ACRYLIC (METH) PURIFICATION PROCESS

Abstract

The present invention relates to the production of (meth)acrylic acid in a (meth)acrylic acid recovery process based on the use of two distillation columns in the absence of an external solvent. More particularly, it relates to the evacuation of the finishing column using a liquid ring vacuum pump, the liquid being a solution containing (meth)acrylic acid. The invention also relates to an installation adapted for implementing this process.

Description

Title of the invention: PROCESS FOR PURIFYING (METH)ACRYLIC ACID Technical field

[0001] The present invention relates to the production of (meth)acrylic acid in a (meth)acrylic acid recovery process based on the use of two distillation columns in the absence of external solvents. External solvents include azeotropic solvents, absorption solvents, and extraction solvents. More particularly, the present invention relates to the evacuation of the finishing column using a liquid ring vacuum pump, the liquid being a solution containing (meth)acrylic acid. The invention also relates to an installation adapted for carrying out this process. TECHNICAL BACKGROUND AND TECHNICAL PROBLEM

[0002] The acrylic acid synthesis process used on a large industrial scale involves a catalytic oxidation reaction of propylene in the presence of oxygen.

[0003] This reaction is generally carried out in the gas phase and most often in two steps: the first step carries out the substantially quantitative oxidation of propylene into a mixture rich in acrolein, then, in the second step, carries out the selective oxidation of acrolein into acrylic acid.

[0004] The gaseous mixture from the second step consists, apart from acrylic acid, of untransformed compounds from the reactants involved or of impurities generated during at least one of the 2 reaction steps, namely:

[0005] - of light, non-condensable compounds under the temperature and conditions of pressures usually implemented, essentially: propylene, propane, nitrogen, unconverted oxygen, carbon monoxide and carbon dioxide formed in small quantities by ultimate oxidation;

[0006] - of light condensable compounds, namely essentially: water, non-acrolin converted, light aldehydes such as formaldehyde, glyoxal and acetaldehyde, formic acid, acetic acid or propionic acid;

[0007] - of compounds having a boiling point higher than that of acid acrylic: furfuraldehyde, benzaldehyde, maleic acid and anhydride, benzoic acid, 2-butenoic acid, phenol, protoanemonin.

[0008] Finally, heavy compounds derived from the addition of compounds with nucleophilic properties to the double bond of unsaturated carbonyl monomers, by Michael reaction.

[0009] The complexity of the gaseous mixture obtained in this process requires a series of operations to recover the acrylic acid contained in this gaseous effluent and transform it into a grade of acrylic acid compatible with its final use, for example the synthesis of acrylic esters or the production of acrylic acid polymers and / or acrylic esters.

[0010] Document EP 2 066 613 describes a process for recovering acrylic acid without using an external solvent. This process employs two distillation columns to purify the cooled gaseous reaction mixture:

[0011] a) a dehydration column, and

[0012] b) a finishing column (or purification column) fed by a portion of the bottom flow from the dehydration column

[0013] According to this process, the cooled gaseous reaction stream is subjected to dehydration in the dehydration column. The distilled gas stream at the top of the column is sent to a condenser in which the light compounds are partially condensed and returned to the dehydration column as liquid reflux, the uncondensed gaseous effluent being at least partially returned to the reaction and the remainder being removed.

[0014] The bottom stream of the dehydration column is sent to the finishing column. During the purification / finishing step, a stream rich in heavy compounds is removed at the bottom and a distillate comprising water and light by-products is recovered at the top, which is condensed and then recycled to the bottom of the first dehydration column, thus forming a recirculation loop.

[0015] A stream of purified acrylic acid is recovered in liquid or vapor form by lateral withdrawal from the finishing column. The acrylic acid obtained is of technical grade.

[0016] The temperature and pressure operating conditions for the finishing column are not critical in this process. However, preferably, the purification column is operated at a pressure lower than atmospheric pressure, thus preventing the polymerization of any unsaturated products present and minimizing the formation of heavy byproducts. These compounds are heavy products that reduce the recovery yield by consuming the monomeric acrylic acid.

[0017] In the case of an acrylic acid (AA) production unit, this essentially involves:

[0018] - of derivatives of the addition of acrylic acid to the double bond of another molecule acrylic acid: 3-acryloxypropionic acid also called "acrylic acid dimer" or "AA dimer";

[0019] - of derivatives of acrylic acid addition to the double bond on a molecule of AA dimer, to form the "AA trimer" and other oligomers formed by successive additions of acrylic acid to the double bonds of the preceding AA oligomers,

[0020] - of carboxylic acid addition derivatives formed as by-products of the acid acrylic or water on the double bond of the AA or the oligomers mentioned above.

[0021] Despite the advantages of the process described in document EP 2 066 613, there are still drawbacks related to its implementation. In particular, at the top of the finishing column, which operates under vacuum, the condenser releases residual vapors containing organic impurities that must be eliminated, for example by incineration, which is harmful to the environment.

[0022] Numerous vacuum generating systems are available for reducing the operating pressure of distillation columns (see, for example, Techniques de l'Ingénieur, Production de vide, BM 4270, April 1999 or Techniques de l'Ingénieur, Pompe à vide, B4030, 1983). Two main groups of vacuum pumps can be distinguished: - Pumps that, through one or more compression stages, extract gas or vapor molecules from the columns to be distilled and expel them at a higher pressure. These are called transfer pumps or extraction pumps. - Pumps that fix gas or vapor molecules onto a solid surface by condensation or sorption of these molecules. These are fixation pumps.

[0023] The first group, by far the most important, is divided into two subgroups, according to the physical principles leading to different systems:

[0024] - So-called "positive displacement" pumps, which are pumps in which a A gas-filled volume is cyclically isolated from the inlet and then compressed before being discharged, for example, vane pumps and liquid ring pumps, and

[0025] - So-called "kinetic" pumps, in which it is the flow of a fluid that creates the depression (ejector pumps, steam jet pumps).

[0026] Document EP 1059281 describes the implementation of one or more steam ejectors and their condensation systems to create reduced pressure in a distillation train for obtaining (meth)acrylic acid by a process comprising a reaction section and a reaction gas absorption section with external water addition. To create reduced pressure in these distillation columns or even storage tanks, the vents, consisting essentially of oxygen or air necessary for stabilization and organic vapors and water, are mixed with an external supply of water vapor and introduced into an ejector. The resulting mixture is then condensed, creating a pressure drop in the upstream equipment, and yields a liquid phase consisting of (meth)acrylic acid and water, as well as a gaseous phase. Depending on the desired vacuum level, several ejectors and condensation systems are implemented. The recycling of the various condensates obtained... At different points in the process, depending on their water and (meth)acrylic acid composition, and the recycling of the gas phase to stabilize the distillation columns complete this vacuum system. The number of pieces of equipment required and the management of the various condensates make this vacuum system a complex but suitable solution for a water absorption process of reaction gases as described, since the condensate flow rate, as cited in Example 1, represents approximately 15% of the water flow rate introduced into the water absorption column. Conversely, a vacuum system for a process not using a water absorption column would create a problem in managing these condensates.

[0027] US patent 7288169 aims to limit fouling problems resulting from the polymerization of heat-sensitive products such as acrylic acid at various key points in a distillation column operating under reduced pressure. In particular, it describes how to prevent fouling of the pressure control valve at the top of the distillation column by partially recycling the gases obtained after condensation at the ejector outlet, thereby diluting the vents of the distillation column and consequently reducing the amount of acrylic acid vapors that could foul this pressure control valve. This patent presents the same disadvantage as the one previously described, namely the management of condensate.

[0028] The inventors have now discovered that the use of a volumetric pump, of the liquid ring pump type, implementing a solution containing acrylic acid or methacrylic acid judiciously taken and recycled in a two-column process, allows optimal operation of the finishing column with economic and environmental advantages.

[0029] It also appeared to the inventors that this invention could be applied to: acrylic acid produced from sources other than propylene, methacrylic acid, and to those acids derived from renewable raw materials, which are likely to pose the same purification problems. Summary of the invention

[0030] The present invention relates firstly to a process for recovering (meth)acrylic acid without using an external solvent, from a gaseous reaction mixture comprising (meth)acrylic acid obtained by gas-phase oxidation of a precursor of (meth)acrylic acid, comprising at least the following steps: i. the gaseous reaction mixture is subjected to dehydration in a first column called the dehydration column, leading to a head flow, at least part of which is condensed and returned to the dehydration column as reflux, and to a foot flow; ii. at least part of the bottom stream of the dehydration column is subjected to distillation at a pressure lower than atmospheric pressure in a second column called the finishing column, leading to a top stream, and a bottom stream containing heavy compounds; iii. A stream of purified (meth)acrylic acid is recovered by lateral withdrawal of the finishing column; iv. The top flow of the finishing column after condensation and the flow of the lateral withdrawal after condensation are subjected to at least one negative pressure system,

[0031] said process being characterized in that said depressurization system is a liquid ring pump supplied by a solution containing (meth)acrylic acid.

[0032] According to certain particular embodiments, the invention also has one or, preferably, several of the advantageous features listed below: - said solution is drawn from a return line from the top of the finishing column to the lower third of the dehydration column or from a recirculation loop at the bottom of this column. This solution containing (meth)acrylic acid then becomes the liquid ring of the pump. - The liquid ring stream is fluidically connected to the recycling stream from the finishing column to the dewatering column. "Fluidically connected" or "fluidically connected" refers to a connection via a system of pipes capable of transporting a flow of material. This connection system may include valves, bypasses, heat exchangers, or compressors. - the flow of purified (meth)acrylic acid by lateral withdrawal from the column The finishing product is of technical grade. The term "technical grade acrylic acid" refers to an acrylic acid that contains at least 98.5% acrylic acid by weight, preferably at least 99% acrylic acid, and more preferably at least 99.5% acrylic acid. In addition, the acrylic acid contains less than 0.5% water and less than 0.4% acetic acid, preferably less than 0.3% water and less than 0.2% acetic acid, and more preferably less than 0.15% water and less than 0.075% acetic acid. - At the outlet of the liquid ring pump, a two-phase mixture is obtained which is sent to the lower part of the recirculation loop, which includes a heat exchanger at the bottom of the dehydration column. By "two-phase mixture," we mean that the mixture sent to the dehydration column is partly in gaseous form and partly in liquid form. This The biphasic mixture then comprises, in liquid form, the methacrylic acid solution, the organic products, namely mainly acetic acid, acrylic acid and water which were present in the vents of the finishing column and in gaseous form, air, by oxygen-depleted air at 7% O2, or oxygen injected into the finishing column to allow good effectiveness of the inhibitors used in it. The liquid ring vacuum pump system can be single-stage or two-stage. The supply and discharge of the operating fluid of the liquid ring pump system can be done without recirculation, with a circuit with partial recirculation of the operating fluid. A single liquid ring pump system can be used to vacuum and finish column and side draw line. To operate, this type of pump requires a liquid, in this case, a meth(acrylic) acid solution. Upon startup, this liquid is centrifuged against the walls of the pump body, forming the liquid ring. The pump draws in the lateral condensate flow and / or the condensation flow from the top of the finishing column through the suction port. Due to the eccentricity of the impeller within the pump, this pumped flow is compressed and then discharged through the pump's discharge port along with the meth(acrylic) acid solution. A first liquid ring pump system is used to create a vacuum in the finishing column, while a second liquid ring pump system is used to create a vacuum in the lateral draw-off line. The supply of the meth(acrylic) acid solution to the liquid ring pump, which enables the vacuuming of the lateral withdrawal of the finishing column, is done after condensation of the head of the finishing column by diverting part of the flow returning to the dehydration column or from a tank which will have been put in place on this line connecting the head of the finishing column to the bottom of the dehydration column. The evacuation of the meth(acrylic) acid solution, condensate, and gas (air, oxygen-depleted air, or oxygen) from the vents of the finishing column to the vacuum pump discharge is achieved by returning the working fluid in two-phase form to the dewatering column, into the bottom loop of the recirculation system, or into the section lower of the dehydration column of the return circuit of this recirculation loop. - The liquid ring pump system may optionally include a liquid separator, and / or sealing and isolation systems. - there is no gas-liquid separation at the outlet of the liquid ring pump.

[0033] According to a second aspect, the invention relates to an installation adapted for implementing the (meth)acrylic acid recovery process without using an external solvent, according to the invention, comprising at least:

[0034] a) a dehydration column;

[0035] b) a finishing column connected fluidly at the bottom of said dehydration column;

[0036] c) at least one vacuum system using a liquid ring pump, fluidly connected after condensation at the top of said finishing column.

[0037] d) optionally, a vacuum system using a liquid ring pump, connected laterally fluidly after condensation to said finishing column.

[0038] The present invention overcomes the drawbacks of the prior art. More particularly, it provides a process for obtaining technical-grade (meth)acrylic acid, incorporating a liquid ring vacuum system that recycles the working fluid and impurities, in the form of a two-phase mixture, in the bottom of the dehydration column. The process according to the invention reduces final liquid discharges. It is less fouling than a dry pump system and less energy-intensive than a kinetic pump system.

[0039] Other features and advantages of the invention will become clearer upon reading the detailed description that follows, with reference to the attached Figures 1 to 4, which represent:

[0040] - [Fig. 1]: Diagram of a process for the production of acrylic acid illustrating the process recovery / purification according to the preferred method of the invention.

[0041] - [Fig.2]: Installation adapted to the implementation of the recovery process illustrating one embodiment of the invention.

[0042] - [Fig.3]: Prior art installation including a pump-operated condensation system liquid ring with water as the operating fluid.

[0043] - [Fig.4]: Prior art installation with a condensation system integrating steam ejectors. DETAILED DESCRIPTION OF THE INVENTION

[0044] The present invention overcomes the drawbacks of the prior art. More particularly, it provides a process for obtaining a high-purity technical (meth)acrylic acid, incorporating, as a vacuum system for the finishing column, a liquid ring pump supplied with a solution containing (meth)acrylic acid, said solution being preferably taken from the return line of the head of the finishing column.

[0045] The term "(meth)acrylic acid" refers to acrylic acid or methacrylic acid.

[0046] According to various embodiments, said process comprises the following characteristics, where appropriate combined.

[0047] The process according to the invention may further include other preliminary, intermediate or subsequent steps provided that they do not negatively affect the obtaining of purified (meth)acrylic acid.

[0048] According to one embodiment, the gaseous reaction mixture containing (meth)acrylic acid is obtained from a bio-based precursor. These bio-based precursors are derived from renewable organic matter (biomass) of biological origin (microorganisms, plants or animals), as opposed to raw materials of fossil origin.

[0049] According to one embodiment, the precursor of acrylic acid is acrolein.

[0050] According to one embodiment, acrolein is obtained by oxidation of propylene or by oxidative dehydrogenation of propane.

[0051] According to one embodiment, the precursor of methacrylic acid is methacrolein obtained by oxidation of isobutylene and / or tert-butanol or from oxidative dehydrogenation of butane and / or isobutane.

[0052] According to one embodiment, the gaseous reaction mixture comprising (meth)acrylic acid obtained by gas-phase oxidation of a (meth)acrylic acid precursor comprises carbon of renewable origin.

[0053] According to one embodiment, the precursor of (meth)acrylic acid is derived from glycerol, 3-hydroxypropionic acid or 2-hydroxypropionic acid (lactic acid).

[0054] According to a preferred embodiment of the invention, the gaseous reaction mixture comprises propylene-derived acrylic acid obtained by a two-step oxidation process.

[0055] According to one embodiment, the oxygen used in this process is obtained by electrolysis of water.

[0056] According to one embodiment, the oxygen used in this process is obtained by pressure inversion adsorption (PSA) technology to concentrate oxygen from ambient air.

[0057] According to one embodiment, the oxygen used in this process is obtained by compressing atmospheric air.

[0058] According to one embodiment, the purification is carried out with a process as described in the aforementioned document EP 2066613.

[0059] According to one embodiment, the finishing column is stabilized by an injection at the bottom of the column of pure oxygen, by an oxygen / carbon dioxide mixture, by an oxygen / carbon dioxide / nitrogen mixture, by depleted air (7% O2 in nitrogen), or by air.

[0060] According to one embodiment, the finishing column is a conventional distillation column.

[0061] According to one embodiment, the finishing column is a column with a separating wall.

[0062] According to one embodiment, the finishing column operates under a reduced pressure of 5 to 60 kPa.

[0063] According to one embodiment, a chemical aldehyde reducing agent can be injected into the feed of this column.

[0064] According to one embodiment, the combustion of residual gases is carried out by a regenerative or recuperative thermal oxidizer.

[0065] According to one embodiment, the combustion of residual gases is carried out by a regenerative or recuperative catalytic oxidizer.

[0066] According to the process shown in [Fig. 1], a precursor of acrylic acid stream 1 (such as propylene) and stream 9 obtained after compression by compressor c supplied with recycled gases (stream 8) and air (stream 31) are mixed (Alim R1 / R2), then introduced into the reaction section composed of two reactors RI / R2. The gaseous reaction mixture (stream 2) comprising acrylic acid obtained by gas-phase oxidation of an acrylic acid precursor, after cooling in a condenser El, supplies gases (stream 3), at a temperature between 150°C and 220°C, to a dehydration column C100.

[0067] The dehydration column leads to a headflow 5, at least part of which is condensed in a condenser E2 and returned to the dehydration column as reflux 6, the other part (flow 7) comprising the light non-condensable compounds being generally sent partially or totally to an oxidizer (flow 21) or recycled (flow 8), preferably in a stage located upstream of the compressor c.

[0068] The dehydration step aims to remove, in a headstream, most of the water present in the reaction mixture, as well as the light non-condensable and light condensable compounds. It generates a headstream 5 comprising most of the water and light compounds, along with acrylic acid and heavy compounds in very small quantities, and a 10 foot stream depleted in light compounds and comprising mostly acrylic acid, with heavy by-products.

[0069] The dehydration column generally comprises 5 to 50 theoretical trays.

[0070] Advantageously, the dehydration column operates at pressure atmospheric or slightly higher, up to an absolute pressure of 1.5 x 10⁵ Pa.

[0071] Advantageously, the temperature in the upper part of the dehydration column is at least 40°C. The temperature of the bottom flow of the dehydration column preferably does not exceed 120°C.

[0072] The foot stream 10 of the dehydration column is sent at least in part (stream 11) to the top of a second distillation column C200, called the purification column or finishing column, in which a top stream 12 and a foot stream 13 are separated.

[0073] Part of the liquid flow 10 from the foot of the dewatering column is sent into a heat exchanger E6 which can be a reboiler, and reinjected into the dewatering column (flow 30), so as to constitute a bottom recirculation loop.

[0074] The C200 finishing column is generally a conventional distillation column comprising 5 to 30 theoretical trays, preferably 8 to 25 theoretical trays. This distillation column is connected at the bottom to at least one E7 reboiler and at the top to an E220 condenser.

[0075] Advantageously, the finishing column operates under an absolute pressure ranging from 5 kPa to about 60 kPa, the temperature of the head flow being advantageously between 40°C and about 90°C, and the temperature of the foot flow being between 60°C and 120°C.

[0076] The top gas stream 12 from the finishing column is sent into the condenser E220, and the outgoing liquid stream 12T is returned to the dehydration column, preferably via the reboiler E6.

[0077] The lateral draw-off flow 16, located in the first third of the bottom of the finishing column, preferably 2-3 theoretical trays above the reboiler, comprises technical acrylic acid.

[0078] According to one embodiment, polymerization inhibitors are added. The addition of the polymerization inhibitors can be done at different points, with the introduction of the reagents at the top of the dehydration or finishing columns, at the condensers of the heads of the dehydration column, at the head of the finishing column or on the one present at the lateral withdrawal of the finishing column.

[0079] Examples of usable polymerization inhibitors include phenothiazine (PTZ), copper salts such as copper dibutyldithiocarbamate (CB), hydroquinone (HQ), hydroquinone monomethyl ether (EMHQ), Di-tert-butyl paracresol (BHT), paraphenylenediamine, TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy), di-tert-butylcatechol, or derivatives of TEMPO, such as OH-TEMPO, manganese salts such as manganese acetate alone or in mixtures in any proportion, at concentrations in the reaction medium ranging from 10 ppm to 5000 ppm,

[0080] To make the inhibitors more effective, oxygen, an oxygen / carbon dioxide mixture, an oxygen / carbon dioxide / nitrogen mixture, depleted air (7% O2 in nitrogen), or air can be injected at the bottom of the finishing column. Preferably, the amount of oxygen injected corresponds to a concentration of 0.1% to 0.5% relative to the amount of organic vapor in the column.

[0081] The stream 13 separated at the bottom of the finishing column includes the bulk of the heavy by-products, including Michael addition products such as 3-acryloxypropionic acid, maleic anhydride / acid, benzoic acid, and polymerization inhibitors.

[0082] This stream 13 can be partly recycled in the bottom of the finishing column (stream 15), or sent (stream 14) to an evaporator (not shown), and / or sent to a cracking system.

[0083] The uncondensed overhead gas stream 22 from the heat exchanger E220 is sent at least partially, preferably entirely, to a liquid ring pump (VP) system, shown in [Fig. 1]. The operating fluid for the liquid ring (stream 25) is obtained by partially drawing from the stream 12t, either by connecting to the 12t piping or by installing a tap on a tank supplied by this stream (not shown). The two-phase mixture is discharged from the VP via a line 26 which joins the dehydration column, preferably above the stream 30 (not shown).

[0084] The use of such a VP system incorporating a liquid ring vacuum pump provides a pressure below atmospheric pressure in the finishing column, thus enabling the acrylic acid distillation step to be carried out in the finishing column at a reduced temperature.

[0085] According to one embodiment, the liquid ring pump system VP can also provide a pressure below atmospheric pressure in the lateral draw-off line, thus enabling the gaseous phase sampling of the flow 16. The non-condensed flow 23 which passes through the exchanger E8 then feeds VP.

[0086] According to one embodiment, liquid ring vacuum pumps consist of a cylindrical body in which a rotor rotates in an eccentric position. The rotor has notches into which vanes are inserted to draw in a gas flow. This housing is filled with the operating fluid so as to immerse the vanes of the impeller. With the rotation of the impeller and the centrifugal force The resulting liquid in the casing forms the liquid ring. Due to the eccentric position of the impeller, the volumes of these spaces are constantly changing, causing repeated intake, compression, and expulsion of gases.

[0087] Examples of liquid ring pumps include liquid ring pumps marketed by the companies Sterling, Busch, Edwards or Nash.

[0088] According to one embodiment, the pump body and the parts in contact with the operating fluid are made of 316L stainless steel.

[0089] According to one embodiment, the operating temperature of the vacuum pump is between 20°C and 40°C.

[0090] According to one embodiment, the suction pressure of this vacuum pump makes it possible to obtain a vacuum of 105 to 102 Pa and a discharge pressure equal to or greater than atmospheric pressure up to 2.105 Pa.

[0091] According to the recovery process of the invention [Fig. 2], the uncondensed overhead gas stream 22 from the heat exchanger E220 is sent at least partially, preferably entirely, to a liquid ring pump system VP. The operating fluid for the liquid ring (stream 25) is obtained by partially drawing from the stream 12t, either by connecting to the 12t piping or by installing a draw-off point on a tank supplied by this stream (not shown). The two-phase mixture is discharged from the vacuum pump via a line 26 which joins the dehydration column, preferably above the stream 30.

[0092] A second liquid ring pump system VP2 can also provide sub-atmospheric pressure in the lateral withdrawal line, thus enabling the gaseous withdrawal of the stream 17 by imposing an operating pressure at the lateral withdrawal point between the operating pressure of the finishing column head and the pressure measured at the lateral withdrawal tray. The uncondensed gaseous fluid 19 from the heat exchanger E8 feeds the liquid ring pump VP2. A portion of the liquid stream 17 (not shown) exiting E8 serves as the operating fluid for VP2. The operating fluid, after passing through VP2 and a separator pot (not shown), is mixed with the stream 17. The uncondensed vent 23 exiting VP2 is sent to the oxidizer.

[0093] According to one embodiment, the body of the VP2 pump and the parts in contact with the operating fluid are made of 316L stainless steel.

[0094] According to one embodiment, the operating temperature of the VP2 vacuum pump is between 20°C and 40°C.

[0095] According to one embodiment, the suction pressure of this vacuum pump VP2 makes it possible to obtain a so-called "coarse" vacuum of 105 to 102 Pa and a discharge pressure equal to or greater than atmospheric pressure.

[0096] The following examples illustrate the present invention without however limiting its scope. EXPERIMENTAL SECTION Example 1 (figure 1) - method according to the invention

[0097] Simulations using ASPEN software were used to illustrate the process according to the invention.

[0098] With reference to [Fig. 1], in a continuous production process of acrylic acid from propylene, a reaction mixture 3 was subjected to the recovery / purification process according to the invention.

[0099] In this process, 11,000 kg / h of technical-grade acrylic acid are produced (flow 16), having a purity of 99.8%. The main impurities are acetic acid (0.05%), propionic acid (0.021%), furfural (0.014%), benzaldehyde (0.008%), and maleic anhydride (0.037%).

[0100] The gas stream 22 (67.6 kg / h) from the condenser E220 at the top of the finishing column C200 was subjected to a liquid ring pump VP. This pump provides a pressure of 12 kPa at the top of the column C200. A stream 23 (7.3 kg / h) from the condenser E8 located at the lateral outlet of the technical acrylic acid stream was simultaneously sent to the vacuum pump.

[0101] The flow rate of the stream 25 introduced into the pump to ensure a pressure of 12 kPa is 1000 kg / h.

[0102] The main role of this flow 25 is to constitute a liquid seal necessary for the generation of the vacuum in the pump and to ensure the renewal of the liquid by purging it of condensed impurities.

[0103] The flow exiting the pump is partly liquid and partly gaseous. The two phases are not separated but are sent as a two-phase mixture to the dehydration column.

[0104] This VP system therefore produces at the output a two-phase mixture composed of a gaseous part (52 kg / h), but also of a liquid part (1022.5 kg / h).

[0105] [Tables 1] Mass flow rate (kg / hr) Flow 22 Gaseous flow 2 6 Liquid flow 26 Flow 23 Flow 25 Total 67.6 51.8 1022.5 7.3 1000 n2 4.8230 4.8221 0.0012 0 0.0003 02 30.3200 36.4422 0.0120 6.134 0.0002 H2O 8.1080 1.9044 55.5934 0 49.3897 acetic acid 4.8700 3.0577 239.5665 0.0014 237.7527 acrylic acid 17.3900 4.5406 726.2642 1.181 712.2338

[0106] Table 1 shows the compositions of the liquid ring solution (25), the vents from the top of the finishing column after condensation (22), the lateral withdrawal flow after condensation (23), and especially the two-phase mixture obtained at the outlet of the VP pump, detailed in liquid and gaseous form. It can be seen in particular that the acrylic acid present in vents (22) and (23) has been condensed into liquid form to 99.5%. This quantity of acrylic acid can then be recovered at the bottom of the dehydration column. Examples 2 and 3 (comparatives)

[0107] For comparison purposes, the same flows 22 and 23 were subjected respectively to a liquid ring pump (example 2, [Fig.3]), and to a system integrating vapor ejectors (example 3, [Fig.4]).

[0108] In particular, Example 3 involves a steam ejector vacuum generator technology described in US documents 6,677,482 or US 7,288,169.

[0109] Figure 3 shows a system S2 incorporating a liquid ring pump P240, supplied by a water flow 24. The flow rate of the flow 24 introduced into the pump to ensure a pressure of 12 kPa is 1000 kg / h. This system S2 comprises a pump P240, a heat exchanger E240 and a condensate collection vessel R.

[0110] The P240 pump is supplied on the one hand by the gas streams 22 and 23 from the C200 column and on the other hand by the aqueous stream 27. The main role of this aqueous stream is to act as a liquid seal necessary for generating the vacuum in the pump and to ensure the renewal of the liquid by purging it of condensed impurities. The heat released by the pump is eliminated by cooling the condensed stream through the heat exchanger.

[0111] The output flow from the pump is partly liquid and partly gaseous. The two phases are separated in the container R and a portion of the liquid phase (essentially aqueous) is recirculated to the pump P240 after cooling in the heat exchanger E240.

[0112] This system S2 therefore produces at its outlet a gaseous stream 25 (43.2 kg / h), but also a significant quantity of liquid effluent 26 (1031.7 kg / h). This stream 26, essentially aqueous, contains organic compounds in solution at significant concentrations (mainly 1.8% acrylic acid, 0.5% acetic acid), which render it unsuitable for discharge without further purification treatment.

[0113] Figure 4 shows a system S3 integrating two ejectors in series P240 and P250, supplied respectively by 400 kg / h of water vapor (flow 27) and 600 kg / h of water vapor (flow 28), so as to ensure a pressure of 12 kPa at the top of column C200.

[0114] This S3 system comprises 2 pumps (ejectors) mounted in series which are supplied with steam at a pressure of 1500 kPa and 3 condensers.

[0115] The first ejector P240 is fed on one side by the gas streams 22 and 23 from column C200 and on the other side by the pressurized steam stream. The outgoing gas stream at a temperature of 144°C is cooled to 42°C in a first condenser E240. The liquid condensate 29 is sent to a condensate collection vessel R, and the uncondensed gas vents 30 are sent to the feed of the second ejector P250. At the outlet of this ejector, the gas stream at 162°C is cooled to 42°C in the condenser E250. The condensed liquid stream 31 is sent to the receiver R. The uncondensed vents 32 in this 2nd condenser are cooled to 15°C in the 3rd condenser E260, producing a 3rd liquid condensate collected in the tank R. The uncondensed gas stream 25 in this 3rd condenser is removed.

[0116] This system S3 thus produces at its outlet a gaseous stream 25 (42.8 kg / h), but also a significant quantity of aqueous effluent 26 (1032.1 kg / h). This aqueous stream 26 contains organic compounds in solution at high concentrations (mainly 1.8% acrylic acid, 0.5% acetic acid), which render it unsuitable for discharge without further purification treatment.

[0117] The main constituents of the gas flow 25 and the aqueous flow 26 at the outlet of systems S2 and S3 are shown in Table 2.

[0118] [Tables2] Example 2 (comparative) Example 3 (comparative) Mass flow rate kg / h Gas flow x 25 Aqueous flow 26 Gas flow 25 Aqueous flow 26 n2 4.820E+00 2.845E-03 4.821E+00 l.453E-03 O2 3.642E+01 3.737E-02 3.644E+01 l.931E-02 H2O 8.758E-01 l.007E+03 4.162E-01 l.008E+03 Acetic acid 2.005E-03 4.869E+00 l.634E-05 4.871E+00 Acrylic acid 7.855E-03 l.857E+01 6.686E-05 l.857E+01 TOTAL 43.191 1031.697 42.811 1032.075

[0119] Conventional vacuum systems using a liquid ring pump or steam jet ejectors generate a significant amount of aqueous effluent. This effluent contains organic impurities and must therefore be treated.

Claims

Demands

1. A process for recovering (meth)acrylic acid without using an external solvent, from a gaseous reaction mixture comprising (meth)acrylic acid obtained from a precursor of (meth)acrylic acid, comprising at least the following steps: i. the gaseous reaction mixture (2) is subjected to dehydration in a first column called the dehydration column (C100), leading to a head stream (5) at least a part of which is condensed and returned to the dehydration column as reflux (6), and to a foot stream (10); ii. at least a part of the foot stream (10) from the dehydration column (C100) is subjected to distillation at a pressure below atmospheric pressure in a second column called the finishing column (C200), leading to a head stream (12), and to a foot stream (13) containing heavy compounds; iii.a purified (meth)acrylic acid stream is recovered by lateral withdrawal (16) from the finishing column (C200), iv. the top stream from the finishing column after condensation (22) and the stream from the lateral withdrawal after condensation (23) are subjected to at least one vacuum system, said process being characterized in that said vacuum system is a liquid ring pump (VP) supplied by a solution (25) containing (meth)acrylic acid.

2. A method according to claim 1, wherein said solution (25) is taken from a return line (12T) from the top of the finishing column to the lower third of the dehydration column or from a recirculation loop (30) at the bottom of this column.

3. A method according to any one of claims 1 or 2, wherein at the outlet of the liquid ring pump a two-phase mixture (26) is obtained which is sent to the lower part of the dehydration column (Cl00).

4. A method according to any one of claims 1 to 3, wherein a single liquid ring pump (VP) system is used to put under empty the finishing column (C200) and put the lateral draw-off line (16) under negative pressure.

5. A method according to any one of claims 1 to 3, wherein a first liquid ring pump system (VP) for achieving vacuum is used to create a vacuum in the finishing column (C200) while a second liquid ring pump system is used to create a vacuum in the lateral draw-off line (16).

6. A process according to any one of the preceding claims, wherein the precursor of acrylic acid is acrolein, obtained by oxidation of propylene or by oxidative dehydrogenation of propane.

7. A process according to any one of claims 1 to 6, wherein the precursor of methacrylic acid is methacrolein obtained by oxidation of isobutylene and / or tert-butanol or from oxidative dehydrogenation of butane and / or isobutane.

8. A process according to any one of claims 1 to 6, wherein the precursor of (meth)acrylic acid comprises carbon from renewable sources.

9. A process according to claim 8, wherein the precursor of (meth)acrylic acid is derived from glycerol, 3-hydroxypropionic acid or 2-hydroxypropanoic acid.

10. A method according to any one of the preceding claims, wherein the finishing column is a column with a separating wall.

11. A method according to any one of the preceding claims, wherein the finishing column (C200) is stabilized by an injection at the bottom of the column of oxygen, an oxygen / carbon dioxide mixture, an oxygen / carbon dioxide / nitrogen mixture, oxygen-depleted air, or air.

12. Installation for carrying out the process of any one of claims 1 to 11, said installation adapted to carry out the process of recovering (meth)acrylic acid without using an external solvent, comprising at least: a) a dehydration column (Cl00); b) a finishing column (C200) fluidly connected at the bottom of said dehydration column; c) at least one vacuum system by a liquid ring pump (VP), fluidly connected after condensation at the top of said finishing column. d) Optionally, a vacuum system using a liquid ring pump, connected laterally fluidically after condensation to said finishing column.

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