Method for recovery of solvent used in production of polyarylene sulfide

Abstract: The present invention provides a method for recovery of solvent used in production of polyarylene sulfide according to which deterioration of the solvent is little and recovery rate is high and which comprises subjecting a mixed liquid mainly composed of solvent used in production of polyarylene sulfide and generated from production process of polyarylene sulfide to flash evaporating operation to separate the mixed liquid into flashed vapor (A) and flashed residue (B) and then rectifying the flashed vapor (A).

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a method for recovery of solvent which has been used in the production of polyarylene sulfide. More particularly, it relates to a method according to which a solvent which has been used in the production of polyarylene sulfide can be efficiently and stably recovered. (2) Description of the Related Art In general, solvents used for the production of polyarylene sulfide such as N-methylpyrrolidone are expensive. Therefore, in order to reduce production cost, it is necessary to recover the solvent for the production of polyarylene sulfide from a mixed liquid mainly composed of the solvent and generated in respective steps of the production of polyarylene sulfide. The mixed liquid mainly composed of solvents used which is generated in respective steps of the production of polyarylene sulfide includes, for example, (1) a solvent solution generated at a step of separating polymer and precipitate from a reaction mixture after the polymerization reaction of polyarylene sulfide, (2) a washing liquid generated at a step of washing the polymerized polyarylene sulfide, or (3) a mixture of the solvent solution and the washing liquid. Such a mixed liquid (mainly composed of solvent for the production of polyarylene sulfide) normally contains impurities such as unreacted monomers, oligomers, solvent deterioration products, and, in some case polymerization aids, in addition to the solvent. Therefore, in order to recover the solvent used in the production of polyarylene sulfide from the mixed liquid, it is necessary to remove the above impurities from the mixed liquid. For recovering the solvent used in the production of polyarylene sulfide by removing the impurities from the mixed liquid, a method has been proposed which comprises subjecting a reaction mixture after the polymerization reaction of polyarylene sulfide to adiabatic flashing at high temperature conditions to volatilize the solvent (Japanese Patent Kokai No.61-53324 and U.S.Patent No.3,941,664). However, the above method suffers from the problems that (1) the flashing tank requires special means such as a specific stirring means, evaporation means, heat source, and means for feeding solids such as polymerization aid and thus the recovery cost increases, so that the solvent for the production of polyarylene sulfide cannot be efficiently recovered and besides (2) there remains much of unvolatilized solvent used for the production of polyarylene sulfide and hence the solvent cannot be recovered in high yield. Another method has been proposed which comprises rectifying the mixed liquid by a rectification column to obtain a rectified solvent used in the production of polyarylene sulfide and bottoms (residue) of the rectification column and subjecting the bottoms to an evaporator to further recover the solvent remaining in the bottoms, thereby to increase the recovery rate (Japanese Patent Kokai No.62-253,624). However, this method also has the following problems. (1) Generally, the mixed liquid contains solid matters such as oligomers and polymerization aids which are soluble in the solvent and they are apt to cause blocking of the rectification column and so the solvent cannot be stably recovered. Especially when the mixed liquid contains a polymerization aid, normally solid matters are contained in a larger amount in the mixed liquid and hence, a phenomenon of preventing stable recovery is more conspicuous. (2) When the compositions of the mixed liquid are changed, the operation conditions of the rectification column vary and control thereof takes time and thus the solvent cannot be recovered continually and efficiently. SUMMARY OF THE INVENTION The present invention has been accomplished under the above circumstances. That is, the object of the present invention is to provide a method for efficient and stable recovery of solvent used in the production of polyarylene sulfide, said method being free from the above-mentioned problems of conventional techniques. As a result of intensive research conducted by the inventors to solve the above problems, it has been found that the above object can be attained by carrying out specific treatments in a specific combination and the present invention has been accomplished. That is, the present invention is a method for recovery of solvent used in the production of polyarylene sulfide, characterized in that a mixed liquid which is produced in the steps of production of polyarylene sulfide and which is mainly composed of solvent used in the production of polyarylene sulfide is subjected to flashing evaporation at a temperature of 100-220°C and an internal pressure of 6.66-101.3 kPa (50-760 Torr) to separate the mixed liquid into a flashed vapor (A) and a flashed residue (B) and then the flashed vapor (A) is rectified. In the above recovery method, the flashed residue (B) may be separated into an evaporated vapor (C) and an evaporated residue (D) using an evaporator having the function to discharge solid or slurry of high viscosity and this evaporated vapor (C) may be rectified. Furthermore, in the above recovery method, the flashed vapor (A) and the evaporated vapor (C) may be rectified by the same rectification column and in this case the flashed vapor (A) and the evaporated vapor (C) may be previously mixed and the mixture may be rectified. Furthermore, the flashed vapor (A) and The evaporated vapor (C) may be fed to the same rectification column at different positions depending on their compositions to perform the rectification. BRIEF DESCRIPTION OF THE DRAWINGS Figs.1, 2 and 3 are flow sheets of the recovery of solvents used in the production of polyarylene sulfide in working examples of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention will be explained in the order of flash evaporation operation, separation using an evaporator and rectification. Flash evaporation operation This flash evaporation operation separates a mixed liquid mainly composed of solvent used which is generated in the production steps of polyarylene sulfide into a flashed vapor (A) and a flashed residue (B). The mixed liquid mainly composed of solvent used in the production of polyarylene sulfide and generated in the production steps of polyarylene sulfide includes, for example, a solvent solution obtained by subjecting a reaction mixture after the polymerization reaction of polyarylene sulfide to a solid-liquid separation, a washing liquid generated at the step of washing solid matter containing polyarylene sulfide which is obtained by the solid-liquid separation, a mixed liquid comprising the above solvent liquid and the above washing liquid, a solvent liquid obtained by further subjecting the above washing liquid to a solid-liquid separation, the above solvent liquid or the above mixed liquid, and a mixture of these various solvent liquids. Generally, the mixed liquid mainly composed of the solvent is mainly composed of the solvent used in the production of polyarylene sulfide and in addition contains water, unreacted monomers, polymerization aids, oligomers, solvent deteriorated products, alkali metal salts, and solvents other than the polymerization solvent such as acetone. The solvent used for the production of polyarylene sulfide is normally used for the polymerization of polyarylene sulfide and includes, for example, organic amide compounds, lactam compounds, urea compounds, and cyclic organophosphorus compounds. As examples of the solvent, mention may be made of N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dipropylacetamide, N,N-dimethylbenzoic acid amide, caprolactam, N-methylcaprolactam, N-ethylcaprolactam, N-isopropylcaprolactam, N-isobutylcaprolactam, N-n-propylcaprolactam, N-n-butylcaprolactam, N-cyclohexylcaprolactam, N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-isopropyl-2-pyrrolidone, N-isobutyl-2-pyrrolidone, N-n-propyl-2-pyrrolidone, N-n-butyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, N-methyl-3-methyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, N-ethyl-3-methyl-2-pyrrolidone, N-methyl-3,4,5-trimethyl-2-pyrrolidone, N-methyl-2-piperidone, N-isopropyl-2-piperidone, N-methyl-2-piperidone, N-ethyl-2-piperidone, N-isopropyl-2-piperidone, N-methyl-6-methyl-2-piperidone, N-methyl-3-ethyl-2-piperidone, tetramethylurea, N,N'-dimethylethyleneurea, N,N'-dimethylpropyleneurea, 1-methyl-1-oxosulfolane, 1-ethyl-1-oxosulfolane, 1-phenyl-1-oxosulfolane, 1-methyl-1-oxophospholane, 1-n-propyl-1-oxophospholane and 1-phenyl-1-oxophospholane. The above-mentioned mixed liquid may contain one of these solvents or two or more of them. In many cases, the mixed liquid contains non-protonic organic amides or lactams and among them, N-alkylpyrrolidones, especially N-methylpyrrolidone are contained. The mixed liquid usually contains the solvent used in the production of polyarylene sulfide in an amount of 80-99% by weight and contains substantially no polyarylene sulfide. Especially when the content of the solvent is 90-99% by weight within the above range, the flash evaporation is easy and the effect of the present invention can be more highly exhibited. When the mixed liquid contains unreacted monomers, the content of the unreacted monomer in the mixed liquid is normally 0.1-2.0% by weight. When the mixed liquid contains water, the content of water in the mixed liquid is normally 0.1-5.0% by weight. When the mixed liquid contains solvent deterioration products, the content of the solvent deterioration products is usually 0.1% by weight or less. When the mixed liquid contains oligomers, the content of the oligomers in the mixed liquid is usually 0.1-2.0% by weight. When the mixed liquid contains polymerization aids, the content of the polymerization aids in the mixed liquid is usually 3.5% by weight or less. When the mixed liquid contains alkali metal salts, the content of the alkali metal salts is usually 0.1-5.0% by weight. When the mixed liquid contains solvents other than those used for the production of polyarylene sulfide, the content of these solvents is usually 5.0% by weight or less. In the above flash evaporation operation, the mixed liquid is heated to 100-220°C, preferably 130-180°C and is poured into a flash drum of 6.66-101.3 kPa (50-760 Torr), preferably 6.66-40 kPa (50-300 Torr) in internal pressure. If the elevating temperature of the mixed liquid is lower than 100°C, the separation efficiency between flashed vapor (A) and flashed residue (B) is inferior. On the other hand, if the elevation temperature is higher tham 220°C, the solvent used in the production of polyarylene sulfide is deteriorated. If a heating source is provided in the flashing drum and the internal temperature of the flashing drum is adjusted to 100-220°C, the mixed liquid can be introduced, as it is, into the flashing drum without heating the mixed liquid to 100-220°C. If the internal pressure of the flashing drum is lower than 6.66 kPa (50 Torr), a high performance and expensive apparatus must be used as a vacuum unit used for the flashing drum and the cost is increased. If the internal pressure is higher than 101.3 kPa (760 Torr), the separation effeciency between flashed vapor (A) and flashed residue (B) decreases. The flashed vapor (A) obtained by the flash evaporating operation generally contains solvent used for the production of polyarylene sulfide, unreacted monomers, water, and, in some cases, solvents such as acetone. The content of the solvent used for the production of polyarylene sulfide in the flashed vapor (A) is normally 90-99.9% by weight and especially when it is 95-99.9% by weight, the effect of the present invention can be more highly exhibited. The flashed residue (B) normally contains solvent used for the production of polyarylene sulfide, solvent deterioration products, oligomers, polymerization aids such as alkali metal halides, alkali metal salts and the like. The content of the solvent for the production of polyarylene sulfide in the flashing residue (B) is usually 5-90% by weight. Separation by an evaporator: According to the present invention, the solvent used for the production of polyarylene sulfide can be efficiently recovered by rectifying the flashed vapor (A) obtained by the flash evaporating operation, and it can be more efficiently recovered by rectifying the flashed vapor (A) and an evaporated vapor (C) separated from the flashed residue (B) by separation using this evaporator. The flashed residue (B) is separated into the evaporated vapor (C) and an evaporated residue (D) by the separating operation carried out by this evaporator. A solid or highly viscous slurry is discharged from the evaporator as the evaporated residue (D). The evaporator used preferably has (1) a function to carry a solid or highly viscous slurry produced with decrease of organic solvent in the evaporated residue (D) to an outlet without growing it as deposits (self-cleaning function) or (2) a function to make the amount of useful solvent remaining in the evaporated residue (D) as small as possible by a highly efficient heat transfer and interfacial renewal. As evaporators satisfying these requirement mention may be made of vertical or horizontal thin film type evaporators and single-axis or horizontal rotor evaporators having one or more rotors. The operating conditions of the evaporators are usually a temperature of 100-260°C, preferably 150-220°C and a pressure of 6.66-101.3 kPa (50-760 Torr), preferably 6.66-40 kPa (50-300 Torr). However, higher temperature and higher vacuum than the operating conditions for the above-mentioned flashing evaporation are preferred for an improvement of the efficiency for recovery of solvent used for the production of polyarylene sulfide. The retention time (separation time) is preferably as short as possible to prevent a deterioration of the solvent used for the production of polyarylene sulfide, but usually is 1 minute- 5 hours. The flashed residue (B) is supplied from the flash evaporating operation and normally contains solvent used in the production of polyarylene sulfide, solvent deterioration products, oligomers, polymerization aids such as alkali metal halides, alkali metal salts, etc. The evaporated vapor (C) normally contains solvent used in the production of polyarylene sulfide, solvent, deterioration products, etc. The evaporated residue (D) normally contains oligomers, solvent deterioration products, polymerization aids such as alkali metal halides, alkali metal salts, etc. Rectification: According to the present invention, the solvent used for the production of polyarylene sulfide can be efficiently recovered by rectifying the flashed vapor (A) obtained by the flash evaporating operation. For a further efficient recovery of the solvent used in the production of polyarylene sulfide, it is preferred to rectify the flashed vapor (A) obtained by the flash evaporating operation and the evaporated vapor (C) obtained by the separating operation which uses an evaporator. In this case, the flashed vapor (A) and the evaporated vapor (C) may be rectified separately by rectification columns, but when most of the flashed vapor (A) and evaporated vapor (C) comprise the solvent used in the production of polyarylene sulfide, especially when more than 90% of each of (A) and (C) is the solvent used in the production of polyarylene sulfide, it is preferred to carry out the rectification in the same rectification column for reducing the number of apparatus. When the rectification is carried out in the same rectification column, it is preferred to mix the flashed vapor (A) and evaporated vapor (C) before feeding them to the rectification column and to carry out rectification of this mixture in the rectification column in order to reduce controlling variables. Furthermore, when the compositions of the flashed vapor (A) and that of evaporated vapor (C) are considerable different, it is preferred to feed them at the positions corresponding to respective compositions from the point of rectification efficiency. Others: In many cases, polymerization aids are used in the production (polymerization) of polyarylene sulfide for increasing the molecular weight. As polymerization aids, there may be used alkali metal halides (such as lithium chloride), carboxylic acid metal salts (such as lithium acetate), and the like. Generally, the polymerization aid is used in nearly an equimolar amount to the starting monomer and thus a considerable amount of polymerization aid is present in the solvent and so, if this is fed directly to a rectification column, there often occur troubles such as blocking and sticking. Therefore, the method for recovery of solvent used for the production of polyarylene sulfide according to the present invention is especially effective when polyarylene sulfide is produced using polymerization aids. Moreover, the polymerization aids are generally expensive and must be recovered and reused and since the main component of evaporated residue (D) is the polymerization aid, the present invention also has an effect to make easy the recovery of the polymerization aid. According to the present invention, the solvent used in the production of polyarylene sulfide can be further effectively recovered by connecting the flashing drum in the flash evaporating operation and the evaporator in the separation by an evaporator and providing, for example, a valve between the flashing drum and the evaporator, thereby to enhance the degree of reduction of pressure. The present invention will be explained in more detail by the following examples. Example 1 The recovery of solvent which was used for the production of polyarylene sulfide was carried out according to the steps (operation) as shown in Fig.1. Mixed liquid 1 ○ which was generated in the step of the production of polyarylene sulfide and which was composed of the components as shown in Table 1 and was mainly composed of N-methyl-2-pyrrolidone was heated to 200°C and was fed to a flashing drum 1 (capacity 0.5 m³) having an internal temperature of 160°C and a pressure of 9.33 kPa (70 Torr). and was separated into flashed vapor (A) a ○ and flashed residue (B) b ○. The flashed vapor (A) a ○ was fed to rectification column 3 where it was separated into overhead distillate 3 ○, bottoms 4 ○ and purified NMP 5 ○ which was a side cut fraction. The purified NMP 5 ○ was used as a reaction solvent for polyphenylene sulfide (PPS) and reaction proceeded with no troubles to obtain PPS having the desired properties. No troubles such as blocking occurred in rectification column 3 after continuous operation of several months. The results of analysis of components contained in mixed liquid 1 ○, flashed vapor (A) a ○, flashed residue (B) b ○, and purified NMP 5 ○ are shown in Table 1. The mixed liquid 1 ○ was a part of combined mixed liquids obtained in a plurality of productions of polyarylene sulfide (polyphenylene sulfide; PPS). The mixed liquid 1 ○ in the following Example 2 was the same as that in this Example. One example of the preparation of the above polyphenylene sulfide is explained below. 9.13 kg (54.3 mol) of sodium sulfide pentahydrate, 2.3 kg (54.3 mol) of lithium chloride and 30 liters of N-methyl-2-pyrrolidone were charged in an autoclave and water was distilled off at 160°C under reduced pressure. Thereafter, thereto were added 10 liters of N-methyl-2-pyrrolidone and 7.98 kg (54.3 mol) of p-dichlorobenzene and reaction was allowed to proceed at 260°C for 3 hours to obtain a reaction mixture. This reaction mixture was cooled and filtered off by a stainless steel sieve of 0.105 mm (140 meshes) to obtain granular polyphenylene sulfide. This granular product was washed with 20 liters of N-methyl-2-pyrrolidone to obtain a white polyphenylene sulfide having an inherent viscosity η inh: 0.335 dℓ/g [temperature 206°C, α-chloronaphthalene, concentration 0.4 g/dl] and sodium ion content of 210 ppm in a yield of 92.5%. Example 2 Solvent used for the production of polyarylene sulfide was recovered according to the steps (operation) as shown in Fig. 2. Mixed liquid 1 ○ having the composition as shown in Table 1 and mainly composed of NMP and produced in the respective steps for producing PPS was fed to flashing drum 1 (capacity 0.5 m³) adjusted to an internal temperature of 140°C and a pressure of 13.33 kPa (100 Torr) and was separated into flashed vapor (A) a ○ and flashed residue (B) b ○ . The flashed residue (B) b ○ was fed to a vertical cylindrical evaporator 2 (heat transfer surface area 0.5 m²) provided with a rotor for formation of thin film and a jacket for heating and was separated into evaporated vapor (C) c ○ and evaporated residue (D) d ○ under the conditions of 240°C and 13.33 kPa (100 Torr). Flashed vapor (A) a ○ and evaporated vapor (C) c ○ were mixed and the mixture was fed to rectification column 3 as mixed vapor 2 ○ and separated into overhead distillate 3 ○, bottoms 4 ○ and purified NMP 5 ○ which was a side cut fraction. The purifed NMP 5 ○ was used as a reaction solvent and reaction proceeded with no special troubles to obtain PPS having the desired properties. No troubles such as blocking occurred in rectification column 3 even after continuous operation of several months. The results of analysis of components contained in mixed liquid 1 ○, flashed vapor (A) a ○, flashed residue (B) b ○, evaporated vapor (C) c ○, evaporated residue (D) d ○, and purified NMP 5 ○ are shown in Table 1. Example 3 Solvent used for the production of polyarylene sulfide was recovered by the steps (operation) as shown in Fig.3. In this Example, the procedure was carried out under the same conditions as in Example 2 except that flashed vapor (A) a ○ and evaporated vapor (C) c ○ were not mixed and separately fed to rectification column 3. Even when the compositions of vapor a ○ and that of c ○ were greatly different, the solvent was able to be efficiently recovered by rectification column 3 of same performance. The results are shown in Table 1. According to the present invention, the flash evaporating operation and rectificating operation are combined and hence a method for recoverying solvent used for the production of polyarylene sulfides which has the following merits can be provided: (1) Efficient recovery can be attained because special complicated equipments are not necessary. (2) There occur no troubles such as blocking of rectification column and no deterioration of solvent is caused and the solvent can be stably recovered. (3) Continuous operation of recovery can be easily performed. Furthermore, when the flash evaporating operation, evaporator evaporating operation and rectificating operation are combined, a method for recovery of solvent used for the production of polyarylene sulfides which has the following merits can be provided: (1) Efficient recovery can be attained because special complicated equipments are not necessary. (2) Efficient recovery can be attained because the amount of solvent used for the production of polyarylene sulfides which is not recovered from the mixed liquid (containing the solvent as main component) and is retained is little and thus, the recovery rate is high. (3) There occur no troubles such as blocking of rectification column and stable recovery can be attained. (4) Continuous operation of recovery can be easily performed. (5) Separation and recovery of polymerization aids are easy.

Claims: A method for recovery of solvent used in the production of polyarylene sulfide which comprises subjecting a mixed liquid mainly composed of solvent used in the production of polyarylene sulfide and generated from the production process of polyarylene sulfide to a flash evaporating operation at a temperature of 100-220°C and an internal pressure of 6.66-101.3 kPa (50-760 Torr) to separate the mixed liquid into a flashed vapor (A) and a flashed residue (B) and then rectifying the flashed vapor (A). The method according to claim 1, wherein the content of the solvent in the mixed liquid is 80-99 % by weight. The method according to claim 1, wherein the mixed liquid contains an alkali metal halide or a carboxylic acid metal salt as a polymerization aid. The method according to claim 1, wherein the flashed residue (B) is separated into an evaporated vapor (C) and an evaporated residue (D) by an evaporator and the evaporated vapor (C) is rectified. The method according to claim 4, wherein the operation of the evaporator is carried out at a temperature of 100-260°C and an internal pressure of 6.66-101.3 kPa (50-760 Torr). The method according to claim 4, wherein the flashed vapor (A) and the evaporated vapor (C) are rectified in the same rectification column. The method according to claim 4, wherein the flashed vapor (A) and the evaporated vapor (C) are mixed before rectification and then the mixture is rectified. The method according to claim 6, wherein the flashed vapor (A) and the evaporated vapor (C) are fed to the same rectification column at different positions depending on the composition of respective vapors.

Inventors: KOYAMA YOSHINARI, OMORI MASAO, SHIROTA DAIGO, KOYAMA, YOSHINARI, OMORI, MASAO, SHIROTA, DAIGO
Applicants: IDEMITSU PETROCHEMICAL CO, IDEMITSU PETROCHEMICAL CO. LTD.
Application year: 1989
Main classification: B01D3
IPC codes: C08G75, B01D3