bluuebunny/crossref_metadata_2025_split huggingface dataset:
Transient studies and kinetic modeling of ethylene oxidation on Pt/SiO2
Authors: Sant Ravindra , Kaul David J. , Wolf Eduardo E. 
Published: June 2004

AbstractThis 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

AbstractThe 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

AbstractOrthogonal 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

AbstractThe 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

AbstractThe 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

AbstractA 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

AbstractA 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

AbstractIt 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

AbstractA 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

AbstractAn 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

AbstractThe 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

AbstractSimultaneous 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

AbstractLarge 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

AbstractExperiments 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

AbstractThe 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

AbstractAndritsos 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

AbstractIn 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

AbstractThe 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

AbstractVarious 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

AbstractCalculations 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

AbstractThis 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

AbstractA 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

AbstractPrevious 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

AbstractA 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

AbstractEquilibrium 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.