Optimized preparation method of methionine based on sulfuric acid indirect acidification method

Abstract

The present invention relates to the field of methionine production, and in particular to an optimized preparation method of methionine based on a sulfuric acid indirect acidification method. Compared with the prior art, the optimized preparation method of methionine based on a sulfuric acid indirect acidification method of the present invention is directed to a sodium hydroxide hydrolyzate of hydantoin, ammonia is discharged by adjusting the pH of a sodium sulfate solution to be alkaline, a methionine concentrate is adjusted to be alkaline to decompose a methionine-derived precursor, impurities are extracted into beneficial substances by extraction and gas stripping, and beneficial resources are purified and processed respectively, which is environmentally friendly, economical, safe, and has a wide market value.

Description

Technical Field

[0001] The invention relates to the field of methionine production, and in particular to an optimized preparation method of methionine based on a sulfuric acid indirect 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 investment and operation costs for the main process flow equipment.

[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 an optimized preparation method of methionine based on the sulfuric acid indirect acidification method, by separately treating methionine, sodium sulfate, methionine derivatives, formic acid and oil phase impurities produced in methionine production, to solve the subsequent treatment problem of sodium hydroxide hydrolysis of hydantoin.

[0020] The key to the optimized preparation method of methionine based on the sulfuric acid indirect acidification method is to include the following steps:

[0021] S1. The sodium hydrolyzate of 5-(2-methylthioethyl)-hydantoin is separated into a methionine solution and a sodium sulfate solution using a hydrogen-type cationic resin;

[0022] S2. Adjust the pH of the sodium sulfate solution to >8, filter and remove the residue, and concentrate the filtrate to obtain sodium sulfate and sodium sulfate residue;

[0023] S3. separating the methionine solution into methionine and a crystallization mother liquor by crystallization;

[0024] S4. The crystallization mother liquor is divided into two parts, the first part of the crystallization mother liquor is concentrated to a methionine content of >5wt% and returned to S1 to be combined with the methionine solution, and the second part of the crystallization mother liquor is concentrated and crystallized and separated into secondary methionine and impurity-enriched liquid;

[0025] S5. The secondary methionine is returned to S4, and the second part of the crystallization mother liquor is used to wash to form a secondary mother liquor. Part of the secondary mother liquor is extracted and returned to S1 to be combined with the methionine solution, and the remaining secondary mother liquor is subjected to S4 concentration and extraction of secondary methionine operation;

[0026] S6. After the impurity-enriched liquid is acidified, the oil phase and formic acid are separated, and the aqueous phase is combined with the sodium sulfate residual liquid in S2 and the pH is adjusted to 4-7 to separate the methionine separation liquid and the sodium sulfate separation liquid. The methionine separation liquid is returned to S3 and combined with the crystallization mother liquor, and the sodium sulfate separation liquid is returned to S2 and combined with the sodium sulfate solution.

[0027] Preferably, the hydrogen-type cationic resin in S1 is a carboxylic acid hydrogen-type resin or a sulfonic acid hydrogen-type resin.

[0028] Preferably, S2 is specifically as follows: at 40-80° C., using calcium oxide to adjust the pH of the sodium sulfate solution to >8, filtering to remove calcium sulfate, concentrating the filtrate at 60-120° C. and separating the sodium sulfate, until the methionine in the sodium sulfate residue is 0.5-5wt%.

[0029] Preferably, S3 is a methionine solution that is kept warm at 10 to 40° C. for 10 to 100 minutes for crystallization, and then centrifuged to separate methionine and a crystallization mother liquor.

[0030] Preferably, the mass ratio of the first part of the crystallization mother liquor to the second part of the crystallization mother liquor in S4 is 3:1-7:1.

[0031] Preferably, the pH of the second portion of the crystallization mother liquor in S4 is adjusted to be greater than 8 before concentration.

[0032] Preferably, the second part of the crystallization mother liquor in S4 is concentrated and the pH is adjusted to 6-8.5, and the mother liquor is kept at 5-40° C. for 10-100 minutes to separate into secondary methionine and impurity-enriched liquid.

[0033] Preferably, the secondary mother liquor extracted in S5 is 10-30%wt of the total secondary mother liquor, the extracted secondary mother liquor contains all methionine solids, and the remaining secondary mother liquor does not contain methionine solids.

[0034] Preferably, the impurity-enriched liquid in S6 is adjusted to pH < 3, and the oil phase impurities are removed by static stratification or organic solvent extraction, and formic acid is removed by vacuum distillation. The aqueous phase is combined with the sodium sulfate residual liquid in S2 and the pH is adjusted to 4-7. The methionine separation liquid and the sodium sulfate separation liquid are separated by chromatography, and the methionine crystallization mother liquor and the sodium sulfate solution are returned respectively.

[0035] Preferably, the impurity-enriched liquid in S6 is adjusted to pH < 3, and the oil phase impurities are removed by static stratification or organic solvent extraction, and formic acid is removed by vacuum distillation. The aqueous phase is combined with the sodium sulfate residue in S2 and the pH is adjusted to 4-7. The solution is separated into methionine separation liquid and sodium sulfate separation liquid using electrodialysis technology, and the methionine crystallization mother liquor and sodium sulfate solution are returned respectively.

[0036] Beneficial effects: Compared with the prior art, the optimized preparation method of methionine based on the sulfuric acid indirect acidification method of the present invention is for the sodium hydroxide hydrolyzate of hydantoin, by adjusting the pH of the sodium sulfate solution to be alkaline to discharge ammonia, adjusting the pH of the methionine concentrate to be alkaline to decompose the methionine-derived precursor, extracting impurities into beneficial substances through extraction and gas stripping, and purifying the beneficial resources separately, which is environmentally friendly, economical, safe, and has broad market value. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0038] 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.

[0039] Example 1 Optimized preparation method of methionine based on sulfuric acid indirect acidification method

[0040] S1. The hydrolyzate obtained by hydrolyzing 5-(2-methylthioethyl)-hydantoin with alkaline sodium is subjected to adsorption and separation of a methionine solution with a mass concentration of 5-10%wt using a hydrogen-type cationic resin at 50-100°C, and then the resin is regenerated using a 3-40wt% sulfuric acid solution to obtain a 5-20wt% sodium sulfate solution; in actual operation, the hydrogen-type cationic resin can be a hydrogen-type sulfonic acid resin or a hydrogen-type carboxylic acid resin with a model of D110, D151, D152, D113 or D001, and the temperature can be selected from 50°C, 60°C, 70°C, 80°C, 90°C, and 100°C;

[0041] S2. At 40-80°C, calcium oxide is used to adjust the pH of the sodium sulfate solution to >8, and calcium sulfate is removed by filtration. The filtrate is concentrated and sodium sulfate is separated at 60-120°C until the methionine in the sodium sulfate residue is 0.5-5wt%; in actual operation, the calcium oxide can be selected from calcium oxide or calcium hydroxide, and the concentration temperature can be selected from 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, and 120°C. The addition of phosphoric acid makes the adsorption of pigment impurities more thorough;

[0042] S3. The methionine solution is kept at 10-40°C for 10-100 minutes for crystallization, and centrifuged to separate methionine and crystallization mother liquor; in actual operation, the temperature can be selected from 10°C, 20°C, 30°C, and 40°C; the holding time can be selected from 10min, 20min, 30min, 40min, 50min, 60min, 70min, 80min, 90min, and 100min;

[0043] S4. The crystallization mother liquor is divided into two parts, and the system is kept in normal operation by controlling the split impurity removal ratio, and the quality of methionine does not decrease significantly. The first part of the crystallization mother liquor is concentrated at 60-120° C. to a methionine content of >5wt%, and then returned to S1 to be combined with the methionine solution. The second part of the crystallization mother liquor is adjusted to pH>8, and then concentrated at 60-120° C. to a methionine concentrate with a mass concentration of 10-15wt%. The methionine concentrate is adjusted to pH 6-8.5, and is kept warm at 5-40° C. for 10-100 minutes to separate into secondary methionine and impurity enriched solution. By controlling the pH of the methionine concentrate to 6-8.5, the impurities are dissolved in water in the form of salt, and the methionine extraction rate is higher and the impurity embedding rate is relatively lower. In actual operation, the mass ratio of the first part of the crystallization mother liquor to the second part of the crystallization mother liquor can be 3:1, 4:1, 5:1, 6:1, 7:1;

[0044] S5. The secondary methionine is returned to S4, and the second part of the crystallization mother liquor is used to wash the secondary mother liquor. The leaf filter extracts the secondary mother liquor containing all methionine solids, accounting for 10-30%wt of the total amount, and returns it to S1 to be combined with the methionine solution. The remaining secondary mother liquor without methionine solids is subjected to S4 concentration and extraction of secondary methionine; the second part of the crystallization mother liquor is used to dissolve and dilute the secondary methionine impurities for cleaning, so as to reduce the concentration of impurities and avoid the secondary methionine directly returning to the methionine solution to cause the reflux of impurities, which may lead to unqualified methionine quality; in actual operation, the extracted secondary mother liquor can be 10%wt, 20%wt, or 30%wt of the total amount of the secondary mother liquor;

[0045] S6. The impurity-enriched liquid is adjusted to pH < 3 using 90-98wt% sulfuric acid, and extracted by static stratification or organic solvent until the color of the filtrate changes from black-red to yellow. The aqueous phase after extraction is stripped / gas-stripped using steam / carbon dioxide at 60-100°C, and the outgas is absorbed by alkali solution and returned to the stripping / gas-stripping, and the alkali solution is concentrated to prepare formate. After the aqueous phase after formic acid separation is combined with the sodium sulfate residue in S2, the pH can be adjusted to 4-7, and separated into methionine separation liquid and sodium sulfate separation liquid by chromatography at 50-70°C; the pH can also be adjusted to 4-7, and separated into methionine separation liquid and sodium sulfate separation liquid by electrodialysis at 10-40°C. The methionine separation liquid is returned to S3 and combined with the crystallization mother liquor, and the sodium sulfate separation liquid is returned to S2 and combined with the sodium sulfate solution.

[0046] In actual use, the organic solvent is selected from alcohols with 5 carbon atoms or more, and the removal rate of methylthio-containing impurities can reach more than 98%, with little effect on the methionine content.

[0047] The organic solvent after extraction 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.

[0048] 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.

[0049] Example 2-1

[0050] The hydantoin hydrolyzate obtained by hydrolyzing the 5-(β-methylmercaptoethyl)hydantoin solution with sodium hydroxide was separated into a methionine solution with a methionine content of 7.8 wt% and a sodium sulfate solution with a sodium sulfate content of 14.5 wt% using D101 resin at 80°C. Take 10kg of sodium sulfate solution, add 2g of 90% phosphoric acid and 30g of calcium oxide, stir until the pH is 8.5, filter, concentrate and separate sodium sulfate at 100°C until the sodium sulfate residual liquid has a methionine content of 1.2wt%, a sodium sulfate content of 99.9wt%, and a pure white solid; take 10kg of methionine solution and keep it at 20°C for 50min, then separate to obtain 0.48kg of white methionine with a content of 99.5wt% and 9.5kg of crystallization mother liquor with a methionine content of 3.2wt%; concentrate 7.6kg of crystallization mother liquor to a methionine content of 6.7wt% and return it to the methionine solution, adjust the pH of the remaining 1.9kg of crystallization mother liquor to 8.6 using 0.2kg of hydrolyzate, and then concentrate it to a methionine content of 14.1wt%, at which time the pH is 9.7, use 95wt% sulfuric acid The pH value is adjusted to 8, and the mixture is kept at 20° C. for 60 minutes and centrifuged to separate the secondary methionine and the impurity-enriched liquid; the impurity-enriched liquid is adjusted to pH 2.3 using 95wt% sulfuric acid, 380g of the impurity-enriched liquid after acid adjustment is extracted using 200g of n-hexanol, 5.5g of calcium oxide is added to the extracted n-hexanol to adjust the pH value to 7.2, the n-hexanol is evaporated, and 20g of calcium oxide is added to the residual liquid to solidify it into a solid; formic acid is extracted from the aqueous phase after the impurities are extracted using carbon dioxide at 90° C., and the pH value of the aqueous phase is stabilized at 2.8, and the formic acid removal is completed; 0.23kg of the aqueous phase after formic acid extraction is combined with 1.1kg of sodium sulfate residual liquid to obtain a mixed liquid with a pH value of 5.7, and chromatographically separated at 50-60° C. into 1.3kg of methionine separation liquid and 0.7kg of sodium sulfate separation liquid, which are returned to the crystallization mother liquor and the sodium sulfate solution respectively.

[0051] Example 2-2

[0052] After the process flow of Example 2-1 was cycled five times, 0.1 kg of the aqueous phase after formic acid extraction was taken, water was added to 1 kg, the pH was adjusted to 5.4 using a hydrolyzate, and then kept at 34°C for 60 min. The mixture was filtered and dried to obtain a methionine content of 98.5 wt%, a sodium sulfate content of 0.8 wt%, no methionine derivatives were detected, and the solid was a white solid.

[0053] Comparative Example 1

[0054] The impurity-enriched liquid after acid adjustment in Example 2-1 was taken and the pH was adjusted to 5.4 using a hydrolyzate, then kept at 34° C. for 60 min, filtered and dried to obtain a methionine content of 96.1 wt %, a sodium sulfate content of 1.3 wt %, and a methionine derivative of 0.37 wt %, which was a sticky red solid.

[0055] Comparative Example 2

[0056] Take 2 kg of the sodium sulfate solution with a sodium sulfate content of 14.5 wt % in Example 2-1, and concentrate it until the methionine content of the sodium sulfate residue is 1.2 wt %. The separated sodium sulfate solid content is 99.4 wt %, and the properties are off-white solid mixed with a small amount of light yellow-red impurities.

[0057] 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. Optimized preparation method of methionine based on sulfuric acid indirect acidification method, Features The following steps are involved: S1. The sodium hydrolyzate of 5-(2-methylmercaptoethyl)-hydantoin is separated into methionine solution and sodium sulfate solution by using hydrogen-type cationic resin; S2. Adjust the pH of the sodium sulfate solution to >8, filter and remove the residue, and concentrate the filtrate to obtain sodium sulfate and sodium sulfate residue; S3. separating the methionine solution into methionine and a crystallization mother liquor by crystallization; S4. The crystallization mother liquor is divided into two parts. When the first part of the crystallization mother liquor is concentrated to a methionine content of >5wt%, it is returned to S1 and combined with the methionine solution. After the second part of the crystallization mother liquor is concentrated, it is crystallized and separated into secondary methionine and impurity-enriched liquid; wherein the mass ratio of the first part of the crystallization mother liquor to the second part of the crystallization mother liquor is 3:1-7:1; before the second part of the crystallization mother liquor is concentrated, the pH is adjusted to >8; after the second part of the crystallization mother liquor is concentrated, the pH is adjusted to 6-8.5, and it is kept at 5-40°C for 10-100 minutes to separate into secondary methionine and impurity-enriched liquid; S5. The secondary methionine is returned to S4, and the second part of the crystallization mother liquor is used to wash to form a secondary mother liquor. Part of the secondary mother liquor is extracted and returned to S1 to be combined with the methionine solution, and the remaining secondary mother liquor is subjected to S4 concentration and extraction of secondary methionine operation; S6. After acid adjustment, the impurity-enriched liquid is separated into an oil phase and formic acid. The aqueous phase is combined with the sodium sulfate residual liquid in S2 and the pH is adjusted to 4-7 to separate into a methionine separation liquid and a sodium sulfate separation liquid. The methionine separation liquid is returned to S3 and combined with the crystallization mother liquor, and the sodium sulfate separation liquid is returned to S2 and combined with the sodium sulfate solution.

2. The optimized preparation method of methionine based on the sulfuric acid indirect acidification method according to claim 1, Features The hydrogen-type cationic resin in S1 is a carboxylic acid hydrogen-type resin or a sulfonic acid hydrogen-type resin.

3. The optimized preparation method of methionine based on the sulfuric acid indirect acidification method according to claim 1, Features S2 specifically comprises: adjusting the pH of the sodium sulfate solution to be greater than 8 using calcium oxide at 40 to 80°C, filtering to remove calcium sulfate, concentrating the filtrate at 60 to 120°C to separate sodium sulfate, until the methionine content in the sodium sulfate residue is 0.5 to 5 wt%.

4. The optimized preparation method of methionine based on the sulfuric acid indirect acidification method according to claim 1, Features S3 is specifically that the methionine solution is kept at 10 to 40° C. for 10 to 100 minutes for crystallization, and is separated into methionine and a crystallization mother liquor.

5. The optimized preparation method of methionine based on the sulfuric acid indirect acidification method according to claim 1, Features The secondary mother liquor extracted in S5 is 10-30%wt of the total secondary mother liquor. The extracted secondary mother liquor contains all methionine solids, and the remaining secondary mother liquor does not contain methionine solids.

6. The optimized preparation method of methionine based on the sulfuric acid indirect acidification method according to claim 1, Features The impurity-enriched liquid in S6 is adjusted to pH < 3, and the oil phase impurities are removed by static stratification or organic solvent extraction. The formic acid is removed by vacuum distillation. The aqueous phase is combined with the sodium sulfate residual liquid in S2 and the pH is adjusted to 4-7. The methionine separation liquid and the sodium sulfate separation liquid are separated by chromatography and returned to the methionine crystallization mother liquor and the sodium sulfate solution respectively.

7. The optimized preparation method of methionine based on the sulfuric acid indirect acidification method according to claim 1, Features The impurity-enriched liquid in S6 is adjusted to pH < 3, and the oil phase impurities are removed by static stratification or organic solvent extraction. The formic acid is removed by vacuum distillation. The aqueous phase is combined with the sodium sulfate residual liquid in S2 and the pH is adjusted to 4-7. The electrodialysis technology is used to separate the methionine separation liquid and the sodium sulfate separation liquid, and the methionine crystallization mother liquor and the sodium sulfate solution are returned respectively.

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