Process for the preparation of D,L-methionine or the salt thereof

A process for the preparation of methionine or a salt of methionine by reaction of the components 3-methylmercaptopropionaldhyde, hydrogen cyanide, ammonia and carbon dioxide or components from which the above-mentioned components can be prepared, optionally in the presence of water, to 5-(2-methylmercaptoethyl)-hydantoin and further reaction thereof to methionine or the salt thereof, characterised in that the reaction of the components is initiated by means of at least one premixture in such a way that a first mixture is formed containing at least the predominant proportion (at least 5/10) of the 3-methylmercaptopropionaldeyde and at least 1/10 of the hydrogen cyanide component or corresponding quantities of components from which these components can be prepared, and less than 5/10 of one of the components ammonia, carbon dioxide or of the components from which ammonia or carbon dioxide can be prepared, and in that this first mixture is combined with the other component(s) for the reactive conversion to 5-(2-methylmercaptoethyl)-hydantoin, wherein this (these) latter component(s) may be premixed in one or more other mixtures.

CROSS-REFERENCE TO RELATED APPLICATIONS 
This application is based on DE 195 47236.5 filed on Dec. 18, 1995 in 
Germany, the entire content of which is hereby incorporated by reference. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for the preparation of D,L-methionine or 
a salt of D,L-methionine, starting from the components 
3-methylmercaptopropionaldehyde, hydrogen cyanide, ammonia and carbon 
dioxide or 5-(2-methylmercaptoethyl)-hydantoin or a salt of methionine or 
those components from which the above-mentioned components can be 
prepared, optionally in the presence of water. 
The synthesis steps can be illustrated by the following reaction equations: 
##STR1## 
M stands for alkali metal, alkaline earth metal, ammonium, particularly 
potassium. 
Advantageously, the process steps of 5-(2-methylmercaptoethyl)-hydantoin 
formation, methioninate formation and methionine liberation may be carried 
out continuously, advantageously in successive stages which are integrated 
in a process which, as a whole, takes place continuously, in particular. 
Particularly advantageously, the components ammonia and carbon dioxide are 
recycled in accordance with the process possibilities, i.e. used again in 
the preceding process stages. Particularly when potassium is used, all the 
agents containing alkali metal are recycled to the process, if possible. 
2. Background Information 
The formation of 5-(2-methylmercaptoethyl)-hydantoin is known in principle. 
Generally speaking, the starting materials are either the components 
described under 1) or components from which these components can be 
prepared. These are, in particular, alkali metal- or ammonium salts in the 
case of the components hydrogen cyanide, ammonia and carbon dioxide, and 
acrolein and methyl mercaptan in the case of the 
3-methylmercaptopropionaldehyde component, as described further below. 
Chem. Rev. 46 (1959) 422-425 describes the preparation of substituted 
hydantoins by reaction of the corresponding aldehydes and ketones with 
alkali metal cyanides and ammonium carbonate. The reaction is carried out 
either using stoichiometric proportions of the substances at 80.degree. C. 
and 3 bar or using several times the stoichiometric proportion of ammonia 
at temperatures up to 60.degree. C. and at normal pressure (DT-PS 11 66 
201). It is also known to prepare 5-(2-methylmercaptoethyl)-hydantoin from 
3-methylmercaptopropionaldeyde, ammonium carbonate and cyanides. The 
reaction is carried out initially at 40.degree.-120.degree. C., the 
reaction mixture is then adjusted to a pH below 4 and the reaction is 
ended at 50.degree.-100.degree. C. (U.S. Pat. No. 2,557,913). Moreover, it 
is known to prepare 5-(2-methylmercaptoethyl)-hydantoin by providing a 
solution which was prepared by dissolving 3-methylmercaptopropionaldehyde 
in an aqueous solution of ammonia, carbon dioxide and hydrocyanic acid or 
the salts thereof and in which, optionally, a reaction to hydantoin has 
taken place partially or wholly, and introducing into this solution an 
aqueous solution of ammonia, carbon dioxide and hydrocyanic acid or the 
salts thereof and, separately, the 3-methylmercaptopropionaldehyde, and 
carrying out the reaction by heating the mixture to a temperature up to 
100.degree. C. at normal pressure (DT-OS 16 20 332). In Japanese patent JP 
48-005763, 3-methylmercaptopropionaldehyde is reacted with hydrogen 
cyanide or the salts thereof and ammonium carbonate in the presence of 
ammonia and at 80.degree. C. in 1.5 hours to form 
5-(2-methylmercaptoethyl)-hydantoin in a 98.5% yield. Added metal ion 
complexing agents in the presence of water lead to a 97.8% yield (JP 
48-004465). A similar reaction in the presence of organic solvents at 
50.degree.-200.degree. C. under pressure in the liquid phase is described 
in patent JP 40-36676. A one-pot variant starting from acrolein, 
methylmercaptan, hydrogen cyanide and ammonium carbonate in water at 
50.degree.-70.degree. C. leads within 2 hours to a hydantoin which is 
saponified to D,L-methionine (JP 50-004018). 
The reaction described in JP 52-027768 takes place in a similar manner, but 
with the addition of amino acids such as methionine, threonine, glycine, 
alanine, or leucine. 3-Methylmercaptopropionaldehyde, carbon dioxide, 
ammonia, hydrogen cyanide and caustic alkali at 80.degree. C. lead to 97% 
5-(2-methylmercaptoethyl)-hydantoin within 2 hours (JP 50-018467). 
3-Methylmercaptopropionaldehyde, sodium cyanide and ammonium carbonate in 
water in the presence of potassium thiosulphate or potassium carbonate 
give 5-(2-methylmercaptoethyl)-hydantoin (SU 740770). In the single-stage 
reaction of acrolein with methyl mercaptan, hydrogen cyanide and ammonium 
carbonate, 5-(2-methylmercaptoethyl)-hydantoin is produced in an 85% yield 
(Asahi Chem. Ind., Agric. Biol. Chem. 52, 589 (1988). The Chinese patent 
CN 85 1085905 also describes a single-stage reaction, but with the 
addition of methionine in acetic acid with 91% conversion to 
5-(2-methylmercaptoethyl)-hydantoin. 
A 5-(2-methylmercaptoethyl)-hydantoin prepared by the known methods is 
contaminated to a considerable extent by 
5-(2-methylmercaptoethyl)-hydantoic acid, 
5-(2-methylmercaptoethyl)-hydantoic acid amide, methioninamide, methionine 
nitrile and methylmercaptopropionaldehyde cyanohydrin, iminonitrile and 
polymers. Whilst the first three compounds mentioned, like the hydantoin, 
are converted to methionine during alkaline hydrolysis, the other 
compounds or the saponification products thereof enter the saponification 
solution and the methionine to be isolated later, where they can be 
separated only with great difficulty. This is particularly so when the 
methionine is prepared from the hydantoin and separated from the reaction 
mixture using carbon dioxide and the mother liquor recycled. The 
methionine obtained is discoloured and has poor storage stability. 
Alkaline hydrolysis of 5-(2-methylmercaptoethyl)hydantoin is by no means 
new. U.S. Pat. No. 2,527,366 and U.S. Pat. No. 2,557,913 describe the 
hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin in an aqueous barium 
hydroxide solution under pressure and at elevated temperature. These 
processes require substantial quantities of expensive barium hydroxide, 
however; moreover, the barium has to be separated again as a neutral salt. 
It is known from U.S. Pat. No. 2,557,920 that .alpha.-amino acids are 
produced by saponification of hydantoins using sodium hydroxide. In these 
processes, however, at least 3 moles of sodium hydroxide per mole of 
hydantoin are required. The situation is similar when potassium hydroxide 
is used. 
Moreover, it is known from U.S. Pat. No. 4,272,631 that a mixture of alkali 
metal and alkaline earth metal hydroxide may be used to saponify 
5-(2-methylmercaptoethyl)-hydantoin. In these processes, however, the 
alkaline earth metal ions first have to be separated during the liberation 
of methionine, so maximum yields of only 80.5% are obtained. 
In U.S. Pat. No. 4,259,925, the hydrolysis of 
5-(2-methylmercaptoethyl)-hydantoin is carried out under pressure at 
105.degree. to 230.degree. C. in a medium that contains a metal hydroxide 
and an alcohol with a boiling point of 125.degree. to 130.degree. C. A 
disadvantage is that the high-boiling alcohol has to be recovered. 
Moreover, the yield is only 65%. 
The hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin using an aqueous 
solution of alkali carbonate and/or alkali hydrogen carbonate is described 
in DE-PS 19 06 405. Ammonia and carbon dioxide are removed continuously 
during hydrolysis. The preferred alkali metal carbonate is potassium 
carbonate; a molar ratio of hydantoin to alkali metal of 1:1 to 1:5 is 
used. Hydrolysis is carried out under pressure at 120.degree. to 
220.degree. C. The continuous pressure apparatus is composed of three 
rotary evaporators set up at great expense in series. The alkali metal 
methioninate solution is used to liberate D,L-methionine with carbon 
dioxide; the mother liquor obtained from the separation of the methionine 
that crystallises out is recycled, optionally with the removal of 1-2%, 
and used again for the hydrolysis of hydantoin. 
DE-AS 15 18 339 describes a process in which gaseous reaction products 
(ammonia and carbon dioxide) produced during hydrolysis are removed from 
the reaction in order to shift the reaction equilibrium towards the amino 
acid, as a result of which the yield is increased. In order to achieve 
this, however, a complex arrangement of apparatus is required to regulate 
the pressure of the gas streams. 
Japanese patent 49/116 008 describes a process in which the hydrolysis of 
5-(2-methylmercaptoethyl)-hydantoin is carried out in the presence of 
vanadic acids, molybdic acids, tungstic acids or the derivatives thereof. 
The yields are about 70%. The separation of the catalyst is difficult. In 
order to prepare a highly concentrated methionine-containing solution, 
alkali metal e.g. a potassium compound, must be added. 
It is known from the Japanese patent application 75/106 901 (C.A. 84,44666k 
(1976)) that methionine is prepared by hydrolysis of 
5-(2-methylmercaptoethyl)hydantoin in the presence of about 1.2 equ. of 
sodium hydroxide and about 9 equ. of ammonia at 180.degree. C. The sodium 
methioninate solution obtained as an intermediate with this mode of 
operation necessarily contains not only sodium methioninate, however, but 
also sodium carbonate which precipitates whilst this reaction is being 
carried out and is therefore troublesome, particularly in a continuous 
process. The same is true of the use of potassium hydroxide and of the 
method of operating according to DE-PS 19 06 405. 
DE 26 14 411 A describes the hydrolysis of 
5-(2-methylmercaptoethyl)-hydantoin with water in the presence of 
imidazole at 160.degree. C. The yields are low and here, too, an alkali 
metal compound has to be added in order to obtain a high solution 
concentration. 
Japanese patent applications JP 03/95145 and JP 03/95146 describe the 
hydrolysis of hydantoins with water at elevated temperature and elevated 
pressure in the presence of metal oxides or oxide mixtures, for example 
ZrO.sub.2, TiO.sub.2, Nb.sub.2 O.sub.5 or TiO.sub.2 --Nb.sub.2 O.sub.5 ; 
the yields are only 65 to 66%, however. These solutions must be 
neutralised with an alkali compound. 
All these processes bring either low yields or have the disadvantage that 
methionine or salts such as carbonates are precipitated during the 
process, as a result of which further process steps are brought about and 
industrial-scale processes in particular, more particularly continuous 
processes, are scarcely possible. 
The liberation of methionine from the alkali metal salt is generally known. 
According to the principle that strong acids liberate weaker acids from 
their salts, free D,L-methionine is precipitated e.g. with hydrochloric 
acid, sulphuric acid, phosphoric acid or a strongly acid ion exchanger (DE 
21 40 506 C; DE 21 22 491 C; DE 29 12 066 A; BE 877 200, U.S. Pat. No. 
3,433,832, FR 1 532 723). The alkali metal salt obtained as a by-product 
must then, however, be separated. As the acid used is not generally 
recovered, this method of operating is unsuitable for a continuous 
economic and environmentally acceptable production process. 
As described for example in the patents U.S. Pat. No. 2,557,913, DE-PS 19 
06 405 and JP 42/44056, D,L-methionine was therefore precipitated 
advantageously from the hydrolysis solutions of 
5-(2-methylmercaptoethyl)-hydantoin with carbon dioxide in aqueous 
solution. In this method, D,L-methionine is usually obtained in the form 
of thin leaves or as flakes. This causes the product to be difficult to 
filter and prevents the crystal cake being washed out; moreover, a 
D,L-methionine with poor flow properties and a tendency to form lumps is 
obtained. In order to counter these disadvantages, according to Japanese 
patent JP 42/44056, additives such as casein or water-soluble, high 
molecular weight cellulose derivatives are added during the precipitation 
of methionine with carbon dioxide. 
SUMMARY OF THE INVENTION 
The object of the present invention is a process for the preparation of 
methionine or a salt of methionine in which as few by-products as possible 
are produced and the individual components of which can be separated 
easily; in particular, when a product is precipitated, it should be easy 
to filter. It should be possible to recycle or regenerate individual 
components to a large extent. Moreover, the methionine obtained should 
have little discoloration and should be stable in storage. It should be 
possible to carry out the process on an industrial scale in particular, 
and continuously. 
According to the invention, this object is achieved with a process for the 
preparation of methionine or a salt of methionine by reaction of the 
components 3-methylmercaptopropionaldehyde, hydrogen cyanide, ammonia and 
carbon dioxide or those components from which the above-mentioned 
components can be prepared, optionally in the presence of water, to 
5-(2-methylmercaptoethyl)-hydantoin and further reaction thereof to 
methionine or the salt thereof, wherein the reaction of the components is 
initiated by means of at least one premixture in such a way that a first 
mixture is formed containing at least the predominant proportion (at least 
5/10) of the 3-methylmercaptopropionaldehyde and at least 1/10 of the 
hydrogen cyanide component or corresponding quantities of components from 
which these components can be prepared, and less than 5/10 of one of the 
components ammonia, carbon dioxide or of the components from which ammonia 
or carbon dioxide can be prepared, and that this first mixture is combined 
with the other component(s) for the reactive conversion to 
5-(2-methylmercaptoethyl)-hydantoin, wherein this (these) latter 
component(s) may be premixed in one or more other mixtures. 
Components from which the above-mentioned components can be prepared 
include, for example, salts of hydrogen cyanide, ammonia and carbon 
dioxide, for example sodium- or potassium cyanide, ammonium carbonate or 
bicarbonate, sodium or potassium carbonate or bicarbonate and, of course, 
in particular the solutions thereof in water. Corresponding components for 
3-methylmercaptopropionaldehyde are acrolein and methyl mercaptan. In 
principle, it is preferable to use any metal salts used in this reaction 
in less than stoichiometric quantities, i.e. the aldehyde component and/or 
the cyanide component is in stoichiometric excess to any metal ion 
component used. Particularly preferably, no metal salt e.g. sodium or 
potassium cyanide is used here if possible, the above-mentioned compounds 
being used in preference. In a continuous process in particular, however, 
other metal salts may be present in addition, e.g. catalysts or the like. 
The metered quantity thereof does not come under the preferred metal ion 
restriction given above as regards the reaction components. 
In this context and hereinafter the stated components are also taken to 
mean their altered form of use under the given conditions of use. For 
example, when ammonia and carbon dioxide are used in water, a part of 
these components is present as ammonium (hydrogen) carbonate. 
According to this process, the 5-(2-methylmercaptoethyl)-hydantoin is 
obtained as a colourless product in a practically quantitative yield and 
is free to such a large extent from impurities that a methionine or the 
salt thereof may be obtained from it in a continuous process with mother 
liquor recycling and is characterised by extraordinary storage stability 
in terms of discoloration and lump formation. 
Preferably, the first mixture contains at least 5/10 of the hydrogen 
cyanide component, particularly 9/10 and particularly at least 99/100 or 
corresponding proportions of the component from which hydrogen cyanide can 
be prepared. The fractions above (e.g. 5/10) and hereinafter always relate 
to the stated component itself (not to the stoichiometry of the process). 
Irrespective of the stoichiometry, therefore, the total quantity of a 
component used is 1/1. 
In particular, it is advantageous in the process if ammonia and carbon 
dioxide or those components from which ammonia or carbon dioxide can be 
prepared are each used in the first mixture in quantities of less than 
5/10 or their use quantities, preferably at most 1/10 and in particular at 
most 1/100. 
It is also advantageous if the water optionally added to the reaction is 
contained in the first mixture in a quantity of at most 5/10, preferably 
at most 1/10 and in particular at most 1/100. 
It is also advantageous if, at the beginning of the reaction, all the 
components are present in their entirety in their intended use quantity, 
i.e. that no component is metered in afterwards. It is also advantageous 
if all the components are combined in a total of two premixtures which are 
then mixed together for the reaction. 
In all the reactions it is particularly favourable if the individual 
components or premixtures are mixed together rapidly and, if possible, 
intimately. 
The reaction mixture preferably contains no organic solvents but if organic 
solvents are used then their quantity is advantageously less than 20 parts 
by weight, particularly less than 10 parts by weight, based on 100 wt. % 
of water. 
It is particularly advantageous if the premixture(s) and optionally 
individual components (one or more premixtures may be caused to react; in 
the case of two premixtures, optionally all the individual components may 
be contained therein) are introduced into a reaction mixture already 
obtained and containing 5-(2-methyl-mercaptoethyl)-hydantoin. It is 
particularly advantageous in this case if all the premixture(s) and 
optionally individual components are introduced into the reaction mixture 
either adjacently or at staggered intervals (in the case of batchwise 
preparation) or flow-dependent intervals (in the case of continuous 
preparation) of at most 30 s. 
The reaction in the method of operating described above is carried out 
preferably at a temperature above 80.degree. C.; moreover, the reaction is 
carried out preferably at a pressure above atmospheric, particularly 
preferably at a pressure greater than the equilibrium pressure carried 
(reaction pressure), particularly at a pressure of at least 3 bar and 
particularly preferably above 10 bar. 
The process described above with its variants is particularly suitable for 
continuous operation. 
In a process for the preparation of methionine or a salt of methionine, in 
particular two of the process measures described above are by themselves 
particularly advantageous, so the invention also relates to a process for 
the continuous preparation of methionine or a salt of methionine, with the 
reaction of the components 3-methylmercaptopropionaldehyde, hydrogen 
cyanide, ammonia and carbon dioxide or of those components from which the 
above-mentioned components can be prepared, optionally in the presence of 
water, to 5-(2-methylmercaptoethyl)-hydantoin and further reaction thereof 
to methionine or the salt thereof, wherein the components are introduced 
into a reaction mixture which is formed from the above-mentioned 
components and already contains at least 1/10 of the quantity of 
5-(2-methylmercaptoethyl)-hydantoin that can theoretically be formed, and 
wherein the reaction is carried out at a pressure above atmospheric, 
preferably at a pressure of at least 3 bar. 
It is particularly advantageous if operations are carried out at a pressure 
of at least 7 bar and particularly at a pressure of at least 10 bar. 
Moreover, it is particularly advantageous if the components are fed into 
the reaction mixture immediately or with a flow-dependent pre-reaction 
time of at least 30 s, i.e. if the individual components are in reactive 
contact for at most 30 s before being introduced into the reaction 
mixture. 
Moreover, the individual advantageous measures described above can also be 
carried out with this process variant. 
The processes described hitherto are carried out particularly preferably as 
one process, a corresponding summary description of this process being 
given below. The other specifications given herein, e.g. regarding the 
stoichiometry, temperatures etc., are also each by themselves applicable 
advantageously to the processes described in a general manner above. 
According to the invention, 5-(2-methylmercaptoethyl)-hydantoin is prepared 
in a particularly advantageous manner by preparing a solution of hydrogen 
cyanide in 3-methylmercaptopropionaldehyde and a solution of ammonia and 
carbon dioxide in water and mixing these solutions rapidly and intimately 
and causing them to react. Ammonia and carbon dioxide may be returned from 
the hydrolysis stage of this hydantoin. The solution of hydrogen cyanide 
in 3-methylmercaptopropionaldehyde is advantageously adjusted such that it 
comprises equimolar proportions of hydrogen cyanide and 
3-methylmercaptopropionaldehyde, or contains excess proportions of 
hydrogen cyanide. Even so, it is advantageous to select a proportion of 
hydrogen cyanide in the solution of not more than 1.1 mole per mole of 
3-methylmercaptopropionaldehyde; the solution preferably contains 1.005 to 
1.05 mole of hydrogen cyanide per mole of 3-methylmercaptopropionaldehyde. 
This solution of ammonia and carbon dioxide in water may be a saturated or 
dilute solution; advantageously, the ammonia content is not less than 
about 5 wt. %. Ammonium hydrogen carbonate, ammonium carbonate, carbamic 
acid, ammonium carbamate, cyanic acid or a mixture of these components may 
be present in the solution. The molar ratio of ammonia to carbon dioxide 
is advantageously about 1.2 to 4.0 mole, preferably 1.6 to 1.8 mole of 
ammonia per mole of carbon dioxide. The solution of hydrogen cyanide in 
3-methylmercaptopropionaldehyde is mixed with the solution of ammonia and 
carbon dioxide in water in such a way that a molar ratio of ammonia to 
3-methylmercaptopropionaldehyde of about 1.2-6 to 1.0, preferably 2.0-4.0 
to 1.0, particularly 2.5-3.0 to 1.0 is advantageously present in the 
mixture. 
The reaction is carried out at ambient temperature or above, advantageously 
at temperatures above 60.degree. C., advantageously approximately between 
80.degree. C. and 140.degree. C. In preference, temperatures between 
80.degree. and 130.degree. C., particularly between 90.degree. and 
120.degree. C. are selected. Although the reaction may take place at any 
desired pressure, it is advantageous to operate at elevated pressure; 
pressures of up to 20 bar have proved advantageous, particularly pressures 
that are 2 to 3 bar above the equilibrium pressure of the reaction 
mixture. The reaction time depends on the reaction conditions, 
particularly on the temperature and quantity ratios. 
In the preferred mode of operation, it is particularly advantageous to 
introduce the solution of hydrogen cyanide in 
3-methylmercaptopropionaldehyde and the solution of ammonia and carbon 
dioxide in water into a reaction mixture of these substances, i.e. into a 
mixture obtained beforehand during the reaction of the solutions in which 
the reaction of the hydantoin has taken place wholly or partially, and to 
carry out the reaction in this mixture. 
It is particularly advantageous to select a continuous mode of operation, 
to recycle the reaction mixture to this end, to feed the solutions of 
hydrogen cyanide in 3-methylmercaptopropionaldehyde and of ammonia and 
carbon dioxide in water continuously at two adjacent locations in this 
circuit and to draw off an appropriate proportion of the reaction mixture 
continuously from the circuit at another location. Although the mixing 
ratio between the solutions that are fed into the circuit and the reaction 
mixture which is recycled may be any ratio, it is advantageous to select 
this mixing ratio such that several parts by volume of the reaction 
mixture, optionally 1000 or more, preferably 5 to 100, particularly 10 to 
25 parts by volume of the reaction mixture, are present per part by volume 
of the solutions. It is decidedly advantageous to mix the solutions fed 
into the circuit with the recycled reaction mixture rapidly and 
intimately; this may be brought about optionally by means of a mixing 
nozzle, a static mixer, by means of a high circulation rate, or a 
combination of all the measures. It is possible that the reaction has not 
taken place completely in the proportions of the reaction mixture drawn 
off from the circuit, so it may be advantageous to allow these proportions 
to finish reacting for a certain period in a secondary reactor. If less 
than 5 parts by volume of the reaction mixture are present per part by 
volume of solutions, there is a risk of polymerisation of the starting 
components. The mixture becomes dark in colour, precipitation occurs and 
hence losses of yield and/or technical disturbances. 
According to the present invention, the 5-(2-methylmercaptoethyl)-hydantoin 
prepared as described above or by another manner or means may be reacted 
further to an alkali metal salt of methionine or optionally further to 
methionine. The invention therefore also relates to a process for the 
preparation of methionine or an alkali metal salt of methionine by 
hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin in the presence of an 
aqueous solution containing alkali metal and carbon dioxide, and 
optionally further reaction to methionine, wherein hydrolysis is carried 
out at least at the beginning in the presence of at least 0.1 equ., 
particularly up to 7 equ. of ammonia per equivalent of 
5-(2-methylmercaptoethyl)-hydantoin. The invention also relates to a 
process for the preparation of an alkali metal salt of methionine by 
hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin in the presence of an 
aqueous solution containing alkali metal and carbon dioxide, and 
optionally further reaction to methionine, wherein hydrolysis is carried 
out in the presence of metallic zirconium or a zirconium alloy containing 
at least 10 wt. % of zirconium. 
It is particularly advantageous if both processes according to the 
invention are combined. 
It has become apparent that it is particularly advantageous if hydrolysis 
is carried out from the outset in the presence of metal and carbon 
dioxide, i.e. if in particular a mixture of alkali metal compounds is 
present, particularly alkali hydrogen carbonate, alkali carbonate, alkali 
hydroxide, where alkali stands in particular for potassium and sodium. The 
quantity of alkali metal and carbon dioxide is advantageously at least the 
stoichiometric quantity based on the hydantoin. This quantity may be 
substantially higher than stoichiometric. A molar ratio with an excess of 
about 3:1 based on the hydantoin is particularly advantageous; in 
principle, it must be assumed that an even greater excess is even more 
favourable. For practical purposes, however, ratios of about 1.5:1-2:1 are 
particularly preferred. According to the invention, a little ammonia is 
added in addition, this likewise being present accordingly in the form of 
ammonium compounds. It is particularly advantageous if, at the beginning 
of hydrolysis, max. 7 moles of ammonia (incl. ammonium compounds) are 
present per mole of 5-(2-methylmercaptoethyl)-hydantoin. As a result, 
hydrolysis takes place with practically no by-product formation and in 
good yields and, on the other hand, little or no alkali metal carbonate 
precipitates out. It is particularly advantageous if, during hydrolysis, 
ammonia and/or carbon dioxide, optionally together with water, are removed 
from the reaction system. The reaction conditions can thereby be 
controlled in a particularly favourable manner so that no alkali metal 
carbonate precipitates out and the reaction takes place completely. It is 
particularly advantageous if the hydrolysis apparatus itself contains 
zirconium fittings (made of zirconium or a corresponding zirconium alloy). 
It was ascertained that zirconium has a particularly favourable effect, 
presumably catalytic, on hydrolysis. A favourable secondary effect is that 
this apparatus is highly resistant and thus durable, so the use of 
zirconium apparatus does not bring about any apparatus-dependent 
disadvantages compared with other apparatus. 
The hydrolysis processes are carried out advantageously at a temperature of 
120.degree. to 250.degree. C. and, accordingly, at a pressure of 5 to 30 
bar. Very good conversions and little by-product formation are obtained in 
this range. It is also advantageous if the alkali metal component is used 
at least in an equimolar proportion with regard to 
5-(2-methylmercaptoethyl)-hydantoin. In that case, in addition to complete 
hydrolysis, the corresponding alkali metal salt of methionine is obtained 
practically quantitatively. In preference, the hydrolysis solution already 
contains methionine or the salt thereof at the beginning; this, too, has a 
favourable, presumably autocatalytic effect on hydrolysis. 
With this method of operating, practically all the ammonia and all the 
carbon dioxide may be drawn off from the hydrolysis solution during or 
after hydrolysis, so the hydrolysate may be removed in a form which is 
substantially free from ammonia and carbon dioxide. 
In this case, too, it is particularly advantageous to carry out the process 
continuously. It is more particularly advantageous that the processes 
described hitherto may be coupled together, particularly as a continuous 
process in which carbon dioxide and ammonia may be recycled. 
DETAILED DESCRIPTION OF THE INVENTION 
The invention will be described in more detail below with regard to 
hydantoin hydrolysis; in principle, individual detailed specifications by 
themselves also apply to the general method of operating given above. The 
method of operating reproduced below is described for corresponding 
potassium compounds since this is the most preferred embodiment. 
According to the invention, a potassium methioninate solution is obtained 
by hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin in the presence of 
potassium hydroxide, potassium carbonate and/or potassium hydrogen 
carbonate or a mixture thereof and in the presence of excess ammonia, 
carbon dioxide, carbonic acid, cyanic acid or a mixture thereof in water 
at a temperature of 120.degree. to 250.degree. C. and a pressure of 5 to 
30 bar. Advantageously, hydrolysis of the hydantoin takes place in the 
presence of 1 to 15 equ. of one or more potassium compounds, based on the 
hydantoin (e.g. KOH, KHCO.sub.3, K.sub.2 CO.sub.3, potassium 
methioninate). During or after hydrolysis, it is also advantageous if 
ammonia produced or still present and/or carbon dioxide is separated 
wholly or partially from the reaction system. In principle, any 
5-(2-methylmercaptoethyl)-hydantoin may be used; advantageously, the 
hydantoin used is one which may be obtained in the manner described above. 
Advantageously, the ammonia is in a molar ratio to carbon dioxide of 1.1 to 
8.0 at the beginning of hydrolysis. A molar ratio of ammonia to the 
hydantoin of 0.2 to 5 is also favourable. In the method of operating 
described, it is possible to take the ammonia and the carbon dioxide 
directly from the process described above, hydantoin preparation, so that 
the hydantoin directly from hydantoin preparation together with any 
ammonia and carbon dioxide remaining may be introduced into the hydrolysis 
stage where ammonia and/or carbon dioxide may then be adjusted to 
different concentrations, if desired. 
In the process according to the invention, the hydrolysis of 
5-(2-methylmercaptoethyl)-hydantoin takes place at temperatures of 
120.degree.-250.degree. C., preferably 150.degree.-200.degree. C., 
particularly 160.degree.-180.degree. C.; the pressure during the reaction 
should be 5-30 bar, preferably 5-10 bar, particularly 7-9 bar. 
The process is carried out advantageously in a steam-heated column with 
fittings, in which the internal wall and the fittings are composed of 
zirconium or of a zirconium alloy containing at least 10 wt. % of 
zirconium. Advantageously, the 5-(2-methyl-mercaptoethyl)-hydantoin 
solution is introduced continuously at the top of the column at a rate 
such that the hydrolysis product, potassium methioninate solution, may be 
drawn off in a corresponding amount at the bottom, i.e. hydrolysis at the 
bottom of the column has taken place quantitatively. The gaseous 
constituents water vapour, ammonia and carbon dioxide are removed 
advantageously at the top of the column and may be used advantageously to 
restore the aqueous ammonia/carbon dioxide solution for the preparation of 
5-(2-methylmercaptoethyl)-hydantoin. 
According to the invention, an aqueous solution of potassium hydroxide, 
potassium carbonate and/or potassium hydrogen carbonate with a potassium 
ion content of advantageously 100-200 g, preferably 140-160 g of potassium 
per litre of hydrolysis solution is used for the hydrolysis of 
5-(2-methylmercaptoethyl)-hydantoin, the apparatus walls and fittings 
composed of zirconium exerting a favourable catalytic effect on 
hydrolysis, so that this takes place largely without the formation of 
by-products. Advantageously, in a continuous process, the mother liquor is 
used again at this juncture after methionine solids separation; the mother 
liquor may additionally contain residual methionine--according to the 
solubility--and this has also proved to be advantageous for the process. 
The average residence time of the reaction solution in the hydrolysis 
column is advantageously 10-20 minutes. The molar ratio of the potassium 
ion quantity to the sum of 5-(2-methylmercaptoethyl)-hydantoin+methionine 
is favourably up to 10, preferably 1.3-5, particularly 1.5-2. The 
potassium methioninate yield obtained in the reaction solution is 
typically 99.0-100% with this method of operating. The potassium 
methioninate concentration may be adjusted by making an appropriate choice 
of hydantoin concentration or by diluting or concentrating the solution 
obtained after hydrolysis. 
The present invention also includes the liberation of methionine from 
alkali metal methioninate, advantageously from an aqueous solution of the 
kind that may be obtained by the methods of operating above. The invention 
therefore also relates to a process for the preparation of methionine from 
metal methioninate in aqueous solution by liberation with carbon dioxide, 
wherein a defoaming agent is added to the aqueous solution containing 
alkali metal methioninate before liberation of the methionine. The 
invention also relates to a process for the preparation of methionine from 
alkali metal methioninate in aqueous solution by liberation with carbon 
dioxide, wherein the liberation is carried out in a stirred-cell reactor 
with intensive mixing or in a stirred reactor with quasi ideal mixing. 
These two methods of operating are advantageously combined. All compounds 
that have a foam-inhibiting function are suitable as defoaming agents. The 
defoaming agent is introduced into the solution preferably in the form of 
a dispersion. As a result, a particularly good distribution is obtained in 
the solution rather than a concentration essentially on the surface of the 
aqueous solution. The favourable effect of the defoaming agent on the 
liberation of methionine is thereby promoted, particularly the prevention 
of the formation of thin leaves or flakes. Solid, spherical crystals are 
obtained, predominantly with a diameter of 100 to 200 .mu.m. 
The defoaming agent is added advantageously in a concentration of 1,000 to 
10,000 ppm, based on the total methionine (methionine+methioninate, 
converted to methionine). 
When methionine is liberated from the aqueous solution by means of carbon 
dioxide, it is particularly advantageous if the carbon dioxide is fed into 
the aqueous solution by means of a nozzle device in the region of the 
bottom. This in turn promotes the liberation of methionine. Moreover, 
liberation is carried out advantageously at a pressure of 1 to 30 bar, 
preferably also at a temperature of 0.degree. to 100.degree. C. 
An aqueous solution that is essentially free from ammonia is used in 
particular preference. 
The last method of operating is also carried out continuously in particular 
preference. The method of operating described is advantageously combined 
with the method of operating described above for the preparation of an 
alkali metal salt of methionine, the entire combination of the methods of 
operating described above being possible in particular preference. 
The process for the liberation of methionine is described below on the 
basis of the preferred potassium-D,L-methioninate, although other alkali 
metals e.g. sodium are also possible. The other preferred or general 
process conditions given here also apply accordingly to the general method 
of operating described above. 
During the liberation of D,L-methionine from potassium-D,L-methioninate by 
feeding carbon dioxide particularly into a hydrolysis solution of 
5-(2-methylmercaptoethyl)-hydantoin, it is particularly advantageous if 
the solution is practically free from ammonia. In preference, the solution 
also contains dissolved D,L-methionine. Certain quantities of potassium 
carbonate and potassium hydrogen carbonate may also be present. The 
solution may, if desired, be purified over activated carbon prior to the 
addition of carbon dioxide. The addition of carbon dioxide usually takes 
place at a temperature of 0.degree. to 100.degree. C., preferably at 
20.degree. to 35.degree. C. and usually at a pressure of 1 to 30 bar, 
preferably 2 to 5 bar. The carbon dioxide is fed into the reaction mixture 
preferably until a pH of about 7 to 9, preferably 7.5 to 8.5 is reached 
and/or until the precipitation of D,L-methionine has ended. It is 
particularly advantageous if the carbon dioxide is introduced at the 
bottom of the reactor directly or advantageously in the finely divided 
form by means of a nozzle device. The reactor advantageously takes the 
form of a stirred-cell reactor or a quasi ideal stirred reactor. Moreover, 
particularly in a continuous mode of operation, the defoaming agent may 
additionally increase the throughput. The defoaming agent is usually added 
in a quantity of at least 1,000 and advantageously up to 10,000 ppm, 
preferably 3,000 to 5,000 ppm, based on the total methionine present in 
the reaction solution, particularly as an aqueous emulsion. The methionine 
liberated is advantageously separated from the mother liquor and is 
largely free from dust after drying and characterised by good flow 
properties and a high bulk density. The methionine particles have a 
diameter of predominantly 100 to 200 .mu.m. With this method of operating, 
the yield of isolated D,L-methionine is usually 98 to 100%. The mother 
liquor obtained after D,L-methionine separation, particularly filtration, 
may advantageously be used again for the hydrolysis of 
5-(2-methylmercaptoethyl)-hydantoin, optionally after concentration and/or 
discharge of CO.sub.2. 
The invention is explained in more detail below on the basis of Figures and 
Examples.

EXAMPLES 1-4 
Examples of the preparation of 5-(2-methylmercaptoethyl)-hydantoin 
The general process for Examples 1-4 is shown in FIG. 1. In the continuous 
mode of operation, hydrogen cyanide is mixed by means of a mixing nozzle 1 
with 3-methylmercaptopropionaldehyde and a downstream static mixer 2. A 
solution of ammonia and carbon dioxide in water, which components may be 
recycled from the subsequent stages, is prepared in a mixing reactor 3. 
The two mixtures are fed continuously into the reaction mixture in a 
circulation reactor 20. A good temporary mixing of the solutions with the 
recycled reaction mixture takes place in a mixing device 4. The 
circulating mixture is pumped 6 via a heat exchanger 5 with which the 
desired temperature is adjusted. An appropriate proportion of the reaction 
mixture is drawn off continuously from the circuit. This proportion is 
then fed to a secondary reactor 7 to complete the reaction. The product 
mixture thus obtained may then be conveyed, optionally immediately, to the 
next reaction stage. 
Example 1 
A device as in FIG. 1 was used. Initially, water heated to 90.degree. C. 
was circulated; a solution of 10.5 mole of hydrogen cyanide in 10 mole of 
3-methylmercapto-propionaldehyde and 6.8 1 of an aqueous ammonium 
carbonate solution containing 9.6 wt. % of ammonia and 15.2 wt. % of 
carbon dioxide was then fed hourly into the circuit. The circulation in 
the circuit was 300 l per hour. The temperature was kept at 90.degree. C.; 
the pressure was 14 bar. A volume of reaction mixture corresponding to the 
inflow was drawn off constantly from the circuit and fed to the secondary 
reaction. The average residence time was 10 min in the circuit and 2 h in 
the secondary reaction. The yield of 5-(2-methylmercaptoethyl)-hydantoin 
or of compounds saponifiable to methionine, based on 
3-methylmercaptopropionaldehyde used, was 99.8%. (The details are based on 
3-methylmercaptopropionaldehyde, calculated as 100%). 
Example 2 
The procedure was the same as in Example 1 but an ammonium carbonate 
solution saturated at room temperature and containing 9.6 wt. % of ammonia 
and 15.2 wt. % of carbon dioxide was circulated at the beginning instead 
of water. The yield was 99.7%. 
Example 3 
The procedure was the same as in Example 1, but the temperature in the 
circuit and during the secondary reaction was kept at 115.degree. C. The 
pressure was 16 bar. The circulation in the circuit was 150 l per hour. 
The average residence time was 6 min in the circuit and 20 min in the 
secondary reaction. The yield obtained was 99.9%. 
Example 4 
The procedure was the same as in Example 1 but a solution of 10.1 mole of 
hydrogen cyanide in 10.0 mole of 3-methylmercaptopropionaldehyde and 6.8 l 
of an aqueous ammonium carbonate solution containing 5.5 wt. % of ammonia 
and 8.5 wt. % of carbon dioxide was fed in hourly. The temperature in the 
circuit and during the post-reaction was kept at 115.degree. C. The 
pressure was 16 bar. The flow rate in the circuit was 150 l per hour; the 
average residence time was 6 min in the circuit and 40 min in the 
secondary reaction. The yield was 99.8%. 
Examples 5 and 6 
Examples of the preparation of potassium methioninate solution 
The general process for the preparation of potassium methioninate solution 
is reproduced in FIG. 1. 
Example 5 
Comparison example with a pressure apparatus without zirconium fittings. 
Using pump pressure, a solution of 100 kg of potassium hydrogen carbonate 
in aqueous solution and 41 kg of 5-(2-methylmercaptoethyl)-hydantoin in 
400 l of water is fed hourly into a continuously operating pressurised 
column 8 made of special steel (compare FIG. 2) operated with steam. The 
reaction mixture is heated to 180.degree. C. and has an average residence 
time of about 15 minutes at about 8 bar. Ammonia and carbon dioxide 
liberated is drawn off at the top of the reactor column by means of a 
pressure-maintaining valve. The reaction solution is depressurised at the 
bottom of the pressurised reactor and cooled with the heat exchanger 9. A 
quantity of 41.9 kg per hour of potassium methioninate in solution is 
obtained (94.5% of the theoretical). 
Example 6 
An apparatus according to FIG. 2 is used, comprising a pressurised column 
having zirconium fittings. 
Using a pump, 553 kg of 5-(2-methylmercaptoethyl)-hydantoin in 1600 l of 
reaction solution from hydantoin preparation according to FIG. 1 and 3550 
l of a mixture of potassium carbonate/potassium hydrogen carbonate and 
potassium hydroxide in aqueous solution from mother liquor recycling after 
methionine solids separation with a potassium content of 140 g per litre 
and a residual methionine content of 120 g per litre are fed hourly into 
the top of a pressurised hydrolysis column 8 with zirconium fittings. The 
reaction temperature is 165.degree. C.; the reaction pressure is 7 bar. 
The ammonia and carbon dioxide liberated are removed at the top of the 
column by means of a pressure-maintaining valve and fed to 
5-(2-methylmercaptoethyl)-hydantoin synthesis again. 
At the bottom of the pressure apparatus, 5150 l per hour of an aqueous 
mixture with 96.5 g of potassium per litre and 175 g of methionine per 
litre (corresponding to an increase of 476 kg/h methionine in the system 
as a whole) are obtained (yield: 100%). The reaction solution is cooled by 
means of a heat exchanger 9 and fed to methionine liberation. 
Example 7 
Example of the liberation of D,L-methionine from potassium-D,L-methioninate 
(FIG. 3) 
686 l per hour of an aqueous solution with 83.6 kg of potassium 
D,L-methioninate (hydrolysis solution of 
5-(2-methylmercaptoethyl)-hydantoin) with additionally 39.77 kg of 
recycled methionine and potassium compounds are fed in continuously at the 
top of a stirred reactor 10 with a capacity of 340 l. At the same time, 
carbon dioxide is fed in at the bottom of the reactor so that a pressure 
of 2-3 bar is carried in the reactor. Similarly, 0.38 kg per hour of 
defoaming agent are fed into the reactor in the form of an aqueous 
emulsion; this quantity corresponds to approximately 3940 ppm of defoaming 
agent per kilogram of total methionine. The reaction temperature is kept 
at 25.degree. C. In order to keep a constant level in the reactor, 
quantities of reaction solution corresponding to the inflow are removed 
from the lower part of the reactor. The suspension removed is filtered, 
66.5 kg per hour of solid D,L-methionine (calculated as dry substance) 
being obtained, and the mother liquor with a residual content of 39.77 kg 
of D,L-methionine can be recycled to the hydantoin hydrolysis stage as 
saponification agent. The yield is quantitative.