Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2002-08-05
2004-03-23
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S076000, C560S089000, C560S096000, C560S098000
Reexamination Certificate
active
06710202
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing high-purity bis-&bgr;-hydroxyethyl terephthalate from crude terephthalic acid.
DESCRIPTION OF THE PRIOR ART
Bis-&bgr;-hydroxyethyl terephthalate is widely used as a raw material for polyethylene terephthalate which is an extremely useful polyester in the field of various moldings such as fibers, films and resins.
Polyesters, particularly polyesters comprising polyethylene terephthalate as the main constituent are widely used for various purposes as described above and mainly produced by obtaining an intermediate containing bis-&bgr;-hydroxyethyl terephthalate through direct esterification between terephthalic acid and ethylene glycol or through an ester exchange reaction between a lower alkyl ester of terephthalic acid, especially dimethyl terephthalate, and ethylene glycol and then generally polycondensing the intermediate at a high temperature under a high vacuum. These production processes are currently put to practical use. Due to the recent diversified application of the polyesters based on their excellent properties, requirements for obtaining higher quality are being more and more diversified and advanced. One of the characteristic features of the polyesters is that they can be decomposed into their raw materials which can be polymerized again to obtain new polyesters. Therefore, it can be said that they are excellent in terms of resource saving.
Terephthalic acid as a raw material for terephthalate-based polyesters is generally supplied by oxidizing paraxylene. However, as terephthalic acid obtained by this method has low purity and is colored in most cases, it cannot be used for the production of a polyester as it is. Therefore, it must be purified before it is used for the production of a polyester. However, it is not easy to purify terephthalic acid because of its low solubility in solvents.
That is, terephthalic acid is obtained by air oxidizing paraxylene at about 175 to 230° C. in an acetic acid solvent using a catalyst comprising cobalt, manganese and bromine compounds in accordance with a so-called Amoco process. In this case, paratoluylic acid and 4-carboxybenzaldehyde by-produced in the above oxidation step, metals such as cobalt and manganese used as a catalyst and halogen such as bromine remain in the obtained terephthalic acid, and the existence of a compound such as 3,6-dicarboxyfluorenone as a coloring impurity is also known. Kagaku Binran, Applied Chemistry I, Process, pp. 535 (fifth edition published on Apr. 20, 1992) teaches that high-purity terephthalic acid is obtained by passing a water slurry solution containing 10% or more of the above crude terephthalic acid through a reactor maintained at a temperature of 250° C. or more to purify the crude terephthalic acid by hydrogenation in the above Amoco process. The catalyst used in this case is a precious metal carried on active carbon. It is disclosed that the contents of 4-carboxybenzaldehyde and the coloring component in the formed product thereby become very small. However, as terephthalic acid is generally obtained as a fine particle solid, it is very difficult to handle it. Therefore, it is often supplied as a slurry containing ethylene glycol to an industrial-scale production process. As easily anticipated by people having ordinary skill in the art, it is difficult to remove impurities due to big changes in the properties and reactivity of the slurry caused by non-uniformity in particle diameter.
A process in which terephthalic acid is purified as dimethyl terephthalate which is a functional derivative relatively easy to be purified and a polyester is obtained through an ester exchange reaction between it and ethylene glycol has also been employed. However, in this so-called ester exchange process, it is necessary to recycle methanol which is by-produced inevitably, thereby imposing great restrictions on equipment and handling. In this case, there arises another problem that an ester exchange catalyst is contained in the polyester product. Further, when a polyester obtained by the ester exchange process is molded into a bottle, for example, the bottle is unsatisfactory in terms of color, transparency and moldability.
Such an attempt is also proposed to obtain bis-&bgr;-hydroxyethyl terephthalate from terephthalic acid and purify this bis-&bgr;-hydroxyethyl terephthalate by distillation.
JP-A 48-15846 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a process for obtaining high-purity bis-&bgr;-hydroxyethyl terephthalate by reacting high-purity terephthalic acid with ethylene oxide in the presence of a basic catalyst to obtain bis-&bgr;-hydroxyethyl terephthalate and distilling this bis-&bgr;-hydroxyethyl terephthalate at a temperature of 200 to 350° C. under reduced pressure quickly and teaches that when quick distillation is carried out in this process in the presence of a phosphorus, sulfur or boron compound, the polymerization rate of bis-&bgr;-hydroxyethyl terephthalate can be controlled and bis-&bgr;-hydroxyethyl terephthalate can be obtained at a high yield. However, the results of studies conducted by the present inventor revealed that the purification of bis-&bgr;-hydroxyethyl terephthalate containing various impurity ions cannot be realized because these metal compounds separate out on the evaporation surface, thereby greatly preventing heat transmission with the result that long-term continuous stable operation cannot be continued substantially.
JP-A 49-36646 discloses a process for purifying bis-&bgr;-hydroxyethyl terephthalate by distillation at a temperature of 140 to 190° C. by maintaining the pressure of an output port at a range of 0.01 to 0.1 mmHg. However, the results of studies conducted by the present inventor revealed that the purification of bis-&bgr;-hydroxyethyl terephthalate containing various impurity ions cannot be realized even when this process is carried, because polycondensation takes place with the passage of operation time, thereby making impossible to carry out distillation operation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for producing high-purity bis-&bgr;-hydroxyethyl terephthalate from crude terephthalic acid.
It is another object of the present invention to provide a process for producing high-purity bis-&bgr;-hydroxyethyl terephthalate which makes it easy to remove impurities and possible to purity crude terephthalic acid by removing impurities contained in the crude terephthalic acid after terephthalic acid is converted into bis-&bgr;-hydroxyethyl terephthalate without purifying the crude terephthalic acid.
Other objects and advantages of the present invention will become obvious from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a process for producing high-purity bis-&bgr;-hydroxyethyl terephthalate by carrying out the following steps (1) to (4) in the mentioned order:
(1) the step of esterifying crude terephthalic acid with ethylene glycol and/or ethylene oxide to form a reaction product containing bis-&bgr;-hydroxyethyl terephthalate;
(2) the step of preparing a solution containing bis-&bgr;-hydroxyethyl terephthalate as the main solute and ethylene glycol as the main solvent by mixing the above reaction product obtained in the above step (1) with ethylene glycol as required and deionizing the solution to form a deionized solution of bis-&bgr;-hydroxyethyl terephthalate;
(3) the step of distilling off substances having a boiling point lower than that of bis-&bgr;-hydroxyethyl terephthalate from the above deionized solution obtained in the step (2) to form crude bis-&bgr;-hydroxyethyl terephthalate; and
(4) the step of subjecting the crude bis-&bgr;-hydroxyethyl terephthalate obtained in the step (3) to molecular distillation to distill out bis-&bgr;-hydroxyethyl terephthalate.
The present inventor has paid attention to bis-&bgr;-hydroxyethyl terephthalate obtained by the est
Inada Shuji
Sato Kikuchi
Aies Co. Ltd.
Anderson Kill & Olick
Lieberstein Eugene
Meller Micheal N.
Oh Taylor V.
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