PROCESS FOR PRODUCING POLYOLEFINS

Abstract: A process for producing polyolefins having a bimodal molecular weight distribution, the process comprising producing a first polyolefin fraction in the presence of a catalyst in a first loop reactor, and producing a second polyolefin fraction in the presence of the catalyst in a second loop reactor which is serially connected to and downstream of the first loop reactor, the first and second polyolefin fractions being blended in the second loop reactor to form a polyolefin having a bimodal molecular weight distribution, at least the first loop reactor containing a diluent under supercritical conditions which is circulated around the loop of the reactor, and wherein at least the first loop reactor is provided with a fluff concentrating device communicating with the loop and in which polyolefin fluff of the first fraction is concentrated in the supercritical diluent, and polyolefin fluff of the first polyolefin fraction is transferred together with an amount of supercritical diluent from the fluff concentrating device of the first loop reactor into the second loop reactor.

Description: PROCESS FOR PRODUCING POLYOLEFINS The present invention relates to a process for the production of polyolefms, in particular polyethylene or polypropylene. In particular, the present invention relates TO the production of a polyethylene having a multimodal molecular weight distribution, for example a bimodal molecular weight distribution. It is known ro produce polyethylene in liqu α phase loop reactors in which etnylene monomer, ana optional! an alpha-oleflnic comonomer typically having from 3 to 1C carbon atoms, are circulated under pressure around a loop reactor by a circulation pump. The ethylene monomer and comonomer when present are present in a liquid diluent, such as an alkane, for example isobutane. Hydrogen may also be added to the reactor. A catalyst is also fed to the loop reactor. The catalyst for producing polyethylene may typically comprise a chromium-based catalyst, a Ziegler-Natta catalyst or a metallocene catalyst. The reactants m tne diluent ana tne catalyst are circulated at an elevated polymerisation temperature around the loop reactor thereby producing polyethylene homopolymer or copolymer depending on whether or not a comonomer is present. Either periodically or continuously, part of the reaction mixture, including the polyethylene product suspended as slurry particles in the diluent, together with unreacteα ethylene and comonomer, is removed from the loop reactor. The reaction mixture when removed from the loop reactor may be processed to remove the polyethylene product from the diluent and the unreacted reactants, with the diluent ana unreacted reactants typically being recycled oack into tne loop reactor. CONFIRMATION COFY Alternatively, the reaction mixture may be fed to a second loop reactor serially connected to the first loop reactor where a second polyethylene fraction may oe produced. Typically, when two reactors in series are employed in this manner, the resultant polyethylene product, which comprises a first polyethylene fraction produced in the first reactor and a second polyethylene fraction produced in the second reactor, has a bimodal molecular weight distribution. It is known in the art to operate a loop reactor under conditions of high temperature and pressure such that tne diluent is present under supercritical conditions. Thus tne diluent is at a pressure greater than the critical pressure P c and at a temperature greater than the critical temperature T c . Under tnese conditions, there is no thermodynamic transition between the gas phase and the liquid phase and the homogeneous supercritical fluid has the properties of a dense gas and a low density liquid. For example, O-A-92/12181 discloses a method for nomo- or copolymerising etnene m the presence of a Ciegler-Natta catalyst m a loop reactor under supercritical conditions. The diluent wnich is m the supercritical state is propane. It is disclosed that the use of a propane phase at a supercritical state provides some advantages, namely that the hydrogen content of the reactor may be adjusted within a wide range and no pressure-shock effects occur which would otherwise tend to damage the circulation pump for the diluent, as a result of the high compressibility of tne supercritical fluid. This specification makes clear that propane should be used as a diluent rather than for example isobutane, because the use of propane enables more polymer types to be prepared in the reactor and also tne solubility of polyethylene is lower in propane tnan m isobutane. Tne specification a so discloses that since the boiling point of propane is low, tne hydrocarbons may readily be separated from the polymer particles after the polymerisation. The specification discloses that two loop reactors in series may be employed for making ethylene polymers and/or copolymers having a wide or bimodal molecular weight distribution. EP-B-0517868 also discloses a multi-stage process for producing polyethylene which employs supercritical conditions. It is disclosed that the inert hydrocarbon medium which is employed under supercritical conditions is propane. It is also disclosed that the polyethylene ma\ nave a bimodal molecular wei t distribution . WO-A-96/18662 discloses a process for preparing polyethylene which may have a multimodal molecular weight distribution by using supercritical conditions. Again, it is disclosed to oe advantageous to use propane as the inert hydrocarbon medium under supercritical conditions. O-A-96/34895 discloses a process for manufacturing PΞ polymers again using propane as a reaction medium unαer supercritical conditions. The LLDPE polymers are manufactured using a metallocene catalyst. It is disclosed that the excellent polymer morphology of the products produced with the metallocene catalysts together with the low polymer solubility into tne diluent and relatively low diluent density, especially in tne supercritical conditions, result m very good settling properties of the polymer and thus efficient reactor operation, v .e. diluent flow into tne reactor can oe minimised) . However, there is no disclosure of any specific reactor structure indicating how the operation of the reactor may oe made more efficient. O-A-97/13790 discloses a process for making prop\ lene homo- or copolymers in a loop reactor under supercritical conditions. It is disclosed that a polypropylene having a bimodal molecular weight distribution may be employed using two reactors in series. While the above-specified patent specifications relating to supercritical conditions for the diluent provide advantages of higher hydrogen solubility m tne diluent and easier nydrogen flashing if the reaction has been continued ir the second reactor, combined with reduced swelling of the polymer in tne supercritical diluent and the absence of pressure shocks as a result of tne high compressioility of the supercritical diluent, nevertheless, the use of propane as a diluent tenαs to require the use of comonomers having low carbon numbers, for example butene which militates against the use of high carbon comonomers, for example hexene which would assist in the production of polymers having better properties than if butene were used. Moreover, the use of propane as a diluent tends to require a relatively nigh pressure to be employed above tne critical pressure P for propane. Moreover, the supercritical processes referred tc nereinabove αc net permit a particularly hiα- comonomer concentration tc be employed in tne reactor, particularly for comonomer witn high carbon number, e.g. nexene . In the manufacture of polyoiefins having a bimodal molecular weight distrioution under supercritical conditions employed m serially connected reactors, tne above-identifled specifications suffer from tne disadvantage tnat tnere is no specific disclosure as to how the reaction medium is transferred from tne first reactor to the second reactor. US-A-4754007 discloses a process for copolymerismg ethylene to form LDPΞ copolymers m wr.icn liquid propane s useα as a diluent in a slurry process. It is disclosed that the use of propane diluent provides more economical production of copolymers having more desirable physical properties than slurry processes using isobutane, nexane or other liquid diluents. There is no disclosure of the diluent being under supercritical conditions. EP-A-0649860 discloses a process for the copolymerisation of ethylene in two liquid full loop reactors in series in which the average molecular weight is regulated. A comonomer is introduced into the first reactor and high and low average molecular weight polymers are produced respectively in the first and second reactors. One or more settling legs is provided for the first reactor in order to transfer the high average molecular weight polymer from the first reactor to the second reactor. The reaction is carried out in a diluent, for example isobutane, in a slurry process. This process suffers from the disadvantage that although the use of settling legs for concentrating the fluff between the first and second reactors allows preferential polymerisation of comonomer in the high molecular weight fraction nevertheless the comonomer amounts in the first and second reactors are rattier close because tne reactors do not operate substantially independently. It would be desirable to achieve lower Ce/C^ ratios in the second reactor, thereby yielding improved properties for the resultant polyolefin resin. US-A-4740550 discloses a multi-stage, continuous polymerisation process for the preparation of propylene/ethylene impact copolymers comprising the use of a re-circulating pipe-loop reactor for homopolymerismg propylene, a cyclone separator for removing fines, a gas-pnase fiuidised bed reactor for additional propylene homopolymeπsation, ano a gas-pnase fiuidised bed reactor for propyiene/etnylene copolymerisation. The essence of tne disclosure is tnat since the first reactor is ODerated under slurry conditions and the second reactor is operated under gas phase conditions, a hydrocyclone separator is employed to separate the fine particles from the coarse fluff particles tnat are fed to the gas phase reactor. The slurry phase reactor operates with a liquid diluent and the fine particles are recycled back to the first slurry phase reactor. This process requires the reactors to operate in the liquid and gas phases, and the use of a hydrocyclone which is inconvenient. EP-A-0905153 discloses a process for producing high densιt\ polyethylene in the presence of a Ziegler-Natta catalyst systerr in two liquid full loop reactors series. The reactors are both operated with a liquid diluent, for example isobutane. In a first reactor there is substantially homopolymerisation, optionally with a minor degree of copolymerisation, and hydrogen is introduced into the first reactor to achieve the required homopolymerisation. Copolymerisation is carried out in the second reactor. In order to reduce or prevent hydrogen from entering the second reactor, a hyorogenation catalyst is introduced into tne reactants downstream of the first reactor . This process requires the use of an additional hydrogenaticr catalyst . The present invention aims at least partially to overcome these problems of tne prior art. Accordingly, the present invention provides a process for producing polyoiefms having a bimodal molecular weignt distribution, the process comprising producing a first polyolef fraction in tne presence of a catalyst m a first loop reactor, and producing a second polyolefm fraction the presence of tne catalyst m a second loop reactor wnich is serially connected to and downstream of tne first loop reactor, the first and seconα polyolef fractions being blended in the second loop reactor to form a polyolefm having a bimodal molecular weight distribution, at least the first loop reactor containing a diluent under supercritical conditions which is circulated around the loop of the reactor, and wherein at least the first loop reactor is provided with a fluff concentrating device communicating with the loop and in which polyolefm fluff of the first fraction is concentrated in the supercritical diluent, and polyolef fluff of the first polyolefm fraction is transferred together with an amount of supercritical diluent from the fluff concentrating device of the first loop reactor into the second loop reactor. The polyolefm may comprise polyethylene or polypropylene. Wnen producing polyethylene, the diluent typically comprises at least one Ci to C 4 alkane. When producing polypropylene, the diluent typically comprises propylene. Preferably there is provided a process wherein the fluff concentrating device is selected from one or a combination of a downwardly depending settling leg, a cyclone or hydrocyclone and a centrifuge. More preferably the fluff concentrating device includes a valve for permitting an amount of the polyolef fluff together with an amount of the supercritical diluent periodically to be removed from the fluff concentrating device. In one preferred aspect there is provided a process wherein the diluent m the second loop reactor is operated under supercritical conditions and the second loop reactor is provided with a respective fluff concentrating device. More preferably there is provided a process further compris g recycling back into the first and second loop reactors any diluent removed from the fluff concentrating device of the second loop reactor on removal of polyolefm fluff therefrom. Ir. another preferred aspect there is provided a process wnerem tne diluent in the second loop reactor is operated under liquid conditions and the second loop reactor is provided with a respective fluff concentrating device. Tne present invention further comprises the use, in a pair of serially connected loop reactors for polymerising an o ef in t e presence of a catalyst to produce a polyolefm navmg a bimodal molecular weight distribution and including a first polyolefm fraction produced m a first loop reactor and a second polyolefm fraction produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in at least the first loop reactor for increasing the settling of polyolefm fluff m a respective fluff concentrating device of at least the first loop. Tr.e present invention yet furtner provides tne use, in a pair or serially connected loop reactors for polymerising ethylene in the presence of a catalyst to produce polyethylene havmg a bimodal molecular weignt distribution and including a first polyethylene fraction comprising polyethylene copolymer produced m a first loop reactor and a second polyethylene fraction comprising polyethylene homopolymer produced in a second loop reactor downstream of tne first loop reactor, of a diluent under supercritical conditions in at least the first loop reactor for reducing the amount of comonomer in solution m the diluent transferred from the first loop reactor to the second loop reactor . The present invention also provides the use in a pair of serially connected loop reactors for polymerising ethylene in the presence of a catalyst to produce polyethylene having a bimodal molecular weight distribution and including a first polyethylene fraction comprising polyethylene homopolymer produced m a first loop reactor and a second polyethylene fraction comprising polyethylene copolymer produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in at least the first loop reactor for reducing the amount of hydrogen in solution in the diluent transferred from the first loop reactor to the second loop reactor . The present invention still further provides the use, in a pair of serially connected loop reactors for polymerising an olefin in the presence of a catalyst to produce a polyolef having a bimodal molecular weight distribution and including a first polyolefm fraction produced in a first loop reactor and a second polyolefin fraction produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in the first and second loop reactors for reducing the amount of diluent to be recycled back into the loop reactors following removal of a mixture of polyolefm fluff and diluent from a fluff concentrating device of the second loop reactor. Through this process tne second reactor is maαe more independent from the first one. An embodiment of the present invention will be described, by wa\ of example only, with reference to the accompanying drawings, in which : - Figure 1 is a schematic diagram of a pair of serially connected loop reactors for performing a method for producing polyethylene accordance with an embodiment of tne present invention; and Figure 2 is an enlarged schematic diagram of a settling leg and valve assembly of each loop reactor of the apparatus of Figure 1 showing settling of the polyethylene therein. Referring to Figure 1, there is shown an apparatus designated generally as 2, for the production of a polyolefm, m particular polyethylene. The apparatus 2 comprises a first loop reactor 4 and a second loop reactor 6 seriall y connected thereto b\ a conduit 8. The first loop reactor 4 memoes an inlet port 1. from which ethylene monomer and where appropriate comonomer, sucn as hexene, and hyαrogen, ana diluent are fed into the first loop reactor 4. A port 11 is provided for introducing a catalyst, for example a cnromium-based catalyst, into the reactor 4. The chromium-based catalyst may be employeα together with a cocatalyst. Alternative catalysts are Ziegler-Natta catalysts together with a cocatalyst, metallocene catalysts together wιt. n a cocatalyst, ana late transition metal catalysts together wit" a cocatalyst. Ail tnese catalysts max be ore-polymerised to a level of up to 10 gram of polyethylene per gram of the catalyst. The diluent may comprise an alkane, such as a Cχ-C 4 alkane or a mixture thereof or an olefin monomer, sucn as propylene for tne production of polypropylene. Separate inlet ports may be proviαeα for each constituent. The first loop reactor 4 is provided with a pump (not shown) for circulating diluent containing the reactants and the catalyst around tne first loop reactor 4. Tne first loop reactor 4 is also proviαeα at a bottom portion 12 thereof with an outlet port, designateα generally as 14, whicr is provideα with a fluff concentrating device which comprises a downwardly depending settling leg 16 and a valve 18 at the bottom of the settling leg 16. The output siαe of tne valve 18 connects with the conduit 8. The output side of the conduit 8 comprises an inlet port 20 for the second loop reactor 6. A plurality of additional inlets, designated generally as 22, are provided through which, if desired, additional monomer and diluent, and optionally comonomer and/or hydrogen may be fed for introduction into the second loop reactor 6. When supercritical diluent is used in the first reactor 4 in accordance with the invention, the settling in the reactor 4 can be so high that an additional feed of diluent is necessary to push the fluff from the outlet 18 of the concentrating device 14 of the first reactor 4 to the second reactor 6. This additional feed is the total or a part of tne diluent and monomer reed to tne second loop reactor. Thus some of these additional components may be fed via an inlet 23 in the conduit 8 for assisting in transforming the polyethylene fluff through the conduit 8 into the second loop reactor 6. The second loop reactor 6, like the first loop reactor 4, is provided with a pump (not shown) for circulating the diluent containing the reactants and the catalyst around the second loop reactor 6. The second loop reactor 6 is, like the first loop reactor 4, proviαed with an outlet port designated generally as 24, which is proviαed with a fluff concentrating device wnich comprises a downwardly depending settling leg 26 extending from a oottom portion 28 of the second loop reactor 6 and a valve 30 at the bottom of the settling leg 26. The settling legs 16,26 act to concentrate the polyethylene fluff before it leaves the reactor 4,6. The settling leg 16,26 may be vertical or inclined to the vertical, for example at an angle less than 8 π ° , more preferably less than 60°, to the vertical. The settling legs 16,26 may be additionally or alternatively be provided at an external edge of an elbow or bent part of the reactors 4,6, for example in a tangential orientation so as to form a tangential fluff removal pipe for continuous or discontinuous (e.g. periodic) evacuation of polyethylene fluff from one or several lines of the reactor 4,6. Tne fluff concentrating αevice may alternatively comprise a nydrocyclone or a centrifuge. In other embodiments tne fluff concentrating device may comprise a combination of two or more of a settling leg, a hydrocyclone and a centrifuge. For example, a centrifuge may be located downstream of a settling leg/valve assembly. In a particularly preferred embodiment, the outlet of the settling leg of the first loop reactor communicates with a centrifuge. The centrifuge is fed additionally with part or all cf the diluent feed for tne second reactor. Tne centrifuge outputs recycled diluent to the first reactor and fluff and diluent feed to the second reactor. In the first loop reactor 4, in one particular mooe of operation for producing polyethylene copolymer, ethylene, comonomer, typically hexene, hydrogen and the chromium-baseα catalyst are introduced with the diluent, which typically comprises at least one Cι-C alkane, preferaoly a mixture of Ci-C, alkanes witn propane as a major component, into the first loop reactor 4 v±a tne inlet ports 10 and II as described above. The diluent typically comprises propylene when producing polypropylene. The ethylene and the hexene comonomer and also hydrogen when present are dissolved in the diluent. The diluent is under supercritical conditions, i . e . at a pressure above the critical pressure P c and at a temperature above the critical temperature T c . Typically, tne first loop reactor - is operated under supercritical conαitions at a pressure of from 37 to 100 bars and at a temperature of from 70 to 14C°C, more preferably from 80 to 110°C for polyethylene or from 6C°C to 100°C for polypropylene with a Ziegler-Natta catalyst and from 50 to 140°C with a metallocene catalyst . As the ethylene copolymerisation reaction proceeds, polyethylene m the form of fluff is formed the first loop reactor 4 and progressively builds up in the settling leg 16 at the bottom portion 12 of the first loop reactor . The polyethylene fluff settles in the settling leg 16 under the action of gravity. Periodically, for example around every 30 seconds, the valve 18 is opened to permit the polyethylene fluff (comprising a copolymeric first polyethylene fraction of the eventual polyethylene resir to be drawn off from the first loop reactor 4 through tne conαuit 6 and fed into the second loop reactor c via the inlet port 20. An amount of the diluent, together with ethylene and comonomer and hydrogen dissolved therein, is also inevitably transferred on opening of the valve 18 from the first loop reactor 4 tc the second loop reactor 6, as a result of incomplete packing of the polyethylene fluff and because the removed volume fror the settling leg tends to include at the top thereof a layer of diluent with unsettled fluff. In the second loop reactor 6, a homopoiymenc second polyethylene fraction is produced. Tne second polyethylene fraction, together with the first polyethylene fraction, progressively settles in the settling leg 26 at the bottom portion 28 of tne second loop reactor 6 under the action of gravity. The valve 30 is periodically opened to permit the polyethylene resm, comprising the blend of the first and second polyethylene fractions, to be removed from the apparatus. Removal of the polyethylene resm from the settling leg 26 inevitably removes additionally some diluent from the second loop reactor 6, the diluent having reactants dissolved therein. The αischarged mixture is sent to a separator 32 where the polyethylene resm fluff is separated from the diluent, >;mcr. is tnen recycled along a line 34. Tne polyethylene resm is recovered via an outlet 36 of the separator 32. In the illustrated embodiment, both the first and second reactors are operated under supercritical conditions. However, the second reactor may be operated under liquid or supercritical conditions. Also, in the illustrated emoodiment, the first supercritical reactor is employed to produce the high molecular weight polyolef fraction and the second supercritical or subcritical reactor is employed to produce the low molecular weight polyolefm fraction. In alternative arrangements, the first ana second reactors ma\ instead proαuce respectively the low and hign molecular weight fractions. In accordance with the invention, the use of supercritical conditions for the diluent enables significantly more efficient polyethylene fluff removal from each of the first and second loop reactors 4,6 which is operated under supercritical conditions. To increase the process efficiency it is desired to remove a maximum amount of polyethylene fluff with a minimum amount of diluent, together with any reactants αissoived therein, from the respective settling leg 16,26 of the first and second loop reactors 4,6. In accordance with the invention, it has been found that the use of supercritical conditions for the diluent in conjunction with a settling leg of the loop reactor, wherein the settling of tne polyethylene fluff occurs under the action of gravity, is significantly increased as compared to the use of a liquid diluent not unαer supercritical conditions. For the settling of polyethylene fluff, tne settling speed is determined by tne difference between tne downwardly directed gravitational force acting or the polyethylene fluff particles suspended tne c_ment and tne upwarαiv directed viscous force of the diluent acting on the polyethylene fluff as it falls downwardly under the action of gravity. The gravitational force is in turn dependent on the difference density between tne polyethylene resm and tne diluent. The use of a supercritical diluent significantly reduces the density of the diluent as compared to the liquid phase. Typically, for isobutane tne density of the isobutane under supercritical conditions is around one third to one half of the density of the same diluent unoer liquid conditions. Moreover, the viscosity of the diluent unαer supercritical conditions is significantly reduced as compared to the viscosity of tne diluent when under subcritical conditions. For example, for isobutane, tne diluent unoer supercritical conditions is around one tenth of the diluent viscosity when unαer subcritical conditions. Thus the viscous force which tends to resist settling of the polyethylene fluff is significantly less under supercritical conditions than unαer subcritical conditions. Moreover, the density difference between the polyethylene and the diluent is significantly increased unαer supercritical conditions as compared to under subcritica, conditions. For example, wher the diluent is isooutane, typically tne density difference oetweεr tne polyethylene and tne diluent under supercritical conditions is around 0.65g/cc whereas the difference between the polyethylene and the diluert under subcritical conditions is only around 0.35g/cc. The increased density difference under supercritical conditions tends to increase the gravitational force acting on tne polyethylene fluff, which in turn tends to increase the settling speeα. Typically, the settling speed for isobutane as a diluent is arounα 20 times faster under supercritical conditions tnan under subcritical conditions. This greatly increases the buiiα- up of fluff the settling legs cr the loop reactors, leading to a mgner rate of recovery of polyethylene resm from tne apparatus. As a result of the increase settling speeα, satisfactory settling of the polyethylene fluff is achieved even with relatively fine fluff particles which otherwise would not settle rapidly enough to be recoverable efficiently. In addition, the supercritical diluent tends to increase the degree of packing of the fluff particles building up m the settling legs 16,26. Typically, the maximum packing or settling under supercritical conditions is around twice that achievable under subcritical conditions. In view of the increased settling in the settling leg 16,26, for any given volume of material removed from tne respective settling leg 16,26 on opening of the respective valve 18,30, that given volume tends to include a smaller amount of diluent under supercritical conditions as compared to under subcritical conditions. Figure 2 shows a diagrammatic representation of a settling leg and a valve assembly of a loop reactor, which may be the settling leg 16,26 of both of the first and second loop reactors 4,6. The settling leg 40 includes a tuoular wall 42 defining an upwardly extending chamber 44 above tne valve 46. The polyethylene fluff 48 settles in the bottommost part 50 of the chamber 44 and progressively the volume of tne settled polyethylene fluff grows upwardly, thereby displacing the diluent 52. For any given volume of material downwardly released through the valve 46 from the settling leg 40, under subcritical conditions not m accordance with the invention, the given volume typιcall\ comprises around 60% by weignt of polyethylene fluff 48 and 40- by weight of the diluent 52, which may have the ethylene monomer and comonomer dissolved therein. In contrast, under supercritical conditions, as a result of the faster settling ana increased packing of the polyethylene fluff 48 at the bottom of tne settling leg, the same volume typically may comprise around 80- by weight of polyethylene and around 20% by weight of the supercritical diluent 52. Accordingly, in order to remove a given amount of polyethylene fluff from either of the first and second loop reactors 4,6, significantly less diluent is additionally removed with the polyethylene resm under supercritical conditions as compared to under subcritical conditions . Accordingly, with regard to the settling leg 16 of tne first loop reactor 4, for transferring a given amount of polyethylene fluff from the first loop reactor 4 to the seconα loop reactor 6, a significantly reduced volume is transferred under supercritical conditions as compared to under subcπtica. conditions. The lower density of the diluent under supercritical conditions as compared to under subcritical conditions also means that for any given volume of diluent transferred from the loop reactor 4 to the second loop reactor 6, resulting from transferring the polyethylene fluff from the first loop reactor 4 to the second loop reactor 6, a considerably reduced weight of the diluent is accordingly transferred from the first loop reactor 4 to the second loop reactor 6 for any given amount of polyetnylene fluff transferreα. Accordingly, this results m a smaller amount in weight of the supercritical fluid being required to transfer a given amount of polyetnylene from tne first loop reactor 4 to tne second loop reactor 6. Since there is a reduced amount of diluent transferred from the first loop reactor 4 to the second loop reactor 6 for any given amount of polyethylene transferred, if comonomer and/or hydrogen is present in the first loop reactor 4, then less comonomer and/or less hydrogen is transferred from the first loop reactor 4 to the second loop reactor 6 in solution in the diluent under supercritical conditions than under subcritical conditions. This thus largely enhances the independence of the two reactors . When the first and second loop reactors 4,6 are employed to produce a polyethylene resm having a bimooal molecular weight distribution, a lo density fraction is produced in the first loop reactor 4 as a result of high comonomer incorporation to produce a lov. density high molecular weight first polyethylene resm fraction, wnereas m the seconα loop reactor 6 a high density low molecular weight polyethylene resm homopolymer fraction is produced with no comonomer oemg deliberately introduced into the second loop reactor 6. Since for any given amount of the first polyethylene fraction transferred from tne first loop reactor 4 to the second loop reactor 6 less di.uer: is transferred, accordingly less comonomer, wnicr is dissolved therein, is also transferreα, thereoy reading to improveα homopolymerisation m the second reactor as a result of reduced comonomer incorporation. Thus the improved settling of the polyethylene fluff in tne first loop reactor 4 tends to provide reduced comonomer transfer from the first xoop reactor 4 to the second loop reactor 6. This m turn enables a larαer difference m density to be achieveα between the low αensity copolymer fraction produces in the first loop reactor 4 and tne relatively high αensitx homopolymer fraction produced in the seconα loop reactor 6, with the of the final composite resm oe g selected as a desired value. This yields a composite polyethylene resm whicr has improveα mechanical properties. Furtnermore, since less diluent is transferreα from the first loon reactor 4 to the second loop reactor 6 for any given amount of polyetnylene transferreα, less impurities tend to be transferred from tne first loop reactor ~ to the second loop reactor 6. This improves the homogeneity of tne polyethylene resir by maximising the activity of tne catalyst grams in the second reactor. Accordingly, in accordance with the invention the use of supercritical conditions for the diluent a loop reactor havmg a settling leg tends to increase the capacity of the reactor because for the same compressor capacity circulating t n e reactants around the loop, significantly more polyethylene car be removed from the reactor as a result of improved settling of polyethylene fluff in the settling leg. This enhanced settling provides increased reactor throughput. The reactor can ce provided with a reduced number of settling legs, reducing tre capital cost of the reactor. In addition, since for any given amount of polyethylene flu.tr removed from the settling leg a reαuced amount of diluent is aisc removed unαer supercritical conditions as compared to suocπtιca_ conditions, there is a reduced amount of diluent having monomer and possibly comonomer, incorporated therein for recycling bac to the loop reactor. The use of supercritical conditions for a diluent in a loop reactor with a settling leg also increases the recycling economy of the reactor system. Furthermore, the reduced amount or diluent also reduces f ~ = amount cf impurities and poisons rr tne reactors whicn enhances catalyst activity. The improved settling speed and pacKing of the fluff particles m the settling leg as a result cf tne use of supercπtιca_ conditions for the diluent tenos to enable settlement tc ce achieved with smaller fluff particles. Such small fluff particles can be used dιrect_\ for rotational moulding. There is a lower swelling tendency for the polyethylene fluff. Tre fluff can be processed at higner temperatures, and has improveα degassing. Furthermore, the achievement of reliaole ana efficient settling cf smaller fmff particles tends to permit tne use of correspondingly smaller catalyst particles, wi corresponding higher catalyst activity. This increases tne efficiency of the polymerisation process. Tne reduced viscosity of tne supercritical diluent as compareα tc the subcritical diluent tends to permit a higher diffusion rate of comonomer through tne diluent, m turn leading to a higher comonomer incorporation in the copolymer. Tne comonomer incorporation is more homogeneous. The higher comonomer incorporation permits a lower comonomer/monomer ratio to be required in the reactor, wnich again reduces the trend tc swelling (i.e. tne tendency for tne polyolef to dissolve tne diluent . In manufacturing the oimodai polyethylene resm in the two loop reactors m series, m order tc improve the mechanical properties of the resm it is desirable to nave a large density difference between the two fractions produced m the two reactors, and this is achieved by copolymerismg ethylene and the comonomer m the first loop reactor, ana nomopclymeπsmg ethylene, the suostantial aosence of comonomer, m the second loop reactor. Improveα settling cf tne fluff m the first loon reactor reduces the amount by weignt of the supercritical fluid needed to transfer a given amount of polyethylene from the first loop reactor to the second loop reactor, thus m turn reducing the comonomer transfer to the second loop reactor from the first loop reactor, the comonomer oemg dissolved the diluent. Alternatively, the reduced comonomer transfer allows an increase in tne traction of the low αensity portion for any prescrioeα final αensity of tne polyetnylene fluff. Thus the use of a supercritical diluent in tne first reactor increases tne decoupling or independence of the polymerisation reaction occurring m the loop reactors. This broadens the range of poivmer oroαucts wnicn can ce produced bv the two seπalL connected reactors . For example, for two loop reactors in series operating not in accordance with the invention under subcritical conditions for the liquid diluent comprising isobutane, the weight ratio in the first loop reactor of the isobutane to ethylene feed is typically around 1.5 and the isobutane has dissolved therein around lwt% ethylene and 5wt% hexene as comonomer giving a hexene/ethylene weight ratio of 5. Typically, for a commercial reactor around 75kg comonomer are removed from the first loop reactor for every ton of polyethylene removed from the first loop reactor. In the second loop reactcr under subcritical conditions, the weight ratio of diluent to ethylene feed is around 1. The ethylene concentration in the diluent is around 2wt%. No additional comonomer is introduced into the second loop reactor but the hexene comonomer is transferred into the second loop reactor together with the diluent from the first loop reactor. Typically, the hexene concentration is around 0.5wt% giving a hexene/ethylene feed weight ratio of around 0.25. This amount cf hexene in the second loop reactor means that some hexene is incorporated into tne polyetnylene fraction produced in tne second loop reactcr, thereby reducing the homopolymeric nature of the second polymeric fraction. In contrast, when. the same apparatus comprising serially connected first and second loop reactors is operated under supercritical conditions, as a result of the reduced density cf the supercritical diluent as compared to the liquid diluent, the density of the supercritical diluent being typically around one third to one half that cf tne liquid diluent, the weight ratio of diluent to ethylene feed in the first loop reactor is typically around 0.3 to 0.5. The ethylene and hexene concentrations in the first loop reactcr and accordingly tne hexene/ethylene feed weight ratio, are the same or lower as for the reactor when operated with liquid diluent as αescπbed above. However, as a result of the cumulative effect cf the improveα settling of the polyethylene fluff in tne first loop reactor and the reduceα weight of any given volume of the diluent under supercritical conditions as a result of the reduced αensity thereof, typically only around 15kg of comonomer are transferred per ton of polyethylene from the first loop reactor to the second loop reactor. Tnis in turn typically reduces the amount of comonomer transferred from the first loop reactor to a seconα loop reactor by a factor of 5. Accordingly, m the second loop reactor, the hexene comonomer is present in an amount of only around about C.lwt% and the ethylene monomer is present in the same amount of around 2wt%, yielding a hexene/ethylene weight ratio in the second loop reactor of only around 0.05%. This is consiαerably reduceα when the diluent is under supercritical conditions as compareα to when the diluent is under liquid conditions. Ideally, tne nexene/etnylene weignt ratio is zero for production cf a pure homopolymer. The reduceα amount cf comonomer tne second roop reactcr permits ar increase in the αensity difference between tne two polyetnylene fractions . In the second loop reactor wnen operateα under supercrιtιca_ conditions, improved settling of tne polyethylene fluff reduces the amount of diluent which is required tc be removed together with the polyethylene resm from the seconα loop reactor. The improveα settling of the polyolefm f ^ uff means that less diluent is removeα from the reactors for any given amount or polyolef fluff recovered wnich m turn reduces the amount of material reσuireα to be recycled pack tc tne first and seconα loop reactors. Typically, the amount of diluent recycled oac< to the loop reactors under supercritical conditions is around one half that as compared to under liquid conditions. The improved settling of the polyethylene fluff enables very fine fluff to be produced and recovered in the first or second loop reactor. Tnis in turn broadens the range of catalysts which ma_ be employee in the polymerisation process. Furthermore, tne improvement settling of the polyethylene fluff increases tne productivity of the first and second loop reactors pecause of tne need of shorter residence times of the reactants in the reactor . Furthermore, since the supercritical fluid has a lower αensιt_ than tne supercritical liquid, the polyethylene fluff has a lower solubility in the supercritical fluid as compared to m tne liquid diluent. This lower solubility enables higher polymerisation temperatures to be employed before considerable solubiiismg of the polyethylene m the diluent occurs. Accordingly , it is possible with the use of supercritical diluent to polymerise at a higher polymerisation temperature than for a liquid di-uuent. This turn provides greater catalyst activit . These two onenomena result m a cumulative increase u me productivity of tne production of polyethylene using supercritical fluid as compared tc a liquid diluent. Furthermore, oy wor mg at a high pressure with the supercrιtrcc_ fluid, this allows tne hydrogen ana etnylene concentrations in the supercritical diluent to oe increased, tnereby increasing tne polymerisation rate for tne production cf polyethylene. For example, unαer supercritical conditions, m the first loco reactor tne nyαrogen may oe present around 2.vol% and m tne second loop reactor the hyorogen may oe present in around 0.1 vol. , yielding a hydrogen volume ratio for the two reactors or 20. In contrast, unαer suocπtical conditions with a lιαu_α diluent in the first loop reactor, which is designed for use under subcritical conditions, the hydrogen content is required to be lower, typically arounα 1 vole, as a result of the lower αesign pressure of the reactcr and in the second loop reactor the hyαrogen may still be present m around 0.1 voll, giving a lower hyαrogen volume ratio for the two reactors of 10. Reduced hyαrogen transfer from the first loop reactor to the second loop reactor enhances the viscosity difference between the two polyethylene fractions. The reduced hydrogen transfer allows an increase in the molecular weight difference between the two fractions or permits a reduceα amount of hydrogenation catalyst, fcr consuming the transferred nydrogen, to be required. In preferred aspects, the critical temperature and the critical pressure of the supercritical fluid can be varied by mixing lower boiling point hydrocarbons, e.g. methane or ethane with propane or isobutane. Propane and ethane have progressively lower critical temperatures, but nrgner critical pressures than isooutane. The supercπtιca_ fluid may comprise a mixture or sucn hydrocarbons so tnat fcr some given apparatus, wnich is ccnfigureα to operate at particular temperatures and pressure, supercriticality of the diluent can oe achieved by altering the composition of tne supercritical fluid. Thus the pressure of the supercritical fluid can oe altered for optimising tne operation of the reactor pump of the loop reactors. If ethane is mixeα witn tne isobutane or with propane, the critical point is reacneα at a lower temperature, reducing any temperature problems caused by melting of the polyethylene polymer and allows the apparatus tc operate unαer lower operating temperatures. In addition, both hydrogen ana etnylene when present in the reactor m tne supercritical fluid affect tne critical temperature and critical pressure cf the supercrιtιca_ f_uια. Thus the composition of the dmuent, as well as tne amounts of nyαrogen and ethylene present m the reactors, can be varied in order to ensure tnat supercπticality is achieved with a given apparatus operating under particular pressure and temperature ranges. The critical point can be fine-tuned by addition of low ooilmg nydrocarbons, including metnane and ethane. The diluent may comprise a mixture of propane, ethane and olefin monomer. For polypropylene the use of a supercritical diluent with settling legs m the first reactor allows to transfer a lot of fluff with a minimum amount of liquid. Accordingly, the second reactor may oe fed with additional cocatalyst and/or electron αonor, whicn can men be different from tne cocatalyst and/or electron donor fed in the first reactor. Tnis allows running of the two reactors quite independently, what is not possible if tne fluff is simply transferred througn, for example, a horizontal pipe, without a concentrating device m accordance with the invention . For making low density polyethylene containing hiαn amounts of incorporated nexene comonomer, the hexene/ethylene ratio m tne reactor has tc oe high. Since the critical temperature Tc of the hexene is high, the nexene concentration will be limited, but can be increased if ethane is added to a propane diluent for example. By mixing hyαrocaroons, the critical point of the diluent can be fine-tuned, and a nigher hexene concentration can be reached without leaving tne supercritical state. B\ lowering the critical point, a higher comonomer concentration can be achieved, with a nigher catalyst productivit . This causes larger polyethylene particle sizes in the reactor to be formed, giving improveα settling. This in turn provides an improved reactcr throughput, with increased economy m recycling. In the supercritical fluid, t~e diffusion rate of tne reactants is significantly higher than in the liquid, typically up to about 200 times faster. This in turn can increase the degree of polymerisation . The present invention will now be illustrated in greater detaι_ with reference to tne following non-limitmg Examples. EXAMPLE 1 and COMPARATIVE EXAMPLE 1 In Comparative Example 1 a bimodal resm was produceα m two reactors (abbreviated Rx in Table 1 and 2 m series under the conditions speciflea m Table 1. Fcr Example 1, tne corresponding conditions were calculated for producing such a bimodal res in tne same reactor system. It will be seen that for both Example 1 and Comparative Example 1, there was a relatively high amount of comonomer, m tne form of hexene, present in the first reactor as compared to that present in the second reactor whereby a low density fraction was produced in the first reactor ana a nigh density fraction was produceα in tne second reactor. In the manufacture of the polyethylene resm or Example 1, rt may oe seen that a reduced hexene concentration m present in the second reactor as compared tc Comparative Example 1, this yielding a greater density differential between tne first and second fractions than for Comparative Example 1. It may be seen that Example I also exhibits increase settling efficiency of the polyethylene fluff as a result of the use of tne supercritical diluent. Moreover, the feeα rate of diluent into the second reactor m less for Example 1 tnan for Comparative Example 1. In addition, the catalyst productivity for Example 1 is significantly nigher than that for Comparative Example i. TABLE 1 The creep properties of the resins was determined using a full notch creep test (FNCT) which is used mainly in Europe by res producers fcr development purposes . Depending on the selected test conditions, the rupture time can be strongly reduced, sucn that information can be obtained c:r highly resistant materials in a short time. The test equipment is simple, being the usual set-up for a tensile creep test. In the test, a sample is immersed in water or a specified srrfactant solution at 80°C or 95°C. A constant load is appiieα ; :c the sample (a small bar - lOxlOxlOOmm) and the sample r. ≡ notched on four sides perpendicularly tc the stress diretricn. The time to rupture is recorded as a function of the appi reα stress. The test method has been standardised in Japan (JIΞ K 6774; . With reference tc the present invention, the conditic :r.s applied were: a 10x10x100mm bar sample notched on four sides with a razor blade to a depth of 1.6mm was immersed in a solution of 2% by weight Arkopal® N-100 (Hoechst commercial product) at 95°C (±0.5°C) and a constant stress load of 4. OMPa applied based on the initial remaining cross section at the place where the notches were introduced . A full notch creep test (FNCT) on Resin 1 gave times before fracture of between 200 and 300 hours. In contrast for the Resm 2, the time before fracture was largely improved and reached 600 tc 1000 hours. This shows that the use of supercritical diluent in accordance with the invention gives resins with improved mechanical properties. EXAMPLE 2 and COMPARATIVE EXAMPLE 2 In these Examples, a bimodal polyethylene resin was produced using the conditions shown m Table 2. These Examples are different from Example 1 and Comparative Example 1 in that the relatively low density polyetnylene fraction is produced in the second reactor, and the relatively higher density polyethylene fraction is produced in the first reactor. In order that the low density fraction having a hign. degree of comonomer incorporation, must be produced in the second reactor, hydrogen needed to be consumed either between the two reactors or in the second reactcr by the use of a costly hydrogenating catalyst. The use of a supercritical diluent in the first reactor allowed a very- significant economy of such a nydrogenation catalyst because very little hydrogen transferred from the first reactor to the second reactor. For metallocene catalysts the process using supercritical diluent permits the production of higher molecular weight in the second reactcr even without the use of special hvdroαenatinα catalysts. Furthermore, if the consumption cf the hydrogen is performed in the second reactor, a second significant amount of hexene unαergoes hydrogenation, which increases the production costs. Accordingly, the minimisation of the amount of hydrogen is thus important for obtaining improved process economics. It may be seen for Example 2 that the settling efficiency is higher, and the diluent feed rate is lower, for the first reactor of Example 2 as compared to that for the first reactor of Comparative Example 2. TABLE 2

Claims: CLAIMS : 1. A process for producing polyolefms having a bimodal molecular weight distribution, the process comprising producing a first polyolefm fraction in the presence of a catalyst n a first loop reactor, and producing a seconα polyolefm fraction m the presence of the catalyst m a seconα loop reactor which is serially connected to and downstream of the first loop reactor, the first and second polyolefm fractions being blended in the second loop reactor to form a polyolefm having a bimodal molecular weight distribution, at least tne first loop reactor containing a αiluent under supercritical conditions wnich is circulated around the loop of the reactor, and wherein at least the first loop reactor is provided with a fluff concentrating device communicating with the loop and in which polyolefm fluff of the first fraction is concentrated m the supercritical diluent, and polyolefm fluff of the first polyolefm fraction is transferred together with an amount of supercritical diluent from the fluff concentrating device of the first loop reactor into the second loop reactor. 2. A process according to claim i wherein the fluff concentrating device is selected from one or a combination of a downwardly depending settling leg, a cyclone or hydrocyclone and a centrifuge. 3. A process according to claim 1 or claim 2 wherein the fluff concentrating device includes a valve for permitting an amount of the polyolefin fluff together with an amount of tne supercritical diluent periodically to be removed from tne fluff concentrating device. 4. A process according to any one of claims 1 to 3 wherein the diluent comprises at least one alkane having from 1 to 4 caroon atoms or propylene. 5. A process according to claim 4 wherein the diluent comprises isobutane . 6. A process according to any foregoing claim wherein tne pressure of the supercritical diluent is from 37 to 100 bars. 7. A process according to any foregoing claim wherein tne temperature of the supercritical diluent is from "0 to 140°C. 8. A process according to any foregoing claim wherein tne diluent in the second loop reactor is operated unαer supercritical conditions and the second loop reactor is provided with a respective fluff concentrating device. 9. A process according to claim 8, further comprising recycling back into tne first and second loop reactors any diluent removed from the fluff concentrating device of the seconα loop reactcr on removal of polyolefin fluff therefrom. 10. A process according to any one of claims 1 to 7 wherein the diluent m the second loop reactor is operated under liquid conditions and the second loop reactor is provided with a respective fluff concentrating device. 11. Use, in a pair of serially connected loop reactors fcr polymerising an olefm in the presence of a catalyst to produce a polyolefm having a bimodal molecular weight distribution and including a first polyolefm fraction produced in a first loop reactor and a second polyolefm fraction produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in at least the first loop reactor for increasing the settling of polyolefm fluff a respective fluff concentrating device of at least the first loop reactor. 12. Use, m a pair of serially connected loop reactors for polymerising ethylene in the presence of a catalyst to produce polyethylene having a bimodal molecular weight distribution and including a first polyethylene fraction comprising polyethylene copolymer produced in a first loop reactor and a second polyethylene fraction comprising polyethylene homopolymer proαuced n a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions m at least the first loop reactor for reducing the amount of comonomer solution in the diluent transferred from the first loop reactor to the second loop reactor. 13. Use, a pair of serially connected loop reactors for polymerising ethylene in the presence of a catalyst to produce polyethylene having a bimodal molecular weight distribution and including a first polyethylene fraction comprising polyethylene homopolymer produced in a first loop reactor and a second polyethylene fraction comprising polyethylene copolymer produced m a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions m at least the first loop reactor for reducing tne amount of hydrogen m solution the diluent transferred from the first loop reactor to the second loop reactor. 14. Use, in a pair of serially connected loop reactors for polymerising an olefm in tne presence of a catalyst to produce a polyolefm having a bimoαai molecular weight distribution and including a first polyolefm fraction produced a first loop reactor and a second polyolefm fraction produced in a second loop reactor downstream of the first loop reactor, of a diluent under supercritical conditions in the first and second loop reactors for reducing the amount of diluent to be recycled back into the loop reactors following removal of a mixture of polyolef fluff and diluent from a fluff concentrating device of the second loop reactor.

Inventors: MARECHAL PHILIPPE, MARECHAL, PHILIPPE
Applicants: FINA RESEARCH, MARECHAL PHILIPPE, FINA RESEARCH S.A., MARECHAL, PHILIPPE
Application year: 2000
Main classification: C08F2
IPC codes: B01J8, B01J19, C08F2, C08F10, C08F110, C08F210