bluuebunny/crossref_metadata_2025_split huggingface dataset:
Axial mixing in modern packings, gas and liquid phases: I. Single‐phase flow
Authors: Macías‐Salinas Ricardo , Fair James R. 
Published: April 2004

AbstractAn experimental study was conducted in a 0.43‐m‐ID packed column to determine the axial mixing properties of air and water under single‐phase flow conditions. The packings used were 25.4‐mm ceramic Raschig rings, one modern random packing, 25.4‐mm metal Pall rings; two structured packings, Sulzer BX and Flexipac 2. The column was operated at ambient conditions with flow rates varying from 0.4 to 4 kg/m2·s for the gas and from 3.25 to 8.5 kg/m2·s for the liquid. Axial mixing was experimentally determined via dynamic response studies based on the pulse injection technique. The diffusion‐type model served to reproduce the experimental response curves satisfactorily and proved to be a suitable means of describing axial mixing in both phases. The results confirm previous observations for first‐generation packings. Axial mixing is much greater in the liquid than in the gas; axial mixing in the gas increases with gas rate, whereas axial mixing in the liquid decreases with liquid rate. It was also found that the two structured packings produce the lowest levels of axial mixing in the gas. Surprisingly, the greatest mixing effects in the liquid were obtained for Flexipac 2 structured packing. Correlations were developed to represent the experimental mixing and liquid holdup data, yielding an average ±16% difference between experimental and correlated data. Mixing measurements in gas or liquid under two‐phase flow conditions will be discussed in Part II.
Pulse‐response study for the humidity effect on sorption of ethyl bromide on clays
Authors: Yaşyerli Nail , Doğu Gülşen , Doğu Timur , McCoy B. J. 
Published: April 2004

AbstractThe effects of relative humidity on sorption and the diffusion of ethyl bromide in kaolinite and montmorillonite pellets were investigated by the one‐sided single‐pellet moment technique. Montmorillonite demonstrated a higher water affinity than kaolinite, causing a sharper decrease of the ethyl bromide adsorption equilibrium constant with an increase of relative humidity. For montmorillonite, most of the active sites are covered by water even at relative humidities of 8%, and adsorption of ethyl bromide on the gas–water interface controls the sorption process. For kaolinite, however, adsorption on gas‐to‐mineral, gas‐to‐water, and water‐to‐mineral surfaces contributes to the sorption process. Effective macropore diffusivity of ethyl bromide in montmorillonite showed a decreasing trend with increase in relative humidity. This is due to high water uptake of montmorillonite and the corresponding change in pore structure. No such change was observed for kaolinite.
Measuring induction period for calcium sulfate dihydrate precipitation
Authors: Lancia Amedeo , Musmarra Dino , Prisciandaro Marina 
Published: April 2004

AbstractHomogeneous nucleation of CaSO4·2H2O (gypsum) based on an optical diagnostic technique was studied within a supersaturation range of 1–4 at 25–90°C. The experiments were carried out using an experimental apparatus consisting of a batch crystallizer with the related measurement devices. Signals of scattered and transmitted light coming from a He–Ne laser source were analyzed to measure the induction period (tind), that is, the time delay necessary for homogeneous nucleation to take place. As expected from theory, it was found that tind decreases when either temperature or supersaturation increase; from the dependence of tind on supersaturation, it was possible to distinguish between the mechanisms of homogeneous and heterogeneous nucleation. From the experimental data relative to homogeneous nucleation, the interfacial tension (γs) between CaSO4·2H2O and the surrounding aqueous solution and the activation energy (Eatt) for CaSO4·2H2O crystallization were evaluated. In particular, the dependence of tind on temperature made it possible to evaluate Eatt at 30 kJ/mol and the dependence of tind on supersaturation offered the value of γs to be about 37 mJ/m2, which does not vary with temperature in the interval explored.
Pattern formation in homogeneous reactor models
Authors: Sheintuch Moshe , Nekhamkina Olga 
Published: April 2004

AbstractThis work analyzes pattern formation mechanisms in the homogeneous model of a fixed catalytic bed for reactions with oscillatory kinetics. Two cases are analyzed: a nonadiabatic reactor with a continuous mass‐supply (either by a preceding reaction or via a membrane wall), and a simple adiabatic or cooled reactor. In the former case, the system may reach asymptotic space‐independent solutions, and when bistability of such solutions exists fronts may be established. Stationary or oscillatory front solutions, oscillatory states that sweep the whole surface, or excitation fronts may be realized then and the reactor behavior can be predicted from the sequence of phase planes spanned by the reactor. In an adiabatic reactor, fronts are formed only for sufficiently small Pe numbers, but these frontlike solutions do not separate different steady states. The patterns that can be realized in this case are quite similar to those in the previous case. The reactor behavior can be predicted by the sequence of phase planes spanned by the reactor, using an approximate finite difference presentation.
Prediction of colloid detachment in a model porous media: Thermodynamics
Authors: Bergendahl John , Grasso Domenico 
Published: April 2004

AbstractDetachment of polystyrene latex colloids was studied in a model porous media system comprised of a packed column of glass beads. The polystyrene colloids were attached and subsequently detached from the glass beads by varying solution pH and ionic strength. It was found that classical‐DLVO theory could not predict detaching conditions. Extended‐DLVO theory was used to determine the median detaching pH for deionized water, and solutions of 0.01 and 0.1 M NaCl. Although both the colloids and porous media were macroscopically homogeneous, detachment was observed at a variety of pH values. The pH distribution of detachment and the energy distribution of the interaction between the polystyrene latex colloids and the glass beads followed gamma distributions. This work has provided a technique to quantitatively predict the aqueous chemistry changes that induce colloid detachment in porous media.
Binary protein adsorption on gel‐composite ion‐exchange media
Authors: Lewus Rebecca Karst , Carta Giorgio 
Published: April 2004

AbstractThe adsorption kinetics of mixtures of lysozyme and cytochrome‐C on S‐HyperD‐M, a cation exchanger composed of a rigid macroporous silica matrix whose pores are filled with a functionalized polyacrylamide gel, was characterized. Single‐component isotherms were found to follow approximately the steric mass‐action law of Brooks and Cramer, and an extension of this model to a binary system was consistent with two‐component uptake equilibrium results. Transient adsorption experiments were done to investigate the kinetics of sequential and simultaneous adsorption of the two proteins. At low protein concentrations, adsorption is essentially noncompetitive, and mass transfer is controlled by the external film resistance. At higher concentrations, equilibrium becomes competitive and coupling of the intraparticle diffusion fluxes occurs. A model based on a Maxwell–Stefan approach, where the driving force for diffusion is expressed in terms of the chemical potential gradient, was in approximate agreement with the experimental results.
Mass transfer during freezing in rat prostate tumor tissue
Authors: Devireddy Ramachandra V. , Smith David J. , Bischof John C. 
Published: April 2004

AbstractCryosurgery for treating prostate cancer is hampered by an incomplete understanding of the mechanisms whereby tissue destruction is achieved during freezing. The two known biophysical mechanisms of injury, intracellular ice formation and cellular dehydration injury (solute effects), have not been quantified within tumor tissue. Freeze substitution microscopy and a differential scanning calorimeter (DSC) were used to quantify freeze‐induced dehydration within Dunning AT‐1 rat prostate tumor tissue. Stereological analysis of histological tumor sections was used to obtain the initial cellular (Vo), interstitial, and vascular volumes of the AT‐1 tumor tissue. A Boyle‐van´t Hoff (BVH) plot was then constructed by examining freeze substituted micrographs of equilibrium cooled tissue slices to obtain the osmotically inactive cell volume, Vb= 0.25Vo. Obtaining dynamic cellular water transport information from the freeze substitution microscopy data proved difficult because of the artifact added by the high interstitial volume (∼35%). Since the DSC technique does not suffer from this artifact, a model of water transport was fit to the DSC water transport data at 5°, 10° and 20°C/min to obtain the combined best fit membrane permeability parameters of the embedded AT‐1 tumor cells, assuming either a Krogh cylinder geometry or a spherical cell geometry. Numerical simulations were also performed to generate conservative estimates of intracellular ice volume (IIV) in the tumor tissue at various cooling rates typical of those experienced during cryosurgery (≤100°C/min). Water transport data in tumor systems with significant interstitial spaces can be obtained by using histology and the low‐temperature microscopy methods to obtain the initial and final tissue cell volumes, respectively, and the DSC technique to obtain the dynamic volume changes during freezing.
Mass transfer of water vapor in a hollow fiber for degassing processes
Authors: Murata Hideyuki , Tomita Yasushi , Miyashita Makoto , Sakai Kazunari , Toda Masayuki , Ohmi Tadahiro 
Published: April 2004

AbstractThe mass‐transfer mechanisms of water vapor in hollow‐fiber membrane modules used to remove gases from water were studied experimentally and theoretically. Hollow‐fiber modules for degassing are widely used in various industries, including ultrapure water production. Although these modules perform extremely well, a general principle for the design has not been established. Recently, very highly efficient asymmetric hollow‐fiber modules were developed, that could reduce the dissolved oxygen to less than 1 μg/L (1 ppb). From our studies of the modules, it was found that the mass transfer of water vapor plays an extremely important role in the flow in the lumen side of the hollow fibers. The establishment of a theoretical design method for degassing modules can be expected from our studies.
Mixed cation zeolites: LixAgy‐X as a superior adsorbent for air separation
Authors: Hutson Nick D. , Rege Salil U. , Yang Ralph T. 
Published: April 2004

AbstractLi‐X zeolite (Si/Al = 1.0) is currently the best sorbent for use in the separation of air by adsorption processes. Silver is also known to strongly affect the adsorptive properties of zeolites and thermal vacuum dehydration of silver zeolites leads to the formation of silver clusters within the zeolite. In this work we synthesized type X zeolites containing varying mixtures of Li and Ag. Adding very small amounts of Ag at the proper dehydration conditions formed silver clusters and enhanced adsorptive characteristics and energetic heterogeneity, as compared to those of the near fully exchanged Li+‐zeolites. The performance for air separation by the best of these sorbents, containing, on average, only one Ag per unit cell, was compared to that of the near fully Li+‐exchanged zeolite using a standard PSA cycle by numerical simulation. The results show that the new sorbent provides a significantly higher (> 10%) product throughput at the same product purity and recovery when compared to that of the near fully Li+‐exchanged zeolite.
Unified network model for adsorption–desorption in systems with hysteresis
Authors: Rajniak Pavol , Soós Miroslav , Yang Ralph T. 
Published: April 2004

AbstractThe problem of equilibrium and kinetics for adsorption–desorption of condensable vapors in porous media is studied experimentally and theoretically. For adsorption, the network model for diffusion based on pore blocking theory with percolation (in the network) added by effective medium approximation is further improved. A new predictive model based on properties of the Bethe lattices is proposed to account for the existence of liquid‐filled “blind” pores that result in a decrease in the total diffusion rate. For desorption, a new “shell and core” (or shrinking core) representation of the network model is proposed. Information from adsorption–desorption equilibria is needed to compute the thickness of the shell in which desorption/evaporation occurs for concentrations higher than the percolation threshold. These models form a unified equilibrium‐kinetics theory for gas‐porous solid systems that exhibit hysteresis. The models are applied to the systems silica gel‐water vapor and Vycor glass‐nitrogen. Concentration‐dependent Fickian diffusivities for these systems have been measured for both adsorption and desorption branches. The adsorption model successfully predicts the experimental data with a maximum in diffusivity. The desorption model correctly predicts the concentration dependence of diffusivity with a steep minimum at the percolation threshold.
Multirate nonlinear state estimation with application to a polymerization reactor
Authors: Tatiraju Srinivas , Soroush Masoud , Ogunnaike Babatunde A. 
Published: April 2004

AbstractA method of multirate nonlinear state observer design, which can use directly a nonlinear process model in the observer design without any linear approximation, is presented. The multirate nonlinear state observer is easy to design and implement, and is computationally efficient. Furthermore, for a number of processes, it is possible to prove the global convergence of the multirate state observer analytically. The design, implementation and performance of the state observer design method are shown by a polymerization reactor in which free‐radical solution polymerization of styrene takes place. The initiator concentration and three leading moments of the molecular weight distribution (MWD) of the polymer product are estimated continuously from: (i) frequent measurements of the reactor temperature, jacket temperature and reacting‐mixture density; (ii) infrequent and delayed measurements of the leading moments of the MWD. Each infrequent measurement has a sampling period and a measurement delay of 0.5 or 1 h. In the presence of model‐plant mismatch and measurement noise, the convergence of the multirate state observer is shown by numerical simulations.
Role of distributed oxygen addition and product removal in the oxidative coupling of methane
Authors: Androulakis Ioannis P. , Reyes Sebastián C. 
Published: April 2004

AbstractModeling and optimization of C2 hydrocarbon production via the oxidative coupling of methane (OCM) were studied. The model includes both homogeneous and heterogeneous reactions. Focusing on the use of detailed reaction networks, previously validated experimentally, and the critical role of oxygen in both methane activation and product degradation, this work systematically explores the use of controlled oxygen addition and product removal schemes that improve OCM performance. Based on a plug flow reactor that is divided into Np stages, within which oxygen is added and/or products are removed, a rigorous optimization algorithm is developed that simultaneously maximizes C2 yields and minimizes O2 consumption. In the absence of catalyst and product removal, the C2 yield is maximized at a fixed O2 /CH4 ratio, but this maximum yield is independent of the form in which the oxygen is added (cofeed or staged). When a catalyst is added, the optimal C2 yields show only gradual improvements with oxygen distribution because the benefits of the lower oxygen reaction order on the catalyst are adversely affected by concomitant surface degradation reactions. The largest yield improvements are obtained when the C2 hydrocarbons are removed at each stage before they undergo oxidation reactions. Thus, when staged oxygen addition is combined with product removal in the presence of a catalyst, C2 yields as high as 87% are achieved in about 20 stages. Such yield values are consistent with experiments in which continuous product separation schemes have been used.
Concentration polarization in cross‐flow reverse osmosis
Authors: Song Lianfa , Yu Sanchuan 
Published: April 2004

AbstractA dimensionless governing equation was formulated for a cross‐flow RO process in which the local variation of concentration polarization was rigorously considered. It was shown in this formulation that the cross‐flow RO process could be fully characterized with a single dimensionless parameter. The coupling between permeate flux and concentration polarization was properly solved and a closed‐form analytical solution was obtained. This analytical solution enabled us to conveniently investigate concentration polarization in the RO process. The significance of local variation of concentration polarization was demonstrated, and the operations of RO under various conditions were simulated and investigated with the newly developed model.
Fluid‐dynamic and mass‐transfer behavior of static mixers and regular packings
Authors: Cavatorta Omar N. , Böhm Ursula , de del Giorgio Ana María Chiappori 
Published: April 2004

AbstractThe fluid dynamics and liquid‐to‐wall mass transfer for spaced and stacked regular packings were studied for forced convection and fluidized beds. The behavior of these configurations in bubble columns and under natural convection conditions is also presented. Flow parameters characterizing structured packings, presented in the literature, were used in the evaluation of results. General equations to predict pressure drop and mass transfer are discussed, as well as the relationship between energy dissipation and mass transfer. In the presence of fluidized particles, single‐phase flow or natural convection conditions, the mass‐transfer behavior of a packing element stacked between other packs or separated from the neighboring elements by liquid layers is almost the same, but differs in bubble columns.
Monte Carlo simulation of the radiation field in a reticulated foam photocatalytic reactor
Authors: Changrani Rajnish , Raupp Gregory B. 
Published: April 2004

AbstractThe 3‐D polychromatic radiation field for an annular packed‐bed photocatalytic reactor using alumina reticulated foams as a monolithic catalyst support was simulated using Monte Carlo methodology. Two distinct methods were used for simulating photon transport: (1) a “spatial” approach that tracks the flight of a photon in a predetermined reticulate structure; (2) a “temporal” approach that generates the random porous structure of the reticulate as the photon flies into it. The two approaches yield almost identical results, although the temporal approach is far more efficient computationally. Simulations for the integral axially‐averaged radial ultraviolet (UV) light profiles agree closely with experimental measurements for titania‐coated 10, 20 and 30 PPI alumina reticulates and near UV lamps. The simulations reveal that the local volumetric rate of energy absorption (LVREA) in the reticulate and the magnitude of the LVREA gradient both increase with decreasing reticulate pore size.
Effect of rheology on coalescence rates and emulsion stability
Authors: Dreher T. M. , Glass J. , O'Connor A. J. , Stevens G. W. 
Published: April 2004

AbstractThe effect of continuous‐phase rheology on the coalescence time of single water drops at an organic/aqueous interface was investigated experimentally. For Newtonian fluids, the coalescence time increases monotonically with the continuous‐phase viscosity at a fixed drop diameter and with the drop diameter at a fixed continuous‐phase viscosity. Elasticity in the continuous phase caused significant increases in the coalescence times for drops of less than 1 mm diameter in the systems studied, but had no discernible effect on the coalescence of larger drops. Estimates of the interfacial shear rates were used to demonstrate that the elasticity of the continuous phase becomes significant in the coalescence process as the drop size decreases. These results indicate that emulsion stability may be increased without the use of surfactants by adding a suitable polymer to the continuous phase and control of the dispersed‐phase drop size.
Tensile stress in a porous medium due to gas expansion
Authors: Benes Nieck E. , Biesheuvel P. Maarten , Verweij Henk 
Published: April 2004

AbstractStress profiles develop in a porous material due to a gas‐phase pressure difference and subsequent gas flow. If stresses become tensile, material failure (explosion and blistering) can occur. Stress profiles are calculated for an asymmetric inorganic porous disk‐like membrane material placed in a pressure vessel, which is depressurized. The stress that develops in the membrane material depends on the gas‐phase pressure and the porosity. The gas‐phase pressure is a function of place, time and characteristics of the membrane, the vessel and the valve. Two regimes are identified for membrane depressurization, and a critical initial pressure is defined below which tensile stresses cannot develop. The theory presented combines the dusty gas model with balances for mass, momentum, and mechanical energy.
Thermodynamic modeling of multicomponent ion‐exchange equilibria of amino acids
Authors: Bellot Jean Christophe , Tarantino Ramon Vega , Condoret Jean‐Stéhane 
Published: April 2004

AbstractA thermodynamic model was developed to describe phase equilibria for aqueous amino acid/polyelectrolyte gel systems. In the gel phase, activities of all exchangeable species (charged and neutral components) are calculated with the generalized Flory‐Huggins model. In the surrounding solution, a numerical procedure is used to allow for partial dissociation phenomena combined with a modified UNIFAC model associated with the Pitzer‐Debye‐Hückel term. The osmotic pressure difference between the two phases is also taken into account in the equations. The model enables the prediction of resin‐phase composition, gel swelling and intraparticle pH. It can be used for various biochemicals in a wide range of concentrations. Its validity was successfully tested using binary and multicomponent exchange equilibria data of several amino acids (phenylalanine, alanine, proline, and glutamate) and HCl on a strong‐acid cation‐exchange resin (Amberjet 1200H).
Heat and mass transport resistances in vacuum membrane distillation per drop
Authors: Bandini Serena , Sarti Giulio C. 
Published: April 2004

AbstractVacuum membrane distillation (VMD) is a separation process based on the use of microporous hydrophobic membranes. The membrane is located between an aqueous phase and a permeate, which is kept under vacuum at pressure values below the equilibrium vapor pressure of the feed. The liquid stream vaporizes at one side of the membrane, and the vapors diffuse through the gas phase inside the membrane pores. The process rate and performance are affected highly by the transport phenomena both in the liquid phase and through the membrane. Heat‐ and mass‐transfer resistance in the liquid phase, as well as mass‐transfer resistance through the membrane, play an important role in determining the process performance. Based on VMD experimental data for several binary aqueous mixtures containing volatile organic compounds, a simple criterion to investigate the role of each transport resistance on the separation efficiency is discussed.