JP2651622B2 - Trioxane purification method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for separating trioxane obtained by heating an aqueous formaldehyde solution. More specifically, the present invention relates to a method for separating a trioxane-containing distillate obtained by heating an aqueous formaldehyde solution from benzene. And a method for separating and purifying trioxane from the benzene solution containing trioxane by continuous distillation. Trioxane is a polymer intermediate of a polyacetal resin, and a method for producing highly purified trioxane is industrially desired.

(Prior art) Trioxane is generally obtained by heating an aqueous formaldehyde solution. As an industrial production method, trioxane obtained by heating and distilling a 30 to 70% by weight aqueous solution of formaldehyde in the presence of an acidic catalyst is used. A method has been proposed in which a distillate having a composition of 10% to 35% by weight, formaldehyde at 10 to 35% by weight and water at 20 to 50% by weight is used as a crude material.

The water contained in such a crude material and the formic acid contained in the crude material as a by-product due to the use of a strongly acidic catalyst in the production reaction are used as chain transfer agents when producing polyoxymethylene by polymerizing trioxane. , Which lowers the degree of polymerization, and is usually removed from trioxane.

As a method of removing water, a method of adding an additive such as isocyanate, sodium dispersion or metal hydride (Japanese Patent Publication No. 40-12900, West German Patent No. 1906846, West German Patent No. 1,288,884) book),
A method of adding an adsorbent such as alumina or zeolite (JP-B-44-17915), a method of distillation in the presence of a solvent that forms an azeotrope with water (JP-B-49-5351, JP-B-49-28197) And Japanese Patent Publication No. 28-518).

However, these methods are not necessarily satisfactory as industrial methods, because they require post-treatment because they involve side reactions, are complicated, or have insufficient effects. I can't say.

On the other hand, as a method of removing formic acid in trioxane, a method of adding a reagent for capturing formic acid such as alkali metals, isocyanates, metal hydroxides and amines and distilling the same (Japanese Patent Publication No. 40-8545, Japanese Patent Publication No. 40-12900, Belgian Patent No. 647355, Japanese Patent Publication No. 45-3340
No. 7) and a method of adsorbing and removing formic acid by using alumina, zeolite, ion exchange resin or the like as an adsorbent (Japanese Patent Publication Nos. Sho 44-17915, 47-7384 and Sho 4)
5-1503) has been proposed. In particular, a concentrated aqueous solution of formaldehyde is heated in the presence of a strongly acidic catalyst to form formaldehyde, the aqueous distillate is extracted with benzene, and the trioxane benzene solution is fractionated to obtain crude trioxane. (JP-B-49-5351) has also been proposed. But,
These methods are not very preferable as industrial methods in that they require special chemicals and special equipment.

The present inventors have conducted intensive studies to develop an industrial method for efficiently obtaining trioxane having a low content of water or formic acid by a simple operation.As a result, trioxane and water obtained by heating and distilling formalin were used. It has been found that a high-quality trioxane can be obtained by extracting a solution consisting of formaldehyde with benzene and distilling this benzene extract under selected conditions, and has already filed a patent application (Japanese Patent Publication No. 64-10513).

The method involves extracting a trioxane-containing distillate obtained by heating a formaldehyde aqueous solution with benzene, supplying the trioxane-containing benzene solution to a distillation column as a raw material, distilling benzene from the top of the column, and trioxane from the bottom of the column. In the method for continuous distillation and separation of trioxane, the trioxane concentration X (% by weight) of the liquid composition in the raw material supply stage of the distillation column is calculated by the formula [Where R is the reflux ratio, γ is the weight ratio of the liquid portion in the feedstock q, and the trioxane concentration in the feedstock is C
(% By weight), the formula A trioxane separation method characterized by maintaining the relationship in the range satisfying the relational expression represented by the following formula:

McCave Chile (Mc
According to the theory of cabe-Thiele), j +
The molar fraction of the steam leaving the first stage, y _{j + 1,} is given by the equation (For example, see Hirata and Yomi, edited by Distillation Engineering Handbook, page 111).

Where L is the molar flow rate of the descending liquid, F is the raw material supply amount, W is the bottom liquid amount, q is the ratio of the liquid portion in the raw material, and X _j is the molar amount of the liquid leaving the j-th stage counted from the top. The fraction, _XW, is the mole fraction of the bottoms.

The gradient γ of the recovery unit operation line in this equation is Then, assuming ideal conditions for completely separating the light-boiling component and the heavy-boiling component of the two-component system, Is established, and these are substituted into equation (4). Becomes

Accordingly, γ can be used as a parameter indicating ideal distillation conditions, but the invention (Japanese Patent Publication No. 64-1051)
The invention of No. 3) uses γ as a basic parameter and specifies an optimum range experimentally.

That is, in the above formula (1), the liquid composition of the raw material supply stage is set on an extension of the ideal recovery section operation line, while gas-liquid separation of benzene and trioxane is possible, and trioxane at the bottom of the distillation column is set. It is experimentally confirmed the allowable range of the liquid composition in the feed column of the distillation column that can reduce the content of the chain transfer agent in the solution.

Here, Is the minimum value of the trioxane concentration in the feed composition of the distillation column, and Indicates the maximum value. As is clear from this, the larger the reflux ratio, the wider the allowable range of the liquid composition in the raw material supply stage.

In the conventional method of separating and separating a benzene solution of trioxane by distillation, it is usual that the liquid composition in the raw material supply stage is made coincident with the intersection of the operation line of the enrichment section and the raw material line (q line) when operating the distillation column. (See, for example, Tokyo Chemical Doujinshi, Shigefumi Fujita, Heiichiro Higashihata, "Chemical Engineering III, 2nd Edition, Mass Transfer Operation," pp. 40-47). That is, when the raw material is a liquid having a boiling point, a basic operation has been to make the trioxane concentration of the liquid composition in the raw material supply stage substantially equal to the raw material concentration. However, this makes the removal of the chain transfer agent insufficient.

On the other hand, the invention of Japanese Patent Publication No. 64-10513 is a powerful method for providing highly purified trioxane.

(Problems to be Solved by the Invention) However, under certain selected conditions, the purification of trioxane may still be difficult even when the method of Japanese Patent Publication No. 64-10513 is adopted. Do you get it. The present invention seeks to improve this point.

In particular, heating a formalin aqueous solution containing a large amount of methanol, and separating benzene from a trioxane-benzene solution obtained by extracting the obtained trioxane with benzene,
It was difficult to obtain trioxane containing almost no water, methanol, formic acid and the like.

Here, the formalin aqueous solution containing a large amount of methanol specifically refers to a solution having a methanol content of about 1% or more. The methanol concentration in the aqueous formalin solution used for the synthesis of trioxane is determined by a method for producing or purifying formalin,
Furthermore, various concentrations can be assumed depending on the recovery and reuse methods of formalin in the industrial process,
In the present invention, an aqueous formaldehyde solution containing 2 to 15% of methanol is used.

(Means for Solving the Problems) As a result of intensive studies on a method for highly purifying trioxane under all conditions, the present inventors have reached the present invention.

That is, the present invention is adapted to any method by adding a process of extracting a benzene solution containing trioxane extracted with benzene with water in the method of the invention of Japanese Patent Publication No. 64-10513. This is a method for purifying trioxane.

The extraction operation of the trioxane-benzene solution with water in the present invention transfers methanol, formic acid, formaldehyde, and the like dissolved in the solution to the aqueous phase, thereby facilitating the purification of trioxane by the subsequent distillation operation. By this extraction operation, the content of impurities such as water, methanol, and formic acid in trioxane is significantly reduced in a subsequent distillation operation. Although the mechanism of this result is not clear, it is assumed that the formaldehyde dissolved in the trioxane-benzene solution is transferred to the aqueous phase by extraction with water, thereby facilitating purification in the subsequent distillation step. Presumed.

The extraction operation with water is a batch extraction method using a tank,
Alternatively, a continuous extraction method using a tower or the like is used. As a preferred method, water is flowed from the upper part of the tower, and a trioxane-benzene solution is flowed from the lower part of the tower, and in the tower, auxiliary substances for improving the interfacial contact between the water and the trioxane-benzene solution with a step or a filler are used. A so-called countercurrent contact system is adopted. The ratio of water and trioxane to benzene solution used is from 0.01: 1 to 1: 1, preferably
It is used between 0.03: 1 and 0.3: 1, more preferably between 0.05: 1 and 0.2: 1. If the amount of water used is small, the extraction effect is insufficient, and if it is too large, loss of trioxane and the like is large and uneconomical.

The formaldehyde concentration in trioxane to benzene after extraction with water varies depending on the target purification degree, but is usually 1% or less, preferably 0.2% or less, particularly preferably 0.05% or less.

The trioxane-benzene solution extracted with water in this manner is then separated from benzene in a distillation column, and becomes trioxane having a small content of impurities such as water from the bottom of the column.

At this time, the operation conditions of the distillation column need to match the liquid composition of the raw material supply stage of the distillation column as shown by the formula (1). That is, when the trioxane concentration of the liquid composition in the raw material supply liquid is kept lower than the range represented by the formula (1), separation of trioxane and benzene becomes difficult,
On the other hand, when the concentration is maintained at a high level, it becomes difficult to lower the content of the chain transfer agent. The trioxane concentration in the feed stage is
It can be controlled by increasing or decreasing the amount of steam rising at the bottom of the column, changing the temperature profile in the column, or other methods.

(Examples) Hereinafter, the present invention will be described by way of examples, but this does not limit the scope of the present invention.

Example 1 A 65% aqueous solution of formalin containing 6% of methanol was distilled under heating in the presence of sulfuric acid to obtain a water-soluble distillate containing 45% of trioxane and the balance being formaldehyde, methanol and water. This aqueous solution was subjected to countercurrent extraction with benzene, and 42%
A benzene solution containing trioxane was obtained. The benzene solution was supplied from the lower part of the packed tower packed with Raschig rings, and water was supplied from the upper part. Here, the ratio of water: trioxane to benzene solution was 0.2: 1 (weight ratio). Here, the formaldehyde in the trioxane-benzene solution extracted and washed was 0.05%. The trioxane concentration was 38%. This trioxane-benzene solution
The mixture was continuously supplied as a liquid having a boiling point to a 65-stage distillation column, rectified under a reflux ratio of 3, and benzene was continuously separated from the top of the column and trioxane was continuously separated from the bottom of the column. As an operating condition at this time, the concentration of trioxane in the supply stage was maintained at 25% according to the above formula (1).

The chain transfer agent in the obtained trioxane has a water content of 1 ppm,
Formic acid was 1 ppm.

Comparative Example 1 The same operation as in Example 1 was performed, except that the trioxane-benzene solution was not extracted and washed with water. 90ppm of water in the obtained trioxane, 60ppm of formic acid
Met.

Comparative Example 2 In Example 1, exactly the same operation as in Example 1 was performed, except that the trioxane concentration in the supply stage was maintained at 38%, which was the same as the raw material, as the operating conditions of the distillation column. The water content in the obtained trioxane was 12 ppm and formic acid was 50 ppm.

Example 2 The same operation as in Example 1 was performed, except that the amount of water to be subjected to countercurrent extraction and the ratio of trioxane to benzene solution were changed to 0.15: 1 (weight ratio). Here, the formaldehyde in the trioxane-benzene solution extracted and washed was 0.11%. The trioxane concentration was 39%.

2ppm water and 3p formic acid after distillation purification
pm.

Example 3 The same operation as in Example 1 was performed, except that the amount of water to be subjected to countercurrent extraction and the ratio of the trioxane to benzene solution were set to 0.1: 1 (weight ratio). Here, the formaldehyde in the trioxane-benzene solution extracted and washed was 0.5%. The trioxane concentration was 40%.

Trioxane after distillation and purification contains 4ppm of water and 4p of formic acid
pm.

(Effects of the Invention) According to the present invention, highly purified trioxane can be obtained. Thereby, trioxane of polymerization grade for polyacetal resin was easily obtained.

Claims (1)

(57) [Claims] 1. A trioxane-containing distillate obtained by heating an aqueous formaldehyde solution containing 2 to 15% of methanol is extracted with benzene, the benzene solution containing the trioxane is supplied to a distillation column as a raw material, and benzene is supplied from the top of the column. In the method for continuously distilling and extracting trioxane from the bottom of a distillation column, A. a benzene solution containing trioxane extracted with benzene is extracted with water, and B. The trioxane concentration X (% by weight) of [Where R is the reflux ratio, γ is the weight ratio of the liquid portion in the feedstock q, and the trioxane concentration in the feedstock is C
(Weight) and the formula Which is a parameter defined by the following formula: