bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulation of vortex core precession in a reverse‐flow cyclone
Authors: Derksen J. J. , Van den Akker H. E. A.
Published: April 2004

A large‐eddy simulation of the single‐phase turbulent flow in a model cyclone geometry on a uniform, cubic computational grid consisting of 4.9×106 cells was performed. The Navier‐Stokes equations were discretized according to a lattice‐Boltzmann scheme. The Reynolds number, based on the inlet velocity and the cyclone body diameter, was 14,000. A standard Smagorinsky subgrid‐scale model with cs = 0.1, including wall‐damping functions, was applied. The 3‐D, average flow field was predicted with a high level of accuracy. Furthermore, the simulations exhibit vortex‐core precession, that is, the core of the main vortex is observed to move about the geometrical axis of the cyclone in a quasi‐periodic manner. The Strouhal number associated with the simulated vortex‐core precession was 0.53, whereas 0.49 was experimentally observed in a similar geometry at approximately the same Reynolds number.
Numerical simulation of transient flow of two immiscible liquids in pipeline
Authors: Fairuzov Yuri V.
Published: April 2004

A two‐fluid model for transient flow of two immiscible liquids in a pipeline was formulated based on two transient continuity equations and a combined momentum equation in a quasi‐steady form. It can be used for simulating both stratified and dispersed flow. Numerical simulations were performed to investigate the behavior of oil–water flow during transients caused by an increase or decrease in the water volume fraction at the entrance to the pipeline. An increase in the water flow rate within the stratified flow regime may result in the formation of a shock (an abrupt change in the water holdup). In transients produced by a decrease in the input water volume fraction, the smoothening of the water holdup wavefront occurs. This behavior is explained by a strong dependence of the speed of the holdup wave propagation in stratified flow on the water holdup. The results obtained for inclined pipelines show that the slope of the pipe affects the flow behavior significantly. The liquid–liquid flow model was used to simulate the transport of corrosion inhibitor in a pipeline conveying an oil–water mixture.
Local bubble‐size distribution in fluidized beds
Authors: Santana D. , Macías‐Machín A.
Published: April 2004

The importance of the rise angle of bubbles on the influence of local bubble‐size distribution in fluidized beds was studied. A new method is proposed for transforming chord‐length distribution to a local bubble‐size distribution, including the effect of the velocity vector direction on the transformation. Monte‐Carlo simulation was used to generate bubbles synthetically with several rise angles, based on the fact that the velocity vector direction distribution of bubbles was a Bingham distribution. The transformation of chord‐length data obtained by an imaginary probe, using the Parzen window as an estimation of the probability density function, gives a good estimation of the local bubble‐size distribution for different rise angles and the new approach offered here, knowing that the direction of the velocity vector direction for any orientation of the bubbles in the fluidized bed gives a superior agreement than the approach where the local bubble‐size distribution is estimated from bubbles with vertical rises.
Robust design of countercurrent adsorption separation processes: 5. Nonconstant selectivity
Authors: Migliorini Cristiano , Mazzotti Marco , Morbidelli Massimo
Published: April 2004

Operating conditions for the separation of a binary mixture using a nonadsorbable eluent through simulated moving‐bed technology were designed. The results obtained using equilibrium theory for adsorption described by Langmuir models lead to the definition of explicit constraints on the operating parameters to operate the unit in the desired regime of separation. A more general approach was able to produce the same result for a larger class of isotherms. The physical and mathematical conditions defining this larger class of isotherms are discussed, as well as the algorithms necessary to calculate the region of complete separation. Applications to the bi‐Langmuir isotherm and the ideal adsorbed solution model, which are more flexible than the Langmuir model and can describe systems where selectivity changes with composition, are discussed. A shortcut method to get an explicit, though approximate, solution is proposed and its accuracy is discussed.
Unified approach for synthesizing crystallization‐based separation processes
Authors: Wibowo Christianto , Ng Ka M.
Published: April 2004

A procedure for synthesizing crystallization‐based separation processes is presented. In an evolutionary manner and without being restricted to a particular crystallization technique, flowsheet alternatives are generated to meet the separation objectives for a given system. This is achieved by recognizing the fact that four basic crystallization‐related movements in composition space are sufficient to represent a variety of crystallization processes. Suitable movements are selected based on relevant features of the phase diagram to construct feasible flowsheets. The procedure consists of six steps. First, separation objectives are defined. Second, a separation core structure is generated. Third, the separation sequence and unit operations are selected, followed by adding other units to the structure for proper plant operations. Fifth, appropriate crystallizer type is selected. Finally, economic evaluation and feasibility checks are performed for the generated alternatives. Rules are provided to aid decision‐making at each step. The procedure is illustrated with various examples including separation of amino acids, p‐ and m‐cresols, chlorobenzoic acids, calcium carbonate and magnesium oxide from dolomite, and the production of salt.
Rigorous modeling of UV absorption by TiO2 films in a photocatalytic reactor
Authors: Zhang Z. , Anderson W. A. , Moo‐Young M.
Published: April 2004

The radiation absorption profiles on the surfaces of TiO2 films in a corrugated‐plate photocatalytic reactor were modeled based on first principles. A new term, the local‐area‐specific rate of energy absorption (LASREA), was adopted to describe the catalyst surface radiation in heterogeneous photoreactors. The LASREA and the energy absorption efficiency were both quite sensitive to the dimensions of the corrugated plates. Due to the multiple photon reflections between the opposing surfaces, corrugated plates possess a superior capability for recapturing longer wavelength photons that would otherwise be reflected out of some reactor designs. This results in higher energy absorption efficiency and more uniform LASREA on the catalyst films. Compared to a flat plate, corrugations are predicted to enhance the energy absorption efficiency by up to 50% for UV‐A fluorescent‐lamp‐powered systems and more than 100% for solar‐powered systems.
Acid‐neutralizing of marine cylinder lubricants: Measurements and effects of dispersants
Authors: Wu Rong Chang , Papadopoulos Kyriakos D. , Campbell Curt B.
Published: April 2004

Considerable amounts of sulfuric acid are produced in marine diesel engines during the combustion of high‐level sulfur fuels, thus causing corrosive wear. The acid‐neutralizing ability of marine cylinder lubricants (MCLs) is therefore a “measuring stick” of their quality. A new experimental protocol was previously published to qualitatively test the acid‐neutralizing ability of marine cylinder lubricants. Using a capillary video‐microscopy technique and measuring the shrinking rate of the size of nitric‐acid drops in MCL, the acid‐neutralizing ability of commercial MCLs, as well as model lubricants, can be determined quantitatively. The shrinking rate of the nitric‐acid drop size in MCL was constant during the reaction period and independent of the initial drop size. Analysis of the experimental results, following an interfacial neutralization reaction mechanism, suggests that the adsorption of overbased reverse micelles on acid‐oil interface is the controlling step in the reaction process. The presence of small amounts of dispersant (1 wt. %) in a model MCL on the neutralization reaction decreased the reaction rate for both nitric and sulfuric acid, while further increase in dispersant concentration (up to about 4%) did not reduce the acid‐neutralizing ability further. For sulfuric acid, dispersants act also to effectively disperse the hydrated calcium sulfate crystals produced by the neutralization reaction with sulfuric acid.
Theoretic information approach to protein stabilization by solvent engineering
Authors: Di Domenico Roberto , Lavecchia Roberto , Ottavi Alessandro
Published: April 2004

A novel approach based on molecular thermodynamics and the information theory is proposed to quantify the influence of water‐miscible additives on protein stability. According to the two‐state mechanism of inactivation, solvent effects are described in terms of perturbation of the equilibrium between the folded and unfolded protein forms. The model provides the dependence of the protein's melting temperature on the additive concentration. Effects of the latter are accounted for by an empirical parameter related to the free energy of transfer of the protein from the pure to the mixed solvent. The model was tested using experimental data relative to the influence of hydroxylic and aminoacidic additives on the thermal unfolding of hen egg lysozyme and erythrocyte carbonic anhydrase. Fitting parameters were correlated in terms of a theoretic information index characterizing the additive's molecule and incorporating an atomic‐composition term and a topological contribution. Model calculations agreed very well with experimental data, suggesting that the molecular information content of the additive can be used effectively to correlate solvent‐induced perturbations of stability. The procedure was also used to predict melting temperatures in systems containing binary mixtures of additives and to reconstruct thermal unfolding curves in the different media.
Semicontinuous, middle‐vessel distillation of ternary mixtures
Authors: Phimister James R. , Seider Warren D.
Published: April 2004

The operation of a semicontinuous, middle‐vessel column (SCMVC) that separates a nearly‐ideal ternary mixture is analyzed. MVCs have a large vessel between the rectification and stripping sections. In traditional batch rectification or stripping, a still is charged and distillate or bottoms products are removed, leaving heavy or light concentrates in the still, respectively. In contrast, the middle vessel of an MVC is charged, and, similar to a continuous process, both distillate and bottoms products are withdrawn. The process separates nearly‐ideal ternary mixtures into three nearly‐pure species, with the intermediate‐boiling species concentrating in the middle vessel, and the light and heavy species concentrating in the distillate and bottoms products, respectively. Shortcut methods are adapted to determine the minimum number of trays and minimum reflux ratio. Alternative control configurations are considered to achieve suitable performance throughout the campaign. Two DB configurations are shown to be suitable for this process, with the middle vessel decoupling the composition control loops and permitting dual‐composition control with little interaction. Simulations confirm that difficulties encountered when using a DB configuration for the dual‐composition control of a continuous column do not occur for a SCMVC.
Synthesis and oxygen‐permeation properties of thin YSZ/Pd composite membranes
Authors: Kim Jinsoo , Lin Y. S.
Published: April 2004

Thin dense yttria‐stabilized‐zirconia (YSZ)/Pd composite (dual‐phase) membranes were fabricated on porous YSZ membranes by a new approach that combines the reservoir method and chemical vapor deposition (CVD) technique. A thin porous YSZ layer was coated on a porous alumina support by dip coating the YSZ suspension. A continuous Pd phase was formed inside pores of the YSZ layer by the reservoir method. The residual pores of the YSZ/Pd layer were plugged with yttria/zirconia by CVD to ensure the gas tightness of the membranes. The oxygen‐permeation fluxes through these composite (dual‐phase) membranes, measured in situ in the CVD reactor, are 2.0×10−8 and 4.8×10−8 mol/cm2·s at 1,050°C when air and oxygen are used as the permeate gases, respectively. These oxygen‐permeation values are about 1 order of magnitude higher than those of pure YSZ membranes prepared under similar conditions. The apparent activation energies for oxygen permeation at 900–1,050°C are 193 kJ/mol for thin YSZ/Pd composite (dual‐phase) membranes and 124 kJ/mol for pure YSZ membranes. These results are explained in terms of relative importance of various mass‐transfer steps for oxygen permeation and microstructure of the YSZ/Pd composite membranes prepared by this technique.
Modeling chromatographic chiral separations under nonlinear competitive conditions
Authors: Migliorini Cristiano , Mazzotti Marco , Zenoni Gianmarco , Pedeferri MariaPia , Morbidelli Massimo
Published: April 2004

The chromatographic separation of Tröger's base enantiomers with ethanol as eluent on microcrystalline triacetate cellulose (CTA) was studied. Binary competitive adsorption equilibrium data measured were interpreted using ideal adsorbed solution and real adsorbed solution (RAS) theory. Both models were used to simulate a set of column breakthrough experiments. The competitive behavior of the two enantiomers is quantitatively discussed and the reliability of the two models is assessed. The results show that the RAS model can describe with good accuracy the complex competitive adsorption behavior exhibited by the CTA/Tröger's base model system. This conclusion is expected to apply to the adsorption of the other enantiomeric mixtures on chiral stationary phases.
Comparison of sorbents and isotherm models for NH3‐gas separation by adsorption
Authors: Helminen Jarkko , Helenius Joni , Paatero Erkki , Turunen Ilkka
Published: April 2004

Adsorption equilibrium isotherms of ammonia gas were measured at temperatures between 298 and 393 K on 13X zeolite, 4A zeolite, alumina, silica gel, and activated carbon. The applicability of these sorbents to ammonia gas separation was compared based on equilibrium data. In the pressure range of 1 to 100 kPa activated carbon has its highest working capacity (5.5 mmol·g−1) at 298 K, and the working capacity drops rapidly with temperature, reaching its lowest point at 393 K. The two zeolites provide almost the same working capacity, 3.0–3.5 mmol·g−1, over the entire temperature range. Silica gel and alumina showed low working capacities. The experimental equilibrium data were fitted to 16 different isotherm models, with accuracy and reliability statistically evaluated. The Langmuir–Freundlich model with the van't Hoff equation for the equilibrium constant and with a thermal expansion equation for the saturation sorbate concentration provided the most accurate fit for the 13X and 4A zeolites. This model was also very accurate for the alumina and silica gel data, even though the Dubinin–Astakhov model gave slightly higher predictions. The Henry and vacancy solution models provided the best fit for activated carbon.
Dynamic data rectification using the expectation maximization algorithm
Authors: Singhal Ashish , Seborg Dale E.
Published: April 2004

Although on‐line measurements play a vital role in process control and monitoring process performance, they are corrupted by noise and occasional outliers (such as noise spikes). Thus, there is a need to rectify the data by removing outliers and reducing noise effects. Well‐known techniques such as Kalman Filtering have been used effectively to filter noise measurements, but it is not designed to automatically remove outliers. A new methodology based on the Kalman filter rectifies noise as well as outliers in measurements. Filter equations were formulated in the form of probability distributions. Then the Expectation‐Maximization algorithm was used to find the maximum‐likelihood estimates of the true measurement values based on a state‐space model, past data, and current observations. This approach was evaluated when the assumption of normally distributed outliers is not valid. The method can be used with any dynamic process model, as shown by integrating it with an extended Kalman Filter and by an augmented linear state‐space model to account for unmeasured disturbances. It also can be used to provide diagnostic information about changes to the process or sensor failures.
Design sensor trajectory for control: Application to sheet‐forming processes
Authors: Chang Der‐Ming , Yu Cheng‐Ching , Chien I‐Lung
Published: April 2004

The conventional scanning sensor generates a zig‐zag pattern for the sheet‐forming process, immediately leading to the problem of unequal measurement update intervals in state estimation. Instead of reconstructing the profile given a zig‐zag pattern, the sensor trajectory was redesigned to better estimation and control. In this work, the characteristics of the conventional scanning sensor were explored as well as the implications in estimation and control. The concept of elliptic sensor contour is presented, deriving analytical expressions for the sensor trajectories. Alternatively, the improved sensor trajectory can be obtained using a variable‐speed scanning sensor under linear sweep. That implies that control performance can be improved by reprogramming the motor of the existing sensor. Moreover, several alternatives for sensor arrangement are also explored. The results for estimation and control can be used as guidelines for the development of future sensor technology.
Passive control design for distributed process systems: Theory and applications
Authors: Alonso Antonio A. , Banga Julio R. , Sanchez Isidro
Published: April 2004

A recently developed theory linking passivity with the second law of thermodynamics was used to develop a robust control design methodology for process systems with states distributed in time and space. Asymptotic stabilization of the infinite dimensional state thus can be accomplished for convection–diffusion processes with nonlinear production terms. Two examples representative of these phenomena were considered: chemical reactors and thermal treatments induced by electromagnetic fields. The first case shows how mixing and reactor size are critical control design parameters. If mixing is complete, at least in some direction, exponential stabilization can be achieved by high gain control. If not, stabilization is still possible for reaction domains smaller than a critical volume. In the second case, the electromagnetic field power supplied can always be manipulated to preserve passivity for any domain size. Two important consequences are that the infinite dimensional state can be reconstructed at arbitrary precision by robust observers and that control of the energy inventory will suffice to provide asymptotic stabilization. Theoretical justification of these findings is given on a general framework and illustrated through simulation experiments.
Spatiotemporal patterns in thermokinetic models of cross‐flow reactors
Authors: Nekhamkina Olga , Rubinstein Boris Y. , Sheintuch Moshe
Published: April 2004

The behavior of stationary and moving spatially‐periodic patterns in a simple cross‐flow reactor was simulated and analyzed for a situation in which reactant is supplied continuously along the reactor and a first‐order exothermic reactor occurs. The Danckwerts boundary conditions for realistic Le and Pe values. While the unbounded (infinitely long) reactor is an asymptotic case used to study the stability of the homogeneous solution, the moving waves that emerge in the convectively unstable unbounded system may be arrested at the boundaries of a bounded system and stationary waves are established above some amplification threshold. Sustained periodic and aperiodic behavior may emerge under certain conditions. The spatial behavior in the bounded system with Pe→∞ is analogous to the temporal behavior of the simple thermokinetic CSTR problem and the behavior of the distributed system is classified according to that of the lumped one.
Ultrasound‐modulated two‐fluid atomization of dilute polymer solutions
Authors: Tsai Shirley C. , Luu Patrick
Published: April 2004

New findings presented can further our understanding of the atomization of dilute polymer solutions. First, contrary to the conventional view that viscoelastic liquids are more difficult to atomize than Newtonian liquids, it was found that aerodynamic conditions required to form uniform drops are less severe for some viscoelastic liquids than for Newtonian liquids. Under similar aerodynamic conditions, using ultrasound‐modulated two‐fluid atomization at 54 kHz, 40‐μm‐dia. uniform drops determined by the third harmonic frequency were obtained for gel‐forming xantham gum solutions, but not for Newtonian liquids. Second, ultrasonic atomization yielded 350–400‐μm‐dia. drops for aqueous solutions of rodlike polyethylene oxides (PEO) with molecular weights of 1‐2×105at 54 kHz, supporting previous predictions that PEO relaxation times may be as short as 10−4s. Third, at smaller molecular weights (up to 2×104), aqueous solutions of PEO behave like Newtonian liquids in accordance with modified Taylor's dispersion relation.
Sorbate/sorbent phase transition during adsorption of p‐xylene in silicalite
Authors: Mohanty S. , Davis H. T. , McCormick A. V.
Published: April 2004

An analytically solvable lattice model was used to study the difference in the physics of p‐xylene adsorption in ORTHO and PARA phases of silicalite. The model predicts that a phase transition of p‐xylene in the pore space does not necessarily require a silicalite‐phase transition. Conditions for the sorbate‐phase transition are identified. The sign of the sorbate‐sorbate interactions is critical to sorbate‐phase transition. It is suggested that although the sorbate‐ and sorbent‐phase transitions can be distinct events, there is strong coupling between the two. It is reasonable to suppose that the fluid‐phase change may drive the zeolite phase change. Finally, this equilibrium study suggests the possibility of hysteresis at higher coverages depending on the kinetics of adsorption.
Progress in the thermal sciences: AIChE Institute Lecture
Authors: Churchill Stuart W.
Published: April 2004

Rather than merely surveying progress in the prediction of heat transfer over the past half century, attention is focused on the factors that have led to significant advances in understanding and practice. An almost one‐to‐one correspondence is demonstrated between advances in heat transfer and those in computer hardware and software. However, the development of specialized algorithms for flow and heat transfer by individual investigators appears in most cases to be an essential ingredient. Direct contributions of experimentation are relatively minor, but the synergy of combined experimentation and numerical analysis is most often the key to discovery and innovation. A quiet revolution has occurred in the form of correlative equations, and the primary and almost sole contribution of closed‐form analysis has been the derivation of asymptotes for that purpose. Finally, progress in heat transfer has occurred primarily in discrete steps, and most often as a consequence of exploratory research and/or the observation and pursuit of the explanation for an anomaly.
Motion in a particle bed agitated by a single blade
Authors: Laurent B. F. C. , Bridgwater J. , Parker D. J.
Published: April 2004

Studies on the agitation of granular materials were conducted in a horizontal cylindrical shell stirred by a single long flat blade located on radial arms fixed to a rotor shaft. Positron emission particle tracking, a noninvasive method of investigating opaque systems, permitted the motion of a single particle to be followed. Axial flow patterns showed two loops of circulation inside each compartment defined by the radial arms of the rotating blades. Results revealed the presence at 20% of fill of a circulation zone in the cross‐sectional plane directly beneath the rotating shaft where little agitation occurred. This zone decreased in size and moved toward the agitator shaft as fill increased from 20% to 60%. Velocity profiles and Fourier analysis of particle displacement showed that particle movement was controlled by the number of blade passes. Torque measurements on the agitator shaft were correlated with distribution of material in the cross section of the mixer and distribution of tangential velocity. This opens the prospect of relating torque to powder flow patterns. The information can now be incorporated into models for heat transfer, chemical reaction, or agglomeration.
Predicting adsorption equilibrium using molecular simulation
Authors: Davies Graham M. , Seaton Nigel A.
Published: April 2004

The capability of molecular simulation, combined with pore‐size distribution (PSD) analysis, to predict adsorption equilibrium in microporous carbon adsorbents, using as a case study the adsorption of methane and ethane, was investigated. The calculations indicate that a wide range of data for the adsorption of pure components and binary mixtures can be predicted successfully. Significantly, only a limited set of experimental data comprising three or four adsorption measurements is required to calculate the PSD. This predictive ability is relevant to both the design of adsorptive separation processes and the design of new adsorbents.
CFD modeling of heat transfer in turbulent pipe flows
Authors: Thakre S. S. , Joshi J. B.
Published: April 2004

Twelve versions of low Reynolds number k‐ϵ and two low Reynolds number Reynolds stress turbulence models for heat transfer were analyzed comparatively. Predictions of the mean axial temperature, the radial and axial turbulent heat fluxes, and the effect of Prandtl number on Nusselt number were compared with the experimental data. The model by Lai and So from the k‐ϵ group and Lai and So from the Reynolds stress group had the best overall predictive ability for heat transfer in turbulent pipe flow. The Lai and So model was attributed to its success in the predictions of flow parameters such as mean axial velocity, turbulent kinetic energy, eddy diffusivity, and the overall energy dissipation rate. The k‐ϵ models performed relatively better than the Reynolds stress models for predicting the mean axial temperature and the Nusselt number. This qualitative and quantitative study found the need for more sophisticated near‐wall experimental measurements and the accuracy of the dissipation (of turbulent energy) and the pressure‐scrambling models.
Statistical and causal model‐based approaches to fault detection and isolation
Authors: Yoon Seongkyu , MacGregor John F.
Published: April 2004

Both fundamental and practical differences between two common approaches to fault detection and isolation are examined. One approach is based on causal state‐variable or parity‐relation models developed from theory or identified from plant test data. The faults are then detected and isolated using structured or directional residuals from these models. The multivariate statistical process control approaches are based on noncausal models built from historical process data using multivariate latent variable methods such as PCA and PLS. The faults are then detected by referencing future data against these covariance models, and isolation is attempted through examining contributions to the breakdown of the covariance structure. There are major differences between these approaches arising mainly from the different types of models and data utilized to build them. Each of them has clear, but complementary, strengths and weaknesses. These are discussed using simulated data from a CSTR process.
Ethylene compressor monitoring using model‐based PCA
Authors: Rotem Y. , Wachs A. , Lewin D. R.
Published: April 2004

Principal component analysis (PCA), which is widely used in process monitoring, performs best when the system variables are linearly correlated. In practice, however the variables are often nonlinearly related and may be subject to periodic forcing, both of which compromise the performance of conventional PCA. In model‐based PCA (MBPCA), multivariate statistics are used to analyze the portion of the observed variance that cannot be predicted using a model of the process and thus significantly enhances the attainable diagnostic resolution. Here, MBPCA is used for fault‐detection monitoring of an ethylene compressor, which operates under a significant periodic disturbance caused by the ambient temperature. An analytical expression is derived to predict the limits of identifiable faults given bounds on the parametric model uncertainty.
Residual activity of sorbent particles with a long residence time in a CFBC
Authors: Abanades Juan Carlos , de Diego Luis Francisco , García‐Labiano Francisco , Adánez Juan
Published: April 2004

To characterize the sulfation capacity of limestone as a sorbent of SO2 in a circulating fluidized‐bed combustor (CFBC), long‐term sulfation tests were carried out in a thermobalance. In order to cover the range of residence times expected in a large‐scale combustor, the experiments lasted 24 h. The residual activity of the sorbent after the first hours of reaction indicated a conversion of ∼0.1, depending on the particle's size and sulfation conditions. The shrinking core model described well these residual activities. An effective gas‐phase diffusion coefficient was derived from plots of the measured conversion vs. time between 3 and 24 h. These diffusivities ranged between 1.7 and 4.1×10−9 m2/s at 850°C and depended only on the limestone. It is argued that this simple description could be incorporated into more complex sulfation models of the first few hours of reaction and also account for the subsequent increases in conversion over longer time scales. It is also stressed that, to predict a sorbent's performance in a boiler, characterization techniques should be expanded to experimental time scales similar to those expected in large‐scale CFBCs.
Low‐flow limit in slot coating: Theory and experiments
Authors: Carvalho Marcio S. , Kheshgi Haroon S.
Published: April 2004

The region of acceptable quality in the space of operating parameters of a coating process is called coating window. Their limits are set by coating defects. For the slot‐coating process the low‐flow limit is important. It corresponds to the maximum web speed at a given film thickness, or the minimum film thickness at a given web speed, at which the coating bead remains stable. The available viscocapillary model is based on the Landau‐Levich equation, which is limited to small Capillary and Reynolds numbers. Under these conditions, the minimum film thickness that can be coated decreases with decreasing coating speed, but many coating processes do not occur at low Capillary numbers. It is important to determine the range of validity of the viscocapillary model and find the low‐flow limit outside this range. The low‐flow limit was determined here theoretically and experimentally. The 2‐D Navier‐Stokes equations with free surfaces describe liquid flow in the coating bead. Theoretical approaches solve the Navier‐Stokes system by either using Galerkin's method with finite‐element basis functions or applying a long‐wave expansion. The minimum layer thickness at a set of parameters was determined by the turning point on the solution path as the thickness was diminished. The minimum film thickness was measured experimentally by determining the flow rate at which the coating bead breaks, leading to stripes of coated and uncoated web. Results show that the low‐flow limit of the coating bead at large Capillary and Reynolds numbers fundamentally differs from that at their low numbers. At large Capillary and Reynolds numbers, the minimum film thickness that can be coated decreases with increasing coating speed. The coating window of the process is much larger than that in the literature, broadening the applicability of this coating method.
Development of volume‐average theory for deep‐bed filtration
Authors: Stephan E. A. , Chase G. G.
Published: April 2004

Over the past 20 years, researchers have attempted, with limited success, to develop a general correlation for permeability loss within a deep‐bed filter. The research presented here yields a unique result by rigorously developing a filtration model that focuses on the effective surface area as the dominant factor causing permeability loss. Both a constitutive relation and a geometric interpretation are developed to represent the effective surface area. The resulting macroscale equations accurately model the permeability reduction by incorporating the lower scale mechanisms. The model was tested using the specific process of water injection into a sandstone formation with experimental variables of salt concentration, pH, and concentration of influent fines. Postulated forms of the constitutive relations were verified through these experiments. By accounting for the changes in effective surface area, the model uniquely and accurately describes the permeability loss within a deep‐bed filter.
Convection‐enhanced mass transfer in aggregated beads for gel chromatography
Authors: Li Qiangliang , Grandmaison Edward W. , Goosen Mattheus F. A. , Taylor David
Published: April 2004

Convection‐enhanced mass transfer in aggregated κ‐carrageenan beads was assessed using experimental and mathematical modeling studies. Aggregated beads, prepared from subsidiary beads in a fluidized bed, were employed for gel chromatography of proteins. The efficiency of a chromatography column was measured by the plate height. The plate height of the aggregated beads was much lower than that of conventional carrageenan beads of the same size. This was attributed to intraparticle convection, which occurred even at relatively low flow rates due to the large intraparticle channels resulting from the low degree of aggregation. A general equation for predicting the reduced plate height was modified by using an apparent diffusivity in order to incorporate intraparticle convection. The modified equation was in good agreement with experimental data. The protein productivity of an aggregated bead column (mg/h/mL column) was estimated to be six times higher than that of a conventional column.
Numerical modeling of helical flow of viscoplastic fluids in eccentric annuli
Authors: Hussain Q. E. , Sharif M. A. R.
Published: April 2004

Helical flow of viscoplastic Herschel‐Bulkley fluids in concentric and eccentric annuli with rotating inner cylinder was investigated numerically. Similar flows occur in drilling operations of oil and gas wells. A finite volume algorithm with a nonstaggered grid system and a nonorthogonal curvilinear coordinate system to handle irregular geometry of an eccentric annulus was used to analyze the problem. Papanastasiou's modification of the Herschel‐Bulkley (Yield‐Power‐Law) constitutive equation was used to model the shear rate‐dependent viscosity of a viscoplastic fluid. For a fixed axial pressure gradient, the axial flow rate increased with increasing rotational speed of the inner cylinder. The discharge, as well as torque required to rotate the inner pipe, increased with increasing eccentricity for a fixed axial pressure gradient and inner cylinder rotational speed. Discharge also increased with increasing axial pressure gradient at a fixed eccentricity and rotational speed of the inner pipe. The flow field in an eccentric annulus is complex because vigorous secondary flow is produced in addition to the primary axial helical flow. Blockage at the narrow part of the eccentric annulus, when present, intensifies this secondary flow, with the discharge decreasing initially, then increasing, and decreasing again with increasing height of the blockage.
Product transfer between plants using historical process data
Authors: Jaeckle Christiane M. , MacGregor John F.
Published: April 2004

Based on the concepts laid out in an earlier article (Jaeckle and MacGregor, 1998), this paper defines the problem of moving the production of a particular product grade from a plant A to another plant B when both plants have already produced a similar range of grades. Since the two plants may differ in size, configurations, and so on, the process conditions required to produce any given product grade may be very different in the two plants. How historical process data on both plants may be utilized to assist in this problem is investigated. A multivariate latent variable method is proposed that uses data from both plants to predict process conditions for plant B for a grade previously produced only in plant A. The approach is illustrated by a simulation example.
Process control structure selection based on economics
Authors: Heath Jonathan A. , Kookos Ioannis K. , Perkins John D.
Published: April 2004

In process systems, selecting suitable sets of manipulated and controlled variables and the design of their interconnection, known as the control structure selection problem, is an important structural optimization problem. The operating performance of a plant depends on the control structure selected, as well as the characteristics of the disturbances acting on the plant. The economic penalty associated with the variability of main process variables close to active constraints was used in this work to develop a quantitative measure for the ranking of alternative control structures. The problem is formulated as a mixed integer nonlinear optimization problem of special structure, which was used for an algorithm to solve this problem to global optimality. The final formulation is a mixed integer linear problem for which effective solution methods are currently available. The validity and usefulness of the method are demonstrated through a number of case studies.
Two‐phase flow model of the cathode of PEM fuel cells using interdigitated flow fields
Authors: He Wensheng , Yi Jung S. , Van Nguyen Trung
Published: April 2004

When interdigitated gas distributors are used in a PEM fuel cell, fluids entering the fuel cell are forced to flow through the electrodes porous layers. This characteristic increases transport rates of the reactants and products to and from the catalyst layers and reduces the amount of liquid water entrapped in the porous electrodes thereby minimizing electrode flooding. To investigate the effects of the gas and liquid water hydrodynamics on the performance of an air cathode of a PEM fuel cell employing an interdigitated gas distributor, a 2‐D, two‐phase, multicomponent transport model was developed. Darcy's law was used to describe the transport of the gas phase. The transport of liquid water through the porous electrode is driven by the shear force of gas flow and capillary force. An equation accounting for both forces was derived for the liquid phase transport in the porous gas electrode. Higher differential pressures between inlet and outlet channels yield higher electrode performance, because the oxygen transport rates are higher and liquid water removal is more effective. The electrode thickness needs to be optimized to get optimal performance because thinner electrode may reduce gas‐flow rate and thicker electrode may increase the diffusion layer thickness. For a fixed‐size electrode, more channels and shorter shoulder widths are preferred.
Molecular and macroscopic modeling of phase separation
Authors: Escobedo Fernando A.
Published: April 2004

Recently proposed pseudoensemble Monte Carlo methods are extended in this work to map out diverse phase diagrams (projections of the phase‐coexistence hypersurface) of multicomponent mixtures required to characterize fluid‐phase equilibrium phenomena and to design separation processes. Within the pseudoensemble framework, the macroscopic models of different equilibrium processes can be readily integrated to the mathematical constraints that specify the thermodynamic state of the system. The proposed Monte Carlo methods allow, for example, the simulation of isopleths and cloud‐point lines to compare experimental to simulation data and to test molecular force fields. Applications of this approach include the study of retrograde phenomena in a model natural‐gas mixture through simulation of dewlines and coexistence lines at constant vaporization fraction. As demonstrated, pseudoensemble simulations can also be used to generate the thermodynamic data necessary to solve problems encountered in continuous and discontinuous distillation processes.
Linear driving force approximation to a radial diffusive model
Authors: Başağaoğlu Hakan , Ginn Timothy R. , McCoy Benjamin J. , Mariño Miguel A.
Published: April 2004

Transient volume‐averaged and spatial intraparticle contaminant concentration in a dissolved phase of spherical immobile aggregates is examined using the linear driving force (LDF) model, a special case of a generalized concentration profile (GCP) approximation. Particular cases are solved when a mass flux occurs between the surrounding bulk concentration and intraparticle concentration. Four different forms for the time‐variant surrounding bulk concentration are considered: linear, exponential, sinusoidal (oscillatory) and step function. Both pointwise and volume‐averaged intraparticle concentrations computed by the LDF model match the numerical and analytical (when available) solutions almost perfectly, when the surrounding bulk concentration is assumed to be changing linearly or exponentially. For the other two cases, however, only volume‐averaged concentrations across the immobile spherical particle and the temporal concentrations at the outer boundary of the particle computed from the LDF approximation match the numerical model results reasonably well.
Quantifying deformation in gel swelling: Experiments and simulations
Authors: Achilleos Evdokia C. , Prud'homme Robert K. , Kevrekidis Ioannis G. , Christodoulou Kostas N. , Gee Kyle R.
Published: April 2004

A technique for the real‐time visualization of material line movement during transparent gel deformation was developed based on caged photo‐activated fluorophores covalently attached to the gel network. It is used to quantitatively monitor the swelling of sodium polyacrylate and poly(acrylamide‐sodium acrylate) gels in quasi two‐dimensional constrained geometries. The experimentally obtained profiles for the Finger deformation tensor and the polymer concentration are compared with transient simulations of a model of transport in polyelectrolyte gels.
Local transitions in flow phenomena through packed beds identified by MRI
Authors: Johns M. L. , Sederman A. J. , Bramley A. S. , Gladden L. F. , Alexander P.
Published: April 2004

Magnetic resonance imaging (MRI) velocity measurement techniques are used to investigate flow of water and a glucose solution, of viscosity 3.89 × 10−3Pa·s, within the interparticle space of a cylindrical packed bed of 5 mm diameter glass ballotini. The experiments were performed over a range of Reynolds numbers from 0.84 to 14.52, spanning the regime where inertial effects begin to play a significant role relative to viscous forces. A transition from creeping to inertial flow occurs in isolated pores at a local Reynolds number, defined for each pore within the interparticle space, of approximately 30. Despite this transition in flow behavior in some pores, the gross features of the flow pattern scale approximately with flow rate, which can be explained by considering the volume of the void space in which the flow is nearly stagnant as largely determining the pressure distribution within the bed.
Bubble formation in high‐pressure liquid–solid suspensions with plenum pressure fluctuation
Authors: Yang G. Q. , Luo X. , Lau R. , Fan L. S.
Published: April 2004

The behavior of bubble formation from a single orifice in a nonaqueous liquid and a liquid–solid suspension with pressure fluctuations in the gas chamber (plenum region) is studied at high pressures (up to 8.3 MPa). An analytical model is developed to account for the initial bubble size in liquid–solid suspensions at high pressures. The model takes into consideration various forces induced by particles, such as suspension inertial force. Both experiments and model predictions indicate that the initial bubble size in the suspension is generally larger than that in the liquid, and that it increases with an increase in the solids concentration. The system pressure has a significant effect on the initial bubble size in liquids or liquid–solid suspensions when bubbles are formed under variable gas flow‐rate conditions, and a negligible effect under constant gas flow‐rate conditions. This model can reasonably describe the initial bubble sizes under high‐pressure conditions measured experimentally in this study and those reported in the literature.
Thermally coupled distillation with reduced number of intercolumn vapor transfers
Authors: Agrawal Rakesh
Published: April 2004

The classical thermal coupling between two distillation columns requires two‐way communication whereby both vapor and liquid streams are transferred in opposite directions between one specific location in each of the distillation columns. A general framework is presented whereby this classical two‐way thermal coupling is converted to a one‐way liquid‐only transfer. This conversion is achieved by adding one or more distillation sections and a reboiler and/or a condenser with each added section. The resulting equivalent configurations coproduce some of the component product streams from more than one distillation column and are quite flexible. Finally, the techniques developed for this framework are applied to reduce the heat duty of the well‐known conventional distillation column configurations with no thermal coupling.
Multieffect distillation for thermally coupled configurations
Authors: Agrawal Rakesh
Published: April 2004

A systematic method presented draws multieffect configurations for any thermally coupled multicomponent distillation scheme. For ternary mixtures, all the resulting double‐effect configurations are presented for each of the five known thermally coupled configurations. For most feed conditions, some of these configurations often require substantially lower heat duty than the double‐effect configurations derived from conventional distillation schemes, such as direct and indirect splits and prefractionator. Essential features of the resulting multieffect configurations are that some of the product streams are produced from more than one distillation column and the transfer of at least one mixture stream takes place between intermediate locations in the distillation columns. This method opens up many more multieffect distillation possibilities for multicomponent feed mixtures.
Extreme conditions in binary reactive distillation
Authors: Lee Jae W. , Hauan Steinar , Westerberg Arthur W.
Published: April 2004

In certain cases, reactive distillation can be used to avoid minimum reflux pinch points and azeotropic compositions. A pinch caused when using minimum reflux is always in the nonreactive section of the column. Its location is affected by the extent of reaction occurring elsewhere in the column. This article shows how to encounter such pinch situations by McCabe‐Thiele and Ponchon‐Savarit diagrams. Reactive distillation columns were designed under the extreme conditions of assuming reaction equilibrium and total reflux. Putting more than one reactive stage in such a column can be harmful to column performance for an azeotropic mixture. As a function of the magnitude of the reaction equilibrium constant, when an azeotrope can be circumvented is described. A binary reactive distillation can be bypassed only under finite reflux conditions, with the reaction equilibrium constant being greater than the reaction equilibrium constant evaluated at the azeotropic composition. A rigorous simulation of actual systems is used to confirm the insights provided by these graphical methods.