A methionine composition and preparation method thereof

Abstract

The present invention provides a storage-stable methionine composition and a method for preparing the same. The methionine preparation method comprises performing a segmented synthesis of 5-(2-methylmercaptoethyl)hydantoin, wherein 2-hydroxy-4-(methylthio)butyronitrile is mixed with excess carbon dioxide, ammonia, and the like at a low temperature, and then heating the mixture to a high temperature. The ammonia concentration in the system is adjusted to control the content of the compound #imgabs0#. The mixture is then subjected to hydrolysis, acidification, crystallization, and separation to obtain a high-quality methionine composition with enhanced storage stability.

Description

Technical Field

[0001] The present invention belongs to the field of nutritional chemicals, and in particular relates to a methionine composition and a preparation method thereof. Background Art

[0002] Methionine, the only essential sulfur-containing amino acid for humans and animals, cannot be synthesized in the body and must be obtained through food. Currently, it is produced on a large scale industrially worldwide. Methionine is used in many fields, such as pharmaceuticals, health and fitness products, and as a feed additive for most farmed animals.

[0003] The quality of methionine is also a focus of attention for consumers and manufacturers. In industrial production, the mother liquors of ammonia, potassium carbonate, potassium bicarbonate, and carbon dioxide are usually recycled, which inevitably causes some impurities to be enriched in the circulating mother liquor. WO2013030068 reports the presence of potassium formate in the circulating mother liquor, formed from residual hydrocyanic acid in the methionine hydantoin solution and the alkaline potassium salt produced by hydantoin hydrolysis. CN1589259 reports the impact of the byproduct methionine dipeptide and another urea-based impurity on the methionine crystallization process. WO2016170252 mentions the use of sulfuric acid to neutralize methionine sodium salt, resulting in the production of a large amount of sodium sulfate as a byproduct. This byproduct has no practical application and must be separated and removed through post-processing, and the product methionine contains a high sodium content. US5770769 reports the use of carbon dioxide to precipitate methionine from the solution. Through recrystallization, filtration, and drying, the resulting methionine has a purity greater than 99% and a potassium content less than 0.5%. Generally speaking, excessive accumulation of byproducts in the hydantoin hydrolysis cycle must be avoided, as this can seriously affect the purity, color, and crystal form of methionine.

[0004] The inventors have discovered that there are currently no patents reporting methods for controlling this substance during the reaction or post-processing stages. Therefore, this substance will enter the product methionine as the methionine salt crystallizes through acidification, and the quality of methionine is often affected by excessive levels.

[0005] In summary, the current methionine market requires composition products with higher quality in terms of storage stability, etc. Summary of the Invention

[0006] The object of the present invention is to provide a method for preparing a methionine composition, which can effectively control the enrichment degree of methionine by-products and obtain a composition product with higher storage stability.

[0007] The inventors discovered that in the synthesis process of methionine, 3-methylthiopropionaldehyde and hydrocyanic acid are usually reacted to first generate 2-hydroxy-4 (methylthio) butyronitrile, which is then reacted with carbon dioxide, ammonia, etc. to generate 5-(2-methylmercaptoethyl) hydantoin. To ensure complete reaction of the raw materials, the molar amount of carbon dioxide and ammonia must be greater than the molar amount of 2-hydroxy-4 (methylthio) butyronitrile, especially the excess of ammonia, resulting in a large amount of the compound of formula 1 being present in the hydantoin reaction solution and the methionine salt solution.

[0008] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing a methionine composition, comprising the following steps:

[0010] S1: When synthesizing 5-(2-methylmercaptoethyl)hydantoin, 2-hydroxy-4(methylthio)butyronitrile is reacted with carbon dioxide and ammonia in a low temperature section to generate the intermediate 2-amino-4(methylthio)butyronitrile. In a high temperature section, carbon dioxide is introduced to replace the ammonia to control the production of compound 1 of formula 1.

[0011] S2: 5-(2-methylmercaptoethyl)hydantoin is hydrolyzed under alkaline conditions to obtain a methionine salt solution;

[0012] S3: introducing carbon dioxide into the solution of S2 for crystallization, and obtaining methionine crystals and methionine mother liquor after separation;

[0013] S4: drying the methionine crystals obtained in S3 to obtain a methionine composition;

[0014]

[0015] Wherein, S1 controls the content of compound 1 in the reaction solution to be 5-500ppm.

[0016] The inventors studied the mechanism of methionine synthesis and found that 2-hydroxy-4 (methylthio) butyronitrile easily reacts with ammonia to form the intermediate 2-amino-4 (methylthio) butyronitrile. However, further addition reaction with carbon dioxide and intramolecular cyclization require higher reaction temperatures. However, the presence of excessive ammonia in this high-temperature process is detrimental to the cyclization process, i.e., it produces a large amount of the compound shown in Formula 1. Therefore, the ammonia content of the high-temperature process needs to be reduced. We also surprisingly found in our research that including a certain amount of the compound of Formula 1 in methionine can significantly improve the stability of methionine during storage. The compound of Formula 1 has certain antioxidant properties, but when present in large quantities, it increases the color of the product.

[0017] In the present invention, the molar ratio of 5-(2-methylmercaptoethyl)hydantoin to carbon dioxide and ammonia in S1 is 1:(1.5-2.5):(1.5-4).

[0018] In the present invention, the temperature of the low-temperature section S1 is 50-90°C, the temperature of the high-temperature section is 110-140°C, and carbon dioxide is introduced into the high-temperature section to control the volume of ammonia in the gas phase system to account for 1-10% of the total volume of ammonia and carbon dioxide.

[0019] In one embodiment, carbon dioxide and ammonia are not limited to pure gaseous raw materials, but can also be materials that can be decomposed to release carbon dioxide and ammonia when heated, such as ammonium carbonate, ammonium bicarbonate and ammonia water.

[0020] In the present invention, the alkaline condition in S2 is formed in the presence of potassium hydroxide and / or potassium carbonate.

[0021] In the present invention, the S2 hydrolysis reaction temperature is 150-200°C.

[0022] In one embodiment, S2 hydrolyzes the 5-(2-methylmercaptoethyl)hydantoin generated by S1 under alkaline conditions of pH ≥ 11, the reaction temperature is 150-200° C., preferably 160-180° C., the treatment time is 0.2-5 h, preferably 0.5-2 h, the alkaline conditions of pH ≥ 11 are achieved by adding potassium carbonate or potassium hydroxide, the molar ratio of 5-(2-methylmercaptoethyl)hydantoin to potassium ions is 1:2-4, and methionine in the resulting hydrolysis reaction solution exists in the form of potassium salt.

[0023] In the present invention, carbon dioxide is introduced into the methionine salt solution in S3 for acidification.

[0024] In the present invention, the temperature of the S3 acidification reaction is 20-50°C.

[0025] In one embodiment, S3 deaminates the hydrolyzed liquid obtained in S2 by removing excess ammonia or ammonia from the ammonium salt by steam stripping or nitrogen stripping until the ammonia content is ≤0.1%. Carbon dioxide gas is introduced into the deaminated hydrolyzed liquid, and a pressure reaction is carried out at a gauge pressure of 0.2-1.0 MPa and a temperature of 20-50° C. until the pH drops to ≤9.0 and the molar ratio of carbon dioxide to potassium methionine is ≥1.5:1, thereby obtaining methionine crystals and a methionine mother liquor.

[0026] In one embodiment, the methionine crystals in S3 and the mother liquor are subjected to solid-liquid separation by a generally adopted method, specifically a vacuum suction filter or a centrifuge.

[0027] In the present invention, the methionine composition described in S4 comprises methionine and compound 1; preferably, the mass content of compound 1 in the methionine composition is 0.1-20 ppm.

[0028] In one embodiment, the methionine crystals in S4 are dried, which can be done by heating in a vacuum or by blowing hot air. The drying temperature is preferably 60-110° C. to obtain a methionine product with a moisture content of ≤0.5%.

[0029] Another object of the present invention is to provide a methionine composition prepared by the above method

[0030] A methionine composition prepared by the above method, comprising methionine and compound 1 of formula 1;

[0031]

[0032] Preferably, the mass content of compound 1 in the methionine composition is 0.1-20 ppm.

[0033] In the present invention, after the methionine composition is stored at 60° C. for 3 months, the content of the newly added impurity methionine dipeptide is ≤0.2 wt %.

[0034] In the present invention, after the methionine composition is stored at 60° C. for 3 months, the color increase value of the aqueous solution of the methionine composition containing 2 wt % is ≤0.1 Hazen.

[0035] Another object of the present invention is to provide a use of the methionine composition.

[0036] A use of a methionine composition, wherein the composition is the methionine composition prepared by the above method, or is the above methionine composition, and the methionine composition is used as an animal feed additive.

[0037] In one embodiment, to obtain a methionine composition with excellent storage stability, the content of the compound represented by Formula 1 obtained in the 5-(2-methylmercaptoethyl)hydantoin synthesis step is controlled to 5-500 ppm. When the content of the compound represented by Formula 1 in the 5-(2-methylmercaptoethyl)hydantoin solution exceeds 500 ppm, more than 20 ppm of this compound often enters the methionine composition, causing a significant increase in color during storage at 60°C. When the content of the compound represented by Formula 1 in the 5-(2-methylmercaptoethyl)hydantoin solution is less than 5 ppm, the content of this compound entering the methionine composition is less than 0.1 ppm, causing rapid growth of impurities during storage at 60°C. The compound represented by Formula 1 has certain antioxidant properties in methionine, and at a certain content, it also improves the color of the product.

[0038] In one embodiment, if the methionine mother liquor needs to be recycled, the convenient method for controlling the content of the compound represented by Chemical Formula 1 in the methionine salt solution includes reducing the amount of methionine mother liquor recycled, oxidizing the methionine mother liquor, and subjecting the methionine mother liquor to high-temperature hydrolysis. Oxidation treatment is preferred.

[0039] In one embodiment, the oxidation treatment is carried out by adding an oxidant, such as hydrogen peroxide, to the recycled methionine mother liquor. The reaction temperature is 30-60° C., the treatment time is 1-4 hours, and the molar ratio of the compound represented by Chemical Formula 1 to the oxidant is 1:2-6.

[0040] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0041] (1) By optimizing the reaction conditions, the amount of by-products generated as shown in Formula 1 was greatly reduced and effectively controlled, thereby improving the yield and purity of methionine.

[0042] (2) The amino acid composition has excellent storage stability, which plays an important role in inhibiting the formation of impurities and maintaining color stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Attachment Figure 1 The figure is a production flow chart of the method for preparing methionine of the present invention. DETAILED DESCRIPTION

[0044] The present invention will be further described below with reference to the embodiments. However, the present invention is not limited to the embodiments listed, but also includes any other known changes within the scope of the rights claimed by the present invention.

[0045] Source of raw materials:

[0046] 2-Hydroxy-4(methylthio)butanenitrile, Wanhua Chemical, >99%;

[0047] Ammonia, Yantai Huasheng Gas, >99.99%;

[0048] Carbon dioxide, Yantai Huasheng Gas, >99.99%;

[0049] Ammonium carbonate, Aladdin, ≥98%;

[0050] Ammonium bicarbonate, Aladdin, ≥98%,

[0051] Potassium carbonate, Inokane, ≥98%.

[0052] The test instrument for the compound of Chemical Formula 1 was a high performance liquid chromatograph, Agilent 1260, column C18, and a mobile phase of acetonitrile and phosphoric acid aqueous solution, with a volume ratio of acetonitrile to water of 95:5 and a mass fraction of phosphoric acid in the aqueous solution of 1.0%.

[0053] The color number is tested using a HACH LICO 500 colorimeter.

[0054] Example 1

[0055] 2-Hydroxy-4 (methylthio) butyronitrile, carbon dioxide, ammonia and water were injected into a low-temperature reactor in a molar ratio of 1:1.5:2:23. The temperature in the low-temperature reactor was 50°C and the average residence time was 1 hour. After the low-temperature reaction was completed, the reactor was injected into a high-temperature reactor. The reaction temperature was 110°C and the average residence time was 1 hour. During the high-temperature reaction, carbon dioxide was continuously introduced to replace the ammonia in the system. The volume fraction of ammonia in the gas phase in the high-temperature reactor was sampled and detected to be 10%. 5-(2-methylmercaptoethyl)hydantoin at the reactor outlet is mixed with the recycled methionine mother liquor in a mass ratio of 3:1 and then introduced into a hydrolysis kettle. At the same time, a potassium carbonate aqueous solution with a mass content of 50% is introduced into the hydrolysis kettle, maintaining a molar ratio of 5-(2-methylmercaptoethyl)hydantoin to potassium ions at 1:2. The mixture is retained at 150° C. for an average of 2 hours. After the reaction is completed, the hydrolysis reaction liquid is introduced into a deamination tower and stripped and deaminated with 2S steam at a deamination temperature of 95-100° C. and a retention time of 2 hours to obtain a potassium methionine aqueous solution.

[0056] The obtained potassium methionine aqueous solution was pumped into an acidification kettle, and carbon dioxide gas was introduced into the reactor. The reaction temperature was maintained at 20°C and the system pressure was 0.2 MPa. When the pH of the reaction solution dropped to 9.0, the reaction was stopped. The molar ratio of the introduced carbon dioxide to potassium methionine was 1.5:1. The obtained solid-liquid mixture was centrifuged to separate methionine crystals and methionine mother liquor. The methionine crystals were dried in a dryer at 60° C. under vacuum to obtain a methionine composition. The methionine mother liquor was heat-treated at 140° C. to remove some water. When the pH of the mother liquor rose to 11.0, heating was stopped. When the temperature was lowered to 30° C., a 27% hydrogen peroxide solution was introduced into the mother liquor, maintaining a molar ratio of the compound represented by Chemical Formula 1 to hydrogen peroxide in the mother liquor at 1:2. The reaction was allowed to proceed for 1 hour, and the content of the compound in the mother liquor was reduced from 2000 ppm to 800 ppm. The treated methionine mother liquor was then reused in the hydrolysis step. The content of the compound in the mixture of 5-(2-methylmercaptoethyl)hydantoin and the recycled methionine mother liquor was tested to be 480 ppm. The content of the compound represented by Chemical Formula 1 in the obtained methionine composition was 19 ppm.

[0057] The methionine composition was stored at 60° C. for 3 months. The methionine dipeptide content increased by 0.12%, and the color number of the 2% aqueous solution increased from 3.0 Hazen before storage to 3.1 Hazen.

[0058] Example 2

[0059] 2-Hydroxy-4 (methylthio) butyronitrile, ammonium bicarbonate, ammonia and water were injected into a low-temperature reactor in a molar ratio of 1:2:3:23. The temperature in the low-temperature reactor was 60°C and the average residence time was 1 hour. After the low-temperature reaction was completed, the reactor was injected into a high-temperature reactor. The reaction temperature was 120°C and the average residence time was 1 hour. During the high-temperature reaction, carbon dioxide was continuously introduced to replace the ammonia in the system. The volume fraction of ammonia in the gas phase in the high-temperature reactor was sampled and detected to be 6%. 5-(2-methylmercaptoethyl)hydantoin at the reactor outlet is mixed with the recycled methionine mother liquor in a mass ratio of 3:1 and then introduced into a hydrolysis kettle. At the same time, a 50% potassium carbonate aqueous solution is introduced into the hydrolysis kettle, maintaining a molar ratio of 5-(2-methylmercaptoethyl)hydantoin to potassium ions at 1:3. The mixture is retained at 160° C. for an average of 1.5 hours. After the reaction is completed, the hydrolysis reaction liquid is introduced into a deamination tower and deamination is performed using 1N gas stripping. The deamination temperature is 95-100° C. and the retention time is 1 hour to obtain a potassium methionine aqueous solution.

[0060] The obtained potassium methionine aqueous solution was pumped into an acidification kettle, and carbon dioxide gas was introduced into the reactor. The reaction temperature was maintained at 30°C and the system pressure was 0.5 MPa. When the pH of the reaction solution dropped to 8.5, the reaction was stopped. The molar ratio of the introduced carbon dioxide to potassium methionine was 1.6:1. The obtained solid-liquid mixture was centrifuged to separate methionine crystals and methionine mother liquor. The methionine crystals were dried in a dryer at 70° C. under vacuum to obtain a methionine composition. The methionine mother liquor was heat-treated at 150° C. to remove some water. When the pH of the mother liquor rose to 11.2, heating was stopped. When the temperature was lowered to 40° C., a 27% hydrogen peroxide solution was introduced into the mother liquor, maintaining a molar ratio of the compound represented by Chemical Formula 1 to hydrogen peroxide in the mother liquor at 1:5. The reaction was allowed to proceed for 3 hours, and the content of the compound in the mother liquor was reduced from 1500 ppm to 500 ppm. The treated methionine mother liquor was then reused in the hydrolysis step. The content of the compound in the potassium methionine aqueous solution obtained by the hydrolysis reaction was tested to be 260 ppm. The content of the compound represented by Chemical Formula 1 in the obtained methionine composition was 16 ppm.

[0061] The methionine composition was stored at 60° C. for 3 months. The methionine dipeptide content increased by 0.15%, and the color number of the 2% aqueous solution increased from 3.0 Hazen before storage to 3.08 Hazen.

[0062] Example 3

[0063] 2-Hydroxy-4 (methylthio) butyronitrile, ammonium bicarbonate, ammonia and water were injected into a low-temperature reactor in a molar ratio of 1:1:1:23. The temperature in the low-temperature reactor was 70°C and the average residence time was 1 hour. After the low-temperature reaction was completed, the reactor was injected into a high-temperature reactor. The reaction temperature was 130°C and the average residence time was 1 hour. During the high-temperature reaction, carbon dioxide was intermittently introduced to replace the ammonia in the system. The volume fraction of ammonia in the gas phase in the high-temperature reactor was sampled and detected to be 3%. 5-(2-methylmercaptoethyl)hydantoin at the reactor outlet is mixed with the recycled methionine mother liquor in a mass ratio of 3:1 and then introduced into a hydrolysis kettle. At the same time, a 30% potassium hydroxide aqueous solution is introduced into the hydrolysis kettle, maintaining a molar ratio of 5-(2-methylmercaptoethyl)hydantoin to potassium ions of 1:3. The mixture is retained at 170° C. for an average of 1 hour. After the reaction is completed, the hydrolysis reaction liquid is introduced into a deamination tower and deamination is performed using 1N gas stripping at a deamination temperature of 95-100° C. and a retention time of 1 hour to obtain a potassium methionine aqueous solution.

[0064] The obtained potassium methionine aqueous solution was pumped into an acidification kettle, and carbon dioxide gas was introduced into the reactor. The reaction temperature was maintained at 50°C and the system pressure was 1.0 MPa. When the pH of the reaction solution dropped to 8.3, the reaction was stopped. The molar ratio of the introduced carbon dioxide to potassium methionine was 1.5:1. The obtained solid-liquid mixture was centrifuged to separate methionine crystals and methionine mother liquor. The methionine crystals were dried in a dryer by passing hot air at 90° C. to obtain a methionine composition. The methionine mother liquor was heat-treated at 160° C. to remove some water. When the pH of the mother liquor rose to 11.3, heating was stopped. When the temperature was lowered to 50° C., a 27% hydrogen peroxide solution was introduced into the mother liquor, maintaining a molar ratio of the compound represented by Chemical Formula 1 to hydrogen peroxide in the mother liquor at 1:4. The reaction was carried out for 2 hours, and the content of the compound in the mother liquor was reduced from 800 ppm to 230 ppm. The treated methionine mother liquor was then reused in the hydrolysis step. The content of the compound in the potassium methionine aqueous solution obtained by the hydrolysis reaction was tested to be 70 ppm. The content of the compound represented by Chemical Formula 1 in the obtained methionine composition was 1.5 ppm.

[0065] The methionine composition was stored at 60° C. for 3 months. The methionine dipeptide content increased by 0.18%, and the color number of the 2% aqueous solution increased from 3.0 Hazen before storage to 3.04 Hazen.

[0066] Example 4

[0067] 2-Hydroxy-4 (methylthio) butyronitrile, ammonium carbonate and water were injected into a low-temperature reactor in a molar ratio of 1:2:4:23. The temperature in the low-temperature reactor was 80°C and the average residence time was 1 hour. After the low-temperature reaction was completed, the reactor was injected into a high-temperature reactor. The reaction temperature was 120°C and the average residence time was 1 hour. During the high-temperature reaction, carbon dioxide was intermittently introduced to replace the ammonia in the system. The volume fraction of ammonia in the gas phase in the high-temperature reactor was sampled and detected to be 1%. 5-(2-methylmercaptoethyl)hydantoin at the reactor outlet is mixed with the recycled methionine mother liquor in a mass ratio of 3:1 and then introduced into a hydrolysis kettle. At the same time, a 50% potassium carbonate aqueous solution is introduced into the hydrolysis kettle, maintaining a molar ratio of 5-(2-methylmercaptoethyl)hydantoin to potassium ions at 1:3. The mixture is retained at 180° C. for an average of 0.5 h. After the reaction is completed, the hydrolysis reaction liquid is introduced into a deamination tower and deamination is performed using 1N gas stripping. The deamination temperature is 95-100° C. and the retention time is 1 h to obtain a potassium methionine aqueous solution.

[0068] The obtained potassium methionine aqueous solution was pumped into an acidification kettle, and carbon dioxide gas was introduced into the reactor. The reaction temperature was maintained at 30°C and the system pressure was 0.7 MPa. When the pH of the reaction solution dropped to 8.3, the reaction was stopped. The molar ratio of the introduced carbon dioxide to potassium methionine was 1.6:1. The obtained solid-liquid mixture was centrifuged to separate methionine crystals and methionine mother liquor. The methionine crystals were dried in a dryer at 100° C. under vacuum to obtain a methionine composition. The methionine mother liquor was heat-treated at 180° C. to remove some water. When the pH of the mother liquor rose to 11.1, heating was stopped. When the temperature was lowered to 50° C., a 27% hydrogen peroxide solution was introduced into the mother liquor, maintaining a molar ratio of the compound represented by Chemical Formula 1 to hydrogen peroxide in the mother liquor at 1:6. The reaction was carried out for 4 hours, and the content of the compound in the mother liquor was reduced from 220 ppm to 30 ppm. The treated methionine mother liquor was then reused in the hydrolysis step. The content of the compound in the potassium methionine aqueous solution obtained by the hydrolysis reaction was tested to be 5 ppm. The content of the compound represented by Chemical Formula 1 in the obtained methionine composition was 0.1 ppm.

[0069] The methionine composition was stored at 60° C. for 3 months. The methionine dipeptide content increased by 0.2%, and the color number of the 2% aqueous solution increased from 3.0 Hazen before storage to 3.02 Hazen.

[0070] Example 5

[0071] 2-Hydroxy-4 (methylthio) butyronitrile, ammonium bicarbonate and water were injected into a low-temperature reactor in a molar ratio of 1:1.5:1.5:23. The temperature in the low-temperature reactor was 90°C and the average residence time was 1 hour. After the low-temperature reaction was completed, the reactor was injected into a high-temperature reactor. The reaction temperature was 140°C and the average residence time was 1 hour. During the high-temperature reaction, carbon dioxide was intermittently introduced to replace the ammonia in the system. The volume fraction of ammonia in the gas phase in the high-temperature reactor was sampled and detected to be 5%. 5-(2-methylmercaptoethyl)hydantoin at the reactor outlet is mixed with the recycled methionine mother liquor in a mass ratio of 3:1 and then introduced into a hydrolysis kettle. At the same time, a 50% potassium carbonate aqueous solution is introduced into the hydrolysis kettle, maintaining a molar ratio of 5-(2-methylmercaptoethyl)hydantoin to potassium ions at 1:4. The mixture is retained at 200° C. for an average of 0.2 h. After the reaction is completed, the hydrolysis reaction liquid is introduced into a deamination tower and deamination is performed using 1N gas stripping. The deamination temperature is 95-100° C. and the retention time is 1 h to obtain a potassium methionine aqueous solution.

[0072] The obtained potassium methionine aqueous solution was pumped into an acidification kettle, and carbon dioxide gas was introduced into the reactor. The reaction temperature was maintained at 40°C and the system pressure was 0.4 MPa. When the pH of the reaction solution dropped to 8.0, the reaction was stopped. The molar ratio of the introduced carbon dioxide to potassium methionine was 1.6:1. The obtained solid-liquid mixture was centrifuged to separate methionine crystals and methionine mother liquor. The methionine crystals were dried in a dryer at 100° C. under vacuum to obtain a methionine composition. The methionine mother liquor was heat-treated at 200° C. to remove some water. When the pH of the mother liquor rose to 11.0, heating was stopped. When the temperature was lowered to 60° C., a 27% hydrogen peroxide solution was introduced into the mother liquor, maintaining a molar ratio of the compound represented by Chemical Formula 1 to hydrogen peroxide in the mother liquor at 1:3. The reaction was allowed to proceed for 1 hour, and the content of the compound in the mother liquor decreased from 1200 ppm to 330 ppm. The treated methionine mother liquor was then reused in the hydrolysis step. The content of the compound in the potassium methionine aqueous solution obtained by the hydrolysis reaction was tested to be 110 ppm. The content of the compound represented by Chemical Formula 1 in the obtained methionine composition was 8.0 ppm.

[0073] The methionine composition was stored at 60° C. for 3 months. The methionine dipeptide content increased by 0.16%, and the color number of the 2% aqueous solution increased from 3.0 Hazen before storage to 3.05 Hazen.

[0074] Comparative Example 1

[0075] 2-Hydroxy-4 (methylthio) butyronitrile, ammonium carbonate and water were injected into a low-temperature reactor in a molar ratio of 1:2:4:23. The temperature in the low-temperature reactor was 80°C and the average residence time was 1 hour. After the low-temperature reaction was completed, the reactor was injected into a high-temperature reactor. The reaction temperature was 120°C and the average residence time was 1 hour. During the high-temperature reaction, carbon dioxide was intermittently introduced to replace the ammonia in the system. The volume fraction of ammonia in the gas phase in the high-temperature reactor was sampled and detected to be 1%. 5-(2-methylmercaptoethyl)hydantoin at the reactor outlet is mixed with the recycled methionine mother liquor in a mass ratio of 3:1 and then introduced into a hydrolysis kettle. At the same time, a 50% potassium carbonate aqueous solution is introduced into the hydrolysis kettle, maintaining a molar ratio of 5-(2-methylmercaptoethyl)hydantoin to potassium ions at 1:3. The mixture is retained at 180° C. for an average of 0.5 h. After the reaction is completed, the hydrolysis reaction liquid is introduced into a deamination tower and deamination is performed using 1N gas stripping. The deamination temperature is 95-100° C. and the retention time is 1 h to obtain a potassium methionine aqueous solution.

[0076] The obtained potassium methionine aqueous solution was pumped into an acidification kettle, and carbon dioxide gas was introduced into the reactor. The reaction temperature was maintained at 30°C and the system pressure was 0.7 MPa. When the pH of the reaction solution dropped to 8.3, the reaction was stopped. The molar ratio of the introduced carbon dioxide to potassium methionine was 1.6:1. The obtained solid-liquid mixture was centrifuged to separate methionine crystals and a methionine mother liquor. The methionine crystals were dried in a dryer at 100° C. under vacuum to obtain a methionine composition. The methionine mother liquor was heat-treated at 180° C. to remove some water. When the pH of the mother liquor rose to 11.1, heating was stopped. When the temperature was lowered to 50° C., a 27% hydrogen peroxide solution was introduced into the mother liquor, maintaining a molar ratio of the compound represented by Chemical Formula 1 to hydrogen peroxide in the mother liquor at 1:8. The reaction was continued for 4 hours, and the content of the compound in the mother liquor was reduced from 220 ppm to 10 ppm. The treated methionine mother liquor was then reused in the hydrolysis step. The content of the compound in the potassium methionine aqueous solution obtained by the hydrolysis reaction was tested to be 2 ppm. The content of the compound represented by Chemical Formula 1 in the obtained methionine composition was less than 0.1 ppm.

[0077] The methionine composition was stored at 60° C. for 3 months. The methionine dipeptide content increased by 0.3%, and the color number of the 2% aqueous solution increased from 3.0 Hazen before storage to 3.02 Hazen.

[0078] Comparative Example 2

[0079] 2-Hydroxy-4-(methylthio)butyronitrile, ammonium carbonate and water are introduced into a reactor in a molar ratio of 1:2:4:23, the temperature in the reactor is 120° C., and the average residence time is 2 hours. 5-(2-methylmercaptoethyl)hydantoin at the reactor outlet is mixed with recycled methionine mother liquor in a mass ratio of 3:1 and then introduced into a hydrolysis reactor. Simultaneously, a potassium carbonate aqueous solution with a mass content of 50% is introduced into the hydrolysis reactor, maintaining a molar ratio of 5-(2-methylmercaptoethyl)hydantoin to potassium ions at 1:3. The mixture is kept at 180° C. for an average residence time of 0.5 hours. After the reaction is completed, the hydrolysis reaction liquid is introduced into a deamination tower, and 1N gas stripping and deamination are performed. The deamination temperature is 95-100° C., and the residence time is 1 hour to obtain a potassium methionine aqueous solution.

[0080] The obtained potassium methionine aqueous solution was pumped into an acidification kettle, and carbon dioxide gas was introduced into the reactor. The reaction temperature was maintained at 30°C and the system pressure was 0.7 MPa. The reaction was stopped when the pH of the reaction solution dropped to 8.3. The molar ratio of the introduced carbon dioxide to potassium methionine was 1.6:1. The obtained solid-liquid mixture was pumped into a centrifuge to separate methionine crystals and methionine mother liquor. The methionine crystals were dried in a dryer at 100°C under vacuum conditions to obtain a methionine composition. The methionine mother liquor was heat-treated at 180°C to remove some water. When the pH of the mother liquor rose to 11.1, heating was stopped. The treated methionine mother liquor was then recycled to the hydrolysis step. As the operation time increased, the content of the compound represented by Chemical Formula 1 in the potassium methionine aqueous solution obtained by the hydrolysis reaction was tested to be 800 ppm. At this time, the content of the compound represented by Chemical Formula 1 in the obtained methionine composition was 70 ppm.

[0081] The methionine composition was stored at 60° C. for 3 months. The methionine dipeptide content increased by 0.1%, and the color number of the 2% aqueous solution increased from 3.0 Hazen before storage to 3.3 Hazen.

[0082] It can be seen from the above examples and comparative examples that a storage-stable methionine composition can be obtained by adopting the reaction process of the present invention.

[0083] Those skilled in the art will appreciate that, based on the teachings of this specification, some modifications or adjustments may be made to the present invention, and such modifications or adjustments should also be within the scope defined by the claims of the present invention.

Claims

1. A method for preparing a methionine composition, characterized in that: The method comprises the following steps: S1: When synthesizing 5-(2-methylmercaptoethyl)hydantoin, 2-hydroxy-4(methylthio)butyronitrile is reacted with carbon dioxide and ammonia in a low temperature section to generate the intermediate 2-amino-4(methylthio)butyronitrile. In a high temperature section, carbon dioxide is introduced to replace the ammonia to control the production of compound 1 of formula 1. S2: 5-(2-methylmercaptoethyl)hydantoin is hydrolyzed under alkaline conditions to obtain a methionine salt solution; S3: introducing carbon dioxide into the solution of S2 for crystallization, and obtaining methionine crystals and methionine mother liquor after separation; S4: drying the methionine crystals obtained in S3 to obtain a methionine composition; Wherein, S1 controls the content of compound 1 in the reaction solution to be 5-500ppm.

2. The method according to claim 1, characterized in that The temperature of the low-temperature section S1 is 50-90°C, and the temperature of the high-temperature section is 110-140°C. Carbon dioxide is introduced into the high-temperature section to control the volume of ammonia in the gas phase system to 1-10% of the total volume of ammonia and carbon dioxide.

3. The method according to claim 1 or 2, characterized in that S2 said alkaline condition is formed in the presence of potassium hydroxide and / or potassium carbonate; And / or, the S2 hydrolysis reaction temperature is 150-200°C.

4. The method according to claim 1, wherein S3: The methionine salt solution is acidified by passing carbon dioxide; And / or, the temperature of S3 acidification reaction is 20-50°C.

5. The method according to claim 1, wherein S4 The methionine composition comprises methionine and compound 1.

6. The method according to claim 5, characterized in that The mass content of compound 1 in the S4 methionine composition is 0.1-20 ppm.

7. A methionine composition prepared by the method according to any one of claims 1 to 6, characterized in that: The methionine composition comprises methionine and compound 1 of formula 1; The mass content of compound 1 in the methionine composition is 0.1-20 ppm.

8. The methionine composition according to claim 7, characterized in that After the methionine composition is stored at 60° C. for 3 months, the content of the newly added impurity methionine dipeptide is ≤0.2 wt %; And / or, after the methionine composition is stored at 60° C. for 3 months, the color increase of an aqueous solution of the methionine composition containing 2 wt % of the composition is ≤0.1 Hazen.

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