bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular, pressure, and thermal diffusion in nonideal multicomponent mixtures
Authors: Ghorayeb Kassem , Firoozabadi Abbas
Published: April 2004

A formalism based on powerful concepts of the thermodynamics of irreversible processes is derived for calculating multicomponent diffusion flux both at the critical point and away from the critical point. The derivations are based on the entropy balance expression combined with the phenomenological equations and Onsager reciprocal relations. The formalism results in a clear expression of the thermal contribution in the diffusion flux for nonideal multicomponent mixtures. The diffusion flux analysis at the critical point showed that, unlike isothermal and isobaric conditions where molecular diffusion flux is zero, molecular diffusion flux is finite and nonzero at the critical point. The thermal contribution in the diffusion flux is also finite at the critical point in multicomponent mixtures. At nonisothermal conditions, as well as nonisothermal and nonisobaric conditions, the composition gradient reaches infinity at the critical point; therefore, the mol fraction plot vs. spatial coordinates has an inflection point.
Theoretical model of thermal diffusion factors in multicomponent mixtures
Authors: Firoozabadi Abbas , Ghorayeb Kassem , Shukla Keshawa
Published: April 2004

Unlike molecular diffusion, neither measured thermal diffusion coefficients nor the theoretical framework exist for the estimation of thermal diffusion coefficients in nonideal multicomponent mixtures. This work derives a theoretical model for thermal diffusion coefficients in ideal and nonideal multicomponent mixtures, based on the thermodynamics of irreversible processes and the molecular kinetic approach incorporating explicit effects of nonequilibrium properties, such as the net heat of transport and molecular diffusion coefficients, and of equilibrium properties of the mixture, which are determined by the Peng‐Robinson equation of state. An interesting feature of this model is that in nonideal multicomponent mixtures thermal diffusion coefficients depend on molecular diffusion coefficients, while in binary mixtures they do not. The model successfully describes thermal diffusion factors of binary mixtures for which experimental data are available, even those in extreme nonideal conditions and close to the critical point. Since experimental data on thermal diffusion factors in multicomponent hydrocarbon mixtures are not available, testing the model's accuracy was not possible. The model, however, successfully predicted spatial variation of composition in a ternary mixture of nC24/nC16/nC12, providing an indirect verification. The six‐component mixture of C1/C3/nC5/nC10/nC16/C2 shows significant dependency of thermal diffusion factors on the distance to the critical point. It also demonstrates for the first time that there is no need to adopt a sign convention for thermal diffusion coefficients in binary and higher mixtures. The thermodynamic stability analysis shows that when the thermal diffusion coefficient is positive, the component should go to the cold region in a binary mixture.
Feasible products for kinetically controlled reactive distillation of ternary mixtures
Authors: Chadda Nitin , Malone Michael F. , Doherty Michael F.
Published: April 2004

A method to determine feasible product compositions from a single‐feed reactive distillation column for finite rates of reaction is presented. Tracking of pinch points to estimate the limit at minimum reflux is combined with performance modeling for the limit at the minimum number of stages. Using two examples it is shown how the product composition regions deform from the known limit of no reaction through intermediate rates of reaction and finally collapse onto the chemical equilibrium curve at extremely fast reaction rates. However, compositions can be achieved at intermediate reaction rates that do not lie between the limits for slow (no reaction) and fast (chemical equilibrium) reactions.
“String‐of‐beads” flow of liquids on vertical wires for gas absorption
Authors: Chinju Hirofumi , Uchiyama Kazunori , Mori Yasuhiko H.
Published: April 2004

A liquid steadily fed onto a thin wire vertically held in a gas stream may flow down the wire, taking the form of a “string of beads,” which is a composite of teardrop‐shaped beads regularly aligned on the wire and thin cylindrical films each coating the portion of the wire between two neighboring beads. The beads slide down at a constant speed, interacting with the films flowing down much more slowly, thereby inducing some mixing motion in the liquid phase. The string‐of‐beads flow exhibited by water or an aqueous monoethanolamine solution was observed, and the mass transfer of CO2 from the surrounding CO2/N2 mixture into water flowing down single wires was studied both experimentally and analytically. The results indicate the potential of on‐wire string‐of‐beads flow devices for innovating gas–liquid contact operations to be used in, for example, removing CO2 from flue gases exhausted at fossil‐fuel‐fired power plants.
Optimal design of reverse osmosis module networks
Authors: Maskan Fazilet , Wiley Dianne E. , Johnston Lloyd P. M. , Clements David J.
Published: April 2004

The structure of individual reverse osmosis modules, the configuration of the module network, and the operating conditions were optimized for seawater and brackish water desalination. The system model included simple mathematical equations to predict the performance of the reverse osmosis modules. The optimization problem was formulated as a constrained multivariable nonlinear optimization. The objective function was the annual profit for the system, consisting of the profit obtained from the permeate, capital cost for the process units, and operating costs associated with energy consumption and maintenance. Optimization of several dual‐stage reverse osmosis systems were investigated and compared. It was found that optimal network designs are the ones that produce the most permeate. It may be possible to achieve economic improvements by refining current membrane module designs and their operating pressures.
Modeling fluid phase transition effects on dynamic behavior of ESDV
Authors: Mahgerefteh Haroun , Saha Pratik , Economou Ioannis G.
Published: April 2004

The characteristics method was employed to elucidate the effects of fluid‐phase transition on the dynamic behavior of emergency shutdown valves following full bore rupture (FBR) of long pipelines containing high‐pressure hydrocarbons. The responses of both check valves and ball valves were simulated. The application of the model to the FBR of the Piper‐Alpha MCP‐01 gas riser reveals that condensation of the gaseous inventory results in a delay in valve activation times, as well as an increase in the amount of inventory released prior to pipeline isolation. However, the magnitude of the pressure surge brought about from having to bring the high‐pressure escaping fluid to rest following emergency isolation was significantly reduced.
Finite element, discrete‐fracture model for multiphase flow in porous media
Authors: Kim Jong‐Gyun , Deo Milind D.
Published: April 2004

A new discrete‐fracture multiphase flow model developed allows incorporation of fractures in a spatially explicit fashion. It is an alternative to conventional dual‐porosity, dual‐permeability models used most often to model fractured subsurface systems. The model was applied to a water flood on a 2‐D random fracture network. A standard Galerkin finite element method was used to discretize the domain; triangular elements were used for matrix and line elements for the fractures. The finite element formulation was validated by using a commercial finite difference simulator. Results from a simple discrete‐fracture model agreed reasonably well with the explicit fracture representation of the same domain. At low permeability contrasts between the matrix and the fractures, the model, as expected, predicted approximately symmetric water advance. At high permeability contrasts, the fracture network played a critical role in determining water advance and oil recovery. Significant oil bypassing was observed, particularly at higher flow rates. The oil recovery was determined by a complex interplay of the absolute matrix permeability, permeability contrast and flow rates. Fracture capillary pressure also played a significant role in determining water penetration in the matrix.
Extractive bioconversion in a four‐phase external‐loop airlift bioreactor
Authors: Sajc Lidija , Vunjak‐Novakovic Gordana
Published: April 2004

The integration of biosynthesis and product separation can increase the productivity of immobilized plant cells in airlift bioreactors. Extractive bioconversion of anthraquinones was studied in an external‐loop airlift bioreactor consisting of a riser, a downcomer, and two horizontal sections, while containing alginate‐immobilized Frangula alnus cells, a continuous aqueous phase (nutrient solution), dispersed solvent phase (n‐hexadecane or silicone oil), and gas bubbles. A simple mathematical model was developed to describe the cocurrent liquid‐liquid extraction in the riser section of the bioreactor and to rationalize the measured product concentrations in the aqueous and solvent phase. The model equations were solved analytically in a dimensionless form and used to study the effects of flow conditions, solvent properties, product formation rate, droplet size, and contactor length on the extraction efficiency and product concentration profiles in the continuous and dispersed phase.
Phase stability with cubic equations of state: Global optimization approach
Authors: Harding S. T. , Floudas C. A.
Published: April 2004

Calculation of phase and chemical equilibria is of fundamental importance for the design and simulation of chemical processes. Methods of minimizing the Gibbs free energy provide equilibrium solutions that are candidates only for the true equilibrium solution, because the number and type of phases must be assumed before the Gibbs energy minimization problem can be formulated. The tangent plane stability criterion was used to determine the stability of a candidate equilibrium solution. The Gibbs energy minimization and tangent plane stability problems are challenging due to highly nonlinear thermodynamic functions. This work develops a global optimization approach for the tangent plane stability problem that provides a theoretical guarantee about the stability of the candidate equilibrium solution with computational efficiency. Cubic equations of state were used due to their ability to accurately predict the behavior of nonideal vapor and liquid phases across a broad range of pressures. The mathematical form of the stability problem was analyzed and nonlinear functions with special structure were identified to achieve faster convergence of the algorithm. This approach, when applied to the SRK, Peng‐Robinson, and van der Waals cubic equations of state, could address a variety of mixing rules. Computational results on problems with two‐eight components are presented.
In situ determination of the viscosity of gas mixtures containing trace quantities of oxygen
Authors: Hellgardt K. , Al‐Musa A. , Cumming I. W. , Mason G. , Buffham B. A.
Published: April 2004

An electrochemical cell was used to add trace quantities of oxygen to a gas stream. The viscosity of these gas mixtures was determined using an established method that measures the rate of change of viscosity with oxygen concentration. Viscosity data are reported for nitrogen/oxygen and helium/oxygen mixtures at very low oxygen concentrations. The data are compared with values for the viscosity of these gas mixtures, interpolated from results reported in the literature.
Pressure drop measurements in a microchannel
Authors: Pfund David , Rector David , Shekarriz Alireza , Popescu Aristotel , Welty James
Published: April 2004

Recent developments in micro‐energy and micro‐chemical systems have produced a need for greater understanding of flow in small channels. Several recent studies of friction factors and transition Reynolds numbers in rectangular microchannels have produced results that differ from classical theory. In this work, friction factors and laminar flow friction constants were determined for water flowing in high aspect ratio channels with depths ranging from 128 to 521 μm. Reynolds numbers were between 60 and 3,450. Pressure drops were measured within the channel itself to exclude entrance and exit losses. Transitions to turbulence were observed with flow visualization. Uncertainties in measured variables were quantified and propagated into the estimated friction constants. Friction factors were also determined in a 1,050‐μm‐deep channel that served as a control. After considering experimental uncertainties and systematic errors, significant differences remained between the results and classical theory.
Multivariable model state feedback: Computationally simple, easy‐to‐tune alternative to MPC
Authors: Mhatre Suraj , Brosilow Coleman
Published: April 2004

Methods for the design and tuning of multivariable model state feedback (MMSF) systems for complete and triangular output decoupling of inherently stable processes are presented. MMSF is a method of implementing multivariable IMC controllers that substantially reduces the complexity of the implementation and compensates for control effort saturation. Complexity is reduced because the controls are formed as a linear combination of past and current model states. Saturation‐induced directionality problems are avoided by temporarily increasing filter time constants to bring the control vector into the constraint set. The possibility that needed future controls will be inadequate to compensate for past control actions is avoided by setting minimum values for the controller filter time constants. The feedback structure of MMSF facilitates on‐line tuning, one process variable at a time. Also, because MMSF is a form of IMC, it can be tuned off‐line to accommodate process uncertainty using H∞ methods.
Wavelet‐based adaptive robust M‐estimator for nonlinear system identification
Authors: Wang D. , Romagnoli J. A. , Safavi A. A.
Published: April 2004

A wavelet‐based robust M‐estimation method for the identification of nonlinear systems is proposed. Because it is not based on the assumption that there is the class of error distribution, it takes a flexible, nonparametric approach and has the advantage of directly estimating the error distribution from the data. This M‐estimator is optimal over any error distribution in the sense of maximum likelihood estimation. A Monte‐Carlo study on a nonlinear chemical engineering example was used to compare the results with various previously utilized methods.
Tuning and design of single‐input, single‐output control systems for parametric uncertainty
Authors: Stryczek Karel , Laiseca Mario , Brosilow Coleman , Leitman Marshall
Published: April 2004

Frequency domain methods, which are for the design and tuning of control systems for processes with uncertain parameters, and are easy to use and interpret, are proposed. The advantages of the proposed methods over other H∞ methods are mainly that they require substantially less expertise on the part of the designer, and that they result in control system performance that closely approximates that which was desired. Several examples illustrate the proposed methods. A stable second‐order lag and dead time process, with gain and dead time uncertainties, provides the vehicle for studying controller designs for single and two degree of freedom control systems. Application of Mp tuning to an uncertain, unstable, first‐order lag, plus dead time process, shows that there can be a significant performance penalty associated with using a simple feedback control system rather than an internally stable model‐based control system. The performance of a controller for an integrating, oscillatory process obtained by Mp synthesis also compares favorably with that of a controller obtained by other investigators using loop shaping.
Engineering design methods for cavitation reactors II: Hydrodynamic cavitation
Authors: Gogate Parag R. , Pandit Aniruddha B.
Published: April 2004

The bubble behavior and hence the pressure generated at the collapse of the cavity for hydrodynamic cavitation depends on the operating conditions and geometry of the mechanical constriction generating cavitation. The effect of operating parameters such as inlet pressure through the system's orifice, initial cavity size, and the indirect effect of the hole diameter (it affects the frequency of turbulence in the vicinity of the orifice) on the bubble behavior was numerically studied. The bubble dynamics were simulated in two stages considering: Rayleigh‐Plesset equation up to the point of bubble wall velocity = 1,500 m/s; then the compressibility of the medium using the equation of Tomita and Shima. An empirical correlation was developed to predict the collapse pressure generated as a function of just mentioned parameters. The trends in the magnitudes of collapse pressure match the observed experimental trends for cavitation‐induced reactions. The work is an extension of the earlier analysis done for the sonochemical reactors. Some recommendations are also suggested for the design of hydrodynamic cavitation reactors based on the simulations.
Global optimization of mixed‐integer nonlinear problems
Authors: Adjiman C. S. , Androulakis I. P. , Floudas C. A.
Published: April 2004

Two novel deterministic global optimization algorithms for nonconvex mixed‐integer problems (MINLPs) are proposed, using the advances of the αBB algorithm for nonconvex NLPs of Adjiman et al. The special structure mixed‐integer αBB algorithm (SMIN‐αBB) addresses problems with nonconvexities in the continuous variables and linear and mixed‐bilinear participation of the binary variables. The general structure mixed‐integer αBB algorithm (GMIN‐αBB) is applicable to a very general class of problems for which the continuous relaxation is twice continuously differentiable. Both algorithms are developed using the concepts of branch‐and‐bound, but they differ in their approach to each of the required steps. The SMIN‐αBB algorithm is based on the convex underestimation of the continuous functions, while the GMIN‐αBB algorithm is centered around the convex relaxation of the entire problem. Both algorithms rely on optimization or interval‐based variable‐bound updates to enhance efficiency. A series of medium‐size engineering applications demonstrates the performance of the algorithms. Finally, a comparison of the two algorithms on the same problems highlights the value of algorithms that can handle binary or integer variables without reformulation.
Generation of microparticles using CO2 and CO2‐philic antisolvents
Authors: Sarkari Marazban , Darrat Inaas , Knutson Barbara L.
Published: April 2004

Precipitation using a compressed antisolvent (PCA) is a demonstrated technology for generation of monodisperse ultrafine particles. Microparticle precipitation using CO2‐philic antisolvents was studied with the ultimate goal of developing fundamental approaches to tailor microparticle generation. The ability to micronize small solutes and polymeric systems using CO2‐philic liquid antisolvents is compared to precipitation with compressed CO2 and traditional antisolvents. Analogies are made between PCA and CO2‐philic antisolvent precipitation based on thermodynamic driving forces and the dynamics of the spray process. Analysis of the interfacial gradient energies of the miscible solvent–antisolvent systems indicates that the spray process is dominated by turbulent mixing and not atomization. The similar microparticle morphologies of amorphous and semicrystalline poly(lactic acid) (PLA) obtained using liquid CO2‐philic antisolvents and compressed CO2 suggest that a similar demixing mechanism dominates microparticle formation at these operating conditions.
Glycosaminoglycan deposition in engineered cartilage: Experiments and mathematical model
Authors: Obradovic Bojana , Meldon Jerry H. , Freed Lisa E. , Vunjak‐Novakovic Gordana
Published: April 2004

Functional cartilaginous constructs for scientific research and eventual tissue repair were cultivated in bioreactors starting from chondrocytes immobilized on polymeric scaffolds. The scaffolds gradually degraded as the cells regenerated tissue matrix consisting of glycosaminoglycan (GAG) and type II collagen. To facilitate data interpretation and optimize cultivation conditions, a mathematical model was developed which yields the concentrations of oxygen and GAG as functions of time and position in growing tissue. Calculated GAG concentrations were qualitatively and quantitatively consistent with profiles measured via high‐resolution image processing of tissue samples cultured at two different oxygen tensions for various periods of time.
Bubble‐column and airlift photobioreactors for algal culture
Authors: Sánchez Mirón Asterio , García Camacho Francisco , Contreras Gómez Antonio , Grima Emilio Molina , Chisti Yusuf
Published: April 2004

Bubble columns and airlift photobioreactors can be useful for culturing phototrophic organisms requiring light as a nutrient. Light availability in bubble columns and airlift devices is influenced by aeration rate, gas holdup, and the liquid velocity (mixing and turbulence). The photosynthetically generated oxygen also needs to be removed, as excessive dissolved oxygen suppresses photosynthesis. Oxygen removal capacity is governed by the magnitude of the overall gas–liquid mass‐transfer coefficient, kLaL. This work characterizes the relevant hydrodynamic and mass‐transfer parameters in three air‐agitated reactors: bubble column, split‐cylinder airlift device and concentric draft‐tube sparged airlift vessel. The reactors are then evaluated for culture of the microalga Phaeodactylum tricornutum. All reactors were about 0.06 m3 in working volume, and the working aspect ratio was about 10. Data were obtained in tap water for a base‐line comparison and in Mediterranean seawater, as a potential medium for algal culture. A theoretical relationship was developed and proved between kLaL and the aeration rate. In addition, a method based on mechanistic relationships was proved for predicting the liquid circulation velocity and kLaL in airlift reactors. Existing correlations applied satisfactorily to gas holdup and kLaL data obtained in the bubble column. Aqueous solution of sodium chloride (0.15 M) closely resembled seawater in terms of its hydrodynamics and oxygen transfer behavior. Under the conditions tested, all three reactors attained a biomass concentration of about 4 kg·m−3 after ∼260 h. The mean maximum specific growth rate was 0.022 h−1 in all cases at a power input of 109 W·m−3.
Patterns of sedimentation from surface currents generated by turbulent plumes
Authors: Zarrebini Mehran , Cardoso Silvana S. S.
Published: April 2004

A theoretical model was developed for the dynamics and deposition pattern from gravity currents generated by axisymmetric particle‐laden plumes. This model incorporates the interaction between the particles in the environment and the continuing plume. This interaction plays a central role in the evolution of the behavior of the plume and on the sedimentation patterns on the surrounding floor. Theoretical predictions for the deposition patterns were successfully compared with data from laboratory experiments for environments of both infinite and finite lateral extent. The evolution of the particle concentration in the environment and in the surface current until steady state is attained was also examined. A simple analytical expression is proposed for the sedimentation rate on the ground in the case of dilute plumes. The application of these new results to estimate the patterns of sedimentation from industrial chimney emissions is also discussed.
Estimation of transport parameters of RO membranes for seawater desalination
Authors: Taniguchi Masahide , Kimura Shoji
Published: April 2004

Membrane transport parameters from a running plant data were studied to understand their changes with time, as well as the causes of membrane fouling under various running conditions. First, the mass‐transfer coefficient of a spiral‐type reverse osmosis (RO) element was measured using a polyamide membrane, designated UTC‐80, and an appropriate correlation equation was obtained. Then, a computer program was developed to obtain membrane transport parameters from flux and rejection data of an actual RO plant. When it became apparent that the temperature corrections should be applied, laboratory experiments were used to determine them. Thus, the time course change of both solution and salt permeabilities and the relation between them could be followed. The permeability of each ion or solute can be obtained by the same method when their analytical data are available.
Two‐step procedure for data‐based modeling for inferential control applications
Authors: Amirthalingam Raja , Sung Su Whan , Lee Jay H.
Published: April 2004

A two‐step procedure for building an inferential control model, which uses both historical operation data and plant test data, is proposed. Motivation for using the two types of data is given, and a systematic way to combine them in the model‐identification step is proposed. Some potential problems associated with the procedure in practice and their solutions are discussed. The efficacy of the procedure is demonstrated in a case study involving a multicomponent distillation column simulated in HYSYS.
Construction and optimization of complex surface‐reaction mechanisms
Authors: Aghalayam Preeti , Park Young K. , Vlachos Dionisios G.
Published: April 2004

A multistep methodology for the quantitative determination of rate constants of a detailed surface‐reaction mechanism is proposed. As a starting point, thermodynamically consistent, coverage‐dependent activation energies and heats of reactions were derived from the application of the unity bond index–quadratic exponential potential formulation, and initial estimates of the preexpontentials were obtained from transition‐state theory or available experiments. Important feature identification analysis was performed to determine key kinetic parameters for various experiments. Model responses were parameterized in terms of these important parameters by polynomials and factorial design techniques, and these parameterized responses were subsequently used in simultaneous optimization through simulated annealing against different sets of experimental data to obtain a quantitative reaction mechanism that is valid over a wide range of operating conditions. The technique was successfully applied to the development of a comprehensive reaction mechanism for H2/air mixtures on polycrystalline Pt.
Dynamic and steady‐state features of a cooled countercurrent flow reactor
Authors: Garg Rohit , Luss Dan , Khinast Johannes G.
Published: April 2004

A countercurrent flow reactor (CFR) is an autothermal reactor, in which the reactants flow countercurrently at equal velocities and flow rates through two compartments, with negligible heat‐transfer resistance between the two sections. Two‐reactor configurations can be used: a “regular” one, in which the two components are fed by two separate feeds; a “folded” configuration, in which a single feed flows first through one compartment and then reverses its direction and flows countercurrently in the second. Temperature and concentration profiles in the regular CFR configuration usually have mirror symmetry around its center. Under cooling, however, asymmetric steady‐state profiles may form. The interaction between the heat removed by the effluents and by cooling through the wall may generate complex periodic or even chaotic states. The temperature and concentration profiles in a folded CFR are the same as those attained in a regular CFR, which are symmetric around the reactor center. The imposed symmetry causes the steady‐state and dynamic features of the folded CFR to be less complicated than those of the regular CFR and is less likely to cause operational problems. Boundaries of parameter regions with qualitatively different features for both CFR configurations were constructed by the method of Khinast et al. (1998a). The analysis of the CFR enhances the understanding of similar dynamics observed in a reverse‐flow reactor.
Engineering approximations for activation energies in hydrogen transfer reactions
Authors: Blowers Paul , Masel Rich
Published: April 2004

Previous investigators have often used the Polanyi relationship, extensions due to Seminov, or the BOC‐MP/UBI‐QEP relationship to estimate activation barriers in complex reaction networks. However, these relationships are not accurate for many reactions. In this article, a new engineering approximation for the estimate of activation barriers is presented. The approximation allows one to estimate barriers analytically, using known heats of reaction and one adjustable parameter, the intrinsic barrier. The equation was fit to the recommended values of activation energies for all 151 independent hydrogen transfer reactions in the NIST tables. All but nine of the reactions are fit within ±4 kcal/mol. Of the nine failures, seven of the data points came from previous estimates, and may not be accurate. By comparison, Seminov's relationship fails for 47 of the reactions even though the Seminov relationship has three adjustable parameters compared to one in our approximation. The BOC‐MP/UBI‐QEP method gives errors of more than 10 kcal/mol for all 151 reactions and errors of more than 30 kcal/mol for many of the reactions. These results show that the new approximation is a significant improvement over the approximations that are currently in use.
High‐resolution adsorption isotherms of supercritical carbon dioxide on activated carbon
Authors: Humayun Raashina , Tomasko David L.
Published: April 2004

Supercritical fluids are attractive solvents for heterogeneous processes, including catalysis and adsorptive separation. However, adsorption processes in the near‐critical region are poorly understood and exhibit unique behavior where the adsorption of the supercritical solvent plays an important role in the solute adsorption. The behavior of supercritical fluids in confined pores has been studied theoretically, but there are few experimental data on their behavior in industrially important microporous materials. The adsorption of carbon dioxide on Calgon F400 activated carbon over a wide range of pressures (0–20 MPa) at temperatures near the critical point of carbon dioxide (30 to 45°C) was studied. Near‐continuous adsorption and desorption isotherms were measured with a new flow gravimetric apparatus with precise control over pressure and temperature. As pressure is increased, the excess adsorption increases sharply at low pressures; then a broad maximum is observed. At temperatures greater than the critical temperature, there is a sharp drop in excess adsorption near the critical region where the density of the bulk fluid increases sharply. A crossover is observed near the critical region where, below a certain pressure, the excess adsorption decreases with temperature, while above the crossover point the trend is reversed. When analyzed as a function of solvent density, the crossover disappears, revealing an anomalous maximum in total adsorption near the critical point similar to the enhanced local density or “charisma” observed in binary solute–supercritical fluid systems. A 2‐D EOS model using the 2‐D Peng‐Robinson EOS was able to qualitatively describe the adsorption behavior over the entire pressure range, but the quantitative agreement was poor in the near‐critical region.
Equilibrium and diffusion of methanol and water in a nafion 117 membrane
Authors: Gates Craig M. , Newman John
Published: April 2004

The uptake concentrations of methanol and water in a Nafion membrane are measured as a function of methanol mol fraction from a liquid methanol–water mixture. The results are fitted by a thermodynamically consistent equation, derived from an expression of the Gibbs energy. The uptake of pure methanol and pure water from the vapor phase is measured as a function of vapor pressure. A discontinuity, known in the literature as Schroeder's paradox, between membrane uptakes of pure methanol and pure water from the liquid and vapor phases is noted. Regions of unconditional instability are enveloped by spinodal loci for the ternary mixture in the membrane when equilibrated with liquid mixtures. The effective diffusion coefficient is measured at 60°C for diffusion from a vapor into a saturated Nafion membrane. The effective diffusion coefficients are found to be on the order of 10−11 m2/s, which is comparable to other measurements made for diffusion of a vapor through the membrane.
Early warning of agglomeration in fluidized beds by attractor comparison
Authors: van Ommen J. Ruud , Coppens Marc‐Olivier , van den Bleek Cor M. , Schouten Jaap C.
Published: April 2004

An enhanced monitoring method, based on pressure fluctuation measurements, for observing nonstationarities in fluidized‐bed hydrodynamics is presented. Experiments show that it can detect small changes in the particle‐size distribution. Such a monitoring method is useful to give an early warning of the onset of agglomeration in a fluidized bed. In contrast to earlier methods, this method is insensitive to small changes in superficial gas velocity and can handle multiple signals, making it relevant to industrial application. By carefully choosing the measurement position, the method becomes also insensitive to small bed mass variations. It uses the attractor reconstructed from a measured pressure signal, which is a “fingerprint” of the hydrodynamics of the fluidized bed for a certain set of conditions. Using this method statistically the reconstructed attractor of a reference time series of pressure fluctuations (representing the desired fluidization behavior) is compared with that of successive time series measured during the bed operation.
Predicting LLE in mixed‐solvent electrolyte systems by an electrolyte EOS
Authors: Zuo Julian Youxiang , Zhang Dan , Fürst Walter
Published: April 2004

This work extends the previously published aqueous electrolyte equation of state (AEEOS) to predict liquid–liquid equilibria (LLE) of mixed‐solvent electrolyte systems. Interaction parameters between ions and organic solvents, and cations and anions were determined by fitting the experimental vapor‐pressure data of binary methanol + halide electrolyte mixtures, and then correlated to the cationic Stokes and anionic Pauling diameters. The focus is on the ionic standard/reference state and the standard Gibbs energy for transferring salts from one solvent to another. The methods applied to predict LLE of several ternary water + organic solvent + salt system are to select: 1. the hypothetical ideal gas at unit mol fraction, the system temperature, and 1 bar as the ionic standard state; 2. the infinite dilution in the solvent mixture as the ionic reference state, whose activity coefficients were converted to those at the infinite dilution in pure water by the ionic standard Gibbs energy of transfer. The predicted LLE results agree well with the measured data without any adjusted parameters in fitting the ternary experimental data. The extended AEEOS is comparable to the model of Zerres and Prausnitz, but the latter requires two adjusted parameters in fitting the ternary experimental data for each ternary system.
Ligand diffusion, receptor binding, and internalization: Zeroth‐order kinetics
Authors: Strieder William
Published: April 2004

The exact, analytical solution for steady‐state diffusion and zeroth‐order surface reaction at a cell‐wall chemoreceptor site is presented. Following Berg (1993), and others, the receptor is modeled as a chemically active circular site on an impermeable, inert cell wall. Explicit values of the dimensionless site‐specific rate constant mark the regions of chemical‐, transitional‐ and diffusion‐controlled kinetics. Particularly when the Michaelis‐Menten constant is small relative to the ligand bulk concentration, this simple form of the receptor‐site reaction current should provide a useful model for cell growth‐factor internalization, nutrient uptake, and certain cellular signaling behaviors. Various biophysical conditions that cause site mass‐transport resistance in cellular site‐reaction kinetic processes are discussed.
Mass‐transfer calculation methods for transient diffusion within particles
Authors: Alopaeus Ville
Published: April 2004

Modeling of diffusion within spherical particles is often an important part of modeling of many processes relevant to chemical engineering such as adsorption. The accurate solution of the differential equations describing diffusion is quite a complicated task involving calculation of infinite series. A rational approximation to the diffusion equation is presented in this article leading to a solution that is both very easy to use and very accurate. The present method was compared to the exact solution for diffusion and adsorption in spheres, and it was found that the present solution is practically indistinguishable from the exact solution. This method is also applicable to multicomponent systems in a straightforward manner.
Influence of impeller type on the flow structure in a stirred reactor
Authors: Revstedt Johan , Fuchs Laszlo , Kovács Tamas , Trägårdh Christian
Published: April 2004

Large eddy simulations (LES) of a stirred reactor with a liquid volume of 0.64 m3 were performed. The tank was stirred by either two standard Rushton impellers or two Scaba 6SRGT impellers. The LES can provide details of the flow field that cannot be obtained with so‐called Reynolds‐averaged equations and the corresponding models. The unsteady spatially filtered incompressible Navier‐Stokes equations were solved together with a transport equation for the concentration of an inert additive using a multigrid finite difference code with a staggered grid. The unresolved turbulent scales were modeled using a scale similarity model. The aim of this investigation has been to study the influence of impeller type on the flow structure and scalar transport. The results show that, compared at equal power input, the center plane velocities and the volume flow in the impeller stream do not differ much for the impeller types. Also, for the 6SRGT the periodic fluctuations associated with the blade passing is less pronounced.
Representation of high‐dimensional, molecular solid‐liquid phase diagrams
Authors: Samant Ketan D. , Berry David A. , Ng Ka M.
Published: April 2004

A generic framework for the representation of multicomponent molecular solid‐liquid phase diagrams in the form of digraphs is presented. Pure components, eutectic points, and saturation points in the solid‐liquid composition space are represented as vertices of the digraph. These vertices are connected to form edges on the basis of the adjacency matrix, which is constructed by following a set of generic rules derived from the Gibbs phase rule. These edges form the boundaries of high‐dimensional saturation varieties. Included in the framework is a procedure for plotting high‐dimensional isobaric and isothermal isobaric phase behavior in the form of projections and cuts. With this framework, solid‐liquid phase behavior of multicomponent systems can be easily represented and studied. Applications include facilitation of experimental development of high‐dimensional phase diagrams and synthesis of crystallization‐based separation processes. The latter is illustrated with the separation of a quinary mixture.
High interaction regime Lockhart‐Martinelli model for pressure drop in trickle‐bed reactors
Authors: Pinna Dario , Tronconi Enrico , Tagliabue Lorenzo
Published: April 2004

Pressure drop and liquid saturation in a cold model representative of trickle‐bed reactors operating in the high interaction regime were measured using air and water at atmospheric pressure. Five different packing similar in shape and dimension to typica1 industrial catalysts (cylindrical pellets, raschig rings, extrudates) were tested, covering a wide range of geometric characteristics. The adequacy of the most recent literature correlations for pressure drop in trickle‐bed reactors was checked. The best predictions are provided by an empirical correlation based on the Lockhart‐Martinelli parameters. A new model of similar accuracy was derived, which provides a theoretical basis for the successful application of the Lockhart‐Martinelli approach.
Level‐by‐level flowsheet synthesis methodology for thermal system design
Authors: Manninen Jussi , Zhu X. X.
Published: April 2004

A new level‐by‐level methodology for flowsheet synthesis presented. In the first level, an initial superstructure is constructed to form a master model. Optimization of the master gives an initial optimal flowsheet, which indicates major structural features. In the second level this initial flowsheet is analyzed using exergy analysis to identify relevent modification options. The Promising options are used to update the intial superstructure, which is optimized to obtain an improved design. The results from the above design process determine the flowsheet of overall system and provide targets for design of subsystems. Design of subsystems forms the third design level. The predetermined of subsystems. Design of subsystems froms the third design level due to use of simplistic models in the previous design levels. The master model is updated and then Optimized again. The overall design process terminates when designs for subsystems can meet the design targets determined by the master model.
Reliable computation of mixture critical points
Authors: Stradi Benito A. , Brennecke Joan F. , Kohn P. , Stadtherr Mark A.
Published: April 2004

The determination of critical points of mixtures is important for both practical and theoretical reasons in modeling phase behavior, especially at high pressure. This article presents the first completely reliable method for locating all the critical points of a given mixture. The method also verifies the nonexistence of a critical point if a mixture of a given composition does not have one. The methodology used is based on interval analysis, in particular an interval Newton/generalized bisection algorithm providing a mathematical and computational guarantee that all mixture critical points are located. The procedure is initialization‐independent and thus requires no a priori knowledge of the number of mixture critical points or their approximate locations. The technique is illustrated using several example problems involving cubic equation‐of‐state models; however, the technique is for general purpose and can be applied in connection with other thermodynamic models.
New method for conversion of gas sonic velocities to density second virial coefficients
Authors: Mossaad Ehab , Eubank Philip T.
Published: April 2004

A new, numerical method was developed to calculate density second virial coefficients B(T) from sonic velocity measurements in gases at low pressures. Unlike existing methods, this procedure requires no model assumption as to the form of the temperature variation of B(T). Rather, it differences the measured accoustic second virial coefficient according to a new mathematical identity. While two higher‐ordered terms in the identity must be ignored to initiate the numerical calculations, the magnitude of these terms can later be found from the initial determination of B(T). This article describes new equations developed, numerical procedures, calculations with Redlich‐Kwong and Peng‐Robinson gases and, with a Lennard‐Jones model gas, to show the accuracy of the method and the magnitude of the two error terms, and the method applied to existing sets of sonic velocity data.
Preparation of enzyme‐in‐polymer composites with high activity and stability
Authors: Kim Jungbae , Kosto Timothy J. , Manimala Joseph C. , Nauman E. Bruce , Dordick Jonathan S.
Published: April 2004

Flash devolatilization was applied to incorporate an enzyme into a polymer matrix, which created a novel and highly active biocatalytic composite suitable for use in both aqueous and organic media. Enzymes were codissolved in toluene with commercial, high‐molecular‐weight polymers, and the solvent was then rapidly removed by injecting the mixture into a vacuum chamber. Subsequent cross‐linking of the enzyme with glutaraldehyde resulted in stable entrapped enzyme within the polymeric matrices. For example, a composite of α‐chymotrypsin and low‐density polyethylene showed no significant loss of enzymatic activity in aqueous buffer over a period of one month. The normalized activity of this biocatalytic material in organic solvents was 3–13 times higher than that of native α‐chymotrypsin lyophilized from aqueous buffer. Washing the composite material with aqueous buffer increased the activity in isooctane an additional ten fold. The composites of α‐chymotrypsin and polyethylene demonstrated the feasibility of obtaining active and stable biocatalytic materials via the application of compositional quenching to a biological system.
Scale effect on porosity and permeability: Kinetics, model, and correlation
Authors: Civan Faruk
Published: April 2004

Variation of porosity and permeability by scale dissolution and precipitation in porous media is described based on fractal attributes of the pores, realization of flow channels as a bundle of uniformly distributed mean‐size cylindrical and tortuous hydraulic flow tubes, a permeability‐porosity relationship conforming to Civan's power law flow units equation, and the pore surface scale precipitation and dissolution kinetics. Practical analytical solutions, considering the conditions of typical laboratory core tests and relating the lumped and phenomenological parameters, were derived and verified by experimental data. Deviations of the empirically determined exponents of the pore‐to‐matrix volume ratio compared to the Kozeny‐Carman equation were due to the relative fractal dimensions of pore attributes of random porous media. The formulations provide useful insights into the mechanism of porosity and permeability variation by surface processes and accurate representation of the effect of scale on porosity and permeability by simpler lumped‐parameter models.
Viscous‐Gravity spreading of oil on water
Authors: Chebbi Rachid
Published: April 2004

Spreading of oil on calm water is analyzed for the second stage of spread, which is controlled by a balance between gravity and viscous forces. The analysis, using the integral boundary‐layer method, agreed well with a numerical solution for the case of unidirectional spread. The analysis is extended to include axisymmetric spread for which there is no sound theoretical treatment presented in the literature. The approximations made in the model are discussed based on the lubrication approximation and order‐of‐magnitude arguments.
Free drainage in aqueous foams: Model and experimental study
Authors: Magrabi S. A. , Dlugogorski B. Z. , Jameson G. J.
Published: April 2004

Free drainage in compressed‐air foams were studied experimentally and theoretically. The time evolution of liquid holdup profiles in a 0.2‐m‐high by 0.29‐m‐diameter foam column was determined at various heights by measuring sonic velocity. A new experimental technique was devised to measure the true drainage rate of surfactant solution leaving the foam column. A drainage model was outlined to predict the discharge rate and evolution of the liquid‐fraction profile in aqueous foams. The model led to the formulation of a nonlinear partial differential equation in which the liquid fraction was used explicitly as a dependent variable. The model was applied with one adjustable parameter to simulate drainage in foams made from fluorocarbon surfactants containing mobile plateau border and film walls. The liquid‐fraction profiles, drainage rates, and final equilibrium liquid profiles depended strongly on the surface mobility of plateau border and film walls; the bubble size; and therefore on the coarsening histoy in the foam under study. Over longer time periods this model needs to be coupled with interbubble gas diffusion to account for coarsening‐induced drainage.