bluuebunny/crossref_metadata_2025 huggingface dataset:
Unsteady heat and mass transfer from a spheroid
Authors: Feng Zhi‐Gang , Michaelides Efstathios E.
Published: June 2004

The unsteady heat‐transfer processes from oblate or prolate spheroids, at the limit of very small Peclet numbers is examined. A perturbation technique for the temperature and the geometry of the particle is used to obtain the rates of heat and mass transfer, first in the Laplace and then in the time domain. A solution to the problem is obtained, including the ϵ2 contribution (ϵ is the eccentricity). The solution reveals the existence of several history terms, which are analogous to the history terms of the creeping flow equation of motion. One of these terms is solely due to the eccentricity of the spheroid. This is an indication that the shape of the particle is a factor of the existence and from of history terms. In addition, an exact expression for the steady‐state heat transfer from a spheroid is obtained using a convenient transformation of the heat‐transfer integral.
Resonant microwave power absorption in slabs and cylinders
Authors: Ayappa K. G. , Davis H. T. , Barringer S. A. , Davis E. A.
Published: June 2004

Resonant power absorption is an important phenomenon during microwave heating. The resonances that occur when plane electromagnetic waves are incident on infinitely long cylinders and slabs are investigated as a function of sample dimensions. For cylinders two kinds of incident waves are studied: TMz when the electric field is oriented along the axis of the cylinder and TEz when the magnetic field is oriented along the axis. At a resonant condition the overage power absorbed by the sample is a local maximum. Due to attenuation within the sample the resonances decrease in intensity as the sample size increases. Using the dielectric properties of water, resonances are found to be a function of the ratio of the sample dimension to the wavelength of radiation, λs, in the sample. For slabs of thickness L and integer values of n, resonances occurred at L/λs = 0.5n; for cylinders of diameter D, resonances occurred at D/λs = 0.5n – 0.257. The generality of these relations to predict resonances in other food samples are shown using existing dielectric data. Resonances in cylinders for both polarizations occurred at similar radii. However, the radius at which the first resonance occurred for the TMz case was absent in the TEz case.
Transient behavior of pulsed particulate fluidized beds
Authors: van der Wielen L. A. M. , Fa A. W. K. G. Sjauw Koen , Potters J. J. M. , Luyben K. Ch. A. M.
Published: June 2004

Transient phenomena in solid–liquid fluidized‐bed systems are important in designing pulsed, countercurrent (multistage) fluidized‐bed contactors of the Cloete‐Streat type at high‐solids flow rate. Of particular interest are the residence times or corresponding velocities of porosity gradients in the bed and the excess or overshoot height of the bed after refluidization. Theory assuming local equilibrium between holdup and velocity of the phases (local‐equilibrium model) for stepwise perturbations in the liquid flow is readily available. It is investigated whether the local‐equilibrium theory can be used for more complex perturbations and whether inertia effects, such as are encountered in countercurrent multistage fluidized‐bed systems, can be ignored. Therefore, the detailed particle‐bed model of Foscolo and Gibilaro, which incorporates inertia effects, was applied to investigate the transient behavior of fluidized‐bed systems. Transient fluidization experiments were performed with a broad range of water‐fluidized particles in a laboratory‐scale multistage fluidized‐bed contactor. The operating conditions corresponded to those for countercurrent contact.

Numerical simulations with the particle‐bed model predict satisfactory experimental results. The “overshoot” heights of the fluidized bed were estimated correctly by the particle‐bed model, whereas the local‐equilibrium model only provides a conservative estimate. However, the local‐equilibrium model allows an analytical solution that is more interesting for design, as it avoids tedious calculations. The residence time of the last perturbation before the fluidized bed relaxes to steady state was estimated with similar accuracy by both models.
Identification of full profile disturbance models for sheet forming processes
Authors: Rigopoulos Apostolos , Arkun Yaman , Kayihan Ferhan
Published: June 2004

In this article we present a method for the on‐line identification and modeling of full profile disturbance models for sheet forming processes. A particular principal components analysis technique called the Karhunen‐Loève expansion is used to adaptively identify the significant features of the profile. In addition, we show how the temporal modes of the reconstructed profile can be modeled using low‐order linear autoregressive (AR) processes. By simulation examples, the effect of the order of the AR model is studied, as well as the window size of the data used in the on‐line application of the KL expansion, the effect of data weighting, the importance of the correct selection of the number of modes, and the frequency of updating the parameters of the AR models. Identified disturbance models can be easily incorporated into model‐predictive control algorithms.
Effects of particle‐phase turbulence in gas‐solid flows
Authors: Hrenya Christine M. , Sinclair Jennifer L.
Published: June 2004

Numerous experimental investigations on the vertical transport of dense gas–solid suspensions indicate that particles tend to segregate toward the tube wall. Although models based on the kinetic theory analogy can predict such patterns for perfectly elastic particle‐particle collisions, the predictive ability of these models breaks down for inelastic collisions. In the present effort, a mathematical model is developed that incorporates two mechanisms that give rise to the lateral segregation of solids: interactions associated with individual particles based on a kinetic theory treatment and interactions associated with collections of particles based on an analogy with single‐phase turbulent flows. Although these two mechanisms have been treated independently by previous workers, their combined contributions to the overall flow behavior have not been thoroughly investigated. The effect of such a treatment on the sensitivity of the model predictions to the inelasticity of particle–particle collisions is explored. A key element in eliminating the undue sensitivity appears to be a consideration of the effects associated with the collective motion of particles on the kinetic theory expressions. The resulting model can predict the expected segregation patterns for systems characterized by inelastic collisions, as well as many of the other salient features of vertical gas–solid flows.
Simulated moving bed under linear conditions: Experimental vs. calculated results
Authors: Yun Tong , Zhong Guoming , Guiochon Georges
Published: June 2004

A simulated moving bed (SMB) was operated for the separation of mixtures of 2‐phenyl ethanol and 3‐phenyl‐1‐propanol on columns packed with Zorbax C18 bonded silica, using a 60:40 (v/v) solution of methanol and water as the mobile phase. Series of four or eight columns were used. The experiments were carried out with low concentration mixtures, that is, under linear conditions. Band profiles of both compounds eluted from one of the columns during successive periods after steady state had been reached were recorded, as were the concentration histories at the extract and raffinate ports. These experimental results are compared to those predicted by two models: the linear ideal and the linear equilibrium‐dispersive models of chromatography, applied to the SMB separator. These two models give excellent agreement between the experimental profiles and those calculated with the model. As expected, the profiles predicted by the ideal and the equilibrium‐dispersive models differ only by the lack of dispersion in the profiles given by the former. The latter model is demonstrated to be a solid, reliable tool for further studies of the SMB design and optimization.
Pulsed electrophoretic filter‐cake release in dead‐end membrane processes
Authors: Bowen W. Richard , Ahmad A. Latif
Published: June 2004

The application of electric fields can be a very effective means of removing deposits from membrane surfaces. Such a means of process improvement has usually been applied to cross‐flow filtration, allowing efficient operation at low cross‐flow velocities. In the limiting case of dead‐end filtration (zero crossflow) it is possible to use electric‐field pulses to release the filter cake for collection. Experimental data are presented for the dead‐end ultrafiltration of silica colloids and the protein bovine serum albumin and for the dead‐end microfiltration of titania dispersions taking as variables the magnitude of the applied field, the pulse interval, the pulse duration, and the feed conditions (pH, ionic strength, concentration). The data identify the conditions when pulsed electric fields can be used as an efficient means of releasing filter cakes for collection. A force balance model is developed to predict the filtration rate at the end of the release process, taking into account electrophoretic, electroosmotic, and hydrodynamic forces. The model shows excellent agreement with the experimental data for ultrafiltration and reasonable agreement with the experimental data for microfiltration. The use of pulsed electric fields to release filter cakes in dead‐end membrane processes is a promising technique that is most likely to find application in the clarification of process feeds containing low dispersed solutes.
Inference in dynamic error‐in‐variable‐measurement problems
Authors: Albuquerque JoãO S. , Biegler Lorenz T. , Kass Robert E.
Published: June 2004

Efficient algorithms were developed for estimating model parameters from measured data, even in the presence of gross errors. In addition to point estimates of parameters, however, assessments of uncertainty are needed. Linear approximations provide standard errors, but they can be misleading when applied to models that are substantially nonlinear. To overcome this difficulty, profiling methods were developed for the case in which the regressor variables are error free. These methods provide accurate nonlinear confidence regions, but become expensive for a large number of parameters. These profiling methods are modified to error‐in‐variable‐measurement models with many incidental parameters. Laplace's method is used to integrate out the incidental parameters associated with the measurement errors, and then profiling methods are applied to obtain approximate confidence contours for the parameters. This approach is computationally efficient, requires few function evaluations, and can be applied to large‐scale problems. It is useful when certain measurement errors (such as input variables) are relatively small, but not so small that they can be ignored.
Integrated flexibility and controllability analysis in design of chemical processes
Authors: Bahri Parisa A. , Bandoni Jose A. , Romagnoli Jose A.
Published: June 2004

A systematic approach for the synthesis of flexible and controllable plants is presented. It provides a new quantitative measure of the flexibility and controllability of a design and allows the designer to systematically evaluate different process structures and/or control systems. An integrated approach based on the dynamic mixed‐integer nonlinear‐programming problem is introduced that consists of two stages in each iteration of the algorithm. In this way, the effect of disturbances on the process design and operation, as well as its ideal performance, under a variety of control schemes can be estimated. The method is illustrated using a mini‐integrated plant as a case study.
LP methods in MPC of large‐scale systems: Application to paper‐machine CD control
Authors: Dave Prashant , Willing Dennis A. , Kudva Gautham K. , Pekny Joseph F. , Doyle Francis J.
Published: June 2004

An application of a linear‐programming based model‐predictive control strategy to the paper‐machine cross‐direction (CD) control problem is presented. The objective of CD control is to maintain flat profiles of variables of interest by minimizing worst‐case deviations from setpoints (defects). These control problems can have as many as 200 actuatiors (inputs) and 400 sensor measurements (outputs). This large size coupled with the stringent real‐time requiremnt of computing a control move in a few seconds poses a very challenging control problem. The LP‐based strategy is particularly well suited for solving such classes of control problems. This strategy has demonstrated its ability to solve large‐scale control problems (over 100 inputs and 100 outputs) in real time and exhibits robustness to model uncertainty.
Design procedure for stable operations of first‐order reaction systems in a CSTR
Authors: Heiszwolf Johan J. , Fortuin Jan M. H.
Published: June 2004

An application‐oriented design procedure is presented for unique and point‐stable operations of first‐order reaction systems in a continuous stirred‐tank reactor (CSTR). For a given set of values of kinetic constants, reaction enthalpy, feed conditions, residence time, and relevant physical properties, two boundary values of the heat‐transfer capacity (St1, St3) and two of the modified coolant temperature (θmc, 2, θmc, 3) are analytically derived after a linearization of the unsteady mass and energy balances. With these boundary values, two separate design conditions are formulated; one for the heat‐transfer capacity (HTC, characterized by St) and one for the modified coolant temperature (MCT, characterized by θmc). Each of these conditions is sufficient to guarantee unique and point‐stable steady‐state operations for a range of St or θmc values. Predicted behaviors of reacting systems are compared with experimental results obtained from five different systems reacting in four bench‐scale and two commercial reactors.
Estimation of nonlinear systems using linear multiple models
Authors: Banerjee A. , Arkun Y. , Ogunnaike B. , Pearson R.
Published: June 2004

This article addresses the problem of estimating the states of a nonlinear plant that operates in multiple regimes and makes transitions between them. It is often difficult to obtain a single nonlinear model that accurately describes the plant in all regimes. Even if a global model is available, it sometimes cannot be used conveniently in an estimator. An alternative approach is presented where local linear models are identified at each different operating point, and estimation is performed by tracking the transitions from one regime to another. This is done by first estimating the validity of the local models on‐line and then constructing a time‐varying global model by interpolating between the local linear models. State and parameter estimation is then performed using this global model in a moving horizon estimator. To demonstrate the effectiveness of this method, it is applied to three test systems: a simple numerical example, a CSTR, and a copolymerization reactor.
Optimal molecular design under property prediction uncertainty
Authors: Maranas Costas D.
Published: June 2004

A systematic method that quantitatively assesses property prediction uncertainty (imprecision) on optimal molecular design problems is introduced. Propety–structure relations are described with specific nonlinear functionalities based on group contribution methods. Property prediction uncertainty is explicitly quantified by using multivariate probability density distributions to model the likelihood of different realizations of the group contribution parameters. Assuming stability of these probability distributions, a novel approach is introduced for transforming the original nonlinear stochastic formulation into a deterministic MINLP problem with linear binary and convex continuour parts with separability. The resulting convex MINLP formulation is solved to global optimality for molecular design problems involving many uncertain group contribution parameters. Results indicate the computtional tractability of the method and the profound effect that property prediction uncertainty may have in optimal molecular design. Specifically, trade‐off curves between performance objectives, probabilities of meeting the objectives, and chances of satisfying design specifications offer a concise and systematic way to guide optimal molecular design in the face of property prediction uncertainty.
Modeling of imperfect mixing and its effects on polymer properties
Authors: Zhang Simon X. , Ray W. Harmon
Published: June 2004

A compartmental model is used to study imperfect mixing and its effects on polymer properties. Examples of imperfect feed mixing caused by a rapidly decomposing initiator are shown for styrene polymerization, high‐pressure ethylene polymerization, and vinyl acetate/methyl methacrylate solution copolymerization. Continuation analysis indicates mixing can change reactor steady states and stability. A composite approach is proposed to construct full molecular weight distribution (MWD) under imperfect mixing. In linear polymerization imperfect mixing broadens MWD; however, in nonlinear polymerization mixing can cause either broader or narrower MWD. By changing the dominant chaintermination mechanism to chain transfer, the influence of imperfect mixing on the MWD can be reduced. Depending on reactivity ratio and monomer composition, mixing also affects copolymer composition and chain sequence length.
Mechanisms of initiation of incrustation
Authors: Vendel Martin , Rasmuson Åke C.
Published: June 2004

Initiaton of incrustation by crystallization mechanisms is studied. An experimental technique was developed by which nucleation of a crystalline phase on a solid surface is investigated at controlled local supersaturation without the interference of particle deposition. Experiments were carried out with different substances and surface materials. The results show that the solid surface may catalyze primary nucleation and hence promote the initiation of incrustation, but this was recorded only at quite high supersaturation. Collision of crystals with the solid surface in a supersaturated solution may initiate incrustation at a reasonably low supersaturation and can be of substantial industrial importance.
Parameterization of pressure‐ and temperature‐dependent kinetics in multiple well reactions
Authors: Venkatesh Prasana K. , Chang Albert Y. , Dean Anthony M. , Cohen Morrel H. , Carr Robert W.
Published: June 2004

It is now well confirmed that the influence of temperature on the fall‐off behavior of dissociation, recombination and chemically‐activated reactions can be dramatic. For single‐well, single‐product dissociation reactions, it is customary to approximate these fall‐off surfaces using extensions of Lindemann's empirical expression. We consider here chemical‐activation and dissociation reactions possessing multiple wells and multiple products. We show that direct approximation of the rate coefficients via Chebyshev expansions yields reliable and accurate representations of their pressure and temperature dependences, which are superior to those from a Lidemann approach to fit the form factor representing the fall‐off surface. The superiority of the method is demonstrated in a study of seven channels corresponding to four different reactions important in combustion chemistry over the ranges 300–3,000 K and 0.02–200 atm.
Crystallization process optimization via a revised machine learning methodology
Authors: Sheikh Ahmad Y. , Jones Alan G.
Published: June 2004

A revised machine learing methodology is applied to a simulated crystallization process flowsheet for continual imporvement of its performance by generating and analyzing process data. The aim is to identify bands of crucial decision variables leading to zones of best average process performance. The methodology comprises two components: symbolic induction and case‐based reasoning. It uses an incremental algorithm to update performance classification rules, which not only improves the efficiency of the symbolic induction of the classification rules by eliminating the need for their periodic reinduction, but also simplifies the case‐based reasoning step. The new concepts and procedures are illustrated by application to a potassium nitrate crystallization process comprising a mixed‐suspension, mixed‐product‐removal crystallizer, a hydrocyclone, and a fines dissolver, By identifying and establishing ranges for the most crucial decision variables, namely, feed concentration, flow rate, and cooling stream temperature, three zones leading to an improvement of nearly 12% on nominal average performance are detected within two generations of the classification rules.
Prototype for intergated hazard analysis
Authors: Kuo Dong‐Hsiung , Hsu Ding‐Shang , Chang Chuei‐Tin , Chen Dong‐Han
Published: June 2004

The prototype of an integrated hazard‐analysis system (IHAS) was developed. Essentially any process can be analyzed with this software if the system topology is correctly supplied by the user. Since all altorithms adopted are digraph‐based, the system digraph must be constructed first with IHAS. The embedded feedforward and feedback loops are then identified and classified. On the basis of this information, three widely accepted hazard‐assessment procedures—FTA, ETA, and HAZOP—can be performed automatically. From the results obtained in practical applications, one can see that the quality. From the results obtained in practical applications, one can see that the quality of hazard analysis is indeed improved if IHAS can be used as an aid to the human experts.
Characterization of hydrocarbon systems by DBE concept
Authors: Korsten Hans
Published: June 2004

Thermochemical data of mineral oil fractions are scarce. Due to the complex structure of natural hydrocarbon mixture, standard heat and Gibbs free energy of formation depend on the sample. A procedure for predicting these properties is presented. Both properties depend on the hydrocarbon type and on its molecular weight. The double‐bond equivalent (DBE) and the number of olefinic double bounds are the important characterization parameters of the model. DBE can basically be calculated from molecular weight and elemental composition. Alternatively, boiling point and atomic H/C ratio can be used if the model is applied to mineral oil fractions from which boiling curves can be obtained. A criterion for testing the reliability of the estimated DBE value is available, because DBE is also a function of structural group distribution.

The relevance of the DBE concept is shown with regard to hydroprocessing of mineral oil fractions. The model can be applied to total average molecules as well as to an arbitrary number of pseudocomponents and continuous mixtures. The difference between feed and product DBE is equivalent to the amount of hydrogen consumed for hydrogenation of unsaturated hydrocarbons. Further properties of the product can be predicted.
Numerical analysis and data comparison of a supercritical water oxidation reactor
Authors: Oh Chang H. , Kochan Robert J. , Beller John M.
Published: June 2004

Supercritical water oxidation of isopropyl alcohol was investigated in a pilot‐scale reactor. A computational fluid‐dynamics model developed reveals the detailed flow field, chemical‐component distribution, temperature distribution, and salt‐particle trajectories in the reactor flow domain. The near‐wall fluid temperature from the numerical analysis was compared with experimental temperature data. The temperature comparison was within a 3% error band. The effect of the chemical kinetic rate was investigated for four different rates. Turbulent salt‐particle trajectories were also calculated to investigate the effect of particle sizes on salt deposit on the wall. Also, a method of calculating the adiabatic reaction temperature was developed to estimate reaction temperatures prior to a full numerical simulation.
Application of neural networks to mass‐transfer predictions in a fast fluidized bed of fine solids
Authors: Zamankhan Piroz , Malinen Pekka , Lepomäki Hannu
Published: June 2004

In this study back‐propagation, feed‐forward neural networks are applied to estimate mass‐transfer parameters in fast fluidized beds of fine solids. These networks are trained to predict mass‐transfer rates using measurements of the sublimation rate of coarse naphthalene balls in fast fluidized beds of fine glass beads at several solid‐to‐gas mass flow rates within the relevant superficial gas‐velocity range. When tested to predict the effective diffusivities from a coarse particle to the bulk of the fast bed of fine solids, trained neural networks calculated the Sherwood number with high accuracy. It is demonstrated that back‐propagation, feed‐forward neural networks provide a more accurate correlation for the mass‐transfer coefficient compared to those obtained by the currently used heuristic models.
Electrochemically modulated complexation process for ethylene/ethane separation
Authors: Terry Patricia A. , Noble Richard D. , Swanson Daniel , Koval Carl A.
Published: June 2004

An electrochemically modulated complexation (EMC) process was designed to selectively extract and concentrate ethylene from a feed‐phase mixture with ethane. Unlike previous EMC processes, where the separation was based on equilibrium solubility differences, this separation was based on differences in the rate of complexing with copper(I) solution between the two gases. The EMC process employed flowthrough electrolysis cells that electrochemically modulate aqueous‐phase copper chlorides between high, copper(I), and low, copper(II), affinity states. The liquid phase in the high affinity form contracts a feed gas phase in a hollow‐fiber membrane module for selective extraction and contacts the receiving gas phase in the low affinity form for concentration. Experimental results and modeling of the process are reported.
In‐situ CIR‐FTIR study of the diffusion of supercritical hydrocarbons in zeolite L
Authors: Süer Murat G. , Dardas Zissis , Lu Yaping , Moser William R. , Ma Yi H.
Published: June 2004

Cylindrical internal reflectance infrared spectroscopy (CIR‐FTIR) was used for the determination of the transport properties of hydrocarbons in zeolite L at high temperatures and pressures. This was the first time that the intracrystalline diffusivity of hydrocarbons in microporous media could be determined under supercritical conditions by using a dynamic technique. The study investigated the self‐diffusion of n‐heptane, the counterdiffusion of n‐hexane and 1‐hexene, and the codiffusion of n‐heptane and pyxlene in zeolite L under subcritical and supercritical conditions. The diffusion time constants obtained for the single and binary systems showed that the diffusion within the pore was reduced significantly under supercritical conditions (i.e., liquidlike behavior) as the hydrocarbon density within the pore was increased.
Polymerization reactor control using autoregressive‐plus Volterra‐based MPC
Authors: Maner Bryon R. , Doyle Francis J.
Published: June 2004

A nonlinear model‐predictive control scheme based on the autoregressive‐plus Volterra model is proposed. A “plant‐friendly,” yet persistently exciting, input sequence is designed for model identification. A second feature of this work is that the control action is determined by solving a less computationally burdensome nonlinear programming problem than the optimization problems associated with Newton‐type controllers and polynomial ARMA model nonlinear MPC. A third contribution is that semiglobal closed‐loop stability conditions are derived and are shown to be less conservative than those previously published. The control scheme is shown to outperform a feedback strategy using proportional integral control and a linear model‐predictive control design for the control of two polymerization reactor case studies. The complex features of the latter case study [multivarible (3 × 3), recycle loop, high‐order (24) dynamics] motivate the applicability of this approach for industrial problems.
Characterization and permeation properties of ZSM‐5 tubular membranes
Authors: Coronas Joaquín , Falconer John L. , Noble Richard D.
Published: June 2004

ZSM‐5 zeolite membranes with reproducible properties were prepared by in‐situ synthesis on porous α‐ and γ‐alumina tubular supports and characterized by XRD, SEM and electron microprobe analysis. Single‐gas permeances for H2, CH4, N2, CO2, n‐butane, and i‐butane increase over some temperature range, but some gases exhibit maxima or minima. The highest ideal selectivities at room temperature are 299 for N2/SF6, 392 for H2/n‐butane, and 2,820 for H2/i‐butane. These membranes can separate n‐butane/i‐butane, H2/n‐butane and H2/i‐butane mixtures. All n‐butane/i‐butane separation selectivities have maxima as a function of temperature and are higher than ideal selectivities because n‐butane inhibits i‐butane permeation. Thus, separation is not by size selectivity, but is due to pore blocking. Temperature depenencies of single‐gas permeances and separation selectivities depend strongly on the location of zeolite crystals and the location is determined by preparation procedure. Ideal selectivities also depend strongly on the preparation procedure. When the zeolite forms a continuous layer on the inside surface of the support tubes, pure i‐butane permeates faster than pure n‐butane so that the single‐gas permeances are not determined just by molecular size. The i‐butane permeance also increases much more with temperature than the n‐butane permeance. The permeation behavior may be the result of permeation through nonzeolitic pores in parallel with zeolite pores. When zeolite crystals are dispersed throughout the pores of α‐alumina supports, permeances are lower and gas permeation and separation properties are quite different. Ideal selectivities are lower, pure n‐butane permeates faster than i‐butane, and the permeances increase much less with temperature. Separation selectivities are lower but can be maintained to higher temperatures.
Mechanism reduction during computer generation of compact reaction models
Authors: Klinke David J. , Broadbelt Linda J.
Published: June 2004

One of the challenges in building reaction mechanisms using algorithms for automated model construction is to describe the essential chemistry and enable prediction of experimental data over wide ranges of reaction conditions while maintaining a manageable model size. Two complementary methodologies for building compact reaction mechanisms were developed and combined with existing algorithms based on graph theory and bond–electron matrix operations. Each strategy was developed using pentadecylbenzene pyrolysis as an illustrative example. The first approach used a radical lumping strategy to group radicals according to their reactivity. The mechanism was reduced from 719 to 215 species and successfully predicted the experimentally observed initial reactivity. However, implementation of the radical lumping criteria alone was insufficient to allow for secondary reactions to higher‐rank products. Therefore, on‐the‐fly sensitivity analysis was incorporated to identify the important and necessary species as the mechanism was generated to guide the mechanism building process. The generic algorithms developed can be applied to generate compact reaction mechanisms for a wide array of higher molecular‐weight reactants.
Clathrate phase equilibria for the water–phenol–carbon dioxide system
Authors: Yoon Ji‐Ho , Lee Huen
Published: June 2004

Clathrate phase equilibria for the ternary water–phenol–carbon dioxide system containing one‐guest and two‐host components were studied. Three four‐phase equilibrium loci for this ternary system were determined over wide temperature and pressure ranges. A quintuple point, thermodynamically unique, and invariant condition appeared in this type of ternary system, was carefully measured, and were 293.7 K and 57.2 bar. At this quintuple point, five individual phases of water‐rich liquid, phenol‐rich liquid, carbon dioxide‐rich liquid, phenol clathrate, and vapor could coexist in equilibrium. A new experimental technique was used to determine the liquid‐phase compositions coexisting with phenol clathrates at the isobaric conditions of 30.0 bar and six temperatures ranging from 278.2 and 303.2 K. A potential applicability of the clathration process to phenol separation from aqueous solutions was demonsrated in terms of phenol concentrations based on carbon dioxide‐free concentration. In particular, phenol separation by using high‐pressure carbon dioxide as a clarthrate‐inducing agent could be applied even to multiphase aqueous solutions over an entire range of phenol concentrations.
Control of semicontinuous emulsion copolymerization reactors
Authors: Saldívar Enrique , Ray W. Harmon
Published: June 2004

The problem of simultaneous control of composition and molecular weight of copolymers produced in semicontinuous emulsion polymerization is addressed. A first‐principles model of emulsion copolymerization is used to produce optimal open‐loop policies, which are then experimentally tested on the system methyl methacrylate/vinyl acetate. In contrast to earlier work in the control of semicontinuous emulsion copolymerization, the control problem addressed has multiple inputs and multiple outputs, and the experimental application is free of empirical correlations and performed on an unseeded system. Optimal open‐loop policies are calculated efficiently by optimizing properties of the instantaneous polymer being formed. The experimental results confirm the feasibility of these techniques. A model‐predictive control scheme, in which optimal policies are recomputed on‐line to account for disturbances, is proposed. The scheme, which exploits the advantages of the efficient calculation of optimal policies, is tested by simulation showing good performance.
Effect of catalyst distribution in a membrane reactor: Experiments and model
Authors: Szegner John , Yeung King Lun , Varma Arvind
Published: June 2004

Inorganic membrane reactors combine reaction and separation operations in a single unit. Preferential permeation of a product species through the membrane enhances equilibrium‐limited reactions beyond the thermodynamic limit. Proper catalyst formulation and spatial distribution also enhance membrane reactor performance. To minimize reactant loss due to high gas permeation, the thickness of the γ‐alumina layer in a composite alumina membrane (Membralox) was increased to 17 μm by slip‐casting with alumina sol. Among the catalysts investigated a formulation of 1.10 wt. % Pt and 1.36 wt. % Sn supported on γ‐alumina pellets yielded the highest activity, selectivity, and stability for the dehydrogenation of ethane to ethylene. Using this catalyst composition, various nonuniform platinum distributions within the pellets were prepared by solution impregnation, while maintaining a uniform distribution of Sn. The effect of nonuniform catalyst distribution on dehydrogenation of ethane in a packed‐bed membrane reactor (PBMR) under well‐mixed, isothermal conditions was evaluated both experimentally and theoretically. Reactor performance is maximized when the catalyst is concentrated near the surface of the support. Supraequilibrium conversions, about 80% beyond equilibrium values, were obtained with the narrow surface–step catalyst pellets. Experimental results agree well with theoretical predictions, obtained without the use of any adjustable parameters. The effect of membrane thickness on reactor performance was also investigated, by comparing the 17‐μm alumina membrane with a 1.2‐mm‐thick porous Vycor glass, using pellets with the narrow surface‐step distribution. For the experimental conditions employed, relatively low permeation through the porous Vycor resulted in conversions near fixed‐bed reactor values.
Mass and thermal transport analysis of oscillating flow with absorbent polymers
Authors: Kalumuck Kenneth M. , Prabhukumar Sankar , Brown Clinton E. , Chahine Georges L.
Published: June 2004

A new concept for removing toxic gases with respirator applications based on inhalation and exhalation through absorbent polymer‐lined tubes was studied. The two‐way oscillatory flow through the respirator element enables capture and expulsion of toxic vapors, which is not feasible with activated charcoal because of the adverse effect of moist breath on the charcoal efficiency. To study the influence of various material, design and operating parameters on the performance of such a respirator, a numerical model that solves the governing equations of mass and heat transport in an oscillatory flow through an absorbent polymer‐lined tube was developed and utilized with experimentally measured property data. Factors investigated include respirator element geometry, polymer and toxic vapor characteristics, and flowthrough speed. Results show a large delay in toxic gas penetration over that obtained by conventional one‐way flow, and define the importance of partition coefficient, flow passage size, and length and velocity. Results are compared with data from simulated breathing experiments.
Polyelectrolyte precipitate formation during miscible displacement in porous media
Authors: De Debnath , Hrymak Andrew N. , Pelton Robert
Published: June 2004

The miscible displacement of aqueous lignin solutions (model black liquor) by water from beds formed from glass beads was studied as a function of the presence of cationic polymer in the wash water, bed structure, lignin concentration, and superficial flow rate. The displacement behaviors in homogeneous beds formed from fine beads (121 μm) were compared with results of experiments using a channel bed consisting of a bed formed from fine beads surrounding a channel of coarse beads (638 μm) running the length of the bed in the flow direction. Washing efficiencies, defined as the fraction of lignin removed when one pore volume of eluate was collected, were 84–93% for homogeneous beds washed with water compared with 31–33% for the channel bed; the range in efficiencies reflects the influence of other variables. The presence of cationic polymer in the wash water enhanced the washing efficiency by 1.7 to 2 times with a corresponding 20 to 35% decrease in the permeability of the central channel in the channel bed. The improved washing with polymer was due to selective plugging of the central channel with precipitate formed from complex formation between anionic lignin in the black liquor and the cationic polymer. Breakthrough curves obtained from 12 microconductivity probes located throughout the bed showed that mixing of lignin in the black liquor and the cationic polymer. Breakthrough curves obtained from 12 microconductivity probes located throughout the bed showed that mixing of lignin solution (high conductivity) with wash water (low conductivity) in the displacement front, as expressed by a mixing length, was a maximum in the coarse bead channel and was decreased when the wash water contained cationic polymer.
Analysis of adsorption selectivity in a one‐dimensional model system
Authors: Talbot Julian
Published: June 2004

The adsorption selectivity, Si, j = (Xi/yi)/(xj/yj), is examined in a 1–D model of a mixture of hard rods adsorbing on a linear substrate. The adsorbed components are characterized by their length, li, and equilibrium constant, ki. We derive an expression for the bulk‐phase pressure Po, for which there is no separation of components i and j, S,i, j = 1, in terms of these parameters. The selectivity of the smaller component always increases with the bulk pressure, which results in a selectivity reversal if, as is usually the case, the larger component is more strongly adsorbed at low surface coverages. In a binary mixture, when P < Po, the selectivity of the smaller component decreases with increasing bulk‐phase mole fraction, y1, while for P > Po, the reverse is true. The effect of varying the molecular parameters on the selectivity is also studied.
Standing wave analysis of SMB chromatography: Linear systems
Authors: Ma Z. , Wang N.‐H. L.
Published: June 2004

The concept of standing concentration waves is introduced to derive design equations for continuous moving bed (CMB) processes. For linear isotherm systems, simple equations are derived from the analysis to link product purity and recovery to zone lengths, bed movement velocity, flow rates, column capacity factors, and mass‐transfer coefficients. Once product purity, recovery and feed flow rate are specified for a given system, the zone flow rates and bed movement velocity that provide the highest throughput and the lowest solvent consumption can be determined from the solutions. In a given system, there is a trade‐off between product purity and throughput. If bed volume and product purities are fixed, the longer the zone lengths, the higher the throughput. Simulations based on a linear driving force model that considers axial dispersion and lumped film and intraparticle diffusion are used to compare the column profiles and effluent histories of CMB and simulated moving bed (SMB). A numerical algorithm is introduced to allow simulation of both CMB and SMB operations using the same program. The comparison shows that the design equations derived for CMB systems are applicable to SMB systems. Finally, the standing wave solutions are used to analyze an experimental SMB system from the literature (Ching et al., 1991). Simulations agree closely with the data and the predictions of the theoretical analysis.
Study of a six‐bed pressure swing adsorption process
Authors: Malek A. , Farooq S.
Published: June 2004

A mathematical model of a six‐bed, ten‐step PSA operation is developed. The process cycle considered resembles an industrial hydrogen recovery process from the refinery fuel gas. Three hydrocarbon impurities (methane, ethane, and propane) are considered in the feed gas. The adsorbent used is activated carbon. The nonisothenmal, bulk separation PSA model adopts the linear driving force approximation for particle uptake and the extended Langmuir isotherm to represent adsorption equilibrium. A transient pressure equation is incorporated to account for the dynamics in uariable pressure steps. The model was verified using experimental results from a computer‐controlled, laboratory‐scale PSA unit. Considering the complexity of operation, comparison of various stream flow rates, concentrations, bed pressure and temperature profiles indicates that the model provides a sufficiently accurate prediction of the PSA performance. Parametric studies further show that its product purity declines relatively quickly with increasing cycle time and decreasing high operating pressure due to the breakthrough of a relatively sharp methane front. There is no real advantage in operating the PSA unit beyond a high operating pressure of about 18.0 bar.
Gas‐filled cavity structures and local void fraction distribution in aerated stirred vessel
Authors: Bombač A. , Žun I. , Filipič B. , Žumer M.
Published: June 2004

The local structural function obtained by a microresistivity probe at different hydrodynamic regimes is examined. The structures, such as the vortex‐clinging structure, the appearance of one and two large cavities, small 3‐3 structure, large 3‐3 structure, and ragged cavities were recognized by frequency transformation of the time‐domain structural function. An optimized phase discrimination in signal processing was used. The distribution of the local void fraction (α) in a pilot‐size stirred tank was experimentally investigated, because almost no such data can be found in the literature. The two‐phase mixture was composed of air and water; α was measured at 190 nodes in the vertical half‐section plane of the vessel. Relative differences smaller than 9% between integrated values of α and measured gas holdups agreed reasonably well under all conditions.
Model for surface diffusion in liquid‐phase adsorption
Authors: Miyabe Kanji , Takeuchi Shigeya
Published: June 2004

Based on the absolute‐rate theory, a consistent interpretation was provided for the dependence of surface‐diffusion coefficient, Ds, on temperature and the amount adsorbed in various liquid‐phase adsorption systems, such as the Langmuir‐, Freundlich‐and Jossens‐type adsorption. It was demonstrated that a restricted molecular diffusion model for surface diffusion was useful for the analysis of the characteristic features of Ds. A formulation of Ds was derived based on the model and was applied to the analysis of surface‐diffusion phenomena in various adsorption systems. The temperature dependence could be interpreted by assuming surface diffusion as an activated process. By taking into account the change in both the logarithmic slope of an adsorption isotherm and an adsorption potential, the concentration dependence of Ds could be interpreted irrespective of the type of the adsorption isotherms.
On‐line nonlinear model‐based estimation and control of a polymer reactor
Authors: Mutha Rajendra K. , Cluett William R. , Penlidis Alexander
Published: June 2004

Polymer reactor control problems often lack frequent measurements of polymer properties, while other reactor environment measurements, such as temperatures, are readily available. The experimental application of a new estimator for handling such multirate measurement systems is presented. The estimator, in conjunction with a detailed nonlinear model and all measurements at their respective sampling rates, is able to provide frequent predictions of the process variables related to the quality (weight‐average molecular weight) and quantity (monomer conversion) of the polymer product. The experimental application of a multivariable, nonlinear model‐based predictive controller using reactor temperature and initiator flow rate as the manipulated variables for controlling weight‐average molecular weight and monomer conversion is presented. The control strategy is designed to handle control nonaffine systems (nonlinear in the manipulated variables), while allowing for the incorporation of rate and amplitude constraints on the manipulated variables. A continuous methyl methacrylate solution polymerization pilot plant was used for the experiment.
Micromachined reactors for catalytic partial oxidation reactions
Authors: Srinivasan Ravi , Hsing I‐Ming , Berger Peter E. , Jensen Klavs F. , Firebaugh Samara L. , Schmidt Martin A. , Harold Michael P. , Lerou Jan J. , Ryley James F.
Published: June 2004

Silicon‐based microfabrication of a novel chemical reactor (microreactor) having submillimeter flow channels with integrated heaters, and flow and temperature sensors is described. The potential application of this reactor to partial‐oxidation reactions is explored by using Pt‐catalyzed NH3oxidation as a model reaction. Investigation of reactor behavior as a function of operating conditions shows that conversion and selectivity behavior of conventional laboratory reactors can be reproduced and demonstrates the feasibility of conducting chemical reactions in microfabricated systems. Ignition‐extinction behavior is explored, along with high‐temperature microreactor materials degradation. Potential applications and scale‐up of microreactors are also discussed.
Deflagration and detonation in solid‐solid combustion
Authors: Viljoen Hendrik J. , Hlavacek Vladimir
Published: June 2004

Solid‐solid combustion becomes self‐sustaining when the preheating of the fresh mixture is high enough to support a spontaneous chemical reaction. These reactions have high activation energies, requiring significant preheating. Traditionally, conduction has been considered as the main form of preheating, and propagation velocities in the order of a few mm to a few cm per second were found. When acoustic equations are included in the analysis, no significant changes occur for traditional SHS reactions. However, the analysis of a 1‐D model propagating at a constant velocity reveals the existence of two other solutions with propagation velocities which are much faster. An SHS deflagration wave is found with combustion temperature lower than the adiabatic value. The propagation velocity is less than the longitudinal sound speed of the medium, but typical Mach numbers vary between 0.6 and 0.95. The third solution is an SHS detonation with temperature above the adiabatic value and supersonic propagation velocity. Since the heat fluxes are extremely high, the hyperbolic conduction model is used.