Solution polymerization processes are carried out at temperatures that are above the melting point of the product polymer. In a typical process, catalyst components, solvent, polymerizable monomers and hydrogen are fed under pressure to one or more stirred tank reactors. Catalyst components may be fed to the reactor as a solution or as a slurry and the temperature of the reactor is controlled by the rate of catalyst addition, the temperature of the catalyst feed stream and/or the use of heat transfer systems. Typical polymerizable monomers for solution phase polymerization processes include ethylene, styrene, propylene and various dienes.
For ethylene polymerization, reactor temperatures can range from about 130° C. to about 250° C. while pressures are generally in the range of from about 500 to about 4000 psi. Although catalyst residence times are generally short (e.g. minutes) due to the harsh reactor conditions, if desired, solution polymerization may be operated under a wide range of process conditions that allow tailoring of the product polymer as well as rapid product swings.
In solution polymerization, product polymer is molten and remains dissolved in the solvent under reactor conditions, forming a polymer solution. After a selected hold-up time (i.e. catalyst residence time), the polymer solution leaves the reactor as an effluent stream and the polymerization reaction is quenched, typically with coordinating polar compounds, to prevent further polymerization. Once quenched, the polymer solution is typically fed to a flash devolatilization system for solvent removal. Flash devolatilization also removes un-reacted monomers from the polymer solution.
Under certain conditions of temperature and pressure, the polymer solution can phase separate into two distinct liquid phases, one which is “lean” in dissolved polymer and one which is “rich” in dissolved polymer. Phase separation occurs at the lower critical solution temperature (LCST), also known as the “cloud point”. Increasing the temperature, or decreasing the pressure at the cloud point leads to further phase separation. The cloud point is determined in part by the pressure, temperature, solution composition and the solvent used for polymerization.
It is generally undesirable to have phase separation occur within the polymerization reactor, and process conditions such as monomer concentration, temperature and pressure are controlled to avoid liquid-liquid phase separation. For example, the polymerization temperature may be kept between the crystallization boundary and the LCST of the polymer solution for a given pressure, solvent and monomer concentration. However, once the polymer solution leaves the reactor, it may be beneficial to promote liquid-liquid phase separation as it can facilitate separation of volatile components from the polymer product.
U.S. Pat. Nos. 3,553,156 and 3,726,843 assigned to du Pont de Nemours describes a process in which the reactor effluent, an elastomeric ethylene copolymer solution, is induced to undergo a liquid-liquid phase separation into “polymer rich” and “polymer lean” fractions through the release of pressure by use of a pressure let down valve. The two liquid phases are decanted from one another in a settlement chamber and the polymer rich phase is fed into a low-pressure separator to boil off residual solvent and un-reacted monomer. The polymer lean phase is recycled to the reactor. The process substantially reduces the energy lost by evaporation of volatiles (i.e. the heat of vaporization) in a devolatilization chamber by separating out the volatiles in a “polymer lean” liquid phase.
In U.S. Pat. No. 4,857,633 assigned to Exxon Research & Engineering, a high temperature solution process is described in which a low molecular weight hydrocarbon is added to a polymer solution to facilitate phase separation of a polymer solution under certain conditions of temperature and pressure.
U.S. Pat. No. 6,881,800 assigned to ExxonMobil, discloses a process and apparatus to separate a polymer solution into polymer rich and polymer lean liquid phases prior to devolatilization. The apparatus includes a pressure source, a polymerization reactor, a pressure let down device, and a separator downstream of one another respectively. In the process, the high pressure source is used to maintain a single liquid phase in the polymerization reactor, while the pressure let-down device facilitates the formation of a two-phase liquid-liquid system having a polymer rich phase and a polymer lean phase. Separation of these phases is accomplished by way of a liquid phase separator that feeds the polymer rich phase to a chamber at lower pressure in order to flash off residual solvent and un-reacted monomer.
Similarly, U.S. Pat. No. 5,599,885 assigned to Mitsui Petrochemicals, describes a solution polymerization process in which phase separation downstream of the reactor is used to facilitate polymer isolation. The reactor effluent is separated into a lower phase that is rich in polymer and an upper phase that is rich in solvent by increasing the temperature of the polymer solution within a “separation zone”. The temperature is raised to more than 180° C. above the upper cloud point temperature of the polymer solution. Polymer is recovered from the lower phase, while the upper phase is in part recycled to the reactor.
In U.S. Pat. No. 4,444,922, an improved phase separation process is described. Temperatures and pressures are moderated to produce “spinodal decomposition” driven phase separation as opposed to “nucleation and growth” driven phase separation. Spinodal decomposition driven phase separation is a form of phase separation that promotes rapid partitioning and settling of the polymer lean and polymer rich phases. The process facilitates separation of the distinct liquid phases by way of a liquid-liquid separator or a decanter.
In a typical devolatilization process, the polymer solution (reactor effluent) is pre-heated in a heat exchanger and then passed into a chamber of reduced pressure. Boiling of solvent and un-reacted monomers occurs and the vapors are sent to a solvent and monomer recovery system and are recycled back to the reactor. Heating the polymer solution upstream of the devolatilization system increases the enthalpy of the product stream, providing high temperatures to the polymer melt after devolatilization. The high temperatures facilitate flow of the polymer melt by reducing its viscosity. The heat exchangers used are most commonly shell and tube type heat exchangers and can increase the temperature of the polymer solution to as high as about 280° C.
U.S. Pat. No. 4,547,473 describes a typical high temperature solution process for the homo- or co-polymerization of ethylene at temperatures in excess of 150° C. using a titanium based catalyst system. Solvent is removed using standard flash devolatilization as described in U.S. Pat. No. 5,708,133.
In PCT application, 98/02471 filed by Dow Chemicals, a solution polymerization process is described in which a two stage devolatilization system is used to remove solvent and un-reacted monomers from an EPDM (ethylene-propylene-diene monomer) polymer solution. In a preferred embodiment a dual reactor system is used in which the temperature of the second reactor is between 90° C. and 120° C. For flash devolatilization, the temperature of the reactor effluent is raised to between 210° C. and 250° C. by passage through a heat exchanger prior to entering the flash chamber that is at lower pressure.
U.S. Pat. No. 5,691,445 assigned to Novacor Chemicals describes a polymer solution devolatilization process in which less than 150 ppm of residual volatiles is retained in the isolated polymer. In the process, the polymer solution leaves the polymerization reactor and travels through a pre-heat exchanger. The pre-heat exchanger heats the polymer solution to temperatures from about 200° C. to 270° C. to increase the vapor pressure of volatiles and to reduce the polymer solution viscosity. In a preferred embodiment, a super-critical fluid is added to the process at a point between the first and second devolatilization chambers to enhance polymer melt foaming.
The efficiency of a heat exchanger is a major consideration when determining the volume of polymer solution that may be adequately heated by a given heat transfer fluid. The overall amount of heat transfer depends on a number of factors, including but not limited to the materials used for construction of a heat exchanger, the area of the heat exchange surface (i.e. the number, length and diameter of tubes in the tube sheet of a shell and tube type heat exchanger), the rate of flow of polymer solution and/or the heat transfer fluid through the tube and shell sides of the heat exchanger respectively, whether the flows are parallel counter-current or parallel co-terminus, the nature of fluid flow (turbulent or Newtonian), and the nature and composition of the exchanging fluids.
Optimization of heat transfer is most commonly addressed though the design and construction of the associated heat exchanger equipment. As a result, significant capital investment may be required for making suitable upgrades such as the installation of inserts to increase turbulent flow within the heat exchanger tubes, the use of larger heat exchangers or the use of heat exchangers with more heat exchange capacity. Alternatively, the heat transfer fluid may be heated to higher temperatures, but this requires significantly higher energy input.
There remains a need for improving the efficiency of heat transfer within the one or more heat exchangers, associated with a solution polymerization process, without requiring large capital investments or increased operating costs.
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