## 3 Non-catalytic Biodiesel Synthesis by Supercritical Fluid Technology  
### General Considerations  
Product separation and isolation along with catalyst recovery are by far the most energy intensive steps in biodiesel production and, therefore, the most economically unfavorable. Supercritical fluid technology may provide some advantages, facilitating these separation and isolation processes. The use of a supercritical fluid phase as solvent/reagent for biodiesel synthesis seems, accordingly, a straightforward approach. Taking into account that the nature of the solvent is controlled by the pressure and temperature, the formation of a single phase between the reagents and the triglycerides can be induced. Although high pressures (>5 MPa) and temperatures (>200 \({}^{\circ}\)C) are needed to reach a homogeneous phase, the transesterification reaction kinetics is favoured under these harsh conditions. Indeed, the reaction can proceed in the absence of any catalyst, leading to non-catalytic biodiesel synthesis. Furthermore, the downstream isolation of the biodiesel from glycerol is simpler since many discrete operations such as catalyst neutralization and separation are not required, leading to products of higher purity, which is also important from a economic point of view. Thus, SCFs technology, even though it requires the use of high temperatures and pressures, may provide distinct advantages in comparison over conventional technologies. For instance, in the conventional alkali-catalyzed method, the final glycerol phase contains methanol, water, alkaline catalyst and soap. Thus, the selling price value of this crude glycerol is extremely low compared with the purified glycerol commonly used in food and pharmaceutical products [34]. The thermal stability of fatty acid methyl esters and biodiesel prepared by non-catalytic SCFs methods and obtained from various plant oils has been studied over a wide range of high temperatures (270-430 \({}^{\circ}\)C) and pressures (17-56 MPa) in order to evaluate the effect of the harsh experimental conditions on the biodiesel fuel quality [35,36]. It was found that although all fatty acid methyl esters, including the poly-unsaturated ones, are stable at low temperatures and pressures, but they partially decomposed with isomerization from _cis_-olefin to _trans_-olefin at high pressure and temperature. From these results, it was suggested that, for high-quality biodiesel production, the reaction temperatures in SCFs processes should be maintained below 300 \({}^{\circ}\)C, preferably at 270 \({}^{\circ}\)C for supercritical methanol (scMeOH), and lower than 360 \({}^{\circ}\)C for supercritical methyl acetate (scMeOAc) with a supercritical pressure higher than 8.09 MPa (see Figure 3). Besides, higher temperatures increase energy consumption and thus operating costs. However, it has been suggested that by selecting the adequate experimental conditions, processing could use less energy than the conventional method. Justification for this claim was based on the absence of the need for catalyst addition and energy intensive purification operations (distillation), making this technology economically competitive with the conventional ones [37]. As regards reactor design, although batch biodiesel production is favoured over continuous processes in many research laboratories and in large scale operations, continuous operation is expected to gain wider acceptance, considering their added advantages that include a higher production capacity and lower operating costs to ensure the long term supply of biodiesel. Additionally, key parameters such as oil concentration in the supercritical phase, and the density of the mixture (depending on temperature, pressure and composition) can be easily controlled and optimized under flow conditions [38; 39; 40; 41]. Alcohols are the most obvious choice for the development of non-catalytic biodiesel synthesis by transesterification reactions in SCFs [42].