bluuebunny/crossref_metadata_2025 huggingface dataset:
Multidimensional modeling of turbulent two‐phase flows in stirred vessels
Authors: Gosman A. D. , Lekakou C. , Politis S. , Issa R. I. , Looney M. K.
Published: June 2004

This article outlines a computational procedure for the prediction of dispersed two‐phase, solid‐liquid and gas‐liquid, turbulent flows in baffled, impeller‐stirred vessels common in the chemical industries. A two‐flow Eulerian model is employed, based on the main assumption of interpenetrating coexisting continua. Mean momentum and mass conservation equations are solved for each phase and turbulent closure is effected by extending the single phase k‐ε turbulence model to two‐phase flows. The resulting set of highly coupled equations is solved by a two‐phase implicit algorithm, PISO‐2P, which allows calculation for a wide range of phase fraction, particle size and phase density rations. Predictions are presented for solid‐liquid and gas‐liquid (bubbly) flows. Comparisons are made with experimental data for the mean phase velocities and volume fraction, mean slip velocity and turbulence quantities.
Lumping nonlinear kinetics in porous catalysts: Diffusion‐reaction lumping strategy
Authors: Ocone Raffaella , Astarita Gianni
Published: June 2004

Nonlinear kinetics in multicomponent mixtures are best described in terms of continuous distribution functions. The quasilinearization procedure that we introduced in 1988 for “uniform” kinetics does not carry over to problems where second‐order derivatives appear, as in all cases where diffusion phenomena are of importance. We analyze a simple diffusion‐reaction problem in a porous catalyst, by making use of a perturbation technique. The general conclusions reached from this specific problem can be generalized to a wider class of diffusion‐reaction problems in complex mixtures.
An axial dispersion model of competing ion etching and mixing of binary solids
Authors: Elias Raul J. , Barteau Mark A.
Published: June 2004

Ion bombardment of solid surfaces produces a number of changes in the nearsurface region. The surface is etched by ejection of material by momentum exchange. The surface concentrations of the components of a multicomponent solid, such as an alloy, are altered by preferential sputtering. Finally, the incident ions penetrate a distance into the solid, producing a damaged region referred to as the “altered layer” in which diffusion is enhanced relative to that in the unaltered bulk. Previous models have not explicitly accounted for the ion penetration depth and the discontinuity of solid diffusivities at this boundary. We demonstrate that this problem is directly analogous to that of a plug‐flow reactor with axial dispersion, and solve for steady‐state and transient concentration profiles in a binary solid, taking into account the different values of the “Peclet number” appropriate for the two regions, the altered layer and the unaltered bulk of the solid.
Pressure drop, liquid holdup, and flow regime transition in trickle flow
Authors: Holub R. A. , Duduković M. P. , Ramachandran P. A.
Published: June 2004

A phenomenological, pore‐scale, hydrodynamic model is developed for representation of the uniform, cocurrent, two‐phase flow in the low interaction regime in trickle bed reactors. Comparison of model predictions with numerous pressure drop and liquid holdup data reveals that phase interaction terms are negligible which results in a simplified model with no adjustable parameters. This model yields improved pressure drop and liquid holdup estimates for the low interaction regime. In addition, a criterion for the prediction of the trickle to pulsing flow regime transition is developed based on Kapitza's (1945) work on laminar film stability. This criterion compares favorably to data and to some other existing models for prediction of the trickle to pulsing flow regime transition.
Representation of excess properties of electrolyte solutions using a new equation of state
Authors: Fürst W. , Renon H.
Published: June 2004

A new equation of state for electrolyte solutions has been developed from an expression of the Helmholtz free energy containing a nonelectrolyte part and a part relative to ions. The nonelectrolyte part is taken from the equations of state (EOS) of Schwartzentruber et al. (1989). The ionic part is composed of an MSA long‐range term to account for electrostatic interactions and a short‐range interaction term specific to ions. Using correlations between parameters and experimental ionic diameters, the model reduce to a one‐parameter model. It has been applied to numerous strong electrolyte systems and extended to ternary systems to test its predictability without mixing parameters for ions. Its results compare well to the results reported for other one‐parameter models (electrolyte EOS). Furthermore, it was found that the cation‐anion interaction parameter could also be correlated to experimental ionic diameters. Then, the osmotic coefficients of 28 alkaline and alkaline‐earth halide systems may be represented with a root mean square relative deviation of 2.9% using only six correlation parameters. This result has been extended to other systems, with the conclusion that the model with all parameters correlated may also be applied to systems other than halide solutions. The resulting model is predictive. The quality of the prediction was tested by determining osmotic coefficients relative to six systems without any parameter adjustment. The deviations of the predicted values range from 2.0 to 5.4%. The quality of the representation of mixed salts systems without mixing parameters was evaluated using experimental osmotic coefficients of 30 ternary systems.
Ordinary and transition regime diffusion in random fiber structures
Authors: Tomadakis Manolis M. , Sotirchos Stratis V.
Published: June 2004

The problem of bulk, transition and Knudsen regime diffusion in structures of freely overlapping fibers of various orientation distributions was numerically investigated, and the interrelation of the resulting effective diffusivities was examined. Fibers were randomly positioned and oriented in d = 1, 2, or 3 directions. A Monte Carlo simulation scheme was employed to determine the effective diffusivities from the mean‐square displacement of random walkers traveling in the interior of the porous structure. The effective diffusivity was found to depend strongly on the orientational distribution of the fibers, porosity of the fibrous structures, and Knudsen number. The tortuosity factor decreased in general with increasing porosity, approaching at the limit of dilute beds the lower bound derived for each direction of diffusion from variational principles. The simulation results agreed well with experimental values of the bulk tortuosity of fibrous beds from the literature. It was also found that the reciprocal additivity or harmonic average effective diffusivity expression (Bosanquet formula), commonly used to estimate transition regime diffusivities from the values at the ordinary and Knudsen diffusion limits, provides an excellent approximation for the effective diffusivity of fibrous beds, except for that parallel to the fibers of a unidirectional structure.
An experimental study of bubble deformation in viscous liquids in simple shear flow
Authors: Canedo Eduardo L. , Favelukis Moshe , Tadmor Zehev , Talmon Yeshayahu
Published: June 2004

An experimental study of the deformation of gas bubbles in viscous Newtonian liquids subjected to simple shear flow has been undertaken as a first step toward understanding bubble dynamics in polymer processing. Measurements of the shape, interfacial area, and volume of air bubbles suspended in the gap between two coaxial, counterrotating cylinders are presented and discussed as function of capillary number. At high capillary numbers (high deformations), the experiments confirm the theoretical model of Hinch and Acrivos.
Settling behaviors of iron oxide suspensions
Authors: Glasrud G. G. , Navarrete R. C. , Scriven L. E. , Macosko C. W.
Published: June 2004

This article shows the settling behaviors of flocculated, magnetic and nonmagnetic iron oxide suspensions. It is unique, since in the literature on settling of model submicron flocculated particulate suspensions there is no direct visualization of the structures and how they change during settling. Channeling, cracking, and a novel collapse phenomenon were detected during settling. Causes were investigated using lapse videorecording for side and top views. The effect of air bubbles was shown to contribute to the accelerated settling of the suspension by creating channels that conduct oil to the settling front. Settling heights were recorded for long periods of time (up to a year) for iron oxide suspensions of different concentrations. Decreasing tube diameters delayed settling, and magnetic interparticle forces produced smaller final settling heights. The effect of magnetic forces was interpreted in terms of a floc model. The model finds magnetic flocs 1.3 to 1.4 times denser than their nonmagnetic analogs. Finally, a mechanism is put forth to interpret the settling behavior observed.
Improving hydraulics and efficiencies with the T‐By sieve tray
Authors: Tedder D. William , Bravo Jose L. , Parker Byron M. , Parker Trent J.
Published: June 2004

A modified sieve (that is, T‐By) tray uses a system of weirs and risers to approximate liquid plug flow over a cross‐flow, circular‐section tray. Vapor‐liquid contact is improved using a system of curtain‐pattern vapor holes and intermediate weirs to form cells across the tray that promote liquid mixing into discrete, wellmixed pools and stabilize froth. A theoretical model is used to analyze experimental data, guide tray design, and suggest new strategies for improving performance. These results indicate that tray efficiencies and stabilities can be increased without capacity losses. Pressure drop can also be manipulated to enhance performance.
Acceleration of chemical reaction in boiling polymer solutions
Authors: Chen L. , Hu G. H. , Lindt J. T.
Published: June 2004

This article discusses the effect of boiling inert hydrocarbon solvent on the model second‐order transesterification reaction of ethylene/vinyl acetate copolymer with 3‐phenyl 1‐propanol. In the presence of boiling, the overall reaction rate increases substantially due to mass‐transfer effects in the proximity of the vapor‐liquid interface. The present experiments and theory indicate that during the lifetime of a bubble in the boiling medium, the concentrations of reactants and the catalyst increase significantly near the bubble surface due to the rapid diffusion of the inert solvent. After the bubble rupture, the concentration differences persist and continue affecting the chemical reaction rate. The combination of these effects is responsible for the apparent increase in the overall reactivity.
High‐pressure vapor‐liquid equilibria in the propane‐1‐propanol system
Authors: Mühlbauer Andre L. , Raal J. David
Published: June 2004

High‐pressure isothermal vapor liquid equilibrium data were measured for the propane‐1‐propanol system at 81.6, 105.2, and 120.1° C in a static equilibrium cell with liquid‐phase sampling by a piston‐driven sampling rod and homogenizing the sample with a static jet mixer. The vapor phase was sampled by releasing it into an evacuated manifold, and the gas chromatograph was calibrated with a new variable volumetric device. Satisfactory modeling was achieved with the combined method (Wichterle, 1978b) using the UNIQUAC equation with equations of state: the group contribution EOS (Skjold‐Jorgenson, 1986), Peng‐Robinson EOS (Peng Robinson, 1976) or the two‐parameter Virial EOS. Differences between the measured and calculated vapor‐phase mole fractions, however, were signficant for the lower pressure regions of the 81.6 and 120.1° C isotherms.

UNIQUAC parameters aij, hitherto unavailable, with fairly strong temperature dependence in the 81.6 to 120.1° C range are proposed for the system. The covariance matrix indicated a significant correlation among the parameters. The classical mixing rule interaction parameters, σij, required for the original Peng‐Robinson EOS in the combined method were obtained using the direct method (Wichterle, 1978a) and were temperature‐independent for the isotherms for which the propane was supercritical. The possibility of propane/1‐propanol immiscibility was theoretically examined according to the criteria of Baker et al. (1982). The plots of Gibbs energy of mixing vs. phase mole fractions did not indicate liquid‐phase splitting, but the inferences are EOS‐dependent and must await visual confirmation. Our earlier vaporphase thermodynamic consistency test (1991a) indicated the data for all three data sets not to be inconsistent.
Effectiveness of calcium magnesium acetate as an SOX sorbent in coal combustion
Authors: Levendis Yiannis A. , Zhu Wenqi , Wise Donald L. , Simons Girard A.
Published: June 2004

A fundamental study was conducted on the effectiveness of the chemical calcium magnesium acetate (CMA) as a sulfur capture agent during combustion of pulverized coal. It was based on high‐temperature laboratory‐bench experiments with the scope of exploring the use of CMA as a “dry scrubbing” medium for in‐boiler injection. Two methods of CMA introduction in the furnace were considered: dry‐spraying fine powders of the chemical and wet‐spraying aqueous solutions to generate fine aerosols. It considered conditions pertinent to post‐flame in‐boiler injection of CMA to identify optimum temperatures and residence times. In addition to the versatility of the water‐soluble CMA to enable spray drying injection and therefore eliminate grinding costs, there are other attractive features. Mainly, its ability to form highly cenospheric, “popcorn” ‐like, oxide particles on heating to high temperatures. These cenospheres possess thin, porous walls with blowholes that enable penetration of the SO2 in the interior of the particle which promotes high sorbent utilization. SO2 captures in the order of 90% were achieved with dry‐injection of the chemical at furnace gas temperatures of about 1,000° C, a Ca/S ratio of 2, and particle size of ≈ 50 μm. Moreover, CMA was superior(by over 40%) to either CaCO3 or Ca(OH)2 in sulfur capture effectiveness per unit mass of calcium. This commercially obtained CMA was even superior to reagent‐grade calcium acetate (by as much as 30%), again per unit mass of calcium. The utilization of CMA and calcium acetate depended on the cenosphere wall thickness, rather than the particle size and, thus, outperformed other sorbents regardless of the size of the resulting oxide particles.
Simplification of network theory for polymer melts in nonlinear oscillatory shear
Authors: Jeyaseelan Ranjit S. , Giacomin A. Jeffrey , Oakley Jason G.
Published: June 2004

Giacomin and Oakley (1991) have shown that an upper convected Maxwell model with a structure‐dependent relaxation spectrum incorporating the three‐parameter kinetic rate equation proposed by Mewis and Denn (1983) works remarkably well for a molten LDPE in large‐amplitude oscillatory shear (LAOS). In a search for a simpler framework for interpreting LAOS based on network theory, four nonlinear, differential constitutive equations, each with less than three parameters, were evaluated. These models are compared with the Mewis‐Denn model, and a two‐parameter model has been found to work just as well for molten IUPAC LDPE X. The LAOS behavior of molten LLDPE is also adequately described by this model.
NMR imaging of multiphase flow in porous media
Authors: Chen Songhua , Qin Fangfang , Kim Kyung‐Hoe , Watson A. Ted
Published: June 2004

Nuclear magnetic resonance (NMR) imaging techniques are used to determine porosity and saturation distributions. Difficulties in quantitation which arise from the short transverse relaxation times exhibited by fluids in porous media are addressed. Procedures are presented whereby transverse relaxation during the signal acquisition process is modeled to obtain accurate estimates of intrinsic magnetization intensity, and thus porosity and saturation. Special consideration is given to saturation/position dependence and multiexponential behavior of transverse relaxation. These procedures are validated with results for longitudinal profile images. The use of thin‐slice cross‐sectional profile images are demonstrated by observing fluid displacement in a rock sample with structural heterogeneities.
Wavelength and depth of ribbing in roll coating and its elimination
Authors: Hasegawa Tomiichi , Sorimachi Kazunori
Published: June 2004

Experiment has been made on ribbing induced on a film coated over a roll surface by a two‐roll system: one roll rotated with a speed and the other fixed or rotated with relatively different speeds. The two rolls have the same diameters and are placed parallel with each other by keeping a gap. Test fluids are Newtonian (glycerin/water) and viscoelastic [polyacrylamide (Separan AP30)/glycerin/water] solutions. Wavelength and depth of ribbing are measured by a newly developed technique utilizing an image of a straight bar placed above the roll and reflected on the ribbing film surface. Nondimensionalized wavelength is correlated with capillary number for Newtonian liquids, but higher for dilute Separan solutions and lower for nondilute Separan solutions than for Newtonian liquids. The depth of ribbing increases for dilute Separan solutions with nearly the same value for nondilute Separan solutions as those for Newtonian liquid. This behavior observed for the ribbing seems to have some relation to the existence of a line of vortices that occurs visibly for nondilute Separan solutions in the entrance region of the gap between the rolls. Increase in the ratio of the velocities of two rolls has an effect of increasing both the wavelength and depth of ribbing for glycerine and nondilute Separan solutions, but it has the effect of decreasing them for dilute Separan solutions. Addition of Separan in glycerine solution promotes the generation of ribbing.

A string spanned near over the gap to touch the surface of the liquid pool between the gap has the effect of eliminating the ribbing. This is successful in various combinations between the speeds of the two rolls and the diameters of the string.
Chromatographic study of alkanes in silicalite: Transport properties
Authors: Hufton Jeffrey R. , Danner Ronald P.
Published: June 2004

Micropore diffusion coefficients for infinitely‐dilute methane, ethane, propane, n‐butane, and isobutane in silicalite were measured via concentration‐pulse chromatography. Two different samples of large laboratory‐synthesized crystals were investigated. Analysis of van Deemter plots clearly indicated the presence of a significant micropore mass‐transfer resistance for isobutane. Diffusion coefficients evaluated from these plots were consistent with results measured by the membrane technique. The linear alkane data differed completely in that the micropore masstransfer resistance for these adsorbates was very small, implying that the true diffusion coefficients were too large to be measured with this system. Thus, effective diffusion coefficients evaluated from these data represent a lower limit of the true values. Comparison of this limit with nuclear magnetic resonance data was favorable, since the latter are significantly larger in magnitude. Inconsistency was observed with diffusion coefficients obtained from other macroscopic techniques including the membrane method, zero‐length and tracer‐pulse chromatography, and some frequency response experiments.
Some complements in combinatorics of sharp separation systems synthesis
Authors: Floquet P. , Domenech S. , Pibouleau L. , Aly S. M.
Published: June 2004

The analysis of combinatorics of sequences of sharp separation systems is a problem extensively studied in chemical engineering. Thompson and King (1972) first presented a closed‐form expression for determining a number of possible sequences for separating an n‐component mixture into pure products by using simple (one input and two outputs) sharp separators. Then, Shoaei and Sommerfeld (1986) showed that this determination could be interpreted in term of Catalan numbers; recently Wahl and Lien (1990) derived this formula from a generating function. This article presents a closed‐form expression for a number of different possible separation sequences when complex (one input and three or more outputs) separators are used.
Numerical simulation of Case II transport
Authors: Fu Thomas Z. , Durning Christopher J.
Published: June 2004

A nonlinear conservation equation for the fluid concentration field during onedimensional viscoelastic diffusion is derived by retaining the concentration dependencies of physical properties in the fluid flux expression from the nonequilibrium thermodynamic treatment of Durning and Tabor (1986). The result is specialized to the limit of small, but finite, diffusion Deborah numbers to give a model essentially the same as that by Thomas and Windle (1982) for Case II transport (TW model). Orthogonal collocation on Hermite cubic trial functions together with a stiff ordinary differential equations integrator were used to solve the TW model for integral sorption in a dry film. The solutions show good agreement with previous analyses and confirm that strong nonlinearities in both the fluid diffusivity and the mixture viscosity are essential for prediction from the model of wave‐like concentration profiles associated with the Case II process. Also, the numerical results partially confirm the analytical asymptotic analysis of Hui et al. (1987b) for the Case II wave‐front velocity, v. They show that Hui et al.'s formula for the dependence of v on characteristic physical properties does not depend on the details of how the diffusivity and viscosity change with fluid content, as long as both are strongly nonlinear.
A dew point technique for limiting activity coefficients in nonionic solutions
Authors: Trampe Diane B. , Eckert Charles A.
Published: June 2004

A new measurement technique for activity coefficients at infinite dilution has been developed involving the construction of a very dilute vapor phase and the accurate determination of its dew point. This method is especially applicable to systems of low solute relative volatility, precisely where other methods such as differential ebulliometry and headspace gas chromatography become less precise. Data are reported for γ∞ for N, N‐dimethylformamide, dimethylsulfoxide, and N‐methyl pyrrolidone in water over a range of temperatures.
Quasicritical behavior of dense‐gas solventsolute clusters at near‐infinite dilution
Authors: O'Brien James A. , Randolph Theodore W. , Carlier Claude , Ganapathy Shankar
Published: June 2004

We Compare molecular dynamics and electron paramagnetic resonance spectroscopic experiments on very dilute solutions of nitroxide free radicals in nearcritical and supercritical ethane. The measurement of the effective local density of solvent in the cybotactic region of the probe solute shows that maxima in local density enhancements occur in both computer and spectroscopic experiments at about half of the critical density, well removed from the maximum in the solvent's isothermal compressibility.

Remarkably, at the bulk density where the maximum enhancement occurs, we observed long‐lived clusters in the vicinity of the solute molecule. Geometrically defined clusters persist for at least 100 picoseconds of simulated time. The persistence of these clusters is corroborated by the behavior of the solute diffusion coefficient, which shows fluctuating values over simulated time periods as long as 1.3 nanoseconds. We argue that fluctuations in the local solvent density about a solute molecule are not a long‐ranged solvent critical effect. Rather, they represent a localized, short‐ranged quasicritical phenomenon induced by the solute, which may be described in terms of a transition between weakly attractive and attractive behavior. We discuss the implications of time‐varying microstructure for reactions in supercritical fluid media.
A simple design strategy for fault monitoring systems
Authors: Chang Chuei‐Tin , Mah Kai‐Nan , Tsai Chii‐Shiang
Published: June 2004

A simple strategy is developed for the design of model‐based fault‐monitoring systems. Several new features allow this approach suitable for practical applications. The core of the suggested monitoring system is a parallel parameter estimation method designed to reduce the chance of bias. Two important criteria, diagnostic observability and diagnostic resolution, are adopted to evaluate the performances of alternative designs. By following a systematic procedure, it is possible to determine the best sensor locations in a complex chemical process based only on the structural informations of the system model. After selecting the most appropriate measurement variables, diagnostic resolution can be enhanced further by exploiting additional insights gained from analysis of the physical meanings of the model parameters and by comparing the fault patterns in diagnosis. Several effective on‐line implementation techniques are also proposed to minimize computation for the estimation algorithm. The correctness and efficiency of the suggested design strategy are demonstrated by the simulation results in examples.
Steps in CH4 oxidation on Pt and Rh surfaces: High‐temperature reactor simulations
Authors: Hickman D. A. , Schmidt L. D.
Published: June 2004

The direct oxidation of CH4 to H2 and CO in O2 and in air at high temperatures over alumina foam monoliths coated with high loadings of Pt and Rh has been simulated using a 19‐elementary‐step model of adsorption, desorption and surface reaction steps with reaction parameters from the literature or from fits to previous experiments. The surface reaction model for Pt is in good agreement with previously reported low‐pressure(0.1 to 1 torr) reactor measurements of CH4 oxidation rates at temperatures from 600 to 1,500 K and of OH radical desorption during CH4 oxidation at 1,300 to 1,600 K over polycrystalline Pt foils. The model predictions for both catalysts are also consistent with product selectivities observed over monolithic catalysts in an atmospheric‐pressure laboratory‐scale reactor, and the differences between Pt and Rh can be explained by comparing individual reaction steps on these surfaces. Because of the good agreement between the model and both low‐and atmospheric‐pressure reactor simulations, a complete energy diagram for methane oxidation at low coverages is proposed. The model results show that under CH4rich conditions at high temperatures, H2 and CO are primary products of the direct oxidation of methane via a pyrolysis mechanism.
Simulation of carbon gasification kinetics using an edge recession model
Authors: Kyotani Takashi , Leon Y Leon Carlos A. , Radovic Ljubisa R.
Published: June 2004

An edge recession modeling method is proposed as a new approach to the prediction of carbon gasification kinetics. Using this method, the variations in reactive surface area (RSA) and specific reactivity (R) with conversion were simulated for several polynuclear aromatic molecules adopted as models of carbon crystallites. The effects of crystallite size and shape, edge site reactivity and vacancies on the changes in RSA and R with conversion were investigated. As a result, RSA and R were found to be essentially independent of crystallite shape, but to depend on crystallite size and the presence of vacancies. Good agreement was obtained between model predictions and experimental data for several kinds of model carbon crystallites. In the case of structurally disordered carbons (such as chars), simultaneous gasification of model crystallites of two different sizes gave better predictions than that of single‐size crystallites. The edge recession model is proposed as an alternative, or at least complementary, approach to the more conventional (and, arguably, less physically meaningful) pore structure development models.
Heat transfer and cure in pultrusion: Model and experimental verification
Authors: Batch Gibson L. , Macosko Christopher W.
Published: June 2004

Though pultrusion has existed for nearly 40 years, factors affecting temperature and resin reactivity are not completely understood. A model is tested to help explain the effects of die temperature, line speed and initiator concentration on curing inside a pultrusion die. The model includes radio frequency preheating, heat conduction, heat of cure, changing thermal properties with temperature and degree of cure, and hindered heat transfer due to shrinkage of the profile away from the die. The analysis is compared to experimental temperature measurements for a 1‐in. (25‐mm) round die, where die temperature, line speed, and initiator concentration were varied. Temperature predictions by the model are in good agreement with the data for many of the conditions tested. Considerable disagreement still persists, however, under processing conditions with faster line speeds and lower die temperatures.
Polydisperse mixture adsorption kinetics
Authors: Muralidhar Ramachandran , Talbot Julian
Published: June 2004

We present a theoretical study of the adsorption kinetics of particles with a continuous distribution of sizes. If the particles interact with short‐ranged forces, the diminishing rate of adsorption with increasing surface coverage can be attributed to a geometrical blocking effect of the preadsorbed particles. We exploit the fact that at low coverages the blocking effects result from isolated adsorbed particles to develop an analytic description of the adsorption kinetics using a moment expansion approach. We evaluate the time‐dependent surface coverage, density and the average size of the adsorbed particles and compare these quantities with those of the bulk phase. The effect of desorption is studied by introducing a size‐independent desorption constant, and the corresponding adsorption equilibria properties, which are valid at low coverages, are derived. Simulation results for irreversible adsorption are obtained and compared with the theoretical predictions.
Gross error detection when variance‐covariance matrices are unknown
Authors: Rollins D. K. , Davis J. F.
Published: June 2004

Equations introduced here identify measurement biases and process leaks, when gross errors exist in measured process variables and the variance‐covariance matrix of the measurements, Σ, is unknown. Σ is estimated by the sample variance, S, using process data.

For an unknown Σ, the global test statistic is the well‐known Hotelling T2 statistic. Its power function has a noncentral F‐distribution. For component tests used for specific identification of measurement biases and nodal leaks, two tests are presented with Σ unknown. The first test is independent of the number of component tests, k, and is given by a statistic with an F‐distribution. The second test depends on k and has a student t‐distribution. The power functions for both component tests are provided. Process examples and a Monte Carlo simulation study presented demonstrate the use and performance of these statistical equations in identifying biases and process leaks.
Mathematical modeling of wax deposition in oil pipeline systems
Authors: Svendsen John A.
Published: June 2004

Deposition of wax on the wall of oil pipelines is often regarded as a problem since the tube diameter is reduced. Consequently, more power is needed to force the same amount of oil through the system. A mathematical model for quantitative prediction of wax deposition for each hydrocarbon component has been developed. Each component is characterized by weight fraction, heat of fusion, and melting point temperature. A model explains how a phase transition in the flow from liquid oil to waxy crystals may create a local density gradient and mass flux, which depends on the local temperature gradient. The model predicts that wax deposition can be considerably reduced even when the wall temperature is below the wax appearance point, provided the liquid/solid phase transition, expressed by the change in moles of liquid with temperature, is small at the wall temperature. Deposition as function of time has been obtained as a solution of differential equations derived from the principles of mass and energy conservation and the laws of diffusion.
Kinetics of hydrogen absorption and desorption in LaNi5‐xAlx slurries
Authors: Snijder E. D. , Versteeg G. F. , van Swaaij W. P. M.
Published: June 2004

The kinetics of hydrogen absorption and desorption in LaNi4.8Al0.2,LaNi4.9Al0.1 and LaNi5, suspended in cyclohexane and LaNi5 in ethanol have been investigated. The absorption process can be described in terms of mass transfer and reaction resistances in series. The rate‐limiting steps for this process are dissolution of hydrogen in the solvent and the reaction of hydrogen with the metal alloy. Over a broad range of hydrogen loadings, the reaction of hydrogen with the metal follows the rate equation:

This type of kinetic equation indicates that during hydride formation in a slurry a surface process is the rate‐determining step, which was confirmed by the observed influence of the solvent. Desorption of hydrogen can be described best with a shrinking core relation:

The phase transformation at the β/α boundary appears to be the rate‐determining step. Alloys with higher aluminum contents produce a lower reaction rate constant.
Residence time distribution in screw extruders
Authors: Chen Liqin , Pan Zaoqi , Hu Guo‐Hua
Published: June 2004

A companion article (Chen and Hu, 1993a) discussed a statistical theory that for a system consisting of two closed subsystems, the residence time density (RTD) functions of the two subsystems will be statistically independent, if a two‐dimensional perfect mixing exists at the subsystem boundary. In this case, the overall RTD function is related to that for individual subsystems through the convolution integral. This theory has been validated experimentally using two die‐screw combinations.

Based on this theory, a predictive RTD model for an intermeshing counterrotating twin‐screw extruder has been developed. The screw in the longitudincl direction has been treated as C‐chambers in series. The overall RTD of leakage flows has been calculated from the RTDs of these individual chambers and then converted into the RTD of the extruder. The predicted RTD has been tested against experimental results with success.
Spatiotemporal temperature patterns during hydrogen oxidation on a nickel disk
Authors: Lane Samuel L. , Graham Michael D. , Luss Dan
Published: June 2004

Spatiotemporal temperature patterns on a polycrystalline nickel disk were recorded using infrared video imaging during atmospheric hydrogen oxidation and characterized by the proper orthogonal decomposition pattern analysis technique. The system was studied at two different residence times, 3.2 s and 6.4 s. At moderate feed temperatures, steady‐state multiplicity and rate oscillations were found. Oscillations at a residence time of 6.4 s were periodic and essentially spatially uniform. At a residence time of 3.2 s, however, the surface temperature became nonuniform, and rate oscillations occurred via traveling temperature waves which emanated from “pacemakers” (locally active regions) on the edge of the catalyst. During periodic oscillations, the waves were emitted synchronously from the pacemakers, while during chaotic oscillations, the pacemakers were desynchronized and emitted waves independently of each other. Nonlocal gas‐phase coupling between distance surface elements caused spatial desynchronization during rate oscillations.
Transport of gases in miscible polymer blends above and below the glass transition region
Authors: Li R. J. , Hsu W. P. , Kwei T. K. , Myerson A. S.
Published: June 2004

The study of diffusion and permeation of methane, nitrogen, and helium in miscible blends of poly(styrene‐stat‐acrylonitrile) (PSAN) and poly (methyl methacrylate) (PMMA) at 35–140°C shows that permeation coefficients (P) of helium followed the linear logarithmic mixing rule. Positive deviations from the linear logarithmic mixing rule were observed for permeation and diffusion coefficients (D) of methane and nitrogen below the glass transition temperature (Tg). The deviations decreased with increasing temperature and disappeared above Tg. The experimental results were analyzed by free volume and activated state theories. The Arrnenius plots of log D or log P vs. the reciprocal of temperature exhibited discontinuities in the glass transition region for all gases and blend compositions. The discontinuities are caused by large thermal expansion coefficient differences between the rubbery and glassy states of PSAN and PMMA. The sorption of methane in 50/50 PSAN/PMMA has dual‐mode characteristics below Tg.
Use of lagrangian statistics to describe slurry transport
Authors: Binder J. L. , Hanratty T. J.
Published: June 2004

Lagrangian statistics are explored as a means of describing the transport of solids in a turbulently flowing liquid. The essential feature of the approach is to represent the concentration field as resulting from a distribution of sources of particles. It is argued that this provides a better framework to understand the physics than the Eulerian analysis currently being used. Fully developed concentration fields are calculated, using the assumptions of homogeneous turbulence and plug flow. It is found that the configuration of solids and the suspended load depend primarily on the ratio of the settling velocity to the friction velocity, which is a measure of the relative importance of turbulence and of settling in depositing particles. The analysis emphasizes the need for a better understanding of the mechanism and the rate of entrainment of particles into a turbulently flowing liquid.
Effect of chemical reactions on turbulent diffusivities
Authors: Toor H. L.
Published: June 2004

A mixing length model describes the recently observed (Bilger et al., 1991) effect of chemical reactions on turbulent diffusivities. From this viewpoint, the coupling between microscale reactions and macroscale mixing occurs because the reaction rate is a quasitransferable property which causes more reaction in fluctuating fluid lumps leaving regions of high rate than low rate. The net effect, which raises or lowers reactant diffusivities, depends on a characteristic macromixing time and the rate at which the mean reaction rate changes with mean concentration:

The model shows correct qualitative behavior and appears to agree quantitatively with much of the existing data. It adds an apparent flux, ετM(drA/dy), to the convective diffusion equations which can enhance or hinder the normal turbulent fluxes, but is likely to have little effect on the mean concentration field.
A combined heat‐transfer analysis of a single‐fiber CVD reactor
Authors: Kassemi M. , Gokoglu S. A. , Panzarella C. H. , Veitch L. C.
Published: June 2004

A combined‐conjugated heat‐transfer and fluid‐flow analysis is presented for coating fibers by CVD in a vertical cylindrical quartz reactor. The numerical model focuses on radiation and natural convection. Three case studies are performed, and the wall temperature predictions are compared to experimental measurements. In the first case, the flowing gas is hydrogen, and conduction is more important than both radiation and convection, in which case measured and predicted wall temperatures agree excellently. In the second, hydrogen is replaced by argon, thus making radiation heat transfer more important than the previous situation. Three radiation models with increasing degrees of sophistication are compared: an approximate nongray model (no wavelength dependence of emissivity), an approximate semigray model, and a rigorous semigray model with view factor calculations. Comparison with experiments suggest that a semigray radiative analysis is needed for correct determination of wall temperatures. The third involves argon at a lower flow rate, where natural convection effects are more pronounced. Checking the validity of the Boussinesq approximation by incorporating the explicit dependence of density on temperature in the model shows a slight difference between the velocity fields predicted using the Boussinesq approximation and those obtained using the explicit dependence of density on temperature. However, there is negligible difference between the temperature fields predicted in the two cases.
Self‐similar collapse of stationary bulk foams
Authors: Patzek Tad W.
Published: June 2004

In bulk foams, the dependence of bubble size on time can be deduced from a hypothesis of statistical self‐similarity and the scaling characteristic of the volume change rate of a foam bubble. If this rate, \documentclass{article}\pagestyle{empty}\begin{document}$ {\dot v} $\end{document}, scales as the mean bubble volume v to the power α, the total surface area of the foam decreases as an inverse of time to the power 1/[3 (1 − α)]. Coarsening of polyhedral foam scales with α = 5/6, when molecular diffusion limits gas transport across lamellae and liquid drainage through Plateau borders limits lamella thinning. Excess liquid is released by disappearing small bubbles and flows into the lamellae and Plateau borders of growing large bubbles. If none of this liquid accumulates in the foam, coarsening is exponential and α = 1. When resistance to mass transfer at the lamella surfaces is the rate‐limiting step, polyhedral foam coarsens with α = 1/3. Coarsening of slowly draining, spherical‐bubble foam scales with α = 0. The theory is compared with nine measurements of the total surface area of polyhedral and spherical‐bubble foams pregenerated from aqueous solutions of sodium lauryl sulfate, hexadecyltrimethylammonium bromide, alpha‐olefin sulfonate with alkyl chain lengths from C14 to C16, and two shaving creams. The theory also proves that the collapse of a two‐dimensional, polygonal foam is self‐similar and scales with α = l. In all cases, our theory agrees well with experiment and numerical calculations.
Electrochemical measurements of mass transfer at surfaces with discrete reactive areas
Authors: Juhasz Nikola M. , Deen William M.
Published: June 2004

The limiting current technique was used to measure area‐averaged mass‐transfer coefficients for surfaces containing many small reactive areas arranged in various patterns. Partially masked platinum electrodes were fabricated using photolithography, and mass‐transfer measurements were performed with a rotating disk apparatus. The average mass‐transfer coefficient was sensitive to the fraction of the electrode area exposed (ε), declining from values near that for a fully exposed surface for ε = 0.5 to less than 1% of the fully exposed value for ε = 0.001. For any given ε, the mass‐transfer coefficient declined with increased spacing between reactive sites. The results were relatively insensitive to details of the site distribution, such as whether the sites were arranged in regular arrays (square or hexagonal lattices) or distributed randomly over the surface. For all conditions studied, the mass‐transfer coefficient greatly exceeded that predicted by conventional models which apply the stagnant film approximation to the fluid surrounding a representative active site. This finding is qualitatively consistent with recent computational results, which suggest that convective transport enhances mass transfer at partially active surfaces to an extent not accounted for by adjustments in the effective film thickness.
Initiation of slugs in horizontal gas‐liquid flows
Authors: Fan Z. , Lusseyran F. , Hanratty T. J.
Published: June 2004

Experiments were conducted with air‐water flow in a horizontal 0.095‐m pipeline at atmospheric pressure to examine the mechanism by which slugs form in a stratified flow. A specially designed entrance box was used to avoid disturbances. In these experiments, at superficial gas velocities less than 3 m/s, the slugs are found to evolve from waves, with a length of about 0.085 m, that are generated by a Jeffreys mechanism. These waves grow in height and eventually double in wavelength by a nonlinear resonance mechanism. Depending on the height of the liquid, the growth can lead to a breaking wave or to a wave that fills the whole pipe cross section. At superficial gas velocities equal to or greater than 4 m/s capillary‐gravity waves with a wide range of lengths are generated by a linear Kelvin‐Helmholtz mechanism. These rapidly evolve into long waves outside the range of linear instability. If the liquid height is large enough, these waves can form slugs through a nonlinear Kelvin‐Helmholtz instability that is aided by wave coalescence.
Adsorption characteristics of octadecylsilyl‐silica gel in gaseous systems
Authors: Miyabe Kanji , Suzuki Motoyuki
Published: June 2004

Adsorption rates and thermodynamic parameters were determined for the gasphase adsorption of n‐alkanes and benzene derivatives on octadecylsilyl‐silica gel (ODS) by chromatographic measurement and moment analysis. Thermodynamic characteristics of adsorption phenomena on ODS were similar to those on the surface. The contribution of intraparticle diffusion to mass‐transport resistance was dominant in the ODS column, and the role of surface diffusion was significant for the intraparticle diffusion. The values of the surface diffusion coefficient were of the order of 10−5 cm2/s and could be estimated by assuming that the surface migration on ODS is a tracer diffusion of an adsorbate molecule in n‐octadecane.
Thermophoretic and evaporational losses of ultrafine particles in heated flow
Authors: Shimada Manabu , Seto Takafumi , Okuyama Kikuo
Published: June 2004

The transport, wall deposition and evaporation of nanometer‐sized aerosol particles are studied using a nonisothermal laminar flow in a pipe. The test aerosol particles, monodisperse Ag and NaCl particles of 7–40 nm dia., are introduced into a vertical circular pipe with wall temperature distribution in the axial direction. The measured particle loss during transport through the pipe increased with the wall temperature difference. The enhancement of particle loss due to diffusive and thermophoretic deposition is examined by the gas flow, temperature, and particle concentration distribution equations. Predicted particle losses agreed with the experimental results when the local variation of gas properties, particle Brownian diffusion, and thermophoretic migration are considered. Sudden increases in the particle loss due to particle evaporation were also observed at very high temperatures (about 700°C for Ag particles and 400°C for NaCl particles). The measured reductions of particle diameter due to evaporation showed good agreement with calculation based on aerosol evaporation theory for free‐molecule regime.
Parametric study on the bulk hydraircooling of spherical food products
Authors: Rao K. V. Narasimha , Narasimham C. S. V. L. , Murthy M. V. Krishna
Published: June 2004

A numerical study of the bulk hydraircooling of spherical food products is performed using the technique of spraying chilled water to form a thin film over the food products while blowing cold unsaturated air in the countercurrent or cocurrent direction. This precooling process is found to combine the advantages of hydro‐and air cooling.

Governing equations describing the simultaneous heat and mass transfer occurring in the hydraircooling process for both counterflow and parallel‐flow arrangements are solved using finite‐difference numerical methods. A parametric study is performed to determine the effect of important processing parameters on the process time. It is generally observed that higher quantities of spray water, air‐mass flow rate and product thermal conductivity, as well as lower values of water temperature will result in faster rates of cooling. For the same operating conditions, parallel‐flow arrangement leads to higher plant capacities.