bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics of nucleation from aqueous solution
Authors: Mohanty Rita , Bhandarkar Suhas , Estrin Joseph
Published: June 2004

Homogeneous nucleation is achieved in aqueous systems by creating adequately high levels of supersaturations. This process can be fully described by considering depletion of supersaturation due to simultaneous growth of the nuclei. Nielsen (1964) formulated this by integrating the growth kinetics with classical nucleation theory. This paper uses his combined nucleation‐growth approach, as well as more recently developed mechanisms of adding monomers to developing nuclei (Chiang et al., 1988). The experimental data considered suggest that the systems fall into two categories: one involving the same mechanism for nucleation and growth and the other involving different mechanisms. A dimensionless parameter which evolves in the development permits the comparison of the rate of nuclei formation with that of nuclei growth. The inferences from this approach suggest further that the surface energy parameter is an indicator of the category to which a solution system belongs.
Mercury‐sensitized photochemical vapor deposition of amorphous silicon
Authors: Albright D. E. , Saxena N. , Fortmann C. M. , Rocheleau R. E. , Russell T. W. F.
Published: June 2004

A reaction engineering model has been developed to describe the mercury‐sensitized photochemical vapor deposition of hydrogenated amorphous silicon (a‐Si:H) semiconductor thin films. Model equations governing the gas‐phase generation, transport, and surface reactions of SiH3 and H film precursor radicals are solved to predict film growth rate and bonded hydrogen content. Behavior of the model has been studied as a function of deposition conditions (pressure, temperature, feed composition, and flow rates) and has been verified by comparison with experimental results.
Analysis of liquid chromatography with nonuniform crystallite particles
Authors: Lin Y. S. , Ma Y. H.
Published: June 2004

A mathematical analysis for the pulse responses of a liquid chromatographic column packed with crystal powders having a particle size distribution and a nonlinear adsorption isotherm is presented. The mathematical model is solved numerically by the orthogonal collocation method. Based on the parametric analysis of the model, the effects of a symmetrical and moderately asymmetric PSD on the LC responses are shown to be negligible in comparison with the effects of other parameters, such as isotherm nonlinearity, whose effects are much more profound. The simulated responses are compared with the experimental response data for an LC column packed with silicalite crystals, and a good agreement is found between the theoretical and experimental results. Using the nonlinear LC model, the simultaneous determination of nonlinear adsorption isotherms and intraparticle diffusivities from LC pulse responses is demonstrated for liquids in porous adsorbents.
Application of the group contribution lattice‐fluid EOS to polymer solutions
Authors: High Martin S. , Danner Ronald P.
Published: June 2004

A new group contribution lattice‐fluid equation of state (GCLF‐EOS) is described that can accurately predict the activities of solvents in polymer solutions. This equation of state is a modification of the equation of state derived by Panayioutou and Vera (1982), which is based on the lattice statistics developments of Guggenheim (1952). The group contribution modification permits the prediction of solvent activity, given only the structure of the polymer and solvent involved. The GCLF‐EOS can accurately predict solvent activities in polystyrene, polyethylene, poly(ethylene oxide), poly(ethylene glycol), poly(propylene oxide), and poly(vinyl chloride). The model does not perform as well for solvent activities in polyisobutylene due to the inaccurate group contribution values for the quaternary carbon group.
NMR imaging of saturation during immiscible displacements
Authors: Mandava Shanthi S. , Watson A. Ted , Edwards Carl M.
Published: June 2004

The use of magnetic resonance imaging (MRI) for monitoring multiphase displacement experiments for quantitative characterization of fluid saturation is demonstrated. Displacements are conducted with one fluid phase in a porous medium being immiscibly displaced by another. Our objective is to accurately measure porosity and saturation distributions corresponding to one spatial dimension. Measures for the accuracy and resolution, with which the properties are identified, are developed.
Vector processing strategies for chemical process flowsheeting
Authors: Vegeais James A. , Stadtherr Mark A.
Published: June 2004

The potential for effective vectorization of the sequential‐modular, simultaneous‐modular, and equation‐based approaches to process flowsheeting is assessed. The last appears to be the most promising, but the required sparse matrix techniques currently used do not vectorize well. Different approaches to vectorizing the sparse matrix calculation are considered. With a good matrix reordering scheme, the most promising sparse matrix strategy for the vectorization of flowsheeting problems is the frontal method, a technique currently used primarily on finite‐element problems.
Diffusion‐controlled surfactant adsorption studied by pendant drop digitization
Authors: Lin Shi‐Yow , McKeigue Kevin , Maldarelli Charles
Published: June 2004

Pendant drop tensiometry enhanced by video‐image digitization is shown to be a useful tool for the experimental measurement of the relaxation in interfacial tension due to the adsorption of surfactant at a fluid interface. Using this method, profiles of the relaxation in surface tension of a diffusion‐controlled, nonionic polyethoxy surfactant were measured. A diffusion coefficient was computed by comparing these profiles with numerical solutions of the bulk surfactant diffusion equation and a Frumkin equilibrium adsorption isotherm. This comparison was made for the entire relaxation period. This method establishes a more reproducible diffusion coefficient than current techniques that utilize only the short‐ or long‐time parts of the relaxation spectrum. In addition, lower bounds on the kinetic constants for the sorption process are inferred for the polyethoxy surfactant used by comparing numerical solutions of mixed diffusion and surface kinetic transfer with the diffusion‐limited result.
CO oxidation on Pt: Variable phasing of inputs during forced composition cycling
Authors: Graham William R. C. , Lynch David T.
Published: June 2004

A combined experimental and theoretical investigation of the effect of forced feed composition cycling for CO oxidation on platinum has been performed. A novel approach to forced composition cycling was examined, in which the phase angle between the two input streams was varied. Reaction rate enhancement is shown to occur, and by varying the phasing of the feed streams it is possible to achieve a global maximum in the time‐average reaction rate. This phenomenon can be explained quantitatively by a model based on an adsorbate‐induced phase change of the Pt surface combined with CO adsorption self‐exclusion. This mathematical model can also quantitatively describe the complex steady‐state behavior (uniqueness‐multiplicity transitions) observed for this reaction. The predictions of the model have been validated further through a detailed experimental study of the effects of feed flow rate, temperature, size of catalyst charge, and cycling frequency on the instantaneous and time‐average conversions during forced cycling of the feed composition.
Microcomputer‐automated reactor for synthesis of 13C‐labeled Monosaccharides
Authors: Stafford Ulick , Serianni Anthony S. , Varma Arvind
Published: June 2004

For less labor‐intensive, larger‐scale, and more efficient production of D‐[1‐13C] glucose and other labeled monosaccharides using the cyanohydrin reduction reaction, an automated reactor was designed, constructed, and tested. A suitable vessel and all ancillary and control equipment were fabricated, or otherwise obtained, and the apparatus was controlled with a microcomputer program written in BASIC. Running without manual intervention, the yields for the condensation reaction of D‐arabinose with KCN and the recovery of the excess cyanide were high (>90%), and the resultant aldononitriles were reduced to product aldoses with little reduction of the intermediate imines to amines. The reduction rate could be improved by controlling pH between 2–4 and by agitation speeds >250 rpm, while no improvement resulted from the use of high hydrogen pressures [> 129 kPa (4 psig)]. Reduction reactions conducted in the presence of formic acid (to regulate pH) suggest that formic acid acted as a coreducing agent.
Thermodynamics of Lewis acid‐base mixtures
Authors: Economou Ioannis G. , Ikonomou George D. , Vimalchand P. , Donohue Marc D.
Published: June 2004

The associated perturbed‐anisotropic‐chain theory (APACT) has been applied to treat multicomponent mixtures, in which components exhibit Lewis acid‐base interactions. Mixtures of amphoteric molecules (such as alcohols), acidic molecules (such as chloroform), and basic molecules (such as ketones) as well as nonassociating components (diluents) are treated. The acid‐base version of the perturbed‐anisotropic‐chain theory (ABPACT) is a closed form equation of state that takes into account explicitly dispersion interactions, polar interactions, and hydrogen bonding interactions between components that self‐associate or solvate. The equation fits binary VLE mixtures quite accurately. Calculated results also are compared with those obtained from the Peng‐Robinson equation of state, UNIFAC and the original APACT. In all cases where there are mixtures of associating components, the ABPACT gives a better fit of experimental data than these other equations.
Interrelating excess thermodynamic functions using fusion properties
Authors: Hansen Allen R. , Eckert Charles A.
Published: June 2004

A new model that interrelates the liquid‐phase excess Gibbs energy, enthalpy and volume, called the GHV model, is developed and discussed. The model is based on the fact that most mixtures are immiscible in solid phases and, therefore, have steep solid‐phase boundaries. This single assumption allows the model to be applied potentially to all types of mixtures, including organic, metallic, electrolytic and polymeric solutions. The model, which predicts that excess properties are related to one another through various fusion properties, pure component volumes, and enthalpies of fusion in particular, has been applied to a wide variety of organic and liquid metal mixtures. The results show at least qualitative validity without adjustable parameters and more quantitative ability with “effective” fusion properties. The model is also shown to rationalize some sign, magnitude and shape phenomena.
Void wave propagation phenomena in two‐phase flow (Kern Award Lecture)
Authors: Lahey R. T.
Published: June 2004

A detailed linear and nonlinear analysis of void wave phenomena in two‐phase flow is presented. A state‐of‐the‐art, one‐dimensional two‐fluid model was used as the basis for this analysis. It is shown that void wave dispersion is strongly influenced by phasic slip. In particular, the smaller the slip the more dispersion occurs. It has also been found that nonlinear wave forms, such as shocks and solitons, may occur. Moreover, the virtual mass force was found to be an important parameter in the modeling of void waves. It is shown that void wave analysis is an excellent means of assessing the closure assumptions used in two‐fluid models. It is also implied that carefully taken void wave data are needed for this purpose.
Kinetic invariant model of dissolution with chemical reaction of large particles
Authors: Bhaskarwar Ashok N.
Published: June 2004

The kinetic invariant model has been reformulated and an approximate analytical form of a kinetic function is obtained for the dissolution accompanied by a chemical reaction of large solid particles. A restriction on the analytical solution is provided in the form of a sufficient condition for the convergence of the series involved. Expressions for the concentration of unreacted, dissolved solid reactant, the time of complete conversion, and the ratio of driving forces available for the reaction and mass transfer steps are presented. Solutions for the rate‐limiting regimes are deduced as special cases of the general solution. The small‐particle solution is recovered as a special case in the limit of a vanishingly small Reynolds number. Rigorous numerical solution has been obtained to assess the accuracy of the pseudostationary‐state solution and to discern the validity of the approximations.
Effect of dispersion quality on particulate magnetic recording disk properties
Authors: Shah M. J. , Karis T. E. , Cuka G. M.
Published: June 2004

The production of particulate magnetic disks or tapes requires consistent and uniform magnetic characteristics over the entire disk or tape to have as few defects as possible. To achieve this goal, the magnetic particle suspension used to coat the disk must be well dispersed. This article describes the use of a rheomagnetic instrument that provides an indirect measurement of the dispersion quality prior to coating application. The instrument was designed, tested, and put on a manufacturing scale line to characterize the relationships between the measured dispersion quality and functional characteristics of the magnetic coating. The results of tests done during scaleup for production yielded a qualitative correlation between the in‐situ measurements made on the magnetic particle suspension at the coater with the surface roughness due to flocs, the on‐disk orientation ratio, and the signal to noise ratio. The current results are the first demonstration of the dispersion quality measurement in a practical manufacturing environment.
Curvature and parametric sensitivity in models for adsorption in micropores
Authors: Saito A. , Foley H. C.
Published: June 2004

The sensitivity of the calculated micropore size of zeolite Y in a fluidized cracking catalyst based on empirical models for argon adsorption has been tested by examining the effect of curvature and by systematically verifying the magnitude of physical constants in the model equations. With a consistent set of physical parameters the slit model provided a pore size value of 0.45 nm, while the new cylindrical models provided values of 0.69 and 0.74 nm. The latter values are found to correspond well with the known aperture size of zeolite Y, 0.74 nm. By separately varying the magnitudes of five of the physical constants in the model over a range of ±30%, it was concluded that the diameter of the oxide ion at the surface had a large effect on the calculated pore size, while the other parameters had only moderate to small effects. Preliminary application of the cylindrical pore model to isotherms of argon on other zeolites and molecular sieves leads to promising results, especially for medium to large pore zeolites. These results suggest that the cylindrical pore model is a useful means for the transformation of argon adsorption data on a zeolite into a micropore size distribution.
Prediction of the three‐dimensional turbulent flow in stirred tanks
Authors: Kresta Suzanne M. , Wood Philip E.
Published: June 2004

Efforts to model the turbulent flow in stirred tanks require accurate boundary conditions at the tip of the impeller, not just of velocities, but of the turbulence quantities k and ε. Kolar's (1982) phenomenological, swirling radial jet model of the impeller region is extended by using a two‐equation k – ε turbulence model to obtain direct estimates of k and ε on the impeller periphery. The model is extended and clarified, so that the number of parameters required for its application is reduced to two: the rotational speed and the diameter of the impeller. Three‐dimensional simulations allow a realistic treatment of the baffles. Agreement of the modeling results with recently published experimental data is excellent. This is particularly true in the important impeller discharge zone, where details of the predicted behavior of the turbulence kinetic energy and dissipation rate are in quantitative agreement with the available data. Based on these results, average values of ε are calculated, along with the zones over which the apply. For the impeller discharge zone, the dimensionless, volume‐averaged ε is 0.19.
Methods of deterministic chaos applied to the flow of thin wavy films
Authors: Lacy C. E. , Sheintuch M. , Dukler A. E.
Published: June 2004

The structure of thin, wavy falling films was studied to evaluate whether the random‐appearing wave structure is a result of deterministic chaos or a purely stochastic process. The time‐varying film thickness was obtained at different spatial locations near the point of wave inception for flow rates in the range of Re=3–10. Under all conditions the wave structure was aperiodic in nature and displayed none of the known transitions to chaos. However, the power spectra followed an exponential decay law at high frequencies that is characteristic of chaotic systems. The estimated attractor dimension, used to characterize the complexity of a chaotic system, was much higher than those of known model chaotic systems. It is demonstrated that these high values could be explained due to small levels of noise present in experimental situations. Since experimental data are seldom noise free, a basic limitation in applying these methods to experimental measurements is demonstrated.
Multiple steady states in ideal two‐product distillation
Authors: Jacobsen Elling W. , Skogestad Sigurd
Published: June 2004

Simple distillation columns with ideal vapor‐liquid equilibrium may display multiple steady‐state solutions. Two fundamentally different sources for the multiplicity are presented. Both bring about the unexpected result that increasing reflux makes separation worse in the top part of the column. It corresponds to an unstable operating point.

The first type of multiplicity is found for columns with mass or volume inputs (e.g., mass reflux and molar boilup). Even for constant molar flows, the transformation from the actual input units to molar units may become singular (corresponding to a pitchfork bifurcation point), resulting in multiple steady‐state solutions. The results are highly relevant in practice, as industrial columns usually have inputs on a mass or volume basis. The second type for specifications on a molar basis (e.g., molar reflux and molar boilup) depends on the presence of an energy balance in the model. The multiplicity is caused by interactions between flows and compositions in the column.
Effects of protein aggregation in isocratic nonlinear chromatography
Authors: Whitley Roger D. , Van Cott Kevin E. , Berninger James A. , Wang N.‐H. Linda
Published: June 2004

A versatile reaction‐separation (VERSE) model was developed to quantitatively simulate the behavior of chromatographic separations coupled with reactions. Detailed mass transfer and reaction mechanisms are considered. Aggregation data of myoglobin and β‐lactoglobulin A verified the model. The effects of concentration, equilibrium distribution, reaction rate, convection rate, particle radius, and relative affinity are shown for a dimerizing system. When the aggregation rate is relatively slow compared with convection and mass transfer rates, the individual forms behave as separate species in frontal, elution, and displacement chromatography. For rapid aggregation rates, the individual forms behave as a single component with an average affinity. The wave asymmetry and increased spreading due to aggregation depend on relative affinity differences. Serious error may result if aggregation is overlooked in parameter estimation using frontal or pulse analysis. The dimensionless group principles developed here are useful in scaling and predicting when peak or wave splitting or merging will occur in reaction chromatography systems.
Extent of reaction in open systems with multiple heterogeneous reactions
Authors: Friedly John C.
Published: June 2004

The familiar batch concept of extent of reaction is reexamined for systems of reactions occurring in open systems. Because species concentrations change as a result of transport processes as well as reactions in open systems, the extent of reaction has been less useful in practice in these applications. It is shown that by defining the extent of the equivalent batch reaction and a second contribution to the extent of reaction due to the transport processes, it is possible to treat the description of the dynamics of flow through porous media accompanied by many chemical reactions in a uniform, concise manner. This approach tends to isolate the reaction terms among themselves and away from the model partial differential equations, thereby enabling treatment of large problems involving both equilibrium and kinetically controlled reactions. Implications on the number of coupled partial differential equations necessary to be solved and on numerical algorithms for solving such problems are discussed. Examples provided illustrate the theory applied to solute transport in groundwater flow.
Mass transfer and axial dispersion in a Kühni extraction column
Authors: Dongaonkar Kishor R. , Pratt H. R. Clive , Stevens Geoffrey W.
Published: June 2004

Concentration profiles were determined in both phases for the system methyl isobutyl ketone (MIBK)—water‐acetic acid in a Kühni 72.45‐mm‐dia. Extraction column of 25 stages. Mass transfer and backmixing parameters were estimated numerically from the profiles in terms of the backflow model. The results indicated that backmixing of the continuous phase was appreciable, while that of the dispersed phase was negligible. Independent values of the continuous‐phase backmixing coefficient, αc, were also determined by both unsteady‐state and continuous tracer injection, in the former case both with and without mass transfer. The results indicated that αcis increased by mass transfer, and that the tracer values, both with and without mass transfer, are greater than those obtained from the profile data.
Nonlinear wave theory for dynamics of binary distillation columns
Authors: Hwang Yng‐Long
Published: June 2004

A major difficulty in control of distillation columns stems from their nonlinear behavior, especially of columns producing high‐purity products. Based on a previous development that adapted fixed‐bed theories to simple countercurrent processes, this study has established a nonlinear wave theory for the nonlinear and distributed dynamics of binary distillation columns by including the effects of reflux, reboil, and combination of sections. This theory clearly elucidates the intriguing dynamic behavior of distillation columns such as high steady‐state gains, large response lags, strong dependence of dynamics on the magnitudes and directions of disturbances, and asymmetric dynamics of the transitions between two steady states. It also underlines the inadequacy of conventional linearized models for high‐purity columns. With fairly simple mathematics, this nonlinear wave approach may be useful for model development, control system design, dual‐composition control, and feedforward control.
Slit devolatilization of polymers
Authors: Maffettone Pier Luca , Astarita Gianni , Cori Laura , Carnelli Lino , Balestri Franco
Published: June 2004

A model of the process of polymer devolatilization in a heated slit is presented. Momentum, heat, and mass transfer in the slit are taken into consideration, and only one adjustable parameter is used. The model predicts pressure and temperature profiles, and residual volatile contents at the exit which are in good agreement with those measured in a single instrumented slit.
Flow characteristics of waxy crude oils: Application to pipeline design
Authors: Wardhaugh L. T. , Boger D. V.
Published: June 2004

Waxy crude oils are highly non‐Newtonian materials known to cause handling and pipelining difficulties and whose flow properties are time‐ and history‐dependent. Experimental techniques are described that enable reproducible steady‐state flow property data to be obtained from rotational viscometers. The flow properties are shown to depend strongly on the shear rate applied during cooling (shear history effect). This leads to a definable minimum operating point below which flow in a waxy crude oil pipeline would cease. Modified pipeline design techniques are presented for both laminar and turbulent flow at temperatures below the pour point, and it is shown that existing techniques overestimate the flow rate in laminar flow by the order of 100%. The modified design techniques can be used to quantitatively assess the performance of flow improver (pour point depressant) additives under steady‐state conditions.
Schedule optimization with simultaneous lot sizing in chemical process plants
Authors: Musier Reiner F. H. , Evans Lawrence B.
Published: June 2004

Operational problems are of increasing importance as the process industries turn to the batch manufacture of high‐value specialty products. A method is presented to minimize stockout for multiunit plants where customer orders may constitute nonintegral batch multiples. Product‐ and unit‐dependent processing times and batch sizes, sequence‐dependent changeover times, and product‐to‐unit assignment constraints are modeled for nonidentical parallel processes. Arbitrary initial conditions are also modeled. The algorithm determines the number of production runs of each product, run length (lot sizing), the assignment of run to units, and run sequence on each unit.

Scheduling with simultaneous lot sizing has not previously been addressed in the literature. The evolutionary method presented here obtains near‐optimal (and often globally optimal) solutions on industrial‐sized test problems. These results are significant since near‐optimal scheduling of many industrial process plants now becomes possible, although problems of this complexity are intractable using exact methods.
A Monte Carlo analysis of acid hydrolysis of glycosidic bonds in polysaccharides
Authors: Pinto José‐Henrique Q. , Kaliaguine Serge
Published: June 2004

Kinetic experiments for the hydrolytic depolymerization of cellobiose and amylose were performed in acidic medium. In the case of amylose, only the production rate of glucose was monitored. Then, a Monte Carlo procedure was developed to simulate long‐chain saccharide acid hydrolysis using the kinetic information obtained from the model compound, cellobiose. This stochastic procedure, which is applicable to the homogeneous acid hydrolysis of highly amorphous polysaccharides of sufficient degree of polymerization, provides estimated curves for the time evolution of all intermediate products. Literature data on the hydrolysis of oligoxylans were also compared to the results obtained from the Monte Carlo procedure. The depolymerization of linear polysaccharides, which should normally be viewed as a nonhomogeneous Markov chain, could be considered as a homogeneous Markov chain, provided that the chain is visited at specifically determined time intervals.
Model for spherical cavity radii and potential functions of sorbates in zeolites
Authors: Baksh M. S. A. , Yang R. T.
Published: June 2004

A spherical pore model for calculating the zeolite cavity radius and the potential energy of interactions of noble gases (Ar, Kr, Xe) is presented. The model utilizes the gas adsorption isotherm and equates the free energy change upon adsorption to the potential energy of interaction of the gas molecules with the zeolite cavity. The results obtained for the cavity radius using Ar, Kr, and Xe adsorption isotherms are in excellent agreement with those of the cavity radius for 5A, 13X, and Y zeolites obtained from crystallographic data. Also, the average potential energies of interaction of these gases with the surface atoms of the zeolite cavities, calculated from the isotherms and the spherical pore model, compare reasonably well with the internal energies determined from the experimental heats of adsorption.
Comparative analysis of qualitative models when the model changes
Authors: Grantham S. D. , Ungar L. H.
Published: June 2004

An important application of the recently developed techniques in qualitative mathematical modeling is to qualitatively predict how changes in the operation of chemical units affect their behavior. Weld (1987, 1988a,b) has developed a series of comparative techniques that analyze the effect of perturbations to the parameters of a given qualitative unit model. In this article we demonstrate a system, based on the qualitative process theory of Forbus (1984), which extends comparative analysis in two ways. First, it predicts the effects of changes in the qualitative equations and in parameter values. Secondly, it compares physical descriptions, rather than comparing models directly, and builds and compares the associated models automatically.
Periodic forcing of the CSTR: An Application of the generalized II‐criterion
Authors: Sterman Leah E. , Ydstie B. Erik
Published: June 2004

Nonlinearities give the opportunity to influence the time average performance of a system by periodic forcing of external parameters. In this article, this concept is applied to the following reactions occurring in a continuous stirred tank reactor: A → B, A → C; A ⇋ B; A → B → C; and A → B, A + B → C. It is shown, using a generalized II‐criterion, that if the activation energies satisfy the constraint E1 > E2, then the yield of B is improved using high‐frequency periodic perturbations in the temperature regardless of reaction order and operating conditions. Conversely, if E1 > E2, then high‐frequency forcing always leads to a deterioration of the yield. There exist regions of parameter space where low‐frequency forcing may lead to an improvement. Results valid for low and intermediate frequencies are developed and can be used to predict additional areas of parameter space of interest for periodic operation.
Axial mixing in a reciprocating plate column with very small density gradients
Authors: Baird Malcolm H. I. , Rao N. V. Rama
Published: June 2004

Axial dispersion coefficients have been measured in single‐phase flow conditions in a 5.08‐cm‐dia. reciprocating plate extraction column. The measurements were done under steady‐state conditions by analyzing temperature profiles (for hot and cold water mixing) and concentration profiles (for mixing of water and salt solutions). The results confirm previous published data of Holmes et al. (1991) in showing that axial dispersion is increased strongly in the unstable situation, where liquid density increases with height. The earlier work is extended to the condition where buoyant energy dissipation is exceeded by mechanical energy dissipation by four orders of magnitude. Even in this case, axial mixing is increased significantly by density gradient‐induced instability. Results have been correlated by a simple equation based on a mixing length model.
Knudsen diffusivities and properties of structures of unidirectional fibers
Authors: Tomadakis Manolis M. , Sotirchos Stratis V.
Published: June 2004

Effective Knudsen diffusion coefficients are presented for fibrous structures consisting of parallel, nonoverlapping or partially overlapping fibers. They are computed by means of a Monte Carlo simulation scheme which is employed to determine the mean square displacement of molecules travelling in the interior of the porous medium for large travel times. The results show that structures of paralle, non‐overlapping fibers have smaller effective diffusion coefficients parallel to the fibers than structures of parallel, randomly overlapping fibers of the same porosity and fiber radius, but larger in directions perpendicular to the fibers. Partially overlapping fiber structures are found to exhibit behavior intermediate to those of the two extreme cases. Molecular trajectory computations are also used to obtain results for the structural properties of partially overlapping fiber structures (e.g., porosity and internal surface area, accessible porosity and internal surface area, and percolation threshold), which are compared with some results of the literature for the equivalent problem of partially overlapping disks on a plane.
Transient deformation of freely‐suspended liquid droplets in electrostatic fields
Authors: Haywood R. J. , Renksizbulut M. , Raithby G. D.
Published: June 2004

The problem of freely‐suspended liquid droplets deforming due to an applied electrostatic field is examined. Developed is a numerical model capable of predicting the complete transient histories of droplets in systems with a wide range of dispersedand continuous‐phase densities, viscosities, relative permittivities, and electric field strengths. For liquid/gas systems, the predictions of the numerical model demonstrated that the critical field strength and critical permittivity ratio during actual transient breakup are not necessarily the same as those predicted by steady‐state theories. An approximate analytical model of transient droplet deformation is also developed, which is able to predict the deformation time histories for large Ohnesorge number, small‐deformation liquid/liquid systems. The approximate analytical model agreed well with the results of the complete numerical model.
Multicomponent adsorption and chromatography with uneven saturation capacities
Authors: Gu Tingyue , Tsai Gow‐Jen , Tsao George T.
Published: June 2004

In chromatographic separations involving elutes with large differences in the molecular size, the adsorption saturation capacities of the elutes may differ because of the differences in the degree of size exclusion. With uneven saturation capacities, isotherm crossovers may occur, which often results in selectivity reversal. In this work, a new multicomponent isotherm has been developed for this kind of system. The isotherm is an extension of the common multicomponent Langmuir isotherm and introduces no or a very limited number of new parameters for its construction. The isotherm crossover conditions have also been derived. Simulations based on a general rate model using the new isotherm have successfully demonstrated the phenomena of peak reversal and crossover of breakthrough curves.
A series solution for mass transfer in laminar flow with surface reaction
Authors: Bašić Andjelko , Duduković Milorad P.
Published: June 2004

We present an explicit analytical solution for the Lévêque's problem with the boundary condition of the third kind. This solution is applicable to problems of mass (heat) transfer with surface reaction (surface resistance) in the entry region of fully developed flow fields of power law fluids, and to the developing boundary layer flows that admit Falkner‐Skan solutions, provided that the Schmidt (Prandtl) number is large. The series form of the solution developed by inversion of the Laplace transform has excellent convergence properties within the concentration (temperature) boundary layer in contrast to the integral forms that are usually reported for problems of this type. An efficient computational algorithm for evaluation of the surface concentration is presented, as well as accurate approximate formulas in the form of simple algebraic expressions for the local and average mass (heat) transfer coefficients and the surface concentration (temperature).
Modeling and simulation of a honeycomb reactor for high‐severity thermal cracking
Authors: Heynderickx G. J. , Froment G. F. , Broutin P. S. , Busson C. R. , Weill J. E.
Published: June 2004

A new steam cracking technology has been developed that uses honeycomb‐type ceramic reactors with high surface to volume ratios and is heated by flue gas. “Rectangular” process gas temperature profiles are possible with such a reactor. An optimal combination of temperature profile and residence time leads to a very significant increase in olefins yield with respect to conventional cracking units. Benchscale results are compared with predictions based on a reactor model, which accounts in great detail for the geometry of the structure and the associated heat transfer, combined with a rigorous kinetic model based on radical reaction mechanisms.
Equation of state for pure fluids and mixtures based on a truncated virial expansion
Authors: Anderko Andrzej , Pitzer Kenneth S.
Published: June 2004

An accurate, yet simple, equation of state was developed for pure fluids ranging from nonpolar to H2O and their mixtures. The equation consists of a virial expansion truncated after the fourth virial coefficient and a closed‐form term approximating higher coefficients. For nonpolar or weakly polar compounds, the equation is generalized using critical temperature, critical pressure, and acentric factor. Despite its simplicity, the equation represents the P‐V‐T and saturation properties of pure fluids with a very good accuracy, which is comparable to that of multiparameter, generalized BWR‐type equations. For mixture calculations, mixing rules were formulated based on the rigorously known composition dependence of virial coefficients. The equation represents vapor‐liquid, gas‐gas and liquid‐liquid equilibria, as well as volumetric properties of mixtures containing nonpolar components and/or water. Binary parameters can be generalized for series of mixtures containing a common component.
Baffle effects on performance of catalytic circulating fluidized bed reactor
Authors: Jiang Peijun , Bi Hsiaotao , Jean Rong‐Her , Fan Liang‐Shih
Published: June 2004

Baffle effects on the performance of a catalytic circulating fluidized bed reactor were examined experimentally. The circulating fluidized bed reactor or riser was 102 mm in diameter and 6.32 m in height. Reaction was the catalytic decomposition of ozone using FCC particles with a mean diameter of 89 μm, impregnated with ferric oxide as catalysts. Four ring‐type baffles, mounted horizontally around the riser wall, were used in this study. Ozone concentrations were measured in both axial and radial directions under various operating conditions in a riser with and without baffles. Experimental results showed that in a riser with baffles, the ozone concentration in the radial direction was more uniform and the ozone conversion was higher than that in a riser without baffles except at the lowest gas velocity used. A mathematical model developed accounted for the gas‐phase ozone conversion under various operating conditions. The gas‐solid contact efficiency in the riser was discussed in light of the model.
Molecular thermodynamics of solubilities in gas antisolvent crystallization
Authors: Dixon David J. , Johnston Keith P.
Published: June 2004

An expanded liquid molecular thermodynamic model is developed to predict the solubilities of pure solids in a liquid expanded with a gaseous antisolvent. Experimental data are presented for systems containing naphthalene, phenanthrene, and a mixture of both in toluene expanded with a gas antisolvent, CO2. The pressure range is 1 to 64 bar and the temperature is 25°C. The data are predicted accurately with regular solution theory up to moderate pressures, but not at the higher pressures where the liquid phase is nearly pure CO2. In contrast, the new expanded liquid equation of state model describes the wide range of behavior from the nearly ideal liquid solution at ambient pressure to the highly nonideal compressible fluid at elevated pressures. As a result, it predicts solubilities accurately over three orders of magnitude by using only binary interaction parameters. The implications of the phase behavior on fractional crystallization with a gas antisolvent are discussed.
Nonisothermal photochlorination of methyl chloride in the liquid phase
Authors: Cabrera María I. , Alfano Orlando M. , Cassano Alberto E.
Published: June 2004

A model for the photochlorination of methyl chloride in the liquid phase was developed, with numerical solution and experimental verification. The reaction was carried out under pressure, using carbon tetrachloride as a solvent with polychromatic radiation in a nonisothermal tubular reactor. Momentum, thermal energy, mass and photon balances were established rigorously and solved numerically. Reaction constants were taken from the known reaction mechanistic kinetics in the gas phase. Theoretical results agreed well with bench‐scale experiments performed inside an elliptical reflector in the prediction of both reactant conversion and product selectivities. The validated model was used to show the existence of significant temperature and concentration gradients inside the reactor, both in the radial and in the axial directions.
Model of particle‐vapor codeposition with application to ceramic materials synthesis
Authors: Hurt Robert H. , Allendorf Mark D.
Published: June 2004

The simultaneous deposition of particles and material from the vapor phase gives rise to surface deposits having characteristic structure. A mathematical model of the particle‐vapor codeposition process is developed that predicts the structure and properties of these deposits as well as their growth rate over a wide range of conditions. A random sphere formulation is developed that provides the framework for a unified treatment of both subprocesses: ballistic particle deposition and porous media densification through vapor deposition. The model is applied to particleenhanced chemical vapor deposition (PECVD) processes for the production of ceramic materials. In PECVD, particles are introduced into a CVD process, either in the form of an aerosol created in situ by a controlled amount of gas‐to‐particle conversion or in the form of independently charged powder. The model identifies PECVD conditions that produce deposition rates substantially higher than those achievable by conventional CVD.
Stability of SISO quadratic dynamic matrix control with hard output constraints
Authors: Zafiriou Evanghelos , Marchal André L.
Published: June 2004

The inclusion of output constraints in the on‐line optimization problem solved by quadratic dynamic matrix control (QDMC) can result in a unstable, closed‐loop system, even when the corresponding unconstrained algorithm is stable. The presence of constraints in the optimization problem produces a nonlinear, closed‐loop system, although the plant and model dynamics are assumed linear. This article quantifies the effect of output constraints on both nominal and robust stability for single‐input/single‐output processes. Stability conditions provided can be used to select the QDMC parameters and constraint window. Examples demonstrate that these conditions can capture the nonlinear effect of the constraints and that tuning rules developed for the unconstrained case should not be used with output constraints. An example with dead time error illustrates the use of this framework to guarantee the robustness of constrained QDMC with respect to modeling error.
An efficient design method for binary, azeotropic, batch distillation columns
Authors: Diwekar Urmila M.
Published: June 2004

Using rigorous models to simulate batch distillation involves the solution of many stiff differential equations. The dimensionality of the problem also increases with an increase in the number of plates and components, which imposes limitations on the use of rigorous models. Furthermore, problems in design, optimization, synthesis, and control involve iterative procedures, and considerable computational efforts would be required to solve them rigorously. As an alternative, the short‐cut method (Diwekar and Madhavan, 1991a) provided reasonably accurate solutions without much computational effort. This method in the present form, however, cannot be applied to azeotropic systems because of the impassable barrier of the azeotropic point and needs to be modified. The azeotropic approximation proposed by Anderson and Doherty (1984) was used to modify the short‐cut method for binary azeotropic systems. The modified algorithm presented here was tested extensively with a number of binary azeotropic systems for both constant reflux and variable reflux modes of operation, and the results were shown to compare well with the rigorous models. The method requires significantly less computational efforts and is very useful in analyzing feasible region of operation.
Linear oscillations of a drop in uniform alternating electric fields
Authors: Yang Wenrui , Carleson Thomas E.
Published: June 2004

Oscillations of a conducting drop immersed in a dielectric fluid in an alternating electric field were modeled to understand the enhancement of transport processes by the electric field. Numerical solutions for oscillation amplitude, velocity distribution, resonant frequency and streamlines were obtained. The effects of viscosity and density on the resonant frequency and the velocity distribution were investigated. It was found that the resonant frequency of viscous fluids was always smaller than the free oscillation frequency of the same droplet. The predicted scanning frequency response curve and the streamlines agree well with the experimental observations.
Diffusion in ion‐exchanged clinoptilolites
Authors: Ackley Mark W. , Yang Ralph T.
Published: June 2004

The two‐dimensional channel structure of clinoptilolite has been altered systematically by ion exchange to study the effects of cation type, size, location, and distribution on the diffusion of N2 and CH4 probe molecules. Concentration‐dependent diffusion time constants (D/L2) were determined from gravimetric uptake measurements for fully‐exchanged K+, Na+, and H+ clinoptilolites, and highly‐exchanged Ca2+ (89%) and Mg2+ (72%) clinoptilolites. Both plane sheet and parallel channel diffusion models were developed from the one‐dimensional plane sheet diffusion equation and fit to the uptake data. Resulting values of D/L2 varied by a factor of more than 1,000 for both N2 and CH4, while kinetic selectivity spanned nearly two orders of magnitude for this group of modified clinoptilolites. Achieving this range in performance for the difficult N2/CH4 separation demonstrates the excellent potential for tailoring clinoptilolite by cation manipulation for the kinetic separation of other gas mixtures.
Nonideal multicomponent distillation: Use of bifurcation theory for design
Authors: Fidkowski Z. T. , Malone M. F. , Doherty M. F.
Published: June 2004

A method for the design of distillation systems for nonideal multicomponent mixtures is described, offering an alternative to extensive performance simulations. The basis is an analysis of the pinches or “fixed points” in the model equations and the results are widely applicable because approximations such as constant volatilities and ideal solutions are unnecessary. Tangent pinches are represented naturally as turning points in the diagram of pinch composition vs. reflux ratio, giving a basis for designs that alleviate the effects of tangent pinches in certain nonideal mixtures. An efficient computational approach for design is demonstrated in examples for homogeneous liquid mixtures, containing as many as four components, with and without azeotropic behavior. The results are in excellent agreement with detailed performance simulations.
Particle‐to‐bubble adhesion in gas/liquid/solid slurries
Authors: Vinke H. , Hamersma P. J. , Fortuin J. M. H.
Published: June 2004

In gas/liquid/solid slurries, adhesion of small particles to gas bubbles is often encountered, for example, during flotation of finely divided minerals and in slurry reactors where particle‐to‐bubble adhesion may result in an enhanced gas absorption rate (Wimmers, 1987; Lindner, 1988). The adhesion of small particles to gas bubbles in water is studied with a modified bubble pick‐up method. The extent of the particle‐to‐bubble adhesion is revealed in the angle αmax, by which the gas‐bubble surface is covered by adhering particles under static conditions. It is shown that αmax depends on the modified Eötvös number, the ratio of the particle and bubble radii, and the effective contact angle θE. It is also found that θE is affected by the curvature of the three‐phase contact ring and by the type of the gas.

A “particle‐to‐bubble adhesion” model, indicated by PBA model, based on a balance of forces under static conditions, is developed to calculate the value of θE from the measured values of αmax. The measured values of cos(θE) appeared to be a linear function of the curvature 1/r of the three‐phase contact ring which is in accordance with the extended Young‐Dupré equation.