CN111377801B - Method and system for refining low carbon alcohol - Google Patents

Abstract

The invention relates to the field of refining of alcohol, in particular to a method and a system for refining low-carbon alcohol. (1) Rectifying the low-carbon alcohol raw material mixture to obtain organic wastewater and a first fraction; (2) Performing first extractive distillation on the first fraction by using a first extracting agent to further remove water and obtain a second fraction; (3) Rectifying the second fraction II to obtain a refined methanol product and a third fraction; (4) Performing second extractive distillation on the third fraction by using a second extractant to obtain a refined ethanol product and a fourth fraction; (5) Carrying out III rectification on the fourth fraction to obtain a second extracting agent and a fifth fraction; (6) Performing IV rectification on the fifth fraction to obtain a crude isopropanol material flow and a sixth fraction; (7) And (3) rectifying the sixth fraction V to obtain a refined n-propanol product and a seventh fraction. The method can obtain refined low-carbon alcohol products with higher purity at low cost and high efficiency.

Description

Method and system for refining low-carbon alcohols

technical field

The present invention relates to the refining field of alcohol, in particular to a method and system for refining lower alcohol.

Background technique

Low-carbon alcohols refer to unit alcohols containing less than 6 carbon atoms. Low-carbon alcohols are important chemical basic raw materials and chemicals, and are widely used in energy, chemical industry, medicine and other fields. Among them, methanol and ethanol are used to prepare ether. It is an important raw material for substances such as aldehydes and esters; propanol and isopropanol are commonly used industrial solvents; isobutanol and sec-butanol can be used as plasticizers, dye dispersants and anti-emulsifiers.

Among the low-carbon alcohols synthesized by different routes, the content of methanol and ethanol is often high, and the sum of the two accounts for more than 70% of the total low-carbon alcohol, and the content of other low-carbon alcohols is relatively low. Methanol and water do not azeotrope, and the product can be obtained by ordinary rectification methods, while ethanol and water form a low-boiling azeotrope, which is difficult to separate. The separation of other low-carbon alcohols is similar to that of ethanol.

CN104788284A discloses a continuous production method for producing low-carbon alcohol from synthesis gas, comprising the following steps: a) the synthesis gas raw material is at least divided into two strands; b) the first strand of synthesis gas raw material enters a reactor, and is contacted with a catalyst to obtain a gas-phase product logistics; c) after the gas-phase product flows through the gas-gas heat exchanger for heat exchange, it enters the condenser, and after condensation, uncondensed gas and condensate are obtained; d) the second synthesis gas feedstock enters the hydrocarbon evaporator, and the described The hydrocarbons in the condensed liquid are stripped to obtain the stripped condensed liquid and the gas-phase stream containing hydrocarbons; e) the stripped condensed liquid is decompressed by the decompression equipment and then enters the rectifying tower, and the rectifying tower The upper part obtains the liquid phase stream containing methanol, the side line obtains products C2, C3 alcohol, and the tower still obtains the liquid phase stream containing C4 ⁺ alcohol; Described methanol-containing liquid phase stream and the liquid phase stream containing C4 ⁺ alcohol are returned to the reactor; f) The uncondensed gas and the hydrocarbon-containing gas phase stream are compressed through a compression device, and after heat exchange with the gas phase product stream, are recycled back to the reactor. The method aims to improve the yield of C2 and C3 alcohols, and does not provide a method for alcohol purification.

CN105130749A discloses a process for producing low-carbon alcohol from syngas combined with coke oven gas, which is characterized by comprising the following steps: (1) introducing coke oven gas and syngas into a gas cabinet to fully mix, forming a process that meets the requirements of low-carbon alcohol synthesis hydrogen (2) The crude raw material gas is compressed to carry out low-temperature methanol cleaning, and impurities such as hydrogen sulfide and carbon dioxide are removed to obtain pure gas for synthesizing low-carbon alcohols, and the extracted hydrogen sulfide is desulfurized and recovered. The removed carbon dioxide is discharged at high altitude; (3) the purified gas is subjected to the reaction of synthesizing low-carbon alcohols, and the obtained gas-liquid two-phase product is subjected to gas-liquid separation, and the separated gas mainly includes _CO , H and _CO and part of it is sent to low-temperature methanol Washing cycle, the other part is sent to the torch for combustion discharge as purge gas, the separated liquid phase product is an alcohol-water mixture, and the alcohol-water mixture is separated from alcohol and water to obtain water and low-carbon alcohol products; the volume percentage of the coke oven gas is composed of H ₂ : 50-60%, CH ₄ : 20-28%, CO+CO ₂ : 10-18%, N ₂ : 3-8%, the volume percent composition of syngas is CO: 45-60%, H ₂ : 20-28%, CO ₂ : 8-18, CH ₄ : 7-15%, H ₂ S: 0.15-0.35%; the operating temperature of the low-temperature methanol washing is -35～-55 ℃, and the operating pressure is 2.0 -6.5MPa, H ₂ S <0.1ppm, CO ₂ <20ppm in the synthesis gas after low temperature methanol washing; the alcohol-water separation adopts benzene azeotropic distillation technology, ion exchange resin, membrane separation technology, molecular sieve dehydration technology One, the operating conditions are: the temperature is 80-150 ℃, and the pressure is normal pressure or reduced pressure; the low-carbon alcohol reaction adopts one or more series-connected fixed-bed reactors, and the reaction process conditions are: the reaction pressure is 3-10MPa , the reaction temperature is 240-410 ℃, the hydrogen-carbon molar ratio is H ₂ /CO=2.0-5.0, and the space velocity is 5000-50000h ^-1 to carry out the synthesis of low-carbon alcohol; the catalyst used in the synthesis of the low-carbon alcohol In order to use the MoS 2-MK catalyst of DOW Chemical Company of the United States, the modified Cu-Zn-Al-K series catalyst of Lurgi Company of Germany, the Cu-Co-Cr-K catalyst of the French Petroleum Institute, the Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences One or more of the Zn-Cr catalyst or the Japanese Cu-Ni-Zr-Na catalyst; the mass percentage of the low-carbon alcohol product is composed of 20-30% methanol, 40-60% ethanol, and 12% propanol. -20%, other alcohols 8-20%. However, this patent application does not provide a specific alcohol refining method.

The master's thesis "Research on the separation process technology of low-carbon alcohol" proposed a process for the separation of low-carbon alcohol. The low-carbon alcohol comes from the hydrogenation product of acetate. The low-carbon alcohol mixture contains: methyl acetate 7.97%, methanol 44.88% , ethyl acetate 3.01%, ethanol 35.19%, water 4.82%, a small amount of other alcohols. Various types of azeotropes are easily formed in this raw material: low-carbon alcohols (except methanol) and water, methyl alcohol and acetate all form binary minimum azeotropes. In addition, ethanol, water and ethyl acetate also form a ternary low-boiling azeotrope system. Because this raw material contains a certain amount of methyl acetate and ethyl acetate, the composition of the raw material is quite different from the low-carbon alcohol mixture obtained by the synthesis gas reaction, which is not suitable for the purification and separation of the low-carbon alcohol obtained by the synthesis gas reaction. And this process uses DMSO+NMP composite extractant, and the price of extractant NMP is relatively high, close to 20,000 yuan / ton, the extractant in the extraction and rectification process will be lost during the operation process, which will lead to the investment and cost of this process. Operating costs are high.

CN104529704B discloses a combined production device for synthesis and separation of low-carbon mixed alcohol, which is characterized by comprising: a low-carbon mixed alcohol synthesis subsystem and a low-carbon mixed alcohol separation subsystem; wherein, the low-carbon mixed alcohol synthesis subsystem It includes a raw material gas processing module, a low-carbon alcohol synthesis module and a product processing module that are connected in sequence; the raw material gas processing module includes a raw material gas compressor group (11) for pressurizing the raw material gas and a raw material gas compressor set (11) for heating the raw material A raw material gas heating unit for gas, the gas outlet of the raw gas compressor unit (11) is connected to the air inlet of the raw gas heating unit; the low-carbon alcohol synthesis module includes a low-carbon alcohol synthesizer (21), so The air inlet of the low-carbon alcohol synthesizer (21) is communicated with the air outlet of the raw material gas heating unit; the product processing module comprises a low-carbon alcohol separation part, and the low-carbon alcohol separation part comprises a low-carbon alcohol separator (311), wherein the low-carbon alcohol separator (311) comprises an inlet of a low-carbon alcohol separator, and the inlet of the low-carbon alcohol separator is connected to the gas outlet of the low-carbon alcohol synthesizer (21), and the The low-carbon alcohol separation part has a low-carbon mixed alcohol outlet for diverting low-carbon mixed alcohol products; the low-carbon mixed alcohol separation subsystem includes: a methanol rectification tower (100), which has a low-carbon mixed alcohol outlet connected to the low-carbon mixed alcohol outlet. The methanol tower feed port, the methanol tower top discharge port and the methanol tower bottom discharge port; the methanol tower top discharge port is connected with the methanol storage tank (110); the ethanol rectification tower (200) has the same An ethanol tower feed port connected to a methanol tower bottom discharge port, and an ethanol tower top discharge port and an ethanol tower bottom discharge port; an extractive distillation column (300), having an extraction rectification tower connected to the ethanol tower top discharge port The inlet of the tower, and the inlet of the extraction agent, the outlet of the top of the extraction tower and the outlet of the bottom of the extraction tower; wherein the inlet of the extraction tower is located below the inlet of the extraction agent; the top of the extraction tower The discharge port is connected with the ethanol storage tank (310); the n-propanol rectification tower (400) has the n-propanol tower feed port connected with the discharge port at the bottom of the ethanol tower, and the n-propanol tower top discharge mouth and n-propanol tower bottom discharge port; described n-propanol tower top discharge port is connected with n-propanol storage tank (410); described n-propanol tower bottom discharge port and mixed butanol storage tank (420) connected, the low-carbon alcohol separation part further includes a first-stage separator (23), the first-stage separator (23) includes a separator inlet, a first outlet of the separator and a second outlet of the separator, the separation The inlet of the separator is communicated with the outlet of the low-carbon alcohol synthesizer (21), the first outlet of the separator is connected to the inlet of the low-carbon alcohol separator (311), and the second outlet of the separator is connected to To the wax product device of the combined production device, the product processing module further includes a light oil product separation part, the light oil product separation part is connected to the low-carbon alcohol separation part, and the low-carbon alcohol separator (311 ) also includes the first outlet of the low-carbon alcohol separator and the second outlet of the low-carbon alcohol separator, and the inlet of the low-carbon alcohol separator (311) is connected to the first separator of the first-stage separator (23). an outlet, the first outlet of the low-carbon alcohol separator and the light oil The inlet of the product separation part is connected, and the second outlet of the low-carbon alcohol separator is used for outputting low-carbon alcohol products. However, the alcohol products obtained by the system or method are of low purity, and products with desired purity must be obtained by combining with other methods such as membrane separation and purification, and membrane separation methods are limited by efficiency and cost in practical production applications; Since water can form azeotropes with other low-carbon alcohols (ethanol, propanol, butanol, amyl alcohol, etc.) except methanol, and the system or method fails to fully separate water from the system before alcohol refining, water The circulation in the subsequent refining process not only increases the difficulty of refining and separation of low-carbon alcohols, but also increases the energy consumption.

Therefore, there is an urgent need for a method for purifying low-carbon alcohols with low cost, high efficiency and high product purity.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and system for purifying low-carbon alcohols in order to overcome the problems of the prior art, such as narrow application of raw materials, insufficient product purity, and high cost.

In order to achieve the above object, a first aspect of the present invention provides a method for refining low-carbon alcohol, the method comprising:

(1) low-carbon alcohol raw material mixture is carried out the 1st rectification, obtains organic waste water and the first fraction;

(2) the first fraction is subjected to a first extractive distillation using a first extractant to further remove water, and a second fraction is obtained;

(3) carrying out the Ⅱ rectification with the described second fraction to obtain refined methanol product and the third fraction;

(4) using the second extractant for the third fraction to carry out the second extractive rectification to obtain a refined ethanol product and the fourth fraction;

(5) carrying out the Ⅲ rectification with the described 4th fraction, obtains the 2nd extractant and the 5th fraction;

(6) the fifth fraction is carried out the IV rectification to obtain the crude isopropanol stream and the sixth fraction;

(7) The sixth fraction is subjected to the V rectification to obtain the refined n-propanol product and the seventh fraction.

A second aspect of the present invention provides a system for refining low-carbon alcohols, the system comprising:

The first rectification column is used for removing organic waste water, and has the first rectification column inlet for receiving the mixture of low-carbon alcohol raw materials, the first rectification column top outlet and the first rectification column bottom. outlet;

The first extractive rectification tower is used for further removing water, and has the first extractive rectification tower feed port, the first extractant feed port, the first A material outlet at the top of the extractive rectification tower and a material outlet at the bottom of the first extractive rectification tower;

The second rectification tower is used for refining methanol, and has a feed port of the second rectification tower connected with the outlet of the top of the first extractive distillation tower, a methanol outlet and a bottom outlet of the second rectification tower. material mouth;

The second extractive distillation column is used for refining ethanol, and has a second extractive distillation column feed port, a second extractant feed port, and a second extractive distillation column, which are communicated with the bottom discharge port of the second distillation column A discharge port at the top of the distillation tower and a discharge port at the bottom of the second extractive distillation tower, and the discharge port at the top of the second extractive distillation tower outputs ethanol;

The third rectification column is used for recovering the second extractant, and has a feed port of the third rectification column communicated with the discharge port at the bottom of the second extractive rectification column, and a discharge port at the top of the third rectification column. And the discharge port at the bottom of the third rectification tower, the discharge port at the bottom of the third rectification tower outputs the second extractant;

The fourth rectification tower has a feed port of the fourth rectification tower communicated with the discharge port at the top of the third rectification tower, the discharge port at the top of the fourth rectification tower and the discharge port at the bottom of the fourth rectification tower. The feed port, the outlet of the top of the rectifying tower IV outputs the crude isopropanol stream; and

The fifth rectification tower has a feed port of the fifth rectification tower communicated with the outlet of the bottom of the fourth rectification tower, the discharge port of the top of the fifth rectification tower and the bottom outlet of the fifth rectification tower The feed port, the outlet of the top of the fifth rectifying tower outputs n-propanol.

In actual production and application, the method of the present invention is easy to operate, has low cost, and the obtained product has high purity. For a higher isopropanol product, the purity of the obtained isopropanol product can reach more than 99.5% by weight.

Description of drawings

FIG. 1 is a schematic diagram of an embodiment of the system provided by the present invention.

2 is a schematic diagram of an embodiment of the isopropanol recovery and purification subsystem provided by the present invention.

Description of reference numerals

1 The first rectification column 2 The first extractive rectification column

3 The first extraction agent recovery column 4 The second rectification column

5 The second extractive distillation column 6 The third distillation column

7 Rectification column IV 8 Rectification column V

21 The third extractive distillation column 22 The third extractant recovery column

23 The fourth extractive distillation column 24 The sixth distillation column

101 Low-carbon alcohol raw material mixture 103 The first fraction

104 Organic wastewater 109 The first extractant

111 Second fraction 112 Non-condensable components

113 Refined methanol products 114 Third fraction

115 Ethanol Products 116 Fourth Fraction

117 Second extractant 118 Fifth fraction

119 Crude isopropanol stream 120 Sixth fraction

122 n-Propanol products 123 Seventh fraction

203 The third extractant 201 The eighth fraction

202 Ninth fraction 208 Fourth extractant

205 Crude Ethanol Products 206 Tenth Fraction

207 Isopropyl alcohol products

Detailed ways

The endpoints of ranges and any values disclosed herein are not limited to the precise ranges or values, which are to be understood to encompass values proximate to those ranges or values. For ranges of values, the endpoints of each range, the endpoints of each range and the individual point values, and the individual point values can be combined with each other to yield one or more new ranges of values that Ranges should be considered as specifically disclosed herein.

A first aspect of the present invention provides a method for purifying low-carbon alcohol, the method comprising:

(1) low-carbon alcohol raw material mixture is carried out the 1st rectification, obtains organic waste water and the first fraction;

(2) the first fraction is subjected to a first extractive distillation using a first extractant to further remove water, and a second fraction is obtained;

(3) carrying out the Ⅱ rectification with the described second fraction to obtain refined methanol product and the third fraction;

(4) using the second extractant for the third fraction to carry out the second extractive rectification to obtain a refined ethanol product and the fourth fraction;

(5) carrying out the Ⅲ rectification with the described 4th fraction, obtains the 2nd extractant and the 5th fraction;

(6) the fifth fraction is carried out the IV rectification to obtain the crude isopropanol stream and the sixth fraction;

(7) The sixth fraction is subjected to the V rectification to obtain the refined n-propanol product and the seventh fraction.

The “first extractant”, “second extractant”, “third extractant” and “fourth extractant” in the present invention are only used to distinguish the extractant used in each step, and there is no order, main second.

In the present invention, the "first fraction", "second fraction"..."seventh fraction"..."tenth fraction" are only used to distinguish the fractions produced in each step, and there is no priority or priority. point.

In the present invention, lower alcohols refer to unit alcohols containing less than 6 carbon atoms.

In the present invention, the lower alcohols refer to unit alcohols containing less than 6 carbon atoms. The low-carbon alcohol mixture may be a product synthesized by various processes, for example, a mixture of products obtained by producing low-carbon alcohol from synthesis gas or by other processes such as acetate hydrogenation. The low-carbon alcohol raw material mixture contains water, methanol, ethanol, n-propanol, isopropanol, butanol, etc., wherein the content of water is 30-80% by weight, the content of low-carbon alcohol is 20-70% by weight, and the content of methanol is 30-80% by weight. The sum of ethanol and ethanol accounts for more than 70% of the total amount of low-carbon alcohols.

According to the method of the present invention, in step (1), the low-carbon alcohol raw material mixture is subjected to the first rectification to obtain organic waste water and the first fraction, and the conditions of the first rectification include: the temperature at the top of the tower is 60- 80°C, the column top pressure is 0-10kPa, and the reflux ratio is 0.5-1; preferably, the column top temperature is 65-75°C, the column top pressure is 4-6kPa, and the reflux ratio is 0.7-0.9; The top temperature was 70°C, the top pressure was 5kPa, and the reflux ratio was 0.8. In one embodiment, the organic waste water is discharged from the bottom of the column, and the first fraction is discharged from the top of the column. The organic wastewater contains most of water, acids, heavy alcohols (alcohols with a carbon number greater than 6), and the like. In a preferred embodiment, the organic waste water is heat-exchanged with the low-carbon alcohol raw material mixture before being discharged to recover heat. In the present invention, unless otherwise specified, all pressures are gauge pressures.

According to the method of the present invention, in step (2), the first fraction is subjected to a first extractive rectification using a first extractant to further remove water, and a second fraction is obtained, wherein the first extractant is At least one selected from ethylene glycol, furfural, dimethylformamide, and dimethyl sulfoxide, preferably, the first extractant is a composite solvent composed of ethylene glycol and dimethyl sulfoxide. The conditions of the first extractive rectification include: the column top temperature is 50-70°C, the column top pressure is 0-10kPa, and the reflux ratio is 0.35-0.65; preferably, the column top temperature is 55-65°C, and the column top pressure is 55-65°C. is 4.5-5.5kPa, and the reflux ratio is 0.4-0.6; more preferably, the column top temperature is 62° C., the column top pressure is 5kPa, and the reflux ratio is 0.5. In one embodiment, the first extractant is added from the top of the tower, and the first extractant is a composite solvent of ethylene glycol and dimethyl sulfoxide, and the volume ratio of the two (ethylene glycol/dimethyl sulfoxide) sulfone) is (0.5-2):1, preferably (1-2):1, more preferably 1.5:1; the feed ratio of the first extractant to the first fraction is 2-4, preferably 2.5-3.5 , more preferably 3.0. In one embodiment, after the first extractive rectification, the mixture of the first extractant and water is discharged from the bottom of the column, and the second fraction is discharged from the top of the column. In a preferred embodiment, the second fraction contains 20-60 wt% methanol, 10-40 wt% ethanol, 0.1-1 wt% water, 10-30 wt% n-propanol, 1 -10% by weight of isopropanol, 0.1-5% by weight of butanol. In one embodiment, the second fraction also contains 0.1-3% by weight of methyl ether. Preferably, the second fraction contains 40-50 wt% methanol, 20-30 wt% ethanol, 0.1-0.5 wt% water, 10-20 wt% n-propanol, 1-7 wt% isopropyl alcohol Propanol, 0.1-3 wt% butanol and 0.1-2.5 wt% methyl ether.

In an embodiment of the method of the present invention, the method further comprises a first extractant recovery step, which comprises rectifying the mixture of the first extractant and water obtained at the bottom of the column in the first extractive rectification step. Distillation separation, waste water is obtained at the top of the column and removed, and the first extractant is obtained at the bottom of the column. The conditions for the rectification for recovering the first extractant include: the column top temperature is 40-50° C., the column top pressure is 5-15kPaA (here the pressure is absolute pressure), and the reflux ratio is 1-3; The top temperature was 45°C, the top pressure was 10 kPaA (here the pressure was absolute pressure), and the reflux ratio was 2. In a preferred embodiment, the first extractant obtained at the bottom of the column is recycled to the first extractive distillation step after heat recovery in the heat exchange device 9.

According to the method of the present invention, in step (3), the second fraction is subjected to the second rectification to obtain a refined methanol product and the third fraction, and the conditions for the second rectification include: the temperature at the top of the column is 35- 50°C, the column top pressure is 0-10kPa, and the reflux ratio is 3-7; preferably, the column top temperature is 38-48°C, the column top pressure is 4-6kPa, and the reflux ratio is 5-6; The top temperature was 44°C, the top pressure was 5kPa, and the reflux ratio was 5.6. The purity of the refined methanol product can reach more than 99.5% by weight, and even reach 99.9% by weight. The recovery rate of methanol reaches more than 95%, even more than 98.5%. The second rectification step also produces non-condensable components such as dimethyl ether, which are withdrawn overhead for further processing.

According to the method of the present invention, in step (4), the third fraction is subjected to a second extractive rectification using a second extractant to obtain a refined ethanol product and a fourth fraction; The conditions include: the top temperature is 70-90°C, the top pressure is 0-10kPa, and the reflux ratio is 1-3; preferably, the top temperature is 75-85°C, the top pressure is 4-6kPa, and the reflux ratio is 1.5-2.5; more preferably, the column top temperature is 80°C, the column top pressure is 5kPa, and the reflux ratio is 2. In one embodiment, the second extractant is added from the upper part of the column, and the feed ratio of the second extractant to the first fraction is 2-4, preferably 3.3. The second extraction agent is selected from at least one of ethylene glycol, furfural, dimethylformamide and dimethyl sulfoxide, preferably, the second extraction agent is composed of ethylene glycol and dimethyl sulfoxide The volume ratio of the two (ethylene glycol/dimethyl sulfoxide) is (0.5-2): 1, preferably (1-2): 1, more preferably 1.5: 1. The purity of the purified ethanol product can reach more than 99.5% by weight, and even reach 99.9% by weight. The recovery rate of ethanol reaches more than 95%, even 97.5%. In one embodiment, after the second extractive rectification, the fourth fraction comprising the second extractant and the lower alcohol is discharged from the bottom of the column, and the refined ethanol product is discharged from the top of the column.

According to the method of the present invention, in step (5), the fourth fraction is subjected to the third rectification to obtain the second extractant and the fifth fraction; the conditions of the third rectification include: the top temperature is 35-55 ℃, the top pressure is 5-15kPaA (here the pressure is absolute pressure), the reflux ratio is 0.5-1.5; preferably, the top temperature is 41 ℃, the top pressure is 10kPaA (here the pressure is absolute pressure) ), the reflux ratio is 1. In one embodiment, after the third rectification, the second extractant is discharged from the bottom of the column, and the fifth fraction is discharged from the top of the column. Preferably, the second extractant removed at the bottom of the column is subjected to heat exchange and then recycled to the second extractive and rectification step.

According to the method of the present invention, in step (6), the fifth fraction is subjected to the IV rectification to obtain a crude isopropanol stream and the sixth fraction; the conditions of the IV rectification include: column top temperature is 75-90°C, the column top pressure is 3-7kPa, and the reflux ratio is 3-7; preferably, the column top temperature is 82°C, the column top pressure is 5kPa, and the reflux ratio is 5. In one embodiment, after the IV rectification, the crude isopropanol stream is withdrawn from the top of the column and the sixth fraction is withdrawn from the bottom of the column. The crude isopropyl alcohol stream mainly contains isopropyl alcohol, and also contains a small amount of ethanol and water, preferably, the content of isopropyl alcohol in the crude isopropyl alcohol stream is 75% by weight to 95% by weight, preferably 85% by weight %.

According to the method of the present invention, in step (7), the sixth fraction is subjected to the Ⅴ rectification to obtain a refined n-propanol product and the seventh fraction; the conditions of the Ⅴ rectification include: tower top The temperature is 90-105°C, the column top pressure is 0-10kPa, and the reflux ratio is 1-3; preferably, the column top temperature is 95-100°C, the column top pressure is 4-6kPa, and the reflux ratio is 1.5-2.5; more Preferably, the column top temperature is 98° C., the column top pressure is 5 kPa, and the reflux ratio is 2. In one embodiment, after the fifth rectification, the refined n-propanol product is discharged from the top of the column, and the seventh fraction is discharged from the bottom of the column. In a preferred embodiment, the purity of the n-propanol product reaches more than 99.5% by weight, more preferably 99.9% by weight, and the recovery rate of n-propanol reaches more than 90%, preferably more than 95.5%. In one embodiment, the resulting n-propanol product can provide heat to an upstream step such as the reboiler 11 of the second rectification step by heat exchange. The seventh fraction is a crude butanol stream. In one embodiment, the content of butanol in the crude butanol stream is 80-95% by weight, preferably 89.3% by weight.

In an embodiment of the method of the present invention, the method further comprises performing the third extractive rectification on the crude isopropanol stream obtained in step (6) using a third extractant to obtain the eighth fraction; The eighth fraction is subjected to the fourth extractive rectification using the fourth extractant to obtain crude ethanol product and the tenth fraction; the sixth fraction is subjected to the sixth rectification to obtain the refined isopropanol product.

In a preferred embodiment of the method of the present invention, the conditions of the third extractive rectification include: a column top temperature of 75-90° C., a column top pressure of 0-10 kPa, and a reflux ratio of 2-4; Preferably, the top temperature is 80-85°C, the top pressure is 4-6kPa, and the reflux ratio is 3-4; more preferably, the top temperature is 83°C, the top pressure is 5kPa, and the reflux ratio is 3.2. In a preferred embodiment, the feed ratio of the crude isopropanol stream to the third extractant is 2-6, preferably 4. The third extraction agent is selected from at least one of ethylene glycol, furfural, dimethylformamide, and dimethyl sulfoxide. Preferably, the third extraction agent is composed of ethylene glycol and dimethyl sulfoxide. The volume ratio of the two (ethylene glycol/dimethyl sulfoxide) is (0.5-2): 1, preferably (1-2): 1, more preferably 1.5: 1. In one embodiment, after the third extractive rectification, the eighth fraction is discharged from the top of the column, and the ninth fraction is discharged from the bottom of the column, and the ninth fraction is a mixture of the third extractant and water.

In a preferred embodiment of the method of the present invention, the conditions of the fourth extractive rectification include: the temperature at the top of the column is 70-90° C., the pressure at the top of the column is 0-10kPa, and the reflux ratio is 3-7; Preferably, the top temperature is 75-85°C, the top pressure is 4-6kPa, and the reflux ratio is 4-6; more preferably, the top temperature is 80°C, the top pressure is 5kPa, and the reflux ratio is 5. In a preferred embodiment, the feed ratio of the eighth fraction to the fourth extractant is 2-10, preferably 6. The fourth extractant is selected from at least one of ethylene glycol, furfural, dimethylformamide, and dimethyl sulfoxide. Preferably, the fourth extractant is composed of ethylene glycol and dimethyl sulfoxide. The volume ratio of the two (ethylene glycol/dimethyl sulfoxide) is (0.5-2): 1, preferably (1-2): 1, more preferably 1.5: 1. In one embodiment, after the fourth extractive rectification, the crude ethanol product is discharged from the top of the column, and the tenth fraction is discharged from the bottom of the column. In a preferred embodiment, the purity of ethanol in the crude ethanol product is above 95% by weight, preferably 97.5% by weight.

In a preferred embodiment of the method of the present invention, the conditions of the sixth rectification include: the top temperature is 30-50°C, the top pressure is 12-18kPaA (here the pressure is absolute pressure), The reflux ratio is 0.2-1.5; preferably, the column top temperature is 41° C., the column top pressure is 15 kPaA (the pressure here is absolute pressure), and the reflux ratio is 1. In one embodiment, after the VI rectification, the refined isopropanol product is discharged from the top of the column, and the fourth extractant is discharged from the bottom of the column. Preferably, the fourth extractant removed from the column bottom is recycled to the fourth extractive distillation step.

In a preferred embodiment of the method of the present invention, the method further comprises a third extractant recovery step, the third extractant recovery step comprises subjecting the ninth fraction obtained by the third extractive distillation step to VII rectification, the conditions of the VII rectification include: the column top temperature is 40-55°C, the column top pressure is 12-18kPaA (absolute pressure here), and the reflux ratio is 1-5; preferably, the column top The temperature was 46° C., the pressure at the top of the column was 15 kPaA (absolute pressure here), and the reflux ratio was 3. In one embodiment, after the VII rectification, the waste water is discharged from the top of the column, and the third extractant is discharged from the bottom of the column. Preferably, the third extractant removed at the bottom of the column is recycled to the third extractive rectification step.

The method of the invention removes almost all of the water in the low-carbon alcohol raw material mixture through the first rectification and the first extractive rectification step, reduces the separation difficulty of the subsequent separation steps, and makes it easier to obtain an alcohol product with higher purity; The specific combination of rectification and rectification, and through the step-by-step extractive rectification and the setting of the extractant, for example, the composite solvent composed of ethylene glycol and dimethyl sulfoxide is selected as the first, second, third, and fourth extractant. Compared with the use of a single solvent (such as ethylene glycol), the use of composite solvents to separate multi-component low-carbon alcohol systems has the advantages of high product recovery, high product purity, low solvent ratio, and low energy consumption. Cost-effective separation of alcohol species.

The method of the invention can easily obtain methanol, ethanol and n-propanol products with higher purity (purity can reach 99.9% by weight), and can obtain isopropanol products with higher purity (purity can reach 99.7% by weight). The invention reduces the number of column plates for rectification through a specific process combination of rectification and extractive rectification, so that a product with higher purity can be obtained under the condition of a lower number of column plates.

A second aspect of the present invention provides a system for refining low-carbon alcohols, as shown in FIG. 1 is a schematic diagram of an embodiment of the system, the system includes:

The first rectification tower 1 is used for removing organic waste water, and has the first rectification tower feed port, the first rectification tower top discharge port and the first rectification tower tower for receiving the low-carbon alcohol raw material mixture The bottom discharge port, wherein, the first fraction is output from the top discharge port of the first rectification tower, and the organic waste water is output from the bottom discharge port of the first rectification tower;

The first extractive distillation column 2 is used to further remove water, and has the first extractive distillation column feed port, the first extractant feed port, the An extractive distillation column top discharge port and a first extractive distillation column bottom discharge port, wherein the first extractive distillation column feed port receives the first extractive distillation column from the top discharge port of the first distillation column. One fraction, the discharge port at the bottom of the first extractive rectification tower outputs waste water, and the discharge port at the top of the first extractive rectification tower outputs the second fraction;

The second rectification column 4 is used for refining methanol, and has the second rectification column feed port, the methanol outlet and the bottom of the second rectification column communicated with the outlet of the first extractive rectification column. A discharge port, wherein the feed port of the second rectification tower is used to receive the second fraction from the discharge port at the top of the first extractive distillation tower, and the discharge port at the bottom of the second rectification tower outputs the first fraction. Three fractions, the methanol outlet outputs refined methanol;

The second extractive distillation column 5, used for refining ethanol, has a second extractive distillation column feed port, a second extractant feed port, a second extraction agent feed port, and a second extractive distillation column feed port communicated with the bottom discharge port of the second distillation column. The rectification tower top discharge port and the second extractive rectification tower bottom discharge port, wherein the second extractive rectification tower feed port is used to receive the second extractive distillation tower bottom discharge port from the second rectification tower. Three fractions, the outlet at the top of the second extractive rectification tower outputs refined ethanol, and the outlet at the bottom of the second extractive rectification tower outputs the fourth fraction;

The third rectifying column 6 is used for recovering the second extractant, and has a third rectifying column feed port connected with the bottom discharge port of the second extractive rectifying column, and the third rectifying column top discharges material and the outlet of the bottom of the third rectification tower, wherein the inlet of the third rectification tower is used to receive the fourth fraction from the outlet of the bottom of the second extractive distillation tower, and the inlet of the third rectification tower is used to receive the fourth fraction. The discharge port at the bottom of the distillation column outputs the second extractant, and the discharge port at the top of the third rectifying tower outputs the fifth fraction;

The fourth rectifying tower 7 has the fourth rectifying tower feed port, the fourth rectifying tower top outlet and the fourth fourth rectifying tower bottom A material outlet, wherein the inlet of the fourth rectification tower is used to receive the fifth fraction from the outlet of the top of the third rectification tower, and the outlet of the top of the fourth rectification tower outputs crude isopropanol Logistics, the sixth fraction is output from the outlet at the bottom of the fourth rectifying tower;

The 5th rectifying tower 8 has a feed port of the 5th rectification tower communicated with the outlet of the bottom of the said 4th rectification tower, the outlet of the top of the 5th rectification tower and the bottom of the 5th rectification tower The outlet, wherein the inlet of the fifth rectification tower is used to receive the sixth fraction from the outlet of the bottom of the fourth rectification tower, and the outlet of the top of the fifth rectification tower outputs the refined fraction. n-propanol, the seventh fraction is output from the bottom outlet of the V rectifying tower.

In a preferred embodiment of the system of the present invention, the system further includes an isopropanol recovery and refining subsystem, as shown in FIG. 2 , which is an implementation of the isopropanol recovery and refining subsystem of the present invention Schematic diagram of the way. The isopropanol recovery and purification subsystem includes:

The third extractive distillation column 21 has the third extractive distillation column feed port, the third extractant feed port, and the third extractive distillation column top that are communicated with the discharge port at the top of the fourth distillation column. The discharge port and the bottom discharge port of the third extractive distillation column, wherein the third extractive distillation column feed port is used to receive the crude isopropanol stream from the top discharge port of the IV distillation column, and the third The outlet at the top of the extractive distillation tower outputs the eighth fraction, and the outlet at the bottom of the third extractive distillation tower outputs the ninth fraction;

The fourth extractive rectification tower 23 has the fourth extractive rectification tower feed port, the fourth extractive rectification tower feed port, and the fourth extractive rectification tower The top discharge port and the fourth extractive distillation column bottom discharge port, wherein, the fourth extractive distillation column feed port is used to receive the eighth fraction from the third extractive distillation column top discharge port, The fourth extractive rectification tower top discharge port outputs ethanol, and the fourth extractive rectification tower tower bottom discharge port outputs the tenth fraction;

The sixth rectifying column 24 has the sixth rectifying column feed port communicating with the fourth extractive distillation column bottom outlet, the sixth rectifying column top outlet and the sixth rectifying column tower Bottom discharge port, wherein, the feed port of the sixth rectification tower is used to receive the tenth fraction from the bottom discharge port of the fourth extractive distillation tower, and the top discharge port of the sixth rectification tower outputs isopropyl Alcohol, the fourth extraction agent is output from the bottom outlet of the sixth rectifying tower.

In an embodiment of the system of the present invention, the system further comprises a first extractant recovery column 3, which has a first extractant recovery column feed in communication with the first extractive distillation column bottom outlet The first extraction agent recovery tower tower top outlet and the first extraction agent recovery tower tower bottom outlet. In one embodiment, the first extractant is discharged from the bottom discharge port of the first extractant recovery tower, and the waste water is discharged from the top discharge port of the first extractant recovery tower. In an embodiment of the system of the present invention, the system further comprises a third extractant recovery column 22, which has a third extractant recovery column feed in communication with the bottom outlet of the third extractive distillation column port, the third extractant recovery tower top discharge port and the third extractant recovery tower bottom discharge port, the third extractant recovery tower feed port receives the output from the third extractive distillation tower bottom discharge port ninth fraction. In one embodiment, the third extractant is discharged from the bottom discharge port of the third extractant recovery tower, and the waste water is discharged from the top discharge port of the third extractant recovery tower.

In a preferred embodiment of the system of the present invention, the system further comprises a heat exchange device 9 for recovering heat. In a preferred embodiment, the organic waste water discharged from the first rectification tower undergoes heat exchange through a heat exchange device to heat the low-carbon alcohol raw material mixture. In a preferred embodiment, the first extractant obtained from the bottom of the first extractant recovery tower is recycled to the first extractive distillation step after heat recovery in the heat exchange device 9.

The present invention will be described in detail below by means of examples.

Example 1

The low-carbon alcohol raw material mixture used in this example comes from a synthesis device, and the composition of the low-carbon alcohol raw material mixture is shown in Table 1 below.

Table 1 composition of low-carbon alcohol raw material mixture

component Content (wt%) dimethyl ether 0.7% methanol 22.8% Ethanol 13.7% n-Propanol 7.3% isopropyl alcohol 2.7% n-butanol 3.2% amyl alcohol 1.7% Acetic acid 0.5% water 47.4%

The setting parameters of each tower involved in the examples are shown in Table 2.

Table 2

In Table 2, * indicates that the pressure is absolute pressure, and the rest are gauge pressure.

Here, the column top temperature is defined as the saturated condensation temperature of the column top condenser, and the column bottom temperature is defined as the temperature of the column bottom reboiler;

The overhead pressure is defined as the pressure of the overhead condenser, and the bottom pressure is defined as the pressure of the bottom reboiler.

The low-carbon alcohol raw material mixture is separated and purified by the following method.

(1) low-carbon alcohol raw material mixed solution 101 (composition as shown in Table 1) is input into the first rectifying tower 1 after heating, and the control tower top pressure of the first rectifying tower is 5kPa, the tower top temperature is 70 ° C, and the reflux ratio 0.8, the organic waste water 104 (including most of water, acid, heavy alcohol, etc.), etc. is discharged (and removed) from the bottom of the tower through the rectification separation process, and the organic waste water 104 is exchanged with the low-carbon alcohol raw material mixed solution 101 before being discharged. Heat exchange is performed in the vessel 9 to recover heat, and the mixed liquid 101 of the low-carbon alcohol raw material is heated to 51° C.; the first fraction 103 is discharged from the top of the column and enters the first extractive distillation column 2 .

(2) in the first extractive distillation column 2, the first extraction agent 109 (the composite solvent of ethylene glycol and dimethyl sulfoxide, the volume ratio of the two is 1.5:1) is added from the top of the tower, and the first extraction agent The feed ratio of 109 to the first fraction 103 is 2.9, and the conditions for controlling the first extractive distillation in the first extractive distillation column 2 include: the column top pressure is 5kPa, the column top temperature is 62°C, and the reflux ratio is 0.5. In the first extractive distillation process, to further remove the remaining water, the second fraction 111 is obtained at the top of the first extractive distillation column 2 and enters the second distillation column 4, and the mixture 107 of the first extractant and water is obtained at the bottom of the column. Enter the first extractant recovery tower 3;

(3) in the first extraction agent recovery tower 3, control the first extraction agent recovery tower 3 tower top pressure to be 10kPaA (absolute pressure here), tower top temperature 45 ℃, reflux ratio 2, the first extraction agent and water The mixture 107 is obtained through the rectification separation process column top to obtain waste water 108 and is discharged from the system to the downstream device for processing, and the column bottom obtains the first extractant 109 and enters the heat exchange device 9 for heat recovery and then circulates into the upper part of the first extractive distillation column 2, The first extractive distillation column 2 feed (first fraction 103) is heated to 134°C;

(4) the second fraction 111 enters the second rectifying tower 4 to carry out the second rectification, and the pressure at the top of the second rectifying tower 4 is controlled to be 5 kPa, the temperature at the top of the tower is 44°C, and the reflux ratio is 5.6. In the distillation process, the purified methanol product 113 with a purity of 99.9% by weight is obtained at the top of the column, and the recovery rate of methanol is 98.5%. The third fraction 114 enters the second extractive distillation column 5;

(5) the second extractive distillation tower 5 is an ethanol purification tower, and the second extractant 117 (the composite solvent of ethylene glycol and dimethyl sulfoxide, the volume ratio of the two is 1.5:1) adds from the top of the tower, and the second The feed ratio of extractant 117 and the feed of the second extractive distillation column 5 (the third fraction 114) is 3.3, and the control second extractive distillation column 5 column top pressure is 5kPa, and the column top temperature is 80 ° C, and the reflux ratio is 2 , through the second extractive distillation process, an ethanol product 115 with a purity of 99.9% by weight is obtained at the top of the tower, the recovery rate of ethanol is 97.5%, and a fourth fraction 116 is obtained at the bottom of the tower. The fourth fraction contains the second extractant and Low carbon alcohol, the fourth fraction 116 enters the third rectifying tower 6;

(6) In the third rectifying tower 6, the conditions for controlling the third rectification in the third rectifying tower 6 are that the top pressure is 10kPaA (here is absolute pressure), the top temperature is 41°C, and the reflux ratio is 1. Through the third rectification process, the fifth fraction 118 is obtained at the top of the column and enters the fourth rectification column 7; the second extractant 117 is obtained at the bottom of the column and circulates into the upper part of the second extraction and rectification column 5;

(7) in the Ⅳ rectifying tower 7, control the Ⅳ rectifying condition in the Ⅳ rectifying tower 7 that the column top pressure is 5kPaG, the column top temperature is 82 ℃, the reflux ratio is 5, and the fifth fraction 118 is passed through the Ⅳ rectification The distillation process obtains the crude isopropanol stream 119 (mainly with isopropanol, containing a small amount of ethanol and water) at the top of the tower and enters the downstream device for further processing, and the isopropanol content in the crude isopropanol stream 119 is 85% by weight ; Obtain the sixth fraction 120 at the bottom of the tower and enter the Ⅴ rectifying tower 8;

(8) In the fifth rectifying column 8, the conditions for controlling the fifth rectification in the fifth rectifying column 8 include a column top pressure of 5kPa, a column top temperature of 98°C, and a reflux ratio of 2. Through the 5th rectification process, the n-propanol product 122 with a purity of 99.9% by weight is obtained at the top of the column, the n-propanol recovery rate is 95.5%, and the seventh fraction 123 is obtained at the bottom of the column to enter the downstream device for further processing. The butanol content in the fraction 123 is 89.3%, the condensation of the vapor phase 121 at the top of the fifth rectifying tower 8 can provide heat for the reboiler 11 in the second rectifying tower 4, and the condensate 121 is partially refluxed and partially used as a butanol product 122 discharge system.

(9) described thick isopropanol stream 119 enters the 3rd extractive distillation column 21, and the 3rd extractant 203 (the composite solvent of ethylene glycol and dimethyl sulfoxide, the volume ratio of the two is 1.5:1) from the column The top adds, and the feed ratio of the raw material (thick isopropanol stream 119) of the third extractant 203 and the third extractive distillation column 21 is 4, and the condition of the third extractive distillation in the control dehydration extractive distillation column 21 is a column The top pressure is 5kPa, the top temperature is 83°C, and the reflux ratio is 3.2. After the third extractive distillation process, the eighth fraction is obtained at the top of the third extractive distillation column 21, and the eighth fraction 201 enters the fourth extractive distillation. Tower 23; the ninth fraction 202 is obtained at the bottom of the tower, and the ninth fraction 202 is a mixture of the third extractant and water, and enters the third extractant recovery tower 22.

(10) in the 3rd extraction agent recovery tower 22, control the 3rd extraction agent recovery tower 22 tower top pressure to be 15kPaA (being absolute pressure here), tower top temperature 46 ℃, reflux ratio 3, the ninth cut (the third The mixture of extraction agent and water) 202 obtains waste water 204 and the waste water 108 is merged at the top of the rectification separation process and is discharged from the system to go to the downstream device for processing, and the third extraction agent 203 is obtained at the bottom of the tower and circulates into the upper part of the third extraction and rectification tower 21;

(11) in the fourth extractive distillation column 23, the fourth extractant 208 (the composite solvent of ethylene glycol and dimethyl sulfoxide, the volume ratio of the two is 1.5:1) is added from the top of the column, and the fourth extractant The feed ratio of 208 and the fourth extractive distillation column 23 feed (the eighth cut) is 6, and the condition of the fourth extractive distillation in the control the fourth extractive distillation column tower is that the tower top pressure is 5kPa, and the tower top temperature is 80 ℃. ℃, reflux ratio 5. Through the fourth extractive distillation process, a crude ethanol product 205 with a purity of 97.5% by weight is obtained at the top of the tower, and the tenth fraction is obtained at the bottom of the tower to enter, and the tenth fraction (comprising the fourth extractant and isopropanol) 206 enters The VI distillation column 24;

(12) In the sixth rectifying tower 24, the sixth rectifying condition in the control sixth rectifying tower 24 is that the tower top pressure is 15kPaA (absolute pressure here), the tower top temperature is 41°C, and the reflux ratio is 1. The tenth fraction 206 is subjected to the VI rectification process to obtain an isopropanol product 207 with a purity of 99.7% by weight at the top of the column, and a fourth extractant 208 obtained at the bottom of the column is recycled into the middle and upper part of the fourth extractive distillation column 23.

The composition of each material involved in Example 1 is shown in Table 3 below.

table 3

The method of the invention can separate low-carbon alcohols with higher purity, and the obtained methanol products, ethanol products, and n-propanol products all have a purity of 99.9% by weight, and the invention can also obtain higher-purity isopropanol. The purity of the propanol product can reach more than 99.5% by weight.

Example 2

Refer to the method described in Example 1 to separate and purify the raw material mixture of low-carbon alcohols described in Example 1, except that the used extraction agent (the first extraction agent, the second extraction agent, the third extraction agent and the fourth extraction agent) is ethyl alcohol. The composite solvent of diol and dimethyl sulfoxide, the volume ratio of the two is 0.8:1. The resulting product looks like this:

The purity of methanol product is 99.9%; the purity of ethanol product is 99.8%; the purity of n-propanol product is 99.5%; the purity of isopropanol product is 99.2%.

Example 3

Refer to the method described in Example 1 to separate and purify the raw material mixture of low-carbon alcohols described in Example 1, except that the used extraction agent (the first extraction agent, the second extraction agent, the third extraction agent and the fourth extraction agent) is ethyl alcohol. diol. The resulting product looks like this:

The purity of methanol product is 99.4%; the purity of ethanol product is 99.8%; the purity of n-propanol product is 90%; the purity of isopropanol product is 85%.

The preferred embodiments of the present invention have been described above in detail, however, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, a variety of simple modifications can be made to the technical solutions of the present invention, including combining various technical features in any other suitable manner. These simple modifications and combinations should also be regarded as the content disclosed in the present invention. All belong to the protection scope of the present invention.

Claims (12)

1. A method of refining a lower alcohol, the method comprising:

(1) Rectifying the low-carbon alcohol raw material mixture to obtain organic wastewater and a first fraction;

(2) Performing first extractive distillation on the first fraction by using a first extracting agent to further remove water and obtain a second fraction;

(3) Performing second rectification on the second fraction to obtain a refined methanol product and a third fraction;

(4) Performing second extractive distillation on the third fraction by using a second extractant to obtain a refined ethanol product and a fourth fraction;

(5) Carrying out III rectification on the fourth fraction to obtain a second extracting agent and a fifth fraction;

(6) Performing IV rectification on the fifth fraction to obtain a crude isopropanol material flow and a sixth fraction;

(7) Rectifying the sixth fraction V to obtain a refined n-propanol product and a seventh fraction;

the method further comprises the following steps: performing third extractive distillation on the crude isopropanol stream obtained in the step (6) by using a third extractant to obtain an eighth fraction; carrying out fourth extractive distillation on the eighth fraction by using a fourth extractant to obtain a crude ethanol product and a tenth fraction; performing VI rectification on the tenth fraction to obtain a refined isopropanol product;

the first extracting agent, the second extracting agent, the third extracting agent and the fourth extracting agent are composite solvents of ethylene glycol and dimethyl sulfoxide, and the volume ratio of the ethylene glycol to the dimethyl sulfoxide is (0.8-1.5): 1.

2. The method as claimed in claim 1, wherein, in step (1), the conditions of the I rectification comprise: the temperature at the top of the tower is 60-80 ℃, the pressure at the top of the tower is 0-10kPa, and the reflux ratio is 0.5-1.

3. The method as claimed in claim 2, wherein, in the step (1), the conditions of the I rectification comprise: the temperature at the top of the tower is 65-75 ℃, the pressure at the top of the tower is 4-6kPa, and the reflux ratio is 0.7-0.9.

4. The method of any one of claims 1-3, wherein in step (2), the conditions of the first extractive distillation comprise: the temperature at the top of the tower is 50-70 ℃, the pressure at the top of the tower is 0-10kPa, and the reflux ratio is 0.35-0.65.

5. The method according to any one of claims 1 to 3, wherein in step (2), the conditions of the first extractive distillation comprise: the temperature at the top of the tower is 55-65 ℃, the pressure at the top of the tower is 4.5-5.5kPa, and the reflux ratio is 0.4-0.6.

6. A process according to any one of claims 1 to 3, wherein the second fraction comprises 20 to 60% by weight of methanol, 10 to 40% by weight of ethanol, 0.1 to 1% by weight of water, 10 to 30% by weight of n-propanol, 1 to 10% by weight of isopropanol, 0.1 to 5% by weight of butanol.

7. The method according to any one of claims 1 to 3, wherein in step (3), the conditions for the II-th rectification comprise: the temperature at the top of the tower is 35-50 ℃, the pressure at the top of the tower is 0-10kPa, and the reflux ratio is 3-7.

8. The method according to any one of claims 1 to 3, wherein in step (3), the conditions for the II-th rectification comprise: the temperature at the top of the tower is 38-48 ℃, the pressure at the top of the tower is 4-6kPa, and the reflux ratio is 5-6.

9. The method according to any one of claims 1 to 3, wherein in step (4), the conditions of the second extractive distillation comprise: the temperature at the top of the tower is 70-90 ℃, the pressure at the top of the tower is 0-10kPa, and the reflux ratio is 1-3.

10. The method of any one of claims 1 to 3, wherein in step (4), the conditions of the second extractive distillation comprise: the temperature at the top of the tower is 75-85 ℃, the pressure at the top of the tower is 4-6kPa, and the reflux ratio is 1.5-2.5.

11. The method according to any one of claims 1 to 3, wherein in step (7), the rectification conditions of V comprise: the temperature at the top of the tower is 90-105 ℃, the pressure at the top of the tower is 0-10kPa, and the reflux ratio is 1-3.

12. The method according to any one of claims 1 to 3, wherein in step (7), the rectification conditions of V comprise: the temperature at the top of the tower is 95-100 ℃, the pressure at the top of the tower is 4-6kPa, and the reflux ratio is 1.5-2.5.

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