Polymerisation process

0001The present invention is concerned with olefin polymerisation in slurry phase loop reactors, in particular in multiple reactor systems.

0002Slurry phase polymerisation of olefins is well known wherein an olefin monomer and optionally olefin comonomer are polymerised in the presence of a catalyst in a diluent in which the solid polymer product is suspended and transported.

0003This invention is specifically related to polymerisation in at least one loop reactor of a multiple reactor system where the slurry is circulated in the reactor typically by means of a pump or agitator. Liquid full loop reactors are particularly well known in the art and are described for example in <patcit id="pcit0001" dnum="US3152872A"><text>U.S. Patent Numbers 3,152,872</text></patcit>, <patcit id="pcit0002" dnum="US3242150A"><text>3,242,150 </text></patcit>and<patcit id="pcit0003" dnum="US4613484A"><text> 4,613,484</text></patcit>.

0004Polymerisation is typically carried out at temperatures in the range 50-125°C and at pressures in the range 1-100 bara. The catalyst used can be any catalyst typically used for olefin polymerisation such as chromium oxide, Ziegler-Natta or metallocene-type catalysts. The product slurry comprising polymer and diluent, and in most cases catalyst, olefin monomer and comonomer can be discharged intermittently or continuously, optionally using concentrating devices such as hydrocyclones or settling legs to minimise the quantity of fluids withdrawn with the polymer.

0005The at least one loop reactor of a multiple reactor system is of a continuous tubular construction comprising at least two, for example four, vertical sections and at least two, for example four horizontal sections. The heat of polymerisation is typically removed using indirect exchange with a cooling medium, preferably water, in jackets surrounding at least part of the tubular loop reactor. The volume of the at least one loop reactor of a multiple reactor system can vary but is typically in the range 10 to 120 m³ the loop reactors of the present invention are of this generic type.

0006Maximum commercial scale loop reactor plant capacities have increased steadily over the years. Growing operating experience over the last few decades has led to operation of increasingly high slurry and monomer concentrations in reaction loops. The increase in slurry concentrations has typically been achieved with increased circulation velocities achieved for example by higher reactor circulation pump head or multiple circulation pumps as illustrated by <patcit id="pcit0004" dnum="EP432555A"><text>EP 432555</text></patcit> and <patcit id="pcit0005" dnum="EP891990A"><text>EP 891990</text></patcit>. The increase in solids loading is desirable to increase reactor residence time for a fixed reactor volume and also to reduce downstream diluent treatment and recycling requirements. The increased velocity and head requirement of the loop has however led to increasing pump design sizes and complexity, and energy consumptions as slurry concentrations increase. This has both capital and operating cost implications.

0007Historically relatively high circulation velocity has typically been maintained in the reaction loop in order to ensure good thermal, compositional and particle distribution across the reactor cross-section, particularly the avoidance of solids settling, stable flow characteristics, or excessive solids concentrations at the pipe wall rather than reduced to minimise pressure drop/power in the polymerisation loop.

0008Inadequate cross-sectional distribution could lead to increased fouling, reduced heat transfer and reduced polymer productivity and homogeneity. Construction and commissioning of new commercial plants is very expensive and therefore new designs seek to avoid or minimise changes to operating parameters that are seen to increase risk to the successful operation of the new unit.

0009In accordance with the present invention there is provided a process comprising polymerising in at least one loop reactor of a continuous tubular construction of a multiple reactor system an olefin monomer, optionally together with an olefin comonomer, in the presence of a polymerisation catalyst in a diluent to produce a slurry comprising solid particulate olefin polymer and the diluent wherein the average internal diameter of at least 50% of the total length of the reactor is at least 700 millimetres and the solids concentration in the reactor is at least 20 volume %, preferably 25vol%, optionally wherein the at least one loop reactor has a volume of at least 10 preferably at least 25m³. Preferably at least 30wt%, most preferably more than 40wt%, of the polymer produced in the multiple reactor system is made in at least one loop reactor wherein the average internal diameter of at least 50% of the total length of the reactor is at least 700 millimetres.

0010One advantage of the present invention is that the specific energy consumption (i.e. the energy consumed per unit weight of polymer produced) of the loop reactor is reduced whilst maintaining a given reactor residence time and avoiding unacceptable reactor fouling. The invention is especially advantageous when it is desired to design and operate a reactor at high solids loadings when it has previously been considered necessary to use what have now been found to be excessively high loop circulation velocities.

0011This invention relates to a method and apparatus for continuous polymerization of olefins, preferably alpha mono olefins, in an elongated tubular closed loop reaction zone. The olefin(s) is continuously added to, and contacted with, a catalyst in a hydrocarbon diluent. The monomer(s) polymerise to form a slurry of solid particulate polymer suspended in the polymerisation medium or diluent.

0012Typically, in the slurry polymerisation process of polyethylene, the slurry in the reactor will comprise the particulate polymer, the hydrocarbon diluent(s), (co) monomer(s), catalyst, chain terminators such as hydrogen and other reactor additives. In particular the slurry will comprise 20-75, preferably 30-70 weight percent based on the total weight of the slurry of particulate polymer and 80-25, preferably 70-30 weight percent based on the total weight of the slurry of suspending medium, where the suspending medium is the sum of all the fluid components in the reactor and will comprise the diluent, olefin monomer and any additives; the diluent can be an inert diluent or it can be a reactive diluent in particular a liquid olefin monomer; where the principal diluent is an inert diluent the olefin monomer will typically comprise 2-20, preferably 4-10 weight percent of the slurry.

0013The slurry is pumped around the relatively smooth path-endless loop reaction system at fluid velocities sufficient (i) to maintain the polymer in suspension in the slurry and (ii) to maintain acceptable cross-sectional concentration and solids loading gradients.

0014It has now been found that, for high solids loadings, cross-sectional slurry concentration distributions (as evidenced by fouling, flow variations and/or heat transfer) can be maintained within acceptable operating limits whilst increasing the internal diameter of the tubular reactor above that which is conventionally regarded as operationally reliable for at least one of the reactors of a multiple reactor system. This is contrary to what the man skilled in the art would believe to be the case in the light of conventional process conditions where the internal diameter of the reactor is no greater than 600 millimetres and is typically about 500 millimetres.

0015The solids concentration in the slurry in the at least one loop reactor is preferably at least 20 volume %, more preferably at least 25 volume%, and most preferably at least 30 volume%, where volume % is [(total volume of the slurry-volume of the suspending medium)/(total volume of the slurry)] x100. The solids concentration measured as weight percentage which is equivalent to that measured as volume percentage will vary according to the polymer produced but more particularly according to the diluent used. Where the polymer produced is polyethylene and the diluent is an alkane, for example isobutane, it is preferred that the solids concentration is above 30 in particular above 40 weight % for example in the range 40-60 preferably 45%-55 weight % based on the total weight of the slurry.

0016We have found that operation of a polymerisation process in a multiple reactor system where at least one of reactors has an internal diameter of over 700 millimetres - ie a larger diameter reactor than is conventionally used in slurry polymerisation - can be carried out without any significant problems, particularly with regard to fouling at the reactor walls.

0017Preferably greater than 50% of the total length of the loop reactor has an internal diameter over 750mm, for example over 850mm, and preferably between 700 and 800 millimetres. It is preferred that greater than 50%, in particular greater than 70%, for example greater than 85% of the total length of the reactor has an internal diameter over 700 millimetres, in particular over 750 millimetres, for example between 700 and 800 millimetres.

0018A particular advantage of this invention is that high slurry concentrations at relatively low circulation velocities and relatively high reactor loop diameters can be used. Accordingly, the Froude number in the reactor is preferably maintained at or below 30, for example in the range 24 to 0.5, more preferably 20 to 1, parrticularly in the range 15 to 2, with the most preferred ranges being 10 to 3 or 9 to 2. The Froude number is a dimensionless parameter indicative of the balance between the suspension and settling tendencies of particles in a slurry. It provides a relative measure of the momentum transfer process to the pipe wall from particles compared to the fluid. Lower values of the Froude number indicate stronger particle-wall (relative to fluid-wall) interactions. The Froude number (Fr) is defined as v²/(g(s-1)D) where v is the average velocity of the slurry, g is the gravitational constant, s is the specific gravity of the solid in the diluent and D is the internal pipe diameter. The specific gravity of the solid polymer which is the ratio of the density of the polymer to the density of water is based on the annealed density of the degassed polymer after being substantially devolatilised and immediately prior to any extrusion as measured using method ISO1183A.

0019It has been found that reactors can be designed and operated at specific pressure drop both per unit reactor length and per mass of polymer and total pressure drop for the loop less than that taught as being required, particularly at high solids loadings and large reactor diameters. This invention permits total loop pressure drops of less than 1.3 bar, particularly less than 1 bar even for polymer production rates of above 25, even above 45 tonnes per hour. It is possible to employ one or more than one pump in the loop preferably on one or more horizontal sections; these can be located on the same horizontal section or on different sections. The pump or pumps can be of the same diameter or larger or smaller diameter preferably of the same diameter as the internal diameter of the section of the reactor where the pump or pumps are located. It is preferable to employ a single pump and it is a feature of the present invention that requirements for number and power of pump(s) is less onerous than for conventional processes.

0020Reactor size is typically over 10m³, usually over 25m³ in particular over 50m³ for example 75-200m³ preferably in the range 100-175m^3.

0021Use of higher internal diameter reactors for at least one loop reactor of a multiple reactor system as set out herein above enables reactors, for example having volumes of greater than 80 m³, to be built with a reactor length to diameter ratio of less than 500, preferably less than 400 more preferably less than 250. Reduction in reactor length to diameter ratio minimises compositional gradients around the reaction loop and enables production rates of greater than 25 tonnes (per reactor) per hour to be achieved with only a single point of introduction for each reagent around the reaction loop. Alternatively it is possible to have multiple inlets into the loop reactor for reactants (e.g. olefins), catalyst, or other additives.

0022Where a reactor system is designed to operate different catalyst types (eg Ziegler-Natta, chromium and/or metallocene) or a catalyst system where the activity or production ratio required varies significantly between reactors under different operating regimes, reactors of equal volume reactors are typically used in order to accommodate the entire grade range with maximum flexibility. The existence of such externally imposed reactor dimensions places constraints on the operating window available for each reactor, and thus on the ability to balance reactor activity, product quality, throughput and cooling in each reactor. It is generally preferred that such reactors have the same average internal diameter, it is most preferred that the reactors are constructed to the same dimensions.

0023In a further embodiment of the present invention it is preferred that the loop reactor has horizontal and vertical sections, and the vertical sections comprise at least 50% preferably at least 60% more preferably at least 70% of the total length of the reactor.

0024It is also preferred that the density span of the polymer powder particles (defined as the absolute value of the density difference in g/cm³ between the average density of the polymer particles exiting the reactor with particle size above D90 and the average density of the material with particle size below D10) is below 0.005, preferably below 0.003, more preferably below 0.0026, most preferably below 0.0023.

0025Accordingly, a further independent aspect of the invention provides process for controlling a slurry phase (co-) polymerisation process in the presence of a polymerisation catalyst, <b>characterised in that</b> the said process comprises maintaining the density SPAN of the polymer powder particles below 0.005, preferably below 0.003, more preferably below 0.0026, most preferably below 0.0023.

0026D values are measured by sieving granulometry and are expressed in µm, and may be defined as follows: <ul id="ul0001" list-style="none" compact="compact"><li>D5 : value under which 5% by weight of the particles are collected;</li><li>D10 : value under which 10% by weight of the particles are collected;</li><li>D50: value under which 50% by weight of the particles are collected;</li><li>D90 : value under which 90% by weight of the particles are collected.</li><li>D95 : value under which 95% by weight of the particles are collected.</li></ul>

0027A preferred embodiment also maintains a particle size distribution such that (D90-D10)/D50 is less than 2, preferably less than 1.5, more preferably less than 1.2. It is also preferred that D95 is less than 2000µm, preferably less than 1500µm, more preferably less than 1000 µm, most preferably less than 355µm where D95, D90, D50 and D10 are defined such that 95wt%, 90wt%, 50wt% or 10wt% of the polymer particles have a diameter of less than D95, D90, D50 and D10 respectively. The average particle size D50 is preferably between 100 and 1500 microns, most preferably between 150 and 1000 microns.

0028A particularly preferred embodiment has a particle size distribution such that D95 is less than 355µm and (D90-D10)/D50 is less than 1.2, where D95, D90, D50 and D10.

0029A further independent aspect of the invention provides a process comprising polymerising in a loop reactor of a continuous tubular construction an olefin monomer optionally together with an olefin comonomer in the presence of a polymerisation catalyst in a diluent to produce a slurry comprising solid particulate olefin polymer and the diluent wherein the average internal diameter of at least 50% of the total length of the reactor is at least 650 millimeters, the solids concentration in the reactor is at least 15 volume % and having a particle size distribution such that (D90-D10)/D50 is less than 2.

0030A still further independent aspect of the invention provides a process for polymerising olefins in a polymerisation reactor, <b>characterised in that</b> the polymer powder recovered from the reactor has a particle size distribution such that D95 is less than 1500µm and (D90-D10)/D50 is less than 1.5.

0031According to another preferred embodiment of the present invention, it has been found that the claimed control, i. e. the maintenance of the density span below certain values, was highly critical for the metallocene catalysts.

0032A further benefit of the present invention is to improve the speed and safety associated with start-ups and grade transitions. When changing grades, the compositions of monomers and hydrogen and the product properties may be changing continuously, the quantity of absorption of monomers in the polymer will additionally be altered by the changing product properties (principally density) and gas composition.

0033The control of the span according to the present invention enables the speed of start- ups and transitions to be improved and the production of wide-specification material to be minimised.

0034It has particularly been found that stable operation of high capacity reactors, particularly large diameter loop reactors, can be optimised by strictly controlling the density span of the powder within or withdrawn from the reactor.

0035With many catalyst systems the density span can vary significantly with particle size and/or particle size distribution. The principal determinants of the average particle size of the powder are the residence time in the reactor and the catalyst yield. The particle size distribution of the powder can be affected by many factors including the type, and particle size distribution, of the catalyst fed to the reactor, the initial and average catalyst activity, the robustness of the catalyst support and susceptibility of the powder to fragment under reaction conditions. Solids separating devices (such as hydrocyclones) can be used on the slurry withdrawn from the reactor to assist in control of the average particle size and the particle size distribution of the powder in the reactor. The location of the withdrawal point for the concentrating device and the design and operating conditions of the concentrating device system, preferably the at least one hydrocyclone recycle loop, also enables the particle size and particle size distribution within the reactor to be controlled.

0036An alternative is to select a specific catalyst which has been found to give the desired particle size distribution. The advantage of catalyst systems which provide the desired particle size distribution is that they can be used at high productivity: generally the higher the productivity (as measured in g polymer per g of catalyst) the larger the individual polymer particles formed. Thus the present invention permits the use of catalyst systems having productivity of at least 5000g polyolefin/g of catalyst, and typically at least 10000g polyolefin/g of catalyst, most preferably above 15000 g polyolefin/g of catalyst.

0037A pre-polymer of controlled particle size distribution made using any catalyst as described above may also be introduced to the polymerization reactor. The prepolymerisation may be carried out by any suitable process, for example, polymerisation in a liquid hydrocarbon diluent or in the gas phase using a batch process, a semi-continuous process or a continuous process. The conversion to prepolymer is generally carried out by bringing the catalyst into contact with one or more alpha-olefins in amounts such that the prepolymer contains between 0.002 and 10 millimoles of transition metal per gram. The prepolymer particle size may be controlled by sieving, hydrocyclone or elutriation separation of fines or large particles or other known techniques.

0038In these and other embodiments of the present invention comprising a multiple reactor system wherein one of the reactors is a loop reactor, the loop reactor may have one or more characteristics of the loop reactor as set out hereinabove. The second or any subsequent reactor of the multiple reactor system may be another loop reactor or any reactor for the polymerisation of olefins, for example a fluidised-bed reactor. However, it is preferable that the second or any subsequent reactor of the multiple reactor system is another loop reactor which may or may not have one or more characteristics of the loop reactors as set out hereinabove. For example, the loop reactor of the invention may have an internal diameter of at least 50% of its length greater than 700 millimetres, whilst the second or any subsequent loop reactor may have an internal diameter greater than 500 millimetres for example greater than 600 millimetres preferably greater than 700 millimetres.

0039The multiple reactor system can be used to make monomodal or multimodal, preferably multimodal polymers. Preferably the process of the invention comprises making a multimodal ethylene polymer, in particular a bimodal ethylene polymer, in which a low molecular weight (LMW) polymer is made in one reactor, and a high molecular weight (HMW) polymer is made in another reactor, the polymers being made in either order and the second polymer being made in the presence of the first. One or both of the reactors may have an internal diameter of at least 700mm along at least 50% of its length.

0040In one embodiment of the invention, the process in the multiple reactor system comprises making a multimodal ethylene polymer having a density greater than 940 kglm³ and a melt flow index MI₅ of 0.05 to 50 g/10min, said ethylene polymer comprising-from 30 to 70wt%, based on the total weight of the ethylene polymer, of a first polyethylene fraction having a density of at least 950 kg/m³ and a melt flow index MI₂ of at least 10 g/10 min, and from 70 to 30wt%, based on the total weight of the multimodal ethylene polymer, of a second polyethylene fraction comprising units of ethylene and optionally up to 5mol% of at least one other alpha-olefin containing from 3 to 12 carbon atoms, and a melt flow index MI₂ of less than 10 g/10min.

0041An alternative polymer is a multimodal ethylene polymer having a density between 900 and 930 kg/m³ and a melt flow index MI₂ of 0.1 to 20 g/10min, said ethylene polymer comprising: <ul id="ul0002" list-style="none" compact="compact"><li>from 30 to 70wt%, based on the total weight of the ethylene polymer, of a first polyethylene fraction having a density of less than 950 kg/m³ and a melt flow index MI₂ of at least 10 g/10 min, and</li><li>from 70 to 30wt%, based on the total weight of the multimodal ethylene polymer, of a second polyethylene fraction comprising units of ethylene, from 0.1 to 20 mol% of an alpha-olefin containing from 3 to 12 carbon atoms, and a melt flow index MI₂ of less than 10g/10min.</li></ul>

0042The loop reactor utilised in the present invention is the reactor in which either the first or second polyethylene fraction referred to above is made.

0043Although it may be made in more than two reactors, it is most preferred that the polymer is a bimodal polymer made in two reactors in series. One or both of the reactors may have an internal diameter of at least 700mm along at least 50% of its length. The first polymer made in the first reactor may be a low molecular weight (LMW) polymer and the second polymer made in the second reactor may be a high molecular weight (HMW) polymer. In one embodiment, 30-70wt% and more preferably 40-60wt% of a low molecular weight (LMW) polymer is made in the first reactor, and 70-30wt% and more preferably 60-40wt% of a high molecular weight (HMW) polymer is made in the second reactor. The most preferred range of ratios of the HMW and LMW polymers is 45-55wt% to 55-45wt%.

0044In an alternative embodiment, the HMW polymer is made in the first reactor and the LMW polymer in the second reactor, preferably in the same ratios as mentioned above. The following requirements apply to this embodiment only. In this case the first (HMW) reactor preferably has a space time yield (defined as production of polymer in kg/h per unit volume of reactor) greater than 100 kg/m³/h, more preferably greater than 150kg/m³/h, and most preferably greater than 250kg/m³/h. It is also preferred in this case that the ratio of space time yield in the first (HMW) reactor to the second (LMW) reactor is greater than 1, more preferably greater than 1.2, most preferably greater than 1.5. This may be achieved by designing the first (HMW) reactor with a volume that is no more than 90%, preferably between 30-70%, and more preferably approximately 40-60%, of the volume of the second (LMW) reactor. Under the above circumstances, preferably the ratio of length to diameter (L/D) of the first reactor is greater than 350, more preferably between 500 and 3000, most preferably greater than 750. The ratio of L/D of the first reactor to L/D of the second reactor is alternatively or additionally also preferred to be greater than 1.5, most preferably greater than 2.

0045In the HMW-LMW embodiment of the invention, in the case where the reactors differ in volume by no more than 10%, it is preferred to balance the activities between the reactors and the respective cooling capacities by maintaining the temperature of the first reactor between 60°C and 80°C, preferably no higher than 75°C. It is also preferred that the ratio of solids concentration in the first reactor to that in the second reactor is maintained less than 1.0, preferably between 0.6 and 0.8, as this also assists in maintaining the activity balance between the two reactors within the desired range. Generally the solids concentration in the final reactor is at least 35wt%, most preferably between 45wt% and 60wt% and the solids concentration in the HMW reactor is between 20wt% and 50wt%, more preferably between 25wt% and 35wt%. Under these circumstances it is preferred to concentrate the solids transferred from the first reactor to the second reactor using a settling zone and/or hydrocyclone to concentrate the solids concentration. A comonomer free diluent stream is most preferably introduced upstream of the hydrocyclone to minimize the comonomer transferred to the downstream reactor, thus maximizing the density potential in LMW reactor. The solids concentration is based on the weight of polymer in the total weight of the slurry of particulate polymer.

0046When the HMW polymer is produced in a reactor upstream of the LMW polymer reactor it is preferred that the ratio of the average internal diameter of the HMW reactor to the average internal diameter of the LMW reactor is between 0.8 and 1.4, generally less than 1, preferably less than 1.2, most preferably less than 1.4. In this configuration it is preferred that at least 50% of the total length of the LMW reactor is at least 700mm, whereas the average internal diameter of the HMW reactor is less 700 millimetres, preferably less than 600 millimetres.

0047When the LMW polymer is produced in a reactor upstream of the HMW polymer reactor it is preferred that the ratio of the average internal diameter of the HMW reactor to the average internal diameter of the HMW reactor is between 0.6 and 1.4, generally between 0.8 and 1.2, most preferably between 0.9 and 1.1. It is preferred that the reactors have the same average internal diameter, it is most preferred that both reactors are constructed to the same dimensions.

0048Referring generally to the present invention, in the case where the multiple reactor system comprises two loop reactors, it is preferred that the Froude number in at least one loop reactor is no more than 30, and the ratio of the Froude number in that reactor to the Froude number in the other loop reactor is in the range 0. 1 to 10 preferably 0.2 to 5.

0049In a further embodiment of the invention, a slurry containing polymer is withdrawn from the second reactor and transferred to a flash tank operating at a pressure P and temperature such that at least 50mol% of the liquid component of the slurry is withdrawn from the flash tank as a vapour. Preferably the above process enables at least 98mol%, more preferably 98.5mol%, most preferably 98.5mol%, of the vapour withdrawn from the flash tank to be condensed without compression. It is also preferred that at least 80mol%, more preferably 90mol%, most preferably 95mol%, of the liquid component of the slurry is withdrawn from the flash tank as a vapour.

0050The pressure employed in the loop, or loops is preferably sufficient to maintain the reaction system 'liquid full' i.e. there is substantially no gas phase. Typical pressures used are between 1-100 bara, preferably between 30 to 50 bara. In ethylene polymerization the ethylene partial pressure will typically be in the range 0.1 to 5 MPa, preferably from 0.2 to 2 MPa, more particularly from 0.4 to 1.5 MPa. The temperatures selected are such that substantially all of the polymer produced is essentially (i) in a non-tacky and non-agglomerative solid particular form and (ii) insoluble in the diluent. The polymerization temperature depends on the hydrocarbon diluent chosen and the polymer being produced. In ethylene polymerisation the temperature is generally below 130°C, typically between 50 and 125°C, preferably between 75 and 115°C. For example in ethylene polymerisation in isobutane diluent, the pressure employed in the loop is preferably in the range 30-50 bara, the ethylene partial pressure is preferably in the range 0.2-2MPa and the polymerisation temperature is in the range 75-115°C. The space time yield which is production rate of polymer per unit of loop reactor volume for the process of the present invention is in the range 0.1-0.4 preferably 0.2-0.35 ton/hour/m³. Where there are two or more loops, the reaction conditions may be the same or different in different loops.

0051The process according to the invention applies to the preparation of compositions containing olefin (preferably ethylene) polymers which can comprise one or a number of olefin homo-polymers and/or one or a number of copolymers. It is particularly suited to the manufacture of ethylene polymers and propylene polymers. Ethylene copolymers typically comprise an alpha-olefin in a variable amount which can reach 12% by weight, preferably from 0.5 to 6% by weight, for example approximately 1% by weight.

0052The alpha mono-olefin monomers generally employed in such reactions are one or more 1-olefines having up to 8 carbon atoms per molecule and no branching nearer the double bond than the 4-position. Typical examples include ethylene, propylene, butene-1, pentene-1, hexene-1 and octene-1, and mixtures such as ethylene and butene-1 or ethylene and hexene-1. Butene-1, pentene-1 and hexene-1 are particularly preferred comonomers for ethylene copolymerisation.

0053Typical diluents employed in such reactions include hydrocarbons having 2 to 12, preferably 3 to 8, carbon atoms per molecule, for example linear alkanes such as propane, n-butane, n-hexane and n-heptane, or branched alkanes such as isobutane, isopentane, isooctane and 2,2,-dimethylpropane, or cycloalkanes such as cyclopentane and cyclohexane or their mixtures. In the case of ethylene polymerization, the diluent is generally inert with respect to the catalyst, cocatalyst and polymer produced (such as liquid aliphatic, cycloaliphatic and aromatic hydrocarbons), at a temperature such that at least 50% (preferably at least 70%) of the polymer formed is insoluble therein. Isobutane is particularly preferred as the diluent for ethylene polymerisation.

0054The operating conditions can also be such that the monomers (e.g. ethylene, propylene) act as the diluent as is the case in so called bulk polymerisation processes. The slurry concentration limits in volume percent have been found to be able to be applied independently of molecular weight of the diluent and whether the diluent is inert or reactive, liquid or supercritical. Propylene monomer is particularly preferred as the diluent for propylene polymerisation

0055Methods of molecular weight regulation are known in the art. When using Ziegler-Natta, metallocene and tridentate late transition metal type catalysts, hydrogen is preferably used, a higher hydrogen pressure resulting in a lower average molecular weight. When using chromium type catalysts, polymerization temperature is preferably used to regulate molecular weight.

0056In commercial plants, the particulate polymer is separated from the diluent in a manner such that the diluent is not exposed to contamination so as to permit recycle of the diluent to the polymerization zone with minimal if any purification. Separating the particulate polymer produced by the process of the present invention from the diluent typically can be by any method known in the art for example it can involve either (i) the use of discontinuous vertical settling legs such that the flow of slurry across the opening thereof provides a zone where the polymer particles can settle to some extent from the diluent or (ii) continuous product withdrawal. via a single or multiple withdrawal ports, the location of which can be anywhere on the loop reactor but is preferably adjacent to the downstream end of a horizontal section of the loop. Any continuous withdrawal ports will typically have an internal diameter in the range 2-25, preferably 4-15, especially 5-10 cm This invention permits large scale polymerisation reactors to be operated with low diluent recovery requirements. The operation of large diameter reactors with high solids concentrations in the slurry minimises the quantity of the principal diluent withdrawn from the polymerisation loop. Use of concentrating devices on the withdrawn polymer slurry, preferably hydrocyclones (single or in the case of multiple hydrocyclones in parallel or series), further enhances the recovery of diluent in an energy efficient manner since significant pressure reduction and vaporisation of recovered diluent is avoided.

0057It has been found that the slurry concentration in the reactor loop can be optimised by controlling the average particle size and/or the particle size distribution of the powder within the reactor loop. The principal determinant of the average particle size of the powder is the residence time in the reactor. The particle size distribution of the powder can be affected by many factors including the particle size distribution of the catalyst fed to the reactor, the initial and average catalyst activity, the robustness of the catalyst support and susceptibility of the powder to fragment under reaction conditions. Solids separating devices (such as hydrocyclones) can be used on the slurry withdrawn from the reactor loop to further assist in control of the average particle size and the particle size distribution of the powder in the reactor. The location of the withdrawal point for the concentrating device and the design and operating conditions of the concentrating device system, preferably the at least one hydrocyclone recycle loop, also enables the particle size and particle size distribution within the reactor to be controlled. The average particle size is preferably between 100 and 1500 microns, most preferably between 250 and 1000 microns.

0058Where the final reactor of the multiple reactor system is a loop reactor, the withdrawn, and preferably concentrated, polymer slurry is depressurised, and optionally heated, prior to introduction into a primary flash vessel. The stream is preferably heated after depressurisation.

0059The diluent and any monomer vapours recovered in the primary flash vessel are typically condensed, preferably without recompression and reused in the polymerization process. The pressure of the primary flash vessel is preferably controlled to enable condensation with a readily available cooling medium (e.g. cooling water) of essentially all of the flash vapour prior to any recompression. typically such pressure in said primary flash vessel will be 4-25 for example 10-20, preferably 15-17 bara. The solids recovered from the primary flash vessel is preferably passed to a secondary flash vessel to remove residual volatiles. Alternatively the slurry may be passed to a flash vessel of lower pressure than in the above mentioned primary vessel such that recompression is needed to condense the recovered diluent. Use of a high pressure flash vessel is preferred.

0060The process according to the invention can be used to produce resins which exhibit specific density in the range 890 to 930 kg/m³ (low density), 930 to 940 kg/m³ (medium density) or 940 to 970 kglm³ (high density).

0061The process according to the invention is relevant to all olefin polymerisation catalyst systems, particularly those chosen from the Ziegler-type catalysts, in particular those derived from titanium, zirconium or vanadium and from thermally activated silica or inorganic supported chromium oxide catalysts and from metallocene-type catalysts, metallocene being a cyclopentadienyl derivative of a transition metal, in particular of titanium or zirconium.

0062Non-limiting examples of Ziegler-type catalysts are the compounds comprising a transition metal chosen from groups IIIB, IVB, VB or VIB of the periodic table, magnesium and a halogen obtained by mixing a magnesium compound with a compound of the transition metal and a halogenated compound. The halogen can optionally form an integral part of the magnesium compound or of the transition metal compound.

0063Metallocene-type catalysts may be metallocenes activated by either an alumoxane or by an ionising agent as described, for example, in Patent Application <patcit id="pcit0006" dnum="EP500944A1"><text>EP-500,944-A1</text></patcit> (Mitsui Toatsu Chemicals).

0064Ziegler-type catalysts are most preferred. Among these, particular examples include at least one transition metal chosen from groups IIIB, IVB, VB and VIB, magnesium and at least one halogen. Good results are obtained with those comprising: <ul id="ul0003" list-style="none" compact="compact"><li>from 10 to 30% by weight of transition metal, preferably from 15 to 20% by weight,</li><li>from 20 to 60% by weight of halogen, preferably from 30 to 50% by weight</li><li>from 0.5 to 20% by weight of magnesium, usually from 1 to 10% by weight,</li><li>from 0.1 to 10% by weight of aluminium, generally from 0.5 to 5% by weight,</li><li>the balance generally consists of elements arising from the products used for their manufacture, such as carbon, hydrogen and oxygen. The transition metal and the halogen are preferably titanium and chlorine.</li></ul>

0065Polymerisations, particularly Ziegler catalysed ones, are typically carried out in the presence of a cocatalyst. It is possible to use any cocatalyst known in the art, especially compounds comprising at least one aluminium-carbon chemical bond, such as optionally halogenated organoaluminium compounds, which can comprise oxygen or an element from group I of the periodic table, and aluminoxanes. Particular examples would be organoaluminium compounds, of trialkylaluminiums such as triethylaluminium, trialkenylaluminiums such as triisopropenylaluminium, aluminium mono- and dialkoxides such as diethylaluminium ethoxide, mono- and dihalogenated alkylaluminiums such as diethylaluminium chloride, alkylaluminium mono- and dihydrides such as dibutylaluminium hydride and organoaluminium compounds comprising lithium such as LiAl(C₂H₅)₄. Organoaluminium compounds, especially those which are not halogenated, are well suited. Triethylaluminium and triisobutylaluminium are especially advantageous.

0066The chromium-based catalyst is preferred to comprise a supported chromium oxide catalyst having a titania-containing support, for example a composite silica and titania support. A particularly preferred chromium-based catalyst may comprise from 0.5 to 5 wt % chromium, preferably around 1 wt % chromium, such as 0.9 wt % chromium based on the weight of the chromium-containing catalyst. The support comprises at least 2 wt % titanium, preferably around 2 to 3 wt % titanium, more preferably around 2.3 wt % titanium based on the weight of the chromium containing catalyst. The chromium-based catalyst may have a specific surface area of from 200 to 700 m²/g, preferably from 400 to 550 m²/g and a volume porosity of greater than 2 cc/g preferably from 2 to 3 cc/g.

0067Silica supported chromium catalysts are typically subjected to an initial activation step in air at an elevated activation temperature. The activation temperature preferably ranges from 500 to 850°C, more preferably 600 to 750°C.

0068The reactor system can comprise one or more loop reactors connected in series or in parallel preferably in series.

0069In the case of series reactors, a first reactor of the series is supplied with catalyst and the cocatalyst in addition to the diluent and monomer, and each subsequent reactor is supplied with, at least, monomer, in particular ethylene and with the slurry arising from a preceding reactor of the series, this mixture comprising the catalyst, the cocatalyst and a mixture of the polymers produced in a preceding reactor of the series. It is optionally possible to supply a second reactor and/or, if appropriate, at least one of the following reactors with fresh catalyst and/or cocatalyst. However, it is preferable to introduce the catalyst and the cocatalyst exclusively into a first reactor.

0070In the case of at least two reactors in series, the polymer of highest melt index and the polymer of lowest melt index can be produced in two adjacent or non-adjacent reactors in the series. Hydrogen is maintained at (i) a low (or zero) concentration in the reactor(s) manufacturing the high molecular weight components, e.g. hydrogen percentages including between 0-0.1 vol % and at (ii) a very high concentration in the reactor(s) manufacturing the low molecular weight components e.g. hydrogen percentages between 0.5-2.4 vol %. The reactors can equally be operated to produce essentially the same polymer melt index in successive reactors.

0071Particular sensitivity to operating in large diameter reactors (and associated cross-sectional compositional, thermal or particulate gradients) has however been related to production of polymer resins where polymer of either high or low molecular weight resins has been known to lead to increased fouling concerns. Particularly when producing polymers of molecular weights less than 50 kDaltons or greater than 150 kDaltons. These concerns have particularly been confirmed to be accentuated at low polymer solids concentrations in the reactor loop. When producing polymers of molecular weights less than 50 kDaltons or greater than 200 kDa (or melt index below 0.1 and above 50) in large diameter reactors it has however surprisingly been discovered that fouling is decreased when solids loadings are increased to above 20 vol%, particularly above 30 vol%.

EXAMPLES

0072As previously stated, one of the advantages of the invention is that cross-sectional slurry concentration distributions can be maintained within acceptable operating limits at high reactor diameters, even with high solids loadings. The actual cross-sectional slurry concentration distribution is not measurable directly, but other parameters can be measured as an indication of the homogeneity of the slurry.

EXAMPLE 1

0073In the following Example, ethylene was polymerised in two reactors in series. The first reactor had a volume of 96m³ and an internal diameter along over 98% of its length of 730mm. The solids content was 27-28vol%. The volumetric solid content is defined as (Volumetric PE production rate / Volume of slurry exiting the reactor), where: Volumetric PE production rate = (Ethylene mass feed - Ethylene mass flow exiting reactor)/ density of PE] Volume of slurry exiting the reactor = Volume of liquid exiting + volumetric PE production rate Volume of liquid exiting the reactor = [Sum of mass feeds in - PE mass production]/ liquid density The PE density is measured on dry and degassed powder. The density of liquid is calculated based on any suitable model (using composition, temperature and pressure).

Circulation pump power requirement

0074The power requirement of the circulation pump inside the loop reactor is affected not only by the pressure of the flow, but also by the slurry concentration gradient. It is particularly sensitive to concentration gradients because it is located at an elbow in the loop reactor, where there is most likely to be segregation of the flow, which causes variations in the density of the slurry being pumped. Where there is little fouling and good circulation, a relatively constant power demand over time is observed. <figref idref="f0001">Figure 1</figref> shows a plot of power and current requirement of the pump (plotted as a relative value rather than absolute) over a period of one hour for the polymerisation of Example 1. The variation shown here is greater than would be expected for a pure solvent, but is low for a polymer-containing slurry, indicating that little or no segregation of the slurry is occuring.

Reactor temperature

0075The temperature at different points in the reactor during is also affected by the slurry concentration gradient. Absolute differences in measured temperature at different points in a reactor can be due to differences in calibration of the thermocouples, so in order to identify "real" differences in temperature, the variation during a temperature transition is measured. In an inhomogeneous slurry the trend of such temperature transitions would vary. <figref idref="f0002">Figure 2</figref> shows such a transition for the polymerisation of Example 1. <figref idref="f0002">Figure 2</figref> shows that in Example 1 the temperature transition trend is constant throughout the reactor, indictating good homogeneity.

Heat transfer coefficient at the reactor wall

0076A significant effect of fouling of a reactor is a change in the heat transfer coefficient across the reactor wall. The heat transfer coefficient can be measured over a very long period (many months), and an absence of any reduction indicates that there is no build-up of fouling material over this time. <figref idref="f0003">Figure 3</figref> shows the heat transfer coefficient for Example I over a period of 250 days, from which it can be seen that there is no long-term reduction in the coefficient.