hydrocarbons taken from the unreacted olefin - nitrogen separation unit may comprise nitrogen due to partial condensation of nitrogen at these temperatures. In such embodiments, nitrogen may be removed by use of a nitrogen stripper (so that for example below 0.5 mol% nitrogen is achieved).Suitable membranes for use in a membrane separation unit are organic-selective composite membranes that are more permeable to organic components than to nitrogen. Efficient separation can be achieved based on differences in permeability through robust, high-flux membrane based on for example polydimethylsiloxane (PDMS). To provide the driving force for permeation, in operation a pressure difference is maintained across the membranes between the feed (which comprises a mixture of hydrocarbons and nitrogen) and the permeate stream. The pressure difference may be obtained by compressing the feed stream using for example an oil-injected 2-stage screw compressor. The permeate stream may optionally have the small proportion of light hydrocarbons removed to avoid accumulation of light hydrocarbons in the process. Any light hydrocarbons removed may be used for dryer regeneration before it is burned in regenerative thermal oxidizer (RTO), thermal oxidizer (TO), flare or boiler.The purge off gas stream separated from the purge bin may comprise water, unreacted olefins, diluent, hydrogen, heavy hydrocarbons and purge gas (e.g. nitrogen). Water may be separated by compression (e.g. in a compressor) into a liquid phase and separated e.g. for further processing in a water treatment unit. Further, following compression, purge off gas separated from the purge bin may be dried in dryers. Following the drying step, the purge off gas separated from the purge bin can then be cooled to cryogenic temperatures (e.g. in preferred embodiments to around -90 °C or below) to separate the purge gas (e.g., nitrogen) from the unreacted olefins and heavy hydrocarbons. The purge gas (e.g., nitrogen) may then optionally further purified by membrane separation before optionally being recovered for reuse in the purge bin. The unreacted olefins and heavy hydrocarbons may optionally then undergo nitrogen stripping in a nitrogen stripper before passing to a distillation column for recovery of the unreacted olefins and optional diluent. Because the purge off gas separated from the purge bin may be cooled to cryogenic temperatures as low as -90 °C or below, some e.g., nitrogen may also condense or solve Nitrogen can be recovered from the condensed hydrocarbon stream using a nitrogen stripper and the stripped nitrogen can be recycled to the hydrocarbon-nitrogen recovery unit for further purification e.g., prior to reuse in the purge bin.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 bleed using at least one of the distillation columns used to recover unreacted olefin from the withdrawn product stream. Optionally pressurized vapor is fed into distillation columns to recover and recycle unreacted monomer and hydrogen (and where present comonomer) back to the polymerization reactors. Light components (e.g., inert gases and impurities), heavy hydrocarbon components (e.g., waxes and oligomers) and co-catalyst residues may be removed by the recovery area.PrepolymerisationThe 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..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, for example the multistage polymerisation reactor is configured such that the reaction product produced in the prepolymerisation reactor may be fed into at least one loop reactor.Producing the first polymer componentAs discussed above, the process of the present disclosure comprises polymerising, in a 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).Preferably, the first olefin monomer is propylene or ethylene, preferably ethylene.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, ethylene is polymerised in the first reactor to produce ethylene homopolymer as the first polymer component. Alternatively, ethylene may be co-polymerised with at least one alpha olefin comonomer to produce ethylene copolymer as the first polymer component. Examples of suitable alpha olefin comonomer include 1-butene or 1-hexene.In one embodiment, 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.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.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%.The upstream (first) 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.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.Slurry ethylene polymerisation may take place in an inert diluent, typically a hydrocarbon diluent such as, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably the diluent is a low-boiling hydrocarbon having from 3 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 is liquid and acts as monomer and is also the continuous liquid phase to facilitate mixing and heat transfer of the discrete polymerizing particles and reactants (monomer, co-monomer, chain transfer agent, catalyst components). .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.The high pressure reactor operates significantly above atmospheric pressure, and as such there is a need for the olefin monomer to be compressed before being introduced into the high pressure reactor. Optionally, the olefin monomer may be compressed with a compressor.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. The operating pressure of the high pressure reactor is higher than the operating pressure of the downstream gas phase reactor. Where more than one downstream gas phase reactor is present, the operating pressure of the high pressure reactor is higher than the highest operating pressure of all of the downstream gas phase reactors. The operating pressure of the high pressure reactor may be from 50 to 100 barg. For example, 50 to 90 barg. Preferably, the operating pressure may be 52 to 80 barg, more preferably 55 to 75 barg, yet more preferably 57 to 70 barg, most preferably 60 to 69 barg.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.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.Downstream gas phase reactorOnce formed in the upstream reactor(s), the first polymer component may be transferred to a second (i.e. , downstream) reactor, preferably a gas phase reactor.As mentioned above, polymerisation in the downstream gas phase reactor involves polymerising a second olefin monomer and optionally at least one alpha olefin comonomer in the presence of the first polymer component.As discussed above, the second olefin monomer 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.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.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.Preferably, ethylene may be co-polymerised with at least one alpha olefin comonomer to produce ethylene copolymer as the second polymer component.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, 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.The same or different alpha olefin comonomer may bs 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.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%.The downstream polymerisation may take place in one or more gas phase polymerisation reactor(s) to produce the second polymer component.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.The temperature in the gas phase polymerisation may be from 50 to 100 °C, preferably from 65 to 90 °C.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 25 barg, most preferably 15 to 22 barg.The gas phase polymerisation is preferably conducted in a gas-solids fluidized bed(s).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 zone or freeboard in which, due to its expanding diameter compared to the middle zone, the fluidization gas velocity reduces and so to disengage polyolefin powder from the fluidization gas to minimise entrainmentPrior to being reintroduced into the gas phase reactor as or as part of the fluidisation 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 thecirculation 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.Polymer products of this invention may be high-density polyethylene (HDPE) or linear-low- density polyethylene (LLDPE) or polyethylene with any density in between (MDPE) where ethylene is used as the olefin monomer. Polymer products of this invention may be homopolypropylene, random-polypropylene-copolymer, heterophasic polypropylene copolymer or random-heterophasic PP where propylene is used as the olefin monomer. Where different monomers are used in different reactors block copolymers may be produced.Polymerization catalystThe 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.Suitable Ziegler-Natta catalysts preferably contain a magnesium compound, an aluminium compound and a titanium compound supported on a particulate support.The particulate support can be an inorganic oxide support, such as silica, alumina, titania, silica-alumina and silica-titania. In some embodiments, magnesium chloride may also be employed.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.The magnesium compound may be a reaction product of a magnesium dialkyl and an alcohol. The alcohol is a linear or branched aliphatic mono-alcohol. 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.The aluminium compound may be chlorine containing aluminium alkyl. Especially preferred compounds are aluminium alkyl dichlorides and aluminium alkyl sesquichlorides.The titanium compound may be a halogen containing titanium compound, preferably chlorine containing titanium compound. Especially preferred titanium compound is titanium tetrachloride.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 .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 .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.The Ziegler-Natta catalyst can be 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 .The amount in which the activator is used depends on the specific catalyst and activator.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.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 .FiguresThese and other aspects of the present disclosure will now be described, by way of example, with reference to the accompanying drawings. Figure 1 is a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to a comparative example. Figure 2 depicts a schematic drawing of a typical refrigeration unit according to the prior art. Figure 3 is a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to a first embodiment of the present invention. Figure 4 is a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to a second embodiment of the present invention. Figure 5 is a schematic drawing of a hydrocarbon-nitrogen recovery scheme according to a comparative example. Figure 6 is aschematic drawing of a hydrocarbon-nitrogen recovery scheme according to a third embodiment of the present invention.Figure 1 depicts a schematic view of a multistage polymerisation process configuration for carrying out a process according to a comparative example.The configuration comprises a first reactor 1 (for example one or more slurry loop reactors) for producing a first polymer component, and a downstream gas phase reactor 2. Although a single downstream gas phase reactor 2 is depicted, it will be understood that a plurality (e.g., two or three) of gas phase reactors may be used in series (not shown).The configuration also comprises an olefin feed compressor 3, a gas recovery compressor 4, a recovery unit 5, a high pressure separator 6, and a low pressure separator 7. The recovery unit 5 is for recovery of monomer and optional co-monomers and hydrogen, as well as diluent for recycling components to the reactors and removal of by products and prevention of accumulation of inert light components.In operation, monomer 8 comprising an olefin monomer (e.g. ethylene) is pressurised by the olefin feed compressor 3 to produce a pressurised olefin monomer feed 9 (first feed), which is introduced into the first reactor 1. Other feeds 10 are fed to the high pressure reactor 1 such as but not limited to chain transfer agent, diluent, catalyst, co-catalyst and optional co-monomer. The olefin monomer fed through line 9 and any optional alpha olefin comonomer used is polymerised in the first reactor 1 to produce a first polymer component. A high pressure reactor effluent 11 comprising polymer, diluent and unreacted monomers is removed from the reactor 1. The reactor effluent 11 is flashed in the high pressure separator 6, forming a first vapour stream 12 and a polymer transfer stream 13. The vapour stream 12 is fed to the recovery unit 5. Typically the recovery unit 5 may comprise at least one distillation column. Olefin monomer and diluent and optionally chain transfer agent (e.g. hydrogen) and co-monomer, may be recovered, partly purified and fed to the first reactor 1 via the recovery stream 14. For simplicity the first recovery stream 14 is shown as a single stream, however multiple streams from the recovery unit 5 to the first reactor 1 may be employed and may each have different compositions.The polymer transfer stream 13 comprising still active catalyst is introduced into the downstream gas phase reactor 2. In the downstream gas phase reactor 2 high molecular weight polymer is preferably produced. More preferably a higher molecular weight co-polymer is produced. Typically the downstream gas phase reactor 2 is a fluidized bed reactor. In the gas phase reactor 2, the olefin monomer is further polymerised. Fresh olefin monomer 8 (e.g. ethylene) via line 15, optionally a fresh feed comprising co-monomer 16, and recycled monomers and optional comonomers via line 17 are fed to the gas phase reactor 2. Typically the operating pressure is about 15 to 25 barg in the gas phase polymerisation reactor 2. This contrasts with the operating pressure of the first reactor may be about 60 to 75 barg.Product stream 18 comprising unreacted olefin monomer and polymer product is taken out from the gas phase reactor 2 and fed to a low pressure separator 7. The low pressure separator 7 produces a second vapour stream 19 comprising unreacted monomer which is fed to a gas recovery compressor 4. Compressed gas is fed from the gas recovery compressor 4 to the recovery unit 5 via line 21. The polymer stream 20 comprising polymer and absorbed / adsorbed hydrocarbons is withdrawn from the low pressure separator 7.The product stream 20 withdrawn from the low pressure separator 7 may contain entrained gaseous matter, including unreacted olefin monomer and optional alpha olefin comonomer and absorbed / adsorbed co-monomer, diluent and monomer. The product stream 20 may optionally be fed to downstream units for further degassing (e.g. via a purge bin), deactivation, homogenisation, additivation and pelletizing to obtain a final product. Purge bin off gas comprising unreacted monomer from these downstream units (e.g. from the purge bin) may be compressed, further processed and fed into the recovery unit 5.Figure 2 depicts a schematic drawing of a typical refrigeration unit according to the prior art. The figure depicts a refrigeration unit capable of cooling by providing low temperature refrigerant for direct heat exchange with a process stream, and / or by providing low temperature refrigerant for cooling a coolant stream (e.g., water, glycol water, or mixtures of water and an alcohol) that is then used for removing heat from a process stream, reactor or other component of the polymerisation system. The skilled person will recognise that although low temperature refrigerant for direct heat exchange and cooling water are both shown, refrigeration units are commonly configured so that one or other of direct heat exchange or cooling water are used.In Figure 2, refrigerant vapour in a low pressure refrigerant vapour stream 100 is compressed by refrigerant compressor 101 to form a high pressure refrigerant vapour stream 102. The high pressure refrigerant vapour stream 102 is condensed in a condenser 104 to form a condensed refrigerant stream 105. The condensed refrigerant stream 105 isfed to heat exchanger 107, and the process stream is cooled by evaporation of the refrigerant at the desired pressure and so temperature. Alternatively or additionally, the refrigerant stream 105 may be fed to heat exchanger 106 to cool another coolant stream. Where the heat exchanger 106 is used, the heat exchanger 106 may be fed by a return feed of the closed loop coolant water 108 to produce the low temperature refrigerated coolant water supply stream 109. The refrigerated coolant water 109 can be supplied to optional multiple heat exchangers and optional parts of the polymerisation process and then returned to the refrigeration unit in the coolant water return stream 108.Upstream of the refrigerant compressor 101 is a suction vessel 110 is present where the refrigerant vapour is collected, and liquids can be drained.Whether a particular cooling process uses direct heat exchange or refrigerated water utility depends on factors such as the required temperature.Such refrigerant systems may be used to cool process streams or produce refrigerated coolant streams for cooling process steps in the polymerisation process, for example, in recovery unit 5 of Figure 1.Any refrigerant can be used, e.g. propane, cyclopropane, ammonia can be used.Figure 3 depicts a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to a first embodiment of the present invention.The multistage polymerisation process configuration depicted in Figure 3 has a number of structural features in common with the configuration shown in Figure 1 and like numerals have been used to denote like parts. However, in addition to the features shown in Figure 1, this embodiment includes providing a second olefin monomer feed by withdrawing a slip stream 200 containing pressurised olefin monomer from the pressurised olefin monomer feed 9. The slip stream 200 is optionally pre-cooled or optional condensed in heat exchanger 201 and then depressurised using control valve 202. Although compressor 3 is shown in Figure 3, this may not be necessary if the pressure is sufficiently high at the battery limit.When operating at process configuration sites with low temperature cooling water, the cooling of the slip stream 200 in heat exchanger 201 may be carried out using such lowtemperature cooling water. Alternatively or in addition, the slip stream 200 may be cooled by refrigerated coolant water (e.g. produced as described with reference to Figure 2), wherein the refrigerant may be propane or the slip stream 200 cooled by heat integration by a cold process stream.The optionally cooled slip stream 200 is depressurised and hence expanded using expansion valve 202. This depressurisation causes the temperature of the slip stream to be reduced, producing a reduced temperature olefin monomer-containing stream. This depressurised and reduced temperature stream can be used as a coolant in heat exchanger 203 to cool a process stream (not shown) or a coolant stream, such as a coolant water stream. The slip stream 200 is removed from the heat exchanger 203 after heat exchange, and the removed stream provides an vapour olefin return stream 204 that can be fed to the gas recovery compressor 4 and compressed before being eventually reintroduced into the first reactor 1. Alternatively, the olefin return stream 204 may be fed to the downstream reactor 2, in some instances, without further compression. However, compression may be employed if very low temperatures are required.Figure 4 depicts a schematic drawing of a process scheme polymerising olefins in multistage polymerisation process configuration according to a second embodiment of the present invention.The multistage polymerisation process configuration depicted in Figure 4 has a number of structural features in common with the configuration shown in Figure 3 and like parts are denoted with like numerals. Like the process scheme shown in Figure 3, this process scheme depicts a second olefin monomer feed by withdrawing a slip stream 200 containing pressurised olefin monomer from the pressurised olefin monomer feed 9. The slip stream 200 is cooled in heat exchanger 201. In this embodiment, however, a third stream is formed by withdrawing a further slip stream 300 from the slip stream 200. This further slip stream 300 is depressurised via depressurisation valve 301. This depressurisation causes a temperature drop, producing a further depressurised reduced temperature olefin monomer- containing stream 302. This stream 302 is used as a coolant in heat exchanger 201 to cool the slip stream 200 prior to its depressurisation via valve 201 as described with reference to Figure 2. After heat exchange, the depressurised stream 302 is introduced into the gas phase reactor 2 via line 303. In Figure 4, the depressurised stream 204’ emerging from heat exchanger 203 is introduced to the gas phase reactor 2 via line 304. As an alternative (not shown), the depressurised stream 204’ may be introduced into the first reactor 1 after compression via compression 4 and recovery 5.Figure 5 is a schematic drawing for the recovery of hydrocarbons and nitrogen from an off gas stream from a purge bin according to a comparative example. The purge bin 400 is typically fed with a polymer stream 20 (e.g., Figure 1) to the top of the polymer moving bed in the purge bin. Purge gas and optionally steam are fed to a lower part of the purge bin 400 and purge off gas is removed from the top of the purge bin, optional via a filter unit (not shown). The hydrocarbon and nitrogen recovery scheme comprises a compressor 401, optional knock out drum 411, dryer 402, cooler 403, optional separation vessel (not shown), and membrane separation units 404.Waste gas from the hydrocarbon recovery unit is handled by waste gas handling unit 405, e.g. a thermal oxidiser. . The recovered condensed hydrocarbon stream is optional fed to a degasser (406) to remove remaining solved Nitrogen before feeding the recovered condensed hydrocarbons to the recovery unit (407, hence 5 in figure 1))Product (not shown) from the polymerisation process is introduced into the purge bin 400. The product comprises polymer, as well as hydrocarbons, including unreacted olefin monomer, diluent and optional co-monomer. An inert gas (e.g., nitrogen) is introduced into the purge bin to separate the hydrocarbons from the polymer product. The hydrocarbons are separated together with the inert gas as used purge off gas in line 409. Any water 410 condensed from the used purge gas can be separated and treated in water treatment unit 408. The purge off gas is compressed in compressor 401.Once compressed and optionally pre-cooled in knock out water drum 411 to remove free water, the remaining gaseous components of the purge off gas are dried in dryer 402 to remove remaining water and cooled in cooler 403 (optional multiple coolers including heat integration). Cooling in cooler 403 is achieved using prior art refrigerant systems, such as those described with reference to Figure 2. For example, a reduced temperature refrigerant (e.g., propane) may be used to cool the used purge gas stream directly by direct heat exchange. Typically, cooling to approximately -30 °C is achieved using such refrigerants. This causes a proportion of the hydrocarbons in the purge off gas stream to condense and the condensed hydrocarbons are passed to degasser 406 via line 412. In degasser 406, solved nitrogen is removed via stream 414 from the condensed hydrocarbon stream, and stream 414 also comprising valuable hydrocarbons is recovered. The recovered hydrocarbons 417 are fed to the recovery unit 407 for further processing and recycling to the polymerization process (see figure 1 , recovery unit 5)Once cooling in cooler 403 has occurred, the uncondensed components are introduced into the membrane separation unit 404. These components contain the inert gas (e.g., nitrogen) used in the purge bin, and this is partly recovered by membrane separation 404 and recycled to the purge bin via line 415. Uncondensed hydrocarbon may be permeate through the membrane separation unit 404 and returned to the suction side of compressor via line 416 or partly to an interstage of the compressor, while the remaining off gas may be removed from the process to a waste gas handling unit 405. Light hydrocarbons such as ethylene may not be recovered by this process.Figure 6 is a schematic drawing of a hydrocarbon recovery scheme according to another embodiment of the present disclosure. The scheme is similar to that described with reference to Figure 5 and like parts have been numbered with like numerals. In contrast to the scheme of Figure 5, the cooler 403 is cooled using a reduced temperature olefin monomer stream of the present disclosure as coolant. By way of example, such a reduced temperature olefin monomer stream is the stream produced following depressurisation over valve 202 of Figure 3 or Figure 4. In some embodiments, such streams may first be pressurised using existing e.g., any compressors (e.g., compressors 3 in Figures 1 and 3) used to compress the olefin monomer feed to the reactor. Such compressors may be the same or larger than those used in, for example, those depicted in Figure 1.In the scheme of Figure 6, the stream introduced into cooler 403 can be cooled to cryogenic temperatures as described above. Preferably, temperatures as low as -90 °C or below are achieved. The condensed hydrocarbons can be removed from the cooler via line 412 and, because of the cryogenic conditions, some nitrogen may also be condensed together with the hydrocarbons. This nitrogen is stripped in stripper 500 and the recovered nitrogen 501 is recycled upstream of compressor 401 via line 501. Once nitrogen is stripped the recovered hydrocarbons are recovered via recovery unit 407 as described in relation to Figure 5.In the scheme of Figure 6, the low temperatures employed in the cooler 403 allow nitrogen 502 to be recovered in the absence of a membrane separation unit 503, if desired because the nitrogen recovered is of a high purity. However, if a higher degree of nitrogen purity is desired a membrane separation unit 503 may be employed. This can be used to recover hydrocarbon 504 for recycling upstream of compressor 401.Example 1This example uses a multistage polymerisation process configuration according to the configuration shown in Figure 3. A comparative example is also presented in Table 2 which uses a multistage polymerisation process configuration according to the configuration shown in Figure 1. Using this set up and using ethylene as the olefin monomer, the amounts of ethylene per hour that were used are presented below in Table 1.Table 1Table 2In this example it was possible to obtain a refrigerant at low temperature without installing a separate refrigeration unit of the type shown in Figure 2. Further, it was possible to obtain a refrigerant at very low temperature, temperatures of about -94°C were measured for the refrigerant in this example. As a result of this very low temperature, it was possible to recover ethylene and diluent from the off gas of the purge bin without installing additional compressors. This simple configuration without additional compressors can be achieved without significant change to the other process units, as table 2 demonstrates. The data in table 2 show that at most a 10% increase in capacity of the olefin feed compressor 3 and 25% increase in capacity of the gas recovery compressor 4 would be required. This is nonetheless a minor adjustment to the overall process relative to Figure 1 given the improved performance of the hydrocarbon-nitrogen separation step.Example 2Alternatively cooling by ethylene can be obtained, either for producing a refrigerant or direct cooling another process stream, by flashing only till a pressure slightly above the operating pressure of the reactor block R-2. As shown in Figure 4, all ethylene feed to the reactor block R-2 can be used to create a low temperature cooling ‘utility’, especially when the ethylene pressure at battery limit is high enough. A portion of the ethylene can be flashed to slightly above the operating pressure of the gas phase polymerisation, the low temperature due to the flashing can be used to cool the remaining portion of the ethylene feed to the gas phase reaction block, upstream of the flashing step. The pressure of the pre-cooled ethylene is than reduced to obtain a low temperature of about -21°C.Using a feed of 1000 kg / h to the gas phase reaction block with initial pressure of 56 barg, splitting the feed in two portions of about 4 / 5 and 1 / 5, reducing the pressure of the 800 kg / h portion to about 25 barg, causes a temperature drop to about 3°C. This flow is used to cool the second portion of 200 kg / h, still at 56 barg, to about 7-12 °C. The second pre-cooled portion can be flashed to about 25 barg and the heat of evaporation at about -20 / -21°C can be used to cool down a process stream or producing a cold utility stream. The duty is about 10 kW per 1000 kg / h ethylene feed to the gas phase reaction block. Both portions can be directly fed to the gas phase polymerisation.Aspects of the invention will now be described in the following numbered clauses:1. A process for polymerising olefins, said process comprising: introducing a first feed comprising olefin monomer to a first reactor; polymerising the olefin monomer of the first feed in the first reactor to produce a first polymer component; depressurising a second feed comprising olefin monomer such that the temperature reduction on depressurisation produces a reduced temperature olefin monomer- containing stream; and using the reduced temperature olefin monomer-containing stream as a coolant in the process.2. A process as defined in clause 1 , wherein the reduced temperature olefin monomer- containing stream is used as coolant to cool a process stream and / or another coolant stream by heat exchange.3. A process as defined in clause 1 or 2, wherein the depressurisation of the second feed produces a reduced temperature olefin monomer-containing stream having atemperature of below -40 °C, preferably below -50 °C, more preferably below -70°C, yet more preferably below -80°C, and yet more preferably below -90 °C. A process as defined in any one of the preceding clauses, wherein, after the reduced temperature olefin monomer-containing stream is used as a coolant in the process, the depressurised olefin monomer-containing stream is recompressed and reintroduced into the first reactor. A process as defined in any one of the preceding clauses, wherein the second feed of the olefin monomer is withdrawn as a side stream from the first feed of olefin monomer prior to the first feed of olefin monomer being introduced into the first reactor. A process as defined in any one of the preceding clauses, wherein a third feed of olefin monomer source is formed by withdrawing a side stream from the first feed prior to the first feed being introduced into the first reactor or by withdrawing a side stream from the second feed prior to its depressurisation. A process as defined in clause 6, said process further comprising i) depressurising the third feed of olefin monomer such that the temperature reduction on depressurisation produces a further reduced temperature olefin monomer-containing stream; and ii) using the further reduced temperature olefin monomer-containing stream to cool at least a portion of the second feed by heat exchange before the at least said portion of second feed is depressurised to produce the reduced temperature olefin monomer-containing stream. A process as defined in any one of the preceding clauses, which further comprises polymerising olefin monomer in a second reactor downstream of the first reactor at a second pressure that is lower than the first pressure, wherein polymerisation of olefin monomer in the downstream reactor takes place in the presence of the first polymer component. A process as defined in clause 8, wherein, once the reduced temperature olefin monomer-containing stream has been used as a coolant, the stream is fed to the second reactor and / or fed to a gas recovery compressor prior to being fed to a gas recovery unit.A process as defined in any one of the preceding clauses, wherein the first feed of olefin monomer is compressed, optionally, to a pressure higher than the first pressure of the first reactor prior to being introduced into the first reactor A process as defined in clause 10, wherein the second feed is withdrawn as a side stream from the first feed of olefin monomer after the first feed of olefin monomer has been compressed. A process as defined in any one of the preceding clauses, which further comprises i) withdrawing a product stream comprising polymer and hydrocarbons from the process; ii) introducing at least a portion of the withdrawn product stream into a purge bin; iii) using an inert gas, said inert gas preferably comprising nitrogen, to strip a purge off gas comprising hydrocarbons and inert gas from the product stream; iv) separating the purge off gas from the purge bin; and v) cooling the separated purge off gas to condense hydrocarbons from the separated purge off gas. A process as defined in clause 11 wherein the purge off gas is cooled to a temperature of below -40 °C, preferably below -50 °C, more preferably below -70°C, yet more preferably below -80°C, and yet more preferably below -90 °C to condense hydrocarbons from the purge off gas13. ; and / or wherein the purge off gas is cooled using the reduced temperature depressurised olefin monomer-containing stream or a coolant stream that has been cooled by heat exchange with the reduced temperature olefin monomer-containing stream. A process as defined in any one of the preceding clauses, wherein the process is a process for polymerising ethylene and wherein the second feed comprises ethylene. A process as defined in any one of the preceding clauses, which comprises providing an olefin monomer source; obtaining a first portion of the olefin monomer source as the first feed, and obtaining a second portion of the olefin monomer source as the second feed.

Claims

Claims1. A process for polymerising olefins, said process comprising: introducing a first feed comprising olefin monomer to a first reactor; polymerising the olefin monomer of the first feed in the first reactor to produce a first polymer component; depressurising a second feed comprising olefin monomer such that the temperature reduction on depressurisation produces a reduced temperature olefin monomer-containing stream; using the reduced temperature olefin monomer-containing stream as a coolant in the process; and wherein the second feed of the olefin monomer is withdrawn as a side stream from the first feed of olefin monomer prior to the first feed of olefin monomer being introduced into the first reactor.

2. A process as claimed in claim 1, wherein the reduced temperature olefin monomer- containing stream is used as coolant to cool a process stream and / or another coolant stream by heat exchange.

3. A process as claimed in claim 1 or 2, wherein the depressurisation of the second feed produces a reduced temperature olefin monomer-containing stream having a temperature of below -40 °C, preferably below -50 °C, more preferably below -70°C, yet more preferably below -80°C, and yet more preferably below -90 °C.

4. A process as claimed in any one of the preceding claims, wherein, after the reduced temperature olefin monomer-containing stream is used as a coolant in the process, the depressurised olefin monomer-containing stream is recompressed and reintroduced into the first reactor.

5. A process as claimed in any one of the preceding claims, wherein a third feed of olefin monomer source is formed by withdrawing a side stream from the first feed prior to the firstfeed being introduced into the first reactor or by withdrawing a side stream from the second feed prior to its depressurisation.

6. A process as claimed in claim 5, said process further comprising iii) depressurising the third feed of olefin monomer such that the temperature reduction on depressurisation produces a further reduced temperature olefin monomer-containing stream; and iv) using the further reduced temperature olefin monomer-containing stream to cool at least a portion of the second feed by heat exchange before the at least said portion of second feed is depressurised to produce the reduced temperature olefin monomer-containing stream.

7. A process as claimed in any one of the preceding claims, which further comprises polymerising olefin monomer in a second reactor downstream of the first reactor at a second pressure that is lower than the first pressure, wherein polymerisation of olefin monomer in the downstream reactor takes place in the presence of the first polymer component.

8. A process as claimed in claim 7, wherein, once the reduced temperature olefin monomer- containing stream has been used as a coolant, the stream is fed to the second reactor and / or fed to a gas recovery compressor prior to being fed to a gas recovery unit.

9. A process as claimed in any one of the preceding claims, wherein the first feed of olefin monomer is compressed, optionally, to a pressure higher than the first pressure of the first reactor prior to being introduced into the first reactor10. A process as claimed in claim 9, wherein the second feed is withdrawn as a side stream from the first feed of olefin monomer after the first feed of olefin monomer has been compressed.

11. A process as claimed in any one of the preceding claims, which further comprises vi) withdrawing a product stream comprising polymer and hydrocarbons from the process;vii) introducing at least a portion of the withdrawn product stream into a purge bin; viii) using an inert gas, said inert gas preferably comprising nitrogen, to strip a purge off gas comprising hydrocarbons and inert gas from the product stream; ix) separating the purge off gas from the purge bin; and x) cooling the separated purge off gas to condense hydrocarbons from the separated purge off gas.

12. A process as claimed in claim 11 wherein the purge off gas is cooled to a temperature of below -40 °C, preferably below -50 °C, more preferably below -70°C, yet more preferably below -80°C, and yet more preferably below -90 °C to condense hydrocarbons from the purge off gas13. ; and / or wherein the purge off gas is cooled using the reduced temperature depressurised olefin monomer-containing stream or a coolant stream that has been cooled by heat exchange with the reduced temperature olefin monomer-containing stream.

13. A process as claimed in any one of the preceding claims, wherein the process is a process for polymerising ethylene and wherein the second feed comprises ethylene.

14. A process as claimed in any one of the preceding claims, which comprises providing an olefin monomer source; obtaining a first portion of the olefin monomer source as the first feed, and obtaining a second portion of the olefin monomer source as the second feed.

15. A process as claimed in any one of the preceding claims, wherein the first reactor may be a loop and / or slurry reactor, for example, a slurry loop reactor.

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