JPS5811418B2 - How to dry terephthalic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides terephthalic acid and acetic acid and/or
This invention relates to a method for obtaining dry terephthalic acid by removing acetic acid and/or water from a slurry of water.

The general industrial method for producing terephthalic acid is to use baraxylene as a raw material, use a salt of a heavy metal such as cobalt or manganese as a catalyst, and oxidize it with molecular oxygen in the liquid phase using a lower fatty acid such as acetic acid as a solvent. It is true.

Several types of this production method have been carried out with different reaction conditions, and if necessary, a purification step is set up after the production process to perform hydrorefining in an aqueous solvent to obtain terephthalate of a predetermined purity. In some cases, the acid is obtained, and in other cases, the product is obtained immediately without a purification step unless high purity is required.

In any case, in the industrial production of terephthalic acid, terephthalic acid is mixed with acetic acid and/or
A step is essential to separate it from the water and extract it as a dry, powdered substance.

Traditionally, terephthalic acid has been dried by deliquifying it using a centrifuge and then heating it with steam in a rotating dryer in a stream of non-condensable gas such as nitrogen. Ta.

According to this method, the amount of residual liquid in terephthalic acid after drying can be reduced to about 0.1% by weight.

However, since this drying method utilizes the vapor pressure of acetic acid or water at the heating temperature, the drying speed is low, and therefore the residence time in the dryer is long and the equipment becomes large.
There are many problems, such as terephthalic acid sticking to the walls of the pipe through which the heating medium (steam) passes, reducing the heat transfer effect and making maintenance difficult.

In addition, a recycling system for gas such as nitrogen for drying is required, and in addition to a recycling blower, additional equipment such as a scrubber to recover accompanying terephthalic acid and a heat exchanger for gas heating are required.

In terms of operation, it is necessary to maintain a constant rate of liquid removal by the centrifugal separator in order to smoothly supply slurry to the rotary dryer by the screw feeder.

(If the residual liquid rate fluctuates, the supplied terephthalic acid may form into lumps in the screw feeder, increasing resistance and making transfer impossible, forcing the equipment to stop operating.

) Furthermore, there are limits to the sealing of rotary dryers, and leakage of drying gas is unavoidable.

The present invention was made to solve the above-mentioned problems regarding drying of terephthalic acid, and its object is to provide a new drying method that does not use a rotary dryer.

In the method of drying terephthalic acid of the present invention, a slurry consisting of terephthalic acid, acetic acid, and/or water is fed to a relatively small-diameter tubular heating tube that opens into a relatively large-volume separation tank, and the slurry is acetic acid and/or
It consists of obtaining terephthalic acid in dry powder form by removing water, and is characterized by using operating conditions such that the content of terephthalic acid in the slurry is less than the value determined by the following formula.

C=(2,5θ0・4-1-66)α+(3,2θ0・
35+74)(1-α) The symbols in the formula have the following meanings.

C Terephthalic acid content in Nis slurry (wt%) θ: Temperature near the slurry inlet of heating tube (°C) Moisture content in α Nis slurry medium (mol) 1- Acetic acid fraction in α Nis slurry medium (Mole) First, the present inventors replaced the conventionally used thin film evaporator when treating the mother liquor obtained by collecting terephthalic acid from the reaction solution of terephthalic acid production by liquid phase oxidation of paraxylene.
It has already been found and proposed that good results can be obtained by using a drying apparatus which essentially has a relatively large-volume separation tank and a relatively small-diameter heating tube opening into it.

(Patent Application No. 141737/1982, Patent Publication No. 3565/1983)
(Publication No. 6) This time, we researched the use of a similar evaporative separation device to dry terephthalic acid collected by centrifugation of the reaction solution, confirmed that it was possible, and found that it would not cause clogging of the heating tube. The present invention was completed by discovering the critical conditions that allow continuous operation without interruption.

The present invention is a surprising discovery in itself, since it was expected that when feeding a highly concentrated slurry into a heating tube to evaporate the medium, blockage of the tube would inevitably occur.

If the present invention is explained with reference to the drawings, in FIG.
The slurry in the slurry tank 1 is pumped into the heating tube 3 by the slurry pump 2, reaches the inlet end 4, and is heated from the outside with steam (in the case of heating over 100°C) or hot water (
The mixture is heated by means such as heating at 100° C. or lower (5) or electric heating, and as it progresses through the tube, acetic acid and/or water evaporate to form a vigorously flowing mixture consisting of two phases: solid and gas.

The outlet end 6 of the heating tube 3 opens into a separation tank 7 from which the mixture discharged is separated into solid and gas.

The gas phase advances to a condenser 8, where it is cooled and condensed by cooling water 9, and enters a condensate receiving tank 10.

The solid phase, ie terephthalic acid, accumulates in the separation tank as a dry powder and is removed continuously or batchwise through the valve 11 at the bottom.

The separation tank 7 is equipped with a jacket 1 to prevent the gas phase from condensing there.
2 keeps it warm.

It is preferable that the separation tank 7 is connected to a vacuum line 13 and operated under reduced pressure.

Although the degree of pressure reduction is arbitrary, it is particularly effective for the pressure inside the separation tank to be 600 mmHg or less for reasons described later.

The system of terephthalic acid and acetic acid and/or water is
A slurry with a solids content of up to about 70% by weight can be handled as a slurry by conventional methods, and can be pumped using a slurry pump.

Around 75% is the limit that can be handled as slurry, and 8
When it reaches 0%, it loses its fluidity and should be called a wet solid rather than a slurry.

However, in the present invention, high concentrations of 80 to 90% or more can also be treated.

Therefore, in this specification, for convenience, "slurry" is used regardless of the appearance as long as it can be dried according to this method.
It is called.

To transfer a highly concentrated slurry that is difficult to pump using a slurry pump, it can be fed into the heating tube by forcefully injecting a gas 15 such as nitrogen gas or steam from a hopper 14 in FIG.

The pressure of this gas is 1 kg/i added to the pressure of the separation tank.
It is enough.

As mentioned above, if the inside of the separation tank is operated under reduced pressure, it becomes easier to feed the highly concentrated slurry, and press injection using gas pressure may become unnecessary.

In evaporative drying using a heating tube-separation tank combination, if the terephthalic acid content in the slurry to be treated is low, the amount of the slurry medium and the gas produced by its evaporation in the heating tube will be reduced to terephthalic acid solids. It is easily understood that since the amount is relatively large, the action of fluidizing and flushing the terephthalic acid powder is sufficient, and stable operation can be achieved without clogging the pipe.

However, a large amount of medium to be evaporated means that the amount of heat that must be provided from the heating tubes is correspondingly large and the throughput of the apparatus is low, which is of course unfavorable from an operational economic point of view.

(From this point of view, the terephthalic acid content in the slurry to which the method of the invention is applied should be 50% by weight or more.

) On the other hand, if the terephthalic acid content in the slurry is high, contrary to the above, the force of the medium and its gas to push away the terephthalic acid powder is weak, so it will be understood that clogging of the heating tube may occur depending on the conditions.

For industrial practice, it is preferable to process slurries with as high a concentration as possible without risk of clogging, in order to make the best use of the capacity of the equipment and to favor the energy balance.

As a result of investigating various factors that were expected to affect clogging of the heating tube, the present inventors found that the temperature near the slurry inlet of the heating tube, the composition of the slurry medium, and the slurry concentration are important. As a result of studying the apparent relationship between these factors, they found that the relationship expressed by the above formula holds true.

It is known that the pressure of the separation tank is also somewhat related, but 6
It was found that when the pressure was reduced to 00 mmHg or less, the pressure inside the tank did not contribute to the blockage.

Therefore, this degree of pressure reduction is preferable in order to ensure stable operation.

To further explain the above equation, generally speaking, the composition of the slurry medium (α, 1-α) is a condition given by the object of treatment, so if the temperature near the slurry inlet of the heating tube is set to a certain value, , it is sufficient to feed a slurry having a concentration that does not reach the critical concentration calculated from them to the heating tube.

Furthermore, when a slurry having a certain medium composition and slurry concentration is to be treated, it is sufficient to adopt a heating tube slurry inlet temperature that exceeds the temperature at which the above equation holds.

Figure 2 is a graph showing this relationship, and the three curves are α = 0 (in a system where the medium is only acetic acid) α = 0.76.
7 (acetic acid/water = 1/3.3 system) and α = 1.0 (water only system), the critical slurry concentrations corresponding to various temperatures are shown.

If the slurry is dried under the conditions in the region below each curve, the process can be continued without clogging the heating tube.

According to the present invention, drying of terephthalic acid can be advantageously carried out avoiding the problems associated with the use of conventional rotary dryers.

Because high slurry concentrations can be processed, drying is extremely efficient and requires less energy.

As long as the conditions defined by the above formula are met, clogging of the heating tube will not occur and stable operating conditions can be maintained.

Example 1 A terephthalic acid slurry was dried using an apparatus having the configuration shown in the drawings.

A glass viewing window was installed on the top of the separation tank so that the state of the terephthalic acid powder ejected from the opening of the heating tube could be confirmed.

If a blockage occurs, the powder will stop ejecting and you will know right away.

A slurry with a slurry concentration of 80% by weight and all acetic acid as the medium was fed from the hopper 14 through the bottom valve to the heating tube.

The slurry could be smoothly fed by suction to reduce the pressure in the separation tank to 120 mmHg.

The temperature near the slurry inlet of the heating tube was maintained at 80° C. with hot water.

Under the above conditions, operation could continue without blockage.

After 30 minutes, the terephthalic acid powder accumulated at the bottom of the separation tank was taken out.

This was placed in an electric furnace and heated at 130° C. for 2 hours under a nitrogen atmosphere, and the amount of residual liquid was determined by measuring the weight loss, which was 650 ppm.

Comparative Example 1 Drying was carried out under the same conditions as in Example 1 except that the terephthalic acid content in the slurry was 90% by weight.

A blockage occurred approximately 2 minutes after the start of operation. I tried pressurizing the inside of the hopper with gas to 5 kg/cm2abs, but the blockage did not disappear.

Even when the inside of the hopper was pressurized to a pressure of 5 kg/cm2abs and the operation was started, clogging started after 2 to 3 minutes.

In each case, when the heating tube was removed and the inside was inspected, the entire interior of the tube was found to be filled with a mixture of wet and powdered terephthalic acid.

Examples 2 to 10 The concentration of terefucuric acid slurry, medium composition, and heating tube temperature were variously changed, but all of them satisfied the conditions determined by the above formula, and the degree of vacuum in the separation tank was also changed to achieve the same result. The drying operation was repeated using the apparatus.

In both cases, operation could be continued for a long period of time, and dry terephthalic acid powder could be obtained at one time.

Comparison flI'2-7 Using the same apparatus, experiments were conducted by varying the concentration of the slurry, the composition of the medium, and the temperature of the heating tube, but selecting points that deviated from the conditions determined by the above formula.

Occlusion occurred in a short time and recovery was impossible.

The conditions and results for each side are summarized in Tables 1 and 2.

Further, the conditions of each Example and Comparative Example are plotted on FIG. 2 and shown with numbers.

[Brief explanation of drawings]

FIG. 1 is a flowchart outlining the apparatus used to carry out the method of the present invention. FIG. 2 is a graph showing the relationship of critical slurry concentration to temperature provided by the present invention. 1...Slurry tank, 3...Heating tube, 4...Heating tube slurry inlet, 7...
Separation tank, 8...Condenser, 13...
・Vacuum line, 14...Slurry hopper.

Claims (1)

[Claims] 1. A method for obtaining dry terephthalic acid by removing acetic acid and/or water from a slurry consisting of terephthalic acid, acetic acid, and/or water, the method comprising opening a relatively large volume separation tank. The slurry is continuously fed into a relatively small diameter tubular heating tube, the slurry is converted into a mixture of solids and gas within the heating tube, and this mixture is continuously discharged into the separation tank to separate the solids and gas. terephthalic acid is obtained in dry powder form by removing acetic acid and/or water, with operating conditions such that the terephthalic acid content in the slurry is less than the value determined by the following formula: A method characterized by using C=(2,5θQ4+66)α+(32θ0・35+7
4) (1-α) [The symbols in the formula have the following meanings. Terephthalic acid content in the C Nis slurry (wt%) θ: Temperature near the slurry inlet of the heating tube (C) Moisture content in the α Nis slurry medium (mol) 1- Acetic acid fraction in the α Nis slurry medium C mol)]2 The method for drying terephthalic acid according to claim 1, wherein the inside of the separation tank is operated at a pressure lower than normal pressure.