bluuebunny/crossref_metadata_2025 huggingface dataset:
Polydisperse tube diameters compromise multiple open tubular chromatographyw
Authors: Schisla D. K. , Ding Hongbing , Carr P. W. , Cussler E. L.
Published: June 2004

This article demonstrates that even 1% polydispersity in hollow‐fiber diameters can increase plate heights by as much as an order of magnitude. The demonstration includes an analytical extension of the Taylor‐Aris and the Golay dispersion theories, a numerical solution using the measured polydisperity, and successful predictions of the performance of different hollow‐fiber systems over a range of flow rates. All these results show that the effect of polydispersity can dominate column performance, especially in the region where the column efficiency is optimal.
Chromatographic study of alkanes in silicalite: Equilibrium properties
Authors: Hufton Jeffrey R. , Danner Ronald P.
Published: June 2004

Variations of perturbation chromatography were used to measure Henry's constants and equilibrium isotherms of various gases on silicalite. Three different adsorbent samples were analyzed (commercial powder and extruded pellets, and laboratory‐synthesized crystals), and no discernable differences among the measured parameters were noted. Henry's constants for the linear alkanes were determined from isobaric and nonisobaric chromatography experiments. They were correlated successfully with temperature and the number of carbon atoms per adsorbate molecule. Isotherms were measured from concentration pulse or tracer/concentration pulse chromatography techniques. The Flory‐Huggins version of the vacancy solution model was used to correlate pure gas isotherms and predict binary behavior successfully.
Model‐independent algorithms for time‐optimal control of chemical processes
Authors: Lin Yeong‐Iuan , Beard John N. , Melsheimer Stephen S.
Published: June 2004

The new time‐optimal control algorithms presented do not require a‐priori knowledge of the process dynamics. They are applicable to systems that can be described by a second‐order (or first‐order) plus deadtime model and generate a dynamic model as a byproduct. Simulations presented illustrate the use of these algorithms in time‐optimal control and the process models obtained. The algorithms are robust with respect to the measurement noise often encountered in real chemical processes and are inherently adaptive to changes in the process dynamics with time. An experimental application to the heatup of a laboratory fiber spinning apparatus is presented.

Since these time‐optimal control algorithms are easy to apply and give rapid, smooth responses, they will be of value even when obtaining minimum‐time response is not a critical issue. Such applications include startup of continuous and batch processes. Moreover, they can be used to determine first‐ or second‐order process models (as appropriate) from open‐loop step response data without actually implementing time‐optimal control.
Advanced modeling of fluid catalytic cracking riser‐type reactors
Authors: Theologos K. N. , Markatos N. C.
Published: June 2004

A mathematical model is developed that predicts three‐dimensional, two‐phaseflow, heat transfer and reaction inside catalytic cracking riser‐type reactors. The model consists of the full set of partial‐differential equations that describe the conservation of mass, momentum, energy and chemical species for both phases in the reactor, coupled with empirical correlations concerning interphase friction, interphase heat‐transfer and fluid‐to‐wall frictional forces. The cracking reaction is simulated by a simple three‐lump kinetic model, but more realistic kinetic models can be easily included. The model can predict the most important engineering aspects of a riser reactor including pressure drop, catalyst holdup, interphase slip velocity, catalyst acceleration zone, choking behavior and temperature distribution in both phases, and yields distribution all over the reactor. It can also predict other complex engineering three‐dimensional, two‐phase problems realistically using computational fluid dynamics techniques.
A pore‐level scenario for the development of mixed wettability in oil reservoirs
Authors: Kovscek A. R. , Wong H. , Radke C. J.
Published: June 2004

Understanding the role of thin films in porous media is vital to elucidate wettability at the pore level. The type and thickness of films coating pore walls determine reservoir wettability and whether or not reservoir rock can be altered from its initial state of wettability. Pore shape, especially pore wall curvature, is important in determining wetting‐film thicknesses. Yet, pore shape and physics of thin wetting films are generally neglected in flow models in porous rocks. Thin‐film forces incorporated into a collection of star‐shaped capillary tubes model describe the geological development of mixed wettability in reservoir rock. Here, mixed wettability refers to continuous and distinct oil and water‐wetting surfaces coexisting in the porous medium. This model emphasizes the remarkable role of thin films.

New pore‐level fluid configurations arise that are quite unexpected. For example, efficient water displacement of oil (low residual oil saturation) characteristic of mixed‐wettability porous media is ascribed to interconnected oil lenses or rivulets which bridge the walls adjacent to pore corners. Predicted residual oil saturations are approximately 35% less in mixed‐wet rock than in completely water‐wet rock. Calculated capillary pressure curves mimic those of mixed‐wet porous media in the primary drainage of water, imbibition of water, and secondary drainage modes. Amott‐Harvey indices range from −0.18 to 0.36 also in good agreement with experimental values (Morrow et al., 1986; Jadhunandan and Morrow, 1991).
The propagation of surface waves in flow down an oscillating inclined plane
Authors: Weinstein Steven J. , Baumlin Jean‐Marie , Servant Jeanne
Published: June 2004

The propagation of surface waves is investigated theoretically and experimentally for the case of a single layer of viscous liquid flowing down an inclined plane, where the plane is oscillating in the flow direction. This work focuses on the linearized wavemaker problem, where the oscillations create waves which are small perturbations from the undisturbed flow. Downstream from the entrance region to the incline where the fluid is introduced, the undisturbed interface is parallel to the incline surface, and theory predicts that oscillations do not interact with waves that travel along the free surface. These waves grow as if there were no oscillation at all, and their propagation is governed by a dispersion relation between frequency, wavelength, and wave growth for single layer flow down a nonoscillating inclined plane. The entrance region to the incline is therefore responsible for exciting the various wave frequencies which are observed down the incline, as well as the initial amplitude of these waves. Experiments performed verify that waves propagate as predicted. Theory indicates that these conclusions are valid when the oscillations are perpendicular to the incline, as well as for the case of multiple stacked layers.
Approximate simulation of CO2 and H2s absorption into aqueous alkanolamines
Authors: Glasscock David A. , Rochelle Gary T.
Published: June 2004

Rigorous and approximate methods are compared for the simulation of CO2 absorption into aqueous alkanolamine mixtures of methyldiethanolamine and diethanolamine. In addition, data for the mixtures containing monoethanolamine and the simultaneous absorption of CO2 and H2S are presented. For the rigorous approach, the simplified eddy diffusivity theory is used to simulate the liquid‐phase hydrodynamic characteristics. The approximation methods examined are the pseudo‐first‐order approximation, the interpolation approximation of Wellek et al. (1978), the algebraic combined flux (ACFLUX) approximation and the modified combined flux (MCFLUX) approximation. The latter approximation utilizes the reaction zone concept to determine the kinetic preference of the absorbing gas at the gas‐liquid interface. Under the range of conditions studied, the MCFLUX approximation predicts very accurately the CO2 and H2S flux rates in mixed amine systems, as compared with the rigorous solution of the differential equations.
Saturation gradients in drainage of porous media: NMR imaging measurements
Authors: Fordham E. J. , Hall L. D. , Ramakrishnan T. S. , Sharpe M. R. , Hall C.
Published: June 2004

A steady‐state method recently proposed for measuring two‐phase flow properties of a porous medium is supplemented by NMR imaging for measuring the saturation gradients which are allowed to persist in the specimen. The method relies on a strong capillary end effect which is assumed to be enforced by an outlet capillary pressure boundary condition, but direct verification of this has not hitherto been available. For constant flow rates, incremented stepwise, steady‐state profiles of hydrocarbon saturation Soare obtained which agree with predictions that they be monotonic, nonintersecting, and convex.

Using D2O for the aqueous phase allows unambiguous detection of hydrocarbon by NMR, and imaging measurements of transverse relaxation time T2were included in the protocol. T2shows a dependence on Sowhich is mild but not negligible for quantitative NMR imaging.
Model for describing activity coefficients in mixed electrolyte aqueous solutions
Authors: Lu Xiao‐hua , Maurer G.
Published: June 2004

Most models for describing activity coefficients in ionic solutions are only appropriate for single aqueous electrolyte solutions. Extensions to mixed electrolyte solutions are rarely found and are often unreliable, especially at higher electrolyte concentrations. A new model is developed by combining ionic solvation equilibria and physical interaction forces. It is shown that by using the parameters correlated from single electrolyte aqueous systems, the activity coefficients in mixed electrolyte aqueous solutions can be accurately predicted up to the solubility limit, for example, at very high ionic strength.
Dynamic process modeling with recurrent neural networks
Authors: You Yong , Nikolaou Michael
Published: June 2004

A method of nonhlinear static and dynamic process modeling via recurrent neural networks (RNNs) is studied. An RNN model is a set of coupled nonlinear ordinary differential equations in continuous time domain with nonlinear dynamic node characteristics as well as both feedforward and feedback connections. For such networks, each physical input to a system corresponds to exactly one input to the network. The system's dynamics are captured by the internal structure of the network. The structure of RNN models may be more natural and attractive than that of feedforward neural network models, but computation time for training is longer. Our simulation results show that RNNs can learn both steady‐state relationships and process dynamics of continuous and batch, single‐input/single‐output and multiinput/multioutput systems in a simple and direct manner. Training of RNNs shows only small degradation in the presence of noise in the training data. Thus, RNNs constitute a feasible alternative to layered feedforward back propagation neural networks in steady‐state and dynamic process modeling and model‐based control.
Parametric sensitivity study of a CFD‐based coal combustion model
Authors: Smith Joseph D. , Smith Philip J. , Hill Scott C.
Published: June 2004

Parametric sensitivity of a two‐dimensional pulverized‐fuel (PF) combustion model is studied extensively for the effect of parametric uncertainty on model predictions. Results show that error in coal devolatilization/oxidation parameters has the dominant effect on predicted burnout, NOx formation, local gas temperature, and coal‐gas mixture fraction. Uncertainty in the turbulent particle dispersion parameters appears to have a secondary effect, while error in the particle‐gas radiation parameters has little impact on model predictions. Regions of the computational domain exhibiting sensitivity to specific parameters are identified. Specific parameter sensitivity implies the relative importance of various mechanisms in the overall process. Turbulent particle dispersion seems to be important early in the reactor with kinetic processes dominating at and following the predicted ignition point. Radiation appears to be of minor importance. These results indicate the need for a better method of predicting the overall volatiles yield and further understanding of the devolatilization/oxidation mechanism and its role in the overall PF combustion process. The study provides fundamental direction for future comprehensive model development and focuses on experimental work to better quantify critical input parameters.
Pipeline flow of unstable and surfactant‐stabilized emulsions
Authors: Pal Rajinder
Published: June 2004

The literature available on pipeline flow behavior of emulsions is reviewed critically. New results concerning the laminar and turbulent flow behaviors of unstable (without any added surfactant) and surfactant‐stabilized water‐in‐oil emulsions are presented. The unstable emulsions exhibit drag reduction behavior in turbulent flow; the measured friction factors fall well below the values expected on the basis of the laminar flow properties. Unstable water‐in‐oil emulsions exhibit much stronger drag reduction activity than the unstable oil‐in‐water emulsions. The drag reduction activity diminishes (in some cases vanishes completely) upon the addition of a surfactant to the system.
Design and performance of a simulated countercurrent moving‐bed separator
Authors: Fish Barry B. , Carr Robert W. , Aris Rutherford
Published: June 2004

The design, construction and isothermal (298 K) operation of a continuous flow simulated countercurrent moving‐bed chromatographic separator for the gas‐solid separation of binary mixtures is reported. It consists of only four packed columns, with countercurrency simulated by advancing the feedpoint from column to column in the direction of gas flow. The separated components have periodic concentration profiles, with a duty cycle that depends on operating parameters, and concentrations greater than in the feed stream. Test mixtures of propylene and dimethyl ether, which have retention times differing by a factor of 2 on the 60/80 mesh Chromosorb 101 packing, were successfully separated. Optimum feed advancement intervals and maximum product purity were experimentally determined as a function of feed rate. At low feed rates, when the adsorption isotherm is nearly linear, dimethyl ether was nearly completely separated from propylene, but propylene was found to contain varying amounts of dimethyl ether due to the effects of a nonlinear isotherm and heterogeneous adsorption.
Constrained predictive control using orthogonal expansions
Authors: Finn Cory K. , Wahlberg Bo , Ydstie B. Erik
Published: June 2004

In this article, we approximate bounded operators by orthogonal expansion. The rate of convergence depends on the choice of basis functions. Markov‐Laguerre functions give rapid convergence for open‐loop stable systems with long delay. The Markov‐Kautz model can be used for lightly damped systems, and a more general orthogonal expansion is developed for modeling multivariable systems with widely scattered poles. The finite impulse response model is a special case of these models. A‐priori knowledge about dominant time constants, time delay and oscillatory modes is used to reduce the model complexity and to improve conditioning of the parameter estimation algorithm. Algorithms for predictive control are developed, as well as conditions for constraint compatibility, closed‐loop stability and constraint satisfaction for the ideal case. An H∞‐like design technique proposed guarantees robust stability in the presence of input constraints; output constraints may give “chatter.” A chatter‐free algorithm is proposed.
Rate type equations for the diffusion in polymers: Thermodynamic constraints
Authors: Doghieri Ferruccio , Roda Giovanni Camera , Sarti Giulio C.
Published: June 2004

Conditions imposed by the second law of thermodynamics on viscoelastic rate type constitutive equations for the diffusive mass flux are considered. The analysis of three different rate type models proposed in the literature points out that presently physically unrealistic predictions are possible in desorption processes. The thermodynamic analysis of such models, based on the entropy inequality and on the stability requirement of the equilibrium states, leads to precise relationships among relaxation times, diffusion coefficients, and entropy equations of state. In particular, the analysis shows that relaxation times and diffusion coefficients cannot be simply constant numbers. When the thermodynamic constraints imposed on the constitutive equations are introduced, the models do not show physically unrealistic behaviors any more; Fickian diffusion close to the pure penetrant or pure polymer regions is also recovered. Finally, it is shown that the stability requirement for the equilibrium states may introduce very rigid requirements for the model feasibility, well beyond what appears explicitly from the kinetic equations alone.
Direct simulation of turbulent particle transport in electrostatic precipitators
Authors: Soldati Alfredo , Andreussi Paolo , Banerjee Sanjoy
Published: June 2004

The dispersion of particles in turbulent duct flow under the influence of electrostatic fields is studied using direct numerical simulation. In this new approach, particles are moved in the temporally and spatially varying turbulent flow field under the influence of electrostatic and gravitational body forces, as well as fluid dynamic drag. The simulations agree well with previously performed experiments (done in geometries typical of wire‐plate and plate‐plate electrostatic precipitators) not only in the overall collection efficiency of particles, but in particle concentration profiles at various axial locations in the flow direction. This gives confidence in the technique that may be used to study different precipitator geometries and flow field configurations, supplementing costly and difficult experiments. Furthermore, information is obtained at a much more detailed level than is possible via experiments, allowing insights into the mechanisms dominating particle collection.
Optimal estimation of cell movement indices from the statistical analysis of cell tracking data
Authors: Dickinson Richard B. , Tranquillo Robert T.
Published: June 2004

Active cell migration is essential in many physiological processes and in the function of some bioartificial tissues. Therefore, several investigators have recently attempted to quantitatively characterize random cell movement on isotropic substrata in vitro. A popular approach is to fit a theoretical expression for mean‐squared cell displacement deriving from correlated random walk models to cell tracking data, yielding three objective cell movement indices: root‐mean‐squared speed, directional persistence time, and random motility coefficient (analogous to a molecular diffusion coefficient). The data are obtained typically by averaging cell displacements over a cell track composed of cell positions measured at equal time increments and frequently by further pooling such displacement data from tracks of different cells from the same population. We identify pitfalls introduced if an ordinary nonlinear least‐squares regression analysis is used to fit the theoretical expression to the data as is commonly done and propose a generalized least‐squares regression analysis as a remedy. This method estimates the cell movement indices and associated uncertainties much more accurately. It also predicts the precision of the indices based on their assumed true values and provides a means to address such issues as optimal sampling methods for data acquisition from cell tracks and handling errors associated with measuring cell position.
Strategies for equilibrium‐stage separation calculations on parallel computers
Authors: O'Neill Alfred J. , Kaiser Daniel J. , Stadtherr Mark A.
Published: June 2004

When multicomponent, multistage separation problems are solved on parallel computers by successive linearization methods, the solution of a large sparse linear equation system becomes a computational bottleneck, since other parts of the calculation are more easily parallelized. When the standard problem formulation is used, this system has a block‐tridiagonal form. It is shown how this structure can be used in parallelizing the sparse matrix computation. By reformulating the problem so that it has a bordered‐block‐bidiagonal superstructure, it can be made even more amenable to parallezation. These strategies permit the use of a two‐level hierarchy of parallelism that provides substantial improvements in computational performance on parallel machines.
Tubular reactors for emulsion polymerization: II. Model comparisons with experiments
Authors: Paquet D. A. , Ray W. H.
Published: June 2004

A dynamic model for emulsion polymerization in a tubular reactor has been developed and compared with experimental data. The model uses an axial dispersion coefficient to fit experimentally measured residence time distributions and to represent axial mixing during reaction. Very general emulsion polymerization kinetics are assumed so that a wide range of monomers and operating conditions can be handled. Orthogonal collocation on finite elements was chosen as the numerical solution technique. Comparisons show good agreement between model and experiments for two different monomers.
Hydrodynamic behavior of circulating fluidized bed with polymeric particles
Authors: Jiang Peijun , Bi Hsiaotao , Liang Shu‐Chien , Fan Liang‐Shih
Published: June 2004

A systematic study conducted explores the hydrodynamics of a circulating fluidized bed with polymeric particles employing polyethylene resins with a particle density of 660 kg/m3 and size ranging from 90 to 500 μm. The study indicates that polyethylene resins can be fluidized smoothly in the fast fluidization regime. However, the operating range of the fast fluidization regime for these particles is smaller than that for FCC particles. The deviation of the fluidization behavior of polyethylene particles from that of common Group‐A particles is explained considering the interparticle forces. Experiments with fine polyethylene particles are also conducted with coarse particles added in a circulating fluidized bed. Axial profiles of solid holdups in a bed with and without coarse particles, as well as overall fine particle holdup, are studied. The results show a significantly wider operating range of the fast fluidization regime and enhancement of fine particle holdups in a bed with the presence of coarse particles. For comparison, fluidization with FCC particles is also conducted. A mechanistic model considering particle‐particle collision is proposed. The model accounts for the momentum exchange rate between a coarse particle and a cloud of fine particles, which explains the enhancement of fine particle holdups observed experimentally.
Turbulent gas‐particle flow in vertical risers
Authors: Dasgupta Sanjay , Jackson Roy , Sundaresan Sankaran
Published: June 2004

Many experimental studies on the cross‐sectional distribution of particles in vertical risers have revealed marked segregation of particles. These flows are inherently unsteady with large fluctuations in suspension density. In this article, we have analyzed the time‐smoothed equations for the motion of dense suspensions to demonstrate the role of these fluctuations on the occurrence of segregation. It is shown that the particles will congregate in regions where the kinetic energy of fluctuations associated with the particles is small. In the context of transport of small particles such as FCC, a simplified model based on the mixture velocity can be constructed. A speculative K‐ϵ model based on this velocity is analyzed to illustrate the extent of segregation of particles afforded by unsteady fluctuations.
Brownian dynamics of sterically‐stabilized colloidal suspensions
Authors: TeGrotenhuis Ward E. , Radke Clayton J. , Denn Morton M.
Published: June 2004

Using 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

A 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

Hazard 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

Diffusivities 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

This 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

Oxidative 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

In 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

Hysteresis‐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

This 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

Macroscopic 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

A 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

A 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

We 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

Multivariate 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

It 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

The 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

Separating 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

Four 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

The 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.
Chaotic mixing of shear‐thinning fluids
Authors: Niederkorn Thomas C. , Ottino Julio M.
Published: June 2004

This article investigates the effect of shear‐thinning viscosity on chaotic mixing when the kinematics first begin to deviate from Newtonian flow. Computations are done for a 2‐D, time‐periodic flow between eccentric cylinders. The effectiveness of mixing is analyzed by examining the asymptotic coverage of a passive tracer, character and location of periodic points, and the rate of stretching of fluid elements. Small variations in the velocity field associated with non‐Newtonian kinematics produce large effects in the chaotic advection of a passive tracer. The stretching rate remains exponential, but with a long time constant as the shear‐thinning effect increases, often resulting in the birth of new periodic islands and a decrease in the asymptotic coverage of the tracer. Exceptions to these observations are possible: both the stretching rate and the asymptotic coverage might increase as shear thinning increases. Results also indicate that suitable manipulation of operating conditions can produce shear‐thinning flows which mix as well as Newtonian flows. Since significant effects are observed with less than a 4% difference in the velocity fields, the assumption of Newtonian kinematics can lead to large errors in the design and operation of process mixing equipment for non‐Newtonian fluids.