Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1998-11-30
2001-06-12
Barts, Samuel (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S486000, C562S487000
Reexamination Certificate
active
06245939
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improved process for producing highly pure aromatic carboxylic acid from an impure solid acid product, and, more particularly, to a process improvement whereby the solid acid product can be efficiently dissolved in a suitable solvent at relatively low temperatures despite the presence of solid lumps.
In typical commercial processes for producing highly pure aromatic carboxylic acids, and particularly isophthalic and terephthalic acid, a rather insoluble, impure acid is first produced and recovered from an oxidation process. In the oxidation process, which involves catalytic air oxidation of paraxylene in acetic acid solvent in the case of terephthalic acid, the impure acid, usually in the form of solid crystals, is conveyed from a dryer to a holding silo before further processing in a usually separate purification stage to remove impurities. In the purification stage of such a process, crude, i.e., impure, acid crystals are slurried in water, recycled mother liquor or other suitable solvent, and the resulting slurry is pumped through a series of preheaters to raise the slurry temperature and thereby dissolve the slurry particles. The resulting solution is then subjected to hydrogenation at elevated temperature typically in the range of 280°-283° C. under liquid phase conditions in the presence of a Group VIII noble metal hydrogenation catalyst. The purified acid is recovered by crystallizing the acid from the hydrogen treated solution. The principal impurities, which are p-toluic acid (m.p. 180° C.; b.p. 275° C.) derived from the compound
4
-carboxybenzaldehyde (4-CBA) (m.p. 258° C.; b.p. sublimes) and unidentified color bodies, along with some other organic components, such as benzoic acid (m.p. 122.5° C.; b.p. 249° C.) and some residual terephthalic acid, (in the case of terephthalic acid) remain dissolved in the solution.
Depending on the conditions under which the crude acid is dried, its moisture content, and the time during which the acid is held in the holding silo before entering the purification stage, significant lumping of the solid product can occur. These lumps can be as large as 10 cm in average diameter, and, in turn, they can be very difficult to process, can clog and even damage valves and conveying equipment, and can be very difficult to efficiently dissolve in water and other solvents, especially at relatively low initial temperatures, i.e., below about 100° C., for further processing. The present invention provides a method for handling these lumps in the context of a process for producing highly pure aromatic carboxylic acid from a crude crystalline starting material.
SUMMARY OF THE INVENTION
The present invention is an improvement in a process for producing a highly pure aromatic carboxylic acid crystals from an impure solid acid product which includes the steps of:
(a) dissolving the impure solid product in a solvent at elevated temperature to form a solution;
(b) hydrogenating the solution in the presence of a hydrogenation catalyst;
(c) optionally separating the solution from the catalyst; and
(d) cooling the solution to precipitate pure aromatic carboxylic acid crystals. The improvement comprises dissolving the impure solid product in the solvent by metering the impure solid product from its storage silo through an appropriate exit nozzle into a holding tank containing the solvent in response to a feedback signal. It is thereby possible to form an intermediate slurry of impure acid crystals at a relatively low temperature in the range of from 90° C. to 100° C. and at a concentration in a range of from 28% w/w up to 30% w/w while continuously agitating the slurry as it is formed. The slurry is then removed, i.e., pumped, from the holding tank while undesirably large lumps of yet undissolved solid product, i.e., particles having an average size greater than from 24 mm to 28mm, are retained in the holding tank. The density of the slurry is continuously measured at a location downstream from the holding tank, and that measurement is then converted into a feedback signal which is communicated to the metering device to control the rate at which solid impure product and solvent are introduced into the holding tank. Simultaneously as the slurry is being pumped from the holding tank or thereafter via one or via several intermediate steps, the slurry is heated to an appropriately high processing temperature of about 283° C. and the pressure of the slurry is raised to an appropriate level at which substantially all of the acid particles have dissolved prior to hydrogenation and purification.
The process of the present invention can be operated on a continuous basis or batch-wise, and, in a preferred embodiment of the invention, metering of the solid impure product from its storage silo into the slurry holding tank is accomplished using a motor-actuated rotary valve having a control device which is responsive to the feedback signal.
REFERENCES:
patent: 4626598 (1986-12-01), Packer et al.
patent: 4629715 (1986-12-01), Schroeder
patent: 5062709 (1991-11-01), Slee
Hsu Yu-Wei
Lin Chen-Long
Barts Samuel
E. I. Du Pont de Nemours and Company
Krukiel Charles E.
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