KR20090082897A - Method of separating the acrylic acid from benzoic acid contained in a product gas mixture from a heterogeneously catalyzed gas phase partial oxidation of c3-precursor compound of acrylic acid - Google Patents

Abstract

A method of separating acrylic acid-containing acrylic acid and benzoic acid in a product gas mixture from a partial oxidation, in which the acrylic acid and the benzoic acid are first converted into a liquid phase, components lighter than benzoic acid and acrylic acid are boiled away by means of thermal separation methods, and the acrylic acid is separated by crystallization from the remaining liquid phase.

Description

A method for separating acrylic acid from benzoic acid contained in the product gas mixture of the C3-precursor compound of acrylic acid. PHASE PARTIAL OXIDATION OF C3-PRECURSOR COMPOUND OF ACRYLIC ACID}

The present invention relates to a process for separating acrylic and benzoic acid as well as other product gas mixture components present as a main product and by-product in a heterogeneous catalyzed partial gas phase oxidation product gas mixture of C ₃ precursor compounds of acrylic acid, wherein acrylic acid and Benzoic acid is converted to liquid phase P in the product gas mixture with the other components of the product gas mixture having a lower boiling point and higher boiling point than acrylic acid, and the lower boiling point component than benzoic acid and acrylic acid is produced in liquid phase P using one or more thermal separation processes. Is removed, leaving a liquid phase P ^* comprising at least 80% by weight acrylic acid and at least 0.1% by weight benzoic acid (based on the amount of acrylic acid present).

Acrylic acid is an important monomer that is used by itself and / or in the form of alkyl esters to obtain polymers (e.g. water-superabsorbent polymers) used in the sanitary arts (see, for example, WO 02/055469 and WO 03/078378).

Acrylic acid can be prepared by heterogeneously catalyzed partial oxidation of C ₃ precursor compounds (propylene, propane and / or acrolein) (eg EP-A 990636, US-A 5198578, EP-A 1015410, EP-A 1484303, EP-A 1484308, EP-A 1484309 and US-A 2004/0242826). Appropriately from the application point of view, the C ₃ precursor compound used is generally a relatively high purity C ₃ precursor compound (see DE-A 10131297). However, obtaining, for example, crude propylene in high purity is quite difficult and expensive, and generally C ₃ of acrylic acid Several purification steps are included to isolate the precursor compound to high purity (see DE-A 3521458). According to DE-A 12246119 and DE-A 10245585, unless the purpose of the reaction gas mixture generated by the process is in particular from the partial oxidation, compromising the performance of the catalyst impurity C ₄ It should be as minimal as possible. The disadvantage of this process is that the purification step described above is complex and therefore in some cases economically viable crude C _3. Only relatively limited separation operations are applied to obtain precursor compounds (eg crude propylene).

Through thorough in-house research, C ₃ Crude C ₃ used for heterogeneous catalyzed partial oxidation of precursor compounds (eg propylene) It has been found that benzoic acid can be formed as a byproduct of acrylic acid formation if the precursor compound (eg crude propylene) still contains butadiene or a compound that can be converted to butadiene in the partial oxidation process. The cause of benzoic acid formation is probably due to the C ₃ dienophiles present in the reaction gas mixture of heterogeneously catalyzed partial oxidation, such as propylene, acrolein, and the Diels-Alder reaction of acrylic acid and butadiene and the addition of adducts to aromatics This may be due to oxydehydrogenation. Heterogeneous catalyzed oxydehydrogenation is probably catalyzed by the same catalyst as the actual desired gas phase oxidation. If appropriate, Article C ₃ Slight contamination of the precursor compound (eg crude propylene) with o-xylene can cause benzoic acid byproduct formation. The use of multimetal oxide catalysts with increased activity (or catalyst charges whose volume-specific activity increases at least once in the direction of increasing reactant conversion), the additional use of water vapor as an inert diluent gas, and certain oxidation steps (eg , C _{3 to} be partially oxidized in propylene-acrylic acid) Excess molecular oxygen relative to the compound promotes the formation of benzoic acid by-products, as well as elevated reaction temperatures, and the oxidation of propylene in certain oxidation steps (e.g., the first oxidation step of two-step partial oxidation from propylene to acrylic acid). Or oxidation of acrolein in a second oxidation stage of partial oxidation of propylene to acrylic acid, or oxidation of acrolein in independent acrolein partial oxidation to acrylic acid, or propane in one-step oxidation from propane to acrylic acid). _It is found to promote the conversion of the _three precursor compounds. In many cases this assumption is supported by the fact that only benzaldehyde is found as a byproduct of acrylic acid formation and no benzoic acid is found. In contrast, benzoic acid generally involves benzaldehyde as an acrylic acid byproduct.

Both benzaldehyde and benzoic acid are undesired byproducts of acrylic acid. This is a relatively reactive aromatic compound, which is not totally harmless and therefore undesirable (this also applies correspondingly to esters of benzoic acid). When the obtained acrylic acid and / or its alkyl esters are used to prepare polymers for use in hygienic applications (eg, absorbent polymers in diapers), the acrylic acid and / or alkyl acrylates used are absent from benzoic acid and benzaldehyde. Should be.

C ₃ of acrylic acid in combination with different separation processes It is known that acrylic acid can be removed from the product gas mixture of heterogeneous catalyzed partial gas phase oxidation of precursor compounds (eg propylene). The combination applied in the particular case is generally dependent on the type and amount of the second component other than acrylic acid present in the product gas mixture and on the desired purity of acrylic acid which is generally determined by the particular use of acrylic acid.

Essential components of this combination of different separation processes are usually formed by thermal separation processes. The thermal separation process involves gas (rising) and liquid (falling) streams being passed countercurrent in a separation column containing separation inclusions, where heat and mass transfer is achieved by gradients between the streams, which in turn leads to Separation is made.

Examples of such thermal separation processes are (partial) condensation, fractional condensation (see DE-A 19924532) and rectification. The separation action that is caused is based herein on the difference in boiling points of the second component, in particular acrylic acid and other than acrylic acid. A further example is absorption. The separation action is based here in particular on the difference in solubility of acrylic acid and the second component other than acrylic acid in the absorbent liquid. This involves the thermal separation process of stripping (the stripping gas absorbs from the liquid a component with a different affinity present in dissolved form in the liquid) and desorption (reverse process of absorption; the substance dissolved in the liquid phase is removed by lowering the partial pressure). This also applies to poems. The term "thermal separation process" also encompasses azeotropic distillation and rectification (which uses the different tendencies of acrylic acid and the second component (components other than acrylic acid in the reaction gas mixture of partial oxidation) to remove added azeotrope and azeotrope. Forming).

The boiling point difference between acrylic acid (141 at 1 atm) and benzaldehyde (178.1 at 1 atm) is generally insufficient to quantitatively remove benzaldehyde from acrylic acid with an appropriate level of complexity using this thermal separation process. Instead, benzaldehyde removal is usually achieved only by further exploiting the difference in reactivity of the so-called aldehyde scavenger (WO 03/014172), for example hydrogencarbonate aminoguanidine with benzaldehyde and acrylic acid. The reaction of the aldehyde with the aldehyde scavenger results in the formation of a new second component having a boiling point that differs significantly from the boiling point of acrylic acid than in the case of the aldehyde itself, and thus upon completion of the reaction with the aldehyde scavenger, Quantitative removal can be achieved in a simple manner. Alternatively, EP-A 1159249 discloses separation of benzaldehyde and acrylic acid present in the product gas mixture of partial oxidation by a combination of thermal separation processes (fractional condensation) and crystallization (suspension crystallization) (see also DE-A 10247240). Is disclosed.

In contrast to the above, the boiling point difference between acrylic acid (1 atm at 141 ° C.) and benzoic acid (1 atm at 250 ° C.) is very large and the use of a thermal separation process in which the separation operation is based on the boiling point difference of the mixture components It usually involves quantitative separation of acrylic acid and benzoic acid (see DE-A 10336386, DE-A 19740252, DE-A 10247240), (the additional second component is often removed by crystallization after benzoic acid removal). However, a disadvantage of this thermal separation is that the concentrated form fraction containing acrylic acid must be taken off from the separation column comprising the separation inclusion located above the feed point of the mixture to be separated into the separation column. . However, relatively large amounts of energy are consumed for the evaporation of the total amount of acrylic acid (high boiling point, high evaporation enthalpy, multiple evaporation by reflux), which is too much to perform thermally.

The object of the present invention is therefore that C _{3 of} acrylic acid does not require the above energy-intensive separation process. A heterogeneous catalyzed partial gas phase oxidation of the precursor compound is to provide a process for separating acrylic acid and benzoic acid present as the main product and by-product in a gas mixture.

Thus, C _{3 of} acrylic acid A process for separating acrylic acid and benzoic acid present as a main product and by-product in a product gas mixture of heterogeneous catalyzed partial gas phase oxidation of a precursor compound is provided, wherein the components together with other components of the product gas mixture have a lower and higher boiling point than acrylic acid. Acrylic acid and benzoic acid are converted to liquid phase P in the product gas mixture and have a lower boiling point (based on the boiling point of the pure material at 1 atm) than benzoic acid and acrylic acid (benzoic acid free; where "no" is based on the benzoic acid content of liquid phase P respectively) Less than 10 weight percent, preferably less than 5 weight percent, more preferably less than 3 weight percent, or less than 2 weight percent, or less than 1 weight percent, most preferably less than 0.75 weight percent, or less than 0.1 weight percent, or 0 wt%) is removed from the resulting liquid phase P using at least one thermal separation process to provide at least 80 wt% acrylic acid, and At least 0.1 percent per mille (usually at least 0.2 percent, or at least 0.3 percent, or at least 0.4 percent, or at least 0.5 percent, or at least 0.75 percent, or 1 percent) The liquid phase P ^* containing the benzoic acid of the above) remains, which includes separating the benzoic acid from acrylic acid by crystallization from the liquid phase P ^* , and the acrylic acid accumulates in the crystal to be formed, and the benzoic acid remains in the mother liquor.

The process according to the invention is based on the surprising finding that the deletion coefficient A ^BZA (based on the process) associated with crystallization is usually at least 15. Deletion coefficient A ^BZA is generally the quantitative ratio of benzoic acid remaining in the mother liquor to benzoic acid remaining in the crystal (in each case expressed in terms of weight percent based on the total amount of mother liquor and the total amount of crystals). It is generally sufficient for A ^BZA determinations to remove the crystals / mother liquor by more than 90%, preferably more than 95%, or more than 97%, more than 98%, and more than 99% by weight of the total amount thereof. The effect of residual moisture content on the surface is generally negligible). A ^BZA of 15 or greater confirms that substantially no benzoic acid is introduced into the crystal upon formation of the acrylic acid crystal. This forms the basis of the good efficiency of the procedure of the present invention.

For diacrylic acid (A ^DA ), acetic acid (A ^AA ) and propionic acid (A ^PA ) as the second component of acrylic acid, the corresponding deletion coefficient is usually 10 or less. In other words, they are also incorporated into the acrylic acid crystals, which can be difficult to extract from the crystals, for example by suitable washing.

In other words, to crystallize such impurities from acrylic acid, the use of less efficient, more capital-intensive multistage crystallization processes in the form of multistage combinations of dynamic and static crystallization, for example, as recommended in EP-A 616998, It is generally required and requires one or more dynamic crystallizers and one or more static crystallizers as well as multistage processes. Optimally, under certain boundary conditions of crystallization (see especially WO 03/078378 and WO 01/77056), acrylic acid crystals can be formed and subsequently washed with pure acrylic acid melt to remove acetic acid and propionic acid relatively efficiently. The larger the A ^BZA , the easier the crystallization of benzoic acid from acrylic acid.

Based on the experimental findings, the procedure of the present invention is essentially performed by subsequent washing of the crystals with a pre-purified acrylic acid melt in liquid phase P ^* during the crystallization step of the benzoic acid contaminants that hinder its use in the acrylic acid production route suitable for the superabsorbent. Enable quantitative removal

The expressions "acrylic acid as main product" and "benzoic acid as byproduct" herein simply mean that the product gas mixture of partial oxidation contains significantly more acrylic acid than benzoic acid. In other words, acrylic acid is the desired target product of partial oxidation, and benzoic acid is essentially an undesired by-product thereof.

The process according to the invention has an acrylic acid content of at least 85%, or at least 90%, or at least 95%, or at least 96%, or at least 97%, or at least 98%, or at least 99%, Or in particular in liquid phase P ^{* of} at least 99.5% by weight or more.

In this process, the content of benzoic acid, based on the amount of acrylic acid present, in the acrylic acid content of all liquid phase P ^* (which, of course, includes 80% by weight of acrylic acid content in liquid phase P ^* ), is for example in each case from 0.1 to 20 weights, or 0.2 to 15 weights, or 0.3 to 10 weights, or 0.4 to 8 weights, or 0.5 to 6 weights, or 0.75 to 4 weights, or 1 to 3 weights. Generally, the content of benzoic acid is 100 wt% or less, mainly 75 wt% or less, or 50 wt% or less, based on the acrylic acid present in the liquid phase P ^* . Use of the process according to the invention, the liquid phase P ^* the content of the acid of benzaldehyde by weight present in the liquid phase P ^* of (more than 0), 50% by weight or more, or 75 wt% or more, or 100% or more, or 125 It is particularly advantageous when at least by weight, or at least 150% by weight, or at least 175% by weight, or at least 200% by weight, or at least 250% by weight, or at least 300% by weight, or at least 400% by weight, or at least 500% by weight. Naturally the liquid phase P ^* to be treated according to the invention may not contain any benzaldehyde.

The process according to the invention is characterized in that at least a portion of the mother liquor remaining after crystallization removal (from liquid phase P ^* ) is recycled to one or more of the thermal separation processes applied to obtain liquid phase P ^* from liquid phase P and / or gas phase partial oxidation. Of particular importance when carried out in a recycled manner (usually below the top of a separation column containing the inclusions used) in one or more of the processes applied to the combined conversion of acrylic acid and benzoic acid to liquid phase P present in the product gas mixture of Do.

One or more C ₃ The basic structure of the combined use of separate and non-individual separation processes of crystallization for obtaining liquid phase P ^* from the product gas mixture of heterogeneous catalyzed partial gas phase oxidation of precursor compounds is known from DE-A 19606877.

A non-individual separation process is defined as a separation process comprising a concentrated target product and the composition of the phase formed when the separation process is used, which depends largely on the composition of the mixture to be separated, but the crystallization treatment of the present invention is characterized by Individual separation processes to a large degree (ideally completely independent) from the composition of the liquid phase P ^* . In other words, in the individual separation process of crystallization, the desired separation operation is sufficiently achieved by establishing one equilibrium from the thermodynamic point of view, whereas in the non-individual separation process, several equilibriums must be established continuously from the thermodynamic point of view in order to achieve a significant separation operation. (Note: McCabe-Thiele separation step).

In the case of this combination of an individual separation process and one or more non-individual separation processes, the mother liquor contains benzoic acid in concentrated form as a result of the mother liquor recycling upon successive operation of this procedure such that benzoic acid accumulates in the liquid phase P ^* to be treated according to the invention. The process according to the invention is therefore more important. In other words, a relatively low benzoic acid content in the product gas mixture of heterogeneous catalyzed partial gas phase oxidation can be a serious problem, which is preferred due to A ^BZA (depending on the process) which is customarily at least 15 according to the findings of the present invention. Can be solved. If the deletion factor A ^BZA is less than 15, the procedure to be carried out as described above will be extremely inefficient. In the process according to the invention A ^BZA must be customarily at least 15 to obtain the efficiencies required, especially on an industrial scale.

In addition, the above facts provide for the purpose of increasing the yield of the benzoic acid-containing second stream which is generated in further crystallization of the mother liquor obtained in the process according to the invention, or similarly in a non-individual separation process in order to increase the yield. It is also relevant in the crystallization process.

There is in principle no limitation on the crystallization treatment of liquid phase P ^* , including the process of removing the mother liquor from the crystals of the present invention (all mother liquor / crystal removal processes described in the cited prior art literature can be used).

In other words, one or more of the steps can be carried out continuously or batchwise. In particular, it can also carry out by fractional crystallization. Usually in fractional crystallization, every step of producing acrylic acid crystals that are purer than the liquid phase P ^* supplied (especially with a lower benzoic acid content) is known as the purification step, and all other steps are stripping steps. Suitably, the multistage process removes the crystals from the mother liquor after each crystallization at each stage and feeds these crystals of the particular stage to the next highest purity, while the crystallization residues of the particular stage are next to the lowest purity. It operates on the countercurrent principle supplied.

Generally the temperature of the liquid phase P ^* during the crystallization removal process is from -25 ° C to +14 ° C, in particular from +12 ° C to -5 ° C.

For example, the process according to the invention can be carried out by layer crystallization (see DE-A 2606364, EP-A 616998, EP-A 648520 and EP-A 776875). In this crystallization, the crystals are frozen in the form of continuous, tightly attached layers. Since the residual melt flows easily, the precipitated crystals separate from the remaining residual melt (mother liquor). In principle, "static" and "dynamic" layer crystallization processes are distinguished. Characterized in the dynamic layer crystallization of the liquid phase P ^* is a forced convection of the liquid phase P ^*. This allows pumping circulation of liquid phase P ^* through a full-flow tube, introducing liquid phase P ^* as a trickle film (eg according to EP-A 616 998), or introducing an inert gas into liquid phase P ^* Or pulsed.

In a static process, the liquid phase P ^* is stationary (for example in a tube bundle or a plate heat exchanger) and precipitates in the bed due to the slow temperature decrease in the second aspect. The residual melt (mother liquor) is then discharged and the temperature is gradually increased to drain off more contaminated fractions from the crystal bed, followed by melting of the pure product (see WO 01/77056).

However, according to the invention, for all liquid phases P ^* described herein, the crystallization step of the invention will preferably be carried out by suspension crystallization according to the teachings of WO 01/77056, WO 02/055469 and WO 03/078378.

Generally, acrylic acid crystals have a lower benzoic acid content than the liquid phase P ^{* to} be purified, and the residual melt (mother liquor) has a higher benzoic acid content (relative to a specific total amount and relative) than the liquid phase P ^{* to} be purified, suspended acrylic crystals. A crystal suspension containing is obtained by cooling the liquid phase P ^* . Acrylic acid crystals can be grown directly in the suspension and / or precipitated in layers on the cooled wall and then scratched off and resuspended in the residual melt (mother liquor).

All suspension crystallizers and suspension crystallization processes described in detail in WO 01/77056, WO 02/055469, WO 03/078378 and Research Information Database No. 496005 issued August 2005 are useful according to the invention. Generally, the solids content of the resulting acrylic acid crystal suspension is 20 to 40% by weight.

In addition, all processes specified in all of the above WO publications are suitable for the separation of formed suspension crystals and residual mother liquors (eg mechanical separation processes such as centrifugation). According to the invention, separation in the washing tower is preferred. It is preferably a washing tower for forced transport of precipitated acrylic acid crystals. The crystal volume fraction in the crystalline layer generally reaches above 0.5. In general, the washing tower is operated at values of 0.6 to 0.75. The washing liquid used is advantageously a melt of acrylic acid crystals (removed) which have been previously purified in the washing tower. Washing is usually done in countercurrent. The process according to the invention thus comprises in particular a process comprising the following process steps:

a) crystallizing acrylic acid from liquid phase P ^* ;

b) separating the acrylic acid crystals from the residual mother liquor (residual melt, residual liquid phase);

c) at least partially melting the removed acrylic acid crystals, and

d) at least partially recycling the molten acrylic acid crystals to step b) and / or step a).

Preference is given to performing step b) by countercurrent washing with acrylic acid crystals which have been removed beforehand, melted and recycled to step b).

Advantageously, according to the invention (but not necessarily), when using the process of the invention the liquid phase P ^* comprises water, which according to the teachings of WO 01/77056 and WO 03/078378 crystals of acrylic acid in the presence of water This is because the formation of crystals produces a crystalline form which is particularly advantageous for the separation of crystals from subsequent residual mother liquors. This is especially true when the crystallization is carried out as suspension crystallization, more particularly when the subsequent mother liquor removal is carried out in the washing tower, more particularly when the washing liquid used is a melt of acrylic acid crystals already purified in the washing tower.

In other words, the process according to the invention converts, in particular, the liquid phase P ^* to be treated by crystallization into a crystal suspension consisting of acrylic acid crystals and a residual liquid phase (residual melt) under cold conditions, and the weight ratio of benzoic acid in the acrylic acid crystals in liquid phase P ^* Less than the weight ratio of benzoic acid, the weight ratio of benzoic acid in the residual liquid phase (mother liquor) is greater than the weight ratio of benzoic acid in the liquid phase P ^* , a portion of the residual mother liquor is mechanically removed from the crystal suspension if appropriate, and acrylic acid crystals in the residual mother liquor of the washing tower In glass, only

a) the liquid phase P ^* comprises at least 50 ppm by weight, or 0.20 to 20% by weight, often at most 15% by weight, often at most 10% by weight, based on the acrylic acid present therein,

b) The washing liquid used is a melt of purified acrylic acid crystals in the washing tower.

Process.

In addition, according to the invention, the water content of liquid phase P ^* in the above procedure (or quite generally when using the method according to the invention) is 0.2% or 0.4% by weight, based on acrylic acid present in liquid phase P ^* . It is advantageous to be 8% by weight or less, or 10% by weight or less, or 20% by weight or less.

All the above-mentioned substrates apply in particular when the washing tower is a washing tower forcibly transporting acrylic acid crystals, in particular when it is a hydraulic or mechanical washing tower according to WO 01/77056 and operated as detailed in it.

All of the above are especially true when the washing tower is designed and operated in accordance with the teachings of WO 03/041832 and WO 03/041833.

In principle, the heterogeneous catalyzed partial gas phase oxidation can be carried out in a manner known in the art before using the separation process according to the invention (the crude C ₃ precursor compound used to obtain the starting reaction gas mixture is further Only the fact that it has low purity).

In other words, for example, DE-A 10351269, DE-A 10245585, DE 1020050227988, EP-A 1695954, DE-A 10351269, EP-A 990636, EP-A 1106598, DE-A102005010111, DE-A 1020050130399, DE- A 102004025445, DE-A 102004021764, DE-A 10338529, DE-A 10337788, DE-A 10360396, DE-A 10316465, DE-A 10313210, DE-A 10313214, DE-A 10313213, DE-A 10313212, DE 102005062026.4 , DE-A 10313211, DE-A 10313208 and DE-A 10313209 and the prior art described in the literature.

In particular, the process according to the invention is applicable to a product gas mixture of two-step heterogeneous catalyzed partial gas phase oxidation from propyl to acrylic acid. In the first reaction stage, propylene is essentially partially oxidized to acrolein, and in the second reaction stage, the acrolein obtained in the first reaction stage is partially oxidized to acrylic acid. Typically, acrolein, which is a constituent of the product gas mixture of the first reaction stage, is supplemented with molecular oxygen or a mixture of molecular oxygen and an inert gas, where appropriate.

The process according to the invention has a propylene conversion U ^P in propylene partial oxidation of at least 91 mol%, or at least 92 mol%, or at least 93 mol%, or at least 94 mol%, or at least 95 mol%, or at least 96 mol%, Or at least 97 mol%, or at least 98 mol%, or at least 99 mol%.

The process according to the invention is characterized in that the acrolein conversion U ^A in acrolein partial oxidation is at least 96 mol%, or at least 97 mol%, or at least 98 mol%, or at least 98.5 mol%, or at least 99 mol%, or at least 99.5 mol%, Or 99.8 mol% or more.

This means that when the same catalyst system is used, the conversion rate (which is always based on a single pass of the reaction gas mixture through the catalyst bed) is selected for the level at which the reaction temperature is raised in a particular reaction step and / or includes a catalyst with increased activity. This is because it is usually achieved when using a catalyst charge. In both cases it promotes the production of benzoic acid by-products.

This is especially true for the two-step partial oxidation of propylene to acrylic acid described above.

The catalyst layer for heterogeneous catalyzed partial oxidation of the C ₃ precursor compound will generally be a fixed bed. In particular, active (fixed layer) catalysts for propylene partial oxidation have an active composition of 0.1 to 120 m ² / g, or 0.2 to 50 m ² / g, or 1 to 20 m ² / g, or 2 to 10 m ² / It is a multimetal oxide composition which has a specific surface area of g.

In addition, with respect to the activity of the (fixed layer) catalyst of propylene partial oxidation to acrolein, it is preferable that the diameter of the largest number of pores of the multimetal oxide active composition is 0.1 to 1 m.

The increased activity of the (fixed bed) catalyst for the partial oxidation of propylene to acrolein occurs especially when combined with the numerically largest pore diameter and the specific surface area of the multimetal oxide active composition.

The total pore volume of the multimetal oxide active composition (catalyst) with increased activity is also advantageously 0.1 to 1.00 ml / g, usually 0.10 to 0.80 ml / g, or 0.20 to 0.40 ml / g.

Often, even if the catalyst for heterogeneous catalyzed partial oxidation of propylene to acrolein includes said catalyst.

Particularly active (fixed layer) catalysts for acrolein partial oxidation (particularly the second stage of two-stage propylene partial oxidation to acrylic acid) are those in which the active composition has a specific surface area of 0.1 to 150 m ² / g, or 0.2 to 50 m ² / g, or a catalyst that is a multimetal oxide composition of 1 to 20 m ² / g or 2 to 10 m ² / g.

In addition, with respect to the activity of the (fixed layer) catalyst of the acrolein partial oxidation to acrylic acid, it is advantageous that the numerically largest diameter of the multimetal oxide active composition is 0.1 to 1 m.

The increased activity of the (fixed layer) catalyst for the partial oxidation of acrolein to acrylic acid is especially seen when combined with the numerically largest pore diameter and the specific surface area of the polymetal oxide active composition.

The total pore volume of the multimetal oxide polymetal oxide composition (catalyst) with increased activity is also advantageously 0.1 to 0.90 ml / g, usually 0.20 to 0.80 ml / g, or 0.30 to 0.70 ml / g.

Often, a catalyst for heterogeneous catalyzed partial oxidation of acrolein to acrylic acid is sufficient to include the catalyst.

In principle, in the two partial oxidations described above, the volume-specific activity of the catalyst bed (especially the fixed catalyst bed) in the flow direction of the reaction gas mixture can be constant over the length of the flow path. However, the process according to the invention is particularly relevant when the volume-specific activity of the (fixed) catalyst bed in certain cases increases more than one time (continuously or suddenly or stepwise) in the flow direction of the reaction gas mixture. In all these cases, it is advantageous that the active composition does not change over the length of the stream path.

The catalyst can be a coated catalyst or an unsupported catalyst. The shape of the catalyst can in principle be as desired. Spheres, polygons, cylinders or rings can be used. The longest dimension of the shaped catalyst body is suitably 1 to 10 mm, or 2 to 8 mm. This means the longest straight line connecting two points on the surface of the formed catalyst body. Volume-specific activity of the fixed catalyst bed can be achieved by appropriate dilution of the catalyst with an essentially inert shaped body (molding diluent). Its shape may essentially correspond to the shape of the shaped catalyst body. Materials useful for such inert shaped bodies are in principle all materials suitable as support materials for suitable coated catalysts according to the invention. 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 multimetal oxide active composition of the catalyst for propylene to acrolein partial oxidation with increased activity suitably comprises the metal elements Mo, Fe and Bi according to the invention. It has particularly increased activity if its stoichiometry satisfies Formula I:

[In the meal,

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 to 5,

b = 0.01 to 5, preferably 2 to 4,

c = 0 to 10, preferably 3 to 10,

d = 0 to 2, preferably 0.02 to 2,

e = 0 to 8, preferably 0 to 5,

f = 0 to 10, and

n = number determined by the valence and frequency of elements other than oxygen in formula (I).

This is especially true when the active composition has the properties described above in terms of specific surface area and pore volume.

The multimetal oxide active composition for catalysts for partial oxidation of acrolein to acrylic acid with increased activity suitably comprises the metal elements Mo and V according to the invention. This is particularly increased when the stoichiometry satisfies Formula II:

[In the meal,

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 = at least one alkali metal,

X ⁵ = at least one alkaline earth metal,

X ⁶ = Si, Al, Ti and / or Zr,

a = 1 to 6,

b = 0.2 to 4,

c = 0.5 to 18,

d = 0 to 40,

e = 0 to 2,

f = 0 to 4,

g = 0 to 40, and

n = number determined by the valence and frequency of elements other than oxygen in formula (II).

Particularly active embodiments of active polymetal oxides (II) are those encompassed by the definition of the following 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 to 5,

b = 0.5 to 2,

c = 0.5 to 3,

d = 0 to 2,

e = 0 to 0.2,

f = 0 to 1, and

n = number determined by the valence and frequency of elements other than oxygen in formula (II).

Advantageously, C ₃ Heterogeneous catalyzed partial gas phase oxidation of the precursor compound is carried out in a tube bundle reactor. The fixed catalyst bed is disposed in the reaction tube of the tube bundle reactor. To remove the heat of reaction, a liquid heat carrier (usually a metal melt or liquid metal) is flowed around the catalyst tube.

In both cases the propylene or acrolein loading of the (fixed) catalyst bed is at least 70 l (STP) / l · h, or at least 90 l (STP) / l · h, or at least 110 l (STP) / l · h, or 130 L (STP) / L · h or more, 140 L (STP) / L · h or more, or 160 L (STP) / L · h or more, or 180 L (STP) / L · h or more, or 240 L (STP) / L · h or more, or 300 L (STP) / L · h or more. In general, it is 600 l (STP) / l · h or less (loading amount is in each case based on the volume of the fixed catalyst bed excluding any additionally used portion consisting solely of inert material).

In this document, the loading of the starting reaction gas mixture into the (fixed) catalyst bed is expressed in standard liters (L (STP) through 1 liter of the (fixed) catalyst bed per hour; standard conditions, ie corresponding amounts of starting under 25 ° C. and 1 bar). The volume of the starting reaction gas mixture, expressed in liters occupied by the reaction gas mixture. The loading to the (fixed) catalyst bed may also be based on only one component of the starting reaction gas mixture. In this case, the amount expressed in standard liters of this component as a component of the corresponding starting reaction gas mixture is passed through one liter of (fixed) catalyst bed per hour.

In this document, the inert gas is quite generally a gas in which at least 95 mol%, preferably at least 97 mol% and most preferably at least 99 mol% of the partial oxidation process alone does not change chemically. I understand.

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

The working pressure during partial oxidation of the present invention may generally be below standard pressure (eg 0.5 bar or less; the reaction mixture is sucked at this pressure) or above standard pressure. Typically, the working pressure will be 1 to 5 bar, often 1.5 to 3.5 bar. In general, the reaction pressure during partial oxidation of the present invention will not exceed 100 bar.

Useful sources of molecular oxygen required as oxidants during the partial oxidation process are air and air depleted of nitrogen molecules.

Catalysts used for the partial oxidation of propylene (to acrolein) or acrolein (to acrylic acid) generally have a selectivity of target product formation generally of at least 83 mol%, mainly at least 85 mol%, or at least 88 mol% And usually at least 90 mol% or at least 93 mol%.

Typically the starting reaction gas mixture for partial propylene oxidation (to acrolein) may include, for example:

5-12 volume percent propylene,

2 to 15% by volume of water,

From 0 to 10 volume percent propane,

0.1 to 5% by volume of components other than propylene, propane, water, oxygen and nitrogen,

Sufficient molecular oxygen such that the molar ratio of molecular oxygen: propylene is 1: 3,

And molecular nitrogen filling the remaining 100% by volume.

Alternatively, the starting reaction gas mixture for propylene partial oxidation (to acrolein) may comprise:

6 to 10 volume percent propylene,

8-18 volume percent molecular oxygen,

6 to 30 volume percent propane and

32 to 72% by volume molecular nitrogen.

The reaction mixture for propylene-> crolein partial oxidation may also comprise up to 20% by volume H ₂ .

Starting reaction gas mixtures for the partial oxidation of acrolein (to acrylic acid) include, for example:

4-8% by volume of acrolein,

2 to 9 volume percent molecular oxygen,

0-30% by volume propane,

30 to 75 volume percent molecular nitrogen and

5 to 30 volume percent water vapor, or

3-25% by volume of acrolein,

5 to 65% by volume molecular oxygen,

6 to 70 volume percent propane,

0-20% by volume of molecular hydrogen and

5 to 65 volume percent water vapor.

Particularly suitable partial oxidation according to the invention is propylene partial oxidation (to acrolein) or acrolein partial oxidation (to acrylic acid), wherein the product gas mixture is also at least 0 to 4% by volume (eg to 0.5% by volume, or To 1 vol%, or to 1.5 vol%, or to 2 vol%, or to 2.5 vol%).

In principle, the organic content of its components in the liquid phase P ^* can be determined by gas chromatography.

Absorptive and / or condensing means are in principle useful for the conversion of acrylic acid and benzoic acid to the liquid phase P present in the product gas mixture of the partial oxidation to be carried out according to the invention.

Examples of useful absorbents include water, aqueous solutions (which may include, for example, 0.1 to 10 wt% acetic acid, 0.1 to 5 wt% acrylic acid and 80 to 99.8 wt% water) and / or organic (especially hydrophobic) ) Solvents (eg, diphenyl ether, diphenyl and / or dimethyl phthalate). From the point of view of application prior to absorption, the product gas mixture of the partial oxidation can be cooled directly and / or indirectly.

Suitable absorption and / or condensation processes according to the invention are described, for example, in documents DE-A 10336386, WO 01/96271, DE-A 19631645, DE-A 19501325, EP-A 982289, DE-A 19838845, WO 02 /. 076917, EP-A 1695954, EP-A 695736, EP-A 778225, EP-A 1041062, EP-A 982287, EP-A 982288, US 2004/0242826, EP-A 792867, EP-A 784046, EP-A 695736, EP-A 1125912, EP-A 1388533 and the documents cited in connection with this subject in the above documents.

Acrylic acid and benzoic acid present in the product gas mixture can also be liquefied by, for example, condensing the components having higher boiling points than water in the product gas mixture.

Similar to its conversion to liquid phase P ^* , both absorbent and condensable conversion from acrylic acid and benzoic acid to liquid phase P are commonly carried out in a separation column comprising separation inclusions (to increase the exchange surface area). Absorption and condensation can also be applied in addition to one another.

Suitable separation towers in connection with the absorption and / or condensation process are disclosed in particular in DE-A 10336386, EP-A 1125912 and US 2004/0242826 A1.

Useful separation inclusions are in principle all inclusions known to be separation-active. In other words, short, irregular fillers, such as Raschig rings, or regular fillers, such as Sulzer fillers, such as a soot end, double flow end or valve end, may be used as separation inclusions.

In a separation column, the product gas mixture of the partial oxidation, if appropriate previously cooled, is generally delivered in an ascending manner from below. In absorbent condensation, the absorbent is usually moved (delivered) from top to bottom in the separation column.

Liquid phase P ^* comprising acrylic acid and benzoic acid can be recovered from the separation column, from the point of view of the application, via a side drawer, preferably at the bottom of the liquid column or below the feed point of the cooled product gas mixture if appropriate.

Using one or more thermal separation processes, components having a lower boiling point (at atmospheric pressure) than benzoic acid and acrylic acid are now removed from the liquid phase P. Advantageously in accordance with the present invention, one or more thermal separation processes are employed in which, by low boiling point removal, constituents of the liquid phase P with a boiling point below the boiling point of water as pure material and at atmospheric pressure are captured.

Particularly advantageously used thermal separation processes are stripping with gas (eg air, branched nitrogen or other gas). To this end, the gas is passed through liquid phase P (advantageously from the point of view of the countercurrent in a separation column comprising separation inclusions) and strips off the low-boiling components present therein. Advantageously, liquid phase P is preheated for this purpose. Suitably, stripping is carried out at a working pressure below atmospheric pressure.

As a result of stripping, stripping additionally includes desorption thereon. It will also be appreciated that desorption can be used alone to remove low boilers.

As an alternative to or after stripping and / or desorption, rectified low boiler removal may be performed. In particular, from the standpoint of fixing the separation line at the boiling point of water, it is appropriate to perform rectified low boiler removal at least in part as an azeotropic rectification. Examples of suitable azeotropic agents in this regard are heptane, dimethylcyclohexane, ethylcyclohexane, toluene, ethylbenzene, octane, chlorobenzene, xylene or mixtures thereof (e.g. 60 wt% toluene and 40 wt% heptane). Of a mixture). Alternative azeotropic agents that can be used are also methyl isobutyl ketone or isopropyl acetate.

Further suitable azeotropic agents are disclosed in US 2004/0242826, EP-A 778255, EP-A 695736 and the prior art cited in the above document. Typically the distillation is likewise advantageously carried out at working pressures below atmospheric pressure. It will be appreciated that each of the above thermal separation processes may be applied alone or in combination with one or more of the other thermal separation processes described above.

According to the present invention, although the thermal separation process is advantageously used only for low boiler removal purposes, the procedure of the present invention additionally results in particularly low levels of undesired polymers and / or particularly low levels of polymerization inhibitors required. This is accompanied.

However, all process steps detailed in this document are carried out while inhibiting polymerization. This procedure may be as described in the cited prior art. Out of all commercially available acrylic acid process stabilizers, outstanding effects are dibenzo-1,4-thiazine (PTZ), 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4- OHTEMPO) and p-methoxyphenol (MEHQ), which can be part of the mixture to be treated according to the invention, for example, alone or as a two- or three-membered mixture, respectively. Typically, the total amount of polymerization inhibitor present in the liquid phase comprising acrylic acid is from 0.001 to 2% by weight, based on the total amount of acrylic acid present therein.

After the low boiling point removal described above is carried out, a liquid phase P ^* which generally contains at least 80% by weight of acrylic acid and 0.1% by weight of benzoic acid, based on the amount of acrylic acid present, remains. Based on the amount of acrylic acid present in the liquid phase P ^* , it advantageously contains at least 50 ppm by weight, or at least 0.2% by weight, or 0.2 to 15% by weight of water.

Since the liquid phase containing acrylic acid is left to stand, an undesired acrylic acid oligomer (Michael adduct) is formed, so that crystallization removal of the present invention yields liquid phase P ^* and is carried out as soon as possible. Thermal dissociation of the acrylic acid oligomers that similarly accumulate in the mother liquor can recover the acrylic acid present therein and recycle to one of the thermal separation processes applied to obtain the liquid phase P and / or P ^* .

Generally after the process according to the invention the acrylic acid crystals are melted and followed by a free-radical polymerization of the polymer. Often the polymer will be a superabsorbent polyacrylate.

A 2-liter stirred tank was used. The stirring tank is cylindrical in shape and has an internal diameter of 110 mm. Stirring tanks were made of glass. However, the lower part of the stirring tank was made of stainless steel in the form of a jacket. The stainless steel wall thickness was 2 mm. The distance between the two walls was approximately 1 cm. In the space between the two walls is flowed a mixture consisting of 30% by weight water, 40% by weight methanol and 30% by weight ethylene glycol as the cooling medium (75 l / h or more).

The contents of the stir tank were stirred with a twin-paddle stirrer (paddle height 50 mm, diameter 57 mm, rotation speed 500 rpm).

In all experiments, the stirred tank was filled with 1500-1650 g of glacial acrylic acid having an acrylic acid content of at least 99.5% by weight and doped with different amounts of benzoic acid (its temperature was 25 ° C). The water content was 148 ppm by weight. In addition, the glacial acrylic acid was polymerized-inhibited with phenothiazine (PTZ) in a content of approximately 250-270 ppm by weight. The temperature of the cooling medium (in each case proceeds to a starting temperature of 11 ° C.) was continuously reduced to a cooling rate of 15 K / h to 60 K / h.

In each case the crystallization process was stopped when a crystalline layer with a layer thickness of approximately 10 mm and a mass of M precipitated at the bottom of the tank. The stirring tank was rotatably mounted. When the crystallization process was stopped, the stirring tank was inverted to allow residual mother liquor (acid) to flow out. Subsequently, the benzoic acid content was analyzed in the outflowing mother liquor and precipitated crystals, respectively, and the specific deletion coefficient A ^BZA was determined from the analysis results.

The results achieved are listed in the table below. Additionally described is the crystal growth rate W in mm / min and the cooling time t in minutes.

US Provisional Application No. 60/852674 (October 19, 2006) is incorporated herein by reference. In connection with the above teachings, various modifications and variations are possible. Therefore, it can be assumed that within the scope of the appended claims, the present invention can be carried out differently than the particular manner described herein.

Benzoic acid content (weight ‰) Weight% of PTZ Cooling rate (K / h) d (mm) M (g) W (mm / min) T (min) A ^BZA Sample A One 270 15 12.9 161.2 0.152 85 54.1 Sample B 1.5 270 30 7.2 89.9 0.206 35 31.5 Sample C 1.5 270 60 10.8 120.7 0.300 36 30.3

Allyl acrylate, benzaldehyde, coumarone and furfural, which are acrylic acid impurities, similarly have a deletion coefficient of A >> 20. In contrast, the acrylic acid impurities, diacrylic acid, acetic acid and propionic acid, have a deletion coefficient of A <10. The large deletion coefficient seen in benzoic acid is surprising under this background.

Claims (22)

The acrylic acid and benzoic acid are converted to liquid phase P in the product gas mixture together with the other components of the product gas mixture having a lower boiling point and higher boiling point than acrylic acid, and the components having a lower boiling point than benzoic acid and acrylic acid are produced using one or more thermal separation processes. Removed from liquid phase P, leaving liquid phase P ^* comprising at least 80% by weight of acrylic acid and at least 0.1% by weight of benzoic acid (based on the amount of acrylic acid present) and separating benzoic acid from acrylic acid by crystallization of liquid phase P ^* As well as acrylic acid and benzoic acid present as main products and by-products in the heterogeneously catalyzed partial gaseous oxidation product gas mixture of C ₃ precursor compounds of acrylic acid, which comprises accumulating acrylic acid in the crystals formed and accumulating benzoic acid in the residual mother liquor. Room to separate different product gas mixture components . The method of claim 1 wherein the liquid phase P ^* comprises at least 90% by weight acrylic acid. The method of claim 1 wherein the liquid phase P ^* comprises at least 95% by weight acrylic acid. 4. A process according to any one of claims 1 to 3 comprising at least 0.3 wt% of benzoic acid based on the amount of acrylic acid present. 4. A process according to any one of claims 1 to 3 comprising at least 0.5 wt. Of benzoic acid based on the amount of acrylic acid present. Any one of claims 1 to A method according to any one of claim 5, wherein the liquid phase P ^* method the amount of acid is at least 50% by weight of benzaldehyde by weight present in the liquid phase of the P ^*. Any one of claims 1 to A method according to any one of claim 5, wherein at least the liquid phase P ^* 100% by weight of benzaldehyde by weight present in the amount of the acid liquid phase of the P ^*. Any one of claims 1 to A method according to any one of claim 5, wherein at least the liquid phase P ^* 150% by weight of benzaldehyde by weight present in the amount of the acid in the liquid phase of the P ^*. The product gas according to any one of claims 1 to 8, wherein the mother liquor remaining in the crystallization removal is recycled to one or more thermal separation processes applied to obtain the liquid phase P ^* in the liquid phase P and / or the product gas of partial gas phase oxidation. At least partially recycled to at least one process applied for the combined conversion of acrylic acid and benzoic acid to liquid phase P present in the process. 10. The process according to any one of the preceding claims, wherein the crystallization of acrylic acid from phase P ^* consists of one step. 10. The process according to any of claims 1 to 9, wherein the acrylic acid crystallization removal from phase P ^* consists of more than one step. The method according to any one of claims 1 to 11, wherein the crystallization of acrylic acid is made of layer crystallization. The method according to any one of claims 1 to 12, wherein the crystallization removal of acrylic acid consists of suspension crystallization. The method of claim 13, wherein the suspended crystals formed are separated from the remaining mother liquor in the wash tower. 15. The process of claim 14 wherein the wash liquor used is a melt of acrylic acid crystals previously removed from the wash tower. The method according to any one of claims 1 to 15, comprising the following process steps: a) crystallizing acrylic acid from the liquid phase P ^* , b) separating the acrylic acid crystals from the residual mother liquor, c) at least partially melting the removed acrylic acid crystals, and d) at least partially recycling the molten acrylic acid crystals to step b) and / or step a). 17. The method of any of claims 1-16, wherein the liquid phase P ^* comprises at least 150 ppm by weight of water. 18. The process according to any one of claims 1 to 17, wherein acrylic acid and benzoic acid present in the product gas mixture of partial gas phase oxidation are converted by absorption with an aqueous solution. 19. The method of any of claims 1-18, wherein the C ₃ precursor compound is propylene. The method of claim 1, wherein the C ₃ precursor compound is propane. 19. The method of any one of claims 1 to 18, wherein the C ₃ precursor compound is acrolein and the acrolein conversion upon acrolein partial oxidation is at least 99.5 mol%. 22. The process according to any one of claims 1 to 21, wherein the process of subsequently melting the acrylic acid crystals and free-radically polymerizing with a polymer is carried out.