Production of polymer-grade acrylic acid

Abstract

The present invention relates to the purification of acrylic acid containing a low content of aldehyde compounds, which is carried out as follows: distillation in a distillation unit without chemical reagents for treating aldehydes, resulting in a stream of polymer-grade acrylic acid withdrawn from a lateral outlet of the distillation unit. The present invention enables the elimination of the use of products classified as CMR in the manufacture of polymer-grade acrylic acid from industrial-grade acrylic acid, and thus meets the HSE requirements of industrial plants.

Description

Technical Field

[0001] The present invention relates to the production of polymer-grade acrylic acid (commonly known as ice crystal-grade acrylic acid). Background Art

[0002] The process for producing acrylic acid from propylene undergoes the following various purification steps after leaving the propylene oxidation reactor, and these steps may vary in their sequence depending on the process:

[0003] - Removal of non-condensable gases and most very light compounds, especially acrolein (an intermediate in the synthesis of acrylic acid) (crude AA),

[0004] - Dehydration to remove water and formaldehyde (dehydrated AA),

[0005] - Removal of light compounds (especially acetic acid), followed by

[0006] - Removal of heavy compounds (industrial-grade AA).

[0007] Each step thus produces an acrylic acid quality that differs in terms of the content of the residual impurities that will remain therein. These impurities limit the obtained acrylic acid to a specific type of application due to their quantity and nature. When acrylic acid is intended for polymerization processes carried out in different forms (in bulk, in solution, in suspension, or in emulsion), the quality of the acrylic acid (AA) (i.e., the content of its various impurities) will play a major role in the subsequent polymerization process. Subsequently, the manufacturer of this acrylic acid performs additional purification steps to convert industrial-grade acrylic acid (TAA) into the "standard" acrylic acid commonly known as ice crystal-grade acrylic acid (GAA) or polymer-grade acrylic acid.

[0008] In order to obtain the ice crystal-grade acrylic acid quality that enables the synthesis of high molecular weight polymers, it is particularly important to remove certain impurities in industrial-grade AA to extremely low levels. These are especially certain aldehydes (such as furfural (or 2-furaldehyde), benzaldehyde, or acrolein), or other impurities such as protoanemonin (a compound generated during the synthesis of acrylic acid), or non-phenolic polymerization inhibitors (polymerization retarders) such as phenothiazine that can be introduced during the synthesis of acrylic acid. When ice crystal-grade AA is used in polymerization reactions aimed at producing high molecular weight polymers, these compounds have a significant impact on the reactivity of ice crystal-grade AA by slowing down or inhibiting the reaction.

[0009] Therefore, it is generally a problem to convert industrial-grade acrylic acid (which has a purity higher than 98.5 wt% and still contains heavy impurities (such as aldehydes: furfural: <0.05%; benzaldehyde: <0.05%, protoanemonin: <0.05%)) into polymer-grade acrylic acid through purification operations such as distillation in the presence of chemical reagents or through crystallization operations.

[0010] The latter can be characterized as follows:

[0011] - with a purity higher than 99.5% by weight,

[0012] - with less than 0.05% by weight of water,

[0013] - with less than 0.075% of acetic acid,

[0014] - with a furfural content < 2 ppm,

[0015] - with a benzaldehyde content < 2 ppm,

[0016] - with a protoanemonin content < 2 ppm,

[0017] - with a total aldehyde weight content < 10 ppm.

[0018] Processes for obtaining industrial-grade acrylic acid are well-known. Thus, the documents EP 2066 613, WO 2015 / 126704, WO 2008 / 033687 based on "solvent-free" technology describe processes for recovering acrylic acid without using added water or azeotropic solvents. This process uses two distillation columns to purify the cooled gaseous reaction mixture: a) a dehydration column, b) a finishing column supplied with a portion of the bottom stream from the dehydration column. In the context of the present invention, it is not foreseeable to use a solvent-free process as described in these patents to obtain ice crystal-grade acrylic acid. In particular, the finishing column used would then have to separate the light products (water, acetic acid) from the acrylic acid stream and also separate the heavy compounds (furfural, benzaldehyde) from this acrylic acid stream in the same column. The number of separation stages and thus the number of two-way trays would then increase significantly, which would cause an increase in the pressure drop and the temperature at the bottom of the column, which is incompatible with the thermally sensitive nature of acrylic acid.

[0019] To obtain ice crystal-grade acrylic acid, the documents EP 2 066 613 or WO 2008 / 033687 indicate that the industrial-grade acrylic acid obtained by their processes can be subjected to an additional treatment by fractional crystallization, as described by Sulzer in patent US 5504247 or by the applicant's company in the document WO 2011 / 010035 regarding the production of polymer-grade acrylic acid from renewable sources. However, this process requires investment costs and operating costs (power consumption) that can be shown to be higher than those of the distillation process.

[0020] It is also possible to obtain crystal-grade acrylic acid by means of an additional distillation operation combined with a chemical treatment for removing aldehydes. Among the reagents that can be used, amines, and more particularly hydrazine compounds, such as those described in patents US 3,725,208 or US 7,393,976, can be used. Generally, these compounds can be used as such or in the form of their salts or hydrates.

[0021] Document WO 2017 / 050583 supplements the solvent-free processes described in WO 2015 / 126704 and WO 2008 / 033687 by proposing an aldehyde treatment step using chemical reagents, said aldehyde treatment step being carried out by distillation with one or two distillation columns in a purification section comprising a dehydration column, and a finishing column (or purification column) (preferably inside said purification section, or alternatively in an additional purification section), and enabling the achievement of crystal-grade acrylic acid quality.

[0022] All the chemical treatments described in the prior art have the drawback of generating water during the reaction of aldehydes with chemical reagents.

[0023] The presence of water in acrylic acid is harmful for the manufacture of polymers in non-aqueous media. For this reason, it may be advantageous to carry out this aldehyde chemical treatment operation during a distillation step aimed at removing water and light compounds at the top, and before an acrylic acid distillation step aimed at separating heavy compounds, as described for example in document WO 2010 / 031949. However, this method requires the use of at least two distillation columns.

[0024] Another drawback of the chemical treatment for removing aldehydes by means of amino compounds is the reactivity of the amino compounds towards acrylic acid itself, which results in a significant decrease in the stability of the medium.

[0025] Despite the use of inhibitors conventionally used for the distillation of this monomer, polymer deposits have been observed, particularly on the trays of the columns and / or the hot walls of the boilers. The formation of these solid deposits rapidly leads to problems of clogging or alteration of heat exchange, which makes it necessary to shut down (close) the installation for cleaning.

[0026] Another drawback of the processes using chemical treatment of aldehydes lies in the loss of acrylic acid in the residual stream obtained after separation of the purified AA. Since the required reaction of the chemical reagent (e.g. hydrazine) with aldehydes is not selective, a large excess of reagent relative to the aldehydes to be treated must be used; and due to the presence of reaction products of this reagent with aldehydes and with acrylic acid, the residual stream mainly composed of acrylic acid cannot be recycled or used as, for example, a raw material for the manufacture of esters. This thus constitutes a loss in the recovery yield of acrylic acid.

[0027] Finally, one of the main drawbacks of the chemical treatment of aldehydes stems from the nature of the compounds commonly used (such as hydrazine and its derivatives which are classified as products of CMR (Carcinogenic 1B), or aminoguanidine and its derivatives which are also classified as CMR (Reproductive Toxicity 1B)). For health, safety, and environmental (HSE) reasons, it is clear that the use of this type of substance should be avoided in industrial plants whenever possible. If this product is used, strict measures for the containment, treatment, and management of effluents must be deployed in a stringent manner during normal operation or in the event of a leak, resulting in high operating costs.

[0028] Recently, in document WO 2018 / 185423, the use of a dividing-wall column as a purification / finishing column in a process for recovering acrylic acid without the addition of an organic solvent using two distillation columns has been described. The specific configuration of the dividing-wall column (i.e., when the dividing wall is contiguous with the upper vault of the column in the top section and not contiguous with the bottom of the column in the bottom section) enables the improvement of the energy balance of the process while improving the technical quality of the recovered acrylic acid. The industrial-grade acrylic acid taken at the top of the dividing-wall column can be subjected to additional treatment by fractional crystallization or by distillation optionally in the presence of a compound reacting with the residual aldehydes, thus obtaining a polymer-grade acrylic acid quality. Due to the improved technical quality, the further purification for producing polymer-grade is simplified. Further, under certain conditions of using the dividing-wall column, it has been observed that polymer-grade acrylic acid meeting the specifications regarding the residual content of aldehydes (such as furfural or benzaldehyde) and protoanemonin can be directly taken at the top of the dividing-wall column. However, the use of a dividing-wall distillation column remains complex and specific. In fact, the purification described in this document cannot be applied to conventional processes for recovering acrylic acid using an added organic solvent without significant modification, such as those described in documents WO 10 / 031949 and WO 11 / 114051 regarding the synthesis of acrylic acid from glycerol.

[0029] Therefore, there remains a need to provide a method that is simple, fast, and easy to use for removing aldehydes (such as furfural, benzaldehyde, and acrolein) or other impurities (such as protoanemonin) in industrial-grade acrylic acid, resulting in a polymer-grade (or ice crystal-grade) acrylic acid quality without the intervention of chemical reagents for treating aldehydes or the intervention of expensive processes (such as processes using dividing-wall columns).

[0030] The inventors have now found that this need can be met by a simple operation of distilling industrial-grade acrylic acid, which is applied (performed) under specific conditions and is suitable regardless of the quality of the industrial-grade acrylic acid or its production method.

[0031] Furthermore, it has become clear to the inventors that the present invention can be applied to acrylic acid produced from propylene and / or propane and can also be applied to acrylic acid derived from renewable raw materials. SUMMARY OF THE INVENTION

[0032] The present invention relates to a process for manufacturing crystal-grade acrylic acid from industrial-grade acrylic acid containing a low content of aldehyde compounds, the process consisting of: distillation carried out in a distillation unit without chemical reagents for treating aldehydes, which produces a polymer-grade acrylic acid stream withdrawn through a side outlet of the distillation unit, a stream containing substantially light compounds withdrawn from the top of the distillation unit, and a stream of industrial-grade acrylic acid containing heavy compounds recovered at the bottom of the distillation unit.

[0033] Advantageously, the process of the present invention does not use a dividing-wall distillation column.

[0034] According to a first embodiment of the present invention, the distillation unit comprises a single distillation column E12 equipped with a side-stream draw-off. Generally, column E12 comprises 15 to 30, preferably 20 to 25 theoretical plates.

[0035] According to a second embodiment of the present invention, the distillation unit comprises a first distillation column E1 in which the stream generated at the top feeds a second distillation column E2 equipped with a side-stream draw-off. Generally, each of columns E1 and E2 comprises 8 to 15, preferably 10 to 12 theoretical plates.

[0036] According to both embodiments, the acrylic acid subjected to the process according to the present invention is industrial-grade acrylic acid, which has a weight content higher than 99.5%, contains a low content of aldehydes (such as furfural, benzaldehyde, and acrolein), and which may contain light compounds (such as acrolein, acetic acid, or water); the process according to the present invention enables the production, without the addition of compounds for treating aldehydes, of a stream of purified acrylic acid that meets the high-quality criteria allowing its use in the manufacture of high-molecular-weight acrylic polymers.

[0037] In particular, the polymer-grade acrylic acid obtained by the process according to the present invention can be characterized as follows:

[0038] - Total weight content of aldehydes (furfural, benzaldehyde, acrolein): < 10 ppm, preferably < 4 ppm,

[0039] - Content of protoanemonin: < 2 ppm,

[0040] - Weight content of water: < 0.1%, preferably < 0.05%,

[0041] - Weight content of acetic acid: < 0.1%, preferably < 0.08%.

[0042] Thus, by eliminating the use of products classified as CMR in the manufacture of polymer-grade acrylic acid from industrial-grade acrylic acid, the present invention makes it possible to overcome the deficiencies of the prior art processes. As a result, the formation of water associated with the reaction between an amino compound (such as hydrazine) and an aldehyde or unsaturated acid is avoided, and fouling problems caused by the use of chemical reagents do not occur.

[0043] In addition, the residual product recovered at the bottom of the distillation unit resulting from the distillation of industrial-grade acrylic acid for the purpose of obtaining polymer-grade acrylic acid can be advantageously recycled without additional purification to the esterification unit for manufacturing acrylic C1-C8 esters, which additional purification would be necessary if reagents for the chemical treatment of aldehydes were used.

[0044] According to certain specific embodiments, the present invention also has one or preferably more of the following advantageous features:

[0045] - The distillation unit comprises at least 1 top reflux, in particular the reflux at the top of distillation column E12 or column E2 equipped with a side stream withdrawal port.

[0046] - The distillation unit comprises a condenser at the top (which can be a total condenser or a partial condenser), which makes it possible to at least partially condense the distilled stream rich in light compounds. The condensed stream (at least partially) is sent back to the top of column E12 or E2 equipped with a side stream withdrawal port. The non-condensed part is removed (for example by sending it to an incinerator and then finally discharging it to the atmosphere) or it can be recycled upstream of the process (for example recycled to the column for absorbing acrylic acid from the reaction gas). The part of the condensed stream that is not recycled as reflux to the column can be removed or preferably recycled upstream of the purification process or used for manufacturing acrylic esters.

[0047] - The side stream withdrawal of acrylic acid can be carried out in the gas phase or in the liquid phase. It is preferably carried out in the liquid phase to limit the amount of acrolein present therein.

[0048] - The side stream withdrawal port comprises a condenser, which cools the acrylic acid to a temperature of about 30 °C before storage.

[0049] - At least one phenolic polymerization inhibitor, preferably hydroquinone methyl ether (MEHQ), is introduced into the condenser associated with the side stream withdrawal port in an amount suitable for preventing the condensed stream from polymerizing in the condenser, in the storage tank and during transportation before the use of acrylic acid and for meeting the polymerization reactivity requirements.

[0050] - The side stream withdrawal is preferably carried out in the first third at the top of column E12 or E2.

[0051] - At the top of columns E2 and E12, upstream of the condenser, introduce at least one phenolic polymerization inhibitor (preferably MEHQ) to prevent the formation of polymers during the condensation of the distilled gas mixture and in the column due to the presence of this polymerization inhibitor in the liquid reflux returned to the top of the column.

[0052] - Additionally, the polymerization inhibitor (especially non-phenolic polymerization inhibitors) can be sent, individually or in combination, to the plates located below the side stream withdrawal of column E2 or E12 and / or at the top of column E1. The inhibitors used are those employed by those skilled in the art for the purification of acrylic acid.

[0053] - Inject air or lean air at the bottom of the distillation unit (in column E12 or E1), preferably at a volume ratio of 0.1% - 0.5% relative to the total flow rate of the distilled AA.

[0054] - The weight ratio between the stream withdrawn as a side stream and the feed stream is 60% - 90%, preferably 70% - 80%.

[0055] - The weight ratio between the stream withdrawn as the bottoms product and the feed stream is 10% - 40%, preferably 20% - 30%.

[0056] - The stream recovered at the bottom of the distillation unit is advantageously recycled to the esterification unit without additional treatment.

[0057] - The reflux ratio (which can be defined as the rate of recycling from the top of the column to the column relative to the rate of side stream withdrawal) is 1.5 - 4, preferably 2 - 3, for example equal to 2.5. Under these conditions, a good compromise can be obtained between the column size and the number of separation stages to be used and the energy to be used to ensure this separation.

[0058] - The industrial grade acrylic acid subjected to the process according to the invention contains a total aldehyde (furfural, benzaldehyde, and acrolein) content of less than 0.1% by weight.

[0059] - The process according to the invention is carried out in continuous or semi - continuous mode, preferably in continuous mode.

[0060] In reference attachment Figure 1 and 2 After reading the following detailed description, other characteristics and advantages of the invention will become more apparent, Figure 1 and 2 represent

[0061] Figure 1 : Block diagram of the process according to the first embodiment of the invention;

[0062] ​​Figure 2 : Structural diagram of the process according to the second embodiment of the present invention. Detailed implementation mode

[0063] Definition

[0064] In the following description, the terms "polymerization grade" and "ice crystal grade" have the same meaning and indicate that the acrylic acid meets the high-quality standards that enable it to be used in the manufacture of high molecular weight (meth)acrylic acid polymers.

[0065] The term "reagent for chemical treatment of aldehydes" or "chemical reagent for treating aldehydes" means a compound that forms a heavier reaction product with aldehydes and becomes more easily separable from acrylic acid by distillation.

[0066] The term "chemical treatment" is understood to mean treatment using a reagent for chemical treatment of aldehydes.

[0067] In cases where the reactions or complex actions used are not fully confirmed, this type of treatment and compound that can be used are well-known in the prior art. The main purpose of this mode of action is mainly to form a heavier reaction product than the aldehyde to be treated.

[0068] The term "reagent for chemical treatment of aldehydes" excludes polymerization inhibitors. Although polymerization inhibitors may have a minor effect on aldehydes, they are generally introduced solely for the purpose of stabilizing the stream containing acrylic acid derivatives against polymerization, and these polymerization inhibitors may be present in the acrylic acid undergoing the process according to the present invention.

[0069] The term "light" describing by-product compounds refers to compounds whose boiling point is lower than that of acrylic acid under the working pressure under consideration, and by analogy, the term "heavy" refers to compounds whose boiling point is higher than that of acrylic acid.

[0070] The term "externally added organic solvent" refers to any organic compound in which acrylic acid is soluble and whose source is outside the process, and it is used as an adsorption, extraction, or azeotropic distillation solvent.

[0071] The term "azeotropic solvent" refers to any organic solvent having the property of forming an azeotropic mixture with water.

[0072] The term "non-condensable" refers to compounds whose boiling point is lower than 20 °C at atmospheric pressure.

[0073] Production of polymer-grade acrylic acid according to the present invention

[0074] According to the present invention, polymer-grade acrylic acid can be obtained by a simple operation of distilling industrial-grade acrylic acid without the need for chemical treatment of aldehydes. This distillation operation can follow various processes for manufacturing industrial acrylic acid without a solvent (such as those described in the literature EP 2 066 613, WO2015 / 126704, WO 2008 / 033687) or using an added solvent (such as WO 2010 / 031949).

[0075] Furthermore, it has become clear that the present invention can be applied to acrylic acid produced from propylene and / or propane raw materials and can also be applied to acrylic acid derived from renewable raw materials.

[0076] The quality of industrial-grade acrylic acid subjected to the process according to the present invention can be defined as follows (weight content):

[0077] Water < 0.2%, preferably < 0.05%, for example < 0.01%

[0078] Acetic acid < 0.25%, preferably < 0.10%, for example < 0.05%

[0079] Furfural < 0.05%, preferably < 0.03%, for example < 0.005%

[0080] Benzaldehyde < 0.05%, preferably < 0.02%, for example < 0.005%

[0081] Acrolein < 0.02%, preferably < 0.01%

[0082] Protoanemonin < 0.02%, preferably < 0.01%, for example < 0.005%.

[0083] According to the present invention, the stream of the industrial acrylic acid is sent to a single distillation unit, from which acrylic acid with most of the residual aldehydes removed is taken out as a side stream, and this acrylic acid corresponds to the required polymer grade or ice crystal grade as defined below:

[0084] - Purity higher than 99.5% by weight,

[0085] - Water below 0.05% by weight,

[0086] - Acetic acid below 0.075%,

[0087] - Furfural content < 2 ppm,

[0088] - Benzaldehyde content < 2 ppm,

[0089] - Protoanemonin content < 2 ppm,

[0090] - Total aldehyde weight content < 10 ppm.

[0091] Distillation unit

[0092] The process for manufacturing crystal-grade acrylic acid from industrial-grade acrylic acid containing low levels of aldehyde compounds consists of distillation carried out in a single distillation unit without a dividing-wall distillation column.

[0093] With reference to Figure 1 (which represents a first embodiment of the present invention), column E12 is equipped with a side-stream withdrawal port and contains 15 to 30, preferably 20 to 25, theoretical plates. This single column operates at a reduced pressure generally of 20 mmHg to 150 mmHg, preferably 30 mmHg to 100 mmHg.

[0094] Column E12 consists of any type of trays and / or random internals and / or structured packings that can be used for the rectification of mixtures and are suitable for the distillation of polymerizable compounds. It can be a conventional distillation column that can contain: at least one packing (e.g., random packing and / or a combination of sections equipped with random and structured packings) and / or trays (e.g., perforated trays, fixed valve trays, movable valve trays, bubble cap trays, or combinations thereof). Preferably, column E12 is equipped with perforated trays.

[0095] The stabilization of column E12 is generally carried out using stabilizers well-known to those skilled in the art, optionally in the case of injecting air or oxygen-depleted air.

[0096] For column E12, the feed is introduced at the first quarter of the bottom of the column, preferably at trays ranging from tray 2 to 7, preferably from tray 3 to 5.

[0097] In the absence of chemical treatment of the aldehydes, the gaseous fraction rich in light compounds (such as acrolein, acetic acid, and water) is distilled to the top of the column and removed after condensation and optional further treatment, or recycled to the upstream of the purification process, or used for the manufacture of acrylates. When partial condensation is carried out, these uncondensed light compounds can be sent directly in gaseous form to an exhaust treatment unit.

[0098] The polymer-grade acrylic acid is withdrawn in the liquid phase or in the gas phase, preferably from the first third of the top of column E12, particularly between the theoretical plates that are 1 to 5 trays lower than the top of the column. Preferably, the polymer-grade acrylic acid is withdrawn in the liquid phase.

[0099] At the bottom of column E12, the acrylic acid stream separated from the acrylic acid stream fed to column E12 and containing most of the heavy impurities (especially furfural, benzaldehyde, protoanemonin, and non-phenolic inhibitors) can be advantageously recycled as industrial-grade acrylic acid to the esterification unit without additional purification.

[0100] The weight ratio between the stream withdrawn as a side stream and the feed stream to column E12 is 60% to 90%, preferably 70% to 80%.

[0101] The weight ratio between the stream withdrawn as the bottoms product and the feed stream to column E12 is 10% to 40%, preferably 20% to 30%.

[0102] According to a specific embodiment, column E12 is equipped at the top with a condenser and a liquid feeder, which ensure the liquid reflux to the column. The reflux ratio (which can be defined as the rate of recirculation from the top of the column to the column relative to the rate of side stream withdrawal) is 1.5 to 4, preferably 2 to 3, for example equal to 2.5. These conditions allow for the best compromise between the column size / number of separation stages to be used and the energy to be used to ensure efficient distillation.

[0103] The light compounds at the top of the column are preferentially removed in the gas phase after a partial condensation operation.

[0104] Reference Figure 2 (which shows a second embodiment of the present invention), the distillation unit comprises two distillation columns E1 and E2, which are fluidly connected to each other by a gas stream that is distilled to the bottom of column E2 from the top of column E1.

[0105] Each of columns E1 and E2 contains 8 to 15, preferably 10 to 12, theoretical plates.

[0106] Columns E1 and E2 are generally conventional distillation columns, which may comprise at least one packing (such as random packing and / or structured packing) and / or trays (such as perforated trays, fixed valve trays, movable valve trays, bubble cap trays or combinations thereof). Preferably, columns E1 and E2 are equipped with perforated trays.

[0107] Columns E1 and E2 generally use stabilizers well-known to those skilled in the art and are optionally stabilized relative to polymerization in the case of injecting air or oxygen-depleted air.

[0108] Columns E1 and E2 operate at a reduced pressure generally of 20 mmHg to 150 mmHg, preferably 30 mmHg to 100 mmHg.

[0109] Column E1 is preferably fed at the first quarter of the bottom of the column, preferably at trays ranging from tray 2 to 7, preferably from tray 3 to 5.

[0110] The gas phase generated at the top of column E1 is fed to column E2 at the first quarter of the bottom of column E2, preferably at trays located in the range from tray 1 to 3 of the column.

[0111] In the absence of chemical treatment with aldehydes, the gaseous fraction substantially entraining light compounds (such as acrolein, acetic acid and water) is distilled to the top of column E2 and, after condensation, recovered for treatment in a biological unit, or when using only a partial condenser, these light compounds are sent directly to an exhaust gas treatment unit.

[0112] According to one embodiment, at least a portion of the stream condensed at the top of column E2 is sent as reflux liquid to column E2 and thus to column E1.

[0113] The reflux ratio (which can be defined as the rate of recycle from the top of column E2 to column E2 relative to the rate of the side stream withdrawn from column E2) is 1.5 to 4, preferably 2 to 3, for example equal to 2.5. These conditions allow for the best compromise between the column size / number of separation stages to be used and the energy to be used to ensure efficient distillation.

[0114] Polymer-grade acrylic acid is withdrawn in the liquid phase or in the gas phase, preferably from the first third of the top of column E2, especially between the theoretical plates 1 to 5 trays below the top of the column. Preferably, polymer-grade acrylic acid is withdrawn in the liquid phase. The weight ratio between the stream withdrawn as a side stream from column E2 and the feed stream to column E1 is 60% to 90%, preferably 70% to 80%.

[0115] The weight ratio between the stream withdrawn at the bottom of column E1 and the feed stream to column E1 is 10% to 40%, preferably 20% to 30%. The bottom stream from column E2 is sent in liquid form to the top of column E1.

[0116] At the bottom of column E1, the acrylic acid stream separated from the acrylic acid stream feeding column E1 and containing most of the heavy impurities (especially furfural, benzaldehyde, protoanemonin and non-phenolic inhibitors) can advantageously be recycled as industrial-grade acrylic acid to the esterification unit without additional purification.

[0117] The energy consumption in the process according to the invention is generally higher than the energy consumption required in the separation using hydrazine or its derivatives to treat aldehydes. However, this additional energy cost is largely offset by the gains obtained due to the absence of production stoppages and cleaning and maintenance operations after fouling associated with the use of amino compounds, and most importantly by the absence of CMR products and their restrictive industrial environment.

[0118] The invention will be illustrated by the following examples, which are not intended to limit the scope of the invention defined by the appended claims.

[0119] Examples

[0120] Percentages are expressed as percentages by weight.

[0121] The following abbreviations are used in the table:

[0122] AA: Acrylic acid

[0123] ACO: Acrolein

[0124] ACOH: Acetic acid

[0125] Furfural: Furan formaldehyde

[0126] Benzal: Benzaldehyde

[0127] PTA: Protoanemonin

[0128] HZ: Hydrazine hydrate

[0129] Simulations using a thermodynamic model and ASPEN software are used to illustrate the process according to the prior art and the process according to the present invention.

[0130] Protoanemonin does not appear in these examples because this impurity is unstable and not described in existing thermodynamic models. However, experimental distillation of a mixture of acrylic acid containing the impurities furfural (120 ppm), benzaldehyde (80 ppm) and protoanemonin (50 ppm) showed that the volatility of protoanemonin is between the volatility of benzaldehyde (the least volatile compound) and the volatility of furan formaldehyde (the most volatile compound). Therefore, it can be concluded that the concentration of protoanemonin must be lower than the concentration of furan formaldehyde (well described in the thermodynamic model).

[0131] Example 1 (Comparative)

[0132] According to the prior art process, in the presence of hydrazine hydrate, a stream of technical grade acrylic acid is subjected to a distillation operation using two columns E3 and E4 in series.

[0133] The first column E3 (having 12 theoretical plates and operating at a pressure of 45 mmHg) is fed at the 3rd theoretical tray counted from the bottom of the column with a stream of acrylic acid to be purified and hydrazine as the amino compound reacting with the aldehyde. At the top of column E3 (the topping column), light impurities such as water formed during the reaction of hydrazine with the aldehyde and acetic acid are removed. The mass reflux ratio of this column / distillate stream is set between 0.5 and 0.7.

[0134] The bottom stream from the topping column E3 feeds the second distillation column E4 (having 12 theoretical stages and operating at a pressure of 8500 Pa). Column E4 distills purified acrylic acid at the top and removes heavy compounds at the bottom, which particularly contain hydrazine and aldehyde impurities and reaction products with acrylic acid (excess reagent). The column is fed below the first tray at the bottom of the column.

[0135] The following Table 1 collates the weight compositions of the various streams.

[0136] [Table 1]

[0137]

[0138] According to this process carried out in the presence of hydrazine hydrate, acrylic acid meeting the specifications for polymerization grade was obtained at the top of the second column E4, in a proportion of 17510 kg / 22525 kg of the industrial grade acrylic acid provided. The total energy cost for the operation of the two columns has been estimated at 4.54 Gcal / h. The acrylic acid content is 99.9%, and the impurities furfuraldehyde, benzaldehyde, and acrolein are present in a content of less than 1 ppm. The stream from the bottom of column E4 contains compounds resulting from the reaction of hydrazine with aldehydes and from side reactions with acrylic acid. This stream rich in acrylic acid is not suitable for the manufacture of esters.

[0139] Example 2 (Comparative)

[0140] The configuration of the process of Example 1 was used, but the addition of hydrazine hydrate was not carried out.

[0141] The following Table 2 collates the weight compositions of the various streams.

[0142] [Table 2]

[0143]

[0144]

[0145] In the configuration of the process according to the prior art, in the absence of an amino reagent, the acrylic acid distilled at the top of the second column does not meet the specifications for polymerization grade acrylic acid for residual aldehydes, particularly for furfural, which has a content of 85 ppm for a specification of less than 5 ppm.

[0146] Example 3 (According to the invention)

[0147] The configuration as represented in Figure 1 was used to simulate the process according to the invention.

[0148] Column E12 contains 24 theoretical plates and is equipped at the top with a partial condenser and at the 5th plate counted from the top of the column with a side stream withdrawal port. The column is fed with a stream of technical grade acrylic acid without added aldehyde treatment agent. The column operates at a pressure of 100 mmHg. The temperature at the side stream withdrawal port is 90 °C and the temperature at the bottom is 111 °C.

[0149] Table 3 below collates the weight composition of the various streams.

[0150]

[0151] The stream withdrawn as the side stream corresponds to polymer grade acrylic acid.

[0152] For the same production rate as obtained in the conventional process of Example 1, the energy consumption in this configuration is 7.15 Gcal / h at the boiler. This excessive consumption of energy is offset by the gains obtained due to the absence of production downtimes and cleaning operations after fouling associated with the use of amino compounds, and due to savings made in terms of investment in production equipment and its maintenance (columns, exchangers, pumps, filters).

[0153] Furthermore, the stream of acrylic acid recovered at the bottom of column E12 (free of reaction products of aldehydes with amino compounds such as hydrazine) can be upgraded to go directly to the esterification unit without additional purification.

Claims

1. A process for manufacturing crystal-grade acrylic acid from industrial-grade acrylic acid containing a low content of aldehyde compounds obtained by a purification process with or without an added solvent, said process consisting of distilling in an additional distillation unit without chemical reagents for treating aldehydes, which produces a stream of polymer-grade acrylic acid withdrawn through a side outlet of the distillation unit, a stream substantially containing light compounds withdrawn from the top of the distillation unit, and a stream of industrial-grade acrylic acid containing heavy compounds recovered at the bottom of the distillation unit, wherein the distillation unit comprises a single distillation column E12 equipped with a side-stream outlet, the distillation column E12 having 20 to 25 theoretical plates and the side-stream withdrawal being carried out between theoretical plates at a position 1 to 5 plates lower than the top of column E12, wherein the reflux ratio, defined as the flow rate recycled to the top of column E12 relative to the withdrawn flow rate, is 2 to 3, wherein the weight ratio between the side withdrawal and the feed of the column is 70% to 80%, wherein the column E12 is fed in the first quarter at the bottom of the column in the range from plates 3 to 5, wherein the operating pressure is from 4000 Pa to 13333 Pa.

2. The process according to claim 1, characterized in that The distillation unit comprises at least 1 top reflux liquid.

3. The process according to claim 1 or 2, characterized in that The distillation unit comprises a condenser at the top, which can be a total condenser or a partial condenser.

4. The process according to claim 1 or 2, characterized in that The stream recovered at the bottom of the distillation unit is recycled to the esterification unit without additional treatment.

5. The process according to claim 1 or 2, characterized in that The acrylic acid is of petrochemical origin.

6. The process according to claim 5, characterized in that Propylene or propane is used as the raw material.

7. The process according to claim 1 or 2, characterized in that The acrylic acid is at least partially of renewable origin.

8. The process according to claim 1 or 2, characterized in that The acrylic acid is obtained from a purification process comprising extracting acrylic acid by countercurrent absorption in the form of an aqueous acrylic acid solution.

9. The process according to claim 1 or 2, characterized in that The acrylic acid is obtained from a purification process comprising extracting acrylic acid by countercurrent absorption using a heavy hydrophobic solvent.

10. The process according to claim 1 or 2, characterized in that The acrylic acid is obtained from a purification process without an added organic solvent.

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