bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic modeling of chemical vapor infiltration
Authors: Sotirchos Stratis V.
Published: June 2004

A mathematical model is developed to describe the dynamic and pseudosteadystate behavior of chemical vapor infiltration, a process used to fabricate fiber‐reinforced ceramic matrix composites. The three‐parameter dusty‐gas model is used to describe the interaction of mass transport fluxes, partial pressures, and partial pressure gradients in the interior of the porous medium, with the viscous flux related to the total pressure gradient through Darcy's law. The model is applied to study the deposition of SiC via decomposition of methyltrichlorosilane in a porous medium whose structure can be represented by a population of uniformly‐sized, randomly‐overlapping pores. The results show that use of simplified mass transport flux models can lead to significantly different results, even if the concentration of reactants is low and the effects of the products of the reaction on the deposition rate are ignored. It is also shown that operation of chemical vapor infiltration reactors under pressure pulsing can lead to conversion gradients in the densifying structure by a few orders of magnitude smaller than those seen at the same reaction conditions under constant pressure.
Reduction of membrane fouling in the ultrafiltration of binary protein mixtures
Authors: Rodgers V. G. J. , Sparks R. E.
Published: June 2004

It is well known that separation efficiency and solute rejection change when two or more proteins are ultrafiltered. This phenomenon is primarily due to membrane fouling and is not significantly influenced by concentration polarization. If membrane fouling is the dominant resistance, then negative transmembrane pressure pulsing might significantly reduce this barrier.

A study was performed to determine the effect of negative transmembrane pressure pulsing on solute rejection for an albumin and gamma‐globulin mixture in ultrafiltration. Pulsing improved solute flux for all cross‐flow rates investigated including turbulent conditions by as much as two orders of magnitude. Under certain pulsing conditions, substantially higher solute flux was obtained for operations involving increased concentration polarization. This study reveals that negative transmembrane pressure pulsing can be effective in lowering the solute flux resistance that is observed in binary protein mixture ultrafiltration.
Vapor synthesis of titania powder by titanium tetrachloride oxidation
Authors: Akhtar M. Kamal , Xiong Yun , Pratsinis Sotiris E.
Published: June 2004

Formation of titania particles by vapor‐phase oxidation of titanium tetrachloride was studied in an aerosol reactor between 1,200 and 1,723 K. The effect of process variables (reactor residence time, temperature, and reactant concentration) on powder size and phase characteristic was investigated using the differential mobility particle sizer, scanning electron microscopy, and X‐ray diffraction. Titania particles were primarily anatase though the rutile weight fraction increased with increasing reactor temperature. The geometric number average diameter of the particles was between 0.13 and 0.35 μm, and the geometric standard deviation of the particle size distribution was about 1.4. The average particle size increased with increasing temperature, inlet TiCl4 concentration, and residence time. The observed changes in the particle size distribution were compared with those predicted by solving the aerosol dynamic equation by a sectional method and accounting for coagulation and first‐order chemical reaction. While variations in the process variables resulted in discernible changes in the size of the particles, the spread of the distribution remained rather unaffected.
Reactions of polycyclic alkylaromatics: Structure and reactivity
Authors: Smith C. Michael , Savage Phillip E.
Published: June 2004

A family of alkyl‐substituted polycyclic aromatic hydrocarbons was pyrolyzed in microbatch reactors at temperatures between 350°C and 425°C. A general pyrolysis network was deduced for these compounds, and it comprised two major and one minor parallel pathways. The first major pathway resulted in products analogous to the major products observed from alkylbenzene pyrolysis. The second major pathway led to products via the cleavage of the strong aryl‐alkyl CC bond. The third pathway led to small amounts of products, presumably through cyclization and condensation reactions. The relative importance of the two major pathways varied for the different compounds. The rates of aryl‐alkyl bond cleavage differed for the different compounds, and these rates were quantitatively related to the compounds' localization energies through Dewar reactivity numbers.
Supported liquid‐phase catalytic membrane reactor–separator for homogeneous catalysis
Authors: Kim Jae S. , Datta Ravindra
Published: June 2004

A new configuration of a membrane reactor‐separator for homogeneous catalysis is described that is designed not only to encapsulate the catalyst solution, but to simultaneously separate the product from reactant. It is composed of a porous support layer that is impregnated partially or completely with the liquid‐phase catalyst and is sandwiched between two different membranes. Membrane 1 is readily permeable to the reactant but not to the product, whereas membrane 2 allows free permeation of both the reactant and the product while encapsulating the catalyst solution. The performance of a differential stirred‐cell membrane reactor‐separator was theoretically investigated for the case of a first‐order irreversible reaction. The extent of product separation was studied as a function of membrane permselectivity for reactant and product, the difference in resistance of membrane and catalyst layer, liquid loading of the catalyst layer, and the sweep fluid flow rate. The efficacy of the device was experimentally demonstrated using ethylene hydroformylation with hydridocarbonyltris (triphenyl phosphine) Rh(I) catalyst dissolved in dioctyl phthalate solvent.
Generalized kinetic model for wet oxidation of organic compounds
Authors: Li Lixiong , Chen Peishi , Gloyna Earnest F.
Published: June 2004

A generalized kinetic model for wet oxidation (WO) of organic compounds was developed based on a simplified reaction scheme considering acetic acid as the rate‐limiting intermediate. The selectivity of product vs. intermediate formations was quantified by the ratio of the two reaction rate constants. This point selectivity α may be used to characterize the “strength” of the feed stream to be treated. This global model was validated using WO kinetic data reported for temperatures ranging from 150°C to 550°C and pressures varying from 20 bar to 440 bar. Organic conversions predicted by this model, as compared to other models, more accurately reflect the actual performance of WO processes. The model has practical validity for a variety of organic compounds, wastewaters and sludges in both subcritical and supercritical water oxidation processes.
Robust control of homogeneous azeotropic distillation columns
Authors: Jacobsen Elling W. , Laroche Lionel , Morari Manfred , Skogestad Sigurd , Andersen Henrik W.
Published: June 2004

The entrainer feed used in homogeneous azeotropic distillation provides an extra degree of freedom in the steady‐state design compared to simple distillation. In this article, we discuss the control of azeotropic distillation columns in the region close to minimum entrainer feed. Both industrial experience and previous research indicate that this is a difficult task. However, by considering the high‐frequency behavior (initial response) of the column, we show that tight and robust control can be obtained with simple single‐loop PI controllers. The results depend on the presence of high‐frequency phenomena such as flow dynamics.
Propagation of oscillations during ethylene oxidation on a Rh/SiO2 catalyst
Authors: Kellow John C. , Wolf Eduardo E.
Published: June 2004

Infrared imaging is used to visualize nonuniform temperature patterns occurring on a Rh/SiO2 catalyst wafer during the total oxidation of ethylene. Ignition of the catalyst wafer is seen to occur as a propagating reaction front originating at a single point on the wafer. During self‐sustained oscillations, the reaction zone contracts, expands, and changes shape. Thermographic data of the oscillations are analyzed by calculating the size and mean temperature of the reaction zone. This analysis reveals that the observed oscillations in the bulk CO2 concentration correlate mainly with changes in the size of the reacting zone.
Chemical, quasi‐chemical and perturbation theories for associating fluids
Authors: Economou Ioannis G. , Donohue Marc D.
Published: June 2004

Pure fluids and mixtures of species that hydrogen bond behave differently from systems that interact only through dispersion forces. The deviations from classical behavior often are sufficiently large that conventional equations of state and activity models cannot be used without the introduction of large, condition‐dependent empirical parameters. Consequently, three different classes of theories have been developed specifically to treat hydrogen‐bonding systems. The first is based on the assumption that when molecules hydrogen‐bond, they react to form new species and consequently is referred to as “chemical” theory. The second is based on lattice‐fluid theory that is used to describe different types of specific interactions and is known as “quasi‐chemical” theory. The last is based on the solution of integral equations using a potential function that mimics that of a hydrogen bond. It is shown here that these three approaches give essentially equivalent results. This allows one to relate the parameters in the perturbation theory to the equilibrium constant and hence greatly improves its utility for real systems. All three theories are compared with simulation data.
Computation of transition and molecular diffusivities in fibrous media
Authors: Melkote Rajesh R. , Jensen Klavs F.
Published: June 2004

The tortuosities of fibrous media in the heretofore unexplored transition and ordinary regimes are computed using a Monte Carlo scheme based on the Einstein equation for random walkers. The model structure is that of fully penetrable cylinders (FPC) in a unit simulation volume. The mean square displacement technique is combined with the first passage time distribution to accelerate the progress of the walkers at low Knudsen number. The results include the computation of transition regime transport coefficients for the first time. The calculated ordinary tortuosities are approximately equal to the reciprocal of the porosity over a wide range, while the transition tortuosities are shown to deviate from the reciprocal porosity with a simple dependence on Knudsen number. The limits of the transition regime are shown to correspond roughly to Knudsen numbers of 0.50 and 100, respectively. The calculated Knudsen tortuosities are shown to improve on earlier results obtained by the authors using a flux‐based technique.
Circulation and scale‐up in bubble columns
Authors: Geary Nicholas W. , Rice Richard G.
Published: June 2004

The liquid circulation model of Rice and Geary (1990) is extended to include turbulence originating at the wall. Thus, two possible length scales are considered: one originating from rising bubbles and the other emanating from the wall. The appropriate scale for small columns should be based on bubble size, while for larger systems the proper mixing length is proportional to column diameter. It is proposed that the film Reynolds number may be the key in distinguishing the two cases.
Multiplicity features of adiabatic autothermal reactors
Authors: Lovo Marianne , Balakotaiah Vemuri
Published: June 2004

Singularity theory, large activation energy asymptotics, and numerical methods are used to present a comprehensive study of the steady‐state multiplicity features of three classical adiabatic autothermal reactor models: tubular reactor with internal heat exchange, tubular reactor with external heat exchange, and the CSTR with external heat exchange. Specifically, we derive the exact uniqueness‐multiplicity boundary, determine typical cross‐sections of the bifurcation set, and classify the different types of bifurcation diagrams of conversion vs. residence time. Asymptotic (limiting) models are used to determine analytical expressions for the uniqueness boundary and the ignition and extinction points. The analytical results are used to present simple, explicit and accurate expressions defining the boundary of the region of autothermal operation in the physical parameter space.
Statistical hydrodynamics in trickle flow columns
Authors: Crine M. , Marchot P. , L'Homme G.
Published: June 2004

A stochastic model is proposed for the description of the hydrodynamics in trickle flow columns. The gas‐liquid flow is assumed to be controlled by local packing properties which are represented by random variables. The first (microscopic) level of description corresponds to the smallest volume whose averaged geometrical properties are representative of the whole packing. The hydrodynamics at this level is described by Ergun equations. The second level describes the liquid flow maldistribution induced by the partial wetting of the solid. This is the macroscopic level where the hydrodynamics is described by a probability distribution. This approach is used to develop correlations for predicting the wetting efficiency, the dynamic liquid holdup, and the pressure drop. These correlations give very good results when compared to experimental data over a wide range of operating conditions. In addition, we propose to describe the hysteretic behavior of trickle beds by a modification of the loquid flow texture caused by different values of the apparent solid wettability.
Optimal selection of measuring points in complex plants by linear models
Authors: Madron František , Veverka Vladimír
Published: June 2004

For reliable information on operating plants it is essential to design measuring points well by selecting directly measured quantities from the set of all measurable quantities.

This article deals with a new method for optimizing measurement design. It is based on multiple Gauss‐Jordan elimination of the system of linear mathematical model equations and solves the problem of instrumentation design in new plants as well as the problem of optimizing existing measuring systems. Optimization methods for linear objective functions and for objective functions of general type are proposed. The method also offers a complex classification of quantities (observability and redundancy). After the optimization, the problem is presolved and is ready for an optimal processing of measured data. The mathematical model is reduced to the minimum set of equations and quantities relevant to the solution of a given problem. From a numerical standpoint, the solution is efficient.
Catalyst sintering in fixed‐bed reactors: Deactivation rate and thermal history
Authors: Blasco Valero , Royo Carlos , Monzón Antonio , Santamaría Jesús
Published: June 2004

An experimental study has been carried out of the in situ thermal degradation of a commercial dehydrogenation catalyst in a fixed‐bed reactor. The activity of catalyst particles sampled at different reactor positions after several reaction‐regeneration cycles has been related to their time‐temperature history, according to the following kinetics:

The above equation represents the loss of activity under coke‐free conditions, that is, activity loss due to sintering. The results of the study show that important differences in catalytic activity can be obtained for different positions of a fixed‐bed catalytic reactor after several operation‐regeneration cycles.
Experimental studies of restricted protein diffusion in an agarose matrix
Authors: Boyer Philip M. , Hsu James T.
Published: June 2004

The diffusion of proteins in polymer matrices is an important step in the adsorptive and chromatographic processes used for protein purification, as it is often rate‐limiting. Methods for the estimation of the intraparticle diffusivity in polymer gels have been developed and were applied to the diffusion of seven model proteins in agarose particles. The intraparticle diffusivity was not affected by particle diameter. A correlation based on the restricted diffusion model of Ogston et al. (1973) and Cukier (1984) has been proposed. This correlation allows the estimation of protein diffusivity in these matrices based on the molecular weight of the protein and the polymer concentration.
Kinetics of the CO oxidation by O2 and N2O over Cu‐Cr/Al2O3
Authors: Dekker Nico J. J. , Hoorn Johan A. A. , Stegenga Sander , Kapteijn Freek , Moulijn Jacob A.
Published: June 2004

The oxidation of CO by O2 and N2O over an oxidized 10 wt. % Cu‐Cr/Al2O3 catalyst (Cu:Cr=1:1) has been studied by temperature‐programmed reactivity measurements (400–550 K) over a wide range of partial reactant pressures, including inhibition by CO2. The CO oxidation rate is zeroth‐order in oxygen and has orders between 0‐1 in CO and N2O, depending on the gas‐phase composition. Mechanistic information from literature combined with the kinetic data resulted in the selection of an Eley‐Rideal‐type of kinetic model without a priori assumptions on rate‐determining processes. The model consists of the oxidation of reduced sites by O2 and/or N2O, followed by a reaction with CO, yielding a surface intermediate that releases CO2 in a consecutive step. CO2 inhibits both by reversible adsorption on oxidized and reduces sites, the latter under formation of the surface reaction intermediate. Apart from the surface oxidation by O2, the reaction rates of all assumed elementary processes are of the same order of magnitude and, therefore, determine the overall rate. The surface oxidation by oxygen is about four orders of magnitude larger, which explains the zeroth‐order in oxygen and the observation that oxygen first reacts with CO before N2O is able to oxidize CO. The obtained activation energies of the elementary processes agree with values in the literature for corresponding systems.
A mathematical model of mixing and polymerization in a semibatch stirred‐tank reactor
Authors: Tosun Güray
Published: June 2004

Effects of mixing on number‐ and weight‐average degree of polymerization in free radical homopolymerization in solution in a semibatch stirred tank‐reactor have been modeled by use of the “partially segregated feed” model of Villermaux (1989) and the lumped kinetic treatment of addition polymerization. Mixing conditions are described in terms of the dimensionless time constants θM and θX for convective and diffusive mixing, respectively. Results show that DPn is relatively insensitive, while DPw is highly sensitive to mixing conditions. Comparison with ideal mixing results shows that polydispersity (DPw/DPn) can rise greatly as mixing becomes increasingly nonideal, depending on the value of dimensionless flow rate and dimensionless initiator rate constant. Time constants of the mixing model need to be expressed as dimensional correlations before the proposed polymerization model can be compared to experimental measurements. The model may be adapted to melt polymerization in nonideal CSTR such as LDPE.
Kinetics of carbon dioxide and methyldiethanolamine with phosphoric acid
Authors: Cordi E. M. , Bullin J. A.
Published: June 2004

The Kinetics of the reaction between carbon dioxide and methyldiethanolamine (MDEA) were studied in the presence of various amounts of phosphoric acid at 298 K, 308 K, and 318 K. A stirred cell with a known interfacial area was used at atmospheric pressure to measure the volumetric absorption rate of CO2 flowing through the vessel.

The addition of phosphoric acid to an aqueous solution of MDEA resulted in a Bronsted acid‐base neutralization reaction. While virtually all of the acid was consumed by the neutralization reaction, a large part of the MEDA remained in its free form. The absorption of carbon dioxide in these solutions was slower than expected, given the amount of free MDEA present. The rate depression in the presence of hydrogen phosphate anions was attributed to the stabilization of the reaction transition state between MDEA, water, and carbon dioxide through charge dispersal.
Application of neutron radiography to image flow phenomena in porous media
Authors: Jasti J. K. , Fogler H. S.
Published: June 2004

Neutron transmission imaging is a technique ideally suited for imaging various flow phenomena in consolidated porous media because the porous matrix is virtually transparent to thermal neutrons. Hydrogen‐containing fluids, however, provide a contrast in the image due to their large thermal neutron cross‐sections. This technique has uncovered new phenomena in the study of dissolution and precipitation in porous media. Neutron images of acid‐etched porous patterns show that the degree of branching, and the tortuosity of the etched patterns depend on competing effects of acid flow rate, dissolution rate and precipitation rate of the reaction products. The fine structure of etched pathways show that they can propagate against the direction of flow, a phenomenon not previously observed. In a second study, realtime neutron transmission imaging was used to visualize miscible tracer dispersion. It was shown how quantitative information such as the in‐situ spatial distributions of the tracer concentrations can be obtained with this technique.
Theory of precipitation of protein mixtures by nonionic polymer
Authors: Mahadevan Hari , Hall Carol K.
Published: June 2004

A theory is developed to predict the solubility of protein mixtures in solutions containing nonionic polymer. Effective protein‐protein interactions due to polymer are taken to be volume‐exclusion potentials derived using statistical mechanics. Statistical‐mechanical perturbation theory is used to calculate chemical potentials. The effects of protein size, mole fraction and polymer concentration on solubility are explored. The theory is extended to include electrostatic interactions. The excess chemical potential of the proteins due to the charges on all species is calculated using the mean spherical approximation for a mixture of charged hard spheres. The theory predicts: the larger protein is preferentially precipitated over the smaller one; the more concentrated protein is more likely to precipitate; and increasing the charge of a particular protein reduces its ability to precipitate.
Parameter estimation from multiresponse data
Authors: Stewart Warren E. , Caracotsios Mike , Sørensen Jan P.
Published: June 2004

Multiresponse data are familiar outputs of experiments and processes involving multicomponent mixtures, multiple streams or multiple methods of observation. The resulting arrays of data exhibit various structures, including rectangular (no missing values), block‐rectangular, and irregular (missing values with no simple pattern). Methods for modeling such data are described, emphasizing parameter estimation strategy and available software. Experiences in multiresponse modeling are then reviewed for several chemical engineering problems.
Adsorption breakthrough behavior: Unusual effects and possible causes
Authors: Park Illam , Knaebel Kent S.
Published: June 2004

Adsorption of water vapor on silica gel, at influent humidities from 6 to 80% at 25 and 50°C, yielded breakthrough curves of unusual shapes. Breakthrough patterns varied from the expected sigmoidal shape at low humidity to a curve resembling the tangent function, but symmetric about the stoichiometric breakthrough time. Unusual shapes were found to be due to subtle combination of Type‐IV isotherm behavior and heat effects. A mathematical model was developed to simulate the performance. The results show that complex breakthrough behavior need not be ascribed to complicated causes (such as diffusion in bidisperse pores), which require multiparameter fitting of experimental data. In fact, the effects may be predicted from properties measured in simple independent experiments, though some care is required to account for the effects accurately.
Effect of particle size distribution in different fluidization regimes
Authors: Sun Guanglin , Grace John R.
Published: June 2004

The effect of particle size distribution (PSD) on the performance of a fluidized bed reactor was investigated in different hydrodynamic regimes: bubbling, slugging, turbulent and fast fluidization, with three particle size distributions, all with the same mean diameter and nearly the same particle density and BET surface area. Regime transitions were examined by measuring pressure fluctuations. Void sizes tended to be smaller and the transition from bubbling or slugging to turbulent fluidization was achieved earlier for the wide distribution powder than for the narrow PSD. At gas velocities ≤ 0.2 m/s, the conversion and reactor efficiency were not affected greatly by the PSD. However, at higher gas velocities, PSD played a significant role. For particles of wide size distribution, the conversion in the turbulent and fast fluidization regimes was usually higher than in the bubbling fluidization regime at the same dimensionless rate constant, kf′. On the other hand, for particles of narrow size distribution, the dependence of conversion on the regime is small, except for the fast fluidization regime.
Estimation of surface tension of pure fluids using the gradient theory
Authors: Pérez‐López Juán H. , González‐Ortiz Luis J. , Leiva Miguel A. , Puig Jorge E.
Published: June 2004

The modified van der Waals theory of interfaces (gradient theory) is used with the equations of state of Peng‐Robinson and Mohanty‐Davis to predict surface tensions of various nonpolar, quadrupolar, slightly polar and polar substances as a function of temperature. The influence parameter of the gradient theory is estimated as 0.785ab2/3 in terms of the attractive and repulsive van der Waals parameters using the Lennard‐Jones potential. The predictions made with the Mohanty‐Davis equation of state agree better with experimental data than those made with Peng‐Robinson, even for nonpolar fluids.
Caffeine extraction rates from coffee beans with supercritical carbon dioxide
Authors: Peker Hulya , Srinivasan M. P. , Smith J. M. , McCoy Ben J.
Published: June 2004

The extraction of caffeine from whole coffee beans with supercritical carbon dioxide was studied in a continuous‐flow extraction apparatus. Decaffeination rates were determined as a function of CO2 flow rate, temperature and pressure by continuously monitoring the caffeine in the effluent with a flame ionization detector. Soaking the raw beans in water prior to decaffeination enhanced the rate of extraction, which increased markedly with water content. Using CO2 saturated with water also increased the rate of extraction. The rate of decaffeination increased with pressure and temperature and was influenced by both intraparticle diffusion in the water‐soaked beans and external mass transfer. A mathematical model based on a linear‐driving‐force approximation of mass transfer and partitioning of caffeine between the water and the supercritical CO2 describes the time‐dependent process. The partition coefficient for caffeine distributed between water and supercritical CO2, the only parameter determined from the dynamic extraction rate data, increases with temperature and pressure.
Steady‐state traveling waves on the surface of a viscous liquid film falling down on vertical wires and tubes
Authors: Trifonov Yu. Ya.
Published: June 2004

Various nonlinear wavy regimes of a viscous liquid film flowing down vertical wires and tubes were calculated using the integral method. The linear stability analysis of the trivial smooth solution was compared with the results published previously. In the region of linear instability, the competition among the gravity, viscous and capillary forces formed the steady‐state traveling solutions of finite amplitudes. At least two families of waves were shown to be parameterized by the wave number for given values of external parameters (Reynolds number, cylinder radius and physical characteristics of liquid). The basic waves characteristics depended on external parameters and on wave number. The intensity of wavy processes increased with decreasing cylinder radius. The calculations show the catastrophic growth of wave amplitude, when the system flows down a vertical tube of sufficiently small radius and moves into the linear, unstable region.
Intraparticle convection effect on pressurization and blowdown of adsorbers
Authors: Lu Z. P. , Loureiro J. M. , LeVan M. D. , Rodrigues A. E.
Published: June 2004

Pressurization and blowdown steps in pressure swing adsorption (PSA) with binary mixtures of inert and adsorbable species are studied. Modeling involves mass balances for the bulk fluid phase and inside particles, that is, intraparticle diffusion/convection models, momentum balance equations, and linear adsorption equilibrium isotherm. The importance of intraparticle convection in PSA is assessed. Mass transfer inside pores is enhanced by intraparticle convection, which leads to a better efficiency of adsorption (pressurization) and desorption (blowdown) processes. Performance is improved by intraparticle convection and lies between diffusion and equilibrium limits, as shown in propagation profiles of the adsorbable species mole fraction in the bed and inside particles.
Finite element analysis of viscous mixing with a helical ribbon‐screw impeller
Authors: Tanguy P. A. , Lacroix R. , Bertrand F. , Choplin L. , de la Fuente E. Brito
Published: June 2004

A finite element analysis was carried out to study the mixing patterns of a viscous Newtonian fluid into a stirred tank. A helical ribbon screw impeller (HRS) was used as the mixing device. The numerical simulation involved the full three‐dimensional resolution of the equations of change governing the flow in the tank. The circulation time and the torque were determined and compared with experimental data. A good agreement was found, showing the usefulness of the numerical approach for design purposes.
Settling and rheology of suspensions of narrow‐sized coal particles
Authors: Turian Raffi M. , Hsu Feng‐Lung , Avramidis Kostas S. , Sung Dong‐Jin , Allendorfer Robert K.
Published: June 2004

Settling rates, yield stresses, and shear‐stress/shear‐rate dependences were determined as functions of solids concentration for suspensions in water of coal particles of narrow size fractions. Particles and suspensions are characterized by coal analyses, determination of heating value, solid heat capacity and thermal conductivity, particle size analyses, and determination of densities and maximum packing concentrations for each particle size range. Analysis of the settling rate data establishes that the usually strong retardation of (hindered) settling due to concentration is compounded by particle shape effects. In addition, observed particle shapes are subsumed within particle shape classes inferred from the settling data. Shear‐stress/shear‐rate data for concentrated suspensions of the three larger and relatively narrower sized particles over the broad range of shear rates from about 1 to 104S−1 suggest the behavior to be essentially Newtonian, albeit with a marked particle concentration augmentation. Suspensions of the smallest coal particles of broader size distribution were strongly non‐Newtonian, possessing a yield stress at low shear, essentially power‐low shear‐thinning behavior over the intermediate range and approaching high‐shear Newtonian limiting behavior above about 103S−1. Relations developed include viscosity‐and yield stress‐concentration correlations incorporating the maximum packing volume fraction of solids. These are useful generally for suspensions and particularly as the basis for guiding the formulation of coal‐water fuels.
Coking and activity of porous catalysts in supercritical reaction media
Authors: Baptist‐Nguyen Sarah , Subramaniam Bala
Published: June 2004

A mathematical model is presented to describe coking and activity characteristics of porous catalysts in supercritical reaction mixtures. These characteristics are determined by the way in which temperature and pressure affect the simultaneous physicochemical rate processes involving the effective diffusion of reactants in the pore, coke formation, and coke extraction. For a parallel coking reaction, the model predicts that when the reaction mixture density is isothermally increased from subcritical to low‐to‐moderate values, the ensuing higher reaction rates and restricted diffusion lead to pore‐mouth plugging and decreased catalyst effectiveness factors. At dense supercritical conditions, however, the enhanced coke solubilities in the reaction mixture alleviate pore‐mouth restrictions, resulting in a recovery of catalyst activity and increased effectiveness factors. The model predicts an optimum supercritical density at which catalyst activity is maintained at a maximum value. At smaller than optimum density values, the reaction rate is limited by coke extraction and at larger than optimum values, the rate is subject to pore diffusion limitations. These predictions are qualitatively consistent with reported experimental observations.
Numerical calculation of wall‐to‐bed heat‐transfer coefficients in gas‐fluidized beds
Authors: Kuipers J. A. M. , Prins W. , Van Swaaij W. P. M.
Published: June 2004

A computer model for a hot gas‐fluidized bed has been developed. The theoretical description is based on a two‐fluid model (TFM) approach in which both phases are considered to be continuous and fully interpenetrating. Local wall‐to‐bed heat‐transfer coefficients have been calculated by the simultaneous solution of the TFM conservation of mass, momentum and thermal energy equations. Preliminary calculations suggest that the experimentally observed large wall‐to‐bed heat‐transfer coefficients, frequently reported in literature, can be computed from the present hydrodynamic model with no turbulence. This implies that there is no need to explain these high transfer rates by additional heat transport mechanisms (by turbulence).

The calculations clearly show the enhancement of the wall‐to‐bed heat‐transfer process due to the bubble‐induced bed‐material refreshment along the heated wall. By providing detailed information on the local behavior of the wall‐to‐bed heat‐transfer coefficients, the model distinguishes itself advantageously from previous theoretical models. Due to the vigorous solids circulation in the bubble wake, the local wall‐to‐bed heat‐transfer coefficient is relatively large in the wake of the bubbles rising along a heated wall.
Robust stabilization of input/output linearizable systems under uncertainty and disturbances
Authors: Arkun Yaman , Calvet Jean‐Paul
Published: June 2004

In recent years, differential geometric techniques have been used to transform nonlinear systems into linear systems. Once such equivalent linear systems are obtained, classical linear controllers are designed to achieve desired stability and performance properties. A major critism against these techniques is their lack of guarantee for robustness. In particular, the design of controllers for transformed nonlinear systems under the influence of both disturbances and (parametric) modeling errors is not well‐known. This article presents a methodology to design robust stabilizing controllers for such uncertain and perturbed nonlinear systems.

For feedback linearizable systems, the method guarantees that the nonlinear system has nominally linear input/output dynamics and is stable for the given class of bounded parametric uncertainty and disturbances. The new concepts and the proposed design procedure are shown for an isothermal reactor with second‐order kinetics.
Prediction of axial concentration profiles in catalytic fluidized reactors
Authors: van der Vaart D. R.
Published: June 2004

A previous study by Fryer and Potter (1976) of the catalytic decomposition of ozone in a bubbling fluidized‐bed reactor is considered in the light of a recently developed reactor model. It is shown that since in‐bed sampling leads to an averaged concentration that is biased toward the emulsion‐phase gas composition, an increase in the ozone concentration in the sampled gas is predicted as the probe is raised above the bubbling bed. The analysis, which does not have to assume gas backmixing, shows that the minima in the ozone concentration profiles associated with this transition occurs between Hmf and H, the expanded bed height, and may even occur at lower points in the bed at higher excess fluidizing velocities. The model also correctly predicts the overall conversion of ozone as a function of the reaction rate constant and gas velocity without resorting to the fitting of any parameters.
Dynamic analysis of heterogeneous azeotropic distillation
Authors: Widagdo Soemantri , Seider Warren D. , Sebastian Donald H.
Published: June 2004

A model and algorithm are presented for the separation of mixtures where the phase distribution on the trays is extremely uncertain, as it occurs when the mixtures have two partially‐miscible binary pairs and a minimum‐boiling ternary azeotrope. Included are an algorithm for the consistent initialization of index‐I differential/algebric equations, a novel algorithm for branch switching when the phase distribution changes at a real bifurcation point, and a reliable algorithm for phase stability analysis. Open‐loop responses are presented for the dehydration of secbutanol with disecbutylether in single‐stage, 12‐tray, and 33‐tray separators. These simulation results for the 33‐tray tower are in qualitative agreement with experimental measurements for the ARCO SBA‐II tower.
Identification and application of partial chemical equilibria in reactor modeling
Authors: Norval Graeme W. , Phillips M. Jane , Missen Ronald W. , Smith William R.
Published: June 2004

A method is presented for identifying a subset of species (from a set of reacting species) that are within a specified numerical tolerance of equilibrium, as implied by a given experimental composition. The method also determines a linearly independent set of reactions involving these species and the number of stoichoimetric restrictions inherent in the set. This is an aid in reactor modeling, for example, in the computation of concentration profiles, in examining kinetic schemes or reaction mechanisms, and in identifying rate‐controlling processes. An example involving the methanol‐to‐gasoline synthesis is given to illustrate the method.
The curious behavior of homogeneous azeotropic distillation—implications for entrainer selection
Authors: Laroche L. , Bekiaris N. , Andersen H. W. , Morari M.
Published: June 2004

We examine the simplest homogeneous azeotropic distillation sequence of industrial relevance, where an entrainer is added to a binary azeotrope to recover both azeotropic constituents as pure products. Despite its apparent simplicity, such distillation columns can exhibit an unusual behavior not observed in zeotropic distillation:

For some mixtures, separation as a function of reflux goes through a maximum. At infinite reflux, no separation is achieved.

In some cases, achieving the same specifications with a larger number of trays requires a larger reflux.

Sometimes the only feasible separation yields the intermediate component as a pure distillate, while the bottom product contains the light and heavy components.

Sometimes the only feasible separation yields the intermediate component as a pure bottom product while the distillate contains the light and heavy components.

While these unusual features can be regarded as curiosities, they are essential for proper entrainer selection and design. For a minimum boiling azeotrope, the existing and conflicting entrainer selection rules state that one should use a component that introduces no distillation boundary between the azeotropic constituents (Doherty and Caldarola, 1985), and either a low or high boiling component that introduces no additional azeotrope or a component which introduces new minimum boiling azeotropes (Stichlmair et al., 1989). By taking advantage of the curious aforementioned features, as well as our experience involving more than 400 mixtures, we have been able to analyze the assumptions behind these criteria, show when those assumptions break down, and therefore understand the limitations of the criteria.
Parallel processing strategies for chemical process flowsheeting
Authors: Vegeais James A. , Stadtherr Mark A.
Published: June 2004

The potential for using the sequential‐modular, simultaneous‐modular, and equation‐based approaches to process flowsheeting on multiprocessing (parallel processing) computer architectures is assessed. The simultaneous‐modular and equation‐based problem formulations appear to be the most promising, but the sparse matrix techniques currently used, especially for the latter, do not perform well on parallel machines. We consider two different decomposition schemes for parallelizing the sparse matrix problems that must be solved. These are the bordered‐diagonal form and the bordered‐block‐diagonal form. The latter takes advantage of the inherent block structure of the flowsheeting matrix and represents a promising strategy for solving flowsheeting problems on parallel machines, especially for large problems.
A kinetic model for the fungal pellet lifecycle
Authors: Michel Frederick C. , Grulke Eric A. , Reddy C. Adinarayana
Published: June 2004

Many industrially significant microbial products are produced during secondary metabolism by fungal pellets. Most models of mycelial pellet growth, however, consider only the primary growth phase. During secondary metabolism, the whiterot fungus Phanerochaete chrysosporium produces extra‐cellular lignin peroxidases and manganese‐dependent peroxidases which have been implicated in the degradation of lignin and various xenobiotics. This unstructured kinetic model for mycelial pellet cultures of P. chrysosporium describes the culture dry weight, respiration, glucose consumption, nutrient nitrogen uptake, pellet size and oxygen effectiveness factor during the lag, primary growth, secondary metabolic, and death phases. During secondary metabolism, the intraparticle diffusion of oxygen limits metabolism. Parameters for respiration during secondary metabolism are validated using three separate measurements: 1. by measuring oxygen concentration profiles in mycelial pellets using oxygen microelectrodes; 2. by measuring the depletion of oxygen from the culture fluid using a respirometer; and 3. by measuring the daily evolution of carbon dioxide. The physiological state of the mycelial pellets is predicted for various culture conditions and is related to the production of LIP and MNP during secondary metabolism.
Unified approach to solving optimal design‐control problems in batch distillation
Authors: Diwekar Urmila M.
Published: June 2004

This article presents a unified approach to simultaneous solution of optimization and optimal control problems in batch distillation, operating under different modes of operation: variable, constant, or optimal reflux. The simplified, computationally efficient short‐cut method and a novel algorithm to solve the optimal control problems in batch distillation is the basis of this unified approach. The short‐cut method identifies the feasible region of operation essential for optimization and optimal control problems, and provides analytical partial derivatives of the model parameters crucial to the solution.

The new algorithm for the solution of optimal combination of the maximum principle and NLP optimization techniques. It circumvents the problems associated with the maximum principle approach (iterative solution of a two‐point boundary value problem, undounded control variables, and inability to handle the simultaneous optimization and optimal control problem), and the coupled ODE discretization‐NLP optimization scheme for nonlinear models (higher system nonlinearities, multiplicity of solutions, sensitivity of convergence to initial guesses). This algorithm reduces the dimensionality of the problem, and the nature of the algorithm allows a common platform to optimal solutions of different operating conditions. This article also shows that different categories of the optimal control problems in batch distillation essentially involve the solution of the maximum distillate problem.
Chemometric methods for process monitoring and high‐performance controller design
Authors: Kaspar Michael H. , Ray W. Harmon
Published: June 2004

A huge amount of data is collected by computer monitoring systems in the chemical process industry. Such tools as principal component analysis and partial least squares have been shown to be very effective in compressing this large volume of noisy correlated data into a subspace of much lower dimension than the original data set. Because most of what is eliminated is the collinearity of the original variables and the noise, the bulk of the information contained in the original data set is retained. The resulting low dimensional representation of the data set has been shown to be of great utility for process analysis and monitoring, as well as in selecting variables for control. These types of models can also be used directly in control system design. One way of approaching this is to use the loading matrices as compensators on the plant. Some advantages of using this approach as part of the overall control system design include automatic decoupling and efficient loop pairing, as well as natural handling of nonsquare systems and poorly conditioned systems.
Enthalpy change on vaporization of aqueous and methanolic formaldehyde solutions
Authors: Liu Y.‐Q. , Hasse H. , Maurer G.
Published: June 2004

A physicochemical model for vapor‐liquid equilibrium in multicomponent formaldehyde‐containing mixtures (Maurer, 1986; Hasse et al., 1990; Hasse and Maurer, 1991a). is extended to describe enthalpy changes upon vaporization. Predicted enthalpy changes are compared with new experimental results. The measurements were carried out with a thin‐film evaporator flow‐calorimeter (Liu and Maurer, 1991). More than 80 experimental data points for mixtures of formaldehyde + water and formaldehyde + methanol + (small amounts of water) are reported. The data cover the temperature range from 323 to 363 K (formaldehyde + water) and 312 to 347 K (formaldehyde + methanol) at liquid phase formaldehyde mole fractions up to about 0.3 (formaldehyde + water) and 0.6 (formaldehyde + methanol). Comparisons between predicted and experimental results for the enthalpy change reveal deviations of only a few percent. Further improvements are achieved by fitting some model parameters, which originally were estimated from noncalorimetric data.
Estimation of multiple specific growth rates: Experimental validation
Authors: Pomerleau Yves , Perrier Michel
Published: June 2004

The experimental validation of on‐line estimation of multiple specific growth rates for the bakers' yeast fed‐batch process is presented. Pole placement based parameter estimation combined with an asymptotic biomass observer constitute the basic algorithm. The full process model being ill‐conditioned for estimation using the available measured state variables, the use of two partial models related to two different states of the process is suggested. An alternating procedure between two sets of estimation algorithms designed from the partial models is proposed. The performance of the alternating procedure is validated both with simulated and experimental data. The accuracy of the estimates of the three specific growth rates involved in this process is verified according to two criteria based on the respiratory quotient and on the evaluation of the ethanol production/consumption rate.