bluuebunny/crossref_metadata_2025 huggingface dataset:
Simple and controlled extrapolation of vapor pressures toward the triple point
Authors: Růržička Květoslav , Majer Vladimír
Published: June 2004

Although experimental vapor pressures are abundant in the medium‐pressure range (psat ≃ 5–100 kPa), their values are scarce and uncertain at lower pressures due to experimental difficulties in determining the data. Reliable values are, however, required by technology (separation processes dealing with mixtures containing high molar mass compounds) and environmental concerns (transport and fate of high‐boiling pollutants). The ability of frequently used vapor‐pressure equations to extrapolate the data at medium pressures toward the triple‐point temperature was tested on three groups of compounds (polar and nonpolar organic chemicals, n‐alkanes, and 1‐alkanols) for which reliable experimental data in the low‐pressure range were available. Results of a simple extrapolation were compared with those from a simultaneous correlation of vapor pressures in the medium‐pressure range and the differences between heat capacity of an ideal gas and that of the liquid available at lower temperatures. The latter is generally more reliable, as it is controlled by the exact thermodynamic constraints linking vapor pressures and the thermal data. The extent of improvement when using simultaneous correlation was examined, as well as the influence of the quality of input data on the results of extrapolation. Extrapolated values were compared with experimental vapor pressures and calorimetric enthalpies of vaporization that were selected from literature and free of any important systematic errors.
Hydrodynamic characteristics of a three‐phase inverse fluidized‐bed column
Authors: Ibrahim Yasser A. A. , Briens Cedric L. , Margaritis Argyrios , Bergongnou Maurice A.
Published: June 2004

Gas–liquid–solid inverse fluidized beds were studied, in which the gas and the continuous liquid phase flow countercurrently fluidizing particles that are lighter than the liquid. Conductivity and static pressure measurements were combined to provide the vertical profiles of the gas, liquid and solid holdup, in beds of 4‐ or 6‐mm polypropylene particles with a density of about 870 kg/m3. Various bed heights were obtained with different measurement methods. The minimum fluidization velocity was estimated using the bed static pressure gradient. A new transition velocity, called the uniform fluidization velocity, was identified using conductivity measurements. This is the superficial liquid velocity at which the fluidization quality becomes the same throughout the bed.
Estimation of three‐phase flow functions in porous media
Authors: Mejia G. M. , Watson A. T. , Nordtvedt J. E.
Published: June 2004

Several important processes involve the flow of three immiscible fluids through porous media, such as the flow of oil, water and gas in petroleum reservoirs, or water, nonaqueous‐phase liquid and air in underground aquifers. Multiphase flow functions (relative permeability and capillary pressure) are important to simulate the flow in such systems. Unfortunately, there are few, if any, reliable estimates of these flow properties from laboratory experiments. One of the main reasons for this is the use of unsupported simplifications in the procedures for determining property estimates from measured data.

We present a method for simultaneous estimation of three‐phase relative permeability and capillary pressure functions from laboratory experiments. The method is not limited by restrictions in the experimental design or assumptions regarding the saturation dependence or shape of the functions to be estimated. The method is demonstrated with simulated experiments. It is shown that with a suitable experimental design, regions of the functions represented by the measured data can be determined accurately.
Effect of heterogeneous mechanisms during reburning of nitrogen oxide
Authors: Chen Wei‐Yin , Ma Long
Published: June 2004

NO reduction through heterogeneous mechanisms is a major concern when coal or lignite is used as reburning fuel. Reburning was simulated in an alumina flow reactor operated with a synthetic flue gas at a stoichiometric ratio of 1.1, at 1,100°C and with a residence time of approximately 0.2 s. Reburning fuels used include methane, one bituminous coal, two lignites, chars derived from the bituminous coal and two lignites, and the bituminous coal char impregnated with CaO. Reburning with chars indicates that heterogeneous mechanisms are more important than homogeneous mechanisms when lignites are used. This high reactivity renders lignite char an even more effective reburning fuel than the parent lignite, while char from the bituminous coal produces little activities at stoichiometric ratios above 0.75. Bituminous coal char impregnated with CaO demonstrates higher NO reduction efficiency than the original char. Effects of SR on NO reductions imply that oxygen atoms, required for the NO and HCN conversions in homogeneous mechanisms, are not required in heterogeneous mechanisms. In fact, they inhibit the surface NO reduction. The first‐level kinetic analysis indicates that the surface area is not the only contributor to a remarkable increase in NO reduction of lignite char, and chars of different origins seem to follow different mechanisms or to have different controlling steps in the overall reaction scheme.
On the kinetics of phenol oxidation in supercritical water
Authors: Krajnc Matjaž , Levec Janez
Published: June 2004

Phenol oxidation in supercritical water was carried out in a tubular laboratory‐scale reactor operated at a temperature range of 380°C to 450°C and pressures between 230 and 265 bar. The phenol feed concentrations were between 500 and 1,000 mg/L, while oxygen was fed into the reactor at 50 to 1,000% of the stoichiometric amount needed to oxidize phenol completely to carbon dioxide. Phenol conversions from 16 to 96% were attained as the reactor residence times varied from 15 to 203 s. The oxidation obeys a parallel‐consecutive reaction scheme that involves multiring, intermediate products such as phenoxy‐phenol, biphenol, dibenzo‐dioxin, maleic acid, and succinic acid. Experimental results showed that the phenol disappearance rate is represented well by a power‐law kinetic model in which the rate is proportional to the 0.4 power of the oxygen mole fraction and roughly linearly proportional to the phenol mole fraction. The pressure effect on the disappearance rate was appropriately accounted for by introducing the molar volume of the reaction mixture, which was readily calculated by an equation of state. Total organic carbon reduction can be estimated by a lumped kinetic equation. In the P‐T region the activation energy of the phenol disappearance was 124.7 kJ/mol.
Effective viscosities in thin ionic micellar liquid films
Authors: Pollard Maria L. , Radke C. J.
Published: June 2004

Thin liquid films stabilized by surfactants above the critical micelle concentration exhibit stratification or stepwise dynamic thinning. A continuum hydrodynamic model is outlined for stepwise film thinning that incorporates equilibrium micellar structuring through self‐consistent oscillatory disjoining pressures and effective viscosities. Effective viscosities as functions of thickness are evaluated with an extension of the local average density model, considering dilute colloidal suspension shear viscosities and solvent effects. To establish local shear viscosities, structured DFT micellar profiles, coarse‐grained densities, and disjoining pressure are used. Ionic micelles and other colloidal systems with repulsive interactions show structured effective viscosities that are generally less than the corresponding homogeneous solution shear viscosity, bounded by the pure solvent viscosity and that of the bulk micellar solution. For 0.1 and 0.2‐M sodium dodecylsulfate micellar solutions, the effective viscosities are less than 5 and 10%, respectively, below the homogeneous fluid viscosity, except at small thicknesses, indicating that the micellar film thins faster than a pure water film of the same thickness.

Calculated thinning curves closely resemble experimental observations in the stepwise thinning behavior, displaying decreasing slopes and increased step durations at later times. Despite the micellar structuring within the film, the ionic micelles do not contribute appreciably to the viscous resistance of the thinning film. Rather, Reynolds' film thinning is obeyed, with the equilibrium oscillatory disjoining pressures driving the stepwise dynamics. The shear viscosity of the ionic micellar film is well approximated by that of the bulk solution.
Prefractionation to reduce energy consumption in distillation without changing utility temperatures
Authors: Agrawal Rakesh , Fidkowski Zbigniew T. , Xu Jianguo
Published: June 2004

Distillation systems composed of a prefractionator and a main column are introduced. The columns operate at different pressures, but the temperatures of the utilities are the same as for a single column performing the same separation. The energy integration of the columns is realized by condensing a vapor overhead from the prefractionator in an intermediate reboiler located in the stripping section of the main column or by boiling a bottoms liquid from the prefractionator in an intermediate condenser placed in the rectifying section of the main column.

Analysis is performed to determine feasible regions of operational parameters and to minimize vapor flows. A detailed simulation example of the benzene–toluene separation shows that for certain feed compositions it is possible to reduce energy requirements of a conventional binary distillation column by almost one‐third using this new distillation system. This savings is achieved without any change in the temperature of the utilities used.
A model for drying of viscoelastic polymer coatings
Authors: Cairncross Richard A. , Durning Christopher J.
Published: June 2004

Drying of polymeric coatings often occurs under conditions where the relaxation time of polymer molecules is significant with respect to the processing time scales. The nonequilibrium thermodynamic theory of Durning and Tabor (1986) is applied to model 1‐D drying of viscoelastic solutions with concentration‐dependent physical properties. Transport of solvent to the surface of the coating occurs by viscoelastic diffusion down the gradient of a diffusion potential with a relaxing, nonequilibrium contribution. Galerkin's method with finite‐element basis functions and a differential/algebraic equation system solver enable efficient solution of this stiff nonlinear model. Predictions show that elasticity enhances diffusion within the coating. At high Deborah numbers, however, a fall in the surface activity slows the rate of desorption. The coating thickness after a specified time under fixed total driving force is the smallest at intermediate Deborah numbers, showing that a small amount of viscoelasticity actually aids in drying. This can be interpreted as a skinning effect.
Output‐feedback control of nonlinear plants
Authors: Alvarez Jesús
Published: June 2004

For multiinput‐multioutput nonlinear plants whose state‐feedback control problem is solvable with complete or partial output linearization, the output‐feedback problem is addressed by combining the state‐feedback controller with a suitable closed‐loop state inferer (detector). A candidate closed‐loop detector was built to study the stability of the resulting plant‐controller interconnection. As a result, sufficient conditions for closed‐loop asymptotic nominal stability, as well as a systematic and simple design‐tuning procedure, are obtained. The stabilization of an open‐loop, unstable, free‐radical homopolymerization reactor was studied as an application example.
Fluctuation theory of hydrogen bonding in near‐critical fluids
Authors: Veytsman Boris A. , Gupta Ram B.
Published: June 2004

Recent experiments indicate that the number of hydrogen bonds in solutions increases in the critical region of the solvent. This effect can be explained by the density fluctuations near the critical point. A theory of this unusual effect is developed. The theoretical predictions are compared with the experimental data on hydrogen bonding between perfluoro‐tert‐butanol and dimethyl ether in supercritical sulfur hexafluoride. The theory requires no adjustable parameters and is in excellent agreement with the experimental data. Possible applications of this effect for hydrogen‐bonding solutions and polymer mixtures are discussed.
A bidirectional fixed‐bed reactor for coupling of exothermic and endothermic reactions
Authors: Kulkarni Milind S. , Duduković Milorad P.
Published: June 2004

Performance of a bidirectional fixed‐bed reactor subject to both flow reversal and switching between exothermic and endothermic reactions is simulated. During odd semicycles (blows) an exothermic reaction heats the bed, during even semicycles an endothermic reaction cools the bed and produces the desired product in the hot zone. Such an operation is possible and efficient when the inlet gas temperature is lower than the initial bed temperature, leading to the wrong‐way behavior when the temperature front moves with a finite velocity (creep velocity) from the feed end to the exit during a semicycle. Since the dynamic nature of the fixed‐bed reactor changes with each semicycle, the front velocity during an exothermic semicycle differs from the front velocity during an endothermic semicycle and an asymptotic expression is developed for the differential creep (front) velocity that quantifies this difference. Due to a nonzero differential creep velocity, the front exhibits an effective displacement after each cycle. This asymptotic expression for the creep velocity works very well except in the inlet and outlet region of the fixed‐bed reactor. An expression developed for 100% energy efficiency shows that it can be reached only if the differential creep velocity is zero. A relation for the balanced operation of a reactor‐regenerator is discussed as well as differences in reactor performance caused by reactions occurring in the gas or solid phase.
Polymer kirkwood integral equations: Structure and equation of state of polymeric liquids
Authors: Gan Hin Hark , Eu Byung Chan
Published: June 2004

A self‐consistent theory of chain molecular liquids from polymer Kirkwood integral equations is presented. The theory predicts the intramolecular chain distributions and the intermolecular pair correlation functions from which thermodynamic functions can be correctly predicted. The utility of the theory is demonstrated with the chain distributions from self‐consistent equations at zero density for swollen and collapsed states of the chains and numerical results for the structure and equation of state of athermal chain molecular liquids at various chain lengths and densities. Structural and thermodynamic predictions of the theory are compared with Monte Carlo simulation results. The theory agrees satisfactorily with simulation data for monomer packing fractions up to 0.25. The investigation of the chain length dependence shows that a plateau value is reached for the equation of state as the number of monomer units reaches a value between 20 and 50.
Measurement of transverse and axial apparent dispersion coefficients in packed beds
Authors: Tallarek Ulrich , Albert Klaus , Bayer Ernst , Guiochon Georges
Published: June 2004

The axial and transverse apparent dispersion coefficients of three solvents in two packed chromatographic columns were determined by pulsed‐field‐gradient nuclear‐magnetic resonance in a range of mobile phase velocity. The column beds were packed with 5‐ μm particles of porous C18 silica or 30‐μm particles of silica. The solvents used were methanol and acetonitrile (in an 80:20 ACN/water solution) in the former case and acetone in the latter. The coefficients were determined over a range of particle Peclet numbers from less than 0.1 to approximately 10 in the former case and 40 in the latter. The data obtained with short dispersion times were fitted to the correlations suggested by Giddings, Horvath and Lin, Huber, and Knox. These data agreed well only with the Giddings equation.
Synthesis of waste interception and allocation networks
Authors: El‐Halwagi M. M. , Hamad A. A. , Garrison G. W.
Published: June 2004

This article introduces the novel concept of synthesizing waste‐interception networks (WINs) and incorporates it within a mass‐integration framework for the global allocation of pollutants. The essence of this notion is to provide selective interception and rerouting of undersirable species at the heart of the process instead of dealing with the pollutants in the terminal waste streams. It also provides a unified framework for simultaneously tackling gaseous and liquid pollution. Several tools are developed to track the pollutant throughout the process and determine the optimal interception policies. The problem is formulated as an optimization program that seeks to determine the optimum locations for intercepting the pollutants, extent of separation, and separating agents to carry out the interception tasks. Furthermore, the WINs can be used within a mass‐integration scheme for the global allocation of species throughout the plant. These new concepts and associated mathematical formulation are demonstrated using a case study on the removal of a chlorinated hydrocarbon. This approach has benefits over conventional recycle/reuse (such as MEN synthesis).
Mathematical modeling of fluidized‐bed chlorination of rutile
Authors: Zhou L. , Sohn H. Y.
Published: June 2004

A mathematical model was formulated to describe the reaction rate and the particle size distribution of solids in a fluidized bed for the chlorination of rutile by a CO/Cl2 mixture. The bubble assemblage model was applied to describe the gas behavior in the bed. The particle‐size distribution in the bed was determined by a population balance. Predictions of the model gave satisfactory agreement with experimental results. The effects of important variables such as superficial gas velocity, exchange rate between the phases, and the reaction‐rate constant were evaluated.
Optimum jet mixing in a tubular reactor
Authors: Forney Larry J. , Nafia Noureddine , Vo Hanh X.
Published: June 2004

The rapid mixing of two miscible fluids is a precursor to promoting fluid‐phase reactions. The configuration of a turbulent jet in a crossflow or tee‐mixer is the most efficient passive design for rapid mixing. Here the Damkohler number has been defined from available tee‐mixer data where the reactions are slow to fast compared with mixing times. For the available data the Damkohler number has been found to be of secondary importance for optimum design. The numerical simulation of jets in a tubular reactor suggests that large jet‐to‐tube momentum ratios with no backmixing are superior. Moreover, the optimum design of maximum jet entrainment is shown to be synonymous with either a minimum in the relative standard deviation or the skewness. A formula relating the optimum diameter ratio to the jet‐to‐tube flow ratio has also been derived assuming geometrically similar jet trajectories, and the result correlates both numerical and experimental data.
CEOS/AE mixing rules constrained by vdW mixing rule and second virial coefficient
Authors: Twu Chorng H. , Coon John E.
Published: June 2004

A new class of mixing rules, with van der Waals one‐fluid mixing rule and second virial coefficient constraints, was developed for cubic equations of state (EOS) for application to various asymmetric, highly nonideal chemical systems. This approach evolved from previous research in the EOS field. The resulting mixing rule is practical in application, yet more flexible than Huron–Vidal or Wong–Sandler mixing rules and depends only on composition and temperature. It also avoids the problems associated with the Wong–Sandler mixing rules. A cubic EOS with the new mixing rules was applied successfully to complex mixtures. The new rules reduce to van der Waals mixing rules when the parameters in the nonrandom excess Helmholtz free energy are set to zero. Since many times the classic mixing rules worked very well for nonpolar systems, it is extremely desirable that the composition‐dependent mixing rules reduce to the classic mixing rules. Preserving this capability also ensures that the binary interaction parameters for the classic quadratic mixing rules in the DECHEMA Chemistry Data Series for mixtures of low boiling substances and other existing databases for systems involving hydrocarbons and gases can be used directly in the new EOS mixing rules, in addition to liquid‐activity coefficient model parameters currently reported in the Series.
Hydrodynamic model for short contact time circulating fluidized bed
Authors: Tinaburri A.
Published: June 2004

Short contact time circulating fluidized beds (CFBs) are high‐performance reactors with great potential of application for fast endothermic reactions. Optimal kinetic conditions require high operating temperature and very short residence time (∼0.15 s), as well as plug flow in the reaction zone, and fast and efficient solid separation before quenching effluents. Hydrodynamics of dilute CFB operated at high gas velocity was simulated in isothermal condition. The gas‐flow field is described using turbulence closure models developed for the single phase and modified to account for particle presence. Numerical computation was compared with experimental results of Arena et al. (1993) and Tinaburri et al. (1996) for two different riser configurations. A parametric analysis was performed to investigate inlet geometry influence on flow pattern along the riser. Internal tube and circumferential gas inlet produced the most promising configuration to realize optimal flow condition.
Self‐diffusion in liquids predicted by a theory of flow through granular media
Authors: Sęk Jerzy P.
Published: June 2004

A new semiempirical model for prediction of self‐diffusion phenomena in liquids is proposed. Assuming anlogy between the concept of a free volume in liquids and a porosity of a granular medium, a capillary model of the granular bed and the Kozeny‐Carman theory was applied to determine self‐diffusion coefficients in liquids. The dimensionless free volume in liquids was evaluated using a combination of the proposed dependence and an appropriate formula available in the literature. The new model was tested for 89 sets of data concerning self‐diffusion of different chemical substances at several values of a temperature. The average error of a prediction was between ± 1% and about 3% for some data points.
Ultrasound‐modulated two‐fluid atomization of a water jet
Authors: Tsai Shirley C. , Childs Paul , Luu Patrick
Published: June 2004

A new spray technique, called ultrasound‐modulated two‐fluid (UMTF) atomization, is based on resonance between the liquid capillary waves generated by ultrasound and those generated by high‐velocity air. The capillary waves generated by ultrasound on the cone of liquid film issuing from a coaxial two‐fluid atomizer are magnified in amplitude by air blowing around them. Atomization occurs when the amplitude of the capillary waves is too great to maintain wave stability, and the resulting drop sizes are determined by the frequency of the ultrasound. Calculations of the relative amplitude growth for the capillary waves of various wavelengths yield predictions that agree remarkably well with experimental results of drop size and size distribution. Specifically, uniform drops with diameter determined by the third harmonic frequency of the ultrasound are obtained in UMTF atomization at high air velocity and large air‐to‐water mass ratio. In contrast, drop‐size distributions with multiple peaks are obtained in UMTF atomization at low air velocity and small air‐to‐water mass ratio. The use of air also allows the liquid jet to atomize at ultrasonic power levels below and liquid flow rates above the threshold values for ultrasonic atomization without air. These new findings provide not only direct evidence of the capillary wave mechanism but also a means of controlling drop size and size distribution in two‐fluid atomization.
Mixing of granular materials in slowly rotated containers
Authors: McCarthy J. J. , Shinbrot Troy , Metcalfe Guy , Wolf J. Eduardo , Ottino Julio M.
Published: June 2004

Noncohesive granular materials in slowly rotated containers mix by discrete avalanches; such a process can be described mathematically as a mapping of avalanching wedges. A natural decomposition is thus proposed: a geometrical part consisting of a mapping wedge → wedge, which captures large‐scale aspects of the problem; a dynamical part confined to the avalanche itself, which captures details emanating from differences in size/density/morphology. Both viewpoints are developed and comparisons with experiments are used to verify the predictions of the models. In this article, we develop a model of granular mixing and show how to extend the model in order that it may: (1) handle complicated geometries, (2) be applicable for 3‐D mixers, (3) rapidly test mixing enhancement strategies, and (4) incorporate differences in particle properties. In addition, an optimal fill level is determined for several 2‐D mixing geometries, and a novel hybrid—geometrical/dynamical—computational technique is proposed. By merging the geometrical and dynamical viewpoints, this technique reduces the computational time of a typical molecular‐dynamics‐type simulation by a factor of 15. The ultimate goal is to provide fundamental understanding and tools for the rational design and optimization of granular mixing devices.
Salt‐containing model for simulation of salt‐containing extractive distillation
Authors: Fu Jiquan
Published: June 2004

Three binary vapor–liquid equilibrium (VLE) data were measured: tertbutanolethanediol, tertbutanol‐KAc, and ethanediol‐KAc. Experimental data were correlated with the salt‐containing local composition model (SCLCM). For the multicomponent system, VLE data predicted by SCLCM are in agreement with the data in the literature. An industrial plant (3500T/Y) of salt‐containing extractive distillation for the tertbutanol/water/ethanediol/potassium acetate system was simulated by SCLCM and improved the Rose relaxation method. Simulation results agree well with industrial data.
Nonlinear model‐predictive control: Closed‐loop stability analysis
Authors: Sistu Phani B. , Bequette B. Wayne
Published: June 2004

A method to analyze the closed‐loop stability of a system composed of a nonlinear process and a discrete controller is developed. The closed‐loop system is described by a set of difference equations resulting from the discretization of the continuous‐time model. A commonly used method of discretization (forward difference) offers an incorrect relative order compared to exact discretization. The state and input sensitivity equations of the continuous‐time model are used in computing the nominal closed‐loop stability criteria. The nominal stability analysis is extended to the important cases of unmeasured states and uncertain model parameters. A numerical Lyapunov function is used to estimate closed‐loop regions of attraction. A simulation example (a CSTR with input multiplicity) presented illustrates the analysis methods and closed‐loop behavior.
Single‐stage fractionation of poly(ethylene‐co‐vinyl acetate) in supercritical ethylene with saft
Authors: Folie Bernard
Published: June 2004

Using the discrete thermodynamics approach, the single‐stage fractionation of a polydisperse poly(ethylene‐co‐vinyl acetate) in supercritical ethylene and ethylene‐vinyl acetate mixtures is modeled with an EOS rooted in statistical associating fluid theory (SAFT). The simulation results are compared to new high‐pressure coexistence data and size‐exclusion chromatography data obtained on a few selected extracts. The polymer molecular‐weight distribution is optimally represented by ten nearly‐monodisperse pseudocomponents, determined by a nonuniform lumping method. SAFT quantitatively captures the effect of pressure, temperature and solvent composition on the solvent capacity and the extract yield over a broad range of conditions. The ethylene capacity monotonically increases with increasing pressure between 200 bar and the cloud point pressure, with increasing VA concentration in the solvent mixture, and with increasing temperature above ∼ 480 bar. At intermediate pressure (200–480 bar), SAFT predicts that the ethylene capacity behaves nonmonotonically with temperature. The extract and raffinate molecular weight and polydispersity are predicted also from SAFT as a function of pressure and solvent composition for a typical bubble‐ and dew‐point‐type fractionation, thereby illustrating the underlying differences between those two phase disengagement mechanisms.
Elementary step model of ethane oxidative dehydrogenation on Pt‐coated monoliths
Authors: Huff Marylin C. , Schmidt Lanny D.
Published: June 2004

The oxidative dehydrogenation of ethane to ethylene over Pt‐coated foam monoliths in oxygen with varying levels of nitrogen dilution has been simulated using a 24‐stepmodel of adsorption, desorption and surface reactions. Reaction parameters for these elementary steps were obtained from surface science and catalysis literature or fit to previously reported experimental data. The model agrees with the experimental data remarkably well, mimicking the selectivity and conversion trends observed experimentally with changing operating conditions. The model shows that a purely heterogeneous mechanism can be used to simulate the experimental results collected near atmospheric pressure accurately, suggesting that the mechanism we propose is sufficiently accurate to capture the main features of this reaction system. This model can also be used to examine this reaction system under conditions experimentally inaccessible. Application of this predictive ability to operation of this type of reactor under probable industrial conditions suggests that excellent ethylene yields are possible.
Coagulation of colloids retained by porous wall
Authors: Harmant P. , Aimar P.
Published: June 2004

The structure of a colloidal deposit retained by a porous wall is described, accounting for surface interactions and hydrodynamic forces. The balance of forces acting over spherical, charged particles allows the calculation of the interparticular distances inside the cake according to the physicochemical conditions (ionic strength, particle potential, pH, particle size) and to the experimental parameters (flux). The model predicts that beyond a critical mass deposited on the porous wall, the structure of the layers near the membrane changes where the particles are in close contact with each other. Experimental data obtained with latex monodisperse particles filtered over various types of ultrafiltration membranes for various physicochemical and flux conditions are compared to the model predictions. These results explain the existence of irreversible colloidal deposits in filtration and suggest strategies to optimize backflush or pulsed pressure procedures often used to improve the efficiency of ultrafiltration or microfiltration.
Nonequilibrium modeling of condensed mode cooling of polyethylene reactors
Authors: Jiang Yan , McAuley Kim B. , Hsu James C. C.
Published: June 2004

In most industrial gas‐phase polyethylene reactors, heat is removed by cooling the recycle gas stream in an external heat exchanger, where a portion of the vapor is condensed. The condensate evaporates in the reactor to absorb heat released by polymerization reactions, thereby increasing the production capacity of the unit. Nonequilibrium methods of multicomponent condensation are applied to develop a 1‐D model to simulate the cooling unit of an industrial polyethylene reactor system operated in partial condensing mode. Finite difference approximations are used to convert the resulting set of differential equations to algebraic equations. A practical method of solving the equations is to combine the rapid local convergence of Newton's method with a globally convergent strategy. Correlation methods for estimating local heat‐transfer coefficients in the liquid film layer are discussed. Butterworth's method for shear‐stress‐controlled condensate flow gives reasonable agreement between simulation results and industrial data, while Chen et al.'s method can better describe the transition process of condensate flow from laminar to turbulent flow.
Effect of dissociation equilibria on ion‐exchange processes of weak electrolytes
Authors: Jansen Marcel L. , Houwers Joop , Straathof Adrie J. J. , Van Der Wielen Luuk A. M. , Luyben Karel Ch. A. M. , Van Den Tweel Will J. J.
Published: June 2004

Ion‐exchange processes involving weak electrolytes are influenced strongly by the pH via the occurrence of association/dissociation equilibria. This influence was investigated quantitatively through binary displacement experiments with different electrolytes to gain better insight in the role of the pH and to test a fixed‐bed ion‐exchange model used for the description of multicomponent processes. The two major factors determining the ion‐exchange behavior of weak electrolytes are the actual exchange of counterions and the sorption of neutral species. The sorption of neutral species resulted in concentration fluctuations superimposed on the normal ion‐exchange profiles, whereas pH fluctuations inherent to ion‐exchange processes in general also influenced concentration through modified ionic fractions. The relative importance of these contributions is determined by the magnitude of parameters such as resin capacity, distribution coefficients of neutral species, and selectivities of ions, as well as by operating conditions such as concentrations of all species.
Kinetic study of acrylic acid solution polymerization
Authors: Scott Robert A. , Peppas Nikolaos A.
Published: June 2004

The kinetics of acrylic acid (AA) solution polymerizations were characterized under various polymerization conditions using differential scanning calorimetry. The AA polymerization rate Rp decreased with increasing pH over all pH values due to the decreased reactivity of ionized AA relative to the unionized monomer. The rate behavior above pH 6 was suggestive of diffusion‐controlled termination kinetics. The degree of ionization thus impacted not only the reactivity of species in solution but also the diffusive characteristics of propagating macroradicals. As the AA concentration was increased, the change in polymerization rate with pH was substantially less. The temperature dependence of Rp indicated that the overall activation energy for polymerization varied with temperature, due to diffusion‐controlled termination kinetics. The polymerization rate was significantly higher when a multifunctional cross‐linking agent was added, because of the gel effect, even while maintaining a constant double bond concentration.
Dynamics of semibatch polymerization reactors: II. Pilot‐plant study
Authors: Chylla R. W. , Campbell J. David , Teymour F.
Published: June 2004

A semibatch flow scheduling strategy proposed by Teymour and Ray (1989, 1996) is evaluated for a polymerization reaction conducted in a pilot‐plant reactor. The reaction used is the free radical terpolymerization of styrene, α‐methyl styrene, and acrylic acid monomers initiated by an organic peroxide initiator and carried out in the presence of a reactive glycol ether solvent. This strategy was tested in both single batch and sequential semibatch modes. The process was shown to produce polymer of constant molecular weight properties and composition as inferred from acid number and monomer conversion measurements. This process could be used for obtaining polymer products from a semibatch reactor that are of comparable quality to CSTR products. Results indicate success of this process at meeting this objective; however, practical considerations relating to agitation and temperature control need to be properly addressed to ensure this success.
Phase equilibria for the mean‐force potential of globular protein solutions
Authors: Tavares Frederico W. , Sandler Stanley I.
Published: June 2004

When polymers are added to a solution containing proteins, the protein may deposit as an amorphous precipitate or as crystals. Experimental evidence indicates that from a thermodynamic viewpoint, an amorphous precipitate should be treated as a liquid phase dense in the protein and crystals as true solids. To understand the conditions under which protein precipitation or crystallization may occur, Gibbs‐ensemble Monte Carlo simulation and Gibbs–Duhem integration techniques are used to calculate the liquid–liquid and liquid–solid phase boundaries for protein precipitation induced by polymers. In these simulations, the system is modeled using the pseudo‐one‐component approximation with an appropriate potential of mean force. The critical point for fluid–fluid phase equilibrium disappears as the attractive part of the mean‐force potential becomes very short ranged. The accuracy of the second‐order Barker and Henderson perturbation theory is examined by comparing the phase diagram predicted using this method with the simulation results. Perturbation theory is able to predict the general trends observed in the simulations but not with quantitative accuracy. Perturbation theory is then used to examine a broader range of conditions for protein deposition.
Recovery and purification of paclitaxel using low‐pressure liquid chromatography
Authors: Wu D.‐J. , Ma Z. , Au B. W. , Wang N.‐H. L.
Published: June 2004

An efficient and economical low‐pressure liquid chromatography process has been developed for paclitaxel recovery and purification directly from plant‐tissue culture (PTC) broth. PTC broth is first diluted with ethanol to ensure padlitaxel dissolution and then passed through a column packed with a high‐capacity polystyrene divinyl‐benzene sorbent. A step increase in ethanol concentration in the mobile phase (ethanol:water) is used to concentrate and compress the taxane bands to as high as 29‐fold of influent concentrations (about 1 mg/L). A recycle technique is then used to separate the concentrated paclitaxel band from other taxane bands, achieving 95% purity with more than 90% recovery and 99% purity with more than 80% recovery. In this process, the same low‐pressure columns are used to capture, concentrate and purify paclitaxel. Theoretical predictions agree closely with the stepwise elution and recycle chromatography data. After validation, simulations are used to explore various design and operating alternatives. Analysis of the alternatives shows that the process cost can be further reduced by using higher feed concentration, larger loading volume, smaller particle size, and optimal gradient and recycle strategies.
Gas holdup in slurry bubble columns: Effect of column diameter and slurry concentrations
Authors: Krishna Rajamani , De Swart Jeroen W. A. , Ellenberger Jürg , Martina Gilbert B. , Maretto Cristina
Published: June 2004

To study the influence of particle concentration on the hydrodynamics of bubble‐column slurry reactors operating in the heterogeneous flow regime, experiments were carried out in 0.10, 0.19, and 0.38‐m‐dia. columns using paraffinic oil as the liquid phase and slurry concentrations of up to 36 vol. %. To interpret experimental results a generalization of the “two‐phase” model for gas–solid fluid beds was used to describe bubble hydrodynamics. The two phase identified are: a dilute phase consisting of fast‐rising large bubbles that traverse the column virtually in plug flow and a dense phase that is identified with the liquid phase along with solid particles and entrained small bubbles. The dense phase suffers backmixing considerably. Dynamic gas disengagement was experimented in the heterogeneous flow regime to determine the gas voidage in dilute and dense phases. Experimental data show that increasing the solid concentration decreases the total gas holdup significantly, but the influence on the dilute‐phase gas holdup is small. The dense‐phase gas voidage significantly decreases gas holdup due to enhanced coalescene of small bubbles resulting from introduction of particles. The dense‐phase gas voidage is practically independent of the column diameter. The dilute‐phase gas holdup, on the other hand, decreases with increasing column diameter, and this dependence could be described adequately with a slight modification of the correlation of Krishna and Ellenberger developed for gas–liquid systems.
Mechanistic pressure drop model for columns containing structured packings
Authors: Brunazzin Elisabetta , Paglianti Alessandro
Published: June 2004

A mechanistic model was developed to predict pressure drop and flooding in packed columns equipped with corrugated packing of the regular type. It was developed after considerating the interaction of falling liquid film with the gas phase, based on mass‐and momentum‐conservation equations. Among the most common structured packings, the behavior of the Mellapak and BX types was analyzed. The aim of this work is to demonstrate how mechanistic models, developed for simple geometry, can also be used to compute pressure drops in cases where the geometry is more complex, as with a structured packing. This approach, based on the geometric characteristics of the packing and measurable parameters such as liquid holdup, enables the development of a basic model by limiting the number of adjustable parameters, which are numerous in all the available models. Because of its nature, this model is extremely easy to extend to different types of structured packings.
Multimodel approach to mixed‐gas adsorption equilibria prediction
Authors: Maurer Richard T.
Published: June 2004

A thermodynamically consistent, multimodel approach for the prediction of mixture adsorption equilibria from pure‐component adsorption isotherms is described. The ideal adsorbed solution (IAS) model is employed at low surface coverage, where potential attractive forces tend to govern adsorption behavior. An energetically quasi‐homogeneous version of the Langmuir‐Freundlich (LF) model is proposed at high coverage, where lateral adsorbate interactions and finite‐adsorbent geometric constraints tend to homogenize such attractive forces. The classical LF model is used for intermediate coverage, where both energetic attractive and geometric constraining forces contribute significantly to mixed‐gas adsorption behavior. A major benefit of this three‐regime modeling method is that physically realistic adsorptive selectivities can be adhered to at both extremes of coverage, a property not generally shared by either the IAS or LF models taken singly. This three‐regime approach yields better multicomponent equilibria predictions than IAS‐only modeling approaches for many relatively nonideal adsorption systems.
Application of a control‐law nonlinearity measure to the chemical reactor analysis
Authors: Stack Alexander J. , Doyle Francis J.
Published: June 2004

The optimal control structure has been introduced as a design tool to measure the control‐law nonlinearity of a given process design. In this context, control‐law nonlinearity is the optimal degree of nonlinear compensation in the controller, a system property distinct from open‐loop nonlinearity and determined by a performance objective and the region of operation as well as the nature of the open‐loop system. This approach is extended to the analysis of multivariable systems with output feedback through the application of an extended Kalman filter. Coherence estimation is used as a practical method to measure continuous, open‐loop multivariable system nonlinearity. The CSTR with van de Vusse kinetics, a system that features output feedback and a controllaw nonlinearity that changes with operating points, is analyzed. The optimal control structure approach with coherence estimation correctly indicates changes in the controllaw linearity between different operating points and changes as the regions of operation change around a particular operating point.
Rate parameters and gradient correlations for gradient‐elution chromatography
Authors: Luo Robert G. , Hsu James T.
Published: June 2004

Rate parameters and gradient correlations of proteins are very important to engineering investigations of gradient‐elution chromatography. Before carrying out computer simulations for gradient process, these rate parameters and gradient correlations must be determined from experimental data. This work presents a systematic method for estimating and determining these parameters and correlations using experimental data and computer‐simulation results based on isocratic runs. The values determined were then employed in gradient‐elution studies by putting them into the rate model to perform computer simulations. Experimental runs under gradient conditions using the parameters and correlations just referred to were carried out in a DEAE Sepharose CL‐6B ion‐exchange chromatography. Comparisons between the experimental data and the simulation results show good agreement.
Intraparticle diffusion in hydrogenation of 3‐hydroxypropanal
Authors: Zhu X. , Hofmann H.
Published: June 2004

Strong intraparticle diffusion resistance was observed and the apparent effectiveness factors were measured for the hydrogenation of 3‐hydroxypropanal (HPA) to 1,3‐propanediol (PD) over Ni/SiO2/Al2O3 catalyst pellets at 45 to 80°C and 2.60 to 5.15 MPa in a stirred reactor with a spinning basket. A mathematical model was proposed to describe the intraparticle diffusion and to estimate the effective diffusion coefficients of HPA, PD and hydrogen under reaction conditions by a maximum likelihood function of the apparent effectiveness factors of HPA and PD. The calculated concentration distributions of HPA, PD and hydrogen in catalyst particles reveal an excess of hydrogen at 80°C and higher temperatures, but an excess of HPA at 45°C. It results in a weak dependence of the consumption formation rate of HPA and PD on the partial pressure of hydrogen at high temperature.
Mutual solubilities of hydrocarbons and water: III. 1‐hexene; 1‐octene; C10C12 hydrocarbons
Authors: Economou I. G. , Heidman J. L. , Tsonopoulos C. , Wilson G. M.
Published: June 2004

This article completes the analysis of high‐temperature mutual solubilities of hydrocarbons and water. Part I (Tsonopoulos and Wilson, 1983) presented benzene, cyclohexane, and n‐hexane, while Part II (Heidman et al., 1985) extended the investigation to ethylbenzene, ethylcyclohexane, and n‐octane. Part III presents experimental data up to the three‐phase critical end point for C6, C8, and C10 1‐alkenes, n‐decane, n‐butylcyclohexane, m‐diethylbenzene, p‐diisopropylbenzene, cis‐decalin, tetralin, 1‐methylnaphthalene, and 1‐ethylnaphthalene. The thermodynamic analysis of Henry's constants for hydrocarbons in water is extended to the critical temperature of water, while the solubility and volatility of water in hydrocarbons are successfully correlated with several modifications of the Redlich‐Kwong equation of state.
Wall slip in the capillary flow of molten polymers subject to viscous heating
Authors: Rosenbaum Eugene E. , Hatzikiriakos Savvas G.
Published: June 2004

The traditional way of determining the slip velocity of molten polymers is the classic Mooney technique, which utilizes experimental data obtained from a capillary rheometer. However, measurements of the rheological properties of polymer melts in capillary flow at high shear rates are often complicated by viscous heating, which is not taken into account by this method. A data analysis procedure based on a mathematical model for nonisothermal capillary flow of molten polymers is developed. Conduction, convection, and viscous heating are included, together with the effect of wall slip. The technique provides detailed velocity and temperature fields in the die, and can be used to determine the slip velocity at high shear rates corrected for the effect of viscous heating. It is tested for the capillary flow of several polymers, including polystyrene, polypropylene, high‐density, and linear low‐density polyethylenes.