JP2010506881A - Separation of acrylic acid and benzoic acid contained in product gas mixture of heterogeneously catalyzed gas phase partial oxidation of C3 precursor compound of acrylic acid - Google Patents

Abstract

  A method for separating acrylic acid and benzoic acid contained in a product gas mixture of partial oxidation of acrylic acid, first transferring acrylic acid and benzoic acid to a liquid phase, and then thermally separating the liquid phase from the liquid phase Is used to separate components having a lower boiling point than benzoic acid and acrylic acid, and to separate acrylic acid from the remaining liquid phase by crystallization.

Description

The present invention relates to an acrylic contained in a product gas mixture of a heterogeneously catalyzed gas phase partial oxidation of a C ₃ precursor compound of acrylic acid as a main product and a by-product in addition to other product gas mixture components. A method for separating acid and benzoic acid, wherein acrylic acid and benzoic acid are combined with liquid phase P from the product gas mixture together with components of other product gas mixtures having lower and higher boiling points than acrylic acid. At least 80% by weight of acrylic acid and a component having a boiling point lower than that of benzoic acid and acrylic acid, using at least one thermal separation method, from the liquid phase P produced in the process. The present invention relates to a method for separation with a residual liquid phase P ^* containing at least 0.1 parts by mass of benzoic acid with respect to the amount of acrylic acid.

  Acrylic acid is an important monomer used for the preparation of polymers (eg superabsorbent) used in the hygiene area, per se and / or in the form of their alkyl esters (eg WO02 / 055469 and WO03 / 078378).

Production of acrylic acid, for example _{C 3} precursor compounds can be carried out by heterogeneously catalyzed partial oxidation of (propylene, propane and / or acrolein) (e.g. EP-A990636, US-A5198578, EP-A1015410, EP-A1484303, EP-A 1484308, EP-A 1484309 and US-A 2004/024826).
At that time, as _{C 3} precursor compound, the applications technically appropriate, in general, (see DE-A10131297) that relatively high _{C 3} precursor compound of purity is used. However, for example, the production of such pure crude propylene is rather cumbersome and expensive and generally involves several purification steps in order to isolate the C ₃ precursor compound of acrylic acid in high purity. (See DE-A 352,458). According to DE-A 12246119 and DE-A 10245585, in particular the resulting reaction gas mixture for the partial oxidation contains as little C ₄ hydrocarbons as possible as unwanted impurities which impair the catalytic performance. The disadvantage of such a process mode is that the purification steps described above are cumbersome, so that the process mode is partially compared in the sense of an economical crude C ₃ precursor compound (eg crude propylene). Only used with limited separation effects.

Meanwhile, from carefully unique survey conducted, C ₃ precursor compound (e.g., propylene) crude C ₃ precursor compounds used for the heterogeneously catalyzed partial oxidation of (e.g., crude propylene) further butadiene or partial It has been found that benzoic acid can be formed as a byproduct of acrylic acid formation when it contains a compound that changes to butadiene during the oxidation process. As attributed to the cause of benzoic acid formation, the Diels-Alder reaction of butadiene and C ₃ dienophiles, such as propylene, acrolein and acrylic acid, contained in the reaction gas mixture of the heterogeneous catalytic partial oxidation is assumed, Probably followed by a heterogeneously catalyzed oxo dehydrogenation of the adduct to the aromatic compound. It is assumed that it is catalyzed by the same catalyst as the actual target gas phase oxidation. In some cases, slight impurities between the crude C ₃ precursor compound (eg, crude propylene) and o-xylene can also cause benzoic acid by-product formation. The use of a multi-metal oxide catalyst with increased activity (or a catalyst coating whose volume specific activity increases at least once in the direction of increasing starting material conversion), the combined use of steam as an inert diluent gas, and An excess of molecular oxygen relative to the C ₃ compound to be partially oxidized in each oxidation step (eg propylene → acrylic acid) is caused by the respective oxidation stage (eg propylene to acrylic acid in two stages of partial oxidation). Acrolein in the first oxidation stage or acrolein in the second oxidation stage, or acrolein alone in the partial oxidation of propylene to acrylic acid, or propane to acrylic acid. Each time in the propane oxidation stage in the stage partial oxidation) Similar to increase the reaction temperature and conversion rate of the C ₃ precursor compounds, benzoic acid by-product formation is also believed to promote. The above assumptions are often supported by the fact that only benzaldehyde, not benzoic acid, is found as a by-product of acrylic acid formation. In contrast, benzoic acid is generally generated in association with benzaldehyde as an acrylic acid byproduct.

  Both benzaldehyde and benzoic acid are undesirable accompaniments of acrylic acid. They are not desirable since they are not at all problematic as relatively reactive aromatic compounds (this applies correspondingly to esters of benzoic acid). If the acrylic acid produced and / or their alkyl esters are used in the manufacture of polymers that are therefore applied in the hygiene area (eg water-absorbing polymers in diapers), the acrylic acid used and / or used Great care must be taken that the alkyl acrylates do not contain benzoic acid and benzaldehyde.

In general, it is known to separate acrylic acid from a product gas mixture of a heterogeneously catalyzed gas phase partial oxidation of a C ₃ precursor compound of acrylic acid (eg, propylene) by a combination of various separation methods. In general, the combinations applied individually are the types and amounts of subcomponents contained in the product gas mixture, which are different from acrylic acid, as well as the acrylics that are generally adjusted to the respective use of acrylic acid. Depends on the required purity of the acid.

  The essential component of such a combination of various separation methods is usually a thermal separation method. The thermal separation method is a method in which a gaseous (ascending) and liquid (descending) flow is led in a countercurrent flow to a separation column containing an internal structure having a separation action, and a gradient existing between the flows. Because of this, heat exchange and mass exchange occur, which ultimately causes the desired separation in the separation column.

  Examples for such thermal separation methods are (partial) condensation, fractional condensation (see DE-A 19925532) and rectification. The resulting separation action is in this case due to, among other things, the difference in the boiling points of acrylic acid and subcomponents different from acrylic acid. Yet another example is absorption. In this case, the separating action depends in particular on the various solubilities in the absorption liquid of acrylic acid and of secondary components different from acrylic acid. What has been said above is that stripping (stripping gas absorbs components with various affinities contained dissolved in the liquid from the liquid) and adsorption (reverse process for adsorption; in the liquid phase This is also applied to the thermal separation method in which the lysate is separated by lowering the partial pressure. However, on the other hand, the concept of thermal separation method is azeotropic distillation or rectification (they are azeotropic addition in which acrylic acid and secondary components (components different from acrylic acid in the reaction gas mixture of partial oxidation) are added) (Which uses various prominent tendencies to form an azeotrope with the agent).

  In general, the difference in boiling point between acrylic acid (141 ° C at 1 atm) and benzaldehyde (178.1 ° C at 1 atm) is due to the quantitative separation of benzaldehyde from acrylic acid with moderate complexity using the thermal separation method described above. Not enough to do. Rather, it is usually achieved for the first time by additionally utilizing the distinctive reactivity of benzaldehyde and acrylic acid with so-called aldehyde scavengers (WO 03/014172), such as aminoguanidine bicarbonate. The reaction of the aldehyde with the aldehyde scavenger results in a new secondary component whose boiling point deviates far from that of acrylic acid than that seen with the aldehyde itself, so that the reaction with the aldehyde scavenger is Once done, quantitative separation can easily be achieved by the rectification route. As an alternative, from EP-A 1159249, benzaldehyde and acrylic acid contained in the product gas mixture of partial oxidation are combined by a combination of thermal separation (fractional condensation) and crystallization (suspension crystallization). It is known to separate from each other (see also DE-A 10247240).

  Unlike the above, the difference in boiling point between acrylic acid (141 ° C. at 1 atm) and benzoic acid (250 ° C. at 1 atm) is due to the case where a thermal separation method is used whose separation action is based on the various boiling points of the components of the mixture. Usually large enough with quantitative separation of acrylic acid and benzoic acid (see DE-A 10336386, DE-A 1974252, DE-A 10247240) (separation of further subcomponents follows benzoic acid separation, Frequently by crystallization). However, the disadvantage of such thermal separation is that the fraction enriched in acrylic acid contains a fraction of the mixture to be separated from the separation column containing the internal structure with a separating action. It must be taken out at the top of the supply path. However, this is a very complicated procedure thermally. This is because it takes a relatively large amount of energy to evaporate the total amount of acrylic acid (high boiling point, high evaporation enthalpy, multistage evaporation by reflux).

Object of the present invention therefore does not require energy-intensive separation methods described above, C ₃ in the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation of the precursor compounds, other products of acrylic acid The object was to provide a method for separating acrylic acid and benzoic acid contained as main products and by-products in addition to the gas mixture components.

Accordingly, the acrylic gas contained as the main product and by-product in addition to the other product gas mixture components in the product gas mixture of the heterogeneous catalytic gas phase partial oxidation of the C ₃ precursor compound of acrylic acid. A method for separating acid and benzoic acid, wherein acrylic acid and benzoic acid are combined with liquid phase P from the product gas mixture together with components of other product gas mixtures having lower and higher boiling points than acrylic acid. And a component having a boiling point lower than that of benzoic acid and acrylic acid (relative to the boiling point of the pure substance at 1 atm) from the resulting liquid phase P using at least one thermal separation method ( However, it does not contain benzoic acid; here, “does not contain” means here less than 10% by weight, preferably less than 5% by weight, more preferably not more than 3% by weight, based on the benzoic acid content of the liquid phase P each time. Or less than 2% by weight or less than 1% by weight, and most advantageously less than 0.75% by weight, or less than 0.1% by weight or 0% by weight), at least 80% by weight acrylic acid, and At least 0.1 mass ‰ (often at least 0.2 mass ‰, or at least 0.3 mass ‰, or at least 0.4 mass ‰, or at least 0.5 mass ‰), based on the amount of acrylic acid contained Or separation of benzoic acid from acrylic acid by crystallization from liquid phase P ^{* in} a method of separating the residue of liquid phase P ^* containing at least 0.75 mass%, or at least 1 mass ‰) of benzoic acid. Wherein the acrylic acid is enriched in the crystals formed and the benzoic acid is enriched in the remaining mother liquor.

The method according to the invention is based on the unexpected result that the impairment rate A ^BZS (for the process) with crystallization is regularly ≧ 15. In this case, the impairment rate A ^BZS is generally understood as the ratio of the amount of benzoic acid remaining in the mother liquor to the amount of benzoic acid remaining in the crystal (shown as mass% relative to the total amount of the mother liquor or the total amount of the crystal, respectively). ) Separation of crystals / mother liquor above 90%, preferably above 95%, or above 97% or 98%, or above 99% by weight of their total amount is due to the determination of A ^BZS In particular, it is generally sufficient (the effect of residual moisture on the crystals is generally negligible). A ^BZS ≧ 15 ^confirms that essentially no benzoic acid is introduced into the crystal during the formation of acrylic acid crystals. Based on it is the remarkable effect of the method mode according to the invention.

In the case of diacrylic acid (A ^DA ), acetic acid (A ^ES ) and propionic acid (A ^PS ) as an acrylic acid subcomponent, the corresponding depletion rates are usually values of ≦ 10, ie they are acrylic acid crystals Introduced together and can only be extracted from these crystals with difficulty, for example by suitable washing of the crystals.

That is, the crystallization separation of these impurities of acrylic acid is generally not very effective as recommended in the form of a multi-stage combination of dynamic and static crystallization, for example in EP-A 616998. And capital intensive multi-stage crystallization methods are required, in which at least one dynamic and at least one static crystallization apparatus is required in addition to the multi-stage process. In some cases, acrylic acid crystal forms are formed under special ranges of crystallization conditions (see in particular WO 03/078378 and WO 01/77056), from which pure acrylic acid melts are used. Subsequent washing can remove acetic acid and propionic acid relatively well. The larger A ^BZS is, the more attractive is the crystallization separation of benzoic acid from acrylic acid.

The process mode according to the present invention is based on the results of the above-described tests, and in the course of obtaining acrylic acid suitable for the superabsorbent, liquid phase ^* benzoic acid impurities that hinder such use are pre-purified acrylic acid. This opens up the possibility of essentially quantitative separation in the crystallization stage with subsequent washing of the crystals with the acid melt.

  The words “acrylic acid as the main product” and “benzoic acid as a by-product” mean that in this publication the product gas mixture of partial oxidation contains much more acrylic acid than benzoic acid. Means. That is, acrylic acid is the desired end product of partial oxidation, while benzoic acid is essentially its undesired byproduct.

In particular, the process according to the invention has an acrylic acid content of at least 85% by weight, or at least 90% by weight, or at least 95% by weight, or at least 96% by weight, or at least 97% by weight, or at least 98% by weight, or Applicable to liquid phase P ^* of at least 99% by weight, or at least 99.5% by weight or greater.

At that time, (while it appreciated, even in the case of 80 wt% in the liquid phase P ^* for the content of acrylic acid) liquid phase P ^* All of the above case of the acrylic acid content, amount of acrylic acid contained The content of benzoic acid is, for example, 0.1 to 20 mass ‰, or 0.2 to 15 mass ‰, or 0.3 to 10 mass ‰, or 0.4 to 8 mass ‰, or 0.5 each time. -6 mass ‰, or 0.75-4 mass ‰, or 1-3 mass ‰. In general, the benzoic acid content of the liquid phase P ^* is not greater than 100 mass ‰, frequently not greater than 75 mass ‰, or not greater than 50 mass ‰ relative to the acrylic acid contained therein. Moreover, the use of the process according to the invention is particularly advantageous when the content of benzoic acid in the liquid phase P ^* is (as opposed to 0) at least 50% by weight of the mass of benzaldehyde contained in the liquid phase P ^* , or at least 75 wt%, or at least 100 wt%, or at least 125 wt%, or at least 150 wt%, or at least 175 wt%, or at least 200 wt%, or at least 250 wt%, or at least 300 wt%, or at least 400 wt% %, Or at least 500% by weight. Of course, the liquid phase P ^* to be treated according to the present invention may also contain no benzaldehyde.

The method according to the invention is applied to at least one thermal separation method used to produce a liquid phase P ^* from the liquid phase P, at least partly from the mother liquor remaining during the crystallization separation (from the liquid phase P ^* ). And / or return to the at least one method used to transfer the acrylic acid and benzoic acid contained in the product gas mixture of the gas phase partial oxidation together to the liquid phase P (usually used separation This is particularly relevant when carried out in such a way as below the top of the separation column containing the working internal structure.

So combined with at least one C ₃ precursor heterogeneously catalyzed gas phase portion from the product gas mixture of the oxide non-separation method and sharp sharp crystal for making liquid phase P ^* separation of compounds The basic structure of the use is known from DE-A 196087777.

In so doing, a non-sharp separation method is defined as a separation method in which the composition of the phase enriched and containing the desired product formed upon the use of the separation method depends remarkably on the composition of the mixture to be separated. On the other hand, the crystallization process according to the present invention is sharp in that the composition of the acrylic acid crystals formed is substantially independent of the composition of the liquid phase P ^* (ideally completely independent). It is a simple separation method. That is, in the case of a sharp separation method of crystallization, from the thermodynamic point of view, it is sufficient to establish an equilibrium once in order to obtain the desired separation effect, while in the case of a non-sharp separation method, From a dynamic point of view, several successive equilibrium establishments have to be made in order to bring about a significant separation effect (keyword: MC Cabe Thiele method separation stage).

The process according to the invention is such that in the case of such a combination of a sharp separation method and at least one non-sharp separation method, in the continuous operation of such a process mode, in the liquid phase P ^* to be treated according to the invention. Importance increases in that benzoic acid accumulates upon return of the mother liquor (because the mother liquor contains enriched benzoic acid). That is, a relatively small acid content in the product gas mixture of the heterogeneously catalyzed gas phase partial oxidation is also significant problems with Narie, ^(for the process) This problem, A ^BZS according to results from the present invention Can only be reasonably eliminated by regularly ≧ 15. If the impairment rate A ^BZS <15, the method mode to be performed as described is very inefficient. In particular, the process achieves the efficiencies required especially in the large-scale industry only when A ^BZS is regularly ≧ 15 in the process according to the invention.

  However, the situation just mentioned is that the side stream containing benzoic acid occurs when the mother liquor resulting from the process according to the invention is further crystallized for increased yields or in a non-sharp separation process. It is also suitable for the case where crystallization is similarly performed for increasing the yield.

The crystallization treatment according to the invention of the liquid phase P ^* is basically not limited and includes a method of separating the mother liquor from the crystal (in prior art publications cited with regard to crystallization methods considered usable). All mother liquor / crystal separation methods described in the above can be used).

That is, the treatment can be performed continuously or discontinuously in one or more stages. In particular, this can also be carried out as fractional crystallization. Usually, in the case of fractional crystallization, all steps to produce acrylic acid crystals that are more pure than the supplied liquid phase P ^* (especially with respect to benzoic acid) are referred to as purification steps and all other steps. Is called the stripping stage. Depending on the purpose, a multi-step process is then carried out according to the countercurrent principle, in which the crystal is separated from the mother liquor in each step after crystallization, and this crystal at each stage has a somewhat higher purity. While the crystallization residue of each stage is supplied in somewhat lower purity.

In general, the temperature of the liquid phase P ^* is −25 ° C. to + 14 ° C., in particular + 12 ° C. to −5 ° C. during the separation by crystallization.

For example, the method according to the invention may be configured as layered crystallization (see DE-OS 2606364, EP-A 616998, EP-A 648520 and EP-A 768875). In doing so, the crystals are frozen in the form of tightly attached layers that adhere to each other. Separation of the precipitated crystals from the remaining residual melt (mother liquor) takes place by simple outflow of the residual melt. In principle, a distinction is made between “static” and “dynamic” layered crystallization methods. What indicates the character of the liquid phase P ^* dynamic layer crystallization is a forced convection of the liquid phase P ^*. This can be done by pumping the liquid phase P ^* through a full-flow tube, by supplying the liquid phase P ^* as a trickle film (for example described in EP-A 616998) or introducing an inert gas into the liquid phase P ^* . Or by pulsation.

In the case of static processes, the liquid phase P ^* is stationary (for example in a tube bundle or plate heat exchanger) and deposits forming a layer on the secondary side due to a slow temperature drop. Thereafter, the residual melt (mother liquor) is discharged, and with a slow increase in temperature, the more strongly contaminated fraction is removed from the crystalline layer and the pure product is subsequently melted (see WO 01/77056). .

Advantageously according to the invention, the crystallization step according to the invention is suspended in all cases of the liquid phase P ^* described in this publication, however according to the teachings of WO 01/77056, WO 02/055469 and WO 03/078378. Performed as crystallization.

At that time, generally, a crystal suspension containing suspended acrylic acid crystals is produced by cooling the liquid phase P ^* , in which case the acrylic acid crystals have a lower content than the liquid phase P ^* to be purified. The remaining melt (mother liquor) with a higher content of benzoic acid has a higher content of benzoic acid (relative to the total amount of each) than the liquid phase P ^* to be purified. In doing so, the acrylic acid crystals can be grown directly in suspension and / or deposited as a layer on the cooled wall, from which the crystals are subsequently scraped off and the residual melt ( Suspended again in the mother liquor).

  All suspension crystallizers and suspension crystallization methods mentioned in WO01 / 77056, WO02 / 055569, WO03 / 0778378 and Research Disclosure Database Number 497005 (published in August 2005) are contemplated by the present invention. Can be put. Generally, the acrylic acid crystal suspension produced at that time has a solid content of 20 to 40% by mass.

In addition, all methods of separation of the formed suspension crystals and residual mother liquor mentioned in the aforementioned WO publications are taken into account (for example mechanical separation methods such as centrifugation). Advantageously according to the invention, the separation takes place in a washing tower. Preferably it is a washing tower with forced transport of precipitated acrylic acid crystals. The volume fraction of crystals in the crystal layer generally then reaches a value of> 0.5. In the usual case, the washing tower is operated at a value of 0.6 to 0.75. As the washing liquid, preferably a melt of acrylic acid crystals purified (separated) in advance in a washing tower is used. Washing is usually done in countercurrent. The method according to the invention therefore comprises in particular a method comprising the following method steps:
a) Crystallization of acrylic acid from the liquid phase P ^*
b) separation of acrylic acid crystals from the remaining mother liquor (residual melt, residual liquid phase),
c) at least partial melting of the separated acrylic acid crystals and d) at least partial return of the molten acrylic acid crystals to step b) and / or step a).

  In this case, step b) is advantageously carried out by countercurrent washing with the molten pre-separated acrylic acid crystals returned to step b).

Preferably according to the invention (but not in an essential form), the liquid phase P ^* contains water in the application of the process according to the invention. This is because the formation of acrylic acid crystals in the presence of water causes a particularly favorable crystal form in the subsequent separation of the crystals from the remaining mother liquor according to the teachings of WO 01/77056 and WO 03/078378. . This is especially true if the crystallization is carried out as suspension crystallization, and even if the subsequent mother liquor separation is carried out in the washing tower, and even then, already as washing liquid in the washing tower. Applicable when a melt of purified acrylic acid crystals is used.

That is, the method according to the present invention transfers the liquid phase P ^* to be treated by crystallization into a crystal suspension comprising an acrylic acid crystal and a residual liquid phase (residual melt) under the action of a low temperature. (In this case, the mass proportion of benzoic acid in the acrylic acid crystal is smaller than the mass proportion of benzoic acid in the liquid phase P ^* , and the mass proportion of benzoic acid in the residual liquid phase (mother liquor) is the benzoic acid in the liquid phase P ^* . Greater than the acid mass fraction), optionally separating part of the remaining mother liquor from the crystal suspension, and acrylic acid crystals in the washing tower with the following conditions from the remaining mother liquor: Includes methods that are removed:
a) The liquid phase P ^* contains at least 50 ppm by weight or 0.20-20% by weight, often up to 15% by weight, or up to 10% by weight, based on the acrylic acid contained therein, and b) As the washing liquid, a melt of acrylic acid crystals purified in the washing tower is used.

Further, it preferred by the present invention, the liquid phase P ^* of moisture content, in the case of the method the manner described in the previous (or, quite generally, in use of the method according to the present invention), containing a liquid phase P ^* It is a case where it becomes 0.2 or 0.4-8 mass%, or up to 10 mass%, or up to 20 mass% with respect to the acrylic acid made.

  Furthermore, all the above-mentioned methods are applied to the case where the washing tower is a washing tower with forced transport of acrylic acid crystals, and this is especially the case of the hydrodynamics described in WO01 / 77056. Or if it is a mechanical or mechanical wash tower and it is operated as described in the publication.

  Furthermore, all the above-mentioned methods apply in particular when the washing tower is constructed and operated in accordance with the teachings of WO 03/041832 and WO 03/041833.

Basically, the heterogeneously catalyzed gas phase partial oxidation prior to using the separation method according to the invention is carried out as is known from the prior art (the crude C ₃ precursor compound used for the preparation of the reaction gas starting mixture). Aside from having a lower purity).

  That is, it is, for example, DE-A 10351269, DE-A 10245585, DE 1020050227988, EP-A 1695954, DE-A 10351269, EP-A 990636, EP-A 1106598, DE-A 102005010111, DE-A 1020050130399, DE-A 1020040254445, DE-A 102004021764, DE-A 102004021764, DE-A 102004021729, DE-A 10337788, DE-A 10360396, DE-A 10316465, DE-A 10313210, DE-A 10313214, DE-A 10313213, DE-A 10313212, DE 102006202626.4, DE-A 10313208, DE-A 10313208 and DE-A 10313209 It can be performed as described conventional techniques described in these publications to.

  In particular, the process according to the invention can be used for product gas mixtures of two-stage heterogeneous catalytic gas phase partial oxidation from propylene to acrylic acid. In the first reaction stage, propylene is essentially partially oxidized to acrolein, and in the second reaction stage, the acrolein produced in the first reaction stage is partially oxidized to acrylic acid. Usually, acrolein is in this case as a component of the product gas mixture of the first reaction stage, optionally supplemented by molecular oxygen or by a mixture of molecular oxygen and inert gas. To be supplied.

The process according to the invention, inter alia propylene conversion in the propylene partial oxidation ^{U P} is ≧ 91 mol%, or ≧ 92 mol%, or ≧ 93 mol%, or ≧ 94 mol%, or ≧ 95 mol%, or ≧ 96 Suitable when mol%, and ≧ is 97 mol%, or ≧ 98 mol%, or ≧ 99 mol%.

Moreover the process according to the invention, inter alia acrolein conversion ^{U A} in the acrolein partial oxidation ≧ 96 mol%, or ≧ 97 mol%, or ≧ 98 mol%, or ≧ 98.5 mol%, or ≧ 99 mol%, Or ≧ 99.5 mol%, or ≧ 99.8 mol% or higher.

  This is because the conversion rate mentioned above (which is always relative to a single pass of the reaction mixture through the catalyst bed) is usually selected on the basis of the high reaction temperature in each reaction stage when using the same catalyst system, and This is caused by what is achieved when a catalyst coating with a catalyst with increased activity is used. Both promote the formation of benzoic acid by-products.

  In particular, the above applies in the case of the two-stage partial oxidation of propylene to acrylic acid already mentioned.

Catalyst bed for heterogeneously catalyzed partial oxidation of C ₃ precursor compounds are generally fixed bed. Particularly active (fixed bed) catalysts for the partial oxidation of propylene have an active material that is a multi-metal oxide material and a specific surface area of 0.1 to 120 m ² / g, or 0.2 to 50 m ² / g. Or a catalyst that is 1 to ²⁰ m ² / g, or 2 to 10 m ² / g.

  Furthermore, with respect to its activity of the propylene partial oxidation (fixed bed) catalyst for obtaining acrolein, the numerically most commonly indicated pore size of the multimetal oxide active material of the catalyst is 0.1-1 μm. Is advantageous in some cases.

  Accordingly, the presence of enhanced (fixed bed) catalyst activity for the partial oxidation of propylene to acrolein is most commonly indicated in the multi-metal oxide active material as described above. This is a case where the pore diameter and the specific surface area described above exist in combination.

  The total pore volume of the above-mentioned multimetal oxide active material (catalyst) with increased activity is then preferably from 0.1 to 100 ml / g, usually from 0.10 to 0.80 ml / g, or 0.20 to 0.40 ml / g.

  If the catalyst for the heterogeneous catalytic partial oxidation of propylene to acrolein includes the above-mentioned catalyst, it is often already sufficient.

Particularly active (fixed bed) catalysts for partial oxidation of acrolein (especially as the second reaction stage of the two-stage propylene partial oxidation to obtain acrylic acid) are those whose active material is a multi-metal oxide material. The catalyst has a specific surface area of 0.1 to 150 m ² / g, or 0.2 to 50 m ² / g, or 1 to ²⁰ m ² / g, or 2 to 10 m ² / g.

  Furthermore, with regard to its activity of the (a fixed bed) catalyst for partial oxidation of acrolein to obtain acrylic acid, the numerically most commonly indicated pore size of the catalyst multimetal oxide active material is 0.1-1 μm. It is advantageous in some cases.

  Accordingly, the activity of the (fixed bed) catalyst for the partial oxidation of acrolein to acrylic acid is present in the multi-metal oxide active material, which is most commonly indicated numerically. This is a case where the pore diameter and the specific surface area described above exist in combination.

  The total pore volume of the above-mentioned multimetal oxide active material (catalyst) having increased activity is then preferably simultaneously 0.1 to 0.90 ml / g, usually 0.20 to 0.80 ml / g. Or 0.30 to 0.70 ml / g.

  It is often sufficient already if the catalyst for the heterogeneous catalytic partial oxidation of acrolein to acrylic acid includes the above-mentioned catalyst.

  In principle, in the case of the two partial oxidations mentioned above, the volumetric specific activity of the catalyst bed (in particular the fixed catalyst bed) in the direction of flow of the reaction gas mixture can be constant over the length of the flow path. However, the special significance of the process according to the invention is that this volumetric specific activity of the catalyst (fixed) bed in the flow direction of the reaction gas mixture is at least once (continuously or steeply or stepped) in each case. In the case of increase). In all the above cases it is further preferred if the active material composition does not change over the length of the flow path. The above catalyst may be a shell catalyst or an unsupported catalyst. The geometry of the catalyst can in principle be arbitrary. Not only spherical, polygonal and solid cylindrical shapes but also ring shapes are taken into account. The most extended part of the catalyst compact is suitably 1-10 mm, or 2-8 mm. It is understood as the longest line connecting two points on the catalyst compact. The volume specific activity of the fixed catalyst bed can be brought about by a corresponding damping of the catalyst and the shaped body which behaves essentially inert (damped shaped body). Its geometric shape essentially corresponds to that of the catalyst compact. As materials for such inert shaped bodies, in principle, all materials which are also suitable as support materials for shell catalysts suitable according to the invention are taken into account. Such materials are in particular nonporous oxides such as aluminum oxide, silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide, silicates such as magnesium silicate or aluminum silicate or steatite.

The catalytic multi-metal oxide active material with enhanced activity for the partial oxidation of propylene → acrolein suitably contains the metal elements Mo, Fe and Bi according to the invention. They have a particularly enhanced activity are those of the formula I:
^{_{Mo 12 Bi a Fe b X 1}} c X 2 d X 3 e X 4 f O n (I)
[that time,
X ¹ = nickel and / or cobalt X ² = thallium, alkali metal and / or alkaline earth metal,
X ³ = zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and / or tungsten,
X ⁴ = silicon, aluminum, titanium and / or zirconium,
a = 0.5-5,
b = 0.01-5, preferably 2-4,
c = 0 to 10, preferably 3 to 10,
d = 0-2, preferably 0.02-2
e = 0-8, preferably 0-5,
f = 0 to 10 and n = number determined by the valence and frequency of an element different from oxygen in (I)]
This is the case when the stoichiometry is satisfied.

  The above applies in particular when the active material has the properties already mentioned with regard to specific surface area and pore volume.

The catalytic multi-metal oxide active material with enhanced activity for the partial oxidation of acrolein → acrylic acid suitably contains the metal elements Mo and V in terms of application technology. They have a particularly enhanced activity:
^{_{Mo 12 V a X 1 b X}} 2 c X 3 d X 4 e X 5 f X 6 g O n (II),
[that time,
X ¹ = W, Nb, Ta, Cr and / or Ce,
X ² = Cu, Ni, Co, Fe, Mn and / or Zn,
X ³ = Sb and / or Bi,
X ⁴ = one or more alkali metals,
X ⁵ = one or more alkaline earth metals,
X ⁶ = Si, Al, Ti and / or Zr,
a = 1 to 6,
b = 0.2-4,
c = 0.5-18,
d = 0 to 40,
e = 0-2,
f = 0-4,
g = 0 to 40 and n = number determined by the valence and frequency of an element different from oxygen in (II)]
This is the case when the stoichiometry is satisfied.

A particularly active embodiment of the active multimetal oxide (II) is achieved by the following definitions of variants of the general formula II:
X ¹ = W, Nb and / or Cr,
X ² = Cu, Ni, Co and / or Fe,
X ³ = Sb,
X ⁴ = Na and / or K,
X ⁵ = Ca, Sr, and / or Ba,
X ⁶ = Si, Al and / or Ti,
a = 1.5-5,
b = 0.5-2,
c = 0.5-3,
d = 0-2,
e = 0-0.2,
It is determined by the valence and frequency of an element different from oxygen at f = 0-1 and n = II.

Preferably, the heterogeneously catalyzed gas phase partial oxidation of C ₃ precursor compounds is carried out in tube bundle reactors. The catalyst bed is present in the reaction tube of the tube bundle reactor. To exhaust the reaction heat, a heat transfer fluid (generally a metal melt or a liquid metal) is circulated through the catalyst tube.

The loading of the catalyst (fixed) bed with propylene or acrolein is in each case ≧ 70 Nl / l · h, or ≧ 90 Nl / l · h, or ≧ 110 Nl / l · h, or ≧ 130 Nl / l · h, or ≧ It may be 140 Nl / l · h, or ≧ 160 Nl / l · h, or ≧ 180 Nl / l · h, or ≧ 240 Nl / l · h, or ≧ 300 Nl / l · h.
In general, however, the load is ≦ 600 Nl / l · h (the load value is in each case relative to the volume of the catalyst bed, except in the case of an optional combination, in each case). .

  The loading of the catalyst (fixed) bed with the reaction gas starting mixture is defined in this specification as standard liters (Nl; volume in liters taking up the corresponding amount of reaction gas starting mixture at standard conditions, ie at 25 ° C. and 1 bar. ) Is understood as the amount of the reaction gas starting mixture described, which is passed to 1 l of catalyst (fixed) bed per hour. The catalyst (fixed) bed load may be for only one component of the reaction gas starting mixture. The load is then the amount of this component, described in standard liters, which is passed as a component of the corresponding reaction gas starting mixture to 1 l of catalyst (fixed) bed per hour.

  The inert gas is very generally used in this specification in the course of partial oxidation, in each case at least 95 mol%, preferably at least 97 mol% and very particularly preferably 99 mol%. % Or higher and is understood to be a gas that remains unchanged chemically.

  The reaction temperature is generally 270 to 450 ° C. or 280 to 420 ° C., preferably 300 to 380 ° C., in the partial oxidation of propylene → acrolein. The reaction temperature is generally 200 to 370 ° C. or 200 to 320 ° C., preferably 220 to 300 ° C. in the partial oxidation of acrolein → acrylic acid.

  In the case of partial oxidation according to the invention, the working pressure can generally be below the standard pressure (for example up to 0.5 atm, the reaction gas mixture is suction filtered) or above the standard pressure. Typically, the working pressure is 1 to 5 atm, frequently 1.5 to 3.5 atm. Usually, the working pressure does not exceed 100 atm during the partial oxidation according to the invention.

  As a source of molecular oxygen required as an oxidant within the range of partial oxidation, not only air but also air depleted for molecular nitrogen is taken into account.

  The selection of the catalyst to be used for the partial oxidation according to the invention from propylene (to acrolein) or from acrolein (to acrylic acid) is generally such that the selectivity for the desired product formation (acrolein or acrylic acid) is Generally, it is carried out to be ≧ 83 mol%, frequently ≧ 85 mol%, or ≧ 88 mol%, often 90 mol%, or ≧ 93 mol% or more.

A typical reaction gas starting mixture for partial oxidation of propylene (to obtain acrolein) is for example:
5-12 vol% propylene,
2-15% by volume of water,
Propane ≧ 0-10% by volume,
Ingredients different from propylene, propane, water, oxygen and nitrogen ≧ 0.1-5% by volume,
It may contain molecular oxygen in an amount such that the molar ratio of molecular oxygen to propylene is 1 to 3, and molecular nitrogen as the remaining amount, which is 100% by volume of the total amount.

Alternatively, the reaction gas starting mixture for propylene partial oxidation (to obtain acrolein) is:
6-10% by volume of propylene,
Molecular oxygen 8-18% by volume,
Propane 6-30% by volume and molecular nitrogen 32-72% by volume
May be contained.

The reaction gas mixture for propylene → acrolein partial oxidation may also contain up to 20% by volume of H ₂ .
The reaction gas starting mixture for acrolein partial oxidation (to obtain acrylic acid) is for example:
Acrolein 4-8% by volume,
2-9% by volume of molecular oxygen,
0-30% by volume of propane,
Molecular nitrogen 30-75 vol% and water vapor 5-30 vol%,
Or 3-25% by volume of acrolein,
5 to 65% by volume of molecular oxygen,
6-70% by volume of propane,
Molecular hydrogen 0-20% by volume and water vapor 5-65% by volume
Containing.

  Particularly suitable according to the invention, the product gas mixture is still ≧ 0-4% by volume (for example up to 0.5% by volume, or up to 1% by volume, or up to 1.5% by volume, or 2% by volume) Such partial oxidation from propylene (to acrolein) or from acrolein (to acrylic acid) containing up to or up to 2.5% by volume of molecular oxygen.

Basically, the organic content of that component in the liquid phase P ^* can be determined by gas chromatography.

  In principle, absorption and / or condensation measures are taken into account for the transition of the acrylic acid and benzoic acid contained in the product gas mixture of the partial oxidation to be carried out according to the invention to the liquid phase P. .

  As absorbents, for example, water, aqueous solutions (which may contain, for example, 0.1 to 10% by weight acetic acid, 0.1 to 5% by weight acrylic acid and 80 to 99.8% by weight water) Organic (especially hydrophobic) solvents (for example diphenyl ether, diphenyl and / or dimethyl phthalate) are taken into account. Prior to absorption, the product gas mixture of the partial oxidation may be subjected to further direct and / or indirect cooling depending on the application purpose.

  Absorption and / or condensation processes more suitable according to the invention are described, for example, in the publications DE-A 10336386, WO 01/96271, DE-A 1963 645, DE-A 1950 325, EP-A 982289, DE-A 198 845, WO 02/076917, EP-A 1695954, EP -A695736, EP-A778225, EP-A1041062, EP-A982287, EP-A982288, US2004 / 0242826, EP-A792867, EP-A7884046, EP-A6959573, EP-A11259912, EP-A13885533 and these in the literature References cited with respect to

  However, the liquefaction of acrylic acid and benzoic acid contained in the product gas mixture can also be carried out by condensation of components of the product gas mixture having a higher boiling point than water.

The transfer of acrylic acid and benzoic acid to liquid phase P not only by absorption but also by condensation, as well as their transfer to liquid phase P ^* , is usually an internal structure with a separating action (to increase the exchange surface area) In a separation column containing Absorption and condensation can also be applied to each other.

  Separation columns suitable for the absorption and / or condensation methods mentioned above are disclosed in particular in DE-A 10336386, EP-A 1125912 and US 2004/0242826 A1.

  In doing so, basically all internal structures known as having a separating action are taken into account. That is, trays such as bubble cap trays, dual flow trays, or valve trays, or packings such as Raschig rings or regular packings, such as Sulzer packs, can also be used as internal structures with a separating action.

  In the separation column, the product gas mixture of the partial oxidation, which has been optionally cooled in advance, is then led generally upwards from below. Within the range of absorption condensation, the absorbent is usually moved (guided) from top to bottom in the separation column.

A liquid phase P ^* containing acrylic acid and benzoic acid is optionally produced from the separation tower, depending on the application purpose, from the liquid bottom or from the liquid side discharge. It can be taken out below the supply part of the product gas mixture.

Under the use of at least one thermal separation method, here a component having a lower boiling point (relative to atmospheric pressure) than benzoic acid and acrylic acid is separated from the liquid phase P.
Preferably, according to the invention, at least one thermal separation method is used so that the low boiler separation extends to the components of the liquid phase P as pure substances and whose boiling point at atmospheric pressure is below the boiling point of water. The

  A thermal separation method that can be used particularly advantageously is stripping with a gas (for example air, molecular oxygen or other gases). For this purpose, the gas is led to the liquid phase P (preferably in terms of applied technology, preferably in countercurrent in a separation column containing an internal structure with a separating action) and in this case the low content contained therein Boiling components are stripped out of the liquid phase during flow. Preferably, the liquid phase P is preheated for this purpose. Suitably, the stripping is performed at a working pressure that is below ambient pressure.

  By the latter measure, the stripping is superimposed on the adsorption. Of course, adsorption on the other hand can also be used by itself for low boiler separation.

  As another option for stripping and / or adsorption, or in succession, low boiler separation by rectification may be performed. In particular, to determine a separation line at the boiling point of water, it is suitable for the purpose to carry out low boiler separation by rectification at least partly as azeotropic rectification. Suitable azeotropic additives in this connection are, for example, heptane, dimethylcyclohexane, ethylcyclohexane, toluene, ethylbenzene, octane, chlorobenzene, xylene or mixtures thereof (for example from 60% by weight toluene and 40% by weight heptane). ). However, methyl isobutyl ketone or isopropyl acetate can also be used as other azeotropic additives.

  Still other suitable azeotropic additives are disclosed in US 2004/0242826, EP-A 778255, EP-A 695736 and the prior art cited in these publications. Usually, the rectification described above is likewise carried out at a working pressure which is preferably below the atmospheric pressure. As will be appreciated, all the thermal separation methods described above can be used not only by themselves, but also in combination with one or more of the other listed thermal separation methods.

  Preferably, according to the present invention, the thermal separation process is used only for low boiler separation, so that the process mode according to the present invention has as a further advantage, inter alia, only a slight degree of unwanted polymer formation. And / or the amount of polymerization inhibitor required is particularly small.

  However, it should be understood that all process steps mentioned in this publication are also carried out with polymerization inhibition. In doing so, it may be carried out as described in the cited prior art. Of the total amount of acrylic acid process stabilizer that can be used, the most prominent positions are dibenzo-1,4-thiazine (PTZ), 4-hydroxy-2,2,6,6-tetramethylpiperidine-1. -Oxyl (4-OH-TEMPO) and p-methoxyphenol (MEHQ), each of which, alone or in pairs, or as a ternary mixture, especially the mixture to be treated according to the invention, It may be a liquid phase component. Usually, the whole quantity of the polymerization inhibitor contained in a liquid phase containing acrylic acid is 0.001-2 mass% with respect to the whole quantity of acrylic acid contained.

After carrying out the described low boiler separation, generally the liquid phase P ^* containing at least 80% by weight of acrylic acid and at least 0.1% by weight of benzoic acid relative to the amount of acrylic acid contained is Remains. For acrylic acid contained in the liquid phase P ^* , this preferably contains at least 50 ppm by weight or at least 0.2% by weight or 0.2-15% by weight of water.

If left unattended, undesired formation of acrylic acid oligomers (Michael adducts) occurs in the liquid phase containing acrylic acid, so the crystallization separation according to the present invention can be performed as much as possible after the preparation of the liquid phase P ^*. Applied immediately. Similarly, by the reverse thermal desorption of acrylic acid oligomers enriched in the mother liquor, the acrylic acid contained in these is recovered and used for obtaining the liquid phase P and / or P ^*. Can be returned to one of the thermal separation methods.

  In general, following the process according to the invention, a process is carried out in which an acrylic acid crystal is melted and a polymer is obtained by radical polymerization. Frequently, the aforementioned polymers are superabsorbent polyacrylates.

Example A stirred tank with an internal volume of 2 liters was used. The geometric shape of the stirring vessel was cylindrical and had an inner diameter of 110 mm. The stirring tank was made from glass. However, the bottom surface of the stirring vessel was made of stainless steel with a jacket. The thickness of the stainless steel was 2 mm. The distance between the two walls was about 1 cm. In the space between the two walls, a mixture consisting of 30% by weight of water, 40% by weight of methanol and 30% by weight of ethylene glycol was allowed to flow as a cooling medium (at least 75 l / h).

  The contents of the stirring tank were stirred using a paddle type stirring device equipped with two stirring blades (the paddle height was 50 mm, the diameter was 57 mm, and the rotation speed was 500 rpm).

  In all tests, the stirred tank was filled with 1500-1650 g of purified acrylic acid having an acrylic acid content of ≧ 99.5% by weight and doped with benzoic acid of various contents (its temperature was 25 ° C. Met). Its water content was 148 mass ppm. In addition, the purified acrylic acid was polymerized by phenothiazine (PTZ) having a content of about 250 to 270 mass ppm. With a cooling rate of 15 K / h to 60 K / h, the temperature of the cooling medium (in each case starting from a starting temperature of 11 ° C.) was continuously reduced.

The crystallizing process was interrupted each time after a crystal layer having a mass M of about 10 mm having a mass M was deposited on the bottom of the tank. The stirring tank was rotatably attached. Along with the interruption of the crystallization process, the stirring tank was rotated at the top to discharge the remaining mother liquor (mother acid). Subsequently, the discharged mother liquor and the precipitated crystals were analyzed with respect to their benzoic acid contents, and the respective impairment rates A ^BZS were examined from the analysis results.

  The results obtained are shown in the following table. The table additionally shows the crystal growth rate W in mm / min and the cooling time t in min.

  US Provisional Patent Application No. 60 / 852,674 filed Oct. 19, 2006 is incorporated into this application by reference. Many modifications and variations of the present invention are possible in light of the above teachings. Accordingly, it may be assumed that the invention may be practiced otherwise than as specifically described herein within the scope of the appended claims.

Claims (22)

Acrylic acid and benzoic acid contained in the product gas mixture of heterogeneously catalyzed gas phase partial oxidation of C ₃ precursor compound of acrylic acid as main product and by-product in addition to other product gas mixture components Wherein acrylic acid and benzoic acid are transferred from the product gas mixture to the liquid phase P together with other product gas mixture components having lower and higher boiling points than acrylic acid, And the component which has a boiling point lower than benzoic acid and acrylic acid from the liquid phase P which arises in the use of at least 1 thermal-separation method, at least 80 mass% acrylic acid, and the amount of acrylic acid contained respect to a method of separating a liquid phase P ^* remaining under containing benzoic acid of at least 0.1 wt ‰, crystallization from a liquid phase P ^* the separation of the acid from acrylic acid Performed more, time, method of acrylic acid is enriched in the crystal to be formed, and benzoic acid, characterized in that the enrichment in the mother liquor remaining. 2. Process according to claim 1, characterized in that the liquid phase P ^* contains at least 90% by weight of acrylic acid. 2. Process according to claim 1, characterized in that the liquid phase P ^* contains at least 95% by weight of acrylic acid. The method according to any one of claims 1 to 3, characterized in that the liquid phase P ^* contains at least 0.3 mass benzoic acid with respect to the amount of acrylic acid contained. The method according to any one of claims 1 to 3, characterized in that the liquid phase P ^* contains at least 0.5 parts by mass of benzoic acid with respect to the acrylic acid contained. The content of benzoic acid in the liquid phase P ^*, characterized in that at least 50% by weight of the benzaldehyde contained in the liquid phase P ^*, the method of any one of claims 1 to 5 . The content of benzoic acid in the liquid phase P ^*, characterized in that at least 100% by weight of the benzaldehyde contained in the liquid phase P ^*, the method of any one of claims 1 to 5 . The content of benzoic acid in the liquid phase P ^*, characterized in that at least 150% by weight of the benzaldehyde contained in the liquid phase P ^*, the method of any one of claims 1 to 5 . Contain at least partly the mother liquor remaining in the crystallization separation, in at least one thermal separation method used to produce the liquid phase P ^* from the liquid phase P and / or in the product gas of the gas phase partial oxidation 9. The process as claimed in claim 1, wherein the acrylic acid and benzoic acid are returned to at least one process used for transferring them together to the liquid phase P. 10. The process as claimed in claim 1, wherein the crystallization separation of acrylic acid from the phase P ^* is carried out in one stage. 10. The process as claimed in claim 1, wherein the crystallization separation of acrylic acid from the phase P ^* is carried out in multiple stages.   12. The method according to claim 1, wherein the crystallization separation of acrylic acid is performed by layered crystallization.   The process according to claim 1, wherein the crystallization separation of acrylic acid is carried out by suspension crystallization.   14. Process according to claim 13, characterized in that the suspension crystals formed are separated from the remaining mother liquor in a washing tower.   15. The process according to claim 14, characterized in that a melt of acrylic acid crystals previously separated in the washing tower is used as the washing liquid. The following method steps:
a) a step of crystallizing acrylic acid from the liquid phase P ^* ,
b) separating the acrylic acid crystals from the remaining mother liquor,
c) at least partially melting the separated acrylic acid crystals and d) at least partially returning the molten acrylic acid crystals to step b) and / or step a). Item 16. The method according to any one of Items 1 to 15. 17. A process according to any one of the preceding claims, characterized in that the liquid phase P ^* contains at least 150 ppm by weight of water.   18. The process according to claim 1, wherein acrylic acid and benzoic acid contained in the product gas mixture of the gas phase partial oxidation are transferred by absorption with an aqueous solution. C ₃ wherein the precursor compound is propylene, any one process of claim 1 to 18. C ₃ wherein the precursor compound is propane, any one process of claim 1 to 18. The method according to claim 1, wherein the C ₃ precursor compound is acrolein and the acrolein conversion rate in the acrolein partial oxidation is ≧ 99.5 mol%.   The method according to any one of claims 1 to 21, characterized in that the method of melting an acrylic acid crystal and obtaining a polymer by radical polymerization is subsequently carried out.