In this study, electrochemical impedance spectroscopy (EIS) is evaluated as a novel application in the biodiesel purification process. EIS is proposed as a tool to study the behavior of charge carriers produced from side reactions or dissociations of some impurities during the gravitational separation of glycerol dispersed in crude biodiesel after the transesterification process. In the present study, EIS combined with a simple capacitor of two parallel plates made of stainless steel ASTM 304 with an area of 7.0 cm2 and a gap between the plates of 0.8 mm is demonstrated to be a promising technique to monitor the biodiesel/glycerol separation as a function of time. From the analysis of the impedance data, collected during the separation process, it was found that the biodiesel resistance increases up to a maximum value and then decreases to a value which corresponds to the final equilibrium state. Considering that the maximum resistance represents the less contaminated biodiesel as a criterion, the proper average biodiesel/separation time of 4 h 14 min ± 1 h 11 min was determined. Another finding from the EIS results suggests that a growing absorbed film exists on the plate surface of the capacitor used as an impedance sensor.

The goal of the present study was to determine the potential use of thermal analysis for the measurement of the light crude oil (LCO) content of standard biodiesel mixtures. Standard samples of biodiesel/LCO blends were prepared with different ratios of biodiesel; LCO: 1:0 (biodiesel only), 1:3, 1:1, 3:1, 0:1 (LCO only). Thermogravimetry (TG) and differential scanning calorimetry were used to determine the usefulness of thermal analysis for quantification of the LCO in the mixtures. Proton nuclear magnetic resonance (1H NMR) spectroscopy was also used to confirm the composition of the samples. It was found that thermogravimetric curves were not appropriate for the LCO measurements. However, the DSC signal of wax crystallization exhibited a good linear (R2 = 0.99) correlation with the LCO content in the blend. Furthermore, it was shown that aliphatic protons (-CH2-)n in biodiesel and light crude oil can also be used to determine LCO in biodiesel/LCO blends.

Biodiesel, unlike diesel, is highly susceptible to autoxidation. This autoxidation is a major concern for a biodiesel producer. Various methods have been developed to estimate the degree of oxidative stability of biodiesel. These methods are useful to compare and estimate the oxidative stability between different biodiesel samples. However, an absolute value of the oxidative state will be more useful. In this study, we describe a method of estimating a figure that can be used as an indicator of the oxidation level. This indicator was determined from the thermal decomposition curve of biodiesel, as obtained from the thermogravimetric test. Biodiesel from waste cooking oil was oxidized at 80, 100 and 120 ºC using different exposure areas (19.6, 63.6, and 153.9 cm2). The percentage of oxidation, measured from low (80 ºC, 19.6 cm2) to highly aggressive conditions (120 ºC, 153.9 cm2), was in the range of 0.4 ± 0.1 and 23.5 ± 3.3. Kinematic viscosity was also measured for the oxidized samples, and a strong correlation (R2=0.96) was observed between the percentage of oxidation products and kinematic viscosity. In addition, the oxidative effect of temperature and area of exposure on the biodiesel samples was confirmed by ultraviolet spectroscopy.