bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrodynamics in a cocurrent gas‐liquid trickle bed at elevated pressures
Authors: Wammes W. J. A. , Middelkamp J. , Huisman W. J. , DeBass C. M. , Westerterp K. R.
Published: June 2004

Data on design and operation of trickle beds at elevated pressures are scarce. In this study the influence of the gas density on the liquid holdup, the pressure drop, and the transition between trickle and pulse flow has been investigated in a tricklebed reactor operating up to 7.5 MPa and with nitrogen or helium as the gas phase. Gas–liquid interfacial areas have been determined up to 5.0 MPa by means of CO2 absorption from CO2/N2 gas mixtures into amine solutions.

A comparison of the results from nitrogen as the gas phase to those of helium shows that at equal gas densities the hydrodynamic states are the same. The gas‐liquid interfacial area increases when operating at higher gas densities. When the determined dimensionless interfacial areas agl/as are all within the range 0.25–0.8, the trickle‐bed reactor is suggested to operate in the trickle‐flow regime. The gas density has a strong influence on the liquid holdup. Due to the higher pressure gradients at elevated gas densities, the liquid holdup decreases noticeably. Besides, the boundary between the trickle‐flow and pulse‐flow regime shifts toward higher liquid throughputs: the region for trickle‐flow operationg becomes larger. For the liquid holdup and the pressure gradient in the trickle‐flow regime, correlations derived based on dimensionless numbers can be applied to high‐prssure trickle beds.
Generalized approach toward modeling resist performance
Authors: Ziger David H. , Mack Chris A.
Published: June 2004

A generalized technique for modeling resist performance is outlined. In this approach, the fraction of resist remaining after development as a function of incident dose, or characteristic curve, is related to the development rate which is assumed to be a power law of a dominant soluble species. Soluble species are either photochemically consumed for negative resists or generated for positive. Expressions for the dependence of characteristic curves on exposure dose and chemistry are derived for various resist systems, which are consistent with current models. For similar chemical kinetics, negative resists yield fewer lumped parameters to describe their development rate and characteristic curves than positive.

Under conditions of negligible surface inhibition, lumped parameters can be extracted from characteristic curves and used to simulate lithography. A generalized method to correct for absorption in the resist and reflections is outlined. Exposure latitude was accurately predicted for a commercial negative chemically amplified resist. However, prediction of linewidths from characteristic curves of positive resists is complicated by surface inhibition effects.
Continuous process improvement through inductive and analogical learning
Authors: Saraiva Pedro M. , Stephanopoulos George
Published: June 2004

This article presents a methodology for the continuous detection and definition of process performance improvement opportunities, especially as these pertain to the quality of operations (such as product quality). The problem is first reduced to an essentially pattern recognition formulation, for which an integrated and adaptive methodology combining analogical reasoning and symbolic induction is developed. The resulting classification of past records of data is used to support the construction of a decision support system for the generation/selection of operating suggestions leadin to performance improvements. The overall approach complements the usual set of statistical tools, commonly employed to address quality management problems. The basic methodology is also extended to handle fuzzy class definitions and function learning formulations. Case studies, covering both simulated and real industrial situations, illustrate the concepts and their practical utility.
A new scheme combining neural feedforward control with model‐predictive control
Authors: Lee Moonyong , Park Sunwon
Published: June 2004

A new control scheme is presented for feedforward control of unknown disturbances in the model‐predictive control (MPC) scheme. In this control scheme, a neural network is connected in parallel with the MPC controller and trained online by minimizing the MPC controller output corresponding to the unmodeled effect. It is applied to distillation column control and nonlinear reactor control to illustrate its effectiveness. The result shows that the neural feedforward controller can cope well with strong interactions, time delays, nonlinearities, and process/model mismatch. The controller also offers such advantages as fault tolerance, generalization capability by interpolation, and learning capability by randdom input patterns.
Investigation of amorphous oxide film‐electrolyte junctions by AC techniques
Authors: Piazza S. , Sunseri C. , Di Quarto F.
Published: June 2004

Current AC (alternating current) techniques are used often to characterize the energetics at a semiconducting solid phase/electrolyte interface. For thin layers having a strongly disordered or amorphous structure (such as oxide‐passive layers anodically grown on valve metals), interpretative models currently used for crystalline semiconductors may produce misleading data.

A new interpretation of the admittance data, based on recent models for amorphous semiconductors (a‐Sc) Schottky barriers, is presented for passive films of Nb, W and Ti. The physical bases of the model are presented as well as its advantages and disadvantages. The new theory views the solid/electrolyte interface more satisfactorily and provides information on the solid‐state properties and the electronic structure of the electrode useful for interpreting the electron exchange between the solid phase and redox couples in solution.
Kinetics of COS with primary and secondary amines in aqueous solutions
Authors: Littel R. J. , Versteeg G. F. , van Swaaij W. P. M.
Published: June 2004

The reaction between COS and aqueous solutions of primary and secondary amines has been studied by means of the stirred cell technique. Kinetic experiments at temperatures 283 to 333 K were carried out with MEA, DGA, DEA, DIPA, MMEA, AMP, and MOR. All kinetic experiments could be described by a zwitterion reaction mechanism similar to the mechanism proposed by Caplow (1968) for the reaction between CO2 and secondary amines:

Analysis of concentrated amine solutions at high COS concentrations by various analytical techniques confirmed the conclusions from the kinetic experiments. For all amines except for MEA, the overall reaction rate was found to be determined entirely by the zwitterion deprotonation rate.
Temperature oscillations and vibrations of AC resistively heated thin filaments
Authors: Viljoen H. J. , Hlavacek V.
Published: June 2004

Resistively heated filaments are used to deposit diamond or to act as a substrate in the manufacture of ceramic fibers. This article analyzes the effects of an AC power source, in particular the oscillation in filament temperature and its influence on the kinetic rates, as well as the vibration of the filament, when it is mounted in a state of tensile stress. These effects diminish when the filament gauge is large enough and the analysis can help to decide whether an AC or a DC power source should be used.
Kinetics of phenol oxidation in supercritical water
Authors: Thornton Thomas D. , Savage Phillip E.
Published: June 2004

Aqueous solutions of phenol were oxidized in a flow reactor at temperatures between 300 and 420°C (0.89 ⩽ Tr ⩽ 1.07) and pressures from 188 to 278 atm (0.86 ⩽ Pr ⩽ 1.27). These conditions included oxidations in both near‐critical and supercritical water. Reactor residence times ranged from 1.2 to 111 s. The initial phenol concentrations were between 50 and 330 ppm by mass, and the initial oxygen concentrations ranged from 0 to 1,100% excess. The oxidation experiments covered essentially the entire range of phenol conversions. Analysis of the kinetics data for phenol disappearance using a combination of the integral method and the method of excess revealed that the reaction was first order in phenol and 1/2 order in oxygen, and influenced by pressure. The global reaction order for water was taken to be nonzero, and the global rate constant was assumed to be independent of pressure so that the only effect of pressure was to alter the water concentration and hence the reaction rate. This approach led to a global reaction rate law that was 0.7 order in water and had a rate constant with an activation energy of 12.4 kcal/mol. The implications of these rate laws to the design of a commercial supercritical water oxidation reactor are also explored.
Transfer processes in turbulent pipe flow described by the ERSR model
Authors: Fortuin Jan M. H. , Musschenga Edward E. , Hamersma Peter J.
Published: June 2004

A model is presented for a quantitative prediction of the transfer rates of momentum, heat and mass in turbulent pipe flow. In this so‐called extended random surface renewal (ERSR) model, the tube wall is assumed to be covered by a mosaic of fluid elements of random age and laminar flow with unsteady profiles of axial velocity, temperature or concentration.

Both the age distribution and the mean age of the fluid elements at the tube wall, predicted by the ERSR model, quantitatively agree with experimental results obtained from velocity signals measured with a laser‐Doppler anemometer at 5·103⩽Re⩽43·103. The time‐averaged radial profiles of the axial velocity, the temperature and the concentration in the wall region, and the heat‐ and mass‐transfer coefficients derived with the ERSR model agree with empirical results presented in literature. Furthermore, the ERSR model provides a basis for explaining the Chilton‐Colburn analogy.
Single‐pellet cell for the measurement of intraparticle diffusion and convection
Authors: Lu Z. P. , Loureiro J. M. , Rodrigues A. E.
Published: June 2004

Dynamic analysis of single‐pellet cell for the measurement of diffusion and convection in porous catalysts is revisited. A complete mathematical model for the “diffusion‐convection” cell is solved, and the relation between zero‐and first‐order moments of the cell response and operating parameters is derived. The validity region of the “simplified” boundary condition at the bottom of the pellet is quantified. The effect of the operating parameters on the cell time response to a tracer impulse is analyzed to check the practical range of operation of the system. It also is compared with the chromatographic technique.
CVD of solid oxides in porous substrates for ceramic membrane modification
Authors: Lin Y. S. , Burggraaf A. J.
Published: June 2004

The deposition of yttria‐doped zirconia has been experimented systematically in various types of porous ceramic substrates by a modified chemical vapor deposition (CVD) process operating in an opposing reactant geometry using water vapor and corresponding metal chloride vapors as reactants. The effects of substrate pore dimension and structure, bulk‐phase reactant concentration, reactant diffusivity in substrate pores and deposition temperature are experimentally studied and explained qualitatively by a theoretical modeling analysis. The experimental and theoretical results suggest a reaction mechanism which depends on water vapor and chloride vapor concentrations. Consequently, the diffusivity, bulk‐phase reactant concentration, and substrate pore dimension are important in the CVD process. Effects of deposition temperature on the deposition results and narrow deposition zone compared to the substrate thickness also suggest a Langmuir‐Hinshelwood reaction mechanism involved in the CVD process with a very fast CVD reaction rate. Gas permeation data indicate that whether deposition of solid in substrate pores could result in the pore‐size reduction depends strongly on the initial pore‐size distribution of the substrate.
Spin coating of colloidal suspensions
Authors: Rehg Timothy J. , Higgins G.
Published: June 2004

The coupled, unsteady Navier‐Stokes, convective diffusion, and thermal energy equations that describe spin coating of colloidal suspensions are solved numerically. The theoretical model, absent of any adjustable parameters, is used to explore the effects of angular velocity, initial solvent weight fraction, solvent properties and spin coating protocol on the evolution of temperature and concentration profiles in the liquid film during spin coating. The predicted coated film thickness is found to be in excellent quantitative agreement with spin coating experiments performed with both hard‐sphere and nonhard‐sphere suspensions of monodisperse latex particles in water. The coated film thickness, determined by ellipsometry, is shown to depend on the inverse square root of the angular velocity except at high ionic strength when the dependence on angular velocity is weaker. Timescales that characterize spin coating of colloidal suspensions are shown to be quite different from those that characterize spin coating of polymer solutions, and consequently simple models for predicting the coated film thickness of polymer solutions (Bornside et al., 1991; Lawrence, 1989) are shown to be inadequate for colloidal suspensions. Rapid substrate acceleration, high rotation rates, partial saturation of the overlying gas phase, and high initial solids concentration are identified as spin coating protocols that suppress a convective instability that produces radial striations in the coated film.
Phase equilibria in the system water‐methyldiethanolamine‐propane
Authors: Carroll John J. , Jou Fang‐Yuan , Mather Alan E. , Otto Frederick D.
Published: June 2004

Aqueous solutions of alkanolamines are commonly used to strip acid gases (CO2 H2S) from hydrocarbon streams. Processes for stripping acid gases from gaseous streams are well understood; however, the application to liquids is not as advanced. Experimental data available are scarce for systems containing aqueous amine solutions and the constituents of liquefied petroleum gases. To this end, new data are presented for the equilibria in the system propane–water–methyldiethanolamine (MDEA). A knowledge of the phase behavior in this system is required to model the equilibria encountered in the more complex systems of industrial importance.

This work is a comprehensive study of the phase equilibria in the system propane–3 M MDEA, including vapor–liquid, liquid–liquid, and vapor–liquid–liquid equilibria. Experimental measurements were made for temperatures between 0 and 150°C and pressures up to 20 MPa. The data were correlated using the Stryjek‐Vera modification of the Peng‐Robinson equation of state. In addition, a composition‐dependent mixing rule was required to fit the data adequately.
Relative disturbance gain array
Authors: Chang Jin‐Wen , Yu Cheng‐Ching
Published: June 2004

Disturbance rejection capabilities of different controller structures, for example, diagonal, block diagonal or full multivariable controller, are discussed. A generalized version of Relative Disturbance Gain, Generalized Relative Disturbance Gain (GRDG), is defined to evaluate the disturbance rejection capabilities of all possible controller structures. Furthermore, the relative disturbance gain array (RDGA) is introduced. Basic properties of RDGA are derived. An important one is: GRDG of all possible controller structures can be calculated directly from the array. Therefore, with RDGA, the synthesis of the controller structure can be done in a straightforward manner. Physical implications and quantitative analyses of GRDG are given. These form the basis for the synthesis. Finally, frequency‐dependent GRDG is developed which evaluates the performance further based on dynamic information. Several examples are used to illustrate the synthesis of the controller structure. The results show that better disturbance rejection can be achieved by selecting appropriate controller structure.
Natural tracking PID process control for exponential tracking
Authors: Grujić Ljubomir T. , Mounfield William Pratt
Published: June 2004

To cope with modeling uncertainties and randomness of external disturbances, a new tracking control called the natural control concept is designed. Its implementation is completely independent of the internal dynamics of a controlled system, its desired output and external disturbances. The design algorithm established ensures a prespecified exponential quality of output tracking. The theory presented in this article is applied effectively to the design of natural tracking control for a chemical reaction process described by the fourth‐order, linear, state‐space mathematical model.
Unbiased estimation of gross errors in process measurements
Authors: Rollins D. K. , Davis J. F.
Published: June 2004

A new approach to gross error detection provides unbiased estimates and 100(1‐α)% simultaneous confidence intervals of process variables when biased process measurements and process leaks exist. Presented in this article are estimation equations for process variables, as well as equations that help identify biased measurements and process leaks. These equations include the power function for a global test, and two types of α‐level component tests and their power functions. Important strengths and weaknesses of this approach are compared to those of the serial compensation strategy, in particular, by varying the significance level (α), the variance‐covariance matrix (Σ), the size of measurement bias (δ), the number of biased variables, and the sample size (N). Accuracy of δ estimation and performance in detecting the presence of process leaks (γ) are also evaluated and compared. The proposed approach has unique features that can provide a basis for improving the reconciliation of variables in process operations.
Correlation of fractionation tray performance via a cross‐flow boundary‐layer model
Authors: Young Thomas C. , Stewart Warren E.
Published: June 2004

The nonequilibrium tray model proposed by Young and Stewart (1990) is used to correlate published fractionation data from sieve tray columns 0.45 m to 1.2 m in diameter. The model contains a dimensionless coefficient a00 to describe the interphase heat and mass transfer, and a Peclet number Pe to describe the lateral mixing of the liquid on the tray. The coefficient a00 is investigated as a function of tray geometry and hydrodynamic variables, using an error‐in‐variables extension of a multiresponse parameter estimation algorithm. Data from several sources are well correlated by a simple dimensionless relation for a00, with Pe predicted from the results of Bennett and associates (1983, 1991).
Influence of tube and particle diameter on heat transport in packed beds
Authors: Borkink J. G. H. , Westerterp K. R.
Published: June 2004

Influence of the tube and particle diameter and shape, as well as their ratio, on the radial heat transport in packed beds has been studied. Heat transport experiments were performed with four different packings in three wall‐cooled tubes, which differed in inner diameter only. Experimental values for the effective radial heat conductivity and wall heat‐transfer coefficient for the pseudo‐homogeneous two‐dimensional model and the overall heat‐transfer coefficient for the one‐dimensional model are presented. Values were obtained for glass spheres, alumina cylinders, and alumina Raschig rings. The effective radial heat conductivity and wall heat‐transfer coefficient can both be correlated as a linear function of the gas flow rate. The Bodenstein number for heat at fully developed turbulent flow is influenced strongly by the shape of the packing: 10.9 for glass spheres, 7.6 for alumina cylinders, and 4.2 for alumina Raschig rings. For the same packing, no significant influence is found of the tube diameter on the effective radial heat conductivity or on the wall heat‐transfer coefficient. The overall heat‐transfer coefficient can be described very well by the so‐called “lump equation,” which gives the relations among the overall heat‐transfer coefficient, effective radial heat conductivity, and wall heat‐transfer coefficient. The “lump factor,” as used in the lump equation, has a best‐fit experimental value of 7.4.
Venting of runaway reactions with gas generation
Authors: Leung Joseph C.
Published: June 2004

Pressure relief venting of a runway reaction with gas generation is examined in terms of the overall system response and the associated venting requirement. This article presents an exact formulation as well as an approximate analytical approach. The latter is shown to be particularly useful in vent‐sizing applications. Using an aqueous hydrogen peroxide decomposition example, the analytical results are demonstrated to agree well with the exact numerical results over a wide range of overpressure. In addition, the analytical result is shown to reduce to the correct limit for a pure vapor system and offers a useful vent‐sizing equation for a pure‐gassy system.
Mechanism of heat transfer in bubbly liquid and liquid‐solid systems: Single bubble injection
Authors: Kumar Samir , Kusakabe K. , Raghunathan K. , Fan L.‐S.
Published: June 2004

Local instantaneous changes in heat‐transfer coefficients due to the passage of gas bubbles in liquid and liquid‐solid systems are measured. A special heat‐transfer probe is developed and located within the bed to trace the instantaneous local heat‐transfer rate during the passage of single gas bubbles. A microfoil heat flow sensor is attached to a foil heater, and the sensor‐heater probe assembly can accurately measure the heat flux and the surface temperature over a small area. Signals from the sensor are amplified and interfaced with the microcomputer data acquisition system. Simultaneous visualization is performed using a high‐speed video camera and a borescope to establish the correspondence between the visual and sensor signals, and hence relate the local instantaneous hydrodynamics to the heat‐transfer rate. Local heat‐transfer coefficient vs. time traces are analyzed in conjunction with visual signals. The heat‐transfer coefficient exhibits a sharp peak in the bubble wake. In both liquid and liquid‐solid systems, the observed local maximum in heat‐transfer coefficient behind a rising bubble is due to the bubble‐wake‐induced surface renewal. Enhancement in heat transfer due to the bubble increases with the size because of increased surface renewal caused by larger bubble wake and stronger vortices. The local maximum in heat transfer, however, is more pronounced in liquid than in liquid‐solid systems.
Zirconium transport dynamics in the Purex solvent extraction process
Authors: DeMuth Scott F. , Counce Robert M.
Published: June 2004

The primary objective of this study was to develop and validate experimental and mathematical procedures that can be used to determine the fundamental mechanisms of the Purex (plutonium and uranium) extraction process. The mechanisms of this solvent extraction process have been established by classical chemical engineering principles.

In the past, modeling of transport phenomena for the solvent extraction of spent nuclear fuel by the Purex process has typically (1) relied solely on diffusion, (2) relied solely on chemical reaction, (3) combined diffusion and chemical reaction in a semiempirical fashion, or (4) combined diffusion and chemical reaction in a nonclassical chemical engineering manner.

This study was conducted with the single Purex species, aqueous Zr4+, to simplify the chemistry and modeling mathematics. The validated procedures and model are applicable to most all Purex species.
Uranium adsorption characteristics of a circulating fluidized‐bed adsorber
Authors: Ito Yoshiro , Nakamura Susumu , Shirakashi Masataka , Kanno Masayoshi
Published: June 2004

A new type of fluidized bed, called a circulating fluidized‐bed adsorber (CFBA), was developed for processing a large quantity of water with high efficiency. Its fluid mechanical characteristics are reported, as well as recovery of uranium from natural seawater using a small CFBA. Using hydrous titanium oxide, granulated with polyacrylonitrile (PAN‐HTO), as an adsorbent, CFBA ran stably for six days, the period required for the adsorbent to collect a practical amount of uranium. To predict the uranium adsorption performance of a plant‐size CFBA, a numerical model simulating the adsorption kinetics was developed. The result of the numerical calculation by the model was in good agreement with the experimental data.
Modeling and scale‐up of multiwafer LPCVD reactors
Authors: Badgwell Thomas A. , Edgar Thomas F. , Trachtenberg Isaac
Published: June 2004

The deposition of thin films in a hot‐wall multiwafer low‐pressure chemical vapor deposition (LPCVD) reactor is an important unit operation in the manufacture of modern integrated circuits. In this article, our previously published model for the multiwafer LPCVD reactor has been combined with in‐situ temperature measurements to accurately predict the axial and radial film thickness distributions for a polysilicon deposition process. The model describes in detail multicomponent mass transport, the reactor's thermal environment based on in‐situ temperature measurements, and the reactor geometry including inlet and outlet sections as well as downstream injectors. Model predictions were compared with experimental data from two industrial‐scale polysilicon reactors at SEMATECH and from a smaller research reactor. Approximate scale‐up rules for the important special case of larger wafers were derived from the model equations and tested by simulation. The rules compare well with the results from a nonlinear program in which the axial variation of film growth rate was minimized.
Continuous kinetic lumping of catalytic cracking processes
Authors: Cicarelli Paolo , Astarita Gianni , Gallifuoco Alberto
Published: June 2004

Processes, in which a multicomponent mixture of chemically similar compounds undergoes cracking reactions governed by nonlinear kinetics, are analyzed. Cracking reactions in general imply cleavage of chemical bonds between monomeric units. The article focuses on the case of the “uniform” kinetics. To obtain an analytical solution of the kinetic equations describing this process, we present an approach based on a continuous description of the mixture. The resulting nonlinear integrodifferential equation has been linearized by a warped time scale, and a formal solution has been obtained via a regular perturbation method. The perturbation parameter is twice the inverse of the initial average number of monomeric units in excess of the minimum number that can be cracked by the reactions considered.
Global volumetric phase fractions in horizontal three‐phase flows
Authors: Lahey Jr. R. T. , Açikgöz M. , França F.
Published: June 2004

Generalized drift‐flux expressions for horizontal three‐phase (gas/water/oil) conduit flows have been derived. A detailed set of phasic volume fraction data were measured for the various flow regimes observed in air/water/oil flows. This article shows that drift‐flux techniques can be used to predict the phasic volumetric fractions for horizontal three‐phase flows, such as those which may occur in off‐shore oil well lines.
Effect of column inclination on the performance of three‐phase fluidized beds
Authors: Del Pozo M. , Briens C. L. , Wild G.
Published: June 2004

Inclining a fluidized bed column by as little as 1.5 degrees greatly affects the bed characteristics. The bed contracts, the particle‐liquid mass‐transfer and heat‐transfer coefficients increase by up to 30%, and the gas‐liquid mass‐transfer coefficient can either be increased by up to 15% or decreased by up to 20%.
Creeping axisymmetric flow around a solid particle near a permeable obstacle
Authors: Michalopoulou A. C. , Burganos V. N. , Payatakes A. C.
Published: June 2004

The hydrodynamic interaction between a solid particle and a porous obstacle, both of spherical shape, the former moving slowly along the line of their centers and the latter held stationary in an external axisymmetrical flow field, is analyzed. Owing to the linearity of the creeping motion equations and the boundary conditions, this general problem can be decomposed into two simpler problems: I. the motion of the solid sphere relative to the porous one in a fluid at rest; II. an axisymmetrical streaming flow past the two spheres held stationary. The solution to problem II requires further decomposition into the problem of undisturbed flow in the absence of the two spheres and that of the two spheres following each other in a fluid at rest (problem III). The above component flow problems are solved analytically using the stream function formulation in bispherical coordinates. The flow and pressure fields, and the drag forces exerted on both spheres are determined as functions of the permeability, the slip factor, the gap length, and the relative size of the two spheres. In problem I it is found that the drag force exerted on the solid particle increases with decreasing permeability for any value of the gap length. The opposite behavior is observed in problems III (and II). In all cases, however, the drag force exerted on the porous sphere increases as the permeability decreases for any separation distance. In the region of very small separation distances the drag forces on the two spheres in problem I attain a weak maximum at a critical gap length which is a function of the obstacle permeability and the sphere size ratio. The behavior of the drag forces in problems II (and III) is more complicated and depends strongly on the sphere size ratio. The effects of the slip velocity and the particle to streaming velocity ratio (in the composite problem) on the values of the drag forces are also examined.
Fluid flow in compressible porous media: I: Steady‐state conditions
Authors: Jönsson K. Ann‐Sofi , Jönsson Bengt T. L.
Published: June 2004

This is the first of two articles dealing with fluid flow in compressible porous media. In this article a model describing fluid flow and pressure‐induced variations in porosity under stationary conditions is developed. In a forthcoming article the dynamic behavior during filtration and wet pressing of compressible porous media are presented.

Fluid flow through rigid porous media is generally described by Darcy's law. The corresponding expression for compressible materials is derived in this article. This expression, the steady‐state flow (SSF) equation, allows the steady‐state flow to be easily calculated, either numerically, or by using the approximative analytical solutions that are also presented here. In the SSF equation optional empirical and/or theoretical permeability and compressibility relationships may be combined. Further, a new compressibility model which also applies for viscoelastic materials is presented. The influence of the compressibility of the material and the influence of precompression is illustrated.
Unique adsorption properties and potential energy profiles of microporous pillared clays
Authors: Baksh M. S. A. , Yang R. T.
Published: June 2004

The adsorption isotherms of O2, N2, CH4, CO2, SO2, and NO on five pillared clays (Zr, Al, Cr, Fe, and Ti‐PILCs) are measured. The equilibrium selectivity of CH4/N2 on Al‐PILC is greater than 5.0, which exceeds all known sorbents by a large margin. In addition, high SO2/CO2 equilibrium selectivities are observed on these pillared clays. The sorption characteristics of these pillared clays (PILCs) exhibit characteristic trends that are better understood with the aid of the potential energy profiles. A new semi‐empirical approach is presented for the calculations of the potential energy profiles of PILCs. This approach requires the adsorption isotherms and an isotherm equation that accounts for the structural heterogeneity of the adsorbents. A comparison of the energy profiles obtained using the semi‐empirical approach with the corresponding results obtained via the Kirkwood‐Muller formalism, where only dispersion forces are taken into account, provides a measure of the importance of the electrostatic forces in the sorption characteristics of these PILCs. Sizable differences are observed for the potential energy profiles, indicating that the electrostatic forces are not negligible, and can significantly enhance the adsorption potential, resulting in large increases in the amounts adsorbed on these PILCs.
Magnetic determination of average catalyst temperatures in fluidized beds
Authors: Cale Timothy S. , Merson J. A.
Published: June 2004

A magnetic method to measure the time and spatial average temperature of supported nickel catalysts in fluidized beds is detailed. It is an extension of a magnetic method developed in this laboratory to study fixed beds. The RMS output voltage of an AC permeameter is used to follow changes in the magnetic moment, hence temperature, of the catalyst upon starting or stopping ethane hydrogenolysis. The theory of the thermometry in fluidized beds is detailed and the observed RMS output voltage vs. time traces for typical experiments are explained. Simulations indicate that signals generated by the moving particles are largely averaged out, since the direction of the magnetic moment of each catalyst particle follows the applied field and is independent of movement. Filtering in the frequency domain can be used to further reduce the effects of particle movement. Calibrations needed to convert AC permeameter output voltage changes to changes in sample temperature during experiments on two distinctly different fluidized beds are the same as for the respective fixed beds. Depending somewhat on the sample moment, the average solid temperature can be determined with a repeatability of a few tenths of a degree.
A hybrid neural network‐first principles approach to process modeling
Authors: Psichogios Dimitris C. , Ungar Lyle H.
Published: June 2004

A hybrid neural network‐first principles modeling scheme is developed and used to model a fedbatch bioreactor. The hybrid model combines a partial first principles model, which incorporates the available prior knowledge about the process being modeled, with a neural network which serves as an estimator of unmeasured process parameters that are difficult to model from first principles. This hybrid model has better properties than standard “black‐box” neural network models in that it is able to interpolate and extrapolate much more accurately, is easier to analyze and interpret, and requires significantly fewer training examples. Two alternative state and parameter estimation strategies, extended Kalman filtering and NLP optimization, are also considered. When no a priori known model of the unobserved process parameters is available, the hybrid network model gives better estimates of the parameters, when compared to these methods. By providing a model of these unmeasured parameters, the hybrid network can also make predictions and hence can be used for process optimization. These results apply both when full and partial state measurements are available, but in the latter case a state reconstruction method must be used for the first principles component of the hybrid model.
Inaccessible pore volume formation during sulfation of calcined limestones
Authors: Sotirchos Stratis V. , Zarkanitis Solon
Published: June 2004

A mathematical model for transport, reaction and structure evolution in gas‐solid reactions with solid product is used to analyze a set of experimental data for the sulfation of calcines derived from three stones of high (>95%) calcium carbonate content. The analysis of the experimental data places emphasis on the formation of inaccessible pore space and its effects on the efficiency of calcined limestones as sorbents for SO2 removal. Our results show that the ultimate capacity of calcined limestones for SO2 uptake is influenced strongly by the formation of inaccessible pore space, which in turn is determined by their pore‐size distribution and the connectivity (degree of interaction) of the pores. For the three specimens used in our study, the ultimate conversion (measured experimentally) increased and the inaccessible pore space formed during pore closure (predicted by the mathematical model) decreased with decreasing grain size in the rock precursors, indicating that the morphological texture of the precursors plays a major role in determining the topological features of the pore structure of the resulting calcines.
Analysis of effective axial diffusion in branching networks
Authors: Federspiel William J.
Published: June 2004

Diffusional transport in a network of branching conduits is considered. The branching network is idealized as an ensemble of identical branching segments. Starting with the general species continuity equation, multiscale perturbation analysis is used to derive a one‐dimensional, effective transport equation for species concentration. The macrotransport equation contains an effective diffusion coefficient, D*, which arises naturally from the analysis. D* can be computed from a local diffusion problem posed on an individual branching segment of the ensemble network. The local relations defining D* bear clear similarity to their counterparts for transport in spatially periodic porous media. Although this study was originally directed toward describing gas diffusion in the peripheral airspaces of the lung, the results provide insight for other transport processes occurring in branching networks.
Modeling the formation of boron carbide particles in an aerosol flow reactor
Authors: Xiong Yun , Pratsinis Sotiris E. , Weimer Alan W.
Published: June 2004

The formation of submicron crystals of boron carbide (B4C) by coagulation and sintering by the rapid carbothermal reduction of intimately mixed carbon‐boron oxide powders in an aerosol flow reactor at temperatures above the boiling point of boron oxide is investigated. High heating rates (105K/s) force rapid evaporation of boron oxide and suboxides from the precursor powder, resulting in its rupture and formation of boron carbide molecular clusters that grow to macroscopic particles by coagulation. Consequently, the formation and growth of B4C particles are described by simultaneous interparticle collision and coalescence using a two‐dimensional particle‐size distribution model that traces the evolution of both size and shape characteristics of the particles through their volume and surface area. In addition to the coagulation term, the governing population balance equation includes a coalescence contribution based on B4C sintering law. The predicted evolution of the two‐dimensional particle‐size distribution leads to a direct characterization of morphology as well as the average size and polydispersity of the powders. Furthermore, model predictions of the volume and surface area of boron carbide particles can be directly compared with experimental data of B4C specific surface area and an effective sintering rate of B4C is deduced.
Prediction of the effective diffusivity in pore networks close to a percolation threshold
Authors: Zhang Lin , Seaton Nigel A.
Published: June 2004

The problem of diffusion in pore networks that are close to a percolation threshold is considered. Such networks arise: if the network is poorly connected; if the diffusing molecules are of a comparable size to the pores so that the fraction of the network accessible to the molecules is close to the percolation threshold; and in solids with multimodal pore‐size distributions in which the pores belonging to the largest mode form a network close to the percolation threshold. We have investigated diffusion in such networks, using a new method in which the mass balance equations for diffusion on a regular lattice are solved using a Monte Carlo approach, coupled with a renormalized effective medium approximation. The method is accurate both close to and far from the percolation threshold, and is fast enough for routine calculations, such as in catalyst design applications.
Chemically active liquid membranes in inorganic supports for metal ion separations
Authors: Yi Jongheop , Tavlarides Lawrence L.
Published: June 2004

Ceramic membranes with pores filled with organic chelation acid show promise as the basis for inorganic membrane separator units for metal ion separations from dilute aqueous streams. The performance of such membranes in disk geometry is evaluated experimentally for a model system. The proposed model describes the metal ion flux through the membrane, and a batch or continuous operated separator using these membranes is analyzed. Copper ion fluxes through α‐alumina/silica disks impregnated with 2‐hydroxy‐5‐nonylacetophenone oxime were determined from a rotating diffusion cell apparatus for feed solution concentration of copper 0.03 mol/L, feed solution pH 3, strip solution pH of 0.3, and rotation speed of 130 rpm. Flux values range from 5.4 × 10−6 to 2.2 × 10−5 mol/cm2·h and compare well with the reported values for metal ion transport through polymeric supporting membranes (Largman and Sifniades, 1978; Babcock et al., 1981; Teramoto and Tanimoto, 1983; Kojima and Miyauchi, 1981).

The proposed steady‐state flux model describes the metal flux through chemically active liquid membranes in inorganic supports. The model includes film resistance to mass transfer in both liquid films, complexation and stripping reactions at the liquid‐liquid interface in the membrane pores on both surfaces of the membrane, and metal ion/complex diffusion through the membrane. Separately designed experiments were executed using a precise pH‐static technique to obtain the physical and chemical parameters of diffusivities, kinetic coefficients and equilibrium coefficients. Calculated fluxes compare well with experimentally determined values from batch experiments (average standard error, 11%).

The proposed lumped parameter linear kinetic rate model for the surface reaction rates interprets the kinetic data for the copper chelation reaction. The model is based on a multistep reaction mechanism and has the following form for the extraction reaction for the above mentioned conditions:

A similar expression is presented for the strip reaction.

An analytical solution for the mathematical model predicts the performance of the chemically active liquid membrane reactor for the batch or continuous operation of the rotating diffusion cell. Calculated results describe expected trends and agree well with the experimental results.
Brownian diffusion of hard spheres at finite concentrations
Authors: Selim M. Sami , Al‐Naafa M. A. , Jones M. C.
Published: June 2004

We investigated the effect of concentration on the Brownian diffusion of uncharged rigid spheres. Monosize silica spheres were prepared according to the method of Stöber (1968). The particles were sterically stabilized by chemisorption of stearic alcohol at their surface by the method developed by van Helden (1981).Particle radius was 14.5 nm from electron micrographs of the coated particles. Osmotic pressure measurements of the sterically stabilized particles dispersed in cyclohexane showed that the particles behaved as hard spheres. The measurements agreed well with predictions from the Carnahan‐Starling equation over the concentration range 0.0458 < ϕ < 0.37 where ϕ is the volume fraction of the particles in the suspension. Viscosity measurements of silica dispersions were made over the concentration range 0 < ϕ < 0.25. The relative viscosity over the range 0 < ϕ < 0.2 was fitted by ηr = 1 + 2.4ϕ + 7.1ϕ2. The coefficients 2.4 and 7.1 in this equation are in good agreement with the theoretical values of 2.5 and 6.2 obtained by Einstein(1906) and Batchelor(1977), respectively. The Brownian diffusion coefficient of the particles dispersed in cyclohexane was measured over the concentration range 0.0055 < ϕ < 0.248 using Taylor's hydrodynamic stability method. A laser fiber‐optic system was used to measure the transient concentration profile along the capillary as indicated by a fluorescent dye. This technique offers the advantage of being direct and nonintrusive. The experimental diffusivity data were found to be well described by the generalized Stokes‐Einstein equation(Batchelor, 1976) over the entire concentration range studied.
Polymeric materials formed by precipitation with a compressed fluid antisolvent
Authors: Dixon David J. , Johnston Keith P. , Bodmeier Roland A.
Published: June 2004

Polymer microspheres and fibers are formed with a versatile new process, precipitation with a compressed fluid antisolvent. By spraying a 1 wt. % polystyrene in toluene solution into CO2 through a 100‐μm nozzle, microspheres are formed with diameters from 0.1 to 20 μm as the CO2 density decreases from 0.86 to 0.13 g/cm3. The uniform submicron spheres produced at high CO2 density are due in part to the rapid atomization produced by the large intertial and low interfacial forces. Fibers, with and without microporosity, are obtained at higher polymer concentrations where viscous forces stabilize the jet. The effect of CO2 density and temperature on the size, morphology and porosity of the resulting polymeric materials is explained in terms of the phase behavior, spray characteristics, and the depression in the glass transition temperature.
Stability of multislit devolatilization of polymers
Authors: Ianniruberto Giovanni , Maffettone Pier Luca , Astarita Gianni
Published: June 2004

Slit devolatilization of polymers has been considered in a recent work by Maffettone et al. (1991), who have presented a successful model for single slit devolatilization. In this article, the stability of operation of a realistic multislit process is considered. It is shown that the flow rate per slit which can be stably operated may be significantly less than the optimal one for single slit operation. Stability improves as the concentration of volatiles in the feed solution decreases. The non‐Newtonian character of the feed solution at high shear rates is discussed in qualitative terms.
2‐Chlorophenol oxidation in supercritical water: Global kinetics and reaction products
Authors: Li Ruokang , Savage Phillip E. , Szmukler David
Published: June 2004

2‐Chlorophenol (2CP) was oxidized in near‐critical and supercritical water in a high‐pressure plug‐flow reactor. The global kinetics for 2CP disappearance were described by a rate law that was 0.88±0.06 order in 2CP, 0.41±0.12 order in O2, and 0.34±0.17 order in water. The activation energy was 11.0±3.8 kcal/mol, and the Arrhenius pre‐exponential factor was 102.0±1.2 M−0.63 S−1. The uncertainties represent 95% confidence intervals. The products of 2CP oxidation included CO, CO2, HCl, other chlorophenols, chlorohydroxybenzaldehydes, dichlorophenoxyphenols, dichlorobiphenols, and chlorinated dibenzodioxins and dibenzofuran. The molar yields of the organic products were determined for a set of experiments at 380°C and 278 atm. The most abundant products were 2CP dimers such as dichlorophenoxyphenols and dichlorobiphenols, and the highest yield observed for any individual product was 0.6%. Although the yields of these products were low, their selectivities were high. For example, at 3.6 s, the shortest residence time studied under these conditions, about 50% of the carbon in the 2CP that reacted appeared in 2CP dimers, 18% appeared as CO2, and the balance (32%) was presumably in single‐ring and ring‐opening products. A reaction pathway analysis using the Delplot methodology revealed that the evolution of products from 2CP oxidation in supercritical water was consistent with a reaction network comprising two parallel primary reactions. One primary reaction path led to dichlorophenoxyphenols and dichlorobiphenols whereas the second primary reaction led to single‐ring and ring‐opening products. The 2CP dimers were convereted to single‐ring and ring‐opening products, which were, in turn, ultimately oxidized to CO2.