JPH0345057B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a corresponding unsaturated carboxylic acid by catalytic gas phase oxidation in the presence of water vapor using an olefin or unsaturated aldehyde having three or more carbon atoms in one molecule as a raw material. In particular, the purpose is to reduce the amount of raw steam and cooling water required at that time by using waste heat, and to reduce the amount of wastewater containing organic matter to facilitate post-treatment. be. The main raw materials are propylene and acrolein having 3 carbon atoms, isobutylene having 4 carbon atoms, methacrolein, and the like. Catalytic gas phase oxidation yields acrylic acid from propylene or acrolein, methacrylic acid from isobutylene or methacrolein, and so on.
Reactions that produce unsaturated carboxylic acids corresponding to the raw materials used are well known. In the following explanation, the case of producing acrylic acid from propylene will be described as a representative example, but the present invention is of course applicable to other cases as well. When producing acrylic acid by catalytic gas-phase oxidation of propylene, it is natural to use a catalyst with excellent conversion rate and selectivity, but it is also necessary to use a catalyst with excellent conversion rate and selectivity. In order to increase safety and selectivity to the target product, acrylic acid, a method in which a large amount of water vapor is present in the raw material gas has been used. This method is useful both in the case of directly producing acrylic acid from propylene in one stage, and in the case of a two-stage reaction in which acrolein is produced from propylene in the first stage and acrylic acid is produced from acrolein in the second stage. The amount of water vapor added is 1 to 1 molar ratio to the raw material propylene.
It is said to be about 15 times, preferably 2 to 7 times, and accounts for a considerable proportion of the utility cost. On the other hand, since the acrylic acid produced is a highly polymerizable substance, it is necessary to immediately quench the reaction product gas in order to prevent polymerization of the acrylic acid. Usually, a method is used in which the reaction product gas is cooled to a temperature close to its dew point, then introduced into a quenching tower and brought into countercurrent contact with water. At this time, the gas is rapidly cooled with water, and at the same time the water vapor in the gas is condensed, and the generated acrylic acid is recovered in a state dissolved in the condensed water. In the next purification step, most of the water is first removed from this condensed water by azeotropic distillation, solvent extraction, etc., and then further purified by distillation to obtain a product. Catalytic gas phase oxidation is a significantly exothermic reaction, and in the quenching tower, a part of this reaction heat must be removed together with the latent heat of the steam added for the above-mentioned reason and the steam generated by the oxidation reaction. The bottom liquid temperature of the quenching tower is 90℃ to prevent polymerization of acrylic acid.
It is desirable to keep the amount of acrylic acid below, but if you try to remove that much heat with just newly supplied water, not only will you need a huge amount of water, but the concentration of acrylic acid in the obtained condensed water will be diluted, and the remaining Since this places a burden on the purification process, the conventional method is to cool the condensed water with an indirect cooler and then circulate it to a quenching tower. The above-mentioned conventional method will be explained with reference to the attached FIG. Note that in the accompanying drawings, illustrations of the pump and compressor are omitted, and the flows of liquid and gas are indicated by arrows. 1 and 2 are contact gas phase oxidation reactors, 3 is a quenching tower, and 4 is an indirect cooler for condensed water (quenching tower bottom liquid). If necessary, an intermediate liquid indirect cooler 5 of the quenching tower may be provided. 6, 7, and 8 are supply lines for air (molecular oxygen-containing gas), water vapor, and propylene, respectively. The raw material gas, which is a mixture of these three components, undergoes catalytic gas phase oxidation in reactors 1 and 2 to become a high-temperature product gas containing acrylic acid, which is produced in the product gas indirect cooler 21.
After being cooled to a temperature close to the dew point of the gas, it is sent to the lower part of the quenching tower 3. The water supplied from the water supply line 9 to the upper part of the quenching tower 3 comes into countercurrent contact with the high temperature produced gas to cool it, absorbs and dissolves acrylic acid in the produced gas, and passes through the condensed water discharge line 1
0 to the outside of the quenching tower. 11 is an off-gas discharge line. A portion of the high-temperature condensed water discharged through line 10 is sent to the purification process through line 12, while the rest is branched to line 13, cooled by indirect condensed water cooler 4, and circulated to quench tower 3. .
This cooler 4 may be located not on the branch line 13 but on the main condensate discharge line 10. The condensate sent to the purification process first removes most of the water, and then goes to a higher-order purification unit to reduce the boiling point and
High boiling point impurities are removed to produce purified acrylic acid. Methods for removing most of the water include azeotropic distillation and solvent extraction. Figure 1 is a conceptual diagram of the solvent extraction method, in which condensed water sent from line 12 to extraction tower 14 comes into contact with the extractant from line 15, and acrylic acid is transferred to the extractant layer. The extractant separated therefrom is sent to a purification unit 16 and recycled to line 15. The aqueous layer separated in the extraction tower 14 has the dissolved extractant recovered in a stripping tower 17, and is then discharged as waste water through a line 18 and sent to a post-treatment process.
The recovered extractant is recycled via lines 19 and 15. Line 20 is purified acrylic acid. The wastewater from line 18 contains by-products of the catalytic gas phase oxidation reaction, such as formaldehyde, acetic acid, maleic acid, etc., and a small amount of acrylic acid, but this contains water vapor added to the reaction system and reaction product water.・Since almost all of the quenched water is collected, the amount of water is large and the concentration of organic matter is low. Therefore, when using an underwater combustion method as a post-treatment to render this wastewater harmless, it is necessary to use a large amount of combustion improver. There is an inconvenience that it is necessary. This also applies to the case where condensed water is collected in an azeotropic distillation column to recover acrylic acid at the bottom of the column instead of the solvent extraction method shown in the example. The aqueous solution containing the product is discharged as wastewater. In addition, in the above method in which high-temperature condensed water is cooled using cooling water from outside the system in the indirect cooler 4, most of the reaction heat, the steam added to the reaction system, and the latent heat of the steam generated in the oxidation reaction end up being Since the amount of water is transferred to the cooling water of the indirect cooler, the amount of water is still enormous, but the temperature of the cooling water after heat exchange does not become so high because the bottom temperature of the quenching tower is kept below 90°C. In other words, although the total amount of heat energy is large, the potential is low, so it cannot be used effectively and has no choice but to be discarded. The present invention overcomes the above-mentioned drawbacks of the conventional methods, namely the energy consumption, which requires, on the one hand, a large amount of water vapor to be supplied and, on the other hand, a large amount of cooling water for heat removal.
The purpose is to reduce utility costs by eliminating waste in terms of balance and the waste of generating a large amount of waste water and consuming additional energy for subsequent treatment. The structure of the present invention involves indirect heat exchange between high-temperature condensed water produced by cooling and/or absorption of the gas produced by the catalytic gas phase oxidation reaction with water for humidity control, and heated humidity control water. After conditioning the humidity by contacting the water with a molecular oxygen-containing gas for reaction, the conditioned gas is fed into the reaction system, and the remaining liquid after separating useful components from the condensed water is passed through a reverse osmosis membrane. The method is characterized in that the permeated water obtained by the treatment is used as the humidity control water. The present invention will be explained with reference to the attached FIG. However, explanations of parts that overlap with the conventional method shown in FIG. 1 will be omitted. The high-temperature product gas containing acrylic acid resulting from the catalytic gas phase oxidation reaction in the reactors 1 and 2 is cooled to a temperature close to the dew point of the gas in the product gas indirect cooler 21, and then sent to the lower part of the cooling tower 3. The high temperature condensed water produced by countercurrent contact with water from line 9 is transferred to line 1.
0 and 13 to the indirect heat exchanger 30, where it exchanges indirect heat with humidity control water having a low temperature. The heated humidity control water is sent to the humidity control tower 23 via the line 22.
It is brought into countercurrent contact with reaction air (molecular oxygen-containing gas) fed from line 6'. Note that the contact between the humidity control water and the air does not necessarily have to be in countercurrent flow, and may be in cocurrent flow or cross flow. In this process, the humidity control tower 23
The air flowing out is conditioned to a saturated water vapor pressure corresponding to its temperature. After the humidity of the reaction air is adjusted in this manner, it is fed into the reaction system via line 24 together with propylene from line 8. The humidity control water, whose temperature has been lowered by contacting air in the humidity control tower and being deprived of latent heat of vaporization, is extracted from the bottom of the humidity control tower and circulated to the indirect heat exchanger 30 via line 22' to be reheated. Reference numeral 25 denotes a water supply line for humidity control, but it does not necessarily have to be located at the position shown in the figure, and may be provided at any position in the water circulation system for humidity control. The amount of water vapor added to the reaction air in the humidity control tower to control the humidity varies depending on the temperature and amount of air flowing out from the humidity control tower, but as shown below, it is several times the molar ratio to the raw material propylene. Therefore, there is no need to further add water vapor from the outside.

[Table] That is, the theoretical amount of oxygen required to convert all 1 mole of propylene into acrylic acid is 1.5 moles, so the amount of air is approximately 7.1 moles. Therefore, the amount of water vapor entrained by the humidity control is 7.1 times the number in the last column of Table 1, and the amount of water vapor is 50℃ and 1 mole of propylene.
Approximately 1.0, 1.7, 3.2 at 60℃, 70℃, and 80℃, respectively.
It becomes 6.3 moles. Since gas phase catalytic reactions usually use a slightly larger amount of air than the theoretical amount, it is clear that a humidity control tower alone can supply the necessary amount of water vapor. In this way, while the humidity control tower plays the role of supplying the necessary water vapor to the reaction air, from the other side, it serves as a recooling tower for the humidity control water that is working as a cooling medium for the condensed water in the indirect heat exchanger 30. It will also play a role. As the indirect heat exchanger used here, since a sufficient temperature difference cannot be taken, it is preferable to use a countercurrent type heat exchanger such as a plate heat exchanger or a spiral heat exchanger. Various types of humidity control tower structures can be used, such as packed towers, plate towers, and lattice towers.
It is best to select and use one with as little resistance as possible. Even when the raw materials or reaction products are different, or when the purification methods are different, the residual liquid after separating useful components from the condensed water is sent to the reverse osmosis membrane treatment device 26 via the line 18. Before that, if necessary, the residual liquid is cooled to below the upper limit temperature for use of the reverse osmosis membrane using the indirect cooler 27. As the reverse osmosis membrane, for example, polyether-based, polybenzimidazolone-based, polyacrylonitrile-based, etc. are used. This permeated water is used as replenishment water for humidity control in the line 25. In this way, a cycle of water for humidity control - steam added to the reaction system - condensed water - residual liquid from the purification system - permeated water - water for humidity control can be created, and in terms of calorific value, the heat of condensation of the added steam in the quench tower and humidity control can be created. Since the heat of vaporization of the humidity control water in the tower is balanced, the addition of moisture from outside the system and the addition and removal of heat can be reduced, resulting in savings in utility and reduction in the amount of waste water. Most of the organic substances can be removed by treating the residual liquid from the acrylic acid purification system with a reverse osmosis membrane. The main organic substances in the obtained permeated water are acetic acid and formalin, but by repeating the number of reverse osmosis membrane treatments, it is possible to obtain permeated water that is even closer to pure water. The present inventors investigated the effects on the oxidation reaction of propylene and the acrylic acid purification system when acetic acid and formalin were mixed into humidity control water, and found that more than 90% of formalin was burned and disappeared in the oxidation reactor. It was also found that low concentration acetic acid was discharged from the oxidation reactor with almost no effect on the oxidation catalyst. However, if the ratio of acetic acid supplied from humidity conditioning water to the oxidation reaction system increases compared to the acetic acid produced by side reactions from propylene, the utility required to remove acetic acid in the acrylic acid purification system increases. There is. The optimal concentration of acetic acid in humidity conditioning water purified by reverse osmosis membrane treatment is 1.0% by weight or less, preferably 0.5% by weight or less. Non-permeated water in reverse osmosis membrane treatment is sent as wastewater to a post-treatment process for detoxification through line 28.
The concentration of organic matter is approximately 10 to 20% by weight, which is several times higher than when directly treating wastewater without reverse osmosis membrane treatment, so when using the underwater combustion method for detoxification, the amount of combustion improver is significantly reduced. A reduction can be expected. In addition to the invention detailed above, as shown by line 29 (dotted line) in FIG. A part of the permeated water thus obtained may be used as quenching water for a quenching tower. In this way, no water enters the system from outside the system, so that ultimately only the reaction product water of the catalytic gas phase oxidation must be treated as wastewater. The idea of supplying the necessary water vapor to the reaction system by bringing reaction air into contact with hot water is described in JP-A-51-103664, but this method uses acrylic acid as hot water. Wastewater from the purification process is used as is, and high-boiling side reaction products are condensed in the wastewater, and when they come into contact with air, they are entrained in the air and react depending on their respective partial pressures. Since it circulates through the system, it tends to have an unfavorable effect on reaction control.The available heat is only the sensible heat of the wastewater, which is not necessarily sufficient to obtain the required amount of steam, and requires the introduction of an additional heat source or live steam. There are drawbacks such as: This is because the emphasis of this invention is on the concentration of wastewater, so saving on utilities has become secondary. In contrast, in the present invention, by exchanging heat with the condensed water of the quench tower, a part of the reaction heat and the latent heat of the steam supplied to the reaction system and the steam generated by the oxidation reaction are indirectly utilized. , there is no shortage of heat. Example 1 The reaction product gas obtained by the oxidation reaction of propylene with air was cooled with water in a quenching tower, and the resulting acrylic acid aqueous solution was extracted using isobutyl acetate as an extractant. The acid was separated and recovered. The aqueous layer separated by extraction was passed through a stripping tower to collect dissolved isobutyl acetate at the top of the tower, and the bottom liquid was purified by reverse osmosis membrane treatment. The reverse osmosis membrane is a synthetic composite membrane consisting of a polysulfone support layer and an ultra-thin polyether film layer.
110) was used. The permeated water obtained by performing reverse osmosis membrane treatment at a temperature of 25° C. and a pressure of 70 Kg/cm ² (gauge) was further subjected to reverse osmosis membrane treatment to purify the steam tower bottom liquid, that is, the residual liquid from the acrylic acid purification system. The results are shown in Table 2.

[Table] According to the results in Table 2, the removal rate (absolute value) of high boiling point impurities in permeated water reached 86% for acetic acid and 98.5% for maleic acid. In addition, non-permeable water contains 11 organic substances.
% by weight, and by using a small amount of combustion improver, it has become possible to process by underwater combustion method. The above permeated water was used as humidity control water. An oxidation reaction of propylene was carried out according to the flow shown in FIG. The humidity control tower 23 is at normal pressure, and the permeated water is passed through the reverse osmosis membrane treatment device 26 from the top of the tower at a rate of 12.2% per hour.
Kg, and air was supplied from the bottom of the tower at 31.5 Nm ³ per hour. In order to control the temperature of the humidity-controlled air from the top of the tower to 71°C, 500 kg of water is extracted from the bottom of the tower per hour and supplied to the indirect heat exchanger 30 of the quenching tower to exchange heat with the acrylic acid aqueous solution. circulated to the top of the tower. The temperature of the circulating water is
It was 76℃. The humidity control tower, indirect heat exchanger, and related lines were sufficiently kept warm to prevent temperature drop due to heat radiation. Hourly amount of humidity-controlled air coming out of the humidity-control tower
46.5Nm ³ is boosted by compressor and 6.55Kg/hour
of propylene and fed to the first reactor.
Analysis of the mixed gas revealed propylene 7.0 and water.
It was 30.0% by volume. The outlet gas of the second reactor
After cooling to 200°C, it was supplied to the lower part of the quenching tower 3. The quenching tower was operated at a bottom temperature of 85°C and an off-gas temperature of 35°C, and the bottom liquid was supplied to an indirect heat exchanger 30, cooled to 66°C, and circulated to the middle stage of the quenching tower. The amount of circulating fluid was 460 kg/hour. In the quenching tower, in order to further cool the reaction gas and recover acrylic acid, the liquid is extracted from the middle stage of the quenching tower and guided to the intermediate liquid indirect cooler 528.
After cooling with cooling water at ℃, it was circulated to the top of the tower. The amount of circulating liquid is 360Kg/hour, the outlet temperature of the intermediate liquid indirect cooler is 32℃, and the temperature of the off-gas discharged from the quenching tower is
At 35°C, there was virtually no acrylic acid present. The bottom liquid of the quenching tower was distilled at a rate of 24.4 kg per hour and contained 40.6% by weight of acrylic acid. The cooling water supplied to the intermediate liquid indirect cooler was 360 kg/hour. Continuous operation was carried out in this manner for one month, but the oxidation catalyst was not affected and continuous operation was possible. Comparative Example 1 A humidity control tower was not used, and steam at a gauge pressure of 7 Kg/cm ² was used to supply steam to the reactor. The two indirect coolers of the quenching tower were cooled with cooling water at 28°C, and the reaction product gas was cooled and condensed to recover an aqueous acrylic acid solution. Other conditions were the same as in Example 1. At this time, the cooling water consumption of the quenching tower was 780 kg/hour, which was about 2.2 times that of Example 1. Table 3 shows a comparison of the reaction raw material steam and cooling water consumption of the quenching tower for Example 1 and Comparative Example 1. The economic advantages of the process according to the invention in industrial implementation are obvious.

【table】 [Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a conventional method, and FIG. 2 is an explanatory diagram showing an example of the method of the present invention. 1...First reactor, 2...Second reactor, 3...
Quenching tower, 4... Condensed water indirect cooler, 5... Quenching tower intermediate liquid indirect cooler, 6, 6'... Air supply line,
7...Steam supply line, 8...Raw material supply line, 9...Quick cooling water supply line, 10...Condensed water discharge line, 11...Off gas discharge line, 12
... Purification system transfer line, 13 ... Condensed water circulation line, 14 ... Extraction column, 15 ... Extractant circulation line, 16 ... Purification unit, 17 ... Stripping column, 18 ... Waste water line, 19 ... Recovery Extractant circulation line, 20...Product (purified acrylic acid),
21... Produced gas indirect cooler, 22, 22'...
Humidity control water circulation line, 23... Humidity control tower, 24...
Humidity control air supply line, 25...Humidity control water supply line, 26...Reverse osmosis membrane treatment device, 27...Indirect cooler, 28...Non-permeated water (reverse osmosis membrane treated wastewater),
29...Quick cooling water circulation line, 30...Indirect heat exchanger.

Claims (1)

[Claims] 1 by molecular oxygen-containing gas in the presence of water vapor.
A method for producing a corresponding unsaturated carboxylic acid by catalytic gas phase oxidation of an olefin or unsaturated aldehyde having 3 or more carbon atoms in the molecule,
The high-temperature condensed water produced by cooling and/or absorbing the gas produced by the catalytic gas phase oxidation reaction with humidity control water is indirectly heat-exchanged, and the heated humidity control water is injected with molecular molecules for reaction. After conditioning the humidity by contacting oxygen-containing gas, the conditioned gas is sent to the reaction system, and the residual liquid after separating useful components from the condensed water is treated by a reverse osmosis membrane method. A method characterized in that the permeated water is used as the moisture conditioning water.