bluuebunny/crossref_metadata_2025_split huggingface dataset:
Statistical hydrodynamics in trickle flow columns
Authors: Crine M. , Marchot P. , L'Homme G. 
Published: June 2004

AbstractA stochastic model is proposed for the description of the hydrodynamics in trickle flow columns. The gas‐liquid flow is assumed to be controlled by local packing properties which are represented by random variables. The first (microscopic) level of description corresponds to the smallest volume whose averaged geometrical properties are representative of the whole packing. The hydrodynamics at this level is described by Ergun equations. The second level describes the liquid flow maldistribution induced by the partial wetting of the solid. This is the macroscopic level where the hydrodynamics is described by a probability distribution. This approach is used to develop correlations for predicting the wetting efficiency, the dynamic liquid holdup, and the pressure drop. These correlations give very good results when compared to experimental data over a wide range of operating conditions. In addition, we propose to describe the hysteretic behavior of trickle beds by a modification of the loquid flow texture caused by different values of the apparent solid wettability.
Optimal selection of measuring points in complex plants by linear models
Authors: Madron František , Veverka Vladimír 
Published: June 2004

AbstractFor reliable information on operating plants it is essential to design measuring points well by selecting directly measured quantities from the set of all measurable quantities.This article deals with a new method for optimizing measurement design. It is based on multiple Gauss‐Jordan elimination of the system of linear mathematical model equations and solves the problem of instrumentation design in new plants as well as the problem of optimizing existing measuring systems. Optimization methods for linear objective functions and for objective functions of general type are proposed. The method also offers a complex classification of quantities (observability and redundancy). After the optimization, the problem is presolved and is ready for an optimal processing of measured data. The mathematical model is reduced to the minimum set of equations and quantities relevant to the solution of a given problem. From a numerical standpoint, the solution is efficient.
Catalyst sintering in fixed‐bed reactors: Deactivation rate and thermal history
Authors: Blasco Valero , Royo Carlos , Monzón Antonio , Santamaría Jesús 
Published: June 2004

AbstractAn experimental study has been carried out of the in situ thermal degradation of a commercial dehydrogenation catalyst in a fixed‐bed reactor. The activity of catalyst particles sampled at different reactor positions after several reaction‐regeneration cycles has been related to their time‐temperature history, according to the following kinetics:

The above equation represents the loss of activity under coke‐free conditions, that is, activity loss due to sintering. The results of the study show that important differences in catalytic activity can be obtained for different positions of a fixed‐bed catalytic reactor after several operation‐regeneration cycles.
Experimental studies of restricted protein diffusion in an agarose matrix
Authors: Boyer Philip M. , Hsu James T. 
Published: June 2004

AbstractThe diffusion of proteins in polymer matrices is an important step in the adsorptive and chromatographic processes used for protein purification, as it is often rate‐limiting. Methods for the estimation of the intraparticle diffusivity in polymer gels have been developed and were applied to the diffusion of seven model proteins in agarose particles. The intraparticle diffusivity was not affected by particle diameter. A correlation based on the restricted diffusion model of Ogston et al. (1973) and Cukier (1984) has been proposed. This correlation allows the estimation of protein diffusivity in these matrices based on the molecular weight of the protein and the polymer concentration.
Kinetics of the CO oxidation by O2 and N2O over Cu‐Cr/Al2O3
Authors: Dekker Nico J. J. , Hoorn Johan A. A. , Stegenga Sander , Kapteijn Freek , Moulijn Jacob A. 
Published: June 2004

AbstractThe oxidation of CO by O2 and N2O over an oxidized 10 wt. % Cu‐Cr/Al2O3 catalyst (Cu:Cr=1:1) has been studied by temperature‐programmed reactivity measurements (400–550 K) over a wide range of partial reactant pressures, including inhibition by CO2. The CO oxidation rate is zeroth‐order in oxygen and has orders between 0‐1 in CO and N2O, depending on the gas‐phase composition. Mechanistic information from literature combined with the kinetic data resulted in the selection of an Eley‐Rideal‐type of kinetic model without a priori assumptions on rate‐determining processes. The model consists of the oxidation of reduced sites by O2 and/or N2O, followed by a reaction with CO, yielding a surface intermediate that releases CO2 in a consecutive step. CO2 inhibits both by reversible adsorption on oxidized and reduces sites, the latter under formation of the surface reaction intermediate. Apart from the surface oxidation by O2, the reaction rates of all assumed elementary processes are of the same order of magnitude and, therefore, determine the overall rate. The surface oxidation by oxygen is about four orders of magnitude larger, which explains the zeroth‐order in oxygen and the observation that oxygen first reacts with CO before N2O is able to oxidize CO. The obtained activation energies of the elementary processes agree with values in the literature for corresponding systems.
A mathematical model of mixing and polymerization in a semibatch stirred‐tank reactor
Authors: Tosun Güray 
Published: June 2004

AbstractEffects of mixing on number‐ and weight‐average degree of polymerization in free radical homopolymerization in solution in a semibatch stirred tank‐reactor have been modeled by use of the “partially segregated feed” model of Villermaux (1989) and the lumped kinetic treatment of addition polymerization. Mixing conditions are described in terms of the dimensionless time constants θM and θX for convective and diffusive mixing, respectively. Results show that DPn is relatively insensitive, while DPw is highly sensitive to mixing conditions. Comparison with ideal mixing results shows that polydispersity (DPw/DPn) can rise greatly as mixing becomes increasingly nonideal, depending on the value of dimensionless flow rate and dimensionless initiator rate constant. Time constants of the mixing model need to be expressed as dimensional correlations before the proposed polymerization model can be compared to experimental measurements. The model may be adapted to melt polymerization in nonideal CSTR such as LDPE.
Kinetics of carbon dioxide and methyldiethanolamine with phosphoric acid
Authors: Cordi E. M. , Bullin J. A. 
Published: June 2004

AbstractThe Kinetics of the reaction between carbon dioxide and methyldiethanolamine (MDEA) were studied in the presence of various amounts of phosphoric acid at 298 K, 308 K, and 318 K. A stirred cell with a known interfacial area was used at atmospheric pressure to measure the volumetric absorption rate of CO2 flowing through the vessel.The addition of phosphoric acid to an aqueous solution of MDEA resulted in a Bronsted acid‐base neutralization reaction. While virtually all of the acid was consumed by the neutralization reaction, a large part of the MEDA remained in its free form. The absorption of carbon dioxide in these solutions was slower than expected, given the amount of free MDEA present. The rate depression in the presence of hydrogen phosphate anions was attributed to the stabilization of the reaction transition state between MDEA, water, and carbon dioxide through charge dispersal.
Application of neutron radiography to image flow phenomena in porous media
Authors: Jasti J. K. , Fogler H. S. 
Published: June 2004

AbstractNeutron transmission imaging is a technique ideally suited for imaging various flow phenomena in consolidated porous media because the porous matrix is virtually transparent to thermal neutrons. Hydrogen‐containing fluids, however, provide a contrast in the image due to their large thermal neutron cross‐sections. This technique has uncovered new phenomena in the study of dissolution and precipitation in porous media. Neutron images of acid‐etched porous patterns show that the degree of branching, and the tortuosity of the etched patterns depend on competing effects of acid flow rate, dissolution rate and precipitation rate of the reaction products. The fine structure of etched pathways show that they can propagate against the direction of flow, a phenomenon not previously observed. In a second study, realtime neutron transmission imaging was used to visualize miscible tracer dispersion. It was shown how quantitative information such as the in‐situ spatial distributions of the tracer concentrations can be obtained with this technique.
Theory of precipitation of protein mixtures by nonionic polymer
Authors: Mahadevan Hari , Hall Carol K. 
Published: June 2004

AbstractA theory is developed to predict the solubility of protein mixtures in solutions containing nonionic polymer. Effective protein‐protein interactions due to polymer are taken to be volume‐exclusion potentials derived using statistical mechanics. Statistical‐mechanical perturbation theory is used to calculate chemical potentials. The effects of protein size, mole fraction and polymer concentration on solubility are explored. The theory is extended to include electrostatic interactions. The excess chemical potential of the proteins due to the charges on all species is calculated using the mean spherical approximation for a mixture of charged hard spheres. The theory predicts: the larger protein is preferentially precipitated over the smaller one; the more concentrated protein is more likely to precipitate; and increasing the charge of a particular protein reduces its ability to precipitate.
Parameter estimation from multiresponse data
Authors: Stewart Warren E. , Caracotsios Mike , Sørensen Jan P. 
Published: June 2004

AbstractMultiresponse data are familiar outputs of experiments and processes involving multicomponent mixtures, multiple streams or multiple methods of observation. The resulting arrays of data exhibit various structures, including rectangular (no missing values), block‐rectangular, and irregular (missing values with no simple pattern). Methods for modeling such data are described, emphasizing parameter estimation strategy and available software. Experiences in multiresponse modeling are then reviewed for several chemical engineering problems.
Adsorption breakthrough behavior: Unusual effects and possible causes
Authors: Park Illam , Knaebel Kent S. 
Published: June 2004

AbstractAdsorption of water vapor on silica gel, at influent humidities from 6 to 80% at 25 and 50°C, yielded breakthrough curves of unusual shapes. Breakthrough patterns varied from the expected sigmoidal shape at low humidity to a curve resembling the tangent function, but symmetric about the stoichiometric breakthrough time. Unusual shapes were found to be due to subtle combination of Type‐IV isotherm behavior and heat effects. A mathematical model was developed to simulate the performance. The results show that complex breakthrough behavior need not be ascribed to complicated causes (such as diffusion in bidisperse pores), which require multiparameter fitting of experimental data. In fact, the effects may be predicted from properties measured in simple independent experiments, though some care is required to account for the effects accurately.
Effect of particle size distribution in different fluidization regimes
Authors: Sun Guanglin , Grace John R. 
Published: June 2004

AbstractThe effect of particle size distribution (PSD) on the performance of a fluidized bed reactor was investigated in different hydrodynamic regimes: bubbling, slugging, turbulent and fast fluidization, with three particle size distributions, all with the same mean diameter and nearly the same particle density and BET surface area. Regime transitions were examined by measuring pressure fluctuations. Void sizes tended to be smaller and the transition from bubbling or slugging to turbulent fluidization was achieved earlier for the wide distribution powder than for the narrow PSD. At gas velocities ≤ 0.2 m/s, the conversion and reactor efficiency were not affected greatly by the PSD. However, at higher gas velocities, PSD played a significant role. For particles of wide size distribution, the conversion in the turbulent and fast fluidization regimes was usually higher than in the bubbling fluidization regime at the same dimensionless rate constant, kf′. On the other hand, for particles of narrow size distribution, the dependence of conversion on the regime is small, except for the fast fluidization regime.
Estimation of surface tension of pure fluids using the gradient theory
Authors: Pérez‐López Juán H. , González‐Ortiz Luis J. , Leiva Miguel A. , Puig Jorge E. 
Published: June 2004

AbstractThe modified van der Waals theory of interfaces (gradient theory) is used with the equations of state of Peng‐Robinson and Mohanty‐Davis to predict surface tensions of various nonpolar, quadrupolar, slightly polar and polar substances as a function of temperature. The influence parameter of the gradient theory is estimated as 0.785ab2/3 in terms of the attractive and repulsive van der Waals parameters using the Lennard‐Jones potential. The predictions made with the Mohanty‐Davis equation of state agree better with experimental data than those made with Peng‐Robinson, even for nonpolar fluids.
Caffeine extraction rates from coffee beans with supercritical carbon dioxide
Authors: Peker Hulya , Srinivasan M. P. , Smith J. M. , McCoy Ben J. 
Published: June 2004

AbstractThe extraction of caffeine from whole coffee beans with supercritical carbon dioxide was studied in a continuous‐flow extraction apparatus. Decaffeination rates were determined as a function of CO2 flow rate, temperature and pressure by continuously monitoring the caffeine in the effluent with a flame ionization detector. Soaking the raw beans in water prior to decaffeination enhanced the rate of extraction, which increased markedly with water content. Using CO2 saturated with water also increased the rate of extraction. The rate of decaffeination increased with pressure and temperature and was influenced by both intraparticle diffusion in the water‐soaked beans and external mass transfer. A mathematical model based on a linear‐driving‐force approximation of mass transfer and partitioning of caffeine between the water and the supercritical CO2 describes the time‐dependent process. The partition coefficient for caffeine distributed between water and supercritical CO2, the only parameter determined from the dynamic extraction rate data, increases with temperature and pressure.
Steady‐state traveling waves on the surface of a viscous liquid film falling down on vertical wires and tubes
Authors: Trifonov Yu. Ya. 
Published: June 2004

AbstractVarious nonlinear wavy regimes of a viscous liquid film flowing down vertical wires and tubes were calculated using the integral method. The linear stability analysis of the trivial smooth solution was compared with the results published previously. In the region of linear instability, the competition among the gravity, viscous and capillary forces formed the steady‐state traveling solutions of finite amplitudes. At least two families of waves were shown to be parameterized by the wave number for given values of external parameters (Reynolds number, cylinder radius and physical characteristics of liquid). The basic waves characteristics depended on external parameters and on wave number. The intensity of wavy processes increased with decreasing cylinder radius. The calculations show the catastrophic growth of wave amplitude, when the system flows down a vertical tube of sufficiently small radius and moves into the linear, unstable region.
Intraparticle convection effect on pressurization and blowdown of adsorbers
Authors: Lu Z. P. , Loureiro J. M. , LeVan M. D. , Rodrigues A. E. 
Published: June 2004

AbstractPressurization and blowdown steps in pressure swing adsorption (PSA) with binary mixtures of inert and adsorbable species are studied. Modeling involves mass balances for the bulk fluid phase and inside particles, that is, intraparticle diffusion/convection models, momentum balance equations, and linear adsorption equilibrium isotherm. The importance of intraparticle convection in PSA is assessed. Mass transfer inside pores is enhanced by intraparticle convection, which leads to a better efficiency of adsorption (pressurization) and desorption (blowdown) processes. Performance is improved by intraparticle convection and lies between diffusion and equilibrium limits, as shown in propagation profiles of the adsorbable species mole fraction in the bed and inside particles.
Finite element analysis of viscous mixing with a helical ribbon‐screw impeller
Authors: Tanguy P. A. , Lacroix R. , Bertrand F. , Choplin L. , de la Fuente E. Brito 
Published: June 2004

AbstractA finite element analysis was carried out to study the mixing patterns of a viscous Newtonian fluid into a stirred tank. A helical ribbon screw impeller (HRS) was used as the mixing device. The numerical simulation involved the full three‐dimensional resolution of the equations of change governing the flow in the tank. The circulation time and the torque were determined and compared with experimental data. A good agreement was found, showing the usefulness of the numerical approach for design purposes.
Settling and rheology of suspensions of narrow‐sized coal particles
Authors: Turian Raffi M. , Hsu Feng‐Lung , Avramidis Kostas S. , Sung Dong‐Jin , Allendorfer Robert K. 
Published: June 2004

AbstractSettling rates, yield stresses, and shear‐stress/shear‐rate dependences were determined as functions of solids concentration for suspensions in water of coal particles of narrow size fractions. Particles and suspensions are characterized by coal analyses, determination of heating value, solid heat capacity and thermal conductivity, particle size analyses, and determination of densities and maximum packing concentrations for each particle size range. Analysis of the settling rate data establishes that the usually strong retardation of (hindered) settling due to concentration is compounded by particle shape effects. In addition, observed particle shapes are subsumed within particle shape classes inferred from the settling data. Shear‐stress/shear‐rate data for concentrated suspensions of the three larger and relatively narrower sized particles over the broad range of shear rates from about 1 to 104S−1 suggest the behavior to be essentially Newtonian, albeit with a marked particle concentration augmentation. Suspensions of the smallest coal particles of broader size distribution were strongly non‐Newtonian, possessing a yield stress at low shear, essentially power‐low shear‐thinning behavior over the intermediate range and approaching high‐shear Newtonian limiting behavior above about 103S−1. Relations developed include viscosity‐and yield stress‐concentration correlations incorporating the maximum packing volume fraction of solids. These are useful generally for suspensions and particularly as the basis for guiding the formulation of coal‐water fuels.
Coking and activity of porous catalysts in supercritical reaction media
Authors: Baptist‐Nguyen Sarah , Subramaniam Bala 
Published: June 2004

AbstractA mathematical model is presented to describe coking and activity characteristics of porous catalysts in supercritical reaction mixtures. These characteristics are determined by the way in which temperature and pressure affect the simultaneous physicochemical rate processes involving the effective diffusion of reactants in the pore, coke formation, and coke extraction. For a parallel coking reaction, the model predicts that when the reaction mixture density is isothermally increased from subcritical to low‐to‐moderate values, the ensuing higher reaction rates and restricted diffusion lead to pore‐mouth plugging and decreased catalyst effectiveness factors. At dense supercritical conditions, however, the enhanced coke solubilities in the reaction mixture alleviate pore‐mouth restrictions, resulting in a recovery of catalyst activity and increased effectiveness factors. The model predicts an optimum supercritical density at which catalyst activity is maintained at a maximum value. At smaller than optimum density values, the reaction rate is limited by coke extraction and at larger than optimum values, the rate is subject to pore diffusion limitations. These predictions are qualitatively consistent with reported experimental observations.
Numerical calculation of wall‐to‐bed heat‐transfer coefficients in gas‐fluidized beds
Authors: Kuipers J. A. M. , Prins W. , Van Swaaij W. P. M. 
Published: June 2004

AbstractA computer model for a hot gas‐fluidized bed has been developed. The theoretical description is based on a two‐fluid model (TFM) approach in which both phases are considered to be continuous and fully interpenetrating. Local wall‐to‐bed heat‐transfer coefficients have been calculated by the simultaneous solution of the TFM conservation of mass, momentum and thermal energy equations. Preliminary calculations suggest that the experimentally observed large wall‐to‐bed heat‐transfer coefficients, frequently reported in literature, can be computed from the present hydrodynamic model with no turbulence. This implies that there is no need to explain these high transfer rates by additional heat transport mechanisms (by turbulence).The calculations clearly show the enhancement of the wall‐to‐bed heat‐transfer process due to the bubble‐induced bed‐material refreshment along the heated wall. By providing detailed information on the local behavior of the wall‐to‐bed heat‐transfer coefficients, the model distinguishes itself advantageously from previous theoretical models. Due to the vigorous solids circulation in the bubble wake, the local wall‐to‐bed heat‐transfer coefficient is relatively large in the wake of the bubbles rising along a heated wall.
Robust stabilization of input/output linearizable systems under uncertainty and disturbances
Authors: Arkun Yaman , Calvet Jean‐Paul 
Published: June 2004

AbstractIn recent years, differential geometric techniques have been used to transform nonlinear systems into linear systems. Once such equivalent linear systems are obtained, classical linear controllers are designed to achieve desired stability and performance properties. A major critism against these techniques is their lack of guarantee for robustness. In particular, the design of controllers for transformed nonlinear systems under the influence of both disturbances and (parametric) modeling errors is not well‐known. This article presents a methodology to design robust stabilizing controllers for such uncertain and perturbed nonlinear systems.For feedback linearizable systems, the method guarantees that the nonlinear system has nominally linear input/output dynamics and is stable for the given class of bounded parametric uncertainty and disturbances. The new concepts and the proposed design procedure are shown for an isothermal reactor with second‐order kinetics.