## 3 Non-catalytic Biodiesel Synthesis by Supercritical Fluid Technology
### Two-step SCFs Process for Non-Catalytic Biodiesel Synthesis  
An alternative non-catalytic process in supercritical fluids for the synthesis of biodiesel fuel consists in a two steps process. In this method, triglycerides are firstly hydrolyzed to FFAs, which, in a second consecutive step, are esterified to the corresponding esters. The first example developed according to this concept is known as the \"Saka-Dadan\" SCF method. In the first step the unique properties of scH\({}_{2}\)O are exploited. It is well known that the dielectric constant of water decreases from \(\varepsilon=80\) at standard T and P to \(\varepsilon=31\) at 225 \({}^{\circ}\)C, P = 10.0 MPa, and finally to \(\varepsilon=6\) at the critical point, \(Tc=374.15\)\({}^{\circ}\)C, \(Pc=22.1\) MPa, due to the steady decrease in the effectiveness of the hydrogen bonds with increasing temperature [73]. Thus, the reduction of the dielectric constant with increasing temperature promotes the miscibility between water and the oil, favouring a hydrolysis process. Furthermore, at a constant pressure of 25 MPa, the ionic product of water increases with temperature until it reaches a maximum (\(10^{-11}\)) at 250 \({}^{\circ}\)C. This allows acid- and base-catalysed reactions to be performed in high temperature pressurized water with no catalyst. Hence, scH\({}_{2}\)O is used as both the solvent and the catalyst for the hydrolysis step at 270 \({}^{\circ}\)C, 7 MPa and a volumetric water/triglyceride ratio of 1:1 [74]. These conditions are milder than those reported for the original one step in scMeOH (350 \({}^{\circ}\)C and 20-50 MPa), thus reducing the energy consumption. After hydrolysis, two layers are formed, the upper portion containing fatty acids and the lower portion water with glycerol. In a second step, FAMEs are produced in supercritical methanol after 20 min at 270 \({}^{\circ}\)C and 7 MPa (see Figure 5). Figure 4: Biodiesel fuel synthesis using supercritical methyl acetate (scMeOAc). Reaction equations. Alternatively, a two-step supercritical dimethyl carbonate method, named the \"_Saka and Ilham Process_\" has been proposed. This consists of the hydrolysis of triglycerides to fatty acids in subcritical water (270 \({}^{\circ}\)C and 27 MPa) followed in a second step by the esterification of fatty acids to FAME in supercritical dimethyl carbonate (300 \({}^{\circ}\)C and 9 MPa) [65]. Various parameters affecting the yield of fatty acids and FAMEs have been studied and a production scheme has been proposed. The supercritical dimethyl carbonate method is applicable to oils with a high free fatty acid contents such as _Jatropha curcas_ oil, with 13.6 wt% free fatty acids. The glycerol produced in the first step was valorized in the second step, obtaining glycerol carbonate in supercritical dimethyl carbonate (280-300 \({}^{\circ}\)C and 9-12 MPa) without any catalyst [75]. The glycerol showed higher rate of conversion to glycerol carbonate due to its high purity. This glycerol conversion process was coupled with hydrolyzed fatty acids conversion into FAMEs to establish the two-step supercritical dimethyl carbonate method (Saka and Ilham Process). The non-catalytic two-step supercritical dimethyl carbonate method is a good process to produce high yields of FAMEs, even from oils with high levels of unsaturated fatty acids, together with a value added by-product, glycerol carbonate, under non-acidic and mild reaction conditions [68]. The same group has also reported a \"two-step\" process, in which first step is a transesterification of the oil to triacetin carried out in either subcritical or supercritical acetic acid, followed by supercritical methanol treatment. A transesterification reaction occurs between acetic acid and the triglycerides under subcritical conditions without the addition of catalysts. It was clear that fatty acids and triacetin could be obtained at high yields (see Figure 6). The reaction rate in acetic acid was much higher than when either methyl acetate or methanol was used in the transesterification step.