ADDITIVE USED IN METHIONINE PREPARATION PROCESS, AND METHIONINE PREPARATION METHOD

Abstract

The present disclosure relates to an additive used in a methionine preparation process, and a methionine preparation method. The additive provided by the present disclosure is a mixture containing components A, B, and C; component A has a structure represented by the following general formula (1); component B has a structure represented by the following general formula (2); component C is silicone oil; RCON(CH.sub.3)CH.sub.2CH.sub.2SO.sub.3Na (1). The methionine preparation method provided in the present invention comprises subjecting methionine to crystallization and/or recrystallization in the presence of the additive provided by the present disclosure. The additive provided by the present disclosure results in uniform emulsification, has good stability, can be used stably for a long time, and is suitable for a continuous crystallization process. The prepared methionine crystal has a good crystal form, a large bulk density, and good flowability. In addition, according to the methionine preparation method of the present disclosure, a crystallization system can operate continuously and stably for a long time without obvious foaming, and the crystallization process of the methionine product can proceed smoothly.

##STR00001##

Claims

1. An additive for use in a methionine preparation process, wherein the additive is a mixture comprising components A, B and C, the component A has a structure represented by the following general formula (1):
RCON(CH.sub.3)CH.sub.2CH.sub.2SO.sub.3Na  (1) in formula (1), R is a saturated or unsaturated C.sub.7-C.sub.36 hydrocarbyl group; the component B has a structure represented by the following general formula (2): ##STR00004## in formula (2), X and Y are each an integer of 1 to 30, R.sub.1 to R.sub.4 are the same as or different from one another, and each independently represent H, an C.sub.1-C.sub.3 alkyl group, a saturated or unsaturated aliphatic hydroxyl group, or a saturated or unsaturated polyether group, provided that at least one of R.sub.1 to R.sub.4 represents the saturated or unsaturated polyether group; and the component C is silicone oil.

2. The additive for use in a methionine preparation process according to claim 1, wherein the component A is one or more of sodium methyl octanoyl taurate, sodium methyl decanoyl taurate, sodium methyl lauroyl taurate, sodium methyl myristoyl taurate, sodium methyl palmitoyl taurate, sodium methyl stearoyl taurate, sodium methyl oleyl taurate, sodium methyl linoleoyl taurate, sodium methyl linolenoyl taurate, sodium methyl erucoyl taurate, sodium methyl cocoyl taurate, sodium methyl palmitoyl taurate, sodium methyl soybean oil fatty acyl taurate, sodium methyl peanut oil fatty acyl taurate, sodium methyl sesame oil fatty acyl taurate, sodium methyl mustard oil fatty acyl taurate, sodium methyl hardened tallow fatty acyl taurate, and sodium methyl hardened vegetable oil fatty acyl taurate.

3. The additive for use in a methionine preparation process according to claim 1, wherein the component B has a molecular weight of from 1,000 to 10,000.

4. The additive for use in a methionine preparation process according to claim 1, wherein the component B has an HLB value of from 7 to 15.

5. The additive for use in a methionine preparation process according to a claim 1, wherein the component B is polyether-grafted silicone oil.

6. The additive for use in a methionine preparation process according to claim 1, wherein the component C comprises one or more of dimethyl silicone oil, hydroxyl silicone oil, and hydrogen-containing silicone oil.

7. The additive for use in a methionine preparation process according to claim 1, wherein the additive is an aqueous mixture comprising the components A, B and C, and based on a total mass of the additive, the content of the component A is from 1 to 8 wt %, and the content of the component B is from 0.5 to 8 wt %, and the content of the component C is from 0.5 to 4 wt %.

8. A method for preparing methionine, comprising: subjecting methionine to crystallization and/or recrystallization in the presence of the additive according to claim 1.

9. The preparation method for methionine according to claim 8, comprising the steps of: (1) subjecting an aqueous methionate solution obtained from hydrolysis of 5-(2-methylthioethyl)-hydantoin to reaction crystallization in the presence of an additive and carbon dioxide, to obtain a crude methionine crystal product; and (2) forming a methionine suspension from the crude methionine crystal product, water and/or crystallization mother liquor, adding an additive, and recrystallizing an additive-containing methionine solution, to obtain a methionine crystal product.

10. The preparation method for methionine according to claim 9, wherein an amount of the additive in the step (1) is from 50 ppm to 500 ppm based on a total mass of the aqueous methionate solution.

11. The preparation method for methionine according to claim 9, wherein the methionine suspension in the step (2) has a concentration of from 8 to 15 wt %.

12. The preparation method for methionine according to claim 9, wherein an amount of the additive in the step (2) is from 100 ppm to 1,000 ppm based on a total mass of the methionine suspension.

13. The preparation method for methionine according to any claim 9, wherein in the step (2), the crystallization is carried out by means of refrigerative cooling or evaporative cooling; and when the crystallization is carried out by the evaporative cooling, vapor generated by evaporation is pressurized by a vapor compressor to raise temperature, and is then used in a heating and dissolving process of the methionine suspension.

14. The preparation method for methionine according to claim 9, wherein a crystallizer used in the step (2) includes a stirring crystallization kettle, a forced-circulating crystallizer, an OSLO crystallizer, and a draft-tube-baffled crystallizer.

15. The additive for use in a methionine preparation process according to claim 1, wherein the component B has a molecular weight of from 3,000 to 6,000.

16. The additive for use in a methionine preparation process according to claim 1, wherein the component B has an HLB value of from 9 to 12.

17. The additive for use in a methionine preparation process according to claim 1, wherein the component C has a dynamic viscosity at 25° C. of 90 mm.sup.2/s to 1,500 mm.sup.2/s.

18. The additive for use in a methionine preparation process according to claim 1, wherein the polyether group is a group represented by the following general formula (3):
—C.sub.3H.sub.6O(C.sub.2H.sub.4O).sub.a(C.sub.3H.sub.6O).sub.bR.sub.0  (3) in formula (3), a represents an integer of 0 to 50, b represents an integer of 0 to 50, and R.sub.0 represents H, an alkyl group, an acyloxyl group, or an alkoxyl group.

19. The additive for use in a methionine preparation process according to claim 1, wherein the content of the component A is from 2 to 6 wt %, the content of the component B is from 2 to 6 wt %, and the content of the component C is from 1 to 3 wt %.

20. The preparation method for methionine according to claim 9, wherein an amount of the additive in the step (1) is from 70 ppm to 300 ppm based on a total mass of the aqueous methionate solution.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] FIGS. 1 to 5 show microscope photographs of methionine crystals corresponding to Examples 1 to 5, respectively.

DETAILED DESCRIPTION

[0051] The additive used in the preparation process of methionine provided in the present disclosure is a mixture comprising components A, B, and C. Moreover, in the preferred embodiments of the present disclosure, the additive is used in the form of a water-containing mixture.

[0052] The component A is a crystal growth regulator, and has a structure represented by the following general formula (1):

RCON(CH.sub.3)CH.sub.2CH.sub.2SO.sub.3Na  (1) [0053] in formula (1), R is a saturated or unsaturated C.sub.7-C.sub.36 hydrocarbyl group, preferably a C.sub.7-C.sub.36 alkyl or alkenyl group.

[0054] Specific examples of the component A include one or more of sodium methyl octanoyl taurate, sodium methyl decanoyl taurate, sodium methyl lauroyl taurate, sodium methyl myristoyl taurate, sodium methyl palmitoyl taurate, sodium methyl stearoyl taurate, sodium methyl oleyl taurate, sodium methyl linoleoyl taurate, sodium methyl linolenoyl taurate, sodium methyl erucoyl taurate, sodium methyl cocoyl taurate, sodium methyl palmitoyl taurate, sodium methyl soybean oil fatty acyl taurate, sodium methyl peanut oil fatty acyl taurate, sodium methyl sesame oil fatty acyl taurate, sodium methyl mustard oil fatty acyl taurate, sodium methyl hardened tallow fatty acyl taurate, and sodium methyl hardened vegetable oil fatty acyl taurate.

[0055] Preferably, the component A is one or more of sodium methyl lauroyl taurate, sodium methyl myristoyl taurate, sodium methyl palmitoyl taurate, sodium methyl stearoyl taurate, sodium methyl cocoyl taurate, and sodium methyl palmitoyl taurate.

[0056] Generally, specific compounds of the above-mentioned component A are commercially available.

[0057] In the additive provided in the present application, the component A is featured with extremely low irritation, non-toxicity, and easy-biodegradation, and has good biodegradation. This substance can be decomposed into fatty acids and amino acids by enzymes in animal bodies, which can be utilized in animal bodies. Further, compared with the ester additives used in the prior art, the additive of the present disclosure has better hydrolytic stability, and thus it can be circulated in the system for a long time, and can stably function as a crystal growth regulator.

[0058] The above-mentioned component B is an emulsified foam inhibitor, which mainly has a foam inhibitory effect and an emulsification effect, and has a structure represented by the following general formula (2):

##STR00003## [0059] in formula (2), X and Y are each an integer of 1 to 30, R.sub.1 to R.sub.4 are the same as or different from one another, and each independently represent H, an C.sub.1-C.sub.3 alkyl group, a saturated or unsaturated aliphatic hydroxyl group, or a saturated or unsaturated polyether group; when at least one of R.sub.1 to R.sub.4 represents the saturated or unsaturated polyether group, the polyether group being more preferably a group represented by the following general formula (3):

—C.sub.3H.sub.6O(C.sub.2H.sub.4O).sub.a(C.sub.3H.sub.6O).sub.bR.sub.0  (3) [0060] where a represents an integer of 0 to 50, b represents an integer of 0 to 50, and R.sub.0 represents H, an alkyl group, an acyloxyl group, or an alkoxyl group; and [0061] the component B has a molecular weight of from 1,000 to 10,000, preferably from 3,000 to 6,000, and has an HLB value of from 7 to 15, preferably from 9 to 12.

[0062] The component B is preferably polyether-modified silicone oil. In a more preferred embodiment, the component B is allyl polyoxyalkyl ethers-grafted silicone oil. The component B is even more preferably polyoxyethylene polyoxypropylene ether-grafted silicone oil.

[0063] The component B can be self-emulsified in water or miscible with water in an arbitrary ratio. Thus, during crystallization and filtration of methionine, almost all of the component B enters the water phase without attaching to the surface of methionine products. As a result, its defoaming action can be maintained stably for a long time. The component B is considered to have the defoaming, foam inhibitory and emulsifying functions when used under the conditions of the present disclosure, and it plays a main role in foam inhibition and emulsification.

[0064] In the preparation process of methionine, a large amount of foams is generated due to a need to charge carbon dioxide into the crude methionine crystal product, and meanwhile, the addition of the component A during the recrystallization of methionine also results in lots of foams. These foams have impacts on the stable operation of the system, and on the morphology and bulk density of the product, so there is a need to inhibit the foams using the component B. In the presence of the component B can the component A and the component C be in an emulsified state and be dispersed in the system.

[0065] The component C is silicone oil with an intensified defoaming action. It eliminates foams generated in the crystallization process, stabilizes the crystallization environment, improves the smoothness of crystals, and imparts good flowability to the products.

[0066] The component C comprises dimethyl silicone oil, hydroxyl silicone oil, or hydrogen-containing silicone oil. Preferably, the component C has a dynamic viscosity at 25° C. of 90 mm.sup.2/s to 1,500 mm.sup.2/s.

[0067] By using the additive of the present disclosure during hydrolysis of 5-(2-methylthioethyl)-hydantoin and acidification with carbon dioxide for reaction crystallization and recrystallization, under the synergistic effect of components A, B and C, it is possible to produce powdery methionine crystal products with a higher bulk density and better flowability, and enable the crystallization system to operate continuously and stably for a long time without obvious foaming, and the crystallization process of methionine products to proceed smoothly and steadily. Also, the additive according to the present disclosure can exhibit its effects on defoaming, foam inhibition, and crystal growth promotion when added in a small amount.

[0068] The total content of the components A, B and C is 2% to 20%, preferably 5% to 15% of the total weight of the additive in the form of an aqueous mixture, and the remaining is water.

[0069] Preferably, for the sake of allocation and uniform distribution of the additive, the additive of the present disclosure is used in the form of an aqueous mixture. Moreover, the above-mentioned components A, B and C are each present in the additive in a specific proportion. Specifically, based on the total mass of the additive, the content of the component A is from 1 to 8 wt %, the content of the component B is from 0.5 to 8 wt %, and the content of the component C is from 0.5 to 4 wt %; and preferably, the content of the component A is from 2 to 6 wt %, the content of the component B is from 2 to 6 wt %, and the content of the component C is from 1 to 3 wt %. If the content of the components A, B and C is not within the above-mentioned ranges, it is impossible to form a stable emulsifier, and the components cannot be uniformly dispersed in the system, and thus cannot achieve the desired effects. If the content of any one of the three components is not within the above-mentioned range, the balance of synergism of the additive will be upset, thereby weakening the desired effects.

[0070] The method for preparing methionine provided in the present disclosure comprises subjecting methionine to crystallization and/or recrystallization in the presence of the additive according to the present disclosure.

[0071] Preferably, the preparation method of methionine according to the present disclosure comprises the steps of: [0072] (1) subjecting an aqueous methionate solution obtained from hydrolysis of 5-(2-methylthioethyl)-hydantoin to reaction crystallization in the presence of the additive and carbon dioxide, to obtain a crude methionine crystal product; and [0073] (2) forming a methionine suspension from the crude methionine crystal product, water and/or crystallization mother liquor, and recrystallizing an additive-containing methionine solution obtained by dissolving the methionine suspension together with the additive, to obtain a methionine crystal product.

[0074] To be specific, the preparation method of methionine according to the present disclosure comprises: [0075] (1) adding the additive according to the present disclosure to an aqueous methionate solution obtained from hydrolysis of 5-(2-methylthioethyl)-hydantoin, charging carbon dioxide while stirring, precipitating a methionine crystal by means of reaction crystallization, and filtering the methionine crystal to obtain a crude methionine crystal product; and [0076] (2) mixing the aforesaid crude methionine crystal product with water and/or crystallization mother liquor to obtain a methionine suspension, adding the additive, and completely dissolving the crude methionine crystal product by stirring at a temperature of 90° C. to 110° C. to obtain an additive-containing methionine solution; and recrystallizing the additive-containing methionine solution at a temperature of 25° C. to 65° C., and subjecting the resulting crystallization slurry to filtration, washing and drying to obtain a methionine crystal product.

[0077] The crystallization mother liquor may be crystallization mother liquor resulting from filtration after the recrystallization in step (2) in the preparation method for methionine according to the present disclosure, or mixed liquor of the crystallization mother liquor resulting from filtration after the crystallization in step (1) and the crystallization mother liquor resulting from filtration after the recrystallization in step (2) in the preparation method for methionine according to the present disclosure.

[0078] In the preparation method for methionine provided in the present disclosure, the 5-(2-methylthioethyl)-hydantoin used in step (1) is commercially available.

[0079] In step (1), the method for precipitating a methionine crystal product by producing an aqueous methionate solution from hydrolysis of 5-(2-methylthioethyl)-hydantoin, and then charging carbon dioxide is commonly employed in the art, and the operation conditions may be those commonly used in the art. For example, the method may be a method for obtaining potassium methionate by hydrolyzing 5-(2-methylthioethyl)-hydantoin at 120 to 250° C. and 5 to 30 bar in the presence of potassium hydroxide, potassium carbonate and/or potassium bicarbonate or a mixture thereof, and then liberating a methionine crystal from the aqueous potassium methionate solution with carbon dioxide.

[0080] Further, the amount of the additive in the step (1) is from 50 ppm to 500 ppm, preferably from 70 ppm to 300 ppm, based on a total mass of the aqueous methionate solution. In the step (1), if the addition amount of the additive exceeds the upper limit of the above-mentioned range, the defoaming agent will be accumulated, leading to a decrease in bulk density and an increase in cost; if it is less than the lower limit of the above-mentioned range, the additive does not work as intended.

[0081] According to the preparation method for methionine provided in the present disclosure, in step (2), the process of obtaining the additive-containing methionine solution by dissolving the crude methionine together with the additive and water and/or crystallization mother liquor is usually carried out in a stirring vessel; thereafter, the additive-containing methionine solution is fed into the crystallizer preferably in a continuous state, and is crystallized therein continuously.

[0082] In step (2), the concentration of the additive-containing methionine solution is from 8 to 15 wt %, preferably from 10 to 13 wt %. If the concentration of the additive-containing methionine solution is too low, plenty of water needs to be removed by evaporation in the subsequent stages, resulting in higher costs; if the concentration is too high, the crude methionine crystal cannot be fully dissolved, and as a result, the crystal growth during recrystallization cannot achieve the desired effect.

[0083] In step (2), the addition amount of the additive is from 100 ppm to 1,000 ppm, preferably from 200 ppm to 500 ppm, based on the total mass of the methionine suspension. In the step (2), if the addition amount of the additive exceeds the upper limit of the above-mentioned range, the defoaming agent will be accumulated, leading to a decrease in bulk density and an increase in cost; if it is less than the lower limit of the above-mentioned range, the additive does not work as intended.

[0084] In step (2), the crystallization is carried out preferably by refrigerative cooling or evaporative cooling, more preferably by the evaporative cooling. Besides, when the crystallization is carried out by the evaporative cooling, vapor generated by evaporation may be used in the heating and dissolving process of the methionine suspension, after being pressurized by a vapor compressor to raise temperature, so as to achieve the effect of energy reuse.

[0085] The crystallizer used in step (2) may be of various forms suitable for continuous crystallization, and it is not particularly limited. For example, the crystallizer may be a stirring crystallization kettle, e.g., a stirring crystallization kettle with external circulation and a horizontal stirring crystallizer, a forced-circulating crystallizer (FC crystallizer), an OSLO crystallizer (OSLO crystallizer), and a draft-tube-baffled crystallizer (DTB crystallizer); and preferably, the continuous crystallization is carried out in an FC crystallizer, an OSLO crystallizer, and DTB crystallizer.

[0086] The present disclosure will be further illustrated with reference to the examples, but the examples described below are not intended to limit the scope of protection for the present disclosure.

Example 1

[0087] The additive used in this example was an aqueous mixture of 6 wt % of sodium methyl stearoyl taurate, 6 wt % of polyoxyethylene polyoxypropylene ether-grafted silicone oil having a molecular weight of 3,000 and an HLB value of 10, 3 wt % of hydroxyl silicone oil having a dynamic viscosity at 25° C. of 90 mm.sup.2/s, and the remaining being water.

[0088] (1) To the aqueous methionate solution obtained from hydrolysis of 5-(2-methylthioethyl)-hydantoin, 300 ppm of an additive based on the aqueous methionate solution was added, and carbon dioxide was charged while stirring to precipitate a methionine crystal by means of reaction crystallization. The methionine crystal was filtered to obtain a crude methionine crystal product.

[0089] (2) The above crude methionine crystal product was fed into a stirring vessel, and an appropriate amount of water and/or crystallization mother liquor was added thereto to formulate a methionine suspension having a solid content of 11 wt %. 500 ppm of the additive based on the methionine suspension was added to the vessel, stirred and heated till the temperature in the vessel reached 100° C. and held for a period of time until the crude methionine crystal product was completely dissolved, to produce an additive-containing methionine solution with a concentration of 10 wt %. The above additive-containing methionine solution was continuously fed at 500 L/h into a 1000-L DTB crystallizer. By evaporating water under reduced pressure (vacuum degree: −0.092 MPa), the temperature in the crystallizer was maintained at 25° C. for continuous crystallization, and at the same time, the crystallization slurry was continuously discharged while controlling the liquid level constant. The crystallization slurry was filtered, washed and dried to give, at 3.6 Kg/h, a methionine crystal product (FIG. 1 shows a microscope photograph of its crystal) with a bulk density of 786 g/L.

[0090] After compressed by a vapor compressor to an absolute pressure of 0.09 MPa, the water vapor evaporated during crystallization in step (2) could be used as a heating medium for warming up when dissolving the crude methionine crystal product. Furthermore, the crystallization mother liquor obtained from the filtration of methionine crystals in the step (2) and in step (1) was used to formulate a solution of crude methionine crystal product.

[0091] The crystallization system operated continuously for 15 days without observing obvious foaming, and the crystallization process was smooth and steady.

Example 2

[0092] The additive used in this example was an aqueous mixture of 2 wt % of sodium methyl cocoyl taurate, 2 wt % of polyoxyethylene polyoxypropylene ether-grafted silicone oil having a molecular weight of 6,000 and an HLB value of 12, 1 wt % of dimethyl silicone oil having a dynamic viscosity at 25° C. of 1,500 mm.sup.2/s, and the remaining being water.

[0093] (1) To the aqueous methionate solution obtained from hydrolysis of 5-(2-methylthioethyl)-hydantoin, 150 ppm of an additive based on the aqueous methionate solution was added, and carbon dioxide was charged while stirring to precipitate a methionine crystal by means of reaction crystallization. The methionine crystal was filtered to obtain a crude methionine crystal product.

[0094] (2) The above crude methionine product was fed into a stirring vessel, and an appropriate amount of water and/or crystallization mother liquor was added thereto to formulate a methionine suspension having a solid content of 8 wt %. 200 ppm of the additive based on the methionine suspension was added to the vessel, stirred and heated till the temperature in the vessel reached 90° C. and held for a period of time until the crude methionine crystal product was completely dissolved, to produce an additive-containing methionine solution with a concentration of 11 wt %. The above additive-containing methionine solution was continuously fed at 500 L/h into a 1000-L OSLO crystallizer. By evaporating water under reduced pressure (vacuum degree: −0.088 MPa), the temperature in the crystallizer was maintained at 40° C. for continuous crystallization, and at the same time, the crystallization slurry was continuously discharged while controlling the liquid level constant. The crystallization slurry was filtered, washed and dried to give, at 1.9 Kg/h, a methionine crystal product (FIG. 2 shows a microscope photograph of its crystal) with a bulk density of 802 g/L.

[0095] After compressed by a vapor compressor to an absolute pressure of 0.1 MPa, the water vapor evaporated during crystallization in step (2) could be used as a heating medium for warming up when dissolving the crude methionine product. Furthermore, the crystallization mother liquor obtained from the filtration of methionine crystals in the step (2) and in step (1) was used to formulate a solution of crude methionine crystal product.

[0096] The crystallization system operated continuously for 15 days without observing obvious foaming, and the crystallization process was smooth and steady.

Example 3

[0097] The additive used in this example was an aqueous mixture of 3 wt % of sodium methyl lauroyl taurate, 4 wt % of polyoxyethylene polyoxypropylene ether-grafted silicone oil having a molecular weight of 5,300 and an HLB value of 11, 3 wt % of dimethyl silicone oil having a dynamic viscosity at 25° C. of 1,100 mm.sup.2/s, and the remaining being water.

[0098] (1) To the aqueous methionate solution obtained from hydrolysis of 5-(2-methylthioethyl)-hydantoin, 70 ppm of an additive based on the aqueous methionate solution was added, and carbon dioxide was charged while stirring to precipitate a methionine crystal by means of reaction crystallization. The methionine crystal was filtered to obtain a crude methionine crystal product.

[0099] (2) The above crude methionine product was fed into a stirring vessel, and an appropriate amount of water and/or crystallization mother liquor was added thereto to formulate a methionine suspension having a solid content of 13 wt %. 400 ppm of the additive (3 wt % of sodium methyl lauroyl taurate, 4 wt % of polyoxyethylene polyoxypropylene ether-grafted silicone oil having a molecular weight of 5,300 and an HLB value of 11, 3 wt % of dimethyl silicone oil having a dynamic viscosity at 25° C. of 1,100 mm.sup.2/s, and the remaining being water) based on the methionine suspension was added to the vessel, stirred and heated till the temperature in the vessel reached 110° C. and held for a period of time until the crude methionine crystal product was completely dissolved, to produce an additive-containing methionine solution with a concentration of 12 wt %. The above additive-containing methionine solution was continuously fed at 500 L/h into a 1000-L FC crystallizer. By evaporating water under reduced pressure (vacuum degree: −0.07 MPa), the temperature in the crystallizer was maintained at 65° C. for continuous crystallization, and at the same time, the crystallization slurry was continuously discharged while controlling the liquid level constant. The crystallization slurry was filtered, washed and dried to give, at 2.5 Kg/h, a methionine crystal product (FIG. 3 shows a microscope photograph of its crystal) with a bulk density of 798 g/L.

[0100] After compressed by a vapor compressor to an absolute pressure of 0.15 MPa, the water vapor evaporated during crystallization in step (2) could be used as a heating medium for warming up when dissolving the crude methionine product. Furthermore, the crystallization mother liquor obtained from the filtration of methionine crystals in the step (2) and in step (1) was used to formulate a solution of crude methionine crystal product.

[0101] The crystallization system operated continuously for 15 days without observing obvious foaming, and the crystallization process was smooth and steady.

Example 4

[0102] The additive used in this example was an aqueous mixture of 3 wt % of sodium methyl palmitoyl taurate, 3 wt % of polyoxyethylene polyoxypropylene ether-grafted silicone oil having a molecular weight of 4,200 and an HLB value of 9, 2 wt % of dimethyl silicone oil having a dynamic viscosity at 25° C. of 510 mm.sup.2/s, and the remaining being water.

[0103] (1) To the aqueous methionate solution obtained from hydrolysis of 5-(2-methylthioethyl)-hydantoin, 100 ppm of an additive based on the aqueous methionate solution was added, and carbon dioxide was charged while stirring to precipitate a methionine crystal by means of reaction crystallization. The methionine crystal was filtered to obtain a crude methionine crystal product.

[0104] (2) The above crude methionine product was fed into a stirring vessel, and an appropriate amount of water and/or crystallization mother liquor was added thereto to formulate a methionine suspension having a solid content of 10 wt %. 250 ppm of the additive based on the methionine suspension was added to the vessel, stirred and heated till the temperature in the vessel reached 105° C. and held for a period of time until the crude methionine crystal product was completely dissolved, to produce an additive-containing methionine solution with a concentration of 13 wt %. The above additive-containing methionine solution was continuously fed at 500 L/h into a 1000-L DTB crystallizer. By evaporating water under reduced pressure (vacuum degree: −0.082 MPa), the temperature in the crystallizer was maintained at 50° C. for continuous crystallization, and at the same time, the crystallization slurry was continuously discharged while controlling the liquid level constant. The crystallization slurry was filtered, washed and dried to give, at 2.0 Kg/h, a methionine crystal product (FIG. 4 shows a microscope photograph of its crystal) with a bulk density of 791 g/L.

[0105] After compressed by a vapor compressor to an absolute pressure of 0.13 MPa, the water vapor evaporated during crystallization in step (2) could be used as a heating medium for warming up when dissolving the crude methionine product. Furthermore, the crystallization mother liquor obtained from the filtration of methionine crystals in the step (2) and in step (1) was used to formulate a solution of crude methionine crystal product.

[0106] The crystallization system operated continuously for 15 days without observing obvious foaming, and the crystallization process was smooth and steady.

Example 5

[0107] The additive used in this example was an aqueous mixture of 5 wt % of sodium methyl myristoyl taurate, 5 wt % of polyoxyethylene polyoxypropylene ether-grafted silicone oil having a molecular weight of 3,600 and an HLB value of 10.5, 2 wt % of dimethyl silicone oil having a dynamic viscosity at 25° C. of 270 mm.sup.2/s, and the remaining being water.

[0108] (1) To the aqueous methionate solution obtained from hydrolysis of 5-(2-methylthioethyl)-hydantoin, 200 ppm of an additive based on the aqueous methionate solution was added, and carbon dioxide was charged while stirring to precipitate a methionine crystal by means of reaction crystallization. The methionine crystal was filtered to obtain a crude methionine crystal product.

[0109] (2) The above crude methionine product was fed into a stirring vessel, and an appropriate amount of water and/or crystallization mother liquor was added thereto to formulate a methionine suspension having a solid content of 9 wt %. 400 ppm of the additive based on the methionine suspension was added to the vessel, stirred and heated till the temperature in the vessel reached 95° C. and held for a period of time until the crude methionine crystal product was completely dissolved, to produce an additive-containing methionine solution with a concentration of 14 wt %. The above additive-containing methionine solution was continuously fed at 500 L/h into a 1000-L DTB crystallizer. By evaporating water under reduced pressure (vacuum degree: −0.09 MPa), the temperature in the crystallizer was maintained at 30° C. for continuous crystallization, and at the same time, the crystallization slurry was continuously discharged while controlling the liquid level constant. The crystallization slurry was filtered, washed and dried to give, at 2.4 Kg/h, a methionine crystal product (FIG. 5 shows a microscope photograph of its crystal) with a bulk density of 795 g/L.

[0110] After compressed by a vapor compressor to an absolute pressure of 0.095 MPa, the water vapor evaporated during crystallization in step (2) could be used as a heating medium for warming up when dissolving the crude methionine product. Furthermore, the crystallization mother liquor obtained from the filtration of methionine crystals in the step (2) and in step (1) was used to formulate a solution of crude methionine crystal product.

[0111] The crystallization system operated continuously for 15 days without observing obvious foaming, and the crystallization process was smooth and steady.