bluuebunny/crossref_metadata_2025 huggingface dataset:
Selecting the best distillation control configuration
Authors: Skogestad Sigurd , Lundström Petter , Jacobsen Elling W.
Published: June 2004

The selection of an appropriate control configuration (structure) is the most important decision when designing distillation control systems. The steady state RGA is commonly used in industry for selecting the best structure. One counterexample to the usefulness of this measure is the DB configuration, which has infinite steady state relative gain array (RGA) values, but still good control performance is possible. This is indicated by high‐frequency RGA values close to 1. In this paper it is stressed that decisions regarding controller design should be based on the initial response (high‐frequency behavior) rather than the steady state.

Based on a frequency‐dependent RGA analysis and optimal Pl controller designs of four different configurations, the (L/D) (V/B) configuration is found to be the best choice for two‐point composition control. The conventional LV configuration performs poorer than the above system, but is preferable if one‐point control is used.
Determination of flow profiles in porous media using shifts in gamma spectra
Authors: Jasti Jay K. , Fogler H. Scott
Published: June 2004

A new technique was developed to determine the tracer location and fluid velocity in porous media nonintrusively. This technique exploits the competitive effects between the photoelectric interaction and the Compton scattering phenomenon to determine the distance between a radioactive tracer in a porous medium and an externally positioned detector. The photon energy distribution shifts toward lower photon energies as the tracer moves away from the detector. The shift in the energy distribution can be quantified by the ratio of the scattered photon intensity to the full energy photon intensity. A convective‐dispersion model was used to determine the spatial distribution of the radioactive tracer. An analog Monte Carlo method was developed to simulate photon transport in porous media. Comparison between experimental data and the model shows that in‐situ tracer velocities can be accurately predicted.
Mass transport with relaxation in polymers
Authors: Camera‐Roda Giovanni , Sarti Giulio C.
Published: June 2004

The non‐Fickian mass transport behavior observed in polymeric materials is here analyzed through a viscoelastic constitutive equation for the diffusive flux, endowed with concentration‐dependent relaxation time and diffusivity. The present model thus overcomes the limitations shown by Neogi's model and by the Cattaneo equation used by Camera‐Roda and Sarti. In addition to the Fickian behaviors at both small and high Deborah numbers, the model accounts for case II behavior and anomalous diffusion at intermediate Deborah number, possibly with the presence of a shock wave in the concentration profile which moves at a concentration‐dependent velocity. Weight uptake accelerations and overshoots are also accounted for at intermediate Deborah numbers. The dimensionless problem has been numerically solved and the role of the relevant dimensionless parameters is discussed for both sorption and desorption.
Development of circulating fluidized bed adsorber
Authors: Nakamura Susumu , Yoshimuta Shigeharu , Shirakashi Masataka , Kanno Masayoshi , Ito Yoshiro
Published: June 2004

A new fludized bed which is divided in a looplike shape by a partition wall has been developed for recovering trace elements from a large quantity of industrial water. The bed, a circulating fluidized bed applicable to a liquid‐solid two‐phase flow, is called a circulating fluidized bed absorber (CFBA). Adsorbent particles are mixed and carried along with water in the CFBA. At the upper part of the bed, the absorbent particles are separated from the water flow and recycled in the bed; the depleted water flows out of a water outlet. The CFBA is operated at two to three times higher velocity than conventional liquid‐solid contacting systems. The pressure drop in the CFBA is in proportion to the amount of adsorbent suspended in the contacting section. Characteristics of the CFBA, including design considerations, are described.
Methanol synthesis in a trickle‐bed reactor
Authors: Pass Geoffrey , Holzhauser Craig , Akgerman A. , Anthony R. G.
Published: June 2004

A novel approach to methanol production from coal‐derived synthesis gas is cocurrent gas and mineral oil feed flowing over a packed bed of catalyst in the trickle flow regime. Production rates of 0.7 to 2 kg/h · kg cat were obtained for a H2/(CO + CO2) ratio of 1 and at space velocities of 2,000 to 25,000 L/h · kg cat. Slurry reactor and bubble column productivities were substantially less for H2/(CO + CO2) ratios of 0.55 to 2.3 at similar conditions as the trickle‐bed reactor. Reaction temperature was 250°C in three types of reactors but 240°C in gas‐liquid phase Berty reactor; the pressure in the slurry and bubble column reactors was 52‐70 atm and in Berty Reactor 77.5‐100 atm, whereas 70 atm pressure was used in the trickle bed. Differences in production rates and conversions can be explained by the extent of backmixing in trickle‐bed and slurry reactors operating at the same conditions.
Microelectrode study of gallium deposition from Chlorogallate melts
Authors: Verbrugge Mark W. , Carpenter Michael K.
Published: June 2004

Microelectrodes are used to extract thermodynamic, transport, and kinetic data for the gallium trichloride/1‐methyl‐3‐ethylimidazolium chloride room‐temperature molten salt that has been used by the authors to form GaAs thin films. The gallium electrodeposition process, which must be understood for the analysis of the GaAs deposition process, is simulated numerically. A plausible gallium deposition mechanism is identified. Microcylinder electrodes are shown to be advantageous for the acquisition of physicochemical constants. Microdisk electrodes are employed to investigate current oscillation phenomena that appear to be related to metal crystallization processes.
The Benzylphenylether thermolysis mechanism: Insights from phase behavior
Authors: Wu Benjamin C. , Klein Michael T. , Sandler Stanley I.
Published: June 2004

The thermolysis of benzylphenylether (BPE) was examined at 320°C in liquid, vapor, and supercritical methanol phases as a probe of the reaction mechanism. Toluene and phenol were the major products in all cases, but observed selectivities were dependent upon the reaction phase. Isotopic labeling experiments demonstrated that methanol participated in the reaction network as a hydrogen donor through a free‐radical mechanism. These results are consistent with free‐radical steps for neat BPE primary pyrolysis that include BPE fission followed by hydrogen abstraction and radical recombination reactions. A detailed set of 21 free‐radical steps quantitatively explains the dependence of the selectivity and conversion on the phase behavior.
Heat transfer and population characteristics of dispersed evaporating droplets
Authors: Core K. L. , Mulligan J. C.
Published: June 2004

The direct‐contact heat transfer to a dispersed population of immiscible evaporating droplets is analyzed. Also studied is a population balance formulation for the distribution of two‐phase bubbles, which is similar to that utilized in particle and droplet dispersion analysis and is capable of including bubble coalescence and break‐up. It is shown that the method of classes is useful particularly in solving such problems when the growth functions are size‐ and time‐dependent and nonquadratic. The method is applied to a liquid cool‐down, representing the initial chilling stage of a direct‐contact batch crystallizer or cold‐storage unit wherein a vessel containing a liquid is chilled by injecting a dispersion of refrigerant droplets. Transient bubble population characteristics, volumetric heat transfer coefficient, total heat transfer, and liquid temperature are predicted, along with the liquid refrigerant holdup.
Simultaneous synthesis of mass‐exchange and regeneration networks
Authors: El‐Halwagi Mahmoud M. , Manousiouthakis Vasilios
Published: June 2004

A systematic procedure is developed for the simultaneous synthesis of primary transfer mass‐exchange networks and their associated mass‐exchange regeneration networks. The purpose of the primary transfer network is to preferentially transfer certain species from a set of rich streams to a set of lean streams. The regeneration network aims at regenerating any recyclable lean stream. The proposed procedure deals with the problem in two stages. In the first stage, a mixed‐integer nonlinear program is solved to minimize the cost of mass‐separating and regenerating agents. The solution of this program provides the location of the pinch points as well as the optimal flow rates of all the lean and the regenerative streams without any prior commitment to the network structure. In the second stage, a mixed‐integer linear program is solved to minimize the number of exchangers in both networks. An example problem with industrial relevance is solved to elucidate the merits offered by the devised synthesis procedure.
Displacement separations by continuous annular chromatography
Authors: De Carli Joseph P. , Carta Giorgio , Byers Charles H.
Published: June 2004

Continuous displacement chromatography separations are obtained using a continuous annular chromatograph (CAC). The apparatus consists of a rotating annular bed. At fixed positions at the top of the bed, four solutions are continuously supplied: the feed mixture to be separated, a displacer solution, a regenerant, and a rinse. The four solutions flow concurrently through the bed with little mixing. If the bed is sufficiently deep, a fully developed displacement train forms and the separated components are continuously recovered at various angular locations at the bottom of the bed. Mixtures of amino acid were separated by cation exchange displacement chromatography in a laboratory‐scale CAC. Separation and concentration of dilute mixtures was obtained with a continuous separation performance comparable with the performance of a conventional chromatograph. A mathematical model is developed to describe the separation and provide guidance in design and optimization.
Kinetic separation by pressure swing adsorption: Method of characteristics model
Authors: Ackley Mark W. , Yang Ralph T.
Published: June 2004

A theoretical model using the method of characteristics is presented for a Skarstrom‐type pressure swing adsorption (PSA) cycle using carbon molecular sieve for the kinetic separation of nitrogen/methane mixtures. Good agreement with experimental results was obtained. The solution of the reduced nonlinear ordinary differential equations along characteristics, a variable computational grid from the changing bed velocity, and a constant product flow boundary condition are complications that prohibited using the method of characteristics in favor of finite difference or orthogonal collocation methods. These complications have been overcome in the present study to develop a theoretical model, incorporating nonlinear isotherms and a linear driving force rate for diffusion, which requires less computer time and is particularly amenable for studying transient bed dynamics. The effects of feed pressure and concentration, half‐cycle time, and product flow rate on kinetic PSA performance are also illustrated.
Homogeneous nucleation in supercritical fluids
Authors: Debenedetti Pablo G.
Published: June 2004

When a supercritical solution is rapidly expanded, large solute supersaturations can be attained, and small particles are formed. The evolution of the homogeneous nucleation rate, work of nucleus formation, and critical nucleus size along different expansion paths is investigated here for the model system phenanthrene‐carbon dioxide. Nucleation rates are the result of the competition among solvent expansion, cooling due to depressurization, and high supersaturation. Although supersaturations can reach very high values (> 106), relatively flat nucleation rate profiles result due to cooling and expansion. For an interfacial tension of 0.02 N/m, computed nucleation rates never exceed 104 s−1 · cm−3. A substantial fraction of the maximum nucleation rate is attained with partial decompression to pressures above 1 bar.
Intensification of pressure swing adsorption processes
Authors: Rota Renato , Wankat Phillip C.
Published: June 2004

Scaling rules have been developed which are able to change the dimension of a pressure swing adsorption plant keeping its main performance constant. Such scaling rules result from a system of algebraic equations which arises by constraining the solution of a fairly general dimensionless mathematical model to be the same in the old and new plant configuration. Different heat and mass transfer mechanisms, purification, and multicomponent bulk separation processes, both isothermal and nonisothermal, have been considered. Furthermore, the influence of different equilibrium isotherms and the main practical limitations have been discussed. The application of these rules shows that column size can be reduced significantly by using smaller particles in shorter beds with rapid cycles.
Optimization of multiple‐fraction batch distillation by nonlinear programming
Authors: Farhat S. , Czernicki M. , Pibouleau L. , Domenech S.
Published: June 2004

Batch distillation processes are very attractive for the recent development of the chemical industry: multipurpose, flexible plants and fine chemistry. For many separations of high‐added value products, even a modest change in operating conditions has a significant economic impact—there is an important challenge for optimizing such processes.

Short‐cut and dynamic models are the two classical approaches to the simulation of batch distillation columns. For problems without holdup, an intermediate procedure based on a decoupling method is validated.

For a multifraction separation problem with fixed final time, the reflux policies for each period and the period switching times constitute the set of decision variables. For predefined reflux policies, we apply an engineering approach to the solution of a such constrained variational problem, based on its transformation into a nonlinear programming problem. In this computer‐implementable algorithm, the gain in distillate for the optimal linear or exponential reflux policies is significant (about 10%) compared with the optimal constant reflux policy.
Heat transfer under multiple slot jets impinging on a permeable moving surface
Authors: Polat S. , Douglas W. J. M.
Published: June 2004

Local and average heat transfer were measured for a system of multiple jets impinging on a moving permeable surface at which there may be throughflow. Multiple jets were confined by a hood, as is required industrially for thermal efficiency. Exhaust ports located symmetrically between the jet nozzles eliminated crossflow, otherwise a strongly detrimental effect. Impingement surface motion decreases average heat transfer, by 17% at industrially relevant values of the surface motion parameter,Mvs. Enhancement of impingement heat flux by throughflow is linearly additive. Expressed as ΔSt, this enhancement depends only on the throughflow parameter,Mus, with ΔSt/Mus= 0.17. For typical operating conditions, withdrawal as throughflow of only 10% of the jet flow increases mean Nusselt number by over 50%. Industrial design modeling for the potential process of combined impingement and throughflow drying of wet webs such as paper is demonstrated.
Hydrodynamics of particulate motion in sinusoidal pores via a singularity method
Authors: Nitsche Ludwig C. , Brenner Howard
Published: June 2004

Axisymmetric motions of suspended spheres and dumbbells through sinusoidally corrugated capillaries are considered as an illustrative model of particulate or macromolecular transport in porous media. Numerical simulations are carried out using a least‐squares singularity method, which is well suited to simulating creeping flows in the complicated, time‐evolving geometries associated with particle motion through nonrectilinear pores. The numerical method is applied to a representative closure problem, whose solution yields effective transport coefficients describing particulate flow in porous media. With reference to polymer‐induced mobility control in enhanced oil recovery, a hydrodynamic mechanism of mobility reduction is studied using a rigid dumbbell polymer model.
Bubble coalescence and break‐up in air‐sparged bubble columns
Authors: Prince Michael J. , Blanch Harvey W.
Published: June 2004

A Phenomenological model is proposed for the rates of bubble coalescence and bubble break‐up in turbulent gas‐liquid dispersions. Bubble coalescence is modeled by considering bubble collisions due to turbulence, buoyancy, and laminar shear, and by analysis of the coalescence efficiency of collisions. Bubble break‐up is analyzed in terms of bubble interactions with turbulent eddies. A method for the measurement of coalescence and break‐up events in turbulent systems is described and used to test the validity of the proposed model. The measurement technique relies on the mixing of tracer gases within bubbles upon coalescence, in conjunction with Monte‐Carlo simulations of coalescence events. Both distilled water and salt solutions are examined. Favorable agreement is found between the model and the individual coalescence and breakage rates, as well as with data obtained for the average bubble size and bubble size distribution.
Unified thermodynamic analysis of sorption in rubbery and glassy materials
Authors: Lipscomb G. Glenn
Published: June 2004

A thermodynamic framework for the analysis of gas sorption in glassy polymers is established in the limit of low sorption levels. The results represent an extension of the Flory‐Huggins theory to materials with nonzero internal energy changes due to deformation; the Flory‐Huggins theory is recovered in the limit of zero polymer bulk modulus. The sorption isotherm is expressed in terms of the penetrant vapor‐phase activity, polymer physical properties, the penetrant partial molar volume, and the heat of mixing. The qualitative form of the isotherm is shown to be unrelated to the presence of excess free volume in the polymer; only quantitative predictions are influenced. The downward curvature of the isotherm is due to the thermodynamics of solid deformation. Explicit relationships between the parameters of this work and the dual‐mode model are given for the low‐sorption/high‐bulk modulus limit.
Kinetics of Carbon Dioxide with tertiary Amines in aqueous solution
Authors: Littel R. J. , Van Swaaij W. P. M. , Versteeg G. F.
Published: June 2004

The reaction of CO2 with TEA, DMMEA, and DEMEA has been studied at 293, 303, 318 and 333 K. All the kinetic experiments were carried out in a stirred cell reactor operated with a flat, smooth and horizontal gas‐liquid interface. A numerical method, which describes mass transfer accompanied by reversible chemical reactions, has been applied to infer rate constants from the experimental data. It is argued that the contribution of the CO2 reaction with OH− to the observed reaction rate may have been overstimated in most literature on tertiary amine kinetics as serious depletion of OH− toward the gas‐liquid interface usually occurs.

For all the amines studied, the reaction order in amine was found to be about one for each temperature investigated. This is in good agreement with the base catalysis mechanism proposed by Donaldson and Nguyen (1980). All kinetic data could be summarized reasonably well in one Brønsted relationship.
Growth rate dispersion in batch crystallization
Authors: Shiau Lie‐Ding , Berglund Kris A.
Published: June 2004

A method for describing the crystal size distribution of a batch crystallizer in the presence of growth rate dispersion is presented. The analysis is based on the assumption that individual crystals have inherent constant growth rates, but the growth rate may vary from crystal to crystal, resulting in a distribution of growth rates. The method requires the availability of growth rate, growth rate dispersion, and nucleation rate expressions for prediction of the crystal size distribution. These required rate expressions can be recovered by the use of simple linear regressions from the CSD moment data. Prediction of the unsteady‐state CSD was demonstrated using rate expressions for the sucrose‐water system. Batch fructose experiments were analyzed to demonstrate the recovery of the growth and nucleation rates from the size distribution data.
Modeling the SO2‐slurry droplet reaction
Authors: Newton Gerald H. , Kramlich John , Payne Roy
Published: June 2004

A model of the reaction between SO2 and calcium‐based slurries is presented. The model is coupled with an extensive model of the evaporation process and considers external SO2 mass transfer and liquid‐phase mass transfer of dissolved sulfur and calcium species in both the film surrounding the calcium particles and the bulk liquid phase. The model assumes that particles do not circulate within slurry droplets and that evaporation results in an accumulation of particles at the droplet surface that causes a shift in the controlling mechanisms. Agreement between the model and data available in the literature is excellent.
Flow in a converging channel at moderate reynolds numbers
Authors: James David F.
Published: June 2004

An analytical solution is found for flow in a converging channel for Reynolds numbers in the range of 102 to 103. The solution applies to an axisymmetric channel, for which the shape is given by R2z=constant, where R is the channel radius at the axial distance z. This shape produces a center‐line or core velocity that increases linearly with z, thus making the extensional rate constant. The analytical solution is a pseudosimilarity solution of the axial momentum equation, and its accuracy was gauged by comparison to other results. Comparisons of velocity and pressure distributions to experimental data and to finite‐element results indicate that the analytical solution is accurate to about 5%.
Development of the AVEC model for associating mixtures using NMR spectroscopy
Authors: Karachewski Anne M. , Howell Wayne J. , Eckert Charles A.
Published: June 2004

Common models for excess Gibbs energy account primarily for physical forces and do not represent strong specific interactions. However, purely chemical models neglect strong nonspecific forces and combined chemical‐physical treatments involve an unreasonable number of adjustable parameters. One way to ameliorate this limitation is to make independent measurements of the parameters by a nonther‐modynamic technique. We report an investigation of hydrogen bonding, a localized and very directional specific interaction, conducted in various alcohol mixtures using Fourier transform nuclear magnetic resonance to measure the number of complexed species in solution. A chemical‐physical association model with variable equilibrium constants has been developed. The distinguishing feature of this model is that the species in solution do not have equal probability of forming yet require only a single chemical interaction parameter determined from fitting NMR data and a physical interaction parameter determined from infinite dilution activity coefficients. The AVEC model successfully predicts vapor‐liquid equilibrium and excess enthalphy as well as liquid‐liquid equilibrium, an especially stringent test.
Gas‐liquid two‐phase flow in symmetrically dividing horizontal tubes
Authors: Lightstone L. , Osamusali S. I. , Chang Jen‐Shih
Published: June 2004

The time‐averaged void fraction, pressure drop and flow regime transition behavior of horizontal air‐water two‐phase flows is studied experimentally and numerically for 2‐cm‐inner‐diameter tubes with various flow dividing junctions at its end. The time‐average void and pressure drop behavior along the channel is simulated using a two fluid separated flow model. The results show that two‐phase behavior (flow regime, void fraction, and pressure drop) is affected strongly by the presence of a flow division in the system. These effects extend far upstream of the junction for low‐momentum flows and far downstream for high‐momentum flows. Both numerical and experimental results show that there occurs a large increase in void just downstream of the junction owing to the halving of the fluid volume flow rates and the liquid deceleration.
Modeling of supercritical desorbers with an equation‐of‐state‐based isotherm
Authors: Akman Ugur , Sunol Aydin K.
Published: June 2004

An equation‐of‐state‐based sorption isotherm is developed and used in modeling isothermal supercritical fluid desorption in fixed beds. The conventional fixed‐bed desorber model when coupled with density‐dependent sorption isotherm predicts the temperature, pressure, and density effects on desorber dynamics and regeneration efficiencies that are experimentally observed in supercritical fluid regeneration of fixed beds.

Experimentally observed phenomena such as the presence of pressure‐dependent optimal regeneration temperature, high‐pressure‐favored regeneration, and the reversal of temperature dependence of sorption isotherm—which formerly were attributed to supercritical fluid density and viscosity, and to the endothermicity of sorption isotherm—were fully predicted and explained in terms of a density‐dependent sorption isotherm.

The proposed method also promises prediction of regeneration efficiencies in cosolvent‐aided supercritical fluid desorption.
Use of Hammerstein models in identification of nonlinear systems
Authors: Eskinat Esref , Johnson Stanley H. , Luyben William L.
Published: June 2004

The utility of the Hammerstein model, which is composed of a static nonlinear element in series with a linear dynamic part, was investigated to represent the dynamics of nonlinear chemical processes. Different methods to identify the parameters of Hammerstein models were tested. The methods were applied to the identification of simulated distillation columns and to an experimental heat exchanger process. The results show that the dynamics of such processes can be better represented by Hammerstein‐type models than by linear models.
Growth and transformation of TiO2 crystallites in aerosol reactor
Authors: Kobata Atsuo , Kusakabe Katsuki , Morooka Shigeharu
Published: June 2004

Rate process concerning the formation of TiO2 fine crystalline particles by the gas‐phase reaction of TiCl4 and O2 are studied using aerosol reactors. Chemical reaction of TiCl4, sintering of particles, mixing of reactants, and transformation from anatase to rutile are evaluated as the system parameters of the simulation model proposed. The crystallite size in the range of 55–65 nm at 1,273 K is predicted well by a model that assumes the maximum fusible particle size, 15 nm in this case.

The defect concentration in the TiO2 crystallites strongly affects the transformation rate, and anatase particles produced at 1,173 K are transformed to rutile more rapidly than those produced at 1,373 K. The transformation is simulated quantitatively by the model with the coordinates for elapsed time, particle size and rutile fraction. The model can be applied to such particle production processes as collision, sintering and crystal transformation occurring simultaneously.
Plasma deposition and etching of diamond‐like carbon films
Authors: David Moses , Padiyath Raghunath , Babu Suryadevara V.
Published: June 2004

Diamond‐like carbon films have been deposited from ternary mixtures of butadiene, hydrogen and argon in a parallel plate plasma reactor at constant pressure and power. These films have been etched in O2 and CF4/O2 plasma discharges. A new linear relationship between the composition of the deposition gas mixture and a dimensionless number (EN) defined in terms of etch and deposition rates and the bias voltage during deposition has been derived. EN is a function of the ion flux during deposition. Electron‐impact ionization processes are considered for relating the ion flux to the feed gas composition and ionization potentials. The etch and deposition rate data follow this linear relationship very well. The proportionality constant in this linear relationship varies with composition for CF4/O2 etching data. Film hardness and failure modes on silicon and glass substrates are also described.
Trickle‐bed reactor flow simulation
Authors: Anderson David H. , Sapre Ajit V.
Published: June 2004

A steady‐state, two‐dimensional model for cocurrent gas and liquid flow in a trickle‐bed reactor is described. The model includes gas‐liquid interaction and isothermal, incompressible flow without phase change or chemical reaction. The momentum equation proposed by Saez and Carbonell is used, along with their expressions for relative permeabilities of gas and liquid phases. Capillary pressure equations determine the difference in the pressures of flowing phases. The results of the model simulation agree with experimental data on flow distribution, liquid spreading, and phase segregation in a two‐dimensional trickle bed with air‐water flow in both low‐ and high‐interaction flow regimes.
Airlift bioreactors: Analysis of local two‐phase hydrodynamics
Authors: Young Mark A. , Carbonell Ruben G. , Ollis David F.
Published: June 2004

Local two‐phase flow measurements were obtained in a pilot‐scale, external‐loop airlift bioreactor using hot‐film anemometry and resistivity probe techniques. The radial dependence of both gas and liquid velocities and of the void fraction was substantial. Developing flow effects were pronounced, as evidenced by distinct changes in the radial profiles of fluid flow properties with axial position. For high gas flow rates, liquid acceleration effects near the sparger resulted in greatly reduced slip velocities in a substantial portion of the riser. A significant reduction in mass transfer may occur under such conditions.

The point equations of continuity and motion were used to develop a differential, two‐fluid model for two‐phase flow in airlift risers. The only empirical parameters in the model represent frictional effects. The developing two‐phase flow characteristic of airlift risers was observed to create significantly higher frictional effects at the wall than is routinely observed for fully‐developed flow. Model predictions were compared to our own experimental results as well as those of Merchuk and Stein (1981). Agreement between the predicted and measured values was typically within 10% for both cases.
Inversion and zero dynamics in nonlinear multivariable control
Authors: Daoutidis Prodromos , Kravaris Costas
Published: June 2004

This work concerns general multiple‐input/multiple‐output (MIMO) nonlinear systems with nonsingular characteristic matrix. For these systems, the problem of inversion is revisited and explicit formulas are derived for the full‐order and the reduced inverse system. The reduced inverse naturally leads to an explicit calculation of the unforced zero dynamics of the system and the definition of a concept of forced zero dynamics. These concepts generalize the notion of transmission zeros for MIMO linear systems in a nonlinear setting. Chemical engineering examples are given to illustrate the calculation of zero dynamics. Input/output linearization is then interpreted as canceling the forced zero dynamics of the system, and precise internal stability conditions are derived for the closed‐loop system.
Pressure buildup and internal stresses during binder burnout: Numerical analysis
Authors: Tsai Dah‐Shyang
Published: June 2004

The interstitial pressure buildup and the internal stresses on the skeleton of a cylindrical green body during burnout are analyzed numerically. The intrinsic kinetics of pyrolysis is coupled with the Carman‐Kozeny equation or the slip‐flow model of Wakao and Smith to evaluate the pressure distribution. The stress distributions before failure are estimated from elasticity theory. The stresses are tensile, and their maxima are located at the center of the cylinder. The tangential stress is larger than the radial stress. The effects of green body size, specific surface area, and pressurized atmosphere are discussed. The approximate solution is accurate for small green bodies. The pressurized atmosphere reduces the interstitial pressure and the internal stresses effectively.
Separation of azeotropic organic liquid mixtures by pervaporation
Authors: Dutta Binay K. , Sikdar Subhas K.
Published: June 2004

We have shown that ionomeric membranes of perfluorosulfonic acid (PFSA) polymer allow selective pervaporation of polar organic compounds from their azeotropic mixtures with less polar compounds. A composite membrane of a thin PFSA polymer film, cast on a porous Teflon support, provided desirable permeant fluxes with good selectivity. Model binary azeotropes, composed of alcohols and hydrocarbons, were separated with total pervaporation fluxes of up to 9.5 kg/h·m2 using a stirred membrane permeation cell, the product side of which was exposed to vacuum. Fluxes increased with increasing alcohol content of the feed and with temperature. But the selectivity of the more permeating component remained reasonably constant around the azeotropic point and also over the range of temperature used in the study (25–55°C). Total pervaporation fluxes depended little on downstream pressure up to 6.66 kPa (50 torr). These separation data were explained by a mathematical model based on a solution‐diffusion mechanism.
Spray‐to‐bubbly transition for distillation systems containing two liquid phases
Authors: Davies Bruce , Ali Zafar , Porter Kenneth E.
Published: June 2004

The tray hydrodynamics were determined for a system containing water and kerosene as immiscible liquids. Plates with hole diameters ranging from 3.18 to 12.70 mm were used in a 44.5‐cm2 perspex air‐water‐kerosene simulator. Experiments were also carried out in a 50‐mm‐ID column using different depths of oil and water mixtures to study the drop and bubble mechanisms. A spray‐to‐bubble transition occurred for the two liquid‐phase system experiments. The liquid holdup at the transition increased directly with gas velocity and hole diameter, and decreased with increasing free area. At the same hole velocity, the presence of two liquid phases caused the transition to occur at different liquid holdups than for the single pure liquid. Two different modes of coalescence were observed in the small column work. New correlations have been proposed for the liquid holdup at the transition which allow for the presence of two liquid phases.
Mathematical model of the hydrocyclone based on physics of fluid flow
Authors: Hsieh K. T. , Rajamani R. K.
Published: June 2004

A mathematical model of the hydrocyclone, based on the physics of fluid flow, has been developed. The model equations are solved in a computer code that takes as input the hydrocyclone dimensions and feed slurry characteristics. The output of the computer code is the velocity profiles of the fluid and the separation efficiency curve. To validate the model, an LDV was used to measure the velocity profiles inside a 75‐mm hydrocyclone. Pure water and glycerol‐water mixture were used as the working media to simulate the increase of slurry viscosity in the presence of solid particles. The predicted velocity profiles agree well with the experimental measurements. Dilute limestone slurry was also classified with the same hydrocyclone, and predicted the separation efficiency curve shows good agreement with experimental observation.
Prediction of gas adsorption in 5a zeolites using Monte Carlo simulation
Authors: Razmus David M. , Hall Carol K.
Published: June 2004

Adsorption of air in 5A zeolites was studied using Monte Carlo simulations in the grand canonical ensemble (μ, V, T constant). Site‐site potentials were used to model the adsorbate‐zeolite and adsorbate‐adsorbate interactions. The potential model contains one adjustable parameter that was fit to a single experimental isotherm data point. Adsorption isotherms and heats of adsorption were determined for pure argon, oxygen, and nitrogen at 203.15 K, 233.15 K, and 297.15 K from 0.1 bar to 4.0 bar. Multicomponent adsorption isotherms were determined for binary mixtures of oxygen and nitrogen at 203.15 K. The results for the pure‐component isotherms are in excellent agreement with experimental data. The results for the heat of adsorption are in good agreement with experimental data for argon and oxygen, but not for nitrogen. The results for multicomponent adsorption isotherms are qualitatively correct; however, the simulation was not able to quantitatively predict mixture data.
Electrostatic interactions in colloidal suspensions: Tests of pairwise additivity
Authors: Reiner Eric S. , Radke C. J.
Published: June 2004

The electrical double‐layer free energy is computed numerically for a model colloidal suspension of charged, hexagonally‐packed monodisperse cylinders in a symmetric electrolyte solution using the Poisson‐Boltzmann equations. Comparison is made to the energies estimated from sums of pair interactions and from two versions of a radially‐symmetric cell model: one in which the outer boundary of the cylindrical cell is chosen to make the particle volume fraction Φ in the approximate model agree with that of the full lattice and the other in which the position of the outer boundary is chosen as half the nearest neighbor separation in the lattice. Calculations are performed for ranges of particle volume fractions from infinite dilution to 98% of closest packing, ionic strengths (in terms of the ratio of Debye length to particle radius λ/a) from 0.1 to 10.0, and dimensionless surface potentials and charge densities from 0.1 to 10.0.

The pair approximation is valid only when typical interparticle spacings are greater than the Debye length. The first implementation of the cell model proves quite accurate, while the second is only qualitatively correct. The magnitude of particle surface charge density or potential has little effect on these conclusions.
On‐line inference of polymer properties in an industrial polyethylene reactor
Authors: McAuley K. B. , MacGregor J. F.
Published: June 2004

A major difficulty affecting the control of product quality in industrial polymerization reactors is the lack of suitable on‐line polymer property measurements. In this article a scheme is developed to predict melt index and density in a fluidized‐bed ethylene copolymerization reactor. Theoretically‐based models are derived to predict quality variables from the available on‐line temperature and gas composition measurements. Adjustable parameters in these models are updated on‐line using infrequent laboratory measurements and a recursive parameter estimation technique. The application of this methodology is illustrated using operating data from an industrial reactor. It is shown that both melt index and density can be successfully predicted. Knowledge of product property deviations from desired targets is required so that manufacturers can take corrective actions to reduce the quantity of off‐grade material made and produce a consistent product.
Kinetics of deactivation of bifunctional Pt/Al2O3Cl catalysts by coking
Authors: Beltramini Jorge N. , Wessel Timothy J. , Datta Ravindra
Published: June 2004

The catalytic activity, product selectivity, and coke deposition were studied during methylcyclopentane (MCP) reforming on bifunctional Pt/Al2O3Cl reforming catalysts with different metal loadings, but constant metal dispersion as well as constant Cl loading. Surprisingly, the overall conversion of MCP was found to decrease as the metal content of the catalyst increased. Analysis of the coke by temperature programmed oxidation (TPO) revealed two distinct peaks, one associated with coke on the metallic function and the other associated with coke on the acidic function. The change of activity of the metal and acid functions with time was monitored by following the rates of hydrogenolysis and hydrocracking, respectively. A mechanistically‐based dual‐site model for the kinetics of coke formation and the resulting deactivation was developed. The model successfully correlates the experimental data and is an improvement over existing models, since it explicitly accounts for the deactivation of both the metal and the acid functions.
Modeling of reaction‐induced flow maldistributions in packed beds
Authors: Stroh Friedemann , Balakotaiah Vemuri
Published: June 2004

This work examines reaction‐induced flow maldistributions in adiabatic, downflow packed beds. Using linear stability analysis it is shown that for the case of a constant heat source, the uniform flow loses stability when a certain dimensionless group, the Darcy buoyancy number, exceeds a critical value. Center manifold theory is used to analyze the local bifurcation picture for the case of a simple and double zero eigenvalue. It is found that for large Peclet numbers, all the bifurcations from the uniform solution are subcritical in nature and are unstable locally. Orthogonal collocation and continuation techniques are combined with the local theory to determine the various branches of bifurcating solutions. The temperature and flow distributions of stable and unstable solution branches are presented for several aspect ratios and Peclet numbers. Numerical simulations predict direct transitions from uniform flow to periodic or chaotic flows. It is also found that there is a wide range of the Darcy buoyancy number in which the uniform and maldistributed flows are stable and coexist.
Simulation studies of diffusion‐limited coarsening in two dimensions
Authors: Wong Tang H. , O'Brien James A.
Published: June 2004

The kinetics of particle coarsening are studied by means of contour dynamics simulation in two dimensions. The current method is free from the usual restrictions, such as small area fraction, constant particle shape, and local particle motion. Growth or shrinkage of particles in a given realization is found to depend on the local environment of the particles. The concept of critical size central to the classical theories such as the LSW theory, therefore, appears to be inappropriate. Surprisingly, the ensemble‐averaged macroscopic behavior is consistent with mean‐field theory. The particle‐size distribution (PSD) tends toward an asymptotic shape, different from the mean field result. The normalized width of the PSD seems to be insensitive to volume fraction. Interfacial transport limitation can be significant for small particles, and it has the effect of altering the width of the PSD. The coarsening rate constant is found to be a function of the area fraction.
An internal model control strategy for nonlinear systems
Authors: Henson Michael A. , Seborg Dale E.
Published: June 2004

An internal model control (IMC) strategy for nonlinear single‐input single‐output systems is proposed. The controller is designed to provide nominal performance, and a nonlinear filter is added to make the controller implementable and to account for plant/model mismatch. An important advantage of the new approach is that the assumption of full‐state feedback inherent in most input‐output linearization schemes is eliminated. However, the proposed IMC strategy is restricted to open‐loop stable systems with stable inverses. Under mild assumptions, the closed‐loop system possesses the same stability, perfect control, and zero offset properties as linear IMC. Simulation results for a continuous fermentor illustrate the advantages of the nonlinear IMC strategy.
A transformation approach to nonlinear process control
Authors: Khambanonda Tanes , Palazoglu Ahmet , Romagnoli Jose A.
Published: June 2004

This paper focusses on the approximate modeling and control of nonlinear processes. A polynomial expansion method helps develop successively more accurate models valid for the whole operating region and also provides a means to quantify the uncertainty associated with the approximation. A stability analysis can be performed in a straightforward fashion to identify stable operating regions as well as stable directions for the output variable moves. This permits the testing of linear and nonlinear compensators and the practicality of the method is demonstrated with a case study of heat‐exchanger control.
Equivalent annular model of a multitubular shell‐side (MSS) fixed‐bed reactor
Authors: Li Haoming , Hudgins Robert R. , Chang Kun Soo
Published: June 2004

For a multitube catalytic reactor in which catalyst occupies the shell side of the cooling tubes, a two‐dimensional model is developed to determine the nonuniform radial distribution of the temperature and concentration. The cross‐section of the fixed bed is divided into a number of equivalent portions proportional to the number of cooling tubes. Each portion is approximated by either an equivalent annular fixed bed or a transformed circular fixed bed in the complex plane. Enthalpy and mass balances are performed on the equivalent annular fixed bed while the radially varying flow distribution and the transport properties are considered in a transformed circular fixed bed. A correlation is developed for the equivalent annular model based on the relationship of the two different approximations. The model can also account for variations in radial porosity, radial velocity, and transport properties.
Mathematical model of a gas diffusion electrode bonded to a polymer electrolyte
Authors: Bernardi Dawn M. , Verbrugge Mark W.
Published: June 2004

A mathematical model for an ion‐exchange membrane attached to a gas‐fed porous electrode is derived and discussed. The model is applied to simulate the oxygen electrode of a polymer‐electrolyte fuel cell. Our discussion focuses on cell polarization characteristics, water transport, and catalyst utilization—all of which must be considered for fuel‐cell design. Calculated polarization behavior is shown to compare favorably with published experimental data. Our results indicate that if the membrane maintains full saturation, its contribution to the total cell resistance is most significant at higher operating current densities (greater than 200 mA/cm2). Polarization resistance due to the oxygen reduction reaction appears to be important for all practical current densities. Water transport, driven by pressure and electric‐potential forces, is shown to be a complicated function of the cell operating conditions. The utilization and distribution of noble‐metal catalyst is discussed.
A molecular mechanism for gas hydrate nucleation from ice
Authors: Sloan E. D. , Fleyfel F.
Published: June 2004

Induction phenomena are explained that exist in the only two reproducible sets of kinetic data in the literature for gas hydrate formation from an agitated ice surface. The previously unexplained data are interpreted using recent crystal diffraction results. The induction explanation is validated through cyclopropane hydrate kinetic experiments. The induction reasoning forms the basis of an hypothesis for a molecular mechanism of hydrate formation from ice. The hypothesized mechanism is quantified and given physical interpretation. The hypothesis may have significant implications for inhibiting hydrate formation.
Nucleation and growth of succinic acid in a batch cooling crystallizer
Authors: Qiu Yanfeng , Rasmuson Åke C.
Published: June 2004

A method to evaluate nucleation and crystal growth rates from batch cooling crystallization experiments is presented. Solute concentration and suspension temperature are recorded during the experiment and the product crystal size distribution is analyzed. The crystal growth and nucleation rates at any instant are calculated by solving the population balance equation using the method of characteristics, together with the mass balance equation. Based on a set of different cooling crystallization experiments, kinetics for succinic acid are determined. Applying these kinetics in process simulation allows for a reasonably accurate prediction of the product weight mean size.