GB2053891A - Improvements in or relating to the production of terephthalic acid - Google Patents

Abstract

The mother liquor obtained from the separation of precipitated terephthalic acid in the liquid phase oxidation of p-xylene is treated to separate volatile components, mainly composed of acetic acid, and solid components, composed of heavy metal salt(s) catalyst, residual terephthalic acid and reaction by- products, by using a combination of a tubular type heater having at least one heating tube of a relatively small diameter and a separation chamber into which the heating tube(s) discharge(s) at one end thereof.

Description

SPECIFICATION Improvements in or relating to the production of terephthalic acid The present invention relates to a method of treating the mother liquor from the liquid phase oxidation of paraxylene in the production of terephthalic acid.

The industrial production of terephthalic acid is generally effected by the oxidation of paraxylene with molecular oxygen in the presence of a salt or salts of heavy metal(s) (such as Co or Co + Mn) as catalyst in a liquid medium of a tower aliphatic acid, usually acetic acid. The terephthalic acid formed precipitates in the reaction solution and is recovered from the solution by separation, such as filtering or centrifuging. The mother liquor remaining contains, in addition to the acidic reaction medium (e.g. acetic acid), the catalyst, terephthalic acid which is dissolved or suspended therein, and reaction by-products such as, for example, water, paratoluic acid, 4carboxybenzaldehyde, isophthalic acid, benzoic acid and high polymers.

Heretofore, the mother liquor has been treated by firstly evaporating to recover a major portion of the solvent, e.g. acetic acid, and secondly, further evaporating the resultant concentrated slurry in a thin-film evaporator to recover the rest of the solvent. The thick slurry obtained solidifies after cooling, and heavy metal components in the solid are extracted and regenerated to reuse as the catalyst.

In general, the operation of a thin-film evaporator is delicate and troublesome; particularly in the above described terephthalic acid production process, as it is difficult to effect smooth operation, probably due to the presence of some component(s) of the material to be treated.

Consequently, operation of the process, including the oxidation step, often has to be interrupted.

Moreover, the thick slurry obtained from the thinfilm evaporator solidifies on cooling to form a solid mass, which must be crushed and finely ground to enable extraction of the heavy metal components.

In order to overcome these problems, it has been proposed to add an aliphatic ketone to the mother liquor (see Japanese Patent Publication No. 136634/1976), or to partially concentrate the mother liquor and then add an aromatic hydrocarbon with heating to distill off acetic acid and to obtain solid components (see Japanese Patent Publication No. 136635/1976). According to these improved methods, it is possible to more efficiently effect solid-liquid separation and to obtain a solid which can be treated easily.

However, the introduction of substances of no direct necessity in the process brings about a new probiem of separation of the substances.

It is the object of the present invention to provide a method of treating the mother liquor in terephthalic acid production, which avoids the above problems, but does not employ a thin-film evaporator and avoids the use of further substances in the process.

Thus, according to the present invention there is provided a method of treating the mother liquor obtained in the production of terephthalic acid by the oxidation of paraxylene with molecular oxygen in the presence of a catalyst of a salt of a heavy metal in an acetic acid medium, wherein the terephthalic acid formed and precipitated in the reaction solution is separated by filtrating or centrifuging to leave said mother liquid, which method comprises the steps of a) feeding the mother liquor through a tubular type heater having at least one heating tube of a relatively small diameter which discharges at one end into a separation chamber of a relatively large capacity, b) heating the liquor in said heating tube to a temperature above the boiling point of acetic acid under the operating pressure so as to form a substantially two-phase soiid-gas mixture, c) discharging said mixture into the separation chamber thereby to separate solid and gas phases, and d) removing the gas phase from the separation chamber and condensing it by cooling, and removing the solid phase from the bottom of the separation chamber in the form of a discrete powder or easily crushable mass.

The gas phase obtained in step b) of the method of the invention mainly comprises acetic acid, while the solid phase from step b) comprises the salt(s) of the heavy metal catalyst, a portion of terephthalic acid and reaction by-products.

Prior to feeding into the heating tube, the mother liquor may, if desired, be treated in a preliminary evaporator to evaporate a portion of acetic acid in the liquor. The preliminary evaporation may be readily carried out using a conventional thermosiphon reboiler type evaporator. The preliminary evaporation is advantageous because the amount of evaporation required in the heating tube and the separation chamber may be reduced, and as a result the capacity of the separation chamber may also be reduced. However, if the slurry in the heating tube is too thick blocking of the heating tube may result, and hence the preliminary evaporation should be to an extent advantageous for subsequent evaporation in the heating tube.

The practical conditions of evaporation in the heating tube may be selected from the heating temperature, pressure and flow rate, particularly at the discharge end of the tube. The particular conditions are generally chosen having regard to the composition of the mother liquor or concentrated mother liquor, the diameter of the heating tube, and the pressure in the separation chamber. Among these factors, the heating temperature and the flow rate are generally of most importance.

The temperature in the heating tube should of course be above the boiling point of acetic acid at the operating pressure, and is preferably selected from a range at least 200C higher than this boiling point. If the temperature difference between the operating temperature and the boiling point is too small to give sufficient superheating, the evaporation in the heating tube may be insufficient to form the solid-gas mixture in a favorable form. Thus, if the temperature is too low, the solid component may not be in a powder form or at least in the form of a mass capable of being readily crushed, due to incomplete separation of the volatile component from the solid component.

On the other hand, too high a temperature causes adhesion of solid to the inner walls of the heating tube, and accumulation of the solid on the walls may eventually cause blocking of the tube. Thus, the particular operating conditions including the heating temperature should preferably be selected experimentally under the given conditions. Such selection may be readily made by those skilled in the art.

Although atmospheric pressure inside the separation chamber may be employed, a reduced pressure, such as, for example, 6.7 x 103 to 2.67 x 104 N.m-2 ( 50 to 200 torr) is preferred. In order to obtain a sufficiently high flow rate to form the solid-gas two phase mixture in the heating tube without adhesion of the solid component to the inner walls of the tube, it is favorable to use at least a partial vacuum in the separation chamber. When the pressure in the separation chamber is low, the pressure in the heating tube may be low, and consequently, the boiling point of acetic acid under these pressures is low. Thus, a relatively low heating temperature can be used, which is advantageous from the viewpoint of easy process control, low energy consumption and long life of apparatus.However, if the pressure difference between the heating tube and the separation chamber is too large, a significant temperature decrease due to adiabatic expansion in the separation chamber may occur.

Such a temperature decrease may result in, in addition to the originally low operation temperature, an undesirably low temperature for the complete separation of volatile components.

Thus, the particular pressure is generally experimentally determined depending on the practical situation.

The flow rate of the mixture at the discharge end of the heating tube is advantageously at least 10%, and preferably 50% or more, of the speed of sound.

The method according to the invention makes it possible to obtain continuously the solid components in the mother liquor obtained in the production of terephthalic acid as a powder or a readily crushable mass, which contains a very small amount of acetic acid and water.

Accordingly, the percentage of acetic acid which may be recycled and reused may be increased to a miximum, and it becomes relatively easy to recover and reuse the heavy metal components in the solid residue. The reduction of problems associated with the treatment of the mother liquor ensures a stable operation of the main step, i.e.

the catalytic oxidation, and thus contributes remarkably to the industriai production of terephthalic acid.

The following Examples serve to illustrate the invention: EXAMPLE 1 Paraxylene was oxidized with air by using cobalt acetate and manganese acetate as catalysts and sodium bromide as a promotor, in an acetic acid medium. The reaction was regarded as complete when oxygen absorption ended, and the reaction mixture was then centrifuged to separate the terephthalic acid formed. The solid substance obtained was washed with acetic acid, and the washings combined with the solution remaining following centrifuging.

The mother liquor thus obtained consisted of 76.2% by weight of volatile components (acetic acid and water) and 23.8% by weight of nonvolatile components (terephthalic acid, organic byproducts and catalyst components).

The mother liquor was introduced into a heating tube which was heated with steam to 151 0C, and which discharged into a separation chamber, the inside of which was maintained at a pressure of 1.33 x 104 n.m~2 (100 mmHg). The flow rate at the discharge end of the heating tube was about 80% of the speed of sound. The evaporated components were fed to a condensor and liquefied, and solid components were removed through a valve at the bottom of the separation chamber.

The solid was a discrete powder, which contained only 0.5% by weight of residual acetic acid. No blocking of the heating tube was observed.

EXAMPLE 2 A mother liquor obtained as in Example 1, was discharged into a heating tube heated with steam to 1 51 OC and then into a separation chamber at a pressure of 1.33 x 104 N.m-2 (100 mmHg). The flow rate at the discharge end of the tube was chosen to be about 14% of the speed of sound. No blocking occurred. The product obtained was a mixture of powder and easily crushable masses.

Residual acetic acid: 3.5%.

EXAMPLE 3 The mother liquor obtained as in Example 1 was treated in a normal pressure evaporator to remove a portion of the acetic acid and water. The slurry obtained contained 51.5% of volatile components and 48.5% of nonvolatile components.

The slurry was then introduced into a heating tube which was heated at 1 800C and which discharged into a separation chamber at a pressure of 2.67 x 104 N.m-2 (200 mmHg). The flow rate under these conditions was about 34% of the speed of sound. No trouble with blocking was experienced, and the resulting solid component was also a discrete powder. Residual acetic acid: 1.1%.

Claims (7)

1. A method of treating the mother liquor obtained in the production of terephthalic acid by the oxidation of paraxylene with molecular oxygen in the presence of a catalyst of a salt of a heavy metal in an acetic acid medium, wherein the terephthalic acid formed and precipitated in the reaction solution is separated by filtrating or centrifuging to leave said mother liquor, which method comprises the steps of a) feeding the mother liquor through a tubular type heater having at least one heating tube of a relatively small diameter which discharges at one end into a separation chamber of a relatively large capacity, b) heating the liquor in said heating tube to a temperature above the boiling point of acetic acid under the operating pressure so as to form a substantially two-phase solid-gas mixture, c) discharging said mixture into the separation chamber thereby to separate solid and gas phases, and d) removing the gas phase from the separation chamber and condensing it by cooling, and removing the solid phase from the bottom of the separation chamber in the form of a discrete powder or easily crushable mass.

2. A method as claimed in claim 1 wherein the mother liquor is fed to the heating tube after first being concentrated by evaporating a portion of acetic acid from the liquor.

3. A method as claimed in either of claims 1 and 2, wherein the heating of the mother liquor in the heating tube is conducted to a temperature at least 200C higher than the boiling point of acetic acid under the operating pressure.

4. A method as claimed in any one of claims 1, 2 or 3, wherein step b) is carried out such that the flow rate of the mixture at the discharge end of the heating tube is at least 10% of the speed of sound.

5. A method as claimed in claim 4 wherein the flow rate of the mixture at the discharge end of the heating tube is 50% or more of the speed of sound.

6. A method as claimed in any one of claims 1 to 5, wherein the pressure in the separation chamber is lower than atmospheric pressure.

7. A method of treating the mother liquor obtained in the production of terephthalic acid substantially as herein described in any one of the Examples.