Preparation method of N-methylmorpholine

Abstract

The invention discloses a preparation method of N-methylmorpholine, which belongs to the technical field of purification of medical intermediates and comprises the following steps: desalting wastewater containing N-methylmorpholine hydrochloride through electrodialysis equipment, sequentially adding inorganic salt and an extracting agent into the obtained desalted liquid, and performing low-temperature extraction and distillation to obtain high-purity N-methylmorpholine. The technological process provided by the invention has good feasibility, and is green, safe and environment-friendly; the preparation process is simple, convenient, environment-friendly and energy-saving; according to the method, the wastewater containing the N-methylmorpholine hydrochloride generated in the dapagliflozin production process can be treated, and the recycling of the N-methylmorpholine is realized, so that the cost is effectively saved.

Description

Technical Field

[0001] The invention belongs to the technical field of pharmaceutical intermediate purification, and particularly relates to a method for preparing N-methylmorpholine. Background Art

[0002] Dapagliflozin belongs to the class of adrenal cortex hormone drugs, which are suitable for the treatment of type 2 diabetes and chronic kidney disease (CKD). The synthesis process of this drug uses glucono-δ-lactone as the starting material. First, the hydroxyl group of glucono-δ-lactone is silylated and protected. N-methylmorpholine is used as an acid binding agent during the reaction. In the subsequent treatment steps, purified water is used to quench the reaction, resulting in a large amount of N-methylmorpholine hydrochloride in the discharged wastewater, which increases the difficulty of wastewater treatment.

[0003] N-methylmorpholine is a colorless liquid with an ammonia smell. It is air-sensitive and miscible with a variety of solvents, but it is flammable, corrosive and slightly toxic. Its hydrochloride is well soluble in water, but its presence in industrial wastewater will increase the chemical oxygen demand, and because it is difficult to recover, it increases production costs.

[0004] As N-methylmorpholine is a valuable chemical raw material, if it can be recycled and reused, it will not only reduce the content of pollutants in the discharged wastewater, reduce environmental pollution and the difficulty of wastewater treatment, thus bringing social benefits, but also save costs and achieve economic benefits. Several existing recycling methods in the prior art have defects to varying degrees:

[0005] Patent CN118108683A discloses a method for separating and purifying N-methylmorpholine and N-ethylmorpholine. The method uses triethylamine as an extractant to extract N-methylmorpholine and N-ethylmorpholine in an aqueous phase. However, it should be noted that triethylamine has the smell of ammonia and has a certain irritation, and the extraction agent recovery tower distillation and other operations make the process more complicated.

[0006] Patent CN117924214A discloses a method for recovering N-methylmorpholine from N-methylmorpholine-containing wastewater, which involves mixing the wastewater with a solid base and adjusting it to an alkaline environment, followed by an extraction and liquid separation process. Next, a dehydrating agent is added thereto, and N-methylmorpholine is recovered through a distillation process. However, the technical process is cumbersome, involves multiple steps, and solid waste is generated during the dehydration process, which is not conducive to environmental protection.

[0007] Patent CN104230851A "Method for recovering N-methylmorpholine from valnemulin hydrochloride synthesis wastewater" discloses a method for recovering N-methylmorpholine. The method includes the steps of adjusting pH, extracting with an organic solvent, drying and distilling. The technical process is complex and includes multiple steps. It is necessary to use alkaline reagents such as sodium hydroxide, potassium hydroxide or ammonia water to free N-methylmorpholine and then extract it into an organic phase.

[0008] Therefore, it is necessary to develop a method for preparing N-methylmorpholine to overcome the shortcomings of the prior art. Summary of the invention

[0009] In order to solve the above problems, the present invention provides a method for preparing N-methylmorpholine.

[0010] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0011] A method for preparing N-methylmorpholine comprises the following steps:

[0012] Step 1, desalting the wastewater containing N-methylmorpholine hydrochloride through an electrodialysis device to obtain a desalted solution and a concentrated chamber solution;

[0013] Step 2, adding inorganic salt to the desalted solution obtained in step 1 and stirring;

[0014] Step 3, the material in step 2 is pressed into the extraction tank, and an extractant is added to perform low-temperature extraction at a temperature of -10 to -5°C, and then filtered;

[0015] Step 4, collecting the extract in step 3, and adding a desiccant to remove water from the extract;

[0016] Step 5: The dehydrated extract obtained in step 4 is heated and fractionated, and the fraction at 115° C. is collected to obtain N-methylmorpholine.

[0017] Furthermore, in step 1, the working parameters of the electrodialysis equipment during desalination are: working current ≤23A, working voltage ≤15V, waste liquid ion migration flow rate 80-100L / h, and when the working current drops to 4-5A, the electrodialysis equipment is stopped.

[0018] Furthermore, in step 2, the inorganic salt is any one of sodium chloride and potassium chloride, and the mass volume ratio of the inorganic salt to the wastewater is 1 to 2:5 (kg / L).

[0019] Furthermore, in step 3, the extractant is any one of tetrahydrofuran, n-butanol, and n-valeronitrile, and the ratio of the extractant to the wastewater volume is 1L:5L.

[0020] Furthermore, the upper part of the extraction tank is a low-temperature extraction section, and the lower part is a solid-liquid separation section. An agitator is provided inside the low-temperature extraction section, and a temperature-control jacket is provided outside the low-temperature extraction section for controlling the internal feed and liquid temperature; a discharge port with a control valve is provided on the bottom partition of the low-temperature extraction section for controlling the discharge to the solid-liquid separation section; a sintered filter plate is provided in the middle of the solid-liquid separation section, and the pore size of the filter holes on the sintered filter plate is 0.22 μm, which is used to achieve solid-liquid separation of the feed and liquid; the bottom outlet pipeline of the solid-liquid separation section is connected to a pneumatic diaphragm pump, and the discharge pipe of the pneumatic diaphragm pump is connected to the feed port of the low-temperature extraction section.

[0021] Furthermore, the discharge pipe of the pneumatic diaphragm pump is connected to the turnover tank, and the discharge port of the turnover tank is connected to the feed port of the low-temperature extraction section through another pneumatic diaphragm pump and a feed pipe.

[0022] Furthermore, in step 3, the low-temperature extraction process is as follows:

[0023] 1) Add the extractant to the extraction tank and start stirring to fully mix it with the desalted liquid;

[0024] 2) Control the temperature of the external temperature control jacket of the extraction tank at -10 to -5°C to obtain a solid-liquid mixture;

[0025] 3) Start the pneumatic diaphragm pump connected to the bottom of the extraction tank, the solidified part of the desalted liquid is intercepted by the sintered filter plate with a pore size of 0.22 μm and retained in the extraction tank, and the liquid extractant passes through the sintered filter plate and is separated from the desalted liquid to obtain an extract containing N-methylmorpholine;

[0026] 4) The extract flows out from the bottom of the extraction tank to the turnover tank and serves as the extractant for the next extraction operation;

[0027] Repeat the above steps 1) to 4) for 3 to 5 times.

[0028] Furthermore, in step 4, the desiccant is any one of anhydrous calcium chloride, anhydrous sodium sulfate, and anhydrous magnesium sulfate, and the mass volume ratio of the desiccant to the wastewater is 1:10 (kg / L).

[0029] Compared with the prior art, the present invention has the following technical advances:

[0030] The present invention is to recover high-purity N-methylmorpholine by subjecting wastewater containing N-methylmorpholine hydrochloride in the production process of dapagliflozin through the steps of electrodialysis desalination, low-temperature extraction, distillation, etc. The process provided by the present invention has good feasibility, is green, safe and environmentally friendly; and its preparation process is simple, environmentally friendly and energy-saving; the present invention can treat wastewater containing N-methylmorpholine hydrochloride generated in the production process of dapagliflozin, realize the recycling of N-methylmorpholine, thereby effectively saving costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0032] In the attached picture:

[0033] Figure 1 This is a schematic diagram of the structure of the extraction tank in the present invention;

[0034] In the figure: 1- low temperature extraction section, 2- solid-liquid separation section, 3- agitator, 4- temperature control jacket, 5- sintered filter plate, 6- pneumatic diaphragm pump, 7- turnover tank. DETAILED DESCRIPTION

[0035] The following specific embodiments may be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments. The present invention will be described below in conjunction with specific embodiments.

[0036] The present invention provides a method for preparing N-methylmorpholine, comprising the following steps:

[0037] Step 1: Desalting the wastewater containing N-methylmorpholine hydrochloride through an electrodialysis device to obtain a desalted solution and a concentrated chamber solution.

[0038] The working parameters of the electrodialysis equipment during desalination are: working current ≤23A, working voltage ≤15V, waste liquid ion migration flow rate 80-100L / h, and the electrodialysis equipment is stopped when the working current drops to 4-5A.

[0039] Step 2: adding an inorganic salt to the desalted liquid obtained in step 1 and stirring; the inorganic salt is any one of sodium chloride and potassium chloride, and the mass volume ratio of the inorganic salt to the wastewater is 1 to 2:5 (kg / L).

[0040] Step 3, the material in step 2 is pressed into an extraction tank, and low-temperature extraction treatment is performed at a temperature of -10 to -5°C using an extractant, and then suction filtration is performed; the extractant is any one of tetrahydrofuran, n-butanol, and n-valeronitrile, and the ratio of the extractant to the wastewater volume is 1L:5L.

[0041] The specific structure of the extraction tank is as follows Figure 1 As shown, the upper part of the extraction tank is a low-temperature extraction section 1, and the lower part is a solid-liquid separation section 2. The interior of the low-temperature extraction section 1 is provided with a stirrer 3, and the exterior of the low-temperature extraction section 1 is provided with a temperature control jacket 4 for controlling the internal feed liquid temperature; a discharge port with a control valve is provided on the bottom partition of the low-temperature extraction section 1 for controlling the discharge to the solid-liquid separation section 2; a sintered filter plate 5 is provided in the middle of the solid-liquid separation section 2, and the aperture of the filter hole on the sintered filter plate 5 is 0.22 μm, which is used to achieve solid-liquid separation of the feed liquid; the bottom outlet pipeline of the solid-liquid separation section 2 is connected to a pneumatic diaphragm pump 6, and the discharge pipe of the pneumatic diaphragm pump 6 is connected to the feed port of the low-temperature extraction section 1. In order to facilitate the turnover of materials, the discharge pipe of the pneumatic diaphragm pump 6 is connected to the turnover tank 7, and the discharge port of the turnover tank 7 is connected to the feed port of the low-temperature extraction section 1 through another pneumatic diaphragm pump 6 and a feed pipe.

[0042] The method of the above-mentioned low temperature extraction treatment is as follows:

[0043] 1) Add the extractant to the low-temperature extraction section of the extraction tank, turn on the agitator to stir, and mix it with the desalted liquid thoroughly;

[0044] 2) Control the temperature of the external temperature control jacket of the extraction tank at -10 to -5°C to obtain a solid-liquid mixture;

[0045] 3) Start the pneumatic diaphragm pump connected to the bottom of the extraction tank, the solidified part of the desalted liquid is intercepted by the sintered filter plate with a pore size of 0.22 μm and retained in the extraction tank, and the liquid extractant passes through the sintered filter plate and is separated from the desalted liquid to obtain an extract containing N-methylmorpholine;

[0046] 4) The extract flows out from the bottom of the extraction tank to the turnover tank and serves as the extractant for the next extraction operation (and then circulates to the feed port of the extraction tank through another pneumatic diaphragm pump);

[0047] Repeat the above steps 1) to 4) for 3 to 5 times.

[0048] Step 4: Collect the extract in step 3, and add a desiccant to remove water from the extract; the desiccant is any one of anhydrous calcium chloride, anhydrous sodium sulfate, and anhydrous magnesium sulfate, and the mass volume ratio of the desiccant to the wastewater is 1:10 (kg / L).

[0049] Step 5: The dehydrated extract obtained in step 4 is heated and fractionated, and the fraction at 115° C. is collected to obtain N-methylmorpholine.

[0050] The working principle of the present invention is as follows: hydrochloric acid is separated from waste water containing N-methylmorpholine hydrochloride by electrodialysis technology to obtain an aqueous solution of N-methylmorpholine. N-methylmorpholine is transferred to an extractant, the extractant is separated by distillation, and N-methylmorpholine is recovered. The present invention has the following advantages:

[0051] 1. The method for preparing N-methylmorpholine of the present invention is simple to operate, and high-purity N-methylmorpholine is obtained by means of desalting, extraction and distillation, and the product yield is high.

[0052] 2. The present invention utilizes electrodialysis technology to effectively separate N-methylmorpholine from hydrochloric acid. The desalted liquid is an aqueous solution of N-methylmorpholine, and the concentrated liquid is an aqueous solution of hydrochloric acid. At the same time, the N-methylmorpholine and hydrochloric acid are recycled and reused.

[0053] 3. The present invention uses low-temperature extraction to mix the extractant with the desalted liquid. Under specific temperature conditions, a solid-liquid mixture is formed (the desalted liquid solidifies, while the extractant remains in liquid form). N-methylmorpholine is transferred from the desalted liquid to the extractant, and solid-liquid separation is achieved through a 0.22 μm pore size sintered filter plate. After multiple extraction processes, the extraction is more complete. This method effectively improves the separation and extraction efficiency and ensures the yield of the recovered product.

[0054] 4. The present invention solves the problems of high COD and ammonia nitrogen content in wastewater containing N-methylmorpholine hydrochloride, high cost of three waste treatment, and inability to recycle N-methylmorpholine. The present invention has a simple preparation process, good process feasibility, environmental protection and energy saving, thereby reducing production costs, and is very suitable for large-scale industrial production.

[0055] The present invention is explained below by three specific examples and six comparative examples.

[0056] Example 1

[0057] 5L of waste water (containing about 0.94kg of N-methylmorpholine hydrochloride) was introduced into a waste water tank and desalted through an electrodialysis device. 5L of purified water was introduced into an acid water tank for receiving separated hydrochloric acid. The working current and voltage of the electrodialysis device were controlled to be 23A and 15V, respectively. The ion migration rate of the waste liquid was 80L / h. When the pH value of the waste liquid was greater than 9, the working current decreased. When the working current decreased to 4A, the electrodialysis device was stopped to obtain a desalted solution of N-methylmorpholine aqueous solution and a hydrochloric acid aqueous solution.

[0058] Add 1 kg of sodium chloride to the desalted liquid, stir to dissolve, transfer to an extraction tank, add 1 L of tetrahydrofuran, stir for 30 minutes, lower the temperature of the temperature-control jacket to -10°C, separate the extract, press it back into the extraction tank through the turnover tank, repeat the operation 3 times, add 0.5 kg of anhydrous calcium chloride to the extract, stir for 30 minutes, filter, introduce the filtrate into a distillation device, heat up and collect the fraction at 115°C, with a purity of 99.7% and a molar yield of 91.7%.

[0059] Example 2

[0060] Take 5L of waste water (containing about 0.94kg of N-methylmorpholine hydrochloride) and introduce it into the waste water tank, desalinate it through the electrodialysis equipment, take 5L of purified water and introduce it into the acid water tank to receive the separated hydrochloric acid, control the working current to 23A, the working voltage to 15V, the waste liquid ion migration flow rate to 100L / h, observe that when the waste liquid pH is greater than 9, the working current decreases, and when it drops to 5A, stop the electrodialysis equipment to obtain the desalted liquid.

[0061] Add 2kg of potassium chloride to the desalted liquid, stir to dissolve, transfer to an extraction tank, add 1L of n-butanol, stir for 30 minutes, lower the temperature of the temperature-control jacket to -5°C, separate the extract, press it into the extraction tank again through the turnover tank, repeat the operation 5 times, add 0.5kg of anhydrous sodium sulfate to the extract, stir for 30 minutes, filter, introduce the filtrate into a distillation device, heat up and collect the fraction at 115°C, with a purity of 99.6% and a molar yield of 93.0%.

[0062] Example 3

[0063] Take 5L of waste water (containing about 0.94kg of N-methylmorpholine hydrochloride) and introduce it into the waste water tank, desalinate it through the electrodialysis equipment, take 5L of purified water and introduce it into the acid water tank to receive the separated hydrochloric acid, control the working current to 23A, the working voltage to 15V, the waste liquid ion migration flow rate to 90L / h, observe that when the waste liquid pH is greater than 9, the working current decreases, and when it drops to 5A, stop the electrodialysis equipment to obtain the desalted solution.

[0064] Add 1.5 kg of sodium chloride to the desalted liquid, stir to dissolve, transfer to an extraction tank, add 1 L of n-valeronitrile, stir for 30 min, lower the jacket temperature to -7 ° C, separate the extract, press it into the extraction tank again through the turnover tank, repeat the operation 4 times, add 0.5 kg of anhydrous magnesium sulfate to the extract, stir for 30 min, filter, introduce the filtrate into a distillation device, heat up and collect the fraction at 115 ° C, with a purity of 99.5% and a molar yield of 92.3%.

[0065] Comparative Example 1 (reference patent CN118108683A using triethylamine as extraction agent)

[0066] Take 5L of wastewater (containing about 0.94kg of N-methylmorpholine hydrochloride) and pass it through the electrodialysis equipment for desalination. Control the working current to 23A, the working voltage to 15V, and the waste liquid ion migration flow rate to 90L / h. When the pH of the waste liquid is greater than 9, the working current decreases. When it drops to 5A, stop the electrodialysis equipment to obtain the desalted liquid.

[0067] Add 1.5kg sodium chloride to the desalted solution, stir to dissolve, then add 5L triethylamine and stir for 30min. Let stand for 30min, separate the phases, and keep the upper solution. Add 0.5kg anhydrous magnesium sulfate to the upper solution, stir for 30min, filter by suction, and introduce the filtrate into the distillation equipment, heat up and collect the fraction at 115℃, with a purity of 86.5% and a molar yield of 50.9%.

[0068] Comparative Example 2 (Conditions of Reference Patent CN104230851A)

[0069] Take 10L of saline wastewater from the protection reaction of dapagliflozin, adjust the pH to 11 with 20% sodium hydroxide, extract with benzene, extract three times, and use 2.5L, 1.3L, and 1.3L of benzene each time. After the extraction is completed, the benzene phases are combined and dried with anhydrous magnesium sulfate in an amount of 0.3kg. Filter to remove the desiccant, heat the benzene phase, use a Weiss distillation column for distillation, and collect the 115-116°C fraction at the top of the column. Finally, the molar yield of N-methylmorpholine recovered is 75.3% and the purity is 95.6%.

[0070] Comparative Example 3 (Electrodialysis desalination method is not used, and the remaining steps refer to Example 1)

[0071] 5 L of waste water (containing about 0.94 kg of N-methylmorpholine hydrochloride) was introduced into a waste water tank, and sodium hydroxide was used to liberate N-methylmorpholine to obtain an N-methylmorpholine aqueous solution.

[0072] Add 1.5 kg of sodium chloride to the desalted liquid, stir to dissolve, transfer to an extraction tank, add 1 L of tetrahydrofuran, stir for 30 minutes, reduce the jacket temperature to -10°C, separate the extract, press it back into the extraction tank through the turnover tank, repeat the operation 3 times, add 0.5 kg of anhydrous calcium chloride to the extract, stir for 30 minutes, filter, introduce the filtrate into a distillation device, heat up and collect the 115°C fraction, with a purity of 99.6% and a molar yield of 81.2%.

[0073] Comparative Example 4 (low temperature extraction method is not used, and the remaining steps refer to Example 1)

[0074] Take 5L of waste water (containing about 0.94kg of N-methylmorpholine hydrochloride) and introduce it into the waste water tank, desalinate it through the electrodialysis equipment, take 5L of purified water and introduce it into the acid water tank to receive the separated hydrochloric acid, control the working current to 23A, the working voltage to 15V, and the waste liquid ion migration flow rate to 80L / h. When the pH of the waste liquid is greater than 9, the working current decreases. When it drops to 4A, the electrodialysis equipment is stopped to obtain desalted N-methylmorpholine water and hydrochloric acid aqueous solution.

[0075] Add 1.5 kg of sodium chloride to the desalted liquid, stir to dissolve, transfer to an extraction tank, add 1 L of tetrahydrofuran, stir for 30 min, let stand for 30 min, separate the phases, add 0.5 kg of anhydrous calcium chloride to the organic phase, stir for 30 min, filter, introduce the filtrate into a distillation device, heat up and collect the fraction at 115 ° C, with a purity of 75.6% and a molar yield of 72.5%.

[0076] Comparative Example 5 (the extractant is different from the extractant in the present invention, and the remaining steps refer to Example 1)

[0077] Take 5L of waste water (containing about 0.94kg of N-methylmorpholine hydrochloride) and introduce it into the waste water tank, desalinate it through the electrodialysis equipment, take 5L of purified water and introduce it into the acid water tank to receive the separated hydrochloric acid, control the working current to 23A, the working voltage to 15V, and the waste liquid ion migration flow rate to 80L / h. When the pH of the waste liquid is greater than 9, the working current decreases. When it drops to 4A, the electrodialysis equipment is stopped to obtain desalted N-methylmorpholine water and hydrochloric acid aqueous solution.

[0078] Add 1.5 kg of sodium chloride to the desalted liquid, stir to dissolve, transfer to the extraction tank, add 1 L of toluene, stir for 30 minutes, reduce the jacket temperature to -10 ° C, turn on the vacuum pump at the bottom of the tank, separate the extract, and then press it into the feed port of the extraction tank. Repeat the operation 3 times, add 0.5 kg of anhydrous calcium chloride to the extract, stir for 30 minutes, filter, and introduce the filtrate into the distillation equipment. Heat up and collect the 115 ° C fraction with a purity of 86.7% and a molar yield of 75.9%.

[0079] Comparative Example 6 (parameter values ​​different from those in this application, steps refer to Example 1)

[0080] Take 5L of waste water (containing about 0.94kg of N-methylmorpholine hydrochloride) and introduce it into the waste water tank, desalinate it through the electrodialysis equipment, take 5L of purified water and introduce it into the acid water tank to receive the separated hydrochloric acid, control the working current to 23A, the working voltage to 15V, and the waste liquid ion migration flow rate to 80L / h. When the pH of the waste liquid is greater than 9, the working current decreases. When it drops to 4A, the electrodialysis equipment is stopped to obtain desalted N-methylmorpholine water and hydrochloric acid aqueous solution.

[0081] Add 0.1 kg of sodium chloride to the desalted liquid, stir to dissolve, transfer to an extraction tank, add 0.5 L of tetrahydrofuran, stir for 30 min, reduce the jacket temperature to 2 ° C, separate the extract, press it into the extraction tank again through the turnover tank, add 0.1 kg of anhydrous calcium chloride to the extract, stir for 30 min, filter, and introduce the filtrate into a distillation device. Heat and collect the fraction at 115 ° C, with a purity of 88.6% and a molar yield of 51.3%.

[0082] The quality of 3 batches of examples and 6 batches of comparative samples was tested, and the data are shown in Table 1 below:

[0083] batch number purity(%) Yield (%) Process standards ≥99.5 ≥90 Example 1 99.7 91.7 Example 2 99.6 93.0 Example 3 99.5 92.3 Comparative Example 1 86.5 50.9 Comparative Example 2 95.6 75.3 Comparative Example 3 99.6 81.2 Comparative Example 4 75.6 72.5 Comparative Example 5 86.7 75.9 Comparative Example 6 88.6 51.3

[0084] According to the data in Table 1, it can be determined that the N-methylmorpholine prepared by the present invention has a product quality that meets the relevant standards for recycling and reuse, has good process reproducibility, and is in line with the green, safe and environmentally friendly development direction.

[0085] The liquid phase data of the N-methylmorpholine prepared in Example 1 for the silylation protection step of dapagliflozin are shown in Table 2 below:

[0086]

[0087] Note: Silanization reaction temperature is 25±2℃, reaction time is 12 hours;

[0088] According to the data in Table 2, it can be determined that the dapagliflozin intermediate 1 prepared in Example 1 has high purity, and the impurities RT0.96 intermediate state and RT1.01 did not increase much during the experiment, and the quality is stable, so it can be used for the preparation of dapagliflozin products.

[0089] It should be noted that the upper and lower limits and interval values ​​of the process parameters of the present invention (such as multiples, temperature, pH range, crystal growth time concentration, etc.) can all achieve the present invention, and the embodiments are not listed one by one here.

[0090] In summary, the method for preparing N-methylmorpholine of the present invention is simple to operate, and high-purity N-methylmorpholine is obtained by methods including electrodialysis desalination, low-temperature extraction and distillation, and the product yield is high.

[0091] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein with equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the scope of protection of the claims of the present invention.

Claims

1. A method for preparing N-methylmorpholine, characterized in that: The following steps are involved: Step 1, desalting the wastewater containing N-methylmorpholine hydrochloride through an electrodialysis device to obtain a desalted solution and a concentrated chamber solution; Step 2, adding inorganic salt to the desalted solution obtained in step 1 and stirring; Step 3, the material in step 2 is pressed into the extraction tank, and an extractant is added to perform low-temperature extraction at a temperature of -10 to -5°C, and then filtered; Step 4, collecting the extract in step 3, and adding a desiccant to remove water from the extract; Step 5: The dehydrated extract obtained in step 4 is heated and fractionated, and the fraction at 115° C. is collected to obtain N-methylmorpholine.

2. A method for preparing N-methylmorpholine according to claim 1, characterized in that: In step 1, the working parameters of the electrodialysis equipment during desalination are: working current ≤ 23A, working voltage ≤ 15V, waste liquid ion migration flow rate 80-100L / h, and the electrodialysis equipment is stopped when the working current drops to 4-5A.

3. A method for preparing N-methylmorpholine according to claim 1, characterized in that: In step 2, the inorganic salt is any one of sodium chloride and potassium chloride, and the mass volume ratio of the inorganic salt to the wastewater is 1 to 2:5 (kg / L).

4. A method for preparing N-methylmorpholine according to claim 1, characterized in that: In step 3, the extractant is any one of tetrahydrofuran, n-butanol, and n-valeronitrile, and the ratio of the extractant to the wastewater volume is 1L:5L.

5. A method for preparing N-methylmorpholine according to claim 1, characterized in that: In step 3, the upper part of the extraction tank is a low-temperature extraction section and the lower part is a solid-liquid separation section. An agitator is provided inside the low-temperature extraction section, and a temperature-control jacket is provided outside the low-temperature extraction section to control the internal feed and liquid temperature; a discharge port with a control valve is provided on the bottom partition of the low-temperature extraction section to control the discharge to the solid-liquid separation section; a sintered filter plate is provided in the middle of the solid-liquid separation section, and the pore size of the filter holes on the sintered filter plate is 0.22 μm, which is used to achieve solid-liquid separation of the feed and liquid; the bottom outlet pipeline of the solid-liquid separation section is connected to a pneumatic diaphragm pump, and the discharge pipe of the pneumatic diaphragm pump is connected to the feed port of the low-temperature extraction section.

6. A method for preparing N-methylmorpholine according to claim 5, characterized in that: The discharge pipe of the pneumatic diaphragm pump is connected to the turnover tank, and the discharge port of the turnover tank is connected to the feed port of the low-temperature extraction section through another pneumatic diaphragm pump and a feed pipe.

7. A method for preparing N-methylmorpholine according to claim 6, characterized in that: In step 3, the method of low-temperature extraction treatment is as follows: 1) Add the extractant to the extraction tank and start stirring to fully mix it with the desalted liquid; 2) Control the temperature of the external temperature control jacket of the extraction tank at -10 to -5°C to obtain a solid-liquid mixture; 3) Start the pneumatic diaphragm pump connected to the bottom of the extraction tank. The solidified part of the desalted liquid is intercepted by the sintered filter plate with a pore size of 0.22 μm and retained in the extraction tank. The liquid extractant passes through the sintered filter plate and is separated from the desalted liquid to obtain an extract containing N-methylmorpholine. 4) The extract flows out from the bottom of the extraction tank to the turnover tank and serves as the extractant for the next extraction operation; Repeat the above steps 1) to 4) for 3 to 5 times.

8. A method for preparing N-methylmorpholine according to claim 1, characterized in that: In step 4, the desiccant is any one of anhydrous calcium chloride, anhydrous sodium sulfate, and anhydrous magnesium sulfate, and the mass volume ratio of the desiccant to the wastewater is 1:10 (kg / L).

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