System and method for co-producing ethanol and hexanoic acid through synthesis gas fermentation

Abstract

The invention provides a system and a method for co-producing ethanol and hexanoic acid through synthesis gas fermentation, and belongs to the field of synthesis gas fermentation. The system comprises a first-stage reactor, a second-stage reactor and a third-stage reactor, a material inlet of the thallus separation device is communicated with a material outlet of the primary bioreactor; a material inlet of the distillation device is communicated with a clear liquid material outlet of the thallus separation device; a material inlet of the second-stage reactor is communicated with a residual distilled water outlet of the distillation device. According to the system, metabolites such as ethanol and acetic acid can be produced through primary fermentation, and after an ethanol product is extracted through distillation, acetic acid is used as a substrate of a secondary fermentation system for secondary fermentation to produce caproic acid. Therefore, the system can prevent acetic acid generated in the primary fermentation process from entering a rear-end sewage system, the sewage treatment load is reduced, meanwhile, low-value products are converted into high-value products, and economic benefits are improved.

Description

Technical Field

[0001] The present application relates to the technical field of syngas fermentation, and particularly to a system and method for co-producing ethanol and caproic acid by syngas fermentation. Background Art

[0002] In the existing syngas fermentation technology for producing ethanol, there is mainly the syngas fermentation technology using Clostridium autoethanogenum. The continuous gas bioconversion of Clostridium autoethanogenum is completed through a specific bacterial metabolic pathway - the reductive acetyl-CoA pathway (Wood-Ljungdahl pathway). Microbial cells utilize CO, H 2 / CO 2 through anaerobic fermentation to produce ethanol.

[0003] In the current syngas fermentation technology, the fermented raw material gas is sent to a fermentation tank after being pressurized and purified, and metabolites such as acetic acid, ethanol, and 2,3-butanediol are synthesized under the action of acetic acid-producing bacteria such as Clostridium autoethanogenum. The fermentation system uses a membrane circulation system to retain the strains, realizing independent and stable control of the biomass of the strains and the concentration of metabolites. When the ethanol in the fermentation broth reaches a certain concentration, the broth containing bacteria and the broth without bacteria enter the subsequent distillation system and protein drying system to extract ethanol and protein. After extracting ethanol and protein, the fermentation broth still contains a large amount of nutrients required for the fermentation of the strains and the metabolite acetic acid. A part of it is recycled to the fermentation system for secondary conversion, and the rest enters the downstream sewage treatment system. When the carbon source entering the fermentation system is insufficient, it will cause an increase in the metabolite acetic acid. High-concentration acetic acid will have a certain toxicity to the strains; the acetic acid produced by the fermentation system is recycled to the fermentation with the water after distilling and extracting ethanol, which will further cause the accumulation of acetic acid and affect the activity of the strains. In addition, the acetic acid in the fermentation residue water entering the sewage treatment system will also cause waste of the carbon source. Summary of the Invention

[0004] The present application provides a system and method for co-producing ethanol and caproic acid by syngas fermentation to solve the following technical problems: how to avoid acetic acid generated in the primary fermentation process from entering the subsequent sewage system, thereby reducing the sewage treatment load.

[0005] In a first aspect, an embodiment of the present application provides a system for co-producing ethanol and caproic acid by syngas fermentation, and the system includes:

[0006] A primary reactor, into which the raw material gas is introduced to perform primary fermentation on the raw material gas to obtain a bacterial liquid containing ethanol and acetic acid;

[0007] A cell separation device, the inlet of which is communicated with the outlet of the primary reactor to separate the strains in the bacterial liquid to obtain a fermentation supernatant;

[0008] A distillation device, the feed inlet of which is communicated with the clear liquid discharge port of the bacterial cell separation device, for distilling the fermentation clear liquid to obtain an ethanol product and acetic acid-containing residual distillate water respectively; and

[0009] A secondary reactor, the feed inlet of which is communicated with the residual distillate water outlet of the distillation device, for performing secondary fermentation on the acetic acid-containing residual distillate water to obtain a mash containing caproic acid.

[0010] Optionally, the system further includes:

[0011] A concentration device, the feed inlet of which is communicated with the discharge port of the secondary reactor, for concentrating the mash to obtain a caproic acid product.

[0012] Optionally, the concentration device includes:

[0013] A filtration section, the feed inlet of which is communicated with the discharge port of the secondary reactor;

[0014] A vacuum concentration section, the feed inlet of which is communicated with the clear liquid discharge port of the filtration section;

[0015] A rectification section, the feed inlet of which is communicated with the discharge port of the vacuum concentration section.

[0016] Optionally, the system further includes:

[0017] A nutrient solution supply device, which is communicated with the primary reactor and the secondary reactor respectively.

[0018] Optionally, the bacterial cell separation device is a membrane filtration device or a centrifuge.

[0019] In a second aspect, the present application provides a method for co-producing ethanol and caproic acid by syngas fermentation, the method being adapted to the system described in any one of the embodiments in the first aspect, and the method including:

[0020] Performing primary fermentation on the raw material gas to obtain a bacterial liquid containing ethanol and acetic acid;

[0021] Separating the bacterial species in the bacterial liquid to obtain a fermentation clear liquid;

[0022] Distilling the fermentation clear liquid to obtain an ethanol product and acetic acid-containing residual distillate water respectively;

[0023] Performing secondary fermentation on the acetic acid-containing residual distillate water to obtain a mash containing caproic acid;

[0024] Concentrating the mash to obtain a caproic acid product.

[0025] Optionally, the bacterial species used in the primary fermentation is acetic acid-producing bacteria.

[0026] Optionally, the strain used in the secondary fermentation is caproic acid-producing bacteria.

[0027] Optionally, in the bacterial liquid containing ethanol and acetic acid, the concentration of ethanol is 35 g / L to 45 g / L, and the concentration of acetic acid is 5 g / L to 10 g / L.

[0028] Optionally, the purity of the ethanol product is ≥95%, and the purity of the caproic acid product is ≥95%.

[0029] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0030] The embodiment of the present application provides a syngas fermentation system for co-producing ethanol and caproic acid. This system integrates multiple unit operations such as a primary reactor, a cell separation device, a distillation device, and a secondary reactor, forming a complete technological process for syngas fermentation to co-produce ethanol and caproic acid. This system can achieve the production of metabolic products such as ethanol and acetic acid in the primary fermentation. After the ethanol product is extracted by distillation, acetic acid is used as the substrate for the secondary fermentation system to produce caproic acid in the secondary fermentation. Thus, this system can avoid acetic acid generated in the primary fermentation process from entering the backend sewage system, reduce the sewage treatment load, and at the same time convert low-value products into high-value products, improving economic benefits. Description of the Drawings

[0031] The drawings here are incorporated into the specification and constitute a part of this specification, showing embodiments consistent with the present application, and are used together with the specification to explain the principles of the present application.

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without creative efforts.

[0033] Figure 1 It is a schematic structural diagram of a syngas fermentation system for co-producing ethanol and caproic acid provided by the embodiment of the present application;

[0034] Figure 2 It is a schematic flow diagram of a method for syngas fermentation to co-produce ethanol and caproic acid provided by the embodiment of the present application;

[0035] Reference Signs:

[0036] 1 - Primary reactor, 2 - Bacterial cell separation device, 3 - Distillation device, 4 - Secondary reactor, 5 - Concentration device, 51 - Filtration section, 52 - Vacuum concentration section, 53 - Rectification section, 6 - Nutrient solution supply device, 10 - Inlet gas pipeline, 20 - Bacterial liquid pipeline, 30 - Fermentation supernatant pipeline, 40 - Residual distillate water pipeline, 50 - Ethanol pipeline, 60 - Mash pipeline, 70 - Caproic acid pipeline, 80 - Nutrient solution pipeline. Detailed implementation manners

[0037] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the following will clearly and completely describe the technical solutions in the embodiments of this application with reference to the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are some, but not all, of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in this application without creative efforts shall fall within the scope of protection of this application.

[0038] The various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be construed as a rigid limitation on the scope of this application; therefore, it should be considered that the described range description has specifically disclosed all possible sub - ranges and single values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub - ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within that range, such as 1, 2, 3, 4, 5, and 6, and this applies regardless of the range. Additionally, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0039] In addition, in the description of the specification of this application, terms such as "include" and "comprise" mean "include but not limited to". In this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this text, "and / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B can be singular or plural. In this text, "at least one" means one or more, and "a plurality" means two or more. "At least one kind", "at least one of the following items" or similar expressions refer to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both mean: a, b, c, a - b (that is, a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively. The "parts representation method" such as weight parts and mass parts represents the proportional relationship between each component. In the proportional relationships involved in this text, the parameters that need to be described by proportion should be understood as the antecedents of the proportion formula in the order of description, and the proportional numbers should be understood as the consequents of the proportion formula. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one by one with the proportional numbers in the proportion formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0040] Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment, etc. used in this application can be obtained through market purchase or can be prepared by existing methods.

[0041] Figure 1 It is a schematic structural diagram of a system for co-producing ethanol and caproic acid by syngas fermentation provided for the embodiments of this application.

[0042] As Figure 1 shown, this application provides a system for co-producing ethanol and caproic acid by syngas fermentation, and the system includes:

[0043] A primary reactor 1, into which a raw material gas is introduced to perform primary fermentation on the raw material gas to obtain a bacterial liquid containing ethanol and acetic acid;

[0044] A bacterial cell separation device 2, the feed inlet of which is communicated with the discharge port of the primary bioreactor to separate the bacterial species in the bacterial liquid to obtain a fermentation supernatant;

[0045] A distillation device 3, the feed inlet of the distillation device 3 is communicated with the clear liquid discharge port of the bacterial separation device 2, for distilling the fermentation clear liquid to obtain ethanol products and acetic acid-containing residue water respectively; and

[0046] A secondary reactor 4, the feed inlet of the secondary reactor 4 is communicated with the residue water outlet of the distillation device 3, for subjecting the acetic acid-containing residue water to secondary fermentation to obtain a caproic acid-containing mash.

[0047] The primary reactor 1 converts the purified raw material gas (mainly composed of CO or mainly composed of CO, H2, CO2) into ethanol and acetic acid through a fermentation reaction, while realizing the growth and reproduction of the bacterial strain itself. The raw material gas enters the primary reactor 1 through the inlet pipeline 10, and appropriate temperature, pressure and pH value are maintained in the reactor to promote the growth and metabolism of the bacterial strain.

[0048] The bacterial separation system uses membrane filtration or a centrifuge to separate the bacterial strain in the bacterial liquid to obtain a bacteria-free fermentation broth, so as to facilitate subsequent distillation and secondary fermentation. The bacterial liquid enters the bacterial separation system from the outlet of the primary reactor 1, and through the action of membrane filtration or a centrifuge, the separation of the bacterial strain and the fermentation broth is realized.

[0049] The distillation device 3 extracts ethanol products from the bacteria-free fermentation broth and simultaneously obtains acetic acid-containing residue water. After the fermentation broth enters the distillation device 3, through heating and decompression, ethanol evaporates and is condensed and collected to obtain ethanol products. The residue water in the bottom of the tower contains acetic acid and other components and is used for subsequent secondary fermentation.

[0050] The secondary reactor 4 uses caproic acid-producing bacteria to convert the acetic acid-containing residue water into caproic acid. After the acetic acid-containing residue water enters the secondary reactor 4, appropriate temperature, pH value and nutrient conditions are maintained in the reactor to promote the growth and metabolism of the caproic acid-producing bacteria. The produced caproic acid-containing mash enters the subsequent concentration system.

[0051] In some embodiments, the system further includes:

[0052] A concentration device 5, the feed inlet of the concentration device 5 is communicated with the discharge port of the secondary reactor 4, for concentrating the mash to obtain caproic acid products.

[0053] In some embodiments, the concentration device 5 includes:

[0054] A filtration section 51, the feed inlet of the filtration section 51 is communicated with the discharge port of the secondary reactor 4;

[0055] A vacuum concentration section 52, the feed inlet of the vacuum concentration section 52 is communicated with the clear liquid discharge port of the filtration section 51;

[0056] Rectification section 53, the feed inlet of the rectification section 53 is communicated with the discharge outlet of the vacuum concentration section 52.

[0057] The caproic acid concentration system extracts high-purity caproic acid products from the caproic acid-containing mash through filtration, vacuum concentration, and rectification. The caproic acid-containing mash first removes impurities through filtration and then enters the vacuum concentration section 52 for concentration treatment. The concentrated caproic acid solution enters the rectification section 53 for further purification, and finally, high-purity caproic acid products are obtained.

[0058] In some embodiments, the system further includes:

[0059] Nutrient solution supply device 6, the nutrient solution supply device 6 is respectively communicated with the first-stage reactor 1 and the second-stage reactor 4.

[0060] The nutrient solution supply system provides necessary nutrients for the first-stage and second-stage reactors 4 to support the growth and metabolism of the bacterial strains. The nutrient solution is transported to the first-stage reactor 1 and the second-stage reactor 4 through pipelines. The nutrient solution contains nutrients such as iron, potassium, magnesium, trace metal elements, and vitamins to meet the growth and metabolic needs of the bacterial strains.

[0061] In some embodiments, the bacterial cell separation device 2 is a membrane filtration device or a centrifuge.

[0062] The embodiments of the present application can achieve the production of metabolic products such as ethanol and acetic acid in the first-stage fermentation. After the ethanol product is extracted by distillation, acetic acid is used as the substrate of the second-stage fermentation system to produce caproic acid in the second-stage fermentation. This system can prevent acetic acid generated in the first-stage fermentation process from entering the backend sewage system, reduce the sewage treatment load, and at the same time convert low-value products into high-value products, improving economic benefits.

[0063] Figure 2 It is a process schematic diagram of a method for co-producing ethanol and caproic acid by syngas fermentation provided by an embodiment of the present application.

[0064] Based on a general inventive concept, as Figure 2 shown, the present application provides a method for co-producing ethanol and caproic acid by syngas fermentation. The method is adapted to the system described in any one of the above embodiments. The method includes:

[0065] S1. Perform first-stage fermentation on the raw material gas to obtain a bacterial liquid containing ethanol and acetic acid;

[0066] S2. Separate the bacterial strains in the bacterial liquid to obtain a fermentation supernatant;

[0067] S3. Distill the fermentation supernatant to obtain ethanol products and acetic acid-containing residual distillate water respectively;

[0068] S4. Perform secondary fermentation on the acetic acid-containing residual distillate water to obtain a mash containing caproic acid;

[0069] S5. Concentrate the mash to obtain a caproic acid product.

[0070] In some embodiments, the strain used in the primary fermentation is an acetic acid-producing bacterium.

[0071] Acetic acid-producing bacteria are a type of bacteria that can use CO 2 +H 2 or CO as the sole carbon source and energy source under anaerobic conditions, and convert these substrates into organic acids and alcohols such as acetic acid through the Wood-Ljungdahl pathway. These bacteria play a key role in converting inorganic carbon sources into organic products in syngas fermentation. The main acetic acid-producing bacteria include: Clostridium aceticum, Moorella thermoacetica, and Acetobacterium woodii.

[0072] In some embodiments, the strain used in the secondary fermentation is a caproic acid-producing bacterium.

[0073] Caproic acid-producing bacteria can convert organic substances such as ethanol and acetic acid into higher fatty acids such as caproic acid. In a syngas fermentation co-production system, these bacteria can be used in combination with acetic acid-producing bacteria to achieve the co-production of ethanol and caproic acid. The main caproic acid-producing bacteria include: Clostridium kluyveri.

[0074] In some embodiments, in the bacterial liquid containing ethanol and acetic acid, the concentration of ethanol is 35 g / L to 45 g / L, and the concentration of acetic acid is 5 g / L to 10 g / L.

[0075] In some embodiments, the purity of the ethanol product is ≥95%, and the purity of the caproic acid product is ≥95%.

[0076] Through fine fermentation and purification processes, the concentrations and purities of ethanol and caproic acid have reached relatively high levels, meeting the requirements of industrial production. This method not only realizes the effective utilization of resources but also avoids the waste of by-products such as acetic acid and environmental pollution problems. At the same time, by co-producing two high-value products, ethanol and caproic acid, the overall economic benefit is improved. Exemplarily, the concentration of ethanol can be 35 g / L, 37 g / L, 39 g / L, 40 g / L, 42 g / L, 44 g / L, 45 g / L, etc., and the concentration of acetic acid can be 5 g / L, 6 g / L, 7 g / L, 8 g / L, 9 g / L, 10 g / L, etc.

[0077] In summary, the system and method for co-producing ethanol and caproic acid by syngas fermentation provided by the embodiments of the present application have significant advantages in many aspects:

[0078] (1) Efficient resource utilization: Through the primary and secondary fermentation processes, the system realizes the efficient conversion of syngas (the main components are CO, H 2 , CO 2 ) into valuable ethanol and caproic acid products, avoiding the waste of by-products such as acetic acid in traditional methods and improving the overall resource utilization rate.

[0079] (2) Environmentally friendly: The system design reduces wastewater discharge, especially avoids the entry of harmful substances such as acetic acid into the backend sewage system, and reduces the sewage treatment load. By co-producing high-value products, environmental pollution is reduced, which conforms to the concept of green and sustainable development.

[0080] (3) Significant economic benefits: Co-producing two high-value products, ethanol and caproic acid, improves the overall economic benefits. At the same time, through fine fermentation and purification processes, the high concentration and high purity of the products are ensured, meeting the requirements of industrial production.

[0081] (4) Advanced technology: Advanced fermentation technologies are adopted, including the screening and application of acetic acid-producing bacteria and caproic acid-producing bacteria, as well as the optimized control of fermentation conditions. The design of the distillation and concentration devices also reflects the advanced nature of the technology, ensuring the efficient extraction and purification of the products.

[0082] (5) Flexibility and scalability: The system design is flexible and can adjust the fermentation conditions and product ratios according to actual needs. The production capacity can be expanded by increasing the number or scale of reactors and concentration devices.

[0083] (6) High product quality: The purity of both ethanol and caproic acid products can reach over 95%, meeting the market demand for high-quality products. The high-purity products contribute to improving the quality and performance of downstream processed products.

[0084] The following further elaborates the present application in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods without specific conditions noted in the following embodiments are usually determined according to industry standards. If there is no corresponding industry standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0085] Example 1

[0086] As Figure 1 shown, through the purified raw material gas, generally in the form of CO / H 2 / CO 2Taking [main component] as the main component, it enters the first-stage reactor 1 through the inlet pipeline 10. Through the fermentation reaction in the first-stage reactor, a bacterial liquid containing ethanol and acetic acid is obtained; the ethanol concentration reaches 40 g / L, and the acetic acid concentration is 5 g / L to 10 g / L. The bacterial liquid containing ethanol and acetic acid is transported to the cell separation device through the bacterial liquid pipeline 20, and a ceramic membrane device is used as the cell separation device 2 to achieve the improvement of cell concentration. After the cell separation device intercepts the strains, the fermented clear liquid after filtering the strains enters the distillation column 3 through the fermented clear liquid pipeline 30. The temperature at the bottom of the column is in the range of 110°C to 120°C, the pressure at the top of the column is 60 kPa, and the ethanol product with a purity of 95% is withdrawn from the top of the column and collected through the ethanol pipeline 50. The acetic acid concentration in the acetic acid-containing bottom residue water after ethanol extraction is in the range of 5 g / L to 10 g / L, and it enters the second-stage reactor 4 through the residue water pipeline 40. The second-stage reactor 4 uses caproic acid-producing bacteria for fermentation reaction, and the caproic acid-containing mash produced enters the subsequent concentration device 5 through the mash pipeline 60, and finally a caproic acid product with a purity of 95% is obtained and collected through the caproic acid pipeline 50. Through the nutrient solution supply system 6, the nutrient solution required for fermentation, generally including iron, potassium, magnesium, trace metal elements, vitamins, etc., is transported to the first-stage reactor 1 and the second-stage reactor 4 through the nutrient solution pipeline 80.

[0087] In addition, one or more technical solutions in the embodiments of the present application at least further have the following technical effects or advantages:

[0088] In the embodiments of the present application, the acetic acid inhibition effect of the existing solution can be solved. At the same time, a brand-new high-value product - caproic acid can be obtained, and the sewage treatment load can be reduced.

[0089] The above are only the specific implementation manners of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A system for co-producing ethanol and caproic acid by syngas fermentation, the system comprising: A primary reactor (1), wherein a raw material gas is introduced into the primary reactor (1) to perform primary fermentation on the raw material gas to obtain a bacterial liquid containing ethanol and acetic acid; A bacterial cell separation device (2), wherein the feed inlet of the bacterial cell separation device (2) is connected to the feed outlet of the primary reactor (1) and is used to separate the bacterial species in the bacterial liquid to obtain a fermentation clear liquid; A distillation device (3), wherein the feed inlet of the distillation device (3) is connected to the clear liquid discharge outlet of the bacterial cell separation device (2), and is used to distill the fermentation clear liquid to obtain an ethanol product and acetic acid-containing residual distilled water respectively; as well as A secondary reactor (4), wherein the feed inlet of the secondary reactor (4) is connected to the residual distilled water outlet of the distillation device (3), and is used for performing secondary fermentation on the residual distilled water containing acetic acid to obtain a mash containing caproic acid.

2. The system according to claim 1, characterized in that The system further comprises: A concentration device (5), wherein the feed inlet of the concentration device (5) is connected to the discharge outlet of the secondary reactor (4), and is used to concentrate the mash to obtain a caproic acid product.

3. The system according to claim 2, characterized in that The concentration device (5) comprises: A filter section (51), wherein an inlet of the filter section (51) is connected to an outlet of the secondary reactor (4); A reduced pressure concentration section (52), wherein the feed inlet of the reduced pressure concentration section (52) is connected to the clear liquid discharge outlet of the filtering section (51); A distillation section (53), wherein the feed inlet of the distillation section (53) is connected to the discharge outlet of the vacuum concentration section (52).

4. The system according to claim 1, characterized in that The system further comprises: A nutrient solution supply device (6), wherein the nutrient solution supply device (6) is connected to the primary reactor (1) and the secondary reactor (4) respectively.

5. The system according to claim 1, characterized in that The bacterial cell separation device (2) is a membrane filtration device or a centrifuge.

6. A method for co-producing ethanol and caproic acid by syngas fermentation, the method being adapted to the system according to any one of claims 1 to 5, the method comprising: The raw gas is subjected to primary fermentation to obtain a bacterial liquid containing ethanol and acetic acid; Separating the bacterial species in the bacterial liquid to obtain a fermentation clear liquid; Distilling the fermentation supernatant to obtain an ethanol product and acetic acid-containing residual distilled water; subjecting the acetic acid-containing residual distilled water to secondary fermentation to obtain a mash containing caproic acid; The mash is concentrated to obtain caproic acid product.

7. The method according to claim 6, characterized in that The bacteria used in the primary fermentation are acetic acid producing bacteria.

8. The method according to claim 6, characterized in that The bacteria used in the secondary fermentation are caproic acid producing bacteria.

9. The method according to claim 6, characterized in that In the bacterial solution containing ethanol and acetic acid, the concentration of the ethanol is 35 g / L to 45 g / L, and the concentration of the acetic acid is 5 g / L to 10 g / L.

10. The method according to claim 6, characterized in that The purity of the ethanol product is ≥95%, and the purity of the caproic acid product is ≥95%.

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