bluuebunny/crossref_metadata_2025 huggingface dataset:
Organic microporous materials made by bicontinuous microemulsion polymerization
Authors: Burban J. H. , He Mengtao , Cussler E. L.
Published: June 2004

Microporous solids made by polymerizing the organic phase of bicontinuous microemulsions stabilized with didodecyldimethylammonium bromide show surface areas as high as 70 m2/g. Small‐angle X‐ray scattering measurements on microporous copolymers containing polymethylmethacrylate show that these areas result from structures larger than 250 Å that are generated during polymerization. These X‐rays measurements also show that the 17‐Å structures characteristics of the original microemulsion are retained during polymerization, but destroyed when the detergent is extracted and the solid is dried.
Controlled drop tensiometer for measuring dynamic interfacial and film tension
Authors: Nagarajan R. , Koczo Kalman , Erdos Elemer , Wasan Darsh T.
Published: June 2004

A dynamic interfacial tensiometer described measures the tension of stressed liquid‐liquid interfaces. In this accurate and controlled method, the time‐dependent pressure difference across the curved interface of a spherical drop, immersed in a second immiscible phase, is measured as a function of time. Dynamic experiments are conducted by varying the drop size, therby stressing the interface. The technique is eminently suited for the measurement of static and dynamic interfacial tensions, the study of the diffusion, adsorption, and desorption kinetics of surfactants, and the rheological behavior of liquid films. These capabilities are demonstrated for n‐dodecane contacted with an aqueous solution of a nonionic surfactant. To explain the phenomena, diffusion models are developed that compare well with the experimental data. The noninvasiveness and versatility make this instrument a useful tool.
Computer simulation of protein refolding pathways and intermediates
Authors: Gupta Prashant , Hall Carol K.
Published: June 2004

Computer simulation studies of refolding pathways and the formation of intermediates for a simple, 2–D lattice protein model are presented. The sequence of the 20–bead model protein chain is chosen so that hydrophobic beads will reside in the protein interior in the native state. Nonbonded hydrophobic beads attract each other with strength ϵ decreasing the |ϵ/kT| mimics increasing the concentration of the denaturant. Dynamic Monte Carlo simulations and exhaustive conformational searches have been performed on an isolated model protein sequence at different levels of |ϵ| (different denaturant concentrations). As the denaturant is withdrawn, the model protein exhibits a transition from a random coil state to a compact native state with a hydrophobic core. The refolding process is observed to be cooperative in that the chain does not start folding until the middle section has folded correctly, and proceeds along preferred pathways that are populated by distinct, partially folded intermediates.
Salting out of aqueous proteins: Phase equilibria and intermolecular potentials
Authors: Coen C. J. , Blanch H. W. , Prausnitz J. M.
Published: June 2004

Salting‐out phase equilibria are reported for lysozyme and α‐chymotrypsin from concentrated ammonium‐sulfate solutions. Supernatant and dense‐phase protein concentrations and the resulting protein partition coefficients are given as a function of solution pH and ionic strength. Phase equilibria with a trivalent salt (sodium citrate) confirm that ionic strength, rather than salt concentration, is the appropriate variable describing phase equilibria. The salting‐out behavior of a mixture of an aqueous lysozyme and α‐chymotrypsin is independent of the presence of the other protein.

Parameters for a molecular‐thermodynamic description of salting‐out behavior are obtained from low‐angle laser‐light scattering (LALLS). Osmotic second virial coefficients from LALLS are reported over a range of pH for dilute chymotrypsin concentrations in aqueous electrolyte solutions at 0.01 and 1.0 M ionic strengths. Effective Hamaker constants, regressed from experimental osmotic second virial coefficients, are determined for models of the protein – protein potential of mean force. In addition to excluded volume, dispersion, and shielded charge – charge potentials, the description of protein ‐ protein interactions includes attractive charge – dipole and dipole – dipole potentials as well as an osmotic‐attraction potential that becomes important at high salt concentrations. Protein dipole – dipole potentials are required to account for the observed pH dependence of osmotic second virial coefficients, especially at low ionic strength.
Molecular thermodynamic model for Helix‐Helix docking and protein aggregation
Authors: Chen Chau‐Chyun , King Jonathan , Wang Daniel I. C.
Published: June 2004

The formation of aggregates, rather than correctly folded polypeptide chains, is a pressing problem in biotechnology that has been difficult to approach quantitatively. The competition between folding and aggregation has been carefully analyzed for bovine growth hormone (bGH) and can be attributed to incorrect helix‐helix docking for this four‐helix bundle protein. An extended molecular thermodynamic model reported here represents Gibbs energy changes associated with intramolecular and intermolecular helix‐helix dockings occurring during protein folding and protein aggregation. The model incorporates (1) a semiempirical local composition Gibbs energy expression to account for the helix‐helix hydrophobic interactions, which favor helix‐helix docking and aggregation and (2) a Flory‐Huggins‐type Gibbs energy expression to describe the configurational entropy of the polypeptide backbone conformation, which favors disaggregation. For the folding and aggregation of bGH, the molecular thermodynamic model provides estimates for the Gibbs energies and thermodynamic stabilities of various conformations of bGH and qualitatively accounts for the competition between aggregation and productive folding. It also successfully describes the inhibition of aggregation found with peptides corresponding to bGH helical sequences and the effect of site‐directed mutagenesis.
Multiple roll systems: Steady‐state operation
Authors: Benjamin Dean F. , Anderson Todd J. , Scriven L. E.
Published: June 2004

Roll coaters for applying liquid coating to continuous strip or web are, with some two‐roll exceptions, systems of three or more rolls in which liquid passes through two or more gaps or nips between rolls. Yet most of the literature on roll coating is devoted to some of the 11 distinct flows in individual gaps or nips. This article analyzes how the final coated layer thickness in several types of forward roll transfer and reverse roll coating systems depends, at steady state, on the number of rolls, their speeds, the gaps between roll pairs, and the doctoring of recycle films from the rolls. The inputs to the analysis are elementary mass balances at the gaps, and simple gap performance equations that approximate well the available experimental and theoretical findings about flow rates and film‐splits at individual gaps. The results are fundamentals‐based means of understanding, comparing, predicting, and ultimately designing performance of multiple roll systems.
Aspects of flow of power‐law fluids in porous media
Authors: Shah C. B. , Yortsos Y. C.
Published: June 2004

Non‐Newtonian fluid flow in porous media is encountered in a variety of applications. Aspects of single‐phase flow of power‐law fluids in porous media are examined. First, homogenization theory is used to derive a macroscopic law. It is shown that the single‐capillary power law between flow rate and pressure gradient also applies at the macroscopic scale, provided that the Reynolds number is sufficiently small. Homogenization theory confirms the validity of the use of pore network models to describe flow of power‐law fluids, although not necessarily of fluids of a more general rheology. Flow in pore networks is next used to explore various pore geometry effects. Numerical simulations show that approaches based on an effective medium or on the existence of a critical path, which carries most of the flow, are valid in narrow‐ or wide‐pore‐size distributions, respectively. The corresponding expressions agreed well with the numerical results in the respective ranges. An analysis presented for Bethe lattices leads to closed‐form expressions in two limits: for an effective medium and near percolation. The behavior near percolation generalizes the results of Stinchcombe (1974) for the linear (Newtonian) case.
Convective diffusion and adsorption in a swarm of spheroidal particles
Authors: Coutelieris F. A. , Burganos V. N. , Payatakes A. C.
Published: June 2004

The problem of mass transfer from a Newtonian fluid to a swarm of spheroidal adsorbers under creeping flow conditions is considered using the spheroid‐in‐cell model to represent the swarm. The flow field within the fluid envelope for the Kuwabara type of boundary conditions is obtained form the analytical solution of Dassios et al. (1994). The complete convective diffusion equation is used to describe mass transport within the envelope so that moderate and strong diffusional terms can be taken into account. A new set of boundary conditions is used that respects mass flux and concentration continuity across the outer surface of the cell and maximizes the applicability of the spheroid‐in‐cell model in the convection‐to‐diffusion transition regime. The resulting elliptic problem in two dimensions is solved numerically. Results for the upstream and downstream concentration profiles reveal that tangential diffusion is very significant and should not be neglected for moderate and low Peclet number values. Also, the classical Levich‐type of formulation, which is theoretically valid for very weak diffusional terms only, can in practice be modified to predict with fair accuracy the overall Sherwood number and the adsorption efficiency of prolate and oblate spheroids‐in‐cell even in moderate Peclet number cases.
Solvent concentration dependence of solute distribution coefficient
Authors: Ching C. B. , Liu X. P. , Hidajat K.
Published: June 2004

A simple method for modeling the solvent concentration dependence of solute distribution coefficient is described. Ideal and modified distribution models are proposed based on the mechanism of multicomponent competitive adsorption and the theory of solvent association. The system cresol‐methanol/water‐β‐CD‐silica is tested using the models, which compared with other experimental results show that the ideal distribution model fails at higher water concentrations and that the modified model can describe well the observed solute distribution behavior over the entire concentration range. The relative impact of nonideal factors in the adsorbed phase and the solvent association in the mobile phase on the distribution dependence shows that the limitation of the ideal model at higher water concentrations results mainly from uneven saturation capacities of the solute and solvent components for the system studied here.
Copper selective adsorption with a microemulsion‐based resin
Authors: Yang James H.‐K. , Burban John H. , Cussler E. L.
Published: June 2004

Cross‐linked terpolymers of allyl acetylacetone, 2‐hydroxyethyl methacrylate, and ethylene glycol dimethacrylate were synthesized in the oil phase of a bicontinuous microemulsion stabilized with didodecyldimethylammonium bromide. The resulting material is porous, with a surface area of 58 m2/dry g. It selectively adsorbs copper, picking up 0.07 meq/dry g from a 0.1‐M solution. This is less than the total apparent capacity of 0.65 meq/dry g inferred from titration with base and much less than the amount in conventional, nonselective resins, which is typically 5 meq/dry g. Breakthrough curves in packed beds and across membranes of this material seem consistent with uptake controlled by chemical reaction compromised by dispersion. Nonporous membranes of this material do not show facilitated transport.
Numerical studies of multicomponent chromatography using pH gradients
Authors: Frey Douglas D. , Barnes Andrew , Strong John
Published: June 2004

A general numerical method is developed for multicomponent chromatography for the case where a pH gradient occurs and several buffering species are present that become adsorbed together with the components being separated through an ion‐exchange mechanism. Acid‐base equilibrium relations are used to determine the ionic compositions in the liquid and adsorbed phases and are solved using a single‐variable Newton's method. The differential material‐balance relations are integrated numerically using the method of characteristics. The transport relations are incorporated using a matrix analog of the linear‐driving‐force approximation. The resulting method accounts for the adsorption of each ionic form of each buffering species, for multicomponent diffusional interactions arising from induced electric fields, for volume and concentration overloading of proteins, and for changes in the adsorption capacity caused by pH variations. Numerical calculations illustrate factors govening the selection of the adsorbent and buffer components for use in separating mixtures of proteins using retained, internally generated pH gradients.
Salt extraction from hydrogen‐sulfide scrubber solution using electrodialysis
Authors: Jamaluddin A. K. M. , Kennedy M. W. , McManus D. , Nazarko T. W.
Published: June 2004

The buildup of undesirable sulfur compounds(sulfates and thiosulfates) reduces the scrubbing effectiveness of the LO‐CAT I autocirculation sulfur recovery process from acid‐gas stream. Among various processes, withdrawing and disposing of a portion of the scrubber solution and replacing this “blowdown” with fresh solution have been the practice in the industry. The application of the electrodialysis system to recycle the blowdown is presented. Experiments were carried out using electrodialysis to separate salts (sulfates and thiosulfates) from the LO‐CAT I autocirculation scrubber solution containing organic chelating agents, iron, and various alkali‐metal inorganic salts. The results indicated that the electrodialysis was successful in removing 50% of the salts from the scrubber solution with less than 8% loss of organic and 8% loss of carbonates. The fluxes of the undesired salt species were high even at low current densities (200 to 400 A/m2).
Transient temperature model for reaction injection molding
Authors: Peebles Samuel M. , Corripio Armando B.
Published: June 2004

A model of the temperature response for reaction injection molding (RIM) operation is developed that assumes uniform mold temperature and a very fast reaction with slow heat transfer from the reacting mass to the mold and negligible heat losses to the surroundings. The model parameters are fit to data from a number of RIM experiments that agree well with published values of the physical constants for the reactants. The simple model can be fitted on‐line to data from the RIM apparatus and the results used to detect and correct deviations of the part properties from its desired values which could otherwise result in the production of many defective parts.
Tortuosity of bubble rise path in a liquid‐solid fluidized bed: Effect of particle shape
Authors: Tsuchiya Katsumi , Furumoto Akihiko
Published: June 2004

Effects of particle properties on the rise of single bubbles in a liquid‐solid fluidized bed ared studied by focusing on the particle‐shape effect at various bed voidages near incipient fluidization of glass beads and sand particles. Experiments, covering the bubble‐size range 6–37 mm and solids holdups as high as 0.94 times that in the packed state, are conducted in a 2‐D column to directly observe the possible relationship between the rate and tortuous path of bubble rise. The shape effect, being significant when the close‐range (surface‐to‐surface) interactions between particles are predominant, is appreciable for the relative solids holdups exceeding 0.9. Marked reduction in the bubble rise velocity is observed as the bubble size is decreased below 8 and 12 mm for the spherical glass and irregular sand particles, respectively. This anomalous reduction stems partly from the tortuousness in the rise path and partly from the “hesitation” in the net vertical movement. The former, in particular, is quantified in terms of the tortuosity of the 2–D rise path. The results support the peculiar trend in the rise velocity observed, especially for the sand particles.
PSA performance of densely packed adsorbent beds
Authors: Shirley Arthur I. , LaCava Alberto I.
Published: June 2004

The separation of air by pressure swing adsorption is improved by filling the interparticle voids in kineticaly selective adsorbent beds with fine particles of the adsorbent or inert material. He ratio of the average diameter of the coarse adsorbent particles to the average diameter of the fine particles, the size of the fine particles themselves, and the percent of volume of the fine particles in the bed are all critical to optimum PA performance.
Modeling chemically active liquid membranes in tubular inorganic supports
Authors: Yi Jongheop , Tavlarides Lawrence L.
Published: June 2004

A numerical model is presented to evaluate the dynamic behavior of mas transfer in the shell‐and‐tube‐type membrane modules for metal ion removal from aqueous streams. The analysis that considers the effects of axial‐flow velocity on the radial pressure difference across the membrane wall permits the specifications of this operating parameter to avoid expression of impregnated carrier from the membranes adn to prevent membrane wall rupture. The model accounts for effects of boundary layer mass‐transfer and kinetic rate resistances at the interfces on membrane flux. The performance of different size membrane modules was estimated for two operating modes of a shell‐and‐tube‐type module: one for the flow of feed solution inside the membrane tube and strip solution flow in the annular region and the other for the flow of strip solution inside the membrane tube and feed solution flow in the annular region of the module. A case system evaluated is copper ion extraction from acidic solutions using 2‐hydroxy‐5‐nonylacetophenone oxime impregnated in α‐alumina/silica ceramic membranes. The performance of shell‐and‐tube‐type membrane modules depends on the sizes and operating methods. In these calculations, the operation condition of no‐pressure difference across the membrane was imposed on the design. This condition is achieved by adjusting the ratio of the flow velocities of the feed and strip solutions within the inside and annular regions of the modules.
Gas separations in hollow‐fiber adsorbers
Authors: Gilleskie Gary L. , Parker Jennifer L. , Cussler E. L.
Published: June 2004

The adsorption of ethane from helium was measured in beds packed with 2.5 μm zeolite crystals and containing either a single hollow fiber or multiple fibers. THe single‐fiber experiments indicate that the mass‐transfer rate in bends containing zeolite 13X is limited by diffusion across the fiber wall and through macropores. FOr adsorption in single‐fiber beds packed with zeolite 4A, mass transfer is limited by micropore diffusion within the particles. Breakthrough curves from beds containign 13X are adequately described with the linear driving force model, while curves from beds containing 4A are consistent with the Rosen model.

Breakthrough curves from beds packed with zeolite 13X and containing multiple fibers can be predicted from the experiments with single‐fiber beds when the fibers are evenly spaced. When fibers are unevenly spaced, the breakthrough curves are more disperse. Unevenly spaced fibers are the normal case. Even when fibers are evenly spaced, the productivity of hollow‐fiber beds is expected to be no greater than that in conventional beds.
Numerical model for the vacuum pyrolysis of scrap tires in batch reactors
Authors: Yang Jin , Tanguy Philippe A. , Roy Christian
Published: June 2004

A quantitative model for scrap tire pyrolysis in a batch scale reactor developed comprises the following basic phenomena: conduction inside tire particles; conduction, convection, and radiation between the feedstock particles or between the fluids and the particles; tire pyrolysis reaction; exothermicity and endothermicity caused by tire decomposition and volatilization; and the variation of the composition and the thermal properties of tire particles. This model was used to predict the transient temperature and density distributions in the bed of particles, the volatile product evolution rate, the mass change, the energy consumption during the pyrolysis process, and the pressure histoiy in a tire pyrolysis reactor with a load of 1 kg. The model predictions agree well with independent experimental data.
Isothermal bistability due to remote control: A model for selective catalytic oxidation
Authors: Rebitzki Timm , Delmon Bernard , Block Jochen H.
Published: June 2004

Special kinetic behaviors in isothermal reactions on heterogeneous catalysts composed of two components with different roles are studied with the focus on hi stabilities that could occur in stirred‐tank flow reactors when control of catalytic sites on one component by mobile (spillover) species produced by the other operates. The comprehensive kinetic model involves: (1) generation of spillover species on the second phase, their transfer to the potentially catalytic phase, and their reaction with this acceptor to generate selective sites; (2) the kinetics of the catalytic reaction. We developed a model for selective catalytic oxidation, where catalyst sites change from nonselective to selective under the influence of the control by spillover oxygen. The model is based on the system of differential equations. By integration and iteration to the steady state for each value of the external control parameter under investigation, the oxygen and hydrocarbon reactant consumption, as well as the state of the catalyst, is calculated. These calculations predict bistable selectivity for certain ranges of concentration in the feed. Data on the boundaries of the hysteresis loops as a function of catalyst composition and oxygen partial pressure are discussed, as well as the variation of these hysteresis boundaries as a function of the internal parameters.
Transient profiles in ion‐exchange displacement chromatography
Authors: Gadam Shishir D. , Gallant Stuart R. , Cramer Steven M.
Published: June 2004

A theoretical and experimental study of displacement development in cation‐exchange displacement chromatography is carried out. Simulated displacement chromatograms are generated using a numericl model of displacement chromatography, which employs the steric mass action (SMA) formalism to describe the multicomponent nonlinear adsorption of proteins and polyelectrolytes. Simulated profiles of transient displacement chromatography are compared with experimentla displacement over a ranve of operating conditions, and the model accurately predicts transient displacement behaviour. The success of the model indicates that the SMA formalism is an appropriate tool for prechromatograms indicate that significantly higher throughputs can be achieved by operating these protein displacement systems under nondeveloped conditions. In addition, displacement development proceeds faster at lower salt concentration, indicating that a reduction of the displacement systems. This work sets the stage for the optimization of protien displacement chromatography.
Binary kinetics in the Y‐Ba‐Cu system: 2. Nanosized particles
Authors: Wang Hong , Thomson William J.
Published: June 2004

The kinetics of binaty mixtures of Y2O3 – BaCO3–CuO (Ba–Cu) and Y2O3 –CuO(Y–Cu) prepared by citrate sot‐gel techniques were studied in both air and helium. Particle sizes resulting from the sol – gel preparation were between 5 and 100 nm, and the prevailing kinetics differed significantly from those observed in the companion study of micron‐sized particles. That is, nucleation‐growth kinetics of the Avrami‐Erofe'eu type adequately model the kinetics over the entire conversion range as opposed to the diffusion mechanisms that describe the kinetics in larger sized particles. An alternate nucleation‐growth model, which neglects overlapping volumes during reaction (Austin – Rickett model), was also adequate in the Ba–Cu system where particle size/spacing was larger, but was only applicable in other reaction systems at lower conversions. Activation energies obtained from the Avrami‐ Erofe'ev were consistently lower than those from the Austin ‐ Rickett model, because the former model contains two parameters that are temperature‐sensitive.
An underwood‐like model for a finite mass exchanger
Authors: Gupta Ashish , Manousiouthakis Vasilios
Published: June 2004

A mathematical model is presented for a multicomponent, two‐stream, countercurrent mass exchanger with a finite number of stages. The model relates the outlet compositions to the inlet compositions and the flow rate ratio of the two streams going through the exchanger, as well as the number of stages in the exchanger. An iterative solution procedure employing this model is proposed for the mass exchanger design problem. The model requires solution of a polynomial equation whose roots may be complex. Bounds to the real roots of the polynomial have also been established. Two examples illustrating how the method is employed are presented. The second example demonstrates that the iterative procedure converges even in the complex domain, and that complex roots exist for a real column.
Design of robust constrained model‐predictive controllers with volterra series
Authors: Genecili Hasmet , Nikolaou Michael
Published: June 2004

Sufficient conditions are developed in this work for robust stability and pefformance of nonlinear model‐predictive control systems that use an end‐condition and second‐order Volterra model with parametric uncertainty in the time domain. The robust stabilfly conditions involve the lengths of the prediction and control horizons, as well as the coefficients of the control move suppression terms in the on‐line objective function. These conditions may be used for both analysis and synthesis purposes. A case study of a chemical reactor is presented to elucidate these issues.
Redundant sensor network design for linear processes
Authors: Ali Yaqoob , Narasimhan Shankar
Published: June 2004

Measurements of process variables have a considerable impact on the control, optimization, safety, and reliability of chemical processes. In a 1993 article, Ali and Narasimhan developed a systematic graph‐theoretic procedure for optimally locating a minimum number of sensors in linear steady‐state processes. The sensor network was designed to maximize the probability of estimating variables when sensors are likely to fail. This article extends that procedure for the optimal design of a redundant sensor network for linear processes. The algorithm proposed also accounts for specifications of measurable and important variables. The efficiency and robustness of the proposed algorithm are demonstrated on realistically large processes.
Wetting of fiber mats for composites manufacturing: I. Visualization experiments
Authors: Chen Yung‐Tin , Davis H. Ted , Macosko Christopher W.
Published: June 2004

Wetting of glass fibers was visualized using an oil with the same refractive index. With both a video‐enhanced microscope and a high‐magnification video camera we followed the flow front and qualitatively measured the entrapped air bubbles by image analysis. Due to different permeabilities between the fiber mats and bundles, air bubbles are entrapped. Two major kinds of air bubbles are observed: small cylindrical micro voids between fiber filaments (inside fiber bundles) and spherical macro voids outside fiber bundles. These air bubbles can be described by three major mechanisms initial liquid bypassing/air trapping, later capillary invasion of disordered fiber bundles, and air bubble mobilization. Both random and unidirectional fiber mats were used in this study. Vacuum as well as different fluid viscosities and surface tensions were also investigated, which led us to guidelines of optimum processing parameters of fiber wetting for resin transfer molding and structural reaction injection molding. They are low viscosity, vacuum, high mold temperature, and high pressure.
Wetting of fiber mats for composites manufacturing: II. Air entrapment model
Authors: Chen Yung‐Tin , Macosko Christopher W. , Davis H. Ted
Published: June 2004

In this work the experimental observations are explained with the aid of a simple air‐entrapment model based on the concept of two levels of porosity of fiber mats. A simple model that includes liquid bypassing with initial air trapping, subsequent capillary invasion of regular fiber bundles with air compression, and finally mobilization is proposed to explain air‐entrapment phenomena. The simple model successfully rationalizes the observed air trapping and compression during initial liquid–fiber contact. An empirical equation for the mobilization efficiency is adapted to the model to estimate void content. The velocity dependence of the trapped void content predicted by the model is in good agreement with observations.
Nitridation and CVD reactions with hydazine
Authors: Vogt Kirkland W. , Kohl Paul A. , Abys Joseph A.
Published: June 2004

The low‐temperature nitridation of gallium arsenide, silicon and transition metals was investigated using hydrazine. Gallium nitride films were grown on gallium arsenide (GaAs) by direct reaction of the semiconductor surface layers with hydrazine et 200‐400°C. Auger electron spectroscopy and X‐ray photoelectron spectroscopy (XPS) analyses show that the films are primarily gallium nitride with a small oxide impurity. Thin nitride films (∼15 Å) were grown on silicon by reaction with hydrazine at 300–500°C. Ellipsometry results suggest that the film growth goes through different phases following linear, parabolic and logarithmic functions with time. XPS analysis shows that the nitride films could be formed at much lower temperatures than possible with ammonia (300 vs. 600°C). The formation of numerous transition metal nitrides (Co, Cr, Fe, Mo, Si, Ta, Ti, V, and W) by reaction with hydrazine at 400°C is demonstrated, as well as the chemical vapor deposition of boron nitride films from diborane and hydrazine reactants. The temperature at the mixing point was critical in determining the final composition of the film. A 1‐D transport model suggests that the reaction rate at 400°C was kinetically limited. The results also agree qualitatively with thermodynamic equilibrium calculations.
Properties of pulsing flow in a trickle bed
Authors: Tsochatzidis N. A. , Karabelas A. J.
Published: June 2004

An experimental study of pulsing flow in trickle beds is presented, based on data from two measuring techniques. New evidence on pulse arrangement and propagation in the bed, and data on basic pulse characteristics (frequency, celerity, length, duration) as well as liquid holdup and pressure drop measurements are included. Some of these data, such as the length of the liquid‐rich zone of pulses, are not currently available. Two flow regions exhibit different trends of pulse characteristics, “mild” and “wild” pulsing for relatively small and large liquid flow rates, respectively. New findings are compared with previously available data and correlations. An effort to develop new or modify existing generalized correlations is made for the aforementioned quantities.
Fractional crystallization: Design alternatives and tradeoffs
Authors: Dye Susan R. , Ng Ka M.
Published: June 2004

Guildlines for the design of fractional crystallization processes to separate two‐and three‐solute mixtures are presented. By using solvent addition/removal, stream combination, and cooling/heating, these processes bypass regions of multiple saturation in the phase diagram and recover purse solutes. Design equations are formulated, and the constraints on the design variables are identified. also included is a discussion of the effect on recyle flows of changes in the design variables and an estimate of the cost of a fractional crystallization separation.
Optimal liner regualtion with hard constraints
Authors: Ajbar Hamid , Keenan Michael R. , Kantor Jeffrey C.
Published: June 2004

An l∞ approach to the design of linear multivariable controllers for descrete‐time systems with hard time‐domain constraints is presented. The notion of polar of the set of the exogenous inputs is used to parameterize the set of closed‐loop transfer functions that meet regulation constraints. The constraints may include magintude and rate bounds on all relevant process variables, including the control inputs. Solutions for optimal l∞ design are found by solving a linear program for the impulse‐response coefficients of the controllers, or for the coefficients of an ARMA controller model. Using these formulation, s and analytical frame work is provided for delineating the tradeoffs that govern design of linear control systems.
Axial dispersion of solid particles in a continuous rotary kiln
Authors: Kohav Tamar , Richardson James T. , Luss Dan
Published: June 2004

Six algorithms following single particle trajectories are used to predict the axial dispersion in rolling or slumping flow in a continuous rotary kiln. Models incorporating different physical phenomena show that axial dispersion is affected by the Froude number, L/D ratio, solid fill level, and rolling or slumping layer thickness. The main cause of axial dispersion is segregation of the nonuniform particle size, density and shape. In a rolling bed with uniform particles the rolling layer thickness and the time of roll have to be accounted for in the prediction of the dispersion. The Peclet numbers computed for sold particles with uniform physical properties are of the order of 104. The sold segregated motion may lead to Peclet numbers of the order of 10 – 103, a much larger dispersion. The main obstacle for a‐priori prediction of the axial dispersion is lack of a reliable relation between the segregated roll or slump distance and the variation in particles properties, as well as the change in segregation as the flow properties of kparticles are changed during calcination.
Reaction rate modeling in noncatalytic gas‐solid systems: Species transport and mechanical stress
Authors: Rode Hitmar , Orlicki Dariusz , Hlavacek Vladimir
Published: June 2004

A detailed model to describe the overall reaction rate of the oxidation of titanium is developed. The mathematical model consists of two facets, the first of which involves a detailed description of species transport that accounts for the formation of charged species. This is augmented by a description of the occurrence of mechanical stress due to a Pilling ‐ Bedworth ratio that differs significantly from I as well as differences between precursor and product thermal expansion coefficients. A self imposed electric field is formed across the oxide layer due to different mobilities of the species considered. This field opposes the transport of electrons and enhances the transport of anion vacancies, thus increasing the overall reaction rate compared to a pure diffusion process, while also ensuring that electrical charge is conserved. Large growth stresses result from the unmatched precursor and product volumes, significantly affecting the overall process. These results show that the incorporation of a consistent treatment of mechanical stress forms a necessay part of any accurate description of the overall behavior of a reacting particle.
Studies of aggregation effects on SOx removal by limestone powder
Authors: Kobayashi Yoshio
Published: June 2004

To analyze effects of aggregation in powder/gas reactions quantitatively, particle packing models were applied to aggregate structures, and theoretical equations of the reaction rate, the reaction‐equivalent particle radius, the acid gas concentration exponent and the particle radius exponent were deduced. While confirming the validity of these equations by comparing experimental values (data on SOx removal in aerosol reactions and stationary reactions) to these equations, other experimental data were analyzed using these equations. The results show that: the dependency of lime conversion on the particle size decreases sharply as the aggregation of particles in the aerosol advances; injecting limestone powder into the furnaces, using a high rate dispersion nozzle, reduces the degree of aggregation and elevates the lime conversion; and in thermobalance analysis of SOx/limestone reactions, bulk limestone powder behaves like a single giant particle, and the diffusion process inside the aggregates is controlling. It is concluded that hexagonal closest packing is the optimal packing model for the particles constituting aggregates in aerosol.
An integral‐spectral approach for reacting Poiseuille flows
Authors: Arce Pedro , Locke B. R. , Trigatti I. M. B.
Published: June 2004

An integral‐spectral formulation for laminar reacting flows in tubular geometry (tubular Poiseuille flows) is introduced and performed within an operator–theoretic frame‐work where the original convective‐diffusive differential transport problem coupled with reaction is inverted to give an integral equation. This equation is of second kind and of the Volterra type with respect to the axial coordinate of the tube with a kernel given by Green's function. Green's function is identified by a methodology that gives the Mercier spectral expansion in terms of eigenvalues and eigenfunctions of the Stürm–Liouville problem in the radial variable of the tube. Eigenvalue problems for both Dirichlet and von Neumann boundary conditions are solved in terms of analytical functions (Poiseuille functions) and compared with the values found in the literature. The groundwork is set for future applications of the methodology to solving a wide variety of problems in convective–diffusive transport and reaction. Examples with wall and bulk chemical reaction are given to illustrate the technique.
Recuperative parametric pumping in adsorptive membranes
Authors: Agrawal Akhil , Burns Mark A.
Published: June 2004

Recuperative parametric pumping in adsorptive membranes, a cyclical adsorptive separation process, can lead to two distinct mechanisms of separation: rejection and preferential transport. During rejection, the adsorptive membrane rejects an adsorbing solute while permitting the transport of nonadsorbing solutes. During preferential transport the reverse occurs; the adsorptive membrane selectively transports an adsorbing solute while preventing the transport of nonadsorbing solutes. Switching from rejection to preferential transport can be accomplished by merely decreasing the ratio of the stroke volume (cyclical volume of solution pumped into and out of the membrane) to the membrane void volume. Preferential transport, earlier called an “inverse separation,” results from the adsorbing solute crossing over from an adsorbing to a desorbing region in the interior of the membrane and is governed by the shape of the equilibrium isotherms and by process variables. In experiments with lysozyme and a membrane chromatography cartridge, selectivities obtained by preferential transport were on the order of 5 and agreed with theoretical predictions. Theoretical predictions suggest that to further improve separation by preferential transport, experiments need to be conducted on low dispersion membranes (Pe > 104) and that both rejection and preferential transport can lead to continuous concentrated streams of the adsorbing solutes.
Physically meaningful modeling of the 3‐zone and 4‐zone simulated moving bed processes
Authors: Hotier G.
Published: June 2004

This article is intended as a complement to a recent article in this journal, “Comparative Study of Flow Schemes for a Simulated Countercurrent Adsorption Separation Process” by Ching et al. (1992). These authors carried out experiments on continuous separation of three carbohydrate mixtures with both the usual four‐section scheme and a three‐section scheme: fructose dextran (MW ≌ 9,400), raffinose dextran (MW = 6,000), and fructose‐raffinose. These two schemes were modeled as equivalent true countercurrent systems. From the experiments and model results they conclude that the equivalent true countercurrent model is fully adapted to represent the four‐section scheme, but a major discrepancy arises with the three‐section scheme: the raffinate concentration is not correctly predicted.

From these results it may be concluded that when a physically meaningful modeling of the equivalent true countercurrent is realized, the apparent discrepancy disappears. From another standpoint, such a mismatch between the experiments and model predictions would not have been observed with a model that considers the periodic port motion. Such a model foresees a cyclic steady state where extract and raffinate compositions vary throughout the period T; they can be considered as constant only when averaged over the period. It is worth comparing the results of these two models, especially when the equivalent countercurrent representation is not very close to the physical phenomenon it should represent.

We agree with their major finding that a three‐zone scheme represents necessarily a higher desorbent demand than a four‐zone scheme.
Ion‐exchange kinetics: Heterogeneous resin‐phase model
Authors: Hasnat Abul , Juvekar Vinay A.
Published: June 2004

A new approach to cation‐exchange kinetics views ion‐exchange resin as an aggregate of uniform cylindrical pores. Ion exchange occurs at the pore surface. The counterions dissociate only partially from the fixed sites, and the charge created on the pore surface is balanced by the counterions in the electrical double layer. Thus, any cross section of a pore consists of an annular double‐layer region and a central core region. Electroneutrality prevails in the core region through an influx of coions. Diffusion of ions through both regions is considered. The complete model incorporating these ideas for kinetics of monovalent cation exchange was tested with our experimental data as well as with the reported data. The pore diffusion coefficients used in the model are free ionic diffusion coefficients corrected for the tortuosity factor of the pores. Unknown parameters of the model are the degree of dissociation of counterions from the fixed sites of the pore (fσ), and the film thickness (δ). Both parameters are insensitive to the type of counterions, solution concentration, and the direction of exchange. Further, fσ appears to be a characteristic property of resin alone. Superiority of the developed model over the previous models is established.
Stable adaptive control of a class of continuous‐flow bioreactors
Authors: Bošković Jovan D.
Published: June 2004

A stable adaptive control strategy is suggested for a class of continuous‐flow bioreactor processes described by Monod kinetics with two unknown parameters, one of which appears nonlinearly. Similarly, as in the case of the previously reported adaptive controllers, the parametrization of the process model, in conjuction with the adaptive exponential feeding strategy and corresponding adaptive algorithms, results in a stable system in which the convergence of the output errors to zero is guaranteed. In the former, however, two major problems are encountered: (1) both output errors were used to adjust the controller parameters, which may yield unacceptable performance of the resulting adaptive system; (2) conditions under which the process output can assume only positive values are difficult to derive. Hence, a design of a stable adaptive controller is suggested, whose parameters are adjusted using only one of the output errors and that yields acceptable performance of the control system. With this method, conditions under which the process outputs can assume only positive values can be readily derived. These conditions in turn guarantee that the control input saturation at value zero cannot occur. In this context, two adaptive controllers are suggested, such that the resulting adaptive systems are stable and the control objective is met. The adaptive controller design relies on a convenient coordinate transformation, while the proof of stability is based on suitably chosen Lyapunov functions. The performance of the adaptive system is evaluated through computer simulations.
Dynamics of propene metathesis: Physisorption and diffusion in heterogeneous catalysis
Authors: Gomes Vincent G. , Fuller O. Maynard
Published: June 2004

Simultaneous effects of physisorption, diffusion, and reaction were characterized by dynamic experiments for propene metathesis over rhenium oxide/γ‐alumina catalyst in a Berty reactor. Single‐component sorption isotherms from steady‐state experiments for ethene, propene, and 2‐butene were used to predict multicomponent physisorption by the ideal adsorbed solution theory. Effective diffusivities were determined from molecular, Knudsen, and surface‐diffusion coefficients, including bimodal pore‐size effect and tortuosity factors. Transient sorption diffusion experiments were conducted with inactivated catalysts. The reaction rate expression, derived from elementary steps of carbene mechanism, was identified by solving the inverse problem with step test data. The direct problem was solved by orthogonal collocation within the method of lines. Model predictions agreed with responses from dynamic experiments carried out under conditions different from those used for estimating model parameters.
Aggregate behavior and lumped kinetics of many reactions in backmixed and plug‐flow reactors
Authors: Ho Teh C.
Published: June 2004

The long‐time behavior of a reaction mixture containing infinitely many species in isothermal continuous stirred tank reactors (CSTRs) is analyzed and contrasted with that in a plug‐flow reactor (PFR). The reaction kinetics considered are irreversible first‐order, irreversible coupled pseudo first‐order, and reversible first‐order. Asymptotic lumped kinetics for the mixture as a whole are developed as a function of feed properties and reactor type. Such lumped kinetics in many cases are of the power law form, with the exponent for a CSTR being lower than that for a PFR. Conditions under which non‐power‐law kinetics arise are given. Due to the wide spread of reactor residence times, the mixture in CSTRs admits a much richer variety of asymptotic possibilities than in the PFR. The results uncover some behavioral differences and relationships between CSTRs and a PFR for converting complex mixtures such as petroleum distillates or coal liquids.
Thermodynamics of wax precipitation in petroleum mixtures
Authors: Lira‐Galeana C. , Firoozabadi A. , Prausnitz John M.
Published: June 2004

A thermodynamic framework is developed for calculating wax precipitation in petroleum mixtures over a wide temperature range. The framework uses the experimentally supported assumption that precipitated wax consists of several solid phases; each solid phase is described as a pure component or pseudocomponent that does not mix with other solid phases. Liquid‐phase properties are obtained from an equation of state. Calculated wax‐precipitation data are in excellent agreement with experimental results for binary and multicomponent hydrocarbon mixtures, including petroleum.