bluuebunny/crossref_metadata_2025 huggingface dataset:
Real-time focusing using an ultrasonic one channel time-reversal mirror coupled to a solid cavity
Authors: Quieffin N. , Catheline S. , Ing R. K. , Fink M.
Published: April 2004

Focusing and beam steering is achieved by using a time-reversal process and a single transducer coupled to a solid cavity that is immersed in water. This low-cost technique makes it possible to focus acoustic energy anywhere on a 3D domain with a spatio-temporal resolution comparable to that of multiple transducers array. A short pulse is emitted from a transducer stuck at the surface of the solid cavity. The multiple-scattered field is measured in front of the solid cavity using a hydrophone needle at a reference point. This signal is then time reversed and remitted by the transducer. Around the reference point, one can observe a spatio-temporal recompression. The sidelobe level as well as the focal width no longer depend on the transducer aperture but on the dimensions of the solid cavity and the multiple paths covered by the acoustic waves in the solid. Moreover, it is shown how the experimental impulse responses on the front face of the cavity can be used to control the emitting ultrasonic field. This “synthetic time-reversal” technique is shown to be as powerful as a real time-reversal process.
Factors affecting speech understanding in gated interference: Cochlear implant users and normal-hearing listeners
Authors: Nelson Peggy B. , Jin Su-Hyun
Published: April 2004

Previous work [Nelson, Jin, Carney, and Nelson (2003), J. Acoust. Soc. Am 113, 961–968] suggested that cochlear implant users do not benefit from masking release when listening in modulated noise. The previous findings indicated that implant users experience little to no release from masking when identifying sentences in speech-shaped noise, regardless of the modulation frequency applied to the noise. The lack of masking release occurred for all implant subjects who were using three different devices and speech processing strategies. In the present study, possible causes of this reduced masking release in implant listeners were investigated. Normal-hearing listeners, implant users, and normal-hearing listeners presented with a four-band simulation of a cochlear implant were tested for their understanding of sentences in gated noise (1–32 Hz gate frequencies) when the duty cycle of the noise was varied from 25% to 75%. No systematic effect of noise duty cycle on implant and simulation listeners’ performance was noted, indicating that the masking caused by gated noise is not only energetic masking. Masking release significantly increased when the number of spectral channels was increased from 4 to 12 for simulation listeners, suggesting that spectral resolution is important for masking release. Listeners were also tested for their understanding of gated sentences (sentences in quiet interrupted by periods of silence ranging from 1 to 32 Hz as a measure of auditory fusion, or the ability to integrate speech across temporal gaps. Implant and simulation listeners had significant difficulty understanding gated sentences at every gate frequency. When the number of spectral channels was increased for simulation listeners, their ability to understand gated sentences improved significantly. Findings suggest that implant listeners’ difficulty understanding speech in modulated conditions is related to at least two (possibly related) factors: degraded spectral information and limitations in auditory fusion across temporal gaps.
FUTURE EXPERIMENTS WITH NEUTRINO SUPERBEAMS, BETA-BEAMS, AND NEUTRINO FACTORIES
Authors: HARRIS D. A.
Published: April 2004

This report describes the goals of the next generations of accelerator-based neutrino experiments, and the various strategies that are being considered to achieve those goals. Because these next steps in the field are significantly different from the current or previous steps, novel techniques must be considered for both the detectors and the neutrino beams themselves. We consider not only conventional neutrino beams created by decays of pions, but also those which could be made by decays of beams of relativistic isotopes (so-called "β-beams") and also by decays of beams of muons (neutrino factories).
EXCITATION OF SURFACE CAPILLARY WAVES BY SPATIALLY PERIODICAL LASER RADIATION
Authors: HAKOBYAN R. S. , ZELDOVICH B. YA.
Published: April 2004

In this paper we report theoretical results on thermocapillary hydrodynamic waves induced by travelling spatially periodical distribution of light intensity. The phase of the local elevation in capillary waves is close to the phase of interference pattern when the frequency is far from zero and from eigenfrequency of free capillary waves. This phase difference is exactly π/2 for zero frequency. Predicted thermocapillary mechanism of induced surface waves and boundary disturbances can have an important role for surface relief grating in the polymer-dispersed liquid crystals.
Effect of ambient temperature on cardiovascular parameters in rats and mice: a comparative approach
Authors: Swoap Steven J. , Overton J. Michael , Garber Graham
Published: April 2004

Ambient air temperatures (Ta) of <6°C or >29°C have been shown to induce large changes in arterial blood pressure and heart rate in homeotherms. The present study was designed to investigate whether small incremental changes in Ta, such as those found in typical laboratory settings, would have an impact on blood pressure and other cardiovascular parameters in mice and rats. We predicted that small decreases in Ta would impact the cardiovascular parameters of mice more than rats due to the increased thermogenic demands resulting from a greater surface area-to-volume ratio in mice relative to rats. Cardiovascular parameters were measured with radiotelemetry in mice and rats that were housed in temperature-controlled environments. The animals were exposed to different Ta every 72 h, beginning at 30°C and incrementally decreasing by 4°C at each time interval to 18°C and then incrementally increasing back up to 30°C. As Ta decreased, mean blood pressure, heart rate, and pulse pressure increased significantly for both mice (1.6 mmHg/°C, 14.4 beats·min−1·°C−1, and 0.8 mmHg/°C, respectively) and rats (1.2 mmHg/°C, 8.1 beats·min−1·°C−1, and 0.8 mmHg/°C, respectively). Thus small changes in Ta significantly impact the cardiovascular parameters of both rats and mice, with mice demonstrating a greater sensitivity to these Ta changes.
Geldanamycin treatment inhibits hemorrhage-induced increases in KLF6 and iNOS expression in unresuscitated mouse organs: role of inducible HSP70
Authors: Kiang Juliann G. , Bowman Phillip D. , Wu Brian W. , Hampton Nyasa , Kiang Andrew G. , Zhao Baiteng , Juang Yuang-Taung , Atkins James L. , Tsokos George C.
Published: April 2004

The aim of this study was to determine whether hemorrhage affects the levels of a variety of stress-related proteins and whether changes can be inhibited by drugs reported to provide protection from ischemia and reperfusion injury. Male Swiss Webster mice were subjected to a 40% hemorrhage without resuscitation. Western blot analysis indicated that c-Jun (an AP-1 protein), Kruppel-like factor 6 (KFL6), and inducible nitric oxide synthase (iNOS) were upregulated sequentially in that order. Pretreatment of mice with geldanamycin (GA) 16 h before hemorrhage effectively inhibited the expression of the proteins KLF6 and iNOS, whereas caffeic acid phenethyl ester did not. GA pretreatment increased inducible heat shock protein (HSP) 70 but not HSP90 in both sham and hemorrhagic tissues. The overexpressed inducible HSP70 formed complexes with KLF6 and iNOS. These results suggest that GA may be therapeutically useful for reducing hemorrhage-induced injury when used as a presurgical treatment or when added to resuscitation fluids.
Effect of Concentration, Acid, Temperature, and Metal on Competitive Reaction Pathways for Decarbonylation and Decarboxylation of Formic Acid in Hot Water
Authors: Wakai Chihiro , Yoshida Ken , Tsujino Yasuo , Matubayasi Nobuyuki , Nakahara Masaru
Published: April 2004

Hydrothermal reaction of formic acid has been studied by varying the following conditions: temperature (275 to 350 °C), concentration of formic acid (0.1 to 1 M), addition of HCl, and presence of metal alloy powders. In hot water, formic acid decomposes to CO and CO2. The product ratio of CO to CO2 strongly depends on the conditions. At higher concentrations and lower temperatures, the yield of CO gets larger. In most of the conditions studied here, the main product is not CO2 but CO in contrast to previous works. Acid enhances the CO production and metal powders catalyze the decarboxylation pathway.
KCN-Catalyzed C–C Bond Formation between Imine and gem-Difluoroalkene Moieties: A Facile Synthesis of 2,4-Disubstituted 3-Fluoroquinolines
Authors: Mori Takashi , Ichikawa Junji
Published: April 2004

Intramolecular cyclization of β,β-difluoro-o-methyleneaminostyrenes leading to 3-fluoroquinolines is efficiently promoted by a catalytic amount of KCN and a small excess of K2CO3. The reaction proceeds via (i) the generation of intramolecular carbon nucleophiles from imine moieties and (ii) the cyclization by their substitution for the vinylic fluorine.
Neuroscience and education
Authors: Goswami Usha
Published: April 2004

Neuroscience is a relatively new discipline encompassing neurology, psychology and biology. It has made great strides in the last 100 years, during which many aspects of the physiology, biochemistry, pharmacology and structure of the vertebrate brain have been understood. Understanding of some of the basic perceptual, cognitive, attentional, emotional and mnemonic functions is also making progress, particularly since the advent of the cognitive neurosciences, which focus specifically on understanding higher level processes of cognition via imaging technology. Neuroimaging has enabled scientists to study the human brain at work in vivo, deepening our understanding of the very complex processes underpinning speech and language, thinking and reasoning, reading and mathematics. It seems timely, therefore, to consider how we might implement our increased understanding of brain development and brain function to explore educational questions.
Fostering reading comprehension in fifth grade by explicit instruction in reading strategies and peer tutoring
Authors: Van Keer Hilde
Published: April 2004

Background: Research reveals that explicit reading strategies instruction and engaging students in interaction about texts promote students' reading comprehension ability. The present intervention study combines both aspects.

Aims: The study examines the educational benefits of explicit reading strategies instruction, followed by practice in (a) teacher‐led whole‐class activities (STRAT), (b) reciprocal same‐age (STRAT + SA), or (c) cross‐age peer tutoring activities (STRAT + CA) on fifth graders' reading comprehension achievement.

Sample: Twenty‐two fifth‐grade teachers and their 454 students from 19 different schools throughout Flanders (Belgium) participated.

Method: A quasi‐experimental pretest post‐test retention test design was used with three experimental (STRAT, STRAT + SA, and STRAT + CA) and a matched control group. The experimental interventions were implemented during an entire school year.

Results: Multilevel analysis revealed that the STRAT and STRAT + CA condition made a significantly larger pretest to retention test progress than the control group. The significant major progress was especially situated from pretest to post‐test, during which the intervention took place. Concerning the STRAT + SA condition no significant differences with regard to the control group were detected. Pairwise comparisons of the experimental conditions indicated that the STRAT + CA condition's progress from pretest to retention test also exceeded the STRAT + SA condition's advancement significantly.

Conclusion: The study corroborated the efficacy of the STRAT and STRAT + CA conditions' interventions as feasible tools to enhance fifth graders' reading comprehension achievement.
Genome Scans of Type 2 Diabetes Mellitus
Authors: Rutherford Sue , Shuldiner Alan R. , Mitchell Braxton D.
Published: April 2004

Type 2 diabetes mellitus affects approximately 5–8% of the industrial world. Although a genetic basis to the disease is widely accepted, identification of the specific mutations involved is difficult because of the heterogeneous and multifactorial nature of the disease. With the availability of large‐scale genotyping techniques, genome‐wide linkage approaches are now commonly used to localize regions along the chromosome likely to harbor diabetes susceptibility genes. This chapter presents an overview of the genome scan approach and a summary of linkage results reported from a variety of populations.
Molecular Genetics of Type 1 Diabetes
Authors: Kelly M. Ann , Barnett Anthony H. , Bain Stephen C.
Published: April 2004

Approximately one third of susceptibility to type 1 diabetes is determined by genetic factors. These factors are termedsusceptibility genesas they modify the risk of diabetes but are neither necessary nor sufficient for disease to develop. More than 20 different regions in the human genome have been suggested to contain diabetes susceptibility genes. TheIDDM1locus, which encompasses the human leukocyte antigen (HLA) gene complex on chromosome 6p, is the major genetic risk factor, accounting for approximately 40% of inherited susceptibility. TheHLA‐DQgenes within this region are thought to be primary risk determinants ofIDDM1, although other genes also contribute. TheIDDM2locus, which maps to a variable number of tandem repeats region close to the insulin gene on chromosome 11p, contributes approximately 10% of genetic predisposition. The precise location and identity of other diabetes susceptibility genes remain unknown. These loci, however, are likely to make much smaller contributions to disease risk. Further research is necessary to identify all the genetic susceptibility factors for type 1 diabetes and to determine how they interact, with each other and with environmental factors, to influence disease risk.
Artificial and Bio‐artificial Pancreas Systems
Authors: Reach Gérard , Dulong Jean-Luc , Legallais Cécile
Published: April 2004

The impact of hypoglycemia, the most common side effect of insulin therapy, and the one that patients with diabetes fear most, cannot be understated. Understandably, this fear of hypoglycemia is a major factor preventing patients from adopting intensified insulin therapy. This is the principal reason why normalization of HbA1cis not possible in the majority of patients, despite intensified insulin therapy. Furthermore, in the clinical setting, improved diabetes control can only be achieved when patients continually adjust insulin dose in response to self‐monitoring of blood glucose. This is difficult to achieve for most patients. Thus the development of an artificial pancreas, amachinethat can continuously administer insulin and be automatically controlled by concomitant blood glucose measurements, has been a long sought after goal. Together with pancreatic tissue transplantation, this has been one of the great anticipated advances for patients with diabetes.
Monitoring of Lipids in Diabetes
Authors: Laker M.F. , Neely R.D.G.
Published: April 2004

Dyslipidemia should be monitored regularly in diabetes mellitus and the assessment should include the measurement of total cholesterol, high‐density lipoprotein cholesterol, and triglyceride concentrations. More detailed investigations may be required if a primary hyperlipoproteinemia is suspected and other causes of secondary hyperlipidemia should be considered. Several physiological factors cause variation in serum lipid concentrations, including age, sex, the menstrual cycle, and diet. Posture, venepuncture technique, concomitant medication, and clinical conditions other than diabetes mellitus also affect lipid levels. This chapter will consider the investigations that are used widely in clinical practice and sources of variation in lipoprotein metabolism.
Diabetes and Public Health in the United States
Authors: Vinicor Frank , Rufo Kathy , Murphy Dara
Published: April 2004

Diabetes mellitus (DM) in the United States is a chronic disease epidemic. Further, the rate of increase in the prevalence of diabetes, year to year, is increasing, and the onset of diabetes is at ever younger ages, with prevalence of type 2 diabetes now increasing at greatest rates among people in their 30s. Should the epidemic of type 2 DM increase, even at its present rate, it is projected that there will be approximately 30 million persons with known DM in the United States by 2050. In essence, there will be more people with diabetes who have a longer duration of disease, with the devastating complications and associated morbidity, mortality, impaired quality of life, and costs; and this could increase substantially during the next two decades. The rise in prevalence observed in the United States is also happening throughout the world, and all countries can anticipate increasing challenges associated with DM.
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

A 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

In 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

Selective 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

Proportional, 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

A 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

Thin (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

A 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

The 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

Experiments 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

Defect‐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

A 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

Reactive 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

Most 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

An 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

The 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

A 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

Hydrodynamic 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

An 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

The 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.
Intensification of instantaneous heterogeneous reaction by simple salts and charged microphase
Authors: Hasnat Abul , Roy Sandip
Published: April 2004

The mass‐transfer rate for the instantaneous heterogeneous reaction between N,N‐di‐ethylaniline (organic phase) and HCl in the aqueous phase was determined in the presence of a number of supporting electrolytes, that is, NaCl, SrCl2, AlCl3, and cetyltrimethyl ammonium bromide surfactant micelle. Both the simple salts and the cationic surfactant enhance the mass transfer of the reactive H+ ion, the effectiveness being in the order of CTAB > AlCl3 > SrCl2 > NaCl. The experimentally observed enhancement factor for the reaction in the presence of the supporting electrolytes ranges from 1.3 to 6. This enhancement is explained on the basis of the “ion‐coupling” effect that augments the diffusive fluxes of the more mobile ionic species in a multi‐ion system. The film‐theory approximation is used to obtain theoretical estimates of the enhancement factor, and these are found to be in reasonably good agreement with the experimentally obtained values.
Near‐surface dynamics of sheared polymer melts using ATR/FTIR
Authors: Wise Geoffrey M. , Denn Morton M. , Bell Alexis T.
Published: April 2004

Using attenuated total‐reflectance infrared spectroscopy (ATR/FTIR), the concentration of deuterated polybutadiene near the surface of a flat zinc selenide crystal was followed as it was replaced by ordinary polybutadiene by flow and diffusion. Experiments were performed in the melt, both below (M ∼ 1,500) and above (M ∼ 15,000) the entanglement threshold. The decay profiles agree well with a finite‐element simulation of the system. In contrast to previous investigations of C16's, the decay profiles of both unentangled and entangled polybutadienes are consistent with a uniform diffusivity in the near‐wall region.
Nonequal reactivity model for biocatalytic polytransesterification
Authors: Chaudhary Apurva K. , Beckman Eric J. , Russell Alan J.
Published: April 2004

The lipase‐catalyzed polytransesterification of divinyl adipate and 1,4‐butanediol is a dynamic process controlled by the relative magnitudes of the rates of transesterification and hydrolysis as the polymerization proceeds. A mathematical model that describes the kinetics of the biocatalytic polytransesterification process is presented. Initial rate studies with model substrates in transesterification, hydrolysis, and esterification were used to provide the initial kinetic parameters that, when combined with the effect of enzyme specificity, were used to predict product compositions under different reaction conditions. A comparison of experimental data and model predictions shows that the product composition is strongly influenced by the selective nature of enzyme catalysis, and this model effectively predicts the results of a step condensation polymerization where reactivity depends on molecular weight.
Imaging stirred‐vessel macromixing using electrical resistance tomography
Authors: Holden P. J. , Wang M. , Mann R. , Dickin F. J. , Edwards R. B.
Published: April 2004

An 8‐plane 16‐sensor ring electrical resistance tomography (ERT) system was designed and installed within a 1.5‐m‐dia. plant‐scale stirred uessel. Results are reported for the unsteady dynamic macromixing of miscible liquids following a pulse addition of brine tracer. Acquired data, afier reconstruction, is conveniently represented as a stack of 2‐D tomograms and 3‐D solid body isosurjaces, thereby providing an equiualent of medical whole‐body scanning for a chemical process unit. The results demonstrate the ability of ERT to distinguish the different mixing patterns from two types of impeller in 3‐D and to monitor plant‐scale miscible liquid mixing processes aperimentally, with good resolution in both space and time.
Feasibility of extractive distillation in a batch rectifier
Authors: Lelkes Z. , Lang P. , Benadda B. , Moszkowicz P.
Published: April 2004

A simple method was developed to study the feasibility of extractive distillation in a batch rectifier. By calculating feasible profiles of different column sections and figuring them in the triangular diagram, the feasibility and limiting values of the most important operational parameters (number of stages, flow rate of entrainer, and reflux ratio) were determined for different steps of the batch extractive distillation. The unusual behavior of extractive distillation was also investigated for this process. An infeasible region can be assigned to the given values of parameters; if the still composition is located within this region, the specified separation cannot be attained. By locating this infeasible region, maximum recoveries attainable in a batch rectifier and in a middle vessel column were compared. The results obtained by this method were verified by pilot‐plant expenments for the mixture acetone + methanol water.
Analysis of instability in an industrial ammonia reactor
Authors: Morud John C. , Skogestad Sigurd
Published: April 2004

The starting point for this study was an incident in an industrial plant, where the ammonia synthesis reactor became unstable with rapid temperature oscillations (limit‐cycles) in the range from about 300°C to 500°C. A simple dynamic model reproduces this behavior. In industry a steady‐state van Heerden analysis is often used to analyze the stability, but a more careful analysis for this reactor system reveals that instability occurs when there still is a positive steady‐state margin, namely as a pair of complex conjugate poles cross the imaginary axis (Hopf bifurcation). This is consistent with the obseruations where the instability manifests itself as oscillations rather than extinction of the reaction. This somewhat unusual behavior can be explained by the presence of an inverse response for the temperature response through the reactor beds combined with the positive feedback caused by the preheater.
Steam reforming of naphthalene on Ni–Cr/Al2O3 catalysts doped with MgO, TiO2, and La2O3
Authors: Bangala Denis N. , Abatzoglou Nicolas , Chornet Esteban
Published: April 2004

Magnesium, lanthanum, and titanium oxide‐doped nickel–chromium/alumina catalysts were prepared and their performances assessed in the steam reforming of naphthalene (T: 1,023 K; water‐to‐naphthalene molar ratio: 16, atmospheric pressure, residence time of 0.55 s; and GHSV = 10,080 h−1). Impregnation of nickel and chromium in MgO‐ and La2O3‐doped alumina gave the most active, stable catalysts. Nickel was active in aromatic ring opening, while chromium inhibited the encapsulation of the nickel clystallites by inactive carbon filaments. The use of MgO had a signifkant effect on the robustness of catalyst due to the formation of MgAl2O4 spinelphase. La2O3 was necessaly to drive the reaction toward total gasification. The addition of TiO2 gave poor activity and a soft catalyst. Some of the catalyst preparations were characterized before and after reaction by XRD, SEM, DTA, and surface measurements. The physical and catalytic properties are discussed in light of these characterizations. The catalyst optimized during this study showed high clystallite dispersion, excellent crushing strength, and good thermal stability. When comparing the data obtained using Ni/Al2O3 with Ni–Cr/Al203·MgO·La2O3, it is clear that the improved catalyst reached higher conversion and gas yield performances.