bluuebunny/crossref_metadata_2025 huggingface dataset:
Equation of state and radial distribution functions of FCC particles in a CFB
Authors: Gidaspow Dimitri , Huilin Lu
Published: April 2004

Kinetic theory of granular flow was verified experimentally for flow of fluid catalytic cracking particles in a vertical pipe. Measurements of particle pressure using a differential transducer and granular temperature with a digital camera as a function of bulk density, determined using an X‐ray densitometer, showed that a relation exists among pressure, temperature, and density, analogous to the ideal gas law. In the limit of zero solids volume fraction: (Solid Pressure)/[(Granular Temperature) × (Bulk Density)] = 1.0. Measurements of radial distribution functions using the digital camera showed that their peak values occur at particle contact and lie between the predictions from the Bagnold equation and Carnahan–Starling equation. The hard sphere model was corrected for a cohesive pressure using the minimum in the measured radial distribution function. The new model agrees with the pressure measurements in the dense regime.
Role of water in formic acid decomposition
Authors: Akiya Naoko , Savage Phillip E.
Published: April 2004

Formic acid decomposes primarily to CO and H2O in the gas phase, but to CO2 and H2 in the aqueous phase. Ab‐initio quantum chemical calculations were performed, using Hartree‐Fock and density functional methods, to seek an explanation for this behavior. The effect of water on the two decomposition pathways and on the isomerization of formic acid was determined. The transition state structures were fully optimized and include up to two water molecules. In the absence of water, dehydration is more favorable than decarboxylation. The presence of water reduces the activation barriers for both decomposition pathways, but decarboxylation is consistently more favorable than dehydration. The water molecules actively participate in the bond‐breaking and bond‐forming processes in the transition state. The reduction in the activation barriers with the addition of water indicates that water acts as a homogeneous catalyst for both dehydration and decarboxylation, whereas isomerization of formic acid occurs independently of water. Water has a strong effect on the relative stability of the formic acid isomers, acid–water complexes, and transition states. The relative stability of the transition states plays an important role in determining the faster decomposition pathway.
Compositional variation in hydrocarbon reservoirs with natural convection and diffusion
Authors: Riley Mike F. , Firoozabadi Abbas
Published: April 2004

The knowledge of horizontal compositional variation is of prime importance in hydrocarbon reservoir delineation. However, the effect of natural convection on this variation is largely unknown. This work examines the effect of natural convection and diffusion (thermal, pressure and Fickian) on a two‐component, single‐phase fluid occupying a horizontal cross‐sectional reservoir in the presence of a prescribed linear temperature field. The behavior is investigated using a method of successive approximations, which iterates on solution of Poisson's equation. This behavior is then incorporated in a simplified perturbation solution (in the form of a cubic equation) which not only gives accurate values of horizontal compositional variation, but also clearly shows the interplay of reservoir/fluid properties. The perturbation and the full solutions indicate that a small amount of convection can cause the horizontal composition gradients to increase until a maximum is reached and then decay as 1/k. An alternative scenario is that the gradient asymptotes to a value where the horizontal density derivative approaches zero. Generalization of this perturbative solution to n‐components apparently requires the simultaneous solution of n − 1 cubic equations.
Novel photocatalytic reactor for water purification
Authors: Ray Ajay K. , Beenackers Antonie A. C. M.
Published: April 2004

A novel photocatalytic reactor design for water treatment is characterized by the use of new extremely narrow diameter lamps, thus allowing for much higher surface area for catalyst coating per unit reactor volume and consequently for much higher specific reactor capacity. Experiments in a reactor containing 21 novel U‐shaped lamps coated with catalyst showed a 695% increase in efficiency of the reactor performance in comparison with a classical annular reactor and 259% in comparison with a slurry reactor. Both a classical annular reactor and a slurry reactor cannot be scaled up for large‐scale applications due to the low values of illuminated catalyst surface area per unit volume of liquid treated inside the reactor while this configuration is flexible enough for large‐scale applications.
Design of wire mesh mist eliminators
Authors: Brunazzi Elisabetta , Paglianti Alessandro
Published: April 2004

Knitted wire mesh mist eliminators are used extensively in many industrial plants. Their widespread application is essentially due to the low cost and efficient removal of entrained liquid droplets from vapor and gas streams. Despite the broad range of entrainment removal applications, open literature on this topic is limited. All the available design relations are based on semiempirical equations with an uncertain range of application. In this work, a set of new experimental data was obtained by investigating the performances of commercial eliminators, and a mechanistic model is presented. Comparison between experimental data and the proposed model shows that it can be used to predict separation efficiency both for horizontal and vertical arrangements.
Estimating state probability distributions from noisy and corrupted data
Authors: Johnston Lloyd P. M. , Kramer Mark A.
Published: April 2004

The method of recursive state density estimation (RSDE) is developed for determining the probability distribution of the states of a system from measurements that contain both random noise and gross errors. The technique is based on the expectation maximization algorithm and is iterative in nature. Similar to EM, at each iteration the likelihood of the distribution estimated by the RSDE algorithm is guaranteed to increase, thus arriving at the most likely distribution of the true states, given the measurement data set and the algorithm initial conditions. Convergence of the algorithm to the correct solution, for a simple case where an analytical answer can be derived for comparison, is shown. Two chemical process examples that have more complex distributions are also shown. Once the probability distribution of the states has been determined, many monitoring and statistical process and quality control functions can be performed using the more accurate distributions of the process states, avoiding corruption of the distribution due to faulty measurements.
Role of range and precision of the independent variable in regression of data
Authors: Brauner Neima , Shacham Mordechai
Published: April 2004

Regression of the experimental data of one independent variable, y vs. a linear combination of functions of an independent variable of the form y = Σβjfj(x) is considered. Inherent collinearity among the terms of such functions may prevent obtaining a model of a desired accuracy. Traditional collinearity indicators, condition number of the normal matrix, variance inflation factor, and a new indicator (truncation‐error‐to‐noise ratio) are used to investigate the effects of the range and precision of the independent‐variable data on collinearity among functions in a regression model. Statistical confidence intervals are used to demonstrate harmful effects of collinearity. The harmful effects increase by reducing the range of the independent variable data and/or its precision. Using only independent variable data, the new collinearity indicator allows the identification of the point where the number of terms in a particular regression model becomes larger than can be justified on statistical grounds. The use of the new criterion can improve experimental design in order to minimize the harmful effects of collinearity and enable a rapid screen of correlations published in the literature for identifying those that include more parameters than can be justified.
Phase transfer catalysis: Chemistry and engineering
Authors: Naik Sanjeev D. , Doraiswamy L. K.
Published: April 2004

Phase transfer catalysis (PTC) uses catalytic amounts of phase transfer agents which facilitate interphase transfer of species, making reactions between reagents in two immiscible phases possible. PTC is used widely in the synthesis of various organic chemicals in both liquid‐liquid and solid‐liquid systems. Existing literature on PTC is chemistry‐intensive and a mere handful of recent articles constitute the entire information on engineering analysis. This article reviews the field comprehensively by combining the existing knowledge from chemistry with insights into mechanistic and kinetic analysis and mathematical modeling of soluble and insoluble PTC. By its very nature, PTC involves a series of equilibrium and mass‐transfer steps, beside the two main reactions. Neglect of mass‐transfer effects can grossly overpredict the conversion of a PTC mediated reaction. A practical way of using PTC, which enables easy separation, is to immobilize the catalyst on a solid support. Mass‐transfer limitations and higher costs, however, have precluded its commercial use so far, requiring further analysis of mass‐transfer limitations in these complex three‐phase systems. The use of PTC, combined with other rate enhancement techniques like sonochemistry, microwaves, electroorganic synthesis, and photochemistry, is being increasingly explored. Applications in this area in the manufacture of organic intermediates and fine chemicals seem almost unlimited.
Diffusivities of n‐alkanes in silicalite by steady‐state single‐crystal membrane technique
Authors: Talu Orhan , Sun Matthew S. , Shah D. B.
Published: April 2004

A novel experimental technique that measures the diffusive flux through a single‐crystal membrane (SCM) was developed and tested. Unlike all other macroscopic techniques that depend on a transient response, SCM is used under steady‐state conditions, which results in a wide range of applicability from 10−2 to 10−11 cm2/s. Phenomenological equations for the steady‐state data analysis were developed. The variation of driving force over the diffusion path is included in the model. As required by thermodynamics, the micropore concentration is given as a function of surface‐excess amount adsorbed and gas density. The membrane configuration measures diffusivity in only one crystallographic direction. The micropore difusivities of C1 to C10 normal alkanes through silicalite crystal in the z‐direction were measured at 30, 50 and 70°C. The activation energies for micropore diffusion are also reported. The data agree excellently with the other two studies that measure directional diffusivities. Diffusion and adsorption of hexane and heptane in silicalite display structural heterogeneity induced by the comparable lengths of molecules and silicalite channels rather than diameter of molecule vis‐à‐vis pore diameter.
Mathematical model of combustion synthesis
Authors: Akiyama Tomohiro , Isogai Hiromichi , Yagi Jun‐ichiro
Published: April 2004

Mathematical modeling of the combustion synthesis was studied theoretically and experimentally by taking Mg2Ni synthesis as an example. An unsteady two‐dimensional mathematical model was constructed by reflecting drastic changes in thermo‐physical properties from starting material of magnesium nickel mixture to a product of inter‐metallic compound and measured solid‐phase reaction rate with/without melting. For validating the model experimentally, an end of the cylindrical sample of compressive powders was heated, and local temperatures of the sample were monitored. The comparison between calculated and measured temperatures showed plausible agreement when the model took partial melting effect of a eutectoid phase into the fundamental equations.
Drying kinetics of polymer films
Authors: Guerrier Béatrice , Bouchard Charles , Allain Catherine , Bénard Christine
Published: April 2004

Drying kinetics of a polymer film cast from a polymer–soluent solution is studied. A modelization of the whole drying process is performed, including the difision of soluent through the varnish layer, the moving interface, and the coupled heat and mass transfer between the interface and the drying air. The assumptions and the ualdity domain of the proposed modelization are thoroughly analyzed. In particular, comparison of the varnish temperature with its glass transition temperature all along the process allows us to determine the validity domain of the Fickian difision assumption. Because of the hypodifisiue behavior of polymer solutions, uery large concentration gradients appear in the layer, and the drying kinetics is characterized by two successive regimes. This two‐ stage kinetics is investigated first for typical conditions encountered in the packaging industry, where a film of a few microns is dried in a few seconds, thanks to hot air blown through nozzles. Then, different drying conditions corresponding to evaporation experiments at room temperature of thicker films of the same varnish are studied.
Improved direct and indirect systems of columns for ternary distillation
Authors: Agrawal Rakesh , Fidkowski Zbigniew T.
Published: April 2004

Separation of a ternary mixture into almost pure components is discussed. Systems of distillation columns, with higher thermodynamic efficiency, are developed from a direct sequence (or indirect sequence) of distillation columns by allowing for two interconnecting streams of the same composition and different enthalpy. This increases the reversibility of distillation in the second column, which results in replacing a portion of the high‐temperature boiling duty with a lower‐temperature heat in the direct split case. For the indirect split case, the improvement allows a portion of the low‐temperature condensing duty to be replaced with a higher‐temperature condensation.
Effects of Marangoni surface tension forces on modern distillation packings
Authors: Proctor Sebastian J. , Biddulph Michael W. , Krishnamurthy Krish R.
Published: April 2004

New experimental data presented concern effects of Marangoni surface‐tension forces on the mass‐transfer eficiency of packed distillation columns. Various random packings of sizes 6 and 25 mm, as well as structured packings of sizes 30 mm diameter and 150 mm, were used with the system n‐propanol/water. This system can behave either as a Marangoni positive system at low concentrations of n‐propanol or as a negatiue system at high concentrations of n‐propanol. It was confirmed that small packings show lower eficiencies for Marangoni negative systems, due to the breakup of the film of liquid on the packing surface. In the larger column, howeuer, the situation is reversed with the negative mixtures often showing better eficiencies, especially at heauier loadings. This is attributed to the observation that spray and droplet formation are much more pronounced for the negative mixtures, providing extra surface for mass transfer and, hence, better mass‐transfer eficiency. There are also some indications that the pressure drop is unexpectedly high for these conditions and that premature flooding can occur. This may haue design implications for any mass‐transfer deuice in which vapor and liquid phases are contacted in narrow passageways.
Product and process development using artificial neural‐network model and information analysis
Authors: Chen Junghui , Wong David Shan Hill , Jang Shi‐Shang , Yang Seng‐Lu
Published: April 2004

An experimental design scheme proposed for process and product development integrates the artificial neural network, random‐search algorithm, fuzzy classification, and information theory. An initial batch of experimental data is first collected to construct a neural‐network model. Random search generates a number of candidates for the next batch of experiments. A fizzy classification algorithm is used to find the cluster centers of these candidates. An information free energy index is defined to balance the need for better classification and the relevance of each class in optimization. New experiments are performed at these cluster centers to validate the model. The procedure is repeated until an optimal solution is reached. Case studies using a mathematical model and a real industrial pigment‐blending project illustrate the abilities of this method to locate multiple optima and handle multivariable experimental design.
Design of single‐product campaign batch plants under demand uncertainty
Authors: Petkov Spas B. , Maranas Costas D.
Published: April 2004

A new method is introduced for optimally designing multiproduct batch plants under the single‐product campaign (SPC) production mode. Uncertain future product demands are described with normal probability distributions, and more than one processing unit of equal size are allowed per stage. A t the expense of imposing the normality assumption for product demand uncertainty and the SPC production mode, the original two‐stage stochastic optimization problem is transformed into a deterministic mixed‐integer nonlinear programming problem without relying on implicit or explicit discretization of the uncertain variables. This is accomplished through the explicit solution of the inner problem and the analytical integration overall product demand realizations. This problem representation and solution strategy result in savings of orders of magnitude over existing methods in computational requirements.
Understanding and applying the extrapolation properties of serial gray‐box models
Authors: Van Can Henricus J. L. , Te Braake Hubert A. B. , Dubbelman Sander , Hellinga Chris , Luyben Karel Ch. A. M. , Heijnen Joseph J.
Published: April 2004

There is a need for efficient modeling strategies that quickly lead to reliable models. In the serial gray‐box modeling strategy the inaccurately known terms in the macroscopic balance are modeled with a black‐box model structure such as a neural network. This way one efficiently obtains an accurate model with good extrapolation properties without the need to develop a rigorous white‐box model in which much more knowledge would be involved and without the need to do many identification experiments. Different types of extrapolation are specified and analyzed and they are related to the different parts of the serial gray‐box model structure. The strategy is demonstrated for modeling pH effects on the enzymatic conversion of penicillin G using real‐life data. The resulting serial gray‐box model is compared with a model from a more knowledge‐driven white‐box strategy and with a model from a more data‐driven black‐box strategy. The serial gray‐box modeling strategy is especially advantageous at the medium level of process operation, which is mainly concerned with the calculation of optimal condition for (bio)chemical processes.
Complex dynamic features of a cooled reverse‐flow reactor
Authors: Khinast J. , Gurumoorthy A. , Luss D.
Published: April 2004

A reverse‐flow reactor (RFR) usually attains a symmetric period‐1 state, so that the temperature profile just after a flow reversal is a mirror image of that after the previous flow reversal. Recent simulations show that in certain cases a cooled RFR may attain other states with different types of periodicity or even complex quasi‐periodic states. The maximum temperature of these states often exceeds that of the symmetric states so that it may deactivate the catalyst and/or lead to safety problems. A systematic, numerically efficient method is presented for constructing maps of parameter regions in which a cooled RFR has qualitatively different dynamic features. The technique is applied to determine the dependence of these dynamic features on the cooling capacity and flow‐reversal period. Stable quasi‐periodic and asymmetric period‐1 states exist mainly for short flow‐reversal periods. The quasi‐periodic states usually exist for lower cooling capacities than those for which the asymmetric period‐1 states exist. Stable symmetric and asymmetric period‐1 states exist for the same set of parameters in very narrow regions of the parameter space. The behavior of the RFR in the limit of very fast flow reversals is usually modeled by a countercurrent flow reactor, with equal flow rates in the two compartments. This cooled reactor may attain asymmetric states for certain sets of parameters.
Pseudo‐adiabatic operation and runaway in tubular reactors
Authors: Wu Hua , Morbidelli Massimo , Varma Arvind
Published: April 2004

Critical conditions for thermal runaway in homogeneous tubular reactors with either constant or cocurrent external cooling were investigated through the applications of the generalized sensitivity criterion using reactor axial coordinate as the independent variable, and compared with those predicted by geometry‐based criteria. All criteria are found to provide rather similar results when a hot spot is present inside the reactor. When the hot spot shifts to the reactor outlet, leading to the so‐called pseudo‐adiabatic operation (PAO), the geometry‐based criteria become either invalid or conservative with respect to the generalized one. In addition, when the generalized criterion is applied to the cases where the PA0 is present, it is required to use the axial coordinate rather than reactant conversion as the independent variable to predict the critical conditions, and the obtained runaway boundaries in the reactor parameter plane coincide with the PA0 boundaries.
Molecular‐thermodynamic framework for asphaltene‐oil equilibria
Authors: Wu Jianzhong , Prausnitz John M. , Firoozabadi Abbas
Published: April 2004

Asphaltene precipitation is a perennial problem in producing and refining crude oils. To avoid precipitation, it is useful to know the solubility of asphaltenes in petroleum liquids as a function of temperature, pressure and liquid‐phase composition. In the novel molecular‐thermodynamic framework presented here, both asphaltenes and resins are represented by pseudo‐pure components while all other components in the solution are represented by a continuous medium that affects interactions among asphaltene and resin particles. The effect of the medium on asphaltene‐asphaltent: resin‐asphaltene, resin‐resin pair interactions is taken into account through its density and dispersion‐force properties To obtain expressions for the chemical potential of asphaltene and for the osmotic pressure of an asphaltene‐containing solution, the SAFT model is used in the framework of McMillan‐Mayer theory, which considers hard‐sphere repulsive, association and dispersion‐force interactions. By assuming that asphaltene precipitation is a liquid‐liquid equilibrium process, a variety of experimental observations can be explained, including effects of temperature, pressure, and composition on the phase behavior of asphaltene‐containing fluids. For practical quantitative applications, the model outlined here requires molecular parameters that must be estimated from a few experimental data.
Simulation of flow and mixing of particles in a rotating and rocking cylinder
Authors: Wightman Carolyn , Moakher Maher , Muzzio Fernando J. , Walton O.
Published: April 2004

Granular material in a cylindrical vessel undergoing rotational and rocking motions is modeled using a discrete element method. Rotational motion supplemented by rocking is compared to purely rotational motion via linear density profiles, velocity fields, and axial concentration profiles. The rocking motion, which imparts a time‐dependent flow perturbation to purely rotational motion, dramatically enhanced mixing in laboratory studies. Simulation results appear to agree well with experimental results and observations.
Synthesis of extractive reaction processes
Authors: Samant Ketan D. , Ng Ka M.
Published: April 2004

A systematic method presented synthesizes extractive reaction processes. The objective is to develop liquid‐phase processes involving simultaneous reaction and separation by extraction to achieve improved yield, selectivity to a desired product, and separation of byproducts. The method is based on the equilibrium thermodynamic analysis of multicomponent reactive systems with multiple liquid phases. Using transformed coordinates, systems with three or fewer degrees of freedom can be conveniently analyzed regardless of the number of components in the system. Key features of the reactive phase diagrams relevant to achieving the process objectives are identified. The method is illustrated with several model examples to demonstrate the advantages of extractive reaction over conventional single‐phase processes.
Microhydrodynamics of sharp corners and edges: Traction singularities
Authors: Mustakis I. , Kim S.
Published: April 2004

Sharp corners and edges are ubiquitous in modem technology and found on the tip of an atomic force microscope (AFM) and the vertices and edges of clystalline materials. The microhydrodynamics of particles with sharp corners and edges are special in that geometric discontinuities give rise to traction singularities for viscous flow around the particles. Such traction fields are analyzed with both analytical and numerical solution of a second kind integral equation. The traction field in the vicinity of a vertex has a singular structure that is more intricate than simple superposition of the well‐known edge singularities. The new insights into the structure of the vertex singularities are used to formulate and test spectral element strategies for numerical solution of viscous flow past sharp corners and edges. The best schemes constructed to date show remarkable agreement with the analytical results.
Growth regime map for liquid‐bound granules
Authors: Iveson S. M. , Litster J. D.
Published: April 2004

A regime map of granule growth behavior is proposed based on granule deformation during collision and the granule liquid content measured as the maximum pore saturation. The granule deformability on collision is represented by a deformation number, which is a ratio of granule impact energy to the plastic energy absorbed per unit strain. Granule growth regimes such as steady growth, induction, nucleation, crumb, and slurry are defined. This regime map qualitatively explains the uariatiom in granulation behauior. Laboratoly drum granulation experiments were used to test the regime map. Experiments were peqformed in a 0.3‐m‐dia. drum using three sizes of glass ballotini (19, 31, and 60 μm) with water and glycerol as liquid binders. Increasing granule yield stress by decreasing particle size and increasing binder viscosity caused the system to move from steady growth to induction behauior as predicted by the regime map. Preliminary validation with literature data was also encouraging. More work, howeuer, is required to better quantify the boundaries between different growth regimes and to investigate the effect of process agitation intensiq. This regime map has great potential to help design and control granulation systems, because it is based on properties of the powder/binder qstem that can be measured or estimated without peqforming any granulation tests.
Influence of Nernst‐Planck diffusion on hollow‐fiber mass‐transfer processes
Authors: Qin Yingjie , Cabral Joaquim M. S.
Published: April 2004

Hollow‐fiber membrane reactive extraction and facilitated supported liquid membrane processes for the separation of charged species are analyzed theoretically. A mathematical model incorporating laminar flow, Nemst‐Planck diffusion, buffer effect, ion strength, interfacial reaction kinetics and/or equilibria, diffusivity of species in the membrane phase, shell resistances, and shell concentration is deueloped and solued numerically. When the lumen mass‐transfer resistance is dominant, the predicted remoual rate of a single ion or the separation rate of a mixture of ions by a simplified model with Fickian diffusion deuiates greatly from the prediction with the present model. For instance, when the concentration of triualent ions in the feed decreases to half of its initial ualue, the membrane length calculated using Fickian diffusion is 75% higher than the length predicted by Nemst‐Planck diffusion. Thus, the Leueque equation based on Fick's law does not adequately describe the mass transfer of the charged species in the lumen feed. The effect of buffer ions and diffusiuity of species in the membrane phase on the mass transfer of the species is also discussed.
Adsorptive separations using supercritical frontal analysis chromatography
Authors: Cross William M. , Akgerman Aydin
Published: April 2004

Many food products and pharmuceuticals industries are looking to integrating super‐critical extraction and separations processes to fulfill processing needs due to limitations and regulations concerning the use of organic solvents. One such technology, which may affect these industries, is continuous supercritical adsorptive separation, a processing technology similar to the separation process of xylenes developed at UOP. Many products like pharmaceuticals and other human consumables could be processed more economically and with higher purity by supercritical fluid adsorption/desorption processes. Past studies, however, have focused on single‐component adsorption/desorption involving a supercritical mobile phase, whereas adsorptive separations involving super‐critical fluids are multicomponent systems. Accurate experimental techniques, which can determine both multicomponent solubilities in supercritical fluids and multicomponent adsorption isotherms in the presence of supercritical fluids, were developed to investigate the multicomponent supercritical adsorption phenomenon. A dynamic column model developed takes into account column dispersion as well as mass transfer and diffusive resistances. Experimental isotherm data incorporated into the model can predict the breakthrough profiles. An extension of pulse chromatography was used to identify the important hydrodynamic and transport parameters, and operating conditions are identified for optimal separations.
Hollow‐fiber‐based adsorbers for gas separation by pressure‐swing adsorption
Authors: Feng Xianshe , Pan Chuen Y. , McMinis Curtis W. , Ivory John , Ghosh Dave
Published: April 2004

Hollow‐fiber‐based adsorbers for gas separation by pressure‐swing adsorption (PSA) was studied experimentally. The high efficiency of hollow‐fiber‐based adsorbers for gas separation was illustrated by hydrogen separation using fine‐powder‐activated carbon and molecular sieve as adsorbents. The adsorption equilibrium and dynamics of the hollow‐fiber adsorbers were determined. The pressure drop of the gas flowing through the adsorbers was also examined. The adsorbers were tested for hydrogen separation from nitrogen, carbon dioxide, and a multicomponent gas mixture simulating ammonia syn‐ thesis purge gas. The PSA systems using the hollow‐fiber adsorbers were very effective for hydrogen purification. The high separation efficiency is derived from the fast mass‐ transfer rate and low pressure drop, two key features of hollow‐fiber‐based adsorbers.
Nonlinear model reduction for control of distributed systems: A computer‐assisted study
Authors: Shvartsman Stanislav Y. , Kevrekidis Ioannis G.
Published: April 2004

Model reduction methodologies for the partial differential equations modeling distributed parameter systems constitute an important first step in controller design. A systematic computer‐assisted study illustrating the use of two such methodologies (Approximate Inertial Manifolds and the Karhunen‐Loève expansion) in controlling (stabilizing) a nonlinear reaction‐diffusion problem is presented. The approximation quality of the models, issues of computational implementation of the reduction procedure, as well as issues of closed‐loop stability are addressed.
Cyclic scheduling of continuous parallel‐process units with decaying performance
Authors: And Vipul Jain , Grossmann Ignacio E.
Published: April 2004

The problem of scheduling multiple feeds on parallel units is addressed. The performance of each unit is assumed to decrease with time, therefore requiring planned maintenance to restore its performance. A mixed‐integer nonlinear programming (MINLP) model presented uses the process information regarding the exponential decay in performance with time to find a cyclic schedule for feed processing. Optimization trade‐off is between the performance of the units, and the maintenance costs and the loss in production due to shutdowns. Three main decisions for the optimal schedule are: a) assignment of various feeds to different units; b) processing time of each feed on different units; and c) number of subcycles of each feed on diferent units. Special mathematical properties of the MINLP model are exploited to solve the problem to global optimality with a branch‐and‐bound method. Using several examples advantages of the model are illustrated and performances of different solution algorithms are compared.
Optimal synthesis of stagewise continuous crystallization process networks
Authors: Sheikh Ahmad Y. , Jones Alan G.
Published: April 2004

A new synthesis method based on targeting is presented for the design of stagewise continuous crystallization processes. The methodology determines the optimal flow distribution to each of the crystallizers in addition to other common control variables, including temperature, concentration, and vessel volume. The resulting optimization model yields a boundary‐ualue differential equation system, which is transformed into a set of nonlinear equations through orthogonal collocation and solved numerically. For the synthesis of the continuous KNO3, crystallization process considered here, the predicted optimal network solution shows that the common tradeoff between high yield and large average crystal sizes has been successfilly eradicated by the new design approach.
Effects of imperfect mixing on low‐density polyethylene reactor dynamics
Authors: Villa Carlos M. , Dihora Jiten O. , Harmon Ray W.
Published: April 2004

Earlier work considered the effect of feed conditions and controller configuration on the runaway behavior of LDPE autoclave reactors assuming a perfectly mixed reactor. This study provides additional insight on the dynamics of such reacton by using an imperfectly mixed reactor model and bifurcation analysis to show the changes in the stability region when there is imperfect macroscale mixing. The presence of imperfect mixing substantially increases the range of stable operation of the reactor and makes the process much easier to control than for a perfectly mixed reactor. The results of model analysis and simulations are used to identify some of the conditions that lead to unstable reactor behavior and to suggest ways to avoid reactor runaway or reactor extinction during grade transitions and other process operation disturbances.
Model for the sulfidation of calcined limestone and its use in reactor models
Authors: Heesink A. B. M. (Bert) , Brilman D. W. F. (Wim) , van Swaaij W. P. M. (Wim)
Published: April 2004

A mathematical model describing the sulfidation of a single calcined limestone particle was developed and experimentally verified. This model, which includes no fitting parameters, assumes a calcined limestone particle to consist of spherical grains of various sizes that react with H2S according to the classic shrinking‐core model. The initial size distribution of the grains is derived from mercuiy porosimetiy. The transport of H2S through the bidisperse limestone particle is calculated based on the randompore model of Wakao and Smith, which distinguishes macropore and micropore zones. Knudsen diffusivity inside the micropore zones is calculated according to the dusty‐gas approach. The single‐particle model delivers the value of a new defined utilization factor, which includes effects of external mass‐transfer limitation, pore‐diffusion limitation, and grain‐size distribution on particle reactivity. A correlation derived for a single batch of calcined limestone explicitly expresses this utilization factor as a function of conversion and relevant process parameters. This correlatiori can be easily incorporated into reactor models, as shown for an existing model describing the capture of H2S by a fluidized bed of calcined limestone particles.
Predictive control of sheet and film‐forming processes
Authors: Campbell John C. , Rawlings James B.
Published: April 2004

A constrained infinite‐horizon model‐predictive controller is advocated to improve product properties from sheet‐ and film‐forming processes by feedback control; the proposed model structure captures the main characteristics of the process, including the scanning sensor and large time delay. Model parameters are identified from process data, and symmetry is enforced to improve the parameter estimates. The film thickness is estimated from sparse measurements using a periodic Kalman filter. The covariances of the estimates are used to evaluate scanning patterns. Integrating disturbance models are used to achieve offset free control. Detectability conditions are presented for both models to ensure that the disturbances can be estimated from the measurement. Once the state of the film is reconstructed adequately from the measurement, a control strategy can be implemented to improve film properties. One important control contribution is to enable the controller to handle hard constraints on the actuators without clipping the controller output signal. The target‐tracking problem is discussed, and the uniqueness of the steady‐state solution is established. By simulations an output disturbance model is compared with a measured input disturbance model.
Effect of the shear and volume fraction on the aggregation and breakup of particles
Authors: Serra Teresa , Casamitjana Xavier
Published: April 2004

The aggregation and breakup of particles in a Couette flow system were studied for different values of the shear rate (G) and the volume fraction of particles ϕo. After a certain time, an initial distribution of 2‐μm‐dia. latex particles achieves the steady state, reflecting the balance between coagulation and fragmentation forces. Higher G values shift the steady state to smaller aggregate sizes. For a fixed value of G, when ϕo increases, the steady‐state size increases if the flow is laminar and decreases if the flow is turbulent. The population balance model proposed describes simultuneous coagulation and fragmentation during shear flocculation. A power dependence between the breakage rate coefficient and the shear rate was found. The model is adjusted by using two parameters: the probability of success for collisions α and the effective breakup coefficient for shear fiagmentation B'eff depends on both G and ϕo, while α is nearly constant.
Reliable computation of homogeneous azeotropes
Authors: Maier Robert W. , Brennecke Joan F. , Stadtherr Mark A.
Published: April 2004

It is important to determine the existence and composition of homogeneous azeotropes in the analysis of phase behavior and in the synthesis and design of separation systems, from both theoretical and practical standpoints. A new method for reliably locating any and all homogeneous azeotropes for multicomponent mixtures is presented. The method also verifies the nonexistence of homogeneous azeotropes if none are present. The method is based on interval analysis, in particular an interval‐Newton/generalized‐bisection‐algorithm providing a mathematical and computational guarantee that all azeotropes are located. This general‐purpose technique can be applied in connection with any thermodynamic models. The technique is illustrated in several example problems using the Wilson, NRTL, and UNIQUAC activity‐coeficient models.
Visualization of the UF polarized layer by holographic interferometry
Authors: Fernández‐Torres M. J. , Ruiz‐Beviá F. , Fernández‐Sempere J. , López‐Leiva M.
Published: April 2004

The technique of holographic interferometly used to visualize the ultrafiltration membrane surroundings makes it possible to visualize solute retention and verify whether or not there is cake formation on the membrane surface during the ultrafiltration of bovine serum albumin (BSA) protein using a polyethersulfone membrane. The experimental results indicate the presence of a gelatinous mass or a filter cake during BSA ultrafiltration using the membrane mentioned. This was shown by the small concentration profile obtained from the interferometric information and from direct observation of the membrane environs when the pump stopped, which showed the appearance of a gelatinous material adjacent to the membrane.
Polymorphism of CaCO3, precipitated in a constant‐composition environment
Authors: Tai Clifford Y. , Chen F.‐B.
Published: April 2004

Polymorphism of calcium carbonate was investigated using a constant‐composition method, and three polymorphs—calcite, aragonite, and vaterite—are produced. Factors that affect the formation of polymorphs are studied over a wide range, including solution pH, temperature, concentration ratio of components, supersaturation, ionic strength, and type and concentration of additives. At room temperature, solution pH is the most significant factor, and high yield polymorphs are present at different pH values. At a high temperature, aragonite becomes the major product at a pH below 12. The effects of the additive are complicated, depending on its type and concentration as well as the operating conditions. The influence of concentration ratio and supersaturation on the formation of polymorphs is less significant, Besides, various particle morphologies of CaCO3 polymorphs are observed, depending on the operating conditions.
Process systems and inventory control
Authors: Farschman Chad A. , Viswanath Krishnan P. , Erik Ydstie B.
Published: April 2004

A framework for modeling constraints on the dynamic behavior of systems obeying the laws of thermodynamics is discussed. These constraints induce dissipativity and we call a system obeying such constraints a process system. Passivity theory then shows that low‐dimensional macroscopic analysis can be used for control system design. The inputs and outputs converge to their set points and the state vector converges to a stationary passive state provided one exists. Several examples are developed to support the theory and an extension is proposed for systems with equipment constraints.
Identifiability of kinetic parameters of methanol synthesis in plug‐flow tracing
Authors: Otarod Masood , Happel John
Published: April 2004

The identifability of the fundamental kinetic parameters of a model of a plug‐flow isotope transient tracing of methanol over PdNO/Al was investigated. The values of the parameters were estimated by securing a least‐squares fit between the tracer data and the partial differential equations describing the reaction response in a plug‐flow mode. The results demonstrate that modeling and identifiability of isotope transient tracing data provide for a meaningful assessment of heterogeneous catalytic reaction systems.
Dynamic NMR microscopy of chromatographic columns
Authors: Tallarek Ulrich , Bayer Ernst , Van Dusschoten Dagmar , Scheenen Tom , Van As Henk , Guiochon Georges , Neue Uwe D.
Published: April 2004

Pulsed magnetic field gradients encoded a discrete spatial and temporal motion of the molecules of a fluid percolating through the packed bed of a dynamic radial compression column for liquid chromatography. Two packing materials of porous, chemically bonded C18 silica were used: large irregularly shaped particles (55 – 105 μm) and small spherical particles (6 μm). By combining pulsed field gradient NMR and NMR imaging the distribution of the fluid dynamic displacement probability was determined quantitatively at each point of the static image space. The local axial dispersion coefficient, fluid velocity, and external porosity of the bed were also estimated. The resolution achieved was 250 μm × 250 μm × 2,500 μm per voxel, corresponding to a 250 μm × 250 μm pixel in a 2.5‐mm‐thick slice. Irregularly shaped particles showed slight external porosity heterogeneities over the column cross section, causing a dramatic loss of performance. Spherically shaped particles offered a nearly planar distribution of the transport characteristics across the packed bed, making radial compression of the bed enhance the performance.
Acoustic force distribution in resonators for ultrasonic particle separation
Authors: Woodside Steven M. , Piret James M. , Gröschl Martin , Benes Ewald , Bowen Bruce D.
Published: April 2004

The effectiveness of particle–liquid separation by ultrasonic radiation forces depends on the acoustic energy density distribution in the standing‐wavefield. The energy distribution in an ultrasonic particle‐separation device was analyzed to assist continued optimization and design efforts. Measurements of the energy‐density distribution in the liquid using a microscope‐based imaging system were compared to laser interferometer measurements of the velocity‐amplitude distribution on the transducer and reflector surfaces of the ultrasonic separator. The energy density followed the same trend as the surface velocity, being highest near the resonator center and approaching zero near the walls. The energy density gradients and local gridlike reflector‐amplitude variation had characteristic lengths of 1.4 mm. These results suggest that the energy‐density distribution in the liquid is a defined function of the dimensions, imposed boundary conditions and physical properties of the reflector and transducer. This understanding provides a practical basis for developing a mathematical model of cell aggregation and retention, potentially enabling the design of resonators with predetermined energy density distributions for specific particle aggregation, separation, and fractionation applications.
Robustness metrics for dynamic optimization models under parameter uncertainty
Authors: Samsatli Nouri J. , Papageorgiou Lazaros G. , Shah Nilay
Published: April 2004

Recent research in process systems engineering has focused mostly on the issue of making decisions under uncertainty. Various approaches used over the years include optimizing the expected and worst cases, maximizing the feasibility of operation, and constraining variances of performance measures. The consideration of robustness, that is, guaranteeing a reasonable performance over a wide range of uncertainty, is either implicit or explicit in these approaches, and is certainly receiving more attention. In this article, we argue that mathematical techniques for robust optimization must be capable of capturing different perspectives on risk of different users. We define some general robustness metrics that can represent significantly different robustness objectives simply by modifying functions and parameters. We also describe a solution procedure along with two illustrative examples.
Application of a neural network to evaluation of interactions in a MIMO process
Authors: Ohba Takehiro , Ishida Masaru
Published: April 2004

A neural‐net controller for multivariable systems is presented. The neural network for this controller has a structure in which small neural‐net controllers for SISO systems are assembled and offers a unique path from the controlled variable to the manipulated variable. By using such a structured assembly, the interactions among the controlled and manipulated variables can be evaluated. The simulation results for both a linear three‐input and three‐output system and a crystal‐growth process indicate that the proposed controller has the ability to learn the interactions between the control variables and to disclose the features of the interactions.