Читать книгу Biodiesel Production - Группа авторов - Страница 41

Many researchers opt for this method, in which acid esterification is used for pretreating the oil in order to make it suitable for base‐catalyzed conversion before performing alkali‐catalyzed transesterification, which can completely convert the glycerides into esters, since bases are sensitive to high FFAs (owing to saponification) as well as moisture (owing to hydrolysis) [27]. The process can comprise of either esterification–transesterification steps or hydrolysis and esterification steps (Table 2.2) [34]. Hydrolysis combined with esterification is comparatively more wasteful as generation of FFAs is an energy‐intensive process since high temperatures (exceeding 300 °C) and pressure (exceeding 10 MPa) are required. In the two‐step catalyzed process involving esterification and transesterification, acid catalysts remove almost all of the FFAs through conversion to esters and water, which can be then purified and dried prior to using base catalysts, which convert the glycerides into esters and glycerol [22]. The glycerol and excess alcohol can be removed through washing or by ultracentrifugation before being tested for suitability as fuel. As mentioned in Section 2.4, nonpolar alcohols result in better biodiesel yield compared with polar alcohols, and, thus, they hold great potential for use in biodiesel production [1, 6].