bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of geometry and angular velocity on performance of a rotating disk filter
Authors: Bouzerar Roger , Jaffrin Michel Y. , Ding Luhui , Paullier Patrick
Published: April 2004

The influence of internal geometry and angular velocity on the performance of a rotating‐disk filtration device is described. This module consists of a disk rotating at speeds up to 2,000 rpm inside a cylindrical housing equipped with a fixed flat membrane. The test fluid was a calcium carbonate suspension (4.7‐μm mean particle diameter), and the membranes were in nylon (0.2‐μm mean pore diameter) and PVDF (0.1 and 0.18 μm). The shear stress on the stationary membrane in the laminar boundary‐layer regime was estimated from a similarity solution, and in the turbulent regime from the friction coefficient for a flat plate. Several inlet and outlet configurations were tested. The permeate flux was independent of the axial gap (disk to membrane), but increased when the radial gap (from disk to housing) was raised from 2 to 5 mm. The inviscid fluid core in the axial gap was observed to rotate at 42% of angular velocity with a smooth disk. This factor rose to 64% when the disk was equipped with small vanes. Fouling was limited to the central part of the membrane at 1,500 rpm and disappeared completely for a disk equipped with vanes. Permeate fluxes were consistently much higher than in classic cross‐flow filtration.
Performance assessment of cascade control loops
Authors: Ko Byung‐Su , Edgar Thomas F.
Published: April 2004

The minimum variance control law for a cascade control system is derived for two cases: the primary process has stable and unstable inverses. The expression for the primary output variable under the minimum variance cascade control is shown to be feedback‐invariant. An estimate of the minimum achievable variance in a cascade control loop can be obtained via multivariate time‐series modeling of the primary and secondary measurements collected under normal operation. The utility of the proposed performance assessment method is demonstrated using simulated and industrial data.
Systematic estimation of state noise statistics for extended Kalman filters
Authors: Valappil Jaleel , Georgakis Christos
Published: April 2004

The successful application of model‐based control depends on the information about the states of the dynamic system. State‐estimation methods, like extended Kalman filters (EKF), are useful for obtaining reliable estimates of the states from a limited number of measurements. They also can handle the model uncertainties and the effect of unmeasured disturbances. The main issue in applying EKF remains that one needs to specify the confidence in the model in terms of process noise covariance matrix. The information about the model uncertainties can effectively and systematically calculate the process noise covariance matrix for an EKF. Two systematic approaches are used for this calculation. The first is based on a Taylor series expansion of the nonlinear equations around the nominal parameter values, while the second accounts for the nonlinear dependence of the system on the fitted parameters by Monte Carlo simulations that can easily be performed on‐line. The value of the process noise covariance matrix obtained is not limited to a diagonal form and depends on the current state of the dynamic system. Thus the a‐priori information regarding the uncertainty in the model is utilized and the need for extensive tuning of the EKF is eliminated. The application of these techniques to example processes is also discussed. The accuracy of this methodology is compared very favorably with the traditional methods of trial‐and‐error tuning of EKF.
Internal model control design for transition control
Authors: Chiu Min‐Sen , Cui Shan , Wang Qing‐Guo
Published: April 2004

The problem of controlling processes with a wide range of operating conditions is addressed. The process dynamics is represented by the weighted‐sum of the linear model by using model validity functions. By treating the validity functions as uncertainties in the system, an internal model control (IMC) design strategy is developed for a particular class of nonlinear systems. Robust stability analysis under mixed time‐varying and time‐invariant uncertainties is presented. Simulation results confirm that the proposed IMC design method is indeed superior to its conventional counterpart.
Microbial competition: Study of global branching phenomena
Authors: Ajbar Abdelhamid , Alhumazi Khalid
Published: April 2004

The stability characteristics of a bioreactor with cell recycle involving the competition between microbial cultures are investigated. The unstructured model, based on Andrew's inhibitory kinetics, involves the pure and simple competition between two microorganisms for a single pollutant. The singularity theory used for this study allows an in‐depth analysis of both the static and dynamic bifurcation mechanisms occurring in the system. The hysteresis with five solutions is the highest singularity the system can exhibit. With inhibitory kinetic expressions, the model can also predict self‐sustained oscillations for a wide range of parameters. The analysis of clean feed conditions shows that the model cannot exhibit periodic behavior regardless of the growth kinetics model. Analytical criteria are also derived for the coexistence of the competing cultures and for the prevention of wash‐out conditions. The stability characteristics for Monod kinetics, derived as a limiting case of the inhibitory kinetic expressions, are incorporated in the general framework offered by the singularity theory.
Kinetic mechanism reduction based on an integer programming approach
Authors: Androulakis Ioannis P.
Published: April 2004

An approach toward kinetic mechanism reduction, both in terms of reactions and species, is discussed. The driving force of the approach is to derive reduced kinetic models while maintaining the structural integrity of the detailed mechanisms. The mechanism reduction problem is defined as an integer optimization problem with binary variables denoting the existence/nonexistence of reactions or species. A Branch & Bound framework is implemented for the solution of the resulting mathematical programming problem. Several examples, utilizing a variety of kinetic networks, are presented and the results are analyzed.
Engineering design method for cavitational reactors: I. Sonochemical reactors
Authors: Gogate Parag R. , Pandit Aniruddha B.
Published: April 2004

High pressures and temperatures generated during the cavitation process are now considered responsible for the observed physical and chemical transformations using ultrasound irradiation. Effects of various operating parameters reported here include the frequency, the intensity of ultrasound, and the initial nuclei sizes on the bubble dynamics, and hence the magnitude of pressure generated. Rigorous solutions of the Rayleigh‐Plesset equation require considerable numerical skills and the results obtained depend on various assumptions. The Rayleigh‐Plesset equation was solved numerically, and the results have been empirically correlated using easily measurable global parameters in a sonochemical reactor. Liquid‐phase compressibility effects were also considered. These considerations resulted in a criterion for critical ultrasound intensity, which if not considered properly can lead to overdesign or underdesign. A sound heuristic correlation, developed for the prediction of the pressure pulse generated as a function of initial nuclei sizes, frequency, and intensity of ultrasound, is valid not only over the entire range of operating parameters commonly used but also in the design procedure of sonochemical reactors with great confidence.
Screening multiphase reactors for nonisothermal multiple reactions
Authors: Kelkar Vaibhav V. , Ng Ka M.
Published: April 2004

A screening procedure developed synthesizes a multiphase reactor and the accompanying heat‐transfer devices desired for a given reaction scheme. It is an extension of a procedure, previously described for isothermal systems with a single reaction to multiple reactions involving heat effects. The three key elements to this approach include a generic reactor model, an accompanying sensitivity model, and a knowledge base consisting of heuristics, as well as correlations on hydrodynamics, heat, and mass transport. The generic reactor model is extended to include various topologies and reactor constituent parts for heat and mass transfer. The sensitivity model quantifies the potential impact of various reactor parameters on reactor performance. The knowledge base is expanded to tackle multiple, nonisothermal reactions. In this procedure, reactor attributes and constituent parts are aggregated to form a reactor for the specific reaction under consideration. It is especially useful at the early stage of process development, when there is a great deal of uncertainty regarding reaction kinetics, and potential limitations of heat and mass transfer.
Evolution of primary and aggregate particle‐size distributions by coagulation and sintering
Authors: Tsantilis Stavros , Pratsinis Sotiris E.
Published: April 2004

A sectional model for aggregate aerosol dynamics accounting for gas‐phase chemical reaction, coagulation, and sintering at nonisothermal conditions is presented. The aggregate structure was considered by the implementation of a constant mass fractal dimension on the aggregate collision diameter and an average, but variable, aggregate area within a given volume section. The model showed separately the evolution of the size distribution of the primary and aggregate particles. Model predictions with respect to average primary particle diameters and aggregate particle‐size distributions were evaluated by comparing with experimental data for synthesis of titania by TiCl4 oxidation in a furnace aerosol flow reactor. The effects from sintering time and mass fractal dimension are also discussed.
Thermodynamic micellization model for asphaltene precipitation inhibition
Authors: Pan Huanquan , Firoozabadi Abbas
Published: April 2004

Aromatic solvents and oil‐soluble amphiphiles are recognized as asphaltene precipitation inhibitors in oil production and transportation. In the absence of the model describing the effect of these inhibitors on asphaltene precipitation from crudes, proposed is a thermodynamic micellization model explaining the inhibition mechanism for both aromatic solvents and oil‐soluble amphiphiles. The model shows that aromatic solvents are concentrated in the micellar shell, and the interfacial tension between the asphaltene micellar core and shell is reduced as the micelles becomes stabler. A crude, mixed with a small amount of an oil‐soluble amphiphile, achieves a high micellar stability. The amphiphiles behave like resin species of the crude and coadsorb onto the micellar core with resins. The adsorption enthalpy of an amphiphile onto the micellar core is much higher than that of the resin and, therefore, amphiphiles can be very effective inhibitors. The results suggest that the adsorption enthalpy data can be used to screen the amphiphiles for asphaltene precipitation inhibition. For a given oil‐soluble amphiphile, this model can predict the amount of the amphiphile required to inhibit the precipitation.
Numerical investigation of gas‐driven flow in 2‐D bubble columns
Authors: Pan Y. , Dudukovic M. P. , Chang M.
Published: April 2004

Gas‐liquid bubbly flow in 2‐D bubble columns was studied by numerical simulation. A Eulerian‐Eulerian two‐fluid model used describes the time‐dependent motion of liquid driven by small, spherical gas bubbles injected at the bottom of the columns. Such equations, numerically implemented in this work, were derived by Zhang and Prosperetti. A distinctive feature of this method is the derivation of the disperse‐phase momentum equation by averaging the particle (here, the bubble) equation of motion directly, not the macroscopic equation for the particle phase. Both the time‐averaged quantities and dynamic characteristics of the macroscopic coherent structures agree with the experimental data of Lin et al. and Mudde et al. The comparison of simulated results with data demonstrates that this physical model and numerical approach can provide the key features of the time‐dependent behavior of dispersed bubbly flows qualitatively with reasonable quantitative accuracy. Effects of the number of injectors, magnitude of bubble‐induced viscosity, and various parameters in the interphase momentum exchange were also studied by simulating various cases and comparing with measurements. The applicability of different boundary conditions and the sensitivity to the mesh system used are also examined.
Molecular tagging velocimetry characterization of rapid KDP crystal growth
Authors: Maynes D.
Published: April 2004

Measurements of the tangential and axial velocities near the prism faces of a rotating mock crystal mounted on a rotating platform are presented. These measurements were made using molecular tagging velocimetry and are significant to the rapid growth of KDP crystals because they provide a description of the flow field and the evolution of the relative tangential and axial velocities near the prism faces and in the bulk flow region. These data represent the first measurements of fluid velocities around rotating crystal geometries, and thus provide a benchmark for future computer simulations of the crystal growth process at higher Re than previously obtainable. The measurements provide a temporal and spatial description of the evolving velocity field in the vicinity of a rotating crystal. Rotational conditions and locations on the prism faces where low shear rates are likely to occur, based on the measurements presented, are discussed qualitatively.
Hydrodynamics of slug flow applied to cross‐flow filtration in narrow tubes
Authors: Mercier‐Bonin Muriel , Maranges Claude , Lafforgue Christine , Fonade Christian , Line Alain
Published: April 2004

This article focuses on the characterization of slug‐flow hydrodynamics in two sizes of tubular membrane diameters (6 and 15 mm) to quantify the main mechanical phenomena involved in the limitation of particle fouling during cross‐flow filtration of suspensions. By using a conductance probe technique, the flow structure was accurately identified for two geometries, and noticeable differences were observed in terms of void fractions, velocities and lengths of Taylor bubbles, and liquid slugs. This characterization allowed some data to be theoretically estimated (wall shear stress, “falling” film velocity, film thickness) thanks to the application of a phenomenological model initially developed for oil pipes. The results obtained in a 15‐mm tube showed that the ultrafiltration flux improvement, experimentally achieved with bentonite and yeast suspensions, was partly due to the increase in the wall shear stress, induced by continuous gas sparging inside the tubular filtration module. Other hydrodynamic phenomena linked to the quasi‐periodic succession of Taylor bubbles and liquid slugs were also involved in the control of the particle entrainment: intermittency frequency, reversal of the wall shear stress, instantaneous pressure variations in the long bubble wake with a higher level of turbulence, and an enhanced local mixing.
Performance analysis of axial‐flow mixing impellers
Authors: Wu Jie , Pullum Lionel
Published: April 2004

Theoretical formulations for impeller performance were evaluated based on a blade‐element theory. These enable the calculation of the head and power vs. flow‐rate curves of axial‐flow impellers. The technique uses the lift and drag coefficients of the blade section of an impeller to calculate the spanwise swirl‐velocity distribution. Using the angular‐momentum equation, it is possible to calculate the corresponding spanwise distribution of the energy head of the impeller. Integration of these distributions of head and torque gives the impeller's performance. Parameters including the flow number, the power number, the thrust force number, and the swirl velocity can be found at the impeller operating point, determined using the head curve and an experimentally calibrated resistance curve. A laser Doppler velocimetry (LDV) system was used to measure the velocity distribution for different axial flow impellers in mixing tanks. Calculated flow and power numbers agreed well with the experimental results. Using the blade's spanwise head distribution and a set of calibrated flow‐resistance data, it is also possible to estimate an impeller's outlet axial‐velocity distribution. Predictions compared well with LDV experimental data. The effect of impeller‐blade angle, number of blades, blade camber, and blade thickness on the performance of axial‐flow impellers was investigated using the Agitator software.
Granular flow in a rotating cone partly submerged in a fluidized bed
Authors: Janse Arthur M. C. , Biesheuvel P. Maarten , Prins Wolter , van Swaaij Wim P. M.
Published: April 2004

When a rotating cone with supply openings near the bottom is partly inserted into a fluidized bed, solid particles are taken up and conveyed spirally over the inner surface. This principle for particle transport was used in a novel reactor for the flash pyrolysis of biomass with several distinct advantages. Particle velocities (up to 5 m/s) were measured, as well as the solids flow rate (up to 1.6 kg/s), as a function of the cone rotational velocity, cone size and cone top angle, the fluidization velocity, and the depth of the cone in the fluidized bed. Three hydrodynamic regimes were recognized, each with its own, specific flow characteristics influencing the particle residence time (distribution) on the cone wall and the solids flow rate. The transition between the regimes could be predicted using a force balance for a single particle. The particle velocity and residence time were modeled successfully for one of the regimes.
Pressure swing adsorption cycles for improved solvent vapor enrichment
Authors: Liu Yujun , Ritter James A. , Kaul Bal K.
Published: April 2004

A pressure swing adsorption (PSA)–solvent vapor recovery (SVR) process simulator was used to investigate new PSA cycle configurations designed for higher solvent vapor enrichment. These cycles were modifications of the four‐step Skarstrom cycle used commercially for PSA‐SVR and include the addition of a cocurrent blowdown step, and combinations of cocurrent blowdown and continuous/batch reflux steps. The recovery of gasoline vapor from tank filling operations was simulated with n‐butane, n‐heptane, and nitrogen as representatives of the light and heavy components in gasoline vapor, and carrier gas, respectively. Adding a cocurrent blowdown step increased the solvent vapor enrichment, depending mainly on the step ending pressure, not the step time. Both the continuous and batch reflux steps also increased the solvent vapor enrichment, but at the expense of an increased bed capacity factor. For similar increases in the solvent vapor enrichment, batch reflux led to significantly smaller bed capacity factors compared to continuous reflux and was thus superior for PSA‐SVR. Overall PSA‐SVR process performance improved markedly by adding cocurrent blowdown and batch reflux steps compared to the conventional four‐step cycle.
Design of a carousel process for cesium removal using crystalline silicotitanate
Authors: Hritzko Benjamin J. , Walker D. Douglas , Wang N.‐H. Linda
Published: April 2004

A three‐column carousel process based on a pelletized form of CST powder was designed to remove radioactive 137Cs+ from SRS nuclear wastes. A multicomponent ion‐exchange equilibrium model was used to generate cesium loading data, which were fit to the Langmuir equation to obtain effective single‐component cesium isotherm parameters for representative wastes. Mass‐transfer parameters were estimated by analyzing breakthrough curves for two simulated wastes. Simulations based on a pore‐diffusion rate model were performed to determine the lengths of the mass‐transfer zone for different feed compositions and linear velocities. The length of a single segment in the carousel process was the constant‐pattern mass‐transfer zone to ensure consistent high column utilization during startup and cyclic steady state. Analysis of the dimensionless groups in the mass balance equations revealed that the mass‐transfer zone length is proportional to the particle Peclet number. The proportionality constant is a function of the waste composition and the Cs+ concentration in the waste. A higher distribution coefficient and isotherm nonlinearity result in a smaller proportionality constant. With this analysis designs can be easily adjusted for variations in isotherms, feed concentration, particle size, linear velocity, and intraparticle diffusivity.
Double‐slit model for partially wetted trickle flow hydrodynamics
Authors: Iliuta I. , Larachi F. , Al‐Dahhan M. H.
Published: April 2004

A double‐slit model developed can predict the frictional two‐phase pressure drop, external liquid holdup, pellet‐scale external wetting efficiency, and gas–liquid interfacial area in cocurrent downflow trickle‐bed reactors operated under partially wetted conditions in the trickle flow regime. The model, an extension of the Holub et al. (1992, 1993) mechanistic pore‐scale phenomenological approach, was designed to mimic the actual bed void by two inclined and interconnected slits: wet and dry slit. The external wetting efficiency is linked to both the pressure drop and external liquid holdup. The model also predicts gas–liquid interfacial areas in partially wetted conditions. An extensive trickle‐flow regime database including over 1,200 measurements of two‐phase pressure drop, liquid holdup, gas–liquid interfacial area and wetting efficiency, published in 1974–1998 on the partial‐wetted conditions, was used to validate the modeling approach. Two new improved slip‐factor functions were also developed using dimensional analysis and artificial neural networks. High‐pressure and ‐temperature wetting efficiency, liquid holdup, pressure drop, and gas–liquid interfacial area data from the literature on the trickle‐flow regime using conventional monosized beds and catalyst bed‐dilution conditions were successfully forecasted by the model.
Modeling constant voltage electrophoretic deposition from a stirred suspension
Authors: González‐Cuenca Mercedes , Biesheuvel P. Maarten , Verweij Henk
Published: April 2004

In electrophoretic deposition, a voltage difference is applied across a suspension, and a layer (“cast” or coating) is formed on one of the electrodes. Based on first principles, cast growth is described in this work for a well‐stirred suspension, a constant voltage difference across the deposition cell, and a nonionic solution. The influence of cast formation on cast growth is implemented, which is a refinement of the model described in 1999 by Biesheuvel and Verweij. Simulation results are compared with experimental data. From the derivation it follows that a smooth cast layer is formed when the particle permittivity is lower than that of the liquid. Catastrophic cast growth occurs when the particle permittivity is higher than that of the liquid.
Thermal effects in dynamic storage of hydrogen by adsorption
Authors: Lamari Malek , Aoufi Asdin , Malbrunot Pierre
Published: April 2004

Thermal effects in dynamic hydrogen storage by adsorption at room temperature and high pressure are studied theoretically and experimentally. The system of adsorbate–adsorbent used was hydrogen in granular activated carbon. The theoretical analysis was based on heat‐ and mass‐transfer modeling in a packed‐bed adsorber, with particular emphasis on the thermal effects occurring during charge and discharge steps. The influence of gas flow rate and storage pressure (up to 15 MPa) on the total amount stored or delivered was investigated. Operating conditions were compatible with practical application for onboard vehicle storage. The experimental study was carried out in cylindrical 2‐L reservoirs filled with granular activated carbon in which the bed temperature was measured at various positions. The temperature changes during both charge and discharge agreed well with the model predictions.
Mapping the cavitation intensity in an ultrasonic bath using the acoustic emission
Authors: Moholkar Vijayanand S. , Sable Shishir P. , Pandit Aniruddha B.
Published: April 2004

A new method for separate identification and determination of the spatial distribution of the two components of the energy intensity in an ultrasound bath (due to the ultrasound waves and cavitation activity) uses two media—cavitating (water) and noncavitating (silicon oil)—under the conditions of the acoustic field in the ultrasound bath. The variation of cavitation intensity in the frequency domain was obtained by subtracting the acoustic emission spectrum of silicon oil from that of water. Measurements at various locations in the bath revealed significant spatial variations in the cavitation intensity in the bath. The local cavitation phenomena in the bath (stable or transient cavitation) were explained based on the spectral characteristics of acoustic emission. The radial dynamics of the bubbles at the location of cavitation intensity measurements was determined using the Gilmore model of bubble dynamics. The bubbles in the region of highest cavitation intensity underwent a transient motion, while the bubbles in the region of lowest cavitation intensity underwent stable/oscillatory motion. The transient collapse of the bubbles that gives rise to local temperature and pressure maxima is at the root of the observed effects of ultrasound on chemical systems. The more violent the collapse of the bubbles, the higher the local cavitation intensity. It was verified using the spectral characteristics of the acoustic emission and simulation of the radial motion of the bubbles.
Pressure swing permeation: Novel process for gas separation by membranes
Authors: Feng Xianshe , Pan Chuen Y. , Ivory John
Published: April 2004

A novel process for gas separation, called pressure swing permeation, was investigated to elevate the relatively low permeate pressure by pressurization with high‐pressure feed gas, thereby reducing or eliminating additional permeate compression costs where a pressurized permeate is required. This process uses two or more membrane modules and operates in a cyclic fashion, with each module repeatedly undergoing the sequential steps of feed admission and permeation, residual removal, permeate reception, permeate pressurization, and product withdrawal. The unsteady‐state permeation associated with pressure swing permeation was studied parametrically, and a bench‐scale unit compromising two hollow‐fiber membrane modules in parallel was tested for H2/N2 separation to demonstrate the effectiveness of the process. The permeate product at a pressure as high as the feed pressure can be produced without using a compressor. This is impossible with traditional steady‐state processes where a pressure differential across the membrane must be maintained. The pressure swing permeation is analogous to pressure swing adsorption and has the potential to be synergistically integrated with the pressure swing adsorption process for enhanced separation of gases.
Multistability and hysteresis in an industrial ammonia reactor
Authors: Mancusi E. , Merola G. , Crescitelli S. , Maffettone P. L.
Published: April 2004

A model for an industrial ammonia reactor was studied with nonlinear‐analysis tools. This model was recently considered to explain an industrial incident involving the onset of sustained oscillations due to a sudden decrease in the reactor pressure. The analysis was based on the bifurcation diagrams. Both multistability and hysteretic behaviors described here can be useful for an effective plant design, as well as for an adequate plant control and operation.
Fluidization of solids with CO2 at pressures from ambient to supercritical
Authors: Marzocchella Antonio , Salatino Piero
Published: April 2004

Beds of two granular materials belonging to groups A‐B and B of the Geldart classification of powders were fluidized by carbon dioxide at 35°C and at 1 to 80 bar. Within this pressure interval, spanning from subcritical to supercritical conditions, the fluid density changes from the values typical of dilute gases to those of liquids. The hydrodynamic behavior of the bed changes accordingly. The bed behavior was investigated by visual inspection of the bed and by analysis of the time series of the pressure drop across the bed and of the heat‐transfer coefficient between the bed and a hot wire probe. The boundaries between different captive fluidization regimes—incipient fluidization, onset of bubbling, and the incipient and fully established turbulent regime—were mapped in gas superficial velocity vs. fluid density phase planes for the two granular materials investigated.
Transport in nanoporous carbon membranes: Experiments and analysis
Authors: Acharya Madhav , Foley Henry C.
Published: April 2004

Single‐component permeances of six gases were measured on three different supported nanoporous carbon membranes prepared by spray coating and pyrolysis of poly(furfuryl alcohol) on porous stainless‐steel disks. Global activation energies were regressed from data collected as a function of temperature. Permeances and global activation energies were correlated to molecular size, assuming that entropic affects dominated the transport. The permeance was best correlated to the minimum projected area of the molecule computed from first principles. The free‐energy barriers to transport within the membranes were derived from the temperature dependence of the permeance data, after accounting for porosity differences between the membranes and differences in molecular adsorption. Using transition‐state theory and an entropic model derived, the free energy, enthalpy, and entropic barriers to transport within the membrane were examined as a function of molecular size. Computed on the basis of size, the entropic component of this barrier did not account for the large differences in the transition‐state free energies. However, when these entropic barrier values were used to compute the enthalpic portion of the barrier free energies, the minimum projected area of each molecule correlated strongly. Furthermore, these enthalpic components of the barriers were fitted nicely by the Everett‐Powl mean field potential, using only the pore size as the adjustable parameter. These results shed light on the underlying mechanism by which shape‐selective transport takes place in the NPC membranes and small molecules are separated.
Flow measurements on a turbulent fiber suspension by laser Doppler anemometry
Authors: Andersson Sven R. , Rasmuson Anders
Published: April 2004

The flow and transition of fiber suspensions to turbulence in a rotary shear tester were studied. Both torque and LDA measurements were performed on a refractive‐index‐matched glass‐fiber suspension in the 3–20 wt. % concentration range. The flow of such suspensions underwent two transitions: the first one coincided with complete agitation, a sharp increase in torque, and fluctuating velocities close to zero. The mean velocities were similar to those of single‐phase flow above this transition. The fluctuation velocities approached those of single‐phase flow with increasing rotational speed, until they were nearly equal. This second transition was interpreted as fully developed turbulent flow. CFD, assuming single‐phase flow, is a helpful tool for describing the flow behavior in this region. The network strength was useful for the study of flow transitions. LDA measurements of the boundary layer at the bottom wall showed that the velocity profile is linear close to the wall even with fibers present. The presence of the fibers also flattened the profiles, indicating an increased momentum transfer.
Design of complex wire‐mesh mist eliminators
Authors: Brunazzi Elisabetta , Paglianti Alessandro
Published: April 2004

Knitted wire‐mesh mist eliminators have widespread application in many industrial plants. Despite their extensive use, the open literature regarding them is really limited. Some experimental data and mechanistic models have been published for common knitted wire‐mesh mist eliminators formed from a single metal pad. This type of mist eliminator can be used in most distillation and absorption columns, but because of the poor removal efficiency, cannot be used in operations involving acid mist, fine fog resulting from liquid condensation from a saturated vapor, oil mist from compressed gases, and natural‐gas dehydration applications. Moreover, other possible problems may arise when the separator is fed with high liquid and gas flow rates, because these conditions can induce flooding in the mist eliminator. In both of these cases, common wire‐mesh mist eliminators do not perform satisfactorily, and therefore complex wire‐mesh mist eliminators have to be installed to improve separation efficiency or to increase allowable liquid loadings while avoiding flooding phenomena. This article presents a mechanistic model based on a set of new experimental data obtained by investigating performance of commercial complex eliminators.
Electrodiffusional wall shear rate analysis in scraped surface heat exchanger
Authors: Dumont Eric , Fayolle Francine , Legrand Jack
Published: April 2004

An electrochemical technique was used to measure local and instantaneous wall shear rates in a scraped surface heat exchanger (SSHE). The inner cylinder, equipped with two blades symmetrically opposed, was driven by a stepping motor and the outer cylinder was fixed. Highly viscous Newtonian fluids (Emkarox HV45 solutions) and non‐Newtonian model fluids (aqueous solutions of guar and CMC) were used. The reliability of the electrochemical technique was validated for these fluids in an annular space (SSHE without blades). The evolution of the limiting diffusion current was given by the probes in a scraping situation. In the general case of scraping, the flow was unsteady, and very fast large fluctuations of the limiting diffusion current were measured. The shear rate at the wall of the stator of the SSHE appeared fully controlled by the rotation of the blades. Then, electrodiffusion measurements were interpreted using three different wall shear rate calculating procedures. Wall shear rate in SSHE appeared ten to 100 times more important than in annulus, and the lowest values of the clearance between the edge of the blades and the stator was about 50 × 10−6 m.
Hindered diffusion of dextran and polyethylene glycol in porous membranes
Authors: Shao Jiahui , Baltus Ruth E.
Published: April 2004

The effective diffusion coefficients of dextran and polyethylene glycol in track‐etched polycarbonate membranes were measured using membranes with nominal pore sizes of 0.03 μm, 0.05 μm, and 0.1 μm. Experiments were performed using narrow‐size range fractions of each polymer. When the Stokes‐Einstein radius was used to describe solute size, the observed diffusivities for both polymers agreed closely and were larger than values predicted for rigid spheres, as well as for linear polymers when only steric interactions with the pore wall are assumed. These observations cannot be explained by considering electrostatic interactions between the solute and the pore wall, or solute adsorption on the pore wall. The experimentally measured diffusion coefficients agreed well with a model that treats the polymeric solutes as rigid spheres and includes van der Waals attractive interactions between the solute and the pore wall.
Kinetics of high‐pressure removal of hydrogen sulfide using calcium oxide powder
Authors: Chauk Shriniwas S. , Agnihotri Rajeev , Jadhav Raja A. , Misro Santhosh K. , Fan Liang‐Shih
Published: April 2004

Sulfidation reaction of CaO at high pressure (up to 2 MPa) and high temperature (up to 900°C) to remove H2S in a coal‐fired gasifier was studied in a high‐pressure and temperature differential‐bed flow‐through reactor. Experimental conditions selected are typical for pressurized gasifiers. Effects of total pressure, H2S partial pressure, reaction temperature, fuel gas composition, and CaO surface area on the extent of sulfur capture and sorbent conversions were determined. The gasifier pressure affected the in‐situ calcination of calcium carbonate particles through reduction in available surface area and pore volume of CaO formed, thus limiting the sulfidation conversion. Time‐resolved conversion data of CaO sulfidation were analyzed using a modified grain model. The model incorporates external and internal diffusion, surface reaction, product layer diffusion, and the structural changes of the sulfiding CaO particle. The activation energy for the reaction was 37 kcal/mol. The estimated product layer diffusivity was 8×10−15 m2/s at 800°C with an associated activation energy of 38.4 kcal/mol—typical of solid state diffusion of ionic species through the product layer. The extent of conversion increased with increasing initial surface area and porosity of CaO particles. The high‐pressure sulfidation reaction data for CaO will be useful in understanding and optimizing the in‐gasifier H2S capture using calcium‐based sorbents.
Analytical solution to an axial dispersion model for the fixed‐bed adsorber
Authors: Liao Hsin‐Tzu , Shiau Ching‐Yeh
Published: April 2004

An axial dispersion model for the operation of a fixed‐bed adsorber with a linear adsorption isotherm was formulated and solved analytically using the separation of variables method. The asymptotic solution for a large Peclet number and/or a small operation time was also obtained using two limiting cases. The results can be used to correlate or to predict both the mass‐transfer zones and the breakthrough curves of a fixed‐bed adsorber. Influence of both the Peclet number and the retardation coefficient on the operation of a fixed‐bed adsorber was also studied. As an example, the axial dispersion model was applied to the experimental data of the removal of phenol from the solution in the activated carbon and the Amberlite resin XAD‐4 fixed‐bed adsorbers.
Gradient formation in membrane unit for differential precipitation of proteins
Authors: Sookkumnerd Terasut , Hsu James T. , Ito Yoichiro
Published: April 2004

A new protein fractionation technique presented transfers ammonium sulfate to create a gradient of ammonium sulfate inside a membrane unit for differential precipitation of proteins called “centrifugal precipitation chromatography.” Because it does not require any solid support, it should provide a better alternative to the conventional chromatography with solid stationary phase. To understand the phenomena and achieve a better separation, a mathematical model explaining the ammonium sulfate gradient formation inside the stationary membrane unit is investigated. The model is extended with empirical correlation to the centrifugal membrane unit—a new approach for protein purification. The model agreed well with the experiments for both stationary and centrifugal units. Upon using the model to calculate the ammonium sulfate gradient formation in a membrane unit, this new technique can be useful in separating a mixture of proteins whose solubility in ammonium sulfate solution differs. To demonstrate the technique experimentally, mixtures of proteins are loaded into the column; the partial resolution between proteins is achieved with the step concentration switch of the inlet ammonium sulfate solution.
Closed operation of multivessel batch distillation: Experimental verification
Authors: Wittgens Bernd , Skogestad Sigurd
Published: April 2004

The multivessel batch distillation column, as well as conventional batch distillation, can be operated in a closed (total reflux) mode where the products are collected in vessels along the column. A previously proposed and simulated feedback control strategy for the closed operation is to indirectly adjust the vessel holdups by using the reflux flow out of the vessel to control the termperature at some location in the column section below. The feasibility of this scheme is demonstrated here experimentally on a laboratory‐scale multivessel column. The experimental column consists of a reboiler, two intermediate vessels, and an accumulator, where a mixture of methanol–ethanol–propanol–butanol is separated into almost pure components. The first published experimental work on the closed operation of batch distillation is presented, as well as its results on the operation of a multivessel column.
Monte‐Carlo simulation of branching distribution in Ni‐diimine catalyzed polyethylene
Authors: Simon Leonardo C. , Soares Joao B. P. , de Souza Roberto F.
Published: April 2004

Monte‐Carlo simulation was used to study the short‐chain branch formation of polyethylene made with a Ni‐diimine catalyst. Branches are formed through the chain‐walking mechanism without any comonomer addition. The model describes the type of branch, that is, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octly, nonyl, and decyl (or longer branches) and the distribution of these branches in the backbone, showing information that cannot be accessed by 13C NMR. The branching structure of these polymers seems to be determined by two major kinetic steps described by the following model parameters: a beta transfer parameter describing the ratio of monomer insertion to beta hydrogen transfer and an isomerization barrier parameter controlling the ratio among different types of branches. The model showed that methyl branches are favored on the other branches in the chain‐walking mechanism. The branch distribution obtained with the model developed herein agrees well with 13C NMR experimental results.
Calcium carbonate precipitation using new segmented flow tubular reactor
Authors: Vacassy R. , Lemaître J. , Hofmann H. , Gerlings J. H.
Published: April 2004

A new segmented flow tubular reactor for the continuous production of powders was used to synthesize calcium carbonate by precipitation. This new reactor uses a nonmiscible phase (air in most cases) to create individual microvolumes of the reacting mixture or suspension ensuring that a plug‐flow regime is maintained. In a microvolume, poor micromixing can be avoided and the homogeneity in reaction conditions is increased better than in larger volumes, leading to a better control of powder characteristics. Calcium carbonate with controlled characteristics could be precipitated continuously using this new segmented flow tubular reactor. Pure calcite could be obtained in the tubular reactor by segmenting a Ca2+ solution with a reactive CO2/NH3 gas mixture. This synthesis process is very promising for industrial production since the scale‐up should easily be achieved by multiplying the number of tubes working in parallel.
Inverse method to obtain reaction surface area during gas‐solid reactions
Authors: Krammer Gernot F.
Published: April 2004

During noncatalytic gas‐solid reactions, both the chemical composition, and the size and texture of the particle change simultaneously. A new inverse method allows to determine the influence of the reaction parameters (such as temperature) and changes in the specific reaction surface area with conversion. It can also establish the range of conversion, where constant reaction conditions are present at the reaction surface. This method is based on the mathematical treatment of experimental results which include the conversion vs. time together with the corresponding reaction parameters. This method consists of several well established mathematical procedures applicable to the specific situation at gas‐solid reactions: variable conversion and reaction time are separated and inverted, respectively. The ratio of two reaction rates (of different experiments) at the same conversion provides information about the (only time‐dependent) influence of the reaction parameter on the reaction rate. Finally, also the specific reaction surface area as a function of conversion is derived.
Kinetic model of the partial oxidation of methane to synthesis gas over Ru/TiO2 catalyst
Authors: Elmasides Costas , Ioannides Theophilos , Verykios Xenophon E.
Published: April 2004

The kinetic behavior of the Ru/TiO2 catalyst in the partial oxidation of methane to synthesis gas was investigated as a function of temperature and partial pressures of CH4 and O2. It was found that the catalyst promotes, to a large extent the direct formation of CO and H2 from the reaction of methane and oxygen, while the reforming and water–gas shift reactions are negligible under these conditions. A kinetic model, based on formation of CO and H2 as primary products and of CO2 and H2O as secondary oxidation products, describes satisfactorily the observed kinetic behavior. Values of model parameters satisfy thermodynamic constraints and agree well with data derived from surface science techniques and kinetic studies of elementary surface processes that are in the literature.
Dissolution kinetics of polymer powders
Authors: Parker Alan , Vigouroux Florence , Reed Wayne F.
Published: April 2004

The dissolution kinetics of polymer powders as isolated grains or in beds were studied theoretically and experimentally. A simple model developed for the dissolution of a dispersion of isolated grains with an arbitrary size distribution was confirmed experimentally using polysaccharides in water. Based on experimental data for the dissolution of lump‐forming powder beds, modeled using a bimodal population of grains and lumps, a robust technique was developed for analyzing experimental data, which is independent of the details of lump formation. A qualitative picture of lump formation is described, based on the idea that during the initial hydration of a powder bed a “pore blocking” condition quickly develops due to the formation of a gel layer around the dry grains. Once formed, lumps dissolved the same way and with the same kinetic description, as individual, dispersed grains.
Experimental study of wave occlusion on falling films in a vertical pipe
Authors: Dao Eric K. , Balakotaiah Vemuri
Published: April 2004

Experiments were conducted to study wave occlusion on falling films in a vertical pipe with aqueous solutions of glycerine with viscosities from 0.022 to 1.25 kg/(m·s). Pipe diameters were varied from 0.00635 to 0.015875 m. For each case, the critical liquid flow rate was determined at which the free falling wavy annular flow transforms into slug flow due to wave occlusion. The experimental results are correlated in terms of the liquid Reynolds number at occlusion (ReLS = 4q/πνD) and two dimensionless groups influencing the transition: the Kapitza number Ka = σ/[(νg)1/3 μ], and the Bond number Bo = ρgD2/σ. The annular‐slug transition correlation was ReLS = 0.062 Ka4/3 Bo2/3 and valid in the range 0.1<ReLS <200, 0.2<Ka<55 and 7<Bo<50. Experimental results were compared to related published results, and the experimental correlation is explained qualitatively.
Effect of solute concentration on hindered diffusion in porous membranes
Authors: Shao Jiahui , Baltus Ruth E.
Published: April 2004

The effect of solute concentration on hindered diffusion was investigated by measuring diffusion rates of dextran and polyethylene glycol (PEG) in track‐etched polycarbonate membranes at concentrations up to 60 mg/mL. Results were interpreted using a model that incorporates a first‐order virial coefficient to describe the concentration dependence of the equilibrium partition coefficient, as well as the intrapore diffusivity. For dextran, both virial coefficients were positive, indicating that the effective diffusivity increases with solute concentration. Measured values of the virial coefficient for equilibrium partitioning agreed with a model that includes attractive van der Waals interactions between dextran and the pore wall. For PEG, both virial coefficients were negative, indicating that the diffusivity decreases as solute concentration increases. These observations may be due to chain entanglement or aggregation that occurs with increasing PEG concentration.