JPS6042236B2 - Production method of unsaturated nitrile - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for producing unsaturated nitriles by ammoxidation reaction of olefins, such as a process for producing acrylonitrile or methacrylonitrile by ammoxidation reaction of propylene or isobutylene.

The purpose is to propose an extremely energy-efficient manufacturing method that converts the relatively low-temperature, hitherto unusable, manifest or latent heat generated within the manufacturing system into a cold or heating source and utilizes it effectively. The process for producing unsaturated nitriles by the ammoxidation reaction of olefins involves cooling the reactants at a high temperature with water and using a large amount of water (for example,
．． 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.
Energy costs accounted for a high proportion of costs. The conventional process will be specifically explained below using an example of producing acrylonitrile. In the presence of an oxidation catalyst in the reactor 1, continuously supplied propylene, ammonia, and air undergo an ammoxidation reaction in a high-temperature gas phase, producing reaction products such as acrylonitrile, acetonitrile, hydrocyanic acid, and high-boiling organic substances, and A mixture of unreacted substances is produced.

This high-temperature gaseous mixture is passed through a heat exchanger 2 and then passed through a quenching tower 3.
The gas is brought into contact with circulating cooling water flowing down from the top of the tower via line 4, and cooled to low-temperature gas. In this cooling process, high-boiling organic matter 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, is supplied to a recovery tower 13 from a position slightly above it, and is heated by a reboiler 14.

The recovery tower is usually at least 50. Preferably, the column has a tray of 60 to 10 mm, and performs extractive distillation by contacting in countercurrent with the solvent water fed into the upper part of the column via line 15, and crude acrylonitrile vapor containing hydrocyanic acid and water is passed from the top of the column via line 16. After distillation, solvent water is extracted from the bottom of the column via line 17. Note that the acetonitrile in the absorption liquid is collected in the recovery tower 13.
Most of the acetonitrile is separated by distillation in the acetonitrile recovery column 18 attached to the acetonitrile. The solvent water extracted from line 17 and from which most of the acetonitrile has been removed is sent to recovery tower 13 via line 15, while a part of it passes through heat exchanger 12 to cool down and passes through line 9 as absorbed water to absorption tower. 8. The crude acrylonitrile vapor distilled from line 16 is cooled in condenser 19, and the condensate is sent to decanter 2.
Oil and water are separated at 0, and the oil layer is sent to the prussic acid removal tower 21.
The aqueous layer is returned to the recovery tower 13. The prussic acid removal tower 21 is heated by a reboiler 22 and steamed. Hydrocyanic acid is distilled out from the top of the column by distillation, condensed in a condenser 23 to separate hydrocyanic acid vapor, and the bottom liquid is sent to a dehydration column 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. The product column 28 is heated by 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 and cooling sections.

That is, the recovery tower 13, the prussic acid removal tower 21, the dehydration tower 25, and the product tower 28 are provided with reboilers 14, 22, 26, and 2, respectively.
9 and condensers 19, 23, 27, and 30, and a cooler 6 is attached to the quenching tower 3. 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, published patent publication 1972
According to No.-81848, the high temperature (110
A proposal has been made to use solvent water at a temperature of 130° C. as a reboiler heat source for a prussic acid removal tower or a product tower, and then preheat the absorption liquid in a heat exchanger 12 and 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 Fig. 1), and the recovery tower heat source (reboiler 14) is If you need extra heat exchanger 1
2 had the disadvantage 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 quenching tower, and then in an absorption tower. The resulting absorption liquid is subjected to extractive distillation using solvent water in a recovery column to obtain a distillate containing crude unsaturated nitrile and hydrocyanic acid from the top of the column, and from this distillate, hydrocyanic acid, water, etc. Unsaturated nitrile is obtained by distillation separation in distillation columns such as hydrocyanic acid removal tower, dehydration tower, and product tower, and the solvent water obtained by removing acetonitrile from the lower part of the recovery tower is circulated as absorption tower absorption water and recovery tower solvent water. In the method for producing saturated nitrile, the absorption liquid flowing out from the absorption tower is preheated with solvent water after acetonitrile has been removed and then supplied to the recovery tower.
The absorption liquid is preheated by passing it through at least one of a quenching tower circulating water cooler, a dehydration tower, a product tower, or a recovery tower distillate vapor condenser, and the solvent water is used before preheating the absorption liquid. This is a method for producing an unsaturated nitrile, which is characterized in that it is used as a heat source for at least one column selected from distillation columns such as a prussic acid removal column, a dehydration column, and a product column.

In this production method, the low-temperature absorption liquid flowing out from the bottom of the absorption tower is passed through a cooling tower circulating water cooler, a dehydration tower, a product tower, or a distillate vapor condenser of a recovery tower.

As a result, the heat of the cooler or condenser, which was conventionally released outside the system using cooling water, can be absorbed into the absorption liquid and taken into the system, resulting in an increase in the temperature of the absorption liquid and the cooling water. Renewable energy can be saved. Furthermore, since the temperature of the absorption liquid has been raised in advance, the high-temperature solvent water flowing out from the recovery tower is used as a reboiler heat source for the hydrocyanic acid removal tower, dehydration tower, product tower, etc. to lower the temperature before being used to preheat the absorption liquid. Even if the absorption liquid is heated, the absorption liquid can be preheated to the required temperature, and there is no need for an additional recovery column distillation heat source or an increase in the heat exchanger capacity for preheating the absorption liquid. This will be explained below using examples.

Example 1 As shown in FIG. 2, in this example, before the absorption liquid flowing out from the absorption tower 8 is preheated by solvent water in the heat exchanger 12, the liquid is passed through the circulating water cooler 6 of the quenching tower 3. 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 bottom of the recovery tower 13 is used as a heat source for the reboiler 29 of the product column 28, and then passed through the heat exchanger 12 for preheating the absorption liquid to preheat the absorption liquid and transfer the absorption liquid to the recovery tower. 13 was supplied. 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 example, 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.
In addition, it may be used as a heat source for one or more distillation columns such as the prussic acid removal column 21 and the dehydration column 25.

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, after the high temperature solvent water flowing out from the bottom of the recovery tower 13 is used as a heat source for the reboiler 29 of the product column 28, the liquid is passed through the heat exchanger 12 for preheating the absorption liquid to preheat the absorption liquid, and the absorption liquid is transferred to the recovery tower. 13 was supplied. 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 heat source of the recovery tower can be There was no need for any extra.

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, one of the distillation columns such as the hydrocyanic acid removal column 21 and the dehydration column 25 is used.
It may be used as one or more heat sources.

Further, in this example, the absorption liquid was preheated by the distilled steam from the recovery tower, but the same effect can be obtained 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.
For this reason, almost no equipment cost is required to increase the capacity of the heat exchanger for preheating the absorption liquid.

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 ... line, 5 ... line, 6 ... chiller, 7 ... line, 8
...Absorption tower, 9...Line, 10...
・Line, 11...Line, 12...Heat exchanger,
13...Recovery tower, 14...Reboiler, 15...Line, 16...Line, 17...
Line, 18...Acetonitrile recovery tower, 19
...Condenser, 20 ...Decanter, 21
...... Hydrocyanic removal tower, 22 ... Reboiler, 2
3...Condenser, 24...Line, 25...
... Dehydration tower, 26 ... Reboiler, 27 ...
... Condenser, 28 ... Product tower, 29 ... Reboiler, 30 ... Condenser.

Claims (1)

[Claims] 1. A reaction mixture containing unsaturated nitriles, saturated nitriles, and hydrocyanic acid produced by the ammoxidation reaction of olefins is cooled with water in a quenching tower, absorbed into absorption water in an absorption tower, and the resulting absorption liquid is transferred to a recovery tower. Extractive distillation is carried out using solvent water at the top of the column to obtain a distillate containing crude unsaturated nitrile and hydrocyanic acid, and from this distillate, hydrocyanic acid, water, etc. are removed in distillation columns such as a hydrocyanic acid removal tower, a dehydration tower, and a product tower. In the method for producing unsaturated nitrile, in which unsaturated nitrile is obtained by distillation separation, and the solvent water from which acetonitrile is removed from the bottom liquid of the recovery column is circulated as absorption column absorption water and recovery column solvent water, the absorption liquid flowing out from the absorption column is Before preheating with solvent water after acetonitrile removal and supplying it to the recovery tower, the absorption liquid is passed through at least one of the cooling tower circulating water cooler, the dehydration tower, the product tower, or the recovery tower distillate vapor condenser in advance. and the solvent water is used as a heat source for at least one column selected from distillation columns such as a prussic acid removal column, a dehydration column, and a product column, before preheating the absorption liquid. Method of manufacturing nitrile.