JPS5989651A - Recovery and purification of methacrylonitrile - Google Patents

Abstract

PURPOSE:To recover and purify methacrylonitrile, economically, by contacting ammoxidation reaction gas with cooling water, distilling the condensed oil layer after separating from the water layer, and stripping the cooling water separated from the oil layer, thereby separating the dissolved methacrylonitrile. CONSTITUTION:In the process for the recovery and purification of methacrylonitrile (abbreviated as MAN) from the ammoxidation reaction gas composed mainly of methacrylonitrile, the reaction gas is made to contact with cooling water in the cndensation column 18, the condensed gas is extracted together with the cooling water from the bottom of the column and separated in the oil-water separator 20 into an oil layer and a cooling water layer containing MAN and other reaction components, the oil layer is fed to the following distillation column 21 for the removal of cyanic acid and water, and the water is returned to the stripper 15 of the stripping column 13 to recover MAN, and reused as the cooling water. The oil layer is further distilled in the product column 23, and the objective MAN is obtained from the top 24 of the column.

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 simplify the recovery and purification process and propose a method for the recovery and purification of MAN that can reduce equipment costs and expenses. It is produced by an ammoxidation reaction. This reaction product contains MAN as a main component, 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. That is, the ammoxidation reaction gas whose temperature has been lowered to about 200° C. is cooled in the quenching bath 1 by cooling water circulating through the circulation path 2, and then fed to the bottom of the absorption tower 3. Absorbed water is sent to the top of the absorption tower 3 through a conduit 4, and is 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 unabsorbed inert gas is discharged from the top of the tower as an off-gas. In the recovery tower 6, the absorption liquid is subjected to extractive distillation along with the solvent water sent through the Hiroshi conduit 7 to remove acetonitrile, and the top distillate is fed to the hydrocyanic acid/dehydration tower 8 to obtain a bottom liquid containing acetonitrile. is extracted to the stripping column 9 and distilled to separate acetonitrile and the like from the top of the column, and the bottom liquid mainly composed of water is passed through the conduit 4 or 7 and used as absorbed water or solvent water. 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 the product MAN from. High-boiling substances concentrated at the bottom of the column are extracted and drained 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 requires at least 39 tons of absorbed water (MANI)
(Amount of water required for saturated dissolution of AN)
is collected as an aqueous solution and recovered by extractive distillation using water as a solvent. At the same time, acetonitrile, which has a boiling point close to that of MAN, is removed and transferred to a subsequent distillation purification method to obtain product M.
It was intended to obtain an AN. For this reason, the amount of process liquid increases, and especially the equipment related to the absorption tower and the recovery tower are 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 a condensate forming an oil layer is separated and recovered from a 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 through a conduit 14. The reaction gas rising in the column first comes into contact with the aqueous sulfuric acid solution falling at the bottom of the column to remove unreacted ammonia and high-boiling substances. The remaining reaction gas enters the stripper part 15 in the center of the tower, contacts the absorbed water in which MAN is dissolved and falls from the top, and becomes absorbed water (3
) The MAN in the reactor is stripped and extracted into the reaction gas, which enters the cooling section 16 at the top of the column. In the cooling section 16, the reaction gas comes into contact with circulating and falling cooling water, is cooled to about 40.degree. 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-absorbed water is stripped, the MAN vapor is taken in, and the cooled reaction gas rises inside the tower from the bottom of the condensing tower 18, while the stripping tower 18
In step 5, the stripped absorbed water is cooled by a cooler 19 and is allowed to fall from the top of the condensation tower 18 into the tower 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, the oil layer is separated from the cooling water in which MAN and other reaction components are dissolved, and the oil layer is fed to the hydrocyanic acid dehydration tower 21, which is a subsequent distillation purification means.
The absorbed water is returned to the stripper section 15 of the stripping tower 13 for stripping to recover the MAN, and the water is regenerated as cooling water.

(4) 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 product MAN is released from a conduit 24 at the top of the tower. get. The high-boiling substances concentrated at the bottom of the column are extracted and blown down.
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 the same as above.
■ The ammoxidation reaction gas is condensed and recovered as an oil layer, ■ The cooling water that has absorbed and dissolved MAN and other reactants is stripped and MAN is recovered, and at the same time it is regenerated as cooling water. ■ The condensate forming the oil layer is The product is purified by distillation to obtain the product MAN.

In the conventional MAN recovery and purification process, the reaction gas is absorbed into water and recovered as a homogeneous aqueous solution. Therefore, 39 tons or more of absorbed water was required per ton of MANI, which corresponds to the amount of saturated dissolution, but this method can be recovered by using about 14 tons of cooling water cooled to about 1° C. per ton of MANI.

In addition, in the conventional method, a homogeneous aqueous solution of MAN was re-stripped by steam heating to recover the MAN and regenerate the absorbed water. can be recovered and used to regenerate cooling water. As a result, steam 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, υacetonitrile 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, acrylonitrile and acetonitrile have boiling points of 78°C and 81°C, respectively, with a boiling point difference of 3°C, and separation by normal distillation requires high equipment costs and is not practical. Therefore, 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 cooling water that has fallen through the condensing section 18 and is separated by the oil-water separator 20 is recycled and used after stripping dissolved MAN. The solubility of MAN in water is about 2.5%, and M
The distribution ratio of MAN between the AN-absorbed water and the high-temperature gas is extremely large on the high-temperature gas side;
Alternatively, if water vapor is used, dissolved MAN can be easily stripped to a concentration of 500 ppm or less, MAN can be recovered, and cooling water can be regenerated at the same time. In addition, the MAN concentration in the circulating cooling water can be extremely low, making it possible to
If used after being cooled to a certain level, the MAN concentration in the off-gas discharged from the top of the condensing section can easily be reduced to 200 ppm.
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 to remove the total amount of MAN. It can be recovered as a condensate and fed to the subsequent distillation purification system for purification, making it convenient to dry. However, stripping may be performed using water vapor instead of the reaction gas.

(7.

In addition, the stripping method using the high-temperature reaction gas is also applicable to the second method.
It is not limited to what is shown in the figure.

FIG. 3 shows another embodiment of the process to which this recovery and purification method is applied. The reaction gas (approximately 200°C) is fed to the ammonia neutralization unit 26 at the bottom of the rapid cooling condensation unit 25, where unreacted ammonia is neutralized, high-boiling substances are removed, and the gas is cooled to approximately 90°C.

This reactant gas then enters the stripper portion 27 and enters the MA
MAN in the absorbed water that absorbed and dissolved N (approx. 2.5 wt%
) stripping. Subsequently, it is fed to the cooling section 28, cooled to 40° C. with cooling water, and enters the condensing section 29.

On the other hand, the absorbed water (M
AN (approximately 500 ppm) is cooled by a cooler 31 (to approximately 40° C.) and fed to the cold heat recovery section 30. Cold heat recovery section 3
0, the outlet gas of the condensing section 29 (approximately 1°C) and the absorbed water (approximately 1°C)
0℃) is heat exchanged, and the absorbed water is cooled to about 30℃,
Subsequently, the reactant gas is finally cooled down to 1°C in the cooler 32 and fed to the condensing section 29. (8) In the condensing section 29, the reaction gas comes into countercurrent contact with the absorbed water at 1°C,
MAN and other organic substances condense. Absorbed water containing condensate is extracted from the bottom of the condensing section 29, an oil-water separator 20 separates the oil-water layer, and the oil layer is sent to the purification system and treated in the same process as shown in FIG. The water layer is recovered by stripping the MAN in the stripper part 27, and at the same time regenerating the absorbed water.

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. can.

Example (1) Column configuration (based on the process shown in Figure 2) ■ Stripping tower-13 lower part ... Sprout V-tower cooling section ... 5 sieve trays ■ Condensing section - 18... Sieve tray 52 stages ■ Hydrocyanic acid dehydration tower - 21 sieve tray 55 stages feed stage ... 34 stages counting from the bottom ■ Low boiling point separation tower - 22 sieve tray 1
00th feed stage... 60th stage counting from the bottom■ Product tower-23... Sieve tray 49th stage feed stage...
12-stage product side cut crotch counting from the bottom 42-stage counting from the bottom (2) Steam consumption of the tower ■ Stripping tower - 13 zero ■ Condensing section - 18 zero ■ Hydrocyanic acid dehydration tower - 210,3T/T-MAN ■ Low boiling point separation column - 221,8T/T-MAN ■ Product column - 231
, 0T/T-MAN total 3.1 T/T-MAN (3) Main flow rate■ Stripping tower-13 feed gas amount 8.9T/
T-MAN ■ Condensing section - 18 absorption water amount 14 T/T-MAN ■ Hydrocyanic acid dehydration tower - 21 feed amount 1.3 T/T-MAN
■ However, 1) Delamination-22 Feed amount 1.2 T
/T-MAN■ Product tower-23 feed amount Medium 1. O.T.
/T-MAN (4) Main temperature■ Stripping column-13 Feed gas temperature...200℃ Lower column bottom temperature...86℃ Stripper part bottom temperature 85℃ Stripper part top 1 81℃ Stripper Partial feed liquid l 70°C Cooling section bottom temperature...50°C Cooling section top temperature...41°C ■ Condensing section-18 Absorbed water temperature...1°C Tower top' ・・・・・・・ 1℃ Tower bottom ′ ・・・・・・ 35℃ ■ Hydrocyanic acid removal tower-21 Tower top temperature ・・・・・・ 30℃ Tower bottom ′ ・・・・・・ 100℃ ■ Low-boiling separation column-22 Top temperature: 54℃ (11) Bottom temperature: 87℃ 4■ Product column-23 Top temperature: 50 ℃ Tower bottom temperature 80℃ (5) Main composition (unit: wt%) (12)

[Brief explanation of the drawing]

Figure 1 is a flow sheet of the conventional recovery and purification process;
The figure is a flow sheet of the recovery and purification process of this invention.
FIG. 3 is a flow sheet showing another embodiment of the present invention. 1...Quiet cooling bath, 2...Circulation system path, 3...Absorption tower, 4...
Conduit, 5... Conduit, 6... Collection tower, 7... Conduit, 8...
Hydrocyanic acid removal/dehydration tower, 9. Stripping tower, 10. Low boiling point separation tower, 11. Product tower, 12. Conduit, 13- Stripping column, 14. Conduit, 15.
Part of stripper, 16...Cooling part, 17...Conduit, 18
... Condensing section, 19.. Cooler, 20.. Oil-water separator, 21.. Hydrocyanic acid removal tower, 22.. Low boiling point separation tower, 23.. Product tower, 24..
- Conduit, 25... Rapid cooling condensation section, 26... Ammonia neutralization section, 27
...Part of stripper, 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 ammoxidation reaction gas containing methacrylonitrile as its main component, the reaction gas is brought into contact with cooling water and condensed, and the condensate forming an oil layer is converted into a cooling water layer. A method for recovering and purifying methacrylonitrile, which comprises separating and 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.