A methionine separation method based on ethanol salting-out coupling adsorption-membrane separation

Abstract

The application relates to the technical field of bio-chemical engineering separation and purification, in particular to a methionine separation method based on ethanol salting-out coupling adsorption-membrane separation, which comprises the following steps: step 1, pretreatment; step 2, ethanol salting-out enrichment; step 3, adsorption impurity removal; step 4, membrane separation refining; step 5, crystallization and drying; and step 6, solvent and salting-out agent recovery. The application provides a green methionine separation method based on ethanol salting-out coupling adsorption-membrane separation, which effectively solves the problems of low methionine recovery rate, insufficient purity, high energy consumption, serious pollution and industrial-grade standard unattainable of by-product sodium sulfate in the prior art.

Description

Technical Field

[0001] This application relates to the field of biochemical separation and purification technology, specifically to a methionine separation method based on ethanol salting-out coupled adsorption-membrane separation. Background Technology

[0002] Methionine is a sulfur-containing essential amino acid widely used in feed, pharmaceuticals, and food. Globally, over 98% of its production capacity comes from the chemical synthesis via the hydantoin process, while microbial fermentation represents a green development direction. Both processes produce crude methionine solutions containing inorganic salts, unreacted substrates, and byproducts. Sodium sulfate is one of the most common inorganic salt impurities (a byproduct of raw material reactions in chemical synthesis and a residue in the culture medium in microbial fermentation). Its solubility is significantly affected by temperature (approximately 19.5 g / 100 g water at 20°C, and only about 4.9 g / 100 g water at 0°C), showing a marked difference in solubility characteristics compared to methionine (approximately 3.5 g / 100 g water at 25°C). However, when the two coexist, they easily form co-crystallization, resulting in a large amount of sodium sulfate residue during methionine production (mainly composed of sodium sulfate crystals, containing 1-3% methionine, polypeptides, and small amounts of unreacted substrates, pigments, and other impurities). If the sodium sulfate residue is discarded directly, it will not only waste methionine resources, but also make it difficult to utilize the waste residue for resource recovery due to methionine residue, and at the same time cause environmental pollution. If the efficient separation of methionine from sodium sulfate residue can be achieved and the recovered sodium sulfate can meet industrial-grade standards, the waste residue can be utilized for resource recovery and economic benefits can be improved. Therefore, the separation and recovery of methionine from sodium sulfate residue and the industrial-grade resource recovery of sodium sulfate have become key issues that urgently need to be solved in the methionine production process.

[0003] Current mainstream technologies for methionine separation and purification and sodium sulfate residue treatment include crystallization, ion exchange resin method, single membrane separation method, adsorption method, and single salting-out method. However, these methods have significant shortcomings in the separation and recovery of methionine from sodium sulfate residue. 1. Crystallization method: This method is the most widely used in industry. However, the methionine content in sodium sulfate residue is low and the residue is highly dispersible. Direct crystallization can easily lead to the recrystallization of methionine and sodium sulfate, resulting in an extremely low methionine recovery rate (usually below 60%). Evaporation crystallization consumes a lot of energy, and methionine is prone to thermal decomposition at high temperatures, producing malodorous substances such as dimethyl disulfide, which affects product quality and the operating environment. At the same time, it cannot achieve efficient separation of sodium sulfate and methionine, making it difficult to ensure that the recovered sodium sulfate meets industrial-grade standards and thus hindering the resource recovery of both.

[0004] 2. Ion exchange resin method: It has a good separation effect and can remove inorganic salts such as sodium sulfate and recover methionine. However, it requires frequent resin regeneration, large consumption of regenerant, and generates a large amount of high-salt wastewater. The process is long and costly, and the adsorption capacity is limited. It has low recovery efficiency for low-concentration methionine in sodium sulfate residue and is prone to methionine loss. At the same time, the regenerated liquid is difficult to treat.

[0005] 3. Single membrane separation method: Ultrafiltration cannot separate methionine and sodium sulfate (both are small molecules); although nanofiltration / reverse osmosis membranes can initially separate them through differences in pore size, the trace impurities remaining in the sodium sulfate residue easily lead to membrane fouling, rapid flux decline, and significant concentration polarization. High-pressure operation is required, energy consumption is high, the recovery efficiency of low-concentration methionine is extremely low, and it is difficult to remove trace impurities from sodium sulfate, making it impossible to make the recovered sodium sulfate meet industrial-grade standards and making it difficult to realize the resource utilization of sodium sulfate residue.

[0006] 4. Adsorption method: It is simple to operate and low in cost, but conventional adsorbents are mainly for pigments and macromolecular impurities. They have poor adsorption selectivity for methionine and are difficult to efficiently adsorb methionine from sodium sulfate residue. Moreover, the methionine after adsorption is difficult to elute and is easy to lose. It cannot be used alone to achieve industrial recycling. Although specific adsorbent materials can improve the adsorption effect, the preparation cost is high and it is difficult to apply on a large scale.

[0007] 5. Single salting-out method: It is simple to operate and low in cost, but the salting-out effect is weak, the methionine precipitation rate is not high, and the sodium sulfate in the sodium sulfate residue will interfere with the salting-out effect, resulting in methionine carrying a large amount of sodium sulfate. Subsequent desalination steps are required, the process is cumbersome, the generated high-salt wastewater is heavily polluted, and it cannot effectively remove impurities in sodium sulfate, making it difficult to make the recovered sodium sulfate meet industrial-grade standards. It is impossible to realize the resource recovery of sodium sulfate residue, and the methionine recovery rate is difficult to improve.

[0008] Existing coupled technologies (ion exchange-membrane separation and crystallization-adsorption) still fail to solve problems such as high energy consumption, high pollution, low recovery rate, and insufficient purity. This is particularly true for the separation and recovery of methionine from sodium sulfate residue, which suffers from incomplete separation, low methionine recovery rate, inability to utilize sodium sulfate residue as a resource, easy decomposition of methionine, and difficulty in achieving industrial-grade standards for recovered sodium sulfate. These drawbacks prevent efficient recovery of methionine and industrial-grade recycling of sodium sulfate, failing to meet green and low-carbon requirements. While existing technologies report on the post-treatment of sodium sulfate mother liquor, they do not address the efficient separation and recovery of methionine from sodium sulfate residue and are not coupled with ethanol salting-out or adsorption technologies. The separation effect is limited, failing to ensure that the recovered sodium sulfate meets industrial-grade standards, thus failing to address the problems of methionine resource waste and environmental pollution.

[0009] Ethanol is a green solvent that is recyclable and low in toxicity. Introducing it into salting-out can enhance phase separation and reduce the amount of salting-out agent required. Simultaneously, by utilizing the difference in the solubility of methionine and sodium sulfate by ethanol, the separation effect between the two can be enhanced. Coupled with ethanol salting-out, adsorption, and membrane separation, an integrated process of "sodium sulfate residue dissolution - methionine enrichment - impurity removal - deep purification - sodium sulfate recovery" can be achieved. This process can efficiently recover methionine from sodium sulfate residue and effectively remove trace impurities from sodium sulfate, ensuring that the recovered sodium sulfate meets industrial-grade standards. This realizes the resource utilization of sodium sulfate and significantly improves separation and recovery efficiency, resource utilization rate, and economic benefits. Currently, there are no publicly available documents or industrialized processes for the coupling of these three processes to separate and recover methionine from sodium sulfate residue and achieve industrial-grade sodium sulfate standards, nor are there any reports of green, efficient, and integrated separation and recovery processes for this type of waste residue. Summary of the Invention

[0010] The purpose of this invention is to provide a green separation method for methionine based on ethanol salting-out coupled adsorption-membrane separation, which solves the problems of low methionine recovery rate, insufficient purity, high energy consumption, high pollution, and the inability of the by-product sodium sulfate to meet industrial-grade standards in the prior art.

[0011] To achieve the above-mentioned objectives, this application adopts the following technical solution: This application provides a method for separating methionine based on ethanol salting-out coupled adsorption-membrane separation, including the following steps: Step 1, Pretreatment: Dissolve the crude sodium sulfate residue containing methionine and sodium sulfate in water, adjust the pH to 6.0-7.0, add flocculant for flocculation and sedimentation, filter, and obtain pretreated clear liquid; Step 2, Ethanol salting-out enrichment: Add ethanol stepwise to the pretreated clear liquid obtained in Step 1 under stirring until the final volume fraction is 30-50%, and carry out salting-out at a temperature of 10-30℃. Centrifuge to obtain coarse crystals and salting-out mother liquor. Step 3, adsorption and impurity removal: Dissolve the crude crystals obtained in step 2 in water, add activated carbon for adsorption, decolorization and impurity removal, filter to remove the activated carbon, and obtain the adsorption and purification solution; Step 4, membrane separation and purification: The adsorption purified liquid obtained in step 3 is sequentially separated and purified by ultrafiltration membrane and nanofiltration membrane to obtain methionine concentrate and nanofiltration permeate; Step 5, Crystallization and Drying: Adjust the pH of the methionine concentrate obtained in Step 4, perform low-temperature crystallization, centrifuge, and vacuum dry to obtain the finished methionine product. Step 6, Solvent and Salting-out Agent Recovery: Combine the salting-out mother liquor from Step 2 with the nanofiltration permeate from Step 4, and distill to recover ethanol. The distillation residue is concentrated and crystallized to recover the salting-out agent and the by-product sodium sulfate.

[0012] In a preferred embodiment, in step 1, the methionine content in the crude sodium sulfate residue is 1-5% by mass; The flocculant is at least one of polyaluminum chloride and polyacrylamide, the flocculant dosage is 0.1-0.5 g / L, and the flocculation time is 10-30 minutes; The filtration uses microfiltration membranes or precision filters with a pore size of 0.1-0.5μm, with an operating pressure of 0.1-0.3MPa and a temperature of 20-40℃.

[0013] In a preferred embodiment, in step 2, the ethanol is ethanol with a volume concentration ≥ 90%; Ethanol is added in 2-4 portions, stirring for 10-20 minutes after each addition. The salting-out temperature is 15-25℃, the stirring speed is 100-300rpm, and the total salting-out time is 30-90 minutes; The centrifugation speed is 3000-5000 rpm.

[0014] In a preferred embodiment, in step 3, the activated carbon is wood-based activated carbon, coal-based activated carbon, or fruit shell activated carbon, and its addition amount is 5-15 g / L of the solution volume. The adsorption temperature is 30-50℃, and the adsorption time is 20-60 minutes.

[0015] In a preferred embodiment, the activated carbon is modified activated carbon, and the modification method is as follows: the activated carbon is soaked in an inorganic acid solution with a mass fraction of 5-10% for 2-5 hours, washed with water until neutral, dried, then soaked in an aminosilane coupling agent with a mass fraction of 2-5% for 1-3 hours, and dried at 80-100℃ for 1-2 hours.

[0016] In a preferred embodiment, in step 4, the ultrafiltration membrane has a molecular weight cutoff of 2000-5000 Da, an operating pressure of 0.2-0.5 MPa, and an operating temperature of 25-40°C. The nanofiltration membrane has a molecular weight cutoff of 100-200 Da, an operating pressure of 0.8-1.5 MPa, and an operating temperature of 25-40℃.

[0017] In a preferred embodiment, in step 5, the pH is 5.6-5.8; the low-temperature crystallization temperature is 0-10℃; and the crystallization time is 12-24 hours. The vacuum drying temperature is 55-70℃, the vacuum degree is -0.08~-0.095MPa, and the drying time is 4-8 hours.

[0018] In a preferred embodiment, in step 6, the distillation temperature is 78-85℃, and the ethanol recovery rate is ≥95%. Salting-out agents recovered from distillation residue through concentration and crystallization include sodium sulfate or potassium chloride, with a recovery rate of ≥90%. After purification, the product meets industrial-grade standards. Compared with the prior art, the present invention has the following outstanding advantages: 1. Significantly improved recovery rate and purity The synergistic effect of ethanol salting-out enrichment, adsorption impurity removal, and membrane separation purification results in a total methionine recovery rate of ≥95% and a finished product purity of ≥99.5%, meeting pharmaceutical-grade standards; the recycling of the crystallization mother liquor further reduces losses.

[0019] 2. Green and environmentally friendly, with near-zero emissions of pollutants. Ethanol recovery rate ≥95%, salting-out agent recycling rate ≥90%, modified adsorbent can be regenerated at high temperature 5–8 times; distillation wastewater and nanofiltration permeate can be reused, with no large amount of high-salt wastewater and solid waste generated; at the same time, the recovered sodium sulfate can reach the industrial grade standard after purification (meeting the requirements of Class I Grade 1 of "Industrial Anhydrous Sodium Sulfate" GB / T6009-2014, sodium sulfate purity ≥99.0%, water insoluble matter ≤0.05%, iron ≤0.002%, moisture ≤0.20%), realizing the resource utilization of waste residue and improving economic benefits.

[0020] 3. Energy consumption and costs are significantly reduced. No high-temperature evaporation and concentration are required; the membrane separation conditions are mild; ethanol, salting-out agent, and adsorbent can be recycled; the overall production cost is reduced by 20%–30%; and energy consumption is reduced by more than 25% compared to traditional processes.

[0021] 4. Mild process, stable product quality The entire process is carried out at low temperature and normal / low pressure to avoid methionine racemization, resulting in stable optical rotation, regular crystal form, and good product consistency.

[0022] 5. Wide applicability and industrialization potential It is compatible with a wide concentration range of 5-50 g / L, and is compatible with crude product solutions from chemical synthesis (Hyin process). The process is simple, easy to scale up, and suitable for large-scale continuous production. Detailed Implementation

[0023] The technical solutions and effects of this application will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely for explaining the invention and are not intended to limit the invention.

[0024] This application discloses a method for separating methionine based on ethanol salting-out coupled adsorption-membrane separation, comprising the following steps: Step 1, Pretreatment: Dissolve the crude sodium sulfate residue containing methionine and sodium sulfate in water, adjust the pH to 6.0-7.0, add flocculant for flocculation and sedimentation, filter, and obtain pretreated clear liquid; Step 2, Ethanol salting-out enrichment: Add ethanol stepwise to the pretreated clear liquid obtained in Step 1 under stirring until the final volume fraction is 30-50%, and carry out salting-out at a temperature of 10-30℃. Centrifuge to obtain coarse crystals and salting-out mother liquor. Step 3, adsorption and impurity removal: Dissolve the crude crystals obtained in step 2 in water, add activated carbon for adsorption, decolorization and impurity removal, filter to remove the activated carbon, and obtain the adsorption and purification solution; Step 4, membrane separation and purification: The adsorption purified liquid obtained in step 3 is sequentially separated and purified by ultrafiltration membrane and nanofiltration membrane to obtain methionine concentrate and nanofiltration permeate; Step 5, Crystallization and Drying: Adjust the pH of the methionine concentrate obtained in Step 4, perform low-temperature crystallization, centrifuge, and vacuum dry to obtain the finished methionine product. Step 6, Solvent and Salting-out Agent Recovery: Combine the salting-out mother liquor from Step 2 with the nanofiltration permeate from Step 4, and distill to recover ethanol. The distillation residue is concentrated and crystallized to recover the salting-out agent and the by-product sodium sulfate.

[0025] The raw material for the above method, crude sodium sulfate residue, contains 1-3% methionine, inorganic salts, unreacted substrates, and pigments.

[0026] After separation and treatment using the above methods, the methionine purity is 99.7%, the recovery rate is 96.2%, and the wastewater is minimal; the recovered sodium sulfate meets the industrial grade standard (meets the requirements of Class I Grade 1 of GB / T6009-2014 "Industrial Anhydrous Sodium Sulfate"), realizing resource recycling.

[0027] Specifically, this application provides the following examples: Example 1 This embodiment provides a methionine separation method based on ethanol salting-out coupled adsorption-membrane separation. The raw material is crude sodium sulfate residue (a byproduct of methionine production by a chemical enterprise using the hydantoin process), which contains 2.8% methionine, 85.5% sodium sulfate, water, and small amounts of peptides, pigments, etc.

[0028] The separation method includes the following steps: Pretreatment: Take 100g of crude sodium sulfate residue, add 900mL of deionized water, stir to dissolve, and adjust the pH to 6.5 with 1M NaOH. Add 0.3g of polyaluminum chloride (PAC) and stir to flocculate for 12 minutes. Use a 0.22μm ceramic membrane for cross-flow filtration at a pressure of 0.2MPa and a temperature of 35℃ to obtain 980mL of pretreated clear liquid (methionine concentration approximately 2.85g / L).

[0029] Ethanol salting-out enrichment: The pretreated solution was placed in a jacketed reactor, stirred at 250 rpm, and the temperature was controlled at 18℃. 95% ethanol was added in three portions, each time to achieve a volume fraction of 15%, 30%, and 40%, respectively, with stirring for 15 minutes after each addition. After the addition was complete, stirring continued for 45 minutes. The solution was centrifuged at 4000 rpm for 15 minutes to obtain 12.6 g (wet weight) of crude crystals and approximately 1500 mL of salting-out mother liquor.

[0030] Adsorption and purification: Dissolve the crude crystals in deionized water to 100 mL (methionine concentration approximately 110 g / L). Add 7 g of wood-based activated carbon (modified by soaking in 8% hydrochloric acid for 3 h, washing with water to neutralize, drying at 108 °C for 2.5 h, soaking in 4% aminosilane for 1.5 h, and drying at 85 °C for 1.5 h), stir and adsorb at 35 °C for 30 minutes, filter, and obtain 95 mL of purified solution.

[0031] Membrane separation purification: The purified solution was sequentially passed through ultrafiltration (molecular weight cutoff 3000 Da, 0.3 MPa, 35℃) and nanofiltration (molecular weight cutoff 150 Da, 1.0 MPa, 35℃). The concentrate volume was 45 mL, and the methionine concentration was 180 g / L. The nanofiltration permeate (approximately 50 mL) was combined with the salting-out mother liquor.

[0032] Crystallization and drying: The concentrated solution was adjusted to pH 5.75 with glacial acetic acid and allowed to stand at 2°C for 18 hours to crystallize. After centrifugation at 4500 rpm for 20 minutes, the crystals were dried under vacuum at 65°C and -0.09 MPa for 5 hours to obtain 4.85 g of white crystalline powder.

[0033] Recovery cycle: Combine the salting-out mother liquor (1500 mL) and nanofiltration permeate (50 mL), distill at atmospheric pressure, collect the fraction at 82 °C, and recover 1460 mL of ethanol (recovery rate 96.5%). Concentrate and crystallize the distillation residue to recover potassium chloride (salting-out agent dissolved during previous salting-out, recovery rate 90.8%), and obtain crude sodium sulfate, which is washed with an ethanol-water mixture and dried to obtain the sodium sulfate product.

[0034] Performance testing: The purity of the methionine product (HPLC external standard method) is 99.7%.

[0035] Total methionine recovery rate: 4.85g / (2.85g / L × 0.98L initial methionine amount ≈ 2.793g) × 100% = 96.2%.

[0036] The purity of sodium sulfate by-product (gravimetric method) is 99.2%, with 0.03% water-insoluble matter, 0.001% iron, and 0.15% moisture, conforming to Class I Grade 1 of GB / T6009-2014.

[0037] Ethanol recovery rate: 96.5%.

[0038] Example 2 This embodiment provides a methionine separation method based on ethanol salting-out coupled adsorption-membrane separation. The raw material is crude sodium sulfate residue (from the same chemical enterprise, different batches), which contains 1.5% methionine, 88.2% sodium sulfate, water, and small amounts of peptides, pigments, etc.

[0039] The separation method includes the following steps: Pretreatment: Take 100g of crude sodium sulfate residue, add 900mL of deionized water, stir to dissolve, and adjust the pH to 6.0 with 1M NaOH. Add 0.2g of polyaluminum chloride (PAC) and stir to flocculate for 15 minutes. Filter through a 0.30μm precision filter at a pressure of 0.25MPa and a temperature of 30℃ to obtain 975mL of pretreated clear liquid (methionine concentration approximately 1.54g / L).

[0040] Ethanol salting-out enrichment: The pretreated solution was placed in a jacketed reactor, stirred at 200 rpm, and the temperature was controlled at 15℃. 95% ethanol was added in four portions, each time to achieve a volume fraction of 10%, 25%, 35%, and 45%, respectively, with stirring for 10 minutes after each addition. After the addition was complete, stirring continued for 60 minutes. The solution was centrifuged at 3500 rpm for 20 minutes to obtain 7.8 g (wet weight) of crude crystals and approximately 1600 mL of salting-out mother liquor.

[0041] Adsorption and purification: Dissolve the crude crystals in deionized water to 70 mL (methionine concentration approximately 75 g / L). Add 0.7 g of unmodified wood-based activated carbon (i.e., omitting the modification step), stir at 40 °C for 30 minutes to adsorb, filter, and obtain 66 mL of purified solution.

[0042] Membrane separation purification: The purified solution was sequentially passed through ultrafiltration (molecular weight cutoff 5000 Da, 0.25 MPa, 30℃) and nanofiltration (molecular weight cutoff 200 Da, 1.2 MPa, 30℃). The concentrate volume was 30 mL, with a methionine concentration of approximately 140 g / L. The nanofiltration permeate (approximately 36 mL) was combined with the salting-out mother liquor.

[0043] Crystallization and drying: The concentrated solution was adjusted to pH 5.70 with glacial acetic acid and allowed to stand at 5°C for 24 hours to crystallize. After centrifugation at 4000 rpm for 20 minutes, the crystals were dried under vacuum at 60°C and -0.095 MPa for 6 hours to obtain 2.43 g of white crystalline powder.

[0044] Recovery cycle: Combine the salting-out mother liquor (1600 mL) and nanofiltration permeate (36 mL), distill at atmospheric pressure, collect the fraction at 82 °C, and recover 1490 mL of ethanol (recovery rate 95.2%). Concentrate and crystallize the distillation residue to recover potassium chloride (recovery rate 88.5%), and obtain crude sodium sulfate, which is washed with an ethanol-water mixture and dried to obtain the sodium sulfate product.

[0045] Performance testing: Methionine purity (HPLC external standard method): 99.5%.

[0046] Total methionine recovery rate: 2.43g / (1.54g / L × 0.975L = 1.5015g) × 100% = 95.8%.

[0047] The purity of sodium sulfate by-product (gravimetric method) is 99.0%, with 0.04% water-insoluble matter, 0.002% iron, and 0.18% moisture, conforming to Class I Grade 1 of GB / T6009-2014.

[0048] Ethanol recovery rate: 95.2%.

[0049] Comparative Example 1 This comparative example provides a method for separating methionine using a single salting-out method. The raw material is the same batch of crude sodium sulfate residue as in Example 1, with a methionine content of 2.8% and a sodium sulfate content of 85.5%.

[0050] The separation method includes the following steps: Pretreatment: Take 100g of crude sodium sulfate residue, add 900mL of deionized water, stir to dissolve, and adjust the pH to 6.5 with 1M NaOH. Add 0.3g of polyaluminum chloride (PAC) and stir to flocculate for 12 minutes. Use a 0.22μm ceramic membrane for cross-flow filtration at a pressure of 0.2MPa and a temperature of 35℃ to obtain 980mL of pretreated clear liquid (methionine concentration approximately 2.85g / L).

[0051] Ethanol salting-out enrichment: The pretreated solution was placed in a jacketed reactor, stirred at 250 rpm, and the temperature was controlled at 18℃. 95% ethanol was added in three portions, each time to achieve a volume fraction of 15%, 30%, and 40%, respectively, with stirring for 15 minutes after each addition. After the addition was complete, stirring continued for 45 minutes. The solution was centrifuged at 4000 rpm for 15 minutes to obtain 12.6 g (wet weight) of crude crystals and approximately 1500 mL of salting-out mother liquor.

[0052] Direct drying: The crude crystals were washed three times with deionized water (50 mL each time), and then dried under vacuum at 65℃ and -0.09 MPa for 5 hours to obtain 5.8 g of pale yellow powder.

[0053] Performance testing: The purity of the methionine product (HPLC external standard method) was 78.4%.

[0054] Methionine recovery rate: 5.8g×78.4% / 2.793g×100%=72.1%.

[0055] Sodium sulfate byproducts: qualified products cannot be obtained, and the washing liquid contains a large amount of methionine, which cannot be utilized as a resource.

[0056] Ethanol recovery: Not performed.

[0057] Comparative Example 2 This comparative example provides a method for methionine separation that involves ethanol-free salting-out enrichment, direct adsorption, and membrane separation. The raw material is the same batch of crude sodium sulfate residue as in Example 1, with a methionine content of 2.8% and a sodium sulfate content of 85.5%.

[0058] The separation method includes the following steps: Pretreatment: Take 100g of crude sodium sulfate residue, add 900mL of deionized water, stir to dissolve, and adjust the pH to 6.5 with 1M NaOH. Add 0.3g of polyaluminum chloride (PAC) and stir to flocculate for 12 minutes. Use a 0.22μm ceramic membrane for cross-flow filtration at a pressure of 0.2MPa and a temperature of 35℃ to obtain 980mL of pretreated clear liquid (methionine concentration approximately 2.85g / L).

[0059] Direct adsorption for impurity removal: 7g of modified wood-based activated carbon (modification method as in Example 1) was added to the pretreated clear liquid (volume approximately 980mL), and the mixture was stirred and adsorbed at 35℃ for 30 minutes. After filtration, 960mL of purified liquid was obtained (methionine concentration approximately 2.70g / L, with an adsorption loss of approximately 5%).

[0060] Membrane separation purification: The purified solution was sequentially passed through ultrafiltration (molecular weight cutoff 3000 Da, 0.3 MPa, 35℃) and nanofiltration (molecular weight cutoff 150 Da, 1.0 MPa, 35℃). Due to the low initial concentration, it needed to be concentrated to a volume of approximately 50 mL, which was time-consuming. 50 mL of concentrated solution was obtained, with a methionine concentration of approximately 48 g / L. The nanofiltration permeate (approximately 910 mL) contained a large amount of sodium sulfate and trace amounts of methionine.

[0061] Crystallization and drying: The concentrated solution was adjusted to pH 5.75 with glacial acetic acid and allowed to stand at 2°C for 18 hours to crystallize. After centrifugation at 4500 rpm for 20 minutes, the crystals were dried under vacuum at 65°C and -0.09 MPa for 5 hours to obtain 1.83 g of white crystalline powder.

[0062] Recovery cycle: The nanofiltration permeate (910 mL) was distilled at atmospheric pressure to recover ethanol (but no ethanol was added in this example, so no ethanol was recovered). The distillation residue contained sodium sulfate, but the purity was insufficient.

[0063] Performance testing: The purity of the methionine product (HPLC external standard method) is 92.3%.

[0064] Total methionine recovery rate: 1.83g / 2.793g×100%=65.4%.

[0065] The purity of sodium sulfate by-product is 86.5% (which does not meet industrial-grade standards).

[0066] Membrane fouling: The flux of the nanofiltration membrane decreased to 30% of its initial value after 4 hours of operation, requiring frequent cleaning.

[0067] Comparative Example 3 This comparative example provides a membrane-free separation method for methionine separation using only ethanol salting-out and adsorption. The raw material is the same batch of crude sodium sulfate residue as in Example 1, with a methionine content of 2.8% and a sodium sulfate content of 85.5%.

[0068] The separation method includes the following steps: Pretreatment: Take 100g of crude sodium sulfate residue, add 900mL of deionized water, stir to dissolve, and adjust the pH to 6.5 with 1M NaOH. Add 0.3g of polyaluminum chloride (PAC) and stir to flocculate for 12 minutes. Use a 0.22μm ceramic membrane for cross-flow filtration at a pressure of 0.2MPa and a temperature of 35℃ to obtain 980mL of pretreated clear liquid (methionine concentration approximately 2.85g / L).

[0069] Ethanol salting-out enrichment: The pretreated solution was placed in a jacketed reactor, stirred at 250 rpm, and the temperature was controlled at 18℃. 95% ethanol was added in three portions, each time to achieve a volume fraction of 15%, 30%, and 40%, respectively, with stirring for 15 minutes after each addition. After the addition was complete, stirring continued for 45 minutes. The solution was centrifuged at 4000 rpm for 15 minutes to obtain 12.6 g (wet weight) of crude crystals and approximately 1500 mL of salting-out mother liquor.

[0070] Adsorption and purification: The crude crystals were dissolved in deionized water to a final volume of 100 mL (methionine concentration approximately 110 g / L). 7 g of modified wood-based activated carbon was added (modification method as in Example 1), and the mixture was stirred at 35°C for 30 minutes for adsorption. After filtration, 95 mL of purified solution was obtained.

[0071] Direct crystallization and drying: The purified solution (95 mL, methionine concentration approximately 105 g / L) was adjusted to pH 5.75 with glacial acetic acid and allowed to crystallize at 2°C for 18 hours. After centrifugation at 4500 rpm for 20 minutes, the crystals were dried under vacuum at 65°C and -0.09 MPa for 5 hours to obtain 3.8 g of white crystalline powder.

[0072] Recovery and recycling: The salting-out mother liquor (1500 mL) was distilled at atmospheric pressure to recover ethanol. The fraction at 82℃ was collected, and 1420 mL of ethanol was recovered (recovery rate 93.7%). The distillation residue was concentrated and crystallized to recover crude sodium sulfate.

[0073] Performance testing: Methionine purity (HPLC external standard method): 94.1% (with trace amounts of sodium sulfate and pigments remaining).

[0074] Total methionine recovery rate: 3.8g / 2.793g×100%=90.5%.

[0075] The purity of sodium sulfate by-product is 81.3% (which does not meet industrial-grade standards).

[0076] Ethanol recovery rate: 93.7%.

[0077] Comparative Example 4 This comparative example provides a method for separating methionine using unmodified activated carbon for adsorption. The raw material is the same batch of crude sodium sulfate residue as in Example 1, with a methionine content of 2.8% and a sodium sulfate content of 85.5%.

[0078] The separation method includes the following steps: Pretreatment: Take 100g of crude sodium sulfate residue, add 900mL of deionized water, stir to dissolve, and adjust the pH to 6.5 with 1M NaOH. Add 0.3g of polyaluminum chloride (PAC) and stir to flocculate for 12 minutes. Use a 0.22μm ceramic membrane for cross-flow filtration at a pressure of 0.2MPa and a temperature of 35℃ to obtain 980mL of pretreated clear liquid (methionine concentration approximately 2.85g / L).

[0079] Ethanol salting-out enrichment: The pretreated solution was placed in a jacketed reactor, stirred at 250 rpm, and the temperature was controlled at 18℃. 95% ethanol was added in three portions, each time to achieve a volume fraction of 15%, 30%, and 40%, respectively, with stirring for 15 minutes after each addition. After the addition was complete, stirring continued for 45 minutes. The solution was centrifuged at 4000 rpm for 15 minutes to obtain 12.6 g (wet weight) of crude crystals and approximately 1500 mL of salting-out mother liquor.

[0080] Adsorption and purification: Dissolve the crude crystals in deionized water to 100 mL (methionine concentration approximately 110 g / L). Add 7 g of unmodified wood-based activated carbon (untreated with hydrochloric acid and aminosilane), stir and adsorb at 35 °C for 30 minutes, filter, and obtain 94 mL of purified solution.

[0081] Membrane separation purification: The purified solution was sequentially passed through ultrafiltration (molecular weight cutoff 3000 Da, 0.3 MPa, 35℃) and nanofiltration (molecular weight cutoff 150 Da, 1.0 MPa, 35℃). The concentrate volume was 45 mL, and the methionine concentration was 172 g / L. The nanofiltration permeate (approximately 49 mL) was combined with the salting-out mother liquor.

[0082] Crystallization and drying: The concentrated solution was adjusted to pH 5.75 with glacial acetic acid and allowed to stand at 2°C for 18 hours to crystallize. After centrifugation at 4500 rpm for 20 minutes, the crystals were dried under vacuum at 65°C and -0.09 MPa for 5 hours to obtain 4.68 g of white crystalline powder.

[0083] Recovery and recycling: Combine the salting-out mother liquor (1500 mL) and nanofiltration permeate (49 mL), distill at atmospheric pressure, collect the fraction at 82 °C, and recover 1455 mL of ethanol (recovery rate 96.0%). Concentrate and crystallize the distillation residue to recover potassium chloride (recovery rate 90.2%), and obtain crude sodium sulfate. After washing with an ethanol-water mixture and drying, obtain the sodium sulfate product.

[0084] Performance testing: The purity of the methionine product (HPLC external standard method) is 98.2%.

[0085] Total methionine recovery rate: 4.68g × 98.2% / 2.793g × 100% = 94.1%.

[0086] The purity of the sodium sulfate by-product is 98.5%, with 0.07% water-insoluble matter, 0.003% iron, and 0.22% moisture, conforming to Class II Grade 1 of GB / T6009-2014.

[0087] Ethanol recovery rate: 96.0%.

[0088] As can be seen from the above examples and comparative examples, the ethanol salting-adsorption-membrane separation coupling process provided by the present invention, especially when combined with modified activated carbon adsorption, low temperature salting-adsorption and flocculation pretreatment, can significantly improve the recovery rate (≥95%) and purity (≥99.5%) of methionine, while enabling the by-product sodium sulfate to reach the industrial grade Class I first-class standard, thus realizing the resource utilization of waste residue.

[0089] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for separating methionine based on ethanol salting-out coupled adsorption-membrane separation, characterized in that, Includes the following steps: Step 1, Pretreatment: Dissolve the crude sodium sulfate residue containing methionine and sodium sulfate in water, adjust the pH to 6.0-7.0, add flocculant for flocculation and sedimentation, filter, and obtain pretreated clear liquid; Step 2, Ethanol salting-out enrichment: Add ethanol stepwise to the pretreated clear liquid obtained in Step 1 under stirring until the final volume fraction is 30-50%, and carry out salting-out at a temperature of 10-30℃. Centrifuge to obtain coarse crystals and salting-out mother liquor. Step 3, adsorption and impurity removal: Dissolve the crude crystals obtained in step 2 in water, add activated carbon for adsorption, decolorization and impurity removal, filter to remove the activated carbon, and obtain the adsorption and purification solution; Step 4, membrane separation and purification: The adsorption purified liquid obtained in step 3 is sequentially separated and purified by ultrafiltration membrane and nanofiltration membrane to obtain methionine concentrate and nanofiltration permeate; Step 5, Crystallization and Drying: Adjust the pH of the methionine concentrate obtained in Step 4, perform low-temperature crystallization, centrifuge, and vacuum dry to obtain the finished methionine product. Step 6, Solvent and Salting-out Agent Recovery: The salting-out mother liquor from Step 2 and the nanofiltration permeate from Step 4 are combined, and ethanol is recovered by distillation. The distillation residue is concentrated and crystallized to recover the salting-out agent and the by-product sodium sulfate.

2. The methionine separation method based on ethanol salting-out coupled adsorption-membrane separation according to claim 1, characterized in that, In step 1, the methionine content in the crude sodium sulfate residue is 1-5% by mass. The flocculant is at least one of polyaluminum chloride and polyacrylamide, the flocculant dosage is 0.1-0.5 g / L, and the flocculation time is 10-30 minutes; The filtration uses microfiltration membranes or precision filters with a pore size of 0.1-0.5μm, with an operating pressure of 0.1-0.3MPa and a temperature of 20-40℃.

3. The methionine separation method based on ethanol salting-out coupled adsorption-membrane separation according to claim 1, characterized in that, In step 2, the ethanol is ethanol with a volume concentration ≥ 90%; Ethanol is added in 2-4 portions, stirring for 10-20 minutes after each addition. The salting-out temperature is 15-25℃, the stirring speed is 100-300rpm, and the total salting-out time is 30-90 minutes; The centrifugation speed is 3000-5000 rpm.

4. The methionine separation method based on ethanol salting-out coupled adsorption-membrane separation according to claim 1, characterized in that, In step 3, the activated carbon is wood-based activated carbon, coal-based activated carbon, or fruit shell activated carbon, and its addition amount is 5-15 g / L of the solution volume. The adsorption temperature is 30-50℃, and the adsorption time is 20-60 minutes.

5. The methionine separation method based on ethanol salting-out coupled adsorption-membrane separation according to claim 4, characterized in that, The activated carbon is modified activated carbon. The modification method is as follows: soak the activated carbon in an inorganic acid solution with a mass fraction of 5-10% for 2-5 hours, wash it with water until neutral, dry it, then soak it in an aminosilane coupling agent with a mass fraction of 2-5% for 1-3 hours, and dry it at 80-100℃ for 1-2 hours.

6. The methionine separation method based on ethanol salting-out coupled adsorption-membrane separation according to claim 1, characterized in that, In step 4, the ultrafiltration membrane has a molecular weight cutoff of 2000-5000 Da, an operating pressure of 0.2-0.5 MPa, and an operating temperature of 25-40℃. The nanofiltration membrane has a molecular weight cutoff of 100-200 Da, an operating pressure of 0.8-1.5 MPa, and an operating temperature of 25-40℃.

7. The methionine separation method based on ethanol salting-out coupled adsorption-membrane separation according to claim 1, characterized in that, In step 5, the pH is 5.6-5.8; the low-temperature crystallization temperature is 0-10℃; and the crystallization time is 12-24 hours. The vacuum drying temperature is 55-70℃, the vacuum degree is -0.08~-0.095MPa, and the drying time is 4-8 hours.

8. The methionine separation method based on ethanol salting-out coupled adsorption-membrane separation according to claim 1, characterized in that, In step 6, the distillation temperature is 78-85℃, and the ethanol recovery rate is ≥95%.

9. The salting-out agents recovered from the distillation residue through concentration and crystallization include sodium sulfate or potassium chloride, with a recovery rate of ≥90%, and after purification, they meet industrial-grade standards.