bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative analysis and computation of two‐dimensional bubble columns
Authors: Lin T.‐J. , Reese J. , Hong T. , Fan L.‐S.
Published: June 2004

Experiments conducted quantify the macroscopic hydrodynamic characteristics of various scale 2‐D bubble columns, which include dispersed and coalesced bubble regimes characterized by two flow conditions (4‐ and 3‐region flow) with coherent flow structures. Hydrodynamic behavior is analyzed based on flow visualization and a particle image velocimetry (PIV) system. Columns operated in the 4‐region flow condition comprise descending, vortical, fast bubble and central plume regions. The fast bubble flow region moves in a wavelike manner, and thus the flow in the vicinity of this region is characterized macroscopically in terms of wave properties. In columns greater than 20 cm in width, the transition from the dispersed bubble flow regime to the 4‐ and then to 3‐region flow in the coalesced bubble regime occurs progressively with gas velocities at 1 and 3 cm/s, respectively. The demarcation of flow regimes is directly related to measurable coherent flow structures. The instantaneous and time‐averaged liquid velocity and holdup profiles provided by the PIV system are presented in light of the macroscopic flow structure in various 2‐D bubble columns. Numerical simulations demonstrate that the volume of fluid method can provide the time‐dependent behavior of dispersed bubbling flows and account for the coupling effects of pressure field and the liquid velocity on the bubble motion. Comparison of computational results with PIV results for two different bubble injector arrangements is satisfactory.
Unified network model for diffusion of condensable vapors in porous media
Authors: Rajniak Pavol , Yang Ralph T.
Published: June 2004

A unified network model is formulated for predicting effective Fickian diffusivities of condensable vapors in porous media where capillary condensation and adsorption‐desorption hysteresis occur. The model unifies the equilibrium theory based on the poreblocking interpretation of hysteresis in the interconnected network of pores and the percolation model of mass transport in the network with randomly interspersed regions for capillary condensation and surface flow. The Bethe network is used to represent the porous medium, and the effective medium theory is employed to obtain the effective diffusivity. Using the information on the connectivity and the positions of the closure points of the hysteresis loop enclosed by the equilibrium primary adsorption and primary desorption isotherms, the concentration dependence of the effective diffusivity is predicted. The model is applied to the systems water vapor‐silica gel, water vapor‐activated alumina, and literature data. It successfully predicts the concentration dependence of the effective diffusivity in the whole range of relative pressures for systems both with and without a peak in the diffusivity.
Disturbance rejection with bounded control action: Loop‐shaping methodology
Authors: Figueroa Jose L. , Agamennoni Osvaldo , Romagnoli Jose A.
Published: June 2004

Suppressing the influence of external disturbances on a process is the most common objective of a controller in a chemical plant. A control system that can keep the controlled variables at or near their set point and the manipulated variables lower than their prespecified maximum bounds in the face of load disturbances is wanted. A method is presented to design controllers with disturbance rejection specifications and limits over the control energy. This methodology is based on a special strategy to shape the structured singular value of a closed‐loop transfer function matrix.
Startup strategy design and safeguarding of industrial adiabatic tubular reactor systems
Authors: Verwijs J. W. , van den Berg H. , Westerterp K. R.
Published: June 2004

The safeguarding methodology of chemical plants is usually based on controlling the instantaneous values of process state variables within a certain operating window, the process being brought to shutdown when operating constraints are exceeded. This method does not necessarily prevent chemical reactors suffering from a runaway during dynamic operations because (a) excessive amounts of unreacted chemicals can still accumulate in the process, and (b) no means are provided to the operating personnel to identify hazardous process deviations. A model‐based startup and safeguarding procedure is developed for an industrial adiabatic tubular reactor to improve process safety during startup. The trajectories of manipulated variables are calculated by minimizing the amount of one of the main reactants in the reactor effuent. It is concluded that proper control of the initial reactor temperature profile is critical for a safe startup while the impact of other manipulated variables is relatively smaller than that of the initial reactor temperature profile.
Characterization of fluid distributions in porous media by NMR techniques
Authors: Liaw Hsie‐Keng , Kulkarni Raghavendra , Chen Songhua , Watson A. Ted
Published: June 2004

Nuclear magnetic resonance (NMR) spin‐lattice relaxation measurements are used to investigate pore structures and fluid phase distributions in porous media. A new method for estimating relaxation time distribution functions from measured relaxation data is presented using a B‐spline basis to represent the distribution function and Tikhonov regularization to stabilize the estimation problem. Surface relaxivity, which is required to convert relaxation time distributions to pore‐size distributions of fluid phase distributions at partial saturations, is determined using pore volume‐to‐surface‐area ratios estimated by NMR diffusion measurements. This approach was validated by analyzing certain model porous media with known pore volume‐to‐surface‐area ratios. The method is demonstrated by determining pore‐size and fluid phase distributions of sandstone and carbonate samples, as well as by comparing the pore‐size distributions of chalk samples obtained by this methodology with those estimated by mercury porosimetry.
A new solution to emulsion liquid membrane problems by non‐Newtonian conversion
Authors: Skelland A. H. P. , Meng Xiquan (Michael)
Published: June 2004

Emulsion liquid membrane separation processes remain excessively vulnerable to one or more of four major problems. Difficulties lie in developing liquid membranes that combine high levels of both stability and permeability with acceptably low levels of swelling and ease of subsequent demulsification for membrane and solute recovery. This article provides a new technique for simultaneously overcoming the first three problems, while identifying physical indications that the proposed solution may have little adverse effect on the fourth problem (demulsification) and may even alleviate it. Numerous benefits of optimized conversion of the membrane phase into suitable non‐Newtonian form are identified, their mechanisms outlined, and experimental verifications provided. These include increased stability, retained (or enhanced) permeability, reduced swelling, increased internal phase volume, and increased stirrer speeds. The highly favorable responsiveness of both aliphatic and aromatic membranes to the new technique is demonstrated.
Extension of a generalized quartic equation of state to pure polar fluids
Authors: Lin Yen‐Liang , Bienkowski Paul R. , Shah Vinod M. , Cochran Hank D.
Published: June 2004

A generalized four‐parameter quartic equation of state (EOS) proposed by Shah and coworkers (1992, 1994) was extended to polar fluids. To use the new generalized quartic EOS for polar fluids, only four characteristic properties of the pure compound are required—critical temperature, critical volume, acentric factor, and dipole moment. For nonpolar fluids, the previous equation is recovered and its superior performance for nonpolar fluids is retained. A new set of coefficients for polar fluids for the EOS was determined from multiproperty regressions using literature data for various physical and thermodynamic experimental properties of 30 pure compounds. These regressed coefficients, which are functions of the critical temperature, critical pressure, acentric factor, and the dipole moment of the fluid, were generalized. The accuracy of this equation of state is evaluated with the new set of regressed coefficients to predict thermodynamic properties of pure polar compounds, including vapor pressure, density, residual enthalpy, enthalpy of vaporization, and second virial coefficient. The capability and accuracy of this quartic equation of state are also compared with the well‐known Peng‐Robinson EOS.
Forces on a large cylindrical bubble in an unsteady rotational flow
Authors: Taeibi‐Rahni Mohammad , Loth Eric
Published: June 2004

The full Navier–Stokes equations were employed with a single‐fluid model and a front tracking scheme to study a large cylindrical bubble in a free shear layer. A general formulation based on work by Auton et al. of the hydrodynamic forces on a finite Reynolds number large bubble in an unsteady, nonuniform and rotational flow was then used to investigate the effects of nonlinear spatial and temporal gradients on dispersion. The resulting bubble dispersion in the full Navier–Stokes solution significantly differ from that by a conventional bubble dynamic equation based on linear spatial gradients and quasi‐steady flow. This was due to the adjunct forces not accounted for by such a formulation, which are related to regions of high nonuniformity and unsteadiness. These adjunct forces in the drag/lift direction were correlated with rapid variations of relative bubble velocity and high gradients of the liquid velocity.
Study of breakup mechanisms in cavity flow
Authors: Kang John , Smith Theodore G. , Bigio David I.
Published: June 2004

Drop breakup mechanisms inside a cavity flow are presented for two immiscible fluids. Due to the nonuniform flow condition of the cavity, the breakup mechanism varied along the streamlines. The streamlines were characterized by stream zones A and B, where zone A possessed a methodical transient breakup governed by Tomotika's breakup via capillary instabilities, and the breakup mechanism of stream zone B consisted of tip streaming breakup, an inefficient breakup mechanism. The flow behavior near flight region had a significant role in the drop breakup mechanisms. The study of the evolution of drop dispersion showed that the matrix viscosity is critical in controlling the transient breakup process and that the shear rate increase had little or no effect on the drop breakup.
Simulation of fluidized beds and other fluid‐particle systems using statistical mechanics
Authors: Seibert Kevin D. , Burns Mark A.
Published: June 2004

The expansion behavior and structural phenomena of fluid‐particle systems was simulated using a method analogous to the Monte Carlo method for molecular systems. Individual particles are moved, and the resulting moves are accepted or declined based on the change in the system's potential energy and the average kinetic energy of the system. Several fluid–particle systems have been successfully predicted with the model including colloidal particle concentration profiles and random packing of uniform spheres. Additionally, predictions of steady‐state fluidized‐bed expansion characteristics for uniformly sized stainless‐steel spheres and narrowly distributed nickel and glass spheres show excellent agreement with the theoretical model used in the simulation and satisfactory agreement with experimental data. Dynamic expansion predictions of both bed height and overall bed structure as a function of time also agree with the experimental data.
New discretization procedure for the agglomeration equation
Authors: Hill Priscilla J. , Ng Ka M.
Published: June 2004

A new discretization procedure for the agglomeration equation is presented. The intrinsic problems caused by discretization of the particle‐size distribution are mitigated by using proper probability functions. Thus, the new discretized equation predicts the same total number of particles as its continuous counterpart and guarantees conservation of mass. This general method can correctly predict any two properties simultaneously and is not limited to number and mass. Simulation results with various agglomeration kernels show that the new method provides significant improvements over the conventional discretization method. Furthermore, the new method allows the user to choose equal‐size intervals or geometric‐size intervals with any geometric ratio to cover the particle‐size range. In addition to flexibility, the latter feature leads to better accuracy in particle‐size distribution predictions.
Process synthesis under uncertainty: A penalty function approach
Authors: Chaudhuri Prosenjit D. , Diwekar Urmila M.
Published: June 2004

With the growing environmental concern, it is necessary to improve process simulation and develop design tools to account for environmental factors in the synthesis of large‐scale chemical processes. A major obstacle in tackling this problem is uncertainties in some of the technical and economic parameters, which lead to uncertainties in design, plant performance, and cost estimates. Further, a conceptual process design involves the identification of an optimal flowsheet structure from many alternatives stituting the “superstructure.” Synthesis and optimization of large‐scale processes involving uncertainties often require considerable computational effort. A novel algorithm presented here is based on simulated annealing for the process synthesis of large‐scale flowsheets having several configurations and considers uncertainties in the process design systematically. This new “stochastic annealing algorithm,” provides an efficient approach to stochastic synthesis problems by incorporating a penalty term in the objective function and balances the trade‐off between accuracy and efficiency based on the annealing temperature. It has been used to study a benchmark synthesis problem in the HDA process. Savings of up to 80% in CPU time has been achieved without significant loss of solution precision with stochastic annealing, compared to simulated annealing with a fixed sample size. It can be applied to analyze efficiently any complex process flowsheet and provide valuable insights into process feasibility based on optimal design, plant performance, and uncertainty issues.
Gage control of film and sheet‐forming processes
Authors: Rawlings James B. , Chien I‐Lung
Published: June 2004

A new methodology for gage control of sheet and film processes is presented. A linear process model is developed that is capable of describing many of the interesting features of the process, including a noisy and moving gage sensor, significant process noise, strong coupling between the gage in different lanes, constraints on the actuators, and significant time delay between the actuators and the sensor. An automatic control of this process given this model form is designed. Novel features of the controller are that it can contend with information from a moving sensor and handle hard constraints on the actuators. Control performance and robustness to noise are demonstrated favorably via simulations.
Closed‐loop identification of a continuous crystallization process
Authors: Eek R. A. , Both J. A. , Van den Hof P. M. J.
Published: June 2004

Identification of low‐order linear multiinput/multiouput models can lead to accurate descriptions of the dynamic behavior of a continuous crystallization process. While open‐loop experiments exhibit an oscillating crystal size distribution, improved experimental conditions can be established through stabilization of the process with a simple single‐loop feedback controller. The resulting closed‐loop identification problem is studied using low‐order linear multivariable input–output models. Two closed‐loop identification methods are applied, one of which was recently introduced to provide accurate approximate models in general closed‐loop process configurations. Identification and validation data are obtained from an evaporative pilot crystallizer, and the identified models are validated in terms of time‐ and frequency‐domain responses. A fourth‐order, three‐input three‐output model is shown to describe accurately the process dynamics. The results are compared with a linearized and reduced first‐principles model.
Phase‐change heat regenerators: Modeling and experimental studies
Authors: Erk Henry F. , Duduković M. P.
Published: June 2004

Modeling of and experiments with heat storage in regenerators packed with phase‐change material (PCM) are discussed, as well as the second‐law thermodynamic efficiency for the ideal phase‐change regenerator (PCR). An algorithm to solve the coupled partial differential equations for heat transfer and storage in the PCR on the bed scale and on the PCM scale is presented. The bed is discretized via the tanks‐in‐series approximation. The PCM scale is solved by orthogonal collocation applied to the equations, transformed to immobilize the melt/solid interface and eliminate the effect of spherical geometry. Parametric studies show the effects of specific dimensionless groups. A novel PCM consisting of n‐octadecane retained by capillary forces in a porous silica support is used in a lab‐scale PCR to verify the model. It visually changes from opaque to semi‐transparent when the wax melts, thereby allowing the melt front within the bed to be tracked. Experiments with heated or cooled CO2 passing through the PCR are described. The measured outlet temperature compares qualitatively with model predictions. The model quantitatively predicts melt front movement in the first 60% of the bed. Discrepancies between the model and experiments are linked to significant heat losses.
MRI investigations of fractures and multiphase flow in fractured media
Authors: Chen Songhua , Yao Xiaoli , Qiao Jinli , Watson A. Ted
Published: June 2004

The new application of nuclear magnetic resonance imaging (MRI) techniques for characterizing fractures and flow in fractured media is investigated. Specifically, a relaxation‐weighted imaging technique is used for selectively highlighting either fracture or porous matrix regions. Many advantages over conventional spin‐density MRI techniques for characterizing fractured media are demonstrated. Its use to speed image acquisition is also demonstrated. In addition, a multislice profile imaging technique is used to investigate imbibition and drainage displacement experiments in fractured porous media. These images demonstrate that the fractures can have profound effects on the fluid distributions in multiphase flow.
Model of crystallization in a confined space applied to iron uptake and release by ferritin
Authors: Courteix Anne , Bergel Alain
Published: June 2004

Many living organisms store iron in solid form, Fe(III), as a crystal in the inner cavity of the ferritin molecule. When iron is needed for biosynthesis, a reducing agent reduces Fe(III) into the soluble form Fe2+ released by ferritin. Crystallization and release processes are reversible, and their rates evolve in an identical way as a function of the number n of iron atoms in the molecule. The rate increases with n, showing a maximum value when n is approximately 1,300, and then stabilizes for the highest values of n, which can reach 4,500. On the other hand, plotting the amount of released iron as a function of time gives curves with a sigmoid shape. The proposed model was based on the theoretical description of different steps involved in crystal growth inside the protein shell: several independent crystals grow freely at the inner protein wall, and then a distribution function takes into account possible overlapping of different crystallite clusters, whose further growth is limited by diminution of the available space inside the cavity. The kinetics derived was then used to calculate the release curve as a function of time. Solving the system of differential mass‐balance equations was simplified by describing the ferritin population as a large discrete distribution of species. The model fully fitted and explained the variation in the crystallization rate with n, and the sigmoid shape of the release curve as a function of time obtained experimentally in a thin‐layer electrochemical cell.
Mathematical modeling of a solid‐state limiting current carbon monoxide sensor
Authors: Tan Y. , Tan T. C.
Published: June 2004

A solid‐state carbon monoxide sensor was fabricated using a 9% yttria‐stabilized zirconia (YSZ) disc sandwiched between two platinum thin‐film electrodes. One of the electrodes was coated with a thin layer of 7 CuO · 10 ZnO · 3 Al2O3 catalyst. The sensor showed limiting current behavior at an applied voltage between 0.5 and 1.2 V. Linear response was observed with carbon monoxide in a nitrogen–oxygen mixture at high temperature and limiting current conditions. The linear carbon monoxide concentration range increased with increase in the operating temperature and its sensitivity increased from 2.437 mA · atm−1 CO at 1,023 K to 10.771 at 1,093 K. These characteristics were adequately described by the proposed mathematical model relating the response to the rate processes occurring in the catalyst layer and in the electrochemical cell under limiting current conditions. The model showed that effective sensing and high sensitivity are best obtained using a catalyst with high catalytic activity toward the test solute and proper design and fabrication of the sensor to ensure its high diffusivity in the catalyst.
Random walks in prefractal porous media
Authors: Sternberg Steven P. K. , Cushman John H. , Greenkorn Robert A.
Published: June 2004

Random walks are used to examine dispersion between miscible fluids in one‐dimensional, prefractal porous medium. Mixing in such media is anomalous and several nonlocal theories have been proposed to describe this non‐Fickian characteristic. This work simulates dispersion via a random walk in a prefractal media by incorporation local and spatially nonlocal effects and compares the results with laboratory experiments. Results suggest that nonlocal models fit experimental data better than local models.
Permeability of gigaporous particles
Authors: Pfeiffer James F. , Chen John C. , Hsu James T.
Published: June 2004

The volumetric flow rate of liquid and gas through small gigaporous particles was measured by a new method that isolates single particles in a test apparatus. To our knowledge, this is the first direct confirmation of flow through gigaporous particles made at pressure drops experienced during normal operation. High‐performance liquid chromatography particles from 30 to 50 μm in diameter, previously reported to exhibit convection‐enhanced intraparticle mass transfer, were studied. Using a CFD model of the test system, the permeability of individual particles was determined from the pressure‐drop‐flow‐rate relationship. The average measured permeability of the particles studied is 7.89 × 10−15 m2 with no dependence on particle size. This is 4 to 17 times greater than values calculated from models currently used to estimate the permeability of these kinds of particles. No other experimentally measured values of permeability have been reported for particles of this size. The results of this study might imply that the intraparticle structure does not behave like a bed of uniformly packed microspheres, but rather as an inhomogeneous assemblage of microparticles. The measured permeability values offer the possibility of developing better models of the intraparticle flow field under normal operating conditions. Knowledge of the intraparticle flow field is an important step in deriving predictive models of convective mass transfer in these types of particles.
Determining porosity profiles during filtration and expression of sewage sludge by NMR imaging
Authors: La Heij E. J. , Kerkhof P. J. A. M. , Kopinga K. , Pel L.
Published: June 2004

The formation of a filter cake during mechanical dewatering (filtration and expression) of sludge is studied using one‐dimensional nuclear magnetic resonance (NMR) imaging of porosity profiles. The experiments clearly showed the formation of a porosity gradient during filtration and the disappearing of this gradient during expression, forming a cake with a uniform porosity. This technique makes it possible to investigate the material behavior of the porous medium, which is needed before any model calculations can be performed. The results indicate that NMR imaging is useful in nondestructive monitoring of the filtration and expression process of sludge.
Damage detection in composites by ZnO sensors
Authors: Viljoen H. J. , van Rensbure N. F. J.
Published: June 2004

The development of composites with self‐diagnostic capabilities is based on the analysis of a mathematical model of the structure. The method is illustrated for a vibrating cantilevered plate and analyzed considering damping. Damage is modeled as an elastic joint, and the spectrum of natural frequencies is calculated for different positions and magnitudes of the elastic joint. Small sensors consisting of tungsten filaments coated with ZnO are embedded in the plate, and their output is modeled. The solution of the inverse problem depends on the position and extent of damage which can be uniquely solved if the system is not overdamped with respect to the primary frequency. Higher modes have a decreasing contribution to the signal output; when the primary mode is overdamped, only the secondary frequency can be detected reliably. The solution to the inverse problem is no longer unique; there are as many as three different solutions (damage position and extent of damage) all of which produce the same signal output.
Backone family of equations of state: 1. Nonpolar and polar pure fluids
Authors: Müller Andreas , Winkelmann Jochen , Fischer Johann
Published: June 2004

The Helmholtz energy F is written as the sum F = FH + FA + FPol, where FH is the hard‐body contribution, FA the attractive dispersion force contribution, and FPol either the quadrupolar or dipolar contribution. Here, a new expression for FA is constructed by a simultaneous correlation of experimental data of methane, oxygen, and ethane. The resulting equations for F with only three or four substance‐specific parameters are tested for several nonpolar, quadrupolar, and dipolar fluids. In the correlation of large data sets good results are obtained. The significance of the approach, however, is in the good to excellent prediction of all thermodynamic properties in the whole fluid region based on a parameter fit to only four experimental data—two vapor pressures and two saturated liquid densities.
Analysis of cell‐target encounter by random filopodial projections
Authors: Buettner Helen M.
Published: June 2004

Dynamic projections from the surface of many motile cell types provide for variable contact with the extracellular environment and can be important in regulating cell migration events. For example, during nerve development and regeneration, the sensory motile tip of the axon exhibits long, slender filopodia projecting from the growth cone periphery. Extension and retraction of these filopodia continually remodel the points of contact between axon and surroundings. Experimental studies show that filopodial contact with specific extracellular features can guide subsequent growth cone migration, suggesting a potentially important means of engineering nerve growth to repair nerve injury or construct biological neural networks. A simulation model is presented of the dynamic filopodial structure on the nerve growth cone based on recent experimental characterization. The model is analyzed to obtain quantitative relationships between average filopodial characteristics, which are commonly measured experimental quantities, and the underlying parameters of individual filopodium dynamics. It is then applied to simulate encounter between a growth cone and its target due to filopodial dynamics alone. Filopodial contribution to growth cone‐target encounter is summarized in terms of a mean encounter time that is reminiscent of a first passage time for a diffusing particle. The parametric relationships in this study provide a basis for further investigation of filopodial‐mediated mechanisms in nerve growth and other cellular processes.
Air purification in a reverse‐flow reactor: Model simulations vs. experiments
Authors: van de Beld L. , Westerterp K. R.
Published: June 2004

The behavior of a reverse‐flow reactor was studied for the purification of polluted air by catalytic combustion. A heterogeneous one‐dimensional model was extended with a heat balance for the reactor wall. An overall heat transport term is included to account for the small heat losses in radial direction.

The calculations are compared to experimental data without using fit parameters. The agreement between simulations and experiments is generally good. Discrepancies can be explained mainly by inaccurate kinetic data and experimental uncertainties. At low gas velocities and for small reactor diameters, the one‐dimensional model failed and a two‐dimensional model must be developed to improve the predictive potential.
Atomization of dilute polyisobutylene/mineral oil solutions
Authors: Smolinski Joseph M. , Gulari Esin , Manke Charles W.
Published: June 2004

Atomization experiments, motivated by the need to suppress misting of machining oils, were conducted on dilute solutions of polyisobutylene (PIB) in mineral oil using a coaxial air blast atomizer and an optical (Fraunhofer diffraction) particle sizer. Polymer concentrations (0.1 to 1.0 kg/m3) and molecular weight (1.0 to 2.2 million) were varied to determine their effects on aerosol drop‐size distributions. The pure oil and PIB ‐ oil solutions were atomized at air/liquid mass ratios near 10 and atomizing air velocities of 150–270 m/s, producing droplet‐size distributions with mass mean diameters (MMD) in the range of 7–15 μm for the pure oil. Under identical atomization conditions, the MMDs of PIB solutions were 20–200% higher than pure oil. These observed increases in MMD correspond to significant reductions in the fraction of droplets falling below 5 μm in diameter that constitute the misting problem in industrial machining applications. Observed effects of PIB on atomization are ascribed to the viscoelastic properties of PIB ‐ oil solutions as characterized by the elongational viscosity ηE. This relationship is examined by correlating the change in MMD caused by PIB addition with the stress‐dependent elongational viscosity of PIB‐oil solutions as predicted by the FENE‐P dumbbell kinetic theory. The increase in MMD due to PIB varies linearly with the predicted ηE at constant atomization tensile stress.
Stability of the liquid‐fluidized magnetically stabilized fluidized bed
Authors: Fee Conan J.
Published: June 2004

Rosensweig's (1979) analysis of the stability of gas‐fluidized magnetically stabilized fluidized beds (MSFBs) with voidage perturbations is modified to include terms dependent on the fluid density. This allows the stability analysis to be applicable to liquid‐fluidized MSFBs, which have potential use in liquid chromatography. Results show that in systems where the fluid density is not insignificant when compared to the solid‐phase density, the difference in density between the two phases should be used when normalizing the magnetization required to achieve stability. Comparisons between theory and experiment show that although the present analysis improves on Rosensweig's analysis, it still does not give reliable, quantitative predictions of stability behavior. It is suggested that for liquid‐fluidized MSFBs, stability to particle velocity perturbations, rather than to voidage perturbations, is more relevant to chromatographic applications wherein the purpose of magnetic stabilization is to prevent particle mixing as opposed to preventing the occurrence of bubbling and slugging observed in gas‐fluidized beds.
Theoretical model for drop and bubble breakup in turbulent dispersions
Authors: Luo Hean , Svendsen Hallvard F.
Published: June 2004

A theoretical model for the prediction of drop and bubble (fluid‐particle) breakup rates in turbulent dispersions was developed. The model is based on the theories of isotropic turbulence and probability and contains no unknown or adjustable parameters. Unlike previous work, this model predicts the breakage rate for original particles of a given size at a given combination of the daughter particle sizes and thus does not need a predefined daughter particle size distribution. The daughter particle size distribution is a result and can be calculated directly from the model. Predicted breakage fractions using the model for the air–water system in a high‐intensity pipeline flow agree very well with the available 1991 experimental results of Hesketh et al. Comparisons of the developed model for specific particle breakage rate with earlier models show it to give breakage‐rate values bracketed by other models. The spread in predictions is high, and improved experimental studies are recommended for verification.
Pore and surface diffusion in multicomponent adsorption and liquid chromatography systems
Authors: Ma Z. , Whitley R. D. , Wang N.‐H. L.
Published: June 2004

A generalized parallel pore and surface diffusion model for multicomponent adsorption and liquid chromatography is formulated and solved numerically. Analytical solution for first‐ and second‐order central moments for a pulse on a plateau input is used as benchmarks for the numerical solutions. Theoretical predictions are compared with experimental data for two systems: ion‐exchange of strontium, sodium, and calcium in a zeolite and competitive adsorption of two organics on activated carbon. In a linear isotherm region of single‐component systems, both surface and pore diffusion cause symmetric spreading in breakthrough curves. In a highly nonlinear isotherm region, however, surface diffusion causes pronounced tailing in breakthrough curves; the larger the step change in concentration, the more pronounced tailing, in contrast to relatively symmetric breakthroughs due to pore diffusion. If only a single diffusion mechanism is assumed in analyzing the data of parallel diffusion systems, a concentration‐dependent apparent surface diffusivity or pore diffusivity results; for a convex isotherm, the apparent surface diffusivity increases, whereas the apparent pore diffusivity decreases with increasing concentration. For a multicomponent nonlinear system, elution order can change if pore diffusion dominates for a low‐affinity solute, whereas surface diffusion dominates for a high‐affinity solute.
Robust unconstrained predictive control design with guranteed nominal performance
Authors: Hrissagis Kostas , Crisalle Oscar D. , Sznaier Mario
Published: June 2004

A systematic and complete method to design robust predictive controllers for unconstrained linear systems is proposed. The synthesis procedure is based on rigorous theoretical foundations, without resorting to approximations or ad hoc design guidelines, yet it remains a viable tool for practical application. A significant feature is that the robust predictive controller retains the servo performance of a nominal predictive controller designed using conventional methods. In addition, the robust predictive controller can be designed to guarantee perfect steady‐state rejection of asymptotically constant disturbances. The robust design method is developed for systems affected by unmodeled dynamics and is based on solving a discrete‐time model‐matching problem. It is shown that the robust controller can be classified legitimately as a predictive controller because it minimizes the same performance functional as the nominal predictive controller. An illustrative design example is given.
Statistical operating strategies for charging batch reactors
Authors: Tsai Chii‐Shiang , Chang Chuei‐Tin
Published: June 2004

Two critical decisions must be made in the charging sequence of batch reactors: target setting and alarm generation. A number of statistics‐based strategies are proposed in this article to perform these tasks. The off‐line and on‐line target setting procedures developed in this work can be adopted to increase the profit margin of any given batch process without sacrificing reliability. A synthesis method for building optimal alarm logic is also described in detail. Monitoring systems constructed according to this suggested approach are effective in reducing the probability of undetected faulty batches. Extensive simulation studies show that the proposed strategies are suitable for application of manufacturing high‐value‐added products, which is a prevailing practice in batch processes.
Three‐phase reactor model for hydrotreating in pilot trickle‐bed reactors
Authors: Korsten Hans , Hoffmann Ulrich
Published: June 2004

A three‐phase reactor model for describing the hydrotreating reactions in a trickle‐bed reactor was developed. It includes correlations for determining mass‐transfer coefficients, solubility data, and properties of the compounds under process conditions. The model, based on the two‐film theory, was tested with regard to the hydrodesulfurization of vacuum gas oil in a new high‐pressure pilot plant operated under isothermal conditions. The sulfur content of the product oil was found to depend strongly on the gas/oil flow ratio within the reactor. This is due to the inhibiting effect of hydrogen sulfide on the chemical reaction rates described by Langmuir–Hinshelwood kinetics. The poor conversion which, in contrast to industrial plants, is often observed in pilot plant reactors can be explained by incomplete catalyst wetting produced by low liquid velocities. The simulation shows a good agreement with the experiments carried out in a wide range of temperature, pressure, space velocity and gas/oil ratio.
Study of instabilities in film blowing
Authors: Ghaneh‐Fard A. , Carreau P. J. , Lafleur P. G.
Published: June 2004

Bubble instabilities observed in film blowing using four different polyolefins are discussed: high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), linear low‐density polyethylene (LLDPE), and polypropylene (PP). Special attention is given to the effect of the frost line height on the bubble stability, an effect mostly ignored in the literature. A video‐camera system was used to record the bubble shape and oscillations. In general, three forms of instabilities and combinations were observed: (1) axisymmetric periodic variations in the bubble diameter; (2) helical motions of the bubble; and (3) variations in the position of the solidification line. The four resins show different stability behaviors. The LDPE has the most stable operating space and the PP is the most unstable one. No correlation was observed between bubble stability and oscillatory shear rheological properties of the resins. Instability is enhanced by increasing take‐up ratio, increasing blow‐up ratio, and decreasing frost line height. Furthermore, for the LDPE, some operating points were not attainable and multiple steady states were observed. Our results are in a poor agreement with the predictions Cain and Denn's 1988 analysis.
Electrophoresis of a colloidal sphere in a circular cylindrical pore
Authors: Keh Huan J. , Chiou Jinn Y.
Published: June 2004

The electrophoretic motion of a dielectric sphere along the centerline of a long circular cylindrical pore is studied theoretically. The imposed electric field is constant and parallel to the nonconducting pore wall, and the particle and wall surfaces are assumed uniformly charged. Electrical double layers adjacent to solid surfaces are assumed to be thinner than particle radius and gap width between surfaces. The presence of the pore wall affects particle velocity: 1. an electroosmotic flow of the suspending fluid exists due to interaction between the electric field and the charged wall; 2. the local electric field on the particle surface is enhanced by the insulated wall, speeding up the particle; and 3. the wall increases viscous retardation of the moving particle. To solve electrostatic and hydrodynamic governing equations, general solutions are constructed from fundamental solutions in both cylindrical and spherical coordinate systems. Boundary conditions are enforced at the pore wall by Fourier transforms and then on the particle surface by a collocation technique. Typical electric‐field‐line, equipotential‐line and streamline patterns for the fluid phase are exhibited, and corrections to the Smoluchowski equation for particle electrophoretic velocity are presented for various relative separation distances between the particle and wall. The presence of the pore wall always reduces the electrophoretic velocity; however, the net wall effect is quite weak, even for very small gap width between the particle and wall.
Surface tension prediction for pure fluids
Authors: Escobedo Joel , Mansoori G. Ali
Published: June 2004

For the surface tension of organic compounds, σ = [ 𝒫·(ρl ‐ ρv)]4 is proposed where 𝒫 = 𝒫0·(1 − Tr)0.37·Tr·exp(0.30066/Tr + 0.86442·T). ρl and ρv are molar densities of liquid and vapor, respectively, Tr = T/Tc, and 𝒫0 is a temperature‐independent, compound‐dependent constant similar to Sugden's parachor. This new expression, derived from statistical‐mechanics, represents the experimental surface tension data of 94 different organic compounds within 1.05 AAD%. We also propose 𝒫0 = 39.6431· [0.22217 − 2.91042 × 10−3·(ℛ*/T)·T/P] as a corresponding‐states expression to correlate the temperature‐independent parameter 𝒫0 for various compounds. Here, ℛ* = ℛm,ref, ℛm is the molar refraction, ℛm,ref is the molar refraction of the reference fluid (methane), and Tbr is the reduced normal boiling point. When this generalized expression is used, surface tensions for all 94 compounds can be predicted within 2.57 AAD% at all temperatures investigated.
Extension of metabolic control analysis to chemostat bioreactors
Authors: Tan Hiong Leong , Delgado Javier , Liao James C.
Published: June 2004

A method of determining control coefficients and pseudo‐first‐order kinetic parameters is presented for cell growth, substrate consumption, and product formation in a chemostat bioreactor. From the equations that relate the control coefficients to process variables, such as feed concentrations of the limiting substrate and the product of interest, a perturbation method is developed to determine control coefficients from steady‐state measurements. This method combined with the transient response analysis provides a practical way for determining control coefficients and estimating kinetic properties in chemostat bioreactor systems. To determine the kinetic parameters, one measures cell mass, concentrations of the limiting substrate and the product of interest following a step change in the feed concentration until the system reaches a new steady state. The time courses of these variables are processed to obtain control coefficients, which yield the pseudo‐first‐order kinetic constants for cell growth, substrate consumption, and product formation. Only steady‐state responses are needed in calculation, if a step change in cell concentration in the feed stream can also be introduced without significantly perturbing cell physiology. This method is useful in characterizing the kinetics of whole cell bioreactions: results from chemostat experiments can be used to design operating strategies for batch or fed‐batch bioreactions. It is generally applicable to continuous‐stir‐tank reactors with interacting parallel reactions.
3‐D flow effects on residence time distribution in screw extruders
Authors: Chen L. , Lindt J. T.
Published: June 2004

This work concerns some selected 3‐D flow effects on residence time distribution (RTD) in a screw extruder. The Newtonian problem is emphasized in which the cross‐ and down‐channel flow components can be formally discoupled and treated as two parallel 2‐D problems: our stream‐function‐based analysis of the Newtonian cross‐channel flow was combined with the existing 2‐D solution for the down‐channel flow. The RTD was determined by tracing the streamlines generated from both velocity components for the industrially important cases of fully‐ and partially‐filled channels. The effect of the cross‐channel flow on the RTD is discussed in connection with the stagnation flows, the down‐channel pressure gradient, and the presence of a free surface in a partially‐filled channel. The cross‐channel pressure distribution is compared to the conventional 1‐D theory. The non‐Newtonian effects were assessed using the commercial software POLYCAD.
Computational modeling of gas/particle flow in a riser
Authors: Samuelsberg Arild , Hjertager Bjørn H.
Published: June 2004

Axial solid velocity, solid volume fraction, and solid shear viscosity were computed in the riser of a circulating fluidized‐bed reactor using a two‐phase 2‐D computational fluid dynamic model. The time‐averaged model predictions agree well with the experimental data of Miller and Gidaspow (1992). The model predicts a core‐annulus flow in the riser, similar to that found experimentally. The maximum velocity in the core agrees well with the measurements, but the downflow in the annulus is somewhat overpredicted. The solid volume fractions profiles agree well in both core and annulus, with discrepancy in the core at the level close to the inlet. The radial profile of solid shear viscosity computed by the turbulent kinetic energy model is ten times lower in the core than that found experimentally, but with a linear function of solid volume fraction in the measurement, the computed profile agrees well with experiments.
Hydrodynamic modeling of gas/particle flows in riser reactors
Authors: Nieuwland J. J. , van Sint Annaland M. , Kuipers J. A. M. , van Swaaij W. P. M.
Published: June 2004

Complex hydrodynamic behavior of circulating fluidized beds makes their scale‐up very complicated. In particular, large‐scale lateral solids segregation causes a complex two‐phase flow pattern which influences significantly their performance. Lateral solids segregation has been attributed to direct collisional interactions between particles as well as to interaction between gas‐phase eddies and dispersed particles. However, these phenomena have not been investigated thoroughly.

This article discusses an advanced 2‐D hydrodynamic model developed for circulating fluidized beds based on the two‐fluid concept. Because theory to model the interaction between gas‐phase eddies and dispersed particles is not available, turbulence was modeled on a macroscopic scale using a modified Prandtl mixing length model. To model the influence of direct particle‐particle collisions the kinetic theory for granular flow was applied based on the Chapman‐Enskog theory of dense gases. For model validation purposes, a cold flow circulating fluidized bed was employed in which sand was transported with air as fluidizing agent. The column is equipped with pressure transducers to measure the axial pressure profile and with a reflective optical fiber probe to measure the local solids concentration and axial solids velocity. Theoretically calculated solids concentration and axial solids velocity agree satisfactorily with experiment, especially when one realizes that the model contains no adjustable parameters. In general, however, the model slightly underpredicted the experimentally observed lateral solids segregation and yielded a more peaked velocity profile compared to its experimental counterpart.
Detection of equipment faults in automatically controlled processes
Authors: Rollins Derrick K. , Cheng Yisun , Chen Victoria C. P.
Published: June 2004

Based on the UBET, a new strategy for identifying faulty equipment for dynamic chemical processes under automatic control is presented. The strategy is designed to distinguish between measurement biases, controller biases, and process leaks. For illustration purposes, application is given to a level control process under pseudo‐steady state. This approach was inspired by the work of Rollins and Devanathan to identify biased measurements under dynamic conditions. Advantages of this method are that it is not computationally intensive, can accurately detect and specifically identify the type of fault, and can accurately determine the time when the fault occurs.