Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-08-22
2002-07-30
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S482000, C562S486000
Reexamination Certificate
active
06426430
ABSTRACT:
FIELD OF THE INVENTION
This invention is generally related to the purification of aromatic dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid. More particularly, this invention is related to a practical method of reducing potassium in the final product to acceptable levels for subsequent polymerization.
BACKGROUND OF THE INVENTION
Polymer grade aromatic dicarboxylic acids are important as starting materials for a number of polyester fibers, polyester films, resins for bottles and containers, and the like. Naphthalene dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid (hereafter referred to as 2,6-NDA) are starting materials for polyethylene naphthalates, which can also be employed in the manufacture of fibers, films, and resins. Presently in the art, it is very difficult to produce polymer grade 2,6-NDA and only dimethyl 2,6-naphthalene dicarboxylate, the methyl ester of the more desirable 2,6-NDA is commercially available for use in making polymers such as polyethylene naphthalate.
When aromatic dicarboxylic acids are produced by a disproportionation reaction, such as described in U.S. Pat. No. 2,823,231 and U.S. Pat. No. 2,849,482, for example, where an alkali metal salt of a mono- or dicarboxylic acid is disproportionated to produce isomers of the salt of the desired dicarboxylic acids, the product often retains undesirable amounts of the alkali metal, most often potassium.
The goal of a number of processes for purification of aromatic dicarboxylic acids, and 2, 6-naphthalene dicarboxylic acid in particular, is to reduce these alkali metals along with other impurities to the lowest levels possible, but there is still a need in the art for methods of reducing impurities and alkali metals to levels acceptable for use in polymers.
Methods of purification of aromatic dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid are known in the art. Where 2,6-NDA is produced by disproportionation, common methods for purification include filtration, acidification and crystallization. See, for example, U.S. Pat. Nos. 2,849,482; 3,631,096; 3,671,578; and 3,952,052. It is possible to remove significant amounts of color bodies and impurities, but it is still difficult to obtain polymer grade 2,6-NDA.
Currently in the art the most common process for making 2,6 NDA starts with relatively expensive o-xylene and butadiene feedstocks, as discussed, for example, in U.S. Pat. No. 5,510,563 and U.S. Pat. No. 5,329,058 and incurs substantial yield losses of these starting materials. Following the synthesis and purification of 2,6 dimethylnaphthalene (2,6 DMN), 2,6 DMN is oxidized to produce crude NDA product which forms as a solid with impurities trapped within. Therefore, in such processes, esterification to naphthalene dicarboxylate (NDC) is necessary to eliminate the impurities, as discussed in U.S. Pat. Nos. 5,254,719 and 4,886,901. Direct purification of the crude NDA via hydrogenation has been suggested by U.S. Pat. No. 5,292,934, but requires a difficult and expensive high temperature hydrogenation in the presence of a solvent. Another proposed purification scheme requires the use of nitrogen containing species. (See U.S. Pat. No. 5,770,764 and U.S. Pat. No. 5,859,294). Crystal size and morphology is important in either case, whereas the novel process disclosed herein can optionally avoid the issue of controlling particle size of the final product.
Currently NDC is commercially available, but NDA is not, presumably because of the difficulty of producing polymerization grade NDA without esterifying to NDC. Ideally, if NDA were available commercially at a competitive price, NDA would be preferred over NDC as the starting monomer for PEN. Alternative routes to NDA based on the rearrangement reaction have been plagued with difficulties associated with handling solids, the inefficient recycling of potassium, and ineffective integration from feedstock through final product. Although various improvements have been suggested over the years, there is still a distinct need in the art for an economical, integrated process for producing polymer grade 2,6-NDA, the preferred monomer for the production of polyethylenenaphthalate (PEN). Copending U.S. application Ser. No. 60/151,577 (Attorney's Docket #TH1432) discloses a novel integrated process for producing the preferred 2,6-naphthalene dicarboxylic acid.
It would represent a distinct advance in the art if a method were available to reduce the levels of alkali metals, particularly potassium in product 2,6-NDA to levels which are acceptable for polymerization to polyethylene naphthalate.
SUMMARY
In accordance with the foregoing, the present invention provides a method for reducing alkali metals in aromatic dicarboxylic acids produced by disproportionation or rearrangement of an alkali salt of a monocarboxylic acid to levels acceptable for polymerization which comprises:
a) Washing the aromatic dicarboxylic acid with water in a ratio of about 5:1, water to acid, at a temperature of about 70-200° C.,
b) Introducing the washed aromatic dicarboxylic acid into a reactor characterized by minimal backmixing, such as a pipe reactor, and reacting the washed aromatic dicarboxylic acid in said reactor in approximately a 5:1 water to acid ratio at about 100-200° C.;
c) Directing the aromatic dicarboxylic acid exiting the pipe reactor to a centrifuge to separate the solid aromatic dicarboxylic acid from water containing contaminants;
d) Optionally, combining the solid aromatic dicarboxylic acid again with water in a ratio of about 5:1 at a temperature of about 100-200° C. to further reduce levels of alkali metals.
In the preferred embodiment, the invention makes it possible to reduce the level of potassium in product 2,6-naphthalene dicarboxylic acid to less than 50 ppm.
DETAILED DESCRIPTION
In the preferred embodiment the aromatic dicarboxylic acid which is treated in the present invention is 2,6-NDA produced by a process which incorporates disproportionation of an alkali metal salt of a mono- or dicarboxylic acid.
Following disproportionation the crude product is in the form of an isomer of a dialkali salt of 2,6 -NDA. Salts formed by the reaction can be transformed into the corresponding acids by acidifying the solution with acids or by introducing carbon dioxide into the solution and then separating the free acids from the acidified solution. The salt mixture produced by the reaction may also be transformed directly into derivatives of the acids, such as, for example, their esters or halides, and these derivatives can be purified, if desired, by fractional distillation.
The product treated in the present invention can be produced by the process described in copending Ser. No. 60/151,577 (Attorney's Docket #1432), incorporated by reference herein in its entirety. In that process, after disproportionation the solid product consisting of K2NDA isomers is washed and the liquid is filtered to remove catalyst and coke particles.
The liquid carrying mixed organic salts is introduced into a two-stage evaporative crystallization section where the K2NDA is selectively precipitated, KHCO
3
is recycled, and the purified K2NDA is redissolved with additional H
2
O. Then the purified K2NDA is passed through an activated carbon bed.
Next, the dipotassium salt of 2,6-NDA, (K2NDA) is selectively precipitated using CO
2
to make the KHNDA solids which are then disproportionated into 2,6 -NDA and K2NDA. The product of disproportionation is centrifuged to yield a 2,6 NDA slurry, and a centrate containing predominantly 2,6 K2NDA and KHCO
3
. The product 2,6-NDA contains from about 60-1000 ppm potassium on a dry basis. It is desirable to reduce the amount of potassium to 50 ppm or less.
The key to obtaining polymer grade product with <50 ppm potassium without using excessive water is the combined use of a 5:1 water wash and a pipeline reactor to drive the reaction to completion and/or remove trace K
+
.
The 2,6-NDA and trace KHNDA are directed into a water wash. In the water wash a ratio of about 3 to 8 parts water is ad
Blackbourn Robert Lawrence
June Raymond Lawrence
Killos Paul J.
Mossi & Ghisolfi Oversears, S.A.
Renner Kenner Greive Bobak Taylor & Weber
Reyes Hector M
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