Production of Biodiesel from Waste Fat and Grease

Processing of waste triglycerides by conventional trans-esterification is problematic due to high free fatty acid (FFA) content. Free fatty acid turns to soap that renders the trans-esterification process inoperable. Clean triglycerides such as soy, corn, and canola oils are low in FFA and easy to process but too expensive as fuel feedstocks. We have been exploring a system that combines thermal cracking of waste triglycerides with acid esterification. Middle distillates (bp 165 degrees C-345 degrees C) obtained by continuous thermal cracking of waste triglycerides were batch-esterified using methanol and Amberlyst 36 (wet). The yields of middle distillate from thermal cracking ranged from 58-63wt % for trap grease and 60-68 wt % for rendered animal fat. More than half of the middle distillates was fatty acids and the rest were conventional hydrocarbons. The esterification of the middle distillate with methanol was done at 90 degrees C for 20h. Although methanol boils at 67 degrees C, there was sufficient methanol left in the liquid phase for esterification at the elevated temperatures. This produced bio-diesel is a mixture of hydrocarbons and methyl-esters.