GB2024810A - Method of obtaining dried terephthalic acid - Google Patents

Abstract

Terephthalic acid (e.g. that produced by oxidizing p-xylene) is obtained in the form of dried powder from a slurry containing terephthalic acid and acetic acid and/or water by introducing the slurry into a tubular type heater having at least one heating tube which discharges at one end into a separation chamber whereby the slurry is converted into a solid-gas mixture in the heating tube, and discharging the mixture into the separation chamber to separate the solid component from the gas component. Critical slurry concentrations for continuous operation are given. <IMAGE>

Description

SPECIFICATION Method of obtaining dried terephthalic acid The present invention relates to a method of obtaining dried terephthalic acid by removing acetic acid and/or water from a slurry containing terephthalic acid, and acetic acid and/or water.

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 lower aliphatic acid, usually acetic acid.

In different methods of utilising this process various reaction conditions are employed, e.g. some of them incorporate, after the oxidation step, a purification step by hydrogenation in an aqueous medium to prepare terephthalic acid of a desired purity; and others, where high purity is not required, produced terephthalic acid without a purification step.

However, in the industrial production of tereph tholic acid, it is essential to provide a step of recover ing terephthalic acid as a dry powder material by separating it from the acetic acid and/or water used as a solvent.

Heretofore, the terephthalic acid has been dried, after its separation in a centrifugal separator, by heating and drying with steam in a rotary drier in a stream of a non-condensing gas such as nitrogen. In this way, it is possible to reduce the residual liquid in the terephthalic acid after the drying to about 0.1% by weight.

However, the rate of drying by this procedure is low because the rate depends on the vapor pressure of acetic acid and/or water at the temperature of heating. There are, therefore, a number of problems in this drying technique, e.g. a long residence period in the dryer (which therefore requires a large scale apparatus), a decreased heat-transfer effect in the dryer due to adhesion of the terephthalic acid to the walls of tubes in which the heating medium (steam) passes, and as a resu It troubiesome maintenance.

Moreover, this drying technique requires a system or recycling gas used in the drying (such as nitrogen), namely, a pump for recycling and attached apparatus such as a scrubber for recovering entrained terephthalic acid and a heat exchanger for heating the drying gas. In addition, the operation of the drying system required the maintenance of a constant rate of draining-off in the centrifugal separator in order to ensure a smooth feed of the slurry to the rotary dryer with a screw feeder. Furthermore, if th residual liquid content varies, the terephthalic acid conveyed by the screw feeder forms solid masses, which prevents transfer and may even result in the interruption of the operation of the drying system. Additionally, a perfect gas seal in the rotary dryer is difficult to achieve and the drying gas inevitably leaks.

It is an object of the present invention to avoid the above mentioned problems in drying terephthalic acid. In particular, it is an object of the invention to provide a method of drying terephthalic acid with significantly reduced operating problems and without using a rotary dryer.

Thus, according to the invention there is provided a method for the preparation of dried terephthalic acid which comprises removing acetic acid and/or water from a slurry containing terephthalic acid and acetic acid and/or water, characterized by a) introducing the slurry into a tubular type heater having at least one heating tube which discharges at one end into a separation chamber whereby the slurry is converted into a solid-gas mixture in the heating tube, and b) discharging the mixture into the separation chamber to thereby separate the solid component from the gas component to thus obtain terephthalic acid in the form of a dried powder or easily crushable masses; the terephthalic acid content of the slurry being less than the value C defined by the formula C = (2.5 804 + 66) a + (3.2 '35 + 74) (1-a) (wherein C is expressed as a % by weight, 0 is the temperature in "C of the heating tube, a is the molar ratio of water in the slurry medium, and (1 -cz) is the molar ratio of acetic acid in the slurry medium).

As it would be expected that blocking would necessarily occur when a highly concentrated terephthalic acid siurry is introduced into a heating tube to evaporate the slurrying medium, the method of the present invention is itself surprising.

Embodiments of the invention are hereinafter described by way of example and with reference to the accompanying drawings in which: Figure lisa flow chart of apparatus which may be employed in the method of preparing dried terephthalic acid according to the invention; Figure 2 shows the relationship between temperature and critical slurry concentration defined by the invention; and Figure 3 shows gas-liquid equilibrium curves of an acetic acid-water system, which gives the relationship between the pressure in the separation chamber and the temperature of the heating tube to be selected in effecting the method of the invention under a high pressure.

In Figure 1, a terephthalic acid containing slurry from slurry tank 1 is fed by a slurry pump 2 into heating tube 3 of a tubular type heater. The slurry is heated in the tube with heating means 5 such as, for example, steam (when heating to a temperature above 100"C), hot water (when heating to a temperature below 100"C), electric heating or some other heating means. As the slurry passes through the heating tube, acetic acid and/or water present therein evaporate to form a vigorously fluidizing mixture consisting of a two-phase solid-gas mixture.

The discharge end 6 of the heating tube 3 opens into the separation chamber 7, and the solid and gas phases separate therein.

The gas phase emerging from the separator either passes to the condenser 8 and is cooled by cooling water 9 to produce a condensate, and this condensate is then received by receiver tank 10, or alternatively, the gas phase is introduced into an acetic acid recovery device to separate the acid from water.

When the gas component obtained in the separation chamber consists of acetic acid containing water in an undesired concentration and it is desired to recycle the recovered acetic acid to a reactor produc ing terephthalic acid by paraxylene oxidation, it is necessary to reduce the water content in the acetic acid gas phase, and this is therefore fed to the acetic acid recovery column 15 so as to recover acetic acid therein. In this case, the gas component may be passed to the recovery column via an acetic acid evaporator 14 which may be present in the production plant for the removal of impurities in the reaction mother liquor. If the gas component is acetic acid containing water to an undesirable degree, it may be cooled to condense acetic acid for recycling and reuse in a step other than the oxidation reaction, such as, for example, washing precipitated terephthalic acid.If the gas component obtained is acetic acid of high purity, it is preferred to condense the acid and reuse it. On the other hand, if the gas mainly consists of water, it should be condensed and passed to a water-treatment step.

The solid phase from the gas-solid mixture, nam ely terephthalic acid, accumulates in the- separation chamber 7 in the form of a dry powder or an easily crushable mass, which may be removed continuously or batchwise through valve 11 at the bottom of the separator 7. The separation chamber 7 may be warmed by a heating jacket 12 to ensure that the gas phase does not condense in the chamber.

Slurries of terephthalic acid and acetic acid and/or water containing up to 70 /O by weight of solid component, may be conveyed in a conventional way such as with a slurry pump. In general, a solids content of about 75% is usually regarded as the limit allowing treatment as a slurry, and a system containing 80% solids has a high viscosity, and hence it should be referred to as a wet solid mass rather than a slurry. According to the method of the present invention, however, systems containing a high solids content such as 80 to 90% solids, or even more, may be treated. Thus, insofar as such a system may be dried in accordance with the method of the invention, for convenience, it is herein referred to as a "slurry".

Transfer of slurries of high concentration which are difficu It to pump with a slurry pump may be effected by forcing under the pressure of a gas, such as nitrogen or steam, into the heating tube.

When evaporating, using the combination of the heating tube and the separation chamber according to the invention, a slurry of low terephthalic acid content, the amount of slurrying medium, and consequently the amount of gas phase generated in the evaporation of the medium is relatively large in comparison with the amount of solid terephthalic acid. Therefore, the gas evaporating has the effect of fluidizing and carrying away powdery terephthalic acid. It will thus be readily understood that, in this case, a continuous operation, without blocking of the heating tube, may be achieved.

However, the larger amount of slurrying medium which evaporates requires a larger amount of heat to be introduced into the heating tube. Thus, it is appa rent that the working capacity of the apparatus is reduced. This may, of course, be disadvantageous for operating economy. From this point of view, the slurry to be treated by the present method preferably contains 50 /O by weight or more of terephthalic acid.

It will also be appreciated that, when the slurry contains a high percentage of terephthalic acid, contrary to the above case, the fluidizing effect of the slurrying medium and the gas therefrom is weak, and therefore, blocking may occur. It is however preferable, for industrial practice, to treat a scurry of the highest concentration which carries little risk of blocking to enable the best utilization of apparatus and energy consumption.

The maximum concentration of terephthalic acid in the slurry is given by the value C determined by the above mentioned formula. The molar ratios of water and acetic acid in the slurry medium (a and I -a) depends on the particular slurry to be treated, and when thetemperature of the heating tube is at a certain level, the slurry to be treated should have a terephthalic acid concentration less than the above noted critical concentration calculated on the basis of the temperature. Alternatively, if it is necessary to treat a slurry having a certain water and/or acetic acid composition and a certain terephthalic acid concentration, the temperature af the heating tube should be higherthanthe level which satisfies the above mentioned equation.

Figure 2 illustrates the above relationship. The three curves indicate the critical slurry concentrations at various temperatures for the systems, a = 0 (i.e. the medium consists only of acetic acid), a = 0.767 (acetic acid/water = 1/3.3) and a = 1.0 (water only). When the treatment of the slurry is effected under conditions in the regions below these curves, the method continues without blocking of the heating tube.

The pressure in the separation chamber may be varied over a wide range. Operation at a pressure lower than atmospheric pressure or in a vacuum is convenient in the case when it is intended to cool and condense the volatile component of the gassolid mixture.

On the other hand, the acetic acid separated from terephthalic acid may be, in many cases, recovered for reuse as the medium forthe paraxylene oxidation step. When the process is conducted with a reduced pressure in the separation chamber, the separated acetic acid or acetic acid containing water may be passed to an acetic acid evaporator or an acetic acid distillation column in a liquid state as a result of cooling and condensation, and then reheated to become gaseous. This cooling and reheating is disadvantageous from the point of energy consumption, and it is therefore advantageous to introduce the acetic acid into the recovery means in the gaseous state. If the pressure in the separation chamber is lowerthan that of the acetic acid recovery column, it its convenient to transfer the gas component containing acetic acid using an ejector Ej.

The driving gas for the ejector may be acetic acid vapor from the reboiler of the acetic acid recovery column.

In order to transfer the acetic acid in the form of a vapor, it is convenient to operate with a higher pressure, e.g. at atmospheric or superatmospheric pressure, in the separation chamber. There is no upper limit to the pressure unless the gas component does not liquefy. Practically, however, it is advisable to use pressures of atmospheric to 588 kN.m~, (6 kg/cm2) or so because of ease in construction of apparatus and operation. Too high a pressure gives no particular merit but is not in itself disadvantageous. The acetic acid recovery means into which the gas component is introduced is usually operated at atmospheric pressure.

When the pressure in the separation chamber is high, the temperature of the heating tube must be accordingly raised. The particular temperature of the heating tube also depends on the composition of the slurry as indicated above.

Figure 3 shows gas-liquid equilibrium curves of systems in which the ratio of acetic acid to water isO 100, 50: 50 and 90: 10 by weight or a = 1.000, o = 0.767, and a = 0.270 by molar ratio of water respectively. The regions below these curves are condensing regions in which the slurry medium is in a liquid phase. Needless to say, it is necessary to use conditions in the regions above these curves. Total separation will be performed at a temperature about 20"C or more higher than the equilibrium temperature, i.e. the dew point of the slurry medium at the pressure in the separation chamber.

It has been found, however, that too high a temperature often degrates the quality of the product terephthalic acid, by, for example, changing color. In order to avoid possible degrading, experimental results indicate that the maximum temperature of the heating tube should be desirably about 240"C.

If it is wished to reduce residual liquid in the pro ductterephthalic acid to a minimum, it is convenient to discharge the terephthalic acid in a powder form or easily crushable mass, as shown in Figure 1, through a valve 11 to a receiver 16, and to keep the receiver under a reduced pressure lower than the pressure in the separation chamber, or to keep it under a vacuum by connecting it to a vacuum device (not shown). It is advantageous to agitate the product in the receiver with an agitator (e.g., by using a ribbon blender). It may also be useful to pass a dry inert gas, such as nitrogen, preferably heated to some extent, through the receiver. The gas throughput is preferably conducted with the reduced pressure operation.

Dried terephthalic acid may be obtained in accordance with the present invention without the aforementioned problems related to hitherto used drying techniques using a rotary dryer. As a slurry of high concentration may me treated in the method of the invention, the drying is performed with a high efficiency and low energy consumption. Provided the procedure is effected in accordance with the conditions defined by the above mentioned formula, no blocking of the heating tube occurs and continuous operation can be achieved.

The following Examples serve to illustrate the method of the invention: Example 1 Terephthalic acid slurry was dried using the apparatus shown in Figure 1. The separation chamber was provided with a glass window through which the ejection of terephthalic acid powder from the open end of the heating tube may be observed.

In the event of blocking, this can be promptly recognized by the termination of the powder ejection.

A slurry which contained 80% by weight of solids and in a medium consisting of acetic acid only, was fed to the heating tube. Feeding of the slurry was smoothly effected by maintaining a reduced pressure (1.6 x 104 N.m-2, i.e. 120 mmHg) in the separation chamber. The temperature of the heating tube was kept at 80"C with hot water.

The operation was effected without any blocking of the tube. After 30 minutes running, the terephthalic acid accumulated at the bottom of the separation chamber was taken out. The residual liquid content of the product was determined by heating in an electric oven under a nitrogen atmosphere for 2 hours to give the observed value, 650 ppm.

Control 1 The same procedure as Example 1 was repeated with a higher terephthalic acid slurry content, 900/c.

About 2 minutes after beginning the operation, blocking occurred. It was then attempted to clear the slurry by pushing with compressed gas at 490 kN.m-2 (5 kg/cm2), but the blockage was not removed.

The Control was repeated by initially pressurizing the slurry tank to a pressure of 490 kN.m-2 (5 kg/cm2) but blocking was also observed after a few minutes.

Subsequent inspection of the heating tube showed that the inside of the tub was totally packed with wet and powdery terephthalic acid.

Examples 2 to 11 The drying of terephthalic acid was repeated in the apparatus of Figure 1 with various terephthalic acid slurry concentrations, medium compositions and heating tube temperatures, which satisfy the conditions defined by the above mentioned formula. The degree of vacuum in the separation chamber was also varied.

In all these Examples operation could be continued over an extended period to give highly dried terephthalic acid powder.

Controls 2 to 7 The drying operation was repeated with various terephthalic acid slurry concentrations, medium compositions and heating tube temperatures, which do not satisfy the conditions specifiied according to the invention.

Blocking of the heating tube occurred after a short period of running, and could not be cleared.

The operating conditions and results of the above Examples and Controls are given in the following Tables I and II respectively.

The temperatures and terephthalic acid slurry concentrations of the Examples and Control Examples are plotted in Figure 2 with reference numbers.

Table I Examples slurry Slurry Temp. of press. in Conti- Residual Run concn. Heating Sep. Chamb. nuous Liquid Medium (Wt.%) Tube ( C) N.m-2 (mmHg) Operation (pom) 1 a=0 80 80 1.6x104(120) yes 650 2 " 92 200 4.0x104(300) " 710 3 " 95 250 4.0x104(300) " 750 4 a = 1.0 75 120 4.0x104(300) " 350 5 " 85 200 4.0x104(300) " 420 6 " 88 280 4.0x104(300) " 480 7 a = 0.767 85 120 4.0x104(300) " 470 8 " 88 200 4.0x104(300) " 460 9 a = 0 90 200 8.0x104(600) " 720 10 a = 1.0 85 200 8.0x104(600) " 650 11 a=0 33 158 4.0x104(300) " 560 Table Il Controls Slurry Slurry Temp. of Press. in Continuous Run Concn. Heating Sep. Chamb.

Medium (Wt.%) Tube( CJ N . m - 2 (mmHg) Operation 1 a = 0 90 80 1.6x104(120) No (blocking after 2 min.) 2 " 95 200 4.0x104(300) No (blocking after 3 min.) 3 a = 1.0 85 120 4.0x104(300) No 4 " 88 200 4.0x1 04(300) No 5 " 95 280 4.0x104(300) No (blocking after 5-6 min.) 6 &alpha;;= 0.767 90 120 4.0x104(300) No 7 " 94 200 4.0x104(300) No Example 12 In the same apparatus as used in Example 1 to which a receiver of the solid terephthalic acid pro duct is attached, a slurry containing 60% by weight of solid in a medium which consists of 90 /O by weight of acetic acid and 10 /O zy weight of water (a = 0.270) was introduced to the heating tube. The heat ing tube was heated with steam to 1300C. The pres sure in the separation chamber was 98 kN.m-2 (1 kg/cm2) or atmospheric pressure.

The above conditions resulted in a continuous operation, without blocking of the heating tube.

After 10 minutes running, the valve at the bottom of the separation chamber was shut, and the terephthalic acid was taken out from the receiver.

Residual liquid was determined by the method mentioned in Example 1 and was found to be 740 ppm.

Examples 13 to 29 and Controls 8 to 13 The procedures of Example 12 were repeated with different slurry media, slurry concentrations, temperatures of the heating tube and pressures in the separation chamber.

The results of Examples 13 to 29 are shown in Table Ill, and the results of Controls 8 to 13 in Table IV.

Table Ill Examples Slurry Slurry Temp. of Press. in Conti- Product Acid Run Concn. Heating Sep. Chamb. nous Resd. Appear kN.m-2 Liq. ance Medium (Wt?/0) Tube ("C) (kgizm) Opern. (ppm) 12 a=0.270 60 130 98 (1) yes 740 good 13 " 60 240 98 (1) " 510 14 " 60 245 98 (1) " 510 yellowish 15 " 60 185 490 (5) " 930 good 16 " 60 205 735 (7.5) " 960 17 " 60 215 980 (10) " 1230 18 " 60 245 1177 (12) " 1250 yellowish 19 " 90 150 98 (1) " 710 good 20 rev = 0.767 60 122 147 (1.5) " 710 21 " 60 245 147 (1.5) " 480 yellowish 22 " 60 175 686 (7) " 860 good 23 " 60 190 980 (10) " 1100 24 " 85 130 98 (1) " 770 25 &alpha;= 1.000 60 130 147 (1.5) " 620 26 " 60 245 147 (1.5) " 450 yellowish 27 " 60 185 686 (7) " 810 good 28 " 60 200 980 (10) " 780 29 " 85 170 196 (2) " 580 Table IV Controls Slurry Slurry Temp. of Press. in Conti- Product Acid Resd.

Run Concn. Heating Sep. Chamb. nuous Liqd. Appear kN.m-2 ance Medium (Wt. O/o) Tube ("CJ (kg/cm2) Opern. (ppm) 8 vex = 0.270 60 120 98 (1) yes not 9 " 60 195 735 (7.5) " deter- contained 10 cow = 0.767 60 115 147 (1.5) " mined wet 11 " 60 185 1030 (10.5) " (large masses 12 a=1.000 60 125 147 (1.5) " amount) 13 " 60 195 980 (10) Example 30 A ribbon blender was used as the receiver of terephthalic acid, and the solid-gas separation was performed under the same conditions as Example 12.

After 10 minutes running, valve 11 was shut, and the pressure in the receiver was decreased to 9.8 kN.m-2 (0.1 kg/cm2). Nitrogen gas was then introduced into the receiver with agitation. Terephthalic acid was taken out after increasing the pressure in the receiver to atmospheric. Residual liquid was determined to be as low as 350 ppm.

Examples 31 and 32 Example 30 was repeated with different temperatures of the heating tube and pressures in the sep aration chamber.

Example 33 Terephthalic acid was separated from volatile material under the same conditions as Example 12.

After 10 minutes running, the valve at the bottom of the separation chamber was shut, and nitrogen gas which was heated to some extent was introduced into the receiver at the rate of 100 1/hr for 5 minutes.

Example 34 Example 31 was repeated except that the pressure in the ribbon blender was 147kN.m-2 (1.5 kg/cm2).

These conditions correspond to the modification of Example 4 in which a receiver, (though not vacuumed), was used under a pressure lower than that in the separation chamber.

The results of the above Examples 30 to 34 are given in Table V.

The temperature-pressure conditions of Examples 13 to 34 and Controls 8 to 13 are plotted in Figure 3.

Table VExamples Slurry Slurry Temp. of Press. in Press. in Produc. Acid Resd.

Run Concn. Heating Sep. Chamb. Receiver Liqd. Appear kN.m-2 kN.m-2 (ppm) ance Medium (Wt. O/O) Tube ( C) (kglcm2) (kgismZJ 30 erg = 0.270 60 130 98 (1) 9.8 (0.1) 350 good 31 " 60 185 490 (5) 9.8 (0.1) 320 32 " 60 215 980 (10) 9.8 (0.1) 370 33 " 60 130 98 (1) 9.8 (0.1) 380 34 " 60 185 490 (5) 147 (1.5) 780

Claims (9)

1. A method for the preparation of dried terephthalic acid which comprises removing acetic acid and/or water from a slurry containing terephthalic acid and acetic acid and/or water, characterized by a) introducing the slurry into a tubular type heater having at least one heating tube which discharges at one end into a separation chamber whereby the slurry is converted into a solid-gas mixture in the heating tube, and b) discharging the mixture into the separation chamber to thereby separate the solid component from the gas component to thus obtain terephthalic acid in the form of a dried powder or easily crushable masses; the terephthalic acid content of the slurry being less than the value C defined by the formula C = (2.5 '4 + 66) a + (3.2 0'35 + 74) (1 -cu) (wherein C is expressed as a % by weight, 8 is the temperature in "C of the heating tube, a is the molar ratio of water in the slurry medium, and (1-a) is the molar ratio of acetic acid in the slurry medium).

2. A method according to claim 1, wherein the pressure in the separation chamber is lower than atmospheric pressure.

3. A method according to claim 2, wherein the gas component separated in the separation chamber is passed by means of an ejector to an acetic acid recovery column, optionally by way of an acetic acid evaporator, to recover acetic acid.

4. A method according to claim 1, wherein the pressure in the separation chamber is at atmospheric or superatmospheric pressure and the temperature at the discharge end of the heating tube is at least 20"C higher than the dew point of the slurry medium at the pressure in the separation chamber

5. A method according to claim 4, wherein the pressure in the separation chamber is in the range from atmospheric pressure to 588 kN.m-2.

6. A method according to either of claims 4 and 5, wherein the gas component separated in the separation chamber is passed to an acetic acid recovery column, optionally by way of an acetic acid evaporator, to recover acetic acid.

7. A method according to any one of claims 4 to 6, wherein the temperature of the heating tube does not exceed 240"C.

8. A method according to any one of claims 4 to 7, wherein the obtained terephthalic acid is received in a receiver by way of a valve at the bottom of the separation chamber, and wherein the receiver is subjected to a reduced pressure lower than the pressure in the separation chamber and/or an inert gas is passed through the receiver so as to further dry the terephthalic acid.

9. A method for the production of dried terephthalic acid substantially as herein described in any of the Examples.