Process for Continuous Solution Polymerization (ExxonMobil)

CATEGORY: POLYMERIZATION
Process for Continuous Solution Polymerization (ExxonMobil)
United States Patent Application 20160362506
Costin; Quintin P. W. ;   et al.   December 15, 2016
Applicant: ExxonMobil Chemical Patents
Abstract
Described herein are methods for continuous solution polymerization. The method may comprise polymerizing one or more monomers and comonomers in the presence of a solvent in a polymerization reactor to produce a polymer solution; determining the composition of the polymer solution exiting the polymerization reactor in an on-line fashion; determining at least one of the critical pressure or critical temperature; comparing the critical pressure and/or critical temperature to the actual temperature of the polymer solution and the actual pressure of the polymer solution; heating or cooling the polymer solution to a temperature within 50.degree. C. of the critical temperature; and passing the polymer solution through a pressure letdown valve into a liquid-liquid separator, where the pressure of the polymer solution is reduced or raised to a pressure within 50 psig of the critical pressure to induce a separation of the polymer solution into two liquid phases.
FIELD OF THE INVENTION
[0002] Provided herein are continuous solution polymerization processes, including methods for improving the separation and recovery of the polymer product from the polymerization reaction solution.
BACKGROUND OF THE INVENTION
[0003] Continuous solution polymerization processes generally involve the addition of a catalyst to a monomer and solvent mixture. For example, PCT Publication WO 94/00500 describes a solution polymerization process using a metallocene catalyst in continuous stirred tank reactors, which may be parallel or arranged in series, to make a variety of polymer products. Upon reaction of the catalyst and monomers, the formed polymer is dissolved in the polymerization medium or solvent, often along with catalyst and unreacted monomer. Often the solution exiting the polymerization reactor has a relatively low polymer concentration, such as from about 3 wt % to 30 wt %. The product mixture is then passed to polymer concentration and finishing stages to separate the solvent and unreacted monomer from the mixture such that the desired polymer can be recovered in a usable form. The separated solvent and monomer can then later be recycled back to the reactor for re-use.
[0004] A polymer solution can exhibit the Lower Critical Solution Temperature (LCST) phenomenon whereby the polymer solution separates into a polymer-rich liquid phase and a polymer-lean liquid phase above a certain temperature. Typically, this separation method involves heating the polymer solution under high pressure, followed by reducing the pressure to a point where two phases (polymer-rich phase and polymer-lean phase) are formed. Of the two phases that are formed, the polymer-lean phase is rich in solvent and contains most of the unreacted monomer and contains very little polymer, whereas the rich phase is polymer rich. The denser polymer-rich phase settles to the bottom of the vessel where it is pressure fed to downstream equipment where the remaining solvent is removed. The solvent-rich phase (polymer-lean phase) overflows out the top of the separation vessel where it is cooled and recycled back to the polymerization reactor for re-use.
[0005] If the LCST separation is not carried out at proper operating conditions, there can be incomplete separation of the polymer from the polymer-lean phase. This can lead to polymer product being carried overhead in the lean phase, where it can plate out and detrimentally foul equipment in the recycle solvent stream. Therefore, there remains a need for methods to optimize the recovery of polymer from a solution polymerization process that can minimize or reduce the potential for recycle solvent fouling.
[0006] Additional background references include U.S. Pat. Nos. 3,553,156; 3,726,843; 5,264,536; 6,204,344; 6,881,800; 7,163,989; 7,650,930; and 8,916,659; U.S. Patent Application Publication No. 2012/0088893; PCT Publications WO 2011/008955; WO 2013/134041; WO 2013/137962; WO 2013/169357; and Michelsen, M., "A Simple Method for Calculation of Approximate Phase Boundaries", Fluid Phase Equilibria, 98: 1-11 (1994).
SUMMARY OF THE INVENTION
[0007] Described herein are methods for continuous solution polymerization. The method may comprise (a) polymerizing one or more monomers and comonomers in the presence of a solvent in a polymerization reactor to produce a polymer solution comprising solvent, polymer, and unreacted monomer and comonomer; (b) determining the composition of the polymer solution exiting the polymerization reactor in an on-line fashion; (c) determining at least one of the critical pressure or critical temperature; (d) comparing the critical pressure and/or critical temperature to the actual temperature of the polymer solution and the actual pressure of the polymer solution; (e) heating or cooling the polymer solution to a temperature within 50.degree. C. of the critical temperature; (f) passing the polymer solution through a pressure letdown valve into a liquid-liquid separator, where the pressure of the polymer solution is reduced or raised to a pressure within 50 psig of the critical pressure to induce a separation of the polymer solution into two liquid phases, a polymer-rich phase and a polymer-lean phase; (g) recovering the polymer-rich phase from the liquid-liquid separator; and (h) devolatizing the polymer-rich phase to obtain polymer.
[0008] The methods described herein allow for more efficient separation of the polymer solution, which in turn can reduce the amount of polymer carryover into the polymer-lean phase of the separation and decrease recycle fouling.
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Homogeneous Polymerization Process Using Evaporative Cooling (ExxonMobil)

CATEGORY: POLYMERIZATION
Homogeneous Polymerization Process Using Evaporative Cooling (ExxonMobil)
United States Patent Application 20160319054
LATTNER; JAMES R. ;   et al.   November 3, 2016
Applicant: Exxonmobil Chemical Patents Inc
Abstract
This invention relates to a continuous homogeneous solution phase polymerization process comprising contacting, in an evaporative reactor system at a temperature of 50.degree. C. or more and a pressure of 100 kPa or more, a catalyst system, optionally, a hydrocarbon diluent, optionally, scavenger, and one or more monomer(s) to form a homogeneous polymerization medium, where the polymer product is dissolved in the reaction medium at 10 wt % or more (based upon the weight of the polymer present in the effluent at the exit of the reactor) and the catalyst system is dissolved in the reaction medium, and where the catalyst system comprises an activator and a coordination catalyst precursor compound and where all or part of the polymerization medium is evaporated during the polymerization.
FIELD OF THE INVENTION
[0002] This invention relates to homogeneous olefin polymerization processes using evaporative cooling, such as in boiling pool reactors.
BACKGROUND OF THE INVENTION
[0003] Polyolefins are versatile materials used on a million-ton per year scale due to their excellent performance/price position. In some applications, particularly in physically demanding applications, the market needs are addressed by blending two or more polyolefin components. Examples include impact copolymers (ICPs) that comprise a polypropylene continuous phase and a rubber, typically ethylene-propylene copolymer, dispersed phase. These blends often combine high-melting stiff polymers and low-melting or amorphous soft polymers to tune the stiffness-toughness balance of the product. Current processes may blend these components in a compounding extruder. Compounding extruders are expensive machines and typically use significant amounts of energy. In other instances, the components are combined in the polymerization process. However, some of the blend components may be subpar due to the compromises forced by the current technology, such as in the case of ICPs in which it is virtually impossible to tune the rubber component to an optimal composition, composition distribution, and/or molecular weight.
[0004] Many current solution processes utilize adiabatic continuous stirred tank reactors (adiabatic CSTRs). These reactors have at least three disadvantages: (1) they require expensive chilling of the feed to absorb the reaction heat and thus allow acceptable single-pass conversion; (2) they are usually limited by agitation considerations to solution viscosity of a few thousand centipoise, which in turn limits single-pass conversion and polymer concentration in the reactor effluent thus increasing the solvent recycle load and cost per unit mass of product; and (3) operate at high pressures to keep all components in the liquid phase and thus the reactor liquid filled. Other conventional reactor designs use internal heat exchangers or external heat exchanger loops to remove the reaction heat. However, these reactors are also expensive to build and operate, and can have solution viscosity limitations and heat transfer surface fouling problems, especially when making high-melting polyolefins, particularly polypropylenes. The manufacturing of such products often requires relatively low temperatures, such as between 85.degree. C. and 140.degree. C. The lower temperature limit takes into account the tendency of the polymer to come out of solution as a solid thus fouling the reactor, the upper temperature takes into account catalyst stability and/or product properties, such as molecular weight and/or tacticity/stiffness/melting properties. Lower temperatures tend to yield higher molecular weight and higher-melting polyolefins; the latter particularly applies to polyolefins of high propylene contents, like polypropylene. These operational limits force the use of large and thus expensive heat exchangers due to the limitations in the temperature of the cooling medium since excessively cold surfaces tend to trigger fouling. The limited temperature difference thus forces the use of increased heat exchange surfaces for removing the reaction heat, which makes the equipment more expensive to fabricate and operate.
[0005] Boiling pool reactors have been disclosed in U.S. Pat. Nos. 2,885,389; 2,918,460; 3,126,460; and GB Patent 826,562. In the processes disclosed, elongated vertical reactors are operated in a slurry mode at a pressure and temperature such that the hydrocarbon continuous phase boils, and is subsequently condensed and returned to the reactor.
[0006] U.S. Pat. No. 6,716,936 discloses the use of two or more light solvent boiling pool reactors in series for the polymerization of ethylene and comonomer(s) to produce bimodal polyethylene copolymers having lower densities than polyethylenes made using conventional stirred tank, slurry loop or gas phase reactor technologies where the polymer is not dissolved in the reaction medium.
[0007] U.S. Pat. No. 7,423,100 discloses polymerization processes to produce polymers utilizing boiling pool reactor systems and diluents including hydrofluorocarbons where the polymer is present in the reactor as solid particles in a slurry.
[0008] U.S. Pat. No. 4,501,865 discloses a process to remove quantities of heat from exothermically running polymerization reactions of vinyl monomers in heterogeneous phase, by adding liquids to the reaction medium for the removal of heat and to regulate the reaction temperature, which liquids do not dissolve the polymer under the reaction conditions and the boiling temperatures of which are lower than or are identical to the technically predetermined reaction temperatures under the polymerization conditions which are applied, and the heat which is released in the reaction system is removed by evaporating these liquids.
[0009] US 2014/0213745 discloses a method to prepare, and compositions pertaining to, an amorphous polymer comprising: at least 95 mol % propylene and 0 to 5 mol % vinyl monomer content, wherein the polymer has a g'.sub.vis of less than 0.95, an M.sub.n of about 200 to about 10,000, an .DELTA.H.sub.f of less than 10 J/g and has greater than 50% allylic chain end functionality. At page 10, paragraph 137, US 2014/0213745 states that "[a]ny bulk, homogeneous solution, boiling pool or slurry process known in the art can be used."
[0010] U.S. Pat. No. 8,058,371 discloses processes for polymerizing propylene, where: 1) about 40 wt % to about 80 wt % propylene monomer, based on total weight of propylene monomer and diluent, and about 20 wt % to about 60 wt % diluent, based on total weight of propylene monomer and diluent, can be fed into a reactor; 2) the propylene monomer can be polymerized in the presence of a metallocene catalyst and an activator within the reactor at a temperature of about 80.degree. C. or more and a pressure of about 13 MPa or more to produce a polymer product in a homogenous system; and 3) about 20 wt % to about 76 wt % (preferably about 28 wt % to about 76 wt %) propylene monomer, based on total weight of the propylene monomer, diluent, and polymer product, can be present at the reactor exit at steady state conditions.
[0011] U.S. Pat. No. 7,807,769 discloses a supercritical process to make isotactic propylene homopolymer having: 1) more than 15 and less than 100 regio defects (sum of 2,1-erythro and 2,1-threo insertions and 3,1-isomerizations) per 10,000 propylene units; 2) an Mw of 35000 g/mol or more; 3) a peak melting temperature of greater than 149.degree. C.; 4) an mmmm pentad fraction of 0.85 or more; 5) a heat of fusion of 80 J/g or more; and 6) a peak melting temperature minus peak crystallization temperature (Tmp-Tcp) of less than or equal to (0.907 times Tmp) minus 99.64 (Tmp-Tcp<(0.907.times.Tmp)-99.64), as measured in .degree. C. on the homopolymer having 0 wt % nucleating agent.
[0012] U.S. Pat. No. 7,812,104 discloses a process for producing a propylene-based olefin homopolymer or copolymer, where a monomer composition comprising propylene is contacted with a polymerization catalyst system under homogeneous polymerization conditions (such as solution, supersolution or supercritical conditions), wherein the polymerization catalyst system includes an activator and a bridged bis-indenyl transition metal (group 4) compound substituted with a carbazole (unsubstituted or substituted) at the 4 position.
[0013] Additional references of interest include: WO 1993/021241 and US 2013/0203946. None of the above disclose processes that can make both high-melting polymers, such as, for example, polypropylenes and soft, low melting polymers, such as, for example, ethylene-propylene or ethylene-(C.sub.4-C.sub.12 alpha olefin) rubbers in an efficient, non-fouling, low-cost process operating with homogeneous polymerization phases and utilizing coordination catalysts (such as single-site catalysts). Thus, there is a need for a clean, efficient, rapid, and low-cost process to make polyolefins and polyolefin blends comprising high-melting and/or low-melting components with enhanced properties in a continuous solution process where the heat of polymerization is removed without causing fouling of the reactor.
[0014] The current invention overcomes the limitations discussed above, thus allowing reduced investment and operating costs while also increasing operation reliability by operating the reactor in such a way (e.g., at or near the boiling point of the reaction medium) as to remove the reaction heat by evaporative means. The heat removal rate can be readily controlled by the rate of reflux, a method typically employed in refinery and petrochemical separations. These isothermal reactors thus operate with a homogeneous reaction medium enabling the optimal deployment of coordination catalysts (such as single-site catalysts). They also operate at lower pressures than the liquid filled conventional reactors making them cheaper to build and operate, and can often operate at higher solution viscosity than other designs owing to the agitation provided by the boiling liquid medium. Thus, the current invention provides a process that can produce polymers at higher concentrations while typically having less fouling than current processes.
[0015] The process of the instant invention also overcomes the deficiencies discussed above for ICP's by producing the stiff and soft components in separate reactors (one or both of which may use evaporative cooling) in which the process conditions and catalysts are optimized for the two different (stiff and soft) components. Further improvement, particularly for the soft component is also achieved by production in a homogeneous solution process utilizing coordination catalysts (such as single-site catalysts) homogeneous catalyst and, preferably evaporative cooling. Such operation mode and catalysts afford precise composition and molecular weight control and narrow molecular weight and composition distribution of the soft component, all useful to achieving the maximum benefit this component provides for the final product blend. A further benefit of the production of the stiff and soft components in the currently-disclosed separate, independently tunable reactors is the ability to precisely control the molecular weight and melt viscosity of the blend components. This in turn allows the development of the desired high and uniform dispersion of the soft product component in the continuous stiff phase affording the best utilization of the soft component for improving the toughness with the minimal degradation of the stiffness of the final product blend.
[0016] Further, the processes disclosed herein have: (1) improved/lower cost cooling due to heat exchange in the absence of polymer, which avoids fouling and increases heat exchange efficiency due to lower viscosity, and (2) higher polymer concentration in the reactor without reducing mixing, and thus without jeopardizing product quality, due to the mixing enhancement from the churn caused by boiling.
[0017] The processes of the instant invention operate with coordination catalysts (such as single-site catalysts) in a homogeneous reaction medium, free of slurry, and free of heterogeneous catalyst.
SUMMARY OF THE INVENTION
[0018] This invention relates to a continuous homogeneous solution phase polymerization process comprising contacting, in an evaporative reactor system at a temperature of 50.degree. C. or more and a pressure of 100 kPa or more, a catalyst system dissolved in the reaction medium, optional hydrocarbon diluent, optional scavenger, and one or more monomer(s) to form a homogeneous polymerization medium, where the polymer product is dissolved in the reaction medium at 10 wt % or more (determined by measuring the polymer wt % in the effluent at the exit of the reactor), and where the catalyst system comprises an activator and a coordination catalyst precursor compound and where all or part of the polymerization medium is evaporated during the polymerization and optionally, returned to the polymerization reactor after its condensation.
[0019] Advantageously, the diluent and/or monomer are partially evaporated during the polymerization to remove the heat of polymerization and the heat of mixing. The evaporated diluent and/or monomer is then condensed and, optionally, further cooled before returning it to the reactor. This creates a continuous recycle (often referred to as reflux in the art of chemical engineering), which effectively removes heat in the form of the heat of evaporation and, optionally, by further cooling of the recycle stream.
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Role(s) of TMA in polymerization

CATEGORY: POLYMERIZATION
Role(s) of TMA in polymerization

Type	Journal Article
Author	C. Ehm
Author	R. Cipullo
URL	http://pubs.rsc.org/en/content/articlelanding/2016/dt/c5dt04895h
Volume	45
Issue	16
Pages	6847-6855
Publication	Dalton Transactions
Date	2016.04.19
Abstract	Quenched-flow data for propene polymerization with rac-Me2Si(2-Me-4-Ph-1-indenyl)2ZrCl2/MAO support a picture where removal of MAO qualitatively changes the kinetic profile from a mainly enthalpic to a mainly entropic barrier. DFT studies suggest that a not previously recognized singly-bridged end-on coordination mode of Me6Al2 to catalytically active centers may be kinetically relevant as a resting state. In contrast, the more traditional doubly-bridged complex of Me3Al is proposed to be more relevant to chain transfer to cocatalyst.