Method for producing acrolein

1. A process for preparing acrolein by catalytic gas phase oxidation, the process comprising:

a) providing a reaction gas comprising propylene and molecular oxygen, and

b) contacting the reaction gas with an oxidation catalyst to form a gas mixture comprising acrolein, wherein the oxidation catalyst is a mixed oxide catalyst comprising at least one base component selected from the group consisting of molybdenum, vanadium, and tungsten, wherein the reaction gas is passed through a reactor tube or a plurality of reactor tubes in a bundle at a temperature of from 350° C. to 450° C.,

wherein the reaction gas comprises from 250 ppm to 5000 ppm by weight of sulphur in the form of a sulphur component and not more than 5000 ppm by weight of at last one unsaturated hydrocarbon selected from the group consisting of C 2 H 4 , C 3 H 4 , C 4 H 8 , C 4 H 6 , C 5 H 10 , and C 5 H 8 ,

wherein the reaction gas is passed through a reactor tube or a plurality of bundled reactor tubes in parallel, and the reactor tube or the plurality of bundled reactor tubes are filled with the oxidation catalyst to a length of at least 2 meters.

2. The process according to claim 1 , wherein the reaction gas comprises more than 50 ppm by weight of the at least one unsaturated hydrocarbon selected from the group consisting of C 2 H 4 , C 3 H 4 , C 4 H 8 , C 4 H 6 , C 5 H 10 , and C 5 H8.

3. The process according to claim 1 , wherein the reaction gas comprises from more than 250 ppm by weight to 1000 ppm by weight of sulphur in the form of a sulphur component.

4. The process according to claim 1 , wherein the reaction gas comprises from more than 250 ppm to 1000 ppm by weight of sulphur in the form of H 2 S and/or SO 2 .

5. The process according to claim 1 , wherein the reaction gas comprises from more than 100 ppm to 1500 ppm by weight of the at least one unsaturated hydrocarbon selected from the group consisting of C 2 H 4 , C 3 H 4 , C 4 H 8 , C 4 H 6 , C 5 H 10 , and C 5 H8.

6. The process according to claim 1 , wherein the reaction gas comprises from more than 150 ppm to 1300 ppm by weight of the at least one unsaturated hydrocarbon selected from the group consisting of C 2 H 4 , C 3 H 4 , C 4 H 8 , C 4 H 6 , C 5 H 10 , and C 5 H8.

7. The process according to claim 1 , wherein the mixed oxide catalyst further comprises at least one additional component selected from the group consisting of bismuth, antimony, tellurium, tin, iron, cobalt, nickel and copper.

8. The process according to claim 7 , wherein the oxidation catalyst is a mixed oxide catalyst of the formula (I) or a mixture of various mixed oxide catalysts of the formula (I):

(Mo 12 Bi a C b (Co+Ni) c D d E e F f G g H h )O x   (I),

C=iron,

D=at least one of the elements from W, P,

E=at least one of the elements from Li, K, Na, Rb, Cs, Mg, Ca, Ba, Sr,

F=at least one of the elements from Ce, Mn, Cr, V,

G=at least one of the elements from Nb, Se, Te, Sm, Gd, La, Y, Pd, Pt, Ru, Ag, Au,

H=at least one of the elements from Si, Al, Ti, Zr,

a=0 to 5.0,

b=0.5 to 5.0,

c=2 to 15,

d=0.01 to 5.0,

e=0.001 to 2,

f=0.001 to 5,

g=0 to 1.5,

h=0 to 800, and

x=number which is determined by the valency and frequency of elements other than oxygen.

9. The process according to claim 1 , wherein the reaction gas is produced by mixing at least refinery grade propylene and air, wherein the refinery grade propylene comprises from 0.05 to 2.0% by weight of ethane and/or from 9 to 40% by weight of propane.

10. The process according to claim 1 , wherein the reaction gas is passed through a reactor tube or a plurality of reactor tubes in a bundle at a temperature of from 370° C. to 430° C.

11. The process according to claim 1 , wherein the reactor tubes are charged with oxidation catalyst to the length of at least 3 meters.

12. The process according to claim 1 , wherein the length the reactor tubes are charged with oxidation catalyst to the length of 1.5 to 5 meters.

13. The process according to claim 1 , wherein the length the reactor tubes are charged with oxidation catalyst to the length of 2.0 to 4.5 meters.

14. The process according to claim 1 , wherein the length the reactor tubes are charged with oxidation catalyst to the length of 2.5 to 4.0 meters.

15. The process according to claim 1 , wherein the internal diameter of each reactor tube is from 1.0 to 3.5 cm.

16. The process according to claim 1 , wherein the internal diameter of each reactor tube is from 1.5 to 3.3 cm.

17. The process according to claim 1 , wherein the internal diameter of each reactor tube is from 2.0 to 3.0 cm.