bluuebunny/crossref_metadata_2025 huggingface dataset:
Oxygen permeation through thin mixed‐conducting solid oxide membranes
Authors: Lin Yue‐Sheng , Wang Weijian , Han Jonghee
Published: June 2004

A new approach for developing fundamental equations of oxygen permeation through thin mixed‐conducting oxide ceramic is presented considering both surface reactions on membrane‐gas interfaces and the diffusion of charged species in the bulk oxide. The essence of this work is the coupling of surface reactions with the bulk diffusion using a novel approach which differs from the conventional Wagner. Theory applicable only to limited cases. With this approach, fundamental equations based on various permeation mechanisms can be derived for oxygen permeation through thin mixed‐conducting oxide membranes, which is impossible using conventional approach. In general, the final results are a complex implicit equation correlating the oxygen permeation flux to the driving force, membrane thickness, and rate constants with physical significance in each step. Somewhat simpler theoretical oxygen permeation equations are obtained for some special cases (mixed‐conducting membranes with a rate‐limiting step, ionic‐conducting membranes, ionic‐conducting membranes with a reducing agent in permeate side). Theoretical results derived using this new approach agree excellently with the experimental oxygen permeation data. It is theoretically and experimentally shown that for ionic conductors, the surface permeation parameter measured by the dynamic permeation method is directly related to the oxygen isotope exchange rate constant measured under equilibrium conditions.
Estimation of crystallization kinetics from batch cooling experiments
Authors: Qiu Yanfeng , Rasmuson Åke C.
Published: June 2004

A direct optimization method to estimate nucleation and crystal growth rate parameters from seeded batch cooling crystallization experiments is evaluated. The experimental information applied comprises the concentration of the solution and the temperature as functions of time and the final product size distribution. Parameters in kinetic equations are determined by nonlinear optimization of a dynamic model of the experiment, by which intermediate approximations of experimental data are avoided. The optimization objective function includes both solution concentration data and product size distribution data. Kinetics are estimated for succinic acid crystallizing in aqueous solutions. Results from several different cooling crystallizations are simultaneously supplied into one single optimization to determine seven parameters.
A novel reactor for determination of kinetics for solid catalyzed gas reactions
Authors: Borman P. C. , Bos A. N. R. , Westerterp K. R.
Published: June 2004

A novel perfectly mixed laboratory reactor for determining kinetics of heterogeneously catalyzed gas‐phase reactions has been developed. Perfect mixing is achieved by circulating the gas in the reactor using an axial flow impeller in a well streamlined enclosure. Pellets are fixed in a rectangular opening in the blades of the impeller. They rotate with the impeller, thus realizing high particle velocities in the reactor. Interparticle mass transfer was studied experimentally by vaporization of naphthalene pellets. The mass‐transfer coefficient in the novel reactor was found to depend on the velocity of a particle in the reactor. Mass‐transfer coefficients in an internal recycle reactor at equal impeller tip speeds are 4–6 times lower than those in the novel reactor, and conditions can be chosen easily where at higher rotational speeds the mass‐ and heat‐transfer rates are 8–10 times higher than in classical recycle reactors. The recycle flow rate in a recycle reactor was found to depend strongly on the resistance to flow caused by the catalyst bed itself. The novel reactor was tested under reacting conditions using the hydrogenation of ethene.
New lattice model for adsorption of small molecules in zeolite micropores
Authors: Van Tassel Paul R. , Davis H. Ted , McCormick Alon V.
Published: June 2004

A new adsorption model is developed for small molecules in zeolites whose form is based on features revealed by molecular simulation. Adsorption is assumed to occur onto a 3‐D polyhedral lattice, and both the energy and entropy of the lattice sites are accounted for using a statistical mechanics approach. Energetic interactions are described by an Ising model with both 2‐ and multibody nearest‐neighbor in‐teractions. Entropic interactions are included by an adsorption site volume term which accounts for the loss of traslational freedom associated with lattice crowding.

The model is applied to a system of small molecules (xenon, methane) adsorbed in idealized zeolite NaA, where adsorption has been shown by computer simulation to occur on finite, cuboctahedral lattices (Van Tassel et al., 1992). The model quantitavely predicts the simulated isotherm over the entire pressure range. Comparison is made with a Langmuir model and a van der Waals gas model which, although valid at low pressures, fail at high pressures due to overestimation of translational entropy and inaccurate portrayal of sorbate‐sorbate interaction energy.
Ignition and extinction of flames near surfaces: Combustion of CH4 in air
Authors: Vlachos D. G. , Schmidt L. D. , Aris R.
Published: June 2004

Ignition and extinction characteristics of homogeneous combustion of methane in air near inert surfaces are studied by numerical bifurcation theory for premixed methane/air gases impinging on planar surfaces with detailed chemistry involving 46 reversible reactions and 16 species. One‐parameter bifuraction diagrams as functions of surface temperature and two‐parameter bifurcation diagrams as functions of equivalence ratio and strain rate are constructed for both isothermal and adiabatic walls. Lean and rich composition limits for ignition and extinction, and energy production are determined from two parameter bifurcation diagrams. For a strain rate of 500 s−1, CH4/air mixtures exhibit hysteresis from ∼ 0.5% up to ∼ 12.5% and from ∼ 5.5% up to ∼ 13.5% near isothermal surfaces and adiabatic walls, respectively. Ignition temperature rises with composition from 1,700 to 1,950 K, without a maximum around the stoichiometric ratio. Under some conditions multiple ignitions and extinctions can occur with up to five multiple solutions, and wall quenching, kinetic limitations, and transport can strongly affect flame stability. Flames near the stoichiometric ratio cannot be extinguished by room temperature surfaces for sufficiently low strain rates. The role of intermediates in enhancing or retarding ignition and extinction is studied, and implications of the effect of catalytic surfaces on homogeneous ignition and extinction are discussed. Removal of H atoms and CH3 radicals by wall adsorption can increase extinction and ignition temperature of 6% CH4 in air by up to 300 K for a strain rate of 500 s−1.
Controlling the morphology of CVD films
Authors: Viljoen Hendrik J. , Thiart Jacob J. , Hlavacek Vladimir
Published: June 2004

The morphology of the gas‐solid interface during typical chemical vapor deposition (CVD) processes is investigated. The dynamic behavior of the interface depends on many factors, including local curvature of the film, reactant diffusion, adsorption equilibrium, surface kinetics, and mobility of adatoms. These factors depend on material properties of the system and reactor conditions, such as the deposition temperature and pressure. A 2‐D model proposed describes the evolution of the interface in Cartesian coordinates under the influence of stabilizing and destabilizing effects. A linear stability analysis is used to predict under which conditions a planar interface becomes unstable. Stability criteria of a simplified 1‐D analysis is not necessarily valid if the real system has more than one dimension. The substrate temperature and reactor pressure are important factors affecting the stability of film growth and thus the morphology of CVD films. An increase in temperature stabilizes planar film growth if the deposition is diffusion‐limited, but destabilizes it if the process is reaction‐controlled. The reactor pressure has a destabilizing effect on planar film growth during a typical CVD process.
Ion adsorption and ion exchange in ultrathin films of fatty acids
Authors: Ahn Dong June , Franses Elias I.
Published: June 2004

Ion exchange equilibria of ultrathin Langmuir and Langmuir‐Blodgett films of stearic and arachidic acids in contact with aqueous electrolyte solutions were studied experimentally and theoretically. A model considering electrochemical and thermodynamic aspects of the adsoption of protons, calcium, and cadmium ions from solution to the film has been developed. Key parameters are the binding constants (K's) of ions and the mixing characteristics (or Flory‐Huggins interaction parameters x's) of ions in the two‐dimensional film lattice plane. The ternary system can be described by the parameter values determined with binary proton‐calcium ion, proton‐cadmium ion, and calcium‐cadmium ion systems: KCa = 2.9 × 102, KCd = 8.6 × 103, KH = 3.6 × 106, χH – Ca = −0.76, χH‐Cd = 0.13, and χCa‐Cd = 1.0. Competitive ion adsorption was measured by FTIR ATR spectroscopy of either collapsed or ordered LB films. Data for the proton‐calcium‐cadmium ion system agreed well with the model predictions. The results show possible future applications of such thin films as ion sensors or ion exchange materials.
Protein denaturation in nonlinear isocratic and gradient elution chromatography
Authors: Whitley Roger D. , Zhang Xiaomang , Wang N.‐H. Linda
Published: June 2004

Protein denaturation, common in hydrophobic adsorption systems, causes misinterpretation of adsorption mechanisms, interferes with analysis in analytical chromatography, and complicates the design of large‐scale adsorption processes. A detailed adsorption model isolates the effects due to denaturation from those due to mass transfer and intrinsic adsorption kinetics. The model is verified using protein gradient elution data. Simulations establish that typical symptoms of denaturation in frontal and elution chromatogrrams include sensitivity to changes in feed composition, column length, particle size, and operating conditions (feed size, flow rate, and column history). When a denatured species adsorbs irreversibly, the elution chromatogram shows decreasing peak area with increasing incubation time and apparent adsorption hysteresis over repeated cycles. In gradient elution, the peak elution order, resolution, and relative peak height depend highly on modulator properties and operating conditions. Interfering species limit solid‐phase induced denaturation by competing for binding sites. Strategies for detecting and minimizing denaturation are proposed.
Competition between TiCl4 hydrolysis and oxidation and its effect on product TiO2 powder
Authors: Akhtar M. Kamal , Vemury Srinivas , Pratsinis Sotiris E.
Published: June 2004

The competition between hydrolysis and oxidation of TiCl4 during vapor‐phase synthesis of titania powders was experimentally investigated. The effects of reactor temperature, reactant mixing and H2O/TiCl4 ratio were studied in a hot wall aerosol flow reactor. The presence of water vapor resulted in rounded particles instead of the faceted ones observed in the absence of water; this effect was most pronounced at lower reactor temperatures. Mixing the water and TiCl4 vapor streams at low temperature resulted in anatase titania. Larger aggregates with smaller primary particles were observed for powders synthesized in the presence of water vapor. The mixing temperature of the water and TiCl4 vapors, as well as the H2O/TiCl4 ratios, did not affect the aggregate size or specific surface area. Analysis of micrographs of titania aggregates made in the presence of water revealed that the fractal dimension increased with process temperature.
Stochastic dynamic approach to transport phenomena
Authors: Laso Manuel
Published: June 2004

A stochastic dynamic approach to the equations of change is introduced here, which first is applied to linear equations and is based on the formal analogy between certain forms of the equations of change and the Fokker‐Planck equation. A stochastic differential equation associated with the Fokker‐Planck equation can be derived from the latter and solved numerically, thus yielding the solution to the original equation of change. The proper treatment of boundary conditions is essential for the success of the method. We show that the method is able to handle the eight fundamental types of boundary conditions (Carslaw and Jaeger, 1959; Crank, 1975). In addition, the stochastic dynamic approach provides a deeper insight in the physical processes underlying transport phenomena than do traditional techniques.
Calculating critical transitions of fluid mixtures: Theory vs. experiment
Authors: Sadus Richard J.
Published: June 2004

The progress in predicting critical transitions in fluid mixtures is reviewed. The critical state provides a valuable insight into the general phase behavior of a fluid and is closely linked with the nature and strength of intermolecular interaction. Calculations of critical equilibria have been confined mainly to binary mixtures. The prediction of binary gas‐liquid critical properties was initially limited to empirical correlations. These techniques have been superseded by rigorous calculations of the critical conditions using realistic models of the fluid or equations of state. All of the known types of critical phenomena exhibited by binary mixtures can be, at least, qualitatively calculated. If an optimal combining rule parameter is allowed, continuous gas‐liquid properties can be calculated accurately for a wide variety of mixtures. Similarly, the pressure and composition dependence of upper critical solution phenomena can be accurately predicted. Progress has been achieved in predicting discontinuous critical transitions in polar and nonpolar binary mixtures. There is increasing interest in calculating the critical properties of ternary and multicomponent mixtures. Although the techniques applied to binary mixtures often can be directly extended to ternary mixture calculations, calculated critical properties of ternary mixtures indicate that their behavior cannot be considered as a simple extension of binary mixture phenomena. Consequently, ternary critical calculations are likely to provide a superior insight into the phase behavior of multicomponent fluids.
Theory of convective heat and mass transfer to spherical‐cap bubbles
Authors: Kendoush Abdullah Abbas
Published: June 2004

The forced convective heat and mass transfer to spherical‐cap bubbles were investigated analytically. Potential flow was assumed outside the bubble except at the bubble surface where the boundary layer approximation was adopted. The present analytical study produced the following exact solution:

for the average Nusselt number where Pe is the Peclet number. The derived equation was compared with the available experimental mass‐transfer results.
Extraction of naphthalene by block copolymer surfactants immobilized in polymeric hydrogels
Authors: Calvert Tammy L. , Phillips Ronald J. , Dungan Stephanie R.
Published: June 2004

Gel/micelle materials, comprising hydrogels containing block copolymer micelles, have the potential to be very useful in a variety of extraction and controlled release applications. It is demonstrated that block copolymer surfactants can be immobilized in calcium alginate gels and that the resulting composite material preferentially solubilizes the model hydrophobic solute naphthalene. Five different polyethylene oxide‐polyropylene oxide‐polyethylene oxide triblock copolymer surfactants are considered, and the qualitatively different properties exhibited are interpreted in light of the properties of these surfactants in solution. Alginate gel concentrations of 22.5 and 52.5 g/L and surfactant concentrations ranging from 20.4 to 136 g/L are considered. The data indicate that micelles can be permanently immobilized for purposes of extraction or controlled release and that the extent of solute uptake or release can be altered by varying gel and surfactant concentration and the type of surfactant employed.
Instability of distillation columns
Authors: Jacobsen Elling W. , Skogestad Sigurd
Published: June 2004

As recently recognized, distillation columns, operating with reflux and boilup as independent inputs, may have multiple steady‐state solutions, even in the ideal binary case. Two fundamentally different sources may cause the multiplicity, and in both cases some operating points are found to be unstable. This article provides evidence for the instability and discusses the effect of operating conditions on stability. Increasing the internal flow rates increases the probability of instability; when flows other than reflux and boilup are used as independent inputs, an operating point may become unstable if the level control is not sufficiently tight. In this case, a limit cycle, usually stable, appears as the steady state goes unstable.
A continuous dilution method for phase diagram determination
Authors: Hartounian H. , Koetitz J. A. , Kaler E. W. , Sandler S. I.
Published: June 2004

A relatively simple continuous dilution method with on‐line spectrophotometric measurement has been used to determine the phase transition boundaries of aqueous two‐phase polymer systems. The observed range of compositions over which the phase transition occurs is dominated by the polydispersity of the polymers, but also is a function of the dilution path followed. The binodal curve obtained by the continuous dilution method agrees well with the results obtained by the traditional method of phase equilibration, separation and analysis of liquid chromatography, which is slower and requires more expensive equipment.
Diffusion‐convection‐reaction in multicomponent mixtures
Authors: Guida Vincenzo , Ocone Raffaella , Astarita Gianni
Published: June 2004

We analyze the mathematical structure of a multicomponent reactive mixture in a plug flow reactor with axial diffusion. Quasilinearity of the kinetic equation assuming “uniformity” does not carry over to the second‐order equations when diffusion is considered and a perturbation expansion method needs to be developed. Perturbation around the limit of a CSTR is regular, which leads to nonhomogeneous second‐order differential equations containing no unknown kinetic term, so that the procedure cascades down to the solution of the CSTR problem. Perturbation around the PFR problem is singular, but the inner (boundary layer) solution is easy. The outer solution leads to a series of integro‐differential equations, which can be reduced to complete Volterra integral equations of the second kind; these are known to admit unique solutions. A formal approach to finding these solutions is discussed.
Effect of diffusion on heterogeneous ethylene propylene copolymerization
Authors: Hoel E. L. , Cozewith C. , Byrne G. D.
Published: June 2004

In heterogenous olefin polymerization with Ziegler catalysts, the influence of monomer mass transport in the growing granule on polymer properties has been extensively modeled, but it has not been possible to clearly establish the importance of diffusion experimentally since the multisited nature of most Ziegler catalysts can produce similar effects. In this study, ethylene‐propylene copolymers were made with single‐sited metallocene catalysts by slurry polymerization in liquid monomers. These copolymers had a relatively narrow molecular weight distribution with a composition distribution (CD) broader than expected for a single‐site catalyst. Data analysis indicates that mass‐transfer limitations in the polymer particles are the most likely explanation for the observed results. For amorphous copolymers, a diffusion/reaction model could predict CD breadth in good agreement with experimental data, but for semicrystalline polymers the model was inaccurate. We postulate that model inadequacies are due to radial gradients in monomer diffusivity during polymerization which the model does not account for.
Effect of kinetics on residue curve maps for reactive distillation
Authors: Venimadhavan Ganesh , Buzad George , Doherty Michael F. , Malone Michael F.
Published: June 2004

A class of models is derived for studying the effects of chemical kinetics on residue curve maps for reactive distillation. Activity‐based rate and phase equilibrium expressions provide an accurate and thermodynamically consistent description of composition changes in nonideal, reacting vapor‐liquid mixtures. For certain strategies of operation, which dictate the rate of product removal, the model equations are nonautonomous, leading to unusual dynamic behavior. However, for a certain special product removal policy, the effects of kinetics can be described by a single parameter, the Damköhler number, which measures the rate of reaction relative to product removal. For small values of the Damköhler number, the nonreactive simple distillation residue curve map is recovered and the singular points are the pure components and azeotropes in the nonreactive mixture. A bifurcation analysis shows the deformation and, in some cases, the disappearance of these singular points as the Damköhler number is increased until the equilibrium reactive residue curve map is recovered at large values. This bifurcation analysis reveals the limitations of the equilibrium analysis. A model problem for the reactive distillation of methyl tertbutyl ether from isobutene and methanol is solved.
Influence of dynamic interfacial properties on droplet breakup in simple shear flow
Authors: Janssen J. J. M. , Boon A. , Agterof W. G. M.
Published: June 2004

The breakup of droplets in an inhomogeneous flow is the key to emulsification. Frequently, the local flow experienced by the drops is a (quasi‐) simple shear flow. The breakup of droplets in a steady, simple shear flow in the absence of emulsifiers has been studied extensively. In the presence of emulsifiers, the droplet interface may acquire viscoelastic properties, which are important in the prevention of coalescence, but their influence on droplet breakup has not been established solidly. This article reports on a phenomenological approach, which links the droplet breakup dynamics to the interfacial viscoelasticity, identifying the latter with the elasticity modulus of a deforming planar interface. Over a viscosity ratio range of three decades, the results have been found to agree with the model, which uses only independently known quantities derived from equilibrium interfacial tension properties.
Experimental study on a four‐bed PSA air separation process
Authors: Chiang A. S. T. , Chung Y. L. , Cheng Chao‐Wen , Hung Te‐Hsien , Lee T. Y.
Published: June 2004

A 200‐s PSA cycle involving both pressure equalization and product backfill steps has been experimentally studied on a four‐bed system, where LINDE 5 A zeolites were used as the adsorbent to separate oxygen from air. This cycle is operated under a pressure ratio of 4.3. During the experiment, the pressure history and flow rates, as well as the concentration of the product stream have been continuously monitored. This is the first time detailed experimental data on a four‐bed system are presented. Under favorable conditions, this system produces better than 90% oxygen at a recovery of 17%. For the low‐pressure ratio, such a recovery could not have been achieved without the pressure equalization step and the reduced purge operation. Recovery and throughput, however, are not as high as one would expect from a linear local equilibrium model. The self‐broadening effect of the purge wave has been identified as the major cause of underperformance.
Strings of liquid beads for gas‐liquid contact operations
Authors: Hattori Kenji , Ishikawa Mitsukuni , Mori Yasuhiko H.
Published: June 2004

A novel device for gas–liquid contact operations is proposed to feed a liquid onto wires (or threads) hanging down in a gas stream is proposed. The liquid disintegrates into beads strung on each wire at regular intervals; if the wire is moderately wettable, a thin film forms to sheathe the wire, thereby interconnecting the beads. Since the beads fall down slowly, which possibly renews the film flowing down even more slowly a sufficient gas‐liquid contact time is available even in a contactor with considerably limited height. An approximate calculation method is developed for predicting the variation in the temperature effectiveness for the liquid (the fractional approach of the liquid exit temperature to the gas inlet temperature) with the falling distance, assuming an applicability of strings‐of‐beads contactors to thermal energy recovery from hot gas streams.
Phenomenological model for dispersed bubbly flow in pipes
Authors: Nikitop Dimitris E. , Michaelides Efstathios E.
Published: June 2004

An analytical approach to the problem of steady‐state, axisymmetrically disperesed, bubbly flow in pipes based on a zero equation turbulence model is discussed. The formulation incorporates recent experimental observations and introduces the effect of bubble size in a rudimentary way. The two‐phase mixture is modeled as a variabledensity single fluid assuming an empirical void distribution family. The turbulent shear stress is formed from the contributions of both the velocity and density variation, and the solution of the resulting Reynolds‐type equation yields the velocity profile of the flow. Predicted void fraction and velocity distributions agree well with experimental measurements. The main objective of the model is to predict the friction multiplier with minimal computational effort. The velocity profiles of this model agree reasonably well with experiments. Predictions for the friction multiplier are compared to six known and widely used correlations, as well as to experimental data. All the correlations severely underpredict the friction multiplier in the disperesed bubbly flow regime, while the results of our model agree well with the measurements, within the range of its validity.
Three‐phase mass transfer: Improved pseudo‐homogeneous model
Authors: Nagy Endre , Moser Anton
Published: June 2004

This new pseudo‐homogeneous model describes the absorption rate in the presence of dispersed second liquid phase or solid catalyst particles. In contrast to previous models, it considers the diffusion process and chemical reaction inside the dispersed phase. The absorption rate is defined in the case of first‒ and zero‐order reactions for both continuous and dispersed phases. The concentration of particles from the bulk phase to the gas‐liquid boundary layer replaces the initial concentration to the balance equation for the liquid boundary layer at the gas‐liquid interface. For mass transport in the bulk phase, steady‐state film theory, while in the gas‐liquid boundary layer, unsteady‐state film‐penetration theory is applied. The effect of mass‐transfer and reaction kinetic parameters, as well as the particle size on the absorption rate, is described as well as the simulated data verified by previous experimental studies.
Production rate of an isotachic train in displacement chromatography
Authors: Zhu Jie , Guiochon Georges
Published: June 2004

The shock layer theory permits the derivation of an equation relating the thickness of the shock layer or mixed zone between successive bands in a fully developed isotachic train and the experimental parameters (mobile‐phase flow velocity, concentration and retention factor of the displacer, column length, and average particle size of the packing material). It also permits the calculation of the amount of each product contained in this mixed zone. The main assumptions made are the use of the competitive Langmuir isotherm, the identity of axial dispersion coefficients, and the mass‐transfer rate constants for different components. These equations make it possible to calculate the production rate and recovery yield achieved with an isotachic train, and to optimize the experimental conditions for maximum production rate. The shock layer thickness increases rapidly and the production rate decreases sharply when α‐1 tends toward 0. In agreement with previous experimental results, there are well defined optimum flow velocity and displacer concentrations which vary depending on feed components. This theory permits an easy access to the optimization of these parameters.
Use of optical probes to characterize bubble behavior in gas‐solid fluidized beds
Authors: Mainland Marlene E. , Welty James R.
Published: June 2004

Optical probes are used to study gas‐solid fluidized‐bed hydrodynamics. The probes each consisting of a light source and photodetector separated by a gap are suitable for use at combustion‐level temperatures. The methodology to process the signal for calculation of bubble properties such as bubble frequency, local bubble residence time, bubble velocity, pierced length, bubble size, and visible bubble flow is presented. The signal processing technique is independent of bed operating conditions. The probe signal processing methodology is validated by comparing calculated bubble properties based on the probe signal with properties observed on videotapes of a 2‐D bed.
Monolith froth reactor: Development of a novel three‐phase catalytic system
Authors: Crynes Lawrence L. , Cerro Ramon L. , Abraham Martin A.
Published: June 2004

The monolith froth reactor, involving two‐phase flow and a monolith catalyst, is developed. The flow within monolith channels, consisting of trains of gas bubbles and liquid slugs, is produced by forming a two‐phase froth in a chamber immediately below the bottom of the monolith. The froth then flows upward into the monolith channels through pressure forces, which differs from previous methods since it may be carried out for a commercial‐scale reactor. Because the liquid film which develops between the gas phase and the surface of the catalyst is extremely thin, two‐phase flow within a monolith can provide reaction rates which are near their intrinsic values. Catalytic oxidation of aqueous phenol over copper oxide supported on γ‐Al2O3 is used as a model reaction for investigating reactor performance. Generation of a froth is confirmed by visual inspection; the average bubble size is approximately that predicted by a force balance. The effect of externally controllable process variables (liquid and gas flow rates, temperature, and pressure) on the rate of phenol oxidation was investigated. Reaction rate increases with temperature or pressure increase and decreases with gas flow rate increase, achieving a maximum with respect to liquid flow rate. The activation energy calculated from the apparent reaction rate measured in the monolith froth reactor is similar to that of intrinsic value, suggesting minimal mass‐transfer limitations.
Model for colloidal fouling of membranes
Authors: Bacchin P. , Aimar P. , Sanchez V.
Published: June 2004

A proposed theoretical model describes colloids deposition on a membrane surface accounting for surface interactions. A mass‐transfer equation links the deposition rate to hydrodynamic conditions (permeation and tangential flow through a boundary layer thickness, δ) and to physicochemical properties of the suspension (diffusion, D, and potential barrier between particles, VB). This equation predicts the existence of a critical flux, Jcrit, for ultrafiltration, reverse osmosis, or microfiltration of large‐size colloids as:

Some of the trends observed when processing protein solutions are explained by this model. Previous experimental data for various colloids or our data with a clay suspension in the presence of electrolytes are also compared to predictions of our model. It explains the “flux anomaly” for particle sizes between 10 nm and 1 μm.
Protein transport in ultrafiltration hollow‐fiber bioreactors
Authors: Patkar Anant Y. , Koska Jürgen , Taylor David G. , Bowen Bruce D. , Piret James M.
Published: June 2004

A 1‐D model, which neglects radial variations, describes the hydrodynamics of cell‐free ultrafiltration hollow‐fiber bioreactors (HFBRs) and the transport of highmolecular‐weight proteins trapped in the extracapillary space (ECS). The profiles of radially‐averaged protein concentrations predicted by this model are identical to those obtained using a model with radial variations. The model predictions agree well with axial profiles of bovine serum albumin (BSA) and human transferrin concentrations measured in transient and steady‐state experiments. The validated model explores the influence of cell culture operating conditions on HFBR protein transport. Increasing protein loading decreases BSA and transferrin polarization in HFBRs operated with unidirectional lumen flow. A relationship developed predicts the protein loading needed to ensure a nonzero steady‐state protein concentration throughout the ECS. This critical protein loading depends only on the lumen pressure drop and the ECS protein osmotic pressure. Periodic reversal of the lumen flow direction also decreases protein polarization. The influence of the flow‐direction switching time and membrane permeability on the ECS protein distribution is investigated.
New look at wave analogy for prediction of bubble terminal velocities
Authors: Maneri Charles C.
Published: June 2004

The analogy between waves on the surface of an infinite fluid and bubbles rising in low‐viscosity fluids of infinite extent, originally proposed by Mendelson for 3‐D bubbles, has been used to predict the terminal velocity of plane bubbles. In terms of its terminal velocity, a plane bubble rising in a rectangular duct of small aspect (spacing‐to‐width) ratio behaves as if it were a 3‐D bubble rising in an infinite medium as long as the end walls (the walls in the widthwise direction) are sufficiently far apart. As the end walls are moved toward each other, a wall effect is found to exist. A general expression for the terminal velocity of a bubble of any size rising in a rectangular duct including this wall effect is also developed based on the wave analogy and shown to compare well with existing data.
Agglomeration during precipitation: II. Mechanism deduction from tracer data
Authors: Ilievski D. , Hounslow M. J.
Published: June 2004

A novel approach using a 2‐D population balance model is developed and applied to the analysis of experimental tracer crystal data. This approach is effective in discriminating among various functional forms of agglomeration kernel and enables estimation of the agglomeration kinetics. At present, the analysis is restricted to three simple agglomeration kernels and shows that the size‐independent kernel best describes the agglomeration of Al(OH)3 crystals during precipitation in caustic aluminate solutions. This agrees with the findings of Ilievski and White (1994). Estimates of the agglomeration kinetic parameters from the tracer data agree well with the experimentally observed values.
Solvent effect on adsorption phenomena in reversed‐phase liquid chromatography
Authors: Miyabe Kanji , Suzuki Motoyuki
Published: June 2004

The effect of solvent and mobile‐phase composition on adsorption characteristics in liquid‐phase absorption is studied. Comparing experimental adsorption data for several organics in both gas‐ and liquid‐phase systems confirms that adsorption equilibrium constant K and isosteric heat of adsorption Qst are smaller than those in corresponding gaseous systems. The logarithm of K almost linearly increases as methanol composition decreases. Solvophobic theory applied quantitatively analyzes these solvent effects. Estimation methods for three parameters influence calculation results: solvent effects on K can be quantitatively analyzed; the adsorbability of adsorbates can be estimated from the value of ΔGsolv for each homologue in a reversed‐phase liquid chromatographic system; and the solvent effect on Qst cannot be satisfactorily interpreted. It is, however, confirmed that Qst is influenced by a solvent and an apparent small value is observed in liquid‐phase adsorption.
Chromatographic study on liquid‐phase adsorption on octadecylsilyl‐Silica Gel
Authors: Miyabe Kanji , Suzuki Motoyuki
Published: June 2004

Adsorption characteristics of reversed‐phase liquid chromatography and influence of solvent composition on the characteristics are studied by the chromatographic method and the moment analysis. Adsorption equilibrium constant increases by adding hydrophobic increments into adsorbate molecules. Contributions of fluid‐to‐particle mass transfer and intraparticle diffusion to mass‐transfer resistance in the octadecylsilyl‐silica gel (ODS) column are almost equally great. Surface diffusion is dominant for the intraparticle diffusion in ODS. The logarithm of the surface diffusion coefficient linearly increases with an increases with an increase in methanol fraction in the range from 40 to 100 vol. %. In liquid‐phase adsorption, activation energy of surface diffusion is greater than the isosteric heat of adsorption and both increase when methanol fraction decreases in the range from 40 to 100 vol. %. An empirical correlation based on experimental data proposed estimates surface diffusion coefficients from physical properties of adsorbates.
Sol‐gel preparation and properties of Alumina adsorbents for gas separation
Authors: Deng S. G. , Lin Y. S.
Published: June 2004

The sol‐gel method is applied to prepare Cu+ or Ag+ containing alumina adsorbents which have potential application in purification or separation of carbon monoxide or olefins containing gases. Wet impregnation and sol‐solution mixing methods are used to incorporate the active species into the sol‐gel derived γ−alumina support. These alumina adsorbents have the characteristics of large specific surface area, high dispersion of active specie on the grain surface, and mesopore size with a uniform pore size distribution. The gas chromatographic technique determines the carbon monoxide adsorption properties on these alumina adsorbents. Adsorption equilibrium constants, adsorption rate constants, axial dispersion coefficients, heat of adsorption, and adsorption activation energy are determined from GC response peaks by using the moment method. The alumina adsorbents prepared by the solgel method appear to be superior to the similar adsorbents prepared by other methods.
Process analysis in the complex domain
Authors: Sridhar L. N. , Lucia Angelo
Published: June 2004

Analytical support for recent numerical work in process simulation in the complex domain is discussed. In particular, the observation that nondegenerate singular points are saddle points of the two‐norm on the complex domain is proved rigorously and its numerical implications are discussed. The isothermal isobaric flash problem admits only real‐valued two‐phase solutions, provided that the feed conditions are real. Both single‐ and multivariable chemical process examples illustrate the theoretical results.
Fractal nature of viscous fingering in two‐dimensional pore level models
Authors: Ferer M. , Sams W. Neal , Geisbrecht R. A. , Smith Duane H.
Published: June 2004

Use of saturation‐dependent relative mobilities leads to linear flow; however, experiment and theory show that, in the limit of very large viscosity ratio, the flow is not linear but fractal. Generally, fractional flows and relative mobilities depend on both saturation and time. Use of a standard pore‐level model of 2‐D flow in the limit of infinite capillary number shows that this flow is fractal for large viscosity ratios (M = 10,000) and the sauration and fractional flows agree with the results of our general arguments. For realistic viscosities (M = 3 → 300), our modeling of the unstable flow shows that, although the flows are initially fractal, they become linear on a time scale, τ increasing as τ = τ0M0.17. Once linear, the saturation front advances as x ≈ v0M0.068 t; the factor M0.068 acts as a 2–D Koval factor.
Analysis of diffusion in polymers using evanescent field spectroscopy
Authors: Fieldson Gregory T. , Barbari Timothy A.
Published: June 2004

A rigorous analysis confirmed by experimental results is presented for attenuated total reflectance, Fourier‐transform infrared (FTIR‐ATR) spectroscopy applied to the study of small molecule diffusion in polymers. FTIR‐ATR Fickian diffusion models are derived for a constant surface concentration and for an adjacent mass‐transfer boundary layer. An FTIR‐ATR Case II transport model demonstrates the ability to discriminate between transport modes. New experimental results for the acetone‐polypropylene, methanol‐polystyrene and methanol‐poly(methyl methacrylate) systems are consistent with gravimetric sorption and nuclear magnetic resonance measurements. An important result is the detection of multiple hydroxyl peaks for methanol during diffusion, indicating different hydrogen bonding states and the calculation of separate diffusion coefficients for each state in polystyrene.
Effects of operating conditions on stability of gas‐phase polyethylene reactors
Authors: McAuley K. B. , Macdonald D. A. , McLellan P. J.
Published: June 2004

Fluidized‐bed polyethylene reactors are prone to unstable behavior and temperature oscillations (Choi and Ray, 1985b). Their work is extended to show the effects of ethylene feed system operation, reactor cooling system design, catalyst properties, and gas composition on reactor stability and dynamics. The analysis is performed using a well‐mixed model, because heat‐ and mass‐transfer resistances between multiple phases are small and are not required to account for the observed bifurcation phenomena. The addition of a gas recycle and heat exchanger system to the model significantly affects dynamic performance, including the formation of limit cycles. The size and dynamics of the heat exchanger, however, have little effect on the overall stability. In contrast, automation of the ethylene feed system to replace the monomer in the reactor as it is consumed leads to substantially different dynamic behavior than if the ethylene feed is maintained at a constant rate. Catalyst properties (multiple sites, activation energy, and deactivation) significantly affect dynamics and stability, whereas comonomer and other gases affect them only mildly. The results confirm that without proper temperature control, gas‐phase polyethylene reactors are prone to instability, limit cycles, and excursions towards unacceptable high‐temperature steady states.
Binding of an antifreeze polypeptide to an ice/water interface via computer simulation
Authors: McDonald Shawn M. , White Angela , Clancy Paulette , Brady John W.
Published: June 2004

The interaction between a winter flounder antifreeze polypeptide and an ice/water interface was studied using Molecular Dynamics computer simulation techniques to study the mechanism of action of this class of antifreeze molecules. Simple Point Charge models were used for the water molecules, and a molecular mechanics program (CHARMM) was used to construct the model for the polypeptide. A (2021) face was exposed on the ice surface, as this is believed to be the experimentally favored ice face for peptide binding. The polypeptide binds strongly to the ice surface even though it was placed with its four polar threonine (Thr) groups pointing away from the ice surface. This tested the previously advanced hypothesis that adsorption occurs primarily between these groups and the ice due to a matching of the spacing between oxygen atoms in the ice lattice and the polar Thr residues. As well as contacts with other polar groups on the peptide, the binding to the ice produces a good steric fit of the peptide with the corrugated ice interface. The presence of the peptide did not induce any melting of the ice at 200 K.
Flow through curved rectangular channels of large aspect ratio
Authors: Targett Matthew J. , Retallick William B. , Churchill Stuart W.
Published: June 2004

Numerical solutions were obtained for the field of velocity in angular forced flow through the annulus between two concentric cylinders of large and infinite aspect ratio with a gap‐to‐inner radius ratio of 0.05 using a finite‐element representation and the FIDAP code. For an infinite aspect ratio, velocity vectors reveal a purely angular motion below a critical Dean number of 37.31 and a secondary motion in the form of pairs of counterrotating vortices above that value. The wavelength of these vortices and the friction factor are correlated in terms of the Dean number. For large but finite aspect ratios a weak secondary motion around the periphery is found to occur below the critical Dean number, while for greater values the vortex at each end of the channel is greatly extended. The computed patterns of flow are in good agreement with prior experimental visualizations as well as with those carried out as part of this investigation. The computed characteristics are also in good agreement with prior theoretical results for limiting cases. The adaptation of the results for flow through an Archimedean spiral is described.
Rapid estimation of cylinder erosion rates in abrasive dust‐laden streams
Authors: Rosner Daniel E. , Tandon Pushkar , Labowsky Michael J.
Published: June 2004

Erosion yield data for particular combinations of target and projectile materials (via laboratory experiments carried out with a narrow‐size distribution over the important range of impact velocities and incidence angles), together with recently developed rational correlations for inertial impaction of suspended particles on a cylinder in high Reynolds number crossflow are used to provide a tractable framework for predicting the erosion rates of, say, heat exchanger tubes immersed in particle‐laden streams of combustion products.“Universal” results are cast in terms of the following accessible parameters: sensitivity of erosion yield to projectile incident velocity and angle, ratio of mean particle size to the threshold size required for impaction on the cylindrical target, spread of the mainstream particle size distribution (here log‐normal), and the characteristic “slip” Reynolds number for the critical size abrasive particle in the mainstream. Applications of the results are illustrated, and several generalizations are discussed.
Modeling of a monolithic catalyst with reciprocating flow
Authors: Wallace K. , Viljoen H. J.
Published: June 2004

Several methods have been proposed to improve the conversion of catalytic oxidation reactions. Gases with low enthalpy contents or gases that contain small amounts of combustible compounds can be converted much more efficiently in a reactor with reciprocating flow. The solid phase exhibits an inertia behavior insofar as it does not reach equilibrium with the gas temperature at the reactor ends. The solid‐phase temperature at the center of the bed is considerably higher than in the unidirectional case. Results for the oxidation of CO at different inlet gas temperatures and switching periods are presented. An optimum period is found at which maximum conversion occurs. An estimate of the maximum temperature in the reactor is derived.
Noncatalytic gas‐solid reactions and mechanical stress generation
Authors: Rode Hilmar , Orlicki Dariusz , Hlavacek Vladimir
Published: June 2004

The reaction rate in gas‐solid systems can be affected by mechanical stresses that arise as the reaction proceeds. Stresses develop due to differences between precursor and product molar volumes and thermal expansion coefficients. Experimental evidence on the interaction of reaction rate and mechanical stress for the Ti/N2 and Ti/O2 systems is provided. A detailed and consistent mathematical model is developed for the reaction taking place at the constrained precursor/product interface. An elastic formulation for the stresses is adopted, and stress generation due to mismatches in linear thermal expansion coefficients and equivalent volume (Pilling‐Bedworth ratio) for the precursor and product are considered. The effect of surface energy, which becomes significant for particle sizes below 1 μm, is also included in the model. Both experimentally and theoretically, conditions exist where the mechanical stresses exceed the strength of the material, leading to mechanical breakdown of the product layer, thus causing a discontinuity in the observed reaction rate. The entire processing history, including the reaction, temperature, and pressure profiles, plays an important role in determining the overall reaction kinetics of the powder.