CN114752635B - A process for recovering hexanoic acid in anaerobic fermentation broth based on forward osmosis technology - Google Patents

Abstract

The application discloses a process for recycling caproic acid in anaerobic fermentation liquid based on forward osmosis technology, which mainly comprises an anaerobic fermentation unit and a forward osmosis concentration unit. The inoculum of the anaerobic fermentation unit is anaerobic sludge after heat treatment, and ethanol and lactic acid are used as composite substrates for fermentation to produce caproic acid; the fermentation liquor is filtered by a microfiltration device and then concentrated by a forward osmosis technology. The forward osmosis concentration unit comprises a concentration tank, a forward osmosis membrane component and a liquid drawing tank, and fermentation liquor is filtered and then injected into the concentration tank. The forward osmosis membrane component comprises an inner cavity and an outer cavity, the inner cavity is connected with the concentration tank, and the outer cavity is connected with the liquid drawing tank. Under the drive of osmotic pressure, water continuously flows from one side of the concentration tank to one side of the liquid drawing tank in a transmembrane way, so that the concentration of the caproic acid fermentation liquid is realized. And further, the pH value of the concentrated solution is regulated to separate out the caproic acid in the form of oily matters, so that the high-efficiency and low-consumption separation of the caproic acid is realized, and a high-concentration crude product of the caproic acid is obtained.

Description

A process for recovering hexanoic acid in anaerobic fermentation broth based on forward osmosis technology

technical field

The invention belongs to the field of efficient recycling of resources, and in particular relates to a process for recovering caproic acid in anaerobic fermentation liquid based on forward osmosis technology.

Background technique

As a means of biomass resource utilization, anaerobic biological treatment can convert organic waste into energy (methane, hydrogen, etc.) or chemicals. Among them, the production of organic acids by anaerobic mixed bacterial fermentation is one of the research hotspots in recent years. Using waste biomass as a substrate, the liquid-phase products (ie, fermentation broth) obtained by anaerobic fermentation of mixed bacterial flora mainly include short-chain fatty acids, ethanol and lactic acid. These products are highly hydrophilic, difficult to separate and purify, and low in energy density. Low.

Based on the reverse β-oxidation pathway (reverse oxidation) of anaerobic microorganisms, the short-chain organic acids/alcohols produced by the anaerobic acidification of organic wastes can be further converted into medium-chain fatty acids such as caproic acid. Compared with short-chain fatty acids, caproic acid has higher energy density, lower water solubility (slightly soluble in water), and is easy to separate from fermentation broth; it can be further processed into fuel substances and other chemical materials with higher added value.

How to effectively recover caproic acid from fermentation broth is very important. Existing studies have used extraction technology to extract caproic acid in fermentation broth. First, a specific extractant and carboxyl group are used to form a complex to extract, and after extraction, caproic acid is recovered from the extract by means of rectification or electrodialysis, and the extraction is regenerated. agent. The current main research direction is to improve the chemical interaction between the solute and the extractant, and to optimize the relevant process conditions. In view of its low toxicity to microorganisms and high extraction rate, trioctylphosphine oxide (TOPO) has been used in the extraction process of hexanoic acid. However, no matter which extraction system or extraction solvent is selected, the traditional extraction process inevitably has some defects. On the one hand, during the transfer of organic acid from one phase to another, it needs to be in full contact with the extractant, and it is difficult to form a stable separation interface. On the other hand, the extraction agent regeneration process is complicated, consumes a lot of energy, and has a certain impact on the environment.

Forward osmosis is a membrane separation technology driven by the osmotic pressure difference between the feed liquid and the draw liquid. Its operation does not require external pressure, and it has low energy consumption, low membrane fouling and high material recovery, etc. Advantages, suitable for water treatment fields such as sewage purification and seawater desalination. In recent years, forward osmosis technology has shown good application potential in product concentration, but there is a lack of relevant research on the separation and recovery of caproic acid fermentation products.

Contents of the invention

Purpose of the invention: In order to solve the deficiencies in the prior art, the present invention provides a process for recovering caproic acid in anaerobic fermentation broth based on forward osmosis technology. First, ethanol and lactic acid are used as a composite substrate to ferment caproic acid, and then the fermentation broth is concentrated by forward osmosis technology; further, by adjusting the pH of the concentrated liquid, caproic acid is precipitated in the form of oil, so as to realize the separation and recovery of the target product. A crude product with high concentration of hexanoic acid was obtained.

The technical scheme adopted by the present invention is to provide a process for recovering hexanoic acid in anaerobic fermentation liquid based on forward osmosis technology. The heat-treated anaerobic sludge is used as the inoculum to ferment ethanol and lactic acid to produce hexanoic acid, and the obtained fermentation liquid is filtered by a microfiltration device and then concentrated by forward osmosis technology. The forward osmosis technology adopts a forward osmosis device, and the forward osmosis device includes a concentration tank, a forward osmosis membrane module and a draw liquid tank. The filtered fermented liquid is collected in the concentration tank, and then continuously pumped into the inner cavity of the forward osmosis membrane module and returned to the concentration tank; the outer cavity of the forward osmosis membrane module is connected with the draw liquid tank, and the draw liquid continues to flow in the forward osmosis The outer cavity of the membrane module is circulated. Driven by osmotic pressure, the hexanoic acid fermentation broth is continuously concentrated, and then the crude product of high-concentration hexanoic acid is obtained through acidification and oil extraction.

In one embodiment of the present invention, comprise the following steps:

S1. Anaerobic caproic acid fermentation: an anaerobic fermentation device is inoculated with heat-treated anaerobic sludge, and caproic acid fed-batch fermentation is performed using ethanol and lactic acid as composite substrates.

S2. Feed to the forward osmosis membrane device: the fermentation liquid flows out from the discharge port of the anaerobic device, and enters the concentration tank of the forward osmosis device after being filtered by the microfiltration device.

S3. Concentration of anaerobic fermentation broth: the inner cavity of the forward osmosis membrane module is connected to the concentration tank, and the anaerobic fermentation broth is continuously pumped into the inner cavity of the forward osmosis membrane module and circulated to the concentration tank; the outer cavity of the forward osmosis membrane module The cavity is connected to the draw liquid tank, and the draw liquid continuously circulates in the outer cavity of the forward osmosis membrane module. Driven by osmotic pressure, the water continuously flows across the membrane from the side of the concentration tank to the side of the draw liquid tank to realize the concentration of the caproic acid fermentation broth.

S4. Recovering the concentrated liquid: the concentrated fermented liquid is discharged through the discharge pipe of the concentration tank to obtain the concentrated liquid.

S5. After the concentrated solution is acidified by adjusting the pH, acid oil is formed and precipitated to obtain a crude hexanoic acid product.

In one embodiment of the present invention, the anaerobic sludge is heat-treated at 121° C. for 10 minutes before inoculation, and the inoculation amount is 20 g/L (calculated as VS).

In one embodiment of the present invention, the anaerobic fermentation temperature is maintained at 35±1° C., the stirring speed is 100 rpm, and the fermentation pH is controlled at 6.5.

In one embodiment of the present invention, the fermentation broth is filtered through a microfiltration device before entering the forward osmosis concentration device, and the pore size of the microfiltration membrane is 0.22 μm.

In one embodiment of the present invention, the feed liquid flows in from the top and exits from the bottom, and the draw liquid enters from the bottom and exits from the top.

In one embodiment of the present invention, the flow rates of the feed liquid and the draw liquid are both 500.0-600.0 mL/min.

In one embodiment of the present invention, the forward osmosis membrane module is a hollow fiber forward osmosis membrane, which can withstand a pH of 3-10 and a maximum operating temperature of 45°C.

In one embodiment of the present invention, the drawing solution is 1-3 mol/L NaCl.

In one embodiment of the present invention, the pH of the feed liquid is 6.5-8.5.

In one embodiment of the present invention, the total acid concentration is 9.5-38.0 g/L.

In one embodiment of the present invention, the concentrated fermentation solution is adjusted to a pH of 2.5-6.5, shaken and left still, and the concentrated solution is separated to obtain a crude hexanoic acid product.

The present invention innovatively proposes to recover caproic acid in anaerobic fermentation broth based on forward osmosis technology. Firstly, caproic acid is efficiently synthesized by using ethanol and lactic acid as a composite substrate; then, the fermentation broth is concentrated by using a forward osmosis device; further, by adjusting the pH of the concentrated liquid, caproic acid is precipitated in the form of oil, so as to achieve high-efficiency and low-cost separation of caproic acid, and obtain High concentration of caproic acid crude product.

Beneficial effect: ethanol and lactic acid are used as composite substrates, effectively improving the production intensity of caproic acid fermentation of anaerobic sludge, and significantly shortening the fermentation period. Through fed-batch fermentation, the substrate was completely consumed within 18 days, and the caproic acid production reached 9.2g/L. Forward osmosis technology efficiently concentrates caproic acid in the fermentation broth. Under the condition that the concentration of NaCl in the draw liquid is 3mol/L, the concentration of caproic acid can be increased to 9.0 times within 15 minutes, and the concentration recovery rate of caproic acid membrane is close to 100.0%. Acidification of the concentrated solution can precipitate caproic acid in the form of oil, and the extraction rate of caproic acid reaches 76.2%-92.0% under the acidification condition of pH 5.5-4.5, and the concentration of caproic acid in the acid oil reaches 486.2-578.2g/L, The purity is 73.5%-66.4%, and the total concentration ratio reaches 52.8-62.8 times.

Description of drawings

Fig. 1 is the structural representation of anaerobic fermentation device and forward osmosis concentration device of the present invention;

Figure 2 shows the concentration effect of forward osmosis with different NaCl concentrations in the draw solution;

Figure 3 is the concentration effect of forward osmosis with different feed liquid pH;

Figure 4 is the concentration effect of forward osmosis with different total acid concentrations;

Fig. 5 is the oil extraction effect of different concentrated solutions pH acidification;

Fig. 6 is the substance composition of gained sour oil under different pH conditions;

In the figure, 1-pH detector, 2-dissolved oxygen detector, 3-temperature detector, 4-liquid level gauge, 5-discharge pump, 6-feed pump, 7-lye pump, 8-stirring paddle , 9-temperature control cushion, 10-exhaust port, 11-lye tank, 12-substrate tank, 13-microfiltration device, 14-concentration tank, 15-draw liquid tank, 16-forward osmosis feed pump , 17-draw liquid pump, 18-forward osmosis membrane module, 19-inner cavity liquid inlet, 20-outer cavity liquid inlet.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments. The present invention can be better understood from the following examples. However, those skilled in the art will readily understand that the specific material ratios, process conditions and results described in the examples are only used to illustrate the present invention, and should not and will not limit the present invention described in detail in the claims .

The embodiment design idea of this application is as follows:

(1) Production of caproic acid by fed-batch fermentation, that is, using ethanol and lactic acid as composite substrates to produce caproic acid through anaerobic fermentation to obtain caproic acid fermentation liquid, which is Example 1.

(2) According to the organic acid components and concentrations in the anaerobic fermentation broth obtained in Example 1, a simulated fermentation broth was configured. Investigate the influence of the NaCl concentration of the draw liquid, the pH of the feed liquid and the total acid concentration on the concentration effect of the forward osmosis device, this is the embodiment 2-8.

(3) After filtering the actual anaerobic fermentation broth obtained in Example 1, under optimal conditions, the fermentation broth was concentrated efficiently through a forward osmosis device, which is Example 9.

(4) After the concentrated fermentation solution of Example 9 was acidified by adjusting the pH, hexanoic acid was precipitated in the form of oil, thereby obtaining a high-concentration crude product of hexanoic acid. This is Example 10-14.

Example 1: Utilizing ethanol and lactic acid as composite substrate anaerobic fermentation to produce hexanoic acid

A full-automatic stirring anaerobic fermentation tank as shown in Figure 1 was adopted, and the total reaction volume was 10 L. The inoculum was anaerobic granular sludge heat-treated at 121°C for 10 minutes, and the inoculum amount was 20 g/L (calculated as VS). The reaction mode is fed-batch fermentation, and the fermentation substrate is a complex composed of ethanol and lactic acid, and the initial concentration is 0.1mol/L. After the composite substrate was completely consumed, the substrate was added again to the initial concentration. During the whole fermentation process, the temperature was kept at 35±1° C., the stirring speed was 100 rpm, and the fermentation pH was controlled at 6.5. The anaerobic reactor was started after nitrogen stripping for 5 minutes.

As shown in Figure 1, the fermentation medium is added in the reactor by the substrate tank 12 through the feed pump 6; The discharge pump 5 pumps the fermentation liquid into the microfiltration device 13, and the filtrate is collected in the concentration tank 14. In the anaerobic fermentation tank, the pH detector 1 is used for online feedback of the pH of the fermentation process; the dissolved oxygen detector 2 monitors the anaerobic state of the fermentation system; the temperature detector 3 is used for online feedback and temperature adjustment; the liquid level gauge 4 monitors the temperature in the tank The liquid level; the stirring paddle 8 is used for material stirring, and the speed is controllable; the exhaust port 10 is used for introducing nitrogen and collecting gas.

Embodiment 2: Optimizing forward osmosis process conditions with simulated fermentation broth

According to the organic acid component and the concentration in the anaerobic fermentation broth, configure the simulated fermentation broth as the forward osmosis feed liquid, investigate the influence of the NaCl concentration of the draw liquid, the pH of the feed liquid and the total acid concentration on the concentration effect of the forward osmosis device (Example 2-8). The present invention adopts a hollow fiber forward osmosis membrane with a pH tolerance range of 3-10 and a maximum operating temperature of 45°C. The membrane module is composed of inner and outer chambers, the inner chamber continuously circulates the simulated fermentation broth, and the outer chamber continuously circulates the draw solution (Figure 1). In this example, the feed amount of the simulated fermentation broth was 1 L, the initial pH was 6.5, and the total acid concentration was 21.0 g/L (acetic acid 2.5 g/L, propionic acid 1.0 g/L, butyric acid 4.0 g/L, pentanoic acid acid 4.0g/L, hexanoic acid 9.5g/L). Embodiment 2 utilizes 1mol/L NaCl draw liquid to carry out concentration. The volume ratio of the drawing liquid and the simulated fermentation liquid is controlled to be 1:1, and the circulation of the drawing liquid is in a circulation mode. As shown in Figure 1, the side of the concentration tank 14 is provided with a feed port and a discharge port, and the anaerobic fermentation liquid is passed into the concentration tank from the feed port after being filtered by the microfiltration device 13 as a feed liquid. The forward osmosis feed pump 16 circulates the fermented liquid in the concentration tank into the inner cavity of the forward osmosis membrane module 18, and the draw solution pump 17 passes the draw solution into the outer cavity of the membrane module. The flow rate of both is 500.0mL/min. for 3 minutes. Driven by osmotic pressure, the water continuously flows from the side of the concentration tank to the side of the draw solution across the membrane to realize the concentration of the hexanoic acid fermentation broth.

Example 3:

Same as Example 2, the difference is that the drawing solution is 2mol/L NaCl.

Example 4:

Same as Example 2, the difference is that the drawing solution is 3mol/L NaCl.

Example 5:

Same as Example 4, the difference is that the forward osmosis feed liquid is adjusted to an initial pH of 7.5 and then concentrated.

Embodiment 6:

Same as Example 4, the difference is that the forward osmosis feed liquid is adjusted to an initial pH of 8.5 and then concentrated.

Embodiment 7-8:

Same as Example 4, the concentration of the organic acid was carried out under the condition that the draw liquid was 3 mol/L NaCl, and the initial pH of the feed liquid was 6.5. The difference is that the total acid concentrations corresponding to Examples 7 and 8 are 9.5 g/L and 38.0 g/L, respectively. The components and concentrations of the forward osmosis feed solutions of Examples 4, 7 and 8 are shown in Table 2.

Table 2 simulated fermentation broth with different concentrations

Embodiment 9: Utilize the forward osmosis process to concentrate the actual acid-producing fermentation broth

Take 5L of the actual fermented liquid in Example 1, the pH is 6.5, pass through the forward osmosis device after microfiltration treatment, and the drawn liquid is 3mol/L NaCl. The difference from Examples 2-8 is that the draw liquid is directly discharged through the outer cavity of the forward osmosis membrane, and does not return to the draw liquid tank. During the concentration process, the flow rates of the feed liquid and the draw liquid were both set at 500.0 mL/min, and the running time was 15 minutes.

Example 10: Extraction of caproic acid by adjusting the pH of the concentrate

After the fermentation broth is concentrated by forward osmosis, the pH will increase. The pH of the concentrated solution is adjusted to be acidic by adding hydrochloric acid, so that caproic acid is precipitated in the form of oil (acidification oil extraction). The concentrated solution in Example 9 was acidified to extract oil, 30 mL of the concentrated solution was taken, hydrochloric acid was added to adjust the pH to 6.5, fully shaken, and the concentrated solution was allowed to stand for 12 hours to extract the acid oil after the concentrated solution was separated.

Example 11:

Same as Example 10, the difference is that hydrochloric acid is added to adjust the pH of the concentrate to 5.5.

Example 12:

Same as Example 10, the difference is that hydrochloric acid is added to adjust the pH of the concentrate to 4.5.

Example 13:

Same as Example 10, the difference is that hydrochloric acid is added to adjust the pH of the concentrate to 3.5.

Example 14:

Same as Example 11, the difference is that hydrochloric acid is added to adjust the pH of the concentrated solution to 2.5.

The result and analysis of embodiment

(1) Ethanol and lactic acid are used as composite substrates to ferment caproic acid

The inoculum was anaerobic granular sludge heat-treated at 121°C for 10 minutes, and the inoculum amount was 20 g/L (calculated as VS). The reaction mode is fed-batch fermentation, and the fermentation substrate is a complex composed of ethanol and lactic acid, and the initial concentration is 0.1mol/L. After the composite substrate was completely consumed, the substrate was added again to the initial concentration. After 18 days, the two batches of substrates were completely consumed, and the hexanoic acid production reached 9.2 g/L. The main organic acid components and concentrations in the resulting anaerobic fermentation broth are shown in Table 1.

Organic acid concentration (g/L) in table 1 anaerobic fermentation broth

(2) Influence of the NaCl concentration of the draw solution on the concentration effect of the forward osmosis system

Adjust the initial pH of the simulated fermentation broth to 6.5, and the total acid concentration is 21.0g/L. Examples 2-4 use 1mol/L, 2mol/L, and 3mol/L NaCl draw solutions to concentrate respectively, and investigate the effect of the salt concentration on forward osmosis. Influence of system concentration effect.

Membrane concentration product recovery rate is calculated according to formula (1) and (2):

The membrane water flux increases with the increase of NaCl concentration in the draw solution, from 175.3mL/min in Example 2 to 266.3mL/min in Example 4. The concentration effect of the organic acid membrane is shown in Figure 2, and the concentration ratio of the feed solution is increased from 2.1 times to 5.0 times. In terms of acid recovery effect, the total acid recovery rate was reduced from 100.0% in Example 2 to 97.1% in Example 4. The hexanoic acid membrane concentration recovery rate dropped from 100.0% in Example 2 to 97.8% in Example 4. The higher osmotic pressure accelerates the flow of water across the membrane and also expands the pore size of the membrane, so that a small part of the organic acid is lost. It can be seen that the increase of the concentration of NaCl in the draw solution can effectively improve the concentration effect, and will not have a significant impact on the product recovery. Therefore, in subsequent Examples 5-10, the NaCl concentration of the draw solution was set to 3 mol/L.

(3) Influence of feed liquid pH on concentration effect of forward osmosis system

Under the condition that the draw solution is 3mol/L NaCl and the total acid concentration is 21.0g/L, in order to clarify the influence of the pH of the feed solution on the concentration effect of forward osmosis, Examples 4, 5 and 6 adjusted the initial pH of the feed solution respectively Concentrate after 6.5, 7.5, 8.5. As shown in Figure 3, under different feed pH conditions, the concentration multiples were 5.0 (pH 6.5), 4.9 (pH 7.5), 4.8 (pH 8.5); the total acid recovery was 97.1%, 98.8%, 99.2% respectively %; Hexanoic acid recoveries were 97.8%, 100%, 100%. Therefore, the feed pH has no significant impact on the concentration effect of the forward osmosis system, and no additional pH adjustment is required. In subsequent examples 7-10, the pH of the forward osmosis feed is 6.5.

(4) Influence of feed organic acid concentration on concentration effect of forward osmosis system

Under the condition that the draw solution is 3mol/L NaCl and the pH of the feed solution is 6.5, the effect of the total organic acid concentration on the concentration effect of forward osmosis is further investigated. The increase of the organic acid concentration will reduce the osmotic pressure difference of the solution on both sides of the membrane, which will cause the concentration driving force of the forward osmosis membrane to decrease. When the total acid concentration increased from 9.5g/L to 38.0g/L, the concentration factor decreased from 9.7 times to 2.7 times. In the three examples, the total acid recovery rate is maintained above 96%, and caproic acid is almost not lost, and the recovery rate is close to 100%. Hexanoic acid concentration reaches 24.5g/L (embodiment 7), 47.3g/L (embodiment 4), 36.4g/L (embodiment 8) respectively in the concentrate, and difference is remarkable. Therefore, the control of the total acid concentration is very critical in the forward osmosis concentration process of the fermentation broth.

(5) Utilize the forward osmosis system to concentrate the actual acid-producing fermentation broth

Under the condition that the draw solution is 3mol/L NaCl and the pH of the feed solution is 6.5, the actual acidogenic fermentation broth is concentrated by forward osmosis. After 15 minutes of continuous concentration, the concentration of total acid and hexanoic acid increased by 9 times, and the average water flux of the forward osmosis membrane was 296.3mL/min. As shown in Table 3, the concentration recovery rate of the total acid film is 99.9%, the concentration recovery rate of the caproic acid film is 100.0%, and only a very small amount of acetic acid and butyric acid flow out with the draw solution. In addition, in the continuous repeated experiments, the forward osmosis system can always maintain a high membrane flux and membrane concentration recovery rate, indicating that the actual fermentation broth can effectively reduce the forward osmosis membrane fouling after removing macromolecules by ultrafiltration.

Table 3 actual fermentation broth continuous concentration effect

(6) Extract hexanoic acid by acidifying the concentrated solution pH

In Examples 10-14, hexanoic acid was precipitated in the form of oil by acidifying the concentrated solution, and the influence of the pH of the concentrated solution on the extraction effect of hexanoic acid was investigated. Sour oil extraction rate is calculated according to formula (3):

Fig. 5 is the actual effect diagram of the acid oil extraction of the concentrate under the conditions of pH 6.5, 5.5, 4.5, 3.5, and 2.5. The initial pH of the concentrated solution was 7.3, and when the pH of the concentrated solution was adjusted to 6.5 in Example 10, the solution became turbid, but no obvious oil film was separated out; and as the pH further decreased, oil layers began to appear in the concentrated solution in Examples 11-14 , and the thickness gradually increases, the oil layer and solution become clearer, and the two-phase separation becomes more obvious.

Table 4 shows the extraction rate of each organic acid component after oil extraction with different pH acidification. As the pH decreases, the extraction rate of each organic acid in the concentrated solution increases gradually, and the extraction rate of the carboxylic acid with a longer carbon chain is higher. When the pH of the concentrated solution was adjusted to 4.5 (Example 12), the extraction rate of hexanoic acid could reach 92.0%. As the pH further decreased, the extraction rate of caproic acid did not change much, but the precipitation of other short-chain carboxylic acids continued to increase, which would reduce the purity of caproic acid in the acid oil.

The extraction rate (%) of each component after table 4 different pH acidification oil extraction

Table 5 shows the concentration of each organic acid in the oil phase after acidification at different pHs. As the pH of the concentrate decreases, the concentration of organic acids in the precipitated acid oil increases gradually. When the pH decreased from 5.5 (Example 11) to 4.5 (Example 12), the caproic acid concentration in the acid oil increased significantly from 486.2 g/L to 578.2 g/L. The effect of further lowering the pH on the precipitation of hexanoic acid was weakened, and the concentrations of hexanoic acid in the acid oils of Examples 13 (pH 3.5) and 14 (pH 2.5) were 583.2 g/L and 600.4 g/L, respectively.

Concentration of each component in acid oil under different pH conditions in table 5

The proportions of substances in the acid oil extracted under different pH conditions are shown in Figure 6, and the proportion of hexanoic acid in the initial concentrate is 51.5%. The purity of hexanoic acid can be increased to 73.5% and 66.4% after acidifying oil at pH 5.5 (Example 11) and pH 4.5 (Example 12). However, further lowering the pH will decrease the purity of hexanoic acid. The purity of hexanoic acid after acidification at pH 3.5 (Example 13) and pH 2.5 (Example 14) was 63.5% and 62.9%, respectively.

The above results show that, based on the physical and chemical properties of caproic acid itself, caproic acid can be effectively extracted by acidifying the forward osmosis concentrate. Considering the extraction rate, caproic acid concentration and purity in the sour oil, the acidification pH can be controlled between 4.5-5.5, the caproic acid extraction rate can reach 76.2%-92.0%, and the caproic acid concentration in the sour oil can reach 486.2-578.2g /L, the purity of hexanoic acid reaches 73.5%-66.4%.

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications are also possible. It should be regarded as the protection scope of the present invention.

Claims (1)

1. A process for recovering caproic acid in anaerobic fermentation liquid based on forward osmosis technology is characterized in that heat-treated anaerobic sludge is used as an inoculum, ethanol and lactic acid are used as composite substrates for caproic acid fermentation, and the obtained fermentation liquid is filtered by a microfiltration device and then concentrated by the forward osmosis technology;

the forward osmosis technology adopts a forward osmosis concentration device, the forward osmosis concentration device comprises a concentration tank, a forward osmosis membrane component and a liquid drawing tank, the fermentation filtrate is collected in the concentration tank, and then is continuously pumped into the inner cavity of the forward osmosis membrane component and returned to the concentration tank; the outer cavity of the forward osmosis membrane component is connected with a liquid drawing tank, liquid drawing continuously circulates in the outer cavity, fermentation liquid is efficiently concentrated under the drive of osmotic pressure, and then high-concentration crude caproic acid is obtained through acidification and oil extraction, and the method comprises the following steps:

s1, anaerobic caproic acid fermentation: inoculating the anaerobic sludge subjected to heat treatment in an anaerobic fermentation device, and carrying out caproic acid batch fed-batch fermentation by taking ethanol and lactic acid as composite substrates; the main organic acid components and the concentration of the anaerobic fermentation liquid are in accordance with the following relation, 20g/L of the anaerobic fermentation liquid is calculated by VS according to the inoculation amount, and the main organic acid components and the concentration of the anaerobic fermentation liquid are as follows: acetic acid 2.7+/-0.1 g/L, propionic acid 1.3+/-0.3 g/L, butyric acid 4.5+/-0.2 g/L, valeric acid 4.2+/-0.1 g/L and caproic acid 9.2+/-0.1 g/L;

s2, feeding by a forward osmosis membrane device: the fermentation liquor flows out from a discharge port of the anaerobic fermentation device, is filtered by the microfiltration device and enters a concentration tank of the forward osmosis device;

s3, concentrating anaerobic fermentation liquid: the inner cavity of the forward osmosis membrane component is connected with the concentration tank, and anaerobic fermentation liquid is continuously pumped into the inner cavity of the forward osmosis membrane component and circularly flows to the concentration tank; the outer cavity of the forward osmosis membrane component is connected with a drawing liquid tank, and drawing liquid continuously circulates in the outer cavity of the membrane component; concentrating the fermentation liquor under the action of osmotic pressure;

the feeding liquid is fed in from top to bottom, the extracting liquid is fed in from bottom to top, and the flow rates of the feeding liquid and the extracting liquid are 500.0-600.0mL/min;

the forward osmosis membrane component is a hollow fiber forward osmosis membrane, the tolerance pH is 3-10, and the maximum operating temperature is 45 ℃;

the NaCl concentration of the drawing liquid is 1-3 mol/L;

the pH value of the feed liquid of the forward osmosis membrane component is 6.5-8.5, and the total acid concentration is 9.5-38.0 g/L;

s4, recovering concentrated solution: discharging the concentrated fermentation liquor through a discharge pipe of the concentration tank to obtain concentrated liquor;

s5, acidizing the concentrated solution to form acid oil after adjusting the pH value to be 4.5-5.5, and separating out the acid oil to obtain a coarse caproic acid product;

heat treatment is carried out for 10 minutes at 121 ℃ before the anaerobic sludge inoculation, and the inoculation amount is 20g/L in terms of VS;

the temperature in the anaerobic fermentation process is kept at 35+/-1 ℃, the stirring speed is 100rpm, and the fermentation pH is controlled at 6.5.