Process for the preparation of an alkanediol and a dialkyl carbonate

1. A process for the preparation of an alkanediol and a dialkyl carbonate comprising:

(a) reacting an alkylene carbonate and an alkanol feedstock in a first reaction zone under transesterification conditions to obtain a product mixture of dialkyl carbonate, unconverted alkanol, the alkanediol, unconverted alkylene carbonate and dimers of the alkanediol;

(b) separating dialkyl carbonate and alkanol from the product mixture to obtain a bottom product stream containing alkanediol, unconverted alkylene carbonate and dimers of the alkanediol;

(c) recovering the dialkyl carbonate; and

(d) separating alkanediol from the bottom product stream to leave a recycle stream comprising unconverted alkylene carbonate and dimers of the alkanediol, which process further comprises

(e) passing at least part of the recycle stream to a second reaction zone in which the dimers of the alkanediol are converted to higher-boiling oligomers of alkanediol, yielding an oligomers-containing effluent;

(f) separating the higher-boiling oligomers from the oligomers-containing effluent yielding an alkylene carbonate-containing remaining stream; and

(g) recycling the alkylene carbonate-containing remaining stream to the first reaction zone.

2. A process as claimed in claim 1 , wherein the reaction in the first reaction zone is conducted in a co-current manner.

3. A process as claimed in claim 2 , wherein the reaction is conducted in a plug flow manner.

4. A process as claimed in claim 1 , wherein a heterogeneous catalyst is used in the transesterification reaction.

5. A process as claimed in claim 1 , wherein the temperature in the first reaction zone is from 40 to 200° C., and the pressure from 0.5 to 50 bar.

6. A process as claimed in claim 1 , wherein unconverted alkanol and the dialkyl carbonate are separated by distillation in one fraction.

7. A process as claimed in claim 1 , wherein the bottom product stream is subjected to a further distillation to achieve a separation of alkanediol and a recycle stream comprising unconverted alkylene carbonate and dimers of alkanediol.

8. A process as claimed in claim 7 , wherein the separated alkanediol is obtained in a stream further comprising unconverted alkylene carbonate.

9. A process as claimed in claim 1 , wherein the dimers of alkanediol are converted to higher-boiling oligomers of alkanediol with an alkylene carbonate.

10. A process as claimed in claim 9 , wherein the dimers of the alkanediol are converted under transesterification conditions as in the first reaction zone.

11. A process as claimed in claim 9 , wherein the dimers of alkanediol are converted to higher-boiling oligomers of alkanediol with an alkylene oxide.

12. A process as claimed in claim 11 , wherein the dimers are converted in the presence of an alkaline catalyst.

13. A process as claimed in claim 11 , wherein the reaction conditions in the second reaction zone include a pressure ranging from 0.5 to 15 bar and a temperature ranging from 70 to 250° C.

14. A process as claimed in claim 11 , wherein the reaction conditions in the second reaction zone include a pressure ranging from 1 to 6 bar, and a temperature ranging from 90 to 180° C.

15. A process as claimed in claim 9 , wherein the molar ratio between and alkylene carbonate or alkylene oxide and the dimer of alkanediol ranges from 1:1 to 1000:1.

16. A process as claimed in claim 1 , wherein the alkylene carbonate is selected from the group consisting of ethylene carbonate and propylene carbonate.

17. A process as claimed in claim 1 , wherein the alkanol is selected from the group consisting of ethanol and isopropanol.

18. A process as claimed in claim 1 , wherein the oligomer is selected from the group consisting of triethylene glycol and tripropylene glycol.