bluuebunny/crossref_metadata_2025_split huggingface dataset:
Wave model for longitudinal dispersion: Analysis and applications
Authors: Westerterp K. R. , Dil'man V. V. , Kronberg A. E. , Benneker A. H. 
Published: June 2004

AbstractAn analysis and applications of the wave model for longitudinal dispersion are pre‐sented. Asymptotic forms of the wave model are considered and analytical solutions of typical linear stationary and nonstationary problems of chemical reactor engineering interest are obtained and compared to those for the Fickian dispersion model. The wave model leads to efficient analytical solutions for linear problems, which in principle differ from the solutions of the Fickian dispersion model; only for slowly varying concentration fields do the soluctions of both models approach each other. Spatial and time moments of the concentration distribution are obtained for pulse‐dispersion problems; the first three spatial moments of the mean, variance, and skewness have exact, large‐time asymptotic forms in the case of Taylor dispersion. Old experiments that could not be explained with the standard dispersion model are reconsidered and explained: the change with time of the variance of a concentration pulse when the flow direction is reversed and the difference in values of the apparent axial dispersion coefficient and the backmixing coefficient in a rotating disk contactor. The experimental determination of model parameters is discussed.
Single‐step thermal method to measure intracrystalline mass diffusion in adsorbents
Authors: Grenier Ph. , Bourdin V. , Sun L. M. , Meunier F. 
Published: June 2004

AbstractA single‐step thermal method is used to measure intracrystalline mass diffusion. Sorption/desorption rates is zeolite samples (large crystals, monolayers of pellets, or even a single pellet) after a pressure of volume step are followed by monitoring the sample surface temperature by infrared detection. For a volume step, the pressure is also measured yielding the adsorbed mass. Sorption rates of water vapor in NaX are measured both on large 100 ‐ μm crystals and pellets. This fast system correcsponds to the limit of the pressure‐step termal method (using 100 ‐μm crystals). Sorption rates of methanol in the same large NaX crystals show good precision by the pressure‐step method. The methanol results show that a surface barrier may occur after thermal regeneration of the sample in the presence of methanol traces. A major advantage of this method is that the shape of response curves can provide useful information on the nature of the masstransfer resistance despite its limits. Sorption rates of methanol vapor on mordenite H (zeolon) pellets prove that the intracrystalline diffusivity may be extracted from pellet measurements for a slow diffusing species.
Acetic acid oxidation and hydrolysis in supercritical water
Authors: Meyer Jerry C. , Marrone Philip A. , Tester Jefferson W. 
Published: June 2004

AbstractAcetic acid (CH3COOH) hydrolysis and oxidation in supercritical water were examined from 425 – 600°C and 246 bar at reactor residence times of 4.4 to 9.8 s. Over the range of conditions studied, acetic acid oxidation was globally 0.72 ± 0.15 order in acetic acid and 0.27 ± 0.15 order in oxygen to a 95% confidence level, with an activation energy of 168 ± 21 kJ/mol, a preexponential factor of 109.9 ± 1.7 and an induction time of about 1.5 s at 525°C. Isothermal kinetic measurements at 550°C over the range 160 to 263 bar indicated that pressure or density did not affect the rate of acetic acid oxidation as much as was previously observed in the oxidation of hydrogen or carbon monoxide in supercritical water. Major products of acetic acid oxidation in (upercriuical water are carbon dioxide, carbon monoxide, methaite, and hydrogen. Trace amounts of propenoic acid were occasionally detected. Hydrolysis or hydrothermolysis in the absence of oxygen resulted in approximately 35% conversion of acetic acid at 600°C, 246 bar, and 8‐s reactor residence time. Regression of the limited hydrolysis runs assuming a reaction rate first‐order in organic gave a global rate expression with a preexponential factor of 104.4 ± 1.1 and an activation energy of 94 ± 17 kJ/moL.
Molecular thermodynamics for salt‐induce protein precipitation
Authors: Chiew Y. C. , Kuehner D. , Blanch H. W. , Prausnitz J. M. 
Published: June 2004

AbstractA molecular‐thermodynamic model is developed for salt‐induced protein precipitation, which considers an aqueous solution of globular protein molecules as a pseudo‐one‐component system containing macroions that interact through Coulombic repulsion, dispersion attraction and hydrophobic interactions, and forces arising from ion‐excluded volume. Forces from ion‐excluded volume take into account formation of ion pairs and ionic clusters at high salt concentrations; they are calculated in the context of the Percus‐Yevick integral‐equation theory. Hydrophobic interactions between exposed nonpolar amino‐acid residues on the surfaces of the protein molecules are modeled as short‐range, attractive interactions between “spherical caps” on the surfaces of the protein polyions. An equation of state is derived using perturbation theory. From this equation of state we calculate liquid ‐ liquid equilibria: equilibrium between an aqueous phase dilute in protein and another aqueous phase rich in protein, which represents “precipitated” protein. In the equation of state, center‐to‐center, spherically symmetric macroion–macroion interactions are described by the random‐phase approximation, while the orientation‐dependent short‐range hydrophobic interaction is incorporated through the perturbation theory of associating fluids. The results suggest that either ion‐excluded‐volume or hydrophobic‐bonding effects can precipitate proteins in aqueous solutions with high salt concentrations.
3‐D Laminar stationary flow over a porous surface with suction: Description at pore level
Authors: Schmitz Philippe , Prat Marc 
Published: June 2004

AbstractThe microhydrodynamics associated with membrane cross‐flow microfiltration processes is analyzed. To this end, numerical computations of the flow near a pore of a spatially periodic model porous surface are performed for various porosities and suction conditions. The structure of the fluid capture tube as well as related fluid dynamics aspects, such as the existence of a recirculating eddy in the pore, the slip velocity at the wall, and the pressure drop across the pore, are investigated. In particular, the influence of the neighbor pores on the structure of the fluid capture tube of a given pore is studied.
Growth of naphthalene crystals from supercritical CO2 solution
Authors: Tai Clifford Y. , Cheng Chuen‐Song 
Published: June 2004

AbstractThe growth phenomena, mechanism, and kinetics of naphthalene crystal from supercritical CO2 solution were studies by using a single‐crystal technique. Distinct growth features were observed, including the development of individuals from the seeds and the sprouting of plates from the individuals. The surface integration proceeded by two‐dimensional nucleation mechanism at face corners and by subsequent spreading of the nuclei. The measured growth rates as functions of supersaturation and solubility were consistent with the derived growth‐rate equations, based on the corner nucleation mechanism. In comparison with crystal growth from conventional media, the growth of naphthalene crystals from supercritical CO2 solution is similar to liquid‐solution growth as far as growth mechanism and kinetics are concerned.
Necessary and sufficient conditions for reactive azeotropes in multireaction mixtures
Authors: Ung Sophie , Doherty Michael F. 
Published: June 2004

AbstractNecessary and sufficient conditions for azeotropy in reactive mixtures derived show that in the space of transformed compositions, they take the same functional form as the conditions for azeotropy in nonreactive mixtures. In general, reactive azeotropes do not correspond to points of equal mole fraction (or mass fraction) across the coexisting equilibrium phases. The relationship between stationary points in the equilibrium temperature or pressure surface and reactive azeotropes is also considered, as well as their relevance to the Gibbs‐Konovalov theorems.
Adsorption kinetics for the case of setp and S‐shape isotherms
Authors: Kruglov Alexey V. , Aris Rutherford 
Published: June 2004

AbstractThe Kinetics of diffusion in a spherical pellet saturated by a sorbate in the presence of an inert component are considered for the case of a step isotherm. A new convective flux inside the pores due to sorption effect has been taken into consideration, and an analytical expression for sorption kinetics is obtained. The convective flux may result in considerable increase in the sorption rate. It is also found that the formula obtained also gives a satisfactory description of sorption kinetics for the case of an S‐shaped isotherm.
Simulation of diffusion in zeolitic structures
Authors: van den Broeke Leo J. P. 
Published: June 2004

AbstractUsing Maxwell‐Stefan equations, experimental and computational results of binary diffusion in pore‐ and cage‐type zeolitic structures are described. In the generalized Maxwell‐Stefan (GMS) formulation, the Fick diffusivity is written as the product of two separate contributions, the GMS or corrected diffusivity and the thermodynamic factor. The concentration dependence of the GMS diffusivity for one‐ and two‐component diffusion in zeolitic structures is investigated. From the Maxwell ‐ Stefan equations, different models for the Fick diffusion coefficient matrix for the description of binary mass transport in molecular sieve materials are derived. Various models used predict binary diffusion in zeolitic structures. First, theoretical predictions of binary apparent diffusivities as a function of the occupancy are compared to results from Monte Carlo simulations. Second, theoretical results of binary uptake profiles are compared to experimental results for the system ethylbenzene/benzene/ZSM‐5. For different zeolitic structures, that is, pore‐ and cage‐type structures, results of the Monte Carlo simulations agree well with the theoretical predictions. In cage‐type structures, the effect of counterexchange between sorbed molecules is demonstrated. Experimental results of transient uptake profiles of a mixture of benzene and ethylbenzene in ZSM‐5 follow predictions of the theoretical single‐file diffusion model.
Finely‐divided powders by carrier solution injection into a near or supercritical fluid
Authors: Schmitt William J. , Salada Michael C. , Shook Gary G. , Speaker Stanely M. 
Published: June 2004

AbstractCarbon dioxide or ethane was used as an antisolvent medium from which organic solids dissolved in a liquid carrier solvent are precipated and collected as finely divided solids. Solid compounds of pharmaceutical interest, which are virtually insolute in the supercritical fluids per se, are efficiently obtained by injecting a solution of the solid into a volume of stirred supercritical fluid or compressed liquefied gas. A free‐flowing powder comprising very small particles ( > 5 μm) is obtained. Both continuous‐flow and batch operations were developed although he continuous‐flow operation is preffered. Process variable including temperature, pressure, stirring rate, injection concentration, and the rate and temperature of the carrier solution injection were studied to see how they affect the physical properties of the resultant product. Recovery (yield), particle size and uniformity, bulk powder density, microscopic apperance, and surface area were measured. Carbon dioxide and ethane antisolvents, several liquid‐phase injection solvents, and different solid compunds were studied. A simple unsteady‐state mathematical model of this CSTR precipitator is also presented.
Foamed gel barriers in porous media: Breakdown and permeability evolution
Authors: Miller Matthew J. , Fogler H. Scott. 
Published: June 2004

AbstractFoamed del has begun to play an important role in permeability modification applications because of the reduced chemical requirements. Foamed gels create impermeable barriers in porous media; however, once a critical pressure differential is exceeded, the permeability increases with increasing pressure. A two‐dimensional network model was developed to estimate foamed gel barrier performance in terms of the maximum pressure a barrier can withstand and the evolution of the foamed gel barrier's permeability. The formation of conductive pathways and the accopanying permeability increase were estimated from a model of the pressure‐induced deformation and repture of individual lenses. The evolution of conductive pathways changed from invasion percolation (high elastic modulus, rigid gel) to a lens rupture chain reaction initiated by teh rupture of a single lens (low elstic modulus gel) as the elastic modulus of the gel was decreased. The apparent fractal dimension of the first conductive channel ranged from 1.89 to 1.06 for high and low wlastic modulus gels, respectively. This dependency of breakthrough and breakdown is unique and produces a large range of breakdown behavior for any degree of microscopic heterogeneity.
Turbulent flow in channels in terms of turbulent shear and normal stresses
Authors: Churchill Stuart W. , Chan Christina 
Published: June 2004

AbstractThe local time‐averaged velocity, the mixed‐mean velocity, and the friction factor for fully turbulent flow between parallel plates and in round tubes and concentric circular annuli can be expressed in terms of integrals of the turbulent shear stress, The pressure distribution across a channel can similarly be expressed in terms of an integral of normal stresses. These formulations, which are simple and exact, can be integrated numerically using experimental data, compyted values, or correlating equations for turbulent stresses. Their greatest merit, however, may arise from the insight they provide with respect to the contributions of the fluctuating components of the velocity. For example, for concentric circular annuli such a formulation identifies a difference between the locations of the maximum in the velocity and the zero in the total shear stress. This difference, which has been overlooked in most experimental and semitheoretical investigations, introduces an error of unknown but possibly significant magnitude into all of the results. It also precludes the application of the mixing‐length, eddy viscosity and k ‐ ϵ models.
Mixing, reaction, and precipitation: Interaction by exchange with mean micromixing models
Authors: Tavare Narayan S. 
Published: June 2004

AbstractAn analysis of interaction by exchange with the mean micromixing model is extended to a process involving elementary chemical reaction between two species and subsequent crystallization of product in a continuous mixed suspension mixed product removal etystallizer. Two specific feed conditions are considered: premixed and anpremixed feeds are considered. The sensitivity of these two cases to several parameterv like the Damköhler number, micromixing parameter, fractional flow rate, and dimensionless inlet concentration of excess reactant is explored. Both reaction and crystallizaLion performance characteristics are significantly influenced by the fred conditions. Results for the case of premixed feeds tend to suggest that the model description may be better suited to a nearly segregated configuration.
Solution of equations describing fluid–soil reactions in packed columns
Authors: Fernandes Neil , Gavalas George R. 
Published: June 2004

AbstractA semianalytical technique is presented frr solving the equations describing an isothermal, irreversible reaction of a trace component in a packed bed of a solid reactant or adsorbent The reaction‐rate expression is assumed to have an arbitrary dependence on the solid reactant concentration, but a first‐order dependence on the trace gas component. The technique relies on an integral transformation that reduces the set of partial differential equations to a set of two coupled ordinary differential equations in the spatial variables. Solving these two equations is simpler than solving the original equations by finite differences or finite elements, especially in the presence of steep concentration gradients. Two examples presented illustrate the technique: a progressive conversion model with reaction occurring at dispersed sites within the adsorbent pellet and a reaction proceeding in the shrinking core mode.
Theoretical aspects of self‐propagating reaction fronts in condensed medium
Authors: Ilyashenko Victor M. , Solovyov Stanislav E. , Pojman John A. 
Published: June 2004

AbstractExothermic chemical reactions are capable otautocatalytic self‐propagation in a condensed reactive medium as a thermal wave. These take place in self‐propagating high‐temperature synthesis, frontal polymerization, curing of epoxy resins, and various solid‐phase reaction processes. The lack of accurate analytical solutions for the front velocity and spatial parameter distributions has led to a period of approximate analytical solutions based on the infinitely narrow reaction zone assumption. A more accurate reaction zone approximation is presented that results in new equations for temperature and concentration spatial distributions as well as the wave velocity. Necessaty conditions for nondegeneration of the combustion wave were also obtained.
Transport of n‐paraffins in zeolite T
Authors: Magalhães Fernão D. , Laurence Robert L. , Conner William Curtis 
Published: June 2004

AbstractMass transport of n‐heptane, n‐nonane, and n‐dodecane in zeolite T was studied using a gravimetric uptake technique. The sigmoidal shape of the uptake curves indicated the existence of a nondiffusional resistance in the transport process. To compute the transport parameters, an approach based on the temporal moments of the uptake curves was developed. A model involving an external resistance dependent on the local slope of the sorption isotherm best represented the data. No evidence of the “window effect” reported by Gorring was found. The measured diffusivities decrease with chain length, while the activation energies remain essentially constant. Both resistances (diffusional and nondiffusional) depend strongly on lattice loading, and their relative contributions for the transport time change as saturation is approached.
Effects of isomorphic compounds on the purity and morphology of L‐isoleucine crystals
Authors: Koolman Hannia Canossa , Rousseau Ronald W. 
Published: June 2004

AbstractL‐Isoleucine and the isomorphic amino acids L‐leucine and L‐valine serve as a model system in the examination of the effects of impurities on the purity and morphology of crystals recovered from batch crystallizations. Factors are determined that influence crystal purity and, concomitantly, crystal size and shape. Characteristics of the L‐isoleucine crystal are determined by X‐ray analysis and compare favorably with the literature. These and literature characteristics of L‐leucine and L‐valine are used in simulations to evaluate possible mechanisms by which lattice substitutions with impurities lead to size and morphological changes.
Scanning force microscopy of gypsum dissolution and crystal growth
Authors: Hall Christopher , Cullen David C. 
Published: June 2004

AbstractDissolution and crystal growth on the (010) cleavage surface of mineral gypsum have been studied by scanning force microscopy, which allows observation of surfaces while they are in contact with aqueous solutions. Etch pits formed on the (010) surface in contact with solutions undersaturated with respect to gypsum are oriented in relation to the [001] zone axis. Edges parallel to [001] are stable or move only slowly. Edges parallel to [100] and [101] move more rapidly. Crystal growth by migration of step edges is observed in contact with supersaturated solutions. The inhibition of crystal growth by phosphonate produces gross changes in the texture of steps and terraces.
Model for flue‐gas desulfurization in a circulating dry scrubber
Authors: Neathery James K. 
Published: June 2004

AbstractA simple model was developed to describe the absorption of SO2 in a circulating dry scrubbing (CDS) process, which is a semidry, lime‐based, flue‐gas desulfurization (FGD) process that utilizes a circulating fluidized bed arrangement for contacting a sorbent with SO2‐laden flue gas under “coolside” conditions. The reaction chemistry is thought to be similar to that of spray‐drying absorption. The liquid‐phase mass‐transfer coefficient was successfully modeled as a function of the sorbent particle spacing on the wetted surfaces. Gas‐phase mass‐transfer resistances were assumed to be insignificant. Due to the high surface area available in a CDS reactor, the evaporation rate of water from the slurry was modeled as “constant‐rate” drying according to classic spray‐dryer theory. However, the “falling‐rate” and “diffusion” evaporation stages were negligible in CDS since sorbent particle bunching at the surface of the slurry is nonexistent.
Separation of branched hexane isomers using zeolite molecular sieves
Authors: Huddersman K. , Klimczyk M. 
Published: June 2004

AbstractA range of small, medium and large pore zeolite, and their modified forms are studied for their ability to separate di‐ from monobranched isomers of hexane. The separation studies are carried out using high‐temperature (250–350°C) gas chromatography. Beta(H,Ba) is found to be the most effective separator of 2,3‐dimethylbutane and 3‐methyl‐pentane and is therefore studied for its sorption capacities toward the two hexane isomers.
Neural network architecture for process control based on the RTRL algorithm
Authors: Chovan Tibor , Catfolis Thierry , Meert Kürt 
Published: June 2004

AbstractNeural‐network‐based control schemes are generally designed by replacing standard elements of the classic control schemes by feedforward neural networks. The introduction of discrete time recurrent networks, which are inherently dynamic systems, into those schemes can simplify the design of neural controllers. The concept of applying recurrent networks in indirect adaptive control schemes is described. A combined network cluster consisting of the control network and the model network is constructed to allow the use of the real‐time recurrent learning algorithm. To demonstrate the feasibility of the method two simulation examples are presented.
Experimental evaluation of designs for the simulated countercurrent moving bed separator
Authors: Tonkovich Anna Lee Y. , Carr Robert W. 
Published: June 2004

AbstractSimulated countercurrent chromatography is a continuous flow method for separation of binary mixtures or for separation of multicomponent mixtures into two fractions. Countercurrent flow is simulated by moving the feed point past several fixed adsorbent beds at a rate between the single column breakthrough time of each component. The number of columns and the location of the inlet and outlet port were varied to investigate product purity and productivity. Concentration profiles of the effluent product streams were measured. The maximum product concentration exceeds the feed concentration during part of the feed‐switching cycle and drops to zero during part of the cycle. Three configurations were tested with the total number of columns varying between three and eight. For the test separation chosen, gaseous propylene and dimethyl ether on Chromosorb 101, high purity (> 99%) product streams were obtained with an optimal four‐column configuration using three desorbent streams.