CS207795B2 - Method of making the dried terephtale 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

The invention relates to a process for the manufacture of dried terephthalic acid by removing acetic acid and / or water from a suspension comprising terephthalic acid, acetic acid and / or water.

In the industrial manufacture of terephthalic acid, a process for the oxidation of p-xylene as a feedstock by molecular oxygen in the presence of a salt or heavy metal salt catalyst, such as cobalt or cobalt and manganese, is generally used in a liquid medium containing lower aliphatic acids. .

There are several types of the process in which different reaction conditions are used. According to some of these, it is considered to have the desired purity as the terephthalic acid of the oxidation stage by hydrogenation in an aqueous medium. In other processes where high purity is not required, terephthalic acid is produced without purification.

In the industrial production of terephthalic acid, it is essential to ensure that terephthalic acid is obtained in the form of dry powdered material by separating it from acetic acid and / or water, which have been used as solvents.

Previously, a method of drying terephthalic acid, after dewatering in a centrifugal separator, by heating and steam drying in a tumble dryer in a non-condensing gas stream, such as nitrogen, has been used. In this way, after drying, the residual liquid content of the terephthalic acid can be reduced to about 0.1% by weight.

However, the drying rate in this process is low since it depends on the vapor pressure of acetic acid and / or water at the heating temperature. There are, therefore, many problems in drying, such as a long dwell time in the dryer, requiring large equipment, a reduced heat transfer effect due to the adhesion of the terephthalic acid to the wall of the tubes through which the heating medium (steam) passes and the associated difficult maintenance. In addition, a drying system, such as nitrogen, in particular a recycle blower and associated equipment, such as a scrubber for regenerating entrained terephthalic acid and a heat exchanger for heating the drying gas, must be used for drying. In addition, due to the operation of the drying system, it is necessary to maintain a constant drainage rate in the centrifugal separator in order to maintain a smooth flow of the slurry feed to the tumble dryer by means of a screw feeder. If the residual liquid content fluctuates, the supplied terephthalic acid turns into solids in the screw feeder, preventing transport and even interrupting the operation of the drying system. Furthermore, it is difficult to ensure perfect gas tightness in the tumble dryer, so that the drying gas inevitably escapes.

The invention has been devised to overcome the above-mentioned problems in drying terephthalic acid.

It is an object of the present invention to provide a new method of drying terephthalic acid, wherein operating difficulties are greatly reduced without having to use drum dryers.

The process for producing dried terephthalic acid by removing acetic acid and / or water from a slurry containing terephthalic acid, acetic acid and / or water according to the invention is carried out by feeding the slurry to a tubular type heater with at least one heated tube that opens at one end. to a separation chamber, in which a mixture of a solid and a gas is formed in the heated tube from the suspension, and the mixture is sent to a separation chamber to separate the solid and gaseous components, thereby obtaining terephthalic acid as a dried powder Wherein the terephthalic acid content of the suspension is less than the C-value defined by the formula

C = (2,5 ° 4 + 66) «+ + (3,2 (9 ³⁵ + 74) (1 - a) in which

C is expressed in% · · mass,

Is the temperature of the heated tube in ° C, and is the molar ratio of water in the suspension medium and (1 - · axis) · ·· is the molar ratio of acetic acid in the suspension medium, and the temperature in the heated tube It is maintained in the range of 8 ° to 24 ° C.

Since it was expected that clogging or clogging would necessarily occur when high concentration suspensions were fed into the heated tube to vaporize the suspension medium, the aforementioned discovery is itself surprising.

The invention is further elucidated with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an apparatus for carrying out a process for producing dried terephthalic acid according to the invention; Figure 2 shows the relationship between temperatures (X-axis at ⁰ ° C) and critical concentrations The slurry (na% Y in% by weight) defined according to the invention and FIG. 3 shows a graph of the equilibrium curves in a gas-liquid system consisting of acetic acid and water, the curves indicating the relationship between the pressures in the separation chamber (X axis) in MPa) and the temperatures in the heated tube (on the Y axis in ° C) selected when carrying out the process at a higher pressure.

According to FIG. 1, the slurry is fed from the slurry tank 1 by means of a sludge pump 2 via a pipe 4 to a heated tube 3, of the tube type heater. The feed suspension is heated in the tube by various heating means, such as steam (above 1 ° C) or hot water (below 1 ° C ° C) or some other heating medium or electric heating. in the heated tube, acetic acid and / or water evaporate to form a strongly fluidized mixture consisting of two phases, solid and gaseous The outlet end 6 of the heated tube 3, through which the mixture is discharged, flows into the separation chamber 7 where separation of solid from gas.

The gas phase enters the condenser 8 and is cooled by the cooling water entering through the conduit 9 to form condensate which is collected in the receiver 10.

As an alternative, the gaseous phase as such is fed to an acetic acid recovery plant to separate the acid from water. In case the gaseous component obtained in the separation chamber is acetic acid with a certain water content and the recovered acetic acid is to be recycled to the p-xylene oxidation reactor, the gaseous component is left, as it is necessary to reduce the water content of the acetic acid, flow to the acetic acid recovery column 15 to obtain acetic acid. In this case, the gaseous component can be fed to the column through an acetic acid evaporator 14 equipped with a terephthalic acid plant to remove impurities from the reaction mother liquors. If the gaseous component is acetic acid with a certain water content, the gas can be condensed and cooled, and the condensate can be recycled and reused in a different stage than the oxidation reaction, for example in washing the precipitated terephthalic acid. If the obtained gaseous component is high purity acetic acid, it is appropriate to recover the acid by condensation and reuse it. On the other hand, if the gas is mainly water, it should be subjected to condensation and introduced to a stage where water is used in the treatment.

The solid phase, namely terephthalic acid, accumulates in the separation chamber 7 in the form of a dry powder or easily, comminutable mass and is collected continuously or batchwise by the valve 11 at the bottom of the separation chamber 7. The separation chamber 7 is heated by the radius of the jacket 12 so that the gas phase in the chamber cannot condense.

A system consisting of terephthalic acid, acetic acid and / or water, if it contains up to 7% by weight of the solids, can be processed as a suspension in a conventional mode of transport, for example by means of a sludge pump. A content of about 75% is a limit that allows processing as a suspension, a system containing 8% solids loses mobility and should therefore be referred to as a solid with a moisture content rather than a suspension. According to the invention, however, such a system can be treated with 80-90% solids or even greater. In this description, the system, when it can be dried by the method of the invention, is referred to as a "suspension".

The transport of the high-concentration slurry, which is difficult to pump by the sludge pump, can be effected by extrusion into a heated tube using a pressurized gas such as nitrogen or steam.

When evaporating using a combination of heated tube and separation chamber, if the slurry to be treated has a low terephthalic acid content, the amount of suspension medium and consequently the amount of gas produced by evaporation of the medium is relatively large compared to the solid terephthalic acid and therefore sufficient effect on fluidization and removal of terephthalic acid powder. Thus, it is easy to understand that in this case, stable operation can take place without blockage of the heated tube. However, a larger amount of the medium to be vaporized requires more heat to be transferred through the heated tube. It is therefore clear that the working capacity of the device remains small. However, this is disadvantageous for the economics of operation. In this regard, the suspension to be treated according to the process of the invention should contain 50% by weight or more of terephthalic acid.

It can also be understood that when the suspension contains a higher percentage of terephthalic acid than in the above case, the compressive force of the medium and the resulting gas is weak and therefore; clogging may occur. This implies that in industrial practice it is preferable to process the suspension with the highest concentration only when there is little danger, given the optimal use of equipment and energy consumption.

As an explanation to the above formula, it should be noted that the composition of the suspension medium (aal-α) is a given condition, depending on the subject of processing. Thus, if the temperature of the heated tube is set to a certain temperature. value, the feed suspension should have a concentration less than the above critical concentration, calculated on a temperature basis. In the latter case, if it is necessary to process a suspension with a certain medium composition and a certain concentration of suspension, the temperature of the heated tube should be higher than the value that complies with the above equation.

Giant. 2 shows the above relationship. The three curves indicate critical suspension concentrations at different system temperatures, with a = 0 (medium being acetic acid only), = 0.767 (acetic acid / water - -1 / 3.3) and a - 1.0 (water only). If processing of the suspension under conditions given by the areas under these curves, the processing can be continued without clogging the heated tube.

The pressure in the separation chamber may vary over a wide range. Operation at a pressure below · atmospheric pressure or · under vacuum is suitable when the target is cooling and condensation of the volatile component. In vacuum operation, there is no theoretical lower pressure limit in the separation chamber. However, from a practical point of view, the value of 2.67 kPa represents essentially the lower limit of this pressure.

In contrast, in many cases acetic acid separated from terephthalic acid is recovered and reused as a medium for the degree of oxidation of p-xylene. When the process is carried out under reduced pressure in a separation chamber, the separated acetic acid or acetic acid containing water enters the acetic acid evaporator or the acetic acid distillation column in the liquid state as a result of cooling and condensation and is reheated to transfer to gaseous condition. Such cooling and reheating is disadvantageous with respect to energy consumption and it is therefore advantageous to introduce acetic acid into the gas recovery plant. If the pressure in the separation chamber is lower than the pressure in the acetic acid recovery column, it is recommended to displace the acetic acid contained in the gaseous component using an ejector 16. Acetic acid vapors from the acetic acid recovery column boilers can be used as propellant for the ejector.

In order to supply acetic acid in the vapor state, it is recommended to operate at a higher pressure in the separation chamber, for example at atmospheric pressure up to several tenths of MPa. There is no upper pressure limit except that the gaseous component will not liquefy. In practice, however, it is advisable to use a pressure in the range of atmospheric pressure to an absolute pressure of 0.6 MPa, or the like, due to the ease of design and operation of the apparatus. Too high a pressure is not a particular advantage. The acetic acid recovery devices into which the gaseous component is introduced usually operate at atmospheric pressure.

If the pressure in the separation chamber is high, the temperature of the heated tube must accordingly be high. The appropriate temperature of the heated tube also depends on the system formed by the suspension medium.

Giant. 3 shows gas-liquid equilibrium curves in which the acetic acid: water weight ratio is 0: 100, 50: 50 and 90: 10, or · the molar water ratio is oc = 1,000, and - 0.767 and a = 0.270. the curves are the condensation regions in which the suspension medium is in the liquid phase. It is understood that it is necessary to work under conditions given by areas above these; curves. The perfect separation is

2077β5 reached - at a temperature - 20 ° C or - more than the equilibrium temperature.

However, it is found that too high a temperature often changes the degree of product quality, i.e., terephthalic acid, for example by changing its color. The experimental results concluded that the maximum temperature of the heated pipe must not exceed 240 ° C to prevent possible deterioration.

If the aim is to reduce - to an extreme value - the residual liquid content of the terephthalic acid product, it is recommended to trap the terephthalic acid - in powder form or in the form of a readily disintegrating mass, as shown in Fig. 1, wherein said acid is passed through a valve 11 into a reservoir 13 and a reduced pressure is maintained in the tank at a value lower than the pressure in the separation chamber or is maintained by a vacuum connection to the vacuum generating device. There is no theoretical limit to the reduced pressure in the reservoir. At a higher pressure difference between the separation chamber and the reservoir or at a lower pressure in the reservoir, a higher ejection effect can be achieved. - For practical operation, however, 2.67 kPa represents the lower pressure limit in the container. It is effective if - when the product is mixed with a mixer - (for example, a mixer - with a belt mixer). In addition, it is beneficial to let dry, inert gas, for example nitrogen, which has previously been heated somewhat, preferably flowed through the container. The gas is allowed to flow preferably under reduced pressure operation.

The process according to the invention can be used to obtain terephthalic acid without any inconvenience compared to the conventional process using drum tumble dryers. Since it is possible to process the suspension with a high concentration, the drying according to the method is carried out with a very high efficiency and low energy consumption. If the process corresponds to the conditions defined by the above formula, the heated pipe does not clog and stable operation can take place.

Examples

Example 1

The terephthalic acid slurry was dried using the apparatus shown in Figure 1. The separation chamber has a glass window through which the outlet of the terephthalic acid powder from the open end of the heated tube can be observed. - If clogging occurs, it is quickly recognized by the powder that the outlet is terminated.

A slurry containing -80% by weight, solids, and a medium consisting of acetic acid only was fed to the heated tube. J ^ i ^] ^ ^<v3ád-ENI suspension was held -no defects aspirated from the separation chamber in which the pressure 16 -udržován -kPa temperature heated - - tube - maintained at .80 ° C hot water.

Operation continued without blocking the pipe. After> 30 minutes, the - terephthalic acid collected at the bottom of the separation chamber was collected. The product was then determined for residual-liquid content by heating in an electric furnace under a nitrogen atmosphere for 2 hours and found to be 650 ppm.

Comparative Example 1

The same procedure as in Example 1 was repeated, but with a higher content of terephthalic acid in the suspension, i.e. 90%.

After 2 minutes from the start of operation - clogging occurred. Then the suspension was extruded with pressurized gas at an absolute pressure of 0.5 MPa, but the clogging was not removed.

When the pressure in the suspension tank was adjusted to an absolute value of 0.5 MPa from the beginning, clogging was also observed in a few minutes.

Examination of the heated tube revealed that the inner side of the tube was completely encased in terephthalic acid in a moist state and in powder form.

Examples - 2 to 10

Drying of terephthalic acid was repeated with varying suspension concentrations, medium components, and heated tube temperatures, which were in accordance with the conditions defined above. The extent of vacuum in the separation chamber also varied.

In all cases, the operation was carried out for a long time and terephthalic acid was obtained in the form of a highly-dried powder.

Comparative Examples 2-7

The drying was repeated with varying concentrations of suspension, medium components and temperatures of the heated tube, these parameters not meeting the conditions characteristic of the process of the invention.

After a short period of operation, the heated pipe clogged and the operation could not be restored.

The operating conditions -and the test results from the above examples - and comparative examples are contained in Tables I and -II.

In addition, the temperature dependence on the suspension concentrations of the examples and the comparative examples is indicated in the graph by FIG. 2 with reference to the example numbers.

Table I (examples)

Example no. Suspension Concentration Temperature Pressure in sepa- Continuous Content medium suspension heated com- monization procedure residual (% by weight) pipes (° C) Greek (kPa) liquids (ppm)

1 and - 0 80 80 16 Yes 650 2 a = 0 92 200 40 Yes 710 3 a = 0 95 250 40 Yes 750 4 a = 1.0 75 120 40 Yes 350 5 a = 1.0 85 200 40 Yes 420 6 and - 1.0 88 280 40 Yes 480 7 and - = 0.767 85 120 40 Yes 470 8 cc - 0.767 88 200 40 Yes 460 9 and - 0 90 200 80 Yes 720 10 a = 1.0 85 200 80 Yes 650 11 «= 0 33 158 40 Yes 560

Table II (comparative examples)

Example no.

Suspension medium

Heated tube temperature (% w / w) (° C)

Pressure in separation chamber (kPa)

Continuous procedure

1 a = 0 90 80 16 no (clogging after * 2 min) 2 a = 0 95 200 40 no (clogging after 3 min) 3 and 1.0 85 120 40 No 4 and - 1.0 88 200 40 No 5 a = 1.0 95 280 40 no (clogging after 5 to 6 min 6 and - 0.767 90 120 40 No 7 a = 0.767 94 200 40 No

Example 12

In the same apparatus as in Example 1 to which the solid product container was attached, a slurry containing 60 wt% solids and a medium consisting of 90 wt%, acetic acid and 10 wt% was fed to the heated tube. water (? = 0.270). The tube was heated with steam to 130 ° C. An absolute pressure of 0.1 MPa or atmospheric pressure was maintained in the separation chamber.

Under the above conditions, the operation of blockage of the heated tube continued. After 10 mih, the valve at the bottom of the separation chamber was closed and terephthalic acid was removed from the reservoir.

The residual liquid content was determined by the above method and was 740 ppm.

Examples 13 to 29 and Comparative Examples 8 to 13

The procedure of Example 12 was repeated with various suspending media, suspension concentrations, heated tube temperatures, and pressures in the separation chamber.

The test results from Examples 13 to 29 are included in Table III and the test results from Comparative Examples 8 to 13 in Table IV.

Table III (examples)

Example Suspension medium Susum concentration Heated temperature Abs. pressure in separation Continuous- Acid no. (% by weight) tubes (° C) chamber (MPa) po Obsah Obsah Obsah Obsah Obsah Obsah zbyt Obsah zbyt Obsah Obsah Obsah Obsah Obsah Obsah Obsah Obsah ppm stup stup stup

O O O CD O CD O O O O O O O O Q O O O rH rH 00 what 00 LO rH rH oo what O CM LO rH 00 00 LO LO ОТ от cm CM xF oo г — 1 Os what oo rx LO rH rH rH

LO LO LO LO rH LO 0 ^ © ~ СМ ~ гН ~ т ^ г ^^ О ~ гН ~ гН rH CM ^ θ 'θ' θ 'θ' θ 'r-Γ rH ο ο ο ο r-Γ ο ο * cd гн <ζΓ

CD O LO Ю Ю LO LO CD CM LO LO O O O LO LO O O 00 00 CD rH LO CM ОТ 00 00 00 O rH CM CM rH CM CM CM rH rH CM rH г-1 rH rH CM rH CM rH

соооооооооооюоооою COCOCOCOCOCOCOCOOTCOCOCOCOOOOQCOCĎCOCOOO

O CD O O O O O CD bx b- O O Cx Γχ O what what what what what CM CM CM CM CM CM CM CM b ^ Ьч

II. II. II. II. II. II. II

53 «S Й« 53 Й Й & $ 53 53 & S S S S S

Table IV (comparative examples)

Example no. Suspension medium Suspension concentration (° / w / w) Heated pipe temperature (° C) Abs. pressure in separation chamber (MPa) Continuous procedure 8 a = 0.270 60 120 0.1 Yes· 9 = 0.270 60 195 0.75 Yes 10 a = 0.767 60 115 0.15 Yes 11 a = 0.767 60 185 1.05 Yes 12 a = 1,000 60 125 0.15 Yes· 13 a = 1,000 60 195 1.0 Yes

Table IV (continued)

Example no. Acid produced Residual liquid content Appearance (PPm) 8 9 10 11 12 13 nestanoven with a high moisture content

Example 30

A mixer with a belt mixer was used as a reservoir for terephthalic acid and the solid-phase separation was carried out under the same conditions as in Example 12.

After 10 min of operation, the valve was closed and the reservoir pressure was reduced to an absolute value of 0.01 MPa. Nitrogen gas was then introduced into the container with stirring. After increasing the pressure in the reservoir to atmospheric pressure, terephthalic acid was removed. The residual liquid content was up to 350 ppm.

Examples 31 and 3 2

The procedure of Example 30 was repeated with different heated tube temperatures and pressures in the separation chamber.

Example 33 volatiles under the same conditions as in Example 12. After 10 min. In operation, the valve at the bottom of the separation chamber was closed and somewhat preheated nitrogen gas was introduced into the terephthalic acid tank at a rate of 100 l / h for 5 minutes.

Example 34

The procedure of Example 31 was repeated except that the absolute pressure in the belt mixer was 0.15 MPa. The conditions corresponded to a modification of Example 4, in which the vacuum tank without vacuum was used but in which the pressure was kept lower than in the separation chamber.

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

The conditions for temperature-pressure dependence of Examples 13 to 34 and Comparative Examples 8 to 13 are shown in the graph of Figure 3.

Terephthalic acid was separated from

Table V (examples)

Example no. Suspension medium Suspension concentration (% w / w) Heated pipe temperature (° C) Abs. pressure in separation chamber (MPa) Abs. tank pressure (MPa) 30 and 0.270 60 130 0.1 0.01 31 Λ = 0.270 60 185 0.5 0.01 32 a = 0.270 60 215 1.0 0.01 33 and - = 0.270 60 130 0.1 0.01 34 = 0.270 60 185 0.5 0.15

Table V (continuation) Example number Acid produced Residual liquid content Appearance (ppm)

30 350 satisfactory 31 320 satisfactory 32 370 satisfactory 33 380 satisfactory 34 780 satisfactory

Claims (6)

OBJECT OF THE INVENTION A method for producing dried terephthalic acid by removing acetic acid and / or water from a slurry comprising terephthalic acid, acetic acid and / or water, characterized in that the slurry is fed into a tubular type heater with at least one heated tube that opens at one end into the separation chamber, the mixture of solid and gas formed from the suspension in the heated tube is sent to a separation chamber to separate the solid and gaseous components and obtain terephthalic acid as a dried powder, the terephthalic acid content of the slurry being less than C, defined by the formula C = (2,5 Θ ⁰ ' ⁴ + + + (3,2 Θ ^ο ' ^35. + 74) (1 →), in which C is expressed in% by mass, Θ is the temperature of the heated pipe in ° C, Is the molar ratio of water in the suspension medium, and (1-a) is the molar ratio of acetic acid in the suspension medium, and the temperature in the heated tube is maintained at 80 to 240 ° C. 2. A process according to claim 1, wherein the pressure in the separation chamber is kept below atmospheric pressure with a lower limit of 2.67 kPa. Method according to claim 2, characterized in that, for the acetic acid recovery, a gas component separated in the separation chamber is fed to the acetic acid recovery column, preferably via an ejector, in a further stage, preferably via an acetic acid evaporator. 4. The method of claim 1, wherein the separation chamber maintains a pressure ranging from atmospheric pressure to an absolute pressure of about 6 bar and a temperature around the open end of the heated tube at least 20 ° C above the dew temperature of the slurry. . media under pressure in the separation chamber. Process according to claim 4, characterized in that in the next step, the gaseous component separated in the separation chamber is introduced into the acetic acid recovery column, preferably via an acetic acid evaporator. Method according to Claims 4 and 5, characterized in that the terephthalic acid powder is collected in a container by a valve. at the lower part of the separation chamber and in the reservoir a reduced pressure is maintained with a value lower than. the pressure in the separation chamber, the lower pressure limit in the container being 2.67 kPa, and / or an inert gas for passing through the container. further drying the terephthalic acid.