bluuebunny/crossref_metadata_2025_split huggingface dataset:
Lumping nonlinear kinetics
Authors: Astarita Gianni , Ocone Raffaella 
Published: June 2004

AbstractLumping of the kinetics of mixtures described by a continuous distribution function of concentration is discussed for the case where individual reactions have nonlinear kinetics. The assumption of independent kinetics, which leads to a paradox, is not used. Functional‐differential equations govern the kinetic behavior of the mixture. Formal solutions are presented for a class of kinetic behavior that includes Langmuir isotherm catalysis and for the special case of bimolecular reactions. A class is defined for which the kinetic functionals can be expanded in a series of integrals for which the kernels are entirely determined. Finally, a thermodynamic analysis is presented to show that the commonly accepted equilibrium theory implies a kinetic approximation of maximal rank for the kinetics at the equilibrium point.
Estimation of the molecular weight distribution in batch polymerization
Authors: Ellis Mark F. , Taylor Tad W. , Gonzalez Victor , Jensen Klavs F. 
Published: June 2004

AbstractA filtering technique is proposed for on‐line estimation of the temperature, monomer conversion, initiator conversion, and the entire molecular weight distribution in a batch methyl methacrylate polymerization reactor. The technique uses a detailed polymerization model combined with on‐line measurements of conversion, temperature, and the molecular weight distribution, taken at different discrete time intervals. The polymerization model includes a chain‐length‐dependent termination rate constant which allows the prediction of the molecular weight distribution for common free‐radical polymerization conditions. Comparisons between modeling and experimental results show that the polymerization model gives good predictions of the monomer conversion and the molecular weight distribution in the polymerization system. The performance of the estimation scheme is tested for cases of strong gel effect conditions leading to a bimodal molecular weight distribution, and poor initial conditions. Finally, off‐line experimental data are used to test the algorithm under actual reactor operating conditions.
Novel microreactor with quench system for kinetic study of propane pyrolysis
Authors: Rastogi Ashutosh , Svreck William Y. , Behie Leo A. 
Published: June 2004

AbstractA novel microreactor with a rapid direct quench system was developed to study fast hydrocarbon pyrolysis reactions. Heating of reactants inside the reactor was achieved by a Curie Point Pyrolyser. The propane pyrolysis reaction was studied at four different temperature levels: 600, 700, 800 and 900 °C. Pyrolysis times were varied from 0 to 2 s between 600 to 800°C and from 0 to 1 s at 900°C. The primary reaction products were methane, ethylene, hydrogen and propylene. Using a developed and very effective quench technique increased the production of olefins and reduced the formation of methane. A first‐order kinetic analysis performed on the experimental data between 600 and 700°C yielded an activation energy of 216 kJ/mol, which is in the range of other researchers.
Theoretical study of interfacial transport in gas‐liquid flows
Authors: Back D. D. , McCready M. J. 
Published: June 2004

AbstractMass transfer in sheared, concurrent gas‐liquid flows is investigated theoretically using solutions to the unaveraged advection‐diffusion equation. For sufficiently thick films, the resistance to mass transfer is shown to be confined completely within a thin region in the liquid near the interface and mass transfer coefficients are accurately predicted by an improved numerical technique that uses a velocity field derived from an Orr‐Sommerfeld equation with the time‐varying velocity computed directly from measurements of interfacial waves. The mass transfer coefficients are shown to depend on the magnitude and frequency content of the velocity fluctuations normal to the interface. As the film thickness decreases, transfer resistance extends throughout the film and turbulent mixing in the middle of the film controls the transfer rates. For this region, limiting values of transfer coefficients are predicted well by analytical solutions to the advection‐diffusion equation, which assume a laminar flow.
Design of parallel cascade control for disturbance‐rejection
Authors: Yu Cheng‐Ching 
Published: June 2004

AbstractA new approach is presented to overcome load disturbance problems. Based on the parallel cascade control structure, the primary controller is designed for the servo response, and the secondary controller is designed for the disturbance‐rejection purpose. A perfect disturbance‐rejection controller is derived for the secondary loop. Simulation studies show that improvement in load responses can be achieved for some cases. However, for some other cases, parallel cascade control simply worsens the load responses as a result of interaction between primary and secondary loops. A new interaction measure for load disturbance (γ) is proposed to determine whether improved load responses is achievable using cascade control structure. More importantly, the interaction measure also quantifies the margin of improvement that can be achieved over conventional single‐loop control. A design procedure for improved load responses was proposed. The simulation results show that the new approach offers a simple and effective alternative for disturbance‐rejection.
Study of the adsorption and desorption of multiple adsorbates in a fixed bed
Authors: Huang Chen‐Chia , Fair James R. 
Published: June 2004

AbstractAn experimental and theoretical study was made of the adsorption of ethane/propane mixtures from nitrogen or helium. The study also covered the desorption of the mixtures using a purge of hot nitrogen or helium. The adsorbent was Witco JXC activated carbon. A nonequilibrium, mechanistic model was developed and was shown to represent both adsorption and desorption processes. The model utilized mixture isotherm data predicted by the Flory Huggins form of the vacancy solution model. Both adsorption and hot purge regeneration were found to be intraparticle rate controlled processes, with surface diffusion dominating the intraparticle mass transfer mechanism. The dynamic model can be used for the simulation of a variety of commercial adsorption and desorption conditions.
Theories of precipitation induced by dissolution
Authors: Kopinsky Joel , Aris Rutherford , Cussler E. L. 
Published: June 2004

AbstractWhen a porous solid is placed in acid, the solid dissolves at its surface. However, the same solid can precipitate or dissolve in pores inside the solid. The internal precipitation is a consequence of a nonlinear solubility product amplified by fast chemical kinetics. The internal dissolution can be enhanced by unequal diffusion coefficients. Both precipitation and dissolution can occur simultaneously but in different regions, mimicking the patterns of mineral found in teeth.
Surface area evolution of calcium hydroxide during calcination and sintering
Authors: Mai M. C. , Edgar T. F. 
Published: June 2004

AbstractA simple one‐dimensional mathematical model validated with high‐temperature entrained‐flow reactor data successfully explained dispersed calcium hydroxide particle surface area evolution resulting from concomitant calcination and sintering. An integrated first‐order calcination rate expression and a second‐order sintering rate form accurately predicted time‐resolved surface area generation and degradation. The effects of water vapor and gaseous carbon dioxide concentration on asymptotic specific surface areas and sintering rates were noted. This methodology for predicting sorbent surface area evolution will enable more accurate sulfation modeling efforts in furnace sorbent injection (FSI) applications, where calcination, sintering, and sulfation can all occur on the same time scale.
Phase plane and bifurcation analysis of thin wavy films under shear
Authors: Sheintuch M. , Dukler A. E. 
Published: June 2004

AbstractA long‐wave equation for film thickness as a function of position is derived for a general case incorporating viscous, surface tension, and interfacial shear effects. The derivation considers both the parabolic and the power‐law velocity profiles. The analysis is aimed at revealing the wave velocity that induces infinitely long (homoclinic) periods as well as substrate thickness and wave peak amplitude. Phase plane analysis shows that at Re ≫ 1, due to time‐scale separation, the homoclinic velocity is near that at the Hopf bifurcation. That enables analytical derivation of the wave characteristics.Comparison with experimental results in the range of Re‐310–3, 100 with countercurrent gas flow, shows encouraging agreement. At very high Re the wave velocity suggests the onset of turbulence, in agreement with theory. Phase plane analysis predicts also that the wave shape consists of a simple peak with a steep front, with short waves riding on the main wave at low Re.
Slurry reactor studies of homogeneous and heterogeneous reactions
Authors: Fu Chen‐Chen , McCoy B. J. , Smith J. M. 
Published: June 2004

AbstractA dynamic model is derived for simultaneous, homogeneous and heterogeneous, first‐order reactions in a slurry reactor. The theory is applied to experimental data for the oxidation (with gaseous oxygen) of aqueous sulfite solutions which is first order in oxygen and zero order in sulfite. The homogeneous reaction is very slow at low pH (2–3) where the sulfur exists as dissolved SO2 or HSO3−, even when cobalt is added as a homogeneous catalyst. At higher pH (7–8.5), considerable homogeneous reaction occurs with and without cobalt ions. Here the predominant species are SO3= and HSO3−. Activated carbon is not a heterogeneous catalyst at pH 7 or 8.5, but is responsible for nearly all the reaction at the low pH values. Analysis of the heterogeneous data indicates that the surface reaction between adsorbed oxygen and SO2 controls the rate rather than adsorption of oxygen.
Diffusivity of polymers in dilute solutions undergoing homogeneous flows
Authors: Öttinger Hans Christian 
Published: June 2004

AbstractIn order to describe the translational diffusion of polymers in dilute solution, an elastic dumbbell model is used for which the effects of both hydrodynamic interaction and a nonlinear connector force are treated in a self‐consistently averaged form. For this model, the diffusion tensor in the presence of a homogeneous flow field is obtained from (i) the mass flux caused by concentration gradients, (ii) the average polymer velocity caused by external forces, and (iii) the mean‐square displacement of a polymer caused by the Brownian forces. From the second and third approaches the same expression for the diffusion tensor is found, whereas a different expression is obtained from the first approach. This means that the Nernst‐Einstein equation cannot be generalized to the case of flowing solutions. The model predictions for the diffusion tensors in steady shear flow are discussed in detail.
Capillary pressure: Centrifuge method revisited
Authors: Ayappa K. G. , Davis H. T. , Davis E. A. , Gordon J. 
Published: June 2004

AbstractThe approximate equations of Hassler and Brunner and of van Domselaar are often used to deduce the capillary pressure curve of a porous medium from centrifuge data. The use of these equations restricts the centrifuge method to short samples. Also, these equations require differentiation of data. We report here methods to determine the capillary pressure curve by the midpoint and least‐squares solution of the fundamental equation, relating the average saturation S1 of liquid in the porous medium to the capillary pressure Pc1 at the end of the sample nearest the axis of rotation. The methods do not require differentiation of data and are not restricted to short samples. We introduce and evaluate an approximation based on an exact result derived by Rajan. This new approximation requires the same inputs as do the Hassler‐Brunner and the van Domselaar approximations, but it is accurate over a wider range of sample sizes. In addition, an approximate solution which can be used to estimate capillary pressure curves for long cores is developed.
Catalyst regeneration kinetics in the presence of pore blockage
Authors: Chang Yuli , Perlmutter D. D. 
Published: June 2004

AbstractA model is developed for fluid‐solid reaction kinetics in a porous solid. The model is based on a convergent‐divergent pore structure and accounts for pore blockage and inside cavities. A numerical study applied the model to the process of coke removal for catalyst regeneration. The calculated results show that in the presence of pore occlusion, the reaction rates are significantly reduced at the beginning of reaction. At higher levels of coke loading the reaction behavior can be especially affected by the volume fraction of the divergent pores. This behavior can be used to select catalysts with pore structures that improve control of the regeneration process.
Fluid mixing with a pipeline tee: Theory and experiment
Authors: Sroka L. M. , Forney L. J. 
Published: June 2004

AbstractThe pipeline mixing of two fluid streams by turbulent jet injection normal to the pipeline has been studied theoretically and experimentally. A simple scaling law for the second moment of the tracer concentration within the pipeline is proposed for the first fifteen pipe diameters down‐stream from the injection point. The similarity solution is derived by assuming that the tracer diffuses in a weak compound jet moving parallel to the pipeline axis. The theoretical results are correlated with all of the available experimental measurements. The results indicate that the second moment of the tracer concentration decreases with increasing jet momentum and distance from the injection point such that M = 0.25. (lm/D)−2 (x/D)−4/3.
Determination of rate constants by the frequency response method: CO on Pt/SiO2
Authors: Li Yao‐En , Willcox David , Gonzalez Richard D. 
Published: June 2004

AbstractA technique based upon transmission FTIR to obtain rate constants for adsorption and desorption over supported metal catalysts has been developed and tested. The technique requires the use of a sinusoidal perturbation function imposed on steady state linearized adsorption‐desorption kinetics. The measurement of a phase lag between the sinusoidal inlet gas phase forcing concentration and the response of surface coverage, together with a measurement of the maximum amplitudes of the forcing function and surface response enable the calculation of the relevant adsorption and desorption rate constants.The technique has been successfully applied to the measurement of both adsorption and desorption rate constants for CO adsorbed on a 1% Pt/SiO2 catalyst. The values obtained for these rate constants at 343 K were: Ka = 0.147 s−1 and Kd = 7.28 × 10−3 s−1. A sticking coefficient corresponding to the adsorption of weakly bonded CO on Pt under conditions of high CO surface coverage was obtained.
Analysis of decentralized control structures for nonlinear systems
Authors: Manousiouthakis V. , Nikolaou M. 
Published: June 2004

AbstractConfiguration of feedback loops between input and output variables of a given plant constitutes a subtask of control system synthesis. In the past, this subtask has been tackled through direct decomposition of either the process or the set of input‐output variables. Linear interaction analysis has been used in both approaches to assess and minimize sub‐system interactions. Since most chemical processes exhibit nonlinear behavior, it is evident that a measure is needed for assessment of interactions in the presence of system nonlinearities. In this paper we focus on input‐output variable set decomposition and introduce the notion of nonlinear block relative gain (NBRG) as a nonlinear interaction measure. Both the statistic and dynamic versions of NBRG are discussed, and a computational procedure is presented for their evaluation. Direct simulations on a CSTR verify the interactions predicted by NBRG for different feedback configurations. Moreover, nonsymmetry of the effect of one loop on another, a fact not captured by the linear BRG, is accurately predicted by NBRG.
Phase transition and structural heterogeneity; Benzene adsorption on silicalite
Authors: Guo Chang‐Jie , Talu Orhan , Hayhurst David T. 
Published: June 2004

AbstractAdsorption equilibria have been gravimetrically measured at 10, 20, 30, 50, and 70°C for benzene on pure crystalline silicalite. Isotherms change from type I to type IV with decreasing temperature, indicating heterogeneous behavior. The transition temperature is determined to be 38 ± 1°C by isobaric experiments. Silicalite is energetically homogeneous since there are no cations present. The observed structural heterogeneity is due to the comparable sizes of benzene molecules and the silicalite pore system. Silicalite is modeled as two types of homogeneous pores where a van der Waals model describes local equilibrium. Model parameters are calculated via nonlinear regressions with prescribed pore distribution determined from crystallographic considerations. Data and model predictions are in good agreement with highly significant parameters.
Pore structure and particle size effects on limestone capacity for SO2 removal
Authors: Zarkanitis Solon , Sotirchos Stratis V. 
Published: June 2004

AbstractA detailed investigation of the dependence of the transient behavior of limestone particles reacting with SO2 in the presence of oxygen on the pore structure properties of the calcined solid is presented. Experimental reactivity and pore structure data were obtained for two highpurity limestones that yield calcines of qualitatively similar distribution of pore size but of different capacity for SO2 removal. Experiments were carried out over a wide range of particle size and temperature so as to investigate the effects of intraparticle mass transport limitations on the process. The results obtained showed strong dependence of the transient behavior of reacting particles of calcined limestone, and consequently of their sorptive capacity for SO2 removal, on their initial pore size distribution and on the resistance for intraparticle mass transport—which is also determined to a certain extent by the pore size distribution. The experimental results were found to be in agreement with the predictions of a mathematical model for gas‐solid reactions with solid product involving porous solids of distributed pore size. The findings point to the conclusion that the evaluation of limestones proposed for use as sorbents for controlling SO2 emissions should be based on the size distribution and interconnectedness of the pores of their calcines and not only on their average structural properties (porosity and surface area).
Feedforward and feedback control of a solution copolymerization reactor
Authors: Congalidis John P. , Richards John R. , Ray W. Harmon 
Published: June 2004

AbstractA control system is designed for a copolymerization reactor using a combination of feedforward, ratio, and feedback control to regulate polymer production rate, copolymer composition, molecular weight, and reactor temperature. The solution copolymerization of methyl methacrylate and vinyl acetate in a continuous stirred tank reactor is used as an illustrative example with the kinetic parameters and reactor operating conditions obtained from the literature. The process includes equipment to recycle unreacted monomers and solvent. The recycle stream introduces disturbances to the reactor feed, which perturb the polymer properties. A feedforward control strategy is proposed to counter these disturbances, and its effectiveness is demonstrated using the model. The mathematical model is used to investigate input/output control pairings in order to identify the fundamental nature of the solution copolymerization control problem and to determine the best control system structure. The combined feedforward/feedback strategy is tested on a nonlinear model of the process for set point changes and compensation of unmeasured reactor disturbances. The performance of the control system design was quite good, and such designs have been found successful in plant operations.
Porosity distributions in a fluidized bed with an immersed obstacle
Authors: Bouillard J. X. , Lyczkowski R. W. , Gidaspow D. 
Published: June 2004

AbstractErosion in bubbling fluidized‐bed combustors is a serious issue that may affect their reliability and economics. Available evidence suggests that the key to understanding this erosion is detailed knowledge of the coupled and complex phenomena of solids circulation and bubble motion. A thin transparent “two‐dimensional” rectangular fluidized bed with an obstacle served as a rough model for a fluidized‐bed combustor. This model was studied experimentally and computationally using two hydrodynamic equation sets. The computed hydrodynamic results agree reasonably well with experimental data. Bubble frequencies and sizes compare well with those obtained from analyzing a high‐speed motion picture frame‐by‐frame. Time‐averaged porosities computed from both models agree with time‐averaged porosity distributions measured with a gamma‐ray densitometer. The principal diferences between the data and the computations in this paper are due to asymmetries present in the experiment and to the simplified solids rheology used in the hydrodynamic models.
Desorption processes: Supercritical fluid regeneration of activated carbon
Authors: Recasens F. , McCoy B. J. , Smith J. M. 
Published: June 2004

AbstractModels are developed for isothermal desorption in a fixed bed of adsorbent particles. While more general models require numerical solution of the governing equations, two practically important cases have analytical solutions: (1) an equilibrium desorption model where the rate of desorption is controlled by external and intraparticle mass transfer; and (2) a kinetic model where external and intraparticle mass transfer and a first‐order irreversible desorption step control the overall process.Recent experimental data (Tan and Liou, 1988) for the desorption of ethyl acetate from activated carbon with supercritical carbon dioxide are found to fit the local‐equilibrium model. Data for the effect of temperature on fractions desorbed indicate a reversal of the temperature dependence of the adsorption isotherm as the supercritical pressure is increased. This phenomenon is analogous to the observed cross‐over region for the effect of temperature on the solubility of a solid at supercritical conditions.
Plasma deposition of silicon nitride films in a radial‐flow reactor
Authors: Yoo Chue‐San , Dixon Anthony G. 
Published: June 2004

AbstractExperimental data on silicon nitride film growth have been obtained from a radial‐flow, plasma‐enhanced chemical vapor deposition reactor. A nonisothermal mathematical model has been developed consisting of Navier‐Stokes, energy, and species balance equations taking into account the temperature dependence of physical properties. The model is predictive at various power input levels for film compositions and growth rates.