bluuebunny/crossref_metadata_2025_split huggingface dataset:
Steady state multiplicity and partial internal wetting of catalyst particles
Authors: Bhatia S. K. 
Published: June 2004

AbstractThe partial internal wetting of spherical catalyst particles exposed to saturated vapor is investigated here by considering the interplay of capillary condensation phenomena and thermal effects for a first‐order irreversible reaction. It is shown that for certain very narrow ranges of parameters steady states can exist in which the macropores are vapor‐filled while micropores in an inner core, or an outer shell, are liquid‐filled. Under certain conditions even two such steady states are found, although only one of them can be stable. In addition to these there exist states in which all the micropores are liquid‐filled or those in which the entire particle is internally dry.
Partially wetted catalyst performance in the consecutive‐parallel network
Authors: Harold Michael P. 
Published: June 2004

AbstractIn this study the impact of the transport processes and the degree of wetting on the overall selectivity of a single catalytic pellet is examined for the commercially important consecutive‐parallel network A(g) + vBB(l) → vR1R(l), A(g) + vR2R(l) → P(l). The one‐dimensional formulation applies to catalysts in which the active component is concentrated in a thin porous surface shell coating an impermeable support. Model modifications provide an approximate description of uniformly active catalysts. The kinetics are assumed to be first and zero order with respect to the volatile (A) and nonvolatile (B, R) components, respectively, as encountered in many olefin hydrogenations. The common literature assumption of volatile reactant limiting reactions is relaxed by accounting for nonvolatile reactant depletion. The depletion can significantly reduce the desired product (R) selectivity below its intrinsic value attained under fully wetted, no‐depletion conditions. A model employing the single limiting reactant assumption cannot predict such selectivity variations. The selectivity is shown to be a complex function of the wetting efficiency, the stoichiometry, and the interacting reactions and mass transport processes. In many cases the intrinsic selectivity is attained for wetting efficiencies exceeding a critical value. Increases in the wetting above this value can reduce significantly the production rate of R because of resistance to A supply through the liquid film. Thus, an optimal range of wetting efficiencies exists for which the selectivity is equal to its intrinsic value and overall rates are high. The significance of the main findings with regard to trickle‐bed reactor performance are discussed.
Kinetics of ion exchange accompanied by neutralization reaction
Authors: Kataoka Takeshi , Yoshida Hiroyuki 
Published: June 2004

AbstractIon exchange accompanied by a neutralization reaction has been analyzed for the general cases where the bulk solution contains a neutral salt and an acid or a base, e.g., [R · H] + (NaCl + NaOH). Both intraparticle and liquid film diffusion are considered. The Nernst‐Planck equation was applied for the fluxes of the ionic species in the liquid film and the resin phase with a mobile reaction plane in the liquid film. When the solution contains acid or base more than 50%, the exchange rate is approximated by the case for pure acid or base solution: the reaction plane is located at the solid/liquid interface. When the solution contains acid or base less than 50% (this situation is generally encountered in the case where a neutral salt flows through a mixed bed of OH‐form and H‐form resins), the mobile reaction plane should be considered. When the ratio of intraparticle diffusion resistance to liquid diffusion resistance defined in the text is larger than 2, the exchange rate can be approximated by intraparticle diffusion control. The larger the ratio, the faster the reaction plane moves to the resin particle surface. Experimental uptake curves for the ion exchange systems [R · H] + (NaCl + NaOH) and [R · OH] + (NaCl + HCl) confirm the above conclusions.
Microdispersive interfacial mixing in fast polymerizations
Authors: Machuga Steven C. , Midje Heather L. , Peanasky John S. , Macosko Christopher W. , Ranz William E. 
Published: June 2004

AbstractInterfacial activity between liquid reaction injection molding (RIM) reactants was observed using light microscopy. Diisocyanates were brought into contact with various diols and diamines in a thin (250 μm) gap. A dynamic phenomenon, rapidly producing a well‐mixed, intermaterial phase, was discovered. It was revealed that the rate of growth of this newly formed region was dependent on both the rate of reaction and the physical properties of the initially formed product species at the reaction interface. Spontaneous interfacial mixing also occurred without chemical reaction when a reaction product was dissolved in “capped” contacting reactant liquids. It is believed that strong interfacial intermolecular forces, inducing flow through the polymerization product layer formed immediately upon reactant contact, are responsible for the initial explosiveness of this microscopic process. The range of eventual thicknesses of the mixed, interfacial region (100–800 μm) for various typical RIM systems indicates that this microscopic subprocess may be significant to the fate of the overall process.
The excess gibbs free energy of aqueous nonelectrolyte solutions
Authors: Shealy Glenn S. , Sandler Stanley I. 
Published: June 2004

AbstractA new thermodynamic model is presented for aqueous solutions of polar nonelectrolytes. The model is based on solute aggregation in an aqueous lattice, which is inferred from spectroscopic, thermodynamic, and physiochemical data. The adjustable model parameters have physical meaning, and are less intercorrelated and temperature‐dependent than are the parameters in other models such as the UNIQUAC and NRTL equations, which apply to a more general class of solutions. The model describes observed extrema in the activity coefficients of aqueous solutions, thus pointing out that the behaviors of aqueous and non‐aqueous solutions are fundamentally different, and that aqueous solutions cannot be well described with nonaqueous solution models over the whole concentration range. The two‐parameter form of the new model can better predict binary vapor‐liquid equilibria from liquid‐liquid equilibrium data than other two‐parameter models, and gives better predictions of ternary liquid‐liquid equilibrium using only binary data. It is also shown that the model can be used to describe aqueous solutions of water‐soluble polymers.
Carbochlorination of dispersed oxides in a molten salt reactor
Authors: Dobbins Michael S. , Burnet George 
Published: June 2004

AbstractReaction mechanism and rates were determined from 803 to 1,073 K for the carbochlorination of Al2O3 and of Al, Si and Fe‐mixed oxides using Cl2 and C slurried in a stirred melt of NaCl—AlCl3. Alumina chlorination rates of 0.33–2.0 × 10−8 kmol/s were obtained using C:melt weight ratios between 1:21 and 1:42. The reaction rate was proportional to C loading at temperatures below 923 K, while gas‐liquid mass transfer was rate‐controlling at higher temperatures. Carbochlorination of the combined oxides in coal fly ash at 923 K and oxide conversions less than about 50% was also gas‐liquid mass‐transfer‐controlled. At higher conversions, dissolution became the rate‐controlling factor. A 40% (molar) AlCl3 melt was more effective than a 48% AlCl3 melt for the selective chlorination of Al2O3 over SiO2.
Enhancement in mass transfer due to motion of a particle: An experimental study
Authors: Verma A. K. , Rao D. P. 
Published: June 2004

AbstractThe understanding of the mechanism of wall‐to‐bed heat and mass transfer in liquid‐fluidized beds requires knowledge of the effect of the motion of individual particles on transfer processes. Experimental studies on the enhancement in mass transfer due to motion of a particle in the proximity of the transfer surface are presented. Mathematical relations based on a mechanistic model are obtained to describe the variation of the enhanced rate with time. The model parameters are correlated in terms of the particle Reynolds number, the wall‐to‐particle distance, and an interaction parameter that accounts for the background fluid motion. The results of this work would be of use in modeling transfer processes in other liquid‐solid systems.
Measurement of droplet interfacial phenomena by light‐scattering techniques
Authors: Taflin Daniel C. , Zhang S. H. , Allen Theresa , James Davis E. 
Published: June 2004

AbstractTwo light‐scattering techniques have been applied to study the kinetics of droplet evaporation and attendant interfacial phenomena. The first technique, which is particularly useful for noisy data, utilizes the fast Fourier transform of phase function data (intensity vs. scattering angle) to obtain the size by comparing the phase function with Lorenz‐Mie theory. The second method applies optical resonance measurements to obtain very precise measurements of size and/or refractive index as functions of time. The techniques have been used to measure the slow evaporation of multicomponent and single‐component organic microdroplets, rapid evaporation of water droplets, and rapid evaporation of droplets of aqueous solutions and surfactant solutions. The experiments were performed in an electrodynamic balance, which was used to suspend single microdroplets in a polarized laser beam. Rapid evaporation rate data yield information on the surface temperature, and data for surfactant solution evaporation show that insoluble monolayer formation at the microdroplet surface reduces the evaporation rate by a factor of 200. In addition, it is shown that droplet explosion due to surface charge effects can occur well below the Rayleigh limit of charge.
Analysis of equilibrium shift in isothermal reactors with a permselective wall
Authors: Mohan Krishna , Govind Rakesh 
Published: June 2004

AbstractThe objective of this study is to obtain a fundamental understanding of the behavior of reactors with a permselective wall (membrane reactors) in terms of: design parameters (reactor length, membrane thickness); operating variables (pressure ratio, feed flow rate); physical properties (rate constant, permeability of fast gas, permselectivity, equilibrium constant); and flow patterns (recycle, cocurrent, countercurrent). Pure feed reacts on the high‐pressure side of the membrane, and the product(s) formed are continuously removed to the low‐pressure side so that thermodynamic equilibrium is never reached. It is shown by simulation that equilibrium shift can be enhanced by: recycling unconverted reactant; shifting feed location to separate products; and maintaining high permeation rates to reduce backreaction. It is also shown that the choice between cocurrent flow and countercurrent flow depends on the system parameters.
Continuum theory for lumping nonlinear reactions
Authors: Chou M. Y. , Ho T. C. 
Published: June 2004

AbstractA lumping procedure is proposed for continuous reaction mixtures whose components undergo nonlinear reactions. A species‐type distribution function is introduced to ensure that the lumped continuous mixture is kinetically consistent with the underlying lumped discrete mixture. It is shown that, if the mixture contains some virtually unreactive species, the lump after a long time will react at a second‐order rate. This is true as long as the underlying unlumped reactions are irreversible nth‐order with n < 2 or reversible first order. When many first‐order reversible reactions are lumped together, the apparent behavior of the lump can qualitatively look like that of a reversible reaction. Practical implications of these predictions are discussed. Some published data are reconciled in light of the present theoretical development.
Transformations and consistency relations of distillation control structures
Authors: Häggblom Kurt E. , Waller Kurt V. 
Published: June 2004

AbstractRigorous analytical methods for consistent modeling of control structures for multivariable processes are presented. The continuous distillation process is used to illustrate the methods, but the basic ideas, and more specifically the transformations and consistency relations derived, are valid in general.If steady‐state operating data and the process gains of an arbitrary control structure are known, it is possible to calculate the process gains of any feasible control structure. A general expression relating the process gaisn of different control structures is derived. In general, the process gains must also satisfy certain consistency relationships which can be derived from first principles, e.g., steady‐state material balances.The usefulness of the results is illustrated by control structure transformations and reconciliation of process gains, by an application to process dynamics, by synthesis of noninteracting control loops, and by derivation of analytical relationships useful in relative gain analysis.
Electrochemical measurement of effective diffusivity in Hall‐Heroult electrolyte
Authors: Burgman John W. , Sides Paul J. 
Published: June 2004

AbstractForced‐convection mass transfer at the metal/bath interface during aluminum reduction from cryolitic melts was studied under reproducible convective conditions. A film of molten aluminum on a rotating molybdenum cylinder was the cathode. Concentration overpotential measured as a function of rotation rate, current density, and bath composition was converted to concentration differences between the bulk and the metal surface. Chosen as the basis for calculation of a mass transfer coefficient was the concentration of aluminum fluoride given by:

where the amounts of indicated compounds are in weight percent, IMP designates impurities, M is molecular weight, ρ is the density of the melt, and CR is the cryolite ratio, the ratio of moles NaF to moles AlF3. Agreement with a correlation for mass transfer to a rotating cylinder allowed the calculation of effective diffusivities for aluminum fluoride species, in alumina‐saturated melts, of: 11.1 ± 1.1 × 10−5 cm2/s at 1.8 CR; 11.4 ± 1.7 × 10−5 cm2/s at 2.3 CR; 5.4 ± 0.8 × 10−5 cm2/s at 3.0 CR; and 4.4 ± 0.9 × 10−5 cm2/s at 4.0 CR.
Kinetics of spent activated carbon regeneration
Authors: Harriott Peter , Cheng Alan Tat‐Yan 
Published: June 2004

AbstractPyrolysis and regeneration kinetics of 11 spent activated carbons were studied using a thermal balance and a fludized bed reactor. The adsorbates were aromatic compounds with different types and number of side groups, and most of the side groups were lost on pyrolysis, leaving a residue about equal to the weight of the aromatic portion of the adsorbate. The residues from alkyl phenols reacted with oxygen at 770‐920 K at rates up to 12 times the rate for the base carbon. There was a similar increase in oxygen chemisorption indicating increased number of active sites. The adsorption capacity of the carbons was mostly recovered by pyrolysis, even though the surface area and pore volume were reduced by the residue. Some oxidative regeneration is probably necessary to restore adsorption capacity for repeated use.
Augmented BACK equation of state II: Polar fluid mixtures
Authors: Lee M. J. , Chao K. C. 
Published: June 2004

AbstractThe augmented BACK equation of state for polar fluids is extended to mixtures by introducing mixing rules separately for the nonpolar and polar interactions. Fluid phase equilibria are calculated for polar + nonpolar, and polar + polar mixtures including mixtures of hydrogen‐bonding substances. Enthalpy calculations are illustrated with enthalpy of H2S + N2 and heats of mixing of steam mixtures, and carbon dioxide mixtures.Cross interaction coefficients are adjusted for the correlation of data on binary mixtures. Both phase equilibrium and enthalpy data are correlated at the same time with two such coefficients for each binary. The cross interaction coefficients are constants, invariant with temperature, density, or phase state.
Redox catalysis of absorption from reactive bubbles near electrodes
Authors: Takahashi Ken M. , Alkire Richard C. 
Published: June 2004

AbstractElectrolyzing sparingly soluble gases as they are absorbed from bubbles can be significantly facilitated by addition of a soluble redox couple which serves to increase both mass and charge transfer rates. A diffusion layer model is used to calculate reaction‐catalyzed mass transfer rates in regions of boundary layer penetration by reactive bubbles for the case of (m, n)‐order irreversible homogeneous reaction. The dissolved redox catalyst is continuously regenerated at the solid electrode surface. Approximate solutions, which relate surface flux to concentration driving force, are in excellent agreement with the numerical solutions under conditions of fast homogeneous reaction and concentrated catalyst (modified Damköhler number and dimensionless catalyst concentration greater than 3). Diffusion layer thickness is calculated from the mass transfer coefficient kp in penetrated regions. A model based on spherical penetrating bubbles is used to determine kp and specific surface area ap of penetrated regions from kpap values that have been measured in gas‐liquid flow through a packed bed. The mass transfer behavior provides useful insight into why the penetration process is so effective in increasing overall reaction rates.
Input/output linearization: A nonlinear analog of placing poles at process zeros
Authors: Kravaris Costas 
Published: June 2004

AbstractThis paper deals with SISO nonlinear processes and their control with state feedback. The Byrnes‐Isidori concept of non‐linear zeros is first reviewed and illustrated with two chemical engineering exemples. This concept is then used to interpret input/output linearizing state feedback as a nonlinear analog of placing poles at the process zeros. This interpretation leads to closed‐loop internal stability conditions for nonlinear processes under input/output linearizing state feedback.
A thermodynamic model for predicting wax formation in crude oils
Authors: Hansen Jens H. , Fredenslund Aa. , Pedersen Karen Schou , Rønningsen Hans Petter 
Published: June 2004

AbstractMost crude oils contain high molecular weight components, which at low temperatures may precipitate as a wax phase. This may cause plugging of pipes and numerous other problems. This paper presents a solid‐liquid equilibrium based model for the description of wax formation. The model for the Gibbs energy contains a contribution based on Flory's theory of multicomponent polymer solutions and a contribution from a metastable subcooled state which oil mixtures may attain. The latter is formulated in terms of the surface tension of the wax phase. Experimental wax appearance points (temperatures), WAP's, are reported for 17 different stabilized North Sea crude oils. The values predicted by the new model are in very good agreement with the experimental WAP's.
Calculation of infinite‐dilution partial molar properties by computer simulation
Authors: Shing Katherine S. , Chung Sang‐Tae 
Published: June 2004

AbstractPartial molar properties at infinite dilution for binary Lennard‐Jones mixtures are calculated using Monte Carlo simulation methods. The variation of these properties with density, temperature, and intermolecular force parameters is systematically studied. Simulation pair correlation functions are used to examine the structure of the mixtures at the molecular level. Results are also compared to predictions of the van der Waals I conformal solution theory.Partial molar volume and internal energy are found to be strongly dependent on the isothermal compressibility of the mixture and on the proximity of the mixture state condition to phase boundaries. The van der Waals I conformal solution theory gave good results for the infinite‐dilution chemical potential μ and reasonable results for the partial molar internal energies and volumes. However, it is suspected that this is limited to the infinite‐dilution case, where the isothermal compressibility of the mixture is well represented by the pure‐fluid equation of state.
Impact of model uncertainty descriptions for high‐purity distillation control
Authors: McDonald Karen A. , Palazoglu Ahmet , Bequette B. Wayne 
Published: June 2004

AbstractThe ways in which modeling uncertainties are described for a particular process critically affects the results obtained in robustness studies. In this paper, four multivariable robust stability methodologies are used to characterize and analyze the effects of model inaccuracy due to non‐linearity in high‐purity distillation processes. The unstructured and structured singular value, numerical range, and a mapping of det(l + GPGc) are compared in terms of their ability to predict the stability of the dual‐composition control system over a wide composition range. The importance of using uncertainty descriptions that include a realistic representation of the phase‐magnitude relationship as well as the corrections between uncertainties in each element of the model is demonstrated. The conservation associated with norm‐bounded uncertainty descriptions reveals itself by the extent of detuning needed to insure stability and the subsequent degradation in control performance.
Forced periodic control of an exothermic CSTR with multiple input oscillations
Authors: Rigopoulos Konstantinos , Shu Xianshu , Çlinar Ali 
Published: June 2004

AbstractInteraction among various oscillating inputs may result in significant modifications in the behavior of a system under forced periodic control. Forced oscillations in the input flow rate and input concentrations of an exothermic continuous stirred tank reactor enable the stabilized operation of the CSTR in the unstable steady state region. Reactor temperature oscillations under forced periodic control are similar to the oscillations resulting from proportional‐integral feedback or nonlinear (push‐pull) feedback control, and under some operating conditions the oscillation amplitude is significantly lower. Theoretical and experimental studies illustrate the effects of forcing frequency and phase shift on reactor behavior.
Partitioning of nonspherical molecules between bulk solution and porous solids
Authors: Limbach Kirk W. , Nitsche Johannes M. , Wei James 
Published: June 2004

AbstractMonte Carlo simulation is used to calculate partition coefficients for rigid, axisymmetric, nonspherical molecules in various pore geometries. Novel quantitative results are presented for a variety of molecule and pore shapes, including pores of nonuniform cross‐section such as the interstitial space within granular and fibrous materials.Quick estimation methods for obtaining partition coefficients are discussed, and the effects of molecule and pore wall curvature are described in detail. A boundary perturbation analysis is used to derive easy‐to‐use formulas for a variety of molecule shapes in circular pores.
Uptake of phenylalanine and tyrosine by a strong‐acid cation exchanger
Authors: Saunders Malcolm S. , Vierow John B. , Carta Giorgio 
Published: June 2004

AbstractThe equilibrium and rate of uptake of the amino acids phenylalanine and tyrosine by Amberlite 252, a strongly acidic, cation‐exchange resin, have been investigated. Uptake of the amino acids by the hydrogen form of the resin occurs primarily by the stoichiometric exchange of hydrogen ions and amino acid cations. The amount of amino acid taken up by the resin can be calculated as a function of solution pH and amino acid concentration from a model that takes into account both solution and ion‐exchange equilibria. The rates of uptake of the two amino acids have been determined experimentally for a closed batch system. The results of experiments in which the resin particle size, the flow rate, and the concentration were varied show that intraparticle transport is dominated by the slow diffusion of amino acid cations through the macroreticular polymer structure of the resin, with some contribution from macropore transport of amino acid cations and zwitterions. An intraparticle diffusion model has been developed to describe these results and predict the performance of fixed‐bed operations.