Process for production of acrylic acid in a multi-tubular reactor

Abstract

Process for converting a liquid feedstock based on hydroxypropanoic acid into acrylic acid, comprising the following steps: a) a step of dispersing said liquid feedstock in "n" tubes, where n≥2, so as to obtain a feedstock in the form of droplets with an average diameter of less than 1 mm; b) a step of vaporizing said liquid feedstock in the form of droplets in "n" tubes, where n≥2, by bringing said feedstock into contact with a hot gas at a temperature of between 350°C and less than 550°C in order to obtain a gaseous mixture; c) a step of transferring said gaseous mixture to step d) in a downward flow for a period of time of less than 20 seconds; d) a step of catalytically dehydrating the gaseous mixture from step c).

Description

[0001] Process for production of acrylic acid in a multi-tubular reactor

[0002] Technical field

[0003] The present invention lies in the field of the production of acrylic acid and / or its derivatives from hydroxypropanoic acid and / or its derivatives in a downflow reactor.

[0004] Prior art

[0005] Acrylic acid and its derivatives, or a mixture thereof, is a very important intermediate involved in the synthesis of numerous polymers, paints, absorbents, detergents, adhesives, coatings, etc. The main industrial process for producing acrylic acid is based on the oxidation of propene. At a time of energy transition, and in the context of an inexorable depletion of fossil resources, the production of a large number of intermediates should be reconsidered. The main alternative is the use of biomass, a chemically complex resource that requires the rethinking of all the reactions involved and the development of the associated industrial processes.

[0006] Many attempts have been made over the past 50 years to make acrylic acid and its biobased derivatives, such as lactic acid (known under the name 2-hydroxypropanoic acid) and glycerine. Among these various biobased sources, it is only lactic acid that is currently produced with a high yield and purity from sugars, and that would make it possible to produce acrylic acid with a competitive cost relative to acrylic acid produced from a petroleum source. This production is performed by dehydration of lactic acid in the gas phase. However, due to the heat-sensitive nature of lactic acid (and of its derivatives) and parallel and secondary reactions, it is difficult to obtain high yields of acrylic acid. This is why the catalyst plays an important role in the orientation of the selectivity. The WO 17 / 040383 patent family claims several generations of catalysts, all composed of a mixture of alkali metal phosphates, including some of formula MxPOy(M = K or Cs).

[0007] To produce acrylic acid and / or its derivatives from liquid hydroxypropanoic acid, and its derivatives, two steps are necessary:

[0008] - a vaporization step in which the solution of hydroxypropanoic acid and / or its derivatives is vaporized and brought to the reaction temperature;

[0009] - a catalytic reaction step in which the hydroxypropanoic acid and / or its derivatives in gas form is converted into acrylic acid on contact with a solid catalyst. Unfortunately, in aqueous solutions of hydroxypropanoic acid and its derivatives, numerous chemical species that are likely to react upstream, inside or downstream of the reactor are also present.

[0010] Indeed, hydroxypropanoic acid and its derivatives degrade in the liquid phase by polymerization to form oligomers, all the more so when the temperature is high. Consequently, when it is vaporized, the residence time of the hydroxypropanoic acid in the liquid phase at high temperature is advantageously reduced so as to not degrade the reactant. On the other hand, hydroxypropanoic acid also degrades in the gas phase by thermal cracking to form byproducts, such as, without this being exhaustive, acetaldehyde and carbon monoxide and dioxide. The kinetics of degradation in the gas phase are dependent on the temperature. The processes described in the literature have yields of acrylic acid and derivatives of less than 70%, low selectivity with significant yields of secondary products, long residence times in the catalytic bed, and a fairly rapid deactivation of the catalyst, linked to deposits of by-products thereon.

[0011] Patent US9452967 proposes a process for conversion by catalytic dehydration of hydroxypropanoic acid and its derivatives into acrylic acid with a short residence time and a low yield of secondary products. This patent mentions the introduction of the liquid feedstock of lactic acid and water into the reactor through an atomization nozzle which generates average droplet diameters of less than 500 pm. A large majority of atomization technologies are cited. Once vaporized, the gaseous mixture is transferred to the catalytic dehydration bed. The dehydration catalyst used comprises the monohydrogen phosphate and dihydrogen phosphate anions described by the formulae [HPO4]2' and [H2PO4]_and at most two cations, such that the [HPO4]2' / [H2PO4]_molar ratio is between 0.1 and 10. However, although the dispersion of the hydroxypropanoic acid solution in the form of droplets allows a rapid vaporization thereof, controlling the residence time distribution in the vaporization chamber is difficult and back-mixing phenomena of the liquid and / or gas phase may be encountered, contributing to lengthening the residence times, and consequently the formation of secondary products.

[0012] The Applicant has developed a process for converting hydroxypropanoic acid into acrylic acid by means of a novel reactor configuration which makes it possible to (i) rapidly disperse and vaporize the liquid feedstock while limiting the degradation of the hydroxypropanoic acid both in the liquid phase and in the gas phase, (ii) rapidly transfer the gaseous mixture into contact with the catalyst in order to limit the residence time and therefore the degradation of the hydroxypropanoic acid in the gas phase. The present invention therefore proposes a process for converting hydroxypropanoic acid and its derivatives into acrylic acid with high yields.

[0013] Subjects of the invention

[0014] A subject of the present invention is a process for converting a liquid feedstock based on hydroxypropanoic acid, on hydroxypropanoic acid derivatives or mixtures thereof into acrylic acid, into acrylic acid derivatives or mixtures thereof by means of at least one co-current downflow reactor, said process comprising at least the following steps: a) a step of dispersing said liquid feedstock in “n” tubes, where n>2, so as to obtain a feedstock in the form of droplets with an average diameter of less than 1 mm; b) a step of vaporizing said liquid feedstock in the form of droplets obtained on conclusion of step a) in “n” tubes, where n>2, by bringing said feedstock into contact with a hot gas chosen from inert gases or water vapour at a temperature of between 350°C and less than 550°C in order to obtain a gaseous mixture; c) a step of transferring said gaseous mixture obtained on conclusion of step b) to step d) of catalytic dehydration in a downward flow for a period of time of less than 20 seconds; d) a step of catalytically dehydrating the gaseous mixture from step c) by bringing said gaseous mixture into contact with a fixed-bed dehydration catalyst at a temperature of between 180°C and 450°C, at a pressure of between 0.1 MPa and 12 MPa and a weight hourly space velocity of between 0.01 IT1and 100 IT1.

[0015] Advantageously, the total duration of step a) and b) is less than 180 milliseconds.

[0016] Advantageously, the duration of step c) is less than 10 seconds.

[0017] Preferably, in step a) said liquid feedstock is dispersed by means of an atomization nozzle placed in the upper part of each “n” tube.

[0018] Preferably, said droplets obtained on conclusion of step a) have an average diameter of less than 500 pm.

[0019] In one or more embodiments according to the invention, step c) is carried out in the “n” tubes used in steps a) and b).

[0020] Advantageously, “n” is between 2 and 10 000.

[0021] In one or more embodiments according to the invention, step d) is carried out in “m” tubes comprising said fixed-bed catalyst, m>2, it being understood that m>n.

[0022] Advantageously, “m” is between 2 and 20 000. In one or more embodiments according to the invention, step d) is carried out in “m” tubes, where m>n.

[0023] In one or more embodiments according to the invention, step d) is carried out in “m” tubes, where m=n.

[0024] In one or more embodiments according to the invention, each “n” tube used in steps a), b) and c) is aligned along the longitudinal axis and superposed on each “m” tube used in step d).

[0025] Advantageously, said liquid feedstock comprises between 55% and 85% by weight of hydroxypropanoic acid, of hydroxypropanoic acid derivatives or mixtures thereof relative to the total weight of said liquid feedstock.

[0026] Preferably, in step d) the concentration of hydroxypropanoic acid, of hydroxypropanoic acid derivatives or mixtures thereof is between 0.5 mol% and 10 mol%.

[0027] Advantageously, the catalyst used in step d) comprises an active phase based on salts of sulfates, of phosphates or a mixture of these salts and a support comprising at least one refractory oxide.

[0028] Description of the figures

[0029] Figure 1 shows a co-current downflow reactor according to a first embodiment comprising a zone for dispersion A and vaporization B in n=4 tubes, a transfer zone C and a zone for catalytic dehydration D of the hydroxypropanoic acid to acrylic acid.

[0030] Figure 2 shows a co-current downflow reactor according to a second embodiment comprising a zone for dispersion A and vaporization B in n=4 tubes, a transfer zone C and a zone for catalytic dehydration D in m=4 tubes, in which each “n” tube is aligned along the longitudinal axis and superposed on each “m” tube.

[0031] Figure 3 shows a co-current downflow reactor according to a third embodiment comprising a zone for dispersion A and vaporization B in n=4 tubes, a transfer zone C and a zone for catalytic dehydration D in m tubes, where m > n.

[0032] Detailed description

[0033] Definitions

[0034] In this text, the conversion is defined as the quantity of hydroxypropanoic acid (and / or its derivatives) converted during the reaction according to the following equation:

[0035] [Math 1]

[0036] Likewise, the yield of and the selectivity for a compound “i” are calculated as follows:

[0037] [Math 2]

[0038] Yield of component i (%) = 100

[0039] [Math 3]

[0040] The weight hourly space velocity (WHSV) is defined as follows:

[0041] [Math 4]

[0042] Mass flow rate of hydroxy acid at the inlet I — j WHSV (s-1) = - - - — - - Admass of cata kg)

[0043] In the text that follows, the average diameter of the liquid droplets of the process according to the invention is measured by the laser particle size analysis technique.

[0044] An objective of the invention is to convert hydroxypropanoic acid and / or its derivatives into acrylic acid and / or its derivatives. The reaction takes place in the gas phase, on contact with a catalyst. The hydroxypropanoic acid feedstock is available in the liquid phase mixed with water and is heated before coming into contact with the catalyst. Several constraints have been identified:

[0045] - hydroxypropanoic acid and its derivatives degrade in the liquid phase by polymerization to form oligomers. Consequently, when it is vaporized, the residence time of the hydroxypropanoic acid in the liquid phase at high temperature is advantageously reduced so as to not degrade the reactant;

[0046] - hydroxypropanoic acid and its derivatives degrade in the gas phase by thermal cracking to form by-products such as acetaldehyde and carbon monoxide and dioxide. The kinetics of degradation in the gas phase are highly dependent on the temperature. Consequently, during the vaporization, the vaporized hydroxypropanoic acid is preferably not brought into contact with an excessively hot gas so as to limit the degradation in the gas phase. In addition, once vaporized, the hydroxypropanoic acid is preferably rapidly brought into contact with the solid catalyst so as to limit its residence time in the gas phase.

[0047] In order to overcome these problems and maximize the yield of acrylic acid and / or derivatives, the Applicant has developed a process which makes it possible to reduce the contact time in the liquid phase and in the gas phase by carrying out the dispersion and the vaporization of the liquid phase in a specific multi-tubular reactor. Specifically, the process according to the invention involves dispersing the liquid hydroxypropanoic acid feedstock in the form of droplets which vaporize on contact with a hot gas. The dispersion and the vaporization take place in a multi-tubular technology so as to promote plug flow and control the residence time distribution of the liquid phase and the gas phase in each tube. Once the hydroxypropanoic acid has vaporized, the gaseous mixture is brought into contact with the catalyst in order to convert the hydroxypropanoic acid into acrylic acid.

[0048] According to the invention, the process for converting a liquid feedstock based on hydroxypropanoic acid, on hydroxypropanoic acid derivatives or mixtures thereof into acrylic acid, into acrylic acid derivatives or mixtures thereof by means of at least one co-current downflow reactor comprises at least the following steps: a) a step of dispersing said liquid feedstock in “n” tubes, where n>2, so as to obtain a feedstock in the form of droplets with an average diameter of less than 1 mm; b) a step of vaporizing said liquid feedstock in the form of droplets obtained on conclusion of step a) in “n” tubes, where n>2, by bringing said feedstock into contact with a hot gas chosen from inert gases or water vapour at a temperature of between 350°C and less than 550°C in order to obtain a gaseous mixture; c) a step of transferring said gaseous mixture obtained on conclusion of step b) to step d) of catalytic dehydration in a downward flow for a period of time of less than 20 seconds; d) a step of catalytically dehydrating the gaseous mixture from step c) by bringing said gaseous mixture into contact with a fixed-bed dehydration catalyst at a temperature of between 180°C and 450°C, at a pressure of between 0.1 MPa and 12 MPa and a weight hourly space velocity (WHSV) of between 0.01 IT1and 100 IT1.

[0049] Steps a), b), c) and d) may be carried out within the same reactor. Preferably, steps a), b), c) and d) are carried out within one single reactor.

[0050] The steps of the process are explained in detail below. the liquid feedstock: According to the invention, the liquid feedstock is dispersed in the form of droplets with an average diameter of less than 1 mm so as to provide a large surface area for exchange between the sum of the droplets and the hot gas in order to achieve high vaporization rates and therefore rapid vaporization. Preferably, the average diameter of the droplets is less than 500 pm, and even more preferably less than 100 pm. These droplets are obtained by any dispersion means known to those skilled in the art (injector, pressurized tube, atomization nozzle, etc.). Preferably, atomization nozzles are used and placed in the upper part of each “n” tube. Without this being exhaustive, mention may be made of pressurized nozzles, two- fluid nozzles, rotating nozzles and vortex nozzles.

[0051] Advantageously, step a) is performed in “n” tubes, where “n” is between 2 and 10 000, preferably between 5 and 5000, and more preferentially between 10 and 1000.

[0052] Step b) of vaporizing the liquid feedstock:

[0053] On conclusion of step a), the liquid feedstock droplets are then vaporized by bringing them into contact with a hot gas in a multi-tubular reactor containing n tubes so as to limit the residence time in the liquid phase and in the gas phase. Advantageously, “n” is between 2 and 10 000, preferably between 5 and 5000, and even more preferably between 10 and 1000. Step b) is preferably carried out in the “n” tubes used in step a).

[0054] The hot gas injected in step b) is chosen from inert gases such as nitrogen, helium, argon, air, carbon monoxide, carbon dioxide, or water vapour; preferably, said hot gas is a mixture of water vapour and an inert gas. Optionally, some of the hot gas comes from the recycling of the gases produced.

[0055] Preferably, the hot gas is injected in step b) at a temperature of between 350°C and 550°C, preferably between 380°C and 500°C, and even more preferably between 400°C and 480°C.

[0056] The total duration of step a) and b) is less than 180 milliseconds, preferably less than 150 milliseconds, and even more preferentially less than or equal to 100 milliseconds so as to reduce the contact time in the liquid and gas phase.

[0057] Step c) of transferring the gas phase:

[0058] The transfer step makes it possible to transfer the gaseous mixture from step b) to the fixed- bed dehydration catalyst of step d). Said step also makes it possible to homogenize the compositions and the temperature of the gases leaving the “n” vaporization tubes.

[0059] In order to reduce the residence time in the gas phase, the duration of step c) is less than 20 seconds, more preferentially less than 10 seconds, even more preferentially less than 1 second, and even more preferably less than or equal to 500 milliseconds. In one or more embodiments according to the invention, step c) is carried out in the “n” tubes used in steps a) and b).

[0060] Optionally, the gaseous mixture in step c) is mixed with an additional hot gas chosen from the inert gases nitrogen, helium, argon, air, carbon monoxide, carbon dioxide, or water vapour so as to bring the gaseous mixture to the temperature of the catalytic process of step d).

[0061] Step d) of catalytic dehydration:

[0062] According to the invention, a step d) of catalytically dehydrating the gaseous mixture from step c) is carried out by bringing said gaseous mixture into contact with a fixed-bed dehydration catalyst at a temperature of between 180°C and 450°C, at a pressure of between 0.1 MPa and 12 MPa and a weight hourly space velocity (WHSV) of between 0.01 IT1and 100 IT1.

[0063] The concentration of hydroxypropanoic acid, of hydroxypropanoic acid derivatives or mixtures thereof, at the inlet of step d), is between 0.5 mol% and 10 mol%, preferably between 0.5 mol% and 5 mol%, and even more preferentially between 1 mol% and 5 mol%.

[0064] In this step d), the weight hourly space velocity of the gaseous mixture is preferably between 0.05 h'1and 10 IT1, more preferentially between 0.1 IT1and 0.3 IT1.

[0065] The temperature is between 180°C and 450°C, preferably between 250°C and 400°C, and even more preferably between 350°C and 390°C.

[0066] The pressure of the catalytic dehydration of step d) is between 0.1 MPa and 12 MPa, preferably between 0.2 MPa and 6 MPa, and even more preferably between 0.5 MPa and 5 MPa.

[0067] The partial pressure of water of the catalytic dehydration of step d) is preferably between 0.1 MPa and 5 MPa, preferably between 0.1 MPa and 3 MPa, and even more preferably between 0.1 MPa and 2 MPa so as to maintain the catalytic activity of the catalyst.

[0068] Optionally, the temperature of the catalytic dehydration zone is regulated by means of a hot heat-transfer fluid so as to compensate for the endothermicity of the catalytic reaction. This fluid may be either liquid or gaseous according to the embodiment.

[0069] The catalyst used in step d) preferably comprises an active phase based on salts of sulfates, of phosphates or a mixture of these salts and a support comprising at least one refractory oxide. Preferably, the catalyst consists of a macroporous material based on at least one potassium phosphate salt and / or at least one caesium phosphate salt, and on silica.

[0070] The catalytic dehydration step is carried out in a fixed bed, either in a single catalytic bed (cf. Figure 1) or in a multi-tubular technology comprising m tubes, where m>2 (cf. Figures 2 or 3). If step d) is carried out in a multi-tubular reactor comprising m tubes, the number m of catalytic dehydration tubes may be either equal to (of. Figure 2) or greater than the number n of vaporization tubes (cf. Figure 3). If the number of catalytic dehydration tubes “m” is equal to the number of vaporization tubes n, the vaporization tubes n may be aligned along the longitudinal axis and superposed on the catalytic dehydration tubes. In this case, the four steps a), b), c) and d) are carried out in the same tubes (cf. Figure 2).

[0071] Advantageously, the walls of the tubes, and catalytic reactors, will be selected from, without this being exhaustive, quartz, titanium, tantalum, stainless steel, Inconel, taking into account the corrosive nature of this type of feedstock but also the reactivity of certain surfaces.

[0072] The feedstock:

[0073] According to the invention, the feedstock supplying the process for converting hydroxypropanoic acid and / or its derivatives into acrylic acid advantageously comprises at least one compound included in the list of hydroxypropanoic acid and its derivatives. Said feedstock advantageously comprises a hydroxypropanoic acid chosen from 2- hydroxypropanoic acid and 3-hydroxypropanoic acid. Preferably, said feedstock comprises 2- hydroxypropanoic acid. Said feedstock preferably comprises between 1% and 99.9% by weight, preferably between 10% and 99.5% by weight, very preferably between 50% and 99% by weight and even more preferably between 55% and 85% by weight of hydroxypropanoic acid and / or its derivatives relative to the total weight of the feedstock. Preferably, said liquid feedstock is based on lactic acid.

[0074] Said feedstock may also comprise impurities linked, in particular, to the processes for obtaining the hydroxypropanoic acid and / or its derivatives, such as fermentation. The content of impurities is preferably less than 10% by weight of said feedstock. The hydroxypropanoic acid(s) and / or its (their) derivatives included in said feedstock may be of any origin, chemical, petrochemical or biobased.

[0075] Said feedstock advantageously comprises between 0.1% and 99% by weight, preferably between 0.05% and 90% by weight, very preferably between 1% and 50% by weight and even more preferably between 2% and 40% by weight of water relative to the total weight of the feedstock.

[0076] The sum of the contents of hydroxypropanoic acid and its derivatives, of water, and of any impurities represents 100% by weight of the feedstock.

[0077] The

[0078] The main products obtained by said process are the unconverted hydroxypropanoic acids and / or their derivatives, and the acrylic acid and / or its derivatives resulting from the dehydration reaction. Without this being exhaustive, the by-products of the reaction are carbon monoxide, carbon dioxide, oxygen, nitrogen monoxide, nitrogen dioxide, acetaldehydes, propanoic acid, acetic acid, pyruvic acid, 1 ,2-pentanedione, hydroxyacetones, oligomer-type acrylic acid derivatives.

[0079] The gaseous products leaving the catalytic bed are then advantageously rapidly condensed by cooling so as to limit the residence time in the gas phase.

[0080] The process according to the invention may be performed in at least one co-current downflow reactor according to several embodiments according to the invention.

[0081] Figure 1 presents a first embodiment in which the liquid feedstock 1 is dispersed in the form of droplets through a dispersion system A in n>2 tubes and then brought into contact with a hot gas 2 in order to be vaporized in the vaporization zone B in these same n>2 tubes. The gaseous mixture from these n>2 vaporization tubes is collected in the homogenization zone C. A single fixed-bed catalytic reactor is used for the step of catalytic dehydration D so as to convert the hydroxypropanoic acid into acrylic acid 3.

[0082] Figure 2 presents a second embodiment in which the dispersion A, the vaporization zone B, the homogenization zone C, and the catalytic dehydration D occur in the same tubes. Specifically, the liquid feedstock 1 is dispersed in the form of droplets through a dispersion system A in n>2 tubes and then brought into contact with a hot gas 2 in order to be vaporized in the vaporization zone B in these same n>2 tubes. The gaseous mixture leaving each vaporization zone is sent, after passing through a homogenization zone C, into m = n catalytic dehydration D tubes for converting the gaseous hydroxypropanoic acid into acrylic acid 3. In this embodiment, the process is performed with n>2 tubes and “m” tubes where m=n. Each “n” tube used in steps a), b) and c) is aligned along the longitudinal axis and superposed on each “m” tube used in step d).

[0083] Figure 3 shows a third embodiment in which the liquid feedstock 1 is dispersed in the form of droplets through a dispersion system A in n>2 tubes and then brought into contact with a hot gas 2 in order to be vaporized in the vaporization zone B in these same n>2 tubes. The gaseous mixture from these n>2 vaporization tubes is collected in the homogenization zone C and then sent into m>n tubes filled with catalyst for the catalytic dehydration D of the hydroxypropanoic acid and / or its derivatives into acrylic acid and / or its derivatives 3.

[0084] Examples In the examples below, the composition of the gas phase is analysed by gas chromatography. The liquid phase is analysed by high-performance liquid chromatography (HPLC) and by measuring the total organic carbon (TOC).

[0085] The catalyst used is produced as follows: a source of colloidal silica sol (12.4%) and a source of potassium dihydrogenphosphate (KH2PO4; Aldrich) (15.2%) powder milled and screened to 100 pm are introduced into and mixed in the container of a planetary centrifugal mixer. The suspension obtained, a Methocel™ (K15M) powder (3%) and a precipitated silica powder (Nyasil20; Nyacol) (72.4%) are introduced into and premixed in a kneader. Water is added dropwise until a paste is obtained. The paste obtained is then extruded using a cylindrical die with a diameter of 1.6 mm. The extrudates are dried at 120°C. Finally, the extrudates are subjected to a calcination temperature of 800°C for 4 hours. A material precursor C2 is obtained.

[0086] Subsequently, 2.8 grams of potassium hydrogenphosphate (K2HPO4, M = 174 g / mol) and 2.1 grams of ammonium hydrogenphosphate ((NH4)2HPO4, M = 132 g / mol) are first of all dissolved in 4.2 ml of distilled water at ambient temperature. After dissolution is complete, the mixture is added dropwise to 20 grams of material precursor C2. The solid is matured at ambient temperature for 40 minutes and then dried under air at 120°C for 10 hours. It is then calcined under air at 450°C for 4 hours in order to prepare the catalyst used.

[0087] Compositional analysis of the above material by XRF gives a content by weight of K of 8.7% and of P of 7.5%. The average equivalent content by weight of KPO3 after calcination is 27%.

[0088] In the examples that follow, a liquid feedstock of 24 tonnes / hour containing 70% by mass of lactic acid and 30% by mass of water is converted into acrylic acid on contact with the solid catalyst described above. The reaction takes place in the gas phase, at 1.1 MPa absolute and at a temperature of 380°C. The concentration of lactic acid at the inlet of the catalyst (at the inlet of step d)) is 2.5 mol%.

[0089] 1 (in accordance with the ii

[0090] This example corresponds to the configuration of the first embodiment of Figure 1 in which the liquid feedstock 1 is dispersed in the form of droplets through a dispersion system A in n>2 tubes and then brought into contact with a hot gas 2 in order to be vaporized in the vaporization zone B in these same n>2 tubes. The gaseous mixture from these n>2 vaporization tubes is collected in the homogenization zone C. A single fixed-bed catalytic reactor is used for the step of catalytic dehydration D of the hydroxypropanoic acid and its derivatives to acrylic acid and its derivatives 3. The liquid feedstock is injected in the form of droplets into each tube via a single-fluid atomization nozzle with a flow rate of 8.8 l / min. The average diameter of the droplets generated is 50 pm and the spray angle is 80°. Consequently, in order to respect the total flow rate of the liquid feedstock, the vaporization zone contains 42 tubes. During the vaporization, the spray diameter is 8 cm. The diameter of each tube is set at 25 cm. Considering a triangular arrangement and a tube spacing of 27 cm, the diameter of the reactor is 4.4 m.

[0091] The heat necessary for the vaporization of the liquid feedstock is provided via a stream of water vapour of 123.8 tonnes / hour at 520°C. Under these conditions, the vaporization time and the length of the spray until vaporization is complete are 100 ms and 10 cm, respectively. The temperature of the gas after vaporization is about 370°C with degradation of the lactic acid into acrylic acid, carbon monoxide and dioxide and other by-products of less than 10%. The height of the tubes is set at 15 cm so as to limit the residence time in the gas phase before the inlet into the catalyst.

[0092] The height of the homogenization zone is 5 cm, meaning a residence time of the lactic acid in the gas phase of less than 100 ms. Based on a WHSV of 0.2 IT1, the mass of catalyst necessary for the reaction is 84 tonnes, distributed in a single catalytic bed, giving a height of the catalytic zone of 6.5 m. Under these conditions, the estimated pressure loss is 0.12 MPa and the final production of acrylic acid is 12 tonnes / hour. A yield of acrylic acid of 89% is obtained.

[0093] 2 (in accordance with the ii

[0094] This example illustrates the second embodiment of Figure 2 in which the dispersion A, the vaporization zone B, the homogenization zone C, and the catalytic dehydration D occur in the same tubes. Specifically, the liquid feedstock 1 is dispersed in the form of droplets through a dispersion system A in n>2 tubes and then brought into contact with a hot gas 2 in order to be vaporized in the vaporization zone B in these same n>2 tubes. The gaseous mixture leaving each vaporization zone is sent, after passing through a homogenization zone C, into m = n catalytic dehydration D tubes for converting the gaseous hydroxypropanoic acid into acrylic acid 3. In this embodiment, the process is performed with n>2 tubes and “m” tubes where m=n. Each “n” tube used in steps a), b) and c) is aligned along the longitudinal axis and superposed on each “m” tube used in step d).

[0095] The liquid feedstock is injected in the form of droplets into each tube via a single-fluid atomization nozzle with a flow rate of 5 l / h. The average diameter of the droplets generated is 50 pm and the spray angle is 80°. Consequently, in order to respect the total flow rate of the liquid feedstock, the vaporization zone contains n=4335 tubes. During the vaporization, the spray diameter is 8 cm. The diameter of each tube is set at 8 cm. Considering a triangular arrangement and a tube spacing of 8 cm, the diameter of the reactor is 10.8 m.

[0096] The heat necessary for the vaporization of the liquid feedstock is provided via a stream of water vapour of 123.8 tonnes / hour at 520°C. Under these conditions, the vaporization time and the penetration of the spray are 100 ms and 10 cm, respectively. The temperature of the gas after vaporization is about 370°C with degradation of the lactic acid into acrylic acid and carbon monoxide and dioxide of less than 10%. The height of the n tubes for the vaporization and transfer step is 20 cm, meaning a residence time of the lactic acid in the gas phase of less than 500 ms.

[0097] Based on a WHSV of 0.2 h’1, the mass of catalyst necessary for the reaction is 84 tonnes (distributed in m tubes=n tubes=4335), giving a height of the catalytic zone of 5 m. Under these conditions, the estimated pressure loss is 0.06 MPa and the final production of acrylic acid is 12 tonnes / hour. A yield of acrylic acid of 89% is obtained.

[0098] Example 3 (in accordance with the invention'):

[0099] This example illustrates the third embodiment of Figure 3 in which the liquid feedstock 1 is dispersed in the form of droplets through a dispersion system A in n>2 tubes and then brought into contact with a hot gas 2 in order to be vaporized in the vaporization zone B in these same n>2 tubes. The gaseous mixture from these n>2 vaporization tubes is collected in the homogenization zone C and sent into m>n tubes filled with catalyst for the catalytic dehydration D of the hydroxypropanoic acid and its derivatives to acrylic acid and its derivatives 3.

[0100] The liquid feedstock is injected in the form of droplets into each tube via a single-fluid atomization nozzle with a flow rate of 8.8 l / min. The average diameter of the droplets generated is 50 pm and the spray angle is 80°. Consequently, in order to respect the total flow rate of the liquid feedstock, the vaporization zone contains n=42 tubes. During the vaporization, the spray diameter is estimated at 8 cm. The diameter of each n tube is set at 25 cm. Considering a triangular arrangement and a tube spacing of 27 cm, the diameter of the reactor is 4.4 m.

[0101] The heat necessary for the vaporization of the liquid feedstock is provided via a stream of water vapour of 123.8 tonnes / hour at 520°C. Under these conditions, the vaporization time and the length of the spray until vaporization is complete are 100 ms and 10 cm, respectively. The temperature of the gas after vaporization is about 370°C with degradation of the lactic acid into acrylic acid and carbon monoxide and dioxide of less than 10%. The height of the tubes is set at 15 cm so as to limit the residence time in the gas phase before the inlet into the catalyst.

[0102] The height of the homogenization zone is 5 cm, meaning a residence time of the lactic acid in the gas phase of less than 100 ms. Based on a WHSV of 0.2 IT1, the mass of catalyst necessary for the reaction is 84 tonnes. By setting a height of the m tubes of the catalytic zone at 7.5 m and a diameter of 8 cm, a number “m” of tubes equal to 2890 is obtained with a pressure loss in the catalytic zone of 0.18 MPa. Under these conditions, the final production of acrylic acid is 12 tonnes / hour. A yield of acrylic acid of 89% is obtained.

[0103] Example 4 (not in accordance):

[0104] In this example, no tubes are used for the vaporization and the catalytic transformation. The liquid feedstock is injected in the form of droplets via 42 single-fluid atomization nozzles each with a flow rate of 8.8 l / min. The average diameter of the droplets generated is 50 pm and the spray angle is 80°. During the vaporization, the spray diameter is 8 cm.

[0105] The heat necessary for the vaporization of the liquid feedstock is provided via a stream of water vapour of 123.8 tonnes / hour at 560°C. In the absence of tubes for carrying out the vaporization, the liquid and gas residence time in the vaporization zone is difficult to control and the presence of back-mixes results in a total vaporization time of 200 ms. The temperature of the gas after vaporization is about 385°C with degradation of the lactic acid into acrylic acid, carbon monoxide and dioxide and other by-products of 25%. The length of the spray until vaporization is complete also increases to 15 cm. The height of the vaporization zone is set at 20 cm.

[0106] The height of the homogenization zone is 5 cm, meaning a residence time of the lactic acid in the gas phase of less than 200 ms. Based on a WHSV of 0.2 IT1, the mass of catalyst necessary for the reaction is 84 tonnes, distributed in a single catalytic bed, giving a height of the catalytic zone of 6.5 m. Under these conditions, the estimated pressure loss is 0.12 MPa and the final production of acrylic acid is 10 tonnes / hour. A yield of acrylic acid of 74% is obtained.

[0107] These examples confirm that the implementation of the process for converting hydroxypropanoic acid and its derivatives into acrylic acid as defined in the invention leads to higher yields of acrylic acid.

Claims

Claims1. Process for converting a liquid feedstock based on hydroxypropanoic acid, on hydroxypropanoic acid derivatives or mixtures thereof into acrylic acid, into acrylic acid derivatives or mixtures thereof by means of at least one co-current downflow reactor, said process comprising at least the following steps: a) a step of dispersing said liquid feedstock in “n” tubes, where n>2, so as to obtain a feedstock in the form of droplets with an average diameter of less than 1 mm; b) a step of vaporizing said liquid feedstock in the form of droplets obtained on conclusion of step a) in “n” tubes, where n>2, by bringing said feedstock into contact with a hot gas chosen from inert gases or water vapour at a temperature of between 350°C and less than 550°C in order to obtain a gaseous mixture; c) a step of transferring said gaseous mixture obtained on conclusion of step b) to step d) of catalytic dehydration in a downward flow for a period of time of less than 20 seconds; d) a step of catalytically dehydrating the gaseous mixture from step c) by bringing said gaseous mixture into contact with a fixed-bed dehydration catalyst at a temperature of between 180°C and 450°C, at a pressure of between 0.1 MPa and 12 MPa and a weight hourly space velocity of between 0.01 h1and 100 IT1.

2. Process according to Claim 1, in which the total duration of step a) and b) is less than 180 milliseconds.

3. Process according to either one of the preceding claims, in which the duration of step c) is less than 10 seconds.

4. Process according to any one of the preceding claims, in which in step a) said liquid feedstock is dispersed by means of an atomization nozzle placed in the upper part of each “n” tube.

5. Process according to Claim 4, in which said droplets obtained on conclusion of step a) have an average diameter of less than 500 pm.

6. Process according to any one of the preceding claims, in which step c) is carried out in the “n” tubes used in steps a) and b).

7. Process according to any one of the preceding claims, in which “n” is between 2 and 10 000.

8. Process according to any one of the preceding claims, in which step d) is carried out in “m” tubes comprising said fixed-bed catalyst, m>2, it being understood that m>n.

9. Process according to Claim 8, in which “m” is between 2 and 20 000.

10. Process according to either one of Claims 8 and 9, in which step d) is carried out in “m” tubes, where m>n.

11. Process according to either one of Claims 8 and 9, in which step d) is carried out in “m” tubes, where m=n.

12. Process according to Claim 11 , in which each “n” tube used in steps a), b) and c) is aligned along the longitudinal axis and superposed on each “m” tube used in step d).

13. Process according to any one of the preceding claims, in which said liquid feedstock comprises between 55% and 85% by weight of hydroxypropanoic acid, of hydroxypropanoic acid derivatives or mixtures thereof relative to the total weight of said liquid feedstock.

14. Process according to any one of the preceding claims, in which in step d) the concentration of hydroxypropanoic acid, of hydroxypropanoic acid derivatives or mixtures thereof is between 0.5 mol% and 10 mol%.

15. Process according to any one of the preceding claims, in which the catalyst used in step d) comprises an active phase based on salts of sulfates, of phosphates or a mixture of these salts and a support comprising at least one refractory oxide.

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