bluuebunny/crossref_metadata_2025_split huggingface dataset:
A numerical study of global bifurcations in chemical dynamics
Authors: Kevrekidis I. G. 
Published: June 2004

AbstractGlobal bifurcations are frequently encountered in the dynamic behavior of chemically reacting systems and their models. They cause dramatic qualitative changes in the system response, such as the birth and death of oscillations and even the onset of chaos. They involve entire regions of the system phase space, and due to their nature they are in general not predictable by standard local bifurcation methods, analytical or numerical. Special methods and algorithms must therefore be developed to locate and analyze them in parameter space. This paper presents such methods and algorithms and illustrates them through standard chemical engineering examples. Test cases include lumped chemical reactor models (homogeneous and heterogeneous, autonomous and periodically forced), a problem of compressible gas flow in porous media, and a case of two coupled oscillators. The phenomena discussed include infinite‐period bifurcations, saddle connections, frequency locking, and the creation and extinction of multifrequency responses through global manifold interactions (homoclinic tangles).
Laboratory studies of binary salt CVD in combustion gas environments
Authors: Liang Baishen , Rosner D. E. 
Published: June 2004

AbstractA flash‐evaporation technique is used to obtain vapor deposition characteristics for the binary alkali sulfates K2SO4 + Na2SO4 at 1 atm above 1, 100 K. This technique gives results of immediate engineering interest, such as dewpoint temperatures, condensate composition and rates of vapor deposition as well as useful data on the system's thermodynamic characteristics. It is concluded that alkali sulfate deposition and vaporization in combustion environments are inevitably influenced by chemical reactions such as hydroxide formation. It is also concluded that solution nonideality is important even for homologous alkali‐salt mixtures.Predictions are made using convective‐diffusion mass transfer theory, accounting for chemical reactions by means of effective volatilities, and assuming regular, nonideal condensate solutions. The predicted dewpoints, condensate compositions and deposition rates are quantitatively consistent with experimental observations. This approach, validated here, can be extended to more extreme conditions of engineering interest, including turbulent, high‐temperature/pressure systems.
Forced convection: IV. Asymptotic forms for laminar and turbulent transfer rates
Authors: Stewart W. E. 
Published: June 2004

AbstractMass transfer rates in laminar and turbulent nonseparated boundary layers are asymptotically expanded for small values of the diffusivity 𝒟AB, with a uniform state on the mass transfer surface. Results for heat transfer follow by analogy. The thermal or binary Nusselt number at small net mass transfer rates is given asymptotically by a generalized penetration expression

for short times, or for boundary layers that duplicate the surface tangential motion. For flows past rigid interfaces, the long‐time average of 〈Nu〉 is given asymptotically by a generalized Chilton‐Colburn relation

in regions of nonrecirculating motion. The measurable functions aij and bij depend only on the system shape and laminar or turbulent velocity field. Formal expressions for a00 are given, and an expression for b00 in steady flows. These results agree well with data on mass transfer operations in tubes, packed beds, and fluid‐fluid contactors.
Pressure drop in gas‐liquid downflow through packed beds
Authors: Sai P. S. T. , Varma Y. B. G. 
Published: June 2004

AbstractPressure‐drop in cocurrent gas‐liquid downflow through packed beds was experimentally measured for nonfoaming, foaming Newtonian, and non‐Newtonian liquids. The variables include the column diameter, packing size and shape, flow rates of the phases, and their physical properties. Unified correlations are presented for the data of the present study as well as data available in literature in terms of Lockhart‐Martinelli parameters and flow variables and packing characteristics. The data were modeled using a dynamic interaction model.
A simple method to study gas phase reactions
Authors: Morrison P. W. , Reimer J. A. 
Published: June 2004

AbstractTo understand better the impact of gas phase reactions on chemical vapor deposition, a new experimental method has been developed to isolate and study homogeneous reactions separately from heterogeneous reactions. This new system heats reactant gases by rapidly compressing them to temperatures greater than 1,000 K; the walls of the reactor remain constant at 500 K or less. Pressure and volume measurements determine the mean temperature of the gas. Results from a series of test reactions and simple models show that this apparatus and measurement method work well. The apparatus is inexpensive and simple to use, and has advantages over shock tubes and static systems.
Data reconciliation and parameter estimation in plant performance analysis
Authors: MacDonald R. J. , Howat C. S. 
Published: June 2004

AbstractResults of a computer study are reported applying data reconciliation and interpretation techniques to a single‐stage flash operation. Measured errors in the process variables are significantly reduced. The techniques are successfully extended to estimate flash efficiency. The flash efficiency is estimated using two methods: (1) a statistically rigorous simultaneous data reconciliation and efficiency estimation procedure, and (2), a faster, nonrigorous sequential procedure. It is found that the simultaneous procedure provides better measured variable and flash efficiency estimates.
High‐pressure solution behavior of the polystyrene‐toluene‐ethane system
Authors: Seckner A. J. , McClellan A. K. , McHugh M. A. 
Published: June 2004

AbstractThe effect of supercritical ethane on the high‐pressure phase behavior of the polystyrene‐toluene system is experimentally investigated. Adding 17.8 wt. % ethane to the polystyrene‐toluene solution lowers the temperature of the lower critical solution temperature (LCST) curve by 162°C, 22.5 wt. % lowers the temperature of the curve by 231°C, and with 24.9 wt. % the curve is shifted to such low temperatures that it merges with the upper critical solution temperature curve. In practice this means that polymer solutions can be separated at lower temperatures where thermal degradation of the polymer is less. Also, more than 99 wt. % of the polystyrene is recovered in the polymer‐rich phase after the solution phase splits. Patterson's corresponding states treatment is used to model the LCST data.
Thermoflow multiplicity in a packed‐bed reactor. II: Impact of volume change
Authors: Lee J. P. , Balakotaiah V. , Luss Dan 
Published: June 2004

AbstractSteady states with different flow rates and temperature profiles may exist in a monolith or multitube packed‐bed reactor operating under a prescribed pressure drop, due to the coupling among the species, energy, and momentum balances, and the change of physical properties with temperature and pressure. Criteria are derived predicting the conditions under which thermoflow multiplicity can occur for a gaseous reaction involving a change in the number of moles. In general, a reaction‐induced volume increase enables thermoflow multiplicity to occur for reactions with a lower adiabatic temperature rise. A surprising finding is that thermoflow multiplicity may be found for an isothermal reaction involving a volume decrease. This point emphasizes the difference in the feedback mechanism leading to thermoflow and thermokinetic multiplicity.
Two‐phase pressure drop in vertical crossflow across a horizontal tube bundle
Authors: Schrage D. S. , Hsu J.‐T. , Jensen M. K. 
Published: June 2004

AbstractAn experimental investigation has been made to evaluate the friction, acceleration, and hydrostatic pressure drops in two‐phase vertical crossflow across a horizontal tube bundle through the measurement of the void fraction and determination of the two‐phase friction multiplier. The void fractions were found to increase with increasing mass velocity for a fixed quality level. The two‐phase friction multiplier increased with increasing mass velocity for a fixed value of the Martinelli parameter in both slug and spray flow and decreased with increasing mass velocity in bubbly flows. The void fraction and two‐phase friction multiplier data were correlated and used to predict with very good results the total pressure drop occurring in simulated diabatic flow tests and in actual diabatic tests using R‐113.
Protein extractions with hollow fibers
Authors: Dahuron Lise , Cussler E. L. 
Published: June 2004

AbstractDifferential protein extractions were measured with two types of extractants: inverted micelles and two‐phase aqueous systems. The results show that hollow‐fiber extractions are substantially faster than those possible in conventional equipment. The extractions are not compromised by loading or flooding because the flows of extractant and raffinate are almost completely independent. Mass transfer coefficients inferred from the measurements both support and extend design equations for these contactors.
Non‐Gaussian dispersion in model smokestack plumes
Authors: Thompson V. L. , Greenkorn R. A. 
Published: June 2004

AbstractThe dispersion of emissions from a plume in a cross stream was studied using both experimental and theoretical techniques. Wind tunnel experiments were done for three jet to cross stream velocity ratios: R = 0.5, 1, and 2. The concentration in the plane perpendicular to the cross stream was measured using light scattering and photographic image analysis. The analysis of the negatives revealed the presence of two counterrotating vortices that greatly affected dispersion of smoke in the plume.Convection and random turbulent dispersion were modeled with a statistical convection simulator on a CYBER 205 computer. The input velocities to the convection model were obtained using experimental measurements. The model revealed the effects of the vortices on dispersion, and the concentration distributions across the plume were determined once the vortices had dissipated. The existence of the counterrotating vortices and their effects upon plume dispersion showed that the Gaussian plume model, which is widely used for modeling pollutant dispersion, is inapplicable in the near range of the model stack.
The continuous‐flow gravity thickener: Steady state behavior
Authors: Landman K. A. , White L. R. , Buscall Richard 
Published: June 2004

AbstractThe equations governing consolidation in a continuous‐flow gravity thickener are developed based on the assumption that a flocculated suspension possesses a compressive yield stress Py(ϕ) that is a function of local volume fraction only. These equations are used to model the steady state operation of a thickener. The bed height required to achieve a given underflow concentration is found to be a relatively sensitive function of the details of the Py(ϕ) function, particle flux through the thickener, and variations in the cross‐sectional area of the thickener. The limiting values of the underflow concentration ϕu for a given flux or the limiting values of flux for a desired ϕu are studied and shown to exist only for cylindrical and converging thickeners.
Stochastic simulation of aerosol deposition in model filters
Authors: Ramarao B. V. , Tien Chi 
Published: June 2004

AbstractAerosol deposition in model filters composed of layers of parallel fibers with the orientation of the fiber of each layer kept arbitrary was simulated stochastically. Particle positions at the inlet of the filter were assumed to be randomly distributed over the inlet plane with the trajectory assumed to be rectilinear because of the particles' high inertia. In the simulation, the track of each particle through the filter was monitored and the possible bounce‐off from collision surfaces was allowed for. Deposition behavior and the structure of deposits were studied with various aerosol flow conditions and filter characteristics.
Heat effects in gas sulfiding of hydroprocessing catalysts
Authors: Reyes S. C. , Ho T. C. 
Published: June 2004

AbstractAn a priori model is developed to assess the heat effects during gas‐phase (H2S/H2) sulfiding of a CoOMoO3/γAl2O3 hydroprocessing catalyst in a small fixed‐bed reactor. The model satisfactorily predicts the observed maximum temperature rise and the speed of the traveling exotherm. It is found that the amount of heat released can be of sufficient magnitude to cause significant hot spotting and local starvation of H2S, both of which need to be avoided for desired catalyst performance. With this model, parametric studies were made to optimize the sulfiding operation. It is expected that the model can be used to guide catalyst synthesis research and to scale up laboratory/pilot‐plant sulfiding procedures.
A priori design of a photoreactor for the chlorination of ethane
Authors: Clariá M. A. , Irazoqui H. A. , Cassano A. E. 
Published: June 2004

AbstractA three‐dimensional radiation model coupled with two‐dimensional mass balances for the intervening chemical species is solved for the photochemical chlorination of ethane. The analysis was carried out using the full mechanistic kinetic sequence and employing polychromatic radiation for a process conducted in a tubular reactor placed inside an elliptical reflector.Theoretical predictions were compared with bench‐scale experiments and showed excellent agreement. Using the validated model, computational experiments were conducted to explore the influence of reactor design and operational parameters upon the degree of chlorination. Dichlorination reactions were also added to the kinetic model to analyze reactor behavior from the viewpoint of selectivity. The a priori design method described can be applied from first principles and requires no experimentally adjustable parameters.
Transient free‐surface flows in injection mold filling
Authors: Mavridis H. , Hrymak A. N. , Vlachopoulos J. 
Published: June 2004

AbstractAdvancing and colliding flow fronts are important fluid mechanical problems in injection mold filling. These are small‐scale, transient free‐surface flows with a significant impact on the molecular orientation of molded parts. This paper describes a numerical technique for the simulation of transient free‐surface flows. The algorithm combines a Galerkin/finite‐element discretization of the governing equations with a predictor‐corrector scheme for integration in time, and determines simultaneously the flow field and the free surface at every time step. The method is applied to the start‐up of a fluid flow initially at rest and impingement of two flow fronts to form a weldline. Comparison of simulation with available experiments on the start‐up problem shows very good agreement. Numerical tracking of fluid elements clarifies various aspects of flow‐induced deformation of the material.
Asymmetric cellulose acetate hollow fibers: Studies in gas permeation
Authors: Sidhoum M. , Sengupta A. , Sirkar K. K. 
Published: June 2004

AbstractGas separation behavior of permeators containing asymmetric cellulose acetate hollow fibers with the dense skin on the outside, facing the feed gas, has been investigated experimentally and theoretically using a sweep gas technique. Two systems were studied: CO2‐N2, and O2‐N2 (air). If the membrane structure is assumed to be symmetric or homogeneous, the model complies with the present data much better, compared to the assumption that the membrane is asymmetric.Separation was carried out with the high‐pressure feed outside the fibers as well as inside the fibers. In both cases the data were well predicted by the homogeneous model. No concentration polarization effects were found when the feed flowed inside the fibers. The inherent membrane separation capability appeared practically the same whether the feed was inside or outside the fibers. Internal pressurization did not damage the membrane performance for the low pressure range used.These findings enable the operation of asymmetric hollow‐fiber permeators with the feed inside the fiber lumen, which gives better separation in high stage cut situations.
Pore structural effects in catalytic gasification of coal chars
Authors: Ballal Girish , Amundson N. R. , Zygourakis Kyriacos 
Published: June 2004

AbstractThe effect of pore structure on the gasification of two chars impregnated with potassium carbonate catalyst is investigated. Well‐characterized char samples were reacted with carbon dioxide, water vapor, and oxygen. The effects of catalyst on the magnitude of the gasification rate as well as on the entire rate vs. conversion pattern are reported. Reactivity and structural data are analyzed to obtain a meaningful comparison of the gasification behavior for all char‐gas pairs. Initial intraparticle catalyst dispersion was strongly influenced by the pore structure of the chars. During gasification with carbon dioxide and water, the catalyst appeared to redistribute itself in accordance to the accepted reaction mechanism, leading to superior catalytic activity. The reported measurements indicate that both the pore structure of the chars and the chemistry of a particular reaction are key factors in determining the overall catalytic performance.
Measurement and prediction of entrained‐flow gasification processes
Authors: Brown B. W. , Smoot L. D. , Smith P. J. , Hedman P. O. 
Published: June 2004

AbstractA detailed mathematical model is used to predict local and effluent properties within an axisymmetric, entrained‐flow gasifier. Laboratory experiments were conducted to provide local properties for four coal types from a gasifier operating at near‐atmospheric pressure. Effects of selected model parameters and test variables were examined and compared with measurements in most cases. The comparison of predictions and measurements provides the first evaluation of capabilities and limitations of a comprehensive model for entrained‐flow gasifiers.
A mathematical model for the AC impedance of semiconducting electrodes
Authors: Bonham D. B. , Orazem M. E. 
Published: June 2004

AbstractA mathematical model is developed to calculate the impedance response of a semiconductor electrode to a sinusoidal current perturbation. The model accounts explicitly for electron and hole transport as well as generation and recombination through band‐to‐band mechanisms and through bulk interband electron acceptors of specified energy. The resistive (real) component of the impedance is shown to be sensitive to the concentration, distribution, and energy level of the bulk sites. The capacitive (imaginary) component, while useful for determining the dopant level and the flat‐band potential of the semiconductor, is relatively insensitive to low concentrations of bulk sites.
Nonlinear inferential control
Authors: Parrish J. R. , Brosilow C. B. 
Published: June 2004

AbstractNonlinear inferential control (NLIC) has been developed as a method for improving control of nonlinear systems. The controller is modelbased, and allows for direct use of available measurements. This paper presents the structure of NLIC and the manner in which it is applied to processes when the controlled variables are measured. Also described is the improvement in the process control using NLIC. Two illustrative examples are presented, a laboratory heat exchanger process and a simulated neutralization process. The results indicate that a substantial improvement in control is possible using NLIC.
Dynamic behavior of supercritical extraction of kerogen from shale
Authors: Triday Jaime , Smith J. M. 
Published: June 2004

AbstractA model is presented for the supercritical reaction‐extraction of a solid component from a bed of porous particles through which the solvent flows. Accounted for are reaction at an intraparticle position, pore diffusion, interphase mass and heat transfer, and flow in a differential reactor. The behavior of the bed is dynamic since both temperature and concentration of extractable solid vary with time.The model is compared with experimental data for the extraction with toluene of kerogen from a bed of Colorado shale particles. Curves of effluent bitumen concentration vs. time were measured and fitted with predicted curves to evaluate three parameters: preexponential factor and activation energy for the conversion of insoluble kerogen to soluble bitumen, and the diffusivity of toluene at the critical point. The predicted curves agreed well with the experimental results and gave reasonable values for the parameters. Rates of reaction‐extraction as well as extraction curves exhibited a sharp dip as the reactor was heated through the critical temperature.
Dynamics of a sodium heat pipe reforming reactor
Authors: Richardson J. T. , Paripatyadar S. A. , Shen J. C. 
Published: June 2004

AbstractA prototype of a sodium heat pipe reactor for steam reforming processes has been developed and tested at energy levels up to 6 kW. Both experimental and model simulation data were obtained. Temperature profiles at nonflow and nonreaction flow conditions were used to adjust the parameters of the model, along with an intrinsic rate equation measured in a differential, bench‐scale apparatus.Very good agreement between experiment and model was found for approximately 50 runs under steady state reaction conditions covering a range of process variables. Advantages of the heat pipe as a reactor energy source were confirmed. These include isothermal wall temperatures, energy flux transformation, adjusted flux profiles, and rapid startup. These measurements and computations have provided a model from which optimized configurations have been determined.
Molecular thermodynamics of partially‐ordered fluids: Microemulsions
Authors: Hu Ying , Prausnitz John M. 
Published: June 2004

AbstractMixtures of water, oil, and surfactant often form microemulsions where tiny drops of oil (or water) are dispersed in a matrix of water (or oil). A microemulsion is a partially‐ordered fluid in which the surfactant molecules exist in an oriented layer at the boundary between the continuous phase and the discontinuous phase. This work proposes a molecular‐thermodynamic model that is useful for calculating a variety of phase diagrams observed in microemulsion‐forming systems.The model is based on a revised form of Widom's abstract representation of a microemulsion; that representation provides a procedure for calculating essential contributions to the configurational entropy. In Widom's representation, all molecules are difunctional dumbbells such that all the hydrophilic ends are situated together in some lattice sites and all the lipophilic ends are situated together in other sites. A form of Guggenheim's quasichemical theory is used to evaluate the combinatorial contributions that correspond to Widom's picture. Chemical association of surfactant molecules and physical interactions are also taken into account. Minimization of the Gibbs energy yields a distribution function for representing micelle sizes.Calculated ternary phase diagrams are in good agreement with experimental results. Different types of diagrams can be obtained upon changing physically significant model parameters. A study of how these parameters affect phase behavior provides insight on microemulsion stability.
The size of turbulent eddies close to a wall
Authors: Lyons Stephen L. , Nikolaides Christos , Hanratty Thomas J. 
Published: June 2004

AbstractCoherent flow‐oriented structures, with a dimensionless spacing of λ+ ≈ 100, observed close to a wall in turbulent flows are believed to control the production of turbulence. A 2 1/2 D computer model developed by Nikolaides is used to explain the dimensionless spacing of these structures. The calculations suggest that the net production of turbulence in the viscous wall region is sensitive to changes in λ+ in the neighborhood of λ+ ≈ 100. They indicate that the net production is negative for λ+ < 85 and that for λ+ = 93 the net production is large enough to supply the turbulent energy dissipated in the outer flow.