bluuebunny/crossref_metadata_2025 huggingface dataset:
Diagnostic nonlinear analysis of Fischer‐Tropsch synthesis in stirred‐tank slurry reactors
Authors: Song Hyun‐Seob , Ramkrishna Doraiswami , Trinh Sinh , Wright Harold
Published: April 2004

Multiplicity of steady states was experimentally observed, somewhat sporadically, for Fischer–Tropsch (FT) synthesis in stirred tank slurry reactors in the reactor originally at the wax‐producing (normal) steady state suddenly jumping to a high‐temperature (methane‐producing) steady state for some time and eventually returning to the normal steady state. A diagnostic nonlinear analysis showed a plausible interpretation of this nonlinear behavior under two settings: 1. assuming that the controlled system is stable and motivated by simplicity, as it obviates the inclusion of an energy balance for the cooler, thus reducing the dimension of the problem; 2. examining the more realistic problem by discarding the foregoing assumption, thus having to increase the analysis level. The first approach showed that the Stanton number for heat transfer (StH) and the Damköhler number (Da) could be two key process parameters accounting for the observed multiplicity characteristics of FT synthesis. The decrease in StH, attributable to deteriorating performance of the cooler, is the likely cause for the sudden jump of the reactor from the normal to high‐temperature steady state. The eventual recovery of the original steady state is, spurred by a decrease in Da, possibly attributable to catalyst deactivation. Depending on the initial conditions and/or startup history from the initial to the target conditions, the multiplicity behaviors of FT synthesis may or may not be observed, which is why the observation of multiplicity is sporadic. The second approach incorporating the cooler's energy balance increases the region of multiplicity over that with the first. Simple proportional control law is applied to the system to examine the effect of the control parameter settings on reactor behavior. Except for this feature, the findings from the two analysis modes are in harmony.
Kinetics of microwave‐assisted oxidative degradation of polystyrene in solution
Authors: Sivalingam G. , Agarwal Nitin , Madras Giridhar
Published: April 2004

The kinetics of the oxidative degradation of polystyrene in dichlorobenzene in the presence of benzoyl peroxide was studied in a microwave reactor. Solutions with peroxide concentration ranging from 15 to 40 kg/m3 were irradiated by microwave of 700 W and frequency of 2.45 MHz with a different cycle heating period ranging from 20 to 45 s. The molecular‐weight distributions were measured as a function of reaction time by gel permeation chromatography. Because the temperature of the system continuously varies with reaction time, a radical mechanism for oxidative degradation based on continuous distribution kinetics with time/temperature‐dependent rate coefficients was proposed. The activation energy of 46.4 kJ/mol for the microwave‐assisted oxidative degradation of polystyrene was obtained by nonlinearly regressing the experimental data.
Spanning the flow regimes: Generic fluidized‐bed reactor model
Authors: Abba I. A. , Grace J. R. , Bi H. T. , Thompson M. L.
Published: April 2004

Probabilistic averaging is used to model fluidized‐bed reactors across the three fluidlization flow regimes most commonly encountered in industry (bubbling, turbulent, and fast fluidization), extending earlier work, which introduced this approach to bridge the bubbling and turbulent regimes of fluidization. In extending this concept to the fast fluidization regime, the probabilities of being in each of the three regimes are represented as probability density functions derived from regime boundary transition data. The three regime‐specific models—a generalized version of a two‐phase bubbling bed model at low gas velocities, a dispersed flow model for turbulent beds at intermediate velocities, and a generalized version of a core‐annulus model at higher velocities—are employed, leading to improved predictions compared with any of the individual models, while avoiding discontinuities at the regime boundaries. Predictions from the new integrated model are in good agreement with available ozone decomposition data over the full range of applicability covered elsewhere.
Reconstruction of the O2 uptake rate and CO2 evolution rate on a time scale of seconds
Authors: Bloemen H. H. J. , Wu L. , van Gulik W. M. , Heijnen J. J. , Verhaegen M. H. G.
Published: April 2004

The reconstruction is addressed of the dynamics of the microbial oxygen uptake rate (OUR) and carbon dioxide evolution rate (CER) from off‐gas concentration measurements and dissolved oxygen measurements during highly dynamic conditions, encountered, for example, during a substrate‐pulse experiment. The reconstruction of the OUR and CER is done by fitting a model for the mass transport to the measured data using smoothing techniques. Most of the parameters of the mass‐transport model, including the sensor dynamics, are derived from physical knowledge. A few remaining parameters that cannot be derived from physical knowledge are computed by fitting the model to an identification data set. It is demonstrated that the net gas production or consumption can have a significant impact on the estimated OUR, and, therefore, is included in the model. Besides the reconstruction of the OUR and the CER, the reconstruction algorithm also incorporates the monitoring of the mass‐transfer coefficient kla.
Microwaves in soil remediation from VOCs. 1: Heat and mass transfer aspects
Authors: Acierno Domenico , Barba Anna A. , d'Amore Matteo
Published: April 2004

A novel technique presented performs in situ remediation of soils contaminated by volatile organic compounds (VOCs) based on the use of electromagnetic fields for heating operations. Attention is focused on heat and mass transfer occurring in porous‐moisturized media like a soil matrix. The microwave induced steam distillation process is investigated. Measurements are reported of temperature, humidity, residual contaminant concentration, and permittivity during the process. The crucial role is elucidated that the changes of the dielectrical properties of the soil matrix play on the electromagnetic field propagation. A mathematical model of the remediation process relating all the parameters above is proposed and validated.
Efficient simulation of time‐dependent flows: Application to a twin screw extruder
Authors: Raut Janhavi S. , Naik Vijay M. , Jongen Thibauld R.
Published: April 2004

An efficient, versatile technique was developed for detailed simulation of flows in complex processing equipment with multiple moving parts. It uses a suitably modified finite‐volume CFD simulation algorithm to deal with the problems posed by the relative motion of moving parts in processing equipment. A fixed grid is used to discretize the mass and momentum transport equations for the contents of the entire equipment volume. The motion of the solid moving parts within this domain is accounted for by assigning velocities to nodes of the grid representing the location of these parts at any moment of time. The technique was first validated for a single screw extruder by comparing the results predicted by conventional techniques involving body fitted mesh and a rotating frame of reference. Because of its inherent nonbody fitted mesh characteristics, the fixed grid method resulted in a rough, time‐dependent boundary for the moving parts. The extent of error arising from this artifact was also evaluated. The benefits of this technique were demonstrated by simulating complex three‐dimensional (3‐D) flow in an industrially important processing equipment with multiple moving parts—an intermeshed counterrotating twin screw extruder. The time required for geometry generation and meshing using this approach was very small, and the relevant geometric parameters including those associated with the moving parts can be varied by simply modifying values in the analytical expressions used to define them. This technique does not require recreating or remeshing the entire geometry, thus making it easy to optimize the design of complex equipment via quick parametric studies.
Electrochemical measurement of velocity gradient at the wall of a helical tube
Authors: Galier Sylvain , Issanchou Sébastien , Moulin Philippe , Clifton Michael J. , Aptel Philippe
Published: April 2004

An electrochemical technique was used for measuring the wall velocity gradient in two coiled pipes: a torus and a helix. An electrochemical oxidation reaction was employed with its rate limited by mass transfer. Measurement of the diffusion‐limited current at a microelectrode embedded flush with the wall allows the wall shear rate to be determined. The flow velocity was varied through both the laminar and turbulent regimes. Measurements were made for different electrode positions on the pipe circumference and, as expected, the highest shear rate was found at the outside of the curve, with the lowest value at the inside. In the case of the helical pipe, maxima and minima in shear rate were shifted slightly away from the positions observed for the torus.
Effect of density homogeneity on the dynamic response of powder beds
Authors: Yanagida T. , Matchett A. J. , Coulthard J. M. , Asmar B. N. , Langston P. A. , Walters J. K.
Published: April 2004

Homogeneous and inhomogeneous powder beds subjected to low‐magnitude vibration are compared in terms of the dynamic response. The inhomogeneous samples were segregated into two phases: loose and dense phases, layering the two phases horizontally or vertically. An apparent mass, defined as a ratio of the base force to base acceleration, was measured. Comparison of homogeneous and segregated data demonstrated a significant density gradient dependence on the apparent mass data. First, homogeneous systems showed a resonant peak, which gave the longitudinal elastic modulus of the bed via the velocity of longitudinal stress wave propagation. Second, vertically segregated systems exhibited two significant peaks at low frequencies, corresponding to the resonance of each phase. In addition, the apparent mass values at the two peaks were related to the quantity of each phase. Third, horizontally segregated systems exhibited a resonant peak, whose frequency was approximately equal to homogeneous data, but the apparent mass value at the peak differed from homogeneous data. A model based on the fourth‐power scaling law, two‐phase theory and Rayleigh's energy method gave an interpretation for the insensitivity of the peak frequency to the density gradient in the vertical direction.
Predicting adsorption of water/organic mixtures using molecular simulation
Authors: Jorge Miguel , Seaton Nigel A.
Published: April 2004

The use of Monte Carlo simulation to predict the adsorption of mixtures of polar and nonpolar species on activated carbon was investigated using water and ethane on BPL carbon as a prototype system. The structure of the adsorbent was modeled by an array of slit‐shaped pores, characterized by a pore‐size distribution. The chemical heterogeneity of the carbon was taken into account by including oxygen‐containing sites on the surface of the pores. The pore‐size distribution was obtained from pure‐ethane adsorption on the same carbon sample, while the concentration and distribution of surface sites were determined by analyzing pure‐water adsorption. Model predictions agree well with experimental multicomponent data.
Parameter estimation for industrial polymerization processes
Authors: Bindlish Rahul , Rawlings James B. , Young Robert E.
Published: April 2004

The parameter estimation problem and its application to industrial data obtained from polymerization processes are analyzed. The polymerization process model consists of the material balances with the polymerization kinetics for the reactor and a simplified dynamic model for the downstream separator. The estimable kinetic parameters in the process model are obtained for plant data by analyzing the associated optimization problem. Two kinetic parameters are estimable for the copolymer data and the homopolymer data based on the information in the available industrial plant data. The identified process model forms the basis for a monitoring and feedback control system for these processes.
Coordinating feedback and switching for control of hybrid nonlinear processes
Authors: El‐Farra Nael H. , Christofides Panagiotis D.
Published: April 2004

A robust hybrid control strategy for a broad class of hybrid nonlinear processes with actuator constraints and uncertain dynamics is proposed. These variable‐structure processes comprise a finite family of constrained uncertain continuous nonlinear dynamical subsystems, together with discrete events that trigger the transition between the continuous subsystems. The proposed control strategy is predicated on the idea of coordinating the hierarchical tasks of lower‐level feedback‐controller synthesis and upper‐level switching logic design. Using multiple Lyapunov functions, a family of bounded robust, nonlinear feedback controllers are initially designed to robustly stabilize the constituent modes of the hybrid process, subject to uncertainty and constraints. The region of guaranteed closed‐loop stability is then explicitly characterized for each mode in terms of the magnitude of actuator constraints and the size of the uncertainty. A set of stabilizing switching laws that track the energy evolution of the constituent modes are then derived to orchestrate safe transitions between the stability regions of the constituent modes and their respective controllers, in a way that respects actuator constraints and guarantees robust stability of the overall uncertain hybrid closed‐loop system. This hybrid control method is applied through computer simulations to robustly stabilize an exothermic chemical reactor with switched dynamics, model uncertainty, and actuator constraints at an unstable steady‐state and to design a fault‐tolerant control system for chemical reactors through switching between multiple constrained control configurations.
Feedback control of growth rate and surface roughness in thin film growth
Authors: Lou Yiming , Christofides Panagiotis D.
Published: April 2004

A method for multivariable feedback control of surface roughness and growth rate in thin film growth using kinetic Monte‐Carlo (MC) models is proposed. The method is applied to the process of thin film growth in a stagnation flow geometry including atom adsorption, desorption, and surface migration as the three processes that shape film microstructure and determine film growth rate. A multiscale model involving coupled partial differential equations to model the gas phase and a kinetic MC simulator, based on a large lattice, for the modeling of film growth, is used to simulate the process. A roughness and growth rate estimator constructed allowed computing estimates of the surface roughness and growth rate at a time‐scale comparable to the real‐time evolution of the process. The estimator involves kinetic MC simulators based on small lattice models, adaptive filters used to reduce stochastic fluctuations of the outputs of the small lattice MC simulator outputs, and measurement error compensators used to reduce the errors between the estimates and measurements. The interactions between the inputs (substrate temperature and inlet precursor mole fraction) and outputs (growth rate and surface roughness) in the closed‐loop system are studied and found to be significant. A multivariable feedback controller, which uses the state estimator and explicitly compensates for the effect of input/output interactions, is designed to simultaneously regulate the growth rate and surface roughness by manipulating substrate temperature and inlet precursor mole fraction. Application of the estimator/controller structure to the multiscale process model demonstrates successful regulation of the surface roughness and growth rate to the desired set point values. This approach is shown to be superior to control of the growth rate and surface roughness using two independent feedback control loops.
General hybrid multizonal/CFD approach for bioreactor modeling
Authors: Bezzo F. , Macchietto S. , Pantelides C. C.
Published: April 2004

A critical issue in the modeling of aerobic bioreactors is the close interaction between fluid flow and the biological reactions. In particular, shear rate has a large effect on the broth viscosity which, in turn, affects the rate of mass transfer of oxygen from the gas to the liquid phase. We demonstrate how a generic hybrid multizonal/computational fluid dynamics (CFD) modeling approach can be applied to take account of these interactions. The approach to multizonal modeling presented characterizes the flow rates between adjacent zones, and also the fluid mechanical quantities, such as the shear stress, that have important effects on the process behavior within each zone, by means of steady‐state CFD calculations. An unstructured model for xanthan gum production in a batch aerobic bioreactor is used for this purpose. The hybrid modeling approach is also applied to structured models involving distributions of cell mass within each zone.
Effect of active species mobility on soot‐combustion over Cs‐V catalysts
Authors: Fino Debora , Russo Nunzio , Badini Claudio , Saracco Guido , Specchia Vito
Published: April 2004

The soot combustion process promoted by some of the most promising diesel particulate combustion catalysts (Cs2O·V2O5, Cs4V2O7, CsVO3) is discussed on the grounds of a theoretical analysis and an experimental test campaign involving reaction runs in microreactors (either in isothermal or in temperature‐programmed conditions) and in a DSC apparatus. Such investigations, beyond the prevalent kinetic parameters (activation energy, oxygen reaction order, preexponential kinetic constant), led to a definition of the role of catalyst‐to‐carbon contact conditions on the conversion rate. Particularly, the role of formation of eutectic liquid phases, which dominates the catalytic behavior of the Cs2O·V2O5 catalyst, was fully elucidated on the grounds of either DSC runs or visual inspection through a heated optical microscope.
Electrohydrodynamic mixing in microchannels
Authors: Tsouris C. , Culbertson C. T. , DePaoli D. W. , Jacobson S. C. , de Almeida V. F. , Ramsey J. M.
Published: April 2004

Electrohydrodynamic flow generated by the motion of charge carriers in an electric field was investigated for rapid mixing in microchannel devices. A “tee” geometry of channels ranging from 40 to 110 μm in width and 30 to 60 μm in depth was used to contact two miscible fluids, such as alcohols. A fluorescent dye (Rhodamine B) was used to quantify mixing. A potential difference ranging from 0 to 900 V was applied between two electrodes separated by a distance of 450 μm, resulting in field strengths ranging from 0 to 2 × 106 V/m. Results show that when no electric field is applied, mixing of the two streams is driven by diffusion and is, therefore, slow. The mixing length, at which the concentration across the channel becomes essentially uniform, decrease significantly as the applied voltage was increased. There seems to be a threshold of applied voltage below which mixing is not enhanced by the electric field under the conditions studied. In strong electric fields, the mixing length was less than 150 μm, while in the absence of an electric field, the mixing length was greater than 5,000 μm.
Aqueous speciation of arsenic in sulfuric acid and cupric sulfate solutions
Authors: Casas J. M. , Etchart J. P. , Cifuentes L.
Published: April 2004

Physicochemical data for arsenic speciation calculations, and the development and application of an equilibrium model were determined numerically and experimentally to simulate the speciation of arsenic and copper in aqueous sulfuric acid solutions. Results show that concentrations of dissolved species depended highly on temperature, acidity, and solution concentration. A range of species can be formed, depending on pH and temperature. In acidic solutions, the principal ones are: HSO4−, H+, Cu2+, CuSO4(aq), H3AsO3(aq), and H3AsO4(aq); arsenic ions are present in low concentrations. Association degrees increase with temperature and concentration, the solutes being partially dissociated. Calculations of metal solubilities, pH, and ionic conductivity showed good agreement with experimental data. The model can be applied to study the aqueous speciation in industrial copper hydrometallurgical solutions, as well as to define and to evaluate purification strategies for the treatment of industrial effluents.
Dynamics of isoelectric precipitation of casein using sulfuric acid
Authors: Hofland G. W. , Berkhoff M. R. , Witkamp G. J. , van der Wielen L. A. M.
Published: April 2004

The acid precipitation of casein from skim milk is an interesting and complex operation, as it involves several mechanistic steps occurring simultaneously. Research has focused so far on the influence of static variables such as temperature and final pH on the curd properties. Here the dynamics of the process was investigated. From a characteristic times analysis it was concluded that the most important mechanistic steps in the acid precipitation of casein were acid mixing, aggregation/breakup, and transport in/out of the precipitate particles. Experiments in a fed batch setup using sulfuric acid as the precipitant showed how the precipitation influenced the pH profile and demonstrated the large influence of process variables, such as the acid addition rate, the mixing intensity, and aging time on particle‐size distributions and the release of minerals from the casein.
Chemical processing and micromixing in confined impinging jets
Authors: Johnson Brian K. , Prud'homme Robert K.
Published: April 2004

Rapid processes such as certain organic reactions or precipitations at high supersaturation require the rapid mixing provided by jet mixers. Micromixing in a confined impinging jets (CIJ) mixer was characterized employing the Damköhler number to correlate processing. A scaling theory for the characteristic micromixing time, proportional to momentum diffusion starting at the Kolmogorov microscale, is shown as sufficient to express the micromixing performance of the CIJ mixer. A recently characterized second‐order competitive reaction set is used as a “chemical ruler” to assign an absolute value to the mixing time in the CIJ mixer. A wide range of characteristic time (320 to 5 ms) is evaluated with hydrochloric acid competing for sodium hydroxide neutralization or 2,2‐dimethoxypropane acid catalyzed hydrolysis. This reaction set was sensitive enough to detect the onset of a turbulent‐like flow at a Re of 90 and was able to demonstrate a decrease in undesired products up to the highest Re tested, 3,800 or a jet velocity of 19 m/s. It represents a significant advancement to the reaction sets and techniques used for previous mixing studies, which are reviewed. Experiments verify the characteristic mixing time in a CIJ mixer scales as the inverse of the jet velocity to the three halves power, and the “mesomixing volume” (the volume over which the majority of flow energy was dissipated) is best approximated as proportional to the internozzle separation cubed. For each of the different jet diameters, chamber diameters and outlet configurations tested, the selectivity of the reaction scaled linearly with the Damköhler number, as determined from the known reaction kinetics and the calculated Kolmogorov diffusion time. The first full characterization is provided of micromixing in impinging jets that allows the prediction of mixing performance, reaction selectivity, and scale‐up criteria.
Thermophoresis of an aerosol sphere parallel to one or two plane walls
Authors: Keh Huan J. , Chen Po Y.
Published: April 2004

The steady thermophoretic motion of a spherical particle in a gaseous medium located in an arbitrary position between two infinite parallel‐plane walls is studied theoretically in the absence of fluid inertia and thermal convection. The Knudsen number is assumed small to describe the fluid flow by a continuum model with a temperature jump, thermal slip, and frictional slip at the particle surface. The imposed temperature gradient is constant and parallel to the two plane walls, which may be insulated or prescribed with the far‐field temperature distribution. The presence of neighboring walls causes two basic effects on the particle velocity: the local temperature gradient on the particle surface is enhanced or reduced by the walls, to speed up or slow down the particle; the walls increase viscous retardation of the moving particle. A boundary collocation method is used to solve the thermal and hydrodynamic governing equations of the system. Numerical results for the thermophoretic velocity of the particle relative to that under identical conditions in an unbounded gaseous medium are presented for various relative thermal conductivity and surface properties of the particle, as well as relative separation distances between the particle and two plates. For thermophoretic motions of a spherical particle parallel to a single plate and on the central plane of a slit, the collocation results agree well with the approximate analytical solutions obtained by a method of reflections. The presence of lateral walls can reduce or enhance the particle velocity, depending on the relative thermal conductivity and surface properties of the particle, the relative particle‐wall separation distances, and the thermal boundary condition at the walls. In general, the boundary effect on thermophoresis is complicated and weaker than that on sedimentation.
Slip velocity and drag law in a liquid‐liquid homogeneous dispersed flow
Authors: Augier Frédéric , Masbernat Olivier , Guiraud Pascal
Published: April 2004

Local measurements of phase velocities and phase fraction in a dense liquid‐liquid homogeneous flow were done based on an application of PIV with a refractive index matching technique. This technique allowed to measure simultaneously both phase velocities and phase fraction fields, as well as the drop mean diameter in a vertical cocurrent flow of n‐heptane dispersed in an aqueous solution of glycerin. The evolution of the relative velocity as a function of the phase fraction (0–0.4) was studied with moderate particle Reynolds numbers (10–100) for drops behaving as spherical rigid particles. A local drag coefficient was derived from these measurements. The calculated mean relative velocity as a function of the local phase fraction with existing classical drag coefficient laws showed significant discrepancies with the measurements overestimated. Thus, the drag coefficient variation rate as a function of the phase fraction was underestimated by the models. The evolution of the normalized velocity (by the terminal velocity) as a function of the phase fraction was predicted correctly with mixture viscosity models in a creeping flow regime.
Hydrodynamic similarity of solids motion and mixing in bubbling fluidized beds
Authors: Sanderson John , Rhodes Martin J.
Published: April 2004

Solids motion and solids mixing behavior are important in many fluidized‐bed applications, and yet, in the evaluation of hydrodynamic similarity criteria, relatively few studies have considered these phenomena directly. This work reports on a series of physical experiments involving solids motion and solids mixing undertaken in a number of freely bubbling fluidized beds scaled using the “simplified” scaling criteria. In the first set of experiments, the motion of large neutrally buoyant particles in bubbling beds of a finer material was compared using a simple float‐tracer experiment in beds that were 300 mm, 600 mm, and 1,560 mm in dia. The distribution of circulation times for the float‐tracers was found to scale well across the majority of gas velocities considered when the scaling law was followed, but discrepancies were noted when either the bulk bed material or large tracer particle sizes were mismatched. These results demonstrate that the circulation of large neutrally buoyant particles within the correctly scaled bubbling beds is indeed a “scaleable” phenomenon across a wide range of bed sizes, and this has useful consequences for fluidized‐bed applications involving floating bodies. In the second set of experiments, solids downflow velocities were compared in scaled beds that were 146 mm and 300 mm in dia. using a novel application of electrical–capacitance tomography (ECT). The measured solids downflow velocities showed qualitative agreement with the scaling law and correlations in the literature. The novel tracer technique was only moderately successful in this application, but is worthy of further investigation, as the approach may serve to enhance the capabilities of existing ECT equipment.
Beyond log‐normal distributions: Hermite spectra for solving population balances
Authors: Hamilton Robert A. , Curtis Jennifer S. , Ramkrishna Doraiswami
Published: April 2004

The oft‐used log‐normal distribution for solving many population‐balance problems is in fact a degenerate case of a Hermite function expansion of the solution in log particle‐size coordinate. Corrective capabilities of such an expansion constitute vast improvements not only over those using the log‐normal distribution, but also over discretization methods in that moments other than those designed for in the latter are predicted with much higher accuracy, although at greater computational cost. The Hermite spectral method is compared with known analytical results and other computational techniques for particle dynamic processes involving agglomeration, breakage, and growth. This method is extremely accurate and flexible, as evidenced by the wide range of problems it can solve, both transient and steady state, along with perfectly mixed or convection dominant problems. Particle distributions are allowed to evolve according to the physics of the process, not constrained by restrictive assumptions inherent in the prespecified form of the distribution.
Distillation in hollow fibers
Authors: Zhang Guoliang , Cussler E. L.
Published: April 2004

Distillation in hollow fibers can give better, more productive separations than distillation in random or structured packing. Since the hollow fibers used have little resistance to mass transfer, the selectivity of the separation depends on the relative volatility, just as in conventional distillation. Because the fibers are nonporous, the distillation can operate at high flows which normally cause flooding, and at low flows which normally compromise loading. As a result, the turndown ratio is nearly infinite and the height of a transfer unit can be as small as desired. Experiments do give values consistent with correlations developed for other membrane modules, although the practical value of hollow fiber distillation is uncertain because of the materials limitations.
Liquid‐phase mass transfer in spray contactors
Authors: Yeh Norman K. , Rochelle Gary T.
Published: April 2004

The liquid‐phase mass transfer in sprays has been measured with carbon dioxide desorption by collecting and analyzing samples of the spray. Experiments were conducted with laboratory‐ (0.009 to 0.13 L/s) and pilot‐scale (6.4 to 12.8 L/s) centrifugal hollow‐cone spray nozzles at pressure drops from 34 kPa to 138 kPa. Significant mass transfer occurred during sample collection, and a quench sampling method was developed to minimize this effect. The number of liquid‐phase transfer units (NL) due to spray impact onto walls and liquid pools was often as much as the spray NL. Approximately 60% of the spray NL occurs in the liquid sheet before droplet formation, and the droplet region can account for less than half of the total NL of the spray.
Effect of the size and amount of ZrO2 addition on properties of SrCo0.4Fe0.6O3−δ
Authors: Yang Li , Tan Liang , Gu Xuehong , Qi Hong , Jin Wanqin , Zhang Lixiong , Xu Nanping
Published: April 2004

The effects of ZrO2 addition with various particle sizes (1, 3, and 56 μm) and various amounts (0, 1, 3, 5, 7, 9 wt. %) on structure, oxygen permeation, and stability of SrCo0.4Fe0.6O3−δ (SCF) were investigated. XRD and EDX analysis revealed that the dissolution of Zr cation into the B site of SCF phase occurred with the addition of ZrO2, resulting in a lattice expansion of SCF at elevated temperatures. The dissolution amount of Zr in the SCF phase increased with a decrease of ZrO2 particle size and an increase in the amount of ZrO2 added, and, therefore, resulted in a decrease of oxygen permeation flux of the membranes and an increase in their structural stability in a helium atmosphere. This study indicated that adding a certain amount of Zr cation in SCF oxide was an effective route to sustain the structural stability of SCF in low oxygen partial pressure, and the optimum ZrO2 addition was 3 wt.% with a particle size of 1 μm, which greatly improved the structural stability without significantly changing the oxygen permeability of SCF.
Hierarchical multiobjective strategy for particle‐size distribution control
Authors: Immanuel Charles David , Doyle Francis Joseph
Published: April 2004

A multiobjective optimization formulation is presented to track a desired particle‐size distribution (PSD) trajectory in semi‐batch emulsion polymerization. This is done in a hierarchical framework, controlling the individual processes of nucleation, growth, and coagulation that determine the PSD. An evolutionary technique, the Nondominated Sorting Genetic Algorithm, is employed as the optimization tool. Experimental implementation of these open‐loop optimal recipes yield promising results, but simultaneously highlight the importance of feedback corrective action. Preliminary state/parameter estimation studies are performed, which show an ability to estimate the full distribution and related states from the available measurements (relative distribution, latex density, and feed rates), and also elucidate the requirements in the strategy to be employed for estimation. The potential for on‐line feedback is examined using experimental data, in view of the irreversibilities and process limitations.
Screening for profitable retrofit options of chemical processes: A new method
Authors: Uerdingen Eric , Fischer Ulrich , Hungerbühler Konrad , Gani Rafiqul
Published: April 2004

A new systematic methodology for screening retrofit options to improve the economics of a continuous chemical process is organized in three stages: 1. base case analysis; 2. generation of retrofit options; and 3. rough economic evaluation of the retrofit options. In stage 1, the flowsheet of a process, based on the steady‐state mass and energy balances of a representative process operating state (base case), is decomposed into open and cycle component path flows with their corresponding flow rates. In the path flow assessment, variable process costs are then allocated to these component flows through two economic indicators representing raw material consumption, and energy and waste cost. Additional physico‐chemical path flow indicators characterizing the degree of nonideality of the process are also calculated. In stage 2, the results of the path flow assessment are used to identify possible retrofit options with the help of recommended retrofit actions. In stage 3, the impact of retrofit options on variable costs of the base case is estimated and discussed in terms of its technical feasibility. This method has been applied to the hydro‐dealkylation process for the toluene production. Several promising retrofit options were identified through this method. The results demonstrate how the new method supports the systematic identification of economically beneficial retrofit options for chemical processes.
Simple and safe method for determining explosion limits at elevated pressures
Authors: Chen Jenq‐Renn , Liu Kensin
Published: April 2004

A simple and safe method for determining the explosion limits at elevated pressures and temperatures is developed by using a small explosion test cell with a pressure balancing and containment design. Tests at elevated pressure are achieved by balancing the test‐cell pressure and the containment‐vessel pressure. If the vapor in the test cell is flammable and ignites, the overpressure will rupture the test cell but remain confined safely by the containment vessel. The method successfully achieved explosion‐limit measurement at pressures up to 5.5 MPa with all the equipment components rated at a pressure of only 13.9 MPa. Results are presented for hydrogen and methane at ambient condition and elevated pressures, and shown to be in good agreement with the data in the literature. Preliminary results on the upper explosion limits of cyclohexane at cyclohexane oxidation conditions are also reported. The method will greatly benefit the study of explosion limits at elevated pressures and the safety of hydrocarbon/air oxidation processes.
High shear skim milk ultrafiltration using rotating disk filtration systems
Authors: Ding Lu‐Hui , Akoum Omar , Abraham Antoine , Jaffrin Michel Y.
Published: April 2004

The ultrafiltration (UF) of UHT (sterilized at ultrahigh temperature) skim milk with two prototypes rotating disk systems of 15.5 and 26 cm i.d. using the same 50 kDa PES membrane is investigated. The larger stainless steel unit had a maximum rotation speed of 1,500 rpm. The smaller nylon unit had a maximum speed of 3,000 rpm. Disks equipped with radial vanes of various heights were used in the tests. The large module yielded a stabilized permeate flux of 276 L h−1m−2 at 45°C and a transmembrane pressure (TMP) of 1,000 kPa when fitted with a 2‐mm vanes disk rotating at 1,500 rpm, confirming the reduction of concentration polarization by high shear rate. The mean shear rate on membrane γm was shown to be proportional to (kω)1.8, where ω was the disk angular velocity and k was a coefficient depending upon vanes height. Permeate fluxes observed with the two modules at various speeds and with different disks were well correlated by a single function of mean shear rate, given in Lh−1m−2 by J = 0.301 γm0.552. In concentration tests with a 6‐mm vanes disk, the permeate flux remained constant until a volume reduction ratio (VRR) of 3, then decayed to a theoretical VRR maximum of 9.2 at zero flux. It can thus be concluded that rotating disk filtration devices are very efficient in UF as high shear reduces concentration polarization. The specific power consumed per m3 of permeate was found to be less for the smooth disk than for a disk with vanes.
Single drop impaction on a solid surface
Authors: Park Heungsup , Carr Wallace W. , Zhu Junyong , Morris Jeffrey F.
Published: April 2004

An experimental and theoretical study is presented on spreading and retracting of a single drop impacting on a smooth surface at room temperature. The experimental study showed the influence of kinetic energy, liquid‐solid interaction, and energy dissipation on the impact process. The results are reported for Reynolds number from 180 to 5,513, Weber number from 0.2 to 176, four different liquids (distilled water, n‐Octane, n‐Tetradecane, and n‐Hexadecane), and four different surfaces (slide glass, uncoated silicon wafer, HMDS coated silicon wafer, and Teflon film). A theoretical model based on an energy balance was developed to predict the maximum spreading ratio at low impact velocity. The key novel feature of this model is that the shape of the drop is assumed to be a spherical cap during the spreading process. When compared to models in the literature, the present model not only gives better predictions for low drop impact velocities, but also in most cases gives predictions that are within 10% of the experimental data at high impact velocities.
Nonequilibrium capillary pressure and relative permeability curves of porous media
Authors: Tsakiroglou C. D. , Theodoropoulou M. A. , Karoutsos V.
Published: April 2004

History matching of unsteady immiscible displacement experiments performed on model porous media reveals that nonequilibrium capillary pressure and relative permeability functions are sensitive to the dominating flow pattern. As the capillary number increases, the network displacement pattern changes gradually from invasion percolation cluster to stable displacement. For primary drainage, the capillary pressure and relative permeability curves are increasing functions of the capillary number. The dependence of the relevant macroscopic parameters on the capillary number is consistent with power laws derived from scaling arguments of gradient percolation theory, which describes the growth of the displacement front. The material coefficient of the thermodynamic theory of capillary pressure changes mildly with fluid saturation and is a decreasing function of the capillary number.
Modeling adsorption of cane‐sugar solution colorant in packed‐bed ion exchangers
Authors: Broadhurst Hugh A. , Rein Peter W.
Published: April 2004

The removal of cane‐sugar solution colorant by packed‐bed ion exchangers was modeled using a linear driving‐force adsorption model. Adsorption of colorant is a complex subject, since color is an indiscrete mixture of many components, making it difficult to measure and even more challenging to model. Three resins were investigated: a strong‐acid cation (SAC), a weak‐base anion (WBA), and a standard sugar industry strong‐base anion decolorizing resin. Batch testing of the resins produced linear isotherms, indicating that the colorant is dilute. Results from column testing showed that a plug‐flow model with a constant linear isotherm was sufficient in all cases except the SAC resin. The SAC adsorption parameter decreased sharply as the pH increased, causing the colorant to be desorbed from the resin. This situation must be avoided if optimal decolorization is to be achieved.
Multivariable terrain methods
Authors: Lucia Angelo , Feng Yang
Published: April 2004

A novel geometric terrain methodology was recently presented for finding all physically relevant solutions and singular points to chemical process simulation problems based on intelligently moving along special integral curves of the gradient vector field to guide both uphill and downhill movement. The terrain methodology is extended here to include multivariable problems and integral curve bifurcations. The integral curves of interest are those associated with valleys and ridges and characterized as a collection of extrema in the norm of the gradient over a set of level curves. It is shown that integral curves can undergo tangent, pitchfork, and other types of bifurcations and that terrain methods are clearly superior to differential arc homotopy‐continuation methods on problems that exhibit parametric disconnectedness. Several examples, including a pair of CSTR problems, a retrograde flash calculation, and the task of finding all azeotropes for a heterogeneous ternary mixture, are used to show that terrain methods represent a reliable, efficient, and global way of solving multivariable process engineering simulation and optimization problems. Geometric illustrations are used whenever possible to clarify underlying ideas.
New maximum‐likelihood method for reduction of distribution‐coefficient data
Authors: Marshall Simon L.
Published: April 2004

The sampling distributions for the activity coefficient γ and distribution ratio (or K‐value) in binary fluid mixtures are derived from the assumptions that the pressure errors are negligible, and that the vapor and liquid composition measurements from which they are derived are normally distributed with known mean and variance. The distribution of K is shown to be not normal, with deviations from normality becoming particularly pronounced at the extremes of the composition range. The dispersion of this distribution is considerably greater than that predicted by the classic error‐propagation formula. Deviation from normality invalidates the most important assumption implicit in the use of the least‐squares criterion as the basis for parameter estimation from activity‐coefficient data. This has important implications for the estimation of Henry's law constants for solutes in very dilute solutions, as well as for Raoult's law‐based infinite‐dilution activity coefficients. The use of the exact distributions of γ and K as the basis for a more rigorous maximum‐likelihood method for parameter estimation in excess Gibbs energy models based on either Henry's law or Raoult's law standard states is demonstrated.
Correlating polymer resin and end‐use properties to molecular‐weight distribution
Authors: Hinchliffe Mark , Montague Gary , Willis Mark , Burke Annette
Published: April 2004

The prediction of polymer resin and end‐use properties for a dual‐reactor solution polyethylene process is investigated. Polymer molecular‐weight distribution (MWD) is highly influential in achieving the desired properties, but the extent of the importance and key areas of distribution to achieve specific properties are less well understood. The best empirical approach for resin and end‐use property prediction using the entire MWD along with other influential variables as inputs is investigated. Two modeling methods are considered: partial least squares (PLS) and a novel strategy that uses the weight fraction of polymer in a given molecular‐weight range (referred to as a binning technique). Both linear and nonlinear variants of the two algorithms are used. The intention is to develop a model that facilitates the analysis of the simultaneous influences of process operating conditions and resin characteristics such as MWD on a specified set of end‐use properties. Results demonstrate that the nonlinear variant of the binning technique provides the highest accuracy as well as indicating regions of MWD that are of greatest influence. Such information is particularly useful for the control of the polymerization reactors.
Identification of an ideal reactor model in a secondary combustion chamber
Authors: Bass Charles A. , Barat Robert B. , Lemieux Paul M.
Published: April 2004

Tracer analysis was applied to a secondary combustion chamber of a rotary kiln incinerator simulator to develop a computationally inexpensive networked ideal reactor model and allow for the later incorporation of detailed reaction mechanisms. Tracer data from sulfur dioxide tracer experiments were reconstructed using a least‐squares approximation that eliminated the intrinsic response of the measurement device and produced a residence time distribution between various points of the combustor. A model was chosen based on the best fit to tracer data, and consistency with physical geometry and temperature measurements. The resulting model showed that the active path was 1 s shorter than the mean residence time calculated from the total volumetric flow. The analysis found that complete mixing between the kiln gas and burner stream occurs farther downstream than originally expected. Results underscore the importance of turbulent mixing in reactor design and suggest that different design criteria are needed to ensure complete combustion.
Solubility of CO2 in (H2O+piperazine) and in (H2O+MDEA+piperazine)
Authors: Kamps Álvaro Pérez‐Salado , Xia Jianzhong , Maurer Gerd
Published: April 2004

A thermodynamic model for describing the phase equilibrium for the system CO2+methyldiethanolamine+water has been extended, allowing for the presence of piperazine. Model parameters are estimated from new experimental results for the solubility of carbon dioxide in aqueous solutions of piperazine (for piperazine molalities up to 4 mol per kilogram of water, temperatures between 313 and 393 K, and pressures up to about 9.6 MPa). The model is tested by comparing predicted and experimental carbon dioxide solubilities in aqueous mixtures of both amines (2 molal MDEA+2 molal piperazine, at 353 K, and pressures up to about 6.4 MPa).