bluuebunny/crossref_metadata_2025_split huggingface dataset:
Gas separations in hollow‐fiber adsorbers
Authors: Gilleskie Gary L. , Parker Jennifer L. , Cussler E. L. 
Published: June 2004

AbstractThe adsorption of ethane from helium was measured in beds packed with 2.5 μm zeolite crystals and containing either a single hollow fiber or multiple fibers. THe single‐fiber experiments indicate that the mass‐transfer rate in bends containing zeolite 13X is limited by diffusion across the fiber wall and through macropores. FOr adsorption in single‐fiber beds packed with zeolite 4A, mass transfer is limited by micropore diffusion within the particles. Breakthrough curves from beds containign 13X are adequately described with the linear driving force model, while curves from beds containing 4A are consistent with the Rosen model.Breakthrough curves from beds packed with zeolite 13X and containing multiple fibers can be predicted from the experiments with single‐fiber beds when the fibers are evenly spaced. When fibers are unevenly spaced, the breakthrough curves are more disperse. Unevenly spaced fibers are the normal case. Even when fibers are evenly spaced, the productivity of hollow‐fiber beds is expected to be no greater than that in conventional beds.
Numerical model for the vacuum pyrolysis of scrap tires in batch reactors
Authors: Yang Jin , Tanguy Philippe A. , Roy Christian 
Published: June 2004

AbstractA quantitative model for scrap tire pyrolysis in a batch scale reactor developed comprises the following basic phenomena: conduction inside tire particles; conduction, convection, and radiation between the feedstock particles or between the fluids and the particles; tire pyrolysis reaction; exothermicity and endothermicity caused by tire decomposition and volatilization; and the variation of the composition and the thermal properties of tire particles. This model was used to predict the transient temperature and density distributions in the bed of particles, the volatile product evolution rate, the mass change, the energy consumption during the pyrolysis process, and the pressure histoiy in a tire pyrolysis reactor with a load of 1 kg. The model predictions agree well with independent experimental data.
Isothermal bistability due to remote control: A model for selective catalytic oxidation
Authors: Rebitzki Timm , Delmon Bernard , Block Jochen H. 
Published: June 2004

AbstractSpecial kinetic behaviors in isothermal reactions on heterogeneous catalysts composed of two components with different roles are studied with the focus on hi stabilities that could occur in stirred‐tank flow reactors when control of catalytic sites on one component by mobile (spillover) species produced by the other operates. The comprehensive kinetic model involves: (1) generation of spillover species on the second phase, their transfer to the potentially catalytic phase, and their reaction with this acceptor to generate selective sites; (2) the kinetics of the catalytic reaction. We developed a model for selective catalytic oxidation, where catalyst sites change from nonselective to selective under the influence of the control by spillover oxygen. The model is based on the system of differential equations. By integration and iteration to the steady state for each value of the external control parameter under investigation, the oxygen and hydrocarbon reactant consumption, as well as the state of the catalyst, is calculated. These calculations predict bistable selectivity for certain ranges of concentration in the feed. Data on the boundaries of the hysteresis loops as a function of catalyst composition and oxygen partial pressure are discussed, as well as the variation of these hysteresis boundaries as a function of the internal parameters.
Transient profiles in ion‐exchange displacement chromatography
Authors: Gadam Shishir D. , Gallant Stuart R. , Cramer Steven M. 
Published: June 2004

AbstractA theoretical and experimental study of displacement development in cation‐exchange displacement chromatography is carried out. Simulated displacement chromatograms are generated using a numericl model of displacement chromatography, which employs the steric mass action (SMA) formalism to describe the multicomponent nonlinear adsorption of proteins and polyelectrolytes. Simulated profiles of transient displacement chromatography are compared with experimentla displacement over a ranve of operating conditions, and the model accurately predicts transient displacement behaviour. The success of the model indicates that the SMA formalism is an appropriate tool for prechromatograms indicate that significantly higher throughputs can be achieved by operating these protein displacement systems under nondeveloped conditions. In addition, displacement development proceeds faster at lower salt concentration, indicating that a reduction of the displacement systems. This work sets the stage for the optimization of protien displacement chromatography.
Binary kinetics in the Y‐Ba‐Cu system: 2. Nanosized particles
Authors: Wang Hong , Thomson William J. 
Published: June 2004

AbstractThe kinetics of binaty mixtures of Y2O3 – BaCO3–CuO (Ba–Cu) and Y2O3 –CuO(Y–Cu) prepared by citrate sot‐gel techniques were studied in both air and helium. Particle sizes resulting from the sol – gel preparation were between 5 and 100 nm, and the prevailing kinetics differed significantly from those observed in the companion study of micron‐sized particles. That is, nucleation‐growth kinetics of the Avrami‐Erofe'eu type adequately model the kinetics over the entire conversion range as opposed to the diffusion mechanisms that describe the kinetics in larger sized particles. An alternate nucleation‐growth model, which neglects overlapping volumes during reaction (Austin – Rickett model), was also adequate in the Ba–Cu system where particle size/spacing was larger, but was only applicable in other reaction systems at lower conversions. Activation energies obtained from the Avrami‐ Erofe'ev were consistently lower than those from the Austin ‐ Rickett model, because the former model contains two parameters that are temperature‐sensitive.
An underwood‐like model for a finite mass exchanger
Authors: Gupta Ashish , Manousiouthakis Vasilios 
Published: June 2004

AbstractA mathematical model is presented for a multicomponent, two‐stream, countercurrent mass exchanger with a finite number of stages. The model relates the outlet compositions to the inlet compositions and the flow rate ratio of the two streams going through the exchanger, as well as the number of stages in the exchanger. An iterative solution procedure employing this model is proposed for the mass exchanger design problem. The model requires solution of a polynomial equation whose roots may be complex. Bounds to the real roots of the polynomial have also been established. Two examples illustrating how the method is employed are presented. The second example demonstrates that the iterative procedure converges even in the complex domain, and that complex roots exist for a real column.
Design of robust constrained model‐predictive controllers with volterra series
Authors: Genecili Hasmet , Nikolaou Michael 
Published: June 2004

AbstractSufficient conditions are developed in this work for robust stability and pefformance of nonlinear model‐predictive control systems that use an end‐condition and second‐order Volterra model with parametric uncertainty in the time domain. The robust stabilfly conditions involve the lengths of the prediction and control horizons, as well as the coefficients of the control move suppression terms in the on‐line objective function. These conditions may be used for both analysis and synthesis purposes. A case study of a chemical reactor is presented to elucidate these issues.
Redundant sensor network design for linear processes
Authors: Ali Yaqoob , Narasimhan Shankar 
Published: June 2004

AbstractMeasurements of process variables have a considerable impact on the control, optimization, safety, and reliability of chemical processes. In a 1993 article, Ali and Narasimhan developed a systematic graph‐theoretic procedure for optimally locating a minimum number of sensors in linear steady‐state processes. The sensor network was designed to maximize the probability of estimating variables when sensors are likely to fail. This article extends that procedure for the optimal design of a redundant sensor network for linear processes. The algorithm proposed also accounts for specifications of measurable and important variables. The efficiency and robustness of the proposed algorithm are demonstrated on realistically large processes.
Wetting of fiber mats for composites manufacturing: I. Visualization experiments
Authors: Chen Yung‐Tin , Davis H. Ted , Macosko Christopher W. 
Published: June 2004

AbstractWetting of glass fibers was visualized using an oil with the same refractive index. With both a video‐enhanced microscope and a high‐magnification video camera we followed the flow front and qualitatively measured the entrapped air bubbles by image analysis. Due to different permeabilities between the fiber mats and bundles, air bubbles are entrapped. Two major kinds of air bubbles are observed: small cylindrical micro voids between fiber filaments (inside fiber bundles) and spherical macro voids outside fiber bundles. These air bubbles can be described by three major mechanisms initial liquid bypassing/air trapping, later capillary invasion of disordered fiber bundles, and air bubble mobilization. Both random and unidirectional fiber mats were used in this study. Vacuum as well as different fluid viscosities and surface tensions were also investigated, which led us to guidelines of optimum processing parameters of fiber wetting for resin transfer molding and structural reaction injection molding. They are low viscosity, vacuum, high mold temperature, and high pressure.
Wetting of fiber mats for composites manufacturing: II. Air entrapment model
Authors: Chen Yung‐Tin , Macosko Christopher W. , Davis H. Ted 
Published: June 2004

AbstractIn this work the experimental observations are explained with the aid of a simple air‐entrapment model based on the concept of two levels of porosity of fiber mats. A simple model that includes liquid bypassing with initial air trapping, subsequent capillary invasion of regular fiber bundles with air compression, and finally mobilization is proposed to explain air‐entrapment phenomena. The simple model successfully rationalizes the observed air trapping and compression during initial liquid–fiber contact. An empirical equation for the mobilization efficiency is adapted to the model to estimate void content. The velocity dependence of the trapped void content predicted by the model is in good agreement with observations.
Nitridation and CVD reactions with hydazine
Authors: Vogt Kirkland W. , Kohl Paul A. , Abys Joseph A. 
Published: June 2004

AbstractThe low‐temperature nitridation of gallium arsenide, silicon and transition metals was investigated using hydrazine. Gallium nitride films were grown on gallium arsenide (GaAs) by direct reaction of the semiconductor surface layers with hydrazine et 200‐400°C. Auger electron spectroscopy and X‐ray photoelectron spectroscopy (XPS) analyses show that the films are primarily gallium nitride with a small oxide impurity. Thin nitride films (∼15 Å) were grown on silicon by reaction with hydrazine at 300–500°C. Ellipsometry results suggest that the film growth goes through different phases following linear, parabolic and logarithmic functions with time. XPS analysis shows that the nitride films could be formed at much lower temperatures than possible with ammonia (300 vs. 600°C). The formation of numerous transition metal nitrides (Co, Cr, Fe, Mo, Si, Ta, Ti, V, and W) by reaction with hydrazine at 400°C is demonstrated, as well as the chemical vapor deposition of boron nitride films from diborane and hydrazine reactants. The temperature at the mixing point was critical in determining the final composition of the film. A 1‐D transport model suggests that the reaction rate at 400°C was kinetically limited. The results also agree qualitatively with thermodynamic equilibrium calculations.
Properties of pulsing flow in a trickle bed
Authors: Tsochatzidis N. A. , Karabelas A. J. 
Published: June 2004

AbstractAn experimental study of pulsing flow in trickle beds is presented, based on data from two measuring techniques. New evidence on pulse arrangement and propagation in the bed, and data on basic pulse characteristics (frequency, celerity, length, duration) as well as liquid holdup and pressure drop measurements are included. Some of these data, such as the length of the liquid‐rich zone of pulses, are not currently available. Two flow regions exhibit different trends of pulse characteristics, “mild” and “wild” pulsing for relatively small and large liquid flow rates, respectively. New findings are compared with previously available data and correlations. An effort to develop new or modify existing generalized correlations is made for the aforementioned quantities.
Fractional crystallization: Design alternatives and tradeoffs
Authors: Dye Susan R. , Ng Ka M. 
Published: June 2004

AbstractGuildlines for the design of fractional crystallization processes to separate two‐and three‐solute mixtures are presented. By using solvent addition/removal, stream combination, and cooling/heating, these processes bypass regions of multiple saturation in the phase diagram and recover purse solutes. Design equations are formulated, and the constraints on the design variables are identified. also included is a discussion of the effect on recyle flows of changes in the design variables and an estimate of the cost of a fractional crystallization separation.
Optimal liner regualtion with hard constraints
Authors: Ajbar Hamid , Keenan Michael R. , Kantor Jeffrey C. 
Published: June 2004

AbstractAn l∞ approach to the design of linear multivariable controllers for descrete‐time systems with hard time‐domain constraints is presented. The notion of polar of the set of the exogenous inputs is used to parameterize the set of closed‐loop transfer functions that meet regulation constraints. The constraints may include magintude and rate bounds on all relevant process variables, including the control inputs. Solutions for optimal l∞ design are found by solving a linear program for the impulse‐response coefficients of the controllers, or for the coefficients of an ARMA controller model. Using these formulation, s and analytical frame work is provided for delineating the tradeoffs that govern design of linear control systems.
Axial dispersion of solid particles in a continuous rotary kiln
Authors: Kohav Tamar , Richardson James T. , Luss Dan 
Published: June 2004

AbstractSix algorithms following single particle trajectories are used to predict the axial dispersion in rolling or slumping flow in a continuous rotary kiln. Models incorporating different physical phenomena show that axial dispersion is affected by the Froude number, L/D ratio, solid fill level, and rolling or slumping layer thickness. The main cause of axial dispersion is segregation of the nonuniform particle size, density and shape. In a rolling bed with uniform particles the rolling layer thickness and the time of roll have to be accounted for in the prediction of the dispersion. The Peclet numbers computed for sold particles with uniform physical properties are of the order of 104. The sold segregated motion may lead to Peclet numbers of the order of 10 – 103, a much larger dispersion. The main obstacle for a‐priori prediction of the axial dispersion is lack of a reliable relation between the segregated roll or slump distance and the variation in particles properties, as well as the change in segregation as the flow properties of kparticles are changed during calcination.
Reaction rate modeling in noncatalytic gas‐solid systems: Species transport and mechanical stress
Authors: Rode Hitmar , Orlicki Dariusz , Hlavacek Vladimir 
Published: June 2004

AbstractA detailed model to describe the overall reaction rate of the oxidation of titanium is developed. The mathematical model consists of two facets, the first of which involves a detailed description of species transport that accounts for the formation of charged species. This is augmented by a description of the occurrence of mechanical stress due to a Pilling ‐ Bedworth ratio that differs significantly from I as well as differences between precursor and product thermal expansion coefficients. A self imposed electric field is formed across the oxide layer due to different mobilities of the species considered. This field opposes the transport of electrons and enhances the transport of anion vacancies, thus increasing the overall reaction rate compared to a pure diffusion process, while also ensuring that electrical charge is conserved. Large growth stresses result from the unmatched precursor and product volumes, significantly affecting the overall process. These results show that the incorporation of a consistent treatment of mechanical stress forms a necessay part of any accurate description of the overall behavior of a reacting particle.
Studies of aggregation effects on SOx removal by limestone powder
Authors: Kobayashi Yoshio 
Published: June 2004

AbstractTo analyze effects of aggregation in powder/gas reactions quantitatively, particle packing models were applied to aggregate structures, and theoretical equations of the reaction rate, the reaction‐equivalent particle radius, the acid gas concentration exponent and the particle radius exponent were deduced. While confirming the validity of these equations by comparing experimental values (data on SOx removal in aerosol reactions and stationary reactions) to these equations, other experimental data were analyzed using these equations. The results show that: the dependency of lime conversion on the particle size decreases sharply as the aggregation of particles in the aerosol advances; injecting limestone powder into the furnaces, using a high rate dispersion nozzle, reduces the degree of aggregation and elevates the lime conversion; and in thermobalance analysis of SOx/limestone reactions, bulk limestone powder behaves like a single giant particle, and the diffusion process inside the aggregates is controlling. It is concluded that hexagonal closest packing is the optimal packing model for the particles constituting aggregates in aerosol.
An integral‐spectral approach for reacting Poiseuille flows
Authors: Arce Pedro , Locke B. R. , Trigatti I. M. B. 
Published: June 2004

AbstractAn integral‐spectral formulation for laminar reacting flows in tubular geometry (tubular Poiseuille flows) is introduced and performed within an operator–theoretic frame‐work where the original convective‐diffusive differential transport problem coupled with reaction is inverted to give an integral equation. This equation is of second kind and of the Volterra type with respect to the axial coordinate of the tube with a kernel given by Green's function. Green's function is identified by a methodology that gives the Mercier spectral expansion in terms of eigenvalues and eigenfunctions of the Stürm–Liouville problem in the radial variable of the tube. Eigenvalue problems for both Dirichlet and von Neumann boundary conditions are solved in terms of analytical functions (Poiseuille functions) and compared with the values found in the literature. The groundwork is set for future applications of the methodology to solving a wide variety of problems in convective–diffusive transport and reaction. Examples with wall and bulk chemical reaction are given to illustrate the technique.
Recuperative parametric pumping in adsorptive membranes
Authors: Agrawal Akhil , Burns Mark A. 
Published: June 2004

AbstractRecuperative parametric pumping in adsorptive membranes, a cyclical adsorptive separation process, can lead to two distinct mechanisms of separation: rejection and preferential transport. During rejection, the adsorptive membrane rejects an adsorbing solute while permitting the transport of nonadsorbing solutes. During preferential transport the reverse occurs; the adsorptive membrane selectively transports an adsorbing solute while preventing the transport of nonadsorbing solutes. Switching from rejection to preferential transport can be accomplished by merely decreasing the ratio of the stroke volume (cyclical volume of solution pumped into and out of the membrane) to the membrane void volume. Preferential transport, earlier called an “inverse separation,” results from the adsorbing solute crossing over from an adsorbing to a desorbing region in the interior of the membrane and is governed by the shape of the equilibrium isotherms and by process variables. In experiments with lysozyme and a membrane chromatography cartridge, selectivities obtained by preferential transport were on the order of 5 and agreed with theoretical predictions. Theoretical predictions suggest that to further improve separation by preferential transport, experiments need to be conducted on low dispersion membranes (Pe > 104) and that both rejection and preferential transport can lead to continuous concentrated streams of the adsorbing solutes.
Physically meaningful modeling of the 3‐zone and 4‐zone simulated moving bed processes
Authors: Hotier G. 
Published: June 2004

AbstractThis article is intended as a complement to a recent article in this journal, “Comparative Study of Flow Schemes for a Simulated Countercurrent Adsorption Separation Process” by Ching et al. (1992). These authors carried out experiments on continuous separation of three carbohydrate mixtures with both the usual four‐section scheme and a three‐section scheme: fructose dextran (MW ≌ 9,400), raffinose dextran (MW = 6,000), and fructose‐raffinose. These two schemes were modeled as equivalent true countercurrent systems. From the experiments and model results they conclude that the equivalent true countercurrent model is fully adapted to represent the four‐section scheme, but a major discrepancy arises with the three‐section scheme: the raffinate concentration is not correctly predicted.From these results it may be concluded that when a physically meaningful modeling of the equivalent true countercurrent is realized, the apparent discrepancy disappears. From another standpoint, such a mismatch between the experiments and model predictions would not have been observed with a model that considers the periodic port motion. Such a model foresees a cyclic steady state where extract and raffinate compositions vary throughout the period T; they can be considered as constant only when averaged over the period. It is worth comparing the results of these two models, especially when the equivalent countercurrent representation is not very close to the physical phenomenon it should represent.We agree with their major finding that a three‐zone scheme represents necessarily a higher desorbent demand than a four‐zone scheme.