Other interesting option for producing biodiesel from feedstocks with high concen­tration of impurities such as water and FFA is the transesterification of triglycerides with supercritical methanol, which is receiving a lot of attention [17, 58, 59]. This process is catalyst-free and it is able to obtain full conversion of the triglycerides in a matter of minutes [58], with the possibility of continuous operation mode [26].

The operation is also simpler, as the transesterification of triglycerides and methyl esterification of fatty acids occurs simultaneously without using any cata­lyst. Because no catalyst is used and has to be recovered, the downstream process­ing is much simpler, and soap-free glycerol can be obtained [89] . Other advantage this process presents is the insensitivity to the presence of impurities in the vegeta­ble oil, such as water and FFA [59]. The presence of moisture is not only negligible, but it can also be advantageous in this process [6] .

The reaction is carried out in supercritical methanol (or ethanol), in which feedstocks react with the alcohol under conditions of high pressure (above 100 atm) and high temperature (more than 276°C). At these conditions, the alcohol is in a supercritical gaseous state and the triglycerides are somewhat dissolved in a single phase. The reasons for this behaviour are not yet fully understood, but are certainly related to the high solubility of triglycerides in supercritical alcohol and solvent effects [65]. Also, some authors have observed a dependence on the type of alcohol and triglyceride used [4].

Notwithstanding its clear advantages over other processes, significant hurdles remain for the full scale implementation of supercritical production units. First of all, high temperatures and pressures are necessary to ensure that the alcohol is in super­critical state, requiring the utilization of special equipment designed to support these conditions. This will lead to high equipment and operational costs, making the pro­cess economics not so attractive when compared to other options. Also, the excess of methanol used in the reaction is much larger when compared to the conventional pro­cess; normally an alcohol/oil molar ratio of 42:1 is used, which needs to be recovered and recycled back to the reactor this way complicating the process design [58].

Alternatively, Cao et al. [23] proposed the supercritical methanol process, using propane as co-solvent, which decreases the reaction temperature and pressure, as well as the alcohol to oil molar ratio. This is because propane decreases the critical point of methanol allowing the supercritical reaction to be carried out under milder conditions than those of 424 atm and 350°C reported by Kusdiana and Saka [58]. In this case, the optimal reaction conditions are a temperature of 280°C, a pressure of 126 atm, an alcohol to oil molar ratio of 24:1, and propane to oil molar ratio of 0.05:1. At these conditions, 98% of oils are converted to biodiesel for a reaction time of 10 min. Kasteren and Nisworo [53] performed an economic analysis of this pro­cess, considering the industrial production of biodiesel from waste frying oil and concluded that it can compete with the existing alkali and acid-catalyzed processes.