Alkaline synthesis of fatty acids iso-propyl esters

Abstract

Fatty acid alkyl esters are widely used products. Most of them are used as renewable transport fuel named “biodiesel”. Production of fatty acid iso-propyl esters mainly based on acid process, but using of alkaline catalysts may give good yields also. Alkaline catalysts have some advantageous such as low corrosivity and higher reaction rate. In current work the effectivity of potassium hydroxide and treated potassium hydroxide solution as catalyst for transesterification was compared. It was shown that using of KOH solution in iso-propyl alcohol after special treatment gives almost twice higher yields (95-96 %) from refined sunflower oil triglycerides than over KOH under the same conditions. Yield of fatty acids iso-propyl esters from wasted frying oil stabilized after 1-1.5 hours of reaction over both catalysts. Using 1.8 and 2.0 % treated catalyst at 90 °C leads to yield of about 86-88 % at 9:1 alcohol-to-oil ratio. Reaction temperature has significant impact on a yield wich decreases with temperature reduce in the range from 30 to 90 °C. During reaction proceeding the alkali saponification and thus loss the catalytic activity, which displayed in stopping the yield rising. The lower yield of esters from wasted oil comparing to the refined oil may be caused by presence of heavy polymerized triglycerides components formed during frying. Such components cannot be fully converted into monoalkylesters and gives also the oligomerized esters, which is not visible in standard gas chromatographic analysis of biodiesel. Indirect confirmation of the presence of such compounds in wasted frying oil sample is the sufficiently larger mass of the cube residue in vacuum distillation. For refined oil amount of such residue was only 5.4 %, while for wasted oil it was three time higher (14.9 %). In case of wasted frying oil as raw stuff, even after full conversion and effective self-separation conventional purification methods (like water washing or dry washing with adsorbents) may not provide the necessary purity of resulted biodiesel due to the presence of heavy oligomeric admixtures. In such cases vacuum distillation should be included as necessary final purification stage.