4.2.2. The in-situ method
This involves contacting the raw-milled seed directly with the alcohol in the presence of a catalyst (acid/alkali) instead of the pre-extracted oil. The advantages of the process include reduced reaction time, higher yield, smaller waste water formation and lower consumption of reagents [12], [30] and [42]. Abdulkadir et al. [30] and Widayat et al. [56]employed in-situ method and obtained optimum biodiesel yields of 75 wt.% and 91.05 wt.% respectively during RSO transesterification.
4.2.3. The enzymatic method
This method uses enzymes such as lipases as biocatalysts. Though characterized by low reaction rates, when the enzymes are immobilized, they can be re-used several times [12]. The process is FFA friendly and the milder conditions allowed biodiesel yield of at least 90% [60] and can be used to transesterify RSO to biodiesel.
4.2.4. The supercritical method
This process is tolerant to high FFA and moisture content. It is therefore recommended by researchers for variety of feeds such as RSO and waste vegetable oils [4]. Under the supercritical conditions, the alcohol becomes “super acid” thereby eliminating the use of catalyst. A single phase is created, low reaction time (2–4 min) and mass transfer limitations are reduced. The high temperature (300–400 °C), high pressure (80–100 atm) and high molar alcohol/oil ratio (42:1) are some of its inherent disadvantages [5].