bluuebunny/crossref_metadata_2025_split huggingface dataset:
High‐purity Nitrogen by pressure‐swing adsorption
Authors: Shirley Arthur I. , Lemcoff Norberto O. 
Published: June 2004

AbstractThe separation of air for nitrogen production can be carried out by pressure‐swing adsorption over a carbon molecular sieve. The separation is kinetically based, since the equilibrium adsorption of both oxygen and nitrogen is very similar, but the oxygen is adsorbed faster. Several theoretical and experimental studies have been reported, but mainly dealing with nitrogen purities below 99.9%. The objective of this article is to study experimentally the effect of different process variables on the performance of a rate‐induced PSA process in the high‐purity region. The effect of cycle time on both the yield and the specific product per unit cycle become more significant as the purity increases. On the other hand, the specific feed per unit cycle is almost insensitive to product purity, particularly as the cycle time increases. These results agree very well with predictions from a theoretical model.
Granulation of sol‐gel‐derived nanostructured alumina
Authors: Deng S. G. , Lin Y. S. 
Published: June 2004

AbstractA sol‐gel granulation process was developed to prepare porous nanostructured γ‐alumina granules as supports for catalysts and adsorbents. The process, which starts with an aqueous sol of gelatinous boehmite, involves droplet formation, gelation in paraffin oil, conditioning in ammonia solution, and drying and calcination in air under controlled conditions. The γ‐Al2O3granules prepared are 1–3 mm‐dia. spherical particles with large surface area (380 m2/g) and pore volume (0.5 cm3/g), uniform pore‐size distribution (20–60 Å), and controllable average pore size (35 Å). These sol‐gelderived granules have excellent mechanical properties with crush strength (>100 N per granule) and attrition resistance (<0.01 wt. %/h), much better than the commercial alumina and zeolite granules. Supported CuO sorbents were prepared on these granules for SO2removal applications. The alumina‐supported CuO sorbents contain higher loading of well‐dispersed CuO and better sulfation properties than similar sorbents reported in the literature.
Cosolvent tuning of tautomeric equilibrium in supercritical fluids
Authors: Dillow Angela K. , Hafner Kellye P. , Yun S. L. Jimmy , Deng Fenghua , Kazarian Sergei G. , Liotta Charles L. , Eckert Charles A. 
Published: June 2004

AbstractCosolvents offer a sensitive tool to tailor equilibria, rates or yields of chemical reactions in supercritical fluid solvents. The tautomeric equilibrium of a Schiff base was chosen as a model system, and small amounts of protic cosolvents shift the position of tautomeric equilibria. The equilibrium was tuned from essentially one tautomer to another by modifying the solvent of pure SCF ethane with less than 2 mol % hexafluoroisopropanol cosolvent. The equilibrium constant was a function of cosolvent concentration and mixture density. A chemical‐physical model of equilibrium constants deviates from the measured values in the near‐critical region, which may have been caused by local composition enhancement of cosolvent around the Schiff base. Decrease in the degree of hydrogen bonding with pressure or density affects the keto‐enol equilibria by decreasing the amount of the keto formation. Thus, solution density, as well as the degree of hydrogen bonding, is manipulated to tune the position of tautomeric equilibria.
Effluent concentration profiles in centrifugal partition chromatography
Authors: van Buel M. J. , van der Wielen L. A. M. , Luyben K. Ch. A. M. 
Published: June 2004

AbstractCentrifugal partition chromatography (CPC) is a relatively new preparative chromatographic technique. To understand, predict, and optimize CPC separations a model is needed, describing the effluent concentration profile as a function of the phenomena that determine the separation efficiency (mass transfer, mixing, and partitioning). The model presented in this article describes experimental effluent concentration profiles accurately. Partition coefficients, Stanton numbers, and Péclet numbers were obtained by comparing model simulations to experimental pulse‐response data. The fitted partition coefficients agree well with those obtained from shake‐flask experiments. Mass‐transfer limitation is the major reason for peak broadening. The inverse mass‐transfer coefficient is a linear function of the partition coefficient. The model will be a valuable tool in determining the influence of mass transfer as a function of various experimental conditions.
Nonlinear control of a high‐purity distillation column using a traveling‐wave model
Authors: Balasubramhanya Lalitha S. , Doyle Francis J. 
Published: June 2004

AbstractThe trade‐off between model accuracy and computation tractability for model‐based control applications is well known. While nonlinear models are needed to capture the detailed behavior of many chemical processes, the resultant structures may not lead to straightforward control implementation. The current work advocates the use of low‐order nonlinear models based on wave propagation that are mathematically concise and capture the essential nonlinear behavior of a process. In this work, differential geometry is invoked to achieve input–output linearization of a high‐purity distillation column based on a traveling‐wave model. A Kalman filter is used to recursively update the parameter values. Comparison with a linear controller based on a two‐time constant model shows that the nonlinear controller outperforms the linear controller in tight control of both the overhead and the bottoms composition.
Modeling and optimization of a semiregenerative catalytic naphtha reformer
Authors: Taskar Unmesh , Riggs James B. 
Published: June 2004

AbstractModeling and optimization of a semiregenerative catalytic naphtha reformer has been carried out considering most of its key constituent units. A detailed kinetic scheme involving 35 pseudocomponents connected by a network of 36 reactions in the C5–C10 range was modeled using Hougen‐Watson Langmuir‐Hinshelwood‐type reaction‐rate expressions. Deactivation of the catalyst was modeled by including the corresponding equations for coking kinetics. The overall kinetic model was parameterized by bench‐marking against industrial plant data using a feed‐characterization procedure developed to infer the composition of the chemical species in the feed and reformate from their measured ASTM distillation data. For the initial optimization studies, a constant reactor inlet temperature configuration that would lead to optimum operation over the entire catalyst life cycle was identified. The analysis was extended to study the time‐optimal control profiles of decision variables over the run length. In addition, the constant octane case was also studied. The improvement in the objective function achieved in each case was determined. Finally, the sensitivity of the optimal results to uncertainty in reactor‐model parameters was evaluated.
Microwave heating of the continuous flow catalytic reactor in a nonuniform electric field
Authors: Plazl I. , Pipus G. , Koloini T. 
Published: June 2004

AbstractTemperature profiles in the continuous‐flow reactor with a fixed bed placed in a nonuniform microwave field are predicted by considering specific on – off regulation of the microwave power and local distribution of the electric‐field intensity. The spatial and temperature dependence of the electric‐field intensity affects the heating appreciably, as shown by indirect measurements. Results of numerical calculations agree with experimental data of outlet temperature and explain temperature oscillations in the outlet flow. The model developed allows an accurate description of microwave heating in the continuous‐flow reactor associated with specific regulation of microwave power. The hydrolysis of sucrose catalyzed by the strongly acidic cation‐exchange resin Amberlite 200C in R‐H form was chosen as the reaction system. Calculated conversions of sucrose based on predicted temperature profiles agree with experimental data.
Degradation kinetics of polymers in solution: Dynamics of molecular weight distributions
Authors: McCoy Benjamin J. , Madras Giridhar 
Published: June 2004

AbstractPolymer degradation occurs when macromolecular chains are broken under the influence of thermal, mechanical or chemical energy. Chain‐end depolymerization and random‐ and midpoint‐chain scission are mechanisms that have been observed in liquid‐phase polymer degradation. Here we develop mathematical models, unified by continuous‐mixture kinetics, to show how these different mechanisms affect polymer degradation in solution. Rate expressions for the fragmentation of molecular‐weight distributions (MWDs) govern the evolution of MWDs. The governing integrodifferential equations can be solved analytically for realistic conditions. Moment analysis for first‐order continuous kinetics shows the temporal behavior of MWDs. Chain‐end depolymerization yields monomer product and polymer molecular‐weight moments that vary linearly with time. In contrast, random‐ and midpoint‐chain scission models display exponential time behavior. The mathematical results reasonably describe experimental observations for polymer degradation. This approach, based on the time evolution of continuous distributions of chain length or molecular weight, provides a framework for interpreting several types of macromolecular degradation processes, particularly how bimodal MWDs can evolve during degradation.
Crystal unfolding and chain disentanglement during semicrystalline polymer dissolution
Authors: Mallapragada Surya K. , Peppas Nicholas A. 
Published: June 2004

AbstractMathematical models developed describe the unfolding of polymeric crystals in the presence of a solvent followed by their subsequent disentanglement. A thermodynamic model considering the free energy changes during crystal unfolding was proposed to obtain an expression for the unfolding rate. A simplified version of this expression, assuming uniform crystal size, was incorporated into a continuum model to predict the dissolution kinetics of a semicrystalline polymer slab. The model yielded predictions of the crystalline and solvent volume fractions as a function of position within the slab, in addition to changes in the fraction of polymer dissolved as well as the degree of crystallinity of the polymer as a function of time. The degree of crystallinity of the polymer decreased with time and the plot of the fraction of the polymer dissolved as a function of time exhibited Case II behavior. The model predictions agreed will with experimental results obtained during dissolution of semicrystalline poly(vinyl alcohol) in water.
Unifying approach to modeling granule coalescence mechanisms
Authors: Adetayo A. A. , Ennis B. J. 
Published: June 2004

AbstractA novel, physically based kernel for population balance modeling of granule growth by coalescence is presented. This kernel is size‐independent in that all collisions with an effective average granule size less than a critical value are successful. Simulations based on this kernel show that a variety of contradictory experimental observations can be modeled. In the limiting case of viscoelastic collisions, the kernel can be related to the governing group of the Stokes number (Ennis et al., 1991), representing the ratio of granule collisional kinetic energy to viscous dissipation brought about by the binder. In more general cases, material properties that control deformability, such as interparticle friction, binder viscosity, and liquid content, strongly affect this critical size. The kernel clearly demonstrates the three regimes of drum granulation originally proposed by Kapur and Fuerstenau in 1964 and compares favorably with the two‐stage sequential kernel developed by Adetayo et al. in 1995 for the drum granulation of fertilizers.
Multivessel batch distillation
Authors: Skogestad Sigurd , Wittgens Bernd , Litto Rajab , Sørensen Eva 
Published: June 2004

AbstractThe multivessel batch column consists of a reboiler, several column sections, intermediate vessels and a condenser vessel. This configuration provides a generalization of previously proposed batch‐distillation schemes, including the inverted column and the middle‐vessel column. The total reflux operation of the multivessel batch‐distillation column was presented recently, and the main contribution of this article is to propose a simple feedback control strategy for its operation. We propose to adjust the vessel holdups indirectly by manipulating the reflux flow out of each vessel to control the temperature at some location in the column section below. The feasibility of this strategy is demonstrated by simulations.
Parallel frontal solver for large‐scale process simulation and optimization
Authors: Mallaya J. U. , Zitney S. E. , Choudhary S. , Stadtherr M. A. 
Published: June 2004

AbstractFor the simulation and optimization of large‐scale chemical processes, the overall computing time is often dominated by the time needed to solve a large sparse system of linear equations. A new parallel frontal solver that can significantly reduce the wallclock time required to solve these linear equation systems using parallel/vector supercomputers is presented. The algorithm exploits both multiprocessing and vector processing by using a multilevel approach in which frontal elimination is used for the partial factorization of each front. Results of several large‐scale process simulation and optimization problems are presented.
New simplified models and formulations for turbulent flow and convection
Authors: Churchill Stuart W. 
Published: June 2004

AbstractExpression of the time‐averaged equations for the conservation of momentum and energy in terms of the fractions of the local shear stress and heat flux density, respectively, due to turbulence is shown to result in greatly simplified representations for fully developed flow and convection in round tubes and parallel‐plate channels as compared to those in terms of the eddy viscosity, eddy conductivity, or mixing length. On the other hand, the turbulent Prandtl number is shown to be a fundamental characteristic of flow and convection rather than simply an artifact of the eddy diffusional model. The new simplified representations provide the basis for improved predictions of the velocity distribution, the friction factor, the temperature distribution, and the Nusselt number. The adaption of these new representations for other geometries and for developing convection is discussed briefly.
Wavelet‐based density estimation and application to process monitoring
Authors: Safavi A. A. , Chen J. , Romagnoli J. A. 
Published: June 2004

AbstractAn application of wavelets and multiresolution analysis to density estimation and process monitoring is presented. Wavelet‐based density‐estimation techniques are developed as an alternative and superior method to other common density‐estimation techniques. Also shown is the effectiveness of wavelet estimators when the observations are dependent. The resulting density estimators are then used in defining a normal operating region for the process under study so that any abnormal behavior by the process can be monitored. Results of applying these techniques to a typical multivariate chemical process are also presented.
Circulation in bubble columns
Authors: Burns L. F. , Rice R. G. 
Published: June 2004

AbstractLiquid circulation in bubble columns influences many system properties such as mass transfer and catalyst dispersion. Conditions favorable for liquid circulation often exist only for highly turbulent slurries. Experiments under conditions of lowered surface tension proved conclusively that uniform circulation can exist even in the bubbly‐flow regime. Moreover, the measured circulating velocity profile is more plug‐shaped in the center core than traditional models, which take a parabolic shape. These observations led to the application of a circulation theory based on an energy‐dissipation model for turbulent eddy viscosity, which can be easily computed as a simple function of superficial‐gas velocity. Two length scales were used in model calculations depending on flow regime: bubble diameter for bubbly flow and column diameter for churn turbulence. The liquid‐velocity profile contained a tuned dimensionless proportionality constant, which resulted in two different average values, k = 2.51 for bubbly flow and 0.0242 for churn turbulence, when tuned to experimental data. The integral average eddy viscosity is numerically equivalent to the eddy dispersion coefficient for bubbly flow. Predicted liquid velocity compared very favorably with new measured values under conditions of low surface tension and with literature values.
Turbulence structure and prediction of interfacial heat and mass transfer in wavy‐stratified flow
Authors: Lorencez C. , Nasr‐Esfahany M. , Kawaji M. 
Published: June 2004

AbstractThe turbulence structure of the liquid phase near a wavy gas‐liquid interface in stratified flow was experimentally investigated in a 50 mm (H) × 100 mm (W) rectangular duct. The characteristic parameters of the organized motion under the liquid waves such as the frequency of appearance and length scale were estimated by using the variable‐interval time‐averaging and the photochromic dye activation techniques. These characteristics were used in a hybrid surface renewal‐eddy cell model to predict the interfacial heat‐ and mass‐transfer coefficients in stratified two‐phase flow. The predictions for cocurrent and countercurrent flows agreed reasonably well with experimental data available in the literature.
Evaluation of gas‐solids mixing chamber through cross correlation and Hurst's analysis
Authors: Briens C. L. , Mirgain C. , Bergougnou M. A. , Del Pozo M. , Loutaty R. 
Published: June 2004

AbstractFast circulating fluidized‐bed reactors are used for many applications, both in riser and downer modes. Typical resisdence times are in the order of a few hundreds of milliseconds. A good understanding of initial gas‐solids contact must be reached to realize the full potential of such reactors. Effective and stable gas‐solids mixing chambers were identified from the singals of simple and robust probes, which could be used in industrial and pilot plants. Criteria based on cross correlation or Hurst analysis provided consistent results. Although this study used simplified momentum probes, the criteria it developed can be applied to the signal of any fast‐response sensor which is affected by local hydrodynamics. Temperature, capacitance, ultrasonic, or γ‐ray absorption measurements could be used. Gas or liquid‐liquid mixing chambers could also be optimized with the same criteria.
NMR measurements and numerical simulation of fluid transport in porous solids
Authors: Tessier J. J. , Packer K. J. , Thovert J.‐F. , Adler P. M. 
Published: June 2004

AbstractPulsed magnetic field gradient stimulated echo NMR measurements are performed for diffusion and flow of an aqueous phase both within a sample of packed spherical beads and within a 25% porosity Fontainebleau sandstone. The stimulated echo dependence on the gradient pulse area q is used to derive the displacement probability distributions PΔ(X) for fixed observation times Δ. The shape of PΔ(X) as a function of Δ is simulated for computer‐generated porous media, and a good agreement is obtained between the experimental NMR data and the simulations.
Reaction rates in fractal vs. uniform catalysts with linear and nonlinear kinetics
Authors: Gavrilov C. , Sheintuch M. 
Published: June 2004

AbstractPore fractal objects are expected to be optimal catalysts, since material is supplied to the narrower pores, which are also shorter through the larger pores where diffusion resistance is smaller. To demonstrate this, diffusion and reaction were simulated on Sierpinski‐gasket‐type fractal objects and on the corresponding nonfractal uniform‐pore structures of the same size, porosity and reactive area. Positive order reactions limited by Knudsen diffusion were shown to exhibit larger rates in fractal than in uniform‐pore objects. Fractal catalysts also exhibited a new intermediate domain in which the rate depends only weakly on the kinetics parameters. In nonmonotomic kinetics the branching point (bifurcation point) was extremely sensitive to the pore structure.
Use of a novel autoassociative neural network for nonlinear steady‐state data reconciliation
Authors: Du Yang‐Guang , Hodouin Daniel , Thibault Jules 
Published: June 2004

AbstractA novel autoassociative neural‐network‐based estimator for nonlinear steady‐state data reconciliation was developed, which is a modified autoassociative feedforward neural network. The main difference between them lies in the minimization of an objective function that includes material imbalance terms of flow rates and compositions as well as the traditional least‐square prediction term. Accordingly, this neural network, with the material balance‐related equations included in the objective criterion, can perform simultaneously the following basic functions necessary for proper steady‐state data rectification: (1) eliminate the nonrandom errors, such as the biases and gross erros, from measurements; (2) filter out the random errors from measured data; and (3) estimate the values of unmeasured process variables, provided data redundancy prevails. This novel neural‐network‐based reconciliation method is demonstrated on a flotation system.
Dehydrogenation of propane using a packed‐bed catalytic membrane reactor
Authors: Weyten Herman , Keizer Klaas , Kinoo Annemieke , Luyten Jan , Leysen Roger 
Published: June 2004

AbstractIn a high‐temperature catalytic membrane reactor, a plug‐flow reactor is combined with a gas‐separative membrane. By selectively removing one of the reaction products, the reaction mixture is prevented from reaching equilibrium, and a higher conversion can be obtained. This concept is only valid for reactions that are limited by the thermodynamic equilibrium, such as the direct dehydrogenation of propane to propene. A tubular H2‐selective silica membrane was characterized [αH2/C3H8 = 70–90 at 500°C] and used as the gas‐separative membrane. The membrane reactor was filled with a chromia/alumina catalyst. The kinetics of the catalyst was studied. At 500°C the deactivation of the catalyst is slow, and the propene yield is almost constant for at least 10 h of operation. Under well‐chosen process conditions, the propene yield is at least twice as high as the value obtained at thermodynamic equilibrium in a conventional reactor.
Polymer trace devolatilization: I. Foaming experiments and model development
Authors: Yang Chi‐Tai , Smith Theodore G. , Bigio David I. , Anolick Colin 
Published: June 2004

AbstractVolatile species are removed from polymers by devolatilization. Foaming experiments using corn syrup and low‐density polyethylene were carried out to investigate effects of volatile content, operating vacuum level and stripping agent in devolatilization. When foaming and rupture occur, a limiting foam volume growth was observed for a polymer regardless of the volatile content and the addition of stripper bubbles. A mathematical model was also developed to study the effect of stripper bubbles in polymer devolatilization. Based on the cell model for bubble growth, a bimodal model for foam growth is developed as the first step to describe the initial formation and growth of the volatile and stripper bubbles in polymer during devolatilization. When foaming and breakup occur at a critical foam volume expansion, a film model for mass transfer is used to model the second step of devolatilization. A numerical factorial study shows that the devolatilization section should be designed and operated to provide the screw channel sufficient free volume and residence time for the desired foam expansion.