Method for producing formaldehyde gas

Abstract

To provide an efficient formaldehyde production process with a rapid dehydration rate and decomposition rate.SOLUTION: A method for producing formaldehyde gas includes steps of: reacting (A) an alcohol and (B) an aqueous formaldehyde solution to generate an aqueous hemiformal solution, evaporating and dehydrating the aqueous hemiformal solution to separate the same into (C) a hemiformal concentrate and (D) water; and thermally decomposing the hemiformal concentrate, wherein the alcohol (A) has at least two OH groups per molecule and a carbon number of 4 or less per molecule.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing formaldehyde gas. [Background technology]

[0002] A known method for producing formaldehyde gas is the hemiformalization method, which is carried out as follows: First, an aqueous solution of formaldehyde is reacted with an alcohol to obtain an aqueous solution of hemiformal as a reaction product. Next, water is removed from the aqueous solution of hemiformal to obtain a hemiformal concentrate. Finally, the hemiformal concentrate is thermally decomposed to obtain formaldehyde gas (Patent Document 1).

[0003] The alcohol used in this hemiformalization method is regenerated by thermal decomposition of the hemiformal concentrate, making it a rational process that can be recovered and reused. However, in the hemiformalization method, a hemiformal molecule and another alcohol molecule condense to form a formal bond, which is known to produce a formal compound. The resulting formal compound is not decomposed by heat, which reduces the reaction rate of formaldehyde.

[0004] In addition, the hemiformalization method sometimes uses alcohols with multiple OH groups, such as diols, triols, and polyols, to efficiently react with formaldehyde. However, with these alcohols, the formation of formal bonds can occur repeatedly, resulting in the production of highly viscous, high-molecular-weight components. The alcohols that produce these highly viscous components can cause problems such as clogging, reduced dehydration efficiency, and a slower decomposition rate in the decomposition process, making it necessary to extract and purify the resulting formal compounds or add new alcohol. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 01-216950 Summary of the Invention [Problem to be solved by the invention]

[0006] As described above, in the method for producing formaldehyde gas by the hemiformalization method, it is necessary to efficiently carry out the reaction between alcohol and formaldehyde and the dehydration of the aqueous hemiformal solution.

[0007] However, when the number of OH groups in the hemiformal compound is increased in an attempt to solve the above problem, there is a problem that the viscosity increases due to the generation of formal compounds, and the dehydration efficiency and decomposition rate decrease significantly, and further improvement is desired.

[0008] Therefore, an object of the present invention is to provide a method for producing formaldehyde gas that is excellent in dehydration efficiency and can suppress an increase in the viscosity of alcohol. [Means for solving the problem]

[0009] As a result of extensive investigations into solving the above problems, the present inventors discovered that by using an alcohol with a specific structure, it is possible to efficiently react with formaldehyde and the viscosity is less likely to increase, and thus completed the present invention.

[0010] That is, the present invention is as follows. [1] A method for producing formaldehyde gas, comprising the steps of: (A) reacting an alcohol with (B) an aqueous formaldehyde solution to produce an aqueous hemiformal solution; (C) separating the aqueous hemiformal solution into a hemiformal concentrate and (D) water by evaporating and dehydrating the aqueous hemiformal solution; and (D) thermally decomposing the hemiformal concentrate, A method for producing formaldehyde gas, wherein the alcohol (A) has two or more OH groups per molecule and four or less carbon atoms per molecule. [2] The method for producing formaldehyde gas according to [1], wherein the alcohol (A) has a value obtained by dividing the number of oxygen atoms per alcohol molecule by the number of carbon atoms per alcohol molecule (number of oxygen atoms / number of carbon atoms) of 0.75 or more. [3] The method for producing formaldehyde gas according to [1] or [2], wherein the alcohol (A) is a diol. [4] The method for producing formaldehyde gas according to [3], wherein the alcohol (A) is ethylene glycol. [Effects of the Invention]

[0011] The method for producing formaldehyde gas of the present invention is excellent in dehydration efficiency and can suppress an increase in the viscosity of alcohol. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail below. The present invention is not limited to the following embodiment, but can be practiced in various modified forms within the scope of the invention.

[0013] <Alcohol> The alcohol used in the method for producing formaldehyde gas of this embodiment is at least one type of alcohol selected from the group consisting of alcohols having two or more OH groups per molecule and four or less carbon atoms per molecule. One or more types of alcohol may be used in combination. Use of such alcohols can suppress the generation of high molecular weight components, prevent scaling during the reaction process, and prevent a decrease in the dehydration rate and decomposition rate.

[0014] The number of OH groups per molecule of the alcohol must be 2 or more from the viewpoint of suppressing scaling in the reaction step, and is preferably 2 from the viewpoint of suppressing an increase in the viscosity of the alcohol. Note that when an alcohol having 5 or more carbon atoms is used, the molecular weight of the resulting acetal increases, which is thought to result in an increase in viscosity.

[0015] Furthermore, the value obtained by dividing the number of oxygen atoms of the alcohol by the number of carbon atoms (number of oxygen atoms / number of carbon atoms) is preferably 0.75 or more, from the viewpoint of suppressing scaling in the reaction step.

[0016] Furthermore, the alcohol is preferably a diol, and among them, ethylene glycol is more preferred.

[0017] The method for producing the alcohol is not particularly limited, and the alcohol can be produced by a conventionally known method. Alternatively, commercially available alcohols may be used.

[0018] <Formaldehyde aqueous solution> The formaldehyde content of the aqueous formaldehyde solution used in the method for producing formaldehyde gas of this embodiment is not particularly limited, but is preferably 1% by mass or more and 80% by mass or less.

[0019] The mixing ratio of the alcohol to the aqueous formaldehyde solution is preferably 0.3 to 5.0 in terms of the molar ratio of hydroxyl groups in the alcohol to the formaldehyde (hydroxyl groups / formaldehyde). A ratio of 0.3 or more is preferred because it reduces the amount of formaldehyde contained in water, while a ratio of 5.0 or less is preferred because it increases the amount of formaldehyde that reacts per unit weight of alcohol, which is cost-effective. A ratio of 0.5 to 2.0 is more preferred.

[0020] <Hemiformalization method> The hemiformalization method used in the method for producing formaldehyde gas of this embodiment includes a step (reaction step) of reacting an aqueous formaldehyde solution with an alcohol to form an aqueous hemiformal solution, a step (dehydration step) of removing water from the aqueous hemiformal solution to obtain a hemiformal concentrate, and a step (decomposition step) of thermally decomposing the hemiformal concentrate to obtain an alcohol and formaldehyde gas.

[0021] The alcohol obtained in the decomposition step can be recycled to the reaction step, or may be purified by distillation or the like before being recycled to the reaction step. Furthermore, the formaldehyde gas obtained in the decomposition step may be subjected to a purification step in which impurities such as water, methanol, and formic acid are removed by countercurrent contact with a hydrophilic solvent.

[0022] (Reaction step) The reaction step is a step for reacting an aqueous formaldehyde solution with an alcohol to obtain an aqueous hemiformal solution. The alcohol used in the reaction step may be recovered from the alcohol separated in the decomposition step.

[0023] The apparatus used in the reaction step may be a conventionally known tank-type reactor. The reactor may be equipped with a temperature-controlling device or a stirring mechanism. The reactor may be used in a batch system or a continuous system, or multiple reactors may be connected together.

[0024] The reaction conditions for the reaction step are not particularly limited as long as they are conditions that produce an aqueous hemiformal solution.

[0025] The reaction temperature is not particularly limited as long as it is a temperature at which the purified hemiformal does not decompose, but from the viewpoint of energy efficiency, a lower temperature is preferable. For example, the reaction temperature is preferably from room temperature (about 20°C) to 90°C. The reaction time is set appropriately depending on the progress of the reaction, etc.

[0026] The alcohol in the present invention has a good affinity with formaldehyde and produces a hemiformal compound at a high rate, so that an aqueous hemiformal solution can be obtained without causing scaling even at room temperature.

[0027] In the reaction step, the produced hemiformal compound further reacts with another alcohol molecule to produce a formal compound. The alcohol used in the present invention produces a formal compound with a lower viscosity than conventionally used alcohols. This suppresses a decrease in the reaction rate in the reaction step and a decrease in the dehydration rate in the dehydration step, resulting in an overall efficient process.

[0028] The resulting aqueous hemiformal solution is then sent to a dehydration step.

[0029] (Dehydration process) The dehydration step is a step in which the reaction product containing hemiformal and water (including unreacted alcohol and aldehyde) produced in the reaction step is separated into a hemiformal concentrate and water by evaporation and dehydration. The alcohol remaining unreacted is contained in the hemiformal concentrate, and the formaldehyde is contained in both the hemiformal concentrate and the water.

[0030] The alcohol used in the present invention is less likely to cause an increase in viscosity of the hemiformal compound produced compared to conventionally used alcohols, and therefore the dehydration rate remains constant, allowing the dehydration efficiency to be maintained even after repeated use.

[0031] The apparatus used in the dehydration step may be a conventionally known evaporator, distillation column, or dehydrator. The dehydrator may be a multi-stage type in which a plurality of dehydrators are connected together.

[0032] The conditions for the dehydration step are not particularly limited. However, since the temperature and pressure affect the amount of water contained in the hemiacetal concentrate, the conditions for the dehydration step are appropriately adjusted taking this into consideration.

[0033] (decomposition process) The decomposition step is a step in which the hemiformal concentrate is thermally decomposed to generate formaldehyde gas.

[0034] The alcohol used in the present invention is less likely to cause an increase in viscosity of the hemiformal compound produced compared to conventionally used alcohols, and therefore the decomposition rate remains constant, allowing the dehydration efficiency to be maintained even after repeated use.

[0035] The decomposition step may be carried out using a conventionally known apparatus. For example, the apparatus may be equipped with a heating unit for applying heat to the hemiformal concentrate and a control unit for controlling the heating conditions. It may also be equipped with a device for recovering the formaldehyde gas and alcohol produced by the decomposition. [Example]

[0036] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples in any way.

[0037] [Example 1]

[0038] (Synthesis of Hemiacetals) 30 mL of ethylene glycol and 67 wt% aqueous formaldehyde solution were mixed in a 100 mL recovery flask so that the molar ratio of the OH groups in the alcohol to the molar ratio of formaldehyde in the aqueous formaldehyde solution was 1:1. An air-cooled tube was attached and the flask was heated at 90°C for 2 hours to carry out a hemiformalization reaction. After the reaction was completed, 10 g of the hemiformal compound was weighed out and placed in another 100 mL recovery flask, and a dehydration reaction was carried out in an evaporator while reducing the pressure to 50 mmHg, to obtain an aqueous hemiformal solution with a water content of 15 wt %.

[0039] <Evaluation and measurement> (1) Dehydration speed 1 g of the aqueous hemiformal solution with a water content of 15 wt% obtained by the above procedure was weighed into another 100 mL recovery flask and subjected to a dehydration reaction at 70°C while reducing the pressure to 50 mmHg in an evaporator to obtain a hemiacetal concentrate. The time required for the water content to reach 1 wt% was measured, and this time is shown in Table 1 as the dehydration rate. The water content was measured using a Karl Fischer moisture meter.

[0040] (2) Decomposition rate of hemiacetal concentrate 1 mg of the hemiacetal concentrate with a water content of 1 wt% obtained by the above procedure was weighed into a 100 mL recovery flask and heated to 120°C while stirring to carry out the decomposition reaction. The formaldehyde generated by the decomposition immediately volatilized, leaving alcohol and undecomposed hemiformal in the flask. The weight was measured after 10 minutes to calculate the amount decomposed per gram. The results are shown in Table 1.

[0041] (3) Evaluation of viscosity increase rate The viscosity increase rate was calculated by the following procedure. (3-1) Measurement of moisture content First, the water content of the hemiacetal concentrate having a water content of 1 wt % obtained by the above-mentioned procedure was measured using the following apparatus. Apparatus: CA-200 (Mitsubishi Chemical Analytical Co., Ltd.) Anolyte: Aquamicron® AKX Catholyte: Aquamicron (registered trademark) CXU Measurement method: Approximately 1 g of sample was weighed into a syringe and injected into the device. After the sample was injected, the total amount of water detected until the detection speed reached 0.02 μg / sec was taken as the amount of water contained in the injected sample. (3-2) Viscosity of Hemiacetal Concentrate The hemiacetal concentrate with a water content of 1 wt% obtained by the above procedure was heated to 60°C and the saturation temperature was measured. This value was designated as A. The following equipment was used for the measurement. Device: SV-10 (manufactured by A&D) (3-3) Calculation of viscosity increase rate 30 mL of ethylene glycol and a 67 wt% aqueous formaldehyde solution were added to a pressure vessel so that the ratio of the moles of OH groups in the alcohol to the moles of formaldehyde in the aqueous formaldehyde solution was 1:1. This vessel was heated at 90°C for 120 minutes and then at 170°C for 30 minutes, a cycle repeated five times, and the hemiacetalization reaction and decomposition reaction were repeated inside the pressure vessel. After cooling to room temperature, the resulting hemiacetal mixture was transferred to a 100 mL recovery flask and subjected to a dehydration reaction at 70°C while reducing the pressure to 50 mmHg in an evaporator to obtain a hemiacetal concentrate. The viscosity of the concentrate was measured and designated B. The value obtained by dividing B by A (B / A) is the viscosity increase rate, which is shown in Table 1.

[0042] <Example 2> The same procedure as in Example 1 was carried out except that methoxyethylene glycol was used as the alcohol.

[0043] <Comparative Example 1> The same procedure as in Example 1 was carried out except that 30 g of trimethylolpropane was used as the alcohol.

[0044] <Comparative Example 2> Although 1,5-pentanediol was used as the alcohol, the hemiformalization did not proceed sufficiently by the same procedure as in Example 1, and an aqueous hemiformal solution could not be obtained.

[0045] [Table 1]

[0046] From Table 1, it can be seen that Examples 1 and 2 have a lower viscosity increase rate and a faster dehydration rate than Comparative Example 1. [Industrial Applicability]

[0047] According to the present invention, an efficient process for producing formaldehyde with high dehydration and decomposition rates can be provided.

Claims

1. A method for producing formaldehyde gas, comprising the steps of: reacting (A) an alcohol with (B) an aqueous formaldehyde solution to produce a hemiformal aqueous solution; evaporating and dehydrating the aqueous hemiformal solution to separate it into (C) a hemiformal concentrate and (D) water; and thermally decomposing the hemiformal concentrate, A method for producing formaldehyde gas, wherein the alcohol (A) has two or more OH groups per molecule and four or less carbon atoms per molecule.

2. 2. The method for producing formaldehyde gas according to claim 1, wherein the alcohol (A) has a value obtained by dividing the number of oxygen atoms per alcohol molecule by the number of carbon atoms per alcohol molecule (number of oxygen atoms / number of carbon atoms) of 0.75 or more.

3. 3. The method for producing formaldehyde gas according to claim 1, wherein the alcohol (A) is a diol.

4. 4. The method for producing formaldehyde gas according to claim 3, wherein the alcohol (A) is ethylene glycol.

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