bluuebunny/crossref_metadata_2025 huggingface dataset:
Multicomponent separation using a column‐switching chromatographic method
Authors: Ching C. B. , Chu K. H. , Hidajat K.
Published: June 2004

A cyclic chromatographic process based on the method of column switching was used to separate a three‐component carbohydrate mixture. It permits continuous introduction of the feed and is designed to produce higher product concentration compared to a normal elution cyclic process. A fixed‐bed model incorporating axial dispersion and a linear driving force for mass transfer was successfully used to estimate optimum operating conditions (switch times and fluid‐flow rates). Its experimental setup can be easily constructed by modifying the apparatus of a simulated countercurrent process since both systems comprise segmented columns equipped with on‐off valves for fluid‐flow control. This allows one to conduct either multicomponent separation based on elution operation or binary separation based on countercurrent operation using essentially the same set of experimental apparatus with minor modification.
Video technique for measuring dynamics of liquid‐liquid dispersion during phase inversion
Authors: Pacek A. W. , Moore I. P. T. , Nienow A. W. , Calabrese R. V.
Published: June 2004

Studies on the dynamics of phase inversion available offer limited information due to the difficulty of following the transient mean and drop‐size distribution. A new technique developed provides such data. A stereo microscope with a very shallow depth of field attached to a video camera gives sharp images of droplets in intensely‐agitated, immiscible liquid dispersions by using a Strobotach pulsing at the camera framing rate. Droplets from 40 μm upward at concentrations up to 70% by volume dispersed phase can be measured accurately. Droplets of continuous organic phase in aqueous drops can be seen. The pictures can be analyzed semiautomatically using a computer and in‐house software to give, using a variety of discretizations, cumulative and frequency distributions to any base and any mean size. Means and distributions are a function of time for phase inversions generated in three ways. The technique gives a powerful tool for understanding fast, complex dispersion processes.
Experimental study and parameterization of gas absorption by water drops
Authors: Amokrane H. , Saboni A. , Caussade B.
Published: June 2004

Liquid‐phase mass‐transfer coefficients are determined for the absorption of sulfur dioxide by water drops larger than 1.1 mm in dia. A local model based on the large eddy interfacial model proposed by Fortescue and Pearson (1967) is obtained by the characteristic interfacial scaling. In particular, the agitation process of the liquid phase in the interfacial region is characterized by the interfacial liquid friction velocity. Experiments of sulfur dioxide absorption and desorption from large individually free‐falling water drops are also carried out in a 5‐m rain shaft under various environmental conditions. These experimental results agree well with those from the local model characterizing the interfacial process in water drops greater than 1.1 mm in dia.
Local effective diffusivity in a pellet deactivated by multilayer coking
Authors: Lee Hong H.
Published: June 2004

A relationship is developed for the change in local effective diffusivity with coking using the models available for effective diffusivity. When this relationship is combined with the earlier multilayer kinetic model, the local effective diffusivity becomes dependent solely on the fraction of catalyst deactivated. Experimental procedures developed determine the relationship for the local effective diffusivity. These results provide a framework for a consistent and systematic treatment of the complex deactivation‐diffusion problem caused by coking and coke growth. Unique aspects of coking are accounted for through the fraction of catalyst deactivated, which is the basic quantity for catalytic activity, rather than the coke content.
Dynamic simulation and nonlinear‐model‐based product quality control of a crude tower
Authors: Chung Chang‐Bock , Riggs James B.
Published: June 2004

The dynamic stagewise adiabatic flash algorithm is applied to the dynamic simulation of a crude column. This algorithm is based on the implicit integration of the model equations of each stage and the explicit combination of the stage models to form the column model. Stable, efficient integration for the crude column model is observed with simulation CPU requirements that are 20 times faster than real time using a 50 MHz 486 P.C. Specific dynamic of a crude column are identified such as one‐way interaction and inverse action. The dynamic crude column model is used to study product quality control of a crude column using nonlinear‐process‐model‐based control (PMBC) which was tested using two different steady‐state controller models for setpoint tracking and disturbance rejection. Both nonlinear PMBC controllers show good control performance, but are limited by the inherent inverse action of the crude column. Control results for a set of single‐loop PI controllers is also presented as a point of reference.
Impact of process design on the multiplicity behavior of a jacketed exothermic CSTR
Authors: Russo Louis P. , Bequette B. Wayne
Published: June 2004

Research on exothermic reactor operation has been based mostly on the classic two‐state continuous stirred tank reactor model, implicitly assuming that the cooling jacket temperature dynamics are negligible. In this case, the cooling jacket temperature is the manipulated input instead of the cooling jacket flow rate for feedback control of reactor temperature. Adding a cooling jacket energy balance results in much more complex behavior than a simple lag effect. A stabilizing inner‐loop cascade controller is assumed in the two‐state CSTR model, because the three‐state model incorporating cooling jacket temperature dynamics may be open‐loop unstable when the two‐state model is open‐loop stable. The influence of design parameters on the multiplicity behavior of a three‐state model is considered. Elementary catastrophe theory is used to study the effect of process parameters such as the cooling jacket flow rate, heat‐transfer coefficient, heat of reaction, and cooling jacket feed temperature on the steady‐state multiplicity of the three‐state model. This multiplicity analysis is particularly relevant for control because the primary bifurcation parameter is the cooling jacket flow rate, the manipulated input for feedback control in the three‐state model. This multiplicity analysis guides improvements in process design and/or operation to eliminate difficult operating regions associated with steady‐state multiplicities; the presence of multiple steady states results in safety and operation problems due to ignition/extinction phenomena. Reactor scale‐up affects the presence of these infeasible reactor operating regions. Certain design parameter changes that remove multiplicities in the two‐state model cannot remove multiplicities in the three‐state model.
Hydrogen ion activities and species distribution in mixed metal sulfate aqueous systems
Authors: Filippou Dimitrios , Demopoulos George P. , Papangelakis Vladimiros G.
Published: June 2004

The activity of the hydrogen ion (H+) is estimated in sulfuric acid aqueous solutions of concentrations 0.25 to 2.0 mol.L−1 at temperatures 298 to 373 K (25 to 100°C). This is done by following an existing “extended Debye‐Hückel” equation, which was originally developed only for the case of pure H2SO4 solutions at ambient temperature. The same equation is applied for the evaluation of the activity of H+ in mixed metal sulfate solutions, particularly for the aqueous system H2SO4‐ZnSO4‐FeSO4‐Fe2(SO4)3. The estimation of the activity of H+ by this method requires the knowledge of the molality of free H+ ions and the true ionic strength (in molal scale). Consequently, an algorithm is advanced for the calculation of species concentrations in such mixed solutions. Estimated pH values agree very well with experimentally measured ones.
Effect of solute‐solute correlations on rapid reactions in supercritical fluids
Authors: Ganapathy Shankar , O'Brien James A. , Randolph Theodore W.
Published: June 2004

Brownian and molecular dynamics simulations are used to study rapid bimolecular reactions at near‐infinite dilution in near‐critical and supercritical fluids. We probe the dynamics of both nonreactive and reactive collisions and measure rate constants for reaction and collision. Collision rate constants are nearly independent of bulk solvent density, but affected by local solute‐solute density enhancements at a given density: their magnitudes depend on the length scale for molecular encounters (cybotactic radius) in the reaction through the equilibrium solute‐solute radial distribution function. In contrast, reaction rate constants asymptotically approach the gas‐kinetic limit at low densities and the Smoluchowski liquid‐like limit at high densities. They also display the same radial dependence as collision rate constants at lower densities and a direct dependence on the cybotactic radius at higher densities (as in the Smoluchowski theory). Their behavior is explained in terms of a transition from a collision‐limited regime at low densities to a diffusion‐limited regime at higher densities. The transition between these regimes depends on the cybotactic radius and the density of the system, the interplay of which causes shifts in the transition region which depend not only on the properties of the near‐critical solvent: they differ for different reactions, even at the same solvent density. This explains some of the apparent inconsistencies among previous experimental and computational studies of reactions in supercritical fluid media.
Henry's law and synergism in dilute near‐critical solutions: Theory and simulation
Authors: Munoz F. , Li T. W. , Chimowitz E. H.
Published: June 2004

An asymptotic analysis based on Taylor series expansions is used for first‐order correction terms to the Henry's law approximation to describe solvation phenomena in multiple solute‐multiple solvent systems. The magnitude of these correction terms in solvent systems very near their critical points is of particular concern, as shown in model fluid calculations with the aid of integral equation theory. The results clearly demonstrate that close proximity to the critical point in pure and mixed solvent systems causes the Henry's law approximation to show large errors in predicting solubilities, especially near the critical azeotrope of a mixed solvent system. Theoretical results also show that cross solubility enhancements in a two solutesupercritical solvent system cause cooperative synergism (both solute solubilities are increased relative to the corresponding single solute situations) or reverse synergism (both depressed relative to the single solute situation). It appears to be consistent with the available data. In computer simulations, the solute's infinitely dilute reference state is often used as a basis for describing solute thermodynamic behavior. These simulations are best achieved in the canonical ensemble because of the weak composition dependence of free energies in terms of characteristic variables of this ensemble.
Analysis of bacterial migration: II. Studies with multiple attractant gradients
Authors: Strauss Ian , Frymier Paul D. , Hahn Christopher M. , Ford Roseanne M.
Published: June 2004

Many motile bacteria exhibit chemotaxis, the ability to bias their random motion to ward or away from increasing concentrations of chemical substances which benefit or inhibit their survival, respectively. Since bacteria encounter numerous chemical concentration gradients simulatneously in natural surroundings, it is necessary to know quantitatively how a bacterial population responds in the presence of more than one chemical stimulus to develop predictive mathematical models describing bacterial migration in natural systems. This work evaluates three hypothetical models describing the integration of chemical signals from multiple stimuli: high sensitivity, maximum signal, and simple additivity. An expression for the tumbling probability for individual stimuli (Brown and Berg, 1974) is modified according to the proposed models and incorporated into the cell balance equation for a 1‐D attractant gradient (Ford and Cummings, 1992). Random motility and chemotactic sensitivity coefficients, required input parameters for the model, are measured for single stimulus responses. Theoretical predictions with the three signal integration models are compared to the net chemotactic response of Escherichia coli to co‐ and antidirectional gradients of D‐fucose and α‐methylaspartate in the stopped‐flow diffusion chamber assay. Results eliminate the high‐sensitivity model and favor the simple additivity over the maximum signal. None of the simple models, however, accurately predict the observed behavior, suggesting a more complex model with more steps in the signal processing mechanism is required to predict responses to multiple stimuli.
Mass and heat transfer in crushed oil shale
Authors: Carley James F. , Ott Linda L. , Swecker Jeanne L.
Published: June 2004

Studies of heat and mass transfer in packed beds, which disagree substantially in their findings, have nearly all been done with beds of regular particles of uniform size, whereas oil‐shale retorting involves particles of diverse irregular shapes and sizes. We, in 349 runs, measured mass‐transfer rates from naphthalene particles buried in packed beds by passing through air at room temperature. An exact analogy between convection of heat and mass makes it possible to infer heat‐transfer coefficients from measured mass‐trans‐fer coefficients and fluid properties. Some beds consisted of spheres, naphthalene and inert, of the same, contrasting or distributed sizes. In some runs, naphthalene spheres were buried in beds of crushed shale, some in narrow screen ranges and others with a wide size range. In others, naphthalene lozenges of different shapes were buried in beds of crushed shale in various bed axis orientations. This technique permits calculation of the mass‐transfer coefficient for each active particle in the bed rather than, as in most past studies, for the bed as a whole.

The data are analyzed by the traditional correlation of Colburn jD vs. Reynolds number and by multiple regression of the mass‐transfer coefficient on air rate, sizes of active and inert particles, void fraction, and temperature. Principal findings are: local Reynolds number should be based on the active‐particle size, not the average for the whole bed; differences between shallow and deep beds are not appreciable; mass transfer is 26% faster for spheres and lozenges buried in shale than in all‐sphere beds; orientation of lozenges in shale beds has little or no effect on mass‐transfer rate; and for mass or heat transfer in shale beds, log(j·ϵ) = − 0.0747 − 0.6344logNRe + 0.0592log2 NRe.
The effect of gravity degradation on low‐speed centrifuge capillary pressure data
Authors: Chen Zhigang Andy , Ruth Douglas W.
Published: June 2004

The gravity degradation effect has been identified as a significant physical problem in using centrifuge techniques for determining capillary pressure curves. In 1992, the authors characterized the effect by an angle; however, such an evaluation is simplistic. Here, a new model is constructed to characterize the effect quantitatively. The results with simulated data sets show that there exists a pronounced effect of gravitation on the horizontal centrifugal field in low‐speed experiments (≤ 500 rpm on a standard Beckman centrifuge). Gravity affects the production history for some samples, which leads to inaccurate interpretations of capillary pressure information near the threshold pressure.
New sorbents for olefin/paraffin separations by adsorption via π ‐complexation
Authors: Yang R. T. , Kikkinides E. S.
Published: June 2004

New adsorbents for olefin/paraffin separation are synthesized by effective dispersion of Ag(I) and Cu(I) cations on substrates with hydrocarbon‐phobic surfaces. These cations bind olefin molecules by a π‐complexation bond, a weak chemical bond. Ethane/ethylene and propane/propylene separations are considered. Cation exchange resins and CuCl/γ‐Al2O3 are effective substrates. On the Ag(I) resin at 25°C and 1 atm, the equilibrium adsorption ratio for C2H4/C2H6 = 9.2 and C2H4 capacity = 1.15 mmol/g; the corresponding values for C3H6/C3H8 = 10.4 and C3H6 capacity = 1.29 mmol/g. The CuCl/γ‐Al2O3 sorbent shows equally promising results. The sorption rates are pore‐diffusion‐controlled and rapid. The olefin selectivity; capacity, and rates are much higher than all previous attempts and are suitable for applications in cyclic adsorption processes.

The equilibrium data are correlated with an isotherm equation that accounts for both physical adsorption and π‐complexation with energy heterogeneity, using only two true fitting parameters. Molecular orbital calculations using a C6H5SO substrate indicate that the π‐complexation bond is contributed mainly by the donation of olefin π‐bond electrons to the empty s‐orbital of the metal, while the d‐π* back donation contributes only 16%. Moreover, the relative order of the heats of adsorption is correctly predicted.
Agglomeration during precipitation: I. Tracer crystals for Al(OH)3 precipitation
Authors: Ilievski D. , White E. T.
Published: June 2004

Tracer crystals are developed to study the agglomeration, growth and residence time behavior of gibbsite crystals, an industrially important polymorph of Al(OH)3, in continuous and batch precipitation. Satisfactory tracer crystals contain detectable amounts of Zn and have the crystal structure of gibbsite. The tracer crystals behave in the same manner during precipitation as the nontracer Al(OH)3 crystals. The application of the tracer crystals for investigating, both batch and continuous, seeded Al(OH)3 precipitation is demonstrated. The tracer experiments provide information on the growth and agglomeration kinetics of different size fractions. The tracers are a useful tool for the quantitative investigation of solid‐phase residence time distributions within a crystallizer.
Dynamic modeling of suspension crystallizers, using experimental data
Authors: Eek Rob A. , Dijkstra Sjoerd , Van Rosmalen Gerda M.
Published: June 2004

A model for the dynamics of a single‐stage suspension crystallizer is developed, which serves as a basis for process analysis and the design of controllers. A population balance for the dynamics of the crystal size distribution (CSD) with mass and heat balances is described, as well as empirical relations for the separation efficiency of classified particle removal systems, the initial CSD, and the crystallization kinetics. A continuous pilot crystallizer is used that is equipped with a separator and dissolver for fine crystals and a CSD sensor based on forward light scattering. The process and sensor are modeled separately. The sensor model is based on Fraunhofer light scattering theory assuming rectangular‐shaped particles. CSD dynamics data are obtained from startup experiments with the pilot plant at different process conditions. Experimental process data show a strong effect of fines and the slurry retention time on the CSD dynamics. A nonlinear parameter estimation procedure determines the empirical parameters directly from raw sensor data. The model fits accurately to the measured data. Evidence is found for the existence of a population of slow growing crystals with a growth rate approximately ten times lower than the fast growing crystals. A strong correlation is found between the total surface area of crystals with a size larger than 600 μm and the nucleation rate.
Feedback control of nonlinear differential‐algebraic‐equation systems
Authors: Kumar Aditya , Daoutidis Prodromos
Published: June 2004

The output feedback control problem is addressed for a class of nonlinear multivariable high‐index differential‐algebraic‐equation systems in semiexplicit form. Initially, an algorithmic procedure is developed and used to derive an equivalent state‐space relization of the constrained system. An output feedback synthesis problem is then formulated on the basis of the derived state‐space realization and solved through the combination of state feedback and appropriate state observers. The developed methodology is applied to a two‐phase reactor, and its performance and robustness characteristics are evaluated through simulations.
Glucose hydrolysis and oxidation in supercritical water
Authors: Holgate H. Richard , Meyer Jerry C. , Tester Jefferson W.
Published: June 2004

Glucose hydrolysis and oxidation occurred rapidly in supercritical water at 246 bar and at 425 to 600°C. A diverse set of products, present in the liquid‐phase reactor effluent and also subject to hydrolysis, was formed. At 600°C and a 6‐s reactor residence time, glucose is completely gasified, even in the absence of oxygen. In the presence of oxygen, destruction of liquid‐phase products is enhanced, with none found above 550°C at a 6‐s reactor residence time. Major products formed wee acetic acid, acetonylacetone, propenoic acid, and acetaldehyde in the liquid phase, and carbon monoxide, carbon dioxide, methane, ethane, ethylene, and hydrogen in the gas phase. Methane and hydrogen were present among the products at temperatures up to 600°C for reactor residence times of 6 s.
Novel method for noncontact measurement of particle temperatures
Authors: Wagenaar B. M. , Meijer R. , Kuipers J. A. M. , van Swaaij W. P. M.
Published: June 2004

A nonintrusive temperature measurement technique is developed for noncontact measurement of the temperature of single particles with < 200 μm dia. It is based on the temperature dependence of the fluorescence spectrum resulting from irradiation of a certain phosphor mixture with UV light by applying a mixture of two phosphors with fluorescence colors (blue and green) and a color shift from green to blue (with temperature increase from 20 to 280°C). An experimental setup is described for temperature measurement of particles based on the fluorescence color, together with the calibration of this system. The fluoroptic technique is applied to measure the temperature decrease of hot particles flowing down a cold chute. The measurements agree very well with thermocouple measurements. This novel technique can be applied to nonintrusive measurement of particle temperatures in (dense) multiparticle systems as encountered in packed and fluidized beds.
A product distribution paradox on scaling up a stirred batch reactor
Authors: Mann R. , El‐Hamouz A. M.
Published: June 2004

For 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

A 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

A 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

A 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

In 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

The 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

A 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

Phenomenologically 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

Sufficient 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

Composite 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

A 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

The 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

Direct 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

The 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

Diffusion 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

Discrete‐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

Ion‐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

The 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

A 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

Effective 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.
Dynamic contact angles and flow in vicinity of moving contact line
Authors: Shikhmurzaev Yulii D.
Published: June 2004

A previously developed mathematical wetting model is generalized and applied to the following two closely related situations: the spreading of a liquid over a prewet solid surface and the receding contact‐line motion with a microscopic residual film, remaining behind the contact line. An analytical expression for the velocity dependence of the dynamic contact angle is derived. Macroscopic characteristics (the dynamic contact angle and drag force) and the flow field corresponding to the spreading of a liquid over a wet solid surface differ considerably from those calculated for a dry surface. Under certain conditions the flow in the reference frame fixed with respect to the contact line has a region with closed streamlines. The region appears due to the flow‐induced Marangoni effect, the reverse influence of the surface tension gradient along the liquid‐solid interface caused by the flow on the flow, which gives rise to the gradient. The results are compared qualitatively with experimental data.