Acid value (AV) of raw oil was measured using the hot ethanol method as per Animal and Vegetable Fats and Oils-Determination of Acid Value and Acidity (GB/T5530-2005). Saponification value (SV) of raw oil was measured as per Animal and Vegetable Fats and Oils-Determination of Saponification Value (GB/T5534-2008). The average molar mass of raw oil was computed as follows:

where, SV is saponification value [mg KOH/g oil]; AV is acid value [mg KOH/g oil].

KOH/CaO/C supported catalyst was prepared via incipient wetness impregnation: the ground CaO powder and activated carbon at a preset ratio were fully mixed and then impregnated with KOH solution at room temperature, dried and calcinated at last.

With methanol and soybean oil as raw materials, biodiesel was prepared in a 3-neck flask via transesterification, the hot crude products were filtered immediately, and then the catalyst was removed. The liquid phase was transferred to a separating funnel, and put still until it was separated. The upper layer in the funnel was collected and the lower layer of glycerol phase was discharged. 1/2 volume of water at 50˚C was added into the biodiesel, and then it was shaken, and put still for separation. Then the lower layer of glycerol-water phase was discharged. The remaining solution was washed 3 times. The biodiesel phase was reserved and added with anhydrous CaCl₂ for 24 h of drying. The drying agent was filtered out to obtain biodiesel, which was weighed. The yield y of biodiesel was computed as follows:

where, m_B0 is the weight of biodiesel [g]; m₀ is the weight of the raw oil [g].

C/CaO at different mass ratios were mixed, and then impregnated for 20 h in a 25% KOH solution (mass percentage concentration), followed by drying at 105˚C for 3 h and calcination at 500˚C for 5 h. Catalysts were prepared under different conditions and applied to synthesize biodiesel. The synthesis conditions were: methanol-oil ratio was 9:1, catalyst dose was 1% of that of soybean oil, reaction temperature was 60˚C, reaction time was 4 h. The effects of C/CaO mass ratio on biodiesel yield were investigated and the results were listed in Figure 1. Clearly, the biodiesel yield first increases and then declines with the rising C/CaO ratio, and is maximized to be 93.0% at the ratio of 4:6 (Figure 1). This is because the carrier provided less specific surface area and less active components when C/CaO ratio is too small; and the amount of active components decreases at too large C/CaO ratio, which leads to lower catalytic activity.

Activated carbon and CaO (4:6) were mixed and impregnated at room temperature in KOH solutions with varying concentration for 20 h, followed by drying at 105˚C for 3 h and calcination at 500˚C for 5 h. The effects of impregnated amounts of active components on reactivity were investigated and the results were listed in Figure 2 (conditions for synthesis of biodiesel were the same as in section 3.1.1 ). The biodiesel yield first increases and then declines with the rising concentration of KOH, and is maximized at the concentration of 25% (Figure 2). This is because, KOH distributes unevenly on the carrier at too large concentration, leading to the partial dissolution of the carrier, and the amount of active centers decreases and catalytic activity is weakened when KOH concentration is too small.

At C/CaO ratio 4:6, KOH concentration 25%, drying at 105˚C for 3 h, and calcination at 500˚C for 5 h, the impregnation time was changed and its effects on reactivity were investigated. The results were listed in Figure 3 (conditions for synthesis of biodiesel were the same as in section 3.1.1 ). The biodiesel yield increases with the prolonging of impregnation time, but rises slowly after 24 h (yield 94.2%). This is because during the impregnation, the active components diffuse towards the carrier’s internal surface; if the impregnation time is short, KOH is not fully loaded into the activated carbon’s internal pores, leading to too small loading amount and low catalytic activity. The area of the carrier’s internal surface is constant. When the impregnation time is long enough, the active components occupy most of the internal pores and their diffusion towards the carrier's internal surface is slowed down. Thus, the loading amount of active components in the carrier’s internal surface is saturated and the catalytic activity is basically constant.

At C/CaO ratio 4:6, KOH concentration 25%, impregnation at room temperature for 24 h, drying at 105˚C for 3 h, and calcination for 5 h, the calcination temperature was changed and its effects on reactivity were investigated. The results were listed in Figure 4 (conditions for synthesis of biodiesel were the same as in section 3.1.1 ). Clearly, the calcination temperature significantly affects the catalytic activity, as the biodiesel yield first increases and then decreases with the rising calcination temperature, and is maximized at 500˚C. The reason is that the catalyst is structurally at unfixed state and contains a little of adsorbed water at too low calcination temperature, with fewer active centers and thus low activity. On the contrary, when the calcination temperature is too high, the active component would be sintered and the structure of the catalyst collapses, which leads to the lower specific surface area and lower catalytic activity.

At C/CaO ratio 4:6, KOH concentration 25%, impregnation at room temperature for 24 h, drying at 105˚C for 3 h, and calcination temperature 500˚C, the calcination time was changed and its effects on reactivity were investigated. The results were listed in Figure 5 (conditions for synthesis of biodiesel were the same as in section 3.1.1 ). Clearly, the biodiesel yield first increases and then decreases with the prolonged calcination time, and is maximized to be 94.2% at 5 h. The reason is that too short calcination time leads to an incomplete active phase, but too long time leads to sintering on the catalyst’s surface, thus catalytic activity is reduced.

At methanol-oil molar ratio 9:1, catalyst dose 1% of that of soybean oil, and reaction time 4 h, the reaction temperature was changed and its effects on biodiesel yield were explored. The results were showed in Figure 6. Clearly, biodiesel yield is small at low temperature, and increases with the rising temperature, maximized at 65˚C, and then declines. The reason is that transesterification is a reversible endothermic process, and the reaction rate is quickened by raising temperature first, but the amount of by-products increase and the products become darker at too high temperature, thus the yield and quality of final products are reduced. Thus, the optimal reaction temperature is 65˚C.

At methanol-oil molar ratio 9:1, reaction temperature 65˚C, and reaction time 4 h, we changed the dosage of catalyst and explored its effects on biodiesel yield. The results were showed in Figure 7. It could be seen that the biodiesel yield first rises and then declines with the increasing dosage of catalyst, and is maximized to be 96.5% at the dosage of 2% (Figure 7). The reason is that with too large dosage, the catalyst hinders the contact between reactants and too many alkaline centers will cause saponification reaction, which decreases the yield at last.

At methanol-oil molar ratio 9:1, catalyst dose 2% of that of soybean oil, and reaction temperature at 65˚C, we changed the reaction time and explored its effects on biodiesel yield. The results were showed in Figure 8. It could be seen that the biodiesel yield first rises and then declines with the prolonged reaction time, and is maximized to be 97.6% at 5 h (Figure 8). The reason is that with enough time, the reaction is complete and biodiesel yield increases, but the amount of by-products increases with too long time and the yield decreases.

At catalyst dose 2% of that of soybean oil, reaction temperature 65˚C, and reaction time 5 h, we changed the methanol/bean oil ratio and explored its effects on the yield. The results were showed in Figure 9. It could be seen that the biodiesel yield first rises, when the methanol/bean oil ratio is between 9:1 and10:1, the yield rises indistinctively, and is maximized to be 98% at the ratio of 10:1. After the methanol/bean oil ratio is greater than 10:1, the yield declines with the increasing alcohol-oil ratio (Figure 9). The reason is that when methanol-oil ra-

tio is too small, little fatty acid methyl ester is generated, indicating low yield. But if the methanol-oil ratio is too large or exceeds the normal level in transesterification, the volume of reaction solution becomes larger, which slightly dilutes the concentrations of reactants, and the amount of catalyst is relatively smaller, leading to less opportunity for contact between catalyst and fatty glyceride. These changes less promote the reaction and enhance the polarity of the solution, which leads to slower reaction and lower biodiesel yield.