bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of fluidized‐bed catalyst for partial oxidation of n‐butane
Authors: Wen Xu , Yang Gong
Published: April 2004

Optimization experiments were carried out for preparation of a catalyst precursor. Results show that the acid concentration and reaction temperature in precursor preparation exhibit strong effects on the catalyst behavior. For the catalysts prepared at higher concentrations of acid, the quantity of the V4+ phase (VO)2P2O7 is large, and the n‐butane conversions are all quite high. When decreasing the acid concentration in precursor preparation, the relative amount of the V5+ phase δ‐VOPO4 and the n‐butane conversion increase continuously; at the same time, the selectivity to maleic anhydride increases at the beginning, and then decreases after reaching a maximum value. The catalyst prepared at an acid concentration of 102.3% gave a maximum yield of maleic anhydride of 48.10% at an inlet n‐butane concentration of 4.0% and a space velocity of 500 h−1. The fine particle catalyst in this study offers great flexibility of operation.
Solidification in heat packs: I. Nucleation rate
Authors: Rogerson Mansel A. , Cardoso Silvana S. S.
Published: April 2004

The solidification of sodium acetate trihydrate in a commercial heat pack is investigated in a series of three articles. In the first work, the classic nucleation theory is used to predict the range of temperatures and pressures for which a subcooled solution of sodium acetate solidifies spontaneously. The nucleation temperature was determined experimentally and it was shown that it is insensitive to concentration, at atmospheric pressure. Using the theoretical results for high pressure and the experimental measurements at atmospheric pressure, an equation for the nucleation temperature was derived as a function of pressure. This relation takes into account heterogeneous nucleation on ever‐present impurities suspended in the solution.
Solidification in heat packs: III. Metallic trigger
Authors: Rogerson Mansel A. , Cardoso Silvana S. S.
Published: April 2004

The metallic trigger used in commercial heat packs initiates solidification by releasing minute crystals of solid sodium acetate trihydrate into the subcooled solution. These crystals are harbored in submicron cracks on the disk's surface and are released when the disk is flexed. Using scanning electron microscopy, such seed crystals are observed on the surface of a disk after flexing it. Classic nucleation theory is used to investigate the behavior of crystals residing in the cracks on the metallic disk's surface during heating and cooling. Sodium acetate trihydrate crystals are capable of surviving in a 1‐nm crack or smaller at regeneration temperatures of the order of 353 K. These seed crystals grow to the mouth of the crack at temperatures below the liquidus, but can only promote solidification of the whole solution surrounding the disk at temperatures below 256 K or when the disk is flexed.
Prediction of phase equilibria and excess properties for systems with sulfones
Authors: Wittig Roland , Lohmann Jürgen , Gmehling Jürgen
Published: April 2004

The group contribution method modified UNIFAC (Dortmund) has become very popular because of its broad applications, and reliable predictions for vapor‐liquid equilibria, solid‐liquid equilibria, liquid‐liquid equilibria, activity coefficients at infinite dilution, azeotropic data and excess enthalpies in a wide temperature range. Therefore, the existing parameter matrix for the modified UNIFAC method is continuously extended with the help of the Dortmund Data Bank and by carrying out systematic measurements. The new main group for sulfones, such as that required to describe systems with the selective solvent sulfolane, is introduced, as well as ten new pairs of group interaction parameters for modified UNIFAC.
Laminar mixing in different interdigital micromixers: I. Experimental characterization
Authors: Hessel V. , Hardt S. , Löwe H. , Schönfeld F.
Published: April 2004

Interdigital‐type glass micromixers with alternating feed channels to periodically create liquid multilamellae, were fabricated for basic investigations of hydrodynamics for liquid mixing of two streams. Three interdigital designs developed, rectangular, triangular, and slit‐type, differed in their flow‐through mixing chamber. These designs are based on simple polygon geometries and their combinations. The flow patterns of an aqueous solution dyed with blue and uncolored water were investigated for different interdigital mixers. Since mixing in a nonfocused device, such as the rectangular mixer, was not completed, focusing techniques were applied. Geometric focusing was used to reduce lamellae width and to speed up mixing. In the special version of the triangular mixer, SuperFocus mixer, liquid mixing times are reduced to about 10 ms, as determined by iron‐rhodanide reaction imaging. In the latter case, the lamellae are compressed by a factor of 40, from a width of 160 to 4 μm. For both the triangular and slit‐type micromixer, flow patterns differed from regular multilamination ones. At high flow rates, lamellae tilted. When the flow cross‐sectional area was expanded, jet and associated eddies formed further improved mixing. Mixing investigations also helped develop imaging techniques. The rhodanide imaging complemented water blue contrasting.
Production of carbon nanotubes in a packed bed and a fluidized bed
Authors: Weizhong Qian , Fei Wei , Zhanwen Wang , Tang Liu , Hao Yu , Guohua Luo , Lan Xiang , Xiangyi Deng
Published: April 2004

Carbon nanotubes (CNTs) prepared from ethylene decomposition over the Fe/Al2O3 catalyst are studied in a packed bed (PB) reactor and a nanoagglomerate fluidized bed reactor (NABR), respectively. CNTs sampled at different reaction times are characterized by TEM, Raman spectroscopy, and particle size analysis. The bulk density and agglomerate size of CNTs increase significantly with reaction time in a PB, while it remains at a stable level in NABR. Also, CNTs with good morphology, narrow diameter distribution, and fewer lattice defects are obtained in an NABR, rather than in a PB. In contrast to the unavoidable jamming due to volume increase of CNTs observed in a PB, a continuous CNT growth process is attained in an NABR, even though the amount of CNTs in an NABR is 6–7 times that in a PB. The flow dynamics, available space for growing, and mass and heat transfer can be controlled in an NABR, which favors the large‐scale production of CNTs with uniform properties.
Mechanics of column beds: II. Modeling of coupled stress‐strain‐flow behavior
Authors: Yew Bee Gaik , Ureta Jorge , Shalliker R. Andrew , Drumm Eric C. , Guiochon Georges
Published: April 2004

The rheological behavior of a bed of packing material during its consolidation was investigated using an elastic‐plastic model, the Frictional Material Model. This model takes into account the behavior of the packing material under compression stress, its consolidation, its internal friction, its friction against the column wall, and the dependence of the bed's permeability on the local void fraction. The complexity of the problem arises from the nonlinear behavior of the relationships between the critical parameters controlling the column bed behavior and the external stresses applied to this bed. Solutions of the model were calculated for combinations of axial compression and seepage stresses, the latter corresponding to the flow of the mobile‐phase stream under typical conditions used in HPLC. The results demonstrate the importance of the internal angle of friction of the packing material used on the degree of radial and axial heterogeneity of the beds of chromatographic columns packed with this material.
Positioning of salt gradients in ion‐exchange SMB
Authors: Houwing Joukje , Jensen Thomas B. , van Hateren Stef H. , Billiet Hugo A. H. , van der Wielen Luuk A. M.
Published: April 2004

Salt gradients can be used to improve the efficiency of ion‐exchange separations in simulated moving‐bed systems. The gradient, formed by the use of feed and desorbent solutions of different salt concentrations, introduces regions of increased and decreased affinity of, for example, proteins for the matrix. Several gradient shapes can be formed, depending on the flow‐rate ratios and salt concentrations used. Only some of these effectively increase throughput or decrease desorbent consumption. Correct gradient positioning is essential, but not trivial, because salt is adsorbed in the resin. A procedure developed selects the flow‐rate ratios that allow correct positioning of gradients based on wave theory and incorporates the nonlinear Donnan isotherm of salt on ion‐exchange resins. Predictions are verified by experiments combined with a mathematical equilibrium stage (true moving‐bed) model. Upward and downward gradients are compared with respect to the use of desorbent and salt.
Enhancement of large‐particle gas‐fluidization by adding liquid
Authors: Nagahashi Y. , Lee D. H. , Grace J. R. , Epstein N. , Yokogawa A. , Asako Y.
Published: April 2004

There are circumstances where fluidization of particles by a gas can be promoted by the addition of small quantities of liquid, contrary to many reported cases where liquid impedes gas‐solid fluidization. Enhancement occurs only when the particles are larger than about 3 mm in diameter, the liquid and particles have similar densities (within ∼25%), and the particles are poorly wetted by the liquid. When these conditions are met, the liquid causes the minimum gas fluidization velocity Ugmf to decrease and particle motion to be more vigorous. For low liquid loadings, Ugmf and the pressure drop across the bed are then predicted well by a “modified liquid‐perturbed gas model.”
Performance of immobilized photocatalytic reactors in continuous mode
Authors: Dijkstra M. F. J. , Koerts E. C. B. , Beenackers A. A. C. M. , Wesselingh J. A.
Published: April 2004

A packed‐bed (PBR), fiber (FR), membrane (MR), and tubular (TR) photocatalytic reactor have been modeled to compare their performance on a pilot scale. The influence of radial dispersion in the PBR and mass‐transfer limitation in the PBR, FR, and TR has been taken into account. The oxygen concentration in the liquid has a large influence on the performance and, therefore, air or pure oxygen should be added. The MR and PBR have a better performance than the TR and FR. Only a small degree of mass‐transfer limitation occurs in the PBR. Radial dispersion hardly influences the performance of the PBR. External mass‐transfer limitation influences the conversion in the TR and FR severely. The PBR, MR, and TR show a much better energy efficiency than the FR.
Thermal machines based on surface energy of wetting: Thermodynamic analysis
Authors: Laouir A. , Luo L. , Tondeur D. , Cachot T. , Le Goff P.
Published: April 2004

This work proposes an original thermodynamic‐energetic analysis of the feasibility and ideal performance of thermal machines based on the wetting phenomenon proposed by V. A. Eroshenko. The extension or contraction of a liquid film is taken as a “tutorial” example to introduce the basic thermodynamic relations of this 2‐D transformation. It implies both mechanical and thermal effects, and this coupling allows conversion of heat to work (thermal engine) or conversely to pump heat (refrigeration/heat pump effect). A similar approach is then developed for the interface between a liquid and a highly microporous solid, having a large internal surface area. The thermodynamic behavior of this interface involves as state variables the surface tension of the liquid, the contact angle, and their dependence on temperature. Depending on the relative magnitude and sign of these quantities, and, therefore, on the working couple and the temperature range, a variety of machine cycles are feasible, or excluded, and a method is proposed for a comprehensive inventory. Order‐of‐magnitude calculations of the energy densities are presented based on the existing experimental data for several systems involving water as the fluid. The tentative conclusions are that the energy densities are very small on a mass basis compared to conventional systems based on vaporization, but the contrary is true on a volume basis because the phase transformation (extension of the surface) occurs in a condensed state. There may, therefore, be some niches for thermal machines of this type, but they remain to be identified and validated.
Physical model analysis of foam–TCE displacement in porous media
Authors: Jeong Seung‐Woo , Corapcioglu M. Yavuz
Published: April 2004

A glass micromodel was experimented to study the coupled effects of the apparent viscosity and mobility of surfactant foam (SF) on the displacement of residual trichloroethylene (TCE) during SF flooding. The SF floods operated under moderate interfacial tension (4.9 dyne/cm) of surfactant solution with TCE and pressure gradient (20–200 kPa) conditions showed relatively different foam behaviors to the higher pressure gradient conditions. The apparent viscosity of SF seems to be less affected by changes in surfactant concentration and affected more by the gas fraction in SF or the bubble size of SF. As the total SF flow rate and the gas injection velocity increased, the apparent SF viscosity decreased and the SF mobility increased. The latter characteristic led to an increase in the TCE displacement efficiency although SF under low‐pressure gradients resulted in flow channeling. The experiment results imply that the SF flood for environment application would be affected by operating conditions and distribution of residual contaminants.
CFD Simulations of bubbling/collapsing fluidized beds for three Geldart Groups
Authors: Gelderbloom Susan J. , Gidaspow Dimitri , Lyczkowski Robert W.
Published: April 2004

A multiphase computational fluid dynamics (CFD) model was applied to a commonly used industrial experiment known as the collapsing fluidized‐bed experiment. The experiment involves several hydrodynamic regimes including the bed expansion, bubbling, sedimentation, and consolidation of the fluidized bed. The CFD model is capable of predicting all four of these regimes. The three Geldart Groups, C, A, and B were simulated in a bubbling and subsequently collapsing fluidized bed. Results show that hydrodynamic Models A and B, the use of the modified Ergun equation or the MFIX code drag models, and solids rheology have limited impact on the bubbling‐ and collapsing‐bed simulations. The major finding of this study is that the solids modulus, G, is the controlling parameter during the consolidation regime. The simulated bubble sizes and bed collapse rates for all three Geldart groups were found to be within reported experimental error. The computed high turbulent intensity of Geldart Group A particles also agrees with data in the literature measured using an acoustic shot noise probe. It is also demonstrated that the traditional interpretation of the collapsing bed as consisting of separate bubble escape and sedimentation regimes is incorrect and that, in fact, they occur simultaneously.
Analysis of multicomponent adsorption kinetics on activated carbon
Authors: Ding L. P. , Bhatia S. K.
Published: April 2004

An integrated mathematical model for the kinetics of multicomponent adsorption on microporous carbon was developed. Transport in this bidisperse solid is represented by balance equations in the macropore and micropore phases, in which gas‐phase diffusion dominates the mass transfer in the macropores, with the phenomenological diffusivities represented by the generalized Maxwell–Stefan (GMS) formulation. Viscous flow also contributes to the macropore fluxes and is included in the MS expressions. Diffusion of the adsorbed phase controls the mass transfer in the micropore phase, which is also described in a similar way by the MS method. The adsorption isotherms are represented by a new heterogeneous modified vacancy solution theory formulation of adsorption, which has proved to be a robust method for adsorption on activated carbons. The model is applied to the coadsorption and codesorption of C2H6 and C3H8 on Ajax and Norit carbon, as well as the displacement on Ajax carbon. The effect of the viscous flow in the macropore phase is not significant for the cases studied. The model accurately predicts the overshoot behavior and rollup of C2H6 during coadsorption. The prediction for the heavier compound C3H8 is always satisfactory, though at higher C3H8 mole fraction, the overshoot extent of C2H6 is overpredicted, possibly due to neglect of heat effects.
Thermoresponsive transport through porous membranes with grafted PNIPAM gates
Authors: Chu Liang‐Yin , Niitsuma Takuya , Yamaguchi Takeo , Nakao Shin‐ichi
Published: April 2004

Both thermoresponsive flat membranes and core‐shell microcapsule membranes, with a porous membrane substrate and grafted poly(N‐isopropylacrylamide) (PNIPAM) gates, were successfully prepared using a plasma‐graft pore‐filling polymerization method. PNIPAM was proven to be grafted homogeneously onto the porous membrane substrates, in the direction of both the membrane thickness and surface. Regardless of the solute molecular size, temperature had an opposite effect on diffusion coefficients of the solute across the PNIPAM‐grafted membranes with low graft yields as opposed to those with high graft yields. The PE‐g‐PNIPAM membranes change from positive thermo‐response to negative thermoresponse types with increasing pore‐filling ratios at around 30%. Phenomenological models were developed for predicting the diffusion coefficient of the solute across PNIPAM‐grafted membranes at temperatures, both above and below the lower critical solution temperature (LCST). Predicted diffusional coefficients of solutes across both the PNIPAM‐grafted flat and PNIPAM‐grafted microcapsule membranes fit the experimental values. To obtain an ideal result for the diffusional thermoresponsive controlled release through PNIPAM‐grafted membranes, the substrates strong enough to prevent any conformation changes are more suitable for preparing thermoresponsive membranes than weak ones.
CFD modeling of flow patterns and hydraulics of commercial‐scale sieve trays
Authors: Gesit Getye , Nandakumar K. , Chuang K. T.
Published: April 2004

A computational fluid dynamics (CFD) model was used to predict the flow patterns and hydraulics of a commercial‐scale sieve tray. The model considers the 3‐D two‐phase flow of gas and liquid in which each phase is treated as an interpenetrating continuum having separate transport equations. Interaction between the two phases occurs via an interphase momentum transfer. For the CFD analysis, the commercial packages CFX5.4 and CFX4.4 of AEA Technology were employed. Velocity distributions, clear liquid height, froth height, and liquid holdup fraction in froth were predicted for various combinations of gas and liquid flow rates. Tray geometry and operating conditions were based on the experimental work that Solari and Bell carried out in a 1.22‐m diameter air–water simulator in 1986 at Fractionation Research Inc. Predicted results were found to be in good agreement with the experimental data of these authors. The objective of the work was studying the extent to which CFD can be used as a prediction and design tool for industrial trays. The simulation results are such that CFD can be used as an invaluable tool in tray design and analysis.
Polymer‐zeolite composite membranes for direct methanol fuel cells
Authors: Libby Brett , Smyrl W. H. , Cussler E. L.
Published: April 2004

Direct methanol fuel cells require membranes with the dual properties of high proton conductivity and low methanol crossover. Such membranes have a high selectivity, that is a high ratio of proton conductivity to methanol permeability. This research reports such a membrane made of polyvinylalcohol and loaded with mordenite, a proton conducting, methanol impermeable zeolite. Protons travel directly through both the polymer and zeolite phases, but methanol has a more tortuous path around the zeolite particles. The composite membranes show up to twenty times higher selectivity than Nafion, the current benchmark. The improved behavior, a result of the proper tailoring of the polymer and dispersion phase, is predicted using Maxwell's theory for diffusion in composite media.
Modeling of morphogenesis of polyolefin particles: Catalyst fragmentation
Authors: Grof Zdeněk , Kosek Juraj , Marek Miloš , Adler Pierre M.
Published: April 2004

The growth of polyolefin particles in heterogeneous catalyzed polymerization reactors is simulated often by reaction‐transport models that introduce certain ad hoc effective spatial scales to the description of particle morphology and transport processes inside the particles. Because predictive capabilities of such models with respect to particle morphogenesis are limited, a multiscale model of a growing particle is developed. Maximum relevant information can be systematically extracted from particle microscopy images using the method of reconstructed porous media. The spatially 3‐D replicas of the particle pore space allow to calculate the effective transport and mechanical properties. Two new modeling techniques approximate catalyst particle fragmentation: Delaunay triangulation generalized into a 3‐D space with non‐Euclidean metrics and disconnection of the skeleton representation of the solid phase. Predictive mesoscopic modeling of particle morphogenesis is also discussed.
Aggregation structure and thermal conductivity of nanofluids
Authors: Xuan Yimin , Li Qiang , Hu Weifeng
Published: April 2004

Nanofluids are obtained by suspending metallic nanoparticles in conventional base liquids. Such a new class of heat‐transfer fluid is superior to the base liquids in energy‐transport performance, which depends on the distribution, volume fraction and thermal properties of the suspended nanoparticles. The theory of Brownian motion and the diffusion‐limited aggregation model are applied to simulate random motion and the aggregation process of the nanoparticles. A theoretical model is developed to predict the thermal conductivity of nanofluids. Comparison between the theoretical and experimental results shows the validity and accuracy of the theoretical model.
Transport properties for turbulent dispersion from wall sources
Authors: Mitrovic Bojan M. , Papavassiliou Dimitrios V.
Published: April 2004

The dispersion of a passive contaminant by turbulence is not only intrinsically interesting, but also is fundamental to practical heat‐ and mass‐transfer problems. The mechanism of turbulent dispersion from the wall and effects of the molecular Prandtl number are examined. Transport properties for scalar dispersion from an instantaneous line source at the wall of a parallel‐plate channel in which fully developed turbulent flow occurs, as well as from a continuous line source, are studied. Numerical experiments using a direct numerical simulation (DNS) of the flow, combined with Lagrangian scalar tracking (LST) of trajectories of thermal markers in the flow field, are conducted. The method is applied for a wide range of molecular Prandtl (Pr) or Schmidt (Sc) number fluids (0.1⩽Pr⩽50,000, which corresponds to liquid metals, gases, refrigerants, lubricants and electrochemical fluids). Results from the DNS/LST method are compared with experimental data, which are available for low Pr or Sc fluids. The turbulent diffusivity and turbulent Prandtl number for the plume that results from a line source are calculated, and an appropriate scaling is proposed. Finally, a model is developed for the prediction of the mean temperature or mean concentration profile as a function of Pr or Sc.
Mass‐transfer effects during separation of proteins in SMB by size exclusion
Authors: Houwing Joukje , Billiet Hugo A. H. , van der Wielen Luuk A. M.
Published: April 2004

The chromatographic fractionation of proteins by size‐exclusion chromatography in a simulated moving bed (SMB) is studied. During experimental fractionation of a mixture of bovine serum albumin (BSA) and myoglobin on Sepharose Big Beads, mass‐transfer effects are shown to limit the performance of the SMB. The internal profiles, as well as the extract and raffinate compositions, are described well by a steady‐state equivalent true moving‐bed (TMB) model that incorporates mass‐transfer effects. The selection of the particle size in SMB is a trade‐off between productivity and mass transfer. Based on the equivalent TMB model, the optimum particle size and configuration of the SMB can be selected, at which preset performance criteria (purity, recovery) are met at specified flow‐rate ratios, total column length, and pressure drop. For the current feed and apparatus, an optimal particle size of approximately 145 μm is calculated for achievement of purities and overall recoveries of 95%.
Accounting for batch reactor uncertainty in the nonlinear MPC of end‐use properties
Authors: Valappil Jaleel , Georgakis Christos
Published: April 2004

There are significant incentives in controlling the end‐use properties in batch reactors to reduce the variability in the final product quality. The presence of model errors and other uncertainties poses a problem in achieving this objective using model‐based control. A methodology to account for the model uncertainties in nonlinear model predictive control is presented here. This methodology also allows establishing a trade‐off between robustness and optimality in the controller. This is based on determining the uncertainty in the predicted final values of the properties, in the form of elliptical confidence regions. The controller then ensures that the complete confidence region is within the target range for the product. A semiinfinite programming problem is solved to find the input values for the rest of the batch, utilizing techniques that reduce the complexity of the problem. A Parameter Adaptive Extended Kalman Filter is used to estimate the important unmeasured and observable properties. An emulsion polymerization process for styrene is chosen as a case study and the methods developed here are demonstrated on this example process.
Product‐centered processing: Pharmaceutical tablets and capsules
Authors: Fung Ka Y. , Ng Ka M.
Published: April 2004

A systematic procedure is presented for synthesizing and developing a manufacturing process for pharmaceutical tablets and capsules. The product quality factors—functional, physical and sensorial—are first identified. The dosage form and excipients are then selected, and the process flowsheet is synthesized along with the suitable equipment and operating conditions. Finally, the product and process are evaluated to ensure that the product possesses the desired quality factors. Design heuristics and physical models are provided to assist decision‐making. Examples involving Vitamin C, antacid, and ginseng are provided to illustrate the procedure.
Process uncertainty: Case of insufficient process data at the operation stage
Authors: Ostrovsky G. M. , Datskov I. V. , Achenie L. E. K. , Volin Yu. M.
Published: April 2004

Flexibility analysis is of prime importance in chemical process systems, since it permits the creation of chemical processes, which can satisfy all design specifications in spite of changes in internal and external factors during the operation stage. The modern foundations of flexibility analysis were developed in the 1980s by Grossman and coworkers. They formulated solution approaches for the main problems of flexibility analysis (feasibility test), flexibility index, and the two‐stage optimization problem. All the formulations are based on the supposition that during the operation stage there are enough experimental data to accurately determine uncertain parameter values, but in practice, these assumptions are not likely to be met. This article discusses extensions of the feasibility test and the two‐stage optimization problem (TSOP), which take into account the possibility of accurately estimating some of the uncertain parameters while estimating with less accuracy the remaining uncertain parameters. To solve the TSOP, the split and bound approach was developed based on a partitioning of the uncertainty region and estimation of bounds on the performance objective function. Three computational experiments show the importance of taking into account the possibility (or lack thereof) of obtaining values of greater accuracy for uncertain parameters at the operation stage.
Framework for evaluating the feasibility/operability of nonconvex processes
Authors: Goyal Vishal , Ierapetritou Marianthi G.
Published: April 2004

A large number of publications available on determining the feasible region and the operability limits for a design are all limited due to the assumption that the feasible space is convex, but almost all of the problems appearing in chemical engineering are nonconvex in nature. Determining the region where the design is feasible and safe to operate is of extreme importance. To address this problem, a novel algorithm was developed based on the idea of systematically evaluating the infeasibility areas using the outer approximation procedure introduced earlier, and the simplicial approximation approach to approximate the expanded feasible space constructed with the exclusion of the nonconvex constraints. This approach is general and deals with any nonconvex regions, as long as the nonconvex constraints are either concave or quasi‐convex, as illustrated through a series of example problems. A new metric is introduced to accommodate the need for comparison between various designs.
Modeling and optimization of a novel membrane reformer for higher hydrocarbons
Authors: Chen Zhongxiang , Yan Yibin , Elnashaie Said S. E. H.
Published: April 2004

A novel circulating fast fluidized‐bed membrane reformer efficiently carries out simultaneous steam and oxidative reforming of heptane over nickel catalysts. A plug‐flow reactor model was developed to investigate effects of the operating parameters. The cases with palladium hydrogen membranes and/or dense perovskite oxygen membranes are investigated. The simulations show that high purity hydrogen can be efficiently produced using hydrogen membranes, which “break” the thermodynamic equilibrium barriers. The perovskite oxygen selective membranes supply oxygen along the length (or height) of the reformer for oxidative reforming of hydrocarbons, providing the heat necessary for the highly endothermic steam reforming reaction. The combination of these two membranes with the characteristics of the fast fluidization reformer makes this process not only an efficient hydrogen producer but also an energy efficient process. The fast continuous flow of catalyst makes the effect of carbon deposition on catalyst activity negligible in the novel process. Process optimization for the maximum hydrogen yield is conducted using the flexible tolerance optimization method.
Modeling transient stresses in spherical vessels during blowdown under fire attack
Authors: Falope G. B. O. , Mahgerefteh H.
Published: April 2004

The development of a rigorous mathematical model for simulating the failure risk of pressurized spherical hydrocarbon containing vessels during blowdown under fire attack is described. Accounting for the important thermodynamic and interfacial mass and heat transfer processes taking place during blowdown, the model simulates the thermal and pressure stresses in the vessel wall in both the radial and tangential directions. Failure is assumed to take place when the sum of these two stresses in either direction exceeds the vessel material of construction yield strength. Application of the model to the blowdown of a condensable hydrocarbon mixture indicates that failure occurs in the vapor space due to thermal weakening of the vessel wall. Finally, the risks associated with blowdown during fire attack for a cylindrical as opposed to a spherical vessel of the same volume and prevailing conditions are compared and contrasted.
Modeling the hydration of foodstuffs: Temperature effects
Authors: Weerts A. H. , Lian G. , Martin D. R.
Published: April 2004

To assess the impact of hydration kinetics of dried foodstuffs on their sensory property and release of functional molecules, it is necessary to be able to predict the temperature effect. Based on the dynamics of capillary flow in partially saturated porous media, a finite‐element model is developed to directly predict the infiltration of water into dried food materials as a function of temperature. The transfer properties are derived from physically based constitutive relationships of water adsorption and permeation. This is in contrast to the semiempirically fitted transfer properties often used in the heat‐ and mass‐transfer models of foods based on Fick's diffusion theory. Rehydration of green tea at different temperatures has been measured using the NMR method. The rehydration process also has been simulated using the capillary model, and the predictions are in good agreement with experimental data.
Separation performance predictions of a Stairmand high‐efficiency cyclone
Authors: Derksen J. J.
Published: April 2004

Large‐eddy simulations (LES) were performed on the gas flow in a Stairmand high‐efficiency cyclone at Re = 280,000. The LES realistically represent the 3‐D, time‐dependent flow, including phenomena such as vortex core precession, and vortex breakdown. Quantitatively good agreement with measured velocity profiles is observed, for both the time‐averaged velocities and the velocity fluctuation levels. The single‐phase LES formed the starting point for modeling solid particle motion in the cyclone based on one‐way coupling between the gas flow and the particles. It is shown that some details of the flow in the relatively small region in the vicinity of the inlet have strong influence on the separation process. Due to the long residence times of particles inside the cyclone, the concurrent simulation of gas flow and particle motion is a lengthy computational process. Therefore, three alternative ways of modeling particle motion were explored: a frozen‐field approach, an eddy‐lifetime model, and a periodic‐flow approach. The results of these approaches are compared to the results of the concurrent simulation.
Experimental and simulation studies of heat transfer in polymer melts
Authors: Chen Hongbing , Sundararaj Uttandaraman , Nandakumar K.
Published: April 2004

The heat transfer of a single solid cylindrical poly(ether imide) (PEI) pellet in polyethylene (PE) melt under simple shear flow was studied using experiments and numerical simulations. This included two approximate 2‐D representations of the geometry, and a rigorous 3‐D geometry matching the experimental conditions. A generalized power‐law viscosity model for PE melt, and temperature‐dependent conductivity and specific heat were used for PEI in the models. Viscous dissipation was included in the simulations. From the experimental temperature history, the mean heat‐transfer coefficient h̄ was determined to be 370 W/m2·K, with the assumption of a uniform convective environment. From the simulation, h̄ was determined to be 1,160 W/m2·K and 420 W/m2·K in the 2‐D simulations with circular and rectangular pellet, respectively, and 250 W/m2·K for the 3‐D simulation. Results from the 3‐D simulation proved that the uniform convection assumption used to calculate h̄ for the experiment was not acceptable.
Novel multifunctional optical‐fiber probe: I. Development and validation
Authors: Liu Jinzhong , Grace John R. , Bi Xiaotao
Published: April 2004

By combining aspects of optical fiber probes used to determine local particle concentrations and local particle velocities and by rapid signal processing, it is shown that a single probe can determine local instantaneous particle fluxes. Providing a transparent cover to prevent a “blind zone” is shown to be critical in improving the probe performance and linearity. A simple mechanistic model is successful in predicting the performance of the optical system. The results of the probe are validated using particles glued onto both a rotating disk and a flat vibrating surface.
Attainable reaction and separation processes from a superstructure‐based method
Authors: Linke Patrick , Kokossis Antonis
Published: April 2004

Generic technology for the synthesis and optimization of integrated reaction and separation systems uses rich superstructure formulations comprising two types of generic synthesis units with flexible representation modes. A reactor/mass exchanger unit enables a detailed representation of the reaction and mass exchange phenomena. A conceptual representation of separation systems is facilitated through separation task units. All possible process designs featuring reaction, reactive separation, and separation are embedded in the superstructure formulations as combinations of generic units and their features. The design options are explored using stochastic optimization techniques suitable for this class of problems. The flexible representation framework enables technology applications to general process design, as well as design subproblems including reactor and reactive separator design. Four case studies demonstrate the ability of the methodology to address a wide variety of process systems and to deliver design novelty.
Optimal measurement system design for chemical processes
Authors: Muske Kenneth R. , Georgakis Christos
Published: April 2004

A procedure for the optimal determination of measured variables in chemical processes is presented. This procedure is based on quantifying the trade‐off between the process information that can be obtained and the total cost of the sensors making up the measurement system. The resulting trade‐off is specified as a Pareto optimization problem. The optimal measurement systems that should be considered for installation are determined from the set of solutions to this Pareto optimization problem. The approach is demonstrated using a CSTR process example.
Viscoelastic model for particle fragmentation in olefin polymerization
Authors: Kittilsen Pål , Svendsen Hallvard F. , McKenna Timothy F.
Published: April 2004

The catalyst particle fragmentation process is an important step in determining the final morphology of olefin polymer particles. Understanding the different phenomena that influence it is, therefore, essential in optimizing polymerization processes and in obtaining the desired quality of polymer. A new viscoelastic model of the fragmentation process developed is demonstrated. The model relates the magnitude and generation rate of physical forces and the physical characteristics of the polymer and support. In this way, the effects of different parameters, such as reaction rate, polymer properties, and support characteristics, can be related to changes in the morphology of polymer particles. A faster reaction rate makes the particle become more porous, and a more viscous polymer will make particles with less porosity. It is discussed why larger pores will break before smaller on the same catalyst.
Interfacial protein networks and their impact on droplet breakup
Authors: Jones Daniel B. , Middelberg Anton P. J.
Published: April 2004

Conventional theories for predicting droplet disruption are based on a dimensionless capillary number and fail when a protein emulsifier is present at moderate concentration. In this study, droplet disruption is examined in simple shear and, in parallel, novel equipment is used to define conditions under which an interfacial stress‐transmitting protein network forms at an oil/water interface. Capillary number predictions fail when a rigid interfacial protein network forms. Complete stress‐strain curves, to high material deformation, are presented for these networks. This work demonstrates that droplets destabilized by interfacially adsorbed protein should be viewed as deformable capsules or cells surrounded by a stress‐transmitting network. Deformation and disruption could then be predicted by existing theories for such systems, using the constitutive data provided by the stress‐strain tests. Such an approach is expected to be superior to existing methods based solely on interfacial energy.
Sensitivity study on interfacial closure laws in two‐fluid bubbly flow simulations
Authors: Oey R. S. , Mudde R. F. , van den Akker H. E. A.
Published: April 2004

The two‐fluid formulation with closure models for interfacial forces and turbulence is applied to the evaluation of a meandering plume in a rectangular bubble column. In literature there is some discussion about how precisely the models should be in order to capture the dynamics as is observed experimentally. Furthermore, this type of flow has shown to be quite sensitive for the degree of diffusion (physical and numerical) introduced in the model equations. The focus is on the necessity and influence of certain interfacial closure models in above type of bubbly flow. The influence of the drag, turbulent diffusive, and added mass force will be highlighted. TVD schemes are used for the computations to obtain accurate results against low costs, that is, coarse grid approximation and to reduce the amount of numerical diffusion as much as possible. Still these schemes have to be “tuned” in their properties in order to obtain the desired accuracy. Nonproper choice of certain parameters, that is, “flux‐limiters” introduces errors in the final solution, with the same order of magnitude as physical phenomenon, which are introduced by interfacial forces.
Characterization of asphaltenes aggregation and fragmentation in a shear field
Authors: Rahmani Nazmul H. G. , Masliyah Jacob H. , Dabros Tadeusz
Published: April 2004

Aggregation and breakup of asphaltenes flocs in a Couette flow device are studied for various values of shear rate (G), volume fraction of particles (φ), and solvent composition (ratio of toluene to n‐heptane in the solution Xs). The number average asphaltene aggregate size initially increases before achieving a maximum size and then decreases until reaching a steady‐state value that reflects a balance between aggregation and fragmentation. Increasing the applied shear rate leads to higher aggregation and fragmentation rates, reducing the steady‐state average aggregate size and the characteristic relaxation time required to achieve a steady state. For a fixed value of G, when either φ is increased or Xs is decreased, the steady‐state floc size increases. The kinetics of the aggregation‐fragmentation process and the attainment of steady state by asphaltenes aggregates were modeled using a population balance approach. The model describes the time evolution of the average floc size using fitted parameters (the breakup coefficient for shear fragmentation, b and the porosity of the aggregates ϵ). Binary breakage was included in calculations and collision efficiency was assumed to be equal to one.
Numerical study of the packing of wet coarse uniform spheres
Authors: Yang R. Y. , Zou R. P. , Yu A. B.
Published: April 2004

The discrete‐element method was extended to investigate the effect of liquid addition on the packing of monosized spheres. By explicitly taking into account the interaction forces between particles, such as contact forces and capillary force, the packing of wet spheres can be simulated by the method. Simulations are performed under the so‐called poured packing, where a different amount of water is added to spheres of different sizes. Comparison with data in the literature shows that the simulations yield a satisfactory agreement in the relationship between porosity and moisture content, macroscopically verifying the proposed model. The packing structure is then analyzed in terms of microscopic properties such as the radial distribution function and coordination number and are shown to vary with liquid content for a given particle size. The relationship between porosity and capillary force is finally quantified, which confirms those derived based on macroscopic assumptions.
Flocculation of biological cells: Experiment vs. theory
Authors: Han Binbing , Akeprathumchai S. , Wickramasinghe S. R. , Qian X.
Published: April 2004

Flocculation of biological cells is important in the biotechnology industry, as it could lead to improved efficiencies for bioreactor harvesting operations such as microfiltration. Experimental studies for flocculation of yeast and CHO cells using cationic polyelectrolytes suggest the existence of a steady‐state, self‐similar floc size distribution. The experimentally determined floc size distributions were modeled using a population balance approach. For flocculated yeast suspensions, the variation of the floc volume fraction with dimensionless particle diameter is predicted by the population balance model assuming a binary breakage distribution function. However, the variation of floc number fraction with dimensionless particle diameter is better predicted assuming a log normal fragment distribution function probably due to the presence of submicron‐sized yeast cell debris. For CHO cell flocs, the floc volume and number fractions are predicted using a log normal fragment distribution function. CHO cells are far more fragile than yeast cells. Thus, individual CHO cells in a CHO cell floc can lyse leading to the formation of a number of small particles.
Design of alternative refrigerants via global optimization
Authors: Sahinidis Nikolaos V. , Tawarmalani Mohit , Yu Minrui
Published: April 2004

Accurate and fast enumeration of large, combinatorial search spaces presents a central conceptual challenge in molecular design. To address this challenge, an algorithm is developed that guarantees globally optimal solutions to a mixed‐integer nonlinear programming formulation for molecular design. The formulation includes novel structural feasibility constraints, while the algorithm provides all feasible solutions to this formulation through the implicit enumeration of a single branch‐and‐reduce tree. This algorithm is used to provide the complete solution set to a refrigerant design problem posed elsewhere. In addition to rediscovering CFCs, the proposed methodology identifies a number of novel potential replacements of Freon 12.
Probabilistic model for sensor fault detection and identification
Authors: Mehranbod Nasir , Soroush Masoud , Piovoso Michael , Ogunnaike Babatunde A.
Published: April 2004

A new Bayesian belief network (BBN) model with discretized nodes is proposed for fault detection and identification in a single sensor. The single‐sensor model is used as a building block to develop a BBN model for all sensors in the process considered. A new fault detection index, a fault identification index, and a threshold setting procedure for the multisensor model are introduced. The fault detection index exploits the probabilistic information of the multisensor model, which, along with the proposed threshold setting procedure, leads to effective detection of faulty sensors. The fault identification index uses only the probabilistic information of the faulty sensor to determine in a discretized fashion the size of faults that should be analyzed within a moving time window to identify the fault type. Single‐sensor model design parameters (prior and conditional probability data) are optimized to achieve maximum effectiveness in detection and identification of sensor faults. The single‐sensor model and optimal values of design parameters are used to develop a multisensor BBN model for a polymerization reactor at steady‐state conditions. Its capabilities to detect and identify bias, drift, and noise in sensor readings are illustrated for single and simultaneous multiple faults by several case studies.