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  • Chemical Engineering  (17,980)
  • AERODYNAMICS  (12,790)
  • 101
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An application of a linear-programming based model-predictive control strategy to the paper-machine cross-direction (CD) control problem is presented. The objective of CD control is to maintain flat profiles of variables of interest by minimizing worst-case deviations from setpoints (defects). These control problems can have as many as 200 actuatiors (inputs) and 400 sensor measurements (outputs). This large size coupled with the stringent real-time requiremnt of computing a control move in a few seconds poses a very challenging control problem. The LP-based strategy is particularly well suited for solving such classes of control problems. This strategy has demonstrated its ability to solve large-scale control problems (over 100 inputs and 100 outputs) in real time and exhibits robustness to model uncertainty.
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  • 102
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    AIChE Journal 43 (1997), S. 1032-1040 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: For the simulation and optimization of large-scale chemical processes, the overall computing time is often dominated by the time needed to solve a large sparse system of linear equations. A new parallel frontal solver that can significantly reduce the wallclock time required to solve these linear equation systems using parallel/vector supercomputers is presented. The algorithm exploits both multiprocessing and vector processing by using a multilevel approach in which frontal elimination is used for the partial factorization of each front. Results of several large-scale process simulation and optimization problems are presented.
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  • 103
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    AIChE Journal 43 (1997), S. 1060-1068 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An application-oriented design procedure is presented for unique and point-stable operations of first-order reaction systems in a continuous stirred-tank reactor (CSTR). For a given set of values of kinetic constants, reaction enthalpy, feed conditions, residence time, and relevant physical properties, two boundary values of the heat-transfer capacity (St1, St3) and two of the modified coolant temperature (θmc, 2, θmc, 3) are analytically derived after a linearization of the unsteady mass and energy balances. With these boundary values, two separate design conditions are formulated; one for the heat-transfer capacity (HTC, characterized by St) and one for the modified coolant temperature (MCT, characterized by θmc). Each of these conditions is sufficient to guarantee unique and point-stable steady-state operations for a range of St or θmc values. Predicted behaviors of reacting systems are compared with experimental results obtained from five different systems reacting in four bench-scale and two commercial reactors.
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  • 104
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    AIChE Journal 43 (1997), S. 1041-1059 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A quantitative, model-based approach to the safety verification problem for general processing systems operating in the discrete time domain is presented. It is recognized that the operation of most of these systems involves both discrete and continuous characteristics. Therefore, an appropriate modeling framework is proposed, within which models of purely discrete, purely continuous and hybrid systems of arbitrary complexity can be constructed consistently. The models developed can then be incorporated into a safety verification formulation, which allows the identification of potential hazards that may occur while operating such systems, together with the combinations of events that lead to them. Apart from the dynamic process model, the data required for carrying out the analysis include the space of possible disturbances and the set of operating regimes that are considered to be unsafe or undesirable from the operability point of view. The formulation results in a mixed-integer optimization problem. A number of simple example problems presented illustrate the main ideas of the proposed technique, and the solution of an industrial-scale case study demonstrates its applicability.
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  • 105
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In this work, on-line calorimetry was used to estimate the conversion and the copolymer composition in emulsion polymerization systems. Real-time feedback control of the polymer composition during the semibatch emulsion copolymerization of vinyl acetate and butyl acrylate was carried out. Both homogeneous copolymers and copolymers with previous defined composition profiles were obtained. It was shown that the feedback control was able to avoid monomer accumulation in the reactor, and hence potentially dangerous thermal runaways, without any deleterious effect on the polymer composition, when a sudden inhibition was caused by deliberately adding a solution of hydroquinone. The use of feedback control for the maximization of the production rate under safe conditions in the unseeded emulsion copolymerization of vinyl acetate and VeoVa10 (an alkyl vinyl ester from Shell) is also presented.
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  • 106
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    AIChE Journal 43 (1997), S. 1082-1090 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Industrial practice requires immediate and adequate responses to simple questions asked. For example, does a catalyst particle show any tendency to thermal oscillations in the form of a limit cycle? The root to such cycles is found in the eigenvalues of the Jacobian matrix to the reaction rate vector, applying the concept of reaction invariance, a direct consequence of Avogadro's stoichiometric principle for homogeneous stirred tank reactors. However, this concept does not generally apply to heterogeneous reactor dynamics, because it is found in the heterogeneous transports of heat, reactants and products. The transport is an irreversible phenomenon that usually contributes to shifting the eigenvalues to the left and hence increases the stability. Still, effects of transport mechanisms, both internally and externally, on the catalyst particle are important to assess in such industrial analyses. A practical industrial reactor for methanol production was the subject for stability studies, which concluded that thermal oscillations are not likely to occur. During this study, a number of interesting details were examined such as rank deficiency of the reaction matrix and root loci for the temperature dependence of the Jacobian matrix eigenvalues. A practical consequence of eventual thermal cycling of the catalyst particles is a long-term degradation of the catalyst efficiency, as seen in the ammonia synthesis.
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  • 107
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    AIChE Journal 43 (1997), S. 1100-1107 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: High-pressure homogenization is a key unit operation used to disrupt cells containing intracellular bioproducts. Modeling and optimization of this unit are restrained by a lack of information on the flow conditions within a homogenizer valve. A numerical investigation of the impinging radial jet within a homogenizer valve is presented. Results for a laminar and turbulent (k - ∊ turbulent model) jet are obtained using the PHOENICS finite-volume code. Experimental measurement of the stagnation region width and correlation of the cell disruption efficiency with jet stagnation pressure both indicate that the impinging jet in the homogenizer system examined is likely to be laminar under normal operating conditions. Correlation of disruption data with laminar stagnation pressure provides a better description of experimental variability than existing correlations using total pressure drop or the grouping 1/Y2h2.
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  • 108
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    AIChE Journal 43 (1997), S. 1091-1099 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Two reversible reactions are involved in YBa2Cu3O6+x formation: a reaction between BaCO3 and CuO forming BaCuO2 and CO2, and a reaction of BaCuO2 with Y2O3 and CuO forming YBa2Cu3O6 which undergoes phase transformation to YBa2Cu3O6+x upon cooling. In-situ isothermal time resolved HT-XRD of a thin film was used to quantify the effect of CO2 on the kinetics of the first reaction. Increased CO2 partial pressure shifts the reactions to higher temperatures. At high CO2 partial pressure (〉2 vol. %), the rate of the first reaction becomes essentially a step process with a very high activation energy. Noninstantaneous nucleation of the reaction products occurs at low CO2 partial pressure (0.5-1%) and temperatures (700°C). The data fit a 2-D diffusion-controlled mechanism with a zero nucleation rate for BaCO3 decomposition and a second-order nucleation rate for YBa2Cu3O6 formation. A comparison of the kinetics of a thin film (10 mm) as determined by HT-XRD with those of a thick sample (2 mm) determined by TG revealed that the transport of CO2 within the sample pores and to the ambient gas significantly affect the decomposition of BaCO3. For example, the formation of YBa2Cu3O6 in a thick precursor layer occurs in the 840 to 940°C range, exceeding by about 200°C that in which it is formed in thin films.
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  • 109
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    AIChE Journal 43 (1997), S. 1108-1113 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The storage of natural gas in other light hydrocarbons is one procedure for automotive natural gas usage that reduces the requirement of high-pressure or cryogenic storage. Model solutions of methane in n-butane, propane, and a liquefied-bottled-gas mixture were simulated using the Benedict-Webb-Rubin-Starling equation of state to determine the pressures necessary to maintain a liquid phase with perturbations in both temperature (-1°C to 38°C) and mole fraction (50 to 80 mol% methane). Methane storage in these liquid solutions is between 45 and 93% of an equal “tank” volume of compressed natural gas (CNG) at 21 MPa and 15°C. The simulation results indicate that solutions of this type contain 40 to 67% of the energy of gasoline at pressures that range from 60 to 40% that of CNG at 21 MPa and 15°C.
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  • 110
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    AIChE Journal 43 (1997), S. 1114-1116 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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  • 111
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    AIChE Journal 43 (1997) 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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  • 112
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    AIChE Journal 43 (1997), S. 1117-1121 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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  • 113
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    AIChE Journal 43 (1997), S. 1125-1140 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Expression of the time-averaged equations for the conservation of momentum and energy in terms of the fractions of the local shear stress and heat flux density, respectively, due to turbulence is shown to result in greatly simplified representations for fully developed flow and convection in round tubes and parallel-plate channels as compared to those in terms of the eddy viscosity, eddy conductivity, or mixing length. On the other hand, the turbulent Prandtl number is shown to be a fundamental characteristic of flow and convection rather than simply an artifact of the eddy diffusional model. The new simplified representations provide the basis for improved predictions of the velocity distribution, the friction factor, the temperature distribution, and the Nusselt number. The adaption of these new representations for other geometries and for developing convection is discussed briefly.
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  • 114
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    AIChE Journal 43 (1997), S. 1153-1162 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The behavior of droplet swarms in liquid-liquid dispersions, especially found in extraction columns, are analyzed by a new fluorescence measurement technique, which represents an extension to the capillary suction method. The measuring principle developd benefits from detectable-emission light intensity of a fluorescent dye even in a trace concentration. The addition of this nontransferring dye to the dispersed phase enables in situ determination of bivariate drop-size/drop-concentration distributions, which provides important information on droplet history for the evaluation of coalescence and breakup rates. Furthermore, the measuring method represents a new approach for the measurment of droplet-size-dependent residence times in extraction columns.
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  • 115
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    AIChE Journal 43 (1997), S. 1141-1152 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A dynamic model has been developed to predict the transient behavior of the temperature, the heat removal rate by the in-bed heat exchanger, and the flue-gas oxygen concentration for a circulating fluidized-bed (CFB) combustor. The model was incorporated into a control simulator to reproduce the combustion process within the overall program. The simulator predicts the behavior of the combustor under manual or automatic control to allow testing of control strategies. The model is validated by comparison with step-response tests carried out on a pilot CFB combustor. Discrepancies are attributable to unmodeled disturbances. Further validation, necessary to ensure the applicability of the simulator to control development, is provided by comparing control models identified experimentally using the pilot CFB to those obtained by simulation. Favorable comparison suggests that the dynamic model is suitable for use in control simulation.
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  • 116
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    AIChE Journal 43 (1997), S. 1163-1170 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The hydrodynamic characteristics of a new type of reactor, the immobilized soil bioreactor, were studied. This apparatus is a practical new engineering concept (soil immobilization) based on entrapment of soil particles, which contain pollutant-degrading microorganisms, in the pores of a geotextile to activate the indigenous microorganisms. The soil immobilization is the third on the size scale of immobilization processes, coming after (1) that of molecules in heterogeneous catalysis (in Angstrom) and (2) that of microbial cells and their fragments in immobilized cells and enzymes biocatalysis (in micron). The size of immobilized soil particles is in the range of a millimeter. A mathematical model of liquid flow within the reactor is proposed, which qualitatively explains the distribution of the immobilized soil in space. The dynamics of soil immobilization within the bioreactor has been studied as a function of the particle size, initial slurry concentraion and air flow rate. A mathematical model of the process of soil immobilization was proposed based on deep filter mechanics. The process can be described by a second-order kinetic model. This study will be of great importance for the design of immobilized soil bioreactors for degradation of recalcitrant soil pollutants.
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  • 117
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    AIChE Journal 43 (1997), S. 1171-1179 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A new theoretical approach is proposed for the yield stress of concentrated, flocculated particulate suspensions. Explicit cognizance is taken for the three-dimensional, mechanically rigid particle network held together by interparticle forces wherein the mean coordination number in the assemblage increases and the separation between the particles decreases with an increase in the volume fraction of the solid phase. The Rump-Molerus model relting isotropic normal stress and isotropic normal interparticle force in a bed of single-sized spheres is modified to incorporate the size distribution of particles and extended to the suspension network. The model estimates the yield stress as a function of solids loading for various kinds of size distribution and is in reasonable agreement with experimental data when the surface properties of the particle are held constant.
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  • 118
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    AIChE Journal 43 (1997), S. 1189-1189 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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  • 119
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    AIChE Journal 43 (1997), S. 1180-1189 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Accurate detection of minimum liquid fluidization is essential to the successful operation of gas-liquid-solid fluidized beds, especially when particle or liquid properties evolve. A gas-liqid-solid system of 3-mm glass beads exhibits three distinct flow regimes as the liquid velocity is increased: compacted, agitated and fluidized-bed regmes. Measurements showed that the bed is not fluidized in the agitated bed regime. Pressure gradient and bed height measurements do not provide the minimum liquid fluidization velocity; instead, they offer the velocity between the compacted and agitated bed regimes. Time-averaged signals are not reliable for determining the minimum liquid fluidization velocity. It can be obtained from the standard deviation, the average frequency, the Hurst exponent and the V statistic of the cross-sectional average conductivity, which can be measured under many industrial conditions.
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  • 120
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    AIChE Journal 43 (1997), S. 1190-1193 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The collapse properties of three kinds of fine particles (19, 30, and 59 μm) are studied with a new type of bed-collapse technique, “isolated dilute-phase bed collapse,” to determine the dense-phase properties under high superficial gas velocities. The properties of the emulsion and bubble phases are investigated by a video camera. The influence of particle size on particle collapse properties is also discussed. When gas velocities are high the 30-μm particles take the longest to collapse, have the smallest emulsion density, and have largest bubble volumetric fraction. These observations indicate that the 30-μm particles have a property that the others lack.
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  • 121
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    AIChE Journal 43 (1997), S. 1194-1203 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Equilibrium partitioning of spherical solutes between slit pores and bulk solution is investigated by the Gibbs ensemble Monte Carlo method. Two types of perturbatins are performed in this simulation: a random displacement of solutes that ensures equilibrium within both bulk and pore regions, and random interchanges of solutes that equalize the interaction potentials between the two regions. To study the effects of electrostatic interactions, interaction energies between the solutes and pore walls and between pairs of solutes are evaluated by using a singularity method. Partition coefficients calculated for neutral solutes, which experience purely steric interactions, increase with increasing solute concentration and agree well with existing theoretical results. For pores and solutes of like charge, results for the limit of infinitely dilute solute concentration show a sharp decline in partition coefficient with decreasing ionic strength of solution. As the solute concentration increases, the interplay of solute-wall and solute-solute interactions becomes increasingly important, and the partition coefficient increases accordingly. The density profiles indicate unambiguously that, whether solutes and proes are uncharged or of like charge, solute-solute interaction promotes enhanced concentrations near the wall, causing the partition coefficient to increase. Even at solute concentrations as low as 5%, effects of solute-solute interactions caused by electrostatic charge can more than compensate for sphere-wall repulsive interactions, indicating that concentration effects should be considered at least as important as electrostatic effects in partitioning phenomena.
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  • 122
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    AIChE Journal 43 (1997), S. 1204-1226 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: This article addresses the problem of estimating the states of a nonlinear plant that operates in multiple regimes and makes transitions between them. It is often difficult to obtain a single nonlinear model that accurately describes the plant in all regimes. Even if a global model is available, it sometimes cannot be used conveniently in an estimator. An alternative approach is presented where local linear models are identified at each different operating point, and estimation is performed by tracking the transitions from one regime to another. This is done by first estimating the validity of the local models on-line and then constructing a time-varying global model by interpolating between the local linear models. State and parameter estimation is then performed using this global model in a moving horizon estimator. To demonstrate the effectiveness of this method, it is applied to three test systems: a simple numerical example, a CSTR, and a copolymerization reactor.
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  • 123
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    AIChE Journal 43 (1997), S. 1227-1241 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An application of wavelets and multiresolution analysis to density estimation and process monitoring is presented. Wavelet-based density-estimation techniques are developed as an alternative and superior method to other common density-estimation techniques. Also shown is the effectiveness of wavelet estimators when the observations are dependent. The resulting density estimators are then used in defining a normal operating region for the process under study so that any abnormal behavior by the process can be monitored. Results of applying these techniques to a typical multivariate chemical process are also presented.
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  • 124
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    AIChE Journal 43 (1997), S. 1242-1249 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Measurements such as flow rates from a chemical process violate conservation laws and other process constraints because they are contaminated by random errors and possibly gross errors such as process disturbances, leaks, departures from steady state, and biased instrumentation. Data reconcilation is aimed at estimating the true values of measured variables that are consistent with the constraints, at detecting gross errors, and at solving for unmeasured variables. An approach to constructing sequential principal-component tests for detecting and identifying persistent gross errors during data reconciliation by combining principal-component analysis and sequential analysis is presented. The tests detect gross errors as early as possible with fewer measuremennts. They were sharper in detecting and have a substantially greater power in correctly identifying gross errors than the currently used statistical tests in data reconciliation.
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  • 125
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    AIChE Journal 43 (1997), S. 1250-1264 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A systematic method that quantitatively assesses property prediction uncertainty (imprecision) on optimal molecular design problems is introduced. Propety-structure relations are described with specific nonlinear functionalities based on group contribution methods. Property prediction uncertainty is explicitly quantified by using multivariate probability density distributions to model the likelihood of different realizations of the group contribution parameters. Assuming stability of these probability distributions, a novel approach is introduced for transforming the original nonlinear stochastic formulation into a deterministic MINLP problem with linear binary and convex continuour parts with separability. The resulting convex MINLP formulation is solved to global optimality for molecular design problems involving many uncertain group contribution parameters. Results indicate the computtional tractability of the method and the profound effect that property prediction uncertainty may have in optimal molecular design. Specifically, trade-off curves between performance objectives, probabilities of meeting the objectives, and chances of satisfying design specifications offer a concise and systematic way to guide optimal molecular design in the face of property prediction uncertainty.
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  • 126
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    AIChE Journal 43 (1997), S. 1265-1277 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A compartmental model is used to study imperfect mixing and its effects on polymer properties. Examples of imperfect feed mixing caused by a rapidly decomposing initiator are shown for styrene polymerization, high-pressure ethylene polymerization, and vinyl acetate/methyl methacrylate solution copolymerization. Continuation analysis indicates mixing can change reactor steady states and stability. A composite approach is proposed to construct full molecular weight distribution (MWD) under imperfect mixing. In linear polymerization imperfect mixing broadens MWD; however, in nonlinear polymerization mixing can cause either broader or narrower MWD. By changing the dominant chaintermination mechanism to chain transfer, the influence of imperfect mixing on the MWD can be reduced. Depending on reactivity ratio and monomer composition, mixing also affects copolymer composition and chain sequence length.
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  • 127
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The low-temperature oxidation chemistry of linear and branched alkanes is discussed with the aim of unifying their complex behavior in various experimental systems using a single detailed kinetic model. New experimental data, obtained in a pressurized flow reactor, as well as in batch- and jet-stirred reactors, are useful for a better definition of the region of cool flames and negative temperature coefficient (NTC) for pure hydrocarbons from propane up to isooctane. Thermochemical oscillations and the NTC region of the reaction rate of the low-temperature oxidation of n-heptane and isooctane in a jet-stirred flow reactor are reproduced quite well by the model, not only in a qualitative way but in terms of the experimental frequencies and intensities of cool flames. Very good agreement is also observed for fuel conversion and intermediate-species formation. Irrespective of the experimental system, the same critical reaction steps always control these phenomena. The results contribute to the definition of a limited set of fundamental kinetic parameters that should be easily extended to model heavier alkanes.
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  • 128
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    AIChE Journal 43 (1997), S. 1287-1299 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The fugacity coefficients of the hydroperoxyl radical in supercritical water are estimated through molecular simulations. A potential function for the radical is first derived from ab initio self-consistent field molecular orbital calculations at the unrestricted Hartree-Fock level and from data in the literature. Molecular dynamics simulations of the hydroperoxyl radical are then performed in supercritical water and the fugacity coefficient of the radical is calculated by the free-energy perturbation method using the dynamic coupling parameter window-modification technique. The values of the fugacity coefficients at 773 K differ from unity. This methodology facilitates the incorporation of thermodynamic nonidealities in mechanism-based kinetic models for free-radical reactions in supercritical water.
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    AIChE Journal 43 (1997), S. 1300-1308 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Initiaton of incrustation by crystallization mechanisms is studied. An experimental technique was developed by which nucleation of a crystalline phase on a solid surface is investigated at controlled local supersaturation without the interference of particle deposition. Experiments were carried out with different substances and surface materials. The results show that the solid surface may catalyze primary nucleation and hence promote the initiation of incrustation, but this was recorded only at quite high supersaturation. Collision of crystals with the solid surface in a supersaturated solution may initiate incrustation at a reasonably low supersaturation and can be of substantial industrial importance.
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    AIChE Journal 43 (1997), S. 1309-1318 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The substitution reaction of hexachlorocyclotriphosphazene, N3P3Cl6, with phenol was performed to synthesize the partially substituted (phenoxy) chlorocyclotriphosphazenes, N3P3Cl6 - i(OC6H5)i, i = 1 - 6 by phase-transfer catalysis (PTC) in an organic phase/alkaline solution. The steric hindrance effect plays a crucial role in this reaction. The reaction system was controlled by both chemical kinetics and mass-transfer effects. The mass transfer of the catalyst between two phases was investigated by a pseudosteady-state liquid-liquid PTC (LLPTC) model. Also, the intrinsic reaction-rate constants of the series substitution and the overall mass-transfer coefficient of the catalyst from the organic phase to the aqueous phase were determined by a combined model. In addition, the corresponding energies, enthalpies, and entropies of activation of the series substitution were also estimated.
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    AIChE Journal 43 (1997), S. 1319-1324 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The Ergun equation based on the effective spherical diameter is universally valid for various shapes of granular packings. It, however, was derived from the assumption of infinite tube-to-particle diameter ratio without considering the wall effect. Although some improvements were made by Mehta and Hawley (1969) to correct this, the application of the Ergun equation is still restricted to cylindrical columns with a packing porosity of less than 0.5. To modify the Ergun equation to noncylindrical flow space with or without a wall, it was substituted into the empty tube pressure drop equation by introducing an effective tube diameter so that the pressure drop can be predicted just from the free flow space and the wetted area involved. This treatment offers the basis for a new method in velocity distribution prediction.
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    AIChE Journal 43 (1997), S. 1325-1330 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The effectiveness of applying silent discharge plasmas (SDP) for destroying and removing volatile organic compounds (VOCs) from gas streams is experimentally evaluated with a laboratory-scale reactor. The VOCs selected for study include toluene and methyl ethyl ketone (MEK). Direct collision with energetic electrons and reaction with generated gas-phase radicals are two major mechanisms responsible for destruction and removal of VOCs from gas streams. Operating parameters investigated include applied voltage, gas residence time, and temperature and composition of the gas stream. Experimental results indicate that the removal efficiency of toluene and MEK achieved with SDP can be enhanced by operating the system at a higher gas temperature and applied voltage due to the generation of more energetic electrons and radicals. O2 is essential for removing VOCs from gas streams with SDP. More than 80% removal efficiencies were achieved with this system for both toluene and MEK. SDP can potentially serve as an alternative control technology for removing VOCs from gas streams.
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  • 133
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: It is now well confirmed that the influence of temperature on the fall-off behavior of dissociation, recombination and chemically-activated reactions can be dramatic. For single-well, single-product dissociation reactions, it is customary to approximate these fall-off surfaces using extensions of Lindemann's empirical expression. We consider here chemical-activation and dissociation reactions possessing multiple wells and multiple products. We show that direct approximation of the rate coefficients via Chebyshev expansions yields reliable and accurate representations of their pressure and temperature dependences, which are superior to those from a Lidemann approach to fit the form factor representing the fall-off surface. The superiority of the method is demonstrated in a study of seven channels corresponding to four different reactions important in combustion chemistry over the ranges 300-3,000 K and 0.02-200 atm.
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    AIChE Journal 43 (1997), S. 1357-1361 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1341-1347 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Previous experimental studies have found that surface interactions significantly affect the transport of motile bacteria through small tubes, along solid surfaces, and through porous media. However, the role that hydrodynamic forces play in the interactions between solid surfaces and motile bacteria remains unclear. In this study, the swimming speeds of populations of Escherichia coli bacteria were measured near (〈 10 μm) and far (〉10 μm) from a flat glass surface at four ranges of orientations to the surface (0°-45°, 45°-90°, 90°-135°, and 135°-180°). Populations of bacteria close to the surface and moving in the orientation range most perpendicular (0-45°) to the surface experienced the greatest change in the swimming speed when compared to the population in the same orientation range located far from the surface. The decrease in swimming speed experienced by this population was on the same order as that predicted by hydrodynamic models of bacterial swimming near surfaces.
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    AIChE Journal 43 (1997), S. 1348-1356 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Commercially available biofiltration systems have used natural bioactive materials in packed beds due to low media cost and easy availability. Detailed understanding and modeling of biofiltration systems are lacking in existing literature. Experimental studies on the isopentane treatment in air using peat- and compost-packed beds were conducted with inlet isopentane concentrations of 360 to 960 ppmv, and empty-bed gas-phase residence times of 2 to 10 min. High removal efficiences (〉 90%) were achieved at low contaminant concentrations ( 〈 500 ppmv) and large empty-bed gas-phase residence times ( 〉 8 min). For both peat and compost biofilters, there was an “optimal” water content that gave the highest removal efficiency. For higher water content, mass transfer of isopentane through the liquid phase controlled the biofiltration removal efficiency. At low water content, irreversible changes in the bioactivity of peat and compost occurred, resulting in an irrecoverable loss of removal efficiency. Increases in biofilter bed temperature from 25 to 40°C improved the removal efficiency. A mathematical model incorporating the effect of water content and temperature was developed to describe the packed-bed biofilter performance. Model predictions agreed closely with experimental data.
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    AIChE Journal 43 (1997), S. 1362-1365 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1366-1368 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1369-1371 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1372-1374 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
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    AIChE Journal 43 (1997) 
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    Keywords: Chemistry ; Chemical Engineering
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    AIChE Journal 43 (1997), S. 1377-1389 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Network models are an effective means of incorporating pore-scale heterogeneity into flow models of porous materials. The drawback to these models used to be the inability to obtain quantitative macroscopic parameters representing larger (experimental-scale) media. However, recently developed modeling techniques, combined with more widely available computational resources, make the simulation of macroscopic parameters from a network approach viable. A network model for the slow flow of an incompressible fluid in disordered packed beds is presented. Fundamental fluid mechanics equations are solved at the pore scale and then translated to macroscopic behaviour using a network approach. The results reproduce experimental permeabilities and show excellent quantitative fits to residence time distributions for mechanical dispersion in real beds. Simulations of the RTD are of special interest, because they are definitive links between pore-scale flow behavior and macroscopic responses.
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    AIChE Journal 43 (1997), S. 1390-1402 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Liquid circulation in bubble columns influences many system properties such as mass transfer and catalyst dispersion. Conditions favorable for liquid circulation often exist only for highly turbulent slurries. Experiments under conditions of lowered surface tension proved conclusively that uniform circulation can exist even in the bubbly-flow regime. Moreover, the measured circulating velocity profile is more plug-shaped in the center core than traditional models, which take a parabolic shape. These observations led to the application of a circulation theory based on an energy-dissipation model for turbulent eddy viscosity, which can be easily computed as a simple function of superficial-gas velocity. Two length scales were used in model calculations depending on flow regime: bubble diameter for bubbly flow and column diameter for churn turbulence. The liquid-velocity profile contained a tuned dimensionless proportionality constant, which resulted in two different average values, k = 2.51 for bubbly flow and 0.0242 for churn turbulence, when tuned to experimental data. The integral average eddy viscosity is numerically equivalent to the eddy dispersion coefficient for bubbly flow. Predicted liquid velocity compared very favorably with new measured values under conditions of low surface tension and with literature values.
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    AIChE Journal 43 (1997), S. 1414-1425 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A computed tomographic scanner was developed for imaging gas-holdup distributions in two-phase flow systems such as bubble columns and fluidized beds. The scanner has been used to study the effects of various operating parameters (such as column diameter, superficial gas velocity, and distributor type) on the gas holdup and its distribution in an air-water bubble column. The experimental investigation shows that the column dimensions have no significant effect on the void fraction when the column diameter is greater than 0.15 m. Differences in the holdup distribution due to the kind of distributor used are significant only at low gas flow rates. Surface tension of the liquid has a profound influence on the gas holdup and its distribution.
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    AIChE Journal 43 (1997), S. 1403-1413 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Industrial design of electrostatic precipitators is based on the transport theory developed by Deutsch (1922), which assumes that transverse turbulent mixing is effective enough to maintain the concentration profile uniform throughout the cross section (i.e., turbulent diffusivity is assumed infinite). To improve understanding of turbulent particle dispersion under the influence of electrostatic forces, a database on particle trajectories was first generated, based on the flow field from a direct numerical simulation of a plate-plate precipitator (Soldati et al., 1993). The effect of various parameters, such as particle size, charge and particle migration velocity, on dispersion and collection efficiency was investigated. Results show that particle concentration profiles are not uniform due to finite values of “turbulent diffusion” coefficient. The simulations indicate that the early stages of particle collection are controlled by particle migration velocity, while final stages are controlled by turbulence diffusion mechanisms.
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    AIChE Journal 43 (1997), S. 1426-1435 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The turbulence structure of the liquid phase near a wavy gas-liquid interface in stratified flow was experimentally investigated in a 50 mm (H) × 100 mm (W) rectangular duct. The characteristic parameters of the organized motion under the liquid waves such as the frequency of appearance and length scale were estimated by using the variable-interval time-averaging and the photochromic dye activation techniques. These characteristics were used in a hybrid surface renewal-eddy cell model to predict the interfacial heat- and mass-transfer coefficients in stratified two-phase flow. The predictions for cocurrent and countercurrent flows agreed reasonably well with experimental data available in the literature.
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    AIChE Journal 43 (1997), S. 1436-1447 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Droplet breakup in laminar flows is important in emulsification processes and polymer blending. The influence of surfactants on droplet deformation and breakup in a plane hyperbolic flow was studied experimentally in an opposed-stream device. As in the case of simple shear flow, the inhomogeneity of the surfactant distribution along the droplet interface has a pronounced effect. Our results are qualitatively consistent with other numerical studies for droplet breakup in an axisymmetric elongational flow. Also a striking similarity is noted with other experimental observations for the deformation and breakup of polymeric drops in a quasi-steady-plane hyperbolic flow. The critical capillary number for droplet breakup in the experiments correlates with the interfacial viscoelasticity, and a reformulation of the numerical framework in terms of interfacial elasticity parameters is suggested for future numerical work including other linear flows.
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    AIChE Journal 43 (1997), S. 1448-1457 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A revised machine learing methodology is applied to a simulated crystallization process flowsheet for continual imporvement of its performance by generating and analyzing process data. The aim is to identify bands of crucial decision variables leading to zones of best average process performance. The methodology comprises two components: symbolic induction and case-based reasoning. It uses an incremental algorithm to update performance classification rules, which not only improves the efficiency of the symbolic induction of the classification rules by eliminating the need for their periodic reinduction, but also simplifies the case-based reasoning step. The new concepts and procedures are illustrated by application to a potassium nitrate crystallization process comprising a mixed-suspension, mixed-product-removal crystallizer, a hydrocyclone, and a fines dissolver, By identifying and establishing ranges for the most crucial decision variables, namely, feed concentration, flow rate, and cooling stream temperature, three zones leading to an improvement of nearly 12% on nominal average performance are detected within two generations of the classification rules.
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A cold model of a circulating fluidized bed with a 0.030-m-ID, 2.77-m-high riser was operated in a wide range of operating conditions. Several solids were tested, from 57 μm to 1,830 μm in size and from 1,100 kg/m3 to 2,540 kg/m3 in density. Pressure fluctuations were measured at several points along the riser, and time series were processed to evaluate chaotic invariants (Kolmogorov entropy and correlation dimesion). Axial profiles of average values of pressure and voidage were also evaluated. At fixed operating conditions, the Kolmogorov entropy changed along the riser, which appeared to be a fuction of the local voidage and showed a minimum when voidage decreases from 1.00 until about 0.90. Changes of the Kolmogorov entropy with local voidage were interpreted based on interactions among solids and gas turbulence structure. Three regions were identified in the voidage range investigated: particles-controlled region, clusters-controlled region, and bottom-bed-controlled region.
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    AIChE Journal 43 (1997), S. 1469-1479 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Fast circulating fluidized-bed reactors are used for many applications, both in riser and downer modes. Typical resisdence times are in the order of a few hundreds of milliseconds. A good understanding of initial gas-solids contact must be reached to realize the full potential of such reactors. Effective and stable gas-solids mixing chambers were identified from the singals of simple and robust probes, which could be used in industrial and pilot plants. Criteria based on cross correlation or Hurst analysis provided consistent results. Although this study used simplified momentum probes, the criteria it developed can be applied to the signal of any fast-response sensor which is affected by local hydrodynamics. Temperature, capacitance, ultrasonic, or γ-ray absorption measurements could be used. Gas or liquid-liquid mixing chambers could also be optimized with the same criteria.
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    AIChE Journal 43 (1997), S. 1480-1487 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: This study examines the selective agglomeration of hydrophobic toner particles from a repulped paper fiber slurry by adding an immiscible oil that preferentially wets the toner particles. Effects of cationic starch additives, agitation rate, time, temperature and oil composition are examined experimentally in terms of the dynamic and steady-state evolution of the particle-size distribution produced. A simple, but effective, population balance model is solved numerically to simulate the agglomeration process and provides quantitative relationships between process variables and the effectiveness and rate of agglomeration. These relations are simple and clear. The process is shown to be controlled by the composite effect of the aforementioned variables on the aggregation and breakup rates, and the observed hehavior is understood in these terms. Practical recommendations for agglomeration, a process potentially useful for deinking toner-printed paper, are deduced from the results.
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    AIChE Journal 43 (1997), S. 1488-1493 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Filtration experiments were performed to dewater bentonite slurries at different filtration pressures. The diffusivity of the cake was then determined as a function of void ratio using the measured filtration rates, the final void ratios of the cakes, and the theory of compressible filtercakes. Any filtration process can, in theory, be predicted, once the cake diffusivity is known as a function of void ratio and the void ratio as a function of applied pressure. In particular, the final stage of the filtration process can be modeled and offers an alternative route to determine the cake properties from the experimental results. The final slow approach to equilibrium is difficult to study experimentally; other techniques to determine cake properties are therefore preferable. Final stages of the cake compaction are also markedly slower than predictions based on the intial filtration rates. Possible explanations include a filtercake rheology that is more complicated than assumed in the model or time-dependent effects caused by exclusion of ions from the bentonite filtercake, but such explanations appear unsatisfactory. Filtration models in which the filtercake void ratio and permeability depend solely on the imposed stress, may therefore be too simple for some materials.
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    AIChE Journal 43 (1997), S. 1494-1510 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The prototype of an integrated hazard-analysis system (IHAS) was developed. Essentially any process can be analyzed with this software if the system topology is correctly supplied by the user. Since all altorithms adopted are digraph-based, the system digraph must be constructed first with IHAS. The embedded feedforward and feedback loops are then identified and classified. On the basis of this information, three widely accepted hazard-assessment procedures - FTA, ETA, and HAZOP - can be performed automatically. From the results obtained in practical applications, one can see that the quality. From the results obtained in practical applications, one can see that the quality of hazard analysis is indeed improved if IHAS can be used as an aid to the human experts.
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    AIChE Journal 43 (1997), S. 1511-1518 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The problem of analyzing the stability of chemical process plants is considered. The idea of exchange plants has been extended with more general binary transfer terms and binary equilibrium, resulting in the idea of binary exchange-equilibrium plants. Their structural stability is analyzed using results on conservation matrices. It was found that binary exchange-equilibrium plants with no azeotropes are stable irrespective of the connections of their operating units. The general results are illustrated in examples of practical importance: heat-exchanger networks and binary distillation columns with constant molar flows.
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    AIChE Journal 43 (1997), S. 1529-1544 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Almost-everywhere singular (AES) distributions, usually referred to as multifractal measures, provide an intermediate link between atomic distributions (distributions represented by a countable superposition of Dirac's delta terms) and smooth regular distributions. This article shows how AES distributions can be rigorously treated in connection with distributed-parameter models and presents closed-form expressions and/or recursive, uniformly converging approximation methods for integral transforms (Laplace and Stieltjes). In particular, exact results are obtained and discussed for linear and uniform nonlinear kinetcis and for transport schemes in the presence of continuous mixtures. The physical origin of AES distributions in real systems is also detailed.
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    AIChE Journal 43 (1997), S. 1519-1528 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Back-and-forth-moving temperature fronts separating regions with high and low temperatures (amplitudes up to 100 K) formed on a Pd on alumina ring during the atmospheric oxidation of carbon monoxide. The back-and-forth front movement reflects the intrinsic nonuniformity of the catalytic system and the impact of global coupling. The ignition and extinction fronts usually moved at different velocities, in agreement with theoretical predictions. Complex spatiotemporal patterns were observed due to small-amplitude temperature fronts that moved on the high-temperature region at a much higher velocity than the high-amplitude fronts. The qualitative motion features were affected by changes in the excess oxygen concentration. The nonuniform states existed mainly for feed conditions for which the only uniform state was an extinguished one.
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    AIChE Journal 43 (1997), S. 1545-1550 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Homogeneous thermal cracking of ethane, which is an endothermic and hence energy-intensive process, was carried out in the presence of limited O2 under different process conditions (at 600°-850°C; O2/ethane ratio of 0-0.2; H2O/ethane ratio of 0-3.3; and space velocity of 2,000-11,000 h-1). The influence of a sulfur additive in the feed and reactor material on the process performance was also investigated. Noncatalytic oxycarcking of ethane in the presence of limited O2, greatly enhances not only the total conversion of ethane but also the conversion of ethane by purely thermal cracking. This process also occurs at a much lower contact time than that required to achieve the same conversion for the thermal cracking process. Also, both the exothermic oxidative conversion and the endothermic thermal cracking of ethane occur simultaneously, which makes this process very energy-efficeint, with a drastic reduction in external energy requirement and coke formation. By manipulating the process conditions, particularly the temperature and O2/C2H6 ratio, the overall process can be made almost thermoneutral, mildly exothermic, or mildly endothermic.
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    Keywords: Chemistry ; Chemical Engineering
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The recovery of the acidic structure and activity during the regeneration step by coke combustion of an HZSM-5 zeolite-based catalyst used in the methanol-to-gasoline (MTG) process in reaction-regeneration cycles was studied. The reactivation kinetics (activity us. coke combustion time) was determined as a function of temperature and time on stream of the reaction step. The validity of the reactivation kinetic model was proven by using it for the simulation of the cyclic operation under conditions of catalyst partial reactivation, together with the kinetic model for the MTG process.
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  • 159
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    AIChE Journal 43 (1997), S. 1559-1568 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Thermochemical data of mineral oil fractions are scarce. Due to the complex structure of natural hydrocarbon mixture, standard heat and Gibbs free energy of formation depend on the sample. A procedure for predicting these properties is presented. Both properties depend on the hydrocarbon type and on its molecular weight. The double-bond equivalent (DBE) and the number of olefinic double bounds are the important characterization parameters of the model. DBE can basically be calculated from molecular weight and elemental composition. Alternatively, boiling point and atomic H/C ratio can be used if the model is applied to mineral oil fractions from which boiling curves can be obtained. A criterion for testing the reliability of the estimated DBE value is available, because DBE is also a function of structural group distribution.The relevance of the DBE concept is shown with regard to hydroprocessing of mineral oil fractions. The model can be applied to total average molecules as well as to an arbitrary number of pseudocomponents and continuous mixtures. The difference between feed and product DBE is equivalent to the amount of hydrogen consumed for hydrogenation of unsaturated hydrocarbons. Further properties of the product can be predicted.
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    AIChE Journal 43 (1997), S. 1579-1588 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Parameters of wetting dry fibers with non-Newtonian liquids are studied experimentally. Rheological tests show that seven of the 13 test liquids adhere to Newtonian behavior with a wide range of viscosities 3 〈 μ 〈 30,000 mPa·s. The other six liquids show shear-thinning behavior with different flow indices. Some of the liquids exhibited viscoelastic behavior. Diameters of the fibers are in the range 0.033 〈 D 〈 0.071 cm. Experimental measurements cover contact angles, interfacial depths and diameters, as well as the critical wetting speed of entraining visible small air bubbles in the liquid volume. Dynamic contact angles are strong functions of the apparent viscosity of the liquid at the particular wetting speed regardless of whether the liquid is Newtonian or non-Newtonian. Higher values of contact angles correspond to wetting of fiber with larger diameter for less viscous liquid are generally higher than those for Newtonian liquids. Depressions created in the surface of different test liquids can be classified into three different scales, corresponding to three scales of liquid viscosities.
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    AIChE Journal 43 (1997), S. 1569-1578 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: For the first time, a model for dynamic adsorption and tension of diffusion-controlled systems has been extended to binary premicellar mixtures of nonionic surfactants of different adsorption capacities and nonideal interactions in the monolayer. Local equilibria between subsurface concentrations and adsorbate densitities are modeled using the nonideal adsorbed solution theory, which describes these two elements. The model shows that larger molecules (those with the smaller adsorption capacity) tend to be preferentially adsorbed at low times, if they have equal adsorption equilibrium constants and diffusivities, and smaller moleules at longer times. This adsorption selectivity is reduced when larger molecules have a much larger adsorption equilibrium constant, or when there are negative deviations from ideality in the monolayer. This model's predictions are compared to tension data for two nonionic surfactants, C12E5 and Triton X-100, at 25°C. The data are represented well by the diffusion-controlled model with a finite diffusion-layer thickness, which describes the faster decrease in tension observed with the bubble surfactometer, compared to data with other techniques. With this model, surface coverages and concentration profiles are calculated, thus elucidating the adsorption selectivity of molecules of different adsorption capacities. Synergistic effect in dynamic tension and adsorption can be predicted. Mild synergism in dynamic tension lowering by the preceding nonionic surfactants is also observed.
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    AIChE Journal 43 (1997), S. 1589-1596 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: To reduce the number of adjustable parameters that appear in the thermodynamic modeling of hydrogen-bonding fluids, the use of ab-initio molecular orbital calculations in the form of Hartree-Fock and density functional theories to compute thermodynamic properties are considered. In particular, the enthalpy and entropy changes on hydrogen bonding for water are computed. The theory, the computational methods and the basis sets used, as well as the results for water, are discussed. The results obtained are compared with those of others and with experimental estimates from the literature to determine the validity of the methods that are then used in this extended study. The results obtained here are also used to examine the validity of mixture group contribution results obtained here are also used to examine the validity of mixture group contribution methods for hydrogen-bonding mixtures, to develop simple estimation procedures for model parameters, and to reduce the number of calculations such as those as those described that need to be done.
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    AIChE Journal 43 (1997), S. 1597-1604 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Hartree-Fock theory and density functional theory were used to compute the enthalpy and entropy changes of dimerization for a number of hydrogen-bonding compounds. In Part 1, the calculational methods and procedures use for the water dimer are described, and the results obtained are compared with those of others and with experimental estimates in the literature. Here, a variety of organic compounds that can self associate and/or cross associate are considered. The results obtained for the self-association of these compounds are compared with estimates obtained from various types of experimental data. The results are also used to examine the validity of group-contribution methods for hydrogen-bonding mixtures and to test a simple estimation procedure for cross dimerization that can reduce the number of calculations, such as those described here, that need to be done.
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    AIChE Journal 43 (1997), S. 1605-1610 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A method is presented for a relatively accurate, noniterative, computationally efficient calculation of high-pressure fluid-mixture equations of state, especially targeted to gas turbines and rocket engines. Pressures above 1 bar and temperatures above 100 K are addressed. The method is based on curve fitting an effective reference state relative to departure functions formed using the Peng-Robinson cubic state equation. Fit parameters for H2, O2, N2, propane, methane, n-heptane, and methanol are given.
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    AIChE Journal 43 (1997), S. 1611-1626 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A comprehensive composite database for distillation sieve-tray efficiency is used to develop point efficiency and entrainment correlations based on a model that considers the fluid on the distillation tray to be contained in a liquid-continuous region near the tray deck and a vapor-continuous region on top of the liquid-continuous region. This model allows estimates of the portion of the mass transfer that occurs in each region and the mass-transfer resistance that occurs on the liquid side and vapor side of the interface. For most cases, most of the mass transfer occurs within the liquid-continuous region. The liquid side resistance is often significant.The entrainment correlation is consistent with the work of Bennett et al., which relates entrainment to the ratios of the liquid to vapor density and the froth height to the tray spacing. A simple liquid continuous-only mass-transfer model containing only four empirical parameters correlates the point efficiency data to within 6.4%. Despite a twofold change in vapor Schmidt number, no dependency on vapor Schmidt number is seen. Important dimensionless groupings are the Reynolds number based on the hole velocity, effective froth density, ratio of the liquid inventory to the perforation diameter, and fraction of the tray area perforated. Mathematically simple and accurate methods allow the prediction of the section efficiency for trays operating in cross or parallel flow. They address vapor and liquid mixing, entrainment and a criterion to avoid significant degradation of the tray efficiency due to weeping.
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    AIChE Journal 43 (1997), S. 1641-1648 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1627-1636 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Supercritical water oxidation of isopropyl alcohol was investigated in a pilot-scale reactor. A computational fluid-dynamics model developed reveals the detailed flow field, chemical-component distribution, temperature distribution, and salt-particle trajectories in the reactor flow domain. The near-wall fluid temperature from the numerical analysis was compared with experimental temperature data. The temperature comparison was within a 3% error band. The effect of the chemical kinetic rate was investigated for four different rates. Turbulent salt-particle trajectories were also calculated to investigate the effect of particle sizes on salt deposit on the wall. Also, a method of calculating the adiabatic reaction temperature was developed to estimate reaction temperatures prior to a full numerical simulation.
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    AIChE Journal 43 (1997), S. 1637-1640 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1649-1651 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997) 
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    AIChE Journal 43 (1997), S. 1653-1661 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Pulsed magnetic field gradient stimulated echo NMR measurements are performed for diffusion and flow of an aqueous phase both within a sample of packed spherical beads and within a 25% porosity Fontainebleau sandstone. The stimulated echo dependence on the gradient pulse area q is used to derive the displacement probability distributions PΔ(X) for fixed observation times Δ. The shape of PΔ(X) as a function of Δ is simulated for computer-generated porous media, and a good agreement is obtained between the experimental NMR data and the simulations.
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    AIChE Journal 43 (1997), S. 1662-1674 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Microwave thawing of materials, which melt over a finite temperature range, are analyzed using the effective heat capacity method. The state of the material, solid, liquid or mush is deduced from the equilibrium liquid volume fraction vs. temperature relationship for the substance, and the microwave power is calculated from Maxwell's equations. Using Galerkin finite elements, the microwave power, temperature and liquid volume fractions were obtained for microwave thawing of tylose slabs. Thawing progresses from the inside and/or outside depending on the slab thickness. For slabs ≥ 5 cm, thawing progresses predominantly from the surface of the sample. Resonances, during which the microwave power absorption is high, causes the 2-cm slab to thaw quicker than 1-cm slabs. A power law for the thawing time vs. sample thickness yields an exponent of 1.56 for microwave thawing and 2 for conventional thawing. To control the temperature rise in the liquid regions, simulations were carried out with an on-off control on the microwave power. On-off control results in greater power savings for thick samples.
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    AIChE Journal 43 (1997), S. 1684-1690 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In this study back-propagation, feed-forward neural networks are applied to estimate mass-transfer parameters in fast fluidized beds of fine solids. These networks are trained to predict mass-transfer rates using measurements of the sublimation rate of coarse naphthalene balls in fast fluidized beds of fine glass beads at several solid-to-gas mass flow rates within the relevant superficial gas-velocity range. When tested to predict the effective diffusivities from a coarse particle to the bulk of the fast bed of fine solids, trained neural networks calculated the Sherwood number with high accuracy. It is demonstrated that back-propagation, feed-forward neural networks provide a more accurate correlation for the mass-transfer coefficient compared to those obtained by the currently used heuristic models.
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    AIChE Journal 43 (1997), S. 1675-1683 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In gas-liquid flows through tee junctions, because of the difference in inertia between the phases, the flowing mass fractions for an individual phase at the outlet sides differ from those at the inlet. This phase separation is an important consideration in delivering fluids and energy through pipe networks in the power and process industry. In this work, air-water and steam-water flows through branching and impacting tee junctions are considered. Under certain conditions (when the volumetric fraction of the liquid drops does not change appreciably and their bulk density is much higher than the gas density, and when the flow rates and/or the gas volumetric fraction are high), these flows can be approximated as irrotational flows of incompressible, inviscid fluids and are amenable to potential flow methods, for example, conformal mapping. For the general case of gas-liquid flows through a branching or impacting tee, a CFD code is utilized to conduct transient flow simulations. Predictions of phase separation for both types of tee junction agree well with experimental data.
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    AIChE Journal 43 (1997), S. 1691-1699 
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    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Pore fractal objects are expected to be optimal catalysts, since material is supplied to the narrower pores, which are also shorter through the larger pores where diffusion resistance is smaller. To demonstrate this, diffusion and reaction were simulated on Sierpinski-gasket-type fractal objects and on the corresponding nonfractal uniform-pore structures of the same size, porosity and reactive area. Positive order reactions limited by Knudsen diffusion were shown to exhibit larger rates in fractal than in uniform-pore objects. Fractal catalysts also exhibited a new intermediate domain in which the rate depends only weakly on the kinetics parameters. In nonmonotomic kinetics the branching point (bifurcation point) was extremely sensitive to the pore structure.
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    AIChE Journal 43 (1997), S. 1700-1708 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The shear and compressive properties of aggregated alumina particles are determined as functions of volume fraction and the strength of the interparticle attraction. Over a range of volume fractions, yield stresses, τy, elastic moduli, the strain delimiting the extent of the linear elastic response, and compressive yield stress, Py, are well described by power-law functions of volume fraction, while the role of interparticle attractions can be accounted for by expressing these mechanical properties as (φ/φg - 1)n, where φg captures the strength of particle attractions and n the microstructure. The links between compressive and shear properties are well described by linear elastic models where the Py and τy are a function of Poisson's ratio which, for the suspensions investigated, has a value near 0.49.
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    AIChE Journal 43 (1997), S. 1709-1716 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An electrochemically modulated complexation (EMC) process was designed to selectively extract and concentrate ethylene from a feed-phase mixture with ethane. Unlike previous EMC processes, where the separation was based on equilibrium solubility differences, this separation was based on differences in the rate of complexing with copper(I) solution between the two gases. The EMC process employed flowthrough electrolysis cells that electrochemically modulate aqueous-phase copper chlorides between high, copper(I), and low, copper(II), affinity states. The liquid phase in the high affinity form contracts a feed gas phase in a hollow-fiber membrane module for selective extraction and contacts the receiving gas phase in the low affinity form for concentration. Experimental results and modeling of the process are reported.
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    AIChE Journal 43 (1997), S. 1717-1726 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Cylindrical internal reflectance infrared spectroscopy (CIR-FTIR) was used for the determination of the transport properties of hydrocarbons in zeolite L at high temperatures and pressures. This was the first time that the intracrystalline diffusivity of hydrocarbons in microporous media could be determined under supercritical conditions by using a dynamic technique. The study investigated the self-diffusion of n-heptane, the counterdiffusion of n-hexane and 1-hexene, and the codiffusion of n-heptane and pyxlene in zeolite L under subcritical and supercritical conditions. The diffusion time constants obtained for the single and binary systems showed that the diffusion within the pore was reduced significantly under supercritical conditions (i.e., liquidlike behavior) as the hydrocarbon density within the pore was increased.
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    AIChE Journal 43 (1997), S. 1727-1736 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The magnitude and direction of the ultrasonic radiation forces that act on individual particles in a standing-wave field were determined using a microscope-based imaging system. The forces are calculated from measured particle velocities assuming that the drag force, given by Stokes' law, is exactly counterbalanced by the imposed ultrasonic forces. The axial primary radiation force was found to vary sinusoidally with axial position and to be proportional to the local acoustic energy density, as predicted by theory. The magnitude of the transverse primary force was determined by two independent methods to be about 100-fold weaker than the axial force. Separation concepts exploiting the transverse force for cell retention have been successful despite the great disparity in magnitude between the axial and transvers-force components. This may be explained by the reduced hydrodynamic forces on aggregated particles in transverse flow due to their alignment in the sound field.
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    AIChE Journal 43 (1997), S. 1737-1750 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A systematic method is presented to synthesize reactive crystallization processes. It shows how to selectively crystallize a desired solid product(s) after a reaction step and how to use compound formation to effect separation of a mixture. The method is based on the generation of phase diagrams with liquid-phase reactions. Using transformed coordinates, systems with three or fewer degrees of freedom can be conveniently analyzed, regardless of the number of components and reactions. Features of the solid-liquid phase diagram that are relevant to process synthesis are identified. The method is illustrated with systems with multiple components, complex reactions, and multiple phases.
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    AIChE Journal 43 (1997), S. 1751-1762 
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    Keywords: Chemistry ; Chemical Engineering
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A systematic method to synthesize crystallization-distillation hybrid separation processes is presented. Two classes of hybrids are identified for binary mixtures. The first bypasses azeotropes and tangent pinches, while the second bypasses eutectics. Guidelines for flow-sheet selection are proposed based on an analysis of simple eutectic, constant relative volatility systems. In addition, the hybrids are compared to both extractive and adductive crystallization in order to determine the conditions under which solvent-based crystallization techniques outperform the proposed hybrid configurations. The method is extended to ternary mixture. The use of stream combination and complex distillation columns is also considered.
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    AIChE Journal 43 (1997), S. 1763-1784 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
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    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A nonlinear model-predictive control scheme based on the autoregressive-plus Volterra model is proposed. A “plant-friendly,” yet persistently exciting, input sequence is designed for model identification. A second feature of this work is that the control action is determined by solving a less computationally burdensome nonlinear programming problem than the optimization problems associated with Newton-type controllers and polynomial ARMA model nonlinear MPC. A third contribution is that semiglobal closed-loop stability conditions are derived and are shown to be less conservative than those previously published. The control scheme is shown to outperform a feedback strategy using proportional integral control and a linear model-predictive control design for the control of two polymerization reactor case studies. The complex features of the latter case study [multivarible (3 × 3), recycle loop, high-order (24) dynamics] motivate the applicability of this approach for industrial problems.
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    AIChE Journal 43 (1997), S. 1785-1796 
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    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A novel autoassociative neural-network-based estimator for nonlinear steady-state data reconciliation was developed, which is a modified autoassociative feedforward neural network. The main difference between them lies in the minimization of an objective function that includes material imbalance terms of flow rates and compositions as well as the traditional least-square prediction term. Accordingly, this neural network, with the material balance-related equations included in the objective criterion, can perform simultaneously the following basic functions necessary for proper steady-state data rectification: (1) eliminate the nonrandom errors, such as the biases and gross erros, from measurements; (2) filter out the random errors from measured data; and (3) estimate the values of unmeasured process variables, provided data redundancy prevails. This novel neural-network-based reconciliation method is demonstrated on a flotation system.
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    AIChE Journal 43 (1997), S. 1797-1812 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: ZSM-5 zeolite membranes with reproducible properties were prepared by in-situ synthesis on porous α- and γ-alumina tubular supports and characterized by XRD, SEM and electron microprobe analysis. Single-gas permeances for H2, CH4, N2, CO2, n-butane, and i-butane increase over some temperature range, but some gases exhibit maxima or minima. The highest ideal selectivities at room temperature are 299 for N2/SF6, 392 for H2/n-butane, and 2,820 for H2/i-butane. These membranes can separate n-butane/i-butane, H2/n-butane and H2/i-butane mixtures. All n-butane/i-butane separation selectivities have maxima as a function of temperature and are higher than ideal selectivities because n-butane inhibits i-butane permeation. Thus, separation is not by size selectivity, but is due to pore blocking. Temperature depenencies of single-gas permeances and separation selectivities depend strongly on the location of zeolite crystals and the location is determined by preparation procedure. Ideal selectivities also depend strongly on the preparation procedure. When the zeolite forms a continuous layer on the inside surface of the support tubes, pure i-butane permeates faster than pure n-butane so that the single-gas permeances are not determined just by molecular size. The i-butane permeance also increases much more with temperature than the n-butane permeance. The permeation behavior may be the result of permeation through nonzeolitic pores in parallel with zeolite pores. When zeolite crystals are dispersed throughout the pores of α-alumina supports, permeances are lower and gas permeation and separation properties are quite different. Ideal selectivities are lower, pure n-butane permeates faster than i-butane, and the permeances increase much less with temperature. Separation selectivities are lower but can be maintained to higher temperatures.
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    AIChE Journal 43 (1997), S. 1813-1818 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The catalytic hydrogenation of alpha-methylstyrene to cumene is studied in a laboratory trickle-bed reactor operated at low liquid flow rates. Under this condition, wetting is not complete, vaporization of the liquid phase may occur, and reaction could proceed via liquid-solid and gas-solid catalysis, which increase the hydrogenation rate significantly. To determine and quantify the effect of the gas-solid catalyzed reaction, we propose the use of post-packing sections of increasing lengths. The inert bed acts as an absorber of the gas-phase-produced cumene. The liquid and gas effluent were monitored. Experimental global rates determined from liquid cumene concentrations varied with the post-packing length. A simple one-dimensional model developed shows that theoretical and experimental results agree well.
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    AIChE Journal 43 (1997), S. 1819-1827 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In a high-temperature catalytic membrane reactor, a plug-flow reactor is combined with a gas-separative membrane. By selectively removing one of the reaction products, the reaction mixture is prevented from reaching equilibrium, and a higher conversion can be obtained. This concept is only valid for reactions that are limited by the thermodynamic equilibrium, such as the direct dehydrogenation of propane to propene. A tubular H2-selective silica membrane was characterized [αH2/C3H8 = 70-90 at 500°C] and used as the gas-separative membrane. The membrane reactor was filled with a chromia/alumina catalyst. The kinetics of the catalyst was studied. At 500°C the deactivation of the catalyst is slow, and the propene yield is almost constant for at least 10 h of operation. Under well-chosen process conditions, the propene yield is at least twice as high as the value obtained at thermodynamic equilibrium in a conventional reactor.
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    AIChE Journal 43 (1997), S. 1828-1837 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: One of the challenges in building reaction mechanisms using algorithms for automated model construction is to describe the essential chemistry and enable prediction of experimental data over wide ranges of reaction conditions while maintaining a manageable model size. Two complementary methodologies for building compact reaction mechanisms were developed and combined with existing algorithms based on graph theory and bond-electron matrix operations. Each strategy was developed using pentadecylbenzene pyrolysis as an illustrative example. The first approach used a radical lumping strategy to group radicals according to their reactivity. The mechanism was reduced from 719 to 215 species and successfully predicted the experimentally observed initial reactivity. However, implementation of the radical lumping criteria alone was insufficient to allow for secondary reactions to higher-rank products. Therefore, on-the-fly sensitivity analysis was incorporated to identify the important and necessary species as the mechanism was generated to guide the mechanism building process. The generic algorithms developed can be applied to generate compact reaction mechanisms for a wide array of higher molecular-weight reactants.
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    AIChE Journal 43 (1997), S. 1838-1848 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Opportunities exist to exploit the unique properties of carbon dioxide to process polymers for advanced applications. The diffusion of deuterated water in poly(methyl methacrylate) films swollen by carbon dioxide at 35°C over the pressure range 0 to 90 bar (corresponding density range: 0.0 to 0.66 g/cm3) was measured using in situ FTIR spectroscopy. The mechanism and time scale of the diffusion of deuterated water in PMMA are easily controlled by manipulation of the applied CO2 density. The enhancement of mass transport rate is consistent with the degree of plasticization of the polymer due to the absorption of carbon dioxide.
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  • 189
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    AIChE Journal 43 (1997), S. 1849-1860 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A mathematical model developed for SiO2 deposition in porous Vycor glass using SiCl4 hydrolysis describes reaction, diffusion and evolution of the pore structure due to accumulation of the solid product. The deposition reaction is described by transient heterogeneous kinetics in terms of the concentrations of silanol and chloride groups in the product layer as well as those of the gaseous reactants. For typical deposition conditions the pseudo steady-state approximation for surface species could lead to erroneous predictions. Pore structure evolution is modeled by incorporating results of percolation theory. For this purpose the porous glass is represented by a Bethe lattice with coordination number 3 and alternatively by a decorated Bethe lattice in which each bond is replaced by a composite bond consisting of two bonds in series. The second network can capture the effect of pore radius variation along a single pore. For the decorated lattice, pore connectivity interruption at a higher void fraction leads to thinner deposits and shorter deposition times for pore plugging compared to the corresponding ones for the simple lattice.
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    AIChE Journal 43 (1997), S. 1861-1873 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Volatile species are removed from polymers by devolatilization. Foaming experiments using corn syrup and low-density polyethylene were carried out to investigate effects of volatile content, operating vacuum level and stripping agent in devolatilization. When foaming and rupture occur, a limiting foam volume growth was observed for a polymer regardless of the volatile content and the addition of stripper bubbles. A mathematical model was also developed to study the effect of stripper bubbles in polymer devolatilization. Based on the cell model for bubble growth, a bimodal model for foam growth is developed as the first step to describe the initial formation and growth of the volatile and stripper bubbles in polymer during devolatilization. When foaming and breakup occur at a critical foam volume expansion, a film model for mass transfer is used to model the second step of devolatilization. A numerical factorial study shows that the devolatilization section should be designed and operated to provide the screw channel sufficient free volume and residence time for the desired foam expansion.
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    AIChE Journal 43 (1997), S. 1874-1883 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: In this work, the bimodal/film model is compared with the cell model based on a case study using polystyrene/styrene devolatilization. It shows improvements over the cell model: (1) it requires a lower initial number of bubbles, which is more physically realistic and (2) it takes into account the observation of a limiting foam volume growth for a specific polymer in devolatilization. A stripper dispersion experiment was designed to investigate the effect of dispersed nitrogen stripper bubbles on the foam dynamics and devolatilization efficiency. The variables examined include polymer viscosity, initial volatile concentration, and the initial number and size of dispersed stripper bubbles. Polymer viscosity was the dominant factor in limiting the maximum foam volume expansion. The number and size of dispersed stripper bubbles show positive and negative effects (which are only secondary) on the maximum foam volume expansion, respectively. The initial volatile concentration has no effect on limiting the maximum foam volume expansion.
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    AIChE Journal 43 (1997), S. 1884-1893 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Clathrate phase equilibria for the ternary water-phenol-carbon dioxide system containing one-guest and two-host components were studied. Three four-phase equilibrium loci for this ternary system were determined over wide temperature and pressure ranges. A quintuple point, thermodynamically unique, and invariant condition appeared in this type of ternary system, was carefully measured, and were 293.7 K and 57.2 bar. At this quintuple point, five individual phases of water-rich liquid, phenol-rich liquid, carbon dioxide-rich liquid, phenol clathrate, and vapor could coexist in equilibrium. A new experimental technique was used to determine the liquid-phase compositions coexisting with phenol clathrates at the isobaric conditions of 30.0 bar and six temperatures ranging from 278.2 and 303.2 K. A potential applicability of the clathration process to phenol separation from aqueous solutions was demonsrated in terms of phenol concentrations based on carbon dioxide-free concentration. In particular, phenol separation by using high-pressure carbon dioxide as a clarthrate-inducing agent could be applied even to multiphase aqueous solutions over an entire range of phenol concentrations.
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    AIChE Journal 43 (1997), S. 1894-1903 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The role of swelling clays in hindering the compressional dewatering characteristics of coal-mine tailings is examined. The effects of electrolyte concentration and ion exchange in improving the shear and compressional rheology are compared. Suspensions studies include actual mine tailings (thickener feed and thickener underflow) as well as synthetic clay dispersions made from clay collected from the coal seam. It was shown that the most important parameter in controlling the properties of the tailings suspension is “controlled” dispersion in the presence of a Ca2+ electrolyte concentration in excess of that required to (1) prevent initial swelling and (2) provide full cation exchange of the clay. Under these electrolyte conditions, complete delamination of the clay did not occur, and both the dewatering and handling characteristics of the resultant suspensions improved dramatically.
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    AIChE Journal 43 (1997), S. 1904-1908 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1909-1913 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1914-1920 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1921-1921 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
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    AIChE Journal 43 (1997), S. 1921-1922 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997) 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
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    AIChE Journal 43 (1997), S. 1935-1946 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Explicit algebraic scalar-flux models that are valid for three-dimensional turbulent flows are derived from a hierarchy of second-order moment closures. The mathematical procedure is based on the Cayley-Hamilton theorem and is an extension of the scheme developed by Taulbee. Several closures for the pressure-scalar gradient correlations are considered and explicit algebraic relations are provided for the velocity-scalar correlations in both nonreacting and reacting flows. In the latter, the role of the Damköhler number is exhibited in isothermal turbulent flows with nonpremixed reactants. The relationship between these closures and traditional models based on the linear gradientdiffusion approximation is theoretically established. The results of model predictions are assessed by comparison with available laboratory data in turbulent jet flows.
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