bluuebunny/crossref_metadata_2025_split huggingface dataset:
Continuous and discontinuous models for transport phenomena in polymers
Authors: Ocone Raffaella , Astarita Gianni 
Published: June 2004

AbstractA model for transport phenomena in polymers accompanied by morphological changes is presented that includes the essential features of a variety of seemingly disparate models available in the literature. The latter are classified as special cases of the model considered, and according to whether discontinuities of the morphology are or are not explicitly described. An ordering analysis is presented that indicates under which conditions one or more of the terms appearing in the differential equations can be neglected. A special subcase, termed surface crystallization, is shown to emerge in a well‐defined asymptotic sense from the general model, and is also shown to yield predictions which are closely analogous to those of a model of Astarita and Sarti that has been very successful in correlating experimental data of the type called case II transport. The advantage of the surface crystallization model is that it is not an ad hoc model. The models considered result in hyperbolic differential equations, as is often the case when relaxation phenomena are taken into account. A procedure for numerically solving hyperbolic equations with great ease is presented.
Equilibrium theory for complete adiabatic adsorption cycles
Authors: Davis M. M. , Levan M. D. 
Published: June 2004

AbstractComplete adiabatic fixed‐bed adsorption cycles with adsorption, heating, and cooling steps are analyzed using simple wave theory. Solutions consist of simple waves, shocks, combined waves, and wave interactions with patterns established by a mapping between hodograph and physical planes. The system considered is benzene adsorbed on activated carbon with nitrogen as the carrier gas.
Prediction of multicomponent liquid adsorption equilibria
Authors: Price P. E. , Danner R. P. 
Published: June 2004

AbstractA new method of predicting adsorption equilibria for multicomponent liquid solutions on solids has been developed which combines the thermodynamic and kinetic treatments of liquid adsorption. Multicomponent equilibria can be predicted using parameters obtained from binary adsorption data. The method requires the adsorbent capacities, the bulk and surface phase activity coefficients, and the binary adsorption equilibrium constants. The calculation procedure is simpler than other available methods. Predictions are made for the mixture of benzene, ethyl acetate, and cyclohexane on activated carbon at 303 K. The predictions are in quantitative agreement with published experimental data for this system.
Tracer diffusion measurement in polymer solutions near the glass transition by forced rayleigh scattering
Authors: Huang W. J. , Frick T. S. , Landry M. R. , Lee J. A. , Lodge T. P. , Tirrell Matthew 
Published: June 2004

AbstractForced Rayleigh scattering, a relatively recently developed optical technique, is used to measure tracer diffusion coefficients in polymer‐solvent mixtures near the system glass transition temperature, Tg. The technique has a wider range of potential application than has yet been realized, and so is presented in some detail. The objectives were to obtain data necessary to scrutinize free volume theory, and to understand so‐called anomalous, non‐Fickian diffusion effects observed by others in polymer‐solvent mixtures near Tg. Data on dye tracer diffusion coefficients in the systems polyvinyl acetate‐toluene, polystyrene‐toluene, and polystyrene‐tri‐m‐tolyl phosphate were obtained over a polymer concentration range from infinite dilution to 96 wt. %. Small molecule diffusion coefficients are seen to vary by as much as nine orders of magnitude (10−14 to 10−5 cm2/s) over this concentration range. The data are in reasonable accord with expectations based on the Duda‐Vrentas version of free‐volume theory.
Microflow calorimeter design for heats of mixing
Authors: Raal J. D. , Webley P. A. 
Published: June 2004

AbstractMicroflow calorimeter design for heats of mixing is surveyed. Precise temperature equilibration of the fluids before mixing, the elimination of frictional energy effects from the instrument response, and the absence of flow‐rate and physical‐property‐dependent heat leaks are particularly important. A design using a differential mode of operation is based on a careful analysis of entropy generation in flows with mixing and friction. The compensation for frictional heating and the lack of dependence of measurements on flow rate and mixture physical properties other than hE represents a major advance. Measured data for the welltested cyclohexane‐hexane system had an average deviation of only 0.41%. For the more viscous cyclohexane‐1‐hexanol system, measured data of similar precision are considered the best available.
The periodic, electrochemical codeposition of cadmium and tellurium
Authors: Verbrugge M. W. , Tobias C. W. 
Published: June 2004

AbstractA mathematical model is presented for the codeposition of cadmium and tellurium onto a rotating‐disk electrode. The treatment incorporates the equation of convective diffusion for liquid phase mass transport, Butler‐Volmer expressions for charge‐transfer reactions, and a thermodynamic model for individual component activities in the solid state. Because of the formation of CdTe, a compound that has a large negative free energy of formation, the cadmium deposition reaction occurs at potentials substantially positive to its standard electrode potential (Uθ = −0.40 V). This reaction, along with the deposition of tellurium (Uθ = + 0.55 V), produces an electrodeposit that contains cadmium, tellurium, and cadmium telluride. The model can be used to calculate transient current‐potential relationships, ionic concentration profiles, and deposit compositions. Transport and kinetic parameters for cadmium and tellurium deposition are reported; a multidimensional optimization routine is used to evaluate physiochemical parameters from experimental data for the codeposition process.
Snap‐off of gas bubbles in smoothly constricted noncircular capillaries
Authors: Ransohoff T. C. , Gauglitz P. A. , Radke C. J. 
Published: June 2004

AbstractA corner flow hydrodynamic theory is outlined for the time to snap‐off of a gas bubble moving through a smoothly constricted noncircular capillary as a function of the pore geometry and the capillary number, Ca. Above a transition capillary number the time to snap‐off is independent of Ca, while below it the time to snap‐off is inversely proportional to the capillary number. Thin films of liquid deposited along the capillary walls are shown to play a minor role; they are accordingly neglected in the analysis. The proposed theory is compared to new experimental results for snap‐off in two constricted square capillaries (dimensionless constriction radii of 0.3 and 0.5) over a range of capillary numbers (10−5 to 10−3), wetting‐liquid viscosities (1.0 to 8.5 mPa · s), and surfactant types. Good agreement is found between theory and experiment.
Flow and diffusion of gases through porous substrates
Authors: Allawi Z. M. , Gunn D. J. 
Published: June 2004

AbstractAn experimental study of simultaneous flow and binary diffusion through four porous catalyst substrates was conducted in the range of pressure from 1 to 3 bar absolute. Experiments on the flow of pure gases and isobaric binary diffusion were carried out on the same catalyst substrates. The differential equations describing simultaneous flow and diffusion in the form of the Chapman‐Enskog first‐order approximations were solved analytically, taking into account the variation of viscosity with composition. The parameters for the diffusion and flow processes were found from separate experiments on flow and diffusion, from experiments on nonisobaric flow and diffusion, and from composition transients in fixed beds. There was good agreement between theory and experiment except for permeability experiments at low pressures, where significant differences were found.
Silicon particle growth in a fluidized‐bed reactor
Authors: Hsu George , Rohatgi Naresh , Houseman John 
Published: June 2004

AbstractThis paper describes the overall operation of a 15 cm dia. fluidized‐bed reactor for silicon production via silane pyrolysis. The fluidized‐bed reactor has been characterized by its performance data and parametric effects. A withdrawal system is used to selectively remove big particles. Seed generation is provided by a fluid jet milling device, which avoids metallic grinding contamination. The silicon particle size growth calculated from a scavenging/deposition model agrees with actual size measurements.
Reaction front propagation in nonadiabatic exothermic reaction flow systems
Authors: Gatica J. E. , Puszynski Jan , Hlavacek Vladimir 
Published: June 2004

AbstractA general analysis of the reaction wave propagation in nonlinear exothermic nonadiabatic diffusion‐convection‐reaction systems is presented. The following topics are considered: approximation of temperature profiles based on the idea of infinitesimaly thin reaction front, estimation of maximum temperature, estimation of the conversion at the hot spot, a priori calculation of the front velocity, analysis of the self‐ignition phenomena and their relation to wave propagation, and discussion of the effect of the heat loss parameter on the direction of propagation and shape of traveling waves. A comparison of the analytical work with exact numerical results reveals excellent agreement. The main objective of this paper is to provide a definite analysis of reaction wave propagation in reacting flow systems.
Effect of pore‐distributed coke on catalyst regeneration kinetics
Authors: Chang Yuli , Perlmutter D. D. 
Published: June 2004

AbstractPrior experimental study by Haldeman and Botty has demonstrated that catalyst regeneration rates vary with conversion, showing a nearly constant rate of reaction in the early stages of regeneration and falling off rapidly at higher conversion levels. This peculiar reaction behavior calls for explanation in terms of pore structural changes. In this work a mathematical model is developed based on the interaction between catalyst pore structure and coke distribution in the pores. The model is able to associate overall regeneration kinetics with alternative pore‐distributed coke deposition patterns. Results exhibit a slight shallow rate maximum at an intermediate conversion level, in agreement with the prior experimental report. The model makes coke distribution in the pores accessible to experimental determination. The numerical results indicate that among pore structures with the same mean and overall pore volume, a larger variance offers a higher threshold for coke loading; for a given degree of dispersion, a smaller mean allows a higher coke loading.
Hydration of isobutene in a trickle‐bed reactor: Wetting efficiency and mass transfer
Authors: Leung P. C. , Recasens F. , Smith J. M. 
Published: June 2004

AbstractReaction rates for the hydration of isobutene were measured at 323 K and atmospheric pressure in a trickle‐bed reactor over a sixfold range of liquid rates. Three widely different feed conditions were used to vary the effect of mass transfer and wetting efficiency. The effect of wetting efficiency on the rate was analyzed using the additive procedure of Tan and Smith. The results showed that mass transfer from liquid to particle, wetting efficiency, and intraparticle kinetics all influenced the total rate, but that the gas‐to‐liquid mass transfer resistance was unimportant. Most of the resultant values of wetting efficiency and liquid‐to‐mass transfer coefficients were a little lower than literature values. This may be due to the high surface tension of our aqueous system. The derived total rate equations agreed well with the experimental rates, suggesting that the additive procedure is a satisfactory method of accounting for wetting efficiency.
Thermoflow multiplicity in a packed‐bed reactor Part I: Adiabatic case
Authors: Lee J. P. , Balakotaiah V. , Luss Dan 
Published: June 2004

AbstractSteady states with different flow rates and temperature profiles may exist in a packed‐bed reactor operating under a prescribed pressure drop, due to the coupling among the species, energy, and momentum balances and the change of the physical properties with temperature and pressure. This thermoflow multiplicity may be found even for reactions whose rate is rather insensitive to temperature changes (low activation energy) and may lead to highly undesirable operation of multitube packed‐bed reactors. A hierarchy of models based on different assumptions is used to derive criteria for predicting the conditions under which this thermoflow multiplicity may occur.
Transient free‐surface flows: Motion of a fluid advancing in a tube
Authors: Behrens R. A. , Crochet M. J. , Denson C. D. , Metzner A. B. 
Published: June 2004

AbstractThe fluid motion at a free surface advancing into a mold or duct is appreciably different from its steady state behavior in well‐developed flow; this affects the residence time distribution and structure of macromolecular fluids as they are frozen in injection molding processes. In this work such motion is treated numerically and measured precisely for Newtonian fluids. While the three‐phase contact line represents a special problem conceptually and analytically, a very simple algorithm is seen to represent the fluid motion in this region accurately.Good agreement is found over wide ranges of the governing dimensionless groups (the Reynolds, Jeffrey, and capillary numbers). Since viscous forces are dominant under the circumstances studied, this finding is not surprising but it confirms the applicability of the numerical methods developed herein to the modeling of these flows under conditions of actual interest. As a result, simulations may be made with confidence to predict flow patterns encountered in practice but difficult to reproduce in laboratory experiments.
Chemically driven convection in a porous medium
Authors: Viljoen H. , Hlavacek V. 
Published: June 2004

AbstractThis paper is focused on the analysis of interaction of free convection and exothermic chemical reaction. As a consequence of the chemical reaction, free convection effects can result. It is difficult to perform an analytical bifurcation analysis of the full nonlinear governing equations; however, Fourier expansion combined with a Galerkin approximation results in a small set of ordinary nonlinear differential equations (initial‐value problem) that are amenable to analysis. Conditions for branching of the solution can be determined in an analytical way. A continuation algorithm makes it possible to calculate the branches of stability. The results of the approximative analysis are supported by the numerical integration of the full governing nonlinear equations.
Limitations of the equilibrium theory of countercurrent devices
Authors: Sundaresan S. , Wong J. K. , Jackson R. 
Published: June 2004

AbstractIn the analysis of countercurrent adsorption processes, an equilibrium theory in which local equilibrium between the fluid and the adsorbent phases is assumed has been used in the past to determine the composition of the pinch state that would be obtained for long columns. It is shown that this equilibrium theory is deficient and that certain mass transfer parameters, which are lost in the procedure by which the model equations for the equilibrium theory are formulated, can have a profound effect on the composition of the pinch.
Design of multivariable linear‐quadratic controllers using transfer functions
Authors: Harris T. J. , Macgregor J. F. 
Published: June 2004

AbstractOptimal linear‐quadratic (LQ) controller design is usually associated with state space techniques. However, when one has measurements of the outputs to be controlled, there are many advantages to designing these LQ controllers using input‐output transfer function models. The design procedure leads to a discrete equivalent of the Wiener‐Hopf equation, which can be solved using a spectral factorization approach.In this paper the design procedure is presented and various interpretations of the resulting controllers are discussed. In particular, the controllers are shown to be of the internal model controller (IMC) form, and the Wiener‐Hopf procedure is shown to be a powerful way of selecting approximate model inverses and filters that yield good performance and robustness characteristics. The approach treats the problem of simultaneous disturbance rejection and set‐point tracking, and it easily handles nonsquare systems.The design approach, its performance/robustness trade‐offs, and the structure of the resulting controllers are demonstrated using models for several processes, including a two‐input/one‐output sheet forming process, a (3 × 3) multivariable level control problem, and a (2 × 2) multivariable catalytic reactor.
Application of LQ and IMC controllers to a packed‐bed reactor
Authors: Kozub D. J. , Macgregor J. F. , Wright J. D. 
Published: June 2004

AbstractA practical application of advanced, model‐based, multivariable control to a pilot plant packed‐bed reactor carrying out highly exothermic butane hydrogenolysis reactions is presented. The system is first stabilized by using butane flow rate to control the reactor hot‐spot temperature. Propane production and butane conversion are then controlled using multivariable controllers that manipulate the hot‐spot temperature set point and hydrogen flow rate. The controller designs are based on multivariable transfer function models developed using multivariable time series and process identification methods.Two types of multivariable controllers are applied. The first is an internal model controller (IMC) utilizing a stabilized approximation of the model inverse and a tunable exponential filter. The second is a linear‐quadratic (LQ) controller design using pulse transfer function models to characterize the process dynamics and autoregressive‐moving average models to characterize the disturbances. The optimal control solution was arrived at via a spectral factorization solution to the Wiener‐Hopf equations. The results indicate that both designs, if well tuned, provide good performance and robustness over a wide range of operation.
Clustering in supercritical fluid mixtures
Authors: Kim Sunwook , Johnston K. P. 
Published: June 2004

AbstractThe local compositions of supercritical fluid carbon dioxide and a cosolvent about a solute at infinite dilution have been estimated for the first time using spectroscopic measurements. The solute was phenol blue, the bulk concentrations of the cosolvents n‐octane, acetone, ethanol, and methanol ranged from 0 to 100 mol%, and the pressure ranged from 80 to 300 bar. In each case, the clusters of solvent about the solute are enriched in cosolvent. Solubility data have been measured for phenol blue in CO2 with and without the cosolvent acetone, and regressed using an augmented van der Waals‐density‐dependent local composition model to determine the relevant binary interaction energies. The local compositions calculated from the model agree with those that were esimated spectroscopically. The model was used to predict solubilities of solids in supercritical fluid CO2 with various cosolvents.
A network model for deep bed filtration processes
Authors: Sharma M. M. , Yortsos Y. C. 
Published: June 2004

AbstractEquations for the deep bed filtration problem are obtained as a special case of a general model formulated earlier. The filtration coefficient λ is expressed as an explicit function of system parameters such as fluid flow rate, pore density, and pore size distribution. For a unimodal pore size distribution it is found that λ remains constant both in space and time. In general, however, λ is shown to decrease. Explicit solutions to the problem, including particle density profiles and permeability, are obtained for the two cases of large values of the coordination number Z (capillary tube model), and for a bimodal pore size distribution with a finite coordination number.
High‐pressure particulate expansion and minimum bubbling of fine carbon powders
Authors: Jacob K. V. , Weimer A. W. 
Published: June 2004

AbstractParticulate expansion and minimum bubbling parameters (n, u′t, umB, εmB) are measured for fine carbon powders (dp = 44 and 112 μm) fluidized with synthesis gas (H2/CO = 0.8) at pressures within the range 2,070 < P < 12,420 kPa in an industrial, pilot‐scale fluidized bed. Deviations between minimum bubbling (umB and εmB) and minimum fluidization (umf and εmf) conditions increase with increasing pressure, P. The expansion index, n, decreases with increasing P and always exceeds values recommended by Richardson and Zaki for solid/liquid systems. Particulate bed expansion for the fine powders is well characterized by the equations of Foscolo et al. and Abrahamsen and Geldart. The theory of Foscolo and Gibilaro adequately estimates the onset of bubbling for both powders at all P, provided that experimentally determined values of n and u′t, are applied. For the dp = 112 μm powder, the theory of Foscolo and Gibilaro is applicable with calculated values of ut and experimental values of n.
Modeling the pore structure of coal
Authors: Bhatia S. K. 
Published: June 2004

AbstractThe predictions of the random‐pore model, and various other models formulated here, are compared with experimental data for the uptake of methane by coal. It is found that the most appropriate is a modified model considering coal to be comprised of microporous grains, with the random‐pore model holding for the micropores in the grains. The new model still predicts the same reaction behavior as the random‐pore model.
Studies in the heat integration of chemical process plants
Authors: Viswanathan Murali , Evans L. B. 
Published: June 2004

AbstractA method for solving the problem of heat exchanger network synthesis in chemical process plants is presented based on the use of the out‐of‐kilter algorithm. The method represents an extension of previous approaches based on network flow problem formulations. A new approach to overcome the constraints in forbidden‐match problems, termed the dual stream approach, has been developed based on the use of a stream as both a hot and a cold stream. This approach results in considerable savings in utility costs in certain forbidden‐match problems. An algorithm to deal with heating or cooling utilities that undergo temperature changes, termed non‐point‐temperature utilities, is presented using the transshipment model. The new algorithm determines the utility mass flow rates and pinch points accurately.
Diffusion path analysis of dynamic behavior of oil‐water‐surfactant systems
Authors: Raney K. H. , Miller C. A. 
Published: June 2004

AbstractThe diffusional processes that occur when oil contacts an aqueous surfactant solution have been investigated. These are pertinent to enhanced oil recovery by surfactant flooding and to detergency in certain systems. The theory of diffusion paths was used to solve the diffusion equations for a model pseudoternary system. Although detailed comparison with experimental results was not possible due to a lack of necessary data, calculated diffusion paths and interface velocities were found to be useful for explaining various phenomena observed experimentally in an oil recovery system.Because the surfactant “solutions” studied commonly exist as dispersions of liquid crystal, extension of the theory to allow for initial compositions in two‐phase regions was carried out. Also novel was investigation of the effect of the three‐phase regions that occur in these systems on the position and shape of calculated diffusion paths.
Contact nucleation from aqueous dextrose solutions
Authors: Elankovan P. , Berglund K. A. 
Published: June 2004

AbstractContact nucleation studies were performed with the dextrose‐water system. Under some conditions two crystallographic phases were formed, which were identified in situ using a laser Raman microprobe. In addition, the growth of the contact nuclei was monitored in situ using photomicroscopy. The results are discussed in the context of contact nucleation and growth models.