Читать книгу Biodiesel Production - Группа авторов - Страница 39

The wastewater generated from washing of fuels using acid or base catalysts is not pH neutral, and thus can hamper water bodies connected to the discharge stream if the generated effluent is left untreated before drainage. Enzymes can also be beneficial in conversion of high FFA feedstock, since saponification cannot take place as both glycerides and FFAs are converted to esters [29]. They also offer the following advantages:

 A low alcohol‐to‐oil ratio is needed, with deemulsification conferring reusability of enzymes.

 Low product inhibition and reacting temperature, with easy separation in case of heterogeneous immobilized enzymes.

 Single‐step conversion with appreciable yields and are insensitive to moisture exposure.

However, they do require far longer durations to complete conversion, and since enzymes are very temperature sensitive, the reaction must be closely monitored. Another problem associated with them is their high costs as well as limited reusability due to structural denaturation and moderate conversion efficiencies compared to acid‐ or base‐catalyzed systems [30, 31]. Lipase is the most common enzyme used and is obtained from animals, plants, or microbes, and must not be stereospecific for maximum conversion efficiency. Bacterial and fungal lipases (example being Novozym 435 obtained from Candida antarctica or other enzymes extracted from sources such as Penicillium spp., Rhizopus spp., and Aspergillus niger) used can show maximum yield up to 90%, when operated between 30 and 50 °C for anywhere between 8 and 90 h depending on feedstock [2]. The variety of studies reported by researchers are numerous; a select few of which have been summarily presented in Table 2.2. However, Nelson et al. reported that polar alcohols tend to inactivate enzymes much faster than nonpolar alcohols [28].