bluuebunny/crossref_metadata_2025 huggingface dataset:
Robust design of countercurrent adsorption separation processes: 2. Multicomponent systems
Authors: Mazzotti Marco , Storti Giuseppe , Morbidelli Massimo
Published: June 2004

The adsorption separation of a multicomponent mixture in a four‐section countercurrent separation unit is analyzed. A procedure for the optimal and robust design of the unit is developed in the frame of equilibrium theory, where adsorption equilibria are described through the constant selectivity stoichiometric model, while mass‐transfer resistances and axial dispersion are neglected.

A set of conditions obtained defines a region in the operating parameters space where the unit achieves complete separation. In the case of binary separations, with a desorbent having any adsorptivity with respect to the components to be separated, the boundaries of this region are obtained explicitly. In the general multicomponent case, a numerical procedure is needed. An approximate shortcut method devised allows to obtain explicit and reliable relationships for estimating the boundaries of the exact region.

The results provide a very convenient tool not only to analyze the role of the desorbent in determining the separation performance, but to find both optimal and robust operating conditions. Comparison between model predictions and experimental data assesses the reliability and accuracy of the theoretical findings.
Retention of proteins in cross‐flow UF through asymmetric inorganic membranes
Authors: Pradános P. , Arribas J. I. , Hernández A.
Published: June 2004

The flow and retention of 0.1% w/w aqueous solutions of pepsin, bovine serum albumin (BSA), lipase, γ‐globulin, and invertase with molecular weights of 36,000, 67,000, 80,000, 150,000, and 270,000 dalton (g/mol) are studied when they are tangentially filtered through an inorganic microporous membrane with a nominal pore size of 0.02 × 10−6 m made by Anopore, with transmembrane pressure differences up to 100 kPa. The data were analyzed within the frame of the film layer theory for the concentration polarization phenomenon. This allows the mass‐transfer coefficient to be obtained for the cell used as a function of the feed circulation speed and the molecular weight of the solute. Apparent and true retention curves obtained lead to a size exclusion radius smaller than the nominal one but very similar to that obtained from scanning electronic photographs.
Core‐annular flow of liquid membrane emulsion
Authors: Ho W. S. , Li N. N.
Published: June 2004

A modeling and experimental study of the core‐annular flow (CAF) was carried out with a viscous liquid membrane emulsion as the core and an aqueous solution as the annulus. A hydrodynamic model has been developed for stable CAF with a centered, nondeformable core surrounded by a turbulent aqueous annulus with or without a drag reducer. This model has been verified with data from flow tests with viscous emulsions in a horizontal flow loop of 3/4‐in. (19‐mm) tubing and a fullscale, down‐hole well tubing of 2‐7/8 in. (73 mm). By the use of this model along with the data, the effects of annular size on total and core flow rates and pressure drop are investigated for annuli with and without a drag reducer.
Parallel transport of BSA by surface and pore diffusion in strongly basic chitosan
Authors: Yoshida Hiroyuki , Yoshikawa Motonobu , Kataoka Takeshi
Published: June 2004

The parallel transport of a protein by surface and pore diffusion within a highly porous ion exchanger is studied by measuring equilibria and uptake curves for adsorption of bovine serum albumin (BSA) on two different strongly basic chitosan ion exchangers (hard gels): Ch‐2503 and Ch‐2507 at pH 6.9 and 298 K. Experimental equilibrium isotherms are correlated by the Langmuir equation. Intraparticle effective diffusivities of BSA (Deff) are determined from the homogeneous Fickian diffusion model, increasing with the bulk phase concentration increase of BSA (C0). It suggests the existence of parallel diffusion. The surface diffusivity DS for the parallel diffusion model is determined from the Deff, and DS = 0.47 × 10−13 m2 · s−1 (Ch‐2507) and 2.4 × 10−13 m2 · s−1 (Ch‐2503) were obtained. Pore diffusivities based on the pore diffusion control (D′P) are obtained by matching the shrinking core model with the experimental uptake curves. D′P decreases with increasing C0. Since D′P in Ch‐2507 is constant when C0 ≥ 1 kg·m−3, the constant value 2.7 × 10−11 m2 · s−1 is taken as the accurate pore diffusivity (DP). As D′P in Ch‐2503 does not approach a constant value, DP = 1.0 × 10−11 m2 · s−1 is determined by matching the parallel diffusion model with the uptake data, and the model calculated using experimental values of DS and DP agrees reasonably well with the uptake data for Ch‐2507 and Ch‐2503. This theoretical approach may be applied not only for intraparticle diffusion of proteins but any adsorbates in porous materials.
Model‐based approach to automated hazard identification of chemical plants
Authors: Catino Catherine A. , Ungar Lyle H.
Published: June 2004

A prototype hazard identification system, qualitative hazard identifier (QHI), works by exhaustively positing possible faults, automatically building qualitative process models, simulating them, and checking for hazards. QHI matches a library of general faults such as leaks, broken filters, blocked pipes, and controller failures against the physical description of the plant to determine all specific instances of faults that can occur in the plant. Faults may perturb variables in the original design model or may require building a new model. Fault models are automatically generated using the qualitative process compiler and simulated using QSIM. Hazards including overpressure, overtemperature, controller saturation, and explosion are identified in the reactor section of a nitric acid plant using QHI.
Hindered settling of rod‐like particles measured with magnetic resonance imaging
Authors: Turney Michael A. , Cheung Man Ken , Powell Robert L. , McCarthy Michael J.
Published: June 2004

Magnetic resonance imaging (MRI) is used to measure the time evolution of the volume fraction vs. height profile during batch sedimentation of rod‐like particle suspensions. At any instant during sedimentation, MRI clearly delineates the supernatant, suspension, and sediment regions. The rod‐like particles with a mean aspect ratio of 17.4 exhibit considerably larger hindered settling effects than spherical particles with increasing particle concentrations. This can be attributed in part to the larger increase in interparticle forces and relative viscosities with suspensions of rod‐like particles than with suspensions of spherical particles, as the particle concentration increases. The relative viscosity of the rod‐like particle suspensions is a function of the spin‐spin relaxation time, T2. Since T2 is a measurable MRI quantity, this suggests that MRI may be used as an instrument for rheological measurement.
Monte Carlo network simulation of horizontal, upflow and downflow depth filtration
Authors: Burganos V. N. , Paraskeva C. A. , Payatakes A. C.
Published: June 2004

A test particle trajectory approach is developed for the simulation of deep bed filtration. A 3‐D network of constricted pores represents the pore space of granular filters, network‐scale trajectories of a large number of non‐Brownian test particles are computed, and filter coefficient predictions are obtained for horizontal, down‐and upflow filtration operation. This simulator yields numerical results that agree excellently with our earlier predictions by the pore‐scale trajectory‐based population balance method. The new approach, however, circumvents the cumbersome step of calculating the impacted fraction in each unit cell, which the earlier method required, by providing direct statistical estimates of the local and overall deposition rates for continuous and discrete pore‐size distributions. For large superficial velocities (Vs > ∼1 mm/s) and distributed pore size, downflow filters are more efficient than horizontal flow filters, whereas for small velocities (Vs< ∼ 0.5 mm/s) the opposite is observed. Horizontal flow operation is also favored by uniform packing for almost any value of the external pressure gradient. Upflow operation is the least efficient for the packings considered here over a broad range of superficial velocity and particle‐size values. Observed differences among the three filtration types are maximal for uniform packings and decrease considerably with increasing packing heterogeneity.
Liquid holdup in rotating packed beds: Examination of the film flow assumption
Authors: Bašić Andjelko , Duduković Milorad P.
Published: June 2004

Possibilities of predicting liquid holdup in rotating packed beds are examined using the film flow theory. A hydrodynamic model based on film flow on the particle scale accommodates both laminar and turbulent films in the entry and developed regime. Conductance measurement was used for experimental determination of liquid holdup and estimation of the degree of anisotropy of liquid distribution. Experimental results represent the first data for liquid holdup in the rotating packed bed as a function of operating conditions and liquid properties. They indicate an anisotropic liquid distribution dependent on the operating variables. While the film model can be fitted to the experimental data, such a fit lacks a theoretical basis and the classical theory of film flow on the particle scale cannot explain the liquid flow in rotating packed beds. An empirical expression correlates well the holdup data with the operating parameters.
Continous‐mixture kinetics of coke formation from olefinic oligomers
Authors: McCoy Benjamin J. , Subramaniam Bala
Published: June 2004

A model for interpreting the molecular‐weight distributions is presented for the olefinic oligomers and aromatic coke compounds that are experimentally observed in a catalyzed hexene isomerization process (the primary reaction). Reaction rate expressions for the secondary generation of hexene dimers, trimers and tetramers by irreversible reaction are represented in mass balances for a stirred‐tank reactor. Dehydrogenation of the oligomers to form coke aromatics and their subsequent adsorption and desorption are included. A molecular‐weight moment method allows solution of the model and computation of time dependence of molecular‐weight distributions for all components. The model is consistent with observations of decreased catalyst deactivation rates when coke desorption rates approach coke production rates. Catalyst activity maintenance, which can occur under supercritical conditions, is also predicted by the model.
Molecular dynamics simulation of diffusion in pillared clays
Authors: Yi Xiaohua , Shing Katherine S. , Sahimi Muhammad
Published: June 2004

The thermodynamics and transport properties of Lennard‐Jones particles in pillared catalytic clays are studied by molecular dynamics simulation. The clays are represented by parallel sheets separated by a given distance and connected by pillars of a given size. Two different spatial distributions of the pillars are studied to determine their effect on the properties of the system. Calculations did not indicate a strong dependence of the diffusivity on the spatial distribution of the pillars, except at low porosities. The solvation force increases monotonically with decreasing porosity of the clays and increasing density of the molecules. The percolation threshold φc of the system is estimated from the diffusivity measurements in the limit of infinitely low sorbate densities. Near φc the diffusivity D vanishes according to the power law, D ˜ (φ − φc)n, where φ is the porosity of the system, and n is a universal constant. The simulations yield n ⋍ 1.7. Since 2‐ percolation systems require n ⋍ 1.3 and 3‐D systems n ⋍ 2.0, pillared clays behave as a system with an effective dimensionality between 2 and 3.
Residence time distribution in three‐phase monolith reactor
Authors: Patrick Robert H. , Klindera Theresa , Crynes Lawrence L. , Cerro Ramon L. , Abraham Martin A.
Published: June 2004

The residence time distribution of the liquid phase within a three‐phase monlith reactor is determined using tracer studies. The actual liquid residence time in the monolith, relevant for reactor design purposes, is calculated from overall residence time measurements using deconvolution by Fourier transform. The liquid‐phase residence time decreases as liquid or gas flow rates increase, but the reactor Peclet number remains approximately constant. The residence time distribution and calculated values of the Peclet number reveal that the liquid phase is substantially well‐mixed. Comparison with results from experiments in a single glass capillary reveals that the monolith channels become predominantly liquid‐filled, particularly as the liquid flow rate becomes a significant fraction of the total flow rate.
Convective diffusion from a dissolving polymeric particle
Authors: Ranade Vivek V. , Mashelkar Ramesh A.
Published: June 2004

A comprehensive mathematical model for describing the process of dissolution of a spherical polymeric particle in a convective field is developed. It includes the process of glass transition, reptation of macromolecules, disengagement of these molecules from the gel‐liquid interface, and diffusion in the boundary layer surrounding the gel‐liquid interface. Different controlling regimes for the dissolution process are identified and quantitatively delineated in this moving boundary problem. Analytical solutions for the estimation of dissolution time for various limiting cases are presented. Key predictions from the model are verified by comparison with preliminary experimental data. A novel feature of a particle‐size‐independent dissolution of polymeric particles below a critical size brought out by the model is verified experimentally. The findings have pragmatic implications in diverse areas, such as polymerization, drag reduction, and microlithography.
Reformulation of Wong‐Sandler mixing rule for cubic equations of state
Authors: Orbey Hasan , Sandler Stanley I.
Published: June 2004

The cubic equation of state mixing rule of Wong and Sandler is reformulated in a way that eliminates one of its parameters and so that it can go smoothly from activity coefficient‐like behavior to the classical van der Waals one‐fluid mixing rule merely by variation of its parameters. The parameters in the reformulated mixing rule can be obtained from correlation of vapor–liquid equilibrium data or from the two infinite dilution activity coefficients for each binary pair in the mixture. When these activity coefficients are estimated from the UNIFAC group contribution model, the mixing rule becomes completely predictive. The correlative and completely predictive forms of this mixing rule are tested here and shown to be accurate for five difficult binary systems and one ternary mixture over large ranges of temperature and pressure.
Experimental measurement of diffusion in aqueous polyvinylpyrrolidone solutions
Authors: Metaxiotou Zoe A. , Nychas Stavros G.
Published: June 2004

Diffusivities of aqueous dilute and semidilute polyvinylpyrrolidone solutions Mw = 50,500 and Mw = 867,000 were measured at 25°C. The measurements were performed by Taylor's hydrodynamic stability method and the dynamic light scattering (DLS) method. Taylor's method was compared to DLS and evaluated as an experimental technique to determine the molecular diffusivity in polymer solutions. The agreement between the two methods was very good with Taylor's method proving to be a reliable, simple, and inexpensive technique that provided diffusivity data with a maximum error of 5%.
Continuum random walk simulations of diffusion and reaction in catalyst particles
Authors: Drewry H. P. G. , Seaton N. A.
Published: June 2004

To predict the effect of pore structure on the performance of heterogeneous catalysts, a realistic model of the catalyst particle is required. Lattice‐based models in which the diffusion and reaction phenomena are restricted to sites and bonds within a regular or irregular lattice are widely used. However, for the realistic simulation of diffusion and reaction in amorphous catalyst supports, such as alumina or silica, a continuum model, which does not artificially restrict the domain in which the reactants are allowed to diffuse, is required.

An efficient method based on a “first passage time” approach is developed for the simulation of diffusion and reaction in a supported catalyst. The model catalyst is composed of spheres representing the support and active sites. By varying the algorithm used to generate the model catalyst, a range of structures can be created. The effect of the structure, and the size and distribution of active sites on the reaction rate is studied.
Binary activity coefficients from microdroplet evaporation
Authors: Ray Asit K. , Venkatraman Sankar
Published: June 2004

A method‐based on evaporation of a constant‐composition droplet containing two components that differ markedly in volatility accurately estimates activity coefficients of both components. This new technique is developed to simultaneously determine the evaporation rate and composition of a droplet from intensity peaks observed in the light scattered by the droplet. It has no upper or lower limits on the relative volatility of the system and is particularly suitable for systems containing one relatively nonvolatile component. The data on the glycerol‐ethanol system, obtained from evaporation of glycerol droplets in ethanol vapor and correlated with Margules and Wilson equations, are thermodynamically consistent. The results of this study agree with ethanol activity coefficients calculated from the total pressure vs. solution composition data previously reported.
Molecular thermodynamic properties of protein solutions from partial specific volumes
Authors: Pjura P. E. , Paulaitis M. E. , Lenhoff A. M.
Published: June 2004

Partial specific volumes at 25°C are reported for α‐chymotrypsinogen in aqueous solutions containing NaCl, citrate, and/or polyethylene gycol (PEG) over a range of protein concentrations. The concentration dependence of the partial specific volume can be either positive or negative, depending on the solvent. For example, the partial specific volume increases with increasing protein concentration in NaCl/citrate solutions at high salt concentrations, and decreases with increasing protein concentration in solutions containing PEG. Kirkwood‐Buff solution theory has been applied to interpret these results, and it was found that the concentration dependence is determined by two factors: (1) the effective or solvent‐averaged interactions between protein molecules in solution, and (2) three‐body protein – protein – solvent and protein – solvent ‐ solvent interactions. An approach is proposed for the experimental determination of both contributions that involves measuring osmotic pressures and volumetric properties of dilute to concentrated protein solutions.
Numerical simulation of flow in helical ducts
Authors: Wang Jing‐Wu , Andrews J. R. G.
Published: June 2004

Helical pipes and ducts are used widely in industry. There is a huge literature on flow in curved pipes and bends, but comparatively little has been published on flow in helical pipes. To simplify the problem, most theoretical work on flow in helical pipes has assumed zero pitch: a toroidal pipe following the work of Dean in 1927. Recent theoretical studies on helical pipes have used helical coordinate systems. These systems, however, are not appropriate when studying helical ducts. The following results are reported: a coordinate system appropriate for helical ducts; the detailed equations of continuity and motion in this coordinate system; the numerical method used to find the fully developed laminar flow of a Newtonian liquid in helical ducts; the simulations vs. published experimental data; and effects of the pitch ratio, curvature ratio and pressure gradient on secondary flow in a helical duct.
Influence of adsorbate size and adsorbent heterogeneity of IAST
Authors: Sircar Shivaji
Published: June 2004

The ideal adsorbed solution theory (IAST) can introduce serious errors in the calculation of multicomponent gas adsorption equilibria from the corresponding pure gas adsorption isotherms when the adsorbate sizes and the degree of adsorbent heterogeneity for adsorption of the components differ substantially. The multisite Langmuir and heterogeneous Langmuir models were used to evaluate the extent of these errors. It is shown that large diferences in adsorbate sizes and degrees of heterogeneity of adsorption of the components of a mixture can cause the formation of an adsorption azeotrope that cannot be described by IAST.
Balanced realization for state‐space identification and optimal output regulation
Authors: Ramirez W. Fred , Maciejowski Jan M.
Published: June 2004

A new input ‐ output approach is presented for optimal regulatory control of systems based on balanced state realization. State‐space identification is achieved using input–output data and a balanced realization algorithm. Based on the realized model, an optimal controller design is presented to regXSulate the system outputs at desired levels. The optimal control algorithm is model‐predictive in that predictions are made based on measured disturbances. It also compensates for unmeasured disturbances through integral state feedback. This control concept is tested using a continuous stirred‐tank chemical reactor model.
Analysis of fiber formation during air‐gap wet spinning
Authors: Simon Volker
Published: June 2004

The steady formation of fibers in the air‐gap wet‐spinning process is analyzed, in which solidification of the fiber material is brought about by the diffusional exchange of solvent and nonsolvent in a coagulation bath. The concentration profile within the fiber is determined, and a simple model is given for the change of the material behavior with changing concentration. The material behavior of the uncoagulated spinning solution is described by a constitutive model for viscoelastic liquids that allows the incorporation of a relaxation‐time spectrum. The behavior of the solidified fiber is described by a model for rubberlike elastic solids. The effects of variable fiber temperature and skin friction on the fiber motion are also taken into account. The computed velocity profiles are compared to experimental data. These data were obtained by spinning three samples of nominally identical spinning solutions. Even though the theoretically predicted profiles agree well with those obtained with the first two samples, they completely fail to do so for the third. This failure is attributed to the unexpected extreme sensitivity of the model predictions to the width of the dynamic spectrum that is used to infer the relaxation‐time spectrum. It is also found that the velocity profile depends rather strongly on the initial conditions for the stress.
Ternary microemulsions as model disordered media
Authors: Knackstedt Mark A. , Ninham Barry W.
Published: June 2004

The ternary microemulsion systems alkane/water/DDAB (didodecyldimethylammonium bromide) form ideal model‐disordered media. The static microstructure is described by a simple parameter‐free model that can be predetermined and agrees with SAXS and SANS scattering experiments. The component volume fraction can be varied to exhibit bicontinuous random structures with a predicted percolation transition to disconnected water‐in‐oil droplets. Structural transitions are analyzed in the context of theories of percolative phenomena. Experimental transport properties agree well with model predictions based on an effective medium approximation. Critical exponents that describe the scaling of the transport properties near percolation are consistent with theoretical expectations near a static percolation transition. Through variation of component volume fractions a medium of known microstructure can be prescribed, so that independent measurement of transport and mechanical properties is possible.
Solvation effects on reactions of triplet benzophenone in supercritical fluids
Authors: Roberts Christopher B. , Brennecke Joan F. , Chateauneuf John E.
Published: June 2004

Laser flash photolysis of the hydrogen abstraction reaction of triplet benzophenone (3BP) from 2‐propanol and 1,4‐cyclohexadiene in supercritical ethane and fluoroform was investigate. Bimolecular rate constants based on bulk concentrations decrease with an increase in pressure along both isotherms studied. These results corroborate our previous studies in CO2 that show increased reaction rates due to enhanced local compositions of cosolvent around the 3BP solute. Analysis of the results includes prediction of the thermodynamic pressure effect on the rate constant, which suggests an increase in the rate constant with pressure, as well as the effects of increased local cosolvent concentrations about 3BP. Spectroscopic measurements of the local composition of 2‐propanol about a solute in supercritical CO2 are used to explain the apparent discrepancy between experiment and prediction, providing reasonable evidence that the local environment can influence kinetically controlled reactions in supercritical fluids.
Newtonian drop impact with a solid surface
Authors: Scheller Brian L. , Bousfield Douglas W.
Published: June 2004

The spreading of Newtonian liquid drops after impact with a solid surface is reproted for a range of liquid and surface proeprties, drop diameters, and impact velocities. Results for liquid viscosities up to 300 mPa.s are given. For a given drop diameter and velocity, a range of liquid viscosities result in splashing even for smooth surfaces. The maximum spread radius, made dimensionless with the drop radius, is correlated as Rmax* = 0.61(Re2Oh)0.166, where Re and Oh are the Reynolds and Ohnesorge numbers. respectively. A model is proposed to account for inertial, viscous, and surface tension forces on the maximum spread radius. Good agreement is found between the model and experimental data from several sources.
Dilute turbulent gas‐solid flow in risers with particle‐particle interactions
Authors: Bolio Eduardo J. , Yasuna Jules A. , Sinclair Jennifer L.
Published: June 2004

Earlier work of Sinclair and Jackson that treats the laminar flow of gas‐solid suspensions is extended to model dilute turbulent flow. The random particle motion, often exceeding the turbulent flucutations in the gas, is obtained using a model based on the kinetic theory of granular materials. A two‐equation low Reynolds number turbulence model is modified to account for the presence of the dilute particle phase. Comparisons of the model predictions with available experimental data for the mean and fluctuating velocity profiles for both phases indicate that the resulting theory captures many of the flow features observed in the pneumatic transport of large particles. THe model predictions did not manifest an exterme sensitivity to the degree of inelasticity in the particle‐particle collisions for the range of solid loading ratios investigated.
Efficiency estimation of liquid‐liquid Hydrocyclones using trajectory analysis
Authors: Wolbert D. , Ma B.‐F. , Aurelle Y. , Seureau J.
Published: June 2004

As the use of hydrocyclones for liquid‐liquid separation becomes increasingly common, the need for a satisfactory method to assess their efficiency increases. Curerntly available efficiency theories were developed for liquid‐solid separations, based on the velocity distributions inside a hydrocyclone. These theories, however, appear less suitable for emulsions where the dispersed phase is slightly lighter than the continuous phase, such as oil/water emulsions. An efficiency computation based on the analysis of the trajectories of the droplets is presented. Trajectories are characterized through a differential equation combining models for the three bulk velocity distributions (axial, radial, and tangential) and the settling velocity defined by Stokes' law. From the critical trajectory and given operating conditions, a characteristic droplet diameter d100 can be deduced that corresponds to the smallest droplet with a 100% efficiency. Other efficiencies are obtained by changing the initial condition for the trajectory equation. The efficiency result of different hydrocyclone configurations are compared with experimental results, and residual emulsion distributions are estimated.
Experimental and theoretical investigation of the validity of asymptotic lumped kinetics
Authors: Ho T. C. , White B. S.
Published: June 2004

Continuum approximation is often used to treat problems involving reaction mixtures of a vast number of species. Previous studies predicted that in many situations the lumped kinetics of a continuum of reactions, regardless of their kinetics, are of the power law form after a long time. In this article we present experimental support for such asymptotic power law kinetics. The system considered is catalytic deep desulfurization of a petroleum distillate. In addition, a theoretical analysis is made to establish the condition under which the continuum approximation is valid in the long‐time limit. The condition turns out not to be very stringent for practical purposes.
Continuous kinetics for thermal degradation of polymer in solution
Authors: Wang Ming , Smith J. M. , McCoy Ben J.
Published: June 2004

Continuous kinetics of thermal degradation of poly (styrene‐allyl alcohol) (PSA) in solution are explored theoretically and experimentally. Thermal degradation experiments for PSA of number‐average molecular weight 1,640 in t‐butanol solution were conducted in a steady‐state flow reactor at 6.8 MPa and 403–473 K. The molecular‐weight distributions (MWDs) of the original polymer and its reaction products were measured as a function of residence time by gel permeation chromatography The MWD of the initial PSA is described by a gamma distribution function. Experimental data indicated that the polymer cracks to specific, low‐molecular‐weight (MW) products and degrades by random chain scission. Results are interpreted by a mathematical model based on the continuous kinetics for specific and random degradation processes. Rate coefficients are determined separately from the dynamic data of the MW moments for the specific products and the MWDs for the reacting polymer. Activation energies obtained are 5.9 – 7.4 kcal/mol for specific degradation processes and 35 kcal/mol for the random degradation process. The detailed MWDs are related to conventional lumped (numberaverage molecular‐weight) data.
Taylor dispersion in a polymer solution flowing in a capillary tube
Authors: Vartuli M. , Hulin J. P. , Daccord G.
Published: June 2004

Tracer dispersion in polysaccaride (scleroglucan) solution flowing through 0.56‐mm‐ID capillary tubes was studied experimentally. In contrast with experiments modeling enhanced recovery processes, the polymer concertration remains constant during a given experiment, while the tracer concerntration varies. A preparation procedure giving stable rheological characteristics is described, as well as their dependence on the polymer concentration Cp (100 mg/L ≤ Cp ≤ 2,000 mg/L). Dispersion measurements are performed with ionic tracers at Péclet numbers between, showing that the Taylor dispersion mechanism remains dominant. At a constant flow velocity, K decreases by about 75% when Cp increases from 0 to 2,000 mg/L. This variation is related to the flattening of the velocity profile and can be predicted both with a power‐law rheological model and Monte Carlo simulations. The extension of these measurements to heterogeneous porous media is discussed.
Theoretical prediction of electric field‐enhanced coalescence of spherical drops
Authors: Zhang Xiaoguang , Basaran Osman A. , Wham Robart M.
Published: June 2004

A fundamental study of drop collision, coalescence and growth induced by combined effects of gravitational and electrostatic forces is presented. The focus is on the enhancement of rates of collision and growth of spherical, conducting drops bearing zero net charge in dilute, homogeneous dispersions by an external electric field. By completely accounting for hydrodynamic and electrostatic interactions, a trajectory analysis is used to follow the relative motion of two drops equation is then solved to predict the evolution in time of the size distribution and the average size of drops. The results show that the rates of drop collision and growth can be increased significantly by applying an electric field, in accord with fundamental experiments and patents on electrocoalescence. The enhancement of drop collision and coalescence is especially pronounced when the imposed electric field acts horizontally, that is, in a direction perpendicular to gravity.
Pressure filtration of flocculated suspensions
Authors: Landman Kerry A. , White Lee R. , Eberl Maria
Published: June 2004

From the development of a compressional rheology of a flocculated suspension, a number of separation processes have been modeled. With respect to the two‐stage pressure filtration process of compact formation and then consolidation, the rheological formulation appears at first examination to eb significantly at variance with the conventional engineering approach. The present model may be reconciled to the early engineering model. Moreover, the filtration parameters extracted from experiments can be related to the more fundamental rheological parameters, r(ϕ), the hindered settling factor, which is a function of solids volume fraction ϕ — it takes into account hydrodynamic interactions between local particles, which increase the drag forces. Useful estimates of the compact bed formation resistance and the formation time are provided as a function of applied pressure. The initial time dependence of the consolidation ratio on the elapsed consolidation time is not found to be a square root behaviour as suggested by the conventional modeling.
Detection of gross erros in data reconciliation by principal component analysis
Authors: Tong Hongwei , Crowe Cameron M.
Published: June 2004

Statistical testing provides a tool for engineers and operators to judge the valididty of process measurements and data reconciliation. Univeriate, maximum power and chisquare tests have been widely used for this purpose. Their performance, however, has not always been satisfactory. A new class of test statistics for detection and identification of gross errors is presented based on principal component analysis and is compared to the other statistics. It is shown that the new test is capable of detecting gross erros of smallmaginitudes and has substantial power to correctly identify the variables in error, when the other tests fail.
Reactions at supercritical conditions: Applications and fundamentals
Authors: Savage Phillip E. , Gopalan Sudhama , Mizan Thamid I. , Martino Christopher J. , Brock Eric E.
Published: June 2004

Supercritical fluids posses properties that make them attractive as media for chemical reactions. Conducting chemical reactions at supercritical conditions affords opportunities to manipulate the reaction environment (solvent properties) by manipulating pressure, to enhance the solubilities of reactants and products, to eliminate interphase transport limitations on reaction rates, and to integrate reaction and separation unit operations. Supercritical conditions may be advantageous for reactions involved in fuels processing, biomass conversion, biocatalysis, homogeneous and heterogeneous catalysis, environmental control, polymerization, materials synthesis, and chemical synthesis. Moreover, supercritical fluids can be used profitably in fundamental chemical investigations of intermolecular interactions and their influence on chemical processes. Work on chemical reactions in and with supercritical fluids is reviewed. We discuss both fundamental studies and applications of reactions at supercritical conditions, with focus on work published after 1985.
Numerical study on hydrodynamics of short‐dwell paper coaters
Authors: Cao Zhenlei , Esmail M. Nabil
Published: June 2004

The influence of flow hydrodynamics is studied numerically on the operating parameters of short‐dwell paper coaters. The flow in and around a coater is numerically simulated using the method of finite difference, body‐fitted numerical grid generation, and a new computational technique for the solution of viscous incompressible Navier‐Stokes equations. The result reflect a simultaneous solution of the problem in and around the coating pond. The computational strategy includes the treatment of mathematical singularities at dynamic and static contact points, and the boundary conditions at free surfaces with stagnation points. A dynamic contact angle is calculated simultaneously with the solution. Results include shear rate, centrifugal force, pressure, and velocity distributions in the flow field related to the operating parameters of the coater, the quality of product, and the efficiency of operation.
A reaction network model for phenol oxidation in supercritical water
Authors: Gopalan Sudhama , Savage Philip E.
Published: June 2004

Dilute aqueous solutions of phenol were oxidized in a flow reactor at 420, 440, 460 and 480°C at 250 atm. Phenol disappearance kinetics followed the trends exhibited by previously published data obtained at T < 420°C. By merging the two sets of data, a global rate low for phenol disappearance kinetics valid between 380 and 480°C was determined to be rate = 102.34 exp( −12.4/RT) [ϕOH]0.85 [O2]0.50 [H2O]0.42. Undesired multiring products, whose formation was reported previously at the lower temperatures, continued to form in high selectivities at these higher temperatures. Reaction products were classified into three categories: dimers, gases, and a remainder that included products from ring‐opening reactions. A global reaction network that describes the transformation of phenol into these product groups was developed. Steps in the network are: parallel oxidation paths for phenol that from dimers and ring‐opening and other products, secondary decomposition of dimers of ring‐opening and other products, and oxidation of the ring‐opening and other products to carbon oxides. The experimental products yields were used to determine optimal values for the reaction orders and rate constants for each step in the network. This quantitative reaction model shows that dimerization is the dominant primary path for phenol consumption. High temperatures and long residence times reduce the concentration of dimers in the reactor effluent and maximize the gas yield. High oxygen concentrations also increase the gas yield. The quantitative reaction network model is consistent with previously published product yields for T = 380 – 420°C.
Detailed chemical kinetics model for supercritical water oxidation of C1 compounds and H2
Authors: Brock Eric E. , Savage Phillip E.
Published: June 2004

A detailed chemical kinetics model comprising 148 reversible elementory reactions for the supercritical water oxidation (SCWO) of methane, methanol, carbon monoxide and hydrogen was developed. Rate constants were taken from previous critical evaluations. The Lindemann model, at times modified with a broadening parameter, was used to account for the effects of pressure on the kinetics of unimolecular reactions. Model predictions were compared with published experimental SCWO kinetics data for 450–650°C and 240–250 atm. The model correctly predicted global reaction orders for all four fuels to within their uncertainties. In addition, the model correctly predicted that the global reaction orders for O2 during methanol and hydrogen oxidation were essentially zero, and that the O2 concentration had the greatest effect on the methane oxidation rate. The pseudo‐first‐order rate constants predicted by the model were consistently higher than the experimental values, but the global activation energies were predicted correctly for methane oxidation and for CO and H2 oxidation at high temperatures. The model's predictions generally became worse as the temperature decreased toward the critical point of water. A sensitivity analysis revealed that fewer than 20 elementaty reactions largely controlled the oxidation kinetics for the compounds studied. Nearly half of these reactions involved HO2, which is an important free radical for SCWO. Quantitative agreement with the experimental methane conversions was obtained by adjusting the preexponential factors for three elementary reactions within their uncertainties. It could also be obtained by using the JANAF value (0.5 kcal/mol) for the standard heat of formation of HO2, but this value is lower than other recently recommended values.
Analysis of interface evolution and pattern formation during CVD
Authors: Thiart Jacob J. , Hlavacek Vladimir
Published: June 2004

Morphological aspects of the evolution of a gas ‐ solid interface during typical CVD processes are presented, as well as a continuum model of CVD growth. A linear stability analysis used determines the effect of reactor conditions on the stability of planar growth. The main focus, however, is numerical solution of governing equations under a wide variety of conditions and with different initial interface shapes as starting point. Simplified solutions under specific deposition conditions and the numerical procedure for solving the complete system of equations are presented. The focuses are on the use of a parametrization that eliminates numerical problems encountered with steep interface gradients and the automatic generation of an adaptive mesh for the domain above the interface. Several examples illustrate the numerical solution procedure. To our knowledge, this is the first attempt to simulate interface evolution during CVD for long deposition times from various initial interface shapes. The simulation revealed several morphological phenomena observed experimentally in previous studies, including the formation of occlusions that contributes to film porosity and was clearly shown by the numerical results. Film uniformity strongly depends on the controlling mechanism of deposition. Severe nonuniformities develop under diffusional limitations, while deposition is very uniform under conditions of kinetics control. Film uniformity could be improved by choosing conditions for which a Damköhler number of deposition, Da, would have the lowest value.
Multiphase ozonolysis of organics in wastewater by a novel membrane reactor
Authors: Guha Asim K. , Shanbhag Purushottam V. , Sirkar Kamalesh K. , Vaccari David A. , Trivedi Deven H.
Published: June 2004

Conventional ozonation of recalcitrant organic compounds in wastewater suffers from low transfer rate of ozone into water. The low transfer rate can be enhanced by adding an inert fluorocarbon (FC) liquid immiscible with water but having a much higher solubility for ozone. A novel membrane reactor was studied for the destruction of organic pollutants by using PC liquid simultaneously as a reaction medium and a liquid membrane. The membrane reactor consisted of two sets of microporous and/or non‐porous hollow‐fiber membranes well‐mixed in a cylindrical shell filled with the inert FC liquid. Wastewater was allowed to flow through the lumen of one set of microporous fibers; O3‐containing gas flowed through the bore of the other set. Oxidative degradation products get partitioned back into the two mobile phases. Degradation of such pollutants as phenol, acrylonitrile, nitrobenzene, trichloroethylene, and toluene in such a reactor is presented. The kinetics of degradation of each pollutant in the two‐phase (aqueous‐ FC) system were studied using batch and semibatch experiments. Simulation results based on a first‐order model to predict the behavior of pollutant degradation in such a membrane reactor are compared with experimental data.
Wave model for longitudinal dispersion: Development of the model
Authors: Westerterp K. R. , Dil'man V. V. , Kronberg A. E.
Published: June 2004

A new 1‐D model for longitudinal dispersion is proposed as an alternative to the Fickian‐type dispersed plug‐flow model. Accounting for significant features of longitudinal mixing gives rise to a quasilinear hyperbolic system of two first‐order equations for the average concentration. The model equations are obtained based on minor extensions of the heuristic equilibrium analysis of Taylor. A qualitative, more general derivation of the equations is given on the basis of the simple generalization of Fick's law, taking into account the finite velocity of fluid elements. For linear problems the mean concentration and the dispersion flux obey a hyperbolic equation of the second order. The proposed hyperbolic model contains three parameters that depend only on the flow conditinos, the physical properties of the fluid, and the gemetry of the system. It effectively resolves the well‐known problem of boundary conditions that, for unidirectional flow, are formulated now only at the reactor inlet. The new model eliminates the conceptual shortcomings inherent to the Fickian dispersed plug‐flow model: it predicts a finite velocity of signal propagation and does not involve backmixing in the case of unidirectional flow.
Droplet deposition and momentum transfer in annular flow
Authors: Fore Larry B. , Dukler Abraham E.
Published: June 2004

Entrainment and deposition in gas‐liquid annular upflow are known to account for as much as 20% of the pressure gradient, through droplet acclelerations in the core refion. Momentum is transerred from the core when droplets decelerate upon impactwith the liquid film. It is usually assumed that all of this momentum is transferred to the film, essentially driving the film upwardin conjunction with interfacial friction. New data, abtained for annular gas‐liquid upflow in a 5.08‐cm‐ID tube, are used in a momentum balance analysis to determine the mechanismof momentum transfer from depositing droplets. Measurements include the liquid film thickness, wall shear stress, pressure gradient, entrained liquid fraction, droplet deposition rate, droplet centerline axial velocity, and mass‐average drop size for two gas‐liquid systems. This analysis supports the idea that large droplets displace the film locally and decelerate primarily at the wall, effectively transferring negligible momentum to the liquid film.
Catalytic oxidation of toluene and tetralin in superoritical carbon dioxide
Authors: Zhou Luho , Erkey Can , Akgcrinan Aydin
Published: June 2004

The catalytic total oxidation of toluene and tetrahydronaphthalene (tetralin) in super‐critical carbon dioxide over a 0.5% platinum γ ‐alumina catalyst was studied in a fixed‐bed reactor. Toluene oxidation experiments were performed at 618, 633, 648 and 663 K, and at 7.93, 9.31 and 10.7 MPa. Tetralin oxidation experiments were performed at 573, 598, 623 and 648 K and at 8.96and 10.0 MPa Inlet concentrations of toluene and tetralin were varied in the ranges 569 to 2,858 ppm (mass/mass) and 1,420 to 3,648 ppm, respectively. Reactions were run at excess oxygen with a 25:1 molar ratio of oxygen to toluene and a 15:1 molar ratio of oxygen to tetralin. A stepwise mechanism was postulated for the oxidation reaction to describe the kinetics of toluence and tetralin oxidation in supercritical carbon dioxide, and the reaction orders and activation energies were determined.
Interfacial force field characterization in a constrained vapor bubble thermosyphon
Authors: Dasgupta Sunando , Plawsky Joel L. , Wayner Peter C.
Published: June 2004

Isothermal profiles of the extended meniscus in a quartz cuvette were measured in the earth's gravitational field using an image‐analyzing intefferometer thai is based on computaer‐enhanced video microscopy of the naturally occurring ineetference fringes. These profiles are a function of the stress field. Experimentally, the augmented Young‐Laplace equation is an excellent model for the force field at the solid‐liquid‐vapor intrfeces for heptane and pentane menisci on quartz and tetradecane on SFL6. Effects of refractive indices of the solid and liquid on the measurement techniques were demonsrrauzd. Experimentally obtained values of the disjoining pressure and dispersion constantswere compared to those predicted from the Dzyaloshiniskii‐Lifshitz‐Pitaevskii theory for an ideal surface and reasonableagreements were obtained. A parameter introduced gives a quantitative measurement of the closeness of the system to equilibrium. The nonequilibrium behavior of this parameter is also presented.