bluuebunny/crossref_metadata_2025_split huggingface dataset:
Two‐flux radiation‐field model for an annular packed‐bed photocatalytic oxidation reactor
Authors: Roupp Gregory B. , Nico Jill A. , Annangi Suneetha , Changrani Rajnish , Annapragada Rao 
Published: June 2004

AbstractOne of the remaining challenges in application of heterogeneous photocatalysis for treatment of air streams containing dilute VOCs is to design a cost‐effective photocatalytic reactor that simultaneously allows efficient contact of the contaminated air and solid catalyst while uniformly irradiating the solid catalyst with light. A pseudohomogeneous model was developed to study effects of system parameters on process performance for a gas–solid lamp‐in‐tube annular‐photocatalytic‐oxidation (PCO) reactor in which the annular space is filled with photocatalyst‐coated packing. In this model the flow field is assumed to be uniform and radial diffusion negligible. Homogeneous reactions are neglected. Heterogeneous reaction rates follow Langmuir–Hinshelwood–Houghen–Watson kinetics with rate parameters extracted from independent experiments. A 1:D “two‐flux” incidence submodel is used to account for the radial UV light distribution throughout the reactor annular space. This submodel requires knowledge of the UV lamp radiant emittance, the optical characterstics of the catalytic thin‐film coating, and the UV irradiance at the outer wall of the reactor and contains only a single adjustable parameter—the mean free path between photon–catalyst interactions. The model was validated with experimental performance measurements for destruction of acetone and isopropyl alcohol in a bench‐scale photoreactor. The validated model can be used to predict the optimum catalyst film thickness for given reactor dimensions, packing shape and size, and VOC abatement problem.
First‐order closure theories for series‐parallel reaction in simulated homogeneous turbulence
Authors: Chakrabarti Mitali , Hill James C. 
Published: June 2004

AbstractSimple first‐order closure models for covariances of concentration fluctuations, for use in modeling turbulent flow reactors, were tested by direct numerical simulations. Concentration covariances and other statistical functions were evaluated for a series parallel reaction scheme in decaying, homogeneous turbulent flow. The simulations involve solving the unsteady Navier‐Stokes and mass conservation equations by a pseudo‐spectral method in a 643 wavenumber domain, with initially segregated reactants, for an initial turbulence Reynolds number of 29.9. Simulation results show that covariances of concentration fluctuations normalized with respect to mean concentration values are almost constant and that the time dependence of concentration covariances can be estimated if the mean concentrations are known at any one time after the initial time. Predictions of the first‐order closure models of Bourne and Toor, Brodkey and Lewalle, Li and Toor, and Dutta and Tarbell were compared to simulation results. While none of these closures are satisfactory for all the conditions tested, the Brodkey‐Lewalle closure agrees best with the simulations.
Role of coherent structures on reynolds stresses in a 2‐D bubble column
Authors: Mudde R. F. , Lee D. J. , Reese J. , Fan L.‐S. 
Published: June 2004

AbstractThe hydrodynamics of two‐dimensional bubble columns operated in various flow regimes are studied using particle image velocimetry. Both averaged velocity profiles and Reynolds stress profiles are obtained and discussed in relation to large‐scale structures present in the flow. The normal stresses, dominated by large‐scale structures, are an order of magnitude higher than the shear stress. It is found that the contribution from the bubbles to the shear to the normal stresses is negligible. A time series of the flow field is studied, demonstrating that the flow could be split into a low‐frequency contribution due to the vortical structures and a high‐frequency fluctuating part. The latter gives rise to flat normal stress profiles, and the former is responsible for the original form of the normal stress profiles. The shear stress in the smaller columns investigated can be related to the averaged vertical velocity profile according to a Boussinesq approximation. Data on the eddy viscosity are presented.
Modeling and safety verification of discrete/continuous processing systems
Authors: Dimitriadis V. D. , Shah N. , Pantelides C. C. 
Published: June 2004

AbstractA quantitative, model‐based approach to the safety verification problem for general processing systems operating in the discrete time domain is presented. It is recognized that the operation of most of these systems involves both discrete and continuous characteristics. Therefore, an appropriate modeling framework is proposed, within which models of purely discrete, purely continuous and hybrid systems of arbitrary complexity can be constructed consistently. The models developed can then be incorporated into a safety verification formulation, which allows the identification of potential hazards that may occur while operating such systems, together with the combinations of events that lead to them. Apart from the dynamic process model, the data required for carrying out the analysis include the space of possible disturbances and the set of operating regimes that are considered to be unsafe or undesirable from the operability point of view. The formulation results in a mixed‐integer optimization problem. A number of simple example problems presented illustrate the main ideas of the proposed technique, and the solution of an industrial‐scale case study demonstrates its applicability.
Reaction invariance and stability in catalyst particles for production of Methanol
Authors: Asbjornsen Odd Andreas , Hillestad Magne 
Published: June 2004

AbstractIndustrial practice requires immediate and adequate responses to simple questions asked. For example, does a catalyst particle show any tendency to thermal oscillations in the form of a limit cycle? The root to such cycles is found in the eigenvalues of the Jacobian matrix to the reaction rate vector, applying the concept of reaction invariance, a direct consequence of Avogadro's stoichiometric principle for homogeneous stirred tank reactors. However, this concept does not generally apply to heterogeneous reactor dynamics, because it is found in the heterogeneous transports of heat, reactants and products. The transport is an irreversible phenomenon that usually contributes to shifting the eigenvalues to the left and hence increases the stability. Still, effects of transport mechanisms, both internally and externally, on the catalyst particle are important to assess in such industrial analyses. A practical industrial reactor for methanol production was the subject for stability studies, which concluded that thermal oscillations are not likely to occur. During this study, a number of interesting details were examined such as rank deficiency of the reaction matrix and root loci for the temperature dependence of the Jacobian matrix eigenvalues. A practical consequence of eventual thermal cycling of the catalyst particles is a long‐term degradation of the catalyst efficiency, as seen in the ammonia synthesis.
High‐energy density storage of natural gas in light Hydrocarbon solutions
Authors: Horstkamp S. W. , Starling K. E. , Harwell J. H. , Mallinson R. G. 
Published: June 2004

AbstractThe storage of natural gas in other light hydrocarbons is one procedure for automotive natural gas usage that reduces the requirement of high‐pressure or cryogenic storage. Model solutions of methane in n‐butane, propane, and a liquefied‐bottled‐gas mixture were simulated using the Benedict‐Webb‐Rubin‐Starling equation of state to determine the pressures necessary to maintain a liquid phase with perturbations in both temperature (−1°C to 38°C) and mole fraction (50 to 80 mol% methane). Methane storage in these liquid solutions is between 45 and 93% of an equal “tank” volume of compressed natural gas (CNG) at 21 MPa and 15°C. The simulation results indicate that solutions of this type contain 40 to 67% of the energy of gasoline at pressures that range from 60 to 40% that of CNG at 21 MPa and 15°C.
Unique properties of 30‐μm particles as the catalyst of fluidized‐bed reactors
Authors: Yang Guoqiang , Wei Fei , Jin Yong , Yu Zhiqing , Wang Yao 
Published: June 2004

AbstractThe collapse properties of three kinds of fine particles (19, 30, and 59 μm) are studied with a new type of bed‐collapse technique, “isolated dilute‐phase bed collapse,” to determine the dense‐phase properties under high superficial gas velocities. The properties of the emulsion and bubble phases are investigated by a video camera. The influence of particle size on particle collapse properties is also discussed. When gas velocities are high the 30‐μm particles take the longest to collapse, have the smallest emulsion density, and have largest bubble volumetric fraction. These observations indicate that the 30‐μm particles have a property that the others lack.
Detecting persistent gross errors by sequential analysis of principal components
Authors: Tong Hongwei , Crowe Cameron M. 
Published: June 2004

AbstractMeasurements such as flow rates from a chemical process violate conservation laws and other process constraints because they are contaminated by random errors and possibly gross errors such as process disturbances, leaks, departures from steady state, and biased instrumentation. Data reconcilation is aimed at estimating the true values of measured variables that are consistent with the constraints, at detecting gross errors, and at solving for unmeasured variables. An approach to constructing sequential principal‐component tests for detecting and identifying persistent gross errors during data reconciliation by combining principal‐component analysis and sequential analysis is presented. The tests detect gross errors as early as possible with fewer measuremennts. They were sharper in detecting and have a substantially greater power in correctly identifying gross errors than the currently used statistical tests in data reconciliation.
Comprehensive kinetic model for the low temperature oxidation of hydrocarbons
Authors: Gaffuri Paolo , Faravelli Tiziano , Ranzi Eliseo , Cernansky Nicholas P. , Miller David , d'Anna Andrea , Ciajolo Anna 
Published: June 2004

AbstractThe low‐temperature oxidation chemistry of linear and branched alkanes is discussed with the aim of unifying their complex behavior in various experimental systems using a single detailed kinetic model. New experimental data, obtained in a pressurized flow reactor, as well as in batch‐ and jet‐stirred reactors, are useful for a better definition of the region of cool flames and negative temperature coefficient (NTC) for pure hydrocarbons from propane up to isooctane. Thermochemical oscillations and the NTC region of the reaction rate of the low‐temperature oxidation of n‐heptane and isooctane in a jet‐stirred flow reactor are reproduced quite well by the model, not only in a qualitative way but in terms of the experimental frequencies and intensities of cool flames. Very good agreement is also observed for fuel conversion and intermediate‐species formation. Irrespective of the experimental system, the same critical reaction steps always control these phenomena. The results contribute to the definition of a limited set of fundamental kinetic parameters that should be easily extended to model heavier alkanes.
Analysis of bacterial swimming speed approaching a solid–liquid interface
Authors: Frymier Paul D. , Ford Roseanne M. 
Published: June 2004

AbstractPrevious experimental studies have found that surface interactions significantly affect the transport of motile bacteria through small tubes, along solid surfaces, and through porous media. However, the role that hydrodynamic forces play in the interactions between solid surfaces and motile bacteria remains unclear. In this study, the swimming speeds of populations of Escherichia coli bacteria were measured near (< 10 μm) and far (>10 μm) from a flat glass surface at four ranges of orientations to the surface (0°–45°, 45°–90°, 90°–135°, and 135°–180°). Populations of bacteria close to the surface and moving in the orientation range most perpendicular (0–45°) to the surface experienced the greatest change in the swimming speed when compared to the population in the same orientation range located far from the surface. The decrease in swimming speed experienced by this population was on the same order as that predicted by hydrodynamic models of bacterial swimming near surfaces.
Lagrangian simulation of turbulent particle dispersion in electrostatic precipitators
Authors: Soldati Alfredo , Casal Massimo , Andreussi Paolo , Banerjee Sanjoy 
Published: June 2004

AbstractIndustrial design of electrostatic precipitators is based on the transport theory developed by Deutsch (1922), which assumes that transverse turbulent mixing is effective enough to maintain the concentration profile uniform throughout the cross section (i.e., turbulent diffusivity is assumed infinite). To improve understanding of turbulent particle dispersion under the influence of electrostatic forces, a database on particle trajectories was first generated, based on the flow field from a direct numerical simulation of a plate‐plate precipitator (Soldati et al., 1993). The effect of various parameters, such as particle size, charge and particle migration velocity, on dispersion and collection efficiency was investigated. Results show that particle concentration profiles are not uniform due to finite values of “turbulent diffusion” coefficient. The simulations indicate that the early stages of particle collection are controlled by particle migration velocity, while final stages are controlled by turbulence diffusion mechanisms.
Initial and final stages of compressible filtercake compaction
Authors: Sherwood J. D. 
Published: June 2004

AbstractFiltration experiments were performed to dewater bentonite slurries at different filtration pressures. The diffusivity of the cake was then determined as a function of void ratio using the measured filtration rates, the final void ratios of the cakes, and the theory of compressible filtercakes. Any filtration process can, in theory, be predicted, once the cake diffusivity is known as a function of void ratio and the void ratio as a function of applied pressure. In particular, the final stage of the filtration process can be modeled and offers an alternative route to determine the cake properties from the experimental results. The final slow approach to equilibrium is difficult to study experimentally; other techniques to determine cake properties are therefore preferable. Final stages of the cake compaction are also markedly slower than predictions based on the intial filtration rates. Possible explanations include a filtercake rheology that is more complicated than assumed in the model or time‐dependent effects caused by exclusion of ions from the bentonite filtercake, but such explanations appear unsatisfactory. Filtration models in which the filtercake void ratio and permeability depend solely on the imposed stress, may therefore be too simple for some materials.
Coupling of thermal cracking with noncatalytic oxidative conversion of Ethane to Ethylene
Authors: Choudhary Vasant R. , Mulla Shafeek A. R. 
Published: June 2004

AbstractHomogeneous thermal cracking of ethane, which is an endothermic and hence energy‐intensive process, was carried out in the presence of limited O2 under different process conditions (at 600°–850°C; O2/ethane ratio of 0–0.2; H2O/ethane ratio of 0–3.3; and space velocity of 2,000–11,000 h−1). The influence of a sulfur additive in the feed and reactor material on the process performance was also investigated. Noncatalytic oxycarcking of ethane in the presence of limited O2, greatly enhances not only the total conversion of ethane but also the conversion of ethane by purely thermal cracking. This process also occurs at a much lower contact time than that required to achieve the same conversion for the thermal cracking process. Also, both the exothermic oxidative conversion and the endothermic thermal cracking of ethane occur simultaneously, which makes this process very energy‐efficeint, with a drastic reduction in external energy requirement and coke formation. By manipulating the process conditions, particularly the temperature and O2/C2H6 ratio, the overall process can be made almost thermoneutral, mildly exothermic, or mildly endothermic.
Dynamic adsorption and tension of nonionic binary surfactant mixtures
Authors: Siddiqui Faisal A. , Franses Elias I. 
Published: June 2004

AbstractFor the first time, a model for dynamic adsorption and tension of diffusion‐controlled systems has been extended to binary premicellar mixtures of nonionic surfactants of different adsorption capacities and nonideal interactions in the monolayer. Local equilibria between subsurface concentrations and adsorbate densitities are modeled using the nonideal adsorbed solution theory, which describes these two elements. The model shows that larger molecules (those with the smaller adsorption capacity) tend to be preferentially adsorbed at low times, if they have equal adsorption equilibrium constants and diffusivities, and smaller moleules at longer times. This adsorption selectivity is reduced when larger molecules have a much larger adsorption equilibrium constant, or when there are negative deviations from ideality in the monolayer. This model's predictions are compared to tension data for two nonionic surfactants, C12E5 and Triton X‐100, at 25°C. The data are represented well by the diffusion‐controlled model with a finite diffusion‐layer thickness, which describes the faster decrease in tension observed with the bubble surfactometer, compared to data with other techniques. With this model, surface coverages and concentration profiles are calculated, thus elucidating the adsorption selectivity of molecules of different adsorption capacities. Synergistic effect in dynamic tension and adsorption can be predicted. Mild synergism in dynamic tension lowering by the preceding nonionic surfactants is also observed.
Thermodynamics of hydrogen bonding from molecular orbital theory: 1. Water
Authors: Wolbach Jeffrey P. , Sandler Stanley I. 
Published: June 2004

AbstractTo reduce the number of adjustable parameters that appear in the thermodynamic modeling of hydrogen‐bonding fluids, the use of ab‐initio molecular orbital calculations in the form of Hartree–Fock and density functional theories to compute thermodynamic properties are considered. In particular, the enthalpy and entropy changes on hydrogen bonding for water are computed. The theory, the computational methods and the basis sets used, as well as the results for water, are discussed. The results obtained are compared with those of others and with experimental estimates from the literature to determine the validity of the methods that are then used in this extended study. The results obtained here are also used to examine the validity of mixture group contribution results obtained here are also used to examine the validity of mixture group contribution methods for hydrogen‐bonding mixtures, to develop simple estimation procedures for model parameters, and to reduce the number of calculations such as those as those described that need to be done.
Thermodynamics of hydrogen bonding from molecular orbital theory: 2. Organics
Authors: Wolbach Jeffrey P. , Sandler Stanley I. 
Published: June 2004

AbstractHartree‐Fock theory and density functional theory were used to compute the enthalpy and entropy changes of dimerization for a number of hydrogen‐bonding compounds. In Part 1, the calculational methods and procedures use for the water dimer are described, and the results obtained are compared with those of others and with experimental estimates in the literature. Here, a variety of organic compounds that can self associate and/or cross associate are considered. The results obtained for the self‐association of these compounds are compared with estimates obtained from various types of experimental data. The results are also used to examine the validity of group‐contribution methods for hydrogen‐bonding mixtures and to test a simple estimation procedure for cross dimerization that can reduce the number of calculations, such as those described here, that need to be done.
Gas‐phase reaction in a trickle‐bed reactor operated at low liquid flow rates
Authors: Castellari A. T. , Cechini J. O. , Gabarain L. J. , Haure P. M. 
Published: June 2004

AbstractThe catalytic hydrogenation of alpha‐methylstyrene to cumene is studied in a laboratory trickle‐bed reactor operated at low liquid flow rates. Under this condition, wetting is not complete, vaporization of the liquid phase may occur, and reaction could proceed via liquid‐solid and gas‐solid catalysis, which increase the hydrogenation rate significantly. To determine and quantify the effect of the gas‐solid catalyzed reaction, we propose the use of post‐packing sections of increasing lengths. The inert bed acts as an absorber of the gas‐phase‐produced cumene. The liquid and gas effluent were monitored. Experimental global rates determined from liquid cumene concentrations varied with the post‐packing length. A simple one‐dimensional model developed shows that theoretical and experimental results agree well.
Modeling of SiO2 deposition in porous Vycor: Effects of pore network connectivity
Authors: Tsapatsis Michael , Gavalas George 
Published: June 2004

AbstractA mathematical model developed for SiO2 deposition in porous Vycor glass using SiCl4 hydrolysis describes reaction, diffusion and evolution of the pore structure due to accumulation of the solid product. The deposition reaction is described by transient heterogeneous kinetics in terms of the concentrations of silanol and chloride groups in the product layer as well as those of the gaseous reactants. For typical deposition conditions the pseudo steady‐state approximation for surface species could lead to erroneous predictions. Pore structure evolution is modeled by incorporating results of percolation theory. For this purpose the porous glass is represented by a Bethe lattice with coordination number 3 and alternatively by a decorated Bethe lattice in which each bond is replaced by a composite bond consisting of two bonds in series. The second network can capture the effect of pore radius variation along a single pore. For the decorated lattice, pore connectivity interruption at a higher void fraction leads to thinner deposits and shorter deposition times for pore plugging compared to the corresponding ones for the simple lattice.
Explicit algebraic scalar‐flux models for turbulent reacting flows
Authors: Adumitroaie V. , Taulbee D. B. , Givi P. 
Published: June 2004

AbstractExplicit algebraic scalar‐flux models that are valid for three‐dimensional turbulent flows are derived from a hierarchy of second‐order moment closures. The mathematical procedure is based on the Cayley–Hamilton theorem and is an extension of the scheme developed by Taulbee. Several closures for the pressure–scalar gradient correlations are considered and explicit algebraic relations are provided for the velocity–scalar correlations in both nonreacting and reacting flows. In the latter, the role of the Damköhler number is exhibited in isothermal turbulent flows with nonpremixed reactants. The relationship between these closures and traditional models based on the linear gradientdiffusion approximation is theoretically established. The results of model predictions are assessed by comparison with available laboratory data in turbulent jet flows.