Showing papers in "Aiche Journal in 2002"

Fast solvent screening via quantum chemistry: COSMO‐RS approach

Abstract: COSMO-RS, a general and fast methodology for the a priori prediction of thermophysical data of liquids is presented. It is based on cheap unimolecular quantum chemical calculations, which, combined with exact statistical thermodynamics, provide the information necessary for the evaluation of molecular interactions in liquids. COSMO-RS is an alternative to structure interpolating group contribution methods. The method is independent of experimental data and generally applicable. A methodological comparison with group contribution methods is given. The applicability of the COSMO-RS method to the goal of solvent screening is demonstrated at various examples of vapor–liquid-, liquid–liquid-, solid–liquid-equilibria and vapor-pressure predictions.

Bubble‐Size distributions and flow fields in bubble columns

Abstract: Bubble-size distributions and flow fields in bubble columns are calculated numerically. The population balance is simplified and reduced to a balance equation for the average bubble volume. Models developed predict the rate of bubble breakup and coalescence based on physical principles. The flow fields are numerically calculated for bubble columns with cylindrical cross sections using the Euler-Euler method. The newly derived balance equations for the average bubble volumes are implemented into a commercial CFD code. The solutions of the balance equation for high superficial gas velocities result mainly in two fractions: one for the fraction with small and the other for the fraction with large bubble diameters. Both are considered pseudocontinuous phases, in addition to the liquid phase. The calculated flow fields are characterized by several large-scale vortices. The local volume fractions of gas and liquid are locally inhomogeneous and highly time-dependent. The time-averaged flow field is axisymmetric and stationary. The calculated volume fractions, velocities, and bubble-size distributions agree well with existing and previously published experimental results for bubble columns up to 0.3 m in diameter.

Thermodynamics of adsorption in porous materials

Abstract: Thermodynamic equations are developed for adsorption of multicomponent gas mixtures in microporous adsorbents based on the principles of solution thermodynamics. The conventional spreading pressure and surface area variables, which describe 2-D films, must be abandoned for adsorption in micropores, in which spreading pressure cannot be measured experimentally or calculated from intermolecular forces. Adsorption is divided into two steps: (1) isothermal compression of the gas, (2) isothermal immersion of clean adsorbent in the compressed gas. Thermodynamic functions (Gibbs free energy, enthalpy, and entropy) from solution thermodynamics provide a complete thermodynamic description of the system. Applications are described for characterization of adsorbents, gas storage at high pressure, mixture adsorption, enthalpy balances, molecular simulation, adsorption calorimetry, and shape selectivity in catalysis.

Optimal design for flow uniformity in microchannel reactors

Abstract: Velocity and residence time distributions play a crucial role in the performance of microreactors for chemical synthesis. The specific features of fluid flow through multiplate microchannel reactors are examined by an approximate pressure drop model whose validity is confirmed through comparison with more detailed finite-volume calculations. The model results allow for determination of the influence of the geometrical characteristics of the microchannel structures on the flow distributions and are used to optimize the reactor design for maximum flow uniformity.

Absorption of carbon dioxide in aqueous piperazine/methyldiethanolamine

Abstract: Carbon dioxide absorption in 0.6 M piperazine (PZ)/4 M methyldiethanolamine (MDEA) was measured in a wetted wall contactor. The data were simulated using a model that accounts for chemical reactions and transport effects with the eddy diffusivity theory. PZ/MDEA blends absorb CO 2 faster than monoethanolamine (MEA) or diethanolamine (DEA) blends with MDEA at similar concentrations. The reaction of PZ to form a monocarbamate is dominant at low loading ( 50%) only at the top of the absorber.

Silicon array of elongated through-holes for monodisperse emulsion droplets

Abstract: Monodisperse emulsions are of great significance in both scientific and industrial fields, since they have advantages such as their better stability and simplified physical properties. A novel micromachined emulsification device has been developed for monodisperse emulsion droplets. It has uniformly sized through-holes, called straightthrough microchannel (MC), on a silicon microchip. An oblong straightthrough MC successfully yielded monodisperse oil-in-water (O/W) emulsion droplets with an average droplet diameter of 32.5 μm and a coefficient of variation below 2% by forcing the dispersed phase into the continuous phase through the straightthrough MC. On the other hand, a circular straightthrough MC yielded polydisperse emulsion droplets. Straightthrough MC emulsification revealed that an elongated cross-sectional shape in the oblong straightthrough MC contributes significantly to the spontaneous formation of monodisperse emulsion droplets without any turbulent mixing.

Thermal conversion of biomass: Comprehensive reactor and particle modeling

Abstract: Thermal conversion of biomass is often carried out in packed-bed furnaces. Optimization of thermal efficiency and furnace emissions is an important goal, which requires accurate understanding of all physical and chemical effects in the reactor. A combined transient single particle and fuel-bed model is presented. The fuel-bed model is discretized, and a representative particle is chosen and discretized in a radial direction at each grid point. Mass, momentum and energy balances are solved for the entire system. Drying is modeled using an equilibrium approach, and primary pyrolysis is described by independent parallel-reactions. Secondary tar cracking, homogeneous gas reactions, and heterogeneous char reactions are modeled using kinetic data from literature. Simulations validated for single particles agree well with experimental studies. Simulation results for the combustion of a biomass bed are presented for one set of furnace conditions.

Experimental analysis of hydrodynamics in a radially agitated tank

Abstract: A PIV technique is used to analyze the local hydrodynamics generated by a Rushton turbine. Different types of motion coexist in the tank: the mean flow (or global circulation), the periodic fluctuations (or trailing vortices) induced by the blade rotation in the impeller region, and the turbulent fluctuations (that dissipate the kinetic energy). These three kinds of motion can be estimated after experiments as soon as a triple decomposition of the velocity is performed. The mean velocity, the periodically induced stress, and the Reynolds stress are analyzed in the agitated tank, close to the impeller. These data are used for two purposes: to identify and quantify the transfer of kinetic energy between mean flow, periodic flow, and turbulence; and to estimate the dissipation rate of turbulent kinetic energy (TKE) from the balance of TKE, in which each term will be derived from experiments. Characteristics of turbulence are also presented and discussed.

Mapping of sonochemical reactors: Review, analysis, and experimental verification

Abstract: The erratic behavior of cavitational activity exhibited in a sonochemical reactor poses a serious problem in its design and scale-up. Several previous studies in the past dealt with mapping of sonochemical reactors, which have been critically analyzed and recommended for efficient scale-up strategies. There have been no efforts to link the primary effects (local pressure field) of ultrasound activity with the observed secondary effects (such as chemical reaction). In this work an ultrasonic horn (standard immersion-type reactor), and an ultrasonic bath (rectangular geometry with transducers located at the bottom in triangular pitch) reactors were mapped with the help of local pressure measurement (using a hydrophone), and liberated iodine was estimated using the Weissler reaction, and a quantitative relationship was established between the two. In estimating chemical reaction rates, the effect of microscopic variation in the type of microreactor used (test tube in this case) on the extent of degradation was also investigated. Measured local pressure pulses were used in theoretical simulations of bubble dynamics equations to check the type of cavitation taking place locally, and to estimate the possible collapse of the pressure pulse in terms of the maximum bubble size reached during the cavitation phenomena. A relationship also was established between observed iodine liberation rates and the maximum bubble size reached. The engineers can easily use these unique relationships in an efficient design, since the secondary effect can be directly quantified.

Constrained process monitoring: Moving‐horizon approach

Abstract: Moving-horizon estimation (MHE) is an optimization-based strategy for process monitoring and state estimation. One may view MHE as an extension for Kalman filtering for constrained and nonlinear processes. MHE, therefore, subsumes both Kalman and extended Kalman filtering. In addition, MHE allows one to include constraints in the estimation problem. One can significantly improve the quality of state estimates for certain problems by incorporating prior knowledge in the form of inequality constraints. Inequality constraints provide a flexible tool for complementing process knowledge. One also may use inequality constraints as a strategy for model simplification. The ability to include constraints and nonlinear dynamics is what distinguishes MHE from other estimation strategies. Both the practical and theoretical issues related to MHE are discussed. Using a series of example monitoring problems, the practical advantages of MHE are illustrated by demonstrating how the addition of constraints can improve and simplify the process monitoring problem.

Measuring Asphaltenes and Resins, and Dipole Moment in Petroleum Fluids

Abstract: A petroleum fluid can be divided into three types of species: asphaltenes, resins, and oils. Asphaltenes and resins are polar, while the rest of the so-called oils are either nonpolar or mildly polar. The interaction among these species strongly affect asphaltene precipitation from petroleum fluids. Different measuring methods for asphaltenes in a petroleum fluid give similar results, but different results for the resin content of a petroleum fluid. In addition to the amount affecting precipitation, the polarity of asphaltenes and resins affects precipitation strongly. The Onsager formulation of dipolar moments was used to measure the dipole moment of asphaltenes, resins and the oil species from eight different petroleum fluids from various parts of the world. The dipole moment, a measure of polarity, for resins was measured in this work for the first time. Results showed that resin separated from a petroleum fluid by propane is part of the total resin. Adsorption methods, however, give the total amount of resins. For a given petroleum fluid, asphaltenes had a higher dipole moment than resins. However, resins from one petroleum fluid can have a higher dipole moment than asphaltenes from another petroleum fluid.

Multiobjective optimization of SMB and varicol process for chiral separation

Abstract: The multiobjective optimization of continuous countercurrent chromatography separation units, such as simulated moving bed (SMB) and Varicol, is considered. The Varicol system is based on a nonsynchronous shift of the inlet and outlet ports instead of the synchronous one used in the SMB technology. The optimization problem is complicated by the relative large number of decision variables, including continuous variables, such as flow rates and lengths, as well as discontinuous ones, such as column number and configuration. It is also important to reformulate the optimization problem as multiobjective, since the factors affecting the cost of a given separation process are multiple and often in conflict with each other. A typical example is simultaneous maximization of the productivity of the process and the purity of the corresponding products. A new optimization procedure based on a genetic algorithm allows handling these complex optimization problems. An existing literature chiral separation model was used to illustrate the potential of this optimization procedure. This work also offered a unique opportunity to compare the optimal separation performance achievable with the SMB and Varicol technologies.

On the theory of optimal sensor placement

Abstract: On the Theory of Optimal Sensor Placement An optimal sensor placement is defined as a sensor configuration that achieves the minimum capital cost while observing prespecified performance criteria. Previous formulations of this problem have resulted in the definition of a mixed-integer nonlinear program (MINLP) with dimensions dependent on the value of the integer decision variables. The main contribution of this work is an equivalent reformulation of the design problem such that the dimension of the NLP is independent of all decision variables. Additionally, the traditional sensor-placement problem, based on static process conditions, is extended to linear dynamic processes. The final contribution is the exact conversion of the general NLP into a convex program through the use of linear matrix inequalities. The aggregation of these results show that the sensor-placement problem can be solved globally and eficiently using standard interior-point and branch-and-bound search algorithms.

Population Balance Modeling of Aerated Stirred Vessels Based on CFD

Abstract: A developed model predicts local gas fraction and bubble-size distributions for turbulent gas dispersion in a stirred vessel, based on the population balance equations (PBE), with relations taken from literature for bubble coalescence and breakup derived from isotropic turbulence theory. The transport of bubbles throughout the vessel is simulated by a scaled single-phase flow field obtained by CFD simulations. Model predictions for the gas fractions in pseudoplastic Xanthan solutions are compared with local measurements and agree well qualitatively. This formulation overcomes the necessity of choosing a constant bubble size throughout the domain, as is done in two-fluid models and is, therefore, more reliable in mass-transfer calculations.

Statistical process monitoring based on dissimilarity of process data

Abstract: Multivariate statistical process control (MSPC) has been widely used for monitoring chemical processes with highly correlated variables. In this work, a novel statistical process monitoring method is proposed based on the idea that a change of operating condition can be detected by monitoring a distribution of process data, which reflects the corresponding operating conditions. To quantitatively evaluate the difference between two data sets, a dissimilarity index is introduced. The monitoring performance of the proposed method, referred to as DISSIM, and that of the conventional MSPC method are compared with their applications to simulated data collected from a simple 2 × 2 process and the Tennessee Eastman process. The results clearly show that the monitoring performance of DISSIM, especially dynamic DISSIM, is considerably better than that of the conventional MSPC method when a time-window size is appropriately selected.

COSMOSPACE: Alternative to conventional activity‐coefficient models

Abstract: An analytical solution, COSMOSPACE, to the statistical thermodynamics of a model of pairwise interacting surfaces, is presented. This solution was initially developed for the a priori prediction model COSMO-RS in an implicit form. A comparison of COSMOSPACE with UNIQUAC and with the quasi-chemical theory of Guggenheim reveals the conditions under which the models yield similar results and when they differ very considerably. It is shown that COSMOSPACE is in extremely good agreement with Monte-Carlo simulations for some lattice fluids (where UNIQUAC is particularly poor). The ability of COSMOSPACE to provide good fits to experimental data is shown for three binary mixtures including ethanol-cyclohexane, where UNIQUAC incorrectly predicts a liquid-liquid phase separation.

Modeling intra- and extra-particle processes of wood fast pyrolysis

Abstract: A detailed single-particle model, including a description of transport phenomena and a global reaction mechanism, is coupled with a plug-flow assumption for extraparticle processes of tar cracking, in order to predict the fast pyrolysis of wood in fluid-bed reactors for liquid-fuel production. Good agreement is obtained between predictions and measurements of product yields (liquids, char, and gases) as functions of temperature. Particle dynamics are very affected by the convective transport of volatile products. The average heating rates are on the order of 450–455 K/s, whereas reaction temperatures vary between 770 and 640 K (particle sizes of 0.1–6 mm and a reactor temperature of 800 K). The effects of several factors, such as size, shape, and shrinkage of wood particles, and external heat-transfer conditions are also examined.

Pore-scale modeling of fluid transport in disordered fibrous materials

Abstract: The modeling of fluid transport in fibrous materials is important for many applications. Most models operate at the continuum level, which requires an a priori knowledge of spatially averaged transport parameters. Alternatively, highly detailed models, in which the momentum equations are solved directly, require major simplifying assumptions. Thus, it is desirable to use intermediate-level techniques that model transport using first principles, but that are appropriate for real engineering processes. In this work, pore-scale network modeling is adapted for fibrous materials and tested for a large range of fibrous structures and solid volume fractions. A novel technique is used to generate prototype network structures from Voronoi diagrams. The Voronoi networks are coupled with two different multiphase flow algorithms, enabling the modeling of various displacement processes relevant to engineering. Permeability predictions agree well with known values. Effects of dynamics, wettability, and material structure on displacement were studied. This modeling technique not only allows for better quantification of how microscale properties affect macroscopic transport, but helps reduce the number of experiments required to predict continuum transport parameters for various materials and processes.

Effect of Pore Structure, Randomness and Size on Effective Mass Diffusivity

Abstract: m coatings is analyzed using 2-D network models of connecting arms anderified experi- mentally for a multiple pore length scale coating layer. The network model includes ( effects ofariation in the lattice randomness Voronoi tessellation in the form of Delau- )( ) nay lattice triangulation , pore coordination number, pore size Knudsen effect , and : pore-size distribution on the predicted D. The effect of pore poisoning, resulting in a m pore blockage, is analyzed. Correlations for the porosity and pore-blockage dependency : () of D , as well as relationships for the pore size low-dimensionality and multiple m pore length scale effects, are also discussed. An experiment performed on a catalytic () conerter washcoat segment represented by three pore length scales placed on an oth- erwise impermeable wall of an electrochemical sensor shows a good agreement with the : predicted D based on a multiple pore length scale medium with parallel diffusion m paths.

Gas and solids mixing in a turbulent fluidized bed

Abstract: Gas and solids mixing characteristics in the bubbling and turbulent regimes of a gas-solid fluidized bed are examined using helium and phosphor tracer techniques to obtain the gas and solids dispersion coefficients, respectively. The real time, quasi-3-D flow behavior is qualified and quantified by the electrical capacitance tomography (ECT) technique. The mixing behavior varies significantly with the flow regimes. As the gas velocity increases, axial and radial gas dispersion coefficients increase in the bubbling regime, reach a peak at U c , the transition velocity from the bubbling to turbulent regimes, and then decrease. In contrast, as the gas velocity increases, axial and radial solids dispersion coefficients increase in the bubbling regime and continuously increase through U c to the turbulent regime. Temperature and pressure have little effect on the gas and solids mixing behavior. A small quantity of fine particles is noted to drastically affect the gas and solids mixing behavior in the turbulent fluidized bed. The axial and radial dispersion coefficients of gas and solids reach their maximum when the fines content is about 15%. The ECT measurements illustrate the effect of adding fines on the flow behavior in the bed. A small quantity of fine particles disintegrate the bubble/void phase, significantly modifying bubble/void- and emulsion-phase mixing behavior.

Miniature fuel cells fabricated on silicon substrates

Abstract: Novel miniature fuel cells were fabricated from micromachined silicon wafers. The cells used methanol and air as reactants, and a thin polymer electrolyte as separator. The assembled cells had a working volume of 12 mm3 and could be scaled down in size by three orders of magnitude by simple adjustments of the masking and etching procedures. Electrodeposition of Pt-Ru as the anode catalyst (oxidation of methanol) was successful in lowering the loading to 0.25 mg/cm2 without loss of performance. Cell performance approached that of the best, state-of-the-art, large fuel cells, when scaled for size. In particular, single miniature cells yielded 822 Wh/kg and 924 Wh/L when operated at 70°C. The same chip design was also used for the hydrogen/air system, and the cell current, power, and specific energy density were higher than those of methanol/air, Further tailoring of the chips for spefici fuels could lead to further improvements.

Linearized transport model for nanofiltration: Development and assessment

Abstract: Finite difference linearization of pore concentration gradient in nanofiltration membranes greatly simplifies the solution of a three-parameter model (pore radius, membrane charge, and pore dielectric constant) for electrolyte rejection by removing the requirement for numerical integration of the extended Nernst–Planck equation. The validity of the linearized model is first experimentally tested by comparing with a rigorous characterization of the Desal-DK nanofiltration membrane, the linearized model closely agreeing with the numerical solution of the full model. Investigation of pore concentration profiles showed the assumption of linearity to be valid over a wide range of nanofiltration conditions. The linearized model was also successfully extended to ternary electrolyte mixtures, highlighting its main advantage over analytic solutions. Overall, the model is a powerful tool for characterization of nanofiltration membranes and subsequent prediction of separation performance. Computational demands are modest in terms of time and complexity.

Protein nanoparticles formation by supercritical antisolvent with enhanced mass transfer

Abstract: In recent years, the supercritical antisolvent (SAS) precipitation technique has emerged as a promising method for the formation of fine particles. Despite its numerous advantages, this technique still cannot be used to produce particles in the sub-micron range (<300 nm) for many “soft” materials. A significantly improved SAS process can produce particles of controllable size, up to an order of magnitude smaller than those of the conventional SAS process, with a narrower size distribution. Like the conventional SAS technique, this new supercritical antisolvent with enhanced mass transfer technique utilizes supercritical carbon dioxide as the antisolvent, but the solution jet is deflected by a surface vibrating at an ultrasonic frequency atomizing the jet into much smaller droplets. Furthermore, the ultrasound field generated by the vibrating surface enhances mass transfer and prevents agglomeration through increased mixing. The particle size is controlled by varying the vibration intensity of the deflecting surface, which then can be adjusted by changing the power supplied to the attached ultrasound transducer. It is demonstrated by the formation of lysozyme nanoparticles and microparticles. The biological activity of the protein is retained during the processing.

Thermodynamic equilibrium of organic-electrolyte mixtures in aerosol particles

Abstract: This thermodynamic model describes the phase equilibria of mixtures of electrolytes and organic species in aqueous solutions existing as aerosol particles. The activity coefficient of each species in solution is explicitly related to the chemical composition by treating the (inorganic) ion–water, organic–water and ion-organic interactions with a combined Pitzer–UNIFAC thermodynamic approach. It was parameterized with a new type of multifunctional “meta-group” to better represent measured properties of long-chain monofunctional compounds and short-chain multifunctional compounds. Interactions between dissolved electrolytes and organic species are modeled using the “salting-out” effect of NaCl with organic compounds to predict the hygroscopic growth of particles composed of a salt and diacid. The predicted growth agrees well with laboratory measurements. The presence of 50% malonic acid decreases the growth of pure (NH4)2SO4 by 20% at high relative humidities, while mixtures with 50% succinic acid and 50% glutaric acid cause decreases of 35% and 38%, respectively. The mixing of organic compounds with solubility higher than 4 mol·L−1 with salt can decrease the observed DRH. The mixture of malonic acid and (NH4)2SO4 is predicted to start taking up water at 58%, much lower than the DRH of pure (NH4)2SO4 (80%). Insoluble compounds do not change the observed DRH, while reducing the amount of water taken up. The predicted water contents for internal and external mixtures are largely similar, with small differences predicted for mixtures of soluble organic species. The largest deviation of 10% between the water contents of internal and external mixtures occurs for 50% malonic acid with 50% (NH4)2SO4. For less soluble compounds such as succinic acid and glutaric acid, the two types of growth generally agree within 3%.

Mechanisms of Mixing and Creation of Structure in Laminar Stirred Tanks

Abstract: Although stirred tanks have been the most commonly used fluid mixing devices since the beginnings of the industrial era, little is known about how and why they mix, and, therefore, about how to improve their performance, particularly when operated in the laminar regime. In laminar conditions, chaos is the only route to achieve efficient mixing. From the body of research in 2-D chaotic flows, very minute perturbations in the velocity field can lead to widespread chaos and substantial enhancement of mixing performance, but these observations have not been systematically applied to industrially relevant 3-D flows. Using planar laser induced fluorescence and direct numerical simulations, the mechanisms of creation and evolution of mixing structures were studied in Newtonian and non-Newtonian flows. The observed dynamical behaviors are related to geometric features of the system. In Newtonian flow systems, the passing of the impeller blades triggers the onset of chaos by introducing small perturbations to the underlying regular 2-D flow that is observed when impellers are substituted with discs. In non-Newtonian viscoelastic systems, nonlinearity in the stress field introduced by the fluid rheology drives the system to spontaneous chaos even when concentric discs are used to stir the fluid.

CFD of multiphase flow in packed‐bed reactors: I. k‐Fluid modeling issues

Abstract: The Eulerian k-fluid CFD model was used to simulate the macroscale multiphase flow in packed beds. The geometric complexity of the bed structure is resolved by statistically describing the porosity distribution. The complicated multiphase interactions are computed using the Ergun type of formula developed based on bench-scale hydrodynamic experiments. The work is presented in two sequential articles. Part I discusses implementation issues of the k-fluid CFD model for packed beds. The drag exchange coefficients are obtained from the model of Holub et al. for the particle-fluid interfaces Xks and from the model of Attou et al. (1999) for the gas–liquid interface, Xgl. The effect of particle external wetting on flow distribution was incorporated into the model through the capillary pressure evaluated by either the J-function of Leverett (1941) for air–water or by the expression of Attou and Ferschneider (1999) for other fluids. In the framework of CFDLIB, the choice of the grid size and boundary conditions are discussed. An appropriate relationship between the section size and variance of the sectional porosity distribution was used for flow simulation. Part II discusses the extensive numerical results, and the CFD model is compared with experimental data in the literature.

Pattern Matching in Historical Data

Abstract: For many engineering and business problems, it would be very useful to have a general strategy for pattern matching in large databases. For example, the analysis of an abnormal plant condition would benefit if previous occurrences of the abnormal condition could be located in the historical data. A new pattern-matching strategy is proposed for multivariate time series based on statistical techniques, especially principal-component analysis (PCA). The new approach is both data-driven and unsupervised because neither training data nor a process model is required. Given an arbitrary set of multivariate data, the new approach can be used to locate similar patterns in a large historical database. The proposed pattern-matching strategy is based on two similarity factors: the standard PCA similarity factor and a new similarity factor that characterizes the pattern of alarm violations. An extensive simulation study for a chemical reactor demonstrates that this strategy is more effective than existing PCA methods and can successfully distinguish between 28 different operating conditions.