bluuebunny/crossref_metadata_2025 huggingface dataset:
A discretized population balance for continuous systems at steady state
Authors: Hounslow Michael J.
Published: June 2004

This paper is concerned with the solution of the population balance for continuous systems at steady state, in which the active mechanisms are nucleation, growth and aggregation. A discretized population balance, initially proposed by Hounslow et al. (1988a) for batch systems, is adapted for use with continuous systems at steady state. It is shown that simultaneous nucleation and growth can be described very effectively by the discrete equations. Criteria are developed for the selection of the optimal size domain. A simple modification to the original discrete equations describing growth, permits the modelling of size‐dependent growth effects. Both size‐independent and size‐dependent aggregation are described by the discrete equations with three significant‐figure accuracy. The complete set of discrete equations is used to simulate the nucleation, growth and aggregation of Nickel Ammonium Sulphate. It is shown that analysis by the approximate model must lead to underestimation of the nucleation and growth rates.
Ultrapurification of SiCl4 by photochlorination in a bubble column reactor
Authors: Mixon David A. , Bohrer Michael P. , O'Hara Patricia A.
Published: June 2004

The development and performance of a bubble column photoreactor for ultrapurification of SiCl4 is described. Trichlorosilane (SiHCl3), the principle contaminant in SiCl4 commercially available for use in optical fiber manufacture, is converted to SiCl4 and HCl by UV‐initiated photochlorination. Studies of the homogeneous photochlorination kinetics and of Cl2 gas absorption in a bubble column were undertaken. In addition, an extensive set of data was collected from continuous‐flow bubble column reactor (BCR) runs. The rate of photochlorination in the BCR was found to be mass‐transfer‐limited. A simple model of BCR performance, incorporating the effect of Cl2 mass transfer rate enhancement by chemical reaction, was tested that agrees well with the experimental data. The BCR was found to be operable over a wide range of SiHCl3 concentrations and SiCl4 feed rates, and capable of lowering the concentration of SiHCl3 in product to below measurable levels (<1 ppm).
Unifying indicator and instantaneous reaction methods of measuring micromixing
Authors: Shenoy U. V. , Toor H. L.
Published: June 2004

Instantaneous chemical reactions, either themselves or together with chemical indicators, have previously been used as probes of turbulent micromixing. Both methods obtain statistics of the micromixing from measurement of time mean quantities. The mean reactant concentration, which is the quantity measured in the instantaneous reaction method, is shown here to be the integral with respect to feed concentration ratio of the mean color density, which is the quantity measured in the chemical indicator method. Differentiation of the mean color density gives the scalar probability density while integration gives the mean reactant concentration. Measurements of the color density of bromothymol blue at the centerline of a turbulent jet of base mixing with acid are used as an example.
Restricted diffusion through granular materials
Authors: Limbach Kirk W. , Wei James
Published: June 2004

The restricted diffusion of nickel etioporphyrin is experimentally measured in six granular catalyst support materials. These materials include five γ‐aluminas and one small‐pore silica‐alumina. Pore size distribution is narrow for three of the supports and roughly bimodal for the remaining three. Diffusivities are determined by a transient uptake technique utilizing a Freundlich equilibria adsorption isotherm.

Experimentally determined diffusivities agree well with restricted diffusion theory based on a circular pore model when the appropriate characteristic length scales are used to describe the molecule and pore size. For the restricted diffusion of dilute solute molecules in neutral pores, the appropriate measure of granular pore size is the hydraulic pore radius. Molecule size, in this case, is best described by a mean projected molecule radius for partitioning and a Stokes‐Einstein radius for hydrodynamic aspects of restricted diffusion.
Synthesis of multivariable nonlinear controllers by input/output linearization
Authors: Kravaris Costas , Soroush Masoud
Published: June 2004

This work concerns the synthesis of nonlinear controllers for multivariable nonlinear processes that make the closed‐loop system linear in an input/output sense. Necessary and sufficient conditions for input/output linearizability via static state feedback are derived as well as formulas for the feedback law. Once such a static state feedback is applied to the process, an external multivariable linear controller with integral action can control it to set point. The proposed control methodology is tested through simulations in a semibatch copolymerization reactor example.
Experimental study and parameterization of interfacial gas absorption
Authors: Caussade B. , George J. , Masbernat L.
Published: June 2004

Experiments of simultaneous absorption of carbon dioxide and helium, in transient absorption regimes, have been carried out in a gas‐liquid flume for stratified‐type cocurrent flows. There was a need to improve a method of chromatographic analysis for helium and CO2 titrations in the gaseous phase and the liquid phase; this led us to study simultaneous absorption of CO2 and He independent of dynamic conditions. In addition the dominant wave characteristics were studied and the interfacial friction velocity was measured. It was confirmed that, for two gases presenting a large contrast in diffusivities, gas absorption rates are related directly to the physical properties of the gases. The mass transfer coefficient varies with the − 1/2 exponent of the Schmidt number for a large interfacial stress and a clean interface whereas it varies with the − 2/3 exponent for weaker winds.
Spatial instabilities in the regeneration of diesel‐particulate filters
Authors: Saunders James H.
Published: June 2004

An instability is proposed that can lead to channel formation during the regeneration of monolithic diesel‐particulate filters. This instability, which occurs during the transport‐controlled combustion of the soot bed, is compared to a kinetically driven instability by using a numerical model for channel growth. Both mechanisms are shown to be important and may lead to higher thermal stresses in the filter wall. It is suggested that both mechanisms can be controlled by the ratio of the filter‐plate to soot‐bed flow resistance, lateral heat transfer, and other design parameters.
Computer simulation of isothermal fluidization in large‐scale laboratory rigs
Authors: Richner D. W. , Minoura T. , Pritchett J. W. , Blake T. R.
Published: June 2004

Finite‐difference computer models have been used by several investigators to predict hydrodynamic mixing in fluidized beds. In the present study the finite‐difference model CHEMFLUB is used to simulate cold, isothermal mixing in axisymmetric fluidized beds. The bed geometries and the operating conditions are based upon those of two large‐scale laboratory cold flow rigs. The finite‐difference model is an iterative, implicit formulation to solve continuum equations describing the gas and particle dynamics of fluidization. Comparisons with experimental data are presented. It is shown that the model provides a good prediction of jet penetration height, bubble frequency, and bubble velocity.
Microencapsulation in polyurea shell by interfacial polycondensation
Authors: Yadav S. K. , Suresh A. K. , Khilar Kartic C.
Published: June 2004

A model and a simple measurement technique for time of encapsulation have been developed to study the microencapsulation of butachlor in polyurea shell by means of interfacial polymerization. The model is based on diffusion of the hydrophilic monomer through the polymeric shell with an interfacial reaction at the inner surface while the measurement technique is based on the indirect determination of the concentration of the hydrophilic monomer in the continuous phase by monitoring the pH. Measurements show that capsule sizes ranging from 1 to 20 μm can be produced, and the surface to volume mean size varies only from 2 to 6 μm for a large variation in rpm of the agitator. Time of encapsulation is found to be approximately proportional to the microcapsule size, and it varies from 150 to 300 s. Both the data and the model were used to discern that the process is kinetically‐controlled by and large. It is also shown that time of encapsulation varies with the square of the capsule size in a diffusion‐controlled regime.
Ceramic processing: An overview
Authors: Rice Roy W.
Published: June 2004

Processing of ceramics is reviewed from a broad perspective, with emphasis on the predominant powder‐based methods, but with considerable attention to other methods. Major stages of the powder process, from preparation, modification, and handling, to sintering or pressure densification (or postdensification) are discussed. Nonpowder‐based methods, such as chemical vapor deposition, melt, and polymer pyrolysis processing, are also presented. Processing of ceramic composites is reviewed, noting the shifts in processing technology this entails. Practical aspects, such as part size, shape, volume, and the costs of various processes are also addressed.
Magnetic resonance imaging and modeling of flow in hollow‐fiber bioreactors
Authors: Heath C. A. , Belfort G. , Hammer B. E. , Mirer S. D. , Pimbley J. M.
Published: June 2004

Nuclear magnetic resonance flow imaging (MRFI) was used to measure fluid flow noninvasively in the extracapillary space (ECS) of a hollow‐fiber bioreactor without cells. Agreement between these axial flow measurements in a single hollow‐fiber module and predicted axial velocity contour plots at various image planes along the path length was good.

Flow in a solid‐wall tube (phantom) was first used to calibrate pixel intensities with axial velocities. Flow images at several locations along the permeable hollow fiber length were then obtained in order to observe the well‐known leakage or Starling flow in the ECS. These quantitative spatially dependent velocity measurements were then compared to theoretically derived velocities obtained from a solution of Poisson's equation with a constant pressure gradient and no slip at the solid surfaces. A mathematical transformation was used to simplify the numerical methods. Leakage flow through the ECS of a multifiber bioreactor (40 fibers) was also measured by MRFI, illustrating the applicability of this method for optimizing operational procedures and design of membrane bioreactors and filtration devices.
Pitch‐to‐diameter effect on two‐phase flow across an in‐line tube bundle
Authors: Dowlati Ramin , Kawaji Masahiro , Chan Albert M. C.
Published: June 2004

Void fraction and friction pressure drop measurements were made for an adiabatic, vertical two‐phase flow of air‐water across two horizontal, in‐line, 5 × 20 rod bundles, one with a pitch‐to‐diameter ratio, P/D, of 1.3, the other 1.75. For both bundles the average void fraction showed a strong mass velocity effect and values were significantly less than those predicted by a homogeneous flow model. All void fraction data were found to be well correlated, with no P/D effect, using the dimensionless gas velocity, j*g. The two‐phase friction multiplier data exhibited a strong effect of P/D and mass velocity, however, the data for both bundles could be well correlated with the Martinelli parameter for G > 200 kg/m2s. The correlations developed for void fraction and two‐phase friction multiplier were successfully tested in predicting the total pressure drop in boiling freon experiments.
Conical moving‐bed shale retort model
Authors: Anderson David H. , Sapre Ajit V. , Kennedy Clinton R. , Krambeck Frederick J.
Published: June 2004

This two‐dimensional model of a moving‐bed shale oil retort accounts for the major physical interactions of convective heating and kerogen decomposition coupled with the shale and gas flow distributions. Data in a small‐sale apparatus support a solids flow model of potential flow, although the exiting boundary conditions are uncertain. The calculations show that efficient retorting will only occur if the upper shale surface is shaped so that the shale particle velocity is perpendicular to this surface, and parallel to the entering retort gas flow. Even in the ideal flow case, performance is quite sensitive to gas/solids flow ratio. This model could be used with measured temperature distributions to optimize the surface profile.
Gas phase and catalytic ignition of methane and ethane in air over platinum
Authors: Griffin Timothy A. , Pfefferle Lisa D.
Published: June 2004

The ignition of gas phase combustion by a heated catalytically active surface involves dynamic coupling of transport processes and chemical kinetics. Researchers have observed that surfaces with high catalytic activity require higher surface temperatures to ignite gas phase combustion, compared to nonactive surfaces, and that this difference is greatest for a stoichiometric mixture. In this work an analysis of the ignition of surface‐catalyzed oxidation is developed along with an experimental technique for independently monitoring both surface and gas phase ignition such that the dynamics of these two events can be decoupled. Experiments suggest that the sharp maximum in surface temperature required for gas phase ignition observed in previous work is an artifact caused by transient heating of the surface as ignition occurs. Analysis of surface ignition was also used to investigate the kinetics of methane and ethane oxidation on platinum.
Effects of osmotic pressure and adsorption on ultrafiltration of ovalbumin
Authors: Nabetani Hiroshi , Nakajima Mitsutoshi , Watanabe Atsuo , Nakao Shin‐Ichi , Kimura Shoji
Published: June 2004

The influence of osmotic pressure and solute adsorption on permeate flux during ultrafiltration of a solute that forms a gel‐like layer on membrane surfaces was investigated. Ovalbumin solutions were ultrafiltrated using three different kinds of membranes, polyolefine, polysullfone, and polyacrylonitrile. Flux data were analyzed by three conventional models: gel polarization, osmotic pressure, and resistance in series.

The experimental data were in conflict with all conventional models. Data analysis suggested that the main factors which influence permeate flux decline were osmotic pressure increase at the membrane surface and resistance caused by solute adsorption.

A new model, which takes into account both osmotic pressure and resistance due to solute adsorption, is proposed. Permeate flux declines observed in the experiments fit this model quite precisely. The gel layer that forms on the membrane surface has little influence on permeate flux and can be neglected.
Electrokinetic dispersion in capillary electrophoresis
Authors: Datta Ravindra , Kotamarthi Veerabhadra R.
Published: June 2004

A “Taylor” dispersion‐type model is developed for the electrokinetic dispersion coefficient of a solute in capillary electrophoresis that accounts for the effects of Poiseuille and/or electro‐osmotic flow of the elutant for the case of low ζ potential. The expression obtained for the height equivalent of a theoretical plate is compared with experimental results reported in the literature for the case of neutral, nonretained solutes propelled by electro‐osmotic flow. The results further reveal some interesting and somewhat unexpected interactions of the electro‐osmotic and Poiseuille components of the elutant flow. Superposition of Poiseuille flow on the natural electro‐osmotic flow allows greater freedom in the choice of elutant velocity, without necessarily increasing the dispersion. In fact, it results in lower dispersion under some conditions. Optimum flow conditions are obtained for minimizing the plate height or, equivalently, for maximizing the Peclet number.
Thermal integration of homogeneous azeotropic distillation sequences
Authors: Knapp Jeffrey P. , Doherty Michael F.
Published: June 2004

Existing methods for synthesizing thermally‐integrated distillation sequences fail frequently when applied to nonideal and azeotropic systems, because increasing the column pressures often introduces new azeotropes and distillation boundaries into the mixture, which make some separation tasks infeasible. A new thermal integration procedure is presented that combines a bifurcation and residue‐curve map analysis with the methods of Andrecovich and Westerberg. The procedure is demonstrated with two commercially important separations: ethanol‐water‐ethylene glycol and methanol‐acetone‐water. Energy savings of 65% and 40%, respectively, over the optimized nonintegrated sequences are possible.
Near‐critical phenomena and resolution in supercritical fluid chromatography
Authors: Kelley F. D. , Chimowitz E. H.
Published: June 2004

The analysis in this paper shows the relationship between molecular resolution in supercritical fluid chromatography (SFC), and certain divergent thermodynamic properties of solutes dissolved in near‐critical fluids. This divergence is most pronounced in the near‐critical regime, leading to the hypothesis that this region provides most promise for heightened molecular resolution, which lies at the heart of analytical and separation technologies.

A thermodynamic model for representing capacity factor data in SFC is proposed, spanning a wide density range. The model uses a perturbed hard sphere equation for the fluid phase and lattice concepts for the stationary phase. In addition, data are presented for a number of systems and discussed in terms of the analysis provided. For an isomeric system, the data shows that resolution between species increases with the onset of retrograde behavior, resulting in a distinct maximum point.
Rationalization for the molecular weight distributions of Coal pyrolysis liquids
Authors: Darivakis George S. , Peters William A. , Howard Jack B.
Published: June 2004

A model for coal thermal depolymerization is described and used to predict molecular weight distributions (MWD) of primary coal pyrolysis liquids. The model assumes that rupture of kinetically similar bonds in the coal is the preferred pathway for release of oligomers which ultimately become tars or extractables. The analysis provides the liquids MWD as a probability function of finding different oligomers composed of whole integer numbers of monomers and it is in excellent qualitative agreement with measured MWDs of coal pyrolysis liquids. The predicted MWDs are of the form of the gamma distribution function (GDF), and its parameters are interpreted in terms of the postulated chemistry of coal thermal depolymerization. Consequently, successful empirical correlations of experimental data on coal pyrolysis liquids MWDs with the GDF are better understood. Thus a protocol is provided to predict MWDs of pyrolysis liquids from knowledge of coal molecular structure.
Kinetics of absorption of hydrogen sulfide into aqueous ferric sulfate solutions
Authors: Asai Satoru , Konishi Yasuhiro , Yabu Tadahiro
Published: June 2004

Absorption experiments of H2S into aqueous Fe2(SO4)3 solutions, concerned with a unique process capable of regenerating by biological oxidation the reactant from the products, were performed at 20–40°C in an agitated vessel with a flat interface. The absorbents were aqueous solutions of Fe2(SO4)3 with and without the addition of K2SO4, KOH or H2SO4 of various concentrations, which was added to change the ionic strength or pH of the solutions. The experimental results revealed that the species which reacts with H2S is FeOH2+. While comparing the observed absorption rates with the theoretical predictions, it was found that the reaction is irreversible and first order with respect to H2S and FeOH2+. The reaction rate constants were measured and correlated.
Turbulent mixing with multiple second‐order chemical reactions
Authors: Heeb Thomas G. , Brodkey Robert S.
Published: June 2004

A molecular‐based statistical simulation program was developed to study the covariance terms involved in the mass balance equations for complex chemical reactions during mixing. Several closure theories were compared to the simulations and available experimental data. The simple closure by Brodkey and Lewalle was found to be an extension of Toor's analysis applied to two reactions. This closure does not satisfy the molar fluctuation balance equation and was found only to represent the high Reynolds number data of Li and Toor. This result led to examining other possible closures which were based on Damkoehler numbers, reaction rate constant ratios, and limiting forms of the covariance term. These closures also were inadequate. The second reaction's covariance term varied from the product of the average values for each component to the Brodkey and Lewalle value for the range of Reynolds numbers considered.
Influence of molecular shape on probing mass transfer resistances on zeolites
Authors: Kärger Jörg , Pfeifer Harry , Stallmach Frank , Bülow Martin , Struve Peter , Entner Roland , Spindler Herbert , Seidel Rüdiger
Published: June 2004

Mass transfer resistances in zeolite NaCaA after different hydrothermal pretreatment are probed by NMR tracer desorption studies with the short‐chain‐length hydrocarbons from methane to n‐butane and with xenon. In all cases, the measurements lead to the coinciding result of forming surface barriers, which differ only slightly depending on the pretreatment modes, but lead to a retardation of molecular mass transfer more dramatically than the starting material. The dramatic retardation of mass transfer after hydrothermal pretreatment is confirmed by supplementary uptake measurements with n‐decane.
Statistical‐mechanical model of protein precipitation by nonionic polymer
Authors: Mahadevan Hari , Hall Carol K.
Published: June 2004

A theoretical approach to predict the solubility of proteins in solutions containing nonionic polymers is presented. The effective protein‐protein interaction due to the presence of the polymer is related to the volume‐exclusion potential of Asakura and Oosawa. Statistical‐mechanical perturbation theory, as originally applied by Gast et al. to model colloidal flocculation, is used to calculate free energies, from which solubility curves for varying protein‐polymer diameter ratios are obtained. The theory correctly predicts all the trends observed in experimental studies of these systems. To explain the influence of processes parameters such as the pH and the ionic strength on protein solubility, the intermolecular potential is improved by the addition of an electrostatic interaction term. It is found that theoretical predictions of the variation in protein solubility, both with the solution pH and the ionic strength, are in accordance with experimental observations.
Two‐dimensional numerical study of cross‐flow filtration combustion
Authors: Dandekar Hemant W. , Puszynski Jan A. , Hlavacek Vladimir
Published: June 2004

Transition metal nitrides can be synthesized in the combustion regime using a self‐propagating, high‐temperature synthesis by the process of filtration combustion. A detailed two‐dimensional numerical study for the filtration combustion process in the cross‐flow configuration is presented. The process is modeled by a two‐dimensional pseudohomogenous model. The model equations, which are very stiff and exhibit varying time and length scales, are solved using finite differences on a two‐dimensional orthogonal adaptive mesh. The effects of pressure, bed height and bed porosity on combustion front propagation and final conversion are studied. A series of full simulations for the titanium‐nitrogen system shows good agreement between theoretical predictions and experimental observations.
Revised vapor‐liquid equilibrium model for multicomponent formaldehyde mixtures
Authors: Hasse H. , Hahnenstein I. , Maurer G.
Published: June 2004

Formaldehyde is one of the most important intermediate products of the chemical industry. Due to its high reactivity, it is commonly handled in aqueous or methanolic solutions. In these solutions, formaldehyde is dissolved chemically. Therefore, modeling thermodynamic properties of aqueous and methanolic formaldehyde‐containing mixtures require the consideration of chemical reactions and physical effects. A recently published physico‐chemical model (Maurer, 1986) for the description of vapor‐liquid equilibria of these systems is tested and improved on the basis of about 140 new experimental data points for mixtures containing formaldehyde, water, methanol, and trioxane at temperatures between 320 and 380 K and pressures below 100 kPa. Improvements are achieved by fitting some of those binary interaction parameters, which formerly had to be estimated due to the lack of experimental data. The revised model is able to reliably predict vapor‐liquid equilibria in multicomponent formaldehyde‐containing mixtures.
Adsorption of amine and paints on H‐form resin from electrodeposition waste water
Authors: Yoshida Hiroyuki , Shimizu Kazuyuki , Kataoka Takeshi
Published: June 2004

Wastewater of electrodeposition painting process contained diisopropanolamine, unknown three paints, and solvents (butylcellosolve and/or ethylcellosolve). The recovery of the amine and paints from the real wastewater by adsorption on an H‐form strong acid ion exchanger appeared feasible technically and economically.

Equilibrium isotherms and breakthrough curves have been determined experimentally. The selectivity for the amine adsorption on the resin was fairly high. Solvents were not adsorbed on the resin. The components of the wastewater showed typical breakthrough curves for multicomponent system. Theoretical breakthrough curves and intraparticle effective diffusivities of the amine were determined from the analytic solution for rectangular isotherm in the single‐component system. Experiments of adsorption, elution and regeneration also have been repeated. The amine and paints adsorbed on the resin were eluted completely by NaOH aqueous solution. The adsorption capacity of the amine did not change with repeat times, and the intraparticle effective diffusivity of the amine was constant after the second cycle.
Nonisothermal parison inflation in blow molding
Authors: Poslinski Andrew J. , Tsamopoulos John A.
Published: June 2004

Nonisothermal inflation of a viscous annular parison and its cooling and solidification before and upon wall contact are analyzed by solving the unsteady momentum and energy conservation equations. The algorithm simultaneously determines the flow field and temperature distribution within the parison together with the moving surfaces at every time step. Results of the present study indicate that the inflation rate is increased as a result of higher temperature within the parison. However, the instantaneous shape of the parison and the final thickness variation upon mold contact remain virtually unchanged. Due to the temperature dependence of physical properties, the critical factor during inflation is initial temperature distribution within the parison. Inflation resulting in full attachment to a confining mold wall takes only a small fraction of the time required to cool the material to the ambient gas temperature. The transient calculations illustrate how computer simulations may be used to improve design of blow molding operations, and they are in good agreement with available experiments.
Wave propagation in the flow of shear‐thinning fluids down an incline
Authors: Weinstein Steven J.
Published: June 2004

The effect of shear‐thinning rheology on the spatial growth of waves in multilayered flow down an inclined plane is modeled, utilizing the Carreau viscosity constitutive equation. It is shown that waves associated with the free surface propagate as if they were in a Newtonian system, where the viscosity is some average of the varying viscosities in the shear‐thinning layer. This averaging is due to the global effects of shear thinning, such as changes in velocity profile and film thicknesses. In contrast, waves that are associated with the interfaces between adjacent fluid layers are largely affected by the local interfacial viscosities; wave propagation is not governed by some average Newtonian viscosity across the layer. It is found that shear‐thinning rheology can either increase or decrease the growth of waves associated with a fluid‐fluid interface, compared with a purely Newtonian case.
Kinetics and fixed‐bed reactor modeling of butane oxidation to maleic anhydride
Authors: Sharma Ramesh K. , Cresswell David L. , Newson Esmond J.
Published: June 2004

Selective oxidation kinetics of n‐butane to maleic anhydride in air were studied over a commercial, fixed‐bed vanadium‐phosphor oxide catalyst. The temperature range was 573–653 K with butane concentrations up to 3 mol % in the feed, which is within flammability limits but below ignition temperatures.

The rate data were modeled using power law kinetics with product inhibition and included total oxidation and decomposition reactions. Kinetic parameters were estimated using a multiresponse, nonlinear regression algorithm showing intercorrelation effects. The kinetics were combined with independent measurements of catalyst diffusivity and reactor heat transfer using a one‐dimensional heterogeneous reactor model. Model predictions and observed temperatures and concentrations from non‐isothermal pilot plants were compared up to 115 days on stream. Agreement was acceptable with inlet butane concentrations up to 2.7 mol %. For example, runaway was predicted at a salt temperature 3 K higher than observed. Effectiveness factors around the hot spot were estimated at 0.6 with the catalyst surface temperature 2–3 K higher than the average gas temperature.
Pattern formation in catalytic reactors: The role of fluid mixing
Authors: Arce P. , Ramkrishna D.
Published: June 2004

Fluid‐mediated interaction between catalyst particles alone is shown to yield several interesting and significant phenomena in a catalytic reactor that have been generally attributed in the past to direct interaction between particles. Thus, collaborative interaction between particles and the fluid may enhance or abate steady‐state multiplicity, and reverse stability behavior. From the simple setting of a population of particles in a well‐mixed CSTR, it is shown that the catalyst phase in a catalytic reactor is susceptible to very fine pattern formation in the face of steady‐state multiplicity in single particles, which negates the usual assumption that particles exposed to a given fluid have identical states. In a reactor such variability in behavior must be accompanied by a corresponding variability in conversion and selectivity (in multireaction systems) and may have strong implications for reactor control strategies.
Use of a second endothermic reaction to desensitize a tubular reactor
Authors: Van Der Vaart D. R. , Van Der Vaart H. R.
Published: June 2004

Three parametric sensitivity criteria were examined for application to a pseudohomogeneous tubular reactor system, in which both methanol dehydration (exothermic) and methanol dissociation (endothermic) were catalyzed. The addition of the endothermic reaction was shown to lead to a desensitized reactor. It was illustrated, however, that the relative activities of the two catalysts should be somewhat similar to maintain the overall conversion of the resulting reactor for a given residence time.

The three criteria investigated tended to disagree with one another more, as the system was made less sensitive by the addition of more endothermic catalyst. A potential application of the resulting nonsensitive reactor design would be in methanol‐fueled vehicles.
An improved finite difference method for fixed‐bed multicomponent sorption
Authors: Sun L. M. , Meunier F.
Published: June 2004

A new computational procedure based on the finite difference methods is developed to solve the coupled partial differential equations describing nonisothermal and nonequilibrium sorption of multiple adsorbate systems on a fixed bed that contains bidispersed pellets. In this numerical method, a solution‐adaptive gridding technique (SAG) is applied in combination with a four‐point quadratic upstream differencing scheme to satisfactorily resolve very sharp concentration and temperature variations occurring in the case of small dispersing effects. Furthermore, the method resorts to a noniterative implicit procedure for solving the coupling between the column transport equations and the adsorption kinetics inside the pellets, which may be particularly efficient when the particle kinetics equations are highly stiff.

The numerical model will be tested for one‐, two‐ and three‐transition systems. The results are compared to available analytical and equilibrium theory solutions.
Hollow fiber contained liquid membrane separation of citric acid
Authors: Basu R. , Sirkar K. K.
Published: June 2004

Citric acid was successfully separated from an aqueous solution using the hollow fiber contained liquid membrane (HFCLM) technique. In this technique, the organic liquid membrane is contained in the shell side between two sets of hollow fibers; feed solution flows through one set of fibers and the strip solution flows through the other set. Tri‐n‐octylamine diluted in various organic solvents was used as a complexing agent for facilitated transport of citric acid from an aqueous solution. Pure water and aqueous sodium hydroxide were used as stripping agents. Membrane life and stability problems encountered in supported liquid membranes are eliminated. A mathematical model of facilitated solute transport through HFCLM that accounts for the interfacial reversible reaction kinetics and diffusion process inherent in carrier‐facilitated transport is presented. Experimental data agreed well with the theoretical predictions for permeators achieving almost complete solute recovery.
Design and economic trade‐offs of extractive crystallization processes
Authors: Rajagopal Shankar , Ng Ka M. , Douglas James M.
Published: June 2004

A systematic procedure is formulated for the separation of a binary mixture by means of extractive crystallization. It shous that two flowsheet structures can handle systems with a wide variety of solid‐liquid phase behaviors, including components with simple eutectics, multiple eutectics, and compound formation. Design equations are presented for both flowsheet structures. In addition, design variables and constraints are identified. The economic trade‐offs in extractive crystallization processes are examined through the analysis of an example: the complete separation of paraxylene from meta‐xylene using n‐pentane as the extractive solvent. Comparison of our general methodology with the system‐specific designs reported in the literature shows that the latter are process alternatives of the former.
Concentration gradients in supersaturated solutions: A thermodynamic analysis
Authors: Veverka V. , Söhnel O. , Bennema P. , Garside J.
Published: June 2004

The development of a concentration gradient along the vertical axis of a column of quiescent solution is explained within the framework of nonequilibrium thermodynamics. The formulae derived enabled us to determine explicitly the concentration difference between two vertical levels in a liquid column and specific conditions under which the concentration profile is at equilibrium. The concept of solute cluster formation in a supersaturated solution is consistent with the presented theory although the theory itself does not provide any direct or indirect proof of the occurrence of clustering.
Managing qualitative simulation in knowledge‐based chemical diagnosis
Authors: McDowell James K. , Davis James F.
Published: June 2004

Deep knowledge about process behaviors plays an important role in the diagnosis of chemical processes. Cause‐and‐effect reasoning using deep knowledge is useful especially for interacting malfunctions. This work explores the integration of deep knowledge into task‐specific, knowledge‐based architectures for resolving interacting multiple malfunctions and presents a novel methodology called diagnostically focused simulation (DFS). Invoked in an auxiliary manner, DFS uses deep knowledge and performs qualitative simulation in a highly constrained manner. The close integration with other problem solvers is an evolutionary approach to using qualitative simulation in diagnosis and manages a normally computationally‐explosive procedure. Diagnostic results from the compiled problem solver provide a situation‐specific assessment of the chemical process, identify possible malfunction scenarios, and focus on appropriate levels of process detail. DFS effectively demonstrates a balance between run‐time simulation and compiled problem solving in diagnosis.
Window effects in zeolite diffusion and brownian motion over potential barriers
Authors: Nitsche Johannes M. , Wei James
Published: June 2004

Gorring (1973) reported a phenomenon, the “window effect,” whereby the diffusivities of normal paraffins within zeolite T do not decrease monotonically with increasing carbon number N, as would be expected intuitively. Rather, following an initial decrease with N, the diffusivities exhibit a local minimum at C8 followed by a pronounced local maximum at C12. This article presents a theoretical analysis of the window effect and related phenomena, based on an analogy of the configurational diffusion process with an “equivalent” one‐dimensional Brownian motion of a rod through a periodic sequence of potential barriers. Numerical calculations are found to be in reasonable agreement with Gorring's experimental data and quantify his qualitatively‐stated mechanism, namely that n‐alkanes longer than C8 are too large to fit entirely within the potential wells formed by erionite cages and therefore experience smaller energetic barriers to diffusion.
Adsorption in bidisperse‐pore systems
Authors: Petersen Eugene E.
Published: June 2004

Adsorption of a strongly bound absorbate in a bidisperse‐pore system is modeled using a Turner pore structure. This idealized system leads to an analytical solution for the micropore uptake of adsorption that is valid for a part of the adsorption‐time profile. The analytical solution yields a well‐defined criterion that establishes the boundary between an adsorption process dominated by macropore resistance and one dominated by micropore resistance. The range of the analytical solution is extended to include the entire adsorption‐time profile using an approximate analytical solution and two asymptotic solutions.
Isotherm selection for band profile simulations in preparative chromatography
Authors: Jacobson Stephen , Golshan‐Shirazi Sadroddin , Guiochon Georges
Published: June 2004

Equilibrium adsorption data for the N‐benzoyl derivatives of D‐ and L‐phenyl alanine in water/propanol solutions have been determined on a chiral‐selective stationary phase, bovine serum albumin immobilized on silica. These data are well accounted for by an isotherm equation resulting from the sum of two Langmuir terms. The first term corresponds to the chiral‐selective interactions, and the second to the nonselective molecular interactions between the enantiomers and the stationary phase. The individual band profiles of the two enantiomeric amino acids, either pure or in binary mixtures of various compositions, are predicted exactly by an equilibrium dispersive model using this bi‐Langmuir isotherm. A classical Langmuir isotherm fails to predict these band profiles, illustrating the importance to collect accurate adsorption data and to use a correct isotherm model when calculating chromatographic band profiles.
Characterization of coal char gasification rate
Authors: Adschiri Tadafumi , Nozaki Takao , Furusawa Takehiko , Zi‐bin Zhu
Published: June 2004

The transient kinetic technique was applied to the evaluation of both elemental rate constants, kav and kin, and the amount of reactive carbons, nw, which participate in gasification. The dynamic change in gasification rate with increasing conversion is due to the change in nw. Differences in the gasification rates of ten different chars can be explained mostly by the differences in nw, rather than by those of kav or kin. nw increased and approached a maximum value, N, with increasing partial pressure of carbon dioxide. The maximum value, N, related closely to the carbon structure of char, especially to the crystallite size of graphite.
Oxidative pyrolysis of methane to higher hydrocarbons: Effects of water in feed
Authors: Choudhary Vasant R. , Chaudhari Sopan T. , Rajput Amarjeet M.
Published: June 2004

Direct conversion of methane into higher hydrocarbons (ethane, ethylene, and to small extent C3‐4 hydrocarbons) by its partial oxidation with oxygen in the absence of catalyst has been investigated. It was carried out in a quartz flow reactor at atmospheric pressure under nonflame conditions at different temperatures (1,123‐1,273 K), CH4/O2 ratios (1.7‐9.0), space velocities (500‐5,000 h−1), and water concentrations (0.0‐80 mol %) in the feed. No coke deposition was observed in the reactor, but there was a formation of small amounts of tarry matter in the reaction at ≥ 1,223 K, particularly for the lower CH4/O2 ratios (≤3.5). The product distribution in the process was strongly influenced by the dilution of feed with N2 or water. Addition of water in the feed was highly beneficial to obtaining higher yield and/or to selectivity for the higher hydrocarbons and suppressing almost completely the formation of tarry matter.
Reaction‐driven convection in a porous medium
Authors: Farr William W. , Gabitto Jorge F. , Luss Dan , Balakotaiah Vemuri
Published: June 2004

The onset of three‐dimensional reaction‐driven convection in a porous medium is investigated using linear stability theory. The geometries investigated include a finite cylinder and a rectangular parallelepipped of arbitrary aspect ratios. The analysis determines, among other things, the likely modes (flow patterns) to emerge first as a function of reaction parameters and aspect ratios. The flow fields corresponding to three‐dimensional modes are described in detail. Important qualitative differences are found between reaction‐driven convection and the standard Lapwood or Bénard convection due to a temperature gradient applied to the boundaries of the system.

The second part of the work examines numerically reaction‐driven natural convection in a porous two‐dimensional rectangular box. Orthogonal collocation and continuation techniques are used to determine the conduction and convection branches of solutions as a function of the Rayleigh number (Ra), the Frank‐Kamenetskii number (δ) and the aspect ratio (α). The convective solutions (streamlines and isotherms) corresponding to primary, secondary and tertiary bifurcations are presented. The effect of natural convection (Ra) on the ignition point (critical δ value) is determined for three different aspect ratios.
Emulsion layer model for wall heat transfer in a circulating fluidized bed
Authors: Mahalingam M. , Kolar Ajit Kumar
Published: June 2004

An emulsion layer model is presented which predicts the thickness of a downward‐moving emulsion layer along the wall of a circulating fluidized bed, the mean solids velocity, and the solids flux in the layer. Also presented is a heat transfer model which, in combination with the emulsion layer model, predicts the low‐temperature data very well. An alternate slab model proposed for the radiative component in a high‐temperature circulating fluidized bed agrees well with experimental data. The heat transfer predictions of the overall model for such operating parameters as solid circulation flux, suspension temperature, length of the heat transfer surface, superficial gas velocity, and mean particle size are in good agreement with the published data for long surfaces.
Influence of operating and physical variables on interfacial area determination
Authors: Camacho Fernando , Molina Emilio , Valdés Fernando , Andujar José Manuel
Published: June 2004

The influence of various operating and liquid‐phase physical variables on interfacial area ab in cylindrical and spherical gas‐liquid stirred contactors was studied using the sulfite method. Studied were stirring speed, sparger porosity, gas flow rate, liquid‐phase temperature, surface tension, and pH.

The results show that the influence of operating variables (gas flow rate Qg, stirring speed ω, and the average pore radius of the sparger used rp) on ab, for both types of contactors, can be given by the equation:

Dependency of ab on surface tension observed was believed to be caused by a rising rate of bubble coalescence with increased surfactant concentration. The higher the surfactant concentration, the lower ab, tending to a limit that depends on both the type of surfactant and the average size of sparger pore used. Due to the measurement method used, solution pH and temperature were also observed to influence ab.
Electrostatic potentials and protein partitioning in aqueous two‐phase systems
Authors: Haynes C. A. , Carson J. , Blanch H. W. , Prausnitz J. M.
Published: June 2004

A thermodynamic analysis unambiguously relates interfacial‐electrostatic‐potential differences measured with Ag/AgCl capillary electrodes to the equilibrium properties of an aqueous two‐phase system. Interfacial electrostatic potentials were measured as a function of total α‐cyclodextrin concentration in an aqueous two‐phase system containing 9.1 wt. % poly (ethylene glycol), 6.1 wt. % Dextran T‐70, and 1‐mM potassium iodide. An order‐of‐magnitude increase in the interfacial electrostatic potential was observed as the total concentration of α‐cyclodextrin increased from 0 to 1 mM. Measured partition coefficients for α‐chymotrypsin, lysozyme, and bovine serum albumin depend strongly on α‐cyclodextrin concentration. For example, as the concentration of α‐cyclodextrin rises from 0 to 1 mM, the partition coefficient of lysozyme decreases from 1.7 to 0.55. These measurements are in good agreement with theoretical expectations.
Effects of operating conditions on longitudinal solids mixing in a circulating fluidized bed riser
Authors: Rhodes M. J. , Zhou S. , Hirama T. , Cheng H.
Published: June 2004

Longitudinal solids mixing was studied experimentally in circulating fluidized bed risers of internal diameter 0.152 m and 0.305 m. Superficial gas velocity and mean solids flux used were 2.8–5.0 m/s and 5.0–80 kg/m2·s, respectively, and the bed solids had a surface volume mean diameter of 71 μm and a particle density of 2,456 kg/m3. A sodium chloride tracer was used in impulse injection experiments. A simple, one‐dimensional dispersion model describes measured solids mixing satisfactorily. Peclet numbers (UoL/Dz) found, in the range 1.0–9.0, were correlated with the riser diameter and mean solids flux. The modeling approach described here permits residence time distribution curves to be calculated directly from the knowledge of superficial gas velocity, mean solids flux, and riser diameter. Longitudinal solids mixing in the riser decreased with increasing riser diameter. The results are consistent with recent hydrodynamic studies.
Direct numerical simulations of reacting flows in homogeneous turbulence
Authors: Gao Feng , O'Brien Edward E.
Published: June 2004

A direct numerical simulation (DNS) code has been modified to investigate chemically reacting flows in a stationary, homogeneous turbulence. Single and multispecies reactions as well as complicated reaction schemes such as parallel/consecutive reactions are studied. The effects of some parameters are discussed. The numerical data are used to test a simple first‐order closure model, proposed by Dutta and Tarbell (1989), for turbulent reacting flows. It is shown that the DNS technique provides us with an effective tool to isolate the effects of different parameters and is likely to be useful for proposing and testing closure models.