A citric acid clean production process

Abstract

This invention discloses a clean production process for citric acid. The clean production process of this invention is as follows: Raw materials are mixed with condensate and the pH is adjusted to obtain a mixed solution; two enzyme additions and two spray liquefaction processes are performed to obtain a liquefied solution; a portion of the liquefied solution is filtered through a plate and frame filter to obtain a sugar solution; condensate is added to another portion of the liquefied solution to prepare a seed culture medium, and Aspergillus niger spores are inoculated to prepare a seed solution; the remaining liquefied solution is mixed with the above sugar solution, and condensate is added to prepare a fermentation culture medium, and the seed solution is added to cultivate to obtain a fermentation broth; the fermentation broth undergoes solid-liquid separation and extraction to obtain citric acid and extraction waste liquid; the extraction waste liquid is heated, the pH is adjusted, and it is evaporated to obtain a concentrated solution, while simultaneously collecting the condensate. The method of this invention effectively reduces the volatile acid content of the condensate generated during the concentration process, avoids the inhibition of citric acid production by Aspergillus niger by condensate reuse, and achieves water resource recycling and heat energy recovery while simultaneously preparing citric acid from organic wastewater.

Description

Technical Field

[0001] This invention relates to the technical field of microbial fermentation, and more particularly to a clean production process for citric acid. Background Art

[0002] Citric acid is the world's largest producer of edible organic acids and a key chemical in the emerging bioeconomy, with a wide range of applications. It is primarily used in the food industry as an acidulant, antioxidant, and gelling agent, and also has extensive applications in the pharmaceutical, animal feed, chemical, electronics, and textile industries, with market demand increasing year by year.

[0003] However, the citric acid industrial production process generates a large amount of high-concentration organic wastewater in the citric acid extraction section. This wastewater is rich in nutrients, including amino acids, proteins, polysaccharides, monosaccharides, organic acids, and fats, with a COD concentration as high as 15,000–25,000 mg / L. To avoid environmental pollution, anaerobic or aerobic biological methods are typically used to treat the citric acid wastewater to meet standards before discharge. This requires a large area of ​​land and significant equipment investment, and also results in a substantial waste of water resources and nutrients.

[0004] Currently, technologies for the resource utilization of citric acid organic wastewater have been developed. For example, patent CN108947118A discloses a method for the resource utilization of citric acid fermentation wastewater. This method uses calcium oxide or calcium hydroxide to adjust the pH to alkaline, adds a flocculant, and removes nitrogen and phosphorus through sedimentation and filtration. The filtrate is then concentrated and used as a supplementary carbon source for biological nitrogen and phosphorus removal processes in chemical or municipal wastewater treatment. Patent CN110810635A introduces a citric acid organic nutrient solution for livestock and poultry farming and its preparation method, which involves recycling and concentrating the waste liquid from citric acid production to obtain the citric acid organic nutrient solution. However, the above-mentioned literature only focuses on the resource utilization of nutrients, neglecting the recycling of condensate generated during the concentration process or the impact of volatile acids in the condensate. Studies have found that the condensate generated during the concentration process contains a high concentration of volatile acids, which significantly inhibits citric acid production by Aspergillus niger, increases the fermentation lag period, limits the recycling of condensate, and causes a significant waste of water and energy. Therefore, how to reduce the volatile acid content in the concentrated condensate while preparing citric acid organic wastewater for resource utilization, and thus achieve water resource recycling, is an urgent problem to be solved. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a clean production process for citric acid. By modifying the citric acid production process, particularly by heating, adjusting the pH, and evaporating and concentrating the extraction waste liquid, and by limiting process conditions, this invention reduces the content of volatile acids in the condensate, while simultaneously enabling the recycling of the condensate. This avoids the inhibition of citric acid production by Aspergillus niger by volatile acids, and also prevents the waste of water and energy.

[0006] The technical solution of the present invention is as follows:

[0007] A clean production process for citric acid includes the following steps:

[0008] (1) Mix starchy raw materials with condensed water, then add alkaline solution to adjust the pH to 5.0-6.5 to obtain a mixed solution;

[0009] (2) The mixture from step (1) is subjected to two enzyme additions and two spray liquefaction treatments, and an iodine test is performed to obtain a qualified liquefied liquid; 60-80% of the liquefied liquid is filtered through a plate and frame filter to remove the filter residue and obtain a sugar solution.

[0010] (3) Add cooling water to the liquefied liquid prepared in step (2) to prepare a seed culture medium, adjust the pH to 6.0-7.0, sterilize and cool, and then inoculate with Aspergillus niger spores to obtain seed liquid;

[0011] (4) Take the liquefied liquid prepared in step (2), mix it with the sugar solution, add cooling water, prepare the fermentation medium, sterilize and cool it, add it to the seed liquid obtained in step (3), adjust the pH to 4.5-5.5, and culture to obtain the fermentation liquid;

[0012] (5) The fermentation broth from step (4) is separated by solid-liquid separation to remove the bacterial cells, and then extracted by the calcium hydrogen method to obtain citric acid, while the extraction waste liquid is discharged.

[0013] (6) Heat the extracted waste liquid from step (5) to 65-80°C and store it in a storage tank. Add alkaline substances and volatile acids to neutralize the liquid. Control the pH at the end of the reaction to 6.0-7.0. Then evaporate the liquid to obtain a concentrated liquid and collect the condensate. The concentrated liquid is used as an organic fertilizer substrate or a carbon source additive for wastewater treatment. The condensate is returned to steps (1), (3), and (4) for recycling.

[0014] Further, in step (1), the starchy raw material is at least one of corn flour, wheat flour, cassava flour, and sorghum flour; the mass ratio of the starchy raw material to the condensed water is 1:2.5 to 1:3.5; and the alkaline solution is a calcium hydroxide solution.

[0015] Further, in step (2), the specific method of the two enzyme additions and two spray liquefaction treatments is as follows: high-temperature α-amylase is added to the mixture at a ratio of 9 to 18 U / g starch raw material, and the first spray liquefaction is carried out at 95 to 105°C; then, high-temperature α-amylase is added to the first spray solution again at a ratio of 6 to 12 U / g starch raw material, and the second spray liquefaction is carried out at 120 to 130°C.

[0016] Furthermore, in step (3), the total sugar content of the seed culture medium is 100-180 g / L, and the C / N ratio is 15-30.

[0017] Further, in step (3), the concentration of Aspergillus niger spores is 1×10⁻⁶. 5 ~9×10 5 CFU / mL.

[0018] Further, in step (4), the volume ratio of the liquefied liquid to the sugar solution is 1:3.5 to 1:5.

[0019] Further, in step (4), the total sugar content of the fermentation medium is 150-180 g / L, and the C / N ratio is 55-90.

[0020] Furthermore, in step (5), the extraction waste liquid refers to the waste sugar water generated during the extraction of citric acid using the calcium hydrogen method.

[0021] Further, in step (6), the alkaline substance includes sodium hydroxide, calcium hydroxide, calcium oxide or potassium hydroxide; the evaporation temperature is 75-95°C and the vacuum degree is 0.2-0.6 bar.

[0022] Furthermore, in steps (1), (3), (4) and (6), the temperature of the condensate is 50-75°C, the pH value is 4.5-6.0, and the total amount of volatile acids in the condensate is <100mg / L.

[0023] Furthermore, in step (6), the heating of the extracted waste liquid can be achieved by means of plate heat exchanger, jet heater and direct steam heating.

[0024] Furthermore, in step (6), the optimal method for heating the extracted waste liquid is to use a plate heat exchanger, which utilizes the waste heat from other production stages for heat exchange, saving energy consumption and avoiding material dilution caused by steam contact.

[0025] Furthermore, in step (6), the evaporation is performed using mechanical vapor recompression (MVR), multi-effect evaporation, or low-temperature vacuum evaporation.

[0026] The beneficial technical effects of the present invention are:

[0027] This invention reduces the generation of volatile acids by using high-temperature inhibition during the storage stage of the extracted waste liquid, and converts the volatile acids into non-volatile salts through a neutralization reaction before evaporation, further reducing the volatile acid content. At the same time, it reduces the inhibitory effect of volatile acids on the growth and acid production of Aspergillus niger by increasing the initial pH of the seed and fermentation culture. By combining source control, process treatment and tail-end reduction, this invention avoids the inhibition of citric acid production by Aspergillus niger by the reuse of condensate. It achieves water resource recycling while preparing citric acid organic wastewater. Detailed Implementation

[0028] The present invention will now be described in detail with reference to the embodiments.

[0029] This invention reveals that volatile acids are primarily produced by microorganisms during the storage stage of the extraction waste liquid, and these volatile acids enter the condensate during the evaporation and concentration of the extraction waste liquid. To prevent the generation of volatile acids, the first consideration should be inhibiting the growth of microorganisms at the source. The results of this invention show that raising the storage temperature of the extraction waste liquid to above 65°C can effectively inhibit microbial growth and significantly reduce the generation of volatile acids.

[0030] In one embodiment of the present invention, the storage temperature of the extracted waste liquid is 65-80°C.

[0031] To further reduce the volatile acid content in the condensate, in one embodiment of the present invention, before evaporation and concentration, an alkaline substance is added to the extraction waste liquid for a neutralization reaction, so that the volatile acid is converted into non-volatile salts, thereby preventing the volatile acid from entering the condensate.

[0032] In one embodiment of the present invention, after optimization, the alkaline substances in the extraction waste liquid are neutralized, and the pH at the neutralization endpoint is controlled at 6.0 to 7.0. At this time, only a very small amount of volatile acid enters the condensate.

[0033] Meanwhile, the study found that volatile acids significantly inhibited the growth and acid production of Aspergillus niger when the initial fermentation pH was low. Conversely, the inhibition of Aspergillus niger growth and acid production by volatile acids weakened when the initial fermentation pH increased. Therefore, in order to further eliminate the inhibitory effect of volatile acids on the growth of Aspergillus niger and citric acid fermentation, the initial pH of both seed culture and fermentation culture should be appropriately increased.

[0034] In one embodiment of the present invention, the initial pH of the seed culture is optimized to be 6.0–7.0, and the initial pH of the fermentation culture is 4.5–5.5.

[0035] The present invention will be further illustrated below through specific embodiments.

[0036] All raw materials and reagents used in the following examples are commercially available. *Aspergillus niger* was obtained from the China Industrial Microbial Culture Collection Center (CICC), accession number CICC 40021. Total sugar and reducing sugar were determined using Fehling's titration method; nitrogen source was determined using the Kjeldahl method; citric acid was determined using 0.1429 mol / L NaOH titration; spore counting was performed using a hemocytometer. Unless otherwise specified, commonly used equipment and processes in the art were employed.

[0037] Example 1

[0038] A clean production process for citric acid includes the following steps:

[0039] (1) The corn flour that has passed through a 60-mesh sieve is mixed with 50℃ condensed water at a mass ratio of 1:2.5. The pH of the mixture is adjusted to 5.0 using Ca(OH)2.

[0040] (2) The resulting mixture was subjected to two enzyme additions and two spray liquefaction treatments. The first high-temperature α-amylase addition ratio was 18 U / g corn flour, and the first spray temperature was 105℃. The second high-temperature α-amylase addition ratio was 12 U / g corn flour, and the second spray temperature was 130℃. After the iodine test turned light brown, the qualified corn liquefaction liquid was obtained. Then, 60% of the corn liquefaction liquid was filtered through a plate and frame filter to remove the filter residue, and the corn sugar solution was obtained.

[0041] (3) Add cooling water to the corn liquefaction liquid to make the total sugar content 180 g / L and the C / N ratio 15, prepare a seed culture medium, adjust the pH to 6.0, sterilize and cool, and then inoculate with Aspergillus niger spores (the concentration of Aspergillus niger spores is 1×10⁻⁶). 5 Seed culture was obtained by culturing (CFU / mL).

[0042] (4) Mix corn liquefaction liquid and corn sugar solution at a ratio of 1:3.5 and add cooling water to make the total sugar 180 g / L and C / N 55. Prepare a fermentation medium, sterilize and cool it, inoculate it with seed liquid, adjust the pH to 4.5, and obtain fermentation liquid after fermentation. The acidity is 179.3 g / L, the residual total sugar is 22.6 g / L, the residual reducing sugar is 5.3 g / L, the conversion rate is 99.6%, and the cycle is 64 h.

[0043] (5) The fermentation broth is separated by solid-liquid separation to remove the bacterial cells, and then citric acid is obtained by calcium hydrogen extraction. At the same time, the extraction waste liquid is discharged.

[0044] (6) The extracted waste liquid was heated to 65°C with direct steam and stored. Then, calcium hydroxide and volatile acids were added for neutralization. The pH of the reaction endpoint was controlled at 6.0. The concentrate was then obtained by evaporation at 75°C under a vacuum of 0.6 bar. Condensate was collected at the same time. The volatile acid content in the condensate was 80 mg / L. The condensate was returned to steps (1), (3), and (4) for water recycling. After 10 batches of recycling, the citric acid fermentation water was normal.

[0045] Example 2

[0046] A clean production process for citric acid includes the following steps:

[0047] (1) The corn flour that has passed through a 60-mesh sieve and the condensed water at 65℃ are mixed evenly at a mass ratio of 1:3.0, and the pH of the mixture is adjusted to 5.8 with Ca(OH)2.

[0048] (2) The resulting mixture was subjected to two enzyme additions and two spray liquefaction treatments. The first high-temperature α-amylase addition ratio was 12 U / g corn flour, and the first spray temperature was 100℃. The second high-temperature α-amylase addition ratio was 8 U / g corn flour, and the second spray temperature was 125℃. After the iodine test turned light brown, the qualified corn liquefaction liquid was obtained. Then, 70% of the corn liquefaction liquid was filtered through a plate and frame filter to remove the filter residue, and the corn sugar solution was obtained.

[0049] (3) Add cooling water to the corn liquefaction liquid to make the total sugar content 140 g / L and the C / N ratio 20, prepare a seed culture medium, adjust the pH to 6.5, sterilize and cool, and then inoculate with Aspergillus niger spores (the concentration of Aspergillus niger spores is 5 × 10⁻⁶). 5 (CFU / mL), to obtain seed culture.

[0050] (4) Mix corn liquefaction liquid and corn sugar solution in a ratio of 1:4 and add cooling water to make the total sugar 165 g / L and C / N 65. Prepare a fermentation medium, sterilize and cool it, inoculate it with seed liquid, adjust the pH to 5.0, and the acidity at the end of fermentation is 165.7 g / L, the residual total sugar is 20.5 g / L, the residual reducing sugar is 4.9 g / L, the conversion rate is 100.4%, and the cycle is 58 h.

[0051] (5) The fermentation broth is separated by solid-liquid separation to remove the bacterial cells, and then citric acid is obtained by calcium hydrogen extraction. At the same time, the extraction waste liquid is discharged.

[0052] (6) The extracted waste liquid was heated to 70°C using a plate heat exchanger and stored. Then, calcium oxide and volatile acids were added for neutralization. The pH of the reaction endpoint was controlled at 6.5. The concentrate was then evaporated at 85°C under a vacuum of 0.4 bar to obtain a concentrated liquid. Condensate was collected at the same time. The volatile acid content in the condensate was 50 mg / L. The condensate was returned to steps (1), (3), and (4) for water recycling. After 10 batches of recycling, the citric acid fermentation water was normal.

[0053] Example 3

[0054] A clean production process for citric acid includes the following steps:

[0055] (1) The corn flour that has passed through an 80-mesh sieve is mixed with 75°C condensed water at a mass ratio of 1:3.5. The pH of the mixture is adjusted to 6.5 using Ca(OH)2.

[0056] (2) The resulting mixture was subjected to two enzyme additions and two spray liquefaction treatments. The first high-temperature α-amylase addition ratio was 9 U / g corn flour, and the first spray temperature was 95℃. The second high-temperature α-amylase addition ratio was 6 U / g corn flour, and the second spray temperature was 125℃. After the iodine test turned light brown, the qualified corn liquefaction liquid was obtained. Then, 80% of the corn liquefaction liquid was filtered through a plate and frame filter to remove the filter residue, and the corn sugar solution was obtained.

[0057] (3) Add cooling water to the corn liquefaction liquid to make the total sugar content 100 g / L and the C / N ratio 30, prepare a seed culture medium, adjust the pH to 7.0, sterilize and cool, and then inoculate with Aspergillus niger spores (the concentration of Aspergillus niger spores is 9 × 10⁻⁶). 5 (CFU / mL), to obtain seed culture.

[0058] (4) Mix corn liquefaction liquid and corn sugar solution at a ratio of 1:5 and add cooling water to make the total sugar 150 g / L and C / N 90. Prepare a fermentation medium, sterilize and cool it, inoculate it with seed liquid, adjust the pH to 5.5, the acidity at the end of fermentation is 151.8 g / L, the residual total sugar is 18.9 g / L, the residual reducing sugar is 4.5 g / L, the conversion rate is 101.2%, and the cycle is 52 h.

[0059] (5) The fermentation broth is separated by solid-liquid separation to remove the bacterial cells, and then citric acid is obtained by calcium hydrogen extraction. At the same time, the extraction waste liquid is discharged.

[0060] (6) The extracted waste liquid was heated to 80°C using a sprayer and stored. Then, potassium hydroxide and volatile acids were added for neutralization. The pH of the reaction endpoint was controlled at 7.0. The concentrate was then obtained by evaporation at 95°C under a vacuum of 0.6 bar. Condensate was collected at the same time, and the volatile acid content in the condensate was 10 mg / L. The condensate was returned to steps (1), (3), and (4) for water recycling. After 10 batches of recycling, the citric acid fermentation water was normal.

[0061] Comparative Example 1

[0062] A citric acid production process includes the following steps:

[0063] (1) The corn flour that has passed through a 60-mesh sieve is mixed with deionized water at 65℃ at a mass ratio of 1:3.0. The pH of the mixture is adjusted to 5.8 using Ca(OH)2.

[0064] (2) The resulting mixture was subjected to two enzyme additions and two spray liquefaction treatments. The first high-temperature α-amylase addition ratio was 12 U / g corn flour, and the first spray temperature was 100℃. The second high-temperature α-amylase addition ratio was 8 U / g corn flour, and the second spray temperature was 125℃. After the iodine test turned light brown, the qualified corn liquefaction liquid was obtained. Then, 70% of the corn liquefaction liquid was filtered through a plate and frame filter to remove the filter residue, and the corn sugar solution was obtained.

[0065] (3) Add deionized water to corn liquefaction liquid to make the total sugar 140 g / L and C / N 20, prepare seed culture medium, do not adjust pH, pH 5.0, sterilize and cool, and inoculate with Aspergillus niger spores to obtain seed liquid.

[0066] (4) Mix corn liquefaction liquid and corn sugar solution in a ratio of 1:4 and add deionized water to make the total sugar 165 g / L and C / N 65. Prepare a fermentation medium, sterilize and cool it, and then inoculate it with seed liquid. Do not adjust the pH. The pH is 3.8. The acidity at the end of fermentation is 162.4 g / L, the residual total sugar is 20.3 g / L, the residual reducing sugar is 4.8 g / L, the conversion rate is 98.4%, and the cycle is 60 h.

[0067] (5) The fermentation broth is separated by solid-liquid separation to remove the bacterial cells, and then citric acid is extracted by the calcium hydrogen method. At the same time, the extraction waste liquid is discharged. The extraction waste liquid is treated by anaerobic and aerobic biological methods and then discharged into the municipal sewage treatment plant.

[0068] Comparative Example 2

[0069] A citric acid production process includes the following steps:

[0070] (1) The corn flour that has passed through a 60-mesh sieve and the condensed water at 65℃ are mixed evenly at a mass ratio of 1:3.0, and the pH of the mixture is adjusted to 5.8 with Ca(OH)2.

[0071] (2) The resulting mixture was subjected to two enzyme additions and two spray liquefaction treatments. The first high-temperature α-amylase addition ratio was 12 U / g corn flour, and the first spray temperature was 100℃. The second high-temperature α-amylase addition ratio was 8 U / g corn flour, and the second spray temperature was 125℃. After the iodine test turned light brown, the qualified corn liquefaction liquid was obtained. Then, 70% of the corn liquefaction liquid was filtered through a plate and frame filter to remove the filter residue, and the corn sugar solution was obtained.

[0072] (3) Add cooling water to the corn liquefaction liquid to make the total sugar 140g / L and C / N 20, and prepare seed culture medium. Do not adjust the pH, the pH is 5.0. After sterilization and cooling, inoculate with Aspergillus niger spores to obtain seed liquid.

[0073] (4) Mix corn liquefaction liquid and corn sugar solution in a ratio of 1:4 and add cooling water to make the total sugar 165 g / L and C / N 65. Prepare fermentation medium, sterilize and cool it and inoculate it with seed liquid. Do not adjust the pH, the pH is 3.8. The acidity at the end of fermentation is 160.5 g / L, the residual total sugar is 20.8 g / L, the residual reducing sugar is 5.0 g / L, the conversion rate is 97.3%, and the cycle is 69 h.

[0074] (5) The fermentation broth is separated by solid-liquid separation to remove the bacterial cells, and then citric acid is obtained by calcium hydrogen extraction. At the same time, the extraction waste liquid is discharged.

[0075] (6) The extracted waste liquid is not subjected to any controlled treatment and is stored at 50℃ with a pH of 4.5. It is then evaporated at 85℃ to obtain a concentrated liquid, and condensate is collected at the same time. The volatile acid content in the condensate is 1600 mg / L. The condensate is returned to steps (1), (3), and (4) for water recycling. The volatile acid content in the condensate gradually increases, the lag period of citric acid fermentation increases, the cycle is prolonged, the conversion rate decreases, and the fermentation results are seriously affected.

[0076] Comparative Example 3

[0077] A citric acid production process includes the following steps:

[0078] (1) The corn flour that has passed through a 60-mesh sieve and the condensed water at 65℃ are mixed evenly at a mass ratio of 1:3.0, and the pH of the mixture is adjusted to 5.8 with Ca(OH)2.

[0079] (2) The resulting mixture was subjected to two enzyme additions and two spray liquefaction treatments. The first high-temperature α-amylase addition ratio was 12 U / g corn flour, and the first spray temperature was 100℃. The second high-temperature α-amylase addition ratio was 8 U / g corn flour, and the second spray temperature was 125℃. After the iodine test turned light brown, the qualified corn liquefaction liquid was obtained. Then, 70% of the corn liquefaction liquid was filtered through a plate and frame filter to remove the filter residue, and the corn sugar solution was obtained.

[0080] (3) Add cooling water to the corn liquefaction liquid to make the total sugar 140g / L and C / N 20, and prepare seed culture medium. Do not adjust the pH, the pH is 5.0. After sterilization and cooling, inoculate with Aspergillus niger spores to obtain seed liquid.

[0081] (4) Mix corn liquefaction liquid and corn sugar solution in a ratio of 1:4 and add cooling water to make the total sugar 165 g / L and C / N 65. Prepare a fermentation medium, sterilize and cool it, and then inoculate it with seed liquid. Do not adjust the pH. The pH is 3.8. The acidity at the end of fermentation is 161.2 g / L, the residual total sugar is 20.6 g / L, the residual reducing sugar is 4.9 g / L, the conversion rate is 97.7%, and the cycle is 64 h.

[0082] (5) The fermentation broth is separated by solid-liquid separation to remove the bacterial cells, and then citric acid is obtained by calcium hydrogen extraction. At the same time, the extraction waste liquid is discharged.

[0083] (6) The extracted waste liquid was heated to 70°C using a plate heat exchanger and stored without neutralization. The pH of the extracted waste liquid was 4.7. Then, it was evaporated at 85°C to obtain a concentrated liquid, and condensate was collected at the same time. The volatile acid content in the condensate was 300 mg / L. The condensate was returned to steps (1), (3), and (4) for water recycling. The volatile acid content in the condensate gradually increased, the lag period of citric acid fermentation increased, the cycle was prolonged, the conversion rate decreased, and the fermentation results were seriously affected.

[0084] Comparative Example 4

[0085] A citric acid production process includes the following steps:

[0086] (1) The corn flour that has passed through a 60-mesh sieve and the condensed water at 65℃ are mixed evenly at a mass ratio of 1:3.0, and the pH of the mixture is adjusted to 5.8 with Ca(OH)2.

[0087] (2) The resulting mixture was subjected to two enzyme additions and two spray liquefaction treatments. The first high-temperature α-amylase addition ratio was 12 U / g corn flour, and the first spray temperature was 100℃. The second high-temperature α-amylase addition ratio was 8 U / g corn flour, and the second spray temperature was 125℃. After the iodine test turned light brown, the qualified corn liquefaction liquid was obtained. Then, 70% of the corn liquefaction liquid was filtered through a plate and frame filter to remove the filter residue, and the corn sugar solution was obtained.

[0088] (3) Add cooling water to the corn liquefaction liquid to make the total sugar 140g / L and C / N 20, and prepare seed culture medium. Do not adjust the pH, the pH is 5.0. After sterilization and cooling, inoculate with Aspergillus niger spores to obtain seed liquid.

[0089] (4) Mix corn liquefaction liquid and corn sugar solution in a ratio of 1:4 and add cooling water to make the total sugar 165 g / L and C / N 65. Prepare fermentation medium, sterilize and cool it and inoculate it with seed liquid. Do not adjust the pH, the pH is 3.8. The acidity at the end of fermentation is 162.3 g / L, the residual total sugar is 20.5 g / L, the residual reducing sugar is 4.9 g / L, the conversion rate is 98.4%, and the cycle is 61 h.

[0090] (5) The fermentation broth is separated by solid-liquid separation to remove the bacterial cells, and then citric acid is obtained by calcium hydrogen extraction. At the same time, the extraction waste liquid is discharged.

[0091] (6) The extracted waste liquid was heated to 70°C using a plate heat exchanger and stored. Then, calcium oxide and volatile acids were added for neutralization. The final pH of the reaction was controlled at 6.5. The concentrate was obtained by evaporation at 85°C, and condensate was collected. The volatile acid content in the condensate was 50 mg / L. The condensate was returned to steps (1), (3), and (4) for water recycling. The volatile acid content in the condensate gradually increased, the lag period of citric acid fermentation increased, the cycle was prolonged, the conversion rate decreased, and the fermentation results were seriously affected.

[0092] Test Example 1

[0093] The technical effects of Examples 1-3 and Comparative Examples 1-2 were compared, and the results are shown in Table 1.

[0094] Table 1. Technical effects of Examples 1-3 and Comparative Examples 1-2

[0095]

[0096]

[0097] Table 1 shows that, in Comparative Example 1, without condensate recycling, the fermentation conversion rate was 98.4%, the fermentation cycle was 60 hours, the fermentation intensity was 2.71 g / L / h, and the fermentation water consumption was as high as 4.85 m³ / h. 3 / t citric acid.

[0098] Comparative Example 2 does not control the volatile acid in the condensate and directly reuses it, reducing the fermentation water consumption to 1.92m³. 3 / t citric acid, but the volatile acid content in the condensate is as high as 1600mg / L, the final fermentation conversion rate drops to 97.3%, the fermentation cycle is extended to 69h, and the fermentation intensity drops to 2.33g / L / h, which seriously affects the fermentation results.

[0099] Examples 1-3 involved controlling the volatile acids in the condensate and reusing it. Compared to Comparative Example 2, the most significant improvement was reducing the volatile acid content in the condensate to 10-80 mg / L, thus avoiding its impact on fermentation acid production; the fermentation conversion rate increased to 99.6-101.2%, and the fermentation intensity increased to 2.80-2.92 g / L / h. Compared to Comparative Example 1, the most significant improvement was reducing the fermentation water consumption to 1.35-1.68 m³ / h. 3 / t citric acid.

[0100] The results show that the method of the present invention effectively reduces the volatile acid content of the condensate produced during the concentration process while preparing citric acid organic wastewater, avoids the inhibition of citric acid production by Aspergillus niger by the reuse of condensate, and realizes water resource recycling; at the same time, the reuse of high-temperature condensate realizes the recovery and utilization of heat energy. The method of the present invention has an important role in promoting the technological improvement of the citric acid industry.

[0101] Test Example 2

[0102] The technical effects of Example 2 and Comparative Examples 2-4 were compared, and the results are shown in Table 2.

[0103] Table 2. Technical effects of Example 2 and Comparative Examples 2-4

[0104]

[0105]

[0106] As shown in Table 2, in Comparative Example 2, no treatment measures were taken. The extracted waste liquid was stored at 50℃, and the condensate produced by concentration was directly reused. The test results showed that the volatile acid content in the condensate was as high as 1600 mg / L, the final fermentation conversion rate was low, the fermentation cycle was long, and the fermentation intensity was low.

[0107] In Comparative Example 3, the extraction waste liquid was heated to 70°C and stored, and then the condensate generated from the concentration was reused. Compared with Comparative Example 2, after increasing the storage temperature of the extraction waste liquid, the volatile acid content in the condensate decreased to 300 mg / L, indicating that high-temperature storage has a significant effect on controlling volatile acids and the fermentation results are improved.

[0108] In Comparative Example 4, the extracted waste liquid was heated to 70°C and stored. Calcium oxide was then added to neutralize the volatile acids, and the final pH of the reaction was controlled at 6.5. The condensate produced by concentration was then reused. Compared with Comparative Example 3, the volatile acid content in the condensate further decreased to 50 mg / L, indicating that the neutralization treatment had a significant effect on further controlling volatile acids, and the fermentation results were further improved.

[0109] Compared with Comparative Example 4, the fermentation results of Example 2 were further improved after further fermentation and adjustment of the initial pH, indicating that increasing the initial pH can alleviate the negative impact of volatile acids on fermentation.

[0110] The results show that storage temperature and the final pH of the neutralization reaction significantly affect the volatile acid content in the condensate; the seed pH and the initial pH of fermentation have a significant inhibitory effect on the elimination of volatile acids. Therefore, these conditions need to be controlled within appropriate ranges to completely eliminate the influence of volatile acids and achieve safe and economical recycling of condensate.

[0111] The above description is merely a preferred embodiment of the present invention, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations that are directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.

Claims

1. A clean production process for citric acid, characterized in that, The clean production process includes the following steps: (1) Mix starchy raw materials with condensed water, then add alkaline solution to adjust the pH to 5.0-6.5 to obtain a mixed solution; (2) The mixture from step (1) is subjected to two enzyme additions and two spray liquefaction treatments, and an iodine test is performed to obtain a qualified liquefied liquid; 60-80% of the liquefied liquid is filtered through a plate and frame filter to remove the filter residue and obtain a sugar solution. (3) Take the liquefied liquid prepared in step (2), add cooling water, prepare seed culture medium, adjust the pH to 6.0-7.0, sterilize and cool, and inoculate with Aspergillus niger spores to obtain seed liquid; (4) Take the liquefied liquid prepared in step (2) again, mix it with sugar solution, add cooling water, prepare fermentation medium, sterilize and cool it, add it to the seed liquid obtained in step (3), adjust the pH to 4.5-5.5, and culture to obtain fermentation liquid; (5) The fermentation broth from step (4) is separated by solid-liquid separation to remove the bacterial cells, and then extracted by the calcium hydrogen method to obtain citric acid, while the extraction waste liquid is discharged. (6) Heat the extracted waste liquid from step (5) to 65-80°C and store it in a storage tank. Add alkaline substances and volatile acids to neutralize the liquid. Control the pH at the end of the reaction to 6.0-7.

0. Then evaporate the liquid to obtain a concentrated liquid and collect the condensate. The concentrated liquid is used as an organic fertilizer substrate or a carbon source additive for wastewater treatment. The condensate is returned to steps (1), (3), and (4) for recycling.

2. The clean production process according to claim 1, characterized in that, In step (1), the starchy raw material is at least one of corn flour, wheat flour, cassava flour, and sorghum flour; the mass ratio of the starchy raw material to the condensed water is 1:2.5 to 1:3.5; and the alkaline solution is calcium hydroxide.

3. The clean production process according to claim 1, characterized in that, In step (2), the specific method of the two enzyme additions and two spray liquefaction treatments is as follows: add high-temperature α-amylase to the mixture, the addition ratio of high-temperature α-amylase is 9-18 U / g starch raw material, and perform the first spray at 95-105℃; then, add high-temperature α-amylase to the first spray solution again, the addition ratio of high-temperature α-amylase is 6-12 U / g starch raw material, and perform the second spray at 120-130℃.

4. The clean production process according to claim 1, characterized in that, In step (3), the total sugar content of the seed culture medium is 100-180 g / L, and the C / N ratio is 15-30.

5. The clean production process according to claim 1, characterized in that, In step (3), the concentration of Aspergillus niger spores is 1×10⁻⁶. 5 ~9×10 5 CFU / mL.

6. The clean production process according to claim 1, characterized in that, In step (4), the volume ratio of the liquefied liquid to the sugar solution is 1:3.5 to 1:

5.

7. The clean production process according to claim 1, characterized in that, In step (4), the total sugar content of the fermentation medium is 150-180 g / L, and the C / N ratio is 55-90.

8. The clean production process according to claim 1, characterized in that, In step (5), the extraction waste liquid refers to the waste sugar water generated during the extraction of citric acid using the calcium hydrogen method.

9. The clean production process according to claim 1, characterized in that, In step (6), the alkaline substance includes sodium hydroxide, calcium hydroxide, calcium oxide or potassium hydroxide; the evaporation temperature is 75-95°C and the vacuum degree is 0.2-0.6 bar.

10. The clean production process according to claim 1, characterized in that, In steps (1), (3), (4) and (6), the temperature of the condensate is 50-75℃, the pH value is 4.5-6.0, and the total amount of volatile acid in the condensate is <100mg / L.

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