bluuebunny/crossref_metadata_2025 huggingface dataset:
Contact nucleation from aqueous dextrose solutions
Authors: Elankovan P. , Berglund K. A.
Published: June 2004

Contact nucleation studies were performed with the dextrose‐water system. Under some conditions two crystallographic phases were formed, which were identified in situ using a laser Raman microprobe. In addition, the growth of the contact nuclei was monitored in situ using photomicroscopy. The results are discussed in the context of contact nucleation and growth models.
A numerical study of global bifurcations in chemical dynamics
Authors: Kevrekidis I. G.
Published: June 2004

Global bifurcations are frequently encountered in the dynamic behavior of chemically reacting systems and their models. They cause dramatic qualitative changes in the system response, such as the birth and death of oscillations and even the onset of chaos. They involve entire regions of the system phase space, and due to their nature they are in general not predictable by standard local bifurcation methods, analytical or numerical. Special methods and algorithms must therefore be developed to locate and analyze them in parameter space. This paper presents such methods and algorithms and illustrates them through standard chemical engineering examples. Test cases include lumped chemical reactor models (homogeneous and heterogeneous, autonomous and periodically forced), a problem of compressible gas flow in porous media, and a case of two coupled oscillators. The phenomena discussed include infinite‐period bifurcations, saddle connections, frequency locking, and the creation and extinction of multifrequency responses through global manifold interactions (homoclinic tangles).
Laboratory studies of binary salt CVD in combustion gas environments
Authors: Liang Baishen , Rosner D. E.
Published: June 2004

A flash‐evaporation technique is used to obtain vapor deposition characteristics for the binary alkali sulfates K2SO4 + Na2SO4 at 1 atm above 1, 100 K. This technique gives results of immediate engineering interest, such as dewpoint temperatures, condensate composition and rates of vapor deposition as well as useful data on the system's thermodynamic characteristics. It is concluded that alkali sulfate deposition and vaporization in combustion environments are inevitably influenced by chemical reactions such as hydroxide formation. It is also concluded that solution nonideality is important even for homologous alkali‐salt mixtures.

Predictions are made using convective‐diffusion mass transfer theory, accounting for chemical reactions by means of effective volatilities, and assuming regular, nonideal condensate solutions. The predicted dewpoints, condensate compositions and deposition rates are quantitatively consistent with experimental observations. This approach, validated here, can be extended to more extreme conditions of engineering interest, including turbulent, high‐temperature/pressure systems.
Forced convection: IV. Asymptotic forms for laminar and turbulent transfer rates
Authors: Stewart W. E.
Published: June 2004

Mass transfer rates in laminar and turbulent nonseparated boundary layers are asymptotically expanded for small values of the diffusivity 𝒟AB, with a uniform state on the mass transfer surface. Results for heat transfer follow by analogy. The thermal or binary Nusselt number at small net mass transfer rates is given asymptotically by a generalized penetration expression

for short times, or for boundary layers that duplicate the surface tangential motion. For flows past rigid interfaces, the long‐time average of 〈Nu〉 is given asymptotically by a generalized Chilton‐Colburn relation

in regions of nonrecirculating motion. The measurable functions aij and bij depend only on the system shape and laminar or turbulent velocity field. Formal expressions for a00 are given, and an expression for b00 in steady flows. These results agree well with data on mass transfer operations in tubes, packed beds, and fluid‐fluid contactors.
Pressure drop in gas‐liquid downflow through packed beds
Authors: Sai P. S. T. , Varma Y. B. G.
Published: June 2004

Pressure‐drop in cocurrent gas‐liquid downflow through packed beds was experimentally measured for nonfoaming, foaming Newtonian, and non‐Newtonian liquids. The variables include the column diameter, packing size and shape, flow rates of the phases, and their physical properties. Unified correlations are presented for the data of the present study as well as data available in literature in terms of Lockhart‐Martinelli parameters and flow variables and packing characteristics. The data were modeled using a dynamic interaction model.
A simple method to study gas phase reactions
Authors: Morrison P. W. , Reimer J. A.
Published: June 2004

To understand better the impact of gas phase reactions on chemical vapor deposition, a new experimental method has been developed to isolate and study homogeneous reactions separately from heterogeneous reactions. This new system heats reactant gases by rapidly compressing them to temperatures greater than 1,000 K; the walls of the reactor remain constant at 500 K or less. Pressure and volume measurements determine the mean temperature of the gas. Results from a series of test reactions and simple models show that this apparatus and measurement method work well. The apparatus is inexpensive and simple to use, and has advantages over shock tubes and static systems.
Data reconciliation and parameter estimation in plant performance analysis
Authors: MacDonald R. J. , Howat C. S.
Published: June 2004

Results of a computer study are reported applying data reconciliation and interpretation techniques to a single‐stage flash operation. Measured errors in the process variables are significantly reduced. The techniques are successfully extended to estimate flash efficiency. The flash efficiency is estimated using two methods: (1) a statistically rigorous simultaneous data reconciliation and efficiency estimation procedure, and (2), a faster, nonrigorous sequential procedure. It is found that the simultaneous procedure provides better measured variable and flash efficiency estimates.
High‐pressure solution behavior of the polystyrene‐toluene‐ethane system
Authors: Seckner A. J. , McClellan A. K. , McHugh M. A.
Published: June 2004

The effect of supercritical ethane on the high‐pressure phase behavior of the polystyrene‐toluene system is experimentally investigated. Adding 17.8 wt. % ethane to the polystyrene‐toluene solution lowers the temperature of the lower critical solution temperature (LCST) curve by 162°C, 22.5 wt. % lowers the temperature of the curve by 231°C, and with 24.9 wt. % the curve is shifted to such low temperatures that it merges with the upper critical solution temperature curve. In practice this means that polymer solutions can be separated at lower temperatures where thermal degradation of the polymer is less. Also, more than 99 wt. % of the polystyrene is recovered in the polymer‐rich phase after the solution phase splits. Patterson's corresponding states treatment is used to model the LCST data.
Thermoflow multiplicity in a packed‐bed reactor. II: Impact of volume change
Authors: Lee J. P. , Balakotaiah V. , Luss Dan
Published: June 2004

Steady states with different flow rates and temperature profiles may exist in a monolith or multitube packed‐bed reactor operating under a prescribed pressure drop, due to the coupling among the species, energy, and momentum balances, and the change of physical properties with temperature and pressure. Criteria are derived predicting the conditions under which thermoflow multiplicity can occur for a gaseous reaction involving a change in the number of moles. In general, a reaction‐induced volume increase enables thermoflow multiplicity to occur for reactions with a lower adiabatic temperature rise. A surprising finding is that thermoflow multiplicity may be found for an isothermal reaction involving a volume decrease. This point emphasizes the difference in the feedback mechanism leading to thermoflow and thermokinetic multiplicity.
Two‐phase pressure drop in vertical crossflow across a horizontal tube bundle
Authors: Schrage D. S. , Hsu J.‐T. , Jensen M. K.
Published: June 2004

An experimental investigation has been made to evaluate the friction, acceleration, and hydrostatic pressure drops in two‐phase vertical crossflow across a horizontal tube bundle through the measurement of the void fraction and determination of the two‐phase friction multiplier. The void fractions were found to increase with increasing mass velocity for a fixed quality level. The two‐phase friction multiplier increased with increasing mass velocity for a fixed value of the Martinelli parameter in both slug and spray flow and decreased with increasing mass velocity in bubbly flows. The void fraction and two‐phase friction multiplier data were correlated and used to predict with very good results the total pressure drop occurring in simulated diabatic flow tests and in actual diabatic tests using R‐113.
Protein extractions with hollow fibers
Authors: Dahuron Lise , Cussler E. L.
Published: June 2004

Differential protein extractions were measured with two types of extractants: inverted micelles and two‐phase aqueous systems. The results show that hollow‐fiber extractions are substantially faster than those possible in conventional equipment. The extractions are not compromised by loading or flooding because the flows of extractant and raffinate are almost completely independent. Mass transfer coefficients inferred from the measurements both support and extend design equations for these contactors.
Non‐Gaussian dispersion in model smokestack plumes
Authors: Thompson V. L. , Greenkorn R. A.
Published: June 2004

The dispersion of emissions from a plume in a cross stream was studied using both experimental and theoretical techniques. Wind tunnel experiments were done for three jet to cross stream velocity ratios: R = 0.5, 1, and 2. The concentration in the plane perpendicular to the cross stream was measured using light scattering and photographic image analysis. The analysis of the negatives revealed the presence of two counterrotating vortices that greatly affected dispersion of smoke in the plume.

Convection and random turbulent dispersion were modeled with a statistical convection simulator on a CYBER 205 computer. The input velocities to the convection model were obtained using experimental measurements. The model revealed the effects of the vortices on dispersion, and the concentration distributions across the plume were determined once the vortices had dissipated. The existence of the counterrotating vortices and their effects upon plume dispersion showed that the Gaussian plume model, which is widely used for modeling pollutant dispersion, is inapplicable in the near range of the model stack.
The continuous‐flow gravity thickener: Steady state behavior
Authors: Landman K. A. , White L. R. , Buscall Richard
Published: June 2004

The equations governing consolidation in a continuous‐flow gravity thickener are developed based on the assumption that a flocculated suspension possesses a compressive yield stress Py(ϕ) that is a function of local volume fraction only. These equations are used to model the steady state operation of a thickener. The bed height required to achieve a given underflow concentration is found to be a relatively sensitive function of the details of the Py(ϕ) function, particle flux through the thickener, and variations in the cross‐sectional area of the thickener. The limiting values of the underflow concentration ϕu for a given flux or the limiting values of flux for a desired ϕu are studied and shown to exist only for cylindrical and converging thickeners.
Stochastic simulation of aerosol deposition in model filters
Authors: Ramarao B. V. , Tien Chi
Published: June 2004

Aerosol deposition in model filters composed of layers of parallel fibers with the orientation of the fiber of each layer kept arbitrary was simulated stochastically. Particle positions at the inlet of the filter were assumed to be randomly distributed over the inlet plane with the trajectory assumed to be rectilinear because of the particles' high inertia. In the simulation, the track of each particle through the filter was monitored and the possible bounce‐off from collision surfaces was allowed for. Deposition behavior and the structure of deposits were studied with various aerosol flow conditions and filter characteristics.
Heat effects in gas sulfiding of hydroprocessing catalysts
Authors: Reyes S. C. , Ho T. C.
Published: June 2004

An a priori model is developed to assess the heat effects during gas‐phase (H2S/H2) sulfiding of a CoOMoO3/γAl2O3 hydroprocessing catalyst in a small fixed‐bed reactor. The model satisfactorily predicts the observed maximum temperature rise and the speed of the traveling exotherm. It is found that the amount of heat released can be of sufficient magnitude to cause significant hot spotting and local starvation of H2S, both of which need to be avoided for desired catalyst performance. With this model, parametric studies were made to optimize the sulfiding operation. It is expected that the model can be used to guide catalyst synthesis research and to scale up laboratory/pilot‐plant sulfiding procedures.
A priori design of a photoreactor for the chlorination of ethane
Authors: Clariá M. A. , Irazoqui H. A. , Cassano A. E.
Published: June 2004

A three‐dimensional radiation model coupled with two‐dimensional mass balances for the intervening chemical species is solved for the photochemical chlorination of ethane. The analysis was carried out using the full mechanistic kinetic sequence and employing polychromatic radiation for a process conducted in a tubular reactor placed inside an elliptical reflector.

Theoretical predictions were compared with bench‐scale experiments and showed excellent agreement. Using the validated model, computational experiments were conducted to explore the influence of reactor design and operational parameters upon the degree of chlorination. Dichlorination reactions were also added to the kinetic model to analyze reactor behavior from the viewpoint of selectivity. The a priori design method described can be applied from first principles and requires no experimentally adjustable parameters.
Transient free‐surface flows in injection mold filling
Authors: Mavridis H. , Hrymak A. N. , Vlachopoulos J.
Published: June 2004

Advancing and colliding flow fronts are important fluid mechanical problems in injection mold filling. These are small‐scale, transient free‐surface flows with a significant impact on the molecular orientation of molded parts. This paper describes a numerical technique for the simulation of transient free‐surface flows. The algorithm combines a Galerkin/finite‐element discretization of the governing equations with a predictor‐corrector scheme for integration in time, and determines simultaneously the flow field and the free surface at every time step. The method is applied to the start‐up of a fluid flow initially at rest and impingement of two flow fronts to form a weldline. Comparison of simulation with available experiments on the start‐up problem shows very good agreement. Numerical tracking of fluid elements clarifies various aspects of flow‐induced deformation of the material.
Asymmetric cellulose acetate hollow fibers: Studies in gas permeation
Authors: Sidhoum M. , Sengupta A. , Sirkar K. K.
Published: June 2004

Gas separation behavior of permeators containing asymmetric cellulose acetate hollow fibers with the dense skin on the outside, facing the feed gas, has been investigated experimentally and theoretically using a sweep gas technique. Two systems were studied: CO2‐N2, and O2‐N2 (air). If the membrane structure is assumed to be symmetric or homogeneous, the model complies with the present data much better, compared to the assumption that the membrane is asymmetric.

Separation was carried out with the high‐pressure feed outside the fibers as well as inside the fibers. In both cases the data were well predicted by the homogeneous model. No concentration polarization effects were found when the feed flowed inside the fibers. The inherent membrane separation capability appeared practically the same whether the feed was inside or outside the fibers. Internal pressurization did not damage the membrane performance for the low pressure range used.

These findings enable the operation of asymmetric hollow‐fiber permeators with the feed inside the fiber lumen, which gives better separation in high stage cut situations.
Pore structural effects in catalytic gasification of coal chars
Authors: Ballal Girish , Amundson N. R. , Zygourakis Kyriacos
Published: June 2004

The effect of pore structure on the gasification of two chars impregnated with potassium carbonate catalyst is investigated. Well‐characterized char samples were reacted with carbon dioxide, water vapor, and oxygen. The effects of catalyst on the magnitude of the gasification rate as well as on the entire rate vs. conversion pattern are reported. Reactivity and structural data are analyzed to obtain a meaningful comparison of the gasification behavior for all char‐gas pairs. Initial intraparticle catalyst dispersion was strongly influenced by the pore structure of the chars. During gasification with carbon dioxide and water, the catalyst appeared to redistribute itself in accordance to the accepted reaction mechanism, leading to superior catalytic activity. The reported measurements indicate that both the pore structure of the chars and the chemistry of a particular reaction are key factors in determining the overall catalytic performance.
Measurement and prediction of entrained‐flow gasification processes
Authors: Brown B. W. , Smoot L. D. , Smith P. J. , Hedman P. O.
Published: June 2004

A detailed mathematical model is used to predict local and effluent properties within an axisymmetric, entrained‐flow gasifier. Laboratory experiments were conducted to provide local properties for four coal types from a gasifier operating at near‐atmospheric pressure. Effects of selected model parameters and test variables were examined and compared with measurements in most cases. The comparison of predictions and measurements provides the first evaluation of capabilities and limitations of a comprehensive model for entrained‐flow gasifiers.
A mathematical model for the AC impedance of semiconducting electrodes
Authors: Bonham D. B. , Orazem M. E.
Published: June 2004

A mathematical model is developed to calculate the impedance response of a semiconductor electrode to a sinusoidal current perturbation. The model accounts explicitly for electron and hole transport as well as generation and recombination through band‐to‐band mechanisms and through bulk interband electron acceptors of specified energy. The resistive (real) component of the impedance is shown to be sensitive to the concentration, distribution, and energy level of the bulk sites. The capacitive (imaginary) component, while useful for determining the dopant level and the flat‐band potential of the semiconductor, is relatively insensitive to low concentrations of bulk sites.
Nonlinear inferential control
Authors: Parrish J. R. , Brosilow C. B.
Published: June 2004

Nonlinear inferential control (NLIC) has been developed as a method for improving control of nonlinear systems. The controller is modelbased, and allows for direct use of available measurements. This paper presents the structure of NLIC and the manner in which it is applied to processes when the controlled variables are measured. Also described is the improvement in the process control using NLIC. Two illustrative examples are presented, a laboratory heat exchanger process and a simulated neutralization process. The results indicate that a substantial improvement in control is possible using NLIC.
Dynamic behavior of supercritical extraction of kerogen from shale
Authors: Triday Jaime , Smith J. M.
Published: June 2004

A model is presented for the supercritical reaction‐extraction of a solid component from a bed of porous particles through which the solvent flows. Accounted for are reaction at an intraparticle position, pore diffusion, interphase mass and heat transfer, and flow in a differential reactor. The behavior of the bed is dynamic since both temperature and concentration of extractable solid vary with time.

The model is compared with experimental data for the extraction with toluene of kerogen from a bed of Colorado shale particles. Curves of effluent bitumen concentration vs. time were measured and fitted with predicted curves to evaluate three parameters: preexponential factor and activation energy for the conversion of insoluble kerogen to soluble bitumen, and the diffusivity of toluene at the critical point. The predicted curves agreed well with the experimental results and gave reasonable values for the parameters. Rates of reaction‐extraction as well as extraction curves exhibited a sharp dip as the reactor was heated through the critical temperature.
Dynamics of a sodium heat pipe reforming reactor
Authors: Richardson J. T. , Paripatyadar S. A. , Shen J. C.
Published: June 2004

A prototype of a sodium heat pipe reactor for steam reforming processes has been developed and tested at energy levels up to 6 kW. Both experimental and model simulation data were obtained. Temperature profiles at nonflow and nonreaction flow conditions were used to adjust the parameters of the model, along with an intrinsic rate equation measured in a differential, bench‐scale apparatus.

Very good agreement between experiment and model was found for approximately 50 runs under steady state reaction conditions covering a range of process variables. Advantages of the heat pipe as a reactor energy source were confirmed. These include isothermal wall temperatures, energy flux transformation, adjusted flux profiles, and rapid startup. These measurements and computations have provided a model from which optimized configurations have been determined.
Molecular thermodynamics of partially‐ordered fluids: Microemulsions
Authors: Hu Ying , Prausnitz John M.
Published: June 2004

Mixtures of water, oil, and surfactant often form microemulsions where tiny drops of oil (or water) are dispersed in a matrix of water (or oil). A microemulsion is a partially‐ordered fluid in which the surfactant molecules exist in an oriented layer at the boundary between the continuous phase and the discontinuous phase. This work proposes a molecular‐thermodynamic model that is useful for calculating a variety of phase diagrams observed in microemulsion‐forming systems.

The model is based on a revised form of Widom's abstract representation of a microemulsion; that representation provides a procedure for calculating essential contributions to the configurational entropy. In Widom's representation, all molecules are difunctional dumbbells such that all the hydrophilic ends are situated together in some lattice sites and all the lipophilic ends are situated together in other sites. A form of Guggenheim's quasichemical theory is used to evaluate the combinatorial contributions that correspond to Widom's picture. Chemical association of surfactant molecules and physical interactions are also taken into account. Minimization of the Gibbs energy yields a distribution function for representing micelle sizes.

Calculated ternary phase diagrams are in good agreement with experimental results. Different types of diagrams can be obtained upon changing physically significant model parameters. A study of how these parameters affect phase behavior provides insight on microemulsion stability.
The size of turbulent eddies close to a wall
Authors: Lyons Stephen L. , Nikolaides Christos , Hanratty Thomas J.
Published: June 2004

Coherent flow‐oriented structures, with a dimensionless spacing of λ+ ≈ 100, observed close to a wall in turbulent flows are believed to control the production of turbulence. A 2 1/2 D computer model developed by Nikolaides is used to explain the dimensionless spacing of these structures. The calculations suggest that the net production of turbulence in the viscous wall region is sensitive to changes in λ+ in the neighborhood of λ+ ≈ 100. They indicate that the net production is negative for λ+ < 85 and that for λ+ = 93 the net production is large enough to supply the turbulent energy dissipated in the outer flow.
Steady state multiplicity and partial internal wetting of catalyst particles
Authors: Bhatia S. K.
Published: June 2004

The partial internal wetting of spherical catalyst particles exposed to saturated vapor is investigated here by considering the interplay of capillary condensation phenomena and thermal effects for a first‐order irreversible reaction. It is shown that for certain very narrow ranges of parameters steady states can exist in which the macropores are vapor‐filled while micropores in an inner core, or an outer shell, are liquid‐filled. Under certain conditions even two such steady states are found, although only one of them can be stable. In addition to these there exist states in which all the micropores are liquid‐filled or those in which the entire particle is internally dry.
Partially wetted catalyst performance in the consecutive‐parallel network
Authors: Harold Michael P.
Published: June 2004

In this study the impact of the transport processes and the degree of wetting on the overall selectivity of a single catalytic pellet is examined for the commercially important consecutive‐parallel network A(g) + vBB(l) → vR1R(l), A(g) + vR2R(l) → P(l). The one‐dimensional formulation applies to catalysts in which the active component is concentrated in a thin porous surface shell coating an impermeable support. Model modifications provide an approximate description of uniformly active catalysts. The kinetics are assumed to be first and zero order with respect to the volatile (A) and nonvolatile (B, R) components, respectively, as encountered in many olefin hydrogenations. The common literature assumption of volatile reactant limiting reactions is relaxed by accounting for nonvolatile reactant depletion. The depletion can significantly reduce the desired product (R) selectivity below its intrinsic value attained under fully wetted, no‐depletion conditions. A model employing the single limiting reactant assumption cannot predict such selectivity variations. The selectivity is shown to be a complex function of the wetting efficiency, the stoichiometry, and the interacting reactions and mass transport processes. In many cases the intrinsic selectivity is attained for wetting efficiencies exceeding a critical value. Increases in the wetting above this value can reduce significantly the production rate of R because of resistance to A supply through the liquid film. Thus, an optimal range of wetting efficiencies exists for which the selectivity is equal to its intrinsic value and overall rates are high. The significance of the main findings with regard to trickle‐bed reactor performance are discussed.
Kinetics of ion exchange accompanied by neutralization reaction
Authors: Kataoka Takeshi , Yoshida Hiroyuki
Published: June 2004

Ion exchange accompanied by a neutralization reaction has been analyzed for the general cases where the bulk solution contains a neutral salt and an acid or a base, e.g., [R · H] + (NaCl + NaOH). Both intraparticle and liquid film diffusion are considered. The Nernst‐Planck equation was applied for the fluxes of the ionic species in the liquid film and the resin phase with a mobile reaction plane in the liquid film. When the solution contains acid or base more than 50%, the exchange rate is approximated by the case for pure acid or base solution: the reaction plane is located at the solid/liquid interface. When the solution contains acid or base less than 50% (this situation is generally encountered in the case where a neutral salt flows through a mixed bed of OH‐form and H‐form resins), the mobile reaction plane should be considered. When the ratio of intraparticle diffusion resistance to liquid diffusion resistance defined in the text is larger than 2, the exchange rate can be approximated by intraparticle diffusion control. The larger the ratio, the faster the reaction plane moves to the resin particle surface. Experimental uptake curves for the ion exchange systems [R · H] + (NaCl + NaOH) and [R · OH] + (NaCl + HCl) confirm the above conclusions.
Microdispersive interfacial mixing in fast polymerizations
Authors: Machuga Steven C. , Midje Heather L. , Peanasky John S. , Macosko Christopher W. , Ranz William E.
Published: June 2004

Interfacial activity between liquid reaction injection molding (RIM) reactants was observed using light microscopy. Diisocyanates were brought into contact with various diols and diamines in a thin (250 μm) gap. A dynamic phenomenon, rapidly producing a well‐mixed, intermaterial phase, was discovered. It was revealed that the rate of growth of this newly formed region was dependent on both the rate of reaction and the physical properties of the initially formed product species at the reaction interface. Spontaneous interfacial mixing also occurred without chemical reaction when a reaction product was dissolved in “capped” contacting reactant liquids. It is believed that strong interfacial intermolecular forces, inducing flow through the polymerization product layer formed immediately upon reactant contact, are responsible for the initial explosiveness of this microscopic process. The range of eventual thicknesses of the mixed, interfacial region (100–800 μm) for various typical RIM systems indicates that this microscopic subprocess may be significant to the fate of the overall process.
The excess gibbs free energy of aqueous nonelectrolyte solutions
Authors: Shealy Glenn S. , Sandler Stanley I.
Published: June 2004

A new thermodynamic model is presented for aqueous solutions of polar nonelectrolytes. The model is based on solute aggregation in an aqueous lattice, which is inferred from spectroscopic, thermodynamic, and physiochemical data. The adjustable model parameters have physical meaning, and are less intercorrelated and temperature‐dependent than are the parameters in other models such as the UNIQUAC and NRTL equations, which apply to a more general class of solutions. The model describes observed extrema in the activity coefficients of aqueous solutions, thus pointing out that the behaviors of aqueous and non‐aqueous solutions are fundamentally different, and that aqueous solutions cannot be well described with nonaqueous solution models over the whole concentration range. The two‐parameter form of the new model can better predict binary vapor‐liquid equilibria from liquid‐liquid equilibrium data than other two‐parameter models, and gives better predictions of ternary liquid‐liquid equilibrium using only binary data. It is also shown that the model can be used to describe aqueous solutions of water‐soluble polymers.
Carbochlorination of dispersed oxides in a molten salt reactor
Authors: Dobbins Michael S. , Burnet George
Published: June 2004

Reaction mechanism and rates were determined from 803 to 1,073 K for the carbochlorination of Al2O3 and of Al, Si and Fe‐mixed oxides using Cl2 and C slurried in a stirred melt of NaCl—AlCl3. Alumina chlorination rates of 0.33–2.0 × 10−8 kmol/s were obtained using C:melt weight ratios between 1:21 and 1:42. The reaction rate was proportional to C loading at temperatures below 923 K, while gas‐liquid mass transfer was rate‐controlling at higher temperatures. Carbochlorination of the combined oxides in coal fly ash at 923 K and oxide conversions less than about 50% was also gas‐liquid mass‐transfer‐controlled. At higher conversions, dissolution became the rate‐controlling factor. A 40% (molar) AlCl3 melt was more effective than a 48% AlCl3 melt for the selective chlorination of Al2O3 over SiO2.
Enhancement in mass transfer due to motion of a particle: An experimental study
Authors: Verma A. K. , Rao D. P.
Published: June 2004

The understanding of the mechanism of wall‐to‐bed heat and mass transfer in liquid‐fluidized beds requires knowledge of the effect of the motion of individual particles on transfer processes. Experimental studies on the enhancement in mass transfer due to motion of a particle in the proximity of the transfer surface are presented. Mathematical relations based on a mechanistic model are obtained to describe the variation of the enhanced rate with time. The model parameters are correlated in terms of the particle Reynolds number, the wall‐to‐particle distance, and an interaction parameter that accounts for the background fluid motion. The results of this work would be of use in modeling transfer processes in other liquid‐solid systems.
Measurement of droplet interfacial phenomena by light‐scattering techniques
Authors: Taflin Daniel C. , Zhang S. H. , Allen Theresa , James Davis E.
Published: June 2004

Two light‐scattering techniques have been applied to study the kinetics of droplet evaporation and attendant interfacial phenomena. The first technique, which is particularly useful for noisy data, utilizes the fast Fourier transform of phase function data (intensity vs. scattering angle) to obtain the size by comparing the phase function with Lorenz‐Mie theory. The second method applies optical resonance measurements to obtain very precise measurements of size and/or refractive index as functions of time. The techniques have been used to measure the slow evaporation of multicomponent and single‐component organic microdroplets, rapid evaporation of water droplets, and rapid evaporation of droplets of aqueous solutions and surfactant solutions. The experiments were performed in an electrodynamic balance, which was used to suspend single microdroplets in a polarized laser beam. Rapid evaporation rate data yield information on the surface temperature, and data for surfactant solution evaporation show that insoluble monolayer formation at the microdroplet surface reduces the evaporation rate by a factor of 200. In addition, it is shown that droplet explosion due to surface charge effects can occur well below the Rayleigh limit of charge.
Analysis of equilibrium shift in isothermal reactors with a permselective wall
Authors: Mohan Krishna , Govind Rakesh
Published: June 2004

The objective of this study is to obtain a fundamental understanding of the behavior of reactors with a permselective wall (membrane reactors) in terms of: design parameters (reactor length, membrane thickness); operating variables (pressure ratio, feed flow rate); physical properties (rate constant, permeability of fast gas, permselectivity, equilibrium constant); and flow patterns (recycle, cocurrent, countercurrent). Pure feed reacts on the high‐pressure side of the membrane, and the product(s) formed are continuously removed to the low‐pressure side so that thermodynamic equilibrium is never reached. It is shown by simulation that equilibrium shift can be enhanced by: recycling unconverted reactant; shifting feed location to separate products; and maintaining high permeation rates to reduce backreaction. It is also shown that the choice between cocurrent flow and countercurrent flow depends on the system parameters.
Continuum theory for lumping nonlinear reactions
Authors: Chou M. Y. , Ho T. C.
Published: June 2004

A lumping procedure is proposed for continuous reaction mixtures whose components undergo nonlinear reactions. A species‐type distribution function is introduced to ensure that the lumped continuous mixture is kinetically consistent with the underlying lumped discrete mixture. It is shown that, if the mixture contains some virtually unreactive species, the lump after a long time will react at a second‐order rate. This is true as long as the underlying unlumped reactions are irreversible nth‐order with n < 2 or reversible first order. When many first‐order reversible reactions are lumped together, the apparent behavior of the lump can qualitatively look like that of a reversible reaction. Practical implications of these predictions are discussed. Some published data are reconciled in light of the present theoretical development.
Transformations and consistency relations of distillation control structures
Authors: Häggblom Kurt E. , Waller Kurt V.
Published: June 2004

Rigorous analytical methods for consistent modeling of control structures for multivariable processes are presented. The continuous distillation process is used to illustrate the methods, but the basic ideas, and more specifically the transformations and consistency relations derived, are valid in general.

If steady‐state operating data and the process gains of an arbitrary control structure are known, it is possible to calculate the process gains of any feasible control structure. A general expression relating the process gaisn of different control structures is derived. In general, the process gains must also satisfy certain consistency relationships which can be derived from first principles, e.g., steady‐state material balances.

The usefulness of the results is illustrated by control structure transformations and reconciliation of process gains, by an application to process dynamics, by synthesis of noninteracting control loops, and by derivation of analytical relationships useful in relative gain analysis.
Electrochemical measurement of effective diffusivity in Hall‐Heroult electrolyte
Authors: Burgman John W. , Sides Paul J.
Published: June 2004

Forced‐convection mass transfer at the metal/bath interface during aluminum reduction from cryolitic melts was studied under reproducible convective conditions. A film of molten aluminum on a rotating molybdenum cylinder was the cathode. Concentration overpotential measured as a function of rotation rate, current density, and bath composition was converted to concentration differences between the bulk and the metal surface. Chosen as the basis for calculation of a mass transfer coefficient was the concentration of aluminum fluoride given by:

where the amounts of indicated compounds are in weight percent, IMP designates impurities, M is molecular weight, ρ is the density of the melt, and CR is the cryolite ratio, the ratio of moles NaF to moles AlF3. Agreement with a correlation for mass transfer to a rotating cylinder allowed the calculation of effective diffusivities for aluminum fluoride species, in alumina‐saturated melts, of: 11.1 ± 1.1 × 10−5 cm2/s at 1.8 CR; 11.4 ± 1.7 × 10−5 cm2/s at 2.3 CR; 5.4 ± 0.8 × 10−5 cm2/s at 3.0 CR; and 4.4 ± 0.9 × 10−5 cm2/s at 4.0 CR.
Kinetics of spent activated carbon regeneration
Authors: Harriott Peter , Cheng Alan Tat‐Yan
Published: June 2004

Pyrolysis and regeneration kinetics of 11 spent activated carbons were studied using a thermal balance and a fludized bed reactor. The adsorbates were aromatic compounds with different types and number of side groups, and most of the side groups were lost on pyrolysis, leaving a residue about equal to the weight of the aromatic portion of the adsorbate. The residues from alkyl phenols reacted with oxygen at 770‐920 K at rates up to 12 times the rate for the base carbon. There was a similar increase in oxygen chemisorption indicating increased number of active sites. The adsorption capacity of the carbons was mostly recovered by pyrolysis, even though the surface area and pore volume were reduced by the residue. Some oxidative regeneration is probably necessary to restore adsorption capacity for repeated use.
Augmented BACK equation of state II: Polar fluid mixtures
Authors: Lee M. J. , Chao K. C.
Published: June 2004

The augmented BACK equation of state for polar fluids is extended to mixtures by introducing mixing rules separately for the nonpolar and polar interactions. Fluid phase equilibria are calculated for polar + nonpolar, and polar + polar mixtures including mixtures of hydrogen‐bonding substances. Enthalpy calculations are illustrated with enthalpy of H2S + N2 and heats of mixing of steam mixtures, and carbon dioxide mixtures.

Cross interaction coefficients are adjusted for the correlation of data on binary mixtures. Both phase equilibrium and enthalpy data are correlated at the same time with two such coefficients for each binary. The cross interaction coefficients are constants, invariant with temperature, density, or phase state.
Redox catalysis of absorption from reactive bubbles near electrodes
Authors: Takahashi Ken M. , Alkire Richard C.
Published: June 2004

Electrolyzing sparingly soluble gases as they are absorbed from bubbles can be significantly facilitated by addition of a soluble redox couple which serves to increase both mass and charge transfer rates. A diffusion layer model is used to calculate reaction‐catalyzed mass transfer rates in regions of boundary layer penetration by reactive bubbles for the case of (m, n)‐order irreversible homogeneous reaction. The dissolved redox catalyst is continuously regenerated at the solid electrode surface. Approximate solutions, which relate surface flux to concentration driving force, are in excellent agreement with the numerical solutions under conditions of fast homogeneous reaction and concentrated catalyst (modified Damköhler number and dimensionless catalyst concentration greater than 3). Diffusion layer thickness is calculated from the mass transfer coefficient kp in penetrated regions. A model based on spherical penetrating bubbles is used to determine kp and specific surface area ap of penetrated regions from kpap values that have been measured in gas‐liquid flow through a packed bed. The mass transfer behavior provides useful insight into why the penetration process is so effective in increasing overall reaction rates.
Input/output linearization: A nonlinear analog of placing poles at process zeros
Authors: Kravaris Costas
Published: June 2004

This paper deals with SISO nonlinear processes and their control with state feedback. The Byrnes‐Isidori concept of non‐linear zeros is first reviewed and illustrated with two chemical engineering exemples. This concept is then used to interpret input/output linearizing state feedback as a nonlinear analog of placing poles at the process zeros. This interpretation leads to closed‐loop internal stability conditions for nonlinear processes under input/output linearizing state feedback.
A thermodynamic model for predicting wax formation in crude oils
Authors: Hansen Jens H. , Fredenslund Aa. , Pedersen Karen Schou , Rønningsen Hans Petter
Published: June 2004

Most crude oils contain high molecular weight components, which at low temperatures may precipitate as a wax phase. This may cause plugging of pipes and numerous other problems. This paper presents a solid‐liquid equilibrium based model for the description of wax formation. The model for the Gibbs energy contains a contribution based on Flory's theory of multicomponent polymer solutions and a contribution from a metastable subcooled state which oil mixtures may attain. The latter is formulated in terms of the surface tension of the wax phase. Experimental wax appearance points (temperatures), WAP's, are reported for 17 different stabilized North Sea crude oils. The values predicted by the new model are in very good agreement with the experimental WAP's.
Calculation of infinite‐dilution partial molar properties by computer simulation
Authors: Shing Katherine S. , Chung Sang‐Tae
Published: June 2004

Partial molar properties at infinite dilution for binary Lennard‐Jones mixtures are calculated using Monte Carlo simulation methods. The variation of these properties with density, temperature, and intermolecular force parameters is systematically studied. Simulation pair correlation functions are used to examine the structure of the mixtures at the molecular level. Results are also compared to predictions of the van der Waals I conformal solution theory.

Partial molar volume and internal energy are found to be strongly dependent on the isothermal compressibility of the mixture and on the proximity of the mixture state condition to phase boundaries. The van der Waals I conformal solution theory gave good results for the infinite‐dilution chemical potential μ and reasonable results for the partial molar internal energies and volumes. However, it is suspected that this is limited to the infinite‐dilution case, where the isothermal compressibility of the mixture is well represented by the pure‐fluid equation of state.
Impact of model uncertainty descriptions for high‐purity distillation control
Authors: McDonald Karen A. , Palazoglu Ahmet , Bequette B. Wayne
Published: June 2004

The ways in which modeling uncertainties are described for a particular process critically affects the results obtained in robustness studies. In this paper, four multivariable robust stability methodologies are used to characterize and analyze the effects of model inaccuracy due to non‐linearity in high‐purity distillation processes. The unstructured and structured singular value, numerical range, and a mapping of det(l + GPGc) are compared in terms of their ability to predict the stability of the dual‐composition control system over a wide composition range. The importance of using uncertainty descriptions that include a realistic representation of the phase‐magnitude relationship as well as the corrections between uncertainties in each element of the model is demonstrated. The conservation associated with norm‐bounded uncertainty descriptions reveals itself by the extent of detuning needed to insure stability and the subsequent degradation in control performance.
Forced periodic control of an exothermic CSTR with multiple input oscillations
Authors: Rigopoulos Konstantinos , Shu Xianshu , Çlinar Ali
Published: June 2004

Interaction among various oscillating inputs may result in significant modifications in the behavior of a system under forced periodic control. Forced oscillations in the input flow rate and input concentrations of an exothermic continuous stirred tank reactor enable the stabilized operation of the CSTR in the unstable steady state region. Reactor temperature oscillations under forced periodic control are similar to the oscillations resulting from proportional‐integral feedback or nonlinear (push‐pull) feedback control, and under some operating conditions the oscillation amplitude is significantly lower. Theoretical and experimental studies illustrate the effects of forcing frequency and phase shift on reactor behavior.
Partitioning of nonspherical molecules between bulk solution and porous solids
Authors: Limbach Kirk W. , Nitsche Johannes M. , Wei James
Published: June 2004

Monte Carlo simulation is used to calculate partition coefficients for rigid, axisymmetric, nonspherical molecules in various pore geometries. Novel quantitative results are presented for a variety of molecule and pore shapes, including pores of nonuniform cross‐section such as the interstitial space within granular and fibrous materials.

Quick estimation methods for obtaining partition coefficients are discussed, and the effects of molecule and pore wall curvature are described in detail. A boundary perturbation analysis is used to derive easy‐to‐use formulas for a variety of molecule shapes in circular pores.
Uptake of phenylalanine and tyrosine by a strong‐acid cation exchanger
Authors: Saunders Malcolm S. , Vierow John B. , Carta Giorgio
Published: June 2004

The equilibrium and rate of uptake of the amino acids phenylalanine and tyrosine by Amberlite 252, a strongly acidic, cation‐exchange resin, have been investigated. Uptake of the amino acids by the hydrogen form of the resin occurs primarily by the stoichiometric exchange of hydrogen ions and amino acid cations. The amount of amino acid taken up by the resin can be calculated as a function of solution pH and amino acid concentration from a model that takes into account both solution and ion‐exchange equilibria. The rates of uptake of the two amino acids have been determined experimentally for a closed batch system. The results of experiments in which the resin particle size, the flow rate, and the concentration were varied show that intraparticle transport is dominated by the slow diffusion of amino acid cations through the macroreticular polymer structure of the resin, with some contribution from macropore transport of amino acid cations and zwitterions. An intraparticle diffusion model has been developed to describe these results and predict the performance of fixed‐bed operations.