Papers by V. S. Moholkar, PhD, CEng, MIChemE, FRSC, MAIChE (Sr)
Abstract
An attempt has been made to present a unified theoretical model for the cavitating flow... more Abstract
An attempt has been made to present a unified theoretical model for the cavitating flow in a hydrodynamic cavitation reactor using the nonlinear continuum mixture model for two-phase flow as the basis. This model has been used to describe the radial motion of bubble in the cavitating flow in two geometries in hydrodynamic cavitation reactors, viz., a venturi tube and an orifice plate. Simulations of the bubble dynamics in a venturi flow demonstrate the stable oscillatory radial bubble motion due to a linear pressure gradient. Due to an additional pressure gradient due to turbulent velocity fluctuations the radial bubble motion in case of an orifice flow is a combination of both stable and oscillatory type. The results of numerical simulations have been explained on the basis of analogy between hydrodynamic cavitation and acoustic cavitation.
Keywords
Bubble dynamics; Two-phase flow; Hydrodynamic cavitation; Acoustic cavitation; Process Intensification
This paper addresses the matter of intensification of the sonochemical degradation of phenol from... more This paper addresses the matter of intensification of the sonochemical degradation of phenol from mechanistic point of view by coupling the experimental results to a mathematical model that takes into account essential physics and chemistry of cavitation bubbles. We assess the relative influence of extent of radical production by the cavitation bubbles and radical scavenging (or conservation) on the overall degradation of phenol. We have used a molecular species (oxygen) and an ionic species (Fe 2+ ions) for scavenging of radicals produced by cavitation bubbles. Four different gases, viz. argon, oxygen, nitrogen and air have been used to provide cavitation nuclei in the bulk medium. It is revealed that sonochemical degradation of phenol is governed by the extent of radical scavenging, both inside and outside the bubble. Phenomenon of radical scavenging influences the probability of interaction between the phenol molecules and radicals. The extent of degradation in the presence of Fe 2+ ions has been much higher. This result has been attributed to higher reactivity and uniform concentration of the Fe 2+ ions in the medium, as a result of which they can effectively scavenge the radicals generated by cavitation bubbles.
Energy, 2010
In recent years, biomass gasification has emerged as a viable option for decentralized power gene... more In recent years, biomass gasification has emerged as a viable option for decentralized power generation, especially in developing countries. Another potential use of producer gas from biomass gasification is in terms of feedstock for FischereTropsch (FT) synthesis e a process for manufacture of synthetic gasoline and diesel. This paper reports optimization of biomass gasification process for these two applications. Using the nonestoichometric equilibrium model (SOLGASMIX), we have assessed the outcome of gasification process for different combinations of operating conditions. Four key parameters have been used for optimization, viz. biomass type (saw dust, rice husk, bamboo dust), air or equivalence ratio (AR ¼ 0, 0.2, 0.4, 0.6, 0.8 and 1), temperature of gasification (T ¼ 400, 500, 600, 700, 800, 900 and 1000 C), and gasification medium (air, airesteam 10% mole/mole mixture, airesteam 30% mole/mole mixture). Performance of the gasification process has been assessed with four measures, viz. molar content of H 2 and CO in the producer gas, H 2 /CO molar ratio, LHV of producer gas and overall efficiency of gasifier. The optimum sets of operating conditions for gasifier for FT synthesis are: AR ¼ 0.2e0.4, Temp ¼ 800e1000 C, and gasification medium as air. The optimum sets of operating conditions for decentralized power generation are: AR ¼ 0.3e0.4, Temp ¼ 700e800 C with gasification medium being air. The thermodynamic model and methodology presented in this work also presents a general framework, which could be extended for optimization of biomass gasification for any other application.
Polymer, 2010
This paper attempts to get a physical insight into the sonochemical emulsion copolymerization usi... more This paper attempts to get a physical insight into the sonochemical emulsion copolymerization using butyl acrylate (BA) and methyl methacrylate (MMA) as model monomers at low to moderate ultrasound intensity. The principal physical mechanism underlying beneficial effects of ultrasound on emulsion polymerization system is cavitation, which affects the system in both chemical (i.e. generation of radicals that can initiate/propagate polymerization process) as well as physical (i.e. emulsification of reaction mixture) way. By taking dual approach of coupling experiments with simulations of cavitation bubble dynamics, we have tried to justify the trends in experiments results. The role of cavitation in the present study is found to be only physical. Quite interestingly, the chemical effect of cavitation is found to have no role to play. Reactivity ratios of both monomers for applied experimental conditions have been found to be less than 1, which hints at moderately alternating behavior of copolymerization. Theoretically calculated copolymer composition using the reactivity ratios of copolymers matched well with experimental values. The copolymer composition for all monomer feed ratios is rich in MMA, due to higher reactivity of MMA than BA. The molecular weight of the copolymer reduced with greater fraction of MMA in the reaction mixture. This effect is attributed to nature of termination of the BA (i.e., combination) and MMA (i.e., disproportionation) monomer radicals.
Aiche Journal, 1997
Harnessing the energy associated with cavitation for a number of physical and chemical changes ha... more Harnessing the energy associated with cavitation for a number of physical and chemical changes has been pursued enthusiastically by sonochemists using ultrasound equipment of various sizes, shapes and forms. The effect of cavitation as observed in ultrasonic equipment has been attributed to the transient form of cavitation. Hydrodynamically generated cavities are believed to behave in a stable cavitation mode and, hence, are not very useful for the desired sonochemical effects.
Ultrasonics Sonochemistry, 1999
A comparative study of hydrodynamic and acoustic cavitation has been made on the basis of numeric... more A comparative study of hydrodynamic and acoustic cavitation has been made on the basis of numerical solutions of the Rayleigh-Plesset equation. The bubble/cavity behaviour has been studied under both acoustic and hydrodynamic cavitation conditions. The effect of varying pressure fields on the collapse of the cavity (sinusoidal for acoustic and linear for hydrodynamic) and also on the latter's dynamic behaviour has been studied. The variations of parameters such as initial cavity size, intensity of the acoustic field and irradiation frequency in the case of acoustic cavitation, and initial cavity size, final recovery pressure and time for pressure recovery in the case of hydrodynamic cavitation, have been found to have significant effects on cavity/bubble dynamics. The simulations reveal that the bubble/cavity collapsing behaviour in the case of hydrodynamic cavitation is accompanied by a large number of pressure pulses of relatively smaller magnitude, compared with just one or two pulses under acoustic cavitation. It has been shown that hydrodynamic cavitation offers greater control over operating parameters and the resultant cavitation intensity. Finally, a brief summary of the experimental results on the oxidation of aqueous KI solution with a hydrodynamic cavitation set-up is given which supports the conclusion of this numerical study. The methodology presented allows one to manipulate and optimise of specific process, either physical or chemical.
Chemical Engineering Science, 2007
Hydrodynamic cavitation has been increasingly used as a substitute to conventional acoustic (or u... more Hydrodynamic cavitation has been increasingly used as a substitute to conventional acoustic (or ultrasonic) cavitation for process intensification owing to its easy and efficient operation. In this paper, we have put forth conceptual design of a new kind of hydrodynamic cavitation reactor that uses a converging-diverging nozzle for generating pressure variation required for driving radial motion of cavitation bubbles. Moreover, the reactor uses externally introduced bubbles of a suitable gas (argon or air) for cavitation nucleation. This design differs from earlier designs used by researchers where an orifice plate is used for creating cavitating flow. The new design offers a good control over two crucial parameters that affect the cavitation intensity produced, viz. rate of nucleation and nature of pressure variation driving bubble motion. Using numerical simulations of bubble dynamics and associated heat and mass transfer, trends in cavitation intensity produced in the reactor are assessed with varying design parameters. The results of simulation show that the externally introduced bubbles undergo transient motion in the flow through the nozzle generating moderate cavitation intensity. On the basis of results of simulation, some recommendations have been made for the effective design and scale up of the new kind of hydrodynamic cavitation reactors using concept introduced in this paper. ᭧
Chemical Engineering Science, 2001
An attempt has been made to present a uniÿed theoretical model for the cavitating ow in a hydrody... more An attempt has been made to present a uniÿed theoretical model for the cavitating ow in a hydrodynamic cavitation reactor using the nonlinear continuum mixture model for two-phase ow as the basis. This model has been used to describe the radial motion of bubble in the cavitating ow in two geometries in hydrodynamic cavitation reactors, viz., a venturi tube and an oriÿce plate. Simulations of the bubble dynamics in a venturi ow demonstrate the stable oscillatory radial bubble motion due to a linear pressure gradient. Due to an additional pressure gradient due to turbulent velocity uctuations the radial bubble motion in case of an oriÿce ow is a combination of both stable and oscillatory type. The results of numerical simulations have been explained on the basis of analogy between hydrodynamic cavitation and acoustic cavitation. ?
Aiche Journal, 2000
A new method for separate identification and determination of the spatial distribution of the two... more A new method for separate identification and determination of the spatial distribution of the two components of the energy intensity in an ultrasound bath (due to the ultrasound waves and cavitation activity) uses two media—cavitating (water) and noncavitating (silicon oil)—under the conditions of the acoustic field in the ultrasound bath. The variation of cavitation intensity in the frequency domain was obtained by subtracting the acoustic emission spectrum of silicon oil from that of water. Measurements at various locations in the bath revealed significant spatial variations in the cavitation intensity in the bath. The local cavitation phenomena in the bath (stable or transient cavitation) were explained based on the spectral characteristics of acoustic emission. The radial dynamics of the bubbles at the location of cavitation intensity measurements was determined using the Gilmore model of bubble dynamics. The bubbles in the region of highest cavitation intensity underwent a transient motion, while the bubbles in the region of lowest cavitation intensity underwent stable/oscillatory motion. The transient collapse of the bubbles that gives rise to local temperature and pressure maxima is at the root of the observed effects of ultrasound on chemical systems. The more violent the collapse of the bubbles, the higher the local cavitation intensity. It was verified using the spectral characteristics of the acoustic emission and simulation of the radial motion of the bubbles.
Fuel
... 29] has studied effect of dilute acid treatment of ... Ethanol production from wheat straw us... more ... 29] has studied effect of dilute acid treatment of ... Ethanol production from wheat straw using acid pretreatment and enzymatic saccharification using recombinant strain of E. coli ... Detoxification of hydrolyzate obtained from acid pretreated biomass increased yield with simultaneous ...
Industrial & Engineering Chemistry Research, 2010
In this paper, we have attempted to make a comparative assessment of the three techniques for ext... more 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 ...
Renewable & Sustainable Energy Reviews, 2010
Industrial & Engineering Chemistry Research, 2009
Acceleration of the transesterification reaction for synthesis of biodiesel by application of ult... more Acceleration of the transesterification reaction for synthesis of biodiesel by application of ultrasound is known. This paper tries to establish the mechanism of this enhancement by discriminating between physical and chemical effects of ultrasound. Experiments with different conditions have been coupled to a bubble dynamics model. It is revealed that influence of ultrasound on transesterification reaction is of purely physical nature. Formation of fine emulsion between oil and alcohol due to microturbulence generated by cavitation bubbles generates enormous interfacial area, which accelerates the reaction. For the power input used in the present experiments, the temperature peak reached in transient collapse of cavitation bubble in methanol is found to be too low to produce any radical species, which can induce transesterification reaction. The yield of the reaction is found to have an optimum with respect to alcohol to oil molar ratio. This result is attributed to the difference in intensity of microturbulence produced by cavitation bubbles in oil and methanol.
Aiche Journal, 2007
In this study, a mechanistic approach has been taken to enhance yield of a sonochemical reaction.... more In this study, a mechanistic approach has been taken to enhance yield of a sonochemical reaction. Formation of highly reactive free radicals due to the transient collapse of cavitation bubbles is the primary mechanism of a sonochemical reaction. A physical (reduction in dissolved gas concentration) and a chemical (increasing the reactant concentration) technique is used for enhancing yield of a sonochemical reaction using those techniques, which influence the phenomenon of radical formation by the cavitation bubbles. A bubble dynamics model is used for explaining the sonochemical phenomena. In a degassed medium, the ultrasound wave undergoes lesser attenuation; moreover, equilibrium size of a bubble shrinks due to rectified diffusion. Because of this, a bubble undergoes more violent collapse, resulting in greater production of radicals that give higher yield. On the other hand, increasing the initial reactant concentration shows an adverse effect on the sonochemical yield. This is ascribed to reduction in water vapor flux in the bubble due to reduction of vapor pressure of the medium. This study, therefore, demonstrates as how macroscopic manifestation (the sonochemical yield) of the microscopic phenomena (transient collapse of cavitation bubble) is a complicated function of several physical processes. The results of this study shed light on the complex and multifaceted physical mechanism of a sonochemical reaction, which may be useful in maximization of yield of other sonochemical systems. © 2007 American Institute of Chemical Engineers AIChE J, 2007
Ultrasonics Sonochemistry, 2010
This paper addresses the physical features of the ultrasonic cavitational synthesis of zinc ferri... more This paper addresses the physical features of the ultrasonic cavitational synthesis of zinc ferrite particles and tries to establish the relationship between cavitation physics and sonochemistry of the zinc ferrite synthesis. A dual approach of coupling experimental results with simulations of radial motion of cavitation bubbles has been adopted. The precursors for the zinc ferrite, viz. ZnO and Fe 3 O 4 are produced in situ by the hydrolysis of Zn and Fe(II) acetates stimulated by Å OH radicals produced from the transient collapse of the cavitation bubbles. Experiments performed under different conditions create significant variation in the production of Å OH radicals, and hence, the rate of acetate hydrolysis. Correlation of the results of experiments and simulations sheds light on the important facets of the physical mechanism of ultrasonic cavitational zinc ferrite synthesis. It is revealed that too much or too little rate of acetate hydrolysis results in smaller particle size of zinc ferrite. The first effect of a higher rate of hydrolysis leads to excessively large growth of particles, due to which they become susceptible to the disruptive action of cavitation bubbles. Whereas, the second effect of too small rate of hydrolysis of Zn and Fe(II) acetates restricts the growth of particles. It has been observed that the initial reactant concentration does not influence the mean particle size or the size distribution of zinc ferrite particles. The present investigation clearly confirms that the rate-controlling step of zinc ferrite synthesis through ultrasonic cavitational route is the rate of formation of Å OH radicals from cavitation bubbles.
Ultrasonics Sonochemistry, 2010
This paper addresses the matter of mechanistic features of ultrasound-assisted permanganate oxida... more This paper addresses the matter of mechanistic features of ultrasound-assisted permanganate oxidation of organic compounds in aqueous phase. This reaction system is essentially a liquid-liquid heterogeneous one, which is limited by the mass transfer characteristics. Previous research has established that ultrasound irradiation of reaction mixture enhances the kinetics and yield of permanganate oxidation. The principal physical effect of ultrasonic cavitation is formation of fine emulsion between immiscible phases that eliminates the mass transfer resistance, while principal chemical effect is production of radicals through transient collapse of cavitation bubbles, which accelerate the reaction. In this paper, we have tried to discriminate between these physical and chemical effects by coupling experiments with different conditions (which alter the nature of cavitation phenomena in the medium) to simulations of cavitation bubble dynamics. It is revealed that in absence of radical conserving agent, the enhancement effect is merely physical. Diffusion of radicals towards interface between phases, where the oxidation reaction occurs is the limiting factor in contribution of chemical effect of ultrasonic cavitation towards enhancement of oxidation. Enhancement of total radical production in the aqueous phase (by degassing of the medium) increases the overall oxidation yield, but only marginally. On the other hand, addition of a radical conserver such as FeSO 4 Á7H 2 O results in marked enhancement in oxidation yield, as the conserver assists deeper penetration of radicals in the aqueous medium and diffusion towards interface.
Environmental Technology, 2010
In this study, we have attempted to reveal the physical or mechanistic features of the sonochemic... more In this study, we have attempted to reveal the physical or mechanistic features of the sonochemical degradation of 2,4‐dichlorophenol (2,4‐DCP). The principal physical phenomenon underlying sonochemical effects is radial motion of cavitation bubbles and production of radicals from transient collapse of these bubbles. We reveal some important physical facets of sonochemical degradation of 2,4‐DCP by adopting dual approach of coupling experimental results with simulations of radial motion of cavitation bubble. First, the location of the degradation is predominantly the interfacial region between bubble and bulk medium, and secondly, the extent of degradation is controlled by conservation – and not the production – of oxidizing radicals that affects the probability of radical–pollutant interaction.
Chemical Engineering Journal, 2009
This paper addresses the matter of intensification of the sonochemical degradation of phenol from... more This paper addresses the matter of intensification of the sonochemical degradation of phenol from mechanistic point of view by coupling the experimental results to a mathematical model that takes into account essential physics and chemistry of cavitation bubbles. We assess the relative influence of extent of radical production by the cavitation bubbles and radical scavenging (or conservation) on the overall degradation of phenol. We have used a molecular species (oxygen) and an ionic species (Fe 2+ ions) for scavenging of radicals produced by cavitation bubbles. Four different gases, viz. argon, oxygen, nitrogen and air have been used to provide cavitation nuclei in the bulk medium. It is revealed that sonochemical degradation of phenol is governed by the extent of radical scavenging, both inside and outside the bubble. Phenomenon of radical scavenging influences the probability of interaction between the phenol molecules and radicals. The extent of degradation in the presence of Fe 2+ ions has been much higher. This result has been attributed to higher reactivity and uniform concentration of the Fe 2+ ions in the medium, as a result of which they can effectively scavenge the radicals generated by cavitation bubbles.
Aiche Journal, 2008
This work considers the mechanistic features of sonochemical degradation of volatile (chlorobenze... more This work considers the mechanistic features of sonochemical degradation of volatile (chlorobenzene) and nonvolatile (phenol) organic pollutants. The sonochemical degradation of pollutant occurs in two pathways: thermal pyrolysis and hydroxylation. By coupling experimental results to bubble dynamics model, we have tried to establish relative contribution of these pathways to degradation of two kinds of pollutants. It is revealed that degradation of volatile pollutants occurs primarily by thermal pyrolysis while hydroxylation is the predominant mechanism of degradation of nonvolatile pollutants. The simulation results also help explain some interesting trends observed in degradation with reaction parameters such as initial pollutant concentration and salt addition to solution. These parameters are found to influence both pathways of degradation that leads to enhancement in degradation. However, the extent of this influence on hydroxylation and pyrolysis pathways is different for phenol and chlorobenzene. This is attributed to different partitioning behavior and solubility of the two pollutants. © 2008 American Institute of Chemical Engineers AIChE J, 2008
Chemosphere, 2008
This article attempts to discern the physical (or mechanistic) features of the sonochemical degra... more This article attempts to discern the physical (or mechanistic) features of the sonochemical degradation of two major and ubiquitous nitroaromatic pollutants, viz. nitrobenzene and p-nitrophenol. The fundamental physical phenomenon behind sonochemical degradation of pollutants is radial motion of cavitation bubbles. This study implements a dual approach to the problem, i.e. results of the experiments under different conditions have been coupled to a mathematical model that addresses physics and chemistry of the cavitation bubbles. Various experimental techniques applied in this study influence important physical parameters related to cavitation phenomenon in the liquid medium such as extent of radical production from the bubble, thickness of the liquid shell surrounding the bubble that gets heated up during transient collapse, the concentration of the pollutant in the interfacial region and extent of radical scavenging in the medium. Concurrent analysis of the experimental and simulation results reveal that overall degradation of the pollutant achieved for a given combination of experimental conditions is a function of competing (and sometimes conflicting) effect of these parameters. A semi-quantitative account of the relative influence of these parameters and the interrelations between them is presented.
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Papers by V. S. Moholkar, PhD, CEng, MIChemE, FRSC, MAIChE (Sr)
An attempt has been made to present a unified theoretical model for the cavitating flow in a hydrodynamic cavitation reactor using the nonlinear continuum mixture model for two-phase flow as the basis. This model has been used to describe the radial motion of bubble in the cavitating flow in two geometries in hydrodynamic cavitation reactors, viz., a venturi tube and an orifice plate. Simulations of the bubble dynamics in a venturi flow demonstrate the stable oscillatory radial bubble motion due to a linear pressure gradient. Due to an additional pressure gradient due to turbulent velocity fluctuations the radial bubble motion in case of an orifice flow is a combination of both stable and oscillatory type. The results of numerical simulations have been explained on the basis of analogy between hydrodynamic cavitation and acoustic cavitation.
Keywords
Bubble dynamics; Two-phase flow; Hydrodynamic cavitation; Acoustic cavitation; Process Intensification
An attempt has been made to present a unified theoretical model for the cavitating flow in a hydrodynamic cavitation reactor using the nonlinear continuum mixture model for two-phase flow as the basis. This model has been used to describe the radial motion of bubble in the cavitating flow in two geometries in hydrodynamic cavitation reactors, viz., a venturi tube and an orifice plate. Simulations of the bubble dynamics in a venturi flow demonstrate the stable oscillatory radial bubble motion due to a linear pressure gradient. Due to an additional pressure gradient due to turbulent velocity fluctuations the radial bubble motion in case of an orifice flow is a combination of both stable and oscillatory type. The results of numerical simulations have been explained on the basis of analogy between hydrodynamic cavitation and acoustic cavitation.
Keywords
Bubble dynamics; Two-phase flow; Hydrodynamic cavitation; Acoustic cavitation; Process Intensification