bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of molecular association on diffusion in binary liquid mixtures
Authors: McKeigue Kevin , Gulari Erdogan
Published: June 2004

A predictive equation for the mutual diffusion coefficient is developed for binary solutions which contain a self‐associating component. The solution is modeled as a nonideal multicomponent mixture consisting of the solvent molecules and various association complexes. The model provides for a unique mobility for each of the species present and accounts for both physical and chemical contributions to the solution thermodynamics and their effects on the diffusive fluxes. The predictions of the theory were evaluated for n‐alcohols in nonpolar solvents using data for six systems at temperatures ranging from 293 to 363 K. In each case, the predictions of this theory are found to be superior to other currently available methods.
Hydrodynamics of a three‐phase fluidized bed containing low‐density particles
Authors: Tang Wen‐Tzung , Fan Liang‐Shih
Published: June 2004

Experiments were conducted to study the hydrodynamics of a gas‐liquid‐solid fluidized bed containing low‐density particles. The density and size of the particles employed were comparable to those ordinarily encountered in fluidized‐bed bioreactors for either wastewater treatment or fermentation. A dual resistivity probe and a conductivity probe were employed to study the axial holdup of gas and liquid phases and the bubble size distribution. The behavior of the axial holdup distribution for solid, gas, and liquid phases is reported. A mechanistic model, based on solids entrainment and deentrainment mechanisms by bubbles, is developed to describe the behavior of the axial solids holdup distribution. The model is shown to satisfactorily account for the experimental data.
Thermoflow multiplicity in a packed‐bed reactor: Conduction and cooling effects
Authors: Pita Jorge , Balakotaiah Vemuri , Luss Dan
Published: June 2004

Steady states with different flow rates and temperature profiles may exist in different tubes of a multitube packed‐bed reactor due to the coupling among the species, energy and momentum balances, and the change of physical properties with temperature and pressure. Under typical operating conditions if no axial conduction of heat occurs thermoflow multiplicity can be found only for unrealistically high heats of reaction and/or over a very narrow range of exit pressures. However, accounting for the finite but small heat conduction in the reactor tubes leads to multiplicity for typical parameters and a reasonable range of exit pressures. Cooling tends to decrease the region of parameters for which the multiplicity occurs.
Cell transport in the millipore filter assay
Authors: Buettner Helen M. , Lauffenburger Douglas A. , Zigmond Sally H.
Published: June 2004

During inflammation, leukocytes cross the blood vessel wall and migrate to the inflammatory site in response to gradients of diffusible chemical attractants produced there. This directed migration response to a chemical gradient, termed chemotaxis, can be studied experimentally in the Millipore filter assay. We have applied a mathematical model to analyze cell population migration in the assay in terms of two parameters analogous to molecular transport coefficients. The random motility coefficient, μ, reflects the cell response to uniform concentrations of chemical attractant, while the chemotaxis coefficient, χ, reflects the response to a concentration gradient. We have measured μ and χ by comparing theoretical cell density profiles to those measured in the assay. Both parameters vary as a function of the attractant concentration; μ ranges from 10−10–10−9 cm2/s and χ ranges from 10–100 cm2/s.M for the attractant tested. These values agree with ones predicted from a priori theoretical relationships for μ and χ. Quantitation of the Millipore filter assay provides a framework for the quantitation of analogous cell transport systems such as a composite assay simulating cell migration across the vessel wall.
A group contribution method for second virial coefficients
Authors: Abusleme J. A. , Vera J. H.
Published: June 2004

A semitheoretical group contribution method for predicting pure‐compound second virial coefficients is proposed. An expression for the second virial coefficient, expressed as the product of a nonpolar and a polar contribution, is derived using statistical thermodynamic concepts and an approximate form for the radial distribution function. For polar compounds the nonpolar contribution is assumed to be given by the Tsonopoulos correlation with the nonpolar acentric factor defined by Thompson and Braun. The polar contribution is represented by an energetic term expressed in terms of group contributions. Second virial coefficient predictions for strong polar and associating systems are comparable with those obtained with the correlations of Tsonopoulos and Hayden and O'Connell. Cross‐second virial coefficients for mixtures obtained using well‐defined molecular mixing rules compare well with literature values.
Structural changes in surfactant‐modified sorbents during furnace injection
Authors: Kirchgessner David A. , Jozewicz Wojciech
Published: June 2004

A calcium hydroxide [Ca(OH)2] sorbent modified by the addition of calcium lignosulfonate has recently been developed for use in the Environmental Protection Agency's limestone injection multistage burner process. The increased reactivity with sulfur dioxide (SO2) displayed by this modified sorbent has been shown to be caused, in part, by its decreased particle (agglomerate) size compared to conventional Ca(OH)2. Subsequent work has shown that surfactant‐modified Ca(OH)2 also undergoes significantly different structural changes during furnace injection. For a given reactor temperature and residence time, the modified sorbent calcines to a greater extent than unmodified sorbent. It also loses surface area more slowly, and retains more of its porosity, suggesting that it sinters more slowly than conventional sorbent. Therfore, in addition to reducing the particle size of Ca(OH)2 in some applications, calcium lignosulfonate also to inhibit one or more of the diffusion mechanisms responsible for the process of sintering.
Thermodynamics of high‐pressure aqueous systems containing gases and salts
Authors: Harvey Allan H. , Prausnitz John M.
Published: June 2004

This work provides a new method for calculating phase equilibria for systems where conventional activity‐coefficient models are not suitable. A procedure is presented for superimposing ionic effects on a conventional equation of state for nonelectrolytes. A modification of Born's equation is used to describe charging the ions; ion‐ion interactions are described with the Mean Spherical Approximation, coupled with an adjustable salt/solvent parameter that is obtained from osmotic‐coefficient data at room temperature. The equation of state is used to predict gas solubilities in aqueous salt solutions at high pressures. While this method produces qualitative agreement with the experimental solubility data, quantitative agreement requires a salt/ gas parameter obtained from Setchenow‐constant data. When Setchenow constants are used, agreement with experiment is good except at high salt concentrations, where salting‐out is usually underpredicted. Good results are obtained for phase equilibria in a natural‐gas/brine system at high pressures.
Process optimization via simulated annealing: Application to network design
Authors: Dolan W. B. , Cummings P. T. , LeVan M. D.
Published: June 2004

Simulated annealing is a multivariable optimization technique based on the Monte Carlo method used in statistical mechanical studies of condensed systems and follows by drawing an analogy between energy minimization in physical systems and costs minimization in design applications. In this paper, simulated annealing is introduced and reviewed. The utility of the method for optimization of chemical processes is illustrated by applying it to the design of pressure relief header networks and heat exchanger networks.
Hollow‐fiber liquid chromatography
Authors: Ding Hongbing , Yang Ming‐Chien , Schisla David , Cussler E. L.
Published: June 2004

Modules containing from 120 to 27,000 parallel hollow fibers 100 μm in diameter can separate mixtures by reversed phase chromatography. The mixtures separated include aqueous solutions of ketones using alkanes as the stationary phase, and aqueous solutions of the proteins myglobin and cytochrome‐c using an octane solution of reversed micelles as the stationary phase. The dispersion observed in these separations is comparable to that predicted from the Aris‐Taylor theories. Modules of these fibers promise a lower pressure drop and a greater reproducibility than columns of spheres of equivalent surface area per volume. Such modules can facilitate scale‐up of liquid chromatography.
Holdup and interfacial area measurements using dynamic gas disengagement
Authors: Patel Snehal A. , Daly James G. , Bukur Dragomir B.
Published: June 2004

The dynamic gas disengagement technique is discussed comprehensively in this paper. Also outlined is the procedure for estimating holdup structures and bubble rise velocities in a bubble column, which is extended to include the methodology for estimating the Sauter mean bubble diameter and therefore the specific gas‐liquid interfacial area. The error limits for the estimated quantities are established using two extreme cases to describe the disengagement process: constant rate disengagement and interactive disengagement of bubbles. The analysis is done assuming a bimodal bubble size distribution; however, generalized equations for a multimodal bubble size distribution are also presented. Sensitivity of results to the two cases is illustrated using results obtained from experiments conducted with the air‐tap water system in two bubble columns (0.05 and 0.23 m in diameter, 3 m tall). Sauter mean bubble diameters and specific interfacial areas estimated using the two approaches provide lower and upper limits for values reported in the literature.
Simulation of microstructure/mechanism relationships in particle deposition
Authors: Tassopoulos M. , O'Brien J. A. , Rosner D. E.
Published: June 2004

The important connection between particulate deposit properties and deposition mechanism remains poorly understood and only scarcely studied. Accordingly, in this research, we develop a discrete stochastic model to simulate particulate deposition processes resulting from realistic combinations of deposition mechanisms. Particle motion is assumed to be determined by the superposition of a deterministic force that acts toward the collecting surface and a random force, which produces Brownian diffusion. We characterize the resulting deposit microstructure via porosity and pore size/area distribution, surface area; and we examine the evolution of these descriptors with time (number of particles deposited) for different deposition mechanisms. We also examine the effect of particle polydispersity, spatial orientation (for nonspherical particles), and mean‐free‐path on the resulting deposit structure.
Experimental data for the crossover process in a model supercritical system
Authors: Kelley F. D. , Chimowitz E. H.
Published: June 2004

This paper presents the experimental technique and data acquired in the evaluation of a separation process in the crossover region of a model supercritical fluid system consisting of benzoic acid, 1–10 decanediol, and carbon dioxide. The results show that by exploiting the crossover effect it is possible to obtain essentially pure benzoic acid (>99.5% purity) with a single temperature cycle. A number of other issues are also discussed including the tradeoff between yield and purity and the manner in which the process variables are specified, which involves the interplay of experiment and model calculations.
Role of porosity loss in limiting SO2 capture by calcium based sorbents
Authors: Newton Gerald H. , Chen Shih Lin , Kramlich John C.
Published: June 2004

The extent of high temperature (900‐1,300°C), short time (<1 s) SO2 capture was found to be limited by temperature‐dependent losses in the porosity of calcium based sorbents. At 970°C these porosity losses were caused by CO2‐activated sintering. Sulfation of the sorbents either prevented or reduced the extent of porosity losses. Differences in SO2 capture between hydroxides from different commercial sources, and significantly lower levels of capture by calcium carbonates compared to hydroxides were attributed to differences in particle size and the degree of porosity loss.
Thermal integration of processes with heat engines and heat pumps
Authors: Swaney Ross E.
Published: June 2004

An extended transportation array representation is developed for the design of process heat recovery networks incorporating heat engines and heat pumps. The formulation automatically accounts for the relocation of pinches in the integrated system. The optimal heat flow pattern is determined based on simple economic criteria. Simplified models of heat engine and heat pump systems are developed that permit integration options to be identified on the basis of heat flows without requiring specific cycle designs. The solution array identifies design alternatives and provides a decision framework to guide in selecting the final design topology.
Dynamics and control of quasirational systems
Authors: Ramanathan S. , Curl R. L. , Kravaris C.
Published: June 2004

Systems having transfer functions of the form

where P1(s), P2(s) and Q(s) are polynomials, are called quasirational distributed systems (QRDS). They are encountered in processes modeled by hyperbolic partial differential equations. QRDS can have an infinity of right half‐plane zeros which causes large phase lags and can result in poor performance of the closed‐loop system with PID controllers. Theory on the asymptotic location of zeros of quasipolynomials is used to predict the nonminimum phase characteristics of QRDS and formulas are presented for factoring QRDS models into minimum and non‐minimum phase elements.

A generalized Smith predictor controller design procedure for QRDS, based on this factorization, is derived. It uses pole placement to obtain a controller parameterization that introduces free poles which are selected to satisfy robustness specifications. The use of pole placement allows for the design of robust control systems in a transparent manner. Controller selection is generally better, simpler and more direct with this procedure than searching for optimal PID controller settings.
Adjustable solute distribution between polymers and supercritical fluids
Authors: Shim Jae‐Jin , Johnston Keith P.
Published: June 2004

The sorption of toluene, dilute in CO2, and likewise the distribution coefficient of toluene between silicone rubber and CO2 have been explored from gaseous to supercritical fluid conditions. Sorption and desorption isotherms were determined by frontal analysis using a new inverse supercritical fluid chromatography technique at 35 and 70°C up to 250 bar. Complementary swelling data are presented for pure CO2 in silicone rubber up to 315 bar. A new result is that the sorption of toluene goes through a maximum and is highly adjustable over a continuum in the highly compressible region of carbon dioxide. This behavior is explained physically and predicted quantitatively with the Flory equation and the Peng‐Robinson equation of state using only information from binary systems. These results are useful for a wide variety of applications including impregnation of polymers with pharmaceuticals, fragrances and other additives, and polymer purification.
Partial internal wetting of catalyst particles with a distribution of pore size
Authors: Bhatia S. K.
Published: June 2004

The recent model of partial internal wetting of catalyst particles (Bhatia, 1988) is extended to allow for pore‐size distribution, multicomponent diffusion, and nonlinear kinetics for spherical particles exposed to condensible vapor undergoing an exothermic reaction. Profiles of liqid filling of the pores are computed, and the influence on mass transfer and effectiveness factor is determined. Under certain circumstances, the effectiveness factor has a maximum with respect to the mole fraction of condensible component suggesting an optimum vapor‐phase composition. As the bulk vapor approaches saturation the effectiveness factor and particle temperature can drop sharply because of precipitious increase in liquid filling. However, a significant fraction of the pore space is still dry in contradiction to prior models assuming complete internal wetting of catalyst particles. The new model is more realistic than earlier attempts and lays the framework for proper representation of the physical phenomena involved.
Phase equilibria for supercritical fluid process design
Authors: Brennecke Joan F. , Eckert Charles A.
Published: June 2004

Supercritical fluids (SCF's) are powerful solvents with many unique properties. They have great potential for many extraction processes, but reliable and versatile mathematical models of the phase equilibrium thermodynamics are needed for their use in process design and economic feasibility studies. This paper reviews existing experimental data and analytical methods and attempts to delineate their strengths and limitations. Also discussed are new data needs and possible new directions for a better fundamental understanding of the molecular processes in SCF solutions.
A MIMO cautious self‐tuning controller
Authors: Papadoulis Apostolos V. , Svoronos Spyros A.
Published: June 2004

This paper presents a simple linear model based self‐tuning controller (STC) for MIMO systems. The controller, namely the cautious selftuning controller (CSTC), features enhanced robustness characteristics in the case of unmodeled nonlinearities and / or dynamics. The controller structure is that of a linear quadratic, minimizing a single‐step cost function, and with time varying input penalization. The CSTC can be applied to open‐loop stable and minimum‐phase systems. Application to a simulated distillation column demonstrates its effectiveness.
Gas‐particle flow in a vertical pipe with particle‐particle interactions
Authors: Sinclair J. L. , Jackson R.
Published: June 2004

A theory is presented for the fully‐developed flow of gas and particles in a vertical pipe. The relation between gas pressure gradient and the flow rates of the two phases is predicted, over the whole range of cocurrent and countercurrent flows, together with velocity profiles for both phases and the radial concentration profile for the particles. The gas and the particles interact through a drag force depending on their relative velocity, and there are mutual interactions between pairs of particles through inelastic collisions. This model is shown to account for marked segregation of gas and particles in the radial direction, and the predicted relation between the pressure gradient and the flow rates of the two phases is surprisingly complex.
A phenomenological correction to an equation of state for the critical region
Authors: Chou Grace F. , Prausnitz John M.
Published: June 2004

This work presents a phenomenological correction to improve a classical equation of state for representing phase equilibria and densities in the vapor‐liquid critical region. This correction consists of two steps. The first step is a volume translation which locates the correct critical point; this volume translation also improves density predictions for pure fluids and mixtures. The second step provides a near‐critical contribution to the residual Helmholtz energy which accounts for aomalous behavior near the critical point. For pure fluids, the near‐critical contribution flattens the coexistence curve and pressure‐density isotherms near the critical point. For mixtures, the near‐critical contribution has only a small effect on the calculated coexistence curve; this effect is often masked by the choice of binary parameters in the classical equation which have a more profound effect on the calculated results.
Deadtime compensation for nonlinear processes
Authors: Kravaris Costas , Wright Raymond A.
Published: June 2004

Many industrially important processes feature both nonlinear system dynamics and a process deadtime. Powerful deadtime compensation methods, such as the Smith predictor structure in state space for linear systems is presented first and then directly extended to nonlinear systems. When combined with input/output linearizing state feedback, this Smith‐like predictor makes a nonlinear system with deadtime behave like a linear system with deadtime. The control structure is completed by adding an external linear controller, which provides integral action and compensates for the deadtime in the input/output linear system, and an open‐loop state observer. Conditions for robust stability with respect to errors in the deadtime and more general linear unstructured multiplicative uncertainties are given. Computer simulations for an example system demonstrate the high controller performance that can be obtained using the proposed method.
A reaction engineering approach to the problem of concrete carbonation
Authors: Papadakis V. G. , Vayenas C. G. , Fardis M. N.
Published: June 2004

Steel bars in reinforced concrete are protected from corrosion by the high pH environment of the surrounding concrete. This alkaline environment is destroyed by the reaction of atmospheric CO2 with the Ca(OH)2 of the concrete mass. When this process, called carbonation of concrete, reaches the reinforcing bars, corrosion of the latter may commence. In this paper, the physiochemical processes in this phenomenon are presented and modeled mathematically. The mathematical model is fairly complex, but certain simplifying assumptions are possible, which lead to the formation of a “carbonation front” and to a simple analytical expression for the evolution in time of this front, in terms of the composition parameters of cement and concrete and of the environmental conditions. This simple expression is in very good agreement with experimental results obtained in this and in previous studies. The effect of some parameters on the carbonation front propagation is also discussed.
Diffusion in zeolites containing mixed cations
Authors: Yeh Y. T. , Yang R. T.
Published: June 2004

Experimental results are reported for equilibrium adsorption and diffusivities of CH4, C2H6 and CO2 in type A zeolite containing mixed Na and K cations ranging from 0 to 100% in composition, at two temperatures and 1 atm. Two simple theoretical solutions are given for the diffusivity. One is for diffusion with pore (aperture) blocking using the effective medium approximation, which is readily available from the literature on percolation theory. The other is a kinetic solution derived in this work for cavities containing heterogeneous sites, which is a case where the percolation theory is not applicable. The equilibrium adsorption results show that K+ has a preference for the aperture (or pore‐blocking) sites (site type II), and these results are interpreted by the bond percolation theory. The diffusivity results are successfully interpreted by the poreblocking theory in zeolites containing less than 25% K+ and by the kinetic theory for zeolites with higher K+ contents.
Suspension‐to‐Surface heat transfer in a circulating‐fluidized‐bed combustor
Authors: Wu Richard L. , Grace John R. , Lim C. Jim , Brereton Clive M. H.
Published: June 2004

Heat transfer coefficients were measured for ciculating beds of sand particles of mean size 222 to 299 μm at temperatures of 340–880° C. Transfer coefficients were obtained for both a 1.22‐m‐long, 12.7‐mm‐OD vertical tube and a 1.59‐m‐long, 148‐mm‐wide membrane wall near the top of a 152‐mm‐square by 7.32‐m‐tall combustion column. For both surfaces and all temperatures, average heat transfer coefficients increased almost linearly with local suspension density which ranged from 0 to 70 kg/m3. Radiation played a significant role, especially at high temperatures and low suspension densities. Heat transfer coefficients also varied significantly with the lateral position of the tube. The vertical length of heat transfer surface is shown to be an important parameter allowing seemingly discrepant published results to be reconciled.
Aggregation kinetics in salt‐induced protein precipitation
Authors: Przybycien Todd M. , Bailey James E.
Published: June 2004

The kinetics of protein aggregation in salt‐induced precipitation processes were studied as a function of salt type, salt concentration, temperature and protein concentration. α‐chymotrypsin (αCT) was used as a model protein. Stopped‐flow turbidimetry was used to monitor the progress of precipitations. Analysis of the linear portions of the turbidity trajectories indicates that temperature and salt concentration effects are related to protein solubility; the protein concentration dependence is well described by the Smoluchowski collision equation. The aggregation kinetics of partially‐inhibited αCT exhibit poisoning behavior, underscoring the importance of dimerization and monomer addition in the precipitation of αCT. Solute particle radius distributions determined via dynamic laser light scattering for low salt and supernatant αCT solutions indicated that significant aggregation does not occur in the absence of supersaturation. A detailed population balance model was proposed that accounted for specific and nonspecific interactions and monomer addition. The model is expected to find general application to protein aggregation phenomena, in particular for proteins that have specific quaternary interactions.
The pressure field in an unsteady‐state fluidized bed
Authors: Foscolo Piero U. , Felice Renzo Di , Gibilaro Larry G.
Published: June 2004

An assumption commonly, but by no means universally, made in the stability analysis of the homogeneously fluidized state is that the pressure field may be well approximated by its gravitational component alone. This assumption is critically examined: the results indicate that it is valid for most cases of practical interest but can lead to significant discrepancies for liquid fluidization of low‐density particles.
Gas diffusion in random‐fiber substrates
Authors: Melkote Rajesh R. , Jensen Klavs F.
Published: June 2004

A Monte Carlo (MC) technique involving explicit calculation of molecular trajectories in fibrous media is used to obtain effective transport coefficients in the Knudsen regime of gas diffusion. A fully penetrable cylinders (FPC) model is used to represent a fibrous substrate during the course of densification, as in composite fabrication processes such as chemical vapor infiltration (CVI). The calculated Knudsen permeabilities are in excellent agreement with available data. Accessible porosities are computed as a function of total porosity and are shown to depend on the fiber number density. The nonfiber phase is shown to percolate at ϕ ≈ 0.095, but the threshold is sensitive to the boundary conditions employed.
Modeling phenol degradation in a fluidized‐bed bioreactor
Authors: Livingston Andrew G. , Chase Howard A.
Published: June 2004

A study was made of phenol degradation by bacteria immobilized onto particles of calcined diatomaceous earth in a draft‐tube, three‐phase fluidized‐bed reactor.

A mathematical model is used to describe simultaneous diffusion and reaction of oxygen and phenol in the reactor. Kinetic parameters for the growth of nonsupported cells were obtained in batch and chemostat experiments. Liquid‐solid mass transfer coefficients were determined experimentally and showed good agreement with literature values for conventional three‐phase fluidized beds. Experimental steady‐state degradation data were used to calculate biofilm substrate diffusivities. These were found to decrease as the biofilm density increased.

The transition from phenol to oxygen‐limiting biofilm kinetics predicted by the model was shown to exist experimentally. A critical ratio of phenol/dissolved oxygen concentration was found at which this transition occurred. This provides a criterion for establishing whether increased aeration will increase the volumetric degradation rate.
Simulation of ceramic particle formation: Comparison with in‐situ measurements
Authors: Zachariah M. R. , Semerjian H. G.
Published: June 2004

Ceramic particle formation processes have been studied using SiO2 as a model compound. Silica particles have been synthesized in a counterpropagating diffusion flame reactor, in which in‐situ measurements of particle size and number density have been made. In addition, the time‐temperature history of the particle field has been calculated from a flame simulation. Numerical simulations using moment and sectional methods for particle formation have been applied and compared to the experimental measurements. The simulations for the particle formation assume a kinetically‐constrained approach, allowing a simple representation of nucleation, surface growth and coagulation. The results suggest that, if the source rates are known well enough, particle formation of low vapor pressure species can be predicted. Both models do well in predicting the gross features of particle formation (number density and mean particle size), although the moment solution does a poor job of predicting the polydispersity effects during periods of high monomer generation rates.
Closure models for turbulent reacting flows
Authors: Dutta A. , Tarbell J. M.
Published: June 2004

In this paper, a simple procedure based on fast and slow reaction asymptotics has been employed to derive first‐order closure models for the nonlinear reaction terms in turbulent mass balances from mechanistic models of turbulent mixing and reaction. The coalescence‐redispersion (CRD) model, the interaction by exchange with the mean (IEM) model, the three‐environment (3E) model, and the four‐environment (4E) model have been used to develop closure equations. The closure models have been tested extensively against experimental data for both single and multiple reactions. The closures based on slow asymptotics for the CRD, 3E and 4E models provide very good predictions of all of the experimental data, while other models available either in the literature or derived here are not adequate. The simple new closure equations developed in this paper may be useful in modeling systems involving turbulent mixing and complex chemical reactions.
Calculation of the compression zone height in continuous thickeners
Authors: Font R.
Published: June 2004

A method to calculate the compression zone height in continuous thickeners is presented. With this method, it is necessary to know the variations of the pulp‐supernatant interface and the sediment height vs. time in a batch test. This method is discussed considering the following aspects: relationship between the settling rate of solids and their concentration in the settling zone; and the compression of solids due to the squeeze transmitted by the upper layers—unbuoyed weight of particles minus force of friction due to the Darcian flow. When the variation of the sediment height vs. time becomes linear, it is possible to calculate the maximum solids concentration which can be reached by sedimentation. The change of the solids matrix permeability and its influence on the method proposed are also analyzed.
Fluidized bed drying of solids
Authors: Chandran A. N. , Rao S. Subba , Varma Y. B. G.
Published: June 2004

A kinetic model is developed for the drying of solids in fluidized beds, assuming a falling rate period following a constant rate period. Experimental data obtained using batch and continuous single and spiral fluidized beds are satisfactorily matched with the assumed drying kinetics and the residence time distribution of solids appropriate for the type of dryer.
The backfill cycle of the pressure swing adsorption process
Authors: Liow Jong‐Leng , Kenney C. N.
Published: June 2004

The backfill cycle in the pressure swing adsorption (PSA) separation of air to produce an enriched oxygen product using a zeolite 5A molecular sieve was studied theoretically and experimentally. The effects of the backfill cycle, cycle configuration, backfill rate, pressurisation rate, and product rate were studied. The theory agreed well with the experimental results in predicting the product oxygen concentration over a wide range of backfill pressures, thus giving a basic understanding of the bed dynamics of the backfill cycle. The theory showed that the backfill rate and adsorption capacity of the adsorbent were the most important parameters governing the efficiency of the PSA process. We showed that, given an adequate backfill pressure, a separate pressurization step could be omitted by incorporating it with the product release step with no detrimental effect on the product oxygen concentrations while increasing adsorbent productivity.
The stability analysis of nonlinear feedback systems
Authors: Khambanonda Tanes , Palazoglu Ahmet , Romagnoli Jose A.
Published: June 2004

A stability condition is developed for multivariable, nonlinear feedback systems. The method is based on a modified sector condition and is combined with a polynomial expansion of the nonlinear system to create viable approximations that can be exploited within the sector bound setting. An example demonstrates the utility of the method in defining stable operating regimes for chemical processes.
The fluid dynamics of reverse roll coating
Authors: Coyle D. J. , Macosko C. W. , Scriven L. E.
Published: June 2004

The flow in the metering gap of a reverse roll coater is examined by experiments and finite element solutions of the Navier‐Stokes equations. At high speed ratios and capillary numbers, the metered film flow deviates strongly from predictions of lubrication theory: the wetting line moves through the gap center and the metered film thickness passes through a minimum. The two flow instabilities found are ribbing, a sinusoidal cross‐web waviness extending smoothly down‐web; and cascade, an irregular V‐shaped cross‐web wave, repeated quasiperiodically down‐web. Experimental operability diagrams define parameter ranges where these instabilities and the steady two‐dimensional flow are encountered. Ribbing behavior is understood by consideration of the pressure gradient at the free surface. The mechanism of cascade is the intrusion through the gap of the wetting line, which causes the metered film to thicken and eventually reattach to the metering roll in a cyclical manner.
Estimation of multiple specific growth rates in bioprocesses
Authors: Pomerleau Yves , Perrier Michel
Published: June 2004

An on‐line estimation technique for multiple specific growth rates is proposed. Time‐varying tuning parameters are used to obtain constant estimation error dynamics in spite of strongly varying process dynamics commonly encountered in bioprocesses. This approach simplifies the tuning of the algorithm to a pole placement procedure. Application of the method is illustrated through the study of the baker's yeast fedbatch process. Guidelines for selection of measured state variables are given in relation to the performance of the estimation method.
Thermoflow multiplicity in a cooled tube
Authors: Stroh Friedemann , Pita Jorge , Balakotaiah Vemuri , Luss Dan
Published: June 2004

The coupling between the momentum and energy balances and the change of physical properties with temperature can give rise to situations in which the pressure drop vs. flow rate curve is nonmonotonic. This can lead to thermoflow multiplicity—the existence of different flow rates in a tube under the same overall pressure drop. A two‐dimensional model is used to analyze the conditions leading to thermoflow multiplicity for an incompressible non‐Newtonian fluid flowing in a cooled tube. First, the multiplicity features of various limiting models are determined. These results are later used to gain an understanding of the asymptotic multiplicity features of the general model. The results show that the temperature sensitivity of the viscosity necessary for thermoflow multiplicity to occur decreases with increasing Brinkman number or β (modified Stanton number), or with decreasing cooling temperature, Biot number, or power law parameter. Multiple flow rates for a prescribed pressure drop are unlikely to occur in heat exchangers in which the Brinkman numbers are usually low and Biot numbers are high but may be found in polymer processing applications.
A bubbling fluidization model using kinetic theory of granular flow
Authors: Ding Jianmin , Gidaspow Dimitri
Published: June 2004

Detailed knowledge of solids circulation, bubble motion, and frequencies of porosity oscillations is needed for a better understanding of tube erosion in fluidized bed combustors. A predictive two‐phase flow model was derived starting with the Boltzman equation for velocity distribution of particles. The model is a generalization of the Navier‐Stokes equations of the type proposed by R. Jackson, except that the solids viscosities and stresses are computed by simultaneously solving a fluctuating energy equation for the particulate phase. The model predictions agree with time‐averaged and instantaneous porosities measured in two‐dimensional fluidized beds. Observed flow patterns and bubbles were also predicted.
Correlation of aqueous Henry's constants from 0°c to the critical point
Authors: Harvey Allan H. , Sengers J. M. H. Levelt
Published: June 2004

Recent theoretical results (Japas and Levelt Sengers, 1989) for the temperature dependence of Henry's constant near the solvent's critical point are used to obtain a linear expression which, for aqueous solutions of nonpolar gases, fits experimental Henry's constant data at temperatures from water's critical point down to roughly the normal boiling point. A small correction with only one additional adjustable parameter extends the correlation to 0°C. The final result is a three‐parameter correlation, covering the entire range of temperatures, which fits the available data as well as or better than existing four‐parameter empirical expressions and better than a recently proposed three‐parameter expression. Since the correct near‐critical behavior is built in, the new correlation should be especially useful for estimating Henry's constants in systems where little or no data exist at high temperatures. Fitted parameters are given for ten nonpolar gases in H2O and six nonpolar gases in D2O.
Study of restricted diffusion in porous catalysts by NMR
Authors: Cheng W. C. , Luthra N. P. , Pereira C. J.
Published: June 2004

The diffusion of 1,3,5 tri‐tert‐butylbenzene (TTB) into cylindrical γ‐alumina extrudates of varying micropore diameter and macropore volume has been studied by NMR spectroscopy. The proton NMR signal of the methyl proton of TTB in the micropores of the alumina extrudate was found to be well resolved and shifted upfield relative to that in the bulk liquid by 0.55 ppm. The area under the shifted peak is proportional to the liquid concentration of TTB in the micropores. The change in the intensity of this peak as a function of time and measurements of the amount of TTB absorbed on the surface alumina at equilibrium were used to calculate the effective diffusivity of TTB in each extrudate. Measured values of the effective diffusivity are in reasonable agreement with predictions using correlations in the literature. These observations suggest a new method for measuring the liquid‐phase effective diffusivity in porous materials.
Elastohydrodynamics of blade coating
Authors: Pranckh F. R. , Scriven L. E.
Published: June 2004

Blade coating is the most common method of coating paper, and it is applied in coating magnetic suspensions and adhesives. Typically, the blade's upstream edge is clamped into a rigid holder so that the blade and substrate form a converging wedge. The translating substrate drags liquid into the wedge, where liquid forces develop and deflect the elastic blade, but it is loaded externally to counteract the deflection. Liquid dragged past the blade and thus the coating thickness depend on the elastohydrodynamic interaction among blade, liquid, and loading. Shell theory and lubrication flow theory describe the interaction by means of differential equations that are shown to be equivalent to Saita and Scriven's (1985) earlier formulation but that lead to a computationally more efficient analysis. Predictions computed agree with those of the earlier formulation and with experiments. Moreover, the new predictions illustrate effects of normal force loading and broaden the theory of blade coating.
Axial dispersion for turbulent flow with a large radial heat flux
Authors: Beckman Lawrence V. , Law Victor J. , Bailey Raymond V. , Von Rosenberg Dale U.
Published: June 2004

In 1954, G. I. Taylor derived an expression for an apparent axial dispersion coefficient for mass transfer in turbulent flow under the assumption of no mass transport between the tube wall and the fluid inside the tube. Taylor's expression has been verified in a number of experimental efforts for both mass and heat transfer.

This work extends the results of Taylor to the case where there is a significant transport of heat from the tube wall to the fluid in the tube, such as in a double‐pipe heat exchanger. Using an analysis similar to Taylor's, it is shown that the apparent axial dispersion coefficient is about one order of magnitude larger when a large radial gradient is present. Experimental data on a double‐pipe heat exchanger show significantly improved correspondence with a dispersion‐based model of the system when the larger dispersion coefficients are used.
Dynamics of pulsing flow in trickle beds
Authors: Dankworth David C. , Kevrekidis I. G. , Sundaresan S.
Published: June 2004

A macroscopic model of two‐phase flow in packed beds, based on the volume‐averaged equations of motion for the gas and liquid phases, was analyzed in an attempt to understand the onset and evolution of fully‐developed pulsing flow in trickle beds. By assuming that solutions take the form of waves travelling at constant speed, periodic solutions to these equations are found which can be associated with long‐time, asymptotic behavior of pulses in a very long bed. Families of one‐dimensional waves which exist at a particular set of mass fluxes can be characterized by infinite period bifurcations in the travelling wave frame. We numerically follow these bifurcations as the fluxes are changed, generating bifurcation diagrams for the original model. The results predict that properties of one‐dimensional pulses should correlate with the total superficial velocity through the bed. A hysteresis in the trickling‐pulsing transition is also predicted.
Collocation analysis of a boundary‐layer model for crossflow fractionation trays
Authors: Young Thomas C. , Stewart Warren E.
Published: June 2004

A nonequilibrium model for fractionation trays with lateral dispersion is analyzed by a collocation method. Mobile‐interface turbulent boundary‐layer theory is used to describe the local heat and mass transfer rates with allowance for vapor superheat, liquid subcooling, and net molar transfer between the phases. Multicomponent transport is incorporated by a linearized matrix method. Numerical integration of the gradients through the tray permits use of local flows and states, as well as local phase equilibrium expressions. A collocation scheme solves the model efficiently; two grid points normally suffice to represent a tray. The model contains just two adjustable coefficients, a00 and Pe; the dependence of these on tray geometry and hydrodynamics remains to be investigated.
Hydrodynamics of gas‐agitated liquid‐liquid dispersions
Authors: Hatzikiriakos Savvas G. , Gaikwad Rajendra P. , Nelson Philip R. , Shaw John M.
Published: June 2004

Gas‐agitated liquid‐liquid dispersions arise in applications as diverse as direct hydrogenation processes for bitumen and coal, and the manufacture of iron and steel. The transfer of gas‐phase constitutents to the dispersed liquid phase and/or elution of dispersed‐phase drops have been identified as potential limiting phenomena in these processes. Consequently, mean drop size and drop size distribution are key design variables. In this paper, the impact of gas flux and the physical properties of dispersed‐phase constitutents on the steady‐state size distribution of liquid drops in lean liquid‐liquid dispersions is quantified. The physical properties of the dispersed phase are shown to have a significant impact on drop size and drop‐size distribution at low gas fluxes. Sauter mean drop size is correlated using theoretical models for drop break‐up and coalescence. All results are compared with stirred tank analogues.
Stability of multilayer extrusion of viscoelastic liquids
Authors: Anturkar N. R. , Papanastasiou T. C. , Wilkes J. O.
Published: June 2004

A linear stability analysis of multilayer plane Poiseuille flow of Oldroyd‐B liquids with shear rate dependent viscosities is performed for an arbitary number of layers. Asymptotic solutions at long wavelengths and numerical solutions at wavelengths of O(1) are obtained for two‐dimensional infinitesimal disturbances. The asymptotic solutions are identical for viscoelastic and Newtonian liquids in two‐ and three‐layer flows, except for nearly geometrically symmetric configurations in three‐layer flows. Multilayer flows of viscoelastic liquids can be stable at all wavelengths; thus, operating diagrams of stable flows can be constructed. Symmetric and nearly symmetric configurations in three‐layer flows are unstable when the core layer is more viscous than the cuter layers. For highly elastic liquids, stability is not influenced by elasticity, whereas shear thinning always destabilizes the flow. The analysis provides guidelines to avoid interfacial instabilities, which originate inside dies of multilayer extrusion.
A kinetic model for industrial gas‐phase ethylene copolymerization
Authors: McAuley K. B. , MacGregor J. F. , Hamielec A. E.
Published: June 2004

A dynamic kinetic model describing gas‐phase olefin copolymerization using a multiple active site Ziegler‐Natta catalyst is presented. This model is capable of predicting production rate, molecular weight, and copolymer composition changes observed in an industrial reactor. The model also explains how broad molecular weight distributions and bimodal copolymer compositions can occur as has been observed for commercial linear polyethylenes.
RF plasma synthesis of amorphous AIN powder and films
Authors: David M. , Babu S. V. , Rasmussen D. H.
Published: June 2004

Amorphous powder and thin films of aluminum nitride (AIN) are deposited in a parallel‐plate RF plasma reactor starting from trimethylaluminum (TMA) and ammonia. Film deposition is favored at lower pressures and NH3/TMA ratios; particle deposition occurs under higher pressures and NH3/TMA ratios. Electron microscopy, x‐ray diffraction, IR spectroscopy, and UV reflectance measurements are used for product characterization. Films of AIN grown at 250°C are transparent, pure, and have high UV reflectivity. AIN powder is amorphous and has a particle size of 10–20 nm and surface area of about 85 m2/g. Amine impurities present in the as‐deposited powder are removed by heat treatment at 600°C. The IR spectra of as‐deposited powders do not show the characteristic Al‐O bonding peak found in powders deposited by other methods.
Effective thermal conductivity of composites containing spheroidal inclusions
Authors: Lu Shih‐Yuan , Kim Sangtae
Published: June 2004

The effective thermal conductivity of composite materials containing perfectly‐conducting, perfectly‐aligned, spheroidal inclusions has been computed to O(c2), where c is the volume fraction of the inclusions, by incorporating two‐particle interactions in a rigorous fashion. Relevant two‐particle problems were solved by boundary collocation and method of reflections in the near and far fields, respectively. Our results for the effective thermal conductivity are rigorous extensions of Maxwell's theory in the form of an expansion in small cℓα, where ℓ is the spheroid aspect ratio (α = 1 for oblate and α = 2 for prolate spheroids). The influence of microstructure was examined with two ad hoc model distribution functions: 1. a uniform distribution and 2. a stretched hard‐sphere pair distribution. The latter yielded superior results over a greater range of c, as judged by comparisons with Willis' (1977) bounds developed from variational methods.