PROCESS FOR PREPARING POLYOLEFINS

Abstract: The invention relates to a process for the production of polyolefins in a reactor system which comprises at least one gas-phase reactor (11) wherein at least one monomer is polymerized. According to the process, there is fed into the gas-phase reactor the monomer to be polymerized and possibly a polymer-containing fluid. According to the invention, before being fed into the gas-phase reactor (11) the fluid is subjected to a phase change in order to separate uncondensed compounds from liquid and possibly solid compounds. The gas including the uncondensed compounds is recovered, and hydrogen is separated from the gas by membrane separation. The invention can be applied to the removal of hydrogen from the circulation gas of a gas-phase reactor, for example in order to change the polyolefin grade to be produced.

Description: Process for preparing polyolefins The present invention relates to the production of polyolefins. The invention relates in particular to a process according to the preamble of Claim 1 for the production of polyolefins in a reactor system which comprises at least one gas-phase reactor wherein at least one monomer is polymerized. According to the process, hydrogen and possibly other gaseous or vaporized light components are separated from the fluid being directed to the gas-phase reactor. The invention is applicable to the separation of light components from the stream between two polymerization reactors and to the removal of volatile (uncondensable) components from the circulation gas of a gas-phase reactor used for the preparation of polyolefins. Olefins are polymerized into polyolefins in a gas-phase reactor in the presence of, for example, Ziegler-Natta type catalysts or metallocene catalysts. The forming polymer mass, which contains an active catalyst, is fluidized in a fluidized bed reactor by using a hydrocarbon stream. This stream contains a monomer (typically ethylene or propylene) and possibly a comonomer (C 2 -C 10 olefins or diolefins, preferably ethylene, propylene, 1- butene or hexene), as well as hydrogen, which controls the molecular weight. The hydrocarbon stream may also contain high concentrations of a substance inert in the reaction (typically nitrogen or propane). During the polymerization there is withdrawn from the gas-phase reactor a gas stream which contains at least some amount of unreacted monomer and which is returned to the gas-phase reactor as circulation gas. Most ofthe heat of reaction is removed by cooling this circulation gas. In order to ensure sufficient cooling efficiency and to achieve suitable fluidization velocities, the circulation gas stream is typically quite large. From the viewpoint ofthe functioning ofthe condenser it is preferable to remove from the stream the components with the lowest molecular weights, in particular hydrogen. Nowadays hydrogen is removed from the gas-phase reactor either from the circulation gas stream or from the recovery stream together with the purge stream. 2 There are considerable disadvantages associated with prior art solutions. Thus the most typical method of lowering the concentration of hydrogen in the reactor is to increase the off-gas stream of accumulating inerts to a level close to the maximum capacity ofthe gas removal system. The gas removed is directed, for example, to the flare for burning or as feed to cracking. These methods are bad, expensive and slow, since both chemicals and production capacity are lost, which is due to the fact that a product of deficient quality is produced. A further disadvantage ofthe prior art is that, when a typically relatively low concentration of hydrogen is lowered, the gases leaving together with the hydrogen cause a large financial loss to the polyolefin plant. The accumulation of hydrogen and other light components in the gas streams ofthe polymerization process constitutes a problem also when polyolefins are prepared in a plant where two reactors are coupled together. Owing to the light components, such as hydrogen and inert compounds, present in the stream between the reactors, all products cannot be prepared by directing the mixture emerging from one reactor directly to the following gas- phase reactor. The reactor product is therefore directed to a product separator, where its pressure is lowered, whereupon the liquid possibly arriving together with the product vaporizes at least in part, or the gas expands and is recyclable to the first reactor. The pressure ofthe recycled stream is raised by using a compressor. Owing to the recycling, inert components tend to accumulate in the reactor. In both ofthe cases mentioned above, particularly significant among the light components are hydrogen and lower alkanes, such as methane, ethane and propane, as well as nitrogen and other inert gases. Hydrogen is used for controlling the molar mass ofthe polymer, and the amount used varies according to the grade ofthe polymer. Reducing the proportion of hydrogen in the gas-phase reactor feed is important in situations in which it is desired to prepare long-chain polymers in a gas phase. The object ofthe present invention is to eliminate the disadvantages ofthe prior art and to provide a novel process for the preparation of polyolefins in a reactor system having at least one gas-phase reactor. 3 The invention is based on the basic idea that hydrogen and possible other light components are separated by membrane separation from the fluid (liquid or gas stream) being fed into the gas-phase reactor, which fluid can comprise the circulation gas stream ofthe reactor or the effluent ofthe preceding reactor. The use of membrane systems for the separation of hydrogen from process streams of polyolefin reactors is known per se from JP published application 08/151.413. In the said prior known process, gas to be separated under a high pressure from a gas-phase reactor is treated in a high-pressure membrane system to remove hydrogen, whereafter the gas stream is returned to the reactor. The known technical solution has the disadvantage of a low degree of separation. Since the gas is separated under a high pressure, the polymer product will, even after the first separation step, contain volatile compounds, such as polymerization diluent and unreacted monomer, which have to be removed from the polymer separately. Also, no mention ofthe use ofthe process in connection with a grade change in the gas-phase reactor can be found in the publication, and evidently it is not applicable for this purpose, since in a separation carried out under a high pressure, relatively large amounts of monomer are left in the product polymer. There are also other previously known membrane solutions. Thus, US patent publication 4,740,550 describes a process for the preparation of a propylene-ethylene copolymer so that the propylene is first polymerized in two reactors and is then fed together with ethylene into a third reactor. According to the publication, hydrogen is removed from the circulation of the third reactor (i.e. the last gas-phase reactor) by using a hydrogen-selective hydrogen-removing membrane. In this known solution, the gas coming from the reactor and containing, for example, hydrogen, ethylene and propylene, is directed to a gas scrubber, to which ethylene and propylene which have condensed in the scrubber and have thereafter been cooled are recycled. The separation ofthe hydrogen-containing gas from the condensed components is thus based on the adding of cold liquid into the scrubber. The present invention differs from the prior art in that the fluid to be fed into the gas-phase reactor is subjected, before being fed into the reactor, to a phase change in order to separate uncondensable compounds from liquid .and possibly solid compounds. After at least a 4 partial vaporization or condensation ofthe fluid, the gas made up of uncondensed and possibly vaporized compounds is recovered and directed to membrane separation. When it is desired to treat the stream between two reactors by using the option according to the invention, in accordance with prior art the product ofthe first reactor is directed to separation cyclones or corresponding apparatus for separating the reaction medium. According to the present invention, instead of vaporizing any possible liquid and directing it to a compressor, as in the prior art, a portion ofthe possible liquid is vaporized or the gas is allowed to expand in one or more steps, and the obtained gases are directed through a membrane system, whereby most ofthe hydrogen is removed. The hydrocarbons separated from the hydrogen are returned into the exit stream from the first reactor, and the said stream (fluid) is then fed into the gas-phase reactor. Thus it is possible to prepare from the effluent ofthe first reactor an intermediate product which can as such be fed into the second reactor (gas-phase reactor). In the treatment ofthe circulation gas ofthe gas-phase reactor, there is withdrawn from the reactor a first gas stream, which is cooled to a first condensing temperature to condense it at least in part and to separate the liquid from the gas. Including the first gas stream, uncondensed compounds are separated in a membrane separator unit from the gas obtained from its condensation. The components obtained from the membrane separation are treated separately, and they are returned to the gas-phase reactor or are directed to distillation or a similar further treatment. More specifically, the present invention is mainly characterized in what is stated in the ch.aracteriz.ing part of Claim 1. Considerable advantages are gained by means ofthe present invention. Thus, the production capacity ofthe gas-phase reactor will increase by at least 20 %, i.e. to correspond to the capacity of a reactor system equipped with a cooling circulation operating with partial condensation. Control ofthe hydrogen concentration is sensible .and economical. When the circulation gas compressor stops, the gas-phase reactor can be emptied rapidly in order to prevent the foπnation of agglomerates, since components can be removed from the hydrogen control cycle in a liquid phase. The off-gas is easily distilled, since it contains very little hydrogen. If the gas-phase reactor feed is in a liquid 5 phase, there is saved in the polymer feed system (including pneumatic conveying) the flash tank, the removal of dust from the gas and the recovery compressor, and the distillation columns are reduced. If partial vaporization is installed in their place, even then the required gas treatment system will be substantially smaller and more economical than in the prior art. This is the case because hydrogen can be removed effectively from the gas- phase circulation. The invention is especially advantageously applicable to situations in which the diluent used in the gas-phase reactor is a heavy hydrocarbon such as butane, and/or a heavy comonomer such as 1-butene or 1-octene. The condensation can be made more affective through the removal of light components, in particular hydrogen, from the circulation stream ofthe gas-phase reactor. By means ofthe present invention, the change of grade ofthe polyolefin prepared in the reactor can be sped up. Although membranes usually do not make possible as precise a separation as, for example, distillation, a sufficiently high degree of separation in terms of polymer grade change can be achieved by the solution according to the invention. At the same time the costs of investment in equipment can be maintained low. By means ofthe invention, at least approx. 50 %, preferably approx. 80-90 %, ofthe hydrogen present in the gas stream subjected to treatment is removed. In connection with the present invention it has been observed that a result especially advantageous in terms of grade change is achieved by feeding into the membrane separator system a gas stream the composition of which substantially corresponds to the composition in the gas-phase reactor as regards small-molecular components. By "small-molecular" components is meant hydrogen and/or nitrogen, as well as monomers and/or diluent residues, which in a gas-phase reactor are adsorbed to polymer particles. Such a gas stream is obtained either by withdrawing a gas stream from the top ofthe gas-phase reactor or by lowering the pressure ofthe reactor effluent and by separating the gas phase from the solid phase. The proportion of small-molecular components remains substantially unchanged. In the above-mentioned JP published application, the system is placed at a point after a product separator operating under high pressure, in which case it has been necessary to provide a separate cycle for the system. In the present case, the membrane system is combined with a cycle necessary even otherwise; preferably the membrane system is used 6 for separating hydrogen from the circulation-gas cooling stream or the recovery stream of the gas-phase reactor. This option improves the operation ofthe condenser in the circulation gas stream, if the membrane system is fitted in the circulation-gas cooling stream. If the membrane system is placed in the recovery stream, a compressor even otherwise needed is utilized, and thus a good operating environment is created for the membrane system. By both methods the speed of polymer grade change is increased, and thereby the profitability ofthe plant is improved. By means ofthe invention the costs incurred from a separate cycle constructed for the membrane system are avoided. In an operation according to the JP publication, the composition ofthe gas introduced into membrane separation differs substantially from the small-molecule composition in the gas- phase reactor, since, when the product separation takes place substantially under the reactor pressure, heavy components ofthe reaction mixture leave the reaction gas in a larger proportion by volume. In a preferred embodiment according to the present invention, product separation takes place under a pressure substantially lower than the reactor pressure. Thereby the reaction gas leaving together with the product polymer is decreased, and the change in its composition is very slight, since the comonomers and heavy diluent components are substantially entirely vaporized, .and only the possibly oligomeric byproducts ofthe reaction remain in part unvaporized. The mutual proportions ofthe light, i.e. small-molecular components, such as hydrogen, lower alkanes and alkenes, in particular methane, ethane and ethene, are substantially the same as in the stream obtained from the reactor. The purity ofthe product is additionally maximized through the placement ofthe membrane system at a point after the low-pressure product separator. At the same time the reactive components are exploited maximally by being returned to the polymerization reactor (i.e. gas-phase reactor), where a product is prepared therefrom. The membrane separates from the stream the lightest components and thereby prevents them from accumulating in the circulation. In the present invention it is therefore possible to operate so that the reactor pressure is lowered (if hydrogen is separated from the polymer product and the membrane system is located in the recovery stream). 7 According to another preferred embodiment, the separation is carried out from the circulation gas ofthe reactor. In this respect the invention to some degree resembles the process of US patent publication 4,740,550; however, the composition ofthe gas directed to the membrane system is completely different, since in the present invention the condensed lower hydrocarbon compounds are not used for scrubbing the gas. Considerable advantages are achieved when the present invention is applied to a process made up of at least one loop reactor and a gas-phase reactor. Thus, a process implemented according to the invention is economically better than current systems, since the cooling need ofthe second reactor is smaller. The second reactor is easier to control, and grade changes are more rapid and easier, whereupon less product of a deficient quality is produced. The current compressor between the two successive reactors can be replaced with a considerably smaller one. The removal of hydrogen at a point between the reactors is especially advantageous in a case in which the latter reactor is operated in such a manner that from the top ofthe reactor there is withdrawn a gas stream which, after partial or total condensation, is recycled to reactor feed. The invention will be examined below in greater detail with the help of a detailed description, the accompanying drawings and a few embodiment examples. Figures 1 and 2 depict the principles of a hydrocarbon-selective and, respectively, a hydrogen-selective membrane system; Figure 3 depicts a process flow diagram for the first preferred embodiment ofthe invention, wherein the uncondensed portion ofthe circulation gas stream is treated using two membranes arranged in series; Figure 4 depicts a simplified process flow diagram of an option in which the condensed liquid is returned as a separate stream to the gas-phase reactor; Figure 5 depicts a diagram ofthe principle of an apparatus configuration suitable for grade change, in which configuration a condenser and a membrane separator have been added to the circulation gas stream, in which case the stream is divided into an uncondensed stream which is directed to the membrane separator and a condensed stream which is at least in part returned to the gas-phase reactor; and 8 Figure 6 depicts a diagram of a process according to the invention, wherein the membrane separation is arranged between the first polymerization reactor ofthe reactor system and the gas-phase reactor. As is evident from the above, the invention relates to a novel process for the production of polyolefins in a gas-phase reactor. According to the first preferred embodiment ofthe invention, the system is applied to a case in which hydrogen is removed from the circulation gas of a gas-phase reactor by carrying out, in the desired order, a partial condensation ofthe circulation gas and a treatment ofthe gas in a membrane separator system. According to the second preferred embodiment ofthe invention, the system is applied to a process in which the hydrogen leaving together with the effluent, i.e. the polymerization product, of a first polymerization reactor or polymerization reactor series operated in liquid, vapour, gas or supercritical conditions is separated from the product in the exit stream ofthe preceding reactor by using a membrane separator before the said stream is fed into the subsequent reactor, which operates in vapour, gas or supercritical conditions. The process according to the present invention is suitable for use for the preparation of polyolefins, in particular polyethylene and polypropylene, the monomers to be polymerized including ethylene and propylene and the comonomers including propylene and respectively ethylene and, for example, 1-butene, 4-methyl-l-pentene, 3 -methyl- 1- butene, vinylcyclohexane, cyclopentene, 1-hexene, 1-octene and 1-decene, as well as dienes. The polymerization reaction is carried out in a gas-phase reactor at en elevated pressure and at an elevated temperature. Thus the pressure is approx. 10-40 bar and the temperature respectively approx. 60-110 °C. The polymerization is typically carried out in the presence of a heterogeneous catalyst, in which case, for example, transition metal catalysts such as Phillips-type chromium catalysts, Ziegler-Natta catalysts or metallocene catalysts are used in the production of polyethylene. Metallocene catalysts and Ziegler-Natta catalysts, such as MgCl 2 *TiCl 4 , are also suitable for the polymerization of polypropylene. 9 In the reactors the medium may be in a liquid, vapour, gas or supercritical state, and the polymers of olefins in a solid or liquid state. In the first embodiment ofthe invention, there is withdrawn from a gas-phase reactor a gas stream, which is hereinafter also called the "first gas stream," and a portion, at least 20 %, preferably approx. 40-95 %, of its condensable compounds are condensed. It is possible to direct to condensation the entire circulation gas stream obtained from the top ofthe gas- phase reactor, or a portion thereof. As will be described below in greater detail, certain advantages are achieved by withdrawing from the reactor at least two circulation gas streams, of which only one is treated in a membrane unit. If two circulation streams are taken for treatment, the second stream can be returned after cooling directly to the reactor, possibly combined with the reactor feed. The flow rate ofthe stream treated in the membrane unit is in general approx. 0J-500 %, typically approx. 1-50 %, ofthe amount of the second stream. The pressure ofthe circulation gas stream is preferably raised before membrane separation in order to promote this separation. A blower, a compressor or a corresponding pressure- raising apparatus can be used for raising the pressure. Preferably the light components are separated from the gas stream only after condensation and the separation ofthe uncondensed components. By means of condensation, heat is removed from the circulation gas stream and the temperature ofthe gas stream is reduced at least to below the condensation temperature ofthe heavy components. Typically the gas stream is cooled to approx. 20-70 °C, e.g. approx. 25-60 °C. Thus there are obtained a liquid stream which contains the condensed compounds and a gas stream which contains light components such as hydrogen, nitrogen and lower alkanes such as ethane and/or methane. The uncondensed gas stream directed to membrane separation is heated, when so desired, before the membrane separation. Preferably the gas stream is heated to a temperature 5- 45 °C higher than the condensation temperature in order to inhibit condensation at the membrane. This option is suitable in particular for hydrogen-selective polymer membranes. Since together with the gas stream there may come fines formed in polymerization, the fines disturbing membrane separation, solids are separated from the gas stream, for 10 example by filtration, at a point before the membrane unit. Preferably the uncondensed portion ofthe first gas stream from the gas-phase reactor is treated in two successive membrane separator units, in which case the gas stream which is obtained from the first membrane separator unit, and from which uncondensed compounds present in it have been separated, is cooled to a second condensation temperature in order to separate condensable compounds from the gas. This uncondensed portion is directed to the second membrane separator unit, where uncondensed compounds are separated from it. The uncondensed portion is again heated to a temperature preferably 5-45 °C higher than the second condensation temperature, in particular if hydrogen-selective polymer membranes are used in the second separator unit. The membrane separator units connected in series may have different or similar membrane materials (see below for greater detail). In the first membrane separator unit, hydrogen, nitrogen and lower alkanes, in particular ethane and/or methane, are removed from the first gas stream or from its uncondensed portion. Preferably there are removed from the uncondensed portion ofthe first gas stream 20-99 % ofthe hydrogen, nitrogen and ethane and/or methane present therein. The propane or other diluent and possibly propylene are incorporated into the condensing portion ofthe first gas stream. In the present embodiment, membrane separator units the membranes of which contain polyamide, polyimide or silicone rubber are used. Especially preferably the above-mentioned two-stage membrane separation comprises, combined with polymerization, the following steps: polymerizing the monomers in a gas-phase reactor in the presence of a catalyst; withdrawing a first gas stream containing at least some amount of unreacted monomer from the gas-phase reactor; directing the first gas stream, for example via a blower, to a heat exchanger, where it is divided into a second gas stream which contains uncondensed gases and a liquid stream; separating uncondensed compounds from the second gas stream in a membrane 11 separation unit. Preferably 20-95 % ofthe uncondensed compounds (including hydrogen) ofthe second gas stream are removed. As a result ofthe membrane separation there is thus obtained a third gas stream, which contains 20-95 % ofthe uncondensed compounds ofthe second gas stream, and a fourth gas stream. At least a portion ofthe third gas stream can be fed into the gas-phase reactor. The third gas stream can, for example, be condensed in part, whereupon the uncondensed portion is fed into the gas-phase reactor, whereas the condensed portion is fed at least or only in part into the gas-phase reactor. Preferably the condensed portion is combined with a liquid stream before this stream is fed into the gas-phase reactor. The liquid stream may comprise the liquid stream from the polymerization reactor preceding the gas-phase reactor, the liquid stream condensed from the first gas stream or the liquid stream condensed from the fourth gas stream. Preferably the liquid stream is heated before it is fed into the gas-phase reactor. Depending on its composition the condensed portion, and respectively the uncondensed portion, can also be directed somewhere else. Thus they may be directed to distillation in order to separate the hydrogen and/or the inerts. They may also be directed to distillation, to the fuel-gas network or to the cracking unit, or be recycled to the polymerization reactor (or another reactor). The uncondensed portion ofthe third gas stream is directed to membrane separation. The uncondensed portion ofthe fourth gas stream may be combined with it in order to produce a combined gas stream. According to a preferred embodiment, the pressure of this combined stream is raised by using a compressor, whereafter it is directed to a condenser in order to divide the stream into a fifth gas stream, which contains uncondensed compounds, and a condensed liquid stream. The fifth gas stream is directed to membrane separation in order to separate hydrogen from hydrocarbons, the hydrogen-containing stream being returned to the polymerization reactor preceding the gas-phase reactor. To summarize the options presented above, the process according to the present invention comprises, among others, the following combinations: 12 - condenser/compressor/membrane (+condenser/membrane); - compressor/condenser/membrane (+condenser/membrane); - condenser/membrane/compressor/condenser/membrane; - condenser/membrane/compressor/membrane/condenser; and - condenser/membrane/condenser/membrane. The operation ofthe membrane unit can be improved by using sweeping streams. As the sweeping gas it is possible to use nitrogen, polymerization diluents, and/or (pure) monomers or comonomers. When, for example, there is propane on one side of a hydrogen-selective membrane, the partial pressure of hydrogen drops and the amount of hydrogen separating in the unit increases. It is possible to combine the gas stream from the product separation ofthe fluidized-bed reactor with the first gas stream (i.e. the circulation gas stream ofthe reactor). This sixth gas stream is obtained from the separation tank where the reactor effluent is divided into a product stream, which contains the polymer, and a sixth gas stream, which contains gaseous compounds. The solution according to the present invention can be applied, for example, to grade change in a gas-phase reactor. In general, in grade change the procedure can be that a gas stream is separated from the exit stream of a gas-phase reactor at a pressure substantially lower than the pressure in the gas- phase reactor. The gas stream is directed to membrane separation. Preferably hydrogen is removed in this case mainly from the circulation gas ofthe gas-phase reactor or from the recovery stream ofthe product separator. The pressure ofthe feed gas ofthe membrane unit may, when necessary, be increased if this proves to be economically profitable. The need for pressure increase is dependent both on the selectivity and the permeability ofthe membrane film, and on the mutual economy ofthe pressure increase and the membrane surface area. In grade change the membrane system is advantageously combined with the circulation gas cooling stream; the membrane system is preferably placed at a point after the circulation- gas blower, before the circulation-gas condenser. Thereby the advantage is gained that in the membrane apparatus the lightest portion is removed from the gas stream, and thus the cooling ofthe remaining (most ofthe gas stream) stream is facilitated. At the same time the 13 advantage is gained that with the exiting gas the hydrogen content ofthe circulation gas is decreased and the grade change is sped up. Usually a part ofthe circulation gas is fed into the membrane system. Most preferably the gas may be removed from the recovery stream, in which case the product coming from the gas-phase reactor is directed, after a lowering ofthe pressure, to a product separator. From the separator the uncondensed gases are directed to a recovery compressor. In terms ofthe invention it is essential in this case that the product separator is operated under a pressure considerably lower than the gas-phase reactor. The lowering of the pressure results in that it is possible to separate from the product a maximal amount of light gases, which are directed to the recovery compressor. If the product separator operates under substantially the same pressure as the gas-phase reactor, the quantity of light components traveling along with the product polymer is large. In this case they must be separated from the product later. It is important for the entire polyolefin process that the product can be purified well, which is possible if the product separator is operated under as low a pressure as possible. Combined with the treatment ofthe circulation stream, grade change is carried out by cooling the circulation gas stream at a point after the blowers but before the membrane separation, in which case only the uncondensed portion ofthe circulation stream is directed to membrane separation. By combining the membrane separator with the cooling cycle it is possible to concentrate in the gas stream the components to be separated, and thus grade change can be sped up by using quite small a membrane surface area. The gases can be directed to the gas phase reactor either with the liquid or as a separate stream, or they may be directed for further treatment (e.g. distillation). As was pointed out above, membrane separation can also be used for the separation of hydrogen and other light components from the fluid between two reactors. The reaction product which comes from a liquid-phase reactor, or from a reactor in which supercritical conditions prevail, typically comprises a medium having a monomer and comonomer concentration above 60 %. The reaction medium possibly also contains an inert hydrocarbon (diluent), which usually vaporizes when the pressure is lowered. Hydrogen used for the control ofthe molar mass is released from the reaction product in the product separator. 14 Corresponding gaseous compounds are obtained from the product separators of gas-phase and liquid-phase reactors. The procedure according to the invention is that from the effluent, i.e. the exit stream, of the first reactor/reactor system there is separated by vaporization the portion which contains unreacted light hydrocarbons and hydrogen. From the exit sfream, or more precisely from its fluid portion, there is preferably separated approx. 0J-90 %, advantageously approx. 5-50 %, most preferably approx. 10-40 % ofthe volatile com- pounds present in it. All compounds except the polymerization or copolymerization product are deemed to be volatiles. From the reject it is possible to separate portions which contain light hydrocarbons and hydrogen several times between the reactors. According to one preferred embodiment, the hydrocarbon effluent from the first separator is directed to a second separator, wherein there is again separated from it a portion, wliich is directed to a membrane separator. The hydrocarbon effluent from the second separator can be fed into the reactor or it can be directed to a third separator. In each separator it is possible to separate from the hydrocarbons approx. 5-30 %, and thus in total approx. 15-90 % ofthe effluent ofthe first reactor has been treated in a membrane separator before being fed into the second reactor. The said fluid portion ofthe effluent contains the gaseous and liquid substances, such as unpolymerized monomer, hydrogen and possibly comonomer and diluent. For separating the gaseous fraction from the effluent ofthe first reactor or reactor system it is possible to use as the separator a cyclone or a so-called flash tank. If the preceding reactor is a bulk reactor, a portion ofthe volatile components present in the exit stream is vaporized either by lowering the pressure ofthe effluent or by raising its temperature. In this case the pressure is lowered in the flash tank preferably by approx. 5-90 %, most preferably by approx. 20-70 %. The pressure level in the separation tank, of course, varies according to the pressure in the preceding reactor. If the pressure is lowered significantly (e.g. by 10-60 %), the temperature can either be raised or even be lowered by approx- 1- 40 °C, preferably by approx. 5-20 °C. If the separator is preceded by a gas-phase reactor, the vapour fraction is separated from its effluent most preferably by condensing a portion ofthe hydrocarbons in a condenser. The temperature ofthe effluent is in this case lowered 15 by, for example, approx. 1-50 °C, preferably by approx. 10-40 °C. In the second separator and the separators subsequent thereto, a portion ofthe hydrocarbons is separated by vaporizing the liquid hydrocarbon effluent from the preceding separator fed into it. Typically the temperature is raised by approx. 1-20 °C and/or the pressure is lowered by approx. 5-20 %. The vaporous sfream obtained from each separator is directed to a membrane system, whereby it can be divided into a hydrogen-containing hydrocarbon stream and one or more hydrocarbon streams from which a substantial proportion ofthe hydrogen has been removed. The uncondensed gases are directed to the membrane unit preferably via a heater. Heating is carried out in particular in the case of polymer-based hydrogen-selective membranes in order that there should not occur condensing at the membrane unit. In order to make the separation more effective, the pressure ofthe gas stream directed to a membrane may be increased, for example in a compressor, before membrane separation. According to the invention, there may be a plurality of membrane separator modules in one and the same membrane separation unit. These modules may be arranged in parallel and/or in series. Each membrane separator unit has preferably 2-30 membrane separator modules. Hydrogen-selective and hydrocarbon-selective membrane units can be combined together according to the process needs. One preferred embodiment comprises an option which has a plurality of membrane separator units arranged in series, hydrocarbon-selective membrane separation being carried out in the first and hydrogen-selective membrane separation in the second. The operation of a membrane unit, in particular the separation of hydrogen, can be improved by means of sweeping streams. The sweeping gas used is, for example, nitrogen, polymerization diluent and/or monomer and/or comonomer. The retentate/permeate of some other membrane unit is also usable. When there is an inert hydrocarbon, such as propane, on one side of a hydrogen-selective membrane, the partial pressure of hydrogen decreases and the amount of hydrogen to be separated increases. The separated hydrogen can further be directed for reuse to other process units where it is 16 needed or, in an extreme case, be burned in the flare. When most ofthe hydrogen has been removed, the hydrocarbon-containing stream can be condensed at least in part and be returned to the reactor cycle. Membrane separator units are typically operated at a temperature of 20-90 °C and under a pressure of 0.5-5 MPa. However, these values do not represent any absolute limits to the process according to the invention. The membranes used in the separation of gases can be classified into inorganic membranes, carbon fiber membranes, polymer membranes and liquid membranes. All of these types can be used in the present invention. Ceramic membranes can be mentioned as an example of inorganic membranes. Polymer membranes constitute an especially advantageous option. The operation ofthe liquid membranes used in the invention is based on the different rates of dissolution, in the liquid membrane concerned, ofthe substances to be separated. The stability ofthe liquid in the membrane is a problem, as is its relatively low permeability, which leads to a large mass transfer surface area required and thereby a high price. In practice the polymer membranes used can be further divided into two categories according to whether the polymer is in a rubbery or a glassy state. By rubbery polymers are meant here amorphous polymers, which are used at temperatures above their softening or glass transition temperatures. The mechanical properties of rubbery polymers can be improved by combining polymers, such as styrene-butadiene-styrene (SBS) or silicone- polycarbonate. Especially well suited for the separation of hydrogen and light hydrocarbons is silicone rubber, i.e. poly(dimethyl-siloxane), PDMS; selectivity and permeability values of membranes made from it have been presented in the literature. The mechanical properties ofthe polymer can be improved by preparing from the material copolymers, e.g. siloxane-polyimide segment polymers. Polyacetylene membranes are usually classified as glassy materials, and of them it is possible to use, for example, poly(l-(trimethylsilyl)-l-propene), PTMSP, for which selectivity and permeability values have also been presented in the literature. This membrane can also be used for preparing substituted polyacetylene membranes, for example, PtB 17 poly(t-butylacetylene) and PFM poly(o-(trifluoromethyl)phenylacetylene). One further option is to use membranes made from polyvinyltrimethylsilane, PNTMS. Polyimides are glassy membranes which have good mechanical properties and heat resistance. Polyimides which can be used in the present invention include PMDA, BPDA, BTDA, 6FDA, pp'-ODA, mp'-ODA, APAP, BAHF, BAPHF and BATPHF (acronyms and properties: Tanaka et. al. Permeability and permselectivity of gases in flourinated and non- flourinated polyimides, Polymer, 33 (1992), 3). The polyimides mentioned above can be used for preparing numerous substituents which are, according to their properties, usable in the process defined by the present invention. Polyamides are also used commonly for the separation of light gases. For example, the following materials can be used in the present invention: IP/6F, TB.I/SO 2 , TB.I/6F, PII/SO 2 and IP/SO 2 (acronyms and properties: Morisato et. al, Gas separation properties of aromatic polyamides containing hexafluoroisopropylidene groups, Journal of Membrane Science, 97 (1994)). Other glassy membrane materials include polyphenyloxide, bisphenol A polysulfone (PSF), teframethylbisphenol A polysulfone (TMPSF), polycarbonates [bisphenol A polycarbonate (PC), teframethylbisphenol A polycarbonate (TMPC), hexafluorobisphenol A polycarbonate (HFPC) and tetramethylhexafluorobisphenol A polycarbonate (TMHF- PC)], various grades of polyethylene, cellulose acetate, and copolymers of polytetrafluoroethylene and Teflon AF 2400. The operation ofthe membrane system varies according to the material from which it is made. The operation of hydrocarbon-selective and respectively hydrogen-selective membrane units is depicted in Figures 1 and 2. Figure 1 depicts a system which uses silicone rubber and which functions so that the hydrocarbons in the feed ofthe membrane unit permeate more rapidly through the membrane 1 and hydrogen is concentrated in the reject stream (i.e. the portion which does not pass through the membrane). In this case the membrane is hydrocarbon-selective. When other materials are used (see Figure 2), hydrogen concentrates in the permeate stream (i.e. the portion which has passed through the membrane), i.e. the membrane 2 is 18 hydrogen-selective. In the system according to the invention, hydrogen and the lightest hydrocarbons are removed from the gas mixture emerging from a gas-phase reactor. The quantity of hydrocarbons is considerably higher than that of hydrogen. Thus it is advantageous to use a membrane system based on a hydrogen-selective membrane material or combinations which have both hydrogen-selective and hydrocarbon-selective membranes. The use of polyamide, polysulfone and polyimide membranes is especially advantageous, since their selectivity with respect to hydrogen is known to be good, and additionally their mechanical properties enable an advantageous membrane system to be made for this application. Membrane systems are usually made up of a plurality of membrane modules. At the manufacturing stage usually either an even sheet or hollow fibers are formed from the membrane material, and these are packed into different module structures to provide a large mass transfer surface area. A module is a small and independent building block ofthe membrane system, and it includes one or more membrane elements and the flow spaces surrounding them. The membrane modules generally the most suitable for the present application and the separation of gases are hollow-fiber and spiral modules. The sheet-like membranes used in spiral modules have in general a higher permeability than hollow-fiber membranes made from the same material, since it is difficult to make a fibrous structure as thin as a sheet- like structure. The advantage of hollow- fiber modules is their clearly lower manufacturing cost. Commercial membrane systems and their modules are described in, for example, Handbook of Separation Process Technology (Ronald W. Rousseau Ed., Wiley- Interscience, John Wiley & Sons, New York 1987, 866-869). Process configurations of preferred embodiments ofthe invention are illustrated in accompanying Figures 3-6. In the case according to Figure 3, there is withdrawn from a gas-phase reactor 11 a conventional circulation gas stream the amount of which is 1-50 % smaller than that of a 19 normal circulation gas stream. Alongside the circulation gas stream there is withdrawn from the reactor a second gas stream 101, which is directed to a condenser 12, where it is cooled in order to condense the condensable compounds. There is obtained a gas stream 102 which contains hydrogen and light hydrocarbons, and a stream 103 which contains condensed compounds and is returned to the reactor. The gas stream 102 is directed to a membrane 13. The first membrane used is a hydrogen-selective membrane, which is, for example, ofthe silicone rubber type. Hydrogen .and the lightest hydrocarbons concentrate in the retentate stream 105 ofthe membrane and are directed to a second membrane system 14. The permeate is directed to a two-stage compressor 15, where its pressure is raised from 280 kPa to 2850 kPa. Between the compressor stages the gas is cooled 16. After the raising ofthe pressure the gas sfream is cooled in a condenser 17, from which the uncondensed components are directed, together with the retentate ofthe first membrane, to a second compressor 18, where the pressure ofthe combined gas stream is raised from 2800 kPa to 3950 kPa. The product ofthe second compressor is cooled 19, and the condensed components are directed, together with the components condensed in the first and the third condensers 12, 17, in a liquid phase back to the gas-phase reactor 11. From the second condenser 19 the uncondensed gases are directed to the second membrane system 14, where a hydrogen-containing permeate and an almost hydrogen- free retentate are separated using a hydrogen-selective membrane. The permeate may, for example, be recycled to the preceding reactor. The retentate may be directed to the recovery of light components, as distillation column feed. The 1 -butene used as a diluent can be fed directly into the gas-phase reactor 11. This is what is also done in Example 1 below. Figure 3 shows an alternative point for feeding in 1- butene, namely the condenser 17. The said option promotes the condensation ofthe stream taken from the gas phase. In Example 2 presented below, the advantage provided by the process is exploited by increasing the production capacity by increasing the partial pressure of ethylene in the gas-phase reactor. The most significant advantage ofthe embodiment depicted in Figure 3 is that by the method energy is removed from the gas-phase reactor in a manner which enables the capacity to be increased without enlarging the reactor. 20 The process can be used for separating the hydrogen for recycling it to the loop reactor. The process can be used for preparing a sfream (No. 114) which is directed to the recovery system for light gases and which at the same time serves as a removal system for the inert components entering the gas-phase reactor and produced therein. In the case according to Figure 4 the invention has been implemented using slightly simpler means than in the preceding examples. From a gas-phase reactor 21 a stream is taken via a condenser 22 to a compressor 23, as before, but the condensed liquid is returned via a pump 24 as a separate stream to the gas- phase reactor. The uncondensed portion is compressed and fed into a membrane system 25. The membrane system is based on the use of a hydrocarbon-selective film (e.g. silicone rubber). Hydrogen is concentrated in the retentate, but the retentate also contains a large amount of hydrocarbons. It is directed to a loop reactor. The permeate which has passed through the membrane is directed either directly to the gas-phase reactor and is combined with the circulation gas stream ofthe gas-phase reactor or is directed together with it to the gas-phase reactor. It is possible that a portion or all ofthe permeate is directed via a condenser 26 to a pump 27, from where it is further pumped to the gas-phase reactor in accordance with what is stated above. Those gases which do not condense in the condenser can be directed, for example, to the distillation unit for light gases. Figure 5 depicts an embodiment relating in particular to grade change in a gas-phase reactor 31. A membrane system 34 has been placed in the circulation-gas sfream at a point after a blower 32 and a condenser 33. The placing ofthe membrane system in the manner according to Figure 5 in the cooling cycle ofthe gas-phase reactor 31, makes it possible to separate the accumulating components from the cycle. At the same time the efficiency of the membrane separation is increased, since the components to be separated are concentrated in the uncondensed portion obtained from the condenser. Preferably the membrane separator is operated at a temperature approx. 5-45 °C higher than the condensation temperature ofthe condenser. When necessary, a filter or the like may be 21 mounted between the condenser 33 and the membrane separator 34 in order to remove solids from the gas stream. The hydrocarbon product ofthe membrane separator may be recycled to the reactor 31, but the process depicted in Figure 5 may also be implemented as a two-stage process so that after the membrane separator 34 there is arranged a condenser and a second membrane separator, by means of which a portion ofthe off-gas ofthe first separator is condensed. In the case according to Figure 6, the polymer-containing product stream of a reactor 43 operating in supercritical conditions (in this case a loop reactor) is directed to a product separator 44, from which the gases are directed via filtration to a first membrane system 46. The separator membrane is a hydrocarbon-selective membrane, for example ofthe silicon rubber type. When necessary, a heater 45 may be arranged at a point before the membranes to ensure that the permeate ofthe membrane system is not condensed. Condensation is possible, since the heaviest hydrocarbons present in the feed are concentrated in the permeate sfream 206, and thus the boiling point ofthe permeate is higher than that ofthe feed. Hydrogen and the lightest hydrocarbons concentrate in the retentate stream 205, and they are directed to a second membrane system 47. In the second membrane system 47, hydrogen is separated from the light hydrocarbons and is recycled to the loop reactor or possibly to the polymerization reactor preceding it (sfream 223). The separating membrane is a hydrogen-selective material, typically a polymer. In the example, the permeate stream 215 from a third membrane 48 is directed to the permeate side 207 ofthe second membrane. Thereby the hydrogen concentration on the permeate side is diluted and the amount of hydrogen going to membrane 47 is increased. Respectively, the permeate ofthe third membrane system is used as a sweeping gas 216 for the retentate side of a fourth membrane system. The retentate 208 ofthe second membrane system is directed to a second product separator 50, but it may also be directed to the flare. The membrane units 46, 47, 48 and 49 may comprise a plurality of so-called modules. The modules may be placed either in series and/or in parallel. Typically there are 5-20 modules. The surface area required for separation depicts the total surface area ofthe modules required, and a suitable number of modules is selected according to the manufacturer and the membrane type. Usually there are a few spare membranes. For example, 50 membranes 22 are in use at a time and five in reserve so that one battery contains 5 membranes and there are three batteries. The polymer-containing product from the first product separator 44 is directed to the second product separator 50, from where the gases (sfream 210) are directed together with the permeate from the first membrane (stream 206) to the third membrane system 48, the hydrogen-containing permeate 215 of which is directed to the permeate sides ofthe second and the fourth membrane systems 47, 49. The permeate 215 contains only a little hydrogen, since it is obtained from a hydrocarbon-selective membrane in the permeate of which hydrocarbons are concentrated, while hydrogen is concentrated in the retentate. The peimeate can be used as a sweeping gas in other membrane modules (47 and 49). In that case it lowers the partial pressure of hydrogen on the permeate sides ofthe membranes 47 and 49 and thereby increases the flow of hydrogen through the said membranes. The third membrane system 48 has a hydrocarbon-selective membrane, and the fourth membrane system 49 has a hydrogen-selective membrane through which hydrogen permeates more easily than do light hydrocarbons. The retentate (stream 214) ofthe third membrane system is directed to the fourth membrane system (49), the permeate of which is a hydrogen- containing stream 218, which is directed back to the loop reactor (stream 223) or possibly to the polymerization reactor preceding it. The retentate from the fourth membrane 217 is directed to the third product separator 51. The polymer-containing stream 211 exiting from the second product separator 50 is further directed to the third product separator 51 , from which the gases are directed to a fifth membrane unit 52, from which there is obtained as the retentate of its hydrocarbon- selective membrane an off-gas 221 , which is directed, for example, for fuel gas. The permeate stream 222 contains hydrocarbons (and only a little hydrogen) and thus it can be directed, for example, to the recovery compressor of a gas-phase reactor 53 or be fed back to the loop reactor. The polymer-containing bottom sfream 220 ofthe third product separator 51 is fed into the gas-phase reactor 53. The following computational examples illustrate the invention: 23 Examples 1 and 2 Polyethylene is prepared using the process configuration according Figure 3. The membrane material used in the membrane separator 13 is a hydrocarbon-selective membrane ofthe silicone rubber type. The membrane ofthe membrane separator 14 is of a polymer material (polyimide). Tables 1 and 2 depict the compositions ofthe streams depicted in the figure. Table 1 ϊi o Example 1: Normal et! hylene concentration in the feed Stream 101 102 103 104 105 106 107 108 109 110 111 1 12 113 1 14 115 116 Pressure, kPa 2050.0 2050.0 2050.0 2050.0 205.0 660.0 640.0 2070.0 2050.0 2050.0 3000.0 3000.0 3000.0 3200.0 200.0 2900.0 Temp., °C 75.0 44.7 44.7 42.0 42.0 87.5 60.0 110.7 25.0 25.0 63.2 25.0 25.0 45.0 45.0 35.0 Tot., kg/h 103365 9441.9 93923 6214.2 3227.7 3227.7 3227.7 3227.7 57.9 3169.8 6272.1 460.4 5812.3 450.7 9.7 0 Component flow rate, kg/h H 2 12.4 9.1 3.4 8.3 0.710 0.710 0.710 0.710 0.288 0.422 8.6 5.8 2.8 1.0 4.7 0 Ethylene 8661.7 1982.0 6679.4 1498.3 483.7 483.7 483.7 483.7 23.6 460.1 1521.9 232.4 1289.2 228.9 3.6 0 t Ethane 335.6 61.6 274.0 46.6 15.0 15.0 15.0 15.0 0.540 14.5 47.1 5.5 41.6 5.5 0.035 0 Propane 77520 6615.9 70904 4230.2 2385.7 2385.7 2385.7 2385.7 31.4 2354.3 4261.6 206.2 4056.2 204.9 1.3 0 1 -Butene 2078.8 88.6 1990.2 49.3 39.2 39.2 39.2 39.2 0.219 39.0 49.6 1.1 48.5 1.1 3.45E-3 0 n-Butane 179.5 6.7 172.8 3.7 3.0 3.0 3.0 3.0 0.014 3.0 3.7 0.071 3.7 0.071 2.25E-4 0 i-Butane 14577 678.1 13899 377.7 300.4 300.4 300.4 300.4 1.9 298.5 379.5 9.3 370.4 9.2 0.029 0 13 O H - o-- o Table 2 vo ;o *- π o w Example 2: High ethyl lene concentration in t he feed Stream 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 Pressure, kPa 2880.0 2880.0 2880.0 2880.0 288.0 1070.0 1050.0 2880.0 2880.0 2880.0 4500.0 4450.0 4450.0 4450.0 200.3 2900.0 Temp., °C 75.0 30.0 30.0 42.0 42.0 106.9 60.0 114.4 25.0 25.0 67.6 25.0 25.0 45.0 45.0 35.0 Tot., kg/h 100067 19575 80503 13817 5758.6 5758.6 5758.6 5758.6 3176.0 3301.4 16993 12970 4022.3 12513 456.6 720.0 Component flow rate, kg/h H 2 25.2 21.8 3.3 20.1 1.8 1.8 1.8 1.8 1.7 0.065 21.8 21.0 0.757 1.9 19.1 0 Ethylene 35068 13189 21869 9902.3 3287.2 3287.2 3287.2 3287.2 2274.0 1014.3 12176 10028 2148.1 9626.4 401.9 0 Ethane 6013.8 1863.5 4149.5 1399.1 464.4 464.4 464.4 464.4 289.7 174.8 1688.8 1332.8 356.0 1310.6 22.2 0 Propane 22.0 3.3 18.8 2.1 1.2 1.2 1.2 1.2 0.465 0.736 2.6 1.7 0.898 1.6 0.028 0 1 -Butene 37172 2729.0 34456 1512.9 1216.1 1216.1 1216.1 1216.1 422.3 1512.1 1935.2 975.1 959.7 966.9 8.2 720.0 n-Butane 145.3 9.3 136.0 5.2 4.2 4.2 4.2 4.2 0.809 3.3 6.0 2.9 3.1 2.8 0.024 0 i-Butane 21611 1758.8 19870 975.1 783.7 783.7 783.7 783.7 187.0 596.1 1162.0 608.2 553.7 603.1 5.1 0 o H ϊ VO-i VO ©o 26 As can be seen from the table, the capacity ofthe reactor can be increased by the process according to the invention. At the same time, the examples show that the 1 -butene used as the comonomer and the n-butane and iso-butane used as diluents can be fed into the condensers shown in the examples. Thereby the operation ofthe system is promoted by condensing as large as possible a portion ofthe stream (No. 101) withdrawn from the gas- phase reactor. Example 3 The efficiency of a membrane separator fitted in the cooling cycle of a gas-phase reactor is examined below by comparing it in two different cases to grade change without condensation and with partial condensation. The object was in both cases to halve the amount of hydrogen. In the first case, run 2, case 4 of WO application publication 94/25495 was repeated. The conditions were as follows: Reactor pressure 2.17 bar temperature 98 °C feed temperature 29.4 °C gas amount lO t gas flow 25600 mVh Membrane unit 1750 m 2 feed 3000 kg/h permeate pressure 300 kPa 27 Table 3 Initial situation Partial condensation/29.4 °C Without Duration 132 min condensation Duration 300 min Composition mol-% Uncondensed Final Final situation, gas, mol-% situation, mol-% mol-% H 2 17.7 21.2 8.7 8.8 CH 4 1 1.2 1 1 C 2 H 4 40.5 45.1 40.5 40.5 C 2 H 6 8.5 9.2 8.5 8.5 1-C 4 H 8 0 0 0 0 i-C 5 H 12 3.7 1.2 4.7 3.7 1-C6H 12 0.6 0J 0.8 0.6 n-CβHπ 10 1.0 13.7 9.9 n-C 8 H 18 0.5 0.0 0.6 0.4 N 2 17.6 21.0 21.5 26.7 100 100 100 100 Density, kg/m 3 23.4 27.9 25.1 Total 154 298 permeate, kg Ethylene loss in 86 kg 163 kg permeate, kg 28 Table 4 Initial situation Partial condensation 36.4 °C Without Duration 144 min condensation Duration 272 min Composition mol-% Uncondensed Final Final situation, gas, mol-% situation, mol-% mol-% H 2 12.5 15.3 6.2 6.2 CH 4 0 0 0 0 C 2 H 4 29.1 32.7 29.1 29.1 C 2 H 6 16.4 9.2 8.5 8.5 1-C 4 H 8 0 0 0 0 i-C 5 H 12 13.3 4.6 14.3 13.1 l-C f rHπ 0 0 0 0 n-C 6 H 14 0 0 0 0 n-C 8 H 18 4.3 0.8 4.8 4.2 N 2 24.4 30.0 29.2 31.0 100 100 100 100 Density, kg/m 3 29.0 30.8 29.6 Total permeate, 186 334 kg Ethylene loss in 99 kg 159 kg permeate, kg In the second case, Example 9 of WO application publication 94/28032 was repeated. The conditions were as follows: Reactor pressure 24 bar temperature 103 °C feed temperature 36.4 °C gas amount lO t gas flow 20700 m 3 /h 29 Membrane unit 2500 m 2 feed 3000 kg/h permeate pressure 300 kPa As is evident from the foregoing examples, by the use of membrane separators it is possible to speed up considerably grade change in a gas-phase reactor equipped with a cooling cycle and to reduce ethylene loss. The examples also show that feeding the gas stream directly, without partial condensation, into the membrane unit is not advantageous, since in that case the stream fed into the unit has the same composition as has the stream exiting from the gas-phase reactor. When partial condensation is used, the sfream fed into the membrane system is considerably lighter, i.e. a stream from which the heaviest compounds have been removed. With this procedure, for example, the concentration of hydrogen in the membrane unit feed is higher, and this makes a more rapid removal of hydrogen possible. However, by each method hydrogen can be removed well as compared with removal carried out without a membrane. In the last-mentioned case the halving of hydrogen without condensation would require the removal of one half of the amount ofthe gas-phase reactor + circulation. If, for example, the inventory is 10 1, this would require 5 t of purge. When membranes are used, there will not be more than a few hundred kg. The gas stream obtained from partial condensation has a composition different from that of the circulation gas stream. A large proportion of C 5 and heavier hydrocarbons have been re- moved from it. This is significant when a membrane unit is used, but without membrane separation, partial condensation does not improve the situation substantially. This is due to the fact that in the gas coming from partial condensation the hydrogen to ethylene concentration ratio remains mainly unchanged even if a purge stream is withdrawn from the gas. Thus almost the same amount of ethylene is removed with hydrogen from the system as in a case in which partial condensation is not used. 30 Example 5 Polyethylene is produced by using the process configuration according to Figure 6. The membrane material used in membrane separator systems 46, 48 and 52 is a hydrocarbon- selective membrane ofthe silicone rubber type. The membranes in the membrane separators 47 and 49 are polyimides. Table 5 shows the compositions ofthe streams shown in the figure. 31 Table 5 Stream 201 202 203 204 205 206 207 208 209 Pressure, kPa 6000.0 3200.0 3200.0 3000.0 3000.0 2600.0 1000.0 3000.0 400.0 Temperature, °C 8 5 .0 62.5 62.5 80.0 80.0 80.0 80.0 80.0 80.0 Total kg/h 10982 2239.7 8742.3 2239.7 2068.4 171.3 642.6 2035.0 675.9 Component mass flows H2 21.5 17.3 4.2 17.3 16.4 0.851 0.194 0.220 16.4 CH4 0.035 0.020 0.015 0.020 0.019 1.11E-03 7.07E-04 0.017 2.17E-03 Ethylene 515.0 198.3 316.7 198.3 184.4 13.9 23.3 180.7 27.0 Ethane 6.6 2.2 4.4 2.2 2.1 0.145 0.227 2.0 0.243 Propane 9744.7 1941.1 7803.6 1941.1 1790.9 150.2 589.4 1777.7 602.6 1 -Butene 463.5 56.3 407.2 56.3 52.0 4.3 20.4 51.8 20.6 Isobutene 38.2 4.7 33.5 4.7 4.3 0.361 1.7 4.3 1.7 c-2-Butene 19.9 2.1 17.8 2.1 1.9 0.162 0.800 1.9 0.807 t-2-Butene 23.6 2.6 21.0 2.6 2.4 0.199 0.969 2.4 0.977 n-Butane 38.3 4.2 34.1 4.2 3.9 0.325 1.6 3.9 1.6 i-Butane 79.5 10.3 69.2 10.3 9.5 0.789 3.6 9.4 3.7 n-Hexane 9.6 0.302 9.3 0.302 0.277 0.024 0.217 0.276 0.218 n-Octane 21.4 0.187 21.2 0.187 0.172 0.015 0.159 0.172 0.159 N2 0.225 0.163 0.063 0.163 0.157 5.58E-03 1.26E-03 0.130 0.028 Stream 210 211 212 213 214 215 216 217 218 Pressure, kPa 2800.0 2800.0 2600.0 2600.0 2600.0 1000.0 1000.0 2600.0 400.0 Temperature, °C 69.0 69.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0 Total kg h 3263.5 5478.8 3263.5 3434.8 2149.6 1285.2 642.6 2130.7 661.5 Component mass flows H2 3.7 0.479 3.7 4.6 4.2 0.388 0.194 0.192 4.2 CH4 0.011 3.95E-03 0.011 0.012 0.011 1.41E-03 7.07E-04 9.92E-03 1.44E-03 Ethylene 184.7 132.0 184.7 198.6 151.9 46.7 23.3 149.3 25.9 Ethane 2.3 2.1 2.3 2.5 2.0 0.454 0.227 2.0 0.241 Propane 2921.2 4882.4 2921.2 3071.3 1892.6 1178.8 589.4 1880.6 601.4 1 -Butene 105.4 301.9 105.4 109.7 68.9 40.8 20.4 68.7 20.6 Isobutene 8.7 24.8 8.7 9.1 5.7 3.4 1.7 5.7 1.7 c-2-Butene 4.1 13.7 4.1 4.3 2.7 1.6 0.800 2.7 0.809 t-2-Butene 5.0 16.0 5.0 5.2 3.3 1.9 0.969 3.3 0.979 n-Butane 8.2 25.9 8.2 8.5 5.3 3.2 1.6 5.3 1.6 i-Butane 18.7 50.5 18.7 19.5 12.3 7.3 3.6 12.2 3.7 n-Hexane 0.784 8.5 0.784 0.809 0.375 0.434 0.217 0.374 0.218 n-Octane 0.577 20.6 0.577 0.592 0.274 0.318 0.159 0.274 0.160 N2 0.053 0.010 0.053 0.058 0.056 2.52E-03 1.26E-03 0.047 9.74E-03 Stream 219 220 221 222 223 Pressure, kPa 2400.0 2400.0 2200.0 400.0 400.0 Temperature, °C 58.0 58.0 80.0 80.0 50.0 Total kg h 1076.9 8567.7 171.4 905.5 2243.0 Component mass flows H2 0.633 0.258 0.446 0.187 20.8 CH4 0.013 0.018 7.96E-03 5.06E-03 8.67E-03 Ethylene 113.6 348.5 39.2 74.5 127.4 Ethane 1.2 4.9 0.561 0.687 1.2 Propane 924.5 7616.2 125.7 798.8 2002.8 1 -Butane 25.7 396.7 3.8 21.8 63.1 Isobutene 2.1 32.6 0.317 1.8 5.2 c-2-Butene 0.950 17.3 0.141 0.809 2.4 t-2-Butene 1.2 20.5 0.173 0.993 2.9 n-Butane 1.9 33.2 0.283 1.6 4.8 i-Butane 4.7 67.4 0.697 4.0 11.3 n-Hexane 0.124 9.0 3.71E-03 0.120 0.556 n-Octane 0.071 21.0 2.14E-03 0.069 0.388 N2 0.112 0.175 0.094 0.018 0.056 32 A comparison ofthe compositions of streams 201 and 220 in the table shows that with the help ofthe invention it is possible to remove 99 % ofthe hydrogen of stream 201 and, of the accumulating lower hydrocarbons, approx. one-half of the methane and over 30 % of the ethane.

Claims: 33 Claims 1. A process for the production of polyolefins in a reactor system which comprises at least one gas-phase reactor (11; 21; 53) wherein at least one monomer is polymerized, in which case according to the method the monomer to be polymerized and possibly a polymer-containing fluid are fed into the gas-phase reactor (11; 21; 53), characterized in that prior to being fed into the gas-phase reactor (11 ; 21 ; 53) the fluid is subjected to a phase change in order to separate uncondensed compounds from liquid and possibly solid compounds, the gas made up of uncondensed compounds is recovered and hydrogen is separated from the gas by membrane separation. 2.The process according to Claim 1, characterized in that the fluid fed into the gas-phase reactor is the circulation gas ofthe gas-phase reactor. 3. The process according to Claim 2, characterized in that the monomers are polymerized in the gas-phase reactor ( 11 ; 21 ) in the presence of a catalyst, a first gas stream containing at least some amount of unreacted monomer is withdrawn from the gas-phase reactor (11; 21), the first gas stream is cooled to a first condensation temperature in order to condense it at least in part and to separate the liquid from the gas, - uncondensed compounds are separated from the first gas sfream or its uncondensed portion in a membrane separation unit (13, 14; 25), and at least a portion ofthe liquid is returned to the gas-phase reactor (11; 21). 4. The process according to Claim 3, characterized in that at least 20 % ofthe condensable compounds in the first gas sfream are condensed. 5. The process according to Claim 4, characterized in that 40-95 % ofthe condensable compounds in the first gas stream are condensed. 34 6. The process according to any of Claims 2-5, characterized in that light gases are removed from the first gas stream before the condensation. 7. The process according to any of Claims 2-5, characterized in that the uncondensed gas stream obtained from the condensation is directed to membrane separation. 8. The process according to Claim 7, characterized in that before the membrane separation the uncondensed gas stream is heated preferably to a temperature 5-45 °C higher than the condensation temperature. 9. The process according to any of Claims 2-8, characterized in that the solids are separated from the gas stream to be directed to membrane separation. 10. The process according to any of Claims 2-9, characterized in that the uncondensed portion ofthe first gas stream is treated in two successively arranged membrane separator units (13, 14). 11. The process according to Claim 10, characterized in that the gas stream obtained from the first membrane separator unit (13), from which stream uncondensed compounds present in it have been separated, is cooled to a second condensation temperature in order to separate condensable compounds from the gas, and the uncondensed portion is directed to the second membrane separation unit (14), where uncondensed compounds are separated from it, or to distillation. 12. The process according to Claim 11, characterized in that the uncondensed portion is heated to a temperature 5-45 °C higher than the second condensation temperature. 13. The process according to any of Claims 10-12, characterized in that the membrane separator units have different membrane materials. 14. The process according to Claim 13, characterized in that the uncondensed portion of the first gas stream is treated first by using a hydrocarbon-selective membrane and then by 35 using a hydrogen-selective membrane. 15. The process according to any of Claims 2-14, characterized in that from the first gas stream or from its uncondensed portion hydrogen, nitrogen and lower alkanes, in particular ethane and/or methane are removed. 16. The process according to any of Claims 2-15, characterized in that from the first gas stream or from its uncondensed portion 20-99 % ofthe hydrogen, nitrogen and ethene and/or methane present in it are removed. 17. The process according to Claim 15 or 16, characterized in that the propane or other diluent is incorporated mainly into the condensing portion ofthe first gas stream. 18. The process according to any of Claims 2-17, characterized in that such membrane units are used the membranes of which contain polyamide, polyimide or silicone rubber. 19. The process according to any of Claims 2-18, characterized in that the gas obtained from a membrane separation unit is at least in part returned to the gas-phase reactor. 20. The process according to any of Claims 2-19, characterized in that the pressure ofthe stream being directed to a membrane separator unit is raised before the membrane separation. 21. The process according to Claim 1 or 2, wherein - a first gas stream containing at least some amount of unreacted monomer is withdrawn from the gas-phase reactor (11; 21), the first gas stream is cooled to a first condensation temperature in order to condense it at least partly and to separate the liquid from the gas, and at least a portion ofthe liquid is returned to the gas-phase reactor (11 ; 21), characterized in that the gas stream withdrawn from the gas-phase reactor is directed to a heat exchanger (11; 21), wherein it is divided into a second gas stream, which contains uncondensed gases, and a liquid stream and 36 uncondensed compounds are separated from the second gas sfream in a membrane separation unit (13, 14; 25). 22. The process according to Claim 21, characterized in that 20-95 % ofthe uncondensed compounds present in the second gas stream are separated therefrom and a third gas stream is formed of them. 23. The process according to Claim 21 or 22, characterized in that the second gas stream is divided by membrane separation into a third gas stream, which contains 20-95 % ofthe uncondensed compounds ofthe second gas stream, and a fourth gas sfream. 24. The process according to any of Claims 21-23, characterized in that the pressure of the first gas sfream is raised before the stream is directed to the heat exchanger. 25. The process according to any of Claims 21-24, characterized in that the pressure of the second gas stream is raised before the membrane separation. 26. The process according to any of Claims 21-25, characterized in that the pressure of the third gas stream is raised after the membrane separation, before the stream is directed to further treatment. 27. The process according to any of Claims 22-26, characterized in that at least a portion ofthe third gas stream is fed into the gas-phase reactor. 28. The process according to any of Claims 22-27, characterized in that the third gas sfream is condensed in part. 29. The process according to any of Claims 22-28, characterized in that at least a part of the condensing portion ofthe third gas stream is fed into the gas-phase reactor. 30. The process according to Claim 29, characterized in that the condensing portion is combined with a liquid sfream before the liquid stream is fed into the gas-phase reactor. 37 31. The process according to Claim 30, characterized in that the liquid stream comprises the liquid stream from the polymerization reactor preceding the gas-phase reactor, the liquid stream condensed from the first gas stream or the liquid stream condensed from the fourth gas sfream. 32. The process according to Claim 31, characterized in that the condensing portion is directed to distillation for the separation of light hydrocarbons and hydrogen. 33. The process according to Claim 22-29, characterized in that the uncondensed portion ofthe third gas stream is directed to membrane separation. 34. The process according to any of Claims 22-33, characterized in that the uncondensed portion ofthe fourth gas stream is combined with the third gas stream in order to produce a combined gas stream. 35. The process according to Claim 34, characterized in that the pressure ofthe combined gas stream is raised using a compressor, whereafter it is directed to a condenser in order to divide the sfream into a fifth gas stream which contains uncondensed compounds and a condensed liquid stream. 36. The process according to Claim 35, characterized in that the fifth gas stream is directed to membrane separation in order to separate hydrogen from hydrocarbons, whereupon the hydrogen-containing stream is returned to the polymerization reactor preceding the gas-phase reactor. 37. The process according to any of Claims 22-29, characterized in that the uncondensed portion ofthe third gas stream is directed to distillation, the fuel gas network, a cracker or another reactor. 38. The process according to any of Claims 2-37, characterized in that the liquid sfream is heated before it is fed into the gas-phase reactor. 39. The process according to any of Claims 2-38, characterized in that a sixth gas stream, 38 which is obtained from the product separation ofthe fluidized-bed reactor, is combined with the first gas stream. 40. The process according to Claim 39, characterized in that the product of the polymerization reactor is directed to a separator, wherein the product is divided into a product stream which contains polymer and a sixth gas stream which contains gaseous compounds, and the sixth gas sfream is combined with the first gas sfream withdrawn from the reactor. 41. The process according to Claim 1, characterized in that the polyolefins are produced in a reactor system made up of at least two reactor units (43, 53), at least one ofthe reactors being a gas-phase reactor (53) to which the effluent ofthe preceding reactor unit (43) is directed, hydrogen being removed from the effluent before the stream is fed into the gas- phase reactor (53). 42. The process according to Claim 41, characterized in that the pressure ofthe effluent is lowered, whereby possible liquid is partly vaporized to gas and whereby the gas present in the effluent or the liquid vaporized to gas contains light hydrocarbons and hydrogen, and hydrogen is separated from the gas or the vaporized portion by membrane separation (46, 47; 48, 49; 52). 43. The process according to Claim 42, characterized in that the pressure is lowered by at least 1 bar, preferably by at least 10 % ofthe pressure ofthe preceding reactor. 44. The process according to Claim 43, characterized in that a) a portion which containing unreacted light hydrocarbons and hydrogen is vaporized out from the effluent, and b) hydrogen is separated from the vaporized portion by membrane separation (46, 47; 48, 49; 52). 45. The process according to Claim 44, characterized in that process steps a and b are 39 repeated at least once. 46. The process according to Claim 44 or 45, characterized in that OJ-50 % by weight, preferably 1-40 % by weight, ofthe fluid portion ofthe effluent is vaporized. 47. The process according to Claim 46, characterized in that the fluid portion ofthe effluent contains unpolymerized monomer, hydrogen, and possibly comonomer and diluent. 48. The process according to any of Claims 44-47, characterized in that a portion ofthe fluid portion ofthe polymerization reactor (43) effluent is vaporized in at least one vaporizer (44), the vaporized portion is directed to membrane separation (46, 47), and the stream from membrane separation, from which hydrogen has been removed, is combined with the unvaporized portion ofthe effluent, obtained from the vaporizer (44). 49. The process according to Claim 48, wherein at least two vaporizers (44, 50) arranged in series are used, characterized in that the uncondensed portion from the preceding vaporizer (44), from which hydrogen has been removed, is combined with the unvaporized portion ofthe effluent ofthe subsequent vaporizer (50). 50. The process according to Claim 48 or 49, characterized in that the sfream obtained from the membrane separation is cooled before it is combined with the unvaporized portion ofthe effluent. 51. The process according to any of Claims 41-50, characterized in that the pressure of the vaporized portion is raised in a compressor before the membrane separation. 52. The process according to any of Claims 41-51, characterized in that the reactor unit (53) from which the polyolefin product is recovered is a gas-phase reactor. 53. The process according to any of Claims 41-52, characterized in that the reactor (43) 40 the effluent of which is directed to the gas-phase reactor is a liquid-phase reactor or a reactor in which there prevail supercritical conditions. 54. The process according to any of Claims 41-53, characterized in that the separated hydrogen is recycled to polyolefin production. 55. The process according to Claim 1 for changing the composition ofthe gas in the gas- phase reactor in order to change the grade ofthe polyolefin being prepared, characterized in that - a gas stream is withdrawn from the gas-phase reactor, a hydrogen-containing stream is separated from the gas stream by means of a membrane separator, there being fed to the membrane separation a gas stream the composition of which corresponds to the composition in the gas-phase reactor as regards the small-molecular components, and - the gas stream thus treated is returned to the gas-phase reactor. 56. The process according to Claim 55, characterized in that the gas sfream directed to the membrane separation contains at least hydrogen and lower alkanes, and possibly nitrogen and unreacted monomers. 57. The process according to Claim 56, characterized in that the composition ofthe gas stream introduced to the membrane separation corresponds to the composition of that in the gas-phase reactor, at least as regards the mutual concentrations of hydrogen and lower alkanes. 58. The process according to any of Claims 55-57, characterized in that the effluent ofthe gas-phase reactor is divided into a polymer product and a reject containing gaseous compounds, the reject is separated from the polymer product at a pressure substantially lower than that in the gas-phase reactor, and a hydrogen-containing stream is separated form the reject by using a membrane separator system. 41 59. The process according to Claim 58, characterized in that the pressure ofthe effluent stream ofthe gas-phase reactor is lowered using a pressure-lowering valve, and the reject is separated from the polymer product in a separator. 60. The process according to any of Claims 55-59, characterized in that from the top of the gas-phase reactor a gas stream is withdrawn, from which gas stream hydrogen is separated using a membrane separator system. 61. The process according to any of Claims 55-60, characterized in that the solids are separated before the membrane separation. 62. The process according to any of Claims 55-61, characterized in that the pressure is raised before the membrane separation. 63. The process according to any of Claims 59-62, characterized in that the reject obtained from the separator, containing gaseous compounds, is combined with the gas stream withdrawn from the top ofthe reactor. 64. The process according to Claim 63, characterized in that hydrogen is separated from the reject before the reject is combined with the gas sfream withdrawn from the top ofthe reactor. 65. The process according to any ofthe preceding claims, characterized in that the efficiency ofthe membrane separation is increased by using sweeping gases. 66. The process according to Claim 65, characterized in that the sweeping gas used is nitrogen or polymerization diluent, monomer or comonomer.

Inventors: AITTAMAA JUHANI, JAERVELIN HARRI, NYMAN TIMO, AITTAMAA, JUHANI, JAERVELIN, HARRI, NYMAN, TIMO
Applicants: AITTAMAA JUHANI, BOREALIS AS, JAERVELIN HARRI, NYMAN TIMO, AITTAMAA, JUHANI, BOREALIS A/S, JAERVELIN, HARRI, NYMAN, TIMO
Application year: 1999
Main classification: C08F2
IPC codes: B01D53, B01J19, C01B3, C08F2, C08F10, C08F210