Terephthalic acid purge filtration rate by controlling % water in filter feed slurry

1. A terephthalic acid purge process comprising:

(a) subjecting an oxidizer purge stream formed in a terephthalic acid synthesis process comprising a carboxylic acid, a metal catalyst, impurities, water and a solvent comprising acetic acid to evaporation in a 1 st evaporator zone to produce a vapor stream and a concentrated purge slurry; and

(b) adding water to said concentrated purge slurry in a 1 st mix zone to produce a water rich concentrated purge slurry;

(c) subjecting said water rich concentrated purge slurry to evaporation in a 2 nd evaporator zone to produce a solvent rich stream and a super concentrated purge slurry wherein said second evaporator zone comprises an evaporator operated at a temperature of about 20° C. to about 70° C.; wherein said super concentrated purge slurry has a water content of about 5 wt % to about 25 wt %

(d) filtering said super concentrated purge slurry in a solid-liquid separation zone to form a filter cake and mother liquor

(e) washing said filter cake with a wash feed in said solid-liquid separation zone to form a washed filter cake and a wash liquor.

2. The process according to claim 1 wherein at least 80% of said metal catalyst from said super concentrated purge slurry is recovered through said separation zone into said mother liquor and said wash liquor cumulative.

3. The process according to claim 2 wherein about 75 wt % to about 95 wt % of said solvent and water is removed by evaporation from said oxidizer purge stream in step (a) and step (c) combined.

4. The process according to claim 3 where in at least a portion of said washed filter cake is routed to a stream comprising terephthalic acid downstream of oxidation zone.

5. The process according to claim 1 wherein said super concentrated purge slurry has a water content of about 8 wt % to about 23%.

6. The process according to claim 1 wherein said super concentrated purge slurry has a water content of about 11 wt % to about 21%.

7. The process according to claim 1 wherein said super concentrated purge slurry has a water content of about 13 wt % to about 18%.

8. The process according to claim 1 wherein the temperature of said concentrated purge stream routed from said 1 st evaporation zone to said mix zone is maintained above 80 C.

9. The process according to claim 1 wherein the residence time in said second evaporation zone ranges from 30 minutes to 180 minutes.

10. The process according to claim 1 wherein the filtration rate of said super concentrated purge slurry in said solid liquid separation zone is higher for water content range of said super concentrated purge slurry ranging from 13% to 19% than in any water concentration outside said range.

11. A terephthalic acid purge process comprising:

(a) subjecting an oxidizer purge stream formed in a terephthalic acid synthesis process comprising a carboxylic acid, a metal catalyst, impurities, water and a solvent comprising acetic acid to evaporation in a 1 st evaporator zone to produce a vapor stream and a concentrated purge slurry; and

(b) adding water to said concentrated purge slurry in a 1 st mix zone to produce a water rich concentrated purge slurry;

(c) subjecting said water rich concentrated purge slurry to evaporation in a 2 nd evaporator zone to produce a solvent rich stream and a super concentrated purge slurry wherein said second evaporator zone comprises an evaporator operated at a temperature of about 20° C. to about 70° C.;

(d) filtering said super concentrated purge slurry in a solid-liquid separation zone to form a filter cake and mother liquor

(e) washing said filter cake with a wash feed in said solid-liquid separation zone to form a washed filter cake and a wash liquor;

wherein the mass of water added in the mix zone in step (b) is manipulated to result in the super concentrated purge slurry of step (c) having a water content of from 12.5% to 18.9% water.

12. The process according to claim 11 , wherein at least 80% of said metal catalyst from said super concentrated purge slurry is recovered through said separation zone into said mother liquor and said wash liquor cumulative.

13. The process according to claim 12 , wherein about 75 wt % to about 95 wt % of said solvent and water is removed by evaporation from said oxidizer purge stream in step (a) and step (c) combined.

14. The process according to claim 13 , wherein at least a portion of said washed filter cake is routed to a stream comprising terephthalic acid downstream of oxidation zone.

15. The process according to claim 11 , wherein the temperature of said concentrated purge stream routed from said 1 5 evaporation zone to said mix zone is maintained above 80 C.

16. The process according to claim 11 , wherein the residence time in said second evaporation zone ranges from 30 minutes to 180 minutes.

17. The process according to claim 11 , wherein the filtration rate of said super concentrated purge slurry in said solid liquid separation zone is higher for water content range of said super concentrated purge slurry ranging from 13% to 19% than in any water concentration outside said range.