bluuebunny/crossref_metadata_2025_split huggingface dataset:
Two‐phase flow model of the cathode of PEM fuel cells using interdigitated flow fields
Authors: He Wensheng , Yi Jung S. , Van Nguyen Trung 
Published: April 2004

AbstractWhen interdigitated gas distributors are used in a PEM fuel cell, fluids entering the fuel cell are forced to flow through the electrodes porous layers. This characteristic increases transport rates of the reactants and products to and from the catalyst layers and reduces the amount of liquid water entrapped in the porous electrodes thereby minimizing electrode flooding. To investigate the effects of the gas and liquid water hydrodynamics on the performance of an air cathode of a PEM fuel cell employing an interdigitated gas distributor, a 2‐D, two‐phase, multicomponent transport model was developed. Darcy's law was used to describe the transport of the gas phase. The transport of liquid water through the porous electrode is driven by the shear force of gas flow and capillary force. An equation accounting for both forces was derived for the liquid phase transport in the porous gas electrode. Higher differential pressures between inlet and outlet channels yield higher electrode performance, because the oxygen transport rates are higher and liquid water removal is more effective. The electrode thickness needs to be optimized to get optimal performance because thinner electrode may reduce gas‐flow rate and thicker electrode may increase the diffusion layer thickness. For a fixed‐size electrode, more channels and shorter shoulder widths are preferred.
Molecular and macroscopic modeling of phase separation
Authors: Escobedo Fernando A. 
Published: April 2004

AbstractRecently proposed pseudoensemble Monte Carlo methods are extended in this work to map out diverse phase diagrams (projections of the phase‐coexistence hypersurface) of multicomponent mixtures required to characterize fluid‐phase equilibrium phenomena and to design separation processes. Within the pseudoensemble framework, the macroscopic models of different equilibrium processes can be readily integrated to the mathematical constraints that specify the thermodynamic state of the system. The proposed Monte Carlo methods allow, for example, the simulation of isopleths and cloud‐point lines to compare experimental to simulation data and to test molecular force fields. Applications of this approach include the study of retrograde phenomena in a model natural‐gas mixture through simulation of dewlines and coexistence lines at constant vaporization fraction. As demonstrated, pseudoensemble simulations can also be used to generate the thermodynamic data necessary to solve problems encountered in continuous and discontinuous distillation processes.
Linear driving force approximation to a radial diffusive model
Authors: Başağaoğlu Hakan , Ginn Timothy R. , McCoy Benjamin J. , Mariño Miguel A. 
Published: April 2004

AbstractTransient volume‐averaged and spatial intraparticle contaminant concentration in a dissolved phase of spherical immobile aggregates is examined using the linear driving force (LDF) model, a special case of a generalized concentration profile (GCP) approximation. Particular cases are solved when a mass flux occurs between the surrounding bulk concentration and intraparticle concentration. Four different forms for the time‐variant surrounding bulk concentration are considered: linear, exponential, sinusoidal (oscillatory) and step function. Both pointwise and volume‐averaged intraparticle concentrations computed by the LDF model match the numerical and analytical (when available) solutions almost perfectly, when the surrounding bulk concentration is assumed to be changing linearly or exponentially. For the other two cases, however, only volume‐averaged concentrations across the immobile spherical particle and the temporal concentrations at the outer boundary of the particle computed from the LDF approximation match the numerical model results reasonably well.
Quantifying deformation in gel swelling: Experiments and simulations
Authors: Achilleos Evdokia C. , Prud'homme Robert K. , Kevrekidis Ioannis G. , Christodoulou Kostas N. , Gee Kyle R. 
Published: April 2004

AbstractA technique for the real‐time visualization of material line movement during transparent gel deformation was developed based on caged photo‐activated fluorophores covalently attached to the gel network. It is used to quantitatively monitor the swelling of sodium polyacrylate and poly(acrylamide‐sodium acrylate) gels in quasi two‐dimensional constrained geometries. The experimentally obtained profiles for the Finger deformation tensor and the polymer concentration are compared with transient simulations of a model of transport in polyelectrolyte gels.
Local transitions in flow phenomena through packed beds identified by MRI
Authors: Johns M. L. , Sederman A. J. , Bramley A. S. , Gladden L. F. , Alexander P. 
Published: April 2004

AbstractMagnetic resonance imaging (MRI) velocity measurement techniques are used to investigate flow of water and a glucose solution, of viscosity 3.89 × 10−3Pa·s, within the interparticle space of a cylindrical packed bed of 5 mm diameter glass ballotini. The experiments were performed over a range of Reynolds numbers from 0.84 to 14.52, spanning the regime where inertial effects begin to play a significant role relative to viscous forces. A transition from creeping to inertial flow occurs in isolated pores at a local Reynolds number, defined for each pore within the interparticle space, of approximately 30. Despite this transition in flow behavior in some pores, the gross features of the flow pattern scale approximately with flow rate, which can be explained by considering the volume of the void space in which the flow is nearly stagnant as largely determining the pressure distribution within the bed.
Bubble formation in high‐pressure liquid–solid suspensions with plenum pressure fluctuation
Authors: Yang G. Q. , Luo X. , Lau R. , Fan L. S. 
Published: April 2004

AbstractThe behavior of bubble formation from a single orifice in a nonaqueous liquid and a liquid–solid suspension with pressure fluctuations in the gas chamber (plenum region) is studied at high pressures (up to 8.3 MPa). An analytical model is developed to account for the initial bubble size in liquid–solid suspensions at high pressures. The model takes into consideration various forces induced by particles, such as suspension inertial force. Both experiments and model predictions indicate that the initial bubble size in the suspension is generally larger than that in the liquid, and that it increases with an increase in the solids concentration. The system pressure has a significant effect on the initial bubble size in liquids or liquid–solid suspensions when bubbles are formed under variable gas flow‐rate conditions, and a negligible effect under constant gas flow‐rate conditions. This model can reasonably describe the initial bubble sizes under high‐pressure conditions measured experimentally in this study and those reported in the literature.
Thermally coupled distillation with reduced number of intercolumn vapor transfers
Authors: Agrawal Rakesh 
Published: April 2004

AbstractThe classical thermal coupling between two distillation columns requires two‐way communication whereby both vapor and liquid streams are transferred in opposite directions between one specific location in each of the distillation columns. A general framework is presented whereby this classical two‐way thermal coupling is converted to a one‐way liquid‐only transfer. This conversion is achieved by adding one or more distillation sections and a reboiler and/or a condenser with each added section. The resulting equivalent configurations coproduce some of the component product streams from more than one distillation column and are quite flexible. Finally, the techniques developed for this framework are applied to reduce the heat duty of the well‐known conventional distillation column configurations with no thermal coupling.
Multieffect distillation for thermally coupled configurations
Authors: Agrawal Rakesh 
Published: April 2004

AbstractA systematic method presented draws multieffect configurations for any thermally coupled multicomponent distillation scheme. For ternary mixtures, all the resulting double‐effect configurations are presented for each of the five known thermally coupled configurations. For most feed conditions, some of these configurations often require substantially lower heat duty than the double‐effect configurations derived from conventional distillation schemes, such as direct and indirect splits and prefractionator. Essential features of the resulting multieffect configurations are that some of the product streams are produced from more than one distillation column and the transfer of at least one mixture stream takes place between intermediate locations in the distillation columns. This method opens up many more multieffect distillation possibilities for multicomponent feed mixtures.
Extreme conditions in binary reactive distillation
Authors: Lee Jae W. , Hauan Steinar , Westerberg Arthur W. 
Published: April 2004

AbstractIn certain cases, reactive distillation can be used to avoid minimum reflux pinch points and azeotropic compositions. A pinch caused when using minimum reflux is always in the nonreactive section of the column. Its location is affected by the extent of reaction occurring elsewhere in the column. This article shows how to encounter such pinch situations by McCabe‐Thiele and Ponchon‐Savarit diagrams. Reactive distillation columns were designed under the extreme conditions of assuming reaction equilibrium and total reflux. Putting more than one reactive stage in such a column can be harmful to column performance for an azeotropic mixture. As a function of the magnitude of the reaction equilibrium constant, when an azeotrope can be circumvented is described. A binary reactive distillation can be bypassed only under finite reflux conditions, with the reaction equilibrium constant being greater than the reaction equilibrium constant evaluated at the azeotropic composition. A rigorous simulation of actual systems is used to confirm the insights provided by these graphical methods.
Analysis of single‐stage nonequilibrium separation process problems
Authors: Sridhar Lakshmi N. , Maldonado Carlos , Garcia Anamarie , de Frias Atilio 
Published: April 2004

AbstractEquations constituting the nonequilibrium rate‐based separation process problem involving binary and ternary mixtures were analyzed. Multiplicity occurring in the nonequilibrium stage resulted from multiplicity taking place in the equilibrium calculations at the interface. This is achieved by demonstrating a one‐to‐one correspondence between the bulk and interface compositions in a nonequilibrium stage. The analysis of the film model for the mass‐transfer problem where the fluxes are obtained using the Maxwell‐Stefan equations showed solution uniqueness for this equation set. It also showed that the imposition of the mass‐transfer equations does nothing to cause additional multiplicities.
Nonisothermal synthesis of homogeneous and multiphase reactor networks
Authors: Mehta Vipul L. , Kokossis Antonis C. 
Published: April 2004

AbstractA new optimization method for the synthesis of nonisothermal reactor networks offers major challenges concerning the lack of robustness in the existing methods and the numerous local and near‐optimal solutions that characterize these problems. The approach offers a reliable and widely tested methodology that upholds a profile‐based search. The search concentrates on targets rather than network details. The results illustrate ways to avoid unnecessarily complicated networks and develop simpler and transparent designs. The single‐phase examples explain merits over past studies and other conventional synthesis methods. Multiphase examples are presented for the first time in the literature, as well as results from an industrial example.
Selectivity effects on series reactions by reactant storage and PSA operation
Authors: Kodde Adriaan J. , Fokma Ymke S. , Bliek Alfred 
Published: April 2004

AbstractThis work evaluates adsorptive reactors used to improve the operation of a sequential reaction scheme,

, for the total removal of A from a stream with an excess of B. In the adsorptive‐reactor concept, the reactor is filled with a physical mixture of catalyst and an adsorbent, the latter being thermodynamically selective toward primary reactant A. In this case, the sorbent is periodically regenerated using the principles of pressure swing adsorption and purged with secondary reactant D. This concept is restricted to low temperatures to have sufficient adsorption capacity. Improved reaction selectivity arises from the accumulation of A in the unit. The reaction of A maximizes the driving force for regeneration and thus accelerates the regeneration half‐cycle. The adsorptive reactor is compared to a conventional plug‐flow reactor (PFR) and to PSA and PFR units in series. Reaction selectivity improved and pure B recovered over these alternative reactors under realistic conditions. The volume‐based productivity is lower than that of PFR, but higher than that of PSA. The purge‐gas flow rate can be manipulated to balance the sorption flux and reaction rate, thereby maximizing the conversion of A. The influence of differences in sorption kinetics is discussed and the required sorbent characteristics are identified.
Examination of stochastic dispersion theory by MRI in aperiodic porous media
Authors: Irwin N. C. , Greenkorn R. A. , Altobelli S. A. , Cushman J. H. 
Published: April 2004

AbstractMagnetic resonance imaging techniques were applied to obtain concentration and velocity field measurements during flow in an aperiodic heterogeneous porous medium. These measurements were used to evaluate the applicability of a stochastic perturbation theory and the validity of the assumptions underlying its derivation. A comparison of experimental moment data to the first and second moments of simulated mean concentration distributions showed that the theory did not match the experimental data. While the results showed general agreement, the stochastic model appeared to slightly overpredict the experimentally observed mixing behavior. Discrepancies between experimental and numerical results were attributed to the assumption that triplet correlation terms involving fluctuating velocities and fluctuating concentration are insignificant relative to terms containing doublet cross‐correlations. Measured velocity covariances were compared to the velocity covariance determined from the first‐order solution to the flow equation. The first‐order relation agreed generally with the measured covariance, but did not accurately predict the detailed covariance structure in the aperiodic heterogeneous model.
Permeation of drug and swelling agent through polymeric membranes
Authors: Wang Da‐Ming , Wang Ching‐Ya , Chu Chwen‐Yen , Yeh Hsiu‐Min 
Published: April 2004

AbstractThe permeation of drug (benzocaine) and swelling agent (ethanol) through the polymeric [poly(ethylene‐co‐vinylacetate) and polyurethane] membranes was investigated theoretically and experimentally. Based on the Flory‐Huggins and Yasuda theories, the swelling effect on permeant diffusivity was described by expressing the permeant diffusivity in powers of the activity of the swelling agent. The experimental results suggest that the leading‐order term (first order of activity) is accurate enough to describe the diffusivities of benzocaine and ethanol in the poly(ethylene‐co‐vinylacetate) and polyurethane membranes. The linear dependence of permeant diffusivity on swelling‐agent activity was further used to analyze the drug diffusion in a membrane subject to a swelling gradient. With the knowledge of the dependence of benzocaine and ethanol diffusivities on ethanol activity, the activity profile of ethanol across the membrane can be calculated, and the effective diffusivity of benzocaine can then be predicted. Experiments carried out to determine the variation in the effective benzocaine diffusivity corresponding to different swelling gradients agreed well with the calculated results.
Mass transport in a novel two‐fluid taylor vortex extractor
Authors: Baier Gretchen , Graham Michael D. , Lightfoot Edwin N. 
Published: April 2004

AbstractFlow instabilities occurring in rotating flows can be exploited as a new approach to liquid–liquid extraction. Two immiscible liquids are radially stratified by centrifugal force in the annulus between corotating coaxial cylinders. When the inner cylinder is rotated above a critical speed, Taylor vortices form in one or both of the fluids. Although the flow pattern yields a relatively small amount of interfacial surface area, the surface is highly active for interphase mass transfer due to the local vortex motion. By adding countercurrent axial flow, efficient continuous processing is also possible. This flow yields a viable extraction process, particularly for fluid pairs that are easily emulsifiable and therefore have limited processing options with the current equipment commercially available. This article demonstrates that two‐fluid Taylor‐Couette flow with countercurrent axial flow is achievable in practice and explores, experimentally and computationally, the mass‐transfer characteristics of the flow. Experimentally, when the vortices first appear, axial dispersion decreases and the interphase mass transfer starts to increase. Upon further increase in differential rotation rate, the extraction performance continues to improve, with the mass‐transfer coefficient proportional to the strength of Taylor vortices. This suggests that very high extraction efficiencies can be obtained with even larger relative rotation rates. Furthermore, mass‐transfer boundary‐layer theory, in combination with computational fluid dynamics, provides a reliable method for predicting the extraction performance.
IDEAS approach to process network synthesis: Application to multicomponent MEN
Authors: Wilson Stevan , Manousiouthakis Vasilios 
Published: April 2004

AbstractA novel method for the globally optimal solution of the general process network synthesis problem is first presented and then applied to the solution of the minimum utility cost (MUC) problem for mass exchange networks (MEN) with multicomponent targets (MT). Previous approaches that address the MUC MEN MT synthesis problem are discussed, and the novel conceptual framework, Infinite DimEnsionAl State‐space (IDEAS), is then introduced. IDEAS allows the formulation of general process network synthesis problems as infinite convex (linear) programs whose local solutions are guaranteed to be globally optimal. In addition, an MUC MEN MT problem where conventional methods fail is probed. Then, the optimal network generated using IDEAS is discussed, together with the wide‐ranging potential impact and flexibility of IDEAS.
Effect of number of fractionating trays on reactive distillation performance
Authors: Al‐Arfaj Muhammad A. , Luyben William L. 
Published: April 2004

AbstractSneesby et al. recently suggested that adding trays in the stripping and rectifying sections of a reactive distillation column can degrade performance. This effect, if true, is not only counterintuitive, but very disturbing because it suggests that the design of reactive distillation columns cannot use conservative estimates of tray numbers, that is, we cannot simply add excess trays, as in conventional distillation. The problem is compounded by the uncertainty in vapor–liquid equilibrium data and tray efficiencies. This implies that developing reactive distillation columns would require extensive experimental work at the pilot‐plant and plant stages to find the numbers of stages offering the best performance. Such a scenario would mean long and expensive development programs. This article explores the effect of the number of trays in the rectifying and/or stripping sections of reactive (catalytic) distillation columns. Three reactive distillation systems are used: an ideal hypothetical system, the ETBE system, and the methyl acetate system. Contrary to the published results, it is demonstrated that additional trays do not degrade performance. Two degrees of freedom available in all cases must be carefully chosen for fair comparisons.
Recycling titanium from Ti‐waste by a low‐temperature extraction process
Authors: Yang Fenglin , Hlavacek Vladimir 
Published: April 2004

AbstractIn the commercial chloride process, titanium is extracted from Ti concentrates at high temperatures (800–1,500°C), and the high‐purity rutile is the primary raw material. Chlorination at high temperatures results in high‐energy consumption, rapid corrosion of equipment, pipelines and control system, and agglomeration of the solid bed by the liquid byproducts. The high‐purity rutile is expensive and its natural deposit is being depleted. Therefore, a low‐temperature Ti extraction process using Ti‐waste as the primary raw material is highly desirable. Thermodynamic equilibrium simulation of the reaction system shows that the extraction of Ti from Ti waste is feasible at a temperature as low as 200°C. In this study, a simple technology was used to remove diffusion barriers, and a low‐temperature chloride process was developed. The chlorination reaction operates at 300–350°C, and Ti‐waste can be used to replace the expensive rutile. Up to 80% of the titanium can be recycled in 5 min at 350°C. The extraction of other components has a relatively low extent, so a selective extraction of Ti can be achieved. A fluidized‐bed reactor was used for the chlorination process. The formation of an activated TiO2–C–Cl complex on the TiO2/C interface accounts for the gas‐solid‐solid reaction mechanism.
Impact of confinement on zeolite cracking selectivity via Monte Carlo integration
Authors: Macedonia Michael D. , Maginn Edward J. 
Published: April 2004

AbstractMonte Carlo integration methods employing a classic molecular mechanics force field were used to probe the impact of confinement on the selectivity of the n‐hexane/3‐methylpentane test reaction in 12 different zeolites. The simulations estimated relative cracking rates and predicted the constraint index (CI) selectivity for both monomolecular and bimolecular cracking mechanisms. By comparing with experimentally measured values of the CI, information on the dominant mechanism responsible for the observed selectivity was obtained. For small‐pore zeolites, the monomolecular mechanism dominated, with the measured CI attributed to reactant selectivity based on preferential adsorption. For large‐pore zeolites, the CI selectivity was attributed to confinement effects imposed on the bimolecular transition states. For zeolite structures with intermediate‐sized pores, such as MTW and TON, the simulations indicated that both reaction mechanisms may be operative. The experimentally observed reduction in the CI with temperature in ZSM‐5 can be explained in terms of the relative importance of steric and entropic factors in the stabilization of the bimolecular transition state. A change in mechanism from bimolecular to monomolecular, as postulated in the literature, is not necessary to explain the experimentally observed temperature dependence of the CI.