MX2008010653A - Reducing method of water from reactor outlet gas in the oxidation process of aromatic compound - Google Patents

Abstract

The present invention relates to a method of reducing water from the reactor outlet gas in the oxidation process of an aromatic compound, for example, in the preparation of terephthalic acid by oxidation of xylene in an acetic acid solvent. As water is removed from the reactor outlet gas at the first absorption tower, the amount of water inflow to the dehydration tower is reduced and, thus, the amount of steam required to separate water and acetic acid at the dehydration tower is reduced and the load of the dehydration tower can be decreased. Further, by carefully controlling the flow amount of the reactor outlet gas to the first absorption tower, the operation of the dehydration tower may be made unnecessary.

Description

METHOD OF REDUCTION OF WATER FROM THE GAS OUTPUT OF A REACTOR IN THE PROCESS OF OXIDATION OF AROMATIC COMPOUNDS TECHNICAL FIELD The present invention relates to a method for reducing the water coming from the exhaust gas of a reactor in the process of oxidation of aromatic compounds, by passing the gas from the outlet of the reactor through a first absorption tower and a condenser. As the amount of water influx to the dehydration tower decreases, the amount of steam required to separate water and acetic acid is reduced, and the dehydration tower load can also be reduced.

BACKGROUND OF THE INVENTION In general, the terephthalic acid preparation process comprises an oxidation step of p-xylene with air in the presence of a catalyst such as cobalt, manganese and bromide, and a distillation step to recover the acetic acid solvent of the reactor and to eliminate the water. Typically, conventional distillation, azeotropic distillation, etc., are used to separate and collect acetic acid from water. Figure 1 illustrates the conventional process of collecting acetic acid through azeotropic distillation using an azeotropic agent. With reference to the figure, the conventional apparatus for collecting acetic acid through azeotropic distillation using an azeotropic agent, comprises a dehydration tower (1) for separating acetic acid from water through azeotropic distillation, a condenser (2) to condense the exhaust gas from the top of the dehydration tower (1), an organic-water separation tank (3) to separate the liquid organic materials passing through the condenser (2) of the water, a heater (4) to supply steam to the dehydration tower (1) and a heat exchanger (5) to cool the acetic acid discharged to the bottom of the dehydration tower. This conventional technology is advantageous since, by the addition of an azeotropic agent to a mixture of water and acetic acid, the energy consumption of the dehydration tower (1) can be reduced since the resulting azeotrope boils at a lower temperature than the boiling point of water. However, because the steam supply is necessary to collect the acetic acid, additional energy is required to remove the water. The reactor outlet gas formed during the preparation of the terephthalic acid is hot at 180 ° C or higher, and includes non-compressible gases, for example nitrogen, acetic acid, p-xylene and water. The reactor outlet gas is passed through several heat exchangers containing cooling water for heat exchange, in order to gradually lower the temperature of the reactor outlet gas. The condensed acetic acid and some of the water are returned to the reactor and the remaining water is sent to the dehydration tower for discharge. The gas that has passed through the final heat exchanger includes a small amount of acetic acid and p-xylene. The gas is sent to a high pressure absorption tower, where the p-xylene is collected as it is trapped by the acetic acid, the acetic acid is collected as it is trapped by the water, and the non-condensable gases that include the nitrogen are sent to a gas discharge unit and processed there. The liquid mixture of acetic acid and the water that has passed through the final heat exchanger and has been condensed, is sent to the dehydration tower, where the acetic acid is discharged at the bottom of the dehydration tower and the water is discharged at the top of the dehydration tower.

DESCRIPTION OF THE TECHNICAL PROBLEM OF THE INVENTION Although the method of separating acetic acid from water by gradually lowering the temperature of the gas leaving the reactor using several heat exchangers is advantageous since the steam can be generated from the hot gas leaving the reactor , the consumption of energy is inevitable since the water has to be removed from the acetic acid in the dehydration tower using steam.

Technical Solution The present invention has been made to solve this problem, and an object of the present invention is to provide a method to reduce the water of the gas leaving the reactor in the process of oxidation of the aromatic compounds, without additional energy consumption. To achieve the objective, the present invention provides a method to reduce the water of the gas leaving the reactor in the process of oxidation of an aromatic compound, which comprises: flowing the gas leaving the reactor towards the lower entrance of a first absorption tower in which the tray or packing is equipped as a means to increase the gas-liquid contact surface; the supply of an aromatic compound at the upper entrance of the first absorption tower for the purpose of collecting the carboxylic acid selected from the group consisting of acetic acid, propionic acid and acrylic acid, and included in the reactor outlet gas, and to recover it through the bottom outlet of the first absorption tower; and the discharge of the water included in the reactor outlet gas together with the aromatic compound through the upper outlet of the first absorption tower, condensing the water and the aromatic compound using a condenser, separating the water from the aromatic compound using a Separator of organic compound-water and discharging the water as waste water. The aromatic compound or the aromatic compound and part of the water separated by the organic compound-water separator are recycled to the first absorption tower and the gas discharged from the organic compound-water separator is transferred to a second absorption tower. The aromatic compound is selected from the group consisting of o-xylene, m-xylene, p-xylene, benzene and toluene. Preferably, p-xylene is used.

Advantageous Effects Firstly, since the water is removed from the reactor outlet gas, the amount of water removed in the dehydration tower is reduced, and the amount of steam used to separate water and acetic acid can also be reduced. Second, as the amount of water removed in the dehydration tower is reduced, the dehydration tower load decreases, thereby increasing the capacity of the dehydration tower. Third, since the xylene compounds including p-xylene are used as aromatic compounds, the mixture discharged in the first absorption tower can be recycled to the reactor for the oxidation of the aromatic compounds, without using special separation devices. .

Brief Description of the Figures Figure 1 illustrates the conventional process of acetic acid collection through azeotropic distillation using an azeotropic agent. Figure 2 illustrates the process of reducing water from the reactor outlet gas of the terephthalic acid oxidation process according to one embodiment of the present invention.

BEST METHOD FOR CARRYING OUT THE INVENTION Under the present, a specific detailed description is given of the method of reducing water from the reactor outlet gas in the process of oxidation of aromatic compounds according to the present invention, for a process of reduction of water from the reactor outlet gas in the oxidation process of p-xylene to prepare terephthalic acid. During oxidation p-xylene with air in the presence of a catalyst such as cobalt, manganese and bromide in the process of preparation of terephthalic acid, acetic acid, which is used as a solvent and the water that is generated during the reaction, they are discharged in the form of hot gas together with nitrogen and other gases. The inventor found that, by removing water from the reactor outlet gas, it is possible to reduce the amount of water influx to the dehydration tower, and the amount of steam required to separate the water and the acetic acid in the dehydration tower. Consequently, the loading of the dehydration tower can be reduced or it becomes unnecessary to use the dehydration tower. The liquid mixture of acetic acid, p-xylene and water that is discharged at the bottom outlet of the first absorption tower is recycled to the reactor for the subsequent production of terephthalic acid. Although variable depending on the degree of reaction, the liquid mixture comprises, in general, 10 to 60% by weight of acetic acid, 10 to 60% by weight of p-xylene, and 3 to 40% by weight of water.

In general, terephthalic acid is prepared by the oxidation with air of p-xylene in the presence of a catalyst, as seen in the following reaction scheme 1. Approximately 65 parts by weight of p-xylene are required to prepare 100 parts by weight. weight of terephthalic acid. Figure of Chemistry 1 The water included in the reactor outlet gas is discharged at the upper outlet of the first absorption tower together with the excess p-xylene. The gas including p-xylene and water is condensed by the first heat exchanger in a current of about 1002C, condensed by the second heat exchanger in a current of about 402C, and is transferred to the organic-water separation unit . The p-xylene is separated from the water and collected there and the water is discharged as waste. The gas discharged by the separated organic-water is a gas of approximately 40 SC or less and is transferred to the second absorption tower, which is a high-pressure absorption tower, common and processed there as in the conventional method. The gas leaving the reactor is blown into the first absorption tower directly or passing through several heat exchangers. For example, if the gas passes through 4 heat exchangers, the hot gas from the reactor outlet of 120 to 190 SC that has passed through the second and third heat exchangers, is blown towards the lower entrance of the first absorption tower and the water is removed from the reactor outlet gas as described above. Preferably, the amount of gas leaving the reactor flowing to the lower entrance of the first absorption tower by division is determined depending on the amount of water that is to be removed in the entire terephthalic acid preparation process. The amount of water that is to be removed in the complete preparation process of the terephthalic acid is from 21 to 22 parts by weight per 100 parts by weight of the terephthalic acid produced and is calculated from 2 (moles of water) x 18 ( molecular weight of water) / 166 (molecular weight of terephthalic acid) x 100 (amount of terephthalic acid). For example, if 500,000 tons of terephthalic acid are produced per year and the amount of production per hour is 62.5 tons, the amount of water required per hour becomes 13.5 tons. Provided that the air supplied for the reaction comprises 23% oxygen and 77% nitrogen, and that all the supplied air is oxidized, the required amount of oxygen is 3 (moles of oxygen) x 32 (molecular weight of the oxygen) / 166 (molecular weight of terephthalic acid) x 62.5 (production of terephthalic acid per hour) = 36 tons and the amount of nitrogen discharged is 36 x 77/23 = 121 tons. However, considering the oxidation efficiency of the reactor and the evaporation of liquid caused by the heat of reaction, the quantity of the total gas leaving the reactor is determined between 300 and 700 tons. Preferably, the means for increasing the gas-liquid contact surface are constructed in the form of trays or packings. The terms and words used in this specification and in the claims do not have to be interpreted in common or literal meanings. Based on the principle that an inventor can adequately define a meaning of the terms and words to better describe his own invention, they would be interpreted in the meaning and context conforming to the spirit of the present invention.

Accordingly, the embodiment presented in this description and the appended figures is only one example of the most preferred embodiment of the present invention. It will be appreciated by those skilled in the art that changes and modifications may be made without departing from the principles and spirit of the present invention, the scope of which is defined in the appended claims and their equivalents. Figure 2 illustrates the process to reduce the water of the reactor outlet gas, in the process of oxidation of terephthalic acid according to one embodiment of the present invention. With reference to Figure 2, the apparatus for removing water from the preparation of terephthalic acid according to one embodiment of the present invention comprises a reactor (100), a first absorption tower (110), a first heat exchanger ( 120), a second heat exchanger (130), an organic-water separator (140) and a second absorption tower (150). In one embodiment of the present invention, the first absorption tower (110) may be equipped with a means for increasing the gas-liquid contact surface, which may be constructed in the form of trays or packings, but the present invention is not limited by these.

The first heat exchanger (120) cools the hot gas discharged into the upper outlet of the first absorption tower (110). The first heat exchanger (120) can produce a low temperature steam of about 100 ° C. The second heat exchanger (130) condenses the gas that has passed through the first heat exchanger (120) and separates it into a liquid mixture of water and p-xylene and a nitrogen-containing gas. From the liquid mixture, the p-xylene is separated by the organic-water separator and the water is processed as the waste water through a pipe (L7). And the gaseous material is transferred to the second absorption tower (150), a common high pressure absorption tower, and processed there. Next, the process of removing water from the gas leaving the reactor in the process of preparing the terephthalic acid according to an embodiment of the present invention will be described in detail. First, a reactor outlet gas, discharged from a reactor (100) for the preparation of terephthalic acid, is flowed to the lower entrance of a first absorption tower (110) via a tube (L2) directly, and which passes through several heat exchangers.

The reactor outlet gas is cooled to 120-1902C by a heat exchanger (not shown) before flowing to the first absorption tower (110). The reactor outlet gas comprises nitrogen, acetic acid, water and a small amount of organic materials. In general, the reactor outlet gas comprises 60 to 95% by weight of nitrogen, 1 to 18% by weight of acetic acid, 2 to 36% by weight of water and a small amount of organic material, which can be acetate. methyl, p-xylene, etc. At the upper entrance of the first absorption tower (110), the p-xylene aromatic compound is flowed inwardly by means of the pipes (L4, IOL). The amount of p-xylene is preferably 2 to 5 equivalents by weight of the water to be removed. If the amount of p-xylene is outside this range, the acetic acid can be discharged at the top of the absorption tower along with the gas, without being sufficiently collected or the excess p-xylene can be discharged at the bottom of the absorption tower, which results in certain problems. Of the pipes (L4, IOL) through which the p-xylene is flowed towards the upper entrance of the first absorption tower (110), the pipe (L4) is one through which a stream from the organic-water separator (140) to be described later. Next, the reactor outlet gas flowing to the lower inlet of the first absorption tower (110) rises towards the top of the first absorption tower (110) and the p-xylene is flowed to the upper entrance of the first absorption tower (110) and descends towards the bottom of the first absorption tower (110). Within the first absorption tower (110) there is a means to increase the gas-liquid contact area, built in the form of trays or packaging. Passing through the trays or packings, the reactor outlet gas makes contact with p-xylene, during which process acetic acid and some of the water included in the reactor outlet gas is absorbed by p-xylene and condensate to be collected at the bottom of the first absorption tower (110). In this process, p-xylene acts as the solvent that absorbs the acetic acid and the water included in the gas leaving the reactor and, therefore, it is possible to recycle the liquid mixture discharged at the bottom of the first absorption tower (110) to the reactor (100) for the preparation of the terephthalic acid via the pipe (L9), without the need for solvent removal. And if the amount of p-xylene included in the liquid mixture discharged at the bottom of the first absorption tower is greater than the amount of p-xylene required for the preparation of the terephthalic acid, then the p-xylene included in the liquid mixture can be separated by an organic-water separator to recycle it towards the upper entrance of the first absorption tower (110). Water that has not been condensed at the bottom of the first absorption tower (110) is discharged to the upper outlet of the first absorption tower (110) together with the excess p-xylene, and is condensed by the condensers ( 120, 130), which can be heat exchangers. In the organic-water separator (140), the p-xylene is separated from the water and the water is processed as waste water. In this process, the gas is discharged by the organic-water separator (140), which comprises nitrogen, acetic acid and p-xylene, is transferred to the second absorption tower (150), a conventional high-pressure absorption tower, and processed there. Subsequently, a mixture comprising 50 to 90% by weight of nitrogen, 5 to 30% by weight of p-xylene, 2 to 15% by weight of water, 5 to 500 ppm of acetic acid and a small amount of organic materials is discharged from the upper outlet of the first absorption tower (110) via a pipe (L5). The mixture is cooled while passing through the first heat exchanger (120).

The temperature of the mixture flowing to the first heat exchanger (120) is from about 110 ° C to 180 ° C. The temperature of the mixture that has been cooled while passing through the first heat exchanger (120) is about 1002C. In the first heat exchanger (120), a low pressure steam can be generated. From the mixture that has been cooled while passing through the first heat exchanger (120), the water and p-xylene included in the mixture are condensed as the mixture passes through the second heat exchanger (130) . At the end, the mixture that has passed through the second heat exchanger (130) is separated by the organic-water separator (140). In the organic-water separator (140), the nitrogen, the uncondensed p-xylene and a small amount of acetic acid are collected towards the second absorption tower (150), which is a conventional high-pressure absorption tower, in gaseous state via a pipe (L8) and the water is transferred to a waste water processing system by means of a pipe (L7). The p-xylene separated by the organic-water separator (140) is recycled to the upper part of the absorption tower (110) via the pipe (L4).

Also, although not illustrated in the figure, some of the water can be recycled to the first absorption tower (110) by means of another pipeline, in order to decrease the concentration of the acetic acid that is discharged by the organic separator. water (140), together with the water. According to the present invention, the amount of water that is to be removed in the dehydration tower is decreased because some of the water is discharged at the upper outlet of the first absorption tower (110). Therefore, the energy consumption by the dehydration tower is reduced. In addition, by carefully controlling the amount of gas flow from the reactor to the first absorption tower (110), the operation of the dehydration tower can be made unnecessary.

Modality for the Invention The practical and currently preferred embodiments of the present invention are illustrative as shown in the following example. However, it may be appreciated by those skilled in the art, in consideration of this disclosure, that modifications and improvements may be made within the spirit and scope of the present invention.

EXAMPLE The flow-in and flow-out conditions in the first absorption tower are given in Table 1 below. The absorption tower had an internal diameter of 40 mm and the means to increase the gas-liquid contact area were constructed in the form of a random packing with a height of 1.5 m. In Table 1, PX means p-xylene and KGa means kg / cm2 (absolute pressure).

Table 1 Flow inlet and outlet conditions in the absorption tower As shown in table 1, 189.9 g / hr of water (H20) were flowed through the pipeline (L2) and 112.3 g / hr of water were discharged into the bottom of the first absorption tower (110) via the pipe (L9) and collected in the reactor (100). Consequently, the amount of water that is going to be removed in the dehydration tower is reduced by 77.6 g / hr and the amount of steam that will be used by the dehydration tower decreases. In this way, the energy consumption and the load of the dehydration tower decrease.

Possibility of Industrial Application As is apparent from the foregoing description, the method for reducing water from a reactor outlet gas in the aromatics oxidation process according to the present invention, decreases the amount of steam required to separate the water and acetic acid in the dehydration tower, and the loading of the dehydration tower. In addition, by carefully controlling the amount of gas flow from the reactor outlet to the first absorption tower, the operation of the dehydration tower can be made unnecessary. Those skilled in the art will appreciate that the specific concepts and embodiments described in the foregoing description can be readily used as a basis for modifying or designing other embodiments to accomplish the same purposes of the present invention. Those skilled in the art will also appreciate that such embodiments and equivalents do not depart from the spirit and scope of the present invention as described in the appended claims.

Claims (7)

1. A method for reducing the water of the gas leaving the reactor in the oxidation process of an aromatic compound, characterized in that it comprises; flowing the gas leaving the reactor towards the lower entrance of a first absorption tower in which a means is provided for increasing the gas-liquid contact surface; the provision of an aromatic compound at the upper entrance of the first absorption tower, for the purpose of collecting the carboxylic acid selected from the group consisting of acetic acid, propionic acid and acrylic acid, and included in the reactor outlet gas at through the lower outlet of the first absorption tower; and the discharge of the water included in the reactor outlet gas, together with the aromatic compound, through the upper outlet of the first absorption tower, the condensation of the water using a condenser, the separation of the water from the aromatic compound using a organic separator-water and the discharge of water as waste water.

2. The method for reducing the water of a gas leaving the reactor in the oxidation process of an aromatic compound according to claim 1, characterized in that the mixture of carboxylic acid, aromatic compound and water discharged at the lower outlet of the The first absorption tower is recycled to the reactor for the preparation of terephthalic acid.

3. The method for reducing the water of a gas leaving the reactor in the oxidation process of an aromatic compound according to claim 1, characterized in that the aromatic compound or the aromatic compound and some of the water separated by the organic separator- Water is recycled to the first absorption tower, and the gas discharged by the organic-water separator is transferred to a second absorption tower.

4. The method for reducing the water of a gas leaving the reactor in the oxidation process of an aromatic compound according to claim 1, characterized in that the gas leaving the reactor that is flowed to the first absorption tower is a hot gas of 120 to 1902C and is flowed directly or through several heat exchangers.

5. The method of reducing the water of a gas leaving the reactor in the process of oxidation of an aromatic compound according to claim 1, characterized in that the quantity of gas leaving the reactor that is flowed towards the first tower of absorption by division, corresponds to the amount of water that is going to be removed from the oxidation reaction.

6. The method of reducing the water of a gas leaving the reactor in the oxidation process of an aromatic compound according to claim 1, characterized in that the means for increasing the gas-liquid contact surface are constructed in the form of trays or packaging.

7. The method of reducing the water of a gas leaving the reactor in the oxidation process of an aromatic compound according to any of claims 1 to 6, characterized in that the aromatic compound is selected from the group consisting of o-xylene , m-xylene, p-xylene, benzene and toluene.