Method for separating (METH)acrylic acid

Abstract

The invention relates to a method for separating (meth)acrylic acid from a gas mixture which is present as a raw product of a catalytic gas phase oxidation of at least one C3- / C4 precursor of (meth)acrylic acid, wherein the gas mixture is depleted of high-boiling components, the gas mixture depleted of high-boiling components is condensed in a separating column having installations which have a separating effect, and a liquid mixture containing the (meth)acrylic acid is guided out of the separating column, the liquid mixture being adjustably fed to a rectification column in order to further purify the (meth)acrylic acid and / or directly to an esterification system, without further purification, in order to convert the (meth)acrylic acid into (meth)acrylic acid alkyl ester.

Description

[0001] Methods for separating (meth)acrylic acid

[0002] The present invention relates to a method and a system for separating (meth)acrylic acid and a use of the (methacrylic acid) obtained.

[0003] The term "(Meth)acryl-" is used in this document as a shortened form of "acryl- or methacryl-".

[0004] (Meth)acrylic acid is generally known. One route for the large-scale production of (meth)acrylic acid is the heterogeneously catalyzed partial gas-phase oxidation of suitable C3 / C4 precursors (e.g., propylene, acrolein, isobutene, or methacrolein) with molecular oxygen (see, e.g., WO 2010 / 012586 A, US 5,198,578 A, EP 1 710 227 A, EP 1 015 410 A, EP 1 484303 A, EP 1 484 308 A, EP 1 484 309 A, US 2004 / 0242826 A, WO 2006 / 136336 A, DE 10 028 582 A, and WO 2007 / 074044 A). However, this process does not yield pure (meth)acrylic acid, but a gas mixture which, in addition to (meth)acrylic acid, contains acetic acid as a byproduct, the separation of which from (meth)acrylic acid, especially by rectification, is complex (see, e.g., DE 19814449 A and DE 19814421 A).

[0005] To separate (meth)acrylic acid from the product gas mixture of the heterogeneously catalyzed partial gas-phase oxidation of a C3 / C4 precursor compound, a combination of different separation methods is typically employed to achieve a purity level appropriate for the subsequent use of the (meth)acrylic acid in the most economical way possible. The specific combination used depends on the type and quantity of components other than (meth)acrylic acid contained in the product gas mixture, and also on the subsequent use. The purity requirements for (meth)acrylic acid used in esterification and subsequent polymerization of the ester are generally lower than for the direct use of this (meth)acrylic acid in a polymerization reaction, e.g., for use as a superabsorbent.

[0006] A feature common to essentially all possible combinations of separation processes for separating (meth)acrylic acid from the product gas mixture of a heterogeneously catalyzed partial gas phase oxidation of a C3 / C4 precursor compound is that, optionally after direct and / or indirect cooling of the aforementioned product gas mixture, the (meth)acrylic acid contained in the product gas mixture is transferred to the condensed, in particular liquid, phase in a basic separation step.

[0007] The existing processes for the production of acrylic acid are described in the PEP Review 2016-10 “Acrylic Acid Process Summary”. 11The process is presented in detail in the I HS Chemical publication. All four processes presented there have in common that the condensed acrylic acid is drawn off from the bottom of the absorber, and that this condensed acrylic acid also contains all the high-boiling elements formed in the reaction. Therefore, the condensed, acrylic acid-containing mixture must undergo complex rectification in all four processes, even if only the low purity requirements of the acrylic acid for subsequent esterification need to be met. The mandatory rectification, due to the frequent necessary flushing shutdowns (WO 2017 / 025391 A), leads to low plant availability and high energy consumption during rectification.

[0008] To produce high-purity acrylic acid, either further fractional crystallization (US 5 504 247, WO2011 / 0010035) with high investment costs and electricity consumption or further rectification (US 2022 / 0169589) with high energy costs and low plant availability is required.

[0009] WO 1999 / 014181 A discloses a process in which high-purity acrylic acid is obtained by suspension crystallization and subsequent separation of the crystals from the mother liquor. Since the purification of the acrylic acid-containing liquid mixture is carried out crystallizatively at low temperatures, high plant availability is achieved. However, the high investment costs and electricity consumption are disadvantages. The process only enables the production of high-purity acrylic acid; the purity requirements for using acrylic acid in the production of esters are exceeded, resulting in increased costs.

[0010] The object of the present invention was to provide an improved process for the production of (meth)acrylic acid. In particular, the process should have low investment requirements, high availability, and low energy consumption, and should provide both acrylic acid of lower purity for subsequent esterification and acrylic acid of high purity for subsequent polymerization to produce superabsorbents.

[0011] The problem is solved by a process for separating (meth)acrylic acid from a gas mixture which is present as a crude product of a catalytic gas-phase oxidation of at least one C3 / C4 precursor of (meth)acrylic acid, wherein a) the gas mixture is depleted of high-boiling components, b) the gas mixture depleted of high-boiling components is condensed in a separation column with separation-effective internals, and c) a liquid mixture containing the (meth)acrylic acid is removed from the separation column, characterized in that the liquid mixture is adjustable to a rectification column for further purification of the (meth)acrylic acid and / or, without further purification, is fed directly to an esterification plant for the conversion of the (meth)acrylic acid to (meth)acrylic acid alkyl esters.According to the invention, the "adjustable feed of the liquid mixture into the esterification plant and / or the rectification column" refers to the ability to adjust the proportion of the liquid mixture fed into the esterification plant or the rectification column. Thus, for example, the entire liquid mixture can be fed into either the esterification plant or the rectification plant, or a corresponding ratio can be set.

[0012] According to the invention, feeding the liquid mixture "directly into the esterification plant without further purification" means that no further thermal separation process is used to purify the (meth)acrylic acid between the separation column and the esterification plant. The term "thermal separation process" is intended to express that heat must be added to or removed from the system to achieve the desired separation effect (cf. DE-A 10 2008 041573 and DE-A 10 2008 8040799). Examples of thermal separation processes include desorptive, distillative, stripping, rectifying, azeotropic distillative, azeotropic rectifying, and crystallizing processes.

[0013] Crystallizative separation processes can be carried out in the manner described in documents DE-A 102008041573, DE-A 102008040799 and WO 2007 / 074044 A as well as DE-A 102007029053.

[0014] Non-crystallizing separation processes are generally thermal separation processes in which gaseous (ascending) and liquid (descending) streams, or two liquid streams, are passed countercurrently through separation columns containing separation-effective internals. Due to the gradients between the streams, heat and mass exchange occurs, ultimately resulting in the desired separation effect in the column. Examples of such non-crystallizing thermal separation processes include rectification, azeotropic rectification, extraction, desorption, stripping, distillation, azeotropic distillation, and adsorption. Such thermal separation processes (e.g.,All thermal separation processes described in WO 2011 / 000808 A2, DE-A 10 10336386, DE-A 19924532, DE-A 19924533, and DE-A 102007004960 are preferably carried out according to the invention in devices that comply with the recommendations of US 6441228 B2 and US 6966973 B2.

[0015] According to the invention, a high-boiling component or compound is understood to have a higher boiling point than (meth)acrylic acid. According to the invention, a low-boiling component or compound is understood to have a lower boiling point than (meth)acrylic acid.

[0016] Separation columns and separating internals are known to those skilled in the art and can be found in textbooks on process engineering, particularly on thermal separation (for example, M. Baerns et al., Technische Chemie, 2006, WILEY-VCH, Weinheim). In the context of the present invention, a separation column is preferably understood to be an apparatus for the thermal separation of mixtures. Examples include rectification columns, absorption columns, and condensation columns, with condensation columns being preferred. The column body is typically designed as a cylindrical tube insulated to prevent heat loss. For the evaporation of the mixture to be separated, or a portion thereof, the lower end of the column can include an evaporator, or an evaporator can be implemented as an external apparatus adjacent to the column.Almost all conceivable heat sources can be used for evaporation, such as hot water, electricity, microwaves, or waste heat from other processes or other process steps within the present process. For condensation of the mixture to be separated, or a portion thereof, the column can include a condenser at the top, or the condenser can be implemented as an external unit adjacent to the column. Various operating media can be used for cooling in the condenser, such as water, refrigerants, air, or even media from other processes or other parts of the present process that need to be heated. To improve heat and mass transfer within the column, the column body can contain internals such as screens, bubble caps, or valve trays, packing materials such as Raschig rings, or structured packings.Experts know that (horizontal) separating elements in such columns are generally referred to as trays. In principle, columns with rotating inserts, so-called rotary columns, can also be used, which spray the reflux liquid in droplets.

[0017] In the context of the present invention, dual-flow trays and cross-flow trays are preferably used as separating internals. Dual-flow trays as such are known to those skilled in the art. In the context of the present invention, these are horizontal internals that are installed in the column at specific intervals and have openings through which vapor and liquids can pass in counterflow. These openings can be, for example, holes or slots, the opening ratio being adjustable via the number of openings. Typically, dual-flow trays do not have a drain pipe connecting them to the next tray. Naturally, each dual-flow tray can be flush with the walls of the rectification column, but can also be connected to it via webs.

[0018] Crossflow trays are also known to those skilled in the art. In the context of the present invention, these are horizontal internals installed at specific intervals in the column, whereby liquid is fed onto the tray at one side, flows over the tray, and comes into contact with the rising gas. The liquid then flows to a discharge chute, is collected there, and fed to the tray below. Crossflow trays can, for example, contain sieve holes, movable valves, bells, or tunnels. Preferably, dual-flow trays and / or crossflow trays are used as separation-effective internals in the separation column according to the invention. Furthermore, the separation column can include additional inlets and / or outlets, for example at the top, to, for example, withdraw lighter components and / or return components.

[0019] The operating pressure prevailing in the separation column 100 is preferably 0 to 5 bar, more preferably 0 to 3 bar and even more preferably 0 to 1.6 bar.

[0020] The liquid mixture removed from the separation column in step c) is commonly referred to as crude acrylic acid. Crude acrylic acid streams, obtained during the purification of acrylic acid, are well known to those skilled in the art. It is also known to those skilled in the art that such a crude acrylic acid stream typically does not represent a pure product, but rather contains acrylic acid as its largest component, as well as (small) amounts of other components, such as water, lower aldehydes (e.g., furfurals, acrolein, benzaldehyde), lower carboxylic acids (e.g., acetic acid, propionic acid), and diacrylic acid.

[0021] Furthermore, the process of heterogeneously catalyzed partial gas-phase oxidation for the production of acrylic acid can be carried out as described in the prior art, as is known per se.

[0022] If the Cs precursor compound is, for example, propylene and / or acrolein, the heterogeneously catalyzed partial gas-phase oxidation can proceed, for example, as described in the literature.

[0023] WO 2005 / 042459 A, WO 2005 / 047224 A and WO 2005 / 047226 A are described as being carried out.

[0024] If the Cs precursor compound is, for example, propane, the heterogeneously catalyzed partial gas-phase oxidation for the production of acrylic acid can be carried out, for example, as described in documents EP-A 608 838, DE-A 198 35 247, DE-A 102 45 585 and DE-A 102 46 119.

[0025] If the Cs precursor compound is, for example, glycerol, the heterogeneously catalyzed partial gas-phase oxidation for the production of acrylic acid can be carried out, for example, as described in WO 2007 / 090991.

[0026] A, WO 2006 / 114506 A, WO 2005 / 073160 A, WO 2006 / 114506 A, WO 2006 / 092272 A or WO 2005 / 073160 A are described.

[0027] Methods for producing high-purity acrylic acid are known to those skilled in the art, as described above.

[0028] The esterification of (meth)acrylic acid is known to those skilled in the art and can be found in various forms in the prior art. For example, EP 4 015 498 A1 and EP 1 129 061 A1 describe the acid-catalyzed esterification of acrylic acid with n-butanol to n-butylacrylate. In this process, n-butylacrylate is produced continuously by reacting acrylic acid with n-butanol in a solvent-free phase at elevated temperature and with the addition of an acid, particularly sulfuric acid, as an acidic esterification catalyst. The acrylic acid, n-butanol, and esterification catalyst are fed into a reaction zone. During the residence time in the reaction zone, the water formed, as a component of a mixture containing n-butanol, is separated from the reaction mixture in a first rectification unit placed on top of the reaction zone.The resulting distillate is separated into an n-butanol-containing organic phase and an aqueous phase containing water, and the organic phase is returned to the first rectification unit. The aqueous phase is optionally fed, in whole or in part (e.g., a proportion of 50 to 100 wt.%, particularly 80 to 100 wt.%, most especially 95 to 100 wt.%, and particularly 100 wt.%), to a stripping unit. The reaction mixture containing n-butyl acrylate, which is drawn off from the reaction zone, is subjected to pre-purification. In a first pre-purification stage, the majority of the esterification catalyst is extracted by water washing. In a second pre-purification stage, the acidic components are neutralized and extracted with an aqueous alkaline solution by reactive extraction.Optionally, in a third pre-purification stage, residual salts and aqueous foreign phase components can be extracted with water from the organic reaction residue mixture remaining after the second pre-purification stage. The remaining organic first reaction residue mixture is then fed into a separation zone comprising a further rectification unit, where the n-butyl acrylate formed is separated by feeding the remaining first reaction residue mixture into a second rectification unit. In this unit, the remaining first reaction residue mixture is rectified into a low-boiling product containing n-butyl acrylate and components with a lower boiling point than n-butyl acrylate, and into a second reaction residue mixture containing n-butyl acrylate and components with a higher boiling point than n-butyl acrylate.The lower-boiling product is preferably fed into a further rectification unit, in which the lower-boiling components are distilled off and the bottoms product is recycled (see EP 4 015 498 A1). The second reaction residue mixture is fed into a third rectification unit, in which the n-butyl acrylate is separated from the components with higher boiling points than n-butyl acrylate.

[0029] It was surprisingly discovered that the process according to the invention, involving the processing of a partial stream in a rectification column, offers the following advantages over the prior art: firstly, crude (meth)acrylic acid can be taken directly from the separation column after removal of heavy components and fed to the esterification process, without the need for complex additional purification of the crude (meth)acrylic acid and / or the resulting (meth)acrylic acid alkyl ester; and secondly, (meth)acrylic acid can be obtained with high purity. This reduces the equipment requirements, saves costs and CO2 emissions, and increases plant availability.

[0030] A crude acrylic acid stream of sufficient purity for esterification can only be achieved if a removal of the heavier components is carried out beforehand in the separation column. Without this step, operation of the esterification process would not be possible due to heat exchanger fouling.

[0031] Furthermore, as described in EP 0 169 254 A1, problems with inhibition and discoloration would occur when using n-butyl acrylate for the production of polymer dispersions. Additionally, the alternative or cumulative work-up must be carried out via rectification in the rectification column and not, for example, via crystallization, since the recycling of the inevitably generated mother acid, which contains a high proportion of both heavy and light boiling elements, does not guarantee sufficient quality of the crude acrylic acid stream for esterification.

[0032] A further advantage of the present invention is that crude (meth)acrylic acid for esterification can continue to be produced during a necessary flushing shutdown to remove evaporator and bottom fouling from the rectification column. For this purpose, the rectification column is isolated from the rest of the plant. Due to the decoupling of the rectification column's operation from the rest of the plant, high availability and economic efficiency of the overall process are ensured.

[0033] The method according to the invention is shown schematically in an exemplary embodiment in Figure 1. Details not essential to the invention and already well known to those skilled in the art have been omitted for clarity.

[0034] 100 separation column

[0035] 101 Synthesis gas stream (gas mixture) in separation column

[0036] 102 Column floor

[0037] 103 Reflux fluid flow

[0038] 104 Crude acrylic acid stream (liquid mixture) from separation column

[0039] 105 Rectification column

[0040] 106 Stream of high-purity acrylic acid

[0041] 107 evaporators

[0042] 108 coolers

[0043] 109 Esterification plant

[0044] An acrylic acid-containing synthesis gas stream 101 (gas mixture), e.g., from a reactor for the heterogeneously catalyzed partial gas-phase oxidation of propene, is introduced into the separation column 100. In the separation column 100, which has separation-effective internals 102, e.g., column trays, the acrylic acid is condensed or purified. The quality of the crude acrylic acid stream 104 (liquid mixture) is ensured by controlling the reflux liquid stream 103. The crude acrylic acid stream 104, which still contains impurities and is discharged from the separation column, is fed without further purification to an esterification unit 109 and / or a rectification column 104. The proportion of the crude acrylic acid stream 104 from the separation column 100 that enters the esterification unit 109 and / or the rectification column 105 is adjustable.For example, the entire crude acrylic acid stream 104 can be fed into the esterification plant 109 or into the rectification column 105, or any ratio can be set, for example 1 to 1, 1 to 2, 1 to 3, etc. and vice versa.

[0045] High purity acrylic acid 106 is obtained by sufficient energy input in the evaporator 107 and generation of reflux in the condenser 108.

[0046] Below the point where the crude acrylic acid stream 104 is withdrawn from the separation column 100, the removal of the heavy-boiling components takes place, and between the point where the crude acrylic acid stream 104 is withdrawn and the point where the reflux liquid stream 103 is added, the acrylic acid condensation takes place.

[0047] A crude acrylic acid stream of sufficient purity for esterification can be obtained without energy input. Neither heat input, as in rectificative processing, nor electricity, as in crystallization processing, is necessary.

[0048] The reflux rate 103 in the separation column 100 can preferably be adjusted via the amount of reflux at the top of the separation column 100 or at a position far above the side outlet from which the liquid mixture, the crude acrylic acid, is drawn off. The reflux into the separation column is preferably obtained by condensing the low-boiling components of the reaction gas and consists essentially of water. Alternatively, water or other low-boiling components can also be used as reflux. The condensation of the low-boiling components preferably takes place by cooling following the (meth)acrylic acid condensation according to the invention, either in a separation column section located above or in a separate separation column.

[0049] Preferably, the liquid mixture is discharged via a side outlet of the separation column. Alternatively, the heavy-boiling components can also be separated in a separate separation column prior to the acrylic acid condensation. In this case, the liquid mixture could be drawn off from the bottom of the acrylic acid condensation column.

[0050] Preferably, the separation column is operated such that the temperature profile within the column exhibits an inverse sigmoidal shape, particularly when the liquid mixture is fed at least partially or exclusively to the esterification unit. If a partial stream of the crude acrylic acid cannot be fed directly into the esterification unit, e.g., due to a planned or unplanned shutdown of the esterification process, it is advantageous to operate the separation column with a higher reflux flow than required for an inverse sigmoidal temperature profile. The temperature profile of the acrylic acid condensation then begins to drop relatively quickly and subsequently decreases only very slowly. This results in lower acrylic acid losses in the exhaust gas without compromising product quality, as this is ensured during rectification. The temperature profile, or rather its shape, is inverse sigmoidal and can be essentially determined by the function where n = soil number, c = soil with the greatest temperature change, and a, b, d, g = adjustment constants. This means that initially the temperature decreases slowly (the curve is flat) and then gradually begins to fall until a sharp or exponential drop in temperature occurs (the curve is very steep). Subsequently, the rate of decrease slows again, and the temperature continues to fall slowly (the curve is flat). In summary, the shape of the inverse sigmoidal curve is flat in the upper part, steep in the middle, and flat again in the lower part.

[0051] The temperature profile in the distillation column or in a distillation column section cannot be adjusted by a conventional ratio control of the column reflux, e.g., in relation to the (meth)acrylic acid flow rate in the reaction gas. The temperature profile is very sensitive. It is influenced, among other things, by the reaction gas temperature, the acrylic acid concentration in the reaction gas, the reflux temperature, and the reflux composition. The temperature profile is preferably adjusted by measuring the temperature in the distillation column, e.g., using a temperature sensor. The temperature measurement is preferably taken, or the temperature sensor is preferably located, where the inverse sigmoidal shape of the temperature profile exhibits the steepest or exponential drop (steep curve), i.e., where the temperature change is greatest. For this purpose, a temperature setpoint is preferably defined.If the measured temperature (actual temperature) is above the setpoint, the reflux rate in the separation column is increased; if the measured temperature (actual temperature) is below the setpoint, the reflux rate in the separation column is reduced. The inverse sigmoidal temperature profile preferably extends from the sampling plate of the liquid (meth)acrylic acid-containing mixture to the plate onto which the reflux is added.

[0052] Preferably, a temperature measurement is taken in the section of the column with the greatest temperature change, and the reflux rate is controlled as a function of the measured temperature (actual temperature). As preferably described above, the temperature profile is controlled by increasing the reflux rate when the temperature setpoint is exceeded and decreasing it when the temperature setpoint is undershot.

[0053] The reflux rate in the distillation column can preferably be adjusted by the amount of reflux at the top of the column or at a point above the steep temperature drop above the side outlet from which the liquid mixture, the crude acrylic acid, is drawn. The reflux into the distillation column is preferably obtained by condensing the low-boiling components of the reaction gas and consists essentially of water. Alternatively, water or other low-boiling components can also be used as reflux. The condensation of the low-boiling components is preferably carried out by cooling following the (meth)acrylic acid condensation, either in a distillation column section located above the column or in a separate distillation column.

[0054] Preferably, a purified (meth)acrylic acid with a purity of at least 98 wt.%, preferably at least 98.5 wt.%, more preferably at least 99 wt.%, and particularly preferably at least 99.5 wt.%, is removed from the rectification column, based on the total amount of purified (meth)acrylic acid. If the purified acrylic acid is to be used for the production of superabsorbents, the concentration of benzaldehyde, acrolein, and 2-furfural must be < 2 ppm. If, on the other hand, the purified acrylic acid is to be used for the production of esters in a process without acetate separation, an acetic acid content of < 2000 ppm is required.

[0055] Preferably, during a shutdown of the rectification column, the process for separating (meth)acrylic acid is continued and the liquid mixture is completely fed to the esterification plant.

[0056] Preferably, the liquid mixture is treated with at least one chemical reagent for the conversion of any aldehydes present before being fed into the rectification column. This facilitates the separation of the aldehydes in the rectification column, as they can be separated in their converted form as high-boiling compounds. Suitable chemical reagents include hydrazine and hydrazine derivatives, such as aminoguanidine and its salts, as described in EP 270999 B1.

[0057] Preferably, the power output of a bottom evaporator in the rectification column to the input liquid mixture is between 0.2 and 1 kWh / kg. The lower value is advantageous for using the purified acrylic acid to produce esters in a process without acetate separation, and the upper value is advantageous for using the purified acrylic acid to produce superabsorbents.

[0058] Preferably, the purified (meth)acrylic acid is discharged via a side outlet of the rectification column. This allows for the simultaneous separation of the heavy and light components in a single column. Preferably, at least part of the bottoms product from the rectification column is recycled back into the separation column. This facilitates the separation of the heavy components in the rectification column.

[0059] Preferably, a distillate or overhead product from the rectification column is at least partially recycled back into the separation column above the point where the liquid mixture is drawn off. This promotes the removal of the lighter components in the rectification column without increasing the lighter component concentration in the crude acrylic acid.

[0060] Preferably, the liquid mixture from the separation column is fed to the rectification column below the side outlet of the rectification column. This allows for a more efficient separation of the heavy elements.

[0061] Preferably, the number of trays in the rectification column above the side outlet is at least 8, more preferably at least 10, and more preferably at least 12.

[0062] Preferably, the 2-furfural content of the liquid mixture discharged from the separation column is less than 350 ppm, more preferably less than 300 ppm, and more preferably less than 250 ppm, in each case based on the total amount of liquid mixture.

[0063] Preferably, the separation column for removing the heavy elements has at least 3, preferably at least 5, and more preferably at least 7, theoretical stages. The number of trays is obtained by multiplying the number of theoretical stages by the reciprocal of the tray efficiency.

[0064] Another object is the use of the (meth)acrylic acid purified in the rectification column according to the process of the invention for the production of superabsorbents. Superabsorbents are water-swellable polyacrylic acids obtained by polymerization of (methyl)acrylic acid or at least partially neutralized (methyl)acrylic acid. The production of superabsorbents is described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, Volume 35, pages 73 to 93.

[0065] Furthermore, the invention relates to a plant for separating (meth)acrylic acid from a gas mixture which is present as a crude product of a catalytic gas-phase oxidation of at least one C3 / C4 precursor of (meth)acrylic acid, comprising a separation column, an esterification plant, and a rectification column, characterized in that the separation column is connected to the esterification plant and the rectification column via at least one liquid line and the at least one liquid line is designed such that the liquid supply to the rectification column and to the esterification plant is adjustable.

[0066] Preferably, the feed to the rectification column is regulated to a fixed setpoint using a valve to ensure consistent column operation and esterify the remaining crude acrylic acid from the separation column. A buffer tank between the separation column and the esterification unit would be conceivable to compensate for any fluctuations.

[0067] Such a system setup according to a preferred embodiment is shown schematically in Figures 1 and 2.

[0068] Preferably, at least one temperature sensor is arranged in the separation column, configured to measure the temperature within the column. The temperature sensor is preferably positioned where the inverse sigmoidal temperature profile in the separation column is expected to exhibit a steep or exponential drop (steep curve). Preferably, the reflux rate in or into the separation column is controlled based on this measured temperature. For this purpose, a temperature setpoint is preferably defined. If the measured temperature is above the setpoint, the reflux rate in the separation column is increased; if the measured temperature is below the setpoint, the reflux rate in the separation column is reduced. The reflux rate is preferably controlled by a suitable control unit, which, for example, controls the opening degree of a valve.

[0069] Figure 2 shows a schematic representation of an exemplary system according to a preferred embodiment. Details not essential to the invention and already well known to those skilled in the art have been omitted.

[0070] 200 separation column

[0071] 201 Synthesis gas stream (gas mixture) in separation column

[0072] 202 Crude acrylic acid stream (liquid mixture)

[0073] 203 Reflux fluid flow

[0074] 204 Esterification plant

[0075] 205 Temperature sensor

[0076] 206 column floor

[0077] 207 Rectification column

[0078] 208 Spray coolers

[0079] 209 Countercurrent extraction column

[0080] 210 Residue column 211 Desorber

[0081] 212 Bottom evaporator of the rectification column

[0082] An acrylic acid-containing synthesis gas stream (gas mixture) 201, e.g., from a reactor for the heterogeneously catalyzed partial gas-phase oxidation of propene, is introduced into the separation column 200. In the separation column 200, which has separation-effective internals 206, e.g., column trays, the acrylic acid is purified, and a crude acrylic acid stream (liquid mixture) 202, which still contains impurities, is withdrawn and fed without further purification into an esterification unit 204 and / or a rectification column 207. The temperature in the separation column 200 is measured by a temperature sensor 205, and the reflux liquid stream 203, i.e., the reflux rate, is adjusted into the column 200 depending on the measured temperature.

[0083] Below the extraction point of the crude acrylic acid stream 202, the removal of heavy substances takes place; between the extraction point of the crude acrylic acid stream 202 and the addition point of the reflux liquid stream 203, the acrylic acid condensation takes place.

[0084] The features disclosed for the process shall also be deemed disclosed for the plant, mutatis mutandis, and vice versa.

[0085] Example 1 (according to the invention)

[0086] The process for separating acrylic acid is illustrated by way of example in Figure 2. The product gas mixture was obtained from a heterogeneously catalyzed gas-phase partial oxidation of polymer-grade propylene. Mass flow rate, temperature, and composition are given in Table 1.

[0087] The product gas mixture 201 is cooled to the temperature Tq specified in Table 1 in a co-current spray cooler 208, into which a mass flow of 171 t / h sump liquid is introduced.

[0088] The gas mixture 201 (101 in Fig. 1) is then fed to the separation column 200 (100 in Fig. 1). The separation column 200 has an inner diameter of 7.4 m and features separation-effective internals in the form of different trays 206. Trays 1 to 37 of the separation column 200 are dual-flow trays. Trays 38 to 75 are single-flow Thormann trays.

[0089] A liquid mixture 202 (104 in Fig. 1), the so-called crude acrylic acid, was drawn from a side outlet on the bottom 22 of the separation column 200. The composition of the drawn crude acrylic acid is shown in Table 2. Part of the crude acrylic acid stream (14,687 kg / h) is fed directly to the esterification unit 204 (109 in Fig. 1) without further thermal purification, while the other part (10,707 kg / h) is fed to the rectification column 207 (105 in Fig. 1).

[0090] The temperature profile of the separation column 200 was adjusted by modifying the reflux flow rate 203 so that the temperature measurement on tray 46 reached the target value of 88.1°C. The respective operating conditions are shown in Table 3.

[0091] Trays 1 to 21, i.e., below the side outlet, serve to remove the heavier components from the gas mixture 201. 4193 kg / h of the sump liquid are discharged to a residue column 210 for this purpose. The temperature on tray 10 is 102.5°C.

[0092] Above tray 75 is a collection tray and a sour water condensation unit (trays 76 to 85). A partial flow (570 t / h) of the sour water drawn off the collection tray via a side outlet is cooled by indirect heat exchange and returned to the trays, resulting in a gas temperature of 32°C at the outlet of column 200. Another partial flow of the sour water is returned to tray 75. The flow rate is variable depending on the target temperature on tray 46.

[0093] The residual gas extracted at the top of the separation column 200 is partially recycled as dilution gas (recycled gas) into the gas-phase partial oxidation process and partially fed as stripping gas to a desorber 211 and the residue column 210. The remaining residual gas stream is fed to the exhaust gas aftertreatment system, with the flow rate adjusted to maintain an absolute pressure of 1.2 bar at the top of the separation column 200.

[0094] The difference between the condensed and recirculated acid water is fed to a countercurrent extraction column 209. The packing section of the countercurrent extraction column 209 (MontzPak B1-350) has a diameter of 1.3 m and a height of 19.5 m. The acid water is fed countercurrently below the packing section, and the extraction solvent Palatinol® A is fed countercurrently above the packing section at a mass ratio of 0.97 and a temperature of 66°C.

[0095] The acid-depleted water taken from the head of the countercurrent extraction column 209, which is depleted of acrylic acid, is thermally utilized.

[0096] The extraction solvent, enriched with acrylic acid and taken from the sump of countercurrent extraction column 209, is fed to the top tray of desorber 211. Desorber 211 contains 5 dual-flow trays in the lower section, followed by 15 single-flow cross-flow trays, and has an inner diameter of 2 m.

[0097] Below the lowest tray 206, 10.7 t / h of compressed residual gas at a temperature of 162°C is fed countercurrently to the enriched extraction solvent in desorber 211. The bottoms of desorber 211 are heated to 162°C by heat exchange. For this purpose, a stream of 88 t / h of the bottoms is heated and returned to desorber 211 on tray 5. The loaded stripping gas is discharged from the top of desorber 211, mixed with 20.2 t / h of compressed residual gas at a temperature of 162°C, and fed below the first tray of the residue column 210. This column has an inner diameter of 2.4 m and 50 dual-flow trays. The bottoms liquid for the separation column 200 is fed onto tray 8.

[0098] The sump liquid is heated to 162°C by a forced-circulation evaporator. The mass flow rate through the forced-circulation evaporator is 550 t / h. A residue stream of 914 kg / h is discharged and sent for thermal treatment.

[0099] A gas mixture is extracted from the head of the residue column 210, fed to the direct circulation evaporator and mixed there with reaction gas and bottom contents.

[0100] A spray cooler is located at the top of residue column 210. A flow rate of 99 t / h is taken from the collecting tray located above the last dual-flow tray, indirectly cooled, and then sprayed into the gas stream, resulting in a liquid temperature of 69°C. A partial flow rate of 4000 kg / h of the liquid from the collecting tray is returned as reflux to tray 50 of residue column 210.

[0101] The n-butyl acrylate was produced using the procedure described in Example 1 of EP 4 015 498 A1. N-butyl acrylate meeting specifications and with a purity > 99.5 wt% was obtained. The butanol content was not increased within the measurement accuracy (+ / - 0.5%).

[0102] Rectification column 207 has 75 dual-flow trays and a diameter of 2.5 m. The crude acrylic acid stream 202 is added on tray 18. The capacity of a bottoms evaporator 212 is 6.4 MW. The mass flow rate through the bottoms evaporator 212 is 550 t / h. A partial stream of the bottoms product (1,156 kg / h) is recycled to tray 6 of the residue column 210. The gas phase is condensed at the top of rectification column 207. The top pressure of rectification column 207 was set to 100 mbar absolute by a vacuum pump. A partial stream of the top condensate (943 kg / h) was returned to rectification column 207 on tray 50.

[0103] At the side outlet of rectification column 207 on tray 66, 8600 kg / h of high-purity acrylic acid were obtained, meeting all requirements for the production of superabsorbents. The concentrations of benzaldehyde and acrolein were undetectable. The concentration of 2-furfural was < 2 ppm.

[0104] The plant availability for the production of crude acrylic acid for esterification was 95.8%, the availability of rectification column 207 due to flushing shutdowns was 88.9%.

[0105] Example 2 (comparative example)

[0106] The process for producing acrylic acid is illustrated by way of example in Figure 3.

[0107] Figure 3 shows a schematic representation of an exemplary system according to a non-inventive embodiment. Details that are not essential and are well known to those skilled in the art have been omitted.

[0108] 300 separation column

[0109] 301 Synthesis gas stream (gas mixture) in separation column

[0110] 302 Crude acrylic acid stream (liquid mixture)

[0111] 303 Reflux fluid flow

[0112] 304 Esterification plant

[0113] 305 Temperature sensor

[0114] 306 Column floor

[0115] 307 Cooling disc crystallizer

[0116] 308 Spray cooler

[0117] 309 Countercurrent extraction column

[0118] 310 Rear Column

[0119] 311 Desorber

[0120] 312 Washing column

[0121] The product gas mixture 301 was obtained from a heterogeneously catalyzed gas-phase partial oxidation of polymer-grade propylene. Mass flow rate, temperature, and composition are given in Table 1 and are comparable to those of Example 1. The product gas mixture 301 is cooled to the temperature Tq specified in Table 1 in a co-current spray cooler 308, into which a mass flow rate of 171 t / h of bottom liquid is fed.

[0122] The mixture is then fed to the separation column 300. Separation column 300 is identical to separation column 200 from Example 1.

[0123] A liquid mixture 302, the so-called crude acrylic acid, was drawn from a side outlet on tray 22 of the separation column 300. The composition of the drawn crude acrylic acid 302 is shown in Table 2. Part of the crude acrylic acid stream (14,687 kg / h) is fed directly to the esterification unit 304 without further thermal purification, while the other part (34,400 kg / h) is fed to a cooling disk crystallizer 307. This part is significantly higher than in Example 1, since the crystallizate content in suspension crystallization is limited to 25% of the feed.

[0124] A portion of the crude acrylic acid stream was used directly for the production of n-butyl acrylate. The procedure was the same as described in Example 1 of EP 4 015498 A1. It was not possible to produce n-butyl acrylate meeting specifications with a purity > 99.5 wt%. The decisive factor was that the concentration of furfural in the final product (n-butyl acrylate) was 35% above the acceptable maximum value. As described in EP 0 169 254 A1, this leads to problems with inhibition and discoloration when using n-butyl acrylate for the production of polymer dispersions. The butanol content was not increased within the measurement accuracy (+ / - 0.5%).

[0125] The temperature profile of column 300 was adjusted by modifying the reflux rate so that the temperature measurement on tray 46 reached the target value of 88.1 °C. The respective operating conditions are shown in Table 3.

[0126] Trays 1 to 21, i.e., below the side outlet, are used for the removal of heavy elements from the gas mixture. 4193 kg / h of the sump liquid are diverted to the residue column 310 for this purpose. The temperature on tray 10 is 102.4°C.

[0127] Above tray 75 is a collection tray and the sour water condensation stage (trays 76 to 85). A partial flow (570 t / h) of the sour water drawn off the collection tray via a side outlet is cooled by indirect heat exchange and returned to the trays, resulting in a gas temperature of 32°C at the outlet of column 200. Another partial flow of the sour water is returned to tray 75. The flow rate is variable depending on the target temperature on tray 46.

[0128] The residual gas extracted at the top of the separation column 300 is partially recycled as dilution gas (recycled gas) into the gas-phase partial oxidation process and partially fed as stripping gas to a desorber 311 and the residue column 310. The other residual gas stream is fed to the exhaust gas aftertreatment system, with the flow rate adjusted to maintain an absolute pressure of 1.2 bar at the top of the separation column 300.

[0129] The difference between the condensed and recirculated acid water is fed to the countercurrent extraction column 309. The design and operation of the countercurrent extraction column 309 and the desorber 311 are identical to Example 1.

[0130] The design and operation of the residue column 310 are also identical to example 1.

[0131] 34,400 kg / h of the extracted crude acrylic acid are cooled to a temperature of 16 °C in several stages by indirect heat exchange (including heat-integrated heat exchange with mother liquor, which is returned to the separation column 300) and then fed into the cooling disk crystallizer 307 (see WO 2006 / 111565). This crystallizer is a trough in which 24 weighted circular cooling plates (each internally perforated by a cooling medium) are suspended one behind the other at equidistant intervals of 30 cm (plate diameter = 3.3 m). The cooling medium flows countercurrently to the crystallizing mixture through the crystallizer 307. The total cooling medium flow rate of the crystallizer 307 is 200 t / h. The inlet temperature of the cooling medium (the brine) is +2.5 °C. The outlet temperature is 2.5 °C higher. Wiping the cooling plates suppresses the formation of a crystal layer.The crude acrylic acid is continuously passed from back to front through the crystallizer 307. During this process, the crude acrylic acid thickens (residence time 2.5 h) into a two-phase suspension containing acrylic acid crystals as the solid phase, at a temperature of 8°C and with a solids content of approximately 25 wt% at the outlet. The electricity consumption for cooling is 110 kWh / t acrylic acid.

[0132] Subsequently, in a washing column 312, the crystallizate is separated from the liquid (mother liquor) and the crystallizate is purified from the mother liquor and melted (see WO 2006 / 111565).

[0133] Purifying and melting the crystal saturate yielded 8600 kg / h of high-purity acrylic acid, meeting all requirements for the production of superabsorbents. The concentrations of benzaldehyde, acrolein, and 2-furfural were undetectable (less than 1 ppm).

[0134] The mother liquor was heated in countercurrent flow to the crystallizer 307 containing the crude acrylic acid and returned to the separation column 300 via tray 21. This recirculation of the mother liquor stream during the crystallization process leads to an increased furfural and benzaldehyde content in the crude acrylic acid stream 302, making its direct use for esterification without further purification impossible.

[0135] When using a rectification column (Example 1), this negative effect on the performance of the separation column does not occur.

[0136] The plant availability was 95%.

[0137] Example 3 (comparative example)

[0138] The process for producing acrylic acid is illustrated by way of example in Figure 4. It is identical to Example 1 of the invention, with the difference that the condensed acrylic acid (PEP Review 2016-10 “Acrylic Acid Process Summary”, IHS Chemical) is taken directly from the bottom of the distillation column, without prior removal of the heavy components, and fed to the rectification column. Since no prior removal of the heavy components takes place, the crude acrylic acid in the bottom contains all the heavy components present in the reaction gas. The maleic anhydride content of the crude acrylic acid would therefore be 0.8 wt%, the benzaldehyde content 0.08 wt%, and the 2-furfural content 0.032 wt%, and thus significantly higher than in the process according to the invention (factor 56 maleic anhydride, factor 3.4 benzaldehyde, factor 1.4 2-furfural).Esterification of this crude acrylic acid is not possible because the concentration of 2-furfural and benzaldehyde in the final product (n-butyl acrylate) is significantly higher than the acceptable maximum values. As described in EP 0 169 254 A1, this leads to problems with inhibition and discoloration when using n-butyl acrylate for the production of polymer dispersions. Furthermore, the high maleic anhydride content would cause deposits in the evaporators and coolers of the n-butyl acrylate plant, severely reducing its availability.

[0139] The condensed, acrylic acid-containing mixture must therefore undergo rectification, even though only the low purity requirements of the acrylic acid for subsequent esterification need to be met. The mandatory rectification, due to the frequent necessary purging shutdowns (WO 2017 / 025391 A), leads to low plant availability and high energy consumption during rectification. Table 1: Reaction gas

[0140] *) Comparative example Table 2: Compositions of crude acrylic acid

[0141] ') Comparative example

[0142] Table 3: Operating conditions of the separation column

[0143] *) Comparative example

[0144] **) Unit Ratio = Amount of propene used [kg] / Amount of acrylic acid obtained [kg]

[0145] ***) Synthesis gas temperature = temperature at which the synthesis gas leaves the reactor

Claims

Patent claims 1. A process for separating (meth)acrylic acid from a gas mixture present as a crude product of a catalytic gas-phase oxidation of at least one C3 / C4 precursor of (meth)acrylic acid, wherein a) the gas mixture is depleted of high-boiling components, b) the gas mixture depleted of high-boiling components is condensed in a separation column with separation-effective internals, and c) a liquid mixture containing the (meth)acrylic acid is removed from the separation column, characterized in that the liquid mixture is adjustable to a rectification column for further purification of the (meth)acrylic acid and / or, without further purification, is fed directly to an esterification plant for the conversion of the (meth)acrylic acid to (meth)acrylic acid alkyl esters.

2. Method according to claim 1, characterized in that the separation column is operated in such a way that the temperature profile in the separation column has an inverse sigmoidal curve when the liquid mixture is at least partially fed to the esterification plant.

3. A method according to claim 1 or 2, characterized in that a purified (meth)acrylic acid with a purity of at least 99 wt.% is extracted from the rectification column.

4. Method according to one of claims 1 to 3, characterized in that during a shutdown of the rectification column the process for the separation of (meth)acrylic acid is continued and the liquid mixture is completely fed to the esterification plant.

5. A method according to any one of claims 1 to 4, characterized in that the liquid mixture is treated with at least one chemical reagent for the conversion of contained aldehydes before being fed into the rectification column.

6. Method according to one of claims 1 to 5, characterized in that the ratio of the power of a bottom evaporator of the rectification column to the supplied liquid mixture is between 0.2 and 1 kWh / kg.

7. Method according to any one of claims 1 to 6, characterized in that the purified (meth)acrylic acid is removed via a side draw-off of the rectification column.

8. Method according to claim 7, characterized in that a bottoms product of the rectification column is at least partially recycled to the separation column.

9. Method according to claim 7 or 8, characterized in that a distillate from the rectification column is at least partially recycled back into the separation column above the point of withdrawal of the liquid mixture.

10. Method according to one of claims 7 to 9, characterized in that the liquid mixture from the separation column is fed to the rectification column below the side draw-off of the rectification column.

11. Method according to one of claims 7 to 10, characterized in that the number of trays in the rectification column above the side draw-off is at least 10.

12. Method according to any one of claims 1 to 11, characterized in that the 2-furfural content of the liquid mixture discharged from the separation column is less than 300 ppm, based on the total amount of liquid mixture.

13. Method according to one of claims 1 to 12, characterized in that the separation column for removing the heavy elements has at least 5 theoretical stages.

14. Use of a purified (meth)acrylic acid obtained according to the process of any one of claims 1 to 13, for the production of superabsorbents.

15. Plant for separating (meth)acrylic acid from a gas mixture, which is present as a crude product of a catalytic gas-phase oxidation of at least one C3 / C4 precursor of (meth)acrylic acid, comprising a separation column, an esterification plant, and a rectification column, characterized in that the separation column is connected to the esterification plant and the rectification column via at least one liquid line and the at least one liquid line is designed such that the liquid supply to the rectification column and to the esterification plant is adjustable.

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