REACTOR WITH FOAM SHEARING MEANS FOR SOLUTION POLYMERIZATION PROCESS

Abstract: A reactor for the solution polymerization of monomers equipped with a foam breaker (31) in the vapor space (28) thereof to prevent foam accumulation and fouling of the reactor.

Description: REACTOR WITH FOAM SHEARING MEANS FOR SOLUTION POLYMERIZATION PROCESS The present invention relates to a process and reactor design for the polymerization of compounds in a nonaqueous system. More particularly the present invention relates to a reactor which is operated partially full. That is, a portion of the volume of the reactor is vapor space. The vapor space additionally contains an agitator which aids in reducing the build up of foam under certain reaction conditions. The reactor is particularly suited for use in an anionic polymerization process. Processes for the polymerization of compounds, especially for the anionic polymerization of monovinyl- idene aromatic monomers and mixtures thereof with conjugated dienes are well known in the art. Because of the exothermic nature of most polymerizations and anionic polymerizations in particular, it is generally necessary to remove heat from the reaction mixture. If the temperature becomes excessive, undesirable chain transfer, premature termination, or side reactions may occur. At reduced temperatures, the time for each polymerization step to occur increases significantly, thereby lowering productivity. Therefore, lower and upper reaction temperature limits are normally chosen for optimum results. Because of such temperature constraints, it is generally necessary to provide a control system to regulate the temperature of polymerization mixtures. In United States Patents 3,231,635; 3,681,304; and 3,801,555 there are disclosed reaction processes for the anionic polymerization of monomers that employ ebullient temperature control systems. In these 10 processes, a relatively low boiling solvent is employed and the heat of vaporization of the reaction mixture is used to remove heat from the reaction medium. A condenser in operative communication with the reactor is -.Γ- employed to cool and condense the vaporized components and return the same to the reaction mixture. The latter two U.S. Patents particularly note the problems associated with foam generation. Boiling reactors may also be utilized in other polymerizations such as in the 20 free radical polymerization of monovinylidene aromatic monomers and/or alkenyl nitriles, especially in the preparation of styrene/acrylonitrile copolymers. Ebullient cooling methods are highly efficient 25 so that only small amounts of solvent need be used. Disadvantageously however, the previously known ebullient cooling processes have been limited in their ability to polymerize reaction mixtures due to the formation of a tenacious foam under certain reaction 30 conditions which occupies the available vapor space and enters the condensation equipment, thereby fouling the heat exchanger and rendering the system inoperable. In other solution polymerization systems, such as free radical, ring opening, cationic, condensation or coordination polymerizations, foaming of the polymerization mixture may be equally undesired even in the absence of a condenser. Accordingly, there remains a need to provide a suitable reactor for use in solution polymerizations, particularly those operating under ebullient cooling conditions, that provides a means to control the level of foam. It would also be desirable if there were provided a polymerization process, particularly a process employing ebullient cooling conditions, that is characterized by a means for the control of foam production. According to the present invention there is provided a reactor suitable for use in the polymerization of polymerizable compounds, said reactor comprising a vessel fitted with an inlet, outlet, agitation means immersed in the liquid reaction mixture, and condenser means for the condensation and return of volatile reaction components to the reactor, said reactor characterized by the presence of a mechanical means for imparting shearing forces to foam accumulating in the vapor space of the reactor. Also provided according to the present invention is a process for the polymerization of polymerizable compounds conducted in a reactor equipped with vapor condensation means and operating at least in part under ebullient cooling conditions with the generation of foam, characterized in that shearing agitation is imparted to the foam in the vapor space of the reactor sufficient to prevent fouling of the reactor condensation means. The invention is further illustrated by reference to Figure 1 where there is illustrated a vertically disposed reactor, comprising top, 1, and bottom, 3, fitted with inlet, -i , outlet, 5, and a heating jacket, 7; for circulation of a heat transfer fluid for thermal control. Also fitted to the reactor is a condenser, 9, supplied with circulating cooling fluid through connections at 11 and 13• The condenser is in operative communication with the reactor by means of an inlet, 15, which allows vapors to enter the condenser and discharge, 17, which allows condensate to return to the reactor. It is understood that a pressure regulating means, 16, may be employed to improve the condensing system. Such pressure regulating means may be a means to control the pressure of the reactor contents such as a source of vacuum or venting, preferably to a monomer scavenging and environmental control system. Alternatively however, the pressure regulating means may be a control system to provide an increased or reduced condensation rate by the condenser. Additional monomer, initiator or other reaction components may be added to the returning condensate through line, 18, in operative communication with a drop tube, 19, for discharging condensate, initiator, and other reaction components beneath the surface of the reaction mixture, 29, in order to obtain rapid dispersal thereof. The interior of the reactor is fitted with agitation means, 21, turned by means of a shaft, 23, connected to a source of rotational energy such as a motor driven gear reduction unit, 25, to provide sufficient mixing of the reaction mixture to ensure substantial homogeneity thereof. A foam shearing means, 31, in the embodiment of the invention illustrated in Figure 1, is also fitted to the shaft, 23, and moves in the vapor space, 28, above the surface of the reaction mixture to contact and cause shear degradation of foam that forms in the vapor space. Baffles, 27, on the inside surface of the reactor and in the vapor space serve to improve mixing of the reactor contents and to retard movement of foam in the vapor space to increase the differential velocity between the foam which is retarded by the baffles and the foam shearing means, thereby increasing the shearing force imparted to the foam. The foam shearing means (also referred to as a foam breaker) comprises elongated members, 33, that are spatially ordered and activated so as to provide shearing forces to the foam that accumulates in the vapor space. By disrupting the structure of foam formed in the reaction, vapor is released from the accumulated bubbles and may thereafter be drawn into the condenser, and drainage of liquid back to the reaction mixture is improved. The elongated members of the foam breaker assembly are desirably arranged to provide bracing and rigidity, and the foam breaker is symmetrically balanced with respect to the axis of rotation to reduce vibrational forces when in use. The elongated members preferably include bars of polygonal cross-section (thereby providing edges for greater shear forces) projecting radially from the same shaft used to activate the agitator of the reactor as well as bars arranged parallel to such shaft. The bars are joined together by welding, mechanical fastening or other means into a unitary structure. The arrangement of the individual elongated members is not critical to success, however, in a preferred arrangement for use in a substantially cylindrical shaped reactor, the foam breaker preferably includes bars that are positioned parallel to the axis of rotation of the foam breaker and in close proximity with the reactor wall or baffles. Preferably the clearance between the foam breaker and the outer wall, or baffles located on the outer wall, is from 1 to 50 mm, more preferably from 5 to 15 mm. The best shearing effect on foam is obtained at clearances within this range. While the foam breaker may be located in any portion of the vapor space of the reactor, most desirably it is located in the region closest to the surface of the reaction mixture without intervening mechanical devices separating the foam breaker from the surface of the reaction mixture. This arrangement has been found to be particularly effective because controlling foam generation in close proximity to the surface of the reaction mixture allows monomers to return rapidly to the liquid reaction mixture. Polymer uniformity is improved by such rapid return of monomers to the liquid reaction mixture. It should further be understood that whereas the present invention has been illustrated in a preferred embodiment by a common shaft for activating both the agitator and foam breaker as previously described, separate means for powering the foam breaker and agitator may also be employed. It should be noted, however, that the foam breaker of the present invention should be sized appropriately to generate a swept area substantially equal to the free and unimpeded cross-sectional area of the reactor at the surface level of the reaction mixture. By the term "unimpeded" is meant the area available for rotation of the foam breaker that is unimpeded by drop tubes and baffels located near the reactor wall. Preferably, such swept area is from 50 to 99 percent, most preferably 80 to 99 percent of such unimpeded cross-sectional area. The reactor and associated equipment, .including the foam breaker are constructed from steel, stainless steel, glass lined steel, or similar materials of construction. The foam breaker is rotated generally from 10 rpm to 300 rpm, preferably from 20 to 200 rpm, so as to provide effective reduction in foam buildup within the reactor due to shear induced foam collapse. Suitably, the foam breaker is rotated to provide a tangental velocity of the foam breaker of from 0.5 to 10 M/sec. Process conditions for polymerizations, including free radical, cationic, anionic, condensation and coordination polymerizations are well known in the art. Solvents particularly useful for the practice of ebullient cooling are inert hydrocarbons or mixtures of hydrocarbons having a boiling point at or near the desired temperature range for the polymerization. Preferred solvents are butane, pentane, isopentane, cyclopentane, hexane, cyclohexane, toluene, and mixtures thereof. The polymerization may be conducted at a wide range of temperatures. Preferred temperatures are from 30°C to 110°C, most preferably from 45°C to 100°C. The temperature may be adjusted by controlling the pressure of the reactor to induce boiling of the reactor contents. Once the reactor is at equilibrium, minor adjustments to the reactor pressure as small as 0.1 psi (700 Pa) are generally sufficient to regulate the boiling behavior. Additional heating and cooling can be incorporated into the reactor design, if desired. For example a jacket enclosing a circulating heat transfer fluid may be employed for partial control of temperature, or for use when ebullient cooling is not desired. Preferred polymerization processes for which the present invention is especially suited are anionic polymerizations, especially such polymerizations utilized to prepare block copolymers of vinylaromatic- and conjugated diene monomers. In the preparation of 10 such block copolymers any of three well known anionic polymerization techniques: use of multifunctional initiators, sequential polymerization or coupling of living polymer anions, may be used. All of the monomer -.-- may be added to the reactor before initiation of polymerization or monomer may be added continuously or incrementally during all or some of the polymerization. The anionic initiator employed in the anionic 2 Q process is not critical. Lithium alkyl compounds having from 2 to 6 carbons in the alkyl group, especially sec- butyl lithium and hydrocarbon soluble, difunctional lithium initiators are preferred. Suitable difunctional lithium initiators are well known and have 25 been previously disclosed in the following U.S. Patents: 4,169,115; 4,172,100; 4,172,190; 4,427,837; 4,196,154; and 4,205,016, the teachings of which are herein incorporated by reference. 30 Particularly desirable difunctional lithium containing compounds are selected from the group consisting of compounds corresponding to the formula: wherein R-l is independently each occurrence hydrogen or an inert radical having from 0 to 16 carbon atoms; R 2 is a divalent organic radical having at least 6 carbon atoms, R 2 having at least one aromatic ring and the aromatic ring being directly attached to a carbon which is attached to an aromatic ring of the above formula. R is independently each occurrence selected from the group consisting of alkyl, cycloalkyl, aromatic, mixed alkyl/aromatic, and mixed cyclo- alkyl/aromatic radicals containing from 1 to 20 carbon atoms. Especially preferred are initiating compounds of the formula: wherein R- | and R are as previously defined. By the term "inert" as used in this context is meant substituents that do not interfere with the desired anionic polymerization. In a most preferred embodiment, R -> is selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, aryl and mixtures thereof. Specific examples of difunctional initiators (DFIs) corresponding to the above formula are 1 ,3-phenylene bis(3-methyl-1-phenylpentylidene)bis- (lithium), 1 ,3-phenylene bis(3-methyl-1-(4-methyl- phenyDpentylidene) bis(lithium) , 1 ,3-phenylene bis(3- methyl-1-(4-ethylphenyl)-pentylidene) bis(lithium) , 1,3- phenylene bis(3-methyl-1-(4-(1 , 1-dimethyl- ethyDphenyl)pentylidene) bis(lithium) , and 1,4- phenylene bis(3-methyl-1-(4-dodecylphenyl)-pentylidene) bis(lithium) . Diene monomers suitable for use in the practice of the present invention include conjugated dienes, preferably 1 ,3-butadiene, isoprene and mixtures thereof. In addition to diene monomers, one or more olefin comonomers are additionally suitably employed. Any copolymerizable olefin comonomer may be employed. Preferred olefin comonomers are alkenyl aromatic monomers. By the term alkenyl aromatic monomer is meant a monomer of the formula: R c where n is an integer from 0 to 3. R is an alkyl radical containing up to 5 carbon atoms and R5 is hydrogen or methyl. Preferred alkenyl aromatic monomers are styrene, vinyl toluene (all isomers, alone or in admixture), α-methylstyrene, and mixtures thereof. Particularly preferred alkenyl aromatic monomers are styrene and mixtures of styrene and α-methylstyrene. Monomer and solvent purities are carefully monitored. Purification by contacting with molecular sieves, distillation, degassing, etc. may be employed to remove water, oxygen, and other contaminants. Prior to addition of initiator, reactive impurities may be removed by "blanking", that is, by addition of a small amount of lithium initiator to react with and remove the contaminant, but not enough- to begin polymerization. The polymerization is conducted for time periods suitable to achieve the desired product properties and conversions. Suitable reaction times are from 10 minutes to 3 hours, preferably from 20 minutes to 2 hours. Having described the invention, the following examples are provided as further illustrative and are not to be construed as limiting. Unless stated to the contrary, parts and percentages are based on weight. Example 1 Preparation of Styrene/Isoprene/Styrene Triblock Copolymer A 30 gallon (0.1 m 3 ) reactor substantially according to Figure 1 is loaded with 50.2 kg of solvent comprising of a blend of 65 weight percent cyclohexane and 35 weight percent isopentane. To the solvent was added 8.15 kg isoprene. (All solvents and monomers were purified for the removal of polar impurities such as water and degassed to remove oxygen.) After 20.18 millimoles pentamethyldiethylenetriamine were added, the solution was blanked using dilute 1 ,3-phenylene-bis(3- methyl-1 ,-[4-methylphenyl]pentylidene)bis(lithium) in order to remove any residual impurities that would consume any initiator. Polymerization was then initiated at 48°C and 16.2 psia (111 kPa) with 910 g of a solution of 1,3-phenylene-bis(3-methyl-1 ,-[4- methylphenyl]pentylidene)bis (lithium) (concentration was 0.073 mmole/g in cyclohexane). The agitator/foam breaker speed was set at 150 rpm. The polymerization temperature and boiling were controlled by venting as needed to induce boiling and heat removal. Foam levels were limited to a few centimeters from the solution surface and no condenser fouling occurred. The temperature did not exceed 70°C during the exotherm period. After the isoprene polymerization was completed, about 65 minutes into the run, 1.44 kg of styrene monomer was added in order to prepare the pure triblock polymer. During the second polymerization the reaction was conducted adiabatically, that is, no boiling of the reaction mixture took place. The reaction mixture was cooled and isopropanol added to terminate the active polymer anion. Polymer was recovered by devolatilization. Number average molecular weight was determined by gel permeation chromatography according to the technique of Runyon et al., J. Appl. Poly . Sci., 13, 2359 (1969) to be 146,000. Comparative In experiments similar to that of Example 1 but conducted in reactors without a mechanical foam breaker, the foam formed during the isoprene polymerizations was uncontrollable and fouled the condensers. Example 2 Preparation of Styrene/Butadiene/Styrene Tapered Triblock Copolymer The procedure of Example 1 was substantially repeated using 50.09 kg of solvent comprising a blend of 65 weight percent cyclohexane and 35 weight percent isopentane. 6.28 Kg butadiene, 3-09 kg styrene and 4.96 millimoles of pentamethyldiethylenetriamine were added to the reactor. Polymerization of the mixture of both monomers was initiated at 54°C and 26.4 psia (181 kPa) with 1421 g of a solution of 1 ,3-phenylene-bis(3- methyl-1,-[4-methylphenyl]pentylidene)bis(lithium) (concentration was 0.0758 mmole/g in cyclohexane). The agitator/foam breaker speed was set at 150 rpm. The polymerization temperature and boiling were controlled by controlling pressure through a vent. The foam level was kept within control specifications and the temperature did not exceed 75°C during the period of butadiene polymerization. After the butadiene polymerization was completed (about 65 minutes) the boiling phase was terminated by increasing the pressure. Styrene homopolymerization commenced and the reaction mixture was allowed to increase in temperature. The reaction was terminated by addition of isopropanol and the polymer recovered by devolatilization. The resulting product was a tapered triblock copolymer styrene-butadiene-styrene having Mn of 87,200. Example 3 m. Preparation of Styrene/Isoprene/Styrene Triblock Copolymer The same reactor as was employed in previous Examples was loaded with 49.81 kg of solvent composed of 10 a blend of 85 weight percent cyclohexane and 15 weight percent isopentane. (All solvent and monomers were purified for the removal of polar impurities such as water and degassed to remove oxygen.) The solution was then blanked in order to remove any residual impurities 15 that would consume any initiator. The reactor was heated to 62°C under 21.5 psia (147 kPa) pressure. 45.5 ml of a solution of sec-butyllithium (1.4835 normality in cyclohexane) was added and the agitator/foam breaker speed was set at 150 rpm. 20 Polymerization initiated immediately upon addition of 0.714 kg of styrene. The styrene polymerization was allowed to continue for 36 minutes. At a solution temperature of 64°C, 4.12 kg of isoprene was added. 2~ Ebullient cooling was utilized to control the reaction temperature. Foam generated during the polymerization was controlled by the foam breaker throughout the polymerization so that foam did not enter the condenser. After 16 minutes of polymerization, the temperature 30 reached 68°C. The agitator speed was increased to 200 rpm, and 2.04 kg more isoprene were added. After polymerizing for an additional 25 minutes, the temperature reached 69°C and an additional quantity (1.82 kg) of isoprene was added. After 27 more minutes, the temperature reached 72°C. The pressure was increased so that no further boiling occurred. A second quantity of styrene (0.710 kg) was then added and allowed to polymerize to form a triblock copolymer. Recovery was according to the techniques of Example 1. The polymer's number average molecular weight (Mn) was 168,000.

Claims: Claims: 1. A reactor suitable for use in the polymerization of polymerizable compounds, said reactor comprising a vessel fitted with an inlet, outlet, agitation means immersed in the liquid reaction mixture, and condenser means for the condensation and return of volatile reaction components to the reactor, said reactor characterized by the presence of a mechanical means (foam breaker) for imparting shearing forces to foam accumulating in the vapor space of the reactor. 2. A cylindrical shaped reactor according to claim 1 wherein the foam breaker comprises a rigid structure formed from elongated bars projecting substantially radially from an axially positioned shaft in the vapor space of the reactor and bars arranged parallel to said shaft. 3. A reactor according to claim 2 wherein the shaft additionally is attached to and activates the agitation means. 4. A reactor according to claim 2 wherein the area swept by the foam breaker is from 50 to 99 percent of the unimpeded cross-sectional area at the surface level of the reaction mixture. 5. A process for the solution polymerization of polymerizable compounds conducted in a reactor equipped with vapor condensation means and operating at least in part under ebullient cooling conditions with the generation of foam, characterized in that shearing -- agitation is imparted to the foam in the vapor space of the reactor sufficient to prevent fouling of the reactor condensation means. 6. A process according to claim 5 wherein the 10 shearing agitation is imparted by a mechanical foam breaker. 7. A process according to claim 5 which is an anionic, free radical, ring opening, cationic, ,,- condensation or coordination polymerization. 8. A process according to claim 6 wherein the reactor is vertically disposed and cylindrically shaped and the foam breaker rotates in the vapor space at from 0 10 to 300 rpm. 9. A process according to claim 8 wherein the area swept by the rotating foam breaker is from 50 to 99 percent of the unimpeded cross-sectional area of the c 5 reactor at the surface level of the reaction mixture. 0

Inventors: BREDEWEG CORWIN J, MASS ROBERT O, TUBERGEN PAUL R, BREDEWEG, CORWIN, J., MASS, ROBERT, O., TUBERGEN, PAUL, R.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1991
Main classification: C08F2
IPC codes: B01J19, C08F2, C08G85