bluuebunny/crossref_metadata_2025_split huggingface dataset:
Brownian dynamics of sterically‐stabilized colloidal suspensions
Authors: TeGrotenhuis Ward E. , Radke Clayton J. , Denn Morton M. 
Published: June 2004

AbstractUsing Brownian dynamics, equilibrium and dynamic properties are calculated for colloidal suspensions that are stabilized throug the Milner, Witten and Cates (1988) steric potential. Results are reported for osmotic pressure, radial distribution functions, static structure factors, and self‐diffusion coefficients. The sterically‐stabilized systems are also approximated by equivalent hard spheres, with good agreement for osmotic pressure and long‐range structure. The suitability of the potential to model the behavior of a real system is explored by comparing static structure factors calculated from Brownian dynamics simulations to those measured using SANS (Markovic et al., 1986). Finally, the effects of Hamaker and hydrodynamic forces on calculated properties are investigated.
Drop size distribution and holdup profiles in a multistage extraction column
Authors: Tsouris C. , Kirou V. I. , Tavlarides L. L. , Wang C. Y. 
Published: June 2004

AbstractA population‐balance‐equation model is employed for the analysis of liquid‐liquid extraction columns. This model considers drop breakage, coalescence, and exit phenomena for the drop phase caused by drop‐drop and drop‐continuous phase interactions. Drop breakage and coalescence rates are employed from a previous study on liquid dispersions in stirred‐tank contactors. A drop exit frequency is developed based on a stochastic modeling approach. The model is tested by drop size distribution and dispersed‐phase volume fraction (holdup) data obtained for a multistage column contactor of pilot‐plant scale. Steady‐state drop size distribution and transient holdup measurements are obtained by a photomicrographic technique and an ultrasonic technique, respectively. The model can predict flooding of the column. The effect of mass transfer on the hydrodynamic parameters of the contactor is also examined. The population‐balance‐equation model can be used for the control of extraction columns and can be extended to include mass‐transfer calculations for the prediction of extraction efficiency.
A knowledge‐based framework for automating HAZOP analysis
Authors: Venkatasubramanian Venkat , Vaidhyanathan Ramesh 
Published: June 2004

AbstractHazard and operability (HAZOP) analysis is the study of systematically identifying every conceivable abnormal process deviation, its abnormal causes and adverse hazardous consequences in a chemical plant. HAZOP analysis is a difficult, time‐consuming, and labor‐intensive activity. A automated HAZOP system can reduce the time and effort involved in a HAZOP review, make the review more thorough and detailed, and minimize or eliminate human errors. Towards that goal, a knowledge‐based system, called HAZOPExpert, has been proposed in this article. In this approach, HAZOP knowledge is divided into process‐specific and processindependent components in a model‐based manner. The framework allows for these two components to interact during the analysis to address the process‐specific aspects of HAZOP analysis while maintaining the generality of the system. Process‐general knowledge is represented as HAZOP models that are developed in a process‐independent manner and are applicable to a wide variety of process flowsheets. The important features of HAZOPExpert and its performance on an industrial case study are described.
Diffusion of oxygen, nitrogen and their mixtures in carbon molecular sieve
Authors: Chen Y. D. , Yang R. T. , Uawithya P. 
Published: June 2004

AbstractDiffusivities of oxygen, nitrogen and their mixtures in carbon molecular sieve are measured under conditions used for kinetic separation of air by adsorption (elevated pressures). In binary diffusion, codiffusion enhances fluxes for both components, whereas counterdiffusion has the opposite effects; for both cases, the effects are significantly more pronounced for the fast diffusing component. The multicomponent diffusion theory developed earlier in this laboratory for surface diffusion is reformulated for diffusion in molecular sieves. Multicomponent diffusivities can be predicted from the following pure‐component information: concentration‐dependent diffusivities and diffusional activation energies. Predicted binary diffusion results agree fairly with the experimental data for oxygen and nitrogen in carbon molecular sieve.
Structural analysis and output feedback control of nonlinear multivariable processes
Authors: Daoutidis Prodromos , Kumar Aditya 
Published: June 2004

AbstractThis article concerns general multivariable nonlinear processes, particularly those with singular characteristic matrix. A precise characterization of the structural process properties that give rise to generic singularity of the characteristic matrix is initially developed within a graph‐theoretic analysis framework. An output feedback controller synthesis problem is then formulated for multivariable processes with singular characteristic matrix. A comprehensive solution to this problem is obtained through a combination of dynamic state feedback controllers and state observers. The performance and robustness characteristics of the proposed control methodology are illustrated through simulations in a double‐effect evaporator.
Oxide ion transport for selective oxidative coupling of methane with new membrane reactor
Authors: Nozaki Takao , Fujimoto Kaoru 
Published: June 2004

AbstractOxidative coupling of methane was conducted by using membrane reactors. The nonporous membrane film that consisted of PbO modified by alkaline or alkaline earth compound was supported on porous SiO2‐Al2O3 tube. Higher hydrocarbons were successfully synthesized with high selectivity (about 90%). A kinetic analysis was conducted to clarify whether oxide ion transportation through PbO film participated in the oxidative coupling of methane. The evaluated value of the diffusion coefficient of oxide ion transport based on the methane oxidation agreed well with that of published data. The simulated gradient of the oxide ion concentration through the PbO membrane agreed well with that measured by electron probe X‐ray microanalyzer. A transient response simulated by using kinetic parameters evaluated from steady‐state analysis also agreed well with the experiment. These results prove the validity of the reaction model that consists of surface reactions of methane with oxide ion which is transferred from inside to outside of the membrane reactor. Alkali modifiers on the PbO membrane surface exhibited a promotional effect on the surface reaction of methane coupling. Another membrane reactor containing Bi2O3 showed higher activity than the PbO membrane.
Prediction of the properties of model polymer solutions and blends
Authors: Ghonasgi D. , Chapman Walter G. 
Published: June 2004

AbstractIn this work we present an extension of the statistical associating fluid theory (SAFT) developed for associating fluids by applying this extension to polymer solutions and blends. The polymer molecule is modeled as a flexible chain made of bonded spherical segments. These segments interact with the Lennard‐Jones potential. The extension of SAFT is compared with molecular simulation results for polymer solutions and blends. Since the same force model is used in the simulation and theory, this comparison is a strong test of the assumptions made in deriving the theory. Results are presented for pure polymers of up to 50 segments long, polymer solutions of a polymer 20 segments long dissolved in its own monomer, and blends of polymers of 25 segments each. The theory produces results that are in better agreement with simulation results than the Flory‐Huggins theory for the systems studied.
Hysteresis‐dependent adsorption‐desorption cycles: Generalization for isothermal conditions
Authors: Rajniak Pavol , Yang Ralph T. 
Published: June 2004

AbstractHysteresis‐dependent adsorption‐desorption equilibrium cycles are measured for the systems water vapor‐silica gel at 25°C. Various and distinct types of equilibrium cycles are observed, depending on the relative positions of the limiting partial pressures with respect to the closure points of the main hysteresis loop as well as the history of the cycle. Hysteresis‐dependent cycles are classified accordingly. The simple theoretical model based on the network pore blocking theory for predicting primary and secondary adsorption and desorption isotherms (Rajniak and Yang, 1993) is extended for predicting isotherms of higher orders and equilibrium cycles of all types. Only primary adsorption and desorption isotherms are needed for the predictions. Furthermore, there exists an infinite number of equilibrium cycles between any two partial pressures within the main hysteresis loop; the exact path of the cycle depends on the history before the cycle is initiated—it is memory‐dependent.
Nonlinear control of a polymerization CSTR with singular characteristic matrix
Authors: Soroush Masoud , Kravaris Costas 
Published: June 2004

AbstractThis experimental work concerns the multivariable nonlinear control of a pilotsize continuous polymerization reactor with generically singular characteristic matrix. The control problem is to control conversion and temperature in a continuous stirred tank reactor by manipulating two coordinated flow rates (reactor residence time) and two coordinated heat inputs. A nonlinear controller is synthesized within the framework of the globally linearizing control (GLC) method and is implemented on a microcomputer. Conversion is inferred from on‐line measurements of density and temperature. A key feature of the control problem is that its characteristic matrix is generically singular. Singularity of the characteristic matrix is handled by using a dynamic input/output linearizing state feedback rather than a static feedback. A reduced‐order observer is used to calculate the monomer, initiator, and solvent concentration estimates, which are needed for the calculation of controller action. In the presence of active state and input constraints, the reactor‐startup and setpointtracking performance of the controller is evaluated through experimental runs.
Flow structure in a three‐dimensional bubble column and three‐phase fluidized bed
Authors: Chen R. C. , Reese J. , Fan L.‐S. 
Published: June 2004

AbstractMacroscopic flow structures of 3‐D bubble columns and gas‐liquid‐solid fluidization systems under various operating conditions are studied using particle image velocimetry. Flow visualization is also conducted with the aid of a laser sheeting technique. The refractive index matching technique is used to eliminate the opaqueness of solid particles occurred in the visualization study of gas‐liquid‐solid fluidization. Three flow regimes (dispersed bubble, vortical‐spiral flow, and turbulent flow) are identified. The flow structure is investigated for various operating variables including liquid velocity, gas velocity, and particle holdups. Four flow regions (descending flow, vortical‐spiral flow, fast bubble flow, and central plume) can generally be characterized in the vortical‐spiral flow regime where the gross circulation pattern occurs. A conceptual model for the flow structure in the vortical‐spiral flow regime is discussed. The transition of the flow regimes and structure in the vortical‐spiral flow regime is postulated to be related to the Taylor instability for flow between two concentric rotating cylinders. Similarities between the flow structures of 2‐ and 3‐D beds are also discussed.
Pressure swing adsorption reactors: Simulation of three‐step one‐bed process
Authors: Lu Z. P. , Rodrigues A. E. 
Published: June 2004

AbstractA three‐step (feed, delay, and purge), one‐column pressure swing adsorption reactor has been analyzed. An equilibrium model taking into account species mass balances, overall mass balance, momentum balance, reaction rate, and nonlinear multicomponent adsorption equilibrium isotherms was derived to describe the PSA reactor process. Numerical solution of model equations was obtained by using the method of orthogonal collocation on finite elements, and simulation results showed that 10–20% higher conversion than reaction equilibrium conversion in a fixed‐bed reactor could be obtained from the PSA reactor for a reversible reaction A ⇄ B + C, such as dehydrogenation of ethane to ethylene and hydrogen on cadmium‐exchanged zeolites, where only product B was assumed to be adsorbed. When both reactant A and product B are adsorbed, 10% higher conversion over the reaction equilibrium limit was observed for pure reactant feed. The effects of system and operating parameters on the process performance of the PSA reactor have been studies, as well as the bed dynamics of the PSA reactor at the cyclic steady state.
Computation of holdups in fluidized and trickle beds by computer‐assisted tomography
Authors: Kantzas Apostolos 
Published: June 2004

AbstractA fourth‐generation X‐ray computer‐assisted tomography (CAT) scanner was used for the study of fluidized and trickle beds. A variety of exploratory experiments were performed, and images of density and holdup were obtained in three dimensions as a function of time. The gas/solid fluidized beds were glass bead/nitrogen and polyethylene/nitrogen systems. The trickle bed was a glass bead/nitrogen/water system. The algorithms presented can be used to determine holdup from X‐ray absorption data. These algorithms—extensions of similar algorithms used for the calculation of fluid saturations in porous media—are applied on data of CAT scanner images as they are generated by the scanner and after they are transferred to a workstation. The results of the presented tests demonstrate how detailed holdup calculations can be performed in a chemical reactor. Radial and longitudinal variances of the holdup can be easily defined. The proposed algorithms can assist in the design of chemical reactor prototypes.
Modeling chemical processes using prior knowledge and neural networks
Authors: Thompson Michael L. , Kramer Mark A. 
Published: June 2004

AbstractWe present a method for synthesizing chemical process models that combines prior knowledge and artificial neural networks. The inclusion of prior knowledge is investigated as a means of improving the neural network predictions when trained on sparse and noisy process data. Prior knowledge enters the hybrid model as a simple process model and first principle equations. The simple model controls the extrapolation of the hybrid in the regions of input space that lack training data. The first principle equations, such as mass and component balances, enforce equality constraints. The neural network compensates for inaccuracy in the prior model. In addition, inequality constraints are imposed during parameter estimation. For illustration, the approach is applied in predicting cell biomass and secondary metabolite in a fed‐batch penicillin fermentation. Our results show that prior knowledge enhances the generalization capabilities of a pure neural network model. The approach is shown to require less data for parameter estimation, produce more accurate and consistent predictions, and provide more reliable extrapolation.
Monitoring batch processes using multiway principal component analysis
Authors: Nomikos Paul , MacGregor John F. 
Published: June 2004

AbstractMultivariate statistical procedures for monitoring the progress of batch processes are developed. The only information needed to exploit the procedures is a historical database of past successful batches. Multiway principal component analysis is used to extract the information in the multivariate trajectory data by projecting them onto low‐dimensional spaces defined by the latent variables or principal components. This leads to simple monitoring charts, consistent with the philosophy of statistical process control, which are capable of tracking the progress of new batch runs and detecting the occurrence of observable upsets. The approach is contrasted with other approaches which use theoretical or knowledge‐based models, and its potential is illustrated using a detailed simulation study of a semibatch reactor for the production of styrene‐butadiene latex.
Effect of sample size on the microwave heating rate: Oil vs. water
Authors: Barringer S. A. , Davis E. A. , Gordon J. , Ayappa K. G. , Davis H. T. 
Published: June 2004

AbstractIt is generally believed that oil samples heat faster in a microwave oven than do water samples of the same mass. For sufficiently large and thich samples this conventional wisdom is indeed correct, but this trend can be far from true in smaller samples. In a commercially‐made home microwave oven, we observed that with decreasing sample size the heating rate of a water sample increases much faster than that of an oil sample. At 50 g the heating rate of a water sample is several times greater than that of an oil sample. Additionally, in studies of cylindrical samples in a customized oven having a unidirectional microwave source, the heating rate of water samples smaller than 2.4 cm in radius is greater than that of oil samples and is a strongly oscillatory increasing function of decreasing sample radius. Combining Maxwell's theory of microwave penetration and the heat conduction equation, we show that this previously unreported oscillatory heating behavior results from the added power absorbed by samples due to resonant absorption of microwaves. The added power arises from standing waves produced by internally reflected microwaves. This effect is small for oil because only 3% of the microwave power is reflected at an oil‐air interface. On the other hand, 64% is reflected at a water‐air interface, which causes strong resonant heating. Our findings might prove to be useful for future consumer food product development or oven design.
Mathematical analysis of potentiometric oxygen sensors for combustion‐gas streams
Authors: Baker Daniel R. , Verbrugge Mark W. 
Published: June 2004

AbstractThe mathematical tools necessary to describe quantitatively the chemical processes that dictate the performance of exhaust oxygen sensors are developed. Such sensors are used commonly to monitor exhaust streams generated by internal‐combustion processes. Calculated results compare well with available experimental results, although several open questions are idetified that require more experimental data. The mathematical formalism for describing the transport of gaseous species through the porous spinel structure protecting the platinum electrode on the exhaust side of the sensor is developed based on the Stefan‐Maxwell equations. The kinetic processes occurring at the interface formed by the platinum electrode and the spinel structure, including the oxidation of hydrogen and carbon monoxide and various adsorption‐desorption reactions, enter as boundary conditions for the transport equations. The analysis enables one to calculate the sensor's voltage response as a function of the air‐to‐fuel ratio λ and to investigate phenomena such as the magnitude of the voltage jump in going from rich to lean gas mixtures and the λ value at which this jump occurs.
Displacement chromatography of amino acids: Effects of selectivity reversal
Authors: Carta Giorgio , Dinerman Adam A. 
Published: June 2004

AbstractSeparating mixtures of α‐amino butyric acid (ABA) and isoleucine (Ile) by displacement chromatography with a cation exchange resin and an alkaline displacer leads to the formation of a displacement train, in which only ABA is recovered as a pure component, while Ile is recovered in a mixed band with ABA. The purity of the Ile band depends on the concentration of the displacer, and essentially pure Ile is obtained when the displacer concentration is reduced to a low value. The observed behavior is explained by considering the nonideality of the equilibrium uptake of these amino acids by the resin. An equilibrium model, representing the pure component isotherms, predicts variable selectivity and the occurrence of selectivity reversal in the two‐component system. Analyses using this model based on the theory of coherent waves and a numerical solution of the conservation equations for the displacement chromatography process agree with the observed behavior, predicting the formation of “azeotropic” bands during the development of the displacement train. Such analyses are used to determine under what conditions a complete separation may be obtained for a given experimental system.
Equations of state and activity coefficient models for vapor‐liquid equilibria of polymer solutions
Authors: Kontogeorgis Georgios M. , Fredenslund Aage , Economou Ioannis G. , Tassios Dimitrios P. 
Published: June 2004

AbstractFour noncubic equations of state, (EOS) and five activity coefficient models are applied to binary polymer and solvent solutions. Solvent activities at intermediate concentrations and equilibrium pressures are predicted with the perturbed‐soft‐chain theory (PSCT), group‐perturbed‐soft‐chain theory, (GPSCT), group‐contribution‐lattice fluid (GCLF) EOS, GC‐Flory EOS, UNIFAC‐FV, entropic‐FV and GK‐FV models, “new” UNIFAC, and modified Flory‐Huggins model. Free‐volume activity coefficient models (UNIFAC‐FV, entropic‐FV) are simpler and, when applied to polymer solutions, more accurate than the EOS. Activity coefficient models are restricted to low‐pressure calculations and require accurate values of pure‐component volumes. Mixture parameters for activity coefficient models and GC‐Flory EOS have been previously evaluated from experimental vapor‐liquid equilibrium data for mixtures with only low‐molecular‐weight compounds. The GC‐Flory EOS, though more complicated than activity coefficient models, provides equally good or in some cases better predictions. The application of GC‐Flory EOS developed as an activity coefficient model is restricted to low‐pressure calculations. On the other hand, PSCT and GCLF developed as “true” EOS provide reliable equilibrium predictions using mixture parameters evaluated solely from pure‐component properties together with standard mixing and combining rules. PSCT EOS performs generally better than GCLF EOS for polymer solutions considered in this study.
Chaotic mixing of viscous fluids by periodic changes in geometry: Baffled cavity flow
Authors: Jana Sadhan C. , Tjahjadi Mahari , Ottino J. M. 
Published: June 2004

AbstractThe baffled cavity flow—the first representative of a class of chaotic flows produced by time‐dependent changes of geometries—is introduced and extensions to spatially‐periodic systems proposed. Both experimental and computational results are reported. The goal of the study is to put into evidence some of the steps and considerations when designing a new mixing flow; a related objective is to demonstrate the use of an efficient and accurate numerical technique (BIEM) suitable for mixing studies in time‐dependent geometries. Several parameters that affect the degree of mixing are studied: the mixing protocol—how the baffles are alternated and how many baffles are used in the flow during each piecewise motion; baffle sizes and shapes; geometrical and flow parameters, such as the wall displacement per period; and details of the baffle's motions.