## 2 Supercritical Fluids Technology: Challenges and Limitations
The proposed system assembled four high-pressure separator vessels, where a pressure cascade was produced by back-pressure valves, allowing a continuous recovery of the liquid product at the bottom of each separator, and then the recycling of the non-reacted substrates. Alternatively, membrane reactors constitute an attempt to integrate catalytic conversion, product separation and/or concentration and catalyst recovery into a single operation unit. Thus, enzymatic dynamic membranes, formed by depositing water-soluble polymers (e.g., gelatine, polyethyleneimine, _etc._) on a ceramic porous support, exhibited appropriate properties for continuous butyl butyrate synthesis in scCO\({}_{2}\), along with a high operational stability [27, 28]. The most important environmental factors affecting (bio)catalyst performance in SCFs are pressure, temperature and water content. Indeed, these parameters influence all mass-transfer phenomena and (bio)catalyst activity by changing the rate-limiting steps or modulating their selectivity [29, 30]. As an example, the catalytic activity of immobilized lipase for butyl butyrate synthesis was exponentially increased by the drop in scCO\({}_{2}\) density accompanying different combinations of pressure and temperature [27]. In the same context, for the pressure range of 7.7 to 8.5 MPa, the changes in the conformation of lipase resulted in an increased rate of esterification reactions. Thus, [PERSON] _et al._ obtained good yields (up to 99%) for the enzymatic synthesis of several citronellyl esters at 45 \({}^{\circ}\)C and 8.0 MPa [31]. Temperature influences enzyme activity much more than pressure not only due to the usual increment in reaction rates at higher temperatures, but also because of enzyme thermo deactivation processes. The optimal temperature of enzymatic processes in SCFs is related with pressure as both control solvent properties [29]. Water concentration also greatly influences enzyme activity because proteins require a specific amount of bound water molecules to be active. When the water content is too high or if the water is a product of the reaction in the supercritical medium, the resulting increased humidity may lead to enzyme deactivation. Thus, the actual amount of water needed is specific to each SCF-substrate-enzyme system, and must be maintained constant throughout the process [32]. However, all the unique properties of scCO\({}_{2}\) in its role as green solvent to extract, dissolve and transport chemicals are tarnished by its denaturative effect on enzymes. Several approaches have been developed to protect enzymes against these adverse effects of scCO\({}_{2}\), including covalent attachment on supports coated with hydrophilic polymers, the entrapment of enzymes in silica-aerogels, and the use of cross-linking enzyme aggregates. In spite of the advantages obtained with all these stabilization approaches, the best results for enzyme-catalyzed reactions in scCO\({}_{2}\) were observed when the biocatalyst was applied in suspension or coated with other green solvents, such as ionic liquids, as described below [19]. There are additional challenges and/or weakness that need to be addressed before SCFs technology can play a major role in industrial applications. Those include its cost, energy consumption, and safety issues in the operation process. The SCFs technology requires expensive equipments, such as strong durable reactors, high pressure pumps, efficient control devices, _etc._, and the costs involved in operation and maintenance are also higher than those of conventional processes. Furthermore, the energy consumption to reach supercritical conditions is dependent of the nature of the solvent in question (see Table 1 for critical parameters), and this may be unsustainable for long term industrial application. An example is the biodiesel synthesis by non-catalytic supercritical alcohol technology requiring temperatures up to 200 \({}^{\circ}\)C (the critical temperature of methanol is 239 \({}^{\circ}\)C).