bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrodynamics and mass transfer on three‐phase distillation trays
Authors: Herron Charles C. , Kruelskie Bradley K. , Fair James R.
Published: June 2004

Several oil‐water mixtures were contacted by air in a sieve tray distillation column simulator. The oil was mineral spirits with 183–206°C boiling range and 51.8° API gravity. Two sieve tray designs were used, and both hydraulic and mass transfer measurements were made, the latter utilizing acetone as the transferring solute. It was found that gas agitation caused the mixtures to behave as a homogeneous liquid that followed general correlations for pressure drop, liquid holdup, froth height, downcomer liquid level, and fractional entrainment. No unusual problems with foaming or entrainment were observed. Mass transfer efficiency was influenced by oil‐water ratio, with mild minima occurring at about 50 vol % mixtures. It was concluded that, when the immiscible liquids are both present in significant proportions, designers need not avoid formation of a second liquid phase for fear of unusual hydraulic or mass transfer behavior.
Generalized likelihood ratios for gross error identification in dynamic processes
Authors: Narasimhan Shankar , Mah Richard S. H.
Published: June 2004

Gross error identification in dynamic processes is important in ensuring proper process control. This paper describes the application of a generalized likelihood ratio (GLR) method for identifying gross errors caused by biases in measuring instruments and controllers, process leaks, and failure of controllers. As shown in the application to steady state processes (Narasimhan and Mah, 1987), this method provides a general framework for identifying different types of gross errors whose effect on the process can be modeled. An important feature of the work is the treatment of closed‐loop dynamic processes. The formulation of the hypotheses of the GLR method proposed by Willsky and Jones (1974) is extended for this purpose. For estimating the time of occurrence of the gross error, a simple chi‐square test on the innovations (measurement residuals) is used, which is computationally more efficient than the method used by Willsky and Jones. Through simulation studies of a level control process the appropriate selection of parameters of the GLR method is investigated.
Pressure swing adsorption: An experimental study of diffusion‐induced separation
Authors: Shin H.‐S. , Knaebel K. S.
Published: June 2004

Gas separation based on differences in intraparticle diffusion rates was achieved by four‐step pressure swing adsorption (PSA) process. The application studied was the separation of nitrogen from air using molecular sieve type RS‐10. Systematic PSA experiments were conducted in bench‐scale equipment to study several variables, including duration of steps of the cycle, flow rates, pressure ratio, subatmospheric blowdown and purge, and column geometry.

The test results were compared with predictions of a mathematical model (Shin and Knaebel, 1987). In all cases, the trends of product purity and net recovery of both theory and experiment were in close agreement. Since the theory accurately portrays the overall behavior of the process, the transport mechanisms embodied in the theory may provide insight into diffusion‐induced separation by PSA. In addition, by reviewing the results of the parametric studies of the variables, it has been possible to gain an intuitive understanding of the relations between factors that affect each step, and hence overall performance.
Dispersion in laminar tube flow at low Peclet numbers or short times
Authors: Vrentas J. S. , Vrentas C. M.
Published: June 2004

A new asymptotic solution is developed for the dispersion of a passive solute in a Newtonian fluid in fully‐developed laminar flow through a straight circular tube. A perturbation solution to the convective dispersion problem is constructed by utilizing the pure diffusion solution as an initial approximation in an iterative successive approximation procedure. The derived perturbation solution is shown to be valid at both short and long times for sufficiently low values of the Peclet number and is also valid at any value of the Peclet number for sufficiently small values of time.
Mathematical modeling of boron diffusion from boron oxide glass film sources
Authors: Yeckel Andrew , Middleman Stanley
Published: June 2004

Mathematical models are developed to examine two alternative means of boron doping from boron‐rich glass films. For the planar source diffusion system, in which boron diffusion and glass film growth occur simultaneously at high temperature (900–1,200°C), glass film growth rates and the degree of doping achieved are predicted. In most cases, the solubility limit of boron in silicon is attained at the silicon surface. It is found that depletion of the boron source wafers used in this process may be slowed considerably by their removal from the reactor after a short time. Model predictions for an alternative method, in which the glass film is pregrown at low temperature (300°C) and the diffusion subsequently performed at high temperature, demonstrate a strong dependence of the boron surface concentration on the initial boron content in the pregrown glass film, for concentrations of boron near the solubility limit. The conditions for which a masking film of SiO2 is insufficient to prevent boron diffusion into silicon, known as mask failure, are also predicted by the models.
Pressure swing adsorption: Effects of incomplete purge
Authors: Matz Michael J. , Knaebel Kent S.
Published: June 2004

Theoretical and experimental studies of pressure swing adsorption (PSA) cycles for various extents of purging have been conducted. In both the theoretical and experimental portions, pressurization was accomplished with product rather than with feed. A simple, local equilibrium model was employed, and the results were analyzed by the method of characteristics. It was found that for difficult separations (i.e., closely spaced isotherms and/or heavily contaminated feeds), incomplete purging, e.g., as little as 50% complete, yielded recoveries attainable only at much higher pressure ratios with complete purge. Conversely, for easy separations (i.e., widely spaced isotherms and dilutely contaminated feeds), incomplete purging was less attractive, yielding only slight improvement over complete purge.

Splitting oxygen from air with zeolite 5A is moderately difficult due to the proximity of the isotherms and due to the predominance of the more strongly adsorbed nitrogen in the feed. This case was also modeled and was studied experimentally. The experiments yielded nearly double the recovery of oxygen for minimal purge compared with that available with complete purge. The productivity of oxygen was relatively high, e.g., 0.0378 m3(O2 at STP)/kg (adsorbent) · h. Those experiments validated the trends predicted theoretically.
Mass transfer from dispersed bubbles to electrolyte solution in packed beds
Authors: Takahashi Ken M. , Alkire Richard C.
Published: June 2004

The measurement of gas‐liquid interfacial areas and mass transfer coefficients in concurrent upward bubbly flow (uG from 0.02 to 0.16 m/s, uL from 0.0051 to 0.056 m/s) over a packed bed consisting of 1.2 mm glassy carbon particles is described. Specific surface areas were measured by chemical absorption of O2 into Na2SO3 solutions; volumetric mass transfer coefficients were measured by physical desorption of O2 into N2 bubbles. Surface areas were obtained in the range of 4 × 103 to 2 × 105 m−1, much higher than have been previously reported. High areas are related to the use of electrolytes and small packing particles. Bubble size was found to depend primarily on liquid velocity. Volumetric mass transfer coefficients kLaG are also high (0.1 < kLaG < 2.3 s−1), primarily as a result of high gas‐liquid interfacial areas. Mass transfer coefficients kL are small, however. For the smaller bubble dispersions, mass transport is controlled by diffusion rather than by convection.
Thermodynamics of liquid‐liquid equilibria including the critical region
Authors: De Pablo Juan José , Prausnitz John M.
Published: June 2004

To obtain good representation in the liquid‐liquid critical region, a semitheoretical correction is added to a conventional expression (e.g., van Laar or NRTL) for the excess Gibbs energy of a binary or ternary system along the coexistence curve. This correction is an exponential function of a suitable distance from the critical point; it is significant in the critical region but not elsewhere. In the correction function, the preexponential factor is determined from stability considerations. In the exponential argument, two parameters are determined from theoretical power laws. While the coordinates of the critical point must be known (or estimated), no phase‐equilibrium data in the critical region are required to determine parameters. The correction presented here provides an excellent approximation for phase equilibria along the coexistence curve close to and remote from critical conditions. Further, the correction facilitates simultaneous representation of isothermal vapor‐liquid and liquid‐liquid equilibria including the liquid‐liquid critical region.
Hydrogen transfer in slurries of carbon supported catalyst (HPO process)
Authors: Lindner D. , Werner M. , Schumpe A.
Published: June 2004

The effect of suspended activated carbon on gas/liquid mass transfer is studied for the Pd/C‐catalyzed hydrogenation of nitrate to hydroxylamine (HPO process). In a bubble column and in a stirred autoclave, physical mass transfer (kLa) in the concentrated salt solution is improved by more than a factor of three by the catalyst. Chemical absorption enhancement by the zero‐order catalytic reaction is insignificant at the employed catalyst diameter (40 μm), but hydroxylamine selectivity depends on the absorption regime.
A network model for deep bed filtration of solid particles and emulsion drops
Authors: Rege S. D. , Fogler H. S.
Published: June 2004

A network model has been developed to simulate the flow of emulsions and solid particles through porous media. Particle deposition due to direct interception, as well pore plugging by straining are accounted for in the model. The effects of two important factors—the ratio of particle size to pore size, and the fluid velocity—on particle deposition are also investigated. The strength of the model lies in its ability to predict accurately effluent concentration profiles, permeability changes occurring during deep bed filtration, and the evolution of the filter coefficient with time. Model predictions for different particle and pore size distributions of both solid and emulsion particles are in agreement with experimental data.
Flocculation of colloids suspended in a viscoelastic fluid undergoing shear
Authors: Mifflin R. T. , Schowalter W. R.
Published: June 2004

The effect of viscoelasticity on the kinetics of the shear‐induced flocculation of colloids has been investigated theoretically and experimentally. A numerical technique for such three‐dimensional problems called global residual minimization (GRM) has been developed as an alternative to finite element and finite difference methods. In the GRM method, minimization of the residual of the momentum balance is used as the selection criterion for the velocity‐field solution. Trajectory simulations were carried out for spheres in a corotational Jeffreys fluid at a variety of Weissenberg numbers, We. For We ≤ 0.01, the effect of viscoelasticity is found to be negligible. For We = 1.0, flocculation rates are enhanced. For We = 10.0, a possibly large reduction in flocculation rate is indicated.

Experiments were conducted using 600 nm radius polystyrene spheres in hydroxyethyl cellulose solutions. At shear rates corresponding roughly to We = 10.5 and 15.3, the flocculation rates were found to be at least an order of magnitude less than the rate expected in a Newtonian fluid. It is postulated that this reduction in flocculation rate is caused by the viscoelasticity of the suspending fluid.
A gas‐solid reaction with nonuniform distribution of solid reactant
Authors: Dassori C. G. , Tierney J. W. , Shah Y. T.
Published: June 2004

A pore‐pellet model is used to analyze experimental data for SO2 removal using CuO impregnated on alumina as a sorbent. The effect of structural changes in the active solid (CuO) is included in the analysis and found to be important. The changes are characterized by three parameters: the ratio of solid product (CuSO4) molal volume to solid reactant (CuO) molal volume, the weight fraction of active solid, and the radial distribution of the active solid within the pellet.

The calculated kinetic constants are independent of CuO weight fraction and can be used for pellets with different CuO weight fractions. For the data set used in this work, it is shown that some degree of nonuniformity in the CuO distribution within the alumina matrix is needed in order to obtain a good fit of the data by the model.
Transient free‐surface flows: Fluid advancing through a bed of cylinders
Authors: Behrens R. A. , Crochet M. J. , Denson C. D. , Metzner A. B.
Published: June 2004

The flow of a viscous liquid into a bed filled with a regular array of cylinders was observed experimentally and simulated numerically. Viscous forces were always dominant, as is expected to be the case in many actual resin transfer molding operations.

Excellent agreement on the shape of the free surface was found between experiment and simulation over the full experimental range. The work accordingly validates a very simple algorithm for representing the surface motion of a fluid at its contact line with a solid surface, and extends its use to surfaces with smaller radii of curvature than considered previously. Consequently, this algorithm is expected to be valid for fluid flow into any irregularly shaped empty domain and to be useful in molding operations in inferring the presence of voiding, and of molecular orientations and conformations adjacent to fibers or other solids.
Characterization of fuel cell electrode processes by AC impedance
Authors: Yuh C. Y. , Selman J. R.
Published: June 2004

Alternating current impedance measurements can be used as an in situ method for characterizing rate‐limiting processes in fuel cell electrodes. Theoretical models have been derived and solved analytically. Theoretical analysis indicates that kinetic information can be extracted from the effect of temperature and gas composition on the size of the kinetic loop. The relative importance of kinetics and mass transfer can be inferred from AC impedance analysis. Several examples are given for the case of molten carbonate fuel cell (MCFC) electrodes. Such electrodes are not accessible to classical kinetic investigation techniques due to the high operating temperature and corrosive environment. Relevant kinetic information about MCFC electrodes has been obtained successfully using this in situ technique.
Calculation of the temperature field in injection molding
Authors: Dupret F. , Vanderschuren L.
Published: June 2004

The numerical simulation of nonisothermal effects during the filling stage of injection molding is investigated here. Generalized Newtonian fluid flows are simulated within thin cavities of arbitrary shape. The numerical scheme is based on a hybrid spatial discretization: classical low‐order Lagrangian interpolants are used in the midsurface directions, while full polynomials constitute the approximation in the gapwise direction. Discrete equations are obtained by use of the Galerkin finite element method combined with a collocation procedure. Special attention is devoted to the influence of the fountain flow (occurring at the front) on the temperature field. A correct writing of the front thermal boundary condition is derived in agreement with the Hele‐Shaw simplified form of the equations. Some illustrative results are presented.
Mass‐transfer effects of bubble streams rising near vertical electrodes
Authors: Whitney Gina M. , Tobias Charles W.
Published: June 2004

Mass‐transfer enhancement by a stream of bubbles rising near a planar, vertical surface is resolved spatially and temporally using a micromosaic electrode. A stream of gas bubbles is generated electrolytically, either at a segment directly below and in the plane of the monitoring electrodes, or at a wire tip that can be positioned inside or outside the mass‐transfer boundary layer. The mass‐transfer enhancement resulting from bubbles rising within the mass‐transfer boundary layer is found to be strong and localized, in agreement with trends predicted by a surface‐renewal model. Mass transfer resulting from bubbles rising outside the boundary layer is found to receive a steady, laminar enhancement, correlating well with predictions from an idealization of the bubble stream entraining a cylinder of liquid.
Discrete structural models and their application to gas‐solid reacting systems
Authors: Zygourakis Kyriacos , Sandmann Charles W.
Published: June 2004

Discrete models are developed to simulate the reaction of solids with complex pore structures in the kinetic‐ and diffusion‐controlled regimes. Simulations employ two‐ and three‐dimensional computational grids, and model parameters are directly obtainable from measurable physical properties of the unreacted solid. Model predictions can be displayed pictorially, allowing for direct visualization of the simulated process, while image processing of the computational grids can provide detailed structural information not easily obtainable from other analytical models. The effects of various pore structural parameters on the predicted reactivity patterns are investigated. Several examples are presented to illustrate the use of the models and demonstrate the significance of the pore structure in determining the reactivity of coal‐derived chars.
Transient studies and kinetic modeling of ethylene oxidation on Pt/SiO2
Authors: Sant Ravindra , Kaul David J. , Wolf Eduardo E.
Published: June 2004

This paper describes a reaction‐reactor model, utilizing alternative reaction pathways, developed to explain experimental results from the total oxidation of ethylene on a Pt/SiO2 catalyst. Transient temperature‐programmed reaction and concentration‐programmed reaction experiments were utilized to study the reaction through FTIR spectroscopic and surface temperature measurements combined with bulk measurements of concentration and temperature. Different elementarystep pathways were incorporated into the concentration and energy balances in performing the simulations. One pathway utilized adsorbed CO, formed from adsorbed ethylene, as the route to CO2 formation. This pathway is shown to become important under ethylene‐rich conditions. The second pathway utilized the direct reaction between adsorbed ethylene and adsorbed oxygen as the path to CO2 formation. The latter route is shown to become important under oxygen‐rich conditions. Finally, an example is utilized to show the advantages of the nonequilibrium elementary‐step modeling approach over conventional “equilibrium” models where only one step is taken to be rate‐determining.
Translational‐rotational coupling parameters for mutual diffusion in N‐octane
Authors: Erkey Can , Akgerman Aydin
Published: June 2004

The infinite dilution diffusion coefficients of methane, ethane, n‐propane, n‐pentane, n‐heptane, n‐decane, n‐dodecane, and n‐tetradecane have been measured in n‐octane using the Taylor dispersion technique in the temperature range 304–435 K at 1.72 × 106 N/m2. Using the framework of the rough hard sphere theory, the values of the translation‐rotational coupling parameter have been determined for each solute‐solvent pair at each temperature. The coupling parameter was found to be independent of temperature over the entire temperature range of the experiments performed and it has the same value for solutes ranging from n‐pentane to n‐tetradecane. A calculation scheme is presented for accurate determination of infinite dilution diffusion coefficients for a wide range of solutes in n‐octane at a wide temperature range.
Multicomponent reactive dispersion in tubes: Collocation vs. radial averaging
Authors: Wang J. Chih , Stewart Warren E.
Published: June 2004

Orthogonal collocation is used to analyze the dispersion and chemical conversion of a multicomponent fluid pulse in a laminar isothermal flow in a catalytic tube. Homogeneous and heterogeneous first‐order chemical reactions are included, and a full matrix diffusion law is used. The collocation method represents the dynamics well over wide ranges of system parameters and reaction time. Radial averaging approaches prove less convenient, since the dispersion coefficients thus introduced are complicated functions of time, transport properties and chemical kinetics.
Lumping nonlinear kinetics: Apparent overall order of reaction
Authors: Astarita Gianni
Published: June 2004

The analysis of Astarita and Ocone (1988) of the lumping of nonlinear kinetics in continuous mixtures offers a framework for examining the apparent overall behavior of a multicomponent mixture given the underlying intrinsic kinetics. Specific solutions of the governing equations are given for some special cases of underlying intrinsic kinetics. It is shown that the apparent overall order of reaction may be both larger and smaller than the underlying one, depending on the initial concentration distribution and on the distribution of kinetic parameters.
A hybrid approach to nonlinear system stability and performance
Authors: Nikolaou M. , Manousiouthakis V.
Published: June 2004

The problems of stability and performance for nonlinear feedback systems are examined. A hybrid approach that combines input‐output and state‐space theories is proposed. Within this framework the notions of dynamic gain and dynamic incremental gain of a nonlinear operator are reexamined and their inability to accurately reflect the dynamic behavior of a physical system is demonstrated. The concepts of dynamic gain and dynamic incremental gain over sets are introduced and their connection to a physical system's dynamic behavior is established. Theorems are developed that allow the systematic computation of these gains through nonsmooth optimal control theory. Illustrative examples are provided throughout the paper.
Thermocapillary flows in a rotating float zone under microgravity
Authors: Natarajan Ramesh
Published: June 2004

A Galerkin finite‐element method is used to study the rotating, thermocapillary fluid flow in the cylindrical float zone configuration for crystal growth. Results are presented for a fluid of Prandtl number 1.0 in a zone of unit aspect ratio under microgravity conditions. In the absence of rotation, the thermocapillary flow is responsible for large radial non‐uniformities in the heat transfer, especially at the cold boundary. A sufficiently strong uniform corotation of the end rods leads to a rigid rotation of the fluid in the interior of the zone; in this case, the thermocapillary flows are considerably attenuated and confined to a region near the free surface of the zone. This provides a more uniform thermal environment over the bulk of the crystal‐melt boundary, which is a very desirable processing condition. When the end rods are counterrotated, inertially driven meridional flow cells appear in the zone. The interaction of the rotational forced convection with the thermocapillary flow causes the region of the local heat transfer minimum at the cold boundary to gradually shift away from the axis of the zone toward the meniscus.
A mathematical model for heterogeneous reactions with a moving boundary
Authors: Batarseh Kareem I. , Swaney Glenn P. , Stiller Alfred H.
Published: June 2004

A general mathematical model is presented for the noncatalytic reaction between a porous solid and a gas with solid product formation under conditions controlled by intraparticle diffusion. The model has been formulated in general terms so as to allow the incorporation of specific details pertaining to actual systems. The formulation assumes that the gaseous reactant diffuses through the pore space of the particle followed by diffusion through the product layer. The resulting equations are solved numerically.

This model has been used to predict conversion‐time curves for the atmospheric oxidation of iron disulfide contained in porous solids. The predicted curves are in excellent agreement with the experimental data. In particular, it produces accurate estimates for the conversion rates of iron disulfide at different temperatures.
Gas solubility and Henry's law near the solvent's critical point
Authors: Japas M. L. , Sengers J. M. H. Levelt
Published: June 2004

It has been experimentally observed, for water and nonaqueous solvents alike, that Henry's constant passes through a maximum and then declines as the temperature is raised from the triple point to the critical point. From classical and nonclassical models, we derive exact relations for the value of Henry's constant and its temperature dependence at the solvent's critical point, showing that the decline of this constant is a universal phenomenon. We demonstrate that the limiting temperature dependence of Henry's constant can be predicted from the thermodynamic properties of the pure solvent and the initial slope of the critical line. The validity of our prediction is tested by comparing it with experimental solubility data for several gases in high‐temperature water and benzene. Our predictive model appears valid over a temperature range of at least 15% in temperature below the critical point of the solvent.
Silane pyrolysis in a piston reactor
Authors: Morrison Philip W. , Reimer Jeffrey A.
Published: June 2004

A piston reactor has been developed to examine some aspects of particle formation during the pyrolysis of silane (SiH4). This reactor generates conditions intermediate between a static pyrolysis reactor and a shock tube reactor. It effectively excludes contributions of wall reactions to the pyrolysis experiments. The apparent kinetics for this reactor do not conform to values published in the literature. A model that incorporates silane‐particle reactions can account for the deviations. In addition, emission of visible light accompanies the sooting reactions. The source of this emission seems to be the sum of incandescence from the hot particles and continuum radiation from another source.
Detecting and avoiding unstable operation of autothermal reactors
Authors: Gusciora Paul H. , Foss Alan S.
Published: June 2004

An on‐line procedure is developed that uses estimates of the dominant eigenvalues of a reactor‐exchanger system to detect impending unstable operating behavior. In the event reactor stability is threatened, changes are made in the reactor's nominal operating state through a procedure that ensures the attainment of an accessment is made through estimation of closed‐loop eigenvalues derived from a physically‐based dynamic model. The guiding of the reactor to safe and stable conditions is carried out through a multiloop control system that receives optimal process setpoints found from an interative search. Experiments were made on a laboratory autothermal reactor system to assess the correspondence of the predicted and observed degrees of reactor stability.
Diffusion limitations in fischer‐tropsch catalysts
Authors: Post M. F. M. , Van't Hoog A. C. , Minderhoud J. K. , Sie S. T.
Published: June 2004

The extent of diffusion limitations in the catalytic conversion of synthesis gas to hydrocarbons by the Fischer‐Tropsch reaction has been established for a number of iron‐ and cobalt‐based catalysts. The studies were performed in a fixed‐bed microreactor system at temperatures in the range 473–523 K. Variation of catalyst particle size in the range 0.2–2.6 mm shows that the conversion of synthesis gas decreases considerably when the average particle size is increased. The effects of variation of particle size and pore diameter have been quantified with the Thiele model for diffusion limitations. Evidence has accumulated that the limited mobility of reactant molecules in the liquid‐filled pores of Fischer‐Tropsch catalysts is the main cause of retardation of the reaction rates. The experimentally determined reaction rates with various catalysts operated under different conditions show an excellent fit with the theoretical model.
Surface phenomena and mass transfer rates in liquid‐liquid systems: Part 2
Authors: Javed Khalid H. , Thornton John D. , Anderson Terry J.
Published: June 2004

Simultaneous measurements of mass transfer rates to quasisteady‐state pendant aqueous droplets and rates of surface renewal due to Marangoni effects have been carried out using the water/toluene system with acetone as solute. The surface renewal measurements were made with the photochromic dye tracer technique reported earlier (Thornton and Anderson, 1981; Thornton et al., 1985).

Both the mass transfer coefficients and the rates of surface renewal have been shown to be dependent upon the age of the droplet interface and the solute concentration in the continuous phase. Thus, coefficients of a young droplet are high initially, but decrease with time and approach a constant value, dependent upon solute concentration, as the interface ages.

This parallel decrease in both interfacial (Marangoni) activity and transfer rates with time has also been observed (Rogers et al., 1987) with inorganic solutes such as uranyl nitrate and has implications for contactor design.
Wrong‐way behavior of packed‐bed reactors: Influence of interphase transport
Authors: Chen Y. C. , Luss Dan
Published: June 2004

Large transient temperature excursions may be caused by a sudden reduction in the feed temperature to a packed‐bed reactor operating at an intermediate conversion. When a unique steady state exists for all feed temperatures, the magnitude of the wrong‐way behavior predicted by a two‐phase model is very close to that predicted by a pseudohomogeneous model if PeH is equal to the dimensionless heat transfer coefficient H. The two‐phase model enables a more efficient numerical simulation in such cases. The predictions of these two models may be rather different when steady‐state multiplicity exists for some feed temperatures. In such cases, a two‐phase model, which accounts for the axial dispersion of heat, should be used to simulate the transient behavior. The wrong‐way behavior may lead to an ignition of a low‐temperature state or an upstream propagation of a transient temperature wave.
Tube array heat transfer in fluidized beds a study of particle size effects
Authors: Chung Tae‐Yong , Welty James R.
Published: June 2004

Experiments were performed with an array of horizontal tubes, arranged in a regular equilateral triangular pattern, immersed in a fluidized bed operating at 812 K. Data are reported for heat transfer between the bed and a centrally‐located tube in the array. Both total and radiative heat transfer rates were measured for superficial velocities spanning the range from packed bed conditions to over twice the minimum fluidization velocity. Results are presented for five differentsize particles. Local heat transfer values, measured around the tube periphery, and integrated averages are reported for all test conditions.

Comparisons are also made between the heat transfer behavior of a tube in an array and that for a single tube in a hot fluidized bed under the same overall operating conditions. The results of this comparison suggest that the two mechanisms, gas convection and radiation, are competing effects.
Electrodeposition of composition‐modulated alloys in a fluctuating flow field
Authors: Schwartz D. T. , Stroeve P. , Higgins B. G.
Published: June 2004

The effect of fluctuating flow on the electrodeposition of binary alloys is analyzed by numerically solving the transient Navier‐Stokes and species continuity equations for a rotating disk electrode system. Timeperiodic modulations of the flow field and the electrode potential are studied and shown to induce periodic deposition rates for a mass‐transfer‐limited species and an activation‐limited species, respectively. Dimensionless groups are formulated to characterize the operating regimes, where composition‐modulated alloys electrodeposit from a single electrolyte and where fluctuating flow influences the composition distribution of the deposit. It is shown that changing the phase angle difference between the flow and potential modulation waveforms can modify the composition gradients in the deposited alloy, whereas other features of the alloy, such as the total composition variation, are affected less by fluctuating flow.
Wave height estimation in stratified gas‐liquid flows
Authors: Bontozoglou V. , Hanratty T. J.
Published: June 2004

Andritsos and Hanratty (1978b) have shown that the increase in interfacial drag caused by waves in stratified gas‐liquid flows is related to the wave steepness. Recent analyses of finite amplitude Kelvin‐Helmholtz waves are used to develop a correlation for the ratio of the wave height to wavelength.
Axial dispersion of submicron particles in capillary hydrodynamic fractionation
Authors: Silebi Cesar A. , Dosramos Jose G.
Published: June 2004

In the present investigation, Taylor's analysis of the axial dispersion of a solute in a Newtonian fluid undergoing laminar flow through a circular tube was applied to dispersions of colloidal particles, in which effects of size exclusion, inertial and colloidal forces, and wall retardation must be considered. The results indicate that the product of the particle Reynolds and Peclet numbers determines the importance of the inertial forces on both the effective axial diffusion coefficient and the height of a theoretical plate.

The height of a theoretical plate as a function of the eluant ionic strength and average velocity, particle diameter, and tube diameter was determined experimentally. Close agreement with the numerical calculations from the diffusion equation was obtained. The height of a theoretical plate was found to attain a maximum value when the product of Reynolds and Peclet numbers was approximately 10.5.
Periodic operation of a trickle‐bed reactor
Authors: Haure Patricia M. , Hudgins Robert R. , Silveston Peter L.
Published: June 2004

The influence of periodic water flow on SO2 oxidation in a trickle bed of activated carbon catalyst was investigated, whereby gaseous reactants were introduced into the trickle‐bed reactor continuously, but water was turned on and off. Mean liquid superficial velocities of 0.86 and 1.65 mm/s were used. At the latter, an increase in the oxidation rate of about 30 to 45% was found within a range of cycle periods from 2 to 80 min. A temperature change of up to 7°C was observed in the bed during periodic operation. An explanation of the improved average oxidation rate under periodic operation is developed in terms of the steady‐state rates with and without water flow for symmetrical and asymmetrical cycles.
Design/optimization of ternary heterogeneous azeotropic distillation sequences
Authors: Ryan Peter J. , Doherty Michael F.
Published: June 2004

Various strategies for sequencing heterogeneous azeotropic distillations have been explored. The design and optimization of these systems by simulation‐based methods is generally acknowledged to be a very difficult problem (Kingsley and Lucia, 1988). In this article, a simple and accurate design and optimization procedure has been developed. The method does not involve any recycle convergence loops and has been applied successfully to several test problems. Unlike the simulation‐based methods, our method is robust to changes in the system optimization variables (e.g., reflux ratio in the azeotropic column).

In order to demonstrate the method, we present design and optimization results for the dehydration of ethanol using a benzene entrainer. Our optimal designs consume significantly less energy than state‐of‐the‐art designs reported in the literature.
Calculation of the ionization constant of H2O to 2,273 K and 500 MPa
Authors: Tanger John C. , Pitzer Kenneth S.
Published: June 2004

Calculations of the ionization constant of H2O are presented for densities ranging from those of subcritical water to those of supercritical steam. A recently‐proposed semicontinuum model is used to calculate the standard‐state hydration properties for the ions, H3O+, and OH− (Tanger and Pitzer, 1989). Calculated ionization constants are in good agreement with reported measurements at pressures from vapor‐liquid saturation to 500 MPa and temperatures from 273 to 1,073 K. These measurements cover H2O densities ranging from ∼ 1,000 to 450 kg · m−3. At H2O densities less than 400 kg · m−3, our calculated ionization constants should be more reliable than those obtained from the commonly‐used equation of Marshall and Franck (1981).
Tracer experiments and RTD's in heterogeneous reactor analysis and design
Authors: Shinnar Reuel , Rumschitzki David
Published: June 2004

This paper focuses on the problem of scale‐up of two‐phase (e.g., solid‐fluid) reactors. It outlines a class of such reactors which, for first‐order reactions, scale in the familiar single‐phase manner via an analog of the usual residence time distribution formula. The differences lie in the fact that the appropriate tracer experiment entering the formalism for this reactor class refers to a nonadsorbing tracer, and the analog of the plug flow solution is the solution for a fixed bed with fluid‐phase plug flow. Surprisingly, unlike single‐phase systems, there exist two‐phase reactors, outside of the class defined, that do not scale in this manner, even when, say, the true catalyst surface chemistry is purely first‐order. The paper discusses a few examples and implications for the design of two‐phase reactors, including fluidized beds.
Single‐component mass transfer in a cosurfactant‐water‐surfactant system
Authors: Ma Zhuning , Friberg Stig E. , Neogi P.
Published: June 2004

A mass transfer system has been designed to allow transport of only one component and over small changes in concentration. One of the phases was always lamellar liquid crystals, the other was micellar or its inverse structure. It can be argued that the kinetics of phase transformation at the interphase would be important in such systems; in fact, the measurements of individual phase volumes show overshoots that are not explained by classical mass transfer mechanisms. It is suggested that the intermediate structures that are formed during transport are not thermodynamically favored, and consequently the condition of local equilibrium does not hold. Using developments for such systems in diffusion in polymers, the breakdown of local equilibrium has been shown to be adequate in explaining the observations qualitatively. A parallel development for adsorption has not been undertaken.
Two‐component LDV measurements in a stirred tank
Authors: Mahouast M. , Cognet G. , David R.
Published: June 2004

Previous work has shown that in a standard stirred tank, the three‐dimensional velocity field in the discharge flow of the stirrer is characterized by a strong deviation from homogeneous isotropic turbulence. By means of a two‐component laser Doppler velocimeter (2D‐LDV), the Reynolds stresses are measured in a 6.3‐dm3 standard tank stirred by a six‐flat‐blade Rushton turbine. Obviously, these stresses must be known to better understand the turbulent mixing in such a configuration. Moreover, the 2D‐LDV appears to be an efficient tool for characterizing, from an energy viewpoint, the type of agitator used and, in principle, is confirmed to be useful to accurately obtain the length scales of turbulence which are fundamental parameters for the study of the micromixing processes.
An algorithm for simultaneous chemical and phase equilibrium calculation
Authors: Xiao Wen‐de , Zhu Kai‐hong , Yuan Wei‐kang , Chien Henry Hung‐yeh
Published: June 2004

A new generalized algorithm has been developed for solving simultaneous chemical and phase equilibrium equations of two‐phase systems. By rearranging convergence loops of the algorithm of Sanderson and Chien (1973), this new approach shows improved robustness and speed. Solutions to the material balance and chemical equilibrium equations are obtained with an improved Marquardt method (Zhan, 1976). An esterification reaction and dissociation reactions of electrolytes in phase equilibrium are presented as examples.
Adsorption of gaseous ethylamine on H‐form strong‐acid ion exchangers
Authors: Yoshida Hiroyuki , Ruthven Douglas M.
Published: June 2004

Equilibrium isotherms and kinetic data for sorption of ethylamine on two different H‐form ion exchangers (a homogeneous gel type and a biporous MR type) have been determined experimentally by the gravimetric method. The gaseous amine is adsorbed on the dry resins according to an acid‐base neutralization reaction, and the saturation capacity coincides with the exchange capacity of the resin. For the gel‐type resin, the equilibrium is less favorable and the isotherm is almost linear over the experimental range.

The kinetic data also reflect the structural difference between the two adsorbents. In the MR‐type resin, the sorption rate is controlled by macropore diffusion with rapid equilibration of the adsorbed phase within the microparticles. Since the equilibrium isotherm is highly favorable, approaching the irreversible limit, the uptake curves are well represented by the ‘shrinking core’ model. Diffusion in the gel‐type resin is much slower and the pore diffusivities are smaller by several orders of magnitude, presumably reflecting the smaller effective pore diameter.

These results suggest that the MR‐type resin is a potentially useful adsorbent for removal of traces of light amines from industrial gases.
Precipitation of microsize organic particles from supercritical fluids
Authors: Chang C. J. , Randolph A. D.
Published: June 2004

The precipitation of organic particles from supercritical fluids (SF) by expansion (SFX) has become an interesting alternative to milling without thermal decomposition. Through the rapid expansion process, a dramatic change of the solute supersaturation ratio is created causing subsequent precipitation with a narrow particle‐size distribution. It was found that β‐carotene precipitates from SF ethylene and ethane have the feed material crystallinity. However, SF CO2 reacted with β‐carotene and did not give characteristic β‐carotene X‐ray spectra. The mean particle sizes of these precipitates were in the submicron range (ca 0.3 μm). Increased solubility was obtained by addition of toluene as cosolvent in SF ethylene. It was found that the mean size of β‐carotene particles, generally remained unchanged if the toluene concentration was less than 11/2 mol %. The SFX process appears to be in a single fluid phase when up to 11/2 mol % toluene cosolvent is used.
Multicomponent diffusion and convection in capillary structures
Authors: Sotirchos Stratis V.
Published: June 2004

A previously developed method (Sotirchos and Burganos, 1988) for studying isobaric multicomponent diffusion in a class of capillary networks is extended for application to the problem of convection and diffusion of gaseous mixtures in porous media. The method is used to develop flux models for nonisobaric transport in capillary structures, which are then compared with corresponding “three‐parameter” dustygas models. Results for discrete and continuous pore size distributions show that the predictions of the developed flux models depend strongly, both qualitatively, and quantitatively, on the procedure used to average the dusty‐gas model equations for a single pore over the pore‐size distribution.
Capillary experiments of flow induced crystallization of HDPE
Authors: Titomanlio G. , Marrucci G.
Published: June 2004

Flow‐induced crystallization experiments are made in a capillary apparatus modified with a downstream reservoir under pressure. Capillary length, diameter, and entrance angle are changed, as well as flow rate. The results show that the crystallization temperature is influenced both by the elongational flow at the capillary entrance and by the shear flow along the capillary. The independent effect of the pressure equals that obtained under static conditions. The effect of shear is correlated in terms of shearing work.
Oxygen transport in biofilm electrodes for screening of toxic chemicals
Authors: Goldblum David K. , Holodnick Steven E. , Mancy Khalil H. , Briggs Dale E.
Published: June 2004

A biosensor electrode system with unique configuration and a thin layer of immobilized yeast cells, set on the surface of an amperometric oxygen membrane electrode, was developed for rapid screening of toxic chemicals in a variety of pollution and process control applications. Measurement is based on the instantaneous detection of changes in oxygen respiratory activity of biofilm of yeast cells upon exposure to toxic chemicals.

The design of this electrode system, referred to as biofilm electrodes, was based on a mathematical model of oxygen transport in the biofilm and the electrochemical current response. The biofilm, which consists of three sublayers—boundary layer, filter pad, and yeast cell layer—was modeled as a one composite diffusion layer, or three separate layers in series. While the three layer model is more theoretically complete, the one layer model was more reliable and simpler to use.
A model for volatiles release into a bubbling fluidized‐bed combustor
Authors: Stubington John F. , Chan Shu W. , Clough Stephen J.
Published: June 2004

A volatiles release model has been developed to predict the location and quantity of coal volatiles that are released into a bubbling fluidized‐bed combustor, using overbed, in‐bed or underbed feed systems. This model not only considers time resolution of the simultaneous processes of coal devolatilization and coal particle movement after injection, but also takes account of the stochastic nature of this particle movement. Volatiles release is nonuniform, occurring throughout an industrial‐scale bed, but only in restricted parts of a laboratory‐scale bed. For the industrial‐scale bed, 32% of volatiles are released directly into the freeboard while the particle is devolatilizing at the surface, whereas 24% are released there for the laboratory scale bed. The model predicts the formation of numerous discrete volatiles regions within the bed, in agreement with a new interpretation of experimental measurements from an in‐bed oxygen probe. The model is shown to be consistent with other experimental in‐bed measurements obtained during combustion of “simulated” volatiles.
Consequence of flow nonuniformity on the measurement of effective diffusivity
Authors: Cui L. C. , Schweich D. , Villermaux J.
Published: June 2004

Measurements of effective diffusivity by the chromatographic method are shown to be biased by nonuniformity of the external porosity in a cross‐section of the column. A very simple model is proposed to interpret experimental results. A modified Van Deemter method is given to extract the axial dispersion and effective diffusion coefficients from chromatographic peaks. This method allows one to ignore the external porosity distribution. Experiments show that even though axial dispersion obeys classical correlations, the internal mass transfer parameters may be strongly affected by the external porosity distribution.
Bubble‐size distribution in Fischer‐Tropsch‐derived waxes in a bubble column
Authors: Patel Snehal A. , Daly James G. , Bukur Dragomir B.
Published: June 2004

Bubble sizes were measured for molten wax‐nitrogen systems using photography and dynamic gas disengagement. The effects of operating conditions, system geometry and wax type were studied in 0.05‐ and 0.23‐m‐diameter by 3‐m‐tall bubble columns. Both techniques were used with FT‐300 wax, while only the dynamic gas disengagement technique could be used with reactor waxes due to their dark color. For FT‐300 wax, Sauter mean diameters obtained from photographs taken near the column wall were significantly lower than those obtained from photographs taken near the center. The ds values obtained from dynamic gas disengagement and photographic (near the column center) methods, for this noncoalescing medium, were in the range 0.5–1.6 mm in the large‐diameter column. For reactor waxes, ds values were significantly higher (1–2 mm for Sasol and 1–5.5 mm for Mobil's reactor wax) and are in agreement with results reported in earlier studies with similar waxes (ds = 2–4 mm), where different experimental techniques (light transmission or hot wire anemometry) were employed.