bluuebunny/crossref_metadata_2025 huggingface dataset:
Novel scaling parameter for circulating fluidized beds
Authors: Kehlenbeck Ralf , Yates John , Di Felice Renzo , Hofbauer Hermann , Rauch Reinhard
Published: April 2004

Experimental results of a one‐fifth scale cold laboratory model of a circulating fluidized‐bed pilot plant for biomass gasification are reported here. Measurements revealed that the solid circulation rate is a function of the superficial gas velocity in the riser and the total mass load in the system. A novel dimensionless scaling parameter introduced describes the dimensionless mass turnover. Experiments were carried out varying both particle diameter, 170 μm ≤ d ≤ 860 μm, and density, 1,480 kg/m3 ≤ ρs ≤ 8,900 kg/m3. An excellent prediction for the mass turnover was obtained for mass loads varying from 1 to 15 kg and throughout the whole particle size and density range using the equations derived. Furthermore, the particle‐size distribution is a scaling parameter to be matched between a model and an industrial plant.
Effect of chemical kinetics on feasible splits for reactive distillation
Authors: Chadda Nitin , Malone Michael F. , Doherty Michael F.
Published: April 2004

Feasible direct and indirect sharp splits for multicomponent single‐feed continuous reactive distillation are predicted with a model, in which each column section is represented by a series of cocurrent isobaric flashes. In the limits of no reaction and equilibrium chemical reaction, the model reduces to conventional models for distillation lines, and each column section can be represented by the same equations. At intermediate reaction rates, however, the models for the column sections differ, and new results for fixed points and feasible products are obtained. A bifurcation study shows the limits of feasibility, including the influence of flow rate, catalyst level and holdup. Unlike distillation without reaction, limited ranges of feasibility in all of these variables are found. The method has been applied to five examples, one of which is described in detail. Feasibility predictions are validated by column simulations.
Modular synthesis framework for combined separation/reaction systems
Authors: Ismail Soraya Rahim , Proios Petros , Pistikopoulos Efstratios N.
Published: April 2004

As an extension to our earlier work (1999a), this article presents a systematic approach for the synthesis of combined separation/reaction systems which involves a phenomena‐based representation formulated within a superstructure optimization framework. A multifunctional, mass‐/heat‐transfer‐based process module is utilized as the building block of process alternatives. Within this module, Gibbs‐free‐energy‐based “driving‐force” constraints are introduced to describe the physiochemical phenomena and to ensure feasibility of pure separation, reaction or combined separation/reaction (such as reactive distillation). A superstructure of these multipurpose modules is then systematically constructed to represent process alternatives, which are not explicitly prepostulated, but investigated, as mass and heat exchange network possibilities.
Designing a fast‐igniting catalytic converter system
Authors: Keith Jason M. , Chang Hsueh‐Chia , Leighton David T.
Published: April 2004

Through analysis and numerical simulation, the late lightoff (∼ 3 min) of the current automobile catalytic converter is attributed to a downstream ignition phenomenon that is unstable to flow‐rate fluctuations. A design is proposed which incorporates a stable and thermally efficient leading‐edge ignition. The key feature involves diverting a small portion of exhaust gas through a bypass stream, which contains an electric preheater and a preigniter, during startup. This allows the preheater to utilize a low power input using the existing battery and yet raise the gas temperature beyond a precise minimum that rapidly lights off the entire preigniter. The hot preigniter is then used to light off the main catalytic converter. Removal of the bypass after complete ignition preserves the preigniter catalyst, a major advantage. The optimal design is shown to lightoff within 10 s and reduce pollution by almost 90% over current designs and thus meet ultra low emission vehicle (ULEV) standards.
Simulation of turbulent precipitation in a semi‐batch Taylor‐Couette reactor using CFD
Authors: Marchisio D. L. , Barresi A. A. , Fox R. O.
Published: April 2004

A finite‐mode probability density function method was used with a computational fluid dynamics code to model the semibatch precipitation of barium sulfate from aqueous solutions of barium chloride and sodium sulfate in a Taylor‐Couette reactor. It was simulated in turbulent flow at high Taylor numbers under the assumption of axial symmetry. The flow field was modeled using two different turbulence models, and flow predictions were validated using experimental tracer dispersion measurements. The crystal‐size distribution (CSD) was calculated in terms of the first five moments considering both nucleation and growth, while neglecting the effect of aggregation. The effect of the operating conditions on the CSD was studied, and code predictions were compared with experimental data.
Simulations of gas‐liquid‐solid 3‐phase flow and reaction in FCC riser reactors
Authors: Gao Jinsen , Xu Chunming , Lin Shixiong , Yang Guanghua , Guo Yincheng
Published: April 2004

A 3‐D, three‐phase flow‐reaction model was developed to predict the performance of FCC riser reactors. This model incorporates the effect of feed vaporization into an earlier 3‐D, two‐phase flow‐reaction model (Gao et al., 1999). The synergistic effects of hydrodynamics, heat transfer, and feed vaporization on FCC reactions are elucidated. The results show that a complex phenomenon occurs in the zone up to 4m above the point at the bottom of the riser reactor where feed is injected. The changes in the spray droplet diameter, volumetric concentration and temperature of the feed in the feed injection zone are illustrated. The model was used as a design tool for an operation modification study. Results indicate that FCC reaction selectivity for the desired products (gasoline and light fuel oil) can be improved by reducing initial droplet diameter in the feed spray from 80 to 40 μm.
Mixed hydrates of methane and water‐soluble hydrocarbons modeling of empirical results
Authors: de Deugd R. M. , Jager M. D. , de Swaan Arons J.
Published: April 2004

The hydrate disappearance conditions (H − Lw − V → Lw − V transition) in the water–methane–water‐soluble hydrocarbon system were measured and modeled. The hydrocarbons in the experiments are tetrahydrofuran (THF), 1,3‐dioxolane and tetrahydropyran, and THF, 1,3‐dioxolane and acetone in the modeling study. Experiments and calculations, which correspond qualitatively and quantitatively very well, show a large reduction in the equilibrium pressure upon addition of the hydrocarbons. The minimum pressure is about 5–6 mol % hydrocarbon relative to water, independent of temperature and applied hydrocarbon, with the composition almost equal to the ratio between the number of large cages and water molecules in structure II hydrates (5.88%). Based on this observation, the hydrates are assumed to be structure II methane–hydrocarbon mixed hydrates. The model is also used to calculate the occupancies of the hydrate cavities. No experimental data are available to compare the predicted values, but the results help understand observed phenomena better.
Optimal carbon source switching strategy for the production of PHA copolymers
Authors: Mantzaris Nikolaos V. , Kelley Aaron S. , Srienc Friedrich , Daoutidis Prodromos
Published: April 2004

During polymerization in a nongrowing cell population of Ralstonia eutropha, alternating between two different carbon sources (fructose and fructose/valeric acid) could lead to the production of block copolymers consisting of blocks of homo‐poly‐3‐hydroxybutyrate (PHB) and polyhydroxybutyrate‐co‐valerate (PHBV) copolymer. The problem of finding the optimal number of carbon source switches and corresponding switching times that maximize the final concentration of diblock copolymers (PHB‐PHBV and PHBV‐PHB) was addressed. It was mathematically formulated in the mixed‐integer nonlinear programming (MINLP) framework, which allows the decomposition of the original problem into the primal and master problems. The primal problem corresponds to the original problem for a fixed number of carbon source switches, whereas the master problem consists of finding the number of carbon source switches that maximizes the optimum solutions of all possible primal problems. The global optimum was obtained for 39 carbon source switches. It corresponds to a mass fraction of 50.6% of final diblock copolymer concentration over the final total polymer concentration.
PIV study of small‐scale flow structure around a Rushton turbine
Authors: Sharp K. V. , Adrian R. J.
Published: April 2004

The structure of turbulent motions was measured in a region surrounding the blade tips of a Rushton turbine mixer. The tip vortices responsible for micromixing were created in this region, as was the radial jet that drives the large‐scale circulation responsible for macromixing. Measurements of instantaneous velocity fields in axial–radial planes showed large cycle‐to‐cycle variability relative to the mean. The average measurements of the azimuthal vorticity and the Reynolds stresses in the radial–axial plane showed the growth and motion of the tip vortices as they drift away from the blades. Radial and axial spatial derivatives were measured to assess various methods of computing the turbulent viscous dissipation. The small scales were significantly anisotropic at the Reynolds numbers studied.
Computational analysis of surface‐tension‐driven coating‐defect flow
Authors: Blunk Richard H. J. , Wilkes James O.
Published: April 2004

Bondline readout (BLRO) is a coating defect frequently exhibited on adhesively‐bonded, polymeric automotive body panels painted with high‐glamor/flow clearcoats. BLRO or telegraphing results from Marangoni‐type, surface‐tension‐driven flows. An efficient numerical code was developed to predict temperature‐ and concentration‐induced BLRO flows to use it as an effective tool for screening and developing potential clearcoat systems. Two numerical codes were developed: a 1‐D code based on the lubrication approximation; a 2‐D code based on the SIMPLER (Semiimplicit method for pressure‐linked equations revised) algorithm. The 1‐D code produced BLRO profiles in agreement with those predicted from the 2‐D code and those measured from BLRO flow experiments, thus confirming a proposed BLRO mechanism. Moreover, the 1‐D code was vastly more time‐efficient than the 2‐D code due to a severe time‐step stability limitation found with the latter.
Mass transfer across dilated interfaces
Authors: Tokarz Alexander , Millies Marco , Mewes Dieter
Published: April 2004

The mutual dependence of mass and momentum transport at fluid interfaces was studied theoretically and experimentally. Applying thermodynamics of irreversible processes to systems with interfaces showed, on a theoretical basis, that the dilation of the interface acts as a thermodynamic driving force for the mass transfer across the interface. Mass‐transfer rates and concentration profiles at the dilated interface between toluene and water were measured using holographic interferometry for various transfer components. Experimental study showed that mass‐transfer rates increase with increasing dilational rate, due solely to an enhanced convective transport in the continuous phases. The dilation does not affect the concentrations at the interface. It was demonstrated theoretically that the maximum possible alteration in the interfacial concentrations due to dilation was in the order of magnitude of the experimental accuracy of the interferometer. It was concluded that the thermodynamic driving force due to dilation is—if present—too small to be relevant to technical applications.
Representation of high‐dimensional solid‐liquid phase diagrams of ionic systems
Authors: Samant Ketan D. , Ng Ka M.
Published: April 2004

A generic framework is presented for representation of the solid‐liquid phase behavior of multicomponent ionic systems with and without compound formation. Isothermal isobaric phase behavior is represented in form of digraphs. The invariant points in the composition space are identified as vertices of the digraph. These vertices are connected by edges identified from the adjacency matrix, which is constructed by using a set of generic rules based on Gibbs phase rule. These edges form the boundaries of high‐dimensional saturation varieties, which are identified from the saturation variety matrix. For graphical representation, the number of independent coordinates is calculated and a suitable set of coordinates is developed. Projections are used to view high‐dimensional phase diagrams in their entirety and to simplify the analysis of the phase behavior. Potential applications of this framework include facilitation of experimental development and computer‐aided design of crystallization‐based separation processes.
Gypsum nucleation into sodium chloride solutions
Authors: Prisciandaro Marina , Lancia Amedeo , Musmarra Dino
Published: April 2004

The nucleation of gypsum into a NaCl solution was studied within a supersaturation range of 2–4 at 25–70°C. The experiments were carried out using a lab‐scale apparatus consisting of a batch crystallizer with the related measurement devices. Signals of scattered and transmitted light coming from a He–Ne laser source were analyzed to measure the induction period, that is, the time delay necessary for homogeneous nucleation to take place. Comparison of the experimental results with those previously obtained in the absence of added NaCl in solution showed that NaCl strongly affects gypsum precipitation. They were compared in terms of the activation energy and the interfacial tension between crystals and the surrounding aqueous solution.
Reactions in multiindexed continuous mixtures: Catalytic cracking of petroleum fractions
Authors: Peixoto F. C. , de Medeiros J. L.
Published: April 2004

The classic breakage problem is a population‐balance problem using first‐order kinetics and product distribution rules over a multidimensional continuum, mathematically expressed as linear integrodifferential equations for the multiindexed frequency distribution functions of entities. To model the catalytic cracking of petroleum fractions, population‐balance equations are solved through Galerkin formalism. It focuses on a simplified version of such equations, which was solved analytically through the method of characteristics. Kinetic and stoichiometric parameters were statistically estimated for both models from a set of pseudoexperimental data generated using available traditional lumped models. For this purpose, an arbitrary structural description for a continuous representation of the species also had to be developed.
Hydrogen bonding with aromatic rings
Authors: Brinkley Ray L. , Gupta Ram B.
Published: April 2004

Thermodynamic effects of hydrogen bonding are important in determining the phase behavior in polar fluids. While efforts to understand “strong” donor and “strong” acceptor combinations (H‐bonds involving a formation energy more negative than −20 kJ/mol such as alkanol‐alkanol H‐bonding have been substantial, weak H‐bonds involving double bonds or aromatic rings as proton acceptors) have largely been ignored. Using FTIR spectroscopy H‐bonding between alcohol donor and aromatic ring‐containing acceptor molecules was studied including l‐hexanol and cyclohexanol at low concentrations where self‐association is negligible (strong donors). H‐bonding “weak” acceptors studied include toluene and m‐xylene. Clear spectroscopic evidence existed for the formation of H‐bonds between the alcohol and aromatic molecules, which are much weaker than conventional “strong donor”–“strong acceptor” H‐bonds. Using quantitative FTIR measurements, the percentage of H‐bonded alcohol molecules over a range of aromatic concentrations was determined. Ab initio calculations also showed that alcohol–aromatic H‐bonds are much weaker than alcohol–alcohol H‐bonds. This H‐bonding, though weak, will contribute significantly to the chemical potential of the molecules.
Comparison of Hg0 capture efficiencies of three in situ generated sorbents
Authors: Lee Tai Gyu , Biswas Pratim , Hedrick Elizabeth
Published: April 2004

Three different sorbent materials (Ti‐, Si‐, and Ca‐based) were compared for their mercury (Hg0) capture efficiencies in an entrained flow reactor. Agglomerated particles with a high specific surface area were generated in situ by injecting gas‐phase sorbent precursors into a high‐temperature furnace reactor. Titania particles in the presence of UV irradiation were most effective at mercury capture (>98%). In situ generated CaO particles had a capture efficiency of 33% (without any UV irradiation), while SiO2 was completely ineffective at Hg0 capture. The efficiency of elemental Hg capture of both CaO and TiO2 particles decreased with increasing SO2 concentration. The increase in the feed rate of the Ti sorbent resulted in the recovery of the higher Hg0 capture efficiency due to the availability of more active sites. The in situ generated titania sorbents were also effective at Hg0 capture (>87%) at elevated temperatures (160°C).
Estimating polymer/solvent diffusion coefficient by optimization procedure
Authors: Doumenc Frédéric , Guerrier Béatrice
Published: April 2004

Solvent diffusion in polymer solutions is one of the main parameters that determine the drying kinetics of polymer films, especially in the concentrated domain. As is well known for polymer/solvent solutions, solvent diffusion coefficients decrease by several orders of magnitude when the solvent concentration decreases. Direct measurements of these coefficients are complex and time‐consuming, and most often dedicated to well‐characterized “model” systems. In this study, another approach is developed that is based on simple gravimetric experiments coupled with the modelization of the drying kinetics and an optimization iterative procedure. The sensitivity of the method to various errors (measurement errors and uncertainties on a priori known parameters) is thoroughly investigated. The method accurately estimates diffusion coefficients on a large concentration domain, provided a suitable experimental strategy is used.
An engineering approach to an unsolved problem in multicomponent diffusion
Authors: Nauman E. Bruce , Savoca Joseph
Published: April 2004

Both practical and conceptual problems arise in standard theories of multicomponent diffusion. They include the sometimes arbitrary and even artificial selection of one component as the solvent, the lack of explicit dependence on the properties of the solvent, and the difficulty in formulating compositionally dependent diffusion coefficients that satisfy material‐balance constraints. Two alternative approaches based on tracer diffusion coefficients for various components are physically reasonable and overcome these difficulties. Both approaches predict cross‐diffusion, that is, diffusion of component A as a result of a gradient in component B. Either approach can be used in systems where component diffusivities differ greatly in magnitude and where the number of chemical species is large, as in polymerizations.
Gas–solid fluidization in a centrifugal field
Authors: Qian Gui‐Hua , Bágyi István , Burdick Ian W. , Pfeffer Robert , Shaw Henry , Stevens John G.
Published: April 2004

The nature of fluidization and the behavior of particles in a centrifugal field are discussed. Powders are classified into four groups (A, B, C, and D) following Geldart based on their fluidization behavior. A simple general model, agreeing with conventional fluidized beds and usable for variable “g,” is proposed to determine the transition boundary between group A and C particles. The general model for conventional fluidized beds by Foscolo and Gibiloro was used to determine the boundary between group A and B particles, and the semiempirical equation of Molerus for the boundary between group B and D particles in a centrifugal field. Theoretical analysis showed that group A particles can shift to group B, and group C particles can shift to group A under a centrifugal force. Therefore, certain group C particles can be fluidized in rotating fluidized beds. For very high “g,” however, such particles shift to group D and cannot be fluidized. Experiments in a horizontal rotating fluidized bed under different “g” forces to check the theoretical analysis confirmed that Geldart group C particles (7 μm alumina), which cannot be fluidized in a conventional fluidized bed, can fluidize in a rotating fluidized bed operating at a sufficiently high rotating speed to shift them into group A or B. Similarly, in a series of experiments in a rotating fluidized bed, group A particles behaved as group B.
Suppression of gas separation membrane plasticization by homogeneous polymer blending
Authors: Bos A. , Pünt I. , Strathmann H. , Wessling M.
Published: April 2004

Plasticization is a phenomenon frequently encountered in the application of glassy polymeric materials for solution–diffusion membranes. Conventional methods for stabilizing the membrane are either annealing or cross‐linking, which hardly influence the selectivity of the membrane, but decrease the permeability. For single‐gas experiments, the literature shows that plasticization can be stabilized by blending a polymer with high plasticization tendencies with one that is hardly affected by the sorbed molecules. Most permeation experiments are carried out with pure CO2, but little is known about the transport properties determined from mixed gas experiments. Stabilization of plasticization in mixed‐gas experiments for polymer blends of the polyimide Matrimid and polysulfone is reported, as well as transport properties of a new homogeneous polymer blend based on Matrimid and the copolyimide P84. Experimental results show that the material is stabilized against carbon dioxide plasticization and selectivity for a carbon dioxide/methane mixture significantly improves.
Contributions to electrostatic interactions on protein transport in membrane systems
Authors: Burns Douglas B. , Zydney Andrew L.
Published: April 2004

Several recent studies have demonstrated the importance of electrostatic interactions on the transport of charged proteins through porous membranes. The objective of this study is to compare predictions of available hydrodynamic and partitioning models with experimental data for the sieving coefficient of ovotransferrin through both positively and negatively charged membranes over a range of solution conditions. Model calculations were performed with all parameters evaluated from independent experimental measurements. The membrane pore‐size distribution was determined from dextran sieving data, and the membrane surface charge was evaluated from streaming potential measurements. The model properly predicts the complex effects of solution pH and ionic strength on protein transmission in the presence of both attractive and repulsive electrical interactions. Model simulations were performed to highlight the key contributions to protein transport.
Concentration polarization in stirred ultrafiltration cells
Authors: Johnston Scott T. , Smith Kenneth A. , Deen William M.
Published: April 2004

Laminar boundary‐layer theory was used to quantify concentration polarization in ultrafiltration systems in which the membrane forms the base of a stirred, cylindrical container. The flow was approximated as a rigid‐body rotation above a stationary surface (Bödewadt flow), with a filtration velocity dependent on the osmotic pressure of the retained solute, and therefore varying with radial position on the surface. Because the analysis was limited to moderate solute concentrations and filtrate velocities, physical properties were assumed to be constant. The axisymmetric convective‐diffusion problem was solved by a finite difference method. Boundary‐layer results were compared with predictions based on the common assumption of a uniform stagnant film and with a hybrid approach of applying locally the stagnant‐film relationships but allowing the film thickness to vary with position. In general, both of the approximate models underestimated polarization effects, with the hybrid approach yielding smaller errors. In one set of simulations on the osmotic reduction in filtration rate caused by a completely retained solute, predicted errors in filtration rates with the models were moderate, with a maximum discrepancy of 21% (for the stagnant‐film model). In the second set of simulations concerning polarization effects on apparent sieving coefficients for permeable solutes, the error in the predicted ratio of true‐to‐observed sieving coefficient was as much as 78% with the stagnant‐film approach, while with the hybrid model it was no more than 15%. Thus, the stagnant‐film approach was much less satisfactory for inferring sieving coefficients than for predicting mean filtrate velocities. The predictive capability of the boundary‐layer model was tested using previous filtration data for bovine serum albumin in two ultrafiltration cells. The agreement was excellent, provided that an appropriate value was selected for the angular velocity of the bulk fluid.
Dean vortex stability using magnetic resonance flow imaging and numerical analysis
Authors: Mallubhotla Hanuman , Belfort Georges , Edelstein William A. , Early Thomas A.
Published: April 2004

Magnetic resonance flow imaging using flow encoding with spin warp imaging was used in three dimensions to measure velocity profiles and the dynamic behavior of centrifugally‐induced (Dean) vortices in curved tube flow. Experimental measurements were compared with numerical simulations obtained from the solution of Navier‐Stokes and continuity equations using a commercial finite‐element package. Effects of flow rate and geometry, and the ratio of the tube radius to that of curvature on the stability of Dean vortices were studied. Twisting and bifurcation of vortices increased with increasing flow rate and radius ratio. A six‐vortex pattern was measured experimentally and predicted numerically. Additional wall shear rates, due to Dean vortices, were estimated from velocity measurements in the cross‐sectional plane. A phase diagram was constructed to establish conditions for the existence of two, four or six vortices as a function of flow rate and curvature. Experimental observations were compared with numerical results obtained from three types of finite‐element grids. The full‐tube grid without symmetry planes was most predictive for vortex bifurcation, while the pseudo‐cylindrical full‐tube grid with a plane of symmetry gave best results for shear rates. Results from the numerical analysis agreed qualitatively well with the MRI measurements.
Mixed‐gas adsorption
Authors: Siperstein Flor R. , Myers Alan L.
Published: April 2004

The prediction of multicomponent adsorption equilibria from single‐component data is one of the most challenging and important problems in adsorption. The chief obstacle to progress is a scarcity of accurate and consistent experimental data over a wide range of temperature and loading for testing theories. Several binaries and one ternary system on two types of zeolites (silicalite and faujasite) were studied in a combination calorimeter‐volumetric apparatus. Activity coefficients and excess functions for enthalpy, free energy, and entropy were extracted from the binary data using a three‐constant equation to represent nonidealities. The successful correlation of binary excess functions with pure‐component properties for type I isotherms on zeolites is a first step toward predicting multicomponent adsorption from single‐gas adsorption and a major advancement over the theory of ideal adsorbed solutions.
Microkinetic models and dynamic Monte Carlo simulations of nonuniform catalytic systems
Authors: Dooling David J. , Broadbelt Linda J.
Published: April 2004

The combined insight gained from laboratory and computational experiments has historically provided detailed information about the complex interactions present in many heterogeneous catalytic systems. Although this information has been effectively used to interpret and explain experimental observations, it unfortunately has not, in general, been incorporated into kinetic models of the reacting systems. In particular, catalytic nonuniformity is one such phenomenon which is often ignored when kinetic models are constructed despite there being much experimental and computational evidence indicating its presence. Catalytic nonuniformity has been therefore incorporated into both microkinetic models and dynamic Monte Carlo simulations of catalytic reacting systems, and the performance and applicability of each solution method has been investigated for several model systems exhibiting different causes of nonuniform behavior.
Thermodynamic modeling of binary and ternary adsorption on silica gel
Authors: Steffan Dirk G. , Akgerman Aydin
Published: April 2004

Qualitative and quantitative characterization and prediction of multicomponent adsorption equilibria are essential for design of environmental remediation processes such as soil vapor extraction, bioventing, and thermal desorption. Single component, binary‐ and ternary‐mixture isotherms on silica gel were measured by the frontal analysis chromatography technique. Binary organic mixtures were composed of environmentally relevant VOCs, hexane/benzene (nonpolar/nonpolar), and hexane/trichloroethylene (nonpolar/slightly‐polar). Water binaries were water with hexane, benzene, or trichloethylene. Organic mixtures exhibited enhanced adsorption, where the adsorbed amount for each component was increased due to the second component. Water/hexane and water/benzene mixtures showed noncompetitive adsorption, where water adsorbed onto polar sites and n‐hexane or benzene adsorbed onto nonpolar sites. Competition for polar adsorption sites was observed for the trichloroethylene adsorption in water. Adsorption data of n‐hexane/benzene/water showed that n‐hexane and water were independently adsorbing onto different adsorption sites. Furthermore, n‐hexane and benzene with water showed the same adsorption behavior as without water. For the mixture of n‐hexane/trichloroethylene/water, only the amount of water adsorbed was reduced compared to the pure species adsorption of water. The adsorption equilibria of n‐hexane and trichloroethylene were not influenced in comparison to their pure species isotherms. Mixture isotherms were modeled with IAST, MSAM, and RAST. The nonideal adsorbed phase was characterized using activity coefficients.
Separating solvation from molecular diffusion in polymers
Authors: Elabd Yossef A. , Barbari Timothy A.
Published: April 2004

The diffusion of methyl ethyl ketone (MEK) in two vinyl alcohol/vinyl butyral (VA/VBu) copolymers (11‐ and 19‐wt. % VA) was studied at low vapor activities using Fourier transform infrared‐attenuated total reflectance (FTIR‐ATR) spectroscopy. MEK has the ability to interact through hydrogen bonding to sites in the polymer and was chosen to study the effect of penetrant–polymer solvation on molecular diffusion. The assumption of local equilibrium was verified by examining the time‐evolved concentrations of hydrogen‐bound and free MEK determined from the carbonyl (C = O) stretching bands. A mathematical model that accounted explicitly for solvation during the diffusion process was developed. Solvation hindered the diffusion of MEK in the VA/VBu copolymer by factors of 2.0 (11‐wt. % VA) and 2.6 (19‐wt. % VA). After separating solvation from diffusion, true diffusion coefficients of MEK were compared to those of methylene chloride (CH2 Cl2 ), a noninteracting penetrant of similar size to MEK. Within experimental error, true diffusion coefficients for MEK were the same as those for CH2 C12 over the concentration range studied.
CFD modeling of gas‐fluidized beds with a bimodal particle mixture
Authors: van Wachem B. G. M. , Schouten J. C. , van den Bleek C. M. , Krishna R. , Sinclair J. L.
Published: April 2004

A computational fluid dynamics model was developed for gas‐solid fluidized beds containing a mixture of two particle species. To calculate stresses of the solid phase, the kinetic theory of granular flow was extended to consider a binary mixture of smooth, nearly elastic, spheres. The developed model was simulated to demonstrate key features of binary mixture fluidization. Bed expansion with a binary mixture of different size particles, but with identical densities, was much higher than that of a system consisting of mono‐sized particles of the same mean size as the bimodal mixture. Minimum fluidization velocity for the binary particle system was significantly lowered. The mixing behavior of the binary mixture of particles, characterized by the mixing index, increased with increasing superficial gas velocity. For a binary mixture of particles of larger size with lower density and smaller size with higher density, larger, lighter particles segregated to the top of the fluid bed, while smaller, heavier particles segregated to the bottom. With increasing fluidization velocity, this segregation pattern reversed and “inversion” occurred. The drag and gravity force difference between small, heavy particles and large, light particles was dominant at low gas velocities. With an increase in gas velocity, however, the gradients in granular temperature and pressure became dominant terms in the equations for the relative force and thus velocity between two different particle species.
Stochastic modeling of particle coating
Authors: Alonso M. , Alguacil F. J.
Published: April 2004

Dry coating of powders, in forming a layer, consists of fine particles of one component onto the surface of coarser particles of another component. In the process, fine particles are transferred among colliding coarse particles (carriers) until a steady‐state distribution of fines on the carriers surface is attained. A stochastic model was developed for the kinetics of fines transfer based on a birth‐death population balance including theoretically‐derived one‐step transition probabilities. First, the population balance equation is solved under steady‐state conditions leading to the result that, at equilibrium, the number of fines per carrier follows a Bernoulli distribution. Based on the obtained equilibrium distribution, an approximate transient solution proposed agrees fairly well with the numerical solution of the birth‐death equation. Model predictions were compared qualitatively with earlier experimental results.
Flow pattern transition in a large jetting fluidized bed with double nozzles
Authors: Guo Qingjie , Tang Zhi , Yue Guangxi , Liu Zhenyu , Zhang Jiyu
Published: April 2004

An experimental study was conducted in a 500 mm ID, 8,000 mm high jetting fluidized bed with double vertical nozzles using frame‐by‐frame analysis of experimental videos. Three different flow patterns (separated jets, flow transition, and jet coalescence), with the two jets always coalesce within the penetration depth, were first observed in the jetting fluidized bed. Radial profiles in the fluidized bed were measured with a PC‐4 optical fiber probe. Effects of jet gas velocity and nozzle distance on the radial and axial voidage distribution were studied. The bed hydrodynamic behavior was characterized using a deterministic chaotic theory to analyze effects of jet gas velocity, static bed height, and nozzle distance on the correlation dimension, which increased with increasing jet gas velocity and static bed height.
Performance assessment of constrained model predictive control systems
Authors: Ko Byung‐Su , Edgar Thomas F.
Published: April 2004

A constrained minimum variance controller is derived based on a moving horizon approach that explicitly accounts for hard constraints on process variables. A procedure for the performance assessment of constrained model predictive control systems is then developed based on the constrained minimum variance controller. The performance bound computed using the proposed moving horizon approach converges to the unconstrained minimum variance performance bound when the constraints on process variables become inactive. The utility of the proposed method in the performance assessment of constrained model predictive control systems is demonstrated through a simulated example.
Distributed models in plantwide dynamic simulators
Authors: Martinson W. S. , Barton P. I.
Published: April 2004

Modeling support for dynamic simulation of chemical‐process flowsheets, which is of significant value for plantwide dynamic simulation using differential–algebraic model formulations, is to date very limited when one or more unit models include partial differential equations. Several new techniques that provide modeling support for such simulations are presented. These techniques are based on a generalized characteristic analysis and a differentiation index analysis of partial differential–algebraic models. They can be used to uncover systems that cannot be solved as part of a dynamic simulation, and to determine whether or not the initial and boundary conditions supplied by the modeler form a well‐posed problem. In a network flow context, they can further be used to select, enforce, and adapt the boundary conditions as required to maintain automatically a mathematically well‐posed problem. Each of these provides time‐saving support to the system modeler.
Design of fuel additives using neural networks and evolutionary algorithms
Authors: Sundaram Anantha , Ghosh Prasenjeet , Caruthers James M. , Venkatasubramanian Venkat
Published: April 2004

It is difficult and challenging to design high‐performance fuel additives in an industrial‐design setting where data are sparse and noisy, and fundamental knowledge is often limited. An automated framework is presented for the design of such fuel‐additive molecules that minimize the intake‐valve deposit in the automobile. A hybrid model that combined functional descriptors from a first‐principles degradation model with a statistical/neural‐network model was developed to predict additive performance, given the additive structure. The results of the predictive model are discussed for different real industrial case studies. An evolutionary method using specialized representation and constrained operators to enforce formulation constraints was used to generate optimal additive molecules that meet desired performance criteria. The evolutionary design strategy in combination with the hybrid prediction model was successful in identifying novel additive molecules that also possess good synthesis potential.
Verification of scaling laws for degrading PLA‐b‐PEG‐b‐PLA hydrogels
Authors: Metters Andrew T. , Bowman Christopher N. , Anseth Kristi S.
Published: April 2004

Several macroscopic properties of degrading poly(lactic acid)‐b‐poly(ethylene glycol)‐b‐poly(lactic acid) (PLA‐b‐PEG‐b‐PLA) hydrogels were compared to one another as functions of degradation time. The specific hydrogel properties measured were the compressive modulus (K), the volumetric swelling ratio (Q), and the normalized compressive modulus (KNORM = KQ1/3). Their comparisons were used to evaluate the applicability of fundamental thermodynamic theories developed for nondegradable hydrogels on a degradable system. The equations predict scaling factors of −2, −5/3, and 6/5 for the dependence of K on Q, KNORM on Q, and K on KNORM, respectively. These important experimental data for a degrading hydrogel system initially show good agreement with the scaling predictions. Further examination, however, reveals the scaling factors to change over the course of the degradation process. These changes are attributed, at least in part, to the changing chemical, mechanical, and ionic properties of the degrading copolymer networks.
Application of sieve‐tray air strippers to the treatment of surfactant‐containing wastewaters
Authors: Kibbey Tohren C. G. , Pennell Kurt D. , Hayes Kim F.
Published: April 2004

Using sieve‐tray air stripping as a means of removing contaminants from surfactant solutions was studied to determine if tray air stripping can be applied successfully to treating surfactant‐containing wastewaters and, if so, to develop a quantitative approach for designing tray air‐stripping systems for treating surfactant‐containing wastewaters. High‐concentration surfactant solutions were prepared and contaminated with five different volatile organic compounds. Removal of the organic compounds was evaluated using a commercial sieve‐tray air stripper. Overall, the air stripper was able to efficiently remove the contaminants from solution, and correction of Henry's Law constants for micellar solubilization‐induced volatility reduction allowed tray air‐stripper design equations to quantitatively predict air‐stripper removal for all contaminants. Foaming within the air stripper could be controlled with low concentrations of a silicone‐polymer antifoaming agent. Although adding antifoaming agents may be undesirable for some applications, other applications can benefit from the operational and performance advantages of sieve‐tray air strippers. Issues related to air‐stripper design for low‐volatility compounds are also discussed.
Validation of a new filtration technique for dewaterability characterization
Authors: Usher Shane P. , De Kretser Ross G. , Scales Peter J.
Published: April 2004

Characterization of the compressibility and permeability of flocculated suspensions is a time‐consuming experimental process that often takes days to perform. A new pressure filtration method developed characterizes a sample only in hours. Using stepped pressure filtration, the compressive yield stress Py (ϕ) and a hindered settling function R(ϕ) of flocculated suspensions are determined as a function of the solids volume fraction ϕ. Traditional pressure filtration experiments involve the pressure filtration of a particulate suspension in which the time of filtration t is monitored as a function of piston height from which the specific volume of filtrate V is determined. Py (ϕ) is determined from the equilibrium solids volume fraction. The gradient d(t/V)/dV is traditionally used to determine R(ϕ) using a suspension filtration theory developed by Landman and White. The new method uses only one stepped pressure compressibility filtration test and one stepped pressure permeability filtration test to determine Py (ϕ) and R(ϕ) for multiple solids volume fractions and substantially decreases the time required for sample characterization. To eliminate the influence of a stepped pressure on calculated permeabilities, the analysis of experimental results was modified using the gradient dt/dV2. Results are presented for a zirconia suspension comparing both single and stepped pressure filtration.
Detection and identification of faulty sensors in dynamic processes
Authors: Qin S. Joe , Li Weihua
Published: April 2004

A novel method proposed detects and identifies faulty sensors in dynamic systems using a subspace identification model. A consistent estimate of this subspace model was obtained from noisy input and output measurements by using errors‐in‐variables subspace identification algorithms. A parity vector was generated, which was decoupled from the system state, leading to a model residual for fault detection. An exponentially weighted moving average (EWMA) filter was applied to the residual to reduce false alarms due to noise. To identify faulty sensors, a dynamic structured residual approach with maximized sensitivity is proposed which generates a set of structured residuals, each decoupled from one subset of faults but most sensitive to others. All the structured residuals are also subject to an EWMA filtering to reduce the noise effect. Confidence limits for filtered structured residuals were determined using statistical inferential techniques. Other indices like generalized likelihood ratio and cumulative variance were compared to identify different types of faulty sensors. The fault magnitude was then estimated based on the model and faulty data. Data from a simulated 4 × 4 process and an industrial waste‐water reactor were used to test the effectiveness of this method, where four types of sensor faults, including bias, precision degradation, drift, and complete failure, were tested.
Kinetics for hydrocracking based on structural classes: Model development and application
Authors: Martens G. G. , Marin G. B.
Published: April 2004

A kinetic model for hydrocracking of an industrial feedstock, fully incorporating the carbenium ion chemistry, was developed. Individual hydrocarbons in the reaction network were relumped into 8 lumps per carbon number: n‐alkane, mono‐, di‐ and tribranched alkanes, mono‐, di‐, tri‐ and tetraring cycloalkanes. The rate coefficient of a reaction in the relumped network resulted from the product of the rate coefficients of the elementary reaction steps with lumping coefficients. The former were obtained from regressions on gas‐phase hydrocracking data of model components. Lumping coefficients were calculated based on the assignment of all (cyclo)alkanes and the corresponding carbenium ions to structural classes comprising species with identical thermodynamic properties. The simulation of an industrial reactor with vacuum gas‐oil feed revealed the relative unimportance of transfer limitations for hydrocracking of saturated hydrocarbons and the model's ability to detail the influence of process conditions on the product composition.
Microfabricated packed‐bed reactor for phosgene synthesis
Authors: Ajmera Sameer K. , Losey Matthew W. , Jensen Klavs F. , Schmidt Martin A.
Published: April 2004

A silicon micropacked‐bed reactor for phosgene synthesis is demonstrated as an example of the potential for safe on‐site/on‐demand production of a hazardous compound. Complete conversion of chlorine is observed for both a 2:1 CO/Cl2 feed at 4.5 std. cm3/min and a 1:1 feed at 8 std. cm3/min. The latter gives a projected productivity of ∼100 kg/yr from a 10‐channel microreactor, with the opportunity to produce significant quantities through operating many reactors in parallel. The versatility of silicon microfabrication technology for producing reactors for corrosive gases is demonstrated by a protective oxide coating formed during reactor fabrication. The increased heat and mass transfer inherent at the submillimeter reactor length scale provides a larger degree of safety, control, and suppression of gradients than is available in macroscale systems. These advantages are also explored in the extraction of chemical kinetics from microreactor experiments. The preexponential factor and apparent activation energy for phosgene formation are determined demonstrating the utility of micropacked‐bed reactors as laboratory research tools.
Theoretical calculation of polymer deposition thickness on a cylindrical substrate
Authors: Shiflett Mark B. , Foley Henry C. , Yokozeki Akimichi
Published: April 2004

A theoretical model developed predicts the thickness of a polymer film, coated on a rotating and translating cylindrical support. A model for the deposition process can be used to determine optimal operating conditions. It was validated experimentally and shows that the film‐thickness morphology is quite sensitive to certain operating parameters. For example, particular sets of operating parameter values can produce highly nonuniform film thicknesses. Selected examples of the model calculations are discussed, as well as their implications.