JPS6133815B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for recovering and purifying methacrylonitrile (hereinafter abbreviated as MAN). The purpose is to propose a MAN recovery and purification method that can simplify the recovery and purification process and reduce equipment costs and expenses. MAN is produced by a gas phase contact reaction between isobutylene or tertiary butyl alcohol, etc., ammonia and oxygen, that is, an ammoxide reaction. This reaction product is mainly composed of MAN and contains by-products such as methacrolein, acetonitrile, acrylonitrile, hydrocyanic acid, isobutyronitrile, and propionitrile, and is obtained in the form of a high-temperature gas. The method for recovering and purifying MAN from this reaction gas has conventionally been carried out in accordance with the process for acrylonitrile, as shown in FIG. i.e. 200
The ammoxidation reaction gas whose temperature has been lowered to about .degree. C. is cooled by cooling water circulating through a circulation path 2 in a quenching tower 1, and is fed to the bottom of an absorption tower 3. Absorbed water is sent to the top of the absorption tower 3 through a conduit 4 and brought into contact with the reaction gas rising from the bottom of the tower, so that the reactants are absorbed and collected by the absorbed water. The absorption liquid is fed from the bottom of the tower through a conduit 5 to a recovery tower 6, and the unabsorbed inert gas is discharged from the top of the tower as an off-gas. In the recovery tower 6, the absorbed liquid is sent through the conduit 7 and subjected to extractive distillation together with the solvent water to remove acetonitrile, and the top distillate is fed to the prussic acid removal/dehydration tower 8, and the bottom liquid containing acetonitrile is sent to the stripping tower. 9 is extracted and distilled to separate acetonitrile and the like from the top of the column, and the bottom liquid mainly composed of water is used as absorbed water or solvent water via conduit 4 or 7. The process liquid from which acetonitrile has been removed is distilled in a hydrocyanic acid removal/dehydration tower 8 to remove hydrocyanic acid and water, and then low-boiling substances are removed in a low-boiling separation tower 10, further distilled in a product tower 11, and passed through a conduit 12 at the top of the tower. Get product MAN from. High-boiling substances concentrated at the bottom of the column are extracted and blown down, and trace amounts of low-boiling substances remaining are extracted from the top of the column and removed. The conventional MAN recovery and purification process is as described above, and at least one ton of MAN absorbs water.
It requires more than 39 tons (the amount of water required for saturated dissolution of MAN), and MAN is collected as an aqueous solution, which is then recovered by extractive distillation using water as a solvent.At the same time, acetonitrile, which has a boiling point close to MANN, is removed, and the subsequent distillation is performed. It was to be transferred to a refining means and obtain a product MAN. For this reason, the amount of process liquid increases, and especially the equipment related to the absorption tower and the recovery tower becomes large, resulting in a large burden of equipment costs and expenses. This invention was made in view of the above circumstances, and its gist is that in the process of recovering and refining MAN from an ammoxidation reaction gas containing MAN as a main component, the reaction gas is brought into contact with cooling water and condensed, A MAN characterized in that the condensate forming the oil layer is separated and recovered from the cooling water layer and purified by being fed to a subsequent distillation purification means, and the cooling water separated from the oil layer is stripped to recover dissolved MAN.
This is a recovery and purification method. An example of a process to which this recovery and purification method is applied is shown in FIG. 2. In this process, an ammoxidation reaction gas whose temperature has been lowered to about 200° C. is fed to the bottom of a stripping column 13 from a conduit 14. The reactant gas rising in the tower first comes into contact with the sulfuric acid aqueous solution falling at the bottom of the tower, and unreacted ammonia,
High boilers are removed. The remaining reaction gas enters the stripper section 15 at the center of the column, contacts the absorption water in which MAN falling from the top is dissolved, strips the MAN in the absorption water, extracts it into the reaction gas and takes it into the stripper section 15 at the top of the column. It enters the cooling section 16. Cooling section 16
In this process, the reaction gas comes into contact with the circulating cooling water and is cooled to about 40° C., and is sent from the top of the tower through a conduit 17 to a condensation tower 18. In the stripping tower 13, the MAN absorption water is stripped, the MAN vapor is taken in, and the cooled reaction gas rises in the tower from the bottom of the condensation tower 18.
On the other hand, in the stripper section 15, the stripped absorption water is cooled by a cooler 19 and falls from the top of the condensing tower 18 as cooling water. In the condensation tower 18, the reaction gas comes into contact with the cooling water and is condensed, and together with the cooling water, it is extracted from the bottom of the tower and enters the oil-water separator 20, and the inert gas that is not condensed is discharged from the top of the tower. In the oil-water separator 20, an oil layer and
Cooling water in which MAN and other reaction components are dissolved is separated, and the oil layer is fed to the hydrocyanic acid dehydration tower 21, which is a subsequent distillation purification means, and the absorbed water is returned to the stripper section 15 of the stripping tower 13 for stripping.
The MAN is recovered and the water is recycled as cooling water.
After removing hydrocyanic acid and water from the oil layer in a hydrocyanic acid removal tower 21, low-boiling substances are removed in a low-boiling separation tower 22, and further distilled in a product tower 23, and a product MAN is obtained from a conduit 24 at the top of the tower. High-boiling substances concentrated at the bottom of the column are extracted and blown down, and trace amounts of low-boiling substances remaining are extracted from the top of the column and removed. The process using this recovery and purification method is as described above. The ammoxidation reaction gas is condensed and recovered as an oil layer, and the cooling water that has absorbed and dissolved MAN and other reactants is stripped.
MAN is recovered and regenerated as cooling water at the same time.
The condensate that forms the oil layer is purified by distillation to produce the product MAN.
The purpose is to obtain In the conventional MAN recovery and purification process, the reaction gas is absorbed into water and recovered as a homogeneous aqueous solution. Therefore, more than 39 tons of absorbed water was required per 1 ton of MAN, which corresponds to the amount of saturated dissolution, but this method can be recovered by using approximately 14 tons of cooling water cooled to about 1°C per 1 ton of MAN. Also,
In the conventional method, a homogeneous aqueous solution of MAN was stripped by steam heating to recover the MAN and regenerate the absorbed water.
The cooling water can be regenerated by stripping around 10% of the cooling water and recovering the MAN. As a result, the amount of steam used for regenerating absorbed water can be saved, and at the same time, the conventional attached equipment such as boilers and condensers around the stripper (recovery tower) can be eliminated or replaced with other small-scale equipment. In this method, acetonitrile is separated and removed from the condensate collected as an oil layer by simple distillation without performing extractive distillation (stripping) using water as a solvent. In the recovery and purification of acrylonitrile, the boiling points of acrylonitrile and acetonitrile are 78°C and 81°C, respectively, with a boiling point difference of 3°C, and separation by ordinary distillation requires high equipment costs and is impractical. For this reason, it was necessary to perform water extractive distillation using the difference in solubility in water. However, MAN has a boiling point of 90°C, and the difference in boiling point from acetonitrile is 9°C, so it can be separated by distillation under economically viable conditions. In this method, the condensation tower 18 is dropped and the oil/water separator 20
The cooled water separated is stripped of dissolved MAN and recycled for use. The solubility of MAN in water is approximately 2.5%, and the distribution ratio of MAN between the MAN-absorbed water and the high-temperature gas is extremely large on the high-temperature gas side, and it is easy to solve this problem if a reaction gas of 85°C or higher or water vapor is used. It is possible to strip the MAN dissolved in water to a concentration of 500 ppm or less, recover the MAN, and regenerate cooling water at the same time. Also, MAN in the cooling water used for circulation.
Since the concentration can be made extremely low, if the MAN concentration in the off-gas discharged from the top of the condensing tower can be easily reduced to 200 ppm if it is used after being cooled to about 1°C.
Can be kept below. If the absorbed water in which MAN is dissolved is stripped using a high-temperature reaction gas using this recovery and purification method, the heat of the reaction gas can be used effectively, and the stripped MAN can be extracted into the reaction gas and the entire amount of MAN can be converted into condensate. It is convenient because it can be recovered as a liquid and fed to the subsequent distillation purification system for purification. However, stripping may be performed using water vapor instead of the reactant gas.
Further, the form of the strip formed by the high-temperature reaction gas is not limited to that shown in FIG. 2 above. FIG. 3 shows another embodiment of the process to which this recovery and purification method is applied. Reactant gas (approx.
200°C) is fed to the ammonia neutralization section 26 at the bottom of the quenching condensation tower 25, neutralizes unreacted ammonia, removes high-boiling substances, and cools to about 90°C. This reaction gas then enters the stripper section 27,
MAN in the absorbed water that absorbed and dissolved MAN (approximately 2.5wt
%). Subsequently, it is fed to the cooling section 28, cooled to 40° C. with cooling water, and then enters the condensing section 29. On the other hand, the absorbed water (MAN about 500 ppm) stripped by the stripper section 27 is cooled by a cooler 31 (to about 40° C.) and fed to the cold heat recovery section 30. In the cold heat recovery section 30, the outlet gas of the condensing section 29 (approximately 1
℃) and absorbed water (40℃) are heat exchanged, and the absorbed water is approximately
It is cooled down to 30° C., then finally cooled down to 1° C. in the cooler 32, and fed to the condensing section 29. In the condensing section 29, the reaction gas comes into countercurrent contact with the absorbed water at 1° C., and MAN and other organic substances are condensed. Absorbed water containing condensate is extracted from the bottom of the condensing section 29,
The oil-water separator 20 separates the oil-water layer, and the oil layer is sent to a refining system and treated in a process similar to that shown in FIG. The water layer is MAN at the stripper part 27.
The water is stripped and recovered, and at the same time the absorbed water is regenerated. This recovery and purification method has the above-mentioned configuration. Compared to the conventional method, this method reduces the amount of cooling water (absorbed water) and the number of towers required for recovery and purification, reducing equipment costs and expenses. . Example (1) Column configuration (based on the process shown in Figure 2) Stripping tower-13 Lower part......Spray tower stripper section 15 10-stage sieve tray Cooling section......5-stage sieve tray Condensing tower-18......Sieve tray 52 Stages Hydrocyanic acid removal dehydration tower-21 Feed stage with 55 sieve trays...34 stages counting from the bottom Low-boiling separation tower-22 Feed stage with 100 sieve trays...60 stages counting from the bottom Product tower-23......Sieve tray 49 Stage feed stage... 12 stages counting from the bottom Product side cut stage 42 stages counting from the bottom (2) Steam consumption of the tower Stripping tower - 13 Zero Condensing tower - 18 Zero Hydrocyanic acid removal tower - 21 0.3T/T- MAN Low-boiling separation column-22 1.8T/T-MAN Product column-23 1.0T/T-MAN Total 3.1T/T-MAN (3) Main flow rate Stripping column-13 Feed gas amount
8.9T/T-MAN Condensing tower-18 Absorbed water amount 14T/T-
MAN Hydrocyanic acid dehydration tower-21 Feed amount
1.3T/T-MAN Low-boiling separation column-22 Feed amount 1.2T/
T-MAN Product tower-23 Feed amount ≒1.0T/T
-MAN (4) Main temperature Stripping column -13 Feed gas temperature......200℃ Lower column bottom temperature......86℃ Stripper section bottom temperature 85℃ Stripper section top temperature 81℃ Stripper section feed liquid temperature 70℃ Cooling section Bottom temperature: 50°C Cooling section top temperature: 41°C Condensing tower-18 Absorbed water temperature: 1°C Tower top: 1°C Bottom: 35°C Hydrocyanic acid dehydration tower -21 Column top temperature......30℃ Column bottom 〃......100℃ Low-boiling separation column-22 Column top temperature......54℃ Column bottom temperature......87℃ Product column-23 Column top temperature...... 50℃ Bottom temperature...80℃ (5) Main composition (unit: wt%)

【table】

[Brief explanation of the drawing]

FIG. 1 is a flow sheet of a conventional recovery and purification process, FIG. 2 is a flow sheet of a recovery and purification process according to the present invention, and FIG. 3 is a flow sheet showing another embodiment of the present invention. 1...Quick cooling tower, 2...Circulation system path, 3...Absorption tower, 4...Pipe, 5...Recovery tower, 7...Pipe, 8
... Hydrocyanic acid removal/dehydration tower, 9 ... Stripping tower, 10 ... Low boiling point separation tower, 11 ... Product column, 12 ... Conduit, 13
... Stripping tower, 14 ... Conduit, 15 ...
Stripper section, 16... Cooling section, 17... Conduit, 18... Condensing tower, 19... Cooler, 20...
Oil/water separator, 21... Hydrocyanic acid removal dehydration tower, 22... Low boiling point separation tower, 23... Product column, 24... Conduit, 25
...Quick cooling condensation tower, 26...Ammonia neutralization section, 2
7... Stripper section, 28... Cooling section, 29...
...Condensing section, 30...Cold heat recovery section, 31, 32...
cooler.

Claims (1)

[Claims] 1 In the process of recovering and refining methacrylonitrile from an ammoxide reaction gas containing methacrylonitrile as the main component, the reaction gas is brought into contact with cooling water and condensed, and the condensate forming an oil layer is separated from the cooling water layer. 1. A method for recovering and purifying methacrylonitrile, which comprises recovering methacrylonitrile, feeding it to a subsequent distillation purification means for purification, and stripping the cooling water separated from the oil layer to recover dissolved methacrylonitrile.