bluuebunny/crossref_metadata_2025 huggingface dataset:
Circumferential tube skin temperature profiles in thermal cracking coils
Authors: Heynderickx G. J. , Cornelis G. G. , Froment G. F.
Published: June 2004

Uniform temperature distributions in cracking furnaces favor the run length of the production cycle of the furnace and the tube metal life. The coupled simulation of the furnace and the reactor tubes by means of the zone method is extended to permit the calculation of circumferential nonuniformities under reaction conditions. Circumferential tube skin temperatures were found to vary over 50°C and more, due to “shadow effects.” As a result, nonuniform coking rates and coke layers are obtained. The model and computational scheme presented here can be used as a guide for plant operators and as a tool for designing.
Flow of dilute gas‐particle suspensions
Authors: Rhodes Martin , Zhou Shihui , Benkreira Hadj
Published: June 2004

A semi‐empirical model of the radial segregation of solids in upward flow of dilute gas‐particle suspensions in riser systems is presented on the basis of a reduced form of the fundamental two‐phase flow governing equations and experimental evidence concerning the solids concentration at the wall. The following simple expression for the radial solids concentration profile is obtained:

and is in agreement with experimental data over a wide range of operating conditions: superficial gas velocity from 1.4 to 15.3 m/s, riser diameter from 0.032 to 0.40 m, imposed solids flux from 6.60 to 207 kg/m2·s, mean particle size from 32 to 120 μm and particle density from 1,000 to 3,500 kg/m3.

The model confirms the existence of the core‐annulus flow structure in gas‐particle suspensions reported in riser reactors, circulating fluidized beds, and the freeboard of bubbling fluidized beds.
Analytical and knowledge‐based redundancy for fault diagnosis in process plants
Authors: Fathi Zohreh , Ramirez W. Fred , Korbicz Jozef
Published: June 2004

The increasing complexity of process plants and their reliability have necessitated the development of more powerful methods for detecting and diagnosing process abnormalities. Among the underlying strategies, analytical redundancy and knowledge‐based system techniques offer viable solutions. In this work we consider the adaptive inclusion of analytical redundancy models (state and parameter estimation modules) in the diagnostic reasoning loop of a knowledge‐based system. This helps overcome the difficulties associated with each category. The design method is a new layered knowledge base that houses compiled/qualitative knowledge in the high levels and process‐general estimation knowledge in the low levels of a hierarchical knowledge structure. The compiled knowledge is used to narrow the diagnostic search space and provide an effective way of employing estimation modules. The estimation‐based methods that resort to fundamental analysis provide the rationale for a qualitatively‐guided reasoning process. The overall structure of the fault detection and isolation system based on the combined strategy is discussed focusing on the model‐based redundancy methods which create the low levels of the hierarchical knowledge base. The system has been implemented using the condensate‐feedwater subsystem of a coal‐fired power plant. Due to the highly nonlinear and mixed‐mode nature of the power plant dynamics, the modified extended Kalman filter is used in designing local detection filters.
In‐situ measurement of interfacial pH using a rotating ring‐disk electrode
Authors: Hessami Sholeh , Tobias Charles W.
Published: June 2004

A nonintrusive technique for the in‐situ measurement of interfacial pH and current efficiency has been developed. A ring electrode, operated potentiometrically at opencircuit, is used to measure the pH change at a rotating disk electrode. The technique takes advantage of the well‐characterized hydrodynamics at the rotating disk electrode and has the added advantage that the pH probe, the ring electrode, is not interfering with the flow field and the current distribution on the disk. To determine the pH at the disk electrode by measuring the potential of the ring, the radial transport of hydronium ions across the insulating gap and on the ring is analyzed taking into account the effect of homogeneous dissociation reactions of water and metal‐hydroxide ion complexes. Shifts in the ring potential caused by hydrogen super‐saturation and ohmic drop are also evaluated. A platinized ring electrode in a hydrogensaturated electrolyte is shown to provide a stable and reproducible hydrogen ion sensor with a Nernstian response to the changes in the bulk pH. Performance of the ring is evaluated by generating hydrogen at the disk electrode from a dilute acid solution, in the absence of other electrochemical reactions. The technique is then applied to determine the interfacial pH of nickel, iron and nickel‐iron alloy electrodeposition with concurrent hydrogen evolution. This method was also used to measure the current efficiency of nickel electrodeposition in a fast, nonintrusive and in‐situ manner.
Circulation model for absorption and dispersion in cocurrent bubble columns
Authors: Rice Richard G. , Geary Nicholas W. , Burns Louis F.
Published: June 2004

Liquid circulation and a finite entrance zone are accounted for in modeling gas absorption into cocurrent flows of pure oxygen and CMC solutions. The new models lead to a Taylor‐like correction factor to account for circulation in both absorption and transient dispersion experiments. Steady‐state experiments for viscosities of 1, 12, 25 and 50 mPa·s were conducted by measuring dissolved gas concentration profiles along the column axis. Liquid mixing experiments were performed using the transient acid‐base neutralization method, leading to measured values of axial dispersion coefficients which depended on viscosity to the power 0.75. Measurements suggest that circulation was minuscule owing to uniform gas injection by Flexisparger in the present column.
Effect of energy distribution on sorption kinetics in bidispersed particles
Authors: Hu Xijun , Rao Gade N. , Do Duong D.
Published: June 2004

In this article, we present a model to describe sorption kinetics of gaseous adsorbates onto bidispersed heterogeneous microporous particles. This model describes adsorption and desorption of gaseous adsorbates with allowance for the energy distribution of adsorption site for both equilibrium isotherm and diffusion of the adsorbed species. The adsorbed species of all energy level diffuse in two directions inside the particle. One direction is the direction along the particle coordinate while the other is the direction perpendicular to the particle coordinate (that is, the direction into the grain of the solid). This model is more fundamental than models previously proposed by Do and coworkers. Effects of energy distribution on the adsorption and desorption dynamics are investigated. Extensive experimental analysis of adsorption and desorption dynamics of ethane and propane onto Ajax activated carbon are carried out to validate this theory.
Model predictive control with linear models
Authors: Muske Kenneth R. , Rawlings James B.
Published: June 2004

This article discusses the existing linear model predictive control concepts in a unified theoretical framework based on a stabilizing, infinite horizon, linear quadratic regulator. In order to represent unstable as well as stable multivariable systems, the standard state‐space formulation is used for the plant model. The incorporation of a nominally stabilizing constrained regulator eliminates the current requirement of tuning for nominal stability. Output feedback is addressed in the well‐established framework of the linear quadratic state‐estimation problem. This framework allows the flexibility to handle nonsquare systems, noisy inputs and outputs, and nonzero input, output, and state disturbances. This formulation subsumes the integral control schemes designed to remove steady‐state offset currently in industrial use. The online implementation of the controller requires the solution of a standard quadratic program that is no more computationally intensive than existing algorithms.
Dispersion coalescence: Kinetic stability of creamed dispersions
Authors: Lobo Lloyd , Ivanov Ivan , Wasan Darsh
Published: June 2004

A Model is developed to predict the coalescence behavior in liquid‐liquid dispersions. Coalescence times are based on the lifetimes of the single films that are formed between adjoining drops in a creamed or sedimented dispersion. The model is developed for smaller drop sized dispersions (<100 μm) in which the drops are not substantially deformed due to gravitational forces. The model predicts that coalescence occurs between the emulsion drops without the gross separation of the dispersed phase (that is, the dispersion does not separate into distinct oil and water phases). As a result of interdrop coalescence, the mean drop size of the dispersed phase increases along with an increase in the polydispersity of the drop size. The model's predictions are used to develop a quantitative relationship between the lifetime of the single film and the rate of increase of the mean drop size of the dispersion. The model also accounts for the effect on dispersion stability via the models of single film drainage in the presence of surfactant.
Adaptive nonlinear cell mass state estimator for a continuous yeast fermentation
Authors: Ramseir Michael E. , Agrawal Pramod , Mellichamp Duncan A.
Published: June 2004

On‐line measurement of the important state variables in fermentations, particularly cell mass concentration, remains a difficult problem. However, a number of secondary or environmental variables can be measured conventionally and on‐line, such as pH, and CO2 and O2 in the exhaust gas. Stephanopoulos and San (1984) have developed a modeling approach, based on species balances, that provides relations between the environmental and important state variables. Using such a model, the important state variables can be estimated in principle from more easily accessible on‐line measurements.

In this article, a new adaptive estimator is developed, incorporating as its basis an underlying nonlinear model so as to utilize the best possible a priori process knowledge. Base addition rate and CO2 offgas concentration are measured on‐line and periodically. Cell mass measurements are incorporated infrequently and even at irregular sampling periods, thus providing a very flexible scheme. Only a single adapted parameter is required to match the model to the plant operating characteristics. This simple but rigorous model form results in an estimator that is easy to implement and to tune and which exhibits long‐term robustness due to its multirate feedback structure. Experimental results from a laboratory‐scale continuous fermentor show that such a cell mass estimation scheme yields excellent performance both open‐loop (without control) and as a part of conventional and nonlinear adaptive control approaches.
Memory‐integral mass‐transfer models for adsorption process simulation
Authors: Harriott George M.
Published: June 2004

The memory integral based on convolution of the unit step response to loading a clean pellet with concentration history at the pellet surface is proposed for computations of mass transfer in adsorption process simulation. Although rate laws are widely used to describe diffusion and sorption in pellets for slow (Glueckauf, 1955) and moderate (Kim, 1989) rates of mass transfer, these models fail to describe mass transfer at short contact times because the concentration history experienced by the pellet is not accounted for. To facilitate memory integral computations, approximations to the unit step response to loading a clean pellet are derived based on a single moving finite element and by asymptotic matching in time. Numerical evaluation of the memory integral is demonstrated on simple cycles which show the merits of this approach.
Surface diffusion of multilayer adsorbed species
Authors: Chen Y. D. , Yang R. T.
Published: June 2004

A model based on kinetic theory is derived for surface diffusion of multilayer adsorbed species on a homogeneous surface. A simple solution is obtained for expressing the concentration dependence of surface diffusivity. For monolayer adsorbate, the solution is reduced to the classical HIO model. This solution, in principle, requires no fitting parameters and is capable of predicting the unique concentration dependent behavior exhibited by multilayer surface diffusion, where the diffusivity first increases with surface concentration to a maximum followed by a decrease. Seventeen sets of experimental data from the literature are used to test the model, with fair results. It is also shown that heats of adsorption can be calculated from multilayer surface diffusivity data by using this model.
Polymer‐solvent diffusion and equilibrium parameters by inverse gas‐liquid chromatography
Authors: Romdhane Ilyess Hadj , Danner Ronald P.
Published: June 2004

The solubility and diffusion coefficients of low‐molecular‐weight penetrants in a polymeric stationary phase were determined by use of inverse gas chromatography (IGC). Previously reported applications of packed‐column IGC to the measurement of the stationary‐phase diffusion coefficient have been carried out via the van Deemter approach. We present here a new mathematical model to describe the chromatographic process in packed columns. A moment analysis procedure was developed from this model to analyze the chromatographic peaks eluting from packed columns. The first moment (mean retention time) is determined to depend only on the equilibrium partition coefficient, whereas the second moment (variance) is related to both the thermodynamic and transport properties of the polymer‐solute system. The validity of the technique was demonstrated by measuring the diffusivity and solubility of toluene and benzene in polystyrene above the glass transition temperature of the polymer. Free‐volume theory was used to interpret and correlate the diffusivity data obtained at infinite dilution of the solute. One can use the results obtained at infinite dilution to predict reliably via the free‐volume theory the concentration and temperature dependence of polymer‐solvent diffusion coefficients.
Mean velocity and turbulence fields inside a β=0.50 orifice flowmeter
Authors: Morrison Gerald L. , Deotte Robert E. , Nail Gregory H. , Panak David L.
Published: June 2004

The flow field inside an orifice flowmeter with a beta ratio of 0.50 operating at a Reynolds number of 91,100 has been studied using a three‐color, 3‐D laser Doppler anemometer system. Mean velocity measurements show large radial velocities leading into the orifice, the fluid separating from the orifice plate at the throat, the presence of a vena contracta farther downstream, flow reattachment to the pipe wall 5.3 pipe radii (R) downstream, the presence of a small upstream recirculation zone, and both a primary and secondary recirculation zone downstream of the orifice plate. The static wall pressure distribution attains a minimum pressure at 1.5 R, which does not coincide with the location of the vena contracta (0.75 R). Distributions of the entire Reynolds stress tensor are presented along with calculated values of turbulence kinetic energy, turbulence kinetic energy production, vorticity, and turbulence induced accelerations. These data are analyzed to interpret the complex turbulent flow field.
Thermal method of measuring concentration during turbulent reactive mixing
Authors: Singh Madan , Toor H. L.
Published: June 2004

When the total reactant diffusivities are equal, the time‐average concentration fields which result from turbulently mixing reactants can be measured thermally if the temperature difference between the feed streams is suitably chosen. Each reactant profile is obtained from measurements of the temperature profile in an experiment in which the inlet temperature of that reactant is less than that of the other reactant by an amount equal to the adiabatic reaction temperature change of the measured reactant. The method is demonstrated in a liquid phase confined jet reactor.
Control of high‐purity distillation column using a nonlinear wave theory
Authors: Han Myungwan , Park Sunwon
Published: June 2004

A new model‐based controller for profile position control of high‐purity distillation columns is presented. The controller has been developed by incorporating nonlinear wave model into the generic model control framework. An observer based on the nonlinear wave theory is also proposed to determine the profile position. The performance of the nonlinear control scheme has been tested in simulation experiments of high‐purity binary distillation columns. Tight control of profile position, which leads to fast stabilization of product compositions, has been achieved. Since the profile position control alone produces a slight offset from the desired product concentration, the composition/profile position cascade system has been used to remove the offset. The control scheme can handle significant disturbances and model‐plant mismatch.
Sensor network design for maximizing reliability of linear processes
Authors: Ali Yaqoob , Narasimhan Shankar
Published: June 2004

A new concept of reliability of estimation of variables is introduced which relates to the estimability of variables in the presence of sensor failures. Based on this concept, a method for optimal location of sensors in a pure flow process is developed. A graph‐theoretic algorithm, SENNET, developed for this purpose, is shown to perform robustly and give globally optimum solutions for realistic processes.
Combined effects of mass transfer and inaccessible porosity in gasification reactions
Authors: Delikouras Evangelos A. , Perlmutter Daniel D.
Published: June 2004

The effect of inaccessible porosity on the rate of gasification reactions is illustrated under conditions where intraparticle and extraparticle resistances to mass transfer are significant. The mathematical model developed here is based on a mass balance for the reactant fluid in the porous particle, coupled with an appropriate structural model developed for the kinetics‐controlled case (Delikouras and Perlmutter, 1991). It is demonstrated numerically that the effects of inaccessible porosity generally diminish with increased diffusional resistances. In agreement with the prior kineticscontrolled case, it is shown that the effects of inaccessible porosity are more pronounced at lower values of the structural parameter ψ and at greater levels of the inaccessible fraction.
Lattice thermodynamics for binary closed‐loop equilibria: Ordinary and polymer systems
Authors: Hino Toshiaki , Lambert Stephen M. , Soane David S. , Prausnitz John M.
Published: June 2004

The incompressible lattice‐gas model by ten Brinke and Karasz is adopted to introduce the effect of specific interactions into a recently‐presented Monte‐Carlobased lattice expression for the Helmholtz energy of nonrandom mixing. While the lattice remains incompressible, intermolecular forces consist of two types: London dispersion forces and specific (chemical) forces. The specific interactions between similar components, as well as those between dissimilar components, are incorporated in a systematic manner. Closed‐loop temperature‐composition phase diagrams are obtained. The theory is compared with experimental data for several binary systems, including polymer solutions, which exhibit closed‐loop coexistence curves. Theoretical and experimental results are in good agreement.
Hydrodynamic cleaning of a viscous film from the inside of a long tube
Authors: Mickaily Elizabeth S. , Middleman Stanley
Published: June 2004

A simple fluid dynamic model is developed which predicts the thinning rate of a viscous film of liquid, in contact with the smooth interior surface of a long cylindrical tube, due to the shearing action of a turbulent flow of an immiscible fluid through the tube. Experiments indicate the degree to which the model provides an accurate means of predicting viscous film removal from the tube. Similar experiments performed with electrochemically roughened tubes elucidate the limitation of this cleaning process due to surface roughness. The observed limitation is consistent with an analysis based on numerical solutions of the equations for shear flow over a rough surface.
Dynamic effects of vaporization with exothermic reaction in a porous catalytic pellet
Authors: Watson Paul C. , Harold Michael P.
Published: June 2004

A single catalytic pellet reactor was used to investigate dynamic multiphase reaction‐transport interactions. The experiments permitted the simultaneous measurement of pellet weight(liquid holdup) and center temperature under conditions which mimic the unsteady, local events within a fixed‐bed multiphase reactor. In the liquid‐prefilled pellet experiments, the pellet (Pd/Al2O3‐clay) was prefilled with liquid reactant (AMS or cyclohexene) and then exposed to a flowing gas containing hydrogen. In the gas‐prefilled pellet experiments, the pellet was prefilled with hydrogen and then contacted by a liquid reactant pulse of prescribed flow rate and duration. The liquid‐prefilled experiments revealed an acceleration in the drying rate due to the exothermic reaction. The pore emptying process was accompanied by a significant increase in the pellet temperature. The considerably slower dynamics observed for the AMS system compared to the cyclohexene system are attributed both to differences in liquid‐phase volatility and to self‐oligimerization of the AMS. In the gas‐prefilled experiments, a pore filling with reaction process precedes the pore emptying step. The degree of catalyst overheating is highest for a liquid pulse of intermediate duration. A phenomenological mechanism of drying with exothermic reaction built on the well‐accepted theory of drying of granular media is offered to explain the trends in the data. Implications of the findings with regards to hot spot formation in the catalytic reactor are discussed.
The effect of solid interaction forces on pneumatic handling of sorbent powders
Authors: Lee Rhonda J. , Fan L.‐S.
Published: June 2004

This study shows that a comparison of powder characteristics–‐particle morphologies, particle size distributions, and static dielectric and Hamaker constants–‐can be used to interpret differences in dispersion and transport behavior between powders. These differences are attributed to the relative values of the solid‐solid interaction forces experienced by each powder in the process. The static dielectric constants of the powders are used as the material properties related to the relative magnitudes of the electrostatic forces. Similarly, the Hamaker constants are the material properties used to indicate the relative magnitudes of the van der Waals forces. The effects of differences in particle morphologies and size distributions are used to evaluate the dispersibility and efficiency of transport of four calcium‐based powder materials used as sorbents in flue‐gas desulfurization.
A model for gas‐liquid slug flow at reduced gravity conditions
Authors: Janicot Antoine , Dukler Abraham E.
Published: June 2004

The gas‐liquid slug flow pattern exists over a broad range of flow rate space at reduced gravity conditions and for that reason is expected to be widespread in many space applications. The objective of this work was to develop a model predicting the characteristics of slug flow at reduced gravity including the bubble length, shape, velocity and other properties of the slug flow. Observations derived from low‐gravity experiments suggested the use of a boundary layer‐type approach. Multiple solutions exist, and a criterion for selecting the solution which corresponds to experimental observation is indicated.
Momentum transfer across fluid‐fluid interfaces in porous media: A network model
Authors: Goode P. A. , Ramakrishnan T. S.
Published: June 2004

Two‐phase flow in porous media is described based on the extended form of Darcy's law, which ignores momentum transfer at fluid‐fluid interfaces. Two forms of corrections to this simple description have been proposed in the literature: one on the relative permeability dependence on viscosity ratio; the other on the velocities assumed to be proportional to both phase pressure gradients and so introducing an additional saturation‐dependent cross coefficient.

In this article, to identify the correct form of transport equations, a simple cubic network model of 30 × 30 × 30 bonds is used. The cross section of the bonds is that of a four‐cusp duct. The fluid interface in each duct is located by capillary equilibrium. The duct hydraulic conductances are then obtained as a function of viscosity ratio and phase volume fraction using a finite element calculation. These individual duct results are used in the network calculations for which a percolation algorithm is applied to simulate nonwetting phase displacing wetting phase, a process also known as initial drainage. Flow calculations show that both the nonwetting‐phase relative permeability and the cross coefficient are strong functions of saturation and viscosity ratio. Also, the off‐diagonal terms may contribute to a nonnegligible fraction of the flow. The proposed generalization of the Darcy equations is applicable to all problems involving multiphase flow in porous media. The current practices for relative permeability measurements and reservoir simulation may have to be reexamined in the context of the proposed transport equations.
CO2/CH4 separation by facilitated transport in amine‐polyethylene glycol mixtures
Authors: Davis Richard A. , Sandall Orville C.
Published: June 2004

In this work, the separation of carbon dioxide from methane by reactive liquid membranes consisting of the secondary amines, diethanolamine, and diisopropanolamine in polyethylene glycol with an average molecular weight of 400 was studied. A mathematical model was developed to describe the transport process. This model employs the zwitterion mechanism for the CO2 amine reaction kinetics. Membrane flux experiments utilizing a flat plate device were carried out to verify the model predictions. It was found that large permeabilities and separations were achieved for low CO2 feed concentrations. The results also show that DEA based solutions give better separation than DIPA based solutions.
Simulation of a multiple substrate reactor for chemical vapor infiltration of pyrolytic carbon within carbon‐carbon composites
Authors: McAllister P. , Wolf E. E.
Published: June 2004

A model for a pilot‐scale chemical vapor infiltration (CVI) reactor in which multiple substrates are densified simultaneously by carbon deposition from a pyrolyzing hydrocarbon is presented. Kinetic expressions and parameters and transport properties for carbon deposition from propylene determined from experimental studies and a single substrate CVI model are incorporated into the pilot reactor model. The two‐dimensional transport equations for heat, mass and momentum transfer are solved for the entire reactor to simulate the effects of varied operating conditions on substrate densification throughout the reactor during the CVI process. Of the conditions simulated, the reactor temperature was found to have the most significant impact on both the uniformity of densification and the process time required. As the temperature increased, the uniformity of densification decreased and the time at which pore blockage at the outer surface occurred was shortened. Increasing the hydrocarbon feed concentration shortened the process time, but did not affect the final level of densification, while the feed rate only affected CVI at intermediate times. Comparison between experimental pilot reactor results and model predictions were satisfactory at short and long times, but agreement was less satisfactory at intermediate times.
Nucleation mechanisms and particle size distributions of polymer colloids
Authors: Giannetti Enzo
Published: June 2004

A theoretical model is developed for the time evolution of volume particle size distributions (PSDs) of polymer colloids during the nucleation stage. Analytic expressions for the PSD and related cumulants are presented. Negatively skewed distributions are observed only when the formation rate of mature latex particles is a monotonically decreasing function of time. When this model is tested against experimental PSD data of styrene polymerization at 50°C, agreement between theory and experiment is found only assuming a monotonically decreasing nucleation rate. The fitting is completely missed when the production rate of mature latex particles is predicted according to the coagulative nucleation scheme. Theory and experiment can combine introducing an extended coagulative nucleation model. According to this approach, the nucleation time is much shorter than predicted by earlier theories. As a consequence, most of the mechanistic information convoluted with early‐time PSDs is lost.
Gas‐Particle flow in a duct of arbitrary inclination with particle‐particle interactions
Authors: Ocone R. , Sundaresan S. , Jackson R.
Published: June 2004

Earlier work of Sinclair and Jackson on the flow of gas‐particle suspensions in vertical pipes is extended to the case of ducts of arbitrary inclination. As a result of the compaction due to gravity, it is necessary to take into account forces transmitted between particles at points of sustained, rolling and sliding contact, and a simple frictional model of this contribution to the stress is introduced. The resulting theory is shown to predict fully developed flows with the qualitative features to be expected, even in horizontal ducts. The effects of flow rates, duct inclination, and duct width on the solution are explored.
Static instability analysis of circulating fluidized beds and concept of high‐density risers
Authors: Bi Hsiaotao , Zhu Jingxu
Published: June 2004

An instability analysis has been carried out to elucidate the unsteady flow conditions encountered in the typical circulating fluidized bed units in light of the conveyor‐solids feeder interaction. The results successfully predict the critical velocity and the maximum solids circulation rates reported in the literature and explain the origin of such unstable conditions. Furthermore, the simulation, for the first time, reveals the importance of unit structure in improving the performance of circulating fluidized bed systems. Finally, the concept of a high‐density circulating fluidized bed is proposed.
Unbiased estimation in dynamic data reconciliation
Authors: Rollins Derrick K. , Devanathan Sriram
Published: June 2004

A computationally fast technique accurately estimates process variables when conditions are dynamic due to changes in steady states. The process variable estimators are unbiased and have known distributions. Thus, confidence intervals for true values of process variables are provided. The formulation of this technique was motivated by a recursive, dynamic data reconciliation technique that obtains very accurate estimators. These two techniques are compared in terms of computational speed and accuracy of estimators. The proposed technique is computationally faster, but not as accurate when variances of process measurements are large. However, the accuracy of the proposed estimators is shown to approach that of the recursive technique by iteratively recalculating estimates and when measurement variances decrease.
Reactions of polycyclic alkylaromatics: 5. pyrolysis of methylanthracenes
Authors: Smith C. Michael , Savage Phillip E.
Published: June 2004

1‐, 2‐ and 9‐methylanthracene were pyrolyzed neat at temperatures between 350 and 450°C for batch holding times up to 300 min. The pyrolysis proceeded through three parallel primary reaction pathways: one led to anthracene via demethylation; the second to dimethylanthracenes through methyl addition; and the third to methyl‐9,10‐dihydroanthracenes through hydrogenation. The relative importance of these three paths varied for different methylanthracene isomers. The presence of these primary pathways can be rationalized in terms of recently elucidated hydrogentransfer mechanisms and other aspects of the developing free‐radical chemistry of polycyclic alkylarenes. The demethylation rate at 400°C for the methylanthracenes and seven other methylarenes was correlated with Dewar reactivity numbers, which provide a measure of the localization energy, as ln rate (arene yield/min) = 3.7– 7.1 Nts, where Nts is the Dewar reactivity number for the perpheral aromatic carbon atom bearing the methyl substituent. This correlation may be useful in molecular‐based reaction models for the conversion of heavy hydrocarbon resources such as coals and heavy oils.
Continuous‐mixture kinetics and equilibrium for reversible oligomerization reactions
Authors: McCoy Benjamin J.
Published: June 2004

Reversible oligomerization of distributions of molecules yields distributions of monomers, dimers, trimers, tetramers and higher‐order oligomers. The distributions can be functions of molecular weight, carbon number, or other variables that may be considered continuous. Rate expressions are formulated for the rates of generation of oligomers and loss of reactants due to combination (fusion) reactions and for the generation of fragments and loss of oligomers due to cracking (fission) reactions. Two types of cracking reactions are proposed: proportioned fission where products are precisely proportioned to yield monomers, dimers or trimers of given molecular weight; random fission where the fission products are distributed over all molecular weights up to that of the reactant. The zero moment of a distribution is the species concentration, which satisfies molar equilibrium and kinetics relationships. The temporal evolution of molecular‐weight moments is expressed in terms of differential equations for continuous‐flow and batch stirred reactors. By setting forward and reverse rates equal, equilibrium relations are developed for zero‐ and higher‐order moments, thus generalizing the mass‐action equilibrium concept. Equilibrium states for proportioned and random fission manifest infinitesimally narrow and infinitely broad distributions.
Effect of temperature on the phase equilibrium of aqueous two‐phase polymer systems
Authors: Hartounian H. , Floeter E. , Kaler E. W. , Sandler S. I.
Published: June 2004

We report experimental data showing the effect of temperature on polymer partitioning in the dextran (DEX500)‐polyethylene glycol (PEG8000)‐water system. Increasing temperature increases the concentration of PEG in the top phase and decreases the concentration of DEX in the bottom phase. A solution thermodynamic model based on local compositions with temperature‐dependent parameters correlates the experimental data well, and an efficient Gibbs‐free energy minimization algorithm for phase equilibrium calculations is described. The partial molar enthalpies and entropies of each of the solutes, calculated with the model, are negative and decrease in magnitude as temperature increases.
Computer simulation and theory for free energies in dilute near‐critical solutions
Authors: Li T. W. , Munoz F. , Chimowitz E. H.
Published: June 2004

Computer simulation results are presented for the residual chemical potential μ, local solvent density enhancement Δρloc, and average solute potential energy u1 of a solute molecule immersed in afluid very close to its critical point. The simulation algorithm involves the Monte Carlo method in conjunction with free energy perturbation ideas. A number of ideas related to the feasibility of computer simulations in near‐critical solvents are investigated. Due to long‐range correlations near the critical point, it is not clear that simulations in this regime can be expected to yield accurate results using ensemble sizes of the magnitude typically used in simulations. Examination of system size and number of sampling steps on the simulations shows that the accuracy of the results depend to a large extent on the nature of the thermodynamic property being investigated. The solute chemical potential is suited particularly to simulation since it depends primarily on short‐range structure in the system. A comparison of the simulation results with integral equation theory calculations shows both approaches agree well with each other.
Heat‐transfer analysis of the basal melting of antarctic ice shelves
Authors: Minale Mario , Astarita Gianni
Published: June 2004

Basal melting of Antarctic ice shelves is an important element in the overall balance of Antarctic ice. A heat‐transfer model for the basal melting of the Drygalski Ice Tongue is presented. The model does not contain any adjustable parameter. The calculated basal melting rate agrees very well with the value estimated from an overall ice balance on the ice Tongue. It is concluded that relatively simple concepts of transport phenomena may be used to model some important features of the dynamics of the Antarctic ice sheet.
u′v′ Reynolds stress in the viscous sublayer over a wide range of Re numbers
Authors: Rosén Christer , Trägådh Christian
Published: June 2004

To test the ability of different turbulence models to predict the turbulent momentum transport in the viscous sublayer, measurements are performed with a laser Doppler velocimeter (LDV). The main objective of the experiments is to measure the u′v′ correlation in the near wall region. The results support the correlation: −u′v′+ = Aky+3. The results of 2‐D computer simulations using four different low Reynolds number k‐ϵ turbulence models are compared with the experimental LDV data. The models tested are the original low Reynolds number k‐ϵ model of Jones and Launder (1972, 1973) and three later modifications by Chien (1980), Lam and Bremhorst (1981), and Nagano and Hishida (1987). The u′v′ correlation was found to be underestimated with all four models, but the Chien model was superior, with only a 40% underestimation in the viscous sublayer.
Rate enhancement and multiplicity in a partially wetted and filled pellet: Experimental study
Authors: Watson Paul C. , Harold Michael P.
Published: June 2004

Phase transition during an exothermic multiphase reaction was studied experimentally using a single catalytic pellet reactor. Cyclohexene hydrogenation (to cyclohexane) and disproportionation (to benzene and cyclohexane) on Pd/Al2O3 comprised the test reaction system. The steady‐state behavior of the pellet exposed on part of its surface by a flowing liquid rivulet containing the liquid reactant (cyclohexene) and the other part by a flowing gas containing the gaseous reactant (hydrogen) was examined. Measurements included the pellet weight (liquid holdup), degree of external wetting, center and surf ace temperature and overall reaction rates. Two regimes observed are: a low‐rate regime for all hydrogen gas‐phase concentrations in which the partially wetted pellet is filled mostly with liquid and nearly isothermal; a high‐rate regime for hydrogen concentrations exceeding a critical value in which the pellet is filled only partially with liquid and the pellet temperature rise is considerable. Benzene formation was observed in this state. The difference in overall cyclohexane formation rates between the two states was as high as a factor of 20 for the same bulk conditions. Over the range where multiple states were observed, the steady state of the pellet depended on whether the pellet was pref tiled with the reactive gas mixture or with liquid cyclohexene. The range over which the high‐rate state was sustained was the largest for the most active catalyst and declined as the catalyst slowly deactivated. Data features are interpreted using the theoretical foundations of the half‐wetted catalytic slab model (Harold and Watson, 1993).
Determination of vapor pressure in vapor‐induced puffing
Authors: Johnny Wu P. , Schwartzberg Henry G.
Published: June 2004

Water vapor pressure for cornstarch and popcorn grits at a temperature of between 100 and 180° C was determined by measuring vapor pressures generated at various moisture contents in a heated, closed system. At a higher temperature, fugacity of water increased with increasing temperature probably because of melting or partial melting of starch. The apparent heat of sorption for cornstarch changed significantly with moisture content and temperature. Noncondensible gases were produced by heating popcorn grits to temperatures above 150°C. It is not accurate to estimate vapor pressure based on the heat of sorption or extrapolation of currently used empirical isotherm equations to a temperature above 100°C. Vapor pressure inside popcorn at the instant of popping are. roughly 758 to 827 kPa (110 to 120psia).
Removal of VOCs from groundwater using membrane‐assisted solvent extraction
Authors: Hutter Joseph C. , Vandegrift George F. , Nuñez Luis , Redfield David H.
Published: June 2004

A membrane‐assisted solvent extraction (MASX) system coupled to a membrane‐assisted distillation stripping (MADS) system for use in decontaminating ground‐water is evaluated. Volatile organic compounds (VOCs) in the groundwater are extracted in the MASX unit using a sunflower oil solvent. In the MADS unit, VOCs are stripped from the sunflower oil, which is recycled to the MASX unit. Thermodynamic data for the sunflower oil‐water‐VOCs system were measured. The results were used along with published membrane mass‐transfer data to design MASX and MADS modules.
Hydrodynamic assist of dynamic wetting
Authors: Blake Terence D. , Clarke Andrew , Ruschak Kenneth J.
Published: June 2004

Dynamic wetting speeds are limited by the gross entrainment of air between the liquid and the moving substrate. We present experimental data for the curtaincoating method, in which liquid impinges at high speed on the substrate. We also show that air entrainment is strongly affected by macroscopic hydrodynamics and is subject to hysteresis. Using boundary‐layer theory, a simple hydrodynamic model is developed for the flow field in the impingement zone away from the dynamic wetting line. The model approximately accounts for the shear thinning of polymer solutions and for the influence of surface tension. We apply the molecular kinetic theory of dynamic wetting, modified to account for hydrodynamic stress, to the immediate vicinity of the wetting line. The main result is a correlation for airentrainment data.

The experimentally calibrated model predicts that the hydrodynamic assist of wetting is greatest when the dynamic wetting line is located beneath the impinging curtain. Flow visualization supports this physical picture.
Optimizing experimental conditions for minimum production cost in preparative chromatography
Authors: Felinger Attila , Guiochon Georges
Published: June 2004

Optimum experimental conditions and column design parameters are calculated for most economical separation of binary mixtures by preparative liquid chromatography. The solvent consumption and capital costs are regarded as the two major cost contributions. Calculations were performed by using the numerical solution of the equilibrium‐dispersive model of chromatography and a nonlinear simplex algorithm. Solvent consumption depends only on the column efficiency, the retention factor, and the loading factor. The production rate depends on the particle size, the column length, the mobile‐phase flow velocity, the retention and the loading factors, and can be optimized with or without recovery yield constraints. A hybrid objective function was constructed to find the tradeoffs between the production rate and the solvent consumption.
Solubility of HFC‐134a refrigerant in glycol‐type compounds: Effects of glycol structure
Authors: Tseregounis Spyros I. , Riley Maynard J.
Published: June 2004

Environmental concerns have dictated the replacement of CFC‐12 refrigerant with HFC‐134a in automotive air‐conditioning (A/C) systems. Since polyglycols are synthetic compounds compatible with HFC‐134a and considered as lubricants for the A/C compressor, interations of HFC‐134a with glycol‐type compounds and thermodynamic properties of the solutions are important in designing an A/C system. In this work, the solubility of HFC‐134a in four glycol‐type compounds was measured at −5 to 80°C and 90 to 960 kPa. HFC‐134a had the greatest solubility in tetraethylene glycol dimethyl ether. HFC‐134a was less soluble in hexylene glycol and tetraethylene glycol and least soluble in triethylene glycol. Mixtures of HFC‐134a with TRIG or TGDE showed phase separation. Solubility data were used to calculate the activity coefficient of HFC‐134a in glycol solutions. An equation of the form, lnγr = (1 − xr)[A + Bxr], was found to correlate γr to the mole fraction of HFC‐134a in the solution where A and B are constants which may depend on the temperature. Based on a model assuming a specific interaction between molecules and using the activity of HFC‐134a, a higher degree of interaction is expected between HFC‐134a and tetraethylene glycol dimethyl ether molecules than between the molecules of HFC‐134a and other glycols. In general, glycols with free ‐OH groups may have less affinity for HFC‐134a. Solubility is discussed in terms of its parameters and chemical structure of the glycols, as well as the effect of temperature on the solubility, the activity coefficient, and the thermodynamic properties of the HFC‐134a/glycol solutions.
Structure of falling film heat and mass transfer on a fluted tube
Authors: Conlisk A. T.
Published: June 2004

The falling liquid film has become a popular means of transferring heat and mass from a vapor to a binary liquid, especially in gas‐fired heat pump systems. Ideally, the required amount of heat and mass transfer can be accomplished by using a simple cylindrical tube; however, increasingly stringent size and weight requirements for the machine prohibit use of the simple cylindrical surface, and other more complex surfaces with higher absorption capacities have been sought. In this article, absorption of a single component and condensation of a binary mixture on an axially fluted tube is considered. The solution to the problem hinges on the energy equation, although the entire energy transfer process is mass‐transfer‐limited. Significant mass transfer is limited to a thin layer near the liquid‐vapor interface. Solutions to the energy equation are obtained for both the conduction‐ and convection‐dominated regimes. In the latter, significant heat transfer occurs within a thin layer near the liquid‐vapor interface which contains the mass transfer layer; this “boundary layer” structure does not appear to have been recognized in previous work in this area. Using the present results, the capacity of a given tube may be predicted as a function of governing geometrical and physical parameters. The principal objective of this work is to develop the theoretical tools from which computations may be carried out during a design process. The theoretical results may be applied to mixtures typical of application in the absorption heat pump industry.
Factors affecting coalescence frequency of droplets in a stirred liquid‐liquid dispersion
Authors: Wright H. , Ramkrishna D.
Published: June 2004

Liquid‐liquid dispersions are commonly used for separations and reaction systems, because the added interfacial area due to dispersion aids in mass transfer and chemical reaction rates. Transient coalescence experiments have been performed for a wide range of dispersed‐phase fractions and impeller speeds. A similarity transformation of the experimental drop‐size distribution is possible for most of the experiments. The drop‐size‐dependent coalescence frequency is obtained from an inverse problem procedure previously developed. Over the range of drop sizes encountered in our experiments, the coalescence frequency increases with increases in the size of the coalescing drops, the dispersed‐phase fraction, and the impeller speed. For a dispersephase system of neutrally buoyant droplets of benzene/carbon tetrachloride in water, the coalescence frequency is K(v, v′)= 3.72 (10−3)ϕ1.42N(v1/2+v′1/2) as a function of dispersed‐phase fraction, ϕ, impeller speed, N2, and volumes of the coalescing droplets, v and v′. The results are compared with previous models for the coalescence frequency. Models predicting an eventual decrease in coalescence rates with drop size are disqualified unless this decrease occurs beyond the sizes observed in our experiments.