JPS624391B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for purifying trioxane for producing high molecular weight polyoxymethylene polymers or copolymers. Generally, trioxane is synthesized by heating an aqueous formaldehyde solution in the presence of an acid catalyst. An industrially advantageous method for producing trioxane is to thermally distill an aqueous formaldehyde solution containing approximately 40 to 70 wt% in the presence of an acid catalyst to produce trioxane of 20 to 55 wt%, formaldehyde of 15 to 40 wt%,
A known method is to obtain a distillate with a composition of 20 to 50 wt% water, then extract this distillate liquid-by-liquid with a solvent that is insoluble or poorly soluble in water, and rectify the extract to separate trioxane. . In addition, many methods have been proposed for purifying the obtained trioxane, such as a method of purifying trioxane by distillation in the presence of an anion exchange resin or basic amines, a method of purifying trioxane with activated alumina or molecular sieves, Alternatively, there may be a method of removing impurities using a chemical reaction. However, although these purification methods can remove impurities such as water, formic acid, and formaldehyde in trioxane, polyoxymethylene dimethoxide (hereinafter abbreviated as PODM) of the following general formula CH ₃ O (CH ₂ O ) nCH ₃ (in the formula, n represents an integer of 2 or more. n of those contained in trioxane is usually 2 to 7) cannot be removed, and therefore trioxane suitable as a raw material for polymerization is I couldn't get it. According to the knowledge of the present inventors, the trioxane used as a polymerization raw material should contain PODM in a content of 700 ppm or less, preferably 500 ppm or less. However, this PODM, especially the one with n=2 in the above general formula, has a boiling point close to that of trioxane and has chemical properties similar to trioxane, so it cannot be purified by the conventional distillation, adsorption, extraction, reaction, etc. mentioned above. It could not be removed by law. There is also a known method (Japanese Patent Publication No. 55-23855) in which PODM is removed by cooling crude trioxane containing PODM and allowing it to exist in an open system in an inert gas or air atmosphere in a crystalline state. There is also a problem in handling when crystallizing the material and allowing it to exist in an atmosphere such as an inert gas. Methods of preventing PODM formation by shortening the reaction residence time are also known (Japanese Patent Laid-Open No. 50-35177, etc.), but there are problems with the materials of the thin film evaporator and distillation column, making this method industrially and economically difficult. There is a side. The inventors of the present invention have conducted intensive studies to effectively remove PODM, particularly the compound where n=2 in the general formula above, from trioxane. Although it was believed that PODM could not be separated and removed by distillation, the present inventors discovered that PODM could be removed by distilling crude trioxane under specific conditions, leading to the present invention. That is, in the present invention, crude trioxane containing PODM is supplied to the middle part of the distillation column, and PODM is extracted from the top of the column.
In the distillation method in which a trioxane solution rich in polyoxane is distilled off and purified trioxane is extracted from the bottom of the column, a higher concentration of PODM in the distillate is preferable because the loss of trioxane is smaller, but above a certain concentration, the removal of PODM is The present invention has been completed by discovering the fact that the rate is significantly reduced and remains in the bottom liquid.
Crude trioxane containing PODM is heated at a reflux ratio in a distillation column with 15 or more theoretical plates, preferably 20 or more plates.
This is a method of distilling at a PODM concentration of 20 or more, preferably 25 or more, and a PODM concentration in the distillate of 30% or less, preferably 25% or less. Note that a distillation column with actual plates of 25 or more plates, preferably 30 plates or more is required. The concentration of PODM in the distillate increases as the number of theoretical plates or the reflux ratio of the distillation column increases. Therefore, the PODM concentration in the distillate during distillation is controlled by changing the reflux ratio, but in reality, the amount of distillate withdrawn (distillation rate) or the bottom liquid It is controlled by increasing or decreasing the heating amount and changing the reflux ratio. That is, if the distillation rate is increased or the amount of heating of the column bottom liquid is decreased, the reflux ratio is decreased and the PODM concentration in the distillate is decreased; When the amount of heating of the bottom liquid is increased, the reflux ratio increases and the PODM concentration in the distillate increases. The concentration of PODM in trioxane can be determined by various known analytical methods, but gas chromatography is preferred.For example, a column containing polyethylene glycol PEG-6000 as a liquid phase and silicone SE-30 is Gas chromatography is preferred, using a column packed with a mixture of fillers and a liquid phase, a flame ionization detector (FID) as a detector, and a sample of trioxane containing PODM dissolved in ether. be. As for the type of distillation column, general tray columns such as bubble cap type, perforated plate type, and flexi-tray type, and packed columns such as Raschig ring, McMahon, and helipack types can be used. Although PODM can be removed by batch distillation, continuous distillation is preferred industrially. In this case, the raw material supply stage varies depending on the PODM concentration in the crude trioxane, the composition of the distillate and bottoms, and the number of stages in the distillation column, but the composition inside the distillation column and the composition of the feed liquid are almost the same. It is better to supply it to a place. Since trioxane is easily decomposed in the presence of oxygen, in order to prevent oxygen from entering the system, it is preferable to flow an inert gas such as nitrogen through the gas outlet of the reflux section connected to the atmosphere. In the method of the present invention, the removal rate of PODM is high, and crude trioxane with a high PODM content, such as that produced from general formalin with a high methanol content, can be used as a feedstock. If there are large amounts of impurities with boiling points lower than trioxane, such as water, formic acid, methyl formate, formaldehyde, and extraction solvents that are insoluble or poorly soluble in water used in the production of trioxane, it will be difficult to remove PODM. Total amount of boiling point impurities is 1% by weight of trioxane
It is preferable that the amount is below, and if it is contained in a large amount, it is preferable to remove it in advance by means such as distillation. For example, these impurities can be removed together with the solvent by distilling the extraction solvent solution of trioxane in the above-mentioned known method. According to the method of the present invention, PODM in trioxane obtained from the bottom of the distillation column can be lowered to 500 ppm or less, and it can be used as a polymerization raw material as it is. The amount of distillate extracted from the distillation column varies depending on the PODM content in the crude trioxane supplied, but it is 500~
In the case of trioxane containing 10,000 ppm of PODM, it is about 0.3 to 3% of the supplied amount, and the loss of trioxane is also small. In addition, although the reflux ratio is high, the amount of distillate is small, so the amount of steam used is small, and since it is a basic unit operation of distillation, it can be easily implemented industrially. In addition, as mentioned above, crude trioxane with a high PODM content can be used, so general formalin with a high methanol content can be used as a raw material when producing trioxane, and methanol removal equipment for raw formalin can be used, or formalin with a low methanol content can be produced. There is no need to adopt a special formalin production method for this purpose. Specific examples will be shown below in Examples. Note that the PODM concentration was analyzed by gas chromatography under the following conditions. Equipment: Shimadzu Corporation GC-6A (detector FID) Column packing material: Chromos orb
An equal weight mixture of a packing material in which 20% by weight of silicone (SE-30) is supported on WHP) and a packing material in which 20% by weight of polyethylene glycol (PEG-6000) is supported in Celite (Celite 545SK).Column temperature: 60℃ Vaporization chamber temperature: 200℃ Carrier gas and flow rate: Nitrogen 40ml/min Sample: Used as an ether solution Example 1 1800g of crude trioxane containing 1523ppm of PODM was charged into flasks 1 and 2, the flask was heated with a mantle heater, and the distilled vapor was transported by helicopter. The distillation column was introduced into a glass distillation column with an internal diameter of 30 mm and an external heating and insulation device, which was packed with 50 theoretical plates. The vapor distilled from the distillation column was introduced into a hot water condenser, and the condensate was completely refluxed for 3 hours.
493 ml of the above crude trioxane is extracted from the center of the distillation column.
It was fed at a rate of g/hr. The amount extracted from the top of the tower
The reflux ratio was adjusted to 5.4 g/hr and the reflux ratio was 117, and purified trioxane was extracted from the bottom of the column so that the liquid level in the flask remained constant. Nitrogen gas was flowed through the gas outlet of the reflux section to prevent oxygen from entering. The distillate and column bottom liquid were sampled and analyzed at regular intervals, and continuous distillation operation was performed until steady operation was achieved. PODM in trioxane bottom liquid during steady operation is
The content was 40 ppm, formalin 1 ppm, and formic acid 4 ppm, and was of high purity that could be used as a raw material for direct polymerization of polyoxymethylene. Furthermore, PODM in the distillate
is 13.5%, and the distillate volume relative to the feed volume is
It was 1.1%. Example 2 Crude trioxane 2000 containing 1680 ppm PODM
g into flask No. 2, heat the flask with a mantle heater, and introduce the distilled vapor into a glass distillation column (with external heating and insulation) with 50 theoretical plates and a 30 mm inner diameter filled with a helipad, and distill it under a nitrogen atmosphere. I did this. The vapor distilled from the distillation column was introduced into a hot water condenser, and the condensate was completely refluxed for 5 hours, followed by batch distillation at a reflux ratio of 50. Distillation was stopped after distilling off 41.1 g. PODM in the distillate was 7.3%, PODM in the trioxane bottom liquid was 176 ppm, formic acid 3 ppm, formaldehyde 2 ppm. Examples 3 to 5 Table 1 shows the results of continuous distillation as in Example 1 while varying the distillation conditions and crude oxymethylene composition (PODM content). Comparative Examples 1 and 2 For comparison, when the number of theoretical plates and reflux ratio of the distillation apparatus were set outside the range of the present invention, and by lowering the distillation rate and increasing the reflux ratio, the PODM concentration in the distillate was lowered to the original value. The results for cases outside the scope of the invention are listed in Table 1 as Comparative Examples 1 and 2.

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Claims (1)

[Scope of Claims] 1. When crude trioxane containing polyoxymethylene dimethoxide is distilled, the reflux ratio is set to 20 or more in a distillation column having 15 or more theoretical plates, and the polyoxymethylene dimethoxide concentration in the distillate is A method for purifying trioxane, characterized by distilling it while adjusting the value to be 30% or less. 2. Claim 1 describes distillation in a distillation column having 20 or more theoretical plates while controlling the reflux ratio to 25 or more and the concentration of polyoxymethylene dimethoxide in the distillate to 25% or less. the method of.