METHOD FOR PREPARING ALPHA-HYDROXYCARBOXYLIC ACID ESTERS IN WHICH AMMONIA IS RECYCLED

Abstract

The present invention relates to a process for preparing alpha-hydroxycarboxylic esters proceeding from hydrogen cyanide, wherein the ammonia formed in the step of alcoholysis of the corresponding alpha-hydroxycarboxamide is recycled into a hydrogen cyanide preparation process after a purification step.

Claims

1. A process for preparing an alpha-hydroxycarboxylic ester, comprising: preparing hydrogen cyanide, alcoholysis of an alpha-hydroxycarboxamide to obtain said alpha-hydroxycarboxylic ester wherein ammonia formed in the alcoholysis of the alpha-hydroxycarboxamide, after a purification step, is recycled into the hydrogen cyanide preparation process still comprising at least one alkylamine.

2. The process according to claim 1, wherein a concentration of a total amount of the alkylamines based on ammonia is 1-100 000 ppm.

3. The process according to claim 1, wherein the alpha-hydroxycarboxylic ester is methyl hydroxyisobutyrate.

4. The process according to claim 1, wherein the alkylamine is trimethylamine.

5. The process according to claim 1, wherein the hydrogen cyanide preparation is according to an Andrussow process.

6. The process according to claim 1, wherein the ammonia is purified by an absorbent by passage through a solid.

7. The process according to claim 1, wherein the ammonia is purified in a continuously operated adsorber bed.

8. The process according to claim 1, wherein the ammonia is purified by activated carbon.

9. The process according to claim 1, wherein the alcoholysis reaction of the alpha-hydroxycarboxamide is effected in a liquid phase or in a gas phase.

10. The process according to claim 1, wherein the purification is effected within a temperature range from 0° C. to 150° C.

11. The process according to claim 1, wherein the purification is effected within a pressure range from 0.05 to 5 bar.

12. The process according to claim 1, comprising: a) feeding a reactant stream comprising the alpha-hydroxycarboxamide and an alcohol into a pressure reactor containing a catalyst, to obtain a reaction mixture, b) converting the reaction mixture in the pressure reactor at a pressure in the range of 0.1-100 bar, to obtain a product mixture, c) depleting the product mixture of alcohol and ammonia that arises from b), d) separating alcohol and ammonia containing at least trimethylamine and e) purifying ammonia containing at least trimethylamine by activated carbon before said ammonia into the hydrogen cyanide preparation process.

Description

COMPARATIVE EXAMPLES 1-2, EXAMPLE 1

[0092] These are conducted in a plant analogous to FIG. 1. In Comparative Example 1, pure ammonia is used; in Comparative Example 2, 34% by weight of ammonia of reaction from the HIBA alcoholysis and a catalyst are added thereto. In both cases, the ammonia feed is not conducted through the adsorber bed. In Inventive Example 1, Comparative Example 2 is repeated, except that the entire ammonia feed is passed through the adsorber bed, which is filled with Hydrafin CC 12×40 activated carbon. The results are shown in Tab.1 for Comparative Example 1 and Example 1 after a TOS (time on stream) of 14 d and, for Comparative Example 2, after a TOS of 1 h. Likewise shown are the amine impurities in the ammonia of reaction prior to introduction into the adsorber bed.

TABLE-US-00001 TABLE 1 Influence of activated carbon Comparative Comparative Example 1 Example 2 Example 1 Ammonia (99.9%)/g/min 2.506 0.906 0.799 Ammonia of reaction/g/min 0 1.600 1.600 NH.sub.3 (% by wt.) 92.0 MeOH (% by wt.) 1.5 TMA (% by wt.) 5.5 DME (% by wt.) 1.0 Methane (Linde 3.8) (g/min) 2.25 2.096 1.95 Air (g/min) 21.82 19.74 19.90 N total/C total (mol/mol) 1.049 1.034 1.032 NH.sub.3/CH.sub.4 (l/l) 1.049 1.069 1.080 Air/(NH.sub.3 + CH.sub.4) (l/l) 2.63 2.51 2.72 HCN/NH.sub.3 yield (mol %) 63 13 65

[0093] The yield of HCN is significantly increased with activated carbon-purified ammonia feed containing ammonia of reaction compared to unpurified, and is comparable to pure ammonia.

EXAMPLES 2-4

[0094] Example 1 was repeated, and the adsorption was conducted at different adsorber bed temperatures. The activated carbon used was Alcarbon PH 55×8C from Donau Carbon. The results are shown in Tab. 2.

TABLE-US-00002 TABLE 2 Temperature dependence Example 2 Example 3 Example 4 Adsorber bed temperature (° C.) 20° C. 70° C. 100° C. NH.sub.3 (% by wt.) 95.783 95.084 95.084 TMA (% by wt.) 2.097 2.512 2.512 DME (% by wt.) 0.792 0.753 0.753 MeOH (% by wt.) 1.328 1.65 1.65 HCN/NH.sub.3 yield mol % 63.26 62.57 62.51

[0095] Treatment with activated carbon achieves good results in terms of HCN yield over a wide temperature range.