A method for separating and purifying medium-chain carboxylic acids

Abstract

This invention provides a method for the separation and purification of medium-chain carboxylic acids, belonging to the field of chemical separation and purification technology. The method includes pretreatment, alkalization concentration, acidification extraction, and vacuum distillation. In the pretreatment stage, anaerobic fermentation is used to lengthen the carbon chain, followed by filtration. The clarified fermentation broth is then alkalized and concentrated. In an alkaline environment, the carboxylic acid is converted into a non-volatile carboxylate, thus minimizing loss during concentration. After centrifugation to remove precipitated solids, the pH is adjusted to acidity, and extraction is performed using non-polar and / or weakly polar solvents, which improves the extraction rate of the target medium-chain carboxylic acid, the recovery rate of short-chain carboxylic acids, and the purity of the product. Using this separation and purification method, this invention separates and purifies n-hexanol from baijiu (Chinese liquor) lees, achieving a hexanoic acid yield of 77%, a mass purity of 87%, and a total acid purity of 97%.

Description

Technical Field

[0001] This invention belongs to the field of chemical separation and purification technology, specifically relating to a method for separating and purifying medium-chain carboxylic acids. Background Technology

[0002] In recent years, research on carbon chain elongation technology has been steadily increasing, becoming a hot topic in the synthesis of medium-chain carboxylic acids. However, while attention has increased to product separation and purification techniques, the level of focus remains relatively limited. Hexanoic acid, as a typical representative of medium-chain carboxylic acids, faces similar technical challenges in its separation and purification as other compounds in this class. Therefore, research on the separation and purification of hexanoic acid should fully draw upon the extraction techniques of short-chain and medium-chain carboxylic acids with similar properties.

[0003] Current technologies for separating and purifying short-chain and medium-chain carboxylic acids mainly include liquid-liquid extraction, precipitation, ion exchange, membrane separation, and electrochemical membrane technology. However, these technologies also have many drawbacks. For example, liquid-liquid extraction suffers from emulsification, high solvent costs, product loss, and environmental pollution; precipitation methods result in complex products, making subsequent purification difficult; ion exchange suffers from limited material lifespan and high chemical consumption during desorption; although electrodialysis and membrane electrolysis reduce the use of chemical reagents, they still face challenges such as membrane fouling and high energy consumption. Therefore, existing technologies actually lack methods for separating and purifying medium-chain carboxylic acids, especially hexanoic acid. Summary of the Invention

[0004] This invention provides a method for separating and purifying medium-chain carboxylic acids, comprising pretreatment, alkalization concentration, acidification extraction and vacuum distillation steps, resulting in medium-chain carboxylic acids with high yield and purity.

[0005] This invention provides a method for separating and purifying medium-chain carboxylic acids, comprising the following steps: (1) anaerobic fermentation of biomass organic waste as raw material, followed by centrifugation and filtration of the fermentation broth to obtain a clear solution; (2) After adjusting the clarified solution in step (1) to alkalinity, concentrate it to obtain a concentrated solution; (3) After adjusting the concentrated solution in step (2) to acidity, extract it using a non-polar solvent and / or a weakly polar solvent to obtain the extract phase; (4) The extract phase obtained in step (3) is subjected to vacuum distillation. The residue contains medium-chain carboxylic acids, wherein the carbon chain of the medium-chain carboxylic acid has 6 to 12 carbon atoms.

[0006] In one specific embodiment of the present invention, the biomass organic waste in step (1) includes at least one of the following: distiller's grains, kitchen waste, crop straw and mushroom production waste.

[0007] In one specific embodiment of the present invention, the anaerobic fermentation includes fermentation using the microbial community of the liquor lees itself, and maintaining a pH value of 5.0 to 6.0 during the anaerobic fermentation, while supplementing electron donors.

[0008] In one specific embodiment of the present invention, the concentration method in step (2) includes vacuum decompression concentration.

[0009] In one specific embodiment of the present invention, during vacuum concentration, the pH value is 7~14, the temperature is 39~55℃, and the vacuum degree is -0.094 MPa.

[0010] In one specific embodiment of the present invention, the pH value of the acid in step (3) is 2.0~5.5.

[0011] In one specific embodiment of the present invention, the nonpolar and / or weakly polar solvent in step (3) includes at least one of the following: cyclohexane, n-hexane and ethyl acetate.

[0012] In one specific embodiment of the present invention, the extraction in step (3) is performed 1 to 3 times.

[0013] In one specific embodiment of the present invention, the temperature of the vacuum distillation in step (4) is higher than 90°C and the vacuum degree is -0.093 MPa.

[0014] In one specific embodiment of the present invention, the time for vacuum distillation in step (4) is 0.5 to 2 hours.

[0015] Beneficial Effects: This invention provides a method for the separation and purification of medium-chain carboxylic acids, comprising steps such as pretreatment, alkalization concentration, acidification extraction, and vacuum distillation. In the pretreatment stage, anaerobic fermentation is used to elongate the carbon chain, followed by filtration. The clarified fermentation broth is then alkalized and concentrated. In an alkaline environment, the carboxylic acid is converted into a non-volatile carboxylate, thus minimizing losses during concentration. After centrifugation to remove precipitated solids, the pH is adjusted to acidity, and extraction is performed using non-polar and / or weakly polar solvents, which improves the extraction rate, recovery rate, and product purity of the target medium-chain carboxylic acid.

[0016] In one embodiment of the present invention, hexanol is separated and purified using baijiu lees as raw material. During the extraction stage, ethyl acetate is used for three extractions, achieving a hexanoic acid extraction rate of up to 99%. The extracted phase is then subjected to vacuum distillation, yielding hexanoic acid with high yield and purity. The total acid purity of the hexanoic acid product is significantly higher than its mass purity, indicating the possible presence of impurities with boiling points higher than hexanoic acid. After the complete process, the hexanoic acid yield is 77%, the mass purity is 87%, and the total acid purity is 97%. The resulting hexanoic acid product exhibits high yield and purity, reaching the highest level in the international field. Attached Figure Description

[0017] Figure 1 This is a diagram illustrating the effect of carboxylic acid concentration via direct rotary evaporation. Figure 2 The concentration effects of alkalization and rotary evaporation on carboxylic acids at different concentration ratios are shown in the diagram. Figure 3 The extraction efficiency of n-hexane, cyclohexane, and ethyl acetate on short-chain and medium-chain carboxylic acids is shown in the graph. Figure 4 Graphs showing the extraction effects of ethyl acetate on short-chain and medium-chain carboxylic acids at different extraction times; Figure 5 The extraction effects of short-chain and medium-chain carboxylic acids under different pH conditions are shown in the figure. Figure 6 The graph shows the yield and purity of hexanoic acid under different vacuum levels during molecular distillation. Figure 7 The graph shows the results of the yield and purity of hexanoic acid in the simulated distillate at different distillation temperatures; Figure 8 The graph shows the yield and purity of hexanoic acid at different distillation temperatures. Figure 9 The graph shows the yield and purity of hexanoic acid at different distillation times. Figure 10 This is a graph showing the results of the carboxylic acid material balance calculation for the entire separation and purification process. Detailed Implementation

[0018] This invention provides a method for separating and purifying medium-chain carboxylic acids, comprising the following steps: (1) anaerobic fermentation of biomass organic waste as raw material, followed by centrifugation and filtration of the fermentation broth to obtain a clear solution; (2) After adjusting the clarified solution in step (1) to alkalinity, concentrate it to obtain a concentrated solution; (3) After adjusting the concentrated solution in step (2) to acidity, extract it using a non-polar solvent and / or a weakly polar solvent to obtain the extract phase; (4) The extract phase obtained in step (3) is subjected to vacuum distillation, and the residue contains medium-chain carboxylic acids containing 6 to 12 carbons.

[0019] This invention uses biomass organic waste as raw material. The biomass organic waste undergoes anaerobic fermentation, followed by centrifugation and filtration to obtain a clear solution. This invention does not specifically limit the type of biomass organic waste; it can include distiller's grains, kitchen waste, crop straw, and mushroom production waste. One embodiment uses baijiu (Chinese liquor) distiller's grains as an example, but this should not be considered as the entire scope of protection of this invention.

[0020] This invention utilizes the microbial community of baijiu (Chinese liquor) lees for anaerobic fermentation. In this embodiment, the anaerobic fermentation is conducted in a 20 L total mixed anaerobic fermentation reactor (CSTR) with a stirring speed of 40 r / min. The working volume of the reactor is set to 15 L. Baijiu lees are used as the fermentation raw material, and fermentation is carried out using its inherent microbial community. Due to the high solids content in the batch fermentation experiment, which makes reactor stirring difficult, the solids content of the fermentation system is set at 6%. The pH is adjusted to 5.5 at start-up and then maintained between 5.0 and 6.0. Lactic acid is added during fermentation to supplement electron donors. The anaerobic fermentation described in this invention can be a sequential batch or continuous fermentation process at different process scales. The main criterion for the end of the fermentation experiment is that the concentration of hexanoic acid produced by the system is basically stable and no longer significantly increases, indicating that the system has fully demonstrated the maximum efficiency of microbial carbon chain elongation, and the reactor operation can be stopped.

[0021] In this invention, the fermentation broth is centrifuged and filtered. In one embodiment, the centrifugation parameters are 6000 r / min for 10 min to allow solid impurities to settle and obtain a supernatant. Subsequently, the supernatant is filtered through a 0.22 μm hollow fiber membrane to obtain a clear solution.

[0022] This invention involves alkalizing the clarified solution and then concentrating it under reduced pressure. The alkalization can be achieved using a pH adjuster to a pH of 7-14. This invention does not specifically limit the type of pH adjuster, as long as it can adjust the pH to alkaline levels. Examples include NaOH, KOH, NaHCO3, or Na2CO3. One embodiment uses a 6 mol / L NaOH solution adjusted to pH 10 for illustration, but this should not be considered the entire scope of protection of this invention.

[0023] The present invention concentrates the alkalized solution by vacuum concentration. In one embodiment, for small-scale laboratory operations, the alkalized solution is placed in a rotary evaporator and rotary evaporated at a temperature of 39~55 ℃ and a vacuum of -0.094 MPa to complete the concentration. The volume ratio before and after concentration is (2~6):1.

[0024] In this invention, after concentration, the solution is filtered, the precipitate is discarded, and the concentrate is extracted. The extraction is carried out in an acidic environment with a pH value of 2.0 to 5.5. For example, in one embodiment, the pH value is designed to be 2, 4, 4.5, 4.8, 5.0, and 5.5, respectively. At these values, the extraction of medium-chain carboxylic acids can be completed. In one embodiment, when hexanoic acid is the target product, in order to improve the extraction rate of hexanoic acid while reducing the extraction of short-chain carboxylic acids, 4.5 is selected as the optimal pH.

[0025] In the extraction process described in this invention, a nonpolar and / or weakly polar solvent is used as the extractant, which includes at least one of the following: cyclohexane, n-hexane, and ethyl acetate. Regarding the overall extraction characteristics of the extractants described in this invention, all three extractants exhibit an increasing extraction rate with the length of the carboxylic acid carbon chain. However, considering the extraction rate of n-hexanoic acid, ethyl acetate is selected as the optimal extractant for the extraction process.

[0026] The extraction rates of butyric acid, valeric acid, and hexanoic acid in this invention all show an increasing trend with the increase of extraction times. Therefore, the extraction described in this invention can be performed multiple times, such as 1 to 3 times. In one embodiment, in order to reduce the loss of hexanoic acid during the extraction process, the 3 extractions with the highest extraction efficiency were selected as the optimal number of extraction times.

[0027] The present invention involves vacuum distillation of the extract phase obtained after extraction, wherein the vacuum distillation temperature is higher than 90°C, the vacuum degree is -0.093 MPa, and the time is 0.5~2h.

[0028] To further illustrate the present invention, the following detailed description of a method for separating and purifying medium-chain carboxylic acids provided by the present invention is provided in conjunction with embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0029] Unless otherwise specified, the materials used in the embodiments of this invention can be obtained through conventional channels in the art, such as commercial channels. The lees used in the embodiments mainly come from the lees produced in the production of Luzhou Laojiao and Hengshui Laobaigan.

[0030] Example 1 1. Pretreatment Fermentation experiments were conducted in a 20 L total mixed anaerobic fermentation reactor (CSTR) with a stirring speed of 40 r / min. The working volume of the reactor was set to 15 L. Baijiu (Chinese liquor) lees were used as the fermentation feedstock, utilizing its inherent microbial community. Due to the high solids content in the batch fermentation experiment, which made reactor stirring difficult, the solids content of the fermentation system was set at 6%. The pH was adjusted to 5.5 at start-up and then maintained between 5.0 and 6.0. Lactic acid was added during fermentation to supplement electron donors. The main criterion for ending the fermentation experiment was that the concentration of hexanoic acid produced by the system remained relatively stable and no longer showed a significant increase, indicating that the system had fully demonstrated the maximum efficiency of microbial carbon chain elongation, and the reactor could be shut down.

[0031] First, the fermentation broth was centrifuged at 6000 r / min for 10 min to allow solid impurities to settle and obtain the supernatant. Then, the supernatant was filtered through a 0.22 μm hollow fiber membrane to obtain a clear solution.

[0032] 2. Concentration Method 2.1 Direct Rotary Evaporation Concentration Effect A simulated fermentation broth was prepared based on the actual fermentation broth concentration. The carboxylic acid components and concentrations were 2.18 g / L acetic acid, 0.34 g / L propionic acid, 1.36 g / L butyric acid, 0.15 g / L valeric acid, and 5.05 g / L hexanoic acid. A certain volume was measured and placed in a rotary evaporator flask, and concentration experiments were carried out under two sets of conditions.

[0033] Experiment 1 involved rotary evaporation at a temperature of 35 ℃ and a vacuum of -0.094 MPa to concentrate the fermentation liquid volume to 39% of its original volume.

[0034] Experiment 2 involved rotary evaporation at 60 ℃ and a vacuum of -0.08 MPa to concentrate the fermentation broth to 71% of its original volume. The liquid in the evaporation flask (i.e., the concentrated liquid) was then removed, its volume measured, and a sample taken for testing.

[0035] The results of Experiment 1 and Experiment 2 are as follows Figure 1 As shown, the yields of hexanoic acid after concentration were both low, at 31% and 45%, respectively. The experimental results indicate that among several carboxylic acids, the concentration yield decreases with increasing carbon chain length. In both experiments, only the simulated fermentation broth volume was concentrated to approximately 39% and 71% of its original volume, while the loss rate of the target product, hexanoic acid, exceeded 50%.

[0036] 2.2 Effect of rotary evaporation concentration after alkalization The pH of the fermentation broth was adjusted to 10 using 6 mol / L NaOH solution, and a certain volume was measured and placed in a rotary evaporation flask. Rotary evaporation was carried out at a temperature of 55 ℃ and a vacuum of -0.094 MPa to concentrate the fermentation broth to approximately 50%, 39%, and 15% of its original volume, respectively. The liquid (concentrated phase) in the evaporation flask was removed, its volume was measured, and samples were taken for analysis.

[0037] The results are as follows Figure 2 As shown, after alkalization, the fermentation broth was concentrated at different ratios, and the yields of acetic acid, propionic acid, butyric acid, valeric acid, and hexanoic acid remained at high levels. Even when the volume of the fermentation broth was concentrated to 15% of its original volume, no significant loss of carboxylic acids was observed, indicating that this concentration method has the ability to highly concentrate the fermentation broth.

[0038] 3. Research on extraction methods 3.1 Effects of different extractants on the extraction efficiency of carboxylic acids Cyclohexane, n-hexane, and ethyl acetate were selected as the three extraction solvents. Equal volumes of fermentation broth were measured into a separatory funnel, and n-hexane, cyclohexane, and ethyl acetate were added in a 1:1 volume ratio, respectively. The mixture was shaken for 1 min to mix, and then allowed to stand for 5 min to allow for layering. After layering, the lower aqueous phase was discharged from the bottom of the separatory funnel, and the upper organic phase was collected by pouring it out from the top. The organic phase was collected, and the extraction process was repeated for the aqueous phase. This process was performed three times, and the aqueous phase obtained from the last extraction was the residue.

[0039] The extraction effects of n-hexane, cyclohexane, and ethyl acetate on various carboxylic acids are shown in the figure. Figure 3 Overall, the extraction efficiency of all three extractants increased with the length of the carboxylic acid carbon chain. Hexane and cyclohexane showed significantly lower extraction efficiencies for acetic acid, propionic acid, butyric acid, and valeric acid than ethyl acetate, which reduces the co-extraction of short-chain carboxylic acids and facilitates obtaining higher purity hexanoic acid. However, the extraction efficiencies of hexane and cyclohexane for the target product hexanoic acid were also relatively low, at 88% and 90%, respectively. In contrast, ethyl acetate achieved the highest extraction efficiency for hexanoic acid, reaching 99%. Therefore, ethyl acetate, with the highest extraction efficiency, was selected as the optimal extractant for the extraction process.

[0040] 3.2 Effect of extraction times on the extraction efficiency of carboxylic acids The effect of ethyl acetate extraction times on carboxylic acid extraction efficiency was studied by setting the extraction times to 1, 2, and 3 times. Equal volumes of fermentation broth were measured into a separatory funnel, and ethyl acetate was added at a 1:1 volume ratio. The mixture was shaken for 1 min to mix, and then allowed to stand for 5 min to allow for layering. After layering, the lower aqueous phase was discharged from the bottom of the separatory funnel, and the upper organic phase was collected by pouring it out from the top. The organic phase was collected, and the extraction operation was repeated for the aqueous phase. This process was performed 1, 2, and 3 times, and the aqueous phase obtained from the last extraction was the residue.

[0041] like Figure 4 As shown, the extraction rates of butyric acid, valeric acid, and hexanoic acid all increased with the number of extractions. Among them, the extraction rates of hexanoic acid were 82%, 84%, and 87% when the number of extractions was 1, 2, and 3, respectively. Therefore, in order to reduce the loss of hexanoic acid during the extraction process, the three extractions with the highest extraction efficiency were selected as the optimal number of extractions.

[0042] 3.3 Effect of fermentation broth pH on carboxylic acid extraction efficiency To investigate the effect of pH on extraction efficiency, pH values ​​of 2, 4, 4.5, 4.8, 5.0, and 5.5 were set. The pH of the fermentation broth was adjusted to the target value using 1 mol / L hydrochloric acid solution and 1 mol / L sodium hydroxide solution, respectively. Equal volumes of fermentation broth were placed in a separatory funnel, and ethyl acetate was added at a 1:1 volume ratio. The mixture was shaken for 1 min to mix, and then allowed to stand for 5 min to allow for layering. After layering, the lower aqueous phase was discharged from the bottom of the separatory funnel, and the upper organic phase was collected by pouring it out from the top. The organic phase was collected, and the extraction operation was repeated for the aqueous phase. This process was performed three times, and the aqueous phase obtained from the last extraction was the residue.

[0043] The extraction effects of carboxylic acid under different pH conditions are shown in the figure. Figure 5 At pH 2.0, the extraction rates of various carboxylic acids were the highest, all exceeding 95%, with hexanoic acid reaching 100%. However, at this pH, there was no significant difference in the extraction results for propionic acid, butyric acid, valeric acid, and hexanoic acid. At pH 4.0, the extraction rate of hexanoic acid still reached 100%, and the extraction rates of short-chain carboxylic acids were also at a high level. At pH 4.5, the extraction rate of hexanoic acid remained as high as 99%, not significantly different from the previous two conditions, but the extraction rates of short-chain carboxylic acids decreased significantly. When the pH was further increased to 4.8, 5.0, and 5.5, although the extraction efficiency of short-chain carboxylic acids continued to decline, the extraction rate of hexanoic acid also decreased significantly, to 87%, 79%, and 42%, respectively, resulting in a large loss of the target product. Therefore, to achieve a high extraction rate of hexanoic acid and reduce the extraction of short-chain carboxylic acids, pH 4.5 was selected as the optimal pH.

[0044] In summary, the optimal extraction conditions were determined to be: using ethyl acetate as the extractant, adjusting the pH of the fermentation broth to 4.5, and performing three extractions at a 1:1 volume ratio. These conditions effectively reduce the extraction of short-chain carboxylic acids while efficiently recovering hexanoic acid, resulting in preliminarily purified hexanoic acid.

[0045] 4. Research on distillation methods 4.1 Purification effect of molecular distillation on hexanoic acid To verify the feasibility of purifying hexanoic acid by molecular distillation, a simulated distillation solution was prepared. Targeting the highest concentrations of acetic acid, butyric acid, and hexanoic acid in the fermentation broth, a mixed solution was prepared according to the relative proportions of these three acids in the fermentation broth. The simulated distillation solution consisted of a mixture of 10 mL of acetic acid, 10 mL of butyric acid, and 30 mL of hexanoic acid. The mixed solution entered through the feed inlet at the top of the molecular distillation apparatus, and the condensation temperature was set to 8 °C. Since acetic acid, butyric acid, and hexanoic acid are volatile, a lower heating temperature of 40 °C was chosen. Pressures of 65 mbar, 55 mbar, 45 mbar, 35 mbar, 25 mbar, 15 mbar, and 5 mbar were set to test the feasibility of purifying hexanoic acid by molecular distillation. Samples were taken from the distillation rephase, and the volume was measured to determine the concentration.

[0046] The yield and purity results of hexanoic acid under different vacuum conditions are as follows: Figure 6 As shown, the yield of molecular distillation fluctuates and decreases with changes in vacuum level, while the purity fluctuates between 55% and 65%, showing no obvious pattern with pressure changes and no significant increase. Therefore, molecular distillation is not suitable for the purification of hexanoic acid in this experiment. A possible reason is that the liquid flow rate is too fast in short-path molecular distillation, resulting in insufficient material residence time and failure to achieve effective separation of hexanoic acid.

[0047] 4.2 Purification effect of vacuum distillation on hexanoic acid A simulated distillation solution was prepared by mixing 10 mL of acetic acid, 10 mL of n-butyric acid, and 30 mL of n-hexanoic acid. This simulated distillation solution was used in every experiment. The simulated distillation solution was placed in an evaporating flask, the vacuum was adjusted to -0.093 MPa, the rotation speed was set to 35 r / min, and the reduced-pressure distillation time was set to 3 h to ensure thorough distillation. Therefore, oil bath temperatures of 90℃, 100℃, 110℃, 120℃, 130℃, and 135℃ were set to test the feasibility of purifying n-hexanoic acid by reduced-pressure distillation. The volume and concentration of the distilled rephase were measured. The yield and purity of n-hexanoic acid under different temperature conditions are shown below. Figure 7 As shown, the yield of hexanoic acid gradually decreased with increasing temperature, but the purity of hexanoic acid showed a significant upward trend, reaching 97% and 98% at 130 ℃ and 135 ℃, respectively. Therefore, the experimental results of the simulated distillation solution indicate that vacuum distillation has the potential to obtain high-purity hexanoic acid from mixtures containing short-chain carboxylic acids and hexanoic acid, and can be used for the purification of hexanoic acid.

[0048] To obtain hexanoic acid with high purity and yield, the vacuum distillation conditions were optimized. The effects of different temperatures and times on the purification effect of hexanoic acid by vacuum distillation were investigated. Vacuum distillation experiments were conducted using the second batch of fermentation broth under different temperature and time conditions. To simplify the process and ensure quantifiable material before and after distillation, acetic acid, propionic acid, butyric acid, valeric acid, and hexanoic acid were added to the second batch of fermentation broth according to the concentration of each acid when the fermentation broth volume was concentrated to 10% (ignoring the loss of carboxylic acid during the concentration process), and this was recorded as a simulated concentrate. Subsequent vacuum distillation experiments under different temperature and time conditions all used the same volume of simulated concentrate.

[0049] 4.2.1 Effect of different distillation temperatures on the purification effect of hexanoic acid The pH of the simulated concentrate was adjusted to 4.5, and then extracted to obtain an ethyl acetate phase solution. This solution was placed in an evaporating flask, and the vacuum degree was adjusted to -0.093 MPa. The rotation speed was set to 35 r / min, and the reduced pressure distillation time was set to 2 h. The oil bath temperatures were set to 90 ℃, 100 ℃, 110 ℃, 120 ℃, and 130 ℃, respectively. The liquid in the evaporating flask was recorded as the distilled heavy phase. The volume and concentration of the distilled heavy phase were measured. 1 mL of the distilled heavy phase was weighed.

[0050] The changes in yield and purity of hexanoic acid at the same temperature are shown in the figure. Figure 8 Overall, the yield of vacuum distillation decreased while the purity increased with increasing temperature. The highest purity of hexanoic acid was achieved at 130 °C, reaching 92% by mass and 99.08% by total acid purity. However, the yield of hexanoic acid decreased significantly to only 74% at 130 °C. Considering both yield and purity, 120 °C was selected as the optimal distillation temperature to achieve a higher yield of hexanoic acid while maintaining high purity.

[0051] 4.2.2 Effect of different distillation times on the purification effect of hexanoic acid The pH of the simulated concentrate was adjusted to 4.5, and then extracted to obtain an ethyl acetate phase solution. This solution was placed in an evaporating flask, and the vacuum degree was adjusted to -0.093 MPa. The rotation speed was set to 35 r / min, and the oil bath temperature was set to 120 ℃. The vacuum distillation times were set to 0.5 h, 1 h, 1.5 h, and 2 h, respectively. The liquid in the evaporating flask was recorded as the distilled heavy phase. The volume of the distilled heavy phase was measured, and its concentration was then determined. The mass of 1 mL of the distilled heavy phase was weighed.

[0052] The changes in yield and purity of hexanoic acid under different distillation times are shown in the figure. Figure 9 Under different distillation times, the yield of hexanoic acid did not vary significantly, but the purity generally showed an increasing trend, reaching its highest level at 2 hours. Therefore, 2 hours was selected as the optimal distillation time.

[0053] Based on the comprehensive experimental results, the optimal conditions for the distillation process were set as follows: distillation method: vacuum distillation; vacuum degree: -0.093 MPa; rotation speed: 35 r / min; distillation temperature: 120 ℃; and distillation time: 2 h.

[0054] Example 2 To test the extraction efficiency of hexanoic acid by the separation and purification methods and conditions determined in Example 1, a complete process flow experiment was conducted, including pretreatment, alkalization concentration, acidification extraction, and vacuum distillation. The specific experimental methods are as follows.

[0055] First, the fermentation broth was centrifuged at 6000 r / min for 10 min to coagulate the solids and separate them from the liquid. Then, it was filtered through a 0.22 μm hollow fiber membrane to obtain a clear solution. The pH of the fermentation broth was adjusted to 10 with 6 mol / L sodium hydroxide solution, and a sample was taken, its volume measured, and placed in an evaporating flask. Vacuum distillation was performed at 55 ℃ to concentrate the fermentation broth (concentration ratio 6:1). The liquid in the evaporating flask (concentrated phase) was collected, its volume measured, and 900 μL was sampled for analysis. The pH was adjusted to 4.5 with 5 mol / L hydrochloric acid, and the broth was extracted three times with ethyl acetate at a 1:1 volume ratio. The ethyl acetate phase solution obtained after extraction was placed in an evaporating flask, the vacuum was adjusted to -0.093 MPa, the rotation speed was set to 35 r / min, the oil bath temperature to 120 ℃, and the distillation time to 2 h. The liquid in the evaporating flask is the distilled heavy phase. The volume of the distilled heavy phase was measured, and its concentration was determined. 1 mL of the heavy phase was weighed.

[0056] Depend on Figure 10 It is evident that a small amount of each acid is lost during the concentration process, with a loss rate of approximately 5%. In the extraction stage, the loss of hexanoic acid is mainly due to the detection of the concentrate, with a loss rate of less than 1%, and the extraction yield can reach 100%. During extraction, propionic acid, butyric acid, and valeric acid are partially unextracted, with extraction rates of 75%, 95%, and 99%, respectively. During vacuum distillation, propionic acid is completely distilled off from the evaporating flask; however, 69.15 mg of butyric acid and 25.53 mg of valeric acid remain undistilled. Simultaneously, hexanoic acid experiences a distillation loss of 1287.96 mg, a loss rate of 17%. After completing the entire process, the hexanoic acid yield is 77%, the mass purity is 87%, and the total acid purity reaches 97%. This process yields hexanoic acid with high yield and purity.

[0057] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for separating and purifying medium-chain carboxylic acids, characterized in that, Includes the following steps: (1) After anaerobic fermentation of biomass organic waste as raw material, the fermentation liquid is centrifuged and filtered to obtain a clear solution; (2) After adjusting the clarified solution in step (1) to alkalinity, concentrate it to obtain a concentrated solution; (3) After adjusting the concentrated solution in step (2) to acidity, extract it using a non-polar solvent and / or a weakly polar solvent to obtain the extract phase; (4) The extract phase obtained in step (3) is subjected to vacuum distillation. The residue contains medium-chain carboxylic acids, wherein the carbon chain of the medium-chain carboxylic acid has 6 to 12 carbon atoms.

2. The separation and purification method according to claim 1, characterized in that, The biomass organic waste mentioned in step (1) includes at least one of the following: distiller's grains, kitchen waste, crop straw and mushroom production waste.

3. The separation and purification method according to claim 2, characterized in that, The anaerobic fermentation involves using the microbial community of the liquor lees itself for fermentation, maintaining a pH of 5.0 to 6.0 during the anaerobic fermentation, and supplementing electron donors.

4. The separation and purification method according to claim 1, characterized in that, The concentration method described in step (2) includes vacuum decompression concentration.

5. The separation and purification method according to claim 4, characterized in that, During the vacuum concentration process, the pH value is 7~14, the temperature is 39~55℃, and the vacuum degree is -0.094 MPa.

6. The separation and purification method according to claim 1, characterized in that, The acidic pH value in step (3) is 2.0~5.

5.

7. The separation and purification method according to claim 1, characterized in that, The nonpolar solvent and / or weakly polar solvent in step (3) includes at least one of the following: cyclohexane, n-hexane and ethyl acetate.

8. The separation and purification method according to claim 1 or 7, characterized in that, The extraction in step (3) is performed 1 to 3 times.

9. The separation and purification method according to claim 1, characterized in that, The vacuum distillation in step (4) is carried out at a temperature higher than 90°C and a vacuum degree of -0.093 MPa.

10. The separation and purification method according to claim 1 or 9, characterized in that, The time for vacuum distillation in step (4) is 0.5 to 2 hours.

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