Process for the dissociation of MTBE

1. A process for preparing isobutene by dissociation of MTBE in the gas phase, comprising:

a) fractional distillation of an MTBE-containing stream comprising feed MTBE (I) and a recycle stream (VIII) to give an MTBE-containing overhead stream (II) and a bottom stream (III) having a boiling point higher than MTBE,

b) catalytic dissociation of the overhead stream (II) obtained in step a) to give a dissociation product (IV),

c) separation by distillation of the dissociation product (IV) obtained in step b) into an overhead stream (V) comprising more than 90% by mass of isobutene and a bottom stream (VI) comprising diisobutene, MTBE and more than 50% of the methanol present in the dissociation product (IV),

d) fractional distillation of the bottom stream obtained in step c) under conditions under which the methanol is obtained as bottom product (VII) and more than 99% of the MTBE is obtained in the overhead product (VIII), and

e) recirculation of the overhead product (VIII) to step a).

2. The process according to claim 1 , wherein the fractional distillation in step a) is carried out so that the stream (II) has a concentration of 2-methoxybutane of less than 2500 ppm by mass, based on MTBE.

3. The process according to claim 1 , wherein the dissociation in step b) is carried out under conditions under which the conversion of MTBE is greater than the conversion of 2-methoxybutane.

4. The process according to claim 1 , wherein the dissociation in step b) is carried out over a catalyst which has an activity with respect to the dissociation of MTBE which is at least 1% greater than the activity with respect to the dissociation of 2-methoxybutane.

5. The process according to claim 1 , wherein the dissociation in step b) is carried out in a single pass at a conversion of MTBE of from 50 to 98%.

6. The process according to claim 1 , wherein industrial MTBE is used as stream (I).

7. The process according to claim 1 , wherein in a further process step f), methanol is separated off by extraction and dimethyl ether is separated off by distillation from the isobutene-containing overhead stream (V) obtained in process step c).

8. The process according to claim 1 , wherein an MTBE-containing stream which is entirely or partly obtained by removing low boilers from an MTBE-containing stream (Ia) is used as stream (I).

9. The process according to claim 1 , wherein step a) is carried out at a pressure of from 0.1 to 2.0 MPa (abs) .

10. The process according to claim 1 , wherein step b) is carried out at a temperature of from 200 to 400° C.

11. The process according to claim 1 , wherein step c) is carried out at a pressure of from 0.1 to 2.0 MPa (abs) .

12. The process according to claim 1 , wherein step c) comprises a reactive distillation for dissociating MBTE to isobutene and methanol.

13. The process according to claim 1 , wherein said reactive distillation is carried out at a temperature of from 60 to 140° C.

14. The process according to claim 1 , wherein said reactive distillation is carried out at a pressure of from 0.1 to 1.2 MPa (abs) .

15. The process according to claim 1 , wherein step d) is carried out at a pressure of from 0.05 to 1.0 MPa (abs) .

16. The process according to claim 7 , wherein step f) is carried out at a temperature of from 15 to 50° C.

17. The process according to claim 1 , wherein the fractional distillation (a) is carried out in a distillation column having a bottoms temperature higher than a top temperature.

18. The process according to claim 1 , wherein the overhead stream (V) and the overhead product (VIII) each comprise methanol.