K. S. Gandhi

Platinum-based alloys as oxygen-reduction catalysts for solid-polymer-electrolyte direct methanol fuel cells

Electrocatalytic activities of various carbon-supported platinum-based binary, namely, Pt–Co/C, Pt–Cr/C and Pt–Ni/C, and ternary, namely, Pt–Co–Cr/C and Pt–Co–Ni/C, alloy catalysts towards oxygen reduction in solid–polymer–electrolyte direct methanol fuel cells were investigated at 70°C and 90°C both at ambient and 2bar oxygen pressures. It was found that Pt–Co/C exhibits superior activity relative to Pt/C and other alloy catalysts.

Breakage of viscous and non-newtonian drops in stirred dispersions

A model of breakage of drops in a stirred vessel has been proposed to account for the effect of rheology of the dispersed phase. The deformation of the drop is represented by a Voigt element. A realistic description of the role of interfacial tension is incorporated by treating it as a restoring force which passes through a maximum as the drop deforms and eventually reaching a zero value at the break point. It is considered that the drop will break when the strain of the drop has reached a value equal to its diameter. An expression for maximum stable drop diameter, dmax, is derived from the model and found to be applicable over a wide range of variables, as well as to data already existing in literature. The model could be naturally extended to predict observed values of dmax when the dispersed phase is a power law fluid or a Bingham plastic.

Modelling of a batch sonochemical reactor

Ultrasonication of aqueous KI solution is known to yield I2 due to reaction of iodide ions with hydroxyl radicals, which in turn are generated due to cavitation. Based on this conceptual framework, a model has been developed to predict the rate of iodine formation for KI solutions of various concentrations under different gas atmospheres. The model follows the growth and collapse of a gas—vapour cavity using the Rayleigh—Plesset bubble dynamics equation. The bubble is assumed to behave isothermally during its growth phase and a part of the collapse phase. Thereafter it is assumed to collapse adiabatically, yielding high temperatures and pressures. Thermodynamic equilibrium is assumed in the bubble at the end of collapse phase. The contents of the bubble are assumed to mix with the liquid, and the reactor contents are assumed to be well stirred. The model has been verified by conducting experiments with KI solutions of different concentrations and using different gas atmospheres. The model not only explains these results but also the existence of a maximum when Ar---O2 mixtures of different compositions are employed.

Alternative mechanisms of drop breakup in stirred vessels

Kumar el al. (1991, Chem. Engng Sci. 46, 2483-2489) have shown that in a stirred vessel, size of the largest stable drop, d(max), first increases with phi (holdup of the dispersed phase) at low phi, but decreases with phi at high phi. They have proposed two additional mechanisms of breakage-in shear and elongational flow regions in the front of the impeller blade-that operate along with the hitherto accepted mechanism due to turbulent fluctuations, and conclude that d(max) at high phi is controlled by breakage in shear flows in the range of parameters investigated by them. We show in this paper that their model is deficient on various counts. The new model proposed here overcomes these deficiencies. It predicts that at high phi, d(max) is controlled by breakage in the accelerating flow in the tip region of a rotating blade. The model predicts the data of Kumar er at. (1991) and Boye et al. (1996, Chem. Engng Commun. 143, 149-167). New experiments were also conducted to discriminate between the two proposed mechanisms. The experiments independently confirm that drop breakage at high phi is indeed controlled by accelerating flow. The model could predict the new experimental data also quite well

A new model for the breakage frequency of drops in turbulent stirred dispersions

A model of drop breakage in turbulent stirred dispersions based on interaction of a drop with eddies of a length scale smaller than the drop diameter has been developed. It predicts that, unlike the equal breakage assumed by earlier models, a large drop reduces in size due to stripping of smaller segments off it through unequal breakage. It is only when the drop nears the value of the maximum stable drop diameter that it breaks into equal parts. This new model of drop breakage, coupled with the pattern of interaction of drops with eddies of different sizes existing in the vessel, has been used to evaluate not only the breakage frequency, but also the size distribution of the daughter droplets(which was hitherto assumed). The model has been incorporated in the population balance equation and the resulting cumulative size distributions compared with those availble in the literature.

A new model for coalescence efficiency of drops in stirred dispersions

A model for coalescence efficiency of two drops embedded in an eddy has been developed. Unlike the other models which consider only head-on collisions, the model considers the droplets to approach at an arbitrary angle. The drop pair is permitted to undergo rotation while they approach each other. For coalescence to occur, the drops are assumed to approach each other under a squeezing force acting over the life time of eddy but which can vary with time depending upon the angle of approach. The model accounts for the deformation of tip regions of the approaching drops and, describes the rupture of the intervening film, based on stability considerations while film drainage is continuing under the combined influence of the hydrodynamic and van der Waals forces. The coalescence efficiency is defined as the ratio of the range of angles resulting in coalescence to the total range of all possible approach angles. The model not only reconciles the contradictory predictions made by the earlier models based on similar framework but also brings out the important role of dispersed-phase viscosity. It further predicts that the dispersions involving pure phases can be stabilized at high rps values. Apart from explaining the hitherto unexplained experimental data of Konno et al. qualitatively, the model also offers an alternate explanation for the interesting observations of Shinnar.

Prediction of separation factor in foam separation of proteins

A phenomenological model has been developed for predicting separation factors obtained in concentrating protein solutions using batch-foam columns. The model considers the adsorption of surface active proteins onto the air-water interface of bubbles, and drainage of liquid from the foam, which are the two predominant processes responsible for separation in foam columns. The model has been verified with data collected on casein and bovine serum albumin (BSA) solutions, for which adsorption isotherms are available in the literature. It has been found that an increase in liquid pool height above the gas distributor and the time allowed for drainage result in a better separation. Further, taller foam columns yield poorer separation at constant time of drainage. The model successfully predicts the observed results.

Calorimetric and Other Interaction Studies on Mineral-Starch Adsorption Systems

Abstract Studies have been made on adsorbabilities of starch, phosphorylated starch and starch constituents — amylose and amylopectin — on hematite and also calcite. The adsorption process is slow, partially irreversible, non-physical and appreciably exothermic. For a small starch concentration, Δ H on hematite reaches a maximum of 314 J/g starch adsorbed, and then falls off at higher concentration, probably due to partial deanchorage and reorientation of adsorbed molecules initially lying flat on the surface. Adsorption isotherm patterns, including pH-dependence behaviour, are similar for starch and amylopectin. The magnitude of adsorption of amylose, compared to that of amylopectin (nearly 20 times bulkier), is larger on a molar basis but smaller on a g/cm 2 basis. Thus, interpretation of calcite—starch adsorption data should be made in terms of amylose as well as amylopectin. For the hematite—starch system, adsorption of amylopectin is of crucial importance. Conductometric data and IR spectrograms point to specific chemical interactions between starch constituents and ions such as Fe 2+ and Ca 2+ . These are evidence of the existence of chemisorption bonds of amylose as well as of amylopectin interacting on calcite and hematite surfaces.

A model for static foam drainage

A model for static foam drainage, based on the pentagonal dodecahedral shape of bubbles, that takes into account the surface mobility of both films and Plateau border walls has been developed. The model divides the Plateau borders into nearly horizontal and nearly vertical categories and assigns different roles to them. The films are assumed to drain into all the adjacent Plateau borders equally. The horizontal Plateau borders are assumed to receive liquid from films and drain into vertical Plateau borders, which in turn form the main component for gravity drainage. The model yields the liquid holdup values for films, horizontal Plateau borders and vertical Plateau borders as functions of height and time. The model has been tested on static foams whose cumulative drainage was measured as a function of time. The experimental data on the effect of foam height, initial holdup, surface viscosity, etc. can be explained by the model quantitatively.

Modelling of sonochemical oxidation of the water-KI-CCl4 system

The sonolysis of KI solution containing CCl4 as a separate phase results in the formation of I2, but shows characteristics which are different from those observed when KI solution alone is sonicated. By the addition of CCl4, the rate goes up by two orders of magnitude, the rate becomes independent of KI concentration, the effect of gas atmosphere becomes less pronounced, and the rate becomes time-dependent. Further the average rate passes through a maximum as the dispersed phase hold-up is increased. These results are explained on the basis of a model which treats cavitation bubbles as microreactors which generate fragmented products from their contents and release them into the liquid phase at the end of collapse phase. The composition of the products of the microreactors is calculated assuming attainment of reaction equilibria. The significant increase in the oxidation rate has been found to be due to release of Cl2, Cl, and HOCL which act as separate source of reactants to yield I2. As all these quantitatively react in the reactor with KI, the rate becomes independent of KI concentration. The gas atmosphere here is found to continuously change because of formation of CO2 and O2. This results in the change in the composition of gas bubble with time, resulting in reduced effect of initial gas atmosphere used. The presence of a dispersed phase reduces the number of bubbles because of attenuation and scattering but increases them due to interfacial cavitation, thus yielding a maximum at a specific hold-up. The model takes these factors into account and is able to not only explain the different observed trends but also predict them quantitatively.