Fatty Acids & Glycerine

Glycerine from petrochemical feedstock

Synthesis from Propene

Three processes are known for the production of glycerol from propene [115-07-1], involving the following intermediate stages (Fig . 9):

allyl chloride - epichlorohydrin acrolein - allyl alcohol - glycidol propene oxide - allyl alcohol - glycidol Only the first of these is important industrially.

Production from Allyl Chloride

The first synthetic glycerol was produced in 1943 by I.G. Farben in Oppau and Heydebreck and in 1948 by Shell in Houston, Texas. This method became available once the high-temperature chlorination of propene to allyl chloride [107-05-1] could be controlled properly (® Allyl Compounds A 1, p. 427). The allyl chloride produced is oxidized with hypochlorite to dichlorohydrin, which is converted without isolation to epichlorohydrin [106-89-8] by ring closure with calcium or sodium hydroxide (® Epoxides, A 9, p. 539). Hydrolysis to glycerol is carried out with sodium hydroxide or sodium carbonate [18]-[20].

Chlorination of propene to allyl chloride:

H₂C=CH-CH₃ +Cl₂ ---> H₂C=CH-CH₂Cl + HCl

Hypochlorination:

Dehydrochlorination:

Hydrolysis of epichlorohydrin to glycerol:

Epichlorohydrin is hydrolyzed at 80-200°C with a 10 - 15 % aqueous solution of sodium hydroxide or sodium carbonate at atmospheric or overpressure. The residence time in one or a series of several closed, continuously operating reactors amounts to several minutes or several hours depending on the plant concerned. The yield of dilute (10-25%) glycerol solution is > 98 %. The solution contains 5 - 10 % sodium chloride and less than 2 % of other impurities (Fig. 10). This aqueous glycerol solution containing sodium chloride is evaporated in a multistage evaporation plant under vacuum to a glycerol concentration of > 75 %; precipitated sodium chloride is separated at the same time (Fig. 11). The glycerol solution is then distilled under high vacuum (ca. 0.5 - 1.0 kPa) (Section 3.1.3., Fig. 6 and 7); codistilled water is separated by fractional condensation. Residual inorganic salts and higher oligomers of glycerol remaining after the evaporation must be worked up further or discarded. The glycerol, practically free of water, is treated further to remove colored impurities and odorous material; this can be performed, for example, with activated carbon.

Figure 11. Three -stage evaporation plant for aqueous glycerol solutions: Buss glycerol evaporation process [12] a) Evaporator vessel; b) Reboiler; c) Cyclone; d) Vacuum pump; e) Centrifuge; f) Stirred tank; g) Storage tank for glycerol solution