bluuebunny/crossref_metadata_2025 huggingface dataset:
Gas–liquid two‐phase flow through packed beds in microgravity
Authors: Motil Brian J. , Balakotaiah Vemuri , Kamotani Yasuhiro
Published: April 2004

Experimental data on flow pattern transitions, pressure drop, and flow characteristics for cocurrent gas–liquid flow through packed columns in microgravity is analyzed. The pulse flow regime exists over a wider range of gas and liquid flow rates under microgravity conditions. Furthermore, maps used to predict transition boundaries in normal gravity do not apply in microgravity. The flow regime data are compared to the widely used Talmor map and a new transition criterion between bubble and pulse flow in microgravity is proposed. The pressure‐drop data clearly show that interfacial effects can increase the pressure drop by as much as 300% compared to those predicted by the single‐phase Ergun equation. A two‐phase friction factor is correlated to the superficial gas and liquid Reynolds numbers and the Suratman number. New data are also presented on the influence of gravity on the pulse amplitude and frequency.
Promotion of deactivated sintering by dry‐particle coating
Authors: Mohan Michelle Ramlakhan , Dave Rajesh N. , Pfeffer Robert
Published: April 2004

Dry‐particle coating was used to examine the feasibility of increasing the sintering temperatures of different particulate materials (hosts) by applying a discrete coating of a highly refractory material (guest). Increasing the sintering temperature reduces the sintering rate of the material, and hence the phenomenon is called “deactivated sintering.” Several host particles, such as glass beads, PMMA, γ‐alumina, and an alumina–silica composite, were coated with submicron‐size silicon carbide in three different dry‐particle coating devices: magnetically assisted impaction coating, mechanofusion, and hybridizer. The coated products were characterized using several techniques, including particle‐size analysis, SEM, EDX, BET, optical microscopy, and dilatometry studies to compare the coating performance of each device. By coating the host particles with silicon carbide, it was possible to appreciably increase the sintering temperatures of all four of the particulate host materials. Simple phenomenological sintering models based on volume diffusion for crystalline materials and viscous flow for amorphous materials were combined with dilatometry data to explain the phenomenon of deactivated sintering. For γ‐alumina, there was a significant reduction in the effective diffusivity due to the addition of a surface coating of SiC, whereas for SiC‐coated glass beads and PMMA, an increase in the effective viscosity was postulated as the cause of deactivated sintering.
Optimal region for design and control of ternary systems
Authors: Cheng Yu‐Chang , Yu Cheng‐Ching
Published: April 2004

The trade‐off between steady‐state economics and dynamic controllability of recycle plants is analyzed for ternary systems. For this analysis, a boundary is first established in the composition space to identify the correct flow sheet, and then optimal trajectories for recycle plants with direct and indirect sequences are derived as the conversion varies. In the optimality regions, at any conversion, the correct separation sequence, as well as optimal reactant distribution, can also be obtained. For dynamic controllability, the reachable composition space is identified as the recycle ratios vary. The results clearly indicate that trade‐off between steady‐state design and dynamic control is not likely to occur, if the plant is designed along the optimal trajectories with the correct flow sheet. Rigorous nonlinear simulations also reveal that no single control structure works well over the entire composition space.
2‐D wavelet‐based adaptive‐grid method for the resolution of PDEs
Authors: Santos J. C. , Cruz P. , Magalhães F. D. , Mendes A.
Published: April 2004

An efficient interpolating wavelet‐based adaptive‐grid numerical method is described for solving systems of bidimensional partial differential equations. The grid is dynamically adapted in both dimensions during the integration procedure so that only the relevant information is stored, saving allocation memory. The spatial derivatives are directly calculated in a nonuniform grid using cubic splines. Numerical results for five typical problems presented illustrate the efficiency and robustness of the method. The adaptive strategy significantly reduces the computational times and the memory requirements, as compared to the fixed‐grid approach.
Three‐liquid phase equilibria in water+benzene+caprolactam+(NH4)2SO4 mixtures
Authors: Huan Zhou , Van Bochove Gerard H. , De Loos Theo W.
Published: April 2004

Liquid–liquid–liquid equilibrium (LLLE) data are reported for the water+benzene+caprolactam+ammonium sulfate system at 293.1 K, 313.1 K, and 328.1 K. The experiments show a distinct pattern of phase behavior with increasing salt concentration and temperature. The tricritical point of the system was not determined, but was estimated to be around 332 K. The results are modeled using a modification of the extended electrolyte NRTL model. The model was able to represent the LLLE data satisfactorily.
Maximum carrying capacity and granular temperature of A, B and C particles
Authors: Gidaspow Dimitri , Mostofi Reza
Published: April 2004

The observed cyclic operation of a riser near choking was calculated using a computational fluid‐dynamic model used previously to predict its flow regimes. An approximate analytical expression for the maximum carrying capacity of Geldart A, B and C particles was developed and compared to data in the literature. In the new theory, an unknown parameter is the pseudosonic velocity of the particles. To estimate this quantity, new granular temperatures were measured and compared to data from the literature for Geldart A, B and C particles. In the riser flow regime (2% to 20% solid volume fraction), the turbulent intensity of the particles is approximately 0.5, in agreement with the theory. The theoretical value involves the ratio of particle viscosity to the granular conductivity.
Kinetics of particle formation in the gas antisolvent precipitation process
Authors: Elvassore Nicola , Parton Tiziana , Bertucco Alberto , Di Noto Vito
Published: April 2004

An original experimental setup based on UV‐vis spectroscopy was developed to study the precipitation kinetics of a biodegradable polymer by gas antisolvent processes (GAS). Poly(L‐lactide) acid (PLA) precipitations were carried out in a high‐pressure optical cell equipped with sapphire windows, working at 1–80 bar and 301.15–307.15 K. The particle formation and precipitation kinetics were investigated in situ by measuring UV‐vis absorbance of polymeric particles at a wavelength of 600 nm. They were measured in a batch system at different pressurization rates (different supersaturation conditions). To rationalize the precipitation kinetics in GAS processes, a population balance model was developed considering particle nucleation, growth, aggregation, and settling. Nucleation and growth were represented by the McCabe model, whereas both independent‐ and nonindependent‐kernel Smoluchowski's coagulation equations were used for aggregation. Settling was approximated by a first‐order kinetic. Absorbance measurements were related to the second moment of the simulated particle‐size distribution, and the kinetic parameters were estimated based on spectroscopic data. The model gave a correct phenomenological representation of all experimental data and fairly predicted the particle‐size distribution of the precipitated PLA microparticles.
Comparing true countercurrent and simulated moving‐bed chromatographic reactors
Authors: Lode Florian , Mazzotti Marco , Morbidelli Massimo
Published: April 2004

Simulated Moving Bed reactors (SMBR) combine chemical reaction and adsorptive separation within one single continuous and countercurrent unit. This integration promises substantial improvements in process performance, especially when applied to equilibrium‐limited reactions involving such heat‐sensitive products as fine chemicals and pharmaceuticals. In this work, the interplay among the relevant process design parameters (dimensionless ratios of the fluid and solid flow rates, and the Damköhler numbers for each section of the unit) is investigated. For this, an analytical solution of differential mass‐balance equations for the corresponding true countercurrent process (TCC), using as a model system the reaction A⇌B+C with each species exhibiting linear adsorption behavior, was developed. Based on this solution, criteria were derived for the optimum process design with respect to productivity and solvent consumption. Comparing these results with numerical simulations of an SMBR unit shows that the TCC model does not apply to SMBR units with a finite number of columns per section, that is, units of practical relevance, because the two units exhibit different residence time distributions and, hence, lead to different degrees of conversion.
Core–shell structure and segregation effects in composite droplet polymer blends
Authors: Reignier Joël , Favis Basil D.
Published: April 2004

Core–shell morphology formation within the dispersed phase was studied for composite droplet polymer‐blend systems comprising a high‐density polyethylene matrix, polystyrene shell and different molecular weights of poly(methyl methacrylate) core material. The blends were prepared in the melt using an internal mixer, and the morphology was analyzed by electron microscopy. Changing the viscoelastic properties of the core in the dispersed phase dramatically affects PS‐PMMA segregation within the dispersed composite droplet itself. A high‐molecular‐weight‐PMMA core contains a large quantity of occluded PS inclusions, while the low‐molecular‐weight PMMA results in a perfectly segregated PS shell and PMMA core. These phenomena were attributed to the viscosity of the PMMA. Using the latter system, a direct microscopic study of the shell formation process demonstrates unambiguously that under conditions of perfect segregation, the onset of complete shell formation corresponds to a shell thickness that is close to two times the radius of gyration of polystyrene. Thus, the thinnest possible shell in such a system possesses a molecular‐scale thickness. The system with the high‐molecular‐weight‐PMMA core demonstrates an onset of complete shell formation that is displaced to higher concentrations due to the poor segregation effect. By counterbalancing the effects of viscosity ratio and interfacial effects on the composite droplet size, it is possible to generate perfectly segregated core–shell dispersed‐phase morphologies of almost identical size with a controlled shell thickness ranging from 40 to 300 nm.
Kinetics of microbubble–solid surface interaction and attachment
Authors: Yang Chun , Dabros Tadeusz , Li Dongqing , Czarnecki Jan , Masliyah Jacob H.
Published: April 2004

Microbubble–solid surface interaction and attachment under the influence of hydrodynamic and physicochemical forces were studied experimentally and theoretically. An impinging‐jet technique was developed to measure bubble‐attachment flux onto a flat solid surface in an impinging‐jet stagnation flow. A video imaging system enables direct observation of the attachment behavior of hydrogen microbubbles onto two different collector surfaces: hydrophilic untreated glass and hydrophobic methylated glass. Experimental results showed that the attachment flux depends on both hydrodynamic flow and electrolyte concentration. A mass‐transfer model developed computes bubble‐attachment flux, considering hydrodynamic convection, Brownian diffusion, migration under gravitational buoyancy, and DLVO surface forces (that is, van der Waals and electric double‐layer forces). At high flow rates, the numerical predictions for attachment rates onto methylated glass generally agreed well with the experimental data. However, a difference exists between theoretical and experimentally determined attachment rates for both untreated and methylated glass when the Reynolds number of the flow is low. Several mechanisms are proposed to account for this discrepancy.
Retrograde melting behavior in polyolefin + solvent + antisolvent solutions
Authors: Adidharma Hertanto , Radosz Maciej , Luszczyk Marek
Published: April 2004

The SAFT1 equation of state captures the melting retrograde behavior for a ternary system of poly(ethylene‐co‐octene‐1) + isooctane + ethylene. The melting temperature minimum depends on the polymer branch density, solvent composition, polymer concentration, and polymer molecular weight, in decreasing order of sensitivity. Furthermore, the effects of the solvent/antisolvent ratio and polymer concentration on the melting temperature are nonmonotonic, that is, their sign may change in going from low to high pressures. Finally, the sign of the slope of the melting curve depends only on the difference between the partial molar volume of polymer in solution and the molar volume of pure solid polymer.
Population balance equations for cell and microbial cultures revisited
Authors: Fredrickson A. G.
Published: April 2004

The first chemically structured model of growth of a microbial or cell population proposed in 1967 by Fredrickson et al. took account of the particulate nature of the population. This model assumed implicitly that cells passed through the cell cycle continuously without transitions between recognizable cell cycle phases. This article discusses how to generalize or revise the first model so that passages of cells through a series of recognizable cell cycle phases can be accounted for. The 1967 article assumed that cell fission can occur anywhere in state space, although with high probability only in a limited domain of that space. Extension of this concept to cell cycle transitions leads to a model that violates a principle of determinism. Revised models that subdivide state space into nonoverlapping subdomains associated with each phase of the cell cycle are developed to obviate this unacceptable situation. The new models introduce the need for information about cell behavior that does not currently exist and thereby provide motivation for an experimental program aimed at supplying the data necessary for their identification and validation.
Using MR techniques to probe permeability reduction in rock cores
Authors: Johns M. L. , Sederman A. J. , Gladden L. F. , Wilson A. , Davies S.
Published: April 2004

Polymer solutions are used commonly in oil‐recovery applications to reduce water production. While the polymer treatment is known to modify the permeability of the rock, the mechanism by which this occurs is poorly understood, and, hence, the ability to optimize either the polymer solution or its application is restricted. In this work, the pulsed‐gradient spin‐echo nuclear magnetic resonance technique is used to determine displacement propagators (distributions) for brine solution flowing through a Bentheimer sandstone rock core, both before and after treatment of the rock core with a polyacrylamide polymer solution. A novel simulation strategy employing the lattice–Boltzmann method is then proposed to interpret the differences in these propagators in terms of the location of polymer retained within the pore structure of the rock following treatment. The results suggest that the polymer is preferentially trapped in comparatively low permeability pores.
Bed nonhomogeneity in turbulent gas‐solid fluidization
Authors: Du Bing , Warsito W. , Fan Liang‐Shih
Published: April 2004

Bed dynamic behavior in a gas‐solid fluidization system is measured in real time using ECT based on the NNMOIRT recently developed. The ECT results reveal the time‐averaged and transient characteristics of the bubble/void phase and the emulsion phase, which is of value to the verification of the 3‐D computation codes with real physical systems. In the turbulent regime, the time‐averaged solids holdup distribution exhibits radial symmetry, which is not the case for the bubbling regime. The standard deviations of the fluctuations of the cross‐sectional, averaged bubble/void phase fraction, and the cross‐sectional, averaged solids holdups in the bubble/void phase and in the emulsion phase, all peak at the transition velocity from the bubbling to the turbulent regimes Uc. The addition of 10% fine particles decreases the solids holdup in the emulsion phase due to the breakup of large bubbles. The ECT study indicates that optical fiber probe measurement of the bubble/void phase fraction may yield significantly different results depending on the threshold level selected for the measured solids holdup distribution. The bubble/void phase fraction measured by the optical fiber probe does not vary with bed temperature up to 400°C; the emulsion phase voidage is observed to increase as the bed temperature increases.
Simulation and optimization of pressure‐swing adsorption systems for air separation
Authors: Jiang Ling , Biegler Lorenz T. , Fox V. Grant
Published: April 2004

Over the past 20 years, PSA processes have gained increasing commercial acceptance as an energy‐efficient separation technique. After a startup time, the system reaches cyclic steady state (CSS), at which the conditions in each bed at the start and end of each cycle are identical. Contrary to the traditional successive substitution method, we apply a direct determination approach using a Newton‐based method with accurate sensitivities to achieve fast and robust convergence to CSS. Trust region methods and scaling are used to handle ill‐conditioning and model nonlinearities. When design specifications are imposed, this approach is easily extended to include design constraints. This eliminates trial‐and‐error experiments and determines all the operating parameters simultaneously. In addition, we modify a standard flux limiter in order to deal with nondifferentiable terms and ensure computational accuracy and efficiency. Optimal PSA processes are designed by means of state‐of‐the‐art rSQP‐based optimization algorithms. The simultaneous tailored approach incorporates detailed adsorption models and specialized solution methods. Here, CSS convergence is achieved only at the optimal solution and the time‐consuming CSS convergence loop is eliminated. Applications of several nonisothermal VSA O2 bulk gas separation processes are presented to illustrate all of these approaches.
Quadrature method of moments for population‐balance equations
Authors: Marchisio Daniele L. , Pikturna Jesse T. , Fox Rodney O. , Vigil R. Dennis , Barresi Antonello A.
Published: April 2004

Although use of computational fluid dynamics (CFD) for simulating precipitation (and particulate systems in general) is becoming a standard approach, a number of issues still need to be addressed. One major problem is the computational expense of coupling a standard discretized population balance (DPB) with a CFD code, as this approach requires the solution of an intractably large number of transport equations. In this work the quadrature method of moments (QMOM) is tested for size‐dependent growth and aggregation. The QMOM is validated by comparison with both Monte Carlo simulations and analytical solutions using several functional forms for the aggregation kernel. Moreover, model predictions are compared with a DPB to compare accuracy, computational time, and the number of scalars involved. Analysis of the relative performance of various methods for treating aggregation provides readers with useful information about the range of application and possible limitations.
Steam reforming of methane and water‐gas shift in catalytic wall reactors
Authors: Venkataraman K. , Wanat E. C. , Schmidt L. D.
Published: April 2004

Catalytic wall reactors permit high heat‐transfer rates between exothermic and endothermic reactions taking place catalytically on opposite sides of a thin wall, because they eliminate resistance to heat transfer in thermal boundary layers, thus making them compact and efficient. A parallel plate catalytic wall reactor was built in which exothermic methane combustion on platinum and endothermic methane steam reforming on rhodium occurred on walls in alternate channels. This reactor gave 95% conversion of methane to synthesis gas with a residence time of ∼70 ms at a steam/methane ratio of 1/1 with a thermal efficiency of ∼60%. A preheat pass was added on the combustion side that enabled heat exchange between hot combustion products and cold combustion inlet gases to increase temperature upstream and decrease it downstream. This two‐pass reactor gave H2/CO ratios of ∼14/1 with a residence time of ∼170 ms at a steam/methane ratio of 4/1. To increase the H2/CO ratio, the endothermic channel length was also extended, with a platinum‐ceria wall coating on the extended region to further reduce downstream temperatures and promote water‐gas shift. This reactor gave downstream temperatures as low as 200°C, and produced H2/CO ratios as high as 42/1 with a residence time of ∼300 ms at a steam/methane ratio of 4/1. The extended reactor shows good potential for producing high H2/CO ratio product streams suitable for preferential oxidation and subsequent use in fuel cells in a scalable configuration.
Formation process of three‐dimensional arrays from silica spheres
Authors: Ishikawa Keiji , Harada Hironori , Okubo Tatsuya
Published: April 2004

Elucidating the formation process of 3‐D arrays from submicrometer‐sized particles is a key to creating an optimal fabrication process of photonic crystals. In this research, the spontaneous sedimentation of silica spheres is imaged by in situ confocal laser scanning microscopy, and the dynamics is studied. When the pH is 7.0 and 10, the rate of the formation process of a hexagonal structure is attributed to the transport of spheres in the early stage of precipitation, and can be reproduced by a simple equation for Langmuir‐type adsorption, ignoring the desorption part. In a later stage, however, the rate becomes smaller. In the first layer, even after drying, the nearest two spheres are separated, while maintaining a hexagonal structure. From the results, the formation process of three‐dimensional arrays from silica spheres is illustrated.
Methane hydrate nonstoichiometry and phase diagram
Authors: Huo Zhongxin , Hester Keith , Sloan E. Dendy , Miller Kelly T.
Published: April 2004

A previous study discovered that, when formed from different ethylene oxide (EO) feed solutions, hydrates had different small‐cage occupancies with essentially total occupation of the large cavities of sI hydrate. An EO + H2O isobaric phase diagram was proposed with a solid solution range to explain this phenomenon. Since methane and CO2 also occupy both small cages and large cages of sI hydrate, new phase diagrams were proposed which allowed the methane or CO2 hydrate composition to vary as a function of feed composition at fixed p‐T conditions. Raman spectroscopy experiments now show that H2O + CH4 hydrates formed at the vapor–liquid interface have a different composition than dendrites growing into the water below the interface. Interfacial hydrates were typical laboratory samples formed with excess gas, while dendrites coexisted with excess water and were similar to seafloor hydrate. Integrated peak intensities were used to calculate relative cage occupancies in Raman data analysis. Interfacial hydrates had an average of 95.4% small cages occupied at 30 MPa and 275.15 K, while dendrite hydrates had only 82.8% small‐cage occupancy at the same conditions. This result suggests that hydrates in the ocean floor may have lower methane concentrations than hydrates formed under normal laboratory conditions at the same pressure and temperature.
Modeling wet‐gas annular/dispersed flow through a venturi
Authors: van Werven M. , van Maanen H. R. E. , Ooms G. , Azzopardi B. J.
Published: April 2004

A theoretical model for gas–liquid annular/dispersed flow through a venturi meter is reported. It is based on an earlier model developed for venturi scrubbers. Changes implemented are based on new research and on the different physics between the two cases. The predictions of the model have been tested using information from recent experiments on venturi meters employed for measuring wet‐gas flows with a liquid volume fraction up to 10%. The model gives good predictions with appropriate values of a small set of input variables.
Population balance PSD model for emulsion polymerization with steric stabilizers
Authors: Immanuel Charles D. , Doyle Francis J. , Cordeiro Cajetan F. , Sundaram Sekhar S.
Published: April 2004

A detailed population balance model is developed for the particle‐size distribution (PSD) in the emulsion copolymerization of vinyl acetate and butyl acrylate, with nonionic poly(ethylene oxide) surfactants and a redox initiator pair (t‐butyl hydrogen peroxide and sodium formaldehyde sulphoxylate). The model accounts for the effects of the nucleation, growth, and coagulation events on the evolution of the PSD. The study addresses the modeling of the coagulation kernel (time‐varying and size‐dependent intrinsic coagulation rate) under nonionic surfactants. The coagulation‐inclusive model results show significant differences from the results generated by a coagulation‐free model (even under low solids conditions). More importantly, the coagulation‐inclusive model predicts to a reasonable accuracy the experimental data on the full PSD.
Novel multifunctional optical‐fiber probe: II. High‐density CFB measurements
Authors: Liu Jinzhong , Grace John R. , Bi Xiaotao
Published: April 2004

The novel parallel three‐fiber optical probe described in Part I of this article is applied to determine simultaneously local instantaneous solids volume concentration, velocity, and flux in multiphase suspensions. Radial distributions of local particle concentration, velocity, and flux and their fluctuations in a high‐density circulating fluidized bed (HDCFB) are presented. A strong correlation exists between fluctuations of local particle velocity and particle concentration. The optical probe is shown to provide a means of evaluating alternative measurement techniques that have commonly been employed in earlier studies.
Optimal design of interconnected bioreactors: New results
Authors: Harmand Jérôme , Rapaport Alain , Trofino Alexandre
Published: April 2004

The optimal design of two interconnected continuous stirred bioreactors in which a single reaction occurs is presented. The term “optimal” should be understood here as the minimum of the total volume of the reactors required to perform a given conversion rate, given a quantity of matter to be converted per time unit. In determining the optimal volume, it is assumed that the input flow may be distributed among the tanks and also that a recirculation loop can be used. The analysis of the optimal configurations is investigated, and the concept of “steady‐state equivalent biological system” is highlighted. The class of growth functions for which the results of this study hold is quite wide and includes, in particular, Monod, Haldane, and Aiba kinetics.
Shortcut method for kinetically controlled reactive distillation systems
Authors: Lee J. W. , Brüggemann S. , Marquardt W.
Published: April 2004

A geometric‐based shortcut method for reactive distillation is addressed. The rectification body method for nonreactive distillation, the concept of critical Damköhler numbers, and the geometric design method for reactive distillation are combined with a new eigenvector analysis of pinch points. This shortcut method provides a minimum or reasonable Damköhler number for a given heat duty, as well as the design implication of how to effectively distribute reaction zones inside a column. This method can be used for a fast screening of process design alternatives and for an initialization of rigorous optimization.
Theory of trickle‐bed magnetohydrodynamics under magnetic‐field gradients
Authors: Iliuta Ion , Larachi Faïçal
Published: April 2004

External inhomogeneous magnetic fields exert a body force (magnetization force) on electrically nonconducting and magnetically permeable fluids. The force acts on both paramagnetic and diamagnetic fluids and can be used to compensate for or to amplify the gravitational body force, unlike the Lorentz braking force that manifests only for electrically conducting liquids, such as liquid metals. The ability to influence two‐phase flows through porous media by the application of external inhomogeneous magnetic fields is of interest in the operation of trickle‐bed reactors for catalytic process intensification, particularly in oxidation catalysis, where the O2 paramagnetic properties may be propitious to macrogravity operation for enhancing liquid holdup and wetting efficiency. An isothermal 1‐D two‐fluid magnetohydrodynamics model based on volume‐average mass and momentum balance equations to describe the gas‐liquid downflow under a spatially uniform magnetic‐field gradient. The slit model approximation was used for the derivation of drag force closures intervening in the momentum equations. The evolution of the trickle‐bed magnetohydrodynamics was theoretically investigated in terms of liquid holdup and pressure drop under four gas‐liquid combinations. Advantages of this novel approach of process intensification were rationalized in terms of catalytic reactions.
Modeling structure effects on aggregation kinetics in colloidal dispersions
Authors: Sandkühler Peter , Sefcik Jan , Lattuada Marco , Wu Hua , Morbidelli Massimo
Published: April 2004

The time evolution of the cluster‐mass distribution (CMD) during colloidal aggregation can be modeled using population balance equations, given that the matrix of the aggregation rate constant (kernel) is known. Although numerous aggregation kernels have been proposed, their validity is still a major open problem, particularly when the role of the internal structure of the aggregates is referred to. A procedure is presented for the discrimination among possible kernel expressions including the structure effect. For aggregation processes in the submicron range, information about size and structure of aggregates can be obtained by dynamic and static light‐scattering measurements, for example, in terms of the average hydrodynamic (Rh) and gyration (Rg) radii. These quantities can also be calculated from the cluster‐mass distribution when accounting for the aggregate structure by the fractal concept and for the angular and rotational diffusion dependence of Rh. Since Rh and Rg represent different averages of the CMD, their simultaneous fitting is a severe test for a given kernel due to its inclusion of information on the average and width of the distribution. This procedure allows differentiation among several types of kernels proposed in the literature for DLCA and RLCA based on their ability to describe experimental data.
Hydrogen storage in carbon nanotubes: Residual metal content and pretreatment temperature
Authors: Lueking Angela , Yang Ralph T.
Published: April 2004

Hydrogen storage in MWNT was enhanced via the catalytic activity of NiMgO. The magnitude of the hydrogen to metal ratio for the MWNT/NiMgO system, combined with temperature programmed adsorption and desorption studies, showed hydrogen spillover from the catalyst to the carbon surface. Metal doping combined with temperature activation studies showed that both nickel and magnesium are active in the catalytic process. The yield, quality, and carbon‐metal contact were shown to affect hydrogen uptake. Higher pretreatment temperatures enhanced uptake, for both low‐ and high‐pressure measurements, due to increased carbon‐metal contact and activation of the catalyst. At 69 bar (or 1,000 psia), the hydrogen adsorption and desorption of the MWNT/NiMgO system were 3.7% and 3.6%, respectively.
Comparing flow‐reversal and inner recirculation reactors: Experiments and simulations
Authors: Ben‐Tullilah Moshe , Alajem Einat , Gal Rina , Sheintuch Moshe
Published: April 2004

The operation of reactors with flow reversal operate similar to a reactor with internal recirculation, which the feed enters through one (say, inner) reactor and then turns around and flows out through (the outer) another, when the heat‐transfer coefficient between the tubes is large. In this study, we compare the behavior of a packed‐bed reactor operating in flow‐reversal or internal‐recirculation modes, using ethylene oxidation on Pt/Al2O3 as a model reaction. The reactor was built from two concentric tubes (with 28.5 and 42.5 mm in diameter), both packed with a 20 cm catalytic bed and 10 cm inert beds (of alumina‐pellets) on each side. An adjustable opening between the tubes allowed for an internal recycle mode and the whole system could be operated with periodic flow reversal. The reactor can be employed then either as a simple once‐through bed or as a bed with flow reversal in the inner tube or as bed with internal recirculation flowing from the inner to outer tube, or in the opposite direction, as well as an internal‐recirculation reactor with flow reversal. Due to heat losses, the latter two modes were inferior to the others. The experiments, backed by simulations using a homogeneous model with independently determined parameters, showed that the technically‐simpler inner‐outer internal‐recycle reactor operated better at low flow rates, than that with flow reversal, but the conclusion is reversed at high flow rates. The domain where the internal‐recirculation reactor is superior depends on the heat‐transfer coefficient between the streams. By lowering the feed concentration, the extinction point was determined for each mode highlighting again the conclusions drawn above that inner‐recirculation operation may be superior to flow reversal at low flow rates. Simulations revealed also the existence of solutions with stationary fronts or oscillatory fronts.
Surfactants effects on mass transfer during drop‐formation and drop falling stages
Authors: Lee Yuh‐Lang
Published: April 2004

Three types of surfactants were used to study their effects on the inhibition of mass transfer in liquid–liquid extraction. A single‐drop extraction apparatus was used to obtain the extraction percentage of acetic acid from dispersed CCl4 droplets to the aqueous phase. By using various lengths of extraction column, the extraction amounts were estimated during the drop‐formation and drop‐falling stages. Effects of these surfactants on the mass transfer during various stages were related to the adsorption behavior of these surfactants. The results of dynamic and equilibrium tensions show that the transport of Triton X‐100 is a diffusion‐controlled mechanism, which shows a fast adsorption kinetic at elevated concentrations. By comparing the effectiveness of the three surfactants in decreasing interfacial tension and in mass‐transfer inhibition, the mass‐transfer inhibition during the drop‐formation and drop‐falling stages can be related, respectively, to the properties of dynamic and equilibrium interfacial tensions. For surfactant concentrations below CMC, a surfactant that is most effective in decreasing the interfacial tension always exhibits the highest effectiveness in mass‐transfer inhibition. When the surfactant concentration is small (about 10−3 mM), the mass‐transfer inhibition of SDS and DTMAC is significant in the drop‐formation stage, but not in the drop‐falling stage. As the concentration is increased, the effectiveness of Triton X‐100 increases more quickly than the others in both the formation and falling stages. The mass‐transfer rate increased slightly when the Triton X‐100 concentration is elevated near CMC, due to the surface remobilization caused by the fast adsorption–desorption rate of Triton X‐100 at elevated concentrations. The mass‐transfer inhibition of surfactant also was caused mainly by the hydrodynamic effect in the drop‐formation stage, but from the obstruction of the barrier layer in the drop‐falling stage.
Modeling particle‐size distribution dynamics in a flocculation system
Authors: Zhang Jian‐jun , Li Xiao‐yan
Published: April 2004

Flocculation dynamics accounting for both particle coagulation and aggregate breakage was simulated mathematically by using modified sectional modeling techniques. The methodological improvement included the use of a continuous‐size density function, instead of a characteristic size for each size section, the applications of a comprehensive curvilinear model for the coagulation kinetics, and the fractal scaling relationship for particle aggregates. Simulation results demonstrated that a flocculation system could arrive at a dynamic steady state after a period of flocculation when coagulation and breakage counterbalanced each other, resulting in a stationary size distribution with a unique peak mass concentration. Three distinct breakage distribution functions—binary, ternary, and normal distribution—did not differ considerably based on the simulation results of the steady‐state size distributions. A lower shear rate, breakage rate constant, a higher collision efficiency, and initial particle concentration would result in larger aggregates in a flocculation system. The numerical simulations compared well with the results of the jar‐test flocculation experiments using latex microspheres, suggesting the applicability of the curvilinear–fractal–breakage modeling system for the process simulation of the flocculation units used in water and wastewater treatment.
Thermodynamic solution model for trona brines
Authors: Haynes Henry W.
Published: April 2004

A thermodynamics solution model has been developed for concentrated trona brines over temperatures ranging from about 0°C to 200°C. The model was constructed by regressing on solubility data for the temperature‐dependent parameters in Pitzer's activity coefficient model. In addition, the model incorporates published thermochemical data for the solid salts and aqueous species. In some cases it was necessary to estimate solid salt heat capacities, but otherwise no adjustments were made in the thermochemical parameters. With one exception, the model provided an excellent fit to the data. The existence of a high‐temperature trona phase reported earlier cannot be reconciled with the model and is believed to be in error. The phase‐equilibrium diagram generated by the model appears suitable for design purposes.
Heat transfer at the subcooled‐scraped surface with/without phase change
Authors: Qin Frank G. F. , Chen Xiao Dong , Russell Andrew B.
Published: April 2004

Continuous heat extraction is important for the process of freeze concentration of aqueous solutions, in which water is removed as solid ice. Three typical unsteady heat transportation patterns were distinguished at the subcooled surface of a scraped‐surface heat exchanger (SSHE) in this study. They were found in different stages of freeze concentration. Experimental measurement of the heat‐transfer coefficient in an SSHE showed that the overall heat‐transfer coefficient of stage III, which was characterized by ice formation on the cooler surface, was about 1.5 times higher than stage I, where no ice formed. Although the ice layer (also known as ice fouling) on a heat exchanger surface may be considered disadvantageous for heat transfer, the initial ice formation actually “boosted up” the heat transportation in an SSHE. The mechanism analysis and mathematical modeling of this phenomenon, however, have not been found in the literature. A mathematical model is developed and a unified expression of the heat‐transfer coefficient in an SSHE with/without phase change is presented. The model predicts a step increase of heat transfer occurs at the onset of ice formation and the maximum heat‐transfer coefficient exists in a narrow range right after reaching the freezing point. These are consistent with the experimental results of this study.
Flow between two coaxial rotating cylinders with a highly water‐repellent wall
Authors: Watanabe Keizo , Takayama Tsukasa , Ogata Satoshi , Isozaki Seiji
Published: April 2004

The torque and velocity profile of a flow between two coaxial cylinders with a highly water‐repellent wall were measured experimentally. Two kinds of highly repellent wall were tested to clarify the surface condition effect of the cylinder on laminar drag reduction with fluid slip. The results of torque measurement showed that laminar drag reduction does not occur for a highly water‐repellent wall with no fine grooves at the wall surface. Fluid slip of Newtonian fluids was clarified by measuring the velocity profile close to the highly water‐repellent wall in a Couette flow by a tracer method for flow visualization. The maximum slip velocity is less than 20% of the wall velocity of the rotating inner cylinder. The result of the drag reduction ratio for the torque was calculated by applying to the slip velocity, and agrees with the experimental result quantitatively. Consequently, the gas trapped in the fine grooves plays an important role in laminar drag reduction with respect to fluid slip. From the flow visualization of laminar Taylor–Couette flow, the intervals of Taylor cells become slightly irregular in the case of the cylinder with a highly water‐repellent wall. The calculation result for the Taylor cell applying the fluid slip boundary condition agrees with the experimental result.
Theoretical analysis of two‐phase bubble formation in an immiscible liquid
Authors: Chen W. B. , Tan Reginald B. H.
Published: April 2004

Two‐phase bubble formation coupled with phase change at a submerged nozzle is studied theoretically. The whole bubble consists of a vapor and a liquid phase. The inner vapor phase is assumed to be surrounded by a thin condensate layer, in which the vapor condenses partially as the two‐phase bubble grows. The interface element approach is applied to describe the dynamics of bubble formation. The effect of heat transfer from bulk vapor within the bubble to the surrounding bulk liquid is related to pressure analysis of vapor within the two‐phase bubble via the mass flux of condensation. The results computed from this model agree well with the experimental data.
Mathematical analysis of physicochemical processes on catalytic surfaces
Authors: Lele Tanmay , Lauterbach Jochen , Ramkrishna Doraiswami
Published: April 2004

The traditional approach followed for mathematically modeling physico‐chemical processes on catalytic surfaces involves the choice of an infinitesimal surface area on the catalyst and formulation of mass balances involving adsorbate surface concentrations. Such a strategy is inadequate when the catalytic surface itself changes dynamically with respect to adsorbate‐dependent surface arrangement of its catalytic atoms (and hence) its characteristic kinetics. A rigorous mathematical framework to model such processes is presented. The basic postulates of the theory are the availability of (1) a length scale over which the local infinitesimal area is of one surface type of another and (2) a time scale in which changes in fractional coverage occurring on the length scale in (1) are deterministically describable by continuous variables. A combination of probability and area‐averaging is used to arrive at a deterministic set of partial differential equations for surface concentrations. The resulting equations include reaction and surface diffusion, and new terms such as dilution/augmentation of surface concentration of species brought about by phase transformation. Such terms are significant in predicting the nonlinear behavior of the system and in extracting the kinetics of surface reactions from dynamic data. An application of the theoretical framework to CO oxidation on Pt(100) is demonstrated and dilution/augmentation terms were identified in the purely temporal model. These terms are shown to be significantly important by simulation.
Bubble injector effect on the gaslift efficiency
Authors: Guet S. , Ooms G. , Oliemans R. V. A. , Mudde R. F.
Published: April 2004

An experimental investigation has been made of the influence of the (initial) bubble size and bubble concentration distribution on the gaslift efficiency for a vertical air–water flow through a pipe of 72‐mm dia. and 18‐m height. Three significantly different bubble injectors were used. It was found that with decreasing bubble size, the liquid production increases at constant gas flow rate and, hence, the gaslift efficiency increases. Also, the initial concentration distribution can be significant, as this distribution can influence the coalescence of bubbles close to the injector. With decreasing bubble size, the transition from bubbly flow to slug flow can be postponed. This is one of the main reasons for the beneficial effect of the bubble size on gaslift efficiency. A theoretical model has been developed to predict the influence of the bubble size on the gaslift efficiency. The agreement is reasonable.
Investigation of impinging‐jet crystallization with a calcium oxalate model system
Authors: Hacherl Jean M. , Paul Edward L. , Buettner Helen M.
Published: April 2004

An impinging‐jet crystallizer was investigated in this work to assess its operational sensitivity and reproducibility for the production of small, monodisperse crystals using calcium oxalate, a model system capable of forming multiple hydrates. The impinging‐jet mixer provides rapid mixing of the reactant solutions through the impingement of two narrow reactant streams at high velocity. Impinging jet linear velocity and postjetting conditions were studied, with the jet operated in nonsubmerged mode. Hydrate form and crystal‐size distribution (CSD) were determined using optical microscopy and image analysis techniques. The impinging jet consistently produced small, monodisperse crystals. However, at a high level of supersaturation, slight variations in the CSD were observed for apparently identical conditions, suggesting a degree of sensitivity in the system that could lead to difficulty in its application. An apparent trend between impinging‐jet linear velocity and crystal size and number was observed, with more small crystals produced at higher linear velocity.
Testing the relative importance of mass and heat resistances in solid catalysts
Authors: Petrus Roman , Szukiewicz Mirosław
Published: April 2004

Criteria for isothermal conditions and/or absence of diffusion resistances in catalytic reaction were derived using approximate profiles of reagents concentration and temperature for the important reaction νAA+νBB→νRR and for a power‐law type of the kinetic equation, which are quite often applied in practice. The method can be easily adapted to another type of kinetic rate equation and/or more complicated reaction schemes. The developed criteria were applied to some data from the literature with good result.
Novel method for the fabrication of gradient‐index plastic optical fibers
Authors: Sohn In‐Sung , Park Chang‐Won
Published: April 2004

A novel coextrusion method has been introduced for the fabrication of gradient‐index plastic optical fibers (GI‐POFs). In this method, the radial diffusion of a refractive‐index‐modifying dopant in a polymer matrix is enhanced significantly by a new die design concept. A theoretical analysis indicates that this new method can substantially reduce the material residence time required to obtain a smooth refractive‐index profile, thereby increasing the output rate to a much higher level than possible with the conventional methods. An experiment using a poly(methyl methacrylate) (PMMA) with diphenyl sulfide (DPS) and diphenyl sulfoxide (DPSO) as the refractive‐index‐modifying dopants has shown an excellent agreement with the theoretical prediction, suggesting that the new coextrusion method is a viable way to manufacture a GI‐POF with the bandwidth higher than 700 Mbits/s at a distance of 100 m.