A process for the production of methionine is disclosed in FR 2772026 where methionine amide is hydrolyse in the presence of sodium hydroxide. The resulting product stream comprises the sodium salt of methionine. It is necessary to isolate the methionine. This patent application discloses the use of a cation exchange resin wherein the product stream is contacted with the resin such that the sodium ion is exchanged with the resin, thus liberating the methionine.

We have developed a process for the production of methionine which utilises a titanium catalyst instead of sodium hydroxide and in particular we have found that the process is particularly efficient when a resin is used in association with the titanium catalysed process as the resin eliminates the sodium salt contained in the synthesis amide stream and the catalyst hydrolyses the amide.

Accordingly the present invention provides a process for the production of methionine which comprises (a) a first step of contacting hydroxymethylthiobutyronitrile with ammonia or a solution of ammonia to produce a product comprising 2-amino methylthiobutyronitrile, (b) a second step of removing any excess ammonia or solution of ammonia from the product, (c) a third step of reacting the 2-amino methylthiobutyronitrile with a ketone and an alkali metal hydroxide to produce a product stream comprising methionine amide and an alkali metal salt of methionine, (d) a fourth step of removing any unreacted ketone and excess ammonia from the product stream (e) a fifth step of contacting the product stream of step (d) with an ion exchange resin, to carry out an exchange process of the alkali metal on the resin, and thereby liberating free methionine,

(f) a sixth step of hydrolising the methionine amide in the presence of a catalyst comprising titanium to produce ammonium methioninate, and (g) a seventh step of liberating methionine from the ammonium methioninate salt.

The process of the present invention provides the advantage over the prior art processes in that it the process can be operated at a lower temperature and can treat a greater concentration of substrate. The methionine salt is limited in quantity after the aminoamide synthesis and thus less resin is required and as compared to the process of FR 2772026. Furthermore, as the exchange step is carried out prior to the hydrolysis of the amide, the process can be carried out at a lower temperature and without dilution of the stream.

A further advantage of the present process is that methionine obtained at the end of the process, is obtained in solution without any mineral salt thus the process for recovering solid methionine is very simple compared to the known prior art processes where complicated separation processes must be used.

In the first step of the process of the present invention, hydroxymethylthiobutyronitrile is contacted with ammonia or a solution of ammonium and water, to produce a mixture containing 2-amino methylthiobutyronitrile. The molar amount of ammonia relative to hydroxymethylthiobutyronitrile is suitably from 3 to 10, preferably from 4 to 7.

Where it is desired to use an aqueous solution of ammonia, the solution is suitably at a concentration greater that 25% by weight, preferably greater than 60% by weight. Preferably, the hydroxymethylthiobutyronitrile is contacted with pure ammonia.

This first step of the process is suitably carried out at a temperature of from 40 to 80°C, preferably from 70 to 75°C and under a pressure ou from to 30 bar, preferably from 15 to 25 bar. The reaction may be carried out in a stirred or tubular reactor with, in particular, a plug flow reactor with a calorific exchange system.

At the end of the reaction of the first step it is likely that there exists excess unreacted ammonia. The excess ammonia is removed from the reactor.

This may be implemented by flash depressurisation or by entrainment with an inert gas such as nitrogen. The temperature during this separation step is suitably below 60°C, preferably between 10 and 40°C. The pressure can be atmospheric

pressure or below atmospheric pressure. Preferably a pressure of from 0.1 to 0.5 xl 05 Pa is used. The ammonia recovered from the reaction may then be condensed or recuperated by any other suitable process and mixed with additional ammonia and recycled into the reactor.

The 2-amino methylthiobutyronitrile produced in the first step of the process is then hydrated in the presence of a ketone and a catalytic amount of alkali metal hydroxide to produce methionine amide. The ketone is suitably present in a concentration of from 0.1 to 1, preferably 0.2 to 0.5 equivalent of ketone. The alkali metal hydroxide is suitably present in a catalytic concentration of from 0.05 to 0.5, preferably from 0.1 to 0.25 equivalent of alkali metal hydroxide. Preferably the ketone is acetone. Suitably the alkali metal hydroxide is potassium hydroxide or sodium hydroxide, especially sodium hydroxide.

The hydration reaction is suitably carried out at a temperature of from 10 to 40°C, preferably from 25 to 35°C. Suitably the reaction is carried out under atmospheric pressure. The reaction may be carried out in a stirred or in a tubular reactor or in a column packed with suitable packing material with a calorific exchange system.

By-products to this reaction include the alkali metal salt of methionine, residue aminomethylthiobutyronitrile, imidazolidinone (2,2-dimethyl-5 (2- (methyl thio) ethyl)-4-imidazolidinone), aqueous ammonia, unreacted ketone and the alkali metal hydroxide.

The unreacted ketone and the aqueous ammonia in the product stream are then separated from the other components. To facilitate this separation step, the product stream may be distilled or stripped or by any other suitable separation technique. The ketone and the ammonia may be recycled back to the reactor.

The product stream devoid of the ketone and ammonia is then contacted with a resin wherein the alkali metal of the alkali metal methioninate salt is retained on the ion-exchange resin, thereby providing a solution containing methionine, free of alkali metal ions. Suitable resins are sulphonic resins.

Commercially available resins sold under the trade names Rohm & Haas IMAC C16P and Fluka Amberlist 15 may be used. Also suitable, are carboxylic acid resins wherein the pKa of the acid is less than 6.2. Suitable resins are resins such as those sold under the trade name Fluka Duolite C464 or Rohm & Haas IRC50.

It is preferred to use a carboxylic acid resin.

Suitably, the stream comprising the alkali metal methioninate salt is passed continuously over the resin. When the resin is saturated with the alkali metal ion, the resin is suitably regenerated by displacing the metal ions. The metal ions may be displaced by treatment in acidic medium for example with a strong inorganic acid, such as sulphuric acid or hydrochloric acid. Molar amounts of inorganic acid corresponding to 2 to 14 mol, preferably 3 to 6 mol of acid per kg of resin may be used. The carboxylic acid resin may alternatively be regenerated by treating the resin with carbon dioxide in an aqueous medium under pressure of typically 10 to 25 bar. The regeneration is suitably carried out with a molar amount of acid corresponding to 2 to 14, preferably from 3 to 6 mol acid per kg of resin. The eluate of the resin containing the alkali metal sulphate or chloride formed is itself free of methionine. The inorganic salt may then be easily crystallised and separated.

The next step in the process of the present invention is the hydrolysis of the methionine amide to produce ammonium methioninate. The stream comprising the amide is, of course, now substantially devoid of alkali metal salt.

The hydrolysis step is catalysed using a titanium based catalyst, for example TiO2.

A mixture of titanium and at least one other metal may be also used, for example Ti-W, Ti-Mo, Ti-Si-W, Ti-Nb, Ti-Nb-W, Ti-Nb-Mo, Ti-Zr, Ti-Al, Ti-Cr, Ti-Zn and Ti-V. Preferably, the catalyst is TiO2 The catalyst may be used in the powdered form, suitably in a concentration of from 0.1 to 2g of catalyst per gram of aminoamide, preferably from 0.5 to 1. 5g of catalyst per gram of aminoamide. Alternatively, the catalyst may be used in the form of pellets. Preferably, the catalyst is in the form of pellets and used in a continuous process.

The catalysed hydrolysis of the methionine amide is suitably carried out at a temperature of from 50 to 150°C, preferably from 80 to 130°C, and under a pressure of from atmospheric pressure to 10 bar, preferably from 1 to 5 bar.

Water may be added to the process at any appropriate stage during the process. Suitably, water is added before or after step (e), namely before or after contact with the resin, or before hydrolysis of the aminoamide, namely before step (f), or before removal of ammonia, namely before step (g). Preferably, water is introduced into the reaction stream immediately before hydrolysis of the aminoamide. Where water is added, it is added in an amount so as to have a molar

amount of free methionine after step (g) from 0.5 to 1.5, preferably from 0.7 to 1 mol/kg of free methionine.

The components of the product stream from the hydrolysis step are then separated by any suitable separation technique, for example a stripping process.

The liberated ammonia is withdrawn, leaving an aqueous solution comprising free methionine and minor amounts of unreacted aminoamide and imidazolinone. This solution of course does not contain ammonia or alkali metal salt.

The final resulting product stream comprising free methionine in the liquid form may be used as is or optionally it may be further treated to recover solid methionine. This may be achieved by separating the methionine using any suitable separation method, for example by simple crystallisation after concentration or by atomisation after partial concentration, crystallisation and grinding, or by granulation after concentration.

The present invention will now be illustrated with reference to the following examples: Synthesis of Methionine The overall reaction can be represented by the scheme shown in Figure 1 wherein the compositions of the streams at each stage of the reaction are given in Table 1.

2-hydroxymethylthiobutyronotrile is reacted with ammonia in reactor (A) to provide a mixture comprising 2-aminomethylthiobutyronotrile (composition 1).

The excess ammonia is separated from the product stream and passed to a recovery vessel (B) for recycling back to the reactor (A) after further treatment in the recovery block. The treated stream (composition 2) is passed to tank (C).

Acetone, water and sodium hydroxide are fed into tank C and the resulting mixture passed to reactor (D). The resulting product stream comprising methionine amide (composition 3) is distilled to separate the unreacted acetone and ammonia. Water is added to the resulting amide solution (composition 4) and the solution (composition 5) is then continuously contacted with the resin.

Additional water added to the treated product which does not comprise sodium salts and the resulting stream (composition 6) are contacted with the titanium catalyst in reactor (E). The product stream comprising ammonium methioninate (composition 7) is treated to liberate ammonia and isolate the free methionine by

stripping in an ammonium stripper (F). The liquid free methionine (composition 8) may be treated further to obtain solid methionine.

Table 1 N° 1 N°2 NO3 N°4 N°5 N°6 N°7 N°8 Aminonitrile Aminonitrile Amide Amide Amide Amide Resulting resulting solution solution after solution solution after solution after solution after solution after solution after before ammonia before removing water contacting contacting separating ammonia separation removing acetone and dilution with resine with Ti02 ammonia separation acetone NH3 and NH3 and water dilution % w/w Hydroxy 0 0 0 0 0 0 0 0 nitrile Amino 50.70 67.10 0 0 0 0 0 0 nitrile Amide 0 0 26.10 30,90 9,90 9,60 0,0 0,0 44 MTN Na 0 0 5.97 7.36 2,36 0 0 0 MTN-NH4 0 0 0 0 0 0 10,72 0 free MTN 0 0 0 0 0 2,06 2,06 12,54 NH3 27. 70 7. 10 7.20 0,15 0,05 0 0 0 acetone 0 0 4.00 0,01 0,00 0 0,03 0 IDZ 0 0 1. 20 1,30 0,42 0,42 0,32 0,26 water 21. 60 25.80 55.53 60.28 87.27 87,92 86.83 87,16 T°C 70 20 30 100 50 50 100 100 pressure 10 1 1 1 1 1 3 1 (bars)