Purification of glycerin obtained as a bioproduct from the transesterification of triglycerides in the synthesis of biofuel
Methods for purifying glycerin contaminated with one or more lower boiling alcohols such as methanol, ethanol, straight, branched or cyclic C3-C6 alcohols, and the like. The methods are particularly useful for purifying crude glycerin phases recovered from the synthesis of biofuels. The present invention uses distillation techniques to strip alcohol contaminants from glycerin. In contrast to conventional methods that carry out distillation either under substantially anhydrous or very wet conditions, the present invention carries out distillation in the presence of a limited amount of water, e.g., from about 0.8 to about 5 parts by weight of water per 100 parts by weight of contaminated glycerin to be purified.
1. A method of purifying crude glycerin, comprising the steps of:
a) providing an alkaline admixture comprising glycerin, soap, a fatty acid ester, and at least one other alcohol, said other alcohol having a lower boiling point than glycerin;
b) adding a sufficient amount of water so that the admixture includes from about 0.8 to about 5 parts by weight water per 100 parts by weight of the admixture;
c) after adding the water, distilling the admixture under conditions effective to strip away substantially all of the other alcohol;
d) after stripping the other alcohol, lowering the pH of the admixture with aqueous acid under conditions effective to convert the soap to free fatty acid and to form a first organic phase comprising the free fatty acid and the fatty acid ester and a second aqueous phase comprising the glycerin; and
e) separating the organic and aqueous phases.
2. The method of claim 1 , wherein step (b) comprises adding from 1 to 4.5 parts by weight of water per 100 parts by weight of the admixture.
3. The method of claim 1 , wherein the other alcohol is selected from methanol, ethanol, and linear, branched or cyclic C3 to C6 alcohols.
4. The method of claim 3 , wherein the other alcohol is methanol.
5. The method of claim 1 , wherein the fatty acid ester is a fatty acid methyl ester.
6. The method of claim 1 , wherein the aqueous acid is selected from HCl, H 2 SO 4 , phosphoric, and citric acid.
7. The method of claim 1 , wherein step (b) comprises lowering the pH to a value in the range from about 2 to about 4.
8. The method of claim 1 , further comprising the step of adding an antifoaming agent to the admixture while step (c) is being carried out.
9. The method of claim 1 , wherein the distillation occurs at a pressure of about 760 mmHg.
10. The method of claim 1 , wherein enough water is added so that the distillation occurs at a temperature of less than 200° C. in order to reduce the amount of the other alcohol to less than 1 weight percent.
11. The method of claim 10 , wherein enough water is added so that the distillation occurs at a temperature of less than 190° C. in order to reduce the amount of the other alcohol to less than 1 weight percent.
12. The method of claim 1 , wherein the admixture has an alkaline pH during at least a portion of step (c).
13. The method of claim 12 , wherein the pH is 10-13.
14. The method of claim 13 , wherein the pH is about 12.
15. The method of claim 1 , wherein the admixture of step (a) is substantially anhydrous.