METHOD FOR PURIFYING DIMETHYL SULFOXIDE.
Patent Information
- Authority / Receiving Office
- MX · MX
- Patent Type
- Patents
- Current Assignee / Owner
- TORAY FINE CHEMICALS CO LTD
- Filing Date
- 2018-06-06
- Publication Date
- 2026-05-19
Abstract
Description
METHOD FOR PURIFYING DIMETHYL SULFOXIDE TECHNICAL FIELD 5 The present invention relates to a method for purifying dimethyl sulfoxide (DMSO). BACKGROUND OF THE ART 10 Dimethyl sulfoxide is widely used in industries as a solvent for polymerization and for spinning polymer fibers. Furthermore, the recovery and reuse of once-used dimethyl sulfoxide are widely carried out in industries and require heating and distillation steps for purification. 15 However, dimethyl sulfoxide is known to be relatively thermally unstable and decomposes slightly when distilled under atmospheric pressure. In the manufacture or recovery of dimethyl sulfoxide by distillation, contamination by a partially decomposed decomposition product reduces the efficiency of dimethyl sulfoxide as a solvent. 20 Therefore, the distillation of dimethyl sulfoxide is often carried out under reduced pressure at 100°C or less.If dimethyl sulfoxide can be distilled, for example, at a high temperature of 110°C or more, it is unnecessary to use a high vacuum in the distillation, and furthermore, no load is applied to a decompression device. Therefore, the distillation setup 25 can be simplified, which is preferable industrially. Conventionally, there are known methods for adding a metal hydroxide such as sodium hydroxide or potassium hydroxide as an inhibitor to the decomposition of dimethyl sulfoxide (see patent documents 1, 2, and 3). The amount of sodium hydroxide, potassium hydroxide, or similar compounds added is limited to 0.003 to 0.5%, and adding 1% or more promotes the decomposition of dimethyl sulfoxide. In patent literature 1, the amount of a decomposition product quantified as formaldehyde after heating to 150 °C for 10 hours was 0.032% when no metal hydroxide was added, 0.054% when 1% potassium hydroxide was added, and 0.052% when 1% sodium hydroxide was added. When dimethyl sulfoxide is purified by distillation, the purified dimethyl sulfoxide is distilled out of the system. Additionally, water contained in a dimethyl sulfoxide-containing liquid, a solvent with a lower boiling point than dimethyl sulfoxide, unreacted monomers from the polymerization, and impurities such as decomposition products of dimethyl sulfoxide are also distilled out of the system. As a result, when a metal hydroxide such as sodium hydroxide or potassium hydroxide is added as an inhibitor of dimethyl sulfoxide decomposition, the metal hydroxide remains at the bottom of the distillation column during the purification process.This increases the concentration of the metal hydroxide in the residual liquid after distillation. When distilling dimethyl sulfoxide, even if a metal hydroxide is added at a low concentration, the decomposition of the dimethyl sulfoxide is actually encouraged when the dimethyl sulfoxide is distilled and the metal hydroxide concentration reaches 1% or more. Therefore, a problem has arisen where purity is reduced due to contamination of the distillate by a decomposition product of dimethyl sulfoxide. There has been a desire for a method of purifying dimethyl sulfoxide that allows for the safe production of high-purity dimethyl sulfoxide, for example, at a high temperature of 110°C or more and even when a decomposition inhibitor is highly concentrated when distilling and purifying the dimethyl sulfoxide. STATE OF THE ART REFERENCES Patent documents Patent Document 1: JP-B-S43-3765 Patent Document 2: JP-B-S38-20721 Patent Document 3: JP-A-2015-145359 DISCLOSURE OF THE INVENTION PROBLEMS THAT THE INVENTION INTENDED TO SOLVE It is an objective of the present invention to solve those problems and provide a method for purifying dimethyl sulfoxide to obtain high-purity dimethyl sulfoxide. MEANS TO SOLVE PROBLEMS The present invention is a method for purifying dimethyl sulfoxide, comprising distilling a liquid containing dimethyl sulfoxide in the presence of sodium carbonate in an inert gas atmosphere to distill the dimethyl sulfoxide, wherein the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in a residual liquid after distillation is 6 times or more the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the liquid containing dimethyl sulfoxide before distillation. EFFECTS OF THE INVENTION The method for purifying dimethyl sulfoxide according to the present invention inhibits the decomposition of dimethyl sulfoxide, even when the concentration of sodium carbonate is increased from low to high, so that high-purity dimethyl sulfoxide can be obtained by distillation. In the method for purifying dimethyl sulfoxide according to the invention, the amount of dimethyl sulfoxide decomposition is small not only in the distillate after distillation (a main distillate and an early distillate) but also in the residue after distillation. The decomposition of dimethyl sulfoxide is small in the residue after distillation or even in the liquid during distillation, so that high-purity dimethyl sulfoxide can be obtained by distillation. In the method for purifying dimethyl sulfoxide according to the invention, the amount of dimethyl sulfoxide decomposition is small, which is calculated from the purity of dimethyl sulfoxide in a mixed distillate liquid (the main distillate and the early distillate) after distillation and the residue liquid after distillation. Therefore, the sodium carbonate used in the dimethyl sulfoxide purification method of the invention effectively inhibits the decomposition of dimethyl sulfoxide. The method for purifying dimethyl sulfoxide according to the invention allows distillation to be carried out at high temperatures, thus eliminating the need for a high-vacuum system and enabling low-cost purification. Since sodium carbonate is safe, the operation can be performed more safely compared to conventional purification methods that use hazardous substances. The dimethyl sulfoxide obtained by the method for purifying dimethyl sulfoxide according to the present invention can be used as a solvent in the polymerization and centrifugation stages of polymers, such as polyacrylonitrile, cellulose, polyimide, polysulfone, and polyurethane, as a stripping liquid for photoprotectants that are electronic materials, as a solvent for synthesizing pharmaceuticals and agrochemicals, as a removal and cleaning liquid for lens molds and the like, or as a paint stripping liquid. METHOD FOR CARRYING OUT THE INVENTION The present invention is a method for purifying dimethyl sulfoxide, comprising distilling a liquid containing dimethyl sulfoxide in the presence of sodium carbonate in an inert gas atmosphere to distill the dimethyl sulfoxide, wherein the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in a residual liquid after distillation is 6 times or more the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the liquid containing dimethyl sulfoxide before distillation. Sodium carbonate can be either an anhydride or a hydrate, and the preferred hydrate is a monohydrate or decahydrate, which are readily available. Sodium carbonate can be added as a powder or solid, or as an aqueous solution. When added as an aqueous solution, sodium carbonate can be automatically and continuously charged to a uniform concentration in a distillation apparatus, which is therefore preferable in terms of safety. The concentration of sodium carbonate prepared as an aqueous solution can be increased to a saturated concentration at the temperature used. When adding sodium carbonate as an aqueous solution, the preferred amount is 0.1 to 35 g, preferably 0.2 to 30 g, still preferably 1 to 30 g, and most preferably 10 to 25 g, relative to 100 g of water. The solubility of sodium carbonate in 100 g of water is 22 g at 20°C, which is high. Consequently, when the concentration of sodium carbonate is increased by distillation and sodium carbonate crystals are deposited, the crystals in the column can be easily removed, even by cleaning with water, due to their high solubility in water. In the present invention, the liquid containing dimethyl sulfoxide may be a liquid free of impurities and 100% dimethyl sulfoxide. Furthermore, in the invention, the liquid containing dimethyl sulfoxide may be a liquid containing a very small or small amount of impurities. Additionally, in the present invention, the liquid containing dimethyl sulfoxide may contain a large amount of a liquid other than dimethyl sulfoxide. In the present invention, since it is costly to distill and remove impurities, the liquid containing dimethyl sulfoxide before distillation preferably contains pure dimethyl sulfoxide in an amount of 10% by weight or more. The pure dimethyl sulfoxide in the invention refers to dimethyl sulfoxide with a purity of 100%. The liquid containing dimethyl sulfoxide before distillation preferably contains pure dimethyl sulfoxide in an amount of 20% by weight or more, and even more preferably contains pure dimethyl sulfoxide in an amount of 30 to 100% by weight. The liquid containing dimethyl sulfoxide before distillation may contain water other than dimethyl sulfoxide. The amount of water in the liquid containing dimethyl sulfoxide before distillation is preferably 0.01 to 900 g, more preferably 0.1 to 400 g, more preferably 1 to 250 g, and more preferably 5 to 100 g, relative to 100 g of pure dimethyl sulfoxide in the liquid. When the liquid containing dimethyl sulfoxide contains an impurity and / or the like that hinders distillation, such as a resin component, an insoluble substance, a component that tends to gel when concentrated, a strong acid or alkali, and / or a component that reacts with dimethyl sulfoxide, it is preferable to remove, separate, deactivate, or neutralize the impurity and / or the like by prior filtration, adsorption and separation, the addition of activated carbon, an ion exchange resin, or a base, or the like. By distilling and purifying the liquid containing dimethyl sulfoxide by adding a decomposition inhibitor, the purified dimethyl sulfoxide is distilled out of the system. Additionally, the water contained in the dimethyl sulfoxide liquid, a solvent with a lower boiling point than dimethyl sulfoxide, unreacted monomers from the polymerization, and impurities such as decomposition products of dimethyl sulfoxide are distilled off in a distillation unit. In the present invention, provided that agitation is possible, the concentration of sodium carbonate in a residual liquid after distillation can be increased. When the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the residual liquid after distillation is less than 6 times the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the liquid containing dimethyl sulfoxide before distillation, the recovery rate of dimethyl sulfoxide is low, thus increasing the cost to purify dimethyl sulfoxide. In the present invention, the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the residual liquid after distillation is preferably 6 to 1000 times, more preferably 10 to 500 times, and even more preferably 20 to 200 times the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the residual liquid before distillation. The amount of sodium carbonate after distillation is preferably concentrated to between 0.01 and 100 g, and more preferably between 0.1 and 85 g, relative to 100 g of pure dimethyl sulfoxide in the residual liquid after distillation. When the amount of sodium carbonate after distillation is 100 g or more, the sludge in the residual liquid may harden at the bottom of the distillation column, which can make stirring difficult. In the present invention, with respect to the timing of adding sodium carbonate to the liquid containing dimethyl sulfoxide, the sodium carbonate may be added before distillation or after distilling an impurity (impurities) having a lower boiling point than dimethyl sulfoxide, in order to carry out the distillation. Furthermore, the sodium carbonate deposited after distillation may be discarded or recovered and reused. In the present invention, dimethyl sulfoxide is distilled in an inert gas atmosphere. The term “inert gas atmosphere” refers to an atmosphere of nitrogen, carbon dioxide, helium, or argon, and may consist of one type of gas or a gas mixture consisting of two or more gases. The preferred inert gas atmosphere is a nitrogen atmosphere. When distilled in air, dimethyl sulfoxide readily decomposes. In the present invention, dimethyl sulfoxide is preferably distilled in an inert gas atmosphere and under pressure ranging from atmospheric to reduced. When there is a small difference in boiling point between an impurity that is to be removed and dimethyl sulfoxide, the degree of pressure reduction is not significantly reduced in order to increase the boiling point difference between the impurity and dimethyl sulfoxide, thereby facilitating the removal of the impurity. In the present invention, when distilling under atmospheric pressure, the temperature during distillation is preferably from 160°C to 200°C, more preferably from 170°C to 195°C, more preferably from 180°C to 194°C, and more preferably from 189°C to 193°C, whereby no load is applied to the apparatus, and the installation for distillation is also simplified, which is industrially preferable. In the present invention, when distilling under reduced pressure, the distillation is preferably carried out from 1.333 kPa to 100 kPa (10 to 750 Torr), and more preferably from 2 to 97.33 kPa (15 to 730 Torr). In the present invention, when distilling under reduced pressure, the temperature during distillation is preferably from 108°C to 180°C, with greater preference being 120°C to 170°C, and even more preferably from 131°C to 160°C. In the present invention, the purity of dimethyl sulfoxide is measured by gas chromatography with a capillary column. The purity of the purified dimethyl sulfoxide in the present invention is analyzed by gas chromatography using a capillary column and is represented as a percentage of the area. The purity is preferably 99.990% or more, more preferably 99.991% or more, and even more preferably 99.992% or more. In the present invention, the amount of decomposition (% by area) of dimethyl sulfoxide is defined as described below. The liquid containing dimethyl sulfoxide before distillation is defined as the “loaded liquid.” A liquid obtained by blending a distillate after distillation (including a main distillate and, where applicable, an early distillate) and a residual liquid after distillation is defined as the “post-distillation blended liquid.” The purity (% by area) of dimethyl sulfoxide in each of the loaded liquid and the “post-distillation blended liquid” was measured by gas chromatography, and the amount of decomposition of dimethyl sulfoxide was obtained using the following calculation equation: Amount of dimethyl sulfoxide decomposition (% of area) = purity of charged liquid (% of area) - purity of mixed liquid after distillation (% of area) In the present invention, the amount of decomposition of dimethyl sulfoxide is preferably 0.009% of the area or less, more preferably 0.008% of the area or less, and still more preferably 0.007% of the area or less. In the method for purifying dimethyl sulfoxide according to the present invention, distillation is applicable to both batch and continuous distillation, and the distillation column may be a single column, a compound column, or a combination of two or more distillation columns. When performing continuous distillation, an aqueous solution of preferably sodium carbonate is supplied continuously before the distillation column(s). In the present invention, with regard to the number of theoretical plates of the distillation column(s), distillation column(s) with from 1 to 50 theoretical plates are preferred, and conveniently, further distillation column(s) with from 3 to 40 theoretical plates are preferred. In the present invention, the liquid containing dimethyl sulfoxide may be a reaction liquid containing dimethyl sulfoxide obtained in a synthesis step by oxidation or similar means of dimethyl sulfide, a waste liquid containing dimethyl sulfoxide used in a polymerization or spinning step of a polymer such as polyacrylonitrile, cellulose, polyimide, polysulfone, or polyurethane, a waste liquid containing dimethyl sulfoxide used as a pickling liquid for a photoresistor which is an electronic material, a waste liquid containing dimethyl sulfoxide used as a solvent for synthesizing a drug or agrochemical, a waste liquid containing dimethyl sulfoxide used as a remover and cleaning liquid for a lens mold or similar, a waste liquid containing dimethyl sulfoxide used as a paint stripper, or similar. EXAMPLES The present invention will be described specifically by the examples below. Various types of measurement values used in the examples and similar examples of the invention were measured using the following measurement methods. (1) Purity of dimethyl sulfoxide (% of area) The purity of dimethyl sulfoxide was measured by gas chromatography under the following conditions. • Apparatus used: GC-2010 (FID) manufactured by Shimadzu Corporation • Column: DB-WAX, 0.25 mm × 60 m, film thickness: 0.25 μm • Carrier gas: He: 165.7 kPa • Column temperature rise conditions: 35°C —► 7°C / min -> 140°C × 10 min 15°C / min -» 250°C × 10 min • Inlet temperature: 200°C • Detection temperature: 250°C • FID: Air: 400 ml / min, H2: 40 ml / min, Composition: 30 ml / min • Split ratio: 14 • Sample preparation: Samples were filtered through a 0.5 μm PTFE syringe filter. • Injection quantity: 1.0 μL (2) Amount of Decomposition of Dimethyl Sulfoxide (% of area) With respect to a liquid containing dimethyl sulfoxide before distillation (a loaded liquid) and a mixed liquid prepared by mixing a post-distillation distillate (which includes a main distillate and, where applicable, an early distillate) and a residual liquid after distillation (a post-distillation mixed liquid), the purities of dimethyl sulfoxide (% of area) were measured in the same way as in (1), and then a decomposition amount of dimethyl sulfoxide was obtained using the following calculation equation: Amount of dimethyl sulfoxide decomposition (% of area) = purity of charged liquid (% of area) - purity of mixed liquid after distillation (% of area) (3) Amount of Additive The amount of an additive before distillation is the amount of the additive relative to 100 g of pure dimethyl sulfoxide in the liquid containing dimethyl sulfoxide. In the chromatographic measurement of the dimethyl sulfoxide gas in (1), neither water nor additive is detected. The amount of pure dimethyl sulfoxide in the loaded liquid was obtained by multiplying a value obtained by subtracting amounts of water and the additive from a total amount of liquid by the purity of the dimethyl sulfoxide in the loaded liquid. The amount of additive after distillation is the amount of additive relative to 100 g of pure dimethyl sulfoxide in the residual liquid after distillation. The amount of pure dimethyl sulfoxide in the residual liquid after distillation was obtained by multiplying a value obtained by subtracting amounts of water and the additive from a total amount of residual liquid in the flask by the purity of the dimethyl sulfoxide in the residual liquid. (4) Concentration rate of the additive before and after distillation The concentration rate of the additive before and after distillation was obtained using the following calculation equation: Ratio of additive concentration between before and after distillation = (amount (g) of additive with respect to 100 g of pure dimethyl sulfoxide in the residual liquid after distillation) / (amount (g) of additive with respect to 100 g of pure dimethyl sulfoxide in the liquid containing dimethyl sulfoxide before distillation) (Example 1) A 1-L four-necked flask equipped with a Dimroth condenser, a distillate receiver, a stirrer, and a thermometer, as required for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.997% by area) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchange water, and, as an additive, 0.0072 g of sodium carbonate (0.001 g relative to 100 g of pure dimethyl sulfoxide). After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 200°C, and the point in time when distillation began was taken as the start time.After collecting 100 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 8 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 0.09 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.997% of the area, as shown in Table 1, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.903% of the area, as shown in Table 1. The dimethyl sulfoxide barely decomposed even when heated for three hours at temperatures ranging from 147 to 170°C (flask temperatures) in the early distillation operation and from 170 to 191°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residual liquid was 0.09 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 90-fold by distilling the dimethyl sulfoxide. As shown in Table 1, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.994% of the area, and the amount of decomposition was 0.003%, extremely small. (Example 2) A 1-L four-necked flask equipped with a Dimroth condenser, a distillate receiver, a stirrer, and a thermometer, as required for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.997% by area) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchange water, and, as an additive, 0.72 g of sodium carbonate (0.1 g relative to 100 g of pure dimethyl sulfoxide). After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 200°C, and the point in time when distillation began was taken as the start time.After collecting 100 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 72 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 1 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.995% of the area, as shown in Table 1, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residue was 99.964% of the area, as shown in Table 1. The dimethyl sulfoxide barely decomposed even when heated for 2.5 hours at temperatures from 147 to 170°C (flask temperatures) in the early distillation operation and from 171 to 191°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residue was 1 g relative to 100 g of pure dimethyl sulfoxide in the residue, the sodium carbonate was concentrated tenfold by distilling the dimethyl sulfoxide. As shown in Table 1, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.991% of the area, and the amount of decomposition was 0.006%, extremely small. (Example 3) A 1-L four-necked flask equipped with a Dimroth condenser, distillate receiver, stirrer, and thermometer, as required for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.997% by area) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchange water, and, as an additive, 7.2 g of sodium carbonate (1 g relative to 100 g of pure dimethyl sulfoxide). After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 200°C, and the point in time when distillation began was taken as the start time.After collecting 100 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 16 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 82 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillation was 99.994% of the area, as shown in Table 1, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.892% of the area, as shown in Table 1. The dimethyl sulfoxide barely decomposed even when heated for 3 hours at temperatures from 147 to 170°C (flask temperatures) in the early distillation operation and from 170 to 191°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residual liquid was 82 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 82-fold by distillation of the dimethyl sulfoxide. As shown in Table 1, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.992% of the area, and the amount of decomposition was 0.005%, extremely small. (Comparative Example 1) A 1-L four-necked flask equipped with a Dimroth condenser, distillate receiver, stirrer, and thermometer, as required for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.997% by area) (pure dimethyl sulfoxide: 719.98 g) and 80 g of ion-exchange water without any additives. After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 200°C, and the point in time when distillation began was considered the start time.After collecting 100 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 8 g (the amount of water in the residual liquid: 0.0 g). The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.988% of the area, as shown in Table 1, indicating a reduction in the purity of dimethyl sulfoxide. The purity of dimethyl sulfoxide in the residual liquid was 99.922% of the area, as shown in Table 1. As shown in Table 1, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.987% of the area, and the amount of decomposition was 0.010%, which was greater than the examples from 1 to 3. (Comparative Example 2) A 1-L four-necked flask equipped with a Dimroth condenser, a distillate receiver, a stirrer, and a thermometer, as required for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.997% by area) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchange water, and, as an additive, 0.144 g of sodium hydroxide (0.02 g relative to 100 g of pure dimethyl sulfoxide). After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 200°C, and the point in time when distillation began was taken as the start time.After collecting 100 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 7 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 2 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.960% of the area, as shown in Table 1, indicating a reduction in the purity of dimethyl sulfoxide. The purity of dimethyl sulfoxide in the residual liquid was 99.568% of the area, as shown in Table 1. The dimethyl sulfoxide decomposed slightly when heated for 3 hours at temperatures from 147 to 170°C (flask temperatures) in the early distillation operation and from 170 to 191°C (flask temperatures) in the main distillation operation. As shown in Table 1, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.956% of the area, and the amount of decomposition was 0.041%, which was greater than examples 1 to 3. (Comparative Example 3) A 1-L four-necked flask equipped with a Dimroth condenser, a distillate receiver, a stirrer, and a thermometer, as required for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.997% by area) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchange water, and, as an additive, 7.2 g of potassium carbonate (1 g relative to 100 g of pure dimethyl sulfoxide). After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 200°C, and the point in time when distillation began was taken as the start time.After collecting 100 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 18 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 67 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.984% of the area, as shown in Table 1, indicating a reduction in the purity of dimethyl sulfoxide. The purity of dimethyl sulfoxide in the residual liquid was 99.617% of the area, as shown in Table 1. The dimethyl sulfoxide decomposed slightly when heated for 3 hours at temperatures from 147 to 170°C (flask temperatures) in the early distillation operation and from 170 to 191°C (flask temperatures) in the main distillation operation. As shown in Table 1, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.975% of the area, and the amount of decomposition was 0.022%, which was greater than examples 1 to 3. (Comparative Example 4) A 1-L four-necked flask equipped with a Dimroth condenser, a distillate receiver, a stirrer, and a thermometer, as required for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.997% by area) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchange water, and, as an additive, 0.216 g of sodium carbonate (0.03 g relative to 100 g of pure dimethyl sulfoxide). Without performing nitrogen substitution, an air-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask's interior from air. The flask was heated in an oil bath maintained at 200°C, and the point in time when distillation began was considered the start time.After collecting 100 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 22 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 0.99 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.982% of the area, as shown in Table 1, indicating a reduction in the purity of dimethyl sulfoxide. The purity of dimethyl sulfoxide in the residual liquid was 99.844% of the area, as shown in Table 1. The dimethyl sulfoxide decomposed slightly when heated for 3 hours at temperatures from 147 to 170°C (flask temperatures) in the early distillation operation and from 171 to 193°C (flask temperatures) in the main distillation operation. As shown in Table 1, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.978% of the area, and the amount of decomposition was 0.019%, which was greater than examples 1 to 3. Table 1 Additive Atmosphere in the system Loaded liquid Main distillate Residual liquid Concentration rate of the additive Purity of the mixed liquid after distillation (% of area) Decomposition rate (% of area) Purity of dimethyl sulfoxide (% of area) Amount of additive (g) Amount of water (g) Pressure Temperatures in the flask Purity of dimethyl sulfoxide (% of area) Purity of dimethyl sulfoxide (% of area) Amount of additive (g) I Sodium carbonate Nitrogen 99 997 ().()() 1 II Atmospheric pressure 170 191 99.997 99.901 0.09 90 99 994 0 0.1 0 2 Sodium carbonate Nitrogen 99.997 0 1 1 1 Atmospheric pressure 171-191 99 995 99.964 1 10 99.991 0.006 O ' Sodium carbonate Nitrogen 99.997 1 II Atmospheric pressure Π0 191 99.994 99 X92 X2 S2 99.992 0.005 Fj Com 1 None Nitrogen 99.997 II Atmospheric pressure |71 192 99.9SX 99.922 99.9X7 0OI0 Fj Com 2 Sodium hydroxide Nitrogen 99.997 0.02 II Atmospheric pressure 170 191 99.960 99 56X 2 100 99.956 0041 Fj Com 3 Potassium carbonate Nitrogen 99.997 1 11 Atmospheric pressure |7() |91 99.9X4 99 61 7 67 67 99.975 0.022 Lj Com 4 Sodium carbonate Air 99.997 0.01 11 Atmospheric pressure 171 191 99.9X2 99.X44 O 99 y; 99.97X 0.019. The amount of additive is the amount of an additive with respect to 100 g of pure dimethyl sulfoxide in the liquid. The amount of water in the loaded liquid is the amount of water relative to 100 g of pure dimethyl sulfoxide in the liquid. In examples 1 to 3, high-purity dimethyl sulfoxide was obtained as the main distillate. On the other hand, the purities of dimethyl sulfoxide in the main distillates obtained in comparative examples 1 to 4 were lower than those in examples 1 to 3. (Example 4) A 10-L four-necked flask fitted with a distillation column packed with structured packing, a distillate receiver, a stirrer, a thermometer, and a Dimroth condenser provided at the top of the distillation column was charged with 4830 g of dimethyl sulfoxide (purity: 99.998% by area) (pure dimethyl sulfoxide: 4829.9 g), 1998 g of ion-exchange water, and, as an additive, 1.4 g of sodium carbonate (0.03 g relative to 100 g of pure dimethyl sulfoxide). After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 145 to 191 °C under a pressure of 2.67 to 96 kPa (20 to 720 Torr), and a point in time when distillation began was considered to be a start time.After collecting 2289 g containing water as an early distillate, a main distillation operation was performed in which the flask was heated in an oil bath from 153 to 192 °C under a pressure of 2.67 kPa (20 Torr), and a main distillate was collected for a period of time when the temperature in the flask varied from 110 to 112 °C, until the amount of residual liquid in the flask reached 688 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 0.2 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 12 hours. The purity of dimethyl sulfoxide in the main distillate was 99.999% of the area, as shown in Table 2, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the waste liquid was 99.975% of the area, as shown in Table 2. Even when heated, the dimethyl sulfoxide barely decomposed. Since the amount of sodium carbonate in the waste liquid was 0.2 g relative to 100 g of pure dimethyl sulfoxide in the waste liquid, the sodium carbonate was concentrated 7-fold by distillation of the dimethyl sulfoxide. As shown in Table 2, the purity of dimethyl sulfoxide in a mixed liquid from the distillate after distillation (the main distillate) and the residue after distillation (a post-distillation mixed liquid) was 99.992%, and the amount of decomposition was 0.006%, extremely small. In Example 4, high-purity dimethyl sulfoxide was obtained as the main distillate. Table 2 Additive Liquid charged Main distillate Residual liquid Additive concentration rate Purity of mixed liquid after distillation (% of acreage) Amount of decomposition (% of acreage) Purity of sodium sulfoxide (1% of acreage) Amount of additive (g) Amount of water (g) Pressure Temperatures in flask (°C) Purity of dimethyl sulfoxide (% of acreage) Purity of diethyl sulfoxide (% of acreage) Amount of additive (g) Sodium carbonate 99.99X 0.03 41 Reduced pressure 110 112 99.999 99.975 0.27 99.992 0.006 The amount of additive is the amount of an additive with respect to 100 g of pure dimethyl sulfoxide in the liquid. The amount of water in the loaded liquid is the amount of water relative to 100 g of pure dimethyl sulfoxide in the liquid. (Example 5) A 1-L four-necked flask equipped with a Dimroth condenser, a distillate receiver, a stirrer, and a thermometer, as required for simple distillation, was charged with 400 g of dimethyl sulfoxide (purity: 99.995% by area) (pure dimethyl sulfoxide: 399.98 g), 400 g of ion-exchange water, and, as an additive, 0.004 g of sodium carbonate (0.001 g relative to 100 g of pure dimethyl sulfoxide). After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 220°C, and the point in time when distillation began was taken as the start time.After collecting 500 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 9 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 0.044 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.994% of the area, as shown in Table 3, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.932% of the area, as shown in Table 3. The dimethyl sulfoxide barely decomposed even when heated for 3 hours at temperatures ranging from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residual liquid was 0.044 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 44-fold by distillation of the dimethyl sulfoxide. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.992% of the area, and the amount of decomposition was 0.003%, extremely small. (Example 6) Example 6 was carried out in the same manner as Example 5 except that the amount of sodium carbonate added was changed to 0.024 g (0.006 g relative to 100 g of pure dimethyl sulfoxide), and the distillate was collected until the amount of residual liquid in the flask reached 32 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 0.075 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.992% of the area, as shown in Table 3, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residue was 99.964% of the area, as shown in Table 3. The dimethyl sulfoxide barely decomposed even when heated for 2.5 hours at temperatures ranging from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residue was 0.075 g relative to 100 g of pure dimethyl sulfoxide in the residue, the sodium carbonate was concentrated 13-fold by distilling the dimethyl sulfoxide. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.988% of the area, and the amount of decomposition was 0.007%, extremely small. (Example 7) Example 7 was performed in the same manner as Example 5 except that the amount of sodium carbonate added was changed to 0.08 g (0.02 g relative to 100 g of pure dimethyl sulfoxide), and the distillate was collected until the amount of residual liquid in the flask reached 34 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 0.24 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.991% of the area, as shown in Table 3, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residue was 99.965% of the area, as shown in Table 3. The dimethyl sulfoxide barely decomposed even when heated for 2.5 hours at temperatures ranging from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residue was 0.24 g relative to 100 g of pure dimethyl sulfoxide in the residue, the sodium carbonate was concentrated 12-fold by distilling the dimethyl sulfoxide. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.988% of the area, and the amount of decomposition was 0.007%, extremely small. (Example 8) Example 8 was performed in the same manner as Example 5 except that the amount of sodium carbonate added was changed to 0.4 g (0.1 g relative to 100 g of pure dimethyl sulfoxide), and the distillate was collected until the amount of residual liquid in the flask reached 28 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 1.4 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.991% of the area, as shown in Table 3, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residue was 99.956% of the area, as shown in Table 3. The dimethyl sulfoxide barely decomposed even when heated for 2.5 hours at temperatures ranging from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residue was 1.4 g relative to 100 g of pure dimethyl sulfoxide in the residue, the sodium carbonate was concentrated 14-fold by distilling the dimethyl sulfoxide. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.987% of the area, and the amount of decomposition was 0.008%, extremely small. (Example 9) Example 9 was carried out in the same manner as Example 5 except that the amount of sodium carbonate added was changed to 4 g (1 g relative to 100 g of pure dimethyl sulfoxide), and the distillate was collected until the amount of residual liquid in the flask reached 42 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 11 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.995% of the area, as shown in Table 3, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.985% of the area, as shown in Table 3. The dimethyl sulfoxide barely decomposed even when heated for 2.5 hours at temperatures ranging from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residual liquid was 11 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 11-fold by distilling the dimethyl sulfoxide. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.992% of the area, and the amount of decomposition was 0.003%, extremely small. (Example 10) Example 10 was carried out in the same manner as Example 5 except that the amount of sodium carbonate added was changed to 4 g (1 g relative to 100 g of pure dimethyl sulfoxide), and the distillate was collected until the amount of residual liquid in the flask reached 17 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 31 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.994% of the area, as shown in Table 3, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.968% of the area, as shown in Table 3. The dimethyl sulfoxide barely decomposed even when heated for 3 hours at temperatures ranging from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residual liquid was 31 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 31-fold by distilling the dimethyl sulfoxide. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.992% of the area, and the amount of decomposition was 0.003%, extremely small. (Comparative Example 5) Comparative Example 5 was performed in the same manner as Example 5 except for not adding sodium carbonate, and the distillate was collected until the amount of residual liquid in the flask reached 23 g (the amount of water in the residual liquid: 0.0 g). The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.978% of the area, as shown in Table 3, indicating a reduction in the purity of dimethyl sulfoxide. The purity of dimethyl sulfoxide in the residual liquid was 99.961% of the area, as shown in Table 3. The dimethyl sulfoxide decomposed slightly when heated for 3 hours at temperatures from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.979% of the area, and the amount of decomposition was 0.016%, which was higher than in examples 5 to 10. In addition, as a decomposition product of dimethyl sulfoxide, a small amount of polymer, which appeared to be polyacetal, was deposited on the tool after distillation. (Comparative Example 6) Comparative Example 6 was performed in the same manner as Example 5, except that the type of additive was changed to sodium hydroxide and the amount of additive added was changed to 0.08 g (0.02 g relative to 100 g of pure dimethyl sulfoxide). The distillate was collected until the amount of residual liquid in the flask reached 35 g (the amount of water in the residual liquid: 0.0 g), and the amount of additive after distillation reached 0.23 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.980% of the area, as shown in Table 3, indicating a reduction in the purity of dimethyl sulfoxide. The purity of dimethyl sulfoxide in the residual liquid was 99.926% by area, as shown in Table 3. The dimethyl sulfoxide decomposed slightly when heated for 2.5 hours at temperatures from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillate operation. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.971% by area, and the amount of decomposition was 0.024%, which was higher than in examples 5 to 10. (Comparative Example 7) Comparative Example 7 was performed in the same manner as Example 5, except that the type of additive was changed to sodium hydroxide and the amount of additive added was changed to 0.4 g (0.1 g relative to 100 g of pure dimethyl sulfoxide). The distillate was collected until the amount of residual liquid in the flask reached 32 g (the amount of water in the residual liquid: 0.0 g), and the amount of additive after distillation reached 1.3 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.972% of the area, as shown in Table 3, indicating a reduction in the purity of dimethyl sulfoxide. The purity of dimethyl sulfoxide in the residual liquid was 99.910% by area, as shown in Table 3. The dimethyl sulfoxide decomposed slightly when heated for 2.5 hours at temperatures from 108 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillate operation. As shown in Table 3, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.963% by area, and the amount of decomposition was 0.032%, which was higher than in examples 5 to 10. Table 3 Additive Liquid Loaded Main Distillate Residual Liquid Additive Concentration Rate Purity of Mixed Liquid after Distillation (% of area) Amount of Decomposition (% of area) Purity of Diethyl Sulfoxide (% of area) Amount of Additive (g) Amount of Water (g) Pressure Temperatures in Flask (°C) Purity of Dimethyl Sulfoxide (% of area) Purity of Dimethyl Sulfoxide (% of area) Amount of Additive (g) Ex 5 Sodium Carbonate 99.995 0.001 100 Atmospheric Pressure 192 193 99.994 99.932 0.044 44 99.992 0.003 Ex 6 Sodium Carbonate 99.995 0.006 100 Atmospheric Pressure 192 191 99.992 99.964 0.075 13 99.988 0.007 Ex7 Sodium carbonate 99.995 0.02 100 Atmospheric pressure 192 191 99 991 99 965 0 24 12 99.988 3.()07 ExS Sodium carbonate 99.995 0.1 100 Atmospheric pressure 192 193 99.991 99.956 1.4 14 99.987 11.00S Ex 9 Sodium carbonate 99.995 1 100 Atmospheric pressure 192 193 99.995 99.985 11 11 99 992 0.003 Ex I0 Sodium Carbonate 99.995 I 100 Atmospheric pressure 192 193 99.994 99.968 31 31 99.992 3.003 Ej Com 5 None 99.995 100 Atmospheric pressure 192 193 99 978 99 96 1 99 979 3.016 Ej Com 6 Sodium hydroxide 99.995 0.02 100 Atmospheric pressure 192 193 99.980 99.926 0.23 12 99.971 3.024 E.) Com 7 Sodium hydroxide 99.995 0.1 100 Atmospheric pressure 192-193 99.972 99.910 1.3 13 99.963 :).032. The amount of additive is the amount of an additive with respect to 100 g of pure dimethyl sulfoxide in the liquid. The amount of water in the loaded liquid is the amount of water relative to 100 g of pure dimethyl sulfoxide in the liquid. In examples 5 to 10, high-purity dimethyl sulfoxide was obtained as the main distillate. On the other hand, the purities of dimethyl sulfoxide in the main distillates obtained in comparative examples 5 to 7 were lower than those in examples 5 to 10. (Example 11) A 1-L four-necked flask equipped with a Dimroth condenser, distillate receiver, stirrer, and thermometer, necessary for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.995% by area) (pure dimethyl sulfoxide: 719.96 g) and, as an additive, 7.2 g of sodium carbonate (1 g relative to 100 g of pure dimethyl sulfoxide). After filling the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 220°C, and the point in time when distillation began was taken as the start time. The distillate was collected until the amount of residual liquid in the flask reached 56 g (the amount of water in the residual liquid: 0.0 g) and the amount of additive after distillation reached 15 g relative to 100 g of pure dimethyl sulfoxide.The warm-up time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.990% of the area, as shown in Table 4, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.930% of the area, as shown in Table 4. The dimethyl sulfoxide barely decomposed even when heated for 2.5 hours at temperatures ranging from 192 to 193°C (flask temperatures). Since the amount of sodium carbonate in the residual liquid was 15 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 15-fold by distillation of the dimethyl sulfoxide. As shown in Table 4, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.986% of the area, and the amount of decomposition was 0.009%, extremely small. (Example 12) A 1-L four-necked flask equipped with a Dimroth condenser, a distillate receiver, a stirrer, and a thermometer, as required for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.995% by area) (pure dimethyl sulfoxide: 719.96 g). Then, as an additive, 7.2 g of sodium carbonate monohydrate (0.86 g of sodium carbonate relative to 100 g of pure dimethyl sulfoxide) was charged into the same flask. After filling the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 220°C, and the point in time when distillation began was taken as the start time.After collecting 12 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 61 g (the amount of water in the residual liquid: 0.0 g) and the amount of sodium carbonate after distillation reached 11.4 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.5 hours. The purity of dimethyl sulfoxide in the main distillate was 99.990% of the area, as shown in Table 4, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residue was 99.936% of the area, as shown in Table 4. The dimethyl sulfoxide barely decomposed even when heated for 2.5 hours at temperatures from 191 to 192°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residue was 11.4 g relative to 100 g of pure dimethyl sulfoxide in the residue, the sodium carbonate was concentrated 13-fold by distilling the dimethyl sulfoxide. As shown in Table 4, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.986% of the area, and the amount of decomposition was 0.009%, extremely small. (Example 13) A 1-L four-necked flask equipped with a Dimroth condenser, distillate receiver, stirrer, and thermometer, necessary for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.995% by area) (pure dimethyl sulfoxide: 719.96 g). Then, as an additive, 7.2 g of sodium carbonate decahydrate (0.37 g of sodium carbonate relative to 100 g of pure dimethyl sulfoxide) was charged into the same flask. After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 220°C, and the point in time when distillation began was taken as the start time.After collecting 16 ml containing water as an early distillate, a main distillation operation was carried out in which the temperature of the oil bath was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 82 g (the amount of water in the residual liquid: 0.0 g) and the amount of sodium carbonate after distillation reached 3.4 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2.8 hours. The purity of dimethyl sulfoxide in the main distillate was 99.990% of the area, as shown in Table 4, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.951% of the area, as shown in Table 4. The dimethyl sulfoxide barely decomposed even when heated for 2.8 hours at temperatures from 187 to 190°C (flask temperatures) in the early distillation operation and from 191 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residual liquid was 3.4 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 9-fold by distilling the dimethyl sulfoxide. As shown in Table 4, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.986% of the area, and the amount of decomposition was 0.009%, extremely small. (Example 14) A 1-L four-necked flask equipped with a Dimroth condenser, distillate receiver, stirrer, and thermometer, necessary for simple distillation, was charged with 720 g of dimethyl sulfoxide (purity: 99.996% by area) (pure dimethyl sulfoxide: 759.97 g). Then, as an additive, an aqueous solution of sodium carbonate was charged, prepared by dissolving 7.2 g of sodium carbonate (1 g relative to 100 g of pure dimethyl sulfoxide) in 40 g of ion-exchange water. After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 220°C, and a point in time when distillation began was considered as a start time.After collecting 140 ml containing water as an early distillate, a main distillation operation was performed in which the oil bath temperature was increased to 230°C, and the distillate was collected until the amount of residual liquid in the flask reached 56 g (the amount of water in the residual liquid: 0.0 g) and the amount of sodium carbonate after distillation reached 15 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 2 hours. The purity of dimethyl sulfoxide in the main distillate was 99.995% of the area, as shown in Table 4, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.956% of the area, as shown in Table 4. The dimethyl sulfoxide barely decomposed even when heated for 2 hours at temperatures from 162 to 191°C (flask temperatures) in the early distillation operation and from 192 to 193°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residual liquid was 15 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 15-fold by distilling the dimethyl sulfoxide. As shown in Table 4, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.992% of the area, and the amount of decomposition was 0.004%, extremely small. (Example 15) A 1-L four-necked flask equipped with a Dimroth condenser, distillate receiver, stirrer, and thermometer, necessary for simple distillation, was charged with 400 g of dimethyl sulfoxide (purity: 99.996% by area) (pure dimethyl sulfoxide: 399.98 g). Then, as an additive, an aqueous solution of sodium carbonate was charged, prepared by dissolving 0.4 g of sodium carbonate (0.1 g relative to 100 g of pure dimethyl sulfoxide) in 400 g of ion-exchange water. After replacing the inside of the flask with nitrogen, a nitrogen-filled rubber balloon was mounted on top of the Dimroth condenser to seal the flask. Under atmospheric pressure, the flask was heated in an oil bath maintained at 220°C, and the point in time when distillation began was considered the start time. A quantity of 500 ml containing water was collected as an early distillate.Next, as a primary distillation operation, the oil bath temperature was reduced to 170 °C, and under a pressure of 20 to 22.66 kPa (150 to 170 Torr), the distillate was collected until the amount of residual liquid in the flask reached 22 g (the amount of water in the residual liquid: 0.0 g) and the amount of sodium carbonate after distillation reached 1.9 g relative to 100 g of pure dimethyl sulfoxide. The heating time from the start was 3 hours. The purity of dimethyl sulfoxide in the main distillate was 99.997% of the area, as shown in Table 4, so that high purity dimethyl sulfoxide was obtained. The purity of the dimethyl sulfoxide in the residual liquid was 99.938% of the area, as shown in Table 4. The dimethyl sulfoxide barely decomposed even when heated for 3 hours at temperatures ranging from 108 to 193°C (flask temperatures) in the early distillation operation and from 138 to 140°C (flask temperatures) in the main distillation operation. Since the amount of sodium carbonate in the residual liquid was 1.9 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid, the sodium carbonate was concentrated 19-fold by distilling the dimethyl sulfoxide. As shown in Table 4, the purity of dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate and the early distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.989% of the area, and the amount of decomposition was 0.007%, extremely small. (Comparative Example 8) A 1-L four-necked flask equipped with a Dimroth condenser, distillate receiver, stirrer, and thermometer, as required for simple distillation, was charged with 201 g of dimethyl sulfoxide (purity: 99.996% by area) (200.99 g pure dimethyl sulfoxide), without any additives. After filling the flask with nitrogen, a nitrogen-filled rubber balloon was placed on top of the Dimroth condenser to seal the flask. The flask was heated in an oil bath maintained at 220°C, and the point in time when distillation began was recorded as the start time. When the distillate was collected until the amount of residual liquid in the flask reached 50 g (the amount of water in the residual liquid: 0.0 g), the purity of dimethyl sulfoxide in the main distillate was 99.972% of the area, as shown in Table 4.Due to the reduction in the purity of the dimethyl sulfoxide, the distillation was stopped. The heating time from the start was 2 hours. The purity of the dimethyl sulfoxide in the residual liquid was 99.986% of the area, as shown in Table 4. As shown in Table 4, the purity of the dimethyl sulfoxide in a mixed liquid of the distillate after distillation (the main distillate) and the residual liquid after distillation (a post-distillation mixed liquid) was 99.975% of the area, and the amount of decomposition was 0.021%, which was greater than in Example 11. Table 4 Additive Liquid Loaded Main Distillate Residual Liquid Concentration Ratio of Additive Purity of Mixed Liquid After Distillation (or of the area) Amount of Additive (or of the area) Purity of Dimethyl Sulfoxide (% of the area) Amount of Additive (g) Amount of Water Pressure Temperatures in the Flask Purity of Dimethyl Sulfoxide (% of the area) Purity of Dimethyl Sulfoxide (% of the area) Amount of Additive (g) Ex 11 Sodium Carbonate 99.995 l () Atmospheric Pressure 192 19.1 99.990 99.910 15 15 99.9X6 0.OO9 Ex 12 Sodium Carbonate 99.995 0.56 0.14 Atmospheric Pressure 192 19? 99 990 99 916 114 1? 99.9X6 1)009 Ei n Sodium carbonate 99.995 9.17 0.63 Atmospheric pressure 191 191 99.990 99.95 1 1.4 9 99.9X6 0.009 Ej I4 Sodium carbonate 99.996 I 5.1 Atmospheric pressure 192 193 99.995 99 956 15 15 99.992 0.004 Ej I5 Sodium carbonate 99.996 0 1 1 (X) Reduced pressure 11S 140 99 997 99 91X 1.9 19 99 9X9 0.007 Ej Com X None 99 996 0 Atmospheric pressure 192 19.1 99.972 99.9X6 99 975 0 021. The amount of additive is the amount of an additive with respect to 100 g of pure dimethyl sulfoxide in the liquid. The amount of water in the loaded liquid is the amount of water relative to 100 g of pure dimethyl sulfoxide in the liquid. In Examples 11 through 15, high-purity dimethyl sulfoxide was obtained as the main distillate. The purity of the dimethyl sulfoxide obtained in Comparative Example 8 was lower than in Example 11. The previous results showed that the method for purifying dimethyl sulfoxide according to the present invention provided high purity dimethyl sulfoxide as the main distillates. Furthermore, as purification progressed by distilling the liquid containing dimethyl sulfoxide, the decomposition inhibitor remained in the lower part of the distillation column during purification, thus increasing the concentration of the decomposition inhibitor included in the residual liquid during distillation. With the use of sodium carbonate as the decomposition inhibitor, even when the dimethyl sulfoxide was distilled and the sodium carbonate concentration was increased, the decomposition of the dimethyl sulfoxide was not promoted, and the purity of the dimethyl sulfoxide remained high. On the other hand, with the use of sodium hydroxide or potassium carbonate as the decomposition inhibitor, even when added at low concentrations, the decomposition of the dimethyl sulfoxide was promoted when the dimethyl sulfoxide was distilled and the concentration of the decomposition inhibitor was increased.Therefore, a decomposition product of dimethyl sulfoxide was contaminated inside the distilled dimethyl sulfoxide, thereby reducing the purity of the dimethyl sulfoxide. INDUSTRIAL APPLICATION The dimethyl sulfoxide obtained by the method for purifying dimethyl sulfoxide according to the present invention has a high purity and can be used as a solvent in the polymerization and centrifugation stages of polymers, such as polyacrylonitrile, cellulose, polyimide, polysulfone, and polyurethane, as a stripping liquid for photoprotectants that are electronic materials, as a solvent for synthesizing pharmaceuticals and agrochemicals, as a removal and cleaning liquid for lens molds and the like, or as a paint stripping liquid.
Claims
1. A method for purifying dimethyl sulfoxide, characterized in that it comprises distilling a liquid containing dimethyl sulfoxide in the presence of sodium carbonate in an inert gas atmosphere under reduced pressure to distill dimethyl sulfoxide, the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in a residual liquid after distillation being 10 to 500 times the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the liquid containing dimethyl sulfoxide before distillation, wherein the temperature during distillation is from 131°C to 160°C.
2. The method for purifying dimethyl sulfoxide according to claim 1, further characterized in that the distillation is carried out by adding, at the beginning of the distillation, sodium carbonate in an amount from 0.0005 to 1.0 g with respect to 100 g of pure dimethyl sulfoxide to the liquid containing dimethyl sulfoxide.
3. The method for purifying dimethyl sulfoxide according to claim 1, further characterized in that the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the residual liquid after distillation is from 6 to 1000 times the amount of sodium carbonate relative to 100 g of pure dimethyl sulfoxide in the liquid containing dimethyl sulfoxide before distillation.
4. The method for purifying dimethyl sulfoxide according to claim 1, further characterized in that the amount of sodium carbonate in the residual liquid after distillation is from 0.01 to 100 g relative to 100 g of pure dimethyl sulfoxide in the residual liquid after distillation.
5. The method for purifying dimethyl sulfoxide according to any of claims 1 to 4, further characterized in that after water is distilled from the liquid containing dimethyl sulfoxide as a first distillate, the dimethyl sulfoxide is distilled as a main distillate.
6. The method for purifying dimethyl sulfoxide according to any of claims 1 to 5, further characterized in that the purified dimethyl sulfoxide has a purity of 99.990% or more.
7. The method for purifying dimethyl sulfoxide according to any of claims 1 to 6, further characterized in that sodium carbonate is used as an inhibitor for the decomposition of dimethyl sulfoxide.