Showing papers in "Aiche Journal in 1990"
TL;DR: In this paper, a predictive two-phase flow model was derived starting with the Boltzman equation for velocity distribution of particles, which is a generalization of the Navier-Stokes equations of the type proposed by R. Jackson, except that the solids viscosities and stresses are computed by simultaneously solving a fluctuating energy equation for the particulate phase.
Abstract: Detailed knowledge of solids circulation, bubble motion, and frequencies of porosity oscillations is needed for a better understanding of tube erosion in fluidized bed combustors. A predictive two-phase flow model was derived starting with the Boltzman equation for velocity distribution of particles. The model is a generalization of the Navier-Stokes equations of the type proposed by R. Jackson, except that the solids viscosities and stresses are computed by simultaneously solving a fluctuating energy equation for the particulate phase. The model predictions agree with time-averaged and instantaneous porosities measured in two-dimensional fluidized beds. Observed flow patterns and bubbles were also predicted.
1,583 citations
TL;DR: In this article, a Phenomenological model for the rates of bubble coalescence and bubble break-up in turbulent gas-liquid dispersions is proposed, based on the mixing of tracer gases within bubbles upon coalescence, in conjunction with Monte-Carlo simulations of coalescence events.
Abstract: A Phenomenological model is proposed for the rates of bubble coalescence and bubble break-up in turbulent gas-liquid dispersions. Bubble coalescence is modeled by considering bubble collisions due to turbulence, buoyancy, and laminar shear, and by analysis of the coalescence efficiency of collisions. Bubble break-up is analyzed in terms of bubble interactions with turbulent eddies. A method for the measurement of coalescence and break-up events in turbulent systems is described and used to test the validity of the proposed model. The measurement technique relies on the mixing of tracer gases within bubbles upon coalescence, in conjunction with Monte-Carlo simulations of coalescence events. Both distilled water and salt solutions are examined. Favorable agreement is found between the model and the individual coalescence and breakage rates, as well as with data obtained for the average bubble size and bubble size distribution.
1,058 citations
TL;DR: In this paper, the ability of the mixing rules proposed by Michelsen to predict high-pressure phase equilibrium, when used in combination with the parameter table of modified UNIFAC, was investigated.
Abstract: Recent procedures developed by Heidemann (1990) and by Michelsen (1990a, b) enable us to formally incorporate excess Gibbs energy model parameters into a fully consistent equation of state, cith accurate reproduction of the behavior of the excess Gibbs energy model at atmospheric pressure.
This paper investigates the ability of the mixing rules proposed by Michelsen to predict high-pressure phase equilibrium, when used in combination with the parameter table of modified UNIFAC. Considering that a group contribution method is used for the excess Gibbs energy and that model parameters are extrapolated over a 200 K temperature interval, quite satisfactory results are obtained for the mixtures investigated.
357 citations
TL;DR: In this article, the flow regimes of dense-phase vertical pneumatic transport of solids, referred to in the literature as circulating fluidized beds, computed using a generalization of the Navier-Stokes equations for two fluids.
Abstract: This paper reports on flow regimes of dense-phase vertical pneumatic transport of solids, referred to in the literature as circulating fluidized beds, computed using a generalization of the Navier-Stokes equations for two fluids. In the less dense regime corresponding to volume fractions of solids of about 1%, the flow consists of centrally upward moving solids and downward moving clusters. The computations agree with observations made by high-speed motion pictures and with measurements of radial solids concentrations and velocities. In the dense regime, corresponding to volume fractions of about 10%, a core-annulus type of regime is obtained, with solids descending down at the wall.
335 citations
TL;DR: In this article, a new criterion is introduced for the onset of shear thickening in a concentrated dispersion, which follows from the assumption that shear forces overrule the interparticle forces.
Abstract: A new criterion is introduced for the onset of shear thickening in a concentrated dispersion. The criterion follows from the assumption that shear thickening occurs when the shear forces overrule the interparticle forces. A force balance at small interparticle distances is used to predict the dependence of this critical shear rate on the volume fraction. It is deduced that the critical shear rate is proportional to the interparticle distance, has a linear dependence on the magnitude of the stabilizing force, an inverse linear dependence on the dispersion medium viscosity, and an inverse linear dependence on the particle radius. The model is confirmed experimentally with viscosity measurements on various electrostatically stabilized dispersions. The validity of the model is also checked with data obtained from the literature. Experiments indicate that polydisperse dispersions exhibit pronounced shear thickening but with a less dramatic increase in viscosity than monodisperse dispersions.
292 citations
TL;DR: Pendant drop tensiometry enhanced by video-image digitization is shown to be a useful tool for the experimental measurement of the relaxation in interfacial tension due to the adsorption of surfactant at a fluid interface as discussed by the authors.
Abstract: Pendant drop tensiometry enhanced by video-image digitization is shown to be a useful tool for the experimental measurement of the relaxation in interfacial tension due to the adsorption of surfactant at a fluid interface Using this method, profiles of the relaxation in surface tension of a diffusion-controlled, nonionic polyethoxy surfactant were measured A diffusion coefficient was computed by comparing these profiles with numerical solutions of the bulk surfactant diffusion equation and a Frumkin equilibrium adsorption isotherm This comparison was made for the entire relaxation period This method establishes a more reproducible diffusion coefficient than current techniques that utilize only the short- or long-time parts of the relaxation spectrum In addition, lower bounds on the kinetic constants for the sorption process are inferred for the polyethoxy surfactant used by comparing numerical solutions of mixed diffusion and surface kinetic transfer with the diffusion-limited result
279 citations
TL;DR: In this paper, the reaction of CO2 with TEA, DMMEA, and DEMEA has been studied at 293, 303, 318 and 333 K. All the kinetic experiments were carried out in a stirred cell reactor operated with a flat, smooth and horizontal gas-liquid interface.
Abstract: The reaction of CO2 with TEA, DMMEA, and DEMEA has been studied at 293, 303, 318 and 333 K. All the kinetic experiments were carried out in a stirred cell reactor operated with a flat, smooth and horizontal gas-liquid interface. A numerical method, which describes mass transfer accompanied by reversible chemical reactions, has been applied to infer rate constants from the experimental data. It is argued that the contribution of the CO2 reaction with OH- to the observed reaction rate may have been overstimated in most literature on tertiary amine kinetics as serious depletion of OH- toward the gas-liquid interface usually occurs.
For all the amines studied, the reaction order in amine was found to be about one for each temperature investigated. This is in good agreement with the base catalysis mechanism proposed by Donaldson and Nguyen (1980). All kinetic data could be summarized reasonably well in one Bronsted relationship.
201 citations
TL;DR: In this article, a dynamical kinetic model describing gas-phase olefin copolymerisation using a multiple active site Ziegler-Natta catalyst is presented, which is capable of predicting production rate, molecular weight, and copolymers composition changes observed in an industrial reactor.
Abstract: A dynamical kinetic model describing gas-phase olefin copolymerisation using a multiple active site Ziegler-Natta catalyst is presented. This model is capable of predicting production rate, molecular weight, and copolymer composition changes observed in an industrial reactor. The model also explains how broad molecular weight distributions and bimodal copolymer compositions can occur as has been observed for commercial linear polyethylenes.
191 citations
TL;DR: In this article, a percolation model of foam mobilization in porous media is developed, which indicates that there is a minimum pressure gradient or, equivalently, a minimum gas velocity required to initiate mobilization of foam.
Abstract: A percolation model of foam mobilization in porous media is developed. This model indicates that there is a minimum pressure gradient or, equivalently, a minimum gas velocity required to initiate mobilization of foam. As a result, for most foam enhanced oil recovery processes, where the surface tension is not low, deep foam penetration depends on propagation of foam formed at a high pressure gradient near the well. Low surface tension makes mobilization of CO2 foams feasible, however, at pressure gradients found throughout much of the formation in a typical field application. The theory further predicts, and data confirm, that the minimum velocity for foam mobilization during steady flow of liquid and gas decreases as injected liquid volume fraction increases. The theory suggests a better strategy for foam generation: alternate injection of small slugs of liquid and gas.
190 citations
TL;DR: In this article, two approaches are considered to separate the output from measured and unmeasured disturbances in a chemical reactor model, in which the manipulated coolant flow rate appears nonlinearly.
Abstract: Two approaches are considered. The first approach is based on the original process model, while the second is based on an extended model that is control linear. Decoupling the output from measured and unmeasured disturbances is also investigated. The two approaches are evaluated via simulation for a chemical reactor model, in which the manipulated coolant flow rate appears nonlinearly.
168 citations
TL;DR: In this paper, a discretized population balance is adapted for continuous systems at steady state, in which the active mechanisms are nucleation, growth and aggregation, and a simple modification to the original discrete equations describing growth, permits the modelling of size-dependent growth effects.
Abstract: This paper is concerned with the solution of the population balance for continuous systems at steady state, in which the active mechanisms are nucleation, growth and aggregation. A discretized population balance, initially proposed by Hounslow et al. (1988a) for batch systems, is adapted for use with continuous systems at steady state. It is shown that simultaneous nucleation and growth can be described very effectively by the discrete equations. Criteria are developed for the selection of the optimal size domain. A simple modification to the original discrete equations describing growth, permits the modelling of size-dependent growth effects. Both size-independent and size-dependent aggregation are described by the discrete equations with three significant-figure accuracy. The complete set of discrete equations is used to simulate the nucleation, growth and aggregation of Nickel Ammonium Sulphate. It is shown that analysis by the approximate model must lead to underestimation of the nucleation and growth rates.
TL;DR: In this article, a systematic procedure is developed for the simultaneous synthesis of primary transfer mass exchange networks and their associated mass exchange regeneration networks and the solution of this program provides the location of the pinch points as well as the optimal flow rates of all the lean and regenerative streams without any prior commitment to the network structure.
Abstract: A systematic procedure is developed for the simultaneous synthesis of primary transfer mass-exchange networks and their associated mass-exchange regeneration networks. The purpose of the primary transfer network is to preferentially transfer certain species from a set of rich streams to a set of lean streams. The regeneration network aims at regenerating any recyclable lean stream. The proposed procedure deals with the problem in two stages. In the first stage, a mixed-integer nonlinear program is solved to minimize the cost of mass-separating and regenerating agents. The solution of this program provides the location of the pinch points as well as the optimal flow rates of all the lean and the regenerative streams without any prior commitment to the network structure. In the second stage, a mixed-integer linear program is solved to minimize the number of exchangers in both networks. An example problem with industrial relevance is solved to elucidate the merits offered by the devised synthesis procedure.
TL;DR: In this paper, the authors investigated the evolution of the homogeneous nucleation rate, work of nucleus formation, and critical nucleus size along different expansion paths for the model system phenanthrene-carbon dioxide.
Abstract: When a supercritical solution is rapidly expanded, large solute supersaturations can be attained, and small particles are formed. The evolution of the homogeneous nucleation rate, work of nucleus formation, and critical nucleus size along different expansion paths is investigated here for the model system phenanthrene-carbon dioxide. Nucleation rates are the result of the competition among solvent expansion, cooling due to depressurization, and high supersaturation. Although supersaturations can reach very high values (> 106), relatively flat nucleation rate profiles result due to cooling and expansion. For an interfacial tension of 0.02 N/m, computed nucleation rates never exceed 104 s−1 · cm−3. A substantial fraction of the maximum nucleation rate is attained with partial decompression to pressures above 1 bar.
TL;DR: In this article, the synthesis of nonlinear controllers for multivariable nonlinear processes that make the closed-loop system linear in an input/output sense is discussed, and necessary and sufficient conditions for linearizability via static state feedback are derived.
Abstract: This work concerns the synthesis of nonlinear controllers for multivariable nonlinear processes that make the closed-loop system linear in an input/output sense. Necessary and sufficient conditions for input/output linearizability via static state feedback are derived as well as formulas for the feedback law. Once such a static state feedback is applied to the process, an external multivariable linear controller with integral action can control it to set point. The proposed control methodology is tested through simulations in a semibatch copolymerization reactor example.
TL;DR: In this paper, a model for steady polymer melt blowing is developed, which includes the dominant effect that the forwarding air has upon the process and inertial, gravitational and heat transfer effects are also included.
Abstract: A model has been developed for steady polymer melt blowing. This model includes the dominant effect that the forwarding air has upon the process. Inertial, gravitational and heat transfer effects are also included. The model equations are solved numerically with both Newtonian and viscoelastic (Phan-Thien and Tanner) constitutive equations. The predicted results compare favorably with actual experimental data.
TL;DR: In this paper, the concept of relative order of an output with respect to an input, extended to include disturbance as well as manipulated inputs, is used to obtain a characterization of the dynamic interactions among the input and the output variables.
Abstract: This paper concerns general MIMO nonlinear processes, whose dynamic behavior is described by a standard state-space model of arbitrary order, including measurable disturbances. The concept of relative order of an output with respect to an input, extended to include disturbance as well as manipulated inputs, is generalized in a MIMO context and it is used to obtain a characterization of the dynamic interactions among the input and the output variables. A synthesis formula is calculated for a feedforward/state feedback control law that completely eliminates the effect of the measurable disturbances on the process outputs and induces a linear behavior in the closed-loop system between the outputs and a set of reference inputs. The input/output stability and the degree of coupling in the closed-loop system are determined by appropriate choice of adjustable parameters. A MIMO linear controller with integral action completes the feedforward/feedback control structure. The developed control methodology is applied to a continuous polymerization reactor and its performance is evaluated through simulations.
TL;DR: A significant fraction of volatile compounds of toxic metals like lead, mercury, cadmium, arsenic, and selenium, are emitted as vapor or fine particles when wastes containing these metals are incinerated.
Abstract: Since strict regulations have been placed prohibiting the placement of hazardous wastes in landfills, incineration is emerging as the principal and most attractive alternative for disposal of hazardous wastes. Incineration of waste offers various advantages including conversion of toxic organic components of hazardous wastes to harmless or less harmful forms, reduction in volume, and means of energy recovery. Unfortunately, since metals are not destroyed during the incineration process, toxic metal particles or vapors are generated as byproducts when wastes containing these metals are incinerated. A significant fraction of volatile compounds of toxic metals like lead, mercury, cadmium, arsenic, and selenium, are emitted as vapor or fine particles. Nonvolatile toxic metal compounds are released with the ash and also require safe disposal.
TL;DR: In this article, the authors examined the amount of salt required to immobilize the gas-liquid interface of the film between coalescing bubbles and showed that it is not a trivial task.
Abstract: We examine the amount of salt required to immobilize the gas-liquid interface of the film between coalescing bubbles.
TL;DR: In this article, a kinetic model for the drying of solids in fluidized beds, assuming a falling rate period following a constant rate period, was developed for single and spiral fluidized bed.
Abstract: A kinetic model is developed for the drying of solids in fluidized beds, assuming a falling rate period following a constant rate period. Experimental data obtained using batch and continuous single and spiral fluidized beds are satisfactorily matched with the assumed drying kinetics and the residence time distribution of solids appropriate for the type of dryer.
TL;DR: In this paper, the (L/D) (V/B) configuration was found to be the best control configuration for two-point composition control in distillation distillation systems.
Abstract: The selection of an appropriate control configuration (structure) is the most important decision when designing distillation control systems. The steady state RGA is commonly used in industry for selecting the best structure. One counterexample to the usefulness of this measure is the DB configuration, which has infinite steady state relative gain array (RGA) values, but still good control performance is possible. This is indicated by high-frequency RGA values close to 1. In this paper it is stressed that decisions regarding controller design should be based on the initial response (high-frequency behavior) rather than the steady state.
Based on a frequency-dependent RGA analysis and optimal Pl controller designs of four different configurations, the (L/D) (V/B) configuration is found to be the best choice for two-point composition control. The conventional LV configuration performs poorer than the above system, but is preferable if one-point control is used.
TL;DR: In this article, the effect of pressure on the reaction rate of enzymes in supercritical fluids has been investigated and it has been shown that pressure-induced changes in the physical properties of a supercritical fluid solvent affect the rate of enzymatic reaction and if so, which properties are responsible for the change.
Abstract: Three different authors have reported on the use of four different enzymes in supercritical fluids. Lipase carries out transesterification reactions in the presence of supercritical carbon dioxide. Polyphenyl oxidase is active in supercritical CO{sub 2} and fluoroform. It has been shown that alkaline phosphatase and cholesterol oxidase are active in supercritical CO{sub 2}. More recently, an examination of the effect of aggregation of cholesterol on cholesterol oxidase activity in CO{sub 2} using electron paramagnetic resonance (EPR) was done. They found that when cosolvents which promoted aggregation were added, the reaction rate increased in proportion to the amount of aggregation. To date, no data on the effect of pressure on reaction rate have been presented. The objective of this work is to determine whether pressure-induced changes in the physical properties of a supercritical fluid solvent affect the rate of an enzymatic reaction and if so, which properties are responsible for the change.
TL;DR: In this paper, a new thermal integration procedure was presented that combines a bifurcation and residue-curve map analysis with the methods of Andrecovich and Westerberg, which achieved energy savings of 65% and 40% over the optimized nonintegrated sequences.
Abstract: Existing methods for synthesizing thermally-integrated distillation sequences fail frequently when applied to nonideal and azeotropic systems, because increasing the column pressures often introduces new azeotropes and distillation boundaries into the mixture, which make some separation tasks infeasible. A new thermal integration procedure is presented that combines a bifurcation and residue-curve map analysis with the methods of Andrecovich and Westerberg. The procedure is demonstrated with two commercially important separations: ethanol-water-ethylene glycol and methanol-acetone-water. Energy savings of 65% and 40%, respectively, over the optimized nonintegrated sequences are possible.
TL;DR: In this article, a novel approach based on the concept of fractals, has been adopted to analyze these complicated and stochastic characteristics of three-phase fluidized beds that have played important roles in various areas of chemical and biochemical processing.
Abstract: This paper reports on three-phase fluidized beds that have played important roles in various areas of chemical and biochemical processing. The characteristics of such beds are highly stochastic due to the influence of a variety of phenomena, including the jetting and bubbling of the fluidizing medium and the motion of the fluidized particles. A novel approach based on the concept of fractals, has been adopted to analyze these complicated and stochastic characteristics. Specifically, pressure fluctuations in a gas-liquid-solid fluidized bed under different batch operating conditions have been analyzed in terms of Hurst's rescaled range (R/S) analysis, thus yielding the estimates for the so-called Hurst exponent, H. The time series of the pressure fluctuations has a local fractal dimension of d{sub FL} = 2 {minus} H. An H value of 1/2 signifies that the time series follows Brownian motion; otherwise, it follows fractional Brownian motion (FBM), which has been found to be the case for the three-phase fluidized bed investigated.
TL;DR: The architecture addresses each of the shortcomings discussed, and information is derived from an a priori analysis technique used to show the degree to which different faults can be discriminated based on the model equations available.
Abstract: Many recent attempts to use expert systems for process fault diagnosis have included information derived from deep knowledge. This information is generally implemented as a rule-based expert system. Drawbacks of this architecture are a lack of generality, poor handling of novel situations, and a lack of transparency. An algorithm called the diagnostic model processor is introduced; it uses the satisfaction of model equations from process plants to arrive at the most likely fault condition. The method is generalized by the process model and diagnostic methodology being separated. The architecture addresses each of the shortcomings discussed. Experiments show that the methodology is capable of correctly identifying fault situations. Furthermore, information is derived from an a priori analysis technique, which is used to show the degree to which different faults can be discriminated based on the model equations available. The results of this analysis add further insight into the diagnoses provided by the diagnostic model processor.
TL;DR: Nuclear magnetic resonance flow imaging was used to measure fluid flow noninvasively in the extracapillary space (ECS) of a hollow-fiber bioreactor without cells, illustrating the applicability of this method for optimizing operational procedures and design of membrane bioreactors and filtration devices.
Abstract: Nuclear magnetic resonance flow imaging (MRFI) was used to measure fluid flow noninvasively in the extracapillary space (ECS) of a hollow-fiber bioreactor without cells. Agreement between these axial flow measurements in a single hollow-fiber module and predicted axial velocity contour plots at various image planes along the path length was good.
Flow in a solid-wall tube (phantom) was first used to calibrate pixel intensities with axial velocities. Flow images at several locations along the permeable hollow fiber length were then obtained in order to observe the well-known leakage or Starling flow in the ECS. These quantitative spatially dependent velocity measurements were then compared to theoretically derived velocities obtained from a solution of Poisson's equation with a constant pressure gradient and no slip at the solid surfaces. A mathematical transformation was used to simplify the numerical methods. Leakage flow through the ECS of a multifiber bioreactor (40 fibers) was also measured by MRFI, illustrating the applicability of this method for optimizing operational procedures and design of membrane bioreactors and filtration devices.
TL;DR: In this paper, a two-dimensional electrokinetic transport model that incorporates ionic hydration, orientation of solvent molecules by an applied electric field, and solvent dipole-dipole interactions is developed.
Abstract: A two-dimensional, electrokinetic transport model that incorporates ionic hydration, orientation of solvent molecules by an applied electric field, and solvent dipole-dipole interactions is developed. The model is used to simulate equilibrium and transport experiments for perfluorosulfonic acid membranes containing aqueous alkali metal sulfate solutions. The membrane is modeled as an array of cylindrical pores. Solution of the mathematical model requires that the membrane porosity, water partition coefficient, coion partition coefficient, water diffusion coefficients, and coion and counterion diffusion coefficients be known. Membrane coion and counterion diffusion coefficients were determined from free solution equivalent conductance data. All other parameters were determined experimentally for a Nafion (of E. I. du Pont de Nemours Inc.) cation-exchange membrane and five 0.1 M alkali metal sulfate solutions. Experimental radiotracer data for coion absorption as well as for coion and water transport are compared with theoretical predictions to test the accuracy of the model.
TL;DR: In this paper, a macroscopic model of two-phase flow in packed beds, based on the volume-averaged equations of motion for the gas and liquid phases, was analyzed in an attempt to understand the onset and evolution of fully-developed pulsing flow in trickle beds.
Abstract: A macroscopic model of two-phase flow in packed beds, based on the volume-averaged equations of motion for the gas and liquid phases, was analyzed in an attempt to understand the onset and evolution of fully-developed pulsing flow in trickle beds. By assuming that solutions take the form of waves travelling at constant speed, periodic solutions to these equations are found which can be associated with long-time, asymptotic behavior of pulses in a very long bed. Families of one-dimensional waves which exist at a particular set of mass fluxes can be characterized by infinite period bifurcations in the travelling wave frame. We numerically follow these bifurcations as the fluxes are changed, generating bifurcation diagrams for the original model. The results predict that properties of one-dimensional pulses should correlate with the total superficial velocity through the bed. A hysteresis in the trickling-pulsing transition is also predicted.
TL;DR: A modified form of the Flory-Huggins theory of polymer solution thermodynamics is obtained for correlating protein partitioning in the polyethylene glycol (PEG)/dextran aqueous two-phase systems, which will facilitate the selection and engineering scale-up of aqueously two- phase systems for biomolecule purification.
Abstract: Utilizing a modified form of the Flory-Huggins theory of polymer solution thermodynamics, a generalized expression has been obtained for correlating protein partitioning in the polyethylene glycol (PEG)/dextran aqueous two-phase systems. The expression relates the natural logarithm of the partition coefficient of the protein to the polymer concentration difference between the phases. The parameters of the relationship are a function of protein and phase forming polymer molecular weight, protein-water, protein-polymer and polymer-water interaction parameters, and the electrostatic potential difference between the phases. The relationship was verified by partitioning 17 proteins covering a wide range of molecular weight in the tie lines of the above systems, along with data from the literature. This correlation will facilitate the selection and engineering scale-up of aqueous two-phase systems for biomolecule purification.
TL;DR: In this article, an approach for modeling the effects of temperature and pH on the solubility of amino acids in water is presented, where the data employed are activity coefficients and solubilities of the amino acid in neutral water and the dissociation constants for various amino acid ionization reactions.
Abstract: An approach is presented for modeling the effects of temperature and pH on the solubility of amino acids in water. The amino acids studied are alanine, amino-butyric acid, glycine, hydroxy-proline, proline, serine, threonine, and valine. The data employed are activity coefficients and solubilities of the amino acids in neutral water and the dissociation constants for the various amino-acid ionization reactions. Activity coefficients are correlated with the modified UNIFAC group contribution model and with new glycine and proline groups being introduced.
A similar approach is presented for modeling the solubilities of certain antibiotics in mixed nonaqueous solvents. Fusion temperature data and solubilities of the antibiotics in pure organic solvents are used to deduce modified UNIFAC energy interaction parameters between new large antibiotic groups and the standard alkane (CH2), alcohol (OH), aromat (ACH), and ester (CCOO) groups.