bluuebunny/crossref_metadata_2025 huggingface dataset:
Fixed‐bed multisolute adsorption characteristics of nonwet adsorbents
Authors: Rixey William G. , King C. Judson
Published: June 2004

Certain nonfunctional macroreticular polymeric adsorbents are nonwet by suffciently dilute aqueous solutions and enable separations not achievable with adsorbents in the prewet state. Fixed‐bed adsorption data were obtained for a multisolute aqueous solution containing acetic acid, 1,3‐butanediol, and succinic acid, with Porapak Q and Amberlite XAD‐4 as adsorbents in both the prewet and nonwet states. The prewet breakthrough data agree with a model allowing for axial dispersion and a linear rate law, using independently estimated parameters. Break‐through curves for the nonwet case depend strongly upon solute volatility since mass transport within the particles appears to occur by Knudsen diffusion, and is rate limiting. Data for Porapak Q reflect an effective particle diffusivity about ten times lower than for Amberlite XAD‐4.
Methane steam reforming: II. Diffusional limitations and reactor simulation
Authors: Xu Jianguo , Froment Gilbert F.
Published: June 2004

Given the intrinsic kinetics, the tortuosity factor of a Ni/MgAl2O4 catalyst was determined under reaction conditions by minimizing the sum of squares of residuals of the experimental and the simulated conversions. The parallel cross‐linked pore model with uncorrelated pore size distribution and orientation was used in the calculation of the effective diffusivities. A modified collocation method was used to obtain the partial pressure profiles of the reacting components in the catalyst pellet. The simulation of the experimental reactor during the optimization of the tortuosity factor also yielded the effectiveness factors of the reactions. The results of the simulation of an industrial steam reformer are also discussed.
Experimental study of particle to gas heat transfer in fixed‐bed reactors
Authors: Pathangey Balachanderrao , Kovenklioglu Suphan
Published: June 2004

Particle to gas heat transfer studies were carried out for the exothermic benzene hydrogenation and endothermic cyclohexane dehydrogenation reactions on the same catalyst bed. A recirculation type reactor and eggshell type commercial platinum on alumina catalyst were used. When the thermodynamic value for the heat of reaction is used for both reactions in the steady‐state energy balance, local JH factors for the exothermic reaction were found to be, in general, higher than those for the endothermic reaction. This was attributed to product molecules leaving the catalyst surface with excess vibrational energy, which implies that the actual heat of reaction would be different than the thermodynamic value used in the JHfactor calculations. The study also indicated significant variations in the local JH factors; these were attributed to variations in the linear velocity in the radial direction.
CO hydrogenation over alumina‐supported sulfide cluster catalysts
Authors: Thompson Levi T. , Schwank Johannes , Curtis M. David
Published: June 2004

Bimetallic Mo‐Fe and Mo‐Co sulfide clusters were anchored on Al2O3 and used for CO hydrogenation. In addition to methane, significant amounts of dimethyl ether were produced. The reaction orders obtained from power rate laws for methanation indicated that the surfaces of the catalytic ensembles were not completely saturated by CO, in contrast to the observations for most conventional CO hydrogenation catalysts. A kinetic analysis and parameter estimation was performed to identify the rate‐determining step for methanation. A catalytic cycle was postulated that could account not only for the formation of methane and higher hydrocarbons, but also for dimethyl ether as a primary product.
Insights into the hydrodynamics of free falling wavy films
Authors: Wasden Frederic K. , Dukler A. E.
Published: June 2004

Three isolated waves of differing amplitude and shape were selected from experimental measurements of a falling liquid film at Re = 880 for study using an algorithm developed for solution of the Navier‐Stokes equations. The method computes the velocity and pressure fields as well as the velocity of the wave. The results show that large streamwise accelerations exist along with regions of recirculating flow in a moving coordinate system. These features can explain the enhanced rates of heat and mass transfer observed in wavy film flow. Computed wave velocities and wall shear stress were in reasonably good agreement with measurements. Wave velocity is shown to be sensitive to small variations in the wave shape and explains the apparent random variation of wave velocity with amplitude that has been observed experimentally. This numerical experiment points to the shortcomings of the many methods used to model large waves on falling films that have been based on parabolic velocity profiles.
Particle motion in axisymmetric stagnation flow toward an interface
Authors: Stoos J. A. , Leal L. G.
Published: June 2004

Numerical solutions for the creeping motion of a spherical particle in a linear axisymmetric straining flow normal to a deformable interface are presented for a range of viscosity ratios, capillary numbers, and Bond numbers. The parameter ranges investigated have applications in areas of flotation (small interface deformation) and material processing (large interface deformation). The accuracy of previous solutions for flotation problems which neglect interface deformation is considered, along with the magnitude and form of interface deformation “defects” which may appear in material processing applications involving fluids containing bubbles or small particles.
Control of processes with noise and time delays
Authors: Walgama Kirthi S. , Fisher D. Grant , Shah Sirish L.
Published: June 2004

A stable Kalman filter predictor (KFP) is developed which generates minimum variance estimates of the future outputs {y(t + i | t), i = 1, … d} of stochastic, single‐input/single‐output processes with time delay, d. The predicted outputs are used for time delay compensation and in the design of a predictive feedback controller. An innovation model analysis is used to convert the state space formulation to transfer function form and to show the relationship between the KFP, the Smith predictor, and the internal model controller. A modified KFP includes a disturbance model, and eliminates offset due to deterministic disturbances (e.g., steps) and modeling errors. Simulation results show that the modified KFP also predicts the disturbances and gives significantly better performance than the Smith predictor, particularly in the presence of process and measurement noise.
A study of Cr(III)‐polyacrylamide reaction kinetics by equilibrium dialysis
Authors: Hunt James A. , Young Teng‐Shau , Green Don W. , Willhite G. Paul
Published: June 2004

A kinetic model was developed to describe the rate of reaction between chromium(III) and polyacrylamide (PAAm) that leads to the crosslinking or gelation of the polymer. Cr(III) was added directly to solutions of polyacrylamide which had a degree of hydrolysis of approximately 4%. The unreacted Cr(III) was separated from that which had reacted with the polymer by equilibrium dialysis and measured by atomic absorption.

The rate of the reaction was described by an empirical model based on the assumption of invariant polymer group concentration during the reaction. The order of reaction was determined to be 1.32 for Cr(III), 0.8 for the polymer group, and −1.0 for hydrogen ion. Modifications were made to the model to allow for variation during reaction of the concentration of the polyacrylamide reactive group. The modified model accurately described the reaction rate for Cr(III) concentration ranging from 0.19 to 1.2 mM, polyacrylamide concentrations from 6.9 to 69 mM, and pH values from 4.0 to 5.5.
Extension of Chen's (1982) theory to correlate densities of aqueous electrolytes
Authors: Humffray Alan A.
Published: June 2004

The model proposed by Chen et al. for treating nonideality in concentrated electrolyte solutions has been extended to the correlation of the densities of solutions of single electrolytes (1:1, 2:1, 1:2, and 2:2) in terms of Chen's energy interaction parameters τca,m and τm,ca. At any single temperature, only three adjustable parameters, V (the partial molar volume of the solute at infinite dilution), and the pressure derivatives (δτca,m/δp)T and (δτm,ca/δp)T are required for correlating the density as effectively as the conventional treatment employing a power series in (molality)1/2. For some electrolytes, only the latter two parameters are required, as tabulated V values also lead to satisfactory correlations. The treatment can be extended to cover the temperature range 0–100°C, using either τca,m and τm,ca values derived from activity or osmotic coefficients, or constant τm,ca and τca,m values, but the irregular variation with temperature of the pressure derivatives precludes their representation by simple analytical functions, and individual values of the pressure derivatives at each temperature are necessary.
Flow of noncolloidal slurries in pipelines
Authors: Hsu Feng‐Lung , Turian Raffi M. , Ma Tzu‐Wang
Published: June 2004

A multiphase model for the flow of dense, noncolloidal, settling slurries through horizontal pipelines, and an associated Galerkin‐finite element numerical scheme to carry out computer simulations based on the model, were developed. The principal elements of the model are: equations for estimation of the velocity distributions for each component; and those for the concentration distributions. In the former, which are derived from the local volume‐averaged, time‐smoothed conservation of mass and momentum equations, the total stress is comprised of components to account for interparticle and particle‐boundary as well as viscous and turbulent interactions. The governing equations for the concentration distributions are derived similarly from the convective diffusion equation, with the eddy diffusivity modified to account for the presence of the solid particles. Numerical simulations and detailed comparisons with experiment for the flow of gypsum, coal, crushed glass, sand and gravel, covering particle sizes from 38.3 μm to 13,000 μm, and pipe diameters from 4 cm to 49.5 cm, were carried out. The agreement between model prediction and experiment is excellent.
The birefringence problem in optical disk substrates: A modeling approach
Authors: Greener Jehuda , Kesel Raymond , Contestable Beverly A.
Published: June 2004

An analytical model has been constructed to predict the birefringence level in an injuection‐molded disk with direct application to optical disk substrates. The model incorporates simplified analyses for the flow in a center‐gated cavity during the filling stage, the heat transfer during the cooling stage, and the stress relaxation following the cessation of flow at the end of the filling stage of the injection‐molding cycle. The stress relaxation process is analyzed with the integral constitutive equation of Wagner and the residual stresses are converted to retardation via the stress‐optical law. Predictions of normal retardation profiles are in line with experimental data and the general effects of melt temperature, mold temperature, and injection speed are closely captured by the model. By contrast, the observed transverse retardation is not in accord with the predicted response. This discrepancy is attributed to the fact that the transverse retardation is induced by cooling stresses and thus cannot be explained in terms of a flow stress mechanism employed in the present analysis.
Numerical calculation of particle‐laden gas flows past tubes
Authors: Schuh M. J. , Schuler C. A. , Humphrey J. A. C.
Published: June 2004

A numerical study has been conducted for the flow of a dilute particleladen gas moving past one or more tubes undergoing erosion. A nonor‐thogonal body‐fitted coordinate system was used to calculate three tube configurations for laminar and turbulent flow regimes. The assumption of one‐way coupling allows the calculation of individual particle velocities from the fluid flow field. The significant effects of turbulent velocity fluctuations are taken into account by means of the stochastic separated flow model. The particle flow field information is then used to predict circumferential distributions of particle flux and erosion. Predictions of trajectories for the case of two in‐line tubes show that particles with inertia numbers λ > 1 will strike many tubes in a tube bank due to particle rebounding from tube surfaces. By contrast, particles with λ < 1 are entrained in the bulk flow between tubes. In general, the effect of increasing the particle‐gas suspension temperature is to couple the particle‐fluid motion more closely through viscous drag and, thus, to decrease erosion.
A model for unsteady analysis of preform drawing
Authors: Myers Matthew R.
Published: June 2004

A model is introduced for analyzing diameter variations occurring in glass fibers drawn from preforms. The heat transfer is shown to be radiation‐dominated, and approximate expressions for radiative exchange are developed. The expressions are used in the one‐dimensional governing equations for extensional flows to derive a set of linearized disturbance equation as an eigenvalue problem. The sensitivity to an oscillatory take‐up rate is also studied as a boundary‐value problem. Calculated amplitudes of final diameter perturbations are found to agree well with experimental values.
The continuous countercurrent moving‐bed separator
Authors: Fish Barry B. , Carr Robert W. , Aris R.
Published: June 2004

An investigation of the continuous chromatographic separation of an equimolar mixture of 1,3,5‐trimethylcyclohexane and 1,3,5‐trimethylbenzene with a 2.4 m × 1.37 cm ID vertical countercurrent moving bed is reported. The solid adsorbent was 30 mesh Al2O3, and the separation was carried out at 200°C. Steady‐state axial concentration profiles of each species were determined by withdrawing vapor samples for analysis by gas chromatography. The binary mixture was continuously fed through a port located at approximately the midpoint of the column. The relative solids and carrier gas flow rates could be adjusted so that the more strongly adsorbed trimethylbenzene was transported downward, and less strongly adsorbed trimethylcyclohexane moved upward. For low feed rates, high‐purity product streams of each were obtained at the bottom and top of the separator. For sufficiently high feed rates, trimethylbenzene was transported upward as well as downward, and top product purity deteriorated. This behavior is in qualitative accord with the predictions of a dispersionless, one‐dimensional flow, adsorption equilibrium model incorporating a Langmuir isotherm.
Dynamics of packed‐bed reactors loaded with oxide catalysts
Authors: Arnold Ernest W. , Sundaresan Sankaran
Published: June 2004

The lattice of an oxide catalyst can act as a reservoir for oxygen, storing and releasing it for oxidation reactions at the catalyst surface under appropriate transient conditions. The implication of this oxygen storage property on the dynamic response characteristics of packed‐bed reactors loaded with oxide catalysts has been investigated through an experimental study of 2‐butene oxidation over vanadium oxide. Temperature profiles in a wall‐cooled packed‐bed reactor have been measured at steady state and following step changes in the feed butene concentration and in the feed gas flow rate. Thermal reactor runaway was observed experimentally for large step increases in the feed butene concentration following certain catalyst pretreatments. Gas flow rate step increases and less severe butene step increases resulted in the reactor temperature overshooting its final steady‐state profile. These response characteristics are a direct consequence of the oxygen storage property of the vandaium oxide catalyst.
Electrosorption of n‐alcohols on graphite particles
Authors: Zabasajja J. , Savinell R. F.
Published: June 2004

The loading of organic molecules adsorbed on high surface area carbon materials can be influenced by the potential of the surface. This phenomenon can form the basis for carbon regeneration and for separation processes. A theoretical treatment demonstrates that adsorption loading becomes more sensitive to potential change when the double layer capacitance of the interface and the size of the organic species are increased. An experimental method was developed to estimate isotherm parameters. The technique was demonstrated on model adsorbates, pentanol and heptanol. The values of the measured isotherm parameters (B = 3.4–9.1 V−2: Emax = 0.36‐0.5 V vs. Hg/Hg2 SO4) are in the range of expected values based on literature values for similar systems. Although only 5% of activated carbon surface area was shown to be electrochemically accessible, it still has ten times the area available for electrosorption as compared to the graphite, thus making it attractive for industrial application.
An internally staged hollow‐fiber permeator for gas separation
Authors: Sidhoum M. , Majumdar S. , Sirkar K. K.
Published: June 2004

Conventional membrane permeation of gases employ a non‐step process between the feed and the product on two sides of a membrane. By applying the overall driving force change through a two‐step process using two membranes in one device, we illustrate theoretically the possibility of achieving a much higher enrichment without any extra energy consumption. Such an internally staged permeation is experimentally carried out in a hollow‐fiber permeator with two sets of asymmetric cellulose acetate hollow fibers for the systems of O2N2 (air) and CO2N2. The superior enriching performance of the internally staged permeator over the same permeator operated in the conventional mode is demonstrated experimentally. A cocurrent flow pattern is found to be superior to a countercurrent flow pattern for the conditions employed. The simulation model incorporating fiber lumen pressure drop predicts the experimental performance quite well. A further improvement in performance can be achieved by recycling the intermediate pressure shell reject stream to the feed stream. The energy requirement in an internally staged permeator without recycle is less than that of a conventional permeator with permeate recycle.
Controller design for integrating and runaway processes involving time delay
Authors: Quinn Stanley B. , Sanathanan C. K.
Published: June 2004

This paper presents an efficient method for the design of controllers for integrating and runaway processes. The method is based on model matching in the frequency domain. The presence of open‐loop instability as well as pure time delay in the process models make the design task challenging for these classes of processes.

The goal is to achieve low‐order, easily implementable cascade controllers in a unity‐output‐feedback configuration. It is shown that the central problem is in the selection of appropriate reference models. Several key constraints are developed which relate a given process model to a class of reference models for achieving total stability. Typical design examples are presented to clearly illustrate the various mathematical techniques.
Absorbed solution theory models for multicomponent adsorption equilibria
Authors: Gamba Giuseppe , Rota Renato , Storti Giuseppe , Carra Sergio , Morbidelli Massimo
Published: June 2004

A new approach is developed for applying ideal and real adsorbed solution theory models to the description of multicomponent adsorption equilibria at fixed pressure. This allows ignoring single‐component behavior in the Henry's law region, P → O, but introduces new parameters that must be evaluated from binary equilibrium data. Comparisons with various sets of experimental data show that the new approach gives results as good as the classical one, but without requiring knowledge of the single component behavior in the Henry's law region; more over, it removes the sensitivity to the type of model adopted in fitting single‐component equilibrium data.
Laser‐induced ignition in solid‐state combustion
Authors: Dimitriou P. , Hlavacek V. , Valone Steven M. , Behrens Robert G. , Hansen G. P. , Margrave J. L.
Published: June 2004

The ignition process of condensed‐phase combustion by a laser is studied experimentally as well as numerically. Numerical simulations are performed in one, two and three spatial dimensions, and are compared with the experimental observations. The effects of energy‐dissipative mechanisms, such as melting and ablation due to laser irradiation and the effects of laser power output fluctuations, are also studied. We find that the ignition process depends mainly on the energy deposited in the system. The time leading up to ignition is longer for higher light fluxes keeping the average laser power constant, since a larger fraction of the energy is reradiated from the heated surface or consumed during the process of material removal.
Pellet‐model effects on simulation models for fixed‐bed desulfurization reactors
Authors: Sotirchos Stratis V. , Zarkanitis Solon
Published: June 2004

Detailed mathematical models, pore or grain based, describing diffusion, reaction, and pore structure evolution in pellets of porous solids are used to simulate, along with a macroscopic reactor design model, the transient phenomena that take place during desulfurization of coal gas in fixed‐bed reactors of pellets of metal oxide sorbents. Our computer simulation results show that the form of the pore‐ or grain‐size distribution strongly influences the predictions of the overall reactor design model.
Numerical simulation of theories for gas absorption with chemical reaction
Authors: Glasscock David A. , Rochelle Gary T.
Published: June 2004

Steady‐ and unsteady‐state theories for interfacial mass transfer are used to model gas absorption with second‐order, reversible reaction and the absorption of CO2 into aqueous MDEA (methyldiethanolamine). The latter case represents a system of industrial interest, having both finite rate and equilibrium reactions. Steady‐state theories studied are film theory, simplified eddy diffusivity theory, and an approximation to surface renewal theory. Higbie's penetration and Danckwert's surface renewal theories are the unsteady‐state theories reviewed. The Nernst‐Planck equations for diffusion and reaction in ionic systems are solved numerically using orthogonal collocation on finite elements. Comparisons are made between the absorption enhancement factor obtained for all theories. The applicability of approximate methods of solution for absorption with chemical reaction is also discussed.
Laser‐doppler velocimeter measurements in simulated entrained gasifier flows
Authors: Lindsay Jeffrey D. , Hedman Paul O. , Smith Philip J.
Published: June 2004

A laser‐Doppler velocimeter (LDV) was used in a cold‐flow study of a simulated entrained‐flow coal gasifier. The study was designed to provide fundamental information about the flows in such a gasifier and to provide data for the validation of a turbulence submodel used in modeling combustion processes. Measurements in 20 swirling and nonswirling flow cases were made with several levels of replication. This study emphasized the effects of inlet conditions on flow properties within the simulated reactor.

Unsteady flow phenomena with time scales on the order of seconds to minutes were sometimes observed. The unsteadiness was apparently associated with relaminarization‐type flow transitions.

Comparisons were made with model predictions from PCGC‐2, a model for combustion processes based on the k, — ϵ turbulence model. Several areas of weakness in the model results were observed, but the unusual flow regimes measured in this study may be beyond the abilities of practical computer models.
Deposition rates from polydispersed particle populations of arbitrary spread
Authors: Rosner Daniel E. , Tassopoulos Menelaos
Published: June 2004

For particle deposition from log‐normal polydispersed aerosol streams by one or more of several mechanisms described by piecewise‐power‐law mass transfer coefficients, we derive useful relations between actual total mass deposition rates and the ‘reference’ rate one would calculate by imagining that all particles in the mainstream population had the average particle volume v(=ϕp/Np). Included here are diffusion or inertial mechanisms for laminar‐ or turbulent‐boundary layers, free‐molecular or continuum diffusion at high Peclet numbers of dense spherical particles, or fractal agglomerates. The mainstream particle volume distribution is considered to be log‐normal with arbitrary ‘spread’ parameter, thereby generalizing earlier results for “coagulation‐aged” (self‐preserving) distributions. Further generalizations include transitions between important particle transport mechanisms, opening the way to efficient, finite‐analytic methods for predicting mass deposition rates for arbitrary, size‐dependent particle capture efficiencies.
Synthesis of feedforward/state feedback controllers for nonlinear processes
Authors: Daoutidis Prodromos , Kravaris Costas
Published: June 2004

A systematic method for synthesizing feedforward/state feedback controllers for a broad class of SISO nonlinear systems with measurable disturbances is presented. Depending on the structural characteristics of the system, the control law can be static or dynamic. The closed‐loop system is independent of the measurable disturbances and linear with respect to set point changes. The performance of the proposed control scheme is illustrated through an example of composition control in a system of three CSTR's in series.
Manipulating solids CSD in a supercritical fluid crystallizer: CO2‐benzoic acid
Authors: Tavana A. , Randolph A. D.
Published: June 2004

The final properties and end use of many organic solids often depends on their crystal‐size distribution (CSD), e.g., bioavailability for pharmaceutical solids. The objective of this study was to develop an alternate technology for crystallizing and manipulating size distribution of organic solids by crystallization from supercritical fluids. Such a technology might be superior to conventional‐size manipulation techniques, such as milling and recrystallization from organic solvents, which might contaminate the products and affect their physical‐chemical stability. Supercritical carbon dioxide was used as solvent to dissolve and then crystallize benzoic acid. A batch crystallizer was constructed and operated in the supercritical region. Seeded and unseeded crystallization experiments were performed. Different depressurization vs. time profiles for the supercritical fluid were used to produce crystals of various sizes.
Parametric analysis of thermal swing cycles for multicomponent adsorption
Authors: Huang Chen‐Chia , Fair James R.
Published: June 2004

A study was made of the effect of design and operating parameters on the regeneration of a fixed bed of activated carbon. Two adsorbed species were removed by a hot gas purge, making the study relevant to thermal swing cycles used commercially. The analysis utilized direct experimental results as well as simulated results using a model described earlier by the authors (Huang and Fair, 1988).

Regeneration parameters studied were: type of purge gas, purge gas contact time and velocity, regeneration temperature, initial bed loading, and bed pressure. These were related to certain parameters influencing the adsorption step: type of carrier gas, feed concentration, contact time and velocity, pressure, and initial bed temperature. The results are summarized in the form of guidelines for optimal regeneration cycle specification.
Turbulent micromixing parameters from reactive mixing measurements
Authors: Shenoy U. V. , Toor H. L.
Published: June 2004

Previously published conversion data for turbulent diffusion‐controlled acid‐base reactions in two different multijet reactors are used to extract the mixing parameters, tracer probability density function, and variance. The PDF is obtained with this instantaneous reaction method by differentiating twice the reactive mixing plot of mean reactant concentrations. The limiting behavior for the cases of complete segregation and complete mixing is derived and turns out to be piecewise linear on the reactive mixing plot. The variance is shown to be twice the area between the actual reactive mixing curve and the limiting curve for complete mixing; it can be computed by numerical integration without any foreknowledge of the PDF shape. The method, in principle, allows detection of Dirac delta functions (which represent unmixed virgin fluid) and indeed shows significant amounts of virgin fluid close to the inlet of one of the reactors.
Profile equations for film flows at moderate Reynolds numbers
Authors: Kheshgi Haroon S.
Published: June 2004

An approximate equation for the evolution of finite‐amplitude, long disturbances to Newtonian liquid films is found to be accurate over a wide range of Reynolds numbers. A long‐wave expansion leads to a film profile equation asymptotically correct when wave number and Reynolds number are small. Solutions of the film profile equation are compared to exact and other approximate solutions of the Navier‐Stokes system. An alternative form of the film profile equation results in remarkably accurate solutions, when Reynolds numbers are moderate, in the cases of standing or monotonically decaying waves in horizonatal films, rising film flow, final acceleration of a moving film, and film flow emerging from a slot coater.
Effective diffusivity by pulse gas chromatography at elevated pressures
Authors: Sarmah S. K. , Haynes H. W.
Published: June 2004

Pulse gas chromatography experiments at ambient pressure are often limited by an overwhelming contribution of axial dispersion to the observed variance and the inability to obtain a quantitative correction for this factor. Theoretically, the axial dispersion contribution can be reduced and at the same time the contribution due to particle diffusion can be enhanced by operating at elevated pressures. Our experiments on methane/helium diffusion in 0.32‐cm‐diameter commercial catalyst particles confirm this, and a new means of accounting for axial dispersion is demonstrated.
Trickle‐bed reactors: An experimental study of partial wetting effect
Authors: Ring Zbigniew E. , Missen Ronald W.
Published: June 2004

The effect of partial external wetting of catalyst particles on conversion for hydrodesulfurization of dibenzothiophene in a white oil was studied in a 25‐mm trickle‐bed reactor at 10 MPa and 330–370°C with Mo/SiO2 catalyst. The superficial liquid velocity (Lm) was varied from 0.092 to 0.557 kg · m−2 · s−1. Measurements of conversion were augmented by tracer, kinetics and solubility experiments. The objectives were to determine the parameters of a reactor model based on the multizone external wetting (MZEW) model for the overall effectiveness factor, ηo, and to compare the measured and predicted values of ηo. As the wetting efficiency, f, increases, ηo also increases, but, even at the highest Lm, catalyst wetting was not complete and consequently ηo was lower than the value corresponding to f = 1. The MZEW model describes the experimental data better than the simpler two‐zone model, based on a particular interpretation of f. The difference between the two models can be as high as 200%.
The critical properties of n‐alkanes using a low‐residence time flow apparatus
Authors: Rosenthal Daniel J. , Teja Amyn S.
Published: June 2004

A low residence time flow method for the measurement of the critical temperatures and pressures of thermally‐unstable fluids was developed in this work. The low residence time at elevated temperatures minimizes decomposition and other reactions, making it possible to measure the critical properties of many unstable fluids. An extrapolation procedure was developed to obtain the critical properties of rapidly reacting substances, based on the linear behavior of the apparent critical property with residence time. The measured critical properties of 14 n‐alkanes (pentane through octadecane) using this method are reported in this paper. The results extend the available data on the critical properties of the higher alkanes and reveal a possible error in the literature value for the critical pressure of tetradecane.
An extended horizon feedback/feedforward self‐tuning controller
Authors: Kershenbaum Lester S. , Pérez‐Correa J. Ricardo
Published: June 2004

A feedforward/feedback version of a single input/single output self‐tuning controller has been developed and tested both via simulation studies and experiments on a section of an absorption/desorption pilot plant. The algorithm is based on the recursive least squares estimation of parameters for the linear models relating the output to the controlled input and to the disturbances; adaptation is achieved using a variable forgetting factor. The control input at each time interval is calculated using one of several single‐step extended‐horizon control strategies.

The results show that the performance of the algorithm is insensitive to the choice of initial parameters, all of which have a readily identifiable intuitive basis. The algorithm is especially robust against deterministic disturbances (measurable and unmeasurable) and unknown and varying time delays. Computational load beyond that of a feedback‐only version is minimal.
Lumping nonlinear kinetics in a CSTR
Authors: Astarita Gianni , Nigam Abhash
Published: June 2004

The model of uniform nonlinear kinetics recently developed for the continuous description of a multicomponent mixture reacting in a batch or plug flow reactor (Astarita, 1989) is applied to the analysis of a continuous stirred tank reactor (CSTR). It is shown that most of the equations can be solved in general, independently of the specific kinetic mechanism. Some specific mechanisms are discussed. The difference between the behavior in a plug flow reactor (PFR) and a CSTR turns out to be quite dissimilar from what one would expect by considering the single‐component case.
Dynamics of heterogeneous azeotropic distillation columns
Authors: Rovaglio Maurizio , Doherty Michael F.
Published: June 2004

A new dynamic model has been developed for heterogeneous azeotropic distillation columns, which is capable of automatically detecting and accounting for multiple liquid phases on trays at each instant of time. Extensive dynamic simulations have been carried out for the heterogeneous distillation of ethanol, water and benzene. Our results are in general agreement with prior work, which shows that this class of separations can be expected to exhibit multiple steady states, complex dynamic behavior, and parametric sensitivity. One of the most unexpected results was that these systems exhibit parametric sensitivity with respect to small changes in column pressure. For systems vented to the atmosphere, small increases in the barometric pressure lead to loss of interface in the decanter over a 24‐hour period. If small pressure variations of this sort are left uncontrolled long enough, the entire separation will fail. Small decreases in the barometric pressure do not have such a disastrous effect.
A mathematical model of pulse plating on a rotating disk electrode
Authors: Yin Ken‐Ming , White Ralph E.
Published: June 2004

A galvanostatic pulse plating model is presented for the electrodeposition of an alloy on a rotating disk electrode. This model is used to simulate the electrodeposition of nickel/chrome alloys. The mass transport equations used in the model include the effects of diffusion, migration and convection; and the electrode kinetics are described by the Butler‐Volmer equation. It is predicted that the effect of ionic migration is significant and therefore should be included in models of pulse plating.
Viscoplastic‐elastic modeling of tubular blown film processing
Authors: Cao Bangshu , Campbell Gregory A.
Published: June 2004

The blown film process has been modeled through the transition from liquidlike to solidlike behavior at the freeze line. A new constitutive equation has been developed that, when incorporated into the the kinematic and dynamic equations that describe the process, for the first time yields qualitatively correct predictions of all process variables. It is suggested that the demarcation between liquidlike behavior and solidlike behavior be altered from the conventional, kinematically based constraint, dr/dz = O, to a rheologically based constraint, the plastic‐elastic transition (PET). The results are qualitive in nature since the material is modeled as an elastic solid above the PET instead of as a viscoelastic material. The model is tested using the polystyrene data of Gupta (1981).
Catalytic hydroprocessing of chlorobenzene and 1,2‐dichlorobenzene
Authors: Hagh Bijan F. , Allen David T.
Published: June 2004

The hydrodechlorination reactions of chlorobenzene and 1,2‐dichlorobenzene on a NiMo/γAl2O3 catalyst were examined in a differential microflow reactor. The NiMo catalyst had high activity and selectivity for dechlorination; no hydrogenation of the aromatic ring was detected. The dechlorination reactions were examined at temperatures ranging from 275 to 375°C. In this temperature range the rates of adsorption and surface reaction are comparable and neither controls the overall reaction rate. A kinetic model, incorporating adsorption, desorption and surface reaction rates, has been used to model the data.
Prediction of differential sorption kinetics near Tg for benzene in polystyrene
Authors: Mehdizadeh S. , Durning C. J.
Published: June 2004

A recent first principles theory for “anomalous” diffusion, which accounts for the effect of a protracted macromolecular response to a perturbation, is used to quantitatively predict the results from successive differential sorption experiments in the benzene/atactic polystyrene system. The physical constants in the theory are evaluated for the conditions of the experiment without reference to any non‐Fickian diffusion data. The theoretical predictions are in reasonable agreement with the experimental observations.
Critical properties of dilute multicomponent mixtures
Authors: Anselme M. J. , Teja A. S.
Published: June 2004

Equations for the critical properties of dilute multicomponent mixtures have been derived using the Soave‐Redlich‐Kwong equation of state. The utility and limitations of the approach are described using data on 47 mixtures from the literature. New data on three dilute multicomponent mixtures have also been measured in this work and are compared with the predictions of our method. In general, it is possible to predict all three critical properties of dilute multicomponent systems with a satisfactory degree of accuracy. Predictions are as good as those obtained from conventional equation of state methods, which employ a rigorous solution of the Gibbs criteria for the critical point, but offer the advantage that no iterations are required in the calculations.
Diffusion from spheres in a continuous‐flow stirred tank
Authors: Liekhus Kevin J. , Hanley Thomas R.
Published: June 2004

Solvent diffusion from spheres at constant mass‐transfer Fourier number, FoM, in a single continuous‐flow stirred tank (CFST) and in a series of CFSTs is simulated. A transfer efficiency is defined and transfer efficiency curves are constructed as a function of FoM and the residence time distribution of the spheres in the system. The fraction of solvent removed from the spheres in a series of CFSTs can be approximated using the transfer efficiency curves and knowledge of the operating conditions in each tank. The usefulness of the curves to predict solvent removal from polymer spheres under pilot‐plant conditions is examined.
A modified smith predictor with an approximate inverse of dead time
Authors: Huang Hsiao‐Ping , Chen Cheng‐Liang , Chao Yung‐Cheng , Chen Pei‐Lin
Published: June 2004

A compensator that approximates the inverse of dead time at low frequencies in a modified Smith predictor (MSP) control system is proposed, and the design of an MSP control system for it is given. The performance of the MSP with compensator, using a first‐order element as a low‐pass filter, is described. Analysis and simulation results show that the compensator improves performance in the disturbance rejection of the original Smith predictor.
Adaptive‐predictive control of kamyr digester chip level
Authors: Allison Bruce J. , Dumont Guy A. , Novak Lloyd H. , Cheetham William J.
Published: June 2004

Kamyr digester chip level poses a difficult control problem, because the chip column dynamics are complex and it is difficult to measure the chip‐liquor interface. Control of the chip level is required to smooth digester operation and to stabilize the residence time. Conventional methods rely largely on fixed parameter controllers which are unable to compensate for changes in the process dynamics. Manual controller returning is often required.

An adaptive chip level controller based on Generalized Predictive Control (GPC) is presented. Closed‐loop time series identification methods were used to develop the process model and to design the controller. A significant reduction in chip level and P‐number variability has been observed since commissioning the controller on an industrial digester in September 1988. The adaptive controller has eliminated the need for manual returning, reduced the need for operator intervention, and demonstrated the potential to predict hang‐ups.
Control structures for disturbance rejection and decoupling of distillation
Authors: Häggblom Kurt E. , Waller Kurt V.
Published: June 2004

The process gains for different distillation control structures can be calculated through transformations from known gains of one arbitrary structure.

In this paper the transformation is utilized for calculation of a two‐point distillation control structure which, in the steady state, simultaneously rejects disturbances in the feed composition and in the feed flow rate and results in implicit decoupling between the two product control loops. This disturbance rejecting and decoupling (DRD) structure is tested on a model of a 15‐plate pilot‐plant distillation column. It compares favorably with standard structures for dual composition control. Experimental results agree with the simulation results.
Modeling of additive effects on the electroplating of a through‐hole
Authors: Hazlebeck David A. , Talbot Jan B.
Published: June 2004

A mathematical model of copper plating of a through‐hole is developed which relates uniformity of deposition to bulk electrolyte composition, applied potential difference, aspect ratio, through‐hole diameter, and deposition kinetics. The electrochemical transport equations governing plating in a through‐hole are solved assuming that the fluid within the through‐hole is stagnant. Conditions for uniform plating are determined both for Butler‐Volmer kinetics considering the effects of dissociation of bisulfate ions in the electrolytic solution and for kinetics limited by complexation of cupric ions with an adsorbed neutral additive species.
Effective kinetic modeling of multistep homogeneous reactions
Authors: Chern Jia‐Ming , Helfferich Friedrich G.
Published: June 2004

The Bodenstein approximation for trace‐level intermediates is used to reduce multistep reaction‐kinetic networks of arbitrary topology and size to simpler forms with only pseudosingle steps between nodes and to derive explicit equations for formation or consumption rates of end members. Networks with nontrace intermediates or steps of higher orders in intermediates must be broken at the respective steps into portions to which the procedure can then be applied separately. A recipe is provided with which rate and yield‐ratio equations are readily compiled for networks of arbitrary configuration and complexity. The method is illustrated with an example of a reaction of industrial interest.
Estimation of surface diffusion through porous media
Authors: Sircar S. , Rao M. B.
Published: June 2004

A mathematical model is developed to describe steady‐state nonequilibrium surface diffusion of an adsorbing gas through an energetically homogeneous porous medium by assuming that the adsorption isotherm is linear and that the gas‐surface mass transfer can be described by the linear driving force model. Analytical solutions for the model are reported. It is found that the conventional assumption of local thermodynamic equilibrium between the gas and surface phases within the porous medium can seriously underestimate the surface diffusivity when the mass transfer rate is finite. Also, the length of the porous medium becomes a critical variable in that case.

Meaningful estimation of surface diffusivity also requires that the gas phase diffusivity through the porous medium be accurately known. The estimated surface diffusivity is model dependent when local thermodynamic equilibrium is not established instantaneously. Modeling surface diffusion can be very complex when the surface is energetically heterogeneous.
Stirring characteristics in bioreactors
Authors: Vanags J. J. , Rikmanis M. A. , Ushkans E. J. , Viesturs U. E.
Published: June 2004

Distribution data of local values of specific kinetic energy of medium flow in the bioreactor volume made it possible to determine some integral criteria. The relationship of these criteria, microorganism growth and biosynthesis characteristics was studied in 5–5,000‐L bioreactors. Energy‐efficient stirring systems that ensure a minimum damage of cells have been also studied.
A numerical study of mass transfer in free falling wavy films
Authors: Wasden Frederic K. , Dukler A. E.
Published: June 2004

Numerical simulations of mass transfer into falling liquid films, both through the wavy interface and from the wall, have been performed for experimentally measured large waves within which the flow fields have been computed. Experiments have shown that the occurrence of waves on free falling films causes dramatic increases in mass transfer into the film, even under laminar flow conditions. Wave effects have been modeled in several ways, none of which predicts the observed rate of enhancement. The present numerical procedure includes solving the convective‐diffusion equation for wavy films by extending a technique developed for hydrodynamic simulation. The presence of waves is shown to cause significant velocities normal to each interface. In conjunction with recirculation within the large waves, these flow patterns produce transfer rates for large waves that are several times larger than predicted for quasiparallel velocity fields. Experimental wave structure data were used to define the dimensions and frequency of an average large wave and surrounding substrate. Computed transfer rates at both the gas‐liquid interface and the wall for a film composed of a periodic sequence of average waves agree well with published data. These simulations confirm the inadequacy of parabolic, or Kapitza‐type velocity profiles in formulating transport models.
Dynamic structural transformations for distillation control configurations
Authors: Yang Dae R. , Waller Kurt V. , Seborg Dale E. , Mellichamp Duncan A.
Published: June 2004

A control structure transformation is presented that includes the effects of inventory control system dynamics and internal flow dynamics on flows and compositions. This new transformation can be used to predict column dynamic operation for any alternative control structure from the open‐loop model (inventory control only) obtained for a particular control structure (LV). New types of inverse responses are predicted using this technique and verified both by rigorous dynamic simulation and by experimental studies on a pilot‐scale distillation column. These features also can be shown theoretically using a simplified transfer function model.