bluuebunny/crossref_metadata_2025_split huggingface dataset:
A product distribution paradox on scaling up a stirred batch reactor
Authors: Mann R. , El‐Hamouz A. M. 
Published: June 2004

AbstractFor a triplet competitive‐consecutive halogenation sequence forming mono‐, di‐ and trihalogenated products of the form, A + B → R + B → S + B → T, under semibatch operation adding B to A, if perfect mixing could be assumed at all scales, the product distribution would be unchanged on scaling up. However, if the reaction rates are reasonably faster than the mixing rate, the semibatch addition of B to A will be imperfectly backmixed, exhibiting macroscale concentration gradients. This partial segregation of the primary reagents is capable of modifying the selectivity and corresponding appearance of R, S and T in the course of the batch. Imperfect mixing is quantified using the networks‐of‐zones model. The effect of scaling up at equal tip speed is examined for a lab‐scale 0.3‐dm3 reactor, a semitech 30‐dm3 reactor, and a production‐scale 3,000‐dm3 vessel. The intensity of partial segregation is weak at the lab scale, but very severe at the production scale. The lab‐scale reactor is therefore close to perfectly backmixed, and the primary, secondary and tertiary products appear in sequence. At the semitech scale the increased partial segregation causes the final product to initially precede the secondary product paradoxically but lag the initial product. At the large scale the more severe segregation between A and B gives an even greater paradox, whereby the final product appears ahead of both the primary and secondary ones. The segregated concentration fields of A and B are visualized as sectional image reconstructions for networks comprising on the order of 1,000 zones. Localized intensive plumes of B emanating from the addition point cause the paradoxical reversals of product sequences. The calculations are directly relevant to real industrial miscible liquid halogenations for which product distribution paradoxes have been observed (Haywood, 1990).
Mathematical model on the sensing behavior of a biooxidation biosensor
Authors: Chen Y. , Tan T. C. 
Published: June 2004

AbstractA phenomenological model developed describes the steady‐state sensing characteristics of a biosensor based on biooxidation of organic solutes by dissolved oxygen. The model is experimentally verified using a recently developed dopamine sensor that enzymatically oxidizes dopamine by polyphenolase in apple tissues. It adequately describes its steady‐state sensing characteristics, including the effects of mass of immobilized apple fines in the biofilm, temperature, and pH of the substrate solution. The parameters provide a basis for evaluating the suitability of different bioactive materials or the same bioactive material from different sources for biosensor fabrication. At zero thickness of the membranes and biofilm, the model describes the sensing characteristics of a dissolved oxygen probe in which the oxygen diffusivity through the Teflon membrane is 9.83 × 10−11m2/s, comparable with a previously reported value. It can also describe the sensing behavior of any nonbiocatalyzed oxidation‐related sensor in a two‐substrate system simply by replacing the governing faradaic equations for the dissolved oxygen probe with those appropriate to the type of probe used.
Modeling molecular weight development of gas‐phase α‐olefin copolymerization
Authors: Xie Tuyu , McAuley Kim B. , Hsu James C. C. , Bacon David W. 
Published: June 2004

AbstractA comprehensive kinetic model developed for molecular weight calculations of ethylene axsnd α‐olefin copolymerizations in the context of a terminal model accounts for multiple‐type active centers of the catalyst, detailed elementary chemical reactions, and catalyst composition. The moments of copolymer chain distributions are defined considering molecular weights of comonomer units so that copolymer molecular weight averages can be directly calculated from those moments. A double Z‐transformation is introduced for the derivation of differential equations of the moments. Model simulations are carried out based on ethylene and 1‐butene copolymerizations in a gas‐phase fluidized‐bed reactor. Polydisperity of accumulated copolymer depends on catalyst composition and kinetic characteristics of the catalyst. For a catalyst with specified kinetic characteristics, the polydispersity depends on the mole fraction of each type of active center. For a catalyst with two types of active centers, the maximum polydispersity of copolymer occurs at 50 wt. % of the total copolymer if polydispersities of the copolymers generated at each active site are the same. Polydispersity of accumulated copolymer is sensitive to propagation reactions and chain transfer to hydrogen reactions. Differences in chain transfer to cocatalyst and monomers and in spontaneous deactivation rates for different types of active centers may play minor roles in controlling molecular weight development in the presence of hydrogen. This model can be used for catalyst composition design, simulation of commercial olefin copolymerization processes, and kinetic parameter estimation.
Bypassing eutectics with extractive crystallization: Design alternatives and tradeoffs
Authors: Dye Susan R. , Ng Ka M. 
Published: June 2004

AbstractA systematic design method reported here uses extractive crystallization to separate a three‐component mixture despite the presence of eutectics. Phase behavior can be classified into six basic types, and six flowsheet structures can handle systems with any of these solid‐liquid‐phase behaviors. Design equations are formulated for these flowsheet structures, and design variables and constraints are identified. In addition, design issues, such as the choice of solvent, the effect of design variables on recycle flows, and the magnitude of the costs, are discussed.
Circulating fluidized bed as a catalytic reactor: Experimental study
Authors: Ouyang S. , Li X.‐G. , Potter O. E. 
Published: June 2004

AbstractIn recent years, the circulating fluidized bed as a reactor has experienced increasing application in industry. A circulating fluidized bed (CFB) has some unique features as a chemical reactor. The performance of the CEB reactor was investigated in a O.25‐m‐dia. Riser system with ozone decomposition in the reactor. Both axial and radial profiles of ozone concentration are presented, as well as overall conversions under various reaction conditions. The effects of the operating conditions on the performance of the CFB reactor were examined. The experimental results show that the performance of a circulating fluidized bed as a reactor is much nearer to that of a well‐mixed system than that of a plug‐flow system. The effectiveness factor of the reactor seems to decrease with increase of solids holdup in the reactor.
Binary kinetics in the Y‐Ba‐Cu system: 1. Mixed powders
Authors: Sobolik Jeffrey L. , Thomson William J. 
Published: June 2004

AbstractThe kinetics of the reactions between mixed powders BaCO3 and CuO, as well as BaCO3 and Y2O3, have been studied using DXRD techniques as a function of particle size, temperature, and CO2 pressure. Except for intial nucleation phenomena, the reaction ration rates are governed by shrinking core bahavior for BaCO3 particle sizes between 6 and 33 μm. During the initial stages of the reactions, the surface reaction kinetics are gare governing, whereas the diffusion of CuO, Y2O3, and CO2 are limiting factors at later stages in the reactions. Quantitative conversion dat were used to determine the values of the activation energies and the pertinent diffucivities in these systems.
A nonequilibrium model for dynamic simulation of tray distillation columns
Authors: Kooijman Hendrik A. , Taylor Ross 
Published: June 2004

AbstractA nonequilibrium model for the dynamic simulation of distillation columns is described. The nonequilibrium model includes the direct calculation of the rates of mass and energy transfer and is better able to model the actual physical processes occurring on a real distillation tray than is the conventional equilibrium stage model. Example calculations show that heat‐transfer limitations and the vapor holdup above the froth cannot be neglected at elevated pressures. Back‐computed Murphree tray efficiencies are not constant over time, which implies that the equilibrium model should not be used for dynamic simulations.
A mechanistic analysis of sieve tray froth height and entrainment
Authors: Bennett Douglas L. , Kao Alan S. , Wong Lisa W. 
Published: June 2004

AbstractPhenomenologically based model for forth height was developed using the assumption of both vapor‐and liquid‐continuous zones within the total froth height. The model demonstrates the importance of liquid and vapor rates and determines that the Weber number (and therefore droplet size and surface tension) has little effect on froth height.A composite air/water database including entrainment data for both the spray and froth regimes is used to develop an entrainment correlation as a function of the ratio of tray spacing to froth height (Ts/h2ϕ) and the average froth density (ϕ2ϕ = hL).For conditions resulting in the ratio of liquid inventory to the perforation diameter (hL/dH) <4, the height, equation requires an empirical correction for froth height, and entrainment depends onϕ2ϕ.This results from the predominantly vapor‐continus nature of the froth. For large values of hL/dH, the theoretical value for h2ϕ appears adequate and entrainment is essentially independent of ϕ2ϕ, since most of the liquid is contained in the liquid‐continuous region located far away from the underside of the tray above. In such cases, entrainment is from a light spray on top of the liquid‐continuous region, and therefore entrainment depends much less on ϕ2ϕ.The differen dependency of entertainment on ϕ2ϕ can be used as a method to identify the spray‐froth transition. For hL/dH<2, a spraylikecondition exists; for hL/dH > 2, a frothlikecondition exists.The reported dependency of air/water system entrainment on major geometry and operating conditions is described well by the new correlation. Alternate correlations for the non‐air/water entrainment data, however, are required. New methods for estimating the impact of entrainment on efficiency are given.
Performance bounds for robust quadratic dynamic matrix control with end condition
Authors: Vuthandam Prernkiran , Genceli Hasmet , Nikolaou Michael 
Published: June 2004

AbstractSufficient conditions for robust stability of multivariable quadratic dynamic matrix controllers with an end condition (EQDMC) are developed, and the effect of these conditions on closed‐loop performance is examined. Hard and soft constraints on process inputs and outputs, and process modeling uncertainty are present Modeling uncertainty is quantified in the time domain as upper and lower bounds on the coefficients of a finite pulse response process model. The robust stability conditions that we develop involve the prediction and control horizon lengths, and a set of inequalities that the move‐suppression coefficients in the on‐line EQDMC objective function must satisfy. These conditions imply that for processes with modeling uncertainty, the move‐suppression coefficients could be quite large and quite sensitive to the control horizon length and process modeling errors. This could make the EQDMC controller conservative, thus significantly deteriorating performance. To determine the optimal control horizon length corresponding to the best robust performance of the EQDMC controller for a class of disturbances, follow the proposed EQDMC controller design methodolo∼. To illustrate this methodology simulations on an SISO example and a 2 × 2 subsystem of the Shell Standard Control Problem are presented.
Novel preparation techniques for thin metal‐ceramic composite membranes
Authors: Lun Yeung King , Varma Arvind 
Published: June 2004

AbstractComposite metal membranes obtained by supporting thin metallic films on ceramic substrates have good thermal and mechanical stability. The use of a thin metal film increases transmembrane flux, while retaining high permselectivity that is characteristic of metallic membranes. Novel techniques have been developed for preparing metal‐ceramic composite membranes. By the appropriate use of osmotic pressure, the microstructure, porosity, and thickness of the deposited metal can be systematically manipulated. Three new procedures are described for film densification and fabrication: (1) the osmotic pressure is used to densify an existing supported metal membrane; (2) densification and growth of the film is managed under the influence of osmotic pressure by using a dilute plating solution; and (3) films of varying porosity are deposited on the ceramic membrane by combining electroless deposition and osmotic pressure. Silver, palladium, and palladium‐silver films prepared by these techniques on a commercial alumina membrane (Membralox) are thermally more stable than similar films deposited by conventional electroless plating.
Modeling of polymer erosion in three dimensions: Rotationally symmetric devices
Authors: Göpferich Achim , Langer Robert 
Published: June 2004

AbstractA three‐dimensional erosion model was developed to simulate the erosion of polymer cylinders. Erosion is regarded as a random event. After contact with the erosion medium small polymer parts are randomly assigned lifetimes from a first‐order Erlang distribution using a Monte Carlo sampling technique. The model takes into account the partially crystalline nature of polymers as well as the geometry of the devices. It allows investigation of the effect of slow or nonerodible coatings. It was found that by partially coating cylinders, erosion periods of polyanhydride polymers can be expanded substantially. This is very important for using polyanhydrides as drug carriers where the polymer erosion period controls drug release.
First‐order corrections to infinite dilution fugacity coefficients using computer simulation
Authors: Li T. W. , Chimowitz E. H. , Munoz F. 
Published: June 2004

AbstractThe relative merits of various approaches using computer simulation methods for estimating first‐order correction terms to Henry's law in dilute supercritical mixtures are investigated. Three thermodynamic formulations are provided for the calculation of these properties. One method, termed the fluctuation integral approach, requires the explicit calculation of solute‐solvent pair correlation functions at the infinite dilution limit, which is a difficult task for simulations given the conflicting demands of both small numbers of solute species (for approximating the infinite dilution limit) as well as the need for large numbers of solute species to improve simulation statistics. A second approach, called the pressure gradient method, does not require the explicit, determination of these solute‐solute functions and, as a result, is not as sensitive to the choice of system size and other difficulties associated with establishing an adequate ensemble size and/or number of solute species to be used in the simulations. The third approach uses the exact formulation for the property of interest using Kirkwood‐Buff theory. This approach, however, requires all the solute pair correlation functions to be established, making it the most sensitive to issues concerning number of solute species used in the simulations, and so on. An examples is given showing simulation results for these approaches, illustrating their respective strengths and drawbacks.
Direct numerical simulation of chemical selectivity in homogenous turbulence
Authors: Chakrabarti Mitali , Kerr Robert M. , Hill James C. 
Published: June 2004

AbstractDirect numerical simulations were used to study chemical selectivity in a series‐parallel reaction scheme in a decaying, homogenous turbulent flow, where A, B, R, and S represent chemical species with R the principal product and S the secondary product. These simulations involve solution of the unsteady Navier‐Stokes and mass conservation equations by a pseudo‐spectral method in a 643 wavenumber domain. Reactants A and B were initially spatially segregated, corresponding to a nonpremixed system. The effect of turbulence Reynolds number and other physical parameters on selectivity was determined. Turbulence increases the formation of primary product R over byproduct S compared to the case of no fluid motion, as expected. It was also found that any mechanism promoting homogenization of reactants favors the formation of R, Whereas any mechanism sustaining segregation favors the formation of S.
Sensitivity analysis of tubular packed‐bed reactor by pseudohomogeneous 2–D model
Authors: Juncu Gh. , Floarea O. 
Published: June 2004

AbstractThe sensitivity behavior of the fixed‐bed catalytic reactors is examined using the pseudohomogeneous two‐dimensional model with radial dispersion for the first‐order irreversible reaction. The sensitivity criteria developed are tested. The relations between self‐similarity and thermal runaway are studied. The pseudoadiabatic type of operation is taken into consideration. The boundaries of the sensitivity region in the space Da – Pe are compared with those predicted by the standard one–dimensional model and the criterion of Hagan et all The sensitivity behavior of the system in the space α – Bi is also discussed.
Barrier membranes with tipped impermeable flakes
Authors: Eitzman Diana M. , Melkote R. R. , Cussler E. L. 
Published: June 2004

AbstractDiffusion of carbon dioxide through silicone‐polycarbonate membranes containing oriented mica flakes is proportional to the square of the volume fraction of the flakes, to the square of the aspect ratio, and to the square of the cosine of the angle at which the flakes are oriented. These experiments are consistent with Monte Carlo simulations. The simulations allow investigation of more complex geometries, including other flake orientations and polydisperse flake sizes.
Discrete‐time nonlinear feedback control of multivariable processes
Authors: Soroush Masoud , Kravaris Costas 
Published: June 2004

AbstractDiscrete‐time nonlinear feedback control laws are derived for multivariable nonlinear processes, whose “delay‐free” parts are minimum phase. These include mixed error‐and state‐feedback, error‐feedback, and mixed error‐ and output‐feedback laws, which can induce linear input–output closed‐loop response. This study is carried out within the framework of the discrete‐time globally linearizing control. The broader class of nonlinear processes, in which controlled outputs as well as some other process variables are measured, are also considered. A mixed error‐ and output‐feedback control law is derived for this class of processes. The conditions under which the mixed error‐ and output‐feedback can be applied to a process operating at or around an open‐loop unstable equilibrium point are determined. The application and performance of the derived control laws are illustrated by the numerical simulation of a chemical reactor.
Measurement and correlation of ion activity in aqueous single electrolyte solutions
Authors: Khoshkbarchi Mohammad K. , Vera Juan H. 
Published: June 2004

AbstractIon‐selective electrodes were used to measure the activity coefficients at 298.2 K of individual ions in aqueous solutions of NaCl and NaBr up to of 5 molal and of KCl up to 4 molal. The mean ionic activity coefficients of NaCl, NaBr, and KCl, obtained from the values of the activity coefficients of the individual ions, show good agreement with values reported in the literature. The experimental results show that the activity coefficients are different for the anion and the cation in an aqueous solution of a single electrolyte and that, as expected from the ion‐ion and ion‐solvent interactions, the activity coefficient of an ion depends on the nature of its counterion. A modified form of the Pitzer's model, which distinguishes between the activity coefficients of the anion and the cation, was used to correlate the experimental results.
Hydrodynamic characteristics of dual Rushton impeller stirred vessels
Authors: Rutherford K. , Lee K. C. , Mahmoudi S. M. S. , Yianneskis M. 
Published: June 2004

AbstractThe flows generated in vessels stirred by two Rushton impellers were investigated in two vessels of diameter (T) 100 and 294 mm with impellers of diameter D = T/3 using flow visualization, power consumption, mixing time, and 360° ensemble‐averaged and 1° angle‐resolved LDA measurement techniques. The flows depended strongly on the clearance of the lower impeller above the base of the vessel (C1), the separation between the impellers (C2), and the submergence (C3) of the upper impeller below the top of the liquid column height (H). When these distances were varied, three stable and four unstable flow patterns were observed. Comparisons between the two LDA techniques showed that while the 360° ensemble‐averaged measurements are useful for characterizing the overall flow structure and turbulence levels in the vessel, care must be exercised when interpreting such data, since in the impeller region they include periodic variations in the mean velocity in addition to the turbulent fluctuations. The trailing vortex structure and flow periodicity produced by the Rushton impellers is shown to decay significantly within a cylindrical region of height 1.2D and radius 1.0D centered around the middle of the vessel, when C1 = C2 = T/3. The turbulence structure within this region is anisotropic, while outside this region it might be considered mostly isotropic. The main flow features scaled well between the vessels.
Network simulation of steady‐state two‐phase flow in consolidated porous media
Authors: Constantinides George N. , Payatakes Alkiviades C. 
Published: June 2004

AbstractA computer‐aided simulator of steady‐state two‐phase flow in consolidated porous media is developed. The porous medium is modeled as a 3‐D pore network of suitably shaped and randomly sized unit cells of the constricted‐tube type. The problem of two‐phase flow is solved using the network approach. The wetting phase saturation, the viscosity ratio, the capillary number, and the probability of coalescence between two colliding ganglia are changed systematically, whereas the geometrical and topological characteristics of the porous medium and wettability (dynamic contact angles) are kept constant. In the range of the parameter values investigated, the flow behavior observed is ganglion population dynamics (intrinsically unsteady, but giving a time‐averaged steady state). The mean ganglion size and fraction of the nonwetting phase in the form of stranded ganglia are studied as functions of the main dimensionless parameters. Fractional flows and relative permeabilities are determined and correlated with flow phenomena at pore level. Effects of the wetting phase saturation, the viscosity ratio, the capillary number, and the coalescence factor on relative permeabilities are examined.
Compression of chemical process data by functional approximation and feature extraction
Authors: Bakshi Bhavik R. , Stephanopoulos George 
Published: June 2004

AbstractEffective utilization of measured process data requires efficient techniques for their compact storage and retrieval, as well as for extracting information on the process operation. Techniques for the on‐line compression of process data were developed based on their contribution in time and in frequency using the theory of wavelets. Existing techniques for compression via wavelets and wavelet packets are inconvenient for on‐line compression and are best suited for stationary signals. These methods were extended to the on‐line decomposition and compression of nonstationary signals via time‐varying wavelet packets. Various criteria for the selection of the best time‐varying wavelet packet coefficients are derived. Explicit relationships among the compression ratio, local and global errors of approximation, and features in the signal were derived and used for efficient compression. Extensive case studies on industrial data demonstrate the superior performance of wavelet‐based techniques as compared to existing piecewise linear techniques.