Environmentally friendly preparation method of methionine based on carbon dioxide acidification method

Abstract

The present invention relates to the field of methionine production, and in particular to an environmentally friendly preparation method of methionine based on a carbon dioxide acidification method. Compared with the prior art, the environmentally friendly preparation method of methionine based on a carbon dioxide acidification method of the present invention is more environmentally friendly, economical and safe, and increases the single yield by extracting methionine in the crystallization mother liquor, increases the amount of single hydrolysis of hydantoin in subsequent cycles by extracting alkaline potassium twice, and enriches formic acid and oil phase impurities and then extracts them separately, so that the hydrolysis cycle can be performed more times, and impurities in the saponified liquid in the DL-methionine production process are effectively removed, and a large amount of accumulation will not be generated in the continuous production process of methionine, affecting the quality of methionine.

Description

Technical Field

[0001] The invention relates to the field of methionine production, and in particular to an environmentally friendly preparation method of methionine based on a carbon dioxide acidification method. Background Art

[0002] D,L-methionine, as an amino acid feed additive in feed, is an animal growth promoter used for protein synthesis, which can improve feed conversion rate, animal growth rate and poultry egg production rate. It can promote the growth of poultry and livestock, increase lean meat, and shorten the feeding cycle. Animal feed with methionine can help animals grow rapidly in a short time and save about 40% of feed.

[0003] At present, the production methods of D,L-methionine mainly include chemical synthesis, biological fermentation, and enzyme catalysis, among which chemical synthesis is the most common. The known methionine production process is to synthesize 5-(β-methylmercaptoethyl)hydantoin, also referred to as hydantoin or hydantoin, from 3-methylmercaptopropionaldehyde, hydrocyanic acid, ammonia, and carbon dioxide as raw materials. Hydantoin is hydrolyzed with alkali to obtain a hydrolyzate of a mixture of methionine salt and carbonate, and the hydrolyzate is acidified to obtain methionine and filtrate. The filtrate contains a large amount of useful components and must be returned to the cycle after treatment; the difference in acidification medium and alkali determines the difference in the return link of the filtrate, and the filtrate is either returned to the hydrolysis step of hydantoin or returned to the crystallization step.

[0004] Hydantoin is an essential intermediate in the mainstream production process of methionine. Hydantoin hydrolysis can use alkalis such as potassium carbonate, sodium carbonate, sodium hydroxide, calcium hydroxide, potassium bicarbonate, and sodium bicarbonate. The methionine production process is relatively long. No matter which alkali is used to hydrolyze hydantoin, the generation of impurities is inevitable, including formic acid, acrolein polymer derivatives, 3-methylthiopropionaldehyde polymer derivatives, methionine hydroxyl analogs, methionine derivative precursors, and polycarboxylic acid impurities. These impurities are similar to the structure of methionine: either carbon chain or amino or carboxyl, so most impurities have a similar effect of similar miscibility with methionine and are difficult to be separated from methionine more thoroughly. During the cycle, the quality of the extracted methionine will decrease and even affect the yield of the reaction, so the separation of impurities is a necessary operation.

[0005] Patent publication CN106748932A uses a microporous membrane filter to remove polymers from a mixed solution of methionine and bicarbonate, and then uses a cation exchange resin to separate the mixture into a methionine solution and a sulfate solution, and uses electrodialysis or stripping / or an anion exchange resin to separate formic acid from the methionine solution. First, the impurities in the crystallization mother liquor are not enriched, and it is a waste to directly use a cation resin to convert alkaline potassium into neutral potassium; second, the impurities in the crystallization mother liquor are still small molecules and have viscosity, and it is difficult for the microporous membrane filter to filter out these impurities and even cause blockage.

[0006] Patent CN102796033B uses an organic solvent to extract the hydrolyzate, extracts part of the impurities into the organic phase, and the organic phase is recycled after distillation, and the residual liquid is treated as waste liquid. First, the concentration of methionine salts and carbonates in the saponified liquid is high, and the organic solvent mixes part of the saponified liquid, resulting in the waste of part of the product; second, the organic solvent is partially dissolved in the saponified liquid, resulting in the accumulation of subsequent organic solvents; third, the saponified liquid has a low impurity ratio, the extraction efficiency is low, and this part of the impurities is basically 3-methylthiopropionaldehyde polymer and is immiscible with the water phase, and TPMA dimer can be controlled to a very low level in the preparation of 5-(2-methylthioethyl)-hydantoin; fourth, the saponified liquid is strongly alkaline, carboxyl impurities are more soluble in the water phase, and the organic extraction effect is not obvious, but this impurity is one of the main components. Comprehensive organic solvent extraction of saponified liquid can only extract a very small amount of impurities in the saponified liquid, and no operation plan is proposed for the enrichment of formic acid.

[0007] Patent CN110678445A uses electrodialysis to remove anions containing 1 to 5 carbon atoms from the mother liquor of methionine crystallization. Electrodialysis cannot achieve this goal well within the acceptable time and energy consumption of industrial production; methionine will enter the anolyte due to osmotic pressure or potential difference, and the anolyte still needs further treatment; in addition, the impurities in the mother liquor of crystallization are enriched, which may cause blockage of the membrane, and ultimately lead to poor efficiency or even collapse of electrodialysis.

[0008] CN401693082A points out that methionine will produce impurities such as dimethyl disulfide, propyleneamine, and 3-methylthiopropylamine during heating, but does not mention the heating time, number of times, and temperature. Alkali metals or alkaline earth metals are used in the hydrolysis of hydantoin. The salt and methionine mixture generated by acidification have different contents and solubilities, so there is a limit on the number of times of subsequent concentration, so it has to be concentrated repeatedly. In order to reduce the number of concentrations, methionine practitioners have proposed the following solution.

[0009] In CN107531621A, the hydrolyzate is first concentrated to extract part of the sodium carbonate and then used for the hydrolysis of hydantoin, and then acidified with sulfuric acid to extract methionine, which can reduce the consumption of alkaline sodium and sulfuric acid for hydrolysis and also reduce the production of sodium sulfate. However, sodium carbonate is easily soluble in water, and it is not easy to concentrate the hydrolyzate to a high multiple and then filter it. Concentration requires a lot of energy, and there is no mention of impurity treatment.

[0010] In CN104693082A, chromatographic separation technology is used to separate a mixed solution of sodium sulfate and methionine, and in CN106432018A, chromatographic separation technology is used to separate a mixed solution of potassium bicarbonate and methionine. However, there are the following defects: the chromatographic separation technology requires the introduction of a large amount of water, which reduces the concentration of the methionine solution; the chromatographic separation investment is large, the separation efficiency is low, and the most important thing is that the impurities generated in the preparation of methionine are not processed, and these impurities (relative to the impurities generated by thermal decomposition) are the ones that need to be processed in the first place; in addition, the separation effect of the chromatographic separation on bicarbonate and methionine is worth considering.

[0011] In CN109734637A, CN103922980A, CN103933861A, CN106432020B, CN104130169A, CN103922980A, and CN105671587A, bipolar membranes are used to separate methionine from carbonate / bicarbonate mixed solutions into methionine and alkali solution. However, there are also the following defects: first, methionine has limited solubility and must be diluted with water, and the separated methionine solution needs to be concentrated and crystallized or crystallized at low temperature; second, the mixed solution contains a lot of impurities, which affects the electronic utilization efficiency of the bipolar membrane and the life of the membrane itself; third, the low concentration of alkali produced by the bipolar membrane still needs to be concentrated for use; fourth, the generated carbon dioxide is a neutral molecule, which leads to an increase in solution resistance, low electrodialysis efficiency, and high operating costs for the main process flow equipment investment.

[0012] In CN1923807A, CN101602700A, CN101602701A and CN110536881A, the hydrolyzate is acidified with carbon dioxide to extract methionine and mother liquor, a part of the mother liquor is concentrated and then a lower alcohol is added to separate the mixture of methionine and potassium carbonate and returned to the hydrolysis of hydantoin, and the secondary mother liquor is further treated and discarded. However, the introduction of lower alcohols makes the system have residual recovery, which is also a problem.

[0013] In summary, the treatment can be divided into two ideas: extracting impurities or extracting methionine / salt. Because methionine and salt have a large solubility in water, extracting impurities is an option. However, the known disclosed patents still have the following problems:

[0014] (1) The treatment of impurities generated during methionine production is not clearly mentioned;

[0015] (2) Using high-cost and high-energy-consuming equipment and technology to separate methionine and salts in order to reduce the small amount of impurities generated in subsequent processing;

[0016] (3) Treatment of methionine crystallization mother liquor by introducing lower alcohol or coagulant;

[0017] (4) After the methionine crystallization mother liquor is treated, a portion of the crystallization mother liquor still needs to be discarded, and the beneficial substances cannot be recovered;

[0018] (5) Part of the crystallization mother liquor is discarded and processed into low-value substances using beneficial resources, which increases the cost twice. Summary of the invention

[0019] In order to solve the problems mentioned in the background technology, the purpose of the present invention is to provide a method for preparing methionine, which solves the subsequent treatment problem of alkaline potassium hydrolysis of hydantoin by separately treating methionine, potassium carbonate, potassium bicarbonate, methionine derivatives, formic acid and oil phase impurities produced in methionine production.

[0020] The key to the environmentally friendly preparation method of methionine based on the carbon dioxide acidification method is to include the following steps:

[0021] S1. The alkaline potassium hydrolyzate of 5-(2-methylmercaptoethyl)-hydantoin is acidified with carbon dioxide and separated into crude methionine and crystallization mother liquor;

[0022] S2. The crystallization mother liquor is concentrated, and when the degree of dissociation of potassium bicarbonate in the system is 0.3-0.6, part of the concentrate is diverted, and the remaining concentrate is returned to 5-(2-methylthioethyl)-hydantoin hydrolysis;

[0023] S3. The concentrated liquid of the split stream is cooled and crystallized to separate the filtrate and potassium bicarbonate;

[0024] S4. The filtrate is acidified with carbon dioxide and separated into crude methionine and impurity-enriched liquid.

[0025] Preferably, water is added to the crude methionine in S1 for mixing and beating, and methionine is obtained by separation and drying, and the beating water is combined with the crystallization mother liquor.

[0026] Preferably, the concentration temperature of the crystallization mother liquor in S2 is 100-180°C.

[0027] Preferably, the concentrated liquid diverted in S2 accounts for 10-30% of the total concentrated liquid.

[0028] Preferably, the remaining concentrated liquid in S2 is thermally decomposed at 160-200° C., potassium bicarbonate is thermally decomposed into potassium carbonate and potassium methionine, and then returned to hydrolyze 5-(2-methylmercaptoethyl)-hydantoin.

[0029] Preferably, the mass ratio of potassium methionine to water in the concentrated solution diverted in S3 is a, 0.15≤a≤1; more preferably, 0.25≤a≤1.

[0030] Preferably, the crystallization conditions in S3 are: keeping warm at 10-30° C. for 30-360 min.

[0031] Preferably, the crude methionine of S4 is returned to the crystallization mother liquor of S1 to form a secondary mother liquor, a portion of the secondary mother liquor not containing methionine solids is extracted to repeat S3-S4, and the remaining secondary mother liquor containing methionine solids is returned to the acidification of the hydrolyzate in S1.

[0032] Preferably, the impurity-enriched liquid in S4 is separated into methionine solution and potassium sulfate using a hydrogen-type cationic resin.

[0033] Preferably, after the methionine solution is acidified, impurities in the oil phase are removed by layering or organic solvent extraction, formic acid is removed by reduced pressure distillation, and finally the methionine solution, potassium hydroxide solution and sulfuric acid solution are separated from the deacidified aqueous phase.

[0034] Beneficial effects: Compared with the prior art, the environmentally friendly preparation method of methionine based on the carbon dioxide acidification method of the present invention is more environmentally friendly, economical and safe. By extracting methionine from the crystallization mother liquor to increase the single yield, by extracting alkaline potassium twice to increase the amount of single-cycle hydrolysis of hydantoin, by enriching formic acid and oil phase impurities and then extracting them separately, the hydrolysis cycle can be performed more times, and the impurities in the saponification liquid in the production process of DL-methionine are effectively removed, and a large amount of accumulation will not be generated in the continuous production process of methionine, which will affect the quality of methionine. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0036] The present invention is described in detail below by specific examples. It is pointed out that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above invention content. Unless otherwise specified, the parts described in the present invention are all weight parts, the percentages described are all mass percentages, and the concentrations described are mass percentage concentrations.

[0037] Example 1 Environmentally friendly preparation method of methionine based on carbon dioxide acidification method

[0038] S1.5-(2-methylmercaptoethyl)-hydantoin is hydrolyzed with alkaline potassium to obtain a hydrolyzate, and carbon dioxide is introduced at a pressure of 3 to 10 bar and a temperature of 10 to 40°C for 10 to 100 minutes. In actual operation, the pressure can be 3 bar, 4 bar, 5 bar, 6 bar, 7 bar, 8 bar, 9 bar, and 10 bar, the temperature can be 10°C, 20°C, 30°C, and 40°C, and the insulation time can be 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, and 100 min. The hydrolyzate is centrifuged to separate into methionine and a crystallization mother liquor, and the methionine is pulped with water and then separated and dried, and the pulping water is incorporated into the crystallization mother liquor; the potassium bicarbonate embedded deep in the methionine is washed out by pulping, and the methionine content is greater than 99wt% after separation and drying;

[0039] S2. The crystallization mother liquor is concentrated at 100-180°C. According to the actual production situation, the concentration temperature can be selected from 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, and 180°C. When the degree of dissociation of potassium bicarbonate in the system is detected to be 0.3-0.6, 10-30%wt of the concentrated solution is diverted, and the system is kept in normal operation by controlling the split ratio of the concentrated solution. The yield of hydantoin hydrolysis and the quality of methionine do not decrease significantly; During actual operation, the split ratio can be selected from 10%wt, 20%wt, and 30%wt, and the remaining concentrated solution continues to be thermally decomposed at 160-200°C. The thermal decomposition temperature can be selected from 160°C, 170°C, 180°C, 190°C, and 200°C, potassium bicarbonate is thermally decomposed into potassium carbonate and potassium methionine, and then returned to 5-(2-methylmercaptoethyl)-hydantoin hydrolysis;

[0040] S3. Detect the water content of the shunt concentrate. Since potassium bicarbonate has a high solubility, the water content of the shunt concentrate is controlled so that it can only dissolve potassium methionine, thereby maintaining a high precipitation rate of potassium bicarbonate. When the mass ratio of potassium methionine to water is a, 0.15≤a≤1, the temperature is lowered to 10-30°C and kept warm for 30-360min, and the filtrate and potassium bicarbonate are obtained by crystallization separation. In actual operation, the mass of potassium methionine / water can be controlled to be 0.15-0.45, 0.35-0.65, 0.55-0.85, 0.75-1, the temperature can be selected to be 10°C, 20°C, 30°C, and the insulation time can be selected to be 30min, 60min, 90min, 120min, 160min, 200min, 240min, 280min, 320min, 360min; the ratio of potassium bicarbonate extracted to supplement alkaline potassium. In actual operation, the concentrated solution to be regenerated is used to dissolve it and then regenerate it, which can significantly increase the molar ratio of alkaline potassium / methionine in the regenerated solution, thereby hydrolyzing more hydantoin in a single time;

[0041] S4. The filtrate is kept warm at 2-5 bar and 5-30° C. for 10-100 min, and the crude methionine and impurity-enriched liquid are obtained by acidification extraction using carbon dioxide. In actual operation, the pressure can be selected from 2 bar, 3 bar, 4 bar, and 5 bar, the temperature can be selected from 5° C., 10° C., 15° C., 20° C., 25° C., and 30° C., and the insulation time can be selected from 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, and 100 min;

[0042] The crude methionine product of S4 is returned to the crystallization mother liquor of S1 to form a secondary mother liquor, and the potassium bicarbonate in the crude methionine product is dissolved in the crystallization mother liquor. In the specific implementation, the secondary mother liquor is divided into two parts, and the secondary mother liquor without methionine is extracted by a leaf filter for subsequent steps S2-S4. The secondary mother liquor containing methionine is mixed with the next hydrolyzate acidification system and then separated. The ratio of the two parts of the crystallization mother liquor can be selected: 1:9, 2:8, 3:7 or 4:6; the crude methionine product is a mixture of potassium bicarbonate and methionine, which is washed by the crystallization mother liquor and returned to the circulation system, which can reduce the impurity concentration and increase the molar ratio of alkaline K / methionine in the circulating solution, so that more hydantoin can be hydrolyzed in a single cycle, reducing energy consumption and reducing impurities due to the increase in the number of cycles;

[0043] At 50-100°C, the impurity-enriched liquid is separated into a methionine solution and a potassium-type resin by a hydrogen-type cationic resin. The potassium-type resin is regenerated into a hydrogen-type cationic resin and a potassium sulfate solution by using sulfuric acid. The potassium sulfate solution is concentrated to obtain potassium sulfate. In actual use, hydrogen-type sulfonic acid or carboxylic acid resins of models D110, D151, D152, D113 or D001 can be selected to separate the impurity-enriched liquid;

[0044] The methionine solution is adjusted to pH < 3 using sulfuric acid, and the adjusted methionine solution is extracted using an organic solvent. In actual use, the organic solvent can be selected from alcohols with 5 carbon atoms or more. The removal rate of methylthio impurities can reach more than 98% by extraction, and the methionine content is basically unaffected.

[0045] After extraction, the organic solvent is evaporated to extract pure solvent for recycling, and the residual liquid is treated. In actual use, the pH of the organic solvent after extraction is adjusted to neutral or weakly acidic with alkali, which greatly reduces the corrosion of equipment during evaporation and improves the purity of the evaporated solvent. The methionine solution after extraction is stripped / gas-stripped with steam / carbon dioxide at 60-100°C, and the gas containing formic acid is absorbed by alkali solution and then recycled for gas stripping. The absorbed liquid can be concentrated to prepare formate;

[0046] The methionine solution after the removal of formic acid is separated into methionine solution, sulfuric acid solution and potassium hydroxide solution at 10-45°C using bipolar membrane electrodialysis technology. The electrodialysis operation ends when the pH of the feed liquid chamber becomes 4-6.

[0047] The methionine solution after the removal of formic acid can also be adjusted to pH 4-7 with alkali, and concentrated and separated into methionine and potassium sulfate; it can also be adjusted to pH 4-7 with alkali, and chromatographically separated into methionine solution and potassium sulfate solution at 50-70°C; it can also be adjusted to pH 4-7 with alkali, and electrodialyzed at 10-40°C to separate into methionine solution and potassium sulfate solution; in actual operation, the alkali can be potassium hydroxide, potassium carbonate, potassium bicarbonate, calcium oxide, calcium hydroxide and calcium carbonate. The methionine solution after the removal of formic acid can also be precipitated with alkali for sulfate, and the filtrate after filtration and separation is returned to the hydrolyzate of hydantoin. In actual operation, the alkali can be calcium oxide, calcium hydroxide and calcium carbonate.

[0048] The present invention concentrates and splits the crystallization mother liquor, extracts methionine and alkaline potassium in the split mother liquor, increases the single yield of methionine, increases the amount of hydantoin hydrolyzed in a single cycle, and enriches and removes formic acid and oil phase impurities generated in the continuous production process of methionine, so that the hydrolysis cycle can be performed more times, and the continuous production quality of methionine can be stabilized, and the impurities will not be accumulated due to repeated use of the crystallization mother liquor, thereby affecting the stability of the methionine quality.

[0049] The following provides a specific embodiment of production practice using the technical solution provided by the present invention, and the practical operations are performed in sequence to simulate the process of industrial continuous production using the present invention.

[0050] Example 2-1

[0051] 100 kg of hydrolyzate of 5-(β-methylthioethyl)hydantoin solution hydrolyzed by alkaline potassium, wherein the methionine content is 15.4 wt % and the alkaline potassium ion content is 11 wt %, is introduced into carbon dioxide at 3 bar and 25° C. for 80 min, and then centrifuged to obtain 10.7 kg of methionine and 94 kg of crystallization mother liquor; 10.7 kg of methionine is stirred with 20 kg of water at 25° C. and then separated and dried to obtain white methionine with a content of 99.4 wt %; 94 kg of crystallization mother liquor is concentrated at 150° C. to obtain 44.3 kg of concentrated solution, the potassium bicarbonate dissociation degree is detected to be 0.42, and the potassium methionine / water mass ratio is 0.093, 15 kg of the shunt concentrated solution is shunt, and the remaining 29 kg of the concentrated solution is regenerated and used for 5-(2- 15kg of the split concentrate was further concentrated to 5.6kg, and 0.8kg of potassium bicarbonate and 4.8kg of the filtrate were centrifuged at 20°C for 50min, and the filtrate was acidified by passing carbon dioxide at 20°C and 3bar for 60min, and then centrifuged to obtain 3.5kg of crude methionine and 1.5kg of impurity-enriched liquid; 1.5kg of the impurity-enriched liquid was separated into methionine solution and potassium sulfate solution using D101 resin, the methionine content in the methionine solution was 6wt%, the methionine solution was adjusted to pH 2.3 using 98wt% sulfuric acid, extracted with 0.5kg of isoamyl alcohol, and then stripped with carbon dioxide at 90°C to remove formic acid, and the formic acid removal was completed when the pH was stabilized at 2.7; titration and chromatography were used to detect the acid-adjusted system and the methionine solution, and the removal rate of methylthio impurities was 97%, and the methionine solution changed from black-red to light yellow.

[0052] Example 2-2

[0053] 100 kg of hydantoin hydrolyzate obtained by hydrolyzing 5-(β-methylmercaptoethyl)hydantoin solution with alkaline potassium, wherein the methionine content is 15.4 wt %, the alkaline potassium ion content is 11 wt %, and carbon dioxide is introduced at 3 bar and 25 ° C for 80 min, and then centrifuged to separate into 11 kg of methionine and 90 kg of crystallization mother liquor; 90 kg of crystallization mother liquor is mixed with 17.7 kg of pulping water of Example 2-1, 3.5 kg of crude methionine of Example 2-1 is added and stirred until the potassium bicarbonate in the mixture is completely dissolved to form a secondary mother liquor, 100 kg of secondary mother liquor is extracted (10 kg of secondary mother liquor not extracted is incorporated into the next hydrolyzate acidification system), and concentrated at 140 ° C to 42 kg of concentrated solution, the potassium bicarbonate dissociation degree is detected to be 0.47, and the potassium methionine / water mass ratio is 0.11, 15 kg of the shunt concentrate is shunt, and the remaining 27 kg of the concentrated solution is further shunted. After production, the product is used for the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin, the split concentrate is further concentrated to 6.4 kg, and 1.8 kg of potassium bicarbonate and 4.6 kg of filtrate are centrifuged at 20° C. for 50 min. The filtrate is acidified by passing carbon dioxide at 20° C. and 3 bar for 60 min, and then centrifuged to separate 3.3 kg of crude methionine and 1.6 kg of impurity-enriched liquid. The impurity-enriched liquid is separated into a methionine solution and a potassium sulfate solution using D001 resin, and the pH of the methionine solution is adjusted to 2.3 using 98wt% sulfuric acid. The methionine solution is extracted with 0.6 kg of n-hexanol, and then stripped with carbon dioxide at 90° C. to remove formic acid. When the pH is stabilized at 2.6, the formic acid is completely removed. The acid-adjusted system and the methionine solution are tested by titration and chromatography, and there is substantially no loss of methionine, the removal rate of methylthio impurities is 98%, and the methionine solution changes from red to light yellow.

[0054] Example 2-3

[0055] 100 kg of hydantoin hydrolyzate obtained by hydrolyzing 5-(β-methylthioethyl)hydantoin solution with alkaline potassium, wherein the methionine content is 15.4 wt %, and the alkaline potassium ion content is 11 wt %, is introduced with carbon dioxide at 3 bar and 25° C. for 90 min, and then mixed with 10 kg of the secondary mother liquor of Example 2-2, and centrifuged to obtain 13 kg of methionine and 102 kg of crystallization mother liquor, 13 kg of methionine is slurried and stirred with 20 kg of water at 25° C., and then separated and dried to obtain white methionine with a content of 99.5 wt %; 102 kg of the crystallization mother liquor and 19 kg of Example 2-2 are mixed. The mixture was mixed with pulping water, 3.3 kg of crude methionine of Example 2-2 was added and stirred until the potassium bicarbonate in the mixture was completely dissolved to form a secondary mother liquor, 117 kg of the secondary mother liquor was extracted (the 7 kg of the secondary mother liquor not extracted was incorporated into the next hydrolyzate acidification system), and concentrated at 150 ° C to 50 kg of concentrated solution. The dissociation degree of potassium bicarbonate was detected to be 0.49, and the mass ratio of potassium methionine / water was 0.107. 13 kg of the shunt concentrate was shunt, and the remaining 37 kg of the concentrated solution was regenerated and used for the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin. The shunt concentrate was further concentrated to 5.2 kg, and 2 kg of potassium bicarbonate and 3.2 kg of filtrate were centrifuged at 10 ° C for 50 min. The filtrate was heated at 20 ° C, The method comprises the following steps: introducing carbon dioxide at 3 bar for acidification for 40 minutes, and then centrifuging to obtain 2.7 kg of crude methionine and 0.6 kg of impurity-enriched liquid; using D001 resin to separate the impurity-enriched liquid into a methionine solution and a potassium sulfate solution, and adjusting the pH of the methionine solution to 2.1 using 98 wt % sulfuric acid; extracting the methionine solution using 1 kg of n-hexanol, and then stripping the formic acid using carbon dioxide at 70° C., and the formic acid removal is completed when the pH is stabilized at 2.5; using titration and chromatography to detect the acid-adjusted system and the methionine solution, it is found that there is substantially no loss of methionine, the removal rate of methylthio impurities is 98.5%, and the methionine solution changes from red to light yellow.

[0056] Embodiment 2-4

[0057] 100 kg of hydantoin hydrolyzate obtained by hydrolyzing 5-(β-methylthioethyl)hydantoin solution with alkaline potassium, wherein the methionine content is 15.4 wt % and the alkaline potassium ion content is 11 wt %, is introduced with carbon dioxide at 4 bar and 35° C. for 70 min, and then mixed with 7 kg of the secondary mother liquor of Example 2-3, and centrifuged to obtain 13 kg Methionine and 99kg crystallization mother liquor, 13kg methionine is slurried and stirred with 20kg water at 30°C, separated and dried to obtain white methionine with a content of 99.6wt%; 99kg crystallization mother liquor is mixed with 21kg slurry water of Example 2-3, 2.7kg methionine crude product of Example 2-3 is added and stirred until potassium bicarbonate in the mixture is completely dissolved to form a secondary mother liquor, 110kg secondary mother liquor is extracted (13kg secondary mother liquor not extracted is incorporated into the next hydrolysis liquid acidification system), concentrated at 130°C to 48kg concentrate, the potassium bicarbonate dissociation degree is detected to be 0.44, and the mass ratio of potassium methionine / water is 0.11, 14kg of the diverted concentrate is diverted, and the remaining 34kg of the concentrate is regenerated and used for the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin, and the diverted concentrate is diverted. The concentrate is further concentrated to 6 kg, and 1.7 kg of potassium bicarbonate and 4.3 kg of filtrate are centrifuged at 10° C. for 30 min. The filtrate is acidified by passing carbon dioxide at 15° C. and 3 bar for 20 min, and then centrifuged to obtain 3.3 kg of crude methionine and 1 kg of impurity-enriched liquid. The impurity-enriched liquid is separated into a methionine solution and a potassium sulfate solution by using D001 resin, and the pH of the methionine solution is adjusted to 2.3 by using 98 wt % sulfuric acid. The methionine solution is extracted with 0.5 kg of n-hexanol, and then gas stripping is performed with carbon dioxide at 80° C. to remove formic acid, and the removal of formic acid is completed when the pH is stabilized at 2.7. The acid-adjusted system and the methionine solution are tested by titration and chromatography, and substantially no methionine loss is observed, a removal rate of methylthio-containing impurities is 99.4%, and the methionine solution changes from red to light yellow.

[0058] Embodiment 2-5

[0059] 100 kg of hydantoin hydrolyzate obtained by hydrolyzing 5-(β-methylthioethyl)hydantoin solution with alkaline potassium, wherein the methionine content is 15.4 wt %, and the alkaline potassium ion content is 11 wt %, is introduced with carbon dioxide at 4 bar and 35° C. for 60 min, and then mixed with 13 kg of the secondary mother liquor of Example 2-3, and centrifuged to obtain 14 kg of methionine and 104 kg of crystallization mother liquor, and the 13 kg of methionine is slurried and stirred with 20 kg of water at 30° C., and then separated and dried to obtain white methionine with a content of 99.9 wt %; the 104 kg of crystallization mother liquor and 20 kg of Example 2-4 are mixed. The mixture was mixed with pulping water, 3.3 kg of crude methionine of Example 2-4 was added and stirred until the potassium bicarbonate in the mixture was completely dissolved to form a secondary mother liquor, 120 kg of the secondary mother liquor was extracted (the 7 kg of secondary mother liquor not extracted was incorporated into the next hydrolyzate acidification system), and concentrated at 150 ° C to 49 kg of concentrated solution. The dissociation degree of potassium bicarbonate was 0.52 and the mass ratio of potassium methionine / water was 0.12. 9 kg of the diverted concentrated solution was diverted, and the remaining 40 kg of concentrated solution was regenerated and used for the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin. The diverted concentrated solution was further concentrated to 4 kg, and 1.3 kg of potassium bicarbonate and 2.7 kg of filtrate were centrifuged at 10 ° C for 80 min. The filtrate was heated at 10 ° C and 4 bar. The method comprises the following steps: acidifying the solution with carbon dioxide for 100 minutes and then centrifuging to obtain 1.9 kg of crude methionine and 0.8 kg of impurity-enriched liquid; separating the impurity-enriched liquid into a methionine solution and a potassium sulfate solution by using D001 resin, and adjusting the pH value of the methionine solution to 2.8 by using 98 wt % sulfuric acid; extracting the methionine solution by using 0.3 kg of n-hexanol, and then performing gas stripping with carbon dioxide at 90° C. to remove formic acid, and the removal of formic acid is completed when the pH value is stabilized at 3.1; and detecting the acid-adjusted system and the methionine solution by using titration and chromatography, which shows that there is substantially no loss of methionine, the removal rate of methylthio impurities is 99%, and the methionine solution changes from red to light yellow.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. An environmentally friendly method for preparing methionine based on carbon dioxide acidification, characterized in that The following steps are involved: S1. The alkaline potassium hydrolyzate of 5-(2-methylmercaptoethyl)-hydantoin is acidified with carbon dioxide and separated into crude methionine and crystallization mother liquor; S2. The crystallization mother liquor is concentrated, and when the degree of dissociation of potassium bicarbonate in the system is 0.3-0.6, part of the concentrate is diverted, and the remaining concentrate is regenerated and used for 5-(2-methylmercaptoethyl)-hydantoin hydrolysis; S3. The concentrated solution of the split stream is cooled and crystallized to separate the filtrate and potassium bicarbonate; S4. The filtrate is acidified with carbon dioxide and separated into crude methionine and impurity-enriched liquid.

2. The environmentally friendly method for preparing methionine based on carbon dioxide acidification according to claim 1, characterized in that The crude methionine in S1 is added with water for mixing and beating, separated and dried to obtain methionine, and the beating water is combined with the crystallization mother liquor.

3. The environmentally friendly method for preparing methionine based on carbon dioxide acidification method according to claim 1, characterized in that The concentration temperature of the crystallization mother liquor in S2 is 100-180°C.

4. The environmentally friendly method for preparing methionine based on carbon dioxide acidification method according to claim 1, characterized in that The concentrated liquid diverted in S2 accounts for 10-30% of the total concentrated liquid.

5. The environmentally friendly method for preparing methionine based on carbon dioxide acidification method according to claim 1, characterized in that The remaining concentrated liquid in S2 is thermally decomposed at 160-200° C., and potassium bicarbonate is thermally decomposed into potassium carbonate and potassium methionine, which are then used for hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin.

6. The environmentally friendly method for preparing methionine based on carbon dioxide acidification method according to claim 1, characterized in that The mass ratio of potassium methionine to water in the concentrated solution diverted from S3 is a, 0.15≤a≤1.

7. The environmentally friendly method for preparing methionine based on carbon dioxide acidification method according to claim 1, characterized in that The crystallization conditions in S3 are: keeping warm at 10-30°C for 30-360 min.

8. The environmentally friendly method for preparing methionine based on carbon dioxide acidification method according to claim 1, characterized in that: The crude methionine in S4 is returned to the crystallization mother liquor in S1 to form a secondary mother liquor. Part of the secondary mother liquor without methionine solid is extracted to repeat S2-S4, and the remaining secondary mother liquor containing methionine solid is returned to the acidification of the hydrolyzate in S1.

9. The environmentally friendly method for preparing methionine based on carbon dioxide acidification method according to claim 1, characterized in that The impurity-enriched liquid in S4 is separated into methionine solution and potassium sulfate using hydrogen-type cationic resin.

10. The environmentally friendly method for preparing methionine based on carbon dioxide acidification method according to claim 9, characterized in that: After the methionine solution is acidified, impurities in the oil phase are removed by layering or organic solvent extraction, formic acid is removed by reduced pressure distillation, and finally the methionine solution, potassium hydroxide solution and sulfuric acid solution are separated from the deacidified water phase.

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