bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of a complex reaction network: I. Experiments in a high‐pressure trickle‐bed reactor
Authors: Khadilkar M. R. , Jiang Y. , Al‐Dahhan M. , Duduković M. P. , Chou S. K. , Ahmed G. , Kahney R.
Published: April 2004

A high‐pressure trickle‐bed reactor was used to achieue high productivity and selectivity for the manufacture of a key herbicide intermediate (α‐aminomethyl‐2‐furanmethanol) (amino alcohol, AA) from α‐nitromethyl‐2‐furanmethanol (nitro alcohol, NA). Raney Nickel catalysts of varying activity were prescreened for suitability in tricklebed operation. The effect of operating parameters such as reactant feed concentration, liquid mass uelocity, and temperature on the yield of amino alcohol (A) for RNi‐a are discussed. The superiority of trickle‐bed reactors ouer others such as semibatch and batch sluny systems is demonstrated. The AA yield increases with decrease in feed reactant concentration and liquid mass velocity, as well as with lowering of the operat ing temperature. A maximum product yield of 90.1% was obtained at 8.3 wt. % feed concentration of nitroalcohol (NA), while at the highest feed concentration of 40 wt. % NA, the maximum product yield was 58%. The volumetric productivity of AA was significantly higher at higher reactant feed concentrations, even though the yield was lower under these conditions. The operating temperature was also an important parameter, with a lower temperature being preferable for trickle‐bed experiments. Bed dilution with inert fines improued catalyst utilization and increased the AA yield and productivity in the laboratory‐scale trickle‐bed reactor.
Investigation of a complex reaction network: II. Kinetics, mechanism and parameter estimation
Authors: Jiang Y. , Khadilkar M. R. , Al‐Dahhan M. , Duduković M. P. , Chou S. K. , Ahmed G. , Kahney R.
Published: April 2004

Conventional strategies for discrimination of intrinsic and apparent kinetics from crushed‐ and whole‐catalyst‐pellet experimental data, respectively, do not yield satisfactory results for the reaction network in the manufacture of (α‐aminomethyl‐2‐furanmethanol) (aminoalcohol) from αnitromethyl‐2‐furanmethanol (nitroalcohol). Laboratory trickle‐bed reactor tests in the range of concentration and product yield of commercial interest are utilized to yield a reasonable set of kinetic parameters, which are otherwise unobtainable. This is accomplished by proposing a reaction network, a plausible mechanism, and optimizing the kinetic parameters based on the reactor‐model‐generated peformance data to fit experimental output concentrations of all species for the entire set of experiments. A complex reaction network for the key reactions in the system is developed based on the reaction scheme in Part I. Fitting of trickle‐bed reactor data to this model is attempted to yield an insight into the actual kinetics. The results show promise of obtaining an overall network kinetic model, even with the limited data available.
Structural stability of kinetic models: Anomalies due to irreversible adsorption
Authors: Nibbelke Rob H. , Hoebink Jozef H. B. J. , de Croon Mart H. J. M. , Marin Guy B.
Published: April 2004

The assumption of irreversible adsorption of a reactant can lead to a structurally unstable kinetic model. The structural instability is caused by the fact that two steadystate solutions intersect in the bifurcation diagram o j for example, the degree of surface coverage of one of the adsorbates us. the partial pressure of one of the reactants. One of the two solutions is a trivial solution, corresponding to a catalyst surface completely covered with the irreversibly adsorbing reactant. The intersection point is a transcritical bifurcation point. A structurally stable kinetic model is obtained i f an arbitrarily small desorption rate coefficient is introduced. Two examples are discussed in which a transcritical bifurcation point is encountered, that is, the total oxidation of ethene in three‐way catalysis and the chemical vapor deposition of refractory metals. The consequences of the existence of a transcritical bifurcation point for kinetic parameter estimation and reactor simulation are emphasized.
Study of the morphological hysteresis in immiscible polymer blends
Authors: Minale Mario , Mewis Jan , Moldenaers Paula
Published: April 2004

The effect of processing conditions (essentially the flow conditions) on the mophology of incompatible, immiscible polymer blends was investigated. In simple shear flow the steady‐state mophology, that is, droplet size, of such blends is a single‐valued finction of the shear rate whenever a critical shear rate is exceeded. Below this critical value it becomes uely dificult to reach a dynamic equilibrium between droplet breakup and coalescence, which gives rise to a hysteresis region for the size of the inclusions. Such regions were identified and studied for model‐blend systems. The critical shear rate decreases with increasing concentration of the droplet phase. A large ratio of droplet over matrix viscosity seems to have the opposite effect. The hysteresis region is bounded by the critical curues for droplet breakup and coalescence. The accessibility of this re gion can therefore be used to evaluate models for the said phenomena in nondilute systems. Droplet breakup is nearly independent of droplet concentration, while elasticity of the dispersed phase seems to facilitate breakup. As expected, coalescence is enhanced by higher concentrations. Droplet elasticity induces a certain degree of “rigidity” to the interface, and therefore the mobile interface model for coalescence is not suitable for such systems.
Droplet size evolution during coalescence in semiconcentrated model blends
Authors: Vinckier I. , Moldenaers P. , Terracciano A. M. , Grizzuti N.
Published: April 2004

The droplet‐size evolution in polymer blends, due to flow‐driven coalescence, is nvestigated both experimentally and theoretically. In the experiments, preshearing at a high rate is applied to generate a fine morphology in the blend; subsequently, the shear rate is suddenly decreased to induce coalescence, and the resulting growth in droplet size is measured. It is demonstrated that rheological as well as microscopic techniques can be used for an in‐situ determination of the size evolution of the droplets. Effects of shear rate, step‐down ratio, and blend concentration were studied systematically. A master curve for the droplet growth at each step ratio can be obtained by plotting the relative increase in droplet diameter us. the strain after step‐down in shear rate, multiplied by the fraction of the disperse‐phase squared. To model the droplet‐size evolution the strategy of Chesters is followed. It is implemented here for semiconcentrated blends, resulting in a one‐parameter model equation that correctly describes the experimental observations.
Modeling and simulation of shell and tube heat exchangers under milk fouling
Authors: Georgiadis Michael C. , Rotstein Guillermo E. , Macchietto Sandro
Published: April 2004

A mathematical model for single shell and tube heat exchangers under milk fouling is presented. A fouling model based on a reaction/mass‐transfer scheme is detailed in which the main factors during milk heat treatment are quantified in a formal way. This model is coupled with a detailed dynamic model of a shell‐and‐tube heat exchanger where both radial and axial domains are taken into account. An analytical procedure for the calculation of key parameters provides the means to achieve more accuracy. The simulation results agree well with available experimental work. Four different heat exchanger arrangements are then considered to illustrate their impact on the fouling behavior. The results are encouraging enough to validate current operating industrial techniques for fouling mitigation. For a given thermal duty, short heat exchangers are more prone to fouling due to high‐temperature requirements and milk should be heated 11s gradually as possible to minimize fouling. The results show that there are main tradeoffs between design and operation issues.
Filtration of particulate suspensions in acoustically driven porous media
Authors: Gupta Sanjay , Feke Donald L.
Published: April 2004

Retention of suspended solids within a porous medium subjected to resonant ultrasonic fields was studied experimentally. In laboratory tests, 325 mesh polystyrene particles were collected with high (typically > 80%) single‐pass efficiency using a polyester foam mesh with pores more than two orders of magnitude larger than the particles retained. Deactivation of the acoustic field allows the retained particles to be flushed from the porous medium. The effects on filtration efficiency of processing variables (suspension flow rate, solids concentration in the feed, intensity of the acoustic field, and frequency of the acoustic field) are reported. A simplified analysis of particle trajectories toward cylindrical collectors in response to acoustic and hydrodynamic forces provides insight into the fundamental physical phenomena that govern the acoustically enhanced filtration process.
Mixing and particle dispersion in the wavy vortex regime of Taylor–Couette flow
Authors: Rudman Murray
Published: April 2004

Fluid flow and particle dispersion were investigated numerically in the wavy vortex regime of Taylor‐Couette flow. The flow field for wavy vortex flow is stationary when viewed in a frame rotating with the azimuthal wave velocity. These steady flow fields are used to track fluid particles and to estimate the effective axial difision resulting from chaotic fluid aduection. The effective diffusion coefficient for a fixed wave state is a function of the Reynolds number. Particle dispersion is a strong function of wave state, showing that a universal relationship between dispersion and Reynolds number cannot be found in this regime of cylindrical Taylor‐Couette flow. The effectiue Schmidt number of chaotic advection is less than unity for all wavy vortex flows examined, indicating that chaotic advection plays an important role in fluid mixing in these flow regimes. Fluid particle retention in the cores of the wavy vortices is also predicted for some parameter regimes, although not all. Particles trapped in vortex cores are only poorly mixed within the core and play no role in global mixing. A preliminary examination of inertial particle settling suggests that the fluid flow does not prevent settling in the mean, although the magnitude of the settling velocity significantly affects the dispersion of inertial particles.
Critical criteria for ignition of combustible fluids in insulation materials
Authors: Brindiey J. , Grifiths J. F. , Zhang J. , Hafiz N. A. , McIntosh A. C.
Published: April 2004

The possibility of a combustion hazard, when a flammable liquid leaks into insulation materials, is determined by the surface temperature, the volatility and loading of the fluid, the exothermicity of the combustion process, and the porosity of the matrix. Whether or not there is asymmetric heating is also influential. The oxygen diffusion rate or a reduction in its ambient concentration have a limited effect. A dimensionless, analytical, criticality criterion (u∞) for ignition of flammable liquids dispersed within a hot inert matrix is compared with a dimensionless parameter (u∞) based on the classic Frank‐Kamenetskii ignition criterion. The relationship between u∞, and uFK is explored numerically over wide ranges of conditions and the switch as a monitor of safe operating practice are calculated assuming uniform surface heating. The application of u∞, is most appropriate with a high loading of a relatively high volatility fluid dispersed within a matrix that presents a very high internal surface area, and the reaction is not very exothermic. The application of uFK is appropriate to other circumstances. It was also found that certain classes of compounds may be capable of bonding to insulation in a way that suppresses their normal evaporative loss. This may signify a hitherto unrecognized problem that certain combinations of liquids and pipe insulating materials enhance combustion hazards.
Large‐scale DAE optimization using a simultaneous NLP formulation
Authors: Cervantes A. , Biegler L. T.
Published: April 2004

The differential‐algebraic equation (DAE) optimization problem is transformed to a nonlinear programming problem by applying collocation on finite elements. The resulting problem is solved using a reduced space successive quadratic programming (rSQP) algorithm. Here, the variable space is partitioned into range and null spaces. Partitioning by choosing a pivot sequence for an LU factorization with partial pivoting allows us to detect unstable modes in the DAE system, which can now be stabilized without imposing new boundary conditions. As a result, the range space is decomposed in a single step by exploiting the overall sparsity of the collocation matrix; which performs better than the two‐step condensation method used in standard collocation solvers. To deal with ill‐conditioned constraints, we also extend the rSQP algorithm to include dogleg steps for the range space step that solves the collocation equations. The performance of this algorithm was tested on two well known unstable problems and on three chemical engineering examples including two reactive distillation columns and a plug‐flow reactor with free radicals. One of these is u batch column where an equilibrium reaction takes place. The second reactive distillation problem is the startup of a continuous column with competitive reactions. These optimization problems, which include more than 150 DAEs, ure solved in less than 7 CPU minutes on workstation class computers.
Product design through multivariate statistical analysis of process data
Authors: Jaeckle Christiane M. , Macgregor John F.
Published: April 2004

A methodology is developed for finding a window of operating conditions within which one should be able to produce a product having a specified set of quality characteristics. The only information assumed to be available is that contained within historical data on the process obtained during the production of a range of existing product grades. Multivariate statistical methods are used to build and to invert either linear or nonlinear empirical latent variable models of the existing plant operations to obtain a window of operating conditions that are capable of yielding the desired product and that are still consistent with past operating procedures and constraints. The methods and concepts are illustrated using a simulated high‐pressure tubular reactor process for producing low‐density polyethylene.
Activated carbon production from low ash subbituminous coal with CO2 activation
Authors: Teng Hsisheng , Lin Hung‐chi
Published: April 2004

Activated carbon was prepared by physical activation in CO, from a low‐ash subbituminous coal. The preparation process consisted of carbonization of the coal in N, followed by activation of the resulting chars in CO2. The activation temperature ranged from 700 to 950°C. Experimental results revealed that the surface area, pore volume and average pore diameter of the activated carbon generally increased with the amount of carbon burnoff carried out at the same temperature. These surface characteristics were also influenced by the variation of the activation temperature. It was found that the porosity of the carbon passed through a maximum at a temperature of 750°C. The activation energy for the carbon gasification in CO2 was calculated to be in a range of 130 180 kJ/mol, based on the assumption that the reaction rate was proportional to the surface area of the carbon. The apparent order with respect to CO2 partial pressure for the gasification was found to be within the range of 0.15–0.25.
Thermodynamic consistency test for equilibria involving cross‐linked polymers
Authors: Hillaire A. , Favre E.
Published: April 2004

The implications of the peculiarities of equilibria involving a cross‐linked polymer network and a solvent were examined in the derivation of a thermodynamic consistency test. While strict application to a binary system achieves no tangible result, it is shown that a simple expression can be obtained from the Gibbs‐Duhem equation, if several sets of data, including both binary and ternary, are combined. The expression derived was applied to previously reported multicomponent data. The results and further developments of the newly proposed test are discussed.
Trailing vortices around a 45° pitched‐blade impeller
Authors: Schäfer M. , Yianneskis M. , Wächter P. , Durst F.
Published: April 2004

The trailing vortex system near impeller blades has been identified as the major flow mechanism responsible for mixing and dispersion in stirred vessels, and high turbulence levels in the vortices have an important impact on such phenomena as drop breakup and cell damage in bioreactors. Numerical computations of the flows require more detailed information on the velocity characteristics generated by different impeller designs than is available in the literature. Our study on the mean flow and turbulence structure generated by a pitched‐blade turbine with four 45° inclined blades found that single trailing vortex is formed around each turbine blade. The vortex axis spread out radially by less than 0.0015 T and was inclined at 20° to the horizontal plane. The vortices merged into the bulk flow structure at around 135° behind each blade. Periodicity of the mean flow due to the crossing of the individual blades and high levels of kinetic energy of turbulence (k) are contained within a radial distance of around r/T = 0.23 from the axis and a vertical distance of z/T = 0.07–0.46 from the bottom of the vessel. The k levels decay to nearly‐uniform and low values outside this region. The results are compared with earlier investigations, and their implications for mixing processes and CFD predictions of the flows are discussed. The data identify flow regions accurately where intense turbulence is present and thus give useful indications for the optimization of mixing processes.
Model elastic liquids with water‐soluble polymers
Authors: Dontula P. , Macosko C. W. , Scriven L. E.
Published: April 2004

Model liquids with nearly constant viscosity and adjustable elasticity are needed to resolve the role of elasticity in coating and other free‐surface flows. Available Boger liquids are not well suited to free‐surface flows, because they are solutions in organic solvents and their viscosities exceeding 1 Pa · s fall on the high side. Aqueous liquids are preferred in laboratory studies partly due to environmental hazards. Aqueous polymer solutions with constant shear viscosity and adjustable elasticity were prepared by adding small amounts of a high‐molecular‐weight polymer to a more concentrated aqueous solution of the same polymer but of a much lower molecular weight. Up to 0.2 wt. % of high‐molecular weight poly(ethylene oxide) (PEO, Mw from 400,000 to 4 million g/mol) was added to almost inelastic solutions of low‐MW polyethylene glycol (PEG, Mn = 8,000 g/mol). PEG concentrations in these solutions varied between 16.7 and 42.9 wt. %. Shear viscosities of these solutions ranged from about 0.02 to 0.3 Pa · s and were constant up to shear rates of 100 s−1. The stress ratio is one measure of the elasticity of the liquid. Stress ratios up to 0.2 were estimated from small‐amplitude oscillatory measurements. Terminal behavior (elastic modulus rising with the square of the frequency) was not observed even at 10−2 rad/s. Viscosity and elasticity of the liquids can be manipulated over a wide range by varying the amounts and molecular weights of PEG and PEO within the unentangled and dilute regions of the concentration–molecular weight diagram, respectively. Fits of experimental data to candidate differential and integral constitutive equations are also discussed.
Modeling multicomponent gas separation using hollow‐fiber membrane contactors
Authors: Coker D. T. , Freeman B. D. , Fleming G. K.
Published: April 2004

A model developed for multicomponent gas separation using hollow‐fiber contactors permits simulation of cocurrent, countercurrent, and crossflow contacting patterns with permeate purging (or sweep). The numerical approach proposed permits simulation to much higher stage cuts than previously published work and provides rapid and stable solutions for cases with many components, with widely varying permeability coefficients. This new approach also permits the rational and straightforward incorporation of effects such as permeate sweep, pressure‐dependent permeability coefficients, and bore side pressure gradients. Simulation results are presented for separation of commercially significant multicomponent gas mixtures using polymer permeation properties similar to those of polysulfone. The effect of permeate purging on separation performance is explored for air separation. The influence of pressure ratio on hydrogen separation performance for a refinery stream is presented. Air is modeled as a four‐component mixture of O2, N2, CO2, and H2O and the refinery stream contains five components: H2, CH4, C2H4, C2H6, and C3H8. In air separation, permeate purging with a small fraction of the residue stream provides a very effective method for improving module efficiency for drying but is not efficient for improving nitrogen purity or recovery. In multicomponent mixtures, maxima in the compositions of components of intermediate permeability may be observed as a function of distance along the hollow fiber. This result suggests the use of membrane staging to capture these components at their maximum concentration.
Efficient use of an intermediate reboiler or condenser in a binary distillation
Authors: Agrawal Rakesh , Herron D. Michael
Published: April 2004

The impact of an intermediate reboiler or condenser on the distillation of ideal binary mixtures into pure product streams is studied using a simplified model. The advantage of heuristics derived from this study is that they can quickly tell a process engineer if an intermediate reboiler or condenser is going to be effective in improving the efficiency and, of the two options, which one would be more effective. The heuristics simply states that if the actual fraction of liquid in a given feed is less than that with the maximum thermodynamic efficiency for distillation with no intermediate reboiler or condenser, then an intermediate condenser not only substantially improves the thermodynamic efficiency but is also more effective than an intermediate reboiler. An analogous heuristics exists for the intermediate reboiler when the fraction of liquid in the feed is greater than the optimum. Quick identification of cases that can achieve a substantial improvement in efficiency provides an incentive to search for the proper utilities needed for the intermediate reboiler or condenser. When relatively pure feed streams (concentration of either component greater than 90%) are distilled, the extremely low efficiencies of distillation can be remarkably improved by using an intermediate reboiler or condenser.
Adsorption dynamics of a layered bed PSA for H2recovery from coke oven gas
Authors: Yang Jaeyoung , Lee Chang‐Ha
Published: April 2004

The adsorption dynamics of a layered bed packed with activated carbon and zeolite 5A were studied experimentally and theoretically through breakthrough experiments and two‐bed pressure swing adsorption (PSA) processes by using coke oven gas (56.4 vol. % H2; 26.6 vol. % CH4; 8.4 vol. % CO; 5.5 vol. % N2; and 3.1 vol. % CO2). The results of breakthrough curves of a layered bed showed an intermediate behavior of those of zeolite‐5A bed and activated carbon bed, because each concentration front propagates with its own wavefront velocity in each layer by a different adsorption equilibrium. Since a fast and dispersed mass‐transfer zone of CO in the zeolite layer of a layered bed leads to a long leading front of the N2wavefront, controlling the leading wavefront of the N2plays a very important role in obtaining a high‐purity product and in determining the optimum carbon ratio of a PSA process for H2recovery from cokeoven gas. The layered bed PSA process was simulated in a simplified form of two single‐adsorbent beds linked in series. The dynamic model incorporating mass, energy, and momentum balances agreed well with the experimental data. Concentration profiles inside the adsorption bed were also investigated.
Permeation dynamics of small molecules through silica membranes: Molecular dynamics study
Authors: Takaba Hiromitsu , Mizukami Koichi , Kubo Momoji , Fahmi Adil , Miyamoto Akira
Published: April 2004

The permeation of He, CO2, and N2 through a thin silica membrane was investigated using molecular dynamics. The permeance of He follows Knudsen flow. The permeation of CO2 is higher than the estimated value from Knudsen theory. Inside the pore, CO2, tends to be parallel to the pore wall. This orientation is suitable for diffusion. In contrast, N2, shows lower permeance compared to the value from Knudsen theory. The molecules present perpendicular orientations inside the pore wall, which reduces their diffusion. In mixed CO2/N2, gas simulation, the selective permeation of CO2 relative to N2 was observed.
Sorption and diffusion of chlorinated methanes in moist clay
Authors: Cabbar H. Canan , Varol Nesrin , McCoy Benjamin J.
Published: April 2004

The dynamic behavior of chlorinated methanes in clays exposed to relative humidities (RH) from 0 to 80% was investigated with the single‐pellet pulse‐response moment technique. Values of effective diffusion coefficients obtained from zeroth moment analysis differed only slightly for dry and moist clay pellets. When the clay pellet was dry, equilibrium adsorption constants of dichloromethane (D), trichloromethane (T), and carbon tetrachloride (C) at the gas‐mineral interface were determined from first moments to be 118, 92, and 90, respectively. Dryness factors (dry fraction of surface area) for clays with RH of 5, 20, 40, and 60% were also calculated from first moments as 0.5, 0.1, 0.04, and 0.01, respectively. When the pore surfaces were covered with the water film (dryness factor was 0 for the pellet at RH 80%), overall equilibrium adsorption constants for the same tracers (D, T, C) at the gas‐water and water‐mineral interfaces were determined to be 7.5, 7.2, and 3.6, respectively.
Simplification techniques for EKF computations in fault diagnosis: Model decomposition
Authors: Chang Chuei‐Tin , Hwang Jung‐Ing
Published: April 2004

The extended Kalman filter (EKF) is one of the most popular model‐based techniques for fault detection and diagnosis. In this study, the suboptimal EKF technique is utilized to enhance computation efficiency without sacrificing diagnostic accuracy. In particular, three simple strategies are proposed to decompose the filter model according to the precedence order of the state/parameter estimation process. The computation load needed in fault identification can be reduced significantly by implementing all or part of these decomposed EKFs on‐line. Extensive simulation results are also presented to demonstrate the effectiveness of these proposed techniques.
Discrimination and goodness of fit of multiresponse mechanistic models
Authors: Stewart W. E. , Shon Y. , Box G. E. P.
Published: April 2004

Criteria are derived for objective choice among rival models of multiresponse processes. Formulas are given for the relative posterior probabilities of candidate models and for their goodness of fit as tested on a common data set with Normally distributed errors. The formulas are demonstrated with examples from chemical kinetics and catalysis.
Hydrodynamic model for gas‐lift reactors
Authors: Sáez A. Eduardo , Márquez Marco A. , Roberts George W. , Carbonell Ruben G.
Published: April 2004

The hydrodynamic model of Young et al. (1991) for external‐loop gas‐lift reactors was modified to account for buoyancy forces in the gas phase. The revised model is based on macroscopic balances for the gas‐liquid separator and external downcomer and spatially‐averaged, 1‐D mass and momentum balances in the riser. Using only the physical properties of the gas and liquid phases, the reactor dimensions, and the gas superficial velocity, the model predicts gas holdup profiles, gas and liquid velocity profiles, and pressure profiles in the riser for bubbly flow. Empirical correlations are used to represent frictional and drag effects, but there are no adjustable parameters in the model. The system of equations that must be solved to predict hydrodynamic behavior is simpler than that of Young et al. The sensitivity of the model to choice of drag coefficient correlation is analyzed. The model predictions match experimental data of previous works.
Kinetics of 2‐hexanol and 3‐hexanol oxidation reaction over TS‐1 catalysts
Authors: Gallot J. E. , Kapoor M. P. , Kaliaguine S.
Published: April 2004

A kinetic investigation of the catalytic oxidation of 2‐(3‐) hexanols to 2‐(3‐) hexanones over a series of titanium silicalites [TS‐1] is reported. This study considers the biphasic character of the reaction medium and uses the mathematical theory of linear graphs to derive rate equations for both hexanols' oxidation and hydrogen peroxide's decomposition. The 2‐hexanols' oxidation is usually considered as the secondary reaction in the n‐hexane oxyfunctionalization process. Comparing the kinetic constants obtained in this study and those generated by the analysis of previously published n‐hexane oxyfunctionalization rate data allows one to conclude that in the latter case, hexanones are also produced by a direct primary oxidation of n‐hexane.
Thermodynamics of fluid mixtures near to and far from the critical region
Authors: Lue Leo , Prausnitz John M.
Published: April 2004

A theory for an equation of state for simple fluid mixtures valid both near to and far from critical points is presented. The base equation of state obtained from integral‐equation theory using the mean‐spherical approximation is used to compute the contribution of short‐wavelength fluctuations to the free energy of the fluid mixture. Wilson's phase space cell approximation, as extended by White, is used to compute the contribution of long‐wavelength fluctuations. The resulting theory possesses nonclassic critical exponents similar to those observed experimentally. Far from the critical region, where long‐wavelength fluctuations are not important, the theory reduces to that corresponding to the base equation of state. The complete theory is used to represent the thermodynamic properties and phase behavior of binary mixtures of methane, carbon dioxide, and n‐butane. In the critical region, agreement with experiment is dramatically improved upon, adding to the base equation of state corrections from long‐wavelength fluctuations.
Model of stress‐induced defect formation in drying polymer films
Authors: Christodoulou K. N. , Lightfoot E. J. , Powell R. W.
Published: April 2004

Photographic films are multilayer polymer structures coated from solution onto a relatively rigid substrate, chilled into a solid gel, and then dried into a permanent coating. This model predicts stress growth and defect formation during drying of such films that remain above their glass transition temperature, so that Fickian difision persists. The stress in the drying gel is taken from mixture theory; the polymer network is a nonlinear elastic or hypoelastic solid, whereas the solvent is an ideal fluid. Conservation laws and constitutive equations are formulated in terms of the polymer network velocity and discretized by the mixed method of Guenette and Fortin on an elliptically genereted, independently evolving finite‐element mesh. Results include evolution of polymer velocity, free surfaces, solvent concentration, and residual stress. Various model problems are solved for drying of uniformly coated films, suspended films (as in tenter‐frame ovens) and around rigid particle inclusions in suspended films and in multilayer photographic coatings causing particle migration and drying defects.
Bubble formation in a coflow configuration in normal and reduced gravity
Authors: Bhunia Avijit , Pais Salvatore C. , Kamotani Yasuhiro , Kim Iee‐Hwan
Published: April 2004

A study of bubble generation for constant gas‐flux condition by single‐nozzle injection in a coflowing liquid is reported. Focusing on single‐bubble generation in the dynamic and bubbly flow regime, the onset condition for bubble coalescence is investigated. The role of various forces involved in the bubble formation process is studied, and an overall force balance describing bubble dynamics is developed. Gas‐momentum flux and buoyancy in normal gravity enhance, while the surface‐tension force at the nozzle rim inhibits bubble detachment. On the other hand, liquid drag and inertia can act both as attaching or detaching forces, depending on the relative velocity of the bubble with respect to the surrounding liquid. Predictions of the theoretical model compare well with the present reduced‐gravity experiment and available normal‐gravity experiments. Effects of the fluid properties, injection geometry, and flow conditions on bubble size are investigated.
Screening procedure for synthesizing isothermal multiphase reactors
Authors: Kelkar Vaibhav V. , Ng Ka M.
Published: April 2004

A procedure is developed to identify a multiphase reactor for carrying out a given reaction at the early stage of process development. There are three elements to this screening approach: a generic reactor model, an accompanying sensitivity model, and a knowledge base consisting of heuristics, plus correlations on hydrodynamics, heat, and mass transport. In synthesizing a generic reactor model, the conuentional multiphase reactors are decomposed into common attributes and constituent parts, and reaggregated for the reaction under consideration. Thus, in addition to choosing one among conventional reactor types, the procedure prouides insights for the creation of novel reactors. The sensitivity model quantifies the relative impact of the model parameters on the reactor performance. The knowledge base provides heuristics, estimates, and reality checks in the decision‐making process. While this article focuses on isothermal systems with a single reaction, the framework is applicable for general multiphase reactions.
Multiscale PCA with application to multivariate statistical process monitoring
Authors: Bakshi Bhavik R.
Published: April 2004

Multiscale principal‐component analysis (MSPCA) combines the ability of PCA to decorrelate the variables by extracting a linear relationship with that of wavelet analysis to extract deterministic features and approximately decorrelate autocorrelated measurements. MSPCA computes the PCA of wavelet coefficients at each scale and then combines the results at relevant scales. Due to its multiscale nature, MSPCA is appropriate for the modeling of data containing contributions from events whose behavior changes over time and frequency. Process monitoring by MSPCA involves combining only those scales where significant events are detected, and is equivalent to adaptively filtering the scores and residuals, and adjusting the detection limits for easiest detection of deterministic changes in the measurements. Approximate decorrelation of wavelet coefficients also makes MSPCA effective for monitoring autocorrelated measurements without matrix augmentation or time‐series modeling. In addition to improving the ability to detect deterministic changes, monitoring by MSPCA also simultaneously extracts those features that represent abnormal operation. The superior performance of MSPCA for process monitoring is illustrated by several examples.
Molecular‐weight distribution control in emulsion polymerization
Authors: Echevarría Antonio , Leiza José R. , De La Cal José C. , Asua José M.
Published: April 2004

Emulsion polymers with well‐defined molecular‐weight distributions (MWD) were obtained by using a control strategy based on on‐line measurements of both unreacted monomer and chain‐transfer agent (CTA). The control strategy includes a nonlinear model‐based controller that calculates the feed rates of monomer and CTA needed to obtain the desired MWD. This requires a mathematical model for the MWD that was developed based on independent measurements. The control strategy was assessed by computer simulation and experimentally verified by producing polymers with widely different MWDs in the styrene emulsion polymerization using CCl4, as CTA.
Solution viscosity increase in fusion synthesis of Ca/Mg‐resinates
Authors: Sundqvist S. , Paatero E. , Klemola A. , Tenhola H.
Published: April 2004

The effect of Ca/Mg‐resinate concentration on solution viscosity was studied in toluene. The viscosity was observed to increase drastically above a specific concentration value. The change in viscosity dependence on concentration implies a transition from a dilute to a concentrated regime, where viscous dissipation is governed by resinate molecular interactions and excluded volume effects. The concept of critical resinate concentration, ccrit, was applied and used to explain the nonlinear solution‐viscosity increase during the resinate synthesis. Ca/Mg‐resinates were synthesized by the fusion method at 265°C in a laboratory‐scale batch reactor. During the syntheses the increase in 50 wt. % toluene solution viscosity of the resinate turned almost exponential at a specific reaction time, denoted by t50. Based on the apparent reaction kinetics, a semiempirical model with two estimated parameters was derived for the solution viscosity to be used for the prediction of the t50, time of resinate syntheses.
Modeling and optimization of drug release from laminated polymer matrix devices
Authors: Lu Sanxiu , Fred Ramirez W. , Anseth Kristi S.
Published: April 2004

The design and modeling of drug delivery devices using laminated layers to produce spatially nonuniform matrix devices leading to desired release rates were investigated with the focus on their optimal laminated hydrogel matrices for the diffusion‐controlled release of dissolved drugs. The model consists of polymer layers laminated together through a photopolymerization process to form matrices with a spatially nonuniform initial drug distribution and/or nonuniform drug diffusivities. The model solution establishes that the drug diffusion behavior, especially the early time release behavior, can be manipulated by altering spatially nonuniform initial drug distributions and drug diffusivities in the assembly. Furthermore, optimal control theory and calculus of variation were used to determine a set of initial drug concentrations in the layers to attain a system that exhibits a drug release profile as close to required profile as possible for all time.
Enhanced photon‐migration methods for particle sizing in concentrated suspensions
Authors: Jiang Huabei
Published: April 2004

This article presents a new method for determining particle size distribution in undiluted suspensions from spectral measurement of transport‐scattering coefficients using continuous‐wave photon migration techniques. Particle size distribution (PSD) for concentrated suspensions is determined by a regularized inverse algorithm, which makes it possible to incorporate a synthesized scheme of Marquardt and Tikhonov regularizations. Spectral measurements of dense TiO2, suspensions with three different particle concentrations were used to recover the suspensions' size distributions. Our results are in excellent agreement with the size distribution measured with X‐ray sedimentation. These reconstructions of PSDs are obtained both with and without a priori distribution function assumptions.
Molecular simulation of adsorption of short linear alkanes and their mixtures in silicalite
Authors: Du Zhimei , Manos George , Vlugt Thijs J. H. , Smit Berend
Published: April 2004

Configurational‐bias Monte Carlo simulations in the grand‐canonical ensemble are employed to compute adsorption isotherms of methane, ethane, propane, butane and binary mixtures of methane–ethane, methane–propane, ethane–propane, and methane–butane in the zeolite silicalite. Comparison of the simulation results with the limited expetiniental data available shows good agreement. For ethane at room temperature, a small inflection point in the isotherm was observed due to a surprising ordering of the ethane molecules in the zeolite. For the simulation of a methane–ethane mixture, at low pressure ethane is preferentially adsorbed, while at high pressures methane replaces ethane due to entropic effects.
Diffusion and reaction in whole wheat grains during boiling
Authors: Stapley A. G. F. , Fryer P. J. , Gladden L. F.
Published: April 2004

The boiling of whole wheat grains was studied using magnetic resonance imaging (MRI) and pulsed‐gradient‐spin‐echo nuclear magnetic resonance (NMR) (PGSE NMR). MRI was used to image moisture distributions across a central section of grains boiled for different times at either 100 or 120°C. A moisture front gradually moved into the grain, although some moisture did penetrate ahead of the front. PGSE NMR, which measures self‐diffusion, indicated that two pools of water with differing diffusion characteristics may exist within the grain. Diffusion models suggested from the PGSE results were incorporated into finite‐element simulations along with the Thomas‐Windle model for diffusion/relaxation in polymers. Simulation results were compared to the MRI data, and the best fit was achieved using the Thomas‐Windle model with a step‐change in diffusivity above a critical moisture content. This behavior is similar to that of diffusion in polymers undergoing a glass transition, a phenomenon widely accepted to occur when starch gelatinizes.
Diafiltration by nanofiltration: Prediction and optimization
Authors: Richard Bowen W. , Wahab Mohammad A.
Published: April 2004

A predictive model for the performance of a nanofiltration membrane in separating the components of a dye/salt solution was developed. It is based on the extended Nernst‐Planck equation with incorporation of concentration polarization for mixtures of charged ions. Excellent agreement between predicted and experimental rejections of anions and cations was obtained for batch nanofiltration at different ratios of dye/salt assuming that the membrane charge density (Xd) depends on the total concentration of negative charges in the solutions. Prediction for diafiltration experiments was also excellent by allowing for variation in Xd as the salt concentration in the solution changed as a function of time. The model was used to investigate optimization of the processing conditions. The overall diafiltration process operated best in a two‐phase mode, in which the dye solution was preconcentrated to the final required dye concentration before diafiltration.
Subspace approach to multidimensional fault identification and reconstruction
Authors: Dunia Ricardo , Joe Qin S.
Published: April 2004

Fault detection and process monitoring using principal‐component analysis (PCA) and partial least squares were studied intensively and applied to industrial processes. The fundamental issues of detectability, reconstructability, and isolatability for multidimensional faults are studied. PCA is used to define an orthogonal partition of the measurement space into two orthogonal subspaces, a principal‐component subspace, and a residual subspace. Each multidimensional fault is also described by a subspace on which the fault displacement occurs. Fault reconstruction leads to fault identification and consists of finding a new vector in the fault subspace with minimum distance to the principal‐component subspace. The unreconstructed variance is proposed to measure the reliability of the reconstruction procedure and determine the PCA model for best reconstruction. Based on the fault subspace, fault magnitude, and the squared prediction error, necessary and sufficient conditions are provided to determine if the faults are detectable, reconstructable, and isolatable.
Structured approach to storage allocation for improved process controllability
Authors: Kelly J. D. , Forbes J. F.
Published: April 2004

An optimization‐based approach is presented that determines how plant feedstocks should be allocated to storage when there are fewer storage vessels than feedstocks. It is assumed here that material from the storage vessels will be subsequently blended for processing in downstream processes. The objective of the feedstock allocation strategy is chosen to ensure maximum flexibility for downstream process operation. Given the stated objective for feedstock allocation and the physical constraints, the feedstock storage allocation problem is posed in optimization form. The solution of the resulting singular‐value optimization problem is discussed in terms of semidefinite programming techniques. The ideas presented are illustrated using a crude‐oil storage case study. Finally, a number of observations regarding useful extehsions to the proposed methods are presented.
Boiling‐slurry reactor: Feasible operation and stability
Authors: Khinast J. , Luss D. , Leib T. M. , Harold M. P.
Published: April 2004

In this study the feasible operation and stability of a special boiling‐slurry reactor (BSR), in which a single exothermic chemical reaction occurs, are determined. Its feed consists of a nonvolatile liquid reactant dissolved in an inert solvent and gaseous reactants, while the effluent consists only of gaseous components. The BSR attains a unique steady state within a bounded range of operating conditions, the boundaries of which are defined by simple algebraic expressions. While the unique steady states are usually locally stable, they are not always globally stable. For example, when the start‐up temperature is too low, the reactor may not reach the steady state and instead reach a “fill‐up” state. In that state the liquid volume continuously increases, while the concentrations and temperature remain essentially constant. For some sets of start‐up temperatures two distinct fill‐up states may exist. When the start‐up liquid volume and temperature are very high, a “dry‐up” state may exist so that the liquid continuously decreases, while the reactor temperature and concentration remain essentially constant. This state eventually will shift the reactor to the unique steady state. Dry‐up, fill‐up, and a steady state coexist in some parts of the feasible operation region, thus requiring a special start‐up procedure. The BSR may exhibit sustained oscillations for some parameter sets.
Fischer‐tropsch synthesis in near‐critical n‐hexane: Pressure‐tuning effects
Authors: Bochniak David J. , Subramaniam Bala
Published: April 2004

For Fe‐catalyzed FischerTropsch (FT) synthesis with near‐critical n‐hexane (Pc, = 29.7 bar; Tc, = 233.7°C) as the reaction medium, isothermal pressure tuning from 1.2–2.4 Pc, (for n‐hexane) at the reaction temperature (240°C) significantly changes syngas conversion and product selectivity. For fixed feed rates of syngas (H2/C0 = 0.5; 50 std. cm3/g catalyst) and n‐hexane (1 mL/min), syngas conversion attains a steady state at all pressures, increasing roughly threefold in this pressure range. Effective rate constants, estimated assuming a first‐order dependence of syngas conversion on hydrogen, reveal that the catalyst effectiveness increases with pressure implying the alleviation of pore‐diffusion limitations. Pore accessibilities increase at higher pressures because the extraction of heavier hydrocarbons from the catalyst pores is enhanced by the liquid‐like densities, yet better‐than‐liquid transport properties, of n‐hexane. This explanation is consistent with the single α ( =0.78) Anderson‐Schulz‐Flory product distribution, the constant chain termination probability, and the higher primary product (1 ‐olefin) selectivities ( ∼ 80%) observed at the higher pressures. Our results indicate that the pressure tunability of the density and transport properties of near‐critical reaction media offers a powerful tool to optimize catalyst activity and product selectivity during FT reactions on supported catalysts.