Sivamohan N. Reddy

Ignition of hydrothermal flames

Supercritical water oxidation is one of the most promising technologies for complete oxidation of complex organic compounds. Flames in supercritical water, often referred to as hydrothermal flames, improve the oxidation rates of reactants in an organic waste stream. The ignition and control of flames in supercritical water could potentially be used to reduce the reaction time (from seconds to milliseconds) and enhance the thermochemical decomposition rates of recalcitrant molecules without the release of any harmful intermediates. This provides a platform to design compact reactors for processing complex organic waste followed by their conversion to valuable compounds. This paper reviews some notable work focused on the ignition and qualitative observations of hydrothermal flames as an environmentally friendly technology. More specifically, the review highlights the classification and characterization of hydrothermal flames with several demonstrations of laboratory scale (e.g., visual flame cell) and pilot scale (e.g., transpiring wall reactor) reactor configurations. The process parameters such as feed concentration, reaction temperature, oxidant temperature, oxidant flow rate, and transpiration flow properties (in the case of transpiring reactors) are comprehensively discussed for their influence on the ignition and stability of hydrothermal flames, and total organic carbon removal. In addition, the impact of these parameters on the performance of various flame reactors is presented. Finally, the paper also outlines some wide-ranging applications and challenges concerning the industrial utilization of hydrothermal flames.

Applications of Supercritical Fluids for Biodiesel Production

The direction to opt for renewable fuels has become essential due to the overconsumption of fossil fuels and their emissions challenging the safe and clean environment. Biodiesel, preliminarily the fatty acid alkyl esters, is resultant of transesterification of oils and fats with alcohols. Owing to the problems associated with the conventional biodiesel production in the presence of basic catalysts, non-catalytic supercritical processes eliminates mass transfer resistances, enhancing reaction rates approaching near complete conversion. Supercritical transesterification processes require temperature greater than critical temperature and pressure to attain the desirable biodiesel yields. High alcohol-to-oil ratio with temperatures higher than 300 °C and residence times within minutes are used to maximize the biodiesel yields with methanol/ethanol for different oil feedstocks. Green technologies by implementing enzymatic catalysts such as lipases in supercritical carbon dioxide (SCCO2) have received attention due to enhanced interactions between the reactant molecules. The key findings of enzymatic transesterification in SCCO2 and their combination with ionic liquids are presented in this chapter along with the operating parameters at maximum biodiesel yields. To improve the biodiesel economy, products superior to glycerol can be synthesized by replacing conventional alcohols with other solvents. Solvents such as methyl acetate (MeOAc) and dimethyl carbonate (DMC) are applied to get the triacetin and glycerol carbonate, which have more economic value. The challenges involved in enzymatic and non-catalytic supercritical processes both in terms of operation and economics are discussed.