bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electrostatic interactions in colloidal suspensions: Tests of pairwise additivity
Authors: Reiner Eric S. , Radke C. J. 
Published: June 2004

AbstractThe electrical double‐layer free energy is computed numerically for a model colloidal suspension of charged, hexagonally‐packed monodisperse cylinders in a symmetric electrolyte solution using the Poisson‐Boltzmann equations. Comparison is made to the energies estimated from sums of pair interactions and from two versions of a radially‐symmetric cell model: one in which the outer boundary of the cylindrical cell is chosen to make the particle volume fraction Φ in the approximate model agree with that of the full lattice and the other in which the position of the outer boundary is chosen as half the nearest neighbor separation in the lattice. Calculations are performed for ranges of particle volume fractions from infinite dilution to 98% of closest packing, ionic strengths (in terms of the ratio of Debye length to particle radius λ/a) from 0.1 to 10.0, and dimensionless surface potentials and charge densities from 0.1 to 10.0.The pair approximation is valid only when typical interparticle spacings are greater than the Debye length. The first implementation of the cell model proves quite accurate, while the second is only qualitatively correct. The magnitude of particle surface charge density or potential has little effect on these conclusions.
On‐line inference of polymer properties in an industrial polyethylene reactor
Authors: McAuley K. B. , MacGregor J. F. 
Published: June 2004

AbstractA major difficulty affecting the control of product quality in industrial polymerization reactors is the lack of suitable on‐line polymer property measurements. In this article a scheme is developed to predict melt index and density in a fluidized‐bed ethylene copolymerization reactor. Theoretically‐based models are derived to predict quality variables from the available on‐line temperature and gas composition measurements. Adjustable parameters in these models are updated on‐line using infrequent laboratory measurements and a recursive parameter estimation technique. The application of this methodology is illustrated using operating data from an industrial reactor. It is shown that both melt index and density can be successfully predicted. Knowledge of product property deviations from desired targets is required so that manufacturers can take corrective actions to reduce the quantity of off‐grade material made and produce a consistent product.
Kinetics of deactivation of bifunctional Pt/Al2O3Cl catalysts by coking
Authors: Beltramini Jorge N. , Wessel Timothy J. , Datta Ravindra 
Published: June 2004

AbstractThe catalytic activity, product selectivity, and coke deposition were studied during methylcyclopentane (MCP) reforming on bifunctional Pt/Al2O3Cl reforming catalysts with different metal loadings, but constant metal dispersion as well as constant Cl loading. Surprisingly, the overall conversion of MCP was found to decrease as the metal content of the catalyst increased. Analysis of the coke by temperature programmed oxidation (TPO) revealed two distinct peaks, one associated with coke on the metallic function and the other associated with coke on the acidic function. The change of activity of the metal and acid functions with time was monitored by following the rates of hydrogenolysis and hydrocracking, respectively. A mechanistically‐based dual‐site model for the kinetics of coke formation and the resulting deactivation was developed. The model successfully correlates the experimental data and is an improvement over existing models, since it explicitly accounts for the deactivation of both the metal and the acid functions.
Modeling of reaction‐induced flow maldistributions in packed beds
Authors: Stroh Friedemann , Balakotaiah Vemuri 
Published: June 2004

AbstractThis work examines reaction‐induced flow maldistributions in adiabatic, downflow packed beds. Using linear stability analysis it is shown that for the case of a constant heat source, the uniform flow loses stability when a certain dimensionless group, the Darcy buoyancy number, exceeds a critical value. Center manifold theory is used to analyze the local bifurcation picture for the case of a simple and double zero eigenvalue. It is found that for large Peclet numbers, all the bifurcations from the uniform solution are subcritical in nature and are unstable locally. Orthogonal collocation and continuation techniques are combined with the local theory to determine the various branches of bifurcating solutions. The temperature and flow distributions of stable and unstable solution branches are presented for several aspect ratios and Peclet numbers. Numerical simulations predict direct transitions from uniform flow to periodic or chaotic flows. It is also found that there is a wide range of the Darcy buoyancy number in which the uniform and maldistributed flows are stable and coexist.
Simulation studies of diffusion‐limited coarsening in two dimensions
Authors: Wong Tang H. , O'Brien James A. 
Published: June 2004

AbstractThe kinetics of particle coarsening are studied by means of contour dynamics simulation in two dimensions. The current method is free from the usual restrictions, such as small area fraction, constant particle shape, and local particle motion. Growth or shrinkage of particles in a given realization is found to depend on the local environment of the particles. The concept of critical size central to the classical theories such as the LSW theory, therefore, appears to be inappropriate. Surprisingly, the ensemble‐averaged macroscopic behavior is consistent with mean‐field theory. The particle‐size distribution (PSD) tends toward an asymptotic shape, different from the mean field result. The normalized width of the PSD seems to be insensitive to volume fraction. Interfacial transport limitation can be significant for small particles, and it has the effect of altering the width of the PSD. The coarsening rate constant is found to be a function of the area fraction.
An internal model control strategy for nonlinear systems
Authors: Henson Michael A. , Seborg Dale E. 
Published: June 2004

AbstractAn internal model control (IMC) strategy for nonlinear single‐input single‐output systems is proposed. The controller is designed to provide nominal performance, and a nonlinear filter is added to make the controller implementable and to account for plant/model mismatch. An important advantage of the new approach is that the assumption of full‐state feedback inherent in most input‐output linearization schemes is eliminated. However, the proposed IMC strategy is restricted to open‐loop stable systems with stable inverses. Under mild assumptions, the closed‐loop system possesses the same stability, perfect control, and zero offset properties as linear IMC. Simulation results for a continuous fermentor illustrate the advantages of the nonlinear IMC strategy.
A transformation approach to nonlinear process control
Authors: Khambanonda Tanes , Palazoglu Ahmet , Romagnoli Jose A. 
Published: June 2004

AbstractThis paper focusses on the approximate modeling and control of nonlinear processes. A polynomial expansion method helps develop successively more accurate models valid for the whole operating region and also provides a means to quantify the uncertainty associated with the approximation. A stability analysis can be performed in a straightforward fashion to identify stable operating regions as well as stable directions for the output variable moves. This permits the testing of linear and nonlinear compensators and the practicality of the method is demonstrated with a case study of heat‐exchanger control.
Equivalent annular model of a multitubular shell‐side (MSS) fixed‐bed reactor
Authors: Li Haoming , Hudgins Robert R. , Chang Kun Soo 
Published: June 2004

AbstractFor a multitube catalytic reactor in which catalyst occupies the shell side of the cooling tubes, a two‐dimensional model is developed to determine the nonuniform radial distribution of the temperature and concentration. The cross‐section of the fixed bed is divided into a number of equivalent portions proportional to the number of cooling tubes. Each portion is approximated by either an equivalent annular fixed bed or a transformed circular fixed bed in the complex plane. Enthalpy and mass balances are performed on the equivalent annular fixed bed while the radially varying flow distribution and the transport properties are considered in a transformed circular fixed bed. A correlation is developed for the equivalent annular model based on the relationship of the two different approximations. The model can also account for variations in radial porosity, radial velocity, and transport properties.
Mathematical model of a gas diffusion electrode bonded to a polymer electrolyte
Authors: Bernardi Dawn M. , Verbrugge Mark W. 
Published: June 2004

AbstractA mathematical model for an ion‐exchange membrane attached to a gas‐fed porous electrode is derived and discussed. The model is applied to simulate the oxygen electrode of a polymer‐electrolyte fuel cell. Our discussion focuses on cell polarization characteristics, water transport, and catalyst utilization—all of which must be considered for fuel‐cell design. Calculated polarization behavior is shown to compare favorably with published experimental data. Our results indicate that if the membrane maintains full saturation, its contribution to the total cell resistance is most significant at higher operating current densities (greater than 200 mA/cm2). Polarization resistance due to the oxygen reduction reaction appears to be important for all practical current densities. Water transport, driven by pressure and electric‐potential forces, is shown to be a complicated function of the cell operating conditions. The utilization and distribution of noble‐metal catalyst is discussed.
A molecular mechanism for gas hydrate nucleation from ice
Authors: Sloan E. D. , Fleyfel F. 
Published: June 2004

AbstractInduction phenomena are explained that exist in the only two reproducible sets of kinetic data in the literature for gas hydrate formation from an agitated ice surface. The previously unexplained data are interpreted using recent crystal diffraction results. The induction explanation is validated through cyclopropane hydrate kinetic experiments. The induction reasoning forms the basis of an hypothesis for a molecular mechanism of hydrate formation from ice. The hypothesized mechanism is quantified and given physical interpretation. The hypothesis may have significant implications for inhibiting hydrate formation.
Nucleation and growth of succinic acid in a batch cooling crystallizer
Authors: Qiu Yanfeng , Rasmuson Åke C. 
Published: June 2004

AbstractA method to evaluate nucleation and crystal growth rates from batch cooling crystallization experiments is presented. Solute concentration and suspension temperature are recorded during the experiment and the product crystal size distribution is analyzed. The crystal growth and nucleation rates at any instant are calculated by solving the population balance equation using the method of characteristics, together with the mass balance equation. Based on a set of different cooling crystallization experiments, kinetics for succinic acid are determined. Applying these kinetics in process simulation allows for a reasonably accurate prediction of the product weight mean size.
Dynamic modeling of chemical vapor infiltration
Authors: Sotirchos Stratis V. 
Published: June 2004

AbstractA mathematical model is developed to describe the dynamic and pseudosteadystate behavior of chemical vapor infiltration, a process used to fabricate fiber‐reinforced ceramic matrix composites. The three‐parameter dusty‐gas model is used to describe the interaction of mass transport fluxes, partial pressures, and partial pressure gradients in the interior of the porous medium, with the viscous flux related to the total pressure gradient through Darcy's law. The model is applied to study the deposition of SiC via decomposition of methyltrichlorosilane in a porous medium whose structure can be represented by a population of uniformly‐sized, randomly‐overlapping pores. The results show that use of simplified mass transport flux models can lead to significantly different results, even if the concentration of reactants is low and the effects of the products of the reaction on the deposition rate are ignored. It is also shown that operation of chemical vapor infiltration reactors under pressure pulsing can lead to conversion gradients in the densifying structure by a few orders of magnitude smaller than those seen at the same reaction conditions under constant pressure.
Reduction of membrane fouling in the ultrafiltration of binary protein mixtures
Authors: Rodgers V. G. J. , Sparks R. E. 
Published: June 2004

AbstractIt is well known that separation efficiency and solute rejection change when two or more proteins are ultrafiltered. This phenomenon is primarily due to membrane fouling and is not significantly influenced by concentration polarization. If membrane fouling is the dominant resistance, then negative transmembrane pressure pulsing might significantly reduce this barrier.A study was performed to determine the effect of negative transmembrane pressure pulsing on solute rejection for an albumin and gamma‐globulin mixture in ultrafiltration. Pulsing improved solute flux for all cross‐flow rates investigated including turbulent conditions by as much as two orders of magnitude. Under certain pulsing conditions, substantially higher solute flux was obtained for operations involving increased concentration polarization. This study reveals that negative transmembrane pressure pulsing can be effective in lowering the solute flux resistance that is observed in binary protein mixture ultrafiltration.
Vapor synthesis of titania powder by titanium tetrachloride oxidation
Authors: Akhtar M. Kamal , Xiong Yun , Pratsinis Sotiris E. 
Published: June 2004

AbstractFormation of titania particles by vapor‐phase oxidation of titanium tetrachloride was studied in an aerosol reactor between 1,200 and 1,723 K. The effect of process variables (reactor residence time, temperature, and reactant concentration) on powder size and phase characteristic was investigated using the differential mobility particle sizer, scanning electron microscopy, and X‐ray diffraction. Titania particles were primarily anatase though the rutile weight fraction increased with increasing reactor temperature. The geometric number average diameter of the particles was between 0.13 and 0.35 μm, and the geometric standard deviation of the particle size distribution was about 1.4. The average particle size increased with increasing temperature, inlet TiCl4 concentration, and residence time. The observed changes in the particle size distribution were compared with those predicted by solving the aerosol dynamic equation by a sectional method and accounting for coagulation and first‐order chemical reaction. While variations in the process variables resulted in discernible changes in the size of the particles, the spread of the distribution remained rather unaffected.
Reactions of polycyclic alkylaromatics: Structure and reactivity
Authors: Smith C. Michael , Savage Phillip E. 
Published: June 2004

AbstractA family of alkyl‐substituted polycyclic aromatic hydrocarbons was pyrolyzed in microbatch reactors at temperatures between 350°C and 425°C. A general pyrolysis network was deduced for these compounds, and it comprised two major and one minor parallel pathways. The first major pathway resulted in products analogous to the major products observed from alkylbenzene pyrolysis. The second major pathway led to products via the cleavage of the strong aryl‐alkyl CC bond. The third pathway led to small amounts of products, presumably through cyclization and condensation reactions. The relative importance of the two major pathways varied for the different compounds. The rates of aryl‐alkyl bond cleavage differed for the different compounds, and these rates were quantitatively related to the compounds' localization energies through Dewar reactivity numbers.
Supported liquid‐phase catalytic membrane reactor–separator for homogeneous catalysis
Authors: Kim Jae S. , Datta Ravindra 
Published: June 2004

AbstractA new configuration of a membrane reactor‐separator for homogeneous catalysis is described that is designed not only to encapsulate the catalyst solution, but to simultaneously separate the product from reactant. It is composed of a porous support layer that is impregnated partially or completely with the liquid‐phase catalyst and is sandwiched between two different membranes. Membrane 1 is readily permeable to the reactant but not to the product, whereas membrane 2 allows free permeation of both the reactant and the product while encapsulating the catalyst solution. The performance of a differential stirred‐cell membrane reactor‐separator was theoretically investigated for the case of a first‐order irreversible reaction. The extent of product separation was studied as a function of membrane permselectivity for reactant and product, the difference in resistance of membrane and catalyst layer, liquid loading of the catalyst layer, and the sweep fluid flow rate. The efficacy of the device was experimentally demonstrated using ethylene hydroformylation with hydridocarbonyltris (triphenyl phosphine) Rh(I) catalyst dissolved in dioctyl phthalate solvent.
Generalized kinetic model for wet oxidation of organic compounds
Authors: Li Lixiong , Chen Peishi , Gloyna Earnest F. 
Published: June 2004

AbstractA generalized kinetic model for wet oxidation (WO) of organic compounds was developed based on a simplified reaction scheme considering acetic acid as the rate‐limiting intermediate. The selectivity of product vs. intermediate formations was quantified by the ratio of the two reaction rate constants. This point selectivity α may be used to characterize the “strength” of the feed stream to be treated. This global model was validated using WO kinetic data reported for temperatures ranging from 150°C to 550°C and pressures varying from 20 bar to 440 bar. Organic conversions predicted by this model, as compared to other models, more accurately reflect the actual performance of WO processes. The model has practical validity for a variety of organic compounds, wastewaters and sludges in both subcritical and supercritical water oxidation processes.
Robust control of homogeneous azeotropic distillation columns
Authors: Jacobsen Elling W. , Laroche Lionel , Morari Manfred , Skogestad Sigurd , Andersen Henrik W. 
Published: June 2004

AbstractThe entrainer feed used in homogeneous azeotropic distillation provides an extra degree of freedom in the steady‐state design compared to simple distillation. In this article, we discuss the control of azeotropic distillation columns in the region close to minimum entrainer feed. Both industrial experience and previous research indicate that this is a difficult task. However, by considering the high‐frequency behavior (initial response) of the column, we show that tight and robust control can be obtained with simple single‐loop PI controllers. The results depend on the presence of high‐frequency phenomena such as flow dynamics.
Propagation of oscillations during ethylene oxidation on a Rh/SiO2 catalyst
Authors: Kellow John C. , Wolf Eduardo E. 
Published: June 2004

AbstractInfrared imaging is used to visualize nonuniform temperature patterns occurring on a Rh/SiO2 catalyst wafer during the total oxidation of ethylene. Ignition of the catalyst wafer is seen to occur as a propagating reaction front originating at a single point on the wafer. During self‐sustained oscillations, the reaction zone contracts, expands, and changes shape. Thermographic data of the oscillations are analyzed by calculating the size and mean temperature of the reaction zone. This analysis reveals that the observed oscillations in the bulk CO2 concentration correlate mainly with changes in the size of the reacting zone.
Chemical, quasi‐chemical and perturbation theories for associating fluids
Authors: Economou Ioannis G. , Donohue Marc D. 
Published: June 2004

AbstractPure fluids and mixtures of species that hydrogen bond behave differently from systems that interact only through dispersion forces. The deviations from classical behavior often are sufficiently large that conventional equations of state and activity models cannot be used without the introduction of large, condition‐dependent empirical parameters. Consequently, three different classes of theories have been developed specifically to treat hydrogen‐bonding systems. The first is based on the assumption that when molecules hydrogen‐bond, they react to form new species and consequently is referred to as “chemical” theory. The second is based on lattice‐fluid theory that is used to describe different types of specific interactions and is known as “quasi‐chemical” theory. The last is based on the solution of integral equations using a potential function that mimics that of a hydrogen bond. It is shown here that these three approaches give essentially equivalent results. This allows one to relate the parameters in the perturbation theory to the equilibrium constant and hence greatly improves its utility for real systems. All three theories are compared with simulation data.
Computation of transition and molecular diffusivities in fibrous media
Authors: Melkote Rajesh R. , Jensen Klavs F. 
Published: June 2004

AbstractThe tortuosities of fibrous media in the heretofore unexplored transition and ordinary regimes are computed using a Monte Carlo scheme based on the Einstein equation for random walkers. The model structure is that of fully penetrable cylinders (FPC) in a unit simulation volume. The mean square displacement technique is combined with the first passage time distribution to accelerate the progress of the walkers at low Knudsen number. The results include the computation of transition regime transport coefficients for the first time. The calculated ordinary tortuosities are approximately equal to the reciprocal of the porosity over a wide range, while the transition tortuosities are shown to deviate from the reciprocal porosity with a simple dependence on Knudsen number. The limits of the transition regime are shown to correspond roughly to Knudsen numbers of 0.50 and 100, respectively. The calculated Knudsen tortuosities are shown to improve on earlier results obtained by the authors using a flux‐based technique.
Circulation and scale‐up in bubble columns
Authors: Geary Nicholas W. , Rice Richard G. 
Published: June 2004

AbstractThe liquid circulation model of Rice and Geary (1990) is extended to include turbulence originating at the wall. Thus, two possible length scales are considered: one originating from rising bubbles and the other emanating from the wall. The appropriate scale for small columns should be based on bubble size, while for larger systems the proper mixing length is proportional to column diameter. It is proposed that the film Reynolds number may be the key in distinguishing the two cases.
Multiplicity features of adiabatic autothermal reactors
Authors: Lovo Marianne , Balakotaiah Vemuri 
Published: June 2004

AbstractSingularity theory, large activation energy asymptotics, and numerical methods are used to present a comprehensive study of the steady‐state multiplicity features of three classical adiabatic autothermal reactor models: tubular reactor with internal heat exchange, tubular reactor with external heat exchange, and the CSTR with external heat exchange. Specifically, we derive the exact uniqueness‐multiplicity boundary, determine typical cross‐sections of the bifurcation set, and classify the different types of bifurcation diagrams of conversion vs. residence time. Asymptotic (limiting) models are used to determine analytical expressions for the uniqueness boundary and the ignition and extinction points. The analytical results are used to present simple, explicit and accurate expressions defining the boundary of the region of autothermal operation in the physical parameter space.