Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1998-06-25
2001-06-26
Shaver, Paul F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S600000, C203S039000, C203S075000, C203SDIG002
Reexamination Certificate
active
06252110
ABSTRACT:
This invention relates to methods for recovering acrylic acid. More particularly, the invention relates to methods for recovering acrylic acid from high boiling impurities containing acrylic acid dimer, acrylic acid and maleic acid. Still more particularly, the invention relates to methods for efficiently and stably recovering acrylic acid from high boiling impurities containing acrylic acid dimer, acrylic acid and maleic acid, which are obtained as bottom liquid of high boiling impurities separation column in a process for producing high purity acrylic acid comprising contacting an acrylic acid-containing gas resulting from gas-phase catalytic oxidation of propylene and/or acrolein with water to trap the acrylic acid as an aqueous acrylic acid solution, distilling said aqueous solution in the presence of an azeotropic solvent to recover crude acrylic acid, and purifying the so obtained crude acrylic acid with a high boiling impurities separation column.
Conventionally, production of high purity acrylic acid through gas-phase catalytic oxidation of propylene and/or acrolein has been practiced by a method comprising contacting an acrylic acid-containing gas from the oxidation step with water to collect acrylic acid in the form of an aqueous acrylic acid solution (collection step); extracting acrylic acid from the aqueous acrylic acid solution using an appropriate extraction solvent (extraction step); separating the solvent from the resulting extract to recover crude acrylic acid (solvent separation step); and thereafter separating from the crude acrylic acid such high boiling impurities contained therein as acrylic acid dimer, maleic acid and the like, to obtain high purity acrylic acid (purification step).
Thus separated high boiling impurities contain, however, besides acrylic acid, acrylic acid dimer and therefore it is uneconomical to discard them as waste liquid as a whole.
Hence, methods have been proposed by JP Sho 45-19281 B2, JP Sho 51-91208A, JP Sho 61-35977 B2 and JP Sho 61-36501 B2, etc., to increase recovery ratio of acrylic acid by pyrolyzing the acrylic acid dimer in said high boiling impurities into acrylic acid and recovering the same.
Recently, however, instead of the above-described solvent extraction method which uses an extraction solvent for recovering acrylic acid from aqueous acrylic acid solution, azeotropic separation method using azeotropic solvents, i.e., distilling an aqueous acrylic acid solution in the presence of an azeotropic solvent which forms an azeotrope with water to cause distillation of an azeotrope of water with the solvent from the top of an azeotropic separation column and recovering acrylic acid from the bottom part of the same column, is becoming the mainstream practice.
Thus, the production method of high purity acrylic acid in the recent years normally consists of an oxidation step for producing acrylic acid through gas-phase catalytic oxidation of propylene and/or acrolein; a collection step of contacting the acrylic acid-containing gas with water and collecting the acrylic acid in the form of an aqueous acrylic acid solution; an azeotropic separation step of distilling the aqueous acrylic acid solution in an azeotropic separation column in the presence of an azeotropic solvent and recovering crude acrylic acid from bottom part of said column; and a purification step of purifying the crude acrylic acid. This purification step is normally conducted using a high boiling impurities separation column for removing high boiling impurities in the crude acrylic acid and optionally an acetic acid separation column for further removing acetic acid.
However, in such acrylic acid production process using the azeotropic separation method as described above, still the high boiling impurities separated in the purification step contain, besides acrylic acid, acrylic acid dimer, maleic acid and the like.
It is therefore desirable to recover acrylic acid from said high boiling impurities also in the high purity acrylic acid production process using the azeotropic separation method. As the recovery means, application of those methods as described in the earlier listed JP publications, which have been proposed in respect of the high purity acrylic acid production process following the solvent extraction method, may be considered.
However, when for example the apparatus for destructive distillation of acrylic acid dimer as described in JP Sho 61-36501 B2 (the first step) is used to simultaneously conduct decomposition of acrylic acid dimer and distillation of the acrylic acid formed upon said decomposition as well as that which is initially contained in the high boiling impurities, it has been found that such a problem as that the impurities mix into the product acrylic acid to reduce purity of the product is encountered.
We have discovered that the problem has its cause in the maleic acid contained in the high boiling impurities. Maleic acid is substantially completely separated and removed in the solvent extraction step in the processes using solvent extraction method and therefore does not remain in the resulting crude acrylic acid, but in processes using azeotropic separation method, it does remain in the crude acrylic acid and is mixed into the high boiling impurities separated in purification of this crude acrylic acid in a proportion of a few percent.
Such maleic acid is distilled off with acrylic acid from aforesaid apparatus for destructive distillation of acrylic acid dimer. When the distillate is recirculated to an earlier step, eg., to a high boiling impurities separation column in the purification step, with the view to recover acrylic acid therefrom, maleic acid is entrained by the distillate and condensed at the bottom of the high boiling impurities separation column to eventually mix into the product acrylic acid to reduce the latter's purity.
Maleic acid also changes into fumaric acid in the step of recovering acrylic acid from the high boiling impurities containing acrylic acid dimer, acrylic acid and maleic acid, said fumaric acid precipitates in said recovery step to interfere with stable operation.
Accordingly, therefore, the object of the present invention is to solve the above problem, that is, to provide a method for efficiently and stably recover acrylic acid from high boiling impurities containing acrylic acid dimer, acrylic acid and maleic acid, in particular, the high boiling impurities containing acrylic acid dimer, acrylic acid and maleic acid as obtained as bottom liquid of high boiling impurities separation column in a process for producing acrylic acid using azeotropic separation method.
We have discovered that the above object can be accomplished by the use of an acrylic acid recovery apparatus and an acrylic acid dimer decomposition apparatus, in place of aforesaid apparatus for destructive distillation (the first step) of acrylic acid dimer. More specifically, we have discovered that acrylic acid can be efficiently recovered from high boiling impurities without reduction in purity of product acrylic acid, by introducing said high boiling impurities into an acrylic acid recovery column (which preferably is a distillation column equipped with a thin film vaporizer), in which separating maleic acid and taking it out from the bottom of the column and recovering acrylic acid of markedly reduced maleic acid content from the column top; introducing the bottom liquid containing acrylic acid dimer, acrylic acid and maleic acid into a pyrolyzing tank, where at pyrolyzing the acrylic acid dimer; and then recirculating a part of the decomposition product into said acrylic acid recovery column.
Thus, according to the present invention, a method for recovering acrylic acid is provided, which is characterized by comprising, in recovering acrylic acid from high boiling impurities containing acrylic acid dimer, acrylic acid and maleic acid,
(1) introducing said high boiling impurities containing acrylic acid dimer, acrylic acid and maleic acid, into an acrylic acid recovery column and recovering acrylic acid as it is distilled
Takeda Takahiro
Uemura Masahiro
Ueoka Masatoshi
Nippon Shokubai Co Ltd
Shaver Paul F.
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