bluuebunny/crossref_metadata_2025_split huggingface dataset:
Novel route to submicrometer tungsten carbide
Authors: Welham Nicholas J. 
Published: April 2004

AbstractA mixture of graphite and scheelite (CaWO4) have been mechanically milled together for 40 h. The resultant powder and an unmilled powder were examined by thermal analysis, isothermal annealing, and X‐ray diffraction to determine the effect of milling on the reduction of scheelite to tungsten carbide. The milled powder underwent a rapid reduction reaction at 800°C–300°C lower than the unmilled powder. The reduction sequence to WC was shown to differ for the two powders with W2C intermediate in the milled powder and W in the unmilled powder. The milled powder showed complete reduction to WC in 1 h at 1,000°C, whereas the unmilled powder was incompletely reduced, even after 1 h at 1,200°C. Separation of the final phases was readily achieved by a simple acid leach, leaving WC grains around 200 nm in size.
Axial mixing in modern packings, gas, and liquid phases: II. Two‐phase flow
Authors: Macías‐Salinas Ricardo , Fair James R. 
Published: April 2004

AbstractAxial mixing measurements of air and water under two‐phase flow conditions were made in a large‐scale packed column (0.43 m diameter) using tracer experiments. Part I of this article dealt with single‐phase mixing in the same column, with the same internals. Four packings were studied: 25.4‐mm ceramic Raschig rings, 25.4‐mm metal Pall rings, Sulzer BX structured packing, and Flexipac 2 structured packing. Air and water flowed countercurrently through the column at atmospheric pressure and at gas rates varying from 0.25 kg/m2·s up to the flooding point, and liquid rates from 3.25 to 8.5 kg/m2 ·s. A diffusion‐type model served to reproduce the experimental response curves obtained for both phases. The results confirmed previous observations for first‐generation packings: axial mixing in the gas increases with both gas and liquid rates, whereas liquid‐phase axial mixing is a decreasing function of liquid rate and is insensitive to gas rate up to the flooding point. It was also found that the BX packing produces the least mixing in both phases. The largest mixing effects in the gas phase are found for the Raschig rings, and the largest mixing effects for the liquid phase are found for Flexipac 2. Correlations were developed to reproduce the results, yielding an average ±22% difference between experimental and correlated data.
Separation of hydrogen from a H2 H2 OHI gaseous mixture using a silica membrane
Authors: Hwang Gab‐Jin , Onuki Kaoru , Shimizu Saburo 
Published: April 2004

AbstractHydrogen separation characteristics of silica membranes prepared by chemical‐vapor deposition (CVD)‐in an H2H2O HI gaseous mixture were evaluated for the application to hydrogen iodide decomposition in the thermochemical iodine–sulfer (IS) process. Porous alumina tubes with a pore size of 10 nm and different gas‐permeating‐portion lengths [20 mm (S membranes) and 100 mm (L membranes)] were modified by CVD using tetraethoxysilane as the Si source. Pore closure was controlled by CVD to maintain high hydrogen permeance while lowering only HI permeance. At a certain point, CVD was stopped using He/N2 selectivity as the indicator of pore closure. H2 N2 selectivities of the modified membranes, which were measured by a single‐component permeation experiment, showed 53, 9.2, 4.1, 135, and 6.6 at 600°C for S1, S2, S3, L1, and L2 membranes, respectively. Separation experiments of the H2H2OHI gaseous mixture (a molar composition of 0.09:0.78:0.13) in the modified membranes were carried out at 300–600°C. Hydrogen permeance was almost the same as the single‐component permeance. The separation factors of H2H2O and H2HI were over 3 and 150, respectively. The S3 membrane showed the highest hydrogen permeance (on the order of 10−7 mol·Pa−1·m−2·s−1) and the highest separation factor of H2HI (650 at 450°C).
Entropic and energetic selectivity in air separation with microporous materials
Authors: Rallabandi Premkumar S. , Ford David M. 
Published: April 2004

AbstractMaterials such as zeolites, carbon molecular sieves, and polymers are used increasingly in the separation of air, based on the difference in diffusion rate between oxygen and nitrogen through the material. The design of improved materials requires knowledge of the molecular‐level phenomena responsible for the separation, particularly relative roles of energetic and entropic (confinement) effects. This issue is difficult to resolve experimentally, as evidenced by the wide range in reported literature values reviewed here. A complementary approach is taken based on a combination of molecular modeling, statistical mechanics, and transition‐state theory. Selectivities for molecular models of oxygen and nitrogen in microporous structures are calculated using a Monte Carlo technique and resolved into entropic and energetic components, for a range of pore window sizes. Atomic‐level flexibility (vibration) is considered as well. The calculated entropic selectivities are significantly lower than reported theoretical results, but still consistent with experimental data. The energetic selectivity is very sensitive to the window dimensions and flexibility, but the entropic contribution is much less affected. This also contradicts some previous assumptions in the literature.
Systems approach for evaluating dynamics and plantwide control of complex plants
Authors: Groenendijk Alexander J. , Dimian Alexandre C. , Iedema Piet D. 
Published: April 2004

AbstractRecycle loops strongly affect dynamics and control of complex plants. The stability of control structures for component inventories may be influenced by recycle interactions and by connectivity, which leads to flowsheet alternatives possessing different control properties. A simulation‐based methodology for evaluating these phenomena and finding the best flowsheet structure is reported from a controllability point of view. Steady‐state and dynamic simulations are combined with controllability analysis tools, both steady state and in the frequency domain, which extracts more value from simulation than the usual sensitivity studies. The case of a vinyl chloride monomer (VCM) plant with impurities handling as a main issue demonstrates the power of the approach. It leads to an unconventional design alternative where an extra impurity‐destroying reactor reduces bad dynamic characteristics, since it prevents impurity buildup in the recycle. The steady‐state analysis of the VCM case is confirmed at low frequencies of disturbances, but difficulties may occur at higher frequencies, where the disturbances' period and time constants of the distillation columns are of the same order of magnitude. Controller interactions play a significant role in disturbance rejection. Nonlinear dynamic simulations with closed plantwide control loops confirm the analysis results.
Robust control of particulate processes using uncertain population balances
Authors: Chiu Timothy Y. , Christofides Panagiotis D. 
Published: April 2004

AbstractA general method is proposed for the synthesis of robust, nonlinear controllers for spatially homogeneous particulate processes described by population balances including time‐varying uncertain variables. The controllers are synthesized via Lyapunov's direct method on the basis of finite‐dimensional approximations of the population balances which are obtained by using the method of weighted residuals. The controllers enforce stability in the closed‐loop system and attenuation of the effect of uncertain variables on the output, and achieve particle‐size distributions with desired characteristics. The robustness of the controllers with respect to unmodeled dynamics is also addressed within the singular perturbation framework. The controllers enforce the desired stability and performance specifications in the closed‐loop system, provided that the unmodeled dynamics are stable and sufficiently fast. The proposed control method is applied to a continuous crystallizer with fines trap in which the nucleation rate and the crystal density change arbitrarily with time, and the actuator and sensor dynamics are explicitly considered in the process model, but not included in the model used for controller synthesis. Simulation runs of the closed‐loop system clearly demonstrate that the controller attenuates uncertainty, achieves a crystal‐size distribution with desired characteristics, and is superior to nonlinear controllers that do not account for the presence of uncertainty.
Identification of the state of a fluidized bed by pressure fluctuations
Authors: Trnka O. , Veselý V. , Hartman M. , Beran Z. 
Published: April 2004

AbstractA novel approach was used for the on‐line characterization of the state of a fluidized bed. The treatment technique using the Fast Fourier Transform of measured pressure fluctuations was developed to create plots describing the evolution of bed behavior from fixed bed to pneumatic transport. A new triad of quantities proposed are: divide ratio (degree of dominancy) of amplitude spectrum, square root of spectrum power of pressure fluctuations, and gas velocity. The technique was demonstrated on gas fluidization of solids having a wide size distribution composed of irregularly shaped solids.
Characterization of downflowing high velocity fluidized beds
Authors: Cao Chunshe , Weinstein Herbert 
Published: April 2004

AbstractA downer‐riser circulating high velocity fluidization apparatus was developed to study the fundamentals of downflowing gas‐solid particle mixtures. The acceleration and deceleration of solids due to the influences of the entrance and exit sections result in a relatively uniform axial solids distribution. Radial solid density profiles detected with an X‐ray imaging system in the downer show the existence of a core‐annulus flow with a dilute core surrounded by a denser wall region. Local solids flux profiles were obtained with an aspirating probe device and the solid velocity profile obtained from the two measured quantities. These confirm that the majority of solids segregates in a wall region that flows faster than the dilute core region. Thus, the shorter residence time in the high‐speed downer wall region is coupled with faster reaction rates due to the accompanying high concentration of catalyst, while the dilute core has slower reaction rates with longer residence time due to the lower catalyst concentration and flow velocity. This results in much more uniform reaction extent over the cross‐sectional area of the downer and, therefore, should improve the product selectivity.
Coalescence of deformable granules in wet granulation processes
Authors: Liu L. X. , Litster J. D. , Iveson S. M. , Ennis B. J. 
Published: April 2004

AbstractIn this work, the coalescence of deformable granules in wet granulation processes is modelled. The model accounts for both the mechanical properties of the granules and the effect of the liquid layer at the granule surface. It is an extension to the model of Ennis et al. (1991) to include the possibility of granule plastic deformation during collisions. The model is written in dimensionless groups such as viscous and deformation Stokes numbers and the ratio of granule dynamic yield strength to granule Young's modulus (Yd/E*). These variables are bulk parameters of the powder‐binder mixture and also functions of the process intensity. The model gives the conditions for two types of coalescence–type I and type II. Type I coalescence occurs when granules coalesce by viscous dissipation in the surface liquid layer before their surfaces touch. Type II coalescence occurs when granules are slowed to a halt during rebound, after their surfaces have made contact. The model explains some of the trends observed in the literature, are preliminary validation of the coalescence criterion with drum granulation data is encouraging. An extension is also made to the case of surface dry granules, where liquid is squeezed to the surface during granule deformation.
Temperature effect on transport performance by inorganic nanofiltration membranes
Authors: Tsuru Toshinori , Izumi Shuhei , Yoshioka Tomohisa , Asaeda Masashi 
Published: April 2004

AbstractThe effect of temperature on nanofiltration performance was examined using three inorganic membranes with a molecular‐weight cutoff of approximately 200, 600, and 2,000, respectively. The inorganic porous membranes were prepared from silica‐zirconia colloidal sols and used in nanofiltration experiments for neutral solutes over a temperature range of 20 to 60°C. The rejection of solutes decreased with an increase in temperature for the membranes, while the permeate volume flux increased. Three transport coefficients—refection coefficient, solute permeability, and water permeability—were obtained using the Spiegler‐Kedem equation, which accounts for the contribution of convection and diffusion to solute flux. As a result, the reflection coefficient corresponding to the fraction of solutes reflected by the membrane in convective flow was almost constant, irrespective of experimental temperature. Solute permeabilities, however, increased with temperature. The dependency was larger for larger solutes and membranes with smaller pore diameters. Therefore, the hindered diffusion of solutes through micropores was indicative of an activated process. Moreover, pure water permeability, after correction for the temperature effect on viscosity, also increased with experimental temperature.
Nonlinear process control using contraction theory
Authors: Lohmiller Winfried , Slotine Jean‐Jacques E. 
Published: April 2004

AbstractContraction theory is a recently developed nonlinear control system tool based on an exact differential analysis of convergence. Contraction theory is applied to stability analysis and control system design for nonlinear chemical processes. Simple designs with explicit stability and convergence guarantees are obtained by taking advantage of the monotonicity of the reaction rates and the linear ambiguity in the choice of the chemical state.
Measuring anisotropic thermal conduction in polyisobutylene following step shear strains
Authors: Iddir Hadjira , Venerus David C. , Schieber Jay D. 
Published: April 2004

AbstractThe connection between polymer chain orientation and several macroscopic properties in a polymer melt was studied using mechanical and optical techniques. Anisotropic thermal conductivity following shear deformation was measured using forced Rayleigh light scattering, the refractive index tensor is followed using birefringence measurements, and the stress was measured mechanically in a parallel‐plate rheometer. The thermal diffusivity measured in the flow and neutral directions increased and decreased, respectively, immediately following the deformation. These quantities then relaxed to the equilibrium value on the time‐scale of the stress‐relaxation memory. Comparison of the difference between measured flow and neutral direction thermal diffusivities with the analogous flow‐induced birefringence in the same deformation provided indirect evidence for a linear relation between stress and thermal diffusivity at two different values of strain. Mechanical measurements were used to characterize the memory of the fluid.
Supported zeolite membrane by vapor‐phase regrowth
Authors: Tsay C. S. , Chiang A. S. T. 
Published: April 2004

AbstractA new method for preparing supported zeolite membrane is proposed. A layer of colloidal MFI zeolite is deposited on a porous ultrafiltration support, on which a silica barrier layer has been precoated. The composite is then heated at 100°C under saturate water vapor for 30 h to obtain a very thin zeolite membrane with a glassy look. Under SEM, the zeolite membrane exhibits obvious void when the particles deposited were 100 × 150 nm in size. The particles are packed much closer if 80 × 100‐nm particles were used. With smaller (60 × 80 nm) particles, the boundary between particles fuses together, and a continuous zeolite membrane could therefore be prepared. The membrane thickness could also be reduced by depositing less colloidal zeolite for regrowth to obtain a continuous zeolite membrane as thin as 0.5 μm on top of a multilayer porous alumina support. Some preferred orientation of the elliptical zeolite particles is observed on the surface of the membrane, but no preferred crystallography orientation is confirmed. The reproducibility of this preparation method was checked by room temperature permeation measurements of several gases. Nearly identical results were obtained on two separately prepared membranes. Permeation results also confirmed that there were very few, if not free of, pinholes in the zeolite membrane so prepared.
Validating a prediction method of mean residence time spatial distributions
Authors: Baléo Jean‐Noël , Le Cloirec Pierre 
Published: April 2004

AbstractDetermining residence time is of major interest in the characterization, design or modeling of most chemical engineering processes, where a proper and homogeneous fluid distribution is often essential. A numerical method was used to obtain theoretical predictions of residence time spatial distributions and was validated by comparing with experiments. It was simulated first by solving continuity and momentum conservation equations, coupled with turbulence quantity transport equations, using the finite volume method. After calculating the flow configuration, the simulation was done by solving the transport equation of a passive scalar, the local mean age of the fluid or local mean residence time, which is the average time of a fluid particle to reach any point of the domain from a supply inlet. The result obtained is a spatial distribution of the local mean age of the fluid, which may be displayed as isocontours in the space domain considered. It was compared with measurements of mean residence times of tracers in a reactor to concretely assess the relevance of this method.
Capillary flow in a noncircular tube
Authors: Turian Raffi M. , Kessler Frederick D. 
Published: April 2004

AbstractThe 1‐D axial, laminar, Newtonian flow through a tube of uniform, but noncircular, cross‐section is analyzed. The tube boundary is r = R[1 + εf(θ)], in which R is a reference radius, ε is a small parameter, and the function f(θ) of the polar angle is general, albeit subject to minimal constraints ensuring single‐valuedness and quadrant symmetry. All results are determined as asymptotic expansions complete to terms O(ε2), which include the velocity distribution, average velocity, volume flow rate, ratio of flow rates in the noncircular to that in a circular pipe of the same cross‐sectional area, the friction factor‐Reynolds number dependence, the permeability of packed beds comprising noncircular capillaries, kinetic‐energy and momentum‐flux correction factors, the capillary penetration rate under the influence of the capillary pressure and the equilibrium value of the capillary rise. Expressions are derived in terms of the general boundary function f(θ) and, for special cases, when f(θ) = sin2kθ (k = 1, 2, 3) and f(θ) = sin2kθ with k a positive integer. The results provide quantitative measures of the effect of tube shape on flow properties and on imbibition and drainage from noncircular capillaries.
Pressure drop in gas slugs in vertical tubes and flooding instability
Authors: de Carvalho J. R. F. Guedes , Ferreira M. J. F. 
Published: April 2004

AbstractFast‐response differential‐pressure transducers were used to continuously monitor the pressure difference between two nearby points along single gas slugs rising in long vertical tubes filled with liquid. Operating pressures in the 0.6 to 6.0‐MPa range gave slugs of argon with densities in the 10 to 100‐kg/m3 range, rising through mixtures of glycerol and water with viscosities in the 10−3‐ to 305 × 10−3‐Pa·s. range. The transducer records suggest that, for all the liquids tested, there was flooding instability of the film flowing around the slugs when the gas density was raised sufficiently. With the assumption that flooding instability is due to the force on the waves moving along the gas–liquid interface, dimensional analysis shows that the observations with gas slugs correlate well with flooding data obtained in wetted wall tubes operating at atmospheric pressure. The work reveals strong limitations of current empirical correlations for the prediction of flooding in wetted wall tubes.
Corrected Horváth‐Kawazoe equations for pore‐size distribution
Authors: Rege Salil U. , Yang Ralph T. 
Published: April 2004

AbstractThe Horváth‐Kawazoe (HK) model is a widely used method for determining pore‐size distribution in a microporous material from a single adsorption isotherm. The original model, however, suffers from conceptual flaws. The total interaction energy of the adsorbate is underestimated since the adsorbate‐adsorbate interaction is incorrecty calculated. New corrected HK models proposed here for three pore geometries (slit, cylindrical, and spherical) can overcome these defects. Two other improvements have also been made in the new models. These assume that a filled micropore is composed of layers of adsorbate molecules which interact only with the molecular layers in the immediate vicinity. A better estimate of adsorbate‐adsorbate‐adsorbent interaction is obtained by utilizing actual distances between interacting molecules. The average interaction energy is calculated by a population‐weighted average of the energy potential of the layers rather than by integration. This average potential approaches a nonzero value at large pore size, unlike that in the original model. Pore‐size distributions predicted by the corrected HK models agreed significantly better with crystallographic data compared to the original model for both microporous and mesoporous sorbents.
Microwave radiation's influence on sorption and competitive sorption in zeolites
Authors: Turner Michael D. , Laurence Robert L. , Conner W. Curtis , Yngvesson K. Sigfrid 
Published: April 2004

AbstractThe influence of microwave radiation on adsorption selectivity was studied using the adsorption and desorption of cyclohexane and methanol on high‐silica zeolites. The amount of microwave energy absorbed depends on the specific system of adsorbent and adsorbate. The adsorbent, a high‐silica zeolite, is effectively “transparent” to microwave radiation, while the two adsorbates reflect high (methanol) and low (cyclohexane) absorption of microwave energy. The measured system temperatures required for desorption by microwave energy were lower than those required for conventional heating. Further, microwave radiation can change the sorption selectivity; the adsorbate with the greater microwave absorptivity is desorbed selectively. It is concluded that the surface and adsorbed species can be heated selectively since the rate of microwave energy absorption can be greater than the rate of heat transfer from the surface.
Extraction in thin liquid films generated by impinging streams
Authors: Berman Yuli , Tamir Abraham 
Published: April 2004

AbstractA new device for liquid–liquid extraction was developed, tested, and modeled, based on two immiscible thin films, flowing one on the top of the other while exchanging mass. The films are formed by impinging streams: collision of two immiscible jets flowing one toward the other on the same axis. Thus, the flow of the films is in a direction perpendicular to the initial flow of the jets. Two liquid systems (diluent–solute–solvent) were tested: water–iodine–kerosene and aqueous solution of HCl–pure water. In the latter case, HCl is transferred from its initial solution comprising one film to the other film of pure water flowing on the top of it. The experiment determined geometrical parameters of the thin film, mass‐transfer coefficients of the solute between the films, as well as power input. For a relatively identical power input, the measured mass‐transfer coefficients were higher by a factor of 10–200 compared to conventional devices. In addition, the mean residence time in the new device is shorter than in conventional devices due to minimal dispersion of the phases during the extraction stage, making the settling stage relatively simple and short.