Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Patent
1995-01-13
1996-12-10
Barts, Samuel
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
562486, 562485, C07C 5143
Patent
active
055832547
DESCRIPTION:
BRIEF SUMMARY
This application was filed under U.S.C. 371 from the application filed May 19, 1993.
This invention relates to the production of terephthalic acid.
Purified terephthalic acid (PTA) is commonly produced by purification of crude terephthalic acid (CTA) obtained by the liquid phase air oxidation of p-xylene. It may also be produced by the hydrolysis of a polyalkylene terephthalate, for example polyethylene terephthalate.
Such purification typically comprises adding an aqueous medium to the CTA to form a slurry thereof which is then heated to dissolve the CTA in the medium to provide an aqueous solution of terephthalic acid. This solution is then passed to a reduction step in which the solution is contacted with hydrogen under reducing conditions in the presence of a heterogeneous catalyst to reduce chemically organic impurities, for example 4-carboxybenzaldehyde (4-CBA). The hydrogenated solution is passed to pressure let-down vessels in which PTA crystals form to provide a slurry of PTA in aqueous medium. PTA is recovered from the aqueous medium.
It is known to recover PTA from the aqueous medium by separating the PTA and medium in a decanter centrifuge in a first separation stage, reslurrying the PTA with fresh water, separating the reslurried PTA and fresh water in a decanter centrifuge in a second separation stage to produce a PTA cake and then drying the cake in a drier.
These steps involve special equipment which incur significant capital costs and variable costs of operation. The recovery stage requires large quantities of water in the centrifuge and reslurry steps. Therefore a large quantity of aqueous waste effluent must be treated and may present problems of disposal. Further, dissolved PTA and other valuable materials in the waste effluent stage may be lost.
The present invention seeks to provide improvements in the production of terephthalic acid.
According to one aspect of the present invention there is provided a process for the purification of crude terephthalic acid, comprising: a terephthalic acid-containing solution; conditions and at elevated temperature and pressure to reduce chemically at least some of the impurity present in the crude terephthalic acid; crystallised pure terephthalic acid in aqueous medium; aqueous medium by filtration of the slurry to remove aqueous medium through a filter surface to obtain a wet mass of crystals; reslurrying said mass and supplying aqueous wash liquor to said mass while effecting filtration whereby the wash liquor is displaced through the mass and through a filter surface in the second zone; and least one of said zones such that, on the lower pressure side of the filter surface, the pressure is substantially the same as or greater than the pressure prevailing following step (c).
Step (c) will normally be carried out in a series of crystalliser stages, in which the pressure and temperature is reduced progressively. Thus, the pressure differential of step (f) will therefore be such that, on the lower pressure side of the filter surface, the pressure is at least equal to the pressure prevailing in the final crystalliser stage, which will normally be superatmospheric, e.g. within the range 1.5 to 15 bara, more preferably 3 to 10 bara.
Preferably the pressure differential will be such that, on the lower side of the filter surface in each of said zones, the pressure is at least equal to the pressure prevailing following step (c).
Advantageously steps (d), (e) and (f) are carried out by discharging said slurry on to a filter material which is movable to transport the terephthalic acid through said first zone in which filtration of said aqueous medium from the slurry is effected, to the second zone in which said mass of crystals is washed by displacement of said aqueous wash liquor through said mass.
In this way, reslurry of the terephthalic acid is avoided and by effecting filtration through a filter surface in such a way that the lower pressure side of the filter surface is at a pressure no less than said superatmospheric pressure, liquid removal from the te
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Hindmarsh Eric
Milne Ian P.
Parker David
Turner John A.
Barts Samuel
Sheehan John M.
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