JPH0244302B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a continuous acetonitrile recovery process.

The production of acrylonitrile by catalytic ammoxidation of propylene with ammonia and oxygen is known to produce a crude acetonitrile by-product. This material usually consists of about 52% acetonitrile, 43.6% water, 2.5% HCN, 0.5% acrylonitrile, and 1.3% organics such as oxazole, allyl alcohol, acetone, propionitrile, by weight.

Traditionally, this crude acetonitrile by-product has simply been disposed of by incineration. However, recently the crude acetonitrile by-product has been processed to recover acetonitrile as a valuable by-product.

In this process, crude acetonitrile is first distilled to remove HCN. Next, this HCN
The free material is distilled to yield an acetonitrile/water azeotrope containing approximately 25% water, which is then slurried with anhydrous calcium chloride. Anhydrous calcium chloride absorbs most of the water in the azeotrope to create an acetonitrile/water mixture containing approximately 3-5% water, which is distilled to yield a pure acetonitrile product.

This conventional acetonitrile recovery process produces relatively large amounts of waste calcium chloride that must be disposed of, and is also carried out in batch operations leading to various operational difficulties.

It is therefore an object of the present invention to provide a new technique for the recovery of acetonitrile from a crude acetonitrile process stream that is simple to implement and can be carried out in a continuous manner.

These and other objects are achieved by the present invention, which is based on the finding that acetonitrile can be recovered with a purity of more than 99% by a continuous distillation operation carried out in three stages at three different pressures.

Therefore, the present invention uses acetonitrile, water,
A novel continuous recovery method for high-purity acetonitrile from crude acetonitrile containing HCN, heavy organic matter is provided, which method comprises: (1) distilling the crude acetonitrile in a first distillation zone at a first pressure of 1 atm or more; hand,
removing HCN to obtain a first acetonitrile/water azeotrope and a water-containing first bottoms product; (2) distilling the first in a second distillation zone at a second pressure of less than 1 atmosphere;
distilling the azeotrope to separate the first azeotrope into a water-containing second bottoms product and a second acetonitrile/water azeotrope having an acetonitrile concentration greater than the first azeotrope; (3) 1 distilling the second acetonitrile/water azeotrope in a third distillation zone at a third pressure greater than atmospheric pressure to form a third acetonitrile/water azeotrope comprising substantially all of the water in the second azeotrope; It consists in obtaining a third bottoms product consisting of acetonitrile and heavy organics and a side stream consisting of high purity acetonitrile.

In accordance with the present invention, crude acetonitrile is processed to recover high purity acetonitrile as a valuable by-product. "High purity" acetonitrile means acetonitrile having a purity of at least 95%.

Typically, the present invention can produce acetonitrile with a purity as high as 99.9%, with the balance being water and very small amounts of organic matter. The crude acetonitrile treated according to the invention can be any acetonitrile/water mixture containing at least 15% water. Therefore, the method of the present invention can be applied to the treatment of various water/acetonitrile azeotropes. However, the present invention has the widest applicability to the treatment of crude acetonitrile streams produced by ammoxidation of propylene with oxygen and ammonia for the production of acrylonitrile. As noted above, such crude acetonitrile streams typically contain approximately 52% acetonitrile, 43.6% water, 0.5% acrylonitrile,
Contains 2.5% HCN and 1.3% other minor impurities like oxazole, allyl alcohol, acetone, propionitrile.

Crude acetonitrile recovered from an acrylonitrile unit and having the above composition can be conveniently processed according to the present invention according to the flowchart shown in the figure. According to this system, crude acetonitrile is supplied via inlet line 10 to crude acetonitrile holding tank 12. Sulfuric acid is charged into the crude acetonitrile tank 12 through the line 14, and the crude acetonitrile is
Acidify the pH to stabilize cyanohydrin and neutralize free ammonia.

After acidification, the neutralized crude acetonitrile is charged via line 14 to head column 16 where it is distilled into three phases at a pressure of about 18 psig. The light components in the crude acetonitrile, namely HCN, acrylonitrile, oxazole and acetone, are withdrawn as vapor from the head column 16 and discharged via line 18 for incineration. Water is recovered from the bottom of head column 16 and discharged via line 20 for disposal. Approximately 70% acetonitrile, 30% water,
HCN500ppm, 1st containing very small amount of heavy organic matter
of the acetonitrile/water azeotrope is recovered as a side draw stream.

The first acetonitrile/water azeotrope is then transferred via line 22 to digester 24. Consists of an aqueous solution of sodium hydroxide and formaldehyde
The HCN digester is added to the digester 24 via line 26 to decompose the HCN in the first azeotrope.
This HCN cooking process is described in more detail in US Pat. No. 4,328,075, which is hereby incorporated by reference. Another acetonitrile/water azeotrope (third azeotrope) is supplied to the digester 24 via line 28.
This azeotrope contains about 22% water.

The HCN-free acetonitrile/water mixture leaving the digester 24 passes through line 30 to a drying tower 32.
be prepared for. Furthermore, a stream consisting of acetonitrile containing small amounts of heavy impurities is also fed to the drying column 32 via line 34. In the drying column 32, the acetonitrile/water mixture is distilled at a pressure of less than 1 atmosphere, e.g. 3.4 psig, to produce a bottoms product consisting of water and heavy organics, which is discharged for disposal via line 36; It also produces a gaseous overhead stream consisting of a second acetonitrile/water azeotrope, the second azeotrope containing approximately 10% water.

A second acetonitrile/water azeotrope is charged via line 38 to condenser 40 where the azeotrope is condensed and passed via line 42 to heat exchanger 44 where it is heated and then via line 46 to form a Load it into tower 48. In product column 48, a second acetonitrile/water azeotrope is heated at high pressure, e.g.
Distill into three phases at 50 psig. The bottoms product consisting of acetonitrile containing heavy impurities is sent to product column 4.
8, and passes through line 34 to drying tower 3.
Recirculated to 2. The third acetonitrile/water azeotrope is withdrawn from the top of product column 48 and recycled via line 28 to digester 24 where it is mixed with the first acetonitrile/water azeotrope produced in head column 16. Ru. Since product column 48 is operated at high pressure, the second
All the water in the acetonitrile/water azeotrope is transferred to the overhead stream of product column 48, i.e., the third
recovered in an acetonitrile/water azeotrope of
High purity acetonitrile remains in the product column. This high purity acetonitrile (99.8% by weight acetonitrile) is removed from column 48 as a fluid side stream via line 50 and, after cooling in heat exchanger 44, is discharged as product via line 52.

From the above, it can be seen that the present invention can produce highly pure acetonitrile very simply and easily by distillation without using calcium chloride for water removal. Thus, the waste disposal problems associated with the prior art are significantly eliminated.

Although only one embodiment of the invention has been described, many modifications are possible without departing from the spirit and scope of the invention. For example, instead of an aqueous solution of sodium hydroxide and formaldehyde, HCN is supplied to the digester 24 with other chemicals that decompose the HCN without adversely affecting the acetonitrile product.
Can be used as a cooking agent. Additionally, pressures other than those listed above can be used in the triple distillation column of the present invention. for example,
The pressure of the head tower, drying tower, and product tower is 15~
The pressure in the product column is preferably at least 5 psig greater than the pressure in the head column. More preferably, the pressures in the head column, drying column, and product column are 16 to 20 psig, 3 to 4 psig, and 40 to 55 psig, respectively. All such variations are intended to be included within the scope of this invention.

[Brief explanation of drawings]

The accompanying drawings are schematic illustrations of the invention.

Claims (1)

[Scope of Claims] 1 (i) Crude acetonitrile containing acetonitrile, water, HCN, and heavy organic matter is distilled in a first distillation zone at 1 atm or above 1 atm to remove HCN. (ii) distilling the first azeotrope in a second distillation zone at a second pressure of less than 1 atmosphere to obtain an acetonitrile/water azeotrope of 1 and a water-containing first bottoms product; separating the boiling mixture into a water-containing second bottoms product and a second acetonitrile/water azeotrope having a greater acetonitrile concentration than the first azeotrope; (iii) the second acetonitrile/water azeotrope; The mixture is distilled in a third distillation zone at a third pressure greater than 1 atmosphere to form a third acetonitrile/water azeotrope containing substantially all of the water in the second azeotrope and acetonitrile and heavy organics. 1. A continuous method for recovering high-purity acetonitrile from crude acetonitrile, characterized in that it is separated into a third bottoms product consisting of a third bottoms product consisting of: