Method for increasing oxidation reactor production capacity

1. A process for increasing the production capacity of a conventional back-mixed oxidation reactor for catalytic liquid phase, air oxidation of paraxylene to terephthalic acid, said method comprising:

(a) positioning a first reaction zone upstream of a second zone comprising said conventional back-mixed oxidation reactor;

(b) feeding acetic acid, oxidation catalyst, paraxylene, and a supply of oxygen to said first reaction zone to form a reaction medium in which the acetic acid:paraxylene mass ratio is in the range of from 13–16:1 and the operating pressure is at least about 2,500 kPa;

(c) limiting the uptake of oxygen within the reaction medium in said first reaction zone to a value which is less than that required for full conversion of the paraxylene present to terephthalic acid, wherein said terephthalic acid produced in the reacton medium in the first reaction zone remains in solution; and then

(d) feeding the reaction medium to said conventional back-mixed oxidation reactor in said second reaction zone while simultaneously reducing the pressure of the reaction medium to a value in the range of from about 500 kPa to less than 2,500 kPa;

(e) vaporizing a portion of the acetic acid present in said conventional back end reactor in said second zone;

(f) removing the vapor from the reactor overhead;

(g) condensing the vapor; and

(h) recycling some or all of the condensate to the first reaction zone.

2. The process of claim 1 in which said first reaction zone is a plug flow reactor or a back-mixed reactor.

3. The process of claim 2 , wherein less than 10 percent by weight of the terphthalic acid precipitates as a solid in the first reaction zone.

4. The process of claim 3 , wherein less than 1 percent by weight of the terephthalic acid precipitates as a solid in the first reaction zone.

5. The process of claim 4 , wherein there is no precipitation of the terephthalic acid as a solid in the first reaction zone.

6. The process of claim 1 , wherein the oxygen uptake within the reaction medium in said first reaction zone is limited to a value less than 70 percent of that required for full conversion of the paraxylene to the terephthalic acid.

7. The process of claim 6 , wherein the oxygen uptake within the reaction medium in said first reaction zone is limited to a value in the range of from 40 to 60 percent of that required for full conversion of the paraxylene to the terephthalic acid.

8. The process of claim 1 which includes the additional step of diverting a portion of the paraxylene feed from the first reaction zone directly to the convention back-mixed oxidation reactor whereby the resulting acetic acid:paraxylene mass ratio in the reaction medium in the first reaction zone is adjusted upwardly in response to that portion of the paraxylene feed which bypasses the first reaction zone to achieve a corresponding value in excess of 13:1.

9. The process of claim 2 which includes the additional step of diverting a portion of the paraxylene feed from the first reaction zone directly to the conventional back-mixed oxidation reactor whereby the resulting solvent:paraxylene mass ratio in the reaction medium in the first reaction zone is adjusted upwardly in response to that portion of the paraxylene feed which bypasses the first reaction zone to achieve a corresponding value in excess of 25:1.

10. A process for increasing the production capacity of a conventional back-mixed oxidation reactor for catalytic liquid phase, air oxidation of paraxylene to terephthalic acid, said method comprising:

(a) positioning a first reaction zone upstream of said conventional back-mixed oxidation reactor;

(b) forming a feed stream comprising acetic acid and oxidation catalyst at a pressure of at least 2,500 kPa;

(c) oxygenating the feed stream;

(d) continuously and simultaneously feeding (1) the oxygenated feed stream and (2) paraxylene to said first reaction zone to form a reaction medium in which the acetic acid:paraxylene mass ratio is in the range of from 13–16:1;

(e) limiting the uptake of oxygen within the reaction medium in said first reaction zone to a value which is less than that required for full conversion of the paraxylene present to terephthalic acid, wherein said terephthalic acid produced in the reaction medium in the first reaction zone remains in solution;

(f) feeding the reaction medium to a said conventional back-mixed oxidation reactor in said second reaction zone while simultaneously reducing the pressure of the reaction medium to a value in the range of from about 500 to less than 2,500 kPa;

(e) vaporizing a portion of the acetic acid present in said conventional oxidation reactor in said second reaction zone;

(f) removing the vapor from the reactor overhead;

(g) condensing the vapor; and

(h) recycling some or all of the condensate to the feed stream.

11. The process of claim 10 , wherein less than 10 percent by weight of the terephthalic acid precipitates as a solid in the first reaction zone.

12. The process of claim 11 , wherein less than 1 percent by weight of the terephthalic acid precipitates as a solid in the first reaction zone.

13. The process of claim 12 , wherein there is no precipitation of the terephthalic acid as a solid in the first reaction zone.

14. The process of claim 10 , wherein the oxygen uptake within the reaction medium in said first reaction zone is limited to a value less than 70 percent of that required for full conversion of the paraxylene to the terephthalic acid.

15. The process of claim 14 , wherein the oxygen uptake within the reaction medium in said first reaction zone is limited to a value in the range of from 40 to 60 percent of that required for full conversion of the paraxylene to the terephthalic acid.

16. The process of claim 10 which includes the additional step of diverting a portion of the paraxylene feed from the first reaction zone to said conventional reactor whereby the resulting acetic acid:paraxylene mass ratio in the reaction medium in the first reaction zone is adjusted upwardly in response to that portion of the paraxylene feed which bypasses the first reaction zone to achieve a corresponding value in excess of 13:1.