Method for obtaining mixture containing methionine and potassium bicarbonate

Abstract

The invention provides a method for obtaining a mixture containing methionine and potassium bicarbonate from an aqueous solution or suspension containing titratable potassium in the form of potassium bicarbonate and / or potassium carbonate, methionine and 4.5 to 12.0 wt.% of methionine-methionine, characterized in that the methionine and the methionine-methionine are mixed in the form of potassium bicarbonate and / or potassium carbonate, the process comprises supplying CO2 (carbonation) to the solution or suspension used at a temperature of 15-60 DEG C to precipitate a mixture containing methionine and potassium bicarbonate as a precipitate containing no more than 6.5% by weight of methionine-methionine on average, and separating the precipitate from the mother liquor.

Description

Technical Field

[0001] The present invention relates to a process for obtaining a mixture containing methionine and potassium bicarbonate and optionally potassium carbonate from an aqueous solution or suspension containing methionine, potassium carbonate, potassium bicarbonate and methionyl-methionine, in particular to a process for recovering methionine and potassium bicarbonate from a methionine mother liquor rich in methionyl-methionine. Background Art

[0002] The amino acid methionine is used in many fields, such as pharmaceuticals, health and fitness products, but especially as a feed additive in many feeds for various livestock. On an industrial scale, methionine is produced chemically by the Bucherer-Bergs reaction, which is a variant of the Strecker synthesis. The starting materials 3-methylmercaptopropionaldehyde (NMP, prepared from 2-propenal and methyl mercaptan), hydrocyanic acid (hydrogen cyanide), ammonia and carbon dioxide react to give 5-(2-methylmercaptoethyl)hydantoin (methionine hydantoin), which is then alkaline hydrolyzed with alkali metal hydroxides and / or alkali metal carbonates and alkali metal bicarbonates, such as potassium hydroxide and / or potassium carbonate and potassium bicarbonate, to give alkali metal methionine salts (e.g. potassium methioninate) (see Formula 1). Methionine is finally released from its alkali metal salt by acidification, such as with carbon dioxide (carbonation) (see Formula 2), and filtered out as a precipitate from a mother liquor containing alkali metal carbonates and alkali metal bicarbonates (e.g. potassium carbonate and potassium bicarbonate). The filtrate, ie the mother liquor (first mother liquor) containing alkali metal bicarbonate (eg potassium carbonate and potassium bicarbonate), is recycled for saponification of methionine hydantoin according to the Degussa potassium cycle process (see, for example, EP 780370 A2).

[0003] Formula 1: Methionine hydantoin saponification

[0004]

[0005] Formula 2: Carbonation

[0006]

[0007] Like all recovery processes, this process also requires discharge of mother liquor to prevent the formation of by-products exceeding the tolerable amount. However, the discharged mother liquor still contains the product methionine, alkali metal carbonate and alkali metal bicarbonate (such as potassium carbonate and potassium bicarbonate), which can be used for the saponification of methionine hydantoin. In order to recover as much methionine and potassium bicarbonate as possible from the mother liquor, the mother liquor to be discharged can be subjected to a second carbonation.

[0008] There are various methods for obtaining methionine and potassium bicarbonate from the first mother liquor by secondary carbonation, for example as described in DE 2421167 A1 and EP 839804 A2. The cited EP specifications use water-soluble solvents to precipitate methionine, which introduces additional complexity and possible impurities.

[0009] It is more difficult to further obtain methionine and saleratus from mother liquor (for example, from the mother liquor (second mother liquor) of the second carbonation). This is because the impurities in these mother liquors are obviously more enriched, and these impurities can interfere with subsequent processing steps. Therefore, EP 1760074 A1 discloses a method for reclaiming methionine and saleratus from the mother liquor of the second carbonation by the 3rd carbonation. The precipitated mixture of gained methionine and saleratus is separated by filtering, and can be directly recycled to hydantoin saponification. However, as described in EP 2133328A2 and EP 2186798A1, in this method, the precipitate of the 3rd carbonation is difficult to filter. The reason is that the byproduct methionyl-methionine (met-met) is significantly enriched in the mother liquor of the second carbonation, and methionyl-methionine has a negative effect on the filterability of the 3rd carbonation precipitate. Therefore, seek to reduce the content of methionyl-methionine in the second mother liquor. Therefore, in EP 2133328 A2 and EP 2186798 A1, the mother liquor of the second carbonation is concentrated at first, is heated to high temperature, for example 180 ℃ subsequently.The result of this processing is that the methionyl-methionine in the second mother liquor is partly cracked into methionine.Then, the mother liquor obtained thus is suitable for reclaiming methionine and saleratus that wherein exist by the 3rd carbonation, for example described in EP 2133328 A / EP 2186798A1, perhaps as described in EP 2186797A1 by recycling mother liquor to the second carbonation.

[0010] A kind of alternative method for reducing methionyl-methionine content is disclosed in EP 2133329 A2.According to EP 2133329 A, when being hydrolyzed in unstirred reactor (no backmixing), the methionyl-methionine formed by methionine hydantoin saponification is less, after removing CO2 and water, remaining hydrolysis solution is further heated in a separate reactor.Therefore, this needs two separate reactors, and also can't avoid heating alkaline solution at high temperature, which is equally disadvantageous.Using auxiliary polyvinyl alcohol is also disadvantageous, which can bring extra complexity and cost, and causes the corresponding residue in the product.

[0011] It is also known from EP 1840119 A2 that the above conditions required for the cleavage of methionyl-methionine in an alkaline methionine mother liquor are very corrosive (high concentrations and temperatures), so that expensive materials (e.g. zirconium or super duplex steel) are required for the reactor. In addition, the above conditions for the cleavage of methionyl-methionine to methionine are very harsh. These conditions may also lead to the decomposition of methionine, which is also disadvantageous, as is the increased energy requirement of this additional process step.

[0012] Therefore, there is a need for new processes for recovering methionine and potassium bicarbonate from methionyl-methionine-rich mother liquors (e.g. mother liquors from the second carbonation) which processes avoid or only show to a reduced extent the disadvantages of the processes described in the prior art.

[0013] The core objective of the process for recovering valuable substances from the discharged mother liquor by carbonation and separation is to separate the precipitate produced in the carbonation, which contains in particular the valuable substances potassium bicarbonate and methionine, and the mother liquor, which is enriched in secondary components such as formate and methionyl-methionine. The precipitate is recycled into the process, the remaining mother liquor (e.g. the second or third mother liquor) constituting the waste product.

[0014] If this solid-liquid separation is not effective, i.e. when the precipitate is difficult to separate, difficult to filter and can therefore only be recycled into the process together with a large amount of mother liquor, this can lead to many secondary components also returning to the hydrolysis step - hydantoin saponification - and thus accumulating in the process, with the result that more first mother liquor must be discharged in order to remove these by-products from the circuit again. The filterability of the precipitate therefore has a decisive influence on the throughput of the discharged first mother liquor in the overall treatment process. The filterability is therefore decisive for the economy of the overall treatment process.

[0015] In the process of recovering methionine and bicarbonate from a methionine mother liquor rich in methionyl-methionine, as described in the specifications of the above-mentioned patents, it is precisely the filtration that is problematic. According to the above-mentioned specifications, this is due to the higher concentration of methionyl-methionine in the second mother liquor. Therefore, these also describe several methods for reducing the methionyl-methionine content in the mother liquor in order to improve the filterability of the precipitate.

[0016] Thus, for example, before carbonation and filtration, the second mother liquor is first heated to a high temperature (eg 160-180° C.) for 0.3 to 10 hours (EP 2186797A1) to cleave methionyl-methionine to methionine. This facilitates subsequent filtration.

[0017] However, the disadvantages of this solution are that expensive additional equipment is required (e.g. corrosion-resistant reactors and heat exchangers with sufficient residence time), additional energy input is required, and the valuable substance methionine is also partially decomposed. This can reformulate secondary components.

[0018] Technical Problems Solved by the Invention

[0019] The problem addressed by the present invention was therefore to provide a process for recovering a mixture containing methionine, potassium bicarbonate and optionally potassium carbonate from an aqueous solution or suspension containing methionine, optionally potassium carbonate, potassium bicarbonate and methionyl-methionine, in particular from a mother liquor of methionine production which is significantly enriched in methionyl-methionine, in which the methionine, potassium bicarbonate and optionally potassium carbonate present precipitate in the highest possible proportion and are as easily filterable as possible, i.e. exhibit the best possible filterability.

[0020] A further problem directly related hereto is to provide an improved process for producing methionine, wherein only small amounts of waste are generated / need to be disposed of. Summary of the invention

[0021] The present method solves the problem of poor filterability of the precipitate produced in the carbonation of the methionyl-methionine mother liquor in a manner different from the prior art. According to the present invention, the precipitate produced by carbonation contains the substances methionine, saleratus and the relatively small proportion of potassium carbonate that optionally exists, and the accompanying mother liquor also contains the methionyl-methionine of considerable amount. The discovery that the present invention is based on is that as long as 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 is dissolved in the mother liquor, the methionyl-methionine that is rich in will not obviously destroy filtration. Only when methionyl-methionine is precipitated in a large number in carbonation and is therefore present in the precipitate with crystals, filtration will become difficult.

[0022] Therefore, the above object is achieved by providing a method for obtaining a mixture containing methionine, potassium bicarbonate and optionally potassium carbonate, said mixture being obtained from an aqueous solution or suspension containing:

[0023] preferably 6.0-18.0% by weight of titratable potassium in the form of potassium bicarbonate and / or potassium carbonate, preferably 2.5-8.0% by weight of methionine, and

[0024] 4.50-12.0 wt.-% methionyl-methionine, characterized in that the method comprises supplying CO2 (carbonation) to the solution or suspension used at a temperature of 15-60° C. to precipitate a mixture containing methionine, potassium bicarbonate and optionally potassium carbonate as a precipitate, the precipitate containing on average not more than 6.5 wt.-%, preferably not more than 0.01 wt.-% to 5.0 wt.-%, in particular not more than 0.01 wt.-% to 3.0 wt.-% methionyl-methionine. For example, the desired methionyl-methionine content can be continuously determined from a sample extracted from the separated precipitate using HPLC.

[0025] This achieves particularly good filterability, thereby overcoming the above-mentioned disadvantages of the prior art methods. The alkaline pH of the aqueous solution or suspension used is about 11 to 12, which is reduced by carbonation, preferably a pH of 7.8 to 9.5, particularly preferably 8.3 to 9.5, especially 8.4 to 9.5, and very particularly preferably 8.4 to 9.0, the pH being measured at a determined temperature in each case with a glass electrode. The methionyl-methionine content in the precipitate is significantly dependent on the pH of the carbonation mother liquor, so this can also be used to control the methionyl-methionine content in the precipitate.

[0026] Separation of the precipitate can be for example carried out by filtering or centrifugation, particularly using corresponding equipment known to those skilled in the art, which can also be used on an industrial scale, which are preferably used in a continuous or semi-continuous manner, thereby can operate the product throughput commonly used in industry. In order to control and regulate pH, thereby also indirectly obtain the technical effect of good filterability, can regularly sample and measure these samples with a glass electrode (referring to the embodiments), or can select continuous pH measurement, to realize very easily regulating the required pH value, and then regulate filterability. In order to directly control and regulate methionyl-methionine content, thereby also for the technical effect of good filterability, can regularly sample from mother liquor, and measure by HPLC (referring to the embodiments). However, the HPLC determination of the methionyl-methionine content favourable according to the present invention can also be carried out in an automated manner.

[0027] Preference is given to using the following method, characterized in that it comprises bringing the content of the solution or suspension used to, optionally by concentrating:

[0028] 6.0-18% by weight of titratable potassium in the form of potassium carbonate and / or potassium bicarbonate,

[0029] 2.5-8.0 wt. % methionine, and

[0030] 4.5-12.0 wt. % methionyl-methionine, then

[0031] CO2 is supplied at a temperature of 15-60°C, preferably 25-55°C (carbonation), until a pH of 7.8 to 9.5, preferably 8.3 to 9.5, particularly preferably 8.4 to 9.5, in particular 8.4 to 9.0, measured with a glass electrode at a determined temperature, is reached, to precipitate a mixture containing methionine, potassium bicarbonate and optionally potassium carbonate as a precipitate, and to separate the precipitate from the mother liquor, wherein the precipitate contains on average not more than 6.5% by weight, preferably not more than 0.01% to 5.0% by weight, in particular not more than 0.01% to 3.0% by weight of methionyl-methionine.

[0032] Therefore, the present invention provides a method in which the method parameters (particularly the carbonation) are selected such that during the carbonation process as much methionine and potassium bicarbonate as possible are precipitated, but at the same time the methionyl-methionine in the precipitate does not exceed a critical value, i.e. remains largely dissolved in the mother liquor, thereby making it possible to recover methionine and potassium bicarbonate from the methionine mother liquor enriched with methionyl-methionine.

[0033] In the present invention, it is no longer necessary to cleave methionyl-methionine before carbonation, thereby achieving considerable energy and cost savings. The method according to the invention can even improve the filterability of the carbonated precipitate in its above-mentioned variants. For example, compared with the 1.6*10 9 m / kg or 0.64*10 described in Example 1 (third crystallization) of EP 2133328 A2. 10 Compared with the value of m / kg, the filtration resistance of Example 1 of the present invention is significantly lower, which is 1.2*10 6 m / kg.

[0034] According to the present invention, the second mother liquor of the second carbonation is concentrated. Low pressure (e.g., 300 mbar absolute pressure) is selected to achieve gentle concentration of the mother liquor at low temperatures. This avoids the cracking of methionine and methionyl-methionine. For example, the mother liquor is concentrated by 2 times. For example, the concentration of free potassium is 13.4% by weight, and for example, the concentration of methionine is 4.9% by weight. Methionyl-methionine is then also enriched; its concentration is, for example, 7% by weight (see Example 1). Then by adding CO2, the solution is carbonated. The pressure is 1-6 bar absolute pressure, preferably 1.5-2.5 bar absolute pressure, to reduce the foaming of the suspension during its decompression. The temperature is 15-60°C, preferably 25-35°C, to avoid using cold water, but to allow the use of cooling water that is more energy-favorable. The residence time in the CO2 supply is 20 to 180 minutes, preferably about 60 minutes. In the reactor, the pH of the suspension is measured at the corresponding temperature with a glass electrode. This is regulated by adding CO2. The pH of the concentrated mother liquor is about 11. At this point, the mother liquor in the reactor is not fully carbonated (i.e., has reached equilibrium at the corresponding pressure). In contrast, only by adding enough CO2 can a pH of 7.8 to 9.5, such as 8.5, be achieved.

[0035] The inventor finds that the solubility of methionyl-methionine in this matrix depends particularly on temperature and pH value.The target pH value in the reactor (and therefore CO amount) is advantageously selected so that only a small portion of methionyl-methionine is present in the precipitate, i.e. no more than 6.5 weight %, preferably no more than 0.1 weight % to 5.0 weight %, particularly preferably no more than 0.1% weight % to 3.0 weight %.Methionyl-methionine content is usually measured by HPLC at first filtering moist, subsequently on the precipitate sample washed with acetone and dried.This is naturally the cross-sectional value of the methionyl-methionine ratio of crystallization in the precipitate and the methionyl-methionine ratio in the mother liquor attached to the precipitate.But this summation / mean value has also indirectly weighed the relevant crystallization ratio of methionyl-methionine, therefore can be used as the manipulated variable of regulating filterability.

[0036] Therefore, the preferred target pH depends on:

[0037] ·Methionyl-methionine concentration in the second carbonated filtrate

[0038] The concentrations of potassium, methionine and methionyl-methionine after concentration, which are in each case related to the concentration factor, which is usually between 1.5 and 2.5

[0039] Temperature in the reactor

[0040] After carbonation, the precipitate is usually filtered. Since the mother liquor has not yet been carbonated to equilibrium, the precipitate contains only a relatively small amount of methionyl-methionine in addition to methionine and potassium bicarbonate. The precipitate is recycled into the process, preferably upstream of the hydantoin saponification (the reaction step includes hydrolysis of 5-[2-(methylthio)ethyl]imidazolidine-2,4-dione). Any methionyl-methionine present therein is cracked into methionine in the hydantoin saponification reaction. Similar to the recycling of methionine present in the precipitate, this increases the output of the methionine process. As a result of the concentration, the secondary components in the filtrate are enriched by about 2 times, and are preferably discarded. Since the concentration factor is about 2, the amount of the filtrate is also reduced by about 2 times. This significantly reduces the liquid waste stream of the process.

[0041] The process is advantageously operated so that the pressure is from 1 to 6 bar absolute, preferably from 1.5 to 2.5 bar absolute.

[0042] Typical residence times are from 20 to 180 minutes, preferably about 60 minutes.

[0043] As aqueous solution or suspension containing methionine, potassium carbonate, potassium bicarbonate and methionyl-methionine according to the process of the invention, preference is given to using the mother liquor from the isolation of methionine in a process for the production of methionine by saponification of alkaline methionine hydantoin, as described, for example, primarily in EP 780370 A2.

[0044] The mother liquor is preferably the mother liquor filtered after repeated carbonation, especially the third carbonation, because here the advantage of recovering other lost valuable substances is greatest.

[0045] The invention also provides a mixture containing methionine, potassium bicarbonate and optionally potassium carbonate produced according to the above method, and a mixture containing methionine, KHCO3 and methionyl-methionine, wherein the content of methionyl-methionine does not exceed 6.5% by weight, as produced according to the method of the invention.

[0046] The invention further provides for the use of this mixture for the production of methionine, in particular as a saponifying agent and additional source of methionine. This has the great advantage of improving process economics by saving resources and increasing yields, thus having a significant impact on typical plant outputs of about 100,000 tons of methionine per year.

[0047] The present invention also provides a general method for producing methionine, comprising the following steps (1) to (6) (see also Figure 1 , principle process flow chart):

[0048] (1) a reaction step comprising hydrolyzing 5-[2-(methylthio)ethyl]imidazolidine-2,4-dione in the presence of a basic potassium compound to obtain a hydrolysis product containing potassium methionine and potassium methionylmethionine;

[0049] (2) a first crystallization step, comprising introducing 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, and precipitating methionine and separating the suspension into a first precipitate containing methionine and a first mother liquor containing methionyl-methionine;

[0050] (3) concentrating the first mother liquor obtained in step (2) (first concentration step),

[0051] (4) recycling a first portion of the concentrated first mother liquor from (3) to the reaction step (1);

[0052] (5) a second crystallization step, which comprises introducing carbon dioxide into a second portion of the concentrated first mother liquor from (3) (second carbonation) to precipitate methionine and potassium bicarbonate, and separating the resulting suspension into a second precipitate and a second mother liquor;

[0053] (6) a third crystallization step, which comprises 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 method, so that the third precipitate contains an average of not more than 6.5% by weight, preferably not more than 0.01% by weight to 5% by weight, and particularly preferably not more than 0.01% by weight to 3% by weight of methionyl-methionine.

[0054] Potassium hydroxide, potassium carbonate and / or potassium bicarbonate and, at least in addition, the above-mentioned precipitate from the carbonation step comprising potassium bicarbonate and, if appropriate, potassium carbonate are preferably used as basic potassium compound.

[0055] By providing an overall process according to the invention, the problem of providing an improved process for producing methionine can be solved, wherein the yield of methionine is increased, more basic potassium is recovered from the mother liquor, and thus the amount of waste generated / needing to be disposed of is significantly reduced. Against the background of a typical output of about 100 000 tons per year in a typical methionine plant, this has a high economic and ecological value, especially in view of the resource-saving advantages.

[0056] Also preferred are processes in which the third precipitate is recycled into the hydrolysis step (1) or into the further steps (2) to (4), wherein due to its high potassium content the precipitate is used to reduce the need for replacement potassium and to recycle the methionine present.

[0057] It is also advantageous to combine the first mother liquor and the third precipitate and subsequently recycle them to the hydrolysis step (1) or the concentration step (3), which has the advantage of reducing the complexity of the process.

[0058] The third mother liquor obtained as waste in the process can be simply discarded or further separated into valuable substances, in particular recycled potassium bicarbonate and methionine, the final product of the process. Example

[0059] Methods used

[0060] 1. High Performance Liquid Chromatography (HPLC)

[0061] Chromatographic studies (methionine and methionyl-methionine) were performed on suitable RP columns using an HPLC instrument from Jasco, followed by UV detection at 210 nm. The mobile phase was an acetonitrile-water mixture acidified with phosphoric acid. 10 μl of the corresponding sample solution was injected at a flow rate of 1 ml / min. The system was pre-calibrated by injecting appropriate calibration solutions of appropriate reference compounds in the methionine / methionyl-methionine process and evaluated by peak area comparison using an external standard method. The procedures of standard methods are known to those skilled in the art.

[0062] Sampling for determination of methionyl-methionine in precipitates:

[0063] Collect one volume of filtered sediment from carbonation on a laboratory filter funnel, wash with twice the volume equivalent of acetone, and decompress and suck out in a water jet vacuum. The remainder of the mother liquor still attached to the sediment (the sediment is slightly soluble in acetone) is mostly removed. Suction for another 5 minutes, and then in a drying oven the sediment is dried to constant weight, thereby removing remaining acetone. Subsequently, weigh an appropriate proportion of the sediment through this pretreatment, dissolve it in the HPLC eluent, and dilute to an appropriate concentration with more HPLC eluent, and inject the sample solution obtained through this into an HPLC instrument as described above.

[0064] 2. Potassium titration

[0065] Basic potassium salts, such as potassium carbonate, potassium bicarbonate and potassium methioninate, are present in almost all process solutions of the methionine process and are covered here by the term "titratable potassium". A titrimetric determination of the "titratable potassium" aqueous solution is carried out, in particular in process solutions, as a collective parameter. This is a potentiometric titration of the relevant potassium-containing alkaline process aqueous solution with 0.1 molar HCl up to pH 4.6. In acid / base titrations, the acid consumption is reported as "titratable potassium".

[0066] For example, according to the reaction with KHCO3, 1 ml of 0.1 mole of HCl corresponds to 3.91 mg of K + Weight equivalent:

[0067] KHCO3+HCl→KCl+H2O+CO2

[0068] Note: When the term equivalent (eq) is used, it should be understood to mean molar equivalent (also called moleq).

[0069] 3. Measurement of filter cake resistance as a measure of suspension filterability

[0070] The so-called filter cake resistance is determined according to VDI guideline VDI 2762 page 2 (December 2010). The filter cake resistance is given in 1 / m 2 Report. Area-based filter cake resistance α H In units of 1 / m 2 Report, mass-based filter cake resistance α M Reported in units of m / kg. These two parameters are determined by the density of the dry filter cake (ρ s , unit is kg / m 3 ) to convert: α H =α M * s The higher the cake resistance value, the worse the filterability. The filterability is also determined empirically by observing and comparing different filter tests. The following filterability ratings are awarded:

[0071] Very good - Good - Acceptable - Poor - Very poor.

[0072] Therefore, the embodiments of the present invention were rated as "very good", "good" or "acceptable".

[0073] 4. Prepare the methionine hydantoin saponification solution as the starting solution of the example substantially according to EP 780370 A2

[0074] Initially, a methionine hydantoin solution is prepared from MMP, HCN and ammonia and CO2 in the form of an ammonium carbonate solution, essentially according to Example 1 (page 10) of EP 780370 A2, and from this a methionine hydantoin saponified solution (hydrolysate) is prepared by saponification using an aqueous solution of titratable potassium, in particular a K2CO3-containing solution, essentially according to Example 6 of EP 780370 A2. Analogously to Example 7 of EP 780370 A2, the hydrolysate is an alkaline solution, in particular containing potassium methioninate and potassium methionylmethioninate, neutralized primarily with CO2 (first carbonation). As described above, the first precipitate thus precipitated, in particular containing methionine, is filtered out from the accompanying first mother liquor (1st moli).

[0075] The first mother liquor is again carbonated (second carbonation), and the resulting precipitate is filtered out from the remaining second mother liquor. In Examples 1 to 12, the resulting second mother liquor is further processed as initial solution 1, 2 and 3, and the composition in each case is as shown in Table 1.

[0076] The respective initial solution (second mother liquor) was concentrated to 1.6 to 1.9 times, and the concentrated second mother liquor thus obtained was in each case fed to a third carbonation. The precipitate thus formed was filtered off in each case by a filtration device and analyzed for titratable potassium K. + , methionine and methionyl-methionine. The filterability was evaluated and in some cases the filter cake resistance was determined (see Table 1).

[0077] Table 1 provides an example summary of carbonation and filtration results to illustrate the dependence of filtration on the methionyl-methionine concentration in the filter cake.

[0078] Table 1: Summary of adjustment parameters and results of Examples 1 to 12

[0079]

[0080]

[0081] Table 1 Note: *) According to the following formula, by dividing by the dry filter cake density ρ s [kg / m 3 ], based on the filter cake area α H [1 / m 2 The initial experimentally determined filtration resistance of ] is converted to the filtration resistance α based on the mass of the dry filter cake M [m / kg]:

[0082] α M [m / kg] = α H [1 / m 2 ] / ρ s[kg / m 3 ] (see Examples 1 and 2), using the values ​​determined by the following experiments:

[0083] α H =1*10 9 *1 / m 2 , wherein ρ in Example 1 s 720kg / m 3 , α H =1*10 10 *1 / m 2 , wherein ρ in Example 2 s 720kg / m 3 .

[0084]

[0085]

[0086] Table 1 Comments

[0087] As is evident from Examples 1 and 2 in Table 1, the presence of methionyl-methionine in the precipitate can be avoided by increasing the pH to the optimum range. Increasing the temperature or reducing the concentration factor in the carbonation step can also prevent the precipitation of methionyl-methionine, thereby improving filterability. However, this variation of these parameters also severely reduces the amount of methionine and potassium in the precipitate. This only occurs to a reduced extent when the pH is increased. Therefore, surprisingly, regulation by pH has proven to be a very effective and also simple, cost-effective and therefore economically most advantageous solution.

[0088] Comparison of Examples 3-5 illustrates the effect of pH on the concentrations measured in the filtrate and the precipitate. The methionyl-methionine concentration in the concentrated second mother liquor was 5.9 wt %. In Example 3, the mother liquor was carbonated to pH 8.9 at 30° C. The methionyl-methionine concentration in the resulting third mother liquor increased to 6.1 wt %. The reason for this is that the methionine and KHCO3 components have partially precipitated due to the potassium concentration in the third mother liquor dropping from 13.1 wt % to 8.2 wt %, and the methionine concentration dropping from 4.6 % to 3.4 %. The precipitation of these components can lead to an increase in the methionyl-methionine concentration, provided that the methionyl-methionine does not precipitate as well. It was found that the precipitate contained only 1.2 wt % of methionyl-methionine. Concentration measurements in the filtrate and filter cake showed that no methionyl-methionine precipitated here.

[0089] The same effect was evident in Example 4. In the third mother liquor, as the pH was lowered to 8.5, the concentrations of methionine and potassium further decreased, and the methionyl-methionine concentration further increased to 6.2 wt%.

[0090] Only in Example 5, when carbonating to pH 8.1, did the methionyl-methionine concentration in the filtrate fall below the initial concentration of 5.9% by weight to a concentration of 5.3% by weight. Moreover, the methionyl-methionine concentration in the precipitate was 5.3% by weight here. The filterability of the filter cake deteriorated significantly: from "very good" to "acceptable".

[0091] The comparison of Examples 6, 7 and Comparative Example 8 illustrates once again the influence of the pH value. All three examples start from the same initial solution (the second mother liquor after concentration) and are carbonated at the same temperature of 30° C. The pH values ​​determined were 9.0, 8.5 and 8.2, respectively. The solubility and thus the concentration of the valuable substances potassium and methionine and the by-product methionyl-methionine in the third mother liquor decrease with pH. In Comparative Example 8, the proportion of methionyl-methionine precipitated in the precipitate is 6.8% by weight, which is already sufficiently high, but the filterability is poor compared to the very good filterability in Examples 6 and 7.

[0092] The comparison of Example 5 with Example 8 illustrates the influence of the concentration factor on the methionyl-methionine concentration of the concentrated second mother liquor, which concentration depends directly on this factor. In Example 8, the precipitate already has a "poor" filterability at a pH of 8.25. The reason for this is that the methionyl-methionine concentration of the initial solution is relatively high (here 7.5 weight %, relative to 5.9 weight % of Example 5). Therefore, at higher pH values, the solubility limit of methionyl-methionine has been exceeded, so that at higher pH values, there is already a considerable methionyl-methionine ratio in the precipitate, which correspondingly reduces filterability. The higher the methionyl-methionine concentration in the initial solution, the higher the pH, and above this pH, the methionyl-methionine ratio in the precipitate will significantly increase. The lower the methionyl-methionine concentration in the initial solution, the lower the pH resulting in a significant methionyl-methionine ratio in the precipitate. When the methionyl-methionine concentration of initial solution was lower than 4.5 % by weight, solution (under normal pressure and temperature) can be carbonated to CO saturated (i.e. achievable minimum pH under corresponding pressure), and can not produce the methionyl-methionine ratio that obviously meets expectation in throw out.Because the content of methionyl-methionine in the mother liquor of the first and second carbonation of producing before is still lower than 4.5 % by weight usually, the problem of poor filterability after carbonation does not occur here.With respect to titratable potassium and methionine, the second mother liquor is rich in methionyl-methionine, because these two materials are removed from solution by carbonation and subsequent filtration.In order that saleratus and / or salt of wormwood and methionine are also precipitated in throw out in the 3rd carbonation, the second mother liquor must first be concentrated.Therefore, the concentrated initial solution (concentrated second mother liquor) that is used for the 3rd carbonation has reached higher methionyl-methionine concentration with respect to previous concentration, therefore has especially adopted the solution of raising throw out filterability of the present invention herein.

[0093] A comparison of Examples 4, 10 and 11 illustrates the influence of temperature. In all three examples, the pH was about 8.5. The solubility and thus the concentration of the valuable substances potassium and methionine in the third mother liquor decreases with temperature.

[0094] The influence of temperature is particularly well demonstrated by the comparison of Examples 4 and 11. In both examples, the concentrations of the initial solutions were similar and the pH values ​​determined during the carbonation were almost identical, whereas the temperature was 30° C. in Example 4 and 50° C. in Example 11. However, the concentration of 1% by weight of methionyl-methionine in the precipitate in Example 4 was at a similarly low level as the concentration of 1.3% by weight in Example 11. The amount of methionyl-methionine that undergoes precipitation in the precipitate increases with decreasing temperature. Here, too, the concentrations of the valuable substances methionine and potassium decrease with temperature.

[0095] However, it is clear from the measurement results in Table 1 that only a small amount of 2.6% by weight of methionyl-methionine was found in the precipitate under the carbonation condition of pH 9.0 according to Example 6. The methionyl-methionine concentration in the third mother liquor was higher than that in the concentrated second mother liquor, and the methionyl-methionine concentration in the precipitate was lower, at 2.6% by weight. This 2.6% weight may also be caused by the adhesion of the mother liquor.

[0096] In the case of carbonation to pH 8.2, the methionyl-methionine concentration in the third mother liquor was lower than that in the concentrated second mother liquor, and the methionyl-methionine concentration in the precipitate was 6.4 wt %. This indicates the presence of methionyl-methionine in the precipitate.

[0097] The cake resistance during carbonation was measured at pH 8.7 (1.2*10 6 m / kg, see Example 1) and pH 8.2 (about 2.3*10 9 m / kg, see Example 2 (Comparative)). The filter cake resistance of the precipitate obtained at pH 8.2 is about 2000 times higher than the filter cake resistance at pH 8.7. Therefore, the filterability of the precipitate at pH 8.7 is very good and therefore significantly better than the only acceptable filterability at pH 8.2.

[0098] From the implementation it can be concluded that the solubility of the components methionine, potassium (mainly in the form of potassium bicarbonate) and methionyl-methionine decreases with temperature and pH. In particular, the concentration factor ensures a large difference in the concentrations in the concentrated second and third mother liquors, thus ensuring a greater mass of precipitate (= valuable substance) and a small amount of third mother liquor (= by-product stream). BRIEF DESCRIPTION OF THE DRAWINGS

[0099] The figure shows a scheme of the methionine process according to the invention.

[0100] The scheme includes the following steps, wherein the figure numbers are shown in Table 2 below.

[0101] Table 2: Figure 1 List of reference numerals

[0102]

[0103]

Claims

1. A method for obtaining a mixture containing methionine and potassium bicarbonate, wherein the mixture is obtained from an aqueous solution or suspension containing: titratable potassium in the form of potassium bicarbonate and / or potassium carbonate, Methionine, and 4.5-12.0% by weight of methionyl-methionine, It is characterized in that The method comprises supplying CO2 to the solution or suspension used at a temperature of 15-60°C (carbonation) to precipitate a mixture containing methionine and potassium bicarbonate as a precipitate, the precipitate containing an average of not more than 6.5% by weight of methionyl-methionine, and separating the precipitate from the mother liquor.

2. The method for obtaining a mixture containing methionine and potassium bicarbonate according to claim 1, wherein the mixture is obtained from an aqueous solution or suspension containing methionine, potassium bicarbonate and methionyl-methionine, characterized in that: The method comprises bringing the solution or suspension used, optionally by concentrating, to the following contents: 6.0-18.0% by weight of titratable potassium in the form of potassium bicarbonate and / or potassium carbonate, 2.5-8.0% by weight of methionine, 4.5-12.0 wt. % methionyl-methionine, and CO2 is supplied at a temperature of 15-60°C to precipitate a mixture containing methionine and potassium bicarbonate as a precipitate, the precipitate containing not more than 6.5% by weight of methionyl-methionine on average, and the precipitate is separated from the mother liquor.

3. The method for obtaining a mixture containing methionine and potassium bicarbonate according to claim 1 or 2, wherein the mixture is obtained from an aqueous solution or suspension containing methionine, potassium bicarbonate and methionyl-methionine, characterized in that: The method comprises bringing the solution or suspension used, optionally by concentrating, to the following contents: 6.0-18.0% by weight of titratable potassium in the form of potassium bicarbonate and / or potassium carbonate, 2.5-8.0% by weight of methionine, 4.5-12.0 wt. % methionyl-methionine, and CO2 is supplied at a temperature of 15-60°C (carbonation) until a pH of 7.8-9.5 is reached, as measured with a glass electrode at a determined temperature, to precipitate a mixture containing methionine and potassium bicarbonate as a precipitate, the precipitate containing an average of not more than 6.5% by weight of methionyl-methionine, and the precipitate is separated from the mother liquor.

4. The method according to any one of claims 1 to 3, characterized in that The method comprises supplying CO2 at a temperature of 25-55°C until the pH reaches 8.3-9.5 to precipitate a mixture containing methionine and potassium bicarbonate as a precipitate, the precipitate containing not more than 0.01-5.0 wt% of methionyl-methionine on average, and separating the precipitate from a mother liquor.

5. The method according to any one of claims 1 to 4, characterized in that The method comprises supplying CO2 at a temperature of 25-55°C until the pH reaches 8.4-9.5 to precipitate a mixture containing methionine and potassium bicarbonate as a precipitate, the precipitate containing not more than 0.01-3.0 wt% of methionyl-methionine on average, and separating the precipitate from a mother liquor.

6. The method according to any one of claims 1 to 5, characterized in that The method comprises supplying CO2 until a pH of 8.4-9.0 is reached.

7. The method according to any one of claims 1 to 6, characterized in that The absolute pressure during the CO2 supply is 1-6 bar.

8. The method according to any one of claims 1 to 7, characterized in that The residence time in the CO2 supply is 20-180 minutes.

9. The method according to any one of claims 1 to 8, characterized in that As the aqueous solution or suspension containing methionine, potassium bicarbonate and methionyl-methionine, a mother liquor obtained by isolating methionine in a method for producing methionine by saponification of alkaline methionine hydantoin is used.

10. The method according to claim 9, characterized in that The mother liquor is the mother liquor obtained by filtration after repeated carbonation.

11. A mixture comprising methionine and potassium bicarbonate prepared according to any one of claims 1 to 10.

12. The mixture according to claim 11, comprising methionine, potassium bicarbonate and methionyl-methionine, wherein the content of methionyl-methionine does not exceed 6.5% by weight.

13. Use of the mixture according to claim 11 or 12 for preparing methionine.

14. A method for preparing methionine, comprising the following steps (1) to (6): (1) a reaction step comprising hydrolyzing 5-[2-(methylthio)ethyl]imidazolidine-2,4-dione in the presence of a basic potassium compound to obtain a hydrolysis product containing potassium methionine and potassium methionylmethionine; (2) a first crystallization step, comprising introducing carbon dioxide into the hydrolyzate obtained in step (1) (first carbonation) to obtain a suspension containing methionine, methionyl-methionine and potassium bicarbonate, and precipitating methionine, and separating the suspension into a first precipitate containing methionine and a first mother liquor containing methionyl-methionine; (3) concentrating the first mother liquor obtained in step (2) (first concentration step), (4) recycling a first portion of the concentrated first mother liquor from (3) into the reaction step (1), (5) a second crystallization step, which comprises introducing carbon dioxide into a second portion of the concentrated first mother liquor from (3) (second carbonation) to precipitate methionine and potassium bicarbonate, and separating the resulting suspension into a second precipitate and a second mother liquor, (6) a third crystallization step, which comprises 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 and potassium bicarbonate and a third mother liquor containing methionyl-methionine according to any one of claims 1 to 9, so that the third precipitate contains an average of not more than 6.5% by weight of methionyl-methionine.

15. The method according to claim 14, characterized in that The third precipitate contains an average of no more than 0.01-5.0 wt. % methionyl-methionine.

16. The method according to claim 14 or 15, characterized in that The method includes recycling the third precipitate to any one of steps (1) to (4).

17. The method according to claims 14 and 15, characterized in that The method comprises combining the first mother liquor and the third precipitate and then recycling them to the reaction step (1) or the concentration step (3).

18. The method according to any one of claims 14 to 16, characterized in that: The method comprises disposing of the third mother liquor or supplying it to further separation of potassium bicarbonate and / or methionine.

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