SU349169A1 - - Google Patents

Description

METHOD OF CLEANING ACRYLONITRILE FROM ACROLEIN

The invention relates to a method for the purification of acrylonitrile from acrolein by chemical heat treatment in an aqueous medium. Acrylonitrile is widely used in the production of synthetic acrylic fiber, which necessitates a high degree of its purification from the by-products formed during the preparation process. The most significant amount in crude crude acrylonitrile contains hydrocyanic acid, acetonitrile, acrolein, acetaldehyde, as well as a small amount of a number of other impurities.

A known method for the purification of acrylonitrile from acrolein by the absorption of crude acrylonitrile, washed from ammonia, with water, followed by separation by distillation. Distillation is carried out with or without the presence of water. Acrylonitrile is obtained with a relatively high content of acrolein, about 0.05%, which does not meet the usual standards for the so-called "spin brand.

The aim of the invention is to obtain high purity acrylonitrile from acrolein, the content of which in the target product does not exceed 0.0005%. This goal is achieved in that the absorption of crude acrylonitrile, washed from ammonia, is carried out in a medium with a pH of 7.5-11, followed by heating the alkaline solution to a temperature of 70-150 ° C and distillation.

According to the invented 1H, the products of the catalytic reaction of iroiylene, ammonia and oxygen, containing, in addition to acrylonitrile, hydrogen cyanide, acetonitrile, acrolein, acetone, acetaldehyde and other impurities in small quantities, are sent to a packed column, irrigated with diluted sulfuric acid, to recover unreacted, in order to recover unreacted, which was removed by the reactor. . The gas mixture, freed from ammonia, is fed to the second packed column, in countercurrent to water, the pH of which is 7.5-11. The resulting aqueous solution is heated to a temperature of 70-150 ° C and fed to the appropriate apparatus, where the solution

Keep at this temperature for 1-60 minutes. After this time, the solution contains only traces of acrolein. Next, the solution is sent for distillation and further purification from other impurities. As a result of all subsequent operations, the content of acrolein in acrylonitrile is reduced to a value below 0.0005%, i.e., to the tolerance specified by the standards.

The pH of the solution containing the products of the absorbed gas mixture, freed from ammonia, in accordance with the invention is between 7.5 and 11, preferably 8.5-10. The temperature to which the aqueous solution of the absorbed products must be heated is 70-150 ° C, preferably 75-90 ° C. The duration of heating the aqueous solution of the crude acrnlonitrile is from 1 to 60 minutes, preferably from 5 to 30 minutes.

The water absorbing the reaction gas is made alkaline by the addition of alkaline compounds, which can be used as hydroxides or alkali metal carbonates, better dissolved in an aqueous solution; alkali metal bicarbonates can also be used, since in the process of heating and boiling, they are converted to the corresponding carbonates. The amount of alkalizing agent is determined depending on its nature and on the desired pH.

The pressure and temperature of absorption are not critical, but since the product being absorbed by acrylonitrile is volatile, it is better to conduct absorption at a temperature close to room temperature.

The drawing shows a flow diagram of the process, explaining the proposed method.

Example 1. Products of a catalytic gas-phase reaction between propylene, ammonia and air, containing, in addition to acrylonitrile, acetonitrile, hydrogen cyanide, acrolein, acetaldehyde, acetone, water, carbon dioxide and carbon monoxide, as well as oxygen, nitrogen, propylene and ammonia, are first used for generating steam by part of their physical heat, after which they are cooled to a temperature of about 200 ° C.

Through pipe 1, these products are routed to the bottom of the scrubber column 2, where ammonia is completely removed. To remove ammonia, an aqueous solution containing 1-2% sulfuric acid and 30-40% ammonium sulfate is fed to column 2, but pipe 3. This solution circulates through column 2 with a temperature of 85-90 ° C.

Pipe 4 continuously or periodically withdraws such an amount of solution that is necessary to release ammonium sulphate that is simultaneously formed, so that its concentration in the cycle remains constant corresponding to the above.

The gaseous reaction products, freed from ammonia, having a temperature of 85-90 ° C and saturated with water vapor, from column 2 through pipe 5 enter the bottom part of the absorption column 6, where they are washed in countercurrent with water that absorbs and dissolves organic products, cyanide hydrogen and a portion of carbon dioxide, while gaseous compounds — nitrogen, oxygen, carbon monoxide, carbon dioxide residue, and propylene — are discharged through pipe 7.

Wash water at a temperature of 20-35 ° C enters column 6 from desorption column 8 through pipe 9. Water has a pH of 10 and contains in solution about 0.2 g / l of sodium in

as carbonate or bicarbonate.

In the bottom of the column 6, a dilute aqueous solution of the reaction products is collected, the pH of the solution is 7-7.5. The bottom solution has the following composition (in%): 1.32 acrylonitrile, 0.11 acetonitrile, 0.10 hydrogen cyanide, 0.02 acrolenine, as well as small amounts or traces of other nocturnal products.

A solution containing these products,

discharged from column 6 through pipe 10 and at the point and an aqueous solution of sodium hydroxide is continuously added to it, so as to obtain a solution pH of 8.5. The alkalized solution is heated in aparate 12 to 85 ° C due to ten. Exchange with boiling solution coming from desorption column 8. Pgretny solution is passed through annarate 13, and the feed rate of the solution is adjusted so as to ensure the residence time of the solution in this apparatus for 15 minutes .

In accordance with the invention, the absorption of the reaction products with an alkaline solution and its subsequent heating in the apparatus 13 significantly reduces the content of acrolein in it.

The solution with a sufficiently reduced amount of acrolein flows from above into the column 8. In the bottom part of this column serves a sharp batch in an amount sufficient to displace

from the solution of all volatile organic compounds dissolved in it. These volatile organic compounds are synthesized and, together with the water deposit, but pipe 14 enters the apparatus 15, where they condense. The condensate in the settler 16 is divided into two phases: the upper — organic and lower — aqueous. The aqueous phase is returned to the column 8 through the pipe-cleaned product of the following composition (in%): acrylonitrile 84.0, acetonentril 7.0, hydrogen cyanide 4.8, acrolein 0.02, acetone 0.17, water 4.0, and traces of other impurities.

Thus, the acrolein content in acrylonitrile is reduced from 1.5 to 0.024%, i.e., 60 times. The residual low content of acrolein is no longer significant, since with further purification of acrylonitrile from other impurities, an additional spontaneous decrease in the concentration of acrolein to a value below 0.0005% occurs.

Example 2-5. The experiments were carried out using the flow chart shown in the drawing, following the procedure, as well as using the equipment specified in Example 1. The effect of the pH of the solution, the temperature and the duration of its heating on the degree of decrease in acrolein concentration is studied.

The aqueous solution of the reaction products has the following composition (in%):

Acrylonitrile1,27

Acetonitrile0.09

Acrolein 0.028

Cyanic voorodorod0,10

Acetaldehyde 0,012

Acetone 0.001 The table shows the conditions of the experiments and their results.

Subject invention

The method of purification of acrylonitrile from acrolein by absorption by water and subsequent distillation, characterized in that, in order to increase the degree of purification of the target product, absorption is carried out in (pH 7.5-11) and then heated the resulting solution to a temperature of 70-150 ° WITH.