JPS5839812B2 - Terephthalene - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing terephthalic acid by continuous liquid phase oxidation of baraxylene, and more particularly to a reaction containing terephthalic acid oxidized in a reactor by continuous oxidation. When the product is transferred to a crystallization tank, molecular oxygen is made to exist in the liquid feeding line so that the oxygen concentration is higher than in the reactor, and unreacted substances and reaction intermediates are reacted in the liquid feeding line. The present invention relates to a method for producing pure terephthalic acid.

In the method of producing terephthalic acid by liquid-phase oxidation of free xylene, raw material free xylene, a solvent, and a catalyst are all charged into a reactor before the reaction, oxygen-containing gas is introduced, and the reaction product is removed after the reaction is completed. Batch method that repeats the extraction operation,
There is a semi-continuous method in which raw xylene and an oxygen-containing gas are continuously introduced into a reactor filled with a solvent and a catalyst, and the reaction product is taken out after the reaction is completed. Among the above-mentioned methods, the present invention particularly relates to the continuous method, which includes a continuous method in which the reaction product is continuously extracted from the bottom of the reactor while a mixed solution of the reactor and the catalyst is also continuously introduced into the reactor.

In recent years, in the production of polyester, the so-called direct polymerization method, in which terephthalic acid is directly polymerized with glycols, has attracted attention, instead of the process of partially esterifying terephthalic acid to form dimethyl terephthalate, which is easy to purify. As a result, quality requirements for terephthalic acid have become extremely strict.

However, terephthalic acid obtained by liquid-phase oxidation of baraxylene contains reaction intermediate products such as 4-carboxybenzaldehyde and para-toluic acid, as well as side reaction products that cause coloration, resulting in poor purity and hue. This is the cause of the decline.

In particular, a continuous system in which raw material xylene solvent and catalyst are continuously charged into a reactor and oxidation reaction products are taken out has the advantage of increasing the yield per unit time and per reactor. The proportion of raw materials that do not remain in the reaction system for a sufficient period of time and exit the reactor as unreacted products or reaction intermediates increases compared to the batch method or semi-continuous method, which increases the amount of terephthal produced. The disadvantage is that the quality of the acid deteriorates.

As a method for obtaining highly pure terephthalic acid by completing such an insufficient oxidation reaction, there are methods disclosed in Japanese Patent Publication No. 12695/1982 and Japanese Patent Application No. 31947/1983.

In the method of Japanese Patent Publication No. 12695/1973, after the oxidation reaction is completed,
The reaction mixture was heated to a temperature higher than the reaction temperature (200-300
In 'C), catalytic oxidation is carried out using an oxygen-containing gas for an additional hour or so, and after the reaction is completed, the product is treated with an oxygen-containing gas for a long time at a high temperature, which has the disadvantage that coloring of the product due to the occurrence of a reaction is unavoidable. .

In addition, the method disclosed in JP-A No. 47-31947 involves extracting a certain amount of the oxidation reaction product from the reactor intermittently, introducing a certain amount of oxygen-containing gas into the reactor, subjecting it to contact treatment, and then extracting it to the next vessel. This method involves repeating the process repeatedly, but since a batch extraction method is adopted for a continuous reaction, there are drawbacks in terms of operability and process stability.

The present invention provides a method for producing terephthalic acid with good operability and process stability in a continuous oxidation reaction of baraxylene, and with a high purity that cannot be achieved by conventional methods.

That is, when baraxylene is continuously oxidized with molecular oxygen in the presence of a solvent and an oxidation catalyst, and the resulting reaction product is sent from the reactor to the crystallization tank via a liquid feed line,
Surprisingly, the temperature and pressure of the liquid feeding line are higher than those of the crystallization tank, and the oxygen concentration in the oxidation reaction product passing through the liquid feeding line is higher than the oxygen concentration of the reaction liquid in the reactor. It has been discovered that terephthalic acid of much higher purity can be produced continuously compared to conventional methods such as methods in which the reaction is completed by introducing molecular oxygen into the reactor.

In the present invention, the oxygen concentration refers to the volume percentage of oxygen in a dry gas (that is, excluding water vaporized acetic acid vapor) at normal temperature and normal pressure.

In the present invention, in order to make the oxygen concentration in the reaction liquid passing through the liquid feeding line higher than the oxygen concentration in the reactor, the oxygen-containing gas is introduced near the outlet in the reactor, and the oxygen-containing gas is introduced into the liquid feeding line. A method of introducing gas is used.

Although a conventionally used oxygen-containing gas blowing pipe can be installed near the outlet, it is preferable to install a new oxygen-containing gas blowing pipe near the outlet.

The oxygen concentration of the reaction solution in the reactor can be determined by measuring the oxygen concentration of the exhaust gas.

Generally, when terephthalic acid is obtained by liquid phase air oxidation of baraxylene, the oxygen concentration is 1.0 to 4. OVo 1%
becomes.

The oxygen concentration in the liquid feeding line is measured by sampling and deaerating the gas-liquid solids in the liquid feeding line.

The oxygen contained in the reaction liquid decreases as it passes through the liquid feeding line, but in the present invention, the oxygen concentration in the oxidation reaction product passing through the liquid feeding line is equal to the oxygen concentration of the reaction liquid. Higher means that the oxygen concentration in the reaction liquid at least at the time when the reaction liquid is fed into the liquid feeding line is higher than the oxygen concentration in the reactor.

Even if oxygen-containing gas is introduced into the liquid extracted into the crystallization tank, there is little effect.

This is because when producing terephthalic acid, once the reaction has finished, that is, once the oxygen in the reaction liquid has disappeared, almost no reaction occurs even if oxygen is introduced again, and if you try to force it to occur, impurities may conversely occur. This results in an increase in

It is sufficient that the amount of oxygen-containing gas introduced is at least the amount necessary to oxidize unreacted substances, reaction intermediate products, and side reaction products.

If the amount of oxygen-containing gas introduced is too large, the oxygen concentration in the gas phase will increase and there will be a risk of explosion, which is not preferable.

The liquid feeding line used in the method of the present invention needs to be maintained at a higher pressure and temperature than the crystallization tank.

The pressure and temperature of the liquid feeding line are not related to the pressure and temperature of the reactor, but generally the reaction mixture is fed due to the pressure difference between the reactor and the crystallization tank, so the pressure and temperature of the liquid feeding line is Advantageously, the temperature is equal to or slightly lower than the reactor pressure and temperature.

The method of the present invention can be applied to any method that continuously produces terephthalic acid by liquid phase oxidation of baraxylene, but it is not applicable to the production of high-purity terephthalic acid used in direct polymerization. is particularly effective.

Further, preferable conditions for the oxidation reaction are as follows.

The reaction temperature is preferably in the range of 80 to 250°C.

Furthermore, since the reaction is carried out in a liquid phase, it is necessary to pressurize the raw material xylene and the solvent to keep them in a liquid phase at the reaction temperature.

Lower aliphatic carboxylic acids such as acetic acid, propionic acid, and butyric acid are used as the solvent.

It is sufficient that the amount of the solvent is about twice or more by weight relative to the raw material.

Heavy metal salts such as cobalt and manganese and bromine compounds are used as catalysts.

The heavy metal compounds such as cobalt and manganese are preferably inorganic salts, naptenates, lower fatty acid salts, etc., and the bromine compounds are preferably inorganic salts such as ammonium salts, sodium salts, or potassium salts, or hydrogen bromide.

It is also possible to use other reaction aids such as ketones and altehydes.

It is economical and advantageous to use air as the oxygen-containing gas.

The present invention will be explained below with reference to Examples.

Example 1 A titanium-lined pressure-resistant reactor (inner volume: 13 ml) equipped with a reflux cooling device, a stirrer, a heating device, a raw material inlet, an oxygen-containing gas inlet, a reactant outlet, an R-wire liquid level gauge, and a reaction A titanium-lined, pressure-resistant crystallization tank with an internal volume of 9rT1, which has a reflux cooling device, a stirrer, a heating device, a raw material inlet, and a reactant outlet, as well as a second crystallization tank, a third crystallization tank, and a centrifugal separator. The reaction was carried out using an apparatus consisting of a dryer.

First, 4.0 tons of acetic acid and 6.0 tons of cobalt acetate tetrahydrate were placed in a reactor. Okg, manganese acetate tetrahydrous solution 4. Okg, charged with 5.2 kg of sodium bromide, pressure 21 ~, temperature 21
maintained at 0°C, parakylene 1.0 T/H and acetic acid,
A solution of cobalt acetate, manganese acetate, and sodium bromide (composition ratio is the same as the first charged solution) was fed in at 4.0 T/H, and air was fed in at 4.2 N moromi/-mono-bara xylene.

On the other hand, the reaction product is sent to the crystallization tank based on the signal from the liquid level gauge.

The pressure in the crystallization tank is 8.5'! The reaction product was maintained at a temperature of 485°C and was sent to the second crystallization tank and then to the third crystallization tank based on the liquid level gauge, and after being brought to normal pressure, it was separated into solid and liquid using a centrifuge and washed. The solid portion of terephthalic acid was continuously dried in a dryer.

Here, 3ONM is added to the reaction product outlet in the reactor.
3/H air was blown.

The oxygen concentration in the reactor was 3.0%, and the temperature was 210°C and 20%.
The oxygen concentration at the inlet of the liquid feeding line maintained at 5G was 4.0% at a temperature of 7.0.

As a result, the properties and yield of the obtained terephthalic acid were as shown in Table 1.

Comparative Example 1 In Example 1, the reaction product outlet in the reactor was
Example 1 except that NM3/H air was not blown.
reacted in exactly the same way.

As a result, the properties and yield of the obtained terephthalic acid were as shown in Table 1.

The oxygen concentration in the crystallization tank was 0.1.

The oxygen concentration at the inlet of the liquid feeding line was 2.0%.

Comparative Example 2 In Example 1, 3ON was applied to the reaction product outlet in the reactor.
185℃, 8.5% instead of blowing M3/H air
A line was created to blow air into the crystallization tank maintained at G, and 30 NM3/H of air was blown through this line.

Otherwise, the reaction was carried out in exactly the same manner as in Example 1.

As a result, the properties and yield of the obtained terephthalic acid are shown in Table 1.
It was as shown in .

The oxygen concentration in the crystallization tank was 6%.

Compared to Comparative Example 1, in which the reaction was completed in the reactor, air was blown in the crystallization tank, as shown in Comparative Example 2, the yield and properties of terephthalic acid were improved, and the reaction was considerably completed. I know that there is.

However, according to the method of the present invention, although the yield according to Example 1 is almost the same as that of Comparative Example 2, the properties of terephthalic acid are significantly improved, and the effects of the method of the present invention are clearly manifested.

Claims (1)

[Claims] 1. When baraxylene is continuously oxidized with molecular oxygen in the presence of a solvent and an oxidation catalyst, and the resulting reactant is sent from the reactor to the crystallization tank via a liquid feeding line, the liquid feeding line is maintained at both temperature and pressure. A liquid feeding line which is higher than the crystallization tank and which leads the oxygen-containing gas to the vicinity of the outlet in the reactor so that the oxygen concentration in the oxidation reaction product passing through the liquid feeding line is higher than the oxygen concentration of the reaction liquid in the reactor. A continuous production method for terephthalic acid characterized by allowing oxygen-containing gas to enter the terephthalic acid.