JPS6056150B2 - Production method of high purity phthalic anhydride - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing highly pure phthalic anhydride by purifying crude phthalic anhydride obtained by catalytic gas phase oxidation of ortho-xylene.

Specifically, the present invention relates to a method for producing highly pure phthalic anhydride by efficiently removing impurities, such as putalad, contained in the crude phthalic acid and difficult to separate.

Generally, catalytic gas phase oxidation of orthoxylene as a raw material using a vanadium catalyst to obtain phthalic anhydride is widely practiced industrially.

The main impurity in the crude phthalic anhydride obtained by this method is the by-product pthalad, which cannot be sufficiently separated using distillation equipment commonly used in industry. It is well known that significant amounts of putalad and other impurities remain in the process, which tends to reduce the quality of the product. Therefore, in the production process of phthalic anhydride, the catalytic gas phase oxidation reaction of ortho-xylene must be operated in such a way that the amount of putalad, which is an impurity in crude phthalic anhydride, is reduced as much as possible and kept at a low level. Become.

In other words, it is generally necessary to raise the reaction temperature in order to suppress the decrease in the conversion rate of ortho-xylene and the increase in the amount of the intermediate by-product, putalade, due to the decrease in the activity of the catalyst, and this will shorten the life of the catalyst. leading to a decrease. Even if this is not the case, the presence of by-product Pthalad in the crude phthalic anhydride is unavoidable, and various methods for removing this Pthalad have been proposed in the industrial production of phthalic anhydride.・For example, Special Public Interest Publication 1976-10
According to the specification of Publication No. 333, potassium bisulfite (K
A method for treating crude phthalic anhydride with alkali metal sulfur compounds such as potassium pyrosulfite (K2S2O5) and potassium pyrosulfite (K2S2O5) is disclosed, US Pat. No. 4,165,324
According to the specification, a method of treatment using an alkali metal hydroxide such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) is disclosed. However, it is pointed out that these methods have the following drawbacks and are difficult to be advantageously employed industrially. First, in the former method, sulfur compounds coexist in the residue during the purification process of phthalic anhydride, so a large amount of sulfur oxide is generated during the treatment of this residue (for example, incineration), causing pollution problems. In addition, the sulfur compound itself tends to cause corrosion of the distillation equipment, and solving these problems requires a large amount of cost. Furthermore, in the latter method, since the alkali metal hydroxide itself has high reactivity, it can be extremely dangerous depending on the crude phthalic anhydride to be added. That is, for example, when potassium hydroxide is added to liquid crude phthalic anhydride, the maleic anhydride contained therein polymerizes explosively.
Forms a coke-like polymer. This reaction is extremely rapid and cannot be ignored in the operation of the phthalic anhydride production equipment, and it also causes clogging of piping, valves, distillation column trays, etc. by the generated coke-like polymer. Furthermore, when potassium hydroxide reacts rapidly with maleic anhydride, it causes a decomposition reaction of phthalic anhydride and other impurities, which reacts with phthalic anhydride and generates a tar-like polymer. It is also pointed out that this results in a significant decrease in the yield of phthalic acid. The purpose of the present invention is to overcome the drawbacks of such conventional methods, and more specifically, the residue discharged from a phthalic anhydride purification device does not contain polluting substances such as sulfur compounds, and moreover, in order to reduce phthalide. ．． The purpose of the present invention is to provide a process for producing high-purity phthalic anhydride with an extremely reduced phthalide content, which is safe and easy to control, and which does not generate coke-like polymers or tar-like polymers in the treatment of phthalic acid. do.

That is, the present invention can be specified as follows.

(1) Crude phthalic anhydride obtained by catalytic gas phase oxidation of ortho-xylene in the presence of an alkali metal salt of at least one carboxylic acid selected from the group consisting of maleic acid, succinic acid and benzoic acid. A method for producing high-purity phthalic anhydride, which comprises contacting it with a molecular oxygen-containing gas at high temperatures and then subjecting it to a distillation operation.

(2) At least 0.001 mol/mol of molecular oxygen-containing gas converted into oxygen gas per 1k9 of the crude phthalic anhydride.
The method according to (1) above, characterized in that the contact is carried out at 0.002 to 0.01 mol/hour, preferably from 0.002 to 0.01 mol/hour.

(3) Contact treatment at 150-300°C, preferably 250°C
The method described in (1) or (2) above, which requires a temperature range of -300°C and a treatment time of 0.5 to 3 hours. (4) The amount of alkali metal salt added is 10 to 10,000 p as alkali metal atoms to crude phthalic anhydride.
pm (by weight), preferably in the range of 20 to 2000 ppm (by weight), the method described in (1), (2) or (3) above.

The method of the present invention will be explained in more detail below.

The crude phthalic anhydride obtained by catalytic gas phase oxidation of ortho-xylene contains 0.05 to 1.0 phthalide impurities.
It is usually contained in % by weight. Industrially, this crude phthalic anhydride is generally supplied to the refining process in a molten state at 130-150°C, and after adding and mixing chemical agents,
It is heat treated at a temperature of 150 to 30 degrees and then distilled. In the method of the present invention, alkali metal salts of maleic acid, succinic acid, and benzoic acid are added to crude phthalic anhydride containing phthalide impurities, and the mixture is heated to contain molecular oxygen at a temperature of 150 to 30σC for 0.5 to 3 hours. After contacting with gas to oxidize most of the phthalide content,
By subjecting it to a commonly used distillation operation, crude phthalic anhydride containing almost no phthalide can be obtained.

It is thought that the molecular oxygen blown in at this time effectively acts as an oxidizing agent for phthalide and compounds near its boiling point (estimated to be aldehydes), together with the alkali metal carboxylate salt. The amount of molecular oxygen blown into the crude phthalic anhydride depends on the amount of phthalide impurities contained in the crude phthalic anhydride, but is at least 0.001 mol/hour per 1k9 of the crude phthalic anhydride, preferably 0.002~
It is in the range of 0.01 mol/hour. Furthermore, according to the findings of the present inventors, organic acid salts other than the alkali metal salts of carboxylic acids, such as alkali metal salts of toluic acid, phthalic acid, and fumaric acid, all have a certain degree of effect in reducing impurity phthalide. Although some of them are less effective than the carboxylic acid salts mentioned above. The reason is not clear, but
It is presumed that this is due to the fact that the reaction affinity for phthalides, such as oxidation activity, is inferior to that of the compounds according to the present invention. The alkali metal carboxylic acid salts according to the present invention are usually obtained by the reaction of a carboxylic acid with an alkali metal hydroxide compound in an aqueous solution, are themselves white crystals, and are extremely stable compounds.

Low explosive and flammability. In the carboxylic acid alkali metal salts of the present invention, examples of the alkali metal include sodium, potassium, lithium, cesium, rubidium, etc., but potassium is most preferred.

The most preferred carboxylic acid is maleic acid. The amount of carboxylic acid alkali metal salts used in the present invention depends on the amount of phthalide contained in the crude phthalic anhydride;
0000ppm (weight), preferably 20-2000p
pm (weight), more preferably 50 to 1000 ppm (
weight).

The present invention has been described above, and the advantages obtained by the present invention are as follows.

(a) The alkali metal carboxylic acid salts used are extremely stable compounds with low explosiveness and ignitability.

It is also less corrosive than potassium hydroxide. (b) Since a small amount of carboxylic acid salts is effective, the amount accumulated in the device is also small.

Moreover, since it is easily dissolved in crude phthalic anhydride, there is no clogging accident in the equipment, there is no risk of generating tar-like or coke-like polymers, and a high yield of high-purity phthalic anhydride is obtained. can be achieved. (c) By oxidizing crude phthalic anhydride with molecular oxygen and the above-mentioned alkali metal carboxylate salt, the purification yield of phthalic anhydride is improved.

It is thought that the phthalide content is converted to phthalic anhydride, contributing to an improvement in the purification yield. In addition, as the amount of impurity phthalide is reduced, the compounds near the boiling point are denatured and distillation separation becomes easier, which is thought to improve product quality. The present invention will be explained in more detail below using Examples and Comparative Examples.

Example 1 Crude phthalic anhydride obtained by catalytic gas phase oxidation of ortho-xylene had the following composition.

This crude phthalic anhydride 500k9 was prepared with an inner diameter of 80C1φ,
It has a height of 120d and has 4 baffle boards with a width of 8cm inside.
It was placed in a vertical reaction tank and heated to 275°C.

At the bottom of this reaction tank, a stirrer having four fan-turbine type blades each having a size of one-third of the inner diameter of the tank is installed, and a gas distribution plate is fixed at the bottom of the stirrer. Add 50f of dipotassium maleate (equivalent to 100ppm) to the reaction tank, and add 1200ml of mixed gas of 5% oxygen by volume and the remaining nitrogen from the bottom.
/ hour (equivalent to 0.003 mol/hour of oxygen) while blowing 2
Heat treatment was carried out at 75°C with stirring for two periods, and then a distillation column (inner diameter 32wnφ, height 500m
) under a pressure of 55° Hg (absolute pressure) and a reflux ratio of 0.
5 to obtain crude phthalic anhydride with an APHAlOl freezing point of 131.17C. The phthalide content of this purified product is 0.
001% by weight or less, and the thermal stability upon heating at 250° C. for 2 hours was also that of APHA2O. The purification yield is 99
．． The weight was %. Moreover, no tar-like or coke-like deposits were observed in the equipment used. Comparative Example 1 The same operation was carried out as in Example 1 except that 50f (corresponding to 1000 ppm) of potassium hydroxide was used instead of potassium maleate, and APHAl5, freezing point 13
0. Purified phthalic anhydride was obtained.

The phthalide content in this purified product was 0.43% by weight, and the thermal stability after heating for 2 hours was APHA9O. The purification yield was 98.1% by weight. Tar-like deposits were observed on the reaction stirring blades and walls, and coke-like deposits were observed at the bottom of the distillation column, resulting in a decrease in purification yield.

Examples 2 to 19 and Comparative Examples 2 to 3 1k9 of each crude phthalic anhydride having the same composition as that used in Example 1 was taken, and in a flask, the treatment agent, amount added, temperature at the time of addition, and The amount of phthalide in phthalic anhydride after the treatment was measured by changing the heat treatment temperature and treatment time, and the results shown in Table 1 were obtained.

Claims (1)

[Scope of Claims] 1. Crude phthalic anhydride obtained by catalytic gas phase oxidation of ortho-xylene is treated with at least one alkali metal carboxylic acid selected from the group consisting of maleic acid, succinic acid and benzoic acid. A method for producing high-purity phthalic anhydride, which comprises contacting it with a molecular oxygen-containing gas at high temperatures in the presence of a salt, and then subjecting it to distillation. 2. The method according to claim 1, wherein 1 kg of the crude phthalic anhydride is contacted with at least 0.001 mol/hour in terms of molecular oxygen-containing gas. 3 Contact treatment temperature range of 150~300℃, 0.5~
3. The method according to claim 1, wherein the method requires a processing time of 30 hours. 4 Claims 1 and 2 characterized in that the amount of the alkali metal salt added is in the range of 10 to 10,000 ppm (weight) as alkali metal atoms based on crude phthalic anhydride.
Or the method described in 3.