A process for recovering methanol from methyl chloride and increasing the purity of methyl chloride

Abstract

The present application belongs to the technical field of chloromethane production, and particularly relates to a process for recovering methanol in chloromethane and improving the purity of chloromethane. First, a rectifying tower is used to condense and recover methanol in gaseous chloromethane after acid water separation; the rectifying tower uses liquid-phase chloromethane condensed after compression by a compressor as reflux; then, gaseous chloromethane enters a sulfuric acid absorption tower from the bottom, so that the chloromethane cannot be condensed. The full utilization rate of sulfuric acid can be improved, and the amount of waste sulfuric acid can be reduced; the amount of sulfuric acid used and waste acid produced can be reduced by more than 50%; the amount of sulfuric acid used in the original process is 60 kg of concentrated sulfuric acid per ton of chloromethane, and now it can be reduced to less than 30 kg of concentrated sulfuric acid per ton of chloromethane; the service life of the sulfuric acid absorption tower is ensured, and the quality of the chloromethane product is improved; the service life of the sulfuric acid absorption tower can be extended by 5 to 10 years; the purity of the recovered methanol reaches more than 99.5%, which can be directly sent to the reaction system buffer tank for continued reaction.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of methyl chloride production, and particularly relates to a process method for recovering methanol in methyl chloride and improving the purity of methyl chloride. BACKGROUND

[0002] At present, the mixed gas such as methyl chloride generated by the reactor of domestic chemical enterprises is sent to a water washing tower and an alkali washing tower after being separated by acid water to absorb most of the unreacted hydrogen chloride gas and methanol, and then enters a sulfuric acid absorption tower to absorb the water and methanol in the crude methyl chloride at normal temperature. The high temperature in summer easily causes the temperature in the tower to be too high, and the increase of the saturated vapor pressure of the water and methanol in the acid will cause the sulfuric acid absorption effect to be poor, and the unabsorbed water and methanol will enter the subsequent system together with the methyl chloride. The product quality cannot be guaranteed. In addition, the service life of the sulfuric acid absorption tower equipment in the existing post-treatment process is short, and a large amount of sulfuric acid is consumed, which produces a large amount of waste acid.

[0003] CN 108440244 A discloses a process and device for recovering and utilizing methanol in methyl chloride production, which processes the object of alcohol-containing dilute hydrochloric acid, and does not involve how to improve the purity of methyl chloride and how to recover the consumed sulfuric acid. SUMMARY

[0004] In view of the poor sulfuric acid absorption effect and large consumption in the production process of methyl chloride at the present stage, especially in summer, the application provides a process method for recovering methanol in methyl chloride and improving the purity of methyl chloride. The gas-phase methyl chloride after acid-water separation is first recovered by a rectifying tower, and then the remaining methanol is removed by a sulfuric acid absorption method, and the purity of methyl chloride is improved.

[0005] The technical scheme of the application is as follows:

[0006] A process method for recovering methanol in methyl chloride and improving the purity of methyl chloride, comprising the following steps:

[0007] (1) The methanol in the gas-phase methyl chloride after acid-water separation is condensed and recovered by a rectifying tower. The liquid-phase methyl chloride after condensation is compressed by a compressor as reflux of the rectifying tower.

[0008] (2) The gas-phase methyl chloride enters a sulfuric acid absorption tower from the bottom to ensure that the methyl chloride does not condense.

[0009] Preferably, the outlet temperature of the rectifying tower in step (1) should be controlled below 10℃.

[0010] Further, the outlet temperature of the rectifying tower in step (1) should be controlled at 0-5℃.

[0011] Preferably, the temperature in the sulfuric acid absorption tower of step (2) is controlled below 25℃, and the pressure is controlled between 0.1Mpa and 0.21Mpa.

[0012] Further, the temperature in the sulfuric acid absorption tower of step (2) is 10-20℃, and the pressure is 0.13Mpa-0.19Mpa.

[0013] Further, the sulfuric acid absorption tower tray is a bubble cap tray, and the bubble cap trays are placed in pairs; the mass ratio of sulfuric acid to dimethyl ether is controlled between 3 and 7.

[0014] Further, the concentration of sulfuric acid is 96%.

[0015] The process of the present application mainly comprises a rectifying tower, a reboiler, a sulfuric acid drying tower, and corresponding conveying pumps and other devices, connecting pipelines, instruments, etc. Specifically, the gaseous chloromethane after acid-water separation is introduced into a rectifying tower within a certain pressure range. The rectifying tower is compressed by a chloromethane compressor to condense the liquid phase chloromethane as reflux, so as to reduce the temperature of the gaseous chloromethane inside the rectifying tower, condense the remaining most of the methanol, and then enter the sulfuric acid absorption tower from below the bottom tray of the sulfuric acid absorption tower in the form of low temperature. Under the corresponding temperature and pressure in the sulfuric acid absorption tower, the chloromethane is still higher than its dew point and will not condense. Therefore, the crude chloromethane vapor starts to flow upwards through the sulfuric acid absorption tower tray. The tray causes the crude chloromethane vapor to fully contact with the liquid sulfuric acid flowing down the tray, so that the trace amount of methanol, dimethyl ether and water in the crude chloromethane are absorbed by the sulfuric acid. After the methanol and dimethyl ether are absorbed by the sulfuric acid, the methanol at this time will react to generate dimethyl ether and water in the presence of sulfuric acid, and then react with sulfuric acid to generate dimethyl sulfate and hydrogen methanesulfate, which are discharged from the bottom of the tower in the form of waste acid.

[0016] Advantages of the present application

[0017] 1. The utilization rate of sulfuric acid can be improved, and the amount of waste sulfuric acid can be reduced; the amount of sulfuric acid used and the amount of waste acid produced can be reduced by more than 50%; the original process uses 60kg of concentrated sulfuric acid per ton of chloromethane, and now it can be reduced to less than 30kg of concentrated sulfuric acid per ton of chloromethane.

[0018] 2. The service life of the sulfuric acid absorption tower is guaranteed, and the quality of the chloromethane product is improved; the service life of the sulfuric acid absorption tower can be extended by 5-10 years; the purity of the recovered methanol reaches more than 99.5%, which can be directly sent to the reaction system buffer tank for further reaction. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The figure is a flowchart of the process method of the present application; as shown in the figure: 1, rectifying tower; 2, rectifying tower reboiler; 3, sulfuric acid absorption tower; 4, waste sulfuric acid conveying pump. DETAILED DESCRIPTION

[0020] The application will be further described in the following specific examples. It should be noted that the following description is only for the purpose of explaining the application and does not limit the content thereof.

[0021] Example 1

[0022] A process for recovering methanol in chloromethane and improving the purity of chloromethane, comprising the following steps:

[0023] The crude chloromethane feed amount is 10000 kg / h, which enters the lower side of the bottom plate of rectification column 1. The liquid-phase chloromethane from the outlet of the chloromethane compressor is 2900 kg / h as the reflux of the top of rectification column 1. The reflux flow is controlled by maintaining the column top temperature to ensure that the crude chloromethane is well separated in the rectification column 1. The column top temperature is controlled at about 10℃, and the column bottom temperature is controlled at about 55℃ by reboiler 2. The crude chloromethane at the outlet of rectification column 1 enters the lower side of the bottom plate of sulfuric acid absorption column 3 under the condition of low temperature and 0.1 Mpa pressure, and the temperature in the sulfuric acid absorption column 3 is controlled at about 25℃. At this temperature and pressure, the chloromethane is still higher than its dew point and will not condense. At this time, the crude chloromethane vapor starts to flow upward through the plates. The plates of the sulfuric acid absorption column are bubble plates, and the bubble plates are placed in pairs. The plates make the crude chloromethane vapor contact with the 300 kg / h liquid sulfuric acid flowing down the plates, so that the sulfuric acid absorbs the methanol, dimethyl ether and water in the chloromethane product. The mass ratio of sulfuric acid to dimethyl ether is 5. The gaseous chloromethane at the top of the sulfuric acid absorption column 3 enters the compression system for compression, and the waste sulfuric acid solution at the bottom is delivered to the sulfuric acid treatment system by sulfuric acid delivery pump 4. Finally, the dimethyl ether content in chloromethane is about 60 ppm.

[0024] Example 2

[0025] A process for recovering methanol in chloromethane and improving the purity of chloromethane, comprising the following steps:

[0026] The crude methyl chloride feed is fed at 15000 kg / h to the lower side of the bottom tray of distillation column 1. Liquid phase methyl chloride from the outlet of the methyl chloride compressor is used as reflux at the top of distillation column 1 at a rate of 4400 kg / h. The reflux rate is controlled by maintaining the temperature at the top of the column to ensure good separation of the crude methyl chloride in distillation column 1. The temperature at the top of the column is controlled at about 1°C, and the temperature at the bottom of the column is controlled at about 50°C by reboiler 2. The crude methyl chloride at the outlet of distillation column 1 is fed in a low temperature state to the lower side of the bottom tray of sulfuric acid absorption column 3 at a pressure of 0.21 MPa, and the temperature in sulfuric acid absorption column 3 is controlled at about 16°C. At this temperature and pressure, methyl chloride is still above its dew point and will not condense. At this time, the crude methyl chloride vapor starts to flow upward through the trays. The trays of the sulfuric acid absorption column are bubble trays, and the bubble trays are placed in pairs. The trays cause the crude methyl chloride vapor to contact 450 kg / h of liquid phase sulfuric acid flowing down the trays, so that the sulfuric acid absorbs the methanol, dimethyl ether and water in the methyl chloride product. The mass ratio of sulfuric acid to dimethyl ether is 7. The gaseous phase methyl chloride at the top of sulfuric acid absorption column 3 is compressed by a compression system, and the waste sulfuric acid solution at the bottom is pumped by sulfuric acid pump 4 to a sulfuric acid treatment system. The dimethyl ether content in the methyl chloride is finally measured to be about 48 ppm.

[0027] Example 3

[0028] A process for recovering methanol in methyl chloride and improving the purity of methyl chloride, comprising the following steps:

[0029] The crude methyl chloride feed is fed at 20000 kg / h to the lower side of the bottom tray of distillation column 1. Liquid phase methyl chloride from the outlet of the methyl chloride compressor is used as reflux at the top of distillation column 1 at a rate of 6000 kg / h. The reflux rate is controlled by maintaining the temperature at the top of the column to ensure good separation of the crude methyl chloride in distillation column 1. The temperature at the top of the column is controlled at about 5°C, and the temperature at the bottom of the column is controlled at about 60°C by reboiler 2. The crude methyl chloride at the outlet of distillation column 1 is fed in a low temperature state to the lower side of the bottom tray of sulfuric acid absorption column 3 at a pressure of 0.19 MPa, and the temperature in sulfuric acid absorption column 3 is controlled at about 20°C. At this temperature and pressure, methyl chloride is still above its dew point and will not condense. At this time, the crude methyl chloride vapor starts to flow upward through the trays. The trays of the sulfuric acid absorption column are bubble trays, and the bubble trays are placed in pairs. The trays cause the crude methyl chloride vapor to contact 580 kg / h of liquid phase sulfuric acid flowing down the trays, so that the sulfuric acid absorbs the methanol, dimethyl ether and water in the methyl chloride product. The mass ratio of sulfuric acid to dimethyl ether is 3. The gaseous phase methyl chloride at the top of sulfuric acid absorption column 3 is compressed by a compression system, and the waste sulfuric acid solution at the bottom is pumped by sulfuric acid pump 4 to a sulfuric acid treatment system. The dimethyl ether content in the methyl chloride is finally measured to be about 55 ppm.

[0030] The above describes the specific embodiments of the present application in combination with the drawings, but is not a limitation on the scope of protection, and those skilled in the art should understand that various modifications or changes made by those skilled in the art on the basis of the technical solutions of the present application without creative labor are still within the scope of protection of the present application.

Claims

1. A process for recovering methanol from chloromethane and improving the purity of chloromethane, characterized in that, For summer use, the following steps are included: (1) The gaseous chloromethane after acid-water separation is introduced into the distillation column from the bottom of the distillation column, and the liquid chloromethane after compression and condensation by the compressor is introduced into the top of the distillation column as reflux to reduce the temperature of the gaseous chloromethane in the distillation column, so that the outlet temperature of the distillation column should be controlled below 10°C, so that methanol can be condensed and recovered. (2) The gaseous chloromethane that comes out from the top of the distillation column and has a temperature below 10°C enters the sulfuric acid absorption tower from the bottom. Under the conditions of a temperature of 10~20°C and a pressure of 0.13Mpa~0.19Mpa, the trace amounts of methanol, dimethyl ether and water in the gaseous chloromethane are absorbed by sulfuric acid with a concentration of 96% to ensure that the chloromethane does not condense.

2. The process for recovering methanol from chloromethane and improving the purity of chloromethane according to claim 1, characterized in that, The outlet temperature of the distillation column in step (1) should be controlled at 0~5℃.

3. The process for recovering methanol from chloromethane and improving the purity of chloromethane according to claim 1 or 2, characterized in that, The sulfuric acid absorption tower uses bubble cap trays, which are placed in pairs; the mass ratio of sulfuric acid to dimethyl ether is controlled at 3-7.

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