bluuebunny/crossref_metadata_2025 huggingface dataset:
Vapor–liquid equilibria of mixtures containing alkanes, carbon dioxide, and nitrogen
Authors: Potoff Jeffrey J. , Siepmann J. Ilja
Published: April 2004

New force fields for carbon dioxide and nitrogen are introduced that quantitatively reproduce the vapor–liquid equilibria (VLE) of the neat systems and their mixtures with alkanes. In addition to the usual VLE calculations for pure CO2 and N2, calculations of the binary mixtures with propane were used in the force‐field development to achieve a good balance between dispersive and electrostatic (quadrupole–quadrupole) interactions. The transferability of the force fields was then assessed from calculations of the VLE for the binary mixtures with n‐hexane, the binary mixture of CO2/N2, and the ternary mixture of CO2 /N2/propane. The VLE calculations were carried out using configurational‐bias Monte Carlo simulations in either the grand canonical ensemble with histogram–reweighting or in the Gibbs ensemble.
Simultaneous breakup of multiple viscous threads surrounded by viscous liquid
Authors: Knops Yolanda M. M. , Slot Johan J. M. , Elemans Pierre H. M. , Bulters Markus J. H.
Published: April 2004

A scenario is proposed for the simultaneous breakup process of an arrangement of parallel viscous threads. At very close distances, threads will break up via an “in‐phase” mode, that is, droplets will be formed at the same axial positions along the threads. In the case of threads that are further apart, breakup will occur via an “out‐of‐phase” mode, that is, on neighboring threads droplets will be formed at axial positions that are shifted by one‐half of a wavelength. The smaller the viscosity ratio between the thread phase and the continuous phase, the closer the threads must be to break up via this in‐phase mode. This scenario agrees with recent experimental results.
Rapid filtration measurement of dewatering design and optimization parameters
Authors: de Kretser Ross G. , Usher Shane P. , Scales Peter J. , Boger David V. , Landman Kerry A.
Published: April 2004

To characterize and optimize the compressive dewatering of suspensions, rapidly determinable material parameters are required. A mathematical framework exists for characterization of dewatering using three parameters: the compressive yield stress Py (ϕ), the hindered settling function r(ϕ), and a solids diffusivity D(ϕ). A new variation on traditional constant pressure, batch filtration tests is described along with a computer‐controlled filtration apparatus that enables complete characterization of a suspension with respect to dewatering in a matter of hours. The testing methods are validated experimentally on flocculated zirconia suspensions along with calculation of the material dewatering parameters. Application of the calculated parameters to modeling of a real filtration test shows excellent agreement. Practical implications of the modeling are also discussed.
Predicting liquid flow profile in randomly packed beds from computer simulation
Authors: Wen X. , Shu Y. , Nandakumar K. , Chuang K. T.
Published: April 2004

Our previous work on the simulation of packing processes was extended to study the liquid trickle flow down randomly packed beds. An algorithmic procedure with detailed geometrical description of packing elements was developed for tracking the liquid flow and holdup distributions in packed beds. Unlike the conventional continuum‐based models such as the diffusion model, it goes into the subparticle scale to capture the geometrical characteristics of the packed bed. Three mechanisms of the trickle flow—film flow, dripping flow, and splash—were observed in packed beds, and the first two were captured in the model. At each point within a packed bed, liquid flow is simplified to two possible directions: vertically down and horizontally in the direction of the negative gradient of the packing surface. The 3‐D geometric model constructed with the packing process simulation was used to determine the direction of the horizontal flow and fraction of the flow rate in each direction. By assuming that the liquid flow is uniform free surface flow, the Manning formula was used to predict the liquid holdup with the width of the liquid rivulet on a packing surface calculated by the Shi and Mersmann correlation. Metal Pall rings and metal Raschig rings were simulated with water as the fluid. Results of liquid flow distributions and average liquid holdup were validated against experimental data and semiempirical correlation from Billet et al.
Model‐based synthesis of nonlinear PI and PID controllers
Authors: Wright Raymond A. , Kravaris Costas , Kazantzis Nikolaos
Published: April 2004

PI and PID controllers continue to be popular methods in industrial applications. It is well known that linear PI and PID controllers result from the application of model‐based controller design methods to linear first‐ and second‐order systems. It is shown that nonlinear PI and PID controllers result from the application of nonlinear controller design methods to nonlinear first‐ and second‐order systems. As a result, the controllers resulting from nonlinear model‐based control theory are put in a convenient form, more amenable to industrial implementation. Additionally, the quantities used in the controller are useful for monitoring the process and quantifying modeling error. Chemical engineering examples are used to illustrate the resulting control laws. A simulation example further demonstrates the performance of the nonlinear controllers, as well as their useful process monitoring quantities.
Hamiltonian view on process systems
Authors: Hangos Katalin M. , Bokor József , Szederkényi Gábor
Published: April 2004

The thermodynamic approach of analyzing structural stability of process plants was extended to construct the simple Hamiltonian model of lumped process systems. This type of model enables us to design a nonlinear PD feedback controller for passivation and loop shaping. This approach is applicable for lumped process systems where Kirchhoff convective transport takes place together with the transfer and sources of various types, and the manipulable input variables are the flow rates. Systems with constant mass holdup and uniform pressure in every balance volume satisfy these conditions. General results are shown by simple examples of practical importance: on a bilinear heat exchanger cell and on an isotherm CSTR with nonlinear reaction.
Design of liquid‐liquid phase transfer catalytic processes
Authors: Samant Ketan D. , Singh Divya J. , Ng Ka M.
Published: April 2004

Issues related to the conceptual design of liquid‐liquid phase transfer catalytic (LLPTC) processes are discussed. Based on a generic mathematical model, effects of reaction kinetics and mass transfer on the performance of LLPTC processes are captured in terms of Damköhler numbers. The effects of thermodynamic parameters on the process performance are also discussed. Elucidation of the basic mechanisms leads to the synthesis of two classes of flowsheets for LLPTC processes with recovery and recycle of the phase transfer catalyst. Salient features of operation are identified for both classes. The use of the model in making judicious choices of the reactor phase attributes, the catalyst recovery scheme, the catalyst, and the organic solvent is demonstrated.
Analysis and control of particulate processes with input constraints
Authors: El‐Farra Nael H. , Chiu Timothy Y. , Christofides Panagiotis D.
Published: April 2004

A general methodology is proposed for the analysis and control of spatially‐homogeneous particulate processes with input constraints modeled by population balance equations. A nonlinear model reduction procedure based on the method of weighted residuals is used for the construction of finite‐dimensional ordinary differential equation (ODE) systems that accurately reproduce the dominant dynamics of the particulate process. These ODE systems are used to analyze the limitations imposed by input constraints on the ability to modify the dynamics of the particulate process, leading to an explicit characterization of the set of admissible set points that can be achieved in the presence of constraints. This information, together with the derived ODE systems, is then used as the basis for the synthesis of practically‐implementable nonlinear bounded output feedback controllers with well‐characterized constraint‐handling capabilities. The designed controllers enforce exponential stability in the closed‐loop system and achieve particle‐size distributions with desired characteristics in the presence of active input constraints. Precise closed‐loop stability conditions are given and controller implementation issues are addressed. This method is successfully used to regulate a continuous crystallizer with constrained control action at an open‐loop, unstable equilibrium point.
Binary adsorption equilibrium of organics and water on activated carbon
Authors: Linders M. J. G. , van den Broeke L. J. P. , Kapteijn F. , Moulijn J. A. , van Bokhoven J. J. G. M.
Published: April 2004

Adsorption isotherms of hexafluoropropylene (HFP), methanol and ethanol were measured under dry and humid conditions using Norit R1 carbon. The presence of water lowered the HFP capacity over the whole pressure range as compared to the single component isotherm, while increasing the methanol and ethanol capacity. The Dubinin‐Radushkevich equation was applicable for methanol, and ethanol, and the Dubinin‐Astakhov equation gave a satisfactory description for water. The ideal adsorbed solution (IAS) theory satisfactorily described the binary, miscible systems of methanol and ethanol with water. Because water and HFP are not miscible, the IAS theory cannot be used. This binary system was described well by the independent co‐adsorption of the two components, that is, HFP vapor is adsorbed in the micropore volume left free by water.
Banded liquid–liquid Taylor‐Couette‐Poiseuille flow
Authors: Zhu Xiaoyan , Vigil R. Dennis
Published: April 2004

It has been reported that several distinct flow patterns may result when two immiscible liquids undergo Taylor vortex flow in the annular gap between a fixed outer cylinder and a rotating concentric inner cylinder. One such flow pattern (banded flow) apparently arises when less dense disperse phase droplets migrate to Taylor vortex cores due to the centrifugal force associated with the rotation of the vortices. Using the hypothesis that centrifugally driven droplet migration to vortex cores competes with turbulent diffusion, a simple criterion for predicting when banded flow patterns arise shows consistency with numerical simulations and experiments.
Microdivers to study sedimentation in polydisperse, concentrated colloidal suspensions
Authors: Biesheuvel P. Maarten , Verweij Henk , Breedveld Victor
Published: April 2004

A small amount of polymer particles with diameters in the 100 μm range can be used as an ensemble of “microdivers” to study sedimentation velocities of polydisperse submicron particles in concentrated opaque suspensions. This tracer technique, based on Berg's (1941) diver method, is simple, measures velocities directly, and does not require underlying theory beyond conservation of momentum. It confirms that the polymer tracers reside at the position in the suspension where the suspension density corresponds to their own density. Aqueous inorganic suspensions of 250–750 nm α‐alumina particles were studied, which is electrostatically stabilized with nitric acid in water, using ∼100 μm polymethylmetacrylate and polystyrene particles as tracers. Experiments were compared with predictions by state‐of‐the‐art continuum models for multicomponent sedimentation to underpin the validity of the tracer technique.
Tracer studies of high‐shear granulation: I. Experimental results
Authors: Pearson J. M. K. , Hounslow M. J. , Instone T.
Published: April 2004

Previous work has shown that attempts to determine the rates of processes active during granulation inevitably confound the effects of granule age and size; since as an experiment progresses, both the age and size of granules increases. In the current work, tracer experiments have been developed, and verified, to unconfound those effects. Qualitative analysis of the tracer data shows that granule breakage is occurring at a significant rate and that the breakage, or selection rate, appears to depend on granule age, but not on granule size.
Gas‐solids flow behavior: CFB riser vs. downer
Authors: Zhang H. , Huang W.‐X. , Zhu J.‐X.
Published: April 2004

Comparisons are made in a circulating fluidized‐bed riser/downer system between a 15.1 m high, 0.10 m ID riser and a 9.3 m high, 0.10 m ID downer, based on the measurements of the radial distributions of the local solids holdups and local particle velocities along the two columns. Although the core‐annulus flow structures exist in both the riser and downer, the radial flow structure in the downer differs largely from that in the riser. The radial distributions of solids holdup and particle velocity in the downer are much more uniform than those in the riser, thus ensuring the low back mixing and the narrow particle residence time distribution in the downer. The axial flow structure in the downer is also more uniform than that in the riser. Due to the high particle acceleration and the high particle velocity in the downer, the overall solids holdup is significantly lower than that in the riser. The microflow structure in the downer, characterized by the low intermittency indices, is also more uniform than that in the riser. These key properties of the downer make it a very promising candidate for industrial applications where short reaction times and high product selectivity are required.
Predicting membrane flux of CH4 and CF4 mixtures in Faujasite from molecular simulations
Authors: Sanborn Martin J. , Snurr Randall Q.
Published: April 2004

The Fickian transport diffusivities of binary mixtures of methane and CF4 in the zeolite faujasite were investigated as a function of composition and loading. Using equilibrium molecular dynamics simulations, transport diffusivities of the mixtures were calculated from linear response theory. At low loadings, the diffusive cross terms are an order of magnitude smaller than the diffusive main terms, but at higher loadings they are on the same order of magnitude. The main term diffusivities are weak functions of concentration. The transport diffusivities were fitted as a function of adsorbate concentrations and used in the Fickian flux equations to predict codiffusion, counterdiffusion, and osmotic diffusion through a perfectly crystalline faujasite membrane. The osmotic and counterdiffusion cases show that it is critical to include both the cross term and main term diffusivities to predict the correct behavior. The permeances of methane and CF4 through the faujasite membrane are reported.
Fructose–glucose separation in a SMB pilot unit: Modeling, simulation, design, and operation
Authors: Azevedo Diana C. S. , Rodrigues Alírio E.
Published: April 2004

Glucose was separated from fructose experimentally using a simulated moving‐bed (SMB) adsorber. A strongly acid cationic resin of gel type (Ca2+ form) Dowex Monosphere (dp = 320 μm) was used in a pilot SMB unit of twelve 26 × 300 mm (ID × length) columns. A recently proposed design procedure was applied to overcome the inherent strong mass‐transfer resistance present in this kind of adsorbent. The fluid/solid velocity ratios in SMB sections 1, 2 and 3 leading to at least 90% product purity were followed by simulation and plotted in a 3‐D parameter space. The design methodology, called “separation volume analysis,” also considered the geometric parameters, as well as allowable working flow rates, temperature and pressure of the plant. Operating conditions from this procedure were used to operate the SMB unit, and the expected performance was achieved experimentally. Simulation strategies based on a true countercurrent and a real SMB were used and the predicted performance of both agreed well with experimental data. Furthermore, experimental results confirmed predictions of the separation volume analysis, which shows the potential of the technique for optimizing existing SMB equipment.
Identification of dynamic process systems with surrogate data methods
Authors: Barnard Jakobus P. , Aldrich Chris , Gerber Marius
Published: April 2004

Identifying the underlying dynamics of chemical process systems from experimental data is complicated, owing to a mixture of influences that cause erratic fluctuations in the time series. These influences can be notoriously difficult to disentangle. The development of process models is usually subject to considerable human judgment and can therefore be very unreliable. This is especially the case when the model priors are unknown and the model is validated empirically, such as with cross‐validation or holdout methods. A case study shows that more reliable identification of systems is possible by using surrogate methods to classify the data, as well as to validate models derived from these data.
Ceramic membrane reactor for synthesis gas production
Authors: Ritchie J. T. , Richardson J. T. , Luss Dan
Published: April 2004

Membrane reactors enable synthesis‐gas production from methane and air while avoiding the need for separation of the nitrogen either before or after the reaction. A stable membrane was developed by spray deposition of a dense thin film of La0.5Sr0.5Fe0.8Ga0.2O3 − δ on a high‐purity porous α‐alumina tube. The oxygen permeation rate at 850°C was 2.5 × 10−7 mol·cm−2·s−1. A quartz tube was placed coaxially around the membrane and the shell filled with a rhodium catalyst. Air was fed to the tube and methane to the shell. At 850°C the methane conversion was 97% and the selectivity to carbon monoxide approached 100%. Rapid radial mixing of the oxygen in the shell is essential to prevent coking and undesirable reactions. The membrane decomposes at 780°C in pure CH4, but remains stable up to 970°C in a mixture of 90‐mol % CH4 and 10‐mol % CO2.
Calcium carbonate catalysis of alcohol oxidation in near‐critical water
Authors: Suppes G. J. , Roy S. , Ruckman J.
Published: April 2004

The near‐critical water oxidations (NCWO) of phenol, methanol, ethanol, n‐propanol and n‐butanol were evaluated at temperatures between 200 and 374°C and pressures between 2 and 220 bar. Reactions were conducted in packed‐bed flow reactors with ZrO2 and CaCO3 packings with a stoichiometric amount of HOOH. The effectiveness of CaCO3 as an oxidation catalyst increased markedly between 200 and 374°C enhancing oxidation rates > 100 × at 374°C for phenol. Hydrolysis was observed for phenol, ethanol, and methanol over CaCO3 at 374°C and was largely nondetectable in the absence of CaCO3. Despite being inexpensive and environmentally benign, CaCO3 exhibited catalytic abilities equal to or better than many of the best published performance of NCWO catalysts containing noble or transition metals.
Existence of a critical carbon number in the aging of a wax‐oil gel
Authors: Singh Probjot , Youyen Arm , Fogler H. Scott
Published: April 2004

A fundamental study was carried out to understand the aging (or hardening) of the wax‐oil gels formed in the subsea pipelines during the flow of crude oil from offshore wells to shore. The aging process is a counterdiffusion phenomenon where there exists a critical carbon number (CCN), and wax molecules with carbon numbers greater than the CCN diffuse into the gel matrices and vice versa. Using a careful analysis of carbon number distributions of gel deposits, collected from a cold finger after various deposition time intervals, the CCN for the wax‐oil system was obtained. A mathematical model, based on a modified version of UNIQUAC model, was developed to predict the CCN for wax‐oil systems. The size of the interaction units for n‐alkanes in the solid‐phase UNIQUAC model was found to be a strong function of the mean carbon number in the solid phase.
Extensive validation of computed laminar flow in a stirred tank with three Rushton turbines
Authors: Zalc J. M. , Alvarez M. M. , Muzzio F. J. , Arik B. E.
Published: April 2004

High‐resolution CFD results, supplemented by extensive experimental validation, are presented for Newtonian laminar flow fields in a stirred tank equipped with three Rushton turbines. Flow fields are computed using the ORCA software suite for Reynolds numbers ranging from 20 to 200 with an unstructured tetrahedral mesh containing roughly 2 million tetrahedra. Each of the flow solutions takes less than 8 h to converge when running in parallel on eight desktop workstations. Excellent agreement is obtained between computed velocity fields and planar velocity vectors obtained using particle image velocimetry. Planar laser‐induced fluorescence was used to expose persistent poor‐mixing regions, in excellent agreement with numerical results. The computational results used illustrate strong flow compartmentalization and significant spatial heterogeneity with respect to local deformation rates within the vessel.
Fourier spectrum to recover deterministic and stochastic behavior in stirred tanks
Authors: Kovács T. , Trägårdh C. , Fuchs L.
Published: April 2004

Velocity data from the impeller region of Rushton and 6SRGT pilot‐scale impellers were analyzed both in the frequency domain and with respect to periodicity. The four characteristic frequencies and their combination were detected from the Fourier spectrum, and the deterministic part of the signal was recovered by making the amplitude of the other frequencies zero. The deterministic part was then analyzed with tools from nonlinear theories. For the two turbine‐types, the dynamics of the deterministic velocity parts were almost identical. The turbulent part of the signal could also be estimated in a similar manner.
Axial and radial solids distribution in a long and high‐flux CFB riser
Authors: Pärssinen J. H. , Zhu J.‐X.
Published: April 2004

Radial solids concentration profiles were determined with a fiber‐optic probe on eight axial levels in a 76‐mm‐ID, 10‐m riser, at solids flux (solids circulation rate) up to 550 kg/m2·s and superficial gas velocity up to 10 m/s. Radial concentration profiles at high solids fluxes of over 300 kg/m2·s are less uniform than lower fluxes of less than 200 kg/m2·s. Under all operating conditions, the flow development in the riser center is nearly instant, with the solids concentration remaining low at the riser center throughout the riser. In the wall region, increasing solids flux significantly slows down the flow development, with the solids concentration near the wall decreasing all the way toward the riser top at high fluxes. In the high‐flux circulating fluidized bed (HFCFB), a middle section with intermediate solids holdups of about 7 to 20% was between the bottom dense section and top dilute section, with its length increasing as the solids circulation rate increases and as the gas velocity decreases. Flow conditions in this section resemble those of the dense suspension upflow under high‐density operating conditions by Grace et al. (1999). When its length extends to the riser top, a high‐density circulating fluidized bed for which an HFCFB is a necessary but not a sufficient condition, forms.
Effect of fiber variation on staged membrane gas separation module performance
Authors: Liu B. , Lipscomb G. G. , Jensvold J.
Published: April 2004

The effect of fiber property variations on the performance of a two‐stage system for gas separation was studied theoretically and experimentally. Variations in fiber inner diameter, slow gas permeance, or selectivity are considered. Variability in any of these properties is detrimental to performance, as characterized by product flow rate and product recovery (fraction of the feed recovered as a product). The performance decline increases as either property variation or product purity increase. The two‐stage system can enhance performance significantly over a single sage. Permeate mixing is also beneficial to performance. Like a single stage, however, some fibers may stop producing a retentate product and consume product produced by other fibers for sufficiently large variation in size and permeance. The theory agrees well with experiments for the production of nitrogen from air.
Evaporative crystallization of anhydrous sodium carbonate at atmospheric conditions
Authors: Oosterhof Harald , Witkamp Geert‐Jan , van Rosmalen Gerda M.
Published: April 2004

A new process for the production of superdense anhydrous soda ash uses a mixture of water and a high‐boiling second solvent to lower the transition point at which anhydrous (Na2CO3) and monohydrous sodium (Na2CO3·H2O) carbonate are in equilibrium to below the atmospheric boiling point. The stable conditions for anhydrate were first established by measuring the water activity in saturated mixtures of water and ethylene glycol. With the results, fed‐batch evaporative crystallization experiments were carried out. Both the water activity and the crystallization measurements showed that anhydrous soda was stable in boiling mixtures containing more than 22.5 wt. % ethylene glycol (on a salt‐free basis). A subsequent continuous evaporative crystallization experiment produced anhydrous soda ash with a bulk density of 1,550 kg/m3, which is substantially higher than that of any other atmospherically crystallized soda.
Cost‐effective bypass design of highly controllable heat‐exchanger networks
Authors: Yan Q. Z. , Yang Y. H. , Huang Y. L.
Published: April 2004

Design of a cost‐effective and highly controllable heat‐exchanger network (HEN) has drawn a great deal of attention for years. One of the key issues in such a design is how to effectively minimize undesirable disturbance propagation in a network with minimum cost increment. Design options in this regard include the derivation of a superior network structure and the selection of bypasses associated with heat exchangers. A unique system modeling approach is developed to predict disturbance propagation and to reject disturbances using bypasses. A novel mathematical representation scheme for a HEN is introduced to facilitate system analysis and design. A relative gain‐array approach is extended to the analysis of nonsquared systems. In addition, an iterative design procedure is introduced to determine optimal bypass locations and nominal fractions for complete disturbance rejection, while economic penalty reaches the minimum. The efficacy of the model‐based approach is demonstrated by designing three HENs where bypasses are optimally placed, and the control schemes are simultaneously developed.
Distribution kinetics of thermolytic macromolecular reactions
Authors: Sterling W. Jerome , McCoy Benjamin J.
Published: April 2004

Essential elements of thermolytic reaction kinetics for distributions of polymers are reviewed. Bond scission, occurring randomly along a chain backbone or at the chain end, is the major reaction process in polymer thermolysis. Underlying this bond scission are free radical mechanisms, including initiation and termination, hydrogen abstraction, and beta scission. Population‐balance equations for the molecular‐weight distributions of macromolecules or their radicals describe the dynamics of the reactions. The governing integrodifferential equations for continuous distributions can be solved by moment methods, similarity techniques, other analytical procedures, or numerical methods. The approach has been applied to analytical thermolysis and pyrolysis, polymer degradation for stability characterization or plastics recycling, and coal thermolysis to produce fuels and feedstocks. Experiments have demonstrated the usefulness of the continuous distribution kinetics approach for polymer thermolysis with additives, including hydrogen donors, peroxides, and other polymers. Molecular‐weight distributions and their moments can be measured by size exclusion chromatography. Experimental data can be interpreted by evaluating rate parameters, which may be composites of rate coefficients for elementary steps in complex reaction mechanisms.
Impact of 2‐D bubble dynamics on the selectivity of fast gas–liquid reactions
Authors: Khinast Johannes G.
Published: April 2004

The impact of mass transfer and bubble‐wake dynamics on the selectivity of fast gas–liquid reactions was studied for a parallel‐consecutive reaction network using numerical simulations. Depending on bubble size and shape, the bubble wake can be closed or open. Spherical bubbles have only closed wakes without recirculation, while all other bubble types can exhibit recirculation or vortex shedding depending on their shape and the Reynolds number. Although the importance of local mixing on the selectivity of complex reactions was studied by many research groups, there exist no studies addressing the effect of local mixing patterns (bubble‐wake dynamics) close to single bubbles on fast gas–liquid reactions, that is, reactions that occur close to the gas–liquid interface. To study this class of reactions, a 2‐D bubble model was developed, which accounts for liquid flow around the bubble, mass transfer, and reactions. It was found that different residence times in the bubble wake and at the bubble roof can lead to the formation of different products; recirculation in the bubble wake acts as a transport barrier for the liquid‐phase reactants; and vortex shedding causes a qualitatively different mixing pattern than that of a closed wake, leading to a different product distribution in the case of mixing‐sensitive reactions. Since bubble shapes and sizes can be controlled by changing operating conditions or design parameters, this analysis can be applied to actual reaction systems to enable a rational design, control, scale‐up, and optimization of existing and new processes.
Nonlinear model reduction of chemical reaction systems
Authors: Vora Nishith , Daoutidis Prodromos
Published: April 2004

A nonlinear model reduction method for nonisothermal reaction systems that exhibit dynamics in two different time scales owing to the presence of fast and slow reactions was developed. The method systematically identifies the independent algebraic constraints that define the low‐dimensional state space where the slow dynamics of the reaction system are constrained to evolve. It also derives state‐space realizations of the resulting differential algebraic system that describes the slow dynamics. This method is illustrated through the classic Michaelis‐Menten reaction system, and is applied to an ozone decomposition reaction system and a reaction mechanism for esterification of carboxylic acid.
Molecular‐weight distribution kinetics for ultrasonic reactions of polymers
Authors: Madras Giridhar , McCoy Benjamin J.
Published: April 2004

Ultrasonic degradation of polymers is distinguished from thermal degradation by: midpoint rather than random chain scission; a lower limit of molecular weight (MW), xf, below which the polymer will not undergo midpoint scission; and possible biomodal distribution evolving with time. The molecular‐weight distribution (MWD) is governed by a population‐balance equation whose time dependence is deduced from MW moments. To examine if ultrasonically induced reactions have similarity solutions, the moment equations were considered in terms of the time evolution of generalized gamma distribution parameters. The rate coefficient for chain scission is assumed to depend on MW, x, as κ (x − xf), where xf is the limiting MW. The MWD approaches a similarity solution (with a polydispersity of 1.5) only when limiting MW xf is zero. This study also provides an exact analytical solution for the MWD, showing how the MWD can evolve, with reaction time, from an unimodal distribution to a bimodal distribution. Numerical solutions are presented for cases when a polymer undergoes a reversible chain‐end scission along with midpoint chain scission. The rate coefficients for chain‐end scission and polymerization are assumed independent of MW. The theory satisfactorily explains experimental observations.
Streaming potential through multilayer membranes
Authors: Szymczyk A. , Labbez C. , Fievet P. , Aoubiza B. , Simon C.
Published: April 2004

The fundamental and practical importance of electrokinetic characterization of porous membranes is now well recognized. However, not enough attention has focused on the fact that the most practically interesting membranes have a multilayer structure. Then, standard electrokinetic methods, such as streaming potential measurement performed through these multilayer membranes, lead to a global signal that does not necessarily reflect the electrokinetic behavior of the skin layer that rules the membrane selectivity. The relative influence of membrane layers with different physical (pore length, porosity, pore radius) and electrical (zeta potential) properties on the global streaming potential arising across a two‐layer membrane is studied. Each porous layer plays a role in the streaming potential process, but in a wide range of length fraction of support layer the skin layer contributes more than the support layer to the global signal. The contribution of the skin layer increases as its porosity and pore radius decrease, but is little affected by its zeta potential. Nevertheless, in many practical situations, the support layer is likely to play a nonnegligible role on the global signal due to its huge thickness with respect to the skin layer. Therefore, electrokinetic measurements performed with multilayer membranes should be interpreted carefully.
Design of flexible reduced kinetic mechanisms
Authors: Sirdeshpande Avinash R. , Ierapetritou Marianthi G. , Androulakis Ioannis P.
Published: April 2004

Reduced mechanisms are often used in place of detailed chemistry because the computational burden of including all the species continuity equations in the reactor model is unreasonably high. Contemporary reduction techniques produce mechanisms that depend strongly on the nominal set of problem parameters for which the reduction is carried out. Effects of variability in these parameters on the reduced mechanism are the focus of this work. The range of validity of a reduced mechanism is determined for variations in initial conditions. Both sampling approaches and quantitative measures of feasibility, such as the flexibility index and the convex hull formulation, are employed. The inverse problem of designing a reduced mechanism that covers the desired range of initial conditions is addressed using a multiperiod approach. The effect of the value of a user‐defined tolerance parameter, which determines whether the predictions made by the reduced mechanism are acceptable, is also assessed. The analytical techniques are illustrated with examples from the literature.
Process analyzer location and performance assessment for optimal process monitoring
Authors: van den Berg Frans W. J. , Boelens Hans F. M. , Smilde Age K. , Hoefsloot Huub C. J.
Published: April 2004

The influence of process analyzer location and performance on plant‐wide process monitoring was evaluated using five uncertainty contributions to the estimation error: measurement error/uncertainty, analysis frequency, sample size/grab error, analyzer memory effect/response correlation, and delay time. The choice of both the location and performance characteristics of different process analyzers can be evaluated using a measurability factor M, ranging from zero to one, where one indicates perfect monitoring capabilities. Due to the unifying nature of the measurability factor, this factor can be used to make a rational decision between very different process analyzers. This allows for finding optimal process analyzer configurations for existing processes or for processes in the design phase. A tubular reactor simulation model was used for the partial oxidation of benzene to maleic anhydride to demonstrate the use of the measurability factor.
Surface diffusion and adsorption of hydrocarbons in activated carbon
Authors: Do H. D. , Do D. D. , Prasetyo I.
Published: April 2004

Adsorption and diffusion in a porous media were studied theoretically and experimentally with a differential transient permeation method. The porous medium is allowed to equilibrate at some specified loading, and then the time trajectory of the permeation process is followed after a small difference between the pressures at the end faces of the porous medium is introduced at time t = 0 +. Such a trajectory vs. time would contain adsorption and diffusion characteristics of the system. By studying this for various surface loadings, pore and surface diffusions can be fully characterized. Mathematical modeling of transient permeation is detailed for pure gases or vapors diffusion and adsorption in porous media. Effects of nonlinearity of adsorption isotherm, pressure, temperature and heat effects were considered in the model. Experimental data of diffusion and adsorption of propane, n‐butane and n‐hexane in activated carbon at different temperatures and loadings show the potential of this method as a useful tool to study adsorption kinetics in porous media. Validity of the model is best tested against the transient data where the kinetics curves exhibit sigmoidal shape, which is a result of the diffusion and adsorption rate during the initial stage of permeation.
Cavitation reactors: Efficiency assessment using a model reaction
Authors: Gogate Parag R. , Shirgaonkar Irfan Z. , Sivakumar M. , Senthilkumar P. , Vichare Nilesh P. , Pandit Aniruddha B.
Published: April 2004

Acoustic and hydrodynamic cavitation can be used for a variety of applications ranging from biological applications such as cell disruption to chemical reactions such as oxidation of organic pollutants in aqueous effluents, including biorefractory toxic chemicals. Different equipment used for cavitational effects was compared based on a model reaction (decomposition of potassium iodide resulting into iodine liberation). A correlation was developed for the prediction of the cavitational yield in terms of the cavity collapse pressure. This correlation, when used with earlier correlations for the pressure amplitude generated during the violent collapse of cavities, will help design engineers to choose particular equipment, operating conditions, and geometric parameters to achieve a desired chemical change. The developed equation relating the macroscopic reaction rates with the collapse pressure is the first of its kind reported in the literature. Pilot‐plant‐scale hydrodynamic cavitation orifice plate setup is most energy‐efficient, with significantly higher cavitational yields for the model reaction.
Enhancing performance of three‐phase catalytic packed‐bed reactors
Authors: Wilhite B. A. , Wu R. , Huang X. , McCready M. J. , Varma A.
Published: April 2004

Our previous theoretical work predicted the possibility of enhancing three‐phase packed‐bed reactor performance by operating in the pulsing‐flow regime. This article deals with the experimental study of the beneficial effect of pulsing flow on reaction outcome. Hydrogenation of phenylacetylene, dissolved in n‐tetradecane over Pt/alumina catalyst, was chosen as the experimental reaction system. This is a triangular reaction, with styrene and ethylbenzene as the desired intermediate and final products, respectively. With properly designed experiments, the reaction performance in pulsing flow and trickling‐flow regimes was compared directly. The effects of process variables such as temperature, feed flow rates, and reactant concentration on reaction behavior were studied. A simplified model to describe the qualitative trends was also developed. Both experiments and calculations show that the yield of styrene is higher in pulsing flow than in trickling flow, which confirms the advantages of pulsing‐flow operation predicted by the theoretical work.
Spectroscopic measurement of solid solubility in supercritical fluids
Authors: Ngo Truc T. , Bush David , Eckert Charles A. , Liotta Charles L.
Published: April 2004

Different in situ spectroscopic techniques, including infrared, ultraviolet, and fluorescence, were developed to measure the solubility of organic solids in supercritical carbon dioxide (scCO2). These techniques are applicable over a wide range of concentrations, as low as 10−6 or 10−7 mol fraction, where the conventional flow method is ineffective and less accurate. No separate calibration is required; in fact, the molar absorptivity is determined at the saturation point as an additional benefit. While this technique requires more time per data point, it is more accurate and unbiased than traditional methods at lower concentrations. The Patel‐Teja equation of state was used to correlate the data and expand the data for the ternary system. Data are reported for anthracene, 1,4‐naphthoquinone, and 2‐naphthol in scCO2 and CO2 with methanol cosolvent at 313 K and a pressure range from 7 to 21 MPa.
Thermodynamics of paint‐related systems with engineering models
Authors: Lindvig Thomas , Michelsen Michael L. , Kontogeorgis Georgios M.
Published: April 2004

Paints are complex materials composed of polymers (binders) dissolved in one or more solvents, pigments, and other additives. The thermodynamics of such systems is essential, for example, for selecting improved solvents and understanding a number of phenomena related especially to adhesion to solid surfaces and drying. Many engineering models have been applied over the last decades for solutions with commoditity polymers. In this work, the performance of some of these models is investigated for paint‐related systems, focusing on those drying by the so‐called “lacquer mechanism” (evaporation of solvents). These are the Entropic‐FV, UNIFAC‐FV, GC‐Flory, and the Flory‐Huggins models using various ways for estimating the FH parameter via solubility parameters. Achievements and shortcomings of these models are discussed in conjunction with difficulties encountered in such calculations. We conclude that, despite the uncertainties involved, several models yield reasonably accurate activity coefficients, even at infinite dilution. Thus, engineering models may be useful for solvent selection via semiempirical rules of thumb, which are based on thermodynamic considerations.
Hybrid SNCR‐SCR technologies for NOx control: Modeling and experiment
Authors: Wendt Jost O. L. , Linak William P. , Groff Paul W. , Srivastava Ravi K.
Published: April 2004

The hybrid process of homogeneous gas‐phase selective noncatalytic reduction (SNCR) followed by selective catalytic reduction (SCR) of nitric oxide (NO) was investigated through experimentation and modeling. Measurements, using NO‐doped flue gas from a gas‐fired 29 kW test combustor, provided data that allowed a fundamentally based catalyst monolith model, adapted from the literature, to be calibrated. The SCR model, which included pore and gas film diffusion (molecular and Knudsen) and Langmuir‐Hinshelwood surface reaction kinetics, used published values for the catalyst pre‐exponential factor and activation energy, and experimental measurements to determine the previously unavailable ammonia (NH3) adsorption coefficient on the catalyst surface. Then, with no further adjustment, a combined SNCR‐SCR model was developed using literature values for the reaction rate coefficients of 112 elementary homogeneous reactions. This combined model agreed well with combined SNCR‐SCR data. The combined model was then extrapolated to practical coal combustion conditions, and the economic benefits of a combined SNCR‐SCR process were explored. With a desired total NO reduction of 95% and NH3 slip in the exhaust constrained not to exceed 5 ppm, adding SNCR led to only modest decreases in the required SCR catalyst volume. Estimated system capital and annualized costs were not economically viable unless the SNCR portion by itself reduced more than 60% of the NO.
Coalescence analysis through small‐angle light scattering
Authors: Priore Brian E. , Walker Lynn M.
Published: April 2004

Flow‐induced coalescence in complex flow fields is studied using an in situ, real‐time small‐angle light scattering technique. Coalescence in a model polyisobutene–polydimethylsiloxane immiscible blend is triggered through step‐downs in the flow rate, creating a transition from a breakup‐controlled morphology to one controlled by coalescence. Inducing droplet breakup, by increasing the flow rate during the coalescence process, is a reliable probe of the structural changes occurring during the step‐down. The coalescence rate, duration of the transient structure, and final droplet size depend strongly on the blend viscosity ratio. Coalescence occurs at extremely dilute droplet concentrations, and an inverse relationship between concentration and time to steady state is observed. Models, developed for simple shear flow, qualitatively describe the coalescence and breakup processes, indicating their potential for predicting these phenomena in processing flows.
Experimental study of turbulent mixing in a tee mixer using PIV and PLIF
Authors: Pan Gang , Meng Hui
Published: April 2004

Turbulent scalar mixing in the near field of a tee mixer (transverse jet in a pipe) was studied using particle image velocimetry and planar laser induced fluorescence. Two jet‐to‐pipe velocity ratios r = 3.06 and r = 5.04 were investigated for a turbulent pipe flow at a Reynolds number of 20,850. Laser measurement techniques are described, typical flow structures in the near field of tee mixer are examined, and turbulence statistics and scalar statistics are calculated. The general behavior of mixing is analyzed from the jet expansion and the concentration decay along the jet centerline, and the flow condition for jet impingement on the pipe wall is estimated. The mixing mechanism in the near field is revealed from the variation of the scalar PDF across the jet. The mixing on the upstream side of the jet is dominated by large‐scale structures and on the downstream side by small eddies and turbulent diffusion, where the β‐PDF is an excellent approximation.
Drag and stability of objects in a yield stress fluid
Authors: Jossic Laurent , Magnin Albert
Published: April 2004

The drag force exerted on objects in a yield stress fluid was measured when the velocities become infinitely slow. In these quasi‐static conditions, yield stress effects are predominant. Particular care was paid to determining the yield stress and checking interface conditions when characterizing these fluids from a rheometric standpoint. Drag coefficients could then be determined for interesting objects of various shapes. The important role of fluid‐object interface effects was also highlighted. On the basis of these results, a stability criterion is proposed for the object in the fluid, to estimate the yield stress needed to balance buoyancy forces.
Simple model for power consumption in aerated vessels stirred by Rushton disc turbines
Authors: Paglianti A. , Takenaka K. , Bujalski W.
Published: April 2004

Accurate prediction of power consumption of impeller in an aerated vessel is crucial for a successful scale‐up, operation, and design of reactors in general. A mechanistic model with a correlation that can accurately predict power consumption was developed. Taking into account both geometrical parameters and physical properties of the working fluids, this model reduces the number of adjustable parameters better than published empirical relations. It has the advantage of allowing the user to easily scale up agitated vessels and to evaluate power consumption even for large‐scale equipment. It has been tested only for Rushton turbines, however, because it is simply based on mass balances, energy balances and continuity equations, it could also be implemented for use with other impellers with very few modifications.
Dispersion of fine powder agglomerates under microgravity
Authors: Pyatenko A. , Takeuchi H. , Chiba S. , Ohyama Y.
Published: April 2004

Dispersion is a very important step in powder handling. Microgravity environments were applied to study the process of powder dispersion with an orifice for low transportation gas velocities and for widely sized particle agglomerates. Particle‐size distributions were measured for particles after passing through the orifice, and the results were compared with the size distributions of primary particles. Data analysis showed that the main factor determining the dispersion efficiency is the inertial parameter. Two approaches are proposed for estimating the inertial parameter in a wide range of Reynolds numbers.