Thermal Degradation of Long Chain Fatty Acids

The thermal hydrolysis of saturated (C16:0 and C18:0) and unsaturated fatty acids (C16:1, C18:1, and C18:2) was investigated at 90 °C to 160 °C for 30-min and 8-hr durations. Hydrolysis efficiencies were calculated based on mass yield (i.e., mg/g parent compound), which accounted for all C2-C24 by-products.Very little degradation (less than 1%) of LCFAs was observed from 30-min thermal hydrolysis. At 140 and 160 °C for 8 hours, only 1% of C16:0 and C18:0 were degraded to volatile fatty acids (VFAs) and medium chain fatty acids (C8:0-C12:0). Saturated fatty acids degraded uniformly to C2 to C14. The results indicated that thermal treatment does not readily break alkane carbon bonds. Saturated fatty acids tended to convert to alkanes (1.5-2.0% of total fatty acids) instead of fatty acids at the longer thermal treatment (8 hours).At an 8-hr thermal hydrolysis duration, the temperature range from 90 to 160 °C did not significantly affect unsaturated LCFA degradation. Approximately 1% or less of the unsaturated LCFAs were degraded to C2 to C14 by-products; higher amounts of C6, C7, and C8 than other intermediates were produced. The unsaturated by-products seen were due to β-scission at allylic or vinylic positions. Thermal hydrolysis of LCFAs with digested sludge was also investigated. The amount of VFAs and LCFAs in primary and secondary sludge at 140 and 160 °C was approximately 30-60% higher than at 90-120 °C. The increase in fatty acids was thought to be from lipid degradation in the sludge mixture.Thermal hydrolysis of fatty acids with different catalysts (high acidity, high alkalinity, metals, and hydrogen peroxide) was also investigated. While saturated LCFAs were stable under all catalytic conditions, unsaturated LCFAs were nearly completely degraded to carbon dioxide (CO2) via an oxidation process when hydrolyzed with hydrogen peroxide and activated carbon or copper sulfate.