DE1920761A1 - Process for the separation of acrylonitrile and acetonitrile - Google Patents

Description

OIL EMISSIONS Limited liability company

FARBENFABRIKEN BAYER AKTIENGESELLSCHAFT

Cologne, the

D / HM

Process for the separation of acrylonitrile and acetonitrile

The present invention relates to the separation of acrylonitrile and acetonitrile from a gas mixture that occurs during production of acrylonitrile is obtained by the catalytic oxidation of propylene in the presence of ammonia.

, In the known process for the production of acrylonitrile the catalytic oxidation of propylene in the presence of ammonia always gives rise to considerable amounts as by-products of acetonitrile and hydrocyanic acid. The separation of hydrogen cyanide from acrylonitrile by distillation does not present any technical difficulties, on the other hand, the separation of acrylonitrile and acetonitrile by distillation is associated with considerable effort; she will carried out in the known processes with the aid of extractions or extractive distillations. The greatest practical So far, processes that use extractive distillation have been important. For this purpose, the one leaving the reaction part is used and the reaction gas freed from excess ammonia is washed with water in an absorption column (I). Included all acetonitrile and partly acrylonitrile and hydrocyanic acid or all products are converted from the gas washed out. The nitrile-free reaction gases still contain residual propylene as well as propane and carbon oxides. To the Eliminating these substances, the gases are burned in a mixture with air in packets or ovens.

Le A 12 183

EC 44 - 1 -

0098 46/1892

The aqueous solutions of the nitriles get into a second one Column (II) which is heated at the bottom and has an inlet of acetonitrile-free water at the top.

Acrylonitrile and hydrogen cyanide are separated by extractive distillation, the gaseous go overhead, and acetonitrile, which is drawn off as an aqueous solution at the bottom and in a third column (III) is distilled off from the water. The bottom discharge from the third column consists of water that has cooled and is fed to columns (I) and (II).

A disadvantage of this process is that a very large amount of water with relatively little acetonitrile flows into the acetonitrile stripper (less than 1 Grew .- ^) and that the steam consumption in this column to drive off acetonitrile is correspondingly high, because the washing out of the acetonitrile from the vapor stream in the extractive distillation only water can with traces of acetonitrile used werden.Dieser effort is sehx ⁴ uneconomical because the expelled acetonitrile must then be destroyed by burning partially or completely, in the previously known methods.

A sufficient use for as by-products at the Acrylonitrile process produces large amounts of acetonitrile does not exist yet.

It has now been found that the process for separating acrylonitrile from gas mixtures which, in addition to acrylonitrile, also contain hydrocyanic acid, acetonitrile, propiononitrile, water and inert gases, can be carried out much more economically if the above reaction products are washed out in a scrubber (A) obtained aqueous solution of an Abitreifkolona "(B)" uleittt, at the head of the loloma "15 ° feie 75 ° 0 warate Waiser ierart" uftthrt, ia · on Acet · -

Le A 12 183

EO 44 - 2 -

009846/1892

nitrile is washed out of the rising vapor mixture in the column, whereas acrylonitrile, hydrocyanic acid, propionitrile are drawn off at the top of the column, in the lower part of the column (B) in the area of the 12th - 25th tray a portion of the water with the total acetonitrile as side stream is withdrawn, this leads as washing water to the top of the scrubber (A), there expels the acetonitrile contained therein with the acrylnitrilfreien gas, to subtract the residual amount of water at the bottom of column (B) and after cooling to 15 ° - 75 ⁰ C completely or partially returned to the top of column (B).

One embodiment of the method according to the invention is on the A "fig. 1 explains:

Acrylonitrile, acetonitrile, hydrocyanic acid and propionitrile are washed out of a gas mixture with inert gases in a wash column (A). The aqueous discharge from (A) is led into the stripper (B) * There, with the help of extractive distillation, acrylonitrile, hydrocyanic acid and propionitrile are driven in gaseous form along with the appropriate amount of water over the top of the column. In the lower part of column (B) in the area of the 12th to 25th tray, a portion of the water is drawn off with all of the acetonitrile and returned to the top of column (A). In the upper part of the column (A), the acetonitrile is largely expelled from the wash water by the reaction gases flowing through the column and, for example, passed to an incinerator. In the lower part of column (A) ( 70-90 generally of the entire length of the column) the reaction gas flowing in from below is washed with the washing water flowing in from above and freed from the bulk of the acetonitrile, so that all nitriles are removed therefrom. In the upper part (10-30 $ of the entire column length), a desorption zone, the acetonitrile introduced with the washing water is expelled from the nitrile-free washed gas in an amount that corresponds to that formed during the reaction. At the bottom of the column (B), the remaining amount of water is without acetonitrile

Le A 12 183

EC 44 ■ - 3 -

009846/1892

withdrawn and, after cooling to 15-75 ^° C., completely or partially returned to the top of column (B) as extraction water to remove acetonitrile from the vapors rising in the column.

In the process according to the invention, the stripping section of column (B) is formed in such a way that it has 12-25 more trays contains as necessary for separating acrylonitrile and acetonitrile and for drawing off an aqueous acetonitrile solution from the sump were. This ensures that the liquid flowing through the column is not only at the bottom tray, but rather 10-25 floors above that are already free of acrylonitrile. These "12-25 trays in the lower part of the column are used to make acetonitrile to drive out of the swamp; it accumulates in the area of the 12th - 25th floor and can there with part of the die Water passing through the column can be withdrawn in the side stream, while the rest of the water with no or only traces of Acetonitrile runs off at the bottom of the column and is returned to the top of the column (B) as extraction water.

When using this process one does not have to - according to the state of the art - draw off all the water with all the acetonitrile at the bottom of the column (B) and pass it through an acetonitrile stripper (C), but by attaching 12 - 25 trays in the bottom of the column (B) so that all of the acetonitrile can be drawn off with, for example, half the water in the side stream. The steam consumption in the column (B) is not higher than it would be if all of the acetonitrile were withdrawn with all of the water in the bottom of the column. This means that you save the entire steam input for operating the acetonitrile stripper (C) according to the prior art; that is 3,500 to 4,500 kg of steam per 1,000 kg of acrylonitrile produced.

Le A 12 183 ' ^:

EC 44 - 4 -

009846/1892

The mixture of all of the acetonitrile and, for example, half the amount of water drawn off in the side stream in the region of the 12th to 25th tray of column (B) is fed to the top of the scrubber (A). The top of the washer (A) contains 11-20 dividers more than would be necessary to remove nitriles by washing with water. This ensures that the reaction gases rising in the scrubber are free of acrylonitrile, acetonitrile, hydrocyanic acid and propionitrile not only at the top tray, but 11-20 floors below. These 11-20 trays in the upper part of the column are used to drive acetonitrile out of the washing water flowing in overhead with the aid of the rising gas. This removal of acetonitrile is largely successful; the temperature of the incoming washing water is between 30 and 70 ^° G. The inert gases leaving the scrubber (A) lead the acetonitrile to a flare or an incinerator.

■ "■ o" -

It was surprising and not foreseeable that with the low expenditure of only 12-25 soils in the swamp of the abortionist (B) completely remove the acetonitrile from the water running off at the bottom and withdraw it in the side stream without the im The extractive distillation taking place at the top of the column is adversely affected, i.e. without any at the top of the column Acetonitrile breakthrough takes place without increasing the amount of heating steam. Furthermore, it was surprising and unforeseeable that with the low cost of only 11-20 practical Separating trays in the desorption zone of the scrubber (A) the introduced Acetonitrile can be largely removed from the wash water and blown off without the absorption of Acrylonitrile, hydrocyanic acid, acetonitrile and propionitrile from the reaction gas in the lower part of the column is impaired. at The presence of large amounts of acetonitrile in the wash water is sufficient for washing the reaction gas by this method, the same amount of water as when using pure water.

In the method according to the invention, by combining the

Le A 12 183

E G 44 - 5 -

009846/1892

two procedural peculiarities that differ greatly from the prior art achieved that installation and operation of one separate acetonitrile stripper (C), which, according to the state of the art, requires a very high amount of steam, is superfluous will.

If you don't want to burn all of the acetonitrile, but want to extract some of it, then you can do this The method according to the invention can be modified in the form illustrated in Fig. 2:

fc In a wash column (A), acrylonitrile, acetonitrile, Hydrocyanic acid and propionitrile washed out of a grass mixture with inert gases. The aqueous drain from (A) leads through line (5) into the aborter (B). There, with the help of extractive distillation, acrylonitrile, hydrocyanic acid and Propionitrile in gaseous form in addition to the corresponding amount of water / water driven over the top of the column. In the lower part of the column (B) is In the area of the 12th to 25th tray, the entire amount or a partial amount of the water-acetonitrile mixture is drawn off via line (9), fed to a stripper (G) and there is removed from acetonitrile freed. The acetonitrile distilled off can be drawn off at the top of column (C) via line (10). The sump drain from the aborter (C) leads to the via line (2)

f head of the wash column (A) closed. If the entire mixture of acetonitrile and water withdrawn at the 12th 25th tray of column (B) has not been passed to the syrup (C), then the untreated portion is also passed to the top of the washing column (A). In the upper part of column (A) (10 to 30% of the entire column length), the acetonitrile is largely driven out of the wash water by the reaction gases flowing through the column. In the lower part of column (A), the washing water flowing in from above via line (2) and freed from the bulk of the acetonitrile is used to wash the reaction gas flowing in from below and to remove all nitriles. Because this is added to the top of the column (A) with the washing water, Ee A 12 183

BO 44 - 6 -

009846 / 1Ö92

led acetonitrile in the upper part of the column is desorbed from the gas via line (4), it is not necessary that the bottom outlet of the column (G) completely free of acetonitrile is. You only need to distill off most of the acetonitrile in (C) and a remainder can be removed from the bottom drain lead to the head of (A). The steam consumption in column (C) for this procedure is much lower than for the complete separation of acetonitrile from the water in the bottom of (C). In this way, a portion or almost all of the acetonitrile can be obtained at will and via line Pull off (10).

Example 1:

The information given in the following examples refer to Fig. 1.

99,200 kg / h of a reaction gas, which in addition to the inert gas components approximately

1,307.0 kg / h: hydrocyanic acid 7,402.2 kg / h: acrylonitrile 610.0 kg / h: acetonitrile

3.7 kg / h: propiononitrile and 32,764.8 kg / h: water

contains, are introduced through line (1) into the bottom of a wash column (A) with 65 practical exchange trays, of which the upper 15 form an acetonitrile desorption zone; the temperature (T) is 28 ⁰ C. 65. At the bottom of this column (A) through the line (2) from the 25th tray of a stripping column (B) after prior cooling in a heat exchanger (D) of 109 ° C given up to 57 ° C 109,000 kg / h of raw wash water with 627 kg / h of acetonitrile. The entire liquid of 100,000 kg / h with only 17 kg / h of acetonitrile is drawn off from the 50th floor of the washer (A) through line (3), in a heat exchanger (C) from 22 ^° C. to that for the washing process necessary

Le A 12 183

EC 44 - 7 -

0 0 9 8 4 6/1892

Cooled temperature of 5 ⁰ C and given up on the 49th floor of the washer (A) as washing water. The inert gas components leave the line (4) with:

15.0 kg / h: hydrogen cyanide

7.0 kg / h: acrylonitrile, 609.9 kg / h: acetonitrile and 8,179.8 kg / h: water

the washer (A); the temperature is 47 ° C. From the swamp the wash column (A)

1,297.0 kg / h: hydrogen cyanide

W 7,395.2 kg / h: acrylonitrile

627.1 kg / h: acetonitrile

3.7 kg / h: propiononitrile

and 132,958.0kg / h: water

withdrawn through line (5), preheated in the heat exchangers (D) and (E) from 15 ° C to 78 ⁰ C and the 64th plate "a stripping column (B) with 95 practical exchange plates, of which the 25 lower the expulsion of the Serve acetonitrile from the sump drain.

berkolonne (B) containing no analytically detectable acetonitrile, are kg through line (6) after prior cooling in the heat exchanger '(E) of 11O ⁰ C to 71 ⁰ C 80,000 / h of water 95th bottom of this column (B) fed, the bottom of which is heated via an evaporator (H) with 22.1 t / h of steam and via the top via line (8)

1,292.0 kg / h: hydrogen cyanide

7,395.2 kg / h. : Acrylonitrile

0.1 kg / h: acetonitrile

3.7 kg / h: propiononitrile

and 585.0 kg / h: water

are distilled off. The excess with, the reaction gases

Le A 12 183> - · _: · _; ν rV-

EC 44 - 8 -

0 0 9 8 4 6/1892

Water introduced is removed through line (7).

Example 2;

The same amount of gas with the same composition as in example 1 is worked up according to the procedure described in example 1. The stripping column (B) has 83 practical exchange trays, of which the 13 lower trays are used to drive the acetonitrile out of the bottom drain. Are fed 70,000 kg / h of washing water mit'1,0 kg / h of acetonitrile and a temperature of 45 ⁰ C to the bottom of the stripping column 83. (B) via line (6). Contains the distillate removed via line (8)

1,292.0 kg / h: hydrogen cyanide

7,395.2 kg / h: acrylonitrile

1.5 kg / h: acetonitrile

3.7 kg / h: propiononitrile

and 585.0 kg / h: water

109,000 kg / h of crude wash water with 625.5 kg / h of acetonitrile and a temperature of 57 ⁰ O are fed to the 60th tray of the wash column (A) via line (2). The gas leaving the scrubbing column (A) through line (4) contains, in addition to the inert gas components

15.0 kg / h: hydrogen cyanide

7.0 kg / h: acrylonitrile

608.5 kg / h: acetonitrile

and 8,179.8 kg / h ί water

The temperature is again 47 ° 0. Via line (3) withdrawn from the bottom of the wash column 50. (A) the liquid of 100,000 kg / h 17.0 kg / h of acetonitrile at a temperature of 22 ⁰ O. After cooling in the heat exchanger (C) at 5 ⁰ C this liquid is applied as washing water to the 49th floor of the washer (A).

Le A. 12 183

EC 44 - 9 -

009846/1892

Claims (1)

Claim t Process for separating acrylonitrile from grass mixtures which, in addition to acrylonitrile, also contain hydrocyanic acid, acetonitrile, propiononitrile, water and inert gases, characterized in that the aqueous solution obtained by washing out the above reaction products in a washer (A) is fed to a stripping column (B), at the top of the column 15 ° to 7ff £> supplies warm water in such a way that only acetonitrile is washed out of the vapor mixture rising in the column, whereas acrylonitrile, hydrocyanic acid, propionitrile are drawn off at the top of the column, in the lower part of the column (B) in the area of the 12th - 25th tray removes a portion of the water with the entire acetonitrile as a side stream, leads this as wash water to the top of the scrubber (A), there drives off the acetonitrile contained therein with the acrylonitrile-free gas, at the bottom of the column (B) subtract the residual amount of water and after cooling to 15 ° - 75 ⁰ O fully or partially to the top of the column (B) returns. le A 12 183 - 10 - BC 44 009846/189 2