bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics of oil dispersion in the absence and presence of block copolymers
Authors: Polat H. , Polat M. , Chander S.
Published: April 2004

A phenomenological model proposed describes droplet breakup in the turbulently agitated lean oil‐in‐water dispersions and provides a correlation between the median droplet size in an agitated vessel of standard geometry and the time of dispersion. It was assumed that the droplet breakup takes place in the dispersion‐only region and coalescence is negligible. The model described the data from this study and the literature quite satisfactorily under these conditions. The effect of adding triblock PEO/PPO/PEO copolymeric surfactants on the dispersion kinetics of oil was also investigated. Addition of surfactant reduced the median oil droplet size significantly, and the extent of this reduction was a strong function of surfactant concentration. Application of the model on these data demonstrated that the change in the median droplet size could be divided into two distinct regions. The breakage rate was high initially, most probably due to continuous adsorption of surfactant molecules at the oil/water interface. A lower breakage rate was attained at longer times, as the surfactant molecules were depleted from the solution. The time of transition between the two was affected strongly by the concentration of the surfactant added. Furthermore, the time of addition of the surfactant did not affect the final droplet‐size distribution in the system.
Process boundary approach to separations synthesis
Authors: Pressly Thomas G. , Ng Ka M.
Published: April 2004

A method for synthesizing separation processes for multicomponent mixtures is presented which identifies the regions, boundaries, and the lower dimensional varieties in an N‐dimensional space. Both thermodynamic and equipment boundaries, which may block or limit a proposed separation step, are considered. To facilitate process synthesis, process boundaries are approximated as linear hyperplanes if appropriate, while nonlinear boundaries are represented using a set of linear hyperplanes. In conjunction with the available separation task selection methods, a step‐by‐step procedure is proposed to generate complete flowsheet alternatives, which allow bypassing both thermodynamic and equipment boundaries to achieve a given separations objective. Several example problems are solved using a spreadsheet program to illustrate the simplicity and applicability of this approach.
Robust PI stabilization of a class of continuously stirred‐tank reactors
Authors: Alvarez‐Ramirez Jose
Published: April 2004

This article deals with the stabilization of continuously stirred‐tank reactors in the face of control input saturations and uncertain chemical kinetics. Using ideas from modeling‐error compensation techniques, a first‐order compensator is designed, and its ability to stabilize the reactor temperature at an arbitrary setpoint is shown. The dynamic part of the proposed controller is interpreted as a type of robust integral action to accommodate the modeling error.
Visualization test for neutralization of acids by marine cylinder lubricants
Authors: Wu Rong Chang , Papadopoulos Kyriakos D. , Campbell Curt B.
Published: April 2004

The combustion of high‐level‐sulfur (2–5%) fuels generates large amounts of sulfuric acid in slow‐speed crosshead marine diesel engines. In this study, a video‐microscope imaging technique is used to develop an experimental protocol for ranking the ability of marine cylinder lubricants to neutralize sulfuric acid within microcapillaries. The technique was applied to several oils with known rates of acid‐neutralization ability to predict these rates. The optical observations allow one to propose a plausible mechanism by which the overbased detergent additive system performs the task of acid neutralization. According to this mechanism, the overbased sulfonate reverse micelles are the site for the neutralization reaction, which occurs at the oil‐acid interface. The results confirm that the detergent surfactants are chemically bound to the micellar core. In the case of overbased calcium‐carbonate detergents reacting with sulfuric acid, the formation of solid crystals was observed which may form a protective antiwear film on engine metal surfaces.
Variable structure NARX models: Application to dissolved‐oxygen bioprocess
Authors: Sargantanis Ioannis G. , Karim M. N.
Published: April 2004

Selecting the best structure of nonlinear autoregressive models with exogeneous inputs (NARX) is important for good parameter estimation and control. If the process is also time varying, the best structure selection procedure should be done in an on‐line fashion. Simultaneous model selection and parameter estimation can be achieved by employing the Givens forward selection with exponential windowing (GFEX) algorithm. An on‐line implementation yields a variable‐structure NARX‐model identification, which can be used for control of a dissolved‐oxygen (DO) process. The results of an adaptive DO control scheme, which uses a fixed model structure and was applied to a batch Bacillus subtilis fermentation, were used in this study. Time‐dependent patterns of selected terms were observed that roughly corresponded to different fermentation stages. Fitting and prediction capabilities for the adaptively selected NARX model were compared with fixed ARX and NARX models. The effect of the forgetting factor and the model order was investigated. Performance is satisfactory for short‐step‐ahead prediction, and avoiding overparametrization can stabilize the long‐step‐ahead prediction.
Improvement and validation of the fluoroptic temperature‐measurement method
Authors: Janse Arthur M. C. , de Jong Xander A. , Prins Wolter , van Swaaij Wim P. M.
Published: April 2004

The fluoroptic measurement method, recently developed by Wagenaar and coworkers to determine in a contactless manner the temperature of moving particles, has been improved. It now has a higher accuracy (approximately 5%) and can be used for particles that move at higher speeds. In addition, the technique has become much less labor‐intensive. Validation has been realized by comparing experimentally determined heat‐transfer coefficients for a cylinder in crossflow with predictions of several well‐known correlations from the literature. A consistent deviation of less than 10% is observed. This comparison therefore shows the reliability of the fluoroptic measurement method.
Hierarchical approach for simulation of binary adsorption in silicalite
Authors: Czaplewski Kenneth F. , Snurr Randall Q.
Published: April 2004

A hierarchical molecular modeling approach presented predicts adsorption thermodynamics of single components and binary mixtures in zeolites. Atomistic simulations that capture details at the molecular level are performed to calculate the free energies of molecules at discrete adsorption sites. Then a more coarse‐grained lattice model is used to calculate the equilibrium loading of adsorbates based on these free energies, and thermodynamic properties can be predicted. Sorbate–sorbate free energies beyond nearest‐neighbor interactions are introduced and their impact on the lattice model is investigated. By adding these free energies, the model is better able to accurately describe single‐component and binary adsorption of sulfur hexafluoride and neopentane in silicalite. The results from the lattice model agree well with full atomistic grand canonical Monte Carlo simulations performed for the same systems, but the hierarchical approach saves an order of magnitude of computational effort.
Evaluation of a locus of azeotropes by molecular simulation
Authors: Pandit Sandeep P. , Kofke David A.
Published: April 2004

The technique proposed here for the evaluation of azeotrope lines by molecular simulation builds on ideas used to devise the Gibbs‐Duhem integration (GDI) technique for evaluating phase equilibria by molecular simulation. Beginning with a known azeotropic state point, the method integrates a differential equation for the locus of azeotropes with a single semigrand‐ensemble molecular simulation performed at each integration state point. Unlike the standard GDI method, fluctuation quantities are needed to conduct the integration. Although these quantities are measured less precisely than simple ensemble averages, the integration is not adversely affected by this difficulty. The method is demonstrated by applying to three model Lennard‐Jones binaries. Among these mixtures is one in which the molecular diameter of one species is varied along the azeotrope line. This application shows how details of the intermolecular interactions affect azeotropic behavior. Such an understanding might be useful in formulating additives to break an azeotrope.
Predicting geometrical properties of random packed beds from computer simulation
Authors: Nandakumar K. , Shu Yiqiang , Chuang K. T.
Published: April 2004

Using advances in computational geometry and collision‐detection algorithms, an algorithm was developed to analyze and predict the geometrical properties of a randomly packed structure using packing objects of arbitrary shape. A 3‐D polygonal model was used to describe an arbitrarily complex packing object and a simple container object. The dynamics of the packing process is not simulated, but a search algorithm finds stable equilibrium positions for the packing objects of arbitrary shapes using a collision‐detection algorithm in a 3‐D space. A modified conjugate gradient optimization method determines the packing object's final packing location and orientation. Once the bed is packed, both macroscopic quantities like the overall porosity, the specific surface area, and the number of packing objects per unit volume and microscopic properties like the porosity variation in any direction could be determined. For accurate porosity calculation inside a given 3‐D polygonal sample space, the Sutherland‐Hodgman polygon clipper algorithm was used. Predicted results are validated against available experimental data for spheres, Raschig rings, Pall rings, and cascade minirings.
Growth and aggregation rates for calcite and calcium oxalate monohydrate
Authors: Collier Alan P. , Hounslow Michael J.
Published: April 2004

Seeded batch experiments were conducted at low magma densities and moderate ionic strength to determine the growth and aggregation rate constants for calcite precipitation in a stirred vessel. The growth was found to be surface‐reaction‐limited, and the growth rates agreed broadly with other studies of calcite precipitation. Aggregation rates have rarely been presented for calcite. The aggregation rate constant studied here was directly proportional to the instantaneous growth rate and inversely proportional to the average shear rate in the vessel. The results were also compared with similar work carried out on calcium oxalate monohydrate. It was found that the relationship between the aggregation rate constant and stirrer speed was virtually identical in the two cases, despite almost an order of magnitude difference in the growth rate constants.
Entry mass transfer in axial flows through randomly packed fiber bundles
Authors: Bao L. , Liu B. , Lipscomb G. G.
Published: April 2004

An analysis of the effects of nonuniform fiber packing on external mass‐transfer coefficients for axial flows through bundles of parallel, axially oriented fibers is presented in the entry mass‐transfer limit. In this limit, one can obtain an analytic solution to the mass‐transfer boundary‐layer equations in terms of the velocity gradient on the fiber surface for either a constant wall flux or constant wall concentration. To explicitly calculate mass‐transfer coefficients, a numerical approximation for the velocity gradient is obtained from the conservation of momentum equations using the boundary‐element method. Results indicate that the effective mass‐transfer coefficient depends strongly on fiber packing. Regions of higher fiber packing have lower flows and lower mass‐transfer coefficients than regions of lower packing. The net effect is a dramatic decrease in overall mass‐transfer coefficient relative to mass‐transfer coefficients in regularly packed fiber bundles.
Radial reactor for trichloroethylene steam reforming
Authors: Moates F. Craig , McMinn Timothy E. , Richardson James T.
Published: April 2004

A ceramic foam radial reactor was used to convert trichloroethylene by steam reforming, using 0.5 wt. % Pt as a catalyst. With a quartz enclosure heated externally by infrared lamps, the inlet temperature to the catalyst bed was low enough to suppress pyrolysis, but high conversions (0.99999+) were achieved at the exit. Stable operation up to 600 h with a space velocity of 5.6 × 104 h−1 was achieved, but reactant breakthrough then occurred, and the catalyst quickly deactivated. Although the deactivated catalyst was regenerated with carbon burning, activity decline was more rapid due to platinum sintering and washcoat degradation. Measured temperature profiles and model calculations indicated a large gradient in the bed and suggested that stable operation could be extended at lower space velocities. Axial temperature profiles were not uniform, since preferential flow occurred in the middle and lower regions of the radial bed. Potential improvements for future designs are suggested.
Mass‐transfer parameters in gas‐solid reactive media to identify permeability of IMPEX
Authors: Lu Hui‐Bo , Mazet Nathalie
Published: April 2004

Lu et al.'s modeling of gas‐solid reaction demonstrated the significant influence of mass transfer on the global transformation of consolidated porous reactive media with reactive gas at low pressure and low permeability. Their model coupled mass transfer with chemical kinetics at the grain level and heat transfer with mass transfer at the pellet level. The comparison between simulation results and recent experiments demonstrated that the value of the material permeability varies during the reaction process. This study presents the permeability decrease during the synthesis process. A correlation representing the nonlinear evolution of permeability was identified for different reactive materials used for experiments. These mechanisms can satisfactorily explain both the experimental measurements of global advancement and local temperature profiles.
Residence‐time distribution model for twin‐screw extruders
Authors: Gao Jun , Walsh Gregory C. , Bigio David , Briber Robert M. , Wetzel Mark D.
Published: April 2004

A model for the residence‐time distribution (RTD) is validated by the analysis of data collected from the extrusion of polyethylene on a 30‐mm Krupp Werner and Pfleiderer (W&P) corotating twin‐screw extruder. Transformation of the RTD to give both the residence‐volume distribution (RVD) and the residence‐revolution distribution (RRD) yields new physical insights into the extrusion process. It is observed that operating conditions with equivalent specific throughput result in an equivalent RVD and RRD, and for a given screw configuration the axial mixing of extrusion material as measured by a tracer is essentially the same for all operating conditions. This allows the experimental RVD curves to be superimposed to form a single master curve for a given screw geometry. These new tools motivate the development of a simple residence model that characterizes the partially filled and fully filled screw sections and is capable of distinguishing between screw configurations and operating conditions. A least‐square error‐fit method used to identify the parameters of the RTD model indicated that the model function is appropriate to describe the RTD experimental data.
Mechanisms of pulsating combustion during synthesis of advanced materials
Authors: Mukasyan Alexander S. , Rogachev Alexander S. , Varma Arvind
Published: April 2004

Gasless combustion is a specific type of combustion, where all initial reactants, as well as intermediate and final products, remain in condensed (solid or liquid) state. This process is of great interest because of its prospective applications in combustion synthesis of advanced materials. The stability of combustion wave propagation is an important factor in determining the quality of materials produced by combustion synthesis. In this work, a digital high‐speed microscopic video recording method is used for direct observation and quantitative characterization of pulsating combustion in several gasless systems. The experimental data quantitatively describe, for the first time, oscillations of local instantaneous velocity at length scale about 10 μm and time scale ∼10;minus;3 s. Three systems investigated were: Ta‐C, Nb‐B, and Ti‐C. Two new mechanisms of pulsating combustion were identified: crack‐induced oscillations occur in Nb + B and Ta + C systems, while ignition‐extinction oscillations take place during reaction in Ti + xC system.
Novel route to submicrometer tungsten carbide
Authors: Welham Nicholas J.
Published: April 2004

A mixture of graphite and scheelite (CaWO4) have been mechanically milled together for 40 h. The resultant powder and an unmilled powder were examined by thermal analysis, isothermal annealing, and X‐ray diffraction to determine the effect of milling on the reduction of scheelite to tungsten carbide. The milled powder underwent a rapid reduction reaction at 800°C–300°C lower than the unmilled powder. The reduction sequence to WC was shown to differ for the two powders with W2C intermediate in the milled powder and W in the unmilled powder. The milled powder showed complete reduction to WC in 1 h at 1,000°C, whereas the unmilled powder was incompletely reduced, even after 1 h at 1,200°C. Separation of the final phases was readily achieved by a simple acid leach, leaving WC grains around 200 nm in size.
Axial mixing in modern packings, gas, and liquid phases: II. Two‐phase flow
Authors: Macías‐Salinas Ricardo , Fair James R.
Published: April 2004

Axial mixing measurements of air and water under two‐phase flow conditions were made in a large‐scale packed column (0.43 m diameter) using tracer experiments. Part I of this article dealt with single‐phase mixing in the same column, with the same internals. Four packings were studied: 25.4‐mm ceramic Raschig rings, 25.4‐mm metal Pall rings, Sulzer BX structured packing, and Flexipac 2 structured packing. Air and water flowed countercurrently through the column at atmospheric pressure and at gas rates varying from 0.25 kg/m2·s up to the flooding point, and liquid rates from 3.25 to 8.5 kg/m2 ·s. A diffusion‐type model served to reproduce the experimental response curves obtained for both phases. The results confirmed previous observations for first‐generation packings: axial mixing in the gas increases with both gas and liquid rates, whereas liquid‐phase axial mixing is a decreasing function of liquid rate and is insensitive to gas rate up to the flooding point. It was also found that the BX packing produces the least mixing in both phases. The largest mixing effects in the gas phase are found for the Raschig rings, and the largest mixing effects for the liquid phase are found for Flexipac 2. Correlations were developed to reproduce the results, yielding an average ±22% difference between experimental and correlated data.
Separation of hydrogen from a H2 H2 OHI gaseous mixture using a silica membrane
Authors: Hwang Gab‐Jin , Onuki Kaoru , Shimizu Saburo
Published: April 2004

Hydrogen separation characteristics of silica membranes prepared by chemical‐vapor deposition (CVD)‐in an H2H2O HI gaseous mixture were evaluated for the application to hydrogen iodide decomposition in the thermochemical iodine–sulfer (IS) process. Porous alumina tubes with a pore size of 10 nm and different gas‐permeating‐portion lengths [20 mm (S membranes) and 100 mm (L membranes)] were modified by CVD using tetraethoxysilane as the Si source. Pore closure was controlled by CVD to maintain high hydrogen permeance while lowering only HI permeance. At a certain point, CVD was stopped using He/N2 selectivity as the indicator of pore closure. H2 N2 selectivities of the modified membranes, which were measured by a single‐component permeation experiment, showed 53, 9.2, 4.1, 135, and 6.6 at 600°C for S1, S2, S3, L1, and L2 membranes, respectively. Separation experiments of the H2H2OHI gaseous mixture (a molar composition of 0.09:0.78:0.13) in the modified membranes were carried out at 300–600°C. Hydrogen permeance was almost the same as the single‐component permeance. The separation factors of H2H2O and H2HI were over 3 and 150, respectively. The S3 membrane showed the highest hydrogen permeance (on the order of 10−7 mol·Pa−1·m−2·s−1) and the highest separation factor of H2HI (650 at 450°C).
Entropic and energetic selectivity in air separation with microporous materials
Authors: Rallabandi Premkumar S. , Ford David M.
Published: April 2004

Materials such as zeolites, carbon molecular sieves, and polymers are used increasingly in the separation of air, based on the difference in diffusion rate between oxygen and nitrogen through the material. The design of improved materials requires knowledge of the molecular‐level phenomena responsible for the separation, particularly relative roles of energetic and entropic (confinement) effects. This issue is difficult to resolve experimentally, as evidenced by the wide range in reported literature values reviewed here. A complementary approach is taken based on a combination of molecular modeling, statistical mechanics, and transition‐state theory. Selectivities for molecular models of oxygen and nitrogen in microporous structures are calculated using a Monte Carlo technique and resolved into entropic and energetic components, for a range of pore window sizes. Atomic‐level flexibility (vibration) is considered as well. The calculated entropic selectivities are significantly lower than reported theoretical results, but still consistent with experimental data. The energetic selectivity is very sensitive to the window dimensions and flexibility, but the entropic contribution is much less affected. This also contradicts some previous assumptions in the literature.
Systems approach for evaluating dynamics and plantwide control of complex plants
Authors: Groenendijk Alexander J. , Dimian Alexandre C. , Iedema Piet D.
Published: April 2004

Recycle loops strongly affect dynamics and control of complex plants. The stability of control structures for component inventories may be influenced by recycle interactions and by connectivity, which leads to flowsheet alternatives possessing different control properties. A simulation‐based methodology for evaluating these phenomena and finding the best flowsheet structure is reported from a controllability point of view. Steady‐state and dynamic simulations are combined with controllability analysis tools, both steady state and in the frequency domain, which extracts more value from simulation than the usual sensitivity studies. The case of a vinyl chloride monomer (VCM) plant with impurities handling as a main issue demonstrates the power of the approach. It leads to an unconventional design alternative where an extra impurity‐destroying reactor reduces bad dynamic characteristics, since it prevents impurity buildup in the recycle. The steady‐state analysis of the VCM case is confirmed at low frequencies of disturbances, but difficulties may occur at higher frequencies, where the disturbances' period and time constants of the distillation columns are of the same order of magnitude. Controller interactions play a significant role in disturbance rejection. Nonlinear dynamic simulations with closed plantwide control loops confirm the analysis results.
Robust control of particulate processes using uncertain population balances
Authors: Chiu Timothy Y. , Christofides Panagiotis D.
Published: April 2004

A general method is proposed for the synthesis of robust, nonlinear controllers for spatially homogeneous particulate processes described by population balances including time‐varying uncertain variables. The controllers are synthesized via Lyapunov's direct method on the basis of finite‐dimensional approximations of the population balances which are obtained by using the method of weighted residuals. The controllers enforce stability in the closed‐loop system and attenuation of the effect of uncertain variables on the output, and achieve particle‐size distributions with desired characteristics. The robustness of the controllers with respect to unmodeled dynamics is also addressed within the singular perturbation framework. The controllers enforce the desired stability and performance specifications in the closed‐loop system, provided that the unmodeled dynamics are stable and sufficiently fast. The proposed control method is applied to a continuous crystallizer with fines trap in which the nucleation rate and the crystal density change arbitrarily with time, and the actuator and sensor dynamics are explicitly considered in the process model, but not included in the model used for controller synthesis. Simulation runs of the closed‐loop system clearly demonstrate that the controller attenuates uncertainty, achieves a crystal‐size distribution with desired characteristics, and is superior to nonlinear controllers that do not account for the presence of uncertainty.
Identification of the state of a fluidized bed by pressure fluctuations
Authors: Trnka O. , Veselý V. , Hartman M. , Beran Z.
Published: April 2004

A novel approach was used for the on‐line characterization of the state of a fluidized bed. The treatment technique using the Fast Fourier Transform of measured pressure fluctuations was developed to create plots describing the evolution of bed behavior from fixed bed to pneumatic transport. A new triad of quantities proposed are: divide ratio (degree of dominancy) of amplitude spectrum, square root of spectrum power of pressure fluctuations, and gas velocity. The technique was demonstrated on gas fluidization of solids having a wide size distribution composed of irregularly shaped solids.
Characterization of downflowing high velocity fluidized beds
Authors: Cao Chunshe , Weinstein Herbert
Published: April 2004

A downer‐riser circulating high velocity fluidization apparatus was developed to study the fundamentals of downflowing gas‐solid particle mixtures. The acceleration and deceleration of solids due to the influences of the entrance and exit sections result in a relatively uniform axial solids distribution. Radial solid density profiles detected with an X‐ray imaging system in the downer show the existence of a core‐annulus flow with a dilute core surrounded by a denser wall region. Local solids flux profiles were obtained with an aspirating probe device and the solid velocity profile obtained from the two measured quantities. These confirm that the majority of solids segregates in a wall region that flows faster than the dilute core region. Thus, the shorter residence time in the high‐speed downer wall region is coupled with faster reaction rates due to the accompanying high concentration of catalyst, while the dilute core has slower reaction rates with longer residence time due to the lower catalyst concentration and flow velocity. This results in much more uniform reaction extent over the cross‐sectional area of the downer and, therefore, should improve the product selectivity.
Coalescence of deformable granules in wet granulation processes
Authors: Liu L. X. , Litster J. D. , Iveson S. M. , Ennis B. J.
Published: April 2004

In this work, the coalescence of deformable granules in wet granulation processes is modelled. The model accounts for both the mechanical properties of the granules and the effect of the liquid layer at the granule surface. It is an extension to the model of Ennis et al. (1991) to include the possibility of granule plastic deformation during collisions. The model is written in dimensionless groups such as viscous and deformation Stokes numbers and the ratio of granule dynamic yield strength to granule Young's modulus (Yd/E*). These variables are bulk parameters of the powder‐binder mixture and also functions of the process intensity. The model gives the conditions for two types of coalescence–type I and type II. Type I coalescence occurs when granules coalesce by viscous dissipation in the surface liquid layer before their surfaces touch. Type II coalescence occurs when granules are slowed to a halt during rebound, after their surfaces have made contact. The model explains some of the trends observed in the literature, are preliminary validation of the coalescence criterion with drum granulation data is encouraging. An extension is also made to the case of surface dry granules, where liquid is squeezed to the surface during granule deformation.
Temperature effect on transport performance by inorganic nanofiltration membranes
Authors: Tsuru Toshinori , Izumi Shuhei , Yoshioka Tomohisa , Asaeda Masashi
Published: April 2004

The effect of temperature on nanofiltration performance was examined using three inorganic membranes with a molecular‐weight cutoff of approximately 200, 600, and 2,000, respectively. The inorganic porous membranes were prepared from silica‐zirconia colloidal sols and used in nanofiltration experiments for neutral solutes over a temperature range of 20 to 60°C. The rejection of solutes decreased with an increase in temperature for the membranes, while the permeate volume flux increased. Three transport coefficients—refection coefficient, solute permeability, and water permeability—were obtained using the Spiegler‐Kedem equation, which accounts for the contribution of convection and diffusion to solute flux. As a result, the reflection coefficient corresponding to the fraction of solutes reflected by the membrane in convective flow was almost constant, irrespective of experimental temperature. Solute permeabilities, however, increased with temperature. The dependency was larger for larger solutes and membranes with smaller pore diameters. Therefore, the hindered diffusion of solutes through micropores was indicative of an activated process. Moreover, pure water permeability, after correction for the temperature effect on viscosity, also increased with experimental temperature.
Nonlinear process control using contraction theory
Authors: Lohmiller Winfried , Slotine Jean‐Jacques E.
Published: April 2004

Contraction theory is a recently developed nonlinear control system tool based on an exact differential analysis of convergence. Contraction theory is applied to stability analysis and control system design for nonlinear chemical processes. Simple designs with explicit stability and convergence guarantees are obtained by taking advantage of the monotonicity of the reaction rates and the linear ambiguity in the choice of the chemical state.
Measuring anisotropic thermal conduction in polyisobutylene following step shear strains
Authors: Iddir Hadjira , Venerus David C. , Schieber Jay D.
Published: April 2004

The connection between polymer chain orientation and several macroscopic properties in a polymer melt was studied using mechanical and optical techniques. Anisotropic thermal conductivity following shear deformation was measured using forced Rayleigh light scattering, the refractive index tensor is followed using birefringence measurements, and the stress was measured mechanically in a parallel‐plate rheometer. The thermal diffusivity measured in the flow and neutral directions increased and decreased, respectively, immediately following the deformation. These quantities then relaxed to the equilibrium value on the time‐scale of the stress‐relaxation memory. Comparison of the difference between measured flow and neutral direction thermal diffusivities with the analogous flow‐induced birefringence in the same deformation provided indirect evidence for a linear relation between stress and thermal diffusivity at two different values of strain. Mechanical measurements were used to characterize the memory of the fluid.
Supported zeolite membrane by vapor‐phase regrowth
Authors: Tsay C. S. , Chiang A. S. T.
Published: April 2004

A new method for preparing supported zeolite membrane is proposed. A layer of colloidal MFI zeolite is deposited on a porous ultrafiltration support, on which a silica barrier layer has been precoated. The composite is then heated at 100°C under saturate water vapor for 30 h to obtain a very thin zeolite membrane with a glassy look. Under SEM, the zeolite membrane exhibits obvious void when the particles deposited were 100 × 150 nm in size. The particles are packed much closer if 80 × 100‐nm particles were used. With smaller (60 × 80 nm) particles, the boundary between particles fuses together, and a continuous zeolite membrane could therefore be prepared. The membrane thickness could also be reduced by depositing less colloidal zeolite for regrowth to obtain a continuous zeolite membrane as thin as 0.5 μm on top of a multilayer porous alumina support. Some preferred orientation of the elliptical zeolite particles is observed on the surface of the membrane, but no preferred crystallography orientation is confirmed. The reproducibility of this preparation method was checked by room temperature permeation measurements of several gases. Nearly identical results were obtained on two separately prepared membranes. Permeation results also confirmed that there were very few, if not free of, pinholes in the zeolite membrane so prepared.
Validating a prediction method of mean residence time spatial distributions
Authors: Baléo Jean‐Noël , Le Cloirec Pierre
Published: April 2004

Determining residence time is of major interest in the characterization, design or modeling of most chemical engineering processes, where a proper and homogeneous fluid distribution is often essential. A numerical method was used to obtain theoretical predictions of residence time spatial distributions and was validated by comparing with experiments. It was simulated first by solving continuity and momentum conservation equations, coupled with turbulence quantity transport equations, using the finite volume method. After calculating the flow configuration, the simulation was done by solving the transport equation of a passive scalar, the local mean age of the fluid or local mean residence time, which is the average time of a fluid particle to reach any point of the domain from a supply inlet. The result obtained is a spatial distribution of the local mean age of the fluid, which may be displayed as isocontours in the space domain considered. It was compared with measurements of mean residence times of tracers in a reactor to concretely assess the relevance of this method.
Capillary flow in a noncircular tube
Authors: Turian Raffi M. , Kessler Frederick D.
Published: April 2004

The 1‐D axial, laminar, Newtonian flow through a tube of uniform, but noncircular, cross‐section is analyzed. The tube boundary is r = R[1 + εf(θ)], in which R is a reference radius, ε is a small parameter, and the function f(θ) of the polar angle is general, albeit subject to minimal constraints ensuring single‐valuedness and quadrant symmetry. All results are determined as asymptotic expansions complete to terms O(ε2), which include the velocity distribution, average velocity, volume flow rate, ratio of flow rates in the noncircular to that in a circular pipe of the same cross‐sectional area, the friction factor‐Reynolds number dependence, the permeability of packed beds comprising noncircular capillaries, kinetic‐energy and momentum‐flux correction factors, the capillary penetration rate under the influence of the capillary pressure and the equilibrium value of the capillary rise. Expressions are derived in terms of the general boundary function f(θ) and, for special cases, when f(θ) = sin2kθ (k = 1, 2, 3) and f(θ) = sin2kθ with k a positive integer. The results provide quantitative measures of the effect of tube shape on flow properties and on imbibition and drainage from noncircular capillaries.
Pressure drop in gas slugs in vertical tubes and flooding instability
Authors: de Carvalho J. R. F. Guedes , Ferreira M. J. F.
Published: April 2004

Fast‐response differential‐pressure transducers were used to continuously monitor the pressure difference between two nearby points along single gas slugs rising in long vertical tubes filled with liquid. Operating pressures in the 0.6 to 6.0‐MPa range gave slugs of argon with densities in the 10 to 100‐kg/m3 range, rising through mixtures of glycerol and water with viscosities in the 10−3‐ to 305 × 10−3‐Pa·s. range. The transducer records suggest that, for all the liquids tested, there was flooding instability of the film flowing around the slugs when the gas density was raised sufficiently. With the assumption that flooding instability is due to the force on the waves moving along the gas–liquid interface, dimensional analysis shows that the observations with gas slugs correlate well with flooding data obtained in wetted wall tubes operating at atmospheric pressure. The work reveals strong limitations of current empirical correlations for the prediction of flooding in wetted wall tubes.
Corrected Horváth‐Kawazoe equations for pore‐size distribution
Authors: Rege Salil U. , Yang Ralph T.
Published: April 2004

The Horváth‐Kawazoe (HK) model is a widely used method for determining pore‐size distribution in a microporous material from a single adsorption isotherm. The original model, however, suffers from conceptual flaws. The total interaction energy of the adsorbate is underestimated since the adsorbate‐adsorbate interaction is incorrecty calculated. New corrected HK models proposed here for three pore geometries (slit, cylindrical, and spherical) can overcome these defects. Two other improvements have also been made in the new models. These assume that a filled micropore is composed of layers of adsorbate molecules which interact only with the molecular layers in the immediate vicinity. A better estimate of adsorbate‐adsorbate‐adsorbent interaction is obtained by utilizing actual distances between interacting molecules. The average interaction energy is calculated by a population‐weighted average of the energy potential of the layers rather than by integration. This average potential approaches a nonzero value at large pore size, unlike that in the original model. Pore‐size distributions predicted by the corrected HK models agreed significantly better with crystallographic data compared to the original model for both microporous and mesoporous sorbents.
Microwave radiation's influence on sorption and competitive sorption in zeolites
Authors: Turner Michael D. , Laurence Robert L. , Conner W. Curtis , Yngvesson K. Sigfrid
Published: April 2004

The influence of microwave radiation on adsorption selectivity was studied using the adsorption and desorption of cyclohexane and methanol on high‐silica zeolites. The amount of microwave energy absorbed depends on the specific system of adsorbent and adsorbate. The adsorbent, a high‐silica zeolite, is effectively “transparent” to microwave radiation, while the two adsorbates reflect high (methanol) and low (cyclohexane) absorption of microwave energy. The measured system temperatures required for desorption by microwave energy were lower than those required for conventional heating. Further, microwave radiation can change the sorption selectivity; the adsorbate with the greater microwave absorptivity is desorbed selectively. It is concluded that the surface and adsorbed species can be heated selectively since the rate of microwave energy absorption can be greater than the rate of heat transfer from the surface.
Extraction in thin liquid films generated by impinging streams
Authors: Berman Yuli , Tamir Abraham
Published: April 2004

A new device for liquid–liquid extraction was developed, tested, and modeled, based on two immiscible thin films, flowing one on the top of the other while exchanging mass. The films are formed by impinging streams: collision of two immiscible jets flowing one toward the other on the same axis. Thus, the flow of the films is in a direction perpendicular to the initial flow of the jets. Two liquid systems (diluent–solute–solvent) were tested: water–iodine–kerosene and aqueous solution of HCl–pure water. In the latter case, HCl is transferred from its initial solution comprising one film to the other film of pure water flowing on the top of it. The experiment determined geometrical parameters of the thin film, mass‐transfer coefficients of the solute between the films, as well as power input. For a relatively identical power input, the measured mass‐transfer coefficients were higher by a factor of 10–200 compared to conventional devices. In addition, the mean residence time in the new device is shorter than in conventional devices due to minimal dispersion of the phases during the extraction stage, making the settling stage relatively simple and short.
Performance analysis of four‐bed H2 PSA process using layered beds
Authors: Park Jong‐Ho , Kim Jong‐Nam , Cho Soon‐Haeng
Published: April 2004

Hydrogen purification from a typical cracked gas mixture (H2/CO2/CH4/CO) by the four‐bed PSA process was studied experimentally and theoretically using layered bed of activated carbon and zeolite 5A. The mathematical model accounts for the mass and heat transfer resistance using the linear driving force approximation for the particle uptake, and the ideal adsorbed solution model represents the multicomponent adsorption equilibrium. Effects of the superficial velocity, heat transfer resistance and the bed layering on the process performance were investigated. Since the recovery sharply decreases above a certain productivity at the given purity, the relation between the productivity and recovery was studied before the process design. Its performance decreased with the increase in heat transfer resistance, but increased with heat transfer resistance in some range. Both the experiment and theory showed optimum carbon‐to‐zeolite ratios. Those optimum ratios depend on such operating variables as the superficial velocity, heat transfer resistance and product purity. For higher product purity more zeolite was used. As the superficial velocity increased, the process performance has its maximum at higher carbon‐to‐zeolite ratio. Except for the adiabatic conditions, the optimum carbon‐to‐zeolite ratio increased with the increase in the heat transfer resistance. At the adiabatic conditions, however, the optimum carbon‐to‐zeolite ratio was lower than that at the nonisothermal conditions.
Scenario‐integrated modeling and optimization of dynamic systems
Authors: Abel O. , Marquardt W.
Published: April 2004

Assessing the consequences of exceptional events, such as failures and sudden changes in product demand or raw‐material prices, is of great importance to the operation of chemical processes. Such an exceptional event and the resulting behavior of the process are called a scenario in this article. Problem formulations for their rigorous incorporation into the optimization of dynamic systems are presented. They allow the determination of operational strategies that are economically optimal and that keep the plant within a chosen regime in the state space, even for a number of possible scenarios. The formulations are based on a unifying model framework containing both continuous and discrete dynamics. For a restricted class of problems a method for the numerical solution of the resulting scenario‐integrated dynamic optimization problems is introduced. The applicability of this technique is demonstrated with two examples of different complexity.
Two‐dimensional heterogeneous model for a reticulated‐foam photocatalytic reactor
Authors: Changrani Rajnish G. , Raupp Gregory B.
Published: April 2004

A 2‐D heterogeneous convection reaction model for a gas–solid lamp‐in‐tube annular photocatalytic reactor is presented. The catalyst (TiO2) is supported on a reticulated‐foam monolithic structure placed in the annular space between the UV lamp and the reactor wall. Mass balances for individual species are coupled through the reaction‐rate expression that appears in the boundary condition at the fluid–solid interface in heterogeneous catalytic reactor models. The heterogeneous reaction rate is modeled using semiempirical Langmuir‐Hinshelwood‐Hougen‐Watson (LHHW) kinetics with adsorption constants for various species. The local volumetric rate of energy absorption (LVREA) term in the rate expression was computed using a 3‐D polychromatic radiation‐field model. The overall system of nonlinear partial differential equations was solved using a combination of the Crank‐Nicolson method and the globally convergent Newton‐Raphson method. The apparent, average quantum yield in the LHHW kinetic rate form is the single adjustable parameter in the model. Isopropanol (IPA) was chosen as the test contaminant to conduct experimental performance measurements for model validation. Model‐predicted radial and axial profiles for bulk and surface concentration reveal that the extent to which mass transport influences the operation of a photocatalytic reactor is determined largely by the local magnitude of the LVREA. Similitude in the scale‐up of an annular lamp‐in‐tube heterogeneous photocatalytic reactor is achieved only when the dimensionless radial radiation profile, as well as corresponding magnitudes of a geometric number, the Peclet number, the Stanton number, and the photocatalytic analog of the Damköhler number, are identical.
Kinetics and product distribution of n‐hexadecane pyrolysis
Authors: Watanabe Masaru , Tsukagoshi Masayuki , Hirakoso Hideyuki , Adschiri Tadafumi , Arai Kunio
Published: April 2004

Pyrolysis experiments on n‐hexadecane (n‐C16) were conducted at 673–723 K and n‐C16 concentrations of 0.07–1.47 mol/L by using batch type reactors. The main products of n‐C16 pyrolysis were n‐alkanes and 1‐alkenes at all the reaction conditions. The 1‐alkene/n‐alkane ratio decreased with increasing n‐C16 concentration at all the reaction temperatures. The rate of n‐C16 pyrolysis increased to a maximum and then decreased with increasing n‐C16 concentration. The activation energy of the overall rate constant of n‐C16 pyrolysis was 196 kJ/mol at 0.07 mol/L of n‐C16 concentration and 263 kJ/mol at 0.22 mol/L. To describe these phenomena, a mathematical model for the pyrolysis that expresses the radical network reaction, including initiation, isomerization, β‐scission, H abstraction, and termination, was developed. The effect of radical size on the rates of bimolecular reactions (H abstraction and termination) was important for a correct quantitative description. Comparison between the experimental data and the model showed that the rates of bimolecular reactions were inversely proportional to the carbon number i of radical Ri. The model can predict product distribution and n‐C16 pyrolysis rate in a wide range of temperatures (603–893 K) and n‐C16 concentrations (6.86 × 10−3 2.48 mol/L). Furthermore, the model can describe the pyrolysis kinetics of n‐C10–n‐C25 by considering the carbon number of the hydrocarbon.
Microencapsulation of proteins by rapid expansion of supercritical solution with a nonsolvent
Authors: Mishima Kenji , Matsuyama Kiyoshi , Tanabe Daisaku , Yamauchi Satoru , Young Timothy J. , Johnston Keith P.
Published: April 2004

A new method—rapid expansion from supercritical solution with a nonsolvent (RESS‐N)—is reported for forming polymer microparticles containing proteins such as lysozyme (from chicken egg white) and lipase (from Pseudomonas cepacia). A suspension of protein in CO2 containing a cosolvent and dissolved polymer is sprayed through a nozzle to atmospheric pressure. The polymers are poly(ethylene glycol) (PEG4000; MW = 3,000, PEG6000; MW = 7,500, PEG20000; MW = 20,000), poly(methyl methacrylate) (PMMA; MW = 15,000), poly(L‐lactic acid) (PLA; MW = 5,000), poly(DL‐lactide‐co‐glycolide) (PGLA; MW = 5,000) and PEG–poly(propylene glycol) (PPG)–PEG triblock copolymer (MW = 13,000). The solubilities of these polymers in CO2 increase significantly with low‐molecular‐weight alcohols as cosolvents. The particles do not tend to agglomerate after expansion, since the pure cosolvent is a nonsolvent for the polymer. The structure and morphology of the microcapsules were investigated by TEM, SEM, and optical microscopy. The thickness of the polymer coating about the protein, as well as the mean particle diameter and particle‐size distribution, could be controlled by changing the feed composition of the polymer.