## 3 Non-catalytic Biodiesel Synthesis by Supercritical Fluid Technology
### Single Step SCFs Processes for Non-catalytic Biodiesel Synthesis  
#### 3.2.1 Supercritical Alcohol  
Supercritical methanol (scMeOH), whose critical parameters are \(\mathrm{T_{c}=239.4~{}^{\circ}C}\), \(\mathrm{P_{c}=8.1}\) MPa, and \(\mathrm{\rho_{c}=0.276~{}g\cdot cm^{-3}}\), was one of the first SCFs evaluated for the production of biodiesel [46]. In light of the promising results obtained with supercritical alcohols, many researchers have studied the effects of various reactions parameters on FAMEs yield. As mentioned, using SCFs it is possible the tuning and control of the solvent polarity through the adjustment of temperature (T) and pressure (P). Indeed, the  
Figure 3: Non-catalytic one-step process for biodiesel synthesis using supercritical dimethyl carbonate. Reaction equations. dielectric constant (\(\varepsilon\)) of methanol decreases from 34 to 5 upon isobaric heating from 25 to 260 \({}^{\circ}\)C at 20 MPa [47]. The hydrogen bond network existing in liquid methanol is broken under SC conditions, leading to a fluid in which only small oligomers exist [48,49]. Reduction of the polarity and the hydrogen bonding can facilitate a much stronger direct nucleophilic attack by the methanol on the carbonyl carbon rendering a catalyst unnecessary. Furthermore, reduced dielectric constant may even lead to a single oil/alcohol phase, increasing the reaction rate [50]. These facts explain the differences observed when the reaction is performed either under sub-critical or supercritical conditions since the reaction kinetics is highly dependent on the homogeneity of the reaction mixture. It seems that operating under supercritical conditions enhances the rate of reaction [51]. The solubility of reactants and transesterification products (homogeneous or heterogeneous) in the alcohol under various pressure-temperature-molar fraction (P-T-x) conditions is a keystone for rapid and complete tranesterification reactions [42]. Liquid-vapor-SC phase transitions for the binary/ternary systems (soybean oil and alcohol at different oil:ethanol molar ratio) were studied by [PERSON] _et al_ using a view cell attached to the outlet of the reactor, which was heated from 26 \({}^{\circ}\)C to 400 \({}^{\circ}\)C at constant volume/density [52]. With two liquid phases in ambient conditions, the system showed increasing homogeneity with increasing temperature and pressure until a single SC phase was reached. Under these conditions, the conversion of triglyceride to FAMEs was very rapid compared to that observed under subcritical conditions [53,54]. The reaction in continuous flow conditions yields above 98% conversion, with a single clear phase at moderate pressures (10-20 MPa), high temperatures (375-400 \({}^{\circ}\)C), low methanol ratios (3:1-6:1) and short reaction times (<3 min). These results are in contrast with those reported by [PERSON] and co-workers, who observed that high conversion can be obtained, in most cases, even when operating in the two-phase region [55]. In this case, the oil transesterification could mainly occur in the light supercritical phase, where the oil and the monoglycerides and diglycerides are partially soluble and the concentration of methanol is high. Moreover, the light-phase transport properties favour a higher reaction rate. The increased methanol/oil ratio leads to a homogeneous supercritical phase by increasing the system temperature while working at moderate pressures of 10-15 MPa, since an increase in the methanol/oil ratio decreases the critical temperature of the system [56,57]. Although a high methanol ratio favours the formation of a homogeneous supercritical phase while also driving the reaction to completion, it also increases costs and energy consumption. The addition of a co-solvent such as propane or CO\({}_{2}\) decreases the critical point of the mixture, allowing the reduction of both temperature and the oil/alcohol ratio [58, 59, 60, 61].