bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modeling mass transfer with enzymatic reaction in electrochemical multilayer microreactors
Authors: Bacha Saliha , Montagné Marielle , Bergel Alain 
Published: June 2004

AbstractElectrochemical biosensors were used to develop a numerical model of transient mass transfer coupled with enzymatic reactions in thin multilayer microstructures. A finite volume method integrates the partial derivative mass‐balance equations making the model very versatile. With a multilayer physical description, general nonlinear enzymatic kinetics and efficient space discretization, the model could be applied to various devices under a broad range of physicochemical conditions. Theoretical results were validated according to experimental data obtained with three different biosensors involving homogeneous or heterogeneous enzymatic catalyses. Model predictions are further discussed for a bienzymatic immobilized enzyme system. The biosensor transient behavior depended strongly on the location of the enzymes in the device and on the mass‐transport features. A transient maximum on the current‐time curves was predicted accurately by the model. This could be used to improve the biosensor performance. The model can also be a useful framework for designing new electrochemical microreactors.
Controlling chaos by periodic perturbations in nonisothermal fluidized‐bed reactor
Authors: Ajbar Abdelhamid , Elnashaie S. S. E. H. 
Published: June 2004

AbstractA two‐phase model of a nonisothermal fluidized‐bed catalytic reactor with consecutive exothermic reactions makes it possible to control the chaotic behavior by small parametric perturbations of an input variable with suitable frequency and amplitude. By periodic forcing of the feed temperature the chaotic behavior of the autonomous system can be controlled and become periodic even for small amplitudes of forcing. For constant frequency with change in forcing amplitudes the behavior of the system alternates between periodic and chaotic regimes via period doubling and period adding bifurcation mechanisms. When the amplitude is fixed, regular patterns also appear with small changes in the forcing frequency. Appropriate selection of the forcing frequency can improve the yield of the desired intermediate component in the consecutive reactions network A → B → C.
Group‐contribution lattice‐fluid EOS: Prediction of LLE in polymer solutions
Authors: Lee Byung‐Chul , Danner Ronald P. 
Published: June 2004

AbstractThe modified group‐contribution lattice‐fluid equation of state (GCLF–EOS) was applied to predict liquid–liquid equilibria (LLE) in polymer solutions. The modified GCLF–EOS is a group‐contribution form of the equation of state by Panayiotou and Vera based on the lattice‐hole theory. Group contributions for the interaction energy and reference volume were developed based only on the saturated vapor pressures and liquid densities of low‐molecular‐weight compounds. For a mixture, group contributions for the binary interaction parameter were developed from the binary vapor–liquid equilibria of low‐molecular‐weight compounds. This modified GCLF–EOS model can be applied to predict LLE behavior in polymer solutions with the same group‐contribution parameters as used for predicting the vapor–liquid equilibria. The only input required for the model is the structure of the molecules in terms of their functional groups.
Radiation heat transfer down an elongated spheroidal cavity
Authors: Fitzgerald Sean P. , Strieder William 
Published: June 2004

AbstractEffects of elongation on the radiation heat transport down a spheroidal cavity, located in a conducting solid with a diffusely reflecting cavity–solid interface, are examined. An effective conductivity λe and a void radiation conductivity λr are obtained as a function of cavity eccentricity α; and surface emissivity ε. To facilitate the calculations and produce readily applicable equations, a rigorous variational principle is used. Exact solutions are generated in the neighborhood of the spherical cavity (α2 → 0) for any ε > 0, a long needle‐shaped void (α2 → 1) for any ε > 0, and a perfect reflector (ε → 0) for arbitrary elongation (0 ≤α2 ≤ 1). Significant differences arising from the shape change are observed. The α2 → 0 edge demonstrates a linear increase in λr with ε. At the opposite edge α2 → 1 and positive ε, λr is a horizontal line independent of ε, much like the long cylinder, whose conductivity is a factor of 32/(9π) (= 1.13) larger. In the neighborhood of ε 0, λr is always zero for any 0 ≤ α2 ≤ 1. The emissivity slope for ε → 0 starts from unity at α2 = 0 and increases monotonically with elongation to a singularity 3π[16(1 – α2)]‐1 as α2 → 1 for the long needle.
Robust design of countercurrent adsorption separation processes: 4. Desorbent in the feed
Authors: Mazzotti Marco , Storti Giuseppe , Morbidelli Massimo 
Published: June 2004

AbstractIn many instances of practical interest, countercurrent adsorption separations operate on feed streams containing not only the components to be separated but also some desorbent. Criteria for the optimal and robust design and operation of these units are developed by extending previous treatments developed for desorbent‐free feedstreams. The effect of the presence of some desorbent (weak, intermediate, or strong) on the location and robustness of the region of complete separation in the operating parameter space is discussed.
Steady‐state multiplicity in cocurrently cooled autothermal reactors
Authors: Pedernera Marisa , Borio Daniel O. , Porras José A. 
Published: June 2004

AbstractThe steady‐state simulation of a cocurrently cooled autothermal fixed‐bed reactor was carried out using a 2‐D heterogeneous mathematical model. The ammonia synthesis was chosen as a case study. Unlike the not‐autothermal cocurrent reactor, which is unconditionally stable, the autothermal cocurrent reactor shows multiple steady states within a broad range of operating conditions. This finding, not reported in the literature, is explained through the mass transport from the bottom to the top of the reactor, associated with a feedback of energy. The feedback of heat, which is inherent to autothermal reactors, leads to an ignition‐extinction phenomenon similar to that found in the countercurrent configuration. The influence of different parameters on the stability of the autothermal cocurrent reactor was analyzed. The regions where steady‐state multiplicity occurs were compared with those presented by the autothermal countercurrent reactor. The influence of an additional heat exchanger on the reactor stability was considered.
Turbulent reactive mixing of reversible reactions
Authors: Toor H. L. 
Published: June 2004

AbstractThe convective diffusion equations were used to obtain mean concentrations and concentration fluctuation covariances during turbulent reactive mixing of fast reversible reactions. The mean concentrations are directly applicable to reactions that are much faster than the mixing, while the covariances can serve as a first approximation closure for slower reactions. The covariances should also be useful for testing more general closure models. Numerical examples are presented for the simple mass‐action rate law with stoichiometric coefficients of unity.
Control of continuous copolymerization reactors
Authors: Padilla Salvador , Alvarez Jesús 
Published: June 2004

AbstractContinuous solution copolymerization is an important industrial process in the manufacture of commodity and engineering plastics. The addition of comonomers and solvent, and the rate of heat exchange must be simultaneously manipulated to maintain safety, operability, and the product quality adequately, yielding a process with nonlinear behavior, strong and asymmetric input–output multivariable coupling, and potential for open‐loop instability and state multiplicity, as shown in earlier dynamics and control studies. Accordingly, the key control objectives of the copolymerization reactor are: the compensation of interaction, the preclusion of input multiplicity and the robustness (i.e., tolerance to modeling and tuning errors) of the controller. In principle, these control issues should be considered within a nonlinear setting. Otherwise, the reactor may have to be operated with a conversion that is conservatively below what can be handled by standard mixing and heat‐exchange equipment. To assess the inherent control possibilities and limitations of a given copolymerization reactor, a methodology to address the control problem is proposed such that the nonlinearity, interaction, input multiplicity, and robustness issues are explicitly confronted. The result is a linear multivariable interaction compensator whose tuning can be done with notions and tools from conventional control. This method is tested with the copolymerization of vinyl acetate with methyl methacrylate, dissolved in ethyl acetate.
Cocurrent membrane reactors vs. PFRs for shifting dehydrogenation equilibrium
Authors: Reo Christopher M. , Bernstein Lewis A. , Lund Carl R. F. 
Published: June 2004

AbstractMembrane reactor performance in reversible reactions like dehydrogenation is often compared to plug‐flow reactor performance. Analysis of cocurrent membrane‐reactor design equations shows that as the Damköhler–Peclet product varies from 0 to ∞, the membrane reactor approaches a fully diluted or undiluted plug‐flow reactor. If its yield is maximized by varying this parameter, the optimum cocurrent membrane reactor is often one of these limiting cases, and a membrane reactor offers no advantage over a plug‐flow reactor. When the optimum membrane reactor does not correspond to one of these limiting cases, there still may be a plug‐flow reactor system that offers equal or higher yields. Analysis of plug‐flow‐reactor performance indicates that there is an optimum degree of dilution; while the optimized membrane reactor yield may be greater than either an undiluted or a fully diluted plug‐flow reactor; it may not be greater than an optimally diluted plug‐flow reactor. When using porous membranes the membrane reactor yield is at most 7% greater than the yield using a plug‐flow reactor. The porous membranes enhance the yields in a regime where the reactor would not be likely to operate. Much larger yield advantages are possible, but only when the membrane is highly permselective, as with dense membranes. Highly permselective membranes also enhance the yields in a regime where the membrane reactor process is likely to be operated.
Molecular thermodynamics of heat‐induced protein unfolding in aqueous media
Authors: Cioci Federico , Lavecchia Roberto 
Published: June 2004

AbstractThe molecular thermodynamic model studied is based on the two‐state mechanism of inactivation, in which only native folded and polymorphous unfolded protein forms are present at equilibrium. The influence of solvent on protein stability is described in terms of perturbation of the protein distribution between the two conformational states. An expression derived for the chemical potential of the protein accounts for conformational changes, ideal mixing effects, and interaction of the protein with the surrounding medium.Thermal unfolding of lysozyme was then studied in the absence or presence of hydroxylic compounds. Ultraviolet difference spectroscopy was used to monitor the conformational changes induced by heating and to determine the melting temperature of the protein. The additives investigated are ethanol, glycols, and natural osmolytes. Media containing ethanol and glycols destabilized lysozyme, whereas sugars increased the conformational stability of the protein. For all of the systems examined the melting temperature was linearly related to the surface tension of the mixed solvent, supporting the ability of the model to describe the influence of the solvent and composition on lysozyme unfolding. Model predictions agreed fairly well with published differential scanning calorimetric data. The influence of hydroxylic additives on protein's conformational stability does not extend to any special property of these components, but to their ability to perturb the surface tension of water. This model can be used to interpret and correlate thermal unfolding data and to solve the problem of protein stabilization.
Colloidally induced smectitic fines migration: Existence of microquakes
Authors: Mohan K. Krishna , Fogler H. Scott 
Published: June 2004

AbstractThe phenomenon of colloidally induced fines migration is a challenge of both scientific and industrial importance. Its occurrence impacts permeability reduction and alteration of flow pathways in porous media, particulate contaminant migration in groundwater flow, and filtration. The release of smectitic fines is a threshold type of process resulting from discontinuous jumps, called microquakes in the interlayer spacing. There is a critical salt concentration at which these microquakes occur and produce fines migration in the porous media. The changes in the microstructure with decreasing salt concentration were analyzed using X‐ray diffraction. The transition between crystalline and osmotic swelling regime is also shown to depend on the type of cation. A mathematical model developed using colloidal principles predicts the swelling behavior of smectites in aqueous solutions. The model can predict the transition of swelling from crystalline to osmotic regimes and explain the effect of different cations on the transition.
Finite‐element simulation of mixing: 1. Two‐dimensional flow in periodic geometry
Authors: Avalosse Th. , Crochet M. J. 
Published: June 2004

AbstractComplex mixing flows and mixing parameters are calculated to evaluate mixing quality on the basis of kinematic parameters together with a statistical analysis. The results allow for a comparative evaluation in terms of geometry and flow parameters. The evaluation is limited to two‐dimensional flows with moving boundaries in periodic motion such as the flow in the cross section of a twin‐cam mixer. The finite‐element calculation requires a remeshing procedure for every time step with special techniques to upgrade mixing variables from one time step to the next. The relative mixing quality of single cam and co: or counterrotating cam devices are compared by evaluating segregation scale, length stretch, and efficiency. The numerical results exhibit a good correspondence with their experimental counterpart.
Finite‐element simulation of mixing: 2. Three‐dimensional flow through a kenics mixer
Authors: Avalosse Th. , Crochet M. J. 
Published: June 2004

AbstractNumerical tools for the analysis of complex 3‐D mixing devices were developed. Calculations are based on the finite‐element method. The flow calculation is combined with that of pathlines for a fairly large number of material points; along these pathlines, relevant kinematic variables, which are then subjected to a statistical treatment, are evaluated. These numerical techniques are applied to the analysis of a Kenics mixer. It is assumed that the geometry is periodic in the axial direction, that is, that the mixer contains an infinite number of blades; the finite‐element analysis may then be completed on a single wavelength of the geometry. To test the accuracy of the three‐dimensional calculation, a Kenics device was mounted in order to mix clays of different colors, and photographs of experimental cross‐sections were compared with numerical results showing concentration maps. A statistical treatment was performed on some 8,000 material points crossing the entry section to evaluate mixing quality.
Steady spontaneous capillary flow in partially coated tubes
Authors: Weislogel Mark M. 
Published: June 2004

AbstractA spontaneous, steady capillary flow is produced for a liquid index in a circular tube that is partially coated with a surface modifier to create a discontinuous wetting condition between upstream and downstream portions of the tube. As a means of demonstrating that the flow configuration may prove useful as a diagnostic tool in studies concerning capillarity and the physics associated with the moving contact line, average steady velocities are predicted and compared against a large experimental data set that includes the effects of tube dimensions and fluid properties. To access a wider range of tube diameters (0.516–9.88 mm), experiments are performed employing “U‐tubes” tested in the low‐gravity environment of a drop tower, in addition to straight capillary tubes tested horizontally in a laboratory. The sensitivity of the steady capillary flow to surface cleanliness is dramatic.
Photon‐migration measurement of latex size distribution in concentrated suspensions
Authors: Sevick‐Muraca Eva , Pierce Joseph , Jiang Huabei , Kao Jeffrey 
Published: June 2004

AbstractFrequency‐domain measurements of photon migration were conducted in concentrated polystyrene suspensions to assess their use for on‐line particle sizing in undiluted process streams. Using a numerical inverse algorithm, particle‐size distributions (PSD) and volume fractions of three latex suspensions with differing distributions were recovered from phase‐shift measurements at 15 or less visible wavelengths. Comparison to dynamic light‐scattering measurements show excellent agreement. Since it is the propagation characteristics of multiply scattered light instead of the amount of light detected that is employed to solve the inverse problem, the measurement technique provides a self‐calibrating (and therefore especially suitable) method for on‐line process monitoring of PSD in the chemical‐based industries.
Solid‐phase creep during the expression of palm‐oil filter cakes
Authors: Kamst G. F. , Bruinsma O. S. L. , de Graauw J. 
Published: June 2004

AbstractFor an adequate model of the processes of compressible cake filtration and mechanical expression, permeability and compressibility data are required. Experimental and modeling results of the creep behavior of palm‐oil filter cakes at constant and time‐dependent pressures are presented. Creep curves of palm‐oil filter cakes at constant pressures cannot be modeled with linear viscoelastic models. Modeling with a modified form of the empirical equation of Nutting gives satisfactory results. This modification does not lead to unrealistic values of the porosity at extreme conditions, contrary to the original form of the equation of Nutting. Creep curves at time‐dependent pressures were modeled with two nonlinear viscoelastic models, which describe the time‐dependent creep behavior as a function of the pressure history and creep curves at constant pressures. Modeling with the strain‐hardening model provides the best porosity predictions.
Permeability of filter cakes of palm oil in relation to mechanical expression
Authors: Kamst G. F. , Bruinsma O. S. L. , de Graauw J. 
Published: June 2004

AbstractPermeability and compressibility data are required for an adequate process model for compressible‐cake filtration and mechanical expression. Experimental and modeling results of the permeability of palm‐oil filter cakes (a highly compressible viscoelastic material) are combined with compressibility data, leading to a model for the expression step. Permeability measurements show that permeability depends strongly on the quantity of fine particles in the cake. Removal of fine particles from the slurry before expression significantly increases the solid‐phase content during expression due to higher permeability. Modeling results of the expression step show that for palm‐oil filter cakes there is a pressure above which the attainable mass fraction of solids becomes independent of pressure. Decrease in specific cake resistance has two effects: a higher mass fraction of solids at the same pressure and a higher pressure at which the mass fraction of solids is not affected further.
Continuous drying of a solid wetted with ternary mixtures
Authors: Vidaurre M. , Martínez J. 
Published: June 2004

AbstractThe influence on drying selectivity of the continuous‐contact mode between a solid wetted with ternary mixtures and a gas stream was theoretically studied. The liquid mixtures, ethanol–isopropanol–water and water–ethanol–acetone, were used. A mathematical model describing a gas‐phase‐controlled process was developed, and the influence of the process variables was studied by simulations. In addition to the inlet composition of the moisture and temperature of the solid, gas composition has the most important effect on selectivity. Small changes of gas composition, either imposed or spontaneous, may modify completely the process trajectory. The extent of these effects depends on the ratio between the flow rates of inlet gas and liquid contained in the solid. Because of their effects on the evolution of temperature and composition, the operating pressure and energy sources other than convection are also useful in controlling the selectivity. Since all these variables determine the composition of the remaining liquid and thereby product quality, their influence should be predictable. The model may be a valuable tool for exploring the process, provided that drying is gas‐phase‐controlled.
Simulation of solids processes accounting for particle‐size distribution
Authors: Hill Priscilla J. , Ng Ka M. 
Published: June 2004

AbstractA prototype computer program SOLIDS has been developed for the simulation of processes with solids processing steps. In addition to heat and mass balances, the equipment units are modeled with discretized population balance equations. Any transformation mechanisms for the particle‐size distribution (PSD)—nucleation, growth, dissolution, agglomeration, and breakage—can be represented. With the sequential modular code, the PSD can be tracked from unit to unit in a complete plant. To provide maximum flexibility for further development, new or improved equipment models and discretization techniques can be included as additional subroutines. Case studies show that accounting for the PSD can lead to improved equipment design and a more complete description of the interactions among the units in the plant.
Mediating operation of catalytic CSTR to stabilize intermediate steady state
Authors: Gol'dshtein Vladimir , Panfilov Vadim 
Published: June 2004

AbstractA novel approach for stabilizing the intermediate steady state of a continuous stirred‐tank reactor (CSTR) is proposed by using a special type of periodic forced operation, the so‐called mediating operation. The mediating operation enables new additional steady states for which one of the new intermediate steady states is stable to be produced. Thus the proposed approach employs a periodic forcing for stabilization of a selected steady state by control of a steady‐state multiplicity. The feed flow rate is considered to be a manipulated variable. Changing CSTR multiplicity is investigated for two‐step control inputs. It is shown, analytically, that under two‐step control CSTR can exhibit at most five steady states with a stable intermediate steady state. A constructive procedure is proposed for finding control parameters corresponding to maximal multiplicity, that is, for stabilizing an intermediate steady state by two‐step control.