The acid catalyst promotes esterification by reacting the carboxylic group of the FFA with methanol to form methylesters and water. In this reaction mixture, the sulfuric acid is more miscible in the water product phase than the methanol phase leading to a pronounced water inhibition effect that decreased biodiesel yield due to reverse hydrolysis. In order to compensate for this effect and increase the biodiesel yield, greater sulfuric acid (10~25% of fat weight) concentrations, higher excess methanol (Methanol/oil =30), and much longer reaction times (24 hrs) were typically required(Dias, Alvim-Ferraz et al. 2009). In addition, the high levels of sulfuric acid were very corrosive, difficult to recover, and required neutralization at the end of the process. These disadvantages also increased operational and capital costs.
A new class of zirconia supported metal oxide catalysts has demonstrated the ability to convert large molecules of free fatty acids such as brown grease. Zirconia supported catalysts can perform simultaneous esterification and transesterification reactions to achieve FAME yields as high as 98% in a single reaction step. This suggests that a wide range of bio-oil feedstocks containing a lot of FFAcan be converted into FAME with this catalyst technology. The heterogeneous nature of the ZrO2 formulations also simplifies the production process while avoiding the problems inherent in homogeneous processes related to separation and washing, corrosive catalyst, and excessive methanol usage.
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