Process for dewatering ethanol

1. A process for preparing relatively low-water ethanol from at least two streams of relatively water-rich ethanol that have a different water content, the process comprising

introducing separately the ethanol streams with different water contents into different membrane units in which the ethanol streams are each separated into relatively low-water ethanol as the retentate and relatively water-rich permeate, and

combining the retentate stream from at least one membrane unit for separating an ethanol stream with a relatively high water content with the feed stream and/or the retentate stream of at least one other membrane unit for separating an ethanol stream, wherein only streams that have a difference in ethanol concentration of not more than 1% by mass absolute are combined.

2. A process according to claim 1 , wherein the dewatering at the membrane is carried out at a temperature of from 20 to 150° C.

3. A process according to claim 1 , wherein the dewatering at the membrane is carried out with a pressure difference of from 0.001 to 2 MPa.

4. A process according to claim 1 , wherein the water removal at the membrane is effected by reverse osmosis, pervaporation or vapor permeation.

5. A process according to claim 1 , wherein a pressure difference is generated by generating a vacuum on the permeate side of the membrane.

6. A process according to claim 5 , wherein the pressure difference is generated by generating a reduced pressure on the permeate side of all membranes used by means of a common reduced-pressure unit.

7. A process according to claim 1 , wherein at least one stream of relatively low-water ethanol which has a water content of below 1% by mass is obtained.

8. A process according to claim 1 , wherein two relatively water-rich ethanol streams with different water contents, whose water concentrations differ by more than 1.0% by mass, are used.

9. A process according to claim 1 , wherein at least one relatively water-rich ethanol stream is fresh alcohol and at least one relatively water-rich ethanol stream stems from an ETBE plant.

10. A process according to claim 1 , wherein the water-rich permeates obtained in the dewatering in the membrane units are worked up separately.

11. A process according to claim 1 , wherein the water-rich permeates obtained in the dewatering in the membrane units are worked up together.

12. A process according to claim 10 , wherein the permeate is separated by distillation into water and at least one distillate consisting of an ethanol/water mixture, and the distillate is recycled into a membrane unit as a relatively water-rich ethanol stream.

13. A process according to claim 10 , wherein the permeate, together with an ethanolic aqueous extract from an ETBE plant, is worked up by distillation, and the ethanol-rich distillate is recycled into a membrane unit as a relatively water-rich ethanol stream.

14. A process according to claim 1 , wherein the retentates, when their water concentrations differ by less than 0.2% by mass, are combined.

15. A process according to claim 1 , wherein two or more retentate streams are combined and are dewatered in a further membrane unit.

16. A process according to claim 1 , wherein the retentate stream from the membrane unit for separating an ethanol stream with a relatively high water content is combined with the feed stream of a membrane unit for separating an ethanol stream with a relatively low water content.

17. A process according to claim 1 , wherein the relatively low-water ethanol prepared in accordance with the invention is used as a reactant in an ETBE synthesis and the excess ethanol from the process of ETBE synthesis is removed by extraction with water from the ETBE process, and this extraction stream is fed, directly or after a distillative workup, as a water-rich ethanol stream to a membrane unit.