bluuebunny/crossref_metadata_2025 huggingface dataset:
Isomerization of 1‐butene on silica‐alumina: Kinetic modeling and catalyst deactivation
Authors: Garcia‐Ochoa F. , Santos A.
Published: June 2004

In the study of 1‐butene isomerization on a silica‐alumina catalyst 448‐523 K, cis‐2‐butene and trans‐2‐butene are detected. Based on BSTR experimental data and zero‐time prediction kinetic models using the Langmuir‐Hinshelwood mechanism are assumed to develop kinetic equations for which a triangular reaction scheme is used. In four different mechanisms, one and two active sites take part in the surface reaction as the controlling step and then the deactivation rate determined considering two types of experimental data from BSTR and by measuring weight changes of a catalyst particle from coke deposition in an electrobalance. A coke precursor is assumed formed by reaction of adsorbed molecules (of any butene isomer) and gas‐phase molecules. Activity‐ and coke‐content‐time data allow us to choose a model whose activation energies of the deactivation kinetic parameter are closer in value. Coke is assumed deposited in a monolayer. The model chosen shows a triangular scheme, kinetic equations of the reaction for fresh catalyst with two active sites in the surface reaction, and the deactivation rate according to a coke formation mechanism in which a precursor is formed by reaction of 3 adsorbed molecules and 1 molecule in the gas phase. It accurately fits both BSTR conversion‐time data and electrobalance coke‐content data. The coke formation mechanism establishes relationships of activity vs. coke content and catalyst acidity which are supported by experimental results.
Nucleation and growth in microcellular materials: Supercritical CO2 as foaming agent
Authors: Goel Satish K. , Beckman Eric J.
Published: June 2004

Bubble growth is a phenomenon encountered in several commercially important processes. A mathematical model presented here describes the growth of bubbles during phase separation of an initially homogeneous polymer‐supercritical fluid mixture, triggered by a sudden pressure drop at constant temperature. It is a modification of the viscoelastic model of Arefmanesh and Advani (1991) in which the polymer is treated as a single relaxation‐time Maxwell fluid. Since properties of the polymer‐fluid mixture vary with the amount of fluid absorbed in the polymer (as a function of fluid pressure), the model needs to be used evaluating system properties as functions of temperature and pressure. The viscosity of polymer/fluid mixture, density of the mixture, diffusivity of CO2 in the mixture, and relaxation time for poly (methyl methacrylate) swollen by supercritical carbon dioxide are, therefore, predicted as functions of CO2 pressure and temperature using appropriate model equations at each step of the bubble growth simulation. The model predicts well the trends in equilibrium cell size vs. saturation pressure and temperature.
Cross‐flow microfiltration with high‐frequency reverse filtration
Authors: Redkar Sanjeev G. , Davis Robert H.
Published: June 2004

A primary method of reducing membrane fouling during cross‐flow microfiltration is periodic reverse filtration. This in situ method of cleaning the membrane forces clear fluid in the reverse direction through the membrane and readjusts the particle or solute accumulation on the retentate side of the membrane. This work focuses on the design of a high‐frequency, reverse‐filtration strategy to maximize the flux for washed yeast suspensions through 0.2‐ μm cellulose acetate flat sheet membranes. Several experiments were conducted with reverse‐filtration times ranging from 0.5–4 s and forward‐filtration times ranging from 1–40 s. For every back‐filtration time, there exists an optimum forward‐filtration time that gives the maximum global average flux. The optimum average flux increases with decreasing back filtration times and feed concentrations, but shows little dependence on cross‐flow velocity and reverse filtration transmembrane pressure. The optimum flux with rapid backflushing is 20 to 30 times higher than the long‐term flux in the absence of backflushing. A theory presented assumes that cake formation during forward filtration follows dead‐end filtration theory and the cake is instantly removed during reverse filtration. The measured average flux per cycle follows the trends predicted by the theory, but the measured values exceed the predictions, presumably due to brief delays in cake removal and cake formation at the start of reverse and forward filtration, respectively, during each cycle.
Habituating control strategies for process control
Authors: Henson Michael A. , Ogunnaike Babatunde A. , Schwaber James S.
Published: June 2004

By “reverse engineering” the functions of a specific biological system, habituating control strategies are developed for process control applications. A habituating control system has the distinguishing property of more more manipulated inputs than controlled outputs; with the inputs differing significantly in their dynamic effect on the outputs and in the relative costs of manipulating each one. A habituating controller coordinates the use of all the available inputs to achieve high‐performance output objectives while simulatneously minimizing the cost of taking control action.

The “baroreceptor reflex,” the control system responsible for short‐term blood pressure regulation, provides the biological paradigm for the analysis and design of the habituating control structure. Its main characteristics are discussed from a process control perspective, indicating that the robust, high‐performance control, characteristic of biological systems is partly due to such habituating control architectures. The broad range of potential process applications is illustrated with two examples. Two basic strategies are presented for the design of habituating controllers for linear systems with two inputs and one output: the direct synthesis approach and the model predictive approach. Compared to previous techniques for multiple‐input, single‐output systems, the direct synthesis strategy is straightforward and systematic. Simulation results demonstrate the superior performance of habituating control compared to conventional techniques for which the number of inputs and outputs are equal.
Shear thinning in ternary bicontinuous and water‐in‐oil microemulsions
Authors: Anklam Mark R. , Prud'Homme Robert K. , Warr Gregory G.
Published: June 2004

Shear thinning of ternary microemulsions of didodecyldimethylammonium bromide (DDAB), water, and dodecane in the range 103 to 106 s−1 is documented over a wide range of compositions. A marked transition in the characteristic shear rate is observed accompanying the structural transition from bicontinuous to water‐in‐oil (w/o) droplets as previously reported from diffusion and small‐angle scattering techniques. Results in the w/o region are consistent with those for a dispersion of hard spheres. The shear thinning of the bicontinuous structure, however, occurs at much higher shear rates, implying a shorter time and length scale for disruption of the equilibrium liquid structure by the flow field.
Diffusion in heterogeneous media: Application to immobilized cell systems
Authors: Riley Mark R. , Muzzio Fernando J. , Buettner Helen M. , Reyes Sebastian C.
Published: June 2004

Transport of small molecules in heterogeneous materials can be an important factor in many engineering and biological applications. This study focuses on the diffusion of cellular nutrients in an immobilized cell system. A Monte Carlo simulation technique is used to describe the diffusion of small molecules in a variety of simulated cellular structures. Diffusivity predictions are in close agreement with experimental values as well as with theoretical bounds reported in the literature. It is revealed that effective diffusivities are highly dependent on the diffusivities of the species in the various phases and on the volume fraction of cells. The spatial arrangement of the cells, however, has no apparent effect on the predicted diffusivity for the range of conditions investigated.
Effectiveness of calcium magnesium acetate as dual SO2 ‐NOx emission Control Agent
Authors: Steciak Judith , Levendis Yiannis A. , Wise Donald L.
Published: June 2004

Calcium magnesium acetate (CMA) is an effective dual SO2‐NOx emission control agent. Laboratory‐scale experiments with dry injected particles at a Ca/S molar ratio of 2 [or (Ca + Mg)/S ratio of 6] resulted in SO2 and NOx removal efficiencies of over 90% at gas temperatures of 950 – 1,250°C in atmospheres containing 12% CO2, 3% oxygen, 2,000 ppm SO2, and 1,000 ppm NOx during a 1‐s residence time in an isothermal cavity followed by injection of after‐fire air. During the experiments, SO2 reacted with the porous, thin‐walled CaO, CaCO3 and MgO cenospheres, formed when CMA calcined, while NOx was reduced by hydrocarbon radicals from the organic acetate decomposition. While Ca was the dominant contributor to SO2 reduction by CMA in the isothermal furnace zone, Mg also contributed to SO2 reduction. At gas temperatures ≤ 950°C, Mg reacted directly with SO2 in the isothermal furnace zone and was the dominant SO2 remover in the post‐furnace quenching zone while indirectly SO2 at higher isothermal zone temperatures by precluding pore‐mouth plugging when CaSO4 formed. Interactions among hydrocarbons, SO2, and NOx are important but not fully understood.
Effect of liquid surface tension on small hole distillation sieve tray pressure drop
Authors: Biddulph Michael W. , Thomas Christopher P.
Published: June 2004

Experimental work on a 229‐mm‐dia. air/liquid distillation simulator shows that the surface tension of a given liquid can affect the total tray pressure drop, which normally consists of the sum of dry tray pressure drop, clear liquid head, and residual pressure drop. Both the dry tray pressure drop and clear liquid head values establish correlations in the literature for their prediction. The residual pressure drop, however, has been regarded as a constant value. This is not the case and the residual pressure drop is a dynamic variable. Three different surface‐tension liquids (water, benzyl alcohol, and n‐Propanol) are used to isolate this effect. Results presented correlate the residual pressure drop in terms of the gas F‐factor, the clear liquid head, and the traditional residual head loss definition.
Optimization and modeling of the crystallization process in a cascade with backmixing
Authors: Jansens Pieter J. , Bruinsma Odolphus S. L. , van Rosmalen Gerda M.
Published: June 2004

Cascades of crystallizers are applied in many industrial processes. The optimization of the filterability of crystals produced in such a cascade is dealt with, since a good filterability is often one of the main product specifications. An integral model description of the crystallization process is developed that allows for the prediction of the crystal size distribution and the filterability in each stage of the cascade, and experiments are performed to support this model. The integral model can be used as a predictive tool in future filterability optimization studies.
Simplified local density model for adsorption over large pressure ranges
Authors: Rangarajan Bharath , Lira Carl T. , Subramanian Ramkumar
Published: June 2004

A mean‐field model is developed that superimposes the fluid‐solid potential on a fluid equation of state to predict adsorption on a flat wall from vapor, liquid, and supercritical phases. A van der Waals‐type equation of state is used to represent the fluid phase, and is simplified with a local density approximation for calculating the configurational energy of the inhomogeneous fluid. The simplified local density approximation makes the model tractable for routine calculations over wide pressure ranges. The model is capable of prediction of Type II and III subcritical isotherms for adsorption on a flat wall, and shows the characteristic cusplike behavior and crossovers seen experimentally near the fluid critical point.
Equal area rule and algorithm for determining phase compositions
Authors: Eubank Philip T. , Hall Kenneth R.
Published: June 2004

The Maxwell equal area rule is a basic construct for determining saturated phase volumes from equations of state. The fundamental idea is that for a pure substance a horizontal line drawn through the trace of an isotherm, which equalizes the van der Waals loops area above and below the line, intersects the isotherm at the saturated volumes.

Another basic construct exists for determining the phase compositions in a binary mixture by establishing the tangent line to the total Gibbs energy curve plotted against composition. We demonstrate that finding the tangent also reduces to an equal area construction for the derivative of the total Gibbs energy plotted against composition. An efficient algorithm suggested effects such equal area determinations. The trial‐and‐error procedure normally converges in one to three steps to the correct solution given any resonable starting guess. Implementation using modern calculators with graphics capabilities is simple and straightforward.
Analysis of zero‐reference‐pressure EoS/GE models
Authors: Kalospiros Nikolaos S. , Tzouvaras Nikolaos , Coutsikos Philippos , Tassios Dimitrios P.
Published: June 2004

An analysis of EoS/GE models, which are based on matching the EoS‐derived expression for the excess Gibbs free energy GE with that from a GE model at zero pressure, is presented, with the focus on the most successful and widely used of these models, MHV2 and PSRK. It shows why zero‐reference‐pressure models do not reproduce exactly the GE models at zero pressure and the ensuing implications on the prediction of asymmetric systems. The development of EoS/GE models that reproduce exactly the GE model is discussed. However, such reproducibility at zero pressure is impossible for systems containing components with reduced temperatures higher than about 0.9. Therefore, extrapolation schemes that allow treatment of such systems are examined.
Editor's note on special section
Authors: Tirrell Matthew
Published: June 2004

The collection of articles in this special topical section were prepared based on presentations made at the 1993 Annual Meeting of AIChE. Theme of the session was the applications of molecular thermodynamics to problems of protein structures, solution properties, transport propertties, separations, and other issues important to chemical engineers in the biotechnology, biomedical, pharmaceutical and food industries. These articles were submitted to and survived the regular Journal reviewing procedure; they were grouped to highlight this important area with a concentration of articles similar to the special section we published last year on new imaging techniques. Further background on this subject is given in the following remarks bu Bramie Lenhoff and Mike Paulaitis.
Development and validation of a simple antigen–antibody model
Authors: Busch N. A. , Chiew Y. C. , Yarmush M. L. , Wertheim M. S.
Published: June 2004

A theoretical model for investigating physical phenomena underlying immune complex formation was developed, based on the statistical mechanical theory of associating fluids that identifies each molecule as a hard sphere with a nested point charge and vector dipole. The interaction between binding molecules (epitope–paratope binding) is represented as a cone truncated by two concentric spheres in which the potential energy is a modified square well with respect to particle separation and a square well with respect to mutual molecular orientation. Equilibrium binding results predicted by the model show good agreement with results obtained experimentally for a model system containing a single antigen and a single monoclonal antibody [bovine serum albumin (BSA) – anti‐BSA antibody]. Moreover, values obtained for the system isothermal compressibility and the second virial coefficient by both the model and light scattering experiments also show good agreement with one another.
Two‐phase flow in the vicinity of an elongated bubble in a fluidized bed
Authors: Levy Yeshayahou , Dorfman Yuri
Published: June 2004

Fluid and solid dynamics inside and outside a stable elongated gas bubble in operating conditions corresponding to those of the channeling regime in a fluidized bed are described. It considers the prolate ellipsoid of revolution as a model of the bubble. The results of the analytical model show a significant increase of the fluid velocity inside the bubble with the growth of its relative height. The maximum values of the fluid velocities exist along the vertical axis of the bubble. For the limiting case of a spherical bubble, the fluid and gas performance, such as the existence of a surface of penetration outside the bubble boundaries and circulation zone inside the bubble, are in good agreement with the existing theory.
Role of porosity in filtration: XII. Filtration with sedimentation
Authors: Tiller Frank M. , Hsyung N. B. , Cong D. Z.
Published: June 2004

Filtration on horizontal surfaces facing upward is accompanied by sedimentation. Material balances that are based solely on the volume of filtrate and neglect sedimentation flux lead to an understatement of the solids deposited in the cake and potentially large errors in calculated values of the average specific resistance αav. In a gravitational sedimentation experiment with kaolin, the value of αav neglecting sedimentation was 3.75 times greater than the value including the effect of sedimentation. In addition to errors due to neglect of sedimentation, CATSCAN studies show that the slurry concentration above the cake increases with time, contrary to usual assumptions. In a manner similar to batch sedimentation in a closed cylinder, characteristics of constant composition arose from the cake surface. Approximate predictions based on a combination of traditional sedimentation and filtration theory were in accord with the CATSCAN data. Existing filtration theory must be substantially modified to account for the effect of sedimentation.
Simultaneous concentration and purification through gradient deformation chromatography
Authors: Velayudhan Ajoy , Hendrickson Richard L. , Ladisch Michael R.
Published: June 2004

Mobile‐phase additives, commonly used to modulate absorbate retention in gradient elution chromatography, are usually assumed to be either linearly retained or unretained. Previous theoretical work from our laboratory has shown that these modulators, such as salts in ion‐exchange and hydrophobic interaction chromatography and organic modifiers in reversed‐phase chromatography, can absorb nonlinearly, giving rise to gradient deformation. Consequently, adsorbate peaks that elute in the vicinity of the head of the deformed gradient may exhibit unusual shapes, form shoulders, and/or be concentrated. These effects for a reversed‐phase sorbent with aqueous acetonitrile (ACN) as the modulator are verified experimentally. Gradient deformation is demonstrated experimentally and agrees with simulations based on ACN isotherm parameters that are independently determined from batch equilibrium studies using the layer model. Unusual adsorbate peak shapes were found experimentally for single‐component injections of phenylalanine, similar to those calculated by the simulations. A binary mixture of tryptophan and phenylalanine is used to demonstrate simultaneous concentration and separation, again in agreement with simulations. The possibility of gradient deformation in ion‐exchange and hydrophobic interaction chromatography is discussed.
New discretization procedure for the breakage equation
Authors: Hill Priscilla J. , Ng Ka M.
Published: June 2004

A new discretization method, applicable for both batch and continuous systems, is developed for the breakage equation. The problem of intrainterval interactions due to discretization is accounted for by matching the zeroth and first moments of the continuous population balance equation with the corresponding moments of the discretized equation, thereby guaranteeing conservation of mass and total number of particles. Without loss of generality, the use of this method is demonstrated with a power law form of the specific rate of breakage, and with both theoretical and empirical breakage functions. The systematic method requires minimum computational efforts by allowing the user to choose either geometric size intervals with any geometric ratio or equal‐size intervals for the particle size range. Simulation results show that the new method significantly improves predictions of the particle size distribution over the discretization method currently in use.
Reducing data dimensionality through optimizing neural network inputs
Authors: Tan Shufeng , Mayrovouniotis Michael L.
Published: June 2004

A neural network method for reducing data dimensionality based on the concept of input training, in which each input pattern is not fixed but adjusted along with internal network parameters to reproduce its corresponding output pattern, is presented. With input adjustment, a property configured network can be trained to reproduce a given data set with minimum distortion; the trained network inputs provide reduced data.

A three‐layer network with input training can perform all functions of a flue‐layer autoassociative network, essentially capturing nonlinear correlations among data. In addition, simultaneous training of a network and its inputs is shown to be significantly more efficient in reducing data dimensionality than training an autoassociative network The concept of input training is closely related to principal component analysis (PCA) and the principal curve method, which is a nonlinear extension of PCA.
Modifications to model IV fluid catalytic cracking units to improve dynamic performance
Authors: Huq Iftikhar , Moran Manfred , Sorensen Ronald C.
Published: June 2004

Model IV fluid catalytic cracking units (FCCUs) differ from other cracking units in that model IV FCCUs do not have slide valves in the catalyst circulation lines to enable direct control of catalyst circulation rate through the unit. Reducing fluctuations in catalyst circulation rate is found to significantly improve closed‐loop performance of the FCCU. Some design and operational modifications that can be made to model IV FCCUs to improve closed‐loop performance at the regulatory level based on this insight are modeled and compared. Closed‐loop performance of a model IV FCCU operated with the weir and standpipe always flooded is examined. The achievable performance is significantly better thah that of the standard model IV FCCU. The closed‐loop performance of the model IV FCCU modified to incorporate slide valves in the catalyst circulation lines is also examined. The performance of the FCCU with slide valves is better than the performance achievable by the FCCU with the weir flooded. It is found that model IV FCCUs are ill‐conditioned owing to the use of the weir and standpipe arrangement in the regenerator section. Both the operational and design modifications studied reduce plant ill‐conditioning appreciably.
Three‐dimensionai temperature and current distribution in a battery module
Authors: Verbrugge Mark W.
Published: June 2004

A method for treating three‐dimensional current and temperature distributions in large‐scale battety modules is described. Simulations are presented for a module consisting of cells formed with a lithium metal anode, a polymer electrolyte, and a vanadium oxide cathode. The calculations illustrate the nonlinear dependence of power output on system temperature, which for the investigated lithium‐polymer battery module is determined primarily by the influence of temperature on the electrochemical reaction rates and ionic conductivity. An appendix is devoted to the estimation of physicochemical parameters, some of which are not available from the current literature, but are nonetheless important model inputs.
Origin of disturbances in cocurrent gas‐liquid packed bed flows
Authors: Krieg D. A. , Helwick J. A. , Dillon P. O. , McCready M. J.
Published: June 2004

Visual, video, pressure, and conductance techniques were used to study time‐varying disturbances in cocurrent flow in packed beds with vertical and horizontal columns. It is found that the trickle‐pulse transition, as defined in previous studies, corresponds to conditions where traveling disturbances finaly become measurable, not the conditions at which infinitesimal disturbances begin to grow. Observations demonstrate that even if the liquid and gas are uniformly distributed initially, segregated, vertical flowing regions with higher or lower than average liquid holdup form after a short distance. Horizontal packed bed experiments, designed to study how regions of differing liquid holdup interact, indicate that the first type of disturbance is infiltration of gas into the liquid region. A simple model suggests that infiltration occurs if the pressure drop exceeds a value necessary to push gas through liquid‐filled pores. Once infiltration is significant enough to form a third “bubbly” phase, traveling wave instabilities form and grow into pulses if sufficient column length is available. A three‐layer Kelving‐Helmholtz stability model is used to interpret the growth of disturbances in horizontal flows. Video obeservations of small‐scale events in the bed failed to detect significant correlations between different regions. Thus it should be possible to describe flow behaviour in these systems with volum‐averaged equations, as long as the presence of segregated regions is considered. Column diameter or thickness significantly affects the frequency of disturbances.
Evaporative cooling of air in impinging streams
Authors: Yao B. , Berman Y. , Tamir A.
Published: June 2004

An experience study was conducted on evaporative cooling of air in an impinging‐stream reactor, in which two droplets gas streams flow in opposite directions and meet in the impingement zone. The air was cooler by evaporation of water droplets. Volumetric heat‐transfer coefficients hv determined enabled us to evaluate the performance of the cooler. Maximum values of hv varied from 4 to 14 kW/m3 K. The effectiveness of impinging streams was evaluated by placing a partition in the implingement zone, which prevented interaction between the streams. A numerical modeling of the droplets' behavior in an impingin‐stream reactor was also performed on the basis of heat and momentum equations. Effects of initial droplets' size and velosity as well as atomization and coalescence on the enhancement by impinging streans have been investigated. Correlation equations are also discussed. The experimental and numerical resluts helped understant the fundamentals mechanisms of an impinging‐stream reactor with a gs‐droplet suspension and furthet verify the effectiveness of the method of impinging streams.
Global optimization for the phase stability problem
Authors: McDonald Conor M. , Floudas Christodoulos A.
Published: June 2004

The Gibbs tangent plane criterion has become important in determining the quality of obtained solutions to the phase and chemical equilibrium problem. The ability to determine if a postulated solution is thermodynamically stable with respect to perturbations in any or all of the phases is very useful in the search for the true equilibrium solution. Previous approaches have focused on finding stationary points of the tangent plane distance function. Obtaining all stationary points, however, cannot be guaranteed due to the complex and nonlinear nature of the models used to predict equilibrium. Simpler formulations for the stability problem are presented for special problems where nonideal liquid phases can be adequately modeled using the NRTL and UNIQUAC activity coefficient equations. It shows how the global minimum of the tangent plane distance function can be obtained for these problems. A global optimization approach is advantageous because a nonnegative solution can be asserted to be the globally stable equilibrium one, unlike available local algorithms. For the NRTL equation, the GOP algorithm of Floudas and Visweswaran (1990, 1993) is used to guarantee obtaining ϵ ‐global convergence to the global minimum. For the UNIQUAC equation, a branch and bound algonthm based on that of Falk and Soland (1969) is used to guarantee convergence to the global solution. Computational results demonstrate the efficiency of both global optimization algorithms in solving various challenging problems.
Avoiding the use of critical constants in cubic equations of state
Authors: Soave Giorgio S. , Bertucco Alberto , Sponchiado Michele
Published: June 2004

A new way to define the two parameters a, b of the Redlich‐Kwong equation of state without using the critical constants is introduced. First, the “shifted” covolume (b ‐ c) of the Peneloux modification of the equation of state is estimated by group contributions (for a wide range of compounds); from it and one vapor pressure‐liquid density pair, the covolume b is then determined. The attractive parameter a is derived, at any temperature, from the covolume and the compound vapor pressure. The proposed method makes it possible to treat substances whose critical constants are not known (heavy or thermally unstable compounds) by using one density value and the vapor pressure curve. Examples are given of vapor‐liquid equilibrium calculations.
Transdermal delivery of peptide and protein drugs: An overview
Authors: Amsden B. G. , Goosen M. F. A.
Published: June 2004

Recently, a large number of new and potentially potent peptide and protein drugs have been developed. Their systemic delivery is difficult because they are rapidly cleared from the bloodstream, are of large molecular size, are vulnerable to proteolytic attack and tend to undergo aggregation, adsorption and denaturation. Controlled release strategies have many advantages over the current delivery method of injection or intervenous infusion. Controlled release allows for prolonged delivery while maintaining the drug concentration within therapeutic limits. It also improves and increases patient compliance by removing the discomfort of repeated applications. The transdermal route of delivery is particularly attractive because it avoids peptide and protein degradation via the gastrointestinal tract and the hepatic first‐pass effect, and delivery can be interrupted by simply removing the device. Its primary problem is that the skin is an excellent barrier to large, hydrophilic, polar compounds. Recent work in the area of transdermal peptide and protein drug delivery is overviewed, including strategies such as prodrugs, chemical enhancement, iontophoresis, electroporation, and ultrasound, with focus on mass‐transport mechanisms of different systems. Of the strategies studied, iontophoretic delivery appears the most promising. It is clear, however that more work remains to be done before transdermalpeptide and protein delivery devices come to market. Chemical engineers can contribute significantly to further research in this exciting area.
Wave model for longitudinal dispersion: Analysis and applications
Authors: Westerterp K. R. , Dil'man V. V. , Kronberg A. E. , Benneker A. H.
Published: June 2004

An analysis and applications of the wave model for longitudinal dispersion are pre‐sented. Asymptotic forms of the wave model are considered and analytical solutions of typical linear stationary and nonstationary problems of chemical reactor engineering interest are obtained and compared to those for the Fickian dispersion model. The wave model leads to efficient analytical solutions for linear problems, which in principle differ from the solutions of the Fickian dispersion model; only for slowly varying concentration fields do the soluctions of both models approach each other. Spatial and time moments of the concentration distribution are obtained for pulse‐dispersion problems; the first three spatial moments of the mean, variance, and skewness have exact, large‐time asymptotic forms in the case of Taylor dispersion. Old experiments that could not be explained with the standard dispersion model are reconsidered and explained: the change with time of the variance of a concentration pulse when the flow direction is reversed and the difference in values of the apparent axial dispersion coefficient and the backmixing coefficient in a rotating disk contactor. The experimental determination of model parameters is discussed.
Single‐step thermal method to measure intracrystalline mass diffusion in adsorbents
Authors: Grenier Ph. , Bourdin V. , Sun L. M. , Meunier F.
Published: June 2004

A single‐step thermal method is used to measure intracrystalline mass diffusion. Sorption/desorption rates is zeolite samples (large crystals, monolayers of pellets, or even a single pellet) after a pressure of volume step are followed by monitoring the sample surface temperature by infrared detection. For a volume step, the pressure is also measured yielding the adsorbed mass. Sorption rates of water vapor in NaX are measured both on large 100 ‐ μm crystals and pellets. This fast system correcsponds to the limit of the pressure‐step termal method (using 100 ‐μm crystals). Sorption rates of methanol in the same large NaX crystals show good precision by the pressure‐step method. The methanol results show that a surface barrier may occur after thermal regeneration of the sample in the presence of methanol traces. A major advantage of this method is that the shape of response curves can provide useful information on the nature of the masstransfer resistance despite its limits. Sorption rates of methanol vapor on mordenite H (zeolon) pellets prove that the intracrystalline diffusivity may be extracted from pellet measurements for a slow diffusing species.
Acetic acid oxidation and hydrolysis in supercritical water
Authors: Meyer Jerry C. , Marrone Philip A. , Tester Jefferson W.
Published: June 2004

Acetic acid (CH3COOH) hydrolysis and oxidation in supercritical water were examined from 425 – 600°C and 246 bar at reactor residence times of 4.4 to 9.8 s. Over the range of conditions studied, acetic acid oxidation was globally 0.72 ± 0.15 order in acetic acid and 0.27 ± 0.15 order in oxygen to a 95% confidence level, with an activation energy of 168 ± 21 kJ/mol, a preexponential factor of 109.9 ± 1.7 and an induction time of about 1.5 s at 525°C. Isothermal kinetic measurements at 550°C over the range 160 to 263 bar indicated that pressure or density did not affect the rate of acetic acid oxidation as much as was previously observed in the oxidation of hydrogen or carbon monoxide in supercritical water. Major products of acetic acid oxidation in (upercriuical water are carbon dioxide, carbon monoxide, methaite, and hydrogen. Trace amounts of propenoic acid were occasionally detected. Hydrolysis or hydrothermolysis in the absence of oxygen resulted in approximately 35% conversion of acetic acid at 600°C, 246 bar, and 8‐s reactor residence time. Regression of the limited hydrolysis runs assuming a reaction rate first‐order in organic gave a global rate expression with a preexponential factor of 104.4 ± 1.1 and an activation energy of 94 ± 17 kJ/moL.
Molecular thermodynamics for salt‐induce protein precipitation
Authors: Chiew Y. C. , Kuehner D. , Blanch H. W. , Prausnitz J. M.
Published: June 2004

A molecular‐thermodynamic model is developed for salt‐induced protein precipitation, which considers an aqueous solution of globular protein molecules as a pseudo‐one‐component system containing macroions that interact through Coulombic repulsion, dispersion attraction and hydrophobic interactions, and forces arising from ion‐excluded volume. Forces from ion‐excluded volume take into account formation of ion pairs and ionic clusters at high salt concentrations; they are calculated in the context of the Percus‐Yevick integral‐equation theory. Hydrophobic interactions between exposed nonpolar amino‐acid residues on the surfaces of the protein molecules are modeled as short‐range, attractive interactions between “spherical caps” on the surfaces of the protein polyions. An equation of state is derived using perturbation theory. From this equation of state we calculate liquid ‐ liquid equilibria: equilibrium between an aqueous phase dilute in protein and another aqueous phase rich in protein, which represents “precipitated” protein. In the equation of state, center‐to‐center, spherically symmetric macroion–macroion interactions are described by the random‐phase approximation, while the orientation‐dependent short‐range hydrophobic interaction is incorporated through the perturbation theory of associating fluids. The results suggest that either ion‐excluded‐volume or hydrophobic‐bonding effects can precipitate proteins in aqueous solutions with high salt concentrations.
3‐D Laminar stationary flow over a porous surface with suction: Description at pore level
Authors: Schmitz Philippe , Prat Marc
Published: June 2004

The microhydrodynamics associated with membrane cross‐flow microfiltration processes is analyzed. To this end, numerical computations of the flow near a pore of a spatially periodic model porous surface are performed for various porosities and suction conditions. The structure of the fluid capture tube as well as related fluid dynamics aspects, such as the existence of a recirculating eddy in the pore, the slip velocity at the wall, and the pressure drop across the pore, are investigated. In particular, the influence of the neighbor pores on the structure of the fluid capture tube of a given pore is studied.
Growth of naphthalene crystals from supercritical CO2 solution
Authors: Tai Clifford Y. , Cheng Chuen‐Song
Published: June 2004

The growth phenomena, mechanism, and kinetics of naphthalene crystal from supercritical CO2 solution were studies by using a single‐crystal technique. Distinct growth features were observed, including the development of individuals from the seeds and the sprouting of plates from the individuals. The surface integration proceeded by two‐dimensional nucleation mechanism at face corners and by subsequent spreading of the nuclei. The measured growth rates as functions of supersaturation and solubility were consistent with the derived growth‐rate equations, based on the corner nucleation mechanism. In comparison with crystal growth from conventional media, the growth of naphthalene crystals from supercritical CO2 solution is similar to liquid‐solution growth as far as growth mechanism and kinetics are concerned.
Necessary and sufficient conditions for reactive azeotropes in multireaction mixtures
Authors: Ung Sophie , Doherty Michael F.
Published: June 2004

Necessary and sufficient conditions for azeotropy in reactive mixtures derived show that in the space of transformed compositions, they take the same functional form as the conditions for azeotropy in nonreactive mixtures. In general, reactive azeotropes do not correspond to points of equal mole fraction (or mass fraction) across the coexisting equilibrium phases. The relationship between stationary points in the equilibrium temperature or pressure surface and reactive azeotropes is also considered, as well as their relevance to the Gibbs‐Konovalov theorems.
Adsorption kinetics for the case of setp and S‐shape isotherms
Authors: Kruglov Alexey V. , Aris Rutherford
Published: June 2004

The Kinetics of diffusion in a spherical pellet saturated by a sorbate in the presence of an inert component are considered for the case of a step isotherm. A new convective flux inside the pores due to sorption effect has been taken into consideration, and an analytical expression for sorption kinetics is obtained. The convective flux may result in considerable increase in the sorption rate. It is also found that the formula obtained also gives a satisfactory description of sorption kinetics for the case of an S‐shaped isotherm.
Simulation of diffusion in zeolitic structures
Authors: van den Broeke Leo J. P.
Published: June 2004

Using Maxwell‐Stefan equations, experimental and computational results of binary diffusion in pore‐ and cage‐type zeolitic structures are described. In the generalized Maxwell‐Stefan (GMS) formulation, the Fick diffusivity is written as the product of two separate contributions, the GMS or corrected diffusivity and the thermodynamic factor. The concentration dependence of the GMS diffusivity for one‐ and two‐component diffusion in zeolitic structures is investigated. From the Maxwell ‐ Stefan equations, different models for the Fick diffusion coefficient matrix for the description of binary mass transport in molecular sieve materials are derived. Various models used predict binary diffusion in zeolitic structures. First, theoretical predictions of binary apparent diffusivities as a function of the occupancy are compared to results from Monte Carlo simulations. Second, theoretical results of binary uptake profiles are compared to experimental results for the system ethylbenzene/benzene/ZSM‐5. For different zeolitic structures, that is, pore‐ and cage‐type structures, results of the Monte Carlo simulations agree well with the theoretical predictions. In cage‐type structures, the effect of counterexchange between sorbed molecules is demonstrated. Experimental results of transient uptake profiles of a mixture of benzene and ethylbenzene in ZSM‐5 follow predictions of the theoretical single‐file diffusion model.
Finely‐divided powders by carrier solution injection into a near or supercritical fluid
Authors: Schmitt William J. , Salada Michael C. , Shook Gary G. , Speaker Stanely M.
Published: June 2004

Carbon dioxide or ethane was used as an antisolvent medium from which organic solids dissolved in a liquid carrier solvent are precipated and collected as finely divided solids. Solid compounds of pharmaceutical interest, which are virtually insolute in the supercritical fluids per se, are efficiently obtained by injecting a solution of the solid into a volume of stirred supercritical fluid or compressed liquefied gas. A free‐flowing powder comprising very small particles ( > 5 μm) is obtained. Both continuous‐flow and batch operations were developed although he continuous‐flow operation is preffered. Process variable including temperature, pressure, stirring rate, injection concentration, and the rate and temperature of the carrier solution injection were studied to see how they affect the physical properties of the resultant product. Recovery (yield), particle size and uniformity, bulk powder density, microscopic apperance, and surface area were measured. Carbon dioxide and ethane antisolvents, several liquid‐phase injection solvents, and different solid compunds were studied. A simple unsteady‐state mathematical model of this CSTR precipitator is also presented.
Foamed gel barriers in porous media: Breakdown and permeability evolution
Authors: Miller Matthew J. , Fogler H. Scott.
Published: June 2004

Foamed del has begun to play an important role in permeability modification applications because of the reduced chemical requirements. Foamed gels create impermeable barriers in porous media; however, once a critical pressure differential is exceeded, the permeability increases with increasing pressure. A two‐dimensional network model was developed to estimate foamed gel barrier performance in terms of the maximum pressure a barrier can withstand and the evolution of the foamed gel barrier's permeability. The formation of conductive pathways and the accopanying permeability increase were estimated from a model of the pressure‐induced deformation and repture of individual lenses. The evolution of conductive pathways changed from invasion percolation (high elastic modulus, rigid gel) to a lens rupture chain reaction initiated by teh rupture of a single lens (low elstic modulus gel) as the elastic modulus of the gel was decreased. The apparent fractal dimension of the first conductive channel ranged from 1.89 to 1.06 for high and low wlastic modulus gels, respectively. This dependency of breakthrough and breakdown is unique and produces a large range of breakdown behavior for any degree of microscopic heterogeneity.
Turbulent flow in channels in terms of turbulent shear and normal stresses
Authors: Churchill Stuart W. , Chan Christina
Published: June 2004

The local time‐averaged velocity, the mixed‐mean velocity, and the friction factor for fully turbulent flow between parallel plates and in round tubes and concentric circular annuli can be expressed in terms of integrals of the turbulent shear stress, The pressure distribution across a channel can similarly be expressed in terms of an integral of normal stresses. These formulations, which are simple and exact, can be integrated numerically using experimental data, compyted values, or correlating equations for turbulent stresses. Their greatest merit, however, may arise from the insight they provide with respect to the contributions of the fluctuating components of the velocity. For example, for concentric circular annuli such a formulation identifies a difference between the locations of the maximum in the velocity and the zero in the total shear stress. This difference, which has been overlooked in most experimental and semitheoretical investigations, introduces an error of unknown but possibly significant magnitude into all of the results. It also precludes the application of the mixing‐length, eddy viscosity and k ‐ ϵ models.
Mixing, reaction, and precipitation: Interaction by exchange with mean micromixing models
Authors: Tavare Narayan S.
Published: June 2004

An analysis of interaction by exchange with the mean micromixing model is extended to a process involving elementary chemical reaction between two species and subsequent crystallization of product in a continuous mixed suspension mixed product removal etystallizer. Two specific feed conditions are considered: premixed and anpremixed feeds are considered. The sensitivity of these two cases to several parameterv like the Damköhler number, micromixing parameter, fractional flow rate, and dimensionless inlet concentration of excess reactant is explored. Both reaction and crystallizaLion performance characteristics are significantly influenced by the fred conditions. Results for the case of premixed feeds tend to suggest that the model description may be better suited to a nearly segregated configuration.
Solution of equations describing fluid–soil reactions in packed columns
Authors: Fernandes Neil , Gavalas George R.
Published: June 2004

A semianalytical technique is presented frr solving the equations describing an isothermal, irreversible reaction of a trace component in a packed bed of a solid reactant or adsorbent The reaction‐rate expression is assumed to have an arbitrary dependence on the solid reactant concentration, but a first‐order dependence on the trace gas component. The technique relies on an integral transformation that reduces the set of partial differential equations to a set of two coupled ordinary differential equations in the spatial variables. Solving these two equations is simpler than solving the original equations by finite differences or finite elements, especially in the presence of steep concentration gradients. Two examples presented illustrate the technique: a progressive conversion model with reaction occurring at dispersed sites within the adsorbent pellet and a reaction proceeding in the shrinking core mode.
Theoretical aspects of self‐propagating reaction fronts in condensed medium
Authors: Ilyashenko Victor M. , Solovyov Stanislav E. , Pojman John A.
Published: June 2004

Exothermic chemical reactions are capable otautocatalytic self‐propagation in a condensed reactive medium as a thermal wave. These take place in self‐propagating high‐temperature synthesis, frontal polymerization, curing of epoxy resins, and various solid‐phase reaction processes. The lack of accurate analytical solutions for the front velocity and spatial parameter distributions has led to a period of approximate analytical solutions based on the infinitely narrow reaction zone assumption. A more accurate reaction zone approximation is presented that results in new equations for temperature and concentration spatial distributions as well as the wave velocity. Necessaty conditions for nondegeneration of the combustion wave were also obtained.