Showing papers in "Aiche Journal in 2000"

Two-phase flow model of the cathode of PEM fuel cells using interdigitated flow fields

Abstract: 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.

Formation and Aging of Incipient Thin Film Wax-Oil Gels

Abstract: A fundamental study of the deposition and aging of a thin incipient wax-oil gel that is formed during the flow of waxy oils in cooled pipes was performed. The solubility of high molecular weight paraffins in naphthenic, aromatic or paraffinic solvents is very low and decreases rapidly with decreasing temperature. This property of the paraffins leads to the formation of gels of complex morphology that deposit on the cold walls of the subsea pipelines during the flow of waxy crudes. This deposition reduces the pipe diameter and decreases the flow capacity of the pipe. These wax-oil gels contain a large fraction of oil trapped in a 3-D network structure of the wax crystals that behaves as a porous medium. After the incipient gel is formed, wax molecules continue to diffuse into this structure, thereby increasing its wax content. A model system of wax and oil mixture was used to understand the aging process of the wax-oil gels, which hardens the wax deposit with time. To understand the physics of the aging process for incipient thin-film deposits, a series of laboratory flow loop experiments was performed. The aging process was a counterdiffusion phenomenon with a critical carbon number above which wax molecules diffuse into the gel deposit and below which oil molecules diffuse out of the deposit. The aging rate of the gel deposit depends on the oil flow rate and the wall temperature. A mathematical model developed predicted the growth and wax content of the gel deposit on externally cooled pipe walls. The theory agreed with experiments excellently for thin gels.

Equations of state for the calculation of fluid-phase equilibria

Abstract: Progress in developing equations of state for the calculation of fluid-phase equilibria is reviewed. There are many alternative equations of state capable of calculating the phase equilibria of a diverse range of fluids. A wide range of equations of state from cubic equations for simple molecules to theoretically-based equations for molecular chains is considered. An overview is also given of work on mixing rules that are used to apply equations of state to mixtures. Historically, the development of equations of state has been largely empirical. However, equations of state are being formulated increasingly with the benefit of greater theoretical insights. It is now quite common to use molecular simulation data to test the theoretical basis of equations of state. Many of these theoretically-based equations are capable of providing reliable calculations, particularly for large molecules.

Robust H∞ glucose control in diabetes using a physiological model

Abstract: A robust H∞ controller was developed to deliver insulin via a mechanical pump in Type I diabetic patients. A fundamental nonlinear diabetic patient model was linearized and then reduced to a third-order linear form for controller synthesis. Uncertainty in the nonlinear model was characterized by up to ± 40% variation in eight physiological parameters. A sensitivity analysis identified the three-parameter set having the most significant effect on glucose and insulin dynamics over the frequency range of interest ω = [0.002, 0.2] (rad/min). This uncertainty was represented in the frequency domain and incorporated in the controller design. Controller performance was assessed in terms of its ability to track a normoglycemic set point (81.1 mg/dL) in response to a 50 g meal disturbance. In the nominal continuous-time case, the controller maintained glucose concentrations within ± 3.3 mg/dL of set point. A controller tuned to accommodate uncertainty yielded a maximum deviation of 17.6 mg/dL for the worst-case parameter variation.

Bubble-column and airlift photobioreactors for algal culture

Abstract: Bubble columns and airlift photobioreactors can be useful for culturing phototrophic organisms requiring light as a nutrient. Light availability in bubble columns and airlift devices is influenced by aeration rate, gas holdup, and the liquid velocity (mixing and turbulence). The photosynthetically generated oxygen also needs to be removed, as excessive dissolved oxygen suppresses photosynthesis. Oxygen removal capacity is governed by the magnitude of the overall gas–liquid mass-transfer coefficient, kLaL. This work characterizes the relevant hydrodynamic and mass-transfer parameters in three air-agitated reactors: bubble column, split-cylinder airlift device and concentric draft-tube sparged airlift vessel. The reactors are then evaluated for culture of the microalga Phaeodactylum tricornutum. All reactors were about 0.06 m3 in working volume, and the working aspect ratio was about 10. Data were obtained in tap water for a base-line comparison and in Mediterranean seawater, as a potential medium for algal culture. A theoretical relationship was developed and proved between kLaL and the aeration rate. In addition, a method based on mechanistic relationships was proved for predicting the liquid circulation velocity and kLaL in airlift reactors. Existing correlations applied satisfactorily to gas holdup and kLaL data obtained in the bubble column. Aqueous solution of sodium chloride (0.15 M) closely resembled seawater in terms of its hydrodynamics and oxygen transfer behavior. Under the conditions tested, all three reactors attained a biomass concentration of about 4 kg·m−3 after ∼260 h. The mean maximum specific growth rate was 0.022 h−1 in all cases at a power input of 109 W·m−3.

Research challenges in process systems engineering

Abstract: Introduction Companies must design and operate chemical processes effectively and efficiently so they may survive in today’s highly competitive world. Providing the methods, tools and people that allow industry to meet its needs by tying science to engineering is a compelling aspect of Process Systems Engineering (PSE). Despite the importance of the PSE, the scope and research of this area are often not well understood. One of the major reasons is that chemical engineering has evolved over the past five decades from being an engineering discipline rooted in the concept of unit operations to one based on engineering science and mathematics, and most recently to one with increasing ties to the natural sciences. This very significant change in emphasis has created a gap between the science-based and the systems-based research in chemical engineering. We argue here that this gap might be closed in two ways: first, by broadening the definition of Process Systems Engineering through the introduction of the concept of the “chemical supply chain,” and second, by gaining a better appreciation of the intellectual research challenges in this area. We address this last issue by discussing the nature and major accomplishments of the PSE area and outlining emerging research directions.

Global optimization of mixed‐integer nonlinear problems

Abstract: Two novel deterministic global optimization algorithms for nonconvex mixed-integer problems (MINLPs) are proposed, using the advances of the αBB algorithm for nonconvex NLPs of Adjiman et al. The special structure mixed-integer αBB algorithm (SMIN-αBB) addresses problems with nonconvexities in the continuous variables and linear and mixed-bilinear participation of the binary variables. The general structure mixed-integer αBB algorithm (GMIN-αBB) is applicable to a very general class of problems for which the continuous relaxation is twice continuously differentiable. Both algorithms are developed using the concepts of branch-and-bound, but they differ in their approach to each of the required steps. The SMIN-αBB algorithm is based on the convex underestimation of the continuous functions, while the GMIN-αBB algorithm is centered around the convex relaxation of the entire problem. Both algorithms rely on optimization or interval-based variable-bound updates to enhance efficiency. A series of medium-size engineering applications demonstrates the performance of the algorithms. Finally, a comparison of the two algorithms on the same problems highlights the value of algorithms that can handle binary or integer variables without reformulation.

Modeling crystal shapes of organic materials grown from solution

Abstract: The shape of a crystalline organic solid has a major impact on its downstream processing and on its end-product quality, issues that are becoming increasingly important in the specialty and fine chemical, as well as the pharmaceutical and life science, industries. Though it is widely known that improved crystal shapes can be achieved by varying the conditions of crystallization (such as solvent type and impurity levels), there is far less understanding of how to effect such a change. Until recently, most methods for predicting crystal shapes were based exclusively on the internal crystal structure, and hence could not account for solvent or impurity effects. New approaches, however, offer the possibility of accurately predicting the effects of solvents. Models for predicting crystal shape are reviewed, as well as their utility for process and product design.

Riser hydrodynamics: Simulation using kinetic theory

Abstract: Computational fluid dynamics (CFD) for fluidization is reaching maturity (Roco, 1998). It has become common practice to compare time-averaged solid velocities and concentrations to measurements of fluxes and densities. The dynamic behavior of the riser, however, has not been previously compared to experiments. This article shows that the dynamics of solids flow in the riser is in the form of clusters, but the time-averaged particle concentrations and fluxes give us the core-annular flow regime in agreement with measurements. The computed clusters, which are essentially compressible gravity waves, produce major frequencies of density oscillations in agreement with measurements. The model and the CFD code compute granular temperature distributions, agreeing qualitatively with data. For volume fraction around 3–4%, which is the average particle concentration in the riser, the computed viscosity agrees with our experimental measurements.

Pyrolysis of sawdust in a conical spouted‐bed reactor with a HZSM‐5 catalyst

Abstract: The effect was studied of using an in-situ catalyst based on a HZSM-5 zeolite in flash pyrolysis with an inert gas (N{sub 2}) of pinus insignis sawdust in a conical spouted-bed reactor in the 400--500 C range and for a gas residence time of 50 ms. The use of the catalyst increases the yield of gases and decreases the yields of liquid and char. Likewise, the yield of CO{sub 2} decreases, whereas the yield of C{sub 4{minus}} hydrocarbons increases (15.9 wt. % at 450 C). The catalyst is efficient for partial deoxygenation of the liquid product.

Pressure drop measurements in a microchannel

Abstract: Recent developments in micro-energy and micro-chemical systems have produced a need for greater understanding of flow in small channels. Several recent studies of friction factors and transition Reynolds numbers in rectangular microchannels have produced results that differ from classical theory. In this work, friction factors and laminar flow friction constants were determined for water flowing in high aspect ratio channels with depths ranging from 128 to 521 μm. Reynolds numbers were between 60 and 3,450. Pressure drops were measured within the channel itself to exclude entrance and exit losses. Transitions to turbulence were observed with flow visualization. Uncertainties in measured variables were quantified and propagated into the estimated friction constants. Friction factors were also determined in a 1,050- μm-deep channel that served as a control. After considering experimental uncertainties and systematic errors, significant differences remained between the results and classical theory

Glycosaminoglycan deposition in engineered cartilage: Experiments and mathematical model

Abstract: Functional cartilaginous constructs for scientific research and eventual tissue repair were cultivated in bioreactors starting from chondrocytes immobilized on polymeric scaffolds. The scaffolds gradually degraded as the cells regenerated tissue matrix consisting of glycosaminoglycan (GAG) and type II collagen. To facilitate data interpretation and optimize cultivation conditions, a mathematical model was developed which yields the concentrations of oxygen and GAG as functions of time and position in growing tissue. Calculated GAG concentrations were qualitatively and quantitatively consistent with profiles measured via high-resolution image processing of tissue samples cultured at two different oxygen tensions for various periods of time.

Modeling the hydrodynamics of multiphase flow reactors: Current status and challenges

Abstract: ew things are more central to chemical engineering than multiphase flow chemical reactors; they are used in industry to produce a variety of chemicals, where economy of scale remains the driving factor. The engineering issues are classical, and the modern trend towards miniaturization will have little impact on these systems. Multiphase reactors for classic applications such as fluid cat- alytic cracking are still evolving. New processes such as slurry bubble column reactors for gas conversion are under development. Reliable multiphase reactor models that can be used with confi- dence for improving existing processes and scale-up of new Coarse-grid simulation of macro-scale flow structures Macro-scale coherent structures in multiphase flow, particularly those involving vortical motion, can be captured only through three-dimensional (3-D) transient simulations. A 3-D simulation of two-phase flow using, say, a million nodes, involves the solution of over nine million nonlinear equations at each time step; the size of the problem becomes much larger when energy and species bal- ances are included. Simulations of such magnitude are not com- mon in process design today, but will become routine in the not- too-distant future. As we shall see, even with this many nodes, the grid structure in simulations of commercial-scale reactors remains coarse; so, the model equations used to simulate commercial reac- tor performance must represent averages at this coarse-grid scale. The drag and stresses influencing the hydrodynamics and effective dispersion coefficients and reaction rates appearing in the energy and species balance equations are affected significantly by the sub- grid scale fluctuations and are grid-size dependent. Yet, this depen- dence is just beginning to be appreciated. Experiments, both labo- ratory and computational, and theory that will lead to accurate scale-dependent closures for these quantities represent important frontiers in this class of problems. The coarse-grid simulation of multiphase flow is conceptually similar to large eddy simulation of single-phase turbulent flow, where one accounts for the effects of unresolved eddies through sub-grid models (Fox, 1996). In single-phase turbulent flow, the flow of energy associated with fluctuations is predominantly from large scale to smaller ones. In the class of multiphase flows consid- ered here, meso-scale structures arise as a result of local instabilities and grow into larger and larger scales, so macro-scale shear is not a requirement for creating and sustaining a chaotic state of flow (unlike in single-phase flow). Because of this difference, we cannot simply adopt the ideas developed in single-phase flow. In this arti- cle, we examine what is known generically about flow structures at different scales in gas-solid, gas-liquid, and gas-liquid-solid sys- tems, and point out where challenges lie in building the hydrody- namic components of the next generation of reactor models.

Engineering design methods for cavitation reactors II: Hydrodynamic cavitation

Abstract: The bubble behavior and hence the pressure generated at the collapse of the cavity for hydrodynamic cavitation depends on the operating conditions and geometry of the mechanical constriction generating cavitation. The effect of operating parameters such as inlet pressure through the system's orifice, initial cavity size, and the indirect effect of the hole diameter (it affects the frequency of turbulence in the vicinity of the orifice) on the bubble behavior was numerically studied. The bubble dynamics were simulated in two stages considering: Rayleigh-Plesset equation up to the point of bubble wall velocity = 1,500 m/s; then the compressibility of the medium using the equation of Tomita and Shima. An empirical correlation was developed to predict the collapse pressure generated as a function of just mentioned parameters. The trends in the magnitudes of collapse pressure match the observed experimental trends for cavitation-induced reactions. The work is an extension of the earlier analysis done for the sonochemical reactors. Some recommendations are also suggested for the design of hydrodynamic cavitation reactors based on the simulations.

Simulation of vortex core precession in a reverse-flow cyclone

Abstract: s damping functions, was applied. The 3-D, aerage flow field was predicted with a high leel of accuracy. Furthermore, the simulations exhibitortex-core precession, that is, the core of the mainortex is obsered to moe about the geometrical axis of the cyclone in a quasi-periodic manner. The Strouhal number associated with the simulated ¤ortex-core precession was 0.53, whereas 0.49 was experimentally obsered in a similar geometry at approximately the same Reynolds number.

Separation of light gas mixtures using SAPO-34 membranes

Abstract: Continuous SAPO-34 membranes were prepared on porous alumina tubular supports, and shown to be useful for light gas separations at low and high temperatures. Single-gas permeances of CO2, N2 and CH4 decreased with increasing kinetic diameter. For the best membrane at 300 K, the He and H2 permeances were less than that of CO2, because He, H2, and CO2 were small compared to the SAPO-34 pore, and differences in the heat of adsorption determined the permeance order. The smaller component permeated the fastest in CO2/CH4, CO2/N2, N2/CH4, H2/CH4 and H2/N2 mixtures between 300 and 470 K. For H2/CO2 mixtures, which were separated by competitive adsorption at room temperature, the larger component permeated faster below 400 K. The CO2/CH4 selectivity at room temperature was 36 and decreased with temperature. The H2/CH4 mixture selectivity was 8 and constant with temperature up to 480 K. Calcination, slow temperature cycles, and exposure to water vapor had no permanent effect on membrane performance, but temperature changes of approximately 30 K/min decreased the membrane's effectiveness.

Low-flow limit in slot coating: Theory and experiments

Abstract: 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.

Coalescence of deformable granules in wet granulation processes

Abstract: In this work coalescence of deformable granules in wet granulation processes is modelled. The model accounts for both the mechanical properties of the granules and the effect of the liquid layer at the granule surface. It is an extension to the model of Ennis ct al. (1991) to include the possibility of grannule plastic deformation during collisions. The model is written in dimensionless groups such as viscous and deformation Stokes numbers and the ratio of granule dynamic yield strength to granule Young's modulus (Y-d/E*). These variables are bulk parameters of the powder-binder mixture and also functions of the process intensify. The model glues the conditions for two types of coalescence - type I and type II. Type I coalescence occurs when granules coalesce by viscous dissipation in the surface liquid layer before their surfaces touch. Type II coalescence occurs wizen granules are slowed to a halt during rebound after their surfaces have made contact. The model explains some of the trends observed in the literature, ale pi preliminary validation of the coalescence criterion with drum granulation data is encouraging. An extension is also made to the case of surface dry granules, where liquid is squeezed to the surface during granule information.

Microencapsulation of proteins by rapid expansion of supercritical solution with a nonsolvent

Abstract: A new method-rapid expansion from supercritical solution with a nonsolvent (RESS-N)-is reported for forming polymer microparticles containing proteins such as lysozyme (from chicken egg white) and lipase (from Pseudomonas cepacia). A suspension of protein in CO 2 containing a cosolvent and dissolved polymer is sprayed through a nozzle to atmospheric pressure. The polymers are poly(ethylene glycol) (PEG4000; MW = 3,000, PEG6000; MW = 7,500, PEG20000; MW = 20,000), poly(methyl methacrylate) (PMMA; MW = 15,000), poly(L-lactic acid) (PLA; MW = 5,000), poly(DL-lactide-co-glycolide) (PGLA; MW = 5,000) and PEG- poly(propylene glycol) (PPG)-PEG triblock copolymer (MW = 13,000). The solubilities of these polymers in CO 2 increase significantly with low-molecular-weight alcohols as cosolvents. The particles do not tend to agglomerate after expansion, since the pure cosolvent is a nonsolvent for the polymer. The structure and morphology of the microcapsules were investigated by TEM, SEM, and optical microscopy, The thickness of the polymer coating about the protein, as well as the mean particle diameter and particle-size distribution, could be controlled by changing the feed composition of the polymer.

Direct simulation Monte Carlo method for particle coagulation and aggregation

Abstract: A Monte Carlo simulation technique developed describes dispersed-phase systems with emphasis on coagulation and aggregation. The method does not use particle trajectories, but is based on the transformation of known collision frequencies into collision probabilities of particle pairs. The particle evolution was computed as a stochastic game, computing the time step after each collision. The simulations were validated by comparing with exact mathematical solutions for aggregation of solid particles and with numerical solutions based on sectional methods for coagulation of droplets. The direct simulation Monte Carlo (DSMC) method is advantageous, because the simulation of complex, multidimensional systems results in very elaborate models when using sectional models and is implemented very easily. Two examples of industrial importance are chemical reaction in coagulating droplets and coating of particles with small solid particles.

Early warning of agglomeration in fluidized beds by attractor comparison

Abstract: An enhanced monitoring method, based on pressure fluctuation measurements, for observing nonstationarities in fluidized-bed hydrodynamics is presented. Experiments show that it can detect small changes in the particle-size distribution. Such a monitoring method is useful to give an early warning of the onset of agglomeration in a fluidized bed. In contrast to earlier methods, this method is insensitive to small changes in superficial gas velocity and can handle multiple signals, making it relevant to industrial application. By carefully choosing the measurement position, the method becomes also insensitive to small bed mass variations. It uses the attractor reconstructed from a measured pressure signal, which is a fingerprint of the hydrodynamics of the fluidized bed for a certain set of conditions. Using this method statistically the reconstructed attractor of a reference time series of pressure fluctuations (representing the desired fluidization behavior) is compared with that of successive time series measured during the bed operation.

Engineering design method for cavitational reactors: I. Sonochemical reactors

Abstract: High pressures and temperatures generated during the cavitation process are now considered responsible for the observed physical and chemical transformations using ultrasound irradiation. Effects of various operating parameters reported here include the frequency, the intensity of ultrasound, and the initial nuclei sizes on the bubble dynamics, and hence the magnitude of pressure generated. Rigorous solutions of the Rayleigh-Plesset equation require considerable numerical skills and the results obtained depend on various assumptions. The Rayleigh-Plesset equation was solved numerically, and the results have been empirically correlated using easily measurable global parameters in a sonochemical reactor. Liquid-phase compressibility effects were also considered. These considerations resulted in a criterion for critical ultrasound intensity, which if not considered properly can lead to overdesign or underdesign. A sound heuristic correlation, developed for the prediction of the pressure pulse generated as a function of initial nuclei sizes, frequency, and intensity of ultrasound, is valid not only over the entire range of operating parameters commonly used but also in the design procedure of sonochemical reactors with great confidence.

Mapping the cavitation intensity in an ultrasonic bath using the acoustic emission

Abstract: A new method for separate identification and determination of the spatial distribution of the two components of the energy intensity in an ultrasound bath (due to the ultrasound waves and cavitation activity) uses two media - cavitating (water) and noncavitating (silicon oil) - under the conditions of the acoustic field in the ultrasound bath. The variation of cavitation intensity in the frequency domain was obtained by subtracting the acoustic emission spectrum of silicon oil from that of water. Measurements at various locations in the bath revealed significant spatial variations in the cavitation intensity in the bath. The local cavitation phenomena in the bath (stable or transient cavitation) were explained based on the spectral characteristics of acoustic emission. The radial dynamics of the bubbles at the location of cavitation intensity measurements was determined using the Gilmore model of bubble dynamics. The bubbles in the region of highest cavitation intensity underwent a transient motion, while the bubbles in the region of lowest cavitation intensity underwent stable/oscillatory motion. The transient collapse of the bubbles that gives rise to local temperature and pressure maxima is at the root of the observed effects of ultrasound on chemical systems. The more violent the collapse of the bubbles, the higher the local cavitation intensity. It was verified using the spectral characteristics of the acoustic emission and simulation of the radial motion of the bubbles.

Effect of mass transfer and catalyst layer thickness on photocatalytic reaction

Abstract: Semiconductor photocatalytic processes hae been studied for nearly 20 years due to their intriguing adantages in enironmental remediation. A rational approach in deter- mining the effect of mass transfer and catalyst layer thickness during photocatalytic reactions is proposed. The reaction occurs at the liquid ) catalyst interface, and therefore when the catalyst is immobilized, both external and internal mass transfer plays signifi- cant roles in oerall photocatalytic processes. Seeral model parameters} external mass-transfer coefficient, dynamic adsorption equilibrium constant, adsorption rate constant, internal mass-transfer coefficient, and effectie diffusiity } were determined either experimentally or by fitting realistic models to experimental results using benzoic acid as a model component. Een though all these parameters are critical to the design and deelopment of photocatalytic processes, they are not aailable in the literature. The effect of the internal mass transfer on the photocatalytic degradation rate oer different catalyst layer thicknesses under two different operating configurations was ana- lyzed theoretically and experimentallyerified. It was obsered that an optimal catalyst layer thickness exists for substrate-to-catalyst illumination.

High-resolution adsorption isotherms of supercritical carbon dioxide on activated carbon

Abstract: Supercritical fluids are attractive solvents for heterogeneous processes, including catalysis and adsorptive separation. However, adsorption processes in the near-critical region are poorly understood and exhibit unique behavior where the adsorption of the supercritical solvent plays an important role in the solute adsorption. The behavior of supercritical fluids in confined pores has been studied theoretically, but there are few experimental data on their behavior in industrially important microporous materials. The adsorption of carbon dioxide on Calgon F400 activated carbon over a wide range of pressures (0–20 MPa) at temperatures near the critical point of carbon dioxide (30 to 45°C) was studied. Near-continuous adsorption and desorption isotherms were measured with a new flow gravimetric apparatus with precise control over pressure and temperature. As pressure is increased, the excess adsorption increases sharply at low pressures; then a broad maximum is observed. At temperatures greater than the critical temperature, there is a sharp drop in excess adsorption near the critical region where the density of the bulk fluid increases sharply. A crossover is observed near the critical region where, below a certain pressure, the excess adsorption decreases with temperature, while above the crossover point the trend is reversed. When analyzed as a function of solvent density, the crossover disappears, revealing an anomalous maximum in total adsorption near the critical point similar to the enhanced local density or “charisma” observed in binary solute–supercritical fluid systems. A 2-D EOS model using the 2-D Peng-Robinson EOS was able to qualitatively describe the adsorption behavior over the entire pressure range, but the quantitative agreement was poor in the near-critical region.

Nonlinear process control using contraction theory

Abstract: Contraction theory is a recently developed nonlinear control system tool based on an exact differential analysis of convergence. Contraction theory is applied to stability analysis and control system design for nonlinear chemical processes. Simple designs with explicit stability and convergence guarantees are obtained by taking advantage of the monotonicity of the reaction rates and the linear ambiguity in the choice of the chemical state.

Theoretical model of thermal diffusion factors in multicomponent mixtures

Abstract: Unlike molecular diffusion, neither measured thermal diffusion coefficients nor the theoretical framework exist for the estimation of thermal diffusion coefficients in nonideal multicomponent mixtures. This work derives a theoretical model for thermal diffusion coefficients in ideal and nonideal multicomponent mixtures, based on the thermodynamics of irreversible processes and the molecular kinetic approach incorporating explicit effects of nonequilibrium properties, such as the net heat of transport and molecular diffusion coefficients, and of equilibrium properties of the mixture, which are determined by the Peng-Robinson equation of state. An interesting feature of this model is that in nonideal multicomponent mixtures thermal diffusion coefficients depend on molecular diffusion coefficients, while in binary mixtures they do not. The model successfully describes thermal diffusion factors of binary mixtures for which experimental data are available, even those in extreme nonideal conditions and close to the critical point. Since experimental data on thermal diffusion factors in multicomponent hydrocarbon mixtures are not available, testing the model's accuracy was not possible. The model, however, successfully predicted spatial variation of composition in a ternary mixture of nC{sub 24}/nC{sub 16}/nC{sub 12}, providing an indirect verification. The six-component mixture of C{sub 1}/C{sub 3}/nC{sub 5}/nC{sub 10}/nC{sub 16}/C{sub 2} shows significant dependency of thermal diffusionmore » factors on the distance to the critical point. It also demonstrates for the first time that there is no need to adopt a sign convention for thermal diffusion coefficients in binary and higher mixtures. The thermodynamic stability analysis shows that when the thermal diffusion coefficient is positive, the component should go to the cold region in a binary mixture.« less

Statistical and causal model-based approaches to fault detection and isolation

Abstract: 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.

Transient fluidization and segregation of binary mixtures of particles

Abstract: Fluidization of binary mixtures of particles belonging to group B of the Geldart classification of powders was studied. Beds tested were prepared by mixing in different proportions particles with almost equal density (≈2,500 kg/m3) and dissimilar size (125 μm silica sand and 500 μm glass beads). Experiments were carried out using a segmented fluidization column equipped with multiple pressure transducers. Experimental procedures included continuous monitoring of pressure drop at different locations along the bed during quasi-steady or stepwise changes of gas superficial velocity, and characterization of particle-size distributions in each segment of the fluidization column after fluidization of the bed for given times. Three ranges of gas superficial velocity were recognized for each solids mixture. At low velocity the bed behaves as a fixed bed. At high velocity, it is fully and steadily fluidized. In an intermediate velocity range, transient fluidization takes place: an initially uniform fluidized bed eventually undergoes segregation, giving rise to a defluidized bottom layer rich in the coarser solids and to a “supernatant” fluidized layer where finer particles prevail. The thresholds between these velocity ranges are rather sharp and were characterized as functions of initial bed composition. Rates at which the defluidized solids layer builds up from initially uniform beds, and the ultimate compositions of the defluidized bottom and fluidized top layers are characterized for beds with different compositions at variable gas superficial velocity.