Polyester precursor purification process

Details Polyester precursor purification process Polyester precursor purification process

2001-03-20

2002-04-30

Boykin, Terressa M. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Treating polymer containing material or treating a solid...

C528S271000, C528S272000

Reexamination Certificate

active

06380352

ABSTRACT:

BACKGROUND OF THE INVENTION
Polyethylene terephthalate (PET) is currently produced by the polycondensation of purified terephthalic acid (PTA) with ethylene glycol. The purchase or manufacture of PTA represents a major cost in raw material for PET manufacture and other PTA containing polyesters and a major portion of the PTA process is devoted to the purification of crude terephthalic acid. Crude terephthalic acid must be purified to be useful for the manufacture of high quality, low color polyesters. Crude terephthalic acid may be purified to produce PTA by the catalytic hydrogenation of colored impurities with simultaneous conversion of 4-carboxybenzaldehyde (CBA) to p-toluic acid and subsequent crystallization of the product (PTA). The purification is carried out by dissolution of CTA in deionized water at 260-280° C. to give a 20 to 30% solution. The resulting solution is treated with hydrogen in the presence of a fixed-bed catalyst (usually Pd/C). The hydrogenated solution is fed to a series of crystallizers where it is carefully cooled to produce crystalline PTA. The resulting PTA is collected by filtration and must be dried before use. Due to the high temperatures involved, the process must be carried out in expensive titanium pressure vessels and is energy intensive. As a result, the purification of CTA to PTA requires a large amount of capital which significantly adds to the cost of PTA. This cost is naturally added to the cost of the final polyester product. Elimination of the capital needed to produce PTA, or the use of CTA in the place of PTA would significantly lower the cost of polyester manufacture.
Several processes that involve hydrogenation of terephthalic acid (TPA) esters to remove color bodies are listed below.
U.S. Pat. No. 3,501,420 discloses the depolymerization of waste PET in an alcohol to give a solution of terephthalic acid esters. The resulting solution is hydrogenated to remove color bodies. In this process, PET is converted to mostly monomeric esters before contact with hydrogen. The decolorized solution of TPA esters can be used to prepare high-quality PET. A related patent not specifically intended to produce high quality PET (unexamined Japanese patent application 50-142537) teaches a process to make cyclohexanedimethanol (CHDM) by the depolymerization and hydrogenation of waste PET. In this process, waste PET is depolymerized in ethylene glycol solution in the presence of hydrogen and a hydrogenation catalyst to convert PET into a solution of ethylene glycol esters of cyclohexanedicarboxylic acid. The product of this step is separated from the catalyst and treated again with hydrogen at high pressure in the presence of a second hydrogenation catalyst. The object of the second step is to convert the esters of cyclohexanedicarboxylic acid into a solution of CHDM.
U.S. Pat. No. 3,487,100 discloses that bis-hydroxyethyl terephthalate (BHET) prepared from crude TPA and ethylene glycol can be decolorized by treatment with hydrogen in the presence of a hydrogenation catalyst. The BHET is dissolved in water and filtered to remove oligomeric species (which are insoluble in water). The filtered solution is treated with hydrogen at a temperature range of 50-100° C. Although the hydrogenation treatment reduces fluorenone impurities, the BHET must be in solution and the product must be crystallized from solution before further use. Another disadvantage is that oligomers cannot be processed by this method.
SUMMARY OF THE INVENTION
We have found that colored impurities can be removed from oligomers produced from crude terephthalic acid (CTA) and ethylene glycol by catalytic hydrogenation in the melt-phase over a supported or suspended catalyst. The process provided by the present invention comprises contacting a polyester oligomer or prepolymer comprising terephthalic acid with a supported or suspended hydrogenation catalyst in the presence of hydrogen at a temperature range of about 200 to about 290° C. (The PET oligomer is reported to undergo a spontaneous exotherm at temperatures greater than 330° C.). The molten oligomer mixture is contacted with a supported catalyst in the presence of hydrogen at a temperature range of about 200 to about 280° C. (depending upon the content of ethylene glycol), preferably about 240 to about 270° C., and hydrogen partial pressure of up to about 55.13 bar gauge (barg; 800 pounds per square inch gauge—psig), preferably a hydrogen partial pressure of about 10.34 to 27.57 barg (150 to 400 psig). The resulting hydrogenation product is polymerized by conventional methods to produce high quality polyester. An advantage of the process is the elimination of the need for purified terephthalic acid as a raw material for the manufacture of polyester. This eliminates the processing steps typically found in conventional terephthalic acid purification processes: slurry mixing, slurry dissolving, hydrogenation, crystallization, separation, drying, and yield recovery. The capital required to produce purified oligomers containing terephthalic acid residues is essentially condensed into one low-pressure hydrogenation bed.
The present invention provides a process for the production of high quality polyester, or polyester pre-polymer, using crude terephthalic acid as raw material in place of purified terephthalic acid. The present invention eliminates the need for the expensive equipment required for producing purified terephthalic acid.
DETAILED DESCRIPTION OF THE INVENTION
Crude terephthalic acid (CTA) is the product of the autoxidation of para-xylene prior to hydrogenation. The major impurity found in CTA is 4-carboxybenzaldehyde (CBA). The colored impurities have been identified as a mixture of mainly dicarboxyfluorenone isomers (with lesser amounts of mono- and tri-carboxyfluorenones) and dicarboxybenzil. These highly-colored carboxyfluorenone compounds may be hydrogenated to colorless carboxyfluorene compounds. Other compounds such as dicarboxybenzophenone and dicarboxybiphenyl have been identified in low concentrations. Although essentially any grade of CTA can be used in the preparation of the oligomers employed as the starting material for the process of the present invention, the CTA used typically contains less than about 4000 ppm of CBA, preferably less than 700 ppm CBA, and most preferably from about 50 to 250 ppm CBA.
CTA typically is reacted with at least one diol at a temperature between about 200 and about 280° C. to produce an oligomer which may be hydrogenated in accordance with the present invention. Suitable esterification pressures include, but are not limited to those up to about 27.6 barg (400 psig) and preferably up to about 13.8 barg (200 psig). The reaction can be self-acid catalyzed or catalyzed with a suitable esterification catalyst such as titanium or organic or inorganic acids. The oligomer mixture typically is produced continuously in a series of one or more reactors. Two reactors are frequently used in commercial practice. Alternatively, the monomer and oligomer mixture may be produced in one or more batch reactors. Suitable reactors for esterification are known in the art and need not be described here. The esterification is generally conducted from about 1 to about 4 hours. It should be understood that generally, the lower the reaction temperature, the longer the reaction time. The product of the esterification reaction is an oligomer mixture having a degree of polymerization (DP) of about 2 to about 20 and containing impurities such as CBA and color bodies. The oligomer preferably has a DP of about 2 to 10, most preferably about 3 to 5, wherein DP is defined as the number average molecular weight of the oligomer divided by the molecular weight of the repeat unit. The oligomers which may be employed in the present invention also may be characterized by the general formula:
HO——[Diol]
x
—[——TPA——Diol——]
y
—H
wherein Diol is a divalent residue of a diol or glycol component such as ethylene glycol, 1,4-cyclohexanedimethanol and the like, TPA is the divalent residue of terephthalic acid, x

Affiliated with

Also associated with

Rating

Polyester precursor purification process does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Polyester precursor purification process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Polyester precursor purification process will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2876517