The usage of a strong acid instead of a strong base is better suited for high acidity feedstocks, a situation normally found in waste oils and fats, making it possible to avoid the oil pre-treatment operation and providing high conversion rates with no soap formation [19, 49]. Nevertheless, it is seldom used due to its longer reaction times and higher temperatures required, when compared to the alkali-catalyzed pro­cess, and it is more corrosive to the process equipment [1, 19]. For example, Kulkarni and Dalai [56] report 88 and 95% conversion obtained, respectively, for 48 and 96 h reaction time.

Also, a higher methanol to oil molar ratio is needed to promote high equilibrium conversions of triglycerides to esters, which generally increases the production costs, due to an increase in the volume needed for the reactor and the separation of glycerol that becomes more difficult. Kulkarni and Dalai [56] report that 98% con­version is obtained for a methanol:oil molar ratio of 30:1 by comparison with a 87% conversion for a molar ratio of 6:1.

Among the several acid catalysts (e. g. sulphuric, sulfonic, phosphoric, or hydro­chloric acid) that can be used, sulphuric acid is the most common. Zhang et al. [101] evaluated economically both the alkali-catalyzed and the acid-catalyzed process, con­cluding that though the first one, using virgin vegetable oil, has the lowest fixed capital cost, the second one, using waste frying oil, is more economically feasible overall.

Kulkarni and Dalai [56] present the effect of various parameters on the acid — catalyzed transesterification, showing that the FFA and moisture content of oils are the parameters that most affect the reaction conversion. For instance, with less than

0. 5% water the conversion is above 90%, and for 3 or 5% of moisture the conversion is, respectively, 32 and 5%. The FFA effect is not so accentuated allowing one to obtain 90, 80, and 60% conversion for 5, 15, and 33% of FFA content, respectively.

Bhatti et al. [13] studied the effect of various parameters in the production of biodiesel from animal fats, concluding that the optimum conditions for 5 g of chicken and mutton tallow are, respectively, a temperature of 50 and 60°C, 1.25 and

2.5 g of H2SO4, and an oil:methanol molar ratio of 1:30 and 1:30, yielding

99.1 ± 0.71% and 93.21 ± 5.07% of methyl esters, after 24 h, in the presence of acid. Gas chromatographic analysis showed a total of 98.29 and 97.25% fatty acids in chicken and mutton fats, respectively.