bluuebunny/crossref_metadata_2025 huggingface dataset:
Fractionation of suspensions using synchronized ultrasonic and flow fields
Authors: Mandralis Zenon I. , Feke Donald L.
Published: June 2004

A fractionation method for fine‐particle suspensions using resonant ultrasonic fields coordinated with bidirectional fluid flow fields is described. The basis for the separation is differences in the speed of response of particles to the imposition of a resonant acoustic field. As such, the method is sensitive to the particle size and the acoustic contrast between the solid particles and their suspending fluid. Both batch and continuous fractionation processes can be developed from a two‐step acoustic‐flow cycle. An analytical model was constructed from equations that describe the trajectories of particles as they respond to the acoustic and flow fields. Model predictions indicate how the fractionation can be controlled through choice of cycle parameters. The method was implemented experimentally. Results for the fractionation of 325‐mesh polystyrene spheres indicate that sharp fractionations can be achieved. The experimental results are generally in agreement with the model predictions.
Liquid‐liquid phase separations in aqueous solutions of globular proteins
Authors: Vlachy V. , Blanch H. W. , Prausnitz J. M.
Published: June 2004

A simple statistical‐mechanical theory, known as the random‐phase approximation, is applied to study liquid‐liquid phase separations in solutions of globular proteins. Phase separation may be induced by addition of nonionic polymer or/and ordinary electrolytes. In this analysis, the osmotic‐attraction mechanism, whereby the depletion of “solvent” particles between two proteins causes an attractive force, is primarily responsible for phase separation. For one‐component models of protein solutions, the theory yields simple algebraic expressions for the equation of state and for the chemical potential of the protein. This analytical theory describes the observed solubility behavior of proteins, including the effect of protein and polymer size, protein charge and concentration, and concentration of simple electrolytes.
Modeling of reverse osmosis in the presence of strong solute‐membrane affinity
Authors: Mehdizadeh H. , Dickson J. M.
Published: June 2004

Modeling of reverse osmosis in the presence of strong solute‐membrane affinity has always been a challenge due to the complexity of the solute‐solvent‐membrane interactions and the resultant effect on membrane performance. Most transport models, including all models treating membranes as nonporous and those based on irreversible thermodynamics, are unable to describe or to predict all of the phenomena associated with this case. Recently, the modified surface force‐pore flow model has been derived and used to describe the performance of reverse osmosis membranes for solutes which are rejected from the membrane. In the present work, this model is extended to a more general form which can describe the solute‐membrane affinity case. For illustration, the extended model, with five adjustable parameters, is used to describe the performance for cellulose acetate membranes and dilute aqueous solutions of toluene, cumene, and p‐chlorophenol (data from literature). The model is reasonably consistent with the data. Simulation results of the extended model are also shown.
Process simulation in the complex domain
Authors: Lucia Angelo , Guo Xinzhou , Wang Xiaofeng
Published: June 2004

The asymptotic behavior of fixed‐point methods in the complex domain is studied. Both direct substitution and Newton's method exhibit stable periodic and aperiodic behavior from real‐ or complex‐valued starting points. Moreover, multiple stable periodic orbits can exist for direct substitution. Traditional trust region (or dogleg) methods, on the other hand, often terminate at singular points, which correspond to nonzero‐valued saddlepoints in the least‐squares function that can be arbitrarily far from a solution. Furthermore, the basins of attraction of these singular points are usually dispersed throughout the basin boundaries in the complex domain, clearly illustrating that singular points (via the dogleg strategy) also attract either real‐ or complex‐valued starting points.

In light of this, an extension of the dogleg strategy to the complex domain, based on a simple norm‐reducing, singular point perturbation, is proposed. This extended trust region method removes all forms of nonconvergent behavior and always terminates at a fixed point, even from critical point (worst‐case) initial values. Many numerical results and geometric illustrations using chemical process simulation examples are presented.
Reactions between calcium oxide and flue gas containing sulfur dioxide at lower temperatures
Authors: Hartman M. , Trnka O.
Published: June 2004

An experimental study has been performed of the reactions occurring when the reactive particles of calcium oxide are exposed to sulfur dioxide bearing flue gas. Using a differential, fixed‐bed reactor, kinetic measurements have been made in the temperature range of 170 and 580°C. A relatively simple correlation based on the amassed data has been proposed. This correlation allows the estimation of the reaction rate as a function of temperature, conversion of the solids and concentration of sulfur dioxide in the gas phase. The proposed rate law equation makes it possible to predict in a straightforward manner the rate of sorption which would be required to design the performance of a desulfurization unit.
Computer simulation of molecular fluid mixtures: Results and two new one‐fluid models
Authors: Sowers Gary M. , Sandler Stanley I.
Published: June 2004

Monte Carlo simulation results for binary mixtures of diatomic molecules with spheres, diatomic molecules of different bond lengths, and triatomic molecules with spheres, all interacting with the site‐site Lennard‐Jones potential, were obtained at three different volume ratios and a range of densities and compositions. We report the configurational internal energy of the mixture and of the 1‐1, 1‐2 and 2‐2 pairs, and the compressibility factors at 195 state points. The site‐site correlation functions also computed are available from the authors. Equations of state derived previously (Sowers and Sandler, 1991, 1992) for pure site‐site Lennard‐Jones fluids from a combination of perturbation theory and a three‐parameter corresponding states principle are extended to mixtures using simple one‐fluid mixing rules. These simple mixing rules allow us to make satisfactory predictions of the compressibilities computed from simulation without the use of binary interaction parameters. We also show that the effect of composition on the perturbation free energy βAp/N not accounted for by one‐fluid mixing rules is quite small, and therefore a more complicated mixing rule is not necessary in our equations of state for the mixtures studied.
Solute retention in electrochromatography by electrically induced sorption
Authors: Rudge S. R. , Basak S. K. , Ladisch M. R.
Published: June 2004

Column chromatography and electrophoresis are combined in electrochromatography, where an electric potential is applied to a chromatography column in the axial direction. These studies utilized a dextran gel stationary phase and an eluent of low ionic strength, which were chosen to minimize electric current and therefore column heating and undesirable dispersion effects. The gel, with a small ion exchange capacity of several microequivalents per mL, turned out to be more conductive than the eluent and was able to concentrate macromolecules in the presence of combined electric and flow fields. The model presented describes solute retention due to electrically induced concentration polarization of solute on the resin surfaces, as well as electrophoresis in the mobile and stationary phases. The polarization effect explains differences between retention of high‐molecular‐weight solutes with exclusion coefficients of less than 1 and that of a charged low‐molecular‐weight solute, which is hypothesized to pass through the gel matrix in the presence of an electric field and does not experience concentration polarization. It also shows the application of this effect for protein separation in a liquid chromatography system with a superimposed electric potential.
Continuous electrochemically modulated complexation separations process
Authors: Jemaa N. , Walls H. J. , Noble R. D. , Wedman D. E. , Koval C. A.
Published: June 2004

A continuous electrochemically modulated complexation (EMC) process was devised to extract and concentrate a class of heterocyclic nitrogen compounds. This design is composed of flow‐through electrolysis cells for redox modulation and cycling of the complexing agent and hollow‐fiber membrane modules for phase contacting. The extent of both extraction and concentration was found to depend mainly on the magnitude of the distribution coefficients for both steps. Experimental results and modeling of this continuous EMC process are reported, as well as potential applications for this process for other separations.
Dense‐gas solvent‐solute clusters at near‐infinite dilution: EPR spectroscopic evidence
Authors: Carlier Claude , Randolph Theodore W.
Published: June 2004

The nitrogen hyperfine splitting constant of ditertbutyl nitroxide radicals was measured with electron paramagnetic resonance spectroscopy at near‐infinite dilution in near‐critical and supercritical ethane, as well as in liquid propane, liquid isobutane, and several nonhydrogen bonding liquid solvents. While the measurements in the liquids are described well by the theory of McRae, large deviations from the liquid behavior are observed in supercritical ethane. The deviations are used as a measure of the effective local density of the solvent around the solute. At the two temperatures investigated, Tr = 1.009 and Tr = 1.084, the local density enhancement, defined as the ratio of local to bulk densities, exhibits a maximum of about 3 occurring around 1/2 the critical density. The maximum is removed well from the critical density, where the maximum of the isothermal compressibility is observed. Local density enhancements are short‐range effects and do not correlate well with the development of long‐range critical phenomena. Local density enhancement data in ethane are compared with the prewetting transition that has been observed in nearcritical ultrapure argon.
Eddy viscosity and velocity distribution in turbulent pipe flow revisited
Authors: Datta Ravindra
Published: June 2004

An algebraic expression involving three universal constants is proposed for the eddy momentum diffusivity in fully developed turbulent pipe flow that adequately simulates its variation throughout the pipe cross‐section. The model permits analytical integration to provide a single universal correlation that, unlike other available expressions, smoothly and accurately describes the entire turbulent velocity distribution from the inner region in the immediate vicinity of the wall all the way to the core region near the pipe axis and compares well with the existing correlations and experimental data. Accurate values of the maximum velocity and other flow variables of engineering interest can also be calculated from the velocity profile expression.
Solid‐State film diffusion for the production of integrated optical waveguides
Authors: Kapila D. , Plawsky J. L.
Published: June 2004

Solid‐state film ion exchange (Ag+‐Na+) in glass substrates has been investigated for the fabrication of passive, integrated, optical waveguides. A new ion exchange mechanism is proposed which couples the oxidation of silver metal to silver oxide and the diffusion of silver ions into the glass substrate. At high electric fields and/or high temperatures, the growth rate of a silver oxide film limits the formation of mobile silver ions and constrains the diffusion of silver into the glass. This new mechanism is able to predict the dopant distribution profile under those conditions and under conditions where a constant surface concentration is appropriate (infinitely thick oxide film). It also explains the characteristics of the current vs. time curve measured during the ion exchange process. The formation and growth of the silver oxide film were confirmed by X‐ray photoelectron spectroscopy and X‐ray diffraction analysis.
Feasibility of separations for distillation of nonideal ternary mixtures
Authors: Fidkowski Z. T. , Doherty M. F. , Malone M. F.
Published: June 2004

The product compositions in single‐feed distillation columns can be specified only in certain regions of the composition space, which depend only on the pressure, feed, and vapor‐liquid equilibrium for the mixture. In nonideal mixtures, even without azeotropes, the regions may allow unusual product distributions. For example, the distillate in a “direct split” is composed primarily of the lightest component. Intuition and experience with relatively ideal mixtures suggest that the next most plentiful component is the intermediate boiler. In nonideal mixtures, however, with or without azeotropes, the next most plentiful component may be the highestboiling species with only trace amounts of the intermediate boiler. For azeotropic mixtures, distillation boundaries may give rise to additional restrictions on the product compositions. We describe how simple distillation boundaries deform into continuous distillation boundaries and, in a limited number of cases, how the simple distillation boundaries can be crossed in continuous columns for certain ranges of the design variables. Unfortunately, such designs may be quite sensitive to model uncertainties or to disturbances in the parameters.
Homogeneous nitrous oxide formation and destruction under combustion conditions
Authors: Hulgaard Tore , Dam‐Johansen Kim
Published: June 2004

N2O decomposition and formation during the oxidation of NH3 and HCN were studied in a quartz flow reactor in the presence of CO, NO and other gases. The emphasis is on the influence of CO and NO. In addition, the homogeneous nitrogen chemistry of fluidized bed combustion and the selective noncatalytic reduction of NO (SNR) are discussed. The rate of N2O decomposition in N2 agrees with a first‐order rate expression. The presence of CO or H2 increases the decomposition rate regardless of the additional presence of O2

For the formation of N2O, HCN oxidation is more efficient than NH3 oxidation. The presence of NO increases the amount of N2O formed during the oxidation of HCN or NH3.CO moves the N2O formation toward lower temperatures. H2O increases the reaction rate where few components are present, whereas H2O has little influence in the presence of large amounts of a combustible component such as CO. There are indications that NO is a necessary intermediate for any significant formation of N2O during the oxidation of NH3 and HCN. NO reduction is obtained when NO is initially present during oxidation of both NH3 and HCN. These results are comparable to the respective SNR results with reductant ammonia and urea.
Partial derivative quantities from phase equilibria relationships for mixtures
Authors: Mullins James A. , Rawlings James B. , Johnston Keith P.
Published: June 2004

A systematic formulation of multicomponent/multiphase phase equilibria as a linear algebra problem in the fugacities, mole fractions, partial molar volumes, and partial molar enthalpies is given. The algorithm takes advantage of the Gibbs‐Duhem relationships for each phase and a modified Gaussian elimination technique to reduce the system of equations. These algorithmic steps allow current symbolic manipulation packages to generate useful partial derivative relationships in terms of measurable thermodynamic quantities. Features of the algorithm are demonstrated by applying a computer implementation of the method to a simple two‐phase/two‐component system and to the more complicated examples of a two‐phase/three‐component supercritical fluid chromatography experiment and a mass‐conserving closed system.
Effect of rheological properties on power consumption with helical ribbon agitators
Authors: Carreau P. J. , Chhabra R. P. , Cheng J.
Published: June 2004

The influence of shear thinning and viscoelasticity on the power required for the mixing of viscous liquids using six different helical ribbon agitators has been investigated. Four Newtonian and 12 non‐Newtonian fluids prepared using several polymers dissolved in varying concentrations in different solvents cover a wide range of rheological properties. By a careful choice of test media, the specific and combined effects of shear thinning and viscoelasticity on the power requirement have been examined. Simple models are proposed to predict the effective shear rate in the tank from the knowledge of the torque or power number. The effective shear rate predictions compared with the effective shear rate estimated using the scheme of Metzner and Otto (1957) show that they slightly depend on the shear thinning properties. Fluid's elasticity increases appreciably the power requirement, and departures from the generalized Newtonian power curve in the laminar regime are observed at smaller Reynolds numbers for viscoelastic fluids. Bottom wall resistance of the mixing vessel makes a negligible contribution to the power consumption.
Simulation of a three‐step one‐column pressure swing adsorption process
Authors: Lu Z. P. , Loureiro J. M. , Rodrigues A. E. , Levan M. D.
Published: June 2004

Separation of a binary mixture by a three‐step one‐column pressure swing adsorption process in isothermal and adiabatic cases has been studied. The separation performance of the process is characterized by three parameters: enrichment, recovery and productivity. It has been compared at various operating conditions. The evolution of the bed performance from cycle to cycle and the bed dynamics at cyclic steady state have also been addressed.
Application of the LCPT model to solid‐liquid equilibria for binary compound‐forming alloys
Authors: Howell Wayne J. , Alger Montgomery M. , Eckert Charles A.
Published: June 2004

The linear chemical‐physical theory (LCPT) model for liquid metal solution thermodynamics has been extended to the determination of the liquidus curves for binary intermetallic compound‐forming systems. The equations developed include corrections to the observed melting point temperature and heat of fusion for compounds that dissociate partially on melting. The primary advantages of the LCPT model for solid‐liquid equilibria are the small number of physically realistic parameters required, ease of implementation, and wide applicability. In addition, the model also permits the incorporation of compounds in modeling the liquidus curves that are not necessary for representing the liquid‐phase thermodynamic properties. For the seven systems studied, the agreement between calculated and experimentally measured liquidus curves is quite good.
Measurements of mass flux in a turbulent liquid flow with a chemical reaction
Authors: Komori Satoru , Nagata Kouji , Kanzaki Takao , Murakami Yasuhiro
Published: June 2004

An original technique for simultaneous measurements of concentration and velocity in a turbulent liquid flow with and without a chemical reaction is developed by combining a two‐component He‐Ne laser Doppler velocimeter with a Ar+ laserinduced fluorescence technique. The mass flux and eddy diffusivity of mass are estimated from the simultaneous measurements of concentration and velocity in a liquid shear‐free mixing layer downstream of a turbulence grid. The results show that the present measuring technique enables us to simultaneously measure velocity and concentration with high resolutions and that the approximation of the mean gradient diffusion causes about 10% error in estimating the mass flux in a reacting liquid flow.
Model for Sr‐Cs‐Ca‐Mg‐Na ion‐exchange equilibria on chabazite
Authors: Perona Joseph J.
Published: June 2004

Chabazite zeolites are used at Oak Ridge National Laboratory to decontaminate wastewaters containing 90Sr and 137Cs. Treatability studies show that chabazite can remove trace amounts of these nuclides from wastewaters containing much higher concentrations of calcium and magnesium. The design of ion‐exchange columns for multicomponent systems requires a method for predicting multicomponent equilibria from binary, ternary and quaternary experiments, since the number of experiments required for an empirical equilibrium model is not generally feasible. Binary interaction parameters for the Wilson equation are used to predict solid‐phase activity coefficients for the five‐component system. The sum of squares of deviations between experimental and predicted solution concentrations for the data points available is calculated. The average deviation per data point for the five‐component system is lower than for some of the ternary‐and four‐component data sets containing calcium or magnesium.
Prediction of polydispersed fluidized bed system properties: A new approach
Authors: Ganguly U. P.
Published: June 2004

A new approach for estimating the system properties of polydispersed liquid‐solid fluidized bed systems, based on the concept of departure from monodispersed systems, is discussed. The proposed generalized correlation based on experimental data on particulate fluidization can predict minimum fluidization velocity, minimum elutriation velocity, onset of semifluidization velocity, and free‐settling velocity with an accuracy of ±25.7%. The correlation is valid for heterogeneous systems comprising solids of various sizes and/or densities.
Shape and extent of the void formed by a horizontal jet in a fluidized bed
Authors: Chen Libin , Weinstein Herbert
Published: June 2004

The introduction of reactant gas into a fluidized‐bed chemical reactor as a jet is a common design practice. However, the shape and extent of the void formed by a jet into a fluidized bed and the manner in which the void breaks up into bubbles have still not been described well.

An experimental study of a horizontal jet into a 15 cm by 38 cm cross‐section fluidized bed was carried out using an X‐ray system. Instantaneous solid fraction averaged along 15‐cm chords across the bed were measured. The mean value and the fluctuating component of solid fraction were determined for two initial jet diameters, 0.64 and 1.27 cm, and three initial jet velocities, 23, 46 and 69 m/s. Maps of the mean solid fraction and of statistical properties of the fluctuating component show that there are three discernible regions in the jet‐influenced area of the bed. These are a coherent void, bubble trains, and a surrounding compaction zone.
Multivariable nonlinear control of a continuous polymerization reactor: An experimental study
Authors: Soroush Masoud , Kravaris Costas
Published: June 2004

This experimental work concerns the multivariable nonlinear control of a continuous stirred‐tank polymerization reactor. The globally linearizing control (GLC) method is implemented to control conversion and temperature in the reactor in which the solution polymerization of methyl methacrylate takes place. Control of conversion and temperature is achieved by manipulating the flow rate of an inlet initiator stream and two coordinated heat input variables. Conversion is inferred from on‐line measurements of density and temperature. A reduced‐order state observer is utilized to estimate the concentrations of monomer, initiator and solvent in the reactor. The concentration estimates are used in the control law. This study demonstrates the considerable computational efficiency of the nonlinear controller, which is implemented on a microcomputer. The experimental results show the excellent performance of the controller in the presence of active state and input constraints. A systematic approach is also given for the synthesis of output feedback controllers within the GLC framework for processes with secondary outputs.
Gravity‐Driven reactive coating flow down an inclined plane
Authors: Qi Quan , Johnson Robert E.
Published: June 2004

The coating flow of a reactive fluid on an inclined plane is studied. The fluid viscosity steadily increases due to a sustained cross‐linking reaction. Using well‐known long wave approximations and a simple kinetic relation, we find that the linear stability of a uniform coating is affected not only by the inclination angle but also by the reaction order. In the weakly nonlinear limit, a generalized Kuramoto‐Sivashinsky (K‐S) equation is derived. Two interesting cases are identified in the limit of creeping flow. One is when surface tension is moderate and a generalized Burger's equation results, and the other is when the surface tension is large and the traditional K‐S equation is recovered when the inclination angle is greater than 90°. The solution in this case represents a shock‐like traveling wave down the incline and it appears to be unique for reacting coating flows.
Excitable waves and spatiotemporal patterns in a fixed‐bed reactor
Authors: Barto Martin , Sheintuch Moshe
Published: June 2004

Spatiotemporal patterns existing in a one‐dimensional fixed‐bed reactor with oscillatory or excitable kinetics are analyzed to develop the methodology of motion identification and classification. The reactor model accounts for a local (solid‐phase) oscillator and a global (gas‐phase) convective interaction. The local oscillator incorporates a fast and diffusing surface temperature and a localized activity as its dynamic variables. Such kinetics admit a traveling pulse solution or homogeneous oscillations in a uniform medium. In a fixed‐bed reactor, the local conditions in the bed (the phase plane character) vary along the system. The response of an excitable bed to local perturbations depends on its location (at inlet or outlet) and the nature of the initial steady state ( ignite or extinguished). The main spatiotemporal‐sustained patterns in the bed are: almost homogeneous oscillations that appear as parallel bands in the time‐space contour map; oscillatory fronts that emerge from the reactor exit, and aperiodic motion that appears as split‐bands. Pattern selection is determined by the phase planes spanned by the reactor and the ratio of the two slowest time scales: front residence time and period of oscillations.
Protein polarization in isotropic membrane hollow‐fiber bioreactors
Authors: Taylor D. G. , Piret J. M. , Bowen B. D.
Published: June 2004

A mathematical model has been developed to predict the coupled hydrodynamics and high‐molecular‐weight protein transport in mammalian‐cell hollow‐fiber bioreactors (HFBRs). The analysis applies to reactors with isotropic ultrafiltration membranes under startup conditions when the extracapillary space (ECS) is essentially unobstructed by cells. The model confirms the experimental finding that secondary ECS flows, engendered by the primary flow in the fiber lumens, can cause significant downstream polarization of ECS proteins at typical mammalian‐cell HFBR operating conditions. It also reveals that the osmotic activity of the proteins, by curtailing transmembrane fluid fluxes, can influence strongly the outcome of the polarization process. In fact, at order‐of‐magnitude higher protein concentrations and/or lower recycle flow rates, the secondary flow velocities can be reduced by as much as six orders‐of‐magnitude throughout the ECS, thereby virtually eliminating the polarization problem. This result has important implications for improved reactor startup procedures.
Random motility of swimming bacteria: Single cells compared to cell populations
Authors: Phillips Bret R. , Quinn John A. , Goldfine Howard
Published: June 2004

The motility of a population of swimming bacteria can be characterized by a random motility coefficient, μ, the operational equivalent of a diffusion coefficient at the macroscopic level and in the absence of interacting chemical gradients. At the microscopic level, random motility is related to the single‐cell parameters: speed, tumbling probability, and index of directional persistence (related to the angle a cell's path assumes following a change in direction). Various mathematical models have been proposed for relating the macroscopic random motility coefficient to these microscopic single‐cell parameters. In separate experiments, we have measured motility at both the cell‐population and single‐cell levels for Escherichia coli. The agreement of these results shows that the macroscopic transport behavior of a population of motile bacteria can be predicted from straightforward microscopic observations on single cells.
Protein precipitation: Effects of mixing on protein solubility
Authors: Iyer Harish V. , Przybycien Todd M.
Published: June 2004

Mixing effects in protein precipitation processes are very poorly understood. They impact apparent protein solubility, protein structure in precipitates, particle size, morphology and recoverable activity. In this work, a model is proposed to describe the impact of mixing on protein solubility in precipitation processes. A series of semibatch‐mode precipitation experiments were performed using bovine liver catalase and ammonium sulfate to test model predictions. The process variables studied include initial protein supersaturation, agitation rate, and rate of addition of salt solution to the initial protein solution. The mixing model parameters were estimated independently with the aid of a series‐parallel dye reaction. The results showed good agreement between model and experiment. The initial protein supersaturation was the most important variable at the 1‐L scale. The influence of mixing on recoverable activity was also examined. Scale‐up guidelines are suggested based on model simulations and experimental results.
Stokes flow through a thin screen with patterned holes
Authors: Wang C. Y.
Published: June 2004

Three‐dimensional Stokes flow through a thin screen which has a regular array of holes with two orthogonal axes of symmetry has been studied. The governing equation is simplified by the Roscoe potential and solved by an efficient eigenfunction expansion and collocation method. The resistance is found for circular and square holes in square arrays, and circular and hexagonal holes in triangular arrays.
Kinetics of H2S absorption into aqueous ferric solutions of edta and hedta
Authors: Wubs Harm J. , Beenackers Antonie A. C. M.
Published: June 2004

Kinetics of the reaction of hydrogen sulfide with ferric chelates of ethylenediaminetetraacetic acid (EDTA) at 22°C and with ferric chelates of hydroxyethyle‐thylenediaminetriacetic acid (HEDTA) from 21°C to 60°C were studied in a stirred cell reactor under industrial conditions. Concentration of the ferric chelates ranged from 40 to 150 mol/m3 and the pH was varied from 4 to 10. Only the hydroxy forms of the ferric chelates appear to react with hydrogen sulfide. Under the conditions applied, the reaction appeared to be first‐order in hydrogen sulfide and first‐order in hydroxy ferric chelate. At 22°C the rate constant for the EDTA complex is 14–21 m3/mol·s. Uncertainty in this constant is due to the characteristics of the EDTA complex at pH values above 7. For the HEDTA complex, the activation energy of the reaction of hydrogen sulfide with the monohydroxy ferric chelate is (26.4 ± 0.6) kJ/mol with k = 1.8 m3/mol·s at 25°C. The rate constant for the reaction with the dihydroxy ferric chelate is (300 ± 150) m3/mol·s in the temperature range investigated.
Synthesis of discrete‐time nonlinear feedforward/feedback controllers
Authors: Soroush Masoud , Kravaris Costas
Published: June 2004

This work concerns the synthesis of discrete‐time feedforward/feedback control systems for general nonlinear processes with stable zero dynamics. Depending on the process under consideration, the derived feedforward/feedback controllers can completely eliminate the effect of measurable disturbances and produce a prespecified linear response with respect to a reference input, or provide integral‐square error optimal response to step changes in the disturbances and a prespecified linear response with respect to a reference input. In either case, the developed feedforward/feedback controllers allow for the asymptotic rejection of unmeasurable disturbances. These controllers are derived within the globally linearizing control frame‐work, first under full state information and then in the absence of state measurements. The internal stability of the closed‐loop system is addressed. The derived controllers are interpreted from a model‐predictive point of view, and their connections with the feedforward internal model control and the model algorithmic control are established. The theoretical results are illustrated through a continuous stirred‐tank reactor example.
Thermal dehydration of magnesium hydroxide and sintering of nascent magnesium oxide
Authors: Hartman M. , Trnka O. , Veselý V.
Published: June 2004

Increasing and constant temperature gravimetric methods were used to measure the thermal decomposition rates of Mg(OH)2 particles. An Arrhenius‐type kinetic equation was proposed and tested. Temperature, exposure time on surface area of the calcined particles and other effects of variables were explored. A fractional‐order sintering rate expression was developed on the basis of collected experimental data. An integrated model is also presented that considers the surface area generation by calcination and the simultaneous surface reduction caused by sintering of the nascent oxide. Obtained results enable specifying the operating conditions needed for the production of the high surface, reactive magnesium oxide.
Foam‐enhanced devolatilization of polystyrene melt in a vented extruder
Authors: Tukachinsky Alexander , Talmon Yeshayahu , Tadmor Zehev
Published: June 2004

Vented extruder devolatilization (DV) of PS melt containing 6,000 ppm styrene was studied by scanning electron microscopy (SEM) and video photography. Vacuum DV of a polymer is accompanied by foaming, which starts instantaneously upon supersaturation of the stretched melt and is enhanced at higher speeds of the vented extruder screw. As the volatiles are removed from the melt, foaming gradually ceases, starting with the pushing flight of the screw. The experimental installation design allowed us to quench the polymer melt in the DV zone at various stages of the process. Samples taken from four areas of the channel width were investigated by SEM. Bubble nucleation in the melt appears to take place mainly in the border area adjoining the gas phase. In the shear field caused by screw rotation, large bubbles become noticeably elongated. Their surface, as well as the free surface of the melt, is covered with blisters, 1–100 μm in size. Microblisters are often concentrated in areas subjected to stretching. Calculations of cooling due to volatile evaporation and of heating due to viscous dissipation near a growing bubble shows that the process of foam‐enhanced DV of a PS/styrene system can be regarded isothermal if the initial volatile concentration does not exceed approximately 1%.
Analysis of bacterial migration: I. Numerical solution of balance equation
Authors: Frymier Paul D. , Ford Roseanne M. , Cummings Peter T.
Published: June 2004

Chemotaxis describes the ability of motile bacteria to bias their motion in the direction of increasing gradients of chemicals, usually energy sources, known as attractants. In experimental studies of the migration of chemotactic bacteria, 1‐D phenomenological cell balance equations (Rivero et al., 1989) have been used to quantitatively analyze experimental observations (Ford et al., 1991; Ford and Lauffenburger, 1991). While attractive for their simplicity and the ease of solution, they are limited in the strict mathematical sense to the situation in which individual bacteria are confined to motion in one dimension and respond to attractant gradients in one dimension only. Recently, Ford and Cummings (1992) reduced the general 3‐D cell balance equation of Alt (1980) to obtain an equation describing the migration of a bacterial population in response to a 1‐D attractant gradient. Solutions of this equation for single gradients of attractants are compared to those of 1‐D balance equations, results from cellular dynamics simulations (Frymier et al., 1993), and experimental data from our laboratory for E. coli responding to α‐methylaspartate. We also investigate two aspects of the experimentally derived expression for the tumbling probability: the effect of different models for the down‐gradient swimming behavior of the bacteria and the validity of ignoring the temporal derivative of the attractant concentration.
Heat‐transfer characteristics in viscous gas‐liquid and gas‐liquid‐solid systems
Authors: Kumar Samir , Fan L.‐S.
Published: June 2004

Local heat‐transfer measurements are performed using a special heat‐transfer probe in gas‐liquid and gas‐liquid‐solid systems with viscous Newtonian liquids as the continuous phase. Effects of viscosity on bubble‐liquid and bubble‐liquid‐solid interactions affecting local heat transfer are studied through heat‐transfer experiments with simultaneous flow visualization in a simplified system involving single bubbles or a chain of gas bubbles moving in viscous liquids and liquid‐solid systems. Effects of viscosity on bubble wake and local heat transfer are examined with reference to heat transfer in freely‐bubbling beds (bubble columns and three‐phase fluidized beds). The kinematic viscosity of the fluid greatly influences the nature of flow in the wake which affects local heat transfer in the bed. The local heat transfer decreases with the viscosity due to the rapid decay in the circulation strength of the bubble wake caused by increased viscous dissipation of vorticity. Local heat transfer due to cyclic/periodic injection of bubbles is significantly enhanced due to increased bubble‐wake interactions which rapidly accelerate bubbles and increase average bubble rise velocity. Heat transfer in simplified liquid and liquid‐solid systems with single‐ and chain‐bubble injections characterizes the local heat‐transfer performance of freely‐bubbling beds (bubble columns and three‐phase fluidized beds). A mechanistic model developed accounts for the heat‐transfer behavior in bubble columns and three‐phase fluidized beds with viscous liquids.
Modeling of supercritical extraction of organics from solid matrices
Authors: Madras Giridhar , Thibaud Catherine , Erkey Can , Akgerman Aydin
Published: June 2004

The desorption profiles of various organics from soil and activated carbon using dense liquid and supercritical carbon dioxide were modeled. The model accounts for effective diffusion of the organics in the solid pores, axial dispersion in the fluid phase, and external mass transfer to the fluid phase from the particle surface. This model, involving partial differential equations, is solved using the orthogonal collocation on finite elements. The model is able to predict the experimental data quite well without any adjustable parameters. The importance of each parameter depended on the system, although adsorption equilibrium and the effective pore diffusion of the organic were the most significant parameters in all the systems investigated.
Optimization of distillation units using collocation models
Authors: Seferlis P. , Hrymak A. N.
Published: June 2004

Reduced‐order models are developed based on orthogonal collocation on finite elements for the steady‐state optimization of multicomponent distillation units. The formulation divides the column sections into subdomains (finite elements) to track irregularities in the column profiles accurately and requies that plates with discontinuities in the flows introduced by feedstreams or sidestreams should be treated as discrete equilibrium stages. Application of the model formulation to a number of examples show that reduced‐order models converge to the same optimal solution as the full‐order, tray‐by‐tray model. Optimization results show that computational time can be reduced significantly using the more compact model formulation, and it is possible to converge to the optimal solution from a larger region for the initial set of variable values.
Food sterilization by electrical heating: Sensitivity to process parameters
Authors: Zhang L. , Fryer P. J.
Published: June 2004

The design of efficient continuous sterilization equipment for flowing solid‐liquid food mixtures requires that temperature and velocity distributions within the fluid are known. In electrical (ohmic) heating, electric current is passed through a two‐sterilize it. This allows rapid sterilization of solid‐liquid mixtures which minimizes quality loss. A computational process model simulates the effect of realistic physical property changes. The effect of variations in physical properties can have a significant effect on the temperature of the product and thus on its sterility. Realistic designs will have to cope with temperature differences between phases. Experimental data for velocities in a food flow have been incorporated into an enthalpy balance model for heat transfer in solid‐liquid mixtures. The effect of inlet temperature on the length of hold tube required to give the necessary process sterility calculated shows the variation in output conditions that might results in real systems.
Optimal fed‐batch control of induced foreign protein production by recombinant bacteria
Authors: Lee Jongdae , Ramirez W. Fred
Published: June 2004

Optimal control strategies for maximizing the production of induced foreign protein by recombinant bacteria were sought by the optimal control theory. Nutrient and inducer feeding rates were selected as key control variables. Since the problem is linear in the control variables, the optimal control is bang‐bang or singular. Singular solutions are shown to exist. The optimal control theory showed that the specific growth rate with respect to nutrient concentration must be kept in its maximum phase and that there exist both a cell growth period and a protein production periods. The optimal control theory calculates exactly the growth and production periods. The glucose concentration is controlled along a singular are to give a maximum specific growth rate. The inducer level is controlled along a separate singular arc.
Equilibrium and kinetics of adsorption of Freon‐12 at infinite dilution
Authors: Golden T. C. , Sircar S.
Published: June 2004

Equilibrium and kinetic data for adsorption of trace CF2Cl2 (Freon‐12) from various carrier gases on BPL activated carbon are reported. Coadsorption of the bulk carrier gas can severely reduce the equilibrium adsorption capacity and adsorptive mass‐transfer coefficient of strongly adsorbed CF2Cl2. The difference in size between CF2Cl2 and the bulk carrier gas molecules plays a major role in establishing the binary or multicomponent equilibrium adsorption properties. The multisite (single and multicomponent) Langmuir model, which accounts for differences in adsorbate sizes, provides a reasonable framework for describing the size effects. The adsorptive mass transfer of CF2Cl2 under the experimental conditions investigated is dominated by surface diffusion into the pores of the activated carbon. The surface diffusivity is a strong function of the extent of coverage and strength of adsorption of the bulk components.
Modeling the spontaneous ignition of coal stockpiles
Authors: Salinger Andrew G. , Aris Rutherford , Derby Jeffrey J.
Published: June 2004

The spontaneous ignition of coal stockpiles is a serious economic and safety problem. This phenomenon is analyzed using the approach of modern reaction engineering, which is made challenging by the nonlinear interactions of chemical reaction, heat transfer, and buoyancy‐driven flows within and around the stockpile. A model developed represents reaction and transport within a realistically‐shaped stockpile and transport and flow in the surrounding air. A new methodology based on the Galerkin finite‐element method (Salinger, 1993b) allows for efficient solution of flows in both porous and open domains. Bifurcation analysis is used to track steady‐state model solutions of relevant parameters, such as the Damköhler number (dimensionless reaction rate), Rayleigh number (dimensionless driving force for buoyant flow), and dimensionless permeability of the stockpile. The solutions provide an understanding of the roles of various transport mechanisms on the ignition behavior and nonlinear coupling between these mechanisms. Results clearly demonstrate the need for incorporating realistic descriptions of flow and transport in the surrounding air into the model.
Products in methane combustion near surfaces
Authors: Vlachos D. G. , Schmidt L. D. , Aris R.
Published: June 2004

Emission of carbon monoxide (CO), formaldehyde (CH2O), and unburned methane (CH4) are calculated for premixed methane/air mixtures impinging on a flat surface as functions of surface temperature, equivalence ratio, and strain rate with detailed chemistry involving 46 reversible reactions and 16 species using numerical bifurcation theory. Multiple solutions with different selectivities to stable products are found. On the extinguished branch unburned CH4, molecular hydrogen (H2), CO, and CH2O dominate, whereas on the ignited branch carbon dioxide (CO2) predominates near the surface. Cold walls can promote the selectivity to CO and CH2O near extinction, and high flow rates can increase considerably the formation of CO, CH2O, and unburned CH4. For example, an ignited stoichiometric methane/air mixture (9.5% CH4 in air) impinging on a surface of 1,000 K is calculated to produce 2% CO, 150 ppm CH2O, and 3% unburned CH4 for a strain rate of 500 s−1. Maximum efficiency of CH4 and minimum selectivity to CH2O occur near the stoichiometric ratio, whereas minimum selectivity to CO occurs for fuel lean mixtures. Comparison of combustion near surfaces with freely propagating flames is also shown.
Electrostatic droplet generation: Mechanism of polymer droplet formation
Authors: Bugarski Branko , Li Qiangliang , Goosen Mattheus F. A. , Poncelet Denis , Neufeld Ronald J. , Vunjak Gordana
Published: June 2004

The mechanism of alginate droplet formation and experimental parameters for producing very small polymer microbeads (less than 100 μm dia.) using an electrostatic droplet generator studied showed that the microbead size was a function of needle diameter, charge arrangement (electrode geometry and spacing) and strength of electric field. Perfectly spherical and uniform polymer beads, 170 μm dia., for example, were obtained at a potential difference of 6 kV with a 26‐gauge needle and an electrode distance of 2.5 cm. Increasing the electric field, and thus the surface charge in the vicinity of the needle, by increasing the applied potential, resulted in needle oscillation, giving a bimodal bead size distribution with a large fraction (30–40%) of microbeads with a mean diameter of 50 μm. The process of alginate droplet formation under the influence of electrostatic forces assessed with an image analysis/video system revealed distinct stages. After a voltage was applied, the liquid meniscus at the needle tip was distorted from a spherical shape into an inverted cone‐like shape. Consequently, alginate solution flowed into this cone at an increasing rate causing formation of a neck‐like filament. When this filament broke away, producing small droplets, the meniscus relaxed back to a spherical shape until flow of the polymer caused the process to start again. A large‐scale multineedle device with a processing capacity of 0.7 L/h was also designed and produced uniform 400 ± 150 μm microbeads.