bluuebunny/crossref_metadata_2025 huggingface dataset:
Self‐propagating solid‐solid noncatalytic reactions in finite pellets
Authors: Varma Arvind , Cao Giacomo , Morbidelli Massimo
Published: June 2004

The method of self‐propagating high‐temperature synthesis (SHS) can be utilized to synthesize certain advanced ceramic materials, such as metal carbides, borides and silicides, directly from the elements in powder form. The method involves mixing the powders, preparing pellets from the mixture, and igniting one end of the pellet. Owing to high exothermicity of the reactions, a combustion wave can propagate through the pellet. The influence of processing conditions on propagation of the combustion wave is investigated extensively in this modeling study for pellets of finite length. It is shown that unlike infinitely long systems, the combustion wave for a finite pellet does not propagate in constant pattern. It is also shown that radiative heat losses play a major role in SHS, while convective heat losses are not relatively important.
Bouncing motions of liquid drops between tilted parallel‐plate electrodes
Authors: Mochizuki T. , Mori Y. H. , Kaji N.
Published: June 2004

The paper reports on experimental and analytic studies on translational motions of liquid drops in an immiscible, dielectric liquid confined by a pair of tilted parallel‐plate electrodes, across which a steady electric field is being applied. With an increase of the strength of the field, drops begin to bounce back and forth between the electrodes while falling down along the axis of tilt of the electrodes. The bouncing motions are found to be predictable with a reasonable accuracy by using a simple model constructed within the framework of conventional fluid mechanics and electrostatics.
On‐line state and parameter Identification of positive photoresist development
Authors: Carroll Thomas A. , Ramirez W. Fred
Published: June 2004

The development phase of the optical photolithography process has long been considered the most crucial, as it is the final image‐forming step. Process monitoring methods have focused primarily on end point detection and have not used other inferable on‐line information. This paper examines the use of mathematical models in conjunction with on‐line development penetration data to determine process changes. An on‐line sequential parameter identification scheme is used to calculate a current rate parameter value for the development model, and a Kalman filter is used to reduce erroneous observations caused by measurement noise. A powerful development monitor system results from the combination of real‐time data, and on‐line parameter and state estimation theory.
Two‐dimensional analysis of a dust explosion
Authors: Aldis David F. , Gidaspow Dimitri
Published: June 2004

Peak pressures and propagation velocities for dust explosions have for many years been studied as a function of particle concentration. There are, however, no multidimensional models in the literature which can be used to compute local pressures and impluses needed to access the potential damage caused by a detonation.

A two‐dimensional two‐phase hydrodynamic computer model was developed in this study to predict pressure propagation in dust explosions. Two cases of detonation of dispersed powders injected into a rectangular cavity were analyzed. The ignition of an idealized, uniformly distributed powder in a box produced a pressure peak that propagated through the powder with an increasing amplitude from the ignition source at one corner. The computational maximum pressure agreed with an approximate calculation of the Chapman‐Jouguet pressure. The pressure propagation for the more realistic nonuniform initial particle distribution produced several peaks not unlike those observed on the ground below the detonation. Computed pressures agreed roughly with the measured average pressure peaks.
Displacement effect in multicomponent chromatography
Authors: Gu Tingyue , Tsao George T. , Tsai Gow‐Jen , Ladisch Michael R.
Published: June 2004

The study of interference effects is of fundamental importance in understanding multicomponent chromatography. In this work, a displacement effect is examined and shown to be able to explain the dominating interference effects in three major modes of chromatography—frontal, elution, and displacement—involving competitive isotherms. It is concluded that the concentration profile of a component usually becomes sharper due to the displacement effect from another component, while the concentration front of the displacer is usually diffused. Five factors that escalate the displacement effect in multicomponent elution were investigated. A binary elution with a competing modifier in the mobile phase was also discussed. This study was carried out using computer simulations based on a general nonlinear multicomponent rate equation model that considers axial dispersion, external mass transfer, intraparticle diffusion, and Langmuir isotherms. The use of the general model helps the visualization of the multicomponent interactions in chromatography under mass transfer conditions.
Percolation theory of creation and mobilization of foams in porous media
Authors: Rossen W. R. , Gauglitz P. A.
Published: June 2004

A percolation model of foam mobilization in porous media is developed. This model indicates that there is a minimum pressure gradient or, equivalently, a minimum gas velocity required to initiate mobilization of foam. As a result, for most foam enhanced oil recovery processes, where the surface tension is not low, deep foam penetration depends on propagation of foam formed at a high pressure gradient near the well. Low surface tension makes mobilization of CO2 foams feasible, however, at pressure gradients found throughout much of the formation in a typical field application. The theory further predicts, and data confirm, that the minimum velocity for foam mobilization during steady flow of liquid and gas decreases as injected liquid volume fraction increases. The theory suggests a better strategy for foam generation: alternate injection of small slugs of liquid and gas.
Analysis of bioproduct separation using gel‐enclosed adsorbents
Authors: Nigam Somesh C. , Siahpush Ali R. , Wang Henry Y.
Published: June 2004

The concept of “gel‐enclosed” adsorbents was recently introduced for whole fermentation broth processing. The term “gel‐enclosed adsorbents” in this paper refers to small adsorbent particles such as the ones used in affinity or ion‐exchange chromatography enclosed within a protective, spherical, semipermeable gel barrier. The barrier can be in the form of an entrapment matrix where a porous gel matrix in itself contains entrapped adsorbent particles, or in the form of an encapsulating membrane where an aqueous spherical core containing the adsorbent particles is enclosed by a gel membrane. Both experimental and theoretical analyses were carried out to elicit a more fundamental view of the diffusion and adsorption phenomena occurring within these forms of adsorbents. Furthermore, the effective porosity of the gel matrix and the effective diffusivities of the species being separated, as well as their adsorption behavior combined result in what is here termed “kinetic selectivity”.
Polymer melt devolatilization mechanisms
Authors: Albalak Ramon J. , Tadmor Zehev , Talmon Yeshayahu
Published: June 2004

Scanning electron microscopy (SEM) was used to study the mechanism of falling‐strand devolatilization of polymer melts. Polystyrene and low‐density polyethylene were enriched with styrene and hexane, respectively, and were subsequently extruded as thin strands at various conditions. The polymer strands were exposed to superheat conditions for preset periods of time ranging from tenths to tens of seconds. The strands were then frozen, fractured, and studied by SEM. Devolatilization was found to proceed through a blistering mechanism, both on the surface of the strands and on the surfaces of volatile bubbles formed within the core of the melt. At all volatile concentrations surface blistering and foaming preceded bubble growth in the core, and occurred before the strands foamed. A mechanism for the observed polymer melt devolatilization is proposed.
Prediction of liquid circulation in viscous bubble columns
Authors: Rice Richard G. , Geary Nicholas W.
Published: June 2004

A new theory to predict liquid circulation was derived and compared with applicable literature data. A single dimensionless group, ε(gRd2/v2c), is shown to control mixing intensity under low‐viscosity conditions, but two separate groups (ε and gRd2/v2c) are necessary for viscous systems. Closure was accomplished using the Prandtl model for Reynolds stress. Locally varying mixing length was taken to be proportional to bubble size and bubble concentration. The model was constructed based on the premise that two distinct zones exist: a mainly turbulent core joined to a thin viscous wall layer.
Feedforward/feedback control of multivariable nonlinear processes
Authors: Daoutidis Prodromos , Soroush Masoud , Kravaris Costas
Published: June 2004

This paper concerns general MIMO nonlinear processes, whose dynamic behavior is described by a standard state‐space model of arbitrary order, including measurable disturbances. The concept of relative order of an output with respect to an input, extended to include disturbance as well as manipulated inputs, is generalized in a MIMO context and it is used to obtain a characterization of the dynamic interactions among the input and the output variables. A synthesis formula is calculated for a feedforward/state feedback control law that completely eliminates the effect of the measurable disturbances on the process outputs and induces a linear behavior in the closed‐loop system between the outputs and a set of reference inputs. The input/output stability and the degree of coupling in the closed‐loop system are determined by appropriate choice of adjustable parameters. A MIMO linear controller with integral action completes the feedforward/feedback control structure. The developed control methodology is applied to a continuous polymerization reactor and its performance is evaluated through simulations.
Stochastic analysis of a three‐phase fluidized bed: Fractal approach
Authors: Fan L. T. , Neogi D. , Yashima M. , Nassar R.
Published: June 2004

Three‐phase fluidized beds have played important roles in various areas of chemical and biochemical processing. The characteristics of such beds are highly stochastic due to the influence of a variety of phenomena, including the jetting and bubbling of the fluidizing medium and the motion of the fluidized particles. A novel approach, based on the concept of fractals, has been adopted to analyze these complicated and stochastic characteristics. Specifically, pressure fluctuations in a gas‐liquid‐solid fluidized bed under different batch operating conditions have been analyzed in terms of Hurst's rescaled range (R/S) analysis, thus yielding the estimates for the so‐called Hurst exponent, H. The time series of the pressure fluctuations has a local fractal dimension of dFL = 2 − H. An H value of ½ signifies that the time series follows Brownian motion; otherwise, it follows fractional Brownian motion (FBM), which has been found to be the case for the three‐phase fluidized bed investigated.
A two‐dimensional model for the free‐settling regime in continuous thickening
Authors: Fitch Bryant
Published: June 2004

All classical models for thickening assume one‐dimensional continuity. However, the free‐settling domain in continuous thickeners is not one‐dimensional. Therefore, a two‐dimensional model is investigated. The two‐dimensional model gives the same values for thickener area demands as the one‐dimensional model, but the relationships between batch and steady‐state thickening are not the same. Free‐settling Kynch characteristics can arise in the continuous operation that do not arise in batch tests. Therefore, design procedures that rely on Kynch theory, such as that of Talmage and Fitch, are not completely valid.

A method is developed, based on extrapolating the Kynch or free‐settling segment of a batch settling, curve, that yields an improved prediction for thickener area demand.
Optimum mixture compositions for measurement of cross virial coefficients
Authors: Eubank Philip T. , Hall Kenneth R.
Published: June 2004

Random error propagation is examined to determine cross second and third virial coefficients from binary gas density measurements by both the indirect and direct methods. Determination of the cross second virial coefficient B12 from Bm at a single composition via the indirect method is optimized when ye1 = ye2, where e1 and e2 are the pure component uncertainties. When all errors are equal, the equimolar mixture is optimum as expected.

Corresponding analysis of the cross third virial coefficients, C112 and C122, show that the optimum compositions are not (frac13;, frac23;) as expected for equal error distribution, but rather (frac14;, frac34;). Equations presented allow determination of the two optimum compositions, when em varies with composition. Minimization of the combined errors in B12, C112, and C122 via mixture measurements at two compositions provides results near (frac13;, frac23;), when all errors are equal. Methods for direct measurement of B12 are reexamined. Optimal compositions are found using third virial coefficient correlations.
Multiple filter methods for detection of hazardous states in an industrial plant
Authors: King R. , Gilles E. D.
Published: June 2004

Model‐based detection systems are presented and classified according to their complexity. The main emphasis is put on multiple Kalman filters. A two‐stage approach including Bayes formula and inhomogeneous Markovian processes is suggested to achieve a fault discrimination. Methods are presented to use the information of nondisjunctive models in this context. To show the proper working of the detection algorithms simulation studies and on‐line laboratory experiments with runaway reactions are considered first. A report on an implementation at a large‐scale industrial process unit concludes the discussion.
Chromatography theory: Application to supercritical fluid extraction
Authors: Erkey C. , Akgerman A.
Published: June 2004

The adsorption equilibrium constants for naphthalene on alumina have been measured in the presence of subscritical and supercritical carbon dioxide by the dynamic tracer response technique. The equilibrium constants decrease with increases in density, and the isochroic temperature dependency is very small at supercritical conditions. Through chromatography theory, an expression has been developed for the density dependency of the capacity factor, a quantity related to the adsorption equilibrium constant by the porosity and density of the particle and the porosity of the bed, in terms of the partial molar volume of the solute at infinite dilution in the fluid phase. The partial molar volumes extracted from the data agree well with the published data. Investigation of the dynamics of the bed shows that a model, including axial dispersion and effective diffusion into the pores, successfully represents the data. The effective diffusivities and the axial dispersion coefficients were also extracted from the second central moments of the response curves at each condition.
Prediction of binary, overloaded elution profiles using the simple wave effect
Authors: Katti A. M. , Ma Z. , Guiochon Georges
Published: June 2004

The separation of binary mixtures in overloaded elution chromatography has been investigated experimentally and theoretically using the Rouchon‐Golshan‐Shirazi implementation of the semiideal model (equilibrium‐dispersive model). A competitive Langmuir isotherm with parameters determined from single‐component data gives good prediction of the experimental data. Improved predictions, however, can be obtained by directly measuring the binary isotherm using the method of the hodograph transform.
High‐pressure vapor‐liquid equilibrium with a UNIFAC‐based equation of state
Authors: Dahl Søren , Michelsen Michael L.
Published: June 2004

Recent procedures developed by Heidemann (1990) and by Michelsen (1990a, b) enable us to formally incorporate excess Gibbs energy model parameters into a fully consistent equation of state, cith accurate reproduction of the behavior of the excess Gibbs energy model at atmospheric pressure.

This paper investigates the ability of the mixing rules proposed by Michelsen to predict high‐pressure phase equilibrium, when used in combination with the parameter table of modified UNIFAC. Considering that a group contribution method is used for the excess Gibbs energy and that model parameters are extrapolated over a 200 K temperature interval, quite satisfactory results are obtained for the mixtures investigated.
Effective Kundsen diffusivities in structures of randomly overlapping fibers
Authors: Tomadakis Manolis M. , Sotirchos Stratis V.
Published: June 2004

Effective Knudsen diffusion coefficients are presented for fibrous structures consisting of overlapping fibers. The fibers are distributed randomly in d (d=2 or 3) directions with their axes perpendicular to one direction (d=2) or in the three‐dimensional space with no preferred orientation (d=3), or they are grouped into d (d=1, 2, or 3) mutually perpendicular bundles of parallel, randomly overlapping fibers. Effective diffusivities are computed using a Monte Carlo simulation scheme to determine the mean square displacement of molecules traveling in the interior of the porous medium for large travel times. Our results show that structures with fibers distributed randomly in d directions have diffusion coefficients identical, within the accuracy of our simulation, to those of d‐directional, parallel fiber structures. Effective Knudsen diffusivities are strongly influenced only by the directionality of the fiber structure, with tridirectional or randomly oriented fiber structures presenting lower percolation thresholds (0.04 vs. 0.11) and higher effective diffusivities than bidirectional or random structures with their axes perpendicular to one direction. The tortuosity factor is in general found to decrease with increasing porosity, approaching for each case, as the porosity goes to unity, the corresponding lower bound that is derived using variational principles.
Influence of chemical and electrostatic gradients on diffusion in microemulsions
Authors: McGreevy Robert J. , Schechter Robert S.
Published: June 2004

Nonequilibrium diffusion behavior in nonionic and ionic surfactant microemulsion systems has been studied experimentally using the open‐ended capillary method. Experimental results for these systems have been compared with a drop theory of diffusion for microemulsions under conditions where large concentration and electrostatic gradients exist. The results show good agreement in concentration profiles between theory and experiment for the microemulsion components—water, benzene, and phenol. Furthermore, under certain conditions the theory predicts that over a limited time interval phenol will diffuse from low‐concentration regions to regions of higher concentration. This phenomenon has been observed.
Nonlinear principal component analysis using autoassociative neural networks
Authors: Kramer Mark A.
Published: June 2004

Nonlinear principal component analysis is a novel technique for multivariate data analysis, similar to the well‐known method of principal component analysis. NLPCA, like PCA, is used to identify and remove correlations among problem variables as an aid to dimensionality reduction, visualization, and exploratory data analysis. While PCA identifies only linear correlations between variables, NLPCA uncovers both linear and nonlinear correlations, without restriction on the character of the nonlinearities present in the data. NLPCA operates by training a feedforward neural network to perform the identity mapping, where the network inputs are reproduced at the output layer. The network contains an internal “bottleneck” layer (containing fewer nodes than input or output layers), which forces the network to develop a compact representation of the input data, and two additional hidden layers. The NLPCA method is demonstrated using time‐dependent, simulated batch reaction data. Results show that NLPCA successfully reduces dimensionality and produces a feature space map resembling the actual distribution of the underlying system parameters.
Analysis of microwave heating of materials with temperature‐dependent properties
Authors: Ayappa K. G. , Davis H. T. , Davis E. A. , Gordon J.
Published: June 2004

Transient temperature profiles in multilayer slabs are predicted, by simultaneously solving Maxwell's equations with the heat conduction equation, using Galerkin finite elements. It is assumed that the medium is homogeneous and has temperature‐dependent dielectric and thermal properties. The method is illustrated with applications involving the heating of food and polymers with microwaves. The temperature dependence of dielectric properties affects the heating appreciably, as is shown by comparison with a constant property model.
Modeling and simulation of NOX absorption in pilot‐scale packed columns
Authors: Suchak N. J. , Jethani K. R. , Joshi J. B.
Published: June 2004

Absorption of nitrogen oxides was studied in three packed columns in series. Two were 254 mm ID and 6 m tall, the third was 800 mm ID and 3 m tall. Absorption was also studied in two packed columns of 800 mm ID, 7 and 10 m tall, operated in series. Solutions of mixed nitric and sulfuric acids were used as absorbents. Specific rates of absorption were measured using a stirred cell with a flat interface.

A mathematical model is developed for an adiabatic operation. The gas phase reactions and equilibria, gas phase mass transfer, interface equilibria, and liquid phase reactions are included in the model. Heterogeneous gas‐liquid equilibria are included in the model for the first time. The variation in the rates of absorption with chemical reaction (of NO2, N2O3, and N2O4) with respect to acid concentration is considered. The formation of nitric acid in the gas phase is also considered in the model. Favorable agreement is shown between the model predictions and the experimental observations.
Effect of unfavorable continuous phase density gradient on axial mixing
Authors: Holmes Timothy L. , Karr Andrew E. , Baird Malcolm H. I.
Published: June 2004

Field experience with several industrial reciprocating plate liquid‐liquid extraction columns indicated that large density gradients in the continuous phase resulted in excessive backmixing. Since there are no published data on the effect of higher density in the upper section compared to lower density in the bottom section of the column, experiments were performed with the calcium chloride brine‐water system in a 76.2 mm diameter reciprocating plate column.

Axial dispersion coefficients, E, were found to be 10–20 times higher than the values in systems with uniform densities. E decreased with increasing agitation to 30% of the value with no agitation. The effect of increasing flow rate of water up the column was to increase both E and the Peclet number. E was correlated with the density difference between any point in the column and tap water. Another correlation was obtained showing the effects of buoyant and mechanical energy input on axial dispersion.
An analytic model of direct‐contact heat transfer in spray‐column evaporators
Authors: Mori Yasuhiko H.
Published: June 2004

The heat transfer to, and the resultant evaporation of, drops of a volatile liquid sprayed upward in an immiscible liquid flowing down in a vertical column are analyzed to enable calculation of the volumetric heat transfer coefficient in the column. For this analysis, a model is contrived that assumes no nucleation delay in initially monodispersed drops and a heat transfer to each of the drops, with simultaneous evaporation, that can be approximated by an empirical correlation for heat transfer to an isolated drop evaporating in a quiescent, sufficiently extended medium. The expression obtained for the volumetric heat transfer coefficient is used to predict its values under some particular column operating conditions, which are then compared with relevant experimental data found in the literature.
Fault diagnosis based on qualitative/quantitative process knowledge
Authors: Yu Cheng‐Ching , Lee Chyuan
Published: June 2004

Recent advances in knowledge engineering have led to develop the qualitative (deep) model based diagnostic systems. As process knowledge accumulated, however, the diagnostic system remains strictly qualitative. This limits the usefulness of such systems. In this work, a framework is developed for integrating quantitative process knowledge into the qualitative model. Once quantitative process information, e.g., steady‐state gains, is available, it can be incorporated into the simplest qualitative process model called the signed directed graph. The quantitative process knowledge is described in terms of membership functions of fuzzy set theory. According to the measurement pattern, the truth values of a hypothesis (e.g., a fault origin) can be calculated based on the fuzzy logic. Consequently, the diagnostic resolution can be improved significantly. Furthermore, the proposed method becomes a strictly qualitative diagnostic system, if no quantitative information is available. A chemical reactor example illustrates the design and performance of the qualitative/quantitative model‐based diagnostic system. The proposed approach can also be extended to the multiple‐fault situations in a straightforward manner.
Ignition and extinction of surface and homogeneous oxidation of NH3 and CH4
Authors: Williams William R. , Zhao Jin , Schmidt Lanny D.
Published: June 2004

Ignition, extinction and autothermal behavior of surface and homogeneous oxidation reactions of CH4, NH3, and mixed NH3/CH4 fuel systems in air over resistively heated Pt foils was studied in an atmospheric pressure flow reactor. Also examined were the effects of varying fuel concentration on the ignition, extinction, and autothermal surface temperatures. Heterogeneous ignition occurred at ∼200°C for NH3 and at ∼600°C for CH4. The mixed fuel systems exhibited two surface ignitions (∼200°C and ∼600°C), implying a sequential ignition of the two fuels. The effects of removing one of the fuels during autothermal operation gave results consistent with this hypothesis.

In homogeneous ignition, two types of flames were observed: a boundary layer flame associated with NH3 systems and an independent flame observed in all systems. Homogeneous ignition of the two fuels and the mixtures occurred at surface temperatures ranging from 1,100°C to 1,600°C and appeared to be dominated by CH4 in the mixed systems. A generic behavior is proposed for homogenous‐heterogeneous combustion of single and mixed fuels.
Partial internal wetting of catalyst particles: Hysteresis effects
Authors: Jaguste D. N. , Bhatia S. K.
Published: June 2004

Condensation‐evaporation hysteresis associated with the internal wetting of catalyst particles is examined here by the Kelvin and Cohan equations. It is shown that in the absence of information on the thermal history, it is difficult to predict the extent of internal wetting and the associated reaction rate, for states with partial internal wetting, with the actual rate lying anywhere between the limits specified by the Kelvin and Cohan equations. Hysteresis effects arising from thermal cycling are discussed, and the predictions are shown to agree with the experimental data of Kim and Kim (1981a). Experimentally observed transitions between states with markedly different extents of internal wetting and with associated sudden evaporation or condensation are also rationalized by the theory.
Pillared clays as a new class of sorbents for gas separation
Authors: Yang R. T. , Baksh M. S. A.
Published: June 2004

High‐surface‐area, Zr‐pillared, layered clays are synthesized and characterized for their adsorption properties. Although large free interlayer spacings are claimed in the literature (as also found in this work, 14.3 Å), the limiting pore size is the narrow interpillar spacing. The distribution of interpillar spacing is determined by molecular probing and adsorption data along with a theoretical framework available from the literature. Interpillar spacing can be tailored by controlling the number density of pillars inserted during the ion exchange (oligomer inserting) step. The following variables in the ion exchange solution result in lowering the pillar density: higher pH, lower oligomer concentration, and introduction of competitive cations. By changing these variables, peak interpillar spacing is shifted by nearly 2 Å (from 5 to 7 Å). The versatility of pillared clays as sorbents for kinetic separation (i.e., separation based on diffusivity differences) has been demonstrated by the separations of air and xylene isomers.
Fractional extraction with hollow fibers with hydrogel‐filled walls
Authors: Ding Hongbing , Cussler E. L.
Published: June 2004

Extractions using hollow fibers can be faster and more efficient than those in conventional equipment. These advantages, due to the large area per volume possible with fibers, can be compromised by accidental convection through the fibers' pores. When these pores are filled with gels of crosslinked polyvinyl alcohol, convection through the pores is stopped but the overall mass transfer is unaltered. The separations across these gel‐filled fiber walls provide excellent yield and purity, especially in the case of fractional extraction.
Experimental study of the tubular multiphase catalyst
Authors: Cini Philippe , Harold Michael P.
Published: June 2004

A new type of multiphase catalyst is developed for transport‐controlled, volatile‐reactant‐limited reactions: the tubular‐supported ceramic membrane. A microporous, catalytically‐impregnated γ‐Al2O3 film (permeable membrane) coats the inside wall of a hollow, macroporous α‐Al2O3 tube. The gas containing the volatile reactant flows through the tube core, and the liquid containing the nonvolatile and dissolved volatile reactants flows on the shell side. Reaction occurs in the film. Results using the hydrogenation of α‐methylstyrene to cumene on Pd/Al2O3 reveal that the tubular catalyst compares favorably to conventional catalyst designs. The results demonstrate that the main benefit is the more efficient supply of the volatile reactant. Apparent activation energy analyses reveal that for catalyst temperatures below 40° C, the rate is kinetically‐controlled and catalyst utilization nearly complete. A reduction in the activation energy above 45°C indicates the onset of transport limitations or heat effects. A comparison between the tubular catalyst and a fully‐wetted pellet reveals rate increases by up to a factor of 20.
Gas‐solid flow in vertical tubes
Authors: Pita Jorge A. , Sundaresan Sankaran
Published: June 2004

A computational study of fully‐developed flow of gas‐particle suspensions in vertical pipes was carried out, using the model proposed recently by Sinclair and Jackson (1989), to understand the predicted scale‐up characteristics. It was shown that the model can capture the existence of steady‐state multiplicity wherein different pressure gradients can be obtained for the same gas and solids fluxes. A pronounced and nonmonotonic variation of the pressure gradient required to achieve desired fluxes of solid and gas with tube diameter was predicted by the model, and this is explained on a physical basis. The computed results were compared with the experimental data. The model manifests an unsatisfactory degree of sensitivity to the inelasticity of the particle‐particle collisions and the damping of particle‐phase fluctuating motion by the gas.
Dynamics of carbon dioxide sorption on activated‐carbon particles
Authors: Gray Peter G. , Do Duong D.
Published: June 2004

A theoretical model comprising the processes of micropore, macropore and surface diffusion was derived and applied to the dynamics of adsorption of CO2 on a single activated‐carbon particle. The effects of a nonlinear isotherm (Langmuir), particle nonisothermality and concentration dependence of the surface diffusivity were also incorporated. The theoretical model was fitted to a preliminary set of adsorption experimental data (two particle sizes, and various temperatures and concentrations) to determine the diffusion coefficients. The validity of the theoretical model was checked by using the model, with fitted diffusivities, to simulate dynamic adsorption curves at conditions different from those of the preliminary fitting data set. These model simulations were found to predict the experimental data in a satisfactory manner.
Zn/Br2 cell: Effects of plated zinc and complexing organic phase
Authors: Kalu Egwu E. , White Ralph E.
Published: June 2004

A model is presented for a zinc/bromine cell that considers the effects of an increase or a decrease in the cathode channel width due to zinc removal on discharge and zinc deposition on charge, respectively. The model also includes the effect of an organic bromine complexing agent (OCA) on the cell performance. Changes in the channel width affect the catholyte velocity, cathode side pressure drop, mass transfer and potential drop in the cell, while the inclusion of the bromine complexing organic phase shows a marked effect on the available bromine in the aqueous phase.

It is shown that during discharge, the release of complexed Bromine by the OCA could degrade the cell performance. A simple equation is derived and used to express the relationship between the total bromine in the organic phase and the bromine in the aqueous phase.
Melt crystallization: Process analysis and optimization
Authors: Gilbert Samuel W.
Published: June 2004

Melt crystallization manufacturing systems are characterized mathematically and optimized. Directional solidification and sweating are modeled, and the results are correlated to plant data. A nonlinear programming algorithm is constructed to determine the optimal design and operation of a production plant. Process constraints include product purity, process yield, capital cost, phase equilibria, kinetic, and operational factors. An objective function is constructed from scaled yield and capital outlay formulae, and is optimized over a variably constrained feasible space. The dependence of the optima on the separation distribution coefficients and on the constraints is given for specific one‐stage examples for three operational regimes. The one‐stage analyses are applied for the solution of the n‐stage problem.
Quantitative description of ultrafiltration in a rotating filtration device
Authors: Holeschovsky Ulrich B. , Cooney Charles L.
Published: June 2004

A correlation was developed to quantitatively describe the flux in a high‐speed rotating filtration device using a minimum set of parameters. The experimental results were found to be consistent with the concentration polarization (CP) model. Beyond a threshold pressure flux ceases to depend on membrane permeability. The CP model was modified to include the concentration dependence of the diffusivity. This approach was found to be consistent with the strong dependence of flux on pH. Protein concentration in the polarized layer adjacent to the membrane surface was estimated using a procedure that corrects for some of the inconsistencies in the methods usually applied. Four dimensionless numbers were necessary to correlate the experiments with good accuracy. Previously‐reported correlations used only three dimensionless numbers. Usage of four numbers could be justified by dimensional analysis. Finally, the performance of rotary or vortex filtration was compared to that of other configurations.
Flow through mixed fibrous porous materials
Authors: Ethier C. Ross
Published: June 2004

Creeping flow through a composite porous material composed of two fiber types has been analyzed for the situation in which the radii of the two fiber populations are highly dissimilar. The composite material has been idealized as a porous matrix that is composed of fine fibers (permeability Kf), into which cylindrical inclusions (radius ac) have been embedded, and flow through the resulting system has been modeled by the Debye‐Brinkman equation. Two different methods of accounting for the random geometry of a real porous material have been considered: a unit cell approach and replacement of the coarse fibers by a periodic fiber lattice. Model results are in good agreement with each other and reduce in the appropriate limit to well‐accepted models for flow in single‐component fibrous materials. Plots of net composite permeability as a function of fiber diameter and packing density are presented. In the parameter ranges of interest, viscous effects at the coarse fiber surfaces lead to a significantly lower overall permeability than that predicted by a simple application of Darcy's law.
A group contribution approach to computer‐aided molecular design
Authors: Gani Rafiqul , Nielsen Bjarne , Fredenslund Aage
Published: June 2004

A flexible and structured methodology for the computer‐aided molecular design (CAMD) by the group contribution approach is presented. The proposed CAMD algorithm takes the following four steps: 1. preselect groups and target properties; 2. generate only a feasible set of compound structures in an optimal fashion; 3. predict properties for the screening of the set of feasible compound structures; and finally 4. select/design the compound. Since the success of any CAMD algorithm depends to a large extent on its ability to predict/compute the needed properties, the importance of the choice and applicability of these methods is considered along with the computational aspects related to the development of a computer program based on the proposed methodology. Finally, the scope of CAMD technology is highlighted using several practical examples.
Experimental evidence for and prediction of micromixing effects in precipitation
Authors: Marcant Bruno , David René
Published: June 2004

The influence of micromixing on precipitation is predicted qualitatively for batch or semibatch precipitations by means of a simplified mixing model, which considers primary nucleation as the main process affected significantly by hydrodynamics. Unwanted micromixing effects are avoided to a maximum in a batch configuration designed to have a volume mixing ratio α close to unity. Using a semibatch precipitator or a batch with a high α value dramatically accentuates mixing interactions. For mixing, changing the feed point location is shown to have much stronger influence than changing the stirring speed and constitutes an efficient test of micromixing. Experimental evidences for these conclusions have been established for calcium oxalate monohydrate batch precipitation. In particular, the early seconds of the reaction (when mainly primary nucleation occurs) have been demonstrated as conditioning the subsequent behavior of the precipitation, even for induction times of a few minutes.
Nonlinear predictive control of uncertain processes: Application to a CSTR
Authors: Sistu Phani B. , Bequette B. Wayne
Published: June 2004

Nonlinear predictive control (NLPC) is an effective strategy for controlling nonlinear chemical processes with constraints and time delays. In this article, a number of important issues in NLPC are addressed, emphasizing continuous stirred tank reactors (CSTR's) with parametric and model structure uncertainty. In particular, the effects of various choices for the initial conditions are discussed. The selection of initial conditions is particularly important in the presence of plant/model mismatch. A nonlinear programming‐based approach is used for process identification. The effect of model structure uncertainty is included in our analysis by using a cascade control structure on the coolant temperature. Our CSTR results indicate that a simple PI (with anti‐reset windup) cascade loop on coolant temperature is adequate for operation over a wide range of operating conditions. A particularly interesting result of this work is that a predictive controller based on an open‐loop observer can be used to stabilize an open‐loop unstable process, although this is not recommended in practice.
Microwave thawing of cylinders
Authors: Pangrle B. J. , Ayappa K. G. , Davis H. T. , Davis E. A. , Gordon J.
Published: June 2004

The finite element method was used to model microwave thawing of pure‐water and 0.1‐M NaCl cylinders. The electromagnetic field was described by Maxwell's equations with temperature‐dependent dielectric properties, while the heat equation, coupled with the Stefan and Robin conditions, was used to describe the thawing process. An additional equation for the frozen volume fraction was used, when necessary, to account for the presence of a mushy region. Two microwave frequencies, 915 MHz and 2,450 MHz, were examined and the microwave radiation was assumed to be radially isotropic and normal to the surface of the cylinder. Results show that a two‐phase mushy region may exist, and an additional thawing front may appear at the center of the cylinder. Salt cylinders have a higher dielectric loss than pure‐water cylinders and therefore thaw more quickly. Internal resonance occurs when the wavelength of the radiation is a harmonic of the cylinder radius. Resonance increases power deposition and expedites the thawing process. The onset of resonance alters thawing times and complicates the development of heuristic rules for microwave thawing.
Microstructural descriptors characterizing granular deposits
Authors: Tassopoulos Menelaos , Rosner Daniel E.
Published: June 2004

Quantitative results are presented on mean coordination number and coordination number distribution, contact normal distribution and fabric tensor of simulated anisotropic granular deposits with resulting solid fractions between ca. 15% for ballistic deposits and 58%, corresponding to a random loose packing. The deposits, generated by the capture of uniform size spherical particles arriving normal to a target, were simulated using a simple algorithmic model.

We focus on microstructural quantities which explicitly take into account the discrete nature of the granules comprising the deposit. Such measures are important in determining the heat transport properties of the deposits for Fourier conduction through the solid phase, as well as their mechanical and sintering properties. The variation of mean coordination number with deposit solid fraction was successfully correlated using a unit‐cell model (Eq. 13). This correlation, in conjunction with entropy maximization arguments (after Nayak and Tien, 1978) has been further used to predict the coordination number distribution. The usefulness of the results reported here is illustrated by computing an upper bound to the deposit effective thermal conductivity (Jagota and Hui, 1990) and comparing it to both ‘exact’ simulation results (Tassopoulos and Rosner, 1991b) and experimental data (Koh, 1971).
Composite membranes for fuel‐cell applications
Authors: Verbrugge Mark W. , Hill Robert F. , Schneider Eric W.
Published: June 2004

One of the major obstacles to overcome for the realization of economical hydrogen‐oxygen, polymer‐electrolyte fuel cells is the high capital cost of the inert perfluorosulfonic acid (PSA) membranes, which provide a pathway for ionic transport between the cell electrodes. It has recently been shown that composite polymer membranes can be synthesized by depositing PSA polymers onto porous poly(tetrafluoroethyene) (PTFE) substrates. The resulting membranes are mechanically durable and quite thin relative to traditional PSA membranes; we expect the composite membranes to be of low resistance and cost. In this experimental study, we examine the composite membrane properties as a function of the membrane composition. Our results allow us to form a conceptual model to explain both the equilibrium and transport characteristics of these materials. For high PSA contents, the membrane behavior is similar to that of the PSA polymer; the water permeability, however, is reduced significantly. For intermediate PSA contents, the membranes have a high porosity and match the thickness of the PTFE substrate (≈50 μm); membranes of this composition range are potentially useful candidates for fuel cells because of their high resistance to water transport and reduced ionic resistance. Composite membranes of very low PSA content demonstrate characteristics similar to the hydrophobic PTFE substrate and are not of interest for fuel cells.
Lattice model for the adsorption of benzene in silicalite I
Authors: Lee Chung‐Kung , Chiang Anthony S. T. , Wu F. Y.
Published: June 2004

A lattice gas model is used to describe the adsorption of benzene in silicalite I. Using the complete equivalence of the lattice gas with an Ising model and the known exact as well as mean‐field results of the latter, we study the thermodynamics of the adsorption and establish criteria on the adsorption energies and site interactions for the existence of a two‐phase region. It is shown that a phase change occurs when zigzag paths connecting interacting adsorption sites become energetically favorable. Our model also leads to a sharp rise of the adsorption heat at the transition, albeit at a level less enhanced than experimentally observed.
Determination of the hindered settling factor for flocculated suspensions
Authors: Landman K. A. , White L. R.
Published: June 2004

The determination of the settling velocity of particles in suspension is a difficult problem due to the complexity of the indirect particle–particle interactions. Formally, a function called the hindred settling factor r (ϕ), which is a function of the solids volume fraction ϕ, is introduced to take into account the increase in the drag forces. The current method for estimating this function is to measure the initial fall rate of the suspension in a batch sedimentation experiment. Here, we propose a steady‐state experiment for flocculated suspensions using vacuum filtration, where the fixed bed height is measured as a function of the liquid flow rate through a porous membrane. Recovering the function r (ϕ) or a related function D (ϕ) involves numerical differentiation of the data. This experiment will also yield the membrane permeability as a function of volume fraction (in cases where clogging or fouling of the membrane occurs). The results using numerically simulated data show good recovery of the assumed r (ϕ) function.
Optimal catalyst activity profiles in pellets: 9. Study of ethylene epoxidation
Authors: Gavriilidis Asterios , Varma Arvind
Published: June 2004

The concept of optimal catalyst activity distribution is studied experimentally for the ethylene epoxidation reaction network on a Ag/α–Al2O3 catalyst in a single‐pellet reactor. The Dirac delta‐type distribution of the catalyst is approximated by a step distribution of narrow width. For a fixed amount of silver, the influence of location and width of the catalytically active layer on the conversion of ethylene and on the selectivity and yield to ethylene oxide is investigated under oxygen‐rich conditions in the temperature range 210–270°C. The results clearly demonstrate that for optimum selectivity and yield to ethylene oxide, the silver catalyst should be placed in a thin layer at the external surface of the pellet.
Modeling of isotropic reacting turbulence by a hybrid mapping‐EDQNM closure
Authors: Frankel Steven H. , Jiang Tai‐Lun , Givi Peyman
Published: June 2004

A hybrid model is developed and implemented for predicting the limiting bound of the reactant conversion rate in an isotropic turbulent flow under the influence of a reaction of the type A + B Products. This model is based on the amplitude mapping closure of Kraichnan for the molecular mixing of a stochastically distributed scalar, and the eddy‐damped quasi‐normal Markovian (EDQNM) spectral closure for the two‐point scalar covariance. The results predicted by this model compare well with available experimental data in both gaseous and aqueous plug‐flow reactors, but point to the need for more detailed measurements in future experimental studies. With the implementation of the mapping closure, a simple analytical expression is obtained for the decay rate of the unmixedness. This expression is very convenient and is recommended for direct practical applications in the modeling and design of plug‐flow reactors.