It was found that the methyl ester contents reached the maximum values when the fresh vegetable oils were used as feedstocks. These results were in accordance with the results found by other researchers. On the other hand, used frying oil provided the lowest methyl ester content for all reaction conditions.
The methyl ester content decreased with the increase in the used frying oil content in the oil feedstock. This similar trend was also obtained for all mixed oil feedstocks. In addition, it was observed that the light gelatinous precipitate(soap) and the emulsion between the biodiesel and washing water layers were occurred during the reaction and washing step, respectively, when the used frying oil and mixed oils were used as feedstocks.
The amount of the gelatinous precipitate increased with further increased in the used frying oil content in oil feedstock. On the other hand, in the case of fresh vegetable oils, the light
gelatinous precipitate and the emulsion formation were not observed during the reaction and washing step, respectively.
These were due to the further increase of the used frying oil content which resulted in the increase of FFA and water contents in the oil feedstocks. The FFA and water caused the soap formation which interfered with the reaction as well as the separation of glycerol from the biodiesel product. It resulted in the low methyl ester content.