Synthesis of biodiesel was made by acid esterification followed by basic transesterification. Esterification conditions varied among experiments whereas the transesterification conditions were kept constant. Regarding the esterification reaction, the following reference conditions were selected according to literature : 55 °C, 2.0 wt% catalyst, 5 h and 6:1 molar ratio of methanol to lard. The variables studied were the catalyst amount, the temperature and the reaction time. When one parameter was studied, all the others were fixed as in the reference conditions.
For the esterification, 120.0 g of the dehydrated lard was weighted and added to the reactor which was set at the reaction temperature (45, 55 or 65 °C). After reaching the reaction temperature, a defined amount of methanol (6:1 molar ratio to lard) pre-mixed with H2SO4 (1.0, 2.0, 3.0 or 4.0% of lard weight) was added to the reactor. At this point the reaction started; the reactor consisted of a 1 L flat-bottom flask immersed in a temperature controlling bath, equipped with a water-cooled condenser and a magnetic stirrer. As soon as the reaction started, samples of 3 mL were taken periodically and the acid value was measured to evaluate the reaction progress (five samples were taken during each experiment). Reaction occurred during 3, 5 or 6 h under vigorous stirring. At the end of the reaction, products were decanted for 0.5 h and two phases could be identified; the upper phase consisted of methanol, H2SO4 and impurities and the lower phase mainly consisted of lard and the esterified fatty acids (Canoira et al., 2008). The phases were separated and excess methanol was recovered using a rotary evaporator under reduced pressure. The esterified product was then washed with distilled water to remove residual H2SO4 and after dried at 90 °C during 1 h in a rotary evaporator at reduced pressure. An alternative method proposed by Issariyakul et al. by using silica gel to remove the water and further using an excess of basic catalyst to neutralise the acid was evaluated, but soap production and impairment of the transesterification reaction were observed. The use of dehydration by heating at 90 °C under reduced pressure and after addition of excess basic catalyst was also evaluated, but results were not different from the previous ones. The washed and dried product was then submitted to the transesterification reaction, performed at 65 °C, 1 wt% NaOH and a 6:1 molar ratio of methanol to lard, according to previously published procedure for synthesis and purification ; the decantation was now maintained overnight and the biodiesel dehydration was performed by evaporation at reduced pressure at 90 °C during 2 h.
After selecting the best pre-esterification conditions, the reaction was repeated for determination of the acid value of the esterified product and also for evaluation of the biodiesel yield.
A mixture of the waste lard with soybean oil was additionally studied as possible raw material for biodiesel production. Aiming to obtain a biodiesel with an iodine value according to EN 14214 (<120 g I2/100 g), the percentage of lard to be incorporated was calculated using a model previously developed (Dias et al., 2008b); to obtain a biodiesel iodine value of 115 g I2/100 g, 25 wt% of waste lard was used.
