CN102690171B - Process for preparing ethanol from synthesis gas via methyl alcohol - Google Patents

Abstract

A process for producing ethanol from synthesis gas through methanol is that the synthesis gas is divided into two parts, one part of the synthesis gas is used for methanol synthesis, and the rest is separated into CO and H ₂ ; Synthetic raw material gas is subjected to methanol synthesis reaction under the action of methanol synthesis catalyst to obtain methanol and methanol synthesis cycle tail gas; after the CO obtained from the synthesis gas separation is mixed with acetic acid synthesis cycle tail gas, it is then carbonylated with methanol to perform acetic acid synthesis reaction , to obtain acetic acid and acetic acid synthesis cycle tail gas; the _H2 obtained by the separation of synthesis gas is mixed with the ethanol synthesis cycle tail gas into the reactor, and the mixed gas and acetic acid contact with the catalyst to undergo hydrogenation for ethanol synthesis reaction to obtain ethanol and by-products and ethanol synthesis cycle The tail gas and the crude product at the outlet of the reactor are separated to obtain ethanol. The invention has the advantages of high ethanol yield, high energy utilization rate of the overall process, low cost and large-scale production.

Description

A process for producing ethanol from synthetic gas through methanol

technical field

The invention relates to a process for preparing ethanol, in particular to a process for producing ethanol from synthesis gas through methanol.

Background technique

The energy consumption structure dominated by coal has made my country face many energy and environmental pressures such as energy security, environmental protection and sustainable development. Using my country's relatively abundant coal resources to develop non-petroleum-dependent energy and chemical technologies and industries to make up for the shortage of petroleum supply It is an important means to ensure China's energy security.

Ethanol is a bulk chemical product, and more importantly, the advantages of ethanol as a fuel additive have become a global consensus. Many countries in the world have or are promoting the use of fuel ethanol. my country has always encouraged and supported the development of ethanol gasoline, and has introduced several support and subsidy policies for the production and use of ethanol gasoline, and issued official documents such as national technical standards. Parts of 10 provinces such as Heilongjiang and Jilin have been used as pilot areas for ethanol gasoline. my country's fuel ethanol production capacity gap is huge, and new large-scale ethanol production technologies are urgently needed.

At present, the vast majority of ethanol production adopts the fermentation method using food and commercial crops as raw materials, which leads to two bottlenecks in the development of the fuel ethanol industry, high cost and threats to national food and cultivated land security. Non-grain route ethanol production technologies include the following: cellulose (mainly biomass straw) fermentation method, syngas synthesis ethanol method, etc. Cellulose fermentation (patent CN101235392) ethanol production technology does not directly use grain and economic crops, which can effectively alleviate the problems of grain consumption and land occupation by grain fermentation. (2) The cost of cellulase is high, and the cost of cellulosic ethanol is still higher than that of grain ethanol. Ethanol synthesis technology using synthesis gas as raw material is an important development direction of fuel ethanol manufacturing technology.

Contents of the invention

The purpose of the present invention is to propose a low-cost, large-scale production of synthesis gas through methanol ethanol process.

Concrete technological process of the present invention comprises as follows:

(1) The synthesis gas (CO+H ₂ ) is divided into two parts, one part of the synthesis gas is used for methanol synthesis, and the rest is separated into CO and H ₂ ;

(2) Synthesis gas and methanol synthesis cycle tail gas are mixed to form methanol synthesis raw material gas, and methanol synthesis raw material gas is subjected to methanol synthesis reaction under the action of methanol synthesis catalyst to obtain methanol and methanol synthesis cycle tail gas;

(3) Mix the CO obtained from the separation of synthesis gas with acetic acid synthesis cycle tail gas, and then carry out carbonylation reaction with methanol to carry out acetic acid synthesis reaction to obtain acetic acid and acetic acid synthesis cycle tail gas;

(4) The _H2 obtained from the separation of synthesis gas is mixed with the tail gas of ethanol synthesis cycle into the reactor, and the mixed gas and acetic acid contact with the catalyst for hydrogenation reaction to generate ethanol and by-products (acetaldehyde and ethyl acetate) and the tail gas of ethanol synthesis cycle. The crude product at the outlet of the reactor is separated to obtain ethanol.

The synthesis gas separation mentioned above adopts the CO and H ₂ separation process disclosed in patents CN102191086A and CN102133498A. The process of CN102133498A is to send the raw material gas from the bottom to the first adsorption tower for adsorption, enrich the bottom of the first adsorption tower to obtain CO product gas, and send the mixed gas obtained from the upper part of the first adsorption tower to the second adsorption tower; the mixed gas After being adsorbed by the second adsorption tower, high-purity hydrogen product gas is obtained from the outlet of the second adsorption tower. CN102191086A is an integrated device for coal-to-synthesis gas co-production of carbon monoxide, methanol, hydrogen, and refined synthesis gas, in which the CO cryogenic separation unit and pressure swing adsorption hydrogen production unit can be used to produce hydrogen and CO.

The methanol synthesis mentioned above adopts the low-pressure methanol synthesis process of ICI multi-stage cold shock tower, the low-pressure methanol synthesis process of Lurgi tubular isothermal synthesis tower or the methanol synthesis process of Linde spiral coil isothermal synthesis tower, etc. Synthesis gas for low-pressure methanol synthesis in Lurgi is prepared by reforming natural gas and water vapor. After purification and cooling, the synthesis gas is sent to a centrifugal turbine compressor, and after being compressed to 4-5 MPa, it is sent to a synthesis tower. Syngas reacts in the presence of a copper catalyst to produce methanol. The heat of reaction generated by synthesizing methanol is used to generate high-pressure steam, which is used as the power of the turbo-compressor. The gas containing methanol at the outlet of the synthesis tower is cooled by heat exchange with the mixed gas, and then cooled by air or water to condense the crude methanol and separate it in the separator. The condensed crude methanol is flashed in the flash tank and then sent to the rectification unit for refinement. The ICI low-pressure methanol synthesis method first converts the synthesis gas composed of H ₂ , CO, CO ₂ and a small amount of CH ₄ to adjust the ratio of CO/CO ₂ , and then uses a centrifugal compressor to boost the pressure to 5 MPa, and the feeding temperature is 270°C In the cold shock reactor, the reacted gas is cooled to separate methanol, and the unreacted gas is compressed and boosted and mixed with fresh raw material gas to enter the reactor again. The Linde isothermal synthesis tower is a new type of single-stage energy-saving isothermal tower. In the tower, the spiral coil is placed in the catalyst bed, the boiler water of 4.5 MPa is introduced from the lower part of the coil, and the medium pressure steam and circulating water are discharged from the top. Syngas enters at the top of the tower, passes through the catalyst bed, and exits at the bottom of the tower.

Methanol synthesis catalysts can be ICI51-1, ICI51-2, ICI51-7 catalysts developed by ICI Company, C79-5GL catalysts of Lurgi Company, C302, C302-1, C302-2, CNJ206 of Southwest Chemical Industry Research and Design Institute, etc.

The synthesis of acetic acid as mentioned above adopts Celanese AO Plus process, BP Cativa process or Chiyoda Acetica process. The Celanese AO Plus process changes the composition of the catalyst by adding high-concentration inorganic iodine (mainly lithium iodide), so that the reactor operates at a low water content of 4% to 5%, which improves the yield and refining capacity of the carbonylation reaction. The total residual iodine content of the product is lower than 5×10 ^-12 . The BP Cativa process changes rhodium-based catalysts into iridium-based catalysts in its traditional process technology. The process uses various rare metals such as rhenium, ruthenium, and osmium as promoters. The catalytic activity of iridium-based catalysts is significantly higher than that of rhodium-based catalysts. When the water content is low, the iridium-based catalysts have high stability and low energy consumption. By-products such as propylene Less, and can be operated under water content ≤ 5%, can greatly improve the traditional methanol carbonylation process, reduce production costs and investment. In addition, CO utilization efficiency is improved and steam consumption is reduced due to the reduced water content. The Chiyoda Acetica process uses a catalyst system combining a heterogeneous rhodium catalyst and polyvinylpyridine resin. This catalyst system can improve the catalytic activity of rhodium and make the yield of acetic acid exceed 99%. Using methyl iodide as a promoter, using a suspended solid rhodium-based composite catalyst, under the conditions of 175 ° C and 2.8 MPa, the reaction is carried out in a bubble column closed-circuit reactor, and the reaction product is flashed, dehydrated, and refined. The final product, the conversion rate of methanol is ≥99%.

The synthesis of ethanol as mentioned above adopts the catalyst and process with patent No. 201210057568.8 or 201210057475.5. The preparation method of the catalyst with the patent number 201210057568.8 is to add one or more compounds of transition metals Ni, Co, Fe, W, V, Mo, Nb, Cr, Ta, etc. and diammonium hydrogen phosphate according to the stoichiometric ratio of the catalyst composition into deionized water, and add citric acid, wherein the molar ratio of citric acid to transition metal is 0.5-3:1. The obtained solution was stirred at 50-90°C for 8-24 h, dried at 100-110°C for 18-24 h, and then the sample was foamed and cured at 120-150°C for 10-20 h, and the cured sample was placed in Roast in air at 400~650℃ for 3-6 hours. Heat the calcined sample to 400-800°C at a rate of 0.5-3°C/min in _H2 atmosphere, keep the temperature constant for 3-8 hours, and the _H2 gas space velocity is 3000-9000 h ^-1 , the sample is heated in _H2 After the atmosphere dropped to room temperature, the catalyst was passivated with O ₂ /N ₂ with an O ₂ content of 0.5-2% for 2-5 h. The preparation method of the catalyst with the patent No. 201210057475.5 is to first mix one or more compounds of transition metals Ni, Co, Fe, W, V, Mo, Nb, Cr, Ta, etc. and diammonium hydrogen phosphate according to the stoichiometric ratio of the catalyst composition Add it into deionized water. For the solution with precipitation, add nitric acid to the solution to dissolve the precipitation, and impregnate the prepared solution with equal volumes in aluminum oxide, molecular sieve, silicon dioxide, titanium dioxide, zirconium oxide or diatomaceous earth Etc. carrier, where transition metal compound: deionized water: carrier = 0.15-8.2 g: 10-30 ml: 10-20 g. The carrier loaded with the transition metal compound is placed at room temperature for 2-5 hours, dried at 30-60°C for 2-5 hours, dried at 100-120°C for 6-12 hours, and then calcined in air at 400-650°C for 3-6 hours. h. The calcined sample was heated to 400-800°C at a rate of 0.5-3°C/min in H ₂ atmosphere, and reduced at this temperature for 3-8 h, and the H ₂ space velocity was 3000-9000 h ^-1 . After the _H2 atmosphere dropped to room temperature, the sample was passivated with _O2 ^/ _N2 with _O2 content of 0.5-2% for 2-5 h to obtain the catalyst.

The reaction temperature for synthesizing ethanol is in the range of 200-300°C, the reaction pressure is 1.0-3.5 MPa, the liquid space velocity of acetic acid is 0.5-3 h ^-1 , and the molar ratio of H ₂ : acetic acid is 5-50. The conversion rate of acetic acid is 100%, and the ethanol selectivity is greater than 90%.

The above-mentioned product separation first adopts simple distillation to separate the acetaldehyde in the product, and then adopts the extractive distillation method disclosed in the patent CN1706799A to separate ethanol, ethyl acetate and water. Patent CN1706799A The steps of adding salt, compound extraction, rectification and separation of ethyl acetate-ethanol-water mixture are: (1) adding 10-20% NaAc to the mixture of ethyl acetate, ethanol and water; (2) extraction and separation of compound extractant For the crude product, ethyl acetate is obtained at the top of the tower, and the temperature at the top of the tower is controlled at 77-78°C; the mixed extractant containing salt flows out of the tower, and the extractant is treated through the regeneration tower, and the fraction at the top of the regeneration tower is treated with the ethanol recovery tower, and the remaining fraction Return it to use, and recycle the salt-containing extractant. After the crude product is separated, 40% of acetaldehyde, more than 99.5% of ethyl acetate and more than 95% of ethanol can be obtained. the

Compared with the prior art, the present invention has the following advantages:

1. The process route provided by the present invention is composed of synthesis gas separation-methanol synthesis-acetic acid synthesis-ethanol synthesis process unit. The methanol synthesis and acetic acid synthesis technologies are mature, the selectivity of the target product is high, and the ethanol yield of the acetic acid hydrogenation synthesis ethanol process is high. The overall process energy utilization rate is high.

2. Low cost and large-scale production.

Description of drawings

Fig. 1 is a schematic diagram of the process flow of syngas to ethanol through methanol in the present invention.

Detailed ways

In order to better understand the present invention, several illustrative rather than restrictive embodiments are given below.

Example 1:

480,000 tons/year ethanol synthesis (based on 300 days of operation per year)

The synthesis gas with H ₂ : CO = 2: 1 and the output of 2.46 million m ³ /day is divided into two parts, the first part is 1.23 million m ³ /day for methanol synthesis, and the second part is 1.23 million m ³ /day for CO and _H2 are separated and used as feedstock for acetic acid synthesis and ethanol synthesis. Using the method disclosed in patent CN102133498A to cryogenically separate CO and H ₂ in synthesis gas, the output of CO after separation is 1.07 million m ³ /day, and the output of H ₂ is 160,000 m ³ /day.

Methanol synthesis adopts Lurgi tubular isothermal synthesis tower low-pressure synthesis process. The amount of raw material synthesis gas is 1.23 million m ³ /day. After heat exchange, the gas entering the tower has a temperature of 225°C and directly enters the reaction tube of the synthesis tower from the upper part, where it reacts to produce methanol under the action of a catalyst. The catalyst for methanol synthesis is C79-5GL copper catalyst from Lurgi Company, the pressure inside the bed is 5 MPa, and the temperature is 250 °C. The heat of reaction released during the reaction is taken away by the vaporization of water in the shell side of the synthesis tower to produce medium-pressure steam. After the reaction gas enters the heat exchanger to be heated and enters the tower gas, the methanol is separated by the methanol condenser and separator and recycled. The ratio of circulating air to fresh air is 5:1. Methanol output is 1160 tons/day.

Acetic acid synthesis adopts AO Plus acetic acid synthesis process of Celanese Corporation, the raw material feed rate of methanol is 1160 tons/day, and CO is 1.07 million m ³ /day. The reaction temperature is 190°C, the pressure is 3 MPa, the reaction uses a rhodium catalyst, and lithium iodide is added to improve the stability of the catalyst, and the water content in the reactor is kept at 5%. Acetic acid The yield of acetic acid is 99%, and the output is 2151 tons/day.

CoNbP catalyst with patent number 201210057568.8 is used for ethanol synthesis, and acetic acid and _H2 synthesized in the carbonylation section are used as raw materials for ethanol synthesis. The reactor temperature is 200°C, the reaction pressure is 1.5 MPa, the molar ratio of _H2 to acetic acid is 50:1, and the conversion rate of acetic acid is 100%. The crude acetic acid hydrogenation product is extracted and distilled using the method disclosed in the patent CN1706799. The crude product is added with 15% NaAc, extracted and separated in a compound extraction tower, and more than 99.5% of ethyl acetate and 95% of ethanol are obtained. Ethanol production is 1584 tons/day.

Example 2:

280,000 tons/year ethanol synthesis (based on 300 days of operation per year)

Divide the synthesis gas with H ₂ : CO = 2: 1 and an output of 1.48 million m ³ /day into two parts, the first part is 740,000 m ³ /day for methanol synthesis, and the second part is 740,000 m ³ /day for CO and _H2 are separated and used as feedstock for acetic acid synthesis and ethanol synthesis. Using the method disclosed in patent CN102191086A to separate CO and H ₂ in the synthesis gas, the output of CO after separation is 640,000 m ³ /day, and the output of H ₂ is 100,000 m ³ /day.

Methanol synthesis adopts ICI multi-stage cold shock tower low-pressure synthesis process, the reaction tower is divided into 4 beds, and the catalyst uses ICI51-7 catalyst. The amount of raw material synthesis gas is 980,000 m ³ /day. The temperature of the gas entering the tower after heat exchange is mixed with part of the cooled gas at 220°C. It enters the first stage of the bed from the top of the tower to react to generate methanol, and at the same time, heat is released to raise the temperature of the reaction gas. to 260°C. The reaction gas leaving the first stage of the catalytic bed is mixed with the chilled feed of the second stage, the temperature drops to 220°C and then enters the second stage of the catalytic bed to continue the reaction, and so on. The operating pressure inside the tower is 6 Mpa, the ratio of recycle gas to fresh gas is 10:1, and the methanol output is 696 tons/day.

The synthesis of acetic acid adopts the Acetica process of Chiyoda Company, and the raw material feed rate is 696 tons/day of methanol and 640,000 m ³ /day of CO. The reaction temperature is 180°C, the pressure is 3.5 MPa, the reaction uses a rhodium catalyst, and the water content in the reactor is 8%. The iodide is separated from the liquid phase product, the yield of acetic acid is 99%, and the output is 1291 tons/day.

The acetic acid synthesized in the carbonylation section and the H ₂ separated from the synthesis gas are used as raw materials, and the FeNb/ZrO ₂ catalyst with the patent number 201210057475.5 is used for ethanol synthesis. The reactor temperature is 240°C, the reaction pressure is 3.5 MPa, the ratio of circulating gas to fresh gas is 30:1, and the conversion rate of acetic acid is 100%. The crude acetic acid hydrogenation product is extracted and distilled using the method disclosed in the patent CN1706799. The crude product is added with 20% NaAc, extracted and separated in a compound extraction tower, and more than 99.5% of ethyl acetate and 95% of ethanol are obtained. The yield of ethanol is 92%, and the output is 920 tons/day.

Example 3:

360,000 tons/year ethanol synthesis (based on 300 days of operation per year)

Divide the synthesis gas with H ₂ : CO = 2: 1 and an output of 1.85 million m ³ /day into two parts, the first part is 925,000 m ³ /day for methanol synthesis, and the second part is 925,000 m ³ /day for CO and _H2 are separated and used as feedstock for acetic acid synthesis and ethanol synthesis. Using the method disclosed in patent CN102133498A to cryogenically separate CO and H ₂ in synthesis gas, the output of CO after separation is 805,000 m ³ /day, and the output of H ₂ is 120,000 m ³ /day.

The methanol synthesis adopts the Linde spiral coil isothermal synthesis tower methanol synthesis process. The amount of raw synthesis gas is 925,000 m ³ /day. After heat exchange, the gas entering the tower has a temperature of 225°C. It enters the reaction tube of the synthesis tower directly from the upper part, and reacts under the action of a catalyst to form methanol. The catalyst for methanol synthesis is CNJ206 catalyst from Southwest Chemical Industry Research and Design Institute. In the reaction tower, a spiral coil tube is placed in the catalyst bed, and 4.5 MPa boiler water is introduced from the lower part of the coil tube, and medium-pressure steam and circulating water are discharged from the top. Syngas enters at the top of the tower, passes through the catalyst bed, and exits at the bottom of the tower. , Methanol output is 870 tons/day.

The acetic acid synthesis adopts the Cativa acetic acid synthesis process of BP Company, and the raw material feed rate is 870 tons/day of methanol and 805,000 m ³ /day of CO. The reaction uses an iridium catalyst, the water content in the reactor is <5%, the yield of acetic acid is 99%, and the output is 1613 tons/day.

Ethanol synthesis adopts FeMoTaP catalyst with patent number 201210057568.8, and acetic acid and _H2 synthesized in the carbonylation section are used as raw materials for ethanol synthesis. The reactor temperature is 230°C, the reaction pressure is 3.5 MPa, the molar ratio of _H2 to acetic acid is 50:1, and the conversion rate of acetic acid is 100%. The crude acetic acid hydrogenation product is extracted and distilled using the method disclosed in the patent CN1706799. The crude product is added with 15% NaAc, extracted and separated in a compound extraction tower, and more than 99.5% of ethyl acetate and 95% of ethanol are obtained. Ethanol production is 1188 tons/day.

Claims (2)

1. a synthesis gas process through methanol ethanol, is characterized in that comprising the steps: (1) The synthesis gas is divided into two parts, one part of the synthesis gas is used for methanol synthesis, and the rest is separated into CO and H ₂ ; (2) Synthesis gas and methanol synthesis cycle tail gas are mixed to form methanol synthesis raw material gas, and methanol synthesis raw material gas is subjected to methanol synthesis reaction under the action of methanol synthesis catalyst to obtain methanol and methanol synthesis cycle tail gas; (3) Mix the CO obtained from the separation of synthesis gas with acetic acid synthesis cycle tail gas, and then carry out carbonylation reaction with methanol to carry out acetic acid synthesis reaction to obtain acetic acid and acetic acid synthesis cycle tail gas; (4) The _H2 obtained from the separation of synthesis gas is mixed with the tail gas of ethanol synthesis cycle and enters the reactor. The mixed gas and acetic acid contact with the catalyst to undergo hydrogenation for ethanol synthesis reaction to obtain ethanol and by-products and the tail gas of ethanol synthesis cycle. The product is separated to obtain ethanol; The methanol synthesis in the step (2) adopts the low-pressure methanol synthesis process of ICI multi-stage cold shock tower, the low-pressure methanol synthesis process of Lurgi tubular isothermal synthesis tower or the methanol synthesis process of Linde spiral coil isothermal synthesis tower; The synthesis gas separation in the step (3) adopts method 1: the raw material gas is sent from the lower part to the first adsorption tower for adsorption, and the CO product gas is obtained by enriching the bottom of the first adsorption tower, and the upper part of the first adsorption tower is obtained The mixed gas is sent to the second adsorption tower; after the mixed gas is absorbed by the second adsorption tower, high-purity hydrogen product gas is obtained at the outlet of the second adsorption tower; or method 2: a coal-to-synthesis gas co-production of carbon monoxide, methanol, and hydrogen , An integrated device for refining syngas, in which the CO cryogenic separation unit and pressure swing adsorption hydrogen production unit can be used to produce hydrogen and CO; The synthesis of acetic acid in the step (3) adopts Celanese AO Plus process, BP Cativa process or Chiyoda Acetica process; The ethanol synthesis of the step (4) adopts method 1: the catalyst preparation method is to combine one or more compounds of transition metals Ni, Co, Fe, W, V, Mo, Nb, Cr, Ta and diammonium hydrogen phosphate Add it into deionized water according to the stoichiometric ratio of the catalyst composition, and add citric acid, wherein the molar ratio of citric acid to transition metal is 0.5-3:1, and stir the resulting solution at 50-90°C for 8-24 h, Dry at 100-110°C for 18-24 h, then foam and solidify the sample at 120-150°C for 10-20 h, and bake the cured sample in air at 400-650°C for 3-6 h; Heat the calcined sample to 400-800°C at a rate of 0.5-3°C/min in _H2 atmosphere, keep the temperature constant for 3-8 hours, and the _H2 gas space velocity is 3000-9000 h ^-1 , the sample is heated in _H2 After the atmosphere drops to room temperature, passivate the catalyst with O ₂ /N ₂ with an O ₂ content of 0.5-2% for 2-5 h to obtain the catalyst; the reaction temperature for synthesizing ethanol is in the range of 200-300°C, and the reaction pressure is 1.0-3.5 MPa, The liquid space velocity of acetic acid is 0.5-3 h ^-1 , and the molar ratio of H ₂ : acetic acid is 5-50; Or method 2: catalyst preparation method is at first one or more compounds of transition metal Ni, Co, Fe, W, V, Mo, Nb, Cr, Ta and diammonium hydrogen phosphate are added to the stoichiometric ratio of catalyst composition In deionized water, for the solution with precipitation, nitric acid is added to the solution to dissolve the precipitation, and the prepared solution is impregnated in equal volume on alumina, molecular sieve, silica, titania, zirconia or diatomaceous earth carrier , where the transition metal compound: deionized water: carrier = 0.15-8.2 g: 10-30 ml: 10-20 g, the carrier loaded with the transition metal compound was placed at room temperature for 2-5 h, and dried at 30-60 °C for 2 -5 h, dry at 100-120°C for 6-12 h, then bake in air at 400-650°C for _3-6 h, heat up the roasted sample at a rate of 0.5-3°C/min to 400-800°C, reduction at this temperature for 3-8 h, H ₂ space velocity is 3000-9000 h ^-1 ; After the H ₂ atmosphere dropped to room temperature, the sample was passivated with O ₂ /N ₂ with an O ₂ content of 0.5-2% for 2-5 h to obtain a catalyst; the reaction temperature for synthesizing ethanol was in the range of 200-300°C, and the reaction pressure was 1.0-3.5 MPa, acetic acid liquid space velocity 0.5-3 h ^-1 , H ₂ : acetic acid molar ratio 5-50; The crude product separation in the step (4) is to firstly separate the acetaldehyde in the product by simple distillation, and then separate ethanol, ethyl acetate and water by extractive distillation; The extraction and rectification method for separating ethanol, ethyl acetate and water is (1) adding 10-20% NaAc to the mixture of ethyl acetate, ethanol and water; (2) extracting and separating the crude product with a compound extractant, and obtaining acetic acid at the top of the tower Ethyl ester, the temperature at the top of the tower is controlled at 77-78°C; the salt-containing mixed extractant flows out of the tower kettle, the extractant is processed through the regeneration tower, the fraction at the top of the regeneration tower is treated with the ethanol recovery tower, and the remaining fractions are returned for use, and the salt-containing extractant recycle. 2. A process for producing ethanol from synthesis gas through methanol as claimed in claim 1, characterized in that the methanol synthesis catalyst in the step (2) uses ICI51-1, ICI51-2, and ICI51-7 catalysts developed by ICI Company , C79-5GL catalyst of Lurgi Company, C302, C302-1, C302-2 or CNJ206 of Southwest Chemical Research and Design Institute.