JPH0345056B2 - - 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 utilizing waste heat. The main raw materials include propylene and acrolein having 3 carbon atoms, isobutylene and methacrolein having 4 carbon atoms, 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 proprene, 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 the condensed water by azeotropic distillation or solvent extraction, and then further purified by distillation to produce 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 and the steam generated in the oxidation reaction for the above-mentioned reasons. It is desirable that the temperature of the liquid at the bottom of the quenching tower be below 90°C to prevent polymerization of acrylic acid, but if we were to remove that much heat with just newly supplied water, a huge amount of water would be required. Not only that, but the concentration of acrylic acid in the obtained condensed water becomes diluted, which puts a burden on the subsequent purification process. Therefore, the usual method is to cool the condensed water with an indirect cooler and then circulate it to the quenching tower. It is taken. 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 the reactors 1 and 2 and becomes a high-temperature product gas containing acrylic acid, which is produced in the product gas indirect cooler 14.
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 rapid cooling 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. In this method, part of the reaction heat and most of the latent heat of the steam added to the reaction system and the steam generated in the oxidation reaction are transferred to the cooling water of the indirect cooler 4, so the amount of water used is still enormous. On the other hand, since the bottom temperature of the quenching tower is kept below 90°C, the temperature of the cooling water after heat exchange does not become so high. In other words, although the amount of total heat energy was large, the potential was low, so it could not be used effectively and the company had no choice but to go out of business. By effectively utilizing waste heat, the present invention solves the above-mentioned drawbacks of the conventional method, namely, the energy balance problem of supplying a large amount of steam and, on the other hand, requiring a large amount of cooling water for heat removal. The purpose is to eliminate waste and reduce utility costs. The structure of the present invention is to indirectly heat exchange the high temperature condensed water produced by cooling and/or absorbing the gas produced by the catalytic gas phase oxidation reaction with the humidity control water, and to generate the heated humidity control water. The method is characterized in that a molecular oxygen-containing gas for reaction is brought into contact with the reactor to adjust the humidity, and then the humidity-adjusted gas is fed into the reaction system. 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 14, and then sent to the lower part of the cooling tower 3, and then passed through the line. The high temperature condensed water produced by countercurrent contact with water from line 9 is transferred to line 1.
The water is sent to an indirect heat exchanger 19 via water pipes 0 and 13, 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 16 via line 15.
It is brought into countercurrent contact with the reactor air (molecular oxygen-containing gas) fed through 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 16
The air flowing out is conditioned to a saturated water vapor pressure corresponding to its temperature. After the humidity of the air for reaction is adjusted in this manner, it is fed into the reaction system via line 17 together with propylene from line 8. The humidity control water, whose temperature has decreased due to contact with air in the humidity control tower and the latent heat of vaporization is removed therefrom, is extracted from the bottom of the humidity control tower, and is circulated through the line 15' to the indirect heat exchanger 19 for reheating. Reference numeral 18 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. As water for humidity control, not only city water but also any water that leaves little evaporation residue and does not contain volatile components harmful to the reaction can be used. 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 add water vapor from 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 at 50°C and 1 mole of propylene is
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 19. 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. 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 experiment was carried out according to the flow shown in FIG. The humidity control tower 16 was at normal pressure, and 12.1 kg of water was supplied per hour from the top of the tower, and 31.5 Nm ³ of air was supplied per hour from the bottom of the tower. In order to control the temperature of the humidified air from the top of the tower to 71°C, air is released every hour from the bottom of the tower.
Take out 500 kg of water and transfer it to the quenching tower indirect heat exchanger 19
The mixture was supplied to the acrylic acid aqueous solution for heat exchange with the aqueous acrylic acid solution, and then circulated to the top of the humidity control tower. The temperature of the circulating water was 76°C. Particular attention was paid to keeping the humidity control tower, indirect heat exchanger, and related lines warm to prevent a drop in temperature due to heat radiation. Saturated air coming out of the humidity control tower every hour
After increasing the pressure by a compressor at a rate of 46.5 Nm ³ , it was mixed with 6.55 Kg of propylene per hour and fed to the first reactor. Analysis of the mixed gas revealed that it contained 7.0% propylene and 30.0% water by volume. The outlet gas of the second reactor was cooled to 200°C and then supplied to the quenching tower 3. The quenching tower is operated at a bottom temperature of 85°C and an off-gas temperature of 35°C, and the bottom liquid is supplied to the indirect heat exchanger 19, and the temperature is 66°C.
It was cooled to a temperature of 100% and then 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 5.
After cooling with cooling water at 28°C, it was circulated to the top of the tower. The amount of circulating liquid was 300 kg/hour, the outlet temperature of the intermediate liquid indirect cooler was 32°C, the temperature of the off-gas discharged from the quenching tower was 35°C, and it contained virtually no acrylic acid. 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 2 shows a comparison of the consumption of reaction raw material steam and cooling water 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 ... Produced gas indirect cooler, 15, 15'
... Humidity control water circulation line, 16 ... Humidity control tower, 17
... Humidity control air supply line, 18 ... Humidity control water supply line, 19 ... 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. A method characterized by controlling the humidity by bringing an oxygen-containing gas into contact with the oxygen-containing gas, and then supplying the humidity-controlled gas to a reaction system.