bluuebunny/crossref_metadata_2025_split huggingface dataset:
Near‐critical phenomena and resolution in supercritical fluid chromatography
Authors: Kelley F. D. , Chimowitz E. H. 
Published: June 2004

AbstractThe analysis in this paper shows the relationship between molecular resolution in supercritical fluid chromatography (SFC), and certain divergent thermodynamic properties of solutes dissolved in near‐critical fluids. This divergence is most pronounced in the near‐critical regime, leading to the hypothesis that this region provides most promise for heightened molecular resolution, which lies at the heart of analytical and separation technologies.A thermodynamic model for representing capacity factor data in SFC is proposed, spanning a wide density range. The model uses a perturbed hard sphere equation for the fluid phase and lattice concepts for the stationary phase. In addition, data are presented for a number of systems and discussed in terms of the analysis provided. For an isomeric system, the data shows that resolution between species increases with the onset of retrograde behavior, resulting in a distinct maximum point.
Rationalization for the molecular weight distributions of Coal pyrolysis liquids
Authors: Darivakis George S. , Peters William A. , Howard Jack B. 
Published: June 2004

AbstractA model for coal thermal depolymerization is described and used to predict molecular weight distributions (MWD) of primary coal pyrolysis liquids. The model assumes that rupture of kinetically similar bonds in the coal is the preferred pathway for release of oligomers which ultimately become tars or extractables. The analysis provides the liquids MWD as a probability function of finding different oligomers composed of whole integer numbers of monomers and it is in excellent qualitative agreement with measured MWDs of coal pyrolysis liquids. The predicted MWDs are of the form of the gamma distribution function (GDF), and its parameters are interpreted in terms of the postulated chemistry of coal thermal depolymerization. Consequently, successful empirical correlations of experimental data on coal pyrolysis liquids MWDs with the GDF are better understood. Thus a protocol is provided to predict MWDs of pyrolysis liquids from knowledge of coal molecular structure.
Kinetics of absorption of hydrogen sulfide into aqueous ferric sulfate solutions
Authors: Asai Satoru , Konishi Yasuhiro , Yabu Tadahiro 
Published: June 2004

AbstractAbsorption experiments of H2S into aqueous Fe2(SO4)3 solutions, concerned with a unique process capable of regenerating by biological oxidation the reactant from the products, were performed at 20–40°C in an agitated vessel with a flat interface. The absorbents were aqueous solutions of Fe2(SO4)3 with and without the addition of K2SO4, KOH or H2SO4 of various concentrations, which was added to change the ionic strength or pH of the solutions. The experimental results revealed that the species which reacts with H2S is FeOH2+. While comparing the observed absorption rates with the theoretical predictions, it was found that the reaction is irreversible and first order with respect to H2S and FeOH2+. The reaction rate constants were measured and correlated.
Turbulent mixing with multiple second‐order chemical reactions
Authors: Heeb Thomas G. , Brodkey Robert S. 
Published: June 2004

AbstractA molecular‐based statistical simulation program was developed to study the covariance terms involved in the mass balance equations for complex chemical reactions during mixing. Several closure theories were compared to the simulations and available experimental data. The simple closure by Brodkey and Lewalle was found to be an extension of Toor's analysis applied to two reactions. This closure does not satisfy the molar fluctuation balance equation and was found only to represent the high Reynolds number data of Li and Toor. This result led to examining other possible closures which were based on Damkoehler numbers, reaction rate constant ratios, and limiting forms of the covariance term. These closures also were inadequate. The second reaction's covariance term varied from the product of the average values for each component to the Brodkey and Lewalle value for the range of Reynolds numbers considered.
Influence of molecular shape on probing mass transfer resistances on zeolites
Authors: Kärger Jörg , Pfeifer Harry , Stallmach Frank , Bülow Martin , Struve Peter , Entner Roland , Spindler Herbert , Seidel Rüdiger 
Published: June 2004

AbstractMass transfer resistances in zeolite NaCaA after different hydrothermal pretreatment are probed by NMR tracer desorption studies with the short‐chain‐length hydrocarbons from methane to n‐butane and with xenon. In all cases, the measurements lead to the coinciding result of forming surface barriers, which differ only slightly depending on the pretreatment modes, but lead to a retardation of molecular mass transfer more dramatically than the starting material. The dramatic retardation of mass transfer after hydrothermal pretreatment is confirmed by supplementary uptake measurements with n‐decane.
Statistical‐mechanical model of protein precipitation by nonionic polymer
Authors: Mahadevan Hari , Hall Carol K. 
Published: June 2004

AbstractA theoretical approach to predict the solubility of proteins in solutions containing nonionic polymers is presented. The effective protein‐protein interaction due to the presence of the polymer is related to the volume‐exclusion potential of Asakura and Oosawa. Statistical‐mechanical perturbation theory, as originally applied by Gast et al. to model colloidal flocculation, is used to calculate free energies, from which solubility curves for varying protein‐polymer diameter ratios are obtained. The theory correctly predicts all the trends observed in experimental studies of these systems. To explain the influence of processes parameters such as the pH and the ionic strength on protein solubility, the intermolecular potential is improved by the addition of an electrostatic interaction term. It is found that theoretical predictions of the variation in protein solubility, both with the solution pH and the ionic strength, are in accordance with experimental observations.
Two‐dimensional numerical study of cross‐flow filtration combustion
Authors: Dandekar Hemant W. , Puszynski Jan A. , Hlavacek Vladimir 
Published: June 2004

AbstractTransition metal nitrides can be synthesized in the combustion regime using a self‐propagating, high‐temperature synthesis by the process of filtration combustion. A detailed two‐dimensional numerical study for the filtration combustion process in the cross‐flow configuration is presented. The process is modeled by a two‐dimensional pseudohomogenous model. The model equations, which are very stiff and exhibit varying time and length scales, are solved using finite differences on a two‐dimensional orthogonal adaptive mesh. The effects of pressure, bed height and bed porosity on combustion front propagation and final conversion are studied. A series of full simulations for the titanium‐nitrogen system shows good agreement between theoretical predictions and experimental observations.
Revised vapor‐liquid equilibrium model for multicomponent formaldehyde mixtures
Authors: Hasse H. , Hahnenstein I. , Maurer G. 
Published: June 2004

AbstractFormaldehyde is one of the most important intermediate products of the chemical industry. Due to its high reactivity, it is commonly handled in aqueous or methanolic solutions. In these solutions, formaldehyde is dissolved chemically. Therefore, modeling thermodynamic properties of aqueous and methanolic formaldehyde‐containing mixtures require the consideration of chemical reactions and physical effects. A recently published physico‐chemical model (Maurer, 1986) for the description of vapor‐liquid equilibria of these systems is tested and improved on the basis of about 140 new experimental data points for mixtures containing formaldehyde, water, methanol, and trioxane at temperatures between 320 and 380 K and pressures below 100 kPa. Improvements are achieved by fitting some of those binary interaction parameters, which formerly had to be estimated due to the lack of experimental data. The revised model is able to reliably predict vapor‐liquid equilibria in multicomponent formaldehyde‐containing mixtures.
Adsorption of amine and paints on H‐form resin from electrodeposition waste water
Authors: Yoshida Hiroyuki , Shimizu Kazuyuki , Kataoka Takeshi 
Published: June 2004

AbstractWastewater of electrodeposition painting process contained diisopropanolamine, unknown three paints, and solvents (butylcellosolve and/or ethylcellosolve). The recovery of the amine and paints from the real wastewater by adsorption on an H‐form strong acid ion exchanger appeared feasible technically and economically.Equilibrium isotherms and breakthrough curves have been determined experimentally. The selectivity for the amine adsorption on the resin was fairly high. Solvents were not adsorbed on the resin. The components of the wastewater showed typical breakthrough curves for multicomponent system. Theoretical breakthrough curves and intraparticle effective diffusivities of the amine were determined from the analytic solution for rectangular isotherm in the single‐component system. Experiments of adsorption, elution and regeneration also have been repeated. The amine and paints adsorbed on the resin were eluted completely by NaOH aqueous solution. The adsorption capacity of the amine did not change with repeat times, and the intraparticle effective diffusivity of the amine was constant after the second cycle.
Nonisothermal parison inflation in blow molding
Authors: Poslinski Andrew J. , Tsamopoulos John A. 
Published: June 2004

AbstractNonisothermal inflation of a viscous annular parison and its cooling and solidification before and upon wall contact are analyzed by solving the unsteady momentum and energy conservation equations. The algorithm simultaneously determines the flow field and temperature distribution within the parison together with the moving surfaces at every time step. Results of the present study indicate that the inflation rate is increased as a result of higher temperature within the parison. However, the instantaneous shape of the parison and the final thickness variation upon mold contact remain virtually unchanged. Due to the temperature dependence of physical properties, the critical factor during inflation is initial temperature distribution within the parison. Inflation resulting in full attachment to a confining mold wall takes only a small fraction of the time required to cool the material to the ambient gas temperature. The transient calculations illustrate how computer simulations may be used to improve design of blow molding operations, and they are in good agreement with available experiments.
Wave propagation in the flow of shear‐thinning fluids down an incline
Authors: Weinstein Steven J. 
Published: June 2004

AbstractThe effect of shear‐thinning rheology on the spatial growth of waves in multilayered flow down an inclined plane is modeled, utilizing the Carreau viscosity constitutive equation. It is shown that waves associated with the free surface propagate as if they were in a Newtonian system, where the viscosity is some average of the varying viscosities in the shear‐thinning layer. This averaging is due to the global effects of shear thinning, such as changes in velocity profile and film thicknesses. In contrast, waves that are associated with the interfaces between adjacent fluid layers are largely affected by the local interfacial viscosities; wave propagation is not governed by some average Newtonian viscosity across the layer. It is found that shear‐thinning rheology can either increase or decrease the growth of waves associated with a fluid‐fluid interface, compared with a purely Newtonian case.
Kinetics and fixed‐bed reactor modeling of butane oxidation to maleic anhydride
Authors: Sharma Ramesh K. , Cresswell David L. , Newson Esmond J. 
Published: June 2004

AbstractSelective oxidation kinetics of n‐butane to maleic anhydride in air were studied over a commercial, fixed‐bed vanadium‐phosphor oxide catalyst. The temperature range was 573–653 K with butane concentrations up to 3 mol % in the feed, which is within flammability limits but below ignition temperatures.The rate data were modeled using power law kinetics with product inhibition and included total oxidation and decomposition reactions. Kinetic parameters were estimated using a multiresponse, nonlinear regression algorithm showing intercorrelation effects. The kinetics were combined with independent measurements of catalyst diffusivity and reactor heat transfer using a one‐dimensional heterogeneous reactor model. Model predictions and observed temperatures and concentrations from non‐isothermal pilot plants were compared up to 115 days on stream. Agreement was acceptable with inlet butane concentrations up to 2.7 mol %. For example, runaway was predicted at a salt temperature 3 K higher than observed. Effectiveness factors around the hot spot were estimated at 0.6 with the catalyst surface temperature 2–3 K higher than the average gas temperature.
Pattern formation in catalytic reactors: The role of fluid mixing
Authors: Arce P. , Ramkrishna D. 
Published: June 2004

AbstractFluid‐mediated interaction between catalyst particles alone is shown to yield several interesting and significant phenomena in a catalytic reactor that have been generally attributed in the past to direct interaction between particles. Thus, collaborative interaction between particles and the fluid may enhance or abate steady‐state multiplicity, and reverse stability behavior. From the simple setting of a population of particles in a well‐mixed CSTR, it is shown that the catalyst phase in a catalytic reactor is susceptible to very fine pattern formation in the face of steady‐state multiplicity in single particles, which negates the usual assumption that particles exposed to a given fluid have identical states. In a reactor such variability in behavior must be accompanied by a corresponding variability in conversion and selectivity (in multireaction systems) and may have strong implications for reactor control strategies.
Use of a second endothermic reaction to desensitize a tubular reactor
Authors: Van Der Vaart D. R. , Van Der Vaart H. R. 
Published: June 2004

AbstractThree parametric sensitivity criteria were examined for application to a pseudohomogeneous tubular reactor system, in which both methanol dehydration (exothermic) and methanol dissociation (endothermic) were catalyzed. The addition of the endothermic reaction was shown to lead to a desensitized reactor. It was illustrated, however, that the relative activities of the two catalysts should be somewhat similar to maintain the overall conversion of the resulting reactor for a given residence time.The three criteria investigated tended to disagree with one another more, as the system was made less sensitive by the addition of more endothermic catalyst. A potential application of the resulting nonsensitive reactor design would be in methanol‐fueled vehicles.
An improved finite difference method for fixed‐bed multicomponent sorption
Authors: Sun L. M. , Meunier F. 
Published: June 2004

AbstractA new computational procedure based on the finite difference methods is developed to solve the coupled partial differential equations describing nonisothermal and nonequilibrium sorption of multiple adsorbate systems on a fixed bed that contains bidispersed pellets. In this numerical method, a solution‐adaptive gridding technique (SAG) is applied in combination with a four‐point quadratic upstream differencing scheme to satisfactorily resolve very sharp concentration and temperature variations occurring in the case of small dispersing effects. Furthermore, the method resorts to a noniterative implicit procedure for solving the coupling between the column transport equations and the adsorption kinetics inside the pellets, which may be particularly efficient when the particle kinetics equations are highly stiff.The numerical model will be tested for one‐, two‐ and three‐transition systems. The results are compared to available analytical and equilibrium theory solutions.
Hollow fiber contained liquid membrane separation of citric acid
Authors: Basu R. , Sirkar K. K. 
Published: June 2004

AbstractCitric acid was successfully separated from an aqueous solution using the hollow fiber contained liquid membrane (HFCLM) technique. In this technique, the organic liquid membrane is contained in the shell side between two sets of hollow fibers; feed solution flows through one set of fibers and the strip solution flows through the other set. Tri‐n‐octylamine diluted in various organic solvents was used as a complexing agent for facilitated transport of citric acid from an aqueous solution. Pure water and aqueous sodium hydroxide were used as stripping agents. Membrane life and stability problems encountered in supported liquid membranes are eliminated. A mathematical model of facilitated solute transport through HFCLM that accounts for the interfacial reversible reaction kinetics and diffusion process inherent in carrier‐facilitated transport is presented. Experimental data agreed well with the theoretical predictions for permeators achieving almost complete solute recovery.
Design and economic trade‐offs of extractive crystallization processes
Authors: Rajagopal Shankar , Ng Ka M. , Douglas James M. 
Published: June 2004

AbstractA systematic procedure is formulated for the separation of a binary mixture by means of extractive crystallization. It shous that two flowsheet structures can handle systems with a wide variety of solid‐liquid phase behaviors, including components with simple eutectics, multiple eutectics, and compound formation. Design equations are presented for both flowsheet structures. In addition, design variables and constraints are identified. The economic trade‐offs in extractive crystallization processes are examined through the analysis of an example: the complete separation of paraxylene from meta‐xylene using n‐pentane as the extractive solvent. Comparison of our general methodology with the system‐specific designs reported in the literature shows that the latter are process alternatives of the former.
Concentration gradients in supersaturated solutions: A thermodynamic analysis
Authors: Veverka V. , Söhnel O. , Bennema P. , Garside J. 
Published: June 2004

AbstractThe development of a concentration gradient along the vertical axis of a column of quiescent solution is explained within the framework of nonequilibrium thermodynamics. The formulae derived enabled us to determine explicitly the concentration difference between two vertical levels in a liquid column and specific conditions under which the concentration profile is at equilibrium. The concept of solute cluster formation in a supersaturated solution is consistent with the presented theory although the theory itself does not provide any direct or indirect proof of the occurrence of clustering.
Managing qualitative simulation in knowledge‐based chemical diagnosis
Authors: McDowell James K. , Davis James F. 
Published: June 2004

AbstractDeep knowledge about process behaviors plays an important role in the diagnosis of chemical processes. Cause‐and‐effect reasoning using deep knowledge is useful especially for interacting malfunctions. This work explores the integration of deep knowledge into task‐specific, knowledge‐based architectures for resolving interacting multiple malfunctions and presents a novel methodology called diagnostically focused simulation (DFS). Invoked in an auxiliary manner, DFS uses deep knowledge and performs qualitative simulation in a highly constrained manner. The close integration with other problem solvers is an evolutionary approach to using qualitative simulation in diagnosis and manages a normally computationally‐explosive procedure. Diagnostic results from the compiled problem solver provide a situation‐specific assessment of the chemical process, identify possible malfunction scenarios, and focus on appropriate levels of process detail. DFS effectively demonstrates a balance between run‐time simulation and compiled problem solving in diagnosis.
Window effects in zeolite diffusion and brownian motion over potential barriers
Authors: Nitsche Johannes M. , Wei James 
Published: June 2004

AbstractGorring (1973) reported a phenomenon, the “window effect,” whereby the diffusivities of normal paraffins within zeolite T do not decrease monotonically with increasing carbon number N, as would be expected intuitively. Rather, following an initial decrease with N, the diffusivities exhibit a local minimum at C8 followed by a pronounced local maximum at C12. This article presents a theoretical analysis of the window effect and related phenomena, based on an analogy of the configurational diffusion process with an “equivalent” one‐dimensional Brownian motion of a rod through a periodic sequence of potential barriers. Numerical calculations are found to be in reasonable agreement with Gorring's experimental data and quantify his qualitatively‐stated mechanism, namely that n‐alkanes longer than C8 are too large to fit entirely within the potential wells formed by erionite cages and therefore experience smaller energetic barriers to diffusion.
Adsorption in bidisperse‐pore systems
Authors: Petersen Eugene E. 
Published: June 2004

AbstractAdsorption of a strongly bound absorbate in a bidisperse‐pore system is modeled using a Turner pore structure. This idealized system leads to an analytical solution for the micropore uptake of adsorption that is valid for a part of the adsorption‐time profile. The analytical solution yields a well‐defined criterion that establishes the boundary between an adsorption process dominated by macropore resistance and one dominated by micropore resistance. The range of the analytical solution is extended to include the entire adsorption‐time profile using an approximate analytical solution and two asymptotic solutions.
Isotherm selection for band profile simulations in preparative chromatography
Authors: Jacobson Stephen , Golshan‐Shirazi Sadroddin , Guiochon Georges 
Published: June 2004

AbstractEquilibrium adsorption data for the N‐benzoyl derivatives of D‐ and L‐phenyl alanine in water/propanol solutions have been determined on a chiral‐selective stationary phase, bovine serum albumin immobilized on silica. These data are well accounted for by an isotherm equation resulting from the sum of two Langmuir terms. The first term corresponds to the chiral‐selective interactions, and the second to the nonselective molecular interactions between the enantiomers and the stationary phase. The individual band profiles of the two enantiomeric amino acids, either pure or in binary mixtures of various compositions, are predicted exactly by an equilibrium dispersive model using this bi‐Langmuir isotherm. A classical Langmuir isotherm fails to predict these band profiles, illustrating the importance to collect accurate adsorption data and to use a correct isotherm model when calculating chromatographic band profiles.
Characterization of coal char gasification rate
Authors: Adschiri Tadafumi , Nozaki Takao , Furusawa Takehiko , Zi‐bin Zhu 
Published: June 2004

AbstractThe transient kinetic technique was applied to the evaluation of both elemental rate constants, kav and kin, and the amount of reactive carbons, nw, which participate in gasification. The dynamic change in gasification rate with increasing conversion is due to the change in nw. Differences in the gasification rates of ten different chars can be explained mostly by the differences in nw, rather than by those of kav or kin. nw increased and approached a maximum value, N, with increasing partial pressure of carbon dioxide. The maximum value, N, related closely to the carbon structure of char, especially to the crystallite size of graphite.