WO2008006977A1

WO2008006977A1 - Process for preparing 2-hydroxy-4-(methylthio)butyronitrile and methionine

Info

Publication number: WO2008006977A1
Application number: PCT/FR2007/001189
Authority: WO
Inventors: Claude Casse
Original assignee: Adisseo France S.A.S.
Priority date: 2006-07-11
Filing date: 2007-07-11
Publication date: 2008-01-17
Also published as: TW200811086A

Abstract

The present invention relates to a process for preparing an aqueous solution of HMTBN, from acrolein, without isolating intermediate products, characterized in that: - in a first step, a reaction (1) is carried out between acrolein (2) and hydrocyanic acid (4) in an HCN/acrolein molar proportion of between 1 and 10, - in a second step, methylmercaptan (MSH) (63) is added in an MSH/acrolein molar proportion of between 1 and 10, said process being carried out at a pH of between 3 and 9. The present invention also relates to a process for preparing methionine from acrolein or from HMTBN.

Description

Process for preparing 2-hydroxy-4- (methylthio) butyronitrile and methionine

The field of the present invention is that of the preparation, from acrolein, of 2-hydroxy-4- (methylthio) butyronitrile (hereinafter called

HMTBN) as an intermediate product or final product. The field of the present invention is also that of the preparation, from HMTBN, of 2-amino-4- (methylthio) butanoic acid (hereinafter referred to as methionine). Finally, the field of the present invention is that of the preparation of methionine from acrolein.

The formulas of these compounds are as follows:

^^^ ⁰ Acrolein

HMTBN

methionine

Generally, the synthesis of HMTBN implements, in a first step, the synthesis of aldehyde-3- (methylthio) propionic acid (or AMTP), by reaction of acrolein with methylmercaptan. In a second step, the AMTP thus obtained is brought into contact with hydrocyanic acid and HMTBN is thus obtained. L ¹ AMTP is an evolutionary product according to the virtues of aldehydes, which requires suitable storage conditions. It is indeed a very unstable product which, in contact with the oxygen of the air, degrades very quickly. In addition, it must generally be kept cold, which has drawbacks during transport.

An object of the present invention is to find an alternative to the synthesis of HMTBN via AMTP. Another object of the present invention is to avoid the storage of intermediate products unstable or likely to present safety problems by their toxicity or flammability.

After synthesis of HMTBN _1, various reaction steps are generally required to access methionine. In particular, the process for the preparation of methionine requires the synthesis of the hydantoin of methionine. The preparation of methionine involves the saponification of the hydantoin of methionine, that is to say its hydrolysis with NaOH or KOH, which has the disadvantage of co-producing sodium salts (Na ₂ Sθ ₄ for example) in large quantities and hardly appreciated.

In addition, it is known that the ammoniacal hydrolysis of methionine hydantoin generates carbon dioxide, which must be removed from the process or recycled. An object of the present invention is to obtain methionine from acrolein or HMTBN with little or no salt co-production. By "little" is meant less than 0.5 equivalent of salt per equivalent of methionine formed.

Another object of the present invention is to require no removal of reaction by-products.

To date, no method for passing directly from acrolein to methionine, without isolating intermediates, has been described.

An object of the present invention is to provide a process for preparing methionine from acrolein without isolating intermediates.

Still another object of the present invention is to provide a process having a high yield at each reaction step relative to the starting acrolein or HMTBN.

The method of the present invention thus has many advantages. Other advantages will become apparent over the following description. The present invention firstly proposes a new process for the preparation of HMTBN, said process passing through an intermediate different from that of the prior art, namely 2-hydroxy-3-butenenitrile (hereinafter cyanohydrin of acrolein). , more stable. The process for preparing an aqueous solution of HMTBN from acrolein, without isolating intermediate products, is characterized in that:

in a first step, (1) is reacted with acrolein (2) with hydrocyanic acid (4) in a molar proportion of HCN / acrolein of between 1 and 10,

in a second step, methyl mercaptan (MSH) (63) is added in an MSH / acrolein molar proportion of between 1 and 10, said process taking place at a pH of between 3 and 9.

Thus, the synthesis of HMTBN from acrolein involves two distinct reaction steps. Said two reaction steps are represented below:

acrolein

Cyrohydrin of acrolein

Acrolein HMTBN Cyanohydrin

A number of documents of the prior art propose the synthesis of cyanohydrin acrolein product of 1 ^st step of the present invention. For information, there may be mentioned US 3,850,976 and FR 1350 752. Also, a number of documents of the prior art relating to the ^2nd stage, for example JP19612 / 71. Nevertheless, none of the documents of the prior art describes the synthesis of HMTBN, from acroiéine, without isolating intermediate products.

The present invention also provides a new methionine preparation process from I ¹ HMTBN. The process, whose starting point is ¹ HMTBN _1, is characterized in that, without isolating intermediate products, NH ₃ and CO 2 are added to the aqueous HMTBN solution, whereby a mixture comprising at least methionine, carbon dioxide and dissolved ammonia.

The process for the continuous preparation of methionine, from HMTBN, according to which ammoniacal hydrolysis (1) of an aqueous solution (2) comprising said HMTBN is carried out, is characterized in that:

(a) the ammoniacal hydrolysis (1) of the HMTBN is carried out continuously in the aqueous phase and without isolation of intermediate products, at a temperature not exceeding 25 ^° C., starting from a molar proportion of NH 3 / HMTBN; between 5 and 50, and a molar ratio CO ₂ / HMTBN between 1 and 10, to obtain an exit mixture (5) comprising at least methionine, carbon dioxide and dissolved ammonia, and derivatives of I ¹ HMTBN; and

(b) recycling said derivatives I ¹ HMTBN, carbon dioxide and ammonia to hydrolysis (a).

The present invention finally proposes a new process for the preparation of methionine. The process, whose starting point is acrolein, is characterized in that, without isolating intermediate products: in a first step, acrolein is reacted with hydrocyanic acid,

in a second step, methylmercaptan (MSH) is added to the reaction mixture of the first step, said two stages being carried out at a pH of between 3 and 9, whereby an aqueous solution of HMTBN is obtained,

in a third step, NH ₃ and CO ₂ are added to the aqueous HMTBN solution, whereby a mixture comprising at least methionine, carbon dioxide and dissolved ammonia is obtained. By "without isolating intermediates" is meant that at no time during the process for preparing HMTBN or methionine is an intermediate product isolated. Indeed, as described above, the processes for preparing HMTBN and methionine from acrolein involve two common reaction steps. One of these reaction stages in particular leads to the cyanohydrin of acrolein. According to the invention and by way of example, this intermediate product is not isolated, that is to say that it is not extracted / withdrawn. In other words, the hydrolysis reaction is carried out in "one-pot". According to the present invention, the process is carried out without intermediate separation or fractionation.

The realization of the processes according to the invention in "one-pot" offers a particular advantage when the processes are conducted at the industrial level.

Process for preparing HMTBN

The process for preparing the HMTBN according to the invention is carried out by two successive reactions, the first involving the addition of hydrocyanic acid (HCN) to the carbonyl group of acrolein to give the cyanohydrin of the acrolein and the second involving the addition of methyl mercaptan (hereinafter referred to as MSH) to the cyanohydrin of acrolein. These two reaction steps take place at a pH of between 3 and 9.

According to one embodiment of the present invention, in the first step, acrolein is reacted with hydrocyanic acid in an HCN / acrolein molar proportion of between 1 and 10, preferably between 1 and 3.

Also, according to another embodiment of the present invention, in the second step, the MSH is added in an MSH / acrolein molar proportion of between 1 and 10, preferably between 1 and 3. The reaction temperature of these two stages is between 10 and 12O ⁰ C. Advantageously, the reaction temperature is between 60 and 100 ^° C.

One of the important characteristics of the first two steps of the process for preparing the HMTBN according to the invention is the control of the pH at which these reactions take place. It is essential that the reactions of the invention take place at a pH of between 3 and 9, preferably between 4 and 8. The person skilled in the art knows how to adjust the pH of the reaction medium. As an indication, it can use buffer solutions. By "buffer solution" is meant according to the invention any solution consisting of a weak acid and a strong base or a strong acid and a weak base, having a pH of between 3 and 9. indicative, these include citric, carbonic and phosphoric buffers.

According to a particularly advantageous embodiment of the present invention, a buffer solution is used for the entire process. The use of a single buffer simplifies the implementation of the method. Indeed, it is sufficient to modify the pH of the solution to obtain the expected product. There is no need to change the buffer type or to perform an intermediate purification.

According to other embodiments of the present invention, the first step takes place / takes place at a pH between 4 and 6 and / or the second step takes place / takes place at a pH between 6 and 8.

Process for the preparation of methionine from HMTBN

The process according to the invention for the preparation of methionine from HMTBN comprises a single step in which NH ₃ and CO ₂ are added to HMTBN, thus resulting in a mixture comprising methionine.

The mixture obtained at the end of this process contains mainly methionine. The mixture also includes carbon dioxide and dissolved ammonia. According to one embodiment of the invention, the carbon dioxide and ammonia of the resulting output mixture are recycled to the beginning of the process in order to be able to use them again in said process.

The reaction temperature during the process according to the invention does not exceed 250 ^° C. In addition, according to one embodiment of the present invention, the procedure is carried out without a catalyst, for example without a solid catalyst, for example without a solid catalyst of the oxide type. metallic.

By "catalyst" is meant any compound not consumed in the hydrolysis reaction and found completely when the reaction is complete. By definition, there is no stoichiometric ratio between the catalyst and the reactants. According to one embodiment of the present In the invention, the process proceeds in the absence of catalyst, which has considerable advantages, industrially speaking.

The process for preparing methionine from HMTBN involves various reaction steps. These different reaction steps consume and produce one mole of NH ₃ and one mole of CO ₂ per mole of methionine produced. By proceeding so that the NH ₃ and CO ₂ products are recycled, in order to be reused in said process, an equilibrium is created between the moles of NH ₃ and CO ₂ produced and those consumed, so that these compounds, although not appearing in the overall assessment of the reaction, can not be considered as catalysts.

The natural recycling of the method for preparing methionine from the HMTBN according to the invention is advantageous.

Recycling can also be carried out by additional treatment, chemical or enzymatic. By way of indication, the recycling can be carried out chemically by homogeneous catalysis, for example by hydrolysis with ammonia or chemically by heterogeneous catalysis. The chemical treatment can be a basic treatment or a lime treatment. Examples of useful bases according to the invention are NH ₃ , NaOH, KOH, EtONa, MeONa and K ₂ CO ₃ . The chemical treatment may also be a retrosynthesis catalyst treatment. Of the catalysts, solid catalysts, for example TiO ₂ or ZrO _{2, will} be preferred.

The recycling can also be carried out enzymatically, for example via an amidase. All the hydrolysis routes of the methionine aminoamide mentioned in WO 00/27809 are applicable to this recycling.

According to one embodiment of the present invention, at the end of the process, the outlet mixture is expanded, then the expanded outlet mixture is separated into a liquid stream enriched with methionine, and a gaseous stream enriched with ammonia and carbon dioxide. .

In another embodiment of the invention, the liquid stream enriched in methionine, in evolved solid methionine and in methionine-depleted mother liquors optionally comprising the derived products is separated, said mother liquors being recycled, after raising their pressure, to the beginning of the process. In another embodiment, the mother liquors are recycled after chemical or enzymatic treatment.

In another embodiment, the mother liquors are recycled to the start of the process in a natural manner, without chemical or enzymatic treatment.

According to another embodiment, the liquid stream enriched in methionine is separated by stepped crystallization to obtain solid methionine and the mother liquors.

According to another embodiment, after condensation, the enriched stream of ammonia and carbon dioxide is stored and recycled to the beginning of the process, after raising of its pressure.

According to one embodiment of the present invention, the hydrolysis, according to the method, is carried out at a temperature ranging between 20 and 220 ^° C. According to another embodiment, the hydrolysis, according to the method, is carried out at a pressure of between 5 and 100 bar.

According to one embodiment of the present invention, it is carried out in a tubular reactor or in a plurality of stirred reactors arranged in series.

Process for the preparation of methionine from acrolein

The process for preparing methionine from acrolein according to the invention is carried out by three successive reactions, the first involving the addition of hydrocyanic acid (HCN) to the carbonyl group of acrolein to give the acrolein cyanohydrin, the latter involving the addition of methyl mercaptan (hereinafter referred to as MSH) to acrolein cyanohydrin, these first two reaction stages taking place at a pH of between 3 and 9, and the third involving the addition of NH ₃ and CO ₂ on HMTBN. According to one embodiment of the present invention, in the first step, acrolein is reacted with hydrocyanic acid in a molar proportion of HCN / acrolein of between 1 and 10.

According to another embodiment of the present invention, in the second step, the MSH is added in an MSH / acrolein molar proportion of between 1 and 10. According to another embodiment of the present invention, in the third step, NH ₃ and CO ₂ are added to the aqueous HMTBN solution starting from an NH ₄ / acrolein molar proportion of between 5 and 50, and from a molar proportion Cθ ₂ / acrolein between 1 and 20. The reaction temperature of the first two stages is between 10 and 120 ^° C. and / or that of the third step does not exceed 250 ^° C.

In addition, according to one embodiment of the present invention, it proceeds without a catalyst. According to one embodiment of the invention, the carbon dioxide and ammonia of the resulting output mixture are recycled to the third stage in order to be able to use them again in said process.

According to an embodiment of the present invention, after the third step, the output mixture is expanded, then the expanded output mixture is separated into a liquid stream enriched with methionine, and a gaseous stream enriched with ammonia and carbon dioxide.

In another embodiment of the invention, the liquid stream enriched in methionine, in evolved solid methionine and in methionine-depleted mother liquors optionally comprising the derived products is separated off, said mother liquors being recycled, after raising their pressure, to the third step.

In another embodiment, the mother liquors are recycled after chemical or enzymatic treatment.

In another embodiment, the mother liquors are recycled to the third step in a natural manner, without chemical or enzymatic treatment.

According to another embodiment, the liquid stream enriched in methionine is separated by stepped crystallization to obtain solid methionine and the mother liquors. According to another embodiment, after condensation, the enriched stream of ammonia and carbon dioxide is stored and recycled to the beginning of the process, after raising of its pressure.

According to one embodiment of the present invention, the hydrolysis, according to the third step of the process, is carried out at a temperature ranging between 20 and 220 ° C. According to another embodiment, the hydrolysis, according to the third step of the process, is carried out at a pressure of between 5 and 100 bars.

According to one embodiment of the present invention, the third stage of the process is carried out in a tubular reactor or in a plurality of stirred reactors arranged in series.

According to the invention, the starting acrolein, both in terms of I ¹ of HMTBN preparation process for which is the methionine preparation process, may be in pure form or be obtained by a conventional method which generally comprises a step of oxidation of propylene and / or propane.

When acrolein is prepared by partial oxidation of propylene, a crude product based on acrolein is obtained. This crude product is in the form of a gaseous mixture comprising acrolein, inert gases, propylene, propane, water, reaction by-products such as acids, aldehydes, alcohols and other compounds.

According to one embodiment of the present invention, acrolein comes from an oxidation of propylene by air in the presence of water. This reaction is gentle, which means that some of the propylene does not react and stays on the side of the synthesized acrolein. It would be largely advantageous to be able to recycle this unreacted starting material.

The method of preparing HMTBN of the prior art prevents the recycling of residual propylene since it has been in contact with sulfur products during the process. An advantage of the present invention is to allow the recycling of propylene as unreacted starting material in the synthesis of acrolein.

According to one embodiment of the present invention, the propylene is recycled to the acrolein synthesis reactor, once the acrolein has been in contact with the hydrocyanic acid during the first step of the process. of preparing HMTBN or methionine from acrolein according to the present invention. According to one embodiment of the present invention, the propylene is recycled after reactive absorption of acrolein and hydrocyanic acid to oxidation.

According to one embodiment of the present invention, after oxidation of propylene by air in the presence of water and before reactive absorption, the acids contained in the product obtained after washing oxidation are removed.

As for acrolein as starting material, hydrocyanic acid can be used in pure form or obtained by an ammoxidation process, that is to say a synthesis from methane and methanol. ammonia in the presence of air and a solid catalyst. In this case, subsequent treatments of the hydrocyanic acid which comes from the synthesis reactor are possible. According to one embodiment of the present invention, the HCN comes from a synthesis from methane and ammonia in the presence of air and a solid catalyst.

According to one embodiment of the present invention, washing is carried out after synthesis of the HCN and before reactive absorption.

Similarly, methyl mercaptan can be used in the process of the present invention in pure or crude form. It can also come from a synthesis, in the presence of methanol, hfeS and a solid catalyst.

The reactors used in the process of the present invention are especially tubular reactors, agitated reactors or "batch".

HMTBN is also an intermediate in the synthesis of 2-hydroxy-4- (methylthio) butanoic acid (HMTBA).

The present invention thus further relates to a method for preparing HMTBA, comprising the method of preparing HMTBN as defined above and a step of obtaining HMTBA. Figures 1 to 14 show, without limitation, different devices for preparing a solution of HMTBN or methionine preparation from HMTBN or acrolein.

Figures 1, 2 and 3 show, without limitation, three devices for preparing a solution of HMTBN from acrolein according to the present invention.

Figures 4 to 6 specify certain parts of the device for preparing the HMTBN of Figure 3. Figures 7 and 8 schematically illustrate two devices for preparing methionine from HMTBN according to the invention.

Figures 9 and 10 schematically illustrate other embodiments of the device for preparing methionine from the HMTBN according to the invention. Figures 11 to 14 schematically illustrate four devices for preparing methionine from acrolein according to the invention.

FIG. 1 represents a device for preparing an HMTBN solution comprising two tabular reactors 1 and 60. The first step of the process, that is to say the synthesis of acrolein cyanohydrin from acrolein and HCN takes place in the tubular reactor 1, while the second step of the process of the invention, namely the addition of MSH on the cyanohydrin of the acrolein obtained in the first step, takes place in the reactor 60 .

Although FIG. 1 represents two tubular reactors, it is quite possible according to the present invention for the process of the invention to take place in a stirred reactor or in a "batch" reactor.

The tubular reactor 1 is supplied with 2 with acrolein and 3 with hydrogen cyanide. The molar proportion HCN / acrolein is between 1 and 10, advantageously between 1 and 3.

There is also provided, at 4, a feed for pH regulation, which as it appears above, is an essential feature of the present invention. The pH regulating solution, for example a buffer solution, is introduced after mixing the other streams, 3 and 2. Hydrocyanic acid is in the form of an aqueous solution of the order of 20 to 80% by weight. This solution can be prepared outside the device, but it is also possible to provide an introduction of water in line. At the outlet of reactor 1, cyanohydrin is recovered from acrolein

61, which is then sent to the tubular reactor 60.

The tubular reactor 60 is further supplied with MSH 63. There is also provided, at 64, a power supply for the regulation of the pH. An aqueous solution of HMTBN is recovered at 62.

FIG. 2 represents a device for preparing a solution of HMTBN comprising two tubular reactors 1 and 60. The first step of the process, that is to say the synthesis of the cyanohydrin of acrolein from acrolein and HCN takes place in the tubular reactor 1, while the second step of the process of the invention, namely the addition of MSH on the cyanohydrin of the acrolein obtained in the first step, takes place in the reactor 60.

There is also provided, at 4, a feed for pH regulation, which as it appears above, is an essential feature of the present invention. The pH regulating solution, for example a buffer solution, is introduced after mixing the other streams, 3 and 2.

Hydrocyanic acid is in the form of an aqueous solution of the order of 20 to 80% by weight. This solution can be prepared outside the device, but it is also possible to provide an introduction of water in line.

After the reactor 1, there is a column 15 which allows the recycling of the HCN gas phase. Such a column also makes it possible to adjust the amount of HCN introduced into the reactor 1.

At the outlet of column 15, cyanohydrin is recovered from acrolein 61, which is then sent to tubular reactor 60. The tubular reactor 60 is further supplied with MSH 63. There is also provided, at 64, a power supply for the regulation of the pH.

After the reactor 60, there is a column 65 which allows the recycling of the MSH gas phase. Such a column also makes it possible to adjust the amount of MSH introduced into the reactor 60.

An aqueous solution of HMTBN is recovered at 62.

The process for preparing HMTBN is now described with reference to Figure 3 and Figures 4, 5 and 6.

The process implements a first reaction step, in this case according to FIG. 3, a first reactive absorption 1. Absorption 1 is fed with a gas stream 2 comprising acrolein. It is further supplied with gas stream 3, comprising hydrogen cyanide. A stream of reaction medium circulates on the absorption column

1. The pH regulation may in particular be made via a feed 4 to this reaction medium stream.

As detailed in FIG. 4, the gas stream 2 comprising acrolein comes from an oxidation 20, followed by a washing 25.

The oxidation reactor 20 is fed with propylene 21, air 22 and water 23. It is a propylene oxidation reactor in the vapor phase.

A crude product 24 is produced at the reactor outlet. This crude product 24 is constituted by a gaseous mixture comprising a high proportion by weight of gas which is difficult to condensate (propane, nitrogen, oxygen, propylene, CO, CO ₂ ), water, acrolein, acrylic acid. and other compounds.

Thus, it is possible to provide, according to one embodiment, of the present invention, after oxidation of propylene 21 by air 22 in the presence of water 23 and before reactive absorption 1, to eliminate the acids contained in the product obtained after washing oxidation 25.

This crude product 24 can then be washed 25 to eliminate the acids. The crude product in the form of gas 24 is introduced into a washing column 25, in which circulates a stream of water 26 and 27. The water 26 entrains the acids. The liquid containing the acids 27 is withdrawn in The purified gas 2 exits at the top of the column 25 and is directed towards the reactive absorption 1.

As detailed in FIG. 5, the absorption 1 is further supplied with a gas stream 3, comprising hydrocyanic acid.

This gas stream 3 comes from a synthesis reactor 30, followed by a washing 35.

The synthesis reactor 30 is fed with NH ₃ 31, methane 32 and air 33. It is an ammoxidation of methane with ammonia, for example in the presence of air and solid catalyst. The reaction is very fast. It takes place at a very high temperature.

A crude product 34, rich in HCN, is produced at the reactor outlet 30.

After cooling, this crude product 34 obtained at the reactor outlet 30 then undergoes a washing before reactive absorption 1. The unreacted ammonia is removed by an aqueous solution of sulfuric acid introduced at the top of the column, and which flows countercurrently in the washing column 35. Ammonium sulfate is withdrawn in the lower part 37 of the column 35.

As is detailed in FIG. 6, the gaseous effluents obtained at the top of the absorption device 1, in which the first step of the synthesis of the HMTBN according to the invention takes place, can be partially recycled in a column of washing 50, in which circulates a current of reaction medium 51. The device 52 separates propylene from other lighter compounds (CO, CO ₂ , nitrogen, H ₂ and HCN). It may be, for example an absorption device in water or a compression and cooling device. The hydrocyanic acid is removed by washing with water (absorption) and the propylene is recovered by condensation and distillation either by absorption in an organic solvent which separates it from incondensables (CO ₂ , CO, Nitrogen, etc.).

The excess propylene is unstained (because it has not been contacted with a sulfur product) and can be recycled, unlike the prior art process which involves bringing acrolein into contact with methyl mercaptan. As previously stated, this is a technical advantage of the present invention. The propylene, thus recycled is then directed to the feed pipe 21 which feeds the oxidation reactor 20 which produces acrolein, via a pipe 53.

Thus, according to one embodiment of the present invention, the propylene is recycled after reactive absorption 1, to the oxidation 20.

The base of the wash column 50 is directed to a distillation column 55 through a conduit 54. The ^one recycled HCN, is withdrawn from the top of the distillation column 55 and is directed to the absorption column 1 by the line 56.

According to Figure 3, the foot of the absorption column 1, which contains the cyanohydrin of acrolein is directed to a second absorption column 60, through the supply line 61. Before introduction into the second column, a device for measuring the pH 65 can be provided. This device makes it possible to adjust the value of the pH in the absorption column 1.

If necessary, it is also possible to provide a storage 66 of the column stand 1, before introduction into the column 60, that is to say before the beginning of the second step.

The absorption column 60 in which the second step of the process takes place is supplied with gas stream 63, containing methyl mercaptan, in liquid form or purified gas.

Similarly to column 1, the reaction medium circulates on the absorption column 60. The pH regulation can in particular be made via a feed 64 on the reaction medium stream of the absorption column.

At the bottom of the absorption column 60, an aqueous solution of HMTBN 62 is withdrawn. This solution can be stored.

Figure 7 shows a device for preparing methionine from an aqueous solution 101, industrial, comprising HMTBN.

The process uses hydrolysis 70 continuously, in aqueous phase. This hydrolysis takes place without isolation or separation of intermediates, as defined above. The feed of the reactor 70 in aqueous solution 101 is carried out via a feed line.

Before introduction into the reactor 70, is added to the aqueous solution 101, the pressure of which has previously been raised by a pump 72, comprising I ¹ HMTBN an aqueous phase 77 containing ammonia and CO ₂ , via the conduit of food.

The aqueous solution of HMTBN has a concentration which is preferably greater than 30%. For example, one can use a solution of HMTBN of concentration greater than or equal to 60%. The solution further comprises water and related byproducts, mainly from the synthesis of HMTBN, for example hydrogen cyanide and sulfuric acid.

According to the invention, this synthesis takes place from an aqueous solution of HMTBN, which has many industrial advantages. In particular, the use of an aqueous HMTBN solution avoids having to use a stripping column (or steam distillation column) in order to recover the solvent. In addition, this has advantages in terms of cost and handling.

According to the present invention, while the temperature is set, the pressure is imposed to have a liquid phase. The temperature is raised in the reactor 70, at a temperature ranging between the temperature of the mixture (ie between 20 ^° C. and 90 ^° C.) and 25 ^° C. According to a preferred embodiment of the invention, the synthesis is carried out at a temperature between 20 and

220 ⁰ C. When operating in aqueous phase, the pressure should be between 5 and 100 bar (preferably about 20 bar).

In the reaction, the temperature in the reactor increases the temperature of the mixture (i.e. between 20 ° C and 90 ° C) to a high plateau.

The molar proportions HMTBN / NH ₃ are between 1/5 and

1/50 for example of the order of 1/20 at the inlet of the reactor 700, and therefore with a very large molar excess of ammonia relative to the HMTBN. The molar proportions HMTBN / CO ₂ are between 1 and 1/20, for example of the order of 1/10 or 1/3 at the inlet of the reactor 70.

An aqueous solution comprising principally methionine, CO ₂ and dissolved NH ₃ is recovered at 73. Figure 8 shows a device for preparing methionine from an aqueous solution 101, industrial, comprising HMTBN.

The feed of the reactor 70 in aqueous solution 101 is carried out via a feed line. Before introduction into the reactor 70, is added to the aqueous solution 101, the pressure of which has previously been raised by a pump 72, comprising HMTBN an aqueous phase 77 containing ammonia and CO ₂ , via the conduit of food.

The aqueous solution of HMTBN has a concentration which is preferably greater than 30%. For example, one can use a solution of HMTBN of concentration greater than or equal to 60%. The solution further comprises water and related byproducts, mainly from the synthesis of HMTBN, for example hydrogen cyanide and sulfuric acid. According to the invention, this synthesis takes place from an aqueous solution of HMTBN, which has many industrial advantages. In particular, the use of an aqueous HMTBN solution avoids having to use a stripping column (or steam distillation column) in order to recover the solvent. In addition, this has advantages in terms of cost and handling.

According to the present invention, while the temperature is set, the pressure is imposed to have a liquid phase. The temperature is raised in the reactor 70, at a temperature ranging between the temperature of the mixture (ie between 20 ^° C. and 90 ^° C.) and 25 ^° C. According to a preferred embodiment of the invention, the synthesis is carried out at a temperature of between 20 and 220 ^° C. For operation in aqueous phase, the pressure must be between 5 and 100 bar (preferably about 20 bar).

During the reaction, the temperature in the reactor increases the temperature of the mixture (ie between 20 ⁰ C and 90 ⁰ C) to a high plateau. The molar proportions HMTBN / NH ₃ are between 1/5 and

1/50 for example of the order of 1/20 at the inlet of the reactor 700, and therefore with a very large molar excess of ammonia relative to the HMTBN. The molar proportions HMTBN / CO ₂ are between 1 and 1/20, for example of the order of 1/10 or 1/3 at the inlet of the reactor 70. An output mixture 73 comprising principally methionine, CO ₂ and dissolved NH ₃ is obtained at the outlet of reactor 70 by hydrolysis of HMTBN.

This mixture is expanded, for example at atmospheric pressure, by an expansion valve 74 before being introduced into a "stripping" column 80 (also called a steam distillation column).

In column 80, the expanded mixture introduced is separated into a liquid stream 82 enriched in methionine and a gaseous stream 81 enriched in NH ₃ and CO ₂ . At the column head 80, the gaseous stream enriched in NH ₃ and CO ₂ and which also comprises water, is condensed into a stream 83. The stream 83 is then stored at 84. The storage 84 optionally comprises a gas purge 85 .

According to the present invention, "ammonia" means a phase containing substantially NH ₃ ; while "ammonia" means a phase containing substantially NH ₄ OH.

Similarly, when a "CO ₂ -containing gas phase" is used, it is meant a phase containing substantially CO ₂ in gaseous form; while when "aqueous phase containing CO ₂ " is used, it is meant a phase containing substantially

H ₂ CO ₃ .

The term "substantially" means that the phase in question comprises an excess of the compound in question greater than 50%, more particularly greater than 70%.

The aqueous phase 77 containing ammonia and CO ₂ is recycled to the hydrolysis reactor 70, via a feed line 77, and after lifting 76 of its pressure. It is also possible according to the present invention to provide a supply line 75, which feeds the pipe 77 with ammonia, CO ₂ and / or water, before raising the pressure. Such a conduct makes it possible to adjust the titer to ammonia, CO ₂ and / or water if necessary.

At the bottom of the stripping column 80, via the withdrawal line 82, a liquid stream enriched with methionine, ie a aqueous solution of methionine, "acidified", that is to say whose pH has been lowered.

This stream is cooled in a heat exchanger 86, which makes it possible to precipitate methionine. But any other means known to those skilled in the art, for example evaporation, is applicable in this case.

The liquor thus obtained is then sent to a separator 90, which makes it possible to separate the methionine in solid form, products derived from HMTBN, unreacted, remaining in aqueous phase, called mother liquors. In 95, the methionine is recovered in solid form, while the mother liquors 92 are returned to the reactor 70, via the supply line 92, after raising of their pressure by the pump 91. This recycling loop is particularly advantageous.

Figure 9 shows another recycling possibility according to the present invention.

The methioninogenic products at 92 can undergo chemical or enzymatic treatment before being recycled. They may be recycled to the hydrolysis reactor 70, to the stripping column 80 or to the withdrawal line.

According to one embodiment of the present invention, the mother liquors 92 are recycled after chemical or enzymatic treatment. The recycling can for example take place towards the column 80, or towards the stream 82.

According to another embodiment of the present invention, said mother liquors 92 are recycled to the reactor 70 in a natural manner, without chemical or enzymatic treatment.

As shown in FIG. 10, according to one embodiment of the present invention, the methionine is synthesized in a tubular reactor or in a plurality of stirred reactors 710, 720, 730 and 740 arranged in series.

The number of agitated reactors is indifferent. Preferably, it is greater than 3, but the maximum number is not limited.

According to one embodiment of the present invention, the methionine-enriched liquid stream 82 is separated by stepped crystallization 91 to obtain, on the one hand, solid methionine 95 and, on the other hand, mother liquors 92. Staged crystallization 91 further generates a gas stream 92, containing water and light products, called "condensates". After condensation 93, the stream 94 is optionally stored.

Figure 11 shows a device for preparing methionine from acrolein. The preparation of the HMTBN solution comprises two tubular reactors 1 and 6. The first stage of the process, that is to say the synthesis of the acrolein cyanohydrin from acrolein and HCN, takes place in the tubular reactor 1, while the second step of the process of the invention, namely the addition of MSH on the cyanohydrin of acrolein obtained in the first step, takes place in the reactor 6.

Although FIG. 11 represents these two reactors in tubular form, it is quite possible according to the present invention that these steps take place in stirred or "batch" reactors or any other suitable reactor.

There is also provided, at 4, a power supply for pH regulation which is an essential feature of the present invention.

According to one embodiment of the present invention, the pH regulation solution, for example a buffer solution, is introduced after mixing the other streams, 2 and 3. The hydrocyanic acid is in the form of an aqueous solution, for example of the order of 5 to 90% by weight. This solution can be prepared outside the device, but it is also possible to provide an introduction of water in line.

At the outlet of reactor 1, cyanohydrin is recovered from acrolein 61, which is then sent to tubular reactor 60.

The tubular reactor 60 is further supplied with MSH 63. There is also provided, at 64, a power supply for the regulation of the pH. An aqueous solution of HMTBN is recovered at 62. The method also implements a third step 70. Before introduction into the reactor 70, to the aqueous solution 62 comprising HMTBN is added an aqueous phase containing ammonia and CO ₂ via the supply line 77.

The aqueous solution containing HMTBN, ammonia and CO ₂ is introduced into the reactor 70, after raising its pressure to 72. A mixture comprising methionine, carbon dioxide and carbon dioxide is recovered at the outlet of the reactor 70. dissolved ammonia 73.

The process for the preparation of methionine from acrolein is now described with reference to FIGS. 12 and 13. These figures differ from FIG. 11 in that they specify the method used.

In a manner similar to the device of FIG. 11, the preparation of the HMTBN solution according to FIG. 12 or FIG. 13 involves two tubular reactors 1 and 60, the first process step taking place in the tubular reactor 1, and the second stage of the process of the invention taking place in the reactor 60.

After the reactor 1, there is a column 15 to which is sent cyanohydrin acrolein and which allows the recycling of HCN gas phase. Such a column also makes it possible to adjust the amount of HCN introduced into the reactor 1. At the outlet of column 15, cyanohydrin is recovered from acrolein

61, which is then sent to the tubular reactor 60.

Similarly, the HMTBN is sent to the column 65 at the reactor outlet 60. This allows the unreacted products to be recycled.

The device of Figure 13 differs from that of Figure 12 in that the third step is specified.

To the aqueous solution 62 comprising HMTBN is added an aqueous phase 77 containing ammonia and CO ₂ . The pressure of the assembly is relieved by the pump 72. The reactor 70 is supplied with an aqueous solution 71, Le. industrial solution, consisting essentially of methionine hydantoin, but also the associated derivatives, methioninogens - hydantoic acid, ureidobutyramide and 2-amino-4- (methylthio) butanamide

(still called AMBTM). By "methioninogen" is meant any compound likely to lead to methionine by ammoniacal hydrolysis. According to this the aqueous solution of hydantoin, which is itself a methioniogenic compound, is furthermore capable of comprising methioninogenic derivatives, and in particular hydantoïque acid, ureidobutyramide and 2-amino-4- (methylthio) butanamide, shown above. By definition, methionine is not included in methioninogenic compounds.

An exit mixture comprising principally methionine, CO ₂ and dissolved NH ₃ is obtained at the outlet of reactor 70.

In column 80, the expanded mixture introduced is separated into a liquid stream 82 enriched in methionine and a gaseous stream 81 enriched in NH ₃ and CO ₂ .

At the column head 80, the gas stream 81 enriched in NH ₃ and CO ₂ and which also comprises water, is condensed and then stored at 84. The storage 84 optionally comprises a gas purge 85.

The gaseous ammoniacal fraction is condensed. The aqueous phase containing ammonia and CO ₂ is recycled to the reactor 70, and after raising its pressure 76.

Thus, according to one embodiment of the present invention, after condensation, the stream 81 enriched with ammonia and carbon dioxide is stored and recycled to the third stage, after lifting 76 of its pressure.

It is also possible according to the present invention to provide a supply line 75 which feeds the ammonia and / or CO ₂ pipe, before lifting the pressure. Such conduct allows the adjustment of the ammonia and / or CO ₂ titer if necessary. At the bottom of column 80, a liquid aqueous stream rich in methionine 82 is withdrawn.

The liquor thus obtained is then sent to a separator 90 which makes it possible to separate the methionine in solid form, products derived from hydantoin, unreacted, remaining in the aqueous phase, called mother liquors.

In 95, the methionine is recovered in solid form, while said by-products are returned to the reactor 70, via the supply line 92, after raising their pressure 91.

This recycling loop is particularly advantageous. The other types of recycling mentioned above are alternative embodiments quite possible. Figure 14 summarizes the whole of a process for the preparation of methionine from acrolein according to the invention. The different blocks are detailed in Figures 4, 5, 6 and 13.

The following examples will make it possible to understand the advantage of the present invention.

Example 1 (NCA18 test)

This Example illustrates a continuous process for the preparation of HMTBN I ¹ according to the present invention.

The equipment used is the following: - "batch" reactor with double jacket;

- refrigerated column;

- pipette for the "bubbling" of MSH;

- pH electrode;

- syringe for introduction and sampling.

The operating conditions are as follows:

The reaction medium is placed under magnetic stirring. The synthesis is conducted at atmospheric pressure. The temperature and the pH are followed. 15.70 g of acrolein 95% are introduced into the reactor. 28.60 g of HCN in aqueous solution at 29.3% by weight are gradually poured into the medium. During this pouring, 17.2 ml of phosphate buffer pH 7 are added. This makes it possible to bring the pH between 4 and 6. The addition of the buffer makes it possible to initiate the synthesis reaction of the cyanohydrin of acrolein. . An exotherm is observed.

When all of the HCN is introduced into the reactor (end of HCN casting), 35.20 g of methyl mercaptan (gas) are added to the reaction medium. This addition allows the continuation of the synthesis.

When all of the methyl mercaptan is added (this corresponds to the end of the synthesis), the reaction medium is acidified to pH = 2 by adding 1 N sulfuric acid at 95% (stopped reaction).

After evaporation under vacuum (4 mmHg) and at a temperature of 50 ^° C., 34.20 g of HMTBN are recovered. The molar ratios are therefore: MSH / acrolein = 2.76 HCN / acrolein ≈ 1, 16

An analytical follow-up makes it possible to determine the overall yield of the reaction. An HPLC analysis is then performed.

The overall yield of the reaction is 95% relative to acrolein. The conversion rate of acrolein is 100%. L ¹ HMTBN has a title of 98%.

Example 2 (NCA test 20)

The material used is identical to that of Example 1.

The reaction medium is placed under magnetic stirring. The synthesis is conducted at atmospheric pressure. The temperature and the pH are followed. 15.0 g of 95% acrolein are introduced into the reactor. 28.0 g of HCN in aqueous solution at 29.3% by weight are gradually poured into the medium. During this casting, 33 g is added to the pH 7 phosphate buffer test. This makes it possible to bring the pH between 4 and 6. The addition of the buffer makes it possible to initiate the synthesis reaction of the cyanohydrin of the acrolein.

/

When all the HCN is introduced into the reactor (end of HCN casting), 21.4 g of methyl mercaptan (gas) are added to the reaction medium.

When all the methyl mercaptan is added, the reaction medium is acidified to pH = 2 by adding 1 N sulfuric acid at 95% (stopped reaction).

After evaporation under vacuum (4 mmHg) and at a temperature of 50 ^° C., 32.6 g of HMTBN are recovered. An analytical follow-up makes it possible to determine the overall yield of the reaction. An HPLC analysis is then performed.

The overall yield of the reaction is 96% relative to acrolein.

Example 3

This example illustrates an industrial process for preparing ¹ HMTBN for which two absorption columns are used.

A crude product based on acrolein is produced at the outlet of a propylene oxidation reactor in acrolein in the vapor phase (300-400 ^° C.). This crude product consists of a gaseous mixture having a temperature of 180 ^° C. and comprising a high proportion by weight of gases which are difficult to condensate (propane, nitrogen, oxygen, propylene, CO, CO ₂ ), water, water or acrolein, acrylic acid and other compounds.

This acrolein-based crude product is then treated to remove the acids. The crude product in the form of gas is introduced into a washing column, in which circulates a stream of water. Water causes acids. The liquid containing the acids is withdrawn at the bottom of the column. The purified gas is withdrawn at the top of the column.

The gaseous phase of acrolein thus obtained is introduced at the head of an absorption column.

This column, maintained at atmospheric pressure, is also fed with a gaseous hydrogen cyanide phase with a flow rate of 1000 kg / h.

The hydrocyanic acid thus introduced is obtained by ammoxidation from NH ₃ , CH ₄ and air in a reactor at elevated temperature (greater than 1000 ^° C.). The mixture is then cooled and introduced into a washing column in which circulates a stream of sulfuric acid, in order to remove excess ammonia.

The reaction between acrolein and hydrocyanic acid in the first absorption column takes place with an excess of hydrogen cyanide. The use of a Absorption column allows a separation of heavy and light products, which makes it possible to recover at the top of the column the most volatile products, in this case I ¹ unreacted HCN which can then be recycled. The gaseous phase of hydrocyanic acid is then sent to a washing column, in which circulates a stream of water. The HCN in aqueous solution, withdrawn at the bottom of the washing column, is then in turn sent to a distillation column.

The HCN thus distilled is returned to the top of the absorption column, thereby generating a descending stream of HCN in aqueous phase in said first absorption column.

It should be noted that a recycling of propylene at the top of the hydrocyanic acid washing column is possible. It is then necessary to separate propylene from CO, CO ₂ , nitrogen and H ₂ .

The acrolein stream in the gas phase condenses within the absorption column and can thus react with the descending stream of HCN in the aqueous phase.

The acrolein cyanohydrin in aqueous phase thus obtained being heavier in vapor pressure, it is drawn off at the foot of the absorption column and is then reintroduced at the head of a second absorption column.

A gaseous phase crude product based on methyl mercaptan is also introduced into the second absorption column. This methyl mercaptan comes from the reaction between methanol CH ₃ OH and hydrogen sulfide H ₂ S.

At the bottom of the second absorption column, an aqueous solution of stable HMTBN (30 to 80% by weight of HMTBN) is withdrawn.

Example 4

Two tests (AME 152 and AME 153) were performed. These tests illustrate the continuous process for preparing the methionine according to the present invention. AME 152 Operating Mode:

In a perfectly stirred reactor are charged: - 59.25 g of a citric acid solution (40%), - 14.85 g of acrolein, and then 40.10 g of a sodium cyanide solution. 5 minutes after the exotherm, 15.30 g of methyl mercaptan is charged. Then, 5 minutes later, 275.2 g of ammoniacal water (from NH ₃ and NH ₄ HCO ₃ ) are added.

The reactor is heated to 150 ^° C.

The total reaction time counted from the introduction of sodium cyanide is two hours.

NB: the citric acid solution and the sodium cyanide solution are used to supply hydrocyanic acid for the synthesis of acrolein cyanohydrin:

Industrially, the HCN would be alone and would not bring salt.

The conversion rate of acrolein is 100%. The yield of methionine is approximately 44%, that of methioninogen + methionine is 74%.

AME 153

Operating mode:

In a perfectly stirred reactor are charged:

58.55 g of a citric acid solution (40%),

13.95 g of acrolein, and then 40.55 g of a solution of sodium cyanide.

5 minutes after the exotherm, 13.40 g of methyl mercaptan is loaded.

Then 5 minutes later, 210.9 g of ammonia water (from NH ₃ and NH ₄ HCO ₃ ) are added.

The reactor is heated to 150 ^° C. The total reaction time counted from the introduction of sodium cyanide is two hours.

NB: The citric acid solution and the sodium cyanide solution are used to provide hydrocyanic acid for the synthesis of cyanohydrin of acrolein.

Industrially, I ¹ HCN would be alone and would not bring salt.

The conversion rate of acrolein is 100%. The yield of methionine is approximately 45%, that of methioninogenic products + methionine is 75%.

Claims

A process for preparing an aqueous solution of 2-hydroxy-4- (methylthio) butyronitrile (HMTBN) from acrolein without isolating intermediates, characterized in that:

in a first step, (1) is reacted with acrolein (2) with hydrocyanic acid (4) in an HCN / acrolein molar proportion of between 1 and 10, - in a second step, methyl mercaptan (MSH) (63) in an MSH / acrolein molar ratio of between 1 and 10, said process occurring at a pH of between 3 and 9.

2. Process according to claim 1, characterized in that the hydrocyanic acid is present in a molar ratio of HCN / acrolein of between 1 and 3.

3. Method according to claim 1 or 2, characterized in that the MSH is present in a molar proportion MSH / acrolein between 1 and 3.

4. Process according to any one of the preceding claims, characterized in that the reaction temperature is between 10 and 120 ^° C.

5. Process according to claims 1 to 3, characterized in that the reaction temperature is between 60 and 100 ^° C.

6. Method according to any one of the preceding claims, characterized in that the process takes place / takes place at a pH between 4 and 8.

7. Method according to any one of the preceding claims, characterized in that the first step takes place / takes place at a pH between 4 and 6.

8. Method according to any one of the preceding claims, characterized in that the second step takes place / takes place at a pH between 6 and 8.

9. Process according to any one of the preceding claims, wherein the acrolein (2) comes from an oxidation (20) of propylene (21) by air (22) in the presence of water (23).

10. The method of claim 9, wherein the recycling (53) of propylene after reactive absorption of acrolein and hydrogen cyanide (1) to the oxidation (20).

11. The method of claim 9 or 10, characterized in that, after oxidation (20) of the propylene (21) by air (22) in the presence of water (23) and before reactive absorption (1), it eliminates the acids contained in the product obtained after washing oxidation (25).

A process according to any one of the preceding claims, wherein the HCN (3) is from a synthesis (30), from methane (32) and ammonia (31) in the presence of air (33) and a solid catalyst.

13. Process according to the preceding claim, characterized in that, after synthesis (30) of the HCN and before reactive absorption (1), washing is carried out (35).

14. Process for the continuous preparation of methionine, from HMTBN, according to which an ammoniacal hydrolysis (70) of an aqueous solution (101) comprising said HMTBN is carried out, characterized in that:

(a) the ammonia hydrolysis (70) of the HMTBN is carried out continuously in the aqueous phase and without isolating intermediate products at a temperature not exceeding 25 ^° C., starting from a molar proportion

NH ₃ / HMTBN between 5 and 50, and a molar ratio CO ₂ / HMTBN between 1 and 10, to obtain an exit mixture (73) comprising at least methionine, carbon dioxide and the dissolved ammonia, and products derived from HMTBN; and (b) recycling said products derived from HMTBN, carbon dioxide and ammonia to hydrolysis (a).

15. The method of claim 14, wherein one proceeds without a catalyst.

Process according to Claim 14 or 15, characterized in that at the end of the process the outlet mixture (73) is expanded (74) and the expanded outlet mixture (80) is separated into a flow (82). ) a liquid enriched in methionine, and a gaseous flow (81) enriched in ammonia and carbon dioxide.

17. Method according to the preceding claim, characterized in that separates (90) the liquid stream (82) enriched in methionine, solid methionine (95), discharged, and mother liquors (92) depleted of methionine and optionally containing the by-products, said mother liquors (92) being recycled, after lifting (91) of their pressure, towards the beginning of the process.

18. Method according to the preceding claim, wherein said mother liquors (92) are recycled after chemical or enzymatic treatment.

19. The method of claim 17, wherein said mother liquors (920) are recycled to the start of the process in a natural manner, without chemical or enzymatic treatment.

20. The method of claim 17, characterized in that separates the stream (82) liquid enriched in methionine by staged crystallization (901) to obtain solid methionine (95) and mother liquors (92).

21. A method according to any one of claims 18 to 22, characterized in that after condensation, is stored (84) the flow (81) enriched with ammonia and carbon dioxide and recycled (77) to the beginning of the process, after lifting (76) of its pressure.

22. Process according to any one of claims 14 to 21, characterized in that the hydrolysis (700), according to the process, is carried out at a temperature ranging between 20 and 220 ^° C.

23. Process according to any one of claims 14 to 22, characterized in that the hydrolysis (700), according to the process, is carried out at a pressure of between 5 and 100 bar.

24. Process according to any one of claims 14 to 23, characterized in that it is carried out in a tubular reactor or in a plurality of stirred reactors (710, 720, 730, 740) arranged in series.

25. Process for the preparation of methionine from acrolein, characterized in that, without isolating intermediate products:

in a first step, acrolein (101) is reacted with hydrocyanic acid (102),

in a second step, methylmercaptan (MSH) (602) is added to the reaction mixture of the first step, said two stages being carried out at a pH of between 3 and 9, whereby an aqueous solution of HMTBN is obtained,

in a third step, NH ₃ and CO ₂ (707) are added to the aqueous HMTBN solution, whereby a mixture (703) comprising at least methionine, carbon dioxide and carbon dioxide is obtained; dissolved ammonia.

26. A process according to claim 25, wherein in the first step, acrolein (101) is reacted with hydrogen cyanide (102) in an HCN / acrolein molar ratio of between 1 and 10.

27. A method according to any one of claims 25 and 26, wherein in the second step, the MSH (403) is added in an MSH / acrolein molar ratio of between 1 and 10.

28. Process according to any one of claims 25 to 27, according to which, in the third step, NH 3 and CO 2 are added to the aqueous HMTBN solution starting from an NH 4 / acrolein molar ratio of between 5 and 5. 50, and a molar proportion Cθ ₂ / acrolein of between 1 and 20.

The process of any one of claims 25 to 28, wherein the carbon dioxide and ammonia of the exit mixture (77) are recycled to the third step.

30. Method according to any one of claims 25 to 29, wherein the reaction temperature of the first two stages is between 10 and 12O ⁰ C and / or that of the third step does not exceed 250 ⁰ C.

31. Process according to any one of claims 25 to 30, according to which acrolein (2) comes from an oxidation (20) of propylene (21) by air

(22) in the presence of water (23).

32. Process according to the preceding claim, according to which recycling is carried out (53) of propylene after reactive absorption of acrolein and hydrocyanic acid (1), towards oxidation (20).

33. The method of claim 31 or 32, wherein, after oxidation (20) propylene (21) by air (22) in the presence of water (23) and before reactive absorption of acrolein and the hydrogen cyanide (1), the acids contained in the product obtained (24) are removed by washing (25).

34. Process according to any one of claims 25 to 33, according to which the HCN (3) comes from a synthesis (30), starting from methane (32) and ammonia (31) in the presence of air ( 33) and a solid catalyst.

35. Process according to the preceding claim, characterized in that, after synthesis (30) of the HCN and before reactive absorption of acrolein and hydrocyanic acid (1), washing is carried out (35).

36. Process according to any one of claims 25 to 35, characterized in that, after the third step, the (74) outlet mixture (74) is expanded (74) and then the expanded outlet mixture (80) is separated into a flux (82) enriched with methionine, and a gaseous flow (81) enriched with ammonia and carbon dioxide.

37. Method according to the preceding claim, characterized in that separates (90) the liquid stream (82) enriched in methionine, solid methionine (95), evacuated, and mother liquors (92) depleted of methionine and optionally containing the by-products, said mother liquors (92) being recycled, after lifting (91) of their pressure, towards the third stage.

38. Process according to claim 37, characterized in that the methionine-enriched liquid stream (82) is separated by staged crystallization (901) to obtain solid methionine (95) and the mother liquors (92).

39. A method according to any one of claims 36 to 38, characterized in that after condensation, the flow (81) is stored (84) enriched in ammonia and carbon dioxide and is recycled (77) to the third step, after lifting (76) of its pressure.

40. Process according to any one of claims 25 to 39, characterized in that the hydrolysis (70), according to the third step of the process, is carried out at a temperature ranging between 20 and 220 ^° C.

41. Process according to any one of claims 25 to 40, characterized in that the hydrolysis (70), according to the third stage of the process, is carried out at a pressure of between 5 and 100 bar.

42. Process according to any one of Claims 25 to 41, characterized in that the third stage of the process is carried out in a tubular reactor or in a plurality of stirred reactors (710, 720, 730, 740) arranged in series.

43. A process for the preparation of 2-hydroxy-4- (methylthio) butanoic acid (HMTBA), comprising the process for preparing HMTBN according to any one of claims 1 to 13.