bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability calculations for nonequilibrium separation processes
Authors: Sridhar Lakshmi N. , Torres Monica
Published: April 2004

Two of the most significant contributions in the last two decades to separation process are the numerical implementation of the tangent‐plane criterion to check the stability of any given mixture and the use of mass‐transfer models to solve separation‐process problems. This work combines the use of the tangent‐plane analysis with mass‐transfer models to solve problems with multiple liquid phases. As in equilibrium‐stage problems, the tangent‐plane analysis works effectively with nonequilibrium‐stage separation‐process problems. An example is presented where the nonequilibrium model, when used in conjunction with the tangent‐plane criterion, predicts the correct number of phases, while the equilibrium model predicts the incorrect number of phases.
Finite‐element methods for steady‐state population balance equations
Authors: Nicmanis M. , Hounslow M. J.
Published: April 2004

A finite‐element algorithm is developed to solve the population balance equation that governs steady‐state behavior of well‐mixed particulate systems. Collocation and Galerkin methods are used to solve several test problems in which aggregation, breakage, nucleation and growth (and combinations of these phenomena) occur. It is shown that the Galerkin method must be used in growth problems to obtain a well‐conditioned system. In all the cases investigated, density distributions and their moments are accurately predicted by the method. In a direct comparison with the discretized population balance (DPB) of Litster et al. (1995) the finite‐element method proves capable of predictions that are typically two orders of magnitude more accurate than those of the DPB. These results were obtained using smaller systems of equations and with considerably less computational power.
Catalytic liquid‐phase nitrite reduction: Kinetics and catalyst deactivation
Authors: Pintar Albin , Berčič Gorazd , Levec Janez
Published: April 2004

Liquid‐phase reduction using a solid catalyst provides a potential technique for the removal of nitrites from waters. Activity and selectivity measurements were performed for a wide range of reactant concentrations and reaction conditions in an isothermal semi‐batch slurry reactor, which was operated at temperatures below 298 K and atmospheric pressure. The effects of catalyst loading and initial nitrite concentration on the reaction rate were also investigated. The Pd monometallic catalysts were found to be advantageous over the Pd‐Cu bimetallic catalyst with respect to either reaction activity or selectivity. Among the catalysts tested, minimum ammonia formation was observed for the Pd(l wt. %)/γ‐Al2O3 catalyst. The proposed intrinsic rate expression for nitrite disappearance over the most selective catalyst is based on the steady‐state adsorption model of Hinshelwood, which accounts for a dissociative hydrogen adsorption step on the catalyst surface and an irreversible surface reaction step between adsorbed hydrogen species and nitrite ions in the Helmholtz layer. Both processes occur at comparable rates. An exponential decay in the activity of Pd(l wt. %)/γ‐Al2O3 catalyst has been observed during the liquid‐phase nitrite reduction. This is attributed to the catalyst surface deprotonation, which occurs due to the partial neutralization of stoichiometrically produced hydroxide ions with carbon dioxide.
Radiation perpendicular to the symmetry axis of prolate and oblate cavities
Authors: Fitzgerald Sean P. , Strieder William
Published: April 2004

Thermal radiation heat transport within prolate and oblate ellipsoidal cavities was examined. The axisymmetric anisotropy of the cavity shape gives rise to a thermal radiation conductivity tensor with principal axes components parallel (λr, ‖) and perpendicular (λr, ⊥) to the symmetry axis. The prolate (λr, ⊥) and oblate (λr, ⊥) transverse components are calculated and compared with well‐known results from the kinetic theory of transport across cylindrical and within slit void geometries. The use of λr, ⊥ and λr, ⊥, along with earlier results for λr, ‖ and λr, ‖, in well‐known effective conductivity equations for spheroidal inclusions within a solid matrix, provides a means to rigorously treat cavity orientation and shape in high‐temperature heat transport across porous materials. To facilitate the calculations and produce readily usable equations, a variational principle is used.
Liquid backmixing in bubble columns and the axial dispersion coefficient
Authors: Degaleesan Sujatha , Duduković Milorad P.
Published: April 2004

Liquid backmixing in bubble columns is commonly described using the 1‐D axial dispersion model, wherein all the contributing mechanisms leading to liquid macromixing are lumped into a single axial dispersion coefficient. No relationship has been reported that allows the quantification of various fluid‐dynamic contributions to the axial dispersion coefficient in bubble columns. In this study, a Taylor‐type analysis was performed to arrive at an expression for the axial dispersion coefficient in terms of the two dominant fluid‐dynamic factors, convective recirculation and eddy diffusion. Experimental measurements of the relevant fluid‐dynamic parameters are used to study the effects of operating conditions on the axial dispersion coefficient and its contributing factors.
Practical model for compressive cake filtration including fine particle invasion
Authors: Civan Faruk
Published: April 2004

Practical and improved ordinary differential models for compressible filter cake buildup involving small particle invasion and deposition inside the cake at static and dynamic conditions are developed by averaging particle and carrier fluid transport equations over the filter cake thickness. The models are simplified by considering that the panicles and carrier fluids involving many practical applications are incompressible. The results of radial and linear filtration cases are compared for constant rate and constant pressure drive filtrations. This thickness‐averaged ordinary differential model reproduces the predictions of the Tien et al. (1997) partial differential model rapidly with less computational effort. Parametric studies indicate that the particle screening efficiency of the filter is an important factor on the filter cake properties and filtration rate, and differences between linear and radial cake filtration performances are more pronounced, and the cake thickness and filtrate volume are smaller for constant pressure filtration than constant rate filtration. The equations of the thickness‐averaged linear and radial filter cake formation models are simple requiring less computational effort over the reported partial differential models for the analysis, design and optimization of the industrial cake filtration processes involving plate and drum filters.
FTIR measurements of acid‐gas–methyldiethanolamine systems
Authors: Rogers William J. , Bullin Jerry A. , Davison Richard R.
Published: April 2004

The standard industrial process for the purification of natural gas is to remove acid gases, mainly hydrogen sulfide and carbon dioxide, by the absorption and reaction of these gases with alkanolamines, but the lack of reliable and accurate vapor–liquid equilibrium (VLB) data impedes the commercial application of more efficient alkanolamine systems. A novel Fourier‐transform infrared (FTIR) technique was developed to make in‐situ VLE measurements of acid‐gas‐aqueous alkanolamine systems and to improve the accuracy of VLE measurements at low hydrogen sulfide and carbon dioxide concentrations. VLE measurements of low carbon dioxide and hydrogen sulfide concentrations in aqueous mixtures of methyldiethanolamine (MDEA) are reported using the new FTIR technique.
Influence of H2O2 on a chloride‐dependent reaction path to chlorine dioxide
Authors: Crump Brian , Ernst William R. , Neumann H. M.
Published: April 2004

Reaction paths governing chlorine dioxide formation from solutions of sodium chlorate, sulfuric acid, hydrogen peroxide and sodium chloride at 25°C were examined. Chlorine dioxide was formed by two paths: the reduction of chlorate by hydrogen peroxide and the reduction of chlorate by chloride. At conditions of this study, the rate of the chloride‐chlorate path predominated and was enhanced by hydrogen peroxide, reaching an upper limit as hydrogen peroxide concentration was increased. A mechanism was proposed that explains the observed behavior. It is based on existing theory, but includes a new step: the rapid reaction of hydrogen peroxide with an intermediate, Cl2O2. It was shown also that chloride was not required to sustain chlorine dioxide formation in the hydrogen peroxide‐chlorate path.
Evaluation of dialysis membranes using a tortuous pore model
Authors: Kokubo Ken‐ichi , Sakai Kiyotaka
Published: April 2004

The pore‐size distribution and diffusive permeability of homogeneous cellulosic dialysis membranes were measured by thermoporometry and by radioisotope and photoabsorption techniques, respectively. A tortuous pore model incorporating membrane pore‐size distribution, in which tortuosity differs with pore size, can predict diffusive permeability for a wide range of solute molecular weights. The tortuosity obtained using the tortuous pore model by fitting the experimentally obtained diffusive permeability varied with molecular weight (Stokes radius). To evaluate the physical meaning of the tortuosity varied with solute Stokes radius, the tortuosity of the tortuous pore model was correlated to permeability parameters of the friction model. Tortuosity is represented by a ratio of intramembrane friction coefficients for solute and water between actual and hypothetical membrane having isodiametric pores for which the straight pore model is valid. The change in the ratio with the solute is attributable to the pore‐size distribution.
Stability and multiplicity approach to the design of heat‐integrated PFR
Authors: Bildea Costin S. , Dimian Alexandra C.
Published: April 2004

The nonlinear behavior of the heat‐integrated plug‐flow reactor, consisting of a feed‐effluent heat exchanger (FEHE), furnace, adiabatic tubular reactor, and steam generator is studied, considering a first‐order, irreversible, exothermic, adiabatic reaction. Bifurcation theory is used to analyze the relationships among design, reaction thermodynamics and kinetics, and state multiplicity and stability. Hysteresis, isola and boundary‐limit varieties are computed, and the influence of the activation energy, reaction heat and FEHE efficiency on the multiplicity region is studied. The double‐Hopf and double‐zero bifurcation points divide parameter space in regions with different dynamic behavior. State multiplicity, isolated branches, and oscillatory behavior may occur for realistic values of model parameters. A design procedure is proposed to ensure a desired multiplicity pattern and a stable point of operation and to avoid high sensitivity. The procedure was applied to three reaction systems with different kinetic and thermodynamic characteristics.
3‐D modeling of SCR of NOx by NH3 on vanadia honeycomb catalysts
Authors: Roduit B. , Baiker A. , Bettoni F. , Baldyga J. , Wokaun A.
Published: April 2004

A novel scheme for simulating the selective catalytic reduction of NO with NH3 on a honeycomb‐type catalyst was developed based on a 3‐D model. New insight was gained by considering such modeling aspects as: correct geometry (square ducts); hydrodynamic entrance effects; occurrence of adsorption phenomena and side reactions; NH3/NO ratio in the feed; influence of the reactant (NO, NH3, O2, H2O) concentrations; interphase diffusion and interphase mass‐transfer processes; and a large range of operating conditions (temperature 140–475°C and gas hourly space velocity 14,800–73,900 h−1). These aspects proved to be crucial for interpreting and quantifying the experimental results and optimizing the catalytic reactor processes. The 3‐D model was validated by comparing manufacturer's set point and experimental data to calculated data, and the external mass‐transfer coefficients derived from the concentration gradients to the mass‐transfer correlations available in the literature.
Radiation heat transfer in catalytic monoliths
Authors: Boehman André L.
Published: April 2004

Numerical simulations of heat‐ and mass‐transfer and heterogeneous reactions in catalytic monoliths are reported with the focus on the influence of radiation heat transfer on the thermal behavior of the monolith. Appropriate heterogeneous kinetics and boundary conditions were calculated for two cases including: an automobile catalytic converter in which carbon monoxide is oxidized over a platinum (Pt) catalyst and a catalytic combustor for gas turbine power generation in which methane is oxidized over a palladium oxide (PdO) catalyst. Surface oxidation rates of carbon monoxide are based on measurements of Pt catalyst activity in a catalytic flow reactor, and rates for methane oxidation are based on reaction kinetics for a supported PdO catalyst assigned from differential reactor measurements. These simulations show that in the high aspect ratio passageways (length to diameter) in catalytic monoliths, radiation heat transfer can play a major role in the energy balance on the catalytic surfaces, which determines the transient warm‐up behavior and the steady‐state location of catalyst light‐off. For gray surfaces, however, the predicted steady‐state and transient behaviors are not sensitive to the emissivity of the monolith walls.
Effect of a nonionic surfactant on the flow dynamics of a model HASE associative polymer
Authors: Tirtaatmadja Viyada , Tam Kam C. , Jenkins Richard D.
Published: April 2004

The interaction between a model hydrophobically modified alkali soluble emulsion polymer and a nonionic surfactant C12EO4 is investigated. A huge synergistic increase in the viscosity over 10,000‐fold is demonstrated when the surfactant concentration is increased to 0.1 M, due to the formation of mixed micelles of surfactant hydrophobic tails and polymer hydrophobic groups. Incorporating surfactant molecules increases the number of mechanically active junctions in the polymer network and thus reduces the functionality of the hydrophobic groups. In the intermediate range of surfactant concentrations (0.001–0.01 M), the parallel superposition of steady shear and dynamic testings reveal that the microstructure of the network is affected by stress even in the low‐shear plateau region of the flow curves. The number of network junctions increases substantially with applied shear stress within the low‐shear viscosity region, although the longest relaxation time of the polymer–surfactant systems is greatly reduced. At very high surfactant concentrations ( > 0.01 M), polymer hydrophobes interact with the surfactant to form a stiff gel‐like structure. Surfactant molecules form a mixed bilayer structure with the polymer hydrophobes. These stronger network junctions cause a larger increase in both the viscosity and longest relaxation time of the system. Thus, adding a nonionic surfactant can effectively enhance flow properties of the associative polymer depending on the amount of surfactant used.
Micromixing in a single‐feed semi‐batch precipitation process
Authors: Phillips R. , Rohani S. , Baldyga J.
Published: April 2004

A mixing‐precipitation model (based on engulfment, the E‐model by Baldyga and Rohani, 1987) is applied to a single‐feed semi‐batch precipitation process to describe the mixing effects on the final particle‐size distribution (PSD) and particle morphology. The model describes mixing on two scales: the microscale (at which the flow directly affects nucleation and growth phenomena in the mixed‐precipitation (MP) zone) and a macroscale (which controls the environment for the MP‐zone). Application of the model is limited to 1≪Sc<4,000 and slow feed addition rates. When the model was applied to experimental results of BaSO4 precipitation, satisfactory agreement between predicted and observed results was obtained. Experiments were carried out in a single‐feed 18‐L Rushton reactor, whereby the effects of various operating conditions such as feed rate, intensity of mixing, mean initial reactant concentration and reactant volume ratio on barium sulfate precipitation process were investigated.
Influence of hydrodynamics on the upper explosion limit of ethene–air–nitrogen mixtures
Authors: Bolk Jeroen W. , Westerterp K. Roel
Published: April 2004

A large pilot plant was constructed to study the upper explosion limit of ethene–air–nitrogen mixtures under conditions of flow in a tube. Experiments were performed at pressures of 0.5, 1.0, and 1.5 MPa, gas temperatures between 298 and 573 K, and with ethene concentrations between 20 and 40 vol. %. A cylinder‐symmetrical 2‐D model developed simulated the experimentally obtained ignition and flame propagation phenomena. The commercial computational fluid dynamics code AEA‐CFX 4.1 was used to solve this model, to which reaction kinetics for a scheme of two consecutive reactions were added. The model predicts the experimental explosion points within 0.5 vol. %. The explosion limit is influenced by the gas velocity: it becomes smaller and shifts to higher oxygen concentrations at increasing flow rates. In practice this means that partial oxidation reactions can safely be operated at high oxygen concentrations, provided the gas is kept flowing at high flow rates.
Electrospun fiber mats: Transport properties
Authors: Gibson P. W. , Schreuder‐Gibson H. L. , Rivin D.
Published: April 2004

No Abstrast.
New insights into nucleation through chiral crystallization
Authors: Botsaris Gregory D. , Qian Ru‐Ying , Barrett Amy
Published: April 2004

No Abstrast.
Axial mixing in modern packings, gas and liquid phases: I. Single‐phase flow
Authors: Macías‐Salinas Ricardo , Fair James R.
Published: April 2004

An experimental study was conducted in a 0.43‐m‐ID packed column to determine the axial mixing properties of air and water under single‐phase flow conditions. The packings used were 25.4‐mm ceramic Raschig rings, one modern random packing, 25.4‐mm metal Pall rings; two structured packings, Sulzer BX and Flexipac 2. The column was operated at ambient conditions with flow rates varying from 0.4 to 4 kg/m2·s for the gas and from 3.25 to 8.5 kg/m2·s for the liquid. Axial mixing was experimentally determined via dynamic response studies based on the pulse injection technique. The diffusion‐type model served to reproduce the experimental response curves satisfactorily and proved to be a suitable means of describing axial mixing in both phases. The results confirm previous observations for first‐generation packings. Axial mixing is much greater in the liquid than in the gas; axial mixing in the gas increases with gas rate, whereas axial mixing in the liquid decreases with liquid rate. It was also found that the two structured packings produce the lowest levels of axial mixing in the gas. Surprisingly, the greatest mixing effects in the liquid were obtained for Flexipac 2 structured packing. Correlations were developed to represent the experimental mixing and liquid holdup data, yielding an average ±16% difference between experimental and correlated data. Mixing measurements in gas or liquid under two‐phase flow conditions will be discussed in Part II.
Pulse‐response study for the humidity effect on sorption of ethyl bromide on clays
Authors: Yaşyerli Nail , Doğu Gülşen , Doğu Timur , McCoy B. J.
Published: April 2004

The effects of relative humidity on sorption and the diffusion of ethyl bromide in kaolinite and montmorillonite pellets were investigated by the one‐sided single‐pellet moment technique. Montmorillonite demonstrated a higher water affinity than kaolinite, causing a sharper decrease of the ethyl bromide adsorption equilibrium constant with an increase of relative humidity. For montmorillonite, most of the active sites are covered by water even at relative humidities of 8%, and adsorption of ethyl bromide on the gas–water interface controls the sorption process. For kaolinite, however, adsorption on gas‐to‐mineral, gas‐to‐water, and water‐to‐mineral surfaces contributes to the sorption process. Effective macropore diffusivity of ethyl bromide in montmorillonite showed a decreasing trend with increase in relative humidity. This is due to high water uptake of montmorillonite and the corresponding change in pore structure. No such change was observed for kaolinite.
Measuring induction period for calcium sulfate dihydrate precipitation
Authors: Lancia Amedeo , Musmarra Dino , Prisciandaro Marina
Published: April 2004

Homogeneous nucleation of CaSO4·2H2O (gypsum) based on an optical diagnostic technique was studied within a supersaturation range of 1–4 at 25–90°C. The experiments were carried out using an experimental 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 (tind), that is, the time delay necessary for homogeneous nucleation to take place. As expected from theory, it was found that tind decreases when either temperature or supersaturation increase; from the dependence of tind on supersaturation, it was possible to distinguish between the mechanisms of homogeneous and heterogeneous nucleation. From the experimental data relative to homogeneous nucleation, the interfacial tension (γs) between CaSO4·2H2O and the surrounding aqueous solution and the activation energy (Eatt) for CaSO4·2H2O crystallization were evaluated. In particular, the dependence of tind on temperature made it possible to evaluate Eatt at 30 kJ/mol and the dependence of tind on supersaturation offered the value of γs to be about 37 mJ/m2, which does not vary with temperature in the interval explored.
Pattern formation in homogeneous reactor models
Authors: Sheintuch Moshe , Nekhamkina Olga
Published: April 2004

This work analyzes pattern formation mechanisms in the homogeneous model of a fixed catalytic bed for reactions with oscillatory kinetics. Two cases are analyzed: a nonadiabatic reactor with a continuous mass‐supply (either by a preceding reaction or via a membrane wall), and a simple adiabatic or cooled reactor. In the former case, the system may reach asymptotic space‐independent solutions, and when bistability of such solutions exists fronts may be established. Stationary or oscillatory front solutions, oscillatory states that sweep the whole surface, or excitation fronts may be realized then and the reactor behavior can be predicted from the sequence of phase planes spanned by the reactor. In an adiabatic reactor, fronts are formed only for sufficiently small Pe numbers, but these frontlike solutions do not separate different steady states. The patterns that can be realized in this case are quite similar to those in the previous case. The reactor behavior can be predicted by the sequence of phase planes spanned by the reactor, using an approximate finite difference presentation.
Prediction of colloid detachment in a model porous media: Thermodynamics
Authors: Bergendahl John , Grasso Domenico
Published: April 2004

Detachment of polystyrene latex colloids was studied in a model porous media system comprised of a packed column of glass beads. The polystyrene colloids were attached and subsequently detached from the glass beads by varying solution pH and ionic strength. It was found that classical‐DLVO theory could not predict detaching conditions. Extended‐DLVO theory was used to determine the median detaching pH for deionized water, and solutions of 0.01 and 0.1 M NaCl. Although both the colloids and porous media were macroscopically homogeneous, detachment was observed at a variety of pH values. The pH distribution of detachment and the energy distribution of the interaction between the polystyrene latex colloids and the glass beads followed gamma distributions. This work has provided a technique to quantitatively predict the aqueous chemistry changes that induce colloid detachment in porous media.
Binary protein adsorption on gel‐composite ion‐exchange media
Authors: Lewus Rebecca Karst , Carta Giorgio
Published: April 2004

The adsorption kinetics of mixtures of lysozyme and cytochrome‐C on S‐HyperD‐M, a cation exchanger composed of a rigid macroporous silica matrix whose pores are filled with a functionalized polyacrylamide gel, was characterized. Single‐component isotherms were found to follow approximately the steric mass‐action law of Brooks and Cramer, and an extension of this model to a binary system was consistent with two‐component uptake equilibrium results. Transient adsorption experiments were done to investigate the kinetics of sequential and simultaneous adsorption of the two proteins. At low protein concentrations, adsorption is essentially noncompetitive, and mass transfer is controlled by the external film resistance. At higher concentrations, equilibrium becomes competitive and coupling of the intraparticle diffusion fluxes occurs. A model based on a Maxwell–Stefan approach, where the driving force for diffusion is expressed in terms of the chemical potential gradient, was in approximate agreement with the experimental results.
Mass transfer during freezing in rat prostate tumor tissue
Authors: Devireddy Ramachandra V. , Smith David J. , Bischof John C.
Published: April 2004

Cryosurgery for treating prostate cancer is hampered by an incomplete understanding of the mechanisms whereby tissue destruction is achieved during freezing. The two known biophysical mechanisms of injury, intracellular ice formation and cellular dehydration injury (solute effects), have not been quantified within tumor tissue. Freeze substitution microscopy and a differential scanning calorimeter (DSC) were used to quantify freeze‐induced dehydration within Dunning AT‐1 rat prostate tumor tissue. Stereological analysis of histological tumor sections was used to obtain the initial cellular (Vo), interstitial, and vascular volumes of the AT‐1 tumor tissue. A Boyle‐van´t Hoff (BVH) plot was then constructed by examining freeze substituted micrographs of equilibrium cooled tissue slices to obtain the osmotically inactive cell volume, Vb= 0.25Vo. Obtaining dynamic cellular water transport information from the freeze substitution microscopy data proved difficult because of the artifact added by the high interstitial volume (∼35%). Since the DSC technique does not suffer from this artifact, a model of water transport was fit to the DSC water transport data at 5°, 10° and 20°C/min to obtain the combined best fit membrane permeability parameters of the embedded AT‐1 tumor cells, assuming either a Krogh cylinder geometry or a spherical cell geometry. Numerical simulations were also performed to generate conservative estimates of intracellular ice volume (IIV) in the tumor tissue at various cooling rates typical of those experienced during cryosurgery (≤100°C/min). Water transport data in tumor systems with significant interstitial spaces can be obtained by using histology and the low‐temperature microscopy methods to obtain the initial and final tissue cell volumes, respectively, and the DSC technique to obtain the dynamic volume changes during freezing.
Mass transfer of water vapor in a hollow fiber for degassing processes
Authors: Murata Hideyuki , Tomita Yasushi , Miyashita Makoto , Sakai Kazunari , Toda Masayuki , Ohmi Tadahiro
Published: April 2004

The mass‐transfer mechanisms of water vapor in hollow‐fiber membrane modules used to remove gases from water were studied experimentally and theoretically. Hollow‐fiber modules for degassing are widely used in various industries, including ultrapure water production. Although these modules perform extremely well, a general principle for the design has not been established. Recently, very highly efficient asymmetric hollow‐fiber modules were developed, that could reduce the dissolved oxygen to less than 1 μg/L (1 ppb). From our studies of the modules, it was found that the mass transfer of water vapor plays an extremely important role in the flow in the lumen side of the hollow fibers. The establishment of a theoretical design method for degassing modules can be expected from our studies.
Mixed cation zeolites: LixAgy‐X as a superior adsorbent for air separation
Authors: Hutson Nick D. , Rege Salil U. , Yang Ralph T.
Published: April 2004

Li‐X zeolite (Si/Al = 1.0) is currently the best sorbent for use in the separation of air by adsorption processes. Silver is also known to strongly affect the adsorptive properties of zeolites and thermal vacuum dehydration of silver zeolites leads to the formation of silver clusters within the zeolite. In this work we synthesized type X zeolites containing varying mixtures of Li and Ag. Adding very small amounts of Ag at the proper dehydration conditions formed silver clusters and enhanced adsorptive characteristics and energetic heterogeneity, as compared to those of the near fully exchanged Li+‐zeolites. The performance for air separation by the best of these sorbents, containing, on average, only one Ag per unit cell, was compared to that of the near fully Li+‐exchanged zeolite using a standard PSA cycle by numerical simulation. The results show that the new sorbent provides a significantly higher (> 10%) product throughput at the same product purity and recovery when compared to that of the near fully Li+‐exchanged zeolite.
Unified network model for adsorption–desorption in systems with hysteresis
Authors: Rajniak Pavol , Soós Miroslav , Yang Ralph T.
Published: April 2004

The problem of equilibrium and kinetics for adsorption–desorption of condensable vapors in porous media is studied experimentally and theoretically. For adsorption, the network model for diffusion based on pore blocking theory with percolation (in the network) added by effective medium approximation is further improved. A new predictive model based on properties of the Bethe lattices is proposed to account for the existence of liquid‐filled “blind” pores that result in a decrease in the total diffusion rate. For desorption, a new “shell and core” (or shrinking core) representation of the network model is proposed. Information from adsorption–desorption equilibria is needed to compute the thickness of the shell in which desorption/evaporation occurs for concentrations higher than the percolation threshold. These models form a unified equilibrium‐kinetics theory for gas‐porous solid systems that exhibit hysteresis. The models are applied to the systems silica gel‐water vapor and Vycor glass‐nitrogen. Concentration‐dependent Fickian diffusivities for these systems have been measured for both adsorption and desorption branches. The adsorption model successfully predicts the experimental data with a maximum in diffusivity. The desorption model correctly predicts the concentration dependence of diffusivity with a steep minimum at the percolation threshold.
Multirate nonlinear state estimation with application to a polymerization reactor
Authors: Tatiraju Srinivas , Soroush Masoud , Ogunnaike Babatunde A.
Published: April 2004

A method of multirate nonlinear state observer design, which can use directly a nonlinear process model in the observer design without any linear approximation, is presented. The multirate nonlinear state observer is easy to design and implement, and is computationally efficient. Furthermore, for a number of processes, it is possible to prove the global convergence of the multirate state observer analytically. The design, implementation and performance of the state observer design method are shown by a polymerization reactor in which free‐radical solution polymerization of styrene takes place. The initiator concentration and three leading moments of the molecular weight distribution (MWD) of the polymer product are estimated continuously from: (i) frequent measurements of the reactor temperature, jacket temperature and reacting‐mixture density; (ii) infrequent and delayed measurements of the leading moments of the MWD. Each infrequent measurement has a sampling period and a measurement delay of 0.5 or 1 h. In the presence of model‐plant mismatch and measurement noise, the convergence of the multirate state observer is shown by numerical simulations.
Role of distributed oxygen addition and product removal in the oxidative coupling of methane
Authors: Androulakis Ioannis P. , Reyes Sebastián C.
Published: April 2004

Modeling and optimization of C2 hydrocarbon production via the oxidative coupling of methane (OCM) were studied. The model includes both homogeneous and heterogeneous reactions. Focusing on the use of detailed reaction networks, previously validated experimentally, and the critical role of oxygen in both methane activation and product degradation, this work systematically explores the use of controlled oxygen addition and product removal schemes that improve OCM performance. Based on a plug flow reactor that is divided into Np stages, within which oxygen is added and/or products are removed, a rigorous optimization algorithm is developed that simultaneously maximizes C2 yields and minimizes O2 consumption. In the absence of catalyst and product removal, the C2 yield is maximized at a fixed O2 /CH4 ratio, but this maximum yield is independent of the form in which the oxygen is added (cofeed or staged). When a catalyst is added, the optimal C2 yields show only gradual improvements with oxygen distribution because the benefits of the lower oxygen reaction order on the catalyst are adversely affected by concomitant surface degradation reactions. The largest yield improvements are obtained when the C2 hydrocarbons are removed at each stage before they undergo oxidation reactions. Thus, when staged oxygen addition is combined with product removal in the presence of a catalyst, C2 yields as high as 87% are achieved in about 20 stages. Such yield values are consistent with experiments in which continuous product separation schemes have been used.
Concentration polarization in cross‐flow reverse osmosis
Authors: Song Lianfa , Yu Sanchuan
Published: April 2004

A dimensionless governing equation was formulated for a cross‐flow RO process in which the local variation of concentration polarization was rigorously considered. It was shown in this formulation that the cross‐flow RO process could be fully characterized with a single dimensionless parameter. The coupling between permeate flux and concentration polarization was properly solved and a closed‐form analytical solution was obtained. This analytical solution enabled us to conveniently investigate concentration polarization in the RO process. The significance of local variation of concentration polarization was demonstrated, and the operations of RO under various conditions were simulated and investigated with the newly developed model.
Fluid‐dynamic and mass‐transfer behavior of static mixers and regular packings
Authors: Cavatorta Omar N. , Böhm Ursula , de del Giorgio Ana María Chiappori
Published: April 2004

The fluid dynamics and liquid‐to‐wall mass transfer for spaced and stacked regular packings were studied for forced convection and fluidized beds. The behavior of these configurations in bubble columns and under natural convection conditions is also presented. Flow parameters characterizing structured packings, presented in the literature, were used in the evaluation of results. General equations to predict pressure drop and mass transfer are discussed, as well as the relationship between energy dissipation and mass transfer. In the presence of fluidized particles, single‐phase flow or natural convection conditions, the mass‐transfer behavior of a packing element stacked between other packs or separated from the neighboring elements by liquid layers is almost the same, but differs in bubble columns.
Monte Carlo simulation of the radiation field in a reticulated foam photocatalytic reactor
Authors: Changrani Rajnish , Raupp Gregory B.
Published: April 2004

The 3‐D polychromatic radiation field for an annular packed‐bed photocatalytic reactor using alumina reticulated foams as a monolithic catalyst support was simulated using Monte Carlo methodology. Two distinct methods were used for simulating photon transport: (1) a “spatial” approach that tracks the flight of a photon in a predetermined reticulate structure; (2) a “temporal” approach that generates the random porous structure of the reticulate as the photon flies into it. The two approaches yield almost identical results, although the temporal approach is far more efficient computationally. Simulations for the integral axially‐averaged radial ultraviolet (UV) light profiles agree closely with experimental measurements for titania‐coated 10, 20 and 30 PPI alumina reticulates and near UV lamps. The simulations reveal that the local volumetric rate of energy absorption (LVREA) in the reticulate and the magnitude of the LVREA gradient both increase with decreasing reticulate pore size.
Effect of rheology on coalescence rates and emulsion stability
Authors: Dreher T. M. , Glass J. , O'Connor A. J. , Stevens G. W.
Published: April 2004

The effect of continuous‐phase rheology on the coalescence time of single water drops at an organic/aqueous interface was investigated experimentally. For Newtonian fluids, the coalescence time increases monotonically with the continuous‐phase viscosity at a fixed drop diameter and with the drop diameter at a fixed continuous‐phase viscosity. Elasticity in the continuous phase caused significant increases in the coalescence times for drops of less than 1 mm diameter in the systems studied, but had no discernible effect on the coalescence of larger drops. Estimates of the interfacial shear rates were used to demonstrate that the elasticity of the continuous phase becomes significant in the coalescence process as the drop size decreases. These results indicate that emulsion stability may be increased without the use of surfactants by adding a suitable polymer to the continuous phase and control of the dispersed‐phase drop size.
Tensile stress in a porous medium due to gas expansion
Authors: Benes Nieck E. , Biesheuvel P. Maarten , Verweij Henk
Published: April 2004

Stress profiles develop in a porous material due to a gas‐phase pressure difference and subsequent gas flow. If stresses become tensile, material failure (explosion and blistering) can occur. Stress profiles are calculated for an asymmetric inorganic porous disk‐like membrane material placed in a pressure vessel, which is depressurized. The stress that develops in the membrane material depends on the gas‐phase pressure and the porosity. The gas‐phase pressure is a function of place, time and characteristics of the membrane, the vessel and the valve. Two regimes are identified for membrane depressurization, and a critical initial pressure is defined below which tensile stresses cannot develop. The theory presented combines the dusty gas model with balances for mass, momentum, and mechanical energy.
Thermodynamic modeling of multicomponent ion‐exchange equilibria of amino acids
Authors: Bellot Jean Christophe , Tarantino Ramon Vega , Condoret Jean‐Stéhane
Published: April 2004

A thermodynamic model was developed to describe phase equilibria for aqueous amino acid/polyelectrolyte gel systems. In the gel phase, activities of all exchangeable species (charged and neutral components) are calculated with the generalized Flory‐Huggins model. In the surrounding solution, a numerical procedure is used to allow for partial dissociation phenomena combined with a modified UNIFAC model associated with the Pitzer‐Debye‐Hückel term. The osmotic pressure difference between the two phases is also taken into account in the equations. The model enables the prediction of resin‐phase composition, gel swelling and intraparticle pH. It can be used for various biochemicals in a wide range of concentrations. Its validity was successfully tested using binary and multicomponent exchange equilibria data of several amino acids (phenylalanine, alanine, proline, and glutamate) and HCl on a strong‐acid cation‐exchange resin (Amberjet 1200H).
Heat and mass transport resistances in vacuum membrane distillation per drop
Authors: Bandini Serena , Sarti Giulio C.
Published: April 2004

Vacuum membrane distillation (VMD) is a separation process based on the use of microporous hydrophobic membranes. The membrane is located between an aqueous phase and a permeate, which is kept under vacuum at pressure values below the equilibrium vapor pressure of the feed. The liquid stream vaporizes at one side of the membrane, and the vapors diffuse through the gas phase inside the membrane pores. The process rate and performance are affected highly by the transport phenomena both in the liquid phase and through the membrane. Heat‐ and mass‐transfer resistance in the liquid phase, as well as mass‐transfer resistance through the membrane, play an important role in determining the process performance. Based on VMD experimental data for several binary aqueous mixtures containing volatile organic compounds, a simple criterion to investigate the role of each transport resistance on the separation efficiency is discussed.
Measurement and theoretical modeling of protein mobility through membranes
Authors: Ho A. K. , Perera J. M. , Dunstan D. E. , Stevens G. W. , Nyström M.
Published: April 2004

The electrophoretic mobilities of hemoglobin and lysozyme were measured through polycarbonate track‐etched membranes of different pore sizes. Together with the zeta potential of the protein‐fouled membranes, and measurements of the free‐solution mobilities, protein sizes, and membrane pore sizes, the theory of Ennis et al. was tested. The presence of the membrane offered little hindrance to protein transfer when the membrane pore size was large in comparison with the protein size and the thickness of the electrical double layers. Under some solution conditions, protein agglomeration was significant and the interactions between the larger particles, and the membrane pore walls caused a more pronounced reduction in the protein mobility from its free‐solution value. Good agreement with the theoretical model was found only for cases where the solution remained as a monodispersed suspension of protein monomers.
Worst‐case performance analysis of optimal batch control trajectories
Authors: Ma David L. , Chung Serena H. , Braatz Richard D.
Published: April 2004

In open‐loop optimal control, inputs to a dynamic system are computed that optimize a specified performance criterion. A novel approach is proposed that quantifies the impact of parameter and control implementation inaccuracies on the performance of open‐loop control policies. Such information can be used to decide whether more laboratory experiments are needed to produce parameter estimates of higher accuracy, or to define performance objectives for lower‐level control loops that implement the optimal control trajectory. The novel features of the approach include (1) computational efficiency; (2) the parametric uncertainty description, which is produced by most process identification algorithms; and (3) addressing of control implementation inaccuracies. The merits of the approach are illustrated on a batch crystallization process.
Crystal growth kinetics of calcite in a dense fluidized‐bed crystallizer
Authors: Tai Clifford Y. , Chien W.‐C. , Chen C.‐Y.
Published: April 2004

The growth kinetics of calcite crystals are studied in a batch‐fluidized‐bed crystallizer, which is maintained at a constant pH. The growth experiments are conducted in the metastable region explored as part of this research. The crystal growth rates are evaluated from the consumption rates of calcium ions, using the cured natural calcite or silica sand as seeding materials. Several operation variables are investigated, including supersaturation, pH, ionic strength, superficial velocity, and particle size and type of seed. The significant factors that affect the crystal growth rate are identified. Then the crystal growth data of constant pH and ionic strength are analyzed by the two‐step growth model. The mass‐transfer coefficients are obtained and compared at various crystal sizes and superficial velocities. Finally, a growth‐rate equation of calcite crystal, which is based on the two‐step growth model, is proposed for design purposes.
Modular representation synthesis framework for homogeneous azeotropic separation
Authors: Ismail Soraya Rahim , Pistikopoulos Efstratios N. , Papalexandri Katerina P.
Published: April 2004

The synthesis and design of separation systems for nonideal azeotropic mixtures traditionally utilize graphical methods, focusing on separation feasibility within a sequential design framework. Once promising systems have been identified, sequencing and design are optimized in a second stage, limited by the separation feasibility region defined in the first stage. A generalized modular representation proposed for the synthesis of nonideal separations can simultaneously explore entrainer and sequencing alternatives. A multipurpose mass/heat exchange module, introduced in earlier work, is employed to investigate separation units and structures as mass‐ and heat‐exchange possibilities. Process units, separation sequences, and entrainers that aid the separation are examined within one separation problem rather than in a sequential manner. Promising separation schemes can be further considered for detailed design optimization.
Optimal‐grade transition strategies for multistage polyolefin reactors
Authors: Takeda Makoto , Ray W. Harmon
Published: April 2004

Polymer‐producing companies normally produce many polymer grades in each polymerization reactor train according to customer demands; therefore, many grade‐transition operations are necessary. However, the determination of these optimal strategies is difficult. To solve this problem, several sample simulations showing the optimal solutions of grade‐transition strategies for multistage olefin polymerization reactors with bimodal products were performed. The methodology utilized a dynamic process simulator, POLYRED, coupled to a sequential quadratic programming, nonlinear optimization algorithm, by using two different objective functions with and without state constraints. It is shown that the most beneficial case of using the optimization technology is the grade transition where the hydrogen content in a reactor must be decreased and/or constraints are imposed on state variables.
Three‐dimensional image analysis of mixing in stirred vessels
Authors: Li Lin , Wei James
Published: April 2004

Several analytical methods were applied to the 3‐D electrical resistance tomography (ERT) image of stirred‐vessel experiments of Holden et al. to characterize the mixing behavior and to measure mixing quantitatively. Based on the time resolved 3‐D ERT results, the centroid and variance of the concentration distribution in three dimensions can be obtained as a time series to track the movement, dispersion, stretch and rotation of the concentration distribution in the tank. By sampling at different scales and locations, the ratios of standard deviations to means and the entropy of corresponding volumes can be calculated to examine the degrees of uniformity. The 3‐D wavelet decomposition provides an effective method of analyzing the mixing process at various scales. The wavelet difference coefficients reflect the concentration gradients of specific volumes along different directions. These methods of analysis promise to guide the design of better mixing schemes and to monitor the process for optimal operation.