Method for recovering methanol in methanol aqueous solution

Abstract

The invention relates to the field of post-treatment of hydrogen peroxide epoxidation products, and discloses a method for recovering methanol in a methanol aqueous solution. The method comprises the following steps: (1) carrying out first rectification on a methanol aqueous solution so that the mass fraction of 3-chloropropene is not higher than 0.1% in a tower bottom discharge material; (2) in the presence of an alkaline aqueous solution, carrying out second rectification on the tower bottom discharged material obtained in the step (1) to respectively obtain methanol and wastewater with the pH value of 5-8; wherein on the basis of the total mass of the methanol aqueous solution, in the methanol aqueous solution, the mass fraction of methanol is 40-50%, the mass fraction of water is 45-55%, the mass fraction of 3-chloropropene is 1-5%, and the mass fraction of chloropropylene glycol monomethyl ether and chloropropylene glycol is 0.1-2%. According to the method, the corrosion of the methanol aqueous solution to equipment in the rectification separation and recovery process can be remarkably reduced, the equipment investment and maintenance cost are reduced, and the risk of safety accidents is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of hydrogen peroxide epoxidation product post-treatment, in particular to a method for recovering methanol from a methanol aqueous solution. BACKGROUND

[0002] CN103772326A discloses a method for producing epichlorohydrin, which comprises an epoxidation reaction process and an extraction process, as well as a first distillation process, a second distillation process and a third distillation process; the epoxidation reaction process comprises synthesizing epichlorohydrin by directly epoxidizing 3-chloropropene with hydrogen peroxide in the presence of a titanium-silicon molecular sieve catalyst and a solvent methanol to obtain an epoxidation reaction product; the extraction process uses an extractant 3-chloropropene and an extractant water to separate the epoxidation reaction product by extraction to obtain a first raffinate phase rich in the solvent methanol and a second raffinate phase rich in epichlorohydrin; the first distillation process distills the first raffinate phase to obtain the solvent methanol and 3-chloropropene, and at least part of the obtained methanol and / or at least part of the obtained 3-chloropropene are recycled to the epoxidation reaction process; the second distillation process distills the second raffinate phase to obtain 3-chloropropene and crude epichlorohydrin, respectively, and at least part of the obtained 3-chloropropene are recycled to the epoxidation reaction process and / or the extraction process; the third distillation process distills the crude epichlorohydrin to obtain an epichlorohydrin product. The first distillation process is preferably carried out at normal pressure (i.e., 1 standard atmosphere), and the distillation temperature can be 60-70℃. The above-mentioned method can obtain a very high epichlorohydrin extraction rate, an epichlorohydrin distillation yield and a high-purity epichlorohydrin product.

[0003] CN105585542A discloses a method for separating epichlorohydrin, which comprises: using 3-chloropropene as raw material and hydrogen peroxide as oxidant to prepare epoxidation reaction product of epichlorohydrin, and using extractant 3-chloropropene and water to separate and obtain raffinate phase containing reaction solvent methanol and extract phase containing epichlorohydrin; feeding the raffinate phase into a first rectification system to separate and recover methanol and discharge waste water, wherein the first rectification system comprises a first rectification tower and a second rectification tower; feeding the extract phase into a second rectification system to separate and obtain epichlorohydrin product and recovered chloropropene, wherein the second rectification system comprises a third rectification tower and a first vacuum rectification tower; by controlling the operating conditions of the first rectification tower and the second rectification tower and the proportion of recovered methanol, the vapor at the top of the second rectification tower can be used as at least part of heat source of the second rectification system. In the first rectification system, the operating conditions of the first rectification tower include: top pressure of 0.5 MPaG to normal pressure, theoretical plate number of 5-60, column bottom temperature of 40-120 DEG C, and reflux ratio of 0.5-3; the operating conditions of the second rectification tower include: top pressure of 0.3-4.0 MPaG, theoretical plate number of 5-60, column bottom temperature of 110-200 DEG C, and reflux ratio of 0.5-3. By using the above disclosed method for separating epichlorohydrin, the energy consumption of rectification separation can be reduced by more than 30%.

[0004] However, the above two patent applications do not consider the problem of serious corrosion of equipment in the rectification process. SUMMARY

[0005] The purpose of the present application is to overcome the problem of serious corrosion of equipment in the prior art, and to provide a method for recovering methanol from methanol aqueous solution. The method can significantly reduce the corrosion of equipment during the rectification separation and recovery of methanol aqueous solution, reduce the investment and maintenance cost of equipment, and reduce the risk of safety accidents.

[0006] In order to achieve the above purpose, the present application provides a method for recovering methanol from methanol aqueous solution, which comprises the following steps:

[0007] (1) performing first rectification on the methanol aqueous solution, and controlling the conditions of the first rectification so that the mass flow rate of methanol in the overhead material accounts for 1-9.9% of the mass flow rate of methanol in the methanol aqueous solution; and the mass fraction of 3-chloropropene in the bottom discharge is not higher than 0.1%;

[0008] (2) performing second rectification on the bottom discharge obtained in step (1) in the presence of an alkaline aqueous solution to obtain methanol and waste water, respectively, and the pH of the waste water is 5-8;

[0009] The mass fraction of methanol in the methanol aqueous solution is 40-50%, the mass fraction of water is 45-55%, the mass fraction of 3-chloropropene is 1-5%, and the mass fraction of chloropropyl glycol monomethyl ether and chloropropyl glycol is 0.1-2%, based on the total mass of the methanol aqueous solution.

[0010] Preferably, the material of the first rectifying tower is selected from at least one of pure titanium TA2, titanium alloy TA8, titanium alloy TA9, and titanium alloy TA10, preferably pure titanium TA2 and / or titanium alloy TA10.

[0011] Preferably, the material of the second rectifying tower is Hastelloy C-276 and / or Hastelloy C-22.

[0012] The inventors of the present application found in the research process that the epoxidation reaction product mainly contains methanol, 3-chloropropene, epichlorohydrin, and water, and also contains a small amount of by-products such as chloropropyl glycol monomethyl ether and chloropropyl glycol, and the pH value of the epoxidation reaction product is generally 3-6, which is slightly acidic. After the epoxidation reaction product is extracted and separated by using the extractant 3-chloropropene and the extractant water, a raffinate phase containing most of the methanol in the epoxidation reaction product is obtained, which generally contains 45%-55% water, 40%-50% methanol, 1%-5% 3-chloropropene, and 0.1%-2% chloropropyl glycol monomethyl ether and chloropropyl glycol, and the pH value of the raffinate phase is generally 2-4, which is relatively strong acidic. The raffinate phase is subjected to atmospheric distillation separation by the first rectifying tower of the first rectifying system disclosed in CN105585542A, 3-chloropropene and at least part of the methanol are recovered from the top of the first rectifying tower, and the methanol aqueous solution obtained from the bottom of the first rectifying tower generally contains 45%-58% water, 40%-50% methanol, and 0.1%-2% chloropropyl glycol monomethyl ether and chloropropyl glycol, the pH value of the 3-chloropropene and methanol mixture discharged from the top of the first rectifying tower is reduced to 0.5-3.5, and the pH value of the methanol aqueous solution discharged from the bottom of the first rectifying tower is also reduced to 1-3, which presents stronger acidity; in the second rectifying tower of the first rectifying system, the pH value of the waste water discharged from the bottom of the second rectifying tower is further reduced to 0.6-3 due to the use of higher distillation separation temperature, which presents stronger acidity, and can seriously corrode the equipment, pipelines, and valves, leading to leakage or even rupture of the equipment, pipelines, and valves, and there is a risk of causing serious safety accidents. The inventors of the present application further found in the research process that the method disclosed in the present application can significantly reduce the corrosion of the methanol aqueous solution to the equipment in the distillation separation and recovery process, reduce the equipment investment and maintenance cost, and reduce the risk of safety accidents. Moreover, the method provided in the present application is easy to implement and suitable for application in large-scale industrial production. DETAILED DESCRIPTION

[0013] The endpoints of the ranges and any values claimed herein are not to be understood as being limited to the exact values recited as implicitly included within the meaning of each range. The ranges should be interpreted in each context as being inclusive of the recited endpoints and all intervening points between the recited endpoints. The endpoints of the ranges of values are not to be understood as being limited to the exact values recited as implicitly included within the meaning of each range. The ranges should be interpreted in each context as being inclusive of the recited endpoints and all intervening points between the recited endpoints and the individual points within the ranges.

[0014] One aspect of the present application provides a method for recovering methanol from a methanol aqueous solution, the method comprising the following steps:

[0015] (1) subjecting the methanol aqueous solution to a first distillation, and controlling the conditions of the first distillation such that the mass flow rate of methanol in the overhead material accounts for 1-9.9% of the mass flow rate of methanol in the methanol aqueous solution, and the mass fraction of 3-chloropropene in the bottom discharge is not higher than 0.1%;

[0016] (2) subjecting the bottom discharge obtained in step (1) to a second distillation in the presence of an alkaline aqueous solution, to obtain methanol and waste water, respectively, and the pH of the waste water is 5-8;

[0017] wherein, based on the total mass of the methanol aqueous solution, the mass fraction of methanol in the methanol aqueous solution is 40-50%, the mass fraction of water is 45-55%, the mass fraction of 3-chloropropene is 1-5%, and the mass fraction of chloropropylene glycol monomethyl ether and chloropropylene glycol is 0.1-2%.

[0018] The methanol aqueous solution according to the present application can also contain a small amount of other impurities, which have no substantial effect on the recovery process of methanol according to the present application, and thus are not described in detail herein.

[0019] In the method according to the present application, the conditions of the first distillation are controlled such that the recovered methanol in the overhead material accounts for only 1-9.9% of the methanol in the methanol aqueous solution, and the mass fraction of 3-chloropropene in the bottom discharge is not higher than 0.1%, which can reduce the corrosion of the overhead material to the equipment, avoid the production of other impurities in the second distillation in the presence of an alkaline aqueous solution, which can affect the quality of the recovered methanol, and reduce the coking of 3-chloropropene at a high temperature in the second distillation, which can affect the long-term operation of the device. Then, the second distillation is performed in the presence of an alkaline aqueous solution, and the pH of the waste water is controlled to be 5-8, which can significantly reduce the corrosion of the methanol aqueous solution to the equipment during the distillation and separation recovery process, reduce the equipment investment and maintenance cost, and reduce the risk of safety accidents.

[0020] The source of the methanol aqueous solution is not particularly limited in the present application, and can be obtained by a conventional method in the art. Preferably, the methanol aqueous solution is a raffinate obtained by extractive separation of a liquid epoxidation product obtained by epoxidation of 3-chloropropene with hydrogen peroxide in the presence of a solvent methanol.

[0021] The specific process of the epoxidation reaction and the extractive separation is not particularly limited in the present application, and can be performed according to a conventional method in the art, as long as the raffinate with the above composition can be obtained. For example, the process can be performed according to CN103772326A.

[0022] Preferably, the extractive separation process of the epoxidation product comprises: sequentially using 3-chloropropene and water as extractants to extractively separate the epoxidation product, to obtain a raffinate.

[0023] According to the present application, preferably, the pH value of the methanol aqueous solution is 2-4.

[0024] In the present application, the mass flow rate of methanol in the overhead of the first rectification accounts for 1-9.9%, preferably 3-9.5%, of the mass flow rate of methanol in the methanol aqueous solution. Specifically, for example, it can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 9.9%, or any value in the range constituted by any two of these point values. By using this preferred embodiment, the corrosion of the overhead material to the equipment can be reduced.

[0025] In the present application, the mass fraction of 3-chloropropene in the bottom discharge of the first rectification is not higher than 0.1%, preferably 0.001-0.1%. Specifically, for example, it can be 0.1%, 0.095%, 0.09%, 0.08%, 0.07%, 0.06%, 0.05%, 0.04%, 0.03%, 0.02%, 0.01%, 0.005%, 0.001%, or any value in the range constituted by any two of these point values. By using this preferred embodiment, the influence of 3-chloropropene on the subsequent second rectification can be effectively avoided, the influence of 3-chloropropene on the rectification separation and recovery of methanol in the presence of an alkaline aqueous solution can be avoided, and the coking of 3-chloropropene at a high temperature in the second rectification, which can affect the long-period operation of the device, can be reduced.

[0026] It can be understood that "the mass fraction of 3-chloropropene in the bottom discharge of the first rectification is not higher than 0.1%" means that, in the bottom discharge of the first rectification, the mass content of 3-chloropropene is not higher than 0.1% based on the total mass of the bottom discharge.

[0027] According to the present application, preferably, the mass flow rate of 3-chloropropene in the overhead of the first rectification accounts for more than 95%, preferably more than 98% of the mass flow rate of 3-chloropropene in the methanol aqueous solution. With this preferred embodiment, it is not only beneficial to make the mass fraction of 3-chloropropene in the bottom discharge not higher than 0.1%, but also to reduce the loss of 3-chloropropene and lower the energy consumption of the first rectification column.

[0028] According to the present application, preferably, the conditions of the first rectification include: the overhead pressure is 0-0.1 MPaG, preferably 0-0.05 MPaG, specifically, for example, it can be 0 MPaG, 0.01 MPaG, 0.02 MPaG, 0.03 MPaG, 0.04 MPaG, 0.05 MPaG, 0.06 MPaG, 0.07 MPaG, 0.08 MPaG, 0.09 MPaG, 0.1 MPaG, and any value in the range constituted by any two of these point values; the overhead temperature is 50-80℃, preferably 50-70℃, specifically, for example, it can be 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, and any value in the range constituted by any two of these point values; the reflux ratio is 0.1-10, preferably 1-5, specifically, for example, it can be 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and any value in the range constituted by any two of these point values. With this preferred embodiment, it is more beneficial to make the overhead and the bottom discharge of the first rectification meet the above conditions.

[0029] In the present application, the pH of the wastewater is 5-8, preferably 5.5-7.5, specifically, for example, it can be 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8, and any value in the range constituted by any two of these point values. With this preferred embodiment, it is possible to reduce the corrosion to the equipment while ensuring that no other impurities are produced in the second rectification to affect the rectification effect.

[0030] According to the present application, preferably, in step (2), the mass flow ratio of the alkaline aqueous solution to the bottom discharge obtained in step (1) is 0.001-0.1, preferably 0.01-0.06, for example, can be 0.001:1, 0.002:1, 0.003:1, 0.004:1, 0.005:1, 0.006:1, 0.007:1, 0.008:1, 0.009:1, 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1, 0.06:1, 0.07:1, 0.08:1, 0.09:1, 0.1:1 and a range value formed by any two of these point values. With this preferred embodiment, it is beneficial to reduce the corrosion of the rectification material to the equipment, and also to avoid the excessive addition of alkali solution to cause material consumption and the generation of other by-products to affect the quality of the methanol recovered by rectification separation.

[0031] According to the present application, preferably, the mass content of the alkaline aqueous solution is 1-50%, preferably 10-30%, for example, can be any value in the range of 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% and any two of these point values. With this preferred embodiment, it is possible to avoid the problem of too low alkali concentration to increase the amount of wastewater discharge, and also to avoid the problem of too high alkali concentration to easily freeze in winter and higher price.

[0032] According to the present application, preferably, the alkaline aqueous solution is selected from at least one of an aqueous solution of an alkali metal hydroxide, an alkaline earth metal hydroxide, ammonia, a quaternary ammonium base and an alkaline salt, more preferably at least one of an aqueous solution of an alkali metal hydroxide, an alkaline earth metal hydroxide and an alkaline salt, and particularly preferably an aqueous sodium hydroxide solution. With this preferred embodiment, it is possible to further avoid the generation of other impurities to affect the rectification effect.

[0033] According to the present application, preferably, the conditions of the second rectification include that the column top pressure is 0-2 MPaG, the column bottom temperature is 100-215℃, and the reflux ratio is 0.5-5. With this preferred embodiment, it is beneficial to more easily achieve the rectification separation and recovery of methanol in the methanol aqueous solution.

[0034] According to the present application, preferably, the first rectification and the second rectification are respectively carried out in a first rectification column and a second rectification column.

[0035] During the research of the rectification separation of the methanol aqueous solution, the inventors of the present application found that for the above-mentioned system, no economically applicable equipment material could be found from the existing corrosion data manual or other public information. The inventors of the present application further found during the research that the adoption of the above-mentioned specific two-step rectification process in combination with the rectification tower made of the specific material can reduce the degree of equipment corrosion. Preferably, the material of the first rectification tower and the second rectification tower is each independently selected from at least one of metal titanium, titanium alloy and nickel-based alloy, and is preferably selected from at least one of pure titanium TA1, pure titanium TA2, titanium alloy TA8, titanium alloy TA9, titanium alloy TA10, Hastelloy C-276 and Hastelloy C-22.

[0036] According to the present application, preferably, the material of the first rectification tower is selected from at least one of pure titanium TA2, titanium alloy TA8, titanium alloy TA9 and titanium alloy TA10, and is preferably pure titanium TA2 and / or titanium alloy TA10. The adoption of the first rectification tower made of the specific material in the first rectification process can further reduce the degree of equipment corrosion and meet the requirement of the design life of the first rectification tower.

[0037] According to the present application, preferably, the material of the second rectification tower is Hastelloy C-276 and / or Hastelloy C-22. The adoption of the second rectification tower made of the specific material in the second rectification process can further reduce the degree of equipment corrosion and meet the requirement of the design life of the second rectification tower.

[0038] The present application will be described in detail below through examples.

[0039] In the following examples, the corrosion experiment test is carried out in a high-pressure stirred tank.

[0040] The raffinate phase methanol aqueous solution in the following examples is the raffinate phase obtained by extraction separation (using 3-chloropropene and water as extractants in sequence) of the liquid epoxidation reaction product obtained by epoxidation reaction of 3-chloropropene with hydrogen peroxide in the presence of solvent methanol.

[0041] Example 1

[0042] The raffinate phase methanol aqueous solution was continuously rectified and separated in two rectifying columns with an inner diameter of 100 mm and a theoretical plate number of 30. The main chemical components and pH value of the methanol aqueous solution are shown in Table 1. The methanol aqueous solution was fed into the first rectifying column made of pure titanium TA2. The mass flow rate of the feed of the first rectifying column was 14.100 kg / h, the top pressure was 0.025 MPaG, the top temperature was 65°C, and the reflux ratio was 3.78. A mixture of 3-chloropropene and methanol with a mass flow rate of 0.780 kg / h was obtained from the top of the first rectifying column, wherein the mass fraction of 3-chloropropene was 58.45%, the mass fraction of methanol was 40.24%, and the pH value was 0.59. The mass flow rate of methanol in the top discharge accounted for 4.9% of the mass flow rate of methanol in the feed, and the mass flow rate of 3-chloropropene in the top discharge accounted for 99.2% of the mass flow rate of 3-chloropropene in the feed. The bottom discharge with a mass flow rate of 13.320 kg / h was obtained from the bottom of the first rectifying column, wherein the mass fraction of 3-chloropropene was 0.026%, the mass fraction of methanol was 43.35%, and the pH value was 2.01. The bottom discharge of the first rectifying column was fed into the second rectifying column after being added with 15 wt% sodium hydroxide aqueous solution, so that the pH value of the feed of the second rectifying column was increased from 2.01 to 12.66. The mass ratio of the mass flow rate of the 15 wt% sodium hydroxide aqueous solution to the mass flow rate of the bottom discharge of the first rectifying column was 0.017. The top pressure of the second rectifying column was 0.80 MPaG, the reflux ratio was 2.70, and the bottom temperature was 175°C. The recovered methanol with a mass flow rate of 6.129 kg / h was obtained from the top of the second rectifying column, wherein the mass fraction of methanol was 98.42%, the mass fraction of water was 1.26%, the mass flow rate of methanol in the top discharge accounted for 99.8% of the mass flow rate of methanol in the feed, and the recovered methanol was recycled back to the epoxidation reaction. The waste water with a mass flow rate of 7.416 kg / h was discharged from the bottom of the second rectifying column, wherein the mass fraction of methanol was 0.12%, the mass fraction of water was 98.49%, and the pH value of the waste water was 5.67. The corrosion rate of the top discharge of the first rectifying column on pure titanium TA2 is shown in Table 2, and the corrosion rate of the bottom discharge of the second rectifying column on different materials is shown in Table 3.

[0043] Table 1

[0044]

[0045] Note: MCPD represents monochloropropanediol methyl ether, and CPD represents chloropropanediol. The remainder in the methanol aqueous solution is other impurities.

[0046] Table 2

[0047] Corrosion test conditions Test material Corrosion rate (mm / a) 65°C, 48h, 150r / min Pure titanium TA2 0.0095

[0048] Note: mm / a is millimeter per year.

[0049] As can be seen from the results in Table 2, the corrosion rate of pure titanium TA2 as the material of the first rectification tower device is less than 0.05 mm / a under the above rectification separation conditions. The first rectification process using the first rectification tower of the above pure titanium TA2 material can meet the requirement of the design service life of the first rectification tower.

[0050] Table 3

[0051]

[0052] As can be seen from the results in Table 3, the corrosion rates of pure titanium TA2, Hastelloy C-276 and Hastelloy C-22 as the materials of the second rectification tower device are all less than 0.05 mm / a under the above rectification separation conditions, which can meet the requirement of the design service life. The corrosion rates of Hastelloy C-276 and Hastelloy C-22 materials are significantly less than that of TA2. The second rectification process using the second rectification tower of the above Hastelloy C-276 and Hastelloy C-22 materials can further reduce the degree of equipment corrosion and better meet the requirement of the design service life of the second rectification tower.

[0053] Comparative Example 1

[0054] The method of Example 1 was followed, except that,

[0055] (1) The material of the first rectification tower was as shown in Table 4; and the material of the second rectification tower was pure titanium TA2;

[0056] (2) No sodium hydroxide aqueous solution was added in the second rectification, specifically: the bottom discharge of the first rectification tower was sent into the second rectification tower, the top pressure of the second rectification tower was 0.01 MPaG, the top temperature was 65°C, the reflux ratio was 2.70, and the mass flow rate of the recovered methanol obtained from the top of the second rectification tower was 6.129 kg / h, in which the mass fraction of methanol was 98.44% and the mass fraction of water was 1.46%. The mass flow rate of methanol in the top discharge accounted for 99.9% of the mass flow rate of methanol in the feed, and the recovered methanol was recycled back to the epoxidation reaction. The waste water discharged from the bottom of the second rectification tower had a mass flow rate of 7.193 kg / h, in which the mass fraction of methanol was 0.11% and the mass fraction of water was 98.73%, and the pH value was 1.87.

[0057] The corrosion rate of the top discharge of the first rectification tower on the material of the first rectification tower was as shown in Table 4.

[0058] Table 4

[0059]

[0060] From the results of Table 4, it can be seen that the corrosion rates of 304 stainless steel and 316L stainless steel as the material of the first rectifying column apparatus are obviously higher than 0.05 mm / a under the above rectifying separation conditions, and cannot meet the requirement of design life.

[0061] Comparative Example 2

[0062] The method of Example 1 was followed, except that no aqueous sodium hydroxide solution was added in the second rectifying, specifically: the bottom discharge of the first rectifying column was fed into the second rectifying column, the pressure at the top of the second rectifying column was 0.80 MPaG, the reflux ratio was 2.70, and the bottom temperature was 175°C, and methanol was obtained from the top of the second rectifying column and was recovered, wherein the mass fraction of methanol was 98.42%, and the mass fraction of water was 1.46%, the mass flow rate of methanol in the top discharge accounted for 99.8% of the mass flow rate of methanol in the feed, and the recovered methanol was recycled back to the epoxidation reaction. Waste water was discharged from the bottom of the second rectifying column, wherein the mass fraction of methanol was 0.13%, the mass fraction of water was 98.73%, and the pH value was 1.14.

[0063] The corrosion rates of the bottom discharge of the second rectifying column on different materials are shown in Table 5.

[0064] Table 5

[0065]

[0066] From the results of Table 5, it can be seen that under the above rectifying separation conditions, without adding an aqueous alkaline solution, the corrosion rates of pure titanium TA2, Hastelloy C-276 and Hastelloy C-22 as the material of the second rectifying column apparatus cannot meet the requirement of design life.

[0067] Example 2

[0068] The continuous rectification separation of the methanol aqueous solution in the raffinate phase was carried out in the same way as in Example 1, except that the main chemical components and pH value of the methanol aqueous solution were as shown in Table 6, the mass flow rate of the feed to the first rectification column was 13.692 kg / h, the column top pressure was 0.02 MPaG, the column top temperature was 64°C, and the reflux ratio was 2.10. A mixture of 3-chloropropene and methanol was obtained from the top of the first rectification column at a mass flow rate of 0.932 kg / h, wherein the mass fraction of 3-chloropropene was 34.10%, the mass fraction of methanol was 62.57%, and the pH value was 1.03. The mass flow rate of methanol in the column top discharge was 9.5% of the mass flow rate of methanol in the feed, and the mass flow rate of 3-chloropropene was 98.1% of the mass flow rate of 3-chloropropene in the feed. The column bottom discharge was obtained at a mass flow rate of 12.760 kg / h, wherein the mass fraction of 3-chloropropene was 0.048%, the mass fraction of methanol was 43.55%, and the pH value was 2.06. The column bottom discharge of the first rectification column was fed into the second rectification column after being mixed with 10% by weight of an aqueous sodium hydroxide solution, so that the pH value of the feed to the second rectification column was increased from 2.06 to 13.01. The mass flow rate ratio of the 10% by weight aqueous sodium hydroxide solution to the column bottom discharge was 0.052. The material of the second rectification column was TA2, and the reflux ratio was 3.00. Recovered methanol was obtained from the top of the second rectification column at a mass flow rate of 5.616 kg / h, wherein the mass fraction of methanol was 98.85%, and the mass fraction of water was 0.88%. The mass flow rate of methanol in the column top discharge was 99.9% of the mass flow rate of methanol in the feed. Waste water was discharged from the bottom of the second rectification column at a mass flow rate of 7.807 kg / h, wherein the mass fraction of methanol was 0.07%, the mass fraction of water was 96.67%, and the pH value of the waste water was 7.41. The corrosion rate of the column top discharge of the first rectification column on pure titanium TA2 material is shown in Table 7, and the corrosion rate of the column bottom discharge of the second rectification column on TA2 material is shown in Table 8.

[0069] Table 6

[0070]

[0071] Note: MCPD represents monochloropropanediol monomethyl ether, and CPD represents chloropropanediol. The remainder in the methanol aqueous solution is other impurities.

[0072] Table 7

[0073] Corrosion test conditions Test material Corrosion rate (mm / a) 64°C, 48h, 150r / min Pure titanium TA2 0.0087

[0074] As can be seen from the results in Table 7, the corrosion rate of pure titanium TA2 as the material of the first rectification column equipment is far less than 0.05 mm / a under the above rectification separation conditions, and can meet the requirement of design life.

[0075] Table 8

[0076]

[0077] From the results of Table 8, it can be seen that the corrosion rates of pure titanium TA2, Hastelloy C-276 and Hastelloy C-22 as the material of the second rectification tower device are all less than 0.05 mm / a under the above rectification separation conditions, which can meet the requirement of design life. The corrosion rates of Hastelloy C-276 and Hastelloy C-22 are significantly less than that of TA2, and the second rectification tower with Hastelloy C-276 and Hastelloy C-22 will have a longer service life.

[0078] Comparative Example 3

[0079] According to the method of Example 2, except that no sodium hydroxide aqueous solution is added to the feed of the second rectification tower, specifically: the bottom discharge of the first rectification tower is sent to the second rectification tower, and the mass flow rate of the recovered methanol obtained from the top of the second rectification tower is 5.609 kg / h, in which the mass fraction of methanol is 98.97% and the mass fraction of water is 1.01%, and the mass flow rate of methanol in the overhead discharge accounts for 99.9% of the mass flow rate of methanol in the feed; the mass flow rate of the wastewater discharged from the bottom of the second rectification tower is 7.151 kg / h, in which the mass fraction of methanol is 0.08% and the mass fraction of water is 97.09%, and the pH value is 1.09.

[0080] The corrosion rate of the bottom discharge of the second rectification tower on TA2 material is shown in Table 9.

[0081] Table 9

[0082] Corrosion test conditions Test material Corrosion rate (mm / a) 175°C, 72h, 600r / min Pure titanium TA2 0.117

[0083] From the results of Table 9, it can be seen that the corrosion rate of pure titanium TA2 as the material of the second rectification tower device is significantly greater than 0.05 mm / a under the above rectification separation conditions, which cannot meet the requirement of design life.

[0084] From the results of Tables 1-9, it can be seen that by using the method provided by the present application, the corrosion rate of the material on the rectification separation device can be effectively reduced in the process of rectification separation and recovery of methanol, the equipment investment and maintenance cost are reduced, and the risk of safety accidents is reduced.

[0085] The preferred embodiments of the present application are described in detail above, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be considered as disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A method for recovering methanol from an aqueous methanol solution, characterized by, The method comprises the following steps: (1) performing first rectification on the methanol aqueous solution, and controlling the conditions of the first rectification, so that the mass flow rate of methanol in the overhead discharge of the first rectification accounts for 1-9.9% of the mass flow rate of methanol in the methanol aqueous solution, and the mass fraction of 3-chloropropene in the bottom discharge is not higher than 0.1%; (2) performing second rectification on the bottom discharge obtained in step (1) in the presence of an alkaline aqueous solution, to obtain methanol and waste water, respectively, and the pH of the waste water is 5-8; wherein, based on the total mass of the methanol aqueous solution, the mass fraction of methanol in the methanol aqueous solution is 40-50%, the mass fraction of water is 45-55%, the mass fraction of 3-chloropropene is 1-5%, and the mass fraction of chloropropandiol monomethyl ether and chloropropandiol is 0.1-2%.

2. The method according to claim 1, wherein, the methanol aqueous solution is a raffinate phase obtained by extractive separation of a liquid epoxidation reaction product obtained by epoxidation reaction of 3-chloropropene with hydrogen peroxide in the presence of a solvent methanol; preferably, the pH of the methanol aqueous solution is 2-4.

3. The method according to claim 1, wherein, the mass flow rate of methanol in the overhead discharge of the first rectification accounts for 3-9.5% of the mass flow rate of methanol in the methanol aqueous solution; preferably, the mass fraction of 3-chloropropene in the bottom discharge of the first rectification is 0.001-0.1%; preferably, the mass flow rate of 3-chloropropene in the overhead discharge of the first rectification accounts for more than 95% of the mass flow rate of 3-chloropropene in the methanol aqueous solution.

4. The method according to any one of claims 1-3, wherein, the conditions of the first rectification include: the overhead pressure is 0-0.1 MPaG, preferably 0-0.05 MPaG; the overhead temperature is 50-80℃, preferably 50-70℃; and the reflux ratio is 0.1-10, preferably 1-5.

5. The method according to any one of claims 1-4, wherein, the pH of the waste water is 5.5-7.

5.

6. The method according to any one of claims 1-5, wherein, in step (2), the mass flow rate ratio of the alkaline aqueous solution to the bottom discharge obtained in step (1) is 0.001-0.1, preferably 0.01-0.

06.

7. The method according to any one of claims 1-6, wherein, the mass content of the alkaline aqueous solution is 1-50%, preferably 10-30%; preferably, the alkaline aqueous solution is selected from aqueous solutions of at least one of alkali metal hydroxides, alkaline earth metal hydroxides, ammonia, quaternary ammonium bases and basic salts, more preferably selected from aqueous solutions of at least one of alkali metal hydroxides, alkaline earth metal hydroxides and basic salts; further preferably, the alkaline aqueous solution is an aqueous sodium hydroxide solution.

8. The method according to any one of claims 1-7, wherein, the conditions of the second rectification include: the overhead pressure is 0-2 MPaG, the bottom temperature is 100-215℃, and the reflux ratio is 0.5-5.

9. The method according to any one of claims 1-8, wherein, the first rectification and the second rectification are respectively performed in a first rectification column and a second rectification column. Preferably, the material of the first and second rectification column is each independently selected from at least one of the group consisting of metallic titanium, titanium alloys and nickel-based alloys, preferably at least one of the group consisting of pure titanium TA1, pure titanium TA2, titanium alloy TA8, titanium alloy TA9, titanium alloy TA10, Hastelloy C-276 and Hastelloy C-22.

10. The method according to claim 9, wherein the material of the first rectification column is selected from at least one of the group consisting of pure titanium TA2, titanium alloy TA8, titanium alloy TA9 and titanium alloy TA10, preferably pure titanium TA2 and / or titanium alloy TA10; and / or the material of the second rectification column is Hastelloy C-276 and / or Hastelloy C-22.

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