FIELD OF THE INVENTION

[0001] The present disclosure provides a process for preparing an ethylene polymer in a sus pension polymerization in a polymerization reactor or a series of polymerization reactors. The present disclosure provides in particular a process for preparing an ethylene polymer in a suspen sion polymerization in a polymerization reactor or a series of polymerization reactors in which the obtained suspension of ethylene polymer particles in a suspension medium is transferred into a solid-liquid separator, in which the suspension is separated in wet ethylene polymer particles and mother liquor and the wet ethylene polymer particles are dried by contacting the particles with a gas stream.

BACKGROUND OF THE INVENTION

[0002] Suspension processes for preparing ethylene polymers are established methods for producing ethylene polymers and, for example, disclosed in EP 0 905 152 A1 or

WO 2012/028591 A1 . Such processes are often carried out in a series of reactors and so allow to set different reaction conditions in the polymerization reactors and thereby to produce different polymer compositions in the individual polymerization reactors. Accordingly produced multimodal ethylene copolymers are characterized by having, for example, a good combination of product properties and processability. Suspension processes for preparing ethylene polymers commonly employ a hydrocarbon or a hydrocarbon mixture as diluent. The suspension medium, which forms the liquid or supercritical phase of the suspension, comprises, besides the diluent as main component, also further components like dissolved ethylene, comonomers, aluminum alkyls, and hydrogen and dissolved reaction products like oligomers and waxes. Principles of producing multi- modal ethylene copolymer in suspension in a series of reactors are, e.g., disclosed in F. Alt et al., Macromol. Symp. 2001 , 163, 135-143.

[0003] For economic and ecological reasons, it is common practice to recycle not consumed components of the reaction mixture like the diluent or non-polymerized monomers or comonomers to the polymerization process. The separation of the produced polyethylene particles from the formed suspension results in wet ethylene polymer particles and separated suspension medium, the so-called mother liquor. The polymer particles are usually dried in a powder dryer using a hot gas stream such as a hot nitrogen stream. The hydrocarbons contained in the hot gas stream are thereafter separated from the hot gas stream and transferred into a liquid hydrocarbon recover stream, which is returned to the polymerization. [0004] The mother liquor can directly be recycled to the polymerization process. However, to remove side products of the polymerization reaction which are soluble in the suspension medium from the polymerization process, a part of the mother liquor has to be worked up. Another reason for working up the mother liquor can be that the suspension obtained after the final reactor of a series of polymerization reactors may contain components which should not be introduced into one of the previous polymerization reactors for producing polyethylenes with specific property combinations. For example, for preparing multimodal ethylene copolymers having a high ESCR, it is often needed that one of the polymerizations in one of the polymerization reactors is an eth ylene homopolymerization and accordingly no comonomer should be present in the respective polymerization reactor. The mother liquor to be worked up is a multi-component mixture. Accord ingly, various separation steps such as evaporation or distillation steps are needed to remove all unwanted components and to split up those constituents which are intended to be recycled to the polymerization process. For example, WO 2010/136202 A1 discloses a process in which a part of the mother liquor is evaporated. The heavy ends of this pre-distillation are fed to a wax removal unit in which the wax-rich hydrocarbon flow is concentrated to polyethylene wax. The overhead stream of the pre-distillation, for example wax-depleted diluent, is distilled in a diluent distillation column. Finally, the diluent is purified in adsorbers and fed back into the process.

[0005] Nonetheless, there is a need to provide a process for working up the mother liquor ob tained by removing the prepared ethylene polymer particles from the produced suspension in an ethylene polymerization process, which work-up process not only gives purified diluent but which can also be carried out in an economical and energy efficient manner and minimizes the amount of mother liquor that is evaporated.

SUMMARY OF THE INVENTION

[0006] The present disclosure provides a process for preparing an ethylene polymer in a sus pension polymerization in a polymerization reactor or a series of polymerization reactors compris ing polymerizing ethylene or copolymerizing ethylene and one or more C3-Ci2-1-alkenes at tempera tures of from 40 to 150°C and pressures of from 0.1 to 20 MPa in the presence of a polymeriza tion catalyst and forming a suspension of ethylene polymer particles in a suspension medium comprising a hydrocarbon diluent, transferring the suspension of ethylene polymer particles into a solid-liquid separator, in which the suspension is separated in wet ethylene polymer particles and mother liquor, transferring a part of the mother liquor into a work-up section comprising an evaporator for pro ducing a wax-depleted portion of the mother liquor, comprising a diluent distillation unit, which produces isolated diluent from the wax-depleted portion of the mother liquor, and comprising a wax removal unit operated by direct steam distillation for removing wax from a hydrocarbon wax solution, which wax removal unit produces a gaseous hydrocarbon/steam mixture which is con densed and then separated, within a first liquid-liquid separator, into an aqueous phase and a hy drocarbon phase, and recycling at least a part of the isolated diluent produced in the diluent distillation unit to the polymerization reactor or the series of polymerization reactors, wherein at least a part of the hydrocarbon phase obtained in the first liquid-liquid separator is transferred to the diluent distillation unit without passing the evaporator for producing the wax-de pleted portion of the mother liquor.

[0007] In some embodiments, the part of the hydrocarbon phase obtained in the first liquid-liq uid separator, which is transferred to the diluent distillation unit without passing the evaporator for producing the wax-depleted portion of the mother liquor, is directly transferred to the diluent distil lation unit.

[0008] In some embodiments, the part of the hydrocarbon phase obtained in the first liquid-liq uid separator, which is transferred to the diluent distillation unit without passing the evaporator for producing the wax-depleted portion of the mother liquor, is transferred to a second liquid-liquid separator and therein separated into a second aqueous phase and a second hydrocarbon phase and the second hydrocarbon phase is transferred to the diluent distillation unit.

[0009] In some embodiments, the second liquid-liquid separator is equipped with a coalescer.

[0010] In some embodiments, the aqueous phase withdrawn from the second liquid-liquid sep aration vessel is returned to the first liquid-liquid separation vessel.

[0011] In some embodiments, the wet ethylene polymer particles obtained in the solid-liquid separator are dried by contacting the particles with a gas stream and thereby forming a gas stream carrying a hydrocarbon load, and thereafter separating the hydrocarbon load from the gas stream and thereby forming a liquid hydrocarbon recover stream, and at least a part of the hydro carbon recover stream is transferred to the diluent distillation unit without passing the evaporator for producing the wax-depleted portion of the mother liquor.

[0012] In some embodiments, the hydrocarbon recover stream transferred to the diluent distil lation unit is passed through a filter before being introduced into the diluent distillation unit. [0013] In some embodiments, the filter is a backwash filter. [0014] In some embodiments, the ethylene polymer particles, which are separated from the hydrocarbon recover stream by the backwash filter, are transferred by a part of the hydrocarbon recover stream to the suspension of ethylene polymer particles upstream of the solid-liquid sepa rator.

[0015] In some embodiments, the hydrocarbon recover stream is formed by contacting the gas stream carrying the hydrocarbon load with a cooled liquid phase in a scrubber and withdrawing the hydrocarbon recover stream from the bottom of the scrubber, and the cooled liquid phase which is introduced into the scrubber for being contacted with the gas stream carrying the hydro- carbon load is a cooled part of the hydrocarbon recover stream withdrawn from the bottom of the scrubber.

[0016] In some embodiments, the prepared ethylene polymer is a multimodal ethylene copoly mer.

[0017] In some embodiments, the preparation of the ethylene polymer is carried out in a series of polymerization reactors and an ethylene homopolymer is prepared in one of the polymerization reactors. [0018] In some embodiments, the ethylene homopolymer is prepared in the first polymerization reactor of the series of polymerization reactors and a copolymer of ethylene is prepared in a sub sequent polymerization reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 shows schematically a set-up for preparing ethylene polymers in suspension.

[0020] Figure 2 shows schematically a set-up for preparing ethylene polymers in suspension according to the process of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present disclosure provides a process for preparing an ethylene polymer in a sus pension polymerization in a polymerization reactor or in a series of polymerization reactors. The ethylene polymers are prepared by polymerizing ethylene or copolymerizing ethylene and one or more C3-Ci2-1-alkenes in the presence of a polymerization catalyst. The C3-Ci2-1 -alkenes may be linear or branched. Preferred C3-C12-I -alkenes are linear C3-C10- I -alkenes such as propylene, 1 -butene, 1 -pentene, 1 -hexene, 1 -heptene, 1 -octene, or 1 -decene or branched C2-C10-I -alkenes such as 4-methyl-1 -penten. It is also possible to polymerize ethylene with mixtures of two or more C3-C12-I -alkenes. Preferred comonomers are C3-C8-I -alkenes, in particular 1 -butene, 1 -pentene, 1 -hexene, 1 -heptene and/or 1 -octene. In prepared ethylene copolymers, the amount of units which are derived from incorporated comonomers is preferably from 0.01 wt.% to 25 wt.%, more preferably from 0.05 wt.% to 15 wt.% and in particular from 0.1 wt.% to 12 wt.%. Particular prefer ence is given to processes in which ethylene is copolymerized with from 0.1 wt.% to 12 wt.% of 1 -hexene and/or 1 -butene and especially from 0.1 wt.% to 12 wt.% of 1 -butene.

[0022] The polymerization can be carried out using all customary olefin polymerization cata lysts. That means the polymerization can be carried out, for example, using Phillips catalysts based on chromium oxide, using titanium-based Ziegler- or Ziegler-Natta-catalysts, using single site catalysts, or using mixtures of such catalysts. For the purposes of the present disclosure, sin gle-site catalysts are catalysts based on chemically uniform transition metal coordination com pounds. Furthermore, it is also possible to use mixtures of two or more of these catalysts for the polymerization of olefins. Such mixed catalysts are often designated as hybrid catalysts. The preparation and use of these catalysts for olefin polymerization are generally known.

[0023] Preferred catalysts are of the Ziegler type, preferably comprising a compound of tita nium or vanadium, a compound of magnesium and optionally an electron donor compound and/or a particulate inorganic oxide as a support material.

[0024] Catalysts of the Ziegler type are usually polymerized in the presence of a cocatalyst. Preferred cocatalysts are organometallic compounds of metals of Groups 1 , 2, 12, 13 or 14 of the Periodic Table of Elements, in particular organometallic compounds of metals of Group 13 and especially organoaluminum compounds. Preferred cocatalysts are for example organometallic alkyls, organometallic alkoxides, or organometallic halides.

[0025] Preferred organometallic compounds comprise lithium alkyls, magnesium or zinc alkyls, magnesium alkyl halides, aluminum alkyls, silicon alkyls, silicon alkoxides and silicon alkyl hal ides. More preferably, the organometallic compounds comprise aluminum alkyls and magnesium alkyls. Still more preferably, the organometallic compounds comprise aluminum alkyls, most pref erably trialkylaluminum compounds or compounds of this type in which an alkyl group is replaced by a halogen atom, for example by chlorine or bromine. Examples of such aluminum alkyls are trimethylaluminum, triethylaluminum, tri-isobutylaluminum, tri-n-hexylaluminum or diethylaluminum chloride or mixtures thereof.

[0026] The process of the present disclosure is a polymerization process carried out in sus pension. Such suspension polymerizations, which are also designated as slurry polymerizations, take place in a medium, the so-called suspension medium, which is in liquid or in supercritical state under the conditions in the respective polymerization reactor and in which the produced eth ylene polymer is insoluble and forms solid particles. The solids content of the suspension is gen erally in the range of from 10 to 80 wt. %, preferably in the range of from 20 to 40 wt. %.

[0027] The suspension medium, which forms the liquid or supercritical phase of the suspen sion, commonly comprises as main component a diluent but also comprises further components like, for example, dissolved monomers or comonomers, dissolved cocatalysts or scavengers like aluminum alkyls, dissolved reaction auxiliaries like hydrogen or dissolved reaction products of the polymerization reaction like oligomers or waxes. Suitable diluents should be inert, i.e. should not decompose under reaction conditions. Such diluents are for example hydrocarbons having from 3 to 12 carbon atoms, and in particular saturated hydrocarbons like isobutane, butane, propane, isopentane, pentane, hexane or octane, or a mixture of these. In preferred embodiments, the dil uent is a hydrocarbon mixture. For producing hydrocarbon mixtures from a raw material, a less high demand for separating the components of the raw material is needed than for producing a specific hydrocarbon, and accordingly hydrocarbon mixtures are economically more attractive as diluent, but show the same diluent performance as specific hydrocarbons. However, hydrocarbon mixtures may have a boiling point range.

[0028] The diluent preferably has a boiling point which is significantly different from those of the monomers and comonomers used in order to make it possible for these starting materials to be recovered from a mixture by distillation. Such diluents are for example hydrocarbons having a boiling point above 40°C or even above 60°C or mixtures comprising a high proportion of these hydrocarbons. Thus, in preferred embodiments of the present disclosure, the polymerization takes place in a liquid suspension medium comprising more than 50 wt.% of saturated hydrocar bons having a boiling point of above 60°C at 0.1 MPa or even comprising more than 80 wt.% of saturated hydrocarbons having a boiling point of above 60°C at 0.1 MPa.

[0029] The process of the present disclosure can be carried out using all industrially known suspension polymerization processes at temperatures in the range from 40 to 150°C, preferably from 50 to 130°C and particularly preferably from 60 to 90°C, and under pressures of from 0.1 to 20 MPa and particularly preferably from 0.3 to 5 MPa. [0030] In preferred embodiments of the present disclosure, the polymerization is carried out in a series of at least two polymerization reactors which are connected in series. These reactors are not restricted to any specific design; preferably these reactors are however loop reactors or stirred tank reactors. There is no limit to the number of reactors of such a series, however preferably the series consist of two, three or four reactors and most preferably of two or three reactors. If a se ries of polymerization reactors is used in the process of the present disclosure, the polymerization conditions in the polymerization reactors can differ, for example by the nature and/or the amount of comonomers or by different concentrations of polymerization auxiliaries such as hydrogen. Preferably, the polymerization is a suspension polymerization in stirred tank reactors.

[0031] The ethylene polymers are usually obtained as powder, that means in form of small particles. The particles which have usually a more or less regular morphology and size, depend ing on the catalyst morphology and size, and on polymerization conditions. Depending on the cat alyst used, the particles of the polyolefin powder usually have a mean diameter of from a few hun dred to a few thousand micrometers. In the case of chromium catalysts, the mean particle diame ter is usually from about 300 to about 1600 pm, and in the case of Ziegler type catalysts the mean particle diameter is usually from about 50 to about 3000 pm. Preferred polyolefin powders have a mean particle diameter of from 100 to 250 pm. The particle size distribution can, for example, ad vantageously be determined by sieving. Suitable technics are e.g. vibrating sieve analysis or sieve analysis under an air jet.

[0032] The density of preferred ethylene polymers obtained by the process of the present dis closure is from 0.90 g/cm ³ to 0.97 g/cm ³ . Preferably the density is in the range of from 0.920 to 0.968 g/cm ³ and especially in the range of from 0.945 to 0.965 g/cm ³ . The density has to be un derstood as being the density determined according to DIN EN ISO 1 183-1 :2004, Method A (Im mersion) with compression molded plaques of 2 mm thickness which were pressed at 180°C, 20MPa for 8 minutes with subsequent crystallization in boiling water for 30 minutes.

[0033] In preferred embodiments, the polyethylenes prepared by the process of the present disclosure have a MFR21.6 at a temperature of 190°C under a load of 21 .6 kg, determined accord ing to DIN EN ISO 1 133:2005, condition G, from 0.5 to 300 g/10 min, more preferably from 1 to 100 g/10 min, even more preferably of from 1 .2 to 100 g/10 min and in particular from 1 .5 to 50 g/10 min.

[0034] The ethylene polymers obtained by the process of the present disclosure can be mono- modal, bimodal or multimodal ethylene polymers. Preferably, the ethylene polymers are bimodal or multimodal ethylene polymers. The term "multimodal" hereby refers to the modality of the ob tained ethylene copolymer and indicates that the ethylene copolymer comprises at least two frac tions of polymer which are obtained under different reaction conditions, independently whether this modality can be recognized as separated maxima in a gel permeation chromatography (GPC) curve or not. The different polymerization conditions can, for example, be achieved by using dif ferent hydrogen concentrations and/or by using different comonomer concentrations in different polymerization reactors. Such polymers can be obtained from polymerizing olefins in a series of two or more polymerization reactors under different reaction conditions. It is however also possi ble to obtain such bimodal or multimodal polyolefins by employing mixed catalysts. In addition to the molecular weight distribution, the polyolefin can also have a comonomer distribution, wherein preferably the average comonomer content of polymer chains with a higher molecular weight is higher than the average comonomer content of polymer chains with a lower molecular weight.

The term "multimodal" as used herein shall include also "bimodal".

[0035] In preferred embodiments of the process of the present disclosure, the polymerization is carried out in a series of polymerization reactors and an ethylene homopolymer, preferably a low molecular weight ethylene homopolymer, is prepared in the first polymerization reactor and an ethylene copolymer, preferably a high molecular weight ethylene copolymer is prepared in a sub sequent polymerization reactor. To be able to prepare an ethylene homopolymer in the first polymerization reactor, no comonomer is fed to the first polymerization reactor, neither directly nor as component of a feed stream or a recycle stream which is introduced into the first polymeriza tion reactor of the series of polymerization reactors. The so obtained multimodal ethylene copoly mers preferably comprise from 35 to 65 % by weight of ethylene homopolymer prepared in the first polymerization reactor and from 35 to 65 % by weight of ethylene copolymer prepared in the subsequent polymerization reactor. If the series of polymerization reactors comprises one are more prepolymerization reactors, the prepolymerization is preferably carried without adding comonomers.

[0036] In other preferred embodiments of the present disclosure, the ethylene polymer is pre pared in a series of three polymerization reactors, i.e. in a first polymerization reactor and two subsequent polymerization reactors, wherein the ethylene polymer prepared in the first polymeri zation reactor is an ethylene homopolymer, preferably a low molecular weight ethylene homopoly mer, the polyethylene prepared in one of the subsequent polymerization reactors is an ethylene copolymer, preferably a high molecular weight copolymer, and the polyethylene prepared in the other subsequent polymerization reactor is an ethylene copolymer of a higher molecular weight, preferably an ultrahigh molecular weight copolymer. The so obtained multimodal ethylene copoly mers preferably comprise from 30 to 60 % by weight, more preferably from 45 to 55 % by weight of ethylene homopolymer prepared in the first polymerization reactor, from 30 to 65 % by weight, more preferably from 20 to 40 % by weight of ethylene copolymer prepared in one subsequent polymerization reactor, and from 1 to 30 % by weight, more preferably from 15 to 30 % by weight of higher molecular weight ethylene copolymer prepared in the other subsequent polymerization reactor. [0037] In the process of the present disclosure, the suspension of ethylene polymer formed in the polymerization reactor or the series of polymerization reactors is transferred into a solid-liquid separator, in which the ethylene polymer particles are separated from the suspension medium. This separation into ethylene polymer particles and mother liquor can be carried out in all suitable separation apparatuses such as centrifuges, decanters, filters or combinations thereof. Prefera bly, the solid-liquid separator is a centrifuge. In preferred embodiments of the present disclosure, the suspension withdrawn from the polymerization reactor or the series of polymerization reactors is first transferred into a separator feed vessel and then conveyed from the separator feed vessel to the solid-liquid separator.

[0038] The term "mother liquor" as used herein refers to the separated suspension medium obtained from the suspension in the solid-liquid separator. Typically, the mother liquor comprises more than 70 wt. % of diluent. Further components of the mother liquor include ethylene, comon omers, aluminum alkyls, and hydrogen and dissolved reaction products like oligomers and waxes. The term "oligomer" as used herein refers to hydrocarbons which have a higher molecular weight than the diluent and the comonomer or the comonomers and which are liquid at standard pres sure and room temperature. The term "wax" as used herein refers to hydrocarbons which are solved in the mother liquor and which are, after evaporation of the diluent and the oligomers, solid at standard pressure and room temperature. In preferred embodiments, the mother liquor com prised from 75 to 99 wt.%, preferably from 80 to 98 wt.% diluent; from 1 to 20 wt.%, preferably from 2 to 10 wt.% oligomers; from 0 to 5 wt.%, preferably from 0.3 to 3 wt.% comonomer or comonomers; from 0 to 3 wt.%, preferably from 0.2 to 2 wt.% waxes, from 0 to 1 wt.%, preferably from 0.001 to 0.1 wt.% ethylene, from 0 to 0.1 wt.% hydrogen and from 0.1 to 2.5 mmol/l alumi num alkyls or reaction products of aluminum alkyls.

[0039] The ethylene polymer particles obtained in the solid-liquid separator still contain sus pension medium, i.e. the ethylene polymer particles are "wet". The wet ethylene polymer particles typically contain from 15 wt.% to 40 wt.% and preferably from 20 wt.% to 35 wt.% of suspension medium, i.e. of mother liquor. In preferred embodiments of the present disclosure, the separated ethylene polymer particles are dried by contacting the particles with a gas stream, thereby forming a gas stream carrying a hydrocarbon load. The drying can occur in any kind of suitable equip ment in which a powder can be contacted, and preferably be countercurrently contacted with a gas stream for taking up suspension medium attached to or included in the ethylene polymer par ticles. Such powder dryers are preferably operated with hot nitrogen which is circulated in a closed loop. The drying is preferably carried out in a two-stage dryer in which residual suspen sion medium is stripped from the ethylene polymer particles in two subsequent stages. The dried ethylene polymer particles are preferably pneumatically conveyed to an extrusion section in which suitable amounts of additives are added and the mixture is molten, homogenized and pelletized.

In preferred embodiments, the dried ethylene polymer particles coming from the powder dryer first pass a degassing unit and/or a powder treatment unit before being conveyed to the extrusion sec tion.

[0040] In preferred embodiments, the hydrocarbon load taken up by the gas stream for drying the wet ethylene polymer particles is subsequently separated from the gas stream, thereby form ing a liquid hydrocarbon recover stream. Such separation can, for example, occur by condensa tion. In preferred embodiments, the hydrocarbon load is separated from the gas stream in a scrubber into which a cooled liquid phase is introduced. In such an embodiment, the liquid phase which has taken up the hydrocarbon load from the gas stream coming from the dryer forms the liquid hydrocarbon recover stream. The liquid hydrocarbon recover stream is withdrawn from the bottom of the scrubber and a part of this stream is returned to the scrubber as cooled liquid phase after having passed a cooler. At least a part of the remaining portion of the liquid hydrocarbon re cover stream withdrawn from the bottom of the scrubber is returned to the polymerization process.

[0041] Preferably, the gas stream coming from the dryer passes a particle removing equip ment such as a filter or a cyclone before the hydrocarbon load is separated from the gas stream.

[0042] Preferably, most of the mother liquor is recycled to the polymerization reactor or the se ries of polymerization reactors. If the polymerization is carried out in a series of polymerization reactors, preferably all polymerization reactors of the series of polymerization reactors are pro vided with recycled parts of the mother liquor. Preferably, from 90 to 99.99 wt.%, more preferably from 95 to 99,5 wt.%, and in particular from 98 to 99 wt.% of the mother liquor are recycled to the polymerization reactor or the series of polymerization reactors. The parts of the mother liquor which are not recycled include, for example, continuous flushings of pumps, off-gas which may be vented to purge gaseous impurities of the feed streams or gaseous by-products of the polymeriza tion process or dissolved reaction products like waxes which are intentionally removed from the mother liquor.

[0043] The mother liquor is preferably first fed to a mother liquor collecting vessel and, for be ing recycled to the polymerization process, the mother liquor is withdrawn from the mother liquor collecting vessel.

[0044] The mother liquor can be recycled to the polymerization reactor or the series of polymerization reactors as such, or the mother liquor can be recycled in worked-up form. In the process of the present disclosure, a part of the mother liquor is worked up in a work-up section for producing worked-up components of the mother liquor. Work-up in the context of the present dis closure means that a composition is separated in one or more separated compositions or that one or more components of a composition are removed from the composition and a purified composi tion void of or at least substantially depleted in the removed component(s) is obtained. A work-up can however even go so far that individual components of the composition are isolated. Such a work-up process can, for example, include removing components of the mother liquor, which have a significantly lower boiling point than the diluent, for example ethylene and/or 1 -butene, from the mother liquor, and/or removing components of the suspension medium, which have a significantly higher boiling point than the diluent, for example oligomers and/or waxes, from the mother liquor. The amount of mother liquor which passes the work-up section is preferably from 1 to 90 wt.%, more preferably from 5 to 80 wt.% of the mother liquor recycled to the series of polymerization re actors. In preferred embodiments of the work-up process, the mother liquor is separated in two or more components which are recycled to the polymerization in individual recycle circuits. After the separation, each of the individual recycle circuits may comprise further purification steps. Compo nents of the mother liquor, which may be recycled to the series of polymerization reactors in indi vidual recycle circuits, can be, besides the diluent, ethylene and comonomers. The worked-up components of the mother liquor can be transferred to any polymerization reactor used for carry ing out the polymerization of the present disclosure.

[0045] The work-up section for working up the mother liquor according to the present disclo sure comprises an evaporator for producing a wax-depleted portion of the mother liquor. In the process of the present disclosure, a part of the mother liquor to be worked up is transferred into the evaporator and separated into lower boiling point components, which are withdrawn in gase ous form from the evaporator, and a remaining part depleted in lower boiling point components. The evaporated part withdrawn from the evaporator is subsequently preferably condensed and transferred as wax-depleted portion of the mother liquor to the diluent distillation unit. The liquid phase depleted in lower boiling point components which is withdrawn from the evaporator is pref erably conveyed as hydrocarbon wax solution to a wax removal unit. The evaporator is preferably a circulation evaporator which is operated by circulating the liquid phase to be evaporated through an external heat exchanger in which the liquid phase is partly vaporized.

[0046] For further working up the wax-depleted portion of the mother liquor, the work-up sec tion comprises a diluent distillation unit, which produces isolated diluent from the wax-depleted portion of the mother liquor. In the diluent distillation unit, preferably lower boiling point compo nents of the mother liquor are separated from the diluent by a distillation step. Typically, lower boiling point components are ethylene, hydrogen, those comonomers, which have a lower or a similar boiling point than the used diluent, and parts of the diluent. This means, when, for exam ple, n-hexane or a mixture of hexane isomers is used as diluent and 1 -butene is used as comono mer, the majority of the 1 -butene comprised in the mother liquor forms a part of the lower boiling point components separated from the diluent in the diluent distillation unit. Preferably, most of the lower boiling point components of the mother liquor are recycled to the polymerizations in individ ual recycle circuits, most preferably after having passed one or more further work-up steps.

[0047] According to the process of the present disclosure, it is also possible to separate higher boiling point components of the mother liquor from the diluent by a distillation step. Such higher boiling point components can be comonomers, which have a boiling point similar to or higher than that of the used diluent. Preferably, most of the higher boiling point components obtained by the distillation are recycled to the polymerizations, most preferably after having passed one or more further work-up steps.

[0048] The isolated diluent produced within the diluent distillation unit is at least partly recycled to the polymerization reactor or the series of polymerization reactors. In preferred embodiments of the present disclosure, the isolated diluent is recycled to a polymerization reactor in which an ethylene homopolymer is prepared.

[0049] Preferably, from 5 to 70 wt.% of the mother liquor, which is recycled to the polymeriza tion reactor or the series of polymerization reactors, are recycled as isolated diluent to the polymerization reactor or the series of polymerization reactors, more preferably from 10 to 60 wt.% and in particular from 15 to 50 wt.% of the mother liquor are recycled as isolated diluent to the polymerization reactor or the series of polymerization reactors.

[0050] In preferred embodiments of the present disclosure, the isolated diluent passes one or more purification steps, thereby obtaining a purified diluent, before being recycled to the polymeri zation reactor or the series of polymerization reactors. These diluent purification steps can be, for example, a purification by adsorption, a purification by absorption, a catalytic hydrogenation, or a purification by a membrane purification process.

[0051] In the process of the present disclosure, the work-up section for working up the mother liquor further comprises a wax removal unit for removing wax from a hydrocarbon wax solution which is operated by direct steam distillation, i.e. by injecting steam directly into a vessel compris ing a hydrocarbon wax solution from which wax is removed. By injecting steam, a gaseous hydro carbon/steam mixture, which comprises primarily diluent and steam, is produced in the wax re moval unit. Wax withdrawn from the polymerization process may be combusted for generating energy or may be sold as by-product of the polymerization process.

[0052] The hydrocarbon wax solution from which the wax is removed is preferably not mother liquor as obtained in the solid-liquid separator but a liquid phase which has been obtained by with drawing lower boiling point components from the mother liquor. This separation of mother liquor in lower boiling point components and a remaining part depleted in lower boiling point compo nents preferably occurs by an evaporating step in which a part of the mother liquor is evaporated. Preferably, the evaporated part of the mother liquor obtained in the evaporating step is subse quently condensed and transferred as wax-depleted portion of the mother liquor to the diluent dis tillation unit. The remaining liquid phase depleted in lower boiling point components is preferably conveyed as hydrocarbon wax solution to the wax removal unit. [0053] The gaseous hydrocarbon/steam mixture obtained in the wax removal unit is withdrawn from the wax removal unit, condensed and transferred into a first liquid-liquid separator, in which the mixture is separated in an aqueous phase and a hydrocarbon phase. The aqueous phase ob tained in the first liquid-liquid separator is preferably discarded.

[0054] Parts of the hydrocarbon phase obtained in the first liquid-liquid separator may be recy cled to a stage of the mother liquor work-up process upstream of the evaporator for producing the wax-depleted portion of the mother liquor, for example by adding to the mother liquor transferred into the work-up section. However, according to the process of the present disclosure, at least a part of the hydrocarbon phase obtained in the first liquid-liquid separator is transferred to the dilu ent distillation unit without passing the evaporator for producing the wax-depleted portion of the mother liquor. By not adding a part of the hydrocarbon phase obtained in the first liquid-liquid separator to the mother liquor upstream of a step, in which the mother liquor is depleted in lower boiling point components, but transferring the hydrocarbon phase obtained in the first liquid-liquid separator to the diluent distillation unit, it is possible to decrease the amount of liquid to be evapo rated in order produce a wax-depleted portion of the mother liquor and hence to operate the pro cess in an economical and energy efficient manner. Furthermore, this also allows to design the evaporator or an equivalent equipment smaller without having a decline in the amount of pro duced isolated diluent. Transferring the hydrocarbon phase obtained in the first liquid-liquid sepa rator to the diluent distillation unit without passing the evaporator for producing the wax-depleted portion of the mother liquor is possible because the hydrocarbon phase obtained in the first liquid- liquid separator has previously been evaporated and was then condensed.

[0055] In preferred embodiments of the present disclosure, the part of the hydrocarbon phase obtained in the first liquid-liquid separator, which is transferred to the diluent distillation unit with out passing the evaporator for producing the wax-depleted portion of the mother liquor, is directly transferred to the diluent distillation unit.

[0056] In other preferred embodiments of the present disclosure, the part of the hydrocarbon phase obtained in the first liquid-liquid separator, which is transferred to the diluent distillation unit without passing the evaporator for producing the wax-depleted portion of the mother liquor, is passed through a second liquid-liquid separator and therein further depleted in water content by separating into a second aqueous phase and a second hydrocarbon phase and the second hy drocarbon phase is transferred to the diluent distillation unit. The aqueous phase withdrawn from the second liquid-liquid separation vessel is preferably returned to the first liquid-liquid separation vessel. By passing the hydrocarbon phase obtained in the first liquid-liquid separator through a second liquid-liquid separator, an improved separator efficiency of the separation step into a hy drocarbon phase and an aqueous phase can be achieved. For example, possible disturbances within the first liquid-liquid separator with regard to the separation performance may be caught up by the second liquid-liquid separator, preventing a breakthrough of too much water into the distil lation unit and hence affecting the quality of the isolated diluent exiting the distillation unit.

[0057] In preferred embodiments, the second liquid-liquid separator is equipped with a coales cer. A coalescer is a technological device which is primarily used to facilitate the separation of emulsions, such as hydrocarbon/water mixtures, into their components.

[0058] In further preferred embodiments, the first liquid-liquid separator or the second liquid- liquid separator or, preferably, the first liquid-liquid separator and the second liquid-liquid separa tor are equipped with level and separation layer measurement devices which use high frequency microwave impulses along a rod sensor which are reflected from the respective layer surface.

The time between sending and receiving is being used for the level indication.

[0059] In preferred embodiments of the present disclosure, the wet ethylene polymer particles obtained in the solid-liquid separator are dried by contacting the particles with a gas stream and thereby forming a gas stream carrying a hydrocarbon load, and thereafter the hydrocarbon load is separated from the gas stream and thereby forming a liquid hydrocarbon recover stream, and at least a part of the hydrocarbon recover stream is transferred to the diluent distillation unit without passing the evaporator for producing the wax-depleted portion of the mother liquor. Such a trans fer of the hydrocarbon recover stream as such to the diluent distillation unit further decreases the amount of liquid to be evaporated in order to produce a wax-depleted portion of the mother liquor, and hence reduces the energy consumption in this unit.

[0060] Preferably, the hydrocarbon recover stream is first passed through a filter before being introduced into the diluent distillation unit in order to prevent fine ethylene polymer particles, which are entrained in the drying gas coming from the powder dryer and end-up in the hydrocarbon re cover stream, from being transferred into the diluent distillation unit. The filter is preferably a backwash filter, more preferably an automatic backwash filter. Such self-cleaning filters may uti lize backwashing to clean the screen without substantially interrupting of the system flow. Eth ylene polymer particles, which are separated from the hydrocarbon recover stream by the back wash filter, are preferably transferred by a part of the hydrocarbon recover stream to the suspen sion of ethylene polymer particles before being fed into the solid-liquid separator, and are more preferably transferred to the separator feed vessel.

[0061] Figure 1 shows schematically a set-up for preparing multimodal ethylene polymers in a series of three polymerization reactors by a suspension polymerization process using hexane as diluent.

[0062] For homopolymerizing ethylene or copolymerizing ethylene with other olefins in a first polymerization reactor (1 ) in suspension, recycled hexane is fed to the reactor (1 ) via feeding line (2). The other components of the reaction mixture like catalyst, ethylene and polymerization auxil iaries and optional components like possible comonomers and/or directly recycled mother liquor are fed to the reactor via one or more feeding lines (3).

[0063] As result of the polymerization in reactor (1 ), a suspension of solid ethylene polymer particles in a suspension medium is formed. This suspension is fed via line (4) to the second polymerization reactor (5) where further polymerization occurs. Fresh comonomer or further com ponents of the reaction mixture can be fed to reactor (5) via one or more feeding lines (6). Di rectly recycled mother liquor can be fed to reactor (5) via line (43).

[0064] The suspension of reactor (5) is thereafter fed via line (7) to the third polymerization re actor (8) in which additional polymerization is carried out. One or more feeding lines (9) allow supplementary feeding of comonomer or further components of the reaction mixture to reactor (8). Within polymerization reactor (8), further polymerization occurs. Directly recycled mother liquor can be fed to reactor (8) via line (44).

[0065] The suspension of ethylene polymer particles in the suspension medium formed in re actor (8) is continuously transferred via line (10) to separator feed vessel (1 1 ). The suspension is then passed via line (12) to centrifuge (13), where the suspension is separated in solid ethylene polymer particles and mother liquor, i.e. recovered liquid suspension medium.

[0066] The isolated wet ethylene polymer particles are transferred via line (20) to a powder dryer (21 ). For removing residual parts of the suspension medium, which is primarily hexane, from the ethylene polymer particles, hot nitrogen is introduced into powder dryer (21 ) through one or more lines (22). Nitrogen, which has taken up the evaporated parts of the suspension medium, primarily hexane, is conveyed via line (23) to a cyclone (24) and then via line (25) to scrubber (26). For recovering hexane from the nitrogen stream conveyed to scrubber (26), a cooled liquid phase is fed into scrubber (26) at position (27). The introduced liquid is then withdrawn from scrubber (26) together with washed-out hexane through line (28) forming a liquid hexane recover stream. A part of this hexane recover stream is recycled by pump (29) via lines (30) and (31 ) through heat exchanger (32) to scrubber (26) for being fed at position (27). The remainder of the hexane recover stream is transferred via line (33) to separator feed vessel (1 1 ). Nitrogen de pleted in hexane leaves scrubber (26) at the top via line (34). Dried ethylene polymer particles are discharged from powder dryer (21 ) via line (36) and conveyed to a vessel for treating the eth ylene polymer particles with a nitrogen/steam gas mixture and finally to a pelletizing unit (not shown).

[0067] The mother liquor obtained in centrifuge (13) is transferred via line (40) to a mother liq uor collecting vessel (41 ). From there, mother liquor can be recycled by means of pump (42) via lines (43) and (44) to polymerization reactor (5) and/or polymerization reactor (8). [0068] For work-up, mother liquor is branched-off line (43) and transferred via line (50) to mother liquor tank (51 ). From there, mother liquor is transferred by means of pump (52) via line

(53) to an evaporator (54). Line (55) which branches off line (50) allows to feed mother liquor di rectly from mother liquor collecting vessel (41 ) to evaporator (54) without passing mother liquor tank (51 ).

[0069] Evaporator (54) is designed as circulation evaporator which is operated by circulating the liquid phase to be evaporated through heat exchanger (56), in which the liquid phase is partly vaporized. The vaporized fractions of the mother liquor are withdrawn from the top of evaporator

(54) via line (57), condensed in heat exchanger (58) and transferred into a distillation column feeding vessel (59). From there, the condensed vaporized fractions of the mother liquor are pumped by means of pump (60) via line (61 ) into a distillation column (62).

[0070] Distillation column (62) is operated by branching off a part of the bottom stream with drawn from distillation column (62) via line (63) and passing the branched-off part of the bottom stream through heat exchanger (64), therein partly vaporizing this stream and returning the heated and partly vaporized part of the bottom stream via line (65) to distillation column (62). The remainder of the bottom stream withdrawn from distillation column (62), which is predominantly hexane, is pumped by pump (66) via line (67) to a hexane purification unit (68) and then via line (69) to a hexane collecting vessel (70). From there, the purified hexane is recycled by means of pump (71 ) via line (2) as diluent to polymerization reactor (1 ). Low boiling point components are withdrawn from the top of distillation column (62) via line (72).

[0071] A liquid phase enriched in higher boiling point fractions of the mother liquor is with drawn from the bottom of evaporator (54) via line (80) and transferred to a wax separator vessel (81 ) which is operated by injecting steam via line (82) directly into wax separator vessel (81 ). Liq uid wax is withdrawn from the bottom of wax separator vessel (81 ) via line (83) and conveyed to an incineration unit (not shown), e.g. for generation of steam, or to a solidifying unit or liquid ship ping unit (not shown) for being sold.

[0072] The gaseous fractions obtained in wax separator vessel (81 ), which comprise primarily hexane and water, are withdrawn from the top of wax separator vessel (81 ) via line (84), con densed in heat exchanger (85) and transferred into a liquid-liquid separation vessel (86). Further condensed hexane/water mixtures obtained at other positions of the polymerization process, for example the condensed hexane/water mixture withdrawn from the powder treatment vessel, can be transferred into the liquid-liquid separation vessel (86) via line (91 ).

[0073] Water is withdrawn from the liquid-liquid separation vessel (86) via line (87) and trans ferred to a waste water system (not shown). The hexane phase is withdrawn from the liquid-liquid separation vessel (86) via line (88) and transferred by means of pump (89) via line (90) to mother liquor tank (51 ).

[0074] Figure 2 shows schematically a set-up for preparing ethylene polymers in suspension according to the process of the present disclosure.

[0075] The process depicted in Figure 2 is a process for preparing multimodal ethylene poly mers in a series of three polymerization reactors using hexane as diluent identical with the pro cess shown in Figure 1 except that the hexane phase withdrawn from the liquid-liquid separation vessel (86) via line (88) is not transferred by means of pump (89) via line (90) to mother liquor tank (51 ) but is transferred by means of pump (89) via a line (92) to a second liquid-liquid separa tion vessel (93). Water is withdrawn from the second liquid-liquid separation vessel (93) via line (94) and returned to the liquid-liquid separation vessel (86). The hexane phase is withdrawn from the second liquid-liquid separation vessel (93) via line (95) and the majority of this hexane phase is transferred via line (96) directly to the distillation column (62). Line (90) offers a possibility to transfer the hexane phase withdrawn from the second liquid-liquid separation vessel (93) to mother liquor tank (51 ).

[0076] The process of the present disclosure allows in an economical and energy efficient manner to work up the mother liquor, which is obtained by removing the prepared ethylene poly mer particles from a suspension obtained in a suspension polymerization process for preparing ethylene polymers, to give purified diluent by minimizing the amount of mother liquor that has to be evaporated.

EXAMPLES

Comparative Example A

[0077] Ethylene was polymerized in a continuous process in three reactors arranged in series as shown in Figure 1 . A Ziegler catalyst component prepared as specified in WO 91 /1 8934, Ex ample 2 and having there the operative number 2.2, was fed into the first reactor (1 ) in an amount of 2.6 mol/h with respect to the titanium content of the catalysts compound, together with 69 mol/h of triethylaluminum, as well as sufficient amounts of hexane as diluent, ethylene, and hydrogen. The amount of ethylene (= 24.1 t/h) and the amount of hydrogen (= 21 .7 kg/h) were adjusted so that the percentage proportion of ethylene and of hydrogen measured in the gas space of the first reactor were 20% by volume of ethylene and 62% by volume of hydrogen and the remainder was a mix of nitrogen and vaporized diluent.

[0078] The polymerization in the first reactor (1 ) was carried out at 84°C. [0079] The suspension from the first reactor (1 ) was then transferred into the second reactor (5), in which the percentage proportion of hydrogen in the gas phase had been reduced to 0.5% by volume, and an amount of 12.8 t/h of ethylene was added to this reactor together with an amount of 397 kg/h of 1 -butene. The amount of hydrogen was reduced by way of intermediate H2 depressurization. 64 % by volume of ethylene, 0.5% by volume of hydrogen, and 6.0% by volume of 1 -butene were measured in the gas phase of the second reactor, the remainder being a mix of nitrogen and vaporized diluent.

[0080] The polymerization in the second reactor (5) was carried out at 85°C.

[0081] The suspension from the second reactor (5) was transferred to the third reactor (8) us ing further intermediate H2 depressurization to reduce the amount of hydrogen to less than 0.08% by volume in the gas space of the third reactor.

[0082] An amount of 10.3 t/h of ethylene was added to the third reactor (8) together with an amount of 476 kg/h of 1 -butene. 72% by volume of ethylene, 0.05% by volume of hydrogen, and 6.8% by volume of 1 -butene were measured in the gas phase of the third reactor, the remainder being a mix of nitrogen and vaporized diluent.

[0083] The polymerization in the third reactor (8) was carried out at a pressure at 84°C.

[0084] The suspension from the third reactor (8) was transferred to separator feed vessel (1 1 ).

[0085] The work-up of the suspension discharged from the separator feed vessel (1 1 ) via line (12) occurred as depicted in Figure 1 . The suspension was separated in centrifuge (13) in solid ethylene polymer particles and mother liquor. The isolated wet ethylene polymer particles were dried in powder dryer (21 ) by a stream of hot nitrogen. Evaporated parts of the hexane, which were removed from the wet ethylene polymer particles in powder dryer (21 ), were recovered by washing in scrubber (26) the gas stream leaving the powder dryer (21 ). The recovered liquid phase was transferred back to separator feed vessel (1 1 ) via line (33).

[0086] The mother liquor obtained in centrifuge (13) was partly recycled to the second and third polymerization reactors (5) and (8) and partly worked up by being transferred via line (53) to an evaporator (54), in which the liquid phase is partly vaporized. Evaporator (54) is designed as circulation evaporator which is operated by circulating the liquid phase to be evaporated through a heat exchanger (56). The vaporized fractions of the mother liquor are transferred via line (57) into a distillation column feeding vessel (59), and from there by means of a pump (60) via line (61 ) into a distillation column (62), in which low boiling point components are separated off. A part of the bottom stream of distillation column (62) which is predominantly hexane is passed through a hex ane purification unit (68) and transferred to the first polymerization reactor (1 ) via line (2). [0087] A liquid phase enriched in higher boiling point fractions of the mother liquor was with drawn from the bottom of evaporator (54) via line (80) and transferred to a wax separator vessel (81 ) which was operated by directly injecting steam through line (82). The gaseous fractions ob tained in wax separator vessel (81 ) were condensed and transferred into a liquid-liquid separation vessel (86) for being separated into water and a hexane phase. The obtained hexane phase was combined with the parts of the mother liquor to be worked up and transferred via line (53) to evap orator (54).

[0088] The flow rates and compositions of selected streams in the mother liquor work-up pro- cess are presented in Table 1 .

Table 1

[0089] For evaporating the fractions of the mother liquor to be transferred into the distillation column (62), heat exchanger (56) had to be provided with steam having a thermal output of 7433 kW. Example 1

[0090] The polymerization of Comparative Example A was repeated, however the work-up of the suspension discharged from the separator feed vessel (1 1 ) via line (12) occurred as depicted in Figure 2.

[0091] In contrast to the work-up carried out in Comparative Example A, the hexane phase withdrawn from the liquid-liquid separation vessel (86) was transferred to a second liquid-liquid separation vessel (93). The hexane phase was withdrawn from the second liquid-liquid separa tion vessel (93) was not transferred completely to mother liquor tank (51 ) but the majority of the hexane phase was transferred via line (96) into distillation column feeding vessel (59) and only a small portion of the hexane phase withdrawn from the second liquid-liquid separation vessel (93) was transferred to mother liquor tank (51 ) via line (90).

[0092] The flow rates and compositions of selected streams in the mother liquor work-up pro- cess are presented in Table 2.

Table 2

[0093] For evaporating the fractions of the mother liquor to be transferred into the distillation column (62), heat exchanger (56) had to be provided with steam having a thermal output of 6910 kW. With respect to Comparative Example A, the reduced flow of mother liquor to heat ex- changer (56) allowed a 7% reduction of energy supplied to heat exchanger (56) without a reduc tion in the throughput of the distillation column (62).