bluuebunny/crossref_metadata_2025 huggingface dataset:
Micromixing effects in a two‐impinging‐jets precipitator
Authors: Mahajan Amarjit J. , Kirwan Donald J.
Published: June 2004

To carry out rapid precipitation on a practical scale, the use of a two‐impinging‐jets (TIJ) mixer was explored in terms of its ability to deliver rapid micromixing. The two‐step Bourne reaction scheme between 1‐naphthol and diazosulfanilic acid was used to characterize micromixing in the TIJ mixer as a function of hydrodynamics and mixer geometry. The characteristic time for micromixing in this device was at least as small as 65 ms. The larger the jet diameter, the higher the jet Reynolds number required to achieve the same micromixing quality. The micromixing time in the TIJ mixer correlated well as a function of a Damkohler number that led to a scale‐up criterion giving an inverse relationship between the micromixing time constant and the 1.5 power of the jet velocity. Finally, the precipitation of Lovastatin was carried out in the TIJ mixer under well and poorly micromixed conditions. The level of micromixing in the precipitator affected the crystal size distribution of the precipitated product only when the time constant for micromixing was comparable to or larger than the induction time for nucleation of the solute.
Temperature polarization in mass transport through hydrophobic porous membranes
Authors: Martínez‐Díez L. , Vázquez‐Gonzàlez M. I.
Published: June 2004

Membrane distillation is a membrane process in which two liquid phases at different temperatures are separated by a microporous hydrophobic membrane. The membrane plays the role of physical support for the vapor‐liquid interface. Pure water transport through PTFE and PVDF flat membranes was studied, as well as the dependence of the phenomena on average temperature and recirculation rate at membrane sides. The influence of these operating conditions on mass transfer is also discussed, taking into account mass and heat transfer within the membrane and adjoining liquid phases. The concept of temperature polarization introduced into the transport equations shows its importance in experimental results.
Effect of pH and concentration on column dynamics of weak electrolyte ion exchange
Authors: Jansen Marcel L. , Hofland Gerard W. , Houwers Joop , Straathof Adrie J. J. , van der Wielen Luuk A. M. , Luyben Karel Ch. A. M. , van den Tweel Will J. J.
Published: June 2004

The column dynamics of simple ion‐exchange processes involving acetic acid and N‐acetylmethionine were studied using a strong‐base anion exchange resin to reveal the complex relation between pH, concentration, equilibrium isotherms, and chromatographic separation in ion‐exchange chromatography of weak electrolytes. Changing either the influent pH or the concentration, while keeping the other variable constant, led to considerable effluent concentration and pH fluctuations, respectively. This behavior is caused mainly by the uptake of undissociated acids. To describe these phenomena quantitatively, a rigorous thermodynamics‐based equilibrium model for ion exchange, including Donnan potential and reaction equilibria, was incorporated into a fixed‐bed model. Experimental results can be described well by this dynamic model, which is superior to conventional models due to its explicit incorporation of coions and partitioning of neutral species. It has great potential for optimizing chromatographic separations of weak electrolyte solutions.
Improved thermodynamic parameters for prediction of structure H hydrate equilibria
Authors: Mehta Ajay P. , Sloan E. Dendy
Published: June 2004

An improved set of all the thermodynamic and molecular properties required for the prediction of the existing 20 systems of Structure H (sH) hydrate phase equilibrium data is presented. The statistical thermodynamics model was based on the van der Waals and Platteeuw theory, and the spherical core Kihara potential was used for guest‐water interactions. Optimized Kihara parameters and reference thermodynamic properties were derived from experimental data of over 20 sH hydrate forming systems. The model could fit all the existing sH hydrate data within an accuracy of ± 6%. Inhibitor predictions were also shown to fit recent data with no adjustable parameters. The feasibility of using hydrate cage occupancies to derive refined Kihara parameters of the guest molecules was investigated. Possible existence of sH hydrates at cryogenic temperatures was also established based on the model.
Boundary effects on the sedimentation and hindered diffusion of charged particles
Authors: Pujar Narahari S. , Zydney Andrew L.
Published: June 2004

A general formalism is developed to examine the sedimentation of a charged particle in a bounded system at small Peclet numbers and small particle‐surface potentials. The excess viscous force is evaluated using a generalized form of the Lorentz reciprocal theorem, eliminating the need to calculate the detailed fluid flow around the charged particle. Specific calculations are provided for the sedimentation of a charged sphere in a concentric (uncharged) spherical cavity. Boundary interactions increase the magnitude of the excess force at small Debye lengths, with the opposite effect seen at very large Debye lengths. A general solution is presented for the sedimentation velocity for an arbitrary particle in both unbounded and bounded systems in the limit of very thin double layers. The excess force under these conditions is proportional to the square of the Debye length, with the proportionality constant being a function of the detailed system geometry. These results provide important insights into the transport of charged particles in porous membranes and chromatographic materials.
A moving horizon‐based approach for least‐squares estimation
Authors: Robertson Douglas G. , Lee Jay H. , Rawlings James B.
Published: June 2004

A general formulation of the moving horizon estimator is presented. An algorithm with a fixed‐size estimation window and constraints on states, disturbances, and measurement noise is developed, and a probabilistic interpretation is given. The moving horizon formulation requires only one more tuning parameter (horizon size) than many well‐known approximate nonlinear filters such as extended Kalman filter (EFK), iterated EKF, Gaussian second‐order filter, and statistically linearized filter. The choice of horizon size allows the user to achieve a compromise between the better performance of the batch least‐squares solution and the reduced computational requirements of the approximate nonlinear filters. Specific issues relevant to linear and nonlinear systems are discussed with comparisons made to the Kalman filter, EKF, and other recursive and optimization‐based estimation schemes.
Optimal design of dynamic systems under uncertainty
Authors: Mohideen M. Jezri , Perkins John D. , Pistikopoulos Efstratios N.
Published: June 2004

Fundamental developments of a unified process design framework for obtaining integrated process and control systems design, which are economically optimal and can cope with parametric uncertainty and process disturbances, are described. Based on a dynamic mathematical model describing the process, including path constraints, interior and end‐point constraints, a model that describes uncertain parameters and time‐varying disturbances (for example, a probability distributions or lower/upper bounds), and a set of process design and control alternatives (together with a set of control objectives and types of controllers), the problem is posed as a mixed‐integer stochastic optimal control formulation. An iterative decomposition algorithm proposed alternates between the solution of a multiperiod “design” subproblem, determining the process structure and design together with a suitable control structure (and its design characteristics) to satisfy a set of “critical” parameters/periods (for uncertainty disturbance) over time, and a time‐varying feasibility analysis step, which identifies a new set of critical parameters for fixed design and control. Two examples are detailed, a mixing‐tank problem to show the analytical steps of the procedure, and a ternary distillation design problem (featuring a rigorous tray‐by‐tray distillation model) to demonstrate the potential of the novel approach to reach solutions with significant cost savings over sequential techniques.
Monte Carlo simulation of carbon gasification using molecular orbital theory
Authors: Kyotani Takashi , Ito Ken‐ichi , Tomita Akira , Radovic Ljubisa R.
Published: June 2004

An attempt was made to simulate carbon gasification with the aid of molecular orbital theory. A simple Hückel method was applied to calculate molecular orbitals of a large polynuclear aromatic molecule, which was employed as a model crystallite for carbon. A free valence at each carbon atom in model crystallites of varying size and shape was determined, and this value was taken as a measure of reaction probability for the Monte Carlo simulation. Simulation of carbon gasification was performed by removing the carbon atoms from the model crystallites according to their corresponding reaction probabilities. This simulation made it possible to visualize how the shape of carbon crystallite is changed during gasification. It was found that the specific gasification rate depends on molecular size, but is essentially independent of the geometry of the starting model crystallite.
Controlled black liquor viscosity reduction through salting‐in
Authors: Roberts Joanna E. , Khan Saad A. , Spontak Richard J.
Published: June 2004

Black liquor viscosity increases exponentially with solids content and therefore causes processing problems for the paper industry by being a limiting factor in the Kraft pulp process. This study investigates a new approach for achieving viscosity reduction by “salting‐in” black liquor through the addition of thiocyanate salts. These salts generally increase the solubility of the polymer constituents in black liquor, leading to a decrease in its viscosity. Several thiocyanate salts capable of reducing liquor viscosity by more than two orders of magnitude have been identified, with viscosity reduction greatest at high solids content. Salting‐in of black liquor depends on the cation paired with the thiocyanate anion, as well as on solution pH and temperature. Comparative studies reveal that GuSCN is the most effective viscosity‐reducing agent of the series examined and that lignin plays an important role in the viscosity behavior of both unmodified and salted‐in black liquor at high solids concentrations. These experimental findings are interpreted in terms of the underlying principles that describe salting‐in and how it affects aqueous solution structure.
Analysis of excess free energy based equations of state models
Authors: Orbey Hasan , Sandler Stanley I.
Published: June 2004

A comparison is made of the capabilities of various equation of state (EOS) + excess free energy (Gex) models in representing the activity‐coefficient models they incorporate. Such EOS models need to reproduce the excess Gibbs free energy behavior of the activity‐coefficient models as closely as possible in order to represent low‐pressure vapor–liquid equilibrium behavior of complex mixtures accurately, and also to make accurate the vapor–liquid predictions at higher temperatures and pressures using only low‐pressure information. It is demonstrated that the EOS models that use the excess Helmholtz free energy instead of the excess Gibbs free energy, and that combine the EOS with activity‐coefficient models at the infinite pressure limit, are successful for calculations of the type just described.
Measurement and modeling of activities of amino acids in aqueous salt systems
Authors: Khoshkbarchi Mohammad K. , Vera Juan H.
Published: June 2004

Activity‐coefficient data were measured for NaCl in the system water–NaCl–DL‐valine at 25°C. The activity coefficients of DL‐valine were then calculated through rigorous thermodynamic relations. The activity coefficients of the amino acids in this and other aqueous systems containing electrolytes were modeled using an expression for the excess Gibbs free energy of the solution. The model combines a contribution for long‐range interactions, given either by the Bromley equation or the K‐V equation, and a contribution of short‐range interactions represented by a ternary NRTL or Wilson expression. The model accurately correlates the activity coefficients of ten amino acids in aqueous solutions and of six amino acids in aqueous electrolyte solutions.
Flow structure of the solids in a 3‐D gas–liquid–solid fluidized bed
Authors: Larachi F. , Cassanello M. , Chaouki J. , Guy C.
Published: June 2004

Local and macroscopic solids flow structure and kinematics in a 3‐D gas–liquid–solid fluidized bed were studied using a noninvasive radioactive‐particle tracking (RPT) technique. Based on the multisite detection of γ radiations emitted from a single radiolabeled tracer particle freely moving in the fluidized bed, RPT permitted to obtain fast sampling of 3‐D trajectories of the tracer, whose physical properties were similar to those of the solids inventory. These trajectories showed the detailed motion sequences of the solid particles as entrained in the bubble wakes, fluctuating randomly or sinking deterministically in the liquid–solid emulsion. Based on measurements done in the vortical‐spiral flow regime, the dynamic solids flow structure inside a three‐phase fluidized bed can be viewed as a three‐zone core–annulus–annulus structure: (1) a central fast‐bubble flow region with the particles swirling upward; (2) a vortical flow region around the velocity inversion point with the particles momentarily captured in emulsion vortices; and (3) a relatively bubble‐free descending flow region where the particles spiral down between the velocity inversion point and vessel walls. Our solids flow structure of dense fluidized beds are similar to the flow structure of liquid and/or solid in lean fluidized beds (observed through laser sheeting imaging). Measured distributions of local ensemble‐averaged particle velocities and turbulence intensities were consistent with the existence of a toroidal recirculatory solids flow pattern in the bed. Measured mean circumferential ensemble‐averaged radial velocity was essentially zero throughout most of the bed. The solids flow turbulence field was nonisotropic, as radial turbulence intensities were generally lower than longitudinal turbulence intensities.
Geometry and heat loss in channel flow of a fluid with temperature‐dependent density
Authors: Bechtel Stephen E. , Cao Dongbu , Hsiao Hsien‐fu
Published: June 2004

Cao et al. previously derived a thermomechanically constrained theory for materials with temperature‐dependent density and applied it to the illustrative problem of plane Poiseuille flow between isothermal walls. Here the theory is applied to geometries and thermal boundary conditions of practical importance in polymer processing: flows through planar, circular and annular dies with heat loss through the die walls. How geometry and thermal boundary conditions combine with material properties, especially thermal expansivity, to effect velocity and temperature profiles, and mass and volume flow rates is explored. A comparison is made with predictions that follow if temperature‐dependence of density is either ignored or handled (as is the standard practice) by a posteriori insertion of a temperature‐dependent expression for density into equations derived for constant density.
Effect of condensation heat on the condensation coefficient
Authors: Chekmarev Sergei F.
Published: June 2004

To account for the effect of condensation heat on the condensation coefficient (α), a semiempirical model is proposed. The model contains two free parameters: a characteristic size of the “hot spot” where the heat release occurs and its lifetime. At reasonable values of the parameters, the model agrees well with experimental and computer‐simulated data for both simple and associating liquids. For nonequilibrium conditions, the model leads to the Hertz–Knudsen equation with α dependent not only on characteristics of the liquid but also on those of ambient vapor. This offers an explanation for the existing scattering of experimental data in α. Among other things, the expression for α obtained shows that for the net evaporation process α is always higher than that for the net condensation, and moreover, α = 1 for all liquids if evaporation occurs in vacuum. The model also indicates that for complex associating liquids like glycerol α may be quite sensitive to peculiarities of the liquid state under consideration.
Process modeling using stacked neural networks
Authors: Sridhar Dasaratha V. , Seagrave Richard C. , Bartlett Eric B.
Published: June 2004

A new technique for neural‐network‐based modeling of chemical processes is proposed. Stacked neural networks allow multiple neural networks to be selected and used to model a given process. The idea is that improved predictions can be obtained using multiple networks, instead of simply selecting a single, hopefully optimal network, as is usually done. A methodology for stacking neural networks for plant‐process modeling has been developed. This method is inspired by the technique of stacked generalization proposed by Wolpert. The proposed method has been applied and evaluated for three example problems, including the dynamic modeling of a nonlinear chemical process. Results obtained demonstrate the promise of this approach for improved neural‐network‐based plant‐process modeling.
Sensor network design for maximizing reliability of bilinear processes
Authors: Ali Yaqoob , Narasimhan Shankar
Published: June 2004

The problem of selecting the variables to be measured in order to maximize process reliability was tackled in our previous articles (Ali and Narasimhan, 1993, 1995). In this article, this approach is extended to the optimal design of sensor networks for bilinear processes. Diverse processes, such as a mineral beneficiation plant, a separation system of a synthetic juice plant, and a crude preheat train of a refinery are used to illustrate the utility of this approach.
Microencapsulation in polyurea shell: Kinetics and film structure
Authors: Yadav S. K. , Khilar Kartic C. , Suresh A. K.
Published: June 2004

Interfacial polycondensation is an important polymerization technique that encapsulates a variety of active principles. Mechanisms governing the reaction and interplay of physical and chemical rate processes need to be understood for both rational design of reaction equipment and the process control to produce capsules with desired characteristics. A theoretical and experimental study of the process is reported here. Kinetic data were obtained over a range of concentrations, monomer mole ratios and polymer film thicknesses, using a technique that relies on the on‐line measurement of pH as a function of time. To understand the mechanisms the reaction was slowed down by reducing the interfacial area. A minimum thickness of the polymer was observed to be necessary so that capsules preserve their integrity and do not break up. The theoretical model developed considers ionic equilibria in the aqueous phase and the resistance due to external mass transfer, diffusion through the polymer, and interfacial reaction. Under the conditions chosen, the resistance due to the chemical reaction is generally more dominant. Values of rate parameters were determined by fitting the model to the experimental data. Observed variations in the diffusivity between different experiments were rationalized through a study of the crystalline structure of polymers.
Continuous lumping model for simulation of hydrocracking
Authors: Laxminarasimhan C. S. , Verma R. P. , Ramachandran P. A.
Published: June 2004

Hydrocracking of vacuum gas oil is an important chemical process involving complex reaction mixtures. The reaction is carried out in a trickle‐bed reactor, considering reaction kinetics along with such hydrodynamic effects as mass transfer, intraparticle diffusion, and partial wetting. Since reaction kinetics is critical to modeling and simulation of a hydrocracking reactor, a modeling approach needs to capture the complex chemistry of the process, along with the elegance of the solution method. The complex chemistry of hydrocarbon is represented by an elegant continuous lumping approach to modeling. The true boiling point of the mixture is used as the characterization parameter. Since the rate constant of hydrocracking is assumed to be a monotonic function of the true boiling point, it is possible to reformulate mass‐balance equations in terms of rate constant as a continuous variable. A novel distribution function p(k, K), which determines the fractional yield distribution of species, was formulated based on data from the cracking patterns of various model compounds. Resulting integrodifferential equations are solved numerically to obtain yields of various fractions as a function of reactor residence time. Model predictions are compared with limited published data to show the utility of the model.
Transport phenomena and kinetics in an extravascular bioartificial pancreas
Authors: Buladi Bora M. , Chang Chen C. , Belovich Joanne M. , Gatica Jorge E.
Published: June 2004

A bioartificial pancreas, consisting of immobilized islets encapsulated within hollow fibers, is investigated as an alternative treatment for insulin‐dependent diabetes. A mathematical model is developed to determine whether this configuration of the bioartificial pancreas can yield an insulin response to a glucose challenge with the appropriate dynamics in diabetic humans. The model consists of the 2‐D mass‐conservation equations for glucose and insulin within the hollow fiber and capillaries. The equations contain terms for insulin‐production kinetics by porcine islets and glucose‐consumption kinetics. The boundary conditions account for transport resistances of the fiber membrane, the tissue surrounding the implant, and a thin film within the capillaries. The equations are coupled to a pharmacokinetic model of the circulatory system. The calculations show that an optimized design with this configuration will be feasible for human use and requires a total volume of 4.6 mL to reach the target insulin concentration in the bloodstream following a glucose challenge. The parameters and processes controlling the system performance are discussed.
Sorption‐enhanced reaction process
Authors: Carvill B. T. , Hufton J. R. , Anand M. , Sircar S.
Published: June 2004

A process for carrying out simultaneous reaction and separation of desired products in a single unit operation is described. It uses a fixed packed column of an admixture of a catalyst and a sorbent that selectively removes a reaction by‐product from the reaction zone. The sorbent is periodically regenerated by using the principles of pressure‐swing adsorption. The process steps allow direct production of the desired product at high purity and at the reaction pressure. High conversion of the reactants to products in an endothermic, equilibrium‐controlled reaction can be achieved while operating the reaction at a substantially lower temperature than would be necessary by a plug‐flow reactor packed with the catalyst alone. The equilibrium‐controlled reverse water‐gas shift reaction for the production of carbon monoxide is experimentally evaluated as a proof of the concept.
Theory for multicomponent adsorption equilibrium: Multispace adsorption model
Authors: Gusev Vladimir , O'Brien James A. , Jensen Craig R. C. , Seaton Nigel A.
Published: June 2004

A new theory for the prediction of multicomponent adsorption equilibrium in non‐crystalline porous solids is presented: the multispace adsorption model (MSAM). This theory allows for the inherent nonuniformity of the adsorbed phase by describing separately the adsorption in “spaces” close to, and far from, the adsorbent surface. The inputs to the calculation are the pure‐species adsorption isotherms and a single parameter characteristic of the adsorbent that can be obtained from very limited mixture data. We evaluate the theory by predicting multicomponent adsorption for a range of carbon adsorbents, making use of our own experimental results and literature data. The MSAM gives quantitative predictions for all the systems studied.
Optimal pressures and temperatures for isobaric, isothermal chemical vapor infiltration
Authors: Ofori John Y. , Sotirchos Stratis V.
Published: June 2004

This study addresses the possibility identifying optimal pressure and temperature pairs that can be used to give the least processing time during isobaric isothermal chemical‐vapor infiltration (CVI) within a window of operating conditions. Optimal pressure—temperature pairs are determined as the solution of an optimization problem that seeks to minimize the processing time predicted by a CVI model under the constraint that the maximum density difference in the preform at the end of the densification does not exceed a preset limit. Results are presented on the dependence of the optimum processing time on the type of structure of the preform, the specifications that the final product has to conform to, and the kinetics of the chemical reaction. The results show that operation at low pressure affects adversely the processing time and that a dramatic reduction in the processing time can be achieved by operating in the vicinity of the optimal pressure.
Data reconciliation and gross‐error detection for dynamic systems
Authors: Albuquerque João S. , Biegler Lorenz T.
Published: June 2004

Gross‐error detection plays a vital role in parameter estimation and data reconciliation for dynamic and steady‐state systems. Data errors due to miscalibrated or faulty sensors or just random events nonrepresentative of the underlying statistical distribution can induce heavy biases in parameter estimates and reconciled data. Robust estimators and exploratory statistical methods for the detection of gross errors as the data reconciliation is performed are discussed. These methods have the property insensitive to departures from ideal statistical distributions and to the presence of outliers. Once the regression is done, the outliers can be detected readily by using exploratory statistical techniques. Optimization algorithm and reconciled data offer the ability to classify variables according to their observability and redundancy properties. Here an observable variable is an unmeasured quantity that can be estimated from the measured variables through the physical model, while a nonredundant variable is a measured variable that cannot be estimated other than through its measurement. Variable classification can be used to help design instrumentation schemes. An efficient method for this classification of dynamic systems is developed. Variable classification and gross‐error detection have important connections, and gross‐error detection on nonredundant variables has to be performed with caution.
Diffusion of additives during in‐situ suspension polymerization
Authors: Cunningham M. F.
Published: June 2004

High value‐added specialty composite materials can now be made by in‐situ suspension polymerization of monomers with additives incorporated to impart a specific property. In these composite particles, the nature of the additive dispersion is often of critical importance in determining final product properties. During polymerization, viscosities in the droplets are sufficiently low that significant diffusion can occur, resulting in dramatic changes to the additive dispersion quality. This article examines the importance of process parameters controlling diffusion lengths of small particle additives during in‐situ polymerizations. A model was developed to provide a basis for devising polymer reactor operating strategies that minimize small‐particle diffusion. Effects of varying polymerization rate, molecular weight and initial conversion were investigated. Model results were validated by comparing to experimental data. Increasing the initial conversion is shown to be highly effective in reducing diffusion of additive particles.
Preferentially oriented submicron silicalite membranes
Authors: Lovallo Mark C. , Tsapatsis Michael
Published: June 2004

An oriented submicron silicalite membrane has been prepared by growing a layer of oriented silicalite crystals on a composite precursor nanocrystalline silicalite/alumina film using controlled secondary growth. The orientation of the crystals at the surface was such that both straight and sinusoidal channel networks run nearly parallel to the membrane surface. The membrane exhibits ideal selectivities for H2 over N2 as high as 60 at 150°C and O2 over N2 as high as 3.6 at 185°C. H2, N2 and O2 permeances are 2.13, 0.05 and 0.17 cm3(STP)/(cm2·min·atm) at 185°C, respectively, and the corresponding apparent activation energies are 11, 26 and 32 kJ/mol. The permeation characteristics are attributed to the preferred orientation of the molecular sieving layer.
Design and operation of convective industrial dryers
Authors: Kiranoudis C. T. , Maroulis Z. B. , Marinos‐Kouris D.
Published: June 2004

Design and operational performance of convective industrial dryers are an important field of chemical engineering, which is still governed by empiricism. This article addresses the design vs. operation problem for three basic types of continuous convective industrial dryers: conveyor‐belt, fluidized bed, and rotary. Design procedures determined the optimal construction and operational characteristics in terms of total annual cost for each type involved and for a given production capacity through appropriate mathematical modeling. All dryer were compared by evaluating optimum configurations for a wide range of product characteristics and production capacity values. Once the dryer configuration was specified, its operational performance was evaluated by comparing the optimum operational cost vs. production capacity for predefined optimum designed structures. Rotary dryers were more expensive to design than fluidized bed dryers. Operationally, however, it is the other way around due to the favored heat transfer achieved in rotary dryers. Conveyor‐belt dryers lie somewhere between producing satisfactory results in terms of both design and operation. Case studies on foods and inorganics are included to demonstrate the performance of each process as well as the effectiveness of the proposed approach.
Model discrimination and criticism with single‐response data
Authors: Stewart Warren E. , Henson Thomas L. , Box George E. P.
Published: June 2004

The inverse probability theorem of Bayes is used, along with sampling theory, to obtain objective criteria for choosing among rival models. Formulas are given for the relative posterior probabilities of candidate models and for their goodness of fit, when the models are fitted to a common data set with Normally distributed errors. Cases of full, partial and minimal variance information are treated. The formulas are demonstrated with three examples, including a kinetic study of a catalytic reaction.
Wave model for longitudinal dispersion: Application to the laminar‐flow tubular reactor
Authors: Kronberg A. E. , Benneker A. H. , Westerterp K. R.
Published: June 2004

The wave model for longitudinal dispersion, published elsewhere as an alternative to the commonly used dispersed plug‐flow model, is applied to the classic case of the laminar‐flow tubular reactor. The results are compared in a wide range of situations to predictions by the dispersed plug‐flow model as well as to exact numerical calculations with the 2‐D model of the reactor and to other available methods. In many practical cases, the solutions of the wave model agree closely with the exact data. The wave model has a much wider region of validity than the dispersed plug‐flow model, has a distinct physical background, and is easier to use for reactor calculations. This provides additional support to the theory developed elsewhere. The properties and the applicability of the wave model to situations with rapidly changing concentration fields are discussed. Constraints to be satisfied are established to use the new theory with confidence for arbitrary initial and boundary conditions.
Kinetics of silica particle formation in nonionic W/O microemulsions from TEOS
Authors: Chang Chia‐Lu , Fogler H. Scott
Published: June 2004

The kinetics of silica particle formation by the ammonia‐catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) in water‐in‐oil (W/O) microemulsions containing a nonionic surfactant was investigated using Fourier‐transform infrared spectroscopy, transmission electron microscopy, and light‐scattering techniques. The results show that TEOS hydrolysis and silica‐particle growth occur at the same rate, indicating the growth of silica particles is rate‐controlled by the hydrolysis of TEOS. The rate of TEOS hydrolysis in microemulsions is first order with respect to the concentration of aqueous ammonia (29 wt. % NH3), but depends weakly on the concentration of water. Based on the fact that TEOS hydrolysis follows a nucleophilic (SN2) substitution of the TEOS's ethoxy group with hydroxyl ion, the kinetic data suggest that both water and ammonia remain predominately in W/O microemulsion droplets. The rate of TEOS hydrolysis also depends on the surfactant concentration that controls the molecular contact between hydroxyl ions and TEOS in the solution. Due to the reaction‐controlled growth mechanism, the silica‐particle size distribution retains virtually the same shape over the growth period. The final average size of silica particles can be varied from 26 to 43 nm by adjusting concentrations of water and surfactant. Increasing the water concentration decreases the average size and uniformity of silica particles. For a given water concentration, the smallest and most uniform silica particles are produced at intermediate water‐to‐surfactant molar ratios (about 1.9). The results are discussed in terms of the effect of water concentration on the stability of the hydrolyzed silica reacting species during the nucleation of particles and of the water‐to‐surfactant molar ratio on the compartmentalization of silica species in microemulsions.
Miscibility of polymer blends with engineering models
Authors: Harismiadis Vassilis I. , van Bergen A. R. D. , Saraiva Ana , Kontogeorgis Georgios M. , Fredenslund Aage , Tassios Dimitrios P.
Published: June 2004

The miscibility behavior of polymer blends that do not exhibit strong specific interactions is examined. Phase equilibrium calculations are presented with the van der Waals equation of state and three group‐contribution models (UNIFAC, Entropic‐FV, and GC‐Flory). Performance of these models is also compared. The van der Waals equation of state was recently shown to accurately correlate and predict vapor–liquid and liquid–liquid equilibria for binary polymer/solven solutions. In this work, it is demonstrated that it correlates the upper critical solution behavior of polymer blends with excellent accuracy using the usual mixing and combining rules and a single temperature‐ and composition‐independent binary interaction parameter. This interaction parameter can be predicted via a generalized expression that uses only the pure component equation‐of‐state parameters. Using this generalized expression, the upper critical solution temperature can be predicted with an average error of less than 45°C. The van der Waals equation of state can correlate the lower critical solution behavior of polymer blends, using an interaction parameter that is a linear function of temperature. The UNIFAC and Entropic‐FV models, in general, are able to predict qualitatively the phase behavior of polymer blends, but quantitative predictions of the critical solution temperatures are not achieved. The GC‐Flory equation of state fails to predict the upper critical solution behavior in polymer blends.
Molecular thermodynamic model for gibbs energy of mixing of nonionic surfactant solutions
Authors: Chen Chau‐Chyun
Published: June 2004

A segment‐based molecular thermodynamic model for the Gibbs energy of polymer solutions or oligomer solutions is used to represent the Gibbs energy of mixing for aqueous nonionic surfactant solutions. In contrast to the mass‐action models and pseudophase models, which are often discussed in the literature, the Gibbs‐energy model provides an explicit account of the solution nonideality, including activity coefficients of monomeric amphiphiles in the aqueous phase. The equilibrium between monomeric amphiphiles and micellar amphiphiles is described by the equal‐activity relationship. The Gibbs‐energy model makes it possible to use a well‐accepted molecular thermodynamic framework to correlate and represent phase behaviors of aqueous nonionic surfactant solutions.
Chaotic mixing of two similar fluids in the presence of a third dissimilar fluid
Authors: Zhang D. F. , Zumbrunnen D. A.
Published: June 2004

Two‐dimensional chaotic mixing of similar Newtonian fluids in the presence of an advected dissimilar minor phase fluid body with specified size, interfacial tension, and viscosity ratio was numerically investigated. Interfacial tension was sufficiently high to allow only small deformations in the dissimilar minor phase body. Mixing was confined to a rectangular cavity with periodically driven upper and lower surfaces. Regions of regular motion (i.e., islands) of comparable size to the minor phase body were eventually destroyed or replaced by the minor phase body. Islands persisted for longer times when the initial separation distance between the minor phase body and island was large or when the viscosity ratio was small. When interfacial tension was small enough to deform the minor phase body more readily, islands showed little indication of instability. Results suggest opportunities for improving mixing uniformity in practical processes and disclose how interactions between dissimilar fluids affect mixing.
Decomposition techniques for the solution of large‐scale scheduling problems
Authors: Bassett Matthew H. , Pekny Joseph F. , Reklaitis Gintaras V.
Published: June 2004

With increased product specialization within the chemical‐processing industries, the ability to obtain production schedules for complex facilities is at a premium. This article discusses ways of quickly obtaining solutions for industrially relevant, large‐scale scheduling problems. A number of time‐based decomposition approaches are presented along with their associated strengths and weaknesses. It is shown that the most promising of the approaches utilizes a reverse rolling window in conjunction with a disaggregation heuristic. In this method, only a small subsection of the horizon is dealt with at a time, thus reducing the combinatorial complexity of the problem. Resource‐ and task‐unit‐based decompositions are also discussed as possible approaches to reduce the problem to manageable proportions. A number of examples are presented throughout to clarify the discussion.
Quartz dissolution kinetics from 100–200°C as a function of pH and ionic strength
Authors: Worley W. G. , Tester J. W. , Grigsby C. O.
Published: June 2004

Of importance to geothermal energy development, oil, gas and mineral recovery, and waste storage is the characterization of the dissolution rate of host reservoir rock as a function of temperature, pressure and liquid‐phase composition. As a major constitutive mineral in natural geologic systems, quartz was selected for study. Dissolution experiments were carried out in a continuous‐flow, titanium autoclave reactor system at 100–200°C in various chemical environments. Acidification to pH 1.1 using nitric acid showed very little effect on the quartz dissolution rate. The effect of hydroxide ion concentration and ionic strength were evaluated in NaOH, NaOH/NaCl and NaOH/Na2SO4 solutions. The fractional‐order dependency of the quartz dissolution rate on hydroxide ion and sodium ion (or ionic strength) concentration was determined in NaOH/NaCl solutions. The results that extend the available range of kinetic data for quartz generally agree with previous work. The observed fractional‐order kinetics were qualitatively described using classical adsorption isotherms. No significant variation in the apparent reaction order of the hydroxide ion with increasing temperature could be determined due to the scatter in the data. Quartz dissolution rates were slower by about 40% in NaOH/Na2SO4 solutions than in NaOH/NaCl solutions at sodium concentrations higher than 0.01 molal. The apparent activation energy from 100 to 200°C in NaOH/NaCl solutions up to 0.01 molal hydroxide ion and 0.1 molal sodium ion was estimated to be 72 (±6) kJ/mol.
Intrinsic nitridation kinetics of high‐purity silicon powder
Authors: Maalmi Mostafa , Varma Arvind
Published: June 2004

The intrinsic nitridation kinetics of high‐purity silicon solid to form silicon nitride were determined by thermogravimetric analysis technique. The initial rate method was used to avoid kinetics falsification due to intraparticle diffusion limitation caused by the formation of silicon nitride layer around the silicon particles. Relatively small sample sizes were used to eliminate the interparticle diffusion limitation (compact effect) that becomes important as the reaction proceeds, owing to the expansion of the individual particles and filling of the initial pores within the compact. The effect of hydrogen addition to the nitriding gas, as well as the influence of the flow rate, on the reaction progress was also investigated.
Miniaturization of hydroprocessing catalyst testing systems: Theory and practice
Authors: Sie S. T.
Published: June 2004

Main factors limiting downscaling of fixed‐bed integral reactors are discussed, which operate with a gas stream or with gas and liquid in trickle flow, as used in hydroprocessing of oil. Criteria developed for a sufficiently close approach to plug flow and for good contacting of the catalyst show that they can be met in very small reactors, both for processes with reactants in the gas phase and for trickle‐flow processes, provided that the catalyst bed is diluted with fine inert material in the latter case. Experimental tests show that microreactors with typical catalyst volumes of 5–10 mL can be used to obtain representative results that can very well match data from industrial reactors. It appears to be feasible to miniaturize catalytic test reactors further to “nanoscale” reactors with as little as 0.2–0.4 mL of catalyst while maintaining the results to be meaningful. Even though there are advantages as well as limitations, miniaturization can enhance safety and reduce manpower.
Diffusion of proteins and nonionic micelles in agarose gels by holographic interferometry
Authors: Kong Donna D. , Kosar T. Fettah , Dungan Stephanie R. , Phillips Ronald J.
Published: June 2004

Diffusion coefficients measured by holographic interferometry are presented for two globular proteins and three nonionic surfactant species in agarose gels. The cloud points for the surfactants in agarose solutions are also given. In all cases, rates of diffusion in the gel are smaller than those in bulk solution, with the hindering effect of the gel increasing both with increasing gel concentration and with increasing solute size. It is shown that the diffusion rate of the surfactant micelles is very similar to that of proteins having similar hydrodynamic radii; it would therefore appear that the size and shape of the micelles in the solution and gel phases are similar. In addition, the measured rates of hindered diffusion agree very well with rates predicted by a rigorous hydrodynamic theory in which the solutes are modeled as hard spheres and the gel fibers are modeled as straight, cylindrical fibers. Hence, for the range of conditions considered, rates of hindered diffusion for both the globular proteins and the micellar species are determined primarily by hard‐sphere hydrodynamic and steric interactions between the solute and the gel matrix.
Analysis of cake growth in cake filtration: Effect of fine particle retention
Authors: Tien Chi , Bai Renbi , Ramarao B. V.
Published: June 2004

Equations were derived that describe the dynamics of cake growth in cake filtration, and methods for their solutions were developed. In deriving the equations, the moving boundary nature of the cake formation process and the effect of fine particle retention were considered. It was shown that fine particle retention may contribute significantly to the decrease of cake permeability and thus alters the performance of cake filtration even if the amount of fine particles involved is small. Through numerical examples, it was also demonstrated that fine particle retention in filter cakes may cause serious errors in determining the constitutive relationships from filter test data.
Synthesis of drowning‐out crystallization‐based separations
Authors: Berry David A. , Dye Susan R. , Ng Ka M.
Published: June 2004

A method is presented to synthesize drowning‐out crystallization‐based separation schemes for binary mixtures. The method tracks how the composition of each process stream changes from unit to unit on a phase diagram by assuming that the system is at equilibrium. Features of phase behavior favorable to drowning‐out processes are identified. Two types of systems are highlighted. In the first, the drowning‐out agent causes the solute to become sparingly soluble. Several process alternatives to separate the feed into pure components are proposed. Dominant costs of each configuration are identified. In the second, the solute is extracted into a phase rich with respect to the drowning‐out agent. The extraction can be performed in a decanter, countercurrent extractor, or fractional countercurrent extractor. Guidelines are given to select an extractor type and to choose between crystallizer‐extractor and extractor‐crystallizer equipment trains.
Analysis of rate enhancement in a periodically operated trickle‐bed reactor
Authors: Gabarain L. , Castellari A. T. , Cechini J. , Tobolski A. , Haure P.
Published: June 2004

When a trickle‐bed reactor (TBR) is operated periodically, the bed is fed with liquid on and off, while the gas phase passes continuously. Rates and conversions could be higher than those corresponding to the steady‐state operation. In the “dry cycles” the heat generated by the reaction can drive the vaporization of the liquid phase, and a much more rapid “gas phase” reaction may occur. There is a trade‐off between the potential for hot‐spot formation and the fact that overall rates are higher during cycling. A qualitative understanding of the phase transition in TBRs is obtained by a simple phenomenological model that takes into account different transport and reaction mechanisms occurring in a catalytic particle under different cycling conditions. A mathematical model is used to predict results.
Vapor–liquid equilibria of the ternary mixture CH4 + C2H6 + CO2 from molecular simulation
Authors: Vrabec Jadran , Fischer Johann
Published: June 2004

The NpT + test‐particle method for the calculation of vapor–liquid equilibria by molecular simulations is extended to ternary mixtures. It is applied to the system methane + ethane + carbon dioxide, for which all binary molecular interaction models are available from previous work. Methane is described as one‐center Lennard‐Jones fluid, ethane as two‐center Lennard‐Jones fluid, and carbon dioxide as two‐center Lennard‐Jones plus point quadrupole fluid. The unlike interactions are treated in the same way as the binary mixtures, using two parameters for each binary interaction. No ternary parameters are introduced. Vapor–liquid phase equilibria are calculated for the ternary mixture at the following temperature–pressure pairs: 233.15 K ‐ 2 MPa; 250.5 K ‐ 2 MPa; and 250.5 K ‐ 3.04 MPa. Comparison of the simulation data with experimental and equation‐of‐state results shows excellent agreement. Bubble and dew densities are also reported.