Hanif A. Choudhury

Mechanistic insight into sonochemical biodiesel synthesis using heterogeneous base catalyst

The beneficial effect of ultrasound on transesterification reaction is well known. Heterogeneous (or solid) catalysts for biodiesel synthesis have merit that they do not contaminate the byproduct of glycerol. In this paper, we have attempted to identify the mechanistic features of ultrasound-enhanced biodiesel synthesis with the base-catalyst of CaO. A statistical design of experiments (Box-Behnken) was used to identify the influence of temperature, alcohol to oil molar ratio and catalyst loading on transesterification yield. The optimum values of these parameters for the highest yield were identified through Response Surface Method (with a quadratic model) and ANOVA. These values are: temperature=62 °C, molar ratio=10:1 and catalyst loading=6 wt.%. The activation energy was determined as 82.3 kJ/mol, which is higher than that for homogeneous catalyzed system (for both acidic and basic catalyst). The experimental results have been analyzed vis-à-vis simulations of cavitation bubble dynamics. Due to 3-phase heterogeneity of the system, the yield was dominated by intrinsic kinetics, and the optimum temperature for the highest yield was close to boiling point of methanol. At this temperature, the influence of cavitation bubbles (in terms of both sonochemical and sonophysical effect) is negligible, and ultrasonic micro-streaming provided necessary convection in the system. The influence of all parameters on the reaction system was found to be strongly inter-dependent.

Ultrasonic biodiesel synthesis from crude Jatropha curcas oil with heterogeneous base catalyst: Mechanistic insight and statistical optimization

This paper reports studies in ultrasound-assisted heterogeneous solid catalyzed (CaO) synthesis of biodiesel from crude Jatropha curcas oil. The synthesis has been carried out in two stages, viz. esterification and trans-esterification. The esterification process is not influenced by ultrasound. The transesterification process, however, shows marked enhancement with ultrasound. A statistical experimental design has been used to optimize the process conditions for the synthesis. XRD analysis confirms formation of Ca(OMe)2, which is the active catalyst for transesterification reaction. The optimum values of parameters for the highest yield of transesterification have been determined as follows: alcohol to oil molar ratio ≈ 11, catalyst concentration ≈ 5.5 wt.%, and temperature ≈ 64°C. The activation energy of the reaction is calculated as 133.5 kJ/mol. The heterogeneity of the system increases mass transfer constraints resulting in approx. 4 × increase in activation energy as compared to homogeneous alkali catalyzed system. It is also revealed that intense micro-convection induced by ultrasound enhances the mass transfer characteristics of the system with ∼ 20% reduction in activation energy, as compared to mechanically agitated systems. Influence of catalyst concentration and alcohol to oil molar ratio on the transesterification yield is inter-linked through formation of methoxy ions and their diffusion to the oil-alcohol interface, which in turn is determined by the volume fractions of the two phases in the reaction mixture. As a result, the highest transesterification yield is obtained at the moderate values of catalyst concentration and alcohol to oil molar ratio.

Mechanistic investigation of the sonochemical synthesis of zinc ferrite

In this investigation, an attempt has been made to establish the physical mechanism of sonochemical synthesis of zinc ferrite with concurrent analysis of experimental results and simulations of cavitation bubble dynamics. Experiments have been conducted with mechanical stirring as well as under ultrasound irradiation with variation of pH and the static pressure of the reaction medium. Results of this study reveal that physical effects produced by transient cavitation bubbles play a crucial role in the chemical synthesis. Generation of high amplitude shock waves by transient cavitation bubbles manifest their effect through in situ micro-calcination of metal oxide particles (which are generated through thermal hydrolysis of metal acetates) due to energetic collisions between them. Micro-calcination of oxide particles can also occur in the thin liquid shell surrounding bubble interface, which gets heated up during transient collapse of bubbles. The sonochemical effect of production of OH radicals and H(2)O(2), in itself, is not able to yield ferrite. Moreover, as the in situ micro-calcination involves very small number of particles or even individual particles (as in intra-particle collisions), the agglomeration between resulting ferrite particles is negligible (as compared to external calcination in convention route), leading to ferrite particles of smaller size (6 nm).

Sonochemical Synthesis of Cobalt Ferrite Nanoparticles

Cobalt ferrite being a hard magnetic material with high coercivity and moderate magnetization has found wide-spread applications. In this paper, we have reported the sonochemical synthesis of cobalt ferrite nanoparticles using metal acetate precursors. The ferrite synthesis occurs in three steps (hydrolysis of acetates, oxidation of hydroxides, and in situ microcalcination of metal oxides) that are facilitated by physical and chemical effects of cavitation bubbles. The physical and magnetic properties of the ferrite nano-particles thus synthesized have been found to be comparable with those reported in the literature using other synthesis techniques.

Physical and Chemical Mechanisms of Ultrasound in Biofuel Synthesis

Physical and chemical mechanisms ultrasound-assisted processes as related to the synthesis of biofuels are reviewed. Ultrasound and its secondary effect of cavitation have physical and chemical effects on a reaction system, which can contribute to enhancement of the kinetics and yield. In this chapter, a mechanistic insight into the ultrasound assisted biofuels synthesis is given by coupling simulations of cavitation bubble dynamics with experimental data. The physical effect of ultrasound and cavitation is through intense micro-convection in the system that gives marked improvements in the mass transfer of the system. The chemical effect is through generation of highly reactive radicals through transient cavitation that induce or accelerate chemical reactions. Chemical effects include thermal decomposition of the solvent vapor molecules in the cavitation bubble resulting in generation of smaller molecular species that also affect chemistry of the process. Raising the static pressure of the reaction system above ultrasound pressure amplitude in the system helps to discriminate between physical and chemical effects of ultrasound and cavitation. Biofuels systems considered in this chapter are the pretreatment of biomass, biodiesel synthesis with acid/base and homogeneous/heterogeneous catalysts, extraction of microalgal lipids, bioconversion of crude glycerol from biodiesel industry to value added products and desulfurization of the fuel. Among the physical effects of ultrasound and cavitation, micro-streaming by ultrasound has a greater influence on reactions than shock waves generated by cavitation bubbles. In some cases, chemical effects of transient cavitation are revealed to have adverse influence on a reaction. Many biofuels systems are limited by their intrinsic characteristics that restrict the effect of ultrasound and cavitation on the reaction system.

Bioenergy from rice crop residues: role in developing economies

Energy demands of industry, agriculture, transport and domestic sectors of a developing nation are primarily in terms of electricity and transportation fuel. Rice is a major crop in many developing countries. The residues of this crop, viz. rice husk, and rice straw have high potential for bioenergy generation. This review article tries to explore potential of this bio-resource and emphasizes its effective utilization for energy production through techno-economic analysis. The structure, properties, and treatment of rice crop residues have been described. A literature review in production of various biofuels through thermo-chemical and biochemical conversion of rice straw and husk has been presented. Finally, brief literature review on economic analysis of production of liquid and gaseous biofuels from rice crop residues through biochemical and thermo-chemical routes has been presented. This analysis reveals that production of different biofuels from rice crop residues is economically viable. This review emphasizes that bioenergy from rice crop residues provides simultaneous solution to issues of energy security and climate change risk in developing nations.

Biomass Gasification Integrated Fischer-Tropsch Synthesis

Abstract With rapid depletion of fossil fuel reserves and environmental concerns of global warming due to greenhouse gas (GHG) emission, energy security, and climate change risks have been the most formidable problems before the world today. A quest is on for a renewable and carbon-neutral liquid transportation fuel that can substitute for fossil fuel-derived gasoline and petrol. Fischer-Tropsch synthesis, which can convert synthesis gas into long-chain and branched hydrocarbons, has been a potential answer to these issues. This technology coupled with biomass gasification (which has been very popular means of decentralized electricity generation) offers an even more attractive solution. In this chapter, we review the scientific, technical, and economic aspects of Biomass Gasification Integrated Fischer-Tropsch (BGIFT) synthesis process. We have given a necessary outline of this technology followed by analysis of a coupled process. The overall conclusion is that the hybrid BGIFT process is economically attractive when used for both fuel and electricity production. With rising international prices of crude oil and the carbon mitigation policy in action (GHG pricing), the BGIFT process will provide a viable and sustainable solution for issues of both energy security and climate change.