bluuebunny/crossref_metadata_2025 huggingface dataset:
Laboratory batch reactor method for kinetic study of chemical vapor deposition
Authors: McConica Carol M. , Bell David A. , Baker Kevin L. , Moss Devin
Published: June 2004

A lab‐scale nonflowing reactor was built to study chemical vapor deposition reactions. Mass spectrometry is used to follow reaction pathways and to determine instantaneous reaction rates throughout film growth. In each experiment, the kinetic rate dependence on concentration for a wide range of concentrations is observed as reactants convert to products. This method of obtaining kinetic data is efficient in terms of sample loading, gas usage, and time, since over 200 instantaneous rate/composition pairs can be determined from one 30‐min deposition. Because the rate is determined from gas‐mass balance, rather than film‐thickness measurements, an unlimited number of rate studies can be made on one sample. As a test case, the SiH4 reduction of WF6, used to deposit tungsten during integrated‐circuit production, was investigated in the 0.64‐L nonflowing laboratory reactor. Gas compositions were measured 2 mm from the growing surface, throughout time, with a mass spectrometer equipped with a capillary sampling tube. Tungsten was deposited on the 95°C surface, and SiHF3 was the primary silicon fluoride reaction product for most tested conditions. A multiple‐regression analysis of 1,975 instantaneous composition/rate pairs gives orders of 1.22 in silane, 0.27 in hydrogen, and −2.17 in WF6. The ratio of SiF4 to SiHF3 stays low and constant until the gas becomes silane‐rich. The evolution of the instantaneous rate over time implies that a minimal level of thermal activation of the reactive gases is necessary for the deposition to be surface‐rate‐limited. Preliminary heat‐transfer models of the wire substrate imply that heat transfer to the gas phase is in the Knudsen regime.
Demulsification of water/oil/solid emulsions by hollow‐fiber membranes
Authors: Tirmizi Neena Pradhan , Raghuraman Bhavani , Wiencek John
Published: June 2004

The demulsification techniques investigated use preferential surface wetting to allow separation of oil and water phases in ultrafiltration and microfiltration membranes. A hydrophobic membrane allows the permeation of an oil phase at almost zero pressure and retains the water phase, even though the molecular weight of the water molecule (18) is much smaller than that of the oil molecule (198 for tetradecane, used in this study). Hydrophobic membranes having pore sizes from 0.02 to 0.2 μm were tested for demulsification of water‐in‐oil emulsions and water/oil/solid mixtures. The dispersed (aqueous)‐phase drop sizes ranged from 1 to 5 μm. High separation rates, as well as good permeate quality, were obtained with microfiltration membranes. Water content of permeating oil was 32–830 ppm depending on operating conditions and interfacial properties. For emulsions with high surfactant content, simultaneous operation of a hydrophobic and hydrophilic membrane, or simultaneous membrane separation with electric demulsification was more efficient in obtaining complete phase separation.
Variable target mass‐exchange network synthesis through linear programming
Authors: Gupta Ashish , Manousiouthakis Vasilios
Published: June 2004

A mass‐exchange network (MEN) synthesis problem is considered with streams whose target compositions are allowed to vary between upper and lower bounds. The design task is to determine the minimum mass separating agent (utility) cost needed for the transfer of a single component from the rich to the lean streams. The mathematical formulation of this synthesis problem leads to a mixed integer nonlinear program. In this work, we propose a novel formulation of the problem that leads to a linear program. Stream decomposition is employed in attaining this novel linear programming formulation and rigorous proofs are presented which establish that the two formulations have the same solution. The linear programming formulation reduces the complexity of the considered MEN synthesis problem, thus making feasible its solution even for large‐scale problems. Two examples, illustrating the procedure, are presented. Both demonstrate that significant utility cost savings can be achieved over the fixed composition MEN synthesis problem.
NMR Imaging of nonaqueous‐phase liquid dissolution in a porous medium
Authors: Baldwin Christopher A. , Gladden Lynn F.
Published: June 2004

It is demonstrated how NMR imaging can be used noninvasively to quantify the volume of nonaqueous phase liquid (NAPL) present in an otherwise water‐saturated porous medium. Data were recorded during the operation of a pump and treat (PT) remediation scheme on a bed of water‐saturated sand contained within a vertical column. The model contaminant used was n‐hexanol, and three different aqueous flow rates were employed. These data were then critically compared with the predictions of three models currently used to describe the dissolution of NAPL during a PT scheme: the linear mass‐transfer model, the pore‐diffusion model, and the shrinking‐core model. The pore‐diffusion model gave best agreement with the experimental data. However, none of the models predicted the observed dependence of mass transfer on Darcy velocity.
Mixed monolayers of acylglycerols on sugar aqueous solutions
Authors: de la Fuente Feria Julia , Patino Juan M. Rodríaguez
Published: June 2004

Structural Characteristics of mono‐ and distearin mixed monolayers spread on aqueous solutions containing sugar (glucose or sucrose) at 0.5 mol/L were studied as a function of temperature and interfacial and subphase compositions. The compression isotherms were obtained using an automated Langmuir‐type film balance over a temperature range of 10 to 40°C. The results show an expansion in the structure of the mixed monolayer with sugar in subphase. Nonideal mixing of the component at interface was observed with positive deviation from the additivity rule. This fact is indicative of the presence of repulsive interactions between the two lipids on sugar aqueous solutions. The mixed monolayers with a monostearin molar fraction less than 0.8 are stable from a thermodynamic point of view. Mixed monolayer stability depends on monolayer composition, temperature, and surface pressure with different order of magnitude.
Spectral analysis of microtubule assembly dynamics
Authors: Odde David J. , Buettner Helen M. , Cassimeris Lynne
Published: June 2004

Microtubules are linear polymers of the cytoskeleton that serve to organize the cytoplasm of eukaryotic cells. Understanding how microtubule polymers self‐assemble is important in biotechnology, including the development of novel cancer therapies and proper guidance of regenerating neurons. The assembly of microtubules occurs by a unique process whereby an individual microtubule undergoes abrupt and apparently stochastic switching between alternating steady states of growth and shrinkage, a phenomenon known as microtubule dynamic instability. To characterize these oscillations spectral (frequency‐domain) analysis, commonly used in engineering for system identification, was applied. Power spectra of the individual microtubule‐length life histories revealed oscillations within growth phases, directly reflecting acceleration and deceleration in the growth process. These fluctuations were not accounted for by the standard two‐state model commonly used in the analysis of microtubule assembly, despite the inclusion of simulated measurement error in the model. Thus, the spectral analysis of microtubule assembly permitted characterization of assembly process dynamics independent of particular assembly models, and as such represents a powerful analytical framework within which to study microtubule dynamic instability and assess its function in vivo.
Model for ER chaperone dynamics and secretory protein interactions
Authors: Robinson Anne Skaja , Lauffenburger Douglas A.
Published: June 2004

Expression of proteins in eucaryotic systems is often the only way to ensure the correct folding and processing necessary for protein function. Heterologous proteins, however, are commonly retained in the secretory pathway, so that secreted product yield is low despite a high level of transcription. A major limiting step in protein secretion is protein folding in the lumen of the endoplasmic reticulum. This process is assisted by accessory macromolecules resident in this compartment, including chaperones such as the hsp70 homologue binding protein (BiP). Although induction of foreign proteins in yeast initially elicits a transient increase in local chaperone concentration, long‐term protein expression lowers both chaperone and secreted product. A mechanistic model that can account for the experimentally observed role of BiP in secretion and the effects of BiP overexpression on the secretory pathway is described here. The model predicts that equimolar synthesis of chaperone and foreign protein should optimize protein secretion.
Meniscus control by string in roll coating experiment
Authors: Decré M. , Gailly E. , Buchlin J.‐M.
Published: June 2004

The ribbing instability hinders the roll coating process, as it produces wavy coating films. The technique studied here prevents ribbing, whereby a string is placed parallel to the rolls, in contact with the unstable meniscus. In this experiment, the fluid is a Newtonian oil. A free‐surface visualization technique allows measurement of the interface profile where the string interacts with the forming films. The ribbing can be eliminated under conditions at least 20 times its natural onset. The influence of the string position on the mass transfer is then examined. Measurements show that the streamwise position of the string controls the total flow rate of the process, while its transverse position accurately controls the relative thicknesses applied on each roll. The string proves to be both an efficient concept to eliminate ribbing, and a reliable way of fixing the coating film thickness, without resorting to traditional gap width and roller speeds selection.
Effectiveness factors for continuous mixtures
Authors: McCoy Benjamin J.
Published: June 2004

Catalyst effectiveness factors provide a quantitative estimate of diffusion resistance for chemical reactions in porous particles. The objective of this research is to develop an approach to calculate liquid‐phase, porous‐particle effectiveness factors, which are based on spherical reactant molecules and cylindrical pores that both have size distributions. Reactants larger than a given pore are excluded from the pore. A first‐order reaction occurs when reactant molecules diffuse into pores that hinder their movement due to steric interactions with the pore wall. The reactant size distribution is described by rectangular or gamma distributions, and the pore‐size distribution is modeled by continuous rectangular or discrete fractal distributions. Effectiveness factors are defined and evaluated for (a) one‐size reactants in one‐size pores, (b) all reactants in one‐size pores, (c) one‐size reactants in all pores, and (d) all reactants in all pores. Most of the discussion deals with equal‐length pores, but the effect of a distribution of pore lengths is also considered.
Vapor–liquid equilibrium of formaldehyde mixtures: New data and model revision
Authors: Albert M. , Hahnenstein I. , Hasse H. , Maurer G.
Published: June 2004

A physicochemical model for vapor–liquid equilibria and enthalpies in formaldehyde‐containing mixtures is revised and extended using recently published NMR spectroscopic data on chemical equilibria in aqueous and methanolic formaldehyde solutions as well as new vapor–liquid equilibrium data for the system formaldehyde–water at temperatures up to 413 K (pressures up to 450 kPa), which were taken in this work with a new apparatus. Furthermore, the model is revised so that it can be extended straight‐forwardly to explicitly account for reaction kinetic effects based on results from other recent NMR spectroscopic studies. The model is tested over the entire temperature range (293–413 K), where reliable experimental data on vapor–liquid equilibria are available, and against calorimetric data. Typical relative deviations between experimental data and the correlation are about 5% for vapor–liquid partition coefficients and 2% for the pressure. Deviations between calculated and measured enthalpies of vaporization are generally below 2%.
Finite‐element analysis of Taylor flow
Authors: Edvinsson Rolf K. , Irandoust Said
Published: June 2004

A finite‐element analysis of Taylor flow in a cylindrical capillary was performed using a commercial FEM program (FIDAP) to solve the fundamental fluid dynamics equations together with the capillary forces at the gas–liquid interface. A moving‐surface formulation was used to calculate the bubble shape. The thickness of the liquid film surrounding the gas bubble, the degree of mixing in the liquid phase, and the slip velocity between the two phases were calculated. These parameters influence the performance of monolith reactors operating in the Taylor flow regime. On comparison with experimental results it was found that the FEM calculation generally predicts a thinner liquid film, which can possibly be explained in terms of a peripheral variation in surface tension. Moreover, the wavelength of the wiggles predicted in the liquid film near the tail end of the bubble was compared to those arising from a simplified mathematical analysis available in the literature. Good agreement was found for Ca < 0.005, while for higher Ca the FEM predicts significantly shorter wavelengths, indicating that the lubrication theory is not valid here.
Rheological properties of mixed polysaccharides and polysaccharide‐thickened emulsions
Authors: Pal Rajinder
Published: June 2004

Steady flow and viscoelastic properties of mixed‐polysaccharides and mixed‐polysaccharide‐thickened oil‐in‐water emulsions are investigated with a cone‐and‐plate system using a controlled‐stress rheometer. The mixed‐polysaccharide system studied is xanthan gum and guar gum. The rheological properties of the mixed‐polysaccharide systems exhibit strong synergistic effects. For a total polysaccharide concentration of 0.75 wt. % investigated, the synergistic effect is maximum at a xanthan:guar ratio of 15:85. The oil‐in‐water emulsions, prepared from a mixed‐polysaccharide system as the suspending medium, are strongly shear thinning and highly viscoelastic in nature. The rheological parameters such as apparent viscosity, storage, and loss moduli increase with an increase in the oil concentration of the emulsion.
Extraction of spray particles with supercritical fluids in a two‐phase flow
Authors: Wagner Henning , Eggers Rudolf
Published: June 2004

A new process for extractive separation with supercritical fluids is described. It is characterized by mixing a liquid feed with a dense gas in a special mixing device and the formation of spray particles when this mixture is injected into an extraction zone where the fluid phase is loaded with the extracts. By dividing the extraction into two process steps, mixing and loading, it is possible to adjust the devices and apparatuses for different media. For example, a static mixer or a fluid‐assist atomizer can be used as mixing devices. Since the viscosity behavior of the mixture and the surface tension in dense gases are important for the formation of spray particles, carbon dioxide was measured at pressures up to 70 MPa and the geometry of the extraction zone was optimized by residence times and fluid dynamics.

Two different materials were tested in an apparatus on a semiindustrial scale with different mixing devices and extraction zones. By measuring fluid loading in the extraction zone, the mass‐transfer parameter of a mathematical extraction model could be adapted. The model considers particle formation in the mixing zone, the fluid dynamic in the loading zone, and the mass transfer between spray particles and fluid phase. Calculated concentration profiles make it possible to determine the residence time and the size of the extraction zone for given geometries and fluid dynamics.
New decline pattern of filtrate flux in cross‐flow microfiltration of dilute suspension
Authors: Murase Toshiro , Ohn Than
Published: June 2004

The cross‐flow microfiltration flux is assumed to fall smoothly with a decreasing slope from startup to the steady state. Under actual operating conditions beyond a critical level, however, an anomalous decline pattern arises; initially cross‐flow medium filtration occurs and then thin‐cake filtration. At the earlier stage, fine particles in polydispersed suspensoids are filtered out on the surface of the membrane and/or captured within the latter, clogging the membrane pores gradually. After the membrane surface is covered by the deposit, further filtration occurs by the buildup of filter cake on the initial deposit. Among the many operating variables, three major controlling factors for the earlier stage are confirmed: solids concentration, relative size of solids and pores, and relative force of sweeping and capturing of suspensoids. Because suspended solids are separated by medium‐ and cake‐law filtration in a series, a method for evaluating the time dependence of flux with the new pattern is derived. Reliable flux analysis is achieved for the transient flux of the dilute suspension forming a low compressibility fouling layer, which helps understand efficient flux performance.
Hollow‐fiber gas‐membrane process for removal of NH3 from solution of NH3 and CO2
Authors: Qin Yingjie , Cabral Joaquim M. S. , Wang Shichang
Published: June 2004

A hollow‐fiber supported gas membrane process for the separation of NH3 from aqueous solutions containing both NH3 and CO2 was investigated theoretically and experimentally. A lumen laminar flow and radial diffusion model was applied to calculate the membrane wall transfer coefficient from the data stripping a single volatile component, NH3 or CO2, from their individual aqueous solutions. Influence of the type of membranes and operating conditions on mass‐transfer rate were discussed, especially the influence of the membrane transfer coefficient on the film mass‐transfer coefficient in the lumen. Appropriate configurations of the hollow‐fiber modules for stripping of a single component were analyzed to optimize mass transfer. To predict the stripping of NH3 from a solution containing NH3 and CO2, a mathematical model incorporating local chemical equilibria and Nernst–Planck diffusion was developed to describe the mass transport. The models described the experimental data fairly well. The experimental results showed that the supported gas membrane process can be used to remove NH3 effectively from aqueous media containing NH3 and CO2.
Surface fractality of wood charcoals through small‐angle X‐ray scattering
Authors: Venkatraman A. , Boateng A. A. , Fan L. T. , Walawender W. P.
Published: June 2004

Geometrical properties of the pore‐solid interfaces of devolatilized charcoals from the pyrolysis of fuel woods in a bench‐scale fluidized‐bed reactor were investigated through small‐angle X‐ray scattering. Specifically, surface morphological features of the interfaces were characterized by a single parameter indicative of the degree of surface roughness, that is, the surface fractal dimension, dSF, at various retention times. The surface fractal dimensions of the original wood samples prior to devolatilization were approximately identical for all species within the range between 2.00 and 2.15, over a length scale from 55 to 600 Å, and those for the respective charcoals from these wood samples were influenced to different degrees by the retention time, τ, in the reactor. The dSF's of the mesopores of charcoals of all the species attained maxima at a value of τ equal to 35 s and decreased subsequently with an increase in τ. The analysis of this unexpected trend was facilitated by the values of the mean radii of gyration of the pores, estimated through the Guinier plots. The results imply that when the retention time of the charcoals in the reactor is prolonged, the mechanism of pore enlargement and smoothening of the interfaces can play a dominant role in altering the surface morphology and thereby influence significantly the kinetics of chemical reactions in heterogeneous porous systems.
Perturbation analysis of Thomas and Windle's model of Case II transport
Authors: Durning C. J. , Edwards D. A. , Cohen D. S.
Published: June 2004

Thomas and Windle's model of Case II transport is analyzed for a semiinfinite medium by a singular perturbation technique. Two adjacent boundary layers separate equilibrated and dry regions. A thin boundary layer of width ∼ O(M−1/2/ln M), where M (≫1) dictates how rapidly the mixture's viscosity decays with liquid concentration, sits next to the equilibrated outer left region. Here, quasi‐steady diffusion balances relaxation. A thicker intermediate layer of width ∼ O(M−1/2) separates the lefthand boundary layer and the dry outer region on the right, where both relaxation and unsteady diffusion participate in the transport. Matching the solutions at leading order specifies the moving front's speed, v: v ∼ M1/2. The analysis indicates that relaxation significantly affects the nearly dry region just ahead of the moving front. This disagrees with the widely accepted view that ordinary diffusion dominates in the nearly dry righthand region. Approximating that ordinary diffusion dominates in this region leads to a stepexponential concentration profile at the front and a simple analytical solution for the front speed, v with the correct M scaling. This approximate result accurately predicts the values of v determined by direct numerical solutions.
Gas‐separation membrane cascades utilizing limited numbers of compressors
Authors: Agrawal Rakesh , Xu Jianguo
Published: June 2004

A systematic procedure is presented to drawn n‐compressor membrane cascades for gas‐separation applications with n ≥ 1. In this method, a parent cascade structure is created that contains the n‐compressor cascades as substructures. These substructures are then revealed by systematically eliminating unwanted membrane stages and recycle compressors from the parent cascade. The procedure is illustrated by generating arrays of one‐ and two‐compressor cascade schemes. It is found that these arrays contain the cascade schemes known from the literature and also lead to some new schemes. This procedure increases the likelihood of finding an economically viable membrane cascade for a given separation application.
Dynamic rectification of data via recurrent neural nets and the extended Kalman filter
Authors: Karjala Thomas W. , Himmelblau David M.
Published: June 2004

The presence of autocorrelated measurement errors and/or measurement bias in process measurements poses serious problems in the rectification of data taken from dynamic processes. The proposed procedure to resolve these problems involves the use of recurrent neural networks (RNN) and the extended Kalman filter (EKF). By interpreting RNNs within a nonlinear state‐space context, a state‐augmented EKF can be used to optimally estimate both the states of the RNNs and noise and bias models. RNN models can be identified off‐line and utilized for data rectification within the extended Kalman filter in process environments in which badly autocorrelated measurement errors exist in the data. The same technique is also used to estimate measurement bias present in both process input and output variables. This approach has the advantage that models developed from “first principles” are not required and that rectification can be performed solely on the basis of the contaminated dynamic data.
Reaction kinetics of photoactive defects in semiconductor dissolution
Authors: Robertson Eric A. , Fogler H. Scott
Published: June 2004

Uniformity and predictability are the principal qualities sought for all wet chemical etches. The establishment of these qualities, however, is hindered by a number of factors, such as nonuniformities in the starting material and random fluctuations in the local temperature and reactant concentration, which can lead to variations in etch rate across the surface of a wafer. The effects of variations in the local etch rate on the morphological development of an etching surface and on the overall etch rate of the semiconductor are discussed. The system studied was an Al0.4Ga0.6As/GaAs heterostructure photodissolved in nitric acid using 730‐nm laser light. Defects in the AlGaAs layer, which etched faster than the surrounding material, were responsible for variations in the etch rate. The defects also exhibited a degree of photosensitivity that has not been previously observed. It was also found that not all of the defects spanned the AlGaAs epilayer. A model for the overall etch rate was based on a system of noninteracting cylindrical defects with a distribution in depths.
Crack propagation in catalytic pellets due to thermal stresses
Authors: Boshoff‐Mostert L. , Viljoen H. J.
Published: June 2004

Catalytic combustion reactions and reacting systems that operate in the parameter sensitivity region of the kinetic regime are likely to experience sudden temperature changes, resulting in the development of thermal stress in the catalyst support. The unified theory of Hasselman is used to study the onset and extent of crack propagation in catalyst support media. This theory is based on the principle that cracks propagate (terminate) when the total energy of the system increases (decreases) with additional crack growth. It is shown that the total energy of the medium (elastic and fracture energy) varies with time and position in the medium, and the extent of crack growth also varies from one point to the other. Within the limitation of the assumption that the crack density is low enough, the model provides valuable insight into the durability and mechanical strength of ceramic support media.
Application of artificial neural networks in modeling limestone–SO2 reaction
Authors: Bandyopadhyay Jayanta K. , Annamalai Senthil , Gauri K. Lal
Published: June 2004

Four varieties of limestone distinguished on the basis of pore‐size distributions were exposed in dynamic 10‐, 20‐ and 50‐ppm SO2 atmospheres for up to 500 h at 25°C and 100% rh. The resulting conversion of the limestone was measured as a function of the reaction product formed and the change in porosity. These conversions could be predicted correctly using either the shrinking unreacted core model or the distributed pore model. An artificial neural network (ANN) was also trained for the purpose. All three approaches predicted conversions that fitted well with the observed data; however, those predicted by ANN were the most accurate. Further, the weights determined for ANN on the basis of three limestone varieties also accurately predicted the conversions of the fourth variety for which no information was supplied in the training phase, showing that ANN can also be used successfully to estimate gas–solid noncatalytic reactions.
Impact of tank geometry on the maximum turbulence energy dissipation rate for impellers
Authors: Zhou Genwen , Kresta Suzanne M.
Published: June 2004

The maximum turbulence energy dissipation rate per unit mass, εmax, is an important variable in dispersion systems, particularly for drop breakup and coalescence, and for gas dispersion. The effect of tank geometry (number of baffles, impeller diameter, and off‐bottom clearance) on εmax for four impellers (the Rushton turbine, RT; the pitched blade turbine, PBT; the fluidfoil turbine, A310; and the high‐efficiency turbine, HE3) is examined. Mean and fluctuating velocity profiles close to the impellers were measured in a cylindrical baffled tank using laser doppler velocimetry. Local and maximum turbulence energy dissipation rates in the impeller region were estimated using ε = Av3/L with A = 1 and L = D/10 for all four impellers. Factorial designs were used to test for the effects of single geometric variables under widely varying conditions and interactions between variables. Several factorial designs were used to ensure that real effects were separated from effects that appeared as an artifact of the experimental design. Results show that the tank geometry has a significant effect on εmax, primarily with respect to variations in impeller diameter and interactions between the off‐bottom clearance and impeller diameter. For the same power input and tank geometry, the RT consistently produces the largest εmax and/or εmax scaled with N3D2.
Investigation of ion separation by microporous nanofiltration membranes
Authors: Rios G. M. , Joulie R. , Sarrade S. J. , Carlès M.
Published: June 2004

Two new nanofiltration membranes, an organomineral (titania‐Nafion) medium and an inorganic (γAl2O3) medium, were studied using single or mixed solutions of monovalent or divalent ions. The experimental results not only confirm previous findings concerning the treatment of electrolytic solutions using charged nanofiltration membranes, but provide additional data notably for microporous membranes. A simple model based on the Nernst–Planck equation is proposed; its simplicity is related to the nature of the two membranes tested, for which the diffusion terms may easily be disregarded. It highlights the importance of the potential difference that forms on the membrane and explains most of the observed rejection results. In addition, ionic plugging of micropores may play a part with regards to rejection and flow depending on working conditions.
Simultaneous optimal bottom linewidth and side‐wall curvature control of wet etching
Authors: Zhou Bo , Ramirez W. Fred
Published: June 2004

Etching is one of the critical steps in the fabrication of microelectronics devices. A time‐optimal control problem with a dual‐term terminal cost including both bottom linewidth and side‐wall curvature is formulated to implement an optimal open‐width design principle. A singular‐arc solution does not exist and the control action is bangbang. The optimal switching time is analytically calculated. The feasibility and effectiveness of this technique are illustrated by simulation studies. This design and control philosophy can easily be applied to other microelectronics manufacturing processes.
Dispersion effects on membrane reactor performance
Authors: Koukou M. K. , Papayannakos N. , Markatos N. C.
Published: June 2004

A mathematical model has been developed that predicts the effects of design parameters, operating variables and physical properties on the performance of a membrane reactor with a permeselective wall. The model consists of the full set of partial differential equations that describe the conservation of mass, momentum and chemical species, coupled with chemical kinetics and appropriate boundary conditions for the physical problem. The solution of this system is obtained by a finite‐volume technique. The model was applied to study the dehydrogenation of cyclohexane. Two membrane types in tubular form were studied: a selective porous glass with low gas permeabilities and a porous alumina with very high gas permeabilities. It is concluded that gas separation and reactor performance are strongly influenced by dispersion effects only in the latter membrane reactor, while in both cases radial concentration profiles do not correspond to those obtained with plug flow. Therefore, simulations of this type of problem should be based on complex dispersion models rather than the existing ideal plug‐flow ones.
General method for the transformation of chord‐length data to a local bubble‐size distribution
Authors: Liu Weidong , Clark Nigel N. , Karamavruc Ali Ihsan
Published: June 2004

Bubble‐size distributions influence the behavior of multiphase systems, but are not readily measured directly using probes. The chord lengths may be transformed into bubble sizes by modeling the bubble shapes as ellipsoids. Previous research on the transformation of chord‐length data into bubble‐size distributions using a numerical backward transformation revealed an instability problem. This problem was overcome by transforming the chord‐length data to a local bubble‐size distribution directly by using a Parzen window function and summing to yield the whole distribution. The best estimate of the local bubble‐size density distribution depends on the Parzen window width that was chosen by proposing a measure of performance. An empirical relationship was also offered to determine the best Parzen window width. Chord lengths were generated using the example of a Rayleigh bubble‐size distribution and Monte‐Carlo simulation. The window approach transformed the chord lengths back into the bubble‐size distribution with good agreement.
Shear‐induced alignment of nonspherical Brownian particles near walls
Authors: Nitsche Johannes M. , Roy Partha
Published: June 2004

A theoretical investigation is presented of the convection‐diffusion of model nonspherical solutes in shear flow over a plane wall. The analysis proceeds by formulating the underlying configuration‐space Brownian transport equation for the distribution over accessible positions and orientations. Geometrical constraints are imposed via boundary conditions preventing wall penetration, and some of the calculations incorporate hydrodynamic interactions with the wall. The analysis is brought to fruition by regular perturbation expansion in the rotary Péclet number, and solution of the resultant boundary‐value problems by a Galerkin technique. Three specific mechanistic conclusions result from the analysis. First, steric constraints imposed by the wall impedes the shear‐induced solute alignment (producing a more nearly uniform distribution over orientations relative to the unbounded‐fluid case) near the wall. Second, although the first effect of flow is to counteract the equilibrium depletion of solute centers near the wall, flow reinforces this depletion at higher order in the shear rate. Third, solute‐wall hydrodynamic interactions act to strengthen the shear‐induced solute alignment near the wall. This last phenomenon occurs because hydrodynamic wall effects significantly decrease the rotary diffusivity, but have little effect on the angular velocity, thereby locally increasing the effective rotary Péclet number (the effective flow strength). Correspondingly, solute‐wall hydrodynamic interactions reinforce the flow effects on the near‐wall depletion just noted. Steric and hydrodynamic wall effects typically are of order 15–20% near the wall.
Analysis of two‐stage heterogeneous separation processes
Authors: Sridhar L. N.
Published: June 2004

In this article, rigorous mathematical analysis is used to prove that the necessary condition for multiple steady‐state solutions to exist in two‐stage separation process problems involving ternary mixtures is the tendency of the mixture to exhibit a second liquid phase. The unrealistic solution multiplicity that exists in these problems, when phase splitting is not included in the model equations used to solve them, vanishes when phase splitting is included. The standard specification of reflux ratio and bottoms flow rate has been investigated.
Fundamental study of the dissolution of calcium phosphonates from porous media
Authors: Browning F. Henry , Fogler H. Scott
Published: June 2004

Phosphonate‐scale inhibitors are commonly used to prevent scale formation in many industrial processes involving high‐salinity brine solutions. To effectively prevent scale formation in these industrial processes, one must have a fundamental understanding of how phosphonates are released into high‐salinity brines. Because phosphonates can precipitate with divalent cations such as calcium, their release into aqueous media is often governed by many dissolution mechanisms. This study focuses on the release of calcium‐phosphonate precipitates from porous media (as related to oil‐field applications) and a mathematical model describing these release mechanisms based on mechanistic studies of pore‐level phenomena. The phosphonate used was 1‐hydroxyethylidene‐1,1‐diphosphonic acid (HEDP). The release of two distinct calcium‐HEDP precipitates from porous media was studied: soluble, fibrous 1:1; insoluble, spherical 2:1. Visual studies using etched‐glass micromodels showed that five distinct regimes govern the release of 1:1 calcium‐HEDP precipitate from porous media. Conversely, the release of 2:1 calcium‐HEDP precipitate was dominated by two distinct regimes. A continuum model developed describes the release of both precipitates from porous media by mathematically describing each of the distinct release regimes and defining conditions under which the transition between release regimes occurred. Experimental data agreed excellently with model simulations for both precipitates.
Modeling mass transfer with enzymatic reaction in electrochemical multilayer microreactors
Authors: Bacha Saliha , Montagné Marielle , Bergel Alain
Published: June 2004

Electrochemical biosensors were used to develop a numerical model of transient mass transfer coupled with enzymatic reactions in thin multilayer microstructures. A finite volume method integrates the partial derivative mass‐balance equations making the model very versatile. With a multilayer physical description, general nonlinear enzymatic kinetics and efficient space discretization, the model could be applied to various devices under a broad range of physicochemical conditions. Theoretical results were validated according to experimental data obtained with three different biosensors involving homogeneous or heterogeneous enzymatic catalyses. Model predictions are further discussed for a bienzymatic immobilized enzyme system. The biosensor transient behavior depended strongly on the location of the enzymes in the device and on the mass‐transport features. A transient maximum on the current‐time curves was predicted accurately by the model. This could be used to improve the biosensor performance. The model can also be a useful framework for designing new electrochemical microreactors.
Controlling chaos by periodic perturbations in nonisothermal fluidized‐bed reactor
Authors: Ajbar Abdelhamid , Elnashaie S. S. E. H.
Published: June 2004

A two‐phase model of a nonisothermal fluidized‐bed catalytic reactor with consecutive exothermic reactions makes it possible to control the chaotic behavior by small parametric perturbations of an input variable with suitable frequency and amplitude. By periodic forcing of the feed temperature the chaotic behavior of the autonomous system can be controlled and become periodic even for small amplitudes of forcing. For constant frequency with change in forcing amplitudes the behavior of the system alternates between periodic and chaotic regimes via period doubling and period adding bifurcation mechanisms. When the amplitude is fixed, regular patterns also appear with small changes in the forcing frequency. Appropriate selection of the forcing frequency can improve the yield of the desired intermediate component in the consecutive reactions network A → B → C.
Group‐contribution lattice‐fluid EOS: Prediction of LLE in polymer solutions
Authors: Lee Byung‐Chul , Danner Ronald P.
Published: June 2004

The modified group‐contribution lattice‐fluid equation of state (GCLF–EOS) was applied to predict liquid–liquid equilibria (LLE) in polymer solutions. The modified GCLF–EOS is a group‐contribution form of the equation of state by Panayiotou and Vera based on the lattice‐hole theory. Group contributions for the interaction energy and reference volume were developed based only on the saturated vapor pressures and liquid densities of low‐molecular‐weight compounds. For a mixture, group contributions for the binary interaction parameter were developed from the binary vapor–liquid equilibria of low‐molecular‐weight compounds. This modified GCLF–EOS model can be applied to predict LLE behavior in polymer solutions with the same group‐contribution parameters as used for predicting the vapor–liquid equilibria. The only input required for the model is the structure of the molecules in terms of their functional groups.
Radiation heat transfer down an elongated spheroidal cavity
Authors: Fitzgerald Sean P. , Strieder William
Published: June 2004

Effects of elongation on the radiation heat transport down a spheroidal cavity, located in a conducting solid with a diffusely reflecting cavity–solid interface, are examined. An effective conductivity λe and a void radiation conductivity λr are obtained as a function of cavity eccentricity α; and surface emissivity ε. To facilitate the calculations and produce readily applicable equations, a rigorous variational principle is used. Exact solutions are generated in the neighborhood of the spherical cavity (α2 → 0) for any ε > 0, a long needle‐shaped void (α2 → 1) for any ε > 0, and a perfect reflector (ε → 0) for arbitrary elongation (0 ≤α2 ≤ 1). Significant differences arising from the shape change are observed. The α2 → 0 edge demonstrates a linear increase in λr with ε. At the opposite edge α2 → 1 and positive ε, λr is a horizontal line independent of ε, much like the long cylinder, whose conductivity is a factor of 32/(9π) (= 1.13) larger. In the neighborhood of ε 0, λr is always zero for any 0 ≤ α2 ≤ 1. The emissivity slope for ε → 0 starts from unity at α2 = 0 and increases monotonically with elongation to a singularity 3π[16(1 – α2)]‐1 as α2 → 1 for the long needle.
Robust design of countercurrent adsorption separation processes: 4. Desorbent in the feed
Authors: Mazzotti Marco , Storti Giuseppe , Morbidelli Massimo
Published: June 2004

In many instances of practical interest, countercurrent adsorption separations operate on feed streams containing not only the components to be separated but also some desorbent. Criteria for the optimal and robust design and operation of these units are developed by extending previous treatments developed for desorbent‐free feedstreams. The effect of the presence of some desorbent (weak, intermediate, or strong) on the location and robustness of the region of complete separation in the operating parameter space is discussed.
Steady‐state multiplicity in cocurrently cooled autothermal reactors
Authors: Pedernera Marisa , Borio Daniel O. , Porras José A.
Published: June 2004

The steady‐state simulation of a cocurrently cooled autothermal fixed‐bed reactor was carried out using a 2‐D heterogeneous mathematical model. The ammonia synthesis was chosen as a case study. Unlike the not‐autothermal cocurrent reactor, which is unconditionally stable, the autothermal cocurrent reactor shows multiple steady states within a broad range of operating conditions. This finding, not reported in the literature, is explained through the mass transport from the bottom to the top of the reactor, associated with a feedback of energy. The feedback of heat, which is inherent to autothermal reactors, leads to an ignition‐extinction phenomenon similar to that found in the countercurrent configuration. The influence of different parameters on the stability of the autothermal cocurrent reactor was analyzed. The regions where steady‐state multiplicity occurs were compared with those presented by the autothermal countercurrent reactor. The influence of an additional heat exchanger on the reactor stability was considered.
Turbulent reactive mixing of reversible reactions
Authors: Toor H. L.
Published: June 2004

The convective diffusion equations were used to obtain mean concentrations and concentration fluctuation covariances during turbulent reactive mixing of fast reversible reactions. The mean concentrations are directly applicable to reactions that are much faster than the mixing, while the covariances can serve as a first approximation closure for slower reactions. The covariances should also be useful for testing more general closure models. Numerical examples are presented for the simple mass‐action rate law with stoichiometric coefficients of unity.
Control of continuous copolymerization reactors
Authors: Padilla Salvador , Alvarez Jesús
Published: June 2004

Continuous solution copolymerization is an important industrial process in the manufacture of commodity and engineering plastics. The addition of comonomers and solvent, and the rate of heat exchange must be simultaneously manipulated to maintain safety, operability, and the product quality adequately, yielding a process with nonlinear behavior, strong and asymmetric input–output multivariable coupling, and potential for open‐loop instability and state multiplicity, as shown in earlier dynamics and control studies. Accordingly, the key control objectives of the copolymerization reactor are: the compensation of interaction, the preclusion of input multiplicity and the robustness (i.e., tolerance to modeling and tuning errors) of the controller. In principle, these control issues should be considered within a nonlinear setting. Otherwise, the reactor may have to be operated with a conversion that is conservatively below what can be handled by standard mixing and heat‐exchange equipment. To assess the inherent control possibilities and limitations of a given copolymerization reactor, a methodology to address the control problem is proposed such that the nonlinearity, interaction, input multiplicity, and robustness issues are explicitly confronted. The result is a linear multivariable interaction compensator whose tuning can be done with notions and tools from conventional control. This method is tested with the copolymerization of vinyl acetate with methyl methacrylate, dissolved in ethyl acetate.
Cocurrent membrane reactors vs. PFRs for shifting dehydrogenation equilibrium
Authors: Reo Christopher M. , Bernstein Lewis A. , Lund Carl R. F.
Published: June 2004

Membrane reactor performance in reversible reactions like dehydrogenation is often compared to plug‐flow reactor performance. Analysis of cocurrent membrane‐reactor design equations shows that as the Damköhler–Peclet product varies from 0 to ∞, the membrane reactor approaches a fully diluted or undiluted plug‐flow reactor. If its yield is maximized by varying this parameter, the optimum cocurrent membrane reactor is often one of these limiting cases, and a membrane reactor offers no advantage over a plug‐flow reactor. When the optimum membrane reactor does not correspond to one of these limiting cases, there still may be a plug‐flow reactor system that offers equal or higher yields. Analysis of plug‐flow‐reactor performance indicates that there is an optimum degree of dilution; while the optimized membrane reactor yield may be greater than either an undiluted or a fully diluted plug‐flow reactor; it may not be greater than an optimally diluted plug‐flow reactor. When using porous membranes the membrane reactor yield is at most 7% greater than the yield using a plug‐flow reactor. The porous membranes enhance the yields in a regime where the reactor would not be likely to operate. Much larger yield advantages are possible, but only when the membrane is highly permselective, as with dense membranes. Highly permselective membranes also enhance the yields in a regime where the membrane reactor process is likely to be operated.