bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nonlinear transport processes in disordered media
Authors: Sahimi Muhammad 
Published: June 2004

AbstractNonlinear transport processes in disordered systems such as porous media and heterogeneous solids are studied, which are represented by two‐ or three‐dimensional networks of interconnected bonds, by a Bethe network (a branching network with no closed loops) of a given coordination number, or by a continuum in which circular or spherical inclusions have been inserted at random. The bonds represent the pores of the pore space, or the conducting and insulating regions of a disordered solid, to which we assign effective properties (radii or conductances) selected at random from a probability density function. Three types of nonlinear transport processes are considered. (1) The relation between the current q and the potential gradient v is of power‐law type (as in, for example, flow of power‐law fluids or the electric current in doped polycrystalline semiconductors). (2) The relation between q and v is piecewise linear, characterized by a threshold (as in flow of Bingham fluids or in mechanical or dielectric breakdown of composite solids). (3) A large v is imposed on the system, so that a linear transport theory is not valid. The behavioral study of the effective transport and topological properties of the system, such as the permeability, conductivity, diffusivity, and the shape of the samplespanning cluster of conducting paths shows that in all cases the concepts of percolation theory play a prominent role, even if the system is well connected and percolation may seem not to play any role. For most cases, new effective‐medium approximations (EMAs) are derived for estimating effective transport properties. Compared to the case of linear transport, new EMAs are considerably more accurate in predicting the scaling properties of the transport coefficients near a critical point such as the percolation threshold. For a power‐law transport process, an exact solution is also derived for the Bethe networks. Using the concepts of percolation theory, scaling laws relating the effective properties to various regimes of transport and to topological properties of the system are also given. A relation between the volumetric flow rate of a power‐law fluid in porous media and the macroscopic pressure drop is derived, which contains no adjustable parameter and is valid at any porosity. To test the accuracy of our analytical predictions, Monte Carlo simulations are carried out for several cases. In most cases, good agreement is found between the simulation results and predictions. The extension of the results to other types of nonlinearities is also discussed.
Physically representative network models of transport in porous media
Authors: Bryant Steven L. , Mellor David W. , Cade Christopher A. 
Published: June 2004

AbstractWe calculate permeabilities for a class of granular porous media derived from a real, disordered packing of equal spheres. The entire structure, including pore space, of these media is completely specified by the radii and spatial locations of the constituent grains. When geometric nearest neighbor grains are grouped together, the structure may be subdivided into pore bodies and pore throats in a natural and unambiguous way. From this subdivision we can establish a network of flow paths whose geometry and topology are completely specified, so that permeability and other transport coefficients can be calculated directly and without any adjustable parameters. The calculations focus on processes that form porous media, rather than on specific examples of such media. Hence, the approach is essentially predictive, rather than correlative. No additional measurements (such as capillary pressure data or pore system data from thin sections) are required, and correlations between permeability and other properties are not used. Predicted permeabilities match measurements on sandstone samples similar to the model perous media studied here over a wide range of porosity. Geometrical attributes of the network representation of the pore space of the model media are found to be spatially correlated. This departure from randomness significantly affects permeability. The agreement between predictions and measurements suggests that spatial correlation is inherent in granular porous media and that uncorrelated network models are therefore unlikely to be physically representative of such media.
Robust design of binary countercurrent adsorption separation processes
Authors: Storti Giuseppe , Mazzotti Marco , Morbidelli Massimo , Carrà Sergio 
Published: June 2004

AbstractThe separation of a binary mixture, using a third component having intermediate adsorptivity as desorbent, in a four section countercurrent adsorption separation unit is considered. A procedure for the optimal and robust design of the unit is developed in the frame of Equilibrium Theory, using a model where the adsorption equilibria are described through the constant selectivity stoichiometric model, while mass‐transfer resistances and axial mixing are neglected.By requiring that the unit achieves complete separation, it is possible to identify a set of implicit constraints on the operating parameters, that is, the flow rate ratios in the four sections of the unit. From these constraints explicit bounds on the operating parameters are obtained, thus yielding a region in the operating parameters space, which can be drawn a priori in terms of the adsorption equilibrium constants and the feed composition.This result provides a very convenient tool to determine both optimal and robust operating conditions. The latter issue is addressed by first analyzing the various possible sources of disturbances, as well as their effect on the separation performance. Next, the criteria for the robust design of the unit are discussed. Finally, these theoretical findings are compared with a set of experimental results obtained in a six port simulated moving bed adsorption separation unit operated in the vapor phase.
Impact of catalyst acid/metal balance in hydroisomerization of normal paraffins
Authors: Degnan T. F. , Kennedy C. R. 
Published: June 2004

AbstractThe hydroisomerization of n‐heptane has been used as a model to study the acidmetal balance requirements for Pt/zeolite catalysts. Experiments were carried out using physical mixtures of zeolite beta and Pt/Al2O3 particles, in which the level of the metal‐containing component was varied to change the acid‐metal balance. Selectivity, in terms of cracking vs. isomerization, varies significantly with the metal loading up to a point beyond which the degree of paraffin isomerization is independent of metal loading. These results are interpreted in terms of a simple dualsite model based on a classical theory of polyfunctional catalysis, first advanced by Weisz (1962). The model is also used to show how an imbalance in hydrogenation and acid functions can even alter the apparent reaction network of the observable chemical species in the system.
Characterization of flow‐through electrode processes by AC impedance
Authors: Yuh C. Y. , Selman J. R. 
Published: June 2004

AbstractAC‐impedance method has been used successfully in characterizing a packed‐bed flow‐through electrode system. A macrohomogeneous model was developed to simulate the effect of structural, physical and flow parameters. The relative importance of kinetics and mass transfer can be inferred from the AC‐impedance analysis. Kinetic information about copper deposition in supported cupric sulfate solution has been obtained successfully using this technique.
Theoretical study of recycling in preparative chromatography
Authors: Seidel‐Morgenstern Andreas , Guiochon Georges 
Published: June 2004

AbstractUsing the equilibrium‐dispersive model of chromatography, we have investigated recycling as a means to improve the recovery yields and possibly the production rates of chromatographic separations. When compared with the optimal values obtained for the conventional single‐cycle regime, our results prove the potential for recycling in preparative chromatography. The analysis performed for the separation of two binary mixtures demonstrates that recycling not only improves the recovery yield but can increase the production rate under certain conditions. Different possibilities of collecting sufficiently separated fractions during recycling are compared. The mathematical model developed can be used effectively for designing optimal recycling strategies in preparative liquid chromatography.
Nonlinear product property control in industrial gas‐phase polyethylene reactors
Authors: McAuley K. B. , Macgregor J. F. 
Published: June 2004

AbstractA nonlinear model‐based scheme is developed for product property control in industrial gas‐phase polyethylene reactors. The controller regulates instantaneous melt index and density, and provides servocontrol during grade changeovers. Hydrogen and butene feed rates are manipulated to force the product properties onto desired trajectories. During grade changeovers, these trajectories are determined from off‐line dynamic optimization studies. Optimal open‐loop policies for reactor temperature, bleed stream flow, catalyst feed rate, and bed level are implemented as part of the changeover strategy.The nonlinear feedback controller design is based on global input/output linearization methods. Disturbances are estimated, and plant/model mismatch is removed using an extended Kalman filter. Simulations on a complex mechanistic model of the process reveal that the nonlinear controller performs well for both regulatory and servocontrol. An analogous linear IMC controller is inadequate for disturbance rejection at different operating conditions and for control during grade changeovers. The simplicity of the nonlinear control algorithm makes it an interesting candidate for industrial application. This single controller can be used to control the properties of many grades of polyethylene made in the reactor and to accomplish near optimal changeovers between these grades.
Activity coefficients in dilute aqueous solutions from free energy simulations
Authors: Lazaridis Themis , Paulaitis Michael E. 
Published: June 2004

AbstractThe free energy perturbation method with Monte Carlo simulations has been used to calculate relative solvation free energies and ratios of activity coefficients of organic solutes at infinite‐dilution in water at 25°C. Systems studied include hydrocarbons, chlorinated hydrocarbons, and alcohols. Analysis of the free energy simulations reveals that a major limitation of current methodologies is our inability to determine a priori the electron density distribution of a molecule in a condensed, polarizing medium. Moreover, currently employed simulation lengths may be inadequate for convergence of the averages in some aqueous systems. These problems do not, however, appear to be insurmountable. In contrast to group contribution methods, free energy simulations are still rather primitive to be applied in a routine fashion for the quantitative prediction of thermodynamic properties, but dramatic improvements in the technique are still to be made.
Computation of tricritical points in ternary systems
Authors: Kohse Bruce F. , Heidemann Robert A. 
Published: June 2004

AbstractAt tricritical points, four equations in the thermodynamic variables must be satisfied. Methods of evaluating the functions and solving these four equations have recently been developed (Michelsen and Heidemann, 1988). A modified approach to the evaluation of these functions and a new solution procedure for the calculation of tricritical points in equation of state models are presented here. The new solution procedure is evaluated in comparison to previous ones, and its application is made using the Peng‐Robinson and Soave‐Redlich‐Kwong equations of state. The new procedure proved capable of locating tricritical points in a variety of systems with complex multiphase behavior. These included ternary systems with single tricritical points, systems with an ordinary critical point occurring at the same composition as a tricritical point and systems capable of four‐phase equilibria with two tricritical points. Comparisons are made with experimental tricritical data for eight systems. The calculations prove to be highly sensitive to pure component and binary parameters in the equation of state models. Features of phase diagrams for tricritical mixtures are discussed as well as the predictions of tricritical points in several homologous series of ternary systems.
Mechanistic model of slug flow in near‐horizontal pipes
Authors: Andreussi P. , Minervini A. , Paglianti A. 
Published: June 2004

AbstractA mechanistic model of slug flow based on a set of mass and momentum conservation equations and a number of empirical closure relations is presented. It is then shown that a simplified version of the model, which physically corresponds to the flow of long liquid slugs(long slug model, LSM), can be immediately derived from the general model. The LSM is much simpler than the general model; at the same time, the predictions obtained in the two cases are very similar, when the comparison is limited to the computation of the pressure gradient and the mean liquid holdup. The LSM successfully correlates a large set of experimental measurements relative to three pipe diameters(18, 50 and 90 mm), four inclinations (0°, ±3°, 0.3°), and two pipe lengths(17 and 34 m).
A simple model of cross‐flow filtration based on particle adhesion
Authors: Stamatakis K. , Tien Chi 
Published: June 2004

AbstractA simple model presented for cross‐flow filtration of liquid suspensions was formulated on the premise that among the particles convected to the filter medium surface in cross‐flow filtration, only a fraction of them become deposited. A criterion based on the interplay of the geometry of the cake‐suspension interface and various forces acting on a particle as it moves toward the interface was established and an expression of the adhesion probability of impacting particles developed. The model was then formulated by incorporating the adhesion probability information together with conventional cake filtration theories. Through sample calculations, the model was found to display behavior consistent with the observed phenomena of cross‐flow filtration and capable of representing experimental results.
Pyrolysis of scrap tires and conversion of chars to activated carbon
Authors: Merchant Akbar A. , Petrich Mark A. 
Published: June 2004

AbstractThe primary objective of this work was to demonstrate the conversion of scrap tires to activated carbon. We have been successful in this endeavor, producing carbons with surface areas greater than 500 m2/g and significant micropore volumes. Tire shreddings were pyrolyzed in batch reactors, and the pyrolysis chars activated by reaction with superheated steam. Solid products of pyrolysis and activation were studied with nitrogen adsorption techniques. We find that the porosity development during steam activation of tire pyrolysis char is similar to that reported for various other chars. A maximum in micropore volume is observed as a function of conversion, but the total surface area increases monotonically with conversion. We suggest that the activation process consists of micropore formation, followed by pore enlargement. The process conditions used in this study are a good starting point from which to optimize a process to convert tires to activated carbon.
Particle dispersion in a binary solid‐liquid fluidized bed
Authors: Asif Mohammad , Petersen James N. 
Published: June 2004

AbstractThe mixing and segregation behavior of liquid fluidized beds containing two different‐sized particle species are described mathematically. This model is based on a mass balance, rather than a volume balance, which has been used by most previous researchers. The model also includes the effects of density gradients that are present in the bed. Particle dispersion coefficients in a binary‐solid fluidized bed are evaluated using this model. Further, a correlation that allows the calculation of these particle dispersion coefficients is presented, which describes not only dispersion coefficients calculated using the above model, but also those presented by other researchers. It is applicable over a broad range of variables of practical interest and is based on relevant dimensionless parameters to account for the influence of important particle and fluid properties on the particle dispersion characteristics.
Size‐Dependent growth and contact nucleation of calcite crystals
Authors: Tai Clifford Y. , Chen Pao‐Chi , Shih Shin‐Min 
Published: June 2004

AbstractThe growth kinetics and contact nucleation of calcite crystals were studied in a pH‐stat crystallizer. Large seed crystals up to 230 μm were successfully prepared by the gel growth technique. The crystal growth data over a wide range of crystal sizes were evaluated from titration curves by an initial slope method and then analyzed by the two‐step model. The mass‐transfer and surface integration rates were found to be functions of crystal size with the exception of mass‐transfer rates of small crystals less than 10 μm. The controlling step of crystal growth process was tested using the effectiveness factor concept. Large seed crystals of calcite induced contact nucleation when a stainless‐steel impeller was used for agitation. The important features of contact nucleation for calcite crystals were explored and compared with those of soluble substances.
Application of integral‐equation theory to aqueous two‐phase partitioning systems
Authors: Haynes C. A. , Benitez F. J. , Blanch H. W. , Prausnitz J. M. 
Published: June 2004

AbstractA molecular‐thermodynamic model is developed for representing thermodynamic properties of aqueous two‐phase systems containing polymers, electrolytes, and proteins. The model is based on McMillan‐Mayer solution theory and the generalized mean‐spherical approximation to account for electrostatic forces between unlike ions. The Boublik‐Mansoori equation of state for hard‐sphere mixtures is coupled with the osmotic virial expansion truncated after the second‐virial terms to account for short‐range forces between molecules.Osmotic second virial coefficients are reported from low‐angle laser‐light scattering (LALLS) data for binary and ternary aqueous solutions containing polymers and proteins. Ion‐polymer specific‐interaction coefficients are determined from osmoticpressure data for aqueous solutions containing a water‐soluble polymer and an alkali chloride, phosphate or sulfate salt.When coupled with LALLS and osmotic‐pressure data reported here, the model is used to predict liquid‐liquid equilibria, protein partition coefficients, and electrostatic potentials between phases for both polymer‐polymer and polymer‐salt aqueous two‐phase systems. For bovine serum albumin, lysozyme, and α‐chymotrypsin, predicted partition coefficients are in excellent agreement with experiment.
Dean vortices in curved tube flow: 5. 3‐D MRI and numerical analysis of the velocity field
Authors: Chung Kun Yong , Belfort Georges , Edelstein William A. , Li Xianming 
Published: June 2004

AbstractThe velocity contour profiles within Dean vortices produced by flowing water in a solid‐walled curved tube are quantitatively measured. Magnetic resonance flow imaging using flow encoding with spin warp imaging is used in two and three dimensions to obtain these velocities. These experimental measurements are then compared with simulations obtained from solutions of the Navier‐Stokes and continuity equations for three axial flow rates. Numerical solutions were obtained with a finite volume method for curved tube Poiseuille flow.
Optimization of the countercurrent moving‐bed chromatographic separator
Authors: Fish Barry B. , Carr Robert W. , Aris Rutherford 
Published: June 2004

AbstractA dispersionless, adsorption equilibrium, plug‐flow model of a countercurrent moving‐bed chromatographic separator operating at steady state is investigated. The flux of the constituents of a binary mixture is examined via concentration‐phase plane plots, which reveal a complex behavior depending on a parameter ó, the ratio of solid‐ and fluid‐phase flow of the mobile phase. When the model is modified by incorporating finite mass‐transfer rates, axial concentration profiles exhibit behavior that is characteristic of experimental observations, rather than the predictions of the ideal model. Numerical simulations with the finite mass‐transfer model reveal an optimum operating condition.
Role of energy distribution in multicomponent sorption kinetics in bidispersed solids
Authors: Hu Xijun , Do Duong D. 
Published: June 2004

AbstractThe heterogeneous macropore, surface and micropore diffusion model recently proposed by Hu et al. (1993) for single‐component systems is exiended to multicomponent systems in the description of sorption kinetics of gaseous adsorbates in bidispersed (macropore and micropore diffusions), heterogeneous microporous particles. This model describes sorption of gaseous adsorbates with allowance for the energy distribution of adsorption site for both equilibrium and diffusion of the adsorbed species. A uniform distribution is used to describe this energy distribution, and the cumulative energy distribution function is assumed the same for all species in the analysis of multicomponent mixtures. Extensive experimental data of ethane and propane in Ajax activated carbon are collected to study the effects of energy distribution on the binary adsorption, desorption, and displacement dynamics.
Output estimation using multiple secondary measurements: High‐purity distillation
Authors: Mejdell Thor , Skogestad Sigurd 
Published: June 2004

AbstractMeasurements of temperatures (secondary outputs) and flows (inputs) are used to estimate product compositions (outputs) in a distillation column. The problem is characterized by strong collinearity (correlation) between temperature measurements and an ill‐conditioned model from inputs to outputs. In a linear study, three estimator methods, the Kalman‐Bucy filter, Brosilow's inferential estimator, and principal component regression (PCR), are tested for performance with μ‐analysis. One can achieve remarkably good control performance with the static PCR estimator, which is almost as good as the dynamic Kalman filter. The quality of the estimate for these two estimators is improved by additional temperature measurements, although the improvement is only minor for more than about five measurements. On the other hand, the performance of the Brosilow inferential estimator may not improve by adding measurements due to sensitivity to modeling errors. For all estimators, the use of flow (input) measurements does not improve the estimator performance and does in fact damage the performance if a static estimator is used.
Analysis of steady‐state reaction fronts in a porous medium
Authors: Escobedo F. , Viljoen H. J. 
Published: June 2004

AbstractA pseudohomogeneous model is used to analyze the steady state of a sharp reaction front in a porous medium. The reaction rate is described by the flame sheet approximation, and an asymptotic matching analysis is used to determine the temperature, conversion and position of the reaction front. Both adiabatic and nonadiabatic operations are considered, and the effect of radiation is included in the adiabatic case. The results show that there exists a maximum molar flux before blowout occurs. The critical molar flux is decreased by higher activation energies, but it is increased by higher rate constants and flame temperatures. Radiation further stabilizes the flame against blowout. Expressions are provided for the flame temperature, reactant conversion and the conditions when blowout occur. It is also shown that under certain conditions the front has two steady‐state positions.