bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ion‐exchange kinetics: Heterogeneous resin‐phase model
Authors: Hasnat Abul , Juvekar Vinay A. 
Published: June 2004

AbstractA new approach to cation‐exchange kinetics views ion‐exchange resin as an aggregate of uniform cylindrical pores. Ion exchange occurs at the pore surface. The counterions dissociate only partially from the fixed sites, and the charge created on the pore surface is balanced by the counterions in the electrical double layer. Thus, any cross section of a pore consists of an annular double‐layer region and a central core region. Electroneutrality prevails in the core region through an influx of coions. Diffusion of ions through both regions is considered. The complete model incorporating these ideas for kinetics of monovalent cation exchange was tested with our experimental data as well as with the reported data. The pore diffusion coefficients used in the model are free ionic diffusion coefficients corrected for the tortuosity factor of the pores. Unknown parameters of the model are the degree of dissociation of counterions from the fixed sites of the pore (fσ), and the film thickness (δ). Both parameters are insensitive to the type of counterions, solution concentration, and the direction of exchange. Further, fσ appears to be a characteristic property of resin alone. Superiority of the developed model over the previous models is established.
Stable adaptive control of a class of continuous‐flow bioreactors
Authors: Bošković Jovan D. 
Published: June 2004

AbstractA stable adaptive control strategy is suggested for a class of continuous‐flow bioreactor processes described by Monod kinetics with two unknown parameters, one of which appears nonlinearly. Similarly, as in the case of the previously reported adaptive controllers, the parametrization of the process model, in conjuction with the adaptive exponential feeding strategy and corresponding adaptive algorithms, results in a stable system in which the convergence of the output errors to zero is guaranteed. In the former, however, two major problems are encountered: (1) both output errors were used to adjust the controller parameters, which may yield unacceptable performance of the resulting adaptive system; (2) conditions under which the process output can assume only positive values are difficult to derive. Hence, a design of a stable adaptive controller is suggested, whose parameters are adjusted using only one of the output errors and that yields acceptable performance of the control system. With this method, conditions under which the process outputs can assume only positive values can be readily derived. These conditions in turn guarantee that the control input saturation at value zero cannot occur. In this context, two adaptive controllers are suggested, such that the resulting adaptive systems are stable and the control objective is met. The adaptive controller design relies on a convenient coordinate transformation, while the proof of stability is based on suitably chosen Lyapunov functions. The performance of the adaptive system is evaluated through computer simulations.
Dynamics of propene metathesis: Physisorption and diffusion in heterogeneous catalysis
Authors: Gomes Vincent G. , Fuller O. Maynard 
Published: June 2004

AbstractSimultaneous effects of physisorption, diffusion, and reaction were characterized by dynamic experiments for propene metathesis over rhenium oxide/γ‐alumina catalyst in a Berty reactor. Single‐component sorption isotherms from steady‐state experiments for ethene, propene, and 2‐butene were used to predict multicomponent physisorption by the ideal adsorbed solution theory. Effective diffusivities were determined from molecular, Knudsen, and surface‐diffusion coefficients, including bimodal pore‐size effect and tortuosity factors. Transient sorption diffusion experiments were conducted with inactivated catalysts. The reaction rate expression, derived from elementary steps of carbene mechanism, was identified by solving the inverse problem with step test data. The direct problem was solved by orthogonal collocation within the method of lines. Model predictions agreed with responses from dynamic experiments carried out under conditions different from those used for estimating model parameters.
Aggregate behavior and lumped kinetics of many reactions in backmixed and plug‐flow reactors
Authors: Ho Teh C. 
Published: June 2004

AbstractThe long‐time behavior of a reaction mixture containing infinitely many species in isothermal continuous stirred tank reactors (CSTRs) is analyzed and contrasted with that in a plug‐flow reactor (PFR). The reaction kinetics considered are irreversible first‐order, irreversible coupled pseudo first‐order, and reversible first‐order. Asymptotic lumped kinetics for the mixture as a whole are developed as a function of feed properties and reactor type. Such lumped kinetics in many cases are of the power law form, with the exponent for a CSTR being lower than that for a PFR. Conditions under which non‐power‐law kinetics arise are given. Due to the wide spread of reactor residence times, the mixture in CSTRs admits a much richer variety of asymptotic possibilities than in the PFR. The results uncover some behavioral differences and relationships between CSTRs and a PFR for converting complex mixtures such as petroleum distillates or coal liquids.
Thermodynamics of wax precipitation in petroleum mixtures
Authors: Lira‐Galeana C. , Firoozabadi A. , Prausnitz John M. 
Published: June 2004

AbstractA thermodynamic framework is developed for calculating wax precipitation in petroleum mixtures over a wide temperature range. The framework uses the experimentally supported assumption that precipitated wax consists of several solid phases; each solid phase is described as a pure component or pseudocomponent that does not mix with other solid phases. Liquid‐phase properties are obtained from an equation of state. Calculated wax‐precipitation data are in excellent agreement with experimental results for binary and multicomponent hydrocarbon mixtures, including petroleum.
Quantitative analysis and computation of two‐dimensional bubble columns
Authors: Lin T.‐J. , Reese J. , Hong T. , Fan L.‐S. 
Published: June 2004

AbstractExperiments 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

AbstractA 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

AbstractSuppressing 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

AbstractThe 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

AbstractNuclear 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

AbstractEmulsion 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

AbstractA 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

AbstractThe 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

AbstractDrop 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

AbstractThe 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

AbstractA 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

AbstractWith 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

AbstractA 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

AbstractIdentification 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

AbstractModeling 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.