bluuebunny/crossref_metadata_2025 huggingface dataset:
Conversion of radiant light energy in photobioreactors
Authors: Cornet J. F. , Dussap C. G. , Gros J. B.
Published: June 2004

The conversion of radiant light energy into chemical affinity by microorganisms in photobioreactors is examined. The kinetics of entropy production in the system is theoretically established from entropy and energy balances for the material and photonic phases in the reactor. A negative chemical affinity term compensated for by a radiant energy term at a higher level of energy characterizes photosynthetic organisms. The local volumetric rate of radiant light energy absorbed, which appears in the dissipation function as an irreversible term, is calculated for monodimensional approximations providing analytical solutions and for general tridimensional equations requiring the solution of a new numerical algorithm. Solutions for the bluegreen alga Spirulina platensis cultivated in photoreactors with different geometries and light energy inputs are compared. Thermodynambic efficiency of the photosynthesis is calculated. The highest value of 15% found for low radiant energy absorption rates corresponds to a maximum quantum yield in the reactor.
Kinetics of p‐xylene liquid‐phase catalytic oxidation
Authors: Cao Giacomo , Servida Alberto , Pisu Massimo , Morbidelli Massimo
Published: June 2004

A Semibatch gas‐liquid reactor model based on a lumped kinetic schem for the liquid‐phase oxidation of p‐xylene to p‐toluic acid catalyzed by cobalt naphtenate is developed. The model accounts for the complex nature of the involved reaction network, as well as for the interphase and intraphase mass transport processes of both reactants and products. The model reliability is tested by comparison with suitable experimental data obtained in a semibatch oxidation reactor, where the role of the composition of both the gaseous and the liquid feed has been investigated. It is shown that the model describes the reactor behavior in any of the regimes which may prevail depending upon the operating conditions and the depletion of liquid reactants in time.
Prediction and correlation of accessible area of large multiparticle aggregates
Authors: Rosner Daniel E. , Tandon Pushkar
Published: June 2004

Aggregates (composed of large numbers of primary particles) are produced in many engineering environments. One convenient characterization is the fractal dimension, the exponent describing how the number of primary particles in each aggregate scales with radial distance from its center of mass. We describe a finite‐analytic pseudo‐continuum prediction of the normalized accessible surface area of an isothermal quasi‐spherical fractal aggregate containing N (≫ 1) primary particles, on the surfaces of which a first‐order chemical process occurs. Results are displayed for specific fractal dimensions (2.5, 2.18, and 1.8) frequently observed in aggregating systems. An effective Thiele modulus is used to develop an efficient and accurate scheme for predicting/correlating the effectiveness factor for an aggregate containing N primary particles in terms of aggregate fractal dimension, reaction probability, and Knudsen number. Our methods now allow calculations of the accessible surface area of populations of aggregates, provided pdf \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--} \rlap{--} ($\end{document}N, Df, …\documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--})$\end{document} is known for the populations of interest.
Determining fluid saturations during multiphase flow experiments by NMR imaging techniques
Authors: Chen Songhua , Qin Fangfang , Watson A. Ted
Published: June 2004

The quantitative determination of porosity and saturation distributions in porous media using unclear magnetic resonance imaging techniques is addresed. A method based on multiple image acquisitions with extrapolation within a model of transverse relaxation was investigated for new experimental situations. The determination of saturation was validated with limestone and sandstone samples. Experimental evidence for dependencies of transverse relaxation and linewidth on saturation is discussed, as well as their impact on image resolution and quantitation. An automatic production measurement device for independent verification of average saturations during dynamic experiments is investigated.
Local velocity and porosity measurements inside Casper sandstone using MRI
Authors: Merrill Melissa Robinson
Published: June 2004

Magnetic resonance imaging is used to make nondestructive experimental measurements of fluid flow velocity and rock porosity inside a brine‐filled Casper sand‐stone sample during brine injection. Two‐dimensional images of these values are obtained in arbitrary cross sections, with pixel sizes on the order of 1 mm2. The data are then statistically analyzed to find the variation of the average interstitial velocity as a function of porosity, as well as frequency distributions for both the velocity and porosity. The results show that as local porosity increases, the local flow velocity increases, according to the empirical relationship, V = 0.0669 ϕ + 0.0055. Thus, higher porosity regions of the core are better contacted by the displacement fluid. Results also imply bypassing of fluid around the perimeter of the core, indicating the need for a coreholder capable of sustaining higher overburden pressures on the order of 225 psi (1.55 MPa).
Model identification and control strategies for batch cooling crystallizers
Authors: Miller Stephen M. , Rawlings James B.
Published: June 2004

The open‐loop optimal control strategy to regulate the crystal‐size distribution of batch cooling crystallizers handles input, output, and final‐time constraints, and is applicable to crystallization with size‐dependent growth rate, growth dispersion, and fines dissolution. The objective function can be formulated to consider solid‐liquid separation in subsequent processing steps.

A model‐based control algorithm requires a model that accurately predicts system behavior. Uncertainty bounds on model parameter estimates are not reported in most crystallization model identification studies. This obscures the fact that resulting models are often based on experiments that do not provide sufficient information and are therefore unreliable. A method for assessing parameter uncertainty and its use in experimental design are presented. Measurements of solute concentration in the continuous phase and the transmittance of light through a slurry sample allow reliable parameter estimation. Uncertainty in the parameter estimates is decreased by data from experiments that achieve a wide range of supersaturation. The sensitivity of the control policy to parameter uncertainty, which connects the model identification and control problems, is assessed. The model identification and control strategies were experimentally verified on a bench‐scale KNO3‐H2O system. Compared to natural cooling, increases in the weight mean size of up to 48% were achieved through implementation of optimal cooling policies.
Automated synthesis of distillation sequences using fuzzy logic and simulation
Authors: Flowers Thomas L. , Harrison B. Keith , Niccolai Marino J.
Published: June 2004

An automated distillation sequencing system (DSEQSYS) is presented, which consists of three components: a control program, a fuzzy heuristic synthesis program, and a process simulator. DSEQSYS, when applied to problems previously reported in the literature, overcomes some of the disadvantages of using heuristics or mathematical programming alone. DSEQSYS can address problems involving nonsharp separations, nonideal chemical behavior, and conflicting heuristics. A simple approach for converting the traditional separation heuristics into corresponding fuzzy heuristics is also demonstrated.
Synthesis of nonequilibrium reactive distillation processes by MINLP optimization
Authors: Ciric Amy R. , Gu Deyao
Published: June 2004

A mixed integer nonlinear programming (MINLP) model is presented for synthesizing reactive distillation columns when chemical reaction equilibrium cannot be assured. The MINLP minimizes the total annual cost subject to a rigorous tray‐by‐tray model. The solution of this MINLP yields the optimal number of trays, the optimal feed rates, and the optimal feed tray locations. The liquid holdup per tray, the reflux ratio, and the temperature and composition profiles within the column are optimized as well. The MINLP is solved using generalized Benders decomposition, and the technique is illustrated with ethylene glycol synthesis.
Dynamic optimization of nonlinear processes by combining neural net model with UDMC
Authors: Chen Qi , Weigand William A.
Published: June 2004

A new dynamic optimization technique presented combines a neural network model with a universal dynamic matrix control (UDMC) algorithm. This technique utilizes a nonlinear‐model‐predictive control technique for on‐line optimization and feedback control by using a dynamic neural net model. This approach offers two important advantages over conventional UDMC. One is that a dynamic neural net model can be developed from process data and used for optimization calculations, thus achieving optimization without a first principle model. This neural‐network‐based optimization approach also produces good performance even with processmodel mismatch. The other is that our neural‐net‐model‐based UDMC algorithm greatly reduces the computation time required for the nonlinear dynamic matrix used for the successive quadratic programming algorithm. The development of this technique also involved an analysis of the effect of network structure on dynamic optimization. A state‐space‐based neural network model which utilizes a priori process knowledge is best suited for optimization calculations. Advantages of this technique are illustrated by simulation for two chemical processes.
Kinetics and mechanism of isobutene formation from T‐butanol in hot liquid water
Authors: Xu Xiaodong , Antal Michael J.
Published: June 2004

Isobutene is the only product of the uncatalyzed and acid‐catalyzed dehydration of tert‐butanol in compressed liquid water at 250°C. The uncatalyzed dehydration reaction is fast: equilibrium is established after about 30 s. Only one of many chemically‐motivated kinetic models is able to fit all the experimental data. This model presumes a heterolytic dehydration mechanism that involves protonated alcohol, carbocation, di‐tert‐butyl ether, and protonated ether as intermediates. The model permits tert‐butanol to dissociate as both an Arrhenius acid and a Bronsted acid while catalyzing its dehydration to isobutene. The pKa of tert‐butanol at 250°C is predicted to be about 9, whereas its value at normal temperature and pressure is 18. This 109 increase in the Ka value of tert‐butanol hints of the promising new world of reaction chemistry in near‐ and supercritical water.
Liquid‐phase capillary condensation and adsorption isothern
Authors: Miyahara Minoru , Kato Masahito , Okazaki Morio
Published: June 2004

A “capillary condensation” in liquid phase was analyzed theoretically and experimentally, in which a solute in solvent could be separated out as a liquid into small pores by capillary effect. The “liquid‐phase capillary condensation” is defined as capillary phase separation. The separated phase is counted as “adsorbed” amount on porous materials. The relation between the concentration and a critical radius of the phase separation was derived. A new method to estimate liquid‐phase adsorption isotherms of adsorbates with limited solubility from pore‐size distribution of solids is based on this concept. The amount absorbed consists of the separated phase and adsorption on surface expressed with a single parameter. While the surface adsorption alone could not explain measured isotherms, this model showed good agreements between experimental and estimated isotherms for adsorption of four kinds of aromatic compounds from aqueous solution onto three categories of porous solids to demonstrate the significance of the capillary phase separation. The applicability of the method to solid solutes is also discussed.
Random and systematic errors in thermophysical property measurements
Authors: Eubank Philip T. , Van Peursem David , Chao Yun‐Peng , Gupta Deepak
Published: June 2004

General procedures are outlined for the simulation and propagation of random and systematic errors in thermophysical property experiments. Density second virial coefficients B(T) from sonic velocity and Joule‐Thomson (J‐T) experiments are examined for error propagation where the connecting thermodynamic identity is a differential equation with missing boundary conditions. A recent controversy is addressed concerning B(T) at subcritical temperatures for pure hydrocarbon gases from direct density measurements vs. new sonic velocity data. Sonic velocity results are more likely correct with adsorption errors causing the problem in the density measurements.

Two new model consistency tests are developed for checking assumed temperature models in the reduction of sonic velocity and J‐T data to B(T). Excellent values of B(T) are then obtained from either type of data when the original experiments are free of errors. Random errors propagate systematically when the connecting equation is a differential equation. Sonic data must be of high precision (± 10 ppm) to generate B(T) to ±1 cm3/mol due to complications in data reduction arising from the temperature model/random error interaction. Except perhaps for adsorption errors, systematic errors in the sonic velocities are unimportant to B(T). J‐T data provide propagation factors near unity with errors in B(T) higher at higher temperature, unlike sonic velocities.
Influence of large‐amplitude oscillations on turbulent drag
Authors: Mao Zhuoxiong , Hanratty Thomas J.
Published: June 2004

The effect of imposed large‐amplitude oscillations on turbulent drag is studied. The system consists of water flow through a straight 5.08‐cm pipe. The velocity gradient at the wall is measured with flush‐mounted electrochemical mass‐transfer probes. Newly developed numerical algorithms are used to analyze the probe performance in the presence of unsteady flows. Sinusoidal oscillations are at large enough frequencies, ω+ = 0.0138–0.0506, that a pseudo‐steady‐state approximation cannot be made. The ratio of the time‐averaged velocity gradient at the wall, with and without oscillations, S̄, varies between 1.00 and 1.03, provided flow reversal does not occur. However, two experiments in which reversed flows existed at the wall for an appreciable period of time show drag reductions of 7 and 13%.

Imposed nonsinusoidal oscillations are also studied for a period of favorable pressure gradient, about twice longer than that of unfavorable, and two sudden changes in the pressure gradient. Experiments at Re = 9,700 with T0 of 2.00, 2.45, and 3.46 s, and at Re = 19,200 with T0 = 3.46 give values of S̄ = 1.04–1.08. At Re = 19,200 and T0 = 2.00, 1.50, 1.00 s, drag reductions are 10–15%. This phenomenon could be associated with the speed with which a flow adjusts to sudden changes in the pressure gradient.
Physical absorption of CO2 and propene into toluene/water emulsions
Authors: Littel R. J. , Versteeg G. F. , Van Swaaij W. P. M.
Published: June 2004

The physical absorption of CO2 and propene into toluene/water emulsions is studied in a stirred cell and laminar film absorber. Experimentally observed masstransfer rates are compared to an absorption model, based on Higbie's penetration theory describing physical gas absorption into an emulsion. For all absorption experiments in a stirred cell absorber (toluene fractions and stirring rates), experimentally observed mass‐transfer rates are considerably higher than the rates predicted by the absorption model. Moreover, the absorption rate decreases with increasing stirring rate, whereas no influence of the stirring rate is predicted by the absorption model. In contradiction to the absorption experiments in a stirred cell absorber, the observed mass‐transfer rates in the laminar film absorber agree with the model simulations. Probable existence of a very thin toluene layer is observed on top of the emulsion for the stirred cell experiments, likely due to minor phase separation. Since in the laminar film absorber gas‐liquid interface and the gravity force are parallel, this phenomenon does not probably occur significantly for absorption experiments in this absorber. The observed mass‐transfer phenomena can be explained, at least qualitatively, from the occurrence of a thin toluene layer.
Automatic synthesis of batch plant procedures: Process‐oriented approach
Authors: Rotstein G. E. , Lavie R. , Lewin D. R.
Published: June 2004

A strategy for the automatic synthesis of plant operating procedures, through the integration of artificial intelligence and mathematical modeling tools is discussed. This “predicative/numerical” modeling paradigm integrates qualitative and quantitative aspects of the problem in a same framework. It uses a “process‐centered” qualitative modeling approach together with MILP optimization techniques to automatically generate operating strategies. The operation procedures network (OP‐Net), formulated in terms of processes from a generic library, describes abstract (hardware‐independent) processing paths leading to the desired operational goals. In this approach, the model assumptions, conservation expressions and data are described at the same level of abstraction, using the same formal representation. This permits the direct, automatic generation and modification of operation procedures in situations involving changes in basic assumptions and initial scenarios. Thus, flexibility, an essential feature of batch processing, is safeguarded.
Fouling of tube surfaces: Modeling of removal kinetics
Authors: Yiantsios S. G. , Karabelas A. J.
Published: June 2004

Fouling of tube surfaces from flowing solutions or suspensions is examined. A new approach to the removal aspects of this process is presented linking removal to the morphological and mechanical characteristics of deposit microstructure. A simplified description of deposit microstructure is employed, involving a collection of geometrically similar roughness elements. A dynamic situation is envisioned whereby these elements grow due to deposition and suffer breakage under the action of hydrodynamic forces. A mathematical model for the macroscopic fouling behavior based on the techniques of population balances is obtained and numerically analyzed. The behavior found is in agreement with experimental evidence.
On‐line terpolymer composition control in semicontinuous emulsion polymerization
Authors: Urretabizkaia Agustín , Leiza José R. , Asua José M.
Published: June 2004

A closed‐loop strategy for terpolymer composition control in semicontinuous emulsion polymerization of high solids content systems is presented. This strategy is based on a nonlinear adaptive controller that calculates the flow rate of the more reactive monomers to be added into the reactor to produce a terpolymer of a given composition. The nonlinear adaptive controller is based on a simplified mathematical model of the process that includes the on‐line estimation of an adjustable parameter. The addition of a conventional feedback proportional‐integral (PI) controller to the prediction of the nonlinear controller was studied by simulation. It is shown that the contribution of the PI controller is negligible. The controller was checked by computer simulation and also experimentally verified during the semicontinuous emulsion terpolymerization of high solids content (55 wt. %) of vinyl acetate, methyl methacrylate and butyl acrylate.
Reducing NOx in diesel exhausts by SCR technique: Experiments and simulations
Authors: Andersson S. Lennart , Gabrielsson Pär L. T. , Odenbrand C. U. Ingemar
Published: June 2004

Results of experiments are compared to simulations of the performance of V2O5/γ‐alumina SCR catalyst used to remove nitrogen oxides from diesel exhausts. The kinetic model is based on Eley‐Rideal kinetics. Temperature programmed desorption (TPD) of the ammonia is used to evaluate adsorption/desorption parameters. Stationary experiments on a 3.6‐dm3 engine with a 4.8‐dm3 honeycomb catalyst is used to evaluate kinetic rate parameters for the reduction of NO with NH3. The desorption activation energy, Ea,des, is best fitted as a function of surface coverage yielding Ea,des = 240* (I −0.44* θ) kJ/mol. A dynamic test (FTP) with a 12‐dm3 engine and24.8‐dm3 honeycomb catalyst is done with astoichiometric injection of ammonia. The NOx conversion is 52%, but during the freeway traffic the conversion reached 72%. The prediction of NOx conversion is good, but there are some deviations for temperature and ammonia slip.
Solubilization of trichloroethylene by polyelectrolyte/surfactant complexes
Authors: Uchiyama Hirotaka , Christian Sherril D. , Tucker Edwin E. , Scamehorn John F.
Published: June 2004

An automated vapor pressure method is used to obtain solubilization isotherms for trichloroethylene (TCE) in polyelectrolyte/surfactant complexes throughout a wide range of solute activities at 20 and 25°C. The polyelectrolyte chosen is sodium poly (styrenesulfonate), PSS, and the surfactant is cetylpyridinium chloride or N‐hexadecylpyridinium chloride, CPC. Data are fitted to the quadratic equation K = K0 (1 − αX + βX2), which correlates the solubilization equilibrium constant (K) with the mole fraction of TCE (X) in the micelles or complexes at each temperature. Activity coefficients are also obtained for TCE in the PSS/CPC complexes as a function of X. The general solubilization behavior of TCE in PSS/CPC complexes resembles that of TCE in CPC micelles, as well as that of benzene or toluene in CPC micelles, suggesting that TCE solubilizes in ionic micelles both within the hydrocarbon micellar interior and near the micellar surface. The presence of the polyelectrolyte causes a small decrease in the ability of the cationic surfactant to solubilize TCE, while greatly reducing the concentration of the surfactant present in monomeric form. PSS/CPC complexes may be useful in colloid‐enhanced ultrafiltration processes to purify organic‐contaminated water.
Control of emulsion polymerization reactors
Authors: Dimitratos John , Eliçabe Guillermo , Georgakis Christos
Published: June 2004

The interest in emulsion polymerization is strong because it offers the possibility of producing products with special properties not easily produced by other types of polymerization processes. Because the main reaction medium is water, not an organic solvent, its importance is steadily increasing with efforts to design environmentally benign processes. The control of emulsion polymerization is a challenging problem due to major limitations encountered in its modeling and on‐line monitoring. Although emulsion polymerization has been studied and used for several decades, progress has been slow. Industrial practice relies heavily on experience while several controversial issues are still being studied. The literature related to this issue is vast and sometimes contradictory, and most deals with experimental investigation of specific emulsion polymerization systems to improve the process understanding and develop a process model. However, universally accepted conclusions are scarce. This article reviews the major issues related to control of emulsion polymerization. The latest contributions in process understanding, mathematical modeling, sensor technology, and process control techniques for emulsion polymerization are discussed. Emphasis is on contributions, which are important in resolving the control of emulsion polymerization processes.
Oxidation of benzyl alcohol using hypochlorite ion via phase‐transfer catalysis
Authors: Asai Satoru , Nakamura Hidemi , Sumita Toshihiko
Published: June 2004

Heterogeneous oxidation of benzyl alcohol using hypochlorite ion with phase‐transfer catalyst is carried out in an agitated vessel with a flat interface. Cetyltrimethylammonium bromide and toluene are chosen as the best catalyst and solvent, respectively. Observed reaction rates are proportional to the interfacial concentration of cetyltrimethylammonium hypochlorite in the organic phase, which is formed by ion exchange between bromide and hypochlorite ions. They are reasonably explained by the proposed theoretical model for the phase‐transfer catalysis. The reaction rate constant evaluated by fitting the rate data to the model prediction is 22.7 m3 /kmol·s at 303 K, independent of the physical properties in the aqueous solutions. The maximum yield of benzaldehyde is about 90% and the selectivity is 100% for the conversion of hypochlorite ion less than 80%.
Analysis of dispersed‐phase systems: Fresh perspective
Authors: Ramkrishna Doraiswami , Sathyagal Arun , Narsimhan G.
Published: June 2004

Dispersed‐phase systems are analyzed with a fresh perspective where the volume fraction of the dispersed phase is emphasized, not particle numbers as in population balance. Such volume fraction balances are more pertinent to engineering because they deal with the amount of the dispersed phase relative to that of the continuous phase. Although it is easy to make detailed volume fraction balances directly or from population balance, many interesting features are identified here with balance equations in terms of volume fraction, which simply characterize the dispersion process and structure the resulting equation. They lead to equivalent “single‐particle” (comprising the entire dispersed phase) processes which can be simulated with great simplicity allowing rapid calculation of quantities associated with the dispersed phase and dispersion. The techniques can solve an inverse problem for mass‐transfer coefficients of individual droplets from (simulated) measurements of the bivariate distribution of drop size and concentration of a transferring solute. Such inverse problem method is important in developing experimental techniques techniques to measure multivariate population distributions such as those of Bae and Tavlarides (1989) and of flow cytometry.
Theory of gas diffusion and permeation in inorganic molecular‐sieve membranes
Authors: Shelekhin A. B. , Dixon A. G. , Ma Y. H.
Published: June 2004

Theoretical models of gas diffusion and permeation in microporous molecularsieve membranes are presented. The effect of the adsorbed diffusant on the total transmembrane flow is insignificant for permanent gases. For highly adsorbable gases the effect of the adsorbed molecules on the total transmembrane flux may be high at low temperatures. The activation energy of diffusion increases when the kinetic diameter of the diffusant increases. The activation energy of gas diffusion compares well with the values calculated based on the Lennard‐Jones potential. Maximum possible permeability coefficients calculated for He in the molecular‐sieve membranes do not exceed ˜30,000 Barrer at room temperature. The experimentally observed value for He permeability is ˜1,000 Barrer (T=30°C) because of the high tortuosity (τ≈︁ 25) and low porosity (θ = 0.22) of the membrane porous structure.
Nucleation, agglomeration and crystal morphology of calcium carbonate
Authors: Tai Clifford Y. , Chen Pao‐Chi
Published: June 2004

A pH‐stat MSMPR crystallizer is used to obtain the nucleation rate equation of calcite formed in the Na2CO3‐CaCl2 reacting system. The experiments are conducted for seeded and unseeded runs. Effects of breakage and agglomeration on the crystalsize distribution are demonstrated. In a continuous operation, the agglomeration of nuclei followed by the breakage of agglomerates occurs at the transient state, while the agglomeration is significant at the steady state. Taking crystal agglomeration into account, the corrected nucleation rate and agglomeration rate are obtained simultaneously. The agglomeration rate is a function of relative supersaturation and magma density, and the corrected nucleation rate is approximately two‒ to five‐fold of the apparent nucleation rate, which is the nucleation rate without considering the agglomeration effect. Discussed also is the formation of calcium carbonate polymorphism.
Separation vector formulation for the synthesis of multicomponent separation sequences
Authors: Kao Yuen‐Koh
Published: June 2004

A new separation system representation uses stream separation vectors in the separation space. The characteristics of a separation sequence are clarified by its separation base vectors that form the sequence basis and those of a separation system by the geometrical properties of separation vectors. The optimal flowsheet of each sequence under separation vector formulation can be obtained as the solution of a linearly‐constrained optimization problem. A set of simple rules determines the minimum separation loads of any sharp separation sequence by inspection. A modified cost measure, which combines the separation cost with the savings due to stream bypass, can be used to select the optimal sequence without the overall cost analysis.

The optimal separation sequence is obtained first by identifying the best sequence configuration according to modified cost measures and then by finding the actual costs of maximum‐bypass and its neighboring sequence configurations. For the nonlinear objective function, the exact optimal flowsheet is determined by solving a linearly‐constrained optimization problem. Since this procedure is a linearly‐constrained optimization problem, the mathematical programming solution is not likely to lead to a local minimum.
Unified excitation and performance diagnostic adaptive control framework
Authors: Hinde Ralph F. , Cooper Douglas J.
Published: June 2004

Model‐based controllers contain two elements that must be adjusted to maintain desired performance: parameters of the process model and a tuning parameter in the controller design equation. A unified framework is presented where vector quantizing networks are used in pattern‐based methods for diagnosing process excitation and controller performance. Excitation diagnostics identify sufficiently excited dynamic process data for model updating. Performance diagnostics analyze set point response data and determine appropriate updates to the tuning parameter. Supervisory adaptation logic enables these two adaptive mechanisms to work together to maintain model accuracy and desired controller performance. The method is general to a number of model‐based control algorithms and process model forms. Demonstrations employ a FOPDT model, as well as both the PI and DMC algorithms for set point tracking and disturbance rejection in a simulation and a bench‐scale process.
Silica gels made by bicontinuous microemulsion polymerization
Authors: Burban J. H. , He Mengtao , Cussler E. L.
Published: June 2004

Microporous silica gels can be made by polymerizing partially hydrolyzed tetramethoxysilane sols present in the aqueous phase of bicontinuous microemulsions stabilized with didodecyldimethylammonium bromide. When vacuum‐dried, the gels made in microemulsions have about twice the specific surface area of conventional vacuum‐dried silica gels. They have 70% of the specific area of supercritically dried gels. Small‐angle X‐ray Scattering measurements in these gels show two characteristic sizes. One size, around 24Å, in the original microemulsion is retained during polymerization, but lost when the detergent is extracted. The second size grows with the square root of time during the polymerization, suggesting diffusion‐controlled gelation.
Diffusion of gases in polymer blends near the lower critical solution temperature
Authors: Li R. J. , Kwei T. K. , Myerson A. S.
Published: June 2004

Gas diffusion through miscible blends of polystyrene (PS) with poly(vinyl methyl ether) (PVME) and poly(styrene‐stat‐acrylonitrile) (PSAN) with poly(methyl methacrylate) (PMMA) is studied from temperatures below the corresponding lower critical solution temperature (LCST) to temperatures above. The Arrhenius equation describes the temperature dependence of diffusion coefficients of pure components. Diffusion coefficients of methane in the 20/80 PS/PVME blend also follow the Arrhenius equation in the temperature ranges above and below their corresponding LCST; however, discontinuity is observed in the LCST region. In contrast to the PS/PVME blend, discontinuity in the LCST region is not obvious for nitrogen and methane in the 50/50 PSAN/PMMA blend. These phenomena are discussed in terms of free volume and morphology changes in the LCST region.
Visualization and simulation of bubble growth in pore networks
Authors: Li X. , Yortsos Y. C.
Published: June 2004

Nucleation and growth of bubbles in porous media are important problems encountered in processes such as pressure depletion and boiling. Experiments and numerical simulations are studied in micromodel geometries to understand their basic aspects. Experiments of bubble growth by pressure depletion are carried out in 2‐D etched‐glass micromodels and in Hele‐Shaw cells. Nucleation of bubbles and subsequent growth of gas clusters are visualized. Contrary to the bulk or Hele‐Shaw cells, gas clusters in the micromodel have irregular and ramified shapes and share many of the features of an external invasion process (such as percolation during drainage). A pore network numerical model developed simulates the growth of multiple gas clusters under various conditions. It is based on the solution of the convection‐diffusion equation and accounts for capillary and viscous forces, which play an important role in determining growth patterns. Numerical simulation agrees well with the experimental results.
Effect of long‐range correlations on transport phenomena in disordered media
Authors: Sahimi Muhammad
Published: June 2004

Three different flow and transport phenomena considered here are hydrodynamic dispersion in heterogeneous porous media and aquifers, transport of passive particles in an oscillating flow field, and miscible displacement processes in heterogeneous reservoirs. At microscales all three phenomena are described by the classical convective‐diffusion equation (CDE). The presence of long‐range correlations at macroscales gives rise to a rich variety of phenomena that cannot be predicted by analyzing the CDE by classical methods. In particular, a new percolation model with long‐range correlations provides a rational explanation for the hitherto unexplained field‐scale experimental data for hydrodynamic dispersion in porous media and aquifers. Moreover, for transport in oscillating flow in convection cells percolation provides a novel relation between the dispersion coefficient and the Péclet number that cannot be predicted by other methods.
Linear eddy modeling of reactant conversion and selectivity in turbulent flows
Authors: Frankel S. H. , McMurtry P. A. , Givi P.
Published: June 2004

The linear eddy model (LEM) is used for statistical predictions of stationary, homogeneous turbulent flows under the influence of isothermal chemical reactions. Nonpremixed reacting systems are considered with two reaction mechanisms: a binary, irreversible single‐step reaction A + B ↦ P; the series‐parallel reaction A + B ↦ R, A + R ↦ P. In both systems, the influence of various flow parameters on the reactant conversion rate is elucidated. For the second reaction scheme, effects of the flow parameters on the “selectivity” are also investigated. The trends predicted by the LEM agree with those produced previously by direct numerical simulation (DNS) at moderate values of the Reynolds number, Schmidt number and Damköhler number. An important feature of the LEM is its capability to extend the parameter range well beyond that currently attainable by DNS. The LEM generated results for a wide range of Schmidt and Damköhler numbers are discussed as well as their effects on the selectivity. These results assess the performance of some of the existing closures for modeling of the selectivity. None of the closures are capable of reproducing the LEM results.
Analysis and shape inequalities for gas‐solid reactions with changing volume
Authors: Cao Giacomo , Strieder William , Varma Arvind
Published: June 2004

For gas‐solid noncatalytic reactions following the sharp‐interface model, some fundamental aspects, not discussed previously in the literature, are presented for particle shapes where diffusion processes are described by a single space dimension. A detailed structure analysis of the solution for monotone nonlinear kinetics reveals interesting differences in the dynamics of the interfacial reaction front, including local maxima and minima in the rate of reaction interface motion, depending on particle shape and expanding or shrinking particle size. Rigorous shape inequalities on the interfacial reactant concentration, the motion of the reaction front and the particle conversion for the sphere and slab, and cylinder and slab geometries are obtained for any nonlinear monotonically increasing reaction kinetics and a wide range of parameter values. By taking advantage of these inequalities, the fraction of original solids of spherical or cylindrical shape, which remains unconverted at the exit of a fluidized‐bed reactor, is bounded by the corresponding quantities related to the slab shape. As an example, a first‐order reaction of the gaseous reactant is considered, where expressions for the reactor are obtained analytically, without any assumptions about possible controlling regimes. These analytical bounds for reactor conversion closely approximate the numerical solutions for design purposes.
Detailed kinetics of titanium nitride synthesis
Authors: Rode Hilmar , Hlavacek Vladimir
Published: June 2004

A thermogravimetric analyzer is used to study the synthesis of TiN from Ti powder over a wide range of temperature, conversion and heating rate, and for two Ti precursor powders with different morphologies. Conversions to TiN up to 99% are obtained with negligible oxygen contamination. Nonisothermal initial rate and isothermal data are used in a nonlinear least‐squares minimization to determine the most appropriate rate law. The logarithmic rate law offers an excellent agreement between the experimental and calculated conversions to TiN and can predict afterburning, which is an important experimentally observed phenomenon. Due to the form of the logarithmic rate law, the observed activation energy is a function of effective particle size, extent of conversion, and temperature even when the intrinsic activation energy remains constant. This aspect explains discrepancies among activation energies obtained in previous studies. The frequently used sedimentation particle size is a poor measure of the powder reactivity. The BET surface area indicates the powder reactivity much better.
Phase transition extraction using solvent mixtures with critical point of miscibility
Authors: Ullmann Amos , Ludmer Zvi , Shinnar Reuel
Published: June 2004

A novel separation process, phase transition extraction, proposed in this article uses solvents that have a critical point of miscibility with the feed liquid to be extracted. Alternate heating and cooling across the coexistence curve leads alternatively to a homogeneous phase or two separate phases. The formation of a homogeneous phase in the mixing stage eliminates the need for intense agitation. Coalescence on cooling in the settling stage is rapid in the presence of impurities that in conventional extraction lead to emulsions. A fermentation broth is extracted without a centrifuge with improved recovery of the antibiotic extracted. The process may have a significant advantage for cases in which extraction is difficult either due to formation of stable emulsions and dispersions or shear sensitivity.
Adjustable discretized population balance for growth and aggregation
Authors: Lister J. D. , Smit D. J. , Hounslow M. J.
Published: June 2004

The discretized population balance (DPB) of Hounslow et al. (1988) is extended to allow the use of an adjustable discretization of the size domain of the form νi+/ν1=2 1/q (where q ≥ 1 and is an integer). All the advantages of Hounslow et al.'s original work are retained: the zeroth and third moments of the particle‐size distribution are correctly predicted, and the DPB is simple to solve with no integration within each size interval. The predicted and analytical particle size distributions are in excellent agreement for the cases of nucleation and growth and aggregation alone in batch and in continuous vessels. High‐order moments and self‐preserving particle‐size distributions are accurately predicted when q ≥ 4.
High‐temperature kinetic study for the reactive ion etching of InP in BCl3/Ar/O2
Authors: Demos Alex T. , Fogler H. Scott , Fournier Jeffrey , Elta Michael E.
Published: June 2004

The reactive ion etching kinetics of InP studied uses BCl3/Ar and BCl3/Ar/O2 as etchants. High‐temperature etching using BCl3 and Ar increases the etch rate negligibly. However, the addition of 30% oxygen in the gas feed increases etch rates by a factor of 10,000 up to 1.5 micron/min at wafer temperatures of 250°C. X‐ray photoelectron spectroscopy analysis reveals that oxygen removes the boron species adsorbing on the InP surface by scavenging the boron to form volatile boron oxides. To study the gas‐phase chemistry, optical emission spectroscopy is used to monitor atomic chlorine intensity at different gas mixtures. The chlorine intensity shows a Gaussian‐type dependence with oxygen addition, which is similar to the etch rate dependence. Two regimes of etching found are: at temperatures below 150°C, the etching is limited by the removal of indium chlorides; above 180°C, the etching is reaction‐limited. The surface morphology shows that the etch profile becomes rougher as a result of increased chemical etching. At high power densities (0.21 W/cm2) and intermediate temperatures (150°C), near vertical wall shapes are obtained. A kinetic model for the high‐temperature etching is developed, as well as a rate law based on the InCl formation reaction. The rate law compares favorably with experimental etch rate results.
Measurement of protein diffusion in Dextran solutions by holographic interferometry
Authors: Kosar T. Fettah , Phillips Ronald J.
Published: June 2004

Theories and experimental measurements related to the diffusion of globular macromolecules and small spheres in polymer solutions are discussed. It is shown that the Kirkwood‐Riseman point scatterer and Brinkman models, two theoretical approaches that lead to hydrodynamic screening, are equivalent. Holographic interferometry is presented as a new method for measuring gradient diffusion of proteins in transparent polymer solutions and gels. This method is used to examine the effect of ionic strength, polymer concentration and polymer molecular weight on the diffusion of bovine serum albumin (BSA) in dextran solutions. The data are interpreted in light of the hydrodynamic screening and Stokes‐Einstein models of diffusion. In particular, it is shown that while the Stokes‐Einstein equation may be appropriate for the diffusion of relatively large latex spheres in polymer solutions, it is inappropriate for predicting diffusion coefficients of BSA and comparable proteins in such solutions.
Modeling membrane filtration of protein and cell suspensions in a vortex flow filtration system
Authors: Mateus M. , Cabral J. M. S.
Published: June 2004

Vortex flow filtration experiments with bovine serum albumin (BSA) solutions and Arthrobacter simplex cell suspensions in a rotary membrane device are modeled in the pressure‐dependent flux and mass‐transfer‐controlled regions. An expression for the concentration‐dependent diffusion coefficients accounting for hydrodynamic and long‐range potential interactions between rigid spherical macromolecules is used for BSA. A shear‐induced diffusion coefficient, strongly concentration‐dependent on the cell concentration, is assumed to describe the diffusion behavior of the cell suspensions. The adjusted model parameters for albumin solutions in the pressure‐dependent region of flux predict the permeation flux behavior for A. Simplex cell suspensions. When the mass‐transfer correlation is calculated using cell diffusion coefficients, based on a volumetric cell fraction that is around half of the average value of the calculated membrane surface fractions, the fluxes are estimated within less than 10.5% relative error. In the mass‐transfer‐controlled region of flux, the volume fractions of cells at the membrane surface are between 0.16 and 0.35, depending on bulk concentration and hydrodynamic conditions.
Oscillatory, creep and steady flow behavior of Xanthan‐thickened oil‐in‐water emulsions
Authors: Pal Rajinder
Published: June 2004

The rheological behavior of xanthan gum–thickened oil‐in‐water emulsions is studied with a cone‐and‐plate system using a constant‐stress rheometer. Xanthan gum solutions and xanthan‐thickened oil‐in‐water emulsions are strongly shear‐thinning and viscoelastic in nature. The effects of polymer and oil concentrations on the rheological behavior of emulsions are investigated. The relative viscosity for the thickened emulsions, at any given oil concentration, increases with an increase in the shear rate, whereas the unthickened emulsions show the opposite trend. The theoretical models give reasonable predictions for the relative viscosity, storage modulus, and loss modulus of xan‐than‐thickened emulsions. The ratio of storage to loss moduli increases considerably with the increase in polymer and oil concentrations. The creep/recovery experiments confirm that the xanthan‐thickened emulsions are highly viscoelastic in nature and that the degree of elasticity increase with the increase in polymer and oil concentrations.
Mathematical model of mass transport through dispersed‐phase polymer networks
Authors: Varelas Charalambos G. , Dixon David G. , Steiner Carol A.
Published: June 2004

A mathematical model of mass transport through dispersed‐phase networks to be used for the sustained release of drugs or other solutes at steady rates is presented. The drug is assumed to be encapsulated within the dispersed microdomains and transported to the bulk by diffusion across the interface. A drug is released to the surroundings by diffusion through the bulk. The results show that the desired steady flux of a drug to the surroundings may be obtained given appropriate values of structural properties of the network. These properties may be manipulated easily in the fabrication of dispersed‐phase networks reported previously.
Estimation of cross‐directional properties: Scanning vs. Stationary Sensors
Authors: Tyler Matthew L. , Morari Manfred
Published: June 2004

Periodic time‐varying Kalman filter equations for problems involving scanning sensors are solved using “lifting” techniques common for multirate systems. The solution to this problem is used to compare the performance of scanning sensors vs. stationary sensors in the estimation of cross‐directional properties. Furthermore, controller performance is examined when the outputs from the Kalman filter are used as inputs to a state feedback control law. Although adding sensors may significantly enhance the estimates of cross‐directional properties, feedback of these improved estimates may translate to lower levels of improvement in cross‐directional variations.
Characterization of structural inhomogeneities in porous media
Authors: Gladden Lynn F. , Hollewand Michael P. , Alexander Paul
Published: June 2004

Recent studies have demonstrated that spatial heterogeneities in voidage and pore size, over lengthscales of 0.1–1 mm, significantly affect the transport of liquids within a given porous medium. Nuclear magnetic resonance imaging is used to probe the structure of porous catalyst support pellets, and four image analysis techniques characterize the degree and nature of heterogeneities observed in the spin‐lattice relaxation‐time images of six pellets taken from the same batch. Power‐spectrum, cluster‐size, and percolation analysis reveal significant differences in spatial correlations and topological characteristics of the six images. The fractal dimension of all the images, however, is the same suggesting that this quantity characterizes the intrinsic heterogeneities associated with the particular batch of porous pellets studied in this work. The results suggest that structural models of such pellets should account for these macroscopic variations in pellet structure, as characterized here.