IMPROVED PROCESS FOR THE PRODUCTION OF HYDRAZINE HYDRATE VIA PYRAZOLINE RECYCLING

Description

[0001] The present invention relates to a process for the preparation of hydrazine hydrate. More specifically, the present invention relates to an improved process for the preparation of hydrazine hydrate from methyl ethyl ketone azine obtained in the presence of methyl ethyl ketone by oxidation of ammonia with hydrogen peroxide and an activator.

[0002] Hydrazine is used in various applications, mainly in the deoxygenation of boiler water (e.g. nuclear power plants) and is used in the preparation of pharmaceutical and agrochemical derivatives.

[0003] There is therefore an industrial need for the preparation of hydrazine hydrate.

[0004] The industrial production of hydrazine hydrate is carried out using the RASCHIG, BAYER or hydrogen peroxide processes.

[0005] In the RASCHIG process, ammonia is oxidized with hypochlorite to obtain a dilute solution of hydrazine hydrate, which must then be concentrated by distillation. This process, which is not very selective, not very productive, and very polluting, is almost no longer used.

[0006] The BAYER process is a variant of the RASCHIG process which consists of shifting a chemical equilibrium by trapping, using acetone, the hydrazine formed in the form of azine with the following formula: (CH 3 ) 2 C = NN = C-(CH 3 ) 2 .

[0007] The azine is then isolated and hydrolyzed to hydrazine hydrate. Yields are improved, but there is no improvement in environmental emissions.

[0008] The hydrogen peroxide process involves oxidizing a mixture of ammonia and a ketone with hydrogen peroxide in the presence of a hydrogen peroxide activator to directly produce the azine, which then only needs to be hydrolyzed to hydrazine hydrate. Yields are high and the process is non-polluting. This hydrogen peroxide process is described in numerous patents, for example US 3,972,878, US 3,972,876, US 3,948,902 and US 4,093,656.

[0009] The hydrolysis of an azine to hydrazine hydrate is described in US patents 4,724,133 SCHIRMANN et al., US patents 4,725,421 SCHIRMANN et al. and GB 1,164,460. This hydrolysis is carried out in a distillation column which is fed with water and azine. The ketone is recovered at the top and a concentrated solution of hydrazine hydrate at the bottom.

[0010] These processes are also described in ULLMANN'S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY (1989), vol A 13, pages 182-183 and the references therein.

[0011] In hydrogen peroxide processes, ammonia is oxidized by hydrogen peroxide in the presence of a ketone and a hydrogen peroxide activator according to the following overall reaction, making an azine:

[0012] The activating agent or activator can be a nitrile, an amide, a carboxylic acid or a derivative of selenium, antimony or arsenic. Then the azine is hydrolyzed to hydrazine and the ketone is regenerated according to the following reaction:

[0013] This hydrolysis is actually carried out in two stages, with the formation of an intermediate hydrazone:

[0014] Whether the azine is produced by a hydrogen peroxide process or another process, methyl ethyl ketone is advantageously used because it is sparingly soluble in an aqueous medium.

[0015] Indeed, in the hydrogen peroxide process, the azine of methyl ethyl ketone is relatively insoluble in the reaction medium which is necessarily aqueous since commercial aqueous solutions of hydrogen peroxide titrating between 30 and 70% by weight are used. This azine is therefore easily recoverable and separable by simple decantation. It is very stable especially in alkaline medium, that is to say in the ammoniacal reaction medium. In current processes, this azine is then purified, then hydrolyzed in a reactive distillation column to release in fine methyl ethyl ketone at the top to be recycled, and above all an aqueous solution of hydrazine hydrate at the bottom, which must contain as few carbonaceous products as possible as impurities and must be colorless.

[0016] However, in these hydrogen peroxide processes using methyl ethyl ketone (also known as MEK), small amounts of by-products are formed, including heterocycles of the pyrazoline family. In particular, these by-products are 3,4,5-trimethyl-5-ethyl-pyrazoline and 3,5-diethyl-5-methyl-pyrazoline, with the following formulae:

[0017] These compounds are isomers of methyl ethyl ketone azine. They are formed mainly in the hydrolysis column and are responsible for the yellow, even brown or red coloration of hydrazine. It is thus possible to obtain at the bottom of the column a solution of hydrazine hydrate that is not colorless, but colored and sometimes strongly colored, depending on the content of heterocycles of the pyrazoline family obtained.

[0018] These by-products can accumulate on the trays of the hydrolysis column, causing the coloring problems described above and operating problems beyond a certain concentration. Preferably, the content of heterocycles of the pyrazoline family is less than or equal to 5% by weight, more preferably between 1% and 3% by weight, relative to the total weight of the liquid phase present on the trays of the column or in the parts of the hydrolysis column where said content is at its maximum. These values ​​are sufficient to avoid coloring the hydrazine hydrate produced.

[0019] There is therefore a need for an industrial process for the preparation of hydrazine hydrate which is improved with regard to the treatment of by-products such as heterocycles of the pyrazoline family, and in particular 3,4,5-trimethyl-5-ethyl-pyrazoline and 3,5-diethyl-5-methyl-pyrazoline.

[0020] WO 99 / 58445 describes the discontinuous purging of pyrazoline heterocycles during the hydrolysis step of azine to hydrazine hydrate. It is mentioned that these compounds are formed in the hydrolysis column.

[0021] However, this document does not give any indication as to any possible treatment of this purge.

[0022] There is therefore a need for an improved process for the preparation of hydrazine hydrate in which the by-products are treated in a more economical and environmentally friendly manner.

[0023] Surprisingly, it was discovered that when the purge of heterocycles of the pyrazoline family is recycled upstream of the hydrolysis step, these by-products return solubilized in the azine at the inlet of the distillation step (e), and are eliminated during this distillation step. The distillation step surprisingly allows the elimination of heterocycles of the pyrazoline family. In addition, the hydrazine precursors (azine and hydrazone) that may be contained in this purge are recovered in the azine stream and return to the hydrolysis column.

[0024] The recycling according to the invention makes it possible to achieve stabilization of the concentration of heterocycles of the pyrazoline family in the hydrolysis column, at a concentration low enough to obtain a colorless hydrazine hydrate solution at the bottom of the hydrolysis column and this in a stable manner when the hydrolysis is carried out continuously.

[0025] The recycling of the purge of heterocycles of the pyrazoline family according to the invention thus presents numerous advantages. It avoids the discharge of an effluent containing azine and hydrazone which represent intermediate products leading to hydrazine, while eliminating the by-products of the process, without adding additional operations or additional costs to the process.

[0026] If the effluent containing said heterocycles were to be discharged, it would be necessary to implement additional treatment, such as incineration or oxidation treatment before its discharge into the environment.

[0027] The method as according to the invention also makes it possible not to lose the hydrazone and azine compounds which may be contained in the purge because the hydrazone and the azine lead by hydrolysis to hydrazine hydrate.

[0028] Thus, the present invention relates to a process for preparing hydrazine hydrate comprising the following steps: (a) reacting ammonia, hydrogen peroxide and methyl ethyl ketone in the presence of a solution comprising at least one activator to form an azine; (b) separating from the reaction mixture from step (a): the aqueous phase comprising the activator(s); and the organic phase comprising the azine formed and optionally the unreacted methyl ethyl ketone; (c) optionally recycling to step (a) the aqueous phase after an optional treatment; (d) washing, preferably countercurrently, the organic phase; (e) distilling the washed organic phase so as to recover the azine; (f) hydrolyzing the azine to obtain hydrazine hydrate and regenerating the methyl ethyl ketone, while purging the heterocycles of the pyrazoline family; (g) optionally recycling to step (a) the methyl ethyl ketone obtained in step (f);(h) recycling the purge of heterocycles of the pyrazoline family obtained in step (f) to at least one of steps (a), (b), (c), (d) or (g).;

[0029] The purging of heterocycles of the pyrazoline family obviously includes said heterocycles, but can also include azine, hydrazone and water. Said purging is notably in aqueous form.

[0030] Preferably, all process steps are carried out continuously.

[0031] The term "heterocycles of the pyrazoline family" means in particular a pyrazoline ring substituted on at least one of its carbon atoms, in particular a pyrazoline ring of the following formula (I): in which Ra, Rb, Rc, Rd and Re can be independently of each other chosen from H, and (C 1 -C 20 )alkyls; at least one of the radicals Ra, Rb, Rc, Rd and Re being a (C 1 -C 20 )alkyl.

[0032] The term "(C 1 -C 20 )alkyl" refers to saturated aliphatic hydrocarbons, which may be linear or branched and comprise from 1 to 20 carbon atoms. Preferably, the alkyls comprise from 1 to 4 carbon atoms, or even from 1 to 2 carbon atoms. Examples include methyl and ethyl. "Branched" means that an alkyl group is substituted on the main alkyl chain.

[0033] It is understood in particular by azine, the azine of methyl ethyl ketone also called mecazine. Step (a): Formation of azine from methyl ethyl ketone

[0034] In step (a), ammonia, hydrogen peroxide and methyl ethyl ketone are reacted in the presence of a solution comprising at least one activator to form an azine, i.e. methyl ethyl ketone azine.

[0035] Ammonia can be anhydrous or in aqueous solution.

[0036] Hydrogen peroxide can be used in its usual commercial form, for example in aqueous solution between 30% and 90% by weight of H 2 O 2 . Advantageously, one or more usual stabilizers of peroxide solutions can be added, for example phosphoric, pyrophosphoric, citric, nitrilotriacetic, ethylenediaminetetraacetic acid or the ammonium or alkali metal salts of these acids. The quantity to be used is advantageously between 10 and 1000 ppm and, preferably, between 50 and 250 ppm of all the reagents and of the solution comprising at least one activator at the inlet of the reactor.

[0037] By "activator" is meant a compound which can activate hydrogen peroxide, i.e. a compound such that azine can be produced from ammonia, hydrogen peroxide and methyl ethyl ketone.

[0038] This activator can be chosen from organic or inorganic oxyacids, their ammonium salts and their derivatives: anhydrides, esters, amides, nitriles, acyl peroxides, or mixtures thereof. Advantageously, amides, ammonium salts and nitriles are used.

[0039] For example, we can cite: (i} amides of carboxylic acids of formula R 5 COOH in which R 5 is hydrogen, a linear alkyl radical having from 1 to 20 carbon atoms, or a branched or cyclic alkyl radical having from 3 to 12 carbon atoms, or a phenyl radical which may be substituted, (ii} amides of polycarboxylic acids of formula R 6 (COOH) n in which R 6 represents an alkylene radical having from 1 to 10 carbon atoms and n being an integer greater than or equal to 2, R 6 can be a single bond, then n is 2.

[0040] The radicals R 5 and R 6 may be substituted by halogens, OH, NO 2 or methoxy groups. Mention may also be made of amides of organic arsenic acids. Organic arsenic acids are, for example, methylarsonic acid, phenylarsonic acid and cacodylic acid.

[0041] Preferred amides are formamide, acetamide, monochloroacetamide and propionamide, and more preferably acetamide.

[0042] Among the ammonium salts, the salts of hydracids, mineral oxyacids, arylsulfonic acids, acids of formulas R 5 COOH or R 6 (COOH) n , R 5 , R 6 and n being defined previously, organic acids of arsenic are advantageously used. The preferred ammonium salts are formate, acetate, monochloroacetate, propionate, phenylarsonate and cacodylate.

[0043] Among the nitriles, advantageously mention may be made of the products of formula R 7 (CN) n , n being able to vary from 1 to 5 depending on the valence of R 7 , R 7 is a cyclic or non-cyclic alkyl having from 1 to 12 carbon atoms or a benzyl or a pyridinyl group. R 7 may be substituted by groups which do not oxidize in the reactor of step (a), for example halogens, carboxylic groups, carboxylic esters, nitro, amine, hydroxy or sulfonic acid.

[0044] The preferred nitriles are acetonitrile and propionitrile.

[0045] The solution comprising at least one activator is formed by dissolving one or more products chosen from organic or inorganic oxyacids, their ammonium salts and their derivatives: anhydrides, esters, amides, nitriles, acyl peroxides, or their mixtures as defined above. Advantageously, the preceding amides, ammonium salts or nitriles are used. Particularly preferably, a single activator, which is acetamide, is used.

[0046] This solution can be aqueous or based on an alcohol or a mixture of alcohol and water. Among the alcohols, saturated aliphatic alcohols having 1 to 6 carbon atoms and, preferably, 1 to 2 carbon atoms are advantageously used.

[0047] Diols are also advantageously used, and more particularly diols having 2 to 5 carbon atoms. Examples include glycol, propylene glycol, 1,3 propanediol, 1,3 and 1,4 butanediol and 1,5 pentanediol.

[0048] According to one embodiment, said solution is an alcoholic solution of an organic acid of arsenic and is described in patent EP 0 070 155.

[0049] According to another embodiment, said solution is an aqueous solution of a weak acid amide and the ammonium salt corresponding to this acid as described in patent EP 0 487 160.

[0050] These weak acid amides are derived from the corresponding carboxylic acids which have a dissociation constant less than 3 x 10 -3< , i.e., acids which have a pK greater than 3 in aqueous solution at 25°C.

[0051] For polycarboxylic acids, these are acids whose first ionization constant is less than 3 x 10 -3< .

[0052] Examples that may be mentioned are carboxylic acids of formula R 8 COOH in which R 8 is a linear alkyl radical having from 1 to 20 carbon atoms, or a branched or cyclic alkyl radical having from 3 to 12 carbon atoms, or a phenyl radical which may be substituted, polycarboxylic acids of formula R 9 (COOH) n in which R 9 represents an alkylene radical having from 1 to 10 carbon atoms and n being a number greater than or equal to 2, R 9 may be a single bond then n is 2. The radicals R 8 and R 9 may be substituted by halogens, OH, NO 2 or methoxy groups. Acetamide, propionamide, n-butyramide or isobutyramide are preferably used.

[0053] The corresponding ammonium salt of acetamide is ammonium acetate.

[0054] It would not be outside the scope of the invention to form the ammonium salt in situ, that is to say by using the corresponding carboxylic acid which gives the ammonium salt by reaction with ammonia.

[0055] The proportions of the amide and the corresponding ammonium salt can vary within wide limits. Usually 1 to 25 parts of the ammonium salt are used for 5 parts of amide and preferably 2 to 10.

[0056] The reagents may be used in stoichiometric quantities. However, 0.2 to 5 moles and preferably 1.5 to 4 moles of methyl ethyl ketone may be used per mole of hydrogen peroxide; 0.1 to 10 moles and preferably 1.5 to 4 moles of ammonia. The quantity of solution comprising at least one activator may be between 0.1 and 2 kg per mole of hydrogen peroxide. This quantity depends on its quality, i.e. its catalytic force or its activity which allows the reagents to be converted into azine. The proportions of the reagents fixed above make it possible to obtain a total conversion of hydrogen peroxide and a production of azine corresponding to more than 50% of the hydrogen peroxide used and which can reach 90%.

[0057] The contacting of hydrogen peroxide, ammonia, methyl ethyl ketone with the solution comprising at least one activator can be carried out in any way.

[0058] The reaction can be carried out in a homogeneous medium or in a medium which ensures at least sufficient solubilization of the reactants to obtain azine. The reaction can be carried out in a very wide temperature range, for example between 0°C and 100°C, and it is advantageous to operate between 30°C and 70°C. Although the reaction can be carried out at any pressure, it is simpler to be at atmospheric pressure, but it can be increased to about 10 bars if necessary to preferably maintain the reaction of step (a) in the liquid phase.

[0059] The reagents can be introduced simultaneously or separately and in any order into the solution comprising at least one activator.

[0060] All kinds of reactors can be used, stirred or non-stirred, or even simple capacities that can be arranged in parallel, in series, co-current or counter-current, or any combination of these possibilities.

[0061] Following the reaction of step (a), a reaction mixture is thus obtained comprising the azine formed, optionally the unreacted methyl ethyl ketone, optionally the activator(s), and optionally other by-products or impurities. Step (b): Phase separation

[0062] The aqueous phase comprising the activator(s) is separated from the organic phase comprising the azine formed and optionally the unreacted methyl ethyl ketone by conventional means such as liquid-liquid extraction, distillation, decantation or any combination of these possibilities. Preferably, decantation is used.

[0063] The resulting organic phase may include unreacted azine and methyl ethyl ketone, activator(s), and possibly impurities. Step (c): Recycling of the aqueous phase

[0064] In step (c), the aqueous phase can be treated before recycling to step (a), conventionally for example by thermal regeneration then possibly concentration.

[0065] Steps (a), (b) and (c) are described for example in patents EP 399 866 and EP 518 728. Step (d): Washing the organic phase

[0066] The step of washing the organic phase obtained in step (b) is a step that can be carried out according to techniques known to those skilled in the art, as indicated for example in document WO 2018 / 065997 (p. 13, “Organic layer processing section”, second paragraph). The washing step makes it possible in particular to recover the activator(s), for example acetamide, which would still be present in the organic phase.

[0067] Washing can be carried out in a countercurrent washing column.

[0068] Preferably, the washing is carried out countercurrently by purging heterocycles of the pyrazoline family carried out in hydrolysis step (f), optionally after adding water. The washing can be carried out by all or part of said purge. Even more preferably, the purge is sufficient to carry out the washing, without adding water, which in particular makes it possible to save an additional water input into the process. For example, the purge of heterocycles of the pyrazoline family as defined above is added at the top of the column and the organic phase to be washed is added at the bottom of the column. The activator(s) which may still be contained in the organic phase thus pass(s) into the aqueous washing phase (i.e. the purge of heterocycles of the pyrazoline family).

[0069] According to one embodiment, after passing through the washing column, the resulting aqueous phase is either recycled to step (a) with the aqueous phase recovered in step (b), or can also be returned directly to the hydrolysis column. Step (e): Distillation of the organic phase

[0070] The step of distillation of the washed organic phase is a step which can be carried out according to techniques known to those skilled in the art, as indicated for example in document WO 2018 / 065997 (p.13, Organic layer processing section), in particular in a distillation column.

[0071] The distillation step separates the azine from the heterocycles of the pyrazoline family as defined above and other heavy, high-boiling impurities. These by-products are recovered at the bottom of the column, for example.

[0072] The distillation step also serves to separate the azine formed in step (a) from the unreacted methyl ethyl ketone, which can be recovered at the top of the column. It is possible to recycle the methyl ethyl ketone thus recovered to step (a) of azine synthesis.

[0073] Thus, at the end of the washing and distillation steps, a purified organic phase is obtained, comprising the azine. Step (f): Hydrolysis of azine, regeneration of MEK and purging of pyrazoline family heterocycles Hydrolysis of azine and regeneration of methyl ethyl ketone

[0074] The hydrolysis step is preferably carried out continuously, under pressure, in a reactive distillation column into which the water and the organic phase comprising the azine from steps (d) or (e) are injected.

[0075] Hydrolysis can be carried out in a packed or tray distillation column, preferably operating at a pressure of 2 to 25 bar and with a bottom temperature between 150°C and 200°C.

[0076] Although conventional packed columns can be suitable, tray columns are generally used. Depending on the residence time allowed on the trays and the pressure, and therefore the temperatures at which they are operated, the number of trays can vary enormously. In practice, when operating at a pressure of 8 to 10 bars, the number of trays required is around 40 to 50.

[0077] Following hydrolysis, we obtain: at the top, methyl ethyl ketone, notably in the form of an azeotrope with water, and at the bottom, an aqueous solution of hydrazine hydrate.

[0078] The hydrolysis of azines is known. For example, EC GILBERT, in an article in the Journal of American Chemical Society vol. 51, pages 3397-3409 (1929), describes the balanced reactions of azine formation and its hydrolysis reactions and provides the thermodynamic parameters of the system in the case of water-soluble azines. For example, the hydrolysis of azine from acetone is described in US 4,724,133. For azines insoluble in aqueous solutions (e.g. azine from methyl ethyl ketone), the hydrolysis must be carried out in a reactive column, so that by continuously separating the methyl ethyl ketone at the top of the distillation column and the hydrazine hydrate at the bottom of the column, complete hydrolysis can be achieved. Of course, this system works best when working continuously as described in patents FR 1 315 348 or GB 1 211 54 7, or US patent 4 725 421.

[0079] In all these patents, the reaction is carried out in a packed or better tray distillation column operating under a pressure of 2 to 25 bars and with a bottom temperature of 150°C to 200°C.

[0080] When working with pure azine, i.e. obtained for example from hydrazine hydrate and methyl ethyl ketone, it is indeed observed by working according to these patents that dilute solutions of hydrazine hydrate are obtained with good yield.

[0081] In this column, the hydrolysis of the azine and the separation of the hydrazine hydrate from the methyl ethyl ketone are carried out. These conditions are known. The person skilled in the art can easily determine the number of trays or the packing height, as well as the azine and water feed points. Solutions containing 30% or even up to 45% by weight of hydrazine hydrate are obtained at the bottom. For example, the water / azine molar ratio at the feed of this column is at least higher than stoichiometry and advantageously between 5 and 30, preferably between 10 and 20. The temperature of the column bottom can be between 150°C and 200°C, preferably between 175°C and 190°C. The pressure is a function of the boiling point of the azine, water and ketone. Such hydrolysis is also described in US 4,725,421 and in WO 00 / 37357. Purging of heterocycles from the pyrazoline family

[0082] According to the invention, the purging of heterocycles of the pyrazoline family is carried out during hydrolysis step (f).

[0083] Depending on the number of trays or the packing height, the position of the azine feed and the position of the water feed, the reflux, the nature of the azine, etc., the person skilled in the art can easily determine in which part of the column the maximum concentration of heterocycles of the pyrazoline family is obtained. It is in fact simpler to purge said heterocycles by withdrawing at the point where their concentration is maximum, for example on one or more trays. Withdrawal can be carried out continuously, or discontinuously, preferably continuously.

[0084] The quantity to be withdrawn can be easily determined by gas chromatographic analysis of the concentration of these heterocycles.

[0085] Preferably, the purging of heterocycles of the pyrazoline family is carried out by withdrawal, preferably by continuous lateral withdrawal. Step (g): Recycling of regenerated methyl ethyl ketone

[0086] The methyl ethyl ketone obtained in step (f) can be recycled to step (a). Step (h): Recycling of the purge of heterocycles from the pyrazoline family

[0087] Preferably, recycling step (h) is carried out continuously. The purge of heterocycles of the pyrazoline family obtained in step (f) can be recycled to at least one of steps (a), (b), (c), (d) or (g) of the process according to the invention, preferably to at least one of steps (c), (d) or (g). Even more preferably, the purge of heterocycles of the pyrazoline family obtained in step (f) is recycled to step (d) of washing the organic phase.

[0088] Thus, the purging of heterocycles from the pyrazoline family can be recycled: either in the aqueous phase resulting from separation step (b) which is recycled to step (a); or with the methyl ethyl ketone recovered at the top of the hydrolysis column and which is recycled to step (a); or in washing step (d). Description of Figures Figure 1 : Process for the preparation of hydrazine hydrate according to the invention

[0089] There Figure 1 represents an example of industrial implementation of the method according to the invention.

[0090] Arepresents step (a) of the synthesis of mecazine, stream 1 comprises ammonia, hydrogen peroxide as well as the necessary additions of acetic acid, ammonium acetate or acetamide or even methyl ethyl ketone as well as the various additives useful during the synthesis step, such as peroxide stabilizers. Stream 13 corresponds to the recycling of the aqueous phase according to step (c), after its thermal regeneration and concentration to eliminate excess water. Stream 8 corresponds to the recycling of the Methyl ethyl ketone regenerated during the hydrolysis step and recovered at the outlet of hydrolysis column E according to step (g).

[0091] Brepresents a decanter at the outlet of step (a) of azine synthesis which receives the reaction mixture 2. It makes it possible to separate the organic phase containing the crude mecazine corresponding to stream 3 and the aqueous phase containing the activator, for example acetamide, and corresponding to stream 4 according to step (b).

[0092] C represents a countercurrent washing column according to step (d). The organic phase, stream 3 is introduced at the bottom of column C and is countercurrently washed by stream 10 corresponding to the purge of the heterocycles of the pyrazoline family extracted from the trays of the azine hydrolysis column, E according to step (h). Stream 12, corresponding to the aqueous phase at the outlet of washing column C, is then returned to section G corresponding to the thermal regeneration step and concentration of the aqueous phase with stream 4.

[0093] The washed organic phase, stream 5, is sent for purification on a distillation column D according to step (e). This column allows the recovery at the top of a little methyl ethyl ketone recycled in A and allows the elimination at the bottom of the heterocycles of the pyrazoline family present in the azine (not shown).

[0094] The organic phase comprising the distilled azine, stream 6, is then sent to hydrolysis column E. Hydrolysis column E is a distillation column operating under pressure. The distilled azine, 6, is introduced into column E along with water, stream 7, necessary for hydrolysis.

[0095] After the hydrolysis step (f), at the top, after condensation of the vapors and decantation in F, stream 8 is obtained, mainly comprising methyl ethyl ketone, water and a little azine. This phase is recycled to the azine synthesis step A.

[0096] The decanted aqueous phase, stream 9, is returned to the hydrolysis column at the top. Stream 11 corresponds to the hydrazine hydrate solution obtained and recovered at the bottom of the column.

[0097] The examples are given for purely illustrative purposes and are not limiting of the invention. EXAMPLES Example 1 : Preparation process according to the invention

[0098] The process as described for the Figure 1 is implemented.

[0099] Column E operates under the conditions described below: Background temperature 178-190°C Head condenser temperature 160°C Head pressure 7.5 to 9.7 bars absolute Reboiling Temperature 200°C, Pressure 16 bars Distilled azine feed 6 5000 kg / hour Water supply 7 10000 kg / hour

[0100] Hydrazine withdrawal at the bottom 12 t / h of an aqueous solution at 22.2% expressed as hydrazine hydrate (or 14.2% expressed as hydrazine N 2 H 4 )

[0101] After the hydrolysis step (f), at the top, after condensation of the vapors and decantation, approximately 6,500 kg / hour of organic phase, stream 8, is withdrawn, mainly comprising methyl ethyl ketone, water and a little azine. This phase is recycled to the azine synthesis step A.

[0102] The decanted aqueous phase, stream 9, is returned to the hydrolysis column at the top.

[0103] A purge, stream 10, is carried out on the trays of the hydrolysis column at a flow rate of 1477 kg / h, at the point where the heterocycles of the pyrazoline family accumulate. This purge is returned to the washing column C to wash the organic phase from separator B.

[0104] The flow rate readings and analyses carried out around washing column C are reported in table 1: Table 1 washing column C of crude azine organic phase aqueous phase entrance exit gain entrance exit gain flow no. 3 5 5 - 3 10 12 12 - 10 kg / h kg / h kg / h kg / h kg / h kg / h DUDE 315 370 0.3 18 MEC-azine 5416 5565 150 75 38 -37 MEC-hydrazone 0 0 89 2 -88 pyrazolines 5 37 32 28 0 -28 Acetamide 46 0 0 49 WATER 49 318 1137 777 total flow 7006 7401 1477 892

[0105] It can be seen that during the washing of the azine stream 3 by the purge of heterocycles of the pyrazoline family 10, the heterocycles of the pyrazoline family contained in the purge 10 are transferred almost quantitatively into the azine stream 5 at the outlet of the washing column.

[0106] We see that 150 kg / h of azine are thus recovered in the organic phase (stream 5) during the washing of stream 3 by the purge of heterocycles from the pyrazoline family 10. Within the analysis uncertainties, this corresponds to the recovery of 37 and 88 kg of azine and hydrazone contained in this purge 10.

[0107] Pyrazolines are removed during the purification of the azine by distillation of stream 5 giving the distilled azine 6, at the bottom of the distillation column with the heavy residues.

[0108] The hydrazine hydrate solution (titrating 22.2% in hydrazine hydrate or 14.1% in hydrazine N2H4) obtained at the bottom of the column (stream 11) only contains a very low concentration of pyrazoline which does not cause any coloring problems in the final hydrazine hydrate solution obtained after concentration in the process.

[0109] The balance is also carried out around the hydrolysis column E and is reported in table 2: hydrolysis column E washed azine azine washed distilled purge pyrazoline MEC recycled WATER Hydrate hydrazine flow no. 5 6 10 8 7 11 kg / h kg / h kg / h kg / h kg / h kg / h DUDE 370 21 0.3 5206 0 MEC-azine 5565 5523 75 404 0 MEC-hydrazone 0 0 89 0 6 pyrazolines 37 3 28 0 0 2 Acetamide 0 0 0 0 0 WATER 318 21 1137 849 11000 7768 N2H4 hydrazine 0 0 0 0 0 1305 impurities div QSP 792 136 0 330 0 104 total flow 7401 5987 1477 6789 11000 9186

Claims

1. Process for preparing hydrazine hydrate, comprising the following steps: (a) reacting ammonia, hydrogen peroxide and methyl ethyl ketone in the presence of a solution comprising at least one activator to form an azine; (b) treating the reaction mixture from step (a) to isolate: - the aqueous phase comprising the activator(s); and - the organic phase comprising the resulting azine and optionally the unreacted methyl ethyl ketone; (c) optionally recycling the aqueous phase to step (a) after optional treatment; (d) washing the organic phase, preferably in countercurrent; (e) distilling the washed organic phase to recover the azine; (f) hydrolyzing the azine to give hydrazine hydrate and regenerate methyl ethyl ketone, with pyrazoline-class heterocycles being purged; (g) optionally recycling the methyl ethyl ketone obtained in step (f) to step (a); (h) recycling the pyrazoline-class heterocycles purge obtained in step (f) to one at least of steps (a), (b), (c), (d) or (g).

2. Process according to Claim 1, wherein the pyrazoline-class heterocycles purge obtained in step (f) is recycled to one at least of steps (c), (d) or (g).

3. Process according to Claim 1 or 2, wherein the pyrazoline-class heterocycles purge obtained in step (f) is recycled to the organic phase washing step (d).

4. Process according to Claim 3, wherein the pyrazoline-class heterocycles purge is sufficient to perform said organic phase washing, without addition of water.

5. Process according to any one of the preceding claims, wherein the pyrazoline-class heterocycles purge is carried out by withdrawal, preferably by a continuous side takeoff.

6. Process according to any one of the preceding claims, wherein the recycling step (h) is carried out continuously.

7. Process according to any one of the preceding claims, wherein the pyrazoline-class heterocycles are 3,5-diethyl-5-methylpyrazoline and 3,4,5-trimethyl-5-ethylpyrazoline.

8. Process according to any one of the preceding claims, wherein the azine hydrolysis and methyl ethyl ketone regeneration step (f) is performed in a packed or tray distillation column, preferably operating at a pressure of 2 to 25 bar and with a bottom temperature of 150°C to 200°C.

9. Process according to Claim 8, wherein the amount of pyrazoline-class heterocycles is not more than 5% by weight, more preferably between 1% and 3% by weight, relative to the total weight of the liquid phase on the trays of the column or in the parts of the hydrolysis column at which said amount is at its maximum.

10. Process according to any one of the preceding claims, wherein said activator is acetamide.