Showing papers in "Aiche Journal in 2005"

Enterprise‐wide optimization: A new frontier in process systems engineering

Abstract: Enterprise-wide optimization (EWO) is a new emerging area that lies at the interface of chemical engineering and operations research, and has become a major goal in the process industries due to the increasing pressures for remaining competitive in the global marketplace. EWO involves optimizing the operations of supply, manufacturing and distribution activities of a company to reduce costs and inventories. A major focus in EWO is the optimal operation of manufacturing facilities, which often requires the use of nonlinear process models. Major operational items include planning, scheduling, realtime optimization and inventory control. One of the key features of EWO is integration of the information and the decision-making among the various functions that comprise the supply chain of the company. This can be achieved with modern IT tools, which together with the internet, have promoted e-commerce. However, as will be discussed, to fully realize the potential of transactional IT tools, the development of sophisticated deterministic and stochastic linear/nonlinear optimization models and algorithms (analytical IT tools) is needed to explore and analyze alternatives of the supply chain to yield overall optimum economic performance, as well as high levels of customer satisfaction. An additional challenge is the integrated and coordinated decision-making across the various functions in a company (purchasing, manufacturing, distribution, sales), across various geographically distributed organizations (vendors, facilities and markets), and across various levels of decision-making (strategic, tactical and operational). © 2005 American Institute of Chemical Engineers AIChE J, 51: 1846 –1857, 2005

Inertial and Interfacial Effects on Pressure Drop of Taylor Flow in Capillaries

Abstract: In a capillary, the two-phase pressure drop in Taylor slug flow was measured. A carefully designed inlet section for the capillary allowed the independent variation of gas bubble and liquid slug length. Gas and liquid superficial velocities were varied from 0.04 m/s to 0.3 m/s. If the slug length was smaller than 10 times the capillary diameter, the length-averaged friction factor for the liquid slug increased drastically from the single phase value (f = 16/Re) due to differences in curvature at the front and the back of the bubble. The use of different liquids allowed the independent variation of Re and Ca. The flow of elongated bubbles in capillaries was simulated using the CFD code FIDAP. It was found both numerically and experimentally that for Re ≫ 1, the extra pressure terms may be taken account using (Ca/Re) as a parameter. The numerical results agreed with the experimental data, provided that Marangoni effects of impurities are taken into account. The results allow the determination of slug length from pressure drop measurements in closed equipment where the slug length cannot otherwise be measured easily, such as monoliths and microreactors. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Two‐scale continuum model for simulation of wormholes in carbonate acidization

Abstract: A two-scale continuum model is developed to describe transport and reaction mechanisms in reactive dissolution of a porous medium, and used to study wormhole formation during acid stimulation of carbonate cores. The model accounts for pore level physics by coupling local pore-scale phenomena to macroscopic variables (Darcy velocity, pressure and reactant cup-mixing concentration) through structure-property relationships (permeability-porosity, average pore size-porosity, and so on), and the dependence of mass transfer and dispersion coefficients on evolving pore scale variables (average pore size and local Reynolds and Schmidt numbers). The gradients in concentration at the pore level caused by flow, species diffusion and chemical reaction are described using two concentration variables and a local mass-transfer coefficient. Numerical simulations of the model on a two-dimensional (2-D) domain show that the model captures the different types of dissolution patterns observed in the experiments. A qualitative criterion for wormhole formation is developed and it is given by Λ ∼ O(1), where Λ = . Here, keff is the effective volumetric dissolution rate constant, DeT is the transverse dispersion coefficient, and uo is the injection velocity. The model is used to examine the influence of the level of dispersion, the heterogeneities present in the core, reaction kinetics and mass transfer on wormhole formation. The model predictions are favorably compared to laboratory data. © 2005 American Institute of Chemical Engineers AIChE J, 2005

An equation-of-state contribution for polar components : Quadrupolar molecules

Abstract: An equation-of-state (EOS) contribution for quadrupolar interactions is developed based on a third-order perturbation theory. Model constants are adjusted to comprehensive molecular simulation data for vapor–liquid equilibria of the two-center Lennard–Jones (2CLJ) plus pointquadrupole fluid from the literature. Molecular elongations L* from 0 (spherical) to 0.8 are covered by the simulation data. The EOS is suited for both, the 2CLJ fluid and the tangent-sphere Lennard–Jones framework. It is applied to pure components and mixtures of real substances with the perturbed-chain statistical associating fluid theory (PC-SAFT) EOS. It is possible to use tabulated values of quadrupolar moments (independently determined) directly in the EOS, so that no additional pure component parameter is introduced. Compared with the case where quadrupolar interactions are not specifically accounted for, the proposed model gives a systematic improvement of pure component properties and of mixture phase equilibria of strong quadrupolar components, such as carbon dioxide, and the absolute value of the required binary interaction parameter is significantly reduced. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Carbon monoxide clathrate hydrates: Equilibrium data and thermodynamic modeling

Abstract: Carbon monoxide occurs in abundance throughout the cosmos, potentially in clathrate form, whereas on Earth, it forms a notable constituent of industrial flue gases. It has been proposed that hydrate technology could be used in CO2 separation from flue gases, and in subsea flue gas CO2 disposal. This—and the likely widespread occurrence of CO clathrates in the cosmos—means it is important that the phase behavior of CO hydrates is known. Here, we present experimental H-L-V (hydrate–liquid–vapor) equilibrium data for CO, COCO2, and COC3H8 (propane) clathrate hydrates. Data were generated by a reliable step-heating technique validated using measured data for CO2 and CH4 hydrates. Data for CO and COC3H8 clathrates have been used in the optimization of Kihara potential parameters for CO, reported here, facilitating the extension of a thermodynamic model to the prediction of CO hydrate equilibria. Model predictions are validated against independent experimental data for COCO2 (structure I) systems, with good agreement being observed. © 2005 American Institute of Chemical Engineers AIChE J, 2005

A New Fault Diagnosis Method Using Fault Directions in Fisher Discriminant Analysis

Abstract: Multivariate statistical methods such as principal component analysis (PCA) and partial least squares (PLS) have been widely applied to the statistical process monitoring (SPM) of chemical processes and their effectiveness for fault detection is well recognized. These methods make use of normal process data to define a tight normal operation region for monitoring. In practice, however, historical process data are often corrupted with faulty data. In this paper, a new process monitoring method is proposed that is composed of three parts: (1) a preanalysis step that first roughly identifies various clusters in a historical data set and then precisely isolates normal and abnormal data clusters by the k-means clustering method; (2) a fault visualization step that visualizes high-dimensional data in 2-D space by performing global Fisher discriminant analysis (FDA), and (3) a new fault diagnosis method based on fault directions in pairwise FDA. A simulation example is used to demonstrate the performance of the proposed fault diagnosis method. An industrial film process is used to illustrate a realistic scenario for data preanalysis, fault visualization, and fault diagnosis. In both examples, the contribution plots method, based on fault directions in pairwise FDA, shows superior capability for fault diagnosis to the contribution plots method based on PCA. © 2005 American Institute of Chemical Engineers AIChE J, 51: 555–571, 2005

Gas fluidization characteristics of nanoparticle agglomerates

Abstract: An experimental study is conducted to determine the effect of different types of nanoparticles on the gas fluidization characteristics of nanoparticle agglomerates. Taking advantage of the extremely high porosity of the bed, optical techniques are used to visualize the flow behavior, as well as to measure the sizes of the fluidized nanoparticle agglomerates at the bed surface. Upon fluidizing 11 different nanoparticle materials, two types of nanoparticle fluidization behavior, agglomerate particulate fluidization (APF) and agglomerate bubbling fluidization (ABF), are observed and systematically investigated. A simple analytical model is developed to predict the agglomerate sizes for APF nanoparticles, and the results agree fairly well with the optical measurements. Using the Ergun equation, the experimentally measured pressure drop and bed height, and the average agglomerate size and voidage at minimum fluidization predicted by the model, the minimum fluidization velocities for APF nanoparticles are calculated and also agree well with the experimental values. Other important fluidization features such as bed expansion, bed pressure drop, and hysteresis effects, and the effects of the primary particle size and material properties are also described. © 2005 American Institute of Chemical Engineers AIChE J, 51: 426–439, 2005

Mixed conducting ceramic hollow‐fiber membranes for air separation

Abstract: Mixed conducting ceramic hollow-fiber membranes, which possess an asymmetric structure, were prepared by a combined phase inversion and sintering technique where precursors of the hollow fibers were first spun using a polymer solution containing suspended LSCF powders and were then sintered at elevated temperature. By controlling the weight ratio of the LSCF powder to the polymer binder, sintering temperature, and time, the LSCF hollow fibers with gastight properties have been prepared and evaluated using an apparatus developed during the course of this study. Using the gastight LSCF hollow fibers, a membrane module was assembled for air separation. The performances of the module for air separation have been studied under various operating modes and at different temperatures and feed flow rates both experimentally and theoretically. The results reveal that the surface exchange reaction at the downstream side is much more important than that at the upstream side, especially for lower operating temperatures. The porous inner surface of the prepared LSCF hollow-fiber membranes substantially favors the oxygen permeation when air is fed in the shell side of the membrane module. At high operating temperatures, oxygen permeation can be enhanced by the countercurrent flow operation. Vacuum operation favors the oxygen permeation kinetically in the LSCF hollow-fiber membrane modules. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Three‐dimensional simulation of bubble column flows with bubble coalescence and breakup

Abstract: Three-dimensional (3-D) Euler/Euler simulations of two-phase (gas/liquid) transient flow were performed using a multiphase flow algorithm based on the finite-volume method. These numerical simulations cover laboratory-scale bubble columns of different diameters, operated over a range of superficial gas velocities in the churn-turbulent regime (8 to 30 cm/s) and at different operating pressures (up to 1 MPa). The bubble population balance equation (BPBE) is implemented in the two-fluid model (TFM) and algebraic slip mixture model (ASMM). Simulated time-averaged axial liquid velocity, turbulence stress, and gas holdup are compared with experimental data of Chen et al., Ong, and Shaikh et al. Moreover, to ensure the experimentally observed lack of dependency of holdup radial profiles on column height in the fully developed region, coalescence rates in the computations had to be enhanced consistently compared to model-predicted values. Quantitative agreement is then obtained between the experimental data and simulations for the time-averaged gas holdup and axial liquid velocity profiles. However, the simulations significantly underestimate the turbulent stress because the velocity field cannot be fully resolved. Simulated bubble size distributions indicate that the volume fraction of small bubbles is uniform in a cross section, whereas that of the large bubbles peaks in the center of the column. The simulation correctly captures the high-pressure effect, which at fixed gas superficial velocity pushes the bubble column flow toward bubbly flow. The simulation also predicts that, although the small bubble volume fraction may not change with superficial gas velocity in the churn-turbulent flow regime, the bubble size distribution still remains single modal but is wider and contains larger bubble sizes when superficial gas velocity is increased. © 2005 American Institute of Chemical Engineers AIChE J, 51: 696–712, 2005

Air drying of milk droplet under constant and time-dependent conditions

Abstract: Spray drying is the prime process for many years for manufacturing food powders. Dairy powders are one of the main products consumed worldwide. There has been a stream of studies published previously on both modeling the drying characteristics of a single milk droplet and the dryer wide simulations incorporating computational fluid dynamics (CFD). In CFD simulations, large numbers of particles of different sizes need be tracked to represent the size distribution; it is desirable to have an accurate yet simple model for drying of a single droplet, which does not require partial differential equation. Here for the first time, two such models are validated. One model is of the characteristic drying rate curve approach and the other (new) model is of the reaction engineering approach. The model predictions are compared against a very wide range of experimental results including isothermal and time-varying temperature conditions. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Design of reactive distillations for acetic acid esterification

Abstract: The reactive distillation provides an attractive alternative for reaction/separation processes with reversible reactions, especially for etherification and esterification. The discrete nature of chemical species and the complexity of phase equilibria seem to cloud the picture in understanding reactive distillation. The esterifications of acetic acid with five different alcohols, ranging from C1 to C5, are studied. First, qualitative relationships between macroscopic process flowsheet and microscopic phase equilibria are established, and the process flowsheets are classified into type I, II, and III for these five systems. Next, a systematic design procedure is devised to optimize the design, based on the total annual cost (TAC) and dominant design variables are identified for different flowsheets. Once quantitative design is available, process characteristic are analyzed and potential problems in process operation are identified. Finally, the economic potentials of these three different flowsheets are explored and explanations are given. The results clearly indicate that it is possible to systemize the design of reactive distillation by qualitatively generating flowsheet from phase equilibria and by quantitatively completing the process flow diagram from a sequential design procedure. Moreover, some of the flowsheets presented in this work cannot be found elsewhere in the open literature. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Dynamics of drop impact on solid surface: Experiments and VOF simulations

Abstract: The process of spreading/recoiling of a liquid drop after collision with a flat solid surface was experimentally and computationally studied to identify the key issues in spreading of a liquid drop on a solid surface. The long-term objective of this study is to gain an insight in the phenomenon of wetting of solid particles in the trickle-bed reactors. Interaction of a falling liquid drop with a solid surface (impact, spreading, recoiling, and bouncing) was studied using a high-speed digital camera. Experimental data on dynamics of a drop impact on flat surfaces (glass and Teflon) are reported over a range of Reynolds numbers (550-2500) and Weber numbers (2-20). A computational fluid dynamics (CFD) model, based on the volume of fluid (VOF) approach, was used to simulate drop dynamics on the flat surfaces. The experimental results were compared with the CFD simulations. Simulations showed reasonably good agreement with the experimental data. A VOF-based computational model was able to capture key features of the interaction of a liquid drop with solid surfaces. The CFD simulations provide information about finer details of drop interaction with the solid surface. Information about gas-liquid and liquid-solid drag obtained from VOF simulations would be useful for CFD modeling of trickle-bed reactors.

Comparison of the Phase Behavior of Some Selected Binary Systems with Ionic Liquids

Abstract: The phase behavior of binary mixtures consisting of two different supercritical fluids, one with no dipole moment and the other one with a strong dipole moment, and an imidazolium-based ionic liquid were studied experimentally. Carbon dioxide (CO2) and trifluoromethane (CHF3), and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) were the selected supercritical fluids and ionic liquid, respectively. A synthetic method was used to measure liquid–vapor (LV) and liquid–liquid–vapor (LLV) boundaries of these binary systems. Results for the LV boundaries are reported for CO2 concentrations ranging from 10.0 to 65.0 mol % and within temperature and pressure ranges of 293.29–363.54 K and 0.59–73.50 MPa, and for CHF3 concentrations ranging from 10.2 to 99.0 mol % and within temperature and pressure ranges of 303.20–363.42 K and 0.58–41.00 MPa, respectively. The LV boundaries of pure CO2 and CHF3 were measured up to their critical points. For the binary systems consisting of either CO2 or CHF3 with [bmim][PF6], the LLV boundaries were similarly measured up to their critical endpoints. The experimental results obtained in this work show that the binary systems of CO2 or CHF3 + [bmim][PF6] have Type III phase behavior according to the classification of Scott and Van Konynenburg. To the extent available, the experimental data obtained for the system CO2 + [bmim][PF6] were compared with literature data. In addition, comparisons were made with literature data of binary systems of either CO2 or CHF3 with other ionic liquids belonging to the same homologous family. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Computational study of a single-phase flow in packed beds of spheres

Abstract: Packed-bed reactors are widely used in petrochemical, fine chemical, and pharmaceutical industries. Detailed knowledge of interstitial flow in the void space of such packed-bed reactors is essential for understanding the heat and mass transfer characteristics. In this paper, fluid flow through the array of spheres was studied using the unit-cell approach, in which different periodically repeating arrangements of particles such as simple cubical, 1-D rhombohedral, 3-D rhombohedral, and face-centered cubical geometries were considered. Single-phase flow through these geometries was simulated using computational fluid dynamics (CFD). The model was first validated by comparing predicted results with published experimental and computational results. The validated model was further used to study the effect of particle arrangement/orientation on velocity distribution and heat transfer characteristics. The simulated results were also used to understand and to quantify relative contributions of surface drag and form drag in overall resistance to the flow through packed-bed reactors. The model and the results presented here would be useful in elucidating the role of microscopic flow structure on mixing and other transport processes occurring in packed-bed reactors.

Uncatalyzed and wall‐catalyzed forward water–gas shift reaction kinetics

Abstract: The kinetics of the high-temperature (1070–1134 K), low- and high-pressure gas-phase forward water–gas shift reaction (fWGSR) were evaluated in an empty quartz reactor and a quartz reactor packed with quartz particles. The power-law expression for the reaction rate was consistent with the Bradford mechanism and was invariant with respect to pressure. The experimental rate constant was lower than that published by Graven and Long, and slightly higher than estimates obtained using the reaction rate expression derived from the Bradford mechanism in conjunction with values of reaction rate constants obtained from the GRI database. Similar experiments conducted using a reactor composed of Inconel® 600, a representative reactor shell material, exhibited substantially enhanced rates of reaction. A simple power-law rate expression was incorporated into a surface-catalyzed plug flow reactor (PFR) model to correlate the results between 600 and 900 K. Palladium and palladium–copper alloy surfaces, representative of hydrogen membranes, were also shown to enhance the fWGSR rate, but not as much as the Inconel® 600 surfaces. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Studies on strongly swirling flows in the full space of a volute cyclone separator

Abstract: The three-dimensional (3-D) strongly swirling turbulent flows in the full space of a volute cyclone separator was measured using laser Doppler velocimetry (LDV), and was simulated using an improved Reynolds stress equation model by modifying the empirical constants in the isotropization of production and convection model (IPCM) + wall pressure–strain term of the Reynolds stress equation, incorporated into the platform of FLUENT 6.0. Predicted Reynolds stress model (RSM) velocities are more reasonable than those obtained previously. The specific features of turbulent flows in the separation space, dust hopper, annular space, and the outlet tube are different. The results show that the time-averaged tangential velocity profiles in the separation space have a typical Rankine-vortex structure. In some regions, such as the entrance, the vicinity of the top of the annular space, the inner vortex-flow region, the vicinity of the discharge port, the vicinity of the wall, the intersection part between the upward and downward flows, the turbulent intensity is very large and changes sharply; the turbulence is anisotropic in most regions, but the magnitudes of three RSM velocity components are of the same order of magnitude. The distribution of time-averaged tangential velocity is asymmetric in the annular space. The longitudinal secondary vortexes exist near the top of the dust hopper and the top of the cyclone. The distribution of time-averaged axial velocity in the exit tube is entirely different from that in the separation space. © 2005 American Institute of Chemical Engineers AIChE J, 51: 740–749, 2005

Air pollution remediation in a fixed bed photocatalytic reactor coated with TiO2

Abstract: In a previous article, the modeling and experimental verification of the radiation field inside a reactor made of several TiO2 coated, parallel, flat glass fiber meshes, bilaterally UV irradiated was accomplished. The degradation of trichloroethylene (TCE) in an air stream is studied using different values of the pollutant feed concentration, relative humidity and light intensity under operating conditions where kinetic control of the process is established. A kinetic model based on a reaction scheme that involves atomic chlorine as an active reaction intermediate is developed for describing concentration dependencies. It includes, explicitly, the effect of the absorbed light intensity on the rate. The interaction of the existing radiation field with the solid semiconductor to generate electrons, and holes in the reaction catalyst activation step is also modeled. All kinetic parameters are estimated from experiments. The obtained kinetic expression gives a first-order dependence with respect to the TCE concentration and accounts for a competitive effect of water vapor and TCE for the catalyst active sites. An additional important feature of the derived expression is its ability to represent both limiting cases concerning the dependence of the reaction rate with the irradiation rate; that is, order 1 or order 0.5, as well as all possible intermediate values. In fact, the equivalent dependence obtained in this work was 0.6, a value closer to the second case. The results show good agreement between predictions derived from the proposed kinetic expression and TCE experimental concentration data at the reactor's exit. The proposed reactor design provides a practical device for the treatment of contaminated air; it permits a fairly uniform irradiation of the catalytic meshes and a good ratio of the irradiated surface area with respect to the volume of the gaseous reacting mixture. With these features, rather high TCE conversions can be obtained. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Enhanced fluidization of nanoparticles in an oscillating magnetic field

Abstract: Some experimental observations on the fluidization characteristics of nanoparticles in the form of agglomerates with magnetic assistance are presented. The nanoagglomerates consist of Degussa Aerosil® R974 fumed silica, with a primary particle size of 12 nm. An oscillating AC magnetic field is used to excite large (mm size) permanent magnetic particles mixed in with the nanoparticle agglomerates, and the fluidization behavior of the nanoagglomerates, including the fluidization regime, the minimum fluidization velocity, the bed pressure drop, and the bed expansion are investigated. It is shown that, with the aid of an oscillating magnetic field at low frequencies, the bed of nanoparticle agglomerates can be smoothly fluidized, and the minimum fluidization velocity is significantly reduced. In addition, channeling or slugging of the bed disappears and the bed expands uniformly without bubbles, and with negligible elutriation. The bed expansion and the minimum fluidization velocity depend on the mass ratio of magnetic particles to nanoparticles, and the intensity and frequency of the oscillating magnetic field. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Distribution of methanol and catalysts between biodiesel and glycerin phases

Abstract: When transesterifying alcohols with fats and oils to produce biodiesel, the phase behavior and respective distribution of catalyst and alcohol between liquid phases can significantly impact both reaction rates and product workup. To better understand this phase behavior, the distribution of methanol and catalysts, potassium hydroxide and sulfuric acid, between the biodiesel and alcohol phases was experimentally investigated and modeled. Experimental vapor-liquid equilibrium data were modeled by using the Wilson activity coefficient model and two temperature-independent model parameters are used to obtain good agreement between calculated and experimental data. The distribution coefficients of methanol between biodiesel and glycerin phases were accurately predicted by the VLE activity coefficients and Wilson model. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Extended statistical associating fluid theory (SAFT) equations of state for dipolar fluids

Abstract: Polar forces have a considerable effect on the thermodynamic and phase equilibrium properties of pure and mixture fluids. In this work, the statistical associating fluid theory (SAFT) and perturbed chain–SAFT (PC-SAFT) are extended to explicitly account for dipole–dipole interactions. A recently proposed perturbation theory for pure dipolar fluids is incorporated in both models and further extended to mixtures. Polar SAFT (PSAFT) and PC-polar SAFT (PC-PSAFT) are applied to alcohols, ketones, water, and other dipolar fluids. Vapor pressure and saturated liquid densities are correlated over a wide temperature range from low temperature up to very near the critical point. Critical constants, second virial coefficients, and monomer fraction predictions are reported. Furthermore, the models are applied to correlate the vapor–liquid equilibria of binary mixtures. A temperature-independent binary interaction parameter is regressed from experimental data. Finally, model predictions for representative polar ternary mixtures are presented. In all cases, very good agreement with experimental data is obtained. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Solubility of carbon dioxide in aqueous piperazine solutions

Abstract: In the present work, new experimental data are presented on the solubility of carbon dioxide in aqueous piperazine solutions, for concentrations of 0.2 and 0.6 molar piperazine and temperatures of 25, 40, and 70°C respectively. The present data, and other data available in the literature, were correlated using a model based on the electrolyte equation of state (EoS), as originally proposed by Furst and Renon. The final model derived, containing only seven adjustable (ionic) parameters, was able to describe the available experimental solubility data (>150 data points for total and/or CO2 partial pressure) with an average deviation of 16%.

Modeling of multitype particle flow using the kinetic theory approach

Abstract: The effects of particle size and/or density on particle interaction and overall behavior of granular flow were studied using the kinetic theory approach. The kinetic theory for granular flow was extended to mixtures of multitype particles assuming a non-Maxwellian velocity distribution and energy non-equipartition. Each type of particles was considered as a separate phase with different velocity and granular temperature. The resulting momentum equation for each particulate phase includes phase interaction arising from collisional pressure and particle–particle drag force. When applied to simple shear granular flow of a binary mixture, this model predicts well the energy non-equipartition and the stresses of particulate systems with different sizes and densities. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Mass transfer and chemical reactions in bubble swarms with dynamic interfaces

Abstract: Detailed, high-resolution numerical simulations have been performed on the buoyancy-driven motion of deformable, chemically reacting bubbles for different operating conditions, that is, Weber, Morton, and Schmidt numbers. In our simulations different bubble shapes and types of bubble wakes were observed. The wake types range from a closed wake without recirculation, to a closed wake with recirculation, to an unsteady wake, leading to vortex-shedding wakes. Two different bubble-rise trajectories were observed for different conditions: straight and zigzag shaped. The mass-transfer rates and the yields and selectivities of liquid-phase chemical reactions were determined for each case. A detailed analysis of the results was carried out, relating the differences in chemical reaction efficiencies to the dynamics of each flow. Furthermore, to obtain a better understanding of the dynamics of the flows inside bubble swarms and their impact on chemical reactions, numerical simulations were performed of multiple bubbles rising in a swarm. Different bubble counts, geometric configurations, and size distributions were considered. Mass-transfer rates and chemical reaction selectivities were determined and a comparison is presented between the results for bubble swarms and single bubbles. It was shown that for mixing-sensitive reaction networks, the hydrodynamics of the bubble swarm may significantly impact the reaction selectivity. Furthermore, it was demonstrated that bubble swarm dynamics differ from the dynamics of single bubbles. © 2005 American Institute of Chemical Engineers AIChE J, 2005

CFD predictions for flow‐regime transitions in bubble columns

Abstract: This work evaluates the ability of multiphase computational fluid dynamics (CFD) models to predict known flow regimes in air–water bubble columns. An initial grid-resolution study shows that grid spacing of 0.25 cm or smaller must be used for adequate resolution. The ability of the two-fluid model to predict homogeneous- and transitional-flow behavior is analyzed next, and the flow predictions are found to be highly dependent on the model formulation (that is, bubble-induced turbulence, drag, virtual mass, lift, rotation, and strain). At low gas velocities, homogeneous flow is observed for only a particular set of force models. At higher gas velocities, the same set of models yields reasonable predictions of transitional flow for small columns. Bubble size and liquid coflow also affect flow structures and flow stability at high gas flow rates. Scale-up to larger column diameters is studied for both the homogeneous- and transitional-flow regimes. In the homogeneous regime, the flow behavior is found to be independent of column diameter. However, because of neglect of coalescence, transition to churn-turbulent flow is not observed at high gas velocities for large column diameters. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Estimation of the hindered settling function R(ϕ) from batch‐settling tests

Abstract: The hindered settling function R(ϕ) is a material function that quantifies the interphase drag of colloidal suspensions for all solids volume fractions ϕ. A method is presented to estimate R(ϕ) from batch-settling tests for solids volume fractions between the initial solids volume fraction, ϕ0, and the solids volume fraction at which the suspension forms a continuously networked structure, ϕg, known as the gel point. The method is based on an analytic solution of the associated inverse problem. Techniques are presented to address initialization mechanics observed in such tests as well as experimental noise and discrete data. Analysis of synthetic and experimental data suggests that accurate estimates of R(ϕ) are possible in most cases. These results provide scope for characterization of suspension dewaterability from batch-settling tests alone. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Recursive estimation in constrained nonlinear dynamical systems

Abstract: In any modern chemical plant or refinery, process operation and the quality of product depend on the reliability of data used for process monitoring and control. The task of improving the quality of data to be consistent with material and energy balances is called reconciliation. Because chemical processes often operate dynamically in nonlinear regimes, techniques such as extended-Kalman filter (EKF) and nonlinear dynamic data reconciliation (NDDR) have been developed for reconciliation. There are various issues that arise with the use of either of these techniques. EKF cannot handle inequality or equality constraints, whereas the NDDR has high computational cost. Therefore, a more efficient and robust method is required for reconciling process measurements and estimating parameters involved in nonlinear dynamic processes. Two solution techniques are presented: recursive nonlinear dynamic data reconciliation (RNDDR) and a combined predictor–corrector optimization (CPCO) method for efficient state and parameter estimation in nonlinear systems. The proposed approaches combine the efficiency of EKF and the ability of NDDR to handle algebraic inequality and equality constraints. Moreover, the CPCO technique allows deterministic parameter variation, thus relaxing another restriction of EKF where the parameter changes are modeled through a discrete stochastic equation. The proposed techniques are compared against the EKF and the NDDR formulations through simulation studies on a continuous stirred tank reactor and a polymerization reactor. In general, the RNDDR performs as well as the two traditional approaches, whereas the CPCO formulation provides more accurate results than RNDDR at a marginal increase in computational cost. © 2005 American Institute of Chemical Engineers AIChE J, 51: 946–959, 2005

Coarsening of immiscible Co-continuous blends during quiescent annealing

Abstract: The quiescent annealing of four different co-continuous polystyrene/high-density polyethylene blends of widely different viscosity ratios were examined at three different temperatures. In addition, the coarsening of a co-continuous poly(methyl-methacrylate)/high-density polyethylene blend was also studied. Since the morphology of co-continuous systems is very difficult to accurately analyze using microscopic techniques, the pore dimensions of the PS and PMMA phase are characterized, after solvent extraction, using mercury porosimetry. The volume average pore diameter is used in order to track the large pores in the system. A significant coarsening effect, as evidenced by the growth of pore size, is observed. For these uncompatibilized systems a direct relationship between pore size R and annealing time t (R ∼ kt) is observed. Using a conceptual model of co-continuity, based on thin and thick rods, it is proposed that the driving force for the coarsening process is a capillary pressure effect. The differences in capillary pressure throughout the co-continuous structure result in the continuous merging of thin parts toward the thick ones. This process is confirmed through the presence of a large number of extremely thin threads in contact with very thick ones after annealing. In order to understand the factors influencing the coarsening rate we have adapted an approach used for phase separation. The thick rod is treated as a cylindrical thread which cannot breakup via a capillary instability due to the numerous branches which continuously feed it. In such a case it is proposed that the rate of growth of the distortion amplitude, dα/dt, taken from Tomotika's analysis for capillary instabilities, can be directly related to the coarsening rate, dR/dt. Since α0/R0 (the ratio of the initial distortion amplitude to the initial thread radius) is found to be constant for all of the co-continuous systems studied, all of the coarsening rates for the various systems are controlled by the interfacial tension, the zero shear viscosity of the surrounding medium and Ω from Tomotika theory. An excellent correlation of this model is demonstrated for all of the systems studied. These results and the proposed mechanism also indicate that the quiescent coarsening of immiscible co-continuous blends can continue over long time periods while still maintaining co-continuity and, hence, can provide an important route toward morphology control in percolated systems. © 2004 American Institute of Chemical Engineers AIChE J, 51:271–280, 2005

Transport processes and large deformation during baking of bread

Abstract: A model of multiphase transport in a porous medium coupled with large deformation of the porous matrix is developed and applied to the process of bread baking. Transport-governing equations are based on energy conservation and mass conservation of water, water vapor, and CO2 produced during baking. Deformation is caused by the pressure gradient from internal evaporation and CO2 generation. Temperature, moisture, and pressure changes in turn are affected by deformation. Bread is assumed to be viscoelastic, mechanical properties of which are functions of temperature. Geometric nonlinear effects are considered in the mechanics problem. Results are compared with those from baking experiments and literature data. Vapor pressure inside the matrix is likely to be lower than the equilibrium vapor pressure. Convective heat transfer is small compared to heat conduction and evaporation–condensation of water vapor promotes heat transfer to the inside. Rate of CO2 generation, mechanical properties of dough, and gravity together determine the final shape of the bread. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Integrated medical feedback systems for drug delivery

Abstract: Drugs are now administered at frequencies and doses based on averages optimized for large populations. Because the optimal frequency and dose for an individual differs, transiently or permanently, from that of a population's average, the dosing is necessarily suboptimal. Feedback loop-based individualized integrated medical systems, comprising an implanted sensor, battery, amplifier, processor, and actuator are now in use in cardiac pacemakers and defibrillators. Drug-delivering medical feedback loops, comprising miniature sensors and drug pumps, would individualize, and thereby improve the effectiveness and safety of drugs. Their sensor would continuously or frequently monitor the effect of the drug and adjust, through a medical control algorithm, its flow to the minimum necessary for effectiveness, reducing thereby side effects and improving the success rate of experimental drugs. The pace of integration of the drug delivering feedback loops depends on the availability of proven miniature components and of medical control algorithms. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Real-time product morphology monitoring in crystallization using imaging technique

Abstract: Many speciality organic chemical products, such as pharmaceuticals are crystals that exhibit multiple morphological forms and habits that are of critical importance not only to the end use properties of the products, but also to their downstream processing, such as in filtration and handling and in transport and storage. It is known that minor changes in operating conditions, such as cooling rates and supesaturation can have significant impact on the product leading to batch-to-batch variation. As a result, precision manufacture of crystalline products demands on-line techniques for real-time measurement of crystal morphology. The use of a non-invasive on-line imaging technique in a batch reactor for monitoring cooling crystallization of (L)-glutamic acid which exhibits two polymorphs, alpha and beta is described. The technique was found to allow real-time observation of some temporal moments that are critical in the crystallization process, in particular the crystallization onset and transformation between the two polymorphs. For validation and benchmarking purpose, an off-line system for particle characterization, the PharmaVision 830, and photo-microscope were also used in the study in parallel with the on-line imaging system. © 2005 American Institute of Chemical Engineers AIChE J, 2005