CN112679397A - Preparation method of DL-methionine - Google Patents
Preparation method of DL-methionine Download PDFInfo
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Abstract
The invention relates to the field of methionine production, and in particular relates to a preparation method of DL-methionine. According to the scheme adopted by the invention, in the process of continuously producing methionine, the concentration of sodium sulfate is improved and the content of methionine is reduced by concentrating the sodium sulfate mother liquor, then the concentrated mother liquor is regulated by using the saponification liquor, the concentrated mother liquor is cooled and crystallized to obtain high-quality mirabilite, and then the crystallization mother liquor is reused in the production process of methionine after impurity removal is carried out according to the impurity content in the crystallization mother liquor after the mirabilite is separated. The crystallization mother liquor can be completely recycled, no waste liquor is generated, and the continuous production process of methionine is ensured not to influence the stability of methionine quality due to impurity accumulation caused by repeated use of the crystallization mother liquor; meanwhile, the precipitation of the sodium sulfate byproduct mirabilite reduces the water content in the crystallization mother liquor and increases the concentration of the crystallization mother liquor methionine, thereby saving the time cost and the energy cost for concentrating the sodium sulfate mother liquor in the methionine production process.
Description
Technical Field
The invention relates to the field of methionine production, and in particular relates to a preparation method of DL-methionine.
Background
Methionine, also known by the name methionine, abbreviated Met, is one of the basic units for the biosynthesis of proteins, also an essential amino acid and a limiting amino acid. Methionine is the only sulfur-containing amino acid in the essential amino acids, the animal can obviously promote growth by taking a small amount of methionine, the feeding period is shortened, the egg milk yield can be increased, if the intake is insufficient, the utilization of other amino acids in the body is insufficient, the unused amino acids are converted into energy molecules and urea through deamination, and the burden of the liver and the kidney is increased. Methionine is not only used in feed industry, but also widely used in the fields of medicine, food, cosmetics and the like. As one of main producing areas and main consuming areas of methionine in China, with gradual release of new production capacity of companies such as winning and creating in 2018 to 2020, Sumitomo and the like, the methionine industry has been fiercely competitive, and the improvement of the methionine preparation process is particularly important.
One of the main processes for preparing Met in the prior art is the leno-planck process, which uses hydrocyanic acid or sodium cyanide to prepare a 5- (beta-methylthioethyl) hydantoin solution, then generates a saponified solution after hydrolysis and deamination of an alkaline compound of sodium, acidifies the saponified solution with sulfuric acid to separate methionine, repeatedly concentrates a sodium sulfate mother liquor containing saturated methionine at high temperature to saturate the sodium sulfate and separate out a sodium sulfate solid, and returns the concentrated residual liquid to the acidification step to finish the preparation of methionine.
On one hand, the process can lead a plurality of chemical reactions to generate malodorous substances to pollute the environment after a large amount of sodium sulfate mother liquor is repeatedly concentrated for a long time at high temperature;
on the other hand, in the treatment process, impurities such as 3-methylmercapto propionaldehyde, 2-acrylic acid and acrolein intermediates and polymers thereof, formate, methionine dipeptide, pigments and the like are accumulated, so that the purity of precipitated sodium sulfate is extremely poor and cannot be recycled, and the concentrated residual liquid also contains a large amount of impurities, so that the quality of the finished methionine product is poor after the concentrated residual liquid is returned to the acidification step, the problem can be solved only by a method of regularly discharging the concentrated residual liquid, and the method is not beneficial to environmental protection and maximization of economic benefits.
In the technical scheme disclosed in publication No. CN104693082A, in order to avoid repeated high-temperature concentration of methionine-containing sodium sulfate mother liquor, a continuous chromatographic separation technique is used to separate methionine and sodium sulfate from the sodium sulfate mother liquor, and sodium sulfate solution is separately subjected to high-temperature concentration and crystallization to obtain sodium sulfate as a byproduct, but the scheme still has the following disadvantages:
firstly, the concentration of methionine and sodium sulfate solution after chromatographic separation is diluted, so that the concentration cost of the sodium sulfate solution is increased;
secondly, the price of a chromatographic separation device is very high, and the efficiency of separating methionine from sodium sulfate is extremely low;
thirdly, the separated sodium sulfate solution does not only contain sodium sulfate, but also contains a small amount of other impurities, and the concentration and crystallization mode finally causes the purity of the sodium sulfate byproduct to be poor.
In the embodiment disclosed in publication No. CN106432020B, carbon dioxide is introduced into the saponified solution to obtain methionine and a potassium bicarbonate aqueous solution containing methionine, and the salt in the potassium bicarbonate aqueous solution is removed by electrodialysis technique from the potassium bicarbonate aqueous solution and recovered for use, thereby treating the saponified solution. On one hand, the electrodialysis technology used in the scheme has extremely high power consumption and high cost; on the other hand, the retention rate and the desalination rate of the methionine by the electrodialysis technology are low, and the desalination requirements of a large amount of solution in industrial production cannot be met; furthermore, the proposal can only remove salt and can not remove impurities generated in the production of methionine, and the recovery and utilization of the methionine solution can cause the accumulation of impurities and influence the quality of the methionine.
The prior art also adopts a treatment method of using ion exchange resin, the saponified solution is separated into a methionine solution and a sodium sulfate solution, the methionine solution is crystallized to obtain a product, and the sodium sulfate solution is concentrated at high temperature to obtain a byproduct.
However, this solution has the following disadvantages:
1. the methionine has low solubility, the saponification liquid must be diluted by adding a large amount of water, and a large amount of time and energy are consumed in the subsequent concentration treatment;
2. the ion exchange resin only can separate sodium sulfate, impurities such as intermediates of 3-methylmercapto propionaldehyde, 2-acrylic acid and acrolein and polymers thereof, formate, methionine dipeptide, pigment and the like are mainly remained in the methionine solution, and the impurities are accumulated after the methionine is crystallized by concentrating the mother liquor, so that the quality of the methionine is deteriorated;
3. sodium sulfate mother liquor contains only a small amount of impurities, but concentration of crystalline sodium sulfate eventually leads to deterioration in the quality of the sodium sulfate by-product.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a preparation method of DL-methionine, which does not generate discharged wastewater, and avoids the accumulation effect of impurities caused by recycling residual liquid to influence the quality of methionine finished products and sodium sulfate byproducts; meanwhile, the cost in the preparation process of methionine can be reduced.
In order to solve the technical problems, the invention provides the following technical scheme:
a preparation method of DL-methionine comprises the following specific steps:
s100; preparing 4-methylthio-2-hydroxybutyronitrile by using hydrocyanic acid and 3-methylthiopropanal, reacting the 4-methylthio-2-hydroxybutyronitrile with ammonia and carbon dioxide to generate 5- (beta-methylthioethyl) hydantoin, hydrolyzing the 5- (beta-methylthioethyl) hydantoin by using an alkali solution of sodium, and deaminating to obtain a saponified solution;
s200; acidifying the saponified solution, cooling, crystallizing and separating to obtain methionine crystals and sodium sulfate mother liquor;
s300; concentrating the sodium sulfate mother liquor, cooling for crystallization, and then carrying out solid-liquid separation to obtain a mixed solid and a concentrated mother liquor;
s350; returning the mixed solid to the step S200, and dissolving the mixed solid into the saponification liquid;
s400; adding the saponification solution into the concentrated mother liquor, then carrying out low-temperature crystallization, and carrying out solid-liquid separation to obtain crystallized mother liquor and mirabilite;
s500; and (4) removing impurities from the crystallization mother liquor, returning the crystallization mother liquor after the impurities are removed to the step S200 to be mixed with the saponification liquor, and then acidifying and separating methionine crystals.
Preferably, the specific steps of step S200 are:
s210; heating the saponification liquid at constant temperature to keep the temperature of the saponification liquid at 60-120 ℃, acidifying the saponification liquid by using sulfuric acid, keeping the pH value of the acidified saponification liquid at 5-7, and keeping the temperature for 10-30 min to obtain an acidified liquid;
s220; reducing the temperature of the acidizing fluid to 35-45 ℃, and preserving the heat for 20-60 min;
s230; and carrying out solid-liquid separation on the acidified liquid to obtain methionine crystals and sodium sulfate mother liquor.
Preferably, the sulfuric acid concentration in the step S210 is 20-98 wt%.
Preferably, the specific steps of step S300 are:
s310; heating the sodium sulfate mother liquor to 90-120 ℃, and concentrating under reduced pressure;
s320; reducing the temperature to 10-60 ℃, and preserving the heat for 10-40 min;
s330; and carrying out solid-liquid separation at 35-45 ℃ to obtain a mixed solid and a concentrated mother liquor.
Preferably, in step S310, the sodium sulfate mother liquor is concentrated under reduced pressure until the sodium sulfate content in the sodium sulfate mother liquor is greater than 20 wt%.
Preferably, the specific steps of step S400 are: adding a saponification solution into the concentrated mother liquor, wherein the mole ratio of sodium carbonate in the added saponification solution to methionine in the concentrated mother liquor is 1-2: 1, adding the saponification solution, then reducing the pH value of the concentrated mother liquor to be more than or equal to 8, reducing the temperature to-10 ℃, preserving the temperature for 20-100 min, and then carrying out solid-liquid separation to obtain a crystallized mother liquor and mirabilite.
Preferably, in step S500, before removing impurities from the crystallization mother liquor, the content of impurities in the crystallization mother liquor is detected, when the ratio of the content of impurities in the crystallization mother liquor to the content of methionine exceeds 3%, the impurities are removed from the crystallization mother liquor, and when the ratio of the content of impurities in the crystallization mother liquor to the content of methionine does not exceed 3%, the crystallization mother liquor is directly returned to step S200 to be mixed with the saponification liquor.
Preferably, in step S500, the crystallization mother liquor is divided into two parts, one of the two parts is subjected to impurity removal, and the impurity-removed crystallization mother liquor is mixed with the other part of the crystallization mother liquor, and then the mixture is returned to step S200 to be mixed with the saponification liquor.
Preferably, in step S500, the specific steps of removing impurities from the crystallization mother liquor are: adding sulfuric acid into the crystallization mother liquor, keeping the pH value of the crystallization mother liquor after adding the sulfuric acid at 2-3, standing for 30min at 10-40 ℃, separating oil phase impurities and water phase liquor, filtering the water phase liquor to remove solid impurities, and removing formic acid by a steam stripping method.
Preferably, the concentration of the sulfuric acid added in the step S500 for removing impurities from the crystallization mother liquor is 20-98%.
The invention has the beneficial effects that:
1. the method comprises the steps of preparing a saponified solution by using hydrocyanic acid and other raw materials, acidifying the saponified solution, cooling and crystallizing, carrying out solid-liquid separation to obtain methionine crystals and a sodium sulfate mother solution, concentrating and cooling the sodium sulfate mother solution, precipitating partial methionine, sodium sulfate and a small amount of impurities in the sodium sulfate mother solution to form a mixed solid, directly dissolving the mixed solid in the saponified solution, carrying out acidification again to extract a methionine finished product, retaining more impurities in the concentrated mother solution, adding the saponified solution into the concentrated mother solution, then precipitating the sodium sulfate in the concentrated mother solution in a mirabilite mode by cooling, after solid-liquid separation, adding sulfuric acid into a filtrate (namely the crystallized mother solution), condensing and separating oil phase impurities in the crystallized mother solution from the concentrated mother solution, and simultaneously filtering partial solid impurities precipitated in an aqueous phase solution. Thereby completing the removal of impurities generated in the DL-methionine preparation process, and ensuring that the impurities do not generate accumulation effect in the continuous DL-methionine preparation process to influence the quality of methionine.
2. The crystallization mother liquor after impurity removal can also be used as an acidifying agent of the saponification liquid after formic acid is removed through stripping, the cost of the acidification process of the saponification liquid is saved, and meanwhile, a large amount of water is brought out due to the precipitation of mirabilite, the time and the energy of the concentration process of the sodium sulfate mother liquor can be saved, and the single extraction rate of methionine can also be improved.
3. The concentrated mother liquor is added into the saponified solution to ensure that the concentrated mother liquor is under the alkaline condition, on one hand, the methionine and the oil phase impurities have higher solubility in the alkaline environment, and the quality of the mirabilite is not influenced in the process of crystallizing and separating out the mirabilite; on the other hand, the alkaline environment does not cause much corrosion to the equipment.
According to the technical scheme provided by the invention, impurities generated in the methionine preparation process can be separated out for treatment, an accumulation effect cannot be generated in the methionine production along with the recycling of the crystallization mother liquor, the quality of a finished methionine product is reduced, the influence of the impurities on the methionine quality is not reduced by means of periodically discharging the crystallization mother liquor, the crystallization mother liquor can be continuously recycled in the methionine production, the economic benefit maximization is achieved, and the pollution to the environment is reduced.
Drawings
FIG. 1 is a schematic flow chart of the present invention.
Detailed Description
In order to facilitate understanding of those skilled in the art, the present invention will be further described with reference to the following examples and drawings, which are not intended to limit the present invention. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention, and that such changes and modifications may be made without departing from the spirit and scope of the invention. The following experimental methods, in which specific conditions are not specified, were carried out according to conventional conditions.
Example 1; a preparation method of DL-methionine comprises the following specific steps:
s100; preparing 4-methylthio-2-hydroxybutyronitrile by using hydrocyanic acid and 3-methylthiopropanal, reacting the 4-methylthio-2-hydroxybutyronitrile with ammonia and carbon dioxide to generate 5- (beta-methylthioethyl) hydantoin, hydrolyzing the 5- (beta-methylthioethyl) hydantoin by using an alkali solution of sodium, and deaminating to obtain a saponified solution;
the specific steps of hydrolyzing the 5- (beta-methylmercapto ethyl) hydantoin by using the sodium alkali solution are as follows:
s110; heating 5- (beta-methylmercapto ethyl) hydantoin to 120-200 ℃, adding sodium alkali solution, wherein the molar ratio of alkaline sodium to 5- (beta-methylmercapto ethyl) hydantoin in the sodium alkali solution is 1.5-4: 1;
s120; and preserving the heat for 10-60 min.
S200; acidifying the saponified solution, cooling, crystallizing and separating to obtain methionine crystals and sodium sulfate mother liquor;
s210; heating the saponification liquid at constant temperature to keep the temperature of the saponification liquid at 60-120 ℃, acidifying the saponification liquid by using sulfuric acid or a mixed liquid of sulfuric acid and the impurity-removed crystallization mother liquid obtained in the step S500, wherein the pH value of the acidified saponification liquid is 5-7, and preserving heat for 10-30 min to obtain an acidified liquid; in actual operation, the concentration of the added sulfuric acid is 20-98 wt% in order to save time and energy in the subsequent concentration process.
S220; reducing the temperature of the acidizing fluid to 35-45 ℃, and preserving the heat for 20-60 min;
s230; carrying out solid-liquid separation on the acidizing solution to obtain methionine crystals and sodium sulfate mother liquor;
according to the precipitation speed and quality of methionine crystals in the mixed mother liquor, the heat preservation time is controlled, more methionine crystals can be precipitated under the condition that time and energy are not wasted, the precipitated methionine crystals are washed and dried to obtain qualified methionine products, and meanwhile, the solubility of sodium sulfate is the maximum under the condition of 35-45 ℃, the solubility of methionine is relatively low, and the separation of the methionine crystals is facilitated.
S310; heating the sodium sulfate mother liquor to 90-120 ℃, and concentrating under reduced pressure;
in specific implementation, the sodium sulfate mother liquor is concentrated until the content of sodium sulfate in the sodium sulfate mother liquor is more than 20 wt%, so that sodium sulfate can be crystallized more in the subsequent steps to form a sodium sulfate byproduct mirabilite.
S320; reducing the temperature to 10-60 ℃, and preserving the heat for 10-40 min;
s330; carrying out solid-liquid separation at 35-45 ℃ to obtain mixed solid and concentrated mother liquor;
s350; returning the mixed solid to the step S200, and dissolving the mixed solid into the saponification liquid; the mixed solid comprises methionine, sodium sulfate and a small amount of impurities, and the mixed solid is re-dissolved into the saponification liquid to continuously purify methionine, so that the yield of methionine is increased.
S400; adding the saponification solution into the concentrated mother liquor, then carrying out low-temperature crystallization, and carrying out solid-liquid separation to obtain crystallized mother liquor and mirabilite; and (3) adding sodium carbonate into the saponification liquid of the concentrated mother liquor and methionine in the concentrated mother liquor in a molar ratio of 1-2: 1, then carrying out heat preservation on the concentrated mother liquor at the temperature of-10 ℃ for 20-100 min, and then carrying out solid-liquid separation to obtain crystallized mother liquor and mirabilite. The pH value of the concentrated mother liquor after the saponification liquid is added is more than or equal to 8, under the condition, methionine exists in the form of sodium methionine, when mirabilite is crystallized at low temperature, methionine cannot be separated out, and meanwhile, oily impurities in the concentrated mother liquor after the saponification liquid is added have high solubility in an alkaline environment, and the quality of the separated mirabilite cannot be influenced.
S500; and detecting the content of impurities in the crystallization mother liquor, removing impurities from the crystallization mother liquor when the ratio of the content of the impurities in the crystallization mother liquor to the content of methionine exceeds 3%, and directly returning the crystallization mother liquor to the step S200 to be mixed with the saponification liquor when the ratio of the content of the impurities in the crystallization mother liquor to the content of methionine does not exceed 3%. When the ratio of the impurity content to the methionine content in the crystallization mother liquor does not exceed 3%, the influence of the impurities on the crystallization quality of the methionine is within an acceptable range, the crystallization mother liquor can be directly recycled without impurity removal, the impurity removal cost of the crystallization mother liquor is saved, and the economic benefit is increased.
And in the concrete implementation, the crystallization mother liquor is divided into two parts, one part of the crystallization mother liquor is subjected to impurity removal, and the crystallization mother liquor after impurity removal is mixed with the other part of the crystallization mother liquor, and then the mixture returns to the step S200 to be mixed with the saponification liquor. The ratio of the crystallization mother liquor without impurities removal to the crystallization mother liquor without impurities removal may be: 1: 9, 2: 8, 3: 7 or 4: 6. The crystallization mother liquor is ensured to be recycled, impurities cannot be accumulated in the continuous production process, and the quality of the produced methionine finished product can be stable under the condition that the methionine is prepared without discharging waste water; meanwhile, partial impurity removal can also reduce the cost of the impurity removal process and increase the economic benefit of methionine production and preparation.
The specific steps of removing impurities from the crystallization mother liquor are as follows: adding sulfuric acid into the crystallization mother liquor, enabling the pH value of the crystallization mother liquor after the sulfuric acid is added to be 2-3, standing for 30min at 10-40 ℃, separating oily impurities and water in the crystallization mother liquor, standing and separating to obtain water phase liquid and oil phase impurities, enabling the oil phase impurities to be subjected to cleaning and burning treatment, enabling a small amount of impurities remaining in the water phase liquid to be crystallized again, and filtering and removing the impurities by using one or more of diatomite, silica gel, activated carbon and activated clay as an auxiliary agent. The filtered aqueous phase is then stripped of formic acid and acidified with sulfuric acid as the acidulant. The crystallization mother liquor is purified, so that the influence on the methionine product is weakened; meanwhile, the stripping removal of formic acid can reduce the water content in the crystallization mother liquor and reduce the cost of concentrating the sodium sulfate mother liquor during continuous production. The concentration of sulfuric acid added during impurity removal of the crystallization mother liquor is 20-98 wt%.
According to the scheme adopted by the invention, in the process of continuously producing methionine, the concentration of sodium sulfate is improved by concentrating the sodium sulfate mother liquor, partial methionine is separated out, then the concentrated mother liquor is regulated by using the saponification liquor, the concentrated mother liquor is cooled and crystallized to obtain high-quality mirabilite, and then the crystallization mother liquor is reused in the production process of methionine after impurity removal is carried out according to the impurity content in the crystallization mother liquor after the mirabilite is separated. The crystallization mother liquor can be completely recycled, waste liquor containing beneficial components cannot be generated, and the continuous production process of methionine is ensured not to influence the stability of methionine quality due to impurity accumulation caused by repeated use of the crystallization mother liquor; meanwhile, the water content in the crystallization mother liquor is reduced by separating out the sodium sulfate byproduct mirabilite, namely, the concentration of sulfuric acid in the crystallization mother liquor is increased, so that the time cost and the energy cost for concentrating the sodium sulfate mother liquor in the methionine production process are saved when the crystallization mother liquor is used as an acidifying agent. Simultaneously, the single extraction rate of methionine is also improved.
The following provides a specific example of production practice using the technical scheme provided by the invention, and practical operation is performed in sequence, so that the process of industrial continuous production using the method is simulated.
Preparing 4-methylthio-2-hydroxybutyronitrile from hydrocyanic acid and 3-methylthiopropanal, reacting 4-methylthio-2-hydroxybutyronitrile with ammonia and carbon dioxide to generate 5000g of 5- (beta-methylthioethyl) hydantoin solution with the concentration of 21.39 wt%, adding 1474g of 30 wt% sodium hydroxide solution, heating to 155-160 ℃ in a high-pressure reaction kettle, preserving heat for 20min, decompressing to atmospheric pressure, then carrying out normal-pressure stripping at 100 ℃ until the ammonia content is less than 50ppm to obtain 5365g of saponified solution, wherein the methionine content in the saponified solution is 16.95 wt%. 4.74
Example 2-a;
taking 1000g of saponified solution, heating at constant temperature to stabilize the temperature of the saponified solution at 70 ℃, dropwise adding 96.7g of 98 wt% sulfuric acid into the saponified solution, and after dropwise adding, keeping the temperature for 20min to obtain acidified solution, wherein the pH value of the acidified solution is 6.5; cooling the acidified liquid to 35 ℃, preserving heat, stirring for 30min, filtering to obtain methionine crystals and sodium sulfate mother liquor, washing and drying the methionine crystals to obtain 149.5g of methionine with the content of 99.1 wt%, wherein the yield is 87.4%; 576g of sodium sulfate mother liquor, wherein the content of methionine is 2.48 wt%, and the content of sodium sulfate is 18.84 wt%. 500g of sodium sulfate mother liquor is taken, reduced pressure distillation is carried out at the temperature of 90 ℃ until the content of sodium sulfate is more than 25 wt%, then the temperature is reduced to 35 ℃, heat preservation is carried out for 30min, and then filtration is carried out to obtain 12g of mixed solid and 251.65g of concentrated mother liquor. The concentrated mother liquor had a methionine content of 1.1 wt% and a sodium sulfate content of 26.42 wt%. Taking 84.9g of saponified solution and concentrated mother solution, uniformly mixing, cooling to 10 ℃ to cool and crystallize for 30min after the pH value of the mixed solution is 8.6, and filtering to obtain 113.7g of mirabilite and 140.8g of crystallized mother solution. 38.21 wt% of sodium sulfate in mirabilite, 61.23 wt% of water, no methionine detected and 76.84% of sodium sulfate yield; the methionine content in the crystallization mother liquor is 6.22 wt%, and the sodium sulfate content is 7.75 wt%.
Example 2-b;
taking 1000g of saponified solution, dissolving the mixed solid in the example 2-a into the saponified solution, heating at constant temperature to stabilize the temperature of the saponified solution at 70 ℃, dropwise adding a mixed solution of sulfuric acid and the crystallization mother solution in the example 2-a into the saponified solution, and preserving heat for 20min after dropwise adding to obtain acidified solution, wherein the pH value of the acidified solution is 6.2; cooling the acidified liquid to 35 ℃, preserving heat, stirring for 30min, filtering to obtain methionine crystals and sodium sulfate mother liquor, washing and drying the methionine crystals to obtain 152.6g of methionine with the content of 99.18 wt%, wherein the yield is 88.6%; the sodium sulfate mother liquor 612g contains methionine 2.35 wt% and sodium sulfate 17.63 wt%. 500g of sodium sulfate mother liquor is taken, reduced pressure distillation is carried out at the temperature of 90 ℃ until the content of sodium sulfate is more than 25 wt%, then the temperature is reduced to 35 ℃, heat preservation is carried out for 30min, and then filtration is carried out to obtain 10.5g of mixed solid and 223.78g of concentrated mother liquor. The concentrated mother liquor had a methionine content of 1.2 wt% and a sodium sulfate content of 26.76 wt%. And (3) uniformly mixing 78.8g of saponified solution with the concentrated mother solution, cooling to 10 ℃ to perform cooling crystallization for 30min after the pH value of the mixed solution is 8.8, and filtering to obtain 109.3g of mirabilite and 131.3g of crystallized mother solution. 38.35 wt% of sodium sulfate in mirabilite, 60.93 wt% of water, no methionine detected and 77.03% of sodium sulfate yield; the methionine content in the crystallization mother liquor was 6.16 wt%, and the sodium sulfate content was 7.57 wt%.
Example 2-c;
taking 1000g of saponified solution, dissolving the mixed solid in the example 2-b into the saponified solution, heating at constant temperature to stabilize the temperature of the saponified solution at 70 ℃, dropwise adding a mixed solution of sulfuric acid and the crystallization mother solution in the example 2-b into the saponified solution, and preserving heat for 20min after dropwise adding to obtain acidified solution, wherein the pH value of the acidified solution is 6.4; cooling the acidified liquid to 35 ℃, preserving heat, stirring for 30min, filtering to obtain methionine crystals and sodium sulfate mother liquor, washing and drying the methionine crystals to obtain 150.23g of methionine with the content of 99.02 wt%, wherein the yield is 87.9%; 608g of sodium sulfate mother liquor, wherein the methionine content is 2.38 wt%, and the sodium sulfate content is 18.23 wt%. 500g of sodium sulfate mother liquor is taken, reduced pressure distillation is carried out at the temperature of 90 ℃ until the content of sodium sulfate is more than 25 wt%, then the temperature is reduced to 35 ℃, heat preservation is carried out for 30min, and then filtration is carried out to obtain 10.7g of mixed solid and 230.34g of concentrated mother liquor. In the concentrated mother liquor, the methionine content was 1.1 wt%, and the sodium sulfate content was 26.83 wt%. And uniformly mixing 80.3g of saponified solution with the concentrated mother solution, wherein the pH value of the mixed solution is 8.7, cooling to 10 ℃, cooling and crystallizing for 30min, and filtering to obtain 110.8g of mirabilite and 139.2g of crystallized mother solution. 38.24 wt% of sodium sulfate in mirabilite, 60.87 wt% of water, no methionine detected and 76.56% of sodium sulfate yield; the methionine content in the crystallization mother liquor is 6.14 wt%, and the sodium sulfate content is 7.48 wt%.
The impurity content in the crystallization mother liquor is detected to exceed 3 percent, so that the yield and the purity of methionine and sodium sulfate are reduced, therefore, the crystallization mother liquor is subjected to impurity removal, 1.4g of 98 wt% sulfuric acid is added into the crystallization mother liquor, the pH value of the concentrated mother liquor is 2.5 after the sulfuric acid is added, liquid separation is carried out to obtain water phase liquid and oil phase impurities, diatomite is used as an auxiliary agent, the water phase liquid is filtered to remove solid impurities, formic acid is removed by using a steam stripping method, 76.5g of crystallization mother liquor after impurity removal is obtained, the methionine content in the crystallization mother liquor after impurity removal is 11.0 wt%, and the sodium sulfate content is 13.1 wt%.
Example 2-d;
taking 1000g of saponified solution, dissolving the mixed solid in the example 2-c into the saponified solution, heating at constant temperature to stabilize the temperature of the saponified solution at 70 ℃, dropwise adding a mixed solution of sulfuric acid and the crystallized mother solution obtained after impurity removal in the example 2-c into the saponified solution, and preserving heat for 20min after dropwise adding to obtain acidified solution, wherein the pH value of the acidified solution is 6.5; cooling the acidified liquid to 35 ℃, preserving heat, stirring for 30min, filtering to obtain methionine crystals and sodium sulfate mother liquor, washing and drying the methionine crystals to obtain 153.3g of methionine with the content of 99.23 wt%, wherein the yield is 90.31%; and sodium sulfate mother liquor 579g, wherein the methionine content is 2.36 wt%, and the sodium sulfate content is 19.39 wt%. 500g of sodium sulfate mother liquor is taken, reduced pressure distillation is carried out at the temperature of 90 ℃ until the content of sodium sulfate is more than 25 wt%, then the temperature is reduced to 35 ℃, heat preservation is carried out for 30min, and then filtration is carried out to obtain 9.8g of mixed solid and 235.7g of concentrated mother liquor. The concentrated mother liquor had a methionine content of 1.4 wt% and a sodium sulfate content of 27.21 wt%. And (3) uniformly mixing 83.2g of saponified solution with the concentrated mother solution, cooling to 10 ℃ to perform cooling crystallization for 30min after the pH value of the mixed solution is 8.8, and filtering to obtain 121.8g of mirabilite and 126.4g of crystallization mother solution. 39.24 wt% of sodium sulfate in mirabilite, 60.68 wt% of water, no methionine detected and 78.35% of sodium sulfate yield; the methionine content in the crystallization mother liquor was 6.21 wt%, and the sodium sulfate content was 7.69 wt%.
Example 2-e;
taking 1000g of saponified solution, dissolving the mixed solid in the example 2-d into the saponified solution, heating at constant temperature to stabilize the temperature of the saponified solution at 70 ℃, dropwise adding a mixed solution of sulfuric acid and the crystallized mother solution obtained after impurity removal in the example 2-d into the saponified solution, and preserving heat for 20min after dropwise adding to obtain acidified solution, wherein the pH value of the acidified solution is 6.2; cooling the acidified liquid to 35 ℃, preserving heat, stirring for 30min, filtering to obtain methionine crystals and sodium sulfate mother liquor, washing and drying the methionine crystals to obtain 151.4g of methionine with the content of 99.21 wt%, wherein the yield is 90.18%; 597g of sodium sulfate mother liquor, wherein the methionine content is 2.35 wt%, and the sodium sulfate content is 18.21 wt%. 500g of sodium sulfate mother liquor is taken, reduced pressure distillation is carried out at the temperature of 90 ℃ until the content of sodium sulfate is more than 25 wt%, then the temperature is reduced to 35 ℃, heat preservation is carried out for 30min, and then filtration is carried out to obtain 10.8g of mixed solid and 230.7g of concentrated mother liquor. The concentrated mother liquor had a methionine content of 1.1 wt% and a sodium sulfate content of 26.79 wt%. And uniformly mixing 80.9g of saponified solution with the concentrated mother solution, wherein the pH value of the mixed solution is 8.7, cooling to 10 ℃, cooling and crystallizing for 30min, and filtering to obtain 117.1g of mirabilite and 135.3g of crystallized mother solution. 39.13 wt% of sodium sulfate in mirabilite, 60.66 wt% of water, no methionine detected and 78.03% of sodium sulfate yield; the methionine content in the crystallization mother liquor was 6.13 wt%, and the sodium sulfate content was 7.49 wt%.
From the above examples 2-a to 2-e, it can be seen that the yield of methionine and the yield of sodium sulfate can be effectively improved in the continuous production process by using the technical scheme provided by the present invention. In example 2-c, which is a continuous production performed three times, the yield of methionine and the yield of sodium sulfate slightly decreased due to the slight accumulation of impurities, and it was detected that the ratio of the content of impurities to the content of methionine in the resulting crystallization mother liquor exceeded 3%, the crystallization mother liquor was once subjected to impurity removal and then used in example 2-d. Therefore, in example 2-d, the yield of methionine was increased to 90 wt% or more, and the yield of sodium sulfate was also increased; in example 2-e, the yield of methionine was also 90 wt% or more. Through setting a planned value of impurities in the crystallization mother liquor or periodically removing the impurities from the crystallization mother liquor, the yield of methionine can be effectively improved and kept stable, the crystallization mother liquor containing methionine is not discharged outside in the production and preparation process of methionine, the economic benefit maximization is ensured, and the pollution to the environment is reduced.
The above embodiments are preferred implementations of the present invention, and other implementations are also included, and any obvious substitutions are within the scope of the present invention without departing from the spirit of the present invention.
Claims (10)
1. A preparation method of DL-methionine is characterized by comprising the following specific steps:
s100; preparing 4-methylthio-2-hydroxybutyronitrile by using hydrocyanic acid and 3-methylthiopropanal, reacting the 4-methylthio-2-hydroxybutyronitrile with ammonia and carbon dioxide to generate 5- (beta-methylthioethyl) hydantoin, hydrolyzing the 5- (beta-methylthioethyl) hydantoin by using an alkali solution of sodium, and deaminating to obtain a saponified solution;
s200; acidifying the saponified solution, cooling, crystallizing and separating to obtain methionine crystals and sodium sulfate mother liquor;
s300; concentrating the sodium sulfate mother liquor, cooling for crystallization, and then carrying out solid-liquid separation to obtain a mixed solid and a concentrated mother liquor;
s350; returning the mixed solid to the step S200, and dissolving the mixed solid into the saponification liquid;
s400; adding the saponification solution into the concentrated mother liquor, then carrying out low-temperature crystallization, and carrying out solid-liquid separation to obtain crystallized mother liquor and mirabilite;
s500; and (4) removing impurities from the crystallization mother liquor, returning the crystallization mother liquor after the impurities are removed to the step S200 to be mixed with the saponification liquor, and then acidifying and separating methionine crystals.
2. The method for preparing DL-methionine as claimed in claim 1, wherein the specific steps of step S200 are:
s210; heating the saponification liquid at constant temperature to keep the temperature of the saponification liquid at 60-120 ℃, acidifying the saponification liquid by using sulfuric acid, keeping the pH value of the acidified saponification liquid at 5-7, and keeping the temperature for 10-30 min to obtain an acidified liquid;
s220; reducing the temperature of the acidizing fluid to 35-45 ℃, and preserving the heat for 20-60 min;
s230; and carrying out solid-liquid separation on the acidified liquid to obtain methionine crystals and sodium sulfate mother liquor.
3. The method of claim 2, wherein the concentration of sulfuric acid in step S210 is 20-98 wt%.
4. The method for preparing DL-methionine as claimed in claim 1, wherein the specific steps of step S300 are:
s310; heating the sodium sulfate mother liquor to 90-120 ℃, and concentrating under reduced pressure;
s320; reducing the temperature to 10-60 ℃, and preserving the heat for 10-40 min;
s330; and carrying out solid-liquid separation at 35-45 ℃ to obtain a mixed solid and a concentrated mother liquor.
5. The method of claim 4, wherein in step S310, the sodium sulfate mother liquor is concentrated under reduced pressure until the sodium sulfate content in the sodium sulfate mother liquor is greater than 20 wt%.
6. The method of claim 1, wherein the step S400 comprises the steps of: adding a saponification solution into the concentrated mother liquor, wherein the mole ratio of sodium carbonate in the added saponification solution to methionine in the concentrated mother liquor is 1-2: 1, adding the saponification solution, then reducing the pH value of the concentrated mother liquor to be more than or equal to 8, reducing the temperature to-10 ℃, preserving the temperature for 20-100 min, and then carrying out solid-liquid separation to obtain a crystallized mother liquor and mirabilite.
7. The method of claim 1, wherein the amount of impurities in the crystallization mother liquor is measured before the crystallization mother liquor is purified in step S500, the purification of the crystallization mother liquor is performed when the ratio of the amount of impurities to the amount of methionine in the crystallization mother liquor exceeds 3%, and the crystallization mother liquor is directly returned to step S200 to be mixed with the saponification liquor when the ratio of the amount of impurities to the amount of methionine in the crystallization mother liquor does not exceed 3%.
8. The process according to claim 7, wherein in step S500, the crystallization mother liquor is divided into two parts, one of the two parts is subjected to impurity removal, and the impurity-removed crystallization mother liquor is mixed with the other part of the crystallization mother liquor, and then the mixture is returned to step S200 to be mixed with the saponification liquor.
9. The method for preparing DL-methionine as claimed in claim 1, wherein in step S500, the specific step of removing impurities from the crystallization mother liquor is: adding sulfuric acid into the crystallization mother liquor, keeping the pH value of the crystallization mother liquor after adding the sulfuric acid at 2-3, standing for 30min at 10-40 ℃, separating oil phase impurities and water phase liquor, filtering the water phase liquor to remove solid impurities, and removing formic acid by a steam stripping method.
10. The method of claim 9, wherein the concentration of sulfuric acid added in the step S500 for removing impurities from the crystallization mother liquor is 20-98%.
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