bluuebunny/crossref_metadata_2025_split huggingface dataset:
Constraint handing and stability properties of model‐predictive control
Authors: de Oliveira Nuno M. C. , Biegler Lorenz T. 
Published: June 2004

AbstractEffects of hard constraints in the stability of model‐perdictive control (MPC) are reviewed. Assuming a fixed active set, the optimal solution can be expressed in a general state‐feedback closed form, which corresponds to a piecewise linear controller for the linear model case. Changes in the original unconstrained solution by the active constraints and other effects related to the loss of degrees of freedom are depicted in this analysis. In addition to modifications in the unconstrained feedback gain, we show that the presence of active output constraints can introduce extra feedback terms in the predictive controller. This can lead to instability of the constrained closed‐loop system with certain active sets, independent of the choice of tuning parameters. To cope with these problems and extend the constraint handling capabilities of MPC, we introduce the consideration of soft constraints. We compare the use of the l2‐(quadratic), l1‐(exact), and l∞‐norm penalty formulations. The analysis reveals a strong similarity between the control laws, which allows a direct extrapolation of the unconstrained tuning guidelines to the constrained case. In particular, the exact penalty treatment has identical stability characteristics to the correspondent unconstrained case and therefore seems well suited for general soft constraint handling, even with nonlinear models. These extensions are included in the previously developed Newton control framework, allowing the use of the approach within a consistent framework for both linear and nonlinear process models, increasing the scope of applications of the method. Process examples illustrate the capabilities of the proposed approaches.
Self‐diffusion coefficients of sol‐gel intermediates
Authors: Ng Li Voon , McCormick Alon V. 
Published: June 2004

AbstractSol‐gel techniques have shown great potential for producing ceramic materials of designed composition and properties. A typical sol‐gel process involves the hydrolysis and polymerization of one or more metal alkoxides in an alcohol solution in a batch reactor to form a homogeneous gel, which is then further treated to provide the type of ceramic desired (such as aerogel, glass, crystal, and coating). Several authors have suggested that diffusion effects may influence the development of the gel microstructure. We investigate what governs the diffusivity of the precursors using the pulsed‐gradient‐spin‐echo NMR technique. This initial report focuses on the early stages of the batch reaction.The system chosen is the acid catalyzed hydrolysis and condensation of tetraethyl orthosilicated in ethanol. 1H PGSE NMR and 29Si NMR show that the diffusivities of the silicate precursors are strong functions both of the molecular weight and of hydrogen bonding between the polar functional groups and the solvent. Moreover, since hydrolysis increases and polymerization reduces the concentration of these polar groups, the mobility of the reactive oligomers changes in a complex manner. Combining our results from PGSE NMR and gas chromatography, we estimate the extent of hydrogen bonding effect on the mobility of various oligomers, providing a model to predict diffusivity trends throughout the batch reaction.
Vapor‐liquid equilibrium of polymer solutions using a cubic equation of state
Authors: Orbey Nese , Sandler Stanley I. 
Published: June 2004

AbstractA recently proposed mixing rule has been combined with a cubic equation of state to correlate the vapor‐liquid equilibria of various polymer + solvent and solvent + long chain hydrocarbon mixtures. We find that the two‐parameter version of this model can correlate the solvent partial pressure in concentrated polymer solutions with high accuracy over a range of temperatures and pressures with temperature‐independent parameters. For the solvent + long chain hydrocarbon systems, for which accurate vapor‐phase concentrations are also important, the model is best used as a three‐parameter correlation. Advantages of this approach are that it extends the application of cubic equations of state to polymer + solvent systems in a simple fashion and that generally only temperature‐independent parameters are required which allow for accurate interpolation and extrapolation of available data.
Structure of the stretching field in chaotic cavity flows
Authors: Liu M. , Peskin R. L. , Muzzio F. J. , Leong C. W. 
Published: June 2004

AbstractStretching of material elements in time‐periodic cavity flows is investigated numerically. The spatial structure of the stretching field is determined not only by nonchaotic islands and by unstable manifolds of hyperbolic periodic points, but also by singularities of the flow field at the cavity corners. For the short time scales interesting to most mixing applications, regions of very high stretching (good local mixing) are determined by unstable manifolds that pass close to the corners of the cavity. Low stretching (poor local mixing) regions are usually found both inside and near islands. In some cases, however, the unstable manifolds wrap themselves around the islands, preventing the formation of segregated low stretching subregions within the chaotic region.
Pressure‐dependent gas heat transport in a spherical pore
Authors: Wolf Jeffrey R. , Strieder William C. 
Published: June 2004

AbstractA mean free path gas kinetic theory is used to model the conductive heat transport of a gas within a void volume enclosed in a Fourier solid. A variational upper bound principle is derived for a void of arbitrary shape and applied to obtain a rigorous upper bound equation for the void gas conductivity in a spherical void. The variational void gas conductivity equation is exact in both the large and small Knudsen number (Kn) limits and provides a means to determine the accuracy of the reciprocal additivity interpolation formula as applied to thermal conductivity rather than diffusive mass transfer (maximum error 6% at Kn = 0.5 and α = 1). Temperature jump will occur even at atmospheric pressures and higher for sufficiently small thermal accommodation coefficients (α < 0.1). Experimental void gas heat conductivities vs. pressure data for H2, He, Ne, N2, CO2, and F12 in a polyurethane foam are compared with theoretical mean free path void gas conductivity vs. inverse Knudsen number curves drawn for various α. Estimates of the thermal accommodation coefficients for the gas‐ polyurethane surface exhibit a maximum with increasing molecular mass of the gas molecules, which qualitatively agrees with the predictions of Baule's classical theory. Results also point to a rather sharp shift of the S curve to higher pressures with decreasing thermal accommodation.
Modeling of coal pyrolsis kinetics
Authors: Liliedahl Truls , Sjöström Krister 
Published: June 2004

AbstractFour different coals were instantaneously introduced into a laboratory‐scale pyrolysis chamber at various temperatures, and their develatilization profiles were analyzed with respect to kinetics. The experimental setup consisted of a gas chromatograph (GC) with a pyrolysis unit, an empty column, and a detector. Detector signals were digitalized and stored for further handling. Normally, 1,024 values were sampled at 250 Hz. For deconvolving the true instantaneous devolatilization from the observed detector response, the residence time distribution was compensated for.An empirical model approach is developed to correlate the total devolatilization rates when instantaneously introducing a particle in a constant temperature environment. The model is also extended to pyrolysis application.
A new reactor for purely homogeneous kinetic studies of endothermic reactions
Authors: Safvi S. A. , Mountziaris T. J. 
Published: June 2004

AbstractA new counterflow jet reactor has been designed to study the purely homogeneous kinetics of endothermic reactions. The reactor consists of two vertical, coaxial, counterflowing, laminar jets and radial‐flow exit region. It can be used to generate a reaction zone near the stagnation point and away from walls, thus eliminating the possibility of surface reactions. One jet is heated and contains only a suitable carrier gas such as hydrogen and nitrogen, while the other is unheated and contains the compound(s) under study diluted in the same carrier gas. A 2‐D model of the process has been used to simulate the thermal decomposition of tertiary‐butyl‐arsine, a precursor for metal‐organic chemical vapor deposition of GaAs films. Performance diagrams based on Reynolds and Demköhler numbers were constructed to identify optimal operating conditions and to demonstrate the feasibility of the technique. This reactor appears to be an attractive choice for studies of the purely homogeneous kinetics of endothermic reactions at pressures close to atmospheric.
Solute‐induced effects on the structure and thermodynamics of infinitely dilute mixtures
Authors: Chialvo Ariel A. , Cummings Peter T. 
Published: June 2004

AbstractA general molecular‐based formalism developed rigorously establishes microscopic bases of the supercritical solubility enhancement in terms of well‐defined molecular correlation function integrals by unambiguously splitting the mixture's properties into short‐(finite) and long‐range (diverging) contributions. Consequently, the short‐range nature of the solute's and solvent's residual chemical potentials is proved and the change of the solvent's local environment around an infinitely dilute solute and its finite contribution to solute mechanical partial molar properties are interpreted in terms of the short‐range solute‐solvent and solvent‐solvent direct correlation function integrals. The solute‐induced effect on the system's microstructure and thermodynamics approaches zero as the mixture approaches ideality or the solute becomes an ideal gas particle. At the solvent's critical conditions, the solute‐induced effect on the solute's partial molar properties shows no compressibility‐driven singularity, though along the critical isotherm it can exhibit a finite extremum or a change of curvature near critical density, depending on the type and strength of solute‐solvent interactions. The utility of the proposed solvation formalism is illustrated using statistical mechanical integral equation calculations for three simple models of infinitely dilute near‐critical mixtures: pyrene‐CO2, diterbutyl nitroxide‐ethane, and Ne‐Xe.
Viscous film flow in the stagnation region of the jet impinging on planar surface
Authors: Yeckel Andrew , Strong Lisa , Middleman Stanley 
Published: June 2004

AbstractA capillary jet of liquid impinges on a planar surface that is normally oriented to the axis of the jet. The surface is initially covered with a thin uniform film of a viscous liquid. The impact and radial spreading of the liquid from the jet cause the underlying viscous film to be removed from the surface. An approximate analysis predicts the thinning rate of the film in the stagnation region of the jet. It uses the shear stress and pressure distribution of the classical Homann flow as boundary conditions for an analytical solution of the Reynolds lubrication equations in this underlying viscous film. A more exact analysis modifies the Homann flow to account for the mobility of the liquid film beneath the spreading jet and sheds light on the limitations of the analytical lubrication analysis. Data presented are in excellent agreement with the theory, subject only to the choice of a value for the hydrodynamic constant that appears in the Homann analysis.
Analysis of steric hindrance effects on adsorption kinetics and equilibria
Authors: Jin Xuezhi , Talbot Julian , Wang N.‐H. Linda 
Published: June 2004

AbstractMost existing adsorption models do not properly consider steric hindrance effects of preadsorbed solutes. As a consequence, the models often fail to represent the adsorption kinetics and equilibria accurately. In this work, we extend the random sequential adsorption concept for irreversible adsorption to analyze reversible adsorption on a continuous surface and a random site surface. Based on simulation results of these processes, general kinetic equations for one‐component adsorption are developed. The equations are used to correlate chromatography frontal curves of lysozyme and isotherm data of ethane adsorption on activated carbon and ethylene adsorption on a molecular sieve. The significance of the equations, as compared with the Langmuir equation, lies not only in their ability to correlate the experimental data more accurately, but in the physical significance of the adsorption parameters such as the maximum adsorption capacity obtained from the correlation. Our study shows that steric hindrance effects alone result in nonlinear Scatchard and Hill plots with negative cooperativity.
Modeling the role of bacteria in leaching of low‐grade ores
Authors: Batarseh Kareem I. , Stiller Alfred H. 
Published: June 2004

AbstractA robustious structural model is developed to describe the role of bacteria in the leaching process of low‐grade ores under conditions controlled by intraparticle diffusion. The main impetus behind developing this model is to provide an insight into such systems, together with a suitable framework for interpreting experimental data. The model is derived in detail with respect to reaction chemistry and the role of bacteria in catalyzing these reactions, specifically the synergism of chemistry, physics and biology in determining the overall behavior of the system. The model is used to simulate the atmospheric oxidation of iron disulfide contained in porous solids in the presence of Thiobacillus ferrooxidans (T. ferrooxidans). The experimental data are predicted well by the model, which demonstrates its applicability and supports the view that the rate of intraparticle diffusion is the controlling mechanism for this system.
Analysis of capillary‐driven viscous flows during the sintering of ceramic powders
Authors: Martínez‐Herrera Jorge I. , Derby Jeffrey J. 
Published: June 2004

AbstractTime‐dependent viscous flows driven by capillarity act to minimize the surface area of a fluid bounded in a plane geometry with initial gradients in surface curvature. These free‐surface flows are solved by a finite‐element model applied to describe the viscous sintering of two‐dimensional ceramic particles. The numerical model is validated by comparison to the analytical solution obtained by Hopper (1990) for the coalescence of two infinite cylinders of equal cross section and is applied to several other geometries pertinent to the study of particle sintering for which analytical results are not available. Details of the flow fields and morphological evolution lend insight to the physical behavior of these systems and provide a basis for the more complete understanding of viscous sintering phenomena.
Robust design of countercurrent adsorption separation processes: 2. Multicomponent systems
Authors: Mazzotti Marco , Storti Giuseppe , Morbidelli Massimo 
Published: June 2004

AbstractThe adsorption separation of a multicomponent mixture in a four‐section countercurrent separation unit is analyzed. A procedure for the optimal and robust design of the unit is developed in the frame of equilibrium theory, where adsorption equilibria are described through the constant selectivity stoichiometric model, while mass‐transfer resistances and axial dispersion are neglected.A set of conditions obtained defines a region in the operating parameters space where the unit achieves complete separation. In the case of binary separations, with a desorbent having any adsorptivity with respect to the components to be separated, the boundaries of this region are obtained explicitly. In the general multicomponent case, a numerical procedure is needed. An approximate shortcut method devised allows to obtain explicit and reliable relationships for estimating the boundaries of the exact region.The results provide a very convenient tool not only to analyze the role of the desorbent in determining the separation performance, but to find both optimal and robust operating conditions. Comparison between model predictions and experimental data assesses the reliability and accuracy of the theoretical findings.
Retention of proteins in cross‐flow UF through asymmetric inorganic membranes
Authors: Pradános P. , Arribas J. I. , Hernández A. 
Published: June 2004

AbstractThe flow and retention of 0.1% w/w aqueous solutions of pepsin, bovine serum albumin (BSA), lipase, γ‐globulin, and invertase with molecular weights of 36,000, 67,000, 80,000, 150,000, and 270,000 dalton (g/mol) are studied when they are tangentially filtered through an inorganic microporous membrane with a nominal pore size of 0.02 × 10−6 m made by Anopore, with transmembrane pressure differences up to 100 kPa. The data were analyzed within the frame of the film layer theory for the concentration polarization phenomenon. This allows the mass‐transfer coefficient to be obtained for the cell used as a function of the feed circulation speed and the molecular weight of the solute. Apparent and true retention curves obtained lead to a size exclusion radius smaller than the nominal one but very similar to that obtained from scanning electronic photographs.
Core‐annular flow of liquid membrane emulsion
Authors: Ho W. S. , Li N. N. 
Published: June 2004

AbstractA modeling and experimental study of the core‐annular flow (CAF) was carried out with a viscous liquid membrane emulsion as the core and an aqueous solution as the annulus. A hydrodynamic model has been developed for stable CAF with a centered, nondeformable core surrounded by a turbulent aqueous annulus with or without a drag reducer. This model has been verified with data from flow tests with viscous emulsions in a horizontal flow loop of 3/4‐in. (19‐mm) tubing and a fullscale, down‐hole well tubing of 2‐7/8 in. (73 mm). By the use of this model along with the data, the effects of annular size on total and core flow rates and pressure drop are investigated for annuli with and without a drag reducer.
Parallel transport of BSA by surface and pore diffusion in strongly basic chitosan
Authors: Yoshida Hiroyuki , Yoshikawa Motonobu , Kataoka Takeshi 
Published: June 2004

AbstractThe parallel transport of a protein by surface and pore diffusion within a highly porous ion exchanger is studied by measuring equilibria and uptake curves for adsorption of bovine serum albumin (BSA) on two different strongly basic chitosan ion exchangers (hard gels): Ch‐2503 and Ch‐2507 at pH 6.9 and 298 K. Experimental equilibrium isotherms are correlated by the Langmuir equation. Intraparticle effective diffusivities of BSA (Deff) are determined from the homogeneous Fickian diffusion model, increasing with the bulk phase concentration increase of BSA (C0). It suggests the existence of parallel diffusion. The surface diffusivity DS for the parallel diffusion model is determined from the Deff, and DS = 0.47 × 10−13 m2 · s−1 (Ch‐2507) and 2.4 × 10−13 m2 · s−1 (Ch‐2503) were obtained. Pore diffusivities based on the pore diffusion control (D′P) are obtained by matching the shrinking core model with the experimental uptake curves. D′P decreases with increasing C0. Since D′P in Ch‐2507 is constant when C0 ≥ 1 kg·m−3, the constant value 2.7 × 10−11 m2 · s−1 is taken as the accurate pore diffusivity (DP). As D′P in Ch‐2503 does not approach a constant value, DP = 1.0 × 10−11 m2 · s−1 is determined by matching the parallel diffusion model with the uptake data, and the model calculated using experimental values of DS and DP agrees reasonably well with the uptake data for Ch‐2507 and Ch‐2503. This theoretical approach may be applied not only for intraparticle diffusion of proteins but any adsorbates in porous materials.
Model‐based approach to automated hazard identification of chemical plants
Authors: Catino Catherine A. , Ungar Lyle H. 
Published: June 2004

AbstractA prototype hazard identification system, qualitative hazard identifier (QHI), works by exhaustively positing possible faults, automatically building qualitative process models, simulating them, and checking for hazards. QHI matches a library of general faults such as leaks, broken filters, blocked pipes, and controller failures against the physical description of the plant to determine all specific instances of faults that can occur in the plant. Faults may perturb variables in the original design model or may require building a new model. Fault models are automatically generated using the qualitative process compiler and simulated using QSIM. Hazards including overpressure, overtemperature, controller saturation, and explosion are identified in the reactor section of a nitric acid plant using QHI.
Hindered settling of rod‐like particles measured with magnetic resonance imaging
Authors: Turney Michael A. , Cheung Man Ken , Powell Robert L. , McCarthy Michael J. 
Published: June 2004

AbstractMagnetic resonance imaging (MRI) is used to measure the time evolution of the volume fraction vs. height profile during batch sedimentation of rod‐like particle suspensions. At any instant during sedimentation, MRI clearly delineates the supernatant, suspension, and sediment regions. The rod‐like particles with a mean aspect ratio of 17.4 exhibit considerably larger hindered settling effects than spherical particles with increasing particle concentrations. This can be attributed in part to the larger increase in interparticle forces and relative viscosities with suspensions of rod‐like particles than with suspensions of spherical particles, as the particle concentration increases. The relative viscosity of the rod‐like particle suspensions is a function of the spin‐spin relaxation time, T2. Since T2 is a measurable MRI quantity, this suggests that MRI may be used as an instrument for rheological measurement.
Monte Carlo network simulation of horizontal, upflow and downflow depth filtration
Authors: Burganos V. N. , Paraskeva C. A. , Payatakes A. C. 
Published: June 2004

AbstractA test particle trajectory approach is developed for the simulation of deep bed filtration. A 3‐D network of constricted pores represents the pore space of granular filters, network‐scale trajectories of a large number of non‐Brownian test particles are computed, and filter coefficient predictions are obtained for horizontal, down‐and upflow filtration operation. This simulator yields numerical results that agree excellently with our earlier predictions by the pore‐scale trajectory‐based population balance method. The new approach, however, circumvents the cumbersome step of calculating the impacted fraction in each unit cell, which the earlier method required, by providing direct statistical estimates of the local and overall deposition rates for continuous and discrete pore‐size distributions. For large superficial velocities (Vs > ∼1 mm/s) and distributed pore size, downflow filters are more efficient than horizontal flow filters, whereas for small velocities (Vs< ∼ 0.5 mm/s) the opposite is observed. Horizontal flow operation is also favored by uniform packing for almost any value of the external pressure gradient. Upflow operation is the least efficient for the packings considered here over a broad range of superficial velocity and particle‐size values. Observed differences among the three filtration types are maximal for uniform packings and decrease considerably with increasing packing heterogeneity.