Showing papers in "Aiche Journal in 1988"

A discretized population balance for nucleation, growth, and aggregation

Abstract: The population balance for batch aggregation of particulate suspensions is recast in a form that may be solved simply and accurately. The transformed equation is deduced with the introduction of only one additional parameter, which is found to be a constant for all cases. The transformed equation is tested by comparison with some analytical solutions with which it is found to be in excellent agreement. In particular, the equation is shown to predict correctly the rate of change of total particle number and volume. Compatible descriptions of linear growth and nucleation are developed with similar success. The method is then applied to modeling the in vitro growth and aggregation of kidney stones (calcium oxalate monohydrate crystals). It is found that these phenomena are well described by McCabe's ΔL law, a size-independent coalescence kernel, and first-order kinetics. Simulated particle size distributions and their moments are in excellent agreement with the experimental results.

Dispersion‐free solvent extraction with microporous hollow‐fiber modules

Abstract: Determination experimentale des coefficients de transfert de matiere pour des modules hydrophobes ou hydrophiles. Mise en place d'une correlation pour decrire les resultats

Pore evolution and channel formation during flow and reaction in porous media

Abstract: A theoretical and experimental study on the dissolution of porous media by flowing acid has been carried out. Dissolution of the media results in an evolution of the pore geometry and the formation of random flow channels. The goal is to predict the range of conditions under which channels will form, and the effects of various parameters on the structure of the channels and on their rate of propagation through the media. A random network model is used to describe the behavior of the stochastic, rootlike channels that form during flow and dissolution in carbonate rock and in other systems. The structure of the flow channels that form as a result of acid attack are characterized and studied using a Wood's metal casting technique. A comparison of model results shows that the rates of channel formation and growth are intimately related to the developing structure of the channels (size of branches and degree of branching), which in turn is controlled by factors such as the fluid velocity and the rate of reaction. Depending on the experimental conditions, the channels range from a single conduit with a minimum of branching to a highly branched, spongy network of channels. The dependence of permeability increase and channel branchedness on injection rates and acid diffusion rates is described by the Damkohler number for flow and reaction.

Theory of transport processes in single crystal growth from the melt

Abstract: The quality of large semiconductor crystals grown from the melt for use in electronic and optoelectronic devices is strongly influenced by the intricate coupling of heat and mass transfer and melt flow in growth systems. This paper reviews the present state of understanding of these processes starting from the simplest descriptions of solidification processes to detailed numerical calculations needed for quantitative modeling of processing with solidification. Descriptions of models for the vertical Bridgman-Stockbarger and Czochralski crystal growth techniques are included as examples of the level of understanding of industrially important methods.

Protein extractions with hollow fibers

Abstract: Differential protein extractions were measured with two types of extractants: inverted micelles and two-phase aqueous systems. The results show that hollow-fiber extractions are substantially faster than those possible in conventional equipment. The extractions are not compromised by loading or flooding because the flows of extractant and raffinate are almost completely independent. Mass transfer coefficients inferred from the measurements both support and extend design equations for these contactors.

Molecular thermodynamics of aqueous two-phase systems for bioseparations

Abstract: Aqueous polymer-polymer two-phase systems provide a powerful method for separating biomolecules by extraction. When a complex mixture of biomolecules (e.g., a fermentation broth or a solution of lysed cells) is added to such a system, biomolecules partition uniquely between the two phases, achieving separation. A thermodynamic framework is presented for optimizing extraction performance in biological separations. First, a molecular-thermodynamic model, based on the osmotic virial equation, is proposed to describe phase equilibria for dilute aqueous mixtures containing polymers and protein. Second, experimental phase-equilibrium data (protein partition coefficients) are reported for a number of model proteins including albumin, lysozyme, and α-chymotrypsin. To interpret and correlate the experimental data, Low-Angle Laser-Light Scattering (LALLS) measurements were made to determine osmotic second virial coefficients for aqueous mixtures containing polymers, proteins, salts (KCl, KH2PO4 and K2SO4 at concentrations of 50 and 100 mM) and several combinations of polymer-polymer and polymer-protein pairs. Combined with electrochemical measurements (differences in potential between the two phases and protein net charge), these data provide parameters for the model to calculate the desired phase equilibria. A comparison of calculated and experimental results indicates that the virial-equation model provides good prediction of binodals and a reliable basis for estimating infinite-dilution protein partition coefficients for biotechnical process design.

A network model for deep bed filtration of solid particles and emulsion drops

Abstract: A network model has been developed to simulate the flow of emulsions and solid particles through porous media. Particle deposition due to direct interception, as well pore plugging by straining are accounted for in the model. The effects of two important factors-the ratio of particle size to pore size, and the fluid velocity-on particle deposition are also investigated. The strength of the model lies in its ability to predict accurately effluent concentration profiles, permeability changes occurring during deep bed filtration, and the evolution of the filter coefficient with time. Model predictions for different particle and pore size distributions of both solid and emulsion particles are in agreement with experimental data.

Qualitative simulation of chemical process systems: Steady‐state analysis

Abstract: Dans une analyse qualitative le mauvais fonctionnement et l'effet des erreurs se propagent par causalite d'une variable a l'autre, en satisfaisant finalement les contraintes du procede en regime permanent. Les contraintes des series d'equations algebriques, sont representees par des equations de confluence stationnaires. La causalite est representee par le graphe oriente signe etendu (Extended Signed Directed Graph: ESDG)

Optimal production of secreted protein in fed‐batch reactors

Abstract: The optimal control policy for the maximization of the secreted heterologous protein in a fed-batch bioreactor was obtained using SEY2102 as a model host yeast and SUC2-s2 as a model secretory protein. A dynamic model for host cell growth, gene expression, and the secretion of expressed polypeptides was formulated. The optimal system trajectory contains multiple singular arcs that are distinct from one another. Optimal control requires transitions between these singular arcs. Optimal transitions between multiple singular arcs with bounded controls are uniquely located in observance with the Minimum Principle of Pontryagin. An iterative numerical search strategy for determining the optimal control showed successful convergence properties.

Onset of pulsing in two‐phase cocurrent downflow through a packed bed

Abstract: In cocurrent downflow of a gas and a liquid through a packed column, a transition from trickling flow to pulsing flow is observed as one increases the flow rates. A simple macroscopic model for the two-phase flow is analyzed to examine whether this regime transition should be viewed as the loss of stability of a steady state or the loss of existence of any solution for the steady-state equations of motion. The former appears to be a more reasonable interpretation and an explicit algebraic criterion for the onset of pulsing is presented.

A thermodynamic model for predicting wax formation in crude oils

Abstract: Most crude oils contain high molecular weight components, which at low temperatures may precipitate as a wax phase. This may cause plugging of pipes and numerous other problems. This paper presents a solid-liquid equilibrium based model for the description of wax formation. The model for the Gibbs energy contains a contribution based on Flory's theory of multicomponent polymer solutions and a contribution from a metastable subcooled state which oil mixtures may attain. The latter is formulated in terms of the surface tension of the wax phase. Experimental wax appearance points (temperatures), WAP's, are reported for 17 different stabilized North Sea crude oils. The values predicted by the new model are in very good agreement with the experimental WAP's.

Critical impeller speed for solid suspension in mechanically agitated contactors

Abstract: Critical impeller speed for the suspension of solid particles, Ncs, has been measured in 0.3, 0.4, 0.57, 1.0, and 1.5 m ID mechanically agitated contactors. Tap water and quartz particles (100, 340, 700, 850, 2,000 μm) were used as liquid and solid phase, respectively. The impeller speed was varied from 3.5 to 13.3 r/s and solid loading from 0 to 50 wt. %. Three types of impellers were employed: disk turbine, pitched turbine downflow, and pitched turbine upflow. The impeller diameter to vessel diameter ratio was varied in the range 0.175 to 0.58 and the impeller blade width to impeller diameter ratio was varied in the range 0.25 to 0.4. The impeller clearance from the tank bottom was varied from 0.5 to 0.167 of tank diameter. The effect of impeller blade thickness was also studied. The pitched-blade impellers were found to be more efficient than a conventional disk turbine, and the pitched turbine downflow type was found to be more efficient than a pitched turbine upflow impeller. An attempt has been made to explain the mechanism of suspension on a rational basis and a correlation has been proposed for the estimation of Ncs that is expected to be useful in design.

Rigorous thermodynamic treatment of gas adsorption

Abstract: Multicomponent adsorption equilibria are predicted from theories based on experimental data for single-gas adsorption isotherms. Theories of multicomponent adsorption differ in the details of their results but should agree in special cases such as low surface coverage and ideal solution behavior. This paper is a summary of rules, limits, and consistency requirements that apply to adsorption of single gases and their mixtures. These rules and limits provide a basis for comparing theories with each other and with experimental data.

A nonrandom factor model for the excess gibbs energy of electrolyte solutions

Abstract: A new approach to represent the deviations from ideality of electrolyte solutions is derived on the basis of the assumptions of specific interactions of Bronsted and of local electroneutrality of Chen. The model consists of two contributions due to long-range forces, represented by the Debye-Huckel theory, and to short-range forces, represented by local compositions through nonrandom factors. With only two adjustable parameters per electrolyte, the model is valid for the whole range of electrolyte concentration, from dilute solution up to saturation. Results are compared with those obtained from two-parameter or one-parameter models of Meissner, Bromley, Pitzer, and Chen et al. The model presented in this work consistently produces better results and reproduces the experimental values from the dilute region up to saturation.

Dispersion of viscous liquids by turbulent flow in a static mixer

Abstract: Drops are stabilized in agitated liquid-liquid systems by both surface and internal viscous forces. The dispersion of an inviscid liquid into a turbulent continuous phase in static mixers has been studied but the effect of dispersed phase viscosity is not well understood. Systematic experiments have been conducted in a Kenics mixer by photographically examining dilute suspensions of viscous oils in water to determine how viscosity and conditions of agitation affect equilibrium mean drop size and size distribution. A semiempirical theory is developed which correlates the mean size data and collapses to the well-known Weber number result in the inviscid limit. A correlation for drop size distribution in terms of cumulative volume frequency is developed by normalization with the Sauter mean diameter D32. Measurements at the mixer entrance indicate that the method of introduction of the dispersed phase should be considered when evaluating mixer performance.

Adsorption of Gas Mixtures: Effect of Energetic Heterogeneity

Abstract: On aborde le probleme theoriquement en utilisant la methode de la solution adsorbee ideale heterogene qui agit idealement sur un site particulier, mais l'heterogeneite energetique provoque une segregation dans la composition de la phase adsorbee. Les resultats theoriques sont compares aux resultats experimentaux anterieurs, on observe une legere discordance

The continuous‐flow gravity thickener: Steady state behavior

Abstract: The equations governing consolidation in a continuous-flow gravity thickener are developed based on the assumption that a flocculated suspension possesses a compressive yield stress Py(ϕ) that is a function of local volume fraction only. These equations are used to model the steady state operation of a thickener. The bed height required to achieve a given underflow concentration is found to be a relatively sensitive function of the details of the Py(ϕ) function, particle flux through the thickener, and variations in the cross-sectional area of the thickener. The limiting values of the underflow concentration ϕu for a given flux or the limiting values of flux for a desired ϕu are studied and shown to exist only for cylindrical and converging thickeners.

Robust nonlinear state feedback under structured uncertainty

Abstract: Mise en place d'un systeme de commande qui permet d'obtenir des limites superieures de la modelisation de l'erreur pour les processus non lineaires, et de concevoir une loi de retroaction qui garantit une stabilite et de bonnes performances pour toute perturbation entrant dans les limites donnees. Ce systeme est applique a la commande d'un reacteur parfaitement agite isotherme ou ont lieu, en phase liquide, des reactions consecutives du type: A⇄B→C ou P→Q→R

Enzyme‐caytalyzed oxidation of cholesterol in supercritical carbon dioxide

Abstract: Supercritical carbon dioxide was studied as a solvent for the enzymatic oxidation of cholesterol with molecular oxygen. Enzymes isolated from Streptomyces sp., Norcardia sp., Pseudomonas sp., and Gloeocysticum chrysocreas are active in supercritical carbon dioxide. For the oxidation of cholesterol to cholest-4-ene-3-one (via cholesterol oxidase) from Gloeocysticum chrysocreas, turnovers are almost two orders of magnitude higher than those found in aqueous solutions; initial rates of the reaction catalyzed by Streptomyces sp. in supercritical carbon dioxide are comparable to or greater than those found in aqueous solutions. Cholesterol oxidase from Gloeocysticum chrysocreas is stable in supercritical carbon dioxide at 100 bar and 35°C, while the enzyme from Streptomyces sp. is not. A trace amount of water is necessary for catalysis in carbon dioxide. The rate of oxidation is increased markedly by addition of small amounts of tert-butanol and iso-butanol, moderately by ethanol, slightly by acetone and n-butanol, and not at all by addition of methanol. Solubility data cannot explain these observations. EPR spectroscopy reveals no large conformational changes in the enzyme from Gloeocysticum chrysocreas as a function of carbon dioxide pressure or cosolvent addition. However, EPR spectroscopy indicates that aggregation of cholesterol molecules in supercritical carbon dioxide is strongly affected by changes in pressure or cosolvent content. The degree of cholesterol aggregation correlates well with observed rate enhancements.

Hindered settling of semidilute monodisperse and polydisperse suspensions

Abstract: Etude experimentale menee sur des solutions mono, bi et tridispersees de particules non colloidales pour de faibles nombres de Reynolds. La correlation de Richardson et Iaki est satisfaisante pour les particules monodispersees. Pour les solutions diluees les valeurs experimentales sont en accord avec la theorie de Batchelor qui predit une decroissance lineaire de la vitesse de sedimentation avec l'accroissement de la concentration en particules

Two‐phase pressure drop in vertical crossflow across a horizontal tube bundle

Abstract: An experimental investigation has been made to evaluate the friction, acceleration, and hydrostatic pressure drops in two-phase vertical crossflow across a horizontal tube bundle through the measurement of the void fraction and determination of the two-phase friction multiplier. The void fractions were found to increase with increasing mass velocity for a fixed quality level. The two-phase friction multiplier increased with increasing mass velocity for a fixed value of the Martinelli parameter in both slug and spray flow and decreased with increasing mass velocity in bubbly flows. The void fraction and two-phase friction multiplier data were correlated and used to predict with very good results the total pressure drop occurring in simulated diabatic flow tests and in actual diabatic tests using R-113.

Separation of solutes from aqueous solutions by contained liquid membranes

Abstract: The hollow fiber contained liquid membrane (CLM) is a thin liquid film contained in the interstices of two sets of intermingled microporous hollow fine fibers. Organic CLM-s have been used here for the separation of solutes from an aqueous feed into an aqueous strip. Solutes studied are phenol and acetic acid. The separations are carried out in either hydrophilic or hydrophobic hollow fiber CLM permeator modules, using a variety of organic liquids (e.g., decanol, methyl isobutyl ketone, xylene) as membranes. First-order models have been developed to predict the overall solute transfer coefficients adequately. The transfer coefficient can be enhanced significantly when a chemical reaction is carried out on the strip side using NaOH. The advantages of the CLM structure include operational stability, independent control of membrane phase pressure, automatic replenishment of the lost membrane liquid, and absence of the need for preequilibration. These features are demonstrated here, even for systems with considerable aqueous-organic mutual solubilities.

Estimation of the molecular weight distribution in batch polymerization

Abstract: A filtering technique is proposed for on-line estimation of the temperature, monomer conversion, initiator conversion, and the entire molecular weight distribution in a batch methyl methacrylate polymerization reactor. The technique uses a detailed polymerization model combined with on-line measurements of conversion, temperature, and the molecular weight distribution, taken at different discrete time intervals. The polymerization model includes a chain-length-dependent termination rate constant which allows the prediction of the molecular weight distribution for common free-radical polymerization conditions. Comparisons between modeling and experimental results show that the polymerization model gives good predictions of the monomer conversion and the molecular weight distribution in the polymerization system. The performance of the estimation scheme is tested for cases of strong gel effect conditions leading to a bimodal molecular weight distribution, and poor initial conditions. Finally, off-line experimental data are used to test the algorithm under actual reactor operating conditions.

Heat capacity and heat of dissociation of methane hydrates

Abstract: The objective of this study was to determine the heat capacity and heat of dissociation of methane hydrates. A technique has been devised which circumvents the two major problems encountered in measuring gas hydrate heat capacity: the need to impose a mechanical pressure during the measurement and the need to have an absolutely pure hydrate sample. The technique was shown to be successful utilizing high-pressure, constant-volume cells in a differential scanning calorimeter.

Computer generation of reaction schemes and rate equations for thermal cracking

Abstract: Reaction schemes for the thermal cracking of paraffins, naphthenes, olefins, and aromatics, and the associated rate equations, are generated by an algorithm based upon Boolean relation matrices. In order to reduce calculation times for the heavier components, a self-learning system is introduced. This self-learning system avoids repetition of identical calculations by recognizing intermediate species, for which the reaction schemes have already been generated. The software includes routines for standardizing the representation of the species, an information retrieval system, and libraries containing the necessary information for recognition of the species and the reaction schemes for these species.

Micellar-enhanced ultrafiltration of chromate anion from aqueous streams

Abstract: Micellar-enhanced ultrafiltration can be used to remove multivalent anions or cations from aqueous streams. In the removal of chromate ions (CrO), the cationic surfactant hexadecylpyridinium chloride is added to the solution, and the chromate ions preferentially adsorb at the surface of the highly charged surfactant micelles. The solution is processed by ultrafiltration, using a membrane with pore sizes small enough to block the passage of the micelles and adsorbed ions. The permeate solution has a chromate concentration less than 0.1% that in the original stream. A new equilibrium model, combining the simple two-phase polyelectrolyte theory of Oosawa with thermodynamic activity, material-balance, and charge-balance equations, successfully correlates ultrafiltration and equilibrium dialysis results for chromate solutions.

Measurement and prediction of entrained-flow gasification processes.

Abstract: A detailed mathematical model is used to predict local and effluent properties within an axisymmetric, entrained-flow gasifier. Laboratory experiments were conducted to provide local properties for four coal types from a gasifier operating at near-atmospheric pressure. Effects of selected model parameters and test variables were examined and compared with measurements in most cases. The comparison of predictions and measurements provides the first evaluation of capabilities and limitations of a comprehensive model for entrained-flow gasifiers.

Effect of viscosity on concentration polarization in ultrafiltration

Abstract: Etude theorique et modelisation pour trois macromolecules couramment rencontrees: BSA, Sucrose et Dextrane