bluuebunny/crossref_metadata_2025 huggingface dataset:
Autocatalytic oxidation of cyclohexane—modeling reaction kinetics
Authors: Suresh A. K. , Sridhar T. , Potter O. E.
Published: June 2004

Liquid phase oxidation of hydrocarbons is a major source of organic chemicals. This paper concerns itself with the oxidation of cyclohexane. The process is one in which gas absorption is accompanied by chemical reaction, but little information has been reported. A major factor has been safety considerations, the Flixborough explosion being a constant reminder. In this paper attention is given to the kinetics and to the interaction of kinetics and mass transfer, showing that in this autocatalytic system the dissolved oxygen level rises to saturation and falls as the rate of reaction increases. Initially zero order in oxygen, the reaction becomes first order at low oxygen levels. Oscillations and enhancement due to reaction within the boundary layer are noted. A kinetic model is developed based on simplification of the accepted free radical scheme and it is reasonably successful in accounting for the results reported. Experiments at temperatures up to 170°C and 70 bar using uncatalyzed reaction are reported from three different reactors: a homogenous batch reactor, a flat interface reactor, and a sparged agitated bubbling reactor.
Autocatalytic oxidation of cyclohexane—mass transfer and chemical reaction
Authors: Suresh A. K. , Sridhar T. , Potter O. E.
Published: June 2004

In an industrial gas‐liquid reactor both mass transfer and kinetics have, in principle, a role to play. This paper describes studies on the oxidation of cyclohexane in two‐phase, gas‐liquid reactors. Regimes of absorption have been clarified by measurements of dissolved oxygen concentrations. The behavior is found to be complex, arising from the fact that the reaction is autocatalytic, and zero order in oxygen over almost the entire absorption range. As reaction velocity increases, due to the autocatalytic kinetics, enhanced physical mass transfer rates arise. Bubble swarms have properties that prevent the simple application of mass transfer and enhancement from flat‐surface systems. A theory is developed here that accounts qualitatively for the enhancement found in bubble swarms in the present case. The theory is also shown to predict quantitatively the appropriate physical mass transfer coefficients.
Simulation of injection mold filling of viscoelastic polymer with fountain flow
Authors: Kamal M. R. , Goyal S. K. , Chu E.
Published: June 2004

Viscoelasticity, nonisothermality, and fountain flow influence the microstructure development during injection mold filling of polymer melts and, therefore, the ultimate properties of a molded article. A comprehensive two‐dimensional mathematical model is developed to evaluate the effects of these factors on the structure of the flow field. Special consideration is given to the shape of the flow front, which is crucial in determining the structure of flow in the fountain region. The effects of slip and crystallization are also considered and their influence on model predictions are analyzed. Simulations using the no‐slip and slip boundary conditions show that a slip boundary condition is necessary to alleviate the singularities in the flow structure. The results also indicate that the predicted stress profiles are significantly influenced by the viscoelasticity of the polymer melt. Furthermore, the nonisothermality and subsequent deposition of a solid layer next to the mold walls considerably alter the moldability parameters and the flow field.
Reaction kinetics in oxygen bleaching
Authors: Hsu C. L. , Hsieh J. S.
Published: June 2004

The oxygen bleaching of wood pulp takes place in a heterogeneous system involving oxygen (gas), water (liquid), and fiber (solid). Inconsistent effects of process variables (reaction temperature, alkali concentration, and oxygen pressure) on the overall oxygen delignification rate of wood pulp in various reacting systems have been reported in the literature.

An apparent intrinsic reaction rate model, excluding the interphase mass transfer effects, has been developed based on oxygen bleaching experiments in an agitated and ultralow‐consistency (solids content) reactor. The apparent intrinsic reaction rate model shows a faster delignification rate than those reported in the literature. The degree of inadequate delignification observed in the literature data is likely to be caused by insufficient mass transfer of bleaching chemicals to fibers.
Compression zone effect in batch sedimentation
Authors: Font R.
Published: June 2004

Theories of batch sedimentation, taking into account the compression zone effect, are reinterpreted and modified. According to the discussion presented and the experimental results shown, characteristics—which are the loci of points representative of layers with the same settling rate and the same solids concentration in the settling zone—rise tangentially to the sediment curve on a height vs. time plot. Mathematical expressions that relate the solids concentration to the variations of the pulp‐supernatant interface height and of the sediment height are presented. In addition, the model presented is proposed for use when it is not possible to measure the sediment height. A procedure for calculating the concentration at the top of the sediment is also indicated.

Calcium carbonate suspensions in water were selected for testing the equations deduced. The relation between the continuous thickener unit area and the underflow solids concentration was obtained from the batch tests, and is compared with that calculated by the graphic methods of Talmadge and Fitch, and Fitch.
Temperature‐programmed desorption: Multisite and subsurface diffusion models
Authors: Leary K. J. , Michaels J. N. , Stacy A. M.
Published: June 2004

Two models are presented that can simulate two peaks in a temperature‐programmed desorption (TPD) spectrum from a high surface area catalyst at atmospheric pressure: a multisite model and a subsurface diffusion model. The multisite model assumes that the two peaks arise from two distinct adsorption sites on the catalyst surface with different activation energies for desorption. The subsurface diffusion model assumes that the high‐temperature peak is produced by adsorbate that diffuses into the subsurface region of the catalyst during heating, and then back to the surface when it becomes depleted by the desorption process. It is shown that one can distinguish between the two models experimentally by measuring the effect of carrier gas flow rate and heating rate on the TPD spectrum.
A novel hollow‐fiber reactor with reversible immobilization of lactase
Authors: Jones C. K. S. , Yang R. Y. K. , White E. T.
Published: June 2004

Experimental and theoretical studies on a backflush hollow‐fiber enzymatic reactor (HFER) were conducted in this work for a lactose/lactase system. An A. niger lactase was chosen, from the four lactases tested, for reversible immobilization in the sponge layers of the fibers. An enzyme loading procedure was developed that allowed reliable and reproducible operation of the hollow‐fiber reactor and produced industrially significant conversions without apparent change in the activity or stability of the lactase used. This reversible immobilization scheme also permitted easy replacement of the enzyme used. The performance of the backflush HFER was investigated and a large number of data concerning its operation were obtained and interpreted. Momentum and mass transports in such a HFER were analyzed, and mathematical models that took the experimental findings into consideration were also developed and solved analytically and/or numerically. Predictions from the computer model developed in this work were found to be in excellent agreement with the experimental data collected, suggesting the possibility of a priori design of a process‐scale backflush HFER. With minor modifications, the models developed are expected to be applicable to hollow‐fiber reactors with a wide selection of immobilized cells, organelles, and other enzymes.
Adsorption of gas mixtures: Effect of energetic heterogeneity
Authors: Valenzuela D. P. , Myers A. L. , Talu O. , Zwiebel I.
Published: June 2004

The ideal adsorbed solution (IAS) theory of adsorption of gas mixtures is extended to the case of energetic heterogeneity. A heterogeneous ideal adsorbed solution (HIAS) behaves ideally on a particular site, but energetic heterogeneity causes a segregation in the composition of the adsorbed phase. Equilibrium properties are obtained by integrating over a multivariate energy distribution based upon perfect positive correlation of site energies. The fact that predictions from HIAS are always an improvement over IAS indicates that heterogeneity is a factor that must be considered in theories of mixed‐gas adsorption. Errors in HIAS predictions may be caused by steric exclusion of the larger molecules from micropores accessible to smaller molecules.
Coating flows in a nip region and various critical phenomena
Authors: Adachi K. , Tamura T. , Nakamura R.
Published: June 2004

When thin films of liquid are produced by roll coaters it is commonly observed that the liquid film is not uniform but shows either regular or irregular patterns resulting from hydrodynamic origins. The appearance of the liquid film has been considered to be governed by the flow geometry and dynamics in a nip region between two adjacent solid planes.

Experimental work was done to observe the flow geometries in the nip region and patterns of liquid film by using a roll‐stationary, transparent flat plate system. Two fundamental mechanisms for rib disappearance in practical high‐speed coatings were recognized. It was also found that the constant rib interval for infinite capillary number depends not only upon the minimum gap size but also upon the physical properties of the liquid. Experimental data are presented in dimensionless form for meniscus position, rib interval, constant rib interval, and for the critical or transient conditions of rib formation, rib disappearance, unstead ribbing, and centrifugal splash.

Comparison of present results for the speed ratio U2/U1 = 0 with previous results for U2/U1 = 1 shows the significance of the speed ratio to the meniscus position and the rib interval.
A theory of design reliability using probability and fuzzy sets
Authors: Kubic William L. , Stein Fred P.
Published: June 2004

All process designs are subject to uncertainties which make it impossible to state with complete certainty that a design will work. This study presents a measure of design confidence which considers the nature of the uncertainty and the operability of the process as a whole. This measure, called design reliability, quantifies the likelihood that a design will work. Detailed analyses are presented for three special cases: designs subject to random uncertainties, designs subject to fuzzy uncertainties, and designs subject to both random and fuzzy uncertainties. Practical procedures for estimating design reliability in these special cases are also presented.
Dispersion of viscous liquids by turbulent flow in a static mixer
Authors: Berkman Paul D. , Calabrese Richard V.
Published: June 2004

Drops are stabilized in agitated liquid‐liquid systems by both surface and internal viscous forces. The dispersion of an inviscid liquid into a turbulent continuous phase in static mixers has been studied but the effect of dispersed phase viscosity is not well understood. Systematic experiments have been conducted in a Kenics mixer by photographically examining dilute suspensions of viscous oils in water to determine how viscosity and conditions of agitation affect equilibrium mean drop size and size distribution. A semiempirical theory is developed which correlates the mean size data and collapses to the well‐known Weber number result in the inviscid limit. A correlation for drop size distribution in terms of cumulative volume frequency is developed by normalization with the Sauter mean diameter D32. Measurements at the mixer entrance indicate that the method of introduction of the dispersed phase should be considered when evaluating mixer performance.
The molecular basis of temperature effects in supercritical extraction
Authors: Debenedetti P. G. , Kumar S. K.
Published: June 2004

The behavior of solute partial molar enthalpies in dilute supercritical mixtures gives rise to the well‐known phenomenon of retrograde solubility (equilibrium solubility decreasing with increasing temperature at constant pressure). A mechanistic interpretation of this phenomenon in terms of the formation of large clusters of solvent molecules around solute molecules is consistent with experimentally observed retrograde behavior. Cluster formation occurs as a consequence of the unbounded increase in the solvent's compressibility arbitrarily close to the latter's critical point. At infinite dilution, the solute's partial molar volume and enthalpy grow linearly with cluster size. This means that the negative divergence of these quantities is simply a macroscopic manifestation of a “condensation” in which an increasing number of solvent molecules cluster around solute molecules. Arbitrarily close to the solvent's critical point, scaling relationships show that the decay of both solvent‐solvent and solute‐solvent correlation functions exhibits the same radial dependence. This functionality is thus solute‐independent, and is determined exclusively by the proximity to the solvent's critical point. The locus along which thermal effects associated with cluster formation are maximized is arbitrarliy close to the solvent's critical isochore as the latter's critical point is approached.
Distribution of volatile species in a refluxing polymer colloid
Authors: Zollars Richard L. , Chen Chein‐Tai , Jones D. Adrian
Published: June 2004

Monomer partitioning between the aqueous and polymer phases in a latex determines many properties of an emulsion polymerization, such as the reaction rate, molecular weight development and, if operated under refluxing conditions, the reaction temperature. A unique two‐chamber vapor‐liquid equilibrium cell was used during this investigation to obtain an in situ separation of the aqueous phase from the latex, thus permitting a direct determination of the compositions of the aqueous, polymer, and vapor phases in a latex, even under refluxing conditions. Thermodynamic analysis of the vapor‐liquid equilibrium problem allows the prediction of the composition of all phases from measured values of the temperature and pressure only. These predictions agreed well with data from other investigations under nonrefluxing conditions and with the aqueous and vapor phase compositions from this investigation. The polymer phase composition data for vinyl acetate polymers with average molecular weights in excess of 106 showed consistently higher polymer volume fractions than predicted. This latter result may be due to crosslinking or strong entanglements resulting from increased chain branching at these high molecular weights.
Augmented back equation of state for polar fluids
Authors: Lee M. J. , Chao K. C.
Published: June 2004

The pressure of water is decoupled into a polar pressure and a nonpolar pressure, the latter being obtained from the BACK equation based on the size and nonpolar energy of the water molecule determined by Jorgensen and coworkers. The polar pressure is obtained upon subtracting the nonpolar pressure from the total pressure given by the steam table equation of Keenan et al. The polar pressure function thus obtained for water is generalized to augment the BACK equation for the general description of polar fluids.

Equation constants are reported for 26 substances. Calculated vapor pressure, liquid molal volume, and pressure‐volume‐temperature (PVT) behavior are compared with data. Mixing rules are introduced to express poles interacting with poles and dispersion modes interacting with dispersion modes, but not poles interacting with dispersion modes. PVT relationship, vapor‐liquid equilibrium, and liquid‐liquid equilibrium in mixtures of water and nonpolar substances are calculated.
Theory of transport processes in single crystal growth from the melt
Authors: Brown Robert A.
Published: June 2004

The quality of large semiconductor crystals grown from the melt for use in electronic and optoelectronic devices is strongly influenced by the intricate coupling of heat and mass transfer and melt flow in growth systems. This paper reviews the present state of understanding of these processes starting from the simplest descriptions of solidification processes to detailed numerical calculations needed for quantitative modeling of processing with solidification. Descriptions of models for the vertical Bridgman‐Stockbarger and Czochralski crystal growth techniques are included as examples of the level of understanding of industrially important methods.
Manufacture of optical waveguide preforms by modified chemical vapor deposition
Authors: Kim Kyo‐Seon , Pratsinis Sotiris E.
Published: June 2004

Manufacture of optical waveguide preforms by modified chemical vapor deposition (MCVD) is theoretically investigated. For the first time, a model accounting for the concurrent heat transfer, chemical kinetics, and silica aerosol dynamics during lightguide preform fabrication by MCVD is presented. Silica particles are formed by high‐temperature oxidation of SiCl4, grow by coagulation, and deposit to the preform walls by thermophoresis and Brownian diffusion. Assuming first‐order SiCl4 oxidation and approximating the aerosol size distribution by a log normal function throughout the process, five partial differential equations describe this process. The emphasis of this study is on the achievement of high process yield (deposition efficiency of MCVD).

Process conditions for operation in the particle transport‐limited and reaction‐limited regimes are quantitatively identified. Operation in the former regime results in complete oxidation of the inlet SiCl4 but only about half of the product SiO2 particles deposit to the tube wall. Operation in the latter regime results in limited oxidation of inlet SiCl4 (about 40%) but almost all product SiO2 particles deposit to the tube wall. This study shows that high process yields and deposition rate can be achieved in lightguide preform manufacture by MCVD by combining operation in the reaction‐limited regime with recycling of the exit gases from the preform tube.
Carbon dioxide absorption into promoted carbonate solutions
Authors: Tseng P. C. , Ho W. S. , Savage D. W.
Published: June 2004

CO2 absorption rates into diethanolamine (DEA)‐promoted carbonate solutions were measured over a wide range of carbonate conversions and temperatures. Analysis of the data showed that the CO2‐DEA reaction rate was controlled by the rate of formation of zwitterion intermediate for carbonate conversions below 30%, and was controlled by the rate of abstraction of proton from the zwitterion intermediate for carbonate conversions greater than 30%. Hydroxyl ion, free amine, and carbonate ion were identified to act as bases to abstract the proton from the zwitterion. The modeling of the CO2 absorption rates and the effects of carbonate conversion and amine concentration on the CO2‐DEA reaction kinetics were investigated.
Parallel algorithms for multicomponent separation calculations
Authors: Wait R. , Landauro J.
Published: June 2004

Tearing and simultaneous correction techniques coupled with an odd‐even cyclic reduction parallel algorithm are used to carry out calculations of multicomponent separation problems. The old‐even algorithm implements effectively on most parallel processors. No noticeable differences are found when numerical results are compared with those obtained using serial algorithms. The results for three example problems are presented to illustrate the method. Calculations were carried out using single precision arithmetic.
Analysis of multicomponent diffusion in pore networks
Authors: Sotirchos Stratis V. , Burganos Vasilis N.
Published: June 2004

Diffusion flux models are developed for isobaric diffusion of a multicomponent gas mixture in pore networks of distributed pore size and length constructed by arranging pore segments around the bonds of a lattice of constant coordination number. Eigenvalue‐eigevector analysis is used to decompose the dusty‐gas model equations, written for each pore segment in the network, into a set of independent single‐species diffusion problems, each of which is then treated by combining the effective medium theory for resistor networks with the smooth field approximation (Burganos and Sotirchos, 1987). Results for a ternary mixture diffusing in a porous slab show that direct application of the smooth field approximation underestimates significantly the mass transport resistance.
A new liquid membrane technique for gas separation
Authors: Majumdar Sudipto , Guha Asim K. , Sirkar Kamalesh K.
Published: June 2004

A new liquid membrane separation technique for gas mixtures has been developed: feed and sweep gases flow through the lumen of two different sets of hydrophobic microporous hollow fibers while a liquid on the shell side acts as the membrane. This membrane is identified as the contained liquid membrane (CLM). Major shortcomings of immobilized liquid membrane techniques are eliminated. The novel separation device is identified as the hollow fiber contained liquid membrane (HFCLM) permeator. Experimental studies are made with different CO2‐N2 feed mixtures and a pure helium sweep stream, with special emphasis on model landfill gas purification. Either pure water or an aqueous solution of K2CO3 is used as membrane. The effects of several variables on the performance of the HFCLM permeator have been studied. A three‐component permeation model, incorporating axial pressure drop, is developed for binary mixture separation in a HFCLM permeator having a sweep gas stream. The experimental data show good agreement with the theoretical predictions.
Minimum reflux behavior of complex distillation columns
Authors: Wachter John A. , Ko Terry K. T. , Andres Ronald P.
Published: June 2004

A continuum model for multicomponent distillation in the minimum reflux limit, first proposed by Nandakumar and Andres, is refined for the special case of constant relative volatility (CRV) and constant molar overflow (CMO) and is extended to columns with multiple feeds and multiple products. A relaxation algorithm for calculating steady‐state conditins in any minimum reflux CRV, CMO column is established. This generalization of Underwood's classic treatment of single‐feed columns is used to explore the characteristics of several single‐feed and multiple‐feed distillation systems. Critical operating points at which the steady‐state composition profile in a minimum reflux column is indeterminate are identified and analytical expressions predicting the location of these points are established.
NMR study of mass transfer in granulated molecular sieves
Authors: Kärger Jörg , Pfeifer Harry , Richter Reinhard , Fürtig Helmut , Roscher Wolfgang , Seidel RÜDiger
Published: June 2004

Nuclear magnetic resonance (NMR) tracer desorption technique in combination with the traditional pulsed field gradient technique is applied to measure directly intracrystalline and long‐range molecular transport, as well as molecular exchange rates, between the individual crystallites in granulated molecular sieves. It is found that the granulation process leads to the formation of transport resistances on the external surface of the crystallites, which may be dramatically enhanced during their technical application. As an example, for granulated zeolite NaCaA the enhancement of such surface barriers under hydrothermal conditions and under the influence of a hydrocarbon atmosphere is studied. Since both intra‐ and intercrystalline transport are found to remain practically unaffected by this treatment, changes in the overall transport behavior are caused by the enhanced transport resistance on the external surface of the zeolite crystallites.
Mass transfer in the absorption of nitrogen oxides in alkaline solutions
Authors: Newman Bernard L. , Carta Giorgio
Published: June 2004

The absorption of mixtures of nitrogen oxides into aqueous solutions of NaOH in the range of partial pressures of 0.004‐ to 0.05‐atm NO and 0.002‐ to 0.015‐atm NO2 was investigated. Absorption experiments were conducted in a gas‐liquid contactor that permitted independent variation of the gas and liquid mass transfer coefficients. The results were analyzed in terms of a model which accounted for diffusion, reaction, and formation of higher oxides and oxyacids in the gas and in the liquid phases. Absorption of both HNO2 and N2O3, formed in the bulk gas and within the gas diffusion film, were found to be significant. Using a rate coefficient of 8.8 × 10−3 mol/s · cm3 · atm3 for the formation of HNO2 in the gas phase (England and Corcoran, 1975), a value of the absorption factor, H √Dk, for N2O3 of 2.5 × 10−3 mol/s cm2 atm was determined at 25°C. This value was found to decrease with temperature and was 1.2 × 10−3 at 40°C. The use of concentrated base as an absorbent solution prevented the formation of nitric acid mist, a problem encountered in many previous studies of NOx absorption.
A condition number scaling policy for stability robustness analysis
Authors: Chanh Nguyen T. , Barton Geoffrey W. , Perkins John D. , Johnston Richard D.
Published: June 2004

The condition number of the process transfer function G(s) has been proposed and used as an indicator of the inherent controllability of a process even though it is a scale‐dependent quantity. A rapid suboptimal scaling policy known as G‐balancing is developed for the case of the l2 norm where the condition number is the ratio of the largest to the smallest singular value of G(s). For 2 × 2 systems, G‐balancing gives the same optimal scaling as an available analytical solution. The higher order examples given compare the results of G‐balancing with the known analytic results for l2 norm optimal scaling.
Critical impeller speed for solid suspension in mechanically agitated contactors
Authors: Raghava Rao K. S. M. S. , Rewatkar V. B. , Joshi J. B.
Published: June 2004

Critical impeller speed for the suspension of solid particles, Ncs, has been measured in 0.3, 0.4, 0.57, 1.0, and 1.5 m ID mechanically agitated contactors. Tap water and quartz particles (100, 340, 700, 850, 2,000 μm) were used as liquid and solid phase, respectively. The impeller speed was varied from 3.5 to 13.3 r/s and solid loading from 0 to 50 wt. %. Three types of impellers were employed: disk turbine, pitched turbine downflow, and pitched turbine upflow. The impeller diameter to vessel diameter ratio was varied in the range 0.175 to 0.58 and the impeller blade width to impeller diameter ratio was varied in the range 0.25 to 0.4. The impeller clearance from the tank bottom was varied from 0.5 to 0.167 of tank diameter. The effect of impeller blade thickness was also studied.

The pitched‐blade impellers were found to be more efficient than a conventional disk turbine, and the pitched turbine downflow type was found to be more efficient than a pitched turbine upflow impeller. An attempt has been made to explain the mechanism of suspension on a rational basis and a correlation has been proposed for the estimation of Ncs that is expected to be useful in design.
Enzyme‐caytalyzed oxidation of cholesterol in supercritical carbon dioxide
Authors: Randolph T. W. , Blanch H. W. , Prausnitz J. M.
Published: June 2004

Supercritical carbon dioxide was studied as a solvent for the enzymatic oxidation of cholesterol with molecular oxygen. Enzymes isolated from Streptomyces sp., Norcardia sp., Pseudomonas sp., and Gloeocysticum chrysocreas are active in supercritical carbon dioxide.

For the oxidation of cholesterol to cholest‐4‐ene‐3‐one (via cholesterol oxidase) from Gloeocysticum chrysocreas, turnovers are almost two orders of magnitude higher than those found in aqueous solutions; initial rates of the reaction catalyzed by Streptomyces sp. in supercritical carbon dioxide are comparable to or greater than those found in aqueous solutions. Cholesterol oxidase from Gloeocysticum chrysocreas is stable in supercritical carbon dioxide at 100 bar and 35°C, while the enzyme from Streptomyces sp. is not. A trace amount of water is necessary for catalysis in carbon dioxide.

The rate of oxidation is increased markedly by addition of small amounts of tert‐butanol and iso‐butanol, moderately by ethanol, slightly by acetone and n‐butanol, and not at all by addition of methanol. Solubility data cannot explain these observations.

EPR spectroscopy reveals no large conformational changes in the enzyme from Gloeocysticum chrysocreas as a function of carbon dioxide pressure or cosolvent addition. However, EPR spectroscopy indicates that aggregation of cholesterol molecules in supercritical carbon dioxide is strongly affected by changes in pressure or cosolvent content. The degree of cholesterol aggregation correlates well with observed rate enhancements.
Qualitative simulation of chemical process systems: Steady‐state analysis
Authors: Oyeleye Olayiwola O. , Kramer Mark A.
Published: June 2004

A method of qualitative simulation for continuous process systems that predicts the steady‐state measurement patterns generated as a result of process malfunctions is presented. The method is based on qualitative versions of the steady‐state process equations and requires no numerical information beyond the signs and relative values of certain groups of parameters. Previous methods of qualitative simulation (Umeda et al., 1980; de Kleer and Brown, 1984; Forbus, 1984; Kuipers, 1986) have tended to generate multiple solutions that do not coincide with any observable system behaviors. In the current method, spurious solutions are reduced using “latent” constraints associated with analytically redundant model equations, and “nonlatent” constraints based on causality, and derived from the extended signed directed graph (ESDG). The ESDG is similar to the signed directed graph (SDG) (Iri et al., 1979), but includes certain nonphysical branches that account for complex dynamics (inverse and compensatory response due to negative feedback). Necessary conditions based on the topology of the SDG for the occurrence of complex dynamics are developed. Based on these criteria, assumptions concerning disturbance propagation are introduced that minimize spurious interpretations of system behavior while guaranteeing inclusion of the steady‐state response. The method is demonstrated by simulating the effects of equipment malfunctions on a model process.
Falling film heat transfer analysis on a bank of horizontal tube evaporator
Authors: Kocamustafaogullari G. , Chen I. Y.
Published: June 2004

Heat transfer analysis of falling film evaporation on a bank of horizontal tubes is investigated in this study. Liquid falling from one tube to the next lower one is considered as a thin liquid jet impinging on the top of the tube with a uniform free falling velocity. The hydrodynamic and thermal solutions are obtained by a finite difference method. In these solutions the heat transfer coefficients are modified to account for waviness.

Liquid film thickness and local and average heat transfer coefficients are obtained for the constant heat flux and isothermal boundary conditions. Convective heat transfer effects are shown to be dominant as Peclet number increases. Heat transfer coefficients for the tubes in a tube bank decrease from the first tube onwards until the fully developed region is reached. The present predictions agree favorably with the reported heat transfer data.
Multicomponent equilibrium theory for ion‐exchange columns involving reactions
Authors: Hwang Yng‐Long , Helfferich Friedrich G. , Leu Rong‐Jin
Published: June 2004

The multicomponent equilibrium theory for ion‐exchange columns (chromatography) has been extended to systems accompanied by reactions in either or both phases. Based on the concept of coherence of concentration waves in the original theory, the proposed approach includes a systematic way to determine the thermodynamic variance and a general mathematical formulation to construct the composition path grid by means of numerical techniques. With proper adjustments, this approach can be applied as well to adsorption columns involving reactions. As a practical example, a comparison of predicted and observed effluent histories for an anion‐exchange system with dissociation‐association reactions is shown.
Rotational effects on natural convection in a horizontal cylinder
Authors: Yang H. Q. , Yang K. T. , Lloyd J. R.
Published: June 2004

A numerical computation is carried out to study the interaction of rotation and natural convection inside a finite horizontal cylinder. The natural convection is due to differential heating on the two ends, while the rotation is along the axis of the horizontal cylinder. The aspect ratio, length of the cylinder to its radius, is 2.0. The Grashof number is fixed at 1.43 × 106 with air as a working fluid. The effect of rotation is examined with Gr/Re2 from 7.0 × 10−2 to ∞, which covers the range from rotation dominated flow to buoyancy dominated flow. It is found that when rotational speed is relatively small or Gr/Re2 » 1, the effect of rotation is to render the spatial heat flux distribution more uniform. As the rotational speed increases, the heat transfer rate distribution on the end surfaces is more homogenous and its level is also reduced. When the rotational speed is sufficiently high (Gr/Re2 « 1.0), heat transfer is by conduction only and the whole fluid inside the enclosure rotates as a rigid body. It is shown that there exists a rotation speed range in which one can have both relatively high and uniform heat transfer rates on the end walls.
Wrong‐way behavior of packed‐bed reactors: II. Impact of thermal dispersion
Authors: Pinjala V. , Chen Y. C. , Luss D.
Published: June 2004

A sudden reduction in the feed temperature of a packed‐bed reactor may lead to a transient temperature rise, referred to as a wrong‐way behavior. As expected, the axial dispersion of heat decreases the magnitude of the temperature excursion and prolongs the transient shift to a new steady state. In addition, the thermal dispersion may enable the wrong‐way behavior to ignite a low‐temperature steady state leading to a disastrous runaway of the reactor. Moreover, it may create a transient high‐temperature wave, which moves initially in the upstream direction. The axial dispersion of heat can lead to some behavioral features which are qualitatively different from those of a model which ignores it. The transient temperature excursion does not exceed a value, which can be estimated by a simple analytical expression.
Fluid mechanics of the planar‐flow melt‐spinning process
Authors: Steen Paul H. , Kent Carpenter J. , Yu Ho
Published: June 2004

The planar‐flow melt‐spinning process which is used to rapidly solidify metals from the molten state is shown to be dominated by the fluid mechanics of the “puddle” region even though heat‐transfer limits the overall thickness of the metal ribbon product. The process is modeled to account for the hydrodynamical forces in the molten region with the influence of heat‐transfer entering through a parameter measuring solidification rate relative to wheel speed. It is shown that this parameter H controls the deviation of the flow behavior from classical coating flow solutions; these solutions are recovered in a limiting case of low solidification rate. A perturbation solution in H distinguishes the melt spinning from the coating process and yields the ribbon thickness as a function of wheel‐speed and the other process parameters for a class of contact‐line conditions. Most interesting of these predictions is the result that under certain conditions there is a window of wheel‐speeds for which there is no steady solution. The relationship of predictions with the limited available data from experiment is briefly discussed.
Separation of solutes from aqueous solutions by contained liquid membranes
Authors: Sengupta A. , Basu R. , Sirkar K. K.
Published: June 2004

The hollow fiber contained liquid membrane (CLM) is a thin liquid film contained in the interstices of two sets of intermingled microporous hollow fine fibers. Organic CLM‐s have been used here for the separation of solutes from an aqueous feed into an aqueous strip. Solutes studied are phenol and acetic acid. The separations are carried out in either hydrophilic or hydrophobic hollow fiber CLM permeator modules, using a variety of organic liquids (e.g., decanol, methyl isobutyl ketone, xylene) as membranes. First‐order models have been developed to predict the overall solute transfer coefficients adequately. The transfer coefficient can be enhanced significantly when a chemical reaction is carried out on the strip side using NaOH. The advantages of the CLM structure include operational stability, independent control of membrane phase pressure, automatic replenishment of the lost membrane liquid, and absence of the need for preequilibration. These features are demonstrated here, even for systems with considerable aqueous‐organic mutual solubilities.
A numerical model of the synthesis of carbon black by benzene pyrolysis
Authors: Ivie J. J. , Forney L. J.
Published: June 2004

A numerical model has been developed to predict the formation of carbon black particles by benzene pyrolysis. The approach, patterned after the work of Jensen, assumes that particle formation is controlled by four steps: gas phase reactions producing radical species, nucleation, growth and coagulation, and oxidation. In the present model, 15 reactions based on the work of Fujii and Asaba are used to describe the gas phase kinetics in which the phenyl radical is treated as the important intermediate in the formation of carbon black. A discrete distribution of 10 particle radii is also used to approximate the simultaneous growth and coagulation of spherical carbon particles. The results of the model show that a self‐preserving log normal distribution of carbon particles develops after an initial nucleation and growth period. The model results also compare favorably with reactor data. In particular, the numerical model predicts a mass median carbon particle diameter to within 10% of measured values.
Rigorous thermodynamic treatment of gas adsorption
Authors: Talu Orhan , Myers Alan L.
Published: June 2004

Multicomponent adsorption equilibria are predicted from theories based on experimental data for single‐gas adsorption isotherms. Theories of multicomponent adsorption differ in the details of their results but should agree in special cases such as low surface coverage and ideal solution behavior. This paper is a summary of rules, limits, and consistency requirements that apply to adsorption of single gases and their mixtures. These rules and limits provide a basis for comparing theories with each other and with experimental data.
Kinetics of the dissolution of alumina in acidic solution in a flow system
Authors: Crawford Myles D. , Baumgart Jerry , Shoaei Mahnoosh , Ernst William R.
Published: June 2004

The kinetics of dissolution of alumina in aqueous solutions of H2SO4 was studied at ambient temperature under continuous‐flow conditions. The results are consistent with a model involving rapid hydration of the surface, adsorption‐desorption of H+ to form precursor sites, and rate‐determining parallel reactions of the precursor sites to form dissolved aluminum and an insoluble surface species. The model also involves renewal of a precursor site after each dissolution step.

The adsorption‐desorption of H+ was represented by a Langmuir isotherm. The dissolution data were correlated by a kinetic equation that was developed by assuming the dissolution step is first order in precursor concentration, and the deactivation step is second order in precursor concentration.
Dispersion coefficient for laminar flow in curved tubes
Authors: Daskopoulos Ph. , Lenhoff A. M.
Published: June 2004

The axial dispersion coefficient in a fluid in laminar flow in a tube is generally smaller in a curved tube than in a straight tube, because of the enhacement of lateral transport by secondary flows. In this paper the extent of this reduction is computed using Horn's modification of Aris's method of moments. Dimensional analysis suggests that the most convenient form in which to represent the results is that obtained empirically by Trivedi and Vasudeva. The results presented here cover the whole laminar flow regime, and show the region of applicability of previously computed dispersion coefficients to be limited. The computed dispersion coefficients agree well with previously reported experimental results for small Schmidt numbers; however, discrepancies are present at large Schmidt numbers. Possible causes are discussed.
Plasma processing in microelectronics manufacturing
Authors: Graves David B.
Published: June 2004

Plasma processing has emerged as an important technology for the deposition and etching of thin solid films in the manufacture of microelectronic devices. In fact, much of the progress achieved to date in device miniaturization would have been much more difficult without the unique characteristics of plasma processes. It is anticipated that future progress will continue to rely heavily on plasma technology. Although plasma chemical reactors are widely used, a lack of fundamental understanding has resulted in heavy reliance on empiricism in process design and control. The complexities inherent in any chemical reactor are compounded in plasma processing by the presence and active participation of the discharge in both gas phase and surface chemical processes. This has resulted in a technology that is sensitive to many design and operating parameters. A large parameter space gives process engineers flexibility in tailoring process conditions to meet objectives. However, without fundamental insight into the key physical and chemical processes, reactor design and control cannot be based on systematic, rational procedures. The purposes of this review are to introduce chemical engineers to plasma processing practice and phenomenology; to outline the progress made to date in fundamental studies of discharge physics and chemistry; to indicate areas in which chemical engineers must expand their traditional training to participate in plasma processing research; and finally to suggest how established chemical engineering methodology can be applied to plasma chemical reactors.
Product yields and kinetics from the vapor phase cracking of wood pyrolysis tars
Authors: Boroson Michael L. , Howard Jack B. , Longwell John P. , Peters William A.
Published: June 2004

The homogeneous vapor phase cracking of newly formed wood pyrolysis tar was studied at low molar concentrations as a function of temperature (773–1,073 K), at residence times of 0.9–2.2 s. Tar conversions ranged from about 5 to 88%. The tars were generated by low heating rate (0.2 K/s) pyrolysis of ∼2 cm deep beds of sweet gum hardwood, and then rapidly conveyed to an adjacent reactor for controlled thermal treatment. Quantitative yields and kinetics were obtained for tar cracking and resulting products formation. The major tar conversion product was carbon monoxide, which accounted for over two‐thirds of the tar lost at high severities. Corresponding ethylene and methane yields were each about 10% of the converted tar. Coke formation was negligible and weight‐average tar molecular weight declined with increasing tar conversion. A first‐order distributed activation energy model more closely correlated tar conversion kinetics over a wider range of reaction conditions than did a single‐reaction model.
Flow in packed tubes with a small tube to particle diameter ratio
Authors: Chu C. F. , Ng K. M.
Published: June 2004

A computer‐generated slim tube randomly packed with spheres of almost uniform size is used to model single‐phase flow in a packed tube with a small tube diameter to particle diameter ratio, R. To obtan a detailed description of its morphology, the slim packed tube is first tessellated into tetrahedra in the interior, and pentahedra near the walls of the tube. Then, the pore space is represented by a network of interconnected circular and triangular sinusoidal flow channels. The size and length of each channel, as well as their interconnectivity, are exactly known. In addition, porosity and solid surface area per unit volume of the porous medium are determined as a function of distance away from a wall. These data suggest that the presence of the walls has two counteracting effects on fluid flow. A higher porosity promotes flow along the walls but a higher surface area per unit volume hinders it. To confirm this prediction, experimental permeability data are obtained for tubes with R between 2.5 and 40. For R > 25, the permeability is the same as that of a large diameter tube, k∞. Between 8 and 25, the permeability can be larger or less than k∞. The way the tube is packed determines whether the higher porosity or surface area dominates in the wall region, and thus higher or lower permeability. Below R = 8, the confining walls cause a marked increase in overall bed porosity and the permeability is always larger than k∞. Theoretical predictions of the permeability of such very slim tubes are in good agreement with experimental data.
Dynamics of Haines jumps for compressible bubbles in constricted capillaries
Authors: Gauglitz P. A. , Radke C. J.
Published: June 2004

The unsteady, impulsive motion of a compressible bubble expanding out of a constricted capillary is quantified with a macroscopic momentum balance. Numerical solution demonstrates the importance of the Ohnesorge number, the geometry of the constriction, the length of the initial gas bubble, and the surface tension, density, and unconstricted capillary radius, which combine to form a characteristic scaling time. Experimental data for the position of the bubble front as a function of time confirm the theoretical result when the time scale for the bubble jump is longer than that required to achieve fully developed parabolic flow. Theory also predicts the capillary number of the bubble jump which, in conjunction with previous theoretical results, determines the time to snap‐off of gas bubbles moving through constricted capillaries. Excellent agreement is found with existing experimental data for Ohnesorge numbers ranging from 5 × 10−3 to 0.3.
A state space formulation for model predictive control
Authors: Li Sifu , Lim Kian Y. , Fisher D. Grant
Published: June 2004

Model predictive control (MPC) schemes such as MOCCA, DMC, MAC, MPHC, and IMC use discrete step (or impulse) response data rather than a parametric model. They predict the future output trajectory of the process {ŷ(k + i), i = 1, …, P}, then the controller calculates the required control action {Δu(k + i), i = 0, 1, …, M − 1} so that the difference between the predicted trajectory and user‐specified (setpoint) trajectory is minimized. This paper shows how the step (impulse) response model can be put into state space form thus reducing computation time and permitting the use of state space theorems and techniques with any of the above‐mentioned MPC schemes. A series of experimental runs on a simple pilot plant shows that a Kalman filter based on the proposed state space model gives better performance that direct use of the step response data for prediction.
Wall‐cooled fixed‐bed reactors: Parametric sensitivity as a design criterion
Authors: Borio Daniel O. , Gatica Jorge E. , Porras José A.
Published: June 2004

Multitubular reactors of the heat exchanger type are commonly used to carry out highly exothermic reactions. Two of the main goals, when designing this type of reactor, are to guarantee a safe operation and to minimize the reactor length. Therefore, these concepts are used here as criteria for comparing the performance of the three most common cooling‐medium flow arrangements. The analysis of the parametric sensitivity behavior, as well as that of some related measures, indicates that, at least from the steady‐state standpoint, the cocurrent arrangement should be considered the most attractive alternative.