Method for producing trioxane

Abstract

To provide a method for producing trioxane from an aqueous formaldehyde solution, which enables reduction of energy consumption and efficient recovery of trioxane.SOLUTION: A method for producing trioxane comprises: (1) a step of contacting an aqueous formaldehyde solution with an acid catalyst to provide an aqueous solution containing trioxane; (2) a step of mixing the aqueous solution containing trioxane derived from step (1) with an extractant to provide an extractant mixture; and (3) a step of separating the extractant from the extractant mixture derived from step (2), wherein the extractant has a water content of 0.10 mass% or less at 90°C and a specific gravity of less than 0.80.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing trioxane.

Background Art

[0002] Trioxane is produced and used as a raw material for producing polyacetal resins.

[0003] As a method for producing trioxane, a method using an aqueous formaldehyde solution as a raw material is known. For example, Patent Document 1 discloses a method for synthesizing trioxane by heating and distilling an aqueous formaldehyde solution of about 30 to 70% by weight in the presence of a liquid acid such as sulfuric acid.

[0004] In addition, there is also a method of using various solid acids as a synthesis catalyst instead of the liquid acid in the above method. For example, Patent Document 2 discloses a method for producing trioxane by bringing an aqueous solution of high-concentration formaldehyde into contact with a solid acid catalyst (strongly acidic ion exchange resin).

[0005] In the method for producing trioxane from an aqueous formaldehyde solution, a step of recovering the produced trioxane from the aqueous formaldehyde solution is required. Regarding this step, for example, Patent Document 1 discloses a method in which an aqueous formaldehyde solution containing the produced trioxane is distilled to obtain a distillate, and then trioxane is extracted from the distillate with a solvent that is insoluble or hardly soluble in water, and this extract is rectified to separate trioxane.

[0006] In addition, Patent Document 2 discloses a method for recovering trioxane by bringing an aqueous formaldehyde solution containing trioxane into contact with m-dichlorobenzene to extract trioxane into m-dichlorobenzene and then separating this by distillation.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] These conventionally disclosed methods have the following problems.

[0009] First, the method disclosed in Patent Document 1 obtains a distillate containing trioxane, formaldehyde, and water, recovers trioxane from the distillate by extraction, and distillation-separates the extract and trioxane. Therefore, there are many distillation-separation operations, and a large amount of energy is consumed for the recovered trioxane.

[0010] Also, in the method disclosed in Patent Document 2, in which trioxane is directly extracted from an aqueous formaldehyde solution containing trioxane and then the extract and trioxane are distillation-separated, the distillation-separation operation is reduced. However, a large amount of water is extracted when extracting trioxane, and when distillation-separating the extractant and trioxane, water mixed in the extractant and trioxane may form an azeotropic mixture, making it impossible to efficiently recover trioxane. In addition, when using an extract that extracts a large amount of water, since it is hydrophilic, it often takes time to separate the extractant layer and the aqueous solution layer when liquid-phase separating the extractant and the aqueous solution.

[0011] The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing trioxane from an aqueous formaldehyde solution that can reduce the energy consumption and efficiently recover trioxane.

Means for Solving the Problems

[0012] The inventors of the present invention have conducted extensive studies to solve the above problems. As a result, they have found that the above problems can be solved by using an extractant having a water content and a specific gravity within a specific range when contacting an aqueous solution containing trioxane, and have thus completed the present invention. That is, the present invention is as follows.

[0013] [1] A step (1) of obtaining an aqueous solution containing trioxane by contacting an aqueous formaldehyde solution with an acid catalyst, A step (2) of mixing the aqueous solution containing trioxane obtained in the step (1) with an extractant to obtain an extractant mixture, and A step (3) of separating the extractant from the extractant mixture obtained in the step (2), wherein the extractant has a water content at 90 °C of 0.10 mass% or less and a specific gravity of less than 0.80 A method for producing trioxane, characterized in that. [2] The method for producing trioxane according to [1], wherein the extractant contains 95 mass% or more of an organic compound. [3] The method for producing trioxane according to [2], wherein the water content of the organic compound at 90 °C is 0.10 mass% or less. [4] The method for producing trioxane according to [2] or [3], wherein the specific gravity of the organic compound is less than 0.80. [5] The method for producing trioxane according to any one of [2] to [4], wherein the organic compound is an aliphatic hydrocarbon compound. [6] The method for producing trioxane according to any one of [2] to [5], wherein the organic compound is at least one selected from the group consisting of tetradecane, pentadecane, and hexadecane. [7] The method for producing trioxane according to any one of [1] to [6], wherein in the step (2), the aqueous solution containing trioxane and the extractant are brought into countercurrent contact. [8] The method for producing trioxane according to any one of [1] to [7], wherein the step (3) is performed in less than 10 minutes.

Advantages of the Invention

[0014] According to the method for producing trioxane of the present invention, the energy consumption for producing trioxane from an aqueous formaldehyde solution can be reduced, and trioxane can be efficiently recovered.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, embodiments for carrying out the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail.

[0016] The method for producing trioxane of the present embodiment includes the following steps. Step (1); A step of bringing an aqueous formaldehyde solution into contact with an acid catalyst to obtain an aqueous solution containing trioxane. Step (2): A step of mixing the aqueous solution containing trioxane obtained in the step (1) with an extractant to obtain an extractant mixture. Step (3): A step of separating the extractant from the extractant mixture obtained in the step (2).

[0017] <Step (1)> In step (1), an aqueous solution containing trioxane is obtained by bringing an aqueous formaldehyde solution into contact with an acid catalyst. More specifically, an aqueous solution containing trioxane is produced by heating an aqueous formaldehyde solution in the presence of an acid catalyst. In the reaction for generating trioxane from formaldehyde, the equilibrium is largely biased toward the formaldehyde side. Therefore, the aqueous solution containing trioxane obtained in step (1) mainly contains water or formaldehyde, and the content of trioxane is low. Hereinafter, the compounds used in step (1) will be described.

[0018] The aqueous formaldehyde solution is not particularly limited as long as it contains formaldehyde and water, and those having various concentrations can be used. When the formaldehyde concentration is too low, the efficiency in the production of trioxane decreases. Therefore, the formaldehyde concentration is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more. On the other hand, when the formaldehyde concentration is too high, the aqueous formaldehyde solution tends to solidify and handling becomes complicated. Therefore, the formaldehyde concentration is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less.

[0019] The aqueous formaldehyde solution may contain methanol in order to avoid the polymerization of formaldehyde or to enhance the solubility of formaldehyde, and such an aqueous formaldehyde solution may be used.

[0020] The aqueous formaldehyde solution can be produced using known methods. For example, the methanol excess method, the air excess method, etc. described on page 626 of the book "Chemical Goods of Edition 2020, 17120" (Chemical Industry Daily) can be used. Also, it may be an aqueous formaldehyde solution obtained by blowing a gas obtained by thermal decomposition of polyacetal into a liquid containing water. Further, for example, an aqueous formaldehyde solution with a formaldehyde concentration of around 37% by mass, called formalin, can also be used.

[0021] The acid catalyst may be a liquid acid catalyst such as sulfuric acid or a solid acid catalyst such as an ion exchange resin. As the solid acid catalyst, either an organic solid acid or an inorganic solid acid may be used, and it may be a single type or a mixture of two or more of these. Examples of the organic solid acid include ion exchange resins, ion exchange membranes, ion exchange fibers, etc. having a sulfonic acid group, a fluoroalkane sulfonic acid group, etc. Examples of the inorganic solid acid include inorganic acid oxide composites such as acid clay, silica, alumina, silica alumina, alumina boria, zeolite, etc., and those obtained by impregnating a solid carrier with sulfuric acid, phosphoric acid, boric acid, etc. Further, an organic-inorganic hybrid solid acid such as a polyorganosiloxane having a sulfonic acid group may also be used. Among them, an organic solid acid having a sulfonic acid group is preferable, and a strong acid type ion exchange resin is preferable.

[0022] As the reactor main body containing these solid acid catalysts, any form such as a packed bed type, a pipe type, a cage type, a fluidized bed type, a tray tower type, etc. may be used.

[0023] The reaction temperature can be arbitrarily selected, but at a low temperature, the aqueous formaldehyde solution may solidify, and preferably it is 85 °C or higher, more preferably 90 °C or higher. On the other hand, at a high temperature, the aqueous formaldehyde solution may become a pressurized system due to boiling, so preferably it is 120 °C or lower, more preferably 115 °C or lower.

[0024] The residence time in the reactor can be arbitrarily set so as to obtain a desired yield. For example, 30 seconds or more is preferable, 1 minute or more is more preferable, and 3 minutes or more is further preferable. On the other hand, when the time is too long, not only does the reactor to be used become large, but since the reaction for generating trioxane from formaldehyde is an equilibrium reaction, there is a limit to the yield of trioxane even if the reaction time is lengthened. Therefore, preferably it is 24 hours or less, more preferably 10 hours or less, further preferably 1 hour or less, even more preferably 40 minutes or less, and particularly preferably 20 minutes or less.

[0025] <Process (2)> The aqueous solution containing trioxane produced in process (1) is mixed with an extractant in process (2).

[0026] In the present disclosure, the extractant is an extractant for extracting trioxane from an aqueous solution containing trioxane. The extractant may be used alone or in combination of two or more.

[0027] The extractant used in this embodiment has a water content of 0.10% by mass or less at 90°C. Generally, when attempting to distillatively separate a mixture of trioxane and water, it is known that trioxane and water form an azeotrope at a temperature lower than the boiling point of trioxane. Therefore, when recovering trioxane from the extractant by distillation separation after step (3), if a large amount of water coexists in the extractant, trioxane may be recovered as an azeotrope with water, making it difficult to recover only trioxane. Therefore, from the perspective of reducing the amount of water contained in the extractant during extraction, the water content of the extractant at 90°C is 0.10% by mass or less, preferably 0.08% by mass or less, and more preferably 0.05% by mass or less. The lower limit value of the water content is not particularly limited and may be 0% by mass or more.

[0028] The specific gravity of the extractant used in this embodiment is less than 0.80. The reasons why such an extractant is preferred are explained below.

[0029] As described above, the equilibrium of the reaction for generating trioxane from formaldehyde is largely biased towards the formaldehyde side. Therefore, an aqueous solution containing trioxane produced from an aqueous formaldehyde solution has water or formaldehyde as the main component, and the trioxane content is low.

[0030] Generally, an aqueous solution containing formaldehyde has a high specific gravity. For example, according to page 22 of "Formaldehyde: Its Chemistry and Applications" (edited by Minoru Imamoto et al., Asakura Shoten, published on June 15, 1965), the higher the formaldehyde content, the greater the specific gravity of the liquid. For example, at a formaldehyde concentration of 40% by mass, the specific gravity is 1.1220, and at a formaldehyde concentration of 50% by mass, the specific gravity is 1.1570.

[0031] Therefore, the extractant used in this embodiment has a large difference in specific gravity from the aqueous formaldehyde solution containing trioxane, so the separation rate is high, and it is advantageous because turbidity at the interface is unlikely to occur. From such a viewpoint, an extractant having a large difference in specific gravity from the aqueous formaldehyde solution containing trioxane, that is, an extractant having a specific gravity of less than 0.80 is used, and it is preferably less than 0.79, and more preferably less than 0.78. Note that generally, since the specific gravity at a high temperature is smaller than that at a low temperature, it is sufficient that the specific gravity of the extractant satisfies the above range at room temperature (25°C).

[0032] The extractant preferably contains 95% by mass or more of an organic compound based on the total amount of the extractant, more preferably 97% by mass or more, still more preferably 99% by mass or more, and may even be 100% by mass. In addition to the organic compound, the extractant may contain compounds such as water and formaldehyde. When the extractant after separating trioxane is reused as described later, it may contain trioxane or the like.

[0033] As described above, the extractant used in this embodiment has a water content of 0.10% by mass or less at 90°C. As the organic compound constituting such an extractant, for example, one having a water content of 0.10% by mass or less at 90°C is used. From the viewpoint of reducing the amount of water contained in the extractant, the water content of the organic compound constituting the extractant at 90°C is preferably 0.10% by mass or less, more preferably 0.08% by mass or less, and still more preferably 0.05% by mass or less.

[0034] Also, as described above, the specific gravity of the extractant is less than 0.80. As the organic compound constituting such an extractant, it is preferably less than 0.80, more preferably less than 0.79, and still more preferably less than 0.78.

[0035] Preferably, the organic compound having a standard boiling point higher than the standard boiling point of trioxane (for example, 114°C) is preferably used. As the standard boiling point of the organic compound used for extraction, when trioxane is recovered from the extractant by distillation separation after step (3), since the distillation separation from trioxane becomes easy, it is preferably 130°C or higher, more preferably 150°C or higher, still more preferably 170°C or higher. On the other hand, when the standard boiling point of the organic compound is too high, the energy required when the distillation purification of the organic compound becomes necessary increases, so it is preferably 320°C or lower, more preferably 300°C or lower, still more preferably 290°C or lower.

[0036] The organic compound is not particularly limited as long as it satisfies the above-mentioned requirements, but among them, aliphatic hydrocarbon compounds are preferably used. As a result of the study by the present inventor, particularly when the compound constituting the extractant contains a halogen atom or a heteroatom such as an oxygen atom, a sulfur atom, or a nitrogen atom, there is often a remarkable tendency to easily contain water. In addition, compounds containing a halogen atom or a heteroatom tend to have a large specific gravity, and it may take time for separation due to a small specific gravity difference from the aqueous formaldehyde solution, or the separated extractant and the aqueous solution may become turbid.

[0037] Such aliphatic hydrocarbons are not particularly limited, and linear or branched ones can be used. For example, n-octane, n-nonane, decane (including isomers), dodecane (including isomers), tetradecane (including isomers), pentadecane (including isomers), hexadecane (including isomers), octadecane (including isomers), etc. can be mentioned.

[0038] Among these, one or more selected from the group consisting of tetradecane (including isomers), pentadecane (including isomers), and hexadecane (including isomers) are preferably used because the amount of water absorbed when extracting trioxane is small and the boiling point difference from trioxane is large.

[0039] The temperature at which step (2) is carried out is not particularly limited. However, when the formaldehyde concentration in the aqueous solution containing trioxane is high, the aqueous solution may solidify at low temperatures. Therefore, it is preferably carried out at 50 °C or higher, more preferably 70 °C or higher, and even more preferably 90 °C or higher. It is also preferable to maintain the reaction temperature of step (1) described above and carry out step (2) at that temperature.

[0040] The apparatus for carrying out step (2) is not particularly limited, and known apparatuses such as a stirring tank, a drum, and line mixing can be arbitrarily used, and these can be combined and carried out as necessary. Step (2) can also be carried out with the apparatus for carrying out step (3) described later.

[0041] The amount of the extractant used in step (2) is not particularly limited. However, in order to extract more trioxane, more extractant should be used with respect to the aqueous solution containing trioxane, and preferably, it is 0.5 times or more, more preferably 1 time or more, and even more preferably 2 times or more in terms of volume ratio with respect to the aqueous solution. On the other hand, if too much extractant is used, when recovering trioxane from the extractant by distillation separation after step (3), the amount of energy used in the distillation separation and the amount of energy used for adjusting the temperature of the extractant increase. Therefore, it is preferably 50 times or less, more preferably 20 times or less, and even more preferably 10 times or less.

[0042] The method of mixing the aqueous solution containing trioxane and the extractant is not particularly limited. For example, a method of contacting them either in countercurrent or in cocurrent can be mentioned, and preferably, they are contacted in countercurrent. Specifically, the aqueous solution can be supplied from the upper part of the apparatus for carrying out step (2), and the extractant can be supplied from the lower part, so that the aqueous solution and the extractant can be contacted in countercurrent. When contacting in countercurrent, the extraction efficiency is improved, and there is a tendency to reach the equilibrium concentration in a short time. Since the descending speed of the aqueous solution and the ascending speed of the extractant in the apparatus depend on the specific gravity difference between the two, the improvement in the extraction efficiency by countercurrent contact is one of the effects obtained by using the extractant of the present embodiment.

[0043] <Process (3)> Step (3) is a step of separating the extractant from the extractant mixture obtained in step (2).

[0044] The temperature at which step (3) is carried out is not particularly limited. However, when the formaldehyde concentration in the aqueous solution containing trioxane is high, the aqueous solution may solidify at low temperatures. Therefore, it is preferably carried out at 50°C or higher, more preferably 70°C or higher, and even more preferably 90°C or higher. It is also preferable to maintain the temperature of step (2) described above and carry out step (3) at that temperature.

[0045] The apparatus for carrying out step (3) is not particularly limited, and extraction apparatuses such as a separating funnel, an oil-water separator having a settler, a mixer-settler, a centrifugal extractor, and a pulsed column can be used. Among them, a separating funnel and an oil-water separator having a settler can be preferably used. Generally, a settler is a device that separates two phases, a light phase and a heavy phase, by static decantation. The light phase and the heavy phase are separated by an agglomeration plate installed inside, the light phase overflows the settler and is sent to the subsequent stage, and the heavy phase is taken out from the lower weir. Since the settler does not use power for phase separation, it is suitable for separating the formaldehyde aqueous solution containing trioxane used in this embodiment and an extractant having a smaller specific gravity than that. Also, similar to the settler, a drum equipped with a weir from the bottom to the middle height inside may be used, and a device that sends the light phase that overflows the weir of the drum to the subsequent stage and recovers the heavy phase from below the weir may be used. In any case, it is a device that can be used by using the extractant of this embodiment.

[0046] Also, step (3) is preferably completed within less than 10 minutes from the time when step (2) is completed. When step (3) is carried out batchwise, the time when the liquid of step (2) is introduced into the apparatus for carrying out step (3) is taken as the time when step (2) is completed, and it is preferable that the separation of step (3) is completed within less than 10 minutes from here. When step (3) is carried out continuously, it can be evaluated by the average residence time of the liquid in the apparatus where step (3) is carried out. For example, the effective internal volume of the apparatus for carrying out step (3) (m 3The time calculated by dividing the unit) by the flow rate in step (2) (the sum of the aqueous solution flow rate and the extractant flow rate, m 3 / min unit) is preferably set to be less than 10 minutes. When steps (2) and (3) are carried out in the same apparatus, it is preferable that the separation in step (3) is completed within less than 10 minutes after the operation in step (2) is completed. If step (3) is completed within less than 10 minutes, the internal volume of the apparatus can be reduced, and the concurrence of unexpected side reactions can also be suppressed, which is preferable. This time is more preferably less than 8 minutes, and even more preferably less than 5 minutes.

[0047] 〈Distillation separation step〉 The extractant after extracting trioxane is separated into trioxane and the extractant, for example, by further distillation separation. The type of distillation column used at this time may be any of a tray column, a bubble-cap column, and a packed column, and various methods can be used according to the extractant to be used and the components and concentrations contained in the extractant.

[0048] The temperature and pressure for distillation separation can be set according to the extractant to be used and the components and concentrations contained in the extractant. However, at a low temperature, formaldehyde contained in trace amounts in the extractant may solidify and precipitate on the inner wall of the distillation column, causing problems such as blockage. Therefore, it is preferably 85°C or higher, more preferably 90°C or higher. On the other hand, at a high temperature, it may become a pressurized system, so it is preferably 120°C or lower, more preferably 115°C or lower. It is also preferable to use the extractant after extracting trioxane in step (2) at the same temperature without heating or cooling. The pressure for distillation separation can be appropriately set according to the components, concentrations, and temperatures contained in the extractant.

[0049] The extractant recovered by distillation separation of the extractant can be reused for the extraction of trioxane from the aqueous formaldehyde solution containing trioxane.

[0050] The distilled trioxane can be directly used as a raw material for producing polyacetal. The distilled trioxane can also be further purified before use.

Examples

[0051] The following examples are described without limiting this embodiment.

[0052] The measurement and evaluation methods used in the examples and comparative examples are as follows.

[0053] <Method for Measuring the Amount of Formaldehyde in Solution> The amount of formaldehyde in the solution was measured under the following conditions using gas chromatography and calculated using a calibration curve prepared in advance. Gas chromatography: GC-2014 (manufactured by Shimadzu Corporation) Column: Porapak T (manufactured by GL Sciences Inc.) Injection port temperature: 180 °C Detector temperature: 180 °C Detector: Thermal conductivity detector Carrier gas: Helium Column temperature: Held at 125 °C for 8 minutes, then heated to 180 °C at a heating rate of 20 °C / min and held for 22 minutes Measurement sample injection volume: 5 μL Method for preparing the measurement sample: Approximately 0.1 g of the sample was dissolved in approximately 1 g of acetone (manufactured by FUJIFILM Wako Pure Chemical Corporation, ultra-dehydrated grade).

[0054] <Method for Measuring the Amount of Trioxane in Solution> The amount of trioxane in the solution was measured under the following conditions using gas chromatography and calculated using a calibration curve prepared in advance. Gas chromatography: GC-2014 (manufactured by Shimadzu Corporation) Column: SH-Stabilwax (manufactured by Shimadzu GLC) Injection port temperature: 240 °C Detector temperature: 240 °C Detector: Hydrogen flame ionization detector Carrier gas: Nitrogen Column temperature: Hold at 50°C for 5 minutes, then increase the temperature to 240°C at a rate of 20°C / min and hold for 5 minutes. Injection volume of measurement sample: 5 μL Method for preparing measurement sample: Dissolve approximately 0.1 g of the sample in approximately 1 g of acetone (manufactured by FUJIFILM Wako Pure Chemical Corporation, ultra-dehydrated grade).

[0055] <Method for measuring water content in liquid> The water content in the liquid was quantified by the Karl Fischer method. Apparatus: CA-200 (manufactured by Mitsubishi Chemical Analytech Co., Ltd.) Anolyte: Aquamicron (registered trademark) AKX Catholyte: Aquamicron (registered trademark) CXU Measurement method: Weigh approximately 1 g of the sample with a syringe and inject it into the apparatus. After injecting the sample, divide the total amount of water detected until the detection rate reaches 0.02 μg / sec by the amount of the injected sample to calculate the water content (mass ppm).

[0056] [Production Example 1] Synthesis of trioxane from aqueous formaldehyde solution Measure 40 mL of ion exchange resin Amberlyst 35 (manufactured by Organo Corporation) with a graduated cylinder and put it into a Teflon (registered trademark) inner cylinder type sealed container with an internal volume of 100 mL, and heat it to 100°C. Pour 50 mL of 67% by mass aqueous formaldehyde solution into this, seal it, and immerse it in an oil bath heated to 100°C. After 24 hours, filter the contents to recover an aqueous solution (reaction solution) containing trioxane. The trioxane concentration of the reaction solution was 5.0% by mass. Repeat the above operation to prepare an aqueous solution (reaction solution) containing trioxane.

[0057] [Example 1] Charge 255.9 g of the reaction solution from Production Example 1 and 767.1 g of n-decane into a jacketed separatory funnel heated to 100°C by oil circulation. Note that the reaction solution and n-decane were pre-heated to 100°C before use. The volume ratio of n-decane to the reaction solution is 4.8 times. The separatory funnel was shaken and then allowed to stand for 5 minutes. After visually confirming that the organic layer and the aqueous layer had been sufficiently separated, liquid separation was started and completed 5 minutes after the start of standing. The water content of the organic layer was 120 mass ppm.

[0058] The organic layer was placed in a 200 mL three-necked flask equipped with a distillation tube and a condenser, and the distillation separation of n-decane and trioxane was carried out. A mixture containing trioxane and water was recovered in the first distillate, and trioxane was recovered in the next fraction. The water content of the first distillate was 26 mass%. The recovery rate of trioxane contained in the reaction solution was 48%, and the water content of the trioxane was 0.1 mass%.

[0059] [Example 2] As shown in Table 1, the same method as in Example 1 was carried out except that hexadecane was used instead of n-decane to obtain trioxane. In the liquid separation operation, the separatory funnel was shaken and then allowed to stand for 5 minutes. After visually confirming that the organic layer and the aqueous layer had been sufficiently separated, liquid separation was started and completed 5 minutes after the start of standing. The recovery rate of trioxane was 42%. The water content of the trioxane was 0.1 mass%. The water content of the organic layer was 210 mass ppm.

[0060] [Comparative Example 1] As shown in Table 1, the same method as in Example 1 was carried out except that a mixed solution of nitrobenzene and trichlorobenzene was used instead of n-decane. In the liquid separation operation, when the separatory funnel was shaken and then allowed to stand for 5 minutes, the separation of the organic layer and the aqueous layer was insufficient. Therefore, liquid separation was started after standing for 10 minutes and completed about 15 minutes after the start of standing. The water content of the organic layer was 1.6 mass%. In the distillation separation, a mixture containing trioxane and water was recovered in the first distillate, and the water content of the first distillate was 24 mass%. The main component of the next fraction was nitrobenzene, and trioxane with a low water content was not recovered.

[0061] [Comparative Example 2] The same method as in Example 1 was carried out except that tetrachloroethane was used instead of n-decane as shown in Table 1. In the liquid separation operation, when the separating funnel was shaken and then allowed to stand for 5 minutes, the separation of the organic layer and the aqueous layer was insufficient. Therefore, liquid separation was started after allowing to stand for 10 minutes, and the liquid separation was completed about 15 minutes after the start of standing. The water content of the organic layer was 0.8 mass%. In the distillation separation, a mixture containing trioxane and water was recovered in the initial distillate, and the water content of the initial distillate was 25 mass%. The main component of the next fraction was tetrachloroethane, and trioxane with a low water content was not recovered.

[0062] The above results are shown in Table 1. Thus, in Example 1 and Example 2 using an extractant with a water content of 0.10 mass% or less and a specific gravity of less than 0.80 at 90 °C, trioxane could be recovered. However, in Comparative Example 1 and Comparative Example 2, the water content of the organic layer increased, and most of the trioxane was recovered as a mixture containing 24 mass% and 25 mass% of water, respectively, during the distillation separation, and trioxane with a low water content could not be recovered.

[0063]

Table 1

Claims

1. Step (1) of bringing an aqueous formaldehyde solution into contact with an acid catalyst to obtain an aqueous solution containing trioxane; Step (2) of mixing the aqueous solution containing trioxane obtained in the above step (1) with an extractant to obtain an extractant mixture; and Step (3) of separating the extractant from the extractant mixture obtained in the above step (2), wherein the extractant has a water content of 0.10% by mass or less at 90° C. and a specific gravity of less than 0.

80. A method for producing trioxane, characterized by the above.

2. The method for producing trioxane according to claim 1, wherein the extractant contains 95% by mass or more of an organic compound.

3. The method for producing trioxane according to claim 2, wherein the water content of the organic compound at 90° C. is 0.10% by mass or less.

4. The method for producing trioxane according to claim 2, wherein the specific gravity of the organic compound is less than 0.

80.

5. The method for producing trioxane according to claim 2, wherein the organic compound is an aliphatic hydrocarbon compound.

6. The method for producing trioxane according to claim 5, wherein the organic compound is one or more selected from the group consisting of tetradecane, pentadecane, and hexadecane.

7. The method for producing trioxane according to claim 1, wherein in the above step (2), the aqueous solution containing trioxane and the extractant are brought into countercurrent contact.

8. The method for producing trioxane according to claim 1, wherein the above step (3) is performed in less than 10 minutes.

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