In this paper, we have attempted to make a comparative assessment of the three techniques for extraction of lipids from microalgal biomass, viz. Soxhlet extraction, the Bligh and Dyer method, and sonication. The approach is mechanistic in the sense that we have tried to determine the physical mechanism of extraction of lipids (cell disruption or diffusion across a cell wall) from microalgae using microscopic analysis of extracted biomass. We have also assessed the relative influence of the solvent (or extractant) selectivity and the intensity of convection in the medium on the overall lipid yield. None of the techniques used produced complete disruption of the cells, not even sonication. Thus, the prominent mechanism of lipid extraction was diffusion across a cell wall. Moreover, the selectivity of the solvent was found to be the most dominating factor in overall lipid extraction by diffusion than the intensity of bulk convection in the medium.

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SUMMARYAmong several liquid alternative fuels, biobutanol has shown great promise because of its very similar properties to gasoline. This review provides an overview of research activities in acetone-butanol-ethanol (ABE) fermentation over the past two and a half decades. We have addressed seven important facets of ABE fermentation, viz. biochemistry, microbial cultures, alternative substrates, solvent recovery, fermentation mode and reactor designs, mathematical modeling, and economics. Development of mutant strains having higher yield, selectivity and tolerance to inhibition, and search for cheap alternative substrates for fermentation are most important thrust areas in biobutanol production. New and efficient processes have been developed for in situ removal and recovery of the ABE solvents. Several rigorous kinetic and physiological models for fermentation have been formulated, which form useful tool for optimization of the process. These research activities have been reviewed in this paper. Finally, we have summarized studies on the economic viability of large-scale ABE fermentation processes employing various process designs, substrates, and microbial cultures. With the use of new strains, inexpensive substrates, and superior reactor designs, economic potential of ABE fermentation has been found to be highly attractive. Research efforts in science, engineering, and economics of ABE fermentation have brought biobutanol close to commercialization as liquid alternate fuel.

The chemical effects induced by acoustic cavitation (popularly known as sonochemical effects) in aqueous medium are well-known and are attributed to the production of various radicals during bubble collapse. Under the influence of pressure variation due to acoustic wave, the bubble expands with the evaporation of water at the gas-liquid interface. This water vapor condenses at the gas-liquid interface during compression. At the final moments of bubble collapse, the dynamics of bubble motion is far more rapid than the diffusion dynamics of water vapor. Therefore, not all the water vapor that has entered the bubble during expansion escapes during compression. The entrapped water molecules are subjected to extremely high temperature and pressure reached during bubble collapse and undergo cleavage to produce various radicals. These radicals are then mixed with the bulk, where they induce various chemical reactions. Similar chemical effects have also been demonstrated by hydrodynamic cavitation, produced because of bubble oscillation and collapse driven by pressure variation in liquid flow. In this work, we try to give a numerical explanation to the sonochemical effects induced by hydrodynamic cavitation. Using a simplified ordinary differential equation (ODE) model for the dynamics of argon bubbles (released because of pressure reduction in the flow) with associated heat and mass transfer, we show that the phenomena of water vapor entrapment and cleavage due to extremes of temperature and pressure at bubble collapse also occur in hydrodynamic cavitation. We also try to investigate the effect of several operating parameters on the extent of water vapor entrapment, the extreme conditions of pressure and temperature generated in the bubble during collapse, and the production of radicals.