bluuebunny/crossref_metadata_2025_split huggingface dataset:
Tracer studies of high‐shear granulation: I. Experimental results
Authors: Pearson J. M. K. , Hounslow M. J. , Instone T. 
Published: April 2004

AbstractPrevious work has shown that attempts to determine the rates of processes active during granulation inevitably confound the effects of granule age and size; since as an experiment progresses, both the age and size of granules increases. In the current work, tracer experiments have been developed, and verified, to unconfound those effects. Qualitative analysis of the tracer data shows that granule breakage is occurring at a significant rate and that the breakage, or selection rate, appears to depend on granule age, but not on granule size.
Gas‐solids flow behavior: CFB riser vs. downer
Authors: Zhang H. , Huang W.‐X. , Zhu J.‐X. 
Published: April 2004

AbstractComparisons are made in a circulating fluidized‐bed riser/downer system between a 15.1 m high, 0.10 m ID riser and a 9.3 m high, 0.10 m ID downer, based on the measurements of the radial distributions of the local solids holdups and local particle velocities along the two columns. Although the core‐annulus flow structures exist in both the riser and downer, the radial flow structure in the downer differs largely from that in the riser. The radial distributions of solids holdup and particle velocity in the downer are much more uniform than those in the riser, thus ensuring the low back mixing and the narrow particle residence time distribution in the downer. The axial flow structure in the downer is also more uniform than that in the riser. Due to the high particle acceleration and the high particle velocity in the downer, the overall solids holdup is significantly lower than that in the riser. The microflow structure in the downer, characterized by the low intermittency indices, is also more uniform than that in the riser. These key properties of the downer make it a very promising candidate for industrial applications where short reaction times and high product selectivity are required.
Predicting membrane flux of CH4 and CF4 mixtures in Faujasite from molecular simulations
Authors: Sanborn Martin J. , Snurr Randall Q. 
Published: April 2004

AbstractThe Fickian transport diffusivities of binary mixtures of methane and CF4 in the zeolite faujasite were investigated as a function of composition and loading. Using equilibrium molecular dynamics simulations, transport diffusivities of the mixtures were calculated from linear response theory. At low loadings, the diffusive cross terms are an order of magnitude smaller than the diffusive main terms, but at higher loadings they are on the same order of magnitude. The main term diffusivities are weak functions of concentration. The transport diffusivities were fitted as a function of adsorbate concentrations and used in the Fickian flux equations to predict codiffusion, counterdiffusion, and osmotic diffusion through a perfectly crystalline faujasite membrane. The osmotic and counterdiffusion cases show that it is critical to include both the cross term and main term diffusivities to predict the correct behavior. The permeances of methane and CF4 through the faujasite membrane are reported.
Fructose–glucose separation in a SMB pilot unit: Modeling, simulation, design, and operation
Authors: Azevedo Diana C. S. , Rodrigues Alírio E. 
Published: April 2004

AbstractGlucose was separated from fructose experimentally using a simulated moving‐bed (SMB) adsorber. A strongly acid cationic resin of gel type (Ca2+ form) Dowex Monosphere (dp = 320 μm) was used in a pilot SMB unit of twelve 26 × 300 mm (ID × length) columns. A recently proposed design procedure was applied to overcome the inherent strong mass‐transfer resistance present in this kind of adsorbent. The fluid/solid velocity ratios in SMB sections 1, 2 and 3 leading to at least 90% product purity were followed by simulation and plotted in a 3‐D parameter space. The design methodology, called “separation volume analysis,” also considered the geometric parameters, as well as allowable working flow rates, temperature and pressure of the plant. Operating conditions from this procedure were used to operate the SMB unit, and the expected performance was achieved experimentally. Simulation strategies based on a true countercurrent and a real SMB were used and the predicted performance of both agreed well with experimental data. Furthermore, experimental results confirmed predictions of the separation volume analysis, which shows the potential of the technique for optimizing existing SMB equipment.
Identification of dynamic process systems with surrogate data methods
Authors: Barnard Jakobus P. , Aldrich Chris , Gerber Marius 
Published: April 2004

AbstractIdentifying the underlying dynamics of chemical process systems from experimental data is complicated, owing to a mixture of influences that cause erratic fluctuations in the time series. These influences can be notoriously difficult to disentangle. The development of process models is usually subject to considerable human judgment and can therefore be very unreliable. This is especially the case when the model priors are unknown and the model is validated empirically, such as with cross‐validation or holdout methods. A case study shows that more reliable identification of systems is possible by using surrogate methods to classify the data, as well as to validate models derived from these data.
Ceramic membrane reactor for synthesis gas production
Authors: Ritchie J. T. , Richardson J. T. , Luss Dan 
Published: April 2004

AbstractMembrane reactors enable synthesis‐gas production from methane and air while avoiding the need for separation of the nitrogen either before or after the reaction. A stable membrane was developed by spray deposition of a dense thin film of La0.5Sr0.5Fe0.8Ga0.2O3 − δ on a high‐purity porous α‐alumina tube. The oxygen permeation rate at 850°C was 2.5 × 10−7 mol·cm−2·s−1. A quartz tube was placed coaxially around the membrane and the shell filled with a rhodium catalyst. Air was fed to the tube and methane to the shell. At 850°C the methane conversion was 97% and the selectivity to carbon monoxide approached 100%. Rapid radial mixing of the oxygen in the shell is essential to prevent coking and undesirable reactions. The membrane decomposes at 780°C in pure CH4, but remains stable up to 970°C in a mixture of 90‐mol % CH4 and 10‐mol % CO2.
Calcium carbonate catalysis of alcohol oxidation in near‐critical water
Authors: Suppes G. J. , Roy S. , Ruckman J. 
Published: April 2004

AbstractThe near‐critical water oxidations (NCWO) of phenol, methanol, ethanol, n‐propanol and n‐butanol were evaluated at temperatures between 200 and 374°C and pressures between 2 and 220 bar. Reactions were conducted in packed‐bed flow reactors with ZrO2 and CaCO3 packings with a stoichiometric amount of HOOH. The effectiveness of CaCO3 as an oxidation catalyst increased markedly between 200 and 374°C enhancing oxidation rates > 100 × at 374°C for phenol. Hydrolysis was observed for phenol, ethanol, and methanol over CaCO3 at 374°C and was largely nondetectable in the absence of CaCO3. Despite being inexpensive and environmentally benign, CaCO3 exhibited catalytic abilities equal to or better than many of the best published performance of NCWO catalysts containing noble or transition metals.
Existence of a critical carbon number in the aging of a wax‐oil gel
Authors: Singh Probjot , Youyen Arm , Fogler H. Scott 
Published: April 2004

AbstractA fundamental study was carried out to understand the aging (or hardening) of the wax‐oil gels formed in the subsea pipelines during the flow of crude oil from offshore wells to shore. The aging process is a counterdiffusion phenomenon where there exists a critical carbon number (CCN), and wax molecules with carbon numbers greater than the CCN diffuse into the gel matrices and vice versa. Using a careful analysis of carbon number distributions of gel deposits, collected from a cold finger after various deposition time intervals, the CCN for the wax‐oil system was obtained. A mathematical model, based on a modified version of UNIQUAC model, was developed to predict the CCN for wax‐oil systems. The size of the interaction units for n‐alkanes in the solid‐phase UNIQUAC model was found to be a strong function of the mean carbon number in the solid phase.
Extensive validation of computed laminar flow in a stirred tank with three Rushton turbines
Authors: Zalc J. M. , Alvarez M. M. , Muzzio F. J. , Arik B. E. 
Published: April 2004

AbstractHigh‐resolution CFD results, supplemented by extensive experimental validation, are presented for Newtonian laminar flow fields in a stirred tank equipped with three Rushton turbines. Flow fields are computed using the ORCA software suite for Reynolds numbers ranging from 20 to 200 with an unstructured tetrahedral mesh containing roughly 2 million tetrahedra. Each of the flow solutions takes less than 8 h to converge when running in parallel on eight desktop workstations. Excellent agreement is obtained between computed velocity fields and planar velocity vectors obtained using particle image velocimetry. Planar laser‐induced fluorescence was used to expose persistent poor‐mixing regions, in excellent agreement with numerical results. The computational results used illustrate strong flow compartmentalization and significant spatial heterogeneity with respect to local deformation rates within the vessel.
Fourier spectrum to recover deterministic and stochastic behavior in stirred tanks
Authors: Kovács T. , Trägårdh C. , Fuchs L. 
Published: April 2004

AbstractVelocity data from the impeller region of Rushton and 6SRGT pilot‐scale impellers were analyzed both in the frequency domain and with respect to periodicity. The four characteristic frequencies and their combination were detected from the Fourier spectrum, and the deterministic part of the signal was recovered by making the amplitude of the other frequencies zero. The deterministic part was then analyzed with tools from nonlinear theories. For the two turbine‐types, the dynamics of the deterministic velocity parts were almost identical. The turbulent part of the signal could also be estimated in a similar manner.
Axial and radial solids distribution in a long and high‐flux CFB riser
Authors: Pärssinen J. H. , Zhu J.‐X. 
Published: April 2004

AbstractRadial solids concentration profiles were determined with a fiber‐optic probe on eight axial levels in a 76‐mm‐ID, 10‐m riser, at solids flux (solids circulation rate) up to 550 kg/m2·s and superficial gas velocity up to 10 m/s. Radial concentration profiles at high solids fluxes of over 300 kg/m2·s are less uniform than lower fluxes of less than 200 kg/m2·s. Under all operating conditions, the flow development in the riser center is nearly instant, with the solids concentration remaining low at the riser center throughout the riser. In the wall region, increasing solids flux significantly slows down the flow development, with the solids concentration near the wall decreasing all the way toward the riser top at high fluxes. In the high‐flux circulating fluidized bed (HFCFB), a middle section with intermediate solids holdups of about 7 to 20% was between the bottom dense section and top dilute section, with its length increasing as the solids circulation rate increases and as the gas velocity decreases. Flow conditions in this section resemble those of the dense suspension upflow under high‐density operating conditions by Grace et al. (1999). When its length extends to the riser top, a high‐density circulating fluidized bed for which an HFCFB is a necessary but not a sufficient condition, forms.
Effect of fiber variation on staged membrane gas separation module performance
Authors: Liu B. , Lipscomb G. G. , Jensvold J. 
Published: April 2004

AbstractThe effect of fiber property variations on the performance of a two‐stage system for gas separation was studied theoretically and experimentally. Variations in fiber inner diameter, slow gas permeance, or selectivity are considered. Variability in any of these properties is detrimental to performance, as characterized by product flow rate and product recovery (fraction of the feed recovered as a product). The performance decline increases as either property variation or product purity increase. The two‐stage system can enhance performance significantly over a single sage. Permeate mixing is also beneficial to performance. Like a single stage, however, some fibers may stop producing a retentate product and consume product produced by other fibers for sufficiently large variation in size and permeance. The theory agrees well with experiments for the production of nitrogen from air.
Evaporative crystallization of anhydrous sodium carbonate at atmospheric conditions
Authors: Oosterhof Harald , Witkamp Geert‐Jan , van Rosmalen Gerda M. 
Published: April 2004

AbstractA new process for the production of superdense anhydrous soda ash uses a mixture of water and a high‐boiling second solvent to lower the transition point at which anhydrous (Na2CO3) and monohydrous sodium (Na2CO3·H2O) carbonate are in equilibrium to below the atmospheric boiling point. The stable conditions for anhydrate were first established by measuring the water activity in saturated mixtures of water and ethylene glycol. With the results, fed‐batch evaporative crystallization experiments were carried out. Both the water activity and the crystallization measurements showed that anhydrous soda was stable in boiling mixtures containing more than 22.5 wt. % ethylene glycol (on a salt‐free basis). A subsequent continuous evaporative crystallization experiment produced anhydrous soda ash with a bulk density of 1,550 kg/m3, which is substantially higher than that of any other atmospherically crystallized soda.
Cost‐effective bypass design of highly controllable heat‐exchanger networks
Authors: Yan Q. Z. , Yang Y. H. , Huang Y. L. 
Published: April 2004

AbstractDesign of a cost‐effective and highly controllable heat‐exchanger network (HEN) has drawn a great deal of attention for years. One of the key issues in such a design is how to effectively minimize undesirable disturbance propagation in a network with minimum cost increment. Design options in this regard include the derivation of a superior network structure and the selection of bypasses associated with heat exchangers. A unique system modeling approach is developed to predict disturbance propagation and to reject disturbances using bypasses. A novel mathematical representation scheme for a HEN is introduced to facilitate system analysis and design. A relative gain‐array approach is extended to the analysis of nonsquared systems. In addition, an iterative design procedure is introduced to determine optimal bypass locations and nominal fractions for complete disturbance rejection, while economic penalty reaches the minimum. The efficacy of the model‐based approach is demonstrated by designing three HENs where bypasses are optimally placed, and the control schemes are simultaneously developed.
Distribution kinetics of thermolytic macromolecular reactions
Authors: Sterling W. Jerome , McCoy Benjamin J. 
Published: April 2004

AbstractEssential elements of thermolytic reaction kinetics for distributions of polymers are reviewed. Bond scission, occurring randomly along a chain backbone or at the chain end, is the major reaction process in polymer thermolysis. Underlying this bond scission are free radical mechanisms, including initiation and termination, hydrogen abstraction, and beta scission. Population‐balance equations for the molecular‐weight distributions of macromolecules or their radicals describe the dynamics of the reactions. The governing integrodifferential equations for continuous distributions can be solved by moment methods, similarity techniques, other analytical procedures, or numerical methods. The approach has been applied to analytical thermolysis and pyrolysis, polymer degradation for stability characterization or plastics recycling, and coal thermolysis to produce fuels and feedstocks. Experiments have demonstrated the usefulness of the continuous distribution kinetics approach for polymer thermolysis with additives, including hydrogen donors, peroxides, and other polymers. Molecular‐weight distributions and their moments can be measured by size exclusion chromatography. Experimental data can be interpreted by evaluating rate parameters, which may be composites of rate coefficients for elementary steps in complex reaction mechanisms.
Impact of 2‐D bubble dynamics on the selectivity of fast gas–liquid reactions
Authors: Khinast Johannes G. 
Published: April 2004

AbstractThe impact of mass transfer and bubble‐wake dynamics on the selectivity of fast gas–liquid reactions was studied for a parallel‐consecutive reaction network using numerical simulations. Depending on bubble size and shape, the bubble wake can be closed or open. Spherical bubbles have only closed wakes without recirculation, while all other bubble types can exhibit recirculation or vortex shedding depending on their shape and the Reynolds number. Although the importance of local mixing on the selectivity of complex reactions was studied by many research groups, there exist no studies addressing the effect of local mixing patterns (bubble‐wake dynamics) close to single bubbles on fast gas–liquid reactions, that is, reactions that occur close to the gas–liquid interface. To study this class of reactions, a 2‐D bubble model was developed, which accounts for liquid flow around the bubble, mass transfer, and reactions. It was found that different residence times in the bubble wake and at the bubble roof can lead to the formation of different products; recirculation in the bubble wake acts as a transport barrier for the liquid‐phase reactants; and vortex shedding causes a qualitatively different mixing pattern than that of a closed wake, leading to a different product distribution in the case of mixing‐sensitive reactions. Since bubble shapes and sizes can be controlled by changing operating conditions or design parameters, this analysis can be applied to actual reaction systems to enable a rational design, control, scale‐up, and optimization of existing and new processes.
Nonlinear model reduction of chemical reaction systems
Authors: Vora Nishith , Daoutidis Prodromos 
Published: April 2004

AbstractA nonlinear model reduction method for nonisothermal reaction systems that exhibit dynamics in two different time scales owing to the presence of fast and slow reactions was developed. The method systematically identifies the independent algebraic constraints that define the low‐dimensional state space where the slow dynamics of the reaction system are constrained to evolve. It also derives state‐space realizations of the resulting differential algebraic system that describes the slow dynamics. This method is illustrated through the classic Michaelis‐Menten reaction system, and is applied to an ozone decomposition reaction system and a reaction mechanism for esterification of carboxylic acid.
Molecular‐weight distribution kinetics for ultrasonic reactions of polymers
Authors: Madras Giridhar , McCoy Benjamin J. 
Published: April 2004

AbstractUltrasonic degradation of polymers is distinguished from thermal degradation by: midpoint rather than random chain scission; a lower limit of molecular weight (MW), xf, below which the polymer will not undergo midpoint scission; and possible biomodal distribution evolving with time. The molecular‐weight distribution (MWD) is governed by a population‐balance equation whose time dependence is deduced from MW moments. To examine if ultrasonically induced reactions have similarity solutions, the moment equations were considered in terms of the time evolution of generalized gamma distribution parameters. The rate coefficient for chain scission is assumed to depend on MW, x, as κ (x − xf), where xf is the limiting MW. The MWD approaches a similarity solution (with a polydispersity of 1.5) only when limiting MW xf is zero. This study also provides an exact analytical solution for the MWD, showing how the MWD can evolve, with reaction time, from an unimodal distribution to a bimodal distribution. Numerical solutions are presented for cases when a polymer undergoes a reversible chain‐end scission along with midpoint chain scission. The rate coefficients for chain‐end scission and polymerization are assumed independent of MW. The theory satisfactorily explains experimental observations.
Streaming potential through multilayer membranes
Authors: Szymczyk A. , Labbez C. , Fievet P. , Aoubiza B. , Simon C. 
Published: April 2004

AbstractThe fundamental and practical importance of electrokinetic characterization of porous membranes is now well recognized. However, not enough attention has focused on the fact that the most practically interesting membranes have a multilayer structure. Then, standard electrokinetic methods, such as streaming potential measurement performed through these multilayer membranes, lead to a global signal that does not necessarily reflect the electrokinetic behavior of the skin layer that rules the membrane selectivity. The relative influence of membrane layers with different physical (pore length, porosity, pore radius) and electrical (zeta potential) properties on the global streaming potential arising across a two‐layer membrane is studied. Each porous layer plays a role in the streaming potential process, but in a wide range of length fraction of support layer the skin layer contributes more than the support layer to the global signal. The contribution of the skin layer increases as its porosity and pore radius decrease, but is little affected by its zeta potential. Nevertheless, in many practical situations, the support layer is likely to play a nonnegligible role on the global signal due to its huge thickness with respect to the skin layer. Therefore, electrokinetic measurements performed with multilayer membranes should be interpreted carefully.
Design of flexible reduced kinetic mechanisms
Authors: Sirdeshpande Avinash R. , Ierapetritou Marianthi G. , Androulakis Ioannis P. 
Published: April 2004

AbstractReduced mechanisms are often used in place of detailed chemistry because the computational burden of including all the species continuity equations in the reactor model is unreasonably high. Contemporary reduction techniques produce mechanisms that depend strongly on the nominal set of problem parameters for which the reduction is carried out. Effects of variability in these parameters on the reduced mechanism are the focus of this work. The range of validity of a reduced mechanism is determined for variations in initial conditions. Both sampling approaches and quantitative measures of feasibility, such as the flexibility index and the convex hull formulation, are employed. The inverse problem of designing a reduced mechanism that covers the desired range of initial conditions is addressed using a multiperiod approach. The effect of the value of a user‐defined tolerance parameter, which determines whether the predictions made by the reduced mechanism are acceptable, is also assessed. The analytical techniques are illustrated with examples from the literature.