Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-05-25
2001-08-28
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
Reexamination Certificate
active
06281386
ABSTRACT:
The present invention relates to the purification of acrylic acid.
The main route for the synthesis of acrylic acid used industrially today is the catalytic oxidation of propylene, which generates acrolein as an intermediate. This reaction, which takes place in the gas phase, generates a gas flow mainly comprising, in addition to acrylic acid, non-condensable gases: unconverted propylene, nitrogen, carbon monoxide and dioxide, “light” organic compounds, that is to say for which the boiling point is less than that of acrylic acid: steam, unconverted acrolein, impurities manufactured by side reactions: formaldehyde, acetic acid, and, finally, heavy compounds: maleic anhydride, furfuraldehyde, benzaldehyde, and the like.
The processes for the purification of this reaction gas which are described in the literature consist in condensing this mixture and in extracting the organic compounds by countercurrent washing using water or heavy solvents.
Processes using an absorption with water exhibit the disadvantage of extracting in a not very selective way virtually all the organic products present in the gas mixture. The purification of the aqueous solution thus composed requires difficult and expensive separations by distillation and/or extraction.
French Patent No. 1,558,432 discloses a process which consists in absorbing the organic compounds present in the reaction gas using esters of aliphatic or aromatic acids with high boiling points or tributyl or tricresyl phosphate. On conclusion of this absorption stage, the light products (acrolein, formaldehyde) are removed at the top of a first distillation column and a second distillation column makes it possible to obtain, at the top, a more concentrated aqueous acrylic acid solution than in the prior art. However, the subsequent purification of the solution obtained, which still comprises acetic acid and water, still requires expensive separations.
The process disclosed in French Patent No. 2,002,126 introduces an improvement by virtue of the use of a mixture of fractions with high boiling points, recovered at the bottom of the columns for the purification of the esters manufactured from acrylic acid, mainly comprising maleates, polyacrylic acids or polyacrylates. This process makes it possible to clear in a single stage, at the top of a distillation column, most of the compounds with low boiling points, such as acrolein, formaldehyde, water and acetic acid. However, this process for the manufacture of acrylic esters is ill-suited to the production of pure acrylic acid, in particular because of the presence, in the initial crude acrylic acid mixture, of esterification derivatives recycled in the absorption stage.
An improvement is introduced into the process using an extraction with the help of heavy hydrophobic solvents, such as disclosed in French Patent No. 2,146,386, German Patent No. 4,308,087 and European Patent No. 706,986, which makes it possible to obtain, on conclusion of the extraction stage, an anhydrous solution which has been freed from a substantial portion of the light organic products which constituted the initial gas mixture (acrolein, formaldehyde, acetic acid), thus substantially facilitating the subsequent purification of the acrylic acid.
French Patent No. 2,146,386 discloses the use of a hydrophobic solvent having a boiling temperature greater than 170° C. at atmospheric pressure and a viscosity of less than 10 cSt in a temperature range of 30-80° C. and in particular the use of a mixture of diphenyl (DP) and diphenyl ether (DPO) as absorption solvent, with in particular approximately 25% by weight of DP and 75% by weight of DPO. This mixture forms a eutectic (at a concentration by mass of 26.5% of DP and 73.5% of DPO) which exhibits the advantage of having a lower solidification point (S=12° C.) than DPO alone (M.p.=27° C.).
It is well known that the distillation of acrylic monomers and in particular of acrylic acid, which readily polymerize under the effect of radicals formed by a thermal effect, for example, requires the use of polymerization inhibitors, particularly during the distillation stages. Compounds typically used for this purpose are, for example, phenol derivatives, such as hydroquinone or hydroquinone methyl ether (p-methoxyphenol), or phenothiazine and its derivatives, or derivatives of the thiocarbamates family, or compounds with nitroso groups, or quinones, or aromatic amines.
In the examples of French Patent No. 2,146,386, the inhibitor involved is phenothiazine. Unfortunately, the application of the process described according to this patent results in the distillation equipment being fouled by polymeric impurities, which finish by causing blockages of the plant.
To reduce this problem, German Patent No. 4,308,087 discloses the use of an absorption solvent composed of the eutectic mixture of DP and DPO, and of dimethyl phthalate (DMP), at a content of 0.1 to 25% by weight in the total mixture.
European Patent No. 706,986 discloses, in these examples, the use of the DP+DPO eutectic mixture and of a mixture of 80% by weight of this binary+20% by weight of DMP.
The mixtures using DMP exhibit two major disadvantages:
DMP is markedly less hydrophobic than the DP+DPO mixture. In the presence of 10% by weight of acrylic acid in the solvent (conditions approximating to the process), the solubility of water in the DP+DPO mixture is approximately 0.1% by weight, against approximately 4% for DMP. Now, the presence of water in the process is inevitable, since this impurity is generated in the initial oxidation reaction of propylene, resulting in acrylic acid. It follows that the flow recovered at the absorption column bottom is much richer in water and in acetic acid since the water, by affinity, increases the absorption of acetic acid in the solvent flow, which complicates the subsequent purifications. This disadvantage is naturally found in the process disclosed in French Patent No. 2,196,986, which discloses the use of carboxylic esters as absorption solvents, since, according to this process, acetic acid and water are absorbed at the same time as acrylic acid;
in the presence of water, DMP is subject to a hydrolysis side reaction which is promoted by the thermal level of the absorption stage. This side reaction results in the formation of novel impurities, such as phthalic anhydride and methanol, which reacts with acrylic acid to form the corresponding ester (methyl acrylate). Besides the additional difficulties related to the separation of these impurities, this esterification reaction results in a loss of acrylic acid.
The same disadvantage, related to the side reaction of hydrolysis of the solvent, exists for all hydrolysable compounds, in particular those of the esters family. The process disclosed in French Patent No. 2,196,986, which claims the use, as absorption solvents, of heavy carboxylic esters having a melting point of less than 30° C. and a boiling point of greater than 160° C., therefore exhibits this same disadvantage.
Another disadvantage of the DP+DPO mixture is the fact that it is composed of two different compounds which possess a slightly different boiling temperature:
DP: B.p.=255° C. at atmospheric pressure,
DPO: B.p.=258° C. at atmospheric pressure.
In a process as disclosed in French Patent No. 2,146,368 and German Patent No. 4,308,087, the hydrophobic heavy solvent is recycled in the absorption stage, after removal of the light impurities absorbed, recovery of the acrylic acid, purging the intermediate heavy compounds (the boiling temperature of which is between that of the solvent and that of acrylic acid) and purging the heavy compounds (polymerization inhibitors and impurities which have a boiling point greater than that of the solvent).
The removal of the intermediate heavy compounds and of the heavier compounds than the solvent can be carried out by washing with water or by distillation.
The first process exhibits the disadvantage of generating an aqueous effluent which is highly polluted with organic compounds, includi
Esch Marc
Fauconet Michel
Laurent Denis
Atofina
Deemie Robert W.
Killos Paul J.
Millen White Zelano & Branigan P.C.
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