JPH0325428B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for synthesizing trioxane from formaldehyde. More specifically, when synthesizing trioxane using a heteropolyacid as a catalyst, trioxane can be synthesized by using two reactors, one high temperature and one low temperature, and using the thermal energy of the former product to supply heat to the latter or to the concentration column. It concerns an economical method of manufacturing. Trioxane is a cyclic trimer of formaldehyde and is mainly used as a raw material for the production of polyoxymethylene. In order to obtain polyoxymethylene with a satisfactory molecular weight, extremely pure trioxane is required, especially when chain transfer agents such as water, formic acid and methanol are used, as well as methylal, methyl formate and low molecular weight polyoxymethylene dimethoxide. It is necessary that the content of so-called by-products be low. Many studies have been conducted on methods of synthesizing trioxane to meet these demands, but the conventional manufacturing method is to convert formaldehyde to acidic catalysts such as sulfuric acid, phosphoric acid, boric acid, benzenesulfonic acid, Trioxane is obtained by heating in the presence of toluenesulfonic acid, an acidic ion exchange resin, or a solid acidic catalyst such as aluminum sulfate or silica. Among them, sulfuric acid is most commonly used because it has a fast reaction rate and is easily available. However, the sulfuric acid method had some practical problems that needed to be solved. For example, when the formaldehyde concentration exceeds 60% by weight, paraformaldehyde is produced as a by-product, and when the sulfuric acid concentration exceeds 8% by weight, by-products such as formic acid and methyl formate are formed, and the yield of trioxane decreases. (Special Publication No. 40-17394) In Japanese Patent Publication No. 46-32274, sulfuric acid 10%
In the above cases, there are many by-products, so it has been proposed to add a dispersant such as di-2-ethylhexyl phthalate to cause the reaction, but adding a third component makes post-processing complicated. I don't think this is an appropriate method. As described above, in the conventional method, 1) there are many by-products, 2) paraformaldehyde scale is likely to occur on the walls of the reactor or distillation column, and 3) it causes corrosion of the reactor and column walls, etc. There were many problems in its application. As a result of intensive research, the present inventors discovered a method that could solve the above problems, and filed an application (Japanese Unexamined Patent Publication No.
118079). According to this method, many advantages are exhibited because a heteropolyacid is used as a catalyst. First of all, reactions using heteropolyacids have high selectivities and conversions. For example, when 10 parts by weight of silicotungstic acid is added to 100 parts by weight of a 60% by weight formaldehyde aqueous solution under normal pressure and the mixture is heated, the by-products methylal, methyl formate,
The total amount of formic acid and methanol is suppressed to 1% or less, and side reactions are significantly lower than with commonly used sulfuric acid catalysts. This means that a large amount of impurities does not accumulate in the reaction product, which is extremely advantageous for separation and purification of trioxane. Furthermore, the trioxane content in the fraction taken out from the reactor is high;
This means that there are fewer unreacted substances, which is not only advantageous for purifying trioxane but also saves energy. Second, even if the concentration of formaldehyde in the raw material used is increased, paraformaldehyde precipitates do not adhere to the reaction system. In the commonly used sulfuric acid catalyst, the formaldehyde concentration during the reaction is
If the concentration is increased above 60% by weight, paraformaldehyde will precipitate in the reactor, and if the sulfuric acid concentration is increased to increase the concentration above 60% by weight, the formation of by-products will increase significantly, making it difficult to implement on an industrial scale. It becomes a problem. As described above, by using a heteropolyacid, trioxane can be synthesized in high yield and high selectivity without precipitation of paraformaldehyde in the reaction system. Originally, in a formaldehyde-trioxane-water system, the conversion rate was low due to equilibrium, and it was necessary to recover unreacted formaldehyde and carry out the contact reaction again. This conversion cannot exceed the so-called equilibrium conversion. In conventional industry, the conversion rate for a single flow is around 20%, and
Even when the heteropolyacid according to Publication No. 118079 is used, it is only a little over 25%. This means that three to four times as much unreacted material as the raw material is recovered and returned to the reactor, and this is a serious problem in the trioxane manufacturing industry. This is because the formaldehyde that is recovered and recycled contains a large amount of water, and the repeated evaporation and condensation consumes a large amount of thermal energy. From the perspective of energy conservation, the present inventors divided the reactor into low-temperature and high-temperature sections, and as a result of intensive research to minimize the heat supply in the reaction-concentration system, they found that a surprising amount of heat could be saved by combining low-temperature reactions. We have discovered something and have come up with the present invention. In the conventional technology (Japanese Unexamined Patent Publication No. 118079/1983), the temperature for trioxane synthesis is set at 60℃ to 200℃, but in order to extract the product, it must be carried out at the boiling point, and industrially, the temperature is It is carried out at around 100℃. Therefore, a low-temperature reaction under boiling is practically impossible and was completely unknown in the past.
This low-temperature reaction is an important part of the present invention, and was discovered for the first time by the inventors. In the past, there have been many cases in which reactions have been attempted at low temperatures below 100°C. For example, extractants for trioxane (Japanese Patent Publication No. 43-29953, Japanese Patent Publication No. 45-1267), inert oily components for emulsion formation (Japanese Patent Publication No. 40-17394), lower fats for azeotropically distilling water. Dihalides of group hydrocarbons (Special public interest
-7029, Japanese Patent Publication No. 46-31867, Japanese Patent Publication No. 47-8826), salts (Japanese Patent Publication No. 44-27390), or high-boiling alcohols and ethers for suppressing the amount of water in the reaction system (Japanese Patent Publication No. 49-32869). Attempts have been made to include a third component such as in the reaction system and react with formaldehyde at low temperatures, but in either case it is difficult to separate the third component from the reaction product, and the reaction In terms of results, the low temperature section is not particularly advantageous. There are also cases where the reaction was carried out at low temperatures without adding a third component (Special Publications 1973-
8545, Japanese Unexamined Patent Application Publication No. 118079/1983), but no advantages have been shown at low temperatures. Furthermore, when trioxane is separated from the reaction mixture by vacuum distillation, there is a description that the reaction is carried out under reduced pressure and boiling, but in that case, the concentration of the raw material formaldehyde must be lowered in order to suppress the by-product and precipitation of paraformaldehyde. , has been shown to be unfavorable (Tokuko Sho 46
-14061). In short, in conventional attempts, reactions at low temperatures are accompanied by precipitation of paraform, which is difficult to operate, and the reaction performance is also less advantageous than reactions at temperatures below the atmospheric pressure boiling point (approximately 100°C). Certain points have not been discovered. The cause of this is due to the conventionally used catalyst. In other words, commonly used acidic catalysts such as sulfuric acid, phosphoric acid, and boric acid have a raw material formaldehyde concentration of 60% by weight.
If the reaction temperature exceeds 100°C, or if the reaction temperature falls below the boiling temperature at atmospheric pressure (approximately 100°C), paraformaldehyde tends to precipitate, making it difficult to continue the reaction. By the way, as a result of intensive research into catalysts to replace sulfuric acid, the present inventors discovered heteropolyacids and filed an application for the invention. (Japanese Unexamined Patent Publication No. 118079/1983) When a heteropolyacid is used, (1) the selectivity and conversion rate are high, and (2) paraformaldehyde is precipitated in the reaction system even if the concentration of formaldehyde in the raw material used is high. However, the present inventors conducted detailed research on the use of catalysts with many such advantages in the low-temperature region, and discovered an effective method. According to this, by using a heteropolyacid catalyst, a stable reaction system can be maintained as long as the relationship between the amount of catalyst and the necessary temperature (boiling point) maintains a constant relationship shown in the following formula. That is, the region in which the reaction system does not solidify and the region in which paraformaldehyde does not precipitate in the reaction system and the reaction can continue is given by the following equation. T≧-18.9logK+129+F Here, T is the reaction temperature (°C), K is the catalyst concentration (parts by weight relative to 100 parts by weight of the raw material), and F is a value that depends on the concentration of the raw material formaldehyde. For example, when 55%, F= It is 0. Usually, it takes a positive value when the concentration is higher than 55%, and a negative value when it decreases. The pressure may be adjusted to fall within this range. Note that the above formula may have slight deviations depending on heat supply, stirring method, reactor type, etc., or may not differ greatly from the above formula. By utilizing such an equilibrium-friendly low-temperature reaction, an energy-saving process for trioxane synthesis has become possible through a completely new combination of steps. The present invention provides a high-temperature reactor in which formaldehyde is heated in the presence of a heteropolyacid in the presence of a heteropolyacid, and a low-temperature reactor in which the reaction is operated at a temperature of at least 10°C lower than the high-temperature reactor. This is a split synthesis method for trioxane, which is carried out using two reactors and uses the generated vapor from the high temperature reactor as a heat source for the low temperature reactor and the concentration column. In the present invention, as a raw material, formaldehyde content
Up to 95% by weight of hydrated formaldehyde can be used. Of course paraformaldehyde and α-
Materials that generate formaldehyde when heated, such as polyoxymethylene, can also be used as raw materials. In particular, in order to carry out the present invention effectively, it is preferable to use an aqueous solution of formaldehyde having a concentration of 30 to 80% by weight. The heteropolyacid used as a reaction catalyst is JE
As described in detail by Keggin in Nature June 24 (1933) p. 908, as shown below, there is a different element (central element) at the center, which shares the oxygen atom and condenses the condensed acid group. It is a heteronuclear condensed acid having a mononuclear or dinuclear complex ion formed, and is generally represented by the following chemical formula. Hn[Mx(M′yO _z )]·mH ₂ O In this structure, the inside of [ ] always maintains a constant structure (Keggin structure), and does not change due to detachment or bonding of crystal water. Therefore, it should be distinguished from structures such as solid acids and complex oxides. Here, M is a central element, M' is a coordination element, and is generally one or more elements selected from W, Mo, V, and Nb, x is 0.1 to 10, and y is 6 ~30, x is a number in the range of about 10 to 80 indicating the number of oxygens in the heteropolyacid, n is the number of acidic hydrogens in the heteropolyacid and is a number greater than 1, and m is The number of moles of water of crystallization ranges from 0 to about 40. When actually used, it is used as an aqueous solution, so the value of m has no direct meaning.
Furthermore, so-called mixed heteropolyacids containing two or more types of coordination elements are also included in the heteropolyacids of the present invention. The central element M in the above compositional formula is generally P,
B, Si, Ge, Sn, As, Sb, U, Mn, Re, Cu,
Consists of one or more elements selected from Ni, Co, Fe, Ce, Th, and Cr. Particularly preferably used are central elements M such as P, Si, B, Ge, Cu, and Sn.
Among these, Si or P is preferred, and the use of silicotungstic acid, silicomolybdic acid, phosphotungstic acid, phosphomolybdic acid, or a mixture thereof is particularly effective. The coordination elements M′ in the above composition formula are W, Mo, V,
It is composed of one or more elements selected from Nb, but particularly preferably used is a coordination element
This is the case when M' is W, Mo or V or a mixture thereof. When these heteropolyacids are used, the concentration of trioxane in the distillate obtained after the reaction is high and the concentration of by-products such as formic acid, methyl formate, methylal and methanol is low. The concentration of the aqueous solution, in other words, the amount of water in the reaction system, can be adjusted as long as the system is homogeneous. If the reaction is to be continued for a long time, the concentration of the aqueous solution, in other words, the amount of water in the reaction system, should be adjusted to an equilibrium level depending on the composition of the feed materials, average contact time, reaction temperature, pressure, etc. It is determined by
It cannot be decided independently of these factors. In any case, if it is in a uniform state, it falls within the scope of the present invention. Further, even if scale generation or solid suspended matter is present, it is chemically outside the reaction system and does not impair the present invention in any way. In the reaction of the present invention, formaldehyde is heated in the presence of a heteropolyacid, and the reaction conditions are as follows. The ratio of the heteropolyacid to the raw materials in the reactor is usually 5 parts by weight or more, preferably 10 to 3000 parts by weight, and more preferably 20 to 900 parts by weight, per 100 parts by weight of the formaldehyde aqueous solution. The temperature of the high-temperature reactor is preferably as high as possible from the point of view of heat reuse, but there are restrictions from the viewpoint of reaction. When the temperature exceeds 100°C, the reaction rate increases due to the pressurized reaction, but the conversion rate decreases in terms of equilibrium theory. In addition, reactions of by-products such as formic acid, methyl formate, and methylal become significant, and the selectivity of trioxane conversion sharply decreases. For this reason, the temperature of the high temperature reaction is 150℃ or less, preferably 130℃ or less.
A temperature below ℃ is desirable. The lower limit of the temperature may be any condition that can maintain a temperature difference from the low-temperature reaction, but usually around 100°C, which is the boiling point at normal pressure, is sufficient. Next, the temperature of the low-temperature reactor is preferably 95°C or lower, which is preferably as low as possible, but in this case, from the standpoint of reaction rate and stability of the reaction system, it is preferably 60°C or higher.
A temperature of 65℃ or higher is recommended. However, this temperature indicates the boiling point in the reactor. Heat recovery of the generated vapor from the high-temperature reactor is performed using a general heat exchanger, with the generated vapor from the high-temperature reactor on the high-temperature side and the formaldehyde mixed solution from the low-temperature reactor on the low-temperature side, or the formaldehyde mixed solution from the concentration column on the low-temperature side. This can be easily achieved by using The temperature difference between the low-temperature reactor and the high-temperature reactor is an important factor, but it also depends on the method by which heat is recovered from the produced steam from the high-temperature reactor. Practically speaking, the temperature is preferably 10°C or higher, preferably 20°C or higher. For example, a high temperature-low temperature reaction is performed at 108℃-85℃ (difference
20℃), 110℃ - 80℃ (difference 30℃), 100℃ - 75℃ (difference
25℃). In this case, the reaction temperature must be at the boiling point, which is realistically achieved by adjusting the pressure between reduced pressure and increased pressure. The concentration column separates unreacted formaldehyde and trioxane from the product, and usually employs reduced pressure. Therefore, the heat generated from the high temperature reactor can be fully utilized. The greatest advantage of the present invention is that the reactors are divided, and this combination was realized for the first time with the discovery of low-temperature reactions. With conventional technology, the boiling point of the synthesis tower is 100℃
Since the temperature of the generated steam is near 100°C, it is difficult to recover heat, and most of the heat supplied to the synthesis tower is lost. Example 1 Commercially available paraformaldehyde was dissolved in distilled water to prepare 55% by weight raw material formalin water. 100 g of this raw material and 100 g of phosphotungstic acid were mixed, charged into a high-temperature reactor, and boiled at 100°C. On the other hand, phosphotungstic acid is also used in the low temperature reactor.
600g and 100g of raw materials were mixed and prepared, and boiled at 80℃. The steam generated from each reactor is supplied to a concentrating column and distilled under reduced pressure, recovering trioxane from the top of the column and unreacted formalin from the bottom of the column.
Mixed with 55 raw materials. In addition, the volume of each reactor was controlled by a liquid level gauge so that it was the same as the charging volume. The process at this time was as shown in Figure 1, and the data at each point on the diagram 50 hours after stable operation was as shown in Table 1. Compared to the case where all raw materials and unreacted materials at the bottom of the column are supplied to the high-temperature reactor,
Heat consumption was reduced by 32.5%. This result demonstrated that it is possible to combine a high temperature reactor, a low temperature reactor, and a concentration column, and that it is possible to operate with a temperature difference of about 20°C.

[Table] Example 2 Tungstic acid was used as a catalyst in a high-temperature reactor and boiled at a temperature of 110°C.
400 parts by weight of phosphotungstic acid was used per 100 parts by weight, and the mixture was boiled at 85°C. The amount of unreacted material at the bottom of the concentration column is 1.3 to the high temperature reactor and low temperature reactor, respectively.
All raw materials were fed to the low temperature reactor. The process at this time is as shown in Figure 2, and the amount of heat could be reduced by 34.8% compared to the case where the raw materials and the unreacted materials at the bottom of the column were all fed to the high-temperature reactor. Although the temperature of the high-temperature reactor was varied up to 135°C (20 parts by weight of catalyst) and the temperature of the low-temperature reactor was varied up to 70°C (900 parts by weight of catalyst), stable operation could be continued.

[Brief explanation of the drawing]

FIG. 1 and FIG. 2 are process diagrams of Examples 1 and 2, respectively. indicates a high temperature reactor, indicates a low temperature reactor, and indicates a concentration column (distillation column). 1... Raw formalin aqueous solution, 2, 10... Unreacted formalin aqueous solution from the concentration column, 3, 4, 11...
Feedstock to the reactor, 5,12...product from the reactor, 6,13...feedstock to the reactor,
7, 9...Product from the reactor, 8,14... Distillate from the concentration column, A, B... Heat exchanger.

Claims (1)

[Scope of Claims] 1. A high-temperature reactor in which the reaction is operated at a temperature of 100°C or higher when formaldehyde is heated in the presence of a heteropolyacid, and a low-temperature reaction in which the reaction is operated at a temperature at least 10°C lower than the high-temperature reactor. 1. A split synthesis method for trioxane, which is carried out using two reactors: a high-temperature reactor, and uses generated vapor from a high-temperature reactor as a heat source for a low-temperature reactor and a concentrating column. 2. The method for dividing trioxane synthesis according to claim 1, wherein the heteropolyacid is phosphotungstic acid, silicotungstic acid, silicomolybdic acid, or phosphomolybdic acid. 3. The method for dividing trioxane synthesis according to claim 1, characterized in that the low temperature reaction is 95°C or lower. 4. The method for dividing trioxane synthesis according to claim 1, wherein a part of the formaldehyde is an unreacted product from the bottom of the concentration column.