Process for obtaining isobutene from a C4-hydrocarbon mixture
The invention relates to a process for obtaining isobutene from an isobutene containing C4-hydrocarbon mixture ( 1 ) in a plant comprising an etherification unit ( 3 ), a first distillation unit ( 5 ), an ether cleavage unit ( 8 ) and a second distillation unit ( 10 ), the process comprising: (a) contacting the C4-hydrocarbon mixture ( 1 ) with a primary alcohol ( 2 ) and reacting the mixture with the primary alcohol in the presence of an acidic catalyst to form the corresponding alkyl tert-butyl ether as an intermediate product and diisobutene as a by-product in the etherification unit ( 3 ); (b) distilling the reaction mixture ( 4 ) from the etherification unit ( 3 ) in the first distillation unit ( 5 ), a C4-hydrocarbon raffinate being withdrawn as the overhead product ( 6 ), the alkyl tert-butyl ether and diisobutene being withdrawn as the liquid or vaporous bottom product ( 7 ), and vaporizing the bottom product ( 7 ) if it is withdrawn as a liquid; (c) reacting the vaporous bottom product ( 7 ) in the presence of an acidic catalyst obtaining isobutene and the primary alcohol as reaction products in the ether cleavage unit ( 8 ); (d) distilling the reaction mixture ( 9 ) from the ether cleavage unit ( 8 ) in the second distillation unit ( 10 ), isobutene being withdrawn as the overhead product ( 11 ), the primary alcohol and diisobutene being withdrawn as the bottom product ( 12 ) and being recycled to the etherification unit ( 3 ); the plant further comprising a byproduct separation unit ( 15 ) being fed by a bottom purge stream ( 13 ) of the first distillation unit ( 5 ) and/or by a part of the bottom product of the second distillation unit ( 10 ), this feed stream ( 14 ) being split up in at least three byproduct streams wherein a first byproduct stream ( 16 ) is rich in diisobutene, a second byproduct stream ( 17 ) is rich in the primary alcohol, and a third byproduct stream ( 18 ) is rich in components with a normal boiling point higher than 110° C.
1. A process for obtaining isobutene from an isobutene containing C4-hydrocarbon mixture in a plant comprising an etherification unit, a first distillation unit, an ether cleavage unit and a second distillation unit, the process comprising:
(a) contacting the C4-hydrocarbon mixture with a primary alcohol and reacting the C4-hydrocarbon mixture with the primary alcohol in the presence of an acidic catalyst to form a corresponding alkyl tert-butyl ether as an intermediate product as an effluent and diisobutene as a by-product in the etherification unit;
(b) distilling the effluent from the etherification unit in the first distillation unit, a C4-hydrocarbon raffinate being withdrawn as the overhead product, the alkyl tert-butyl ether and diisobutene being withdrawn as a liquid or vaporous bottom product, and vaporizing the bottom product if the bottom product is withdrawn as a liquid forming vaporous bottom product;
(c) reacting the vaporous bottom product in the presence of an acidic catalyst obtaining isobutene and the primary alcohol as reaction products in the ether cleavage unit;
(d) distilling the reaction products from the ether cleavage unit in the second distillation unit, isobutene being withdrawn as the overhead product, the primary alcohol and diisobutene being withdrawn as the bottom product and being recycled to the etherification unit;
characterized in that the plant further comprises a byproduct separation unit being fed by a feed stream comprising a bottom purge stream of the first distillation unit and/or by a part of the bottom product of the second distillation unit, this feed stream being split up in at least three byproduct streams wherein a first byproduct stream is rich in diisobutene, a second byproduct stream is rich in the primary alcohol, and a third byproduct stream is rich in components with a normal boiling point higher than 110° C.
2. The process according to claim 1 , wherein the second byproduct stream is recycled to the etherification unit.
3. The process according to claim 1 , wherein the mass fraction of the primary alcohol in the second byproduct stream is at least 90 wt.-%.
4. The process according to claim 1 , wherein the first byproduct stream is further split up in at least two further byproduct streams wherein a fourth byproduct stream is rich in diisobutene and a fifth byproduct stream is rich in alkyl tert-butyl ether.
5. The process according to claim 4 , wherein the mass fraction of diisobutene in the first byproduct stream is at least 30 wt.-% and the mass fraction of diisobutene in the fourth byproduct stream is at least 90 wt.-%.
6. The process according to claim 4 , wherein the fifth byproduct stream is at least partly recycled to the evaporator for evaporation and/or to the ether cleavage unit.
7. The process according to claim 1 , wherein the first distillation unit comprises a distillation column, the bottom product being withdrawn as a side stream from the distillation column at a stage below the feed stage, and a bottom purge stream rich in high-boiling components being withdrawn from the sump of the distillation column.
8. The process according to claim 7 , wherein the bottom purge stream from the sump of the distillation column in the first distillation unit is fed to the byproduct separation unit.
9. The process according to claim 1 , wherein the bottom product from the first distillation unit is vaporized in an evaporator, a bottom purge stream containing high boiling components with a normal boiling point higher than that of the alkyl tert-butyl ether being withdrawn from the evaporator.
10. The process according to claim 9 , wherein the evaporator is a natural circulation evaporator, particularly a Robert type evaporator, and wherein the bottom purge stream is withdrawn from the liquid phase in the bottom of the evaporator.
11. The process according to claim 9 , wherein the bottom purge stream from the evaporator is fed to the byproduct separation unit.
12. The process according to claim 1 , wherein the primary alcohol is isobutanol, and the alkyl tert-butyl ether is isobutyl tert-butyl ether (IBTBE).