JPS6133017B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for producing unsaturated nitriles by the ammoxidation reaction of olefins, such as a process for producing acrylonitrile or methacrylonitrile by the ammoxidation reaction of propylene or isobutylene. The purpose is to propose an extremely energy-efficient manufacturing method that converts the relatively low-temperature sensible or latent heat generated in the manufacturing system into a cold or heating source and uses it effectively.

The process for producing unsaturated nitriles by the ammoxidation reaction of olefins involves cooling the high-temperature reactants with water and adding a large amount of water (for example,
According to No. 41-6571, 17.8 for acrylonitrile.
The unsaturated nitrile is obtained by separating and purifying the unsaturated nitrile from side reactants and unreacted products by repeating heating distillation and cooling condensation of the absorbed liquid. Therefore, the energy required for heating and cooling is extremely large, and the energy cost is high.

The conventional process will be specifically explained below using an example of producing acrylonitrile.

In the presence of an oxidation catalyst in reactor 1, continuously supplied propylene, ammonia, and air undergo an ammoxidation reaction in a high-temperature gas phase, and reaction products such as acrylonitrile, acetonitrile, hydrocyanic acid, and high-boiling organic substances, and unreacted A mixture of substances is produced.
This high-temperature gaseous mixture is sent through a heat exchanger 2 to a quenching tower 3, and is brought into contact with circulating water for cooling flowing down from the top of the tower via a line 4 to be cooled to a low-temperature gas. In this cooling process, high-boiling organic substances and produced water are taken into the circulating water and removed, and if necessary, mineral acid is added to the circulating water to remove unreacted ammonia in the reaction gas. The heated circulating water flows out from the bottom of the tower through a line 5, is cooled by a cooler 6, and is again circulated and supplied to the top of the tower through a line 4, and a portion is discharged to the outside of the system.

The gas introduced into the lower part of the absorption tower 8 from the top of the quenching tower 3 through the line 7 comes into contact with the absorption water supplied through the line 9 flowing down inside the tower from the top of the tower, and produces acrylonitrile, acetonitrile, Hydrocyanic acid is absorbed into absorbed water. This absorption liquid is withdrawn from line 10 as a bottom liquid, and unabsorbed gas is discharged from the top of the column via line 11.

The absorption liquid passes through a heat exchanger 12 to raise its temperature, and then passes through a recovery tower 1.
Reboiler 1 is supplied to Reboiler 3 from a position slightly above it.
It is heated at 4. The recovery column usually has at least 50 trays, preferably 60 to 100 trays, and performs extractive distillation in countercurrent contact with solvent water fed through line 15 to the top of the column, and crude oil containing hydrocyanic acid and water is removed from the top of the column. Acrylonitrile vapor is distilled out via line 16, and then via line 15 to acetonitrile stripping tower 1.
The solvent water stream that does not pass through 8 is withdrawn, cooled through a heat exchanger 12 and fed to the absorption column 8 via line 9 as absorption water. Further, solvent water is extracted from the bottom of the column via line 17 and sent to recovery column 13. The crude acrylonitrile vapor distilled from the line 16 is cooled in a condenser 19, the condensate is separated from oil and water in a decanter 20, the oil layer is sent to a hydrocyanic removal tower 21, and the water layer is returned to the recovery tower 13.

The hydrocyanic acid removal tower 21 is heated by a boiler 22, and hydrocyanic acid is distilled from the top of the tower by distillation, decomposed in a condenser 23 to separate hydrocyanic acid vapor, and the bottom liquid is sent to a dehydration tower 25 through a line 24. The dehydration tower 25 is heated by a reboiler 26, and the steam distilled from the top of the tower is condensed in a condenser 27 to separate oil and water for dehydration, and the bottom liquid is sent to a product tower 28. Product tower 28
is heated in a reboiler 29, and the vapor distilled from the top of the column is condensed in a condenser 30 to obtain product-grade acrylonitrile, and the bottom liquid is drawn out and discharged.

The conventional method for producing acrylonitrile is as described above, and the process incorporates many heating sections and cooling sections. That is, the recovery tower 13, dehydrogenation tower 21, dehydration tower 25, and product tower 28 have reboilers 14, 22, 26, 29 and condensers 19, 23, 27, 30, respectively, and the quencher 3 has a cooler 6. is attached. Various measures have been attempted in the past in order to improve the energy efficiency of these processes that repeat heating and cooling. For example, according to Published Patent Publication No. 1981-81848, after the high temperature (110 to 130°C) solvent water discharged from the recovery tower is used as a reboiler heat source for the prussic acid removal tower and product tower, the absorption liquid is preheated in a heat exchanger 12. However, proposals have been made to use it as absorption water. However, in this method, since the temperature of the solvent water drops, there is insufficient heat to preheat the absorption liquid supplied to the recovery tower 13 (in the heat exchanger 12 in Figure 1), and the heat source of the recovery tower (reboiler 12) is insufficient.
4) or heat exchanger 12
The disadvantage was that the heat transfer area had to be significantly widened.

This invention was made in view of the above circumstances, and its gist is that a reaction mixture containing unsaturated nitriles, saturated nitriles, and hydrocyanic acid produced by the ammoxidation reaction of olefins is cooled with water in a quench tower, and then The resulting absorption liquid is extracted and distilled using solvent water in a recovery column, and a distillate containing crude unsaturated nitrile, hydrocyanic acid and water is obtained from the top of the column, and from this distillate, hydrocyanic acid and Water, etc. is distilled and separated in distillation columns such as a hydrocyanic acid removal column, a dehydration column, and a product column to obtain unsaturated nitrile, and a solvent aqueous stream is obtained by distilling off substantially the fraction containing unsaturated nitrile and hydrocyanic acid in the recovery column. In a method for producing unsaturated nitrile in which a solvent water stream that does not pass through an acetonitrile stripping tower is separated from a recovery tower and returned to the absorption tower as absorbed water for circulation use, the absorption liquid flowing out from the absorption tower is collected in the aforementioned fractionated manner. Before preheating with solvent water and supplying it to the recovery tower, the absorption liquid is preheated by passing it through at least one of the quenching tower circulating water cooler, the dehydration tower, the product tower, or the recovery tower distillate vapor condenser. and, before preheating the absorption liquid, the solvent water is used as a heat source for at least one column selected from distillation columns such as a hydrocyanic acid removal column, a dehydration column, and a product column. It is a manufacturing method.

In this invention, the absorption liquid is quenching tower circulating water,
After raising the temperature by passing the liquid through one or a combination of a cooler, a dehydration tower, a product tower, a product tower, and a recovery tower distillate vapor condenser, it is preheated with solvent water and supplied to the recovery tower. Therefore, even if solvent water is used as a reboiler heat source for a distillation column such as a prussic acid removal tower, dehydration tower, or product tower before preheating the absorption liquid, the absorption liquid cannot be preheated to the required temperature. This eliminates the need for an additional distillation heat source for the recovery column or for increasing the heat exchanger capacity for preheating the absorption liquid.

On the other hand, the absorption liquid flowing out from the absorption tower at a low temperature is passed through the quenching tower circulating water cooler and the condenser of the distillation column and used for cooling the circulating water and condensing the distilled vapor.
The regeneration energy of the cooling water used for these can be saved.

This will be explained below using examples.

Example 1 As shown in FIG. 2, in this example, the absorption liquid flowing out from the absorption tower 8 is passed through the circulating water cooler 6 of the quenching tower 3 before being preheated with solvent water in the heat exchanger 12. The temperature was raised by circulating water that had been raised in temperature. On the other hand, the high temperature solvent water flowing out from the line 15 of the recovery tower 13 is used as a heat source for the reboiler 29 of the product tower 28, and then passed through the absorbent preheating heat exchanger 12 to preheat the absorbent and recover the absorbent. Column 13 was fed.

As a result, the solvent water flowing out of the recovery tower can be used as a heat source for the product column 28, reducing the heat source. At the same time, the absorption liquid can be preheated to an appropriate temperature without increasing the capacity of the heat exchanger 12, and the recovery tower No additional heat source was required. Furthermore, the amount of cooling water in the cooler 6 of the quenching tower 3 was significantly reduced.

In this embodiment, the solvent water flowing out from the recovery tower 13 was used as a heat source for the reboiler 29 of the product tower 28, but in addition to this, the hydrocyanic acid removal tower 21, the dehydration tower 2
5. It may be used as a heat source for one or more distillation columns.

Example 2 As shown in FIG. 3, in this example, before the absorption liquid flowing out from the absorption tower 8 is preheated with solvent water in the heat exchanger 12, it is passed through the condenser 19 of the recovery tower 13. The temperature was raised by sensible heat and latent heat of distilled steam from the recovery tower. On the other hand, the high temperature solvent water flowing out from the line 15 of the recovery tower 13 is used as a heat source for the reboiler 29 of the product tower 28, and then passed through the absorbent preheating heat exchanger 12 to preheat the absorbent and recover the absorbent. Column 13 was fed.

As a result, the solvent water flowing out of the recovery tower can be used as a heat source for the product tower 28, reducing the heat source, and at the same time, the heat exchanger 1
The absorption liquid could be preheated to an appropriate temperature without increasing the capacity of 2, and no additional recovery tower heat source was required. Furthermore, cooling water for the condenser 19 of the recovery tower is no longer necessary.

In this embodiment as well, in addition to using solvent water as a heat source for the product column 28, the hydrocyanic acid removal column 21 and the dehydration column 2
It may be used as a heat source for one or more of the fifth class distillation columns. Furthermore, in this example,
Although the absorption liquid was preheated by the distilled steam from the recovery tower, the same effect can be achieved by preheating it by the distilled steam from the dehydration tower 25 and the product tower 28. Further, preheating may be performed in multiple stages using two or more types of distilled steam.

This invention is as described above, and it is possible to reduce the heat source of distillation columns such as a hydrocyanic acid removal column, dehydration column, and product column, and to save the heat source of a recovery column, thereby increasing the capacity of the heat exchanger for preheating the absorption liquid. Almost no equipment costs are required. In addition, a high degree of energy efficiency can be achieved because cooling water for circulating water in the quenching tower and cooling water for condensing distillation towers such as recovery towers, dehydration towers, and product towers can be saved.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing a conventional acrylonitrile manufacturing process, and FIGS. 2 and 3 are flow sheets showing processes of Examples 1 and 2, respectively. 1... Reactor, 2... Heat exchanger, 3... Quenching tower, 4, 5, 7, 9, 10, 11... Line, 6
...Cooler, 12...Heat exchanger, 13...Recovery tower, 14,22,29...Reboiler, 15,1
6, 17... Line, 18... Acetonitrile stripping tower, 19... Condenser, 20... Decanter, 2
1... Dehydrogenation tower, 23... Condenser, 24... Line, 25... Dehydration tower, 26... Reboiler, 2
7, 30... Condenser, 28... Product tower.

Claims (1)

[Claims] 1 Unsaturated nitriles and saturated nitriles produced by the ammoxidation reaction of olefins,
The reaction mixture containing hydrocyanic acid and hydrocyanic acid is cooled with water in a quenching tower, absorbed into absorption water in an absorption tower, and the obtained absorption liquid is subjected to extractive distillation using solvent water in a recovery tower. A distillate containing water is obtained, and from this distillate, hydrocyanic acid, water, etc. are distilled and separated in distillation columns such as a prussic acid removal column, a dehydration column, and a product column to obtain unsaturated nitrile. and a solvent aqueous stream from which a fraction containing hydrocyanic acid has been substantially distilled off,
In a method for producing unsaturated nitrile in which the solvent water flow that does not pass through the acetonitrile stripping tower is separated from the recovery tower and returned to the absorption tower as absorption water for circulation use, the absorption liquid flowing out from the absorption tower is preheated with the fractionated solvent water. Before supplying the absorption liquid to the recovery tower, the absorption liquid is preheated by passing it through at least one of the quenching tower circulating water cooler, the dehydration tower, the product tower, or the recovery tower distillate vapor condenser, and the solvent is A method for producing an unsaturated nitrile, characterized in that water is used as a heat source for at least one distillation column such as a prussic acid removal column, a dehydration column, and a product column before preheating the absorption.