Process for obtaining a mixture comprising methionine and potassium hydrogen carbonate

Abstract

The present invention is in the form of potassium bicarbonate and / or potassium carbonate, methionine, A process for obtaining a mixture containing methionine and potassium bicarbonate from an aqueous solution or suspension containing titratable potassium and 4.5 to 12.0% by weight of methionyl-methionine, Supplying CO2 to the solution or suspension used at a temperature of 15 to 60 °C (carbonation) to precipitate a mixture containing methionine and potassium bicarbonate as a suspension containing on average 6.5% by weight or less of met-met, Separating the precipitate from the mother liquor, Characterized by comprising.

Description

【Technical Field】 【0001】 The present invention relates to a process for obtaining a mixture comprising methionine, potassium bicarbonate and optionally potassium carbonate from an aqueous solution or suspension comprising methionine, potassium carbonate, potassium bicarbonate and methionyl-methionine, in particular to a method for recovering methionine and potassium bicarbonate from a methionine mother liquor rich in met-met. 【Background Art】 【0002】 The amino acid methionine is used in many fields, such as pharmaceuticals, health and fitness products, and in particular as a feed additive contained in many feeds for various livestock. On an industrial scale, methionine is chemically produced via the Bucherer-Bergs reaction, a variant of the Strecker synthesis. The starting materials 3-methylmercaptopropanal (produced from NMP, 2-propenal and methyl mercaptan), hydrocyanic acid (hydrogen cyanide), ammonia and carbon dioxide are reacted to obtain 5-(2-methylmercaptoethyl)hydantoin (methionine hydantoin), which is then subjected to alkaline hydrolysis with an alkali metal hydroxide and / or an alkali metal carbonate and an alkali metal bicarbonate, such as potassium hydroxide and / or potassium carbonate and potassium bicarbonate, to obtain an alkali metal methionate (e.g., potassium methionate) (see Formula 1). Methionine is finally liberated from its alkali metal salt, for example, by acidification (carbonation) with carbon dioxide (see Formula 2), and filtered off as a precipitate from the mother liquor containing the alkali metal carbonate and the alkali metal bicarbonate (e.g., potassium carbonate and potassium bicarbonate). The filtrate, the mother liquor containing the alkali metal bicarbonate (e.g., potassium carbonate and potassium bicarbonate) (the first mother liquor), is recycled based on the Degussa potassium recycling process for the saponification of methionine hydantoin (see, for example, European Patent Application Publication No. 780370). Formula 1: Saponification of methionine hydantoin [Chem.] Formula 2: Carbonation [Chem.] 【0003】 As with all recycling processes, this process also requires discharging the mother liquor so that the by-products formed do not exceed the allowable amounts. However, the discharged mother liquor still contains useful products such as methionine, alkali metal carbonates and alkali metal hydrogen carbonates (e.g., potassium carbonate and potassium hydrogen carbonate) for the saponification of methionine hydantoin. In order to recover as much methionine and potassium hydrogen carbonate as possible from the mother liquor, the discharged mother liquor may be subjected to a second carbonation. 【0004】 As reported, for example, in German Patent Application Publication No. 2421167 and European Patent Application Publication No. 839804, there are various processes for obtaining methionine and potassium hydrogen carbonate from the first mother liquor via a second carbonation. The listed European patent specification requires a water-soluble solvent to precipitate methionine, which is accompanied by further complexity and the possibility of impurities. 【0005】 It is more difficult to further obtain methionine and potassium hydrogen carbonate from the mother liquor, for example, the mother liquor of the second carbonation (the second mother liquor). This is because impurities become significantly abundant in such mother liquors, and these impurities may disrupt subsequent process steps. Therefore, European Patent Application Publication No. 1760074 discloses a method for recovering methionine and potassium hydrogen carbonate from the mother liquor of the second carbonation via a third carbonation. The resulting precipitate mixture of methionine and potassium hydrogen carbonate is separated by filtration and can be directly reused for the saponification of hydantoin. However, in this method, as reported in European Patent Application Publication No. 2133328 and European Patent Application Publication No. 2186798, the precipitate of the third carbonation cannot be easily filtered. This is because in the mother liquor of the second carbonation, the by-product methionyl-methionine (met-met) becomes significantly abundant, and met-met has an adverse effect on the filterability of the precipitate of the third carbonation. Therefore, it is required to reduce the content of met-met in the second mother liquor. Therefore, in European Patent Application Publication No. 2133328 and European Patent Application Publication No. 2186798, the mother liquor of the second carbonation is first concentrated and then heated to a high temperature, for example, up to 180 °C. As a result of this treatment, met-met in the second mother liquor is partially cleaved to become methionine. The mother liquor thus obtained is suitable for recovering the methionine and potassium hydrogen carbonate present therein by, for example, the third carbonation described in European Patent Application Publication No. 2133328 / European Patent Application Publication No. 2186798, or by reusing the mother liquor for the second carbonation described in European Patent Application Publication No. 2186797. 【0006】 Alternative methods for reducing the content of met-met are disclosed in European Patent Application Publication No. 2133329. According to European Patent Application Publication No. 2133329, when hydrolysis is carried out in a non-stirred reactor (without backmixing) and after removing CO2 and water, the remaining hydrolysis solution is further heated in another reactor, methionine hydantoin is saponified and less met-met is formed. Therefore, this requires two separate reactors and also cannot avoid heating the basic solution at high temperature, which is also disadvantageous. The use of auxiliary polyvinyl alcohol is also disadvantageous as it involves additional complexity and cost and corresponding residues are generated in the product. 【0007】 Also, from European Patent Application Publication No. 1840119, it is known that the conditions required above for cleaving met-met in the basic methionine mother liquor are highly corrosive (high concentration and high temperature), and thus expensive materials (such as zirconium or super duplex steel) are required for the reactor. Furthermore, the above conditions for cleaving met-met to methionine are very stringent. Moreover, these conditions may cause decomposition of methionine, which is also disadvantageous as this also increases the energy demand for this additional process step. 【0008】 Therefore, there is a need for a novel method for recovering potassium bicarbonate from a mother liquor rich in methionine and met-met, such as the mother liquor from the second carbonation, which exhibits the disadvantages of the methods described in the prior art, if any, to a reduced extent only. 【0009】 The process for recovering useful substances from the discharged mother liquor by carbonation and separation aims mainly to separate the precipitate formed in carbonation (especially containing potassium bicarbonate and methionine, which are useful substances) from the mother liquor (which is rich in secondary components such as formate and met-met). The precipitate is reused in the process, and the remaining mother liquor (for example, the second mother liquor or the third mother liquor) becomes waste. 【0010】 If this solid-liquid separation does not function well, that is, if it is difficult to separate the precipitate and difficult to filter, and thus it can only be reused in the process using a large amount of mother liquor, then many secondary components will also be returned to the hydrolysis process, that is, the saponification of hydantoin. As a result, these components accumulate during the process, and as a consequence, more first mother liquor must be discharged to remove these by-products from the circuit again. Therefore, the filterability of the precipitate has a decisive impact on the throughput throughout the post-treatment of the discharged first mother liquor. Therefore, the filterability is extremely important for the economy of the entire post-treatment. 【0011】 As can be seen from the above patent specification, in the recovery of methionine and bicarbonate from a methionine mother liquor rich in met-met, the problem lies precisely in filtration. According to the above specification, this is because the concentration of met-met in the second mother liquor is higher. For this reason, these specifications also describe several processes for reducing the met-met content in the mother liquor to improve the filterability of the precipitate. 【0012】 Therefore, the second mother liquor is first heated to a high temperature (for example, 160 - 180 °C) for a duration of, for example, 0.3 to 10 hours (European Patent Application Publication No. 2186797) before carbonation and filtration, whereby met-met is cleaved to become methionine. This facilitates subsequent filtration. 【0013】 However, the described solutions require expensive additional equipment (e.g., corrosion-resistant reactors and heat exchangers with sufficient residence time), additional energy input, and have the drawback that the useful substance, methionine, also undergoes partial decomposition. As a result, secondary components are reformed. 【Prior Art Documents】 【Patent Documents】 【0014】 【Patent Document 1】 European Patent Application Publication No. 780370 【Patent Document 2】 German Patent Application Publication No. 2421167 【Patent Document 3】 European Patent Application Publication No. 839804 【Patent Document 4】 European Patent Application Publication No. 1760074 【Patent Document 5】 European Patent Application Publication No. 2133328 【Patent Document 6】 European Patent Application Publication No. 2186798 【Patent Document 7】 European Patent Application Publication No. 2186797 【Patent Document 8】 European Patent Application Publication No. 2133329 【Patent Document 9】 European Patent Application Publication No. 1840119 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0015】 Accordingly, the problem addressed by the present invention is to provide a process for recovering a mixture comprising methionine, potassium hydrogen carbonate, and optionally potassium carbonate from an aqueous solution or suspension comprising methionine, optionally potassium carbonate, potassium hydrogen carbonate, and methionyl-methionine, in particular from the mother liquor for producing methionine in which methionyl-methionine is clearly abundant, wherein the methionine, potassium hydrogen carbonate, and optionally potassium carbonate present precipitate at the highest possible rate and are as easily filterable as possible, i.e., exhibit the best possible filterability. 【0016】 A further problem directly related thereto was to provide an improved process for producing methionine with a low amount of waste products generated, i.e., a low methionine / waste requirement. 【0017】 The process of the present invention solves the problem of poor filterability of the precipitate formed in the carbonation of the mother liquor rich in met-met in a manner different from the prior art. According to the present invention, the precipitate formed by carbonation contains the substance methionine, potassium hydrogen carbonate, and optionally a relatively small amount of potassium carbonate, and the accompanying mother liquor also contains a significant amount of methionyl-methionine. The finding on which the present invention is based is that when the precipitation content of methionyl-methionine, i.e., the methionyl-methionine present in the precipitate, does not exceed the critical value, i.e., most of it remains dissolved in the mother liquor, the filter cake is not visibly damaged due to the abundance of met-met. Filtration becomes difficult only when a significant amount of met-met precipitates during carbonation and thus exists as crystals in the precipitate. 【Means for Solving the Problem】 【0018】 Accordingly, the above object is achieved by a titratable potassium of 6.0 to 18.0% by weight, preferably in the form of potassium hydrogen carbonate and / or potassium carbonate, methionine of preferably 2.5 to 8.0% by weight and It is achieved by providing a process for obtaining a mixture comprising methionine, potassium bicarbonate, and optionally potassium carbonate from an aqueous solution or suspension containing 4.50 to 12.0% by weight of methionyl-methionine. The process involves supplying CO2 to the solution or suspension used at a temperature of 15 to 60 °C (carbonation), and precipitating a mixture comprising methionine, potassium bicarbonate, and optionally potassium carbonate as a precipitate containing on average 6.5% by weight or less, preferably 0.01% to 5.0% by weight or less, particularly 0.01% to 3.0% by weight or less of met-met. The desired met-met content can be continuously determined, for example, using a sample from the precipitate separated by HPLC. 【0019】 Thereby, particularly good filterability is achieved, thus overcoming the above-mentioned drawbacks in the prior art processes. The aqueous solution or suspension used has a pH of about 11 to 12, preferably 7.8 to 9.5, particularly preferably 8.3 to 9.5, especially 8.4 to 9.5, very particularly preferably 8.4 to 9.0, which is decreased by carbonation, measured using a glass electrode at the established temperature in each case. Since the met-met content in the precipitate depends significantly on the pH in the carbonated mother liquor, this can also be used to control the met-met content in the precipitate. 【0020】 The separation of the precipitate can be carried out, for example, by filtration or centrifugation, and can be carried out using corresponding devices that are also usable, in particular, on an industrial scale and are known to those skilled in the art. These devices are preferably used continuously or semi - continuously so as to be able to achieve an industrially customary product throughput. To control and adjust the pH and thus indirectly control and adjust the technical effect of good filterability, standard samples are taken and measured with a glass electrode (see the examples), or continuous pH measurement is selected, making it possible to very easily adjust the desired pH and thus the filterability. To directly control and adjust the met - met content and thus the technical effect of good filterability, standard samples can be taken from the mother liquor and measured by HPLC (see the examples). However, the determination of the advantageous met - met content according to the invention by HPLC can also be carried out in an automated manner. 【0021】 The used solution or suspension, optionally depending on the concentration, the content is titratable potassium in the form of potassium carbonate and / or potassium hydrogen carbonate in an amount of 6.0 - 18% by weight, methionine in an amount of 2.5 - 8.0% by weight, and methionyl - methionine in an amount of 4.5 - 12.0% by weight, At an established temperature, while measuring with a glass electrode, carbon dioxide is supplied (carbonated) at a temperature of 15 - 60°C, preferably 25 - 55°C, until a pH of 7.8 - 9.5, preferably 8.3 - 9.5, particularly preferably 8.4 - 9.5, especially 8.4 - 9.0 is achieved, to precipitate a mixture containing methionine, potassium hydrogen carbonate and optionally potassium carbonate as a precipitate, and separating the precipitate from the mother liquor, wherein the precipitate contains on average 6.5% by weight or less, preferably 0.01% by weight - 5.0% by weight or less, particularly 0.01% by weight - 3.0% by weight or less of met - met. A process characterized by including the above steps is preferred. 【0022】 Accordingly, the present invention makes it possible to achieve the recovery of methionine and potassium bicarbonate from a methionine mother liquor rich in met-met by selecting process parameters (especially carbonation), whereby, during carbonation, the maximum possible amount of methionine and potassium bicarbonate precipitates, while at the same time the met-met in the precipitated precipitate does not exceed the critical value, i.e., remains highly dissolved in the mother liquor, and provides a process. 【0023】 In the present invention, there is no longer a need to cleave met-met before carbonation, thus making it possible to achieve a significant energy and cost savings. The process according to the present invention is also capable of achieving an improvement in the filterability of the precipitate from carbonation in its above-described variant. This is evident from the significantly lower filtration resistance of 1.2*10 9 m / kg in Example 1 of the present invention compared to the value of 1.6*10 10 m / kg described in Example 1 (second crystallization) of EP 2186797 A1 or compared to the value of 0.64*10 6 m / kg described in Example 1 (third crystallization) of EP 2133328 A1. 【0024】 According to the present invention, in particular, it is the second mother liquor of the second carbonation that is concentrated. In order to gently concentrate the mother liquor at a low temperature, a low pressure (for example, 300 mbara) is selected. This avoids the cleavage of methionine and met-met. The mother liquor is concentrated, for example, 2-fold. In that case, the concentration of free potassium is, for example, 13.4% by weight, and the concentration of methionine is, for example, 4.9% by weight. Then, met-met is also richly contained, and its concentration is, for example, 7% by weight. (See Example 1). This solution is then carbonated by adding CO2. The pressure ranges from 1 to 6, preferably from 1.5 to 2.5 bara, which reduces the foaming of the suspension during depressurization. The temperature is from 15 to 60 °C, preferably from 25 to 35 °C, which avoids the use of cold water and instead allows the use of more energy-efficient and better cooling water. The residence time in the supply of CO2 is from 20 to 180 minutes, preferably about 60 minutes. In the reactor, the pH of the suspension is measured at each temperature using a glass electrode. This is adjusted by adding CO2. The concentrated mother liquor has a pH of about 11. The mother liquor in the reactor is not completely carbonated at this point (i.e., it reaches equilibrium at each pressure). In contrast, only sufficient CO2 is added to achieve a pH of 7.8 to 9.5, for example 8.5. 【0025】 The inventors have found that the solubility of met-met in this matrix depends particularly on temperature and pH. The target pH in the reactor (and thus the amount of CO2) is preferably selected such that only a very small proportion of met-met, i.e. 6.5 wt% or less, preferably 0.1 wt% to 5.0 wt% or less, particularly preferably 0.1 wt% to 3.0 wt% or less, is present in the precipitate. The met-met content is typically determined by HPLC of a precipitate sample that has first been filtered, wetted, then washed with acetone and dried. This is, of course, a cross-sectional value of the proportion of crystalline met-met in the precipitate and the proportion of met-met in the mother liquor adhering to the precipitate. However, this total / average value is also, indirectly, a measure of the relevant crystal proportion of met-met and can therefore be used as a process variable for adjusting the filterability. 【0026】 Therefore, the preferred target pH depends on the following. · The concentration of met-met in the filtrate of the second carbonation, · The concentrations of potassium, methionine and met-met after concentration, which typically correlate with a concentration factor of 1.5 to 2.5 in each case, · The temperature in the reactor. 【0027】 After carbonation, the precipitate is typically filtered. Since the mother liquor has not been carbonated to equilibrium, the precipitate contains only a relatively small amount of met-met in addition to methionine and potassium hydrogen carbonate. The precipitate is recycled upstream of this process, preferably the saponification of hydantoin (a reaction step including the hydrolysis of 5-[2-(methylthio)ethyl]imidazolidine-2,4-dione). Any met-met present therein is cleaved to methionine during the saponification of hydantoin. Similar to recycling the methionine present in the precipitate, this increases the yield of the methionine process. As a result of the concentration, the filtrate is about twice as rich in secondary components and is preferably discarded. Since it is about twice as concentrated, the amount of filtrate is also reduced by about 1 / 2. This significantly reduces the liquid waste stream of this process. 【0028】 This process is advantageously operated such that the pressure ranges from 1 to 6, preferably from 1.5 to 2.5 bara. 【0029】 Typical residence times are from 20 to 180 minutes, preferably about 60 minutes. 【0030】 In a process for producing methionine, as an aqueous solution or suspension containing methionine, potassium carbonate, potassium bicarbonate and methionyl-methionine in the process according to the invention, for example as described in principle in European Patent Application Publication No. 780370, it is preferred to use the mother liquor from which methionine has been isolated via saponification of alkaline methionine hydantoin. 【0031】 After repeated carbonation, it is particularly preferred that it is the mother liquor filtered after the third carbonation. This is because the mother liquor has the greatest advantage here in recovering useful substances that would otherwise be lost. 【0032】 The present invention also provides a mixture comprising methionine, potassium bicarbonate and optionally potassium carbonate produced according to the above-described method, and a mixture comprising methionine, KHCO3 and met-met, wherein the met-met content is 6.5% by weight or less, as can be produced by the process according to the invention. 【0033】 The present invention further provides the use of such a mixture as a saponifying agent and an additional methionine source for producing methionine. This has the great advantage of improving the economy of the process by resource conservation and increased yield, and thus has a great impact in the typical plant production volume of methionine of about 100,000 tons per year. 【0034】 The present invention further provides the following steps (1) to (6): (1) In the presence of a basic potassium compound, hydrolyze 5-[2-(methylthio)ethyl]imidazolidine-2,4-dione to obtain a hydrolyzate containing potassium methionate and potassium methionylmethionate, including a reaction step of: (2) Introduce carbon dioxide into the hydrolyzate obtained in step (1) (first carbonation) to obtain a suspension containing methionine, methionyl-methionine, potassium bicarbonate, and optionally potassium carbonate, precipitate methionine, and separate this suspension into a first precipitate containing methionine and a first mother liquor containing methionyl-methionine, including a first crystallization step of: (3) A step of concentrating the first mother liquor obtained in step (2) (first concentration step); (4) A step of recycling a first portion of the concentrated first mother liquor of (3) to the reaction step (1); (5) Introduce carbon dioxide into a second portion of the concentrated first mother liquor of (3) (second carbonation) to precipitate methionine and potassium bicarbonate, and separate the resulting suspension into a second precipitate and a second mother liquor, including a second crystallization step of: (6) Concentrating the second mother liquor obtained in step (3), introducing carbon dioxide into the concentrated second mother liquor to precipitate methionine and potassium bicarbonate, and separating the resulting suspension into a third precipitate containing a mixture of methionine, potassium bicarbonate, and optionally potassium carbonate and a third mother liquor containing methionyl-methionine according to the above process, including a third crystallization step, whereby the third precipitate contains, on average, 6.5% by weight or less, preferably 0.01% to 5% by weight or less, particularly preferably 0.01% to 3% by weight or less of met-met. Provide an overall process for producing methionine (see also the block diagram of Figure 1 including the principle process procedure). 【0035】 Preferably, the basic potassium compound used is potassium hydroxide, potassium carbonate and / or potassium hydrogen carbonate, and at least supplementarily, a precipitate containing the above-mentioned potassium hydrogen carbonate and optionally potassium carbonate from the carbonation step. 【0036】 By providing the entire process according to the present invention, the yield of methionine is increased, and more basic potassium is reused from the mother liquor, thus solving the problem of providing an improved process for producing methionine in which a significantly smaller amount of waste products is generated / less amount of waste is required. Against the background of a typical annual production volume of about 100,000 tons in a typical methionine plant, especially considering the advantages of resource conservation, this is of high economic and ecological value. 【0037】 Also preferred is a process in which the third precipitate is reused in the hydrolysis step (1) or further steps (2)-(4), where, due to the high potassium content, this precipitate reduces the demand for replacement potassium and serves to reuse the existing methionine. 【0038】 It is also advantageous to combine the first mother liquor and the third precipitate and then reuse them in the hydrolysis step (1) or the concentration step (3), which has the advantage of reducing the complexity of the process. 【0039】 During the process, the third mother liquor obtained as waste can be simply discarded or supplied for further isolation of useful substances. These are especially recycled potassium hydrogen carbonate and methionine, the final product of the process. 【Examples】 【0040】 Method of use 1. High Performance Liquid Chromatography (HPLC) Investigation by chromatography (methionine and met-met) was carried out using a Jasco HPLC apparatus on a suitable RP column, followed by UV detection at 210 nm. The mobile phase was an acetonitrile-water mixture acidified with phosphoric acid. 10 μl of each sample solution was injected at a flow rate of 1 ml / min. The system was pre-calibrated by injecting suitable calibration solutions of appropriate reference compounds from the methionine / met-met process and evaluated by comparing peak areas using the external standard method. The procedure of the standard method is known to those skilled in the art. 【0041】 Sampling for determination of met-met in the precipitate: From the carbonation, one volume part of the filtered precipitate was collected in a laboratory filtering funnel, washed with two volume equivalents of acetone, and suctioned under reduced pressure in a water jet vacuum. Most of the remaining proportion of the mother liquor still adhering to the precipitate slightly soluble in acetone was removed. Suction was applied for an additional 5 minutes, and then the precipitate was dried to a constant weight in a drying cabinet to remove the remaining acetone. Subsequently, a suitable proportion of the thus pretreated precipitate was weighed, dissolved in HPLC eluent, diluted to a suitable concentration with additional HPLC eluent, and the sample solution thus obtained was injected into the HPLC apparatus as described above. 【0042】 2. Potassium titration Alkaline potassium salts such as potassium carbonate, potassium bicarbonate, and potassium methionate are present in almost all process solutions of the methionine process and are encompassed by the term "titratable potassium" herein. Titrimetric measurements of aqueous solutions of "titratable potassium" were carried out as a collective parameter, especially in process solutions. This was performed as a potentiometric titration with 0.1 mol HCl against the relevant potassium-containing alkaline aqueous process solution up to pH 4.6. In acid / base titrations, the acid consumption is reported as "titratable potassium". 【0043】 1 ml of 0.1 mol HCl corresponds to 3.91 mg of K, for example, by reaction with KHCO3 +corresponds to the weight equivalent. [Chemistry] 【0044】 Note: When the term equivalent (eq) is used, it should be understood to mean molar equivalent (which is also moleq). 【0045】 3. Measurement of cake resistance as a measure of the filterability of the suspension The so-called cake resistance was determined according to VDI Guideline VDI 2762 sheet 2 (December 2010). The cake resistance is reported in 1 / m 2 The area-based cake resistance α H is reported in 1 / m 2 and the mass-based cake resistance α M is reported in m / kg. These two parameters are converted by the density of the dry filter cake (ρ s , unit kg / m 3 ): α H = α M * ρ s . The higher the cake resistance value, the poorer the filterability. The filterability was also determined empirically by observing and comparing different filtration tests. The following filterability ratings, Very good - Good - Acceptable - Poor - Very poor were used. 【0046】 Therefore, the examples of the present invention were given ratings of "very good", "good" or "acceptable". 【0047】 4. Preparation of a methionine hydantoin soap solution as a starting solution for the examples, substantially according to European Patent Application Publication No. 780370 First, a methionine hydantoin solution was produced from MMP, HCN, ammonia, and CO2 in the form of an ammonium carbonate solution, substantially according to Example 1 (page 10) of European Patent Application Publication No. 780370. From this, a methionine hydantoin saponification solution (hydrolysate) was produced by saponification with titratable aqueous potassium, particularly in the form of a K2CO3-containing solution substantially according to Example 6 of European Patent Application Publication No. 780370. This hydrolyzate, an alkaline solution containing, in particular, potassium methionate and potassium methionyl methionate, was mostly neutralized using CO2 (first carbonation), as in Example 7 of European Patent Application Publication No. 780370. The first precipitate thus formed, particularly containing methionine, was filtered off from the accompanying first mother liquor (first moli) as described above. 【0048】 The first moli was carbonated again (second carbonation), and the precipitate obtained was filtered from the remaining second moli. The resulting second moli was subjected to further processing as starting solutions 1, 2, and 3 in Examples 1 to 12, and the compositions for each case are reported in Table 1. 【0049】 Each of these starting solutions (second moli) was concentrated 1.6 to 1.9 times, and the concentrated second moli thus obtained was sent to a third carbonation in each case. The precipitate thus formed was filtered off in each case via a filtration unit and analyzed for titratable potassium K+, methionine, and met-met. The filterability was evaluated, and in some cases, the cake resistance was also determined (see Table 1). 【0050】 Table 1 shows an overview of the examples together with the results of carbonation and filtration to explain the dependence of filtration on the met-met concentration in the filter cake. 【0051】 Table 1: Summary of the adjusted parameters and the results of Examples 1 - 12 【Table 1-1】 【Table 1-2】 【Table 1-3】 【Table 1-4】 【0052】 Explanation of Table 1 It is clear from Examples 1 and 2 of Table 1 that by raising the pH to the optimal range, the generation of met-met in the precipitate can be avoided. By raising the temperature in the carbonation process or by reducing the concentration coefficient, the precipitation of met-met can also be prevented, and thus the filterability can be improved. However, by changing these parameters in this way, the amounts of methionine and potassium in the precipitate also decrease significantly. This is the case where the decrease is only to a certain extent when the pH is raised. Therefore, it has been clarified that the adjustment by pH is surprisingly very effective, simple and cost-effective, and thus the most economically advantageous solution. 【0053】 By comparing Examples 3 to 5, the effect of pH on the concentrations measured in the filtrate and precipitate is explained. The met-met concentration in the concentrated second mother liquor was 5.9% by weight. In Example 3, this mother liquor was carbonated at 30 °C to pH 8.9. As a result, the met-met concentration in the resulting third mother liquor increased to 6.1% by weight. This is because the potassium concentration in the third mother liquor decreased from 13.1% by weight to 8.2% by weight, and the methionine concentration decreased from 4.6% to 3.4%, and the methionine and KHCO3 components partially precipitated. Due to the precipitation of these components, the met-met concentration will increase as long as met-met does not precipitate in the same way. The precipitate was found to contain only 1.2% by weight of met-met. Concentration measurements in the filtrate and cake show that met-met has not precipitated here. 【0054】 The same effect is also evident in Example 4. In the third mother liquor, since the pH was as low as 8.5, the concentrations of methionine and potassium further decreased, and the met-met concentration further increased to 6.2% by weight. 【0055】 Only in the carbonation of Example 5 to pH 8.1 did the met-met concentration in the filtrate decrease to less than 5.9% by weight, the starting concentration, and was 5.3% by weight. Furthermore, the met-met concentration in the precipitate was 5.3% by weight here. The filterability of the cake deteriorated significantly, from "very good" to "acceptable". 【0056】 By comparing Examples 6, 7 and Comparative Example 8, the effect of pH is also explained here. All three examples started from the same starting solution (the second moli after concentration), and these were carbonated at the same temperature of 30 °C. The established pH values were 9.0, 8.5 and 8.2. The solubility, and thus the concentration, of potassium and methionine, the useful substances, and met-met, the by-product, in the third mother liquor decreased with the pH. Compared with the very good filterability of Examples 6 and 7, in Precipitation Comparative Example 8, when the filterability was poor, the proportion of precipitated met-met in the precipitate was 6.8% by weight, which was already high enough. 【0057】 By comparing Example 5 and Example 8, the effect of the concentration coefficient on the met-met concentration of the concentrated second mother liquor, which directly depends on this concentration coefficient, becomes clear. In Example 8, the precipitate already had a filterability that was "poor" at pH 8.25. This is because the met-met concentration of the starting solution was higher (here, 7.5 wt% compared to 5.9 wt% in Example 5). Therefore, at higher pH values, the solubility limit of met-met has already been exceeded, and thus, at higher pH values, a significant proportion of met-met already exists in the precipitate, and accordingly, the filterability decreases. The higher the met-met concentration in the starting solution, the higher the pH, and the clearly increased proportion of met-met in the precipitate. The lower the met-met concentration in the starting solution, the lower the pH, and a significant proportion of met-met is present in the precipitate. When the met-met concentration of the starting solution is less than 4.5 wt%, a significant proportion of met-met in the precipitate is not expected, and the solution can be carbonated to CO2 saturation (i.e., the lowest pH achievable at each pressure) (at typical pressures and temperatures). Since the first and second carbonated mother liquors produced previously generally have a met-met content of less than 4.5 wt%, the problem of poor filterability after carbonation does not occur here. Since potassium and methionine are removed from the solution by carbonation and subsequent filtration, the second mother liquor is rich in met-met compared to these two titratable substances. In order for potassium bicarbonate and / or potassium carbonate and methionine to precipitate in the precipitate also in the third carbonation, it is necessary to first concentrate the second mother liquor. Therefore, the concentrated starting solution (concentrated second mother liquor) for the third carbonation achieves a higher met-met concentration compared to the aforementioned concentration, and thus, the solution of the present invention for improving the filterability of the precipitate is particularly exerted here. 【0058】 By comparing Examples 4, 10, and 11, the effect of temperature is explained. In all three examples, the pH is approximately 8.5. The solubility, and thus the concentration, of potassium and methionine, which are the useful substances in the third mother liquor, decreases with temperature. 【0059】 By comparing Example 4 and 11, the effect of temperature is particularly well demonstrated. In both examples, the starting solutions are equally concentrated and the pH values established in carbonation are substantially the same, but the temperature in Example 4 is 30 °C and the temperature in Example 11 is 50 °C. However, in Example 4, the met-met concentration in the precipitate is at a low level, the same as in Example 11 which is 1.3 wt%. The amount of met-met precipitating in the precipitate increases with the decrease in temperature. Here too, the concentrations of the useful substances, methionine and potassium, decrease with temperature. 【0060】 However, from the measurement results in Table 1, it is clear that in the carbonation at pH 9.0 according to Example 6, only a small amount of 2.6 wt% of met-met was found in the precipitate. The met-met concentration in the third mother liquor is higher than in the concentrated second mother liquor, and the met-met concentration in the precipitate is low at 2.6 wt%. This 2.6 wt% may also have been caused by the adhesion of the mother liquor. 【0061】 In the case of carbonation up to pH 8.2, the met-met concentration in the third mother liquor is lower than that in the concentrated second mother liquor, and in addition, the met-met concentration in the precipitate is 6.4 wt%. This indicates the presence of met-met in the precipitate. 【0062】 The cake resistance was measured for carbonation at pH 8.7 (1.2*10 6 m / kg, see Example 1) and carbonation at pH 8.2 (approx. 2.3*10 9For both Example 2 (comparative) and Example 2 (reference), measurements were taken. The cake resistance of the precipitate obtained at pH 8.2 was approximately 2000 times higher than that at pH 8.7. Therefore, the filterability of the precipitate at pH 8.7 was very good, and thus significantly better than the only acceptable filterability at pH 8.2. 【0063】 From these examples, it can be concluded that the solubility of the components methionine, potassium (mainly in the form of potassium bicarbonate), and met - met decreases with temperature and pH. In particular, according to the concentration factor, the difference in concentration between the concentrated second mother liquor and the third mother liquor is surely increased, and thus, a larger mass of precipitate (= useful substance) and a smaller mass of the third mother liquor (= by - product stream) are surely obtained. 【Brief Description of the Drawings】 【0064】 【Figure 1】 Shows the scheme of the methionine process according to the present invention. 【0065】 This scheme includes the following steps, and the reference numbers are as defined in Table 2 below. 【0066】 Table 2: List of reference numbers in Figure 1 【Table 2】

Claims

[Claim 1] Potassium bicarbonate and / or potassium carbonate, in the form of methionine, A process for obtaining a mixture containing methionine and potassium bicarbonate from an aqueous solution or suspension containing titrable potassium and 4.5 to 12.0% by weight of methionyl-methionine, To the aforementioned solution or suspension, add CO2 at a temperature of 15 to 60°C. ２ By supplying (carbonation), a mixture containing methionine and potassium bicarbonate is precipitated as a suspension containing an average of 6.5% by weight or less of met-met, Separating the precipitate from the mother liquor, A process characterized by including [Claim 2] A process for obtaining a mixture containing methionine and potassium bicarbonate from an aqueous solution or suspension containing methionine, potassium bicarbonate and methionyl-methionine as described in claim 1, wherein the solution or suspension used is optionally used depending on its concentration. Potassium titrable in the form of potassium bicarbonate and / or potassium carbonate, 6.0 to 18.0% by weight, Methionine 2.5-8.0% by weight, The methionyl-methionine content should be 4.5 to 12.0% by weight, CO ２ By supplying a solution, a mixture containing methionine and potassium bicarbonate is precipitated as a precipitate containing an average of 6.5% by weight or less of met-met, Separating the precipitate from the mother liquor, A process characterized by including [Claim 3] A process for obtaining a mixture containing methionine and potassium bicarbonate from an aqueous solution or suspension containing methionine, potassium bicarbonate and methionyl-methionine as described in claim 1, wherein the content of the solution or suspension used is, optionally, depending on the concentration, Potassium titrable in the form of potassium bicarbonate and / or potassium carbonate, 6.0 to 18.0% by weight, Methionine 2.5-8.0% by weight, The methionyl-methionine content should be 4.5 to 12.0% by weight, At an established temperature, measure using a glass electrode and use CO2 at a temperature of 15-60°C until a pH of 7.8-9.5 is achieved. ２ (Carbonation) is supplied to precipitate a mixture containing methionine and potassium bicarbonate as a precipitate containing an average of 6.5% by weight or less of met-met, Separating the precipitate from the mother liquor, A process characterized by including [Claim 4] CO2 at a temperature of 25-55°C until a pH of 8.3-9.5 is achieved. ２ The process according to claim 1, characterized by supplying a mixture containing methionine and potassium bicarbonate as a precipitate containing an average of 0.01% to 5.0% by weight of met-met, and separating the precipitate from the mother liquor. [Claim 5] CO2 at a temperature of 25-55°C until a pH of 8.4-9.5 is achieved. ２ The process according to claim 1, characterized by supplying a mixture containing methionine and potassium bicarbonate as a precipitate containing an average of 0.01% to 3.0% by weight of met-met, and separating the precipitate from the mother liquor. [Claim 6] CO2 until pH 8.4-9.0 is achieved. ２ The process according to claim 1, characterized by comprising supplying. [Claim 7] The aforementioned CO ２ The process according to claim 1, characterized in that the pressure during supply is in the range of 1 to 6 bara. [Claim 8] The aforementioned CO ２ The process according to claim 1, characterized in that the residence time during supply is 20 to 180 minutes. [Claim 9] The process according to claim 1, characterized in that a mother liquor from which methionine is isolated is used as an aqueous solution or suspension containing methionine, potassium bicarbonate, and methionyl-methionine in a process for producing methionine by saponification of alkaline methionine hydantoin. [Claim 10] The process according to claim 9, characterized in that the mother liquor is the filtered mother liquor after repeated carbonation. [Claim 11] A mixture comprising methionine and potassium bicarbonate, prepared as described in any one of claims 1 to 10. [Claim 12] The mixture according to claim 11, comprising methionine, potassium bicarbonate, and met-met, wherein the met-met content is 6.5% by weight or less. [Claim 13] Use of the mixture according to claim 11 for producing methionine. [Claim 14] A process for producing methionine, comprising the following steps (1) to (6): (1) A reaction step comprising hydrolyzing 5-[2-(methylthio)ethyl]imidazolidin-2,4-dione in the presence of a basic potassium compound to obtain a hydrolysate containing potassium methionate and potassium methionylmethioninate, (2) A first crystallization step comprising introducing carbon dioxide into the hydrolysate obtained in step (1) (first carbonation) to obtain a suspension containing methionine, methionyl-methionine, and potassium bicarbonate, precipitating methionine, and separating the suspension into a first precipitate containing methionine and a first mother liquor containing methionyl-methionine, (3) A step of concentrating the first mother liquor obtained in step (2) (first concentration step), (4) A step of reusing the first portion of the concentrated first mother liquor from (3) in the reaction step (1), (5) A second crystallization step comprising introducing carbon dioxide into the second portion of the concentrated first mother liquor of (3) (second carbonation) to precipitate methionine and potassium bicarbonate, and separating the obtained suspension into a second precipitate and a second mother liquor, (6) A third crystallization step comprising: concentrating the second mother liquor obtained in step (3); introducing carbon dioxide into the concentrated second mother liquor to precipitate methionine and potassium bicarbonate; and separating the obtained suspension into a third precipitate containing a mixture of methionine and potassium bicarbonate as described in any of claims 1 to 9 and a third mother liquor containing methionyl-methionine, wherein the third precipitate contains, on average, 6.5% by weight or less of met-met. [Claim 15] The process according to claim 14, characterized in that the third precipitate contains, on average, 0.01% by weight to 5.0% by weight of met-met. [Claim 16] The process according to claim 14, characterized in that the third precipitate is reused in any of steps (1) to (4). [Claim 17] The process according to claim 14, characterized in that it includes combining the first mother liquor and the third precipitate, and then reusing them in a reaction step (1) or a concentration step (3). [Claim 18] The process according to claim 14, characterized by comprising discarding the third mother liquor or supplying the third mother liquor for further isolation of potassium bicarbonate and / or methionine.

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