Analytical Methods
Content of crude oil in each sample was analyzed by referring to GB/T 14488.1-2008 (Chinese National Standard). After each sample (containing < 10% water) was extracted by using petroleum ether for 4 hours in a Bolton-Williams direct drop extractor, the sample-containing filter cartridge was taken out and the absorbed solvent was removed by an evaporator. The first extracted sample was poured out from the filter cartridge and crushed for 7 minutes. The crushed sample was quantitatively transferred to the filter cartridge and then extracted for 4 hours. This step was repeated once. Finally, the sample was weighed after the complete removal of solvent and crude oil content was calculated.
Acid value (AV) of each sample was analyzed by referring to GB/T 5530-2005 (Chinese National Standard). Each sample was dissolved in neutralized solvent and then titrated by using KOH solution.
Peroxide value (PV) of each sample was analyzed by referring to GB/T 5538-2005 (Chinese National Standard). Each sample in acetic isooctane reacted with potassium iodide. The iodine formed was titrated by using sodium thiosulfate solution. PV was finally calculated.