bluuebunny/crossref_metadata_2025_split huggingface dataset:
Oxidative pyrolysis of methane to higher hydrocarbons: Effects of water in feed
Authors: Choudhary Vasant R. , Chaudhari Sopan T. , Rajput Amarjeet M. 
Published: June 2004

AbstractDirect conversion of methane into higher hydrocarbons (ethane, ethylene, and to small extent C3‐4 hydrocarbons) by its partial oxidation with oxygen in the absence of catalyst has been investigated. It was carried out in a quartz flow reactor at atmospheric pressure under nonflame conditions at different temperatures (1,123‐1,273 K), CH4/O2 ratios (1.7‐9.0), space velocities (500‐5,000 h−1), and water concentrations (0.0‐80 mol %) in the feed. No coke deposition was observed in the reactor, but there was a formation of small amounts of tarry matter in the reaction at ≥ 1,223 K, particularly for the lower CH4/O2 ratios (≤3.5). The product distribution in the process was strongly influenced by the dilution of feed with N2 or water. Addition of water in the feed was highly beneficial to obtaining higher yield and/or to selectivity for the higher hydrocarbons and suppressing almost completely the formation of tarry matter.
Reaction‐driven convection in a porous medium
Authors: Farr William W. , Gabitto Jorge F. , Luss Dan , Balakotaiah Vemuri 
Published: June 2004

AbstractThe onset of three‐dimensional reaction‐driven convection in a porous medium is investigated using linear stability theory. The geometries investigated include a finite cylinder and a rectangular parallelepipped of arbitrary aspect ratios. The analysis determines, among other things, the likely modes (flow patterns) to emerge first as a function of reaction parameters and aspect ratios. The flow fields corresponding to three‐dimensional modes are described in detail. Important qualitative differences are found between reaction‐driven convection and the standard Lapwood or Bénard convection due to a temperature gradient applied to the boundaries of the system.The second part of the work examines numerically reaction‐driven natural convection in a porous two‐dimensional rectangular box. Orthogonal collocation and continuation techniques are used to determine the conduction and convection branches of solutions as a function of the Rayleigh number (Ra), the Frank‐Kamenetskii number (δ) and the aspect ratio (α). The convective solutions (streamlines and isotherms) corresponding to primary, secondary and tertiary bifurcations are presented. The effect of natural convection (Ra) on the ignition point (critical δ value) is determined for three different aspect ratios.
Emulsion layer model for wall heat transfer in a circulating fluidized bed
Authors: Mahalingam M. , Kolar Ajit Kumar 
Published: June 2004

AbstractAn emulsion layer model is presented which predicts the thickness of a downward‐moving emulsion layer along the wall of a circulating fluidized bed, the mean solids velocity, and the solids flux in the layer. Also presented is a heat transfer model which, in combination with the emulsion layer model, predicts the low‐temperature data very well. An alternate slab model proposed for the radiative component in a high‐temperature circulating fluidized bed agrees well with experimental data. The heat transfer predictions of the overall model for such operating parameters as solid circulation flux, suspension temperature, length of the heat transfer surface, superficial gas velocity, and mean particle size are in good agreement with the published data for long surfaces.
Influence of operating and physical variables on interfacial area determination
Authors: Camacho Fernando , Molina Emilio , Valdés Fernando , Andujar José Manuel 
Published: June 2004

AbstractThe influence of various operating and liquid‐phase physical variables on interfacial area ab in cylindrical and spherical gas‐liquid stirred contactors was studied using the sulfite method. Studied were stirring speed, sparger porosity, gas flow rate, liquid‐phase temperature, surface tension, and pH.The results show that the influence of operating variables (gas flow rate Qg, stirring speed ω, and the average pore radius of the sparger used rp) on ab, for both types of contactors, can be given by the equation:

Dependency of ab on surface tension observed was believed to be caused by a rising rate of bubble coalescence with increased surfactant concentration. The higher the surfactant concentration, the lower ab, tending to a limit that depends on both the type of surfactant and the average size of sparger pore used. Due to the measurement method used, solution pH and temperature were also observed to influence ab.
Electrostatic potentials and protein partitioning in aqueous two‐phase systems
Authors: Haynes C. A. , Carson J. , Blanch H. W. , Prausnitz J. M. 
Published: June 2004

AbstractA thermodynamic analysis unambiguously relates interfacial‐electrostatic‐potential differences measured with Ag/AgCl capillary electrodes to the equilibrium properties of an aqueous two‐phase system. Interfacial electrostatic potentials were measured as a function of total α‐cyclodextrin concentration in an aqueous two‐phase system containing 9.1 wt. % poly (ethylene glycol), 6.1 wt. % Dextran T‐70, and 1‐mM potassium iodide. An order‐of‐magnitude increase in the interfacial electrostatic potential was observed as the total concentration of α‐cyclodextrin increased from 0 to 1 mM. Measured partition coefficients for α‐chymotrypsin, lysozyme, and bovine serum albumin depend strongly on α‐cyclodextrin concentration. For example, as the concentration of α‐cyclodextrin rises from 0 to 1 mM, the partition coefficient of lysozyme decreases from 1.7 to 0.55. These measurements are in good agreement with theoretical expectations.
Effects of operating conditions on longitudinal solids mixing in a circulating fluidized bed riser
Authors: Rhodes M. J. , Zhou S. , Hirama T. , Cheng H. 
Published: June 2004

AbstractLongitudinal solids mixing was studied experimentally in circulating fluidized bed risers of internal diameter 0.152 m and 0.305 m. Superficial gas velocity and mean solids flux used were 2.8–5.0 m/s and 5.0–80 kg/m2·s, respectively, and the bed solids had a surface volume mean diameter of 71 μm and a particle density of 2,456 kg/m3. A sodium chloride tracer was used in impulse injection experiments. A simple, one‐dimensional dispersion model describes measured solids mixing satisfactorily. Peclet numbers (UoL/Dz) found, in the range 1.0–9.0, were correlated with the riser diameter and mean solids flux. The modeling approach described here permits residence time distribution curves to be calculated directly from the knowledge of superficial gas velocity, mean solids flux, and riser diameter. Longitudinal solids mixing in the riser decreased with increasing riser diameter. The results are consistent with recent hydrodynamic studies.
Direct numerical simulations of reacting flows in homogeneous turbulence
Authors: Gao Feng , O'Brien Edward E. 
Published: June 2004

AbstractA direct numerical simulation (DNS) code has been modified to investigate chemically reacting flows in a stationary, homogeneous turbulence. Single and multispecies reactions as well as complicated reaction schemes such as parallel/consecutive reactions are studied. The effects of some parameters are discussed. The numerical data are used to test a simple first‐order closure model, proposed by Dutta and Tarbell (1989), for turbulent reacting flows. It is shown that the DNS technique provides us with an effective tool to isolate the effects of different parameters and is likely to be useful for proposing and testing closure models.
Diffusive and convective protein transport through asymmetric membranes
Authors: Opong W. Senyo , Zydney Andrew L. 
Published: June 2004

AbstractExperimental data are obtained for bovine serum albumin transport through asymmetric polyethersulfone ultrafiltration membranes of differing molecular weight cutoff in a stirred ultrafiltration device. The actual membrane sieving coefficient is determined from filtrate concentration measurements using a stagnant film model to account for bulk mass transport effects. These sieving coefficients are then used to evaluate the relative contributions of diffusive and convective transport to the overall protein flux. The results are in good agreement with available hydrodynamic models for the hindrance factors for convective and diffusive transport of spherical solutes through well‐defined pores, with the effective solute to pore size ratio evaluated from a partitioning model that explicitly accounts for the ellipsoidal shape of the protein and the membrane pore size distribution. The implications of these results to the analysis of experimental data for membrane sieving and to the design of effective protein fractionation devices are also discussed.
Inaccessible porosity in gasification reactions under kinetic control
Authors: Delikouras Evangelos A. , Perlmutter Daniel D. 
Published: June 2004

AbstractRandom pore models have been successful in interpreting a wide range of experimental observations in gas‐solid reactions; however, their applicability has been limited to cases where the entire porous structure is initially accessible to reacting gases. In many practical cases, porous solids contain significant void regions that are initially inaccessible to gaseous reactants, but become accessible in the course of the reaction. A conceptual and mathematical model is developed here that accounts for the existence of this hidden porosity. It uses the same representation of the porous network as was developed in the earlier random pore models, but introduces physically meaningful expressions for the discovery of hidden pores, and the subsequent growth of the discovered pores. The modified model calls for only one additional parameter, the initial volume fraction of hidden porosity in the total volume. It is demonstrated computationally that neglecting hidden porosity can lead to serious underestimations of conversion and reaction rates.
Geochemical modeling of two‐phase flow with interphase mass transfer
Authors: Novak Craig F. , Lake Larry W. , Schechter Robert S. 
Published: June 2004

AbstractInjecting an acidic aqueous solution into a carbonate‐rich permeable medium can cause the formation of a mobile gas phase through mineral dissolution. The flowing gas can cause significant changes in mineral identities through interaction with the initially present mineral and aqueous species. We have developed a solution to such transport problems based on a finite difference implicit‐pressure/explicit‐saturation formulation for two‐phase flow, using the local equilibrium assumption as calculated with the Villars‐Cruise‐Smith stoichiometric chemical equilibrium algorithm.We illustrate the changes that occur to a calcite/iron (II) hydroxide medium upon injection of hydrochloric acid and the stripping of hydrogen sulfide gas from carbon dioxide gas injected into a siderite‐rich medium. The examples demonstrate that the formation and/or presence of a gas phase can alter resident minerals and that minerals can remove impurities from a gas phase.
Expansion characteristics of tapered fluidized beds
Authors: DiFelice Renzo , Foscolo Piero U. , Gibilaro Larry G. , Wallis Graham B. , Carta Renzo 
Published: June 2004

AbstractThe effects of taper on the steady‐state expansion characteristics of a homogeneous fluidized bed are examined theoretically and experimentally. It is shown that secondary factors that might be thought to influence the expansion (fluid accelerational effects including added mass and particle phase elasticity) are unlikely to be of significance in practical situations.
One‐dimensional numerical model for a spray column heat exchanger
Authors: Summers Steven M. , Crowe Clayton T. 
Published: June 2004

AbstractA numerical model describing performance of a liquid‐liquid spray column heat exchanger is developed. The model is for a disperse packed column operating in steady state. The fundamental equations for the continuous and distributed phases are developed in Eulerian and Lagrangian forms, respectively. The model predicts simultaneously the volume fraction, velocity, and temperature for each phase. Energy and momentum are exchanged between phases through source variables. Empiricism in the model is limited to the specification of droplet drag coefficient and Nusselt number correlations which are utilized to calculate momentum and energy source terms. Application of the computer model to column operation yields predictions that compare closely with experimental results.
Estimation of dissolved carbon dioxide concentrations in aerobic fermentations
Authors: Royce Patrick N. C. , Thornhill Nina F. 
Published: June 2004

AbstractDissolved carbon dioxide and bicarbonate ions in fermentation broths can (independently) inhibit or promote microbial growth and productivity. In research facilities with a large number of fermenters, dissolved carbon dioxide sensors tend not to be used, and as a result this variable will generally go unmonitored, making the meaningful analysis of data more difficult. For aerobic fermentations, mass transfer of carbon dioxide can be described in an analogous way to oxygen transfer. The mass transfer coefficient for carbon dioxide is 0.89 times that for oxygen. The maximum dissolved carbon dioxide concentration as a function of exit gas composition is compared with the concentration obtained by assuming equilibrium between the broth and exit gas. The difference between these two concentrations is typically 20–40% of the equilibrium concentration. In large fermenters, a degree of plug flow behavior in the gas and the generally lower specific aeration rates will serve to produce a better approach to equilibrium than for research fermenters.
Distributor effects in liquid fluidized beds of low‐density particles
Authors: Asif Mohammad , Kalogerakis Nicolas , Behie Leo A. 
Published: June 2004

AbstractThe effect of flow nonuniformities existing near the distributor of a liquid fluidized bed containing very low‐density particles (ρs = 1.05 g/mL) has been investigated experimentally. Immobilized enzyme or cell bioreactors operated as liquid fluidized beds often employ low‐density particles as the support matrix containing the immobilized biocatalyst. With poorly designed multihole distributors, dead zones can be created which seriously undermine the performance of the bioreactor due to the lack of nutrients. Design guidelines are provided for distributors to minimize the inlet flow disturbances, and hence eliminate potential distributor region problems. As a result, the dispersion model, which is normally used for design purposes, can then be applied.
Adaptive control of photolithography
Authors: Crisalle Oscar D. , Soper Robert A. , Mellichamp Duncan A. , Seborg Dale E. 
Published: June 2004

AbstractAdaptive control techniques, with their capability for providing satisfactory control even when the process changes with time, are promising candidates for dealing with common problems encountered in photolithography processing such as batch‐to‐batch variations in resist properties and inconsistencies in resist curing. In this article, an adaptive control strategy for the photolithography process is proposed and evaluated. The design utilizes a reduced‐order lithography model, an on‐line parameter estimator, and a nonlinear model‐inversion controller.The width of the printed resist lines, a crucial output of photolithography, is controlled by automatically adjusting the exposure energy. In the calculation of the appropriate exposure adjustment, the controller uses both measured critical‐dimensions as well as estimated values produced by the process model. The control system is capable of tracking changes in the photolithography process by automatic updating of key model parameters as the process evolves in time. Simulation studies of the closed‐loop adaptive control strategy, using the PROLITH simulation package to represent the lithography process, demonstrate the feasibility of this approach.
Nonminimum‐phase compensation for nonlinear processes
Authors: Wright Raymond A. , Kravaris Costas 
Published: June 2004

AbstractThe design of controllers for nonlinear, nonminimum‐phase processes is one of the most difficult control problems currently faced. Current available control algorithms for nonlinear processes rely implicitly or explicitly on an inverse of the process. Linear control methods for nonminimum‐phase processes are based on a decomposition of the process into a minimum‐phase and a nonminimum‐phase part. Such a decomposition is an open problem for nonlinear systems.In this work, a control structure called the minimum‐phase output predictor for nonlinear, nonminimum‐phase processes is developed. The structure is based on the notion of statically equivalent outputs; a minimum‐phase, statically equivalent output is estimated on‐line and then an available nonlinear control algorithm is used to control it to set point. The advantage of the proposed formulation is that it is based on the calculation of an output function, not on a decomposition of the process dynamics. The proposed control methodology is applied and its performance is evaluated for a chemical engineering example.
Accuracy and convergence of the p‐ and hp‐type finite element methods for the Navier‐Stokes equation
Authors: Talwar Kapil K. , Khomami Bamin 
Published: June 2004

AbstractHigher‐order p and hp finite element algorithms for modeling the flow of New‐tonian fluids have been developed. Numerical computations in three test geometries were carried out to compare the performance of these methods to that of conventional h‐type finite elements. Computations demonstrated that for solutions which are analytic everywhere in the domain of interest, p extensions exhibit an exponential rate of convergence as opposed to the characteristic linear convergence of h‐type finite element procedures. Moreover, for solutions which are analytic everywhere in the domain except at a set of points, hp extensions with geometric mesh refinement based on effective error indicators (individually, the error in the energy norm, local and global deviations from mass and momentum conservation) also give rise to an exponential convergence rate. This rapid convergence of p and hp finite element formulations enables the achievement of the solution of linear Stokes problems with an order of magnitude reduction in degrees of freedom. In nonlinear problems, these computational savings are appreciated by a factor approximately equal to the number of iterations required to obtain converged solutions.
Multiplicity features of nonadiabatic, autothermal tubular reactors
Authors: Lovo Marianne , Balakotaiah Vemuri 
Published: June 2004

AbstractSingularity theory is combined with asymptotic analysis to determine the exact uniqueness‐multiplicity boundary and ignition and extinction locus for the non‐adiabatic, autothermal tubular reactor model. It is found that the steady‐state behavior of the nonadiabatic reactor is described by the two limiting cases of adiabatic and strongly cooled models. The adiabatic case has been examined in a previous study. Here, we develop limiting models to describe the strongly cooled asymptotes. We also classify the different types of bifurcation diagrams of conversion vs. residence time using the results of singularity theory with a distinguished parameter. Analytical criteria are developed for predicting the conditions under which autothermal operation is feasible when heat losses are significant.
Stripping of trihalomethanes from drinking water in a bubble‐column aerator
Authors: Velázquez Carols , Estévez L. Antonio 
Published: June 2004

AbstractThe desorption of trihalomethanes (THMs) in a cocurrent bubble column has been studied experimentally to validate a previously developed model. The experiments were carried out in a 0.2‐m‐diameter, 2.4‐m‐high, plexiglas bubble column, equipped with seven samplers alongside. The phases in contact were tap water and air. Three out of the four THMs normally found in drinking water were spiked in the feed tank to assure their detection in the analyses. After preparing the feed, each run was begun by setting the flow rates of water and air, and waiting until a steady state was attained. Then, samples from each sampler were taken and sent for chemical analysis. A total of 12 runs were performed with varying superficial gas and liquid velocities, and up to four concentration profiles were obtained for each run. The experimental results were then compared to the predictions of the model proposed by Estévez (1991). The agreement between calculated and experimental concentration profiles was quite good, indicating that the model can be used reliably either in the simulation or design of industrial scale equipment.
Hydrodynamic changes of the depletion layer of dilute polymer solutions near a wall
Authors: De Pablo Juan José , Öttinger Hans Christian , Rabin Yitzhak 
Published: June 2004

AbstractThe system studied in this work is a dilute solution of rod‐like molecules under simple shear flow and near a hard wall. The time evolution of the probability density function is described by a diffusion equation; particle trajectories that correspond to this equation are generated by stochastic methods. Several algorithms are presented to handle the constraints imposed by the presence of the wall. In good agreement with recent experimental work on xanthan solutions, for high shear rates we observe an increase in the thickness of the depletion layer near the wall. For low to intermediate shear rates, however, we find a transient decrease of the depletion layer thickness that has not been observed experimentally. Based on the results of our simulations, we present a simple procedure to determine a few, well defined characteristic parameters from the experimental density profiles.
Role of liquid reactant volatility in gas absorption with an exothermic reaction
Authors: Al‐Ubaidi Basil H. , Selim M. Sami 
Published: June 2004

AbstractA film‐theory model is presented for nonisothermal gas absorption with a second‐order exothermic reaction. The model accounts for the volatility of the liquid reactant and heat transfer from the liquid surface to the gas phase. The pertinent equations were solved numerically using B‐spline collocation. Results from this solution show that for intermediate values of Hatta number the liquid‐reactant volatility is deterimental to the enhancement of gas absorption. As Hatta number approaches zero or infinity, however, the effect of liquid‐reactant volatility becomes minor. Heat losses to the gas phase drastically reduce the interfacial temperature rise, which in turn enhances or inhibits the absorption rate depending on the effective activation energy being larger or less than zero, respectively. Approximate expressions for the enhancement factor and the interfacial temperature rise were also developed. Comparisons with the “exact” numerical solution verified the accuracy of these expressions over a reasonable spectrum of parameter values. The model developed was applied to two cases representing real conditions: the chlorination of toluene and the sulfonation of dodecylbenzene. Volatility effects are shown to be important for the former system, while the relatively nonvolatile dodecylbenzene served as a counter example.
Predicting polymer/solvent diffusion coefficients using free‐volume theory
Authors: Zielinski John M. , Duda J. L. 
Published: June 2004

AbstractThe Vrentas/Duda free‐volume diffusion model accurately correlates polymer/solvent diffusion coefficients over wide ranges of concentration and temperature. Currently the model is semipredictive: limited diffusion data are required to estimate model parameters that can then be used to predict diffusion coefficient behavior over sundry conditions. In this work, we present methods for estimating all of the model parameters without any diffusion data and examine the accuracy of the resulting diffusion coefficient predictions. This is the only technique known that predicts polymer/solvent diffusion behavior without any use of any diffusion data.