bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffusive and convective protein transport through asymmetric membranes
Authors: Opong W. Senyo , Zydney Andrew L.
Published: June 2004

Experimental data are obtained for bovine serum albumin transport through asymmetric polyethersulfone ultrafiltration membranes of differing molecular weight cutoff in a stirred ultrafiltration device. The actual membrane sieving coefficient is determined from filtrate concentration measurements using a stagnant film model to account for bulk mass transport effects. These sieving coefficients are then used to evaluate the relative contributions of diffusive and convective transport to the overall protein flux. The results are in good agreement with available hydrodynamic models for the hindrance factors for convective and diffusive transport of spherical solutes through well‐defined pores, with the effective solute to pore size ratio evaluated from a partitioning model that explicitly accounts for the ellipsoidal shape of the protein and the membrane pore size distribution. The implications of these results to the analysis of experimental data for membrane sieving and to the design of effective protein fractionation devices are also discussed.
Inaccessible porosity in gasification reactions under kinetic control
Authors: Delikouras Evangelos A. , Perlmutter Daniel D.
Published: June 2004

Random pore models have been successful in interpreting a wide range of experimental observations in gas‐solid reactions; however, their applicability has been limited to cases where the entire porous structure is initially accessible to reacting gases. In many practical cases, porous solids contain significant void regions that are initially inaccessible to gaseous reactants, but become accessible in the course of the reaction. A conceptual and mathematical model is developed here that accounts for the existence of this hidden porosity. It uses the same representation of the porous network as was developed in the earlier random pore models, but introduces physically meaningful expressions for the discovery of hidden pores, and the subsequent growth of the discovered pores. The modified model calls for only one additional parameter, the initial volume fraction of hidden porosity in the total volume. It is demonstrated computationally that neglecting hidden porosity can lead to serious underestimations of conversion and reaction rates.
Geochemical modeling of two‐phase flow with interphase mass transfer
Authors: Novak Craig F. , Lake Larry W. , Schechter Robert S.
Published: June 2004

Injecting an acidic aqueous solution into a carbonate‐rich permeable medium can cause the formation of a mobile gas phase through mineral dissolution. The flowing gas can cause significant changes in mineral identities through interaction with the initially present mineral and aqueous species. We have developed a solution to such transport problems based on a finite difference implicit‐pressure/explicit‐saturation formulation for two‐phase flow, using the local equilibrium assumption as calculated with the Villars‐Cruise‐Smith stoichiometric chemical equilibrium algorithm.

We illustrate the changes that occur to a calcite/iron (II) hydroxide medium upon injection of hydrochloric acid and the stripping of hydrogen sulfide gas from carbon dioxide gas injected into a siderite‐rich medium. The examples demonstrate that the formation and/or presence of a gas phase can alter resident minerals and that minerals can remove impurities from a gas phase.
Expansion characteristics of tapered fluidized beds
Authors: DiFelice Renzo , Foscolo Piero U. , Gibilaro Larry G. , Wallis Graham B. , Carta Renzo
Published: June 2004

The effects of taper on the steady‐state expansion characteristics of a homogeneous fluidized bed are examined theoretically and experimentally. It is shown that secondary factors that might be thought to influence the expansion (fluid accelerational effects including added mass and particle phase elasticity) are unlikely to be of significance in practical situations.
One‐dimensional numerical model for a spray column heat exchanger
Authors: Summers Steven M. , Crowe Clayton T.
Published: June 2004

A numerical model describing performance of a liquid‐liquid spray column heat exchanger is developed. The model is for a disperse packed column operating in steady state. The fundamental equations for the continuous and distributed phases are developed in Eulerian and Lagrangian forms, respectively. The model predicts simultaneously the volume fraction, velocity, and temperature for each phase. Energy and momentum are exchanged between phases through source variables. Empiricism in the model is limited to the specification of droplet drag coefficient and Nusselt number correlations which are utilized to calculate momentum and energy source terms. Application of the computer model to column operation yields predictions that compare closely with experimental results.
Estimation of dissolved carbon dioxide concentrations in aerobic fermentations
Authors: Royce Patrick N. C. , Thornhill Nina F.
Published: June 2004

Dissolved carbon dioxide and bicarbonate ions in fermentation broths can (independently) inhibit or promote microbial growth and productivity. In research facilities with a large number of fermenters, dissolved carbon dioxide sensors tend not to be used, and as a result this variable will generally go unmonitored, making the meaningful analysis of data more difficult. For aerobic fermentations, mass transfer of carbon dioxide can be described in an analogous way to oxygen transfer. The mass transfer coefficient for carbon dioxide is 0.89 times that for oxygen. The maximum dissolved carbon dioxide concentration as a function of exit gas composition is compared with the concentration obtained by assuming equilibrium between the broth and exit gas. The difference between these two concentrations is typically 20–40% of the equilibrium concentration. In large fermenters, a degree of plug flow behavior in the gas and the generally lower specific aeration rates will serve to produce a better approach to equilibrium than for research fermenters.
Distributor effects in liquid fluidized beds of low‐density particles
Authors: Asif Mohammad , Kalogerakis Nicolas , Behie Leo A.
Published: June 2004

The effect of flow nonuniformities existing near the distributor of a liquid fluidized bed containing very low‐density particles (ρs = 1.05 g/mL) has been investigated experimentally. Immobilized enzyme or cell bioreactors operated as liquid fluidized beds often employ low‐density particles as the support matrix containing the immobilized biocatalyst. With poorly designed multihole distributors, dead zones can be created which seriously undermine the performance of the bioreactor due to the lack of nutrients. Design guidelines are provided for distributors to minimize the inlet flow disturbances, and hence eliminate potential distributor region problems. As a result, the dispersion model, which is normally used for design purposes, can then be applied.
Adaptive control of photolithography
Authors: Crisalle Oscar D. , Soper Robert A. , Mellichamp Duncan A. , Seborg Dale E.
Published: June 2004

Adaptive control techniques, with their capability for providing satisfactory control even when the process changes with time, are promising candidates for dealing with common problems encountered in photolithography processing such as batch‐to‐batch variations in resist properties and inconsistencies in resist curing. In this article, an adaptive control strategy for the photolithography process is proposed and evaluated. The design utilizes a reduced‐order lithography model, an on‐line parameter estimator, and a nonlinear model‐inversion controller.

The width of the printed resist lines, a crucial output of photolithography, is controlled by automatically adjusting the exposure energy. In the calculation of the appropriate exposure adjustment, the controller uses both measured critical‐dimensions as well as estimated values produced by the process model. The control system is capable of tracking changes in the photolithography process by automatic updating of key model parameters as the process evolves in time. Simulation studies of the closed‐loop adaptive control strategy, using the PROLITH simulation package to represent the lithography process, demonstrate the feasibility of this approach.
Nonminimum‐phase compensation for nonlinear processes
Authors: Wright Raymond A. , Kravaris Costas
Published: June 2004

The design of controllers for nonlinear, nonminimum‐phase processes is one of the most difficult control problems currently faced. Current available control algorithms for nonlinear processes rely implicitly or explicitly on an inverse of the process. Linear control methods for nonminimum‐phase processes are based on a decomposition of the process into a minimum‐phase and a nonminimum‐phase part. Such a decomposition is an open problem for nonlinear systems.

In this work, a control structure called the minimum‐phase output predictor for nonlinear, nonminimum‐phase processes is developed. The structure is based on the notion of statically equivalent outputs; a minimum‐phase, statically equivalent output is estimated on‐line and then an available nonlinear control algorithm is used to control it to set point. The advantage of the proposed formulation is that it is based on the calculation of an output function, not on a decomposition of the process dynamics. The proposed control methodology is applied and its performance is evaluated for a chemical engineering example.
Accuracy and convergence of the p‐ and hp‐type finite element methods for the Navier‐Stokes equation
Authors: Talwar Kapil K. , Khomami Bamin
Published: June 2004

Higher‐order p and hp finite element algorithms for modeling the flow of New‐tonian fluids have been developed. Numerical computations in three test geometries were carried out to compare the performance of these methods to that of conventional h‐type finite elements. Computations demonstrated that for solutions which are analytic everywhere in the domain of interest, p extensions exhibit an exponential rate of convergence as opposed to the characteristic linear convergence of h‐type finite element procedures. Moreover, for solutions which are analytic everywhere in the domain except at a set of points, hp extensions with geometric mesh refinement based on effective error indicators (individually, the error in the energy norm, local and global deviations from mass and momentum conservation) also give rise to an exponential convergence rate. This rapid convergence of p and hp finite element formulations enables the achievement of the solution of linear Stokes problems with an order of magnitude reduction in degrees of freedom. In nonlinear problems, these computational savings are appreciated by a factor approximately equal to the number of iterations required to obtain converged solutions.
Multiplicity features of nonadiabatic, autothermal tubular reactors
Authors: Lovo Marianne , Balakotaiah Vemuri
Published: June 2004

Singularity theory is combined with asymptotic analysis to determine the exact uniqueness‐multiplicity boundary and ignition and extinction locus for the non‐adiabatic, autothermal tubular reactor model. It is found that the steady‐state behavior of the nonadiabatic reactor is described by the two limiting cases of adiabatic and strongly cooled models. The adiabatic case has been examined in a previous study. Here, we develop limiting models to describe the strongly cooled asymptotes. We also classify the different types of bifurcation diagrams of conversion vs. residence time using the results of singularity theory with a distinguished parameter. Analytical criteria are developed for predicting the conditions under which autothermal operation is feasible when heat losses are significant.
Stripping of trihalomethanes from drinking water in a bubble‐column aerator
Authors: Velázquez Carols , Estévez L. Antonio
Published: June 2004

The desorption of trihalomethanes (THMs) in a cocurrent bubble column has been studied experimentally to validate a previously developed model. The experiments were carried out in a 0.2‐m‐diameter, 2.4‐m‐high, plexiglas bubble column, equipped with seven samplers alongside. The phases in contact were tap water and air. Three out of the four THMs normally found in drinking water were spiked in the feed tank to assure their detection in the analyses. After preparing the feed, each run was begun by setting the flow rates of water and air, and waiting until a steady state was attained. Then, samples from each sampler were taken and sent for chemical analysis. A total of 12 runs were performed with varying superficial gas and liquid velocities, and up to four concentration profiles were obtained for each run. The experimental results were then compared to the predictions of the model proposed by Estévez (1991). The agreement between calculated and experimental concentration profiles was quite good, indicating that the model can be used reliably either in the simulation or design of industrial scale equipment.
Hydrodynamic changes of the depletion layer of dilute polymer solutions near a wall
Authors: De Pablo Juan José , Öttinger Hans Christian , Rabin Yitzhak
Published: June 2004

The system studied in this work is a dilute solution of rod‐like molecules under simple shear flow and near a hard wall. The time evolution of the probability density function is described by a diffusion equation; particle trajectories that correspond to this equation are generated by stochastic methods. Several algorithms are presented to handle the constraints imposed by the presence of the wall. In good agreement with recent experimental work on xanthan solutions, for high shear rates we observe an increase in the thickness of the depletion layer near the wall. For low to intermediate shear rates, however, we find a transient decrease of the depletion layer thickness that has not been observed experimentally. Based on the results of our simulations, we present a simple procedure to determine a few, well defined characteristic parameters from the experimental density profiles.
Role of liquid reactant volatility in gas absorption with an exothermic reaction
Authors: Al‐Ubaidi Basil H. , Selim M. Sami
Published: June 2004

A film‐theory model is presented for nonisothermal gas absorption with a second‐order exothermic reaction. The model accounts for the volatility of the liquid reactant and heat transfer from the liquid surface to the gas phase. The pertinent equations were solved numerically using B‐spline collocation. Results from this solution show that for intermediate values of Hatta number the liquid‐reactant volatility is deterimental to the enhancement of gas absorption. As Hatta number approaches zero or infinity, however, the effect of liquid‐reactant volatility becomes minor. Heat losses to the gas phase drastically reduce the interfacial temperature rise, which in turn enhances or inhibits the absorption rate depending on the effective activation energy being larger or less than zero, respectively. Approximate expressions for the enhancement factor and the interfacial temperature rise were also developed. Comparisons with the “exact” numerical solution verified the accuracy of these expressions over a reasonable spectrum of parameter values. The model developed was applied to two cases representing real conditions: the chlorination of toluene and the sulfonation of dodecylbenzene. Volatility effects are shown to be important for the former system, while the relatively nonvolatile dodecylbenzene served as a counter example.
Predicting polymer/solvent diffusion coefficients using free‐volume theory
Authors: Zielinski John M. , Duda J. L.
Published: June 2004

The Vrentas/Duda free‐volume diffusion model accurately correlates polymer/solvent diffusion coefficients over wide ranges of concentration and temperature. Currently the model is semipredictive: limited diffusion data are required to estimate model parameters that can then be used to predict diffusion coefficient behavior over sundry conditions. In this work, we present methods for estimating all of the model parameters without any diffusion data and examine the accuracy of the resulting diffusion coefficient predictions. This is the only technique known that predicts polymer/solvent diffusion behavior without any use of any diffusion data.
Modeling of the heat‐transfer process in a differential scanning calorimeter
Authors: Sen B.
Published: June 2004

The model developed predicts a priori potential errors associated with the energy trace recorded by an isoperibol differential power scanning calorimeter in the measurement of heat of adsorption of H2 on Pt and Pd catalysts. The uptake of H2 by the catalyst sample was approximated by a diffusion‐limited quasi‐steady‐state moving boundary model. This approximation is valid only if the parameter [(adsorption capacity of cat. sample)/(inlet conc. of H2)] is extremely large (∼ 24). The effect of flow rate, amount of H2 adsorbed, sink temperature, and the thermal conductivity of the adsorbate mixture was examined. Model predictions indicate that the error in the energy trace recorded by the DSC is appreciable: if a large difference exists between the thermal conductivity of the inert carrier, Ar (K = 0.017 J/m·K·s), and the adsorbate, H2 (k = 0.174 J/m·K·s); if the heat sink temperature is much lower (∼ 90 K) than the measurement temperature. However, these errors can be eliminated by matching the thermal conductivity of the inert carrier and adsorbate, such as He (k = 0.143 J/m·K·s) and H2 (k = 0.174 J/m·K·s). The results agree well with the experimental observations of Vannice et al. (1987) on high‐purity Pt and Pd powder and supported Pt catalysts, if the H2 uptake by the catalyst sample in the calorimeter is small (⩽2 μmol).
Recovery of short‐lived chemical species in a Couette flow reactor
Authors: Ouyang Q. , Swinney Harry L. , Roux J. C. , De Kepper P. , Boissonade J.
Published: June 2004

A new technique for studying and recovering short‐lived chemical intermediate species has been developed using a Couette reactor, which is an open one‐dimensional reaction‐diffusion system. Reaction occurs in the annulus between concentric cylinders with the inner one rotating and the outer one at rest. Fresh reagents are in contact with the ends of the annulus, but there is no net axial flow. The axial transport arising from the hydrodynamic motion is effectively diffusive, but has a diffusion coefficient 3 to 5 orders of magnitude larger than that of molecular diffusion. The oxidant (ClO2−) and reductant (I−) of an autocatalytic reaction are fed at opposite ends of the reactor. The reactants diffuse toward each other and react, forming a steady, sharp chemical front and a stable spatial concentration band of unstable intermediate species (HOCl) in the front region. Unstable intermediate species are thus stabilized at a well‐defined spatial position where they can be recovered and studied. The experiments and numerical simulations demonstrate that the faster the reaction rate, the stabler the chemical front and the more effective the recovery of unstable intermediate species.
Kinetics and column dynamics for adsorption of bulk liquid mixtures
Authors: Sircar S. , Rao M. B.
Published: June 2004

A framework is developed to describe kinetics and column dynamics of ad(de)sorption from bulk liquid mixtures using surface excess as the variable to quantify the extent of adsorption. It is found that the transient rate of change of surface excess with time from a multicomponent liquid mixture can be expressed in terms of a surface excess linear‐driving‐force model. A local‐equilibrium model can be developed to describe the column dynamics of ad(de)sorption from liquid mixtures. Both self‐sharpening and proportionate pattern mass‐transfer zones can be formed depending on the shape of the surface‐excess isotherm and the selectivity of adsorption. Analysis of column dynamics for liquid mixture adsorption can be carried out analogous to that for adsorption from gas mixtures, when a constant pattern mass‐transfer zone is formed. The length of the mass‐transfer zone and the composition‐time column effluent profile can also be derived analytically for such a case. Experimental kinetics and column dynamics data for ad(de)sorption of ethanol‐water mixtures on a large‐pore activated carbon are analyzed using these models.
A kinetic model of membrane formation by CVD of SiO2 and Al2O3
Authors: Tsapatsis Michael , Gavalas George R.
Published: June 2004

Silica and alumina layers deposited onto the walls of porous Vycor tubes by chloride hydrolysis in an opposing reactants geometry have been characterized by scanning electron microscopy and electron microprobe analysis. The layers are asymmetric, having a long tail toward the side of the chloride flow and a sharp boundary at the other side. The deposit thickness is several tenths of microns, while the totally plugged region is of order of 1 micron.

A model has been developed describing reaction, diffusion and evolution of the porous structure in the Vycor substrate due to the accumulation of the solid product. The deposition reaction is described by transient kinetics in terms if the concentrations of silanol and chloride groups in the product layer, as well as the concentrations of the gaseous reactants. The model is capable of generating deposit profiles in good agreement with those measured by electron microprobe analysis.
Modified MWR approach: Application to agglomerative precipitation
Authors: Bhatia S. K. , Chakraborty D.
Published: June 2004

A modified method of weighted residuals approach is presented that combines expansions based on limiting forms of the true solution to solve the population balance equation accurately for continuous agglomerative precipitation. A modified moment method is used that employs fractional moments and is stable for the infinite domain problem. Solutions are obtained for various values of the agglomeration parameter, and the results are fitted to Pade approximations providing simple relationships that can be used for data fitting, thus obviating further need to solve the population balance equation with a constant aggregation kernel. This method is applied to experimental data on the precipitation of nickel ammonium sulfate and calcium carbonate to yield kinetic parameters.
Chromatography of liquid‐phase adsorption on octadecylsilyl‐silica gel
Authors: Miyabe Kanji , Suzuki Motoyuki
Published: June 2004

Intraparticle diffusion and adsorption equilibrium on octadecylsilyl‐silica gel (ODS) were studied. As an adsorbate, p‐tert‐octylphenol (PTOP) was employed. The intraparticle diffusion coefficient was measured by the chromatographic technique and the moment analysis at zero and several surface coverages. Adsorption isotherms can be correlated by the Langmuir equation. The isosteric heat of adsorption and the activation energy of surface diffusion were constant regardless of the amount adsorbed. These results suggest that ODS has an energetically homogeneous surface. The effect of solvent on isosteric heat of adsorption in the liquid system was discussed qualitatively by the solvophobic theory. Surface diffusion coefficient of PTOP on ODS was of the order of 10−7 to 10−6 cm2/s. Surface diffusion was dominant for the intraparticle diffusion in ODS, and its coefficient increased with an increase in the amount adsorbed. Positive concentration dependence of the surface diffusion coefficient was interpreted in terms of diffusion by potential chemical driving force.
Direct catalytic hydrodechlorination of toxic organics in wastewater
Authors: Kovenklioglu Suphan , Cao Zhihua , Shah Dinesh , Farrauto Robert J. , Balko Edward N.
Published: June 2004

The in‐situ catalytic hydrodechlorination of chlorinated hydrocarbons in waste‐water‐generating HCl and a hydrocarbon‐free chlorine is demonstrated as a viable wastewater remediation technique. Catalyst screening studies with a shaker‐type hydrogenation reactor have shown that the commercial Pd/C catalyst is highly effective in hydrochlorinating various chlorinated hydrocarbons in synthetic wastewater at room temperature and near atmospheric pressure. 1, 1, 2‐trichloroethane hydrodechlorination experiments in an autoclave reactor shows that initial rates are well correlated with first‐order dependence of the reactant hydrocarbon adsorbed on carbon. Initial rates are also independent of hydrogen pressure, and adsorption on the carbon support is Langmuir type. Activation energies calculated at different catalyst loadings varied from 29 to 38 MJ/mol.

1,1,2‐trichloroethane hydrodechlorination activity is much lower for Pd/Al2O3 than Pd/C because the reactant hydrocarbon does not adsorb on alumina. When the carbon support does not readily adsorb the reactant hydrocarbon, the hydrodechlorination rates dropped significantly. These results confirm the role of the carbon support in providing the major path to reaction and thereby significantly increasing reaction rates compared to direct adsorption from solution onto the palladium.
Gas‐phase backmixing in bubble‐column reactors
Authors: Shetty S. A. , Kantak M. V. , Kelkar B. G.
Published: June 2004

Gas‐phase backmixing and holdup for the air‐water system were measured in 0.15‐ and 0.25‐m‐dia. bubble columns. Novel experimental and analytical techniques were used to analyze the gas‐phase residence time distribution. A Levenberg‐Marquardt‐type optimization scheme was used to obtain the two reactor model parameters: Peclet and Stanton numbers. Operating variables included the column diameter and the superficial gas‐ and liquid‐phase velocities.

A two‐bubble‐class hydrodynamic model has been proposed to predict the gas‐phase backmixing in bubble column reactors. The model assumes a bimodal distribution of the gas phase with large bubbles flowing in a plug‐flow manner and the small bubbles partially backmixed. The model only needs easily measurable parameters and has been validated by comparing experimental as well as literature data on gas‐phase backmixing with the model predictions. The model is more realistic than the conventional single‐bubble‐class model and provides a rational approach to predict the gas‐phase backmixing in scaling‐up bubble‐column reactors.
Structure H clathrate hydrate equilibria of methane and adamantane
Authors: Lederhos J. P. , Mehta A. P. , Nyberg G. B. , Warn K. J. , Sloan E. D.
Published: June 2004

The first phase equilibrium data are presented for Structure H hydrates. The data represent the initial formation of these hydrates from methane, with adamantane— a previously determined Structure H former. Temperature and pressure conditions are consistent with hydrocarbon production/transportation/processing facilities. Structure H hydrates are shown to contain molecules indigenous to petroleum. which may not be present in natural gas.
Parallel computational microhydrodynamics: Communication scheduling strategies
Authors: Fuentes Yuris O. , Kim Sangtae
Published: June 2004

The behavior of multiparticle systems in a viscous fluid, as governed by the Stokes equations, is computed by the coordinated use of multiple processors on a distributed memory parallel computer. The completed double‐layer boundary integral equation method (CDL‐BIEM) is used to convert the hydrodynamic mobility problems to a fixed‐point problem, amenable to either synchronous or asynchronous iterative solution schemes. Parallel computational strategies, based on assigning particles to processors, are considered, and rules are derived to specify how often processors should exchange information. A spectral communication scheduling strategy, based on the spectral radius in pair‐interaction problems, converges with fewer global iterations and effectively reduces the level of interprocessor communications, suggesting algorithm scalability to massively‐parallel computers with hierarchical access to distributed memories. Stochastic schedules, which specify the probability that the information exchange occurs at every iteration, were also considered. For the same test problems, these strategies performed better than the point Jacobi iterations, but not as well as their deterministic counterparts. Scheduling strategies are extrapolated to larger problems, based on projections of memory and performance capabilities of the next generation of high‐performance parallel supercomputers.
Measurement of individual cell migration parameters for human tissue cells
Authors: DiMilla Paul A. , Quinn John A. , Albelda Steven M. , Lauffenburger Douglas A.
Published: June 2004

We present an approach for determining in vitro the means and distributions of a set of phenomenological parameters, including cell speed and persistence time, which can be used to evaluate the effect of isotropic variations in the extracellular environment on the motility of human tissue cells. Using time‐lapse videomicroscopy and semi‐automated image analysis, we tracked the paths traveled by slow‐moving, isolated human vascular smooth muscle cells over 48 hours on surfaces of petri dishes coated with 10 μg/mL of the adhesive extracellular matrix proteins type IV collagen, fibronectin or laminin. By applying a persistent random walk model to experimental data for mean‐squared displacement as a function of time for these cells, we rigorously distinguished individual cells with different motile characteristics not obvious based on qualitative comparisons between the structures of individual cell paths. We also positively identified the presence of immotile cells. Based on the behavior of 34 to 77 cells on each substrate, we found mean cell speeds and persistence times on the order of 10 micron/h and 3 hours, respectively, on all three ECM substrates, while the fraction of motile cells varied from 65% on laminin to 78% on collagen. On all three surfaces experimental number distributions of speed and persistence time could be described by normal and exponential waiting time distributions, respectively. Our approach provides a framework for addressing questions concerning the mechanistic relationship between cellular and environmental properties and cell motility.
Investigation of the chemistry of a diazo micromixing test reaction
Authors: Wenger Kevin S. , Dunlop Eric H. , MacGilp Iain D.
Published: June 2004

The azo coupling of 1‐naphthol with diazotized sulfanilic acid has been studied in detail focusing on the practical use of this reaction as a micromixing test reaction, as developed by Bourne and coworkers. The reaction is a fast, competitive, consecutive reaction whose final product distribution is affected greatly by mixing. Problems that occur in the isolation of the pure‐dye products and quantification of the product distribution are addressed. Previously unreported information is given about the structure and properties of one of the products as well as the existence of an additional unknown product. The reaction was used to characterize the spatial heterogeneity of micromixing in a 14‐L stirred‐tank fermenter. Results show large differences in the product distribution dependent on the depth and radial position of the feed pipe in the tank.
Modeling of bubble‐size distribution in free rise polyurethane foams
Authors: Niyogi Debdarsan , Kumar R. , Gandhi K. S.
Published: June 2004

The existing models of the foaming process in reaction injection molding are inadequate for predicting the bubble‐size distribution, which influences many properties of interest. This work attempts to develop a model for the bubble‐size distribution under conditions of free rise.

The population balance equation for bubbles, coupled with the system of energy balance and reaction kinetics equations through the blowing agent evaporation rate expression, is solved to obtain the bubble‐size distribution. A simple mass‐transfer coefficient approach for expressing growth rate of bubbles is used, although various growth laws can be accommodated without affecting the solution procedure. Determining factors for the bubble size and its distribution are the foaming period and the growth law. With increase in initial blowing agent concentration, the foaming period increases and bigger bubbles are obtained. The bubble‐size distribution becomes broader as the foaming period increases.
Synthesis of reverse‐osmosis networks for waste reduction
Authors: El‐Halwagi Mahmoud M.
Published: June 2004

The purpose of this work is to introduce the novel notion of synthesizing reverseosmosis networks (RONs) for waste‐reduction applications. The RON design task aims at synthesizing a network of reverse‐osmosis units, booster pumps and energy recovery devices that can separate a set of waste streams into lean (product) streams and rich (retenate) streams at minimum total annualized cost. A systematic and generally applicable procedure for tackling RON synthesis problems is developed. First, a structural representation is devised to embed all potential network configurations. Then, the problem is formulated as an optimization program whose objective is to minimize the total annualized cost of the network while satisfying all environmental and technical requirements. The solution to this program provides the optimal arrangement, types and sizes of the reverse‐osmosis units, the booster pumps, and the energy‐recovery turbines to be employed in the network. Furthermore, the solution also provides the optimum stream distribution, operating conditions, and separation levels. The applicability of the proposed synthesis technique is demonstrated by tackling case studies and comparing the optimal solutions with current industrial practices.
Coalescence of charged droplets in agitated liquid‐liquid dispersions
Authors: Tobin T. , Ramkrishna D.
Published: June 2004

Experimental studies have been performed to assess the effect of drop charge on turbulent drop coalescence. Although it is well known that surface charge can affect coalescence rates of Brownian particles in a quiescent medium, no studies have been found in the literature which explicitly examine charge effects on larger drops in turbulent media.

Results demonstate that drop charge can substantially inhibit coalescence between drops in agitated dispersions. Moreover, this effect can arise solely from latent drop charges, which are attributed to the preferential adsorption of hydroxide ions onto the water‐organic phase interface. Ionic strength and pH thus are shown to play a major role in determining coalescence rates between agitated drops.

Results also demonstrate that reliance on measurement of average drop sizes for the purpose of determining coalescence rate dependencies is a precarious practice. Strong size‐dependent effects such as obtained here have the potential to suggest erroneous conclusions when conclusions are based on measurements of average drop sizes rather than of drop‐size distributions.
Theory of hydrogen bonding in supercritical fluids
Authors: Gupta Ram B. , Panayiotou Constantinos G. , Sanchez Isaac C. , Johnston Keith P.
Published: June 2004

The degree of hydrogen bonding and macroscopic thermodynamic properties for pure and mixed fluids are predicted with the hydrogen bonding lattice fluid (LFHB) equation of state over a wide range in density encompassing the gas, liquid and supercritical states. The model is successful for molecules forming complex self‐associated networks, in this case pure methanol, ethanol, and water, and the mixture 1‐hexanol‐SF6. In supercritical water, significant hydrogen bonding is still present despite all the thermal energy and is highly pressure‐ and temperature‐dependent. A fundamental description of pressure and temperature effects on hydrogen bonding is presented for a well‐defined case, the formation of a complex between a donor and acceptor in an inert solvent, where no self‐association is present. The partial molar enthalpy and volume change on complexation both become pronounced near the critical point, where the density is highly variable with temperature and pressure.
A coupled knowledge based system using fuzzy optimization for advisory control
Authors: Petti Thomas F. , Dhurjati Prasad S.
Published: June 2004

A knowledge based system is described that is designed to generate on‐line advice for operators regarding the proper distribution of hydrogen resources in a refinery. The system uses a coupled architecture incorporating numerical computing in a knowledge based system environment. This arrangement allows for powerful and flexible problem solving. One portion of the coupled system formulates an optimization problem that is subsequently solved by an external routine. This application is particularly concerned with uncertainty that is present in some of the constraints. To deal with this uncertainty, a fuzzy approach to the optimization is taken. A method is presented that solves the fuzzy optimization problem using standard mathematical programming techniques. The results of the fuzzy optimization allow the crisp solution to be expanded into a neighborhood of solutions that is considered acceptable. Although this work examines a specific problem, the concepts presented are general.
Fire and explosion hazards induced by repressurization of air driers
Authors: Schoofs Gregory R.
Published: June 2004

Adsorbers used to dry or purify air typically contain 4A or 13X molecular sieves which are intermittently reactivated by thermal regeneration. This article shows that nitrogen adsorbs preferentially relative to oxygen in freshly reactivated 4A, 5A and 13X molecular sieves during the repressurization step. This produces an oxygen‐enriched, high‐pressure gas within the adsorber vessel which emerges immediately after the air‐drier is brought on‐line. The enriched oxygen concentration widens the flammability range of hydrocarbons, markedly diminishes the minimum autoignition temperature of hydrocarbons, and produces both a higher explosion pressure and a higher rate of explosion pressure rise relative to air at standard temperature and pressure. Thus, the oxygen‐enriched, high‐pressure gas can lead to fires, explosions, and other deleterious effects in and downstream of thermally regenerated air driers which contain large pore molecular sieves. Substituting 3A molecular sieve provides a simple, safe and inexpensive way to prevent transient oxygen enrichment and the problems associated with it. Because activated aluminas and silica gels can also enhance fire and explosion hazards, type 3A molecular sieve becomes the adsorbent of choice for air drying applications.
Accelerated disturbance damping of an unknown distributed system by nonlinear feedback
Authors: Chen Chien‐Chong , Chang Hsueh‐Chia
Published: June 2004

We propose a new control design methodology for distributed systems with unknown nonlinear dynamics. The approach is appropriate for systems whose linearized differential operator possesses an eigenspectrum that can be partitioned into a low‐dimensional slow spectrum and an infinite‐dimensional fast complement. This separation of time scales allows us to utilize center manifold and normal form techniques of modern geometric theories for dynamical systems. It is shown that the convergence to an asymptotically stable equilibrium point after a small‐amplitude transient disturbance quickly (exponentially fast) approaches an invariant manifold W which is locally tangent to the eigenspace of the slow modes and is hence of the same dimension. The nonlinear dynamics on this invariant manifold is much slower than the fast approach and is dominated by the slow modes with the fast modes coupled “adiabatically” to them. The convergence can hence be best accelerated using a slow control with smooth nonlinear feedback involving only the slow modes. Nonlinear feedback is shown to drastically improve the performance of linear feedback. The only required information about the system in our approach is the slow adjoint eigenfunctions which can be easily estimated with a Karhunen‐Loeve scheme for distributed systems. This identification scheme is quite robust to changes in process dynamics and can hence be carried out on‐line in parallel with feedback control. The overall approach is verified by numerical experiments.
Racemic leucine separation by hollow‐fiber extraction
Authors: Ding H. B. , Carr P. W. , Cussler E. L.
Published: June 2004

Racemic leucine can be separated into d‐ and l‐isomers by fractional extraction across microporous hollow fibers. In this extraction, an aqueous solution of the racemate is fed to the lumen of the fibers, and an octanol solution of dodecyl‐l‐hydroxyproline flows countercurrently outside of the fibers. The interface between feed and extractant is stabilized by filling the pores in the hollow‐fiber walls with a cross‐linked polyvinylalcohol gel which offers negligible resistance to mass transfer. The extraction with dodecyl‐l‐hydroxyproline deliberately imitates earlier studies, facilitating comparisons of hollow‐fiber extraction with other techniques. The results show that the isomer yield per equipment volume of racemic separation is 100 times greater than that in a continuously rotating extractor, and 1,000 times greater than that in a conventional packed tower.
Optimal grade transitions in a gas phase polyethylene reactor
Authors: McAuley K. B. , MacGregor J. F.
Published: June 2004

Using gas‐phase technology many grades of polyethylene can be produced in a single reactor. For a series of three polyethylene products, model‐based dynamic optimization is used to determine optimal grade changeover policies. Optimal manipulated variable profiles are determined for hydrogen and butene feed rates, reactor temperature setpoint, gas bleed flow, catalyst feed rate, and bed level setpoint. It is shown that large transitions in melt index are hampered by slow hydrogen dynamics, and that the time required for such a transition can be reduced by manipulating the temperature setpoint and the bleed stream flow. Reduction of the bed level and catalyst feed rates during changeovers can significantly reduce the quantity of off‐specification polymer produced. It is demonstrated that melt index and density are not sufficient to characterize the properties of polymer produced during grade transitions, and that the shape of the cumulative copolymer composition distribution is very sensitive to the grade changeover policy used. Optimal transition policies should not be implemented without feedback control. Disturbances and model mismatch can result in product property trajectories which deviate significantly from the nominal optimal trajectory.
Unsteady axial mixing by natural convection in a vertical column
Authors: Baird M. H. I. , Aravamudan K. , Rao N. V. Rama , Chadam J. , Peirce A. P.
Published: June 2004

Unsteady axial mixing due to addition of a batch of sodium chloride solution at the top of a water‐filled tube (2.63 cm i.d.) has been studied by measuring the developing concentration profile and the advancing front with dye added to the brine. Data have also been obtained with added baffle plates, with the use of a viscous aqueous solution, and in smaller diameter (1.48, 1.91 cm) tubes. Results can be approximately correlated by means of a model based on unsteady one‐dimensional turbulent dispersion. Laminar flow affects the behavior of the advancing front at which the salt concentration is lowest.
Modeling of growth rate dispersion in batch cooling crystallization
Authors: Bohlin Martin , Rasmuson Åke C.
Published: June 2004

The influence of growth rate dispersion on the product‐size distribution of batch cooling crystallization is investigated by computer simulations. The model accounts for primary and magma density‐dependent secondary nucleation, and growth rate dispersion of the constant crystal growth type. The model is solved by a combination of the method of characteristics and moment analysis, by which the entire product‐size distribution is recovered. The study includes three different growth rate activity distributions, and the influence of the corresponding coefficient of variation is analyzed for unseeded and seeded processes. The results show that the effect of growth rate dispersion on the crystal‐size distribution may be significant even at moderate dispersion. At high dispersion, even the actual shape of the growth rate activity distribution may become important.
Gas holdup in stacked expanded metal sheets
Authors: Piovano Stella , Böhm Ursula , Franco Juan I.
Published: June 2004

The gas holdup in different arrays of expanded metal sheets was determined by the conductimetric method.

A model had to be developed for analyzing the conductimetric behavior of the cell in the presence of the metallic sheets. The validity of this model has been verified with single phase flow. The application to two‐phase flow allowed the obtaining of information on gas accumulation within stacked expanded metal sheets, showing a strong influence of the geometry of the array. Depending on the orientation of the stacked sheets and of the large dimension of the expanded metal, the gas holdup within the packing will be equal or larger than in an empty bubble column.
Startup of an industrial adiabatic tubular reactor
Authors: Verwijs J. W. , van den Berg H. , Westerterp K. R.
Published: June 2004

The dynamic behaviour of an adiabatic tubular plant reactor during the startup is demonstrated, together with the impact of a feed‐pump failure of one of the reactants. A dynamic model of the reactor system is presented, and the system response is calculated as a function of experimentally‐determined, time‐dependent, manipulated variables. The values of model parameters are estimated by using the SimuSolv (1991) computer program. The data set collected during the reactor start‐up is used for the parameter estimation procedure. An excellent agreement is obtained between the experimental and the calculated system response. Many continuously‐operated commercial reactors require a complete conversion of one of the main reactants at the reactor exit. It is shown that for an industrial tubular reactor a much higher initial reactor temperature is required during the startup, compared to the reactor inlet temperature during normal steady‐state operation, to ensure a complete reactant conversion. Much more research is necessary to determine whether this is a generally valid rule.
Slow motion of multiple droplets in arbitrary three‐dimensional configurations
Authors: Keh Huan J. , Tseng Yi K.
Published: June 2004

This article presents a combined analytical‐numerical study for the hydrodynamic interactions of an arbitrary finite cluster of spherical droplets at low Reynolds numbers. The droplets may differ in radius, in viscosity and in migration velocity. The theory developed is the most general solution to the mobility or resistance problem of slow motion of an assemblage of fluid spheres in a three‐dimensional unbounded medium. Using a boundary collocation technique, the creeping flow equations are solved in the quasisteady situation, and the interaction effects among the droplets are evaluated for various cases. For the motion of two‐droplet systems, our results for the droplet interaction parameters at all orientations and separation distances agree very well with the exact solutions obtained by using spherical bipolar coordinates or the asymptotic solutions obtained by using the connector algebra. The mobility parameters of linear chains of three droplets have been calculated, demonstrating that the existence of the third sphere can significantly affect the mobility of the other two spheres. The mobility results are also presented for the motion of chains containing up to 101 identical and equally spaced spheres, and the end effect exhibited by a chain of spheres can be found. Finally, our “exact” solutions for the hydrodynamic interaction between two fluid or solid spheres are used to find the effect of volume fractions of particles of each type on the mean settling velocities of the particles in a bounded suspension.