Method for the purification of acrolein

1. A continuous process for the manufacture of 3-(methylthio)propionaldehyde, the process comprising:

(a) subjecting propylene to vapor-phase oxidation using a catalyst, to obtain a crude acrolein-based product;

(b) removing acids present in the crude acrolein-based product;

(c) absorbing the crude acrolein-based product with water to obtain an aqueous acrolein solution;

(d) purifying the aqueous acrolein solution to obtain purified gaseous acrolein by a process comprising:

introducing the aqueous acrolein solution into a distillation column equipped at its base with at least one boiler and at its top with at least one condenser,

withdrawing a liquid mixture essentially comprising water at the base of the distillation column,

withdrawing a gas mixture essentially comprising acrolein and water at the top of the distillation column,

cooling the gas mixture withdrawn at the top of the distillation column in the condenser, to a temperature which makes it possible to obtain, on the one hand, an aqueous condensate and, on the other hand, an acrolein-rich gas mixture, and

withdrawing the acrolein-rich gas mixture; and

(e) reacting the purified gaseous acrolein with methyl mercaptan, to obtain 3-(methylthio)propionaldehyde;

wherein:

noncondensable gases produced in step (a) are separated from the acrolein prior to step (e).

2. The process as claimed in claim 1 , wherein the noncondensable gases produced in step (a) are separated from the acrolein prior to step (d).

3. The process as claimed in claim 2 , wherein the noncondensable gases produced in step (a) are separated from the acrolein prior to step (c).

4. The process as claimed in claim 3 , wherein the noncondensable gases produced in step (a) arc separated from the acrolein in steps (b) and (c).

5. The process as claimed in claim 1 , wherein the noncondensable gases separated from the acrolein are recycled to the vapor-phase oxidation reaction of step (a).

6. The process as claimed in claim 1 , wherein the noncondensable gases separated from the acrolein are discharged and incinerated.

7. The process as claimed in claim 1 , wherein purified gaseous acrolein is reacted with gaseous methyl mercaptan in step (e).

8. The process as claimed in claim 1 , wherein the aqueous acrolein solution has a concentration of acrolein of less than or equal to the solubility limit of acrolein in water.

9. The process as claimed in claim 1 , wherein the distillation column is maintained at a pressure P and the temperature in the condenser is maintained at a value T according to T>21.28*P+32.9.

10. The process as claimed in claim 9 , wherein the distillation column is maintained at atmospheric pressure and the temperature in the condenser is maintained at a value of greater than 54° C.

11. The process as claimed in claim 1 , wherein the acrolein-rich gas mixture has an acrolein concentration ranging from 86 to 95% by weight.

12. The process as claimed in claim 1 , wherein the condensate is at least partially reintroduced into the distillation column.

13. The process as claimed in claim 12 , wherein all of the condensate is reintroduced at the top of the distillation column.

14. A continuous process for the manufacture of 3-(methylthio)propionaldehyde, the process comprising:

(a) subjecting propylene to vapor-phase oxidation using a catalyst, to obtain a crude acrolein-based product;

(b) removing acids present in the crude acrolein-based product;

(c) absorbing the crude-acrolein product with water to obtain an aqueous acrolein solution separated from noncondensable gases,

(d) purifying the aqueous acrolein solution to obtain purified gaseous acrolein by a process comprising:

introducing the aqueous acrolein solution into a distillation column equipped at its base with at least one boiler and at its top with at least one condenser,

withdrawing a liquid mixture essentially comprising water at the base of the distillation column,

withdrawing a gas mixture essentially comprising acrolein and water at the top of the distillation column,

cooling the gas mixture withdrawn at the top of the distillation column in the condenser, to a temperature which makes it possible to obtain, on the one hand, an aqueous condensate and, on the other hand, an acrolein-rich gas mixture, and

withdrawing the acrolein-rich gas mixture, and

(e) reacting the purified gaseous acrolein directly with methyl mercaptan to obtain 3-(methylthio)propionaldehyde.

15. The process as claimed in claim 14 , wherein the methyl mercaptan of step (e) is in the gas phase.

16. A process for the purification of acrolein, the process comprising:

introducing an aqueous acrolein solution into a distillation column equipped at its base with at least one boiler and at its top with at least one condenser,

withdrawing a liquid mixture comprising water at the base of the distillation column,

withdrawing a gas mixture comprising acrolein at the top of the distillation column,

cooling the gas mixture withdrawn at the top of the distillation column in the condenser, to a temperature which makes it possible to obtain, on the one hand, an aqueous condensate and, on the other hand, an acrolein-rich gas mixture, and

withdrawing the acrolein-rich gas mixture,

wherein:

the liquid mixture withdrawn at the base of the distillation column is a nonazeotropic liquid mixture essentially comprising water;

the aqueous condensate is substantially depleted in acrolein; and

the acrolein-rich gas mixture is substantially enriched in acrolein.

17. The process as claimed in claim 16 , wherein the gas mixture obtained at the distillation column top comprises, by volume, between 30% and 70% of water.