EP2118053A1

EP2118053A1 - Method for the synthesis of acrylonitrile from glycerol

Info

Publication number: EP2118053A1
Application number: EP08762109A
Authority: EP
Inventors: Jean-Luc Dubois
Original assignee: Arkema France SA
Current assignee: Arkema France SA
Priority date: 2007-02-16
Filing date: 2008-02-18
Publication date: 2009-11-18
Also published as: CA2676669C; JP5579447B2; CA2676669A1; US20100048850A1; JP2010519191A; BRPI0807736A2; UA104992C2; US8829223B2; FR2912742B1; RU2471774C2; MX2009008746A; FR2912742A1; CN101636381A; CN101636381B; RU2009134501A; WO2008113927A1

Abstract

The invention relates to a novel way to synthesize acrylonitrile from a renewable raw material and more particularly relates to a method for producing acrylonitrile by the ammoxidation of glycerol in gaseous phase. The method can be implemented in a single step, or the glycerol can be previously submitted to a dehydration step. The acrylonitrile thus obtained meets the requirements of green chemistry.

Description

PROCESS FOR THE SYNTHESIS OF ACRYLONITRILE FROM GLYCEROL

The present invention relates to a new synthetic route for acrylonitrile from a renewable raw material and more particularly relates to a process for synthesizing acrylonitrile from glycerol.

Current industrial production of acrylonitrile is based essentially on the Sohio process dating back to 1957. This process consists of catalytic gas phase oxidation of propylene by air in the presence of ammonia, a reaction known as ammoxidation. The reaction is as follows:

CH ₂ = CH-CH ₃ + NH ₃ + 3/2 O ₂ * CH ₂ = CH-CN + 3 H ₂ O The reaction is carried out in a fluidized bed reactor, at a temperature generally of between 400 ^° C. and 500 ^° C., and preferably from 420 ° C. to 450 ^° C. for the most modern catalysts, under a pressure that may range from 20 kPa to 300 kPa, and more often from 150 kPa to 300 kPa. Different catalysts are used, such as bismuth-based complexes, or bismuth molybdate, or mixed molybdates of iron and bismuth, or iron antimonate, or phosphomolybdate, or antimony-uranium combinations. Typically a catalyst optimized for this process has a formulation (K, Cs) oi (Ni, Mg, Mn) ₇₅ (Fe, Cr) ₂₃ Bi ₀ 5Mθi ₂ Ox supported on 50% by weight of silica. It is thus possible to obtain about 1 to 1, 1 kg of acrylonitrile per kg of propylene (reference: "Fundamentals of Industrial Catalytic Processes", CH Bartholomew, RJ Farrauto, 2 ^πd Edition, Wiley Interscience, pages 604-609. ). This method however has many disadvantages:

Acrylonitrile obtained by ammoxidation of propylene contains impurities and by-products which must be separated and which are not always valued. The main by-products include hydrogen cyanide (HCN), acetonitrile (CH ₃ CN) and carbon oxides. They result from the disruption of the propylene CC bond during the ammoxidation reaction carried out at high temperature and in the presence of very active catalysts. The hydrocyanic acid, after extraction and purification, can be used in methyl methacrylate synthesis processes for example. On the other hand, acetonitrile, given its limited applications, is usually destroyed. It therefore appears particularly interesting to use a different raw material, which by nature will lead to the desired product by a different reaction mechanism, and generating fewer byproducts.

The ammoxidation reaction of propylene is highly exothermic and therefore requires reactor technologies to effectively remove the heat of reaction, such as for example multitubular reactors or fluidized beds. Because of the significant runaway risks related to the exothermicity of the reaction, dilute streams are used, in particular, a propylene / air / ammonia mixture is generally used, in which propylene only represents a few%. In this configuration, the reactor is sized according to the total amount of gas in the installation, and therefore oversized with respect to the amount of the propylene reactant, which leads to an additional cost for the reactor.

The cost of the installation is also proportional to the amount of heat that must be exchanged in the process. It is therefore essential to minimize thermal losses by reducing the size of the units. It would therefore be particularly advantageous to be able to produce acrylonitrile by a process that allows for much higher partial pressures of reactants and products than in the conventional case of propylene ammoxidation.

In addition, the current process for the ammoxidation of propylene uses air as an oxidant, thus providing the inert gases (nitrogen) which the process requires. In a process with partial pressures in higher reactants, it becomes possible to consider high concentration oxygen, or oxygen enriched air, as an oxidizer. Air is often considered a free reagent while oxygen is commercially available. However, the air must be purified and compressed to be used, which requires not only an investment in a compressor, but also variable costs in electricity; while oxygen is available under pressure and under good qualities. The use of oxygen can then allow substantial reductions in investment on the industrial unit, but also in operating costs in some cases.

Other routes of synthesis of acrylonitrile by ammoxidation reaction are proposed in the state of the art, especially from aldehydes, more particularly from acrolein, from alkane such as propane, or from alcohols.

The patent GB 709,337 of Distillers discloses a process for the preparation of acrylonitrile by ammoxidation of acrolein, used alone or in admixture with the corresponding alcohol (allyl alcohol) or the corresponding acetal. The preferred catalysts are selected from molybdenum catalysts capable of catalyzing the oxidation reaction of benzene to maleic anhydride or naphthalene to phthalic anhydride. Yields between 25% and 60% are obtained. In his upgrading certificate GB 897 226, the company

Distillers obtains acrylonitrile yields greater than 70% compared to acrolein, using as catalysts compounds based on antimony, tin and oxygen. The reaction can be carried out in the presence of an inert gas, such as nitrogen, water vapor, carbon dioxide or propane, butane or isobutane.

BE 628 287 discloses a process for the ammoxidation of acrolein to produce acrylonitrile in yields greater than 87% with an arsenic catalyst and a significant proportion of water vapor as a diluent. Patent EP 558 424 describes a process for the ammoxidation of propane in the presence of a catalyst comprising vanadium, antimony and at least one metal chosen from iron, gallium or indium. Acrylonitrile selectivities of up to 70% are obtained.

US Pat. No. 4,138,430 describes the ammoxidation of n-propanol. The reaction is carried out in the presence of oxygen and ammonia by passing the gas stream successively on two catalyst beds: a first bed of catalyst consisting of boron phosphate, silica-alumina co-precipitated or co-precipitated alumina-tungsten oxide, on which the dehydration reaction of propanol to olefin and a second catalyst bed, carrying out the olefin ammoxidation reaction, consisting of mixed oxides of Fe, Co, Ni, Bi, P, Mo and K, mixed oxides of Fe, Co, W, Bi, Mo and Mg, mixed oxides of Sb and U, bismuth molybdate or bismuth phosphomolybdate, or a mixture of these compounds. The two stages of the process are carried out at the same temperature, ie at a very high temperature of more than 400 ^° C. The advantage of the process lies in the combination of the endothermic dehydration and the exothermic ammoxidation, which allows a better control of the temperature. However, yields of acrylonitrile are not very high, and the proportion of by-products such as acetonitrile, propionitrile and oxides of carbon is important.

More generally, the ammoxidation reaction of propylene, of aldehydes such as acrolein, ketones or alcohols such as propanol or isopropanol, has been the subject of numerous fundamental studies to determine the reaction mechanisms involved, or to study the effect of various parameters, such as the nature of the catalyst or the presence of water vapor as a diluent.

In particular, the article by H. Oka et al in J. Appl. Chem. Biotehnol (1975), 25, p 663-670, relating to the ammoxidation of acrolein, which concludes that this reaction would be 1000 times faster than the ammoxidation of propylene, assuming that the reaction can be in a chemical regime at 400 ^° C.

Mr. Cathala and JE Germain in Bull. Chem. Soc. No. 6, 1971, p 2167-2174, studied the ammoxidation of acrolein at 460 ^° C. on a Bi-Mo-O catalyst which leads to an acrylonitrile selectivity of 87%. According to the authors, the selectivity of the reaction from acrolein is higher than from propylene on the same catalysts.

In the article Bull. Soc. Chim. No. 3-4 (1975) p 731-734, JE. Germain et al compared the ammoxidation of propylene and acrolein on a Sn-Sb-Fe-O type catalyst at about 450 ° C., with or without steam. The ammoxidation of the isopropanol and n-propanol alcohols is, for its part, compared in Bull. Soc. Chim. No. 5-6 (1979) 173-178. The ammoxidation of isopropanol at 460 ^° C. on a Bi-Mo-O catalyst leads to propylene as initial product which gives, at high contact times, CO + CO2, acetonitrile and acrylonitrile. But the nitrile selectivity is very different from that observed in the ammoxidation of propylene, so the maximum conversion to acrylonitrile from isopropanol is only 17%. On a Sn-Sb-O catalyst, the primary product is mainly acetone which leads to acetonitrile mainly. On the same catalyst, in the case of ammoxidation of n-propanol, the primary product is n-propanaldehyde and not acetone. A notable difference in this case is also the absence of propylene. At a conversion close to 100%, the yield of acrylonitrile is only 7.5%.

The Applicant Company has now surprisingly discovered that it is possible to obtain acrylonitrile by ammoxidation of a renewable raw material, such as glycerol.

Glycerol is derived from the methanolysis of vegetable oils at the same time as the methyl esters which are used in particular as fuels or fuels in diesel and domestic fuel (D. Ballerini and G. Hillion, L'actualité chimique - nov. Dec. 2002 - pages 64-69). It is a natural product that has a "green" aura, is available in large quantities and can be stored and transported without difficulty.

A process for producing acrylonitrile directly from glycerol is therefore particularly advantageous since it is not dependent on a raw material of fossil origin, such as propylene, but uses a renewable raw material. A renewable raw material is a natural resource, animal or vegetable, whose stock can be reconstituted over a short period on a human scale. In particular, this stock must be renewed as quickly as it is consumed. In fact, unlike materials made from fossil materials, materials made from renewable raw materials contain ^14C . All carbon samples taken from living organisms (animals or plants) are in fact a mixture of 3 isotopes: ¹² C (representing ~ 98.892%), ¹³ C (~ 1, 108%) and ¹⁴ C (traces: 1, 2.10 ^"12 %) .The ratio ¹⁴ C / ¹² C living tissues is identical to that of the atmosphere In the environment, ¹⁴ C exists in two main forms: in mineral form, that is to say carbon dioxide (CO2) and in organic form say integrated carbon in organic molecules.

In a living organism, the ¹⁴ C / ¹² C ratio is kept constant by the metabolism because the carbon is continuously exchanged with the environment. The proportion of ¹⁴ C being substantially constant in the atmosphere, it is the same in the body, as long as it is alive, since it absorbs this ¹⁴ C as it absorbs ¹² C. The average ratio of ¹⁴ C / ¹² C is about 1, 2x10 ^"12 .

¹² C is stable, that is to say that the number of atoms of ¹² C in a given sample is constant over time. The ¹⁴ C, it is radioactive (each gram of carbon of a living being contains enough isotope

¹⁴ C to give 13.6 disintegrations per minute) and the number of such atoms in a sample decreases over time (t) according to the law: n = no exp (-at) in which: - no is the number of ¹⁴ C at the origin (at the death of the creature, animal or plant),

n is the number of ¹⁴ C atoms remaining at the end of time t,

- a is the disintegration constant (or radioactive constant); it is connected to the half-life. The half-life (or period) is the time after which any number of radioactive nuclei or unstable particles of a given species are halved by disintegration; the half-life T1 / 2 is related to the decay constant a by the formula aT1 / 2 = In 2. The half-life of ¹⁴ C is 5730 years. Given the half-life (T1 / 2) of ¹⁴ C, the ¹⁴ C content is substantially constant since the extraction of the vegetable oil including glycerol is produced, until the manufacture of acrylonitrile and even until the end of its use.

Acrylonitrile obtained from glycerol consists of 100% organic carbon derived from a renewable resource, i.e. it contains of the order of 10 ^{~ 10%} by weight of ¹⁴ C to the total mass of carbon, which could be certified by determination of the ¹⁴ C content according to one of the methods described in ASTM D6866-06 or ASTM D 7026-04, in particular by mass spectrometry methods or by liquid scintillation spectrometry described in ASTM D8866-06. These methods measure the ratio ¹⁴ CV ¹² C of a sample and compare it with the ratio ¹⁴ CV ¹² C of a reference sample of 100% renewable origin, to give a relative percentage of C of renewable origin in the sample .

Such a process therefore meets the criteria associated with the new concept of "green chemistry", within a broader framework of sustainable development.

Another advantage of this process lies in the fact that it generates fewer impurities, the ammoxidation reaction mechanism being different from that of propylene. In addition, since the glycerol raw material is already partially oxidized, its conversion to acrylonitrile is less exothermic than the ammoxidation of propylene, and consequently it is possible to carry out the synthesis in suitable sizing reactors, with high partial pressures in reagents. , while limiting the risks associated with this synthesis.

An object of the present invention is therefore to respond to sustainable development concerns while overcoming the disadvantages of current processes.

The subject of the present invention is therefore a process for the manufacture of acrylonitrile by reaction of ammoxidation of glycerol in the gas phase.

The reaction mechanism is the following: CH ₂ OH-CHOH-CH ₂ OH + NH ₃ + V * O ₂ → CH 2 = CH-CN + 4H ₂ O (1) According to the present invention, the glycerol is reacted in the vapor phase with ammonia and oxygen in the presence of an acid catalyst, in a one-step process.

Glycerol is used pure, or as a concentrated or dilute aqueous solution. Advantageously, it is possible to use an aqueous glycerol solution with a concentration ranging from 10% to 100% by weight. The concentration should not be too high to avoid side reactions such as the formation of glycerol ethers or reactions between acrylonitrile produced and glycerol.

In addition, the glycerol solution should not be diluted too much because of the energy cost induced by the evaporation of the aqueous glycerol solution. The water vapor resulting from the aqueous glycerol solution can vary within wide limits, but for a good implementation of the process according to the invention, the content of reaction gas, defined as the glycerol-ammonia-oxygen sum. is preferably at least 2%, more preferably at least 4% in the mixture including water vapor and inert gases including nitrogen air when it is used as oxidant.

Inert diluent gases can be used under reaction conditions such as helium, nitrogen or argon.

Within the reaction gas, the respective contents of glycerol, ammonia and oxygen can vary within wide limits.

The ammonia / glycerol molar ratio can vary between 1 and 1.5, and preferably between 1 and 1, 2 and the oxygen / glycerol molar ratio can vary between 0.5 and 10 and preferably between 0.5 and 7.

The reaction temperature is in general between 280 ^° C. and 550 ^° C. and, preferably, between 400 ° C. and 500 ^° C.

The total pressure of the reaction mixture may be greater than or equal to atmospheric pressure. It is generally between 1 and 5 bar and preferably between 1 and 4 bar.

The catalyst used in the glycerol ammoxidation process is an acid catalyst which is not saturated with ammonia at the reaction temperature.

The catalyst may comprise one or more mixed oxides chosen, for example, from molybdenum, bismuth, iron, antimony, tin, vanadium, tungsten, antimony, zirconium, titanium, chromium, nickel, aluminum, phosphorus, gallium.

Those skilled in the art will be able to determine the compromise between the temperature, the gas flow and the precise nature of the catalyst used and the various other parameters of the reaction to obtain good productivity.

According to one embodiment of the invention, the reaction is carried out in a single reactor, that is to say according to a direct ammoxidation of glycerol where all the elementary steps take place in the same reactor.

Any device suitable for ammoxidation or vapor phase oxidation reactions may be used. The process can be carried out continuously or batchwise, using a fixed bed, a fluidized bed, a circulating bed, a plate heat exchanger with a modular catalyst arrangement, a micro-structured reactor or a milli-structured reactor.

According to a preferred embodiment of the invention, the glycerol is subjected before a dehydration step acrolein, the reaction mechanism implementing the following 2 consecutive reactions:

CH ₂ OH-CHOH-CH ₂ OH * CH ₂ = CH-CHO + 2H ₂ O (2)

CH ₂ = CH-CHO + NH ₃ + ^_1/2 O ₂ * CH ₂ = CH-CN + 2 H ₂ O (3) In this case, the reaction is carried out using two catalysts in series, a first acid catalyst to d carry out the dehydration reaction of glycerol to acrolein, and a second catalyst for the ammoxidation of acrolein.

In this embodiment, the ammoxidation of acrolein is advantageously carried out without intermediate purification of acrolein and thus in admixture with the by-products derived from the dehydration step of glycerol, such as acetaldehyde or propanaldehyde. . Indeed, the light aldehydes are converted to preferably unsaturated nitriles simultaneously in the ammoxidation step. Moreover, the dehydration of glycerol produced stoichiometrically

2 molecules of water. In fact, the acrolein produced is naturally diluted in water vapor, and all the more so that the glycerol is diluted in water. When the ammonia and oxygen necessary for the ammoxidation reaction are added, the reaction can be carried out in concentrated reagent medium while remaining outside the flammability limits. Under these conditions of highly concentrated medium, it is possible to economically recycle reagents that have not reacted or have been insufficiently converted.

According to a particular embodiment, the two steps of the process are carried out at the same temperature, preferably between 400 ^° C. and 500 ^° C.

According to another embodiment, the two steps of the process are carried out at different temperatures, thus enabling the optimization of each of the two reactions, in terms of the effectiveness of the catalyst used and the limitation of by-products, such as high temperature glycerol degradation products.

The dehydration step of glycerol is carried out in the gas phase in the presence of a catalyst, at a temperature ranging from 150 ^° C. to 500 ^° C., preferably between 250 ^° C. and 350 ^° C., and a pressure of between 1 ^° C. and 5 bars.

The suitable catalysts are homogeneous or multiphasic materials which are insoluble in the reaction medium and which have a Hammett acidity, denoted H _{0 of} less than +2. As disclosed in US Pat. No. 5,387,720 which refers to the article by K. Tanabe et al in "Studies in Surface Science and Catalysis", Vol 51, 1989, Chapters 1 and 2, Hammett's acidity is determined by amino titration. using indicators or adsorbing a base in the gas phase. The catalysts meeting the acidity criterion Ho of less than +2 may be chosen from natural or synthetic siliceous materials or acidic zeolites; inorganic carriers, such as oxides, coated with inorganic acids, mono, di, tri or polyacids; oxides or mixed oxides or heteropolyacids.

Advantageously, the catalysts are chosen from zeolites, Nafion ^® composites (based on sulfonic acid of fluorinated polymers), chlorinated aluminas, phosphotungstic acid and salts of acids and / or silicotungstic, and various metal oxide type solid such as tantalum oxide Ta2θ _5, niobium oxide iN ^ O _5, alumina Al2O3, oxide TiO2 titanium, Z1O2 zirconia, SnO2 tin oxide, SiO2 silica or SiO2-Al ₂ θ3 silico-aluminate, impregnated with acid functions such as BO3 borate, SO ₄ sulfate, WO3 tungstate, PO ₄ phosphate, SiO ₂ silicate, or MOO3 molybdate . According to the data of the literature, these catalysts all have a Hammett acidity H _{0 of} less than +2.

The preferred catalysts are sulphated zirconias, zirconium phosphates, tungsten zirconias, silicified zirconias, sulphated titanium or tin oxides, phosphated aluminas or silicas.

These catalysts all have a Hammett's acidity of less than +2, the acidity H ₀ can then vary to a large extent, up to values up to -20 in the reference scale with the Hammett indicators. The table given on page 71 of the publication on acid-base catalysis (C. Marcilly) Vol 1 at Technip Editions (ISBN 2-7108-0841-2) illustrates examples of solid catalysts in this range of acidity. . It is possible to add molecular oxygen or a molecular oxygen-containing gas for the dehydration step of glycerol. The amount of oxygen is preferably selected to be outside the range of flammability at any point in the installation. The presence of oxygen makes it possible to limit the deactivation of the dehydration catalyst by coking. In addition, the addition of oxygen improves the reaction efficiency for many catalytic systems.

Preferably, the ammonia is introduced only at the second stage.

The step of ammoxidation of the acrolein acrylonitrile is then carried out on an ammoxidation catalyst at a temperature generally between 300 ⁰ C and 550 ⁰ C and, preferably, between 400 ° C and 500 ⁰ C. and at a pressure generally between 1 and 5 bar and preferably between 1 and 4 bar.

The composition of the reaction mixture, acrolein-ammonia-oxygen, can vary within wide limits and inert diluent gases can be used under the reaction conditions such as helium, nitrogen or argon, or nitrogen. air when it is used as an oxidizer. Ammonia / acrolein and oxygen / acrolein molar ratios can vary widely. The molar ratio ammonia / acrolein may vary between 1 and 1.5, and preferably between 1 and 1, 2 and the oxygen / acrolein molar ratio may vary between 0.5 and 10 and preferably between 0.5 and 7. As As a catalyst for the ammoxidation of acrolein, an acid catalyst which is not saturated with ammonia at the reaction temperature will be used. The catalyst may comprise one or more mixed oxides chosen, for example, from molybdenum, bismuth, iron, antimony, tin, vanadium, tungsten, antimony, zirconium, titanium, chromium, nickel, aluminum, phosphorus, gallium.

As ammoxidation catalysts that may be used, mention may be made especially of mixed oxides based on bismuth molybdate, mixed oxides containing at least Fe and Sb, or at least U and Sb, or at least Sn and Sb, or at least Mo and V, and / or W / Nb / Ti / Ta and / or Te / Sb / Bi, as well as oxynitrides containing at least Al and P.

An advantage of this embodiment is a better choice of the catalyst pair. Indeed, the dehydration reaction requires acid catalysts that can be inhibited by the presence of ammonia if they are too acidic. The separation of the two stages makes it possible to individually optimize the operating conditions of each of the two dehydration and ammoxidation reactions.

According to another preferred embodiment of the invention, illustrated by way of example in the single appended figure, the glycerol is subjected beforehand to a dehydration step in acrolein, and a partial condensation of the water and heavy by-products from the dehydration stage.

The use of a prior step of dehydration of glycerol implemented in the form of aqueous solution has the disadvantage of leading to a flow containing not only acrolein produced and by-products, but also a significant amount of water on the one hand, from the glycerol solution, on the other hand from the water produced by the dehydration reaction. The purpose of the partial condensation step is to condense a portion of the water and products having a higher boiling point than acrolein. It is therefore a simplified separation, which produces two streams, the first containing acrolein, and light by-products, such as acetaldehyde, propanaldehyde, acetone and possibly inert gases, CO and CO2, the second rich in water containing heavy by-products such as phenol, hydroxypropanone, and adducts of acrolein on glycerol (acetals), polycondensation products of glycerol, cyclic or non-cyclic glycerol ethers, acid propionic, acrylic acid. The partial condensation unit may be an absorption column coupled or not to a stripper, a heat exchanger, a condenser, a dephlegmator, as well as any apparatus well known to those skilled in the art, making it possible to perform a partial condensation of an aqueous stream. This unit can also be used to heat the aqueous solution of glycerol, thus optimizing the energy cost of the installation.

After the intermediate condensation step, the acrolein-rich stream is sent to the ammoxidation catalyst, adding the ammonia necessary for the reaction and adjusting the partial pressures of acrolein and oxygen reactants and optionally diluting the reaction gas. with an inert gas.

With reference to FIG. 1, the glycerol is introduced in (1) in the form of an aqueous solution, into a first dehydration reactor (10). Molecular oxygen (2) is also introduced, for example in the form of air or in the form of enriched or depleted air with molecular oxygen. The dehydration reaction is carried out in the gas phase in the reactor (10) in the presence of a dehydration catalyst, at a temperature of between 250 ^° C. and 350 ^° C. and at a pressure of between 1 and 5 bars.

The gaseous flow at the outlet of the reactor (10) consists of a mixture comprising acrolein, water, unprocessed glycerol and by-products. By-products include hydroxypropanone, propanaldehyde, acetaldehyde, acetone, phenol, adducts acrolein on glycerol, polycondensation products of glycerol, cyclic glycerol ethers or not.

This flow is sent to a condensing unit (11), which separates, on the one hand, a mixture (3) rich in water containing the heavy by-products, on the other hand a stream (4) rich in acrolein containing the sub-products. -Light products, such as acetaldehyde, propanaldehyde, acetone and possibly inert gases, CO and CO ₂ .

The stream (3) is sent wholly or partly, either to a rectification column in order to recover the light fraction that could be absorbed in this stream, or to a wastewater treatment station. It can also be sent to a thermal oxidizer, or a portion of this stream is recycled to dilute the glycerol to the desired concentration.

The flow (4), rich in acrolein and freed of heavy by-products and most of the water is sent to the ammoxidation reactor (12) comprising a fixed bed of acrolein ammoxidation catalyst.

The reaction is carried out in the presence of molecular oxygen (6) which may be in the form of air or in the form of air enriched or diluted with molecular oxygen, at a content ranging from 3 to 20% by volume, relative to the incoming flow. and in the presence of a gas mixture (5) comprising inert gases and ammonia. The inert gases required for the process may optionally consist wholly or partly of gas (8) obtained at the top of the absorption column (13).

The ammoxidation reaction is carried out at a temperature of between 400 ^° C. and 500 ^° C. and at a pressure of between 1 and 5 bars. The effluent (7) from the ammoxidation stage, rich in acrylonitrile, is then purified on a separation unit (13) to separate, on the one hand, the light products (8) and the unconverted acrolein, and on the other hand acrylonitrile (9) may still contain traces of heavy byproducts.

According to another embodiment of the invention, the ammoxidation reaction of glycerol is carried out in the presence of a gas containing propylene. The propylene-containing gas can be co-fed with glycerol, or it can be fed after the glycerol dehydration reaction. According to another embodiment of the invention, the ammoxidation reaction of glycerol is carried out in the presence of a thermal ballast, such as for example propane or a gas containing propane, methane, ethane or CO2. Thermal ballast is a heat transfer fluid, which can be used to bring calories necessary for the dehydration reaction, but also to bring calories produced during the ammoxidation reaction. The thermal ballast can be introduced together with glycerol, or when the dehydration and ammoxidation steps are disjointed, it can be introduced between these steps. Thermal ballast such as propane is eventually recovered and recycled.

The products of the reaction can be recovered in the effluent gases by any appropriate means. For example, the effluent gases can pass into a condenser containing dilute sulfuric acid to neutralize unconverted ammonia. The gases can then pass through a refrigerated absorbent column to condense acrylonitrile, acetonitrile and hydrogen cyanide. Acrylonitrile can then be isolated from the byproducts by successive distillation.

By the process of the invention, it is possible to obtain acrylonitrile of high purity, with good productivity, while reducing the dependence on a fossil resource such as propylene. The acrylonitrile obtainable by this process contains ¹⁴ C, advantageously acrylonitrile contains ¹⁴ C at a content greater than ^10-11 % relative to the total carbon, which could be certified by measurement according to one of the methods described in ASTM D6866-06.

The acrylonitrile obtainable according to the process of the invention is used to prepare in particular adiponitrile, precursor of nylon, synthetic polymers such as acrylic fibers, synthetic rubber, elastomers, or resins such as copolymers of acrylonitrile, butadiene and styrene (ABS resins) or copolymers of styrene and acrylonitrile (SAN resins). The products thus obtained contain organic carbon from a renewable resource, responding to the concept of green chemistry.

The following examples illustrate the present invention without limiting its scope.

EXAMPLES

Example 1 (with reference to FIG. 2)

Simulation using the ASPEN software was used to illustrate the process according to the invention. Values are given in kmol / hour. For the sake of clarity, only the main constituents are indicated.

A gaseous flow at 331 ^° C. at 2.0 bar (188 kmol / h glycerol, 963 kmol / h water, 426 kmol / h nitrogen, 113 kmol / h oxygen) is sent to a fixed-bed multitubular reactor (10) containing a heterogeneous dehydration catalyst coupled to a molten salt bath. From this reactor a gas stream (14) leaves at 320 ^° C. under 1.7 bar (1336 kmol / h water, 426 kmol / h nitrogen, 78 kmol / h oxygen, 147 kmol / h acrolein, 19 kmol / h acetaldehyde, 25 kmol / h CO, 13 kmol / h CO2). This stream is cooled to 151 ⁰ C in a heat exchanger (15) and sent at the bottom of an absorption column (11) which has 4 theoretical stages. The gaseous stream (16) which leaves at 102 ^° C. at the head of this absorption column is sent to a partial condenser (17) which cools it to 79 ° C. and then to a flash pot (18) which separates the phase gas (28) of the liquid phase (19). This liquid phase (19) is returned to the top of the absorption column (11). At the bottom of the absorption column, a liquid phase (3) is withdrawn at 103 ^° C. (1067 kmol / h water, 4 kmol / h acetic acid, 4 kmol / h formic acid). This liquid phase (3) is sent to the top of a stripping column (20) comprising 8 trays, in which air (21) is injected at the bottom at 90 ° C. under 1.7 bar. At the bottom of this stripping column, an aqueous stream (22) (55 ° C., 976 kmol / h water, 4 kmol / h acetic acid, 4 kmol / h formic acid) is recovered. The gas stream (23) recovered at the top of the stripping column is mixed with the gas phase (24) of the previously described flash pot (79 ° C, 426 kmol / h N ₂ , 145 kmol / h acrolein, 78 kmol / h oxygen, 270 kmol / h water, 18 kmol / h acetaldehyde, 25 kmol / h carbon monoxide, 13 kmol / h carbon dioxide) and a gaseous flow (6) (923 kmol / h nitrogen, 38 kmol / h oxygen, 134 kmol / h water, 41 kmol / h carbon dioxide, 53 kmol / h carbon monoxide) to which is added an ammonia flow (178 kmol / h). The mixture is heated to 300 ^° C. and then injected into a second multitubular reactor (12) comprising an ammoxidation catalyst. At the outlet of this reactor, a gaseous stream (7) is obtained at 1.4 bar (2030 kmol / h nitrogen, 830 kmol / h water, 118 kmol / h acrylonitrile, 52 kmol / h oxygen, 155 kmol / h carbon, 85 kmol / h carbon monoxide). This stream is cooled to 157 ° C and then injected at the bottom of the absorption column (13). At the top of this column, the gas stream is partially condensed in the heat exchanger (24), then sent to a separator pot (25) which produces a liquid phase (26) and a gas phase (8) (nitrogen, oxygen , water, carbon dioxide, carbon monoxide). The liquid phase (26) is returned to the column (13). The gas phase (8) is partially recycled upstream of the reactor (12) via the flow (6). At the bottom of the absorption column (13), there is obtained a stream (9) concentrated acrylonitrile (116 kmol / h acrylonitrile, 587 kmol / h water and other by-products).

It will be noted that the process makes it possible to remove certain impurities produced in the dehydration reactor (10) from the aqueous phase (22): for example, the flow rates of hydroxypropanone and acetic acid in the gaseous flow at the outlet of the dehydration reactor. (10) are respectively 1 and 4 kmol / h. They are respectively 0.02 and 0.3 kmol / h at the inlet of the oxidation reactor (12).

In the examples which follow, the conversion of glycerol, the selectivity to acrolein and the yields in different products are defined as follows:

Conversion of glycerol (%) = 100 - number of moles of glycerol remaining / number of moles of glycerol introduced.

Yield of acrolein (%) = number of moles of acrolein produced / number of moles of glycerol introduced.

Selectivity in acrolein (%) = 100 ^* number of moles of acrolein produced / number of moles of glycerol reacted. The yield of acetone or hydroxypropanone is calculated as for the yield of acrolein.

Yield of acetaldehyde (%) = 2/3 ^* number of moles of acetaldehyde produced / number of moles of glycerol introduced. Yield phenol (%) = 2 ^* number moles of phenol produced / number of moles glycerol introduced.

All the results are expressed in molar percentages with respect to the incoming glycerol.

Example 2 Preparation of acrolein

A tubular reactor consisting of a 35 cm long tube and an internal diameter of 22 mm was used to carry out the dehydration reaction of glycerol in the gas phase at atmospheric pressure. This reactor is placed in a heated enclosure maintained at the reaction temperature of 300 ^° C., unless otherwise indicated. The catalyst used is milled and / or pelletized to obtain particles of 0.5 to 1.0 mm. 10 ml of catalyst are loaded into the reactor to form a catalytic bed of length 5 cm. This is brought to the reaction temperature for 5 to 10 minutes before the introduction of the reagents. The reactor is fed with an aqueous solution containing 20% by weight of glycerol with an average feed rate of 12 ml / h, and with a flow rate of 0.8 l / h of molecular oxygen for the examples according to the invention. . In this case, the relative proportion O 2 / vaporized glycerol / water vapor is 6 / 4.5 / 89.5. The aqueous glycerol solution is vaporized in the heated chamber and then passes over the catalyst. The calculated contact time is of the order of 2.9 s. After reaction, the products are condensed in a trap refrigerated with crushed ice.

The total mass of input and output products is measured, which makes it possible to perform a mass balance. Similarly, the products formed are analyzed by chromatography. The products thus quantified are unreacted glycerol, acrolein formed, and by-products such as hydroxypropanone, acetaldehyde, propanaldehyde, acetone and phenol. In this example, the catalyst (10 ml) tested is a tungsten zirconia (90.7% ZrO 2 - 9.3% WO 3) from Daiichi Kigenso (supplier reference H1417). The catalyst is characterized by a loss on ignition at 1000 ^° C. of 1.75% and a specific surface area of 47.4 m ² / g (BET, 1 point). The results are shown in the following table:

The acrolein produced does not contain hydroxypropanone or phenol.

Example 3 Synthesis of Acrylonitrile

A Pyrex reactor is charged with a catalyst bed. The reactor is provided with a frit to retain the catalyst. A mass of 6.578 g of propylene oxidation catalyst is first charged with acrolein produced by Nippon Shokubai reference ACF4, used in this example as an ammoxidation catalyst, although not optimized for this reaction, diluted with 7 ml of 0.125 mm silicon carbide grain size. Next, 0.125 mm silicon carbide beds having a particle size of 2 ml and then 7 ml of 0.5 mm. And finally, the reactor is completed with 1.19 mm silicon carbide to the top of the reactor.

The reactor is then connected to the test facility. The temperature of the catalyst is regulated at 420 ^° C. and the WH is adjusted to 1200 h ^-1 The reactor is fed with a gaseous mixture of 4.5% acrolein / 8.7

% oxygen / 5.4% ammonia / (rest) helium-krypton / 15% water. The helium-Krypton gas mixture contains 4.92% krypton which serves as internal standard. The water-acrolein mixture from Example 2, vaporized upstream of the reactor, is used after concentration. The effluents are collected at the outlet of the reactor by a cold ice trap and the acrylonitrile produced is assayed by chromatographic analysis.

The yield of acrylonitrile is 60%.

Claims

1) A process for producing acrylonitrile by glycerol ammoxidation reaction in the gas phase.

2) Process according to claim 1, characterized in that one uses pure glycerol or in the form of aqueous solution of concentration ranging from 10% to 100%.

3) Process according to claim 1 or 2, characterized in that the glycerol is subjected before a dehydration reaction.

4) Process according to claim 3, characterized in that it implements an intermediate step of partial condensation of water and heavy by-products from the dehydration step.

5) Process according to any one of the preceding claims, characterized in that the glycerol ammoxidation reaction is carried out in the presence of a gas containing propylene.

6) Process according to any one of the preceding claims, characterized in that the ammoxidation reaction of glycerol is carried out in the presence of a thermal ballast.

7) Acrylonitrile obtainable by the method according to any one of the preceding claims, characterized in that it contains ¹⁴ C at a content greater than ^10-11 % relative to the total carbon.

8) Use of acrylonitrile according to claim 7 for preparing adiponitrile, synthetic polymers such as acrylic fibers, synthetic rubber, elastomers, or resins such as copolymers of acrylonitrile, butadiene and styrene (ABS resins) or copolymers of styrene and acrylonitrile (SAN resins).