Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-01-07
2001-10-23
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
Reexamination Certificate
active
06307099
ABSTRACT:
This invention relates to the production of aromatic carboxylic acids which are sparingly soluble in acetic acid and water, particularly terephthalic acid.
Terephthalic acid is an important intermediate for the production of polyester polymers which are used typically for fibre production and in the manufacture of bottles. Current state-of-the-art technology for the manufacture of terephthalic acid involves the liquid phase oxidation of paraxylene feedstock using molecular oxygen in a solvent comprising lower C2 to C6 aliphatic monocarboxylic acid, usually acetic acid, in the presence of a dissolved heavy metal catalyst system incorporating a promoter such as bromine. The reaction is carried out in at least one stirred vessel under elevated temperature and pressure conditions, typically 150 to 250° C. and 6 to 30 bara respectively, with air being sparged into the reaction mixture and typically produces terephthalic acid in high yield, e.g. at least 95%. Isothermal reaction conditions are maintained in the oxidation vessel by allowing evaporation of the solvent, together with water produced in the reaction, the resulting vapour being condensed and returned to the reactor vessel as reflux. In the conventional production of terephthalic acid, because terephthalic acid is only sparingly soluble in the solvent, a substantial proportion the product precipitates in the course of the reaction and as a result impurities such as 4-carboxybenzaldehyde (4-CBA) and colour bodies co-precipitate with the terephthalic acid to produce a crude product which, to meet the requirements of many polyester producers, has to be purified to reduce its impurity content. In one purification process, the crude product is dissolved in water and, under elevated temperature and pressure conditions, is contacted with hydrogen in the presence of a hydrogenation catalyst, the purified terephthalic acid thereafter being recovered by crystallisation and solids-liquid separation techniques.
The present invention seeks to provide a process for the production of terephthalic acid in such a way as to afford scope for achieving a sufficiently pure product for subsequent use without necessarily having to carry out an additional purification process.
According to a first aspect of the present invention there is provided a process for the production of terephthalic acid by the liquid phase oxidation of a precursor of terephthalic acid with oxygen in a reaction medium containing the precursor and a solvent under conditions such that substantially all of the terephthalic acid produced in the oxidation reaction zone is maintained in solution during the reaction, characterised in that the oxidation reaction is carried out by passing the reaction medium through the reaction zone in a continuous plug flow reaction regime.
Preferably the reactor is a plug flow reactor or a series of two or more plug flow reactors, preferably operated in a non-boiling mode, although the various aspects ot the invention defined herein are not limited to this particular type of continuous flow reactor. For instance, the reaction may be carried out in a series of non-boiling continuous stirred tank reactors so as to approximate a continuous plug flow regime or in a reaction system comprising one-or more non-boiling continuous stirred tank reactors and one or more plug flow reactors arranged in any sequence.
By “continuous plug flow regime” we mean a reactor in which reactants are introduced and products withdrawn simultaneously in a continuous manner, as opposed to a batch-type reactor. The residence time of the reaction medium within the reaction zone is generally no more than 10 minutes and is preferably no more than 8 minutes, with residence times of 5 minutes or less, e.g. 3 minutes or less, being achievable.
According to a second aspect of the present invention, which may be but is not necessarily used in conjunction with the first aspect of the invention, there is provided a process for the production of terephthalic acid by the liquid phase oxidation of a precursor of terephthalic acid with oxygen in a reaction medium containing the precursor and a solvent under conditions such that substantially all of the terephthalic acid produced in the oxidation reaction zone is maintained in solution during the reaction, characterised in that the oxidation reaction is carried out with substantially all of the oxygen dissolved in the reaction medium.
Thus, in this aspect of the present invention, the liquid phase oxidation reaction is carried out in such a way as to maintain substantially all of the resulting terephthalic acid in solution during the reaction thereby reducing the extent to which the main impurity, 4-CBA, contaminates the recovered terephthalic acid as a result of co-precipitation during the reaction. Substantially all of the oxygen utilised in the process according to this aspect of the present invention is dissolved in the reaction medium. The use of dissolved oxygen in the reaction medium allows the oxygen to be more uniformly distributed throughout the reaction medium. In this manner, oxygen starved regions within the reaction medium can be minimised with consequential reduction in the formation of undesirable reaction by-products such as trimellitic acid, benzoic acid and colour bodies. Overall, this leads to the possibility of producing product with a low level of contamination and without undue solvent burning which, in turn, allows elimination of the purification process conventionally employed in the production of terephthalic acid of sufficient quality for use in high grade polyester manufacture.
Although in the above aspect and other aspects of the invention disclosed herein, it is preferred that all of the terephthalic acid produced in the reaction is maintained in solution during the reaction, we do not exclude the possibility of some precipitation during the reaction, e.g. up to 10%, more usually no more than 5% but desirably no more than about 2% by weight of the terephthalic acid produced may precipitate during the course of the reaction.
Preferably the reaction medium is produced by combining at least two separate liquid phase components and at least part of the oxygen is added to and dissolved in one or more of said liquid phase components before such components are combined to form the reaction medium.
For instance, the separate liquid phase components may include one component consisting of or containing said precursor and a second component consisting of or containing said solvent and at least part of the oxygen required for the reaction may be added to and dissolved in the second component so that reaction between the oxygen and the precursor cannot commence until the components are combined to form the reaction medium.
Usually the solvent is predominantly an aliphatic monocarboxylic acid (preferably containing 2 to 6 carbon atoms) and may for instance be selected from acetic acid, propionic acid, butyric acid, isobutyric acid, n-valeric acid, trimethyl-acetic acid, caprioic acid and mixtures of one of these carboxylic acids with water, which in any event is produced in the course of the reaction. The presently preferred solvent is acetic acid and water. However, we do not preclude the possibility of using other solvents such as benzoic acid, e.g. a mixture of benzoic acid and water.
The water content used in the conventional production of terephthalic acid by liquid phase oxidation of paraxylene is typically such that water comprises between 3 and 10% by weight of the combined solvent-forming carboxylic acid/water supplied to the reaction zone. A feature applicable to the various aspects of the invention disclosed herein is that the water content can be substantially greater than that present in the the total feed to the reaction zone of a conventional terephthalic acid production process; in the various aspects of the invention disclosed herein the reaction medium composition at the time of commencement of the reaction may contain water in an amount of ranging from about 3% up to about 30%, e.g. 12%, or greater (e
Hugall Duncan Stuart
Jones Graham Howard
Royall David John
Turner John Arthur
Woodcock Duncan Charles
E. I. Du Pont de Nemours and Company
Killos Paul J.
Krukiel Charles E.
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