Preparation method of 2-hydroxy-4-methylthiobutyric acid

Abstract

The invention discloses a preparation method of 2-hydroxy-4-(methylthio) butyric acid, and belongs to the technical field of chemical engineering, 2-hydroxy-4-(methylthio) butyronitrile and acidic resin are subjected to a hydration reaction and a hydrolysis reaction, then extractive distillation treatment is performed to recover an extraction agent, and the product 2-hydroxy-4-(methylthio) butyric acid is obtained; according to the invention, the acidic resin is used for replacing free acid, so that the corrosion to equipment is reduced, the possibility of side reaction between concentrated acid and 2-hydroxy-4-methylthio butyronitrile is avoided, the content of impurity polymers is obviously reduced, and the yield of a main product reaches 99.8% or above; the generated resin ammonium salt is about 50% less than the inorganic ammonium salt generated by using free acid, so that the post-treatment cost is reduced; through the process method provided by the invention, the prepared 2-hydroxy-4-methylthiobutyric acid is high in purity, the content of impurities such as residual solvent and resin ammonium salt in the product is low, and efficient separation and purification are realized.

Description

Technical Field

[0001] The present application belongs to the field of chemical technology, and in particular relates to a method for preparing 2-hydroxy-4-methylthiobutyric acid. Background Art

[0002] Methionine hydroxy analogue 2-hydroxy-4-methylthiobutyric acid is also called liquid methionine, abbreviated as MHA or MHA-FA; MHA is a general term for a mixture containing monomers, dimers, oligomers and water, of which monomers account for about 65%, dimers account for about 20%, oligomers account for about 3%, and water accounts for 12%. Its chemical structure is that the amino group on methionine (methionine) is replaced by a hydroxyl group. The structure determines the properties. The change of amino group to hydroxyl group makes liquid methionine have many new nutritional advantages. First, its pH value is less than 1, which is a relatively ideal organic acid that can play an effective bactericidal and antibacterial role; secondly, because the molecule does not contain nitrogen, it can reduce the degradation of MHA by animal rumen microorganisms; under the thermal effect, it has better production performance than powdered methionine, with high feed conversion rate and low mortality. Like methionine, MHA has shown the characteristics of stimulating animal growth in poultry breeding and is valued in the field of feed additives.

[0003] MHA is mainly produced by acid hydrolysis of 2-hydroxy-4-methylthiobutyronitrile, followed by separation and purification to obtain the finished product, accompanied by some ammonium salt byproducts. Patent US4912257A uses a sulfuric acid one-step hydrolysis method without using solvents, the hydrolysis mixture is neutralized with ammonium hydroxide, and ammonium bisulfate is converted into ammonium sulfate to obtain two mobile phases, which are separated and concentrated to obtain liquid methionine and sulfate; Patent CN1067377C uses a two-step hydrolysis method, the first step is to use 60-85% sulfuric acid and sulfuric acid and 2-hydroxy-4-methylthiobutyronitrile at a ratio of 0.5-1:1, hydrolyze at 30-90°C, and the second step is to hydrolyze at 140°C, after the hydrolysis is completed, the whole system is concentrated to make the salt content of the mixture greater than 55%, and then solvent extraction is used; Patent CN1400967A does not use organic solvents, but uses the form of adding alkali metal compounds to the hydrolyzed system to achieve rapid oil-water separation, and the final liquid egg content can reach 89%, and the average yield is about 95%.

[0004] Whether one-step or two-step hydrolysis is used, the process of separation and purification of the reaction system after the reaction is completed cannot be avoided. Broadly speaking, there are two main methods for separation and purification of MHA, one is through organic solvent extraction; the other is direct liquid separation purification. The organic solvent method is the process of achieving liquid-liquid separation by the difference in the solubility of MHA in water and the organic solvent used. The MHA obtained by the organic solvent extraction method has high purity and low salt content, but there will inevitably be a problem of solvent residue. The direct liquid separation method is to use the low solubility of MHA in water and the phenomenon of stratification under salting out without introducing organic solvents, and directly separate the liquid to obtain organic phase and aqueous phase. Compared with the extraction method, the liquid separation method does not introduce organic solvents and there is no problem of solvent residue, which is in line with the concept of green environmental protection. However, this method produces a large amount of wastewater that is difficult to treat, and the product quality obtained is poor, and the product yield is low.

[0005] In summary, the existing methods have the following shortcomings: excessive addition of sulfuric acid in the existing hydrolysis synthesis of liquid methionine causes relatively large corrosion to the equipment, high equipment costs, and excessively high acid concentrations lead to more by-products and impurities that are difficult to separate. In addition, the residence time of the hydrolysis synthesis of liquid methionine is very long, the degree of back mixing increases, the number of impurities increases, and the investment in the separation equipment is relatively large, which requires improvements in terms of process and equipment selection. Summary of the invention

[0006] The purpose of the present application is to provide a method for preparing 2-hydroxy-4-methylthiobutyric acid to solve the technical problems of severe equipment corrosion and a large number of by-product impurities in the synthesis of 2-hydroxy-4-methylthiobutyric acid in the prior art.

[0007] To achieve the above purpose, the technical solution adopted in the present application is: to provide a method for preparing 2-hydroxy-4-methylthiobutyric acid, which specifically comprises the following steps: (i) adding 2-hydroxy-4-methylthiobutyronitrile and acidic resin into a hydration tubular reactor for hydration reaction to obtain 2-hydroxy-4-methylthiobutyramide; transferring 2-hydroxy-4-methylthiobutyramide into a hydrolysis tubular reactor for hydrolysis reaction to obtain a mixture of 2-hydroxy-4-methylthiobutyric acid and resin ammonium salt; (ii) extracting and distilling the mixture of 2-hydroxy-4-methylthiobutyric acid and ammonium salt to obtain an extract phase and a raffinate phase, fractionating the extract phase to obtain pure 2-hydroxy-4-methylthiobutyric acid, and diluting the pure 2-hydroxy-4-methylthiobutyric acid to obtain a finished 2-hydroxy-4-methylthiobutyric acid.

[0008] In one embodiment, In step (i), the temperature of the hydration reaction is 40-70°C, and the residence time is 20-40 min. Preferably, the temperature of the hydration reaction is 65°C, and the residence time is 20 min. The temperature of the hydrolysis reaction is 80-120°C, and the residence time is 2-4 h. Preferably, the temperature of the hydrolysis reaction is 115°C, and the residence time is 3 h.

[0009] In one embodiment, The purity of 2-hydroxy-4-methylthiobutyronitrile in step (i) is 99.95%.

[0010] In one embodiment, Step (i) The acidic resin is one or more of Amberlyst 15, Amberlyst 35 or Amberlyst 16. Preferably, the acidic resin is Amberlyst 15. The diameter of the acidic resin is 0.5-3 mm.

[0011] In one embodiment, The mass ratio of the acidic resin to 2-hydroxy-4-methylthiobutyronitrile is 0.4-0.7: 1, and preferably, the molar ratio of the acidic resin to 2-hydroxy-4-methylthiobutyronitrile is 0.5: 1.

[0012] In one embodiment, Step (i) The hydration tubular reactor and the hydrolysis tubular reactor are both filled with improved flat ring fillers and acidic resin in sections.

[0013] In one embodiment, Step (i) The improved flat ring filler and the acidic resin are installed alternately in the hydration tubular reactor and the hydrolysis tubular reactor.

[0014] In one embodiment, The volume ratio of the improved flat ring filler to the acidic resin in the hydration tubular reactor is 1:1-4, and the volume ratio of the improved flat ring filler to the acidic resin in the hydrolysis tubular reactor is 1:1-8. Preferably, the volume ratio of the improved flat ring filler to the acidic resin in the hydration tubular reactor is 1:2, and the volume ratio of the improved flat ring filler to the acidic resin in the hydrolysis tubular reactor is 1:5.

[0015] In one embodiment, The extraction solvent in step (ii) is C3-C7 ketones, and the C3-C7 ketones are specifically one of acetone, butanone, methyl isobutyl ketone, 3-pentanone, 2-hexanone, cyclohexanone, and 2-heptanone. Preferably, the extraction solvent is cyclohexanone; the volume fraction of C3-C7 ketones is 0.8-0.9%, preferably, the volume fraction is 0.8%.

[0016] In one embodiment, The equipment used in step (ii) extractive distillation is an extractive distillation tower, and the pure 2-hydroxy-4-methylthiobutyric acid is distilled from the middle section of the extractive distillation tower; the temperature of the extractive distillation is 85°C, and the pressure is -0.7~-0.9 Kpa, preferably, the pressure is -0.7 Kpa.

[0017] The present application provides a method for preparing 2-hydroxy-4-methylthiobutyric acid, which uses an acidic resin as a partial filler, and the acidic resin replaces all free acids, so that the total amount of free acids is reduced by 100%, and the resin ammonium salt produced is about 50% less than the inorganic ammonium salt produced by using free acids, thereby reducing the post-processing cost; avoiding the use of free acids can also reduce acid corrosion to equipment, avoid the possibility of concentrated acids increasing side reactions, and reduce the amount of impurities, and the raw material conversion rate can reach more than 99.9%; in the hydrolysis reaction, a filled tubular reactor is used instead of a traditional kettle reactor, and the filler is filled in sections using an acidic resin and an improved flat ring filler. Compared with the prior art, the filler not only utilizes the acidity provided by the acidic resin, but also acts as a filler and the flat ring segmented filling to play a good dispersing effect on the medium, reduces the degree of back mixing, provides more specific reaction active sites, and improves the reaction efficiency. The purity of 2-hydroxy-4-methylthiobutyronitrile is more than 99.95%, and the yield of the main product is more than 99.8%; At the same time, it is unexpected that under the reaction conditions of the present technical solution, the acidic resin can preferentially adsorb and activate the target reactant molecules and promote the main reaction. There is a regular pore structure inside the acidic resin, and only reactant molecules smaller than the pore size can enter the active sites inside the resin for reaction. This blocks the impurity molecules that do not meet the size requirements from being kept outside, thereby reducing the content of impurities in the product. After the resin is finally used, the content of impurity polymers in the product is significantly reduced, such as hydrocyanic acid, 3-methylthiopropionaldehyde self-polymers, etc. The total amount of impurity polymers is reduced from the original 2000-5000 ppm to undetectable. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0019] Figure 1 is the chemical formula of 2-hydroxy-4-methylthiobutyric acid; Figure 2 The chemical reaction formula for preparing 2-hydroxy-4-methylthiobutyric acid; Figure 3is the liquid phase diagram of 2-hydroxy-4-methylthiobutyric acid; Figure 4 This is a diagram of the process equipment.

[0020] Explanation of symbols in the figure: 1. Hydration / hydrolysis tubular reactor; 2. Extraction distillation tower; 3. Raw materials; 4. Improved flat ring filler; 5. Acid resin; 6. Solvent recovery point; 7. Pure 2-hydroxy-4-methylthiobutyric acid; 8. Extraction agent; 9. Raffinate phase. DETAILED DESCRIPTION

[0021] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clear, the present application is further described in detail. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0022] Example 1 (i) Filling a hydration tubular reactor with an improved flat ring filler and an acidic resin Amberlyst 15 (with a diameter of 0.5-3 mm), the filling volume ratio of the improved flat ring filler to Amberlyst 15 being 1:2, adding 2-hydroxy-4-methylthiobutyronitrile with a purity of 99.95% at a mass flow rate of 48.50 kg / h for hydration reaction, the residence time being 20 min, and the reaction temperature being 65°C; at this time, the mass fraction of 2-hydroxy-4-methylthiobutyramide in the mixed system is 68.84%, the mass fraction of water is 25.07%, and the mass fraction of resin ammonium salt is 6.09%. Liquid phase tracking detection is performed to obtain a hydration product, 2-hydroxy-4-methylthiobutyramide, and the cyanohydrin conversion rate is 99.99%; 2-hydroxy-4-methylthiobutyramide was transferred for hydrolysis reaction to obtain a mixture of 2-hydroxy-4-methylthiobutyric acid and resin ammonium salt. The specific steps of the hydrolysis reaction were as follows: 2-hydroxy-4-methylthiobutyramide was heated to 115°C by S3 steam through the jacket of a hydrolysis tubular reactor, the hydrolysis tubular reactor was filled with acidic resin Amberlyst 15 (with a diameter of 0.5-3 mm) and improved flat ring filler, the filling volume ratio of the improved flat ring filler to Amberlyst 15 was 1:5, desalted water was pumped in from the side of the hydrolysis tubular reactor at a mass flow rate of 17.37 kg / h for hydrolysis, the residence time was 3 h, the reaction was completed, and the mixture was transferred to a heat exchanger to obtain a mixture of 2-hydroxy-4-methylthiobutyric acid and ammonium salt, the yield of 2-hydroxy-4-methylthiobutyric acid was 99.8%; The improved flat ring filler and the acid resin are installed alternately in the hydration tubular reactor and the hydrolysis tubular reactor; (ii) In an extractive distillation tower, extractive distillation is performed on a mixture of 2-hydroxy-4-methylthiobutyric acid and ammonium salt of the resin. The mixture of 2-hydroxy-4-methylthiobutyric acid and ammonium salt of the resin enters from the top of the tower, and cyclohexanone with a volume fraction of 0.8% enters from the bottom of the tower at a rate of 105 kg / h. The extractive distillation temperature is 85°C and the extractive distillation pressure is -0.7 Kpa to obtain an extract phase and a raffinate phase. The extractive distillation agent cyclohexanone obtained at the top of the tower can be recycled for use; the raffinate phase at the bottom of the tower has a water content of 50-51%, a resin ammonium salt content of 36-37%, and a 2-hydroxy-4-methylthiobutyric acid content of 0.5-0.6%. The raffinate phase is neutralized and crystallized, neutralized with liquid ammonia, and centrifuged and dried to obtain a resin ammonium salt by-product; the extract phase in the tower is fractionated, and a 98% concentration of pure 2-hydroxy-4-methylthiobutyric acid is distilled from the middle section of the extractive distillation tower, and diluted with water to obtain a 88% concentration of pure 2-hydroxy-4-methylthiobutyric acid. % 2-hydroxy-4-methylthiobutyric acid product, wherein pH ≤ 1, resin ammonium salt content ≤ 0.22%, calculated on the basis of 2-hydroxy-4-methylthiobutyronitrile, 2-hydroxy-4-methylthiobutyric acid yield 99.9%, wherein monomer content 72.87%, dimer content 23.26%, oligomer content 3.86%, structural characterization as shown below Figure 3 As shown; Example 2 (i) Filling a hydration tubular reactor with an improved flat ring filler and an acidic resin Amberlyst 16 (with a diameter of 0.5-3 mm), the filling volume ratio of the improved flat ring filler to Amberlyst 16 being 1:2, adding 2-hydroxy-4-methylthiobutyronitrile with a purity of 99.95% at a mass flow rate of 48.50 kg / h for hydration reaction, the residence time being 20 min, and the reaction temperature being 65°C; at this time, the mass fraction of 2-hydroxy-4-methylthiobutyramide in the mixed system is 67.84%, the mass fraction of water is 25.87%, and the mass fraction of resin ammonium salt is 7.09%. Liquid phase tracking detection is performed to obtain a hydration product, 2-hydroxy-4-methylthiobutyramide, and the cyanohydrin conversion rate is 99.99%; The 2-hydroxy-4-methylthiobutyramide is transferred for hydrolysis reaction to obtain a mixture of 2-hydroxy-4-methylthiobutyric acid and resin ammonium salt. The specific steps of the hydrolysis reaction are: 2-hydroxy-4-methylthiobutyramide is heated to 115°C through the jacket of the hydrolysis tubular reactor with S3 steam, the hydrolysis tubular reactor is filled with acidic resin Amberlyst 16 (with a diameter of 0.5-3 mm) and improved flat ring filler, and the filling volume ratio of the improved flat ring filler to Amberlyst 16 is 1:5, and desalted water is pumped from the side of the hydrolysis tubular reactor at a mass flow rate of 17.37 kg / h for hydrolysis, and the residence time is 3 hours. After the reaction is completed, it is transferred to a heat exchanger to obtain a mixture of 2-hydroxy-4-methylthiobutyric acid and resin ammonium salt, and the yield of 2-hydroxy-4-methylthiobutyric acid is 99.8%; The improved flat ring filler and the acid resin are installed alternately in the hydration tubular reactor and the hydrolysis tubular reactor; (ii) In an extractive distillation tower, extractive distillation is performed on a mixture of 2-hydroxy-4-methylthiobutyric acid and resin ammonium salt. The mixture of 2-hydroxy-4-methylthiobutyric acid and resin ammonium salt enters from the top of the tower, and cyclohexanone with a volume fraction of 0.8% enters from the bottom of the tower at a rate of 105 kg / h. The extractive distillation temperature is 85°C and the extractive distillation pressure is -0.9 Kpa to obtain an extract phase and a raffinate phase. The extractive distillation agent cyclohexanone obtained at the top of the tower can be recycled for use; the raffinate phase at the bottom of the tower has a water content of 50-51%, a resin ammonium salt content of 36-37%, and a 2-hydroxy-4-methylthiobutyric acid content of 0.5-0.6%. The raffinate phase is neutralized and crystallized, neutralized with liquid ammonia, and centrifuged and dried to obtain a resin ammonium salt by-product; the extract phase in the tower is fractionated, and a 98% concentration of pure 2-hydroxy-4-methylthiobutyric acid is distilled from the middle section of the extractive distillation tower, and diluted with water to obtain a 88% concentration of pure 2-hydroxy-4-methylthiobutyric acid. % 2-hydroxy-4-methylthiobutyric acid product, wherein pH ≤ 1, resin ammonium salt content ≤ 0.25%, calculated on the basis of 2-hydroxy-4-methylthiobutyronitrile, 2-hydroxy-4-methylthiobutyric acid yield 99.9%, wherein monomer content 71.92%, dimer content 24.11%, oligomer content 3.96%.

[0023] Example 3 The difference between this embodiment and embodiment 1 is that the temperature of the hydration reaction in step (i) is 40° C., and the other operations are the same.

[0024] Example 4 The difference between this embodiment and embodiment 1 is that the temperature of the hydration reaction in step (i) is 70° C., and the other operations are the same.

[0025] Example 5 The difference between this embodiment and embodiment 1 is that the residence time of the hydration reaction in step (i) is 30 min, and the other operations are the same.

[0026] Example 6 The difference between this embodiment and embodiment 1 is that the residence time of the hydration reaction in step (i) is 40 min, and the other operations are the same.

[0027] Example 7 The difference between this embodiment and embodiment 1 is that in step (i), the mass ratio of the acidic resin Amberlyst 15 to 2-hydroxy-4-methylthiobutyronitrile is 0.4:1, and the remaining operations are the same.

[0028] Example 8 The difference between this embodiment and embodiment 1 is that in step (i), the mass ratio of the acidic resin Amberlyst 15 to 2-hydroxy-4-methylthiobutyronitrile is 0.7:1, and the remaining operations are the same.

[0029] Example 9 The difference between this embodiment and embodiment 1 is that the temperature of the hydrolysis reaction in step (i) is 80° C., and the other operations are the same.

[0030] Example 10 The difference between this embodiment and embodiment 1 is that the temperature of the hydrolysis reaction in step (i) is 120° C., and the other operations are the same.

[0031] Embodiment 11 The difference between this embodiment and embodiment 1 is that the residence time of the hydrolysis reaction in step (i) is 2 h, and the other operations are the same.

[0032] Example 12 The difference between this embodiment and embodiment 1 is that the residence time of the hydrolysis reaction in step (i) is 4 h, and the other operations are the same.

[0033] Example 13 The difference between this embodiment and embodiment 1 is that the pressure of the extractive distillation in step (ii) is -0.8 KPa, and the other operations are the same.

[0034] Embodiment 14 The difference between this embodiment and embodiment 1 is that the volume fraction of cyclohexanone in step (ii) is 0.9%, and the other operations are the same.

[0035] Embodiment 15 The difference between this embodiment and embodiment 1 is that Amberlyst 15 in step (I) is replaced with Amberlyst 35, and the remaining operations are the same.

[0036] Example 16 The difference between this embodiment and embodiment 1 is that Amberlyst 15 in step (i) is replaced with a mixture of Amberlyst 15 and Amberlyst 35, and the mixing ratio of Amberlyst 15 and Amberlyst 35 is 1:1. The other operations are the same.

[0037] Embodiment 17 The difference between this embodiment and embodiment 1 is that the extraction solvent in step (ii) is changed from cyclohexanone to acetone, and the other operations are the same.

[0038] Embodiment 18 The difference between this embodiment and embodiment 1 is that the extraction solvent in step (ii) is changed from cyclohexanone to butanone, and the other operations are the same.

[0039] Embodiment 19 The difference between this embodiment and embodiment 1 is that the extraction solvent in step (ii) is changed from cyclohexanone to methyl isobutyl ketone, and the remaining operations are the same.

[0040] Embodiment 20 The difference between this embodiment and embodiment 1 is that the extraction solvent in step (ii) is changed from cyclohexanone to 3-pentanone, and the remaining operations are the same.

[0041] Embodiment 21 The difference between this embodiment and embodiment 1 is that the extraction solvent in step (ii) is changed from cyclohexanone to 2-hexanone, and the remaining operations are the same.

[0042] Embodiment 22 The difference between this embodiment and embodiment 1 is that the extraction solvent in step (ii) is changed from cyclohexanone to 2-heptanone, and the other operations are the same.

[0043] Embodiment 23 The difference between this embodiment and embodiment 1 is that the volume ratio of the improved flat ring filler to the acidic resin in the hydration tubular reactor in step (I) is changed from 1:2 to 1:1, and the remaining operations are the same.

[0044] Embodiment 24 The difference between this embodiment and embodiment 1 is that the volume ratio of the improved flat ring filler to the acidic resin in the hydration tubular reactor in step (I) is changed from 1:2 to 1:4, and the remaining operations are the same.

[0045] Embodiment 25 The difference between this embodiment and embodiment 1 is that the volume ratio of the improved flat ring filler to the acidic resin in the hydrolysis tubular reactor in step (I) is changed from 1:5 to 1:1, and the remaining operations are the same.

[0046] Embodiment 26 The difference between this embodiment and embodiment 1 is that the volume ratio of the improved flat ring filler to the acidic resin in the hydrolysis tubular reactor in step (I) is changed from 1:5 to 1:3, and the remaining operations are the same.

[0047] Embodiment 27 The difference between this embodiment and embodiment 1 is that the volume ratio of the improved flat ring filler to the acidic resin in the hydrolysis tubular reactor in step (I) is changed from 1:5 to 1:8, and the remaining operations are the same.

[0048] The present application provides a method for preparing 2-hydroxy-4-methylthiobutyric acid, comprising the following steps: adding 2-hydroxy-4-methylthiobutyronitrile and an acidic resin into a hydration tubular reactor for hydration reaction to obtain 2-hydroxy-4-methylthiobutyramide; transferring the 2-hydroxy-4-methylthiobutyramide into a hydrolysis tubular reactor for hydrolysis reaction to obtain a mixture of 2-hydroxy-4-methylthiobutyric acid and resin ammonium salt; extracting and rectifying the mixture of 2-hydroxy-4-methylthiobutyric acid and ammonium salt to obtain an extract phase and a raffinate phase, fractionating the extract phase to obtain a pure 2-hydroxy-4-methylthiobutyric acid product, and diluting the pure 2-hydroxy-4-methylthiobutyric acid product to obtain a finished 2-hydroxy-4-methylthiobutyric acid product; the present application uses an acidic resin as a partial filler of the tubular reactor, and the acidic resin replaces all free acid, so that the total amount of free acid is reduced by 100%. %, which reduces the corrosion to the equipment and avoids the possibility of side reaction between concentrated acid and 2-hydroxy-4-methylthiobutyronitrile; after using the acidic resin, the content of impurity polymers in the product is significantly reduced, such as hydrocyanic acid, 3-methylthiopropionaldehyde self-polymer, etc. The total amount of impurity polymers is reduced from the original 2000-5000 ppm to undetectable, so that the yield of the main product is more than 99.8%; in the hydrolysis reaction, a packed tubular reactor is used instead of a traditional kettle reactor, and the filler is an acidic resin and an improved flat ring filler. Compared with the traditional kettle reactor, the use of this reactor can reduce investment and leakage points, reduce corrosion damage to the equipment, and has relatively low requirements for equipment installation; compared with the traditional filler, the filler not only utilizes the acidity provided by the acidic resin, but also acts as a filler and flat ring segmented filling to play a good dispersing effect on the medium, and the reaction efficiency is also improved, and the raw material conversion rate can reach 99.9%. In this application, all free acids are replaced by acidic resins, and the resin ammonium salt produced is 50% less than the inorganic ammonium salt produced by using free acids. Although the resin has a limited lifespan, two sets of hydration and hydrolysis tubular reactors can be set up in industrial practice, so that one set can be ensured to be in normal use, and the other set can be used for online activation and maintenance of the resin, which is convenient for maintenance.

[0049] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0050] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method for preparing 2-hydroxy-4-methylthiobutyric acid, characterized in that: The specific steps include: (i) adding 2-hydroxy-4-methylthiobutyronitrile and acidic resin into a hydration tubular reactor for hydration reaction to obtain 2-hydroxy-4-methylthiobutyramide; transferring the 2-hydroxy-4-methylthiobutyramide into a hydrolysis tubular reactor for hydrolysis reaction to obtain a mixture of 2-hydroxy-4-methylthiobutyric acid and resin ammonium salt; (ii) extracting and distilling the mixture of the 2-hydroxy-4-methylthiobutyric acid and the resin ammonium salt to obtain an extract phase and a raffinate phase, fractionating the extract phase to obtain a pure 2-hydroxy-4-methylthiobutyric acid product, and diluting the pure 2-hydroxy-4-methylthiobutyric acid product to obtain a finished 2-hydroxy-4-methylthiobutyric acid product.

2. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 1, characterized in that: In step (i), the temperature of the hydration reaction is 40-70°C, and the residence time is 20-40 min; the temperature of the hydrolysis reaction is 80-120°C, and the residence time is 2-4 h.

3. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 1, characterized in that: The purity of the 2-hydroxy-4-methylthiobutyronitrile in step (i) is 99.95%.

4. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 1, characterized in that: In step (i), the acidic resin is one or more of Amberlyst 15, Amberlyst 35 or Amberlyst 16; the diameter of the acidic resin is 0.5-3 mm.

5. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 1, characterized in that: The mass ratio of the acidic resin to the 2-hydroxy-4-methylthiobutyronitrile is 0.4-0.7:

1.

6. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 1, characterized in that: In step (i), the hydration tubular reactor and the hydrolysis tubular reactor are both filled with improved flat ring fillers and acidic resin in sections.

7. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 1, characterized in that: In step (i), the improved flat ring filler and the acidic resin are installed alternately in the hydration tubular reactor and the hydrolysis tubular reactor.

8. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 6, characterized in that: The volume ratio of the improved flat ring filler to the acidic resin in the hydration tubular reactor is 1:1-4, and the volume ratio of the improved flat ring filler to the acidic resin in the hydrolysis tubular reactor is 1:1-8.

9. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 1, characterized in that: The solvent for the extractive distillation in step (ii) is C3-C7 ketones, and the C3-C7 ketones are specifically one of acetone, butanone, methyl isobutyl ketone, 3-pentanone, 2-hexanone, cyclohexanone, and 2-heptanone, and the volume fraction of the C3-C7 ketones is 0.8-0.9%.

10. The method for preparing 2-hydroxy-4-methylthiobutyric acid according to claim 1, characterized in that: The equipment used for the extractive distillation in step (ii) is an extractive distillation tower, and the pure 2-hydroxy-4-methylthiobutyric acid is distilled from the middle section of the extractive distillation tower; the temperature of the extractive distillation is 85°C, and the pressure is -0.7~-0.9 Kpa.

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