bluuebunny/crossref_metadata_2025_split huggingface dataset:
Deformation of a chromatographic bed during steady‐state liquid flow
Authors: Östergren Karin C. E. , Trägårdh A. Christian , Enstad Gisle G. , Mosby Jostein 
Published: April 2004

AbstractA two‐dimensional model was developed that describes the deformation of an open, mechanically stable chromatographic bed during steady‐state flow. It is based on the assumption of pure elastic deformation and the validity of Darcy's law. A control‐volume method was used to solve the mathematical equations. The computed pressure profiles agree well with experimental data. Computer simulations indicate that radial void variations, which will cause significant flow variations, may occur under chromatographic flow conditions. A method of measuring Young's modulus and the Poisson ratio in a water‐saturated sample at very low stress levels ( < 15 kPa) using a cubic plane strain tester, is also presented.
Monitoring the quality of fluidization using the short‐term predictability of pressure fluctuations
Authors: Schouten Jaap C. , van den Bleek Cor M. 
Published: April 2004

AbstractIn various industrial applications of bubbling fluidized beds, defluidizing parts of the bed or even of the complete bed can occur as a result of agglomeration and sintering of particles due to unintentional maloperation of the bed, changes in operating conditions, or variations in gas or solids feed. Defluidization may be prevented by increasing the gas velocity or changing the solids feed, if the change in the “quality” of the fluidized state of particles is detected early enough. An analysis method is proposed that uses the short‐term predictability of time series of (local) pressure fluctuations in the fluidized bed to detect changes early in the hydrodynamic state of the bed. It is based on the comparison of an original time series of pressure fluctuations with successive time series measured during operation of a fluidized bed. The comparison is based on a discriminating statistic computed for the original time series as well as for each successive series. The null hypothesis that the original and successive time series are similar is rejected if the discriminating statistics of both time series significantly differ. Experimental application of the method is illustrated for fluidization at elevated temperatures (ca. 120°C) of agglomerating plastic particles in a 5‐cm‐ID laboratory fluidized bed. The method recognizes the change in the hydrodynamics due to the incipient agglomeration of the particles. In this particular case the time period between the moment of detection of a significant change in the hydrodynamics and the end of the experiment when the bed becomes defluidized seems sufficiently large to take preventive measures. The average bed‐pressure drop is not a sensitive quantity to detect changes early in the fluidization behavior.
Pulsed adsorbent transport in a countercurrent adsorptive reactor
Authors: van der Wielen L. A. M. , van Nunen C. A. P. M. , Luyben K. Ch. A. M. 
Published: April 2004

AbstractSelective countercurrent transport of dense adsorbent particles through pulsed multistage, liquid fluidized beds of light catalyst particles is attractive as the basis for new countercurrent adsorptive reactors. Its applications are expected to demand operation at high solids flows, and thus at high pulsation frequencies and volumes. The effect of high pulsation frequencies and volumes on the flows of liquid and solids is studied by model and experiment. The key parameter determining contactor behavior is the product of pulsation frequency and volume: the pulsed back flow. Its theoretical maximum (flooding condition) equals the liquid flow rate. High pulsation frequencies and volumes greatly affect the degree of liquid and solids mixing. The degree of liquid mixing increases exponentially with increasing pulsed back flow from plug‐flow behavior (Pe = 10–100/stage) to complete mixing near the flooding condition. The solids mixing at low‐to‐moderate solids transport could be described with complete mixing at this stage. At higher solids transport, the extent of mixing slightly decreases toward two tanks‐in series per stage. The relations established allow for a preliminary design of a pulsed multistage fluidized‐bed reactor at any low and high solids transport rate, as well as for the estimation of corresponding operating conditions.
PID controller tuning for desired closed‐loop responses for SI/SO systems
Authors: Lee Yongho , Park Sunwon , Lee Moonyong , Brosilow Coleman 
Published: April 2004

AbstractProportional, integral, and derivative (PID) parameters are obtained for general process models by approximating the feedback form of an IMC controller with a Maclaurin series in the Laplace variable. These PID parameters yield closed‐loop responses that are closer to the desired responses than those obtained by PID controllers tuned by other methods. The improvement in closed‐loop control peformance becomes more prominent as the dead time of the process model increases. A new design method for two degree of freedom controllers is also proposed. Such controllers are essential for unstable processes and provide significantly improved dynamic performance ouer single degree of freedom controllers for stable processes when the disturbances enter through the process.
Analytical control of SISO nonlinear processes with input constraints
Authors: Valluri Sairam , Soroush Masoud 
Published: April 2004

AbstractA new method of controlling nonlinear processes with actuator saturation nonlinearities is presented. Two nonlinear control laws are derived for single‐input, single‐output nonlinear processes. Whether input constraints are present or not, each of these dynamic control laws can minimize the mismatch between the closed‐loop output response and the nominal linear output response that the same control law induces when there are no constraints. The control laws offer great flexibility to obtain a desirable closed‐loop output response in the presence of active input constraints, and they inherently compensate for windup. They allow one to adjust the time period during which an input constraint is active and the decay rate of the mismatch between the constrained output response and the nominal linear unconstrained output response. Conditions under which the constrained closed‐loop system is asymptotically stable are given. The connections between the developed control laws and the modified internal model control are established. The performance of the control laws is demonstrated by numerical simulations of chemical and biochemical reactor examples.
CVD synthesis and gas permeation properties of thin palladium/alumina membranes
Authors: Xomeritakis George , Lin Yue‐Sheng 
Published: April 2004

AbstractThin (0.5–5‐ μm) palladium (Pd) membranes were prepared inside pores or on the suface of mesoporous (pore size 4–6 nm), sol – gel‐derived γ‐Al2O3 supports by the chemical vapor deposition (CVD) method using palladium acetylacetonate and palladium chloride (together with hydrogen) as Pd precursors. When Pd is deposited in the form of metal plugs inside pores of γ‐Al2O3, He permeance of the composite membranes increases exponentially with temperature, indicating activated diffusion through the rnicroporous metal deposit defects, while only modest permselectivity for H2 is observed. When Pd is deposited in the form of a metal film on the surface of γ‐Al2O3, He permeance is governed by Knudsen‐viscous flow through meso/macroporous metal film defects. The H2 permeance of different palladium membranes prepared by CVD appears to increase with palladium crystal grain size. The highest H2 permeance and H2:He permselectivity for membranes prepared in this study are about 1.0–2.0 × 10−7 mol·m−2·s−1·Pa−1 and 200–300 at 300°C, respectively.
Fiber matrix model of sclera and corneal stroma for drug delivery to the eye
Authors: Edwards Aurélie , Prausnitz Mark R. 
Published: April 2004

AbstractA model derived from fiber matrix theory to predict the permeability of the eye's fibrous tissues, namely the sclera and corneal stroma, to water and solutes ranging from low molecular‐weight drugs to macromolecules was developed. The model is based upon the ultrastructure of the cornea and the sclera; all parameters correspond to the geometrical and physicochemical characteristics of the eye and solutes, and are estimated from independent literature data. Comparison of our predictions with a large set of experimental data shows good agreement. The model suggests that important factors controlling diffusion rates across the sclera and stroma are tissue hydration, tissue thickness, and the size and volume fraction of proteoglycans present in these tissues. Applications to ocular drug delivery and treatment of glaucoma are discussed.
Influence of buoyancy forces on the flow of gases through packed beds at elevated pressures
Authors: Benneker A. H. , Kronberg A. E. , Westerterp K. R. 
Published: April 2004

AbstractThe flow of gases through packed‐bed columns at elevated pressures was investigated by displacement experiments with a stepwise change in the tracer concentration. The experiments with different tracers, flow rates, pressures, particle sizes, tube diameters, and flow directions were used to illustrate the effect of buoyancy forces on the apparent fluid mixing in packed beds. The experimental results exhibit a strong influence of the density differences between the tracer and carrier gas on the flow behavior at higher pressures. Axial dispersion significantly increases due to hydrodynamical instabilities when the density increases with the height, and is reduced in the presence of stable density gradients. The effect of density differences on the fluid flow becomes more pronounced with increasing tube diameters. It can be argued that in pressure equipment the usual correlations for mass and heat transfer may not be valid any more and that plug flow in such equipment is hardly possible to achieve.
Quantitative characterization of mixing of dry powders in V‐blenders
Authors: Brone Dean , Alexander Albert , Muzzio F. J. 
Published: April 2004

AbstractExperiments were conducted to determine the effect of rotation rate, fill level, and total number of revolutions on the mixing performance of a V‐blender. The experiments were performed using 66‐μm red and green glass beads. Mixture uniformity was quantified after each experiment by infiltrating the mixtures with a binder, and using image analysis to examine slices of the solidfied mixture. The mixing rate (as a function of total revolutions) was not affected by changing the rotation rate between 8 and 24 rpm. The mixing rate increased significantly as the fill percentage was decreased from 60% to 40% of the total vessel volume. In a symmetric blender such as a V‐blender, the rate of mixing is limited by the rate of mass transfer across the vessel plane of symmetry orthogonal to the axis of rotation. A one‐parameter model based on the amount of material that crossed the plane of symmetry per revolution displayed good agreement with the experimental data.
Defect‐free palladium membranes on porous stainless‐steel support
Authors: Mardilovich Peter P. , She Ying , Ma Yi Hua , Rei Min‐Hon 
Published: April 2004

AbstractDefect‐free Pd membranes were prepared by an electroless plating technique on porous stainless‐steel tubes. The effective surface of Pd membranes was up to 75 cm2. The helium flux was not detected at room temperature and pressure difference of 3 atm. At 350°C hydrogen permeances of up to \documentclass{article}\pagestyle{empty}\begin{document}$ 8\ m^3 /(m^2 \cdot \overrightarrow h \cdot atm^{0.5}) $\end{document} were obtained. A t a pressure difference of 1 atm and 350°C, selectivity coefficients as high as J(H2/JN2) = 5,000 were observed. Fluxes of gases, impermeable through the Pd, were quantitatively described as a combination of Knudsen diffusion and viscous flow through the gaps created in the Pd layer at high temperatures. The stability of the prepared membranes at temperatures up to 700°C was investigated. The membranes were stable at 350°C over a period of 1,100 h, with no significant changes in the steady‐state hydrogen flux and with a recrystallization texture and aggregation of Pd grains.
Continuously stirred decanting reactor: Operability and stability considerations
Authors: Khinast J. , Luss D. , Harold M. P. , Ostermaier J. J. , McGill R. 
Published: April 2004

AbstractA continuously stirred decanting reactor (CSDR) is a well‐mixed vessel fed with two immiscible liquid phases, while its effluent consists of a single phase containing the reaction products. We consider a CSDR in which a desired highly exothermic reaction occurs in one phase, while an undesirable exothermic side reaction occurs in the second extracting phase. The holdup of the two phases in the CSDR is sensitive to its temperature. Thus, the reactor has to be kept in a narrow temperature range to avoid break‐through, that is, with effluent containing both phases, and to maintain a high selectivity of the desired product. Pilot‐plant data for a proprietary selective‐oxidation process verified the predictions of the steady‐state model.A high rate of heat generation may lead to various exotic and undesired periodic and aperiodic oscillations, which may shift the reactor out of the region of safe operation and have deleterious impact on the decantation efficiency. In some cases, the oscillations lead to a breakthrough during part of the period. To avoid these oscillatory states, one may have to maintain a sufficiently small difference between the reactor and coolant temperatures. We show how loci of local and global singular points of codimension 1 and 2 can be used to construct maps of parameter regions with qualitatively different steady‐state and dynamic bifurcation diagrams. These maps clearly describe the desirable regions of operation and point out all the potential stability and operability issues in other regions.
Detrimental influence of excessive fractionation on reactive distillation
Authors: Sneesby Martin G. , Tadé Moses O. , Datta Ravindra , Smith Terence N. 
Published: April 2004

AbstractReactive distillation has been demonstrated to reduce the capital cost and increase reactant conversion in MTBE production. These advantages are potentially transferable to the production of ETBE, TAME, and other tertiary ethers. However, the design of reactive distillation columns is complicated by interaction between phase and reaction equilibrium. Whereas in conventional distillation, an increase in fractionation is always associated with an improvement in process performance (separation of key components), the same does not necessarily apply to reactive distillation. Using the production of ETBE as an example, several reactive distillation columns were designed for various feed compositions and design philosophies. It was found that the best designs incorporated high reflux ratios with a restricted number of theoretical stages, and that increasing the number of theoretical stages could actually be detrimental to process performance.
Runaway limits for adiabatic packed‐bed catalytic reactors
Authors: Christoforatou Evangelia L. , Balakotaiah Vemuri , West David H. 
Published: April 2004

AbstractMost existing criteria for predicting the critical residence time (or flow rate) at which runaway occurs in an adiabatic catalytic packed‐bed reactor do not account for interphase heat‐transfer resistance and intraparticle diffusion. When these transport limitations exist, the critical residence time at which runaway occurs is given by

This simplified equation gives the runaway locus as a function of the feed temperature (To), adiabatic temperature rise (ΔTad), activation energy (E), reaction rate constant at feed temperature [k(To)], fluid – solid heat‐transfer coefficient (h), the interfacial area per unit bed volume (av), and the normalized Thiele modulus (ϕo). The function f(ϕo) accounts for the intraparticle diffusional effects and may be approximated by the two asymptotes, f(ϕo) = 1 when ϕo < 1/2 and f(ϕo) = 2ϕo when ϕo > 1/2. Similar analytical results are presented for other reactor models and also for more complicated reaction networks. A practical example shows the calculation of the runaway boundary for industrial hydrogenation reactions obeying Langmuir–Hinshelwood kinetics.
Mathematical model of a continuous industrial high‐impact polystyrene process
Authors: Estenoz Diana A. , Meira Gregorio R. , Gómez Neidi , Oliva Haydée M. 
Published: April 2004

AbstractAn industrial bulk process for the continuous production of high‐impact polystyrene (HIPS) is mathematically modeled, and the model is validated with actual plant data. The steady states of two commercial HIPS grades were investigated. Polymer samples were taken along the process to measure monomer conversion, styrene (St) grafting efficiency, and free PS molecular weights. The model adequately reproduces the experimental measurements, and it also estimates the molecular macrostructure of the evolving reaction mixture.
Hydrodynamics of suspensions agitated by pitched‐blade turbine
Authors: Pettersson Michael , Rasmuson Åke C. 
Published: April 2004

AbstractThe complete local fluid velocity vector was determined by three‐dimensional phase‐Doppler anemometry, while the velocity vector of suspended process particles was being measured. Three‐dimensional mean and fluctuating velocities, complete Reynolds stresses, 3‐D length macroscales, and turbulent energy dissipation rates are presented. The difference in mean and fluctuating velocities of the process particles and the fluid is also examined. Tangential mean flow opposite to the impeller rotation is observed at certain locations just outside and above the impeller, and impeller fluctuations are found also in the inflow to the impeller. The outflow of the impeller exhibits strong spatial variations. Length macroscales are essentially independent of impeller speed, but vary significantly in the tank. Local energy dissipation rates increase with agitation rate raised to power three. Differences in mean and fluctuating velocities of the two phases are normally small, but increase in the discharge flow, where differences in the direction are also observed.
Shear yield stress of partially flocculated colloidal suspensions
Authors: Scales Peter J. , Johnson Stephen B. , Healy Thomas W. , Kapur Prakash C. 
Published: April 2004

AbstractA general expression was derived for the shear yield stress of a flocculated suspension of particles that is able to describe the effect of particle‐size distribution, solid loading, pH, and hence, electrokinetics of the suspension. The model builds on an earlier model by incorporating the effect of the repulsive interaction between particles. Scaling of the data to the maximum yield stress at a given volume fraction provides a means of removing particle‐size‐ and volume‐fraction‐related effects. The scaling process establishes that to a high level of precision, concentrated dispersions act, in interparticle interaction terms, as the sum of two‐body interactions.
Bubble flow characteristics in bubble columns at elevated pressure and temperature
Authors: Lin T.‐J. , Tsuchiya K. , Fan L.‐S. 
Published: April 2004

AbstractHydrodynamic characteristics of bubble columns are studied by considering both the local/bubble‐scale and global/column‐scale properties of the system, with specific emphasis on their dependency on operating pressure and temperature. The local‐scale properties include single‐bubble characteristics and interactive dynamics involving bubble coalescence and breakup, while the global‐scale properties include gas holdup. Experiments are conducted in a bubble column with a multiorifice ring sparger for pressures as high as 20 MPa and temperatures ranging from 27 to 78°C. Density, viscosity, and surface tension, the liquid phase properties, are measured in situ by using hydrostatic weighing, falling‐ball and emerging‐bubble techniques, respectively. The bubble behavior is examined through direct‐flow visualization. Pressure and temperature affect the bubble rise velocity mainly by varying physical properties of the fluids. Effects of pressure and temperature on the rates of bubble formation, coalescence and breakup, and the initial bubble size and maximum stable bubble size are illustrated. Elevating pressure and/or temperature increases gas holdup; the increasing rate depends on the operating conditions through their effects on the fluid physical properties.
Supercritical devolatilization of polymers
Authors: Alsoy Sacide , Duda J. Larry 
Published: April 2004

AbstractAn increasing number of studies have been reported on the use of supercritical fluids to process polymers. The presence of impurities such as residual solvents, unreacted monomers, catalysts, and side reaction products can adversely affect the end‐use properties of polymeric materials. Therefore, these impurities must be reduced to a level below the maximum permissible limit. Conventional devolatilization techniques for the purification of polymers have limited effectiveness. Devolatilization with supercritical fluids, however, can enhance impurity removal by increasing the thermodynamic driving force and molecular diffusivity. A model for supercritical devolatilization developed incorporates swelling of the polymer film and convection induced by the diffusion that arises because of the nonideal volumetric behavior. The perturbed soft‐chain theory is used to predict the nonideal thermodynamic behavior. Free volume theory is utilized to predict the influence of plasticization and relaxation on impurity mass transfer. The removal of benzene from polystyrene film with supercritical carbon dioxide was simulated. This analysis, which is consistent with experimental results in the literature, indicates that, contrary to conventional wisdom, impurity extraction can be dominated by mass transfer during the quick depressurization step at the end of a supercritical‐fluid extraction process.
Time evolution to similarity solutions for polymer degradation
Authors: Madras Giridhar , McCoy Benjamin J. 
Published: April 2004

AbstractThe theory of polymer decomposition is basic in understanding polymer and plastics stability, durability, characterization, and recycling. Polymers degrade by chain scission occurring randomly along the chain, at the chain midpoint, at the chain end, or by a combination of random chain‐scission and chain‐end scission. Mathematical solutions for polymer degradation were previously constrained to particular stoichiometric kernels (usually random scission) and to specialized distributions (such as the exponential distribution). Because the first term in a Laguerre‐polynomial expansion is the gamma distribution and the initial molecular‐weight distributions (MWDs) of polymers can be represented as superposed gamma distributions, the mathematical solution is represented as the time evolution of generalized gamma distribution parameters. Such solutions show how the first three moments of the MTD evolve in time for each of the chain‐scission models. The rate coefficient for random chain scission is assumed to depend on MW × as xλ, and the stoichiometric kernel is symmetric as in random or midpoint chain scission. Except when λ = 0, the distribution evolves to a similarity solution with the MW and time appearing together in the dimensionless group, xλ/β(t). The case of chain‐end scission, however, has no similarity solution.