Process for the production of acrylic acid

1. A process for the production of acrylic acid (AA) comprising the steps:

(a) subjecting a first gas mixture comprising propylene, oxygen, an inert gas, and steam to a first catalytic oxidation reaction stage thereby converting the propylene in the presence of a catalyst mainly into acrolein being contained in a second gas mixture from said first catalytic oxidation reaction,

(b) subjecting said a second gas mixture from the first catalytic oxidation reaction stage to a second catalytic oxidation reaction stage thereby converting the acrolein in the presence of a catalyst mainly into AA, being contained in a product gas,

(c) subjecting said product gas to a quench tower, wherein said AA is recovered as an aqueous solution comprising AA being contained in the process water, wherein a process vent gas is obtained at the top of said quench tower, and wherein the process vent gas is treated in a subsequent thermal or catalytic combustion unit, and

(d) separating said process water in a subsequent separation unit and process water is fed back into said quench tower so that most parts of the process water vaporized is mixed with the process vent gas leaving the quench tower on top and is treated together with the process vent gas in the thermal or catalytic combustion unit,

wherein

said first gas mixture has a steam/propylene ratio of greater than about 0.3 and less than about 2; and

the amount of said process water is less or equal to the amount of water in said aqueous solution withdrawn from the quench tower; and

wherein said treatment in said thermal or catalytic combustion unit yields a combusted process vent gas and at least a part of said combusted process vent gas is recycled to said first catalytic oxidation reaction stage.

2. The process of claim 1 wherein said catalyst of said first catalytic oxidation stage is a Mo—Co—Bi based catalyst.

3. The process of claim 1 wherein said catalyst of said second catalytic oxidation stage is a Mo—V—W based catalyst.

4. The process of claim 1 wherein a carrier for the catalysts in the catalytic combustion unit include TiO 2 carriers.

5. The process of claim 4 wherein said carrier is in the shape of honeycombs with a low delta pressure of less than about 20 mbar/m 3 .

6. The process of claim 1 wherein the oxygen concentration in the second gas mixture can be increased by the addition of air.

7. The process of claim 6 wherein additional steam is added to the second gas mixture.

8. The process of claim 1 wherein the combusted vent gas is at least partially recycled to the gas mixture being oxidized in the first catalytic oxidation reaction stage.

9. The process of claim 1 wherein the concentration of the propylene in the gas mixture at the first catalytic oxidation reaction stage inlet is at least about 9 Vol.-%.

10. The process of claim 1 wherein said process water is at least a part, of the water from a separation of said AA comprising aqueous solution into water and AA.

11. The process of claim 1 wherein said quench tower comprises a cooling and an absorption section, wherein said product gas is subjected to said cooling section in a product gas portion and a side stream leaves the quench tower in a portion above said product gas portion.

12. The process of claim 1 wherein a side stream of water with AA is taken out in the upper half, wherein said side stream is separated from at least a part of AA in a subsequent separation unit then sent back to the top of the quench tower together with the process water main stream.

13. The process of claim 1 wherein the space velocity of propylene (SVp) in said second catalytic oxidation reactor is at least 160 h −1 or the propylene oxygen ratio is in said second catalytic oxidation reactor is in the range of from about 0.1 to about 0.9 at a propylene conversion of at least about 90 Mol-% at one propylene pass through with a selectivity of acrolein and AA with respect to propylene of at least about 90 Mol-% in said first catalytic oxidation stage and the acrolein conversion in said second catalytic oxidation stage is at least about 95 Mol-% and an overall selectivity of at least about 83 Mol-%.

14. The process of claim 1 wherein the temperature at the top of the quench tower is in the range of from about 30 to about 90° C.

15. The process of claim 1 wherein the pressure at the top of the quench tower is from about 1 to about 8 bar.

16. The process of claim 4 wherein said carrier is in the shape of honeycombs with a low delta pressure of less than about 10 mbar/m 3 .

17. The process of claim 4 wherein said carrier is in the shape of honeycombs with a low delta pressure of less than about 3 mbar/m 3 .

18. The process of claim 1 wherein the concentration of the propylene in the gas mixture at the first catalytic oxidation reaction stage inlet is at least about 11 Vol.-%.

19. The process of claim 1 wherein the concentration of the propylene in the gas mixture at the first catalytic oxidation reaction stage inlet is at least about 14 Vol.-%.

20. The process of claim 1 wherein the temperature at the top of the quench tower is in the range of from about 40 to about 80° C.

21. The process of claim 1 wherein the pressure at the top of the quench tower is from about 1.05 to about 6 bar.

22. The process of claim 1 wherein the pressure at the top of the quench tower is from about 1.1 to about 1.5 bar.