bluuebunny/crossref_metadata_2025 huggingface dataset:
Perturbation solution to the convection–diffusion equation with moving fronts
Authors: Vaidya Durgesh S. , Nitsche J. M. , Diamond S. L. , Kofke David A.
Published: June 2004

Electrophoresis of a solute through a column in which its transport is governed by the convection – diffusion equation is described. Approximate solutions to the convection – diffusion equation in the limit of small diffusion are developed using perturbation methods. The diffusion coefficient and velocity are assumed to be functions of space and time such that both undergo a sudden change from one constant value to another within a thin transition zone that itself translates with a constant velocity. Two cases are considered: (1) the thickness ϵfof the transition zone is negligible compared to the diffusional length scale, so the zone may be treated as a singular boundary across which the diffusion constant and velocity suffer discontinuous changes; (2) the transition zone is considerably wider than the diffusional length scale, so the diffusion coefficient and velocity, although sharply varying, are smooth functions of position and time. A systematic perturbation expansion of the concentration distribution is presented for case 1 in terms of the small parameter ϵ = 1/Pe. A lowest order approximation is given for case 2. A suitably configured system analyzed here can lead to progressive accumulation, or focusing, of the transported solute. The degree of focusing in case 1 scales with ϵ−1, whereas in case 2 it scales with (ϵfϵ)−1/2, and thus increases much more weakly with increasing Pe. A separation based on this concept requires development of materials and devices that allow dynamic tuning of the mass‐transport properties of a medium. This would make it possible to achieve progressive focusing and separation of solutes, such as proteins and DNA fragments, in electrophoretic media with an unprecedented degree of control.
Kinetics of hydrocarbon adsorption on activated carbon and silica gel
Authors: Malek A. , Farooq S.
Published: June 2004

Experimental breakthrough results of methane, ethane and propane in activated carbon and silica gel obtained over a wide range of gas compositions, bed pressures, interstitial velocities, and column temperatures were analyzed using a dynamic, nonisothermal, nontrace column breakthrough model. A linear driving force (LDF) approximation is used for particle uptake, and the Langmuir‐Freundlich isotherm represents adsorption equilibrium. The LDF mass‐transfer‐rate coefficient (and, hence, effective particle diffusivity) and column‐wall heat‐transfer coefficient were determined. The results show that hydrocarbon transport in the activated carbon particles used is essentially by Knudsen and surface flow, while for the silica gel used the transport is primarily by Knudsen flow. For activated carbon, the experimentally derived LDF coefficients for all three sorbates are well correlated using an average effective diffusivity value. With regard to heat transfer, the column‐wall Nusselt number is approximately constant for the range of Reynolds numbers considered. Simulations of multicomponent breakthrough in the activated‐carbon bed based on independently measured single‐component kinetic parameters and the extended Langmuir‐Freundlich isotherm agree very well with experimental results. The computational efficiency gained by adopting the simpler extended Langmuir isotherm model is also investigated.
Current distributions governed by coupled concentration and potential fields
Authors: Yang J. Deliang , West Alan C.
Published: June 2004

Results from steady and unsteady, two‐dimensional simulations of tertiary current distributions at and below the limiting current density are presented. The simulations are based on dilute‐solution theory, with coupling of the concentrations of the ionic species through the electrical field. The electrical field is calculated from the electroneutrality constraint. Results confirm and extend previous theoretical predictions of the electricalmigration enhancement of the limiting current. To demonstrate the potential utility of general current‐distribution solvers, measured temporal variations of the rate of copper deposition in the presence of an oscillating shear flow are measured and simulated. Experiments agree well with simulations.
2‐D time‐dependent viscoelastic flow calculations using connffessit
Authors: Laso Manuel , Picasso Marco , Öttinger Hans Christian
Published: June 2004

Two‐dimensional time‐dependent calculations for a molecular model of finite extensibility in the journal‐bearing geometry are presented. The flow is considered to be incompressible and isothermal. The momentum conservation equation is integrated using a time‐marching procedure in which local ensembles of dumbbells act as stress calculators. The calculations are based on the Calculation of non‐Newtonian flows: finite elements and stochastic simulation technique (CONNFFESSIT) and combine deterministic (finite elements) and stochastic techniques to advance the velocity and stress fields in time. The ability of CONNFFESSIT to treat models for which no closed‐form constitutive equation can be derived is illustrated by performing calculations using FENE dumbbells. Significant differences in the stress field between the true FENE and the linearized FENE‐P are found, especially during the inception period. Steady‐state kinematics are, however, identical within error bars for both FENE and FENE‐P and for the Newtonian fluid. The essential algorithm of 2‐D CONNFFESSIT is detailed, as well as experience gathered from its parallel and vector versions.
Eddy‐viscosity vs. second‐order closures for flows in noncircular ducts
Authors: Younis Bassam A. , Cokjat Davor
Published: June 2004

This article deals with prediction of turbulent flows in ducts of noncircular cross sections and, in particular, assessment of the performance in such flows of two very different models of turbulence. One model is of the two‐equation, eddy‐viscosity type, which is used in conjunction with a non‐linear stress‐strain relationship. The other is a complete Reynolds‐stress transport closure that involves the solution of a differential transport equation for each of the six components of the Reynolds‐stress tensor. The flows considered are characterized by the presence of secondary motions that are largely driven by the turbulence anisotropy and whose prediction remains a severe challenge to turbulence closures. Data from several experiments involving such flows are used here to assess the overall performance of the two models. It is found that the two models yield very similar results that are also of adequate engineering accuracy—an outcome that argues in favor of the use of the nonlinear two‐equation model in practical computations.
Initial deposition of interacting particles by filtration of dilute suspensions
Authors: Koenders M. A. “Curt” , Wakeman Richard J.
Published: June 2004

Data from a range of filtration experiments on dilute suspensions are used to determine the parameters that describe the physics of suspension flow during compaction. The range of solids volume fractions used is 0.00001 < ϕ <0.1; ζ‐potentials vary between 0 and 50 mV. Relevant physical data are extracted from an analysis of the initial stages of experiments at variousϕ and ζ. Theoretical considerations on suspension flow are presented to argue that the physical character of the flow at relatively dense, strongly interacting conditions is significantly different from that of dilute systems. The latter are dominated by fluctuations in the particle velocity near the septum to give gas‐type diffusive behavior, while in the former the particles are more or less localized. This observation has implications for the diffusion coefficient, which is predicted to be quadratic in the filtration pressure for very dilute suspensions and which is roughly independent of pressure for mixtures containing strongly interacting particles. Experiments are described and analyzed, and these reinforce the main theoretical insights.
Breakthrough curves for fixed‐bed adsorbers: Quasi‐lognormal distribution approximation
Authors: Xiu Guo‐Hua , Nitta Tomoshige , Li Ping , Jin Ge
Published: June 2004

The quasi‐lognormal distribution (Q‐LND) approximation was used to predict breakthrough curves in fixed‐bed adsorbers for a linear adsorption system with axial dispersion, external film diffusion resistance, and intraparticle diffusion resistance for slab–, cylindrical–, and spherical‐particle geometries. The exact solution and parabolic profile approximation were also obtained for different particle geometries. Numerical results show that the Q‐LND approximation is a simple and handy solution. It predicts breakthrough curves with an accuracy comparable to the parabolic‐profile approximation over a wide range of parameters; compared with the latter, it only takes less than one hundredth the computation time and does not have a convergence problem in numerical calculations. A criterion for the applicability of the Q‐LND approximation is suggested. The effect of particle geometries on the breakthrough curves is discussed. A criterion is also provided for the Q‐LND approximation to explore the conditions where one should consider this effect on breakthrough curves.
Effect of CO2 concentration on YBa2Cu3O6+x formation
Authors: Kohav T. , Forster K. M. , Richardson J. T. , Luss D.
Published: June 2004

Two reversible reactions are involved in YBa2Cu3O6+x formation: a reaction between BaCO3 and CuO forming BaCuO2 and CO2, and a reaction of BaCuO2 with Y2O3 and CuO forming YBa2Cu3O6 which undergoes phase transformation to YBa2Cu3O6+x upon cooling. In‐situ isothermal time resolved HT‐XRD of a thin film was used to quantify the effect of CO2 on the kinetics of the first reaction. Increased CO2 partial pressure shifts the reactions to higher temperatures. At high CO2 partial pressure (>2 vol. %), the rate of the first reaction becomes essentially a step process with a very high activation energy. Noninstantaneous nucleation of the reaction products occurs at low CO2 partial pressure (0.5–1%) and temperatures (700°C). The data fit a 2‐D diffusion‐controlled mechanism with a zero nucleation rate for BaCO3 decomposition and a second‐order nucleation rate for YBa2Cu3O6 formation. A comparison of the kinetics of a thin film (10 mm) as determined by HT‐XRD with those of a thick sample (2 mm) determined by TG revealed that the transport of CO2 within the sample pores and to the ambient gas significantly affect the decomposition of BaCO3. For example, the formation of YBa2Cu3O6 in a thick precursor layer occurs in the 840 to 940°C range, exceeding by about 200°C that in which it is formed in thin films.
Numerical and experimental study of a homogenizer impinging jet
Authors: Kleinig Andrew R. , Middelberg Anton P. J.
Published: June 2004

High‐pressure homogenization is a key unit operation used to disrupt cells containing intracellular bioproducts. Modeling and optimization of this unit are restrained by a lack of information on the flow conditions within a homogenizer valve. A numerical investigation of the impinging radial jet within a homogenizer valve is presented. Results for a laminar and turbulent (k – ϵ turbulent model) jet are obtained using the PHOENICS finite‐volume code. Experimental measurement of the stagnation region width and correlation of the cell disruption efficiency with jet stagnation pressure both indicate that the impinging jet in the homogenizer system examined is likely to be laminar under normal operating conditions. Correlation of disruption data with laminar stagnation pressure provides a better description of experimental variability than existing correlations using total pressure drop or the grouping 1/Y2h2.
Dynamic modeling for simulation and control of a circulating fluidized‐bed combustor
Authors: Muir James R. , Brereton Clive , Grace John R. , Lim C. Jim
Published: June 2004

A dynamic model has been developed to predict the transient behavior of the temperature, the heat removal rate by the in‐bed heat exchanger, and the flue‐gas oxygen concentration for a circulating fluidized‐bed (CFB) combustor. The model was incorporated into a control simulator to reproduce the combustion process within the overall program. The simulator predicts the behavior of the combustor under manual or automatic control to allow testing of control strategies. The model is validated by comparison with step‐response tests carried out on a pilot CFB combustor. Discrepancies are attributable to unmodeled disturbances. Further validation, necessary to ensure the applicability of the simulator to control development, is provided by comparing control models identified experimentally using the pilot CFB to those obtained by simulation. Favorable comparison suggests that the dynamic model is suitable for use in control simulation.
Fluorescence capillary photometry for characterization of liquid–liquid dispersions
Authors: Weiss Christian , Köhler Volkhard , Marr Rolf
Published: June 2004

The behavior of droplet swarms in liquid–liquid dispersions, especially found in extraction columns, are analyzed by a new fluorescence measurement technique, which represents an extension to the capillary suction method. The measuring principle developd benefits from detectable‐emission light intensity of a fluorescent dye even in a trace concentration. The addition of this nontransferring dye to the dispersed phase enables in situ determination of bivariate drop‐size/drop‐concentration distributions, which provides important information on droplet history for the evaluation of coalescence and breakup rates. Furthermore, the measuring method represents a new approach for the measurment of droplet‐size‐dependent residence times in extraction columns.
Hydrodynamics of soil immobilization in the immobilized soil bioreactor
Authors: Karamanev Dimitar G. , Chavarie Claude , Samson Réjean
Published: June 2004

The hydrodynamic characteristics of a new type of reactor, the immobilized soil bioreactor, were studied. This apparatus is a practical new engineering concept (soil immobilization) based on entrapment of soil particles, which contain pollutant‐degrading microorganisms, in the pores of a geotextile to activate the indigenous microorganisms. The soil immobilization is the third on the size scale of immobilization processes, coming after (1) that of molecules in heterogeneous catalysis (in Angstrom) and (2) that of microbial cells and their fragments in immobilized cells and enzymes biocatalysis (in micron). The size of immobilized soil particles is in the range of a millimeter. A mathematical model of liquid flow within the reactor is proposed, which qualitatively explains the distribution of the immobilized soil in space. The dynamics of soil immobilization within the bioreactor has been studied as a function of the particle size, initial slurry concentraion and air flow rate. A mathematical model of the process of soil immobilization was proposed based on deep filter mechanics. The process can be described by a second‐order kinetic model. This study will be of great importance for the design of immobilized soil bioreactors for degradation of recalcitrant soil pollutants.
Minimum liquid fluidization velocity in gas‐liquid‐solid fluidized beds
Authors: Briens L. A. , Briens C. L. , Margaritis A. , Hay J.
Published: June 2004

Accurate detection of minimum liquid fluidization is essential to the successful operation of gas‐liquid‐solid fluidized beds, especially when particle or liquid properties evolve. A gas‐liqid‐solid system of 3‐mm glass beads exhibits three distinct flow regimes as the liquid velocity is increased: compacted, agitated and fluidized‐bed regmes. Measurements showed that the bed is not fluidized in the agitated bed regime. Pressure gradient and bed height measurements do not provide the minimum liquid fluidization velocity; instead, they offer the velocity between the compacted and agitated bed regimes. Time‐averaged signals are not reliable for determining the minimum liquid fluidization velocity. It can be obtained from the standard deviation, the average frequency, the Hurst exponent and the V statistic of the cross‐sectional average conductivity, which can be measured under many industrial conditions.
Electrostatic partitioning in slit pores by Gibbs ensemble Monte Carlo simulation
Authors: Chun Myung‐Suk , Phillips Ronald J.
Published: June 2004

Equilibrium partitioning of spherical solutes between slit pores and bulk solution is investigated by the Gibbs ensemble Monte Carlo method. Two types of perturbatins are performed in this simulation: a random displacement of solutes that ensures equilibrium within both bulk and pore regions, and random interchanges of solutes that equalize the interaction potentials between the two regions. To study the effects of electrostatic interactions, interaction energies between the solutes and pore walls and between pairs of solutes are evaluated by using a singularity method. Partition coefficients calculated for neutral solutes, which experience purely steric interactions, increase with increasing solute concentration and agree well with existing theoretical results. For pores and solutes of like charge, results for the limit of infinitely dilute solute concentration show a sharp decline in partition coefficient with decreasing ionic strength of solution. As the solute concentration increases, the interplay of solute–wall and solute–solute interactions becomes increasingly important, and the partition coefficient increases accordingly. The density profiles indicate unambiguously that, whether solutes and proes are uncharged or of like charge, solute–solute interaction promotes enhanced concentrations near the wall, causing the partition coefficient to increase. Even at solute concentrations as low as 5%, effects of solute–solute interactions caused by electrostatic charge can more than compensate for sphere–wall repulsive interactions, indicating that concentration effects should be considered at least as important as electrostatic effects in partitioning phenomena.
Estimating radial velocity of fixed beds with low tube‐to‐particle diameter ratios
Authors: Cheng Zhen‐Min , Yuan Wei‐Kang
Published: June 2004

The Ergun equation based on the effective spherical diameter is universally valid for various shapes of granular packings. It, however, was derived from the assumption of infinite tube‐to‐particle diameter ratio without considering the wall effect. Although some improvements were made by Mehta and Hawley (1969) to correct this, the application of the Ergun equation is still restricted to cylindrical columns with a packing porosity of less than 0.5. To modify the Ergun equation to noncylindrical flow space with or without a wall, it was substituted into the empty tube pressure drop equation by introducing an effective tube diameter so that the pressure drop can be predicted just from the free flow space and the wetted area involved. This treatment offers the basis for a new method in velocity distribution prediction.
Destruction and removal of toluene and MEK from gas streams with silent discharge plasmas
Authors: Chang Moo Been , Chang Chun‐Cheng
Published: June 2004

The effectiveness of applying silent discharge plasmas (SDP) for destroying and removing volatile organic compounds (VOCs) from gas streams is experimentally evaluated with a laboratory‐scale reactor. The VOCs selected for study include toluene and methyl ethyl ketone (MEK). Direct collision with energetic electrons and reaction with generated gas‐phase radicals are two major mechanisms responsible for destruction and removal of VOCs from gas streams. Operating parameters investigated include applied voltage, gas residence time, and temperature and composition of the gas stream. Experimental results indicate that the removal efficiency of toluene and MEK achieved with SDP can be enhanced by operating the system at a higher gas temperature and applied voltage due to the generation of more energetic electrons and radicals. O2 is essential for removing VOCs from gas streams with SDP. More than 80% removal efficiencies were achieved with this system for both toluene and MEK. SDP can potentially serve as an alternative control technology for removing VOCs from gas streams.
Biofiltration of isopentane in peat and compost packed beds
Authors: Wang Zhao , Govind Rakesh
Published: June 2004

Commercially available biofiltration systems have used natural bioactive materials in packed beds due to low media cost and easy availability. Detailed understanding and modeling of biofiltration systems are lacking in existing literature. Experimental studies on the isopentane treatment in air using peat‐ and compost‐packed beds were conducted with inlet isopentane concentrations of 360 to 960 ppmv, and empty‐bed gas‐phase residence times of 2 to 10 min. High removal efficiences (> 90%) were achieved at low contaminant concentrations ( < 500 ppmv) and large empty‐bed gas‐phase residence times ( > 8 min). For both peat and compost biofilters, there was an “optimal” water content that gave the highest removal efficiency. For higher water content, mass transfer of isopentane through the liquid phase controlled the biofiltration removal efficiency. At low water content, irreversible changes in the bioactivity of peat and compost occurred, resulting in an irrecoverable loss of removal efficiency. Increases in biofilter bed temperature from 25 to 40°C improved the removal efficiency. A mathematical model incorporating the effect of water content and temperature was developed to describe the packed‐bed biofilter performance. Model predictions agreed closely with experimental data.
Modeling flow in disordered packed beds from pore‐scale fluid mechanics
Authors: Thompson Karsten E. , Fogler H. Scott
Published: June 2004

Network models are an effective means of incorporating pore‐scale heterogeneity into flow models of porous materials. The drawback to these models used to be the inability to obtain quantitative macroscopic parameters representing larger (experimental‐scale) media. However, recently developed modeling techniques, combined with more widely available computational resources, make the simulation of macroscopic parameters from a network approach viable. A network model for the slow flow of an incompressible fluid in disordered packed beds is presented. Fundamental fluid mechanics equations are solved at the pore scale and then translated to macroscopic behaviour using a network approach. The results reproduce experimental permeabilities and show excellent quantitative fits to residence time distributions for mechanical dispersion in real beds. Simulations of the RTD are of special interest, because they are definitive links between pore‐scale flow behavior and macroscopic responses.
Influence of dynamic interfacial properties on droplet breakup in plane hyperbolic flow
Authors: Janssen J. J. M. , Boon A. , Agterof W. G. M.
Published: June 2004

Droplet breakup in laminar flows is important in emulsification processes and polymer blending. The influence of surfactants on droplet deformation and breakup in a plane hyperbolic flow was studied experimentally in an opposed‐stream device. As in the case of simple shear flow, the inhomogeneity of the surfactant distribution along the droplet interface has a pronounced effect. Our results are qualitatively consistent with other numerical studies for droplet breakup in an axisymmetric elongational flow. Also a striking similarity is noted with other experimental observations for the deformation and breakup of polymeric drops in a quasi‐steady‐plane hyperbolic flow. The critical capillary number for droplet breakup in the experiments correlates with the interfacial viscoelasticity, and a reformulation of the numerical framework in terms of interfacial elasticity parameters is suggested for future numerical work including other linear flows.
Oil‐assisted agglomeration for toner deinking: Population balance model and experiments
Authors: Snyder Bret A. , Berg John C.
Published: June 2004

This study examines the selective agglomeration of hydrophobic toner particles from a repulped paper fiber slurry by adding an immiscible oil that preferentially wets the toner particles. Effects of cationic starch additives, agitation rate, time, temperature and oil composition are examined experimentally in terms of the dynamic and steady‐state evolution of the particle‐size distribution produced. A simple, but effective, population balance model is solved numerically to simulate the agglomeration process and provides quantitative relationships between process variables and the effectiveness and rate of agglomeration. These relations are simple and clear. The process is shown to be controlled by the composite effect of the aforementioned variables on the aggregation and breakup rates, and the observed hehavior is understood in these terms. Practical recommendations for agglomeration, a process potentially useful for deinking toner‐printed paper, are deduced from the results.
Application of almost‐everywhere singular distributions in chemical engineering
Authors: Giona Massimiliano , Patierno Oreste
Published: June 2004

Almost‐everywhere singular (AES) distributions, usually referred to as multifractal measures, provide an intermediate link between atomic distributions (distributions represented by a countable superposition of Dirac's delta terms) and smooth regular distributions. This article shows how AES distributions can be rigorously treated in connection with distributed‐parameter models and presents closed‐form expressions and/or recursive, uniformly converging approximation methods for integral transforms (Laplace and Stieltjes). In particular, exact results are obtained and discussed for linear and uniform nonlinear kinetcis and for transport schemes in the presence of continuous mixtures. The physical origin of AES distributions in real systems is also detailed.
Experimental study on wetting of fibers with non‐Newtonian liquids
Authors: Ghannam Mamdouh T. , Esmail M. Nabil
Published: June 2004

Parameters of wetting dry fibers with non‐Newtonian liquids are studied experimentally. Rheological tests show that seven of the 13 test liquids adhere to Newtonian behavior with a wide range of viscosities 3 < μ < 30,000 mPa·s. The other six liquids show shear‐thinning behavior with different flow indices. Some of the liquids exhibited viscoelastic behavior. Diameters of the fibers are in the range 0.033 < D < 0.071 cm. Experimental measurements cover contact angles, interfacial depths and diameters, as well as the critical wetting speed of entraining visible small air bubbles in the liquid volume. Dynamic contact angles are strong functions of the apparent viscosity of the liquid at the particular wetting speed regardless of whether the liquid is Newtonian or non‐Newtonian. Higher values of contact angles correspond to wetting of fiber with larger diameter for less viscous liquid are generally higher than those for Newtonian liquids. Depressions created in the surface of different test liquids can be classified into three different scales, corresponding to three scales of liquid viscosities.
Efficient high‐pressure state equations
Authors: Harstad Kenneth G. , Miller Richard S. , Bellan Josette
Published: June 2004

A method is presented for a relatively accurate, noniterative, computationally efficient calculation of high‐pressure fluid‐mixture equations of state, especially targeted to gas turbines and rocket engines. Pressures above 1 bar and temperatures above 100 K are addressed. The method is based on curve fitting an effective reference state relative to departure functions formed using the Peng‐Robinson cubic state equation. Fit parameters for H2, O2, N2, propane, methane, n‐heptane, and methanol are given.
New correlation for sieve‐tray point efficiency, entrainmnt, and section efficiency
Authors: Bennett Douglas L. , Watson Dana N. , Wiescinski Marie Anne
Published: June 2004

A comprehensive composite database for distillation sieve‐tray efficiency is used to develop point efficiency and entrainment correlations based on a model that considers the fluid on the distillation tray to be contained in a liquid‐continuous region near the tray deck and a vapor‐continuous region on top of the liquid‐continuous region. This model allows estimates of the portion of the mass transfer that occurs in each region and the mass‐transfer resistance that occurs on the liquid side and vapor side of the interface. For most cases, most of the mass transfer occurs within the liquid‐continuous region. The liquid side resistance is often significant.

The entrainment correlation is consistent with the work of Bennett et al., which relates entrainment to the ratios of the liquid to vapor density and the froth height to the tray spacing. A simple liquid continuous‐only mass‐transfer model containing only four empirical parameters correlates the point efficiency data to within 6.4%. Despite a twofold change in vapor Schmidt number, no dependency on vapor Schmidt number is seen. Important dimensionless groupings are the Reynolds number based on the hole velocity, effective froth density, ratio of the liquid inventory to the perforation diameter, and fraction of the tray area perforated. Mathematically simple and accurate methods allow the prediction of the section efficiency for trays operating in cross or parallel flow. They address vapor and liquid mixing, entrainment and a criterion to avoid significant degradation of the tray efficiency due to weeping.
Gas–liquid flow through horizontal tees of branching and impacting type
Authors: Hatziavramidis Dimitri , Sun Bing , Gidaspow Dimitri
Published: June 2004

In gas–liquid flows through tee junctions, because of the difference in inertia between the phases, the flowing mass fractions for an individual phase at the outlet sides differ from those at the inlet. This phase separation is an important consideration in delivering fluids and energy through pipe networks in the power and process industry. In this work, air–water and steam–water flows through branching and impacting tee junctions are considered. Under certain conditions (when the volumetric fraction of the liquid drops does not change appreciably and their bulk density is much higher than the gas density, and when the flow rates and/or the gas volumetric fraction are high), these flows can be approximated as irrotational flows of incompressible, inviscid fluids and are amenable to potential flow methods, for example, conformal mapping. For the general case of gas–liquid flows through a branching or impacting tee, a CFD code is utilized to conduct transient flow simulations. Predictions of phase separation for both types of tee junction agree well with experimental data.
Synthesis of crystallization–distillation hybrid separation processes
Authors: Berry David A. , Ng Ka M.
Published: June 2004

A systematic method to synthesize crystallization–distillation hybrid separation processes is presented. Two classes of hybrids are identified for binary mixtures. The first bypasses azeotropes and tangent pinches, while the second bypasses eutectics. Guidelines for flow‐sheet selection are proposed based on an analysis of simple eutectic, constant relative volatility systems. In addition, the hybrids are compared to both extractive and adductive crystallization in order to determine the conditions under which solvent‐based crystallization techniques outperform the proposed hybrid configurations. The method is extended to ternary mixture. The use of stream combination and complex distillation columns is also considered.
“Tunable” diffusion of D2O in CO2‐swollen poly(methyl methacrylate) films
Authors: Vincent M. F. , Kazarian S. G. , Eckert C. A.
Published: June 2004

Opportunities exist to exploit the unique properties of carbon dioxide to process polymers for advanced applications. The diffusion of deuterated water in poly(methyl methacrylate) films swollen by carbon dioxide at 35°C over the pressure range 0 to 90 bar (corresponding density range: 0.0 to 0.66 g/cm3) was measured using in situ FTIR spectroscopy. The mechanism and time scale of the diffusion of deuterated water in PMMA are easily controlled by manipulation of the applied CO2 density. The enhancement of mass transport rate is consistent with the degree of plasticization of the polymer due to the absorption of carbon dioxide.
Lumen mass transfer in hollow‐fiber membrane processes with constant external resistances
Authors: Qin Yingjie , Cabral Joaquim M. S.
Published: June 2004

Hollow‐fiber membrane processes with a constant external resistance having a constant or variable shell concentration resulting from an operational mode of cocurrent or countercurrent are studied. By solving numerically the continuity mass‐conservation equation with the corresponding boundary conditions, the lumen laminar mass‐transfer coefficients for both cases are correlated. The correlations greatly improve the calculating accuracy of the overall mass‐transfer coefficient and can be used to obtain the lumen mixed‐cup concentration by an algebraic equation substituting the partial differential equation. A separation factor m' is introduced to characterize the effect of the operational mode. Calculation results demonstrate that the lumen mass‐transfer coefficient is independent of the real lumen and shell concentrations, but it is greatly influenced by m'. The countercurrent mode, compared to the cocurrent mode, provides not only a higher mean driving force, but a higher lumen mass‐transfer coefficient. This conclusion is novel and valid for the tube‐shell heat or mass‐transfer processes and is supported by the experimental data in the literature and our gas membrane separation experiments.
Kinetically driven instabilities and selectivities in methane oxidation
Authors: Park Young K. , Vlachos Dionisios G.
Published: June 2004

Ignitions, extinctions, and Hopf bifurcations in methane oxidation were studied as a function of pressure and inlet fuel composition. A continuous stirred‐tank reactor was modeled with numerical bifurcation techniques, using the 177 reaction/31 species mechanism. Sensitivity and reaction pathway analyses were performed at turning points to identify the most important reactions and reactive species. Then, simulations were compared with experimental data. Multiple ignitions and extinctions as well as oscillations that are purely kinetically driven were found. Ignition to a partially ignited state with considerable reactivity of methane indicates possible narrow operation windows with high selectivities to partial oxidation products. At 0.1 atm, we found a selectivity of up to 80% to CO at 70% CH4 conversion. The ignition to a fully ignited branch is associated with high selectivity to CO2 and H2O. The C2 chemistry inhibits the ignition of methane to the partially ignited branch. The methane ignition temperature exhibits two branches with respect to pressure, with only the low‐pressure branch being dominant. Reaction path analysis at ignition conditions shows that the preferred pathway of CH4 oxidation is to form CO and CO2 through CH2O and CH2(s) intermediates. However, at intermediate to high pressures, the recombination of CH3 to C2 H6 also becomes quite significant.
Analytical description of the Lennard‐Jones fluid and its application
Authors: Tang Yiping , Lu Benjamin C.‐Y.
Published: June 2004

A simplified exponential (SEXP) approximation, combining the exponential approximation and the first‐order mean spherical approximation, is proposed to improve the equation of state for the Lennard‐Jones (LJ) fluid. The SEXP approximation, which can be implemented in an analytical manner, yields better radial distribution functions of the LJ fluid. Extensive comparisons with two typical perturbation theories show that the SEXP approximation is more appropriate to describe the behaviors of the LJ fluid. The latest 33‐parameter modified Benedict‐Webb‐Rubin equation, also calculated, is inadequate in the region of phase coexistence. The SEXP approximation is applied to the calculation of methane properties with much better results than the Peng‐Robinson equation of state for saturated liquid densities and second virial coefficients.
Separation of bulk hydrogen sulfide–hydrogen mixtures by selective surface flow membrane
Authors: Parrillo D. J. , Thaeron C. , Sircar S.
Published: June 2004

The separation performance of binary hydrogen sulfide–hydrogen mixtures by a nanoporous carbon membrane called selective surface flow (SSF) membrane is described. The membrane selectively permeates H2S from H2, and a H2‐enriched stream is produced at the feed‐gas pressure. A two‐stage embodiment of the SSF membrane is described for production of high‐purity H2 with high H2 recovery from an equimolar H2S‐H2 feed gas. A novel protocol for operation of the two‐stage membrane is needed to achieve that separation goal.
Implicit model checking of logic‐based control systems
Authors: Park Taeshin , Barton Paul I.
Published: June 2004

Increasing automation in the CPI has led to the growing use of logic‐based control systems (or safety interlock systems) in safety‐related and sequencing operations. The growing complexity and safety‐critical nature of typical applications make developing technologies for the formal verification of logic‐based control systems with respect to their functionality a crucial and urgent issue. It still remains elusive to analysis, primarily due to the exponential growth of the alternatives that must be examined with application size. Implicit model checking is a formal verification technology that can be applied to the verification of large‐scale logic‐based control system. Its primary advantage is to formally verify large‐scale coupled systems, where a novel and compact model formulation makes tractable previously inaccessible problems. Logic‐based control systems are represented compactly as an implicit Boolean state‐space model, and the properties to be verified are represented in the language of temporal logic. Verification is posed as a Boolean satisfiability problem and then transformed into its equivalent integer programming feasibility problem, which allows for efficient solution with standard branch‐and‐bound algorithms. Benefits of the methodology are demonstrated by applying to largescale industrial examples.
Measure of closed‐loop nonlinearity and interaction for nonlinear chemical processes
Authors: Guay M. , McLellan P. J. , Bacon D. W.
Published: June 2004

A new approach for assessing nonlinear interaction effects and closed‐loop nonlinearity in multivariable processes is presented. It is based on a differential geometric interpretation of the relative gain array that leads naturally to systematic procedures for describing interaction effects of higher order and for assessing closed‐loop nonlinearity effects in nonlinear processes. Two types of nonlinear effects associated with the behavior of a process are introduced. Between‐channel nonlinearity is associated with the nonlinear dependence of an output channel on other input ‐ output pairings. Withinchannel nonlinearity is used to identify the nonlinear effects that result from the inherent nonlinearity of an individual output channel. A root‐mean‐squared measure of nonlinearity is introduced and is used to evaluate the significance of local nonlinear effects. Nonlinear interaction measures are derived that provide tools for assessing input ‐ output pairings in a nonlinear process. This new approach extends the standard techniques and provides an estimate of the effect of nonlinearity on closed‐loop interactions.
Bridging the gap between heuristics and optimization: Capacity expansion case
Authors: Liu Ming Long , Sahinidis Nikolaos V.
Published: June 2004

The competition between heuristic and optimization approaches for process synthesis and process operations problems has caused major controversy in recent years. Heuristics offer fast solutions but no guarantee of optimality. Mathematical programming approaches, on the other hand, offer rigor but suffer from combinatorial explosion of computational requirements. The use of analytical investigations is proposed as a theoretical means to characterize the behavior of heuristics and optimization algorithms and produce a framework that combines the strengths of the two approaches while eliminating their weaknesses. The approach contrasts and complements the current practice in process systems engineering, which is entirely empirical in nature. The proposed paradigm is demonstrated in the context of the multiperiod capacity expansion problem for chemical process networks, a problem having much in common with both process synthesis and operations problems. Analytical investigations for this problem lead to the development of a heuristic that is proved to be asymptotically optimal under standard assumptions about the problem parameters. In the more general context of process synthesis and operations, analytical investigations present a large array of opportunities.
Modeling and control of multivariable processes: Dynamic PLS approach
Authors: Lakshminarayanan S. , Shah Sirish L. , Nandakumar K.
Published: June 2004

The issue of modeling and control of multivariable chemical process systems using the dynamic version of a popular multivariate statistical technique, namely, projection to latent structures (partial least squares or PLS) is addressed. Discrete input‐output data are utilized to construct a projection‐based dynamic model that captures the dominant features of the process under study. The structure of the resulting model enables the synthesis of a multiloop control system. In addition, the design of feedforward control for multivariable systems using the dynamic PLS framework is also presented. Three case studies are used to illustrate the modeling and control of multivariable linear and nonlinear systems using the suggested approach.
Ozone transfer with optimal design of a new gas‐induced reactor
Authors: Hsu Yung‐Chien , Huang Chyuan‐Jih
Published: June 2004

The optimal geometric design and the mass transfer of ozone for a new gas‐induced reactor characterized by the installation of a draft tube and dual pitched‐blade turbines were studied. Geometric factors including the impeller diameter, the length and diamter of draft tube, the liquid level, and the impeller speed were investigated to find the optimal gas utilization rate and power consumption for agitation. Furthermore, under the optimal geometric conditions, several variables including the impeller speed, the input gas flow rate, and the liquid level were investigated to reveal the volumetric masstransfer coefficient of ozone. The optimal geometry was suggested by three empirical equations that correlated successfully the onset impeller speed for gas induction and the power consumption post gas induction with geometrical factors and operational variables. Volumetric mass‐transfer coefficients of ozone were 0.8–1.2 min−1 under optimal geometric and proper operational conditions, which are comparable to the cited values in the literature.
Temperature‐dependent behavior of polyethylene oxide in papermaking suspensions
Authors: Khoultchaev Kh. Kh. , Kerekes R. J. , Englezos P.
Published: June 2004

The adsorption of polyethylene oxide (PEO) on chemi‐thermo‐mechanical pulp (CTMP), clay, and chalk suspensions was investigated at 303 and 343 K. These conditions corresponded to points below and above the critical solution temperature (CST) of a 0.005 mass % polyethylene oxide solution in the presence of 2 mol/L of KCl. The PEO adsorption on CTMP and clay particles was also studied at different initial PEO concentrations up to 50 mg/L. PEO adsorbed onto papermaking furnish particles at both temperatures, but the adsorbed amount was found to be larger above the CST in all systems. At 303 K it increased with the increase in PEO added to the system until it reached a plateau, but at 343 K it increased with the increase in PEO added in all the range of PEO concentrations up to 50 mg/L. The state of aggregation of PEO‐clay and PEO‐chalk suspensions was also studied by monitoring fluctuations in the intensity of light transmitted through the suspension. These measurements indicated a strongly temperature‐dependent aggregation. It was concluded that the entropically driven phase separation leads to enhanced aggregation, which in turn favors the retention of fiber fines and clay filler.
Pore modification using the supercritical solution infiltration method
Authors: Wang Zhaohui , Dong Junhang , Xu Nanping , Shi Jun
Published: June 2004

When a supercritical (SC) solution infiltrates through micropores, its pressure will fall off considerably along the flow direction, and significant solute supersaturation can be obtained inside the pores. Thus the solid solute can deposit onto the pore surface due to nucleation. The supercritical solution infiltration (SCSI) method was applied to modify porous ceramic membranes. A mathematical model was used for the membrane poresize change during the SCSI process. Numerical solution of this model showed that for a membrane with initial pore‐size distribution (PSD), its average pore size could be increased or decreased due to dissimilar kinetics of the solute precipitation under different conditions, α‐Al2O3 microfiltration membranes were modified by the SCSI method using SC solution of the aluminum isopropoxide/propane system under different conditions. The PSD change was determined for the modified membranes after hydrolysis and sintering. With the reduction in permeability, the average pore size was reduced when the process conditions were more favorable for homogeneous nucleation and increased under the opposite conditions. Experimental results qualitatively coincided with those by the proposed model.
Screening effects when sampling suspensions in laminar flow through pores
Authors: Jones Francis J. , Weinberger Charles B.
Published: June 2004

Dilute suspensions of neutrally buoyant, uniform‐diameter spheres in a viscous liquid are sampled through pores in the wall of a stirred tank. These studies are performed to investigate screening effects near the sampling orifice that cause samples to be deficient in solids. The ratio of sphere concentration in the sample to that of the original suspension ranges from 0.08 to 1.12. Sample concentration decreases with decreasing sampling pore diameter and increasing stirring rates. The objectives of this work are both to obtain experimental relationships and to develop predictive techniques relating flow fields to sample concentration.

Sample concentration is predicted by calculating particle trajectories in the region near the pore and by accounting for steric exclusion. Particle trajectories can differ from fluid streamlines due to lift forces generated in the flow field. These estimates constitute a priori predictions, since they do not rely on or introduce any empirical parameters. Predicted values of sample concentrations agree closely with data.