bluuebunny/crossref_metadata_2025_split huggingface dataset:
Thermophoretic properties of nonspherical particles and large molecules
Authors: Garcia‐Ybarra Pedro , Rosner Daniel E. 
Published: June 2004

AbstractThe Knp ≫ 1 gas‐kinetic approach of Waldmann (1959, 1961) is extended to predict the thermophoretic properties of nonspherical aerosol particles, and massive nonspherical gas molecules. Calculations are presented for sphero‐cylindrical particles (molecules) of arbitrary aspect ratio, L/R, predicting that in a local temperature gradient grad T they will thermophoretically drift: (i) more rapidly when their major axis is aligned with −grad T; (ii) at a velocity different from that of a sphere of radius R equal to the cylinder radius; (iii) at an angle with respect to −grad T when their major axis is not parallel to, nor perpendicular to, −grad T; and (iv) without a net torque tending to orient the particle with respect to −grad T. Important corollaries are that nonspherical particles in a temperature gradient should also experience new thermophoretically‐induced coagulation mechanisms. We predict that the orientation‐averaged thermal diffusion factor αT should increase approximately linearly with particle (molecule) length.
The optimal design of dynamic experiments
Authors: Espie D. , Macchietto S. 
Published: June 2004

AbstractA robust and efficient algorithm is developed to calculate dynamic inputs for optimal experimental designs. Different objective functions are presented to allow designs for both model discrimination and the improvement of parameter precision. Time‐varying inputs are calculated by reformulating the optimal design problem as an optimal control problem. This approach can provide large improvements in the ability to discriminate among a series of models, and then increase the accuracy of the resulting parameters.
Mass transfer studies of external‐loop airlifts and a bubble column
Authors: Popovic Milan K. , Robinson Campbell W. 
Published: June 2004

AbstractGas holdups (ϵGR) and liquid‐phase volumetric oxygen transfer coefficients (kLaT) were measured for a bubble column (BC) and three different external‐circulation‐loop airlift (ECL‐AL) fermentors of 50 to 60 L working volume, using viscous non‐Newtonian aqueous solutions of various carboxymethylcelluloses. Some measurements also were done with a viscous Newtonian system (51.8 wt. % sucrose solution).Discussed in this paper are correlations of ϵGR and kLaT with riser superficial gas velocity (0.02 ≦ UGR ≦ 0.26 m/s), the ratio of the downcomer and riser crosssectional areas (0[BC] ≦ Ad/Ar ≦ 0.444), and the effective viscosity of the liquid phase (0.02 ≦ ηeff ≦ 0.5 Pa · s), over the parameter ranges indicated. It is shown that both ϵGR and KLaT are highly dependent upon UGR and Ad/Ar. The effective viscosity has a significant effect on KLaT, but has only a relatively weak affect on ϵGR. The KLaT correlation developed for non‐Newtonian systems was extended to include the results obtained for the viscous Newtonian system studied by incorporating the effects of liquid‐phase molecular diffusivity, density, and interfacial tension as determined by Nakanoh and Yoshida (1980).
The rheology of suspensions containing polymerically stabilized particles
Authors: Mewis Jan , Frith William J. , Strivens Trevor A. , Russel William B. 
Published: June 2004

AbstractThe non‐Newtonian viscosities of polymerically stabilized colloidal suspensions are usually predicted and correlated on the basis of data and scaling principles for Brownian hard spheres. Here, the specific effect of the stabilizer layer is investigated using suspensions of monodisperse PMMA particles with a chemically attached stabilizer layer. The ratio between particle radius and stabilizer layer thickness is changed between 5 and 61. At high values of this ratio the data show Brownian hard sphere behavior. At lower values deviations appear. As a first approximation, the “softness” of the particles can be characterized through the concentration at maximum packing. A more detailed comparison with hard sphere data provides a measure for softness that changes with concentration and shear rate. A theoretical estimate of the concentration effect is in line with the experiments. The critical shear stress (or Peclet number) is not a constant but goes through a maximum when the concentration is increased.
Lumping coupled nonlinear reactions in continuous mixtures
Authors: Chou M. Y. , Ho T. C. 
Published: June 2004

AbstractA lumping procedure proposed previously is applied to two types of continuous reaction mixtures: one in which the constituents undergo reactions of a Langmuir‐Hinshelwood (LH) type, the other in which the constituents undergo reactions of a bimolecular type. It is shown that both mixtures initially can behave qualitatively like lumped first‐order reactions. At large times, the behavior of the lumps can be described by power law kinetics, with the bimolecular lump displaying a higher overall reaction order. In particular, if the feed contains a nonzero amount of unconvertible species, the LH lump at large times decays at a second‐order rate, whereas the bimolecular lump decays at a third‐order rate. To complement the asymptotic results, expressions describing the behavior of the lumps at any time are derived.
Reactions in continuous mixtures
Authors: Aris Rutherford 
Published: June 2004

AbstractA continuous mixture is one which is so complex that it is no longer worthwhile to distinguish individual chemical species; instead, an index, such as the simulated boiling point, is chosen and ci, the concentration of the species Ai, is replaced by c(x)dx, the concentration of material with index in the interval (x, x + dx). It has been long known that the total concentration of a suitably distributed mixture, each of whose components disappears by a first‐order reaction with constant k(x), will appear to disappear according to a higher order of reaction. The generalization of this to a mixture that requires two indices for its description is worth considering for three reasons: First, there may well be materials that are so complex as to require this. Second, the second index may be considered to distribute reaction time. Third, this approach seems to answer the mathematical question of how to generalize from the continuum of first‐order reactions to one of parallel Nth‐order reactions.
Model predictive optimal averaging level control
Authors: Campo Peter J. , Morari Manfred 
Published: June 2004

AbstractThe infinite‐horizon, discrete‐time optimal averaging level control problem for surge tanks, with minimization of the rate of change of outlet flow as its objective, is formulated and a solution is presented. A finite moving‐horizon approximation is introduced and analytical solutions are obtained for two important special cases. These results provide a quantitative measure of the impact of a secondary objective, integral action, on flow filtering. The problem is then generalized to include nonconstant level and outlet flow constraints. A model predictive control formulation is presented which addresses the objectives of the generalized problem. The resulting controller minimizes the maximum rate of change of outlet flow, provides integral action, and handles constraints on the tank level and outlet flow rate. The proposed controller includes a single adjustable parameter that directly effects the trade‐off between the incompatible objectives of good flow filtering and rapid settling time. Examples are presented to demonstrate the properties of the model predictive controller. An implementation, involving imbedded feedback, is developed which guarantees internal stability of the model predictive scheme for open‐loop unstable processes (such as integrators).
Interaction measures for nonsquare decentralized control structures
Authors: Reeves D. E. , Arkun Y. 
Published: June 2004

AbstractA block relative gain measure for nonsquare systems has been derived. Evaluated at steady state, this measure serves as a performance‐related tool for evaluating control structures prior to controller design. Both the nonsquare dynamic block relative gain and the relative sensitivities have been presented as dynamic interaction measures that depend on the controller tuning and are consequently applicable to design of nonsquare decentralized controllers. Rigorous relationships between the dynamic block relative gain and the performance and stability of a closed‐loop system have been established.
Hydrodynamics of a lamella electrosettler
Authors: Gidaspow Dimitri , Shih Yang‐Tsai , Bouillard Jacques , Wasan Darsh 
Published: June 2004

AbstractUsing the principles of conservation of mass and momenturn for each phase, a theory is developed for describing quantitatively the sedimentation of colloidal particles in vessels having electrodes that are inclined to the vertical.As a limiting case, a kinematic settling formula was derived that describes the motion of settling fronts of colloidal alumina particles suspended in tetralin as a function of angle of inclination and strength of applied electric field in terms of electrophoretic mobilities. A hydrodynamic computer model shows the flow patterns and settling behavior of such lamella settlers with and without an applied electric field. The predicted motions of the settling fronts and the thicknesses of the clear boundary layers above the upper wall of the inclined vessels globally agreed with the experimental observations.
An experimental and modeling study of softening coal pyrolysis
Authors: Oh Myongsook S. , Peters William A. , Howard Jack B. 
Published: June 2004

AbstractA quantitative model of softening coal pryolysis is developed and tested against observed behavior. The model treats intraparticle transport of gases and metaplast throughout the softening stage of pyrolysis via growth of bubbles uniformly dispersed in the molten coal. The model quantitatively describes the transient softening behavior including the initiation, duration, and magnitude of plasticity and swelling of the particle. The model kinetic parameters are derived from experiments, in which a highly softening Pittsburgh No. 8 bituminous coal was pyrolyzed, and total weight loss, and yields and molecular weight distributions of tars and pyridine extractables of cooled chars were measured. Model predictions of effects of pressure, particle diameter and temperature on tar yields and weight losses, of effects of pressure on swelling ratio, and of effects of heating rate on plasticity are presented. The model provides good fits to the laboratory data and predicts trends in pyrolysis behavior in good qualitative accord with expectations based on published data.
Thermodynamics of aqueous systems containing hydrophilic polymers or gels
Authors: Prange Monika M. , Hooper Herbert H. , Prausnitz John M. 
Published: June 2004

AbstractA quasichemical partition function is applied to represent the thermodynamic properties of aqueous solutions of nonelectrolytes, including linear polymers and crosslinked polymers (gels). The partition function extends conventional lattice theory; to take into account strong specific interactions (hydrogen bonds) as encountered in aqueous solutions, each molecule (polymer segment) may possess three energetically different types of contact sites. We distinguish between sites that interact through dispersion forces and sites that can participate in a hydrogen bond; hydrogen‐bonding sites are divided into electron‐pair donating sites and electron‐pair accepting sites. The Helmholtz energy of the mixture is obtained using an oriented quasichemical approximation. The final equation contains three independent adjustable binary parameters; these are the exchange energies for different types of contact paris. To represent quantitatively upper or lower critical solution phenomena, we include the semitheoretical fluctuation correction recently proposed by de Pablo. Comparison with experimental data indicates that the proposed molecular‐thermodynamic model may be useful for representing phase equilibria for a variety of aqueous systems including swelling equilibria for hydrophilic gels.
Periodic separating reactors: Experiments and theory
Authors: Vaporciyan Garo G. , Kadlec Robert H. 
Published: June 2004

AbstractThe novel combination of a pressure swing adsorber (PSA) with a periodic flow‐forced packed‐bed reactor is explored. The device provides integral component separation and reaction. Feed sequences studied for the periodic separating reactor (PSR) were those of rapid, single‐bed pressure swing adsorption (RPSA). The experimental investigation employed CO oxidation over a packed bed of supported platinum catalyst and molecular sieve adsorbent. A reaction rate limited model is formulated and solved for a variety of irreversible and reversible reactions.The presence of irreversible chemical reaction is shown to greatly enhance the separation achievable by RPSA alone. For a wide range of inlet CO/O2 ratios, CO2 production could be increased up to two times over steady‐state plug‐flow reactor operation, while providing a recycle stream without phase change or extractive procedures. Selectivity and conversion improvements were predicted for multiple reaction systems. Other unusual features of operation, such as separation reversals, were also predicted and observed.
Causality‐based failure‐driven learning in diagnostic expert systems
Authors: Rich Steven H. , Venkatasubramanian Venkat 
Published: June 2004

AbstractIt has been recognized that a diagnostic expert system's ability to learn from past experience will improve its diagnostic efficiency as well as make it acquire new heuristics. In this paper, we propose a failure‐driven learning scheme by which the expert system automatically updates its compiled knowledge by acquiring new heuristics or refining existing heuristics. A heuristic is refined if it hypothesizes the wrong causal origin during a diagnosis. Using its deep‐level knowledge of the process, the expert system draws inductive inferences from causal models to determine why the hypothesis proposed by the heuristic is inconsistent with the current state of the process. The refinement limits the applicability of the heuristic and prevents it from firing if a similar situation were to subsequently arise.
Diffusion and solid dissolution/precipitation in permeable media
Authors: Novak Craig F. , Schechter Robert S. , Lake Larry W. 
Published: June 2004

AbstractWe have developed an analytical solution which describes mineral zonation caused by diffusion in permeable media. For a semiinfinite domain, the species conservation equations transform into ordinary differential equations that yield a closed‐form solution. The solution exhibits shock dissolution/precipitation fronts and gradual (nonshock) precipitation fronts. The solution can exhibit regions (gaps) containing no reactive solids which separate moving dissolution and precipitation fronts. The analysis is, in principle, extendable to include intraaqueous reactions, although the mathematics quickly becomes intractable.Numerical simulation exhibits all of the features of the more restricted analytical solution and is in good agreement with the data on hydroxyapatite dissolution taken by Kim and Cussler (1987).
Laser capillary spectrophotometry for drop‐size concentration measurements
Authors: Bae J. H. , Tavlarides L. L. 
Published: June 2004

AbstractA laser capillary spectrophotometric technique (LCS), based on the method of Verhoff et al. (1969, 1977), is developed for the rapid and accurate determination of the bivariate drop size‐concentration distribution for reactive liquid‐liquid dispersions. The method employs a precision bore capillary, through which a sample is drawn from the mixing vessel by a vacuum pump. An appropriate light source is used to distinguish between the slugs of dispersed phase and continuous phase as the sample passes by two detection points in the capillary. The optical device is designed to measure drop size by the difference of light refraction between the two phases and drop concentration by the light absorbance of the organic phase in the drop. The new technique can automatically measure drop‐size and drop‐concentration distributions of thousands of drops within a few minutes. This technique can provide important information on the state of a reactive dispersion in situ and can provide information on the influence of dispersed‐phase mixing effects on mass transfer and/or reactions in liquid‐liquid dispersions. Further, the LCS technique provides the capability for on‐line monitoring and control of units processing liquid dispersions.
Adaptive extremum control using approximate process models
Authors: Golden Melinda P. , Ydstie B. Erik 
Published: June 2004

AbstractThe proposed on‐line adaptive optimization technique incorporates a priori knowledge in the form of approximate steady‐state models. The steady‐state geometric characteristics of the model are periodically recalculated using a Hammerstein system and recursive least squares. The algorithm is self‐tuning in the sense that it converges to the optimal performance provided that a matching condition is satisfied and that the data are persistently exited. Simulation and experimental studies performed on a continuous fermentation system have been conducted to illustrate the performance of the optimization algorithm and demonstrate the viability of adaptive extremum control.
Competition among flow, dissolution, and precipitation in porous media
Authors: Rege Sunil D. , Fogler H. Scott 
Published: June 2004

AbstractA theoretical and experimental study has been carried out on flow, dissolution, and precipitation in porous media. Flow experiments were performed on linear carbonate cores using acidic ferric chloride solutions. Dissolution of the carbonate by the acid causes an increase in the solution pH, thereby precipitating ferric hydroxide. This precipitate plugs up the pore throats in the medium and increases the resistance to fluid flow. Fluctuations in the permeability ratio were observed during core flood experiments, confirming the competition between channel formation due to dissolution and pore plugging due to precipitation. The evolution of the pore structure was characterized by Wood's metal castings.A network model has also been developed to describe flow and reaction in porous media. The model was used to simulate the ferric chloride system, and pressure oscillations predicted by the model show identical trends to those observed experimentally. Additionally, the evolution of pores in the network were graphically represented.
High‐temperature adsorption of alkali vapors on solid sorbents
Authors: Punjak W. A. , Uberoi M. , Shadman F. 
Published: June 2004

AbstractVapors of alkali metal compounds can be removed from coal combustion and gasification flue gases using high‐temperature aluminosilicate sorbents. The fundamentals of alkali adsorption on kaolinite, bauxite, and emathlite are compared and analyzed both experimentally and through theoretical modeling. The results show that the process is not a simple physical condensation, but a complex combination of diffusion and reaction. The kinetics of adsorption on these sorbents have similarities: the process is diffusion‐influenced, the rate decreases with time, and there is a final saturation limit. There are, however, differences in reaction mechanisms leading to potentially different applications for each sorbent. In adsorbing alkali chloride vapors, kaolinite and emathlite release all the chlorine back to the gas phase while bauxite retains some of the chlorine. Moreover, the products of reaction with emathlite have a melting point significantly lower than those for kaolinite and bauxite. Therefore, emathlite is more suitable for lower‐temperature sorption systems downstream of the combustors/gasifiers, while kaolinite and bauxite are suitable as in‐situ additives.
Synthesis of mass exchange networks
Authors: El‐Halwagi Mahmoud M. , Manousiouthakis Vasilios 
Published: June 2004

AbstractThe purpose of this work is to introduce the notion of synthesizing mass‐exchange networks (MEN's). A systematic, two‐stage, procedure is proposed for the synthesis of cost‐effective MEN's. In the first stage, a thermodynamically‐oriented procedure is used to identify the thermodynamic bottlenecks (pinch points) that limit the extent of mass exchange between the rich and the lean process streams. Preliminary networks, that feature maximum mass exchange, are generated at this stage. The objective of the second stage is to improve the design of these preliminary networks so as to develop a final configuration of the MEN that satisfies the assigned exchange duty at minimum venture cost. This approach is applied to the synthesis of MEN's with single‐component targets as well as multicomponent, compatible targets. An illustrative example on the sweetening of coke‐oven gas is presented to demonstrate the applicability of the proposed synthesis procedure.
Porosity effects in hydrogen reduction of iron oxides
Authors: Fortini Arthur J. , Perlmutter Daniel D. 
Published: June 2004

AbstractThe rates of reduction of hematite to magnetite and of magnetite to iron in hydrogen / nitrogen / water vapor atmospheres were each studied with respect to variation in temperature, gas composition, and solid pore structure. It is shown that at temperatures below 350°C, the reactions are under chemical kinetic control, and diffusional limitations are negligible. The activation energies for the reduction of hematite to magnetite and of magnetite to iron were found to be 185 kJ / mol and 76.6 k J / mol, respectively. Both reactions exhibit first‐order behavior for hydrogen partial pressures less than 76 kPa. The conversion and rate data for each reaction were interpreted in terms of the random pore model of Bhatia and Perlmutter (1980), which takes into account the detailed pore structure of the starting material as well as the changes that occur as the reaction proceeds. The model predictions of conversion with time agree with the measured data to within 2%, and the rate vs. conversion predictions agree to within 5%.
An investigation of CaO sulfation mechanisms in boiler sorbent injection
Authors: Stouffer Mark R. , Yoon Heeyoung 
Published: June 2004

AbstractA laboratory differential reactor study was conducted at temperatures of 975–1,275 K to investigate CaO sulfation mechanisms with calcine (CaO) samples derived from pulverized limestones and dolomites, and hydrated limes. The limestones had 8–115 μm mean particle sizes, a range applicable for boiler sorbent injection for SO2 control. Under conditions applicable for boiler sorbent injection, the sorbent performance, in terms of initial reaction rate and saturation utilization, was limited by the SO2 pore diffusion rate and by premature pore mouth plugging by CaSO4 product and, thus, depended on CaO particle size and pore size. Hydrated lime and Na2CO3‐promoted limestones showed performance superior to that of limestones, because they produced calcines with larger pores. Intrinsic reaction rate as reported by Borgwardt and Bruce (1986) was observed at temperatures below 1, 100 K.
A macroscopic model for countercurrent gas‐liquid flow in packed columns
Authors: Dankworth David C. , Sundaresan S. 
Published: June 2004

AbstractA macroscopic model based on the volume‐averaged equations of motion is presented for countercurrent gas‐liquid flow in a packed bed. The model yields a column‐limited flooding point as the loss of existence of uniform states. It correctly predicts the existence of two uniform states below the flooding point. The lower branch corresponds to the trends commonly observed experimentally. It is shown that the upper branch is made unattainable by the gas distributor/support plate at the bottom of the column. The occurrence of premature flooding induced by the support plate is also explained. It is suggested that the occurrence of spontaneous liquid segregation, necessitating frequent liquid redistribution in columns with large dumped packings and porosities, is a consequence of the loss of stability of the uniform state in the lower branch.