A method and apparatus for purifying crude methanol

Abstract

This invention relates to a method and apparatus for purifying crude methanol. The method includes: separating crude methanol in a first distillation column to obtain a first top fraction, a first bottom fraction, and a first side stream fraction; passing the first side stream fraction into a second distillation column for further separation to obtain a second top fraction, a second bottom fraction, and a second side stream fraction; and passing the first bottom fraction into a third distillation column for further separation to obtain a third top fraction, a third bottom fraction, and a third side stream fraction. This method for purifying crude methanol, according to one embodiment of the invention, can effectively purify methanol-containing byproducts generated during the production of polyglycolic acid (PGA), enabling methanol recovery and reuse. A portion of the separated material can be used as raw material for continued PGA production, reducing production costs.

Description

Technical Field

[0001] This invention relates to the purification of crude methanol, and more particularly to the purification of methanol-containing byproducts generated during the preparation of polyglycolic acid. Background Technology

[0002] To address the problem of "white pollution" (plastic pollution), the country has vigorously developed biodegradable materials in recent years. Polyglycolic acid (PGA), as a high-performance biodegradable material, has been favored by many companies. However, a large amount of methanol byproducts are generated during the prepolymerization process of preparing PGA using methyl glycolate (MG). Because the reaction temperature exceeds 200℃, these byproducts are discharged from the system in a gaseous state. These byproducts include methanol, water, MG, PGA oligomers, and other reaction impurities. Methanol content is particularly high, accounting for approximately 30% of the reactants. Due to the complex composition of these byproducts, if purification is inadequate, the methanol cannot be recycled and must be sold as waste.

[0003] On the other hand, the high moisture content of methanol byproducts is one of the bottlenecks restricting PGA production. High moisture content leads to the hydrolysis of esters into acids. The higher the moisture content, the easier it is to form acids, which severely affect the service life of equipment and instruments, causing problems for the continuous long-term operation of the PGA production process. The existing process purifies the above-mentioned methanol byproducts through a single separation tower, but the purified methanol has low purity. Moreover, after the material enters the separation tower, some of the esters undergo hydrolysis, leading to severe corrosion of equipment and pipelines, affecting the normal operation of the unit.

[0004] In summary, because the existing process cannot purify methanol to the required state, this portion of methanol can only be sold as waste, resulting in low added value and high production costs for polyglycolic acid. Furthermore, the moisture and acidic substances in methanol cannot be removed, and their accumulation and circulation in the polyglycolic acid production system will affect the main process reaction. Summary of the Invention

[0005] To overcome at least one of the defects of the prior art, in a first aspect, one embodiment of the present invention provides a method for purifying crude methanol, comprising:

[0006] Crude methanol is separated by a first distillation column to obtain a first overhead component collected from the top of the first distillation column, a first bottom component collected from the bottom of the first distillation column, and a first side stream component collected from the side stream of the first distillation column; the first side stream component includes methanol and water.

[0007] The first side stream component is passed into a second distillation column for separation, yielding a second overhead component collected from the top of the second distillation column, a second reboiler component collected from the bottom of the second distillation column, and a second side stream component collected from the side stream of the second distillation column; the second side stream component includes methanol; and

[0008] The first column bottom component is fed into the third distillation column for separation to obtain the third column top component collected from the top of the third distillation column, the third column bottom component collected from the bottom of the third distillation column, and the third column side stream component collected from the side stream of the third distillation column.

[0009] The crude methanol is a methanol-containing byproduct produced during the preparation of polyglycolic acid from glycolate.

[0010] Secondly, one embodiment of the present invention provides a process for preparing polyglycolic acid, including the above-mentioned method for purifying crude methanol.

[0011] Thirdly, one embodiment of the present invention provides a crude methanol purification apparatus, comprising:

[0012] The first distillation column is used to separate crude methanol by distillation to obtain the first top component, the first bottom component, and the first side stream component.

[0013] A second distillation column is used to separate the first side-stream component by distillation to obtain a second top component, a second bottom component, and a second side-stream component; and

[0014] The third distillation column is used to separate the first column bottom component by distillation to obtain the third column top component, the third column bottom component, and the third side stream component;

[0015] Wherein, the crude methanol is a methanol-containing byproduct produced during the preparation of polyglycolic acid from glycolate;

[0016] The first distillation column includes a first column top, a first column bottom, and a first sidewall connecting the first column top and the first column bottom. A first side stream outlet is provided on the first sidewall, and the first side stream component is discharged from the first distillation column through the first side stream outlet.

[0017] The second distillation column includes a second column top, a second column bottom, and a second sidewall connecting the second column top and the second column bottom. A second side stream outlet is provided on the second sidewall, and the second side stream component is discharged from the second distillation column through the second side stream outlet.

[0018] The third distillation column includes a third column top, a third column bottom, and a third sidewall connecting the third column top and the third column bottom, which are arranged opposite to each other. A third side stream outlet is provided on the third sidewall, and the third side stream component is discharged from the third distillation column through the third side stream outlet.

[0019] Fourthly, one embodiment of the present invention provides a polyglycolic acid preparation system, including the above-described crude methanol purification device.

[0020] The crude methanol purification method of one embodiment of the present invention can effectively purify methanol-containing byproducts generated during the production of polyglycolic acid, enabling the recovery and reuse of methanol. The separated material can be used as raw material for the continued production of polyglycolic acid, thereby reducing production costs. Attached Figure Description

[0021] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention. Wherein:

[0022] Figure 1 This is a schematic diagram of the structure of a crude methanol purification device according to one embodiment of the present invention;

[0023] The following are the descriptions of the reference numerals:

[0024] 10. First distillation column; 11. First centrifugal pump; 12. First condenser; 13. First buffer tank; 14. Second centrifugal pump; 15. Third centrifugal pump; 20. Second distillation column; 21. Fourth centrifugal pump; 22. Second condenser; 23. Second buffer tank; 24. Fifth centrifugal pump; 25. Sixth centrifugal pump; 30. Third distillation column; 31. Seventh centrifugal pump; 32. Third condenser; 33. Third buffer tank; 34. Eighth centrifugal pump; 35. Ninth centrifugal pump; 40. Neutralization tank; 41. Tenth centrifugal pump. Detailed Implementation

[0025] Typical embodiments embodying the features and advantages of this invention will be described in detail in the following description. It should be understood that the invention can have various variations in different embodiments without departing from the scope of the invention, and the description herein is for illustrative purposes only and not intended to limit the invention. All pressures mentioned herein are gauge pressures.

[0026] Reference Figure 1 As shown, one embodiment of the present invention provides a method for purifying crude methanol, comprising:

[0027] Crude methanol is separated by the first distillation column 10 to obtain a first overhead component collected from the top of the first distillation column 10, a first bottom component collected from the bottom of the first distillation column 10, and a first side stream component collected from the side stream of the first distillation column 10; the first side stream component includes methanol and water.

[0028] The first side stream component is fed into the second distillation column 20 for separation, yielding a second overhead component collected from the top of the second distillation column 20, a second reboiler component collected from the bottom of the second distillation column 20, and a second side stream component collected from the side stream of the second distillation column 20; the second side stream component includes methanol; and

[0029] The first column bottom component is fed into the third distillation column 30 for separation, resulting in the third column top component collected from the top of the third distillation column 30, the third column bottom component collected from the bottom of the third distillation column 30, and the third column side stream component collected from the side stream of the third distillation column 30.

[0030] Crude methanol is a methanol-containing byproduct produced during the preparation of polyglycolic acid from glycolate (e.g., methyl glycolate).

[0031] In one embodiment, crude methanol includes methanol, water, glycolate, and polyglycolic acid oligomer.

[0032] In one embodiment, during the preparation of polyglycolic acid using glycolate, the glycolate undergoes a prepolymerization reaction to generate oligomers and a large amount of methanol. In addition, byproducts include glycolic acid, methyl acetal, dimethyl ether, methyl formate, water, and esters. Therefore, crude methanol may include methanol, water, glycolate, polyglycolic acid oligomers, glycolic acid, methyl acetal, dimethyl ether, methyl formate, and esters.

[0033] In one embodiment, the crude methanol contains 70-90% by mass, for example 75%, 80%, or 85%; methyl glycolate contains 5-15% by mass, for example 8%, 10%, or 12%; and the remaining substances contain 0-5% by mass, for example 0.1%, 0.5%, 1%, 2%, 3%, or 4%.

[0034] In one embodiment, the first distillation column 10 can be used for preliminary separation of crude methanol. The first distillation column 10 can be an atmospheric pressure column (pressure below 50 kPa), and the pressure can be controlled between 10 and 20 kPa. Since the crude methanol has complex components and the boiling points of each component differ greatly, the bottom of the first distillation column 10 can be heated with steam at 1.1 MPa.

[0035] In one embodiment, the bottom temperature of the first distillation column 10 can be 135-140°C, and the top temperature can be 55-65°C, for example, 60°C.

[0036] In one embodiment, the reflux ratio of the first distillation column 10 can be 7.5 to 10.

[0037] In one embodiment, the first distillation column 10 includes a first column top, a first column bottom, and a first sidewall connecting the first column top and the first column bottom, which are disposed opposite to each other. A first side stream outlet is provided on the first sidewall, and the first side stream component is discharged from the first distillation column 10 through the first side stream outlet.

[0038] In one embodiment, the liquid first side-line component comprises methanol, water, and a small amount of light components, wherein the light components may include dimethyl ether and methyl acetal.

[0039] In one embodiment, the methanol mass content (or purity) in the liquid first side stream component can be 95-98%. The extraction rate of the first side stream component can be 4 t / h.

[0040] In one embodiment, a chimney-type collector can be installed inside the first distillation column 10, and the first side stream component can be collected / extracted by controlling the liquid level in the collector.

[0041] In one embodiment, the collection temperature of the first lateral component is 65–70°C.

[0042] In one embodiment, a first centrifugal pump 11 is provided between the first distillation column 10 and the second distillation column 20. The first centrifugal pump 11 can be connected to the first distillation column 10 and the second distillation column 20 respectively, and is used to transport the first side stream component to the second distillation column 20.

[0043] In one embodiment, the first overhead component (containing acid-mixed alcohols) includes methanol, methyl formate, methyl acetal, and dimethyl ether. In the overhead region, the flow rate can be 2 t / h, the reflux flow rate can be 15–20 t / h, and the reflux ratio can be 7.5–10.

[0044] In one embodiment, a first condenser 12, a first buffer tank 13, and a second centrifugal pump 14 are sequentially connected outside the first distillation column 10 near the top of the column. After the components at the top of the first column are discharged from the top of the first distillation column 10, they are fed into the first condenser 12 for cooling to obtain condensed liquid material (containing acid-mixed alcohol) and non-condensable gas. The non-condensable gas can be discharged from the device. The liquid material containing acid-mixed alcohol is fed into the first buffer tank 13. Then, under the action of the second centrifugal pump 14, a portion of the acid-mixed alcohol is returned to the top of the first distillation column 10, and the remaining acid-mixed alcohol is discharged from the device and sent to the external tank area.

[0045] In one embodiment, the first bottom component (heavy component) mainly includes glycolate (e.g., methyl glycolate), polyglycolic acid oligomer, moisture, etc., and the output rate can be 6t / h.

[0046] In one embodiment, a third centrifugal pump 15 is provided outside the reboiler adjacent to the first distillation column 10. The third centrifugal pump 15 can be connected to the first distillation column 10 and the third distillation column 30 respectively, and is used to transport the components from the reboiler of the first column to the third distillation column 30.

[0047] In one embodiment, the second distillation column 20 operates at atmospheric pressure, and the pressure at the top of the column can be 20-30 kPa.

[0048] In one embodiment, the bottom temperature of the second distillation column 20 can be 100-105°C, and the top temperature can be 45-52°C.

[0049] In one embodiment, the feed rate of the first side stream component to the second distillation column 20 can be 4 t / h.

[0050] In one embodiment, the second distillation column 20 includes a second column top, a second column bottom, and a second sidewall connecting the second column top and the second column bottom. A second side stream outlet is provided on the second sidewall, and the second side stream component is discharged from the second distillation column 20 through the second side stream outlet.

[0051] In one embodiment, a chimney-type collector can be installed inside the second distillation column 20, and the second side stream component can be collected / extracted by controlling the liquid level in the collector.

[0052] In one embodiment, the collection temperature of the second side-line component is 63–67°C.

[0053] In one embodiment, the reflux ratio of the second distillation column 20 can be 10 to 12. By controlling the reflux ratio, the methanol from the second side stream outlet can meet the industrial-grade methanol standard.

[0054] In one embodiment, the methanol in the second side-stream component has a purity greater than 99.5%, and can be directly recycled as industrial refined methanol.

[0055] In one embodiment, a fourth centrifugal pump 21 connected to the second side stream outlet is installed outside the second distillation column 20. The second side stream component can be discharged from the second distillation column 20 by the action of the fourth centrifugal pump 21. The extraction rate of the second side stream component can be 3 t / h.

[0056] In one embodiment, the second overhead component can be collected as a light alcohol, and the second overhead component includes methanol, methyl formate, methyl acetal, and dimethyl ether. In the overhead region, the flow rate can be 0.5 t / h, the reflux flow rate can be 5-7 t / h, and the reflux ratio can be 10-12.

[0057] In one embodiment, a second condenser 22, a second buffer tank 23, and a fifth centrifugal pump 24 are sequentially connected outside the second distillation column 20 near the top of the column. After the components from the top of the second column are discharged from the second distillation column 20, they are introduced into the second condenser 22 for cooling to obtain condensed liquid material (light alcohol) and non-condensable gas. The non-condensable gas can be discharged from the device. The liquid material light alcohol is introduced into the second buffer tank 23, and then, under the action of the fifth centrifugal pump 24, a portion of the light alcohol is returned to the top of the second distillation column 20, while the remaining light alcohol is discharged from the device and sent to the external tank area.

[0058] In one embodiment, the second bottom column components include water and glycolic acid, and the output rate can be 0.5 t / h.

[0059] In one embodiment, the second column bottom component can be neutralized in a neutralization tank 40. Furthermore, a sixth centrifugal pump 25 is provided outside the second distillation column 20 adjacent to the bottom, through which the second column bottom component can be transported to the neutralization tank 40 for neutralization.

[0060] In one embodiment, the third distillation column 30 can be operated under negative pressure, with a pressure of approximately 40 kPa.

[0061] In one embodiment, the bottom temperature of the third distillation column 30 can be 130-135°C, and the top temperature can be 70-80°C.

[0062] In one embodiment, the reflux ratio of the third distillation column 30 is 10 to 15.

[0063] In one embodiment, the feed rate of the first column bottom component to the third distillation column 30 can be 6 t / h, and the column bottom can be heated by 1.1 MPa steam.

[0064] In one embodiment, the third distillation column 30 includes a third column top, a third column bottom, and a third sidewall connecting the third column top and the third column bottom, which are disposed opposite to each other. A third sideline outlet is provided on the third sidewall, and the third sideline component is discharged from the third distillation column 30 through the third sideline outlet.

[0065] In one embodiment, the collection temperature of the third lateral component is 90–100°C.

[0066] In one embodiment, a chimney-type collector can be installed in the third distillation column 30, and the third side stream component can be collected / extracted by controlling the liquid level in the collector.

[0067] In one embodiment, the third side stream component comprises water and glycolic acid, and the extraction rate can be 0.8–1 t / h. Further, the third side stream component can be neutralized in a neutralization tank 40.

[0068] In one embodiment, a seventh centrifugal pump 31 is provided between the third distillation column 30 and the neutralization tank 40, through which the third side stream component can be transported to the neutralization tank 40 for neutralization treatment.

[0069] In one embodiment, the third overhead component comprises heavy alcohol, with a flow rate of 0.3–0.5 t / h and a reflux flow rate of 3–5 t / h. Further, the third overhead component or heavy alcohol comprises methyl glycolate, methanol, and water.

[0070] In one embodiment, a third condenser 32, a third buffer tank 33, and an eighth centrifugal pump 34 are sequentially connected outside the third distillation column 30 near the top of the column. After the components from the top of the third column are discharged from the third distillation column 30, they are introduced into the third condenser 32 for cooling to obtain condensed liquid material (heavy alcohol) and non-condensable gas. The non-condensable gas can be discharged from the device. The liquid material heavy alcohol is introduced into the third buffer tank 33, and then, under the action of the eighth centrifugal pump 34, a portion of the heavy alcohol is returned to the top of the third distillation column 30, while the remaining heavy alcohol is discharged from the device and sent to the external tank area.

[0071] In one embodiment, the third bottoms component includes glycolate (MG), water, and polyglycolic acid oligomers, and the third bottoms component can be returned to the prepolymerization reactor for the preparation of polyglycolic acid.

[0072] In one embodiment, a ninth centrifugal pump 35 is provided outside the third distillation column 30 adjacent to the third column bottom, through which the components of the third column bottom can be transported to the polyglycolic acid preparation system (e.g., a prepolymerization reactor).

[0073] In one embodiment, a tenth centrifugal pump 41 is provided outside the neutralization tank 40, which can send the liquid in the neutralization tank 40 to wastewater treatment.

[0074] In one embodiment, by treating the first column bottom component in the third distillation column 30, the first column bottom component can be dehydrated and reused, preventing water from entering the polyglycolic acid preparation system with the reused material, causing water accumulation in the system, hydrolysis of ester substances, and thus affecting the polymerization reaction and corroding equipment and instruments.

[0075] One embodiment of the present invention provides a process for preparing polyglycolic acid, including the above-described method for purifying crude methanol.

[0076] Reference Figure 1 As shown, one embodiment of the present invention provides a crude methanol purification apparatus, comprising:

[0077] The first distillation column 10 is used to separate crude methanol by distillation to obtain the first top component, the first bottom component, and the first side stream component.

[0078] The second distillation column 20 is used to separate the first side-stream component by distillation to obtain the second top component, the second bottom component, and the second side-stream component; and

[0079] The third distillation column 30 is used to separate the components from the bottom of the first column by distillation to obtain the top component, the bottom component, and the side stream component.

[0080] Crude methanol is a methanol-containing byproduct produced during the preparation of polyglycolic acid from glycolate.

[0081] The first distillation column 10 includes a first column top, a first column bottom, and a first sidewall connecting the first column top and the first column bottom, which are disposed opposite to each other. A first side stream outlet is provided on the first sidewall, and the first side stream component is discharged from the first distillation column 10 through the first side stream outlet.

[0082] The second distillation column 20 includes a second column top, a second column bottom, and a second sidewall connecting the second column top and the second column bottom. A second side stream outlet is provided on the second sidewall, and the second side stream component is discharged from the second distillation column 20 through the second side stream outlet.

[0083] The third distillation column 30 includes a third column top, a third column bottom, and a third sidewall connecting the third column top and the third column bottom. A third sideline outlet is provided on the third sidewall, and the third sideline component is discharged from the third distillation column 30 through the third sideline outlet.

[0084] In one embodiment, the first distillation column 10 is connected to the second distillation column 20 through its first side outlet, and the first distillation column 10 is connected to the third distillation column 30 through its first reboiler.

[0085] In one embodiment, the rectification section of the first distillation column 10 adopts a packed column structure, and the stripping section adopts a plate column structure.

[0086] In one embodiment, the second distillation column 20 and / or the third distillation column 30 are packed columns.

[0087] In one embodiment, the second distillation column 20 is a packed column. The packed column has a large gas-liquid mass transfer area, which can effectively carry out mass and heat transfer, improve the purity of methanol, and remove water from methanol.

[0088] In one embodiment, the purification apparatus further includes a neutralization tank 40 for neutralizing the second column bottom component and / or the third side stream component.

[0089] In one embodiment, the purification device further includes a first centrifugal pump 11, a first condenser 12, a first buffer tank 13, a second centrifugal pump 14, and a third centrifugal pump 15, the arrangement and operation of which are adapted to the foregoing limitations.

[0090] In one embodiment, the purification device further includes a fourth centrifugal pump 21, a second condenser 22, a second buffer tank 23, a fifth centrifugal pump 24, and a sixth centrifugal pump 25, the arrangement and operation of which are adapted to the foregoing limitations.

[0091] In one embodiment, the purification device further includes a seventh centrifugal pump 31, a third condenser 32, a third buffer tank 33, an eighth centrifugal pump 34, and a ninth centrifugal pump 35, the arrangement and operation of which are adapted to the foregoing limitations.

[0092] In one embodiment, the purification apparatus further includes a tenth centrifugal pump 41, which can send the liquid in the neutralization tank 40 to wastewater treatment.

[0093] One embodiment of the present invention provides a polyglycolic acid preparation system, including the above-described crude methanol purification device.

[0094] In one embodiment, the polyglycolic acid preparation system includes a prepolymerization unit for carrying out a prepolymerization reaction. The prepolymerization unit can be connected to a crude methanol purification device and further connected to a first distillation column 10 to pass the crude methanol discharged from the prepolymerization unit into the purification device for processing.

[0095] In one embodiment, the prepolymerization unit is also connected to the bottom of the third distillation column 30 so that the bottom components of the third distillation column can be returned to the prepolymerization unit for reuse.

[0096] The methanol byproducts in the preparation of polyglycolic acid have complex components, making separation and purification difficult. One embodiment of the present invention provides a method / apparatus for purifying crude methanol, which employs multiple distillation columns connected in series to achieve efficient methanol purification. The separated components can then be reused in the preparation of polyglycolic acid, reducing the production cost of polyglycolic acid and enabling the recovery of high-purity methanol products.

[0097] The method for purifying crude methanol according to one embodiment of the present invention can effectively remove water from methanol and improve the purity of methanol products to the industrial standard for refined methanol.

[0098] The present invention provides a method for purifying crude methanol, which solves the equipment corrosion problem in the production process of polyglycolic acid, enables methanol to be recycled, reduces production costs, and ensures the safe and stable operation of the equipment.

[0099] Unless otherwise specified, the terms used in this invention have the meanings commonly understood by those skilled in the art.

[0100] The embodiments described in this invention are for illustrative purposes only and are not intended to limit the scope of protection of this invention. Those skilled in the art can make various other substitutions, changes and improvements within the scope of this invention. Therefore, this invention is not limited to the above embodiments, but is only defined by the claims.

Claims

1. A method for purifying crude methanol, comprising: Crude methanol is separated by a first distillation column to obtain a first overhead component collected from the top of the first distillation column, a first bottom component collected from the bottom of the first distillation column, and a first side stream component collected from the side stream of the first distillation column; the first side stream component includes methanol and water. The first side stream component is passed into a second distillation column for separation, yielding a second overhead component collected from the top of the second distillation column, a second reboiler component collected from the bottom of the second distillation column, and a second side stream component collected from the side stream of the second distillation column; the second side stream component includes methanol; and The first column bottom component is fed into the third distillation column for separation to obtain the third column top component collected from the top of the third distillation column, the third column bottom component collected from the bottom of the third distillation column, and the third column side stream component collected from the side stream of the third distillation column. Wherein, the crude methanol is a methanol-containing byproduct generated during the preparation of polyglycolic acid via glycolate esterification; the pressure of the first distillation column is 10-20 kPa, the bottom temperature is 135-140°C, the top temperature is 55-65°C, the reflux ratio is 7.5-10, and the collection temperature of the first side stream component is 65-70°C; the pressure of the second distillation column at the top is 20-30 kPa, the bottom temperature is 100-105°C, the top temperature is 45-52°C, the reflux ratio is 10-12, and the collection temperature of the second side stream component is 63-67°C; The third top column component includes methyl glycolate, methanol, and water; the third bottom column component includes glycolate, water, and polyglycolic acid oligomer; and the third side column component includes water and glycolic acid.

2. The purification method according to claim 1, wherein, The bottom temperature of the third distillation column is 130–135°C, and the top temperature is 70–80°C.

3. The purification method according to claim 1, wherein, The reflux ratio of the third distillation column is 10 to 15.

4. The purification method according to claim 1, wherein, The crude methanol comprises glycolate, glycolic acid, polyglycolic acid oligomer, methanol, and water.

5. The purification method according to claim 4, wherein, The second column bottom component includes water and glycolic acid, and the second column bottom component is neutralized by passing it through a neutralization tank; And / or, The third side-line component is neutralized in a neutralization tank. And / or, The third column bottom component is used for the preparation of polyglycolic acid.

6. The purification method according to claim 1, wherein, The first distillation column includes a first column top, a first column bottom, and a first sidewall connecting the first column top and the first column bottom. A first side stream outlet is provided on the first sidewall, and the first side stream component is discharged from the first distillation column through the first side stream outlet. The second distillation column includes a second column top, a second column bottom, and a second sidewall connecting the second column top and the second column bottom. A second side stream outlet is provided on the second sidewall, and the second side stream component is discharged from the second distillation column through the second side stream outlet. The third distillation column includes a third column top, a third column bottom, and a third sidewall connecting the third column top and the third column bottom, which are arranged opposite to each other. A third side stream outlet is provided on the third sidewall, and the third side stream component is discharged from the third distillation column through the third side stream outlet.

7. The purification method according to claim 6, wherein, The first distillation column is connected to the second distillation column through the first side outlet, and the first distillation column is connected to the third distillation column through the first reboiler.

8. The purification method according to claim 7, wherein the second column bottom component and / or the third side stream component are neutralized in a neutralization tank; The rectification section of the first distillation column adopts a packed column structure, and the stripping section adopts a plate column structure; The second distillation column and / or the third distillation column are packed columns.

9. A process for preparing polyglycolic acid, comprising the purification method of crude methanol according to any one of claims 1 to 8.

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