CN1486968A - An air-lift external circulation bubble tower oxidation device for the production of terephthalic acid - Google Patents

Abstract

The invention provides an air-lift external circulation bubble tower oxidation device for the production of terephthalic acid. The upper and lower ports of the external circulation pipe are connected with the upper and lower parts of the reaction section of the bubble column respectively, and the slurry in the upper part of the tower is introduced into the lower area through the airlift effect through the external circulation pipe to form a circulation of the fluid in the whole tower , for liquid-phase catalytic oxidation of p-xylene. The bubble column adopts a larger height-to-diameter ratio to enhance gas-liquid mass transfer, and uses an air-lift external circulation tube to improve the temperature and concentration distribution in the tower. There are no moving parts in the tower of the present invention, the structure is simple, and the cost and operation cost are low. At the same time, it is applicable to various reaction conditions of air-liquid phase catalytic oxidation of p-xylene in terms of reaction, mass transfer, mixing and heat transfer, and can meet different requirements. Process requirements for the preparation of terephthalic acid.

Description

An air-lift external circulation bubble tower oxidation device for the production of terephthalic acid

Technical field

The invention relates to an air-lift external circulation bubble tower oxidation device for producing terephthalic acid (TA), in particular to a device for air-liquid-phase catalytic oxidation of p-xylene (PX) in the production process.

Background technique

Terephthalic acid is an important raw material for the production of polyester (PET) fibers and resins. At present, it is mainly produced by the p-xylene air oxidation method. Or tetrabromoethane) in the acetic acid solvent, pass into air or enriched oxygen to carry out oxidation, generate solid product terephthalic acid. The typical reaction temperature is 155-205°C, the pressure is 0.5-1.6 MP, and the residence time is 40-120 minutes. The reaction heat is removed by solvent evaporation, and the steam is condensed and returned to the reactor. The resulting slurry is then obtained through subsequent separation and refining processes. Purified terephthalic acid (PTA) products.

The oxidation reactor is the core device of PTA production. The design of the reactor needs to meet the four requirements of gas-liquid mass transfer, liquid phase reaction and crystallization, evaporation heat transfer, and material mixing. At present, the industrialized oxidation reactor mainly adopts a tank structure with stirring paddles. For example, the stirred tank reactors introduced in patent US5211924 and US5102630 have two layers of stirring paddles. The function of the lower paddle is to realize liquid mixing and solid suspension, and the upper paddle is used It is used to disperse air bubbles to meet the requirements of promoting gas-liquid mass transfer and liquid circulation in the tank. The reactor disclosed in patent JP14098/1979, EP0618186A1 and patent US5463113 has a stirring paddle, which is installed at the bottom of the kettle to loosen the solid slurry and prevent scabbing. The main problem of this type of reactor with stirring is that the equipment cost and operating cost are relatively high, because the dynamic sealing and vibration problems of the stirring paddle need special mechanical manufacturing technology to solve, in addition, the power consumption of the equipment is large during operation, and the maintenance cost Also higher.

Patent CN1293184A discloses a non-stirring oxidation device for the production of aromatic carboxylic acids, which is characterized in that the reactor cylinder is equipped with multi-layer vertical and horizontal partitions to divide the space into multiple sub-districts connected up and down, each sub-district is equivalent to a fully mixed In the reactor, raw materials and air are added from the lower part of the tower, and the reacted slurry is discharged from the upper part, and the material flows in a manner close to parallel push, so as to achieve higher conversion and yield. However, the internal structure of the reactor is complicated, and local gas pockets are easily formed when a large amount of solvent evaporates, which is not conducive to the flow of gas and solid-containing slurry.

Patent US2002/0183546A1 discloses a two-stage oxidation process for the production of aromatic carboxylic acids. The first-stage oxidation uses a tower reactor without stirring, at a lower temperature (155-165°C) and pressure (0.52-0.62MP) The second-stage oxidation uses a stirred reactor and operates at a higher temperature (205-225°C) and pressure (6.0-18.5MP). The first-stage oxidation reactor introduced in this patent belongs to the bubble column type, and adopts a large aspect ratio to enhance mass transfer. However, in the case of a large aspect ratio, the problem of poor mixing is prone to occur, and there are large temperature gradients and concentration gradients along the tower height in the reactor, which reduces the volume utilization of the reactor and intensifies the consumption of solvent combustion.

Contents of Invention

The object of the present invention is to provide an air-lift type external circulation bubble column oxidation device for producing terephthalic acid with simple structure, low cost and low operating cost, so as to overcome the defects of the above-mentioned various reactors.

The air-lift external circulation bubble column oxidation device for producing terephthalic acid of the present invention comprises a cylindrical bubble column with an enlarged top section, an external circulation pipe, a gas distributor, a reflux liquid distributor and a condenser. The gas distributor is placed horizontally at the lower part of the reaction section of the bubble column and connected to the gas inlet of the bubble column. The reflux distributor is placed horizontally in the upper expansion section of the bubble column, at 1/2 of the tower height above the gas distributor. There is a raw material feed pipe at 4~1/2, a discharge pipe at the bottom of the tower, a tail gas pipe at the top of the expansion section of the bubble tower, a gas-liquid separator on the upper part of the external circulation pipe, and a gas-liquid separator inlet. It communicates with the upper part of the reaction section of the bubble column, and the lower port of the external circulation pipe is located at 0 to 1 times the diameter of the column above or below the gas distributor, and communicates with the lower part of the reaction section of the bubble column. The tail gas pipe is connected to the gas outlet pipe, the tail gas pipe is connected to the inlet of the condenser, the liquid outlet of the condenser is connected to the raw material feed pipe, the reflux liquid distributor and the subsequent dehydration tower through the condensation pipe, and the gas outlet of the condenser is connected to the subsequent dehydration tower. The exhaust gas treatment unit is connected.

When working, the raw material p-xylene, solvent aqueous acetic acid, catalyst cobalt-manganese-bromine are evenly mixed and then fed into the reactor from the feed pipe, and the air or oxygen-enriched gas enters the gas distributor from the inlet of the tower and is dispersed by the gas distributor. After bubbling through the liquid bed layer, the oxidation reaction with the liquid phase reactant is carried out; the reaction heat is removed from the reactor through the tail gas pipe through the evaporation of the solvent acetic acid and water, and the solvent in the tail gas is condensed by the condenser, and part of it is returned to the tower, and the other part is sent to the reactor. To the subsequent dehydration tower dehydration; maintain a certain liquid level in the reactor so that the inlet of the gas-liquid separator on the upper part of the external circulation pipe is located near the lower part of the liquid level. When the tower is bubbling and inflated, the gas holdup inside the bubble tower is high and the apparent density of the three-phase medium is low, while the gas holdup in the outer circulation pipe is low and the apparent density is high. In this way, the slurry in the circulation pipe will Spontaneously flow from top to bottom under the action of the difference, the slurry with higher temperature in the bad mixing area in the upper part of the reaction section is continuously transported to the well mixed and lower temperature area in the lower part of the tower, forming a circulation of the liquid in the whole tower, which is beneficial to eliminate the slurry in the upper part of the tower. Poor mixing zone and uneven distribution of temperature and concentration along the tower. The terephthalic acid slurry generated by the reaction is output from the bottom discharge pipe to the subsequent separation and refining process for further processing. At the same time, the heat of reaction is removed from the tower through the solvent evaporation, and the steam and the tail gas separated by the external circulation gas-liquid separator are introduced into the condenser. The solid and liquid foam entrained in the tail gas are removed, one stream is mixed with raw materials through the feed pipe and then fed into the tower, and the other stream is sent to the subsequent dehydration tower. The condensed non-condensable tail gas is sent to the subsequent unit for treatment.

There is no stirring paddle and other moving parts in the tower of the device of the present invention, and the slurry in the upper part of the tower is introduced into the lower area through the airlift effect by using the external circulation pipe to form the circulation of the fluid in the whole tower. The structure is simple, the cost and operation cost are low, and it is suitable for Various reaction conditions for the air-liquid-phase catalytic oxidation of p-xylene.

Description of drawings

Fig. 1 is a structural schematic diagram of the device of the present invention.

Detailed ways

With reference to Fig. 1, the air-lift type external circulation bubble column oxidation device of producing terephthalic acid comprises the cylindrical bubble column 1 that has top expansion section, external circulation pipe 2, gas distributor 3, reflux liquid distributor 4 and condenser 5. The gas distributor 3 is placed horizontally at the lower part of the reaction section of the bubble column and connected to the gas inlet of the bubble column. The reflux distributor 4 is placed horizontally in the expansion section at the upper part of the bubble column. 1/4~1/2 of the bubbling tower is provided with a raw material feed pipe 6, a discharge pipe 7 is provided at the bottom of the tower, an exhaust pipe 8 is provided at the top of the expansion section of the bubble column, and the upper part of the outer circulation pipe 2 has a gas-liquid separation Device 2-1, the inlet of the gas-liquid separator communicates with the upper part of the bubble column reaction section, the lower port of the external circulation pipe 2 is located at 0 to 1 times the tower diameter above or below the gas distributor, and communicates with the bubble column reaction section, The top of the gas-liquid separator 2-1 is provided with a gas outlet pipe 10 communicated with the tail gas pipe 8, the tail gas pipe 8 is connected to the inlet of the condenser 5, and the liquid outlet of the condenser 5 is connected to the raw material feed pipe 6 through the condenser pipe 9 , the reflux liquid distributor 4 is connected with the follow-up dehydration tower, part of the condensate passes through the feed pipe 6 and the reflux liquid distributor 4 reflux tower, and the other part is sent to the follow-up dehydration tower for dehydration, the gas outlet of the condenser 5 is connected with the follow-up tail gas The treatment units are connected to send the non-condensable tail gas to the subsequent tail gas treatment unit for further treatment.

The gist of the present invention will be further described below.

1. The cylinder structure and effective volume of the bubble tower

The role of the expansion section of the bubble column is to reduce the gas velocity, splash the buffer liquid surface, and combine the spray of the reflux liquid to reduce the entrainment of solids and liquids in the tail gas. The volume of the reaction section is the effective volume of the bubble column, and the effective volume is determined according to the requirement to ensure that the liquid phase oxidation reaction is fully completed. Since the oxidation of p-xylene is a moderately slow reaction, which is mainly carried out in the liquid phase body, the reaction rate of each step of p-xylene oxidation is not very sensitive to the change of the concentration of the reactant, so as long as the liquid phase has sufficient residence time, the It can achieve extremely high conversion rate of p-xylene (above 99%) and yield of terephthalic acid (above 95%). Usually the diameter ratio of the expansion section and the reaction section is 1.5-2:1, and the height ratio is 1:10-30. The residence time of the liquid phase (= liquid holdup in the tower/outlet liquid flow rate) is kept in the range of 40 to 120 minutes. During operation, the slurry level is controlled at the connection between the reaction section and the expansion section or a slightly lower position.

2. Bubble column aspect ratio

The bubble column promotes mass transfer through the turbulent bubbling of the gas. In order to enhance the mass transfer, the bubble column should adopt a large height-to-diameter ratio to increase the superficial gas velocity, but the increase of the height-to-diameter ratio will easily lead to a decrease in the temperature inside the tower. The inhomogeneous concentration distribution also increases the gas holdup and reduces the liquid phase residence time. The p-xylene (PX) oxidation reactor is an evaporation reactor, and the heat of reaction is removed by evaporation of the solvent, so that near the gas inlet at the bottom of the bubble column, the solvent evaporates violently, the temperature is lower, and the evaporation in the middle and upper parts is less , the temperature is higher, and there is a significant uneven temperature distribution in the lower half of the tower. In addition, since the feed position of PX is in the middle and lower part of the tower, which is closer to the bottom slurry outlet, increasing the aspect ratio will easily cause uneven mixing and form a poor mixing zone in the upper part of the tower. Therefore, the determination of the aspect ratio of the bubble column should take into account the requirements of mass transfer, mixing and reaction. The applicable aspect ratio of the present invention is 6-13, and the preferred aspect ratio is 7-10.

3. External circulation pipe

In the present invention, the upper part of the external circulation pipe has a gas-liquid separator, the inlet of the gas-liquid separator is generally 0.4 to 2 times the tower diameter below the liquid level, and the lower port of the external circulation pipe is located 0 to 1 above the gas distributor. times the diameter of the tower, or 0 to 1 times the diameter of the tower below the distributor (in the case of moving the gas distributor upwards). The gas-liquid separator is a cylinder whose diameter is 1.5 to 2.5 times the diameter of the external circulation pipe, and the ratio of height to diameter is 1.5 to 3. The separated gas enters the tail gas pipe of the bubble tower along the gas outlet pipe upward, and the slurry returns to the tower along the pipe downward. The diameter of the external circulation pipe is determined by the required circulation flow. If the diameter is too small, the circulation flow will be low, which is not enough to improve the unevenness of temperature and concentration in the tower. If the diameter is too large, it will increase the cost of equipment manufacturing and installation, and also reduce the volume. usage efficiency. The diameter of the external circulation pipe provided by the present invention is 1/5-1/15 of the diameter of the reaction section of the bubble column. In order to increase the flexibility of operation, resistance plates or valves can be set at the connection between the gas-liquid separator and the bubble column or in the external circulation pipe to adjust the circulation flow.

4. Paraxylene feed position

Raw material p-xylene is uniformly mixed with solvent and catalyst and then added to the reaction section of the bubble column. The concentration of reactants near the feed inlet is relatively high, and the oxidation is also relatively severe. Since the boiling points of p-xylene and acetic acid are similar, the feed position should not be set too high, otherwise it will increase the evaporation loss of PX, and it should not be set too low to avoid short-circuit loss of PX from the bottom outlet. The suitable feeding position given by the present invention is at 1/4-1/2 of the tower height above the gas distributor. The raw material feed pipe can be a single feed pipe or multiple feed pipes distributed along the height of the bubble column.

5. Gas-liquid distributor

The gas distributor in the present invention is installed at the bottom of the bubble column, and can be a conventional annular or hexagonal perforated plate distributor or a multi-tube distributor, or other commonly used industrial gas distributors, and the injection direction of the gas is upward. The reflux liquid distributor is installed in the expansion section of the bubble column, and the refluxed acetic acid is returned to the tower by spraying through the liquid distributor at the top of the tower. The purpose is to remove the solid and liquid foam entrained in the tail gas. Plate-trough sprinklers or multi-pipe sprinklers. The steam evaporated in the reactor and the tail gas separated from the gas-liquid separator of the external circulation pipe are introduced into the condenser, and part of the condensed liquid is returned to the tower, and part is sent to the dehydration tower to remove water. Due to the large amount of gas evaporation in the tower, the condenser generally adopts 2 to 5 stages of series operation to increase the heat exchange load, and at the same time, superheated steam of different pressures can be produced by-product.

The device of the present invention is applicable to various technological conditions of p-xylene oxidation, for example, reaction temperature 155～205 ℃, pressure 0.5～2.0MP, catalyst total concentration (Co+Mn+Br) 700～3500ppm, feed solvent ratio ( Acetic acid: PX, kg/kg) 3～10: 1, water content 3～15%, reactor residence time 40～120min, will be specifically illustrated by following examples 1～3.

Example 1

The low-temperature oxidation process is used to produce terephthalic acid. The annual production capacity of a single reactor is 200,000 tons of TA, and the annual production time is 7600 hours. The reaction conditions are given in Table 1.1.

Table 1.1 Low temperature oxidation process conditions Middle temperature(℃) PX processing capacity (10 ³ kg/h) Tower top pressure (Mpa, absolute pressure) Tail oxygen concentration (v _O2 , %) Catalyst concentration (mass fraction to HAc, 10 ^-6 ) H ₂ O/HAc (mass percentage) Feed HAc/PX mass ratio co mn Br 165 17.72 0.7190 4.40 942.9 87.9 942.9 7.0 10:1

The corresponding reactor structure and size are given in Table 1.2.

Table 1.2 Reactor structure and size diameter (m) Aspect Ratio (m/m) Circulation pipe diameter (m) Effective volume(m ³ ) PX feed position 3.68 H/D=8 0.368 312.1 0.40H

In the table, D is the diameter of the tower, which is determined according to the production capacity, and H is the height of the reaction section. For a single air-lift external circulation bubble column reactor (ALECBCR) with an annual output of 200,000 tons of terephthalic acid, when the aspect ratio is selected to be 8, D = 3.68m, and the reaction results and related indicators are given in Table 1.3 In the table, the reaction indicators of the bubble column reactor (BCR) without external circulation pipe under the same conditions are also given for comparison.

Table 1.3 Performance comparison between BCR and ALECBCR PX conversion rate (%) TA yield (%) PT concentration (10 ^-6 kg/kgsol) 4-CBA concentration (10 ^-6 kg/kgsol) Tail gas CO ₂ concentration (v _CO2 , %) Maximum temperature difference (°C) PX concentration difference (min/max, %) Residence time (min) BCR 99.9 96.9 8058 1808 1.14 5.55 7.54 100 ALECBCR 99.9 97.5 7315 1506 1.13 2.88 10.40 100

PT and 4-CBA given in the table are respectively the liquid phase concentration (=component quality/solvent acetic acid and the quality of water) of reaction intermediate p-toluic acid and p-carboxybenzaldehyde in the discharge of the reactor, the maximum The temperature difference is the temperature difference between the highest point (the upper part of the tower) and the lowest point (the bottom) of the tower, and the PX concentration difference is the relative ratio between the minimum concentration near the liquid level and the maximum concentration near the feed point, which is the concentration distribution in the tower. A measure of uniformity. From the results given in Table 1.3, it can be seen that both the bubble column and the air-lift external circulation bubble column can complete the process of producing terephthalic acid by oxidation of p-xylene, but the air-lift external circulation bubble reactor ALECBCR Under the same reaction conditions, the distribution of temperature and concentration is more uniform, so the yield is higher than that of bubble column BCR, and the concentration of reaction intermediates PT and 4-CBA is lower. If the PX flow rate of the external circulation bubble column is increased so that the TA yield, PT and 4-CBA concentration in the discharge are consistent with the bubble column, the production capacity can be increased by 4.5% by using ALECBCR than BCR.

Example 2

The medium temperature oxidation process is used to produce terephthalic acid. The annual production capacity of a single reactor is still 200,000 tons of TA, and the annual production time is 7600 hours. The reaction conditions are given in Table 2.1.

Table 2.1 Process conditions of medium temperature oxidation method Middle temperature(℃) PX processing capacity (10 ³ kg/h) Tower top pressure (Mpa, absolute pressure) Tail oxygen concentration (v _O2 , %) Catalyst concentration (mass fraction to HAc, 10 ^-6 ) H ₂ O/HAc(mass percentage) Feed HAc/PX mass ratio co mn Br 185.0 17.72 1.168 3.50 691 406 892 7.52 4.66:1

For the medium temperature oxidation process, when the aspect ratio is selected as 8, D=3.19m, and the structure and size of the reactor are given in Table 2.2

Table 2.2 Reactor structure and size diameter (m) Aspect Ratio (m/m) Circulation pipe diameter (m) Effective volume(m ³ ) PX feed position 3.19 H/D=8 0.319 203.3 1/2H

The reactor output results under the above conditions are listed in Table 2.3. It can be seen that the external circulation bubbling reactor still has better operational performance and reaction effect. Under the condition of maintaining the same TA yield, PT acid and 4-CBA content, the external circulation reactor provided by the present invention can be compared with conventional The Bubble Column Reactor increases capacity by 3.8%

Table 2.3 Performance comparison between BCR and ALECBCR PX conversion rate (%) TA yield (%) PT concentration (10 ^-6 kg/kgsol) 4-CBA concentration (10 ^-6 kg/kgsol Tail gas CO ₂ concentration (v _CO2 , %) Maximum temperature difference (°C) PX concentration difference (min/max, %) Residence time (min) BCR 99.9 97.4 5280 1333 1.60 6.61 1.72 50.3 ALECBCR 99.9 97.8 4773 1221 1.58 3.41 2.80 50.2

Example 3

The high-temperature oxidation process is used to produce terephthalic acid. The annual production capacity of a single reactor is still 200,000 tons of TA, and the annual production time is 7600 hours. The reaction conditions are given in Table 3.1. The structure and size of the reactor are shown in Table 3.2, and the output results of the reactor are listed in Table 3.3. Under the same reaction conditions and output results, the external circulation bubble reactor is 3.2% larger than the conventional bubble column reactor. The effect of the external circulation pipe in the above three cases is slightly different, which is mainly due to the difference in the size of the reactor and the external circulation pipe, resulting in different circulation flow rates.

Table 3.1 Process conditions of high temperature oxidation method Middle temperature(℃) PX processing capacity (10 ³ kg/h) Tower top pressure (Mpa, absolute pressure) Tail oxygen concentration (v _O2 , %) Catalyst concentration (mass fraction to HAc, 10 ^-6 ) H ₂ O/HAc(mass percentage) Feed HAc/PX mass ratio co mn Br 196.2 17.72 1.570 3.60 342 526 949 14.77 3.34:1

Table 3.2 Reactor structure and size diameter (m) Aspect Ratio (m/m) Circulation pipe diameter (m) Effective volume(m ³ ) PX feed position 2.97 H/D=8 0.297 165.0 1/3H

Table 3.3 Performance comparison between BCR and ALECBCR PX conversion rate (%) TA yield (%) PT concentration (10 ^-6 kg/kgsol) 4-CBA concentration (10 ^-6 kg/kgsol) Tail gas CO ₂ concentration (v _CO2 , %) Maximum temperature difference (°C) PX concentration difference (min/max, %) Residence time (min) BCR 99.5 95.4 10339 2588 1.80 6.47 12.0 49.3 ALECBCR 99.7 96.5 9537 2330 1.80 4.49 17.3 49.1

Claims (9)

1. An air-lift type external circulation bubble tower oxidation device for producing terephthalic acid is characterized in that it comprises a cylindrical bubble tower (1) with top expansion section, external circulation pipe (2), gas distribution device (3), reflux liquid distributor (4) and condenser (5), gas distributor (3) is placed horizontally in the lower part of the reaction section of the bubble column, and is connected with the gas inlet of the bubble column, and the reflux liquid distributor ( 4) Horizontally placed in the expansion section of the upper part of the bubble column, a raw material feed pipe (6) is provided at 1/4 to 1/2 of the tower height above the gas distributor (3), and a discharge pipe is provided at the bottom of the tower The pipe (7) is provided with a tail gas pipe (8) at the top of the expansion section of the bubble column, and the upper part of the external circulation pipe (2) has a gas-liquid separator (2-1), and the inlet of the gas-liquid separator reacts with the bubble column The upper part of the section is connected, and the lower port of the external circulation pipe (2) is located at 0 to 1 times the tower diameter above or below the gas distributor, and is connected with the reaction section of the bubble column. There is a gas outlet pipe (10) connected to the tail gas pipe (8), the tail gas pipe (8) is connected to the inlet of the condenser (5), and the liquid outlet of the condenser (5) is connected to the raw material feed pipe through the condenser pipe (9) (6) The reflux distributor (4) is connected to the subsequent dehydration tower, and the gas outlet of the condenser (5) is connected to the subsequent tail gas treatment unit. 2. The air-lift external circulation bubble tower oxidation device according to claim 1, characterized in that the ratio of the height of the reaction section of the bubble tower (1) to the diameter is 6 to 13, and the top of the bubble tower (1) is enlarged The diameter ratio of the section and the lower reaction section is 1.5-2:1, and the height ratio is 1:10-30. 3. The air-lift external circulation bubble column oxidation device according to claim 1, characterized in that the ratio of the reaction section height to diameter of the bubble column (1) is 7-10. 4. The air-lift external circulation bubble tower oxidation device according to claim 1, characterized in that the diameter of the external circulation pipe (2) is 1/5 to 1/15 of the diameter of the reaction section of the bubble tower. 5. The air-lift external circulation bubble column oxidation device according to claim 1, characterized in that the gas-liquid separator connected to the external circulation pipe is a cylinder whose diameter is 1.5 to 2.5 times the diameter of the external circulation pipe , Aspect ratio of 1.5 to 3. 6. The air-lift external circulation bubble column oxidation device according to claim 1, characterized in that a resistance plate or a valve for adjusting the circulation flow is provided at the connection between the gas-liquid separator and the bubble column or in the external circulation pipe. 7. The air-lift external circulation bubble column oxidation device according to claim 1, characterized in that the gas distributor (3) is an annular, hexagonal perforated plate distributor or multi-tube distributor or other industrial gas distributors. 8. The air-lift external circulation bubble column oxidation device according to claim 1, characterized in that the reflux distributor (4) is a perforated plate-trough shower or a multi-pipe shower. 9. The air-lift external circulation bubble column oxidation device according to claim 1, characterized in that the raw material feed pipe (6) is a single feed pipe or multiple feed pipes distributed along the height of the bubble tower.

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