Process for separating a polymer from a solution containing the same

Abstract: In order to recover a polymer, such as styrene butadiene copolymer rubber, from a solution obtained by a solution polymerization of the corresponding comonomers, without degradation or deterioration of the polymer, the solution, ordinarily in 3 to 30% by weight concentration, is separated into the solvent and a more concentrated polymer solution of not less than 50% by weight, by heating the solution under a pressure at such a temperature (A) that the polymer does not decompose, heating a solvent, which is preferably the same as that for the solution, at a higher temperature than (A) above under a pressure, and joining the heated solution and the heated solvent in a mixer to convey to a flash evaporator.

Description: The present invention relates to a process for separating a polymer from a solution containing it. More particularly, it relates to a process in the step for recovery of a polymer from the polymer solution prepared according to a polymerization method known as so-called "solution polymerization". In other words, it relates to a process for the separation and recovery of such a polymer by the removal of a hydrocarbon solvent, such as is generally used in this method, from the polymer solution. The term "solvent" referred to in the description involves the solvent containing any residual or unaltered monomer. Generally, the process for the production of polymers according to a solution polymerization comprises an apparatus to remove the solvent from the polymer solution which has been prepared in a polymerization apparatus. Various kinds of solvent-removing apparatus, have previously been employed. In the case of production of thermoplastic polymers, generaly speaking, the polymer mixture is heated by an indirect heat-exchanging apparatus, and then the solvent is flash-evaporated in a vessel utilizing the heat. However, when a solution of a polymer having a limited thermally durable temperature, such as styrene-butadiene copolymer, is processed by this method, the polymer tends to degrade during the course of the heating step in an indirect heat-exchanging apparatus, thus yielding a deteriorated product. In order to avoid such degradation of polymer and to avoid blockages in the heat-exchanger the degree of the heating in the heat-exchanger should be limited to a moderate temperature. Thus the concentration of the polymer after the flash evaporation remains comparatively low (for example, less than 50% by weight). Accordingly, the method is no more than a preliminary concentration step. These are disadavantages of the conventional process. EP-A-27700 describes a step preparatory to flash evaporation in which an "inert stripping agent" such as a C 1-4 hydrocarbon solvent or, preferably, steam at a temperature equal or close to the heated polymer solution is added to the solution. This invention aims to solve the previous disadvantages, and pertains to a process in the step to recover a polymer from a solution containing the same, for the separation of the solution into the solvent and a more concentrated polymer solution containing not less than 50% by weight, preferably not less than 80% by weight, of the polymer, which process comprises heating a less concentrated solution of the polymer to a temperature at which the polymer remains thermally stable, heating a solvent to a temperature above the temperature to which the said polymer solution has been heated; and mixing the heated solution and the heated solvent together and evaporating the mixture in a flash evaporator, wherein the heated solvent is the same as the solvent of the less concentrated solution. More particularly, the invention pertains to a process in the recovery of a polymer from a less concentrated solution in a C 5-10 hydrocarbon solvent by heating the polymer solution by means of a heat source at a temperature at which the polymer does not degrade and under a pressure sufficient to keep the solution in the liquid phase; heating a solvent which is the same as the solvent of the less concentrated solution to a temperature sufficient to give the necessary heat quantity to obtain the desired concentration of the polymer from a mixture with the less concentrated solution by flash evaporation and under a pressure sufficient to keep the heated solvent in the liquid phase; and joining the flows of the thus-heated and pressurised less concentrated solution and the thus-heated and pressurised solvent and conveying the mixture to a flash evaporator. According to the present invention, a solution of a polymer having a shorter thermo-durable period of time, in a lower concentration, can be converted into a more concentrated solution of the polymer up to 80% by weight or more, with ease and high efficiency, without deterioration of the polymer quality. Polymer concentrate is discharged from a flash evaporator, and, if necessary, is then charged to a vent extruder or a mixing dryer wherein remaining solvent is further vaporized off. The process of the present invention may suitably be applied to the step of separation and recovery of the polymer from the polymer solution prepared according to a solution polymerization of, for example, polybutadiene rubber, styrene butadiene copolymer rubbers (SBR and modified SBR). etc. The present invention will more fully be described with respect to a styrene-butadiene copolymer, which is, however, merely an example. Styrene-butadiene copolymers reduced by solution polymerization are generally obtained by polymerizing the monomers in the hydrocarbon solvent having 5 to 10 carbons, particularly hexane, etc., using a lithium catalyst, such as metallic lithium, an alkyllithium, etc. The polymer solution discharged from the solution polymerization vessel is ordinarly of a concentration of 3 to 30% by weight in case of styrene-butadiene copolymers. Such polymer solution may be subjected, if necessary, to filtration or rinsing with water, an acid or an alkali to remove the catalyst, before the heating. The heating is effected under a pressure sufficient to keep the solvent in the polymer solution in the liquid state, in other words, under a pressure higher than the equilibrium pressure displayed by the solvent of the polymer solution and its vapor at the temperature after the joining of the flow with the flow of the heated solvent. If such pressure is not held, the solvent is vaporized in the heat-exchanger, and the polymer content in the solution is partially increased at the piping, etc. of the heat-exchanger, causing undesirable blockages in the piping. In this case, the heat put in to the polymer solution should not exceed the thermally durable temperature which is determined according to the heated period of time and the kind and amount of the stabilizer added. In other words, it should not exceed the upper limit temperature above which the quality of the product would be deteriorated. Meanwhile, the solvent to be joined with the said solution is heated under a pressure higher than the equilibrium pressure displayed by the solvent and its vapor at the heated temperature. In this case, the heat put in to the solvent should be sufficient to convert an amount of the solvent to vapour, such that the total solvent in the joined liquid is that of the heated polymer solution and the heated solvent, which is eliminated with the solvent accompanying the recovered polymer (namely a more concentrated polymer solution). In view of the characteristics of the present invention, accordingly, the heating temperature for the solvent is higher than the heating temperature for the polymer solution. Besides, the heating temperature for the solvent depends upon the ratio of solvent to polymer solution used and this temperature should be higher as this ratio decreases. The most economical mixing ratio of the solvent is not determined primarily, but it depends upon the thermally durable temperature of the polymer, the kinds of the heating source or heating medium for the solvent, the temperature, the equilibrium vapour pressure of the solvent, and the costs of constructions of heat exchanger and flash evaporator, as a whole. The solvent to be mixed to the polymer solution according to the present invention is the same as the solvent used for the polymer. The solvent separated finally from the polymer may then be reused. The mixing of the heated polymer solution and the heated solvent according to the invention should be conducted while the polymer stays thermally durable. For that purpose, and agitating mixer having a retention time within the thermodurable period of time may be employed. In most cases, a well-known mixing device, such as a line-mixer and so-called static mixer needing no power, is satisfactorily used in general. The kind of such mixer employed may be decided depending upon the ratio of polymer solution to solvent, as well as their fluidal characteristics. The liquid joined in the mixer is then conveyed to a flash evaporator where the liquid is flash-evaporated to separate a polymer solution of a higher concentration and the solvent. The present invention will be described more concretely with respect to the following Example and the Comparative Example, which are, however, presented merely as examples and not for limitative purpose. Figure 1 is a schematic diagram showing the assemblage of apparatus employed in the Example of the present invention. Figure 2 is a schematic diagram showing the assemblage of apparatus employed in the Comparative Example of the present invention. Example An experiment for the separation of a polymer solution into the solvent and a more concentrated polymer solution was conducted with the terms as described below. As the results, a more concentrated polymer solution was obtained without deterioration of the polymer The samples tested: A hexane solution containing 25% by weight of a binary copolymer consisting of 25% by weight of styrene unit and 75% by weight of butadiene unit; and a hexane solvent The apparatus: The assemblage of apparatus as shown in Figure 1 was employed. In the Figure, A is a feeding vessel for the polymer solution; B, a feeding vessel for the solvent; C, a feeding pump for the polymer solution; D, a feeding pump for the solvent; E, a heat exchanger to heat the polymer solution; F, a heat exhanger to heat the solvent; G, a mixer; H, a flash evaporator; I, a flow line for solvent; and J, a flow line for the more concentrated polymer solution. The essential equipments: 1) The mixer 1 ○ The junction of the hexane solution and the hexane solvent: The polymer solution piping (nominal size, 3mm (1/8 inch)) was inserted concentrically (60mm long) into the hexane piping (nominal size, 1.3cm (1/2 inch)). 2 ○ The line mixer: No power loaded. Dimension: nominal size 1.3cm (1/2 inch), length 144mm Interpositions: six elements, each 24mm length, twisting 180°. Material: SUS-304 (a Stainless Steel) 2) The flash evaporator: Consisting of the upper cylindrical portion with 445 mm inner diameter and 650mm height, and the lower conical portion with 400mm height and 130mm bottom diameter, equipped with a heating jacket. Material: SUS-304 (a Stainless Steel) The conditions: 1)The polymer solution feeding rate; 80 kg/ hour 2) The polymer solution heating temperature; 150°C (the heating medium, 0.98 MPa (10 kg/cm 2 ) Guage saturated steam) 3) The hexane feeding rate; 53 kg/hour 4) The hexane heating temperature; 236°C 5) The temperature at the junction of the polymer solution and the hexane solvent; 192°C 6) The pressure in the flash evaporator; 49 kPa (0.5 kg/cm 2 ) Gauge 7) The flash evaporator jacket temperature; 140°C The results: A more concentrated polymer solution containing 85% by weight of the polymer was obtained constantly. The polymer was not thermally degraded, keeping the quality unaltered. Comparative example An experiment of the separation of a polymer solution was conducted with the terms which are deviated from the characteristics of the present invention, as described below. As the results, the experiment has to be discontinued, due to the thermal degradiation of the polymer. The sample tested: A hexane solution containing 15% by weight of the polymer having the constitution as described in the Example. The apparatus: The assemblage of apparatus as shown in Figure 2 was employed. In the Figure, A is a feeding vessel for the polymer solution; C, a feeding pump for the polymer solution; E, a heat exchanger to heat the polymer solution; H, a flash evaporator; I, a flow line for solvent; and J, a flow line for a more concentrated polymer solution. The flash evaporator is the same as that used in the Example above. The conditions: 1) The polymer solution feeding rate; 133 kg/ hour 2) The polymer solution heating temperature; 192°C (the heating medium, 220°C oil) 3) The pressure in the flash evaporator; 0.5 kg/ cm 2 Gauge 4) The flash evaporator jacket temperature; 140°C The results: The experiment could not be continued, because of the increased flow resistance in the heat exhanger to heat the polymer solution. Teardown and inspection of the apparatus revealed that the polymer solution in the heat exchanger and its downstream piping had been thermally degraded, thus the polymer being deteriorated.

Claims: A process for recovery of a polymer from a less concentrated solution of the polymer in a C 5-10 hydrocarbon solvent, comprising heating the less concentrated solution to a temperature at which the polymer remains thermally stable, heating a solvent to a temperature above the temperature to which the less concentrated solution is heated, mixing together the heated solution and the heated solvent and evaporating the mixture in a flash evaporator to produce a more concentrated polymer solution containing not less then 50% by weight of the polymer, characterised in that the heated solvent is the same as the solvent of the less concentrated solution. A process according to claim 1 wherein the less concentrated solution is heated by means of a heat source to a temperature at which the polymer does not degrade under a pressure sufficient to keep the solution in liquid phase, the solvent is heated to a temperature sufficient to give the necessary heat quantity to obtain a desired concentration of the less concentrated solution by flash evaporation when the solvent and solution are mixed under a pressure sufficient to keep the solvent in the liquid phase, and the mixture is conveyed to a flash evaporation step. A process according to claim 1 or 2, wherein the more concentrated solution contains not less than 80% by weight of the polymer. A process according to any one of claims 1 to 3, wherein the polymer is a styrene-butadiene copolymer rubber.

Inventors: INOUE YOSHIO, TAKAO HIROYOSHI, YAMAZAKI HIROTAMI, INOUE, YOSHIO, TAKAO, HIROYOSHI, YAMAZAKI, HIROTAMI
Applicants: SUMITOMO CHEMICAL CO, SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1983
Main classification: C08F6
IPC codes: C08C2, C08F6