bluuebunny/crossref_metadata_2025_split huggingface dataset:
Quantitative description of ultrafiltration in a rotating filtration device
Authors: Holeschovsky Ulrich B. , Cooney Charles L. 
Published: June 2004

AbstractA correlation was developed to quantitatively describe the flux in a high‐speed rotating filtration device using a minimum set of parameters. The experimental results were found to be consistent with the concentration polarization (CP) model. Beyond a threshold pressure flux ceases to depend on membrane permeability. The CP model was modified to include the concentration dependence of the diffusivity. This approach was found to be consistent with the strong dependence of flux on pH. Protein concentration in the polarized layer adjacent to the membrane surface was estimated using a procedure that corrects for some of the inconsistencies in the methods usually applied. Four dimensionless numbers were necessary to correlate the experiments with good accuracy. Previously‐reported correlations used only three dimensionless numbers. Usage of four numbers could be justified by dimensional analysis. Finally, the performance of rotary or vortex filtration was compared to that of other configurations.
Flow through mixed fibrous porous materials
Authors: Ethier C. Ross 
Published: June 2004

AbstractCreeping flow through a composite porous material composed of two fiber types has been analyzed for the situation in which the radii of the two fiber populations are highly dissimilar. The composite material has been idealized as a porous matrix that is composed of fine fibers (permeability Kf), into which cylindrical inclusions (radius ac) have been embedded, and flow through the resulting system has been modeled by the Debye‐Brinkman equation. Two different methods of accounting for the random geometry of a real porous material have been considered: a unit cell approach and replacement of the coarse fibers by a periodic fiber lattice. Model results are in good agreement with each other and reduce in the appropriate limit to well‐accepted models for flow in single‐component fibrous materials. Plots of net composite permeability as a function of fiber diameter and packing density are presented. In the parameter ranges of interest, viscous effects at the coarse fiber surfaces lead to a significantly lower overall permeability than that predicted by a simple application of Darcy's law.
A group contribution approach to computer‐aided molecular design
Authors: Gani Rafiqul , Nielsen Bjarne , Fredenslund Aage 
Published: June 2004

AbstractA flexible and structured methodology for the computer‐aided molecular design (CAMD) by the group contribution approach is presented. The proposed CAMD algorithm takes the following four steps: 1. preselect groups and target properties; 2. generate only a feasible set of compound structures in an optimal fashion; 3. predict properties for the screening of the set of feasible compound structures; and finally 4. select/design the compound. Since the success of any CAMD algorithm depends to a large extent on its ability to predict/compute the needed properties, the importance of the choice and applicability of these methods is considered along with the computational aspects related to the development of a computer program based on the proposed methodology. Finally, the scope of CAMD technology is highlighted using several practical examples.
Experimental evidence for and prediction of micromixing effects in precipitation
Authors: Marcant Bruno , David René 
Published: June 2004

AbstractThe influence of micromixing on precipitation is predicted qualitatively for batch or semibatch precipitations by means of a simplified mixing model, which considers primary nucleation as the main process affected significantly by hydrodynamics. Unwanted micromixing effects are avoided to a maximum in a batch configuration designed to have a volume mixing ratio α close to unity. Using a semibatch precipitator or a batch with a high α value dramatically accentuates mixing interactions. For mixing, changing the feed point location is shown to have much stronger influence than changing the stirring speed and constitutes an efficient test of micromixing. Experimental evidences for these conclusions have been established for calcium oxalate monohydrate batch precipitation. In particular, the early seconds of the reaction (when mainly primary nucleation occurs) have been demonstrated as conditioning the subsequent behavior of the precipitation, even for induction times of a few minutes.
Nonlinear predictive control of uncertain processes: Application to a CSTR
Authors: Sistu Phani B. , Bequette B. Wayne 
Published: June 2004

AbstractNonlinear predictive control (NLPC) is an effective strategy for controlling nonlinear chemical processes with constraints and time delays. In this article, a number of important issues in NLPC are addressed, emphasizing continuous stirred tank reactors (CSTR's) with parametric and model structure uncertainty. In particular, the effects of various choices for the initial conditions are discussed. The selection of initial conditions is particularly important in the presence of plant/model mismatch. A nonlinear programming‐based approach is used for process identification. The effect of model structure uncertainty is included in our analysis by using a cascade control structure on the coolant temperature. Our CSTR results indicate that a simple PI (with anti‐reset windup) cascade loop on coolant temperature is adequate for operation over a wide range of operating conditions. A particularly interesting result of this work is that a predictive controller based on an open‐loop observer can be used to stabilize an open‐loop unstable process, although this is not recommended in practice.
Microwave thawing of cylinders
Authors: Pangrle B. J. , Ayappa K. G. , Davis H. T. , Davis E. A. , Gordon J. 
Published: June 2004

AbstractThe finite element method was used to model microwave thawing of pure‐water and 0.1‐M NaCl cylinders. The electromagnetic field was described by Maxwell's equations with temperature‐dependent dielectric properties, while the heat equation, coupled with the Stefan and Robin conditions, was used to describe the thawing process. An additional equation for the frozen volume fraction was used, when necessary, to account for the presence of a mushy region. Two microwave frequencies, 915 MHz and 2,450 MHz, were examined and the microwave radiation was assumed to be radially isotropic and normal to the surface of the cylinder. Results show that a two‐phase mushy region may exist, and an additional thawing front may appear at the center of the cylinder. Salt cylinders have a higher dielectric loss than pure‐water cylinders and therefore thaw more quickly. Internal resonance occurs when the wavelength of the radiation is a harmonic of the cylinder radius. Resonance increases power deposition and expedites the thawing process. The onset of resonance alters thawing times and complicates the development of heuristic rules for microwave thawing.
Microstructural descriptors characterizing granular deposits
Authors: Tassopoulos Menelaos , Rosner Daniel E. 
Published: June 2004

AbstractQuantitative results are presented on mean coordination number and coordination number distribution, contact normal distribution and fabric tensor of simulated anisotropic granular deposits with resulting solid fractions between ca. 15% for ballistic deposits and 58%, corresponding to a random loose packing. The deposits, generated by the capture of uniform size spherical particles arriving normal to a target, were simulated using a simple algorithmic model.We focus on microstructural quantities which explicitly take into account the discrete nature of the granules comprising the deposit. Such measures are important in determining the heat transport properties of the deposits for Fourier conduction through the solid phase, as well as their mechanical and sintering properties. The variation of mean coordination number with deposit solid fraction was successfully correlated using a unit‐cell model (Eq. 13). This correlation, in conjunction with entropy maximization arguments (after Nayak and Tien, 1978) has been further used to predict the coordination number distribution. The usefulness of the results reported here is illustrated by computing an upper bound to the deposit effective thermal conductivity (Jagota and Hui, 1990) and comparing it to both ‘exact’ simulation results (Tassopoulos and Rosner, 1991b) and experimental data (Koh, 1971).
Composite membranes for fuel‐cell applications
Authors: Verbrugge Mark W. , Hill Robert F. , Schneider Eric W. 
Published: June 2004

AbstractOne of the major obstacles to overcome for the realization of economical hydrogen‐oxygen, polymer‐electrolyte fuel cells is the high capital cost of the inert perfluorosulfonic acid (PSA) membranes, which provide a pathway for ionic transport between the cell electrodes. It has recently been shown that composite polymer membranes can be synthesized by depositing PSA polymers onto porous poly(tetrafluoroethyene) (PTFE) substrates. The resulting membranes are mechanically durable and quite thin relative to traditional PSA membranes; we expect the composite membranes to be of low resistance and cost. In this experimental study, we examine the composite membrane properties as a function of the membrane composition. Our results allow us to form a conceptual model to explain both the equilibrium and transport characteristics of these materials. For high PSA contents, the membrane behavior is similar to that of the PSA polymer; the water permeability, however, is reduced significantly. For intermediate PSA contents, the membranes have a high porosity and match the thickness of the PTFE substrate (≈50 μm); membranes of this composition range are potentially useful candidates for fuel cells because of their high resistance to water transport and reduced ionic resistance. Composite membranes of very low PSA content demonstrate characteristics similar to the hydrophobic PTFE substrate and are not of interest for fuel cells.
Lattice model for the adsorption of benzene in silicalite I
Authors: Lee Chung‐Kung , Chiang Anthony S. T. , Wu F. Y. 
Published: June 2004

AbstractA lattice gas model is used to describe the adsorption of benzene in silicalite I. Using the complete equivalence of the lattice gas with an Ising model and the known exact as well as mean‐field results of the latter, we study the thermodynamics of the adsorption and establish criteria on the adsorption energies and site interactions for the existence of a two‐phase region. It is shown that a phase change occurs when zigzag paths connecting interacting adsorption sites become energetically favorable. Our model also leads to a sharp rise of the adsorption heat at the transition, albeit at a level less enhanced than experimentally observed.
Determination of the hindered settling factor for flocculated suspensions
Authors: Landman K. A. , White L. R. 
Published: June 2004

AbstractThe determination of the settling velocity of particles in suspension is a difficult problem due to the complexity of the indirect particle–particle interactions. Formally, a function called the hindred settling factor r (ϕ), which is a function of the solids volume fraction ϕ, is introduced to take into account the increase in the drag forces. The current method for estimating this function is to measure the initial fall rate of the suspension in a batch sedimentation experiment. Here, we propose a steady‐state experiment for flocculated suspensions using vacuum filtration, where the fixed bed height is measured as a function of the liquid flow rate through a porous membrane. Recovering the function r (ϕ) or a related function D (ϕ) involves numerical differentiation of the data. This experiment will also yield the membrane permeability as a function of volume fraction (in cases where clogging or fouling of the membrane occurs). The results using numerically simulated data show good recovery of the assumed r (ϕ) function.
Optimal catalyst activity profiles in pellets: 9. Study of ethylene epoxidation
Authors: Gavriilidis Asterios , Varma Arvind 
Published: June 2004

AbstractThe concept of optimal catalyst activity distribution is studied experimentally for the ethylene epoxidation reaction network on a Ag/α–Al2O3 catalyst in a single‐pellet reactor. The Dirac delta‐type distribution of the catalyst is approximated by a step distribution of narrow width. For a fixed amount of silver, the influence of location and width of the catalytically active layer on the conversion of ethylene and on the selectivity and yield to ethylene oxide is investigated under oxygen‐rich conditions in the temperature range 210–270°C. The results clearly demonstrate that for optimum selectivity and yield to ethylene oxide, the silver catalyst should be placed in a thin layer at the external surface of the pellet.
Modeling of isotropic reacting turbulence by a hybrid mapping‐EDQNM closure
Authors: Frankel Steven H. , Jiang Tai‐Lun , Givi Peyman 
Published: June 2004

AbstractA hybrid model is developed and implemented for predicting the limiting bound of the reactant conversion rate in an isotropic turbulent flow under the influence of a reaction of the type A + B Products. This model is based on the amplitude mapping closure of Kraichnan for the molecular mixing of a stochastically distributed scalar, and the eddy‐damped quasi‐normal Markovian (EDQNM) spectral closure for the two‐point scalar covariance. The results predicted by this model compare well with available experimental data in both gaseous and aqueous plug‐flow reactors, but point to the need for more detailed measurements in future experimental studies. With the implementation of the mapping closure, a simple analytical expression is obtained for the decay rate of the unmixedness. This expression is very convenient and is recommended for direct practical applications in the modeling and design of plug‐flow reactors.
Design parameters estimation for scale‐up of high‐pressure bubble columns
Authors: Wilkinson Peter M. , Spek Arie P. , van Dierendonck Laurent L. 
Published: June 2004

AbstractExperiments were carried out in bubble columns for a number of liquids at pressures between 0.1 and 2.0 MPa for two column sizes. Based on the experimental results as well as extensive literature data, the extent of the effect column dimensions have on gas holdup were determined, both at low and high pressures (which is of importance to scale‐up). It was also demonstrated that none of the published empirical gas holdup equations incorporate the influence of gas density accurately. Therefore, a new improved gas hold‐up equation is developed that incorporates the influence of gas and liquid properties with an average error of approximately 10%. Finally, it is also discussed to what extent theinfluence of pressure on other important design parameters such as the interfacial area, the liquid volumetric mass transfer coefficient, and gas and liquid mixing, can be estimated on the basis of empirical equations.
The robust global stabilization of a stirred tank reactor
Authors: Adebekun K. 
Published: June 2004

AbstractIn this paper, a program of analysis is carried out for control of a nonlinear stirred tank reactor. By examining the uniqueness and stability properties of the reactor, the zero dynamics are analyzed. Subsequently, a model reference adaptive controller (MRAC) which explicitly takes into account the nonlinearities of the system is designed; the MRAC successfully compensates for parametric uncertainty. It is shown that even when almost all reactor parameters are unknown, global convergence over the entire phase space is achieved.
A theoretically correct mixing rule for cubic equations of state
Authors: Wong David Shan Hill , Sandler Stanley I. 
Published: June 2004

AbstractA new mixing rule developed for cubic equations of state equates the excess Helmholtz free energy at infinite pressure from an equation of state to that from an activity coefficient model. Use of the Helmholtz free energy insures that the second virial coefficient calculated from the equation of state has a quadratic composition dependence, as required by statistical mechanics. Consequently, this mixing rule produces the correct low‐ and high‐density limits without being density‐dependent.As a test, the mixing rule is used for ternary mixtures of cyclohexane + benzene + water, ethanol + benzene + water and carbon dioxide + n‐propane + water, and all the constituent binaries. The new mixing rule and a simple cubic equation of state can be used for the accurate correlation of vapor‐liquid and liquid‐liquid equilibria for binary mixtures. Using the parameters obtained from binary systems, the phase behavior of ternary mixtures can be predicted. Also, unlike previous empirical mixing rules, this theoretically based mixing rule is equally applicable and accurate for simple mixtures containing hydrocarbons and inorganic gases and mixtures containing polar, aromatic and associating species over a wide range of pressures. This mixing rule makes it possible to use a single equation of state model with equal accuracy for mixtures usually described by equations of state and for those traditionally described by activity coefficient models. It is the correct bridge between these two classes of models.
Modeling fixed‐bed coal gasifiers
Authors: Hobbs Michael L. , Radulovic Predrag T. , Smoot L. Douglas 
Published: June 2004

AbstractA one‐dimensional model of countercurrent fixed‐bed coal gasification has been developed, and results have been compared to experimental data from commercial‐scale gasifiers. The steady‐state model considers separate gas and solid temperatures, axially variable solid and gas flow rates, variable bed void fraction, coal drying, devolatilization based on chemical functional group composition, oxidation and gasification of char, and partial equilibrium in the gas phase. Generalized treatment of gas‐phase chemistry and accounting for variable bed void fraction were necessary to predict realistic axial temperature and pressure profiles in an atmospheric fixed‐bed gasifier. Model evaluation includes sensitivity of axial temperature profiles to model options, model parameters and operational parameters. Model predictions agree reasonably well with experimental temperature and pressure profile data for gasification of eight coal types ranging from lignite to bituminous. The relative importance of char oxidation resistances to bulk film diffusion, ash diffusion, and chemical reaction is identified.
Morphology of polymers precipitated from a supercritical solvent
Authors: Lele Ashish K. , Shine Annette D. 
Published: June 2004

AbstractThe precipitation of polymers via the rapid expansion of a supercritical chlorodifluoromethane solution to ambient conditions across a fine diameter capillary has been studied experimentally. The morphology of the polymers precipitated—polycaprolactone, poly(methyl methacrylate) and a styrene/methyl methacrylate block copolymer—is influenced strongly by conditions of the expansion process. Conditions of high temperature, high polymer concentration, low pressure or low capillary L/D ratio enhance the formation of high aspect ratio fibers, while opposite conditions favor the formation of spherical particles of micron size. Each of the conditions favoring fiber formation favors precipitation farther upstream in the expansion process. Based on one‐dimensional compressible flow calculations using a virial equation of state for pure solvent, it is proposed that fiber formation occurs when a polymer‐rich phase is rejected from solution in the entry region to the capillary. The location of precipitation is shown to be crucial in determining the characteristic time scale for the density reduction process, which may be as small as 10−7 s.
Thermal convection, morphological stability and the dendritic growth of crystals
Authors: Koo Kee‐Kahb , Ananth Ramagopal , Gill William N. 
Published: June 2004

AbstractNew data on the dendritic growth and microstructure of ice crystals in quiescent pure water are reported for small subcoolings of 0.035 K <ΔT<1.000 K, where thermal or natural convection is prevalent. Accurate and systematic measurements of the growth velocity VG and the tip radii of the edge and basal planes R1 and R2 were made as functions of time.The central point of this work is that with the harmonic mean of the tip radii Rm as the lengthscale, the intensity of natural convection can be correlated accurately by using the thermal convection analogy, Gr = Re2. On this basis, natural convection has a crucial effect on dendritic growth of ice at ΔT<0.35 K, the region of subcooling in which the tip of the dendrite splits consistently.The experiments show that the morphological stability parameter C* is independent of subcooling and equals 0.075, when the lengthscale is Rm. With the observed values of R1 and R2, the aspect ratio is 28, and the growth velocity for small ΔT is significantly higher than that predicted by the conduction theory of Horvay and Cahn (1961). Thus, the effect of convection on the growth of ice crystals is more important as the subcooling decreases.Moving boundary solutions of the three‐dimensional Navier‐Stokes and energy equations for the dendritic growth of an elliptical paraboloid were obtained here with the Stokes flow approximation. Experimental observations of the quantities, VG, R1, and R2, agree well with predictions of this theory when Gr=Rem2 is based on Rm. In contrast, if convection is neglected in the theory, it does not agree with the experiments and the difference increases significantly as the subcooling is decreased.
Dynamic nonisothermal transport in hygroscopic porous media: Moisture diffusion in wood
Authors: Avramidis Stavros , Englezos Peter , Papathanasiou Thanasis 
Published: June 2004

AbstractA model that predicts heat and moisture transfer in the hygroscopic range of a complex porous material such as wood, was evaluated with unsteady‐state nonisothermal diffusion experimental data. Water chemical potential gradient was taken as the driving force for diffusion, and the derivation of the temperature‐gradient phenomenological coefficient in the mass balance equation was based on the principles of nonequilibrium thermodynamics. The results reveal an excellent prediction of the specimen's average moisture content during desorption in the hygroscopic range. Moreover, a very good agreement was also shown between the specimen's center temperature and the model predictions. The model revealed the existence of a thermal‐diffusion phenomenon during the initial stages of the desorption process. This phenomenon was not predicted by Fick's equation for diffusion.
Identification and cross‐directional control of coating processes
Authors: Braatz Richard D. , Tyler Matthew L. , Morari Manfred , Pranckh Ferdinand R. , Sartor Luigi 
Published: June 2004

AbstractOf special industrial interest is the cross‐directional control of coating processes, where the cross direction refers to the direction perpendicular to the substrate movement. The objective of the controller is to maintain a uniform coating under unmeasured process disturbances. Assumptions that are relevant to coating processes found in industry are used to develop a model for control design. This model is used to derive a model predictive controller to maintain flat profiles of coating across the substrate by varying the liquid flows along the cross direction. Actuator constraints, measurement noise, model uncertainty, and the plant condition number are investigated to determine which of these limit the achievable closed‐loop performance. From knowledge of how these limitations affect the performance we can make some recommendations on how to modify the plant design to improve the coating uniformity. The theory developed throughout the article is rigorously verified through experiments on a pilot plant. The controller rejects disturbances with two sampling times. The proposed controller can reduce the variance in coating thickness by as much as 80% compared to what is possible by manual control or simple control schemes.