Enhanced downstream recovery of olefin polymers from polymer solution

Abstract

The invention involves separating vapor components from an olefin polymer solution through four stages: (i) heating the solution in a first heat exchanger to 70-250°C, (ii) pre-separation by flashing in a first phase separator to remove a first fraction of gaseous components, resulting in a vapor-reduced concentrated solution, (iii) heating this concentrated solution in a second heat exchanger to 120-250°C, and (iv) final separation to remove a second fraction of gaseous components using one or more volatilization chambers operating at decreasing pressure.

Description

FE7656-EP-P1TITLE: “ENHANCED DOWNSTREAM RECOVERY OF OLEFIN POLYMERS FROM POLYMER SOLUTION"THE TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a process for separating vapor components from an olefin polymer solution, an integrated process for the production of olefin polymer, a separation unit for separating vapor components from an olefin polymer solution produced in a solution olefin polymerization reaction step and an integrated unit for the production of olefin polymer.BACKGROUND OF THE INVENTION

[0002] Continuous solution olefin polymerization processes generally entail the preparation of a solution of the olefin monomer(s) in a solvent that is subsequently contacted with a catalyst for polymerization. The liquid reaction medium comprises the olefin monomer(s) and generally, depending on the specific olefin to be polymerized, an inert hydrocarbon solvent useful to favor the solubility of the polyolefin in the liquid medium.

[0003] The polymerization of 1 -butene, as known in the art, can occur in the absence of a polymerization inert solvent, since poly-1 -butene is completely soluble in the liquid monomer at the usual polymerization conditions, thus leading to the formation of a polymeric solution. However other olefins, such as for instance ethylene, propylene or higher alpha-olefins, have so far been polymerized in solution phase in the presence of a suitable inert solvent, such as cyclohexane, n-hexane, n-heptane, 2-methylpentane or Isopar™ E (isoparaffinic hydrocarbon), as the obtained polyolefins are not soluble in the liquid monomer at the usual polymerization conditions.

[0004] Highly viscous polymeric solutions are generally discharged from the polymerization reactor. After the discharge from the polymerization reactor, the polymeric solution has to be transferred to a downstream section, which effects the separation of the obtained polyolefin from the unreacted monomers and the inert solvent. This operation is usually carried out by progressively flashing the reaction bath solution in multiple stages in series. Each stage consists of two consecutive phases: In the first phase, the polymer solution is heated to the devolatilization temperature (70-250°C). In the second phase, pressure expansion leads to the actual volatilization of the solvent and the unreacted monomers. Such process is described in EP 2 072 540 A1.

[0005] One of the most critical aspects is the efficiency of the heating phase of the first devolatilization unit, where the polymer solution is heated up from the reaction temperature to the devolatilization temperature. During this operation, as the polymer solution partially vaporizes, the bubbles that are progressively generated remain entrapped in the surrounding polymer melt, which gets progressively more viscous. This unit operation is therefore characterized by generally poor efficiency.FE7656-EP-P1

[0006] Behind this background, an object of the present invention is how the efficiency of a process for producing olefin polymer, where both the unreacted monomers and the inert solvents are efficiently removed from the polyolefin obtained from the polymerization reactor, can be further improved. More specifically, it was an aim to improve the efficiency for separating vapor components from the olefin polymer solution. Especially the heating-up section upstream of the devolatilization vessels was subject to improvements.DESCRIPTION OF THE INVENTION

[0007] This object is achieved by the present invention, in particular by a process for separating vapor components from an olefin polymer solution as defined in the following, an integrated process for the production of olefin polymer as defined in the following, a separation unit for separating vapor components from an olefin polymer solution as defined in the following and an integrated unit forthe production of olefin polymer as defined in the following. These and further aspects and embodiments of the invention are described in the following.

[0008] According to a first aspect of the present invention, a process for separating vapor components from an olefin polymer solution, comprises stages (i) to (iv):(i) heating the olefin polymer solution in a first heat exchanger to a temperature in a range of from 70 to 250 °C, preferably from 100 to 220 °C more preferably from 120 to 210 °C, to give a heated polymer solution;(ii) subjecting the heated polymer solution to a pre-separation treatment for removing by flashing in a first phase separator, the first phase separator being operated at a pressure in a range of from 5 to 28 barg (1 barg = 0.1 MPag), preferably 8 to 25 barg, more preferably 8 to 20 barg, a first fraction of one or more vapor components, to give a vapor-reduced concentrated polymer solution and a vapor phase stream;(iii) heating the vapor-reduced concentrated polymer solution in a second heat exchanger to a temperature in a range of from 120 to 250 °C, preferably from 130 to 240 °C, more preferably from 150 to 230 °C, to give a heated vapor-reduced concentrated polymer solution; and(iv) subjecting the heated vapor-reduced concentrated polymer solution to a separation treatment for removing a second fraction of the one or more vapor components, such separation treatment being carried out by means of a volatilization chamber or of a sequence of two or more, preferably three or more volatilization chambers operating at a decreasing pressure; wherein the vapor phase stream originated in the first phase separator is sent to the volatilization chamber or to the first of the sequence of two or more volatilization chambers.FE7656-EP-P1

[0009] Preferably, the first volatilization chamber in stage (iv) is operated at a pressure in a range of from 3 to 21 barg, preferably 3 to 18 barg, more preferably 3 to 15 barg.

[0010] The heating-up operation in stage (i), also referred to as “pre-heating”, is carried out in the first heat exchanger. This pre-separation step improves the overall performance of the separation process. Referring to the process known from EP 2 072 540 A1 , volatilization imposes a high duty on the heating step due to the need to obtain a polymer melt, exiting from the final devolatilization stage, which shall have a minimum residual content of hydrocarbons, i.e. of unreacted monomers and solvent. By including a pre-separation step before performing the main-separation on the basis of volatilization, it is possible to improve the separation of the vapor components. Specifically, the required heat exchange surface to vaporize the vapor components such as the solvent and / or residual monomers can be reduced. Further, the overall efficiency of the heat exchange operation increases because less vapor components are entrapped in the polymer melt in the form of bubbles.

[0011] A second aspect of the present invention pertains to an integrated process for the production of olefin polymer. The process comprises the following steps a) and b): a) a solution olefin polymerization reaction step being conducted at a pressure in a range of from 20 to 40 barg, preferably from 30 to 40 barg, more preferably from 30 to 35 barg, producing an olefin polymer solution, and b) a separation step comprising stages (i) to (iv), as defined above.

[0012] In the following, the separation of the vapor components from the olefin polymer solution and embodiments thereof are described. Because the separation is the subject of the first aspect and involved in the second aspect, this description applies to both aspects.

[0013] A key aspect of the present invention is the separation of vapor components from an olefin polymer solution. Vapor components are components that become gaseous under the conditions occurring in any one of stages (i) to (iv). Preferred vapor components include monomers and inert hydrocarbon solvent, which are residual components in the production of polyolefins using a solution olefin polymerization reaction step. When exiting the reactor after the polymerization reaction step, the polymer may still contain undesired low-molecular weight components such as residual monomers, solvents, reaction by-products and water. These component may make the product unusable for further use, may be toxic, may cause bad sensory properties such as an unpleasant smell or worsen the properties of the polymer. It may also be desirable to recycle monomers and solvents to the process. The olefin polymer solution subjected to the separation step thus preferably reflects the reaction product obtained in a solution olefin polymerization reaction step, i.e. a polyolefin dissolved in an inert hydrocarbon solvent.

[0014] A preferred set-up contains, or consists of, two shell-and-tubes type heat exchangers in series and an intermediate phase separator, which allows a first fraction of vapor componentsFE7656-EP-P1 withdrawn from the first heat exchanger to be separated, while the remaining vapor-reduced concentrated polymer solution is sent to the second heat exchanger. In the heat exchangers, the olefin polymer solution flows in the tube side, while the heating medium (e.g., hot oil, molten salt or medium / high pressure steam) flows in the shell. The length of the tubes is a key parameter in the design of such heat exchangers. As the length of the tubes increases, the polymer solution’s vapor phase fraction at the outlet increases, thereby limiting the performance of the heat exchanger. The introduction of the intermediate phase separation advantageously allows the removal of such vapor phase.

[0015] High temperatures are involved in the separation of the vapor components. Hence, it is preferred that the polymer solution is subjected to deactivation of the catalyst system contained therein before stage (i). The catalyst deactivation avoids uncontrolled polymerization or thermal degradation of the polymer during the separation (separation process and separation step (b)). It avoids also uncontrolled reactions of the catalyst residues that can provide by-products affecting the polymer quality (e.g. color). The deactivation of the polymeric solution can be performed in one or more mixing tanks placed in series or, alternatively, in a single deactivation apparatus equipped with a sequence of more mixing stages. In the case of polymerization carried out in the presence of a Ziegler-Natta catalyst, any of the deactivating compounds known in the art can be employed. Suitable deactivating compounds are water or organic compounds with a boiling point higher than 150°C and at least a hydroxy group. Examples of preferred deactivating compounds are water, propylenglycol, dipropylenglycol, glycerol and Atmer™.

[0016] After the optional catalyst deactivation, the polymeric solution is subjected to pre-heating pursuant to stage (i) by flowing it inside a first heat exchanger, in order to give a heated polymer solution. While the precise temperature is bound to the volatility of the specific monomers and inert solvents to be separated in step (b), the temperature at the outlet of the first heat exchanger generally ranges from 70 to 250 °C, preferably from 100 to 220 °C, more preferably from 120 to 210 °C.

[0017] In some applications, it is desirable to carry out a pressurizing stage before the heating stage (i). In the pressurizing stage, a pressure of the polymer solution is increased. For example, the polymer solution can be pressurized up to 60 barg. When the pressure is not sufficiently high, the polymer solution may start to vaporize, and accumulate in the first heat exchanger, deteriorating the performances. Increasing the pressure may avoid formation of an excessive amount of vapor phase, thereby avoiding deterioration of the heat exchanger performance.

[0018] After the optional pressurizing stage, the polymer solution is subjected to heating pursuant to stage (i) by flowing it inside the first heat exchanger, in orderto give the heated polymer solution. While the precise temperature is bound to the volatility of the specific vapor components (e.g., monomers and / or inert solvents) to be separated, the temperature of the first heat exchanger generally ranges from 70 to 250 °C, preferably from 100 to 220 °C, more preferablyFE7656-EP-P1 from 120 to 210 °C. Preferably, the temperature indicated for the heat exchanger is the outlet temperature.

[0019] In some applications, it is preferred to increase the temperature of the polymer solution before the heating stage (i). For example, the polymer solution may be heated by low pressure steam. Thereby, it may be possible to increase economy of the process. Moreover, a low pressure steam pre-heater may also serve as the first heat exchanger in stage (i).

[0020] The heated polymer solution is then guided into the first phase separator of stage (ii). The phase separator is operated at a pressure ranging from 5 to 28 barg, preferably from 8 to 25 barg, more preferably from 8 to 20 barg. It has the function of removing a first fraction of gaseous components, preferably one or more olefin monomers and / or inert solvent, from the heated polymer solution by flashing in the first phase separator. "Flashing" refers to the process of liquid volatile components immediately transitioning into a vapor phase when they move from high to low pressure. A throttling valve may provide for the pressure difference. Removing the first fraction of gaseous components from the heated polymer solution gives a vapor-reduced concentrated polymer solution. The first fraction of gaseous components typically ranges from 0 to 60 % by weight, preferably from 5 to 55 % by weight, more preferably from 10 to 50 % by weight, relative to the weight of the heated polymer solution. That vapor phase stream is not sent to the recovery section, instead it is sent to the downstream devolatilization chamber for further separation.

[0021] Successively, the vapor-reduced concentrated polymer solution is transferred into the second heat exchanger of stage (iii). The vapor-reduced polymer solution is only moderately concentrated with respect to the starting olefin polymer solution. In fact, it can be transferred into the second heat exchanger of stage (iii) without the need of a gear pump. Here, the vapor- reduced concentrated polymer solution is heated to yield a heated vapor-reduced concentrated polymer solution. The temperature at the outlet of the second heat exchanger ranges from 120 to 250 °C, preferably from 130 to 240 °C, more preferably from 150 to 230 °C.

[0022] Stage (iv) aims at separating further gaseous component from the heated vapor-reduced concentrated polymer solution. The separation is achieved by means of one or more volatilization (also referred to as devolatilization) chambers. Starting from the polymer solution volatilization leads to removal of the volatile components from the polyolefin, which is separated in the form of a "polymer melt". The term "polymer melt" denotes an olefin polymer in the molten state. Despite of its usually very high viscosity (of at least 106mPa*s), the polymer melt is preferably still able to be pumped by means of a gear pump.

[0023] In preferred embodiments, a first of the one or more volatilization chambers is operated at a pressure ranging from 3 to 21 barg, preferably from 3 to 18 barg, more preferably from 3 to 15 barg.FE7656-EP-P1

[0024] When two or more volatilization chambers are used, they operate at a decreasing pressure. According to an embodiment of the process described herein, the separation step is performed by means of a sequence of three volatilization chambers operating at a decreasing pressure, as described below.

[0025] In a preferred embodiment of the present invention the separation stage (iv) is performed by means of a sequence of at least three volatilization chambers. The three volatilization chambers may be operated at a decreasing pressure. As mentioned above, the first volatilization chamber may be operated at a pressure in the range of from 3 to 21 barg, preferably from 3 to 18 barg, more preferably from 3 to 15 barg. A second volatilization chamber may be operated at a pressure ranging from 0.8 to 5 bar. A third volatilization chamber may be operated at a pressure ranging from 5 to 100 mbar.

[0026] In a particular preferred embodiment of the present invention, the separation stage (iv) is carried out in three volatilization chambers, and the first volatilization chamber is operated at a pressure ranging from 3 to 21 barg, the second volatilization chamber is operated at a pressure ranging from 0.8 to 5 bar and the third volatilization chamber is operated at a pressure ranging from 5 to 100 mbar.

[0027] At the outlet of the second heat exchanger the polyolefin solution may be introduced in the upper portion of the first volatilization chamber, which may be operated at a temperature ranging from 120°C to 250°C and a pressure ranging from 3 to 21 barg, preferably from 3 to 18 barg, more preferably from 3 to 15 barg. In the first volatilization chamber the different components of the polyolefin solution are separated: the polyolefin falls downward by gravity as a polymer melt, while the volatile components, mostly inert hydrocarbon solvent, olefin monomers and co-monomers, are released from the polyolefin as a gaseous stream, which flows upward to the top of the volatilization chamber. During the vertical drop of the polymer melt along the first volatilization chamber the unreacted (co-)monomers are released away, so that at the discharge (outlet) of the first volatilization chamber the content of unreacted (co-)monomers in the polyolefin melt is generally reduced to less than 40% by weight, preferably to less than 35% by weight, more preferably to less than 30% by weight.

[0028] The second volatilization chamber is preferably operated at a higher temperature than the first chamber. For this reason, the polyolefin melt withdrawn from the outlet of the first volatilization chamber is generally subjected to further heating in a heat exchanger. This heat exchanger has the function of adjusting the temperature of the polymer melt up to the value requested in the second volatilization chamber. In particular, the temperature in the second volatilization chamber is adjusted to a value ranging from 180 to 250°C, while the pressure is maintained close to the atmospheric value, in a range from 0.8 to 5.0 bar, preferably from 1 .0 to 3.0 bar. In view of the temperature increase, and above all, due to the considerable decrease of pressure, the residual amounts of olefin (co-)monomers and a considerable amount of inert solvent are released from the polyolefin melt as a gaseous stream, which flows upward to the topFE7656-EP-P1 of the second volatilization chamber. Simultaneously, the polyolefin melt drops by gravity and settles at the bottom of the second volatilization chamber. In particular, at the discharge (outlet) of the second volatilization chamber the content of unreacted monomers in the polyolefin melt is reduced to less than 12% by weight, preferably less than 10% by weight, more preferably less than 8% by weight.

[0029] Successively, the polyolefin melt withdrawn from the second volatilization chamber is introduced into the third devolatilization chamber. The third volatilization chamber may be operated in the same temperature range as the second volatilization chamber, but under different pressure, specifically vacuum, conditions. A vacuum pump may be arranged on the line transferring the gaseous components from the top of the third volatilization chamber to the monomer recovery section. Said vacuum pump establishes a high degree of vacuum inside the third chamber, in particular a pressure comprised between 5 and 100 mbar, preferably between 10 and 25 mbar. As a consequence of said vacuum conditions, residual amounts of monomers and inert solvent are released away during the vertical drop and settling of the polyolefin melt along the third volatilization chamber. Accordingly, at the outlet of the third volatilization chamber the total content of monomers can be generally lower than 2000 ppm weight, while the content of inert solvent in the polyolefin melt is strongly reduced, up to less than 5000 ppm weight, preferably less than 2500 ppm weight, more preferably less than 2000 ppm weight.

[0030] The polyolefin melt obtained at the bottom of each volatilization chamber is characterized by a very high viscosity, of at least 106cP (1 cP = 1 mPa*s). As a consequence, specific pumps, such as gear pumps, are preferably used to facilitate the efficient extraction of the polymer melt from the bottom of each volatilization chamber. Preferably, the gear pump is directly coupled to the bottom flange of each volatilization chamber in order to prime the pump and to convey the polyolefin melt to the following process steps.

[0031] Splitting the separation in a pre-separation stage (ii) and a separation stage (iv) with an intermediate phase separation stage (ii) in-between serves the removal of low-molecular- weight components with a higher efficiency as compared to the polymer volatilization known from the prior art. A first gaseous component may comprise one or more olefin monomers. A second gaseous component may comprise a hydrocarbon solvent.

[0032] Preferred olefin monomers are selected from the group consisting of ethylene, propylene, 1 -butene, 1 -pentene, 1 -hexene, 4-methyl-1 -pentene, 1 -octene and 1 -decene. Particular preferred monomers are 1 -butene monomers for producing poly-1 -butene, as well as propylene, 1 -butene, 1 -hexene and 1 -octene (co-) monomers for producing ethylene-based copolymers.

[0033] Preferred hydrocarbon solvents are selected from the group consisting of cycloalkanes, such as cyclohexane, methylcyclohexane and cyclopentane; aliphatic hydrocarbons, such as isopentane, hexane, isohexane, isoheptane, isooctane and isononane; aromatic hydrocarbons, such as toluene, benzene and ethylbenzene; and mixtures thereof.FE7656-EP-P1

[0034] Preferably, the first heat exchanger is a shell-and-tubes type heat exchanger. It is further preferred that the second heat exchanger is a shell-and-tubes type heat exchanger. In a particular preferred embodiment, both the first heat exchanger and the second heat exchanger are shell-and-tubes type heat exchangers. The tubes of the first heat exchanger have preferably a length in a range of from 2 to 15 m, preferably 3 to 10 m, more preferably 4 to 8 m, most preferably 5 to 7 m. The tubes of the second heat exchanger have preferably a length in a range of from 2 to 15 m, preferably 3 to 10 m, more preferably 4 to 8 m, most preferably 5 to 7 m.

[0035] A shell-and-tube type heat exchanger with one or more static mixers inserted inside each tube can be used for the heating operations in stages (i) and / or (iii), as described in the Patent Application WO 04 / 000891 . The optional presence of a plurality of static mixing elements inside each tube aims at achieving an intense mixing of the highly-viscous polymeric solution. Said elements cause the mixing between the outermost and the innermost veins of the flowing stream, so as to favor a good heat transfer between the different threads of fluid. As a consequence, a homogenous flow and a homogeneous distribution of temperature is obtained in the polymer solution. Mixing rods may be used as static elements inserted in the tubes of the heat exchanger.

[0036] Next, the solution olefin polymerization step a) is detailed.

[0037] The solution olefin polymerization step is a process where one or more a-olefins of formula CH2=CHR in which R is H or an alkyl radical C1-12, is polymerized to produce a polyolefin soluble in the reaction medium. The process may involve polymerizing said one or more a-ole- fins in a solution phase in the presence of a polymerization catalyst and an inert hydrocarbon solvent having a vapor pressure at 20°C of less than 100 kPa.

[0038] The polymerization step described herein can be carried out at a polymerization temperature (Tp) satisfying the relationship Tp > Tm + 20°C, preferably Tp > Tm + 25°C, more preferably Tp > Tm + 30°C, wherein Tm is the melting temperature of the obtained polymer.

[0039] The polymerization reaction can be carried out in the presence of highly active catalytic systems, such as Ziegler-Natta catalysts, single site catalysts, metallocene catalyst, chromium- based catalysts and vanadium-based catalysts.

[0040] A Ziegler-Natta catalyst system comprises the catalysts obtained by the reaction of a transition metal compound of groups 4 to 10 of the Periodic Table of Elements (new notation) with an organometallic compound of group 1 , 2, or 13 of the Periodic Table of element.

[0041] In particular, the transition metal compound can be selected among compounds of Ti, V, Zr, Cr, and Hf Preferred compounds are those of formula Ti(OR)nXy-n in which n is between 0 and y; y is the valence of titanium; X is halogen and R is a hydrocarbon group having 1 to 10 carbon atoms or a COR group. Among them, particularly preferred are titanium compounds having at least one Ti-halogen bond such as titanium tetrahalides or halogenalcoholates.FE7656-EP-P1Preferred specific titanium compounds are TiCh, TiCk, Ti(OBu)4, Ti(OBu)Cl2, Ti(OBu)2Cl2, Ti(OBu)3CI.

[0042] Preferred organometallic compounds are the organo-AI compounds and in particular Al-alkyl compounds. The alkyl-AI compound is preferably chosen among the trialkyl aluminum compounds such as for example triethylaluminum, triisobutylaluminum, tri-n-butylaluminum, tri- n-hexylaluminum, tri-n-octylaluminum. It is also possible to use alkylaluminum halides, alkylaluminum hydrides or alkylaluminum sesquichlorides such as AIEt2CI and AhEtsC optionally in mixture with said trialkyl aluminum compounds.

[0043] Metallocene catalysis is based on using Zr as a metal, while alkyls are mostly MAO (methyl aluminum oxide) typically dissolved in a solvent, usually toluene, and TIBAL (triisobutylaluminum) as a scavenger (deactivation / killing of any poison). Catalyst and alkyl solution may also include a solvent, preferably cyclohexane or iso-dodecane.

[0044] Particularly suitable high yield Ziegler-Natta catalysts are those wherein the titanium compound is supported on magnesium halide in active form which is preferably MgCh in active form. Particularly for the preparation crystalline polymers of CH2CHR olefins, where R is a Ci to C10 hydrocarbon group, internal electron donor compounds can be supported on the MgCh. Typically, they can be selected among esters, ethers, amines, and ketones. In particular, the use of compounds belonging to 1 ,3-diethers, cyclic ethers, phthalates, benzoates, acetates and succinates is preferred.

[0045] It is also possible to use, besides the electron-donor present in the solid catalytic component, an external electron-donor (ED) added to the aluminium alkyl co-catalyst component or to the polymerization reactor. These external electron donors can be selected among alcohols, glycols, esters, ketones, amines, amides, nitriles, alkoxysilanes and ethers. The electron donor compounds (ED) can be used alone or in mixture with each other. Preferably the ED compound is selected among aliphatic ethers, esters and alkoxysilanes. Preferred ethers are the C2-C20 aliphatic ethers and in particular the cyclic ethers preferably having 3-5 carbon atoms, such as tetrahydrofurane (THF) and dioxane.

[0046] Preferred esters are the alkyl esters of Ci to C20 aliphatic carboxylic acids and in particular Ci to Ca alkyl esters of aliphatic mono carboxylic acids such as ethylacetate, methyl formiate, ethylformiate, methylacetate, propylacetate, i-propylacetate, n-butylacetate, ibu- tylacetate.

[0047] The preferred alkoxysilanes are of formula Ra1Rb2Si(OR3)c, where a and b are integers from O to 2, c is an integerfrom 1 to 3 and the sum (a+b+c) is 4; R1,R2and R3, are alkyl, cycloalkyl or aryl radicals with 1 to 18 carbon atoms. Particularly preferred are the silicon compounds in which a is 1 , b is 1 , c is 2, at least one of R1and R2is selected from branched alkyl, cycloalkyl or aryl groups with 3-10 carbon atoms and R3is a Ci to C10 alkyl group, in particular methyl.FE7656-EP-P1

[0048] Other useful catalysts are the vanadium-based catalysts, which comprise the reaction product of a vanadium compound with an aluminum compound, optionally in the presence of a halogenated organic compound. Optionally the vanadium compound can be supported on an inorganic carrier, such as silica, alumina, magnesium chloride. Suitable vanadium compounds are VCk, VCh, VOCh and vanadium acetyl acetonate.

[0049] Other useful catalysts are those based on chromium compounds, such as chromium oxide on silica, also known as Phillips catalysts.

[0050] Other useful catalysts are single site catalysts, for instance metallocene-based catalyst systems which comprise at least a transition metal compound containing at least one n bond, at least an alumoxane or a compound able to form an alkyl-metallocene cation, and optionally an organo-aluminum compound.

[0051] A preferred class of metal compounds containing at least one n bond are metallocene compounds belonging to the following formula (I):Cp(L )qAMXP(I)

[0052] wherein M is a transition metal belonging to group 4, 5 or to the lanthanide or actinide groups of the Periodic Table of the Elements; preferably M is zirconium, titanium or hafnium; the substituents X, equal to or different from each other, are monoanionic sigma ligands selected from the group consisting of hydrogen, halogen, R6, OR6, OCOR6, SR6, NR62 and PR62, wherein R6is a hydrocarbon radical containing from 1 to 40 carbon atoms; preferably, the substituents X are selected from the group consisting of -Cl, -Br, -Me, -Et, -n-Bu, -sec-Bu, -Ph, -Bz, - CH2SiMe3, -OEt, -OPr, -OBu, -OBz and-NMe2; p is an integer equal to the oxidation state of the metal M minus 2; n is 0 or 1 ; when n is 0 the bridge L is not present; L is a divalent hydrocarbon moiety containing from 1 to 40 carbon atoms, optionally containing up to 5 silicon atoms, bridging Cp and A, preferably L iis a divalent group (ZR72)n; Z being C, Si, and the R7groups, equal to or different from each other, being hydrogen or a hydrocarbon radical containing from 1 to 40 carbon atoms; more preferably L is selected from Si(CH3)2, SiPti2, SiPhMe, SiMe(SiMe3), CH2, (CH2)2, (CH2)3or C(CH3)2; Cp is a substituted or unsubstituted cyclopentadienyl group, optionally condensed to one or more substituted or unsubstituted, saturated, unsaturated or aromatic rings; A has the same meaning as Cp or it is a NR7, -O, S, moiety wherein R7is a hydrocarbon radical containing from 1 to 40 carbon atoms.

[0053] Alumoxanes are considered to be linear, branched or cyclic compounds containing at least one group of the type:FE7656-EP-P1wherein the substituents U, same or different, are defined above.

[0054] In particular, alumoxanes of the formula:can be used in the case of linear compounds, wherein n1is 0 or an integer of from 1 to 40 and where the U substituents, same or different, are hydrogen atoms, halogen atoms, Ci to C20- alkyl, C3 to C2o-cyclalkyl, Ce to C2o-aryl, Ci to C2o-alkylaryl or Ci to C2o-arylalkyl radicals, optionally containing silicon or germanium atoms, with the proviso that at least one U is different from halogen, and j ranges from 0 to 1 , being also a non-integer number; or alumoxanes of the formula:can be used in the case of cyclic compounds, wherein n2is an integer from 2 to 40 and the U substituents are as defined above.

[0055] The catalyst may suitably be employed in the form of a pre-polymer powder prepared beforehand during a pre-polymerization stage with the aid of a catalyst as described above. The pre-polymerization may be carried out by any suitable process, for example, polymerization in a liquid hydrocarbon diluent or in the gas phase using a batch process, a semi-continuous process or a continuous process.

[0056] The solution polymerization step can be performed in one or more continuously stirred tank reactors (CSTR) or one or more static mixer reactors (SMR). A solution of a polyolefin in the reaction medium is obtained from the polymerization step (a). The reaction medium comprises the unreacted monomers and an inert hydrocarbon solvent, which has the function of fostering the solubility of the produced polyolefin.

[0057] Examples of the first olefin monomer of formula CH2=CHR1, wherein R1is hydrogen or a hydrocarbon radical having 2 to 8 carbon atoms, that are polymerized in step (a) describedFE7656-EP-P1 herein are ethylene, propylene, l-butene, 1 -pentene, 1 -hexene, 4-methyl-1 -pentene, 1 -octene and 1 -decene.

[0058] Examples of the second olefin monomer of formula CH2=CHR2, wherein R2is a hydrocarbon radical having 5 to 8 carbon atoms, that are polymerized in step (a) described herein are 1 -pentene, 1 -hexene, 4-methy 1-1 -pentene, 1 -octene and 1 -decene.

[0059] If the main monomer is propylene, the preferred co-monomer is selected from ethylene, 1 -butene, 1 -hexene and 1-octene. If the main monomer is ethylene, the preferred comonomer is selected from propylene, 1-butene, 1-hexene and 1-octene. According to a particular preferred embodiment, the first olefin monomer is ethylene and the second olefin monomer is 1-octene.

[0060] The content of the second olefin monomer in the obtained polymer products is generally comprised between 3 and 50% by weight, preferably between 5 and 45% by weight, more preferably between 7 and 40% by weight.

[0061] Molecular weight regulators, such as hydrogen, can be used to control the molecular weight of the obtained polyolefins. Different operative conditions can be adopted in the polymerization step (a) as regards the concentration of the molecular weight regulator and the different monomers.

[0062] Preferred inert hydrocarbon solvents used in the polymerization step (a) are characterized by a vapor pressure lower than the polymerized olefins, in particular a vapor pressure lower than 100 kPa at 20°C, preferably lower than 50 kPa at 20°C. These low values of vapor pressure imply a different behavior, with respect to the olefin monomers, during the separation step (herein also referred to as volatilization step) (b).

[0063] Taking also into account their good solubilization capacity and their chemical inertia, preferred inert hydrocarbon solvents with the above values of vapor pressure are selected from: cycloalkanes, such as cyclohexane (vapor pressure 10.4 kPa, 20°C), methylcyclohexane and cyclopentane (34.5 kPa, 20°C); aliphatic hydrocarbons, such as isopentane (75.9 kPa, 20°C), hexane (16.0 kPa, 20°C), isohexane, isoheptane, isooctane and isononane; and aromatic hydrocarbons, such as toluene, benzene and ethylbenzene; and mixtures thereof.

[0064] The most preferred hydrocarbon solvents used in step a) are cyclopentane, cyclohexane and methylcyclohexane.FE7656-EP-P1

[0065] Depending on the selected hydrocarbon solvent, catalyst system and olefin monomers, the olefin polymerization of step (a) is generally conducted at temperatures generally higher than 60 °C, preferably in the range of from 60 to 150 °C, more preferably from 80 to 140 °C.

[0066] Also depending on the selected hydrocarbon solvent, catalyst system and olefin monomers, the olefin polymerization of step (a) is generally conducted at pressures lower than 5 MPa, preferably in the range of from 2 MPa to 5 MPa, more preferably from 2.5 MPa to 4 MPa.

[0067] The residence time of the liquid reaction medium inside the polymerization apparatus, which can consist of one or more reactors, may range between 20 minutes and 300 minutes, preferably between 60 minutes and 250 minutes.

[0068] A highly viscous polymeric solution is discharged from the polymerization reactor of step (a). The viscosity of the obtained polymeric solution should not exceed a threshold value, as otherwise it becomes extremely difficult stirring and / or processing the polymeric solution downstream the polymerization section appropriately. The viscosity of the polymeric solution may range between 1000 and 100,000 mPa*s.

[0069] The polymeric solution discharged from the polymerization section is transferred, for instance by means of a volumetric pump, to the downstream separation section, where in step (b) the unreacted monomers and the inert solvent are removed from the obtained polyolefin solution as described further above.

[0070] A separation unit for separating gaseous components from an olefin polymer solution produced in a solution olefin polymerization reaction step is a third aspect of the invention. The separation unit comprises:(i) a first heat exchanger for heating the polymer solution to a temperature in a range of from 70 to 250 °C, to give a heated polymer solution;(ii) a first phase separator, coupled to the first heat exchanger, for removing by flashing a first gaseous component from the heated polymer solution, to give a vapor-reduced concentrated polymer solution and a vapor phase stream;(iii) a second heat exchanger coupled to the first phase separator, for heating the vapor- reduced concentrated polymer solution to a temperature in a range of from 120 to 250 °C, to give a heated vapor-reduced concentrated polymer solution; and(iv) a second phase separator, for removing a second gaseous component from the heated vapor-reduced concentrated polymer solution.FE7656-EP-P1

[0071] The first phase separator of step (ii) is a flashing device. Preferably, also the second phase separator of step (iv) is a flashing device.

[0072] The first phase separator has a top outlet in fluid communication with the second phase separator and a bottom outlet in fluid communication with the second heat exchanger, preferably with no gear pump.

[0073] As already mentioned above, the present invention resides on the innovation that introducing a pre-separation treatment between two heating steps allows to remove part of the vapor phase after the first heating stage. The length of the heat exchanger tubes is a key design parameter. As the length of the tubes increases, the polymer solution’s vapor phase fraction at the outlet increases, leading to deterioration of performance. The introduction of an intermediate phase separator allows the removal of part of such vapor phase. This way, two main advantages are achieved. First, the required specific heat exchange surface to vaporize solvent / residual monomers can be reduced. Second, the overall efficiency of the heat exchange operation can be increased due to decreasing occurrence of vapor phase bubbles entrapped in the polymer solution.

[0074] Preferred first and / or second heat exchangers are shell-and-tubes type heat exchangers. Preferably, both the first heat exchanger and the second heat exchanger are shell-and- tubes type heat exchangers. Preferably, the tubes of the first heat exchanger include one or more static mixers. Preferably, the tubes of the second heat exchanger include one or more static mixers. More preferably, the tubes of both the first heat exchanger and the second heat exchanger include one or more static mixers.

[0075] Preferably, the tubes of the first heat exchanger have a length in a range of from 2 to 10 m, preferably 3 to 9 m, more preferably 4 to 8 m, most preferably 5 to 7 m. Preferably, tubes of the second heat exchanger have a length in a range of from 2 to 10 m, preferably 3 to 9 m, more preferably 4 to 8 m, most preferably 5 to 7 m. More preferably, the tubes of both the first and second heat exchangers have a length in a range of from 2 to 10 m, preferably 3 to 9 m, more preferably 4 to 8 m, most preferably 5 to 7 m.

[0076] According to a preferred embodiment, both the first heat exchanger and the second heat exchanger are shell-and-tube heat exchangers and the number of tubes of the first heat exchanger differs from the number of tubes of the second heat exchanger.

[0077] A fourth aspect of the present invention pertains to an integrated unit for the production of polymer. The unit comprises at least one reaction device, for performing a solution olefin polymerization reaction; and at least one separation unit as described herein.

[0078] A preferred integrated unit comprises two or more separation units (as described herein) connected in series.FE7656-EP-P1

[0079] As regards further features and embodiments, reference is made to the description in relation to the first and second aspects of the invention. These provide corresponding features of embodiments for the third and fourth aspects of the invention.DESCRIPTION OF AN EMBODIMENT OF THE INVENTION:

[0080] An embodiment of the present invention will now be described in detail with reference to the process setup shown in Figure 1 . The process setup of Fig. 1 has to be considered illustrative and not limitative of the scope of the present invention.

[0081] The solution polymerization reaction of the present invention is performed in a continuously stirred tank reactor (not shown). A transition metal compound, optionally supported on a carrier, an aluminum alkyl compound and optionally an electron donor compound are first precontacted in one or more pre-contacting pots (not shown) and then fed to the continuously stirred tank reactor. The pre-contacting pot can be bypassed, with the activated catalyst solution directly fed to the polymerization reactor. A liquid stream containing, as an example, liquid ethylene as olefin monomers, optionally co-monomers, an inert hydrocarbon solvent, such as cyclohexane, is introduced into the polymerization reactor. The olefin monomers and the inert hydrocarbon solvent coming from the monomer recovery section 1 may be recycled to the reactor.

[0082] A high-viscosity solution of polyethylene in inert solvent is discharged from the reactor and is conveyed to a mixing pot (not shown), where it is contacted with a catalyst deactivator, such as water.

[0083] After the catalyst deactivation, the solution of polyethylene in the inert solvent may be pressurized and transferred by means of a screw pump to the first heat exchanger 2 via line 3, where the polymer solution is heated. The first heat exchanger 2 is a shell-and-tubes type heat exchanger with static mixing elements inserted inside each tube. The reaction bath flows in the tube side, while the heating medium (e.g., hot oil or medium / high pressure steam) flows in the shell. By means of heat exchanger 2 the temperature of the polymeric solution is increased to the values requested inside a phase separator 4. The polymer solution exiting from the heat exchanger 2 is transferred into the phase separator 4 through throttling valve 6 via line 5. Expansion results in flashing of a first fraction of volatile monomers and inert solvent form the polymer solution into the gas phase, which can be removed from the top of the phase separator 4 via line 7 and sent through a throttling valve at the top of the first volatilization chamber 10 via line 7.

[0084] This heating-up operation in conjunction with the subsequent phase separation allows to remove a first vapor fraction, thereby enhancing the overall performance.

[0085] Concentrated (vapor- reduced) polymer solution settles on the bottom of the phase separator 4 from which it can be transferred into a second heat exchanger 8 via line 9. In the second heat exchanger 8, the concentrated polymer solution is heated. The second heatFE7656-EP-P1 exchanger s is also a shell-and-tubes type heat exchanger with static mixing elements inserted inside each tube. By means of the second heat exchanger 8 the temperature of the concentrated polymer solution is increased to the values requested by a first (de-)volatilization chamber 10.

[0086] The concentrated polymer solution exiting from the second heat exchanger 8 is guided through a throttling valve 11 into the first volatilization chamber 10 via line 12, so as to favor the separation of a second fraction of unreacted monomers and inert solvent from the polymer.

[0087] The polymeric stream exiting from the second heat exchanger 8 is introduced at the top of the first volatilization chamber 10, which is operated at a pressure ranging from 3 to 21 barg. In said first volatilization chamber 10 the unreacted monomers and the inert solvent are separated from the polymeric components: a polymer melt settles downwards at the bottom of the volatilization chamber 10, while the unreacted monomers and the inert solvent flow upward as a gaseous stream.

[0088] The gas exiting the top of the volatilization chamber 10 may be subjected to partial condensation in a condenser (not shown), and then the obtained gas / liquid mixture may be transferred the monomer recovery section 1 of the polymerization plant. The monomer recovery section 1 may comprise one or more distillation columns to separate the heavy components from the light components, a drying unit, and recycle lines for liquid monomers and inert solvents to the polymerization reactor.

[0089] A polymer melt, still entrapping a certain amount of inert solvent and residual monomers, is withdrawn by means of a gear pump (not shown) from the bottom of the first volatilization chamber 10 and is then transferred via line 13 into a third exchanger (not shown), which adjusts the temperature of the polymer melt to the values requested in the second volatilization chamber (not shown). The polymer melt exiting the third heat exchanger is introduced at the top of the second volatilization chamber, which is operated at a pressure ranging from 0.8 to 5 bar. In the second volatilization chamber, further separation of residual monomers and inert solvent from the polymer is realized.

[0090] A polymer melt, further purified from unreacted monomers and inert solvent, is withdrawn by means of a gear pump (not shown) from the bottom of the second volatilization chamber and is then transferred into a fourth heat exchanger (not shown), which adjusts the temperature of the polymer melt to the values requested by the third volatilization chamber. The polymer melt exiting the fourth heat exchanger is introduced at the top of the third volatilization chamber (not shown), which is operated under vacuum at a pressure ranging from 5 to 100 mbar. In the third volatilization chamber, the residual amounts of unreacted monomers and inert solvent are separated from the polymer melt.

[0091] The polymer melt withdrawn by means of a gear pump (not shown) from the bottom of the third volatilization chamber is conveyed into a static mixer (not shown) to be subjected toFE7656-EP-P1 extrusion. The static mixer has the function of mixing the polymer melt with suitable additives, such as antioxidant compounds, nucleating agents, pigments, etc. As an alternative to the static mixer, a conventional extruder can be used. A side-arm extruder (not shown) can be used for melting and mixing each other the additives used for the polyolefin compounding. The compounded ethylene polymer exiting the static mixer is then passed to an underwater pelletizer (not shown), where it is cut into pellets by the action of rotating knife blades: the pellets are then cooled by means of cooling water.

[0092] The solution polymerization of olefins herewith described is not restricted to the use of any particular family of polymerization catalysts. The invention is useful in any exothermic polymerization reaction employing any catalyst, whether it is supported or unsupported, and regardless of whether it is in pre-polymerized form.EXAMPLE

[0093] The following example shows the comparison between a conventional layout for separation of vapor components from a polymer solution as known from EP 2 072 540 A1 and the separation according to the present invention.

[0094] A polymer solution exiting from the reaction section of a polyolefin elastomer plant is treated in the pre-heating and devolatilization section. The aim is to keep the first volatilization chamber operating pressure at 3 barg and to obtain a certain polymer concentration (70%wt) in the liquid media extracted from the bottom of the first volatilization chamber.

[0095] The inlet stream has the following operating conditions:• Inlet flowrate: 125 t / h• Inlet stream operating temperature: 130 °C• Inlet stream operating pressure: 17 bar• Inlet stream polymer content: 20%wt

[0096] Hot oil is considered as a heating media. Hot oil average temperature is 220°C.

[0097] In both cases, heat exchangers are considered to have tubes with an internal diameter of 21.2 mm, an external diameter of 25.4 mm and a length of 6.1 m. The installation of static mixers (SMXL model) into tubes was considered in each heat exchanger.FE7656-EP-P1

[0099] Case A) Single Heat Exchanger

[0100] The operating conditions of the section have been summarized in the following table.

[0101] Pre-heating section performances have been summarized in the following table.FE7656-EP-P1

[0103] Case B) Two Heat Exchangers in series and Intermediate Phase Separator

[0104] The operating conditions of the section have been summarized in the following table.FE7656-EP-P1

[0106] Pre-heating section performances have been summarized in the following table.

[0107] Comparing results of Case A and Case B, it is evident that the proposed solution allows for a reduction of the overall heat exchange surface that is required for flashing a fixed quantity of solvent / unreacted monomers and a noticeable improvement of the heat exchange performances (i.e. of the overall heat exchange coefficients). Moreover, the design of heat exchangers in Case B shows a better L / D ratio (i.e. length to diameter ratio) than in Case A leading to a simplified scale-up and an improved design.

Claims

FE7656-EP-P1CLAIMS1. A process for separating vapor components from an olefin polymer solution, comprising stages (i) to (iv):(i) heating the olefin polymer solution in a first heat exchanger (2) to a temperature in a range of from 70 to 250 °C, to give a heated polymer solution;(ii) subjecting the heated polymer solution to a pre-separation treatment for removing by flashing in a first phase separator (4), the first phase separator being operated at a pressure in a range of from 5 to 28 barg, a first fraction of one or more vapor components, to give a vapor-reduced concentrated polymer solution and a vapor phase stream;(iii) heating the vapor-reduced concentrated polymer solution in a second heat exchanger (8) to a temperature in a range of from 120 to 250 °C, to give a heated vapor-reduced concentrated polymer solution; and(iv) subjecting the heated vapor-reduced concentrated polymer solution to a separation treatment for removing a second fraction of the one or more vapor components, such separation treatment being carried out by means of a volatilization chamber (10) or of a sequence of two or more volatilization chambers (10) operating at a decreasing pressure; wherein the vapor phase stream originated in the first phase separator (4) is sent to the volatilization chamber (10) or to the first of the sequence of two or more volatilization chambers (10).

2. An integrated process for the production of olefin polymer comprising steps a) and b): a) a solution olefin polymerization reaction step, producing an olefin polymer solution, and b) a separation step comprising stages (i) to (iv):(i) heating the olefin polymer solution in a first heat exchanger (2) to a temperature in a range of from 70 to 250 °C, to give a heated polymer solution;(ii) subjecting the heated polymer solution to a pre-separation treatment for removing by flashing in a first phase separator (4), the first phase separator being operated at a pressure in a range of from 5 to 28 barg, a first fraction of one or more vapor components, to give a vapor-reduced concentrated polymer solution and a vapor phase stream;FE7656-EP-P1(iii) heating the vapor-reduced concentrated polymer solution in a second heat exchanger (8) to a temperature in a range of from 120 to 250 °C, to give a heated vapor-reduced concentrated polymer solution; and(iv) subjecting the heated vapor-reduced concentrated polymer solution to a separation treatment for removing a second fraction of the one or more vapor components, such separation treatment being carried out by means of a sequence of one or more volatilization chambers (10) operating at a decreasing pressure; wherein the vapor phase stream originated in the first phase separator (4) is sent to the volatilization chamber (10) or to the first of the sequence of two or more volatilization chambers (10).

3. The process of claim 1 or 2, wherein in stage (i) the olefin polymer solution is heated to a temperature ranging from 100 to 220 °C, preferably from 120 to 210 °C; and / or wherein in stage (iii) the vapor-reduced concentrated polymer solution is heated to a temperature ranging from 130 to 240 °C, preferably from 150 to 230 °C; and / or wherein the separation stage (iv) is carried out in two or more, preferably three or more, volatilization chambers (10), operating at a decreasing pressure,4. The process of any of the preceding claims, wherein the separation stage (iv) is carried out in three volatilization chambers (10), and the first volatilization chamber (10) is operated at a pressure in a range of from 3 to 21 barg, preferably from 3 to 18 barg, more preferably from 3 to 15 barg and / or wherein the separation stage (iv) is carried out in three volatilization chambers (10) and the second volatilization chamber (10) is operated at a pressure from 0.8 to 5 bar, and / or wherein the separation stage (iv) is carried out in three volatilization chambers (10) and the third volatilization chamber (10) is operated at a pressure from 5 to 100 mbar.

5. The process of any of the preceding claims, wherein the one or more vapor components comprise one or more olefin monomers and / or wherein the one or more vapor components comprise a hydrocarbon solvent.

6. The process of any of the preceding claims, wherein the first heat exchanger (2) and / or the second heat exchanger (8) is a shell-and-tubes type heat exchanger, preferably wherein both the first heat exchanger (2) and the second heat exchanger (8) are shell- and-tubes type heat exchangers.FE7656-EP-P17. The process of claim 6, wherein the tubes of the first heat exchanger (2) and / or the tubes of the second heat exchanger (8) have a length in a range of from 2 to 15 m, preferably 3 to 10 m, more preferably 4 to 8 m, most preferably 5 to 7 m.

8. The process of claim 6 or 7, wherein the tubes of the first heat exchanger (2) and / or the tubes of the second heat exchanger (8) include one or more static mixers.

9. A separation unit for separating vapor components from an olefin polymer solution produced in a solution olefin polymerization reaction step, the separation unit comprising:(i) a first heat exchanger (2);(ii) a first phase separator (4) being configured to be operated at a pressure in a range of from 5 to 28 barg, coupled to the first heat exchanger (2);(iii) a second heat exchanger (8) coupled to the first phase separator (4); and(iv) a second phase separator being configured to be operated at a pressure in a range of from 3 to 21 barg; wherein the first phase separator of step (ii) is a flashing device; preferably wherein the second phase separator of step (iv) is a flashing device; and wherein the first phase separator (4) has a top outlet in fluid communication with the second phase separator and a bottom outlet in fluid communication with the second heat exchanger, preferably with no gear pump.

10. The separation unit of claim 9, wherein the first heat exchanger (2) and / or the second heat exchanger (8) is a shell-and-tubes type heat exchanger, preferably wherein both the first heat exchanger (2) and the second heat exchanger (8) are shell- and-tube heat exchangers.11 . The separation unit of claim 10, wherein the tubes of the first heat exchanger (2) and / or the tubes of the second heat exchanger (8) include one or more static mixers.

12. The separation unit of claim 10 or 11 , wherein the tubes of the first heat exchanger (2) and / or the tubes of the second heat exchanger (8) have a length in a range of from 2 to 10 m, preferably 3 to 9 m, more preferably 4 to 8 m, most preferably 5 to 7 m.

13. The separation unit of any of claims 9 to 12, wherein both the first heat exchanger (2) and the second heat exchanger (8) are shell-and-tube heat exchangers and the number of tubes of the first heat exchanger (2) differs from the number of tubes of the second heat exchanger (8).FE7656-EP-P114. An integrated unit forthe production of olefin polymer, comprising x) at least one reaction device, for performing a solution olefin polymerization reaction; and y) at least one separation unit of any of claims 9 to 13.

15. The integrated unit of claim 14, comprising two or more separation units y) connected in series.

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